Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session J1: Focus Session: Advances in Scanned Probe Microscopy II: High Frequencies & Optical Techniques
Sponsoring Units: GIMSChair: David Goldhaber-Gordon, Stanford University
Room: 203
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J1.00001: Towards Magnetic Resonance Imaging of Semiconducting and Biological Nanostructures D.P. Weber, Fei Xue, P. Peddibhotla, M. Poggio In recent years a technique combining nuclear magnetic resonance (NMR) and sensitive force microscopy has emerged as a viable method for doing magnetic resonance imaging (MRI) on the nanometer scale [1]. This method, known as magnetic resonance force microscopy (MRFM), has the potential to create three-dimensional (3D), non-destructive, sub-surface images of the density of particular nuclear magnetic moments with isotopic contrast. Resolution better than $10\,nm$ has been achieved with ${ }^1\mbox{H}$ in a single virus particle [2]. Here we discuss the application of this technique to nanobiological samples, such as viruses, small bacteria, or cell membranes, and to various semiconductor nanostructures including quantum wells (QWs) and nanowires (NWs). In particular, we focus on the sample preparation challenges presented by these samples. Transfer and attachment of these sub-micrometer samples to our micrometer-sized force sensor includes the use of a focused ion beam (FIB) technique and manual micromanipulators used together with optical microscopy.\\[4pt] [1] Nanotechnology 21, 342001 (2010). \newline [2] Proc. Nat. Acad. Sci. U.S.A. 106, 1313 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J1.00002: Nanoscale MRI of electron spins at millikelvin temperatures Geert Wijts, Andrea Vinante, Oleksandr Usenko, Arthur Den Haan, Laurens Schinkelshoek, Tjerk Oosterkamp Magnetic Resonance Imaging by Atomic Force Microscopy (MRI-AFM) combines the non-destructive subsurface sensitivity of an MRI scanner with the high spatial resolution of an Atomic Force Microscope. It is a powerful technique to detect a small number of spins that relies on force detection by an ultrasoft, magnetically tipped cantilever and selective magnetic resonance manipulation of the spins. In order to minimize the thermomechanical noise of the cantilever and to increase spin polarization, MRI-AFM should be carried out at ultralow temperatures. Therefore, we developed a SQUID-based detection technique, which avoids heating of the cantilever and the spin sample. Using this technique, we demonstrate the manipulation and detection of dangling bond paramagnetic centers on a silicon surface at temperatures as low as 30 millikelvin. The fluctuations of these unpaired electron spins are supposedly linked to $1/f$ magnetic noise and decoherence in SQUIDs as well as in several superconducting and single spin qubits. We find evidence that spin diffusion plays a key role in the dynamics of spins at low temperatures. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J1.00003: ESR induced anomalous polarization in Magnetic Resonance Force Microscopy Lei Chen, Eric Moore, Jonilyn Longenecker, John Marohn Mechanically detecting electron spin resonance has opened up new avenues of performing magnetic resonance detection and imaging to an individual spin-labeled macromolecule. The large gradient field from the magnetic tipped cantilever creates selective resonance conditions for each spin label in the macromolecule. The detection is made through the shifts in the cantilever self-oscillating frequency due to the back action on to the cantilever from the resonating spin polarization. In order to improving the detection sensitivity, great efforts have been made to transfer polarization of electron spins to nearby nuclear spins. Here, we reported an anomalous frequency shift in our mechanically detected ESR experiment. This ESR induced anomalous frequency shift, however is larger in amplitude and slower in relaxation time than ESR frequency shift. We will discuss that this anomalous polarization are potentially due to the dynamic nuclear polarization (DNP) mechanism. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J1.00004: Nanomechanical detection of nuclear magnetic resonance using a silicon nanowire oscillator Invited Speaker: John Nichol ``Bottom-up'' nanomechanical devices such as nanowires, nanotubes, and grapheme oscillators have previously been proposed as next-generation scanning probe force and mass sensors because of their potential for ultralow mechanical dissipation. Here, we report the use of a radio frequency silicon nanowire mechanical oscillator as a nuclear magnetic resonance force sensor to detect the statistical polarization of $^{1}$H spins in polystyrene. Using a specialized scanning probe microscope, as well as a polarization-enhanced fiber-optic interferometer, we operate the nanowire as a force sensor at cryogenic temperatures. Nanowires of the type we study have very low intrinsic dissipation, and they experience negligible increase in dissipation as close as 10 nm from a surface. In order to couple the $^{1}$H spins to the nanowire oscillator, we have developed a magnetic resonance force detection protocol which utilizes a nanoscale current-carrying wire to produce large time-dependent magnetic field gradients as well as the rf magnetic field. Under operating conditions, the nanowire exhibited an ultralow force noise of 1.9~aN$^{2}$/Hz in the measurement quadrature. Further progress toward nanometer scale magnetic resonance imaging using this technique is discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J1.00005: Mechanism for Edge-enhanced Optical Response of Tip Induced Plasmonic Emission Guo Li, Liuguo Chen, Chao Zhang, Zhenchao Dong, Zhenyu Zhang In the investigation of tip induced plasmonic emission supplemented by STM, molecular layers are always viewed as spacers suppressing the induced emission. However, our nanoscale photo mapping of H$_{2}$TBPP/Ag(111) strikingly showed enhanced emission at the molecule-island edge, which is even remarkable higher than that on the bare Ag surface. To understand this intriguing phenomenon, DFT calculations have been carried out. We found that when an organic molecule is absorbed on Ag surface, its HOMO couples with the Ag states around the Fermi level, and forms an inelastic tunneling channel. The involved states of this channel accumulate in the interface and around the phenyl groups. Therefore, when the STM tip locates at the edge of the molecular island, the proportion of the inelastic tunneling current increases and the photo emission is enhanced. We also found that this edge-enhanced photo response is generic; the investigations of other molecule/metal systems demonstrate similar results. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J1.00006: Adiabatic Plasmon Nanofocusing for Ultrashort Pulses and Spectroscopy Joanna Atkin, Samuel Berweger, Robert Olmon, Xiaoji Xu, Markus Raschke The simultaneous control of optical fields on both nanometer spatial and femtosecond time scales would enable direct spectroscopic access to the elementary electronic and vibrational excitations in matter. Here, we utilize adiabatic surface plasmon polariton (SPP) nanofocusing on free-standing 3D tapered metal tips in order to generate nanometer confined field localization at the tip apex. Using the second harmonic generation (SHG) at the tip apex we perform MIIPS pulse optimization and frequency-resolved optical gating (FROG) characterization of the nanofocused pulses. With the combination of high bandwidth coupling using a chirped grating, pulse-shaping, and low-dispersion nanofocusing, we can achieve full optical control on the nanoscale, from $<$ 16 fs pulse duration to arbitrary optical waveforms. This technique enables linear and non-linear plasmon-enhanced spectroscopy, with the simultaneous temporal control over ultrashort pulses opening the possibility for true time-resolved scanning-probe imaging. We demonstrate this capability for background-free probing of individual molecular and nanocrystalline systems. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J1.00007: Near-Field Enhanced UV Resonance Raman Spectroscopy Using an Aluminum Bow-tie Nano-antenna Ling Li, Shuang Fang Lim, Robert Riehn, Hans Hallen An aluminum bow-tie nano-antenna is combined with the resonance Raman effect in the deep UV to dramatically increase the sensitivity of Raman spectra to a small volume of material, such as benzene used here. By carefully choosing the right geometric parameters for the nano-antenna, we achieved a gain of a half million in signal intensity from the near field enhancement due to the surface plasmon resonance in the aluminum nanostructure. The on-line resonance enhancement contributes another factor of several thousands, limited by the laser line width. Thus, an overall gain of billions is achieved. We also demonstrated that the strong electric field gradients inside the bow-tie gap induce gradient-field Raman peaks for several strong IR modes. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J1.00008: Multimodal Imaging of Heterogeneous Materials Wei Liu, Jianyong Yang, Thomas Dieng, Ute Schmidt New materials for highly specified applicaltions can either have mono-atomic flat surfaces or a roughness of several hundred micro- or millimeter. In the past two decades, AFM was the main techniques used to characterize the morphology of nano-materials. On the other hand, Raman spectroscopy is known to be used to unequivocally determine the chemical composition of a material. By combining Raman spectroscopy with high resolution confocal optical microscopy, the analyzed material volume can be reduced below 0.02 $\mu $m$^{3}$, thus leading to the ability to acquire Raman images with diffraction limited resolution. The combination of confocal Raman microscopy with Atomic Force Microscopy (AFM) is a breakthrough in microscopy. Using such a combination, the high spatial and topographical resolution obtained with an AFM can be directly linked to the chemical information provided by confocal Raman microscopy. True Surface Microscopy, allows confocal Raman imaging guided by the surface topography obtained by an integrated non-contact optical profilometer. Large-area topographic coordinates from the chromatic confocal profilometer can be precisely correlated with the large area confocal Raman imaging data. This allows true surface Raman imaging on heavily inclined or rough surfaces, with the sample surface held in constant focus, while maintaining highest confocality. In summary, the combination of confocal Raman microscopy with AFM and true surface microscopy allows the characterization of materials at high, submicron resolution, as well as on mm-rough surfaces across large areas. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J1.00009: Local 2D-2D tunneling in high mobility electron systems Matthew Pelliccione, Adam Sciambi, John Bartel, David Goldhaber-Gordon, Loren Pfeiffer, Ken West, Michael Lilly, Seth Bank, Arthur Gossard Many scanning probe techniques have been utilized in recent years to measure local properties of high mobility two-dimensional (2D) electron systems in GaAs. However, most techniques lack the ability to tunnel into the buried 2D system and measure local spectroscopic information. We report scanning gate measurements on a bilayer GaAs/AlGaAs heterostructure that allows for a local modulation of tunneling between two 2D electron layers. We call this technique Virtual Scanning Tunneling Microscopy (VSTM) [1,2] as the influence of the scanning gate is analogous to an STM tip, except at a GaAs/AlGaAs interface instead of a surface. We will discuss the spectroscopic capabilities of the technique, and show preliminary results of measurements on a high mobility 2D electron system.\newline [1] A. Sciambi, M. Pelliccione \textit{et al.}, Appl. Phys. Lett. \textbf{97}, 132103 (2010).\newline [2] A. Sciambi, M. Pelliccione \textit{et al.}, Phys. Rev. B \textbf{84}, 085301 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J1.00010: Infrared Phonon Fingerprinting of Nanocrystals through Broadband Near-Field Spectroscopy Alexander S. McLeod, Gerardo Dominguez, Priscilla Kelly, Mark Thiemens, Lingfeng M. Zhang, Alex Rodin, Michael M. Fogler, Fritz Keilmann, D.N. Basov Near-field infrared spectroscopy has recently been demonstrated with the capability to resolve optical properties of sub-wavelength sample areas across a broad range of infrared frequencies. This method holds promise for the direct identification of sub-wavelength chemical composition in nanostructured and heterogeneous samples. We apply this technique to the study of phonon-resonant silicon carbide nanocrystals tens of nanometers in size using an apertureless scanning near-field optical microscope (SNOM) coupled to a pulsed broadband infrared laser source and FTIR spectrometer. We present measurements of nanocrystal near-field spectra in the range of 700-1200 cm$^{-1}$ evaluated in comparison with the near-field spectra of bulk silicon carbide, calibrated using ellipsometry. A detailed analytic model of the probe-sample near-field interaction is applied for the identification of nanoscale resonant size effects. These techniques provide a powerful method for identifying and characterizing sub-wavelength nanocrystals in heterogeneous samples via near-field infrared ``phonon fingerprinting.'' [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J1.00011: Creation of Electron Trap States in Silicon Dioxide By Local Electron Injection Dustin Winslow, Clayton Williams Over a decade ago, the Scanning Tunneling Microscope was shown capable of desorbing single hydrogen atoms from the surface of hydrogen terminated silicon.\footnote{T.C. Shen et. al. Science 268, 1590 (1995).} The resultant dangling bonds can act as atomic scale quantum dots.\footnote{M. B Haider et. al. PRL 102, 046805 (2009).} Electrons trapped in such dangling bond states at the surface of crystalline silicon have short retention times at room temperature, due to the proximity of the occupied state energy level to the conduction band. Here we report on a method for creating electron trap states at the surface of a silicon dioxide film by electron injection from a metalized Atomic Force Microscope probe tip. Single Electron Tunneling Force measurements\footnote{E. Bussmann, et. al. Appl. Phys. Lett., 85, 13 (2004).} are employed to examine the existence of trap states in the silicon dioxide surface before and after the electron injection. Evidence for electron trap state creation, without topographic modification of the silicon dioxide surface, will be presented. The trap states created by this process have electron retention times which are greater than one second at room temperature. The methodology for trap state creation and detection will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J1.00012: Nm-scale Mapping of Thermally-Activated Trap Emission in an AlGaN/GaN High Electron Mobility Transistor D. Cardwell, A.R. Arehart, S.A. Ringel, J.P. Pelz AlGaN/GaN high electron mobility transistors (HEMTs) are intrinsically ideal for high frequency and high power applications, but have degraded performance due to charge trapping. Nm-scale AFM-based electrical measurements sensitive to the emission of trapped charge, such as scanning Kelvin probe microscopy (SKPM), can, in principle, be used to determine the energies, cross sections, and densities of electrically-active traps with high spatial resolution. Using SKPM, we obtain nm-scale surface potential transient (SPT) maps over the entire surface of an AlGaN/GaN HEMT. Surprisingly, we find significant SPTs near the edges of the device that are similar to conductance transients, and whose time constants vary with temperature, indicating thermally-activated emission. Comparison of nm-scale measurements and electrostatic simulations will be discussed to quantify the spatial distribution of trapped charge near the edge of the device. Work supported by ONR-DRIFT (P. Maki). [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J1.00013: Stochastic switching of microcantilever motion Warner Venstra, Hidde Westra, Herre van der Zant Fluctuation-induced transitions between two stable states of a strongly driven microcantilever are studied. Intrinsic geometric and inertial nonlinearities of the cantilever give rise to an amplitude-dependent resonance frequency, and at a critical point bifurcation occurs. Two states are stable, represented by vibration at a low and a high amplitude. Adding noise facilitates transitions between the states. The transition rate rises upon increasing noise intensity, as expected for noise-activated escape from a dynamic double well. Further increasing the noise intensity causes a parametric change in the dynamic double well, and results in a decay of the switching rate. Close to the onset of spontaneous transitions, the bistable cantilever is very sensitive. We demonstrate the noise-enhanced detection of weak modulations of the bistable cantilever, resulting in an optimum signal-to-noise ratio at non-minimum noise intensity. [Preview Abstract] |
Session J2: Invited Session: K-12 Science Education: Closing the Gap with the Leading Nations
Sponsoring Units: FEdChair: Paul Cottle, Florida State University
Room: 204AB
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J2.00001: A Framework for K-12 Science Education Invited Speaker: Helen Quinn I will discuss the content and purpose of the NRC study report ``a framework for k-12 science education'' and report on the ongoing project led by Achieve, in which over 20 states are engaged to develop science standards based on this document. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J2.00002: The Globalization of Science Education Invited Speaker: George DeBoer Standards-based science education, with its emphasis on clearly stated goals, performance monitoring, and accountability, is rapidly becoming a key part of how science education is being viewed around the world. Standards-based testing within countries is being used to determine the effectiveness of a country's educational system, and international testing programs such as PISA and TIMSS enable countries to compare their students to a common standard and to each other. The raising of standards and the competition among countries is driven in part by a belief that economic success depends on a citizenry that is knowledgeable about science and technology. In this talk, I consider the question of whether it is prudent to begin conversations about what an international standards document for global citizenship in science education might look like. I examine current practices to show the areas of international agreement and the significant differences that still exist, and I conclude with a recommendation that such conversations should begin, with the goal of laying out the knowledge and competencies that international citizens should have that also gives space to individual countries to pursue goals that are unique to their own setting. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J2.00003: Physics For All -- yes, it's real physics Invited Speaker: Arthur Eisenkraft Can ``all'' students learn ``real'' physics? We want to provide instruction to a wider segment of the population including those students who will not go into STEM (Science, Technology, Engineering, Math) careers. We also want to maintain the integrity of physics, challenge all students, and not compromise the rigor of our courses. Accomplishing this requires a research guided approach to instruction, curriculum and assessment. Physics First and Physics for All have become a success story for thousands of students in urban, suburban, and rural districts. At the same time, the International Physics Olympiad and other competitions have raised the expectation of what the most motivated students can achieve. Many physics educators are exploring ways to set higher goals for our most gifted students while also providing physics instruction to students previously excluded from our physics classes. Many of the same issues that K-12 educators are struggling with are equally important to the college community as colleges try to educate both future physicists and an educated citizenry. Great novels and symphonies are accessible to people of different backgrounds and levels of expertise. We should develop strategies that enable us to share an understanding of physics with all students because everyone deserves an opportunity to reflect on the wondrous workings of our universe. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J2.00004: An Introduction to the new AP Physics algebra-based program: A new focus on best practices Invited Speaker: Gay Stewart Advanced Placement (AP) credit was always designed to represent good college courses. After a call from the NRC, the College Board undertook a redesign of the AP Science courses to improve the quality of teaching and learning in the nation's high schools, modeling best practices at the college level. The Physics Redesign has focused on the AP Physics B course, the equivalent of the algebra-based introductory college physics course. This talk will focus on the background to this undertaking, the process that was followed, and the resulting courses. The impact these changes will have on current teaching practices will be discussed. Currently, Physics B is supposed to follow a preparatory course. Now, the material is divided up and deepened to make each year a stand-alone, rigorous, conceptual and problem-solving course. The significantly deeper conceptual level for the newly designed course allows teachers more time for inquiry-based, student-centered learning. ~ Because of the two-course design, the first year will be accessible to more students. These can be placed flexibly into a school's curriculum; examples will be discussed. Examples from the new curriculum framework for these courses will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J2.00005: The Role of Pre-College Preparation in College Physics Success Invited Speaker: Philip Sadler There is no shortage of, often contradictory, opinions concerning how best to prepare students for college physics, but personal experience is a poor substitute for rigorous investigation. After collecting data from 11,000 science students and their instructors at randomly chosen colleges and universities, we can offer a more universal picture of the middle school to college learning progression in physics. Using epidemiological methods to mine the backgrounds of students taking introductory physics courses, we find predictors of performance and persistence while controlling for demographic differences. I will report on our findings on the value of middle school physical science preparation, lab experiences, technology use, classroom demonstrations, coverage, block scheduling, Advanced Placement, Physics First, project work, and facility in mathematics. [Preview Abstract] |
Session J3: Invited Session: Quantum Computing with Superconducting Circuits
Sponsoring Units: GQI DCMPChair: John Martinis, University of California, Santa Barbara
Room: 205AB
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J3.00001: Continuous High-Fidelity Monitoring of a Superconducting Qubit: From Quantum Jumps to Feedback Invited Speaker: I. Siddiqi Great advances have been made in superconducting qubit technology since the first demonstration of coherent oscillations more than 10 years ago. Continuous, high-fidelity monitoring of the qubit state, however, has remained an elusive target. We demonstrate this functionality by using a wide-bandwidth superconducting parametric amplifier operating near the quantum noise limit to read out the state of a transmon qubit coupled to a linear resonant cavity. Depending on the qubit-cavity detuning and the cavity photon occupation, two different measurement regimes can be accessed. In the strong measurement regime, the qubit states are fully resolved in a time much shorter than $T_{1}$. This permits the observation of quantum jumps between qubit states in real time, and enables the study of the quantum Zeno effect and measurement non-idealities. In the weak measurement regime, information is extracted slowly and the measurement is no longer projective on short time scales. However, the measurement record is still highly correlated with the qubit dynamics and can be used to steer the qubit state using feedback. We demonstrate this idea by phase-locking Rabi oscillations to a classical reference. This allows the oscillations to persist indefinitely, albeit with a reduced amplitude indicative of the efficiency of the feedback protocol. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J3.00002: Observation of the dynamical Casimir effect in a superconducting circuit Invited Speaker: Christopher Wilson Modern quantum theory predicts that the vacuum of space is not empty, but instead teeming with virtual particles flitting in and out of existence. While initially a curiosity, it was quickly realized that these vacuum fluctuations had measurable consequences, for instance producing the Lamb shift of atomic spectra and modifying the magnetic moment for the electron. This type of renormalization due to vacuum fluctuations is now central to our understanding of nature. 40 years ago, Moore suggested that a mirror undergoing relativistic motion could convert virtual photons into directly observable real photons. This effect was later named the dynamical Casimir effect (DCE). Using a superconducting circuit, we have observed the DCE for the first time. The circuit consists of a coplanar transmission line with an \textit{electrical length} that can be changed at a substantial fraction of the speed of light. The length is changed by modulating the inductance of a superconducting quantum interference device (SQUID) at high frequencies ($> 10$ GHz). In addition to observing the creation of real photons, we observe two-mode squeezing of the emitted radiation, which is a signature of the quantum character of the generation process. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J3.00003: Towards a scalable superconducting qubit architecture Invited Speaker: Matthias Steffen I will review IBM's current approach towards building a scalable superconducting qubit architecture. The goal is to build a system using quantum error correction schemes based on two-dimensional surface codes, which are predicted to have a remarkably low fault tolerant threshold. I will briefly outline the particular skew-square lattice configuration and describe the concept of logical qubits and gates. On the experimental side, I will show recent advances towards implementing such surface code. This includes improvements in qubit coherence times, now exceeding T1=5us reliably, achieved by shielding the sample from infrared radiation confirming other recent results. Additional approaches leading to even longer coherence times are reviewed. With improved coherence times we show improved gate fidelities of two-qubit gates based the cross-resonance effect -- an all microwave approach towards implementing two-qubit gates. I will conclude by reviewing some interesting future engineering challenges that should be addressed on our way towards building a quantum computer. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J3.00004: The Photon Shell Game and the Quantum von Neumann Architecture with Superconducting Circuits Invited Speaker: Matteo Mariantoni Superconducting quantum circuits have made significant advances over the past decade, allowing more complex and integrated circuits that perform with good fidelity. We have recently implemented a machine comprising seven quantum channels, with three superconducting resonators, two phase qubits, and two zeroing registers. I will explain the design and operation of this machine, first showing how a single microwave photon $| 1 \rangle$ can be prepared in one resonator and coherently transferred between the three resonators. I will also show how more exotic states such as double photon states $| 2 \rangle$ and superposition states $| 0 \rangle + | 1 \rangle$ can be shuffled among the resonators as well [1]. I will then demonstrate how this machine can be used as the quantum-mechanical analog of the von Neumann computer architecture, which for a classical computer comprises a central processing unit and a memory holding both instructions and data. The quantum version comprises a quantum central processing unit (quCPU) that exchanges data with a quantum random-access memory (quRAM) integrated on one chip, with instructions stored on a classical computer. I will also present a proof-of-concept demonstration of a code that involves all seven quantum elements: (1), Preparing an entangled state in the quCPU, (2), writing it to the quRAM, (3), preparing a second state in the quCPU, (4), zeroing it, and, (5), reading out the first state stored in the quRAM [2]. Finally, I will demonstrate that the quantum von Neumann machine provides one unit cell of a two-dimensional qubit-resonator array that can be used for surface code quantum computing. This will allow the realization of a scalable, fault-tolerant quantum processor with the most forgiving error rates to date. \\[4pt] [1] M. Mariantoni \textit{et al.}, Nature Physics \textbf{7}, 287-293 (2011.)\\[0pt] [2] M. Mariantoni \textit{et al.}, Science \textbf{334}, 61-65 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J3.00005: Realization of Three-Qubit Quantum Error Correction with Superconducting Circuits Invited Speaker: Matthew Reed Quantum computers promise to solve certain problems exponentially faster than possible classically but are challenging to build because of their increased susceptibility to errors. Remarkably, however, it is possible to detect and correct errors without destroying coherence by using quantum error correcting codes. The simplest of these are the three-qubit codes, which map a one-qubit state to an entangled three-qubit state and can correct any single phase-flip or bit-flip error of one of the three qubits, depending on the code used [1]. The fidelity of a process in which errors can occur on all qubits, where there is the possibility of an uncorrectable double or triple error, should therefore decrease only quadratically with error probability. I will first introduce how the three-qubit encoded state can be produced in our superconducting architecture by employing interactions with non-computational qubit states, as previously demonstrated [2]. I will then discuss how these non-computational interactions can be generalized to produce a novel three-qubit conditional-conditional NOT (CCNot) or Toffoli gate, which implements the correcting step of an error correction algorithm. Finally, I will explain how, by combining these ingredients, we have performed a single pass of both quantum bit- and phase-flip error correction and have demonstrated the predicted first-order insensitivity to errors.\\[4pt] [1] M. A. Nielsen and I. L. Chuang, Cambridge University Press, 2000.\\[0pt] [2] L. DiCarlo, et al. Nature 467, 574 (2010). [Preview Abstract] |
Session J4: Non-Equilibrium Dynamics with Quantum Gases
Sponsoring Units: DAMOPChair: Rajdeep Sensarma, University of Maryland
Room: 205C
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J4.00001: Non Equilibrium Quantum Criticality: an intuitive approach Emanuele Dalla Torre, Eugene Demler, Thierry Giamarchi, Ehud Altman Since their discovery in 1976, equilibrium quantum critical points have attracted continuous interest, due to their universality (i.e. the independence from the microscopic details of the systems). In two recent papers [1,2] we have extended these concepts to non-equilibrium systems, by studying the universal properties of quantum systems driven by time-dependent noise. We were able to demonstrated that [1] they can show a new class of non-equilibrium quantum criticality, and [2] small perturbations around the critical point lead to new physical phenomena, such as the spontaneous generation of an effective temperature and an effective dissipation. To this end, we developed a real-time renormalization group (RG) in the Keldysh path-integral formalism, which may however appear cryptic to the non-experts. In this talk, I will show how the main conclusions of the RG approach can be understood by simpler arguments based on circuit theory and fluctuation-dissipation relations. \\[4pt] [1] E.G. Dalla Torre, et al. ``Quantum critical states and phase transitions in the presence of non-equilibrium noise,'' Nature Physics 6, 806 (2010) \\[0pt] [2] E.G. Dalla Torre, et al. ``Dynamics and universality in noise driven dissipative systems,'' arXiv: 1110.3678 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J4.00002: Dynamics of noise correlations of ultracold bosons in an optical lattice Khan W. Mahmud, Eite Tiesinga We study second order correlations of ultracold bosons in an optical lattice for superfluid and Mott insulating phases. Starting with a superfluid ground state, a sudden increase in the lattice depth projects it into a non-equilibrium state. We examine the subsequent dynamics of the system - analyzing noise correlations of the atomic cloud after time-of-flight expansion. We also investigate the effects of three and higher-body interactions on noise correlations in deep lattices. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J4.00003: Stability of Counterflow Superfluidity Noah Bray-Ali, Carl Williams, Eugene Demler We examine the stability of the counterflow superfluid state in two component mixtures of ultracold atoms in optical lattices. Using a Gutzwiller mean-field approach, we find a sharp boundary separating stable counterflow from a dynamically unstable regime. As the inter-component interaction strength increases, the critical counterflow rate drops, falling to zero when interactions are strong enough to induce phase separation of the two components. Going beyond mean-field theory, we compute the decay rate of counterflow within the stable regime due to phase slips. The results agree well with numerically exact simulations and are calculated in a regime of parameters relevant to current experiments on mixtures of ultracold alkali atoms. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J4.00004: Fermion Dynamics from Gross-Pitaevskii--like Equations Michael Forbes, Rishi Sharma The dynamics of condensed fermions (i.e. the Unitary Fermi Gas) play a key role in understanding a range of physical systems, from dynamics in rotating traps of cold atoms, to explaining pulsar glitches in neutron stars. Density functional theory (\textsc{dft}) provides a powerful tool for modeling these dynamics, but unfortunately, simulating even a few vortices requires the use of leadership class computing. This talk will address the efficacy of using modified Gross-Pitaevskii (\textsc{gp}) like equations to model the dynamics of Fermi systems. These \textsc{gp}-like equations are significantly easier to solve, yet still capture much of the relevant physics. We shall advocate an approach of using fermionic \textsc{dft} to adjust the form of the modified \textsc{gp}-like equations, and then using the latter to model more complicated phenomena beyond the capability of the fermionic \textsc{dft}. The dynamics of vortices pinned on defects will serve as an example. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J4.00005: Expansion of Bose-Hubbard Mott insulators in optical lattices Mark Jreissaty, Juan Carrasquilla, F. Alexander Wolf, Marcos Rigol We present a study of the expansion of bosonic Mott insulators in the presence of an optical lattice after switching off a confining potential. We use the Gutzwiller mean-field approximation and consider two different setups. In the first one, the expansion is restricted to one dimension. We show that this leads to the emergence of two condensates with well-defined momenta, and argue that such a construct can be used to create atom lasers in optical lattices. In the second setup, we study Mott insulators that are allowed to expand in all directions in the lattice. In this case, a simple condensate is seen to develop within the mean-field approximation. However, its constituent bosons are found to populate many nonzero momentum modes. An analytic understanding of both phenomena in terms of the exact dispersion relation in the hard-core limit is presented. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J4.00006: Protection of dissipative quantum state preparation by interlacing the control with dynamical decoupling pulses Z.R. Gong, Wang Yao Various dissipative processes have recently be exploited for preparing quantum state with multipartite entanglement between many qubits. Most such schemes are applicable only to an ensemble of identical qubits, and inhomogeneous broadening will reduce the state preparation fidelity. Here we show that by interlacing the dynamical decoupling pulse sequence with the dissipative state preparation control, the errors resulting from the inhomogeneous broadening can be suppressed up to certain order of the pulse interval and the desired entangled states can be prepared with high fidelity. We give two examples where sequence of pi pulses interlaced with dissipative control realize high fidelity preparation of cluster states and many-body singlets of atomic qubits. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J4.00007: ABSTRACT MOVED TO L4.00008 |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J4.00008: Periodically and almost periodically driven quantum system Luca D'Alessio, Anatoli Polkovnikov When a quantum system is driven periodically in time it can display dynamical localization, i.e its energy grows extremely slowly and may saturate at a value smaller than the infinite temperature limit. We show that by making the period of the perturbation longer this phenomenom is destroyed and the energy of the system grows quickly and saturates at the infinite temperature limit. We argue that this process is related to the breaking down of a particular form of perturbation theory (in the duration in of driving) and can be interpreted as a transition from a local to a long-range effective Hamiltonian. We discuss how robust our findings are against small aperiodicity in the driving. We finally discuss how realize this interesting non-equilibrium physics in cold atom experiments. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J4.00009: Cold bosons in noisy optical lattices Johannes Schachenmayer, Hannes Pichler, Peter Zoller, Andrew Daley Cold atoms in optical lattices open the possibility to experimentally study strongly interacting many-body quantum systems with controllable parameters. A key challenge to prepare interesting quantum states in these systems is to achieve sufficiently low temperatures. At these temperatures a deep theoretical understanding of possible heating processes and how they affect the characteristics of the quantum state becomes essential. In every realistic experiment there exist many sources of noise that cause phase and amplitude fluctuations in the standing laser waves that form the optical lattice potential. This classical noise can lead to heating and a significant change of the quantum state. We study the stochastic many-body non-equilibrium dynamics of bosons in an optical lattice and determine how the state changes depending on the characteristics of the noise. We do this by solving time-dependent stochastic many-body Schr\"odinger equations, both analytically and numerically. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J4.00010: Many-body Landau-Zener Transition in Cold Atom Double Well Optical Lattices Yinyin Qian, Ming Gong, Chuanwei Zhang Ultra-cold atoms in optical lattices provide an ideal platform for exploring many-body physics of a large system arising from the coupling among a series of small identical systems whose few-body dynamics are exactly solvable. Using Landau-Zener (LZ) transition of bosonic atoms in double well optical lattices as an experimentally realizable model, we investigate such few to many body route by exploring the relation and difference between the few-body (in one double well) and many-body (in double well lattice) non-equilibrium dynamics of cold atoms in optical lattices. We find the many-body coupling between double wells greatly enhances the LZ transition probability, while keeping the main features of the few-body dynamics. Various experimental signatures of the many-body LZ transition, including atom density, momentum distribution, and density-density correlation, are obtained. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J4.00011: Quasi steady-states, spin statistics, and interaction-induced transport of ultra-cold atoms in 1D optical lattices Chih-Chun Chien, Michael Zwolak, Massimiliano Di Ventra We consider several non-equilibrium scenarios where ultra-cold atoms are initially loaded into the ground state of a 1D optical lattice. The system is then set out of equilibrium either by inducing a density imbalance or by imposing time-dependent inhomogeneous interactions. To monitor the dynamics, we have implemented the micro-canonical approach to transport [1] which has been previously used to study electron dynamics in nanoscale systems. We have found that by removing particles on the right half of the lattice, fermions form a quasi steady-state current, which can be observed as a plateau in the current as a function of time. In contrast, the bosonic current oscillates and decays to zero in the thermodynamic limit [2]. The difference appears in uniform lattices as well as lattices with a harmonic trap. Further, when light-induced interactions are applied to half of the lattice, we have found, using a Hartree-Fock approximation, a conducting-nonconducting transition in the fermionic case as the interaction increases. Our studies are relevant to recent experiments on transport of ultra-cold atoms and address fundamental issues in nanoscale electronic transport. \\[4pt] [1] Di Ventra and Todorov,J. Phys. Cond. Matt. 16, 8025 (2004).\\[0pt] [2] Chien, Zwolak, Di Ventra, arXiv: 1110.1646. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J4.00012: Spin Caloritronics in Noncondensed Bose Gases Clement Wong, Hedwig van Driel, Rakpong Kittinaradorn, Henk Stoof, Rembert Duine We consider coupled spin and heat transport in a two-component, atomic Bose gas in the noncondensed state. We find that the transport coefficients show a temperature dependence reflecting the bosonic enhancement of scattering, and discuss experimental signatures of the spin-heat coupling in spin accumulation and total dissipation. Inside the critical region of Bose-Einstein condensation, we find anomalous behavior of the transport coefficients, and in particular, an enhancement for the spin caloritronics figure of merit that determines the thermodynamic efficiency of spin-heat conversion. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J4.00013: Universal energy fluctuations in thermally isolated driven systems Guy Bunin, Luca D'Alessio, Yariv Kafri, Anatoli Polkovnikov When an isolated system is brought in contact with a heat bath, its final energy is random and follows the Gibbs distribution--this finding is a cornerstone of statistical physics. The system's energy can also be changed by performing non-adiabatic work using a cyclic process. Almost nothing is known about the resulting energy distribution in this set-up, which is in particular relevant to recent experimental progress in cold atoms, ion traps, superconducting qubits and other systems. Here we show that when the non-adiabatic process consists of many repeated cyclic processes, the resulting energy distribution is universal and different from the Gibbs ensemble. We predict the existence of two qualitatively different regimes with a continuous second-order-like transition between them. We illustrate our approach by performing explicit calculations for both interacting and non-interacting systems. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J4.00014: Lie-algebraic Approach to Dynamics of Closed Quantum Systems and Quantum-to-Classical Correspondence Victor Galitski I will briefly review our recent work on a Lie-algebraic approach to various non-equilibrium quantum-mechanical problems, which has been motivated by continuous experimental advances in the field of cold atoms. First, I will discuss non-equilibrium driven dynamics of a generic closed quantum system. It will be emphasized that mathematically a non-equilibrium Hamiltonian represents a trajectory in a Lie algebra, while the evolution operator is a trajectory in a Lie group generated by the underlying algebra via exponentiation. This turns out to be a constructive statement that establishes, in particular, the fact that classical and quantum unitary evolutions are two sides of the same coin determined uniquely by the same dynamic generators in the group. An equation for these generators - dubbed dual Schr{\"o}dinger-Bloch equation - will be derived and analyzed for a few of specific examples. This non-linear equation allows one to construct new exact non-linear solutions to quantum-dynamical systems. An experimentally-relevant example of a family of exact solutions to the many-body Landau-Zener problem will be presented. One practical application of the latter result includes dynamical means to optimize molecular production rate following a quench across the Feshbach resonance. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J4.00015: Visibility of the Amplitude (Higgs) Mode in Condensed Matter and Cold Atomic Systems Daniel Podolsky, Assa Auerbach, Daniel P. Arovas The amplitude mode is a ubiquitous collective excitation in condensed matter systems with broken continuous symmetry. It is expected in antiferromagnets, short coherence length superconductors, charge density waves, and lattice Bose condensates. Its detection is a valuable test of the corresponding field theory, and its mass gap measures the proximity to a quantum critical point. However, since the amplitude mode can decay into low energy Goldstone modes, its experimental visibility has been questioned. Here we show that the visibility depends on the symmetry of the measured susceptibility. We discuss various experimental setups for measuring the scalar susceptibility. We show that the optical conductivity of the O(2) theory (relativistic superfluid) displays a threshold behavior at the Higgs mass. [Preview Abstract] |
Session J5: Focus Session: Interfaces in Complex Oxides - Electronic Structure
Sponsoring Units: DMPChair: Andres Felipe Santander-Syro, Universite Paris-Sud 11
Room: 206A
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J5.00001: Angle resolved photoemission spectroscopy of the electronic structure of complex oxide interfaces Invited Speaker: Kyle Shen Engineering interfaces between complex oxides has proven to be a powerful technique for tuning their electronic and magnetic properties. To fully understand how these interfaces can control these electronic properties, one requires advanced spectroscopic tools to uncover their electronic structure. Angle-resolved photoemission spectroscopy (ARPES) is the leading tool for probing energy and momentum resolved electronic structure. To understand the physics of these complex oxide interfaces, we have developed an approach which combines state-of-the-art oxide molecular beam epitaxy with high-resolution ARPES. As one example, we describe our work on oxide superlattices ([LaMnO$_{3}$]$_{2n}$ / [SrMnO$_{3}$]$_{n}$) with alternating LaMnO$_{3}$ and SrMnO$_{3}$ blocks. Our ARPES measurements reveal that as a function of separation between the LaMnO$_{3}$-SrMnO$_{3}$ interfaces, the interfacial quasiparticle states evolve from a quasi-three-dimensional ferromagnetic metal, to a two-dimensional spin-polarized electron liquid, and ultimately to a pseudogapped ferromagnetic insulator with increasing superlattice thickness, n. I will also describe our work on other oxide thin films SrTiO$_{3}$ and Sr$_{2}$TiO$_{4}$-based interfaces where the quasiparticle interactions can be tuned as a function of dimensionality, and work on cuprate and ruthenate thin films. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J5.00002: Spectral and spatial redistribution at the LaAlO$_{3}$/SrTiO$_{3}$ interface T. Guenter, A. Rubano, T. Fink, D. Paparo, F. Miletto Granozio, U. Scotti di Uccio, L. Marrucci, J. Mannhart, M. Fiebig A conductive two-dimensional electron liquid (2DEL) appears at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface for a LAO thickness of n$\geq$4 unit cells. Despite the tremendous research interest, many questions regarding the origin and characteristics of the 2DEL have to be addressed yet. In particular, this includes the electronic structure of the ``buried interface.'' Optical second harmonic generation (SHG) is an ideal tool for studying interfaces, since it is sensitive to the interfacial symmetry breaking along the stacking direction. Using SHG with frequency-tunable amplified femtosecond laser pulses we obtain information about the structural reorganization of the interfacial STO conduction band for SHG photon energies up to 6.2 eV. A massive spectral weight redistribution is present for n$\geq$3, indicating a global reorganization of the conduction band structure. At low temperatures the spectral resolution is enhanced which alleviates the distinction of interfacial electronic transitions. Additionally, we discuss the modification of the interfacial states by variation of the STO surface termination. Our data are supported by a theoretical framework based on symmetry selection rules that allows a specific assignment of interfacial O(2p)-Ti(3d) transitions to the SHG spectrum. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J5.00003: Electronic properties of LaAlO$_3$/SrTiO$_3$ superlattices: interface charge density, Fermi surfaces, plasmon and optical spectra Se Young Park, Andrew J. Millis We present calculations of the charge density profile, Fermi surface topology, plasmon and optical excitation spectra of the electron gas at the LaAlO$_3$/SrTiO$_3$ interface. The calculations are based on a self-consistent Hartree/RPA approximation and a tight binding parametrization of the band structure. The subband occupancy is determined as a function of polar discontinuity magnitude and dielectric constant. The number of occupied xy bands changes significantly whereas for all reasonable polar discontinuity and dielectric constant profiles only one yz and one xz bands are occupied. These yz and xz band give dominant contributions to the long-distance tail of the interface charge. The plasmon and optical absorption spectra are determined. The results are compared with experimental data. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J5.00004: Direct observations of the band alignment of LaAlO3/SrTiO3 using scanning tunneling microscopy Y.P. Chiu, B.C. Huang, P.C. Huang, V.T. Tra, J.Y. Lin, J.C. Yang, Y.H. Chu In this work, by using cross-sectional scanning tunneling microscopy, local and direct evidence of the electronic information across the LaAlO3/SrTiO3 hetero-interfaces are investigated. A combination of scanning tunneling spectroscopy and analysis with atomic resolution across the hetero-interface reveals how the oppositely charged atomic planes undergo electronic reconstructions and introduce a built-in electric field across the polar LaAlO3 thin films grown on SrTiO3 substrates. With the consideration of the tip-induced band bending effect, the magnitude of the built-in field across LaAlO3, the band bending on SrTiO3 side, and the decay length of the band downshift of SrTiO3 side at the hetero-interface are directly observed. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J5.00005: Multi-orbital subband structure of transition-metal oxide 2DEGs Phil D.C. King, W. Meevasana, H.Y. Hwang, Z.-X. Shen, F. Baumberger We demonstrate the creation and control of two-dimensional electron gases (2DEGs) in the 3d and 5d transition metal oxides SrTiO$_3$ and KTaO$_3$. These 2DEGs, of the form usually generated by interface engineering, are created here at the bare oxide surfaces. This permits their detailed spectroscopic investigation using angle-resolved photoemission (ARPES), and we employ this to directly image the d-orbital subband structure of the 2DEGs. We find that quantum confinement lifts the degeneracy of the bulk band structure, driving orbital ordering of the 2DEG. We measure the resulting ladder of light d$_{xy}$ subbands, which co-exist at lower binding energies with heavy $d_{xz/yz}$-derived states. The electronic structure revealed by ARPES is in quantitative agreement with our model tight-binding calculations. While the strong spin-orbit coupling of KTaO$_3$ promotes substantial orbital mixing, our calculations predict only a small Rashba splitting of the 2DEG states, consistent with our experimental measurements where any spin splitting is too small to be resolved. The polar nature of the KTaO$_3(100)$ surface plays a striking role in mediating formation of the 2DEG as compared to non-polar SrTiO$_3(100)$, reminiscent of the polar catastrophe at LaAlO$_3$/SrTiO$_3$ interfaces. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J5.00006: A Universal Critical Density Underlying the Physics of Electrons at the LaAlO$_{3}$/SrTiO$_{3}$ Interface Arjun Joshua, S. Pecker, J. Ruhman, E. Altman, S. Ilani The two-dimensional electron system formed at the interface between the insulating oxides LaAlO$_{3}$ and SrTiO$_{3}$ exhibits ferromagnetism, superconductivity, and a wide range of unique magnetotransport properties. A key challenge is to find a unified microscopic mechanism that underlies these emergent phenomena. Here we show that a universal Lifshitz transition between d-orbitals lies at the core of the observed transport phenomena in this system. Our measurements find a critical electronic density at which the transport switches from single to multiple carriers. This density has a universal value, independent of the LaAlO$_{3}$ thickness and electron mobility. The characteristics of the transition, its universality, and its compatibility with spectroscopic measurements establish it as a transition between d-orbitals of different symmetries. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed universal transition to a range of reported magnetotransport properties. Interestingly, we also find that the maximum of the superconducting transition temperature occurs at the same critical transition, indicating a possible connection between the two phenomena. Our observations demonstrate that orbital degeneracies play an important role in the fascinating behavior observed so far in these oxides. Ref: arxiv.org/abs/1110.2184 [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J5.00007: Localization of the Two-dimensional Electron Gas in LaAlO$_3$/SrTiO$_3$ Heterostructures Tomas Hernandez, Chung Wung Bark, Chang-Beom Eom, Mark S. Rzchowski We use low temperature magnetotransport measurements to compare the quasi 2-dimensional electron gas (2DEG) at the LaAlO$_3$/SrTiO$_3$ interface in heterostructures grown on (LaAlO$_3$)$_{0.3}$-(Sr$_2$AlTaO$_3$)$_{0.7}$ (LSAT) substrates to the 2DEG at the LaAlO$_3$/single crystal SrTiO$_3$ interface. All heterostructures were grown by pulsed laser deposition with \emph{in-situ} reflection high-energy electron diffraction. For the samples on LSAT, we find that increasing the carrier concentration by growing at lower oxygen partial pressures changes the conductivity mechanism, from strongly localized transport at low carrier concentrations to metallic conductivity with indications of weak localization at higher concentrations. We interpret this as an increasing occupation of Ti 3\emph{d} bands of layers near the interface, changing the spatial extent of the conduction region and its susceptibility to localization by disorder and point defects at the interface. On the other hand, the 2DEG of similarly grown LaAlO$_3$ on single crystal SrTiO$_3$ shows metallic behavior and low temperature measurements display Kohler scaling of the out-of-plane magnetoresistance, consistent with classical orbital transport. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J5.00008: Localized electron states and phase separation at the LaAlO$_3$/SrTiO$_3$ interface S. Satpathy, K.V. Shanavas Even though the 2D electron gas at the polar interfaces of LAO/STO has been studied extensively, an explanation for the observed magnetic centers or the coexistence of magnetism and superconductivity is still lacking. Earlier density-functional calculations have indicated the presence of multi-bands and two types of electrons at the interface [1]. Here we propose that a combination of lattice disorder, octahedral rotations, and Jahn-Teller distortion can lead to some of these electrons to be localized near the interface and form lattice polarons. Evidence for this is presented from detailed density-functional calculations, which indicate that the energy gain associated with JT distortion and impurity or disorder induced local potentials can offset the kinetic energy cost of localization. Our model studies [2] also show the possible existence of a phase separation, thereby providing a natural explanation for the coexistence of magnetism and superconductivity [3].\\[4pt] [1] Z. S. Popovi\'{c}, S. Satpathy and R. M. Martin {\it Phys. Rev. Lett.}, {\bf 101}, 256801 (2008)\\[0pt] [2] B. R. K. Nanda and S. Satpathy {\it Phys. Rev B}, {\bf 83}, 195114 (2011)\\[0pt] [3] L. Li {\it et. al.} {\it Nature Physics}, {\bf 7}, 762 (2011); J. A. Bert {\it et. al.} {\it ibid}, 767 (2011); [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J5.00009: Electronic structure, Fermi surfaces, and electron-phonon coupling in La-doped Sr$_{2}$TiO$_{4}$ and SrTiO$_{3}$ Yuefeng Nie, Shouvik Chatterjee, Bulat Burganov, Eric Monkman, John Harter, Daniel Shai, Che-hui Lee, Darrell Schlom, Kyle Shen Sr$_{2}$TiO$_{4}$ is a quasi-two-dimensional Ruddlesden-Popper structure analogue to SrTiO$_{3}$, and is isostructural with the cuprate parent compound La$_{2}$CuO$_{4}$. Although the electronic structure of SrTiO$_{3}$ has been well-explored due to its importance in oxide electronics, little is known about the electronic properties of Sr$_{2}$TiO$_{4}$. To investigate this, we synthesized epitaxial La doped Sr$_{2}$TiO$_{4}$ and SrTiO$_{3}$ films on (100) LSAT substrates by molecular beam epitaxy (MBE) and investigated the electronic structure using angle-resolved photoemission spectroscopy (ARPES). The electronic structure of 5{\%} La doped Sr$_{2}$TiO$_{4}$ shows a single electron like band with mostly Ti-3d$_{xy}$ character dispersing across the Fermi surface which corresponds well with LDA calculations. This is in contrast to doped SrTiO$_{3}$ where all three t2g bands are degenerate. We also observed signatures of strong electron-phonon coupling in the quasi-two-dimensional Sr$_{2}$TiO$_{4}$ materials which appear to be absent in three-dimensional SrTiO$_{3}$. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J5.00010: Theory of SrTiO3 2DEGs Guru Khalsa, Allan MacDonald There has been much recent interest in oxide 2DEGS - in particular those involving SrTiO3. In spite of the simple conduction band structure of the parent material, there has been no quantitative understanding of the physical properties of these systems. We have built a model, informed by experiment, that is sufficiently realistic to describe defect free surface/interface states in SrTiO3. The model is able to account for non-local dielectric screening and the orbital dependence of quantum confinement effects. Preliminary results of the model will be compared with experiment. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J5.00011: Conducting band structure in LaTiO3/SrTiO3 interfaces Young Jun Chang, Luca Moreschini, Aaron Bostwick, Andrew L. Walter, Karsten Horn, Eli Rotenberg Oxide interfaces between insulating hosts show unexpected conducting carriers, which can be useful for next-generation electronic applications. However, the fundamental understanding of the conducting interfaces remains elusive. Here we report \textit{in situ} angle-resolved photoemission spectroscopy (ARPES) studies in the LaTiO3/SrTiO3 heterostructures, of which layer thicknesses were precisely prepared by pulsed laser deposition in the BL7.0, ALS. We found that the interface generates a high-density electron gas over few unit cells from the junction. We further discuss the orbital characteristics of the interface electronic states with comparison to the recent theoretical calculations. Based on the unit-cell layer resolved electronic structure of the LaTiO3/SrTiO3 interface, we discuss the conducting carriers comparing to the LaAlO3/SrTiO3 interfaces. [Preview Abstract] |
Session J6: Focus Session: van der Waals Bonding in Advanced Materials - Layered Structures & Mechanical Properties
Sponsoring Units: DMPChair: Janice Reutt Robey, University of Maryland
Room: 206B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J6.00001: Improved Description of Soft Layered Materials with van der Waals Density Functional Theory Gabriella Graziano, Jiri Klimes, Felix Fernandez-Alonso, Angelos Michaelides The accurate description of dispersion forces with approaches based on density functional theory has long been a coveted goal and is currently one of the most active areas of research in computational physics/chemistry. We have tested two new functionals, the optimized Becke88 van der Waals (optB88-vdW) and optimized PBE van der Waals (optPBE-vdW) [1, 2] to describe materials where van~der~Waals interactions dominate. Structural (bond length and interlayer distance) and energetic parameters (atomization and interlayer binding energies) of graphite and \textit{hexagonal}-boron nitride have been calculated using these functionals and we show that our calculations are in very good agreement with experiments and higher level theoretical calculations. From these calculations it is possible to conclude that optB88- vdW and optPBE-vdW are promising functionals for the accurate description of systems held together mainly by dispersion forces. \\[4pt] [1] Klime\v{s} \textit{et al.}, J. Phys.: Condens. Matter \textbf{22}, 022201 (2010). \\[0pt] [2] Klime\v{s} \textit{et al.}, Phys. Rev. B \textbf{83}, 195131 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J6.00002: Moir\'e pattern of a single layer MoS$_2$ grown on Cu(111) Duy Le, Talat S. Rahman We present results of first principles calculations of the geometric and electronic structures of a single layer of Molybdenum disulfide (MoS$_2$) on Cu(111) utilizing the van der Waals density functional [1]. The lowest energy Moir\'e structure consists of $(4\times 4)$ MoS$_2$ on $(5\times 5)$ Cu(111), in agreement with experimental observation [2]. Examination of the local density of electronic states and charge redistribution shows that the layer is not purely physisorbed on the surface, rather there exists a chemical interaction between it and the Cu surface atoms. Interestingly the MoS$_2$ film is found to be not appreciably buckled, while the atoms in the top Cu layer gets reorganized and vertically disordered. The sizes of Moir\'e patterns for a single layer of MoS$_2$ adsorbed on several other close packed metal surfaces are estimated by minimizing the lattice mismatch between the film and the substrate. \\[4pt] [1] M. Dion \textit{et al}, Phys. Rev. Lett. \textbf{92}, 246401 (2004) \\[0pt] [2] D. Kim \textit{et al}, Langmuir \textbf{27}, 11650 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J6.00003: Observation of tightly bound charged excitons in monolayer MoS$_{2}$ Keliang He, Kin Fai Mak, Changgu Lee, Jim Hone, Tony Heinz, Jie Shan Recent advances in the development of atomically thin layers of materials have opened up many new research opportunities. In particular, the transition metal-dichalcogenide molybdenum disulfide (MoS$_{2})$ has been shown to cross over from a dark indirect semiconductor to a highly luminescent direct gap material in the limit of monolayer thickness.\footnote{Mak et al. \textit{Phys. Rev. Lett.} \textbf{105}, 136805 (2010); Splendiani et al. \textit{Nano Letters} \textbf{10}, 1271-1275, (2010).} Here we report results of studies of the optical absorption and photoluminescence of a monolayer MoS$_{2}$ field-effect transistor (FET) at 10 K. In the limit of very low doping, the optical properties are dominated by an excitonic feature at $\sim $ 1.9 eV. As the doping density is increased, a new resonance emerges on the low-energy side of the exciton. This feature has been identified as a trion, the bound state of an exciton and an additional electron (or hole). The absorbance and photoluminescence of both the trion and exciton can be tuned by electrostatic doping. A large trion binding energy, exceeding room temperature, is inferred. Our observation can be understood in terms of the dynamical many-body response of a 2D electron gas to the optically created hole and reflects the unusually strong many-body interactions in this 2D system. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J6.00004: Beyond graphene: atomic scale structure of quasi-2D van der Waals materials Christopher Gutierrez, Liuyan Zhao, Kin Fai Mak, Paul Cadden-Zimansky, Dmitri Efetov, Tony F. Heinz, George W. Flynn, Philip Kim, Abhay Pasupathy, Robert J. Cava Graphene, a single atomic layer of graphite, has attracted much attention for its unique electronic and mechanical properties. But what role does reduced dimensionality play in other 2D atomic crystals? In this talk we present scanning tunneling microscopy (STM) measurements of the atomic scale structure and spectroscopy of related quasi-2D materials including the transition metal dichalcogenides (TMDs), and their heterostructures with graphene. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J6.00005: Here to stay: Stability and structure of (6$\sqrt{3}\times$6$\sqrt{3}$)-R30$^\circ$ graphene on SiC(111) by all-electron DFT including van der Waals effects Volker Blum, Lydia Nemec, Patrick Rinke, Matthias Scheffler SiC is a favorite growth substrate for mono- and few-layer graphene by Si sublimation. On the Si side, large ordered graphene areas can be obtained as commensurate (6$\sqrt{3}\times$6$\sqrt{3}$)-R30$^\circ$ (``6$\sqrt{3}$'') periodic films [1]. From a thermodynamic perspective, graphene formation competes with several other phases, including graphite on one side, and several Si-rich reconstructions on the other. The question whether 6$\sqrt{3}$ graphene on SiC(111) is thermodynamically or just kinetically stabilized could be answered by density functional theory (DFT) calculations but for two challenges: (1) The needed surface slab systems are extremely large (up to 2000 atoms in this work), and (2) most standard DFT functionals do not include van der Waals effects. We here show by all-electron DFT including van der Waals effects (PBE+vdW [2]) that the 6$\sqrt{3}$ graphene-like phases on SiC(111) are thermodynamically stable compared to competing surface phases, and obtain the full structure including the substrate-induced graphene corrugation. The impact of strain in smaller-cell approximants and that of possible graphene defects is discussed. [1] K. Emtsev \emph{et al.}, Nature Materials \textbf{8}, 203 (2009). [2] A. Tkatchenko, M. Scheffler, PRL \textbf{102}, 073005 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J6.00006: Edge-Edge interactions in stacked graphene Eduardo Cruz-Silva, Xiaoting Jia, Mauricio Terrones, Mildred Dresselhaus, Vincent Meunier Graphene is often considered as the ultimate material for nanoscale electronics due to its unique structure-dependent properties. It has recently been shown that edge reconstruction and reshaping using combined electron irradiation and Joule heating could be used to control graphene properties towards nanodevice design. For instance, HRTEM experiments on few-layered graphene have revealed the presence of small graphene patches and platelets over larger graphene domains. While these platelets usually move freely over the larger graphene surface, they sometimes get locked in positions close to the edges of the larger sheet, thereby modifying the local electronic environment. We modeled this behavior with extensive density functional calculations using the van der Waals functional of Dion \emph{et al.} Local interactions at the edges are found to be sufficiently strong to explain the presence of stacking configurations (e.g. AA) that are known to be energetically unfavorable in 2D graphene. Our results explain the observed dynamics of these stacked platelets and provide a deeper understanding of graphene edge reconstruction. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J6.00007: Influence of water on the electronic structure of metal supported graphene: Insight from van der Waals density functional theory Ji Feng, Xiao Li, Enge Wang, Sheng Meng, Jiri Klimes, Angelos Michaelides We investigate the interaction between water and metal supported graphene through van der Waals density functional theory calculations. Our results show a systematic increase in the adsorption energy of water on graphene in the presence of underlying metal substrates. In addition, we find that the electronic nature of the graphene-metal contacts behave differently upon water adsorption: in the case of a weak, physical graphene-metal contact, the charge carrier doping level of graphene is tuned by water, resulting in a Fermi level shift on the order of 100 meV. In the case of a strong chemical graphene-metal contact, the ? and ?* bands of graphene are hardly perturbed by water adsorption. These results illustrate the correlated nature of the interactions between water, graphene, and metal substrates, and show that the electronic structure and the doping level of graphene can be controlled by water deposition. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J6.00008: Alkanes adsorbed on graphene: a vdW-DF study Elisa Londero, Emma Karlson, Marcus Landahl, Dimitri Ostrovskii, Jonatan Rydberg, Elsebeth Schroeder Studies of small chains of molecules adsorbed on graphene are important for application of graphene in possible devices, not least its use as a gas sensor for the detection of single molecules. In this work we present a density functional theory study of the first ten n-alkanes adsorbed on graphene using the vdW-DF functional. We compare our adsorption energies to temperature programmed desorption measurements finding a similar linear scaling with the number of carbon atoms in the chain. The presence of an offset when extrapolating to the case of no molecules on the surface is also confirmed. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J6.00009: To wet or not to wet? Dispersion forces tip the balance for water ice on metals Javier Carrasco, Biswajit Santra, Jiri Klimes, Angelos Michaelides For almost 30 years now, density functional theory (DFT) has been used to explore the molecular level details of water-metal interfaces. However, since the typical generalized gradient approximation exchange-correlation functionals used in these studies do not account for van der Waals (vdW) dispersion forces, the role dispersion plays in water adsorption remains unclear. Here, we tackle this issue head on applying a newly developed non-local functional [J. Klime\v{s} {\it et al}., J. Phys.: Condens. Matter {\bf 22}, 022201 (2010)] to two of the most widely studied water-ice adsorption systems, namely water on Cu(110) and Ru(0001). We show that non-local correlations contribute substantially to the water-metal bond and that this is an important factor in governing the relative stabilities of wetting layers and 3D bulk ice [J. Carrasco {\it et al}., Phys. Rev. Lett. {\bf 106}, 026101 (2011)]. Due to the greater polarizability of the substrate metal atoms, non-local correlations between water and the metal exceed those between water within ice. This sheds light on a long-standing problem, wherein common DFT exchange-correlation functionals incorrectly predict that none of the low temperature experimentally characterized ice-like wetting layers are thermodynamically stable. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J6.00010: Understanding the magnetic properties present at hybrid organic-ferromagnetic interfaces and the role of the van der Waals interaction Nicolae Atodiresei, Vasile Caciuc, Predrag Lazi\'c, Stefan Bl\"ugel The design of nanoscale spintronic elements in multifunctional devices relies on a clear theoretical understanding of the physics at the electrode-organic system interfaces and in particular, the functionality of specific molecules in a given organic-metal surface environment. The density functional theory provides a framework where a realistic understanding of these systems with predictive power can be expected. However, only very recent functionals describe the exchange correlation of the organic molecule-metal interface reliably including the van der Waals interaction. We show that this has a great influence in particular on specific flat absorbed $\pi$-conjugated electron systems. Our first-principles calculations performed for several organic molecules containing $\pi(p_z)$ electrons adsorbed onto a magnetic substrate show that the magnetic properties such as the local spin polarization, molecular magnetic moments and their spatial orientation can be specifically tuned by using substituents with different electronegativities. References: [1] N. Atodiresei et al., PRL 102, 136809 (2009); [2] J. Brede et al., PRL 105, 047204 (2010); [3] N. Atodiresei PRL 105, 066601 (2010); [4] C. Busse et al., PRL 107, 036101 (2011). [5] N. Atodiresei et al., PRB 84, 172402 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J6.00011: Simultaneous conductance and mechanical measurements on single molecule junctions reveal enhanced binding due to van der Waals interactions Sriharsha V. Aradhya, Michael Frei, Mark S. Hybertsen, Latha Venkataraman Quantitative measurement of Van der Waals (vdW) interactions at the single molecule level remains challenging in experiments and its accurate inclusion in first principles theory is also complex. Here we report simultaneous measurement of force and electrical conductance across Au-molecule-Au junctions using a conducting atomic force microscope (AFM) for 4,4'-bipyridine (BP) and 1,2-bis(4-pyridyl)ethylene (BPE) molecules. For each of these molecules two distinct molecular junction structures are observed with characteristic conductances, consistent with previous studies utilizing scanning tunneling microscopy (STM). These two structures are found to have very different mechanical properties. Specifically, we find that the higher conductance junctions have a significantly larger rupture force and stiffness than those that show the lower conductance. They also have a larger rupture force than Au point contacts, suggesting multiple points of contact. Density functional theory (DFT) calculations suggest that the rupture force for the low conductance structure is well characterized as arising from N-Au donor acceptor interaction. However, the large rupture force and stiffness of the high conductance structure is most naturally explained as being due to the vdW contributions. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J6.00012: Mechanical properties of van der Waals interactions at compositionally graded interfaces between metals and organic semiconductors Huarui Sun, Vladimir Stoica, Max Shtein, Roy Clarke, Kevin Pipe Interfacial bonding plays an important role in energy transfer (e.g., electrical conduction, thermal transport, or acoustic coupling) in composite materials as well as electronic and optoelectronic devices. In this work we use an ultrafast laser to excite vibrational modes in a thin aluminum film that is in contact with a small molecular organic semiconductor (copper phthalocyanine, CuPc). From the measured acoustic dynamics, we derive the fundamental mechanical properties of the van der Waals bonding at the Al/CuPc interface, and further study how these mechanical properties change in a series of samples as the interface is compositionally graded. The implications of these results are discussed in the contexts of interfacial thermal resistance, organic optoelectronic devices, and thermoelectric energy conversion. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J6.00013: Benchmark data base for accurate van der Waals interaction in inorganic fragments Jan Brndiar, Ivan Stich A range of inorganic materials, such as Sb, As, P, S, Se are built from van der Waals (vdW) interacting units forming the crystals, which neither the standard DFT GGA description as well as cheap quantum chemistry methods, such as MP2, do not describe correctly. We use this data base, for which have performed ultra accurate CCSD(T) calculations in complete basis set limit, to test the alternative approximate theories, such as Grimme [1], Langreth-Lundqvist [2], and Tkachenko-Scheffler [3]. While none of these theories gives entirely correct description, Grimme consistently provides more accurate results than Langreth-Lundqvist, which tend to overestimate the distances and underestimate the interaction energies for this set of systems. Contrary Tkachenko-Scheffler appear to yield surprisingly accurate and computationally cheap and convenient description applicable also for systems with appreciable charge transfer. \\[4pt] [1] S. Grimme, J. Comp. Chem. \textbf{27}, 1787 (2006) \\[0pt] [2] K. Lee, \textit{et al.}, Phys. Rev. B 82 081101 (R) (2010) \\[0pt] [3] Tkachenko and M. Scheffler Phys. Rev. Lett. \textbf{102} 073005 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J6.00014: Interlayer binding energy of graphite: A mesoscopic determination from deformation Zhe Liu, Ze Liu, Quanshui Zheng Despite the interlayer binding energy (BE) being one of the most important material properties of graphite, direct experimental determination is yet to be reported. In this talk, we present a novel experimental method to directly measure the interlayer BE of HOPG. By employing the self-retraction motion of a graphite flake in a graphite island (Phys. Rev. Lett. 100, 067205 (2008)), we assembled a graphite top flake spanning a graphite step, yielding a contact area ($\sim\mu$m$^2$) with a graphite platform. STM scan showed that the interface was atomically smooth. The deformation of the top flake should be determined by the BE with the graphite platform. Thus using a finite element model to simulate the top-flake height profiles measured by AFM, we determine the graphite BE as 0.19($\pm $0.01)J/m$^{2}$, which can serve as a benchmark for other theoretical and experimental works. Our proposed method can be easily extended to measure the BEs between graphite/graphene and other types of substrates. It can also be used in other systems, particularly lamellar materials and thin films. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J6.00015: Helium atom diffraction from a monolayer solid with a beam that penetrates to the substrate Ludwig Bruch, Flemming Hansen Diffraction of a thermal energy helium atomic beam is evaluated in the situation that the target monolayer lattice is so dilated that the atomic beam penetrates to the region between the monolayer and the substrate. Parameters are chosen to be representative of a hypothetical $p(1 \times 1)$ commensurate monolayer solid of H$_2$/KCl(001). Depending on the spacing between the monolayer and the substrate and on the angle of incidence, there are cases where part of the incident beam is trapped in the interlayer region for times exceeding 50 ps. The methodology is a direct extension of the wave-packet propagation used for the scattering of helium from the quantum monolayer solid H$_2$/NaCl(001).\footnote{L. W. Bruch, F. Y. Hansen and F. Traeger, J. Chem. Phys. {\bf 134}, 194308 (2011)} [Preview Abstract] |
Session J7: Focus Session: Graphene Devices
Sponsoring Units: DMPChair: Nina Markovic, Johns Hopkins University
Room: 207
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J7.00001: Graphene-Superconductor based Bolometers Heli Vora, Piranavan Kumaravadivel, Bent Nielsen, Xu Du Due to its small volume and linear energy dependence of electron density of states, graphene has very low electronic heat capacitance compared to what is found in metals with achievable volume. This makes it a promising material for applications requiring bolometric sensing of radiation with a fast response without compromising sensitivity. Here we report fabrication of graphene-superconductor tunnel junctions and characterization of their bolometric response. When radio frequency radiation is shone onto the junction, the electrons in graphene heat up and dynamic resistance within the superconducting gap changes. The relation between absorbed power and temperature rise is used to characterize heat conductance and thermal noise equivalent power. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J7.00002: Microwave transport and noise in graphene devices Bernard Placais, Andreas Betz, Emiliano Pallecchi, Gwendal F\`eve, Jean-Marc Berroir, Christian Benz, Romain Danneau, Antonella Cavanna, Ali Madouri We report on microwave transport and noise in graphene sheets, capacitors and transistors. We achieved a transit frequency of 80 GHz in graphene-on-sapphire transistors [1]. This is close to the state-of-the art for a 200nm gate length. In order to investigate electronic diffusion in graphene we have realized 10 GHz bandwidth experiments in metal-oxide-graphene capacitors. The crossover from capacitor to skin-effect limited cavity regimes provides an accurate and direct measurement of the diffusion constant D($\varepsilon )$. We find D to be energy independent, which points to a mass-disorder scenario [2]. Finally, we have measured the shot noise of CVD graphene in a broad range of bias up to 1V drain-source voltage. In our GHz-frequency measurements we observe a Fano factor exhibiting the three characteristic regimes of electron-impurity, electron-electron and electron-phonon interactions. \\[4pt] [1] E. Pallecchi et al, Appl. Phys. Lett. 99, 113502 (2011) \\[0pt] [2] E. Pallecchi et al, Phys. Rev. B 83, 125408 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J7.00003: Anomalous noise characteristics in graphene p-n junctions Janice Wynn Guikema, Atikur Rahman, Nina Markovic Graphene p-n junctions provide an interesting platform to study the Dirac nature of charge carriers, which gives rise to phenomena such as Klein tunneling and electron lensing. p-n junctions also play an important role in graphene nanodevices, but the ubiquitous low-frequency noise typically limits the device performance. It has been observed that the low-frequency noise for back-gated single layer and bilayer graphene decreases at the Dirac point, even though the resistance is maximal there. This decrease has been attributed to the effects of spatial charge inhomogeneity, and the noise in such devices decreases with decreasing temperature. We will present noise characteristics of dual-gated graphene p-n junction devices. At the Dirac point, the noise shows an ``M'' shape as a function of top gate voltage. Away from the Dirac point the shape changes depending on the back gate and top gate voltage. Unlike the low-frequency noise in graphene nanodevices, noise across the p-n junction decreases with increasing temperature. We also observed that the noise amplitude is larger in p-n-p or n-p-n devices than in p-p-p or n-n-n devices. We will discuss the mechanism that explains this anomalous noise behavior in graphene p-n junction. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J7.00004: Incidence Angle-dependent Transport across a Single Graphene $p-n$ Junction Formed by Buried Split-gates Surajit Sutar, Everett Comfort, Jian Liu, Takashi Taniguchi, Kenji Watanabe, Ji Ung Lee Due to electron chirality effects, carrier transport across Graphene $p-n$ junctions (GPNJ) is predicted to have strong angular dependence [1]. This work reports evidence of such effects in a single GPNJ for various geometries created by the use of buried split-gates (SG). Standard processes are used to fabricate 2-terminal Graphene devices aligned to buried Polysilicon SG at different angles to the junction. Sweeping the SG biases V1 and V2 allows mapping the doping-dependent device resistance (Rt). For doping levels (V1,V2), subtracting the average unipolar resistance Rt(V1,V1) from the bipolar resistance Rt(V1,V2) gives the average junction resistance Rj(V1,V2), subtracting out both contact and channel resistances. For bipolar doping, Rj shows a sharper peak for tilted channels than one that is normal to the junction, the peak being sharpest for 45\r{ }, the largest angle probed. This trend is observed for both exfoliated and CVD Graphene, especially for higher mobility and lower widths, consistent with theory. The ratio of the maximal Rj for 45\r{ } and 0\r{ } devices is about 2.5, significant for the modest Graphene mobilities of our devices. [1] V. Cheianov et al., \textbf{\textit{Science,}} \textbf{315}, 2007, pp 1252. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J7.00005: Gate-Controlled P-I-N Junction Switching Device with Graphene Nanoribbon Shu Nakaharai, Tomohiko Iijima, Shinichi Ogawa, Hisao Miyazaki, Songlin Li, Kazuhito Tsukagoshi, Shintaro Sato, Naoki Yokoyama The concept of a novel graphene P-I-N junction switching device with a nanoribbon is proposed, and its basic operation is demonstrated in an experiment. The concept aims to optimize the operation scheme for graphene transistors toward a superior on-off property. The device has two bulk graphene regions where the carrier type is electrostatically controlled by a top-gate, and these two regions are separated by a nanoribbon which works as insulator. As a result, the device forms a (P or N)-I-(P or N) junction. The off state is obtained by lifting the band of the bulk graphene of the source side and lowering that of the drain side, so that the device forms a P-I-N junction. In this configuration, the leakage current is reduced more effectively than the conventional single gate transistors due to a high barrier height and a long tunneling length in the nanoribbon. The on state is obtained by flipping the polarity of the bias of either top-gate to form a P-I-P or N-I-N junction. An experiment showed that the drain current was suppressed in the cases of P-I-N and N-I-P compared to P-I-P and N-I-N, and all of the behaviors were consistent with what was expected from the device operation model. This research is granted by JSPS through FIRST Program initiated by CSTP. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J7.00006: Fabrication of suspended graphene heterojunctions for exploring the intrinsic transport of Dirac fermions Piranavan Kumaravadivel, Xu Du With linear energy dispersion and chirality, the quasiparticles in graphene differs from the conventional electrons in solid state systems in that the motions of the quasiparticles are governed by the Dirac Weyl equation. High quality of suspended graphene has made it possible to reach lower carrier densities and longer mean free paths and therefore has opened up the possibility of studying some of the unique, intrinsic properties of this Dirac electron system, which cannot otherwise be observed in conventional graphene devices on various substrates. To retain and exploit such properties of suspended graphene in heterojunction devices requires fabricating a contactless air top gate over the suspended graphene channel. Here we report on fabrication of such a device, which enable us to explore further, some interesting physics and transport of these Dirac fermions in graphene. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J7.00007: Transport Properties of Gate Tunable Graphene-Based Tunnel Diodes Damon Farmer, Vasili Perebeinos, Phaedon Avouris Due to its linear dispersion relation and unique physical properties, graphene has become a material of intense experimental and theoretical investigation. There has been rapid progress in the fabrication and understanding of graphene devices, particularly those based on the field effect transistor (FET) configuration. These three-terminal switches rely on a gate field to control electronic transport (diffusive or quasi-ballistic) in the graphene channel, where the field is perpendicular to the current flow. Here, a different type of three-terminal graphene device is demonstrated, one based on quantum tunneling. These devices build upon the convention two-terminal metal-insulator-metal (MIM) tunnel diode configuration by replacing one of the metal electrodes with graphene. Incorporation of a third (gate) electrode allows for modulation of the accessible density of states in the graphene electrode, thereby tuning the threshold voltage for tunneling in the diode. This tunable diode concept, where the gate field is parallel to the tunneling direction, is novel for a purely solid-state system. The device characteristics owing to the unique properties of graphene will be discussed, as will the underlying physics of device operation. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J7.00008: Drain Current Saturation in Sub-$\mu $m Graphene Field Effect Transistors Shu-Jen Han, Dharmendar Palle, Aaron Franklin, Alberto Vlades-Garcia Recently, graphene field-effect transistors (FET) with high cut-off frequencies (fT) were reported; however, the devices showed very weak drain current saturation, leading to an undesirably high output conductance. A crucial figure-of-merit for analog/RF transistors is the intrinsic gain (gm/gds). In this work, we show that by employing an embedded gate structure with an equivalent oxide thickness less than 2 nm, strong drain current saturation can be obtained for graphene FETs with short channels. The mechanism was not entirely due to velocity saturation, but rather the combination of a shift of the Dirac point in the voltage domain and a strong bias-dependent gate capacitance. The mechanisms are also verified with models. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J7.00009: Self-aligned high transconductance CVD graphene transistor Yuchi Che, Alexander Badmaev, Zhen Li, Chongwu Zhou Exceptional electronic properties of graphene make it a highly promising material for high-speed electronics. However, the design and practical realization of graphene transistors are still challenging, which limit the potential of graphene. In this work, we present a novel, practical, and highly scalable method for the fabrication of self-aligned graphene transistors. Large-area single-layer graphene films were grown on copper foil and were transferred onto 12 inch Si wafers. Furthermore, in order to achieve wafer-scale fabrication of graphene transistors with high yield, we developed a self-aligned fabrication approach by standard lithographical methods. The fabricated transistors with gate lengths in the range of 110 to 170 nm showed excellent performance with the peak current density of 1.3 mA/$\mu $m. The peak transconductance reaches 0.5 mS/$\mu $m, which is one of the best transconductance for CVD graphene transistor published up to date. Our novel fabrication method shows great potential toward practical implementation of graphene in high frequency devices and circuits. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J7.00010: Graphene Enabled Vertical Field Effect Transistors Evan P. Donoghue, Maxime G. Lemaitre, Mitchell A. McCarthy, Bo Liu, Bill R. Appleton, Andrew G. Rinzler Vertical field effect transistors (VFETs) using a carbon nanotube source electrode have recently demonstrated state-of-the-art current densities at low operating voltages from comparatively low mobility organic semiconductors. Unlike in conventional thin film transistors, transconductance arises from a gate field modulation of the contact barrier at the organic semiconductor/nanotube interface (with the performance enhanced by nanotube Fermi level shifts allowed by their low density of electronic states). Graphene's low density of states near the Dirac point similarly makes it a good candidate source electrode in VFETs. We have now demonstrated such devices, facilitated by the use of a novel transfer technique that improves areal yields while avoiding both polymeric residue and the particulates that necessitate the use of thicker channel layers in nanotube VFETs. Critical to high performance is a facile method developed for perforating the graphene sheet with holes of a tunable size and density, allowing for a direct assessment of the role of self-screening effects in VFETs. These initial graphene enabled VFETs achieve on-current densities exceeding 250mA/cm$^{2}$ at drive voltage swings of less than 4V with on/off current ratios larger than 10$^{6}$. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J7.00011: Transport/magnetotransport of high-performance graphene transistors on organic molecule-functionalized substrates Shao-Yu Chen, Po-Hsun Ho, Ren-Jye Shiue, Chun-Wei Chen, Wei-Hua Wang We present the transport and magnetotransport of high-quality graphene transistors on conventional SiO2/Si substrates by modification with organic molecule self-assembled monolayers (SAMs). Graphene devices on organic SAM-functionalized substrates exhibit high carrier mobility, low intrinsic doping, suppressed carrier scattering, and reduced thermal activation of resistivity at room temperature. Magnetotransport of graphene devices with pronounced quantum Hall effect and Shubnikov-de Haas oscillations also confirms the high quality of graphene on this ultrasmooth organic SAM-modified platform. The high-performance graphene transistors on the solution-processable SAM-functionalized SiO2/Si substrates are promising for the future development of large-area and low-cost fabrications of graphene-based nanoelectronics. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J7.00012: Quantum transport simulation of graphene transistors Yang Lu, Jing Guo Owing to its unique electrical and thermal properties, graphene has attracted great interests as potential building blocks of next generation electronics, especially for RF applications. Due to lack of band gap, Klein tunneling plays an important role in sub-100nm graphene transistors. Inelastic phonon scattering introduced by intrinsic phonons of graphene and polarized substrate also affect device performance. We show that coupling between inelastic phonon scattering and Klein tunneling leads to different device physics of graphene transistors from common nanoscale transistors. The electron-phonon interaction and quantum transport in graphene transistors are modeled by the non-equilibrium green's function method within the self-consistent Born approximation. We evaluate the effect of inelastic process on both DC and RF performance of graphene transistors. We also briefly discuss the self-heating effects in graphene transistors introduced by inelastic phonon scattering. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J7.00013: Pulse gating on graphene quantum dots Christian Volk, Christoph Neumann, Sebastian Kazarski, Stefan Fringes, Stephan Engels, Bernat Terres, Jan Dauber, Stefan Trellenkamp, Uwe Wichmann, Christoph Stampfer Graphene quantum devices have received increasing attention over the last years. Graphene quantum dots (QDs) are interesting systems for implementing spin qubits. Compared to well-established GaAs-based QDs, their smaller hyperfine and spin-orbit coupling promises more favorable spin coherence times. However, while the preparation, manipulation, and read-out of single spins have been demonstrated in GaAs, research on graphene QDs is still at an early stage. Although Coulomb blockade phenomena and excited state spectroscopy is now well established, experimental signatures allowing the identification of relaxation times have been hard to trace. Here we report on the current status of pulse gating experiments on graphene quantum devices and in particular we will present measurements of the charge relaxation rates in single-layer graphene QDs. The investigated devices consist of an island with a diameter of 120 nm, 4 lateral graphene gates and 2 charge detectors. From so-called diamond measurements we extract a charging energy of 11 meV and excited state level spacings on the order of 2-4 meV. The gates allow to individually tune the tunnelling-in and -out rates down to low MHz regime. Low-bias pulse gate measurements allow finally to extract relaxation rates on the order of 50 ns. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J7.00014: Ultrafast conductivity measurements in CVD graphene James Heyman, Yilikal Ayino, Rolan Manderson-Jones, Jacob Stein We study carrier dynamics in CVD graphene films on Al$_{2}$O$_{3 }$using time-resolved THz spectroscopy (TRTS). Excitation with a 50fs, 800nm pulse produces a conductivity change $\Delta \sigma $which we measure as a decrease in transmission of an ultrafast THz pulse. Ongoing work seeks to investigate TRTS in a magnetic field to directly probe scattering rates of photogenerated carriers. At present we report zero field measurements. We observe different recovery dynamics for low ($\Delta \sigma <<\sigma _0 )$ and high ($\Delta \sigma >\sigma _0 )$ pump powers. For pump fluence $\varphi <10^{12}\mbox{photons cm}^{-2}$ ($\Delta \sigma <<\sigma _0 )$ we find$\Delta \sigma \propto \varphi $ and observe a nearly exponential decay $\Delta \sigma \propto e^{-t/\tau }$with decay time $\tau \approx 2\mbox{ps}$. At higher powers ($\Delta \sigma >\sigma _0 ) \quad \Delta \sigma $ is sublinear in $\varphi $, and the decay rate decreases, with $\tau \approx 4\mbox{ps}$at $\varphi \approx 5\cdot 10^{13}\mbox{photons cm}^{-2}$. Graphene's unusual conductivity relation, $\sigma \propto \sqrt n _{,}$ predicts the observed behavior, since $\Delta \sigma \propto \sqrt {n_0 +n_{photo} } -\sqrt {n_0 } $ is approximately linear for $n_0 >>n_{photo} $, while $\Delta \sigma \propto \sqrt {n_{photo} } $for $n_{photo} >>n_0 $. Here $n_0 $ and $\sigma _0 $ are the equilibrium carrier density and conductivity in these p-type films. At high pump powers we also observe a rapid initial recovery on $\sim $500fs timescale which is not described by this simple model. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J7.00015: Delay Analysis of Graphene Field Effect Transistors and T-Gate Self-Aligned GFETs Han Wang, Allen Hsu, Ki Kang Kim, Jing Kong, Tomas Palacios In this paper, we propose a new method for extracting the carrier transit delays in radio-frequency (RF) graphene field effect transistors (GFETs). This delay analysis not only gives deep physical insight into the carrier transport in the channel, but also provides valuable information that can guide the device engineers in optimizing the design of high performance RF GFETs. The contribution of this work is three-fold. First, GFETs are fabricated on sapphire substrate to reduce the parasitics from the GSG pads. This minimizes the error in measuring the S-parameter of the device and allows small-signal capacitances to be accurately extracted. Second, we present for the first time a detailed delay analysis of high frequency graphene transistors. Lastly, the simple and robust method proposed can accurately extract the intrinsic transit delay of the GFETs - the delay purely associated with the carrier transiting across the intrinsic gate region -- and allows a new method for direct experimental extraction of the average carrier velocity in the channel. Based on the analysis, we propose a self-aligned device structure to minimize the parasitic delays in GFETs. This GFET structure uses a T-shape gate as the mask to allow the source and drain metals to be aligned to the edge of the gate. A T-gate self-aligned GFET with gate head of 300 nm and foot 50 nm is fabricated and its DC and RF characteristics are reported. [Preview Abstract] |
Session J8: Quantum Magnetism, BEC, and Disorder
Sponsoring Units: DCMP GMAGChair: Marcelo Jaime, Los Alamos National Lab
Room: 208
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J8.00001: Finite-$T$ spectral function of the BEC quantum magnets Suhas Gangadharaiah, Sasha Chernyshev We discuss the momentum, frequency, and temperature dependence of the spectral function of interacting 3D bosonic excitations in the vicinity of the BEC quantum critical point. The relaxation rate is demonstrated to have a highly non-trivial $\omega$-dependence with several asymptotic regimes that are studied in some detail. The spectral function is shown to exhibit, in a wide range of temperatures, an asymmetric quasiparticle peak and a shoulder, originating from the behavior of the self-energy. These spectral features should be readily observable in neutron-scattering experiments in the BEC quantum magnets. We also argue that this behavior of the spectral function must persist throughout the strong-coupling limit. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J8.00002: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J8.00003: Theory of magnetic Bose glass in Br-doped dichloro-tetrakis-thiourea-nickel (DTN) Stephan Haas, Rong Yu, Tommaso Roscilde We will review here the microscopic model allowing to describe quantitatively the physics of Br-doped DTN. The magnetic Hamiltonian of Br-DTN describes S=1 spins coupled through bimodally distributed antiferromagnetic bonds, and with a correlated bimodal distribution of single-ion anisotropies. A spin-boson mapping leads to a description in terms of a Bose-Hubbard-like model with random hoppings and random on-site interactions for magnetic quasiparticles, whose density is controlled by the applied magnetic field. This model features an extended gapless and compressible Bose-glass phase in low fields, extending down to zero field, at which the compressibility vanishes, corresponding to a Mott-glass phase. We will present extensive quantum Monte Carlo results for the thermodynamic signatures of the Bose glass, and for the quantum critical signatures of the magnetic ordering transition occurring at stronger field, which corresponds to a remarkable realization of the Bose-glass/superfluid transition. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J8.00004: Magnetization and Specific Heat Investigations of the Bose Glass: Br-doped NiCl$_{2}$-4SC(NH$_{2}$)$_{2}$ Franziska Weickert, Corneliu Miclea, Roman Movshovich, Vivien Zapf, Rong Yu, Tommaso Roscilde NiCl$_{2}$-4SC(NH$_{2})_{2}$ (DTN) is an insulating material, which shows field induced XY-AFM order between $H_{c1}$ = 2.1 T and $H_{c2}$ = 12.6 T. In boson language, the ground state of DTN can be described as a Mott insulator, and the ordered state as a Bose-Einstein condensation of magnons. Bond disorder is introduced by substituting Br atoms on Cl positions, which simultaneously changes the super exchange interaction along the $c$-direction on a local scale and leads to a Mott-glass ground state in zero field. Furthermore, the system develops a gapless Bose glass for magnetic fields 0 $< \quad H \quad < \quad H_{c1}$ and $H \quad > \quad H_{c2}$, followed by a Mott insulating state above the saturation field $H_{sat}$. Note, that the critical fields $H_{c1,2}$ and $H_{sat}$ are shifted compared to those of pure DTN. In this talk, we report on measurements of the magnetization and specific heat at very low temperatures between 50 mK and 3 K in high magnetic fields up to 14 T on an 8{\%} Br-doped single crystal DTN. We compare our data with the local gap model, which reduces the low-temperature and low-field behavior to those of an ensemble of individual three level systems with local magnetization $M_{S}$ = 0, $\pm $1 and a finite energy gap for $H$ = 0. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J8.00005: Magnetic Bose glass in Br-doped NiCl$_{2}$-4SC(NH$_{2})_{2}$: Magneto-Electric Effect Liang Yin, Jian-Sheng Xia, Neil Sullivan, Vivien Zapf, Armando Paduan-Filho NiCl$_{1.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ is a candidate for Bose glass state of the magnetism at low temperatures, which is the bosonic analog of Anderson localization. Here we explore the glassy dynamics of the magnetization taking advantage of magneto-electric coupling in this material. We report measurements of the magneto-electric effect in NiCl$_{1.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ using two different experimental setups: (i) magnetic field-induced polarization and (ii) electric-field-induced magnetization. Both measurements show that the proposed Bose glass state in a high magnetic field is sensitive to the applied frequencies of AC magnetic and electric fields, with a maximum response between 700Hz and 1000Hz. This frequency-dependent behavior could provide information about the characteristic response time and the size of magnetic clusters. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J8.00006: Disorder in a two-dimensional quantum spin liquid Dan Huvonen, Martin Mansson, Severian Gvasaliya, Shuangyi Zhao, Tatiana Yankova, Vasiliy Glazkov, Eric Ressouche, Christof Niedermayer, Mark Laver, Georg Ehlers, Andrey Zheludev We discuss magnetic field induced phase transition to magnon Bose-Einstein condensate state in a disordered two-dimensional spin gap antiferromagnet. Disorder was introduced into piperazinium hexachlorodicuprate (PHCC) by chemically substituting up to 10\% of exchange interaction mediating Cl ions for Br. We present specific heat, magnetization, susceptibility, elastic and inelastic neutron scattering results in fields up to 14T. Data reveals that disorder enlarges significantly the spin gap and induces nonzero susceptibility in the gapped phase. Reduction of magnon bandwidth and lifetime are evident from inelastic neutron scattering measurements. Although the phase transition seems to survive, the condensate wavefunction aquires a history dependence. In contrary to theoretical expectations, the extracted critical exponents show no changes within experimental accuracy. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J8.00007: Dynamics of the Random-Field Ising Model Spencer Tomarken, Daniel Silevitch, Thomas Rosenbaum Ising magnets with long, needle-like domains can be treated as single extended spins which interact via dipole--dipole forces. Typically such dipole interactions cancel out due to spatial symmetry, but the combination of randomly packed grains and the application of a magnetic field transverse to the easy axis of magnetization can break the symmetry. This results in a site-random-magnetic field that points along the easy axis and varies from grain to grain, described by the Random--Field Ising Model (RFIM). We report a series of magnetization measurements in longitudinal and transverse magnetic fields that demonstrate RFIM behavior in the room--temperature, rare--earth ferromagnet Nd$_2$Fe$_{14}$B, and analyze our data in terms of predicted scaling relations. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J8.00008: Strong Disorder Renormalization Group for the Many Body Localization Transition Gil Refael, Vadim Oganesyan, Shankar Iyer The strong disorder renormalization group, originally devised by Ma and Dasgupta to study the random Heisenberg antiferromagnet, has subsequently been used to investigate the low energy physics and quantum phase transitions of a variety of strongly disordered systems. However, recent work by Basko, Aleiner, and Altshuler has focused attention on the many body localization transition, a dynamical quantum phase transition that involves the localization of highly excited eigenstates of a many body system in Fock space. Numerical results from an exact diagonalization study by Pal and Huse suggest that the many body localization transition may exhibit so-called infinite-randomness, a property that implies that a strong disorder renormalization group may be well-suited to study this transition. With the many body localization transition in mind, we therefore outline a strong disorder renormalization procedure that targets the least-localized eigenstate of a model. We then apply this procedure to study disordered quantum Ising and XXZ models. The latter model is similar to the one investigated by Pal and Huse and is expected to contain a dynamical transition between localized and ergodic phases; our principal aim is to use the strong disorder RG to characterize this transition. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J8.00009: Evolution of magnetic order and fluctuations in Ni-V close to the disordered ferromagnetic quantum critical point Almut Schroeder, Ruizhe Wang, S. Ubaid-Kassis, P.J. Baker, F.L. Pratt, S.J. Blundell, T. Lancaster, I. Franke, J.S. Moeller Muon spin rotation and magnetization data of the d-metal alloy Ni$_{1-x}$V$_{x}$ are presented at several vanadium concentrations $x$ below and above the critical $x_c \approx11\%$ where the onset of long-range ferromagnetic (FM) order is suppressed. Bulk magnetization shows signatures of a disordered quantum phase transition (QPT), most clearly in the paramagnetic regime: Above $x_c$ the temperature dependence of the magnetic susceptibility is best described by simple non-universal power laws marking a quantum Griffiths phase. But the deviations from a clean FM in the ordered phase are more subtle to recognize in the bulk magnetization and are noticed only close to $x_c$. Muon data reveal a broad field distribution in the FM regime at even small $x$ $(x \geq 4\%)$. The evolution of the magnetic cluster distribution and dynamics from the pristine FM towards the paramagnetic regime will be presented. This adds new insight in this model system at a disordered QPT. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J8.00010: Unconventional spin-Peierls state in the quantum magnet TiOBr J.-P. Castellan, J.P. Clancy, J.P.C. Ruff, S. Rosenkranz, R. Osborn, B.D. Gaulin, F.C. Chou, S.H. Huang The discovery of spin-Peierls transitions in inorganic materials such as CuGeO$_{3}$ with T$_{sp}$ of 14K allowed for the growth of large single crystals. With the availability of large single crystals the opportunity arises to introduce both magnetic and nonmagnetic impurities and study the resulting perturbations from the ground state. Recently a new class of unconventional spin-Peierls materials were discovered TiOBr and TiOCl. TiOBr and TiOCl have been shown to exhibit dimerized singlet ground states and undergo not one but two successive phase transitions. We have performed x-ray scattering measurements on single crystals of TiOBr. These measurements reveal both commensurate and incommensurate spin-Peierls phases; below T$_{c2}$ $\sim$48K incommensurate super-lattice reflections arise at Q=[H$\pm$$\delta$,K+1/2$\pm$$\epsilon$,L] which persists down to the lock in transition at T$_{c1}$$\sim$27K. We will report on the details of these successive transitions and the destruction of the long-range ordered spin-Peierls state with introduction of magnetic vacancies by doping with Sc. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J8.00011: Ashkin-Teller criticality and pseudo-first-order behavior in a frustrated Ising model on the square lattice Songbo Jin, Arnab Sen, Anders Sandvik We consider the square-lattice frustrated Ising model with first- and second-neighbor interactions, $J_1<0$ and $J_2>0$. Its thermal phase transition to ``stripe'' order when $g=J_2/|J_1|>1/2$ has remained controversial despite many past studies. Using Monte Carlo simulations to investigate the order-parameter distribution and its Binder cumulant, we demonstrate Ashkin-Teller criticality for $g \ge g^*$, i.e., the critical exponents vary continuously between those of the $4$-state Potts model at $g^*$ and the Ising model for $g \to \infty$. The Potts point, below which the transition is first-order, is $g^*= 0.67 \pm 0.01$, much lower than previously believed. The system exhibits {\it pseudo first-order} behavior for $g^* \le g \le g^{\prime}\ (g^{\prime}\approx 0.9)$, which was previously misinterpreted as actual first-order behavior. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J8.00012: Quantum phase diagram of the spin ladder model with next-nearest neighbor interactions Yanchao Li, Haiqing Lin By using the density matrix renormalization group technique, we investigate the quantum phase diagram of a two-leg spin-1/2 ladder with diagonal and in-chain next-nearest neighbor interactions for both anti-ferromagnetic and ferromagnetic frustrated cases. Through analyzing the correlation function and four-site entropy, the existence of the controversial columnar dimer phase is confirmed, and the phase transitions caused by the in-chain next-nearest neighbor interaction are presented for the anti-ferromagnetic frustrated case; meanwhile, for the ferromagnetic frustrated case, we find that the system possesses a tetramer phase, a ferromagnetic phase, and the states I and II, and the spin arrangements for I and II are determined. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J8.00013: Majorana Spin Liquids on a two-leg ladder Hsin-Hua Lai, Olexei Motrunich We realize a gapless Majorana Orbital Liquid (MOL) using orbital degrees of freedom and also an SU(2)-invariant Majorana Spin Liquid (MSL) using both spin and orbital degrees of freedom in Kitaev-type models on a 2-leg ladder. The models are exactly solvable by Kitaev's parton approach, and we obtain long-wavelength descriptions for both Majorana liquids. The MOL has one gapless mode and power law correlations in energy at incommensuare wavevectors, while the SU(2) MSL has three gapless modes and power law correlations in spin, spin-nematic, and local energy observables. We study the stability of such states to perturbations away from the exactly solvable points. We find that both MOL and MSL can be stable against allowed short-range parton interactions. We also argue that both states persist upon allowing $Z_2$ gauge field fluctuations, in that the number of gapless modes is retained, although with an expanded set of contributions to observables compared to the free parton mean field. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J8.00014: Static and dynamic properties of a strong-leg spin ladder David Schmidiger, Sebastian Muehlbauer, Severian Gvasaliya, Andrey Zheludev, Pierre Bouillot, Corinna Kollath, Thierry Giamarchi Static and dynamic properties of the strong-leg S = 1/2 Heisenberg spin ladder system (C$_7$H$_{10}$N)$_2$CuBr$_4$ are studied using inelastic neutron scattering and neutron diffraction experiments, as well as bulk magneto-thermodynamic measurements. The leg-odd excitation channel is dominated by long-lived single-magnon states in the entire Brillouin zone, which supports a symmetric-ladder model for this material [1]. In the leg-even channel, a considerable fraction of the spectral weight is contained in a novel long-lived two-magnon bound state. Contrary to reports by other authors, in applied magnetic fields we observe a Bose-Einstein condensation of magnons that manifests itself if 3D long-range antiferromagnetic ordering. The latter emerges beyond $H_c$=2.8 T and is due to weak inter-chain interactions. The field-temperature phase diagram showing the spin liquid, Tomonaga-Luttinger spin liquid and BEC phases is mapped out. The experimental results are in spectacular agreement with DMRG calculations. The latter provide additional insight on certain spin ladder properties specific to the strong-leg regime. \\[4pt] [1] D. Schmidiger, S. M\"uhlbauer, S. N. Gvasaliya, T. Yankova, and A. Zheludev Phys. Rev. B 84, 144421 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J8.00015: Sign problem free quantum Monte Carlo simulations of spin density wave transitions in metals Erez Berg, Max Metlitski, Subir Sachdev We present a version of the spin-fermion model for spin density wave (SDW) transitions in metals, which can be simulated using the determinant quantum Monte Carlo method with no sign problem. The sign problem is eliminated by generalizing the model to include two orbitals which are coupled through the SDW field. The resulting model has a pseudo-time reversal symmetry that guarantees that the minus signs resulting from integrating out the fermions cancel, resulting in a real effective action for the SDW bosonic field. We present preliminary results for the SDW and pairing susceptibilities and the fermion Green's function near the quantum critical point. [Preview Abstract] |
Session J9: Focus Session: Magnetic Oxide Thin Films And Heterostructures - Multiferroic Thin Films
Sponsoring Units: GMAG DMPChair: Alex Demkov, University of Texas at Austin
Room: 209
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J9.00001: Electric-field induced magnetization reversal using multiferroics Invited Speaker: Morgan Trassin Controlling magnetism using solely electric fields is interesting not only from a fundamental standpoint, but presents great potential for ultimately low energy consumption logic and memory. The evidence of the electrically controllable antiferromagnetic ordering in the multiferroic magnetoelectric bismuth ferrite (BiFeO3) drew an increasing interest in the pursuit for new emerging devices. To use such functionality for device applications, deterministic control not only of antiferromagnetism, but also ferromagnetism is essential. To achieve this goal, a ferromagnet/multiferroic heterostructure has been proposed based on the combination of magnetoelectric coupling in BiFeO3 and exchange coupling between magnetic materials and offers a new pathway for the electrical control of magnetism. By combination of a piezoresponse force microscopy, photoemission electron microscopy and anisotropic magnetoresistance measurements, we demonstrated the non-volatile reversal of a CoFe layer magnetization induced solely by the application of an electric field at room temperature. This 180 degree rotation of the magnetization of the ferromagnetic layer is mediated by a strong interfacial coupling. The correlation between the ferroelectric state in the multiferroic layer and the CoFe ferromagnetic domain architecture is evidenced. The projection of this strong magnetoelectric coupling in an out-of-plane configuration, allowing the reduction by an order of magnitude of voltage required, will be discussed. Our results show the high potential of magnetoelectric-based heterostructures for future low energy consumption data storage devices. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J9.00002: Properties of LuFe$_{2}$O$_{4}$ Films Grown by Molecular-Beam Epitaxy R. Misra, C.M. Brooks, J.A. Mundy, T. Heeg, J. Schubert, Z.K. Liu, D. Muller, D.G. Schlom, P. Schiffer LuFe$_{2}$O$_{4}$ is an exotic material with a simultaneous existence of ferroelectricity and ferrimagnetism at the highest temperature (240 K) of any known material [1]. 25 nm thick films of this unusual multiferroic were grown by MBE on MgAl$_{2}$O$_{4}$, MgO, and SiC substrates. XRD shows that the LuFe$_{2}$O$_{4}$ films are single-phase and epitaxial. Film stoichiometry was regulated using an adsorption controlled growth process by depositing LuFe$_{2}$O$_{4}$ in an iron rich environment at pressures and temperatures where the excess iron desorbs from the film surface during growth. STEM images reveal the layered structure of LuFe$_{2}$O$_{4}$ and a clean substrate-film interface free of second phases. The magnetization data exhibits a rapid increase in magnetization below 240 K consistent with the bulk paramagnetic to ferrimagnetic phase transition. On further cooling, the zero field cooled (ZFC) branch of the magnetization displays a peak at 205 K that is suggestive of a glassy transition, which is also seen in bulk samples. At 100 K and 70 kOe, we observe a saturation magnetization of 2.4 $\mu _{B}$/ f. u. (theoretical value of 3 $\mu _{B}$/ f. u.) \\[4pt] [1] Ikeda \textit{et. al}., Nature \textbf{436} (2005) 1136--1138. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J9.00003: Growth and Characterization of Magnetoelectric Fe$_{2}$TeO$_{6}$ Thin Films Junlei Wang, Peter Dowben, Christian Binek Promising spintronic concepts such as Cr$_{2}$O$_{3}$ based voltage-controlled exchange bias systems [1] employ electric controlled boundary magnetization. Symmetry arguments reveal that equilibrium boundary magnetization is a generic property of magnetoelectric antiferromagnets [2]. However, experimental evidence of boundary magnetization is scarce and microscopic evidence has only been provided for the Cr$_{2}$O$_{3}$ (0001) surface [3]. In order to bring the concept of boundary magnetization into a broader experimental context we prepare the magnetoelectric antiferromagnet Fe$_{2}$TeO$_{6 }$with tri-rutile structure. We use two distinct approaches for the thin film growth, RF sputtering and pulsed laser deposition (PLD). Both methodologies start from targets which we prepare from sintered powder of Fe$_{2}$TeO$_{6}$ produced in a solid-state reaction. We characterize the magnetoelectric thin film of Fe$_{2}$TeO$_{6}$ structurally, magnetically and magnetoelectrically using XRD, SQUID, RHEED, LEED and MOKE. Our investigation aims on an experimental test of the predicted generality of the equilibrium boundary magnetization in magnetoelectric antiferromagnets. \\[4pt] [1] He, Xi \textit{et al.}, Nature Materials 9, 579 - 585 (2010)\\[0pt] [2] Belashchenko, K.D., Phys. Rev. Lett. 105, 147204 (2010)\\[0pt] [3] Wu N. \textit{et al.}, Phys. Rev. Lett. 106, 087202 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J9.00004: Probing of ferroelectric and antiferromagnetic orders of multiferroic YMnO$_{3}$ via second harmonic generation Srinivas Polisetty, Mikel Holcomb, Cameron Keenan, Felio Perez, David Lederman The ferroelectric and antiferromagnetic properties of epitaxial, hexagonal (0001) YMnO$_{3}$ thin films grown on GaN/Al$_{2}$O$_{3}$ substrates were studied using second harmonic generation. A Ti:sapphire laser with a 15 W Nd:YVO$_{4}$ pump was used to generate the second harmonic signal. Above the N\'{e}el temperature, ferroelectric ordering was clearly observed as deduced from angular plots of the incoming and outgoing polarization of the second harmonic generation (SHG) signals. Additional antiferromagnetic order was identified below the N\'{e}el temperature. The ferroelectric-magnetic coupling studied via SHG will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J9.00005: Investigation of Electric Field Control of Antiferromagnetic Domains in Epitaxial BiFeO3 Thin Films Using Neutron Diffraction W. Ratcliff, V. Anbusathaiah, T. Gao, P.A. Kienzle, I. Takeuchi BiFeO3 (BFO) is a multiferroic which displays both ferroelectric and magnetic order at room temperatures.~~ Thin films possess a simple G-type antiferromagnetic order.~ In the bulk, this ordering takes on an additional long-wavelength modulation in the form of a spiral [1].~ Recent neutron diffraction results have revealed that it is possible to recover a modulated magnetic structure in thin films which is strongly dependent on the orientation of the substrate on which the film is grown [1].~ Based on this, BFO thin films were deposited on a vicinal SrTiO3 substrate. PFM measurements demonstrate that a ferroelectric monodomain was achieved.~ New neutron diffraction results show that it is possible to change the population of magnetic domains in such films through the application of an electric field (which also switches the ferroelectric domain state).~~ Based on these results, magneto optic Kerr effect (MOKE) measurements were performed on patterned pads of exchanges coupled Co film deposited on top of the BFO films.~~ The results of these measurements will be discussed in the context of device applications.~~[1]~Lee Seongsu; Choi Taekjib; Ratcliff W. II; et al.,~ Phys. Rev. B 78, 100101 (2008); Ratcliff William II; Kan Daisuke; Chen Wangchun; et al., Advanced Functional Materials 21, 1567 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J9.00006: Antiferromagnetic and structural phase transitions in tetragonal-like BiFeO$_3$ H.M. Christen, G.J. MacDougall, W. Siemons, J.H. Nam, J.L. Zarestky, M.D. Biegalski, S. Liang, E.R. Dagotto, S.E. Nagler The recent observation that strain stabilizes a tetragonal-like (``T-like'') polymorph of BiFeO$_3$ has illustrated how epitaxial constraints can fundamentally alter the properties of this multiferroic material. We performed detailed temperature-dependent neutron and x-ray scattering experiments on epitaxial BiFeO$_3$ films on different substrates (SrTiO$_3$, LaAlO$_3$, YAlO$_3$) to study the nature of the monoclinic crystal structure and of the antiferromagnetic (AFM) order. In agreement with Monte Carlo simulations for a classical Heisenberg model, we observe a much lower N\'eel temperature in the T-like morph (T$_N$ $\simeq$ 325K) than in less-strained (``R-like") films (T$_N$ $\simeq$ 645K), and additionally a low-temperature coexistence of C-type and G-type AFM. Independent of the antiferromagnetic transition, at T $\simeq$ 375K we also observe a structural phase transition from one type of monoclinic distortion to a different one. In combination, these measurements shed light on the complexity of the phase diagram of BFO, and provide routes to explore how the material's properties can be tuned by external parameters. Research supported by the U.S. DOE, BES, MSED (H.M.C., W.S., S.L., E.D., H.M.C) and SUFD (M.D.B., G.J.M, J.L.Z, S.E.N.). [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J9.00007: Reversible switching of magnetic easy axis in Co/BiFeO$_{3}$ thin film heterostructures Tieren Gao, Anbusathaiah Varatharajan, William Ratcliff, Ichiro Takeuchi We are investigating the magnetic properties of a thin Co layer deposited on top of and exchange coupled to (001) BiFeO3 (BFO) thin films. 5 nm Co layer is evaporated and patterned into 100 micron x 100 micron pads. We measure angular dependent magnetic hysteresis loop of the Co layer using the magneto-optical Kerr effect and by sweeping the direction of the external magnetic field. We find that exchange-coupled Co layer always develops a magnetic easy axis along the [100] or [010] direction of BFO. As electric field pulses are applied between the Co pad and the bottom electrode (SrRuO3), non-volatile changes in the magnetic properties are observed in the Co layer. For a particular Co/BFO configuration, application of an electric field corresponding to the ferroelectric coercive field of the BFO film switches the magnetic easy axis by 45\r{ }. Upon application of the opposite electric field pulse, the easy axis switches back to the original direction. Subsequent applications of the alternating electric field pulses result in repeating reversible switching between the initial easy axis direction and the 45\r{ } rotated direction. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J9.00008: Probing interface reconstructions in multiferroic BiFeO$_{3 }$ and charge ordered La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ heterostructures Guneeta Singh-Bhalla, Morgan Trassin, Ying-Hao Chu, Rammamoorthy Ramesh Spurred by the potential for device structures with multiple tuning parameters, recent explorations of carefully engineered oxide interfaces have highlighted intriguing possibilities. A famous example includes the LaAlO$_{3}$/SrTiO$_{3}$ heterostructure where the individual layers are insulating but an electron gas appears at the interface. In similar fashion, the atomically engineered interface between antiferromagnetic BiFeO$_{3}$ and ferromagnetic manganite [La,Sr]MnO$_{3}$ results in the formation of a ferromagnetic state in BiFeO$_{3}$ at the interface. Here we explore the interface between BiFeO$_{3}$ and the charge ordered manganite, La$_{0.5}$Ca$_{0.5}$MnO$_{3}$. The insulating nature of La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and BiFeO$_{3}$ allows us to directly probe the electronic properties of the interface via transport measurements. A combination of capacitance and field effect measurements combined with structural probes shed new light on the charge ordered manganite and multiferroic interface. We explore the effects of cross-plane ferroelectric switching in BiFeO$_{3}$ on charge ordering in La$_{0.5}$Ca$_{0.5}$MnO$_{3}$, and hence the electronic and magnetic properties of La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ near the interface. We discuss our results and implications. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J9.00009: Promise of new multiferroics: Synthesis and characterization of epitaxial NiTiO$_{3}$ films Tamas Varga, Timothy Droubay, Mark Bowden, Scott Chambers, Bernd Kabius, William Shelton, Ponnusamy Nachimuthu, Vaithiyalingam Shutthanandan In a search for new multiferroic materials where the direction of magnetization can be switched by an applied electric field, we have looked for materials in which polarization and magnetization are strongly coupled. Recent theory calculations predicted that the family of compounds MTiO$_{3}$ (M = Mn, Fe, Ni), in a certain polymorphic structure (acentric $R3c)$, are promising candidates where a polar lattice distortion can induce weak ferromagnetism. Guided by these insights, a rhombohedral phase of NiTiO$_{3}$ has been prepared in epitaxial thin film form, whose structure is very close to that predicted to be a multiferroic. The synthesis of such new epitaxial films, their full structural characterization and physical property measurements along with our first-principles DFT calculations to predict the desired NiTiO$_{3}$ structure and its stability are reported. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J9.00010: Structural Control of Magnetic Anisotropy in a Multiferroic EuTiO$_3$ Thin Film J.W. Freeland, X. Ke, P.J. Ryan, J.W. Kim, J.-H. Lee, R. Misra, P. Schiffer, T. Birol, C.J. Fennie, D.G. Schlom Strain control of EuTiO$_3$ has been shown under tensile strain the system converts to a multiferroic groundstate with ferromagnetic and ferroelectric order[1]. Here we present a study of the magnetic order in thin films of EuTiO$_3$ grown on DyScO$_3$(110) substrates by reactive molecular-beam epitaxy (MBE). Neutron scattering and magnetic measurements show the magnetic moment orders with an easy axis along only one of the (110) pseudocubic axis of the unit cell. Such an easy axis is connected to the uniaxial crystal structure that evolves from cubic to tetragonal with octahedral tilting, which agrees well with the strain dependent structure predicted under biaxial tensile strain. The magnetic anisotropy for Eu is attributed to an asymmetric crystal field due to the uniaxial symmetry of the Eu-O coordination. Work at Argonne, including the Advanced Photon, is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [1] J.-H. Lee et al. Nature {\bf 466}, 954 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J9.00011: Growth and Optical Properties of Multiferroic LuFe2O4 Thin Films R.C. Rai, A. Delmont, A. Sprow, B. Cai, M. Nakarmi We present surface, structural, electronic, and optical properties of multiferroic LuFe$_{2}$O$_{4}$ thin films grown on single crystal (0001) sapphire, (111) YSZ, and (001) LiNbO$_{3}$ substrates using electron-beam deposition. LFO thin films have been deposited on substrates at temperature 750 $^{\circ}$C in an oxygen environment and post-deposition annealed at temperatures ranging from 600 to 800 $^{\circ}$C to improve the stoichiometry and the crystal quality. We have used AFM and XRD for surface and structural characterization of the LFO thin films. To investigate the charge order phenomenon and the electronic properties, we carried out variable temperature (78 -- 450 K) optical and resistivity measurements on the LFO thin films. The absorption spectra of LFO thin films show strong electronic excitations with the energy gap of $\sim $2.18 eV at 300 K. Further, the energy gap of LFO displays strong temperature dependence, exhibiting a ferrimagnetic transition at $\sim $240 K and a charge-order transition at $\sim $350 K, respectively. We will also discuss the electronic excitations of the LFO thin film in the energy range 0.5 -- 5.0 eV and their correlations with different magnetic and charge-ordered states. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J9.00012: Exchange bias in BiFeO3/CoFe2O4 heterostructure Yonghang Pei, Jiwei Lu, Ryan Comes, Stuart Wolf Room temperature multiferroics BiFeO3 (BFO), both ferroelectric and antiferromagnetic, has been extensively investigated as a part of exchange bias structures since it promises the electrical field control over the exchange bias. This work focuses on the exchange interaction between ferromagnet CoFe2O4 (CFO) and BFO. Bilayer films CFO and BFO were first grown on SrTiO3 (STO) by Pulsed Electron Deposition (PED) and then field cooled in magnetic field. XRD showed single phase CFO and BFO were grown epitaxially on STO single crystal substrates, and the typical thickness was 21nm for BFO and 3.6$\sim $18nm for CFO. In the bilayer structure, we observed that the coercive field increased 100{\%} at 300K and 30 {\%} at 50K, comparing to these of CFO single layer. Also in the bilayer structure, a noticeable exchange bias field (Hex) increased from $\sim $30 Oe at 300 K to $\sim $ 60 Oe at 50 K. We will discuss the impact of the film thickness and the roughness of interface on exchange bias. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J9.00013: Magnetic and ferroelectric properties of patterned multiferroic CoFe$_{2}$O4-BiFeO$_{3}$ nanocomposites Ryan Comes, Mikhail Khokhlov, Hongxue Liu, Jiwei Lu, Stuart Wolf CoFe$_{2}$O$_{4}$ (CFO) offers unique properties as a magnetoelectric material due to its large magnetoelastic response when strained. Previous work has shown that when CFO is co-deposited with BiFeO$_{3}$ (BFO) nanostructured phase segregation occurs with CFO pillars forming in a BFO matrix and electrical control of the magnetic anisotropy is possible.[1] Such a system offers unique possibilities for an electrically-controlled spintronic logic scheme.[2] We have recently demonstrated the ability to control the location of the CFO pillars in CFO-BFO nanocomposites using e-beam lithography patterning of uniform CFO films grown on Nb-doped SrTiO$_{3}$. Square arrays of pillars with spacings as small as 100 nm have been grown in nanocomposites using pulsed electron deposition. Piezoresponse force microscopy (PFM) measurements show clear ferroelectric response in the BFO matrix. The out-of-plane piezoelectric response, $d_{33}$, has been measured via PFM within the BFO matrix and is in good agreement with published results for BFO. Magnetic force microscopy (MFM) shows in-plane magnetic anisotropy in the pillars due to compressive in-plane strain. [1] F. Zavaliche, et al. \textit{Nano Lett.} \textbf{7} (2007). [2] S.A. Wolf, et al. \textit{Proc. IEEE} \textbf{98} (2010). [Preview Abstract] |
Session J10: Invited Session: How Reliable are Computed Electronic Properties?
Sponsoring Units: DCOMPChair: Stefano Curtarolo, Duke University
Room: 210A
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J10.00001: Validation of Atomic, Molecular and Condensed Matter Calculations Invited Speaker: Eric Shirley The advances in calculational techniques have brought first-principles calculations to a state of frequently having their results for various physical properties agreeing will with measured results for the same properties. Validation of computational results can therefore be an important undertaking in the overall scheme of first-principles work. Along these lines, such validation does not refer to the appropriateness of approximations made in calculations which are necessary to have a practicable methodology. Rather, such validation refers to whether calculations are implemented correctly given whatever approximations one assumes. Three areas are to be discussed in turn to illustrate issues that may arise. These include density-functional total-energy calculations in atoms and ions, in which as many as four atomic-structure programs were tested (and corrected) to permit reasonably high-precision comparisons of results. Second, calculations of dielectric properties and excitation spectra of solids shall be discussed, to illustrate the myriad of choices one might be required to make in terms of approximations, and how various approximations' results can only be compared if numerical methodologies used in conjunction with the respective approximations are sufficiently similar. Third, as a point of reference, brief consideration shall be given to the progress in the calculational validation area made within the quantum chemistry community. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J10.00002: New Tools for the Verification and Validation of Electronic Structure Calculations Invited Speaker: Francois Gygi First-principles electronic structure calculations are playing an increasingly important role in the prediction of materials properties and in the interpretation of experimental data. Numerous simulation codes including various levels of approximations and various numerical approaches are now readily available to users. The complexity of first-principles calculations and the large number of input parameters needed in a simulation make it challenging to obtain high quality, reproducible data. Results obtained with different electronic structure codes are difficult to compare as they involve a multitude of data formats, making the process of verification and validation (V\&V) of electronic structure data complex and error prone. In order to facilitate V\&V activities, we have introduced ESTEST [1,2], a web-based framework that allows for automatic comparison and post-processing of results obtained with six electronic structure codes. Recent developments make it possible to extend this functionality to a decentralized network of servers. We discuss general issues related to the process of verification and validation of electronic structure data and outline requirements for the development of future V\&V tools. \\[4pt] [1] G. Yuan and F. Gygi, Computational Science \& Discovery 3, 015004 (2010) doi:10.1088/1749-4699/3/1/015004G\\[0pt] [2] http://estest.ucdavis.edu [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J10.00003: Single-electron excitations of molecules and solids -- reliability and robustness of density-functional theory and \textit{GW} calculations Invited Speaker: Matthias Scheffler State-of-the-art theory addresses single-electron excitations in molecules and condensed matter by linking density-functional theory (DFT) with many-body perturbation theory. Experimentally such results correspond to measurements by direct or indirect photoemission. In actual calculations it is common to employ the pseudopotential approach, where pseudo-wave-functions enter the calculation of the selfenergy, and the core-valence interaction is treated at the DFT level. Furthermore, calculations are typically not done selfconsistently but as a first-order perturbation on some starting point. The latter may be DFT with LDA, GGA, LDA+$U$, HF, or hybrid functionals. Unfortunately, these different starting points can give noticeably different results. In this talk I will evaluate the various approximations, by comparing and analyzing pseudopotential and all-electron calculations. I will also emphasize the need for selfconsistency either by an iterative solution of the Dyson equation or by properly adjusting the zero-th order exchange-correlation functional. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J10.00004: Role of Validation and Predictions in Modeling: Specific Examples from Semiconductor Industry Applications Invited Speaker: Sadasivan Shankar With the advent of newer non-Silicon materials, using modeling to estimate properties are becoming necessary for process technology development. Since these materials are integrated as part of larger devices, interfaces and material domains are increasingly modulating properties of materials. Unlike in bulk materials, electronic and thermodynamic properties are difficult to characterize in these material structures as the device sizes overlap with material domains. We will illustrate specific cases from semiconductor processing and property estimation on the importance of verification of models for internal consistency and validation with experimental data. Given the discrepancy of scales between predictions and measurement, techniques need to bridge them. In addition, models that are developed need to be modular with open interfaces for cross-checking and integration across scales as indicated in the recently announced Materials Genome Initiative.\\[4pt] [1] President's initiative on Materials Genome Initiative for Global Competitiveness, June 2011\\[2pt] [2] S. Shankar, B. V. McKoy, W. L. Morgan, ``Self-Consistent Modeling of Weakly Ionized Plasmas-Challenges in Quantum and Classical mechanics,'' Sixth U.S. National Congress on Computational Mechanics, U.S. Association for Computational Mechanics, Dearborn, Michigan, (2001) [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J10.00005: Materials by design Invited Speaker: Gerbrand Ceder |
Session J11: Focus Session: Graphene Structure, Stacking, Interactions: Density Functional Theory
Sponsoring Units: DMPChair: Vitor Pereira, National University of Singapore
Room: 210B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J11.00001: Electronic structure, electron-phonon coupling and superconductivity in metal-doped few-layer graphene Serdar Ogut, Xuhui Luo, Taner Yildirim We systematically investigate from first-principles how the electronic properties and electron-phonon (el-ph) coupling change in metal (Ca, Li) doped graphene as we tune the number of graphene layers from single layer to three layers and to the case of bulk graphite. We find that the Fermi level and el-ph constant can be tuned by the number of layers. In particular, the number of graphene layers has a large effect on the inter-layer metal free-electron-like states, which give rise to large el-ph constant in these systems. Surprisingly, the el-ph coupling in Ca intercalated tri-layer graphene system is stronger than one in the superconducting Ca-doped graphite. Our results suggest the possibility of high T$_{c}$ superconductivity in metal doped few-layer graphene for nanodevice applications. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J11.00002: First-principles investigation of lithium doped of bilayer graphene and Lithium Intercalated Carbon Nanotubes Oguz Gulseren, Huseyin Sener Sen We have performed first-principles calculations based on density-functional theory for understanding of the structural and electronic properties of Li doped bilayer graphene and Li intercalated CNTs especially addressing the controversial charge transfer state between Li and C. We have checked the possible adsorption, substitution and intercalation of Li by using a bilayer graphene system both with AB stacking (12 different initial configurations) and AA stacking (8 different initial configurations). All calculations are repeated both with LDA and GGA exchange-correlation potential, even though the values of binding energies are different, their order and corresponding physical picture are same from both of the functionals as well as the stackings. In conclusion, we can summarize that Li prefers the hollow site adsorption geometry and it prefers intercalation but not the substitution. In these adsorption modes, almost 0.9 electron of Li atom is transferred to neighboring carbon atoms network leaving positively charged core behind. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J11.00003: Ab Initio Study of the Interactions between Dopant Atoms in Graphene Igor Vasiliev, Nabil Al-Aqtash, Tarek Tawalbeh, Khaldoun Al-Tarawneh We present a first-principles computational study of the interactions between the boron (B) and nitrogen (N) dopant atoms in graphene. Our calculations are carried out using density functional theory combined with the generalized gradient approximation for the exchange-correlation functional. The total energies, equilibrium geometries, electronic charge distributions, and densities of states of doped graphene sheets are examined in cases of B-B, N-N, and B-N co-doped graphene. We find the B-B and N-N interactions to be repulsive and the B-N interaction to be attractive. In all cases studied, dopant-dopant interactions appear to have a relatively short range. The interaction energy between the two dopant atoms is found to be inversely proportional to the square of the separation distance. We interpret these results in terms of structural relaxation and electronic charge transfer. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J11.00004: Ab Initio Study of the Interactions between Dopant Atoms and Point Defects in Graphene Tarek Tawalbeh, Igor Vasiliev We apply a first-principles computational method based on density functional theory to study the interaction of substitutional boron and nitrogen atoms with Stone-Wales defects and vacancies in graphene. Our calculations are carried out using a pseudopotential technique combined with the generalized gradient approximation for the exchange-correlation functional implemented in the SIESTA electronic structure package. Graphene sheets are modeled by periodic supercells containing up to 160 atoms. The equilibrium geometries, total energies, electronic structures, and densities of states of doped and defective graphene sheets are examined as a function of the separation distance between dopant atoms and point defects in graphene. The results of our study demonstrate the presence of attractive interaction between dopant atoms and point defects in graphene. The interaction energy decreases rapidly with increasing the dopant-defect separation distance. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J11.00005: Many-body study of cobalt adatoms adsorbed on graphene Yudistira Virgus, Wirawan Purwanto, Henry Krakauer, Shiwei Zhang Interest in the adsorption of transition metal adatoms on graphene has grown rapidly. The interaction between magnetic adatoms and graphene may have applications in designing spintronics devices. Several theoretical and experimental studies have examined Co adatoms on graphene. Calculations of Co--graphene systems have largely been done at the density functional theory (DFT) level, with local or semi-local functionals and with an empirical Hubbard on-site repulsion $U$ (LDA+U). We use auxiliary-field quantum Monte Carlo (AFQMC), in combination with DFT and quantum chemistry methods, to examine the effects of electron correlation in Co--graphene systems, without adjustable parameters. Binding energy curves for Co--graphene and model structures will be presented, and their implications on the electronic and structural properties will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J11.00006: Adsorption of group VA atoms in single and double vacancy of graphene Shyam Kattel, Boris Kiefer, Plamen Atanassov Dopants can greatly affect the electronic structure of graphene. In this study we report on the effect of group VA elements associated with single vacancy (SV) and double vacancy (DV) on the electronic structure of graphene. The results of our density-functional-theory (DFT) computations predict strong and exothermic chemisorption of group VA atoms to SV and DV in graphene (M@SV/DV, where M=N, P, As, Sb, or Bi) which leads to large, systematic and diverse effects on its electronic structure. We find that N@SV has a small band gap below Fermi level (E$_{F})$ and P@SV has semiconducting and magnetic properties consistent with previous studies. Our preliminary band structure computations show that As@SV induces $\sim $100{\%} spin polarization close to E$_{F}$ while Sb@SV and Bi@SV are metallic. In contrast, N@DV has a large spin polarization near E$_{F}$ while all other DV defects are predicted to be metallic. These results suggest that especially As@SV and N@DV may have interesting applications in spintronics and nanoelectronics. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J11.00007: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J11.00008: Stability and electronic structure of the functionalized graphene layers Jacek A. Majewski, Karolina Milowska, Magdalena Birowska, Magdalena Woinska We present results of extensive ab initio calculations in the framework of the density functional theory of the graphene layers (GLs) doped with nitrogen and boron, and also functionalized with simple OH, COOH, NH$_{n}$, and CH$_{n}$ molecules. We calculate binding energies, heat of formation, resulting local deformations (characteristic sp$^{3}$ rehybridization of the bonds induced by fragments), electronic structure, elastic properties (Young's modulus and Poisson ratio), and conductance of doped and functionalized GLs as a function of the density of the functionalizing systems. Generally, the stability of the functionalized graphene layers decreases with the growing concentration of functionalizing molecules and we determine the critical density of the molecules that can be chemisorbed on the surface of GLs. We find out that the GLs functionalization leads in many cases to the opening of the graphene band gap and can be therefore utilized in graphene devices. In particular, the zero band gap in K-point of the pristine single GL increases to 0.11, 0.12, 0.25, and 0.24 eV for the GL functionalized with OH, NH, NH$_{2}$, and COOH, respectively. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J11.00009: Band Structures of bilayer graphene with experimentally measured twisting angles Houlong L. Zhuang, Robin W. Havener, Jiwoong Park, Richard G. Hennig Using density functional theory (DFT) and tight binding calculations, we study the band structure of bilayer graphene as a function of twisting angles and compare the results to spectroscopic measurements.To test the accuracy of DFT, we first compare the band structure with DFT and the G$_0$W$_0$ approximation for several bilayer graphene systems with special twisting angles.Our DFT results agree with previous DFT calculations for the weak coupling between the two layers and the band structure.Calculations of the quasiparticle dispersion using the G$_0$W$_0$ approximation show that DFT underestimates the Fermi velocity of bilayer graphene and the bandgap away from the $K$ point.Scaling of the DFT band structure by an empirical parameter that depends on the twist angle accounts for most of the difference between the DFT and the G$_0$W$_0$ approximation. Based on the G$_0$W$_0$ calculations, we fit a set of tight-binding parameters for the interlayer coupling Hamiltonian. Using this tight-binding model we study the band structure of bilayer graphene for twist angles that closely match experimental systems and have larger unit cells. The results of these band structure calculations explain the experimentally observed G band resonance in Raman spectroscopy for specific twist angles. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J11.00010: From graphene to graphite: A first-principles study Bi-Ru Wu The multilayer graphene and graphite are systematic studied via a first-principles method based on density functional theory and general gradient approximation. The hexagonal, Bernal and rhombohedral stacking types are considered. The total energy, the electronic structure and the strength of the multilayer graphene and graphite are analyzed to study the process of graphene to graphite. We predict the number of graphene multilayers shall be greater than 15 layers to behave as a graphite bulk. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J11.00011: Accurate computational studies of carbon doped two-dimensional boron-nitride Hyoungki Park, Amita Wadehra, John W. Wilkins, Antonio H. Castro Neto Advances in development of atomic-layer crystals with a plethora of new materials are greatly extending the range of possible applications of these two-dimensional (2D) materials. One of these materials is the hexagonal structure of boron nitride (h-BN). Hexagonal BN has a wide band gap and a lattice constant similar to that of graphene. We show that even small quantities of C atoms can offer new functionalities and transform h-BN to be an amazing playground for 2D physics. Large-scale accurate density-functional-theory calculations with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional reveal the electronic and the magnetic properties of h-BN with substitutionally embedded carbon atoms. Results of local magnetic moments induced by substitution and their interactions are presented for low C concentrations. We also show the electronic structures of quantum dots made of carbon nano-domains for applications in optics and opto-electronics. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J11.00012: Electronic and Structural Properties of Graphene Dots in h-BN ZhaoHui Huang, James Chelikowsky A major challenge for graphene-based applications is the creation of a tunable electronic band gap as would be present for traditional semiconductor alloys. Since hexagonal boron nitride has a very similar structure to graphene, it is a natural candidate to modify the electronic structure of graphene by forming a hybrid phase sheet containing domains of graphene and hexagonal boron nitride, as has been done experimentally. Here we investigate the properties of such hybrid sheets by using pseudopotential-density functional theory implemented in real space. We find for a graphene dot comparable in size to those observed in experiment, the band gap of the sheet is not significantly modified. Moreover, when the size of graphene dot decreases below $\sim$13\AA, strong structural instabilities of the graphene domain occur. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J11.00013: Hybrid density functional study of 2D graphene-boron nitride (BCN) nanostructures Amita Wadehra, Hyoungki Park, John W. Wilkins, Antonio H. Castro Neto Graphene has attracted enormous research interest in the last few years because of its intriguing physics as well as application potential. Recent synthesis of BCN nanostructures by doping graphene with a wide bandgap insulator boron nitride (BN) has unveiled new possibilities for this material [1]. BCN nanostructures are semiconductors and possess interesting properties that are distinct from the parent compounds. Reliable theoretical estimates can predict the feasibility and usefulness of still largely unexplored BCN nanostructures, and provide a route to engineer their properties. We study electronic structures of a variety of 2D BCN nanostructures using hybrid functional HSE in density functional theory (DFT). We show that their properties can be gradually tuned and are sensitive to composition and the type of configurations. In agreement with experimental observation, a strong tendency to phase-segregate exists for low concentration of BN in graphene. We also investigate magnetic properties of graphene containing substitutional nitrogen atoms, and their suitability for magnetic devices.\\[4pt] [1]. L. Ci et al., Nature Materials 9, 430 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J11.00014: First-principles and Tight-binding Investigations of Graphene-like Systems X.W. Sha, E.N. Economou, D.A. Papaconstantopoulos, M. Pederson, M.J. Mehl, M. Kafesaki We have performed first-principles calculations of graphene-like systems with large unit cells of the order of 100 atoms. These are mostly quasi-circular pieces of graphene with or without hydrogen atoms to passivate dangling bonds. We have also introduced nitrogen replacements of carbon atoms to explore possibilities of creating a quantum-mechanical analog of the split-ring resonator used in negative refractive index metamaterials. In addition, vibrational spectra were calculated to check the stability of the small flakes. Furthermore, we have used the NRLMOL DFT code with extensions to treat an AC magnetic field coupled to spin and orbital moments. We simulate the graphene-like system to a circuit model to show the cancellation of individual loop currents and the emergence of an edge ballistic current. A tight-binding Hamiltonian was fitted to our NRLMOL results using the NRLTB method. The TB approach starts with an exact fit to the benzene molecule and the resulting TB parameters are transferable to the larger molecules matching well the HOMO-LUMO gap found by the DFT. The NRLTB scheme allows to calculate the electronic spectra of much bigger systems, and examples with results for systems with more than a thousand atoms will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J11.00015: Quasiparticle Band Gaps of Graphene and Graphone on Hexagonal Boron Nitride Substrate Neerav Kharche, Saroj Nayak Graphene holds great promise for post-silicon electronics; however, it faces two main challenges: opening up a band gap and finding a suitable substrate material. Graphene on hexagonal boron nitride (hBN) substrate provides a potential system to overcome these challenges. While theoretical studies suggested a possibility of a finite band gap of graphene on hBN, recent experimental studies find no band gap. We have studied graphene-hBN system using the first-principles density functional method and the many-body perturbation theory within GW approximation [1]. A Bernal stacked graphene on hBN has a band gap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits band gaps larger than 2.5 eV. The graphone band gap is due to chemical functionalization and is robust in the presence of misalignment, however, it reduces by about 1 eV due to the polarization effects at the graphone/hBN interface.\\[4pt] [1] N. Kharche and S. K. Nayak, \textit{Nano Lett.}, DOI: 10.1021/nl202725w, (2011). [Preview Abstract] |
Session J12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Strain and Structure
Sponsoring Units: DMPChair: Alain Diebold, University at Albany - SUNY
Room: 210C
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J12.00001: Growth-induced electronic properties of epitaxial graphene Invited Speaker: Phillip First The growth of epitaxial graphene on silicon carbide is challenging to understand and control, yet rife with scientific and technological opportunities. This is due in part to different growth-induced structures such as the ``moire'' alignment of graphene layers in multilayer epitaxial graphene on SiC($000\bar{1)}$ and the formation of sidewall ribbons at natural and lithographically-defined SiC(0001) step-bunches (nanofacets). We apply scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the local energy bands of such growth-induced structures. STS at cryogenic temperatures and large magnetic fields creates a comb of discrete Landau level energies that we use to quantitatively characterize the local electronic properties. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J12.00002: Colossal Corrugations in Freestanding Graphene Observed with STM P.M. Thibado, P. Xu, Y. Yang, S.D. Barber, M.L. Ackerman, J.K. Schoelz, Salvador Barraza-Lopez, L. Bellaiche, Igor A. Kornev The discovery of graphene, a unique two-dimensional electron system with extraordinary physical properties, has ignited tremendous research activity in both science and technology. Graphene interactions with a substrate, such as SiO$_{2}$/Si, are known to significantly degrade the electrical performance of graphene devices. Alternatively, suspending a graphene device eliminates the substrate interaction, thereby yielding a 10-fold increase in mobility. However, a detailed investigation on the microscopic scale explaining the origin of these improvements has yet to be completed. In this talk, we present for the first time atomic-resolution STM images of a freestanding graphene membrane. Samples were prepared by direct CVD growth and by large graphene sheet transfer, both onto a 2000-mesh copper grid. Atomic-scale corrugation amplitudes were observed in perfect registry with, yet 50 times larger than the expected electronic corrugations. Density functional theory revealed that charge localization occurs directly beneath the STM tip due to bond angles rotating away from sp$^{2}$ hybridization as graphene flexes in response to the electrostatic attraction. A detailed model of the 3-way interaction which accounts for the observed behavior will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J12.00003: Melon: A carbon-nitride analog to graphene Joel Therrien, Yancen Li, Daniel Schmidt Although graphene remains the premier 2-D material, many others have been shown to exist. A close analog to graphene would be a two-dimensional sheet composed of carbon and nitrogen, known as melon. Bulk melon, also known as graphitic carbon-nitride, has been successfully synthesized and shown to be an organic semiconductor with a band-gap around 2.7 eV. We report on the successful synthesis of single layer and few layer melon. The physical and electrical characteristics of this close cousin to graphene will be presented along with the synthesis method. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J12.00004: AB-stacked multilayer graphene synthesized via chemical vapor deposition: a characterization by hot carrier transport Debtanu De, Carlos Diaz-Pinto, Viktor Hadjiev, Haibing Peng We report the synthesis of AB-stacked multilayer graphene via ambient pressure chemical vapor deposition on Cu foil. Four-terminal devices were fabricated from such graphene and characterized by hot carrier transport at temperatures down to 240 mK and in magnetic fields up to 14 T. The differential conductance (dI/dV) shows a characteristic dip at longitudinal voltage bias V=0 at low temperatures, indicating the presence of hot electron effect due to a weak electron-phonon coupling. Under magnetic fields, the magnitude of the dI/dV dip diminishes through the enhanced intra-Landau level cyclotron phonon scattering. Our results provide new perspectives in obtaining and understanding AB-stacked multilayer graphene, important for future graphene-based applications. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J12.00005: Signatures of indentation strain in graphene conductance Matthew Barr, Mario Borunda, Eric Heller We investigate effects on the conductance of a graphene sheet of electron scattering from a localized indentation. Strain in graphene creates effective magnetic fields, and the scattering from a radial strain profile is distinct from other short range scatterers. Through tight-binding calculations, we examine the expected conductance in several experimental geometries, including within graphene nano-ribbons and suspended graphene. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J12.00006: Transport in Suspended graphene under strain Hang Zhang, Jhao-Wun Huang, Jairo Velasco, David Dung Tran, Kevin Myhro, Zeng Zhao Adam, Fenglin Wang, yongjin lee, Wenzhong Bao, Chun Ning (Jeanie) Lau Suspended graphene devices with graphene flakes were fabricated using acid etching approach, and strained via application of gate voltage. The stretching procedure was observed and verified by in situ SEM imaging. We observe a change in the devices' minimum conductance and mobility values. Latest experimental results will be compared with theoretical models. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J12.00007: Suppression of Weak-Localization Effect in Strained CVD --grown Graphene Xiaochang Miao, Sefaattin Tongay, Arthur F. Hebard We investigate the magnetic field and temperature-dependent transport properties of CVD-grown graphene subjected to different strains. The graphene is transferred to kapton substrates to which a blending force can be applied. In zero magnetic field, the prefactor to the logarithmic-in-temperature conductivity correction decreases by an approximate factor of 3 for strains as high as 0.6~{\%}. There is also a concomitant decrease in diffusivity by a factor of 7. At 5~K we observe negative magnetoresistance for fields up to 0.2 Tesla followed by positive magnetoresistance at higher fields. We attribute the low field negative magnetoresistance to weak-localization and find that it is well described by theory. The strains resulting from the applied blending force inhibit the intervalley scattering rate more than an order of magnitude, thereby leading to a suppression of weak-localization. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J12.00008: Probing Mechanical Properties of Chemical Vapor Deposition Graphene Membranes Using Indentation Methods Gwan-Hyoung Lee, Ryan Cooper, Sungjoo An, Arend van der Zande, Nicholas Petrone, Sunwoo Lee, Alex Hammerberg, Changgu Lee, Bryan Crawford, Jeffrey Kysar, James Hone Recent experimental studies have shown that two-dimensional pristine graphene is the strongest material ever measured. We used Atomic Force Microscopy (AFM) and Agilent G200 nanoindenter to measure the mechanical properties of graphene films obtained by Chemical Vapor Deposition (CVD). CVD graphene with different grain size and number of layers were produced in controlled synthetic conditions and transferred onto silicon dioxide substrate with holes of various diameters. Nano-indentation measurement revealed that stiffness and fracture strength of CVD graphene membranes are similar to those of pristine graphene membranes under the condition that suspended graphene membrane is within a single grain boundary without defects. Furthermore, elastic modulus and fracture strength of multi-layer graphene membranes increase with respect to the number of layers. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J12.00009: Electron transport measurement of graphene under one-dimensional local strain A. Kanda, H. Tomori, Y. Nukui, Y. Toyota, H. Karube, S. Nihei, Y. Ootuka, K. Tsukagoshi, M. Hayashi, H. Yoshioka Introducing a nonuniform strain is a promising technique for controlling electron transport in graphene. Theories have predicted the formation of band gaps with properly designed strain; however, reports on experimental transport properties of strained graphene are quite limited. In this presentation, we report the measurement of electron transport in graphene under one-dimensional local strain. The local strain was introduced by inserting a one-dimensional dielectric nanorod between a graphene film and its substrate, using a technique reported in [1]. We found that the conductivity across the strained region decreases around the Dirac point in comparison with the unstrained graphene attached to the substrate, although the mobility far from the Dirac point is almost unchanged. The results cannot be explained by the change of the capacitance between the graphene film and the gate electrode, indicating that the strain affects the electron transport. The experimental results on strained and unstrained graphene devices from the same graphene film as well as the numerical results will be discussed. \\[4pt] [1] H. Tomori {\it et al.}, Appl. Phys. Express {\bf 4}, 075102 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J12.00010: Introducing designed local strain to graphene using dielectric nanostructures H. Tomori, A. Kanda, H. Goto, Y. Nukui, Y. Toyota, H. Karube, S. Nihei, Y. Ootuka, K. Tsukagoshi, M. Hayashi, H. Yoshioka Strain engineering is a promising method for controlling electron transport in graphene. In this presentation, we report a simple and easy method for inducing designed local strain in graphene films [1]. Nanostructures made of a dielectric material (electron beam resist) are placed between graphene and the substrate, and graphene sections between nanostructures are attached to the substrate. The strength and spatial pattern of the strain can be controlled by the size and the separation of nanostructures. Application of strain is confirmed by the Raman spectroscopy as well as from scanning electron microscope (SEM) images. The Raman 2D peak shows spatially nonuniform downshift, which corresponds to the positions of the resist nanostructures. From SEM images, the maximum stretch of the graphene film reaches about 20\%. This technique can be applied to formation of band gaps in graphene. \\[4pt] [1] H. Tomori {\it et al.}, Appl. Phys. Express {\bf 4}, 075102 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J12.00011: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J12.00012: Mechanical-Electric-Magnetic Coupling Effects in Black and White Graphenes Wanlin Guo Nanoscale multifield couplings can turn very common materials such as carbon and boron nitride into promising functional materials for many device applications. We recently found that the magnetism in graphene nanoribbons on silicon substrates can be tuned linearly by applied bias voltage (\textbf{\textit{Phys.Rev.Lett}}, \textbf{103}, 187204, 2009), and this novel magnetoelectric effect is robust to material and geometry variations. Adsorbed graphene nanoribbons can also create tunable magnetism on silicon surface (\textbf{\textit{Phys.Rev.B}} \textbf{82}, 235423, 2010). Strain tunable magnetism has also been found in defect graphene (\textbf{\textit{ACS Nano}} \textbf{4}, 2124, 2010; \textbf{\textit{Phys. Rev. B}} \textbf{82}, 085425, 2010). Contrast to the zero-gap graphene, Hexagon-BN layers (white graphene) and rolled-up nanotubes are generally insulating, we show that the wide gap in them can be tuned into semiconducting range, even closed in BN nanoribbons by electric fields and narrowed by reduced tube diameter or local curvature radius (\textbf{\textit{Nano Lett.}} \textbf{10}, 5049, 2010; \textbf{\textit{Phys. Rev. B}} \textbf{82}, 035412, 2010). What is more, our recent experiments have demonstrated that flow-induced-voltage in graphene can be 20 folds higher than in graphite (\textbf{\textit{Appl.Phys.Lett}}. 99, 073103 (2011)). Such extraordinary mechanical-electric-magnetic coupling effects in graphene and BN systems open up new vistas in functional devices compatible with silicon-based technology for efficient energy conversion and novel functional systems. [Preview Abstract] |
Session J13: Focus Session: Magnetic Nanostructures - Thin Films, Surfaces and Interfaces
Sponsoring Units: DMP GMAGChair: Hao Zeng, SUNY Buffalo
Room: 211
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J13.00001: A Symmetry Microscope for Disordered Materials Daniel Parks, Keoki Seu, Run Su, Eric Fullerton, Erik Shipton, Sujoy Roy, Steve Kevan We use position-resolved coherent speckle patterns in concert with rotational x-ray autocorrelation analysis to uncover ``hidden'' ordering at specific length scales in labyrinthine magnetic domains. Central to this measurement is understanding how finite-size effects influence the statistics of rotational cross-correlations. To this end, we present numerical simulations of domain configurations whose speckle patterns display rotational ordering similar to experimental candidates, and examine how the real-space structure of ordered and disordered domains differs. In a scanning measurement, we observe what appears to be an island of ordering in an otherwise disordered perpendicular magnetic thin film. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J13.00002: New Magnetic State In Thin-Film Antiferromagnets and Uncompensated Magnetization Igor V. Roshchin, K.E. Badgley, K.D. Belashchenko, Mikhail Zhernenkov, Michael R. Fitzsimmons We observe a new magnetic state, for which the origin (M(H=0)=0 point) is outside the major hysteresis loop, which means that zero magnetization at zero applied field cannot be reached isothermally. We observe this in antiferromagnet-only, (110)-FeF$_{2}$ grown on MgF$_{2}$, samples. The surface is responsible for the macroscopically broken time-reversal symmetry, uncompensated magnetization (UM) in a nominally compensated antiferromagnet, and, ultimately, for the new magnetic state. We argue that it is an equilibrium state. Magnetometry and polarized neutron reflectivity (PNR) measurements indicate that the UM is present in this AF. We report rather unusual properties of this UM, including the ``intrinsic exchange bias.'' While its manifestation in a shift of the hysteresis loop is similar to that of the ``classical'' exchange bias observed in bilayers, here, it is observed in a single layer material. We discuss the implications of the origin of the UM on the exchange bias mechanism. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J13.00003: Existence of Ferromagnetism in Stacked Bilayers of Pd/C60 Siddhartha Ghosh, Sefaattin Tongay, Arthur F. Hebard, Hasan Sahin, Salim Ciraci We report on an experimental and theoretical study of the magnetic properties of multilayer structures fabricated by alternating layers of sputter-deposited Pd and thermally-sublimated C$_{60}$. Auger Electron Spectroscopy and SEM techniques have been used to characterize samples for which magnetic measurements in a commercially available SQUID have been made. The magnetization measurement reveals ferromagnetism in the Pd/C$_{60}$ system, which has Curie temperature T$_{C} \quad \sim $ 450K, modified Bloch coefficient P $\sim $ 2.7 and a temperature-independent coercive field of 50 Oe. The observed ferromagnetism is surprising since both C$_{60}$ and Pd are non-ferromagnetic in the non-interacting limit. Density functional theory (DFT) calculations show that while the C$_{60}$ molecules are nonmagnetic unless polymerized, Pd films have a degenerate ground state that can become ferromagnetic with a weak perturbation. Though the calculated charge transfers of $\sim $0.06 e between C$_{60}$ and Pd are not the cause of ferromagnetism, DFT shows that ferromagnetism can be associated with Pd clusters or an interaction of C$_{60}$ molecules with sharp edges of the Pd thin film. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J13.00004: Spin-Wave Transmission in Ferromagnetic Thin Films: Flexoelectric Control and Spin-Wave Drag Tianyu Liu, Giovanni Vignale, Michael Flatte Spin waves in insulating ferromagnets have recently emerged as an effective low-dissipation carrier of spin currents. In this work we explore a novel form of control of spin waves by {\it flexoelectric interactions}, which couple an electric field to the spatial gradient of the magnetization. We show that not only the short-wavelength exchange spin waves, but also the long-wavelength magnetostatic spin waves in a thin-film of magnetic insulator can be effectively controlled by an electric field. In fact, the relative electric-field-induced phase shift is even larger for magnetostatic spin waves than for exchange spin waves. We further show that spin waves in an insulating ferromagnetic film can excite a spin current in an adjacent conducting or insulating film, by way of long-ranged dipole-dipole interaction between the layers, in a magnetic analogue of the electronic Coulomb drag. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J13.00005: Impact of preparation conditions on the magnetocaloric properties of Gd thin films H.F. Kirby, D.D. Belyea, J.T. Willman, C.J. Hendryx, C.W. Miller The impact of the deposition temperature and gettering were investigated on Ta(5nm)/Gd(30nm)/Ta(5nm) thin films' magneto caloric(MCE) properties. The samples were grown by magnetron at temperatures up to 600\r{ }C, with and without gettering. Structure of the samples was investigated by X-ray diffraction and ray reflectivity. The isothermal magnetization of the samples was above and below the Curie temperature of the Gd. The entropy change associated with the second order phasewas calculated from M(H,T) using the thermodynamic Maxwell. Increasing the deposition temperature generally improves entropy peak (magnitude, FWHM, and temperature of the peak), but leads to significant oxidation. The ungettered sample grown at00\r{ }C was purely GdO (111). Gettering the chamber by sputtering Tathe walls of the chamber for 30 minutes prior to deposition this oxidation issue, and increased the relative cooling power RCP) of films grown at elevated temperatures. The RCP values of the sample set were increased by as much as 42{\%} over ungettered. Supported by AFOSR and NSF. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J13.00006: Chiral modulations in MnSi thin films Theodore Monchesky, Eric Karhu, Ulrich R\"{o}{\ss}ler, Alexei Bogdanov, Samer Kahwaji, Brian Kirby, Helmut Fritzsche, Michael Robertson, Charles Majkrzak We present an investigation of the magnetic textures present in MnSi thin films grown on Si(111) by molecular beam epitaxy. The magnetic structure is investigated with SQUID magnetometry and polarized neutron reflectometry (PNR). For an out-of-plane magnetic field, a conical phase is formed with a reduced wavelength of $2 \pi / Q =$~13.9 nm that has both left-handed and right-handed chirality due to the presence of inversion domains in the films. We show that the epitaxially induced tensile stress in the MnSi thin films creates an easy-plane uniaxial anisotropy. The magnetoelastic coefficient is obtained from SQUID measurements in combination with transmission electron microscopy and x-ray diffraction data. The agreement between density functional calculations of the coefficient with the experimental value support the conclusion that the uniaxial anisotropy originates from the magnetoelastic coupling. For an in-plane magnetic field, theoretical calculations based on a Dzyaloshinskii model that includes an easy-plane anisotropy predict a variety of modulations to the magnetic order that are not observed in bulk MnSi crystals. Evidence for these states is found in the SQUID and PNR measurements. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J13.00007: Stabilization of surface magnetism by organic-acid adsorption J. G\'azquez, J. Salafranca, N. P\'erez, A. Labarta, S.T. Pantelides, S.J. Pennycook, X. Batlle, M. Varela Magnetically-disordered layers at the surface of nanoparticles and low dimensionality magnetic oxides significantly reduce the magnetization density with respect to bulk values. However, high crystal quality Fe$_3$O$_4$ nanoparticles capped with non-magnetic organic acid molecules display a surprisingly high magnetization, of unknown origin. Here, we present a real space structural, chemical and magnetic characterization of oleic-acid-caped Fe$_3$O$_4$ nanoparticles with nanometer resolution, demonstrating the presence of a strong magnetic surface layer. In combination with theoretical calculations, we establish the key role of the nanoparticle/organic-acid bond. Magnetization is restored in the surface layer because the bonding with the acid's O atoms partially lifts the surface reconstruction, resulting in surface O-Fe atomic configuration and distances close to the bulk values. Our findings have implications for the optimization of magnetic properties of nanoparticles and thin films. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J13.00008: Surface Structure of L1$_{0}$-MnGa(111) Ultra-Thin Films Studied Using Scanning Tunneling Microscopy and First Principles Calculations Reyes Garcia Diaz, Kangkang Wang, Noboru Takeuchi, Arthur Smith Manganese gallium alloys have drawn lots of attention recently for their many desirable properties such as high spin polarization and low damping terms. Furthermore, the magnetic properties depend sensitively on the Mn:Ga stoichiometry, ranging from ferromagnetic for MnGa, to ferrimagnetic for Mn$_{2-3}$Ga, and antiferromagnetic for Mn$_{3}$Ga, giving this material great magnetic tune-ability. In this talk, we will focus on the stoichiometric L1$_{0}$-structured MnGa(111) surface and study its atomic, electronic, and magnetic properties. Ultra-thin MnGa films ($<$ 10 nm) are grown on GaN(0001) substrates using molecular beam epitaxy, and transferred \textit{in-situ} to the analysis chamber for room-temperature scanning tunneling microcopy studies. Atomic resolution images reveal the existence of both 1$\times $1 and 2$\times $2 surface structures. We investigate these structures using periodic, spin-polarized density functional theory with the generalized gradient approximation (GGA), Vanderbilt ultra-soft pseudo-potentials, and a repeated slab geometry. The parameters are optimized to achieve the lowest-energy configuration. Results from these calculations and their comparison with the STM images will be reported. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J13.00009: Surface Ferromagnetism in Pt films Pushkal Thapa, Parashu Kharel, Renat Sabirianov, Mohammad Faiz, Julie Borchers, David Sellmyer, Boris Nadgorny It has long been recognized that Pt as well as some other noble metals are on the verge of being magnetic, with ferromagnetism in these metals readily detected in nanoparticles. Here we report the observation of surface ferromagnetism in Pt thin films. Both sputtered and e-beam evaporated Pt films of various thicknesses (from 10 $\sim $ 100 nm) have been studied. Detailed SQUID measurements indicate that the film magnetization is largely independent of its thickness. This result is consistent with the neutron scattering data, indicating the presence of a magnetic moment near the top Pt surface, and with the spin polarization measurements by point contact Andreev reflection spectroscopy, demonstrating a sizable spin polarization. Our experimental observations are supported by the first principle density functional calculations showing that certain configurations of Pt atoms on Pt (111) surface exhibit a magnetic moment of up to 1.1 $\mu _{B}$ per Pt atom. We argue that magnetism in this system is due to band narrowing, related to reduced coordination numbers of Pt atoms located at the surface. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J13.00010: TiO2 as an electrostatic template for epitaxial growth of EuO on MgO(001) by reactive molecular beam epitaxy Adrian Swartz, Jared Wong, Igor Pinchuk, Roland Kawakami Interfacial electrostatics play a key role in determining epitaxial quality in the heteroepitaxy of ionic rock salt materials. We investigate the initial growth modes and the role of interfacial electrostatic interactions of EuO epitaxy on MgO(001) by reactive molecular beam epitaxy. A TiO2 interfacial monolayer is employed to alleviate electrostatic interactions between the ions of the EuO and MgO to produce high quality epitaxial growth of EuO on MgO(001) with a 45 degree in plane rotation. For comparison, direct deposition of EuO on MgO, without the TiO2 layer, is discussed. A key difference of EuO epitaxy on TiO2/MgO is the ability to form EuO by substrate assisted oxidation and without the introduction of external oxygen to the interface. Such ultrathin films are shown to have bulk like magnetic properties [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J13.00011: Magnetic Thin Films of Inorganic Nanosheets Takashi Yamamoto, Hiroaki Namba, Yasuaki Einaga Molecule-based magnets have been fascinating materials because of the potential applications in information storage, electronic and spintronic devices. However, such applications would require arraying the active materials on a substrate or interfacing with other components. Here, we focus on fabricating multi-functional magnetic films using inorganic nanosheets as a building block. The thin films could be prepared by the modified Langmuir-Blodgett, LB, technique or the layer-by-layer, LbL, method, which are representative wet-processings for film preparation. As the magnetic LB film, we chose semiconductive titania nanosheets and magnetic Prussian Blue. Upon band gap excitation of titania nanosheets, electron injection into Prussian Blue was achieved with scavenging interlayer water molecules, leading to photoreduction to Prussian White. As the magnetic LbL film, we chose magnetic layered double hydroxide, LDH, nanosheets and non-magnetic smectite nanosheets. In powdered LDH, a coercivity increased with expanding the interlayer spacing. On the other hand, despite the larger interlayer spacing for the LbL film, a coercivity was less than that of the comparative powdered LDH. It is indicated LDH nanosheets are integrated in an anisotropic manner in the LbL films. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J13.00012: Suppression and Revival of Magnetic Correlations at Interfaces Axel Euverte, Frederic Hebert, George Batrouni, Simone Chiesa, Richard Scalettar We study a model of metal-insulator interfaces consisting of a multilayer, repulsive Hubbard Hamiltonian in which the interaction is nonzero on one set of layers and zero on another. As the interface hybridization is tuned, the magnetism and spectral functions in the correlated layers undergo an evolution in which the signatures of strong interaction are first reduced and subsequently revived due to the formation of an interfacial spin liquid phase. The penetration into the correlated layers of the suppression of magnetic order is found to be 4-6 layers. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J13.00013: Effects of inhomogeneous strain on the magnetization behavior of magnetic nanostructures in BiFeO$_{3}$/CoFe$_{2}$O$_{4}$ composite Nicolas Aimon, Mark Mascaro, Frank Liu, Markus Buehler, Caroline Ross In CoFe$_{2}$O$_{4}$/BiFeO$_{3}$ (CFO/BFO) nanostructured thin films, where ferromagnetic nanopillars are embedded in a ferroelectric matrix, electric field induced rotation of the easy axis of the ferrimagnetic nanopillars has been demonstrated experimentally [Zavaliche et al. 2005]. However, for applications where the magnetic pillars would be used to store information, electric control of the magnetization has to be acheived at the scale of a single pillar, without disturbing the neighbors, and the effects of local strain on the magnetic reversal is therefore of interest. We carried out finite element simulations of the strain state of an arrangement of CFO pillars when the BFO matrix surrounding one of them is under piezoelectric strain. Because of stress relaxation at the top free surface of the thin film, the strain is highly inhomogeneous along the pillar. The position-dependent strain was imported into a micromagnetic simulation, giving a position-dependant magnetoelasticity, to predict the switching behavior of the CFO pillars and estimate the feasibility of electric control of a single magnetic bit in this system. The reversal of the pillars was found to be highly incoherent, showing that the pillars cannot be treated as a macrospin. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J13.00014: Magnetic field dependencies of dielectric and ferroelectric properties in PbZr0.52Ti0.48O3/La3/8Ca5/8MnO3 multiferroic thin films Yi-ping Yao, Yu-kuai Liu, Si-ning Dong, Xiao-guang Li In this work, the temperature and magnetic field dependencies of dielectric and ferroelectric properties were investigated for Au/PbZr0.52Ti0.48O3/La3/8Ca5/8MnO3 (PZT/LCMO) thin films grown on (001)-oriented SrTiO3 substrates. The results indicate there exists a large magnetodielectric effect up to 30{\%} at T = 220 K, f = 1 MHz and H = 0.8 T in these multi-layer films, which is promising for practical application as compared with conventional multiferroics demanding large magnetic fields and low temperatures. From the electric polarization hysteresis loops, it is found that with increasing temperature at H = 0 T the coercive field Ec of PZT decreases at low temperature range, and then starts to increase till the Curie temperature of LCMO (Tc $\sim $ 220K) where a maximum appears. This peak is obviously suppressed and shifted to a higher temperature with increasing magnetic fields, which may be related to the depolarization field which is affected by the change of the carrier concentration in LCMO. As for the variation of remnant polarization Pr, it increases with increasing temperatures from 50 K to 300 K, but decreases with increasing magnetic fields around Tc. This magnetoelectric effect implies that the strain effect due to the magnetostriction of LCMO may also have some impact on the variation of hysteresis loops. These findings provide potential for multifunctional devices in spintronics. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J13.00015: Tailored inter and intra layer exchange coupled superlattices for optimized magnetocaloric effect Tathagata Mukherjee, S. Michalski, R. Skomski, D.J. Sellmyer, Ch. Binek We explore Magnetocaloric (MC) properties of Fe/Cr superlattices with tailored inter- and intra-layer interaction using simple 3d metals. Our multilayers are fabricated by pulsed-laser deposition with emphasis on maximizing magnetic entropy changes near room temperature. Nanostructuring\footnote{T. Mukherjee, S. Sahoo, R. Skomski, D. J. Sellmyer, and Ch. Binek, Phys. Rev. B~\textbf{79}, 144406 (2009).} allows tailoring ferromagnetic and antiferromagnetic coupling. This in concert with finite size scaling of the ferromagnetic Fe films has the potential to lead to optimized MC materials. Thermodynamic and MC properties of such Fe/Cr superlattices are studied with the help of SQUID magnetometry. Entropy changes are deduced via the Maxwell relation in single phase regions, X-ray diffraction and X-ray reflectivity are used to correlate structural data with the magnetic properties. [Preview Abstract] |
Session J14: Focus Session: Spins in Semiconductors - Spin-Orbit Coupled Electrons and Holes in Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Giovanni Vignale, University of Missouri
Room: 212
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J14.00001: Spin-dependent transport of spin-orbit coupled holes in GaAs nanostructures Invited Speaker: Alex Hamilton In undergraduate physics, we are often taught that holes in the valence band are just positively charged heavy electrons. But valence band holes are spin-3/2 particles, and this gives them very different properties to spin-1/2 electrons, particularly when confined to low dimensions. These differences show up as highly anisotropic spin properties, which can be directly probed with conventional transport measurements. We have fabricated high quality hole quantum wires that show clean and stable quantized conductance plateaus [1]. In contrast to 1D electron quantum systems, the spin-splitting in these hole wires is highly anisotropic [2], and depends only on the orientation of the in-plane magnetic field relative to the quantum wire [3]. However the orientation and $k$-dependence of the spin-splitting cannot be reconciled with existing theories, suggesting that more theoretical work is needed before we understand the physics of spin-3/2 holes, even on ``simple'' (100) surfaces. We have also studied spin-3/2 holes in quantum dots, which show characteristic signatures of Kondo physics. A clear zero-bias peak is observed in the differential conductance, which splits with an applied in-plane magnetic field. The splitting is twice as large as the splitting for the lowest one-dimensional subband, consistent with Kondo physics. Unlike electrons this splitting is highly anisotropic with magnetic field, due to the strong spin-orbit coupling [4]. \\[4pt] [1] O. Klochan \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 092105 (2006).\\[0pt] [2] R. Danneau \textit{et al.}, Phys. Rev. Lett. \textbf{97}, 026403 (2006).\\[0pt] [3] J C H Chen \textit{et al,} New Journal of Physics \textbf{12}, 033043 (2010).\\[0pt] [4] O. Klochan \textit{et al}, Phys. Rev. Lett. \textbf{107}, 076805 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J14.00002: Quantum-confined holes: More spin for your buck! Invited Speaker: Ulrich Zuelicke The physical properties of charge carriers in crystalline solids are dictated largely by the material's band structure. Typically, band electrons from the (lowest) conduction band behave very similarly to free electrons in vacuum, only with parameters such as their mass and $g$-factor adjusted. In contrast, holes from the (upper-most) valence band show a much richer behavior owing to their intrinsic spin-$3/2$ degree of freedom and this spin{'}s strong coupling to the crystal momentum of holes. In a two-dimensional (2D) quantum well, size quantisation induces an energy splitting between heavy-hole (HH) and light-hole (LH) subbands. The existence of this HH-LH \emph{splitting\/} makes it tempting to consider HH and LH degrees of freedom separately, in particular in situations where only the highest (generally HH-like) 2D hole subband is occupied. In my talk, I will focus on how such an approach overlooks intriguing differences in the mesoscopic and many-particle properties exhibited by quantum-confined holes as compared with their conduction-electron counterparts. Recent results on the density response [1] and spin susceptibility of 2D holes will be presented to illustrate the ramifications of ubiquitous HH-LH \emph{mixing\/}. I will also discuss how the temporal modulation of Rashba spin splitting in hole nanostructures generates larger-magnitude spin currents than in corresponding band-electron systems [2]. Finally, Andreev reflection of holes in a p-type-semiconductor~--~superconductor hybrid system will be considered [3], which also exhibits novel behavior due to band mixing. \\[4pt] [1] T. Kernreiter, M. Governale, and U. Zulicke, New J. Phys. \textbf{12}, 093002 (2010).\\[0pt] [2] T. Kernreiter, M. Governale, A. R. Hamilton, and U. Zulicke, Appl. Phys. Lett. \textbf{98}, 152101 (2011).\\[0pt] [3] D. Futterer, M. Governale, U. Zulicke, and J. Konig, Phys. Rev. B \textbf{84}, 104526 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J14.00003: Spin-electric stripes: electric voltage generated by spin current Yuli Lyanda-Geller At the boundaries of the two-dimensional conductor, stripes arise with an electric field transverse to the flowing electric current, and with 100\% electron spin polarization perpendicular to the 2D plane. In these boundary stripes, the magnitudes of spin polarizations are the same, the magnitudes of transverse electric fields are the same, but the directions of electric fields and orientations of spins are opposite. If the spin relaxation is negligible, the magnitudes of the electric fields are directly related to the spin current. The stripes at boundaries are separated by a center-stripe, in which the magnitude and direction of the electric field depend on the ratio of the skew scattering and side jump spin currents. The spin polarization is zero on the centerline and reaches +1 or -1 at the boundaries between the central and periphery stripes if spin relaxation of the z-component of spin normal to the 2D plane is absent. Weak spin relaxation modifies the magnitudes of the spin polarization and electric fields, with +1 or -1 spin polarization persisting at the edges of the sample. Favorable experimental settings, in which electron or hole spin relaxation of the z-component of spin is suppressed but the spin current is not, are discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J14.00004: Spin-orbit or Aharonov-Casher edge states in semiconductor two-dimensional systems L.L. Xu, J.J. Heremans, C.K. Gaspe, S. Vijeyaragunathan, T.D. Mishima, M.B. Santos In semiconductors with spin-orbit interaction we experimentally search for edge states induced by the Aharonov-Casher vector potential or Rashba-type spin-orbit interaction. The Aharonov-Casher effect is electromagnetically dual to the Aharonov-Bohm effect and is predicted to lead to a possibly helical edge state structure at two-dimensional sample edges. We use InGaAs/InAlAs heterostructures patterned into mesoscopic side-gated channel structures, where the edge states can be induced, and where backscattering between edge states can be experimentally measured in the resistance. Sweeping side-gate voltage, low temperature resistances are measured across such mesoscopic closed-path structures at either low applied magnetic field, in-plane or normal to the plane, or at fixed magnetic filling factors of 5, 6, 7, and 8 to obtain states of defined spin. Resistance oscillations are observed at low magnetic fields and around filling factor 6 as function of side-gate voltage, and we analyze the oscillations in the light of the search for the edge states (DOE DE-FG02-08ER46532, NSF DMR-0520550). [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J14.00005: Spin orbit coupling induced splitting in excitations of high mobility 2DESs Albert F. Rigosi, Ursula Wurstbauer, Aron Pinczuk, John Watson, Sumit Mondal, Michael J. Manfra, Ken W. West, Loren N. Pfeiffer Spin orbit interaction (SOI) induces a splitting of the conduction bands in two-dimensional electron systems (2DES) in GaAs. We study the impact of zero-field spin-splitting on excitations of ultra high mobility 2DESs by resonant inelastic light scattering experiments. To distinguish between splitting caused by bulk inversion asymmetry (Dresselhaus) and structure inversion asymmetry (Rashba), we studied symmetric (two-side modulation doped) and asymmetric (single-side modulation doped) quantum wells grown along (001) and (110) crystallographic directions. We probe the excitation modes as a function of transferred momentum for different crystallographic directions in the plane of the QW. At large wave vectors we find a complex splitting of the single-particle intersubband excitation mode that is strongly dependent on the combination of Dresselhaus and Rashba SOI. The observed mode splitting is a result of effective SOI fields in both, ground and first excited subband. Suitable choices of crystallographic orientations yield Dresselhaus and Rashba terms. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J14.00006: Current-induced spin polarization along spin orbit fields in strained InGaAs Benjamin M. Norman, C.J. Trowbridge, J. Stephens, A.C. Gossard, D.D. Awschalom, V. Sih Current-induced spin polarization is a phenomenon in which electron spins undergo a momentum-dependent net spin polarization \footnote{Y. K. Kato, R. C. Myers, A. C. Gossard, and D. D. Awschalom, Phys. Rev. Lett. {\bf93}, 176601 (2004)}, but the mechanism and how material parameters govern the magnitude of this effect remains an open question. Conductive channels are etched into strained n-doped InGaAs samples along the [110], [1$\overline{1}$0], [100] and [010] crystal axes with ohmic contacts at either end to allow control of electrical current. While the spin polarization direction is found to align along the direction of the measured spin-orbit effective magnetic fields \footnote{B. M. Norman, C. J. Trowbridge, J. Stephens, A. C. Gossard, D. D. Awschalom, and V. Sih, Phys. Rev. B. {\bf82}, 081304(R) (2010)}, the magnitude of the spin polarization is not proportional to the magnitude of the spin-orbit fields. Surprisingly, crystal axes with the smallest spin-orbit fields appear to have the largest net spin polarization. Our measurements suggest that the longer spin dephasing time for smaller spin-orbit interactions may play a significant role. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J14.00007: Thermodynamic compressibility and spin-splitting in one-dimensional quantum wires Luke W. Smith, A.R. Hamilton, K.J. Thomas, M. Pepper, I. Farrer, D. Anderson, G.A.C. Jones, D.A. Ritchie We study spin-splitting and the much-debated $0.7$ structure in GaAs quantum wires using compressibility measurements that directly probe the thermodynamic density of states. Two quantum wires are simultaneously defined in the upper and lower well of a GaAs/AlGaAs double quantum well heterostructure, using midline-gated split-gate devices [1]. The lower wire probes the ability of the upper wire to screen the electric field from a biased surface gate. The technique is sensitive enough to resolve spin splitting of the 1D subbands in the presence of an in-plane magnetic field. The compressibility response of the $0.7$ structure is measured, and its evolution with increasing temperature and magnetic field is studied [2]. Despite the sensitivity of our measurements we see no evidence of the formation of the quasibound state predicted by the Kondo model of the $0.7$ structure. Instead our data are more consistent with theories which predict that the $0.7$ structure arises as a result of spontaneous spin polarization. \\[4pt] [1] I.M. Castleton \emph{et al}, Physica B 249, 157 (1998).\\[0pt] [2] L.W. Smith \emph{et al}, Phys. Rev. Lett. 107, 126801 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J14.00008: Spin-orbit effects in two-dimensional hole systems on hydrogen-terminated silicon (111) surfaces Binhui Hu, Tomasz M. Kott, Bruce E. Kane We have studied spin-orbit effects in two-dimensional hole systems (2DHSs) on hydrogen-terminated Si(111) surfaces using Shubnikov-de Haas (SdH) oscillations. The device has a vacuum field-effect transistor structure [1], and the 2DHS is induced on the H-Si(111) surface. Hole concentrations up to $8\times 10^{11}$ cm$^{-2}$ are obtained, and the peak hole mobility is about 15,000 cm$^{2}$/Vs at T = 1.5 K. SdH oscillations show that the heavy hole subband is spin split due to spin-orbit effects. Both frequencies and beating node locations of the SdH oscillations are used to characterize the spin-orbit effects. The spin-splitting energy is measured as a function of the hole concentration, and the underlying physics will be discussed. Heavy-hole effective mass is determined by the temperature dependence of the SdH oscillations, and the relationship between the effective mass and the hole concentration will be presented. [1] K. Eng, R. N. McFarland, and B. E. Kane, Appl. Phys. Lett. \textbf{87}, 052106 (2005) [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J14.00009: Quasiparticle velocities in 2D electron/hole liquids with spin-orbit coupling David Aasen, Stefano Chesi, Bill Coish We study the influence of spin-orbit interactions on quasiparticle dispersions in two-dimensional electron and heavy-hole liquids in III-V semiconductors. To obtain closed-form analytical results, we work within the screened Hartree-Fock approximation, valid in the high-density limit. For electrons having a linear-in momentum spin-orbit interaction, we confirm known results based on the random-phase approximation and we extend those results to higher order in the spin-orbit coupling. For hole systems, with a leading nonlinear-in-momentum spin-orbit interaction, we find two important distinctions. First, the group velocities associated with the two hole-spin branches acquire a significant difference in the presence of spin-orbit interactions, allowing for the creation of spin-polarized wavepackets in zero magnetic field. Second, we find that the interplay of Coulomb and spin-orbit interactions is significantly more important for holes than for electrons and can be probed through the quasiparticle group velocities. These effects should be directly observable in magnetotransport, Raman scattering, and femtosecond-resolved Faraday rotation measurements. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J14.00010: Spin Coulomb drag and optical excitations in low dimensional systems Irene D'Amico, Carsten Ullrich Within the remit of new quantum technologies, an intense effort is devoted to improving our understanding of spin dynamics, with the aim of building novel spintronics devices. In this context the theory of spin Coulomb drag (SCD) was recently developed. It shows that Coulomb interactions are an intrinsic decay mechanism for spin currents. As confirmed by experiments, SCD can be substantial in semiconductors, and it is bound to become one of the most serious issues in spin polarized transport, since, due to its intrinsic nature, it cannot be avoided even in the purest material. More recently the influence of SCD on optical spin-injection and spin-resolved optical experiments has been considered. Here we report on SCD effects on intersubband optical spin excitations in III-V quantum wells, where SCD may contribute substantially to the linewidth of spin plasmons. By going beyond the usual local density functional approximation and properly including the effects due to the inhomogeneity of the system in the growth direction, we show that the quantization of states in the growth direction may strongly reduce the intrinsic plasmon linewidth. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J14.00011: Theory of Helical Fermi liquids Ali Ashrafi, Dmitrii Maslov We extend the Landau Fermi Liquid (FL) theory to include spin-orbit coupling (SOC). In particular, the Rashba SOC is chosen as an example. It is shown that although ``charge-part'' quantities, such as the charge susceptibility and effective mass, are determined solely by the quasi-particles, ``spin-part'' quantities, such as the spin susceptibility, have contributions from the damped states in between the two Fermi surfaces induced by the SOC. However, contributions to the lowest order in the SOC can still be extracted from the theory. The nature of the instabilities of such spin-orbit coupled FL is discussed. [Preview Abstract] |
Session J15: Focus Session: Spins in Metals - Spin Transport in Novel Devices and Structures
Sponsoring Units: DMP FIAP GMAGChair: Peter Metaxas, University of Western Australia
Room: 213
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J15.00001: An Alternative Approach for Fabricating Ring-shaped Spin-transfer Torque RAM Devices Robert Burke, Alan Edelstein Several techniques have been investigated to fabricate spin-torque transfer RAM (STT RAM) devices with ring-shaped geometries. In-plane STT RAM devices with ring-shaped geometries are advantageous in that the magnetization is confined to the ring structure, eliminating the demagnetization field. This, in turn, reduces the critical current needed to switch the magnetization. To date, the most popular fabrication technique of ring-shaped STT RAM involves a combination of electron beam lithography and ion beam milling. A significant issue with ion beam milling, however, involves the redeposition of the MTJ material on the ring-shaped surface, which leads to shorting of the devices. In our study, we seek an alternative method for forming the ring-shaped devices which eliminates the shorting problem. As a result, we are using a nanopillar and cap structure template to define the ring-shaped geometry of the devices. The nanopillar and cap structure template is fabricated using a combination of a bilayer resist and electron beam lithography to either create a mold for the deposition of a nanopillar and cap structure via ion beam deposition or a resist-based nanopillar and cap structure. This study will report on the results of this process and its prospects as an alternative to ion beam milling. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J15.00002: Spin dependent tunneling and possible spin torque effects in magnetic nanoparticle tunnel junctions Chen Wang, Hsin-wei Tseng, Robert A. Buhrman, Daniel C. Ralph It has been predicted that electron tunneling between two magnetic electrodes via a magnetic nanoparticle can exhibit enhanced magnetoresistance at low temperature due to a coherent tunneling mechanism in the Coulomb blockade regime. This has been qualitatively demonstrated for large ensembles of nanoparticles. It has also been predicted that the tunneling current in such a system might exert spin transfer torque with quantum behaviors associated with the discrete energy level of the nanoparticles. Here we report the fabrication and measurement of sub-100 nm nano-pillar tunnel junctions with the layer structure CoFeB/MgO/(CoFeB nanoparticles)/MgO/CoFeB with the dual goal of 1) probing the spin-dependent coherent tunneling associated with the magnetization of a single or very few nanoparticles and 2) exploring the potential for magnetization switching and current-driven dynamics in the nanoparticles induced by spin transfer torque. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J15.00003: What determines the sign of magnetoresistance in a molecular tunnel junction? Subhasish Mandal, Ranjit Pati The recent observations of both positive and negative sign of tunneling magnetoresistance (TMR) in the same organic spin-valve structure has mystified researchers working in organic spintronics. In this article, we resolve this puzzle by exploring the role of interfacial metal-molecule coupling on TMR in a single molecular spin-valve junction. A planar organic molecule sandwiched between two nickel electrodes is used to build a prototypical spin-valve junction. A parameter-free, single particle Green's function approach in conjunction with a posteriori density functional theory involving a hybrid orbital dependent functional is used to calculate the spin-polarized current. The effect of external bias is explicitly included to investigate the spin-valve behavior. Our calculations show that only a 3\% change in the interfacial distance at the metal-molecule junction can alter the sign of the TMR from a positive to a negative value. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J15.00004: Spin-dependent inertial force and spin current in accelerating systems Mamoru Matsuo, Jun'ichi Ieda, Eiji Saitoh, Sadamichi Maekawa In the frontier of spintronics, much attention is paid on the control of spin currents. Due to the recent progress of nanoelectromechanics, mechanical manipulation of spins will increase in importance. We discuss theoretically the generation of spin currents in both rotationally and linearly accelerating systems. The explicit form of the spin-dependent inertial force acting on electrons in accelerating systems in the presence of electromagnetic fields is derived from the generally covariant Dirac equation. It is shown that the force is responsible for the generation of spin currents by mechanical rotation and vibration in the first order of the spin-orbit coupling. We also investigate SU(2) x U(1) gauge theory in accelerating systems, which allows us to extend the spintronic theory in inertial frame to non-inertial frame. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J15.00005: Anomalous Magneto Transport in amorphous TbFeCo thin films Nattawut Anuniwat, Marc Costantine, Jiwei Lu, Manli Ding, Joseph Poon TbFeCo has attracted some interests because of its high perpendicular anisotropy and tunable magnetic properties for nanomagnetic and spintronics application. In this study, we report a strong size dependence of the coercive field in 30 nm-thick Tb$_{30}$Fe$_{63.5}$Co$_{6.5 }$films with MgO capping. Magneto Optical Kerr effect (MOKE) and Vibrating Sample Magnetometer are performed on unpatterned films. The films exhibited strong PMA characteristics with M$_{S} \quad \sim $200 emu/cc, H$_{C}\sim $6000 Oe, and K$_{U} \quad \sim $ 5 x 10$^{6 }$erg/cc. The films were then fabricated into Hall bars with 500 nm, 50 $\mu $m and 500 $\mu $m in width. From anomalous Hall effect (AHE), H$_{C}$ was determined for these patterned films. For Hall bars with the width less than 50 $\mu $m, an increase in the coercivity field ($\sim $ 1.4 Tesla) was observed at room temperature. The temperature dependent of AHE was characterized from 50K to 300K. The thickness and composition dependent will also be studied and discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J15.00006: Anisotropic Magnetoresistance in single-crystalline Ag/NiO/Fe$_{3}$O$_{4}$/MgO(001) sample Jia Li, Ali Sucipt Tan, Jim Son, Eric Jin, Zi Q. Qiu Anisotropic Magnetoresistance (AMR) is a well-known phenomenon in ferromagnetic (FM) materials that the resistivity exhibit different values as the electric current flows parallel and perpendicular to the magnetization direction, respectively. Recognizing that the AMR depends on the spin axis rather than spin direction, we propose that AMR effect should also exist in antiferromagnetic (AFM) materials. In this presentation, we will report the AMR effect in single crystalline Ag/Fe$_{3}$O$_{4}$/NiO/MgO(001) films in which the electrical current is mainly carried by the nonmagnetic Ag film. By changing the FM Fe$_{3}$O$_{4}$ magnetization direction with an external magnetic field, the AFM NiO spin axis direction can be changed through the Fe$_{3}$O$_{4}$/NiO coupling. We observe a non-zero AMR effect in Ag and that the AMR value depends sensitively on the Ag thickness, suggesting that the observed AMR comes from the spin-dependent NiO/Ag interfacial scattering. Moreover, the magnitude of the AMR effect at 1nm thick Ag in Ag/Fe$_{3}$O$_{4}$/NiO/MgO(001) is comparable to the AMR value from single Fe film. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J15.00007: AIP Prize for Industrial Applications of Physics Lecture: Controlled exchange in recording media and spintronic devices Invited Speaker: Eric Fullerton Controlled exchange in recording media and spintronic devices Eric Fullerton University of California, San Diego New functionality and performance advantages have been achieved in magnetic recording media by the formation of complex heterostructures and have helped delay predicted recording limits that arise from thermal instabilities in the media [1]. Example media structures include antiferromagnetically coupled (AFC) media [2] and exchange-spring media structures [3, 4] which emerged in longitudinal and perpendicular recording products and have helped to extend recording densities [1]. In this presentation I will review the implementation of exchange-coupled media, the surprising physics that emerged and its impact on hard-drive products. I will further discuss how similar approaches can be used in emerging recording technologies such as heat assisted magnetic recording [4] and bit patterned media [5, 6] as well as many spintronic devices [7,8]. \newline \newline [1]. A. Moser, et al., J. Phys. D: Appl. Phys. \bf 35\rm, R157 (2002). \newline [2]. E. E. Fullerton, et al., Appl. Phys. Lett. \bf 77\rm, 3806 (2000). \newline [3]. A. Berger, et al., Appl. Phys. Lett. \bf 93\rm, 122502 (2008). \newline [4]. J.-U. Thiele, S. Maat and E. E. Fullerton, Appl. Phys. Lett. \bf 82\rm, 2859 (2003). \newline [5]. S. Li, et al., Appl. Phys. Lett. \bf 94\rm, 202509 (2009). \newline [6]. M. V. Lubarda et al., IEEE Trans. Magn. \bf 47\rm, 18 (2011). \newline [7]. S. Mangin, et al., Nature Materials \bf 5\rm, 210 (2006). \newline [8]. I. Yulaev et al., Appl. Phys. Lett. \bf 99\rm, 132502 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J15.00008: Influence of the exchange bias on the magnetic losses in CoFeB/MgO/CoFeB tunnel junctions Ryan Stearrett, W.G. Wang, Xiaoming Kou, L.R. Shah, J.Q. Xiao, E.R. Nowak We report the influence of the exchange bias on the low-frequency magnetic losses of the reference layer in CoFeB/MgO/CoFeB tunnel junctions near maximum resistance susceptibility. The phase lag associated with the magnetic losses in the reference layer, $\varepsilon $, is field-dependent during its magnetic reversal, being largest around the antiparallel state and slowly decreasing with higher applied fields. Such behavior would indicate a direct influence of the exchange bias strength. Its strength is determined by the magnitude of the reference layer switching field, H$_{ref}$. This is defined as the field at which the magnetoresistance-sensitivity product exhibits its maximum. Devices with the strongest exchange bias tend to have the thickest seed layers and exhibit elevated values for H$_{ref}$ and $\varepsilon $. However, ones with weakened exchange bias due to prolonged annealing show a reduction in H$_{ref}$ and $\varepsilon $ with increased annealing time. A comparison between top and bottom pinning configurations is also discussed along with its impact on double-barrier magnetic tunnel junctions. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J15.00009: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J15.00010: Magneto-optical properties of Fe thin films in an external electric field Kohji Nakamura, Toru Akiyama, Tomonori Ito, Michael Weinert, Arthur Freeman Controlling magnetic properties by an external electric field ($E$-field) is a key challenge in magnetic physics. Previously, from first-principles calculations,\footnote{Nakamura, Shimabukuro, Fujiwara, Akiyama, Ito, Freeman, PRL{\bf 102}, 187201(2009); Nakamura, Akiyama, Ito, Weinert, Freeman, PRB{\bf 81}, 220409R(2010)} we demonstrated the $E$-field-driven magnetocrystalline anisotropy modification in Fe thin films and at the Fe/MgO interface. Here, we extend our investigations to treat the magneto-optical properties of Fe thin films in an $E$-field. Calculations were carried out using the film-FLAPW method\footnote{Wimmer, Krakauer, Weinert, Freeman, PRB{\bf 24}, 864(1981); Weinert, Wimmer, Freeman, PRB{\bf 26}, 4571(1982)},in which an $E$-field is incorporated and the conductivity tensor is obtained by applying the Kubo formula of linear response theory. Results predict that for an Fe monolayer, when the $E$-field is introduced over 1V/{\AA}, the calculated interband conductivity in the low energy region (less than about 2eV from $E_{\rm F}$) are modified compared to that in zero field, due to a magnetization reorientation from out-of-plane to in-plane. The calculated plasma frequency is also found to be reduced by 7\%, which suggests an $E$-field-driven magnetoresistance. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J15.00011: Spin transport in metal-oxide switching devices Curt A. Richter, H.-Jae Jang, Oleg Kirillov, Oana Jurchescu Metal-oxide-based devices in which resistive switching occurs, often referred to as memristors or resistive-RAM (random oxide memory), show promise for use in technologically exciting applications such as high-density non-volatile memories, electronically reconfigurable logic, and neural networks. We report on electron spin transport through electrochemically precipitated copper filaments formed in TaO$_{x}$ memristive devices consisting of Co(60 nm)/TaO$_{x}$ (16nm)/Cu(5 nm)/Py(60 nm) with crossbar-type electrode geometry. These metal-oxide switching devices with ferromagnetic electrodes show memristive behavior having a typical OFF/ON resistance ratio of 10$^{5}$. Magnetoresistance measurements performed by sweeping an external magnetic field display evidence of spin transport in the low-resistance ON-state at 77 K. Spin transport vanishes in the OFF-state. These data are strong evidence that the fundamental switching mechanism in these metal-oxide devices is the creation of Cu filaments in the ON-state that completely span the 16 nm thick TaO$_{x}$ and form a continuous metallic conduction path. In addition to helping elucidate the conduction pathway in these intriguing structures, our findings can advance electronics combining spintronic and electronic functions. [Preview Abstract] |
Session J16: Vanadium Dioxide; Nano and Devices
Sponsoring Units: DCMPChair: Ivan Schuller, University of California, San Diego
Room: 251
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J16.00001: M2 phase in free-standing single-crystalline nanostructures of VO$_2$ at ambient conditions E. Strelcov, A. Tselev, I.N. Ivanov, J. Zhang, J.D. Budai, J.Z. Tischler, S.V. Kalinin, A. Kolmakov A significant drawback of a promising material for realization of an ultrafast switch based on a metal-insulator transition (MIT) - VO$_2$ - is the inherent linkage between the MIT and lattice transformation from tetragonal to monoclinic with a lattice contribution in the band gap formation in the stable monoclinic M1 structure. On the other hand, the metastable M2 phase exhibits a pure Mott MIT and was shown to be driven metallic without the structural change. Existence of this phase at ambient conditions was reported for Cr and Al-doped VO$_2$ and nanostructures doped with oxygen vacancies. Here we report stabilization of the M2 phase in VO$_2$ single-crystalline nanoplatelets (NPls) doped with Al during the growth process via two new methods. Development of these methods came from our recent in situ studies of the NPl growth mechanism. We reconstructed temperature-doping phase diagram for the NPls. Electrical properties of the NPls were also studied as functions of doping level and temperature. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J16.00002: Photocurrent in Vanadium Dioxide T. Serkan Kasirga, Dong Sun, Jae H. Park, James Coy, Xiaodong Xu, David H. Cobden We investigate the photoresponse of VO$_{2}$~using scanning photocurrent microscopy below and above the metal-insulator transition at 67 \r{ }C. To avoid complications of nonuniform strain and twinning boundaries we focus on single-crystal suspended nanobeams, where the strain is either minimal or well controlled. At intermediate temperatures the metallic and insulating phases coexist and a photocurrent and photoconductance associated with the boundary between them is seen. The magnitude and profile of the photoresponse above and below the transition demonstrates a photothermal mechanism, with fast electron-lattice relaxation and absence of built-in electric fields in the insulating phase consistent with strong electron-electron correlations and a short screening length. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J16.00003: Electronic phases in vanadium dioxide nanowires observed by scanning tunneling spectroscopy John Hatch, Luisa Whittaker, Tai-Lung Wu, Adam Stabile, G. Sambandamurthy, Sarbajit Banerjee, Hong Luo The electronic behavior of vanadium dioxide around its metal-insulator transition has been the focus of many experimental and theoretical investigations, concerning transition mechanisms. In this work, we present our experimental investigation of the temperature driven metal-insulator transition in individual single crystal vanadium dioxide nanowires with scanning tunneling spectroscopy. The local density of states is studied at temperatures around the transition temperature. We observe three distinct electronic phases: the insulating phase and two metallic phases with different density of states, both in terms of conductivity and in the complexity of the density-of-states spectra. Vanadium dioxide nanowires doped with tungsten were also studied and will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J16.00004: Current-induced metastable states in single-crystalline VO$_2$ nanoplatelets Alexander Tselev, J.D. Budai, E. Strelcov, J.Z. Tischler, A. Kolmakov, S.V. Kalinin The metal-insulator transition (MIT) in VO$_2$ occurs close to ambient temperature, T$_c$ = 68 $^{\circ}$C, which can be reduced by doping. The phase transition results in a few orders of magnitude change of electrical conductivity and is accompanied by a change of the lattice from tetragonal to monoclinic, which is associated with lattice expansion of $\sim$1\% along the tetragonal c-axis of the metallic phase. We observed that, in suspended single-crystalline VO$_2$ nanoplatelets (NPls) carrying a sufficiently strong electrical current, Joule heating leads to formation of metal-semiconductor domains, which are self-organized in chains providing a path for the current flow. This results in NPl bending depending on the current strength, which can be used for electrically controlled actuator action. The observed domain structures should be interpreted as distinct metastable states in freestanding and end-clamped quasi-1D VO$_2$ samples. We analyze the stability conditions for the heterophase domains patterns and main prerequisites for the realization of current-controlled nanoactuators based on the proposed concept. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J16.00005: Hydrogen stabilization of metallic VO2 in single-crystal nanobeams Jiang Wei, Heng Ji, Wenhua Guo, Andriy Nevidomskyy, Douglas Natelson Vanadium dioxide (VO$_{2})$ is a strongly correlated material with a metal-insulator transition at 67$^{o}$C from a high temperature, rutile metal to a lower symmetry insulating state. Substitutional doping can alter the properties of VO$_{2}$, but is irreversible. Using individual microcrystals and nanobeams, we show that spillover may dope VO$_{2}$ reversibly with atomic hydrogen. Raman and optical microscopy show a stabilized metallic state, consistent with single-crystal electron diffraction and scanning electron microscopy that demonstrate a post-hydrogenation structure similar to the rutile state. Electronic transport shows that the energy band gap of insulating phase can be reversibly tuned towards the metallic phase upon different hydrogenation conditions and the metallic state may be stabilized down to cryogenic temperatures. Electronic structure calculations agree that a hydrogen-containing distorted rutile structure is energetically favored over the monoclinic state. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J16.00006: Identifying the Role of Domains in Metal-Insulator Transitions in Individual Nanowires of Tungsten-Doped $VO_{2}$ Adam Stabile, Sujay Singh, Tai-Lung Wu, Luisa Whittaker, Payam Taherirostami, Sarbajit Banerjee, G. Sambandamurthy Though it is well known that the metal-insulator transition (MIT) in $VO_{2}$ can be achieved by a variety of external parameters, an understanding of how different parameters drive such a transition has remained relatively unknown. We report transport and Raman spectroscopic characteristics on voltage ($V$)- and temperature ($T$)-driven MIT in individual, single-crystal, tungsten-doped $VO_{2}$ nanowires. From transport analyses we discuss the $T$-dependent features in $I$ vs $V$ curves; specifically hysteresis gaps and resistance jump features seen in sub-micron devices. From Raman spectroscopic analyses we discuss the Raman intensity of $A_{g}$ modes while driving the temperature and voltage across the transition. We conclude that driving $T$ supports a slow transition to the rutile (R) metallic phase with a wide temperature range of mixed insulating, monoclinic (M1) and R states due to the population of metallic domains. $V$-driven transition does not appear evolve via the formation of domains, but is activated when $V$ is sufficiently large above a $T$-dependent threshold. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J16.00007: Ionic Liquid Gated Vanadium Oxide Three Terminal Devices: Chemical Stability and Field Effect You Zhou, Zheng Yang, Shriram Ramanathan Understanding electrostatic field effect in correlated oxides is one approach to uncovering mechanisms leading to metal-insulator transition and further is of great interest in oxide-based device technologies. We have fabricated ionic liquid gated VO$_{2}$ three-terminal devices. The VO$_{2}$/IL interface properties were systematically studied with emphasis on electrochemical stability, gate capacitance and charging/discharging using photoelectron spectroscopy, impedance spectroscopy and other electrical characterization. We have observed a large modulation of VO$_{2}$ channel conductance at room temperature with polarity dependence. Interestingly, the conductance modulation also exhibits a time-dependent response to external gate bias and possible mechanisms will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J16.00008: Gating Effect on VO2 nanowire by Ionic Liquid Heng Ji, Jiang Wei, Douglas Natelson VO2 is a well-known strongly correlated material with insulator-to-metal phase transition at 68 \r{ }C. Below this temperature, VO2 is an insulating material whose temperature dependence behaves like a semiconductor with 0.6 eV band gap. However, such material cannot be gated by traditional method. In our experiment, we applied a new technique, using ionic liquids, to provide a much stronger transverse electric field on the VO2 nanowire. We did not observe obvious gating effect on VO2, but in the meantime, we found that VO2 is sensitive to hydrogen. The hydrogen produced by electrochemistry when applying voltage on the electrolyte (an ionic liquid with water contamination, in this case) can dramatically change the conductance of VO2. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J16.00009: Strain control of the metal-insulator transition in vanadium dioxide nanobeams Jae Hyung Park, Serkan Kasirga, James Coy, Xiaodong Xu, David Cobden To be able perform systematic studies at the metal-insulator transition in VO2, we have developed a platform for applying axial strain to suspended single-crystal nanobeams while carrying out optical and transport measurements. The nanobeams are positioned on a piezo-actuated silicon structure using a nanomanipulator. The strain can be used to control the transition temperature, because the metallic and insulating phases have different c-axis lattice constants, or to move the interphase boundary between metal and insulator in coexistence. We report transport, Raman and photoconductance measurements as the transition is tuned in this way. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J16.00010: Investigation of switching behavior of two-terminal devices on VO$_{2}$ Iuliana P. Radu, Koen Martens, Bogdan Govoreanu, Sofie Mertens, Xiaoping Shi, Malgorzata Jurczak, Stefan De Gendt, Andre Stesmans, Jorge A. Kittl, Marc Heyns Vanadium dioxide undergoes an insulator to metal transition at about 68\r{ }C. Two-terminal devices fabricated on VO$_{2}$ show a steep decrease of resistance when the current or voltage applied are large enough. This switching has been largely attributed to a field effect even in two-terminal devices but controversy still exists. We fabricate devices with an array of electrode separations and widths and study how the dc switching voltage and current depend on device size. The data obtained from these devices are most consistent with a Joule heating mechanism governing the switching. Additionally we perform an ac investigation of these devices and find that the switching to the low resistance state can happen faster than 5ns (the time resolution of the measurement set-up) while the switching to the high resistance state is of the order of hundreds of nanoseconds, consistent with the estimated heat dissipation time. In spite of the Joule heating mechanism which is expected to induce device degradation, we find that the devices can be switched for more than 10$^{10}$ cycles making VO$_{2}$ a promising material for memory applications. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J16.00011: Local thermal heating in VO$_2$ electric-field-induced metal insulator transition A. Zimmers, L. Aigouy, A. Sharoni, S. Wang, J.G. Ramirez, I.K. Schuller Over recent years, the insulator to metal transition (IMT) of the vanadium dioxide (VO$_2$) Mott insulator has been revisited revealing an electric-field-induced resistance switching. Whether this feature is purely due to an electrical field effect or due to some Joule heating is still under debate. Here we report a local temperature measurement in a 10$\mu$m and a 20$\mu$m VO$_2$ junction while going through the resistance switching. The sample was placed at $\Delta$T=15K below 340K (the thermally induced insulator to metal transition). When ramping up the voltage across the junction we find that the local heating inside the VO$_2$ junction is close to 15K. This data suggests that in these temperature, current and voltage ranges, the field induced IMT can be explained by local Joule heating. Work supported by the French ANR-09-BLAN-0388-01 and the US DOE and AFOSR. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J16.00012: Electrical properties of vanadium dioxide -- dielectric -- metal structures and the metal-insulator transition Koen Martens, Iuliana Radu, Sofie Mertens, Xiaoping Shi, Marc Schaekers, Stefan De Gendt, Marc Heyns, Jorge Kittl VO$_{2}$ and its metal-to-insulator transition (MIT) are of interest for memory and logic nanoelectronic devices due to its very fast transition, its full volume transition implying good scalability and reliability and relatively large resistance on/off ratio of 2-5 orders of magnitude. In this work the equivalent of the heart of MOS (metal-oxide-semiconductor) technology, the MOS capacitor is investigated for the VO$_{2}$ case in which the Si is replaced by VO$_{2}$. Thermally oxidized VO$_{2}$ with a HfO$_{2}$ or Al$_{2}$O$_{3}$ dielectric grown on top with Atomic Layer Deposition were used to form MOS structures. The MOS capacitor electrical properties are analyzed such as the gate current and capacitance behavior with special regard to the MIT. The influence of the MIT on gate dielectric tunneling is shown and modeled as well as RRAM phenomena and an evaluation of the field effect. Implications for the field-induced metal-insulator transition and for device applications are discussed. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J16.00013: Field-effect modulation of conductance in VO$_2$ nanobeam transistors with HfO$_2$ as the gate dielectric Shamashis Sengupta, Kevin Wang, Kai Liu, Ajay Bhat, Sajal Dhara, Junqiao Wu, Mandar M. Deshmukh Field-effect transistors have been fabricated from VO$_2$ nanobeams using HfO$_2$ as the gate dielectric. When heated up from low to high temperatures, VO$_2$ undergoes an insulator-to-metal transition. We observe a change in conductance (6 \%) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of the hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system involving a timescale of a few minutes. The origin of such slow processes may lie in the coupling between the dipolar arrangement and the strain state of the VO$_2$ crystal. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J16.00014: Metal-to-insulator transition in a columnar nanocomposite oxide Zhaoliang Liao, Peng Gao, Shane Stadler, Xiaoqing Pan, Rongying Jin, E. Ward Plummer, Jiandi Zhang The capability of tuning lattice strain, composition, and dimensionality in epitaxial film growth provide a new avenue of exploring new functionality in correlated electron materials. Here we demonstrated a chemical ratio controlled metal-insulator transition in the nanocomposite films of V$_{2}$O$_{3}$-La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) grown on LaAlO$_{3}$ (111) substrate through alternative deposition of LSMO and V$_{2}$O$_{3}$. A V-Mn ion exchange between V$_{2}$O$_{3}$ and LSMO occurs during the growth and results in the formation of nanoscale and vertically columnar-like insulating Mn$_{2}$O$_{3}$ and metallic La$_{1-x}$Sr$_{x}$VO$_{3}$ composite, as qualitatively revealed by scanning transmission electron microscopy. As determined by transport measurement, a metal-insulator transition is found in the composite films, which depends on the ratio of deposition time of V$_{2}$O$_{3}$ to LSMO. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J16.00015: Avalanches in sub-micron V$_{2}$O$_{3}$ devices Siming Wang, Gabriel Ramirez, Ivan Schuller Systematic resistance versus temperature measurements were performed on micron and sub-micron V$_{2}$O$_{3}$ devices. Instead of smooth R-T curves reported previously, multiple jumps in resistance are observed through the temperature driven metal-insulator transition. These jumps range over 3 orders of magnitude in resistance. A power law distribution of the jump sizes indicates that the metal-insulator transition in V$_{2}$O$_{3}$ occurs through a series of avalanches. The power law exponent in V$_{2}$O$_{3}$ devices is very close to that found in similar VO$_{2}$ devices.\footnote{A. Sharoni, J. G. Ramirez, and I. K. Schuller, Phys. Rev. Lett. \textbf{101}, 026404 (2008).} This indicates that the phase transition in VO$_{2}$ and V$_{2}$O$_{3}$ are similar and occur through phase separation, percolation and avalanches. The effect of magnetic field on the avalanches in V$_{2}$O$_{3}$ will be discussed. [Preview Abstract] |
Session J17: Focus Session: Surfaces and Interfaces in Nonoxide Nanostructures: Growth, Structure, and Characterization - Organic/Inorganic Interfaces & Quantum Dots
Sponsoring Units: DMPChair: Brad Conrad , Appalachian State University
Room: 252A
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J17.00001: Current at Metal-Organic Interfaces Invited Speaker: Klaus Kern Charge transport through atomic and molecular constrictions greatly affects the operation and performance of organic electronic devices. Much of our understanding of the charge injection and extraction processes in these systems relays on our knowledge of the electronic structure at the metal-organic interface. Despite significant experimental and theoretical advances in studying charge transport in nanoscale junctions, a microscopic understanding at the single atom/molecule level is missing. In the present talk I will present our recent results to probe directly the nanocontact between single molecules and a metal electrode using scanning probe microscopy and spectroscopy. The experiments provide unprecedented microscopic details of single molecule and atom junctions and open new avenues to study quantum critical and many body phenomena at the atomic scale. Implications for energy conversion devices and carbon based nanoelectronics will also be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J17.00002: Temperature dependence in metal/organic heteroepitaxy Geoffrey Rojas, Xumin Chen, Donna Kunkel, Mathias Bode, Axel Enders The nucleation and growth of 2D single layers of tetraphenyl porphyrin molecules on Ag(111) are studied with variable temperature scanning tunneling microscopy. The heteroepitaxy of the organic/metal thin film occurs in strict analogy with known processes of metal heteroepitaxy. A similar heirarchy of energetic barriers to diffusion along edges and around corners is established. Temperature is the key component to selectively activating these barriers and determining shape of the adislands, from fractal-like shapes at low temperature to compact shape at high temperatures. Using existing models of metal heteroepitaxy, the terrace diffusion and binding energies of tetraphenyl porphyrin are approximated from measurement of island size as a function of temperature. This study provides evidence of the validity of using existing models of metal heteroepitaxy for the description of organic/metal heteroepitaxial systems. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J17.00003: Scanning Tunneling Microscopy investigation of the assembly of diF-TES-ADT on Ag(111) Shawn Huston, Jiuyang Wang, Marsha Loth, John Anthony, Brad Conrad, Daniel Dougherty Over the past two decades organic molecules have shown increasing promise as active layers in electrical devices such as field effect transistors, organic light emitting diodes, and organic photovoltaic devices. The suitability of organic molecules for use in these devices is governed by several properties, chief among them being the ability to self-organize into a film showing high carrier mobility. Organic thin film transistors (OTFT) partially composed of solution processed 2,8-difluoro-5,11-bis(triethylsilylethynyl)-anthradithiophene have shown high performance. To date these OTFTs have been constructed solely by solution processing. As such, we have chosen to investigate the possibility of vapor deposition as an alternative. Our investigation of the viability of vapor deposition of this promising molecule begins with deposition on Ag(111) as a model system. Preliminary STM results will be presented and discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J17.00004: DFT study of metal/organic interfaces; optimizing morphology and energy levels for maximum Voc Michelle Tomasik, Alexie Kolpak, Jeffrey Grossman Metal/organic interfaces are important for understanding the electronic properties of any device incorporating organic molecules, although these interfaces have been much less studied than their metal/inorganic counterparts. Using density functional theory, we examine the electronic structure of the interfaces of three metals: silver, aluminum, magnesium, and an organic molecule, Alq3, which is utilized in organic light emitting diodes. We calculate properties of interfaces with a clean metal surface as well as ones with small metal particles injected close to the organic. The effects of the different metals are to charge the Alq3 to varying degrees and perturb the energy levels as the metal states mix with the organic molecule. Insights into the energy level alignment and morphology at these interfaces as a function of the electrode workfunction will be discussed, with the goal of maximizing the open circuit voltage through the choice of metal and deposition process. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J17.00005: DFT based modeling of C60/Dichloropentacene/Au OPV heterojunctions Jun Wang, Jian-Ming Tang, Karsten Pohl The co-assembly of functionalized pentacenes (electron-donor materials) and fullerenes (electron-acceptor materials) on metal substrates provides a model for studying the structural and electronic properties for novel organic photovoltaic (OPV) heterojunctions. Our previous STM experimental results show C$_{60}$ to form single, double and triple nano chains on an intact single-domain, brick-wall structured 6,13-dichloropentacene (DCP) monolayer adsorbed on stepped Au(788). Here, we present theoretical DFT calculations of the geometric and electronic structure, and the charge transfer in this interacting three-component system. Our calculations show that single C$_{60}$ molecules prefer to either absorb on top of the DCP molecules (slightly shifted off the Cl center) or in between the DCP rows of the brick-wall structure. When adsorbing chains of C$_{60}$ they will align with either the troughs in between the DCP rows or the top of the DCP rows, in agreement with experiment. Compared to the isolated DCP molecules, the HOMO and LUMO levels move up towards the vacuum level by about 1 eV upon monolayer formation, resulting in charge transfer to C$_{60}$. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J17.00006: Acetophenone on Si(001) with STM and DFT Steven Schofield, Adam Rahnejat, Oliver Warschkow, Daniel Belcher, Marian Radny, Phillip Smith Organic molecules are likely to play an important role in future technologies, e.g., in novel devices where individual molecules are incorporated as active elements, and in extending the functionality of existing technologies. A detailed, atomic-scale understanding of the structural and electronic properties of molecules on surfaces is key to the development of these technologies. Here we present scanning tunnelling microscopy (STM) and density functional theory (DFT) data of the surface binding configurations of acetophenone adsorbed to Si(001). Topographic and spectroscopic tunnelling experiments were performed at 77~K and room temperature in the limit of very low coverage. We find in analogy to other similar molecules such as acetaldehyde~[1], acetone~[2,3] and acetic acid~[4], acetophenone molecules covalently bond to the Si(001) surface in a variety of configurations that can be directly manipulated using the STM tip. In its most stable configuration, the adsorbate stands upright on the surface, attached via the C and O atoms of its acetyl group, producing a geometry that is robust and attractive for molecular electronics applications. [1] JCP 131, 104707 (2010), [2] PCCP 11, 2747 (2009), [3] JACS 129, 11402 (2007), [4] PRB 84, 153302 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J17.00007: Contact Angle Behavior for Fullerene/Porphyrin Mixtures on Si surfaces Miriam Cezza, Romaine A. Isaacs, Qian Shao, Shy-Hauh Guo, Lourdes G. Salamanca-Riba, Janice Reutt-Robey, Raymond J. Phaneuf Fullerene/porphyrin mixtures are of great interest in bulk heterojunction organic solar cells. Here we study the morphology of the phase separation which occurs when [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) and tetranitro-zinc phthalocyanine (tn-ZnPc), are deposited onto silicon (111) substrates, including the individual domain length scales, shapes and wetting angles. tn-ZnPC forms small clusters on the Si surface with a contact angle of approximately 15\r{ }, while PCBM forms compact clusters on broad ($\sim $0.5 um diameter) ``wetting-layer'' disks, with the cluster contact angle of $\sim $19\r{ }. Interestingly, a 50{\%} mixture shows topography qualitatively similar to that for PCBM, but with a larger contact angle of 22\r{ }, indicating that the mixture wets the interface less than either pure component alone. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J17.00008: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J17.00009: Molecular nanostructures on graphite Andreas Riemann, Linda Grabill, Brandon Owens, Eric Krebs The self-assembly of different amino acids on graphite has been studied using Scanning Tunneling Microscopy. Experiments involving the amino acid methionine have shown that the molecules arrange themselves in well-ordered molecular wires with equidistant spacing tunable by the amount of adsorbate concentration on the surface. This behavior can be explained by an attractive interaction of the amino and carboxyl groups with each other, whereas the side chains exhibit repulsive interactions. Experiments using other amino acids with different side chains, like tyrosine and histidine, show adsorption behavior which lead to densely packed films of well-ordered amino acids, but no molecular wire structure. The repulsive interactions of the side chains can not be experimentally observed. This interesting phenomenon of inter-molecular interaction was further investigated using molecular mechanics calculations for these molecules. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J17.00010: Spontaneous Formation of Quantum Height Manganese Gallium Islands and Atomic Chains on N-polar Gallium Nitride (000\underline {1}) Arthur R. Smith, Abhijit Chinchore, Kangkang Wang, Meng Shi, Yinghao Liu Significant interest has been shown over the last 15+ years in the growth of 2-D island nanostructures of special heights on semiconductor surfaces due to quantum size effects, often referred to as `electronic growth.' Recently, there has been much interest in growth of magnetic metal layers on gallium nitride surfaces, but electronic growth in this system was not reported, until now. Surprisingly, we find that deposition of manganese onto gallium-rich, GaN(000\underline {1}) results in the spectacular formation of 2-D quantum-height MnGa island structures. Two unique island heights, differing by just one atomic layer, are observed - one being 0.93 nm (5 atomic layers), the other 1.13 nm (6 atomic layers). The 0.93 nm high islands are unstable against the completion of the next atomic layer (0.93 + 0.20 nm), so a single quantum thickness is preferred (1.13 nm). A row structure at the surface of the islands is also revealed, with atomic resolution images suggesting a mixture of Mn and Ga. Auger electron spectroscopy confirms a significant surface Mn content. In addition, growth of 1-D atomic chains at the surface of the completed 1.13 nm high islands is also seen, indicating strongly anisotropic diffusion. The observed behavior is consistent with a quantum size effect driven growth. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J17.00011: Wavelength tunable high quality positioned InAs quantum dots grown on patterned GaAs (001) substrates Ayesha Jamil, Joanna Skiba-Szymanska, Ian Farrer, Martin Ward, Jonathan Griffiths, Geb Jones, Andrew Shields, Dave Ritchie Semiconductor quantum dots serve as ideal contenders in the domains of quantum optics, quantum cryptography and quantum information processing. The decisive factor about their possible applications is their peak emission energy. Naturally grown quantum dots suffer from the problem of random nucleation behavior and non-uniform dot sizes. Here we report on the growth of site-controlled InAs quantum dots on pre-patterned GaAs(001) substrates with adjustable wavelengths. Interplay of the dot growth parameters, particularly growth temperature and Indium deposition amount, as well as the size of the initial template has been employed. With a 20 nm thick GaAs spacer layer grown between the regrowth interface and the quantum dot layer, uniform arrays of quantum dots have been achieved with emission wavelengths covering a spectral window ranging from 900 nm to 1200 nm. This has been achieved without risking the single dot occupancy per nucleation site measured to be $>$ 60{\%} for all of the investigated samples. To ensure better quality of dots, wafer cleanliness is monitored throughout the process. The dots thus show bright emission lines with no spectral jittering. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J17.00012: Investigation of spatial correlation of type-II ZnTe quantum dots embedded in ZnCdSe barriers Uttam Manna, Ismail Noyan, Gertrude Neumark, Siddharth Dhomkar, Bidisha Roy, Igor Kuskovsky, Richard Moug, Le Peng, Maria Tamargo Doped and undoped multilayered structures of ZnTe type-II quantum dots (QDs) embedded in a ZnCdSe matrix have been grown in order to investigate the formation of an intermediate band, lying within the ZnCdSe band gap, with the aim of absorbing photons with energies below ZnCdSe bandgap. These materials may be useful for intermediate bandgap solar cells. The reciprocal space map (RSM) of the ZnTe/ZnCdSe multilayer QD structure consisting of periodic superlattice peaks in the $q_x$ direction have been studied for two different ZnCdSe spacer thicknesses (d$_A$$\approx$3.5, d$_B$$\approx$1.5 nm). The ZnTe QDs give rise to diffuse scattering in RSM, which is found to be elongated in the $q_x$ direction for both samples indicating a vertical correlation of the QDs. From the widths of the diffuse maxima in the $q_z$ direction, we found that 16\% and 40\% of QDs are correlated vertically for d$_A$$\approx$3.5 nm and d$_B$$\approx$1.5 nm, respectively. With increasing correlation, the pairing probability of the dots increases, leading to a larger average QD size. This conclusion is supported by a smaller blue-shift (26 vs 36 meV) of the photoluminescence peak position with increasing excitation intensity, over five orders of magnitude, for the structure with narrower spacers. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J17.00013: Characterization of quantum dot chains using transmission electron microscopy Tyler Park, John Colton, Jeffrey Farrer, Haeyeon Yang We report on the growth and characterization of InGaAs self-assembled quantum dots which form into chains through an altered Stranski-Krastanov method. The methods we are using to study these quantum dot chains include imaging and chemical analysis using a transmission electron microscope (TEM). In order for the quantum dot chains to be characterized using the TEM, the samples must be cut and thinned to allow enough electrons to pass through the sample for our techniques. We are making cross-section and plan view cuts which allow us to get information about the chemical composition, indium segregation, size and spacing, contaminates and other aspects of the dots. [Preview Abstract] |
Session J18: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- New Fabrication Techniques
Sponsoring Units: DMPChair: Willie Padilla, Boston College
Room: 252B
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J18.00001: Large Area 3D Negative Index Metamaterials Formed by Printing Invited Speaker: John Rogers Negative index metamaterials (NIMS) are man-made structures with values of permittivity and permeability that are simultaneously negative over some range of frequencies. Although advanced lithographic techniques can form the necessary three dimensional (3D) nanoscale features for NIMS, such methods can be applied only over small areas (100's of $\mu $m$^{2})$ on specialized substrates, with low throughput. This talk summarizes a 3D transfer printing method that can yield 3D-NIMs with excellent optical characteristics, in ways that are scalable to arbitrarily large areas and are compatible with manufacturing. We demonstrate 3D-NIMs with 11-layers and sub-micron unit cell dimensions, over areas $>$ 75 cm$^{2}$, corresponding to $>$10$^{5}$x10$^{5}$ unit cells, all with excellent uniformity and minimal defects. These areas and numbers of unit cells both correspond to increases of more than 2x10$^{7}$ times, over previous results. Multiple cycles of printing with a single stamp demonstrate use in a manufacturing mode at throughputs that are $\sim $10$^{8}$ times higher than those possible with state-of-the-art focused-ion beam lithography systems ($\sim $2.5 s per unit cell). Optical measurements show negative index of refraction in the NIR spectral range, with values as large as Re(n) $\sim $ -7 at $\lambda $ = 2.4 $\mu $m and high figures of merit (FOM) of $\sim $8 at $\lambda $ = 1.95 $\mu $m indicating low loss operation Related approaches can be used to form similar classes of 3D-NIMS with operation in the visible regime. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J18.00002: Optical metamaterials with different metals and wedge demonstration of negative refraction with fishnet metamaterials at visible wavelengths Nian-Hai Shen, Thomas Koschny, Maria Kafesaki, Costas M. Soukoulis We investigate the influence of different metals on the electromagnetic response of fishnet metamaterials in the optical regime. We found that, instead of using a Drude model, metals with a dielectric function from experimentally measured data should be applied in order to correctly predict the behavior of optical metamaterials. Through comparison of the performance for fishnet metamaterials made with different metals, i.e., gold, copper, and silver, we found silver is the best choice for the metallic parts compared to other metals, because silver allows for the strongest negative-permeability resonance and, hence, for optical fishnet metamaterials with a high figure-of-merit. We push the negative-index metamaterials to the visible regime and our improved wedge setup provides an unambiguous demonstration of negative refraction for the designed optical metamaterial. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J18.00003: Subwavelength imaging of plasmonic nano-bubble cavity probed by Cathodoluminescence Jun Xu, Hyungjin Ma, Nicholas Fang Concentrating light into a deep-subwavelength volume is big challenge in conventional optics due to the diffraction limit. In recent years, plasmonics, the interaction of light and metallic nanostructures, offers new opportunities in manipulating light-matter interaction at a subwavelength scale. Because of a strong localized resonant response of the, the field can be confined in a plasmonic cavity with an ultra-small mode volume and a high Purcell factor. Plasmonic light sources at the nanoscale have been demonstrated by utilizing an active medium. However, such light sources are characterized based on either a diffraction-limited technique or a spatially-averaged lifetime measurement, neither of which show subwavelength information. Here, we present a method, cathodoluminescence (CL) that shows a subwavelength resolution image of nanoscale air bubbles trapped in between thin amorphous silicon and silver. A novel multiple-fringe pattern, with a strong dependence on the air gap width, is observed due to an enhanced luminescence. A simple model, based on an oscillating electric dipole, is applied to explain the phenomena. Both the plasmonic and conventional cavity effect of the light interacting with the novel nano-bubble system are considered. The plasmonic nano-bubbles may provide a new approach to generate localized light from a continuous thin film layer with high efficiency, for the application in ultra-compact optical device, molecular imaging, etc. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J18.00004: Direct laser writing of three dimensional metal nanostructures using a femtosecond laser and various chemistries SeungYeon Kang, Kevin Vora, Shobha Shukla, Eric Mazur Metal nanostructures play important role in various areas such as catalysts or in plasmonics and especially for metamaterial applications. To generate these structures, most fabrication techniques can allow mass production but are either non-controllable, suffer from high cost and low throughput or are limited in two dimensions. Direct laser writing technique resolves these problems but has been mainly used to fabricate polymeric structures. We direct laser write 3D metal structures of tunable dimensions ranging from hundreds of nanometers to micrometers. With computer-controlled translation stage and by utilizing nonlinear optical interactions between chemical precursors and femtosecond pulses, we can limit the metal-ion photo-reduction process to a focused spot smaller than that of the diffraction-limit to create metal nanostructures in a focal volume. We study the chemistry that effects the photo induced metal growth to generate desirable metal structures. By varying the types of solvent, polymer and the concentration ratios of chemicals, we demonstrate our control over the morphology of the resulting metal structures and other features such as flexibility and conductivity. We plan to create diverse metal nanostructures for a wider range of metamaterial applications. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J18.00005: Self-assembly of metallic nanoparticles into macroscopic, high-density, monolayer films Jake Fontana, Ron Rendell, Jawad Naciri, Banahalli Ratna A vital element of bringing pragmatic optical metamaterials to fruition is the ability to produce and characterize macroscopic, self-assembled, high-density, ensembles of nanoparticles. We have developed a method that functionalizes metallic nanoparticles with thiol-ene ligands, self-assembles the nanoparticles into high-density, monolayer, centimeter size domain films using phase separation, transports the films onto substrates using surface tension gradients, and crosslinks, via click chemistry, the nanoparticles together into a solid film. We have determined the real and imaginary parts of the phase shift for the films using a Mach-Zehnder interferometer and spectrophotometer and compare the measurements to simulations. We discuss the implications of this self-assembly process for the construction of macroscopic optical metamaterials. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J18.00006: Designing Phoxonic Metamaterials with Fractal Geometry Sisi Ni, Cheong Yang Koh, Steve Kooi, Edwin Thomas Recently, the concepts of fractal geometry have been introduced into electromagnetic and plasmonic metamaterials. With their self-similarity, structures based on fractal geometry should exhibit multi-band character with high Q factors due to the scaling law. However, there exist few studies of \textit{phononic }metamaterials based on fractal geometry. We use COMSOL to investigate the wave propagation in two dimensional systems possessing fractal geometries. The simulations of these systems, guided by our recently developed general design framework, help to understand the role of design in determining the phononic properties of the structures. Proposed structures are being fabricated via standard lithographic or 3D printing techniques. The wave behavior of the structures can be characterized using Brillouin Light Scattering, Scanning Acoustic Microscope and Near-field Scanning Optical Microscopy. Due to their sparse spatial distribution, fractal phononic structures show potential fir ``smart skin'', where multifunctional components can be fabricated on the same platform. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J18.00007: Plasmon Enhanced Transparency of a Metallic Film on Silicon Tianyi Sun, Yang Wang, Zhifeng Ren, Krzysztof Kempa Low electrical resistivity and high optical transparency are highly desirable for thin films employed in various applications in electronics, optics, solar photovoltaics. In subwavelength scales, plasmon resonance can help electromagnetic waves to propagate through porous metallic films. In this work, we first employ a theory of effective dielectric response, and then quantitative simulations based on finite-difference-frequency-domain (FDFD) and finite-difference-time-domain (FDTD) methods, to understand and demonstrate physics of this effect. We show, that a nanoscopically perforated, yet continuous planar metallic film on silicon, can be designed to be highly transmissive in the entire visible range. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J18.00008: Enhancing optical gradient forces with metamaterials Vincent Ginis, Philippe Tassin, Irina Veretennicoff The transfer of linear momentum from electromagnetic waves to matter and the associated optical forces allow to dynamically manipulate the geometry of nanophotonic components with electromagnetic fields. In recent years, it has been proposed to use optical gradient forces for all-optical actuation of nanophotonic systems. This would open up the possibility for information processing inside nanoscale devices. Despite many efforts to increase these forces in integrated systems, they remain too small for most practical applications. In this contribution, we demonstrate how optical gradient forces can be enhanced significantly with the use of metamaterials. By implementing the techniques of transformation optics, we show how a metamaterial slab allows for the magnification of optical forces over several magnitudes, even when realistic losses are included. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J18.00009: Terahertz composite right/left-handed transmission-line metamaterial surfaces Zhijun Liu, Philip Hon, Amir Tavallaee, Tatsuo Itoh, Benjamin Williams We present terahertz metamaterial waveguides based on the concept of composite right/left-handed (CRLH) transmission-lines implemented in a metal-dielectric-metal geometry. The waveguides are fabricated with spin-coated Benzocyclobutene sandwiched between a ground plane and photolithographically defined top capacitive metal pads. Angle-resolved reflection spectroscopy measurement is used to map the dispersion of this metamaterial surface, which reveals strong resonant absorptions for both right-handed and left-handed (backward wave) propagating modes within the leaky-wave bandwidth. Tuning of the waveguide dispersion is demonstrated by varying the integrated lumped element capacitive geometry. The incident polarization provides selection of different waveguide modes, exhibiting either fully right/left handed, or right-handed only propagation. Analysis based on full-wave finite element method simulations as well as lumped circuit models will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J18.00010: Metamaterial Single Polarization Grid: Moving Towards the Dynamic Selective Polarizer Corey Shemelya, Thomas Vandervelde Ongoing interest in active metamaterial devices has increased due to their scalability, tunablity, and, more importantly, the ability to turn them on or off. A dynamic metamaterial polarizer has unique applications for identifying manmade objects anywhere in the IR. This work describes a single layer metamaterial polarization grid. The metamaterial grid is immediately scalable to many different wavelengths. The polarization grid has been designed for conversion to a dynamic metamaterial polarizer with simulated on/off ratios of 9 to 1. To this end, samples have been fabricated using varying doping concentrations of Si in GaAs grown epitaxially on Sapphire. Au metamaterials along with contact grids were then patterned and deposited. This design allows for potential dynamic responses as well as monolithic integration to create an active, metamaterial-stack, selective polarizer. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J18.00011: A new theoretical model for enhanced optical transmission through thin films Eli Lansey, Isroel Mandel, Jonah Gollub, David Crouse We present a new theoretical approach for modeling the resonant properties and transmission through subwavelength apertures penetrating metal films. We show that standard cavity mode theory can be applied to an effective resonant cavity whose dimensions are determined by the aperture dimensions in conjunction with the evanescent decay lengths of the of diffracted waves. This method predicts the dependence on variation in both the periodicity of the holes and the film thickness over a wide range of values. It further provides a physical mechanism for the enhanced transmission observed in periodic aperture arrays. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J18.00012: Enhanced second harmonic generation from aperiodic arrays of gold nanoparticles Antonio Capretti, Gary Walsh, Jacob Trevino, Giovanni Miano, Luca Dal Negro Second harmonic generation (SHG) from planar arrays of metal nanoparticles has been investigated in the last years for several configurations of particle shape and excitation-collection directions. In particular L-shaped particles have been employed to remove centrosymmetry. In this work we study SHG dependence on the array geometry, by comparing the generation efficiency of periodic arrays with that of deterministic aperiodic ones, namely Fibonacci and Golden Angle Spiral, lacking planar centrosymmetry. Two excitation-collection configurations are employed to test fabricated arrays. The whole range of average particle separation is explored, from closed spaced particles (plasmonic regime) to far spaced particles (photonic regime). We observe that SHG efficiency can be increased by arranging centrosymmetric particles in aperiodic arrays without inversion symmetry, both in the plasmonic regime and in the photonic one. We also demonstrate that Fibonacci and Golden Angle Spiral arrays perform differently in respect of the average interparticle separation and the collection direction, allowing the tunability of SHG extraction. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J18.00013: Plasmonic Coupling Effects in Large-area High-enhancement, Periodically-Arrayed Nanopillars Francisco Bezares, Joshua Caldwell, Orest Glembocki, Maarit Kariniemi, Jaakko Niinist\"o, Timo Hatanp\"a\"a, Ronald Rendell, Maraizu Ukaegbu, Mikko Ritala, Sharka Prokes, Charles Hosten, Markku Leskela Periodically arrayed Si nanopillars, coated with a thin layer of Ag, have been shown to produce large-area ($\sim $1 mm or more), uniform enhancement of the electromagnetic (EM) field near the surface of such arrays, thus suggesting suitability for the development of next-generation chem/bio-sensors. Although short-range plasmonic coupling effects are expected to increase the enhancement factor of these arrays several orders of magnitude, limitations in current lithographic techniques prohibit the fabrication of closely spaced nanopillars where such coupling effects become significant. Here we show experimentally that the use of Atomic Layer Deposition of Ag allows for the fabrication of Ag-coated Si nanopillar arrays with interpillar spacings of a few nanometers ($\sim $2 nm) resulting in 1-2 orders of magnitude increase in EM enhancement observed throughout the whole array area. Experimental observations that provide insight into the nature of the different coupling phenomena contributing to the overall enhancement of the EM field in these systems will also be discussed. [Preview Abstract] |
Session J19: Invited Session: Strongly Interacting Cold Fermi Gases
Sponsoring Units: DCMP DAMOPChair: John Thomas, Duke University
Room: 253AB
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J19.00001: From ultracold Fermi Gases to Neutron Stars Invited Speaker: Christophe Salomon Ultracold dilute atomic gases can be considered as model systems to address some pending problem in Many-Body physics that occur in condensed matter systems, nuclear physics, and astrophysics. We have developed a general method to probe with high precision the thermodynamics of locally homogeneous ultracold Bose and Fermi gases [1,2,3]. This method allows stringent tests of recent many-body theories. For attractive spin 1/2 fermions with tunable interaction ($^{6}$Li), we will show that the gas thermodynamic properties can continuously change from those of weakly interacting Cooper pairs described by Bardeen-Cooper-Schrieffer theory to those of strongly bound molecules undergoing Bose-Einstein condensation. First, we focus on the finite-temperature Equation of State (EoS) of the unpolarized unitary gas. Surprisingly, the low-temperature properties of the strongly interacting normal phase are well described by Fermi liquid theory [3] and we localize the superfluid phase transition. A detailed comparison with theories including recent Monte-Carlo calculations will be presented. Moving away from the unitary gas, the Lee-Huang-Yang and Lee-Yang beyond-mean-field corrections for low density bosonic and fermionic superfluids are quantitatively measured for the first time. Despite orders of magnitude difference in density and temperature, our equation of state can be used to describe low density neutron matter such as the outer shell of neutron stars. \\[4pt] [1] S. Nascimb\`{e}ne, N. Navon, K. Jiang, F. Chevy, and C. Salomon, \textit{Nature} \textbf{463}, 1057 (2010) \\[0pt] [2] N. Navon, S. Nascimb\`{e}ne, F. Chevy, and C. Salomon, \textit{Science} \textbf{328}, 729 (2010) \\[0pt] [3] S. Nascimb\`{e}ne, N. Navon, S. Pilati, F. Chevy, S. Giorgini, A. Georges, and C. Salomon, Phys. Rev. Lett. \textbf{106}, 215303 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J19.00002: Spin-Imbalance in One and Three-Dimensional Fermi Gases Invited Speaker: Randall Hulet The FFLO modulated superconductor state was independently proposed by Fulde and Ferrell, and Larkin and Ovchinnikov, as a way of accommodating the excess spin in a spin-polarized superconductor. The pairs in the FFLO state have non-zero center-of-mass momentum, which can produce a periodic structure with a spatially-modulated order parameter. Although there is some evidence for FFLO pairing in certain heavy fermion compounds that are able to accommodate both magnetic and superconducting order, conclusive experimental proof remains elusive. Motivated by the search for exotic paired states, we have performed experiments with spin-imbalanced ultracold atomic Fermi gases in both 3D and 1D. We use two hyperfine levels of $^6$Li to emulate the spin-up and down states. The $s$-wave pairing interactions are controlled via a magnetically-tuned collisional (Feshbach) resonance. In 3D, we find that the gas phase separates into an evenly paired BCS-like core, surrounded by the excess spin-up atoms [1]. For the 1D experiment, a two-dimensional optical lattice was used to create an array of 1D tubes that are each filled with $\sim$200 atoms. The weak axial confinement again produces a phase separation, but in contrast to 3D we find a partially-polarized central core surrounded by either fully-paired or fully-polarized wings, depending on the degree of overall spin-polarization [2]. Theory predicts that the partially-polarized phase is an FFLO superfluid. We are currently trying to obtain direct evidence for FFLO pairing, which may be revealed in the pair momentum distribution found by releasing the atoms axially and allowing them to expand in time-of-flight. We are also exploring the 1D-3D dimensional crossover that occurs when the coupling between tubes is reduced. \\[4pt] [1] G.B. Partridge \emph{et al}, Science \textbf {311}, 503 (2006). \\[0pt] [2] Y.A. Liao \emph{et al}, Nature \textbf {467}, 567 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J19.00003: Many-body physics with ultracold alkali-earth fermions in optical lattices Invited Speaker: Jun Ye |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J19.00004: Onset of a Pseudogap Regime in Ultracold Fermi Gases Invited Speaker: Aurel Bulgac We show, using an ab initio approach based on Quantum Monte Carlo technique, that the pseudogap regime emerges in ultracold Fermi gases close to the unitary point. We locate the onset of this regime at a value of the interaction strength slightly to the BCS side of the unitary point. We determine the evolution of the gap as a function of temperature and interaction strength in the Fermi gas around the unitary limit and show that our results exhibit a remarkable agreement with the recent wave-vector resolved radio frequency spectroscopy data. Our results indicate that a finite temperature structure of the Fermi gas around unitarity is complicated and involves the presence of preformed Cooper pairs, which however do not contribute to the long range order. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J19.00005: Strongly Repulsive Quantum Gases Invited Speaker: Tin-Lun Ho Advances in cold atom experiments have shown that the properties of repulsive Fermi and Bose gases are far more intricate than generally expected, as these systems can produce molecules even in the weakly interacting regime. Recent experiments, however, reveal some general yet puzzling properties of these gases in the strongly repulsive regime. In this talk, we show that these properties are direct consequences of statistics, and are fundamental properties of quantum gases [1]. We shall also discuss the related issue of itinerant ferromagnetism, and discuss the physical settings in which ferromagnetism can be found. \\[4pt] [1] V. B. Shenoy and Tin-Lun Ho, arXiv: 1106.0960, to appear in Physical Review Letters [Preview Abstract] |
Session J20: Invited Session: Sexual and Gender Diversity Issues in Physics
Sponsoring Units: COM CSWPChair: Savannah Garmon, University of Toronto
Room: 253C
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J20.00001: The State of Higher Education for STEM LGBTQQ Faculty/Staff Invited Speaker: Susan Rankin It has long been understood---an understanding that has been well supported by research-based evidence---that institutional ``climate'' has a profound effect on any academic community's ability to carry out its tripartite mission of teaching, research, and service (Bauer, 1996; Boyer, 1990; Peterson {\&} Spencer, 1990; Rankin, 1998; 2003; 2010; Rankin {\&} Reason, 2008; Tierney {\&} Dilley, 1996). With the acknowledgment that institutions differ in the level of attention and emphasis on issues campus climate, it is safe to say that a campus climate offering equitable learning opportunities for all students, academic freedom for all faculty, and fairness in employment for all staff and administrators is one of the primary responsibilities of institutions of higher education. The research also suggests that a challenging campus climate exists for LGBTQQ students, faculty and staff. Based on the literature, a challenging climate leads to decreased productivity, decreased sense of value to the community, decreased retention, and negatively influences educational outcomes (Settles, et al. 2006; Trower {\&} Chait (2002); Pascrell {\&} Terenzini, 2005; Whitt, Edison, Pascarella, Terenzini, {\&} Nora, 2001). Little is available in the literature on LGBTQQ faculty in the STEM fields. This program will engage participants in a review of the results of the 2010 project with regard to the experiences of LGBTQQ faculty and staff in the STEM fields. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J20.00002: Shattering the Lavender Ceiling: Sexual Minorities in Physics Invited Speaker: Michael Ramsey-Musolf I will discuss some of the challenges experienced by sexual minorities in physics, from both a personal and broader perspective. I will also comment on the opportunities for the field to become more inclusive, supportive, and scientifically stronger by addressing these challenges. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J20.00003: Why Awareness of LGBT issues in the Physics Community Makes Sense Invited Speaker: Janice Hicks A thriving innovation ecology requires diversity of perspective and knowledge. We want to attract and retain the best possible talent to Science and Engineering, particularly after expensive investments in training of faculty, postdoctoral fellows and students. Participants who bring their authentic identity to work are much more efficient, as it takes a lot of energy to stay in the closet. It is a concern that so few S{\&}E faculty are out of the closet -- we don't know the numbers and they are difficult to obtain. This makes it difficult for the younger generation of students to relate as they do not see sexual orientation as an obstacle. It is also important for LGBT people to be visible in order to benefit from workplace policies such as family leave and other benefits. There are some activities to promote a positive view of LGBT people in S{\&}E. The National Organization of Gay and Lesbian Scientists and Technical Professionals (NOGLSTP) has been in existence since 1983, and holds receptions and symposia at the AAAS meeting and other professional society meetings, as well as a symposium called ``Out to Innovate,'' next to be held October 13-14, 2012 at Ohio State University. The American Chemical Society started an LGBT subdivision of its Division of Professional Relations in 2010. Much more needs to be done to educate leaders so they can speak knowledgably about LGBT issues. Their ability to do so can affect their success in hiring and retaining top talent. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J20.00004: Physics Climate as Experienced by LGBT+ Physicists Invited Speaker: Elena Long In 2009, Elena Long created the LGBT+ Physicists website (http://lgbtphysicists.x10hosting.com) as a warehouse for resources useful for sexual and gender minorities working in physics. This resource has grown to include networking resources, lists of LGBT-friendly universities and localities, recommendations for enacting positive change in physics communities, and out-reach to other STEM-oriented LGBT organizations. This has been possible in large part by the dynamic community of LGBT+ physicists and allies looking to make physics more welcoming towards our community. In 2011, Elena used hir position as Member at Large on the executive committee of the Forum of Graduate Student Affairs (FGSA) to conduct a climate survey that included, among other things, the first serious look at LGBT+ demographics in physics. The survey focused particularly on issues of language heard and harassment experienced by physicists and was broken down into categories based on race, physical and mental ability, gender, and sexuality. Furthermore, it examined the outcomes of experienced harassment and the reasons for when harassment was not reported. Due to the nature of the study, overlapping demographics, especially ``multiple minorities,'' were also explored. This talk will give a brief history of the LGBT+ Physicists resource as well as an overview of the FGSA study. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J20.00005: TBA Invited Speaker: Theodore Hodapp |
Session J21: Focus Session: Search for New Superconductors: New Theories and Experiments
Sponsoring Units: DMPChair: Malcolm Beasley, Stanford University
Room: 254A
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J21.00001: The Challenge of Unconventional Superconductivity Invited Speaker: Michael Norman In the past few decades, several new classes of superconductors have been discovered. Most of these are unconventional in that they do not appear to be related to traditional superconductors where the order parameter is more or less spatially isotropic. As a consequence, it is felt by many (but not all!) that the cause of superconductivity arises from a different source than the electron-ion interactions that are at the heart of conventional superconductivity. But developing a rigorous theory for any of these classes of materials has proven to be a difficult challenge, and will continue to be one of the major problems in physics in the decades to come. This is particularly true in that if history is any guide, even more dramatic discoveries of unconventional superconductors await us in the future. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J21.00002: Transport properties of layered Ba(Pb,Bi)O$_3$ thin films G.W.J. Hassink, K. Munakata, R.H. Hammond, M.R. Beasley Doped BaBiO$_3$ is a 3D oxide superconductor with a maximum T$_{\rm c}$ of 30 K for Ba$_{0.6}$K$_{0.4}$BiO$_3$. There has been a lot of discussion on whether this high T$_{\rm c}$ can be explained purely by electron-phonon coupling with a high coupling constant $\lambda$. In addition, the presence of real-space paired $6s^2$ electrons in the parent compound raise intriguing questions about whether there is an electron-electron coupling interaction as well. This possible negative-U interaction might be used to implement the suggestion by Berg, Orgad and Kivelson [Phys.Rev.B 78, 094509] that for a two-layer system where one layer provides electron pairing interaction and the other layer is conducting, the whole can be superconducting with a high T$_{\rm c}$. Here we discuss the transport properties of BaPbO$_3$/BaBiO$_3$ bilayers, where the BaBiO$_3$ layer is thought to act as the pairing layer, while the BaPbO$_3$ acts as the conducting layer. The transport behavior changes to insulating upon decreasing the metallic BaPbO$_3$ layer thickness at values that single films are expected to still be metallic. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J21.00003: Control of Two-Dimensional Multi-Component Superconductivity in SrTiO$_3$ Heterostructures by Interlayer Coupling Hisashi Inoue, Minu Kim, Christopher Bell, Yasuyuki Hikita, Hiroshi Okamoto, Harold Hwang Two-dimensional (2D) superconductivity (SC) in the clean limit is of particular interest since novel phases such as the Fulde-Ferrell-Larkin-Ovchinnikov state [1,2], and Landau-quantized SC [3] are suggested theoretically. In an attempt to approach this limit experimentally, SC was confined in a narrow $\delta$-doped region of SrTiO$_{3}$ [4]. 2D subbands (SBs) were observed as well as 2D SC. Building on these results, we have explored the coupling between two 2D superconducting layers in structures with two $\delta$-doped layers, with variable interlayer coupling controlled by their spacing. We observe multiple components of the SC in the temperature and angular dependent upper critical field plots. These data suggest that the SB structure in the doped and interlayer regions directly impact the SC, and may play an important role for the design and investigation of multi-component SC. [1] P. Fulde $et$ $al$., $Phys$. $Rev$. $\bf 135$, A550 (1964). [2] A. I. Larkin $et$ $al$., $Sov$. $Phys$. $JETP$ $\bf 20$, 762 (1965). [3] M. Rasolt $et$ $al$., $Rev$. $Mod$. $Phys$. $\bf 64$, 709 (1992). [4] Y. Kozuka $et$ $al$., $Nature$ $\bf 462$, 487 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J21.00004: Electric field induced superconductivity in a layered transition metal chalcogenide J.T. Ye, Y.J. Zhang, Y. Mastuhashi, Y. Iwasa Recent developments in electric double layer transistors (EDLTs) are attracting growing interests because of its stronger field effect orders of magnitude larger than other transistor techniques This method provides unique abilities to reach the high carrier densities required for inducing superconductivity in several kinds of materials. Among them, layered materials are convenient examples to work with since high quality surface suitable for transistor channel could be easily obtained after mechanical cleavage. Especially, after the introduction of graphene techniques, high quality atomically flat surface can be routinely fabricated on a broad range of layered materials. Combining EDL with novel materials processing techniques on layered materials provides new opportunities in manipulating their electronic properties. We can achieve high carrier density up to 10$^{14}$ cm$^{-2}$ electrostatically in layered materials and induce metal insulator transitions. Superconductivity, similar as that shown in ZrNCl EDL transistor, could be observed when we cool down the system to low temperature after inducing a metal insulator transition with large amount of accumulated carriers. The versatility of this combination shows its potential as a protocol to study varieties of layered materials for broader scope of possibilities in accessing their superconductivities. And hopefully, this method could also facilitate to induce superconductivity in new materials. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J21.00005: Fano resonances in multigap Fe based superconductors and complexity for material design Antonio Bianconi The Fano resonance in the superconducting gaps (or ``shape resonance'' or ``Feshabch resonance'' ) in multigap superconductors [A Bianconi \textit{Sol. State Commun.}89, 933 (1994)] has been proposed as the mechanism for high Tc in Fe-based superconductors and related compounds [D Innocenti et al \textit{Supercond. Sci. Technol.}, 015012 (2011)] near the Lifshitz transition for a vanishing Fermi surface in a superlattice of layers or wires, in the proximity of a lattice, electronic, magnetic instability with competing interactions that give complex systems. The multiscale phase separation from nano-scale to micron scale in K0.8Fe1.6Se2. [ A Ricciet al \textit{Phys. Rev.}B 84, 060511 (2011)] has been detected by a mixed real space and momentum space probe: scanning nano focused X-ray diffraction like in La2CuO4+y [M. Fratini, et al \textit{Nature}466, 841 (2010) and [N. Poccia et al \textit{Nature Materials}10, 733 (2011)] showing scale free structural organization of dopants favoring in the high Tc phase. The results on KFeSe show phase separation, percolating superconductivity, competing with percolating magnetism and shape resonances in the superconducting gaps. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J21.00006: Ordering of dopants and potential increase of Tc to near room temperature Stuart Wolf, Vladimir Kresin This talk will describe a novel method to increase the resistive Tc of cuprate superconductors to values that might approach room temperature, especially if applied to the underdoped region of the Tc versus carrier concentration phase diagram. This is the part of the so-called pseudogap region that exhibits energy gaps and a small Meissner effect well above the maximum resistive transition [1]. The method proposed here involves ordering of the dopants that provide the itinerant holes in the copper oxygen planes. These dopants also act as pair breakers since they are defects in the structure. The strategy we are proposing here is to separate the regions with dopants which provide the itinerant carriers in the cuprate planes from the metallic but dopant free regions nearby, but far enough away to be not seriously affected by the proximity effect. If this separation can be carried out appropriately and we will describe how this will be done in the talk, there will be fully connected high Tc regions that can fully span a sample and present a very high temperature resistive transition, approaching room temperature for some of the cuprates. \\[4pt] [1] V.Z. Kresin and S.A. Wolf, ArXiv 1109.0341 [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J21.00007: Inhomogeneous superconducting state and the intrinsic Tc: Near room temperature superconductivity in the cuprates Vladimir Kresin, Stuart Wolf Doped cuprates are inhomogeneous superconductors and exhibit properties strongly affected by this inhomogeneity. The notion of an intrinsic critical temperature whose value greatly exceeds the resistive Tc is supported by a number of experimental studies and these will be reviewed in this talk. In particular the anomalous diamagnetism observed above the resistive transition is a manifestation of the presence of superconducting clusters embedded in a normal metallic matrix. The value of intrinsic critical temperature, that reflects the onset of superconductivity in the highest transition temperature clusters, is in some cuprates near room temperature. The transition to the fully superconducting state is percolative in nature and is strongly dependent on the inhomogeneities. Some consequences of such a system, including the ac response will be described. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J21.00008: A Kohn Luttinger perspective on topological superconductivity Ronny Thomale, Christian Platt, Werner Hanke On the basis of an orbital angular momentum and point group symmetry analysis, we argue that electron-driven fluctuations from the viewpoint of a Fermi surface instability generically provide a propensity towards topological superconductivity when the irreducible lattice representations associated with the Cooper pairs are multi-dimensional. For illustration, we explicate our generation recipe of topological superconductivity for the cases of ruthenates, cobaltates, and graphene doped to van Hove filling, i.e. representatives for square, triangular, and honeycomb lattice pairing. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J21.00009: X-ray photoelectron spectroscopy of Single Crystals of intercalated Bismuth Selenide Somaditya Sen, John A. Dudek, Ying Zou, Mary Severson, Mark Bissen, Prasenjit Guptasarma The possibility of superconductivity in topological insulators is likely to yield new physics, especially because the insulating normal state is highly unconventional. However, before moving on to microscopic theory, it is important to characterize and study basic systems available today. Here, we present XPS studies of intercalated single crystals of Bi$_{2}$Se$_{3}$ and Sb$_{2}$Se$_{3}$. Core level spectroscopy, combined with infra-red and Raman spectroscopy, yield details of the nature of bonding of atoms at the cleaved surface. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J21.00010: Search for Superconductivity in Single Crystals of Topological Insulators Prasenjit Guptasarma, Somaditya Sen, John Dudek, Mary Severson, Mark Bissen Observation of superconductivity in Cu-intercalated Bi$_{2}$Se$_{3}$, and in Bi$_{2}$Te$_{3}$ under pressure, open up the possibility of occurrence of superconductivity in other similarly modified structures. Here, we present detailed studies from a comprehensive search for superconductivity in selenides and tellurides of Bismuth and Antimony. High-quality single crystals were intercalated with a number of different elements using both in situ and ex situ techniques, then scanned for a resistive or magnetic transition to superconductivity. With an intent to understand the nature of intercalation in these compounds, we discuss results from our search for superconductivity, together with crystal structure analysis and detailed optical and x-ray photoelectron spectroscopy studies of the cleaved surface. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J21.00011: Surface Andreev Bound States of Topological Superconducting Phase in Doped Semiconductors: Application to Cu$_x$Bi$_2$Se$_3$ Timothy Hsieh, Liang Fu The recently discovered superconductor Cu$_x$Bi$_2$Se$_3$ is a candidate for three-dimensional time-reversal-invariant topological superconductors, which is predicted to have robust surface Andreev bound states hosting massless Majorana fermions. In this work, we present an analytical and numerical study of the surface Andreev bound state wavefunction and dispersion. We find the topologically protected Majorana fermions at $k=0$, as well as a new type of surface Andreev bound states at finite $k$. We relate our results to a recent point-contact spectroscopy experiment. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J21.00012: Functional renormalization group and variational Monte Carlo studies of the electronic instabilities in graphene near 1/4 doping Fa Wang, Qiang-Hua Wang, Fan Yang, Dung-Hai Lee We study the electronic instabilities of near 1/4 electron doped graphene using the functional renormalization group (FRG) and variational Monte-Carlo method. A modified FRG implementation is utilized to improve the treatment of the von Hove singularity. At 1/4 doping the system is a chiral spin density wave state exhibiting the anomalous quantized Hall effect, or equivalently a Chern insulator. When the doping drops below 1/4, the $d_{x^2-y^2}+i d_{xy}$ Cooper pairing becomes the leading instability. Our results suggest near 1/4 electron- or hole-doped graphene is a fertile playground for the search of Chern insulators and superconductors. [Preview Abstract] |
Session J22: Focus Session: Fe-based Superconductors - Nodal and Nodeless Superconductivity
Sponsoring Units: DMP DCOMPChair: Takashi Imai, McMaster University
Room: 254B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J22.00001: Evolution of symmetry and structure of the gap in Fe-based superconductors with doping and interactions Andrey Chubukov, Saurabh Maiti, Maxim Korshunov, Thomas Maier, Peter Hirschfeld We present a detailed study of the symmetry and structure of the pairing gap in Fe-based superconductors. We treat them as quasi-2D systems, decompose the pairing interaction into $s-$wave and $d-$wave channels and into contributions from scattering between different Fermi surfaces and analyze how each scattering evolves with doping and input parameters. We verify that each interaction is well approximated by the lowest angular harmonics and use this simplification to analyze the interplay between the interaction with and without spin-fluctuation components, the origin of the attraction in the $s\pm$ and $d_{x^2-y^2}$ channels, the competition between them, the nature of angular dependence of the $s\pm$ gaps along the electron Fermi surface, the conditions under which $s\pm$ gap develops nodes, and the origin of superconductivity in heavily electron- or hole-doped systems, when only Fermi surfaces of one type are present. In particular, we find that with increased electron and hole doping, the competition from $d-$wave grows. In the case of strong hole doping, there is some ambiguity over the leading solution, but in the case of strong electron doping, $d-$wave emerges as clear winner. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J22.00002: Non-Fermi liquid behavior in overdoped iron-pnictides Johnpierre Paglione, Kevin Kirshenbaum, Shanta Saha, Tyler Drye Electrical transport, magnetic susceptibility and heat capacity data are presented on a series of single-crystal iron-based intermetallic compounds with the ThCr$_2$Si$_2$ structure with transition metal substitution used to heavily over-dope the system. We will present observations of unusual temperature dependences in transport, susceptibility and electronic specific heat that indicate an unexpected deviation from Fermi liquid behavior that persists to milliKelvin temperatures. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J22.00003: Probing the Superconducting Order Parameter of Co-doped BaFe$_2$As$_2$ by Josephson Interferometry Juan Atkinson, Dale Van Harlingen, Paul Canfield We probe the superconducting order parameter in Co-doped BaFe$_2$As$_2$ (Ba122) single crystals via measurements of the critical current of Josephson junctions fabricated on polished faces orthogonal to the c-axis. The modulation of the critical current as a function of magnetic flux applied along the c-axis is different for junctions fabricated on different points on a circularly polished face of the Ba122 crystal since each junction probes an effective order parameter of the crystal through an angle in k-space. These modulations map the phase anisotropy and test for the proposed s$\pm$ mode pairing symmetry. We will present preliminary results of these studies and compare to existing theoretical models. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J22.00004: Nodal structure and quantum critical point beneath the superconducting dome of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ Invited Speaker: Yuji Matsuda Among BaFe$_2$As$_2$ based materials , the isovalent pnictogen substituted system BaFe$_2$(As$_{1-x}$P$_x$)$_2$ appears to be the most suitable system to discuss many physical properties, because BaFe$_2$(As$_{1-x}$P$_x$)$_2$ can be grown with very clean and homogeneous, as evidenced by the quantum oscillations observed over a wide doping range even in the superconducting dome giving detailed knowledge on the electronic structure. We investigate the structure of the superconducting order parameter in BaFe$_2$(As$_{0.67}$P$_{0.33}$)$_2$ ($T_c=31$\,K) with line nodes by the angle-resolved thermal conductivity measurements in magnetic field. The experimental results are most consistent with the closed nodal loops located at the flat part of the electron Fermi surface with high Fermi velocity. The doping evolution of the penetration depth indicates that nodal loop is robust against P-doping. Moreover, the magnitude of the zero temperature penetration depth exhibits a sharp peak at $x$=0.3, indicating the presence of a quantum phase transition deep inside the superconducting dome.\\[4pt] This work has been done in collaboration with T. Shibauchi, K. Hashimoto, S. Kasahara, M. Yamashita, T. Terashima, H. Ikeda (Kyoto), A. Carrington (Bristol), K. Cho, R. Prozorov, M. Tanatar (Ames), A.B. Vorontsov (Montana) and I.Vekhter (Louisiana). [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J22.00005: London penetration depth in single crystals SrFe$_2$(As$_{0.65}$P$_{0.35}$)$_2$ Charles P. Strehlow, K. Cho, M.A. Tanatar, R. Prozorov, T. Kobayashi, S. Miasaka, S. Tajima In a contrast to a fully-gapped charge-doped (Ba,K)Fe$_2$As$_2$ and Ba(Fe,T)$_2$As$_2$ [1], isoelectron - substituted BaFe$_2$(As,P)$_2$ exhibit nodal superconducting gap [2]. To explore possible universality, low-temperature variation of the London penetration depth, $\Delta \lambda (T)$, was measured in optimally - doped SrFe$_2$(As$_{0.65}$P$_{0.35}$)$_2$ with $T_c$=29~K. $\Delta \lambda (T)$ revealed notable deviations from the exponential temperature dependence, expected for a fully-gapped superconductors. Instead the data are best fit with a power-law function, $\Delta \lambda =AT^n$. The analysis of the data below $1/3 T_{c}$ over a variable fitting temperature range produced exponents $n\le2$ which suggests the presence of nodes in the superconducting gap, similar to the P - doped Ba$122$ compounds. \\[4pt] [1] R. Prozorov and V. G. Kogan, Rep. Prog. Phys. {\bf 74}, 124505 (2011). \\[0pt] [2] K. Hashimoto, {\it et. al.} Phys. Rev. B {\bf 81}, 220501(R) (2010). [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J22.00006: London penetration depth in heavily over-doped Ba(Fe$_{1-x}$Co$_x$)$_{2}$As$_{2}$ Jason Murphy, H. Kim, M.A. Tanata, A. Thaler, P.C. Canfield, U. Welp, W.K. Kwok, R. Prozorov The low-temperature variation of London penetration depth, $\Delta\lambda(T)$, has been previously studied in heavily over-doped Ba(Fe$_{1-x}$Ni$_{x}$)$_{2}$As$_{2}$ [1] and the authors suggested the development of line nodes. Similar conclusion was made from thermal conductivity measurements in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ [2]. However, $\Delta\lambda(T)$ in this system has only been measured for $x \leq x=0.102$ [3], which is not far enough from optimal doping. Here we report tunnel - diode resonator (TDR) measurements in heavily overdoped single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ with Co content of $x=0.108$ ($T_{c}$=14.8~K) and $x=0.127$ ($T_{c}$=9~K. We found a robust power-law behavior of $\Delta \lambda = A T^n$ with $n=2.5$ and $n=2.11$ respectively. To test whether the nodes are symmetry imposed or accidental, samples were irradiated with heavy ions. The produced disorder, leads to a decrease in $T_c$ and of the exponent $n$. These results effects will be discussed in a context of unconventional pairing in Fe-based superconductors. \\[4pt] [1] C. Martin {\it et. al.}, Phys. Rev. B {\bf 81}, 060505 (2010).\\[0pt] [2] J.-Ph. Reid {\it et. al.}, Phys. Rev. B {\bf 82}, 064501 (2010).\\ [0pt] [3] R.T. Gordon et.al. Phys. Rev. B {\bf 82}, 054507 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J22.00007: Specific Heat To H$_{c2}$: Evidence for Nodes or Deep Minima in the Superconducting Gap of Under- and Overdoped BaFe$_{2-x}$Co$_{x}$As$_{2}$ G.R. Stewart, J.S. Kim, K. Gofryk, F. Ronning, A.S. Sefat, K.Y. Choi, K.H. Kim Low temperature specific heat, C, in magnetic fields up to H$_{c2}$ is reported for BaFe$_{1.91}$Co$_{0.09}$As$_{2}$ (underdoped, H$_{c2}\approx $16 T, T$_{c}$=8 K), BaFe$_{1.79}$Co$_{0.21}$As$_{2}$ (overdoped, H$_{c2}\approx $27 T, T$_{c}$=17 K), and - for comparison - BaFe$_{1.95}$Ni$_{0.05}$As$_{2}$, which should have properties similar to the underdoped Co-sample. Previous measurements of thermal conductivity (as a function of temperature and field) and penetration depth on comparable composition samples gave some disagreement as to whether there was fully gapped/nodal behavior in the under-/overdoped materials respectively. The present work shows that the measured behavior of the specific heat $\gamma $ ($\propto $ C/T as T$\to $0, i. e. a measure of the electronic density of states at the Fermi energy) as a function of field obeys $\gamma \approx $H$^{0.6\pm 0.1}$, similar to the Volovik effect for nodal superconductors, over the entire field range for both under- and overdoped Co samples as well as for the underdoped Ni sample. By comparison to theory, the possibility of two bands, one with line nodes and one fully gapped, being present in these materials is discussed. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J22.00008: Nodal versus nodeless order parameters in LiFeP and LiFeAs superconductors T. Shibauchi, K. Hashimoto, S. Kasahara, R. Katsumata, Y. Mizukami, M. Yamashita, H. Ikeda, T. Terashima, Y. Matsuda, A. Carrington There is growing evidence that the superconducting gap structure is not universal in the iron-based superconductors. It is essential to determine experimentally what causes nodal and nodeless states. The 111 materials, LiFeAs and LiFeP provide a unique route to study this problem as both materials are superconducting, nonmagnetic, and importantly very clean, with long electronic mean-free paths. Here we report on high-precision measurements of magnetic penetration depth $\lambda$ in clean single crystals of LiFeAs and LiFeP superconductors, which reveal contrasting low-energy excitations of quasiparticles. In LiFeAs the low-temperature $\lambda(T)$ shows a flat dependence indicative of a fully gapped state, which is consistent with previous studies. In contrast, LiFeP exhibits a $T$-linear dependence of superfluid density $\propto \lambda^{-2}$, indicating a nodal superconducting order parameter. A systematic comparison of quasiparticle excitations in the 1111, 122, and 111 families of iron-pnictide superconductors implies that the nodal state is induced when the pnictogen height from the iron plane decreases below a threshold value of $\sim 1.33$\,\AA. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J22.00009: Inter-Layer Superconducting Pairing Induced c-axis Nodal Lines in Iron-based Superconductors Chandan Setty, Yuehua Su, Ziqiang Wang, Jiangping Hu A layered superconductor with a full pairing energy gap can be driven into a nodal superconducting (SC) state by inter-layer pairing when the SC state becomes more quasi-3D. We propose that this mechanism is responsible for the observed nodal behavior in a class of iron-based SCs. We show that the intra- and inter-layer pairings generally compete and the gap nodes develop on one of the hole Fermi surface pockets as they become larger in the iron-pnictides. Our results provide an explanation of the c-axis gap modulations and gap nodes observed by angle resolved photo emission spectroscopy. In addition, we predict that an anti-correlated c-axis gap modulations on the hole and electron pockets should be observable in the $s\pm$-wave pairing state. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J22.00010: Thermal-transport measurement of the nodal superconductor KFe$_2$As$_2$ Minoru Yamashita, Daiki Watanabe, Takyuya Yamashita, Takasada Shibauchi, Yuji Matsuda, Hideto Fukazawa, Taku Saito, Yoh Kohori, Kunihiro Kiho, Akira Iyo, Hiroshi Eisaki Hole-doped Fe-based superconductors, (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$, possess two different superconducting gap structures; a fully-gapped state near the optimally dope (x$\sim$0.5) and a nodal gap state at the hole-dope end (x = 1). To investigate the detail gap structure, we performed thermal-transport measurements of KFe$_2$As$_2$ with very high purity (RRR$\sim$ 1,600) down to 80 mK. From the temperature dependence of thermal conductivity at zero field, we find a finite residual of $\kappa / T$ in the zero-temperature limit. This residual increases by magnetic field as $\propto \sqrt{H}$ in low fields, followed by a rapid increase near $H_{c2}$. Thermal conductivity measurements of different dopes (x = 0.88, 0.93) will be reported. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J22.00011: Microwave Surface Impedance Measurements of SrFe$_2$(As,P)$_2$ Single Crystals Hideyuki Takahashi, Yoshinori Imai, Atsutaka Maeda, Kentaro Kitagawa, Kazuyuki Matsubayashi, Masashi Takigawa, Yoshiya Uwatoko Various pairing symmetries have been proposed concerning Fe-based superconductors both theoretically and experimentally. It was reported that LaFePO[1] and BaFe$_2$(As,P)$_2$[2] have line nodes in their superconducting gap. It is in sharp contrast to other Fe-based compounds such as LiFeAs[3] and Fe(Se,Te)[4]. To confirm whether line nodes in gap function is a common feature among P doped systems, we measured the microwave surface impedances of SrFe$_2$(As,P)$_2$ single crystals ($T_c\sim30K$). \\ Single crystals were grown by self-flux method. The surface impedances were measured using a cavity perturbation technique. The imaginary part of surface impedance, which is proportional to London penetration depth in the superconducting state, shows a power law, $\lambda(T)-\lambda(0)\propto T^n$. The power law indicates low-energy quasiparticle excitation, and an exponent slightly smaller than 2 does not exclude the possibility of the existence of line nodes. \\ $\left[ 1 \right]$ J. D. Fletcher {\it et al.}, Phys. Rev. Lett. 102 (2009) 147001.\\ $\left[ 2 \right]$ K. Hashimoto {\it et al.}, Phys. Rev. B 81 (2010) 220501(R).\\ $\left[ 3 \right]$ Y. Imai {\it et al.}, J. Phys. Soc. Jpn. 80 (2011) 013704.\\ $\left[ 4 \right]$ H. Takahashi {\it et al.}, Phys. Rev. B 84. (2011) 132503. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J22.00012: Doping-induced line nodes in the superconducting gap of the iron arsenide Ba$_{1-x}$K$_x$Fe$_2$As$_2$ from thermal conductivity R.T. Gordon, J.-Ph. Reid, X.G. Luo, H. Shakeripour, S. Ren\'{e} de Cotret, A. Juneau-Fecteau, N. Doiron-Leyraud, J. Chang, Louis Taillefer, H. Kim, M.A. Tanatar, R. Prozorov, B. Shen, H.-H. Wen The thermal conductivity $\kappa$ of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ was measured down to 50 mK in magnetic fields up to 15 T, for heat current both parallel and perpendicular to the tetragonal c axis, across a range of K concentrations from optimal doping (T$_c$=38 K) down to T$_c$=7 K, deep into the region of coexistence with antiferromagnetic order. From optimal doping down to T$_c\simeq$15 K, well into the coexistence region, there is no residual linear term in $\kappa$(T) as T$\to$0, showing that there are no nodes in the superconducting gap anywhere on the Fermi surface. For concentrations in a narrow range such that 9 K$<$T$_c<$13 K, a large residual linear term appears, signaling the onset of nodes in the superconducting gap, most likely vertical line nodes running along the c axis. For T$_c<$9 K, the gap is again nodeless. We propose that these changes in the superconducting gap structure are triggered by changes in the Fermi surface as it is reconstructed by the growing antiferromagnetic order. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J22.00013: Evidence for line nodes in the energy gap of overdoped Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$: A low-temperature specific heat study Gang Mu, Jun Tang, Yoichi Tanabe, Jingtao Xu, Satoshi Heguri, Katsumi Tanigaki We report the low-temperature specific heat (SH) study on Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals in a wide doping region under different fields. For the overdoped sample, we find the clear evidence for the presence of T$^{2}$ term in the electronic SH data, suggesting the presence of line nodes in the energy gap. This term is absent both for the underdoped and optimal doped samples. Moreover, the field induced electronic SH coefficient $\Delta \gamma $(H) increases more quickly with the field for the overdoped sample than the underdoped and optimal doped ones, showing a large anisotropy of the gap for the overdoped sample. Our results suggest that the energy gap(s) in the present system may have different structures strongly depending on the doping regions. [Preview Abstract] |
Session J23: Phase Changes, Metals, PC Memories
Sponsoring Units: DCMPChair: Paul Kent, Oak Ridge National Laboratory
Room: 255
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J23.00001: Phonon dispersions and structural transitions of CrO$_{2}$ under high pressure Sooran Kim, Kyoo Kim, Chang-Jong Kang, B.I. Min Phonon dispersions of chromium dioxide (CrO$_{2}$) are calculated to investigate the structural phase transitions as a function of pressure. The structural phase transition has been confirmed from the ground state tetragonal CrO$_{2}$ of rutile-type (t-CrO$_{2}$) to the orthorhombic CrO$_{2}$ of CaCl$_{2}$-type (o-CrO$_{2}$). The ferromagnetic and half-metallic property is preserved even in o-CrO$_{2}$. The softening of Raman-active B$_{1g}$ phonon mode, which is relevant to the above structural transition, is also obtained. We will discuss the possible more structural phase transitions from o-CrO$_{2}$ and the related phonon and magnetic properties at much higher pressure. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J23.00002: Optimizing plasmonic properties of supported silver nanocube monolayers Anatoli Ianoul, Adam Bottomley, Nur Ahamad The refractive index (RI) sensitivity of extinction spectra was compared experimentally for silver nanocubes in solution and in supported monolayers prepared by the Langmuir technique. The size of the nanocubes, RI of supporting dielectric substrate, and the monolayer surface pressure were used as variables in refractive index sensing (RIS) optimization. The dipolar plasmon modes of the colloidal nanocubes were found to have the highest RIS values of 176, 361 and 480 nm/RI units for 40, 80, and 130 nm cubes respectively. The largest figure of merit (FOM) of 4.55 was measured for a quadrupolar mode of 130 nm nanocubes. When compared to suspensions, RIS of supported nanocubes were reduced by $\sim $50{\%} and decreased with increasing monolayer surface pressure. RIS of 40 nm cube monolayers appeared to be sensitive to the substrate RI due to the RI dependent plasmon mode hybridization resulting in dipolar (D) and quadrupolar (Q) modes. RIS sensitivities of D and Q modes for silicon- supported nanocube monolayer were found to be 78 and 170 nm/RI units respectively. The FOM for the Q mode appeared to be 5.5. Intensity of this Q peak was found to increase with the angle of incident light. This work shows the use of high refractive index dielectric substrates, a passive molecular spacer, and large angles of incidence can significantly improve the detection of plasmonic response by supported nanocube monolayers. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J23.00003: Isotropic Structural Color of Nanostructured Metal Surfaces Sylvanus Y. Lee, Carlo Forestiere, Alyssa J. Pasquale, Gary F. Walsh, Marco Romagnoli, Luca Dal Negro Engineering angularly insensitive (i.e., isotropic) structural coloration in metals without employing extrinsic materials have been a challenge due to the strong absorption properties of metals in the visible range. In this study, by combining plasmonic resonances of metallic nanoparticles and incoherent light scattering from deterministic arrays with isotropic and diffuse Fourier space, we demonstrate isotropic structural coloration of metal films with spatial uniformity and angular insensitivity. Specifically, we explore the angular scattering properties of aperiodic gold nanoparticle arrays with Pinwheel geometry and their hyperuniform counterpart (Delaunay-triangulated Pinwheel centroid, DTPC). The structures are designed by electromagnetic simulations based on the Coupled Dipole Method in partnership with three dimensional Finite Difference Time Domain modeling of plasmonic resonant nanoparticles on gold films. The experimental characterization is performed by measuring the far-field scattering spectra of the proposed arrays using angle-resolved reflection spectroscopy under white light illumination. The measured radiation diagrams of the fabricated plasmonic arrays demonstrate controllable and isotropic coloration of gold films. The proposed approach can potentially advance plasmonic applications to display, tagging and colorimetric sensing technologies. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J23.00004: Ab-initio study of the free liquid Hg surface Lazaro Calderin, Luis E. Gonzalez, David Gonzalez The free surface of liquid Hg at two temperatures (T=300 and 450 K) has been studied by using first principles molecular dynamics simulations. The calculated longitudinal ionic density profile shows an oscillatory shape extending several atomic diameters into the bulk and its wavelength is in good agreement with experiment. The associated self-consistent valence electronic density profile shows much weaker oscillations which are somewhat out of phase with the ionic ones. The calculated X-ray reflectivity shows a marked maximum at a wavevector transfer of $q_z$ $\approx$ 2.2 \AA$^{-1}$ whose origin is related to the surface layering. Moreover, it shows a good agreement with the experimental reflectivity data. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J23.00005: Noncontact technique for measuring the electrical resistivity and magnetic susceptibility of electrostatically levitated melts G.E. Rustan, N.S. Spyrison, A. Kreyssig, R. Prozorov, A.I. Goldman Over the last two decades the popularity of levitation methods for studying equilibrium and supercooled melts has increased steadily. Measurements of density, viscosity, surface tension, and atomic structure have become well established. In contrast, measurements of electrical resistivity and magnetic susceptibility of levitated melts have been very limited. To fill this void, we have combined the tunnel diode oscillator (TDO) technique with electrostatic levitation (ESL) to perform inductively coupled measurements on levitated melts. A description of the basic operating principles of the TDO and ESL will be given, as well as a description of the implementation and performance characteristics of this technique. Preliminary measurements of electrical resistivity in the solid and liquid state will be presented for samples of Zr, Si, and Ge, as well as the measurements of ferromagnetic transitions in Fe and Co based alloys. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J23.00006: Orientational ordering of tetrahedral clusters in $CaCd_6$ alloy Woosong Choi, Marek Mihalkovic, Chris Henley In icosahedral CaCd quasicrystals and related alloy structures, each icosahedral cluster contains an innermost tetrahedral shell that is loosely coupled with its shell. We extract an effective potential for the pair interaction $V(\Omega,\Omega')$ of the orientations $\Omega$ of neighboring clusters, as mediated by the intervening atoms. Using the EAM potentials of Brommer et al.,~\footnote{Brommer, G\"ahler, and Mihalkovi\v{c}, Phil. Mag. 87, 2671 (2007)}we relax all atoms, with each tetrahedron constrained to a chosen (continuously variable) overall orientation but allowing distortions. Using singular value decomposition, the relaxed energies are represented as $V(\Omega, \Omega')=\sum_j A_j f_j(\Omega) f'_j(\Omega')$ where only a few terms are important and the $f_j$'s have simple sinusoidal forms (which can be understood physically). We thus obtain a fit with only a few parameters, in place of the 46-parameter fit of Brommer et al$^1$ based on 12 discrete orientations. By Monte Carlo simulations with the obtained interaction, we determine the pattern of the orientationally ordered state seen experimentally below $\sim 90$K, and check the ordering transition previously simulated only in small system sizes$^1$. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J23.00007: Electronic structure of Sr$_2$IrO$_4$ and its doped materials measured by angle-resolved photoemission spectroscopy Qiang Wang, Yue Cao, J.A. Waugh, S.R. Park, Tongfei Qi, Oleksandr Korneta, Gang Cao, D.S. Dessau The electronic structure of iridate Sr$_2$IrO$_4$ and its doped materials Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$ and (Sr$_x$La$_{1-x}$)$_2$IrO$_4$ were investigated by angle-resolved photoemission spectroscopy. For Sr$_2$IrO$_4$, which is a J$_{eff}$=1/2 Mott insulator driven by strong spin-orbit coupling, the dispersion of the predominant Ir 5d bands has been successfully resolved and compared with theoretical calculations. The overall band structure is in line with LDA calculations with strong spin-orbit coupling and moderate electron correlation effects included. The detailed electronic structures of isovalent-doped Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$ and electron-doped (Sr$_x$La$_{1-x}$)$_2$IrO$_4$ are also obtained and compared with the electronic structure of the mother compound. These results help reveal the delicate interplay between charge, spin, orbit, and lattice degrees of freedom in iridates and other correlated electron systems. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J23.00008: Electronic Structures of Oxygen-deficient PtO$_{2}$ Yong Yang, Osamu Sugino, Takahisa Ohno We studied the electronic properties of beta-platinum dioxide ($\beta $-PtO$_{2})$, a catalytic material, based on density functional theory. Using the GGA+U method whose predicted band gap is verified by GW calculations, we found that the creation of an oxygen vacancy will induce local magnetic moment on the neighboring Pt and O atoms. The magnetism originates not only from the unpaired electrons that occupy the vacancy induced gap state, but also from the itinerant valence electrons. Because of antiferromagnetic (AF) coupling and the localized nature of gap states, the total magnetic moment is zero for charge-neutral state (V$_{\mbox{O}}^{\mbox{0}}$) and is $\sim $ 1 $\mu _{B}$ for singly-charged states (V$_{\mbox{O}}^{\pm}$). Calculation of grand potential shows that, the three charge states (V$_{\mbox{O}}^{\mbox{0}}$, V$_{\mbox{O}}^{\pm})$ are of the same stability within a small region, and the negatively charged state (V$_{\mbox{O}}^-$) is energetically favored within a wide range of the band gap. On this basis we discussed the implication on catalytic behavior. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J23.00009: Theory of Non-Fermi Liquids Patrick O'Brien The focus of this project is to understand the simplest model needed to explain the physics of the material FeCrAs. The geometry of the FeCrAs crystal is used to perform crystal field splitting calculations, which in turn indicates a model that we will attempt to use to describe two of the main trends in the experimental data collected for the material. The model that we believe captures the physical character of FeCrAs for the resistivity and heat capacity measurements is the double exchange model. Next, we must find a way to describe an antiferromagnetic transition at 125K. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J23.00010: Evolution between metastable strain glass state and stable martensitic phase in TiNi alloy Yuanchao Ji, Xiaobing Ren, Xiangdong Ding Phase transitions occur when the thermodynamic free energy of one phase is not lowest for varying some external conditions, such as temperature, pressure, and others. However, metastable glass states that falls out of equilibrium may be observed on continued cooling. Some frozen metastable states can be further spontaneously transformed into thermodynamic stable ones. The corresponding inverse process, from low temperature stable phase to glass state then to another high temperature stable phase upon heating, seems proving the glass state like the metastable state like in chemical reactions. Here we report the inverse freezing (strain glass state) does not occur for the first time. The lower temperature B19' martensite directly transforms into the higher temperature B2 parent phase and the strain glass also transforms B2 phase in the heating process, although the glass state is in the intermediate between B2 and B19' phases in the cooling process in TiNi single crystal. Completely different transformation sequence reveals the inverse glass transition is not necessary between two stable phases and helps us further deepen the understanding of glass transition. \\[4pt] [1] X. Ren, et. al., Philos. Mag. A 90, 141(2010). \\[0pt] [2] X. Ren, et. al., MRS Bull. 34, 838(2009). [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J23.00011: Ab Initio Study of Phase-Change Materials Doped with magnetic Impurities Riccardo Mazzarello, Yan Li Chalcogenide Phase-change materials are of great technological importance due to their ability to undergo reversible and fast transitions between the amorphous and crystalline phases upon heating. Recently, it was shown experimentally that Ge$_{2}$Sb$_{2}$Te$_{5}$ doped with Fe atoms exhibits phase-change behavior for low concentrations of Fe and that both the amorphous and the crystalline phases are ferromagnetic at low enough temperatures. Moreover, the two phases were found to have different saturation magnetization. This finding opens up the possibility of exploiting the phase-change behavior for fast magnetic switching in e.g. spintronic devices. We have investigated the structural, electronic and magnetic properties of Fe-doped Ge$_{2}$Sb$_{2}$Te$_{5}$ by first-principles simulations based on Density Functional Theory. Both amorphous and crystalline (hexagonal and cubic rocksalt) phases of Ge$_{2}$Sb$_{2}$Te$_{5}$ were considered. Our results show that in the amorphous phase, the magnitude of the magnetic moments of the Fe impurities is reduced with respect to the crystalline phases, due to the different local geometries and chemical environments, which explains the experimentally observed magnetic contrast between the two phases. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J23.00012: Crystallization Times of Ge2Sb2Te5 Nanostructures as a Function of Temperature Faruk Dirisaglik, Kadir Cil, Maren Wennberg, Adrienne King, Mustafa Akbulut, Yu Zhu, Chung Lam, Ali Gokirmak, Helena Silva Phase-change memory is a promising non-volatile memory technology in which a small volume of a phase change material is reversibly and rapidly switched between amorphous and crystalline phases by a suitable electrical pulse. Amorphization is achieved by fast cooling after a melting pulse while crystallization is achieved through electrical breakdown of the amorphous element that leads to heating above the crystallization temperature for a sufficient period. Significant difference between the crystallization behavior of phase-change materials in bulk, thin films and in nanostructures have been observed[1,2]. We have studied the crystallization times of nanoscale Ge2Sb2Te5 line structures using 50-500 ns voltage pulses with a baseline offset in the 500 K to 625 K range under high-vacuum. The baseline voltage allows measurement of resistance before and after the pulse and crystallization time. Current and voltage were recorded with high time-resolution for amorphizing pulse and very long periods of time to observe the crystallization time. The crystallization time decreases for increasing temperature but it remains on the order of seconds even at elevated temperatures. [1] H. Wong et al., Proc IEEE 98, 2201, 2010. [2] T. Zhang et al., Scr. Mater. 58, 977, 2008. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J23.00013: Electrical Resistivity of Liquid Ge2Sb2Te5 in Patterned Nanostructures Kadir Cil, Faruk Dirisaglik, Maren Wennberg, Adrienne King, Mustafa Akbulut, Yu Zhu, Chung Lam, Helena Silva, Ali Gokirmak Phase change memory devices are based on the electrical resistivity contrast between the amorphous and the crystalline phases of chalcogenide materials [1]. Since melting is required to amorphize the material, knowledge of the liquid state properties is critical for device design. Ge2Sb2Te5 (GST) is the most studied phase change material. However, the two experimental values reported to date for electrical resistivity of liquid GST (one on a thin film [2], the other in bulk [3]) differ by an order of magnitude. We have extracted the electrical resistivity of liquid GST from single pulse measurements on a large number of encapsulated GST line structures with varying lengths, widths and thicknesses. Each structure is self-heated to melt by the microsecond voltage pulse while voltage and current are monitored using an oscilloscope. The liquid state resistivity is calculated from slopes of liquid state resistance versus 1/width, fitted as a function of length. The results we obtained for the liquid resistivity of GST in nanostructures are close to those obtained from measurements on bulk GST [3]. 1. H. Wong et al., Proc IEEE 98, 2201, 2010. 2. T. Kato and K. Tanaka, Jap. J. Appl. Phys. 44, 7340, 2005. 3. R. Endo et al., Jap. J. Appl. Phys. 49, 5802, 2010. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J23.00014: 3-D Numerical Study of Switching Dynamics in Nanoscale Phase Change Memory Devices Ibrahim Cinar, Gulsen Kosoglu, Ozgur Burak Aslan, Gulen Aktas, Ozhan Ozatay Phase change memory (PCM) is currently regarded as a strong candidate technology to replace Flash memory in the market. In this work we report a detailed numerical study of the switching process in a nanoscale PCM cell, namely its switching dynamics during SET (turn on) and RESET (turn off) operations. A comprehensive picture of the electrical, thermal and phase change dynamics is obtained using a multiphysics approach with coupled differential equations in the framework of a three dimensional finite element model. The complexity of the problem was handled by constructing separate submodels; an electrical model which involves a temperature and phase dependent electrical conductivity, a thermal model where the joule heating from the electrical current serves as the heat source and involves temperature and phase dependent thermal conductivity and a phase change model. In this presentation we will concentrate on the electrical and thermal submodels in detail. The results of the phase change model taking into account homogeneous and heterogeneous nucleation kinetics will be discussed in another presentation. We will compare the numerical results with experimental data on GST based nanoscale phase change devices with various contact sizes and shapes. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J23.00015: 3-D Simulation Model of Phase Change and Percolation in Phase Change Memory Ozgur Burak Aslan, Ibrahim Cinar, Gulsen Kosoglu, Gulen Aktas, Ozhan Ozatay Even though phase change memory (PCM) appears as a promising nonvolatile solid state memory with its high signal to noise ratio and superior scalability compared to other memory technologies, the complex nature of the phase change process necessitates advanced numerical modeling to optimize the performance of nanoscale memory cells. The phase change and the percolation processes of a nanoscale PCM cell during SET (turn on) and RESET (turn off) operations have been simulated based on a three dimensional finite element model. A multiphysics approach with coupled differential equations is used to observe and understand the phase change and percolation dynamics. The model to represent the PCM is divided into submodels consisting of an electrical, a thermal and a phase change model that affects nucleation kinetics of crystallites. Coupling the submodels in the framework of the multiphysics approach, this model allows us to predict threshold voltage and recrystallization temperature for switching by detecting the critical conditions for the formation of a conductive percolation path in phase change process. These results will be compared to the experimental results to be carried on. The subject of electrical and thermal model will be mentioned in another presentation. [Preview Abstract] |
Session J24: Focus Session: Dopants and Defects in Semiconductors - Si and III-V
Sponsoring Units: DMPChair: Socrates Pantelides, Vanderbilt University
Room: 256
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J24.00001: Dislocation nucleation in Si -- effect of Ge and vacancies Zhi Li, Catalin Picu The presence of dislocations in Si devices is detrimental for their function. However, dislocations may nucleate during device processing from interfaces and from imperfections of the surface. In this work we study the conditions under which this may take place. Specifically, the activation energy for dislocation nucleation in pure Si is evaluated using an atomistic model and considering a corner stress concentrator. At lower temperatures, 60$^{\circ}$ dislocations nucleate in the shuffle set as half-hexagonal loops. The activation energy depends on the step height and on the spacing between steps. The effect of Ge on nucleation is evaluated using the same model and it is observed that the energy barrier decreases slightly in presence of these substitutional atoms. The presence of vacancies in the glide plane also decreases the activation energy. A low concentration of vacancies (0.5{\%}) decreases the barrier for nucleation similarly with a large concentration of Ge (30{\%}). [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J24.00002: Surface Charge Distribution on Hydrogen-Passivated Silicon Measured by UHV Kelvin Probe Microscopy Pavel Nagornykh, Kristen Burson, William Cullen, Bruce Kane A few years ago hydrogen-passivated silicon (H:Si) surfaces were used to observe high-mobility in FETs (100,000 cm2/Vs) [1]. This result is expected since chemical passivation of silicon produces an atomically flat surface with low density of defects. As chemical preparation can lead to creation of fixed charges on H:Si surface, one needs a method to get information about their distribution to further improve the quality of a process. This can be done by measuring electrostatic forces created by such charges with Kelvin Probe Microscopy (KPM), which obtains a voltage distribution map with a resolution of a few nm. We have used UHV KPM for our measurements, since UHV conditions are important for avoiding oxidation of H:Si surface as well as achieving its further depassivation/annealing. By annealing samples above depassivation temperature(~500C), we have obtained data that provides more information about depassivation effect on charges that come from chemical residue on hydrogen and silicon itself. Preliminary results show standard deviations of voltage much smaller (by an order of magnitude) than ones measured previously on silicon oxide, consistent with high quality expected from chemical preparation. \\[4pt] [1] Robert N. McFarland et al., Phys. Rev. B 80, 161310(R) (2009) [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J24.00003: Magnetic resonance characterization of silicon nanowires Marco Fanciulli, Matteo Belli, Antonio Vellei, Carmen Canevali, Davide Rotta, Stefano Paleari, Martina Basini Silicon nanowires (SiNWs) have been extensively investigated in the last decades. The interest in these nanostructures stems from both fundamental and applied research motivations. The functional properties of one- and zero-dimensional silicon structures are significantly different, at least below a certain critical dimension, from those well known in the bulk. The key and peculiar functional properties of SiNWs find applications in nanoelectronics, classical and quantum information processing and storage, optoelectronics, photovoltaics, thermoelectric, battery technology, nano-biotechnology, and neuroelectronics. We report our work on the characterization by continuous wave (CW) and pulse electron spin resonance (CW, FT-EPR) and electrically detected magnetic resonance (EDMR) measurements of silicon nanowires (SiNWs) produced by different top-down processes. SiNWs were fabricated starting from SOI wafers using standard e-beam lithography and anisotropic wet etching or by metal-assisted chemical etching. Further oxidation was used to reduce the wire cross section. Different EDMR implementations were used to address the electronic wave function of donors (P, As) and to characterize point defects at the SiNWs/SiO$_{2}$ interface. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J24.00004: Ab initio study of confinement effects and hyperfine structure in chalcogen doped Silicon nanostructures Alberto Debernardi, Guido Petretto, Andrea Masse, Marco Fanciulli In recent years, increasing interest has been focused on Si nanostructures as building blocks for ultra-scaled electronic devices where quantum mechanic effects are relevant. We have investigated the atomic and electronic structures of a single chalcogen donor in H passivated Si nanostructures of different size by means of the plane-wave pseudopotential techniques we used in Ref. [1] to study Se doped Si (001) nanowires (NW). Our results showed an increase in the gap with diminishing diameter of Si NW. We studied the size dependence of electronic properties and hyperfine constant of single substitutive chalcogen impurity in Si-NWs (001) and (111) oriented, and Si-dots and their dependence on the distance from the NW axis or the centre of the dot. We show that the hyperfine parameters are strongly dependent on the impurity position: we proved that surface effects can lead to strong differences in the hyperfine parameters depending on the chalcogen location inside the nanowire, suggesting a way to determine experimentally the position of the defect on the basis of electron paramagnatic resonance spectra. Preliminary results on chalcogen doped Ge nanowires complete the work. [1] G. Petretto, A.Debernardi, and M. Fanciulli, Nano Letters, Vol. 11, 4509 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J24.00005: Segregation and diffusion of boron dopants in the Si/SiO$_2$ interface Young Jun Oh, Hyeon-Kyun Noh, Geun-myung Kim, K.J. Chang Boron dopants in metal-oxide-semiconductor field-effect transistors exhibit very peculiar behavior such as transient enhanced diffusion, clustering, and segregation. Especially, B segregation to the Si/SiO$_2$ interface significantly affects the dopant distribution and thereby the device performance. However, there is a lack of studies on the mechanism for B segregation and diffusion in the Si/SiO$_2$ interface. In this work, we perform first-principles density-functional calculations to understand how B dopants diffuse and segregate to SiO$_2$. We generate two Si/SiO$_2$ interface structures, in which crystalline alpha-quartz and amorphous SiO$_2$ are placed on Si. Among various B configurations, we find that an interstitial B is energetically more favorable in the oxide, compared with a subsitutional B and a self-interstitial-B complex in Si. We examine the effect of point defects such as a floating bond and an oxygen vacancy in SiO$_2$ on B segregation and also investigate B diffusion pathways across the Si/SiO$_2$ interface. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J24.00006: Hybrid functional calculations for native defects and dangling bonds in $\alpha$-Al$_2$O$_3$ Minseok Choi, Anderson Janotti, Chris G. Van de Walle Al$_2$O$_3$ is a promising material for use as a gate dielectric in III-V-based MOS devices, including in GaN-based transistors. Recent developments indicate that despite the relatively high structural quality, the presence of charge traps and fixed-charge centers near the interface between the oxide and nitride still poses serious problems for device performance. Native defects and dangling bonds in the Al$_2$O$_3$ dielectric or in the vicinity of the interface are the most likely causes. To aid in the identification of these centers, we perform density functional calculations with a hybrid functional for point defects and dangling bonds in $\alpha$-Al$_2$O$_3$. We determine the position of the defect transition levels in the gap of the oxide, and analyze the level positions with respect to the nitride band edges. Our results show that O vacancies and Al dangling-bonds can produce charge traps and Al interstitials act as fixed charges in GaN-based $n$-MOSFETs. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J24.00007: Role of oxygen-related defects in hafnia Manish Jain, James R. Chelikowsky, Steven G. Louie Hafnia has recently received much attention because of its potential application as high-k dielectric material replacing silica in microelectronic devices. Point defects in hafnia - in particular oxygen vacancies and interstitials - can play an important role as traps or sources of fixed charge. In this study, we perform electronic structure calculations on oxygen-related defects in monoclinic hafnia using a combined density functional theory (DFT) and GW formalism. We have previously shown that upon including quasiparticle defect levels and the appropriate electrostatic corrections within a supercell calculation, this formalism corrects for the error in calculating formation energy and charge transition levels using standard DFT. In this study, we calculate the formation energy of these defects as a function of the Fermi level and the chemical potential of oxygen to determine which of these defects are most stable. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J24.00008: Vibrational spectroscopy of cast Si used to fabricate solar cells: microscopic properties of nitrogen and oxygen impurities Haoxiang Zhang, Michael Stavola, Mike Seacrist Cast Si with grain sizes from a few mm to a few cm is commonly used for the fabrication of solar cells. Nitrogen impurities are introduced into cast Si by the SiNx coating of the crucible used for casting. Much is known about N and O centers in single-crystal Si used in microelectronics [1]. We have used vibrational; spectroscopy to probe the concentration and defect configurations of nitrogen centers in cast Si used to fabricate solar cells. The interaction of N with O impurities that are present has also been investigated. The dominant N center in cast Si is a N-N interstitial pair. N-O complexes are also formed. Which defect complexes are present depends on the impurity content of the multi-crystalline Si sample, which can vary widely, and its thermal history. [1] H. Ch. Alt and H. E. Wagner, J. Appl. Phys. \textbf{106}, 103511 (2009) and the references contained therein.. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J24.00009: The chemical trends of a new defect cluster: DDX centers Jie Ma, Su-Huai Wei DX center is a major ``killer'' defect limiting n-type doping in group II-VI and III-V semiconductors. It converts a shallow donor to deep one, which is a major reason for the saturation of free-electron carriers in the doping process. Several structure models of isolated DX centers have been proposed in the literatures, such as the broken-bond model (BB-DX), and the $\alpha $ and $\beta $ cation-cation bond model (CCB-DX). All these DX centers can be stabilized with hydrostatic pressure or reduced dimensionality and size. In group III-V and II-VI semiconductors, it has been common believe that cation-site induced DX centers are easier to form than anion-site induced ones. Because DX centers trap an extra electron, therefore, another defect in the system must donate the electron and form a positive charged defect. We show, using GaAs as an example, that in heavily doped semiconductor, the negative charged DX center and positive charged donor can couple strongly through the Coulomb interaction, forming the dominant DDX center. The DDX centers are still deep level defects. However, unlike the DX center, the DDX centers have different chemical trends, i.e., anion-site DDX center is easier to form than cation-site DDX centers. A simple model is proposed to explain the new trends. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J24.00010: Valence state manipulation of single Fe impurities in GaAs Paul Koenraad, Juanita Bocquel, Victoria Kortan, Michael Flatte, Richard Campion, Bryan Gallagher Cross-sectional STM was used to characterize Fe doped GaAs. We show that, by controlling the tip-induced band bending, Fe atoms can be brought from their isoelectronic state (Fe3+)-3d5 state into their (Fe2+)-3d6 ionized acceptor state. This STM-induced valence manipulation that involves the (de)population of the d-shell of the Fe atom differs from our previous experiments on Mn-acceptors and Si-donors where we changed its charge state by adding or removing a valence band hole or a conduction band electron respectively that is bound by the impurity. In addition for specific tunneling conditions a peculiar contrast is observed under which Fe atoms appear as dark anisotropic features. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J24.00011: Local Environment Effects on Single Zn and Mn Acceptors in Gallium Arsenide Nancy Santagata, Donghun Lee, Jay Gupta The ability to precisely control the properties of single dopants in semiconductors is of interest not only for the improvement of current technologies but also for the development of next generation devices. Recent work in our group has exploited the single atom precision afforded by a scanning tunneling microscope to explore how the properties of dopants in gallium arsenide depend on their local environment. Specifically, we have shown that Zn dopants located within the same layer and occupying identical binding sites exhibit dissimilarities that are dependent upon the proximity to neighboring subsurface Zn acceptors.(1) $^{ }$Further, we demonstrated control of the ionization state of single Mn acceptors by both defect- and tip-induced band bending.(2) Finally, we achieved tunable control over the binding energy of Mn acceptors by varying the proximity to charged As vacancies.(3) This talk will review these findings and elaborate on the application of these techniques to characterize defects in wide bandgap materials, where the origin of properties like ferromagnetism are not yet well understood. 1. Appl. Phys. Lett. 99, 053124 (2011). 2. Nano Lett. 11, 2004 (2011). 3. Science 330, 6012 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J24.00012: Recombination dynamics of excitons bound to nitrogen isoelectronic centers in GaAs Philippe St-Jean, Gabriel \'Ethier-Majcher, John F. Klem, S\'ebastien Francoeur Using time-resolved photoluminescence, we have studied the radiative recombination dynamics of excitons bound to single nitrogen dyads in GaAs. For in-plane dyads of C$_{2v}$ symmetry considered in this work, the lifetime of all four allowed optical transitions, polarized either along or perpendicular to the dyad, decreases with temperature. Over the temperature range studied, the lifetimes of transitions of same polarization remain highly similar, but they are sensitively longer for transitions polarized along the dyad. These results indicate that 1) the spatial orientation of the exciton wavefunction is an important factor in the recombination dynamic, 2) the transfer dynamics between bright states appears negligible, and 3) the transfer to or from dark states is not significant for temperature over 5 K. These findings enhance the understandings of isoelectronic centers which are promising candidates for the implementation of atomic size memories based on a single charge or spin. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J24.00013: First-principles study of InX (X=P,Sb) semiconductors Oscar D. Restrepo, Rohan Mishra, Wolfgang Windl III-V semiconductors are gaining increasing interest for applications due to the possibility of engineering their electronic properties by choosing and combining different elements. Additional design parameters can come from confinement effects which have led to intensified research on nanowires for electronic applications. Among the lesser studied III-V semiconductors with large technological potential are indium-based compounds, where InSb with an extremely small band gap in the infrared range is the staple material for infrared detector devices and InP (with a larger band gap of 1.42 eV) is considered as a promising material for nanowire-based applications. For these materials, many basic questions that have been answered for more-mainstream semiconductors are still unanswered, these include effective masses, optical properties, and the influence of defects on their properties. In order to address some of these questions, we have performed an exhaustive exploration of the defect energetics of InP using first-principles calculations. We also report a detailed comparison of calculated effective masses and optical properties of InSb with experiments. We have used both GGA and HSE06 to treat exchange-correlations. This work was supported by NSF MRSEC DMR-0820414. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J24.00014: Simple intrinsic defects in GaP and InP Peter A. Schultz To faithfully simulate evolution of defect chemistry and electrical response in irradiated semiconductor devices requires accurate defect reaction energies and energy levels. In III-Vs, good data is scarce, theory hampered by band gap and supercell problems. I apply density functional theory (DFT) to intrinsic defects in GaP and InP, predicting stable charge states, ground state configurations, defect energy levels, and identifying mobile species. The SeqQuest calculations incorporate rigorous charge boundary conditions removing supercell artifacts, demonstrated converged to the infinite limit. Computed defect levels are not limited by a band gap problem, despite Kohn-Sham gaps much smaller than the experimental gap. As in GaAs, [P.A. Schultz and O.A. von Lilienfeld, Modeling Simul. Mater. Sci. Eng. 17, 084007 (2009)], defects in GaP and InP exhibit great complexity---multitudes of charge states, bistabilities, and negative U systems---but show similarities to each other (and to GaAs). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J24.00015: Defects in GaSb and InAs/GaSb superlattices Jianwei Wang, Yong Zhang Unintentionally doped GaSb is known to be p-type. One possible explanation for the p-type conductivity is due to the existence of Ga on Sb anti-site defects that behave as acceptors. Such an acceptor-like defect state could potentially impact the performance of an IR detector based on a type II superlattice InAs/GaSb. We use pseudopotential density functional theory to investigate this defect state in both bulk GaSb and the superlattice. We calculate the defect levels with and without spin-orbit interaction and with the p-d band separation and bandgaps corrected. Although the defect might be acceptor-like, its energy level does not necessarily follow the GaSb band edge that is shifted in the superlattice due to quantum confinement. [Preview Abstract] |
Session J25: Focus Session: Simulation of Matter at Extreme Conditions: Shock Compression and Other High-Strain-Rate Phenomena
Sponsoring Units: DCOMP GSCCM DMPChair: Evan Reed, Stanford University
Room: 257A
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J25.00001: Single two-zone elastic-plastic shock waves in solids Vasily Zhakhovsky, Mikalai Budzevich, Nail Inogamov, Ivan Oleynik, Carter White A new regime of shock wave propagation in solids, corresponding to a single two-zone elastic-plastic shock-wave, was discovered using a novel moving window molecular dynamics technique. Both leading low-pressure elastic and trailing high-pressure plastic fronts move with the same speed and have a fixed separation that can extend to several microns. The material in the elastic zone is in a metastable state, having a pressure that substantially exceeds the critical shock strength characteristic of the onset of the well-known split-elastic-plastic, two-wave regime. The single two-zone elastic-plastic shock wave is a quite general phenomenon observed in our simulations for a broad class of crystalline materials, including aluminum, nickel, diamond, and Lennard-Jones crystals. It is the existence of the two-zone, elastic-plastic regime that allows for a consistent explanation of the anomalously high elastic wave amplitudes observed in recent experiments. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J25.00002: The Role of Shock Induced Defect Structure in Spall Failure Justin Wilkerson, K.T. Ramesh Spall failure is a complex multiscale, multirate process. On the macroscale the process involves a period of shock compression followed by dynamic tension set up by the stress wave interactions. During the shock compression, the material undergoes a myriad of shock stress magnitude and pulse duration dependent microscopic processes that may include dislocation multiplication, nucleation, trapping, pile-up, annihilation, recovery, cell evolution, as well as vacancy generation and clustering. In addition to shock hardening the material, this new shock induced defect structure seeds the material with potential void nucleation sites that may be activated during the proceeding period of dynamic tensile loading. Upon nucleation, the voids undergo dynamic growth to coalescence, constrained by inertia and viscoplastic resistance to deformation. A multiscale predictive framework is developed to analyze the role of these time-dependent processes in the experimentally observed spall strength dependence on initial microstructure, preheated temperature, tensile loading rate, pulse duration, and shock stress magnitude. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J25.00003: Physics of hyper-velocity impacts of micrometer and sub-micrometer sized particles Anna Mocker, Klaus Hornung, Zoltan Sternovsky, Keith Drake, Sascha Kempf, Eberhard Gr\"un, Fiege Fiege, Ralf Srama The phenomena occurring during hypervelocity microparticle impact are manyfold and the basis for the variety of applications. The processes of interest are particle fragmentation, impact ionization, impact flashes, and TOF mass spectrometry. To relate the parameters of individual particle impacts with the resulting measured values, a comprehensive program of impact experiments under well known experimental conditions for a wide variety of impact parameters is needed. For this, dust particles are accelerated to hypervelocity speeds with an electrostatic accelerator and the resulting plasma cloud is analyzed with suitable instruments. A detailed investigation using latest analyzing techniques like high-speed cameras and sensitive high-resolution spectrometers promises new instrument concepts and insights into short timescale high-pressure states of matter. Linear TOF mass spectroscopy provides the opportunity to measure the dynamic and thermodynamic properties of the impact ions. Together with a deeper theoretical understanding of the impact process and the subsequent expansion and other experimental approaches, this method can be a powerful tool to investigate the state of the hot compressed matter due to the related residual ion species. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J25.00004: Shock Ejecta Entrainment in Gas Michael Furnish In a continuation of earlier work, paired metal shock ejecta experiments, with and without helium fill, are used to measure shock ejecta motion in gas. The vacuum ejecta experiments use Asay foils and PDV to characterize ejecta properties, and the gas ejecta experiments use PDV. FFT analysis of the PDV signals gives a qualitative indication of the presence of such ejecta and of its motion; this can be ``calibrated'' via the Asay foil data. For modest amounts of ejecta (allowing enough light to reach the free surface and return to the probe to give a strong free surface velocity signal), the FFT amplitudes are roughly proportional to the ejecta areal density, where the proportionality constant depends on the shape and size distribution of the ejecta particles. We assume these are consistent for the two samples in each experiment pair, although limitations to this assumption (e.g. ejecta disruption by the gas) are discussed. An additional caveat is that PDV only measures the motion of ejecta with particle sizes exceeding the 1550 nm light wavelength. Experiments to assess optimal generators of shock ejecta detectable by PDV are also presented. Indium was found to work well in the pressure regime studied. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J25.00005: Why nano-projectiles work differently than macro-impactors- role of plastic flow E.M. Bringa, Christian Anders, Gerolf Ziegenhain, Giles Graham, J. Freddy Hansen, Nigel Park, Nick Teslich, Herbert Urbassek Hypervelocity impacts provide a way to take localized regions of a target to extreme pressure and temperature conditions. Resulting crater features can be challenging for hydrocode simulations and test the validity of constitutive models. We will present atomistic simulation data on crater formation due to hypervelocity impact of nanoprojectiles of up to 55 nm diameter and with targets containing up to ten billion atoms, and compare them to available experimental data on micron-, mm-, and cm-sized projectiles. We show that previous scaling laws do not hold in the nano-regime and outline the reasons: within our simulations we observe that the cratering mechanism changes, going from the smallest to the largest simulated scales, from an evaporative regime to a regime where melt and plastic flow dominate, as it is expected in larger micro-scale experiments. The importance of strain-rate dependence of strength and of dislocation production and motion under these extreme conditions will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J25.00006: Elastic-plastic response and polymorphic phase transition in shock-compressed diamond You Lin, Romain Perriot, Vasily Zhakhovsky, Xiang Gu, Carter White, Ivan Oleynik Shock wave propagation in diamond along the < 110 > crystallographic direction was simulated by molecular dynamics (MD) using the reactive empirical bond order (REBO) potential. In addition to known regimes of shock wave propagation, such as single elastic, split elastic-plastic, and single plastic shock wave, two new regimes were observed: 1) a split elastic-elastic shock wave associated with a polymorphic phase transition; 2) a single two-zone elastic-plastic shock wave with the leading elastic zone followed by the plastic zone. In the case of the split elastic-elastic shock wave, the onset of phase transition occurs at a pressure below the Hugoniot elastic limit (HEL); therefore, the solid-solid transformation takes place in the uniaxially compressed material in the absence of plasticity. Within the single two-zone elastic-plastic shock wave, the material in the elastic zone is in a metastable state at a pressure exceeding the HEL. The metastable elastic state decays into the plastic state within the plastic zone, both elastic and plastic fronts moving with the same speed. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J25.00007: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J25.00008: The Shock Response of Granular and Consolidated Ta2O5 Tracy Vogler, Seth Root, William Reinhart, Gregg Fenton, Dennis Grady The dynamic behavior of granular and nearly fully dense tantalum pentoxide (Ta$_{2}$O$_{5}$) has been explored through planar impact experiments. The experiments span from the compaction regime to the ultra-high pressure range utilizing gas guns and the Z machine. These data provide a valuable data set for the extension of the P-$\lambda$ model to the high-pressure regime. A thermodynamic approach due to Rice and Walsh is employed in the model to treat highly distended materials that can display anomalous compressibility. When the model is calibrated to the gas gun and Z data, we test its applicability against the data of Miller et al. from laser experiment on low-density aerogels. Even in these very different conditions, the model does a credible job of predicting the material behavior, suggesting that the approach may be useful as a general modeling tool for the high-pressure regime. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J25.00009: Optical emission from Gd$_{3}$Ga$_{5}$O$_{12}$ single crystals shock compressed from 40 to 300 GPa Xianming Zhou, Williams Nellis, Jiabo Li, Jun Li, Xun Liu, Xiuxia Cao, Wanguang Zhao, Qiang Wu, Tsutomu Mashimo The question of the shock pressure at which a strong material reaches thermal equilibrium is an important one that depends on strength and has never been answered for any strong material. To answer the range of shock pressures in which shock dissipation is dominated by entropy (damage and disorder) or by shock heating (T), we have performed time-resolved optical emission and transmission measurements on strong GGG single crystals under wide-range uniaxial compression from $\sim $40 GPa to $\sim $300 GPa with a sixteen-wavelength pyrometer. Temperatures T and emissivities e were derived from gray-body fits up to 300 GPa. These data: (i) determine shock pressures at which GGG reaches thermal equilibrium and melting, (ii) essentially confirm previously calculated shock temperatures of GGG, (iii) demonstrate the complete transition from heterogeneous shock heating at lower pressures (small e) to thermally-equilibrated bulk heating (large e) at higher pressure (never before been done in strong materials), and (iv) characterize GGG as an anvil for use in studying metallic fluid hydrogen. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J25.00010: Laser-driven focusing shock waves in a thin liquid layer David Veysset, Thomas Pezeril, Gagan Saini, Steven Kooi, Alex Maznev, Keith Nelson Direct real-time visualization of converging shock waves in a few micron thick liquid layer is demonstrated in an all-optical experiment. The set-up includes an axicon that focuses an intense picosecond excitation pulse into a ring-shaped pattern in a water layer. Optical excitation induces a shock wave that propagates in the plane of the sample and converges toward the center resulting in cylindrical focusing of the shock front. Streak-camera images with a quasi-cw probe beam yield real-time records of the entire shock propagation. Talbot imaging and interferometry with a femtosecond probe pulse are used to obtain full field images at variable delays. Shock pressure values calculated from the velocity of the shock front demonstrate the effect of shock focusing and agree with density profiles obtained by quantitative analysis of interferometric images. The configuration of the experiment provides ample access for optical diagnostics of the shocked material and can be combined with a wide range of spectroscopic probes. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J25.00011: From fingering to fracture in a complex fluid Baudouin Saintyves We present a novel experiment -- a specific Hele-Shaw cell with mobile sides which can be pulled at a prescribed velocity - with which both liquids and solids can be loaded with the same boundary conditions, beyond the small deformation regime. With such a system, one can examine quantitatively the response of a viscoelastic material when the loading rate is varied. In the case of viscous Newtonian liquids, an air bubble is shown to destabilize in a Saffman-Taylor manner, forming a finger which elongates in the direction in which the mobile sides are pulled. In contrast, in a Maxwell liquid, we observe a different kind of instability, which gives rise to more complex patterns. This instability leading to local stress concentrations, it is immediately followed by fracture. The displacement field is evaluated in each case by using tracers and image correlations. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J25.00012: Nonlinear Elasticity as a Guide for Exploring High Pressure/Shear Stability T.W. Wright Recent constitutive representation theorems for nonlinear anisotropic elasticity, Wright [2011], show the characteristics of elastic response for materials in any point group. All anisotropic representations consist of a sum of six terms, and each term consists of a scalar function of anisotropic invariants times a ``tensor generator,'' which has the same invariance under group transformations as the stress itself. Knowledge of these representations shows promise as a guide for exploring material stability under extreme loading conditions. Although much is known both experimentally and theoretically about material stability under high pressure, far less is known about the effect of large shear stress superimposed on high pressure. Stress has six independent components, so study of the effects of pressure alone leaves the other five dimensions unexplored. Rather than random DFT calculations in the five dimensional deviatoric space, the known structure of the six term representations suggests that systematic study of just one additional dimension at a time could be accomplished by following changes in just one additional term in the representation at a time. These ideas will be illustrated in the context of a program designed to explore the effect of shear on amorphization in B4C, a ceramic often used for ballistic protection. \textit{T.W. Wright, Bootstrap elasticity: From linear to nonlinear constitutive representations, accepted for publication, J. Elasticity}. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J25.00013: Multi million-to-Billion Atom Molecular Dynamics Simulations of Cavitation-Induced Damage on a Silica Slab Adarsh Shekhar, Ken-ichi Nomura, Rajiv Kalia, Aiichiro Nakano, Priya Vashishta Cavitation bubble collapse causes severe damage to materials. For example, cavitation erosion is a major threat to the safety of nuclear power plants. The cavitation bubbles may also be utilized for preventing stress corrosion cracking with water jet peening technology. We have performed multi million-to-billion atoms molecular dynamics simulations to investigate the shock-induced cavitation damage mechanism on an amorphous silica slab in water. The system consists of a 60nm thick silica slab immersed in water in an MD box of dimension 285 x 200 x 200 nm3. A nanobubble is created by removing water molecules within a sphere of radius 100 nm. To apply a planar shock, we assign a uniform particle velocity vp on the entire system towards a planar momentum mirror. We have performed the simulation with two kinds of bubbles, an empty bubble and a bubble filled with inert gas. The simulation results reveal nanojet formation during bubble collapse causing damage on the silica surface; however, the damage was significantly reduced in the case of the filled bubble. We will discuss the effect of the presence of inter gas inside the nanobubble on the pressure distribution, the extent of damage, and collapse behavior corresponding the shock front. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J25.00014: Akrology; the physics of the extreme behaviour of metals and energetics Neil Bourne Structures designed for extreme environments must be designed not only for the magnitude of the load that they will experience, but also the time for which that load acts upon them. At the core of the problem lies the loading impulse experienced by materials and the operating deformation mechanisms that are excited. Our experience of materials' physics, gathered by investigating response to mechanical loads, has suggested a series of descriptive constructs within which we build our picture of behaviour. At the highest loadings and the shortest loading times this perception is coloured by experience gathered from historical considerations. This paper suggests a framework by which to interpret data collected on the response of metals and explosives. It suggests that strength is a quantity that decays over time and that fundamentally approaches zero in the limit of infinite time. Controlling this decay is the business of engineering to design structures that will survive in the environments our times of interest define. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J25.00015: Equation of State of a Solid: Potts-Percolation Model Miron Kaufman, H.T. Diep We include stress and strain in a Potts-percolation model of a solid, see J. Phys.: Condens. Matter 20, 075222 (2008) and Phys Rev E80, 031116 (2009). Neighboring atoms are connected by a bond of Lennard-Jones energy. If the energy is larger than a threshold the bond is more likely to fail, while if the energy is lower than the threshold the bond is more likely to be alive. We compute the equation of state: stress as function of strain and temperature by using renormalization group and Monte Carlo simulations. The phase diagram and the equation of state are determined. When the Potts heat capacity is divergent the continuous transition is replaced by a weak first-order transition through the van der Waals loop mechanism. When the Potts transition is first order the stress exhibits a large discontinuity as function of the strain. [Preview Abstract] |
Session J26: Focus Session: What is Computational Physics?
Sponsoring Units: DCOMPChair: Norman Chonacky, Yale University
Room: 257B
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J26.00001: Aneesur Rahman Prize for Computational Physics Lecture: Photonic Crystals and Genetic Algorithms: Adventures of a Computational Physicist Invited Speaker: Kai Ming Ho I will review some of our work in the computation of photonic crystals, focusing on our discovery of the photonic band gap in diamond structures. I will also describe our conception of the cut-and-paste genetic algorithm in materials discovery structure search and discuss applications of the algorithm from early studies of atomic clusters geometries to more recent applications for structures of surfaces, interfaces, nanowires, and bulk crystals. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J26.00002: Computational Science as a Data and Compute Intensive Environment: How do we Prepare the Future for it? Invited Speaker: Nora Sabelli Much has been said about the data- and compute-driven transformations that science is undergoing. Much has also been said about the impacts this transformation will have in society, impacts that are not limited to science professionals. But not much been said about the transformations that will have to take place for this impact on society to be positive. This talk will ask if we know enough about what the multiple institutions that educate society's members should be doing to serve the new needs, and discuss some challenging questions that could inform future action. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J26.00003: New Approaches for Microscopic Hydrodynamics for the Study of Fluid-Structure Interactions Subject to Thermal Fluctuations Paul Atzberger Many problems in fluid mechanics involve the interaction of a hydrodynamic flow with an elastic structure. Recent advances in biology and engineering further motivate such studies at small length and time scales. At such scales traditional continuum mechanics descriptions must be augmented to take into account microscopic phenomena, such as spontaneous thermal fluctuations. This presents a variety of challenges both in formulating appropriate physical models and in computational simulation. In the context of fluid-structure interactions, additional challenges arise from the often subtle interplay between elastic mechanics, hydrodynamic coupling, and thermal fluctuations. In this talk, we present a set of new approaches which address central mathematical, physical, and computational issues for how to incorporate in the description of such fluid-structure interactions thermal fluctuations. We also address important numerical issues in the approximation of the resulting stochastic partial differential equations. We also discuss results for specific illustrative applications including studies of polymeric fluids, vesicles, gels, and lipid bilayer membranes. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J26.00004: Extending the Flux Operator with the Husimi Projection Douglas Mason, Mario Borunda, Eric Heller A common tool among physics in the transport community is the probability flux operator, but connecting this operator to measurement has not received much attention. We have extended the definition of flux by using coherent states, rendering it both measurable and infinitely more useful. For instance, we can use our extended definition to study closed systems and the classical dynamics of individual quantum states, and then connect these dynamics to resonant states interacting with an environment. This analytical technique, based on semi-classics, brings new tools to bear on wavefunction analysis and visualization [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J26.00005: Finite Element Modeling of Open Domain Quantum Scattering A.V. Ilyashenko, A.G. Gianfrancesco, C.R. Boucher, L.R. Ram-Mohan We study quantum scattering in the open domain in 2D using the finite element method. We solve the Schr\"odinger equation in a circular region using an arbitrary triangular mesh with a plane wave source and an arbitrary (finite) scattering potential. We employ perfectly matched layers (PML) around the region of interest to simplify the derivative Cauchy BCs to Dirichlet BCs. With PML, fewer resources are needed to account for the open domain without going to the asymptotic region as is usually done, while obtaining the ``near-field'' evanescent solutions when they are present. The scattering total cross-section, the differential cross-section, and the phase shifts can be determined by performing a partial wave analysis on the computed solution. Examples of this technique and results on multi-component, spin dependent scattering for spintronics will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J26.00006: Solution of a Schroedinger equation containing a general non local potential Joseph Power, George Rawitscher The non locality is given by an integral kernel K(r,r') that is general. We solve the corresponding Lippmann-Schwinger integral equation a) by expanding the wave function into a Fourier series (Galerkin method), and b) by a spectral method involving Chebyshev polynomials (CP). We apply them to the case of the much used Perey-Buck kernel [1], and find that, by using 50 sine functions, method a) requires an hour of computing time, while method b) using 50 CP's takes less than one second and is precise to 1:10$^{5}$. Previously the spectral method could be applied only to a kernel of rank 1, representing for example the knock-on exchange process [2], but with our new procedure we will be able to compare relatively easily the effect of several types of non localities [3]. \\[4pt] [1] F. G. Perey and B. Buck, Nucl. Phys. \textbf{A 32}, 353 (1962). \newline [2] G.H.,Rawitscher, S.Y. Kang, and I. Koltracht, J. Chem. Phys., \textbf{118},9149-9157, (2003). \newline [3] M. I. Jaghoub, M. F. Hassan and G. H. Rawitscher, Phys. Rev. \textbf{C} 84,034618(2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J26.00007: Accurate and fast numerical solution of Poisson's equation for arbitrary, space-filling convex Voronoi polyhedra: near-field corrections revisited Aftab Alam, Brian G. Wilson, Duane D. Johnson We present an accurate and rapid solution of Poisson's equation for space-filling, arbitrarily-shaped, convex Voronoi polyhedra (VP); the method is O(N), where N is the number of distinct VP representing the system. In effect, we resolve the longstanding problem of fast but accurate numerical solution of the near-field corrections (NFC), contributions to each VP potential from nearby VP -- typically involving multipole-type conditionally-convergent sums, or fast Fourier transforms. Our method avoids all ill-convergent sums, is simple, accurate, efficient, and works generally, i.e., for periodic solids, molecules, or systems with disorder or imperfections. We demonstrate the method's practicality by numerical calculations compared to exactly solvable models. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J26.00008: Sonoluminescence and Vacuum Radiation Justin Melunis, Eduardo Flores Sonoluminescence is the generation of light from sound. Our goal is to understand why a bubble trapped in water could generate light from sound. In our work we investigate the contribution of the dynamical Casimir effect to this phenomenon. In previous work researchers have approach this problem as a semi static Casimir effect and have not been able to show a significant contribution of the Casimir effect to sonoluminescence. In our approach, we treat the surface of the bubble as a highly reflecting surface, thus, the electric field of the zero-point modes at the surface is zero. Thus, when the bubble collapses the zero-point modes inside and outside are disturbed. Since the dynamics of zero-point mode fields obey Maxwell equations we can simulate their dynamics using programs like Mathematica. We study the radiation of the excited zero-point mode field. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J26.00009: Second harmonic generation in three-dimensional metamaterials based on homogeneous centrosymmetric spheres Jinying Xu The theory of second harmonic generation in three-dimensional metamaterials consisting of arbitrary distributions of spheres made of centrosymmetric materials is developed by means of the multiple scattering method. The electromagnetic field at both the fundamental frequency and second harmonic, as well as the scattering cross section, are calculated in a series of particular cases such as a single metallic sphere, two metallic spheres, chains of metallic spheres, and other periodic distributions of the metallic spheres. It is shown that the linear and nonlinear optical response of all ensembles of metallic spheres is strongly influenced by the excitation of surface plasmon-polariton resonances. The physical origin for such a phenomenon has also been analyzed. A new class of SHG devices made of such materials is anticipated. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J26.00010: Characterizing the frequency response curve of large rooms in the short and long time regimes Anna Klales, Suzanne Pittman, Matthew Barr, Mario Borunda, Eric Heller Room acoustics can be modeled by real Gaussian statistics, corresponding to randomized ray trajectories and characterized for instance by the reverberation time T60 (free field to decay by 60 decibels) which is independent of position or source point in a room. In his 1954 paper, Manfred Schroeder found universal statistical features of the steady state frequency response curve of large rooms, based upon the assumption of Gaussian probability distributions of the pressure. For example, he found the standard deviation from the mean level is 11 decibels for any concert hall, regardless of the shape of the room or its T60, within reasonable limits. Using semi-classical and numerical methods, we find non-universal (room dependent) corrections to Schroeder's universal results for the statistics of the frequency response curve. Along with corrections to the steady-state frequency response, we present the behavior of the frequency response curve for short to intermediate times. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J26.00011: ABSTRACT WITHDRAWN |
Session J27: Invited Session: International Energy Perspectives
Sponsoring Units: FIP GERAChair: William Barletta, Fermilab
Room: 258AB
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J27.00001: Energy production and use in China Invited Speaker: Mark Levine |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J27.00002: Economics of energy Alternatives Invited Speaker: Carmine Difiglio |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J27.00003: Future Trends ad Issues in Nuclear Power Invited Speaker: Richard Lester |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J27.00004: Science for Sustainable Energy: Recommendations of the 2010 BESAC Report Invited Speaker: Alexis Malozemoff In August 2010, DOE's Basic Energy Sciences Advisory Committee issued a major report on Science for Energy Technology. The report identified opportunities for science to help overcome roadblocks to progress in emerging clean energy technologies and thus to have much needed near-term impact on our energy infrastructure. The report covered diverse areas including solar electricity from photovoltaics, advanced nuclear energy, carbon sequestration, electrochemical energy storage, power grid technologies including power electronics and superconductors, solid state lighting, biofuels, building efficiency, fuel cells and wind power. In addition, mechanisms were suggested to facilitate progress, in particular, by strengthening the link between basic research and industry. The talk will review the highlights of this report. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J27.00005: Fuel from Sunlight: Prospects and Approaches Invited Speaker: Ellen Stechel |
Session J28: Focus Session: Frontiers in Computational Thermodynamics of Materials
Sponsoring Units: FIAP DCOMPChair: Stefano Curtarolo, Duke University
Room: 258C
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J28.00001: The role of Wang-Landau sampling in materials development Invited Speaker: David Landau An understanding of the thermodynamic behavior of materials as well as the prediction of the properties of ``materials by design'' often depends upon knowledge of the free energy of the system under study. Computer simulations offer a powerful tool for such investigations, but traditional methods often suffer from long time scales and metastable states due to the roughness of the free energy landscape. Wang-Landau sampling\footnote{F. Wang and D. P. Landau, Phys. Rev. Lett. 86, 2050 (2001); F. Wang and D. P. Landau, Phys. Rev. E 64, 05610 (2001).} is a powerful alternative to traditional Monte Carlo algorithms which can alleviate many such problems. We will review the Wang-Landau algorithm and discuss various implementations as well as possible application to materials development. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J28.00002: Dissolving the Periodic Table in Zirconia: Data Mining for Insight Bryce Meredig, Chris Wolverton A standard approach to understanding physical phenomena in materials is to manually search for clear trends or correlations in data (i.e., descriptors) governing those materials. Pettifor maps are a classic example of such empirically constructed models. But what if a data set is too large and/or chemically diverse to explain by straightforward human inspection? We present such a case by calculating from first principles the solubility thermodynamics of 70 dopant cations in cubic zirconia. This data set, spanning three charge states and most non-synthetic metals in the periodic table, defies simple ``manual'' explanation. Instead, we employ data mining algorithms and statistical methods to cluster the dopants into distinct classes, and then to build intuitive models for each class' thermodynamics in zirconia. Thus, we show that formal data mining techniques are a powerful means of elucidating meaningful property relationships in complex data sets. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J28.00003: High-throughput discovering half-metals from materials magnetic properties database Junkai Xue, Shidong Wang, Kesong Yang, Chaitanya Das Pemmaraju, Stefano Sanvitos, Stefano Curtarolo Half-metal materials have been found by investigating their magnetism data, such as band gap and spin polarization around the Fermi level. An efficient algorithm has been implemented to create a database to aid discovering materials magnetism properties. With this tool, a series of new half-metals has been obtained. We present this magnetism data as well as how the online database is used. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J28.00004: First Principles Modeling of the Temperature Dependent Ternary Phase Diagram for the Cu-Pd-S System William Huhn, Michael Widom A method for the prediction of temperature dependent phase diagrams using first principles calculations combined with thermodynamics principles will be discussed. Our method allows us to model the phase diagram without any empirical fitting parameters. Due to the importance of sulfidation when dealing with hydrogen separation using copper palladium membranes, we have chosen as our test case the Cu-Pd-S ternary phase diagram which has been experimentally determined. By applying thermodynamic principles and a simple solid solution model, temperature-dependent features of the Cu-Pd-S system can be explained, specifically solubility ranges for substitutions in select crystalline phases. We have also performed electronic density of states calculations to determine the physical origin of the favorability of select substitutions at T=0K. Work is currently underway to use this method to create phase diagrams where no known experimental results exist, specifically the P-Pd-S system. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J28.00005: Wang-Landau Without Binning G. Brown, D. Nicholson, Kh. Odbadrakh, M. Eisenbach, A. Rusanu Results are presented for Wang-Landau calculations on a Heisenberg model of BCC Fe that describe the density of states as function defined for all accessible energies instead of a function tabulated at discrete values of the energy. The density of states function described here is an analytic result valid near the ground state supplemented by a polynomial expansion. The probability density of Wang-Landau random walkers is sampled for a fixed density of states, and that probability density can be used to improve the estimated density of states. Methods for evaluating the convergence of the density of states are discussed along with the diffusion behavior of the random walkers. This work was performed at the Oak Ridge National Laboratory, which is managed by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725, and sponsored by the Laboratory Directed Research and Development Program (ORNL), by the Mathematical, Information, and Computational Sciences Division; Office of Advanced Scientific Computing Research (US DOE), and by the Division of Materials Sciences and Engineering; Office of Basic Energy Sciences (US DOE). Computer resources provided by Florida State University. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 1:15PM |
J28.00006: Thermodynamic Stability of Actinide-Dioxide Solid Solutions and Surface Interactions with Water Invited Speaker: Mark Asta Fluorite-structured actinide dioxides are the most common forms of fuel used in nuclear energy production worldwide. This talk will provide an overview of insights into the energetics of these compounds derived through the combination of density-functional-theory-based computational studies (including Hubbard-U corrections) and calorimetric measurements. The talk will focus on two main topics: the mixing energetics of cation solid solutions, and the energetics of water adsorption on the surfaces of these compounds. For the first topic, we present results for ThO$_{\rm 2}$ and UO$_{\rm 2}$ based solid solutions, highlighting the roles of elastic energy arising from cation size mismatch, electrostatic interactions, and charge-transfer reactions, in governing the sign and magnitude of the mixing energetics. For water adsorption, we contrast results for surface and adsorption energies on two fluorite-structured compounds, ThO$_{\rm 2}$ and CeO$_{\rm 2}$, that are relevant for understanding the behavior of water on actinide oxide surfaces more generally. Through a comparison between calorimetric measurements and computational results we assess the level of accuracy achieved in the computational modeling, and suggest areas where further experimental studies would be particularly useful. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J28.00007: Structure and stability of Al$_2$Fe and Al$_5$Fe$_2$ Michael Widom, Marek Mihalkovic We employ first principles total energy and phonon calculations to address the structure and stability of Al$_2$Fe and Al$_5$Fe$_2$. The observed structure of Al$_2$Fe, which is reported as stable in the assessed Al-Fe phase diagram, is distinguished by an unusually low triclinic symmetry. The initial crystallographic structure determination additionally featured an unusual hole large enough to accommodate an additional atom. Our calculations indicate the hole must be filled, but predict the triclinic structure is unstable relative to a simpler tetragonal structure based on the prototype Mo Si$_2$. This tetragonal structure is interesting because it is predicted to be nonmagnetic, electrically insulating and high density, while the triclinic structure is magnetic, metallic and low density. We reconcile this seeming contradiction by demonstrating a high vibrational entropy that explai ns why the triclinic structure is stable at high temperatures. Finally, we note that orthorhombic Al$_5$Fe$_2$ is also destabilized by the tetragonal structure but may be stabilized at high temperatures, again by vibrational entropy and partial occupancy associated with the diffusion of Al atoms along channels. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J28.00008: Equation of state of paramagnetic CrN from ab initio disordered local moments molecular dynamics Igor Abrikosov, Peter Steneteg, Bj\"orn Alling A first-principles method is suggested for the calculation of thermodynamic properties of magnetic materials in their high temperature paramagnetic phase [1]. It is based on ab-initio molecular dynamics and simultaneous redistributions of the disordered but finite local magnetic moments. We apply this disordered local moments molecular dynamics (DLM-MD) method to the case of CrN and simulate its equation of state. In particular the debated [F. Rivadulla et al., Nat Mater 8, 974 (2009); B. Alling et al., Nat Mater 9, 283 (2010)] bulk modulus is calculated in the paramagnetic cubic phase and is shown to be very similar to that of the antiferromagnetic orthorhombic CrN phase for all considered temperatures. \\[4pt] [1] P. Steneteg, B. Alling, I. A. Abrikosov, arXiv:1110.1331v1 [cond-mat.mtrl-sci] [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J28.00009: Lattice dynamics of anharmonic solids from first principles Olle Hellman, Igor Abrikosov, Sergei Simak An accurate and easily extendable method to deal with lattice dynamics of solids is offered. It is based on first-principles molecular dynamics simulations and provides a consistent way to extract the best possible harmonic--or higher order--potential energy surface at finite temperatures. It is designed to work even for strongly anharmonic systems where the traditional quasiharmonic approximation fails. The accuracy and convergence of the method are controlled in a straightforward way. Excellent agreement of the calculated phonon dispersion relations at finite temperature with experimental results for bcc Li and bcc Zr is demonstrated. In addition to that the bcc-hcp phase diagram for Zr is calculated with high accuracy. arXiv:1103.5590v3 [cond-mat.mtrl-sci] [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J28.00010: Unified cluster expansion method applied to the configurational thermodynamics of cubic Ti$_{1-x}$Al$_{x}$N Bj\"orn Alling, Andrei Ruban, Ayat Karimi, Lars Hultman, Igor Abrikosov We study the thermodynamics of cubic Ti$_{1-x}$Al$_{x}$N using a unified cluster expansion approach for the alloy problem [1]. The purely configurational part of the alloy Hamiltonian is expanded in terms of concentration and volume-dependent effective cluster interactions. By separate expansions of the chemical fixed lattice, and local lattice relaxation terms of the ordering energies, we demonstrate how the screened generalized perturbation method can be fruitfully combined with a concentration-dependent Connolly-Williams cluster expansion method, getting the best out of both two schemes that are traditionally used separately. Utilizing the obtained Hamiltonian in Monte Carlo simulations we access the free energy of Ti$_{1-x}$Al$_x$N alloys and construct the isostructural phase diagram. The results show striking similarities with the previously obtained mean-field results: The metastable c-TiAlN is subject to coherent spinodal decomposition over a large part of the concentration range, e.g., from x 0.33 at 2000 K. \\[4pt] [1] B. Alling, A. V. Ruban, A. Karimi, L. Hultman, and I. A. Abrikosov, PHYSICAL REVIEW B 83, 104203 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J28.00011: Atomistic modeling of thermodynamic equilibrium of plutonium Tongsik Lee, Steve Valone, Mike Baskes, Shao-Ping Chen, Andrew Lawson Plutonium metal has complex thermodynamic properties. Among its six allotropes at ambient pressure, the fcc delta-phase exhibits a wide range of anomalous behavior: extraordinarily high elastic anisotropy, largest atomic volume despite the close-packed structure, negative thermal expansion, strong elastic softening at elevated temperature, and extreme sensitivity to dilute alloying. An accurate description of these thermodynamic properties goes far beyond the current capability of first-principle calculations. An elaborate modeling strategy at the atomic level is hence an urgent need. We propose a novel atomistic scheme to model elemental plutonium, in particular, to reproduce the anomalous characteristics of the delta-phase. A modified embedded atom method potential is fitted to two energy-volume curves that represent the distinct electronic states of plutonium in order to embody the mechanism of the two-state model of Weiss, in line with the insight originally proposed by Lawson et al. [Philos. Mag. 86, 2713 (2006)]. By the use of various techniques in Monte Carlo simulations, we are able to provide a unified perspective of diverse phenomenological aspects among thermal expansion, elasticity, and phase stability. [Preview Abstract] |
Session J29: Focus Session: Quantum Information for Quantum Foundations - Informational Principles and Fundamental Structures
Sponsoring Units: GQIChair: Giulio Chiribella, Perimeter Institute
Room: 259A
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J29.00001: Information Causality as a physical principle Invited Speaker: Valerio Scarani It is known that the physical principle of ``no-signaling'' alone does not single out quantum correlations, and that the post-quantum no-signaling correlations share many of the features that are supposed to define quantum physics (intrinsic randomness, no-cloning, violation of Bell's inequalities...). This talk focuses on the principle of Information Causality (IC), which generalizes no-signaling and has been proved to come close to singling out quantum correlations. I shall review the successes of IC and also the difficulties that the subsequent research is meeting. In particular, I shall emphasize how a generalization of the initial bipartite scenario to a multipartite one is a most urgent necessary step. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J29.00002: Tensor network states for quantum foundations Jacob Biamonte Penrose developed a graphical language to reason about networks of connected tensors and applied these techniques to quantum theory. Here we present the development of a framework and tool set based on Penrose tensor networks that enables one to address certain questions of a foundational nature in quantum theory. A quantum tensor theory is one in which a fixed collection of tensors with clearly defined composition laws defines a physical theory. We show that each element of a universal collection of such tensors gives rise to a physical operation, allowed by the rules of quantum mechanics. Although each of these (possibly) atemporal operations is indeed physical, certain sequences of them could represent processes that violate the rules of quantum theory. The question is to determine when this is the case and we arrive at some perplexing conclusions. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J29.00003: On the Feynman Problem Giacomo D'Ariano Foundations of Quantum Field Theory can be connected to foundations of Quantum Theory if we can derive the former in terms of the latter with two additional postulates of locality and topological homogeneity of interactions between quantum systems, in the hypothesis that a quantum field is ultimately made of a numerable set of quantum systems that are unitarily interacting. But, in order to do that we need to be able to simulate quantumm field by a quantum computer. In the paper ``Simulating Physics with Computers'' (Int. J. Th. Phys, 21 467 (1982)) Richard Feynman raised the problem whether it is possible to simulate Fermi fields by a quantum computer---in short if we can ``qubit-ize'' a Fermi field, keeping the field interaction local on qubits. In this talk I will show how this problem is solved for any space-dimension, upon introducing a new general Jordan-Wigner map between field and qubits, and by adding witnessing auxiliary qubits. I will also derive the unique vacuum for the Fermi fields and for the auxiliary fields. The solution of the Feynman problem allows us to simulate quantum fields by a quantum cellular automata, also providing a kind of Planck-scale version of quantum field theory. Computer simulations will be projected at the end of the talk [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J29.00004: Origin of Feynman's Rules of Quantum Theory and Complementarity Philip Goyal Complex numbers are an intrinsic part of the mathematical formalism of quantum theory, and are perhaps its most mysterious feature. If one considers how to combine experimental arrangements to generate new experimental arrangements, a set of five simple symmetries involving two binary operators naturally arises. Recently, I have shown that these symmetries, together with the probabilistic nature of measurement outcomes and the principle of complementarity (formalized in a novel way as the Principle of Information Duality) naturally lead to Feynman's rules of quantum theory (including their complex nature) [1]. In this paper, I present recent development of this work showing that the assumption of complementarity can be dropped, and, instead, a fundamental theorem from number theory---Frobenius' theorem---can be applied to show that the only formalism compatible with the experimental symmetries are real and complex quantum theory. I shall conclude with a discussion of physical principles which can be used to rule out real quantum theory. \\[4pt] [1] Origin of Complex Quantum Amplitudes and Feynman's Rules, P. Goyal, K. Knuth, J. Skilling, Phys. Rev. A 81, 022109 (2010) [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J29.00005: Partially Ordered Sets of Quantum Measurements and the Dirac Equation Kevin H. Knuth Events can be ordered according to whether one event influences another. This results in a partially ordered set (poset) of events often referred to as a causal set. In this framework, an observer can be represented by a chain of events. Quantification of events and pairs of events, referred to as intervals, can be performed by projecting them onto an observer chain, or even a pair of observer chains, which in specific situations leads to a Minkowski metric replete with Lorentz transformations (Bahreyni \& Knuth, 2011. APS B21.00007). In this work, we unify this picture with the Process Calculus, which coincides with the Feynman rules of quantum mechanics (Goyal, Knuth, Skilling, 2010, arXiv:0907.0909; Goyal \& Knuth, Symmetry 2011, 3(2), 171), by considering quantum measurements to be events. This is performed by quantifying pairs of events, which represent transitions, with a pair of numbers, or a quantum amplitude. In the 1+1D case this results in the Feynman checkerboard model of the Dirac equation (Feynman \& Hibbs, 1965). We further demonstrate that in the case of 3+1 dimensions, we recover Bialnycki-Birula's (1994, Phys. Rev. D, 49(12), 6920) body-centered cubic cellular automata model of the Dirac equation studied more recently by Earle (2011, arXiv:1102.1200v1). [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J29.00006: An Observer-Based Foundation of Geometry Newshaw Bahreyni, Kevin H. Knuth The fact that some events influence other events enables one to define a partially ordered set (poset) of events, often referred to as a causal set. A chain of events, called observer chain, can be quantified by labeling its events numerically. Other events in a poset may be quantified with respect to an observer chain/chains by projecting them onto the chain, resulting in a pair of numbers. Similarly, pairs of events, called intervals, can be quantified with four numbers. Under certain conditions, this leads to the Minkowski metric, Lorentz transformations and the mathematics of special relativity (Bahreyni {\&} Knuth, APS March Meeting 2011). We exploit the same techniques to demonstrate that geometric concepts can be \textit{derived} from order-theoretic concepts. We show how chains in a poset can be used to define points and line segments. Subsequent quantification results in the Pythagorean Theorem and the inner product as well as other geometric concepts and measures. Thus the geometry of space, which is assumed to be fundamental, emerges as a result of quantifying a partially ordered set. More importantly, this proposed foundation of geometry is entirely observer-based, which may provide a natural way toward integration with quantum mechanics. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J29.00007: Quantum state space as a maximal consistent set Gelo Noel Tabia Measurement statistics in quantum theory are obtained from the Born rule and the uniqueness of the probability measure it assigns through quantum states is guaranteed by Gleason's theorem. Thus, a possible systematic way of exploring the geometry of quantum state space expresses quantum states in terms of outcome probabilities of a symmetric informationally complete measurement. This specific choice for representing quantum states is motivated by how the associated probability space provides a natural venue for characterizing the set of quantum states as a geometric construct called a maximal consistent set. We define the conditions for consistency and maximality of a set, provide some examples of maximal consistent sets and attempt to deduce the steps for building up a maximal consistent set of probability distributions equivalent to Hilbert space. In particular, we demonstrate how the reconstruction procedure works for qutrits and observe how it reveals an elegant underlying symmetry among five SIC-POVMs and a complete set of mutually unbiased bases, known in finite affine geometry as the Hesse configuration. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J29.00008: SIC-POVMs and Lie Algebras Hoan Dang, Marcus Appleby, Christopher Fuchs A symmetric informationally complete positive operator valued measure (SIC-POVM) is usually thought of as a highly symmetric structure in quantum state space. However Appleby, Flammia and Fuchs (J. Math. Phys. \textbf{52}, 022202, 2011) have shown that the existence of a SIC-POVM in dimension $d$ is equivalent to a proposition concerning the Lie Algebra $\mathrm{gl}(d,C)$. Related to this they show that there is, associated to each SIC-POVM, a rich and intricate geometric structure in the adjoint representation space of $\mathrm{gl}(d,C)$. In this talk we present a deeper exploration of this structure. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J29.00009: The Galois Group of Symmetric Measurements Marcus Appleby, Hulya Yadsan-Appleby, Gerhard Zauner The problem of proving (or disproving) the existence of symmetric informationally complete positive operator valued measures (SICs) has been the focus of much effort in the quantum information community during the last 12 years. In this talk we describe the Galois invariances of Weyl-Heisenberg covariant SICs (the class which has been most intensively studied). It is a striking fact that the published exact solutions (in dimensions 2--16, 19, 24, 35 and 48) are all expressible in terms of radicals, implying that the associated Galois groups must be solvable. Building on the work of Scott and Grassl (\emph{J. Math. Phys.}\ \textbf{51}, 042203 (2010)) we investigate the Galois group in more detail. We show that there is an intriguing interplay between the Galois and Clifford group symmetries. We also show that there are a number of interesting regularities in the Galois group structure for the cases we have examined. We conclude with some speculations about the bearing this may have on the SIC existence problem. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J29.00010: Quantum Nonlocal Boxes Exhibit Stronger Distillability Jibran Rashid, Peter Hoyer Given the apparent limited distillability of nonlocal boxes (NLBs), we initiate a study of the distillation of correlations for NLBs that output quantum states rather than classical bits. We propose a new non-adaptive protocol for nonlocality distillation which asymptotically distills correlated quantum nonlocal boxes to the value 3.098, whereas in contrast, the optimal non-adaptive parity protocol for classical NLBs asymptotically distills to the value 3.0. The protocol is also proven to be an optimal non-adaptive protocol for 1, 2 and 3 copies by formulating nonlocality distillation as a semi-definite programming optimization problem. Even if we restrict out attention to non-adaptive protocols, qNLBs offer improved distillation over NLBs. A generalization of our SDP approach that allows for adaptive protocols may reveal a similar improvement in general. This may imply distillability for nonlocal correlations that are currently not known to be distillable. As a consequence of the work on nonlocality distillation we provide numerical evidence that correlations with non-trivial marginals which are not known to satisfy the macroscopic locality principle may be distillable even when corresponding correlations with trivial marginals are not. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J29.00011: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J29.00012: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J29.00013: The necessity of entanglement and the equivalency of Bell's theorem with the second law of thermodynamics Ian Durham We demonstrate that both Wigner's form of Bell's inequalities as well as a form of the second law of thermodynamics, as manifest in Carath\'{e}odory's principle, can be derived from the same assumptions. The results suggest that Bell's theorem is merely a well-disguised statement of the second law. It also suggests that entanglement is necessary for quantum theory to be in full accord with the second law. [Preview Abstract] |
Session J30: Open Quantum Systems and Decoherence
Sponsoring Units: GQIChair: Bill Coish, McGill University
Room: 259B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J30.00001: Heisenberg scaling of time-limited quantum metrology with realistic decoherence Maxime Hardy, William A. Coish The prospect of using entanglement to improve various metrology tasks is one of the most promising avenues for a near-term real-world benefit from genuine quantum phenomena [1]. However, in the standard scenario, history-independent Markovian dephasing removes the quantum advantage [2]. We revisit the problem of quantum metrology using the model of trapped ions subject to non-Markovian phase damping decoherence caused by Gaussian noise with finite correlation length and time (a slight generalization of the model used in Ref. [3]). Assuming a fixed available measurement time shorter than the noise correlation time (the non-Markovian limit) and a noise source that is local in space, we recover Heisenberg scaling ($\sim$1/N). This allows one to measure an ``instantaneous'' frequency to a higher precision than the time-averaged noise amplitude and moreover to a higher precision than classically allowed. Interestingly, for this protocol we show that the optimal number of measurements to be performed within the measurement time is three. \\[4pt] [1] V. Giovannetti, S. Lloyd, and L. Maccone Nature Photonics 5, 222 (2011) \newline [2] S. F. Huelga et al. Phys. Rev. Lett. 79, 3865 (1997 \newline [3] T. Monz et al. Phys. Rev. Lett. 106, 130506 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J30.00002: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J30.00003: Entanglement and diffusive behavior of a driven Floquet system coupled to noise Benjamin M. Fregoso, Justin Wilson, Victor Galitski Attempting to improve the persistence of quantum effects in systems interacting with a bath, we consider a periodically-driven quantum system and focus on its quantum dynamics in the Floquet space. As a toy model, we first consider a harmonic oscillator interacting with a bath and investigate the diffusive behavior in the statistics of observables in the presence of a periodic driving force and analyze the system within the Floquet theory. We then extend this analysis to look at the entanglement of two oscillators interacting with a bath, and investigate whether a periodic driving force can improve the persistence of entanglement between the two systems. We discuss possible experimental realization of our exactly-solvable model with trapped ions. We discuss a Lie-algebraic contraction scheme to map the oscillator properties to spin the systems in an environment. Extensions of our theory to more complicated driven quantum systems will also be discussed. This research is supported by JQI-PFC. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J30.00004: A Lie-algebraic approach to decoherence in a quantum spin system Justin Wilson, Benjamin Fregoso, Victor Galitski Quantum spin systems interacting with environment lose quantum coherence and information due to the debilitating effects of the noise. Quantitative description of decoherence even in the simplest spin-1/2 systems is technically complicated and the conventional approach (e.g., calculating $T_1$ and $T_2$) involves a number of strong approximations. In order to go beyond this approximation scheme and identify its regime of validity, we use the Lie-algebraic contraction, which reduces the su(2) algebra into the solvable oscillator algebra to connect the two dynamical systems. We take advantage of the general exact solution to the latter and build a regular expansion in the contraction parameter to describe dissipative quantum dynamics of the spin. This procedure allows for a controlled non-perturbative treatment of the non-Markovian effects. New interesting effects include deviations from pure diffusion due to bath spectrum and non-Markovian effects in both systems. Our approach could shed light on the spin decoherance problem and noise characterization in experiments relevant for quantum computing. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J30.00005: Quantum simulator of an open quantum system using superconducting qubits: exciton transport in photosynthetic complexes Sarah Mostame, Patrick Rebentrost, Alexander Eisfeld, Andrew J. Kerman, Dimitris I. Tsomokos, Alan Aspuru-Guzik In the initial stage of photosynthesis, light-harvested energy is transferred with remarkably high efficiency to a reaction center, with the vibrational environment assisting the transport mechanism. It is of great interest to mimic this process with present-day technologies. Here we propose an analog quantum simulator of open system dynamics, where noise engineering of the environment has a central role. In particular, we propose the use of superconducting qubits for the simulation of exciton transport in the Fenna-Matthew-Olson protein, a prototypical photosynthetic complex. Our method allows for a single-molecule implementation and the investigation of energy transfer pathways as well as non-Markovian and spatiotemporal noise-correlation effects. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J30.00006: Quantum signatures of chaos in quantum tomography Vaibhav Madhok, Carlos Riofrio, Ivan Deutsch We study the connection between quantum chaos and information gain in the time series of a measurement record used for quantum tomography. The record is obtained as a sequence of expectation values of a Hermitian operator evolving under repeated application of the Floquet operator of the quantum kicked top on a large ensemble of identical systems. We find an increase in information gain and hence higher fidelities in the process when the Floquet maps employed increase in chaoticity. We make predictions for the information gain using random matrix theory in the fully chaotic regime and show a remarkable agreement between the two. Finally we discuss how this approach can be used in general as a benchmark for information gain in an experimental implementation based on nonlinear dynamics of atomic spins measured weakly by a the Faraday rotation of a laser probe. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J30.00007: Non-Markovian behavior of small and large complex quantum systems Carlos Pineda, Marko Znidaric, Ignacio Garc\'Ia-Mata The channel induced by a complex system interacting strongly with a qubit is calculated exactly under the assumption of randomness of its eigenvectors. The resulting channel is represented as an isotropic time dependent oscillation of the Bloch ball, leading to non-Markovian behavior, even in the limit of infinite environments. Two contributions are identified: one due to the density of states and the other due to correlations in the spectrum. Prototype examples, one for chaotic and the other for regular dynamics are explored. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J30.00008: Quantum correlation of an optically controlled open quantum system Ching-Kit Chan, L.J. Sham A precise time-dependent optical control of an open quantum system relies on an accurate account of the quantum interference among the system, the photon control and the dissipative environment. In the spirit of the Keldysh non-equilibrium Green's function approach, we develop a diagrammatic technique to precisely calculate this quantum correlation for a fast multimode coherent photon control against slow relaxation, valid for both Markovian and non-Markovian systems. We demonstrate how this novel formalism can lead to a better accuracy than existing approximations of the master equation. We also describe extensions to cases with controls by photon state other than the coherent Glauber state. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J30.00009: Effects of counter rotating terms on quantum discord and entanglement between two atoms in a dissipative cavity Ferdi Altintas, Resul Eryigit We have investigated the role of counter-rotating interaction terms in the Rabi Hamiltonian on the creation and dynamics of entanglement and quantum discord between two identical atoms interacting with a lossy single mode cavity field for a system initially in different product states. For some initial states, the counter-rotating terms are found to lead to steady states in the long time limit which can have high quantum discord, but have no entanglement. The effect of cavity decay rate on these steady states quantum discord has been also investigated, surprisingly the increase in cavity decay rate is found to enhance the steady state quantum discord. Moreover, for certain initial states the effect of counter rotating terms are found to be detrimental to the quantum correlations when counter rotating terms in interaction Hamiltonian are taken into account. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J30.00010: Pointer state engineering in open quantum systems Lorenza Viola, Kaveh Khodjasteh, Viatcheslav Dobrovitski Pointer states have a long history in fundamental quantum theory and a practical relevance as long-lasting high-fidelity states in open quantum systems. For generic dissipative dynamics, however, pointer states need not exist or, when they do, need not coincide with states of interest. I will show how open-loop control procedures may be used to engineer dissipation in such a way that any desired initial pure state can be guaranteed to survive with high minimum fidelity over time and retrieved on demand. Quantitative fidelity bounds and constructive control protocols will be presented, and validated through simulation in paradigmatic single- and two- qubit dissipative scenarios. I will also argued that the state selectivity observed in recent dynamical decoupling experiments can be naturally understood within the pointer state engineering framework. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J30.00011: Crossover from adiabatic to antiadiabatic quantum pumping with dissipation Giuseppe Ernesto Santoro, Franco Pellegrini, Carlotta Negri, Fabio Pistolesi, Nicola Manini, Erio Tosatti Quantum pumping, in its different forms, is attracting attention from different fields, from fundamental quantum mechanics, to nanotechnology, to superconductivity. We investigate the crossover of quantum pumping from the adiabatic to the anti-adiabatic regime in the presence of dissipation, and find general and explicit analytical expressions for the pumped current in a minimal model describing a system with the topology of a ring forced by a periodic modulation of frequency $\omega$. The solution allows following in a transparent way the evolution of pumped DC current from much smaller to much larger $\omega$ values than the other relevant energy scale, the energy splitting introduced by the modulation. We find and characterize a temperature-dependent optimal value of the frequency for which the pumped current is maximal. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J30.00012: Open Quantum Walks and Dissipative Quantum Computing Francesco Petruccione Open Quantum Walks (OQWs) have been recently introduced as quantum Markov chains on graphs [S. Attal, F. Petruccione, C. Sabot, and I. Sinayskiy, E-print: http://hal.archives-ouvertes.fr/hal-00581553/fr/]. The formulation of the OQWs is exclusively based upon the non-unitary dynamics induced by the environment. It will be shown that OQWs are a very useful tool for the formulation of dissipative quantum computing and quantum state preparation. In particular, it will be shown how to implement single qubit gates and the CNOT gate as OQWs on fully connected graphs. Also, OQWS make possible the dissipative quantum state preparation of arbitrary single qubit states and of all two-qubit Bell states. Finally, it will be shown how to reformulate efficiently a discrete time version of dissipative quantum computing in the language of OQWs. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J30.00013: Non-Markovian dynamics of a solid-state charge qubit measured by a quantum point contact Chung-Chin Jian, Po-Wen Chen, Hsi-Sheng Goan We study a system of a charge qubit consisting of an electron in two coupled quantum dots (CQD's) detected by a quantum point contact (QPC). We derive perturbatively the non-Markovian quantum master equation for the CQD's system and calculate the transport current through the QPC (considered as a reservoir) to second order in the system-reservoir interaction. The non-Markovianity of the whole system comes from the energy-dependent tunneling amplitudes and energy-dependent densities of states of the QPC, which are modeled as a spectral density with a Lorentzian shape. In the non-Markovian case, the decay coefficients in the derived master equation and transport current are time-dependent and involve the real and imaginary parts of the contributions from the QPC reservoir correlation functions. In the wide-band limit (WBL), the various Markovian master equations in different parameter regimes are recovered, and the contributions of the imaginary parts are found to vanish. However, in the non-Markovian regime, the contributions of the imaginary parts significantly influence the dynamics of the charge qubit and thus the transport current. Especially, the non-Markovian transient currents through QPC differ significantly from the WBL Markovian counterparts and thus may serve as a witness for the non-Markovian behavior in the QPC-qubit system. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J30.00014: Hierarchical equations of motion: A fundamental theory for quantum open systems YiJing Yan As a powerful alternative to the influence functional path integral formalism, HEOM has been exploited in the study of various systems. In this talk, I will present some recent advancement on the HEOM-based nonlinear/nonequilibrium response theory and efficient implementation methods. Numerical demonstrations include coherent two-dimensional spectroscopy signals of light-harvesting model systems, and transport current noise spectrums through Anderson model quantum dots, operated in high-order co-tunneling regime. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J30.00015: Fidelity of the ground state in adiabatic quantum computation Qiang Deng, Dmitri Averin, Mohammad Amin, Peter Smith The energy gap between the ground and excited states of a qubit register performing an adiabatic quantum computation (AQC) algorithm is expected to provide additional stability against decoherence by environmental noise. However, the precise quantitative magnitude of this effect is still an open question. In this work, we show that fidelity of the ground state provides the ultimate quantitative measure of the AQC stability against decoherence. Even if the qubit register is not driven out of the ground state by the time evolution of the algorithm, the ground state is deformed by the qubit-environment interaction. The extent of this deformation can be characterized by the same noise correlators that determine the relaxation rates in the gate-model QC. We derive finite-temperature expression for the ground-state fidelity and calculate it numerically for the 16-qubit instances of adiabatic quantum optimization. [Preview Abstract] |
Session J31: Focus Session: Topological Insulators: Synthesis and Characterization - Transport
Sponsoring Units: DMPChair: Nitin Samarth, Pennsylvania State University
Room: 260
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J31.00001: Transport in Topological Insulator Thin Films Valla Fatemi, Hadar Steinberg, Jean-Baptiste Laloe, Ferhat Katmis, Lucas Orona, Jagadeesh Moodera, Pablo Jarillo-Herrero We report on electronic transport measurements on Bi2Se3 thin-film devices and show that an ambipolar modulation can be achieved via the electric field effect by using a top-gate with a high-k dielectric insulator. By analyzing the evolution of the weak anti-localization magnetoconductance behavior with respect to gate voltage and temperature, we find that we are able to modulate the effective number of channels, demonstrating that the coherent coupling between the surface and the bulk is tunable. Moreover, we investigate the formation and behavior of tunable p-n junctions on thin-film devices with multiple local top-gates. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J31.00002: Bi$_2$Se$_3$ and Bismuth: A comparison of transport in topological and trivial materials Anthony Richardella, Michel van Maasakkers, Duming Zhang, Joon Sue Lee, Nitin Samarth The simple surface state structure of Bi2Se3 has made it one of the most promising materials for harnessing transport through topologically protected surface states. Identifying unambiguous signatures of transport through these states is difficult however due to residual bulk conductivity and the formation of 2D electron gases near the surface due to band bending. As in bulk bismuth these non-topological states are subject to strong-spin orbit coupling. Very thin films of bismuth are predicted to be 2D topological insulators and thus are interesting themselves for a topological to non-topological transition as a function of thickness. Analysis of signatures such as weak anti-localization and linear magnetoresistance are compared between high quality MBE grown films of these materials to determine what can and cannot be ascribed to transport through protected surface states. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J31.00003: Low-temperature magnetoresistance in electrically gated Bi$_2$Se$_3$ thin films J.S. Lee, A.M. DaSilva, A. Richardella, D.M. Zhang, J.K. Jain, Nitin Samarth Although transport in samples of 3D topological insulators often has a large contribution from bulk conduction, the surface transport can be studied by electrical gating of topological insulator thin films. We have measured thin films of Bi$_2$Se$_3$ grown by molecular beam epitaxy and subsequently photolithographically patterned with a high$-\kappa$ gate dielectric (HfO$_2$) using atomic layer deposition. Gate voltage-dependent Hall effect and magnetoresistance were measured over a temperature range $0.5 \rm{K} \leq T \leq 20$ K in both perpendicular and parallel magnetic fields up to 6 T. Applying a negative gate voltage forms a depletion layer at the top of the thin film and decouples the surface from bulk carriers. We use the weak antilocalization effect at low magnetic field to study the transport contribution of surface and bulk channels. We also discuss scattering mechanisms contributing to surface and bulk conduction. Supported by ONR and NSF-MRSEC. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J31.00004: Thickness Independent Surface Transport of Bi$_{2}$Se$_{3}$ on Al$_{2}$O$_{3}$(0001) Substrates Namrata Bansal, Yong Seung Kim, Matthew Brahlek, Eliav Edrey, Seongshik Oh The key requirement for exploiting the newly emerging three-dimensional (3D) topological insulators (TI) as a novel platform for coherent spin-polarized electronics is TI thin films with dominant surface transport properties. So far, while researchers have been able to observe the existence of surface states locally \textit{in situ}, verification over a wide thickness range outside the growth chamber has not yet been reported. Here, we report large signature of surface transport in TI Bi$_{2}$Se$_{3}$ thin films. The Bi$_{2}$Se$_{3}$ films used for this study were grown on c-axis Al$_{2}$O$_{3}$ substrates with MBE. Hall-effect measurements in the standard Van der Pauw geometry provided clear evidence of two conducting channels for 4QL-2750QL thick samples, with the transport properties for one of the channel being thickness independent and the other varying with thickness. This thickness independent carrier density of $\sim $1.5 $\times $ 10$^{13}$ cm$^{-2}$ has been observed over the entire thickness range down to 2 QL, clearly suggesting that this is due to surface states. Furthermore, another surface transport property directly related to the topological protection mechanism, the weak-antilocalization (WAL) effect, exhibited similar thickness- and bulk-independent characteristics. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J31.00005: Electronic transport in MBE-grown $Bi_{2}Se_{3}$ topological insulator thin film field effect devices Tai-Lung Wu, Jifa Tian, Helin Cao, Yi Xuan, Xinyu Liu, Jack Furdyna, Yong P. Chen Topological insulators (TI), such as $Bi_{2}Se_{3}$ and $Bi_{2}Te_{3}$, have attracted a lot of attention due to their exotic electronic properties. $Bi_{2}Se_{3}$ TI films grown by molecular beam epitaxy (MBE) are promising for studying the nature of topologically protected surface states due to their large size, high quality, the capability to tune the thickness and interface with various semiconductor substrates. In this study, thin films of $Bi_{2}Se_{3}$ have been grown on $GaAs$ (001) semi-insulating substrates in a III-V/II-VI dual chamber MBE system. To study the electronic properties, micrometer scale Hall-bar devices with high-$k$ dielectric ($Al_{2}O_{3}$) top gates have been fabricated. Systematical measurements of temperature dependent and electrical field modulated magnetro-transport are performed to exam the conduction contributed by the surface states. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J31.00006: Weak anti-localization behavior in Bi2(SeTe)3 grown on GaAs (001) substrate Joseph Hagmann, Jacek Furdyna, Malgorzata Dobrowolska, Xinyu Liu A series of Bi2(SeTe)3 thin films were grown by molecular beam epitaxy on GaAs(001) substrates in order to obtain ternary Te-Bi-Se-Bi-Te alloys with large bulk resistivities. X-Ray diffraction data reveals many reflections from only the {\{}003{\}}-type lattice planes, indicative of highly directed c-axis growth of these films despite the very different crystal symmetries of the film and the substrate along the film growth direction. The X-ray data reveal that a wide spectrum of mixed Bi2(SeTe)3 alloys can be obtained by this method [1]. Our studies show that the alloy films are highly uniform, and the crystallinity is comparable to that of films grown on substrates with hexagonal surface structure. In this work, we have carried out comprehensive magneto-transport measurements on a series of Bi2(SeTe)3 thin films. We find that the conductivities of the films are strongly affected by alloy composition, and that insulating samples can be obtained at Se concentrations of $\sim $30{\%}. Moreover, we observe weak anti-localization behavior in all samples, which is also composition-dependent. In order to understand diffusive transport in these topological insulator alloys, both disorder and electron-electron interaction effects are considered in our analysis. [1] X. Liu et al, J. Vac. Sci. Tech. B (submitted). [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J31.00007: Transport in topological insulator films Jian Wang, Ashley DaSilva, Meenakshi Singh, Joon Sue Lee, Jainendra Jain, Nitin Samarth, Moses H.W. Chan, Cui-Zu Chang, Ke He, Xu-Cun Ma, Qi-Kun Xue, Handong Li, Maohai Xie The prediction and the subsequent confirmation of topological insulators is one of the most exciting discoveries in condensed matter physics. In the transport study of topological insulator films, we demonstrate that an excellent agreement between theory and experiment is achieved when both disorder and interaction are taken into account. In addition, measurements under an in-plane magnetic field, along and perpendicular to the bias current show opposite magnetoersistance. Furthermore, Bi2Se3 topological insulator thin films contacted by superconducting (In, Al and W) electrodes show an abrupt resistance upturn when the electrodes become superconducting. Concomitant with the upturn in resistance, there is a significant weakening of the superconductivity of the electrodes. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J31.00008: Magneto-transport study of the topological insulator Bi$_{2}$Te$_{3}$ Pramod Kumar, Aruna Ramanayaka, Ramesh Mani Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator, but have protected surface or edge states. Many materials have been realized as topological insulators, including the HgTe/CdTe superlattice, Bi$_{1-x}$Sb$_{x}$, Bi$_{2}$Se$_{3}$, Sb$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{3}$. Topological insulators are interesting not only because of their fundamental importance but also their great potential for future applications. Here, we examine the magneto-transport properties of exfoliated Bi$_{2}$Te$_{3}$ specimens prepared from Bi$_{2}$Te$_{3}$ single crystals using the scotch tape method. Indium and silver paint contacts were applied to the exfoliated specimens and magneto-transport was examined at liquid helium temperatures at moderate magnetic fields. The results of these experiments will be described here within the context of the ongoing interest in topological insulators. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J31.00009: Theoretical study of carrier transport and screening in topological insulator Bi$_{2}$Se$_{3}$ Shaffique Adam, E.H. Hwang, M.D. Stiles, S. Das Sarma This theoretical work is motivated by two recent experiments on Bi$_{2}$Se$_{3}$ examining the charge inhomogeneity [1-2] close to the topologically protected crossing point of surface bands in these bulk topological insulators. Reminiscent of graphene close to charge neutrality [3-4], the energy landscape becomes highly inhomogeneous, forming a sea of electron and hole puddles, which determine the properties at low carrier density. Here, we show that the induced carrier density fluctuations are of order 1 {\%} of the impurity density, providing a small-parameter with which we can perform a controlled perturbation theory. Analytic results are obtained for the minimum conductivity and puddle auto-correlation length. We also find that the band asymmetry between electron and holes states is a necessary ingredient to understand the aforementioned experiments. \textbf{References: } [1] H. Beidenkopf \textit{et al,} ``\textit{Spatial fluctuations of helical Dirac fermions on the surface of topological insulators},'' Nat. Phys.\textit{ online publ.}, (2011) [2] D. Kim \textit{et al.,} ``\textit{Minimum conductivity and charge inhomogeneity in Bi}$_{2}$\textit{Se}$_{3}$,'' arXiv:1105.1410. [3] S. Adam \textit{et al.}, ``\textit{A self-consistent theory for graphene transport},'' PNAS \textbf{104}, 18392 (2007). [4] S. Das Sarma \textit{et al., ``Electronic transport in 2D graphene},'' \textit{Rev. Mod. Phys.} \textbf{83}, 407 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J31.00010: Minimum conductivity and charge inhomogeneity in Bi$_{2}$Se$_{3}$ in the topological regime Dohun Kim, Sungjae Cho, Nicholas Butch, Paul Syers, Kevin Kirshenbaum, Johnpierre Paglione, Michael Fuhrer Using electrolytic and dielectric dual gating method, we report charge transport measurements of mechanically exfoliated Bi$_{2}$Se$_{3}$ in the topological insulator (TI) regime. We show that the surfaces of thin, low-doped Bi$_{2}$Se$_{3}$ crystals are strongly electrostatically coupled, and a gate electrode can be used to completely remove bulk charge carriers and bring both surfaces through the Dirac point nearly simultaneously with well-defined ambipolar electronic conduction of gapless surface states. In particular, we focus on linear carrier density dependent conductivity away from the Dirac point and a charge-inhomogeneous minimum conductivity region similar to that observed in graphene. An extension of the theory of charge disorder in graphene to Bi$_{2}$Se$_{3 }$explains well the mobility at high carrier density and the doping level at zero gate voltage. We show that the observed minimum conductivity is governed by induced carrier density that is self-consistently determined by the screened, charged impurity potential, as experimentally observed in recent STM study on surfaces of TIs. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J31.00011: Gate-tunable electronic transport in topological insulator Bi$_{2}$Te$_{3}$ thin films synthesized by metal-organic chemical vapor deposition Helin Cao, Rama Venkatasubramanian, Jonathan Pierce, Tai-lung Wu, Jifa Tian, Isaac Childres, Yong Chen Topological insulator is a new state of matter with a nominally insulating gap in the bulk and non-trivial metallic states on the surface. One of the proto-type topological insulator materials, Bi$_{2}$Te$_{3}$, can be synthesized in the form of high quality, wafer scale thin films by metal-organic chemical vapor deposition (MOCVD). Here we present an experimental study of Bi$_{2}$Te$_{3}$ thin films with thickness ranging from a few nm's to 1 $\mu $m synthesized by MOCVD on semi-insulating GaAs (001) substrates. Hall bar shaped devices using atomic layer deposition (ALD) high-k Al$_{2}$O$_{3}$ or HfO$_{2}$ as gate dielectric have been fabricated. We have measured the magneto-transport (including both R$_{xx}$, 4-terminal longitudinal resistance, and R$_{xy}$, the Hall resistance) at various temperatures and gate voltages to probe the possible transport signatures of the topological surface states. We have also studied gate-tunable weak anti-localization in R$_{xx}$(B) for ultra-thin films. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J31.00012: Ambipolar field effect in the ternary topological insulator (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ by composition tuning Yulin Chen, Desheng Kong, Judy Cha, Qianfan Zhang, James Analytis, Keji Lai, Zhongkai Liu, Seung-Sae Hong, Kristie Koski, Sung-Kwan Mo, Zahid Hussain, Ian Fisher, Zhi-Xun Shen, Yi Cui Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping, and these mask the contribution of surface carriers to charge transport in these materials. Our recent work demonstrates that the ternary sesquichalcogenide (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J31.00013: Signature of Topological Insulators in Conductance Measurements Seokmin Hong, Vinh Diep, Supriyo Datta Following the discovery of spin-polarized states at the surface of three-dimensional topological insulators (TI) like Bi$_{2}$Te$_{3}$ and Bi$_{2}$Se$_{3}$, there are intense interests in possible electrical measurements demonstrating unique signatures of these unusual states. A recent interesting proposal suggests that a signature of TI material should be a change in the conductance measured between a normal contact and a magnetic contact when the magnetization of the latter is reversed. However, the generalized Onsager relation suggests that no such change is expected in two-terminal setups and a multi-terminal set up is needed to observe the proposed effect. We present numerical results using a Non-Equilibrium Green Function (NEGF) based model capable of covering both ballistic and diffusive transport regimes seamlessly. Simple expressions based on a semi-classical picture describe some of the results quite well. Finally, we estimate the magnitude of signal expected in realistic samples that have recently been studied experimentally and have shown evidence of surface conduction. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J31.00014: Coupling between 3-D topological insulator and superconductor Vladimir Orlyanchik, Martin Stehno, Christopher Nugroho, Dale Van Harlingen, Namrata Bansal, Matthew Brahlek, Seongshik Oh Topological insulators are band insulators in the bulk with gapless topologically protected surface states. Recently it has been predicted that 3-D topological insulators (TI) can host zero energy modes called Majorana fermions. Many theoretical proposals for observation of the zero energy excitations involve coupling between the surface states of TI and s-wave superconductors (SC). A prerequisite for such experiments is a highly tunable surface which is decoupled from the residual bulk carriers and well established coupling between TI and superconductor. Here we present transport measurements performed in high quality MBE grown thin films of Bi2Se3. Along with presenting evidence for significant contribution of the surface states to the electrical transport, we discuss the dependence of TI-SC coupling on temperature, gate voltage and thickness of TI films. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J31.00015: Aging effect on carrier density and mobility of thin film Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ grown on Al$_{2}$O$_{3}$ Nikesh Koirala, Matthew Brahlek, Namrata Bansal, Seonshik Oh Topological Insulators (TI) are materials with topologically protected metallic surface states and insulating bulk states. Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ are viable TI material as they posses bulk band gap as well as continuous surface band. However, there is bulk conduction as well in these materials due to Fermi surface lying in the bulk conduction band. Natural n-type doping due to ambient oxygen and water vapor and/or creation of Se/Te vacancies create bulk contamination, leading to change in carrier density and motility. We report transport measurement data on epitaxially grown thin films of these materials which show the dependence of charge carrier density and mobility on environmental exposure time of these samples. In addition, we will present our data on samples capped with various materials. [Preview Abstract] |
Session J32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Domain Structures and Switching
Sponsoring Units: DMP DCOMPChair: Peter Maksymovych, Oak Ridge National Laboratory
Room: 261
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J32.00001: Superdomains and Flux Closure Patterns in Ferroelectrics Invited Speaker: Marty Gregg Microstructural complexity has been a recognized feature of ferroelectric materials since the earliest optical microscopy studies on BaTiO$_{3}$ over 60 years ago,\footnote{P. W. Forsbergh, Phys. Rev. 76, 1187 (1949).}$^,$\footnote{W. Merz, Phys. Rev. 95, 690 (1954).} in which rich patterns of domain states were observed. Arlt and Sasko\footnote{G. Arlt and P. Sasko, J. Appl. Phys. 51, 4956 (1980).} famously highlighted the existence of bands of domain structures forming herringbone patterns and, in so doing, helped to establish the notion that domain architectures exist over a number of different length scales: hierarchies naturally develop in which coarse-scale ``superdomains'' are themselves composed of sets of finer-scale ``subdomains.'' What was not widely recognised, was that these ``superdomains'' could act as functional entities in their own right, with net polarisation and spontaneous strain being the vector sum of these quantities from constituent ``subdomains''. In this talk, we will explore the dominant role that superdomain functionality can have on the behaviour of ferroelectrics, at least at the meso and nanoscales. Observations made on single crystal thin film sheets and nanodots of BaTiO$_{3}$ (machined using a Focused Ion Beam Microscope) will be presented and several key points will be made: (i) that boundaries between superdomains generally adhere to the same constraints as those seen in subdomains (for example, that the divergence in the net superdomain polarisation across boundaries is zero); (ii) that superdomains demonstrate the same scaling laws (Landau-Kittel scaling) as were originally developed for simple subdomains; (iii) that ferroelectric switching can be entirely mediated by superdomains rather than subdomains; (iv) that flux closure objects, which extend beyond the nanoscale, require the existence of superdomains. The dynamics of flux closure formation, due to depolarising fields, has also been mapped and will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J32.00002: Superdomain dynamics in single-crystal BaTiO$_{3}$ lamellae Raymond McQuaid, Leo McGilly, Pankaj Sharma, Alexei Gruverman, Marty Gregg In the1950's early studies pioneered by Merz established that 180\r{ } domain reversal in bulk BaTiO$_{3}$ could be described by a 3-stage switching process [see H. L. Stadler, Ferroelectrics 137, 1992]. This involved formation of a reversed domain nucleus, forward growth of a needle shaped domain towards the opposite electrode followed finally by sideways expansion. In a recent study [R. G. P. McQuaid, Nat. Comms. 2, 2011] we used Piezoresponse Force Microscopy (PFM) to monitor the switched domain states that developed in single-crystal BaTiO$_{3}$ slices machined (by Focused Ion Beam) to thin film thicknesses and incorporated into a coplanar geometry. We found that switched states are constrained to exist within a rigid framework of ordered ferroelastic stripe domains. Surprisingly, we see that switching occurs solely by the movement of a complex 180\r{ }-type boundary which separates `bundles' of these domain stripes. We see that it is at the collective `superdomain' level, rather than for individual domains, where the classic Merz nucleation and growth modes are observed. We use PFM imaging to track the realtime boundary position during switching and attempt model fits to understand its field driven dynamics. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J32.00003: Theoretical study of ferroelectric switching of Pt/BaTiO$_{3}$ Slabs Kurt Fredrickson, Alex Demkov BaTiO$_{3 }$(BTO) is a well known ferroelectric perovskite, which is tetragonal at room temperature. The energy barrier for polarization switching is small at 9.7 meV; however, its energy barrier clamped between two metal electrodes is an open question. It has been shown (Junquera {\&} Ghosez, Nature \textbf{422}, 506 (2003)) that the ferroelectric behavior is highly thickness dependent. We examine thin slabs of BTO sandwiched between electrodes of Pt and calculate the polarization barriers using density functional theory to see whether the barrier should be surmountable. We have found that the energy barrier/unit cell reaches the bulk value at only 10 unit cells of BTO. We also examine the polarization of the relaxed slabs and compare them to bulk BTO, and find that the center of the slab exhibits bulk rumpling at 20 unit cells of BTO. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J32.00004: Probing Nanoscale Ferroelectric Domain Switching Mechanisms with Scanning Probe Microscopy Vasudeva Rao Aravind, Senli Guo, Amit Kumar, Stephen Jesse, Sergei Kalinin, Venkatraman Gopalan Ferroelectric domains and domain walls have been a focus topic for research, owing to their applications in memory devices, ultrasonic imaging devices, etc. Recently, ferroelectric domain walls have been demonstrated to exhibit a rich panoply of nanoscale switching behaviors (V. R. Aravind \textit{et al,} Physical Review B \textbf{82}, 024111 (2010)). In this presentation, we report our study of domain reversal and polarization relaxation behavior of ferroelectric domain walls under localized electric field provided by a scanning probe microscope tip. Our studies show the relaxation behaviors differs at different distances from a 180 degree domain wall, throwing light on the microscopic mechanisms of polarization reversal. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J32.00005: In situ TEM studies of interaction between ferroelastic and ferroelastic domains during ferroelectric switching Peng Gao, Christopher Nelson, Jacob Jokisaari, Seung-Hyub Baek, Chung Wung Bark, Chang-Beom Eom, Xiaoqing Pan Dynamic interaction between ferroelastic (90 degree) and ferroelectric (180 degree) domains in lead zirconate titanate during ferroelectric switching was investigated by in situ TEM. It was found that 90 degree domain walls were immobile under applied bias and act as obstacles to 180 degree domain wall motion. Pinning of the incident 180 degree domain wall occurs directly at the leading 90 degree domain wall and results in a charged head-to-head polarization vector configuration and thereby a large surface energy. This causes the roughening of the domain wall which was atomically sharp before switching. Subsequent switching occurs through the nucleation of a new domain on the opposite 90 degree domain wall and ultimately results in 180 degree switching across the film with the 90 degree domain wall still present. Although they were immobile, the 90 degree domains could sometimes be erased by an external electrical field parallel to the normal axis polarization and would return when the field was removed. This suggests that a pre-pulse opposite to the desired writing direction which momentarily erases these domains may improve switching efficiency. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J32.00006: Vortex polarization, strain induced phase transitions and dielectric response in ultra-thin PbTiO$_3$ nanowires from first principles Ghanshyam Pilania, R. Ramprasad Nature of ferroelectricity in nanostructures and the resulting dielectric response are of both fundamental and applied interest. Here, using density functional theory (DFT) based computations, we investigate polarization configurations as a function of axial strain in ultra-thin PbTiO$_3$ [001] nanowires. Our computations involved relaxed and axially strained free-standing nanowires with varying sidewall terminations and diameters. While stress-free nanowires with their sidewalls terminated by PbO surfaces displayed purely rectilinear axial polarization at all sizes, the TiO$_2$-terminated nanowires, at a critical diameter of 16 {\AA}, display a non-rectilinear vortex polarization transverse to the nanowire axis. We discuss the origins of such behavior. We also predict the existence of novel stress-induced phase transitions between the mutually exclusive vortex and the axial polarization states in both the PbO- and TiO$_2$-terminated nanowires. Normal mode vibrational frequency analysis of these nanowires further confirms these results. Furthermore, by employing density functional perturbation theory in combination with effective medium dielectric theory we calculate dielectric permittivity of the ferroelectric nanowires and compare it with the corresponding bulk results. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J32.00007: Domain Wall Nucleation and Propagation within Ferroelectic Nanowires in High Strength Electric Fields Kevin McCash, Arvind Srikanth, Inna Ponomareva Ferroelectric nanowires have attracted a lot of attention recently, thanks to their ability to develop electric polarization at the nanoscale [1]. Such a unique feature could potentially lead to the use of such nanowires in nanoscale, ultra fast, high-density memory elements. Here we take advantage of accurate first-principles-based simulations to study ultra fast polarization reversal in ultra thin ferroelectric nanowires made of PbTi$_{0.6}$Zr$_{0.4}$O$_{3}$ alloy. Our computational experiments reveal that polarization reversal in such nanowires is both qualitatively and quantitatively different from their bulk counterparts and exhibits unique features that could find potential use in nanoscale ferroelectric memory elements. \\[4pt] [1] P.M. R{\o}rvik, T. Grande, and M.-A. Einarsrud (2011), One-Dimensional Nanostructures of Ferroelectric Perovskites. Advanced Materials, 23: 4007-4034 [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J32.00008: Terahertz Frequency Dynamics of Ferroelectric Nanowires Ryan Herchig, Kimberly Schultz, Kevin McCash, Inna Ponomareva A thorough understanding of ferroelectric nanostructures is imperative considering their utility in creating nanoscale devices for the technology of the future. One such ferroelectric nanostructure which may prove useful in the design of nanosensors is the nanowire. We report our study on ferroelectric nanowires of Pb(Zr$_{0.4}$Ti$_{0.6}$)O$_3$ alloy done using classic molecular dynamics with first-principle-based effective Hamiltonian[1] and Evans-Hoover thermostat. We found that 1) the polarization of such nanowires can be reversed and 2) that the nanowires temperature can be controlled by the application of a terahertz electric field pulse. The dependence of these properties on the frequency, width, and amplitude of the pulse is explored and discussed in addition to a possible energy dissipation mechanism. \newline \newline [1] L. Bellaiche {\it et al}, Phys. Rev. Lett. {\bf 8}, 5427 (2000). [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J32.00009: Origin of 90$^{\circ}$ Domain Wall Pinning in Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ Heteroepitaxial Thin Films Dong Su, Qingping Meng, MMyung-Geun Han, Carlos Vaz, Yaron Segal, Matthew Marshall, Fred Walker, Monica Sawicki, Christine Broadbridge, Charles Ahn Researchers have studied the effect of ferroelectric fields in controlling the spin state via electric fields in multiferroic composite structures. For instance, in a bilayer system composed of a ferroelectric perovskite (PbZr$_{0.2}$Ti$_{0.8}$O$_{3})$ and a colossal magnetoresistive (CMR) manganite (La$_{0.8}$Sr$_{0.2}$MnO$_{3}$, LSMO), the spin state in the CMR film can be controlled by switching the ferroelectric polarization state, thereby generating a large magnetoelectric coupling. For this system, the domain's structure and switchability is critically important to the device's performance. We describe transmission-electron-microscopy study of the ferroelectric domains in a epitaxial Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$(PZT) film grown on La$_{0.8}$Sr$_{0.2}$MnO$_{3}$/SrTiO$_{3}$(001). We directly observe the pinning of 90$^{\circ}$ domain walls by pairs of misfit dislocations with Burgers vectors \textbf{\textit{a}}[100] and \textbf{\textit{a}}[001]. Model calculations based on the elastic theory confirm our finding that, in addition to the depolarization field surrounding the dislocation, the strain field of misfit dislocation-pairs plays the primary role in the formation and pinning of \textbf{\textit{a}} domains. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J32.00010: Dynamics of Nanowalls In Ferroelectric Ultrathin Films Qingteng Zhang, Ryan Herchig, Inna Ponomareva Nanoscale ferroelectric films can exhibit nanostripes that are nanoscopic regions of ``up'' and ``down'' polarizations, hence forming domain walls which separate nanodomains with different polarization directions. The dynamical properties of domain walls are of technological importance since they are at the heart of ultradense ferroelectric memory technology and may play an important role in nanoscale ferroelectric sensors, actuators, and others. Here [1] we take advantage of accurate first-principle-based simulations to reveal the intrinsic dynamics of nanodomains in ultra-thin PbTi$_{0.6}$Zr$_{0.4}$O$_3$ films with thickness ranging from 2 to 20 nm. We first demonstrate that the nanodomain walls oscillate under driving AC-field of sub-switching amplitude. Secondly, we reveal that nanowalls can exhibit two types of intrinsic dynamics (resonance and relaxation) at the same frequencies. Thirdly, we prove that at nanoscale the dynamics is determined by the domain size which manifests itself via a unique size-driven transition from relaxational to resonance dynamics.\\[4pt] [1] Q. Zhang et al, Phys. Rev. Lett. 107, 177601 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J32.00011: Nanoscale Ferroelectric Switching in Thin Films by in-situ TEM for Magnetoelectric Applications Christopher Nelson, Peng Gao, Jacob Jokisaari, Colin Heikes, Carolina Adamo, Alexander Melville, Seung-Hyub Baek, Chad Folkman, Benjamin Winchester, Yijia Gu, Yuanming Liu, Kui Zhang, Enge Wang, Jiangyu Li, Long-Qing Chen, Chang-Beom Eom, Darrell Schlom, Xiaoqing Pan The ferroelectric switching along the low-dimensional axis of nanoscale multiferroic BiFeO$_{3}$ thin films is studied in this work using in-situ transmission electron microscopy. With this technique, the atomic scale polarization distribution and the controlling influence of defects on growth kinetics are observed. Despite the inhomogeneous external field applied by a surface probe, nucleation sites are determined by the built-in fields formed within carrier depletion regions at the electrode interfaces. Homogenous full-film switching is often impeded by the pinning of domain growth by such features as point defect assemblies and from independent switching in the near-interface region. This inhomegeneity along the normal film axis has significant implications for the interpretation of surface probe ferroelectric switching measurements and for magnetoelectric applications which require ferroelastic switching at the interface. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J32.00012: Domain dynamics during ferroelectric switching Xiaoqing Pan, Peng Gao, Christopher Nelson, Jacob Jokisaari, Seung-Hyub Baek, Chung-Wung Bark, Chang-Beom Eom, Darrell Schlom Ferroelectric materials are characterized by a spontaneous polarization that can be reoriented by an applied electric field. The ability to form and manipulate domains at the nanometer scale is key to device applications such as nonvolatile memories. The ferroelectric switching is mediated by defects and interfaces. Thus, it is critical to understand how the domain forms, grows, and interacts with defects. Here we show the nanoscale switching of a tetragonal PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ thin film under an applied electric field using \textit{in situ} transmission electron microscopy. We found that the intrinsic electric fields formed at ferroelectric/electrode interfaces determine the nucleation sites and growth rates of domains and the orientation and mobility of domain walls, while dislocations exert a weak pinning force on domain wall motion. We also show that localized 180\r{ } polarization switching initially form domain walls along unstable planes. After removal of the external field, they tend to relax to low energy orientations. In sufficiently small domains this process results in complete backswitching. Our results suggest that even thermodynamically favored domains are still subject to retention loss, which must be mitigated by overcoming a critical domain size. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J32.00013: Modeling Micron Size Multi-domain Ferroelectric Switching using a Massively Parallel Time Domain Phase-Field Model Khalid Ashraf, Sayeef Salahuddin For the study of devices incorporating multi-domain ferroelectric materials, it is necessary to extend the current capabilities of the phase field model up to the micron scale where experiments are typically performed. Also arbitrary electrical and mechanical boundary conditions need to be incorporated relatively easily. In this work, we report a time domain implementation of the 3D phase field model that can simulate multi-domain ferroelectric switching. This massively parallel implementation enables us to study the switching properties of micron size devices with $\sim $10$^{9}$ degrees of freedom. We used a mixed finite difference and finite element grid, for calculating the nonlocal electrostatic and elastic interactions respectively. All the local and non-local interactions are shown to scale linearly up to thousands of processors. The model can take into account arbitrary electrical and mechanical boundary conditions suitable for the study of devices with arbitrary structures. Using this model, we report the simulation results of ferroelectric switching in devices incorporating the multi-ferroic material BiFeO$_{3}$. We explain the domain growth mechanism observed in multiple experiments reported recently on various surfaces of BiFeO$_{3}$. [Preview Abstract] |
Session J34: Focus Session: Impact of Ultrafast Lasers III: Biophysics
Sponsoring Units: DCPChair: Nancy Levinger and Amber Kummel, Colorado State University
Room: 107A
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J34.00001: Probing Electronic Excitations in Molecules by Coherent Multidimensional UV and X Ray Spectroscopy Invited Speaker: Shaul Mukamel Two-dimensional ultraviolet (2DUV) spectra of protein backbone (far UV) and side chains (near UV) provide new insights into the protein structures, dynamics and functions. Simulated chirality-induced 2DUV spectra reveal characteristic patterns of protein secondary structures and allow monitoring the aggregation mechanism of amyloid fibrils and predicting the aggregation propensity of peptides. Time-domain experiments that employ sequences of attosecond x-ray pulses in order to probe electronic and nuclear dynamics in molecules are made possible by newly developed bright coherent ultrafast sources of soft and hard x-rays. By creating multiple core holes at selected atoms and controlled times it should be possible to study the dynamics and correlations of valence electrons as they respond to these perturbations. The stimulated x-ray Raman spectrum of \textit{trans}-N-methylacetamide and Cysteine at the Nitrogen, Sulfur and the Oxygen K-edges in response to two soft x-ray pulses is calculated by treating the core excitations at the Hartree--Fock static-exchange level (STEX) level. The signal is interpreted in terms of the dynamics of valence electronic wave packets prepared and detected in the vicinity of (either the nitrogen or the oxygen) atom. The evolving electronic charge density and as electronic coherences are visualized using a basis set of time-dependent natural orbitals. Effects of orbital relaxation upon core excitations are resolved. A two-dimensional extension of the technique that involves a sequence of three resonant Raman pulses will be presented. Extensions to multidimensional spectroscopy with photoelectron detection are proposed. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J34.00002: Conformation of self-assembled porphyrin dimers in liposome vesicles by phase-modulation 2D fluorescence spectroscopy Alejandro Perdomo-Ortiz, Geoffrey A. Lott, James K. Utterback, Julia R. Widom, Al\'an Aspuru-Guzik, Andrew H. Marcus By applying a phase-modulation fluorescence approach to 2D electronic spectroscopy (PM-2D FS), we studied the conformation-dependent exciton coupling of a porphyrin dimer embedded in a phospholipid bilayer membrane. Our measurements specify the relative angle and separation between interacting electronic transition dipole moments and thus provide a detailed characterization of dimer conformation. PM-2D FS produces 2D spectra with distinct optical features, similar to those obtained using 2D photon-echo spectroscopy. Specifically, we studied magnesium meso tetraphenylporphyrin dimers, which form in the amphiphilic regions of 1,2-distearoyl-sn-glycero-3-phosphocholine liposomes. Comparison between experimental and simulated spectra show that although a wide range of dimer conformations can be inferred by either the linear absorption spectrum or the 2D spectrum alone, consideration of both types of spectra constrain the possible structures to a ``T-shaped'' geometry. These experiments establish the PM-2D FS method as an effective approach to elucidate chromophore dimer conformation. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J34.00003: 2-Dimensional Fluorescence Spectroscopy: Determining the Temperature-Dependent Conformations of Porphyrin Dimers and Nucleic Acids Julia Widom, Alejandro Perdomo-Ortiz, Alan Aspuru-Guzik, Andrew Marcus I will describe spectroscopic studies on a covalently-linked zinc tetraphenylporphyrin dimer embedded in a phospholipid bilayer membrane. Using phase-modulation 2-Dimensional Fluorescence Spectroscopy (2D FS, a fluorescence-detected version of 2D electronic spectroscopy) along with linear absorption and fluorescence spectroscopy, it was found that the dimer adopts two predominant conformations in the membrane, and that the relative populations of these two states change as a function of temperature. Simultaneously fitting the linear absorption spectrum and the 2D FS spectra at four different excitation wavelengths revealed a wealth of information about these two states, including their relative populations, relative fluorescence quantum yields, the strength of the exciton coupling present in each state, and the approximate angles between the electronic transition dipole moments of the two porphyrins. Ongoing analysis focuses on elucidating the relaxation and energy transfer dynamics of this system through the population time dependence of the 2D spectra. Finally, I will present preliminary results from experiments in which 2D FS was performed with ultraviolet excitation to study the conformations of DNA constructs labeled with a fluorescent analogue of guanine. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:51PM |
J34.00004: Probing photosynthetic structure and function using multidimensional spectroscopy Invited Speaker: Jennifer Ogilvie |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J34.00005: Exploring Relaxation Processes in Components of DNA with UV Nonlinear Spectroscopy Andrew Moran, Brantley West, Jordan Womick Underlying photoinduced relaxation in DNA is a complex world of solute-solvent interactions and fluctuations in the geometries of macromolecules. Electronic excitations are rapidly deactivated by nuclear motions through conical intersections, thereby suppressing the formation of lesions (e.g., thymine dimers) known to inhibit cellular function. At the instant following internal conversion, the bases are left in ``hot'' quantum states, wherein a subset of vibrational modes possess a highly non-equilibrium distribution of excitation quanta. The transfer of this energy to the surrounding also involves intriguing fundamental physics. We examine these processes in small components of DNA by conducting femtosecond laser spectroscopies at cryogenic temperatures. Our experiments utilize several recent advances in nonlinear optics. Parametric processes in argon gas are used to generate 25fs pulse durations at 265nm. These short pulses are employed in a variety of measurements (e.g., transient grating, 2D photon echo, fluorescence down-conversion) with the goal of understanding relaxation mechanisms. Our data suggest that excited state deactivation in DNA is quite sensitive to the exchange of vibrational energy between the bases and segments of the backbone. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J34.00006: Early optical response of fluorescent molecules studied by synthetic optically delayed pulses Arkaprabha Konar, Jay Shah, Tapas Goswami, Vadim Lozovoy, Marcos Dantus The early optical response of fluorescent molecules in solution is probed by pairs of collinear pulse replicas. Two approaches are followed. First approach mimics an interferometer, replicating interference as a function of time delay between the pulses. For the second approach, each pulse spans the entire laser bandwidth, sharing no common frequencies with the second pulse, thus no interference is observed between the pulses. In both cases, the pair of pulses is delayed with attosecond resolution to study IR 144. Both fluorescence at right angles or the stimulated emission along the output beam as a function of time delay is monitored. At high intensities when approximately 10{\%} of the dye molecules are excited, the second pulse can stimulate emission from molecules excited by the first pulse, thereby giving rise to interference fringes every 2.66 fs. When the pulse replicas are generated by multiple independent comb shaping, it is evident that the interference fringes for stimulated emission bear an out of phase relationship with those observed from fluorescence and have a maxima at time zero. This is masked with conventional pulse replicas that interfere. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:51PM |
J34.00007: Quantum mechanical light harvesting mechanisms in photosynthesis Invited Speaker: Gregory Scholes More than 10 million billion photons of light strike a leaf each second. Incredibly, almost every red-coloured photon is captured by chlorophyll pigments and initiates steps to plant growth. Last year we reported that marine algae use quantum mechanics in order to optimize photosynthesis [1], a process essential to its survival. These and other insights from the natural world promise to revolutionize our ability to harness the power of the sun. In a recent review [2] we described the principles learned from studies of various natural antenna complexes and suggested how to utilize that knowledge to shape future technologies. We forecast the need to develop ways to direct and regulate excitation energy flow using molecular organizations that facilitate feedback and control--not easy given that the energy is only stored for a billionth of a second. In this presentation I will describe new results that explain the observation and meaning of quantum-coherent energy transfer. \\[4pt] [1] Elisabetta Collini, Cathy Y. Wong, Krystyna E. Wilk, Paul M. G. Curmi, Paul Brumer, and Gregory D. Scholes, ``Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature'' Nature 463, 644-648 (2010).\\[0pt] [2] Gregory D. Scholes, Graham R. Fleming, Alexandra Olaya-Castro and Rienk van Grondelle, ``Lessons from nature about solar light harvesting'' Nature Chem. 3, 763-774 (2011). [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J34.00008: Temperature dependence of proton transfer kinetics in the green fluorescent protein Bridget Salna, Abdelkrim Benabbas, Paul Champion, J. Timothy Sage, Jasper van Thor, Ann Fitzpatrick In green fluorescent protein (GFP), near UV photoexcitation leads to proton transfer from the chromophore phenolic oxygen along a proton ``wire'' consisting of an internal water molecule, Ser205 and Glu222. Using transient absorption kinetics, the complete cycle, including the picosecond excited-state proton transfer, the nanosecond radiative emission, and the slower ground state proton back-transfer reactions have been studied holistically as a function of temperature. This experiment was performed for both the hydrogenated and deuterated forms of GFP. We have extracted the Arrhenius prefactors and activation energy barriers for both the forward and back proton transfer kinetics. A large kinetic isotope effect for the ground state proton back-transfer has been observed at high temperatures suggesting that tunneling plays an important role. At lower temperatures the data suggest a cross-over to a different pathway for the back-transfer reaction. To investigate this hypothesis we studied the E222D mutant of GFP, which substitutes aspartate for glutamate on the proton wire. The H/D kinetics of this mutant explicitly test for the source of proton donors and indicate that proton transfer proceeds along the same pathway in the native protein at room temperature. [Preview Abstract] |
Session J35: Liquids
Sponsoring Units: DCPChair: Donald Truhlar, University of Minnesota
Room: 107B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J35.00001: Ion shape effect on dynamics of ionic liquids Hongjun Liu, Edward Maginn Ionic liquids (ILs) are a group of salts composing of an organic cation and organic or inorganic anion with melting points below 100 $^{\circ}$C and have many suitable properties, such as negligible vapor pressure, low flammability, high ionic conductivity and high thermal stability for various applications. Moreover, a great number of ILs with a variety of physical and chemical properties can be synthesized from a combination of different cations (most differently substituted imidazolium, pyridinium, and quaternary ammonium or phosphonium ions) and anions. One can judiciously select from a multitude of ILs to suit a specific application, where the concept of designer solvent comes from. To expedite the development process of target ILs, it is crucial to understand the relationship between ion shape and dynamics of ILs. We studied a wide range of ILs with different ion shape pairings and found the planar-planar paired ILs have a better dynamics as a whole. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J35.00002: Thermodynamic Phase Diagram of Hydrogen in Nano-porous Silica Eli Van Cleve, Sergei Kucheyev One of the major challenges faced by inertial confinement fusion (ICF) is spherical uniformity of fuel capsules. Most current ignition target designs call for spherically uniform layers of deuterium-tritium (DT) mix in a solid phase. Smooth solid DT layers of high-quality single crystals are necessary, requiring stringent layering protocols. Liquid DT confined in a low-density nano-porous scaffold layer is a possible alternative target, having greatly relaxed layering requirements. Hydrogen in vycor, which has a high density and a relatively uniform pore size distribution, has previously been studied. The porous materials used for this study are silica aerogels. These have ultralow densities and broad pore size distributions. We discuss the thermodynamic phase diagram of hydrogen and deuterium condensed in silica aerogels studied using relaxation calorimetry. We find that crystallization temperatures of both isotopes are suppressed inside the aerogel; however, the freezing takes place over a relatively wide range of temperatures and non-trivially depends on the hydrogen filling fraction. We discuss the correlation of freezing temperatures with the pore size distribution. This work was performed under the auspices of the U.S. DOE my LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J35.00003: ABSTRACT WITHDRAWN |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J35.00004: Quasi-free Electron Energy in Near Critical Point He Yevgeniy Lushtak, Samantha Dannenberg, Cherice Evans, Gary Findley We present the quasi-free electron energy $V_0(\rho)$, where $\rho$ is the density, in He from low density to the density of the gas/He-I/He-II triple point, at various noncritical temperatures and on an isotherm near the critical isotherm. These data, which were obtained using field enhanced photoemission, represent the first extended density overview of $V_0(\rho)$ for He, especially near the critical point. A novel critical point effect is observed and is accurately fitted to the local Wigner-Seitz model, thereby showing that this model, which was developed for attractive systems, can also be applied to repulsive systems.\\[4pt] The experimental measurements reported here were performed at the University of Wisconsin Synchrotron Radiation Center (NSF DMR-0537588). This work was supported by grants from the Petroleum Research Fund (45728-B6) and from the National Science Foundation (NSF CHE-0956719). [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J35.00005: COSMO-3D: Introducing Steric Effects into COSMO Solvation Models Erin McGarrity, Jan van der Eerden, Gerard Krooshof We will present a modified COSMO-based thermodynamical model which can be used for the prediction of solvation in liquid mixtures. The method provides values for the surface-segment interactions and molecular contact areas as functions of the relative positions of all the segment pairings of the constituent molecules in a mixture. These expressions are obtained by placing the complete molecules into contact at each segment pair to determine if the molecules overlap, and using a probing particle to determine the total area of contact. The former expression can be used to discard terms from the COMSO-based segment sums which correspond to overlapping molecular configurations. The areas are used to determine the average contact energies based on Onsager's theory for dielectric screening. Our method allows us to remove three parameters from the original model without any loss of accuracy. We will show the results of our model as applied to binary mixture vapor-liquid equilibrium predictions. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J35.00006: Charge induced hydrophobic/hydrophilic metallic surfaces Luana Pedroza, Adrien Poissier, Marivi Fernandez-Serra Understanding the interaction of water-metal systems in an atomic level is of fundamental importance in many areas, such as catalysis and materials science. We here present a detailed first-principles molecular dynamics study of bulk water molecules confined within Pd(111) surfaces. We show that there is a charge transfer between the substrate and the water inducing an asymmetry in the order of water molecules at Pd surfaces. Our results show that the hydrophobic/hydrophilic character of a metallic surface depend on its charge, which can be controlled by an applied voltage. We also propose a methodology to obtain the dipole moment of each water molecule and show how they are affected by the substrate induced polarization effects. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J35.00007: Pulling adsorbed polymers at an angle: A low temperature theory Gerasim Iliev, Stuart Whittington We consider several partially-directed walk models in two- and three-dimensions to study the problem of a homopolymer interacting with a surface while subject to a force at the terminal monomer. The force is applied with a component parallel to the surface as well as a component perpendicular to the surface. Depending on the relative values of the force in each direction, the force can either enhance the adsorption transition or lead to desorption in an adsorbed polymer. For each model, we determine the associated generating function and extract the phase diagram, identifying states where the polymer is thermally desorbed, adsorbed, and under the influence of the force. We note the different regimes that appear in the problem and provide a low temperature approximation to describe them. The approximation is exact at T=0 and models the exact results extremely well for small values of T. This work is an extension of a model considered by S. Whittington and E. Orlandini. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J35.00008: Low-Energy Electron Induced Reactions in Condensed Methanol Michael Boyer, Mavis Boamah, Kristal Chamberlain, ChanMaye Myae Soe, Andrew Bass, Leon Sanche, Christopher Arumainayagam We investigate the dynamics of low-energy electron-induced reactions in condensed thin films of methanol (CH$_3$OH) through electron stimulated desorption (ESD) and post-irradiation temperature programmed desorption (TPD) experiments. ESD experiments indicate that the anions which desorb from the methanol thin film during electron irradiation are predominantly formed through the dissociation of temporary negative ions formed by electron capture by methanol molecules, a process known as dissociative electron attachment (DEA). However, based on investigation of reaction products remaining in the methanol thin film post-irradiation through TPD experiments, DEA is not the obvious primary mechanism by which methoxymethanol (CH$_3$OCH$_2$OH) and ethylene glycol (HOCH$_2$CH$_2$OH) are formed. Evidence indicates formation of these molecules may be driven by both DEA and electron impact excitation. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J35.00009: The Widom line and noise power spectral analysis of a supercritical fluid Sungho Han, Clare Yu In a typical pressure-temperature phase diagram for a fluid, there is a first order phase transition line between the liquid and the vapor phases that terminates at a critical point. Beyond this critical point lies the supercritical regime where one can go continuously between the liquid and vapor phases. In the supercritical region, there is a line of specific heat maxima, called the Widom line, which is often regarded as an extension of the liquid-vapor coexistence line. Using molecular dynamics simulations of a Lennard-Jones fluid, we find that the noise power spectrum of the density fluctuations on the Widom line of the liquid-vapor transition can be divided into 3 frequency regions. The intermediate frequency region with 1/f noise appears as the temperature approaches the Widom temperature from above or below. Furthermore, we find that the power spectra of both the density and potential energy fluctuations at low frequency have a maximum on the Widom line, suggesting that the noise power can provide an alternative signature of the Widom line. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J35.00010: Disappearance of Widom Line for Liquid-Liquid Phase Transition with Horizontal Coexistence Line Jiayuan Luo, Limei Xu, Sergey Buldyrev, Austen Angell, Gene Stanley The study of spherically symmetric two-scale Jagla model with both repulsive and attractive ramps has been very successful in demonstrating the anomalous behavior of liquids (especially water) and its relation with respect to the existence of a liquid-liquid (LL) critical point. However, the co-existence line of Jagla model shows a positive slope, which is opposite to what has been found in the simulations of water. To more convincingly link the result of the study on Jagla model with that of water, we applied discrete molecular dynamics to Gibson and Wilding's modified Jagla model and found that by shrinking both the attractive and repulsive ramps, the slope of the coexistence line can be reduced to zero. However, at these values of the parameters, the LL critical point becomes completely unstable with respect to crystal and glass. We further studied the Widom line, defined as extreme of response functions and also continuation of the coexistence line into one phase region, and found Widom line disappeared in the case of zero slope of the coexistence line, due to the equal enthalpy of low-density liquid (LDL) and high-density liquid (HDL). [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J35.00011: Effects of pressure on transitions between disordered phases in supercooled liquid Si Alex Antonelli, Karl Garcez In the last 20 years, there has been an increasing interest in liquid-liquid transitions. Substances that exhibit negative melting slopes are considered to undergo such transitions. In this work, we study the pressure effects on the transitions between the disordered phases in supercooled liquid Si through Monte Carlo simulations and efficient methods to compute free energies. Our calculations, using a realistic interatomic potential for Si, indicate that at zero pressure the liquid-liquid phase transition, between the high density liquid and the low density liquid, occurs at a temperature 325 K below melting. We found that the liquid-liquid transition temperature decreases with increasing pressure, following the liquid-solid coexistence curve. As pressure increases, the liquid-liquid coexistence curve approaches the region where the glass transition between the low density liquid and the low density amorphous takes place. Above 5 GPa, our calculations show that the liquid-liquid transition is suppressed by the glassy dynamics of the system. We also found that above 5 GPa, the glass transition temperature is lower than that at lower pressures, suggesting that under these conditions the glass transition occurs between the high density liquid and the high density amorphous. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J35.00012: Thermodynamics of Supercooled Water Vincent Holten, Christopher Bertrand, Mikhail Anisimov, Jan Sengers We present the currently available experimental information for the thermodynamic properties of supercooled ordinary and heavy water and the possibility of modeling these thermodynamic properties on a theoretical basis. Part of the interest into the thermodynamic behavior of supercooled water is caused by an anomalous temperature dependence of the heat capacity, the compressibility and the thermal expansivity. We show that by assuming the existence of a virtual liquid--liquid critical point in supercooled water, the theory of critical phenomena can give an accurate account of the experimental thermodynamic-property data up to a pressure of 150~MPa. In addition, we show that a semi-empirical extension of the theoretical model can account for all currently available experimental data in the supercooled region, up to 400~MPa. Critical-point thermodynamics describes the available thermodynamic data on supercooled water within experimental accuracy, thus establishing a benchmark for any further developments in this area. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J35.00013: Diffusivity crossover and liquid-liquid transition in solutions of hard spheres in Jagla particles Dario Corradini, Paola Gallo, Sergey V. Buldyrev, H. Eugene Stanley We study by discrete molecular dynamics simulations the relation between the thermodynamics and the diffusive behavior in solutions of hard spheres in Jagla particles, close to their liquid-liquid critical point. For comparison, we also show the same properties in the bulk Jagla particles system. The hard spheres and the Jagla model are used as spherically symmetric potentials for small hydrophobic solutes in water. We find that the fragile to strong dynamic transition observed when studying the diffusive behavior is always coupled to the low density to high density liquid transition. Above the liquid-liquid critical pressure the diffusivity crossover happens exactly at the Widom line of the systems, where the thermodynamic response functions show maxima. Below the liquid-liquid critical pressure, the diffusivity crossover corresponds to the crossing of the limit of mechanical stability lines and it shows hysteresis when going from high to low temperatures or vice versa. These findings prove that the strong connection between dynamics and thermodynamics found for bulk Jagla particles persists in hydrophobic solutions for concentrations from low to moderate. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J35.00014: Small-to-Large Polaron Transition in Water Shahriar Shadkhoo, Robijn Bruinsma \noindent Heavier charged particles, such as protons, dissolved in water form a hydration shell, which partially shields the charge and significantly increases their effective mass. On the other hand, electrons do not form a hydration shells in water. Leggett's path-integral formalism can be combined with Feynman's polaron theory to construct a theory for the effective mass of a charged particle in water in terms of the charge structure factor, $S({\bf q},\omega)$ of water, or alternatively, the frequency and wavefunction dependent dielectric function of water. Measurements of $S({\bf q},\omega)$ for density fluctuations indicate that water has a soft mode with a wave number in the range of an inverse Angstrom. By combining these experiments with analytical and numerical models of the dielectric function of water, we discuss the small-to-large polaron transition in water. [Preview Abstract] |
Session J36: Focus Session: New Energy III
Sponsoring Units: DCPChair: Bruce Garrett, Pacific Northwest Research Laboratory and Anders Nilsson, SLAC
Room: 107C
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J36.00001: Tailoring Surface Chemical Properties Using Electronic Structure Theory Invited Speaker: Jens Norskov Electronic structure methods based on density functional theory have reached a level of sophistication where they can be used to describe complete catalytic reactions on transition metal surfaces. This opens the possibility that computational methods can be used to tailor surfaces with desired chemical properties. Recent progress in this direction for transition metal catalysts will be discussed. A series of concepts will be introduced to describe and understand trends in reactivity from one metal surface to the next. It is shown how these concepts can be used to identify the factors determining the catalytic activity of a given transition metal surface, and how this can form the basis for screening of a large number of metals and alloys for catalytic properties. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J36.00002: Ab initio reaction pathways for dissociative adsorption of dioxygen on Al (111) Florian Libisch, Chen Huang, Peilin Liao, Michele Pavone, Emily Carter We investigate the interaction of dioxygen with a clean aluminum (111) surface. The theoretical description of this fundamental process is challenging due to the discrete, abrupt charge transfer (CT) from the metal surface to the molecule. Indeed, experimental investigations suggest a sizeable activation barrier not accounted for by a conventional DFT description, due to exchange-correlation functional errors. We adopt a different approach, embedding a small (~12 atoms) aluminum cluster in a DFT-derived potential simulating the remainder of the Al surface. The interaction between this embedded cluster and an approaching $O_2$ molecule is treated using high-level correlated wave function methods (CASSCF, CASPT2) that allow for a correct description of the CT process involved. We map out the potential energy surface (PES) as a function of dioxygen bond length, orientation, and position. In agreement with experiment, we find an activation barrier of $\sim$500 meV, which corresponds primarily to the cost to induce CT. Additionally, the PES is consistent with oxygen abstraction as the dominant process in the case of incident perpendicular orientation, confirming the mechanism proposed to explain the surprisingly large fraction of single oxygen atoms found in STM measurements. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:39PM |
J36.00003: Alkane and CO$_{2}$ Activation by Modified Oxide Catalysts Invited Speaker: Horia Metiu I will present computational and experimental results concerning the effect of cation or anion doping on the catalytic activity of oxides. We are mainly interested in the activation of alkanes or of CO$_{2}$ and their conversion to useful products. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J36.00004: First-principles calculations of Fischer-Tropsch processes catalyzed by nitrogenase enzymes Joel Varley, Lars Grabow, Jens N{\O}rskov The nitrogenase enzyme system of the bacteria $Azotobacter vinelandii$, which is used in nature to catalyze ammonia synthesis, has been found recently to catalyze the efficient conversion of carbon monoxide (CO) into hydrocarbons under ambient temperature and pressure [1]. These findings indicate that nitrogenase enzymes could inspire more efficient catalysts for electrochemical CO and CO$_2$ reduction to liquid fuels. The nitrogenase variants, in which vanadium substitutes the molybdenum in the active site of the enzyme, show distinct features in their reaction pathways to hydrocarbon production. To compare and contrast the catalytic properties of these nitrogenase enzymes, we perform first-principles calculations to map out the reaction pathways for both nitrogen fixation and for the reduction of CO to higher-order hydrocarbons. We discuss the trends and differences between the two enzymes and detail the relevant chemical species and rate-limiting steps involved in the reactions. By utilizing this information, we predict the electrochemical conditions necessary for the catalytic reduction of CO into fuels by the nitrogenase active sites, analogous to a Fischer-Tropsch process requiring less extreme conditions. \\[4pt] [1] Y. Hu, C.C. Lee, M.W. Ribbe, Science {\bf 333}, 753 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J36.00005: Low-temperature dehydrogenation catalysis of isophorone (C$_9$H$_{14}$O) on Pd(111) Xinguo Ren, Wei Liu, Aditya Savara, Wiebke Ludwig, Karl-Heinz Dostert, Swetlana Schauermann, Alexandre Tkatchenko, Hans-Joachim Freund, Matthias Scheffler Selective hydrogenation and dehydrogenation of hydrocarbons is one of the most extensively used processes in catalysis. In praticular, it is important to design catalysis which can occur at or below room temperature. Here, we investigate the adsorption and dehydrogenation of a model organic/metal catalyst: isophorone (C$_9$H$_{14}$O) on the Pd(111) surface. Density-functional theory (DFT) calculations including van der Waals (vdW) interactions are carried out to elucidate the adsorbate geometry and reaction dynamics. The vdW interactions dramatically modify the potential-energy surface and enhance the binding energy by about 1 eV. The combination of our theoretical results with the analysis of infrared (IR) spectra and temperature-programmed desorption (TPD) experiments leads us to propose a dehydrogenation pathway from the weakly chemisorbed reactant (C$_9$H$_{14}$O) to the strongly chemisorbed product (C$_9$H$_{10}$O), which occurs after four C-H bond cleavages. The obtained low dehydrogenation temperature (130-150 K) from PBE+vdW barriers, including zero-point energy vibrations, is consistent with the TPD analysis. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J36.00006: Interfacial binding in catalytic and photovoltaic materials, and the energetics of elementary surface processes during catalytic fuels production and utilization Invited Speaker: Charles T. Campbell The fundamental understanding of reactivity trends in organic and inorganic chemistry has led to spectacular scientific advances over the past 50 years in synthetic chemistry. At the basis of this understanding is knowledge of the strengths of the relevant chemical bonds being broken and formed. In this regard, surface chemistry is several decades behind. Yet surface chemical reactivity dictates our choices of materials for energy technology, including catalysts for clean fuels production and utilization, fuel cell electrodes, photocatalysts and photovoltaics. This past decade has seen important advances in our ability to measure surface chemical bond energies and to use them to make predictions of relevance to energy technology. We will review those advances here, with a focus on: (1) the interfacial bonding strength between the metal and the support material in metal nanoparticle catalysts, with emphasis on the roles of particle size and the support in catalyst reactivity and stability, (2) the interfacial reactions that occur between the metal electrode and the semiconducting polymer in organic photovoltaic (OPV) devices, and the heats of those reactions, and (3) the adsorption energies of small molecules on Pt(111), with emphasis on intermediates in catalytic steam reforming, oxidation and dehydrogenation, which reveal a strong correlation between the strength with which adsorbates bond to the Pt(111) surface and their bond energy to the H atom in gas-phase molecules. These measurements provide important benchmarks for comparisons with new computational methods designed to improved energy accuracy beyond standard periodic DFT. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J36.00007: Ab initio modeling of EXAFS spectra of nanoparticles Samuel Chill, Aaron Yevik, Anatoly Frenkel, Graeme Henkelman The structure of nanoparticles in the 1-3 nm range is often investigated using extended x-ray adsorption fine structure (EXAFS). Structural information for nanoparticles is typically determined from the fits to EXAFS data using the bulk structure for theoretical calculations. The average coordination numbers, interatomic distances, and their mean squared disorder can be found. The applicability of this procedure is less obvious when the particles are strongly disordered, i.e., when there are significant structural differences between the nanoparticles and the corresponding bulk structure. We use molecular dynamics (MD) simulations of 2 nm nanoparticles to construct EXAFS spectra ab initio, with the atom-by-atom approach, averaged over the time of the MD run. We obtain that the analysis of this data done by conventional procedures reveals significant differences between the actual (time- and configuration- average) structure of the simulated nanoparticles and what was determined from data analysis using a bulk reference. We demonstrate that EXAFS spectra calculated for a variety of theoretical models of nanoparticles can be directly compared to the experiment and thus used to determine the best-fit structure. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J36.00008: Predicting the Catalytic Reactions using Random Phase Approximation J. Yan, T. Olsen, J.J. Mortensen, K.W. Jacobsen, K.S. Thygesen, F. Abild-Pedersen, J.K. Norskov Density functional theory has became the workhorse for simulations of catalytic reactions and computational design of novel catalysis. The generally applied semi-local exchange-correlation functionals have successfully predicted catalytic reaction trends over a variety of surfaces. However, in order to achieve quantitative predictions of reaction rates for molecule-surface systems, in particular where there is weak Van der Waals interactions or strong correlation, it is of vital importance to include non-local correlation effects. The use of random phase approximation (RPA) to construct the correlation energy, combined with the exact, self-interaction free exchange energy, offers a non-empirical way for accurately describe the adsorption energies [1] and dispersion forces [2]. We have recently implemented RPA in the GPAW code [3-4], an electronic structure package using projector augmented wave method and real space grids. In this talk I will present our initial results comparing RPA and generalized gradient functionals for the activation energies and reaction energies for transition metal or metal oxide surfaces. \\[4pt] [1] L. Schimka, et.al, Nature Mat. 9, 741 (2010) [2] T. Olsen, et.al, Phys. Rev. Lett. 107, 156401 (2011) [3] J. Yan, et.al, Phys. Rev. B 83, 245122 (2011). [4] J. Yan, et.al, Phys. Rev. Lett. 106, 146803 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J36.00009: In situ XAS of Pt monolayer model fuel cell catalysts: balance of na-nostructure and bimetallic interactions Daniel Friebel, Venkat Viswanathan, Ask Larsen, Daniel J. Miller, Hirohito Ogasawara, Toyli Anniyev, Christopher P. O'Grady, Jens N{\O}rskov, Anders Nilsson The mechanism of the electrochemical oxygen reduction reaction (ORR) has been well understood based on DFT calculations, but there has been a lack of supporting experimental data, due to the difficulties of probing the electrocatalyst surface in situ. Our new approach using Pt monolayer model catalysts provides true surface sensitivity for - originally bulk sensitive - x-ray absorption spectroscopy (XAS) and, owing to the high resolution of the Bragg analyzer at SSRL beamline 6-2, allows for in situ detection of chemisorbed O and OH, whose stability can be used as a descriptor in predicting the activity of new ORR catalyst materials. Our ability to control the growth mode in the Pt/Rh(111) model system allows us to generate Pt nanostructures with highly different O affinities from identical starting materials. [Preview Abstract] |
Session J37: SPS Undergraduate Research IV
Sponsoring Units: SPSChair: Peter Muhoro, American Physical Society
Room: 108
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J37.00001: Life as a Hiltern Courtney Lemon The Mather Policy Intern Program, conducted through the Society of Physics Students, is an innovative internship encouraging physics students to get involved in science policy. Funded by the John and Jane Mather Foundation for Science and the Arts and the American Institute of Physics, Mather Interns spend a summer at the Capitol, working as congressional interns for a representative or committee. As the first female student inducted into the Mather Policy Intern program, the author presents Life as a Hilltern, detailing her summer working with Representative Rush Holt, the only physicist currently serving in the U.S. House of Representatives. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J37.00002: Zinc Oxide Coated Carbon Nanotubes for Energy Harvesting Applications Austin Mohney, David Stollberg Small scale electrical devices depend on bulky batteries that require recharging or replacement. In biomedical monitoring, where sensors could be implanted inside the body, maintenance of batteries presents a problem. It would be beneficial if small scale devices could generate their own power and alleviate their dependence on batteries. Piezoelectric nanogenerators have proven themselves as a viable means for ambient energy harvesting. Piezoelectric materials, such as zinc oxide (ZnO), produce a voltage difference when subjected to mechanical strain. Manipulation of this voltage can allow for the storage of energy to power small scale devices. The objective of this research is to manufacture a piezo-generator that can transduce mechanical vibrations into electrical energy. Carbon nanotubes, selected for their strong, flexible, and conductive properties, are used as a structural backbone for a ZnO piezoelectric coating and a Ag electrode coating. A Schottky diode interface is used to rectify the current output of the device. The devices yielded an average current output of .79 microAmps. SEM imagining was used to characterize the fabrication process. A Keithley 2700 digital multimeter was used to characterize the current output of the devices. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J37.00003: Influence of carbon nanotubes diameter on thermal conductivity of polyester based nanocomposites Romeo de Coss-Mart\'Inez, Graciela In\'es Qui\~nones-Weiss, Jazm\'In Anely Doporto-Valladares, Caridad Guadalupe Vales-Pinz\'on, Miguel \'Angel Zambrano-Arjona, Jos\'e \'Angel Mendez-Gamboa, Rub\'en Arturo Medina-Esquivel, Juan Jos\'e Alvarado-Gil Carbon nanotubes (CNTs) are considered good candidates to improve the physical properties of polymeric materials. It is well known that CNTs have one of the highest thermal conductivities in nature. However, it has been found that thermal resistance between polymer matrix and CNTs, at nanometric scale, could imply a disadvantage to obtain high thermal conductivity nanocomposites. In this work, the effect of CNTs diameter on the effective thermal conductivity of composites based on polyester resin is studied. In particular, the effects of CNT's diameter and volume fraction are analyzed. The thermal conductivity of the nanocomposites is obtained determining the thermal diffusivity by photothermal radiometry and from the values of their specific heat capacity. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J37.00004: Electrical characterization of field effect transistors made from C-nanotubes covered with poly(3-hexylthiophene) Luis Pomales, Nicholas Pinto, Mitchell Lerner, Alan Johnson Jr. Organic transistors were fabricated using a thin film of regio-regular poly(3-hexylthiophene-2,5-diyl) spun from a 0.5 wt{\%} solution in CHCl$_{3}$ on doped Si/SiO$_{2}$ substrates with and without CNT's. The performance of devices with percolating networks of CVD grown single-wall carbon nanotubes (SWNT's) between the source (S) and drain (D) electrodes and without SWNT's are compared. Nanotubes are used as a way to shorten the effective mean distance between the S/D terminals while retaining the wide spacing macroscopic shadow mask technique for S/D fabrication. We found that devices made with SWNT's do not exhibit S/D current saturation but have a charge mobility of 1.2x10$^{-2}$ cm$^{2}$/V-s and an on/off ratio of 9, while the device without SWNT's show clear saturation with a charge mobility of 8.6x10$^{-4}$ cm$^{2}$/V-s and an on/off ratio of 870. The devices made with nanotubes possess a larger off state current although they show a $\sim $14X increase in the mobility which can thus be increased without using sophisticated lift-off techniques to shorten the S/D distance. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J37.00005: Physical Manipulation of the Na/K Pump David Eve, Clausell Mathis, Wei Chen It has been demonstrated that a well-designed oscillating electric field can synchronize and modulate the functions of the Na+/K+ ATPase pump, a ubiquitous active transporter in the cell membrane. Current work aims to improve this technique in order to synchronize the individual steps in the pumping cycle, allowing all of the individual pumps to run simultaneously. Using frog skeletal muscle fibers and the double Vaseline-gap voltage clamp technique, we are working to design wave pulses, which will isolate the voltage dependent steps of the active ion transport. This will improve the signal-to-noise ratio and provide insight into the protein conformation changes that occur during active transport. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J37.00006: Characterization of the GaN-MgO Transistor Interface: More Power and Efficiency Jose Sanchez, Divine Kumah, Fred Walker In this age of high-energy consumption, the development of more efficient and more reliable devices is indispensable. Gallium nitride (GaN)-based devices are an option in achieving this goal. GaN's wide bandgap of 3.4 eV allows the device to handle large amount of current before leakage makes its energy consumption inefficient. The characteristics of GaN, in conjunction with those of Magnesium oxide (MgO), would allow for improvement of different electronic applications such as mobile phone communication technology. In this work, the fabrication of the GaN/MgO device was done by Molecular Beam Epitaxy. This device was grown under a variety of parameters where the growth temperature, growth chamber pressure, and the rate of material deposition were changed. To determine the optimal growth parameters, current-voltage and capacitance-voltage measurements were conducted on to evaluate the effects of these growth conditions. Atomic Force Microscopy was also used in characterizing the crystallinity and morphology of the samples. A conclusion of the research is that by improving the roughness of the substrate, the breakdown voltage of the MgO layer and the overall performance of the device can be improve, yielding a device with very low energy loss in the current transmission process. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J37.00007: Generating more electricity from water Daniel Moser, Guoping Zhang The Kelvin water drop generator is an ingenious method of using naturally occurring water to induce charge separation in water droplets by allowing them to fall through cross-wired conductors. While being a simple and clean source of energy, the generator has never been considered practical due to its low current output. The goal of this experiment was to discover what conditions were required to obtain maximum voltage. Manipulating where a water stream breaks into droplets inside the conductor, we were able to take advantage of the conductor's geometry and electric field to induce the largest charge separation inside the water droplets. We also explored methods on how to reduce charge cancellation during the operation of the apparatus. [Preview Abstract] |
Session J39: Exciton, Polariton, and Electron Dynamics in two-dimensional semiconductor structures
Sponsoring Units: DCMPChair: Mark Sherwin, University of California, Santa Barbara
Room: 109B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J39.00001: Valence-band Mixing in Strain-based Traps for Indirect Excitons in Coupled Quantum Wells Jeffrey Wuenschell, Nicholas Sinclair, David Snoke, Loren Pfeiffer, Ken West Excitons in a coupled quantum well system under an applied bias are spatially separated, with electrons and holes in opposite wells. The pair can interact via the Coulomb interaction, but recombination is suppressed. Indirect excitons exhibit a long (up to tens of microseconds), tunable lifetime and are of interest to those studying high density excitonic phase transitions in equilibrium. Recent experiments have revealed sharply temperature- and density-dependent transitions in the luminescence pattern of strain-trapped indirect excitons [1]. Localized strain shifts the valence and conduction bands, creating a trap for the case of hydrostatic stretching [2]. Two factors differentiate this technique from electrostatic traps, the splitting between the light- and heavy-hole exciton states varies with stress and the presence of shear strain modifies the symmetry of the ground state. Due to strong dependence of the radiative lifetime on the tunneling rate, the indirect heavy- and light-hole excitons have drastically different recombination rates; meaning that mixing sharply alters the luminescence pattern. We will discuss the modeling of the structure of the single-particle potential and the role that interband mixing plays in the appearance of high density phase transitions. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J39.00002: Dark excitons and Bose-Einstein condensation in strain-trapped excitons in coupled quantum wells Nicholas Sinclair, Jeff Wuenschell, Kenneth West, Loren Pfeiffer, David Snoke It is possible to make excitons in coupled quantum wells with very long lifetime compared to their thermalization, allowing us to study equilibrium behavior. Using a localized stress to create traps for excitons in coupled quantum wells, we have demonstrated that at low temperature, high density, and large stress, the spatial pattern of photoluminescence (PL) from interwell excitons transitions to one with a dimmed center. This pattern emerges despite the center remaining the region of highest exciton density. This darkening is related to a strain-induced interaction between the light hole and heavy hole states. However, while this explanation provides a mechanism to explain many of the features, a few important predictions of this theory are not borne out by experiments. An alternate explanation is possible, utilizing an increasing population of dark (J=2) excitons and a separation of the dark and bright species. It has been proposed that a Bose-Einstein Condensate in this system would occur in a dark state, and this transition is consistent with the onset criteria of the pattern formation and explains how a slight bright/dark energy difference could lead to spatial separation of the species. Experiments employing a magnetic field to turn `dark' excitons slightly `bright' should allow the disambiguation of the role of dark excitons in this system. I will review this pattern formation and discuss data from experiments employing a magnetic field. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J39.00003: Trapping Long-Lifetime Superfluid Polaritons Using a Laser-Generated Barrier B. Nelsen, M. Steger, G. Liu, D. Snoke, K. West, L. Pfeiffer We present results with new microcavity structures that have polaritons that live as long as 100 ps, about two orders of magnitude longer than in previous structures. We show that an exciton reservoir can be used to create a potential barrier for a superfluid polariton gas. In these experiments, we use a non-resonant laser to create a superfluid polariton gas at the same location as a population of excitons. The excitonic component of the polariton can coherently scatter with the excitons. Since the excitons have mass four orders of magnitude larger than the polaritons, they form essentially a static barrier for the polaritons. By resolving the polaritons both in real space and momentum space, we determine that the polaritons roll down the potential barrier created by the excitons. These results are consistent with numerical solutions of the Gross-Pitaevskii equation for the measured parameters of this system. The potential to use the exciton reservoir as a barrier/trap will allow exciting new ways to study macroscopic coherence phenomena such as Josephson oscillations and superfluid vortices. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J39.00004: All Optical Switching Using Exciton-Polariton Renormalization in Microcavities M. Steger, B. Nelsen, C. Gautham, D. Snoke, L. Pfeiffer, K. West We report on a method of all-optical switching based on resonantly pumped exciton-polariton microcavities. Depending on geometry, each optical transistor could be used to make an AND or an AND NOT gate. Optical switching in these samples may be limited by the lifetime of the polaritons, allowing for near THz frequency switching. These gates could be used for fast signal processing or optical computing. Where optical computing calls for signals and gates to be at the same wavelength, we can use a gate beam at an angle, since the dispersion relation of polaritons allows for the absorption of a gate higher in energy than the k=0 state. High exciton and free carrier densities lead to a renormalization of the lower polariton (LP) and increase its energy. If the k=0 LP becomes resonant with the signal beam, then the gate may be used to turn on transmission (or consequently turn off reflectivity) of that signal. We will present achievable on/off ratios and switching speeds as well as discuss modulating an intense signal with a weaker gate. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J39.00005: Two-Dimensional Mott-Hubbard Electrons in an Artificial Honeycomb Lattice V. Pellegrini, M. Gibertini, B. Karmakar, S. Yuan, M. Polini, G. Vignale, M. Katsnelson, A. Pinczuk, L.N. Pfeiffer, K.W. West, A. Singha Artificial crystal lattices can be used to tune repulsive Coulomb interactions between electrons. We trapped electrons, confined as a two-dimensional gas in a gallium arsenide quantum well, in a nanofabricated lattice with honeycomb geometry [1,2]. In our most recent studies [3] we probed the excitation spectrum of electrons in the honeycomb lattice with lattice spacing ranging down to 90nm in a magnetic field identifying collective modes that emerged from the Coulomb interaction in the artificial lattice, as predicted by the Mott-Hubbard model. These observations allow us to determine the Hubbard gap and suggest the existence of a Coulomb-driven ground state [3]. The proposed research promises to further expand current realms of study of quantum simulators. While the experiments are challenging, studies of electrons confined to artificial lattices should provide key perspectives on strong electron correlation in condensed matter science.\\[4pt] [1] M. Gibertini et al. Phys. Rev. B RC 79, 241406 (2009)\\[0pt] [2] G. De Simoni et al. Appl. Phys. Lett. 97, 132113 (2010)\\[0pt] [3] A. Singha et al. Science 332, 1176 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J39.00006: Cooperative carrier dynamics in InGaAs/GaAs quantum wells in high magnetic fields Ji-Hee Kim, Tim Noe, Yongrui Wang, Aleksander K. Wojcik, Stephen A. McGill, Alexey A. Belyanin, Junichiro Kono Ultrafast spectroscopy in strong magnetic fields provides a powerful means for studying quantum coherence in many-body systems. A high magnetic field leads to tunable energy quantization, which in turn results in a substantial enhancement of densities of states and suppression of scattering. Here, we study superfluorescence (SF), i.e., cooperative spontaneous emission of ultradense electron-hole plasmas in InGaAs quantum wells in a perpendicular magnetic field up to 17.5 T. We observe SF both through time-resolved photoluminescence and differential transmission (DT) measurements. We create an ultradense electron-hole plasma with an intense femtosecond laser pulse, and after a certain delay, an ultrashort burst of coherent radiation emerges. At the same time, an abrupt decrease in population from full inversion to zero was observed through DT measurements. Furthermore, the DT signals strongly depended on the probe energy, showing an anomalous negative DT signal (i.e., induced absorption) under certain circumstances. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J39.00007: High-Order Sideband Generation in Quantum Wells Driven by Intense THz Radiation: Electron-Hole Recollisions Benjamin Zaks, Ren-Bao Liu, Mark Sherwin Non-linear mixing of optical beams with intense terahertz beams has been observed in semiconductor heterostructures both as changes to the absorption spectrum and as sideband generation. Sidebands are generated when a material has a non-linear dielectric response that mixes the electric field of the two beams and produces radiation at the optical frequency plus or minus multiples of the THz frequency. Perturbatively generated sidebands have been observed up to fourth order when the optical beam resonantly excites excitons in GaAs quantum wells. We present here our observation of high-order sideband generation (HSG) from excitons in InGaAs QWs. Sidebands of up to 18$^{th}$ order are observed. From the THz intensity dependence and THz polarization dependence (linear-circular) of the sideband generation, we show that the observed phenomenon is non-perturbative. We note that the exciton system driven by an intense THz field is analogous to an atomic system driven by intense optical fields, as is the case for high-order harmonic generation (HHG). The recollision model used to describe HHG in atoms is then extended to describe HSG in excitons. Experimental results indicate that the recollision model is a valid description of the nature of HSG in excitons. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J39.00008: Intersubband electrons coupled to zone-folded coherent acoustic phonons in a GaN/AlN superlattice Cynthia Aku-Leh, Klaus Reimann, Michael Woerner, Eva Monroy, Daniel Hofstetter We present spectrally-resolved resonant pump-probe measurements on a strongly polar GaN/AlN superlattice. The transmitted and reference probe spectra are detected at the same read out rate. Analysis of the normalized probe spectra reveals electronic contributions as well as the first three folded longitudinal acoustic phonons, matching calculated frequency values predicted by the elastic continuum model. The observed modes couple strongly to intersubband electrons and modulate the spectral width and position of the intersuband absorption. We conclude that this electron-phonon coupling takes place via piezoelectric effects and a phonon-induced modulation of the subband effective mass. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J39.00009: Intersubband polariton bleaching observed in ultrafast mid-infrared spectroscopy Simone Zanotto, Riccardo Degl'Innocenti, Ji-Hua Xu, Giorgio Biasiol, Lucia Sorba, Alessandro Tredicucci Strong coupling between intersubband transition in quantum wells and microcavity field leads to the concept of intersubband (ISB) polaritons. Their linear properties have been explored for several years, mainly focusing on optical spectra and electroluminescence. Among the implemented microcavity geometries, the one based on a photonic crystal (Zanotto et. al., Appl. Phys. Lett. 2010) is paricularly interesting as it allows simultaneous access to both polariton branches at anticrossing. Here we report on the nonlinear response of ISB polaritons, investigating their bleaching, an effect already studied in the excitonic framework. When ISB polaritons are probed by an intense mid-infrared laser pulse, the typical double-peaked transmission spectrum is converted in a single-peaked one. By tuning the intensity of the femtosecond pulse that covers both polariton branches, the whole range between weak and strong coupling regimes has been swept. This study reveals that there is a threshold for pumping above which polariton states are destroyed, and restricts the pump intensity range that can be employed in an optically-pumped ISB polariton laser. Moreover, as a consequence of nonlinear optical properties, saturable absorbers and optically bistable devices could be implemented. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J39.00010: Pump-probe experiment in LaMnO3/SrTiO3 superlattices and thin film Purevdorj Munkhbaatar, B. Tsermaa, J.S. Kim, W.S. Choi, S.S.A. Seo, H.N. Lee, K. Myung-Whun We present the time dependent transmittance of LaMnO3 thin film and [(LaMnO3)n/(SrTiO3)8]20 (n=2 and 8) superlattices grown on SrTiO3 substrate. We used the laser pulse pump-probe technique. We observed two phonon oscillation in the LaMnO3 film at 8 THz and at 15 THz. In the superlattices, 8 THz mode seemed obliterated. The phonon oscillation damping time constant was also different. In LaMnO3 thin film, we could observe the oscillation until $\sim$ 1.5 ps. In the superlattices, the damping time constant was smaller: $\sim$ 0.7 ps for n=8 superlattice and $\sim$ 0.3 ps for n=2 superlattice. We will discuss number of phonon mode and the damping time constant in terms of the sample geometry and the electronic struncture. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J39.00011: Excitonics of Hybrid Nanostructures Arranged with Mixed Dimensionality Pedro Ludwig Hernandez Martinez, Alexander O. Govorov, Hilmi Volkan Demir We present a complete study of the F\"{o}rster-type nonradiative energy transfer in hybrid nanostructures composed of nanoparticles, nanowires and quantum wells, and investigate the effects of quantum confinement in different dimensions. We systematically consider all possible combinations in terms of dimensionality for exciton-exciton interactions in these hybrid architectures, and analyze the resulting energy transfer rates for item-to-item excitonic coupling as a function of dimensionality. We derive a full set of analytical expressions and show that the exciton transfer strongly depends on the dimensionality and geometry of the hybrid system. Arrangements of such nanostructures with mixed dimensionality ranging from the low dimensionality to the high offer important high-efficiency applications in photovoltaics [1,2], while in the reciprocal case (from the high dimensionality to the low) in light generation [3] and LEDs [4]. [1] J. Sambur, et al., Science 330, 63 (2010). [2] M. D. Kelzenberg, et al., Nature Materials 9, 239--244 (2010). [3] R. Yan, et al., Nature Photonics 3, 569-576 (2009). [4] H.V. Demir, et al., Nano Today (2011) doi:10.1016/j.nantod.2011.10.006 [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J39.00012: Many-particle effects in the photoluminescent response of silicon quantum-dot solids Joseph B. Miller, Austin R. Van Sickle, Rebecca R. Anthony, Uwe R. Korthshagen, Daniel M. Kroll, Erik K. Hobbie Monodisperse colloidal suspensions of ligand-coated silicon nanocrystals (SiNCs), synthesized through a nonthermal low-pressure plasma reaction, are prepared through density-gradient ultracentrifugation in mixed organic solvents. The SiNC fractions are then self-assembled into close-packed quantum-dot ``solids'' and clusters, and photoluminescent properties of the resulting ordered ensembles are characterized through optical spectroscopy. We find striking manifestations of particle-particle interactions in the measured optical response, and we model these effects using Monte Carlo simulations of the photobleaching kinetics in dense SiNC packings. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J39.00013: Spectral dependence of the Aharonov Bohm effect in the magneto-photoluminescence of layered ZnTe-ZnSe structures Bidisha Roy, Haojie Ji, Siddharth Dhomkar, Le Peng, Richard Moug, Uttam Manna, Maria Tamargo, Fred Cadieu, Igor Kuskovsky Aharonov-Bohm (AB) oscillations in the mangeto-photoluminescence (PL) intensity of multilayered ZnTe/ZnSe structures grown via migration enhanced epitaxy (MEE) using three submonolayer deposition cycles of Zn-Te-Zn sandwiched between ZnSe barriers confirmed the presence of type-II ZnTe-based QDs. These co-exist with isoelectronic centers (ICs) as evident from the PL spectra. The spectral dependence of the transition magnetic field and the magnitude of the AB oscillation in intensity are investigated. A qualitative probing of distribution in the ensemble of QDs and ICs was done. The transition magnetic field changed from a lower value at the lower energy side of the PL emission to a higher value at the higher energy side which confirmed the lateral QD size distribution. AB oscillations at spectral positions dominated by emission from ICs were also observed suggesting that the presence of QDs also affects the ICs although the magnitude of the oscillation in the AB peak decreases at such spectral positions. [Preview Abstract] |
Session J40: Multi-cellular Processes and Development
Sponsoring Units: DBIOChair: Lisa Manning, Syracuse University
Room: 156A
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J40.00001: A Quantitative Analysis of Axonal Growth and Connectivity in Cortical Neurons James White, Elise Spedden, Sawyer Bernath, David Kaplan, Timothy Atherton, Cristian Staii Developing neurons extend processes (axons and dendrites), which are led by a distally positioned growth cone. The growth cone both secretes and senses signaling molecules, that may either attract or repel nearby growing processes. While knowledge on the qualitative effects of several secreted growth factors on axon development (e.g. axon length and number of neurons developing processes) are known, a more detailed mathematical model describing the process of axonal guidance remains to be developed. Towards this end, we have collected time-lapse microscopy data of the axonal development of cortical neurons. Image analysis provides information on the rate of growth, arc length, and curvature of the processes as a function of time and the spatial positioning of the neurons. These results will be discussed in relation to theoretical studies that model axon growth in response to varying gradients of attractive forces, representative of the effect that signaling molecules may have on axon guidance. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J40.00002: Collective cell migration in a periodic potential Thomas Angelini, Jolie Breaux, Steven Zehnder The strength of cell-substrate adhesion can be modulated by the elasticity of substrate material. Thus, a cell culture surface with periodically varying elasticity is analogous to a periodic potential for cell adhesion. Rich collective dynamics emerge in systems of adatoms on crystal lattice surfaces due to the mismatch between the preferred spacing of adatoms and the periodic potential associated with the crystal lattice. Here we explore a biological analog of atoms in a periodic potential; we study the collective dynamics of cells on a substrate with periodically patterned elasticity. Preliminary results will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J40.00003: Collective Cell Migration on Deformable Substrates Adrian Pegoraro, Allen Ehrlicher, Ming Guo, Thomas Angelini, David Weitz In many biological processes, such as wound healing, tumor migration, or embryo development, cell migration is influenced by collective dynamics and coupling between cells. While cell-cell contact is one form of mechanical coupling, long-range interactions mediated by a deformable substrate lead to both spatial and temporal correlations during cell migration that extend over many cell lengths. While it is known that interactions between nearby but not contacted single cells are modified by substrate stiffness, it is not yet clear how changes to the substrate properties affect collective cell migration. This is especially important in the understanding of cancer cell migration since the mechanical properties of these cells change during disease progression; as such we expect that the influence of the substrate to change over time. To investigate this further, we study collective cell migration on deformable substrates of different stiffness to test whether changes in short range interactions between cells are correlated with changes in collective cell migration. Furthermore, we mix cells from different cancer stages and study their migration patterns to test whether correlations exist between different cell types during migration. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J40.00004: Impact of jamming on collective cell migration Kenechukwu David Nnetu, Melanie Knorr, Steve Pawlizak, Thomas Fuhs, Mareike Zink, Josef A. K\"as Multi-cellular migration plays an important role in physiological processes such as embryogenesis, cancer metastasis and tissue repair. During migration, single cells undergo cycles of extension, adhesion and retraction resulting in morphological changes. In a confluent monolayer, there are inter-cellular interactions and crowding, however, the impact of these interactions on the dynamics and elasticity of the monolayer at the multi-cellular and single cell level is not well understood. Here we study the dynamics of a confluent epithelial monolayer by simultaneously measuring cell motion at the multi-cellular and single cell level for various cell densities and tensile elasticity. At the multi-cellular level, the system exhibited spatial kinetic transitions from isotropic to anisotropic migration on long times and the velocity of the monolayer decreased with increasing cell density. Moreover, the dynamics was spatially and temporally heterogeneous. Interestingly, the dynamics was also heterogeneous in wound-healing assays and the correlation length was fitted by compressed exponential. On the single cell scale, we observed transient caging effects with increasing cage rearrangement times as the system age due to an increase in density. Also, the density dependent elastic modulus of the monolayer scaled as a weak power law. Together, these findings suggest that caging effects at the single cell level initiates a slow and heterogeneous dynamics at the multi-cellular level which is similar to the glassy dynamics of deformable colloidal systems. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J40.00005: Self-rheology of cell monolayers Romaric Vincent, Xavier Serra-Picamal, Vito Conte, Ester Anon, Xavier Trepat Collective migration of cell sheets is a central feature of fundamental biological processes including morphogenesis, tissue regeneration, and cancer invasion. The dynamics of such processes are heavily determined by the rheology of the sheet and of the constituent cells. Such material properties have been extensively measured using a broad variety of rheological techniques, but none of these techniques has probed the ultraslow time scales that are central to collective cell migration, and exceed thousands of seconds. Here we present a novel approach we call `Self rheology' that probes cell rheology using the pulses of strain rate that cells spontaneously generate. Using this approach, we show that stress and strain rate are in quadrature, thus indicating that the dominant stresses that govern collective cell migration are elastic. The monolayer's Young modulus is found to be an order of magnitude lower than the stiffness of single cells determined through active micro-rheology techniques at shorter time scales. This elastic behavior is followed by a fluidization regime at higher strains, which we interpret in terms of cell rearrangements. ``Self-rheology'' provides a new approach to study the dynamics of collective cellular processes at ultraslow time scales. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J40.00006: Collective mechanics of undifferentiated cells drives ventral furrow formation in {\sl Drosophila} embryo Ana Hocevar, Primoz Ziherl, Matteo Rauzi, Maria Leptin We propose a 2D mechanical model of ventral furrow formation in {\sl Drosophila} embryo that is based on undifferentiated epithelial cells of identical mechanical properties whose energy resides at their cortex. Depending on the relative tensions of the apical, basal, and lateral sides, the minimal-energy states of the embryo cross-section include circular and buckled furrow shapes. We discuss the possible shape transformation from a circular to an invaginated shape consistent with experimental observations, arguing that generic collective mechanics may contribute to the robustness of tissue shape changes in embryonic development. A small increase of the area of the mesoderm cells is sufficient to pin down the invagination. This agrees with experimental data which show that just before the outset of gastrulation, the apical, basal, and lateral sides of the mesoderm cells indeed are larger than in the rest of the embryo. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J40.00007: Collective Cell Mechanics in 3D Scaffolds Jolie Breaux, Steven Zehnder, Lucas Osterbur, Jennifer Lewis, Thomas Angelini Mechanical cell behavior is influential in tissue health and dynamic cellular processes such as wound healing, and angiogenesis. Traction force microscopy (TFM) is often used to measure cell generated forces while mechanical testing methods such as atomic force microscopy (AFM) are employed to determine materials properties of cells. Extant cell mechanics methods including TFM and AFM are optimal for cells cultured on flat, 2D surfaces. However, the development of new cell mechanics techniques in 3D systems is essential to elucidate the behavior of tissues. In this presentation we introduce results from live-cell time-lapse measurements of mechanical cell behavior in highly ordered 3-D scaffolds. Preliminary data will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J40.00008: Biomechanics of the endothelium substrate influences leukocyte transmigration Kimberly Stroka, Helim Aranda-Espinoza The effects of shear flow and cytokines on leukocyte transmigration are well understood. However, the effects of substrate stiffness on transmigration remain unexplored. We have developed an in vitro model that allows us to study leukocyte transmigration as a function of varying physiological substrate stiffness. Interestingly, leukocyte transmigration increased with increasing substrate stiffness below the endothelium. intercellular adhesion molecule-1 expression, stiffness, cytoskeletal arrangement, morphology, and cell-substrate adhesion could not account for the dependence of transmigration on substrate stiffness. We also explored the role of cell contraction and observed that (1) neutrophil transmigration caused large hole formation in monolayers on stiff substrates. (2) Neutrophil transmigration on soft substrates was increased by decreasing cell-cell adhesion. (3) Inhibition of myosin light chain kinase normalized the effects of substrate stiffness by reducing cell contraction on stiff substrates. These results demonstrate that neutrophil transmigration is regulated by MLCK-mediated generation of gaps at cell borders through substrate stiffness-dependent endothelial cell contraction. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J40.00009: Growth variability in a tissue governed by stress dependent growth Karen Alim, Arezki Boudaoud Cell wall mechanics lie at the heart of plant cell growth and tissue morphogenesis. Conversely, mechanical forces generated at tissue level can feedback on cellular dynamics. Differential growth of neighboring cells is one eminent origin of mechanical forces and stresses in tissues where cells adhere to each other. How can stresses arising from differential growth orchestrate large scale tissue growth? We show that cell growth coupled to the cell's main stress can reduce or increase tissue growth variability. Employing a cell-based two dimensional tissue model we investigate the dynamics of a tissue with stress depending growth dynamics. We find that the exact cell division rule strongly affects not only the tissue geometry and topology but also its growth dynamics. Our results should enable to infer underlying growth dynamics from live tissue statistics. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J40.00010: Emergent mechanical behavior in a minimal model for embryonic tissues Lisa Manning, Marcos Lanio, Jared Talbot, Eva-Maria Schoetz We develop a minimal model for the mechanical properties of embryonic tissues that contains only three parameters and yet accurately reproduces many structural and dynamical features in zebrafish embryonic tissue explants. We verify model predictions for tissue surface tensiometer experiments and fusion assays, and contrast our model with existing models that are either difficult to constrain experimentally or insufficient to explain our experimental observations. The model tracks one degree of freedom per cell and introduces several types of interactions between cells to capture intracellular degrees of freedom, such as single cell viscoelasticity, adhesion, and active force generation. A key observation is that the motion of cells past one another, which must be generated by cells actively exerting tension on contacts, is best described by a special type of structured noise (both multiplicative and colored), instead of the white noise typically used in Brownian dynamics simulations. With such a noise term we can reproduce the glassy, viscoelastic dynamics in the bulk and explain how this new type of active ``droplet'' with very short-range interactions manages to have no ``vapor pressure.'' We discuss how this well-calibrated model can be used to study morphogenesis and pattern formation in developing tissues. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J40.00011: Spatially limited growth of an epithelium Maxime Deforet, Olivier Cochet, Axel Buguin, Pascal Silberzan We present a study dealing with the growth of an epithelium on a spatially limited adhesive substrate. Adhesive patterns (typical size: 50$\mu$m to 500$\mu$m) are created by micro-fabrication techniques: A protein repellent polymeric gel homogeneously grafted on a coverslip is selectively ablated by plasma treatment through a thin layer of photoresist. The technique achieves a high resolution of patterning (around 2$\mu$m). After seeding cells (MDCK) on circular adhesive patterns, we let the monolayer grow for 30 hours after reaching the confluence. We use physical descriptors to describe migration and compaction. Two days after the confluence, we observe and characterize by confocal microscopy, the appearance of a tridimensionnal assembly of cells in the peripherical zone of the adhesive pattern (a ``rim''). Moreover using other patterns, the existence of a tissue line tension and internal pressure is investigated. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J40.00012: Information propagation and nutrient flow in \textit{Physarum polycephalum} Gabriel Amselem, Francois Peaudecerf, Karen Alim, Jacques Dumais, Anne Pringle, Michael Brenner Basal organisms such as slime mold and fungi grow as extended networks that can reach several square meters in size. Despite lacking a central coordination center, these organisms are able to globally reshape their morphology in response to local cues, such as the presence of a patch of nutrient. How are local signals integrated in these organisms, and how do they lead to an overall response? To answer this question, we focus on the flow of nutrients in the slime mold \textit{Physarum polycephalum}. This slime mold exhibits internal flow oscillations, as well as periodic contractions of its veins. Using plastic masks, we constrain network growth to simple geometries. This allows for an experimental characterization of the relationship between the contractions and the flow. We next describe the change in the overall oscillation pattern when a food source is presented locally to the slime mold, and its implication on the internal flow. Internal flows are both inferred from the contraction pattern and experimentally measured using fluorescent markers. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J40.00013: Scaling of Traction Stresses with Size of Cohesive Cell Colonies Aaron F. Mertz, Shiladitya Banerjee, Yonglu Che, M. Christina Marchetti, Valerie Horsley, Eric R. Dufresne We explore the mechanical properties of colonies of cohesive cells adherent on soft substrates. Specifically, we image the spatial distribution of traction stresses exerted by colonies of primary mouse keratinocytes on fibronectin-coated silicone gels. These cells have strong cell-cell adhesions mediated by E-cadherin. We observe that the work performed by a colony on its substrate is concentrated at the colony's periphery. The total work is strongly correlated to the geometrical size of the colony but not to number of cells. In other words, the mechanical output of a large single cell mimics that of a cohesive colony with the same overall size. We compare our findings to a recent theoretical model that treats the cohesive colony as an active gel. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J40.00014: Elasticity of adherent active cells on a compliant substrate Shiladitya Banerjee, Aaron F. Mertz, Eric R. Dufresne, M. Cristina Marchetti We present a continuum mechanical model of rigidity sensing by livings cells adhering to a compliant substrate. The cell or cell colony is modeled as an elastic active gel, adapting recently developed continuum theories of active viscoelastic fluids. The coupling to the substrate enters as a boundary condition that relates the cell's deformation field to local stress gradients. In the presence of activity, the substrate induces spatially inhomogeneous contractile stresses and deformations, with a power law dependence of the total traction forces on cell or colony size. This is in agreement with recent experiments on keratinocyte colonies adhered to fibronectin coated surfaces. In the presence of acto-myosin activity, the substrate also enhances the cell polarization, breaking the cell's front-rear symmetry. Maximal polarization is observed when the substrate stiffness matches that of the cell, in agreement with experiments on stem cells. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J40.00015: Wave transmission through cell-cell coupling Chenlu Wang, Meghan Driscoll, Sagar Chowdhury, S.K. Gupta, Wolfgang Losert We previously found that waves of high boundary curvature travel from the front to the back of individual D. discoideum cells. We investigated the behavior of curvature waves in small groups of adherent cells, in particular, we investigated the transmission of the waves through cell-cell coupling. We analyzed the motion of individual cells in short streams of varying length, which are cells that follow one other. Furthermore, we developed a technique that uses holographic optical tweezers to grip cells indirectly and push them into one another, thereby forming artificial cell-cell contacts. Using that technique, we observed the affect of waves in coupled cells. We also compared the shape dynamics of groups of cells to the shape dynamics of cells within those groups. We extended these methods to suspended cells, which exhibit different wave dynamics. [Preview Abstract] |
Session J41: Vortex Dynamics, Turbulence and Geophysical Flows
Sponsoring Units: DFDChair: Greg Voth, Wesleyan University
Room: 156B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J41.00001: Two point correlations between velocity sums and differences in turbulence Nicholas Rotile, Greg Voth, Susantha Wijesinghe In turbulent flows, the universality of small scales has been a subject of ongoing investigation. Recent work has explored the degree to which small scales are independent of large scales by measuring correlations between velocity differences over a distance r (whose variance is dominated by scales near r) and velocity sums over the same distance (whose variance is dominated by large scales). Some correlations between velocity differences and sums are required by the Navier-Stokes equations (Hosokawa, Prog. Theor. Phys. Lett., 118:169, 2007.) This talk will focus on experimental measurements of correlations between velocity sums and velocity differences in a turbulent flow between oscillating grids. We find that these correlations provide an accurate way to measure the energy dissipation rate that complements existing methods based on the third order structure functions. The correlations which are required by Navier-Stokes dynamics do not appear to violate the assumption of independence between the large and small scales, however there are other correlations in our measurements that show clear dependence of the small scales on the large scales. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J41.00002: Variance of scalar fluctuations using backwards relative dispersion in turbulent channel flows Chiranth Srinivasan, Dimitrios Papavassiliou Temperature fluctuations at a location in a turbulent flow field are brought about by the arrival of particle pairs with different scalar concentrations. Studying backwards relative dispersion can be an alternative way to describe the local variance in scalar fluctuation. This work uses a numerical approach that couples a direct numerical simulation with the tracking of scalar markers to obtain scalar statistics in an infinitely long turbulent channel flow. Focusing on the anisotropic direction perpendicular to the channel walls, the two-particle correlation coefficients are used to determine a Lagrangian material time scale as a function of distance from the wall. Introducing a model that follows Durbin's theory [1], the variance of the temperature fluctuation is calculated by assuming that particle pairs that arrive at a particular location carry with them the mean temperature acquired at the location they were at a previous time. This earlier location is determined by utilizing the Lagrangian backwards timescale. Results obtained from this model are tested at two different Reynolds numbers (at Re$_{\tau }$ = 150 and 300) and for each \textit{Re} case at several different Prandtl numbers (from 0.1 to 1,000). \textbf{References} [1] Durbin, P.A., J. Fluid Mech., 100, 279-302, 1980 [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J41.00003: Measuring anisotropy as a function of scale in turbulence using 3D particle tracking Susantha Wijesinghe, Greg Voth We report the first full 3D experimental measurements of anisotropy as a function of scale in turbulence. From 3D particle tracks obtained with stereoscpic high speed video, we measure the Eulerian structure functions and decompose them into irreducible representation of SO(3) rotation group. This method allows us to quantify the anisotropy in different sectors, specified by $j$ and $m$ of the spherical harmonics $Y_{jm}(\theta,\phi)$, at all scales in the flow. We study a turbulent flow between two oscillating grids in an octagonal tank filled with $1100~l$ of water with $R_{\lambda}=265$. An image compression system processes high-speed video from four cameras in real-time allowing us to acquire huge data sets required for full 3D measurement of anisotropy as a function of scale. Careful selection of a sample of measurements with isotropic orientations is necessary to ensure that anisotropy of the measurement system does not affect the measured anisotropy of the flow. Increasing $j$ sectors show faster decay of anisotropy as scale decreases, consistent with the idea that the small scales should become isotropic at very high Reynolds number. However, conditioning the measured anisotropy on the instantaneous velocity reveals that characterization of anisotropy in an inhom [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J41.00004: Rotation rate of rods in turbulent flow Shima Parsa, Greg Voth We present the first time resolved experimental measurements of the motion of small rod-like particles in turbulent flow. The orientation and position of rods are measured using Lagrangian particle tracking with images from multiple high speed cameras in a flow between two oscillating grids. We work at low particle density so rod-rod interaction can be ignored. The probability distribution of the rotation rate of the rods has extended tails indicating the presence of rare events with large rotation rate. Rods rotation rate is determined by the velocity gradients of the flow, so measurements of the rotation rate provide indirect access to statistics of the velocity gradient of the flow as well as the energy dissipation rate. However, tracer rods preferentially sample the flow since their orientation becomes correlated with the local axes of the velocity gradient tensor. The result is that the typical rotation rate of rods is much smaller than it would be if they were randomly oriented. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J41.00005: Statistics of Macroturbulence from Flow Equations Brad Marston, Thomas Iadecola, Wanming Qi Probability distribution functions of stochastically-driven and frictionally-damped fluids are governed by a linear framework that resembles quantum many-body theory. Besides the Fokker-Planck approach, there is a closely related Hopf functional method\footnote{Ookie Ma and J. B. Marston, J. Stat. Phys. Th. Exp. P10007 (2005).}; in both formalisms, zero modes of linear operators describe the stationary non-equilibrium statistics. To access the statistics, we generalize the flow equation approach\footnote{F. Wegner, Ann. Phys. {\bf 3}, 77 (1994).} (also known as the method of continuous unitary transformations\footnote{S. D. Glazek and K. G. Wilson, Phys. Rev. D {\bf 48}, 5863 (1993); Phys. Rev. D {\bf 49}, 4214 (1994).}) to find the zero mode. We test the approach using a prototypical model of geophysical and astrophysical flows on a rotating sphere that spontaneously organizes into a coherent jet. Good agreement is found with low-order equal-time statistics accumulated by direct numerical simulation, the traditional method. Different choices for the generators of the continuous transformations, and for closure approximations of the operator algebra, are discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J41.00006: Motion of a Thread in Compressible Turbulence Rory Cerbus, Walter Goldburg Particles that float on a turbulent tank of water form a system that is compressible in the two dimensions on which they move. Here we study, with an overhead camera, the snake-like motion of a 10 $\mu$m thread that floats on the surface. The thread, of length much greater than the integral scale $L_I$ of the underlying turbulence, cannot respond to the small-scale turbulent motions at the surface; its Young's modulus is too large. As a result, the mean curvature of the thread is of the order $1/L_I$. Measured properties include velocity structure functions of the thread $S_{n}(r)$ (including the third moment), the local curvature along the thread (a random variable), and ``Richardson diffusion'' of pairs of points along the thread separated by distances $r$. Supported by NSF Grant DMR 0604477 and the Okinawa Institute of Science Technology. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J41.00007: Statistical Equilibria of Turbulence on Surfaces of Different Symmetry Wanming Qi, Brad Marston We test the validity of statistical descriptions of freely decaying 2D turbulence by performing direct numerical simulations (DNS) of the Euler equation with hyperviscosity on a square torus and on a sphere. DNS shows, at long times, a dipolar coherent structure in the vorticity field on the torus but a quadrapole on the sphere\footnote{J. Y-K. Cho and L. Polvani, Phys. Fluids {\bf 8}, 1531 (1996).}. A truncated Miller-Robert-Sommeria theory\footnote{A. J. Majda and X. Wang, \emph{Nonlinear Dynamics and Statistical Theories for Basic Geophysical Flows} (Cambridge University Press, 2006).} can explain the difference. The theory conserves up to the second-order Casimir, while also respecting conservation laws that reflect the symmetry of the domain. We further show that it is equivalent to the phenomenological minimum-enstrophy principle by generalizing the work by Naso et al.\footnote{A. Naso, P. H. Chavanis, and B. Dubrulle, Eur. Phys. J. B {\bf 77}, 284 (2010).} to the sphere. To explain finer structures of the coherent states seen in DNS, especially the phenomenon of confinement, we investigate the perturbative inclusion of the higher Casimir constraints. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J41.00008: Design and Evolution of Shaped Vortices Dustin Kleckner, William T.M. Irvine We present a novel method for generating vortex lines of arbitrary shapes. We then image their dynamics using a high speed scanning technique which provides three-dimensional information at up to 500 volumes per second. We create a variety of configurations and study the effect of geometry on their evolution. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J41.00009: Insights into vortex merger using the core growth model Fangxu Jing, Eva Kanso, Paul Newton We revisit the two vortex merger problem (both symmetric and asymmetric) for the Navier-Stokes equations using the core growth model for vorticity evolution coupled with the passive particle field and an appropriately chosen time-dependent rotating reference frame. Using the combined tools of analyzing the topology of the streamline patterns along with careful tracking of passive fields, we highlight the key features of the stages of evolution of vortex merger, pinpointing deficiencies in the low-dimensional model with respect to similar experimental/numerical studies. The model, however, reveals a far richer and delicate sequence of topological bifurcations than has previously been discussed in the literature for this problem, and at the same time points the way towards a method of improving the model. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J41.00010: Entrainment of solid particles by energetic flow events Manousos Valyrakis The focus of this research study is to investigate the utility and applicability of a recently proposed criterion for the prediction of incipient entrainment of sediment particles. Recently introduced theoretical frameworks and stochastic approaches are presented. At near incipient flow conditions the magnitude of energetic flow events follows a power law distribution, over a wide range of frequencies, similar to many other geophysical phenomena. This implies that highly energetic flow structures, which have a good potential of impinging on an exposed particle and displacing it downstream, occur less frequently. This is in agreement with the intermittent and episodic character of particle entrainment observed from mobile particle flume experiments at low flow stages. Further, analysis of synchronous time series of particle entrainment and local instantaneous flow upstream of it, allows for extraction and characterization of the scales and magnitudes that are relevant to the displacement of individual particles. In addition to having a sound theoretical basis, the modeling approach is shown to perform well in accurately defining the condition of incipient motion and various levels of probability of particle entrainment. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J41.00011: Modeling and dynamics of sand bed vortex ripples Justin Kao, Abigail Koss, Taylor Perron Vortex ripples arise through the instability of a flat sand bed under oscillatory water flow, for example due to wave action at a beach or continental shelf. Fully developed vortex ripples display complex interactions through the mutual influence of fluid flow and bed topography on each other, via sediment transport. We discuss a mechanistic model of ripple dynamics in which the hydrodynamic influence is linearized, and show that this reduced model nonetheless captures many of the ripple dynamics observed in experiments. Cross-sectional profiles of experimentally generated ripples constrain the modeled sediment flux and provide support for our approximations. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J41.00012: The statistics of wind driven ocean currents Yosef Yosef, Golan Bel Ocean currents play an important role is the climate system, yet the properties and origin of their statistics is not fully understood. Using the Ekman layer model we show that the statistical properties of the depth integrated surface currents are associated with the temporal correlations of the wind driving the surface currents---when the temporal correlations of the wind are long the probability distribution of the current magnitude is proportional to that of the wind stress. When the temporal correlations of the wind is short the current approaches zero where each component of the current follows a Gaussian distribution such the current magnitude follows the Rayleigh distribution. Using two idealized cases we show that in between these two limits the second (and higher) moment of the current magnitude reaches a maximal value. The results are validated using an oceanic general circulation model. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J41.00013: Modulation of Atlantic tropical cyclones by El Nino - Southern Oscillation Constantin Andronache North Atlantic tropical cyclones (TC) usually form in the northern hemisphere summer and fall with a maximum of activity in September. El Nino--Southern Oscillation (ENSO) has been shown to impact seasonal levels of Atlantic basin TC activity. ENSO is the strongest year to year climate fluctuation on Earth. It originates in the tropical Pacific through coupled ocean-atmosphere interactions mediated by surface wind stress and sea surface temperature (SST) variations. Understanding the effects of ENSO on the seasonal variations of TC activity has important practical consequences for seasonal forecast of hurricanes in North Atlantic. In this study we use the NOAA Extended Reconstructed Sea Surface Temperature, the TC counts in North Atlantic, and the NCEP/NCAR reanalysis data to investigate the relationship between significant ENSO events and TC activity during the hurricane season. Model calculations show that forecasted ENSO SSTA can be used as predictors of TC in North Atlantic region. Such results are illustrated in the context of current efforts to understand climate predictability relevant to North Atlantic tropical storms. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J41.00014: Fingers and Toes in Miscible Viscous Flows Radha Ramachandran, Irmgard Bischofberger, Sidney Nagel The displacement of a more viscous fluid by a less viscous one in a porous medium produces complex fingering patterns. To study this phenomenon a Hele-Shaw geometry is used in which the gap-averaged equations for flow between two parallel plates has the same form as Darcy's law for flow in porous media. Our experiments use a radial Hele-Shaw cell as well as a two dimensional porous medium of densely packed granular beads between two circular glass plates, to study viscous fingering in miscible fluids. For immiscible fluids it is known that the most-unstable wavelength for interface growth depends on surface tension, viscosity difference, velocity and plate spacing. In contrast, we find that for \textit {miscible fluids} the large-scale structure (i.e., the ratio of finger length to overall size of the pattern) is set entirely by the viscosity ratio rather than the viscosity difference of the two fluids. We further investigate the role played by other dimensionless parameters in determining the fine structure and evolution of these fingering patterns in the two geometries. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J41.00015: Thermal plumes in locally heated vertical soap films Nicolas Adami, St\'ephane Dorbolo, Herv\'e Caps A vertical soap film is maintained by injection of a soap solution from the top. The film is then locally heated. Thermal plumes may be observed to rise in the film, depending on the magnitude of the heating and injected flows. The nearly-2D nature of the system allows to visualize the motion of the plumes using an infrared camera. A model is proposed to describe the growth, emergence, and stationarity of the plumes in the film by taking into account both magnitudes of the heating $\Delta T$ and injected flow $Q$. Oscillatory behaviors of both the full-grown plumes size and direction with respect to the vertical direction may also be observed. Particular soap film thickness dynamics shows to be the origin of those phenomena. [Preview Abstract] |
Session J42: Focus Session: Stochastic Population Dynamics I - Cyclic competition and population stability
Sponsoring Units: DBIO GSNPChair: Michel Pleimling and Uwe Tauber, Virginia Polytechnic Institute and State University
Room: 156C
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J42.00001: Cyclic competition of four or more species: Results from mean field theory and stochastic simulations Invited Speaker: R.K.P. Zia Population dynamics is a venerable subject, dating back two centuries to Malthus, Verhulst, Lotka, Volterra, and many others. Nonetheless, new and interesting phenomena are continually being discovered. For example, the recent discovery of ``Survival of the Weakest'' in cyclic competition between 3 species with no spatial structure (Berr, Reichenbach, Schottenloher, and Frey, Phys. Rev. Lett. 102, 048102 (2009)) attracted considerable attention, e.g., http://www.sciencedaily.com/releases/2009/02/090213115127.htm. Considering a similar system with 4 or more species, we find a more intuitively understandable principle which appears to underpin all systems with cyclically competing species. We will present several interesting aspects of the 4 species system -- from non-linear dynamical phenomena in a deterministic mean-field approach to remarkable extinction probabilities in the stochastic evolution of a finite system. Some insights into the deterministic dynamics, gained from generalizing this system to one with any number of species with arbitrary pairwise interactions, will also be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J42.00002: Varieties of extinction scenarios when four species compete cyclically S.O. Case, C.H. Durney, M. Pleimling, R.K.P. Zia We study a stochastic system with $N$ individuals, consisting of four species competing cyclically: $A+B \longrightarrow A+A$, $\cdots$, $D+A \longrightarrow D+D$. Randomly choosing a pair and letting them react, $N$ is conserved but the fractions of each species evolve non-trivially. At late times, the system ends in a static, absorbing state $-$ typically, coexisting species $AC$ or $BD$. The master equation is shown and solved exactly for $N=4$, providing a little insight into the problem. For large $N$, we rely on simulations by Monte Carlo techniques (with a faster dynamics where a reaction occurs at every step). Generally, the results are in good agreement with predictions from mean field theory, after appropriate rescaling of Monte Carlo time. The theory fails, however, to describe extinction or predict their probabilities. Nevertheless, it can hint at many remarkable behavior associated with extinction, which we discover when studying systems with extremely disparate rates. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J42.00003: The effects of mobility on the one-dimensional four-species cyclic predator-prey model David Konrad, Michel Pleimling The dynamics of a one-dimensional lattice composed of four species cyclically dominating each other is very much dependent on the rates of mobility in the system. We realize mobility as the exchange of two particles located at two nearest neighbor sites with some species dependent rate s. Allowing for only one particle per site, the different species interact cyclically, with species dependent consumption rate k, such that $k + s \leq 1$. When varying the exchange rates, we see vastly different behavior when compared to the three-species model. The patterns of domain growth and decay still show an overall power law behavior, however the fundamental trend of domain growth does not follow the three-species case. We also look at the space-time diagrams to see precisely how the domains form, grow, and decay. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J42.00004: Boundary conflicts and cluster coarsening: Waves of life and death in the cyclic competition of four species Ahmed Roman, Michel Pleimling In the cyclic competition among four species on a two-dimensional lattice, the partner particles, which swap positions on the lattice with some probability, produce clusters with a length that grows algebraically as $t^{1/z}$ where $z$ is the dynamical exponent. Further investigation of the dynamics at the boundary of the clusters is realized by placing one partner particle pair in the upper half of the system and the other pair in the lower half. Using this technique, results about the fluctuations of the interface are obtained. We also observe wave fronts in the case of non-symmetric reaction rates where extinction of a partner particle pair takes place. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J42.00005: Discriminating the effects of spatial extent and population size in cyclic competition among species David Lamouroux, Stephan Eule, Theo Geisel, Jan Nagler Quantifying and understanding the stability and biodiversity of ecosystems is a major task in biological physics as well as in theoretical ecology. From the perspective of game theory, this is highly relevant for questions pertaining to the emergence of cooperation or the coexistence of cyclically competing species. The latter has been recently proposed as a paradigm for biodiversity and it has been shown that the mobility of individuals can support the stability of biodiversity by the formation of spirals. In this contribution, we present a population model for species under cyclic competition that extends earlier lattice models to allow the single cells to accommodate more than one individual by introducing a per cell carrying capacity. We confirm that the emergence of spirals induce a transition from an unstable to a stable regime. This transition however does not appear to be sharp and we find a broad intermediate regime that exhibits an ambiguous behavior. The separation of the two regimes by the usual scaling analysis is thus hampered. The newly introduced carrying capacity offers an alternative way of characterizing the transition. We thus overcome the original limitations by separately analyzing the effect of spatial extent and population size. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J42.00006: Stochastic extinction dynamics of HIV-1 Ira Schwartz, Eric Forgoston, Leor Weinberger We consider an HIV-1 within host model in which T cells are infected by the virus. Due to small numbers of molecules, stochastic effects play an important role in the dynamical outcomes in that two states are observed experimentally: a replication state in which the virus is active, or a dormant state leading to latency in which the virus becomes active after a delay. The two states are conjectured to be governed by the Tat gene protein transcription process, which does not possess two stable attractors. Rather, the active state is stable, while the dormant state is unstable. Therefore the dormant state can only be achieved through the dynamics of stochastic fluctuations in which noise organizes a path to dormancy. Here we use optimal path theory applied to a Tat gene stochastic model to show how random fluctuations generate the dormant state by deriving a path which optimizes the probability of achieving the dormant state. We explicitly show how the probability of achieving dormancy scales with the transition rate parameters. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J42.00007: Experimental observation of critical slowing down before population collapse Lei Dai, Daan Vorselen, Carmel Dudley, Onur Ornek, Kirill Korolev, Jeff Gore Tipping points marking population collapse and other critical transitions in natural systems ($e.g.$ ecosystems, the climate) can be described by a fold bifurcation in the dynamics of the system. Theory predicts that the approach of bifurcations will result in an increasingly slow recovery from small perturbations, a phenomenon called critical slowing down. Here we demonstrate the direct observation of critical slowing down before population collapse using replicate laboratory populations of the budding yeast \textit{Saccharomyces cerevisiae}. We mapped the bifurcation diagram experimentally and found a significant increase in both the size and timescale of the fluctuations of population density near a fold bifurcation, in agreement with the theory. We further confirmed the utility of theoretically predicted warning signals by observing them in two different slowly deteriorating environments. To extend the application of warning signals to spatially extended populations, we proposed and identified several indicators based on the emergence of spatial patterns. Our results suggest that generic temporal and spatial indicators of critical slowing down can be useful in predicting tipping points in population dynamics. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J42.00008: Extinction of Bacterial Populations: A Change of Paradigm? Ingo Lohmar, Baruch Meerson It is now well-established that individual bacteria of many types switch stochastically between two phenotypes: fast-growing ``normals'' susceptible to antibiotics, and slowly-growing ``persisters'' hardly affected by the drug. In the competition of species during exponential growth, persisters are a burden, but they may become beneficial when introducing ``stress'' phases like drug treatment. We suggest to shift the focus to the \emph{persistence} of an established population. Due to fluctuations, the population will (after a long time) eventually go extinct; persisters act as a life insurance against this. We study a simple stochastic model of these processes. Using a WKB approximation, we find the most likely path to extinction and quantify the extinction risk under both favorable and adverse conditions. Analytical results are obtained both in the biologically relevant regime when the switching is rare compared with the birth and death processes, and in the opposite regime of frequent switching. We explain how persisters strongly reduce the extinction risk and show that rare switches are most beneficial to this end. [I. Lohmar and B. Meerson, \textit{Phys. Rev. E} \textbf{84} 051901 (2011)] [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J42.00009: Invasion, Coexistence, and Extinction Driven by Preemptive Competition and Sex Ratio Ferenc Molnar, Thomas Caraco, Gyorgy Korniss We investigate competitive invasion in a simple population dynamics model, where females can differ genetically in the sex ratio of their offspring, and males can differ in mortality. Analyzing of the mean-field dynamics, we obtain conditions for ecological stability of a given sex-ratio allele for any mortality rate parameters. We also found that stable coexistence of the two alleles is possible, but only males can differ; one female phenotype is present. Our results show that the success of invasion is determined by the female birth sex ratio. A lower female ratio never excludes a larger female sex ratio; in case of coexistence, the surviving female phenotype always has the greater female sex ratio. Finally, we identified an interesting invasion-to-extinction scenario: successful invasion followed by extinction occurs when the invader initially propagates with the resident allele, but after excluding the resident, cannot survive on its own. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J42.00010: Vaccine enhanced extinction in stochastic epidemic models Lora Billings, Luis Mier-y-Teran, Ira Schwartz We address the problem of developing new and improved stochastic control methods that enhance extinction in disease models. In finite populations, extinction occurs when fluctuations owing to random transitions act as an effective force that drives one or more components or species to vanish. Using large deviation theory, we identify the location of the optimal path to extinction in epidemic models with stochastic vaccine controls. These models not only capture internal noise from random transitions, but also external fluctuations, such as stochastic vaccination scheduling. We quantify the effectiveness of the randomly applied vaccine over all possible distributions by using the location of the optimal path, and we identify the most efficient control algorithms. We also discuss how mean extinction times scale with epidemiological and social parameters. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J42.00011: Fast stochastic algorithm for simulating evolutionary population dynamics Lev Tsimring, Jeff Hasty, William Mather Evolution and co-evolution of ecological communities are stochastic processes often characterized by vastly different rates of reproduction and mutation and a coexistence of very large and very small sub-populations of co-evolving species. This creates serious difficulties for accurate statistical modeling of evolutionary dynamics. In this talk, we introduce a new exact algorithm for fast fully stochastic simulations of birth/death/mutation processes. It produces a significant speedup compared to the direct stochastic simulation algorithm in a typical case when the total population size is large and the mutation rates are much smaller than birth/death rates. We illustrate the performance of the algorithm on several representative examples: evolution on a smooth fitness landscape, NK model, and stochastic predator-prey system. [Preview Abstract] |
Session J43: Invited Session: Stochastic Geometry and Conformal Invariance in Non-Equilibrium and Disordered Systems
Sponsoring Units: GSNP DFDChair: Walter Goldburg, University of Pittsburgh
Room: 157AB
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J43.00001: Stochastic geometry of turbulence Invited Speaker: Gregory Falkovich Geometric statistics open the window into the most fundamental aspect of turbulence flows, their symmetries, both broken and emerging. On one hand, the study of the stochastic geometry of multi-point configurations reveals the statistical conservation laws which are responsible for the breakdown of scale invariance in direct turbulence cascades. On the other hand, the numerical and experimental studies of inverse cascade reveal that some families of isolines can be mapped to a Brownian walk (i.e. belong to the so-called SLE class) and are thus not only scale invariant but conformally invariant. That means that some aspects of turbulence statistics can be probably described by a conformal field theory. The talk is a review of broken and emerging symmetries in turbulence statistics. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J43.00002: Stochastic geometry in disordered systems, applications to quantum Hall transitions Invited Speaker: Ilya Gruzberg A spectacular success in the study of random fractal clusters and their boundaries in statistical mechanics systems at or near criticality using Schramm-Loewner Evolutions (SLE) naturally calls for extensions in various directions. Can this success be repeated for disordered and/or non-equilibrium systems? Naively, when one thinks about disordered systems and their average correlation functions one of the very basic assumptions of SLE, the so called domain Markov property, is lost. Also, in some lattice models of Anderson transitions (the network models) there are no natural clusters to consider. Nevertheless, in this talk I will argue that one can apply the so called conformal restriction, a notion of stochastic conformal geometry closely related to SLE, to study the integer quantum Hall transition and its variants. I will focus on the Chalker-Coddington network model and will demonstrate that its average transport properties can be mapped to a classical problem where the basic objects are geometric shapes (loosely speaking, the current paths) that obey an important restriction property. At the transition point this allows to use the theory of conformal restriction to derive exact expressions for point contact conductances in the presence of various non-trivial boundary conditions. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J43.00003: Efficient SLE algorithms and numerical pitfalls of the method Invited Speaker: Tom Kennedy We consider a physical experiment or a numerical simulation of a physical phenomena that produces a random family of two-dimensional curves. We would like to know if there is a conformal invariance underlying this stochastic geometry. The Schramm-Loewner evolution (SLE) is a conformally invariant stochastic process which depends on a single parameter $\kappa$. For different values of $\kappa$ it is known to describe the scaling limit of many conformally invariant 2d systems, e.g, percolation, the Ising model, self-avoiding walks and many more. So it is a natural candidate for describing the stochastic geometry of other physical systems. The classical Loewner equation provides a correspondence between curves in the plane and ``driving functions,'' and SLE is obtained by taking the driving function to be a Brownian motion. Given a collection of random curves in the plane one would like to determine if the curves come from an SLE process for some value of $\kappa$. One method is to compute the driving processes of the curves and test if they are a Brownian motion. We discuss algorithms for doing this efficiently and some of the pitfalls in this approach. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J43.00004: Search for Conformal Invariance in Two-dimensional Compressible Turbulence Invited Speaker: - Stefanus We present a viable way of experimentally testing for conformal invariance at the surface of a turbulent fluid. The theory being tested here is related to the behavior of random curves on a plane and is associated with the work of Loewner, Schramm and others. It is usually referred to as Schramm-Loewner evolution (SLE). The scalar random variables that are put to this test are the vorticity and the divergence of the surface flow. Both of these variables display certain characteristics of Brownian motion, but the divergence field does not exhibit the Gaussian behavior required by SLE. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J43.00005: Schramm-Loewner (SLE) analysis of quasi two-dimensional turbulent flows Invited Speaker: Simon Thalabard Quasi two-dimensional turbulence can be observed in several cases: for example, in the laboratory using liquid soap films, or as the result of a strong imposed rotation as obtained in three-dimensional large direct numerical simulations. We study and contrast SLE properties of such flows, in the former case in the inverse cascade of energy to large scale, and in the latter in the direct cascade of energy to small scales in the presence of a fully-helical forcing. We thus examine the geometric properties of these quasi 2D regimes in the context of stochastic geometry, as was done for the 2D inverse cascade by Bernard et al. (2006). We show that in both cases the data is compatible with self-similarity and with SLE behaviors, whose different diffusivities can be heuristically determined. [Preview Abstract] |
Session J44: Focus Session: Dynamics of Polymers: Phenomena Due to Confinement I
Sponsoring Units: DPOLYChair: Zahra Fakhraai, University of Pennsylvania
Room: 157C
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J44.00001: A closer look at the two-layer model Dongdong Peng Previously we have measured the viscosity of short-chain, unentangled polystyrene films coated on silicon covered with a 100 nm thick oxide layer. We found that a two-layer model, portraying the films as being made of a 3 nm thick mobile layer sitting on top of a bulk-like inner layer, was able to describe our data very well. More recently, we extended similar measurements to entangled films with a higher molecular weight of 212 kg/mol. We found that the same two-layer model was able to describe the viscosity measurements, but would imply an unphysically large stretching of the polymer chains in the films or else the near-surface chains would have to be oblate spheroids with an aspect ratio of about 4. In this talk, I will discuss new results we obtain from films with higher molecular weights of up to 2316 kg/mol and also films that are deposited on a different kind of substrate, namely hydrogen terminated silicon. Implications of these results on the two-layer model will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J44.00002: Inter- and intra-chain Proximity in PS and PMMA free-standing thin films Studied by Fluorescence NRET Jie Xu, Gi Xue The reduction in glass transition temperature (Tg) for free-standing thin polymer film has attracted a long term discussion. We found that the inter- and intra-chain coupling/constraint restricts the molecular motion of polymer chains and thus results in the deviation in glass transition dynamics. To observe inter-chain proximity and control the inter-chain interaction, we attached carbazolyl probe (donor) and anthryl probe (accepter) to some side groups of different chains respectively. Meanwhile, we also attached both donor and accepter to one chain to characterize the intra-chain constraint. A close proximity of the donor to the accepter results in a higher NRET efficiency. So the NRET results can provide information about chain proximity and packing density in polymer thin film. With decreasing film thickness h, the density of the films was reduced. The magnitude of the reduction in packing density for PS free-standing films was much larger than that in PMMA, resulted a larger reduction in Tg for PS film. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J44.00003: Surface Dynamics of Partially Tethered Polymer Films Mark Foster, Jin Kuk Lee, Bulent Akgun, Zhang Jiang, Suresh Narayanan, Sushil Satija The surface dynamics of thin polystyrene (PS) melt films can be tailored by tethering some of the chains to a supporting substrate. The relaxation of surface height fluctuations for these films depends on the density of grafting, molecular weight of tethered chains, and extent to which tethered chains extend into the layer. The surface dynamics of ``partially tethered'' thin films have been studied using X-ray photon correlation spectroscopy (XPCS). PS chains have been grafted to substrates with low grafting densities untethered deuterated PS (dPS) chains spun cast on the tethered chains, and the films annealed to create layers containing both tethered and untethered chains. The degree to which the tethered PS extends into the untethered dPS chains has been measured by neutron reflectivity. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J44.00004: Structural characterization of irreversibly adsorbed polymer layers at the polymer/solid interface - In-situ grazing incidence angle x-ray scattering studies Naisheng Jiang, Fen Chen, Xiameng Chen, Zexi Han, Chen Liang, Peter Gin, Mitsunori Asada, Maya Endoh, Tad Koga In recent years, great attention has been paid to irreversibly adsorbed polymer layers formed on solid substrates since they can modify various properties of polymeric materials confined at the nanometer scale. In this talk, by the combined use of in-situ grazing incidence small angle x-ray scattering and x-ray reflectivity techniques, we aim to characterize the detailed structures of the adsorbed layers composed of different homopolymers (polystyrene, polybutadiene, poly (ethylene oxide), and poly (methyl methacrylate)) prepared on silicon substrates. We will highlight the generality/differences in the structures, leading to a better understanding of the formation process of the adsorbed layers at the impenetrable solid interfaces. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J44.00005: Relaxation of Polyisoprene in Nanoscale Confinement Sung A. Kim, Praveen Agarwal, Lynden Archer This talk introduces a simple model of confined polymers in polymer nanoparticle composites. Most studies about confined polymer dynamics have been investigated from the system of polymers in porous media or the polymer thin film structure. This new class of polymer nanoparticle hybrid materials, termed Nanoscale Ionic Materials (NIMs), is synthesized in bulk scale with convenient controllability of diverse properties to create the confined polymers in nanoscale. cis-Polyisoprene (PI), type A polymer whose dipole moments are parallel along the chain backbone, are synthesized by anionic polymerization and then tethered to spherical silica nanoparticles. Broadband Dielectric Spectroscopy measures responses to the applied electric field which are normal mode relaxation indicative of whole chain relaxation, and also segmental relaxation. We show that relaxations of PI are slower when simultaneously confined and tethered. We also show that molecular weight and grafting density have a profound effect on dynamics of the twice-confined PI chains. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J44.00006: Molecular simulations of confined polymer glasses Robert Riggleman, Amit Shavit For many technological applications ranging from semiconductor manufacturing to membranes for separations and flexible displays, the properties of glass-forming materials confined on the nanoscale are of critical importance. For example, common semiconductor manufacturing techniques rely on the mechanical integrity of amorphous polymer nanostructures. Experiments over the past several years have demonstrated that many of the properties of polymer glasses (e.g., the glass transition temperature, Tg, and the elastic constants) can change significantly and in unintuitive ways when confined to dimensions below approximately 100 nm. These confinement effects depend strongly on the detailed polymer chemistry. In this talk, I will describe our recent work using molecular simulations to study the effects of nanoscale confinement on both the dynamic and mechanical properties of a series of coarse-grained polymer models. Our coarse-grained models systematically explore the effects of polymer chemistry by gradually changing the polymer backbone stiffness and analyzing changes in various properties in free-standing thin films. We find that the confinement effects can vary drastically depending on the properties measured, and in some cases we observe significant ordering of our polymer chains [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J44.00007: Structure and Dynamics of Polymer / Single Wall Carbon Nanotube Nanocomposites Nigel Clarke, Argyrios Karatrantos, Karen Winey, Russell Composto We simulate the static\footnote{Karatrantos A., Composto R.J., Winey K.I., Clarke N., Macromolecules, accepted.} and dynamic\footnote{Karatrantos A., Composto R.J., Winey K.I., Clarke N., in preparation.} properties of monodisperse polymers in the presence of a SWCNT using molecular dynamics. The SWCNT has a large aspect ratio and radius smaller than the polymer radius of gyration. We find that although the local chain structure is significantly affected, the overall configuration, as characterized by the radius of gyration, is not perturbed by either the interaction strength between the polymer and the SWCNT or by variations in the SWCNT radius. In contrast, for the dynamics of entangled polymers, we find a significant heterogeneity, with the center of mass polymer diffusion being affected by the strength of the enthalpic interaction between monomers and the SWCNT, as well as the SWCNT radii. In addition, we find that the polymer chain diffusivity perpendicular to the SWCNT is smaller than that parallel to the SWCNT in the case of entangled polymers. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J44.00008: Polymer Conformations in SWCNT/Polystyrene Nanocomposites using SANS Wei-Shao Tung, Nigel Clarke, Russell J. Composto, Karen I. Winey Polymer conformations are a critical factor that affects the performance of polymer nanocomposites. Using small angle neutron scattering, we probe chain conformations and confinement of polymers in SWCNT/polystyrene nanocomposites. We established the contrast matching criteria by measuring nanocomposites with different dPS:PS ratios (79:21 to 63:37) for two different SWCNT loadings and found the best ratio of dPS:PS (72.5:27.5) which gives us a similar scattering strength as SWCNT. Therefore, the scattering signal from SWCNT are screened and contribute little to the total scattering intensity. By making contrast matched samples, we are able to focus on the scattering intensity from polymer chains and determine Rg by fitting the intensity data. We found that the Rg of the polymer chain increases weakly with 1 wt{\%} ($<$ 5{\%}) nanotube and then strongly ($>$25{\%}) with 3 wt{\%} nanotube, while the chain conformation still follows Gaussian statistics. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J44.00009: Structures and Dynamics of Polymers Adsorbed on Supported Cationic Lipid Membrane Chen-Ming Chang, Yuk-Gyn Lau, Jih-Chiang Tsai, Wen-Tau Juan The structures and dynamics of polymers adsorbed on various surfaces have received attentions not only as a subject concerning fundamental polymer physics, but also for their important roles in many industrial and biological processes. In this talk, I will discuss our recent work utilizing single-molecule imaging to study the conformation and dynamics of fluorescent dye labeled $\lambda$-DNA molecules adsorbed on supported cationic lipid membrane. High-magnification snapshots of individual molecules, combined with statistical analyses on their apparent size and anisotropy as well as dynamic analyses on their diffusions, reveal interesting details that could have been overlooked in measurements based on ensemble averaging and theories based on polymers in 2D. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J44.00010: Structure and Dynamics in Hyperbranched Nanohybrids K. Chrissopoulou, S. Fotiadou, S.H. Anastasiadis, B. Frick The structure and dynamics of a hyperbranched polyester-amide (Hybrane$^{\textregistered}$ 1200, Mn=1200, Tg=45$^{\circ}$C) polymer and its nanocomposites with natural montmorillonite (Na$^{+}$-MMT) are investigated to offer a detailed picture of its behavior in bulk and under confinement and reveal its potential use for various applications. The static properties were studied utilizing X-ray diffraction (XRD), while the dynamics using energy-resolved elastic and quasi-elastic neutron scattering (QENS). XRD reveals that the polymer chains reside within the galleries of the Na$^{+}$-MMT producing an intercalated nanocomposite. The elastically scattered intensity for the polymer exhibits two distinct relaxation steps, which are attributed to the methyl group rotation and to the segmental motion. The intensity for the nanocomposite shows the first step broader than the respective of the pure polymer indicating restricted local motion whereas it indicates frozen dynamics under confinement at temperatures higher than the bulk polymer glass transition temperature, Tg. The QENS spectra measured at temperatures covering the regimes below and above Tg are in agreement with the elastic measurements. Sponsored by the Greek GSRT ($\Sigma YNEP \Gamma A \Sigma IA$; $09\Sigma YN-42-580)$ and by the EU (CP-IP 246095-2). [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J44.00011: Large Amplitude Shear Deformations of Nano-Filled Elastomers: Evidence of Glassy Dynamics in the Vicinity of the Fillers Didier Long, Paul Sotta, Samy Merabia, Aurelie Papon, Francois Lequeux, Helene Montes, Laurent Guy Adding fillers in elastomers is known to increase the elastic modulus and the wear resistance of elastomers, but also to increase non-linear dissipation, a phenomenon known as the Payne effect. Indeed, when submitted to deformations of the order of a few per cents or more, the elastic modulus can decrease down to values much smaller than the initial one. On the other hand, when submitted to large amplitude oscillatory shear, frequency analysis shows that the contribution of higher harmonics to the response is quite small. This might appear somehow as a paradox since the non-linear behavior of filled elastomers can be strongly marked. We proposed a possible physical origin of these various features. We do it by comparing experimental results performed on model elastomers to the prediction of a model proposed recently, based on the presence of glassy bridges linking neighboring particles. The non-linear response is a consequence of a shift towards shorter time scales (as compared to the non-perturbed distribution) of the relaxation time distribution when a large amplitude periodic solicitation is applied. The non-harmonic response is the consequence of the fluctuation during one cycle of the distribution of relaxation times and is thus a much smaller effect. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J44.00012: Polymer Diffusion Slows Down in Nanocomposites Containing Hard Nanospheres Russell Composto, Sangah Gam, Karen Winey, nigel clarke, Jeffrey Meth Here, we present two experimental studies of tracer diffusion in polymer nanocomposites (PNCs). Tracer diffusion of deuterated polystyrene (dPS) in a PS matrix containing phenyl-grafted silica (13nm and 29nm) represents a PNC with a weak polymer/particle interaction [1]. However, tracer diffusion of deuterated poly(methyl methacrylate) (dPMMA) in a PMMA matrix containing hydroxyl-terminated silica (13 nm and 28nm) represents a PNC with a strongly attractive polymer/particle interaction. In the former case, the normalized diffusion coefficients fall on a master curve when plotted against the confinement parameter, namely the interparticle separation normalized by the probe size. However in the latter case, the reduced diffusion coefficients in the PMMA matrix with 13nm nanoparticles are less than those with 29nm nanoparticles, suggesting that enthalpic interactions play a role in slowing down diffusion. These experimental studies indicate that a successful model of polymer dynamics must include contributions due to constraints imposed by the nanoparticles as well as interactions between polymer and nanoparticles.\\[4pt] [1] Gam, S., \textit{et. al.}, \textit{Macromolecules} \textbf{44}, 3494-3501, 2011. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J44.00013: Time and Temperature Dependent Surface Stiffness of Poly(alpha-methylstyrene)(PAMS) through Particle Embedment Taskin Karim, Gregory McKenna In the present work, we have used the particle embedment technique with sub-micron particles to study the time dependence surface modulus of poly(alpha-methylstyrene)(PAMS) at different temperature ranging from room temperature to 1.1T$_{g }$of PAMS. The surface was found softer at room temperature and at 1.02T$_{g}$ compared to the bulk film while at 1.1T$_{g }$the surface was found stiffer compared to the macroscopic modulus measured for the same PAMS. The embedment of the particle is determined from atomic force microscope measurements and the modulus was determined using the elastic analysis of Johnson, Kendall and Roberts (JKR) with surface energy estimates of the work of adhesion as the driving force for embedment. REFERENCES 1. K. L. Johnson, K. Kendall and A. D. Roberts, \textit{P. Royal Society of Lonodon A, }\textbf{324}$, $301-313\textbf{\textit{ }}(1971). 2. J. H. Teichroeb and J. A. Forrest, \textit{Physical Review Letter, }\textbf{91}$, $016104 (2003). [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J44.00014: The Calorimetric Glass Transition of Polystyrene Ultrathin Films Siyang Gao, Yung P. Koh, Sindee Simon Although the glass transition behavior of polystyrene ultrathin films has been widely studied, calorimetric measurements are limited due to difficulties in sample preparation. Here we report the use of a rapid scanning calorimeter based on a membrane sensor to measure the rate-dependent glass transition temperature (T$_{g})$ for single ultrathin films. Both microtomed and spin-coated films are investigated. Preliminary results suggest that the magnitude of the T$_{g}$ depression is similar to that observed for freely-standing films. The T$_{g}$ depression also depends on the cooling rate such that at the highest rates used (1000 K/s), the depression is only a few degrees. The kinetics of dewetting are followed, with T$_{g}$ values increasing as a function of time and finally reverting to the bulk values after several hours at 160 \r{ }C. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J44.00015: Dynamics of anchored polymer chains in a tunable micelle confinement system Hengxi Yang, Chelsea Chen, Ga Ram Jun, Peter Green We investigated the dynamics of the polyisoprene (PI) component confined within micelles in the blends of polystyrene-$b$-polyisoprene (PS-$b$-PI) diblock copolymers and polystyrene (PS) homopolymers, using broadband dielectric spectroscopy (BDS). In these blend, the diameter of the PS-b-PI micelle cores, composed of the PI component, increased with increasing molecular weight of the PS host,~$P$, and reached a plateau at high~$P$~regime. The BDS results show that the dynamics of the PI component were fastest within the smallest micelle cores. Additionally, the relaxation intensities were weakest within smallest micelle cores. The local glass transition temperature (Tg) of the PI component was 201 K when its core diameter is $\sim $29 nm, while the Tg dropped to 193 K with a core diameter of $\sim $21 nm. Although faster dynamics with confinement has been previously observed in several systems, few have examined confinement at length scales much greater than that of the typical cooperative rearranging region, which is normally a few nanometers. Therefore, our study may shed new light on understanding the dynamical behavior of confinement, especially in domains formed in microphase separated block copolymer structures. [Preview Abstract] |
Session J45: Padden Award Symposium
Sponsoring Units: DPOLYChair: Karen Winey, University of Pennsylvania
Room: 159
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J45.00001: Field-Theoretic Studies of Nanostructured Triblock Polyelectrolyte Gels Debra Audus, Glenn Fredrickson Recently, experimentalists have developed nanostructured, reversible gels formed from triblock polyelectrolytes (Hunt et al. 2011, Lemmers et al. 2010, 2011). These gels have fascinating and tunable properties that reflect a heterogeneous morphology with domains on the order of tens of nanometers. The complex coacervate domains, aggregated oppositely charged end-blocks, are embedded in a continuous aqueous matrix and are bridged by uncharged, hydrophilic polymer mid-blocks. We report on simulation studies that employ statistical field theory models of triblock polyelectrolytes, and we explore the equilibrium self-assembly of these remarkable systems. As the charge complexation responsible for the formation of coacervate domains is driven by electrostatic correlations, we have found it necessary to pursue full ``field-theoretic simulations'' of the models, as opposed to the familiar self-consistent field theory approach. Our investigations have focused on morphological trends with mid- and end-block lengths, polymer concentration, salt concentration and charge density. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J45.00002: Effect of Confinement on Proton Transport in Nanostructured Block Copolymer/Ionic Liquid Membranes Megan L. Hoarfrost, Madhu S. Tyagi, Rachel A. Segalman, Jeffrey A. Reimer Nanostructured membranes containing structural and proton-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled with high thermal stability. Understanding the effect of nanodomain confinement on proton-conducting properties in such materials is essential for designing new, improved membranes. This relationship has been investigated for a lamellae-forming mixture of poly(styrene-$b$-2-vinylpyridine) (PS-$b$-P2VP) with ionic liquid composed of imidazole and bis(trifluoromethane)sulfonimide (HTFSI), where the ionic liquid selectively resides in the P2VP domains of the block copolymer. Quasi-elastic neutron scattering and NMR diffusion measurements reveal high levels of a fast proton hopping transport mechanism, which we hypothesize is due to changes in the hydrogen bond structure of the ionic liquid under confinement. This, in combination with unique ion aggregation behavior, leads to a lower activation energy for macroscopic ion transport compared to that in a mixture of ionic liquid with P2VP homopolymer. These results portend the rational design of nanostructured membranes having improved mechanical properties and conductivity. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J45.00003: Controlling Au Nanorod Dispersion in Thin Film Polymer Blends Michael J.A. Hore, Russell J. Composto Dispersion of Au nanorods (Au NRs) in polymer thin films is studied using a combination of experimental and theoretical techniques. Here, we incorporate small volume fractions of polystyrene-functionalized Au NRs ($\phi_{rod} \approx 0.05$) into polystyrene (PS) thin films. By controlling the ratio of the brush length (N) to that of the matrix polymers (P), we can selectively obtain dispersed or aggregated Au NR structures in the PS-Au(N):PS(P) films. A dispersion map of these structures allows one to choose N and P to obtain either uniformly dispersed Au NRs or aggregates of closely packed, side-by-side aligned Au NRs. Furthermore, by blending poly(2,6-dimethyl-p-phenylene oxide) (PPO) into the PS films, we demonstrate that the Au nanorod morphology can be further tuned by reducing depletion-attraction forces and promoting miscibility of the Au NRs. These predictable structures ultimately give rise to tunable optical absorption in the films resulting from surface plasmon resonance coupling between the Au NRs. Finally, self-consistent field theoretic (SCFT) calculations for both the PS-Au(N):PS(P) and PS-Au(N):PS(P):PPO systems provide insight into the PS brush structure, and allow us to interpret morphology and optical property results in terms of wet and dry PS brush states. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J45.00004: Influence of Charge and Network Inhomogeneities on the Swollen-Collapsed Transition in Polyelectrolyte Nanogels Prateek Jha, Jos Zwanikken, Fran\c{c}ois Detcheverry, Juan de Pablo, Monica Olvera de la Cruz Polyelectrolyte nanogels are sub-microscopic networks of solvent-permeated polyelectrolyte chains that undergo large reversible volume changes for a range of environmental stimuli. This volume phase transition behavior finds use in targeted drug delivery, optical switching in photonic crystals, and many other applications that require controlled tunability. Although the strength of electrostatic interactions have a strong influence on the nanogel response, these interactions are not well captured by the classical mean-field theories of macroscopic gels. We develop a simplified Poisson-Boltzmann model of spherical gels, that highlights the importance of charge inhomogeneities and the associated Coulomb interactions in determining the response of gels. Our analysis reveals that nanometer-sized gels, collapsed gels, and gels in media with low salinity or high dielectric constant, have large regions of excess charge, and show clear deviations from the classical Donnan picture of polyelectrolyte gels. The detailed swelling-collapse behavior is obtained using the theoretically-informed coarse-grained simulations, which includes the effects of network imperfections and thermal fluctuations. The simulations capture the universal features of volume phase transition in nanogels. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J45.00005: Enhanced electron mobility of n-channel polymer thin film transistors by use of low-k polymer dielectric buffer layer Felix Sunjoo Kim, Samson Jenekhe Understanding the factors that govern charge transport in polymer thin film transistors is of interest in developing high-performance polymer transistors and circuits. Engineering the dielectric properties of the gate insulator of a field-effect transistor represents one of the promising approaches to improving the performance of the devices. We show that insertion of a low-k polymer dielectric layer between a silicon dioxide gate dielectric and poly(benzobisimidazobenzophenanthroline) (BBL) semiconductor of n-channel organic transistors increases the field-effect electron mobility by two orders of magnitude. The enhanced electron mobility was accompanied by increased on/off current ratio, superior multicycling stability with negligible hysteresis, and enhanced durability in air. Systematic studies of a series of polymer dielectrics showed that the electron mobility increased exponentially with decreasing dielectric constant, which can be understood in terms of the reduced energetic expense of charge carrier/dipole interaction. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J45.00006: Imaging three dimensional bicontinuous networks in bulk heterojunction solar cells James T. Rogers, Louis A. Perez, Alan J. Heeger, Hiroshi Jinnai, Guillermo C. Bazan, Edward J. Kramer Highly efficient, solution processable, organic photovoltaics typically consist of a two component donor-acceptor type heterojunction structure comprised of a low bandgap conjugated polymer donor blended with a fullerene acceptor. Efficient charge extraction from these blends demands that donor and acceptor components form nanoscale phase separated percolating pathways to their respective electrodes. Although the existence of this bicontinuous interpenetrating network, termed a bulk heterojunction (BHJ), is hypothesized to be requisite for efficient device operation, attempts to characterize BHJ structures using conventional transmission electron tomography (TEMT) techniques have failed. Energy filtered TEMT (EF-TEMT) is demonstrated to overcome the inadequacies of conventional TEMT, enabling three-dimensional (3D) imaging of high efficiency BHJ structures with nanometer resolution. Considered in combination with x-ray scattering measurements, the 3D chemical maps derived from EF-TEMT are used to offer a plausible mechanism of BHJ formation in devices reaching 7.1{\%} efficiency. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J45.00007: Single molecule dynamics of self-associating polymers Charles Sing, Alfredo Alexander-Katz Recent progress has been made experimentally in understanding the importance of reversible interactions in polymer systems, with systems such as stimuli-responsive gels demonstrating tunable properties. We extend these ideas to single molecules, since this type of behavior is especially important in biological polymers where molecular interactions can be both reversible and long-lived. We have developed simulation models where a Bell-like association model is incorporated into a Brownian Dynamics simulation, so that we represent a self-associating polymer in a coarse-grained fashion. We find that there are exciting implications of these binding properties on the dynamics of polymers both in and out of equilibrium. The introduction of this extra time scale to a dynamic polymer system manifests itself in drastic changes in polymer properties under dynamic loading; in particular, self-associating polymers in shear flow demonstrate large non-monotonic effects and single-molecule pulling scenarios demonstrate pulling behaviors reminiscent of biological molecules. This thus represents a physical model that has intriguing implications in both biological and synthetic polymer systems. [Preview Abstract] |
Session J46: Invited Session: Electrostatic Interactions in Polymeric and Biological Systems
Sponsoring Units: DPOLYChair: Andrey Dobrynin, University of Connecticut
Room: 160ABC
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J46.00001: Ion Specific Effects at Interfaces Invited Speaker: Yan Levin Availability of highly reactive halogen ions at the surface of aerosols has tremendous implications for the atmospheric chemistry. Yet neither simulations, experiments, nor existing theories are able to provide a fully consistent description of the electrolyte-air interface. In this talk a new theory will be presented which allows us to explicitly calculate the ionic density profiles, the surface tension, and the electrostatic potential difference across the solution-air interface [1,2]. The theory takes into account both ionic hydration and polarizability [3]. The theoretical predictions are compared to experiments and are found to be in excellent agreement. Finally, the implications of the present theory for stability of lyotropic colloidal suspensions will be considered [4], shedding new light on one of the oldest puzzles of physical chemistry --- the Hofmeister effect.\\[4pt] [1] Y. Levin, A.P. dos Santos, and A. Diehl, Phys. Rev. Lett. {\bf 103}, 257802 (2009). \\ \noindent [2] A. P. dos Santos, A. Diehl, and Y. Levin, Langmuir {\bf 26}, 10778 (2010)\\ \noindent [3] Y. Levin, Phys. Rev. Lett. {\bf 102}, 147803 (2009) \\ \noindent [4] A. P. dos Santos and Yan Levin, Phys. Rev. Lett. {\bf 106}, 167801 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J46.00002: Solvation of ions in bulk and at interfaces: What can density functional theory teach us? Invited Speaker: Christopher Mundy Insights from molecular simulation have influenced both experiment and theory regarding the understanding of the specific ion effect. Although there seems to be a consensus that large polarizable anions exist at the air-water interface, the understanding of the precise molecular interactions that give rise to surface adsorption remain elusive. I will present our work on the adsorption of iodide at the air-water interface using density functional theory (DFT) based interaction potentials. I will discuss similarities and differences of the results obtained using different descriptions of molecular interaction. Last, we are able to reconcile the results obtained with molecular simulation using standard empirical potentials with the dielectric continuum theory of Levin and co-workers.\\[4pt] In collaboration with Marcel Baer, Pacific Northwest National Laboratory; Douglas Tobias, Abe Stern, University of California, Irvine, CA; and Yan Levin, Instituto de F\'isica, UFRGS, Porto Alegre, RS, 91501-970, Brazil. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J46.00003: Simulation of single molecule stretching experiments on denatured ssDNA Invited Speaker: Mark Stevens We have performed simulations of stretching denatured ssDNA using a bead-spring model to compare with recent single molecule experiments. Each bead represents a singly charged base or monomer in the ssDNA. The salt and counterions are explicitly treated. An equal and opposite force is applied to the two terminal beads, and the force-extension curves are calculated at a range of forces. In denatured ssDNA, the bases of ssDNA are blocked from pairing making the flexible polyelectrolyte model applicable. The recent single molecule stretching experiments\footnote{McIntosh and O.A. Saleh, Macromolecules {\bf 44}, 2328 (2011).} on denatured ssDNA have studied the effect of added salt treating both 1:1 and 2:1 salts. These experiments found force-extensions curves exhibit two regimes: at low forces, the extension $R \sim f^\gamma$, where $\gamma = 0.60 - 0.69 $ and a high force regime where $R \sim \log f$. The force-extension curves can be scaled to produce overlap for the salt dependence. The force at the crossover between the two regimes $f_c$ scales as $c_s^{1/2}$ for 1:1 salt, but as $I^3$ for 2:1 salt, where is $I$ is the ionic strength. The simulation data reproduce the experimental overlap for both salt cases. The two regimes including the logarithmic behavior at large forces are present in the simulation results. The simulation results imply that the behavior is due to a competition between electrostatics, entropy and the applied force and that other molecular interactions can be neglected. Thus, the standard theoretical methods are not missing an important term in the free energy, although approximation level may be an issue. We will present simulation data on the conformations as a function of applied force and discuss the implications for analytic theory. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J46.00004: Charge-regularization effects on polyelectrolytes Invited Speaker: Murugappan Muthukumar When electrically charged macromolecules are dispersed in polar solvents, their effective net charge is generally different from their chemical charges, due to competition between counterion adsorption and the translational entropy of dissociated counterions. The effective charge changes significantly as the experimental conditions change such as variations in solvent quality, temperature, and the concentration of added small electrolytes. This charge-regularization effect leads to major difficulties in interpreting experimental data on polyelectrolyte solutions and challenges in understanding the various polyelectrolyte phenomena. Even the most fundamental issue of experimental determination of molar mass of charged macromolecules by light scattering method has been difficult so far due to this feature. We will present a theory of charge-regularization of flexible polyelectrolytes in solutions and discuss the consequences of charge-regularization on (a) experimental determination of molar mass of polyelectrolytes using scattering techniques, (b) coil-globule transition, (c) macrophase separation in polyelectrolyte solutions, (c) phase behavior in coacervate formation, and (d) volume phase transitions in polyelectrolyte gels. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 2:15PM |
J46.00005: Effect of electrostatic interactions on lubrication in polymeric and biological systems Invited Speaker: Jan-Michael Carrillo Many connective tissues, such as cartilage demonstrate excellent lubrication and wear characteristics. Cartilages in mammalian joints can withstand pressures of the order of ten atmospheres and have remarkably low friction coefficient in the range of 0.001-0.03. The surface of the cartilage is covered with bottle-brush-like polyelectrolyte layer consisting of glycoproteins. This brush layer, which faces a similar layer on the opposing cartilage, is sheared as two surfaces slide passing each other during joint motion. We have performed molecular dynamics simulations of charged and neutral bottle-brush macromolecules tethered to substrates to understand the role of the electrostatic and hydrodynamic coupling between brush layers on the lubricating properties in biological and polymeric systems. Glycoprotein layers were modeled as two opposing layers of highly charged bottle-brush macromolecules composed of Lennard-Jones particles grafted to a substrate. Simulations have shown that charged bottle-brush systems have lower friction under shear and weaker dependence of the disjoining pressure on substrate separation than neutral bottle-brush systems. This was explained by formation of lubricating layer with excess of counterions located in the middle between brush-bearing surfaces. In overlapping brush layers the disjoining pressure between brush-bearing surfaces is controlled by the bottle-brush bending rigidity. Under shear, the main deformation mode of the charged bottle-brush layers is associated with the bottle-brush backbone deformation resulting in backbone deformation ratio and shear viscosity being universal functions of the Weissenberg number. In the case of neutral bottle-brush systems there is coupling between backbone and side chain deformation. This violates universality in backbone deformation ratio and manifests itself in shear viscosity dependence on the shear rate. The shear viscosity as a function of the shear rate for the neutral bottle-brush systems has two plateaus and two shear thinning regimes. [Preview Abstract] |
Session J48: Focus Session: Gelation and Glass Transition in Colloids and Soft Matter Systems II
Sponsoring Units: DPOLYChair: Michael Rubinstein, University of North Carolina
Room: 161
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J48.00001: Kinetic arrest and activated relaxation in dense suspensions of attractive nonspherical colloids Rui Zhang, Kenneth Schweizer The coupled translation-rotation activated dynamics in dense suspensions of uniaxial short-range attractive homogeneous (hDC) and Janus (JDC) dicolloids are studied using the microscopic nonlinear Langevin equation theory based on the concept of a dynamic free energy surface. For larger aspect ratios, three activated regimes, repulsive glass (RG), attractive glass (AG) and gel (G), are predicted where translation and rotation are always strongly coupled. Physical clustering in the JDC system suppresses the re-entrant RG-fluid(F) process. For hDCs of small enough aspect ratio, multiple kinetic arrest transitions, novel re-entrant phenomena, and a dynamical quadruple point emerge associated with F, G, RG, AG, and plastic glass (PG) states. Colloid translational and rotational motions are strongly coupled in the gel, but decoupled (slower translation) in the other activated regimes due to the presence of two distinct hopping processes. Both translation and rotation activated relaxation rates are non-monotonic functions of attraction strength at high volume fractions. Translation-rotation decoupling universally weakens as attraction strength grows, and exhibits a different volume fraction dependence at low, intermediate, and high attraction strengths. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J48.00002: Rearrangements, Vibrations and Microscopic Glassy Dynamics and Structure in Quasi-2D Dense Colloidal Gels Matthew Lohr, Arjun Yodh In this work, we investigate the microscopic dynamics of quasi-2D dense attractive colloidal systems. We confine bidisperse polystyrene spheres between glass coverslips in a suspension of water and 2,6-lutidine; as we increase the temperature of the sample into a critical regime, lutidine wets the colloids, creating a strong attractive interaction ($>$ 4kT). We specifically study suspensions in the ``dense gel'' regime, i.e., at a volume fraction high enough that the attractive particles form a spanning cluster, yet just low enough that there exists some structural heterogeneity larger than the individual particle size. We track the particle locations via bright-field video microscopy and analyze the dynamics of both lower-volume-fraction gel states and higher-volume-fraction glassy states. Specifically, we make correlations between local structure, rearrangement-prone regions, and low-frequency vibrational modes. In doing so, we not only characterize the structural and dynamical differences and similarities between colloidal gels and glasses, but we also gain further insight into the origins of dynamic heterogeneity in glassy systems. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J48.00003: Dynamical Phases and Rheology of Rod-Sphere Nanoparticle Mixtures Ryan Jadrich, Kenneth Schweizer Colloidal mixtures involving sticky nonspherical particles that form glasses and gels are relatively unexplored compared to their single component analogs. We develop and apply a microscopic statistical dynamical approach (mode coupling and nonlinear Langevin equation theories) at the center-of-mass level for dense isotropic mixtures of spheres and rods as a function of (short range) attraction strength, aspect ratio, and composition. Based on the mixture structural pair correlations as input, up to seven dynamical phases (transiently localized states with activated dynamics) are predicted corresponding to fluid, repulsive glass, attractive glass, gel, a mixed coexisting glass-gel state, and several partially localized states. The dynamical complexity increases with aspect ratio, and reflects a rich competition between repulsive force caging, physical bond formation and rod interpenetration. Removal of the nanosphere attraction destroys the double gel state but otherwise has minor consequences. The elastic shear modulus and absolute yield stress are also studied, and order of magnitude changes are found along various trajectories in the dynamical phase diagram. Rods are found to generically impart greater bulk rigidity than spheres. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:27PM |
J48.00004: Phonons and Rearrangements in Disordered Colloidal Glasses Invited Speaker: Arjun Yodh I will describe experiments which explore the behaviors of disordered particle suspensions at high packing fraction (e.g, see references [1-5]). Recent systems studied include two-dimensional glasses composed of temperature-sensitive microgel particles, and colloidal glasses in binary fluid which permit interaction potential to be switched from attractive to repulsive. Displacement correlation matrix techniques are employed to derive phonon modes and phonon density of states, and video microscopy is employed to study particle rearrangements. Connections between phonons and stress-induced rearrangements will be described, and detailed observations of the properties of cooperative rearrangement events in attractive versus repulsive glasses will be presented. \\[4pt] [1] Chen, K., Manning, M.L., Yunker, P.J., Ellenbroek, W.G., Zhang, Z., Liu, A.J., and Yodh, A.G., \textit{Phys Rev Lett} 107, 108301 (2011).\\[0pt] [2] Yunker, P.J., Chen, K., Zhang, Z., and Yodh, A.G., \textit{Phys Rev Lett} 106, 225503 (2011).\\[0pt] [3] Chen, K., Ellenbroek, W.G., Zhang, Z.X., Chen, D.T.N., Yunker, P.J., Henkes, S., Brito, C., Dauchot, O., van Saarloos, W., Liu, A.J., and Yodh, A.G., \textit{Phys Rev Lett} 105, 025501 (2010).\\[0pt] [4] Yunker, P., Zhang, Z.X., and Yodh, A.G., \textit{Phys Rev Lett }104, 015701 (2010).\\[0pt] [5] Yunker, P., Zhang, Z.X., Aptowicz, K.B., and Yodh, A.G., \textit{Phys Rev Lett} 103, 11, 115701 (2009). [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J48.00005: Characterization of local shear zones that govern the deformation of colloidal glasses Katharine Jensen, Nobutomo Nakamura, David Weitz, Frans Spaepen Colloidal glass provides a unique experimental system with which to study the structure, defects, and dynamics of a generic amorphous material. We report experiments on 1.55-$\mu$m-diameter, hard-sphere silica colloidal glasses under conditions of uniform shear. We use confocal microscopy to follow the 3D, real-time trajectories of roughly 100,000 particles during homogeneous deformation and explore the roles of both glass density and applied strain rate. In this way, we probe the elastic, anelastic, and plastic response of the system, with particular focus on identifying specific mechanisms of deformation. In plastic deformation, we directly observe ``shear defects'' or ``shear transformation zones'' (STZs) as clusters of particles that behave as Eshelby inclusions, undergoing highly localized plastic strain. These clusters can be identified as regions that are best fits to the Eshelby strain field throughout the sample. We correlate the development of these regions with local density-related properties, including the Voronoi volume and the free volume of both individual particles and connected clusters of particles. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J48.00006: Elasticity and microstructure of colloidal gels undergoing strain-induced yielding Lilian Hsiao, Richmond Newman, Sharon Glotzer, Michael Solomon The mechanism of yielding in colloidal gels, particularly for high strain-rate deformations that are typical of materials applications, is not fully understood. Here, we examine the oscillatory response and stress relaxation of a colloidal gel formed by short-range depletion interactions to find a correlation between the macroscopic rheological behavior and their microstructural features. We use confocal laser scanning microscopy to directly observe the strain-dependent evolution of 3D gel structure within a shearing device. The gels are made up of monodisperse poly(methyl methacrylate) spheres of different sizes and volume fractions dispersed in refractive index matched solvents. We impose simple shear flows of various strains on the gel and observe the 3D structural change after deformation. This is done using a UV light-activated photopolymer, which allows particle configurations to be locked in place rapidly ($<$0.6s) after yielding. We characterize the transition from a dense network to interconnected clusters using the contact number distribution. Our results show that rigid, stress-bearing clusters play an important part in contributing to the gel elasticity at large strains. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J48.00007: Complex oscillatory yielding in model hard sphere glasses George Petekidis, Nick Koumakis The yielding behaviour of hard sphere glasses under large amplitude oscillatory shear has been studied by experimental rheology and Brownian Dynamics simulations. Here we focus on varying the frequency of oscillation probing the interplay between Brownian motion and shear-induced diffusion. Stress, structure and dynamics are followed by rheology and BD simulations Two frequency regimes are revealed: At low frequencies Brownian motion is dominant, assisting particles to escape their cage during a single step yielding process, identified by a peak of G'' at around the G'=G'' crossover. At high frequencies shear induced particle collisions causes cage breaking with the maximum energy dissipation marked by the G'' peak taking place beyond the G'=G'' crossover. Intermediate frequencies present a complex yielding behaviour influenced by both mechanisms that leads to a double peak in G'' that has not been reported before in HS glasses. The nonlinear response is quantified by the higher harmonics present in the stress signal. While at low frequencies the strength of higher harmonics reaches a constant value at high strains, in the intermediate and high frequency regime a non-monotonic behaviour is detected with characteristic large amplitude strains exhibiting apparent harmonic response. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J48.00008: Interactions of Dislocations with Twin Boundaries in Hard-Sphere Colloidal Crystals Maria Persson Gulda, Eric Maire, David Weitz, Frans Spaepen The interaction of dislocations with twin or grain boundaries is a key factor in understanding the strength and strain hardening of crystals. Confocal tracking of particles in a colloidal crystal allows a detailed look at the mechanisms of such interactions at the particle scale, analogously to what happens on the atomic scale in conventional crystals. The grain boundaries are prepared by sedimentation onto templates in the [110] $\Sigma $3 orientation. A complex set of interactions is observed: additional twinning, the emission of a pair of partial dislocations, and the formation of kinks in the original twin boundary. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J48.00009: Gel formation and aging in weakly attractive nanocolloid suspensions James Harden, Hongyu Guo, Subramanian Ramakrishnan, Robert Leheny We present combined x-ray photon correlation spectroscopy (XPCS) and rheometry studies of the evolution of concentrated suspensions of nanometer-scale colloids undergoing thermo-reversible gelation and aging. After a quench through the gel point, suspensions display a protracted latency period in which they remain fluid followed by a gelation regime in which the shear modulus grows rapidly. The XPCS intermediate scattering function displays two features, a plateau value that provides information about constrained local dynamics and a terminal decay related to relaxation of residual stress. From the wave-vector dependence of the plateau value, a localization length can be extracted. At intermediate colloidal volume fractions ($\phi\simeq 0.20$), the relationship between the localization length and the shear modulus agrees quantitatively with a prediction based on a simplified mode coupling theory, while deviations from the predicted scaling at a higher volume fraction ($\phi\simeq 0.43$) are observed near the gel point. While some features of slow strain from stress relaxation correlate with the evolving rheology, others appear decoupled from the macroscopic behavior. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J48.00010: Phase-Change Dynamics in Attractive, Polydisperse Colloidal Suspensions John Mergo, Itai Cohen, Anthony Dinsmore Understanding the single-particle dynamics of phase changes in polydisperse suspensions is important for predicting the structures that arise in these systems. In this talk, we discuss the results of experiments on the melting and freezing of polydisperse colloidal particles. In these experiments, micron-sized colloidal particles are sedimented in water onto a glass coverslip to form a quasi two-dimensional gas. The particles experience an attractive interaction due to a size-tunable depletant added to the mixture. This allows both melting and freezing to be probed in the same experiment. Optical images with single-particle resolution are recorded and each particle is tracked through the duration of the experiment. Interestingly, we find that the increase in polydispersity may stabilize the fluid phase, which has previously been shown to be metastable in experiments on monodisperse particles. In particular, we explore the stability, structure, and dynamics of this fluid phase and how the structures in this phase solidify over time. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J48.00011: The role of picosecond dynamics in relaxation of molecular glasses Marcus Cicerone, Qin Zhong, Feng Ding, Jack Douglas, Madhusudan Tyagi, David Simmons The importance of the relative amplitude of elemental relaxation processes, specifically of the fast $\beta $ relaxation process ($<$u$^{2}>)$, to long-time relaxation processes have been emphasized in a number of theoretical models. In this talk I will focus on transport properties and their relationship to $<$u$^{2}>$ in supercooled liquids and glasses. In these studies we use antiplasticization to systematically tune molecular packing. These results provide insights into the role that localization and the concomitant emergence of an excess density of states plays in relaxation of and transport in molecular glasses. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J48.00012: AC Impedance Study of the Structural Transformation of Graphite Suspensions Jianjian Wang, Gang Chen Recently graphite suspensions have been demonstrated to possess very high thermal conductivity enhancements with high stability. However, the thermal conductivity is quite sensitive to the internal structures of the suspensions or the different states of the particulates in the liquids. At low graphite loadings, the graphite particulates can only form evenly distributed isolated clusters in the host material. As the graphite loading increases, the clusters will start merging together to form a 3D percolation network and the graphite suspension becomes gel-like. For the first time, we have observed a sharp kink behavior in the thermal conductivity of suspensions at the percolation threshold. Combined microstructural and AC impedance spectroscopy studies suggest that this kink arises from the change in the bonding strength between graphite flakes as the suspensions go through a transition from isolated clusters to percolated structures. Our studies shed light on the heat conduction mechanisms of nanofluids, suspensions and composites. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J48.00013: Large colloidal crystals grown by centrifugation onto a template Daniel Pennachio, Katharine Jensen, David Weitz, Frans Spaepen Colloidal crystals are commonly formed by sedimenting a colloidal solution at 1g onto a patterned template. Slow sedimentation was previously believed to be a requisite for growing large, perfect crystals without crossover to an amorphous sediment. By increasing the relative gravitational force applied to a monodisperse sample of hard-sphere, 1.55$\mu $m diameter silica colloids, we examined the effect of increased sedimentation velocity on the growth of face-centered cubic crystals on a (100) template. We varied relative centrifugal force up to 3000g, time of centrifugation, lattice parameter, and crystal thickness to assess their effect on crystal quality. Single crystals up to 52 $\mu$m thick were grown for all centrifugation speeds. Crystal defects were predominantly stacking faults (bounded by partial dislocations), most of which formed after the critical thickness was reached. The critical thickness, which is a function of the lattice mismatch between crystal and template, was measured directly by varying the crystal thickness. Final stacking fault and vacancy concentrations were independent of centrifugal force and time. We also examined samples centrifuged onto other templates to elucidate the critical role of the template design in directing crystal versus glass formation. [Preview Abstract] |
Session J49: Focus Session: Crystallization in Single-, Multicomponent, and Hybrid Systems I
Sponsoring Units: DPOLYChair: Christopher Li, Drexel University
Room: 162A
Tuesday, February 28, 2012 11:15AM - 11:51AM |
J49.00001: Orientation of conjugated polymers: epitaxy versus mechanical rubbing Invited Speaker: Martin Brinkmann Orientation of conjugated polymers like regioregular poly(3-alkylthiophene)s (P3AT) is of high importance as it can be used to exploit their high intrinsic charge transport anisotropy in the elaboration of devices e.g. OFETs and OLEDs. Orientation has been achieved by two different approaches: i) epitaxy and ii) mechanical rubbing. Herein we describe and compare these two orientation methods. On the one hand side, a large palet of structures, nanomorphologies and orientations can be achieved by controlled epitaxial crystallization of polymers like P3HT, PBTTT or polyfluorenes. Not only can epitaxy afford highly oriented and crystalline films of P3ATs, but also highly oriented fibers with a characteristic shish-kebab morphology. The application of Electron Diffraction analysis (rotation-tilt) on highly oriented polymer layers is an original an powerfull method to unravel the crystal packing of conjugated polymers. On the other hand side, mechanical rubbing of conjugated polymers, especially P3HT can also lead to highly oriented films without the use of an orienting substrate. The mechanism of thin film orientation has been analyzed in detail using Transmission Electron Microscopy, Grazing-Incidence X-ray diffraction and optical spectroscopy. It is demonstrated that the molecular weight Mw of the polymer impacts the maximum orientation achieved by rubbing. The Mw-dependence of orientation is explained in terms of chain folding and entanglements that prevent the reorientation and reorganization of the pi-stacked chains, especially for Mw$\ge $50kDa. Electron diffraction and HR-TEM show that epitaxied and rubbed films differ in terms of \textit{intra-lamellar} order within layers of pi-stacked chains. Whereas the epitaxied P3HT films show a semi-crystalline structure with crystalline domains bearing 3D order, the rubbed P3HT films exhibit rather a 2D nematic-like order. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J49.00002: Solution Assembly of Hybrid Poly (3-hexyl thiophene) and Cadmium Selenide Nanowires Felicia Bokel, Emily Pentzer, Todd Emrick, Ryan Hayward Optimizing morphology of self-assembled systems containing both electron carrying (n-type) and hole carrying (p-type) materials holds promise for the fabrication of improved devices, such as solar cells. In this talk, two routes to formation of hybrid p-n composite fibrils consisting of~crystalline p-type poly(3-hexyl thiophene) (P3HT) nanowires with n-type cadmium selenide (CdSe) quantum dots and nanorods into well-defined structures will be discussed. The first method involves co-crystallization of freely soluble P3HT and P3HT-functionalized CdSe nanorods to form crystalline hybrid nanowires upon addition of a marginal solvent. Transmission electron microscopy reveals that nanorods preferentially orient parallel to and flank the sides of fibers. In a second route to forming hybrid materials, chain-end functionalized P3HT is crystallized into fibrillar nanowires. Introduction of nanoparticles promotes binding at the fibril edge, forming parallel composite pathways or ``superhighways.'' These assembly approaches represent efficient means to organization of conjugated polymers and semiconducting nanostructures, thus offering new opportunities for optoelectronic device design. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J49.00003: Correlating crystallization and ionic conductivity of PEO/graphene oxide nanocomposite Shan Cheng, Derrick Smith, Grace Hsuan, Christopher Li Polyethylene oxide (PEO) is one of the best candidates for solid state electrolyte due to its chemical stability and strong ability to form complex with lithium salts. Crystallization behavior of PEO directly affects the lithium ion transport, and in turn the ionic conductivity of the electrolyte. By adding two dimensional graphene oxide nanosheets into PEO matrix, mechanical property of the latter is significantly strengthened, while the crystallization behavior of PEO is also altered by the graphene oxide sheets. The crystallization of PEO/graphene oxide nanocomposites was studied by differential scanning calorimetry (DSC) and the orientations of graphene oxide and PEO crystal were studied by small angle X-ray scattering and wide angle X-ray diffraction. PEO/graphene oxide nanocomposite doped with lithium salt was further fabricated and characterized by electrochemical impedance spectroscopy. Anisotropic ionic conductivity was observed for the nanocomposite electrolyte due to the orientation of graphene oxide and directional growth of PEO crystals. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J49.00004: Nucleation and Crystallization in nucleated Polymers Christoph Schick, Evgeny Zhuravlev, Andreas Wurm Crystallization is commonly considered as nucleation followed by a growth process. Here we apply the recently developed technique, differential fast scanning calorimetry (DFSC), for a unique, new look at the crystal growth of poly(epsilon-caprolactone) (PCL) and PCL carbon nanotube composites from 185 K, below the glass transition temperature, to 330 K, close to the equilibrium melting temperature. The DFSC allows temperature control of the sample and determination of its heat capacity during temperature treatments by employing cooling and heating rates from 50 to 50,000 K/s. First, the crystal nucleation and overall crystallization half times were determined simultaneously in the range of temperatures where crystallization of PCL occurs. After attempting to analyze the experiments with the classical nucleation and growth model a new methodology is described, which addresses the specific problems of crystallization of flexible linear macromolecules. The structures seem to range from having practically unmeasurable latent heats of ordering (nuclei) to being clearly-recognizable, ordered species with rather sharp disordering endotherms at temperatures from the glass transition to equilibrium melting (increasingly perfect and larger crystals). The mechanisms and kinetics of growth (if any) involve a detailed understanding of the interaction with the surrounding rigid amorphous fraction (RAF) in dependence of crystal size and perfection. E. Zhuravlev, J.W.P. Schmelzer, B. Wunderlich and C. Schick, Kinetics of nucleation and crystallization in poly(epsilon-caprolactone) (PCL), Polymer 52 (2011) 1983-1997. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J49.00005: Spherulite Growth in Polymer-Nanoparticle Blends Goran Ungar, Eunwoo Lee, Ruibin Zhang, Jyongsik Jang Blends of polymers with inorganic nanoparticles (NP) were studied by polarized optical and fluorescence microscopy. Silica nanoparticles with a range of diameters from 7 to 100 nm were used. Neat NPs as well as NPs surface-functionalized with a range of groups from strongly to weakly interacting, were blended with poly(ethylene oxide). A purpose-built T-jump microscopy cell was used allowing rapid temperature equilibration at high supercoolings. Lautitzen-Hoffman type analysis revealed that, although the NPs slow down the standard growth rate $G_0 $in the order PEO - Me-treated SiO2 - untreated SiO2 - COOH-treated SiO2 - NH2-treated SiO2, the surface free energy $\sigma $ decreases in the same order. This suggests that the NPs reduce macromolecular mobility, but at the same time help reduce the secondary nucleation barrier to some extent. Other polymers and NP types, including quantum dots, were also studied. The work also examines the spatial distribution of NPs in the spherulitic polymer nanocomposites. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J49.00006: Synthesis and morphology of hydroxyapatite/polyethylene oxide nanocomposites with block copolymer compatibilized interfaces Ji Hoon Lee, Meisha Shofner In order to exploit the promise of polymer nanocomposites, special consideration should be given to component interfaces during synthesis and processing. Previous results from this group have shown that nanoparticles clustered into larger structures consistent with their native shape when the polymer matrix crystallinity was high. Therefore in this research, the nanoparticles are disguised from a highly-crystalline polymer matrix by cloaking them with a matrix-compatible block copolymer. Specifically, spherical and needle-shaped hydroxyapatite nanoparticles were synthesized using a block copolymer templating method. The block copolymer used, polyethylene oxide-b-polymethacrylic acid, remained on the nanoparticle surface following synthesis with the polyethylene oxide block exposed. These nanoparticles were subsequently added to a polyethylene oxide matrix using solution processing. Characterization of the nanocomposites indicated that the copolymer coating prevented the nanoparticles from assembling into ordered clusters and that the matrix crystallinity was decreased at a nanoparticle spacing of approximately 100 nm. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J49.00007: Confined crystallization in compatibilized Polyamide 6/High Density Polyethylene blends Simona Ceccia, Alexandra Argoud, Lise Trouillet-Fonti, Didier R. Long, Paul Sotta Blending polymers can be considered the easiest way to obtain new materials with tuned properties thanks to the possibility to control blend morphologies. The blend characteristics depend on the properties of each component, on composition and on morphologies developed during polymers processing. In case of semi-crystalline blended polymers, mechanical performances are closely related to the crystalline morphology. Therefore, it is essential that crystallinity is maintained after blending in order to keep or enhance the properties. This may be a challenge when the blends exhibit multiphase morphologies with sub-micrometer domain sizes. In this work, we study the crystallization behavior of compatibilized Polyamide 6/High Density Polyethylene (PA6/PE) blends by means of the Differential Scanning Calorimetry technique. Blends with various morphologies (dispersed, stretched dispersed, fibrillar and co-continuous) are obtained by reactive extrusion and varying blend composition and processing parameters. Blend composition and morphology turn out to greatly affect the bulk crystallization temperatures of both PA6 and PE. When the polymer is confined in domains of a few micrometers the crystallization temperature peak shifts to lower temperatures. Thus, the smaller the domain size the lower the crystallization temperature in case of dispersed morphologies. Moreover, in multi-scale morphologies showing polymer droplets in the nanometer range, fractionated crystallization (multiple crystallization peaks) is observed. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:39PM |
J49.00008: Crystallization and Microphase Separation in Chiral Block Copolymers Invited Speaker: Rong-Ming Ho Block copolymers composed of chiral entities, denoted as chiral block
copolymers (BCP*s), were designed to fabricate helical architectures from
self-assembly. A helical phase (denoted H*) was discovered in the
self-assembly of poly(styrene)-$b$-poly(L-lactide) (PS-PLLA) BCPs*. To examine
the phase behavior of the PS-PLLA, self-assembled superstructures resulting
from the competition between crystallization and microphase separation of
the PS-PLLA in solution were examined. A kinetically controlled process by
changing non-solvent addition rate was utilized to control the BCP*
self-assembly. Single-crystal lozenge lamellae were obtained by the slow
self-assembly (i.e., slow non-solvent addition rate) of PS-PLLA whereas
amorphous helical ribbon superstructures were obtained from the fast
self-assembly (i.e., fast non-solvent addition rate). As a result, the
formation of helical architectures from the self-assembly of the PS-PLLA
reflects the impact of chirality on microphase separation, but the chiral
effect might be overwhelmed by crystallization. Consequently, various
crystalline PS-PLLA nanostructures in bulk were obtained by controlling the
crystallization temperature of PLLA ($T_{c,PLLA})$ at which crystalline
helices and crystalline cylinders occur while $T_{c,PLLA} |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J49.00009: Chain conformation and crystallization in PEO / layered silicate nanocomposites Spiros H. Anastasiadis, K. Chrissopoulou, S. Bollas, K.S. Andrikopoulos, S. Fotiadou, G.A. Voyiatzis The polymer chain conformation under confinement and the polymer morphology are investigated in hydrophilic PEO/Na$^{+}$-montmorilonite nanohybrids synthesized by melt and/or solution intercalation. Intercalated hybrids with mono- and bi-layers of PEO chains are obtained for all compositions covering the complete range from pure polymer to pure clay. For low polymer concentrations, where all the polymer chains are intercalated, PEO is purely amorphous. As PEO concentration increases further, the polymer chains adsorbed on the outer surface of the clay particles remain amorphous as well. It is only when there is large amount of excess polymer outside the completely filled galleries that the bulk polymer crystallinity is abruptly recovered. The conformation of the confined or adsorbed polymer chains, as probed by Raman and FTIR spectroscopies, is found more disordered that the PEO melt even at higher temperatures; this is evident by the dramatic increase of the gauche conformations of the C-C bond along the chain backbone. Sponsored by the Greek GSRT ($\Sigma YNEP \Gamma A \Sigma IA$; $09\Sigma YN-42-580)$ and by the EU (CP-IP 246095-2). [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J49.00010: Lamellar Orientation Inversion under Dynamic Interplay between Simultaneous Crystallization and Phase Separation Charles Han, Weichao Shi Crystallization dynamics and lamellar orientation may be affected under the dynamic interplay between crystallization and phase separation for a two component system. If phase separation is really weak, lamellae grow in the radial direction within spherulites. However, when strong phase separation intervenes, the lamellae growth could be oriented in the tangential direction in a concentric alternating concentration ring pattern. This lamellar orientation inversion is reflected by a birefringence inversion under optical microscopy and will be illustrated by a study of the PEO/PMMA system. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J49.00011: Phase separation of DMDBS from iPP, and controlled crystalline orientation K. Sreenivas, Guruswamy Kumaraswamy, R.S. Basargekar We report an unexpected dependence of DMDBS phase separation temperature on the molecular weight of the matrix isotactic polypropylene (iPP). DMDBS crystallizes out at lower temperatures for iPP with decreasing molecular weight (and correspondingly lower tacticity). This molecular weight dependence is unique to iPP, and is not observed for either syndiotactic PP or for random ethylene-PP copolymers. We show that thermodynamic Flory-type arguments are unable to rationalize the observed results. We also results on extrusion film casting of iPP containing DMDBS and show that flow-alignment of DMDBS networks template the orientation of PP crystals. The modulus and yield strength increase on addition of DMDBS, relative to the neat iPP. Tensile modulus and yield stress of drawn films increase with the degree of orientation, and we are able to achieve a substantial increase even at relatively low draw ratios. [Preview Abstract] |
Session J50: Focus Session: Translocation through Nanopores - Novel Devices and Computational Approaches
Sponsoring Units: DPOLY DBIOChair: Samuel Amanuel, Union College
Room: 162B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J50.00001: Integrated Nanopore Detectors in a Standard Complementary Metal-Oxide-Semiconductor Process Ashfaque Uddin, Chin-Hsuan Chen, Sukru Yemenicioglu, Kaveh Milaninia, Ellie Corigliano, Madoo Varma, Luke Theogarajan High-bandwidth and low-noise nanopore sensor and detection electronics are crucial in achieving single-DNA base resolution. A potential way to accomplish this goal is to integrate solid-state nanopores within a CMOS platform, in close proximity to the biasing electrodes and custom-designed amplifier electronics. Here we report the development of solid-state nanopore devices in a commercial CMOS potentiostat chip implemented in On-Semiconductor's 0.5 micron technology. By using post-CMOS micromachining, a free-standing oxide membrane and electrodes are fabricated utilizing the N+ polysilicon/oxide/N+ polysilicon capacitor structure available in the aforementioned process. Nanopores with sub-5 nm diameter are drilled in the membrane using a Transmission Electron Microscope. The integrity of pores is validated by measuring current-voltage and noise characteristics. DNA translocation experiments are also performed utilizing these on-chip pores. In addition, electrical tests performed on the CMOS potentiostat circuitry show that the post-CMOS micromachining process does not have any detrimental effect on the CMOS circuitry. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J50.00002: Graphene Gating of Solid State Nanopores for DNA Translocation Kimberly Venta, Matthew Puster, Marija Drndic We report on ionic current measurements through gated solid state nanopores. Devices consist of Si$_{3}$N$_{4}$ membranes covered with a graphene sheet connected off-membrane to a gold contact pad. The graphene is insulated from solution with a TiO$_{2}$ layer deposited by atomic layer deposition, and the gold is exposed by an SF$_{6}$ etch. An electron-beam sculpted nanopore below 10 nm in diameter is drilled through the silicon nitride, graphene, and titania. Applying a voltage to the graphene modulates the ionic current through the pore. We measured the current-voltage characteristics for different gate potentials for our devices. We characterized the leakage current from the graphene as well as the ionic current noise in these pores to complement our current-voltage measurements. These results can lead to measurements of the influence on DNA translocation of a potential at the pore and set the groundwork for characterization of graphene-based sensing of DNA at a nanopore. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J50.00003: Nanochannel Device with Embedded Nanopore: a New Approach for Single-Molecule DNA Analysis and Manipulation Yuning Zhang, Walter Reisner Nanopore and nanochannel based devices are robust methods for biomolecular sensing and single DNA manipulation. Nanopore-based DNA sensing has attractive features that make it a leading candidate as a single-molecule DNA sequencing technology. Nanochannel based extension of DNA, combined with enzymatic or denaturation-based barcoding schemes, is already a powerful approach for genome analysis. We believe that there is revolutionary potential in devices that combine nanochannels with nanpore detectors. In particular, due to the fast translocation of a DNA molecule through a standard nanopore configuration, there is an unfavorable trade-off between signal and sequence resolution. With a combined nanochannel-nanopore device, based on embedding a nanopore inside a nanochannel, we can in principle gain independent control over both DNA translocation speed and sensing signal, solving the key draw-back of the standard nanopore configuration. We will discuss our recent progress on device fabrication and characterization. In particular, we demonstrate that we can detect - using fluorescent microscopy - successful translocation of DNA from the nanochannel out through the nanopore, a possible method to 'select' a given barcode for further analysis. In particular, we show that in equilibrium DNA will not escape through an embedded sub-persistence length nanopore, suggesting that the embedded pore could be used as a nanoscale window through which to interrogate a nanochannel extended DNA molecule. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J50.00004: fd Virus as a Model Stiff Polymer for Translocation Experiments with Solid-State Nanopores Angus McMullen, Xu Liu, Mirna Mihovilovic, Derek Stein, Jay Tang We report preliminary experimental results of the translocation of the filamentous virus fd through a solid-state nanopore. fd virus is suitable for translocation and detection in a voltage-biased nanopore because it is highly charged, 880 nm long, and 6.6 nm in diameter. Importantly, fd has a persistence length of $\sim $2 $\mu $m, a forty-fold increase over dsDNA, making fd a model stiff polymer for testing theories of polymer translocation dynamics. fd cannot coil in solution, therefore the dispersion of fd translocation times can test a model by Lu et al. that ascribes DNA translocation velocity fluctuations to the distribution of initial conformations of the DNA coil. That picture is in contrast with an alternative model by Li et al., which attributes the spread of DNA translocation times to thermal velocity fluctuations. The physics of fd capture by a nanopore also differs significantly from DNA since the ends of the virus cannot diffusively search for the pore independently of the middle. As a result, the rate of fd capture from solution may not increase monotonically with the applied voltage across the pore; it is possible for fd to become kinetically trapped against the nanopore membrane by the electric field. We will compare the distribution of translocation times of fd virus to distributions for DNA and discuss the influence of the virus's orientation and interactions with the nanopore on the translocation speed and the measured current blockage. We will also examine the dependence of capture rate on the applied voltage. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J50.00005: Studies of DNA Translocation Dynamics Using Asymmetrical Nanopores Xu Liu, Karri DiPetrillo, Jason Chan, Derek Stein Despite extensive studies of DNA translocations through voltage-biased solid-state nanopores, the influence of the DNA coil on the translocation dynamics remains poorly understood. We investigated this issue experimentally by controlling the separation between the DNA coil and the nanopore. We studied lambda DNA translocations through devices comprising a 400 nm-high, ~2500 nm-wide, disc-shaped cavity bounded from above by a 20 nm-thin silicon nitride membrane with a ~10 nm wide nanopore in the center, and from below by at 400 nm-thick silicon nitride membrane with a 300 nm-wide opening in the center. The asymmetric nanopore-cavity structure introduced an 800 nm gap between the initial DNA coil and the nanopore when a molecule translocated from below, but no gap when it translocated from above. Translocation times were longer and the integrated charge deficit was larger for molecules translocating from below. These results are explained by the viscous drag on the DNA outside the pore, whose importance relative to the drag inside the pore we quantify. We outline a consistent model of DNA translocation speeds that depends on the initial configuration of the DNA coil, similar to the velocity fluctuation model of Lu et al. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J50.00006: First-passage-time analysis of DNA translocation in solid-state nanopores Daniel Ling, Xinsheng Ling We report a DNA translocation experiment using solid-state nanopores and 48 kb lambda DNA samples. As reported previously, the DNA translocation dynamics in such standard solid-state nanopore experiments appear to be complex and multiple folded translocation pathways are observable. We use the translocation events with little or no detectable folded structures to construct a distribution function for the DNA translocation times. We find that the translocation time distribution can be fitted using the first-passage-time probability density function derived by Schrodinger for 1-D Brownian motion with a drift. The voltage dependence of the extracted DNA drift velocity shows excellent agreement with the Stokes' law at high voltages. Deviation from the Stokes' law is found at low voltages, but can be attributed to a systematic error in how different types of folded DNA translocations are sorted. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J50.00007: Some aspects of polymer translocation dynamics through nanopore: comparison of recent the theories with simulation results Invited Speaker: Aniket Bhattacharya Translocation of a flexible poymer chain through a narrow pore has still remained an active field of research. Earlier theoretical studies of Sung and Park,\footnote{W. Sung and P.~J. Park, Phys. Rev. Lett. {\bf 77}, 783 (1996).} Muthukumar,\footnote{M. Muthukumar, J. Chem. Phys. {\bf 111}, 10371 (1999).} Chuang, Kantor and Kardar, Kantor and Kardar\footnote{J. Chuang, Y. Kantor and M. Kardar, Phys. Rev. E {\bf 65}, 011802 (2001); Y. Kantor and M. Kardar, \textit{ibid.} {\bf 69}, 021806 (2004).} for a flexible chain have been complemented by more recent theories of Sakaue\footnote{T. Sakaue, Phys. Rev. E {\bf 76}, 021803 (2007); \textit{ibid.} \textbf{81}, 041808 (2010).} where tension propagation(TP) along the chain backbone at the $cis$ side resulting in a nonuniform stretching of the chain has been proposed to be a key input for theoretical studies. Recently these elements of the TP theory has been incorporated in to a Brownian dynamics (BDTP) scheme and numerical studies of the equations of motion are in excellent agreement with prior simulation studies.\footnote{T. Ikonen, A. Bhattacharya, T. Ala-Nissila and W. Sung (submitted).} A driven translocating chain is essentially \textit{out-of-equilibrium}\footnote{A. Bhattacharya and Kurt Binder, Phys. Rev. E. \textbf{81}, 041804 (2010); A. Bhattacharya \textit{et al.}, Eur. Phys. J. E \textbf{29}, 423 (2009).} which results in \textit{cis-trans} asymmetries both in ocnformations and in dynamics. Therefore, results from theoretical studies should capture these features. In this talk first I will first present results from Langevin dynamics simulation citing several cases where how this \textit{cis-trans} asymmetry affects the chain conformations and the translocation dynamics. Then I will dicuss relevance of these results in the context of exisiting theories. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J50.00008: DNA translocation measurements in solid-state nanopores fabricated using helium-ion microscope Liping Liu, Wang Miao, Chuong Huynh, Quanjun Liu, Xinsheng Ling We report high-quality DNA translocation measurements in solid-state nanopores drilled in free-standing SiN membranes by using a helium-ion beam in a Zeiss helium-ion microscope (HIM). We show that the HIM nanopores have similar performance as the TEM-drilled pores. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J50.00009: Dynamics of Simultaneous, Single Ion Transport through Two Single-Walled Carbon Nanotubes: Observation of a Three-State System Steven Shimizu, Wonjoon Choi, Chang Young Lee, Moon-Ho Ham, Michael Strano The ability to actively manipulate and transport single molecules in solution has the potential to revolutionize chemical synthesis and catalysis. In previous work, we developed a nanopore platform using the interior of a single-walled carbon nanotube (diameter = 1.5 nm) for the Coulter detection of single cations of Li+, K+, and Na+. We demonstrate that as a result of their fabrication, such systems have electrostatic barriers present at their ends that are generally asymmetric, allowing for the trapping of ions. We show that above this threshold bias, traversing the nanopore end is not rate-limiting and that the pore-blocking behavior of two parallel nanotubes follows an idealized Markov process with the electrical potential. Such nanopores may allow for high-throughput linear processing of molecules as new catalysts and separation devices. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J50.00010: Polymer Translocation through a Nanopore Modulated by a Sticky Site Hendrick W. de Haan, Gary W. Slater We examine the translocation of a long polymer that has one ``sticky site'' where a single monomer experiences an attraction to the pore. Using a quasistatic model for the translocation dynamics and employing numerically exact methods to generate results, high precision values for the translocation times are obtained across a wide range of driving forces and sticky site well depths. It is found that the interplay between the sticky site well depth, the driving force, and entropic effects can lead to unexpected results such as a non-monotonic variation of the critical well depth with the driving force and abnormally long translocation times due to the generation of a metastable state at a critical driving force. The dynamics are also found to be strongly dependent on the location of the sticky site: a site near the head of the polymer increases the probability of successful translocation while a site in the middle acts merely as a ``speedbump.'' Treating the sticky site as a perturbation to an otherwise diffusive process (low driving forces) or driven process (high driving forces) yields good agreement with the numerical results. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J50.00011: Translocation dynamics of a semi-flexible chain through a nano-pore Ramesh Adhikari, Andy W.C. Lau, Aniket Bhattacharya We study translocation dynamics of a semi-flexible chain through a nano-pore in two dimensions (2D) using Langevin dynamics simulation. Specifically, we show how the mean first passage time (MFPT) and the probability distribution of the MFPT are both influenced by the bending rigidity of the chain. Furthermore, we monitor the chain conformations both at the cis and the trans sides and relate these results with recent theories and experiments for a translocating chain through a nano-pore. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J50.00012: Characterization of Idealized Helical Repeat Proteins in Silicon Nitride Nanopores Jiali Li, Bradley Ledden, David Talaga, Aitziber Cortajarena, Lynne Regan In this work, we report the measurement of consensus tetratricopeptide repeat (CTPR) proteins with single silicon nitride nanopores. The CTPR proteins were measured in KCl solution at pH below and above its isoelectric point (pI), as well as with and without denaturing agent, Guanidine HCl. When a CTPR protein molecule transits through a nanopore driven by an applied voltage, it partially blocks the ions (K$^{+}$ and Cl$^{-})$ flow in the nanopore and generates a characteristic electric current blockage signal. The current blockage signal reveals information about the size, conformation, and primary sequence of the CTPR protein molecule. Previous translocation studies carried out with DNA have established that higher bias voltages result in shorter duration current blockages indicating that DNA translocates faster at a stronger electric field. However, our CTPR translocation studies show that longer duration current blockades were observed at higher bias voltages. We discuss how the inhomogeneous distribution of the primary charge sequence of the CTPR proteins predicts translocation barriers that are proportional to the bias voltage. Larger barriers at higher bias voltages will result in longer translocation times, consistent with our experimental results. [Preview Abstract] |
Session J51: Liquid Crystals: Smectic, Ferroelectric, Nanocomposites and DNA
Sponsoring Units: DCMP DFDChair: Luz J Martinez-Miranda, University of Maryland
Room: Boston Conference Center 154
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J51.00001: Two-Dimensional Microfluidics: Hydrodynamic Interactions in Ultra-Thin Smectic Liquid Crystal Films Zoom Nguyen, Aaron Goldfain, Cheol Park, Joe Maclennan, Matt Glaser, Noel Clark Hydrodynamics is important in nature and has a wide range of applications in science and industry. Most studies of fluid dynamics have been carried out in 3D systems, but there is an increasing interest in the hydrodynamics in confined geometries. Smectics can form ultra-thin, stable fluid films of uniform but readily variable thickness, a structure which enables the quantitative study of 2D hydrodynamics. Hydrodynamic interactions in 2D extend much further across the fluid than in 3D, and all the dynamics is confined to a well-defined plane, facilitating clean, high-contrast and high-resolution experiments. Here, we explore the hydrodynamic interactions of disk-like smectic islands with other islands and with a straight film boundary acting as a 1D wall. High speed video microscopy confirms that the translational diffusion of an island is anisotropic in the vicinity of another island or the wall, and this anisotropy persists even at large separations many times the island radius. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J51.00002: Two-Dimensional Microfluidics: hydrodynamics of drops and interfaces in flowing smectic liquid crystal channels Zhiyuan Qi, Zoom Nguyen, Cheol Park, Joe Maclennan, Matt Maclennan, Noel Clark The quantization of film thickness in freely suspended fluid smectic liquid crystal film enables the study of the hydrodynamics of drops and interfaces in 2D. We report microfluidic experiments, in which we observe the hydrodynamics of 2D drops flowing in channels. Using high-speed video microscopy, we track the shape of 2D drops and interfaces, visualizing the deterministic lateral displacement-based separation and pinched flow separation phenomena previously observed only in 3D. Finally, we demonstrate techniques for 2D drop generation and sorting, which will be used for 2D microfluidic applications. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J51.00003: Smectic liquid crystal cells with a ``dirty'' substrate Quan Zhang, Leo Radzihovsky I will describe our recent studies of smectic liquid crystal cells with a ``dirty'' substrate. Acting as quenched disorder, such substrate heterogeneity destabilizes long-range smectic order on the surface and in the bulk for arbitrarily weak randomness. We analyze the statistics of the corresponding distortions, their decay into the bulk, topological defects and the role of nonlinear smectic elasticity. We will discuss our predictions in the context of recent experiments on ferroelectric smectic-C liquid crystals. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J51.00004: Structural Reorganization of 4 - Cyano - 4' - octyloxybiphenyl (8OCB) revealed by Fast Scanning Calorimetry Dongshan Zhou, Jing Jiang, Zhijie Huang, Wei Jiang, Gi Xue 4-Cyano-4'-octyloxybiphenyl (8OCB) is a liquid crystal with a few crystalline polymorphic modifications, of which the square plate form is the most elusive. Square plate form was reported to be only solution grown at low temperature and transformed to metastable parallelepiped form immediately even at -20 OC. With the chip calorimeter, we got the smectic glass of 8OCB when it was quenched from the melt with cooling rate of 20, 000 K/s. In the subsequent reheating with rates ranged from 2,000 K/s to 7,000 K/s, we could find two melting peaks located at 310K and 320K, respectively. Under faster heating, the peak at 310K became dominating, while the peak at 320K weakening. At heating rate of 8000 K/s, there was only melting peak of 310K. If further increased the heating rate, the melting peak at 310K would become smaller again because the crystal growth was suppressed until basically invisible at heating rate of about 20,000 K/s. This work shows that the square plate form is the dominating form when grown from the smectic glass, but it starts transforming to the parallelepiped form at heating slower than 8000 K/s. At heating slower than 1000 K/s, the transformation is completed and there is no chance to capture the square plate form. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J51.00005: A Biaxial Banana Liquid Crystal Phase with Short-range Layer Ordering Yongqiang Shen, Tao Gong, Dong Chen, Renfan Shao, Chenhui Zhu, Matthew Glaser, Joseph Maclennan, David Walba, Noel Clark W623, a single-tail, bent-core molecule with a polar termination on one end and a siloxane-terminated tail on the other, exhibits a ferroelectric, orthorhombic, fluid smectic liquid crystal phase, the SmAP$_{F}$. Powder x-ray diffraction (XRD) measurements reveal an exotic structural transition on cooling from the SmAP$_{F}$ to a SmX phase, in which resolution-limited fluid smectic layering reflections give way to four much broader peaks, indicating short-range layer ordering. This behavior points to the kind of internal frustration that gives rise to our recently discovered helical nanofilament phases. We have performed two-dimensional XRD on aligned samples and discovered that one of the four peaks is from the in-plane order. Freeze-fracture transmission electron microscope (FFTEM) measurements confirm that there is two-dimensional short-range order in the SmX phase, with one periodicity in the layer plane and another normal to the layers. The in-plane periodicity can be measured directly from the packing of fibrils to be about 8 nm, consistent with the in-plane x-ray reflection peak. We will present depolarized transmission light microscopy, high-resolution XRD, and FFTEM studies of pure W623, and of mixtures of W623 with the calamitic liquid crystal 8CB. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J51.00006: A Bent -- Shape Leaning Smectic Liquid Crystal Material Cuiyu Zhang, Nick Diorio, S. Radhika, B.K. Sadashiva, Antal Jakli Liquid crystals of bent-shape molecules theoretically can form four types of fluid smectic layer structures: (a) A polar smectic phase (called SmAP) where both molecular plane and the line connecting the end of average molecules (director) are perpendicular to the layer normal; (b) A double tilted chiral polar structure; (c) A single tilted phase (SmCP) where only the molecular plane is tilted with respect to the layer normal. The fourth possibility, where the director is tilted with respect to the layer normal, the ``leaning'' SmLP phase, has never been verified experimentally. Here we present the first bent-core material that forms a SmLP structure, thus proving the reality of all theoretically predicted bent-core smectic phases. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J51.00007: Effective Conductivity due to Continuous Polarization Reorientation in Fluid Ferroelectrics Joseph Maclennan, Yongqiang Shen, Tao Gong, Renfan Shao, Eva Korblova, David Walba, Noel Clark In crystal ferroelectrics, the macroscopic polarization density \textbf{P} is stabilized to a set of discrete orientations by the underlying lattice, and ferroelectricity characterized by field-induced switching of \textbf{P} between these stable states. Fluid ferroelectrics exhibit \textbf{P} with no energy barriers to its reorientation. As a result, \textbf{P} can respond to applied electric field in a continuous fashion. We show here that, due to the reorientation of \textbf{P}, an otherwise insulating fluid ferroelectric behaves electrically as a resistive medium, with conductivity in the semiconducting range. Measurements of cell dynamics are reported for the SmAP$_{F}$ material W623, a bent-core liquid crystal (LC) with large macroscopic polarization that we find to exhibit nearly ideal field-induced block polarization reorientation. We have investigated theoretically the dynamic behavior of block polarization in the SmAP$_{F}$ phase, finding that a reorienting LC polarization block behaves electrically as a resistor. Experimental studies of W623 confirm this behavior, revealing the low resistance of the block-reorienting LC and the corresponding characteristic flat-topped step in the current response. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J51.00008: Microscopic origins of first-order SmA-SmC phase behavior and electro optics in de Vries smectic liquid crystals Z. Kost-Smith, M.A. Glaser, P.D. Beale, N.A. Clark Many de Vries liquid crystals exhibit a first-order SmA-SmC phase transition. The original de Vries hollow cone model, in which molecules in the SmA phase are tilted with respect to the layer normal but are uniformly distributed in azimuthal orientation, $\phi$, has been used successfully to model many properties of de Vries materials, but the microscopic origins of first-order behavior remain obscure. We describe a microscopic mechanism for first-order behavior in de Vries smectics based on the hollow cone model, embodied in a generalized planar spin model where effective interactions between tilted molecules in the smectic layer planes are represented by a nearest-neighbor pair potential, $u(\phi_{ij})$, with a minimum of variable width around $\phi_{ij}=\phi_i-\phi_j=0$. Using mean-field theory and Monte Carlo simulation, we find that the SmA-SmC transition is second order for a relatively broad minimum in the potential, and becomes first-order as the minimum narrows. This reflects the expected behavior due to excluded volume interactions in a tilted smectic, in which increasing cone angle leads to a more steeply varying effective azimuthal potential. This model naturally explains the observed first-order behavior in the framework of a hollow cone model. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J51.00009: Morphology and Dynamics of Liquid Crystalline Molecules Confined in Nano-pores Malgorzata Jasiurkowska, Wilhelm Kossack, Roxana Ene, Ciprian Iacob, Wycliffe Kipnusu, Periklis Papadopoulos, Joshua Sangoro, Maria Massalska-Arodz, Friedrich Kremer Broadband dielectric and Infrared spectroscopy are combined to study the molecular dynamics of the liquid crystalline compounds belong isothiocyanatobiphenyl homologous series (abbreviated as nBT) confined in pores of mean diameters from 4 nm to 10.5 nm. In bulk, the studied substances show only one liquid crystalline phase: the SmE phase with orthorhombic arrangement within the molecular layers. In contrast to well-known bulk dielectric properties of nBTs, confinement leads to modification of the molecular dynamics. Two relaxation processes are detected. The slower process corresponds to molecular reorientation around short axis and it is faster in pores than in bulk. The second process is attributed to a librational motion of the molecules close to the walls. Both processes exhibit an Arrhenius-type temperature dependence. Detailed analysis of the temperature dependent infrared spectra indicates the different impact of confinement on the rigid and flexible molecular units of nBTs. Transition Moment Orientational Analysis is employed to explore the orientational order of molecules in pores. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J51.00010: Temperature Dependence of Smectic Liquid Crystals Mixed With Magnetic Nanoparticles Jefferson W. Taylor, Lynn K. Kurihara, Luz J. Martinez-Miranda We investigate the properties of bulk liquid crystal mixed with a magnetic nanoparticle (CoFe) as a function of temperature. We compare our results to those of a heat capacity measurement of Cordoyiannis et al.\footnote{George Cordoyiannis, Lynn K. Kurihara, Luz J. Martinez-Miranda, Christ Glorieux, and Jan Thoen, Phys. Rev. E \textbf{79}, 011702 (2009)} and compare the way the smectic as a function of temperature the way the nematic behaves. We study how the liquid crystal reorganizes in the presence of the functionalized nanoparticles as a function of temperature and compare it to how it behaves at room temperature.\footnote{L. J. Mart\'inez-Miranda, and Lynn Kurihara, J. Appl. Phys, \textbf{105}, p. 084305 (2009).} The X-rays give rise to three or four peaks whose evolution in temperature varies depending on their origin. In particular the second peak does not seem to vary much with temperature, and can be associated with the first several molecular layers attached to the nanoparticles. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J51.00011: The Role of ZnO Particle Size, Shape and Concentration on Liquid Crystal Order and Current-Voltage Properties for Potential Photovoltaic Applications Luz J. Martinez-Miranda, Janelle Branch, Robert Thompson, Jefferson W. Taylor, Lourdes Salamanca-Riba We investigate the role order plays in the transfer of charges in ZnO nanoparticle - octylcyanobiphenyl (8CB) liquid crystal system for photovoltaic applications as well as the role the nominally 7x5x5nm$^{3}$ or 20x5x5nm$^{3}$ ZnO nanoparticles play in improving that order. Our results for the 5nm nanoparticles show an improvement in the alignment of the liquid crystal with increasing weight percentage of ZnO nanoparticles$^{1}$. Our results for the 7x5x5 nm$^{3}$ sample show that the current is larger than the current obtained for the 5 nm samples. We find that order is improved for concentrations close to 35{\%} wt ZnO for both the 7x5x5 nm$^{3}$ and 20x5x5 nm$^{3}$. We have analyzed the X-ray scans for both the 7x5x5 and the 20x5x5 nm$^{3}$ samples. The signal corresponding to the liquid crystal aligned parallel to the substrate is much smaller than the peak corresponding to the liquid crystal aligned approximately at 70\r{ } with respect to the substrate for the 7x5x5 nm$^{3}$ sample whereas this same peak is comparable or more intense for the 20x5x5 nm$^{3}$ sample. 1. L. J. Mart\'{\i}nez-Miranda, Kaitlin M. Traister, Iriselies Mel\'{e}ndez-Rodr\'{\i}guez, and Lourdes Salamanca-Riba, Appl. Phys. Letts, \underline {97,} 223301 (2010). [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J51.00012: ZnO Nanowire Arrays with Liquid Crystals for Photovoltaic Applications Lourdes Salamanca-Riba, Joshwa Taillon, Luz Martinez-Miranda Liquid crystals are small monodisperse molecules with high mobilities and are easy and cheap to process. In addition, some of their phases exhibit molecular orientation that can provide a path for the electrons, or holes, to move from one electrode to the other. We have added a smectic A liquid crystal (8CB) to ZnO nanowire arrays of different diameters and have observed a photovoltaic effect as a function of the concentration of ZnO in the liquid crystal. The nanowire arrays are covered with 8CB liquid crystal for hole conduction. We have observed an increase in the light absorption of the PV cells as a function of wavelength of the light for the ZnO nanowire cells. We present a detailed study of the structure of the system. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J51.00013: Ligation of Complementary Oligomers in Liquid Crystals of nanoDNA Gregory Smith, David Walba, Noel Clark, Weixian Xi, Tao Gong, Christopher Bowman, Tommaso Fraccia, Giuliano Zanchetta, Tommaso Bellini The chromonic stacking mode of short oligomeric DNA upon forming liquid crystalline phases presents an intriguing possibility for liquid crystal autocatalysis, the promotion, by LC ordering, of chemical synthesis that stabilizes LC ordering. In such a scenario the concentration and physical organization of ligation reactants and the fluidity of the liquid crystal phase promotes the appropriate chemical ligation of oligomers. Because it is a mode of elongation free of other catalysts, this offers a tantalizing means of oligonucleotide self-elongation that might have implications in prebiotic life. We present here work toward elucidating possible catalytic enhancement by liquid crystalline phase formation. Ligation approaches based on water soluble carbodiimide base activation and photopolymerization will be presented. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J51.00014: Liquid Crystal Ordering of Random DNA Oligomers Tommaso Bellini, Giuliano Zanchetta, Tommaso Fraccia, Roberto Cerbino, Ethan Tsai, Mark Moran, Gregory Smith, David Walba, Noel Clark Concentrated solutions of DNA oligomers (6 to 20 base pairs) organize into chiral nematic (NEM) and columnar (COL) liquid crystal (LC) phases. When the oligomer duplexes are mixed with single strands, LC phase formation proceeds through macroscopic phase separation, as a consequence of the combination of various self-assembly processes including strand pairing, reversible linear aggregation, demixing and LC ordering. We extended our investigation to the case of LC ordering in oligonucleotides whose sequences are partially or entirely randomly chosen, and we observed LC phases even in entirely random 20mers, corresponding to a family of 4$^{20} \quad \approx $ 10$^{12}$ different sequences. We have tracked the origin of this behaviour: random sequences pair into generally defected duplexes, a large fraction of them terminating with stretches of unpaired bases (overhangs); overhangs promote linear aggregation of duplexes, with a mean strength depending on the overhang length; LC formation is accompanied by a phase separation where the duplexes with longer overhangs aggregate to form COL LC domains that coexist with an isotropic fluid rich in duplexes whose structure cannot aggregate. [Preview Abstract] |
Session J52: Focus Session: Extreme Mechanics - Plates
Sponsoring Units: GSNP DFDChair: Christian Santangelo, University of Massachusetts
Room: 153C
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J52.00001: Radial stretching of thin sheets: A prototypical model for morphological complexity Benny Davidovitch The complex morphologies of thin sheets consist of wrinkles, crumples, folds, creases, and blisters. These descriptive words may sound lucid -- but do they carry any quantitatively distinguishable content? Following the classical approach of pattern formation theory, we seek to impart a universal meaning to these modes of deformation as distinct types of symmetry-breaking instabilities of a flat, featureless sheet. This idea motivates us to consider the general problem of \textit{axisymmetric stretching} of a sheet. A familiar realization of this problem is the ``map maker's conflict'': projecting a flat sheet onto a foundation of spherical shape. Another representative realization is the Lame' set-up: exerting a radial tension gradient on a sheet, which may be free-standing or resting on a solid or liquid foundation. I will introduce a set of \textit{generic parameters: bendability, confinement, stiffness, adhesiveness, }that span a phase space for the morphology of radially stretched sheets. In this phase space, wrinkling, crumpling, folding, creasing and blistering could be identified as primary and secondary symmetry-breaking instabilities. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J52.00002: Bending of a surface with spontaneous curvature Catherine Quilliet, Philippe Marmottant, Alexandre Farutin, Chaouqi Misbah We interest to curvature deformations that can be described by Helfrich's energy: a quadratic mean curvature term, and a gaussian curvature term. When the surface is not strictly incompressible and presents a nonzero spontaneous mean curvature, we focus on simple cases to show that a priori determination of key features (spontaneous curvature, equilibrium area) may be biased according the expression taken for the energy. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J52.00003: Deformations of 2D Random Elastic Networks Hendrik Florijn, M. van Deen, Henk Imthorn, Martin van Hecke We study the linear and nonlinear behavior of random 2D elastic networks at the desktop scale. We demonstrate how to fabricate random networks and characterize them with the lattice coordination number Z. We investigate experimentally if there is a relation between the mechanical response and the lattice coordination number Z of the network. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J52.00004: Packing with a twist: from Wrinkles to Scrolls Arshad Kudrolli, Julien Chopin We discuss an experimental investigation of a thin elastic sheet in the form of a ribbon with clamped boundary conditions at both ends which is then subjected to a twist by rotating the ends through a prescribed angle. We find that a wrinkling instability appears even at a small twist angle which depends on the aspect ratio of the ribbon, its bending modulus and initial tension. Using x-ray tomography, we show that the pattern of this first instability has an impact on the folding at larger twist angles which can result in ordered configurations including Fermat scrolls. Still further twisting results in a highly compressive packing as in wringing a towel without application of direct radial compression. Implications for developing yarns with novel mechanical and transport properties [Lima, et al., Science 331, 51 (2011)] will be discussed. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J52.00005: Geometry and Mechanics of Chiral Pod Opening Eran Sharon, Shahaf Armon, Efi Efrati, Raz Kupferman We study the geometry and mechanics that drive the opening of Bauhinia seeds pods. The pod valve wall consists of two fibrous layers oriented at $\pm$ 45$^{\circ}$ with respect to the pod axis. Upon drying, each of the layers shrinks uniaxially, perpendicularly to the fibers orientation. This active deformation turn the valve into an incompatible sheet with reference saddle-like curvature tensor and a flat (Euclidean) reference metric. These two intrinsic properties are incompatible. The shape is, therefore, selected by a stretching-bending competition. Strips cut from the valve tissue and from synthetic model material adopt various helical configurations. We provide analytical expressions for these configurations in the bending and stretching dominated regimes. Surface measurements show the transition from minimal surfaces (narrow limit) to cylindrical ones (wide limit). Finally, we show how plants use these mechanical principles using different tissue architectures. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J52.00006: Flat-twisted-helical transition in composed gel sheets and self assembled chiral molecules Shahaf Armon, Eran Sharon, Efi Efrati, Raz Kupferman We recently presented a new chirality creating mechanism in elastic strips. in such frustrated bodies, the chiral configuration is determined in a competition between bending and stretching energies, controlled by a dimensionless parameter $\tilde {w}=w\sqrt {k/t} $, in which $w$ is the strip's width, $t$ -- its thickness and $k$ - the spontaneous curvature. I will show the geometrical and mechanical equivalence between such elastic strips and self assembled molecules made of twisted elements. I will also show experiments in responsive gels, showing how a continuous variation in $\tilde {w}$ yields an ordered shape transition from flat to twisted and helical shapes and to tubes. Similar transitions have been observed in self assembled macromolecules. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J52.00007: Curvature and defects in soft membranes with orientational order Thanh Son Nguyen, Jun Geng, Jonathan V. Selinger Previous research has demonstrated that soft membranes have a coupling between curvature and in-plane orientational order. Defects in the orientational order can induce curvature, and conversely, curvature leads to an effective geometrical potential acting on defects [1]. Recently, our group has done simulations which show that the interaction between curvature and defects depends on several important issues, including the baseline curvature of the membrane (flat, cylinder, sphere, torus), the phase of the defects (radial or tangential), and the relative contribution of in-plane (intrinsic) vs. out-of-plane (extrinsic) variations of the director [2]. To understand the simulations, we develop a theoretical approach that can address those issues. Using this approach, we calculate the energy of defect structures in curved geometries, and determine how the energy varies as a function of the defect position and separation and the membrane distortions. The interaction energy depends on the relative magnitude of intrinsic vs. extrinsic couplings, and on the mechanical properties of the membrane. This approach provides opportunities to design membranes that will relax into selected shapes. [1] AM Turner et al, Rev Mod Phys 82, 1301 (2010). [2] RLB Selinger et al, J Phys Chem B, in press. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J52.00008: Capillary induced buckling of floating sheets Miguel Pineirua, Jose Bico, Benoit Roman, Narayanan Menon When a water droplet is deposited over a thin floating sheet, radial wrinkles appear in the vicinity of the droplet as a result of capillary forces exerted at the contact line [1]. However, determining the stress state at the contact line is still challenging and limits the full description of the wrinkling pattern. In order to avoid this contact line ambiguities, we propose the experimental study of the buckling of a macroscopic annulus floating on the surface of water and submitted to a difference in surface tension between its inner and outer edges. This particular configuration allows to generate radial wrinkles on the membrane with well defined border conditions. The topography of the wrinkled patterns are precisely measured using a synthetic Schlieren technique. Based on the standard buckling theory, we develop scaling laws for the buckling threshold of the annulus as well as for the wave length and radial extension of the wrinkles, which are compared to our experimental results and numerical simulations. \\[4pt] [1] J. Huang, M. Juszkiewicz, W.H. de Jeu, E. Cerda, T. Emrick, N. Menon, and T.P. Russell. Capillary wrinkling of floating thin polymer films. Science, 317(5838):650-653, 2007. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J52.00009: The wrinkle transition of a sheet on a drop Robert Schroll, Benny Davidovitch, Hunter King, Narayanan Menon A thin sheet subject to confinement will wrinkle in order to relieve compressive stress. We discuss the case of a circular sheet living on the surface of a liquid drop. The pressure of the drop forces the sheet to be non-planar, which may induce confinement along the outer edge of the sheet. We show that, in the limit of very thin, highly bendable sheets, the system is governed by a single confinement parameter. This parameter determines if and where wrinkles appear on the sheet. Comparison to experimental results provides the first detailed confirmation of a new far-from-threshold theory to describe such ultra-thin sheets. According to this model, the transition to the wrinkled state represents the loss of axisymmetry in the height field, while the stress field maintains its symmetry. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J52.00010: Transition from Wrinkling to Crumpling in a Sheet Floating on a Drop Hunter King, Narayanan Menon, Robert Schroll, Benny Davidovitch An ultrathin* circular polystyrene sheet floating on the surface of a water drop stretches radially and compresses along its circumference as the curvature of the drop increases. The compression is at first fully relaxed by a wrinkle pattern extending inward from the edge. When the wrinkles occupy too large a fraction of the area of the sheet, sharp, localized, crumpled features continuously emerge. We show that the onset of crumpling is a primary symmetry breaking transition of the stress field. We experimentally characterize this transition from wrinkling to crumpling by studying the distribution of gaussian curvature in the film, measured by optical profilometry. *Typical dimensions are tens of nanometers in thickness and millimeters in lateral size. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J52.00011: Supported conical defects Efi Efrati In this work we study the elastic equilibrium configurations of a hyperbolic conical defect (a flat disc with a single negative Gaussian curvature condensation), supported on a rigid plane. Originating from the study of strictureplasty, this problem which seems to be a natural extension to the D-cone problem displays a distinct behavior. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J52.00012: Stamping and wrinkling of elastic plates Jeremy Hure, Jose Bico, Benoit Roman In classical Euler buckling a beam is found to buckle with the lowest mode as a compressive strain is applied. Higher modes are however observed if the amplitude of the out-of-plane displacement is bounded by geometrical constraints. What is the limit when the maximum amplitude prescribed is decreased to zero? We show that the wavelength tends towards a finite value dictated by the thickness of the beam. This one-dimensional model is used to describe the compression of a circular elastic plate into an hemispherical mold. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J52.00013: Deterministic Wrinkling Patterns of Thin Polymeric Coatings on Soft Substrates Jie Yin, Jose L. Yague, Karen K. Gleason, Mary C. Boyce Wrinkling surface patterns in soft materials have become increasingly important for a broad range of applications including stretchable electronics, microfluidics, thin-film property measurement, wetting and adhesion, and other surface area and topology controlled phenomena. Thermal and swelling mismatch between the thin surface layer and the soft substrate lead to spontaneous formation of buckling-induced disordered labyrinth patterns, which exhibit a mechanistically-determined short wavelength, but an undetermined and highly varied long wavelength. In this paper, analytical and computational models are presented to create deterministic wrinkling patterns through directed buckling methods, which capture the physics of the instabilities governing the formation of multiple wavelength wrinkling patterns, providing a predictive tool for design of deterministic wrinkling patterns. The fabrication of the deterministic patterns is accomplished using novel chemical vapor deposition processes. The role of these patterns in providing multifunctional performance is illustrated and discussed. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J52.00014: Wrinkles in reinforced membranes Atsushi Takei, Fabian Brau, Beno\^It Roman, Jos\'e Bico We study, through model experiments, the buckling under tension of an elastic membrane reinforced with a more rigid strip or a fiber. In these systems, the compression of the rigid layer is induced through Poisson contraction as the membrane is stretched perpendicularly to the strip. Although strips always lead to out-of-plane wrinkles, we observe a transition from out-of-plane to in plane wrinkles beyond a critical strain in the case of fibers embedded into the elastic membranes. The same transition is also found when the membrane is reinforced with a wall of the same material depending on the aspect ratio of the wall. We describe through scaling laws the evolution of the morphology of the wrinkles and the different transitions as a function of material properties and stretching strain. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J52.00015: Mechanics of Graphene Electronics Xuanhe Zhao Graphene, a monolayer of tightly-packed carbon atoms, has demonstrated great academic and industrial promises for integrating superior properties of nanomaterials and nanostructures into novel macroscale devices. Here, we demonstrate a simple method to enable over 200{\%} reversible deformation of continuous large-area graphene sheet (over 1cm x 1cm) on polymer substrates. By patternning large-area graphene on a pre-stretched polymer layer by 200{\%}, the graphene film develops hierachical patterns including wrinkles with wavelengthes on the order 10$\sim $100 nm and delaminated buckles with wavelengths on the the order of 1$\mu$m. If the polymer is stretched again ($<$100{\%}), the wrinkled region relaxes and the graphene on this region becomes flat. As the stretch further increases (over 100{\%}), the graphene on delaminated buckles slides toward the flat regions, decreasing the amplitude of the buckles. The relaxation of the wrinkles and buckles enables the large deformation of graphene electrode without fracture. We further demonstrate potential applications of the graphene electrodes capable of large deformation. For example, a polymer film can be sandwiched between two graphene electrodes. As a voltage is applied between the two graphene electrodes, the polymer can achieve an actuation strain over 200{\%}. [Preview Abstract] |
Session J53: Disordered Systems: Jamming
Sponsoring Units: GSNPChair: Patrick Charbonneau, Duke University
Room: 153B
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J53.00001: The contact percolation transition Tianqi Shen, Corey O'Hern, Mark Shattuck Typical quasistatic compression algorithms for generating jammed packings of athermal, purely repulsive particles begin with dilute configurations and then apply successive compressions with relaxation of the elastic energy allowed between each compression step. It is well-known that during isotropic compression athermal systems with purely repulsive interactions undergo a jamming transition at packing fraction $\phi_J$ from an unjammed state with zero pressure to a jammed, rigid state with nonzero pressure. Using extensive computer simulations, we show that a novel second-order-like, contact percolation, which signals the formation of a system-spanning cluster of mutually contacting particles, occurs at $\phi_P < \phi_J$, preceding the jamming transition. By measuring the number of non-floppy modes of the dynamical matrix, the displacement field between successive compression steps, and the overlap between the adjacency matrix, which represents the network of contacting grains, at $\phi$ and $\phi_J$, we find that the contact percolation transition also heralds the onset of nontrivial response to applied stress. Highly heterogeneous, cooperative, and non-affine particle motion occurs in unjammed systems significantly below the jamming transition for $\phi_P < \phi < \phi_J$, [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J53.00002: Constraint counting for frictional jamming D.A. Quint, S. Henkes, J.M. Schwarz While the frictionless jamming transition has been intensely studied in recent years, more realistic frictional packings are less well understood. In frictionless sphere packings, the transition is predicted by a simple mean-field constraint counting argument, the isostaticity argument. For frictional packings, a modified constraint counting argument, which includes slipping contacts at the Coulomb threshold, has had limited success in accounting for the transition. We propose that the frictional jamming transition is not mean field and is triggered by the nucleation of unstable regions, which are themselves dynamical objects due to the Coulomb criterion. We create frictional packings using MD simulations and test for the presence and shape of rigid clusters with the pebble game to identify the partition of the packing into stable and unstable regions. To understand the dynamics of these unstable regions we follow perturbations at contacts crucial to the stability of the ``frictional house of cards.'' [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J53.00003: Decoupling of Rotational and Translational Diffusion in a 2D Granular Experiment Nabiha Saklayen, Gary L. Hunter, Eric R. Weeks We experimentally study the rotation and diffusion of granular clusters in a 2D binary granular system. Our apparatus vibrates a 2D system of densely packed granular bidisperse disks (to avoid crystallization) containing trackable 3-particle clusters. We use this system to mimic hard-sphere fluids and the clusters probe the system's local translational and rotational dynamics. As the area fraction of the bidisperse disks is increased, diffusion within the sample becomes slower, and above a critical area fraction, the sample behaves as a granular glass. We analyze the rotational and translational motions of the clusters to determine whether they decouple with changing area fraction of the system. As we approach the glass transition, we observe a decoupling between the two motions. [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J53.00004: Characterizing the clogging transition by individual grain behavior Charles Thomas, Douglas Durian Granular media clogs as it flows out of a hopper when the exit hole is appropriately small. However, when the hole enlarged, the grains will never clog: there exists a well-defined transition between these two regimes at a particular critical hole size. To understand the origin of this clogging transition we follow the behavior of individual grains in the bulk. Using a quasi-2D hopper and a fast CCD, we can measure the positions of grains to sub-pixel accuracy. We then use this information to determine various local and global properties, including strain rate, velocity correlations, and dynamical heterogeneity time scales. We study how these quantities depend on the distance to the clogging transition, defined as the difference between the hole size and the critical hole size. This helps to explain the clogging transition and its relationship to the jamming transition. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J53.00005: Jamming around fixed obstacles Amy Bug, Sam Bullard-Sisken, Carl Goodrich, Lisa Manning, Andrea Liu Lattices of obstacles, such as microfluidic arrays, are capable of filtering or sorting particles like emulsion droplets, colloidal particles, and even cells. We study the jamming of soft, bidisperse discs placed within a lattice of fixed obstacles. Such obstacles provide a supporting structure for the jammed configuration, and their ability to alter a jamming threshold is of interest. Conjugate gradient methods are used to find minimum energy configurations, both with and without fixed obstacles. The likelihood of jamming as a function of disc volume fraction is calculated. If a configuration is jammed, the coordination number, energy, pressure, and other parameters of interest are calculated as a function of the obstacle size, number density, and configuration (hexagonal vs. square vs. random lattices). [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J53.00006: Frictional Jammed Packings: Classification, Protocol Dependence and the Phase Diagram Stefanos Papanikolaou, Corey O' Hern, Mark D. Shattuck We probe the nature of the jamming transition in systems of frictional disks, where static friction is modeled geometrically using ``bumpy-particles" with uniform circular asperities on the disks' surface. First, we enumerate and classify the mechanically stable (MS) packings in small systems using exhaustive numerical simulations. We explicitly show that finite friction stabilizes packings that are unstable for frictionless particles, which causes the number of MS packings to increase strongly with the friction coefficient. MS packings for frictional particles are organized into low-dimensional geometric families in configuration space. We then calculate the critical behavior of the structural and mechanical properties near the jamming transition for frictional particles and as a function of protocol and show that friction drastically alters the nature of the transition. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 12:39PM |
J53.00007: Jammed 2D circle packing reconsidered as a jigsaw puzzle Eric Corwin, Kenneth Desmond, Eric Weeks Athermal random packings are inherently non-equilibrium structures. For a bidisperse jammed packing of N disks the global packing structure can be thought of as composed of N jigsaw pieces, each representing the local structure around a disk. We show that we can assign a unique identifier, termed a jigsaw number, to each local packing structure. We find that as the number of disks grows to infinity the number of different jigsaw numbers present in a packing remains finite. We report on the distribution of jigsaw numbers and find that certain local packing structures are more common than others, demonstrating that the non-equilibrium packing structure is incompatible with a flat measure over all configurations. We further report on the correlations present between jigsaw pieces. [Preview Abstract] |
Tuesday, February 28, 2012 12:39PM - 12:51PM |
J53.00008: An experimental test of equilibration of temperature-like variables in jammed granular materials James Puckett, Brian Tighe, Karen E. Daniels Although jammed granular systems are athermal, a number of thermodynamic-like descriptions have been proposed which make predictions about the distributions of volume and stress fluctuations. We perform experiments with an apparatus designed to generate a large number of jammed two-dimensional configurations, which consists of a single layer of photoelastic disks supported by a layer of air driven through a microporous membrane. New configurations are automatically generated by alternately dilating the system (permitting large-scale rearrangements) and compressing it biaxially until a desired volume or pressure is reached. Within each configuration, a bath of $\ga 10^3$ particles surrounds a smaller subsystem of particles with either the same or a different inter-particle friction coefficient than the bath.The use of photoelastic particles permits us to find all particle positions, and to numerically solve for the vector forces at each inter-particle contact. By comparing temperature-like quantities between subsystems, we test whether equilibration is observed under several proposed volume and stress ensembles. [Preview Abstract] |
Tuesday, February 28, 2012 12:51PM - 1:03PM |
J53.00009: Energy decay of freely cooling granular gases in three dimensions Zahera Jabeen, Sudhir N. Pathak, Rajesh R. Freely cooling granular gases, wherein a dilute system of macroscopic particles with uncorrelated initial velocities lose energy through inelastic collisions, have been extensively studied both as a simple model for granular systems as well as a nonequilibrium system showing nontrivial coarsening at late times. As the system cools, inelasticity induces clustering, making the system inhomogeneous. While the form of energy decay ($E(t)\sim t^{-\theta}$) in the initial homogeneous regime is well established by Haff's law ($\theta=2$), the energy decay in the clustered regime is still unresolved in higher dimensions. Within mean field theory, $\theta=2 d/(d+2)$ (where $d$ is the spatial dimension), while a correspondence to Burgers equation implies an exponent $\theta= 2/3 (d=1), d/2 (d>1)$. In one and two dimensions, the two formulae predict the same exponents. By performing extensive event driven molecular dynamics simulations, we show that in three dimensions, the energy decays asymptotically with a power $\approx 1.2$, for all coefficients of restitution $r<1$, consistent with the mean field exponent. However, we argue that the mean field arguments fail due to non local interactions between mass clusters. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J53.00010: The Influence of Topology on Signal Propagation in Granular Force Networks Danielle Bassett, Eli Owens, Karen Daniels, Mason Porter Granular materials exhibit numerous rich and complex behaviours, which have been investigated from both continuum and particulate perspectives. In particular, sound propagation through granular materials is both heterogeneous and complicated, and understanding its features is important not only from the perspective of fundamental physics but also for practical applications such as the characterization and non-destructive testing of such materials. Unfortunately, continuum models of sound propagation have been unable to explain the full range of observed behaviours. Here we represent granular materials as spatially-embedded networks composed of nodes (particles) and weighted edges (contact forces between particles) located in Euclidean space, and we use network science to provide fundamental insights into how sound propagates. Using photoelastic particles, we quantitatively characterise the internal force structure and show that its meso-scale network structure plays a crucial role in sound propagation. These results might help to explain the failure of previous physical models, and illustrate that contact topology alone is insufficient to understand signal propagation in granular materials. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J53.00011: Jamming of Brownian disks in a channel with a constriction Alejandro Bilbao, Jerzy Blawzdziewicz We investigate jamming dynamics of an externally driven system of Brownian particles in a two-dimensional channel with an abrupt constriction. Our numerical simulations reveal a rich dynamical behavior that results from an interplay between external driving forces, thermal excitations, and geometrical constraints due to confinement. In particular, we have observed that channel blockage arising from particle accumulation at the constriction entrance alternates with sudden unjamming events originating from thermal fluctuations. We have also found that the dependence of the average particle flux on the channel width is non-monotonic as a result of strong spatial particle-wall correlations. Under some conditions there exist spontaneous particle ordering and dynamic switching between phases with square and hexagonal symmetry. We expect that similar phenomena can be observed in confined granular flows and in suspension flows in microchannels. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J53.00012: To be or not to be jammed Simon Dagois-Bohy, Brian Tighe, Johannes Simon, Silke Henkes, Martin Van Hecke When are packings of soft athermal spheres jammed? Any experimentally relevant definition must at least require a jammed packing to resist compression and shear. Numerical algorithms usually rely on a global compression monitored by a parameter (like pressure) that signals whether the packing is jammed or not. Here we show that compression is not sufficient to ensure properly jammed packings : some of those packings have positive pressures and bulk moduli, but negative shear moduli, and even for large systems, the number of these ``bad apples'' diverges as the jamming point is approached. We will discuss how to understand this situation and propose as a remedy the boundary relaxation, that is including the boundary shape parameters as variables in the equilibration process; finally we will compare the distribution of shear moduli obtained for both methods. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J53.00013: Jamming, Yielding and Rheology of Weakly Vibrated Granular Media Geert Wortel, Joshua Dijksman, Olivier Dauchot, Martin van Hecke We establish that the rheological curve of dry granular media is nonmonotonic, both in the presence and absence of external mechanical agitations. In the absence of vibrations, the nonmonotonic flow curve governs the yielding behavior of granular media. In the presence of weak vibrations, the nonmonotonic flow curves govern a hysteretic transition between slow but steady and fast, inertial flows. For large agitations, the transition becomes non-hysteretic. We probe the fluctuations near the point where the 1st order transition becomes of 2nd order. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J53.00014: Dynamics near shear-jamming for a dense granular system Jie Ren, Joshua Dijksman, Robert Behringer This talk will present several systematic experimental studies of a two-dimensional, frictional dense granular system subjected to simple shear deformation. The first experiment consists of linear shear for densities smaller than the isotropic jamming point, and examines both the evolution of the average stress and the evolution of force network. These measures reveal three distinguishable regimes of the granular system with increasing shear strain: unjammed, fragile, and shear-jammed regimes. The second experiment uses small amplitude cyclic shear to probe the dynamical response of the states from the first experiment. For fragile or jammed regimes, cyclic shear drives the system through transient states that evolve towards relatively stable forces networks and system-averaged stress. The timescale of the transient increases rapidly as the system moves deeper into the fragile, or shear-jammed regimes. These experiments also involve particle tracking (displacements and rotations) to search for and characterize non-affine motion and spatial heterogeneity. There is a clear increase in particle diffusion with increasing density and shear strain amplitude, even when the system is still unjammed and experiences only minimal stress. When the system is fragile or jammed, the heterogeneity of particle displacements reveals subtle correlations with the force network. [Preview Abstract] |
Tuesday, February 28, 2012 2:03PM - 2:15PM |
J53.00015: Tricriticality in constraint percolation L. Cao, J. M. Schwarz Constraint percolation goes beyond ordinary percolation to include constraints on the occupation of sites/bonds. For instance, $k$-core site percolation implements a geometric constraint requiring each occupied vertex on a network have at least $k$ occupied neighboring vertices. It turns out that the percolation transition in such a model is essentially equivalent to the study of a dynamical glass transition in the Fredrickson-Andersen model, one of the models underlying the kinetically-constrained approach to the glass transition. We study hetereogenous $k$-core bond percolation on a random network with $f$ denoting the probability of a $k=2$-core vertex and $1-f$ the probability of a $k=3$-core vertex. This model corresponds to a hetereogeneous extension of the Frederickson-Anderson model. For $f=1$, the percolation transition is continuous, while for $f=0$, it is discontinuous. Using a master equation approach, we show that there exists a tricritical point at $f=1/2$ with a new order parameter exponent of unity. Our results are consistent with other mean field results obtained via a different method. We also look for tricriticality beyond mean field by investigating another constraint percolation model dubbed force-balance percolation. [Preview Abstract] |
Session J54: Focus Session: Complex and Co-evolving Networks - Empirical Studies of Social Networks
Sponsoring Units: GSNPChair: Gene Stanley, Boston University
Room: 152
Tuesday, February 28, 2012 11:15AM - 11:27AM |
J54.00001: Modeling users' activity on Twitter networks: validation of Dunbar's number Bruno Goncalves, Nicola Perra, Alessandro Vespignani Microblogging and mobile devices appear to augment human social capabilities, which raises the question whether they remove cognitive or biological constraints on human communication. In this paper we analyze a dataset of Twitter conversations collected across six months involving 1.7 million individuals and test the theoretical cognitive limit on the number of stable social relationships known as Dunbar's number. We find that the data are in agreement with Dunbar's result; users can entertain a maximum of 100-200 stable relationships. Thus, the ``economy of attention'' is limited in the online world by cognitive and biological constraints as predicted by Dunbar's theory. We propose a simple model for users' behavior that includes finite priority queuing and time resources that reproduces the observed social behavior. [Preview Abstract] |
Tuesday, February 28, 2012 11:27AM - 11:39AM |
J54.00002: Entropy of dynamical social networks Kun Zhao, Marton Karsai, Ginestra Bianconi Dynamical social networks are evolving rapidly and are highly adaptive. Characterizing the information encoded in social networks is essential to gain insight into the structure, evolution, adaptability and dynamics. Recently entropy measures have been used to quantify the information in email correspondence, static networks and mobility patterns. Nevertheless, we still lack methods to quantify the information encoded in time-varying dynamical social networks. In this talk we present a model to quantify the entropy of dynamical social networks and use this model to analyze the data of phone-call communication. We show evidence that the entropy of the phone-call interaction network changes according to circadian rhythms. Moreover we show that social networks are extremely adaptive and are modified by the use of technologies such as mobile phone communication. Indeed the statistics of duration of phone-call is described by a Weibull distribution and is significantly different from the distribution of duration of face-to-face interactions in a conference. Finally we investigate how much the entropy of dynamical social networks changes in realistic models of phone-call or face-to face interactions characterizing in this way different type human social behavior. [Preview Abstract] |
Tuesday, February 28, 2012 11:39AM - 11:51AM |
J54.00003: The nature and perception of fluctuations in human musical rhythms Holger Hennig, Ragnar Fleischmann, Anneke Fredebohm, York Hagmayer, Jan Nagler, Annette Witt, Fabian Theis, Theo Geisel Although human musical performances represent one of the most valuable achievements of mankind, the best musicians perform imperfectly. Musical rhythms are not entirely accurate and thus inevitably deviate from the ideal beat pattern. Nevertheless, computer generated perfect beat patterns are frequently devalued by listeners due to a perceived lack of human touch. Professional audio editing software therefore offers a humanizing feature which artificially generates rhythmic fluctuations. However, the built-in humanizing units are essentially random number generators producing only simple uncorrelated fluctuations. Here, for the first time, we establish long-range fluctuations as an inevitable natural companion of both simple and complex human rhythmic performances [1]. Moreover, we demonstrate that listeners strongly prefer long-range correlated fluctuations in musical rhythms. Thus, the favorable fluctuation type for humanizing interbeat intervals coincides with the one generically inherent in human musical performances. [1] HH et al., PLoS ONE,6,e26457 (2011) [Preview Abstract] |
Tuesday, February 28, 2012 11:51AM - 12:03PM |
J54.00004: Structural and social aspects of human mobility James Bagrow, Yu-Ru Lin Research on human mobility has been revolutionized by cellular phone data, capturing activity patterns across extensive populations. A number of interesting features have been discovered, including the ultra-slow growth of human mobility patterns, which cannot be reproduced by traditional random-walk models. However, the spatiotemporal flows and detailed microstructure of human mobility have not been well studied. Inferring complex mobility networks from country-wide data from mobile phone data, we find that human mobility is dominated by a small group of frequently visited and dynamically close locations, forming a primary ``habitat'' that captures typical behavior, along with subsidiary habitats representing additional travel. These habitats are both well separated and spatially compact. We find that motion within habitats exhibits distinct temporal scaling and that the time delay to enter subsidiary habitats is a primary factor in the spatiotemporal growth of human travel. Mobility is also coupled with social activity. Interestingly, many phone users possess habitats that occupy single temporal and social contexts and display high temporal and social predictability when occupying subsidiary habitats, revealing new connections between human activity and mobility dynamics. [Preview Abstract] |
Tuesday, February 28, 2012 12:03PM - 12:15PM |
J54.00005: Temporal and spatial regularity of mobile-phone data Philipp Hoevel, Albert-Laszlo Barabasi Network science is a vibrant, interdisciplinary research area with strong connections to a plethora of different fields. As the amount of empirically obtained datasets increases more and more, approaches from network sciences continue to enhance our understanding, for instance, of human dynamics. The available data often consist of temporal as well as spatial information. In our case they originate from anonymized mobile-phone traces, which include information about the timing of the connections between two mobile phones and also their positions. Thus, the data contains an additional social component. In this study, we evaluate patterns of human behavior identifying both temporal and spatial regularity. This leads to a detailed mobility analysis on various timescales and contributes to a general theory of synchronization in complex, real-world networks. [Preview Abstract] |
Tuesday, February 28, 2012 12:15PM - 12:27PM |
J54.00006: Scale-free correlations in the geographical spreading of obesity Lazaros Gallos, Pablo Barttfeld, Shlomo Havlin, Mariano Sigman, Hernan Makse Obesity levels have been universally increasing. A crucial problem is to determine the influence of global and local drivers behind the obesity epidemic, to properly guide effective policies. Despite the numerous factors that affect the obesity evolution, we show a remarkable regularity expressed in a predictable pattern of spatial long-range correlations in the geographical spreading of obesity. We study the spatial clustering of obesity and a number of related health and economic indicators, and we use statistical physics methods to characterize the growth of the resulting clusters. The resulting scaling exponents allow us to broadly classify these indicators into two separate universality classes, weakly or strongly correlated. Weak correlations are found in generic human activity such as population distribution and the growth of the whole economy. Strong correlations are recovered, among others, for obesity, diabetes, and the food industry sectors associated with food consumption. Obesity turns out to be a global problem where local details are of little importance. The long-range correlations suggest influence that extends to large scales, hinting that the physical model of obesity clustering can be mapped to a long-range correlated percolation process. [Preview Abstract] |
Tuesday, February 28, 2012 12:27PM - 1:03PM |
J54.00007: Mining networks of human contact with wearable sensors Invited Speaker: Alain Barrat Due to the development of sensors of various types and the use of digital media and computational devices, we increasingly leave digital traces of our daily activities. The scale at which such data can be gathered and analyzed makes possible a novel, data-driven approach to the investigation of various aspects of human behavior. In this talk, I will focus on the research done within the SocioPatterns project (www.sociopatterns.org), in which we have developed the SocioPatterns sensing platform to obtain longitudinal datasets on face-to-face contact events between individuals in a variety of contexts ranging from scientific conferences to museum, schools or hospitals. The gathered data sets consists in dynamic networks of human contacts, and their analysis reveal interesting similarities and differences of human interaction patterns across contexts. I will also consider the impact of the temporal resolution, which allows to take into account causality constraints, on dynamical processes occurring on networks, such as spreading processes. [Preview Abstract] |
Tuesday, February 28, 2012 1:03PM - 1:15PM |
J54.00008: Are we in our travel decisions self-determined? Christian Schneider, Thomas Couronne, Zbigniew Smoreda, Marta Gonzalez Mobile phone data, as saved by every phone provider worldwide, allows us to extract information about human mobility. It can be mainly used to study the locations and routes of each mobile phone user during entire months. In order to gain deeper understanding of the inherent travel decisions of the daily trips measured by phone data, we compare them statically with those extracted from 10,000 trajectories reported in a travel diary survey. We identify and compare from both data sets the underlying trip decisions networks or motifs. Interestingly, although millions of different motifs are possible, in both data sets we found similar motif distributions. Hence, we develop a simple model, which could reproduce not only the size distribution of the motifs, but the motifs themselves and answer the opening question. [Preview Abstract] |
Tuesday, February 28, 2012 1:15PM - 1:27PM |
J54.00009: A cross-section test of Cobb-Douglass production function between market capitalization and GDP Adam Avakian, Boris Podobnik, H. Eugene Stanley Most classical economic theories imply that both (a) economies with lower output (GDP) per person tend to grow faster in per capita terms and (b) economies with lower capital per person tend to grow faster in per capita terms. It is well-known that the former was found to be wrong. Taking market capitalization as a proxy for physical capital, we analyze a cohort of countries over a 17-year period (1994-2010) and we find the latter statement in agreement with empirical data implying a contradictive result that while capital data worldwide tend to converge, GDP data tend to diverge. However, for the countries analyzed, for which we have both market capitalization and GDP data, we find that even economies with lower output (GDP) per person tend to grow faster in per capita terms. The result that for all countries one obtains divergence while for a group of countries having both market capitalization and GDP data we have convergence is in contrast with the refutation of (a) but our results only apply to countries that have an exchange market, and are thus participating in globalization, indicating the convergent effect of globalization. [Preview Abstract] |
Tuesday, February 28, 2012 1:27PM - 1:39PM |
J54.00010: Statistical Analysis on the Korean Transportation System Okyu Kwon, Gabjin Oh, Woo-Sung Jung We investigate the Korean transportation system including subway, highway and express bus systems. The network topology and traffic pattern of the Korean transportation system is studied. We find the transportation have many complexity features such as scale-free property. We also find the relationship between the traffic flow, and the population and the distance. [Preview Abstract] |
Tuesday, February 28, 2012 1:39PM - 1:51PM |
J54.00011: The Growth Dynamics of Words: How Historical Context Shapes the Competitive Linguistic Environment Joel Tenenbaum, Alexander Petersen, Shlomo Havlin, H. Eugene Stanley Using the massive Google n-gram database of over 10$^{11}$ word uses in English, Hebrew, and Spanish, we explore the connection between the growth rates of relative word use and the observed growth rates of disparate competing actors in a common environment such as businesses, scientific journals, and universities, supporting the concept that a language's lexicon is a generic arena for competition, evolving according to selection laws. We find aggregate-level anomalies in the collective statistics corresponding to the time of key historical events such as World War II and the Balfour Declaration. [Preview Abstract] |
Tuesday, February 28, 2012 1:51PM - 2:03PM |
J54.00012: Voronoi Cell Patterns: Application of the size distribution to societal systems Rajesh Sathiyanarayanan, Diego Luis Gonz\'alez, Alberto Pimpinelli, T.L. Einstein In studying the growth of islands on a surface subjected to a particle flux, we found it useful to characterize the distribution of the areas of associated Voronoi (proximity or Wigner-Seitz) cells in terms of the generalized Wigner surmise\footnote{AP \& TLE, PRL 99 (2007) 226102; PRL 104 (2010) 149602} and the gamma distributions. Here we show that the same concepts and distributions are useful in analyzing several problems arising in society.\footnote{DLG et al., arXiv 1109.3994; RS, Ph.D. dissertation; RS et al., preprint} We analyze the 1D problem of the distribution of gaps between parked cars, assuming that successive cars park in the middle of vacant spaces, and compare with published data. We study the formation of second-level administrative divisions, e.g. French {\it arrondissements}. We study the actual distribution of {\it arrondissements} and the Voronoi tessellation associated with the chief town in each. While generally applicable, there are subtleties in some cases. Lastly, we consider the pattern formed by Paris M\'etro stations and show that near the central area, the associated Voronoi construction also has this sort of distribution. [Preview Abstract] |
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