Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U1: Invited Session: Hidden Order in URu2Si2 and Possibly Related Compounds
Sponsoring Units: DCMP DCPChair: John Mydosh, Kamerlingh Onnes Lab
Room: Ballroom I
Thursday, March 21, 2013 11:15AM - 11:51AM |
U1.00001: Symmetry Breaking in the Hidden-Order Phase of URu$_2$Si$_2$ Invited Speaker: Takasada Shibauchi In the heavy fermion compound URu$_2$Si$_2$, the hidden-order transition occurs at 17.5\,K, whose nature has posed a long-standing mystery. A second-order phase transition is characterized by spontaneous symmetry breaking, and thus the nature of the hidden order cannot be determined without understanding which symmetry is being broken. Our magnetic torque measurements in small pure crystals reveal the emergence of an in-plane anisotropy of the magnetic susceptibility below the transition temperature [1], indicating the spontaneous breaking of four-fold rotational symmetry of the tetragonal URu$_2$Si$_2$. In addition, our recent observation of cyclotron resonance allows the full determination of the electron-mass structure of the main Fermi-surface sheets, which implies an anomalous in-plane mass anisotropy [2] consistent with the rotational symmetry breaking. These results impose strong constraints on the symmetry of the hidden order parameter.\\[4pt] [1] R. Okazaki {\it et al.,} Science {\bf 331}, 439 (2011).\\[0pt] [2] S. Tonegawa {\it et al.,} Phys. Rev. Lett. {\bf 109}, 036401 (2012). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U1.00002: Neutron scattering study of URu$_2$Si$_2$ magnetic properties: from hydrostatic pressure to uniaxial stress Invited Speaker: Frederic Bourdarot Since the discovery of the unusual magnetic and superconducting properties of URu$_2$Si$_2$ in 1985 by Palstra~[1], this heavy fermion has been extensively studied. A ``Hidden Order'' evidences by bulk properties like specific heat, has been found below $T_0$=17.8K. Neutron scattering in this case is an efficient probe for the study of this compound as large magnetic excitations and an irremovable tiny antiferromagnetic moment are present in this sample. Even though the tiny antiferromagnetic moment aligned along the $c$-axis at $Q_0$ is only $\sim0.01 \mu_B$, the magnetic excitations seem to be associated to a large magnetic moment of $\sim$ 1 $\mu_B$ and show two minimums at $Q_0$=(1,0,0) but also at $Q_1$=(0.6,0,0). These magnetic responses have been intensively studied in normal conditions by Broholm~[2,3] and our group[4], but also versus magnetic field~[5], and more recently under hydrostatic pressure~[6]. The result of these experiments seem to indicate that the Hidden Order is linked to the excitation at $Q_0$ and not to the excitation at $Q_1$. We will present the revisited magnetic properties of URu$_2$Si$_2$ under uniaxial stress along the $a$-axis~[7,8]. Both elastic and inelastic contributions have been measured versus the constraints. In the HO state, as the constraint increases, the AF gap excitation at $Q_0$ decreases and the tiny moment increases: it seems also that there is a relation between both parameters. On the other hand, the excitation gap at $Q_1$ is slightly increasing. From our measurement we infer a critical pressure of $\sim$ 0.33~GPa, with a large increase of the antiferromagnetic moment. This behavior is very similar to results under hydrostatic pressure. Combining hydrostatic pressure, uniaxial stress along the $a$-axis and neutron Larmor diffraction measurements, that gives the lattice distribution of our URu$_2$Si$_2$ crystal, we conclude that the magnetic exchange integrals are dominated by the lattice parameter $a$ and not the ratio $c/a$ as usually believed. \\ \noindent[1]~T.~T.~M. Palstra, \textit{et al.}, Physical Review Letters {\bf 55}, 2727 (1985).\\ \noindent[2]~C. Broholm, \textit{et al.}, Physical Review Letters {\bf 58}, 1467 (1987).\\ \noindent[3]~C. Broholm, \textit{et al.}, Physical Review B {\bf 43}, 12809 (1991).\\ \noindent[4]~F. Bourdarot, \textit{et al.}, Journal of the Physical Society of Japan {\bf 79}, 064719 (2010).\\ \noindent[5]~F. Bourdarot, \textit{et al.}, Physical Review Letters {\bf90}, 067203 (2003).\\ \noindent[6]~A. Villaume, \textit{et al.}, Physical Review B {\bf 78}, 012504 (2008).\\ \noindent[7]~M. Yokoyama, \textit{et al.}, Physical Review B {\bf 72}, 214419 (2005).\\ \noindent[8]~F. Bourdarot, \textit{et al.}, Physical Review B {\bf 84}, 184430 (2011). [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U1.00003: The hidden order phase in URu$_2$Si$_2$: Remarkable nesting and spin-orbital hybridization Invited Speaker: Peter Oppeneer Aspects of Fermi surface (FS) nesting properties of URu$_2$Si$_2$ are analyzed with particular focus on their implication for the mysterious hidden order phase which occurs at 17.5~K. We show that there exist two Fermi surfaces that exhibit unusually strong nesting at the antiferromagnetic wavevector, $\mathbf{Q}_0$=(0,\,0,\,1). The corresponding energy dispersions fulfill the relation $\epsilon_{1}(\mathbf{k})$=$- \epsilon_{2} (\mathbf{k}\pm \mathbf{Q}_0)$ at eight FS hotspot lines on the surfaces. Notably, the spin-orbital characters of the involved $5f$ states are {\it different}: $j_z$=$\pm$5/2 {\it vs.} $\pm$3/2, and hence the occurring degenerate Dirac crossings are symmetry protected in the nonmagnetic normal state. Pairing of electrons in these two FSs can commence through interaction with a quasiparticle with wavevector $\mathbf{Q}_0$ and exchange of longitudinal angular momentum $\Delta j_z$. Dynamical symmetry breaking through an Ising-like spin-orbital excitation mode at $\mathbf{Q}_0$ with $\Delta j_z$=$\pm$1 induces a hybridization of the two states, causing substantial FS gapping. Concomitant spin and orbital currents in the uranium planes can give rise to a rotational symmetry breaking. The existence of such specifically nested FSs in URu$_2$Si$_2$ is confirmed in recent experiments.\\[4pt] This work has been performed with S. Elgazzar, J. Rusz, Q. Feng, T. Durakiewicz and J.A. Mydosh. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U1.00004: A Hund's rule mechanism for Hidden Spin-Orbital Density Wave in URu$_2$Si$_2$ Invited Speaker: Peter Riseborough It is proposed that the ``Hidden Order'' state of URu$_2$Si$_2$ corresponds to a combined spin-orbital density wave state, which is stabilized by the inter-orbital Hund's rule coupling. The electronic system is described by the underscreened Anderson Lattice Model, in which there are two-fold degenerate f bands which hybridize with a single conduction band. In the normal state, the bands at the Fermi-energy have almost pure 5f orbital characters in accord with the results of first principles electronic structure calculations. The model Fermi-surface has heavy fermion sheets which exhibit interband nesting and intraband nesting with similar wave vectors. The spin-flip terms of the Hund's rule interaction and the interband nesting produces a second-order phase transition which partially gaps the Fermi-surface, and leads to a state with broken spin-rotational invariance without forming a net ordered magnetic moment. The resulting spin nematic phase is consistent with the magnetic torque experiments of Okazaki {\it et al.}. The similarity of the interband nesting and the intraband nesting conditions leads to an adiabatic continuity between the ``Hidden Order'' and Antiferromagnetic phases for small values of the hybridization. The presence of a nearby hybridization gap results in an asymmetric form of the pseudogap caused by the ``Hidden Order'' transition. Precursor fluctuations of the hidden order parameter, above $T_{HO}$, lead to the formation of ``hot spots'' on the Fermi-surface and a depletion of the density of states in the vicinity of the Fermi-energy as is seen by point contact and optical spectroscopies. The amplitude of the precursor fluctuations increase as $T_{HO}$ is driven towards zero, however, the order of the transition switches from second-order to first-order pre-empting the quantum critical point. These results in accord with the change in the order of the transition inferred by Jaime {\it et al.} from measurements of the specific heat in an applied magnetic field. This model might also be applicable to the enigmatic pseudo-gap phases seen in high-temperature superconductors. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U1.00005: Hastatic Order in URu$_2$Si$_2$ Invited Speaker: Premala Chandra The development of collective long-order via phase transitions occurs by the spontaneous breaking of fundamental symmetries. Magnetism is a consequence of broken time-reversal symmetry while superfluidity results from broken gauge invariance. The broken symmetry that develops below 17.5 K in the heavy fermion compound URu$_2$Si$_2$ has long eluded such identification. In this talk we show that the recent observation of Ising quasiparticles in URu$_2$Si$_2$ results from a spinor order parameter that breaks {\sl double} time-reversal symmetry, mixing states of integer and half-integer spin. Such ``hastatic order'' hybridizes conduction electrons with Ising $5f^2$ states of the uranium atoms to produce Ising quasiparticles; it accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Hastatic order also results in a number of predictions for future experiment: a tiny transverse moment in the conduction sea, a collosal Ising anisotropy in the nonlinear susceptbility and a resonant energy-dependent nematicity in the tunneling density of states. [Preview Abstract] |
Session U2: Invited Session: Topological Insulators: Surface State Transport
Sponsoring Units: DCMPChair: Joel Moore, University of California, Berkeley
Room: Ballroom II
Thursday, March 21, 2013 11:15AM - 11:51AM |
U2.00001: Quantum transport in topological insulator nanowires and thin films Invited Speaker: Jens H. Bardarson Topological insulators have an insulating bulk but a metallic surface. In the simplest case, the surface electronic structure of a 3D topological insulator is described by a single 2D Dirac cone. The transport properties of such a surface state are of considerable current interest; they have some similarities with graphene, which also realizes Dirac fermions, but have several unique features in their response to magnetic fields. In this talk, I give an overview of some of the main quantum transport properties of topological insulator surfaces. I focus on the efforts to use quantum interference phenomena, such as weak anti-localization and the Aharonov-Bohm effect, to verify in a transport experiment the Dirac nature of the surface state and its defining properties. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U2.00002: The strong, weak and anomalous sides of weak topological insulators Invited Speaker: Zohar Ringel Disorder and topology can be thought of as two counter-driving forces. While the former pushes electron wave functions to localize in space, the latter requires them to remain coherent over the entire system. We study the interplay between these two on the surface of a ``weakly'' topological phase- the Weak Topological Insulator. Using arguments based on flux-insertions and locality, we show that such surfaces cannot undergo a localization transition even when the surface is strongly disordered. We also present a numerical study which further quantifies this result. We then reformulate the same notions, in field theory language, using a novel $Z_2$-charge-anomaly. This anomaly generalizes the $Z$-charge-anomaly associated with edges of the Integer Quantum Hall Effect. Besides unifying various aspects of Topological Insulators, the anomaly allows us to calculate new topological properties of TIs in the presence of electric fields. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U2.00003: Surface conduction of topological Dirac electrons in bulk insulating Bi$_{2}$Se$_{3}$ Invited Speaker: Michael Fuhrer The three dimensional strong topological insulator (STI) is a new phase of electronic matter which is distinct from ordinary insulators in that it supports on its surface a conducting two-dimensional surface state whose existence is guaranteed by topology. I will discuss experiments on the STI material Bi$_{2}$Se$_{3}$, which has a bulk bandgap of 300 meV, much greater than room temperature, and a single topological surface state with a massless Dirac dispersion. Field effect transistors consisting of thin (3-20 nm) Bi$_{2}$Se$_{3}$ are fabricated from mechanically exfoliated from single crystals, and electrochemical and/or chemical gating methods are used to move the Fermi energy into the bulk bandgap, revealing the ambipolar gapless nature of transport in the Bi$_{2}$Se$_{3}$ surface states. The minimum conductivity of the topological surface state is understood within the self-consistent theory of Dirac electrons in the presence of charged impurities. The intrinsic finite-temperature resistivity of the topological surface state due to electron-acoustic phonon scattering is measured to be $\sim$60 times larger than that of graphene largely due to the smaller Fermi and sound velocities in Bi$_{2}$Se$_{3}$, which will have implications for topological electronic devices operating at room temperature.~As samples are made thinner, coherent coupling of the top and bottom topological surfaces is observed through the magnitude of the weak anti-localization correction to the conductivity, and, in the thinnest Bi$_{2}$Se$_{3}$ samples ($\sim$ 3 nm), in thermally-activated conductivity reflecting the opening of a bandgap. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U2.00004: Prediction of weak and strong topological insulators in layered semiconductors. Invited Speaker: Claudia Felser We investigate a new class of ternary materials such as LiAuSe and KHgSb with a honeycomb structure in Au-Se and Hg-Sb layers. We demonstrate the band inversion in these materials similar to HgTe, which is a strong precondition for existence of the topological surface states. In contrast with graphene, these materials exhibit strong spin-orbit coupling and a small direct band gap at the point. Since these materials are centrosymmetric, it is straightforward to determine the parity of their wave functions, and hence their topological character. Surprisingly, the compound with strong spin-orbit coupling (KHgSb) is trivial, whereas LiAuSe is found to be a topological insulator. However KHgSb is a weak topological insulators in case of an odd number of layers in the primitive unit cell. Here, the single-layered KHgSb shows a large bulk energy gap of 0.24 eV. Its side surface hosts metallic surface states, forming two anisotropic Dirac cones. Although the stacking of even-layered structures leads to trivial insulators, the structures can host a quantum spin Hall layer with a large bulk gap, if an additional single layer exists as a stacking fault in the crystal. The reported honeycomb compounds can serve as prototypes to aid in the finding of new weak topological insulators in layered small-gap semiconductors. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U2.00005: Manifestation of topological protection in transport properties of epitaxial Bi$_2$Se$_3$ thin films Invited Speaker: Alexey Taskin A topological insulator is a new quantum state of matter which can be realized in some materials with a strong spin-orbit coupling. Due to the spin-momentum locking, massless Dirac fermions residing on the surface of a topological insulator are protected from backscattering and cannot be localized by disorder. However, such protection can be lifted in ultrathin films when the three-dimensionality of the sample is lost due to hybridization between top and bottom surfaces. Recently, using Molecular Beam Epitaxy, we succeeded in growing Bi$_2$Se$_3$ thin films of sufficiently high quality to present quantum oscillations in magnetotransport [1]. By measuring the Shubnikov-de Haas oscillations in a series of high-quality films, we revealed a systematic evolution of the surface conductance as a function of thickness and found a striking manifestation of the topological protection [2]: The metallic surface transport abruptly diminishes below the critical thickness of $\sim$6 nm, at which an energy gap opens in the surface state and the Dirac fermions become massive. At the same time, the weak antilocalization behavior is found to weaken in the gapped phase due to the loss of $\pi$ Berry phase. Our results demonstrate the importance of the spin and momentum coupling in maintaining the topological protection of the surface carriers in topological insulators.\\[4pt] [1] A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando, Adv. Mater. {\bf 24}, 5581 (2012). \newline [2] A. A. Taskin, S. Sasaki, K. Segawa, and Y. Ando, Phys. Rev. Lett. {\bf 109}, 066803 (2012). [Preview Abstract] |
Session U3: Invited Session: Application of the First-Principles and Atomistic Methods to Nuclear Detection Materials
Sponsoring Units: DCOMP DCMPChair: David Beach, Department of Energy, National Nuclear Security Administration
Room: Ballroom III
Thursday, March 21, 2013 11:15AM - 11:51AM |
U3.00001: Point Defect Properties of Cd(Zn)Te and TlBr for Room-Temperature Gamma Radiation Detectors Invited Speaker: Vincenzo Lordi The effects of various crystal defects in CdTe, Cd$_{1-x}$Zn$_x$Te (CZT), and TlBr are critical for their performance as room-temperature gamma radiation detectors. We use predictive first principles theoretical methods to provide fundamental, atomic scale understanding of the defect properties of these materials to enable design of optimal growth and processing conditions, such as doping, annealing, and stoichiometry. Several recent cases will be reviewed, including (i) accurate calculations of the thermodynamic and electronic properties of native point defects and point defect complexes in CdTe and CZT; (ii) the effects of Zn alloying on the native point defect properties of CZT; (iii) point defect diffusion and binding related to Te clustering in Cd(Zn)Te; (iv) the profound effect of native point defects---principally vacancies---on the intrinsic material properties of TlBr, particularly electronic and ionic conductivity; (v) tailored doping of TlBr to independently control the electronic and ionic conductivity; and (vi) the effects of metal impurities on the electronic properties and device performance of TlBr detectors. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U3.00002: First-principles calculations of self-trapping of carriers and excitons in NaI and SrI$_2$ Invited Speaker: Daniel Aberg While the general potential of scintillators as radiation detectors has been demonstrated, one of the current goals is to develop materials with improved energy resolution sufficient to detect fissile materials with a low probability of errors at ports, borders, and airports. The poor resolution has been linked to the non-linear response to the gamma ray energy. Fundamental understanding of this requires detailed knowledge of elementary electronic excitation processes. In particular, in most metal halide scintillators charge carriers and excitations localize and create self-trapped species associated with large effective masses and slow diffusivities. First-principles modeling is essential for providing quantitative understanding of the involved microscopic processes. Here, we present comprehensive ab-initio calculations, with techniques ranging from hybrid DFT+exact exchange to self-consistent GW and Bethe-Salpeter approach, for modeling the electronic structure and mobilities of self-trapped carriers and excitons in metal halides with particular attention given to sodium and strontium iodide. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U3.00003: Multiscale Modeling of Crystal Growth and Microstructural Evolution of CdZnTe Invited Speaker: Charles Henager, Jr. Crystal growth models and modeling tools for CdTe and CZT along with experimental melt-growth data will be presented and discussed. The emphasis will be on creating a multiscale-modeling framework that can be applied to solve portions of the crystal quality and reproducibility problem of CZT crystals grown for high-resolution radiation detectors. The growth models and methods include ab initio models of CdTe, ab initio molecular dynamics (MD) models CdTe, MD of solidification of CdTe, equilibrium growth defects in CdTe, and development of coarser-scale microstructural evolution models using phase field methods. These model and theory results will be discussed in terms of designing a multiscale approach to two relevant problems in CZT crystal growth, namely solid-liquid interface (SLI) stability and concurrent defect generation in the hot but cooling CZT solid. This dovetails with recent experimental research focused on the growth of CdTe from Te-rich melts with an emphasis on SLI instability. Experimental data on SLI instabilities will be featured as well as results of transmitted IR data on Te-particle distributions in as-grown CZT. A new mechanism of Te-particle genesis and spatial arrangement in CdTe and CZT is discussed in terms of a Rayleigh instability mechanism coupled with crystallographic SLI instabilities during growth. However, there are gaps in our capabilities at every length and time scale, plus gaps in building coarse-grained models from fine-scale models, in statistical representations of complex equilibria, and in understanding the complexities of solidification in ternary alloy systems where coupled thermal, concentration, stress, liquid flow, and SLI morphological fields exist. The talk concludes with an assessment of methods and approaches to address desired models and simulations of CZT solidification from the melt. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U3.00004: First-principles Electronic Structure Calculations for Scintillation Phosphor Nuclear Detector Materials Invited Speaker: Andrew Canning Inorganic scintillation phosphors (scintillators) are extensively employed as radiation detector materials in many fields of applied and fundamental research such as medical imaging, high energy physics, astrophysics, oil exploration and nuclear materials detection for homeland security and other applications. The ideal scintillator for gamma ray detection must have exceptional performance in terms of stopping power, luminosity, proportionality, speed, and cost. Recently, trivalent lanthanide dopants such as Ce and Eu have received greater attention for fast and bright scintillators as the optical 5d to 4f transition is relatively fast. However, crystal growth and production costs remain challenging for these new materials so there is still a need for new higher performing scintillators that meet the needs of the different application areas. First principles calculations can provide a useful insight into the chemical and electronic properties of such materials and hence can aid in the search for better new scintillators. In the past there has been little first-principles work done on scintillator materials in part because it means modeling f electrons in lanthanides as well as complex excited state and scattering processes. In this talk I will give an overview of the scintillation process and show how first-principles calculations can be applied to such systems to gain a better understanding of the physics involved. I will also present work on a high-throughput first principles approach to select new scintillator materials for fabrication as well as present more detailed calculations to study trapping process etc. that can limit their brightness. This work in collaboration with experimental groups has lead to the discovery of some new bright scintillators. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U3.00005: DFT Studies of Semiconductor and Scintillator Detection Materials Invited Speaker: Koushik Biswas Efficient radiation detection technology is dependent upon the development of new semiconductor and scintillator materials with advanced capabilities. First-principles based approaches can provide vital information about the structural, electrical, optical and defect properties that will help develop new materials. In addition to the predictive power of modern density functional methods, these techniques can be used to establish trends in properties that may lead to identifying new materials with optimum properties. We will discuss the properties of materials that are of current interest both in the field of scintillators and room temperature semiconductor detectors. In case of semiconductors, binary compounds such as TlBr, InI, CdTe and recently developed ternary chalcohalide Tl6SeI4 will be discussed. Tl6SeI4 mixes a halide (TlI) with a chalcogenide (Tl2Se), which results in an intermediate band gap (1.86 eV) between that of TlI (2.75 eV) and Tl2Se (0.6 eV). For scintillators, we will discuss the case of the elpasolite compounds whose rich chemical compositions should enable the fine-tuning of the band gap and band edges to achieve high light yield and fast scintillation response. [Preview Abstract] |
Session U4: Invited Session: Quantum Reservoir Engineering and Feedback
Sponsoring Units: DCMP GQIChair: Steven Girvin, Yale University
Room: Ballroom IV
Thursday, March 21, 2013 11:15AM - 11:51AM |
U4.00001: Cavity-assisted quantum bath engineering Invited Speaker: Kater Murch In practice, quantum systems are never completely isolated, but instead interact with degrees of freedom in the surrounding environment, eventually leading to decoherence. Precision measurement techniques such as nuclear magnetic resonance and interferometry, as well as envisioned quantum schemes for computation, simulation, and data encryption, rely on the ability to prepare and preserve delicate quantum superpositions and entanglement. The conventional route to long-lived quantum coherence involves minimizing coupling to a dissipative bath. Paradoxically, it is possible to instead engineer specific couplings to a quantum environment that allow dissipation to actually preserve coherence. I will discuss our recent demonstration of quantum bath engineering for a superconducting qubit coupled to a microwave cavity. By tailoring the spectrum of microwave photon shot noise in the cavity, we create a dissipative environment that autonomously relaxes the qubit to an arbitrarily specified coherent superposition of the ground and excited states. In the presence of background thermal excitations, this mechanism increases the state purity and effectively cools the dressed atom state to a low temperature. We envision that future multi-qubit implementations could enable the preparation of entangled many-body states suitable for quantum simulation and computation. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U4.00002: Quantum measurement in action Invited Speaker: Michael Hatridge A quantum system subject to the infinitely-strong measurement of textbook physics undergoes a discontinuous, random state collapse. However, in practice, measurements often involve a finite-strength, continuous process whose iteration leads to a projective evolution only asymptotically. Moreover, if the observation apparatus is fully efficient informationally, the measured system can remain at all times in a pure state. The stochastic evolution of this pure state is trackable from the measurement record. Thus, an initial superposition of states can be usefully transformed by a partial measurement rather than be entirely destroyed. This striking property has been demonstrated in superconducting qubit experiments in which readout is performed by a microwave signal sent through a cavity dispersively coupled to the qubit, and thereafter processed by an amplifier operating at the quantum limit. Such accurate monitoring of a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U4.00003: Quantum feedback control in superconducting qubits: Towards creating and stabilizing entanglement in remote qubits Invited Speaker: Rajamani Vijayaraghavan Recent advances in superconducting parametric amplifiers have enabled quantum limited measurements of superconducting qubits, ushering in a new era of measurement based control using quantum feedback. Quantum entanglement is a key aspect of the measurement process. Measurement creates a pointer state which is entangled with the system being measured. Typically, one analyzes the pointer state which in turn determines the state of the original system. I will discuss experiments where we entangle the state of a 3D transmon qubit with a coherent microwave field (the pointer) using the circuit QED architecture. The use of parametric amplifiers to analyze the microwave field enables us to actually observe this entanglement and the resulting strong correlations between the states of the pointer and the qubit. We reconstruct quantum trajectories of the qubit state as it evolves during measurement and show that the final state of the qubit is consistent with the trajectories. Further, we use quantum feedback to actively steer the state of the qubit and demonstrate Rabi oscillations which persist indefinitely [1]. Finally, I will discuss how we can use the pointer states to generate entanglement between remote qubits and stabilize them using feedback. Applications to quantum computing and quantum error correction will also be discussed.\\[4pt] [1] R.Vijay et al., Nature 490, 77-80 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U4.00004: Quantum Feedback for preparing and stabilizing photon number states of a field stored in a cavity Invited Speaker: Michel Brune The stabilization of complex classical systems requires feedback. A sensor performs measurements of the system's state whose result is fed into a controller, which decides on an action bringing the system closer to a target state. Operating feedback for preparing and stabilizing against decoherence a quantum state is a promising tool for quantum control. It is however much more demanding than its classical counterpart, since a quantum measurement by the sensor changes the measured state. We present the first continuous operation of a closed feedback-loop for preparing and stabilizing photon number states of a microwave field stored in a high Q superconducting cavity. The field is probed by non-resonant Rydberg atoms performing a Quantum Non-Demolition photon counting. The feedback action consists either in the injection of a small coherent field pulse with a controlled amplitude and phase or in the emission and absorption of single photons with individual resonant atoms. The atomic measurement results are fed into a real-time controller, which performs an estimation of the field's state before deciding on the actuator action bringing it closer to the target. We stabilize number states up to 7. We discuss perspectives for the stabilization of mesoscopic quantum superpositions. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U4.00005: Experimental quantum error correction with trapped ions Invited Speaker: Philipp Schindler The computational potential of a quantum information processor can only be utilized if errors occurring during a quantum computation can be controlled and corrected for. Quantum error correction protocols encode the quantum information of a single qubit in a larger register. Errors are then corrected by a quantum-feedback algorithm that is applied repeatedly. We encode a single logical qubit into three physical qubits and perform multiple rounds of error correction with the aid of high-fidelity gate operations and a reset technique for the auxiliary qubits. Furthermore we demonstrate that the same technique can be used to undo a quantum measurement. Full quantum error correction schemes are able to correct for arbitrary errors and enable universal quantum computation, but they require a significant overhead in the number of qubits. This prevents them to be useful for medium-scale systems used for quantum simulation. Therefore, we develop a quantum feedback scheme to reduce the dominant errors in an open-system quantum simulator. Our scheme requires only a single auxiliary qubit regardless of the system size. [Preview Abstract] |
Session U5: Graphene: Transport and Optical Phenomena: Mesoscopics and Harmonic Generation
Sponsoring Units: DCMPChair: Chris Stanton, University of Florida
Room: 301
Thursday, March 21, 2013 11:15AM - 11:27AM |
U5.00001: Resonant inelastic transmission through a time-modulated region in graphene Li Chang, T. L. Liu, C. S. Chu We investigate a number of resonant transmission processes through a time-modulated-potential region in graphene. Incident energies covering both low and high energy regimes are included, and the time-dependent transmission is treated within a tight-binding model. Three main results are obtained. Dip structures in the transmission are obtained when a band edge is involved. It can occur in the low energy regime, if the graphene is gapped, or in the high energy regime, when a graphene band edge is in the energy neighborhood. These dip structures cause significant deviation from Klein-type perfect transmission. Non-typical Fabry-P\'{e}rot interference is observed when, staying upon a dip structure condition, the transmission exhibits an oscillation that has a longer than expected period in $L$, the width of the time-modulated region. Central band refocusing is found in the low energy regime, where the dominance in the transmission by the central-band will occur periodically with $L$. In all these results, we have demonstrated and analyzed detail intricate resonant interplays between sideband processes. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U5.00002: Charge transport across tunable superlattice barriers in graphene Sudipta Dubey, Ajay Bhat, Vibhor Singh, Pritesh Parikh, Tanuj Prakash, Abhilash Sebastian, Padmalekha K.G., Krishnendu Sengupta, Vikram Tripathi, Rajdeep Sensarma, Mandar Deshmukh We create an artificial superlattice structure in graphene using an array of top gate and a bottom gate. A superlattice potential modifies the band structure of graphene, so that extra Dirac points appear in the dispersion periodically as a function of the superlattice barrier height. Tuning the amplitude of the barrier thus gives us control over number of Dirac points generated. We have performed measurements on this superlattice structure. Oscillations in resistance are observed when the charge carrier induced by top gate and back gate are of opposite sign. In this region, the number of oscillations increases with increasing gate voltage. Measurements as a function of temperature show that these oscillations persist even at 70 K. The behaviour of these oscillations in presence of magnetic field is also observed. At low magnetic field we see weak localisation behaviour. At high magnetic field, the superlattice is a small perturbation and quantum Hall effect of pristine graphene is restored. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U5.00003: Graphene under spatially varying external potentials: Landau levels, magnetotransport, and topological modes Si Wu, Matthew Killi, Arun Paramekanti Superlattices (SLs) in monolayer and bilayer graphene, formed by spatially periodic potential variations, lead to a modified bandstructure with extra finite-energy and zero-energy Dirac fermions with tunable anisotropic velocities. We theoretically show that transport in a weak perpendicular (orbital) magnetic field allows one to not only probe the number of emergent Dirac points but also yields further information about their dispersion. or monolayer graphene, we find that a moderate magnetic field can lead to a strong reversal of the transport anisotropy imposed by the SL potential, an effect which arises due to the SL induced dispersion of the zero energy Landau levels. This effect may find useful applications in switching or other devices. For bilayer graphene, we discuss the structure of Landau level wave functions and local density of states in the presence of a uniform bias, as well as in the presence of a kink in the bias which leads to topologically bound `edge states'. We consider implications of these results for scanning tunneling spectroscopy measurements, valley filtering, and impurity induced breakdown of the quantum Hall effect in bilayer graphene. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U5.00004: Confinement, transport gap, and valley polarization from a double barrier structure in graphene Daniel Gunlycke, Carter White Engineering a gap in graphene without degrading its exceptional transport properties is arguably the main obstacle preventing a breakthrough in graphene-based nanoelectronics. To create such a gap, a lot of effort has been devoted to making graphene nanoribbons. Unlike ordinary nanoribbons, we propose a structure formed between two thin parallel transport barriers that is penetrable by electrons in surrounding graphene states. The transport across this railroad track structure is governed by resonant tunneling through quasi-bound states within the confinement. The transport barriers, modeled by chemically decorated line defects, are highly reflective, causing the resonances to form continuous bands closely matching the band structure of a zigzag ribbon. Because boundary-localized states cannot carry any transport, the resonance bands must terminate at the dimensional crossover between extended and boundary-localized states. As the confined region contains no states near the Fermi level extending across the railroad track structure, electrons approaching it experience a transport gap $E_g=2\hbar v_F/W$, where $W$ is the separation between the barriers. In addition to offering confinement and a transport gap, the structure allows for nearly perfect valley polarization. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U5.00005: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U5.00006: Transport Spectroscopy of gate controlled cavity in CVD bilayer graphene transistor Kyunghoon Lee, Yun Suk Eo, Cagliyan Kurdak, Zhaohui Zhong Graphene nanostructure provides an ideal platform for understanding distinctive quantum transport properties such as Klein tunneling and suppression of backscattering due to its chiral nature. Quantum interference of phase coherent electron waves in single-layer graphene has attracted wide attention recently, while few experimental works examine the quantum transport of massive Dirac Fermion in bilayer graphene. To this end, we report the low temperature electrical transport spectroscopy of gate controlled cavity in CVD bilayer graphene transistor. Fabry-Perot like conductance oscillation was observed in both monopolar and bipolar bilayer graphene structures defined by electrostatic gating. Transport comparison between single-layer graphene and bilayer graphene will be also discussed. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U5.00007: Tunable superconductivity in decorated graphene Zheng Han, Adrien Allain, Laetitia Marty, Nedjma Bendiab, Pierre Toulemonde, Pierre Strobel, Johann Coraux, Vincent Bouchiat Graphene offers an exposed bidimensional gas of high mobility charge carriers with gate tunable density. Its chemical inertness offers an outstanding platform to explore exotic 2D superconductivity. Superconductivity can be induced in graphene by means of proximity effect (by depositing a set of superconducting metal clusters such as lead [1] or tin nanoparticles). The influence of decoration material, density or particles and disorder of graphene will be discussed. In the case of disordered graphene, Tin decoration leads to a gate-tunable superconducting-to-insulator quantum phase transition [2]. Superconductivity in graphene is also expected to occur under strong charge doping [3] (induced either by gating or under chemical decoration [4], in analogy with graphite intercalated compounds). I will also show preliminary results showing the influence of Calcium intercalation of few layer graphene and progress toward the demonstration of intrinsic superconductivity in such systems. [1] B. Kessler et al, Phys. Rev. Lett., \textbf{104}, 047001 (2010). [2] A. Allain et al, Nature Materials, \textbf{11 }, 590 (2012). [3] B. Uchoa and A. H. Castro Neto. Phys. Rev. Lett., \textbf{98}, 146801 (2007). [4] G. Profeta, et al., Nature Physics \textbf{8}, 131 (2012). [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U5.00008: Electronic transport experiments on adatom-decorated graphene E.A. Henriksen, J.P. Eisenstein Single-layer graphene is expected to exhibit a wide range of novel behaviors when decorated with a disperse coating of various adatom species. Toward conducting experiments on these systems, we are developing a cryogenic ultra-high vacuum probe with the capability to explore the electronic transport of graphene and other materials that have been cleaned and annealed {\it in situ}, followed by coating via the controlled deposition of sub-monolayer coverages of a range of elements. We will report our progress on the fabrication of such thin layers, and on the characterization of surface-modified graphene devices. This work is supported by the DOE under grant No. DE-FG03-99ER45766 and the Gordon and Betty Moore Foundation. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U5.00009: Quantum interference noise near the Dirac point in graphene Nina Markovic, Atikur Rahman, Janice Wynn Guikema We have studied low-frequency noise characteristics in single layer graphene, focusing specifically on the low-carrier density regime. We show that the 1/f noise at low temperatures is dominated by the time-dependent conductance fluctuations which occur due to quantum interference effects. Close to the Dirac point, the noise is reduced in magnetic field, but the relative noise reduction is larger than what might be expected based on the current theoretical understanding of quantum transport in graphene. The results reflect the inherent symmetry of the system and suggest the importance of additional degrees of freedom. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U5.00010: Noise properties of graphene like systems Avinash Rustagi, C.J. Stanton The unusual electronic properties of graphene and its potential for applications in nanoscale devices motivated us to study the noise properties of materials that have a graphene-like electronic dispersion. For high values of electric field, we find interesting behavior in the noise properties which appear due to hot electron effects. We study the low-frequency noise based on the Boltzmann-Green function method within the relaxation time approximation considering an inelastic scattering term coming from phonon scattering and an elastic scattering term coming from impurity scattering. The steady-state distribution function is evaluated to calculate the average behavior of physical observables like current and energy. We find that as the field strength is increased, the noise \textit{decreases} from the thermal noise value. We have also studied these properties for electronic dispersion with a gap parameter introduced in the Dirac spectrum. The inclusion of gap in the electronic dispersion causes initial heating of the electrons resulting in an increase in noise for intermediate values of field before it decreases at high fields. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U5.00011: Third harmonic generation in graphene Nardeep Kumar, Jatinder Kumar, Chris Gerstenkorn, Rui Wang, Hsin-Ying Chiu, Arthur Smirl, Hui Zhao We report the measurement of optical third harmonic generation from single-layer graphene and few-layer graphite flakes produced by exfoliation. In the measurements, femtosecond near-infrared laser pulses were used to irradiate the samples. The emission observed scales with the cube of the intensity of the incident near-infrared pulse and with one third of the incident wavelength - both are clear evidences of third harmonic generation. We deduced an effective third-order susceptibility for single layer graphene to be on the order of 10$^{-16}$ m$^2$/V$^2$. By measuring a set of flakes with different numbers of atomic layers, we found that for a few layers, the emission scales with the square of the number of atomic layers. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U5.00012: Second Harmonic Generation in a Graphe Armchair Nanoribbon Godfrey Gumbs, Yonatan Abranyos The second order nonlinear optical susceptibility $\chi^{(2)}$ for second harmonic generation is calculated for the 11H transition of a graded double quantum well (DQW) structure of undoped-$GaAs/Al_{x}Ga_{1-x}As$. These results are compared with the single quantum well (QW). Our results show that the values of $\chi^{(2)}$ have optimal magnitudes dependent on the width, depth and separation between the QWs in a DQW structure. When the electric field increases, the dipole moment increases due to the increasing separation between the electron and hole wave functions. On the other hand, the oscillator strength of the 11H transition is reduced as a result of the decrease in the overlap of the electron and hole envelope functions. These two competing factors give rise to optimal conditions for the enhancement of the second order nonlinear susceptibility $\chi^{(2)}$. It is demonstrated that $\chi^{(2)}$ for the DQW structure is more enhanced than for the biased single QW. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U5.00013: Optical Third-Harmonic Microscopy of Graphene Jerry I. Dadap, Sung-Young Hong, Nicholas W. Petrone, Po-Chun Yeh, James C. Hone, Richard M. Osgood, Jr We report strong third-harmonic (TH) generation in monolayer graphene mounted on an amorphous silica substrate using a photon energy that is three-photon resonant with the exciton-shifted van Hove singularity at the M-point of graphene. Our polarization-dependent and azimuthal rotation measurements confirm the expected isotropic symmetry properties for the TH nonlinear optical process in graphene. Since this monolayer graphene TH signal exceeds that of bulk glass by more than two orders of magnitude, the signal contrast permits background-free scanning of graphene and provides structural information that is difficult to obtain via linear optical microscopy. We also discuss the dependence of TH signals on the number of graphene layers and compare the graphene signal strength with that from crystalline Au(111) sample. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U5.00014: Tunable THz Metamaterial Coupled to Graphene Tsung-Ta Tang, Sufei Shi, Long Ju, Feng Wang Metamaterial is a periodic sub-wavelength dielectric structure which can be tailored to have a strong resonance at particular frequencies. However, changing the electromagnetic response of metamaterial often involves changing the design. On the other hand, graphene is an atomic layer of carbon atoms arranged in honeycomb structure and its conductivity in THz regime is highly tunable by changing the Fermi energy of graphene. In our study, we couple graphene to a THz metamaterial device efficiently and demonstrate that the resonance of THz metamaterial can be changed over a wide range by controlling the conductivity of graphene. This graphene-THz metamaterial hybrid device can be used for future THz application such as THz modulator, which can be controlled electrostatically. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U5.00015: Tunable plasmonic resonators in Graphene with extreme light confinement Victor Brar, Min Seok Jang, Josue Lopez, Harry Atwater Graphene plasmons can display a number of interesting properties including small mode volumes, long lifetimes and energies that vary with the sheet charge density. In this work we investigate both experimentally and theoretically the behavior of graphene plasmons in the Mid-IR regime. We find that graphene monolayers that have been patterned with features from 30-100nm can support gate-tunable resonances across the Mid-IR, from 10-5um with charge densities up to 10$^{12}$ e/cm$^{2}$. In our extreme limit, we observe that 30nm sized features cut in graphene can support plasmon resonances for light at 6um wavelengths, indicating mode volumes that are $\sim$ 10$^{6}$ smaller than free space. We further show that these graphene plasmons can couple to phonon polaritons in the supporting dielectric substrate to create multiple new resonances at wavelengths near 10um. These results are analyzed in terms of both analytical calculations and finite element models. [Preview Abstract] |
Session U6: Focus Session: Graphene - Intercalation, Doping, Characterization
Sponsoring Units: DCMPChair: Mark Hybersten, Brookhaven National Laboratory
Room: 302
Thursday, March 21, 2013 11:15AM - 11:27AM |
U6.00001: Analysis of the intercalation of oxygen at the Ru(0001)-Graphene interface Daniel Torres, Mark Hybertsen The process whereby oxygen intercalates at the Ru(0001)-Graphene interface, resulting in systematic electronic decoupling of the graphene layer from the metallic substrate, depends on the interplay between graphene adhesion on the surface and the oxygen adsorption energy. We use density functional theory based calculations, including the effect of van der Waals interaction, to compare the energetics of competing phases in this process. We report three key findings. First, the van der Waals interaction makes a significant contribution to the binding of graphene to Ru(0001). Second, we assess the thermodynamic driving force between uniform oxygen phases on the clean surface and those intercalated at the interface. Third, we consider a series of local 1x1 oxygen patches centered on the raised region of the Ru(0001)-Graphene moir\'e which illustrate a series of stages in the decoupling of graphene from the Ru(0001) surface. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U6.00002: Optical conductivity in bromine-intercalated graphite Zahra Nasrollahi, Sima Saeidi Varnoosfaderani, Sefaattin Tongay, Arthur F. Hebard, David B. Tanner Graphite intercalation compounds have a long and interesting history, with surprising thermal, electrical, and magnetic properties. In this study highly oriented pyrolytic graphite (HOPG) samples were exposed to bromine vapor for times between 20 and 100 minutes. The reflectance was measured using FTIR spectrophotometer, in the~far and mid infrared~at temperatures between~10 K and 300 K. With increasing the bromination time the reflectance in infrared region increases significantly, that gives rise to the increase of optical conductivity of the material calculated by Kramers-Kronig technique. The variation of scattering rate and charge carrier density in different temperatures for different intercalation times can lead to better understanding of the drastic enhancement of electrical conductivity in the material. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U6.00003: Rb-intercalated bilayer graphene studied by high-resolution ARPES James Kleeman, Katsuaki Sugawara, Takafumi Sato, Takashi Takahashi To elucidate the electronic structure at the thinnest limit of the graphite intercalation compound (GIC) C$_8$Rb, we have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) and low-energy electron diffraction (LEED) on Rb-intercalated bilayer graphene fabricated by in-situ evaporation of Rb atoms onto graphene grown epitaxially on SiC. Using LEED, the creation of an intercalated layer with in-plane geometry identical to bulk GICs was confirmed by the observation of a 2x2 spot pattern consistent with Rb intercalation. From ARPES measurement, we found that the Dirac point is at a binding energy of approximately 1 eV, compared to 0.4 eV in pristine epitaxial graphene on SiC [1]. The Fermi surface of this material was also measured. The critical differences between C$_8$Rb, its sister compound C$_8$K, and pristine bilayer graphene will be examined herein.\\[4pt] [1] T. Ohta et al, Science 313, 951-4 (2006). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U6.00004: First principles study of Stage-1 graphene intercalates, IBr and ICl Priyamvada Jadaun, Leonard F. Register, Sanjay K. Banerjee In this study we examine, from a first-principles approach, the properties of 2 graphene intercalant systems namely, iodine monochloride (ICl-GIC) and iodine monobromide (IBr-GIC). These materials are being explored as possible interlayer dielectric candidates for 2D-to 2D-tunnel FETs (TFETs) and Bilayer pseudospin FETs (BiSFETs). To do so we employ density functional theory (DFT). Both these intercalants are stage-1 and acceptor type. We first put forth a structural description of these compounds that intercalate 2 successive layers of graphene, stacked AA type as obtained upon relaxation. Subsequently we describe the electronic structure of ICl-GIC and IBr-GIC and use it to predict the device suitability of these intercalants. It is seen that adding a layer of these GIC's to a single layer of graphene does not disturb graphene electronic spectra except for opening a small gap and introducing doping. With the second graphene layer added, coupling between the graphene layers becomes evident through a small amount of band splitting. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U6.00005: Charge Density Waves on the Graphene Sheets of the Heavily-Doped Superconductor Graphitic Intercalate CaC$_6$ C.F. Hirjibehedin, K.C. Rahnejat, C.A. Howard, N.E. Shuttleworth, S.R. Schofield, K. Iwaya, Ch. Renner, G. Aeppli, M. Ellerby The electronic properties of graphitic materials can be readily tuned by adding charge carriers, and high levels of doping can even lead to superconductivity. We used scanning tunnelling microscopy to investigate the graphene-terminated surface of the superconducting graphitic material CaC$_6$ at temperatures well above T$_c$=11.5K [1]. We find two distinct surface types that show atomic resolution: one exhibits the expected structure of a graphene lattice superimposed on a hexagonal Ca superlattice while the other has stripes with a period three times that of the underlying Ca superlattice. A periodic distortion was found in the Ca atoms matching the periodicity of the electronic contrast on the graphene sheet, though no displacements of the carbon lattice were detected. Spectroscopic measurements reveal an energy gap in the electronic structure that can be directly associated with the stripe periodicity. This provides strong evidence that the stripes correspond to a charge density wave (CDW) in a graphitic system that also superconducts at lower temperatures, offering an excellent test bed for studying the relationship between these two important phenomena. [1] K.C. Rahnejat et al., Nat. Commun. 2, 558 (2011). [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U6.00006: Phonon-mediated superconductivity in graphene by lithium deposition Gianni Profeta, Matteo Calandra, Francesco Mauri Graphene is the physical realization of many fundamental concepts and phenomena in solid-state physics. However, in the list of graphene's many remarkable properties, superconductivity is notably absent. If it were possible to find a way to induce superconductivity, it could improve the performance and enable more efficient integration of a variety of promising device concepts. To this end, we explore, by first-principles DFT calculations, the possibility of inducing superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms [1], in a manner analogous to which superconductivity is induced in graphite intercalated compounds (GICs). As for GICs, we find that the electrical characteristics of graphene are sensitive to the species of adatom used. However, unlike GICs, we find that lithium atoms should induce superconductivity in graphene at a higher temperature than calcium.\\[4pt] [1] G. Profeta, M. Calandra, F. Mauri, Nature Physics 8, 131-134 (2012) [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U6.00007: Si on epitaxial graphene on SiC: intercalation and graphene-SiC transformation Feng Wang, Kristin Shepperd, Alexei Zakharov, Edward Conrad The interface between epitaxial graphene and bulk SiC plays a dominant role in both the growth and transport properties of graphene on SiC. The differences in diffusion of Si through graphene on the two polar SiC surfaces is related to the different nucleation of Si diffusion channels on the two graphene-SiC interfaces. In this work we use LEEM, XPEEM and XPS to study how the excess Si at the graphene-vacuum interface reorders itself at high temperatures. We show that silicon deposited at room temperature onto multilayer graphene films grown on the SiC(000$\bar{1}$) surface rapidly diffuses to the graphene-SiC interface when heated to temperatures above 1020 $^{\circ}$C. The Si that does intercalate into the interface can be removed back out to the graphene-vacuum boundary by heating the sample to 1200 $^{\circ}$C. Most of the Si evaporates at this temperature, however, a significant amount of Si reacts with the graphene at the vacuum interface and form a relative stable reconstructed ($2\times2$) SiC structure. At significantly higher Si concentrations, graphene at the vacuum interface transforms to SiC. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U6.00008: Silicon Layer Intercalation and Interface Properties between Graphene and Metal hosts Yeliang Wang, Jinhai Mao, Lei Meng, Hongjun Gao Graphene is being considered as a contender as the reference material with extraordinary properties for a post-CMOS technology. The availability of high quality and large scale single crystal graphene is fundamental for it to fulfill its promise in electronic applications. Graphene is usually grown on a metallic substrate from which it has to be transferred before it can be used. However, uncontrolled shear and strain, associated with the transfer and the presence of extended domains, lead to unavoidable tearing, rendering it useless for scalable production. We propose a way to overcome this bottleneck and produce high quality, free standing graphene by intercalating Si in graphene epitaxially grown on metals, like Ru(0001) {\&} Ir(111). This G/Si/metal architecture, produced by the silicon-layer intercalation approach (SIA), was characterized by STM/STS, Raman, and angle resolved electron photoemission spectroscopy (ARPES) and proves the high structural and electronic qualities of the new composite. The SIA eliminates the need for the graphene transfer and also allows for an atomic control of the distance between the graphene and the metal. References: 1. Jinhai Mao, Yeliang Wang, H.-J. Gao, et al., Appl. Phys. Lett. 100, 093101 (2012) (Cover). 2. Lei Meng, Yeliang Wang, H.-J. Gao, et al., Appl. Phys. Lett. 100, 083101 (2012). [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U6.00009: Na induced changes in the electronic band structure of graphene grown on C-face SiC Chariya Virojanadara, Chao Xia, Leif Johansson Studies of the effects induced on the electron band structure after Na deposition, and subsequent heating, on a C-face 2 MLs graphene sample will be presented. Na deposition shifts the Dirac point downwards from the Fermi level by about 0.5 eV due to electron doping. After heating at temperatures from around 120 to 300\textordmasculine C, the $\pi $-band appears considerably broadened. Collected Si 2p and Na 2p spectra then indicate Na intercalation in between the graphene layers and at the graphene SiC interface. The broadening is therefore interpreted to arise from the presence of two slightly shifted, but not clearly resolved, $\pi $-bands. Constant energy photoelectron distribution patterns, E(kx,ky);s , extracted from the clean 2MLs graphene C-face sample look very similar to earlier calculated distribution patterns for monolayer, but not Bernal stacked bi-layer, graphene. After Na deposition the patterns extracted at energies below the Dirac point appear very similar so the doping had no pronounced effect on the shape or intensity distribution. At energies above the Dirac point the extracted angular distribution patterns show the flipped, ``mirrored,'' intensity distribution predicted for monolayer graphene at these energies. An additional weaker outer band is also discernable at energies above the Dirac point, which presumably is induced by the deposited Na. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U6.00010: First-Principles Modeling of Low-Energy Electron Diffraction of Few Layer Graphene John McClain, Jiebing Sun, James Hannon, Karsten Pohl, Jian-Ming Tang We present calculations of the low-energy electron microscopy (LEEM) spectra of few layer graphene (FLG) systems using our newly developed theoretical approach based on density-functional theory (DFT). The traditional analysis using multiple scattering off muffin-tin potentials is replaced with a Bloch wave matching approach using self-consistent potentials via DFT to better describe the LEEM spectra, especially in the low energy range. Our calculated results for free-standing FLG exhibit oscillations in reflectivity for energies between 0 and 7 eV, in good agreement with the experimental LEEM spectra of FLG observed on various substrates. The number of oscillations is correlated to the number of graphene layers, a fact often used to determine the number of graphene layers in a sample region. We have calculated FLG on Ni(111)-(1x1) and find that the FLG features dominate those of the bare Ni(111) when two graphene layers are added, as seen in experiments. Our results show that the valleys in the LEEM spectra due to graphene appear only with more than one graphene layer, consistent with our results for free-standing FLG. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U6.00011: Theory of low-energy electron reflectivity from graphene Randall Feenstra, Nishtha Srivastava, Michael Widom, Ivan Vlassiouk We have developed a self-consistent description of low-energy electron reflectivity spectra, yielding results that compare well with experimental data for graphene on SiC and on Cu substrates (obtained by our group as well as by other groups [1]). Our approach utilizes wavefunctions for a thin multilayer graphene slab, computed with a first-principles method. By combining wavefunctions for positive and negative wavevectors, we forms states with only outgoing character on one side of the slab, and hence deduce the electron reflectivity. For free-standing n-layer graphene, we obtain the reflectivity curves that show n-1 reflectivity minima over the energy range 0 - 10 eV. The minima are shown to arise from states with wavefunctions localized between the graphene layers (not on the layers, as previously suggested [1]). For graphene on a substrate, we match the states on one side of the graphene slab to bulk states of the substrate. For graphene on Cu(111) substrates, we find the same set of reflectivity minima as for free-standing graphene, together with an additional minimum whose location varies with the graphene-Cu separation. Hence, this separation can be deduced by comparing experimental and theoretical spectra. [1] H. Hibino et al., Phys. Rev. B \underline {77}, 075413 (2008). [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U6.00012: Bandgap opening in bilayer graphene via molecular doping David Carey, Alexander Samuels We report the emergence of an electronic bandgap in bilayer graphene through the interaction with physisorbed molecules. The bandgap is found to scale linearly with induced carrier density though a slight asymmetry is found between n-type dopants where the bandgap varies as 47 meV/10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ and p-type dopants where the bandgap varies as 38 meV/10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$. The n-type dopant molecules include tetrathiafulvalene (TTF), cobaltocene and decamethylcobaltocene (DMC) and p-type dopant molecules include NO$_{\mathrm{2}}$, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ). Ammonia is found to be weak amphoteric dopant on bilayer graphene, as it is on single layer graphene, where the charge transfer depends on the orientation of the N atom relative to the upper graphene layer. The bandgap opening is explained in terms of the asymmetric charge distributions on the upper graphene layer which is in contact with the molecules. The high binding energy found upon adsorption of some of these molecules results in an attractive way to a permanent bandgap and when combined with a variable external electric field can either close the gap or widen it still further. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U6.00013: Substrate Screening Effects in \textit{ab initio} Many-body Green's Function Calculations of Doped Graphene on SiC Derek Vigil-Fowler, Johannes Lischner, Steven Louie Understanding many-electron interaction effects and the influence of the substrate in graphene-on-substrate systems is of great theoretical and practical interest. Thus far, both model Hamiltonian and ab initio GW calculations for the quasiparticle properties of such systems have employed crude models for the effect of the substrate, often approximating the complicated substrate dielectric matrix by a single constant. We develop a method in which the spatially-dependent dielectric matrix of the substrate (e.g., SiC) is incorporated into that of doped graphene to obtain an accurate total dielectric matrix. We present ab initio GW $+$ cumulant expansion calculations, showing that both the cumulant expansion (to include higher-order electron correlations) and a proper account of the substrate screening are needed to achieve agreement with features seen in ARPES. We discuss how this methodology could be used in other systems. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U6.00014: Electronic Strengthening of Graphene by Charge Doping Chen Si, Zheng Liu, Wenhui Duan, Feng Liu Graphene is known as the strongest 2D material in nature, yet we show that moderate charge doping of either electrons or holes can further enhance its ideal strength by up to $\sim$17\%, based on first principles calculations. This unusual electronic enhancement, versus conventional structural enhancement, of material's strength is achieved by an intriguing physical mechanism of charge doping counteracting on strain induced enhancement of Kohn anomaly, which leads to an overall stiffening of zone boundary K$_{1}$ phonon mode whose softening under strain is responsible for graphene failure. Electrons and holes work in the same way due to the high electron-hole symmetry around the Dirac point of graphene, while over doping may weaken the graphene by softening other phonon modes. Our findings uncover another fascinating property of graphene with broad implications in graphene-based electromechanical devices. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U6.00015: Incremental Tuning of Graphene's Fermi Level by Chemical Doping Kara Berke, Sefaattin Tongay, Arthur Hebard We report a simple, scalable method for fine tuning the Fermi level of CVD-grown graphene, through controlled chemical doping by the addition of the polymer polyethyleneimine (PEI) to the graphene surface. Graphene samples initially showed $p$-type behavior before doping. By dropcasting a low concentration solution of PEI in methanol onto graphene, the hole concentration was lowered. Repeated applications to the same sample shift the Fermi level of the graphene through the Dirac point, yielding an increasingly $n$-type sample. The graphene mobility increases with each application of PEI solution due to charge screening effects. Additionally, the magnetoresistance becomes increasingly linear near the Dirac point, consistent with the existence of charge puddles in neutral graphene. [Preview Abstract] |
Session U7: Focus Session: Carbon Nanotubes: Devices
Sponsoring Units: DMPChair: Junichiro Kono, Rice University
Room: 303
Thursday, March 21, 2013 11:15AM - 11:51AM |
U7.00001: Gate Modulation of Contacts in Carbon Nanotube Devices Invited Speaker: Francois Leonard As the size of electronic devices is reduced, the electrical contacts play an increasingly important role. This is particularly true for contacts to nanomaterials, where new contact phenomena are often observed. In this talk, I will discuss recent numerical simulations to analyze experimental measurements of short-channel carbon nanotube transistors. The results indicate a strong gate modulation of the contact properties, an effect that is distinct from that observed in Schottky barrier nanotube transistors. This modulation of the contacts by the gate allows for the realization of superior subthreshold swings and improved scaling behavior, as observed experimentally. These results further elucidate the behavior of carbon nanotube/metal contacts, and should be useful in the design and optimization of high performance carbon nanotube electronics. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U7.00002: Floating Electrode Transistor based on Single-walled Carbon Nanotube Networks for High Source--drain Voltage Operation Jeongsu Kim, Juhyung Lee, Hyungwoo Lee, Taekyeong Kim, Hye Jun Jin, Juyeon Shin, Youngki Shin, Sangho Park, Yoonho Khang, Seunghun Hong Thin film transistors (TFTs) based on single-walled carbon nanotubes (swCNTs) were reported to exhibit extraordinary characteristics in terms of their conductivity, transparency and flexibility. However, until now, most studies have focused on CNT-TFTs for an operation at a relatively low source--drain voltage below $\sim$ 10 V, while, for some applications such as LCD displays, one needs a rather high source--drain bias voltage. However, such a high voltage bias on source and drain electrodes may reduce the gating effect of conventional CNT-TFT devices by lowering the Schottky barrier and degrade its overall device performance. Herein, we developed floating electrode thin-film transistors (F-TFTs) based on semiconducting swCNT networks for a high source-drain voltage operation. In this device structure, the swCNT network channel was divided into a number of channels connected by floating metallic electrodes. At a high source-drain voltage, the F-TFTs showed a much higher on--off ratio than conventional swCNT-TFTs. This work should provide an important guideline in designing CNT-TFTs for high voltage applications. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U7.00003: High Bias Characteristics of Individual, Suspended Carbon Nanotube p-n Junction Photodiodes Shun-Wen Chang, Kevin Bergemann, Rohan Dhall, Jeramy Zimmerman, Stephen Forrest, Stephen Cronin We have recently investigated p-n junction diodes formed by electrostatic doping of individual, suspended, single-walled carbon nanotubes (CNTs) using two gate electrodes positioned beneath a free standing nanotube that bridges source and drain electrodes. The electrostatic field imposed by the two gates polarizes the nanotube along its length, thereby allowing independent control of the ``doping'' in the nanotube without introducing impurities or defect states. These pn-devices exhibit rectifying diode behavior and finite photoresponse under illumination. Several interesting phenomena are observed at high bias that arise from Schottky contacts formed between the nanotube and its metal contact electrodes and electron tunneling between the n- and p-doped regions. A model is developed explaining this behavior showing evidence for plasmon-induced band gap shrinkage with electrostatic doping. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U7.00004: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U7.00005: Transport Study of Carbon Nanotube Networks with Different Ratios of Semiconducting and Metallic Nanotubes Xuan Wang, Erik H\'aroz, Qi Zhang, Junichiro Kono An important goal of current nanotechnology research is to obtain a quantitative understanding of how electrons drift and tunnel through junctions of nanostructures and how the overall electrical conductivity of networks of nanostructures is determined. Here, we present a comprehensive study of DC transport properties of macroscopic single-wall carbon nanotube (SWCNT) networks with different ratios of metallic and semiconducting nanotubes. The temperature-dependent resistivity shows that when the length of SWCNT is orders of magnitude smaller than the dimensions of the network, the resistance mainly comes from inter-tube junctions. However, the transport mechanism changes from fluctuation-induced tunneling in metallic-enriched networks to variable range hopping in semiconductor-enriched networks. The magneto resistance (MR) of these two networks also show distinct features. In a metallic enriched network, MR is negative up to 10 Tesla below 70 K which can be explained based on weak localization theory. One the other hand, in a semiconductor-enriched network, MR is mostly positive up to 10 Tesla below 10 K, which can be explained based on the shrinking of electron wave function due to the magnetic field. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U7.00006: Electronic durability of flexible transparent coatings from type-specific single-wall carbon nanotubes John M. Harris, Matthew R. Semler, Jeffrey A. Fagan, Erik K. Hobbie The coupling between mechanical flexibility and electronic performance is evaluated for thin flexible coatings of metallic and semiconducting single-wall carbon nanotubes (SWCNTs) deposited on compliant polymer supports. The microstructure, transparency, and electronic properties of the films are independently characterized using a variety of techniques. Cyclic compression experiments suggest that thin films made from metallic SWCNTs show better durability as flexible transparent conductive coatings, which we attribute to a combination of superior mechanical performance and higher interfacial conductivity. We model the role of van der Waals forces in the strain response of the films. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U7.00007: High-frequency performance of scaled carbon nanotube array field-effect transistors Ralph Krupke, Mathias Steiner, Michael Engel, Yu-Ming Lin, Yanging Wu, Keith Jenkins, Damon Farmer, Jefford Humes, Nathan Yoder, Jung-Woo Seo, Alexander Green, Mark Hersam, Phaedon Avouris We report the radio-frequency performance of carbon nanotube array transistors that have been realized through the aligned assembly of highly separated, semiconducting carbon nanotubes on a fully scalable device platform. At a gate length of 100 nm, we observe output current saturation and obtain as-measured, extrinsic current gain and power gain cut-off frequencies, respectively, of 7 GHz and 15GHz. While the extrinsic current gain is comparable to the state-of-the-art, the extrinsic power gain is improved. The de-embedded, intrinsic current gain and power gain cut-off frequencies of 153 GHz and 30 GHz are the highest values experimentally achieved to date. We analyze the consistency of DC and AC performance parameters and discuss the requirements for future applications of carbon nanotube array transistors in high-frequency electronics. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U7.00008: Fabrication and Characterization of Self-Aligned T-gate High-Purity Semiconducting Carbon Nanotube RF Transistors Yuchi Che, Alexander Badmaev, Pyojae Kim, Alborz Jooyaie, Chongwu Zhou We applied the scalable self-aligned T-shaped gate design to semiconducting nanotube RF transistors. In this way, the channel length can be scaled down to 140 nm which enables quasi ballistic transport, and the gate dielectric is reduced to 2-3 nm aluminum oxide, leading to quasi quantum capacitance operation. As a result, our nanotube transistors exhibit excellent on-chip device performance and high linearity with channel length scaling down to 140 nm. With T-shaped gate structure, a cut-off frequency up to 22 GHz and power gain frequency of 10 GHz for separated nanotube transistor are achieved. The T-shaped gate design enables high-yield wafer-scale fabrication with controllable gate length scaling. Furthermore, we also characterized the linearity properties of nanotube transistors, with the 1-dB compression point measurement, in source/load pull setup, with positive power gain to our knowledge, for the first time. Above all, our work reveals that the semiconducting nanotube RF transistor is an interesting and promising direction in high frequency device and circuit exploration. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U7.00009: Carbon Nanotube Thin Film Transistors Using Carbon Nanotube Electrodes Narae Kang, Biddut K. Sarker, Saiful I. Khondaker Carbon nanotubes (CNTs) have attracted a significant attention in recent years due to their exceptional electronics, optical and mechanical properties. In particular, CNT thin film transistors (TFTs) are considered as promising active components in the next-generation flexible, transparent, and invisible electronics. Due to lack of transparency and flexibility, metal electrodes are not suitable for CNT TFTs in their transparent and flexible electronic applications. In this talk, we will discuss the high-performance CNT TFTs where densely aligned array of metallic single walled carbon nanotubes (SWNTs) were used as source and drain electrodes while semiconducting enriched aligned SWNTs (s-SWNT) were used as a channel material. The both metallic SWNTs in the electrodes and s-SWNTs in the channel are aligned via dielectrophoresis using a high quality surfactant-free solution. We show that the performance of the s-SWNT devices with metallic SWNT electrodes is significantly improved than that of the devices with Pd electrodes. In order to find the information about injection barrier between s-SWNT and metallic SWNT interface, we carry out low temperature electron transport measurement of our devices. We will discuss the detailed analysis of the low temperature data. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U7.00010: Dipole induced conductance modulation in chromophore-functionalized single-walled carbon nanotubes Yuanchun Zhao, Changshui Huang, Myungwoong Kim, Padma Gopalan, Mark Eriksson Single-walled carbon nanotubes (SWNTs) are highly sensitive to local electrostatic environments, making SWNT field-effect transistors (FETs) of interest for a number of sensor applications and optoelectronic devices. Here we demonstrate a direct correlation between the conduction of SWNTs and their surrounding dipolar environments. We use azobenzene-based dipolar chromophores, Disperse Red 1 (DR1) and its derivatives to functionalize the sidewalls of SWNTs. The chromophores are coupled with a pyrenebutyric group for realizing noncovalent attachment and to attempt to direct their dipole moments. The functionalizing chromophores produce a dipole field that shifts the threshold voltage (Vth) of the nanotube FET. Under light illumination, these molecules isomerize from the ground trans state to the excited cis state, leading to a decrease of their dipole moments. This dipole moment change acts as an additional gate, causing a shift in Vth. Our results provide a new insight into the photogating mechanisms of the nanotube-chromophore hybrid devices, and they reveal the possibility to modulate optoelectronic properties of nanotube-hybrid devices by designing chromophores with required photosensitive features. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U7.00011: Working cycles of devices based on bistable carbon nanotubes Oleg Shklyaev, Eric Mockensturm, Vincent Crespi Shape-changing nanotubes are an example of variable-shape sp2 carbon-based systems where the competition between strain and surface energies can be moderated by an externally controllable stimuli such as applied voltage, temperature, or pressure of gas encapsulated inside the tube. Using any of these stimuli one can transition a bistable carbon nanotube between the collapsed and inflated states and thus perform mechanical work. During the working cycle of such a device, energy from an electric or heat source is transferred to mechanical energy. Combinations of these stimuli allow the system to convert energy between different sources using the bistable shape-changing tube as a mediator. For example, coupling a bistable carbon nanotube to the heat and charge reservoirs can enable energy transfer between heat and electric forms. The developed theory can be extended to other nano-systems which change configurations in response to external stimuli. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U7.00012: Targeted Placement of Gold Nanoparticles on SWCNT Transistors Using Electrodeposition Yian Liu, Paola Barbara, Makarand Paranjape We present a simple in-situ electrochemical method to target the deposition of gold and other metallic nanoparticles along a single-walled carbon nanotube (SWCNT) field effect transistor (CNTFET). The transistors, fabricated on SiO$_{\mathrm{2}}$/Si substrates, are passivated by a thin layer of poly(methyl-methacrylate), or PMMA. Areas of the PMMA along the carbon nanotube are exposed using electron-beam lithography to target the locations where Au nanoparticles need to be placed. An appropriate potential difference is applied between an in-situ sacrificial gold electrode and the SWCNT, all immersed under a droplet of electrolyte solution. By adjusting the applied voltage and time of deposition, the size of the Au nanoparticle can be controlled from 10 nm to over 100 nm. This method provides better control and is much easier to carry out compared to other site-specific deposition techniques. Such decorated Au nanoparticle/CNTFET heterostructures will allow for a better understanding of single-electron transport behavior, as well as finding application in site-specific biomolecule anchoring for the development of highly sensitive and selective biosensors. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U7.00013: Band Gap Modification in Metallic Nanotubes Due to Nanotube-Substrate Interaction Moh Amer, Adam Bushmaker, Steve Cronin Previous work shows that a small band gap exists in metallic nanotubes. Here we give a detailed comparison between ultra-clean suspended and on-substrate carbon nanotubes (CNTs) in order to quantify the effect of the substrate on the effective band gap of quasi-metallic nanotubes [1]. Individual CNTs are grown across two sets of electrodes, resulting in one segment of the nanotube that is suspended across a trench and the other segment supported on the substrate. A significant change in the conductance of the suspended segment is observed ($\Delta G/G=$0.84) with applied gate voltage. This change is attributed to the existence of the small band gap. The on-substrate segment, however, only shows a change in the measured conductance of $\Delta G/G=$0.11.We used a Landauer model to extract the band gap of these devices. From these fits, the band gaps in the suspended region range from 75 to 100 meV, but are only 5-14.3 meV when the nanotube is in contact with the substrate. The decreased band gap is attributed to localized doping caused by trapped charges in the substrate that result in inhomogeneous broadening of the Fermi energy, which in turn limits our ability to modulate the conductance.\\[4pt] [1] Moh. R. Amer, A.B., and Stephen B. Cronin, The Influence of Substrate in Determining the Band Gap of Metallic Carbon Nanotubes. Nano Letters, 2012. 12: p. DOI:10.1021/nl302321k. [Preview Abstract] |
Session U8: Focus Session: Scanning Tunneling Microscopy of Graphene
Sponsoring Units: DMPChair: Eva Andrei, Rutgers University
Room: 307
Thursday, March 21, 2013 11:15AM - 11:51AM |
U8.00001: Defect engineering of graphene Invited Speaker: Lin He One of the most fascinating aspects of graphene is that its topological features of the electronic states can be fundamentally changed by modifying its local lattice structure. In this talk, I will show how to tune the electronic structures of graphene by defect engineering: (1) we observed superlattice Dirac points and space-dependent Fermi velocity in a corrugated graphene monolayer; (2) we reported angle dependent van Hove singularities (VHSs) of slightly twisted graphene bilayer; (3) we studied the evolution of local electronic properties of twisted graphene bilayer induced by a strain; The strain results in pseudo-Landau levels, which mimic the quantization of massive Dirac fermions in a magnetic field of about 100 T, and valley polarization along a strained graphene wrinkle. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U8.00002: Probing Interactions between Graphene and Cu(111) Surface State Liuyan Zhao, Scott Goncher, George Flynn, Abhay Pasupathy Monolayer graphene and the surface state of Cu(111) are both examples of two-dimensional electronic states. The quasiparticles in monolayer graphene behave as massless Dirac fermions, whereas the ones in the Cu(111) surface state obey the non-relativistic Schrodinger equation. What is the nature of the interactions when these two states are coupled electronically? We probe these interactions using Scanning Tunneling Microscopy/Spectroscopy (STM/S) to investigate how the Cu(111) surface state has been modified with monolayer graphene overlaid on it. In this presentation, we will show that graphene decreases the band width of the Cu(111) surface state and renormalizes the effective mass of the quasiparticles in the Cu(111) surface state. Further, we will show that the modification of the Cu(111) surface state is independent of the registry between Cu(111) and the graphene crystalline orientations. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U8.00003: Visualizing the influence of an isolated Coulomb impurity on the Landau level spectrum in graphene using scanning tunneling microscopy Adina Luican-Mayer, Maxim Kharitonov, Guohong Li, ChihPin Lu, Ivan Skachko, Alem-Mar Goncalves, Eva Y. Andrei Charged impurities play a crucial role in determining the electronic properties of graphene. We report on experiments that elucidate the effect of an isolated charged impurity on the electronic spectrum of graphene in a magnetic field. Using scanning tunneling microscopy and gated graphene devices, we follow the evolution of quantized Landau levels with carrier density and find that the apparent strength of the impurity is controlled by the partial filling of the Landau levels. At low filling the impurity is cloaked and becomes essentially invisible. The cloaking effect diminishes with filling until, for fully occupied Landau levels, the impurity reaches its maximum strength causing a significant perturbation in the local density of states. In this regime we report the first observation of Landau level splitting due to lifting of the orbital degeneracy. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U8.00004: STM/STS study of graphene directly grown on h-BN films on Cu foils Won-Jun Jang, Min Wang, Seong-Gyu Jang, Minwoo Kim, Seong-Yong Park, Sang-Woo Kim, Se-Jong Kahng, Jae-Young Choi, Young Jae Song, Sungjoo Lee Graphene-based devices on standard SiO2 substrate commonly exhibit inferior characteristics relative to the expected intrinsic properties of graphene, due to the disorder existing at graphene-SiO2 interface. Recently, it has been shown that exfoliated and chemical vapor deposition (CVD) graphene transferred onto hexagonal boron nitride (h-BN) possesses significantly reduced charge inhomogeneity, and yields improved device performance. Here we report the scanning tunneling microscopy (STM) and spectroscopy (STS) results obtained from a graphene layer directly grown on h-BN insulating films on Cu foils. STS measurements illustrate that graphene/h-BN film is charge neutral without electronic perturbation from h-BN/Cu substrate. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U8.00005: Kondo quantum criticality in graphene Jinhai Mao, Ivan Skachko, Guohong Li, Eva Andrei The Kondo effect, observed in the presence of coupling between a local magnetic moment and spin degenerate conduction electrons, is a hallmark of the electronic transport in conventional metallic systems. Screening of the local moment gives rise at low temperatures to characteristic signatures in the density of states and electronic spectral function such as the Kondo resonance. Graphene a strictly two dimensional system with carriers whose electronic properties mimic massless Dirac fermions provides a new paradigm for studying interactions in a system where the density of states is linear and can be made vanishingly small by gating, rather than being constant as is the case in standard metallic systems. We study the effect of interactions between the ultra-relativistic electrons in graphene and local magnetic moments introduced by point vacancies in the honeycomb lattice of graphene. Using scanning tunneling spectroscopy and transport measurements we measure the Kondo quantum critical transition and its dependence on carrier density. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U8.00006: Structure and magnetism of cobalt intercalated graphene/Ir(111) via spin-polarized STM Regis Decker, Jens Brede, Nicolae Atodiresei, Vasile Caciuc, Stefan Bluegel, Roland Wiesendanger The presence of intercalation compounds in graphite, i.e. impurities or layer(s) trapped between carbon sheets, can lead to changes in the transport, optical and catalytic properties compared to bulk graphite, or even superconductivity. Here, we present the local structure and magnetic properties of graphene on a magnetic substrate, resolved by spin-polarized STM. The magnetic substrate is obtained by the intercalation of a cobalt layer between graphene and an Ir(111) surface. The atomic structure of the graphene layer is dominated by a highly corrugated Moir\'{e} pattern, which arises due to the incommensurability and/or twisting angle of the graphene lattice and the Co/Ir(111) surface. Within the Moir\'{e} unit cell three different regions, i.e. top, fcc, and hcp regions are identified. Interestingly, these regions show very different electronic and magnetic signatures in the experiments, defining an atomic-scale magnetic Moir\'{e} pattern. The observed spin polarization is compared to density functional theory calculations. The calculations reveal that the bonding between the graphene layer and intercalated Co layer varies from weak to strong within the Moir\'{e} unit cell. Moreover, the interaction between the graphene and the intercalated cobalt layer leads to a spin dependent charge rearrangement, which induces magnetism in graphene as observed in experiment. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U8.00007: STM/STS studies of Ca-intercalated bilayer graphene Ryota Shimizu, Katsuaki Sugawara, Kohei Kanetani, Katsuya Iwaya, Takafumi Sato, Takashi Takahashi, Taro Hitosugi We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) measurements on a two-dimensional Ca-intercalated bilayer graphene epitaxially grown on a 6H-SiC(0001) substrate. The STM topographic images clearly resolve each intercalated Ca atom with graphene-based honeycomb lattice. Furthermore, we found a clear $\times$2.5 modulation in the topography, implying charge density wave or Moir\'{e} pattern originated from the interaction with the SiC substrate. Comparison with ARPES measurements provided us of further insight into the Fermi surface deduced from STS. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U8.00008: Observing Atomic Collapse in Graphene Yang Wang, Dillon Wong, Andrey Shytov, Victor Brar, Sangkook Choi, Qiong Wu, Hsin-zon Tsai, William Regan, Alex Zettl, Roland Kawakami, Steven Louie, Leonid Levitov, Michael Crommie Relativistic quantum mechanics predicts that super-heavy atoms possess unique properties not shared by ordinary atoms. In particular, a very strong electric field around the nucleus should result in ``atomic collapse,'' with an electron component falling onto the nucleus and a positron component escaping to infinity. Predicted by Dirac 80 years ago, atomic collapse has thus far remained experimentally out of reach using accelerator-based techniques. Here we report the observation of atomic collapse on gated graphene devices. The energy and spatial dependence of the atomic collapse state was measured using scanning tunneling microscopy (STM). [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U8.00009: Molecular adsorbates on HOPG: Toward modulation of graphene density of states Michelle Groce, Theodore Einstein, William Cullen Ordered molecular superlattices, particularly those made of planar aromatics with their attendant pi orbitals, have the potential to break the graphene sublattice degeneracy and create a band gap. Trimesic acid (TMA) is a promising candidate due to its self-assembly into symmetry-breaking superlattices nearly commensurate with that of graphene. We have used the graphite (0001) surface as a model system to explore the impact of TMA thin films on band structure. By examining correlations between STM topography and STS maps of corresponding regions, we are able to investigate the effects of TMA on the local density of states. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U8.00010: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U8.00011: Current and voltage dependent interactions between a scanning tunneling microscopy tip and a freestanding graphene sample Kevin Schoelz, Peng Xu, Steven Barber, Matt Ackerman, Paul Thibado The two dimensional nature of graphene gives rise to a number of unique properties. Chief among them are the ability to manipulate the electronic properties using mechanical deformations, opening a new field of ``straintronics.'' Previous work from our group demonstrated the ability to manipulate a freestanding graphene sample with atomic precision using electromagnetic manipulation scanning tunneling microscopy (EM-STM). In the EM-STM technique, the tip bias is ramped over a predetermined range while maintaining a constant tunneling current. The resulting change in height of the tip is then recorded. Typical EM-STM measurements show quick movement of the sample between 0.1-1.0 V, and then slower movement after this point. The height of this final plateau is dependent on the tunneling current. To look for the cause of this current dependence $z(I)$ curves taken at a constant tip bias were examined. It was found that at low tip bias (0.1-0.5 V) the sample drops between 10-20 nm, while at high tip bias (1.0-3.0 V) the sample only drops 2-3 nm. This current dependence is attributed to a drop in the electrostatic force as the tip approaches the sample and holes in the benzene rings become more important. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U8.00012: Gate-controlled modification of molecular electronic structure at the surface of graphene Alexander Riss, Sebastian Wickenburg, Hsin-Zon Tsai, Liang Tan, Miguel Moreno Ugeda, Aaron Bradley, Alex Zettl, Steven G. Louie, Felix R. Fischer, Michael F. Crommie Understanding the behavior of adsorbed molecules on graphene is important for a variety of reasons, including the fact that they can potentially be used to modify the optical, electronic, catalytic, and magnetic properties of graphene devices. Here we show how gate-induced shifting of the Fermi level of a single graphene layer can be used to induce electronic changes in adsorbed molecules. We have used scanning tunneling microscopy and spectroscopy to characterize the structure and electronic properties of 3,3',3''-(Benzene-1,3,5-triyl)tris(2-cyanoacrylonitrile) (BTC) molecules adsorbed onto the surface of a back-gated graphene device. We observe that the energy (with respect to the Fermi level) of the lowest unoccupied molecular orbital (LUMO) of individual BTC molecules can be tuned by application of a gate voltage. These results show the potential to control the physical and chemical properties of adsorbates via electrostatic gating. [Preview Abstract] |
Session U9: Three Dimensional Topological Insulators: Chalcogenides and New Materials
Sponsoring Units: DCMPChair: Phillip King, Cornell University
Room: 308
Thursday, March 21, 2013 11:15AM - 11:27AM |
U9.00001: Spin Control of the topological surface states in 3D topological insulators using polarized light Anna Gura, Jeff Secor, Milan Begliarbekov, Lukas Zhao, Haiming Deng, Lia Krusin-Elbaum The topological surface states of 3D topological insulators (TIs)been shown to interact non trivially with circularly polarized light. Here we report on the study of spin-polarized currents in several $2^{nd}$ generation TIs, such as Sb$_2$Te$_3$, Be$_2$Te$_3$, and Bi$_2$Se$_3$. In particular, to probe the robustness of the helical current surface states we will contrast the polarization dependence of the photocurrent in as grown crystals and crystals with controlled disorder introduced by magnetic and non-magnetic impurities. These result in the development of a gap in the energy spectrum of surface Dirac fermions (DFs), that is DFs acquire mass. The photo-response contrast between massless and massive Dirac fermions studied under electric field gating conditions will be presented. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U9.00002: Highly tunable electron transport in epitaxial topological insulator (Bi$_{1-x}$Sb$_{x})_{2}$Te$_{3}$ thin films Tong Guan, Xiaoyue He, Kehui Wu, Yongqing Li Three dimensional topological insulators (TI) have potential applications in quantum computation and spintronics. These applications often require an insulating bulk and high tunability in chemical potential. Remarkable progresses have been made in synthesizing new TI material with more insulating bulk by alloying the binary compounds Bi$_{2}$Se$_{3}$, Sb$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ in the past couple of years. Here we report the growth of single crystalline (Bi$_{1-x}$Sb$_{x})_{2}$Te$_{3}$ films on SrTiO$_{3}$(111) substrates by molecular beam epitaxy. A full range of Sb-Bi compositions have been studied. Optimal Sb composition for minimum bulk conduction was found to be x $=$ 0.5 $\pm$ 0.1. For the samples (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$, the carrier density can be tuned from n-type to p-type with the help of a back-gate. Linear magnetoresistance has been observed at gate voltages close to the maximum in the longitudinal resistance of (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$ sample. These highly tunable (Bi$_{1-x}$Sb$_{x})_{2}$Te$_{3}$ thin films provide an excellent platform to explore the intrinsic transport properties of the three dimensional topological insulators. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U9.00003: Iodine doping of p-type topological insulators Inna Korzhovska, Lukas Zhao, Haiming Deng, Chen Zhiyi, Lia Krusin-Elbaum We report on the systematic iodine (I) doping of the `intrinsically' \textit{p}-type $2^{nd}$ generation topological insulator (TI) Sb$_2$Te$_3$. Iodine will introduce additional holes into the system and thus pull the Fermi level $E_F$ `down' from the Dirac point. Further, at a sufficient hole density, particle correlation effects are also expected to emerge. Iodine was incorporated into Sb$_2$Te$_3$ using two methods: (i) post-growth vapor exposure of crystals grown by the Vapor-Liquid-Solid (VLS) technique and (ii) \textit{in-situ} doping of crystals grown in a modified Bridgman setup. The first method is self-limiting and only up to 2 at\% iodine is entered, however the \textit{in-situ} doping allowed us to increase iodine content up to 20\%. Detailed XRD Rietveld refinement analysis of the doped crystals doping indicates that for the I-content greater than 10\% the rhombohedral structure is modified to reflect some extent of the I-Te and I-Sb bonding. We find that iodine doping affects the large diamagnetic susceptibility, particularly at low magnetic fields. Our measurements of Hall resistivity confirm that under doping resistivity remains \textit{p}-type. The contrasting effects of the iodine doping into the intrinsically \textit{n}-type Bi$_2$Se$_3$ will be presented. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U9.00004: Nuclear Magnetic Resonance Studies of Bulk States of Bi2Se3 D.M. Nisson, A.P. Dioguardi, P. Klavins, C.H. Lin, K. Shirer, A. Shockley, J. Crocker, N.J. Curro We present $^{209}$Bi nuclear magnetic resonance (NMR) spectra and relaxation rate data on single crystals of Bi$_{2}$Se$_{3}$ grown under various conditions, whose carrier concentrations, resistivities, and Shubnikov-de Haas (SdH) frequencies have been measured. Our NMR data reveal properties of the bulk states, which are influenced by the presence of intrinsic carriers. We find that both the Knight shift and the electric field gradient of the Bi is correlated with carrier concentration, and atypical spectral profiles. Surprisingly, spin-lattice relaxation is not strongly temperature dependent. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U9.00005: Origin of helical spin texture of topological phase transition family materials TlBi(Se$_{1-x}$S$_{x}$)$_{2}$ Justin Waugh, Yue Cao, Koji Miyamoto, Taichi Okuda, Chetan Dhital, Stephen Wilson, Daniel Dessau The unique helically spin-polarized metallic surface states of topological insulators are believed to arise from an odd number of band inversions per unit cell. It is believed that the band inversion in the family of compounds TlBi(Se$_{1-x}$S$_{x}$)$_{2}$ can be removed by replacing Se by S, removing the spin-polarized surface states. Using spin and angle-resolved photoemission spectroscopy we here show that even on the gapped non-topological ``trivial'' side of the phase transition (x=0.7), Dirac-like helical spin polarization still exist, as well as small but finite gaps on the topological side of the phase transition (x=0.3). Additional spin helicity inversions are also present in the bulk bands of both samples. We consider various explanations for this effect, including a superposition of domains, massive Dirac states due to thin domains, and Rashba spin orbit splitting at the surfaces. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U9.00006: Terahertz Quantum Hall Effect of Dirac Fermions in a Topological Insulator A. Pimenov, A. Shuvaev, G. Astakhov, G. Tkachov, Ch. Bruene, H. Buhmann, L. W. Molenkamp Using THz spectroscopy in external magnetic fields we investigate the low-temperature charge dynamics of strained HgTe, a three dimensional topological insulator. From the Faraday rotation angle and ellipticity a complete characterization of the charge carriers is obtained. In resonator experiments, we observe quantum Hall oscillations at THz frequencies. The 2D density estimated from the period of these oscillations agrees well with direct transport experiments on the topological surface state. The Dirac character of the surface state is proven by the observation of a half-integer plateau in the quantum Hall effect. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U9.00007: Various Types of Dirac Cone Materials of Bi$_{1-x}$Sbx Thin Films Shuang Tang, Mildred Dresselhaus The band structure of bismuth antimony thin films varies as a function of stoichiometry, film thickness and growth orientation. Different types of Dirac cone materials can be constructed based on the bismuth antimony thin films system, including single-Dirac-cone, bi-Dirac-cone and tri-Dirac cone materials, and also including exact-Dirac-cone, semi-Dirac-cone and quasi-Dirac-cone materials. The degree of anisotropy of a Dirac cone can be controlled, which range from $\sim$ 2 to $\sim$ 14. Interesting transport phenomena are expected in different Dirac cone materials, which may be optimized for different purposes of applications, e.g. thermoelectrics, electronics etc. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U9.00008: Dirac fermions, Fermi surface and magnetotransport in bulk crystals of layered SrZnSb2 Kefeng Wang, Limin Wang, David Graf, Cedomir Petrovic We report evidence for anisotropic Dirac-like pockets and the large magnetoresistance in the quasi-two-dimensional Sb rectangular layers of bulk SrZnSb2 crystals. Due to the two-fold symmetry of the Sb layers, there are three different Dirac-like pockets as revealed by the calculated Fermi surface. Angular dependent in-plane magnetoresistance and oscillation frequencies indicate the quasi-two-dimensional character of the pockets. This is different from the identical Dirac-cone-like points in the Bi square net of SrMnBi2. The large linear magnetoresistance and magnetothermopower were observed in crystals. The magnetoresistance behavior can be described very well by combining the semiclassical cyclotron contribution and the quantum limit magnetoresistance. Magnetic field also enhances the thermopower. Our results can be well understood by the magnetotransport of Dirac states in the bulk band structure. Work at BNL supported by Office of Basic Energy Sciences, US DOE, under contract No. DE-AC02-98C (K. W, L. W and C. P.). Work at the National High Magnetic Field Laboratory is supported by the DOE NNSA DE-FG52-10NA29659, by the NSF Cooperative Agreement No.DMR-0654118, and by the state of Florida (D. G.) [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U9.00009: $\beta -$Ag$_{2}$Te: A topological insulator with strong anisotropy Lan Wang, Azat Sulaev, Peng Ren, Bin Xia, Qinghua Lin, Ting Yu, Caiyu Qiu, Shuang-yuan Zhang, Ming-Yong Han, Zhipeng Li, Wei Guang Zhu, Qingyu Wu, Yuan Ping Feng, Lei Shen, Shun-Qing Shen We present evidence of topological surface states in $\beta $-Ag$_{2}$Te through first-principles calculations, periodic quantum interference effect and ambipolar electric field effect in single crystalline nanoribbon. Our first-principles calculations show that $\beta $-Ag$_{2}$Te is a topological insulator with a gapless Dirac cone with strong anisotropy. To experimentally probe the topological surface state, we synthesized high quality $\beta $-Ag$_{2}$Te nanoribbons and performed electron transport measurements. The coexistence of pronounced Aharonov-Bohm oscillations and weak Altshuler-Aronov-Spivak oscillations clearly demonstrates coherent electron transport around the perimeter of $\beta $-Ag$_{2}$Te nanoribbon and therefore the existence of topological surface states, which is further supported by the ambipolar electric field effect for devices fabricated by $\beta $-Ag$_{2}$Te nanoribbons. The experimentally confirmed topological surface states and the theoretically predicted isotropic Dirac cone of $\beta $-Ag$_{2}$Te suggest that the material may be a promising material for fundamental study and future spintronic devices. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U9.00010: Topological Surface State Observed in Superconducting (Ir1-xPtx)Te2 Tian Qian, Hu Miao, Gang Xu, Xi Dai, Zhong Fang, Aifa Fang, Nanlin Wang, Hong Ding Topologically non-trivial surface state is the hallmark of 3D topological insulators and topological superconductors, where spin-orbit coupling (SOC) plays an essential role. By Ir site doping of 5{\%} Pt, the huge SOC material IrTe2 becomes a superconductor with maximal Tc $=$ 3K. Our angle resolve photoemission spectroscopy (ARPES) study combined with LDA analysis demonstrate the surface states of (Ir1-xPtx)Te2 is toplogically non-trivial. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U9.00011: Transport properties of crystalline topological insulator Pb$_{1-x}$Sn$_x$Se Tian Liang, Quinn Gibson, Jun Xiong, M.A. Hirschberger, R.J. Cava, N.P. Ong The narrow-band gap semiconductors Pb$_{1-x}$Sn$_x$Se and Pb$_{1-x}$Sn$_x$Te have received considerable attention recently following the prediction~[1] that they are examples of a topological crystalline insulator with surface states characterized by a mirror Chern number. Several ARPES groups have reported evidence for the topological surface states~[2,3]. We have investigated the transport properties of crystals of Pb$_{1-x}$Sn$_x$Se. For Sn content $x$ bracketing 0.23, we observe strong quantum oscillations from bulk carriers (either $n$ or $p$ type) with concentrations near 2$\times 10^{18}$ cm$^{-3}$ and mobilities $\sim $ 3,000 cm$^2$/Vs. The results of experiments to tune the chemical potential into the gap using chemical doping and liquid gating will be reported. References: [1] T. H. Hsieh et al., Nature Commun. {\bf 3}, 982 (2012). [2] P. Dziawa et al., Nature Materials (2012) doi:10.1038/nmat3449 [3] Su-Yang Xu et al., arXiv:1206.2088 [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U9.00012: NMR Studies of the Candidate Topological Superconductor Sn$_{1-x}$In$_{x}$Te: Spin-Triplet Superconductivity Robust against Magnetic Impurities X.R. Lu, L. Ma, J. Dai, P. Wang, B. Normand, W. Yu, R.D. Zhong, J. Schneeloch, Z.J. Xu, G.D. Gu In-doped SnTe is a low-carrier-density semiconductor with strong spin-orbit coupling, and has been proposed to be a topological superconductor. We report nuclear magnetic resonance (NMR) studies of both $^{119}$Sn and $^{125}$Te nuclei, performed on single crystals of Sn$_{1-x}$In$_{x}$Te, where $T_c = 1.8$ K for $x = 0.1$. Under an applied field of 0.33 T, the spin-lattice relaxation rate $1/^{119}T_1$ drops rapidly below 1.2 K, indicating bulk superconductivity. We observe absolutely no change in the Knight shift with temperature when $T < T_c$, which in NMR is normally an indicator of spin-triplet superconductivity. We find no coherence peak below $T_c$ in $1/^{119}T_1$, suggesting an unconventional order parameter but also the possible role of impurities. In the normal state we find that $1/^{119}T_1$ and $1/^{125}T_1$ have Fermi-liquid behavior at high fields, but at low fields show a large Curie-Weiss-type enhancement indicative of magnetic impurity effects. Thus the fact that $T_c$ in our samples is insensitive to the sample purity suggests that superconductivity in Sn$_{1-x}$In$_{x}$Te is robust against magnetic impurities, in contrast to the situation in conventional superconductors. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U9.00013: Electronic Structure Study on a 3D Dirac Semimetal Candidate Y.L. Chen, Z.K. Liu, B. Zhou, S.K. Mo, D. Prabhakaran, Z.J. Wang, Z. Fang, X. Dai, Z.X. Shen, Z. Hussain A family of 3D Dirac semimetals candidates (A3Bi, A$=$alkali metal, B$=$As, Sb, or Bi) have recently been predicted to exist at the phase transition between a topological and a normal insulator when inversion symmetry is preserved. In such a semimetal, the conduction and valence bands touch only at Dirac points around which the dispersion is linear in all directions, leading to distinct physical properties, such as giant diamagnetism and linear quantum magneto-resistance. We used angle resolved photoemission spectroscopy (ARPES) to study a 3D Dirac semimetal candidate, Na3Bi and revealed interesting electronic structures. We will discuss our observation, its possible topological origin and the connection to recent theory investigation. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U9.00014: Competing Orders in the Surface State of Topological Kondo Insulators Jeffrey Botimer, Dae-Jeong Kim, Sean Thomas, Zachary Fisk, Jing Xia The recent discovery of topological (band) insulators (TI) reveals a conceptually new family of quantum materials with novel properties. The bulk energy gap closes at the surface, leading to a gap-less metallic topological surface state. Recently several Kondo insulators have been theoretically proposed in this category, dubbed ``Topological Kondo Insulators'' (TKI). In a TKI, the topological order arises from strong electron correlation and will display new physics. For example, various broken symmetry orders are expected to compete with the topological order. In this talk we will present electrical transport evidence for a high mobility conducting surface state, as well as magneto-optic evidence for broken time reversal symmetry at the surface of several TKI materials. These results suggest that the surface state of the TKI are not only topological but also magnetic, thus providing a convenient system to study topological magneto-electric effects where magnetization can be induced by pure electric field. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U9.00015: Transport Signature of Floquet Majorana Fermions Arijit Kundu, Babak Seradjeh It has been recently predicted that a periodically-driven superconducting quantum wire can support unpaired Floquet Majorana fermions (FMFs), steady-state equal mixtures of electrons and holes bound to the ends of the wire. We further study this proposition and elucidate the range of parameters and drives that give rise to FMFs. We also look for possible transport signatures of FMFs within a non-equilibrium Green's function approach. We analyze the conductance profile for different driving schemes and compare the behavior with that of the static system. We comment on possible experimental setups to observe and exploit FMFs in quantum information processing. [Preview Abstract] |
Session U10: Invited Session: Science in the New Administration
Sponsoring Units: FPSChair: Micah Lowenthal, National Academy of Sciences
Room: 309
Thursday, March 21, 2013 11:15AM - 11:51AM |
U10.00001: The President's Science Policy Agenda Invited Speaker: Gerald Blazey |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U10.00002: DOE Science and the National Agenda Invited Speaker: William Brinkman |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U10.00003: The APS Panel on Public Affairs and Federal Science Policy Invited Speaker: Robert Jaffe The Panel on Public Affairs (POPA) is the organ through which the APS seeks to provide high quality input to the Federal Government on issues with significant physics content, ranging from energy and environment to national security. I will describe POPA's evolving mission, some recent efforts and successes, and look at the agenda for the next few years. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U10.00004: The Role of Science at the State Department in the New Administration Invited Speaker: E William Colglazier |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U10.00005: Science and Policy in the Next Four Years Invited Speaker: Dahlia Sokolov |
Session U11: Invited Session: New Laser Techniques for Imaging and Probing at the Nanoscale
Sponsoring Units: DLSChair: Henry Kapteyn, University of Colorado Boulder
Room: 310
Thursday, March 21, 2013 11:15AM - 11:51AM |
U11.00001: Optical pump-probe microscopy for biomedicine and art conservation Invited Speaker: Martin Fischer Nonlinear optical microscopy can provide contrast in highly heterogeneous media and a wide range of applications has emerged, primarily in biology, medicine, and materials science. Compared to linear microscopy methods, the localized nature of nonlinear interactions leads to high spatial resolution, optical sectioning, and larger possible imaging depth in scattering media. However, nonlinear contrast (other than fluorescence, harmonic generation or CARS) is generally difficult to measure because it is overwhelmed by the large background of detected illumination light. This background can be suppressed by using femtosecond pulse or pulse train shaping to encode nonlinear interactions in background-free regions of the frequency spectrum. We have developed this shaping technology to study novel intrinsic structural and molecular contrast in biological tissue, generally using less power than a laser pointer. For example we have recently been able to sensitively measure detailed transient absorption dynamics of melanin sub-types in a variety of skin lesions, showing clinically relevant differences of melanin type and distribution between cancerous and benign tissue.\footnote{Matthews et al., \textit{Sci. Transl. Med.} \textbf{3}, 71ra15 (2011).} Recently we have also applied this technology to paint samples and to historic artwork in order to provide detailed, depth-resolved pigment identification. Initial studies in different inorganic and organic pigments have shown a rich and pigment-specific nonlinear absorption signature.\footnote{Samineni et al., \textit{Opt. Lett.} \textbf{37}, 1310 (2012).} Some pigments, for example lapis lazuli (natural ultramarine), even show marked differences in signal depending on its geographic origin and on age, demonstrating the potential of this technique to determine authenticity, provenance, technology of manufacture, or state of preservation of historic works of art. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U11.00002: Discovering new physics in magnetic thin films using coherent EUV from high harmonic generation Invited Speaker: Tom Silva The understanding of nanoscale magnetism has become much more critical with recent advances in magnetic data storage applications, as bits on a hard disk are already packed at scales of about 20nm. However, a microscopic model of how spins, electrons, photons and phonons interact does not yet exist. This understanding is fundamentally constrained in large part by our limited ability to observe magnetism on all relevant time and length scales. Until recently, measuring magnetization dynamics used either ultrafast visible-wavelength lasers, or X-rays from synchrotrons and free electron lasers. Our recent work has shown that the fastest dynamics in magnetic materials can be captured using extreme ultraviolet (XUV) harmonics -- with elemental resolution and at multiple atomic sites simultaneously. We first probed with elemental sensitivity how fast the magnetic state can be destroyed in an Fe-Ni alloy. After exciting an Fe-Ni alloy with a fs laser, the spin sublattices randomize on sub-ps timescales. Surprisingly, even in a strongly coupled ferromagnetic alloy, the demagnetization of Ni lags that of Fe by 10 fs [1]. Moreover, we were able to tune this time lag by diluting the alloy with Cu to further reduce the exchange energy. After a time lag characteristic of the exchange energy, the Ni sublattice demagnetizes at the same rate as Fe. This reveals both how the exchange interaction can mediate ultrafast magnetic dynamics in alloys, and how the intrinsic demagnetization process is site-specific such that spins on one sublattice can interact more strongly with the optical field than spins on the other sublattice. In our latest work, we uncovered evidence of giant spin-currents in magnetic multilayers that are generated in the course of the laser-driven ultrafast demagnetization process [2]. By exciting a magnetic multilayer (Fe/Ru/Ni) with a laser pulse, and separately, yet simultaneously, probing the magnetization response of the Ni and Fe layers when the two layers are aligned with an applied magnetic field, we found that optically induced demagnetization of the top Ni layer causes the buried Fe layer to undergo a transient enhancement of the magnetization, of up to 20 percent. This is due to an intense, majority spin-current that enters the Fe layer. \\[4pt] [1] S. Mathias, et al., PNAS 109, (2012).\\[0pt] [2] D. Rudolf, et al., Nat. Comm. 3, (2012). [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U11.00003: Imaging at the X-ray Frontier: Coherent Diffraction Imaging (CDI) for Nano and Bioscience Invited Speaker: Jianwei (John) Miao For centuries, lens-based microscopy, such as light, phase-contrast, fluorescence, confocal and electron microscopy, has played an important role in the evolution of modern sciences and technologies. In 1999, a novel form of microscopy, i.e. coherent diffraction imaging (also termed coherent diffraction microscopy or lensless imaging) was developed and transformed our traditional view of microscopy, in which the diffraction pattern of a noncrystalline object or a nanocrystal is first measured and then directly phased to obtain a high resolution image. The well-known phase problem is solved by the oversampling method in combination with iterative algorithms whose principle can be traced back to the Shannon sampling theorem. In this talk, I will briefly discuss the principle of coherent diffraction imaging and illustrate its broad application in nano and bioscience by using synchrotron radiation, high harmonic generation and X-ray free electron lasers. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U11.00004: Imaging heterogeneous ultrafast exciton dynamics in organic semiconducting thin films Invited Speaker: Naomi S. Ginsberg In solid state semiconducting molecular materials used in electro-optical applications, relatively long exciton diffusion lengths hold the promise to boost device performance by relaxing proximity constraints on the locations for light absorption and interfacial charge separation. The architecture of such materials determines their optical and electronic properties as a result of spacing- and orientation-dependent Coulomb couplings between adjacent molecules. Exciton character and dynamics are generally inferred from bulk optical measurements, which can present a severe limitation on our understanding of these films because their constituent molecules are not perfectly ordered. Rather, films of small organic molecules are composed of multiple microcrystalline domains, and this deposition-dependent microstructure can have profound impacts on transport properties. Using ultrafast transient absorption microscopy, we track the time evolution of excitons, domain by domain, in solid state thin films of TIPS-pentacene, a small soluble molecule that has recently been used in organic semiconducting devices because of its high hole mobility. The results from this spatially-resolved nonlinear optical spectroscopy support our hypothesis that bulk optical measurements deleteriously average over heterogeneities in both spatial and electronic structure; we have revealed significant inhomogeneity in exciton dynamics. Domains that appear homogeneous in linear optical microscopy are shown to have spatial variation and defects, and notable differences in exciton character and behavior are observed at domain boundaries. To interpret the contrast we observe with ultrafast dynamics, we correlate our data to local linear absorption, polarization analysis, profilometry, and atomic force microscopy. With this combined approach, we aim to ultimately understand fundamental structure-function relationship in molecular materials to provide predictive power to material development and device efficiency. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U11.00005: Peering into Cells One Molecule at a Time: Single-molecule and plasmon-enhanced fluorescence super-resolution imaging Invited Speaker: Julie Biteen Single-molecule fluorescence brings the resolution of optical microscopy down to the nanometer scale, allowing us to unlock the mysteries of how biomolecules work together to achieve the complexity that is a cell. This high-resolution, non-destructive method for examining subcellular events has opened up an exciting new frontier: the study of macromolecular localization and dynamics in living cells. We have developed methods for single-molecule investigations of live bacterial cells, and have used these techniques to investigate thee important prokaryotic systems: membrane-bound transcription activation in \textit{Vibrio cholerae}, carbohydrate catabolism in \textit{Bacteroides thetaiotaomicron}, and DNA mismatch repair in \textit{Bacillus subtilis}. Each system presents unique challenges, and we will discuss the important methods developed for each system. Furthermore, we use the plasmon modes of bio-compatible metal nanoparticles to enhance the emissivity of single-molecule fluorophores. The resolution of single-molecule imaging in cells is generally limited to 20-40 nm, far worse than the 1.5-nm localization accuracies which have been attained \textit{in vitro}. We use plasmonics to improve the brightness and stability of single-molecule probes, and in particular fluorescent proteins, which are widely used for bio-imaging. We find that gold-coupled fluorophores demonstrate brighter, longer-lived emission, yielding an overall enhancement in total photons detected. Ultimately, this results in increased localization accuracy for single-molecule imaging. Furthermore, since fluorescence intensity is proportional to local electromagnetic field intensity, these changes in decay intensity and rate serve as a nm-scale read-out of the field intensity. Our work indicates that plasmonic substrates are uniquely advantageous for super-resolution imaging, and that plasmon-enhanced imaging is a promising technique for improving live cell single-molecule microscopy. [Preview Abstract] |
Session U12: Focus Session: Thermoelectrics Materials II
Sponsoring Units: DMP GERA FIAPChair: David Parker, ORNL
Room: 314
Thursday, March 21, 2013 11:15AM - 11:51AM |
U12.00001: Anharmonicity and its application in earth abundant thermoelectrics Invited Speaker: Donald Morelli Recently very exciting improvements in the thermoelectric figure of merit have been reported in bulk nanostructured chalcogenides, mostly due to lattice thermal conductivity suppression by nanoscale-level interfaces. A critical issue in these types of structures is maintaining good electrical conductivity while blocking phonon transport. While so-called ``endotaxial'' nanostructuring, for example, can substantially maintain electron transport across interfaces, generally nanocomposite structures display reduced electrical conductivity which can counteract or in some cases overwhelm the improvements in figure of merit due to thermal conductivity reduction. Additionally, the thermal stability of nanostructured materials at operating temperatures at a significant fraction of the melting point is a concern. Here we describe another approach to reducing lattice thermal conductivity based on designing materials with large lattice anharmonicity. Anharmonic phonon vibrations are the source of intrinsic thermal resistivity in solids and manifest themselves in large Gr\"{u}neisen parameters. We show that one class of compounds, those containing antimony atoms with a lone pair configuration, exhibits a strongly anharmonic phonon spectrum that leads to intrinsically small lattice thermal conductivity. We have applied this concept to ternary copper-antimony-chalcogenide semiconductors and find that the family of compounds based on the tetrahedrite crystal structure can exhibit thermoelectric figure of merit rivaling that of conventional materials like PbTe. The tetrahedrite family is the most widespread sulfosalt mineral on Earth and we show that the mineral itself can be used directly as a source material for earth abundant thermoelectrics. This may pave the way for many new, low cost applications of thermoelectrics in waste heat recovery and power generation. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U12.00002: Phonon lifetime investigation of anharmonicity and thermal conductivity in UO$_2$ Judy Pang, Aleksandr Chernatynskiy, William Buyers, Bennett Larson, Simon Phillpot Understanding low thermal conductivity in UO$_2$ requires a correct accounting for anharmonic phonon-phonon scattering processes. However, over the last five decades there have been remarkably few high-temperature studies of phonon processes in UO$_2$ to underpin its widespread use as a reactor fuel. We have used high-resolution inelastic neutron scattering measurements of individual phonon lifetimes (linewidths) and dispersion at 295 and 1200 K to probe anharmonicity and thermal conductivity in UO$_2$ for individual phonon branches. We found that phonon lifetimes depend strongly on the phonon wave vector and that longitudinal optic phonon modes transport the largest amount of heat, in contrast to recent first principles simulations. The total thermal conductivities calculated using our phonon data demonstrate a quantitative correspondence between microscopic and macroscopic phonon physics. We have also performed density functional theory simulations showing semi-quantitative agreement with phonon lifetimes at 295 K, but larger anharmonicity than measured at 1200 K. These measured phonon dispersion and lifetimes form a benchmark dataset against which numerical simulations including anharmonicity may be assessed. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U12.00003: Determination of elastic constants via phonon- imaging for crystals with low symmetry Tim Head, Elizabeth Carlisle We report progress toward using group velocity surface projections, rather than group velocity surfaces directly, to find elastic constants for low symmetry crystals. Direct determination of elastic constants is difficult in general because of the multi-valued nature of the group velocity surface and a lack of experimentally accessible information about phonon polarizations. Projection of group velocity surfaces onto a plane depend strongly on the elastic constants. We use Monte-Carlo simulations of phonon-images based on continuum elasticity theory to move toward a best-fit algorithm to find elastic constant values for crystals of low symmetry given phonon-imaging data. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U12.00004: Thermodynamic Effects of Na on the Morphology of PbTe-PbS Nanostructured Thermoelectrics Jeff Doak, Jiaqing He, Ivan Blum, Steven Girard, Li-Dong Zhao, David Seidman, Mercouri Kanatzidis, Vinayak Dravid, Chris Wolverton, Hui-Qiong Wang, Jin-Cheng Zheng, Gilberto Casillas, Miguel Jose-Yacaman The creation of nanostructures via phase separation provides a mechanism for decreasing the lattice thermal conductivity and increasing the figure of merit of bulk thermoelectric materials like PbTe-PbS. The addition of Na to PbTe-PbS drastically alters the morphology of PbS precipitates in the system. To see if this change in morphology can be attributed to equilibrium thermodynamics, we use first-principles density functional theory (DFT) calculations to study the energetics of Na partitioning between PbTe and PbS and Na segregation at PbTe/PbS interfaces. We calculate a variety of Na defects in PbTe and PbS and find that the lowest energy defect in both PbTe and PbS is Na substituted for Pb. From the Na defect formation energies, we find the solubility limit of Na in PbTe and PbS, as well as the partitioning coefficient between PbTe and PbS. We find that Na partitions to PbS over PbTe, in agreement with experiment. We calculate Na segregation energies by substituting Na for Pb at the PbTe/PbS interface and find that Na segregates at the PbTe-side of the interface, in qualitative agreement with atom-probe tomography analysis. Applying the Gibbs adsorption isotherm to Na segregation, we find a corresponding decrease in interfacial energy leading to a change in morphology. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U12.00005: Replacement of Ge in GeTe by [Ag$+$Sb] and rare earths: effect on thermoelectric properties E.M. Levin, M. Hanson, R. Hanus, K. Schmidt-Rohr High-efficiency $p$-type Te-Sb-Ge-Ag (TAGS) thermoelectric materials are based on the GeTe narrow-band self-dopant semiconductor where Ge can be replaced by up to 16 at.{\%} [Ag$+$Sb]. To understand the effect of Ge replacement by 4 at.{\%} [Ag$+$Sb] as well as rare earths atoms, we have synthesized and studied XRD, thermopower, electrical resistivity, thermal conductivity, and $^{125}$Te NMR of GeTe and Ag$_2$Sb$_2$Ge$_{\mathrm{46-x}}$R$_{\mathrm{x}}$Te$_{50}$ with R$=$Gd, Dy and $x=$1, 2. At 700 K, GeTe exhibits a thermopower of $+$146 $\mu $VK$^{-1}$ and a large power factor, 42 $\mu $Wcm$^{-1}$K$^{-2}$. Replacement of Ge by [Ag$+$Sb] and rare earths enhances the thermopower, but slightly reduces the power factor due to an increase in electrical resistivity. The thermal conductivity at 300 K of all alloys studied is reduced by a factor of two compared to GeTe. $^{125}$Te NMR spin-lattice relaxation time and resonance frequency reflect changes in carrier concentration. However, decrease of thermal conductivity due to carriers and increase of electrical resistivity are mostly due to a reduction of carrier mobility and indicate strong scattering produced by [Ag$+$Sb] and rare earth atoms. At 700 K, the thermoelectric figure of merit of GeTe is 0.8, whereas that in Ag$_2$Sb$_2$Ge$_{45}$Dy$_1$Te$_{50}$ is much larger, 1.2, due to a reduction in thermal conductivity. Enhancement of thermopower is discussed within a model of energy filtering. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U12.00006: Primary phase alignment in the Mg-Sb system with a 35T DC magnetic field Seth Imhoff, Thomas Ott, Tim Tucker, Jason Cooley Primary phase alignment behavior in the Mg-Sb system is explored by solidification of samples in a 35 tesla DC magnetic field. Compositions with multiple solidification reaction pathways are found to have different phase alignment characteristics.~ In the current study, the orientation of Mg and Sb primary grains do not appear to be strongly influenced, but the $\alpha $-Mg3Sb2 shows a very strong tendency to align with its long axis perpendicular to the field direction. In comparing two compositions that both first nucleate $\alpha $-Mg3Sb2 from the melt, it is found that the volume fraction involved in the primary reaction is a controlling factor for the total degree of alignment throughout the structure. This volume fraction dependence is interpreted as hindering free rotation in the liquid.~ [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U12.00007: Mapping the Fermi Surface in Nb by Tracking Kohn Anomalies with Neutron Scattering Iyad Al-Qasir, Olivier Delaire, Vickie Lynch, Douglas Abernathy, Matt Stone Electron-phonon interaction in metals is a subject of interest for theoretical and experimental investigations. Phonons in Nb show Kohn anomalies due to the electron-phonon interaction. In this work, we are tracking Kohn anomalies in Nb in the full Brillouin zone experimentally and computationally and relating it to the Fermi surface. We measured the 4-dimensional scattering function, $S(\stackrel{\to}{Q},\omega )$ of Nb as a function of temperature, using time of flight inelastic neutron scattering. The 4D data allow us to map phonon dispersion relations along any direction in the full Brillouin zone. In parallel, density functional theory was used to calculate the electronic band structure and Fermi surface, as well as the phonon dispersion relations and line-widths. We present a quantitative comparison, taking into account experimental resolution. These results point to a new avenue of mapping the Fermi surface and electron-phonon coupling in bulk crystals, complementing existing techniques. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U12.00008: Theoretical Study of the Properties of the Type II Clathrates AxB$_{136}$(A=alkali atom;B=Si,Ge,Sn0 $\le $ x $\le $ 24) Dong Xue, Craig Higgins, Charley Myles Type II clathrate semiconductors have cage-like lattices in which Group IV atoms are tetrahedrally-coordinated and sp$^{3}$ covalently bonded. The cages can contain ``guest'' atoms; usually alkali or alkaline earth atoms. These materials are of interest because of their thermoelectric properties. Motivated by recent experimental and theoretical interest [1,2] in the x dependence of properties of the Si and Ge-based Type II clathrate materials A$_{\mathrm{x}}$Si$_{136}$ and A$_{\mathrm{x}}$Ge$_{136}$ (A $=$ alkali atom) we are carrying out a systematic theoretical study of the properties of the Type II clathrate systems A$_{\mathrm{x}}$B$_{136}$(A $=$ alkali atom; B $=$ Si, Ge, Sn). Recent powder X-ray diffraction experiments have found the very interesting result that in Na$_{\mathrm{x}}$Si$_{\mathrm{136}}$, for increasing x in the range 0 $\le $ x $\le $ 8 a lattice contraction occurs and that as x is increased further (8 $\le $ x $\le $ 24), a contrasting lattice expansion results. These observations have motivated us to study the behavior of the lattice constant and other properties as a function of guest concentration in several Type II clathrates. We present results of a density functional based study of the properties of A$_{\mathrm{x}}$B$_{136}$ as a function of x. Results are discussed for the x dependence of the lattice constant and for other structural and electronic properties of these materials. [1] S. Stefanoski and G. Nolas, Cryst. Growth Des. 2011, dx.doi.org/10.1021/cg200756r [2] M. Beekman, E. Nenghabi, K. Biswas, C. Myles, M. Baitinger, Y. Grin, G.S. Nolas, Inorg. Chem. 49 2010, DOI: 10.1021/ic1005049 [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U12.00009: Light Si Based Clathrates For Thermal Energy Conversion: A First Principles Study Yuping He, Fan Sui, Susan Kauzlarich, Giulia Galli Clathrates containing light, earth abundant elements, i.e. Si and Al, are promising materilas for thermoelectric applications, due to their low thermal conductivity, about 2 orders of magnitude smaller than that of bulk Si. However existing Si based clathrates [1] have poor electronic properties for efficient thermal energy conversion. We carried out density functional theory calculations to investigate the electronic and vibrational properties of newly synthesized type I clathrate K$_{8}$Al$_{8}$Si$_{38}$[2]. We predicted that while Al site occupancy does not substantially affect the structure of these systems, it has a strong influence on their electronic and optical properties. In particular, Al occupancy greatly influences the location of the K atoms, and the magnitude and character of the electronic gap of the clathrate (e.g. Whether direct or indirect). Our findings suggest that K$_{8}$Al$_{8}$Si$_{38}$ may have much improved electronic properties, compared to several families of clathrates [2] investigated in the recent literature.\\[4pt] [1] C. L. Condron et al. Inorg. Chem. 2008, 47, 8204.\\[0pt] [2] F. Sui et al. Synthesis and characterization of type I clathrate K$_{8}$Al$_{8}$Si$_{38}$ for thermoelectric application (in preparation) [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U12.00010: High pressure effect on structure, electronic structure and thermoelectric properties of MoS$_2$ Huaihong Guo, Teng Yang, Zhidong Zhang We systematically study high pressure effect on the shape of the unit cell, electronic structure and transport properties of 2H-MoS$_2$, based on density functional calculations and the Boltzmann transport theory. Under pressure, the cross-plane lattice size decreases much faster than the in-plane one, due to the van der Waals interaction, and the size reduction becomes more difficult as external pressure exceeds 20 GPa, agreeing with experimental observation. A conversion from van der Waals to covalent bonding is seen in the calculated charge density and obtial projection of the wave functions. Concurrently, the dependence of band structure on pressure shows that a transition from semiconductor to metal occurs at 25 GPa. Band features close to the Fermi level are found to be advantageous for high values of thermopower. Our transport calculations also find pressure-enhanced electrical conductivities, high values of thermopower (up to a few hundred $\mu$V/K), and significant values of the thermoelectric figure of merit (above 0.10 for high pressure and even up to 0.65 at 25 GPa) over a wide temperature range. Our study supplies a new route to improve the thermoelectric performance of MoS$_2$ and of other transition metal dichalcogenides by applying hydrostatic pressure. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U12.00011: Dimensional crossover and thermoelectric properties in CeTe$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$ single crystals Jong-Soo Rhyee, Kyung Eun Lee, Jae Nyeong Kim, Ji Hoon Shim, Byeong Hun Min, Yong Seung Kwon Several years before, we proposed that the charge density wave is a new pathway for high thermoelectric performance in In$_{\mathrm{4}}$Se$_{\mathrm{3-x}}$ bulk crystalline materials. (Nature v.459, p. 965, 2009) Recently, from the increase of the chemical potential by halogen doped In$_{\mathrm{4}}$Se$_{\mathrm{3-x}}$H$_{\mathrm{0.03}}$ (H$=$Halogen elements) crystals, we achieved high ZT (maximum ZT 1.53) over a wide temperature range. (Adv. Mater. v.23, p.2191, 2011) Here we demonstrate the low dimensionality increases power factor in CeTe$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$ single crystals. The band structures of CeTe$_{\mathrm{2}}$ show the 2-dimensional (2D) Fermi surface nesting behavior as well as a 3-dimensional (3D) electron Fermi surface hindering the perfect charge density wave (CDW) gap opening. By hole doping with the substitution of Sb at the Te-site, the 3D-like Fermi surface disappears and the 2D perfect CDW gap opening enhances the power factor up to x $=$ 0.1. With further hole doping, the Fermi surfaces become 3-dimensional structure with heavy hole bands. The enhancement of the power factor is observed near the dimensional crossover of CDW, at x $=$ 0.1, where the CDW gap is maximized. This research was supported by Basic Science Research Program (2011-0021335), Mid-career Research Program (Strategy) (No. 2012R1A2A1A03005174) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, and TJ Park Junior Faculty Fellowship funded by the POSCO TJ Park Foundation. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U12.00012: Thickness dependent thermoelectric properties of SrTiO$_{3}$/SrLaTiO$_{3}$ and SrZrO$_{3}$/SrLaTiO$_{3}$ heterostructures Masatoshi Ishii, John Baniecki, Robert Schafranek, Kian Kerman, Kazuaki Kurihara Thermoelectric power generators will be required for future sensor network systems. SrTiO$_{3}$ (STO) [1] is one candidate thermoelectric material due to its non-toxicity and comparable power factor to Bismuth telluride. The energy conversion efficiency of SrTiO$_{3}$--based thermoelectric energy conversion elements has been reported to be enhanced by quantum size effects, such as the two dimensional (2D) electron gas in SrTiO$_{3}$/SrTi$_{0.8}$Nb$_{0.2}$O$_{3}$/SrTiO$_{3}$ [2]. Nevertheless, a complete understanding of the mechanisms for the reported increase in efficiency are missing owing to a lack of understanding of the thickness dependence of the transport properties. In the talk, we will present a study of the thickness dependence of the transport properties of SrTiO$_{3}$/SrLaTiO$_{3}$ and SrZrO$_{3}$/SrLaTiO$_{3}$ heterostructures. The SrZrO$_{3}$/SrLaTiO$_{3}$ interface has a large conduction band off-set of 1.9 eV [3] which can be utilized to confine electrons in a 2D quantum well. Characterization of the thermopower, conductivity, and Hall effect will be presented as a function of the SrLaTiO$_{3}$ thickness down to a few unit cells and the implications of the thickness dependence of the transport properties on carrier confinement and increasing the efficiency STO-based 2DEG quantum well structures will be discussed. [1] J. Baniecki et al, Appl. Phys. Lett. 99, 232111 (2011); [2] H. Otha et al., Nature materials, 6, 129 (2007); [3] R Schafranek et al, J. Phys. D: 45 055303 (2012) [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U12.00013: ABSTRACT WITHDRAWN |
Session U13: Topological Insulators: Bi2Se3 and Bi2Te2Se
Sponsoring Units: DCMPChair: Gregory Jenkins, University of Maryland
Room: 315
Thursday, March 21, 2013 11:15AM - 11:27AM |
U13.00001: Topological dangling bonds with large spin splitting and enhanced spin polarization on the surfaces of Bi$_2$Se$_3$ Hsin Lin, Tanmoy Das, Yoshinori Okada, Mike C. Boyer, W. Doug Wise, Michelle Tomasik, Bo Zhen, Eric W. Hudson, Wenwen Zhou, Vidya Madhavan, Chung-Yuan Ren, Hiroshi Ikuta, Arun Bansil We investigate the topological surface state properties at various surface cleaves in the topological insulator Bi$_2$Se$_3$, via first principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS). While the typical surface termination occurs between two quintuple layers, we report the existence of a surface termination within a single quintuple layer where dangling bonds form with giant spin splitting owing to strong spin-orbit coupling. Unlike Rashba split states in a 2D electron gas, these states are constrained by the band topology of the host insulator with topological properties similar to the typical topological surface state, and thereby offer an alternative candidate for spintronics usage. We name these new states ``topological dangling-bond states.'' The degree of the spin polarization of these states is greatly enhanced. Since dangling bonds are more chemically reactive, the observed topological dangling-bond states provide a new avenue for manipulating band dispersions and spin-textures by adsorbed atoms or molecules. Work supported by DOE. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U13.00002: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U13.00003: Transient Surface Photoemission Involving Nonlinear Surface Sheet Polarization Developed on the Doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ Topological Insulator Yukiaki Ishida, Hiroaki Kanto, Walid Malaeb, Shuntaro Watanabe, Chuangtian Chen, Akiko Kikkawa, Yasujiro Taguchi, Yoshinori Tokura, Shik Shin Time- and angle-resolved photoemission spectroscopy is performed on the doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}^{\mathrm{\thinspace }}$topological insulator. We observe unusual variation in the efficiency of photoemission from femto-to-picosecond non-equilibrium particularly when two-dimensional electron gas (2DEG) states are developed on surface, while the surface confinement potential is virtually unchanged. The results indicate that a surface sheet polarization, which is induced nonlinearly by both the photon field and inversion-symmetry-breaking field, grows in magnitude as the 2DEG states become pronounced and opens a so-called surface photoemission channel, \textit{div}\textbf{\textit{A,}} \quad that can be varied transiently. Matrix element effects investigated by linearly-polarized angle-resolved photoemission also supports the presence of \textit{div}\textbf{\textit{A}}. The asymmetric charge distribution developed around vacuum-surface interface is considered as a key to understand and control Rashba splitting of the 2DEG states. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U13.00004: High mobility topological insulator Bi2Se3 exfoliated devices with hexagonal Boron Nitride dielectrics Hadar Steinberg, Valla Fatemi, Lucas Orona, Javier Sanchez-Yamagishi, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero We report electronic transport measurements on double-gated topological insulator Bi2Se3 devices. To obtain both top- and bottom-gating, we exfoliate the Bi2Se3 on standard SiO2-capped Si and coat it with an ultrathin layer of hexagonal Boron Nitride (h-BN), which serves as a dielectric for a top gate. Using both top and bottom gates, we are able to identify the individual contributions of both surfaces and the bulk channel, and show that all three channels have mobilities exceeding 1000 cm2/Vs. Our results suggest that the h-BN transfer technique holds potential for providing a future path for high quality TI density-tunable devices. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U13.00005: Magneto-transport study of magnetically-doped Bi2Se3 Joseph Hagmann, Jonathon Leiner, David Howe, Yongseong Choi, Abdel Al-Asmadi, David Keavney, Richard Rosenberg, Brian Kirby, Xinyu Liu, Margaret Dobrowolska, Jacek Furdyna The interesting properties of topological insulators (TIs) arise from the zero energy gap at the Dirac point characterizing their surface states. These gapless chiral modes are attributed to spin-orbit coupling (typically very strong in TIs such as Bi2Se3), together with time reversal invariance (TRI). The introduction of magnetic dopants into a TI lattice can break TRI, providing a powerful tool for opening the gap in the Dirac cone, and for studying its consequences. In this paper we explore this phenomenon by introducing magnetic ions Mn and Fe into Bi sites in the Bi2Se3 lattice. A series of such magnetically-doped Bi2Se3 layers were grown by molecular beam epitaxy on GaAs (001) substrates, with the intention of studying the effects of such doping on the magnetic and electronic properties of this TI alloy. We discuss the results of magnetization, X-ray magnetic circular dichroism (XMCD), and extensive magneto-transport studies carried out to explore how the presence of magnetic ions in the TI lattice affects the magnetic and the electronic properties of these materials. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U13.00006: Tuning Quantum Oscillations of Dirac Surface States on the Topological Insulator Bi$_2$Te$_2$Se by Ionic Liquid Gating Jun Xiong, Yuehaw Khoo, Shuang Jia, Robert J. Cava, Nai Phuan Ong An \emph{in-situ} method to tune the chemical potential near the Dirac Point (DP) of a topological insulator (TI) would greatly facilitate several key experiments. However, in as-grown crystals of Bi-based TIs, the chemical potential $\mu$ lies high above the DP. Using liquid gating on 50-$\mu$m thick crystals of Bi$_2$Te$_2$Se, we demonstrate that $\mu$ can be tuned by a factor of 6 by observing changes to the Shubnikov-de Haas (SdH) period. A surprise is that the SdH amplitudes increase sharply with gating. Liquid gating allows the n=1 Landau level to be accessed, and the $\pi$-Berry phase to be determined with improved accuracy. We will discuss reversibility of liquid gating, and how we may distinguish the purely gating action from chemical reaction. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U13.00007: Transport studies in topological insulator Bi$_{2}$Te$_{2}$Se Helin Cao, Ireneusz Miotkowski, Jifa Tian, Yong Chen Recently, 3D topological insulators, featuring spin helical topological surface states (SS), have attracted strong attention in condensed matter physics. Although the SS have been directly revealed and intensively studied by surface sensitive measurements, such as ARPES and STM, transport measurements remain challenging due to coexistence of the surface and bulk conduction channels and the sensitivity of sample surfaces to ambient exposure. We have grown high quality Bi$_{2}$Te$_{2}$Se crystals by the Bridgeman method. Resistance showed an insulating behavior followed by saturation at low temperature, indicating surface conduction. Through magnetotransport measurements, we demonstrated high mobility SS on freshly cleaved crystals. The transport signatures of surface Dirac fermions were uncovered from 2D SdH oscillations and non-linear Hall effect. We have also compared transport properties of the samples before and after exposure to air. A giant cusp in magnetoresistance at zero B field was observed after exposure. Our studies may help understand the interplay between the surface and the bulk conduction channels and the degradation of SS due to environmental exposure. We will also present some experimental results of gate tuning and thermoelectric measurements on Bi$_{2}$Te$_{2}$Se. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U13.00008: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U13.00009: Promising topological surface states with persistent high spin polarization across Dirac point in Bi$_{2}$Te$_{2}$Se and Bi$_{2}$TeSe$_{2}$ Koji Miyamoto, Akio Kimura, Taichi Okuda, Hirokazu Miyahara, Hirofumi Namatame, Masaki Taniguchi, Sergey Eremeev, Evgueni Chulkov, Oleg Tereshchenko Topological insulators (TIs) have attracted a great deal of atteion as key materials for spintronics technology. Among the established TIs, Bi$_{2}$X$_{3}$ (X$=$Se, Te) has been mostly studied because of their relatively large energy gap and the simplest topological surface state (TSS) with helical spin texture. However, an absence of the topological natures of TSS below Dirac point (E$_{\mathrm{D}})$ has been shown by spin- and angle-resolved photoemission spectroscopy (SARPES) and scanning tunneling spectroscopy under perpendicular magnetic field. It could be a disadvantage for extending its spintronic applications. Recently, one of the ternary tetradymite compounds, Bi$_{2}$Te$_{2}$Se was shown to be a TI by the ARPES measurement. Importantly, a highly bulk resistive feature in this compound has successfully led to the observation of its surface-derived quantum oscillations in the magnetotransport experiment. We have unambiguously clarified the spin feature of TSS in Bi$_{2}$Te$_{2}$Se and Bi$_{2}$Se$_{2}$Te for the first time by our novel SARPES. The markedly high spin polarization of topological surface states has been found to be 77{\%} and is persistent in the wide energy range across E$_{\mathrm{D}}$ in those compounds. The finding promises to extend the variety of spintoronic applications. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U13.00010: Low frequency noise in exfoliated Bi$_{1.5}$Sb$_{0.4}$Te$_{1.7}$Se$_{1.3}$ field effect devices Mitali Banerjee, Semonti Bhattacharyya, Hariharan N, Suja Elizabeth, Arindam Ghosh Topological insulators are a new class of materials which have emerged as the new paradigm to study the exotic topological phases of matter. Electron transport is studied for field effect devices of Bi$_{1.5}$Sb$_{0.4}$Te$_{1.7}$Se$_{1.3}$ thin films, mechanically exfoliated on Si/SiO$_{2}$ substrates. The resistivity initially decreases with decreasing temperature indicating metallic-like behavior. However the resistivity shows an upturn below 13K which can be associated with the weak localization effect. The resistivity as a function of gate voltage shows hysteresis at low carrier densities and is independent of different sweep rates of the gate voltages. In addition to resistivity measurements, we have investigated low frequency noise or ``1/f'' noise as a function of temperature and gate voltage. The magnitude of 1/f noise increases at lower temperatures and with decreasing carrier densities. At lower carrier densities just like resistivity, noise is also saturated indicating long range disorder in the systems due to selenium vacancies. [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010) [2] E. Rossi, J. H. Bardarson, M. S. Fuhrer, and S. Das Sarma, Phys. Rev. Lett. 109, 096801 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U13.00011: Scanning tunneling microscopy of topological insulator Bi$_2$Te$_2$Se Yingshuang Fu, Tetsuo Hanaguri, Shuhei Yamamoto, Kyushiro Igarashi, Hidenori Takagi, Takao Sasagawa Using scanning tunneling microscopy, we study a prototypical topological insulator Bi$_2$Te$_2$Se having suppressed bulk carrier density. Landau level states of its topological surface state remarkably exhibit hysteresis behavior, which shift in energy controllably with the limits of ramping bias, forming hysteresis loops thereafter. The observed hysteresis behavior is attributed to the interplay between a tip-induced gating effect and an impurity-generated random charging effect. This provides a new avenue to controlling the topological surface state. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U13.00012: Characterization of surface conducting states in Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ topological insulator single crystals Janghee Lee, Joonbum Park, Jae-Hyeong Lee, Jun Sung Kim, Hu-Jong Lee Topologically protected surface state (TSS) of a topological insulator (TI) can be described in terms of a spin-resolved Dirac band with helical-spin texture. In general, however, as-grown TIs are doped so that the surface conduction can be dominated by the bulk conduction. In this study, we minimized the bulk conduction using high-quality Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ TI thin single crystals, with the Fermi level lying in the bulk gap without gating. We confirmed that the weak anti-localization (WAL) effect and universal conductance fluctuations in our samples arose from the top and bottom surfaces. By back-gate tuning the WAL characteristics, we identified the TSS conducting characteristics and the coupling between the TSS and the topologically trivial two-dimensional electron gas (2DEG) states that emerged due to the band bending near the surface. The ambipolar Hall resistivity of the bottom surface was consistent with the back-gate-voltage dependence of the longitudinal resistance of the TSS. This study provides a highly coherent picture of the surface transport properties of TIs by successfully differentiating the transport of the TSS from those of the bulk conducting state and the topologically trivial 2DEG states. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U13.00013: Two Dimensional universal conductance fluctuations in topological insulator Bi2Te2Se microribbons Fengqi Song, Zhaoguo Li, Baigeng Wang, Guanghou Wang The universal conductance fluctuations (UCFs), one of the most important manifestations of mesoscopic electronic interference, have not yet been demonstrated for the two-dimensional surface state of topological insulators (TIs) to date. Even if one delicately suppresses the bulk conductance of TI crystals, the fluctuation of the bulk conductance still keeps competitive and difficult to be separated from the desired UCFs of the surface carriers. Here we report on the experimental evidence of the UCFs of the two-dimensional surface state in the bulk insulating Bi2Te2Se nanoribbons. The solely-B$\bot $-dependent UCF is achieved and its temperature dependence is investigated. The surface transport is further revealed by weak antilocalizations. Such quantum interference unexpectedly survives through the limited dephasing length of the bulk carriers in the ternary TI crystals. Based on the temperature-dependent scaling behavior, the electron-phonon interaction is addressed as a secondary source of the surface state dephasing. (Scientific Reports, 2, 595 (2012)) [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U13.00014: Surface state transport suppression in topological insulators Anjan A. Reijnders, Y. Tian, G. Pohl, I.D. Kivlichan, S.Y. Frank Zhao, Y-J. Kim, S. Jia, R.J. Cava, D.C. Kwok, N. Lee, S.W. Cheong, Kenneth S. Burch An unresolved question in experimental research on topological insulators (TI) is the suppression mechanism of a TI's surface state transport. While room temperature ARPES studies reveal clear evidence of surface states, their observation in transport measurements is limited to low temperatures. A better understanding of this suppression is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this talk, we report the temperature dependent optical properties of the topological insulator Bi$_2$Te$_2$Se (BTS), obtained by infrared spectroscopy and ellipsometry, probing surface and bulk states simultaneously. We see clear evidence of coherent surface state transport at low temperature and find that electron-phonon coupling causes the gradual suppression of surface state transport as temperature rises to 43K. In the bulk, electron-phonon coupling enables the emergence of an indirect band gap transition, which peaks at 43K, and is limited by thermal ionization of the bulk valance band above 43K. For comparison with other resistive TIs, we also discuss the optical properties to BiSbSe$_2$Te. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U13.00015: High-Temperature Andreev Tunneling in the Surface States of a Topological Insulator Parisa Zareapour, Alex Hayat, Shu Yang Frank Zhao, Michael Kreshchuk, Achint Jain, Zhijun Xu, Alina Yang, G.D. Gu, Shuang Jia, Robert Cava, Kenneth Burch Topological insulators (TIs) are materials with high spin-orbit coupling that possess conductive helical surface states. In order to study the exotic properties of the TI surface states, it is favorable to work with TIs that have a low bulk conductivity and exhibit insulating behavior. Bi2Te2Se has been confirmed to have a high bulk resistivity, and it still shows Shubnikov-de Haas oscillations originating from the two-dimensional surface states. We report the observation of coherent Andreev tunneling into the surface states of Bi2Te2Se in high-temperature superconductor (Bi2Sr2CaCu2O8$+\delta)$/Bi2Te2Se junctions fabricated by mechanical bonding method. The differential conductance measurements will be presented in various temperatures and magnetic fields. The characterization of the zero-bias conductance peak observed, suggests that we are tunneling into the surface states of the TI rather than the bulk states. [Preview Abstract] |
Session U14: Focus Session: Quantum Dynamics in Spin Ice
Sponsoring Units: GMAG DMPChair: Satoru Nakatsuji, University of Tokyo
Room: 316
Thursday, March 21, 2013 11:15AM - 11:51AM |
U14.00001: Effective S$=$1/2 Hamiltonians and the Quantum Spin Ice Ground State of Yb$_{2}$Ti$_{2}$O$_{7}$ Invited Speaker: Bruce D. Gaulin New neutron scattering instrumentation offers unprecedented opportunities for mapping out the full dispersion and dynamic susceptibility of magnetic materials. In turn, these measurements can be exploited to determine their microscopic spin Hamiltonians in great detail. We've used these techniques to examine the exotic quantum spin ice ground state of Yb$_{2}$Ti$_{2}$O$_{7}$, a pyrochlore magnet, which can be thought of in terms of spins decorating a network of corner-sharing tetrahedra. In this environment, Yb$^{\mathrm{3+}}$displays a ground state crystal field doublet which is very well separated from its excited states, resulting in an effective S$=$1/2 description for the Yb moments. It's positive Curie-Weiss constant of $\sim$ 0.5 K indicates net ferromagnetic interactions and it displays a g-tensor with XY anisotropy. However strong spin orbit effects give rise to an anisotropic exchange Hamiltonian, which can be understood in quantitative detail by modeling time-of-flight neutron scattering in a high field polarized state with spin wave theory using anisotropic exchange. The resulting Hamiltonian shows strong coupling between local z-components of spin, as in spin ice, but also substantial terms that encourage quantum fluctuations. Armed with the microscopic spin Hamiltonian, the mean field phase diagram and a range of physical properties can be calculated and compared with experiment. We see that any possible ordering is strongly suppressed relative to mean field theory by the presence of geometrical frustration, quantum fluctuations, or both; and the low temperature bulk properties are well accounted for by the effective S$=$1/2 Hamiltonian we determine. \\[4pt] [1] K.A. Ross, L. Savary, B.D. Gaulin and L. Balents, Phys. Rev X, 1, 021022, 2011. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U14.00002: Spin Liquid Regimes at Nonzero Temperature in Quantum Spin Ice: Extension to Finite Temperature of the Phase Diagram of Pyrochlore Magnets Lucile Savary, Leon Balents Many quantum spin liquid theories described so far have not yet benefitted of much attention as regards how they should be interpreted at finite temperature. With growing interest in quantum spin liquid phases and increasingly many material candidates, it is becoming all the more imperative to tackle this matter. Here, we address the finite temperature properties of quantum spin ices, for which quantum spin liquid regimes have been predicted. In particular, we extend to finite temperature the two-dimensional phase diagram found in [PRL 108, 037202 (2012)] using an extension of the gauge mean field theory first introduced in the aforementioned paper. We find that the {\sl quantum} spin liquid features of the $U(1)$ QSL and Coulomb Ferromagnet survive at nonzero temperature and that a first order transition to an entropy-dominated classical spin liquid regime, similar to the classical spin ice liquid, occurs at temperatures lower than a na\"{i}ve scaling with the strength of the interactions might predict. We discuss our results in light of recent experiments on Yb$_2$Ti$_2$O$_7$, where features reminiscent of the well-known classical spin ice phase were reported. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U14.00003: Time domain terahertz study of quantum spin ice Yb$_{2}$Ti$_{2}$O$_{7}$ LiDong Pan, Yuan Wan, Chris M. Morris, Kate A. Ross, S.M. Koohpayeh, Bruce D. Gaulin, Oleg Tchernyshyov, N. Peter Armitage We report the time domain terahertz spectroscopy study of the quantum spin ice material Yb$_{2}$Ti$_{2}$O$_{7}$. Temperature and magnetic field dependence of the transmission spectrum was obtained. Several spin resonance absorption peaks are observed in magnetic field. The results are discussed in comparison with the recently proposed theory of the quantum string excitations in this material. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U14.00004: Hidden order in Yb2TI2O7 Robert D'Ortenzio, Hanna Dabkowska, Sarah Dunsiger, Tatsuo Goko, Jan Kycia, Lian Liu, Teresa Medina, Timothy Munsie, David Pomaranski, Kate Ross, Yasutomo Uemura, Travis Williams, Graeme Luke We report low temperature specific heat and positive muon spin rotation ($\mu$-SR) measurements of both polycrystal and single crystal Yb$_2$Ti$_2$O$_7$. Our zero field (ZF) $\mu$-SR shows little spin relaxation temperature dependence in the polycrystal Yb$_2$Ti$_2$O$_7$, contrast to previously reported results. We observe no collinear ferromagnetic order, rather a hidden order ground state where spin fluctuations remain dynamic down to 16 mK. Single crystal Yb$_2$Ti$_2$O$_7$ zero field $\mu$-SR measurements with the crystallographic $<$111$>$ direction parallel to the initial muon polarization show small but measurable temperature dependence. In addition, our transverse field (TF) $\mu$-SR measurements show the spin susceptibility undergoes a distinct change at temperatures corresponding to the magnetic transition measured in the specific heat. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U14.00005: Single crystals of Yb2Ti2O7 grown by the Optical Floating Zone technique: naturally ``stuffed'' pyrochlores? Kate Ross, Thomas Proffen, Hanna Dabkowska, Jeffery Quilliam, Luke Yaraskavitch, Jan Kycia, Bruce Gaulin In the ``quantum spin ice'' pyrochlore material Yb2Ti2O7, Yb3+ ions are coupled to each other via Ising-like ferromagnetic exchange, creating a situation similar to the highly frustrated classical spin ice compounds, but with significant quantum fluctuations. The ground state of the model resides near two exotic and disordered ``quantum spin liquid'' phases. The experimentally observed ground state of Yb2Ti2O7 is, however, controversial in the literature. Most samples, except one crystal which orders ferromagnetically, show disordered states with varying properties. The controversy is likely to be related to the presence of structural defects of an unspecified type that are known to cause sample-dependence of the low temperature specific heat, particularly in the single crystal samples. Using neutron powder diffraction, we investigated one pulverized single crystal of Yb2Ti2O7 grown by the standard Optical Floating Zone method, and found evidence that 2.3\% excess Yb3+ ions reside on the non-magnetic Ti4+ sites, despite perfect stoichiometry of the starting material. This type of defect lattice is known as a ``stuffed'' pyrochlore structure. The effect of the stuffed spins is an open question which can now be investigated in detail. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U14.00006: Yb$_{2}$Sn$_{2}$O$_{7}$: a quantum critical point approaching the ferromagnetic ordering from the quantum spin liquid side Zhiling Dun, Haidong Zhou, Alannah Hallas, Harlyn Silverstein, Yiming Qiu, John Copley, Jason Gardner, Eunsang Choi, Christopher Wiebe The neutron scattering measurements on pyrochlore Yb$_{2}$Sn$_{2}$O$_{7}$ show no long range ordering down to 0.05 K but appearance of diffuse scattering, low energy spin wave excitations, and temperature-independent relaxation time below 2 K, which indicate the system enters a quantum dynamics region with ferromagnetic interactions. The AC susceptibility further shows that Yb$_{2}$Sn$_{2}$O$_{7}$ enters a ``spin freezing'' region below 0.14 K. Our results suggest that Yb$_{2}$Sn$_{2}$O$_{7}$ sits on a quantum critical point by approaching the ferromagnetic ordering from the spin liquid side. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U14.00007: Not so accidental degeneracies: origin of dimensional-reduction in the Quantum Spin Ice Yb$_2$Ti$_2$O$_7$ Ludovic Jaubert, Han Yan, Owen Benton, Nic Shannon Despite being the best-characterised example of a ``quantum spin ice'' [1], Yb$_2$Ti$_2$O$_7$ remains an enigma. One of its most striking, and puzzling, features are the diffuse, rod-like structures seen in quasi-elastic neutron scattering [2]. These suggest that spin fluctuations in Yb$_2$Ti$_2$O$_7$ decouple into independent Kagome planes, even though magnetic ions occupy a fully three-dimensional pyrochlore lattice [3]. Here, we use a combination of lattice gauge theory, spin-wave calculations and Monte Carlo simulation, to show how the dimensional-reduction seen in Yb$_2$Ti$_2$O$_7$ follows from a two-dimensional branch of excitations ``inherited'' from a nearby phase transition. This analysis sheds new light on ground state selection in a wide range of rare-earth pyrochlore oxides, including the model ``order-by-disorder'' system Er$_2$Ti$_2$O$_7$.\\[4pt] [1] Ross, Savary, Gaulin \& Balents, Phys. Rev. X {\bf 1}, 021002 (2011)\\[0pt] [2] Hodges \textit{et al.}, Phys. Rev. Lett. {\bf 88}, 077204 (2002)\\[0pt] [3] Ross \textit{et al.}, Phys. Rev. Lett. {\bf 103}, 227202 (2009) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U14.00008: Thermodynamic properties of 3-dimensional quantum antiferromagnets Rajiv R.P. Singh, Jaan Oitmaa, Michel J.P. Gingras We present systematic calculations of thermal properties of 3-dimensional quantum antiferromagnets, in the thermodynamic limit, using series expansions. For this purpose, High Temperature Expansions (HTE) are supplemented by Numerical Linked Cluster (NLC) Expansions.\footnote{R. Applegate et al, Phys. Rev. Lett. 109, 097205 (2012); R. R. P. Singh and J. Oitmaa Phys. Rev. B 85, 144414 (2012); R. R. P. Singh and J. Oitmaa Phys. Rev. B 85, 104406 (2012).} These expansions provide essentially exact calculations of thermodynamic properties of the system at (i) all fields at high temperatures and (ii) at all temperatures at high fields. In addition, we show that for classical exchange spin-ice model defined on the pyrochlore lattice, the first order NLC leads to the Pauling approximation, which gives even the zero-field ground state entropy to about one percent accuracy. Thus, these calculations are accurate over a wide parameter range. Results are presented and compared with a variety of experimental systems including pyrochlore materials Yb$_2$Ti$_2$O$_7$ and Er$_2$Ti$_2$O$_7$ and the Hyper Kagome material Na$_4$Ir$_3$O$_8$ [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U14.00009: Seeing the light: Observing photons in quantum spin ice Owen Benton, Olga Sikora, Nic Shannon Spin ice, with its magnetic monopole excitations, is perhaps the best studied example of a classical spin liquid. Quantum mechanical tunnelling between the classical ground states of spin ice leads to an exciting new scenario- a quantum spin liquid ground state with emergent photon excitations [1, 2]. Here we explore how this ``artificial electromagnetism'' would manifest itself in neutron scattering experiments on putative ``quantum spin ice'' materials. Using lattice gauge theory we make explicit predictions for the ghostly, linearly dispersing magnetic excitations which are the ``photons'' of this emergent electromagnetism. We find that ``pinch points,'' which are the signal feature of a classical spin ice, fade away as the system approaches its zero-temperature ground state. The predictions of this field theory are shown to be in excellent quantitative agreement with quantum Monte Carlo simulations at zero temperature~[3].\\[4pt] [1] M. Hermele, M. P. A. Fisher and L. Balents, Phys. Rev. B. {\bf 69}, 064404 (2004).\\[0pt] [2] L. Savary and L. Balents, Phys. Rev. Lett. {\bf 108}, 037202 (2012).\\[0pt] [3] O. Benton, O. Sikora and N. Shannon, Phys. Rev. B. {\bf 86}, 075154, (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U14.00010: Dynamical spectra of quantum strings in quantum spin ice Wesley Fuhrman, Yuan Wan, Oleg Tchernyshyov String-like excitations in quantum spin-ice are a fascinating manifestation of quantum fluctuations and may be observable in materials such as Yb$_2$Ti$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$. We study quantum spin-ice under external magnetic fields on both the checkerboard and pyrochlore lattice for experimentally relevant conditions. We show that excitations in quantum spin ice may be string-like, and that stronger quantum fluctuations reduce string tension and lead to deconfined monopoles. Additionally, we discuss the crossover from strings to magnons in the high-field regime. We provide predictions for observing strings via inelastic neutron scattering and THz spectroscopy. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U14.00011: Quantum Fluctuations in Spin-Ice-Like Pr$_2$Zr$_2$O$_7$ Jiajia Wen, Kenta Kimura, Satoru Nakatsuji, Collin Broholm, Matthew Stone, Eiji Nishibori, Hiroshi Sawa We report the experimental evidence of spin-ice-like correlation and quantum fluctuation in the rare earth pyrochlore Pr$_{2}$Zr$_{2}$O$_{7}$. Low temperature magnetization together with high energy inelastic neutron scattering spectrum reveal the single ion crystal field ground state of Pr$^{3+}$ is a non-Krammer's doublet with local \textless 111\textgreater\ anisotropy. Heat capacity and magnetic susceptibility data show no evidence of long range ordering down to 50 mk. The magnetic interaction energy scale is estimated from AC magnetic susceptibility data where an activation energy gap of 1.6 K is extracted from T-dependent relaxation time. The wave vector dependence of quasi-elastic neutron scattering at 0.1 K resembles that of exchange spin ice, including well-defined pinch points. This indicates the 2-in 2-out ice rule is satisfied over the time scale set by the instrumental energy resolution. In contrast, inelastic scattering with energy transfer of 0.25 meV does not show pinch pints, which suggests these fluctuations break the ice rule. The spectral weight of the elastic scattering accounts for less than 10{\%} of the total scattering from the ground state doublet, providing evidence for the strong quantum fluctuation. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U14.00012: Universal monopole scaling near transitions from the Coulomb phase Stephen Powell Certain frustrated systems, such as spin ice and dimer models, exhibit a Coulomb phase at low temperatures, with power-law correlations and fractionalized monopole excitations. Applied perturbations (external field, pressure, etc.) can drive a transition to a phase where the monopoles become confined. I will present a general analysis of behavior in the vicinity of such critical points, incorporating the effects of a nonzero density of thermal monopoles. Scaling theory allows one to arrive at universal results for the crossover phenomena, which can be tested in numerics or experiment. I will also present Monte Carlo results that confirm these predictions for two particular transitions in spin ice. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U14.00013: Investigation of the Magnetic Properties in the Pyrochlore Pr$_{2}$Sn$_{2}$O$_{7}$ Elizabeth Green, T. Herrmannsd\"{o}rfer, R. Sch\"{o}nemann, Z. Wang, M. Uhlarz, J. Wosnitza, H.D. Zhou Pyrochloric compounds are best known for their remarkable magnetic properties, particularly the possibility to generate magnetic monopoles excitations at low temperatures. Compared to the A$^{3+}$ ions in the spin ice compounds A$_{2}$Ti$_{2}$O$_{7}$ (where A = Ho or Dy), the Pr$^{3+}$ ions in Pr$_{2}$Sn$_{2}$O$_{7}$ have a smaller magnetic moment (2.6 $\mu_{B}$/Pr [1]). This ultimately leads to quantum fluctuations that suppress the spins' ability to freeze [2]. AC susceptibility measurements were performed on a polycrystalline Pr$_{2}$Sn$_{2}$O$_{7}$ sample to probe its dynamic ground state for temperatures down to 11 mK. Preliminary results indicate a narrow distribution of relaxation rates which, as evidenced by neutron experiments [3], are governed by quantum tunneling between states. In addition, relaxation times extracted from isothermal frequency sweeps were found, within error, to be temperature independent below 1 K. Future measurements include specific heat from which the field-dependence of the magnetic monopole densities may be extracted.\\[4pt] [1] K. Matsuhira et al., J. Phys. Soc. Jpn. \textbf{71}, 1576 (2002)\\[0pt] [2] S. Onoda et al., PRL \textbf{105}, 047201 (2010)\\[0pt] [3] H.D. Zhou et al., PRL \textbf{101}, 227204 (2008) [Preview Abstract] |
Session U15: Focus Session: The Physics of Climate
Sponsoring Units: GPCChair: Robert Behringer, Duke University
Room: 317
Thursday, March 21, 2013 11:15AM - 11:27AM |
U15.00001: The New APS Topical Group on the Physics of Climate: History, Objectives and Panel Discussion James Brasseur, Robert Behringer The GPC Chair will introduce the new APS Topical Group on the Physics of Climate (GPC), describe its history and objectives, and introduce the current GPC leadership before opening the floor to a panel discussion. The GPC resulted from two petitions that emerged from the controversy that followed the APS Statement on Climate Change (see APS website). The two proposals were merged and an organization committee formed by the APS leadership. After a long organizational period in 2011, the GPC bylaws were finalized with the following key objective: \textit{The objective of the GPC shall be to promote the advancement and diffusion of knowledge concerning the physics, measurement, and modeling of climate processes, within the domain of natural science and outside the domains of societal impact and policy, legislation and broader societal issues. The objective includes the integration of scientific knowledge and analysis methods across disciplines to address the dynamical complexities and uncertainties of climate physics.} The GPC Invited and Focus Sessions at this March meeting are the inaugural GPC events. The Program Committee Chair will moderate a panel between the attending GPC leadership and audience to solicit suggestions for potential future GPC events that advance the GPC objectives. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U15.00002: Direct Statistical Simulation of Climate Brad Marston Non-equilibrium statistical mechanics opens up the possibility of modeling climate directly,\footnote{E. N. Lorenz, {\it The Nature and Theory of the General Circulation of the Atmosphere}, vol. 218. World Meteorological Organization (1967).} bypassing the traditional approach of accumulating statistics from lengthy numerical simulations. One way to implement such Direct Statistical Simulation (DSS) is by systematic expansion in equal-time cumulants.\footnote{J. B. Marston, E. Conover, and T. Schneider, J. Atmos. Sci. {\bf 65}, 1955 (2008).} Essential physics of the general circulation can be illustrated with idealized 1- and 2-layer models of the atmosphere.\footnote{J. B. Marston, Ann. Rev. Cond. Matt. Phys. {\bf 3}, 285 (2012).} A truncation at second order in the hierarchy of cumulants is equivalent to retaining the interaction between zonal mean flows and eddies. Eddy-eddy interactions appear at higher orders, but care must be taken to keep the higher-order expansions realizable with non-negative probability distribution functions. Live demonstrations of models, and their statistical mechanical solution, will be performed. Possible effects of polar amplification of warming, due to the melting of arctic sea ice, on the mid-latitude jet stream will be illustrated. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U15.00003: Atmospheric Lifetimes and Radiative Forcing of CFC-11 and CFC-12 Kenneth Minschwaner, Lars Hoffmann, Alex Brown, Martin Riese, Rolf M\"uller, Peter Bernath Atmospheric lifetimes for chlorofluorocarbons (CFCs) are important for interpreting their temporal trends and for evaluating their impact on stratospheric chemistry and radiative forcing of climate. The lifetimes of CFC-11 and CFC-12 have been evaluated using global observations of their stratospheric distributions from satellite-based instruments between the period 1992 and 2010. The CFC data sets are from the Cryogen Limb Array Etalon Spectrometer (CLAES), the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA-1 and CRISTA-2), the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), and the Atmospheric Chemistry Experiment (ACE). Stratospheric loss rates were calculated using an ultraviolet radiative transfer code with updated molecular cross section and solar irradiance data. Infrared radiative forcings (net flux changes at the tropopause) were determined using CFC distributions from the satellite observations. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U15.00004: A cloud microphysical mechanism linking solar activity, atmospheric electricity, and climate Brian Tinsley The electrical current density from the ionosphere to the surface changes by tens of percent on the 11-year solar cycle and during transient solar events. This external forcing is accompanied by similar changes due to thunderstorm variability and atmospheric aerosols. The current density deposits space charge in gradients of conductivity associated with stratified clouds and aerosol layers. The space charge, which attaches to droplets and aerosol particles, can be carried deep into clouds by updrafts, and it affects collision rates between droplets and aerosol particles. The most important of these for cloud microphysics are collisions of cloud condensation nuclei (CCN) and ice forming nuclei (IFN) with droplets. These collision rate changes during in-cloud scavenging affect the concentrations of CCN and IFN and the rate of contact ice nucleation. Increases in CCN concentration in deep convective storms have recently been shown to decrease initial precipitation and invigorate the storm with extra release of latent heat of freezing from water not precipitated but carried above the freezing level. The changes in latent heat release account for several sets of correlations of storm vorticity changes with independent inputs that affect the current density. Such dynamical changes can result in regional climate change. A review of models of electrical effects on cloud microphysics, and of observed correlations which support the mechanism, will be presented. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U15.00005: Theory of Arctic Sea Ice Loss: Trends, Noise and Bifurcations John Wettlaufer, Woosok Moon, Sahil Agarwal Within the framework of lower order thermodynamic theories for the climatic evolution of Arctic sea ice we isolate the conditions required for the existence of stable seasonally- varying solutions, in which ice forms each winter and melts away each summer. This is done by constructing a two-season model from a continuously evolving theory and showing that seasonally-varying states are unstable under constant annual average short-wave radiative forcing. However, dividing the summer season into two intervals (ice covered and ice free) provides sufficient freedom to stabilize seasonal ice. Perturbation theory shows that the condition for stability is determined by the timing of when the ice vanishes in summer and hence the relative magnitudes of the summer heat flux over the ocean versus over the ice. This scenario is examined within the context of greenhouse gas warming, as a function of which stability conditions are discerned, and interpreted within the framework of a quantification of the noise extracted from satellite data using multifractal detrended fluctuation analysis. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U15.00006: Coupling of ocean circulation and sea ice D.A. Kurtze, D. S. Comeau, K. Gimre, J.M. Restrepo We propose a simple model of the coupling between oceanic circulation and sea ice dynamics on long time scales. The model begins with a one-dimensional Budyko-Sellers energy balance model of ice-albedo feedback, with a linearized temperature dependence of outgoing longwave radiation. This sits atop a box model of ocean circulation, with conventional thermohaline forcing except that surface heat exchange occurs via the Budyko-Sellers model. The ocean and the ice sheet are coupled via advection and plastic flow of ice, and by the thermodynamics of the ice/seawater interface. We use this model to assess how (and by what mechanisms) ocean circulation and ice sheet dynamics affect one another, primarily to investigate the role played by changes in solar input and greenhouse gas forcing, e.g. in the Snowball Earth scenario. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U15.00007: Ocean Circulation and its Role in Global Warming Invited Speaker: Geoffrey Vallis The surface of the planet is warming because of increased greenhouse gases in the atmosphere. To predict the rate of increase we need to understand how much heat and carbon dioxide are taken up by the ocean. This in turn requires an understanding of both turbulent processes in the upper ocean and the deep, quasi-laminar, overturning circulation. The timescale for the ocean to fully equilibrate to increased greenhouse gases is likely much longer than the timescale on which fossil fuels will still be readily available, and this has important ramifications for what we mean by climate sensitivity. I will discuss these issues with an emphasis on the physical processes of the ocean. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U15.00008: Sea surface temperature and short term climate predictability Constantin Andronache Atmospheric processes have a relatively short memory of initial conditions of about two weeks for detailed daily weather prediction. Nevertheless, skilful seasonal forecast is possible in the presence of slow varying boundary conditions (BC) of the atmosphere, such as sea surface temperature anomalies (SSTA) over large oceanic regions. These conditions typically evolve on a much slower time scale than daily weather events and atmospheric predictability can be increased as long as the future evolution of such BC can be predicted. Given the importance of SSTA in the interaction between the ocean and atmosphere, it is of interest to investigate the nature of temporal persistence of large-scale SSTA in the global ocean. We use the global SSTA and investigate possible sources of predictability at seasonal time scale and its impact in various regions of the ocean. Data used are the NOAA Extended Reconstructed Sea Surface Temperature (SST). We show that: 1) SSTA has a persistence that depends largely on regional location in the global ocean; 2) A given SSTA distribution from a particular month, can have corresponding similar configurations in the past, largely due to the recurrence of ENSO events which affect SSTA distribution over vast regions of the global ocean. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U15.00009: Comparing the Standard Deviation from the Average Seasonal Surface Temperature Signal for Fourteen Years of Hourly Surface Temperature Data as Recorded at Twenty-Five Stations across the United States of America Joseph Trout In this project, Wavelet analysis was used to analyze and filter fourteen years of hourly temperature data recorded at twenty-five stations across the United States of America. The temperature records where filtered using a fast, discrete wavelet transform, keeping the parts of the signal with periods of approximately twelve months. From these filters signals an average seasonal temperature pattern was produced for each station. The standard deviation for each year at every station was then computed. The trends of the standard deviations were examined for each station for evidence of climate change. Wavelet analysis was used because of the ability of wavelet analysis to analyze both periodic and non-periodic behavior at different time or length scales. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U15.00010: Dust shatters like glass: Implications for the climate forcing of mineral dust aerosols Jasper Kok Soil-derived mineral dust aerosols impact climate through interactions with clouds, ecosystems, and radiation, which contributes substantially to uncertainties in understanding past and future climate changes. One of the causes of this large uncertainty is that the size distribution of emitted dust aerosols is poorly understood. In fact, a compilation of measurements indicates that regional and global circulation models overestimate the emitted fraction of clay dust aerosols (\textless\ 2 $\mu $m diameter) by a factor of $\sim$ 2 -- 8. I resolve this discrepancy by deriving a simple theoretical expression for the emitted dust size distribution that is in excellent agreement with measurements. This expression is based on the analogy of dust emission with the scale-invariant fragmentation of brittle materials such as glass. Since regional and global circulation models are usually tuned to the shortwave radiative effect of dust, which is dominated by clay aerosols, these findings suggest that models have substantially underestimated the emission of larger silt (\textgreater\ 2 $\mu $m diameter) aerosols, which tend to produce a net warming effect. I show that this underestimation of silt aerosol emission has implications for the effect of dust on regional and global climate. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U15.00011: Investigation of Solar Cyclic and Climatic Trends in Upper Atmospheric Hydrogen Distributions Susan Nossal, Edwin Mierkiewicz, Fred Roesler, L. Qian, S. Solomon, Alan Burns We will discuss work in progress to better understand solar cyclic and climatic influences on hydrogenous species budgets and distributions from both an observational and modeling perspective. Our Fabry-Perot observations of upper atmospheric hydrogen emissions during solar cycle 23 and during three solar minima (1985, 1997, 2006-2008) establish a reference data set of highly precise, consistently calibrated, thermospheric $+$ exospheric hydrogen column emission observations from Northern mid-latitudes that can be used to compare with future observations and with atmospheric models. We will also discuss use of the National Center for Atmospheric Research's global mean model for sensitivity studies to investigate the response of thermospheric hydrogen to a doubling of carbon dioxide and methane. The results from this study suggest a strong solar cycle dependence and that carbon dioxide cooling may have a greater impact upon the changes in the upper atmospheric hydrogen distribution at solar minimum than do methane increases. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U15.00012: Using multiple equilibria in precipitation to understand self-aggregation of deep tropical convection in a warming climate Sharon Sessions, Stipo Sentic, David Raymond Understanding mechanisms of convective organization is currently an important problem in tropical meteorology. Recent numerical simulations show that the tendency for deep tropical convection to self-aggregate increases as sea surface temperatures (SSTs) increase. This has significant implications for hurricane genesis in a warming climate. Investigating the conditions over which convection self-aggregates requires large domains and is therefore computationally expensive. An alternative approach utilizes the analogy between multiple equilibria in limited domain simulations, and the dry and precipitating regions in a large domain with self-aggregated convection. Multiple equilibria refers to a steady state which either exhibits a completely dry troposphere or persistent precipitating deep convection under identical forcing conditions. The large scale circulation is parameterized based on the assumption that horizontal gradients in temperature are small in the tropics. Understanding the mechanisms which permit multiple equilibria on small domains is a computationally economic approach to understanding self-aggregation. We show how multiple equilibria depend on SSTs, and thus provide insight to self-aggregation in a warming climate. [Preview Abstract] |
Session U16: Focus Session: Magnetic Molecules and Antiferromagnetic Chains
Sponsoring Units: GMAG DMPChair: Andrew Kent, New York University
Room: 318
Thursday, March 21, 2013 11:15AM - 11:51AM |
U16.00001: Transverse Field and Random-Field Ising Ferromagnetism in Mn$_{12}$-acetates Invited Speaker: Pradeep Subedi Single molecule magnets (SMMs) single crystals can exhibit long range ferromagnetic order associated with intermolecular interactions, principally magnetic dipole interactions. With their high spin (S $\sim$ 10) and strong Ising-like magnetic anisotropy, they are model materials to the study of physics associated with Transverse-Field Ising Ferromagnet Model (TFIFM). We have measured magnetic susceptibility of single crystals of the prototype SMM, Mn$_{12}$-acetate, and of a new high-symmetry variant, Mn$_{12}$-ac-MeOH. At zero transverse field the inverse susceptibility of both SMMs is found to accurately follow a Curie-Weiss law with an intercept at a non-zero temperature T$_{cw}$ $\sim$ 0.9 K, indicating a transition to a ferromagnetic phase due to dipolar interactions. With increasing transverse field, the susceptibility and the Curie-Weiss temperature decreases due to increase in spin fluctuations but the nature of the decrease is very different in the two materials. We find that in Mn$_{12}$-ac-MeOH, the suppression of ferromagnetism by the transverse field is consistent with TFIFM, while the suppression of ferromagnetism by the transverse field is considerably more rapid in Mn$_{12}$-acetate. Previous studies show that due to solvent disorder Mn$_{12}$-acetate has an intrinsic distribution of discrete tilts of the molecular magnetic easy axis from the global easy axis of the crystal. Thus with the application of transverse field, the molecules with tilted easy axis experience an additional field along their easy axis and give rise to a distribution of random-fields that further destroys the long-range order, suggesting that this prototypical molecular magnet is a realization of Random-Field Ising Ferromagnet (RFIFM).\\[4pt] [1] \textit{Phys. Rev. B} \textbf{85,} 013441 (2012).\\[0pt] [2] \textit{Phys. Rev. B} \textbf{82,} 014406 (2010).\\[0pt] [3] \textit{Phys. Rev. B} \textbf{82,} 174405(2010) [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U16.00002: Geometric-phase interference in a Mn$_{12}$ single-molecule magnet with four-fold rotational symmetry Spencer Adams, Eduardo H. da Silva Neto, Saiti Datta, John Ware, Christos Lampropoulos, George Christou, Yuri Myaesoedov, Eli Zeldov We study the magnetic relaxation rate $\Gamma$ of the single-molecule magnet Mn$_{12}$-tBuAc as a function of magnetic field component $H_T$ transverse to the molecule's easy axis. When the spin is near a magnetic quantum tunneling resonance, we find that $\Gamma$ increases abruptly at certain values of $H_T$. These increases are observed just beyond values of $H_T$ at which a geometric-phase interference effect suppresses tunneling between two excited energy levels. The effect is washed out by rotating $H_T$ away from the spin's hard axis, thereby suppressing the interference effect. Detailed numerical calculations of $\Gamma$ using the known spin Hamiltonian accurately reproduce the observed behavior. These results are the first experimental evidence for geometric-phase interference in a single-molecule magnet with true four-fold symmetry. Furthermore, the results demonstrate that geometric-phase-interference effects can play a role in the thermally assisted tunneling regime. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U16.00003: Synthesis and spectroscopic characterization of the single molecule magnet Mn$_{12}$-acetate Shi Yuan, Yewon Gim, S.L. Cooper The single molecule magnet [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$]$\cdot$2CH$_{3}$COOH$\cdot$4H$_{2}$O (abbreviated as Mn$_{12}$-acetate) system is currently of great interest because it exhibits a number of fascinating properties, such as quantum tunneling of magnetization and unusual relaxation behavior. High-quality single crystals of Mn$_{12}$-acetate were grown and characterized by X-ray diffraction and magnetization measurements. Room temperature micro-Raman (inelastic light) scattering results on these crystals show phonon spectra consistent with earlier measurements. The frequencies of several Mn-O phonon modes exhibit anomalous behavior as a function of temperature. Studies of the Raman active phonons as functions of magnetic field and pressure are being conducted to better understand the role of different phonons in magnetic quantum tunneling in this system. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U16.00004: B and C doped Cuboctohedral Mn$_{13}$ Clusters with Giant Magnetic Moments Puru Jena, Menghao Wu Using first-principles calculations based on gradient corrected density functional theory we show that an otherwise distorted icosahedric Mn$_{13}$ ferrimagnetic cluster, when doped with six B or C atoms, transforms into a ferromagnetic cuboctahedral cluster with a magnetic moment that is an order of magnitude larger than that of the pure Mn$_{13}$ cluster. The origin of this magnetic transition is attributed to the change in the Mn-Mn interatomic distance resulting from the structural transformation. These doped clusters remain ferromagnetic with giant moments even after removing a B or C atom. However, similar doping with N atom does not lead to ferromagnetic ordering and Mn$_{13}$N$_{6}$ remains ferrimagnetic with a magnetic moment of only 3 $\mu_{\mathrm{B}}$, just as in its parent Mn$_{13}$ cluster. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U16.00005: Geometry and magnetic structure variation in manganese-oxide clusters determined by a self-consistent, LCAO method Kristen Williams, Joseph Hooper \textit{Ab initio} simulations are used to study the variation in geometry and magnetic structure in Mn$_{x}$O$_{y}$ ($x=$3,4; $y=$1,2) clusters. The groundstate wavefunctions for clusters with different magnetic coupling (ferromagnetic, ferrimagnetic and antiferromagnetic) are modeled with linear combinations of atomic orbitals (LCAOs). Self-consistent energies for different spin isomers are calculated by constraining the magnetic moments of Mn atoms constituting each basis AO. The ferrimagnetic and antiferromagnetic ground-state structures of Mn$_{x}$O$_{y}$ are 0.16--1.20 eV lower in energy than their ferromagnetic isomers. The presence of oxygen thus stabilizes low-spin isomers relative to the preferred high-spin ordering of bare Mn$_{3}$ and Mn$_{4}$. Each cluster has a preferred overall magnetic moment, and no evidence is seen of competing states with different spin multiplicities. However, non-degenerate isomags (clusters that possess the same spin multiplicity but different arrangements of local moments) do contribute to peak broadening observed in negative-ion photoelectron spectra. Proper accounting for all possible isomags is shown to be critical for accurate comparison with experimental spectra. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U16.00006: Exploring Magnetic Interactions of an Mo$_3$O$_{13}$ Trimer Containing Compound: La$_5$Mo$_6$O$_{21}$ William Phelan, Rachel Beal, James Neilson, John Sheckelton, Patrick Cottingham, Anna Llobet, Tyrel McQueen When searching for exotic magnetic ground states, it is often useful to seek out materials with certain geometric networks such as: triangular, kagome, and even square lattices with uniform magnetic exchange. Recently, the formation of a condensed valence bond state was proposed to explain the physical properties of LiZn$_2$Mo$_3$O$_8$. This low-temperature ground state emanates from the interactions of one unpaired electron residing on the Mo$_3$O$_{13}$ magnetic subunits. Thus, compounds containing related Mo$_3$O$_{13}$ subunits may prove to be a fertile playground for the study of magnetic interactions between these molecule-like clusters. Earlier structural reports of La$_5$Mo$_6$O$_{21}$ showed that this compound was built from these subunits, as well as, 1-D ``double lambda'' perovskite-like MoO$_6$ octahedra. The Mo atoms residing on the Mo$_3$O$_{13}$ trimers and the double lambda units have oxidation states of 4+ and 5+, respectively. Consequently, the magnetic response and entropy loss ca. 10 K are likely due to the magnetic interactions between the double lambda units and not the Mo$_3$O$_{13}$ trimers. In this presentation, the analysis of the total neutron scattering of La$_5$Mo$_6$O$_{21}$ will be used to draw correlations between the structure and the properties. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U16.00007: Magnetic Response of Mn(III)F(salen) at Low Temperatures J.-H. Park, C.C. Beedle, O.N. Risset, M.J. Andrus, D.R. Talham, M.K. Peprah, E.S. Knowles, M.W. Meisel, M. Shiddiq, S. Hill, A. Podlesnyak, G. Ehlers, S.E. Nagler Due to a report suggesting Mn(III)F(salen), salen $=$ H$_{14}$C$_{16}$N$_{2}$O$_{2}$, is a $S=$ 2 Haldane system with $J/k_{B}=$ 50 K and no long-range order down to 2 K,\footnote{T. Birk \textit{et al}., Inorg. Chem. \textbf{50} (2011) 5312.} we have studied its magnetic response. Torque magnetometry, down to 20 mK and up to 18 T, revealed a feature at 3.8 T when $T$ \textless\ 400 mK. ESR ($\sim $ 200 GHz) studies, using single crystals at 4 K and in 5 T, have not detected any signal. The low-field, high-$T$ susceptibility is unchanged for $P$ \textless\ 1.0 GPa. Using a randomly-oriented, powder-like, deuterated (12 of 14 H replaced by D) sample, neutron scattering data, acquired with the CNCS at SNS, are not consistent with a uniform system consisting of $S=$ 2 Heisenberg antiferromagnetic chains. The INS data show strong, dispersionless excitations, suggesting the possibility of isolated magnetic clusters. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U16.00008: Anisotropic thermal expansion and magnetostructural coupling in CuSb$_{2}$O$_{6}$ Alwyn Rebello, Michael G. Smith, John J. Neumeier Low-dimensional (Quasi-1D or 2D) spin \emph{S}= $1\over 2$ solid-state systems exhibit intriguing electronic and magnetic properties that deserve fundamental attention.\footnote{M. Hase et al., Phys. Rev. Lett. \textbf{70}, 3651 (1993).} Besides, they have long been the subject of intense investigation since the discovery of high-$T_c$ superconductivity in cuprates. Here we present results on anisotropic thermal expansion (TE) and magnetic properties in single crystalline CuSb$_{2}$O$_{6}$ in the temperature range $5 < T < 350$ K. We observe spin-flop transitions for magnetic field applied in $a(b)$ axis, but not in $c$. Our TE data reveals a magnetoelastic coupling in the vicinity of paramagnetic to antiferromagnetic phase transition around $T_{N}$. Also, the temperature dependence of 1D short range magnetic correlations in CuSb$_{2}$O$_{6}$ above $T_{N}$ is reflected in the changes in sample length measured using high resolution dilatometer. Using the scaling of thermal expansion data with the heat capacity data around $T_{N}$,the pressure derivative of $T_{N}$ is obtained as $dT_{N}/dP$= -0.11(1) K/GPa. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U16.00009: Heat conduction in the one-dimensional AF spin chain compound CuSb$_2$O$_6$ Narayan Prasai, Joshua L. Cohn, Michael G. Smith, Alwyn Rebello, John J. Neumeier We report thermal conductivity ($\kappa$) measurements on single crystals of the $S=1/2$ antiferromagnetic spin-chain compound CuSb$_2$O$_6$ over the temperature range $5{\rm K}\leq {\rm T}\leq 300 {\rm K}$. A much larger spin contribution to $\kappa$ is evident along the spin chains ([110] direction) than along [100] and [010]. The possible roles of spin-phonon scattering and twinning will be discussed along with $\kappa$ measurements in applied magnetic field. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U16.00010: Multiple magnetic transitions of the pseudo-1D antiferromagnet CoNiTAC Daniel Teske, John E. Drumheller Magnetic susceptibility and crystal growing methods are reported for the pseudo-one-dimensional antiferromagnet $\left[\left(CH_{3}\right)_{3}NH\right]Co_{1-x}Ni_{x}Cl_{3}\cdot2H_{2}O$ (abbreviated CoNiTAC). For high quality single crystals in the Ni mole fraction range $0.1 < x < 0.6$, two magnetic transitions with transition temperatures separated on the order of 0.1 K were observed. This indicates the possibility of a transition due to a change in the canting angle. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U16.00011: Anomalous transport and thermalization in Heisenberg spin chains Peter Prelovsek, Jacek Herbrych, Robin Steinigeweg In spite of long history 1D spin systems still offer challenging open questions, mostly regarding finite-temperature spin and heat transport as well as the relevance for recent experiments on spin-chain materials. In the talk some recent findings regarding properties of anisotropic spin-1/2 Heisenberg chains, both integrable and nonintegrable, will be presented. Within the Ising-type regime we show that the integrable XXZ model unveils the coexistence of anomalous and normal diffusion resolving in this way conflicting conclusions on Mott insulators. In the gapless regime numerical results in the hydrodynamic regime, consistent with the normal spin diffusion for a nonintegrable model, reveal vanishing current decay rate in the integrable case. The behavior is closely related to the thermalization phenomena in spin-chain systems so that diagonal matrix elements for integrable models show evident deviations from the eigenstate thermal hypothesis. In a weakly perturbed integrable system the finite-size scaling reveals that the crossover between anomalous and normal regime is given by a scale related to the scattering length. The theory of thermal conductivity in spin chains and the relation to recent experiments will be also discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U16.00012: Quench dynamics of the Heisenberg chain Deepak Iyer, Natan Andrei We study the time evolution of the one dimensional Heisenberg chain after a quench from strongly (anti-)ferromagnetic coupling to the isotropic point ($\Delta=1$). We generalize the Yudson integral representation for arbitrary states to the Heisenberg model and use it to study time-evolution of observables and correlation functions. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U16.00013: Quenching Dynamics of Anisotropic Heisenberg Model through a Critical Point Wenshuo Liu, Deepak Iyer, Natan Andrei We study the quenching dynamics of the anisotropic Heisenberg model (XXZ model) with the Yudson contour representation, which is a general method of obtaining the dynamics of integrable models. It replaces the summation over all Bethe eigenstates by integrals over continuous momentum on carefully chosen contours. We begin by applying it to the few-particle case of XXZ model, and then focus on a quenching through the critical point: how a antiferromagnetic phase evolve with time into a spin fluid phase. [Preview Abstract] |
Session U17: Focus Session: Femtoscale Multiferroics
Sponsoring Units: DMP GMAGChair: Sang-Wook Cheong, Rutgers University
Room: 319
Thursday, March 21, 2013 11:15AM - 11:27AM |
U17.00001: Ultrafast Imaging of Real Space Response Functions Yao Wang, Chunjing Jia, Brian Moritz, Thomas Devereaux Understanding the dynamics of spin and charge excitations are critical for the study of correlated materials, such as cuprates. Inelastic X-ray scattering can reveal extensive information related to ultrafast dynamical details about the spin and charge structure factors. To obtain a theoretical understanding, we performed small-cluster exact diagonalization calculations utilizing single-band and three-band Hubbard models with both periodic and open boundary conditions. We demonstrate the ability to track long time behavior; and show that this method can be utilized to study the response dynamics of various materials, such as correlated and chemical systems as well as biological molecules. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U17.00002: Time resolved terahertz and second harmonic investigations in multiferroic \textit{R}MnO$_3$ and \textit{R}Mn$_2$O$_5$ Rolando Valdes Aguilar, Y-M. Sheu, A. Taylor, R.P. Prasankumar, D. Yarotski, E. Abreu, J. Zhang, R. Averitt, S-W. Cheong The dynamical aspects of magnetoelectric interactions has been a very active area of research in multiferroic materials. Through linear far infrared and terahertz spectroscopies it has been shown that electric dipolo active excitations, called electromagnons, exist in some multiferroic materials and complex magnets. An unexplored area of investigation has been the non-linear response of these excitations to strong electromagnetic fields. We investigate the time resolved response of electromagnons in multiferroic materials using high-electric-field terahertz spectroscopy and second harmonic generation at infrared frequencies. We will report results on the well characterized multiferroics TbMnO$_3$, TbMn$_2$O$_5$ and YMn$_2$O$_5$. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U17.00003: Transient magnetic states in the multiferroic frustrated spin chain compound Ca$_{3}$CoMnO$_{6}$ Jae Wook Kim, E.D. Mun, M. Jaime, N. Harrison, D. Rickel, V. Zapf, J.D. Thompson, Y. Kamiya, C. Batista, H. Yi, Y. Oh, S.-W. Cheong We report the discovery of transient magnetic states in a frustrated Ising spin chain system Ca$_{3}$CoMnO$_{6}$ that are observed only within a certain range of magnetic field ($B)$ sweep rates. Spin chains are composed of alternating Co$^{2+}$ and Mn$^{4+}$ spins along the $c$-axis and arranged in a triangular lattice in the \textit{ab}-plane. At zero field, the spins order in a $\uparrow \uparrow \downarrow \downarrow $ configuration that allows for ferroelectric polarization ($P)$. Previous work shows that when DC field is applied along the $c$-axis, a $\uparrow \uparrow \uparrow \downarrow $ spin structure with a 1/2 magnetization ($M)$ plateau is stabilized around $B$ $\sim$ 15 T and $P$ disappears. However, when applying $B$ with various sweep rates using a 60 T shaped-pulse magnet we find transient features in the $M$, $P$, and magnetostriction ($\Delta L$/$L)$. We found one step at 4 T with sweep rate of 75 T/s and another step at 6 T when further increasing the rate to 960 T/s, both below the $M=$1/2 plateau. We attribute this time dependence to the magnetic frustration from both interchain and intrachain exchange interactions between Ising-like Co$^{2+}$ spins which can leads to the creation of magnetic microphases. Thus the evolution of $M$ with external parameters is not a straightforward canting or rotation of spins, but could be a progression through many different ordered microphases that are close in energy. This strongly suggests that an ANNNI-like model is appropriate to describe this system. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U17.00004: Femtoscale magnetically induced lattice distortions in multiferroic TbMnO$_3$ Invited Speaker: Helen Walker Magnetoelectric multiferroics, as exemplified by TbMnO$_3$, exhibit both magnetic and ferroelectric long range order. Whilst the magnetic order is mostly understood, the origin of the ferroelectricity has proved more elusive. Competing models ascribe the ferroelectricity to either charge transfer\footnote{H. Katsura, N. Nagaosa, A. V. Balatsky, \textit{Phys. Rev. Lett.} \textbf{95} 057205 (2005).} or ionic displacements.\footnote{I. A. Sergienko, E. Dagotto, \textit{Phys. Rev. B} \textbf{73} 094434 (2006).} I will review how a new experimental technique, exploiting the interference between charge and magnetic X-ray scattering, enabled our resolution of femtometric ionic displacements\footnote{H. C. Walker \textit{et al., Science} \textbf{333} 1273 (2011).} in TbMnO$_3$. In so doing, I will demonstrate not only that our data provide decisive support for microscopic models attributing $\mathrm{P}$ to ionic displacements, but also the importance of including both symmetric and antisymmetric magnetic interactions in any such models. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U17.00005: Coherent magnon and acoustic phonon dynamics in rare earth doped BiFeO3 multiferroic thin films Kathleen Doig, Frederic Aguesse, Anna-Karin Axelsson, Sam Jones, Ron Synowicki, Neil Alford, James Lloyd-Hughes Magnetoelectric (ME) multiferroics, with coupled electric and magnetic order parameters, exhibit novel physics and have applications in information storage, spintronics and photovoltaics. BiFeO3 is one of the few room temperature multiferroics, but suffers from weak ME coupling. Lanthanide substitution on the Bi site enhances the remnant polarization, saturation magnetization and ME coupling. We investigated the dynamics of ME coupling in the time domain via ultrafast spectroscopy. Coherent magnons and acoustic phonons are impulsively excited and probed in BiLaDyFeO3 thin films using femtosecond laser pulses. Coupling to distinguishable acoustic phonon modes in the film and substrate yields the elastic constants in conjunction with spectroscopic ellipsometry. After substitution of Bi with Dy a rapid magnetoelectric coupling to weak ferromagnetic order creates a magnon oscillation at 75GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31meV. Additional substitution with non-magnetic La suppresses this mode. The behaviour under a magnetic field and correlation with magnetisation studies confirms the assignment of the magnon mode. Our optical approach allows the extraction of parameters otherwise difficult to recover experimentally. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U17.00006: Effect of Ultrafast Thermal Quenching on Nd$_{0.67}$Sr$_{0.33}$MnO$_{3}$ A. Mansour, Kh Ziq, A. Salem, R. Mansour We have successfully performed an ultrafast thermal quenching of Nd$_{1-x}$Sr$_{x}$MnO$_{3}$ (x=0.33) from 1200$^{\circ}$C down to -196$^{\circ}$C in a fraction of a second, at ambient pressure. This allowed us to freeze and investigate the physical properties of the material that have been formed at high temperatures. Resistivity measurements showed a 27 K reduction in the metal-insulator transition (MIT) temperature of the quenched sample compared to the as-grown sample. Whereas magnetic measurements revealed $<$2 K shift in the antiferromagnet-ferromagnet (AFM-FM) transition temperature with a significant broadening in the AFM-FM transition accompanied with a decrease in the low temperature magnetization. Moreover, ultrafast quenching significantly widens the temperature range of the magnetoresistance(MR) from few degrees to over 200 K. Here we present physical interpretations of the results in accordance with X-ray and structural analysis. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U17.00007: Verwey Transition in Magnetite: How fast does an insulator become a metal? Roopali Kukreja, S. de Jong, W.F. Schlotter, J. Turner, W.S. Lee, Y.D. Chuang, H.A. Durr, N. Pontius, T. Kachel, A. Fohlisch, F. Sorgenfrei, M. Beye, W. Wurth, C. Trabant, C.F. Chang, C. Schussler-Langaheine Magnetite (Fe3O4), is the first oxide where a relationship between electrical conductivity and fluctuating/localized charges was observed, with a drop in conductivity by two orders of magnitude at TV=123K. The Verwey transition is accompanied by a structural change from monoclinic to cubic symmetry. Despite decades of research and indications that charge and orbital ordering play an important role, the mechanism behind the Verwey transition is yet unclear. Recently, three-Fe-site lattice distortions called trimerons have been identified as the true microscopic face of electronic order in low temperature insulating phase. We studied the real time response of insulating magnetite to optical excitation with ultrafast soft X-ray scattering. We discover this to be a two-step process. After an initial femtosecond destruction of individual trimerons in the corresponding lattice, we observe a phase separation into residual insulating trimeron and cubic metallic phases on a 1.0 $\pm$ 0.2 picosecond timescale. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U17.00008: Complex Magnetic Interactions in A-site and B-site Doped Multiferroic TbMnO$_{3}$ Margo Staruch, Menka Jain Multiferroic materials have been of great interest in recent years due to a number of potential applications in random access memory or spintronics devices. TbMnO$_{3}$ in particular has attracted attention since the discovery of significant magnetoelectric coupling. The possibility of ferroelectricity in rare-earth chromites has also been examined recently through x-ray diffraction and dielectric measurements. Although several studies have looked at Cr-doped LaMnO$_{3}$, the nature of the Mn--Cr interactions is still controversial and no studies have been performed where the parent compound is multiferroic. In the present work, bulk Tb$_{1-x}$A$_{x}$MnO$_{3}$ (A = Ca$^{2+}$ or Sr$^{2+}$) and TbMn$_{1-y}$Cr$_{y}$O$_{3}$ have been synthesized via solution route. The structural evolution as determined through x-ray diffraction and Raman spectroscopy is consistent with a reduction in the orthorhombic distortion. Magnetic properties distinct from the parent compound, including ferrimagnetism and ferromagnetism, have been observed due to the Mn$^{3+}$--Mn$^{4+}$ or Mn$^{3+}$--Cr$^{3+}$ interactions. These complex interactions between the Mn$^{3+}$/Mn$^{4+}$, Cr$^{3+}$, and Tb$^{3+}$ moments will be discussed in detail. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U17.00009: The domain walls of antiferromagnetic TbMnO$_{3}$ thin films C. Daumont, S. Farokhipoor, C. Magen, D. Rubi, S. Venkatesan, E. Snoek, M. Doeblinger, A. Mueller, C. Scheu, B. Noheda In bulk TbMnO$_{3}$ below 28K, the Mn sublattice orders as an antiferromagnetic cycloidal spin structure. This breaks inversion symmetry and induces a macroscopic electrical polarization: TbMnO$_{3}$ is a multiferroic material with a strong magnetoelectric coupling. Contrary to the bulk, TbMnO$_{3}$ thin films grown on (001)-SrTiO$_{3}$ substrates show ferromagnetic-like behavior with a magnetic moment of 1.5$\mu_{\mathrm{B}}$/f.u. at 15K. However, the thickness dependence of the magnetic moments is not consistent with magnetism homogeneously distributed through the film. Additionally, epitaxial strain enables the stabilization of different symmetries and particular domain configurations at nanometric scales. Large strain gradients and/or lowering of symmetry at the boundaries of these domains allow the appearance of physical responses distinct from those of the domains. In this work we investigate the contribution of the domain walls to the magnetic moment. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U17.00010: Giant magnetoresistance spin valves exchange-biased by ferroelectric BiFeO$_3$ thin films X. Zhang, S. Maruyama, P.J. Chen, G. Feng, T.R. Gao, R.D. Shull, I. Takeuchi The recent demonstrations of electric-field-driven magnetization control in ferromagnet(FM)/BiFeO$_3$ bilayer systems [1,2] have attracted considerable interest because of the potential applications in spintronics. In this study, giant magnetoresistance (GMR) spin valves (Co/Cu/Py/Ta) were fabricated on SrRuO$_3$/BiFeO$_3$ films by magnetron sputtering at a base pressure of 2 $\times$ 10$^{-8}$ Torr and with an external field of 300 Oe. The presence of exchange bias between the BiFeO$_3$ layer and the ferromagnetic Co layer is established by magnetization and electronic transport data. The heterostructure was patterned in a rectangular shape with a width of about 20 $\mu$m and a length up to 100 $\mu$m. The GMR characteristics of the patterned devices were systematically studied and directly compared to that obtained from identically fabricated structures on NiO and SiO$_2$, respectively. How these results relate to the realization of reversible control of the GMR spin valve effect by an electric field will be discussed.\\[4pt] [1] Heron et al., Phys. Rev. Lett 107, 217202 (2011);\\[0pt] [2] Ratcliff et al., submitted]. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U17.00011: Magnetic and magnetoelectric excitations of BiFeO3 Nobuo Furukawa, Masaaki Matsuda, Jason T. Haraldsen, Shin Miyahara, Randy S. Fishman We have determined a model which describes the magnetic and magnetoelectric excitations of multiferroic BiFeO$_3$. Using the full magnetic dispersion relations which are obtained by neutron inelastic scattering measurements [1], parameters for the Heisenberg model with 1st and 2nd neighbor exchange couplings as well as Dzyaloshinskii-Moriya interaction and the single ion anisotropy are estimated. The model also shows excellent agreements with the observed peaks in THz [2] and Raman [3] spectroscopies, which leads to successful assignments of the excitation modes to these peaks. We also discuss that the mode observed at 21.5 cm$^{-1}$ is an electromagnon excitation which should be both magnetic and electric active. This can be verified by the non-reciprocal directional dichroism measurements. {\it REFERENCES: } [1] Matsuda et al., PRL {\bf 109}, 067205 (2012). [2] Talbayev et al., PRB 83, 094403 (2011). [3] Rovillain et al., PRB 79, 180411 (2009). [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U17.00012: Theory of spin-orbit enhanced electric-field control of magnetism in multiferroic BiFeO$_3$ Rogerio de Sousa, Marc Allen, Maximilien Cazayous We present a microscopic theory that shows the importance of spin-orbit coupling in multiferroic compounds with heavy ions. In BiFeO$_3$ (BFO) the spin-orbit coupling at the bismuth ion sites results in a special kind of magnetic anisotropy that is linear in the applied $E$-field. We show how this interaction is capable of disrupting the magnetic cycloid state of bulk BFO, leading to a remarkable level of $E$-field control of magnetism. R. de Sousa, M. Allen, and M. Cazayous, arXiv:1209.6612. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U17.00013: Evolution of the magnetic structure in (Sm,Bi)FeO3 Thin Films William Ratcliff, Amy Poole, Mechthild Enderle, Shingo Maruyama, V. Anbusathaiah, Ichiro Takeuchi BiFeO3 is a multiferroic, which is ordered at room temperature. In this compound, the magnetic and ferroelectric domains are coupled and magnetic domains can be switched with an electric field [1]. It has recently been found that doping Sm onto the Bi site drives the system from rhombohedral to orthorhombic ordering [2]. Furthermore, near the phase boundary, application of an electric field can drive the material between the two structures. It is an open question as to whether the magnetic structure follows. In this talk, I share our recent neutron diffraction results on the magnetic structure of Sm doped BiFeO3 thin films. [1] T. Zhao, A. Scholl, F. Zavaliche, K. Lee, M. Barry, A. Doran, M. P. Cruz, Y. H. Chu, C. Ederer, N. A. Spaldin, R. R. Das, D. M. Kim, S. H. Baek, C. B. Eom, and R. Ramesh, Nature Materials \textbf{5}, 823 (2006). [2] Daisuke Kan, Ching-Jung Cheng, Valanoor Nagarajan, Ichiro Takeuchi \textbf{110}, 014106 (2011) [3] Daisuke Kan, Lucia Palova, Varatharajan Anbusathaiah, Ching Jung Cheng, Shigehiro Fujino, Valanoor Nagarajan, Karin M. Rabe, Ichiro Takeuchi, Adv. Funct. Mater. \textbf{20}, 1108 (2010). [Preview Abstract] |
Session U18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Spin Seebeck and Magneto-optics
Sponsoring Units: GMAG DMP FIAPChair: Berry Jonker, Naval Research Laboratory
Room: 320
Thursday, March 21, 2013 11:15AM - 11:51AM |
U18.00001: Electric Field-Driven Coherent Spin Reorientation of Optically Generated Electron Spin Packets in InGaAs Invited Speaker: Bernd Beschoten Full electric-field control of spin orientations is one of the key tasks in semiconductor spintronics. We demonstrate that electric field pulses can be utilized for phase-coherent 2-pi spin rotation of optically generated electron spin packets in InGaAs epilayers using time-resolved Faraday rotation. Through spin-orbit interaction, the electric-field pulses act as local magnetic field pulses. By the temporal control of these pulses, we can turn on and off electron spin precession and thereby rotate the spin direction into arbitrary orientations in a 2-dimensional plane [1]. Moreover, we apply two subsequent electric field pulses of opposite field polarity to perform spin echo studies of the diffusing spin packet by reversing both the spin precession and the drift direction. In this spin-echo type spin drift experiment we find an unexpected spin rephasing, which is evident by a doubling of the spin dephasing time.\\[4pt] [1] S. Kuhlen et al., Phys. Rev. Lett. 109, 146603 (2012) [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U18.00002: Interacting drift-diffusion theory for photoexcited electron-hole gratings in semiconductor quantum wells Ka Shen, Giovanni Vignale Phase-resolved transient grating spectroscopy in semiconductor quantum wells has been shown to be a powerful technique for measuring such an elusive quantity as the electron-hole drag resistivity $\rho_{eh}$, which depends on the Coulomb interaction between the carriers. In this paper we develop the interacting drift-diffusion theory, from which $\rho_{eh}$ can be determined, given the measured mobility of an electron-hole grating. From this theory we predict a cross-over from a high-excitation-density regime, in which the mobility has the ``normal" positive value, to a low-density regime, in which Coulomb-drag dominates and the mobility becomes negative. At the crossover point, the mobility of the grating vanishes. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U18.00003: Carrier and Spin Dynamics in InAsP Ternary Alloys Michael Meeker, Kelly McCutcheon, Mithun Bhowmick, Brenden Magill, Giti A. Khodaparast, Joe G. Tischler, Sukgeun G. Choi, Chris J. Palmstr{\O}m The recent rapid progress in the field of spintronics involves extensive measurements of carrier and spin relaxation dynamics in III-V semiconductors. In addition, as the switching rates in electronic and optoelectronic devices are pushed to higher frequencies, it is important to understand carrier dynamic phenomena in semiconductors on femtosecond time-scales. In this work, we employed time and polarization-resolved differential transmission measurements in near and mid-infrared, to probe carrier and spin relaxation times in several InAsP ternary alloys. Our results demonstrate the unique and complex dynamics in this material system that can be important for electronic and optoelectronic devices. We present our experimental observations and compare them with the observations in InAs and InP. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U18.00004: Time resolved Magneto-Photoluminescence in $InAs_{x}P_{1-x}$ alloys Travis Merritt, Michael Meeker, Giti A. Khodaparast, Stephen McGill, Joe G. Tischler, Sukgeun G. Choi, Chris J. Palmstr{\O}m Recently, g-factor engineering has attracted much attention for potential applications in spintronics. In the case of $InAs_{x}P_{1-x}$ alloys, a wide range of g-factors, including g=0, can be achieved. In order to probe the band-structure and the dynamics of photo-excited carriers in $InAs_{x}P_{1-x}$ epitaxial films with x=0.13, 0.4, we measured NIR absorption spectra at 4K and 300K, as well as magneto- photoluminescence spectra in both the time and frequency domain for magnetic fields in the range of 0-15T and temperatures in the range of 4-90K. From the temporal measurements, we observed strong tunability in the relaxation dynamics as a function of excitation wavelength. We present these experimental observations and compare them with theoretical calculations. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U18.00005: Probing of the Nature of Carrier Recombination in GaInNAs epilayers using Optical Spin Injection Yutsung Tsai, Biplob Barman, Thomas Scrace, Athos Petrou, M. Fukuda, I.R. Sellers, M.A. Khalfioui Optical pumping experiments have been performed on as-grown and p-type MBE grown GaInNAs epilayers. The PL peak of the nominally undoped as-grown sample exhibits the characteristic S-shaped dependence of dilute nitride material for T \textless\ 60 K [1]. This is associated with carrier recombination via localized states at low temperatures. The reflectance spectra on the other hand map the band-to-band free carrier transition, displaying a Varshni-type behavior. In the p-type material the S-dependence of the PL disappears, and the PL peak coincides with the reflectance spectrum at all temperatures. This indicates band-to-band, rather than localized exciton recombination, in the p-type GaInNAs at all temperatures. This picture was verified by optical pumping experiments. In the undoped sample a large degree of circular polarization was evident only at T \textgreater\ 60 K: below 60 K the polarization is small, and coincident with the reflectance peak. In the p-type samples, on the other hand, non-zero circular polarization, whose maximum matches the peak PL energy, was evident at all temperatures.\\[4pt] [1] A. Polimeni \textit{et al}. Phys. Rev. B. 63, 195320 (2001) [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U18.00006: Dynamic spin Seebeck coefficient and thermo-spin Hall conductivity in systems with Rashba and Dresselhaus spin-orbit coupling Jesus Maytorena, Priscilla Iglesias The generation of spin currents by thermal gradients is a central issue of spin caloritronics. In addition to the recently observed spin Seebeck effect, a transverse thermoelectric effect has been proposed. This is the generation of a spin Hall current by a temperature gradient in a two-dimensional electron gas (2DEG) with Rashba spin-orbit interaction (SOI). We calculate the spin Seebeck coefficient and the thermo-spin Hall conductivity tensor of the spin current response induced by a frequency dependent temperature gradient in a 2DEG with Rashba and Dresselhaus SOI. We consider quantum wells grown in the main crystallographic directions. The spin splitting caused by SOI opens the possibility of resonant effects due to transitions between the spin-split subbands in response to alternating thermoelectric fields and temperature gradients in the THz regime. The spin current response shows characteristic spectral features in notable contrast to the pure Rashba coupling case. Such behavior is caused by the reduced symmetry of the momentum space available for transitions and the presence of critical points. This anisotropic dynamic response could be useful for spin manipulation via thermal means. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U18.00007: Phonon Drag in InSb: Experiment Joseph P. Heremans, Hyungyu Jin, Christopher M. Jaworski, Stewart Barnes A thermoelectric power is reported in a thermocouple in which both arms are made of the same material (n-type InSb) with the same electron concentration, but the phonons have different mean free paths at cryogenic temperatures. This experiment, inspired by [1], isolates the phonon-drag contribution to the thermopower from the diffusion thermopower. The experiment decouples the behavior of the subthermal phonons that drag the electrons, and the thermal phonons that carry most heat. We add data on the contributions of both to the thermal conductivity. This sheds new light on the details of the physical mechanism behind the giant spin-Seebeck effect (GSSE) recently observed [2] on the same material. The GSSE signal was attributed to a combination of electron-phonon drag that pushes the electrons, which are spin-polarized by Zeeman splitting, far from thermal equilibrium, and strong spin-orbit interactions that make the Zeeman splitting sensitive to the electron momentum. Furthermore, we may have found experimental clues about the nature of the phonon force [3]. 1. T. H. Geballe and G. W. Hull, Conference de physique des basses temperatures, p 460, Paris, 1955 2. C.M. Jaworski et al. Nature 487, 210 (2012) 3. S. E. Barnes and S. Maekawa, Phys. Rev. Lett. 98 246601 (2007) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U18.00008: Phonon Drag in InSb: Theory and ``spin''-motive force Stewart Barnes, Joseph Heremans The phonon number operator $\hat n \to \sin^2 \frac{\theta}{2}$ defines the Euler angle $\theta$ and with the phase $\phi$ this maps to a precessing spin. Defined are a ``spin" Berry phase and a ``spin''-motive force (smf)[1]. Unlike an emf, an smf can act upon neutral phonons. Tradition[2] has sub-thermal phonons as central to the thermopower of semi-conductors. The momentum given to these phonons, by the temperature gradient, is transferred to the electrons by ``drag'' where it cancels a Seebeck effect electric field $\vec E$. Here, for InSb at low temperatures, thermal phonons actually relax momentum via boundary and umklapp scattering and energy conservation involves sub-thermal phonons, created by anharmonic effects, with a frequency $\hbar \omega_{\vec q} \sim k_B (dT/dx) \ell$ where $\ell$ is the phonon mean-free-path (mfp). The resulting smf acting upon the thermal phonons produces a ``spin'' voltage $\sim (k_B/e) \Delta T \sim 100\mu$V/K. Via the electron-phonon interaction, the smf, multiplied by the ratio $\ell_{ep}/\ell$, where $\ell_{ep}$ is the electron-phonon mfp, are detected, but not created by the few electrons in our InSb samples. [1] S. E. Barnes and S. Maekawa, Phys. Rev. Lett. {\bf 98}, 246601 (2007) [2] C. Herring, Phys. Rev. {\bf 95}, 954 (1954). [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U18.00009: Planar Nernst effect and Spin dependent Seebeck effect on Py/Ag thin films Priyanga Jayathilaka, Dustin Belyea, Tatiana Eggers, Hillary Kirby, Casey W. Miller We are reporting a systematic study of planar Nernst effect (PNE) and Spin dependent Seebeck effect (SDSE) measurements and their relation to the Anisotropic Magneto Resistance (AMR) on Py thin films grown on SiOx substrates by magnetron sputtering. A 30nm thick Py film was followed by a 15nm of Ag cross electrodes. An in-situ mask exchanging system was allowed the Py and Ag to grow without breaking the vacuum. The sample was placed on top of two thermal baths which were independently controlled by a PID controller. A constant temperature gradient of 15K/cm was applied along the sample and the resultant voltages across the Ag electrodes were measured by nanovoltmeters as the field was swept. In measuring AMR no thermal gradient was applied, and a constant current was applied using a function generator. Both PNE and SDSE showed an AMR like field dependence and angular dependence. SDSE showed a Cos$^{2}$ ($\theta$) angular dependence and PNE showed a Sin (2$\theta$) angular dependence. AMR showed the same angular dependence along the Py film and across the Py film respectively. This suggests both PNE and SDSE behave similar to the AMR in thin films. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U18.00010: Theory of thermal spin-charge coupling in electronic systems Benedikt Scharf, Alex Matos-Abiague, Igor \v{Z}uti\'c, Jaroslav Fabian The interplay between spin transport and thermoelectricity offers several novel ways of generating, manipulating, and detecting nonequilibrium spin in a wide range of materials. Here, we formulate a phenomenological model in the spirit of the standard model of electrical spin injection to describe the electronic mechanism coupling charge, spin, and heat transport and employ the model to analyze several different geometries containing ferromagnetic (F) and nonmagnetic (N) regions: F, F/N, and F/N/F junctions which are subject to thermal gradients (i.e., the spin-dependent Seebeck effect). Furthermore, we study the Peltier and spin-dependent Peltier effects in F/N and F/N/F junctions. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U18.00011: ABSTRACT WITHDRAWN |
Session U19: Metal-Insulator Transitions II
Sponsoring Units: DCMPChair: Rongwei Hu, University of Maryland
Room: 321
Thursday, March 21, 2013 11:15AM - 11:27AM |
U19.00001: MBE synthesis and characterization of charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_3$ thin films Rebecca Sichel-Tissot, Robert Devlin, Philip Ryan, Jong-Woo Kim, Alex Dagg, Steven May La$_{1/3}$Sr$_{2/3}$FeO$_3$ (LSFO) is a transition metal oxide which exhibits strongly correlated electronic behavior. When cooled below 180-190K, an electronic phase transition occurs during which the resistivity abruptly increases. LSFO was deposited on (001) SrTiO$_3$ substrates using molecular beam epitaxy (MBE). The transition temperature T* = 183 K was measured from a sharp increase in the resistivity and confirmed by the appearance of x-ray reflections with wavevectors of q = n/3[111]. Oxygen loss from the film over a period of 8 months was observed to have significant effects on the structural and electronic properties, but was shown to be reversible by annealing in oxygen. This work is supported by the Office of Naval Research under grant number N00014-11-1-0664. Work at the Advanced Photon Source is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences under contract DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U19.00002: Strain dependence of the electronic phase transition in epitaxial La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ films Robert Devlin, Rebecca Sichel-Tissot, Phillip Ryan, Jong-Woo Kim, Steve May The electronic transport properties of La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ thin films were experimentally investigated as a function of epitaxial strain. In bulk, this compound exhibits a first-order electronic phase transition at 198 K accompanied by an abrupt change in resistivity. In order to investigate how different epitaxial strain states affect the abruptness and temperature of the transition, thin La$_{1/3}$Sr$_{2/3}$FeO$_{3}$ films were grown using molecular beam epitaxy on SrTiO$_{3}$ DyScO$_{3}$ and (La,Sr)(Al,Ta)O$_{3}$ imparting $+$0.9{\%} $+$1.8{\%} and -0.05{\%} strain, respectively. The transition temperatures were determined through resistivity measurements as well as synchrotron x-ray diffraction of (4/3 4/3 4/3) peaks, which are a direct signature of an additional ordering below the transition temperature. We find that the transition temperature measured through resistivity and the integrated intensity of the (4/3 4/3 4/3) peaks are in excellent agreement. The variation in transition temperature and the abruptness of the transition will be presented for the films grown on the various substrates. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U19.00003: Electrical Transport in Iron Cobalt Silicide Nanowires Drew Rebar, John DeGrave, Song Jin, John DiTusa Iron silicide is a small gap insulator with fascinating physical properties that can be made metallic and magnetic when doped with cobalt. With the substitution of cobalt for iron, Fe1-xCoxSi, the material undergoes an insulator-to-metal transition becoming a half metal for a wide range of x. The ground state is helimagnetic with distinct itinerant character. It has been demonstrated by others that an exotic intermediate magnetic vortex or skyrmion state exists between the helimagnetic and ferromagnetic phases in small applied fields. Electron transport in bulk Fe1-xCoxSi has been found to be dominated by electron-electron interaction effects similar to what has been found in prototypical semiconductors such as Si:P. Here we probe low temperature electron transport in CVD-grown Fe1-xCoxSi nanowires with x$=$0.05. The reduced dimensionality presents the opportunity to characterize the conductivity where only the phase-coherent contribution may be constrained to one dimension. Results of low temperature transport measurements of these wires will be presented. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U19.00004: Exploring Fe$_{1-y}$Co$_x$Si near the insulator-to-metal transition Yan Wu, Brad Fulfer, Julia Chan, David Young, John DiTusa FeSi is a nonmagnetic narrow gap insulator with interesting temperature-dependent magnetic and optical properties. Doping FeSi with Mn or Co introduces hole or electron a charge carriers as well as additional magnetic moments. Our previous investigations show that for Mn doping near the insulator-metal-transition(IMT) an intriguing field sensitive non-Fermi-Liquid behavior results from the underscreening of the $S=1$ impurity moments. Here we explore the case of electron doping via Co substitution for concentrations very near the IMT. Our magnetic susceptibility measurements indicate an underlying competition between screening of the magnetic moments at low y and ferromagnetic ordering at higher Co-concentrations. Our carrier transport measurements indicate that the IMT occurs near $y=0.01$ and that above 2 K electron-electron interaction effects dominate the magnetoresistance. However, for $T<1$ K, high magnetic fields induce an enhance charge carrier mobility for samples with $y\sim 0.01$. We will present data comparing the magnetotransport of the Co and Mn doped samples in order to compare electron and hole doping in proximity to the IMT. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U19.00005: Doping induced metallization of a narrow gap insulator FeGa$_{3}$ Monika Gamza, Akshat Puri, Jan Tomczak, Jim Quinn, Meigan Aronson Narrow gap semiconductors attract great interest owing to an unusual metallization process which remains poorly understood despite decades of extensive research [1]. Here, we report on the effects of hole doping on properties of a nonmagnetic semiconductor FeGa$_{3}$ with a band gap of 0.4 eV [2]. By means of electrical resistivity, magnetization and specific heat measurements performed on single crystals grown from gallium flux we have found that a substitution of Mn for Fe in Fe$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$Ga3 (0.005\textless\ x \textless\ 0.03) yields an insulating state at high temperatures with residual magnetic moments. With lowering temperature, resistivity deviates from an activation-type behavior and nearly saturates at T\textless 100 K. Finally, it drops by as much as two orders of magnitude at temperature of 6 K, indicating a metal-insulator transition. Magnetization measurements did not show magnetic order associated with the transition. When an external magnetic field is applied, the metal-insulator transition moves to lower temperatures and eventually the resistivity returns to the insulating-type behavior in fields higher then of 5 Tesla. \\[4pt] [1] M. Imada et al, Rev. Mod. Phys., 70, 1039 (1998)\\[0pt] [2] M. Arita et al., Phys. Rev. B 83, 245116 (2011); Y. Hadano et al., J. Phys. Soc. Jpn. 78, 013702 (2009) [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U19.00006: Role of long range Coulomb interaction near the disorder driven metal-insulator transition in Ga$_{1-x}$Mn$_x$As S. Mahmoudian, E. Miranda, V. Dobrosavljevic Surprising signatures of interaction effects on disorder-driven localization have recently been observed by scanning tunneling microscopy of Ga$_{1-x}$Mn$_x$As, where visualizing the electronic wave function near the metal-insulator transition revealed\footnote{A. Richardella {\em et al.}, Science {\bf 327}, 665 (2010).} a pronounced suppression of the local tunneling density of states (LDOS) and enhanced localization only near the Fermi energy. These features highlight the limitation of the non-interacting picture, and point to the crucial importance of the long-range Coulomb interaction. Here, we implement a theoretical approach based on the recently developed Typical-Medium Theory,\footnote{V. Dobrosavljevi\'c, Int. J. Mod. Phys. B {\bf 24}, 1680 (2010).} the conceptually simplest approach to interaction-localization. We show that the presence of long-range Coulomb interaction leads to the simultaneous opening of a soft pseudogap in both the typical (geometrically averaged) and the average (algebraically averaged) LDOS, as the transition is approached. This result is consistent with the experimentally observed features of the STM spectra, suggesting new experiments that should be performed to fully characterize the quantum critical behavior at the metal-insulator transition [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U19.00007: Fitting of Diverging Thermoelectric Power in a Strongly Interacting 2D Electron System of Si-MOSFETs Hyun-Tak Kim The diverging-effective mass (DEM) in a metallic system is evidence of strong correlation between fermions in strongly correlated systems. The identification of the DEM still remains to be revealed The effective mass, m*$=$m$_{\mathrm{o}}$/(1-$\rho^{4})$ [1] where $\rho $ is band filling helps clarify the diverging thermoelectric power, S, measured in inhomogeneous Si-MOSFET systems [2]. As a carrier density n$_{\mathrm{s}}$ decreases, S increases rapidly This is regarded as the metal-insulator transition (MIT) near n$_{\mathrm{c}}\approx $79x10$^{-1}$cm$^{-2}$, where n$_{\mathrm{c}}$ is about 0.02{\%} to n$_{\mathrm{Si}}\approx $3.4x10$^{-14}$cm$^{-2}$ in Si. This can be solved in assuming that $\rho =$n$_{\mathrm{c}}$/n$_{\mathrm{s}}$ increases as n$_{\mathrm{s}}$ decreases. n$_{\mathrm{c}}$ is an excited(doped) carrier density in the semiconductor induced by gate and can be also regarded as a metallic carrier density, that is, n$_{\mathrm{c}}\equiv $n$_{\mathrm{seminon}}=$n$_{\mathrm{metal}}$. n$_{\mathrm{s}}$ is given as n$_{\mathrm{tot}}\equiv $n$_{\mathrm{s}}=$n$_{\mathrm{c}}+$n$_{\mathrm{seminon}}$ where n$_{\mathrm{seminon}}$ is a carrier density in a nonmetallic phase. The carrier density measured by Hall effect is the sum of carriers both induced by gate field and generated by MIT. Moreover, a larger metallic phase is not made due to a conducting path in the field-effect structure after a metallic phase is formed. Thus, increasing n$_{\mathrm{s}}$ indicates increasing n$_{\mathrm{non}}$; this corresponds to an over-doping to increase inhomogeneity. It's fitting is given from S$=(\alpha \pi ^{3}$k$^{2}_{\mathrm{B}}$T/3e)(1/E$_{\mathrm{F}})$ $=(\alpha $8$\pi ^{3}$k$^{2}_{\mathrm{B}}$T/3h$^{2})$(m*/e*n$_{\mathrm{c}})$ $=$S$_{\mathrm{o}}$(1/$\rho )$(1/(1-$\rho^{\mathrm{4}}))$, where e*$=\rho $e [1], $\rho =$n$_{\mathrm{c}}$/n$_{\mathrm{s}}$, T$=$0.8K, m*$=$m$_{\mathrm{o}}$/(1-$\rho^{4})$ [1], $\alpha =$0.6, and S$_{\mathrm{o}}=(\alpha $8$\pi ^{3}$k$^{2}_{\mathrm{B}}$T/3h$^{2})$(m$_{\mathrm{o}}$/en$_{\mathrm{c}})$ $\approx $12.36 are used. The data S [2] are closely fitted by m* [1] Physica C 341-348(2000)259. [2] Phys. Rev. Lett. 109 (2012) 096405. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U19.00008: Metal-insulator and glass transitions in a 2D electron system in Si MOSFETs with a screened Coulomb interaction Ping V. Lin, Dragana Popovi\'c We present a study of conductivity $\sigma$ of a 2D electron system (2DES) in Si MOSFETs with the oxide thickness $d_{ox}=7$~nm. In the low density regime of interest, the average electron-electron ($e$-$e$) separation is larger than $d_{ox}$, so that the $e$-$e$ interaction is screened by the metallic gate. The carrier density $n_s$ was changed at a high temperature $T\approx 20$~K, the 2DES was then cooled to a desired $T$ with a fixed $n_s$, and $\sigma$ was measured as a function of time $t$. At the lowest $n_s$, in the insulating regime, transport occurs via variable-range hopping. Near the critical density $n_c$ on the metallic side of the metal-insulator transition (MIT), the time-averaged $\langle\sigma(T)\rangle$ follows a power-law behavior, giving a reliable extrapolation of $\langle\sigma(n_s, T=0)\rangle$. The critical exponents are discussed and compared to the case of the MIT with long-range Coulomb interactions. The statistical analysis of the fluctuations in $\sigma(t)$ provides evidence for the glassy freezing of electrons for $n_s |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U19.00009: Valence Band Character of NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ using 3p-3d Resonant ARPES Garam Han, Yeongkwan Kim, Yoonyoung Koh, Beomyoung Kim, Dongjoon Song, Jungjin Seo, Wonshik Kyung, Kyungdong Lee, Changyoung Kim Understanding the strong correlated system is one of the most challenging tasks in condensed matter physics. Especially, the metal insulator transition (MIT) has been one of the major topics recent few decades. NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ is known as one of famous material which has MIT. The cubic pyrite NiS$_{2}$ is a charge-transfer (CT) insulator. NiS$_{2}$ attracts particular interest as it easily forms a solid solution with NiSe$_{2}$ (NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}})$ which, while being isoelectronic and isostructural to NiS$_{2}$, is nevertheless a good metal. MIT, induced by Se alloying, is observed at low temperature (T) for x$=$0.45. Perucchi and his collaborators revealed closed relation between MIT and band width through comparison of infrared spectroscopy result and LDA calculation. However, it was only an indirect observation, and is inconsistent with recent proposal that NiS$_{2}$ is not a CT insulator but an insulator due to the bonding-antibonding splitting in the S -- S (Se -- Se) dimers. To reveal the true mechanism in the MIT in NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$, resonant photoemission experiment is essential. According to competing theories (CT insulator and insulator due to bonding-antibonding splitting), it is expected that the character of the main band that is responsible for the MIT should be different. Therefore, we performed 3p-\textgreater 3d resonant ARPES for various Se dopings (x$=$0.43; insulator, x$=$0.5, 0.7, 2.0; metal) and observed a significant change between on- and off-resonances near the MIT. Our experimental result supports that the origin of MIT in NiS$_{\mathrm{2-x}}$Se$_{\mathrm{x}}$ is the CT theory rather than the dimer theory. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U19.00010: Low temperature conductance spectra of STO at the nanoscale Alireza Mottaghizadeh, Qian Yu, Alexandre Zimmers, Herve Aubin The electronic properties of transition metal oxide materials depend on the electronic carrier density, which can be tuned with the oxygen stoichiometry. In binary MOx or ternary perovskite ABOx, it has been shown that upon applying a strong electric field, oxygen vacancies can be created or displaced in the material. This effect is responsible for the memristive behavior recently discovered in TiO2 materials by HP laboratory and launched a worldwide renew interest into ionics. We present a study of oxygen ions vacancies displacement in SrTiO3, the archetype perovskite oxide. For this work, metallic electrodes, separated by distances about 100 -- 300 nm, are deposited on the surface of a STO crystal and ions migration procedures and current-voltage characteristics measurements are done at low temperature, T $\sim$ 260 mK. Upon applying large voltage up to 30 V, oxygen vacancies migration is identified as the apparition of resistance switching events in current-voltage characteristics. Detailed measurements of the junction show that the switching event led to the formation of a nanosized region of highly doped STO, located within the electrodes where the current-voltage characteristics show the presence of the doped in-gap states. This work was supported by the French ANR grants 10-BLAN-0409-01 and 09-BLAN-0388-01. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U19.00011: Crystalline and Magnetic Anisotropy of the 3$d$ Transistion-Metal Oxides Andreas Schr\"on, Claudia R\"odl, Friedhelm Bechstedt The 3$d$ transition-metal oxides (TMOs) are subject of debate since many decades due to their extraordinary properties, such as the formation of an antiferromagnetic ordering AFM2 below their N\'eel temperature. Many studies, both experimental and theoretical, focus only on MnO and NiO, where the crystalline anisotropy is solely driven by exchange striction along the unique symmetry axis in the [111] direction and where the magnetic anisotropy is explained in terms of magnetic dipole interactions. In the other TMOs, FeO and CoO, however, orbital magnetization and spin-orbit interaction play an additional, yet crucial role for both crystalline and magnetic anisotropy. We present density-functional theory (DFT) studies including an on-site interaction $U$ of the crystalline and magnetic anisotropy of the electronic systems with non-collinear spins. The influence of the (semi-)local description of exchange and correlation (XC) by means of the local density approximation (LDA) and generalized gradient approximation (GGA) on the orbital moments in FeO and CoO and the implications on the aforementioned properties is investigated. We discuss the quenching of the orbital magnetization due to the gradient corrections. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U19.00012: Spatially resolved dynamic susceptibilities of disordered two dimensional Hubbard model Nandini Trivedi, Oinam Nganba Meetei We predict the existence of an emergent metallic phase in the disordered two dimensional Hubbard model [1] that has recently been confirmed by experiments on 1T-TaS$_2$ intercalated with Cu. The metallic state has a finite dc conductivity but unusual dynamical properties. We present here a comprehensive analysis of the spatially resolved spin susceptibility, screened charge density, and optical conductivity of the disordered Hubbard model. We develop a new method in which the exact eigenstates from inhomogeneous mean-field theory are used to calculate dynamical susceptibilities within the random phase approximation. By combining the non-perturbative effects of self-consistent mean-field theory with analytical perturbative methods, this approach gives insights about fluctuations near the quantum phase transitions. We make several predictions which can be directly tested in spatially resolved experiments. \\[4pt] [1] D. Heidarian and N. Trivedi, Phys. Rev. Lett. {\bf 93}, 126401 (2004) [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U19.00013: Dual fermion approach for disordered interacting fermion systems Shuxiang Yang, Patrick Haase, Hanna Terletska, Ziyang Meng, Juana Moreno, Mark Jarrell, Thomas Pruschke Understanding the combined effect of electron-electron interaction and disorder is one of the crucial questions in condensed matter physics. There is an obvious need of theoretical tools which allow to treat both these effects on equal footing. To study the intricate interplay of these effects, we generalize our recently proposed dual fermion approach to include both electron-electron interaction and disorder. Since the constraint imposed on the dual-space Feynman diagrams in the disordered case does not apply to those generated due to interactions, it is essential to treat elastic scattering processes due to the disorder separately from the inelastic scattering processes due to the pure interaction and mixed contributions. I will discuss the resulting diagrammatic formalism and an algorithm for its implementation. The possible applications for the Anderson Falicov-Kimball and the Anderson-Hubbard models are also discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U19.00014: Dual fermion method for disordered electronic systems Hanna Terletska, Shuxiang Yang, Zi Yang Meng, Juana Moreno, Mark Jarrell While the coherent potential approximation (CPA) is the most commonly used theoretical method to study disordered systems, it by construction misses non-local correlations and Anderson localization. We have recently extended the dual fermion approach [1] to disordered non-interacting systems using the replica method, which allows one to included such non-local physics. Our method utilizes an exact transform to the dual variables, and includes inter-site scattering via diagrammatic perturbation theory in dual fermion space, with the CPA being a zeroth-order approximation. Analyzing one-particle quantities we demonstrate good agreement between our results and those from the dynamical cluster extension of the CPA. Moreover, by calculating the dc conductivity we show that our approach successfully captures weak localization missing in the CPA. This method as a natural extension of CPA, and presents a powerful alternative to existing cluster extensions of CPA. It can be used in various applications, including systems with disorder and interactions. \\[4pt] [1] A.N. Rubtsov, et. al., Phys. Rev. B 77, 033101 (2008). [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U19.00015: Metal-Insulator Transitions in Crystalline Phase Change Materials Wei Zhang, Alexander Thiess, Peter Zalden, Rudolf Zeller, Peter Dederichs, Jean-Yves Raty, Matthias Wuttig, Stefan Bl\"ugel, Riccardo Mazzarello Phase-change materials are capable of undergoing fast and reversible transitions between amorphous and crystalline phase upon heating and have been exploited in data storage applications based on the strong optical/electrical contrast between the two phases. Recently, compelling evidence for a metal-insulator transition (MIT) solely due to disorder has been observed in the crystalline PCM Ge$_{1}$Sb$_{2}$Te$_{4}$ (GST) and similar compounds: upon annealing at temperatures T below 548K, the system exhibits insulating behavior due to Anderson localization; at higher T, it shows metallic behavior. In contrast to the MITs observed in other systems such as P-doped Si, in GST correlation effects do not play a role and the MIT occurs at fixed stoichiometry. In this work, we present a Density Functional Theory study of this effect. We consider a set of very large models of GST containing one to several thousand atoms and different degree of disorder. We identify the microscopic mechanism that localizes the electron wavefunctions near the Fermi energy in the insulating phase: these states are localized inside regions having large vacancy consequent dissolution of these vacancy clusters. These results could help to develop new device based on multiple resistance states. [Preview Abstract] |
Session U20: Focus Session: Mesoscopics - Preparation, Superconductivity and Magnetism
Sponsoring Units: DMPChair: Jacobo Santamaria, Universidad Complutense
Room: 322
Thursday, March 21, 2013 11:15AM - 11:27AM |
U20.00001: A Novel Nano-Assembly Technique for the Creation of Ultra-Low Disorder, Locally-Tunable One-Dimensional Systems with Carbon Nanotubes Jonah Waissman, Maayan Honig, Sharon Pecker, Avishai Benyamini, Assaf Hamo, Shahal Ilani Carbon nanotubes offer exciting prospects for studies of fundamental physics in one dimension due to their propensity for clean, defect-free growth, and long lengths. Recent technological advances have allowed for the creation of zero-dimensional ultra-clean nanotube devices, leading to new physics. But to date, the full potential of these molecules for full-fledged experiments in extended one-dimensional geometries is still unrealized, owing to fundamental limitations in making complex and clean devices. In this talk, we will describe a new nano-assembly technique to create suspended carbon nanotube devices of large complexity and with extremely low levels of electronic disorder. We demonstrate the creation of devices with multiple electrostatic gates and devices that combine several nanotubes positioned at chosen distances from each other. These capabilities open the door to a wide array of new experiments on the physics of electrons, spins and mechanics in one dimension. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U20.00002: Towards FIB patterning of commercial SiN membranes for sensitive magneto-calorimetry Kurtis Wickey, Thomas Kent, Roberto Myers, Joseph Heremans, Ezekiel Johnston-Halperin Investigating magnetocaloric effects in thin films, spin-thermal coupling, and the heat capacity of 2D materials such as graphene, germanene, and MoS$_{\mathrm{2}}$ requires small (hundreds of microns and less) thermally isolated platforms with sensitivity to comparably small heat capacities. Previously, calorimeters fabricated on amorphous SiNx membranes have been used due to their low thermal conductivity and compatibility with standard fabrication techniques. Here, we use a focused ion beam (FIB) to remove large portions of commercial SiNx membranes, leaving a platform that is thermally isolated from the Si frame by narrow supporting legs. This approach allows the fabrication of the calorimeter around existing samples such as flakes of MoS$_{\mathrm{2}}$, pre-patterned mesas of magnetic thin films, etc. The thermal isolation of the platform ensures uniform temperature without the use of the thermally conducting layer present in unpatterned membrane calorimeters, further improving the sensitivity of our calorimeters. We will discuss our progress towards realizing these calorimeters. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U20.00003: Atomic Calligraphy Matthias Imboden, Flavio Pardo, Cristian Bolle, Han Han, Ammar Tareen, Jackson Chang, Jason Christopher, Benjamin Corman, David Bishop Here we present a MEMS based method to fabricate devices with a small number of atoms. In standard semiconductor fabrication, a large amount of material is deposited, after which etching removes what is not wanted. This technique breaks down for structures that approach the single atom limit, as it is inconceivable to etch away all but one atom. What is needed is a bottom up method with single or near single atom precision. We demonstrate a MEMS device that enables nanometer position controlled deposition of gold atoms. A digitally driven plate is swept as a flux of gold atoms passes through an aperture. Appling voltages on four comb capacitors connected to the central plate by tethers enable nanometer lateral precision in the xy plane over 15x15 sq. microns. Typical MEMS structures have manufacturing resolutions on the order of a micron. Using a FIB it is possible to mill apertures as small as 10 nm in diameter. Assuming a low incident atomic flux, as well as an integrated MEMS based shutter with microsecond response time, it becomes possible to deposit single atoms. Due to their small size and low power consumption, such nano-printers can be mounted directly in a cryogenic system at ultrahigh vacuum to deposit clean quench condensed metallic structures. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U20.00004: Oxidation of atomic scale patterns prepared by scanning probe techniques Kai Li, Namboodiri Pradeep, Joseph Fu, Lei Chen, Richard Silver Scanning probes offer a potential alternative technology pathway in practical atomic scale devices and developing atom-based dimensional standards. However, the process steps, such as atomic scale lithography and subsequent pattern transfer need considerable optimization before the technology can be utilized for manufacturing applications. Nanoscale patterns are prepared in UHV on a hydrogen passivated silicon surface using STM by selectively removing H atoms. These patterns can then be used for further chemical processing such as oxidation and RIE. Conventional Si oxidation processes that require a high temperature and moisture-rich environment are known to damage the hydrogen-protected area. The challenge is to produce a strong SiO$_{\mathrm{2}}$ hard etch mask on the patterned area without affecting the hydrogen passivation layer. Currently we are developing a new low temperature oxidation process that starts with exposing the patterned areas to oxide/moisture at temperatures below H desorption. The presentation will focus on the details of near atomic scale oxide chemistry relevant to processing nanoscale patterns.~We will also present our approach to fabricating stable, atomically defined calibration standards based on the crystal lattice. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U20.00005: Fabrication of Flat Freestanding Silicon Nanomembranes Kyle McElhinny, David Czaplewski, Gokul Gopalakrishnan, Martin Holt, Paul Evans Silicon nanomembranes are suspended single crystal sheets of silicon, tens of nanometers thick, with areas in the thousands of square micrometers. Freestanding nanomembranes provide an ideal system for studying the physics of nanoscale crystalline materials and find application in novel electronic and photonic materials and devices. Challenges in fabrication arise due to buckling in response to stresses in the silicon-on-insulator starting material. In equilibrium, the elastic energy of the membrane is minimized by distributing the buckling distortion across the entire membrane. We demonstrate that flat nanomembranes can be created by utilizing a modification of traditional membrane fabrication procedures. This new scheme produces an elastically metastable structure, in which the buckling is redistributed to a small area near the edges of the membrane. An energetically favorable mechanism for this redistribution will be discussed. Membranes with thicknesses from 315 nm down to 6 nm have been fabricated, showing vertical deviations of less than 10 nm across an area covering 100 $\mu$m $\times$ 100 $\mu$m. X-ray scattering experiments performed on these structures demonstrate the importance of the ability to fabricate crystallographically uniform and flat nanomembranes. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U20.00006: Mesoscopic relaxations in homoepitaxial systems and their effect on oxygen adsorption Oleg O. Brovko, Wuwei Feng, Holger L. Meyerheim, Valeri S. Stepanyuk, J\"urgen Kirschner The importance of mesoscopic relaxations in heteroepitaxial systems has been recognized quite a while ago. Both theoretical predictions and subsequent experimental observations have clearly shown the importance of mesoscopic relaxations for electronic, magnetic and geometric properties of heteroepitaxial nanostructures. The implications of mesoscopic relaxations in \emph{homoepitaxial} systems, however, despite theoretical predictions of their importance, are still not fully understood. In the present joint experimental and theoretical paper, by the example of Fe nanoislands grown homoepitaxially on a p(1x1)O/Fe(001) surface we demonstrate that relaxations at the edges of nanoislands do not only determine the electronic and geometric structures of nanoislands' rims but also govern the oxygen adsorption thereon. Contraction of metallic bonds at the edge of Fe nanoislands leads to a corrugation of the edges and the substrate around, which inevitably leads to a change in adsorption height and electronic structure of oxygen atoms residing on the island. Our results outline the importance of mesoscopic relaxations in homoepitaxial nanostructures for the system's electronic and structural properties and the adsorption of light elements and molecules thereon. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U20.00007: Synthesis of Low Density Metallic Nanowire Network Edward Burks, Chad Flores, Dustin Gilbert, Kai Liu, Thomas Felter, Supakit Charnvanichborikarn, Sergei Kucheyev, Jeffery Colvin Highly porous metallic nanostructures have been shown to possess interesting thermal, electrical and mechanical properties due in part to their high surface areas and low densities. In this work, ion track-etched membranes were used as a template for electrodeposition to realize a low density interconnected copper nanowire network. Polycarbonate membranes (3-6 microns thick) were first irradiated with energetic Xe$^{6+}$ ions at normal incidence and multiple 45 degree azimuthal angles. The total irradiation density was 2x10$^{9}$ tracks/cm$^{2}$. Following a UV/ozone treatment, NaOH was used to preferentially etch the latent tracks of ion damage, creating intersecting nanopores in the polycarbonate matrix. A thin metal layer was then sputtered onto one side of the now-porous membrane to be used as a working electrode. Selected metals such as Cu and Co were then electrodeposited from a sulfate electrolyte into the pores, filling the membrane with an interconnected wire network. The polycarbonate membrane was then folded onto itself several times, and dichloromethane was used to dissolve away the polycarbonate. So far densities as low as 40mg/cm$^{3}$ have been achieved. Structural and magnetic properties of such networks have been investigated. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U20.00008: Superconducting vortex dynamics in nanostructured hybrids based on Fe single-crystal nanotriangles Jose Vicent, Alicia Gomez, Elvira Gonzalez, Miguel Iglesias, Javier Palomares, Nadia Sanchez, Federico Cebollada, Jesus Gonzalez Arrays of Fe single-crystal nanotriangles have been fabricated by Electron Beam Lithography. These arrays are embedded in superconducting Nb thin films. We have studied the superconducting vortex lattice motion on the periodic pinning potentials induced by the magnetic arrays. The vortex dynamics can be controlled through tailoring the magnetic stray field configurations. Which are due to different magnetic remanent states of the Fe single-crystal nanostructures. These configurations have been modified by changing the direction of the saturating applied field and also by using different orientations of the Fe magneto-crystalline easy axes within the triangles. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U20.00009: Suppression of Superconductivity in Small Clusters of Proximity-Coupled Superconducting Islands Malcolm Durkin, Serena Eley, Sarang Gopalakrishnan, Nadya Mason We report transport measurements of proximity-coupled arrays of mesoscopic niobium islands patterned on gold films. We show that superconductivity in the individual islands depends on the number of nearest neighbors, even for island diameters much larger than the superconducting coherence length. We also investigate the length scale where superconductivity in single islands approaches the bilayer approximation. This work is relevant to the understanding of metallic states and quasi-superconductivity in 2D systems [1]. [1] S. Eley, S. Gopalakrishnan, P. Goldbart, and N. Mason, Nature Phys. 8, 59-62 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U20.00010: Electric-field induced superconducting ball formation: new physics of superconductors or a flawed experiment? R.S.B. Ghosh, J.E. Hirsch In 1999, Rongjia Tao, P.W. Anderson and coworkers reported the discovery of a surprising new effect in high temperature superconductors (Phys. Rev. Lett. 83, 5575 (1999)): in the presence of a large electric field, millions of superconducting microparticles spontaneously aggregated into balls of macroscopic dimensions. Subsequently, Tao and coworkers reported that the same effect takes place in low temperature conventional superconductors (Physica C 377, 357 (2002)). If true, this effect would be evidence for novel physics of superconductors, not described by BCS theory. However our experimental studies with high temperature superconductors show that (i) ball formation also occurs in the absence of an applied electric field, and (ii) the phenomenon also occurs at temperatures above the superconducting transition temperature. Possible origins of the phenomenon and implications for theories of superconductivity are discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U20.00011: Electric-Field Induced Formation of Superconducting Balls R. Tao, X. Xu, E. Amr, H. Tang Ghosh and Hirsch recently claimed that many micrometer-size particles in liquid nitrogen, as large as between 25 $\mu m$ and 32 $\mu m$, can be aggregated into balls by shaking. It turns out that they performed their experiments with liquid nitrogen in open air, the moisture condensed on their particle surface leading to ball aggregation by shaking. We repeated their shaking experiment and found that dry BSCCO, YBCO and Pb powders in liquid nitrogen do not form any balls by shaking in a glove bag filled with dry nitrogen gas. No matter how we shake the samples, these powders do not aggregate together. However, when we open the glove bag and let the air come to the samples, BSCCO, YBCO and Pb all form some balls quickly by shaking. Also inside the dry glove bag, when we apply an electric field and slowly increase it, superconducting particles form balls within two critical electric fields, $E_{c1}$ and $E_{c2}$ ($E_{c1} < E_{c2}$), while non-superconducting particles do not form balls at all. The electric field induced superconducting ball formation reveals that the area of interaction between electric field and superconductors requires more investigation. However, the phenomenon can be explained within the BCS theory. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U20.00012: Characterization of spin induced subgap states in superconductor/quantum dot/superconductor junctions Gediminas Kirsanskas, Brian Andersen, Karsten Flensberg, Jens Paaske We examine the emergence of subgap states in a junction consisting of two superconducting leads coupled to spinful Colomb blockaded quantum dot. The system is modeled by an effective Kondo model, which gives rise to so-called Yu-Shiba-Rusinov states inside the gap. We determine the dispersion of these states with an applied phase difference across the junction and study their dependence on an applied magnetic field. Also the effects of coupling asymmetry to the leads and deviation from the particle-hole symmetric point are addressed. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U20.00013: High critical-current superconductor-InAs nanowire-superconductor junctions Simon Abay, Henrik Nilsson, Fan Wu, C.M. Wilson, H.Q. Xu, Per Delsing We report on InAs nanowires coupled to superconducting leads with high critical current and widely tunable conductance. We implemented a double lift-off nanofabrication method to get very short nanowire devices with Ohmic contacts. We observe very high critical currents of up to 800\,nA in a wire with a diameter of 80\,nm. The current-voltage characteristics of longer and suspended nanowires display either Coulomb blockade or supercurrent depending on a local gate voltage, combining different regimes of transport in a single device. In addition, both the conductance and the critical current of the suspended devices increased step-wise as a function of the local gate voltage. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U20.00014: In-Plane Magnetic Field Tolerance of a Nanobridge SQUID Magnetometer Natania Antler, Eli M. Levenson-Falk, Ravi Naik, Shay Hacohen-Gourgy, R. Vijay, I. Siddiqi We describe the operation of a nanobridge SQUID magnetometer subject to an in-plane magnetic field of up to 60 mT. The magnetometer is comprised of a nanobridge SQUID with two aluminum weak links embedded in a 4-8 GHz microwave tank circuit for dispersive readout. We obtain a flux sensitivity of 17 n$\Phi_0/$Hz$^{1/2}$ with 50 MHz of instantaneous bandwidth in zero magnetic field. This effectively corresponds to single spin resolution, within a 1 Hz bandwidth, for nanomagnets placed within 100-200 nm from the nanobridge edge. We find that the effective flux sensitivity only degrades by a factor of $\sim$3 up to 60 mT of applied field. Finally, we describe progress towards magnetization dynamics measurements in different spin species such as Cobalt nanoclusters and Bismuth implanted in Silicon-28. [Preview Abstract] |
Session U21: Nanotechnology Applications: Advances in Sensors and Therapies
Sponsoring Units: FIAPChair: Michael Naughton, Boston College
Room: 323
Thursday, March 21, 2013 11:15AM - 11:27AM |
U21.00001: Protein typing of circulating microvesicles allows real-time monitoring of glioblastoma therapy Huilin Shao, Jaehoon Chung, Leonora Balaj, Ralph Weissleder, Hakho Lee Glioblastomas shed large quantities of small, membrane-bound microvesicles (MVs) into the circulation. While these hold promise as potential biomarkers of therapeutic response, there remain hurdles to their identification and quantitation. Here, we describe a highly sensitive and rapid analytical technique for profiling circulating MVs directly from blood samples of glioblastoma patients. MVs, introduced onto a dedicated microfluidic chip, are labeled with target-specific magnetic nanoparticles and detected by a miniaturized nuclear magnetic resonance system. Compared with current standard assays (e.g., Western blotting, ELISA and flow cytometry), this integrated system has a much higher detection sensitivity, and can differentiate glioblastoma multiforme (GBM) MVs from non-tumor host cell-derived MVs. The system further showed that circulating GBM MVs could serve as a surrogate for primary tumor by reflecting its molecular signature and a predictor of treatment-induced changes. We expect that this converging nanotechnology platform would have a wide range of applications, providing both an earlier indicator of drug efficacy and a potential molecular stratifier for human clinical trials. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U21.00002: Bioelectronic Device Mimicking Human Sensory System based on Nanovesicle-Carbon Nanotube Hybrid Structure Daesan Kim, Hye Jun Jin, San Hun Lee, Tae Hyun Kim, Juhun Park, Hyun Seok Song, Tai Hyun Park, Seunghun Hong We have developed a nanovesicle-based bioelectronic nose (NBN) that could mimic the receptor-mediated signal transmission of human olfactory systems and recognize a specific odorant. The NBN was comprised of a single-walled carbon nanotube (CNT)-based field effect transistor and cell-derived nanovesicles containing human olfactory receptors and calcium ion signal pathways. Importantly, the NBN took advantages of cell signal pathways for sensing signal amplification. It enabled $\sim $100 times higher sensitivity than that of previous bioelectronic noses based on only olfactory receptor protein and CNT transistors. The NBN sensors exhibited a high sensitivity of 1 fM detection limit and a human-like selectivity with single-carbon-atomic resolution. Furthermore, these sensors could mimic a receptor-mediated cellular signal transmission in live cells. This versatile sensor platform should be useful for the study of molecular recognition and biological processes on cell membranes and also for various practical applications such as food conditioning and medical diagnostics. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U21.00003: Fabrication of Wearable Sensors for Human Health Monitoring through Magnetically Directed Assembly Techniques Azar Alizadeh, Jeffrey Ashe, Matthew Misner, Yanzhe Yang, Sheng Zhong, Ming Yin, Joleyn Brewer, Jason Karp Many previous efforts to modify patient monitors for remote or wearable use have suffered from high cost, poor performance, and low medical acceptance. A new technology approach is needed to enable these clinical benefits and to satisfy challenging economic, clinical, and user-acceptance criteria. Here, we present results on our initial efforts aimed at designing and building a prototype multi-wavelength arrayed photoplethysmograph (PPG) by using magnetically directed self-assembly (MDSA). We will discuss novel approaches in magnetic nanomaterial design, synthesis and deposition to enable MDSA based manufacturing. We will also demonstrate that multiple devices can be deposited through heterogeneous MDSA. The novel MDSA technology could make such PPG sensors a reality. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U21.00004: Low-power, fast, selective nanoparticle-based hydrogen sulfide gas sensor Allen Sussman, William Mickelson, A. Zettl We demonstrate a small, low-cost, low-power, highly sensitive, and selective nanomaterials-based gas sensor. A network of tungsten oxide nanoparticles is heated by an on-chip microhotplate while the conductance of the network is monitored. The device can be heated with short pulses, thereby drastically lowering the power consumption, without diminishing the sensor response. The sensor shows high sensitivity to hydrogen sulfide and does not have significant cross sensitivities to hydrogen, water, or methane, gases likely to be present in operation. A sensing mechanism is proposed, and its effect on electronic properties is discussed. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U21.00005: Nanocoax-based molecular imprint polymer for electrochemical biosensor Binod Rizal, Michelle Archibald, Laura Simko, Timothy Connolly, Stephen Shepard, Michael J. Burns, Thomas C. Chiles, Michael J. Naughton We have used molecular imprint polymerization (MIP) on planar, nanopillar, and nanocoax structures to fabricate label-free, all-electronic electrochemical biosensors with high selectivity and sensitivity. MIP-based films of $\sim$ 7 nm thickness are formed on gold-coated surfaces by electropolymerization of a solution containing phenol and a target protein (streptavidin, at 100 $\mu $g/ml, or 1 nanomole concentration) and subsequent removal of exposed target protein, leaving behind its molecular imprint. With its molecular memory, MIP subsequently specifically recognizes and binds target protein with attomolar sensitivity, detected via differential pulse voltammetry. We will discuss and compare the results of MIP for different proteins on planar, nanopillar, and nanocoax structures, along with their respective ultimate sensitivities. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U21.00006: High-Q Gold and Silicon Nitride Bilayer Nanostrings Tushar S. Biswas, Abdul Suhel, Bradley D. Hauer, Alberto Palomino, Kevin S.D. Beach, John P. Davis Nanostrings are attractive for sensing applications due to their small mass and ease of fabrication, yet very high quality factors ($Q$). We have fabricated and measured nanostrings from high stress silicon nitride resulting in high $Q$, and have discerned the dominant dissipation mechanism in our devices. This will provide a method to further improve our devices. In addition, to render our strings chemically sensitive, we decided to deposit a chemically functionalizable layer on the top of our strings. We have shown that the addition of a gold layer for this purpose does not adversely affect the $Q$ of the fundamental mode. As an added bonus, the differences in thermal expansion between different layers make the strings sensitive to temperature changes. This enables actuation of the strings' motion using an alternating current though the gold layer, via a thermoelastic mechanism, and provides integrated actuation that averts any external actuation scheme. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U21.00007: Real-Time Control of Biological Motor Activity using Graphene-polymer Hybrid Bioenergy Storage Device Dong Jun Lee, Kyung-Eun Byun, Dong Shin Choi, Eunji Kim, Daesan Kim, David Seo, Heejun Yang, Sunae Seo, Seunghun Hong Biological motors have been drawing an attention as a key component for highly efficient nanomechanical systems. For such applications, many researchers have tried to control the activity of motor proteins through various methods such as microfluidics or UV-active compounds. However, these methods have some limitations such as the incapability of controlling local biomotor activity and a slow response rate. Herein, we developed a graphene-polymer hybrid nanostructure-based bioenergy storage device which enables the real-time control of biomotor activity. In this strategy, graphene layers functionalized with amine groups were utilized as a transparent electrode supporting the motility of biomotors. And conducting polymer patterns doped with adenosine triphosphate (ATP) were electrically deposited on the graphene and utilized for the fast release of ATP by electrical stimuli through the graphene. Such controlled release of ATP allowed us to control the motility of actin filaments propelled by myosin biomotors in real time. This strategy should enable integrated nanodevices for the real-time control of biological motors to the nanodevices, which can be a significant stepping stone toward hybrid nanomechanical systems based on motor proteins. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U21.00008: Graphene for Biomedical Implants Thomas Moore, Ramakrishna Podila, Frank Alexis, Apparao Rao In this study, we used graphene, a one-atom thick sheet of carbon atoms, to modify the surfaces of existing implant materials to enhance both bio- and hemo-compatibility. This novel effort meets all functional criteria for a biomedical implant coating as it is chemically inert, atomically smooth and highly durable, with the potential for greatly enhancing the effectiveness of such implants. Specifically, graphene coatings on nitinol, a widely used implant and stent material, showed that graphene coated nitinol (Gr-NiTi) supports excellent smooth muscle and endothelial cell growth leading to better cell proliferation. We further determined that the serum albumin adsorption on Gr-NiTi is greater than that of fibrinogen, an important and well understood criterion for promoting a lower thrombosis rate. These hemo-and biocompatible properties and associated charge transfer mechanisms, along with high strength, chemical inertness and durability give graphene an edge over most antithrombogenic coatings for biomedical implants and devices. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U21.00009: Controlling the drug release rate from electrospun phospholipid polymer nanofibers with micro-patterned diamond-like carbon (DLC) coating Soki Yoshida, Terumitsu Hasebe, Tetsuya Suzuki, Atsushi Hotta An effective way of controlling drug release from polymer fibers coated with thin diamond-like carbon (DLC) film was introduced. It is highly expected that electrospinning will produce polymer fiber and useful for drug delivery systems. The drug release rate should be rather precisely controlled in order to prevent side effects due to the burst drug-release from polymers. Our previous research has already revealed that the micro-patterned DLC could control the drug release rate from biocompatible polymer films. In this study, the drug release profile of the polymer fibers with DLC was investigated. Hydrophilic 2-methacryloyloxyethyl phosphorylcholine (MPC) was selected as a typical biocompatible polymer. It is well known that the MPC polymers show good hemocompatibility and that both MPC and DLC are excellent biocompatible materials with antithrombogenicity. The DLC/MPC composites could therefore be extensively utilized for blood-contacting medical devices. The percentile covered area with patterned DLC on MPC fibers containing drug was varied from 0{\%} (without DLC) to 100{\%} (fully covered). It was found that the drug eluting profiles could be effectively controlled by changing the covered area of micro-patterned DLC coatings on MPC. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U21.00010: New Approaches to Targeted Drug Delivery James Cooper, William Oliver, Daniel Fologea For targeted drug delivery, one of the primary drawbacks lies with the inability to design a delivery system that can be loaded with a variety of drugs and biomolecules. Motivated by this challenge, we will present data showing 400 nm liposomes loaded via the novel method of lysenin pores. These pores are approximately 3 nm in diameter and can be closed with divalent and trivalent ions in addition to charged polymers. This new method allows for the controllable passage of large biomolecules such as DNA and protein without the inherent problems common to active and passive loading methods. We will show proof-of-concept results of this method using fluorescent calcein as a drug simulator. Furthermore, data demonstrating current attempts at loading DNA will also be presented. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U21.00011: Nonspecific targeting of iron oxide nanoparticles to the liver, kidney and spleen: A novel approach to achieving specificity Maheshika Palihawadana Arachchige, Amanda Flack, Xuequn Chen, Jing Li, David Oupicky, Y.-C. Norman Cheng, Yimin Shen, Bhanu Jena, Gavin Lawes Recently, there has been significant interest in developing Fe$_{3}$O$_{4}$ nanoparticles for biomedical applications including targeted~drug delivery and magnetic resonance imaging. One of the major problems in these applications is the undesirable filtration of these materials by the mononuclear phagocyte system. Preliminary magnetic resonance imaging and magnetization studies on hyaluronic acid coated nanoparticles injected intravenously into mice confirm that the nanoparticles accumulate in the liver, spleen, and kidneys. To identify whether certain specific proteins are responsible for nanoparticle accumulation in these organs, we exposed hyaluronic acid coated nanoparticles to proteins extracted from the liver, spleen, and kidneys, together with blood plasma proteins, then subsequently used gel electrophoresis and mass spectroscopy to identify the proteins binding to the nanoparticles. We find that the unwanted accumulation of nanoparticles in these organs can potentially be attributed to specific binding by a small number of proteins. By appropriately functionalizing the iron oxide nanoparticles, we expect that the nanoparticles uptake in the liver, spleen, and kidneys will be reduced. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U21.00012: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U21.00013: Interaction of Nucleobases with Semiconducting Nanotubes and Nanocages: Does the Solvent Matter? Zhoufei Wang, William Slough, Haiying He, Ravindra Pandey, Shashi Karna The tremendous advancement in nanotechnology has brought great promise in the area of bio-applications. Nanoscale materials and structures have attracted a lot of interest for their potential applications in biosensing, biorecognition, luminescent probes for DNA, biomedical labeling, drug delivery etc. Gaining fundamental understanding of the interaction of bio-systems with nanomaterials is critical in putting all these applications into full play. Despite the fact that most of these interactions appear in aqueous environment, the solvent effect has often been neglected in previous computational studies. In this talk, we will report our comparison study of nucleobases interacting with BN nanotubes and chalcogenide nanocages with/without considering the aqueous solution, based on first-principles calculations. The results reveal a significant effect from the water solution, which may largely reduce the interaction energy due to the polarization of the dielectric solvent medium. [Preview Abstract] |
Session U22: Optical Properties of Nanowires
Sponsoring Units: DCMPChair: Joe Tischler, Naval Research Laboratory
Room: 324
Thursday, March 21, 2013 11:15AM - 11:27AM |
U22.00001: Exciton Dynamics in Hexagonal InP Nanowires Masoud Kaveh-Baghbadorani, Wolfgang Langbein, Qiang Gao, Chennupati Jagadish, Hans-Peter Wagner We study the exciton dynamics in InP nanowire ensembles by intensity- and temperature-dependent photoluminescence (PL) measurements, time-correlated-single-photon-counting (TCSPC) and heterodyne detected four-wave-mixing experiments (HFWM). The InP nanowires were grown on fused silica substrate by 50 nm gold catalyst metal-organic-vapor-phase-epitaxy at a temperature of 450 $^{\circ}$C resulting in nearly wurtzite type nanowires. The PL measurements at 15 K show a strong emission band at 837 nm and two weak side bands at nearly 820 and 860 nm. The bands are tentatively attributed to trapped, free and zinc-blende related exciton transitions, respectively. With increasing temperature the free-exciton band gains importance relative to the dominating trapped exciton band while the low energy band vanishes. TCSPC measurements show an increasing PL decay rate of all emission bands with increasing temperature most pronounced for the low energy band. The result agrees with the exciton population dynamics obtained from three-beam HFWM measurements. Photon echo experiments at 80 K reveal an ultrafast exciton dephasing time of less than 100 fs which is attributed to scattering with a high carrier background in these nanowires. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U22.00002: Time Resolved Photoluminescence Studies of ZnO and Zn$_{2}$SnO$_{4}$ Nanowires for Solar Cells Applications Baichhabi Raj Yakami, Meg Mahat, Jiajun Chen, Liyou Lu, Wenyong Wang, Jon M. Pikal Sensitized nanowires (NWs) are a promising option for solar cells. They serve as the support structure for the absorbing centers, provide interfacial charge separation, and transport to the anode. Most work has focused on binary oxides, but ternary oxides have advantages due to flexibility in the properties of the oxide. Here we report on the photoluminescence (PL) and Time Resolved PL (TRPL) of Zinc oxide (ZnO) and Zinc Tin Oxide (ZTO) NWs grown by Chemical Vapor Deposition. The ZnO NWs show strong band gap emission and weak but resolvable defect emission peaks. The PL from the ZTO NWs does not show any band gap emission and absorption measurements confirm that these NWs have a direct forbidden transition. The ZTO NWs do have a board visible emission peak, which is usually attributed defects and oxygen vacancies. TRPL of the band gap emission in ZnO NWs yield a carrier lifetime of 1.4ns. The TRPL of the defect peaks in ZTO NWs are more complicated, showing a multi-exponential decay but with an overall decay rate similar to the ZnO NWs. This indicates that the expected increase in carrier lifetime in the ZTO NWs is not currently realized likely due to defect recombination, and additional optimization of the NW growth process may yield improved performance. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U22.00003: Picosecond carrier dynamics within the band structure of single InP nanowires with zincblende and wurtzite symmetries M. Montazeri, Y. Wang, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rice, T. Burgess, H.H. Tan, Q. Gao, C. Jagadish Low temperature transient Rayleigh scattering spectroscopy (TRS) is used to probe the carrier dynamics of single zincblende (ZB) and wurtzite (WZ) InP nanowires (NW). The NWs were MOCVD grown using 50 nm Au-nanoparticles. For ZB NWs, the TRS signal reveals various dynamical processes of the electrons within the conduction band as well as the holes in the degenerate heavy/light bands and the split-off band. The fundamental and the split-off band gaps are measured at 1.423eV and 1.529eV. For WZ NWs, we observe three excitonic resonances associated with the hole bands A at 1.501eV, B at 1.534eV and C at 1.66eV. We also observe clear transitions between the same A and B bands and the second conduction band, resulted from zone folding of the L-valley, which is measured at $\sim$ 230meV higher than the first. The lifetimes of the A, B and C excitons at $\sim$ 800ps, $\sim$ 400ps and $\sim$ 50ps respectively. In addition, a type II transition between electrons confined to small zincblende inclusions and holes confined to the wurtzite is identified which marks the ZB-WZ band-offset. We acknowledge the NSF (DMR-1105362, 1105121), ECCS-1100489 and the ARC. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U22.00004: Electronic and optical properties of InN nanowires from first principles Dylan Bayerl, Emmanouil Kioupakis Group-III-nitride nanowires are promising materials for photovoltaic and solid-state-lighting applications. We use first-principles calculations to investigate the electronic and optical properties of InN nanowires. Density functional theory provides the ground-state properties to which we subsequently apply quasiparticle corrections with the GW method. We thereby accurately predict the electronic band gaps, effective masses, and band dispersions of these nanostructured materials. We further solve the Bethe-Salpeter equation to predict the optical absorption spectra of InN nanowires as a function of cross-sectional dimension and geometry. We demonstrate that quantum confinement can increase the fundamental gap in InN nanowires as high as near-ultraviolet energies. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U22.00005: Stark Effect of Excitons in a Quantum Nano-rod with Parabolic Confinement S.K. Lyo We study the exciton binding energy and the oscillator strength as a function of a DC electric field in a quasi-one-dimensional quantum dot (\emph{i.e.}, nano rod) with parabolic confinements in the conduction and valence bands. The relative importance of the quantum confinement and electron-hole interaction is examined by varying the the linear confinement length (\emph{i.e.}, rod length). We find an abrupt decrease of the oscillator strength, loss of exciton binding energy, and a sudden increase of the root-mean-square average of electron-hole separation as the excitons are dissociated at the threshold field. The field dependence of the effects are also investigated as a function of the rod length and the radius of the nano rod. The numerical results are applied to GaAs and CdSe rods. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U22.00006: Observation of quantum dots in GaAs/AlGaAs core-multishell nanowire quantum well tubes Teng Shi, Howard Jackson, Leigh Smith, Jan Yarrison-Rice, Changlin Zheng, Peter Miller, Joanne Etheridge, Bryan Wong, Qiang Gao, Hark Tan, Chennupati Jagadish We use photoluminescence excitation (PLE) spectroscopy to study the electronic structure of GaAs/Al$_{x}$Ga$_{1-x}$As core-multishell nanowires (NW) which define 4 nm GaAs quantum well tubes (QWTs) embedded inside AlGaAs barriers wrapped around a central 50 nm GaAs core. HAADF-STEM images of NW cross-sections show a GaAs layer wrapped around the hexagonal facets with some tapering. Numerical calculations of this structure show the ground states are localized along the corners of the hexagonal QWT. Because of the strong quantum confinement, localized states can easily be formed through width or alloy concentration fluctuations. By using a hemispherical solid immersion lens, we are able directly observe such localized quantum dots (QDs) and map the emission of QDs with a spatial resolution of 600 nm in a single NW. Excitation and emission light polarized parallel and perpendicular to the NW long axis show multiple QDs along the NW long axis with $\sim$100 micro-eV emission lines. PLE measurements on single dots reveal excited state transitions between confined light or heavy holes to electrons at or above the AlGaAs conduction band barrier. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U22.00007: Photocurrent Spectroscopy of ZB and WZ InP Nanowire Ohmic devices K. Pemasiri, S. Perera, H.E. Jackson, L.M. Smith, J.M. Yarrison-Rise, S. Paiman, Q. Gao, H.H. Tan, C. Jagadish We use photocurrent spectroscopy to study InP nanowire Ohmic devices having either Zincblende (ZB) or wurtzite (WZ) crystal structures at 10K. Photolithography is used to fabricate Ohmic Ti/Al contact pads separated by 5 $\mu$m. Using a tunable Ti-Sapphire laser, photocurrent is measured as a function of bias voltage and excitation energy. At low temperatures (10 K), the ZB device shows strong evidence for excitonic resonance peaks at 1.425eV and 1.539eV relevant to the degenerate heavy and light holes band and the split-off band. The WZ device shows three excitonic peaks at 1.504eV, 1.530eV, and 1.655eV corresponding to the A, B and C valence band energies, respectively. These energies coincide with recent photoluminescence excitation measurements. In some WZ InP nanowire devices, the A, B and C peaks have been observed at 20-30meV higher energies compared to above, suggesting a possible quantum confinement in the nanowires. The polarization dependence of photocurrent spectra measured from 275nm ZB nanowire and 20nm WZ nanowire shows very good agreement with theoretical absorption of light by nanowires as a function of diameter and photon energy. We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489, and the ARC. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U22.00008: Photocurrent spectroscopy of GaAs/GaP hetero-structured nanowires P. Kumar, H.E. Jackson, L.M. Smith, J. Yarrison Rice, J.H. Kang, Q. Gao, H.H. Tan, C. Jagadish We study the photocurrent from photoexcited charge carriers in GaAs/GaP axial and radial hetero-structured nanowires (NWs). These NWs are grown using Metal-Organic Chemical Vapor Deposition (MOCVD) in [111]B direction with Au nano-particles as catalysts. As grown axial GaAs/GaP NWs are sonicated in methanol and dispersed on Si-SiO insulated substrate. Photolithography followed by Ti/Al (20nm/300nm) metal evaporation and lift-off is used to fabricate contacts in Metal-semiconductor-metal across single NW. Spatial imaging of photocurrent at different wavelengths distinguishes the GaP and GaAs regions in these NWs. Peak photocurrent is observed around GaP region for light wavelengths $\sim$ 458nm whereas peak photocurrent is shift towards GaAs region for light wavelength $\sim$ 800nm. Photocurrent measurements in GaAs/GaP strained core-shell NWs are in progress. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U22.00009: Photocurrent spectroscopy of single ZB GaAs and GaAs/AlGaAs core-shell nanowires Bekele Badada, Leigh Smith, Howard Jackson, Jan Yarrison-Rice, Tim Burgess, Chennupati Jagadish We investigate the band structure of single ZB GaAs nanowires and GaAs/Al$_{0.5}$Ga$_{0.5}$As core shell nanowires using photocurrent spectroscopy at room and low temperatures. The single nanowire devices were fabricated photolithographically to define Ti (20nm)/Al (300nm) metal contacts on either end the nanowire. Photocurrent measurements were performed using CW excitation from a tunable CW Ti-Sapphire laser (775nm-890nm) and a broadly tunable (550-960 nm) pulsed excitation from a coherent super continuum photonic crystal fiber. At room temperature we observe an Urbach tail near the absorption edge at 1.42 eV for both GaAs and GaAs/ Al$_{0.5}$Ga$_{0.5}$As core-shell nanowires. In the core shell structure, we also observe the exponential tail from the Al$_{0.5}$Ga$_{0.5}$As superimposed on the GaAs absorption in the core. The 2eV onset is consisant with 50{\%}. At low temperature, 10K, similar measurements were performed and a peak is observed near the band edge $\sim$ 1.50-1.51 eV for both bare and core-shell structure for GaAs reflecting the contribution of excitons to the photocurrent. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U22.00010: Strain-induced piezoelectric field effects on the optical properties of ZnO nanowires Wenhao Guo, Shuigang Xu, Ning Wang, M.M.T. Loy, Shengwang Du In our work, we report the evidence of piezoelectric effects which modifies the spatial distribution of the photo-generated carriers in bent ZnO nanowires. This piezoelectric effect, together with strain-induced changes of the energy band structure, results in redshift of free exction photo-luminescence emission in strained ZnO nanowires. The net redshift is only dependent on the strain, independent on the diameter of the nanowire unless the depth of depletion layer is comparable to the size of nanowire. The experimental results obtained by the near-field scanning microscopy agree well with our numerical simulation. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U22.00011: Tip-enhanced Raman scattering of an InGaN/GaN quantum well on a single GaN nanorod Emanuele Poliani, Markus Wagner, Axel Hoffmann, Janina Maultzsch, Juan Sebastian Reparaz, Martin Mandl, Werner Bergbauer, Martin Strassburg Vertical GaN nanorods with double In0.2Ga0.8N/GaN quantum well were studied by tip-enhanced Raman spectroscopy (TERS). Exploiting the spatial resolution below the diffraction limit, we were able to perform a Raman map of the nanorod top part with 35 nm spatial resolution. Undetectable in the far field, enhanced phonons belonging to InGaN, InN rich regions and GaN were detected and analyzed as Raman shift map. These enhanced spatially resolved phonons revealed an Indium cluster region nucleated at the end of a planar dislocation in the GaN core. The dislocation continues inside the cluster area as an interface between zinclende and wurtzite modification. A narrow localized strain zone was found close to the interface on the zinc blende side surrounding material. On the wurtzite side instead, the Raman map of the GaN surface optical phonon revealed a more extended charge depletion region. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U22.00012: Raman Spectroscopy on GaAs/GaP Nanowire Axial Heterostructures Yuda Wang, Mohammad Montazari, Leigh Smith, Howard Jackson, Jan Yarrison-Rice, Qiang Gao, Jung-Hyun Kang, Chennupati Jagadish We use Raman scattering to study the spatially-resolved strain and stress in Zinc Blende GaAs/GaP axial heterostructure nanowires at room temperature. The nanowires are grown by Metal-Organic Chemical Vapor Deposition in the [111] direction with Au nano particles as catalysts. After initial growth of a 6 $\mu $m-long GaP wire, a short GaAs segment is grown. Since Raman scattering reflects the phonon energies that are in turn related to the stress, we control the polarization of the incident and scattered light to acquire and resolve the TO1 (Transverse Optical) and TO2 phonon modes of both GaAs and GaP. High spatial resolution Raman scans along the nanowires show that the GaAs/GaP interface is clearly identifiable. Within the GaP section of the wire, GaP TO modes are observed at lower energies compared to bulk GaP since it is under tension, while GaAs shell TO modes are at higher energies than bulk GaAs since it is under compression. A strain gradient exists across the interface so the GaP phonon energies shift to lower and GaAs phonon shift to higher energies as one approaches the interface. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U22.00013: Anisotropic surface plasma resonance in self-assembled ErSb quantum nanostructures of tunable shape and orientation Daniel Ouellette, Hong Lu, Sascha Preu, Justin Watts, Ben Zaks, Mark Sherwin, Arthur Gossard Incorporation of erbium during MBE growth of GaSb leads to various self-assembled, semi-metallic ErSb nanostructures. At the lowest concentration, spheres of diameter 4-5 nm are observed. By contrast, at 7-10$\%$ Er, $\sim$5 nm diameter nanowires self-align along the $<001>$ growth direction, and at 15-20$\%$, the nanowires align in the growth plane along the $<\overline{1}10>$ direction. Light polarized along the wires is strongly attenuated over a broad range from THz to near-IR. By contrast, light polarized perpendicular to the wires experiences minimal attenuation apart from a very strong surface plasma resonance at 0.46 eV. Surprisingly, the resonant frequency of the nanospheres is slightly higher than that of the wires, despite the smaller depolarization factor. Motivated by this observation and estimates of the confinement energy, we construct an effective medium theory for the nanostructures which includes a single characteristic intersubband transition. This model provides an excellent description of the IR reflectance and transmittance over the whole range of Er concentration, in contrast to a model which excludes the effect of quantum confinement. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U22.00014: Continuous frequency multiplication in a strongly driven modulated nanowire Kathleen Hamilton, Alexey Kovalev, Amrit De, Leonid Pryadko High-order harmonic generation in a bulk solid strongly driven by a few-cycle pulsed infrared laser has recently been observed [1]. We consider the possibility of observing an analogous effect using a continuously driven, single-band one-dimensional metal. In the absence of phonon scattering, the quantum efficiency of frequency tripling for such a system can be as high as $93\%$. Combining the Floquet quasi-energy spectrum with the Keldysh Green's function technique, we derive the quantum transport equation for strongly and rapidly driven electrons in the presence of weak scattering by phonons. The power absorbed from the driving field is continuously dissipated by phonon modes, leading to a quasi-equilibrium in the electron distribution. We assume terahertz frequency range, and use the Kronig-Penny model with varying effective mass to establish dimensions and modulation periodicity of an InAs/InP nanowhisker. Driving such nanowhiskers could lead to efficient third and higher-harmonic generation. [1] S. Ghimire et al., Nature Physics 7, 138-141 (2011). [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U22.00015: Generation of core-shell structures and segregation of dopants in Si/SiO$_2$ nanowires Sunghyun Kim, Ji-Sang Park, K.J. Chang Oxidized Si nanowires (SiNWs) are usually synthesized by subsequent thermal annealing of as-grown SiNWs. It has been observed that B diffusivity is enhanced during thermal annealing in SiNWs, similar to the phenomena called transient enhanced diffusion or oxidation enhanced diffusion in planar Si/SiO$_2$ interfaces. However, previous theoretical studies have been focused on hydrogen or hydroxyl terminated SiNWs. In this work, we generate realistic atomic models for oxidized SiNWs in which crystalline Si core is sheathed by amorphous SiO$_2$ by using a combined approach of classical molecular dynamics simulations with first-principles density functional calculations. For realistic core-shell structures, we investigate the stability and segregation behavior of B and P dopants. A single substitutional B is more stable in the Si core, with a very small energy variation with the radial position of B. On the other hand, B dopants easily segregate to the oxide shell with the aid of Si self-interstitials generated during thermal oxidation. In contrast to B dopants, P dopants prefer to reside in the Si core even in the presence of Si self-interstitials but tend to aggregate in the Si region near the interface, forming nearest-neighbor donor pairs which are electrically inactive. [Preview Abstract] |
Session U23: Semiconductors: Theory and Spectra I
Sponsoring Units: FIAPRoom: 325
Thursday, March 21, 2013 11:15AM - 11:27AM |
U23.00001: Atomic Multiplets in X-ray Spectroscopies of Solids Bernard Delley, Anne-Christine Uldry The electronic structures of compounds involving open d- and f- shell are studied frequently by X-ray and electron spectroscopies. For a better understanding of the multiplets arising in spectra involving one or more open shells, we have developed recently an easy to use program multiX,\footnote{Systematic computation of crystal field multiplets for x-ray core spectroscopies, A. Uldry, F. Vernay and B. Delley, Phys. Rev. B 85, 125133 (2012).} which is available to download.\footnote{http://people.web.psi.ch/uldry/multiplets/} This first step allows the inclusion of the crystal environment as a crystal field entered simply as positions and charges of a cluster of atoms around the core hole site. This often gives valuable insights in the case of x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray spectroscopy (RIXS) measurements. However, in many cases it is desirable to allow for hybridization of the open shell electrons with the orbitals of neighbor atoms. This requires dealing with a significantly larger active Hilbert space. This is addressed with our recent Lanczos-based procedure to calculate spectra. First results will be discussed. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U23.00002: The Physical Content of Eigenvalues from Density Functional Theory (DFT) D. Bagayoko, L. Franklin, C. Ekuma, Y. Malozovsky The density functional theory (DFT) of Hohenberg and Kohn rests on the energy functional E$_{\mathrm{v}}$[n] assuming its minimum for the \textit{correct density} n(\textbf{r}), with the admissible functions restricted by the condition$N\left[ n \right]=\int {n\left( r \right)} dr=N$, where N is the number of particles in the system under study. We show that, for such a system, there is an infinite number of basis sets (of localized orbitals) for which N is fixed while the density is not necessarily the correct one. Consequently, the eigenvalues obtained with self consistent DFT calculations using a single basis set do not necessarily have any particular physical content. The physical content is ensured only by the search and utilization of \textit{the optimal basis set} that yields \textit{the minima of the occupied energies} and physically meaningful values of low laying unoccupied energies. Further, by virtue of the Rayleigh theorem, there exist many basis sets larger than the optimal one [and that contain it] for which some unoccupied energies are lowered on account of a mathematical artifact. We illustrate these points in the cases of ZnO, TiO$_{2}$, and SrTiO$_{3}$. The calculated band gaps and other properties of these materials are in excellent agreement with experiment. Work funded by in part by the National Science Foundation, through LASiGMA [NSF AwardEPS-1003897, No. NSF (2010-15)-RII-SUBR, and No. HRD-1002541], LONI [Award No. 2-10915], and the Louisiana Space Consortium (LaSPACE). [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U23.00003: Beyond the \textit{GW} approximation: a second-order screened exchange correction Patrick Rinke, Fabio Caruso, Xinguo Ren, Matthias Scheffler, Noa Marom Despite the success of the $GW$ method in describing the photoemission spectra of solids, molecules and clusters, challenges remain. For aromatic molecules for example absolute as well as relative positions of ionisation energies and affinities are not well reproduced in perturbative $G_0W_0$ schemes with different starting points as well as in self-consistent $GW$ [1], sometimes even giving the wrong orbital order. Motivated by renormalized second-order perturbation theory [2] for the ground-state energy, we propose a second-order screened exchange correction (SOSEX) to the $GW$ self-energy. This correction follows the spirit of the SOSEX correction to the random-phase approximation for the electron correlation energy and reduces the self-correlation error. The performance of the $GW$+SOSEX scheme has been benchmarked for a set of molecular systems, including the G2 set, commonly used acceptor molecules, benzene and the azabenzene molecules. We find that the SOSEX correction improves the description of the spectral properties including the orbital order with respect to the different $GW$ schemes, highlighting the importance of reducing the self-correlation error.\\[4pt] [1] N. Marom {\it et al.}, arXiv:1211.0416\\[0pt] [2] X. Ren {\it et al.}, J. Mater. Sci. \textbf{47}, 7447 (2012) [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U23.00004: GW calculations of the bandgap of pyrite under various conditions Brian Kolb, Alexie Kolpak Iron pyrite holds great promise as a solar cell material because of it's near optimal bandgap (0.95 eV) and its high optical absorbance. Nevertheless, real solar cells made from this material suffer from poor performance. In particular, the low open circuit voltage of around 200 meV precludes pyrite's use in effective solar cell devices. Several theories have been proposed to explain this low open-circuit voltage including bulk defects, intrinsic surface states within the gap, and surface defects. Careful DFT calculations have shown that bulk defects are exceedingly rare. Further, the calculations do not exhibit intrinsic surface states within the gap. Researchers disagree about the effect of surface defects, particularly sulfur deficiencies, on the bandgap. This work combines DFT with GW calculations of the bandgap to address some of the most fundamental and important questions about the cause of the low open-circuit voltage of pyrite solar cells including the true role of surface defects, the nature of the interface with metal electrodes, and the effect of phonons on the bandgap. This investigation is undertaken with an eye toward engineering a pyrite-based material that can perform well in real solar cell applications. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U23.00005: $GW$ at the interface: CH$_3$OH and H$_2$O on TiO$_2$(110) Duncan Mowbray, Annapaola Migani, Amilcare Iacomino, Jin Zhao, Hrvoje Petek, Angel Rubio Electronic level alignment at the interface between an adsorbed molecular layer and a semiconducting substrate determines the activity and efficiency of many photocatalytic and photovoltaic materials. However, a quantitative description of the states at the interface remains elusive, due to the computational complexity of quasiparticle $GW$ based algorithms. We compare density functional theory (DFT) calculations and quasiparticle techniques with ultraviolet photoelectron spectra and two photon photoemission spectra to determine the level of theory required to obtain an accurate description of occupied and unoccupied states at the interface. Specifically, we consider GGA DFT, hybrid DFT and $G_0W_0$, $scGW1$, $scGW_0$, and $scGW$ quasiparticle calculations for the interface between rutile TiO$_2$(110) and methanol or water. We find the quasiparticle energy shifts $\Delta$ are linearly dependent on the fraction of the wave function density within the molecular layer $f_{mol}$ and the bulk substrate $f_{bulk}$. For the unoccupied states, the same correlation holds for all the molecular layers studied. This allows one to describe the quasiparticle energy shifts semi-quantitatively for larger molecular layers on TiO$_2$(110) based on more tractable DFT calculations. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U23.00006: \textit{Ab-initio} Calculations of Electronic Properties of InP and GaP Yuriy Malozovsky, Lashounda Franklin, Chinedu Ekuma, Guang-Lin Zhao, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of zinc blende indium and gallium phosphides (InP {\&} GaP) We employed a local density approximation (LDA) potential and implemented the linear combination of atomic orbitals (LCAO) formalism. This implementation followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). This method searches for the optimal basis set that yields the minima of the occupied energies. This search entails methodically increasing the size of the basis set, up to the optimal one, and the accompanying enrichment of angular symmetry and of radial orbitals. Our calculated, direct band gap of 1.398 eV (1.40 eV) for InP, at the $\Gamma $ point, is in excellent agreement with experimental values. We discuss our preliminary results for the indirect band gap, from $\Gamma $ to X, of GaP. We also report calculated electron and hole effective masses for both InP and GaP and the total (DOS) and partial (pDOS) densities of states. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U23.00007: Computational study of the Effect of Sulfur Passivation on GaAs Heterojunction Solar Cells Ted Yu, Ramesh Laghuvamarapu, Liang Yan, Wei You, Diana Huffaker, Christian Ratsch We report DFT calculations that study the effect of sulfur passivation ((NH$_{4})_{2}$S and octanethiol) on GaAs surfaces. Sulfur passivation of GaAs solar cells is an area of interest, as it improves the I-V characteristics of heterojunctions by decreasing the density of surface states. We elucidate the fundamental mechanism of sulfur passivation on GaAs by showing how the sulfur species react with different reconstructed GaAs (100) and (111B) surfaces. Using state of the art hybrid functionals to calculate band structures and density of states, we find that a reconstructed GaAs surface does not have mid-gap surface states. Therefore, we show that sulfur passivation does not reduce surface states on reconstructed surfaces. We also study arsenic vacancies and adatoms on these surfaces to determine the energies of creating these imperfections. They lead to mid-gap surface states that are shown to be energetically plausible in certain GaAs surface reconstruction. We study the most energetically favorable surface reconstructions with As vacancies and show how sulfur passivation plays a role in removing surface states. These results will guide in the selection of passivating agents for GaAs solar cells and lead to a better understanding of such systems. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U23.00008: CsSnX3 (X= Cl, Br, I) band structure calculations by the QSGW method Ling-yi Huang, Walter R.L. Lambrecht CsSnX$_3$ (X=Cl,Br,I) perovskite compounds are of interest because of their strong photoluminescence and their potential application to solar cells. We present quasiparticle self-consistent GW (QSGW) calculations for the cubic ($\alpha$-phase) including spin-orbit coupling and study the changes in band structures from the $\alpha$-phase to the $\beta$- and $\gamma$-phases in LDA. The QSGW gaps are in good agreement with experiment. An analysis of the orbital character of the bands shows that they have an ``inverted'' band structure: the VBM has a non-degenerate s-like character (Sn-s and X-p antibonding), while the (CBM) has Sn-p character. The strongly intra-atomic dipole allowed nature of the direct gap explains the high photoluminescent intensity. The low hole mass indicates high hole mobility in agreement with experiment. The pressure dependence of the gap is found to be anomalous: the band gap decreases when the lattice constant is decreased. Effective masses and the Kohn-Luttinger type Hamiltonian of the CBM are extracted from the band structures and subsequently used to estimate exciton binding energies using our calculated dielectric constants. These indicate a much lower exciton binding energy for CsSnI3 than recently proposed. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U23.00009: Quasiparticle band structures and interface physics of SnS and GeS Brad Malone, Efthimios Kaxiras Orthorhombic SnS and GeS are layered materials made of earth-abundant elements which have the potential to play a useful role in the massive scale up of renewable power necessary by 2050 to avoid unmanageable levels of climate change. We report on first principles calculations of the quasiparticle spectra of these two materials, predicting the type and magnitude of the fundamental band gap, a quantity which shows a strong degree of scatter in the experimental literature. Additionally, in order to evaluate the possible role of GeS as an electron-blocking layer in a SnS-based photovoltaic device, we investigate the band offsets of the interfaces between these materials along the three principle crystallographic directions. We find that while the valence-band offsets are similar along the three principle directions, the conduction-band offsets display a substantial amount of anisotropy. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U23.00010: Ab Initio Study of Quasiparticle and Excitonic Properties of MoS2 Diana Qiu, Felipe Jornada, Steven Louie MoS2 is a layered, transition-metal dichalcogenide that can be cleaved into single-layer sheets, in a manner similar to graphene. Monolayer MoS2 has a direct band gap, strong spin-orbit coupling and strongly enhanced photoluminescence, compared with the bulk. MoS2's interesting electronic and optical properties mean that it could have many applications in single-layer electronic devices, but on the theoretical level, when many-electron interaction effects are included, there is still some uncertainty about the quasiparticle and excitonic properties of MoS2. We use first-principles calculations to study the quasiparticle band structure and optical absorption spectrum of MoS2 at the GW$+$BSE level. We include spin-orbit coupling as a perturbation either before or after the GW calculation of the band structure, and we demonstrate that our calculations are fully converged with respect to the dielectric cutoff and summation over empty bands. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U23.00011: Strain Modulation on electric-optical properties of Graphene and ZnO micro/nanowires Xuewen Fu, Zhimin Liao, Hanchun Wu, Yang-Bo Zhou, Jun Xu, Wanlin Guo, Dapeng Yu Strain increasingly prevails in micro- and nano-structures, and has important influence on the crystal and electronic structures. But its role in these structures remains unclear. The strain dependence of conductance of monolayer graphene has been studied here. The results illustrate the notable transitions: the slight increase, the dramatic decrease, and the sudden dropping of the conductance by gradually increasing the uniaxial strain. The graphene conductance behaves reversibly by tuning of the elastic tensile strain up to 4.5\%, while it fails to recover after the plastic deformation at 5\%. We also investigated the bending strain effect on the photoresponse of ZnO micro/nanowires and found larger photoconductivity and faster rising speed when photo-excitation is localized at the bending region in atmospheric environment, while the rising speeds are almost the same when photo-excitations are localized at the bending and straight regions under vacuum. The bending strain induced improvement of the UV photoresponse in air was well explained by the coupling of piezoelectric effect and surface oxygen adsorption/desorption procedure on the bent ZnO microwire.\\[0pt] [1] Xue-Wen Fu, Zhi-Min Liao, Jian-Xin Zhou, Yang-Bo Zhou, Han-Chun Wu, Appl. Phys. Lett. 99, 213107 (2011). [2] Xue-Wen Fu, Zhi-Min Liao, Jun Xu, Xiao-Song Wu, Wanlin Guo and Da-Peng Yu, Nanoscale, 2013, 5, 916–920. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U23.00012: Electronic Structure of N-doped TiO James Lewis, Barry Haycock, Gary Lander, Binay Prasai, David Drabold Via \textit{ab-initio}density functional theory calculations, we present evidence of the most energetically stable atomic configuration for nitrogen-doped amorphous TiOand analysis of the electronic structure. This material receives much attention in the literature due to it's proposed photocatalytic applications, however synthesis of the crystalline form is an unfavorable process. Nitrogen doping has previously been shown to enable absorption in the visible in crystalline TiO2. As compared to crystalline TiO2, thin films of a- TiOdo not need thermal treatment and have other production advantages such as less dependence on substrate materials. With control of the electronic structure of the amorphous phase via doping the electronic characteristics can be taken advantage of without the costly production of crystalline TiO. N-doping of the amorphous phase introduces tail states to the valance band. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U23.00013: Majorana fermions in vortex lattices Rudro Biswas We consider Majorana fermions tunneling between vortices, within an array of such vortices in a 2D chiral p-wave superconductor. We calculate that the tunneling amplitude for Majorana fermions in a pair of vortices is proportional to the sine of half the difference between the global order parameter phases at the two vortices. Using this result we study tight-binding models of Majorana fermions in vortices arranged in a triangular or square lattice. In both cases we find that this phase-tunneling relationship leads to the creation of superlattices where the Majorana fermions form macroscopically degenerate `flat' bands at zero energy, in addition to other dispersive bands. This finding suggests that in vortex arrays tunneling processes do not change the energies of a finite fraction of Majorana fermions and hence brighten the prospects of topological quantum computing with a large number of Majorana states. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U23.00014: Ultrafast intersystem crossing in nickel porphyrins Javier Fernandez-Rodriguez, Jun Chang, A.J. Fedro, Michel van Veenendaal We study the relaxation dynamics to the metastable state in laser-pumped nickel porphyrins. We use a ligand-field model which takes into account the crystal field created by the porphyrin ring and axial ligands. We propose different decay pathways in terms of charge-transfer and metal-center intermediate states By accounting for the energy redistribution of the lattice vibrations we get an irreversible decay to the metastable state within the order of a few hundred femtoseconds. We show how non-equilibrium time-dependent x-ray absorption at the Ni K-edge measurements can elucidate the nature of the intermediate states involved in the decay. Understanding radiationless transitions in transition-metal complexes is of interest for their relevance for the design of photocatalytic systems and photothermal sensitizers for cancer treatment. [Preview Abstract] |
Session U24: Focus Session: Recent Developments in Density Functional Theory III
Sponsoring Units: DCOMPChair: Pieremanuele Canepa, Wake Forest University
Room: 326
Thursday, March 21, 2013 11:15AM - 11:27AM |
U24.00001: Recovering hidden Bloch character: Unfolding Electrons, Phonons, and Slabs Philip B. Allen, Tom Berlijn, David Casavant, Jose Soler One of the main problems of first principles supercell calculations is the band folding problem. As the supercell gets larger, the bands get folded into a smaller Brillouin zone and cease to give information about the Bloch character of the underlying normal cell. To tackle this problem an unfolding formalism has been implemented in first principles calculations via several techniques [1-5]. Here we will present an extended unfolding formalism for finite systems and exemplify it with first principles calculations of a Si (111) slab.\\[4pt] [1] S. Baroni et al, PRL 65, 84 (1990)\\[0pt] [2] F. Giustino et al, PRL 98, 047005 (2007)\\[0pt] [3] W. Ku et al, PRL 104, 216401 (2010)\\[0pt] [4] V. Popescu et al, PRL 104, 236403 (2010)\\[0pt] [5] M. W. Haverkort, arXiv:1109.4036 [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U24.00002: Exponential supercell convergence of the exact exchange energy via truncated coulomb potentials Ravishankar Sundararaman, T. A. Arias Hybrid density functionals have become increasingly popular as a solution to mitigate the self-interaction error in semi-local density functionals, but widespread application to periodic systems has been limited by computational cost. This cost is exacerbated by poor $k$-point convergence due to the $G\to0$ singularity in the exact exchange energy, in spite of several singularity correction methods such as auxilliary function integration,\footnote{P. Carrier, S. Rohra and A. G\"orling, {\it Phys. Rev. B} {\bf 75}, 205126 (2007)}$^{,}$\footnote{I. Duchemin and F. Gygi, {\it Comp. Phys. Comm} {\bf 181}, 855 (2010)} image subtraction,\footnote{J. Paier et al., {\it J. Chem. Phys.} {\bf 122}, 234102 (2005)} and spherical truncation of the coulomb potential.\footnote{J. Spencer and A. Alavi, {\it Phys. Rev. B} {\bf 77}, 193110 (2008)} We analyze these rather disparate methods in an intuitive formalism based on Wannier function localization, which naturally suggests the truncation of the Coulomb potential on the superlattice Wigner-Seitz cell. We demonstrate that this scheme systematically exhibits the best $k$-point convergence, comparable to that of semi-local functionals, even for low-symmetry and reduced-periodicity systems where previous methods fail. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U24.00003: Large-scale DFT calculations with the ONETEP program on metallic systems Alvaro Ruiz-Serrano, Chris-Kriton Skylaris We present a direct energy minimization method based on the Kohn-Sham formulation of Mermin's extension of density functional theory (DFT) to finite electronic temperature for large-scale calculations on metallic systems. Our approach employs norm-conserving pseudopotentials for the core electrons, whereas the valence electrons are accurately described using a set of localized orbitals, optimized in-situ in terms of a high-resolution periodic-sinc (psinc) basis set equivalent to plane-waves. The localization constraint results in predictable sparsity patterns that simplify the algebraic operations with matrices, while the description in terms of psinc functions allows near-minimal matrix sizes. As a consequence, the traditional computational bottleneck due to diagonalization of the Hamiltonian matrix is greatly reduced, allowing calculations on larger systems. Additionally, we take advantage of available parallel eigensolvers to enhance the efficiency of the method. We present a number of validation results on metallic systems of increasing complexity and size, including calculations on nanoparticles of more than a thousand atoms. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U24.00004: On the Generalization of Homogeneous Coordinate Scaling in Density Functional Theory Lazaro Calderin The scaling properties of functionals find direct applications in the design, testing and use of approximated kinetic-energy and exchange-correlation functionals. Methods, such as the so called orbital-free, benefit from approximations to both functionals, while Kohn-Sham Density Functional Theory approximates only the exchange-correlation functional. In this talk we will introduce a generalization of the uniform scaling of coordinates, that not only embodies all previously known scaling and related results, but also leads to new and important properties of functionals. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U24.00005: Non-linear eigensolver-based alternative to traditional SCF methods Brendan Gavin, Eric Polizzi The self-consistent iterative procedure in Density Functional Theory calculations is revisited using a new, highly efficient and robust algorithm for solving the non-linear eigenvector problem (i.e. H({X})X = EX;) of the Kohn-Sham equations. This new scheme is derived from a generalization of the FEAST eigenvalue algorithm, and provides a fundamental and practical numerical solution for addressing the non-linearity of the Hamiltonian with the occupied eigenvectors. In contrast to SCF techniques, the traditional outer iterations are replaced by subspace iterations that are intrinsic to the FEAST algorithm, while the non-linearity is handled at the level of a projected reduced system which is orders of magnitude smaller than the original one. Using a series of numerical examples, it will be shown that our approach can outperform the traditional SCF mixing techniques such as Pulay-DIIS by providing a high converge rate and by converging to the correct solution regardless of the choice of the initial guess. We also discuss a practical implementation of the technique that can be achieved effectively using the FEAST solver package. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U24.00006: The pole expansion and selected inversion technique for solving Kohn-Sham density functional theory at large scale Lin Lin, Mohan Chen, Weinan E, Lixin He, Jianfeng Lu, Chao Yang, Lexing Ying The standard diagonalization based method for solving Kohn-Sham density functional theory (KSDFT) requires N eigenvectors for an O(N) * O(N) Kohn-Sham Hamiltonian matrix, with N being the number of electrons in the system. The computational cost for such procedure is expensive and scales as O(N$^3$). We have developed a novel pole expansion plus selected inversion (PEXSI) method, in which KSDFT is solved by evaluating the selected elements of the inverse of a series of sparse symmetric matrices, and the overall algorithm scales at most O(N$^2$) for all materials including metallic and insulating systems without any truncation. The PEXSI method can be used with orthogonal or nonorthogonal basis set, and the electron density, total energy, Helmholtz free energy and atomic force are calculated simultaneously and accurately without using the eigenvalues and eigenvectors. Combined with atomic orbital basis functions, the PEXSI method can be applied to study the electronic structure of boron nitride nanotube and carbon nanotube with more than 10,000 atoms on a single processor. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U24.00007: Density Functional Theory of Thermoelectric Phenomena Giovanni Vignale, Florian Eich, Massimiliano Di Ventra We introduce a non-equilibrium density functional theory of local temperatures and associated heat currents that is particularly suited for the study of thermoelectric phenomena. This theory rests on a local temperature field coupled to the energy density operator. We prove the basic theorems of the theory and discuss the construction of approximate functionals. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U24.00008: Simulated non-contact atomic force microscopy based on real space pseudopotentials and density functional theory Minjung Kim, James Chelikowsky Non-contact atomic force microscopy (nc-AFM) is a commonly used technique in surface and nano science owing to its high-resolution and ease of implementation. Theoretical simulations of nc-AFM have been able to facilitate the interpretation of experimental images. However, first-principles AFM simulations can be computationally intensive and problematic if the morphology of the AFM tip is unknown. We introduce an efficient simulation method that does not include an explicit morphology for the tip as suggested by Chan and coworkers.\footnote{T. -L. Chan, C. Z. Wang, K. M. Ho, and James R. Chelikowsky, \textit{Phys. Rev. Lett.} \textbf{102}, 176101 (2009)} Our method is based on a real space implementation of pseudopotentials constructed using density functional theory. We illustrate the method by simulating nc-AFM images for binary semiconducting materials, {\it e.g.}, the GaAs(110) surface, and compare our results to previously performed first principles simulations as well as experimental data. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U24.00009: Dynamical Steps in the Time-Dependent Exchange-Correlation Potential Kai Luo, Neepa Maitra, Peter Elliott, Johanna Fuks, Angel Rubio It was recently demonstrated that the exact correlation potential of time-dependent density functional theory (TDDFT) generically develops step and peak features that have a density-dependence that is non-local in space and time [arXiv:589981]. Usual adiabatic functional approximations fail to capture these steps, yet these same functionals work quite well for excitation spectra. We investigate the role of the steps in the linear response regime. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U24.00010: Time-Dependent Spin-Density Functional Theory for strongly correlated systems Volodymyr Turkowski, Talat S. Rahman We present a formulation of the basic principles for a time-dependent spin-density functional theory (TDSDFT) capable of describing the main properties of strongly correlated systems. Electron-electron correlations are contained in the correlation part of the exchange-correlation (XC) kernel, which we construct using some exact results for the Hubbard model of strongly correlated electrons. The principal feature of the theory is nonadiabaticity of the XC kernel, which corresponds to a local time-resolved (oscillating in time) electron-electron interaction. As in dynamical mean-field theory, in TDSDFT such interaction defines the main properties of correlated systems, including satellite Hubbard peaks in the electronic spectrum. We demonstrate that the corresponding nonadiabatic XC kernel reproduces main features of the spectrum of the Hubbard dimer and infinite-dimensional Hubbard model, some of which are impossible to obtain within the adiabatic approach. We test the theory by applying it to several strongly correlated materials, including calculation of nonequilibrium response of these systems. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U24.00011: Time-dependent density functional theory of extreme environments Rudolph Magyar, Luke Shulenburger, Michael Desjarlais We describe the challenges involved when using time-dependent density functional theory (TDDFT) to describe warm dense matter (WDM) within a plane-wave, real-time formulation. WDM occurs under conditions of temperature and pressure (over 1000 K and 1 Mbar) where plasma physics meets condensed matter physics. TDDFT is especially important in this regime as it can describe ions and electrons strongly out of equilibrium. Several theoretical challenges must be overcome including assignment of initial state orbitals, choice of time-propogation scheme, treatment of PAW potentials, and inclusion of non-adiabatic effects in the potential energy surfaces. The results of these simulations are critical in several applications. For example, we will explain how the TDDFT calculation can resolve modeling inconsistencies in X-ray Thompson cross-sections, thereby improving an important temperature diagnostic in experiments. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U24.00012: Direct calculation of exciton binding energies with time-dependent density-functional theory Zenghui Yang, Carsten Ullrich Excitons are coupled electron-hole pairs below the band gap in bulk semiconductors. They are vital to photovoltaics, but they are hard to obtain in a TDDFT calculation, due to usually employed exchange-correlation kernels lacking the long-range part. Another difficulty comes from the usual method of applying TDDFT on bulk materials which calculate the spectrum - though suitable for continuum excitations, this approach does not upfront yield the binding energy of the discrete excitonic excitations. We develop a method in analog with the Casida equation formalism, in which exciton binding energies are obtained directly. We calculate exciton binding energies for both small- and large-gap semiconductors with this method. We study the recently published 'bootstrap' exchange-kernel within our method, and we extend the formalism to treat triplet excitons. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U24.00013: Nonlocal formulation of spin Coulomb drag in nanostructures: implications for time-dependent current-density-functional theory Carsten A. Ullrich, Irene D'Amico The spin Coulomb drag (SCD) effect occurs in materials and devices where charged carriers with different spins exchange momentum via Coulomb scattering. This causes frictional forces between spin-dependent currents that lead to dissipation and limit spin mobilities. We consider the role of the SCD in the damping of intersubband spin plasmons in semiconductor quantum wells, and show that a local density approximation leads to overdamping. A nonlocal formulation of the SCD is developed which agrees with experimental observations of spin plasmon linewidths. General consequences for using density-functional approaches to describe electronic many-body effects in nanostructures are discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U24.00014: Alternative time-dependent optimized effective potential Vladimir Nazarov The OEP is known as a single-particle potential minimizing the expectation value of a many-body Hamiltonian on the set of eigen-functions of a single-particle Hamiltonian [1]. The time-dependent (TD) OEP can be constructed with the TD quantum stationary-action principle [2]. Very useful conceptually in DFT and TDDFT, both OEPs are not practicable due to the complexity of their implementations. Here we report a TDOEP by minimizing the difference of LHS and RHS of the TD Schr\"{o}dinger equation [3]. If the orbitals are varied, then the TD Hartree-Fock equations are reproduced. Similarly, we now find the OEP. New OMP does not involve the inversion of the density-response function $\chi_s$, which greatly facilitates implementations. Accordingly, the exchange-correlation kernel $f_{xc}$ involves of $\chi^{-1}_s$ only, not its quadratic counterpart. To show the power of this method, we work out the $f^h_{xc}(q,\omega)$ of the homogeneous electron gas to be used with the nearly-free electrons theory, where $f^h_{xc}$ is the main input [4].\\[4pt] [1]. J. D. Talman et al. Phys. Rev. A 14, 36 (1976).\\[0pt] [2]. C. A. Ullrich et al. Phys. Rev. Lett. 74, 872 (1995).\\[0pt] [3]. V. U. Nazarov, Math. Proc. Cambridge Phil. Soc. 98, 373 (1985).\\[0pt] [4]. V. U. Nazarov et al. Phys. Rev. Lett. 102, 113001 (2009). [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U24.00015: Dynamical Hyperpolarizabilities from Real-Time Density Functional Theory Vladimir Goncharov, Kalman Varga The explicitly time dependent wave function obtained in the framework of Real-Space, Time-Dependent Density Functional Theory captures the essential physics and allows a non-perturbative calculations of important observables. We generalize finite-difference method typically used to calculate static hyperpolarizabilities to the dynamical case \footnote{V. A. Goncharov and K. Varga, \emph{J. Chem. Phys.}, 2012, 137, 094111} and compute nonlinear optical response functions to the third order inclusively. The method is simple and free of errors associated with basis function based methods. Comparison with experimental results for a range of molecules including $C_{60}$ is presented. [Preview Abstract] |
Session U25: Superconducting Qubits: Qubit-Field Interactions and Qubit Theory
Sponsoring Units: GQIChair: Jay Gambetta, IBM
Room: 327
Thursday, March 21, 2013 11:15AM - 11:27AM |
U25.00001: Cavity-Mediated Landau-Zener Interferometry Between Two Superconducting Qubits C.M. Quintana, K.D. Petersson, L.W. McFaul, S.J. Srinivasan, A.A. Houck, J.R. Petta Avoided crossings between two energy levels as a function of some external parameter are common to many quantum mechanical systems. In the field of circuit quantum electrodynamics (cQED), the energies of superconducting qubits can be tuned via applied magnetic flux, and a microwave cavity-mediated coupling between two qubits placed in the same resonator leads to an avoided crossing in the system's energy spectrum when the two singly-excited qubit states become degenerate. We utilize such an avoided crossing between two transmon qubits to explore Landau-Zener transition physics, using nanosecond timescale flux bias pulses to non-adiabatically traverse the avoided crossing. We explore the dynamics of single- and double-passage through the resulting ``beam splitter'' of two-qubit states. In particular, we test the general asymptotic Landau-Zener formula for non-adiabatic transition probabilities and demonstrate the creation of two-transmon entanglement via a single passage through the beam splitter. We also study interference phenomena associated with double passage through the avoided crossing (analogous to an optical interferometer), and explore the dependence of the interference fringes on the level velocity with which the passages are made. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U25.00002: First-order sideband transitions with flux-driven asymmetric transmons J.D. Strand, M.E. Ware, Felix Beaudoin, Alexandre Blais, T. Ohki, B. Johnson, B.L.T. Plourde We present data demonstrating first-order sideband transitions between a qubit and a resonator performed with a digitally synthesized waveform coupled to the qubit loop as a magnetic flux. The resulting first-order sideband transitions are much faster (up to 85 MHz in our measurements) than second-order processes and have the potential to create fast quantum gates. The frequency of the red sideband can also be made quite low, typically a few hundred MHz in our experiment, and at these low frequencies expensive microwave generators are not required, simplifying the control electronics and making the process more scalable. We chose to test this process with asymmetric transmons in which one junction is several times larger than the other. This asymmetry creates a shallow flux modulation curve that is optimum for this flux-driven sideband process. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U25.00003: Manipulating Kerr effects in a superconducting cavity via a superconducting qubit Victor V. Albert, Gerhard Kirchmair, Brian Vlastakis, Zaki Leghtas, Mazyar Mirrahimi, S.M. Girvin, R.J. Schoelkopf, Liang Jiang Typically, models of qubit-cavity interactions in superconducting circuits have included terms strictly linear in amplitude of the cavity modes. Due to ever-increasing experimental ability to realize larger coupling strengths, induced nonlinearities in the cavity contribute significantly to the dynamics and thus need to be accounted for. Such nonlinearities include interactions between the photon numbers of two cavity modes (cross-Kerr) and between a mode and itself (self-Kerr). Motivated by the recent experimental demonstration of self-Kerr in superconducting cavities, we investigate quantum control of Kerr effects via a dispersively coupled superconducting qubit, which not only enables us to enhance or suppress the Kerr coupling, but also opens the possibility to investigate higher order Kerr effects. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U25.00004: Giant Cross Kerr Effect via a Superconducting Artificial Atom I.-C. Hoi, C.M. Wilson, G. Johansson, T. Palomaki, T.M. Stace, B. Fan, A. Frisk Kockum, L. Tornberg, P. Delsing We investigate the effective interaction between two microwave fields, mediated by a superconducting artificial atom (transmon qubit) which is strongly coupled to a coplanar transmission line. The interaction between the fields and atom realizes an effective cross Kerr coupling. Using this, we demonstrate average Kerr phase shifts of up to 25 degrees per photon with both coherent microwave fields at the single-photon level. Our results provide an important step towards quantum gates with propagating photons in the microwave regime. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U25.00005: Requirements for Electromagnetically Induced Transparency in a Transmon J.E. Robinson, S. Novikov, Z.K. Keane, B. Suri, F.C. Wellstood, B.S. Palmer In the dressed atom picture, a three-level system can interact with two photons via the Autler-Townes (AT) effect, where the system exhibits two peaks separated by the generalized Rabi frequency of the coupling photon. The system can also exhibit electromagnetically induced transparency (EIT), where the first excited state is made transparent to the probe photon by a strong coupling drive. We examine the results from a multi-tone measurement in a transmon qubit coupled to a 3D cavity, which exhibits an AT splitting, as expected from the dressed atom picture, similar to previous results.\footnote{M. Baur, et al. {\it Phys. Rev. Lett.} {\bf 102}, 243602 (2009).}$^,$\footnote{Mika A. Sillanp\"{a}\"{a}, et al. {\it Phys. Rev. Lett.} {\bf 103}, 193601 (2009).} We will discuss the requirements for a crossover from an AT doublet to an EIT signal, as they relate to the limitations of our device. We will also examine the quantum information implications of realizing EIT in superconducting system. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U25.00006: Probing Electromagnetically Induced Transparency in a Transmon Sergey Novikov, J.E. Robinson, Z.K. Keane, B. Suri, F.C. Wellstood, B.S. Palmer We have designed, fabricated, and measured a transmon made from a single Al/AlOx/Al Josephson-Junction on a sapphire substrate with $f_{01} \sim $ 5 GHz. The transmon was mounted in a 3D microwave cavity (OFHC copper, $f_c \sim $ 7.5GHz), similar to other recent experiments\footnote{Paik, H. \textit{et al.} Phys. Rev. Lett. 107, 240501.}$^,$\footnote{Rigetti, C. \textit{et al.} Phys. Rev. B 86, 100506.}. The observed coherence times were $T_1, T_2^* \sim $ 10$\mu$s allowing us to investigate the possibility of electromagnetically induced transparency (EIT) and other population trapping effects, such as the Autler-Townes (AT) splitting. We will discuss the experiments to look for and distinguish between AT and EIT given the constraints placed by the transmon and the readout limitations imposed by the cavity. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U25.00007: cQED Susceptibility of Superconducting Transmons coupled to a Microstrip Resonator Cavity David Pappas, Martin Sandberg, Jiansong Gao, Michael Vissers, Anton Kockum, Goran Johansson The light-matter interaction of multi-level transmons strongly coupled to a cavity and the external drive field are measured over a wide frequency and power range. The transmons are fabricated from TiN capacitor plates with small Al/AlOx/Al shadow evaporated junctions. The long T1's of these devices, approximately 10 us, allow for a rich spectrum of doubly dressed states to be observed and modeled. Both single- and two-photon absorption features are identified as the drive power is increased. Quantitative agreement of the absorption spectra in both the weak and strong drive limits is obtained using the measured junction properties and the temperature. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U25.00008: Tunable Coupling between Two Resonators Controlled by a Flux Qubit: the Quantum Switch E. Hoffmann, M. Haeberlein, A. Baust, M.J. Schwarz, E.P. Menzel, H. Huebl, F. Deppe, A. Marx, R. Gross, D. Zueco, J.-J. Garcia Ripoll, E. Solano In the field of quantum information processing, superconducting circuits have become a well-established platform. In particular, systems consisting of a few qubits and/or harmonic oscillator circuits have been investigated. When scaling up these systems, it seems practical to aim for active guidance elements allowing for a directed transmission of quantum signals. One way to achieve this is by implementing switchable coupling between two microwave resonators. We show experimental progress on two superconducting transmission line resonators, where a superconducting flux qubit mediates a controllable coupling - the Quantum Switch. We show an experimental characterization of such a device and discuss spectroscopic evidence for the switching behavior.\\[4pt] We acknowledge support from the DFG via SFB~631, the German excellence initiative via NIM, and EU projects CCQED, SOLID and PROMISCE, the Basque Foundation for Science, Basque Government IT472-10, and Spanish MICINN FIS2009-12773-C02-01, DZ granted by ARAID [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U25.00009: Catch-Disperse-Release Readout for Superconducting Qubits Eyob A. Sete, Eric Mlinar, Alexander N. Korotkov, Andrei Galiautdinov, John M. Martinis We analyze a qubit readout scheme for superconducting qubits via controlled capture, dispersion, and release of a microwave field. A tunable coupler is used to decouple the microwave resonator from a transmission line during dispersive interaction with the qubit, thus circumventing the Purcell effect. We show that fast and high-fidelity qubit readout can be achieved for nonlinear dispersive qubit-resonator interaction and for sufficiently adiabatic tuning of the qubit frequency. Interestingly, the Jaynes-Cummings nonlinearity results in quadrature squeezing of the microwave field which leads to a significant decrease in measurement error. The effects of qubit anharmonicity and imperfect quantum efficiency of the microwave amplification on the measurement error are also discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U25.00010: Realizing a Deterministic Teleportation Protocol in Superconducting Circuits Lars Steffen, Markus Oppliger, Matthias Baur, Arkady Fedorov, Andreas Wallraff Teleportation of a quantum state may be used for distributing entanglement between distant qubits in quantum communication and for realizing universal and fault-tolerant quantum computation. Previously, we have demonstrated the implementation of a teleportation protocol, up to the single-shot measurement step, with superconducting qubits coupled to a microwave resonator [1]. Using full quantum state tomography and calculating the projection of the measured density matrix onto the basis states of two qubits has allowed us to reconstruct the teleported state with an average output state fidelity of 86\%. In ongoing experiments we attempt to implement single shot read-out and feed-back to perform full deterministic quantum teleportation.\newline [1] M.~Baur, A.~Fedorov, L.~Steffen, S.~Filipp, M.P.~da~Silva, and A.~Wallraff, Phys. Rev. Lett. \textbf{108}, 040502 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U25.00011: Methods for entanglement in circuit QED Felix Motzoi, Mohan Sarovar, Michael Goerz, Christiane Koch, Birgitta Whaley We discuss some progress in methods of generating entanglement in superconducting qubit architectures. We focus on the minimal time required to generate a perfect entangler in a given system, specifically by combining simultaneously multiple given forms of coupling. Typically the different terms will generate different dynamics and when multiple coupling terms exist one will have a choice about which local equivalence class to use to generate entanglement. Here, we consider the case where we want to simultaneously include the different forms of coupling that will be present in the circuit QED system, such as direct coupling, cavity mediated coupling, or virtual transitions in the multi-qubit space, with similar interaction strengths. No specific gate is targeted, but rather entanglement generation is optimized. Incoherent effects such as measurement/feedback based control can also be included to generate entanglement, even when the qubits are spatially separated (i.e. in different cavities) and no interaction exists. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U25.00012: Tuning from coherent interaction to super- and subradiance with artificial atoms in a 1D waveguide Kevin Lalumi\`ere, Alexandre Blais, Barry C. Sanders, Arjan F. Van Loo, Arkady Fedorov, Andreas Wallraff Taking advantage of the near ideal spatial mode-matching, strong interaction between light and artificial atoms fabricated in a 1D waveguide has been demonstrated experimentally [1]. Here, we study the situation where multiple and possibly un-identical atoms are fabricated in the same waveguide. We find that atom relaxation and Lamb-shift are modified, leading to collective effects. Depending on the distance between the artificial atoms, or equivalently the phase shift accumulated by light traveling from one atom to another, we find that it is possible to tune between a strong modification of individual atomic relaxation with the formation of sub- and superradiant states, and a strong modification of the Lamb-shift leading to a coherent exchange-type interaction between the atoms. These predictions are based on a master equation derived for an inhomogeneous set of atoms coupled to a transmission line. Comparison with experimental results will be discussed.\\[4pt] [1] O. Astafiev et al., Science 327, 840 (2010) [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U25.00013: Quantum dynamics of triplet superconducting circuits David G. Ferguson, Jens Koch, James Sauls We generalize the formalism of ``circuit quantization''~[1] to circuits comprised of spin-triplet superconducting elements. This introduces the dynamics associated with the spin of the Cooper pairs in addition to the phase and charge dynamics. The dynamics of the order parameter for spin-triplet superconductors is encoded in the vector $\vec{d}$ for the spin-projections of the Cooper pairs, which is coupled to the dynamics of the electronic spin polarization, $\vec{S}$. At frequencies below the superconducting gap, $\hbar\omega\ll\Delta$, the classical spin dynamics is described by Leggett's equations for $\vec{d}$ and $\vec{S}$~[2]. Weak spin-orbit coupling ($E_{\mbox{s-o}}\ll\Delta$) leads to frequency shifts of the normal-state spin resonance. Quantization of a spin-triplet superconducting circuit is achieved by including the Hamiltonian that generates Leggett's equations. Analytical and numerical results for the spectra of the quantized Hamiltonians of varrious circuits are reported. As a case study, we highlight the low energy excitation frequencies of two triplet superconductor islands coupled by a Josephson junction.\\[4pt] [1]~M. H. Devoret, Quantum fluctuations in electrical circuits, (Les Houches Session LXIII, 1995).\\[0pt] [2]~A. J. Leggett, Rev. Mod. Phys. 47, 331 (1975) [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U25.00014: A Vector Potential for Flux Qbits Eliot Kapit, Erich Mueller We design a superconducting circuit, based on three junction flux qbits, in which the motion of magnetic flux mimics the behavior of charged lattice bosons hopping in a magnetic field. For realistic device parameters one can reach the strongly interacting bosonic quantum Hall limit where one will find anyonic excitations. We explore the design principles for using these circuits to study many-body physics, for example explaining how the magnitude and phase of the effective hopping matrix elements can be controlled by tuning offset voltages. The circuits could be used for topological quantum computation. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U25.00015: ABSTRACT WITHDRAWN |
Session U26: Focus Session: Semiconductor Qubits - Impurity Complexes
Sponsoring Units: GQIChair: Kai-Mei Fu, University of Washington
Room: 328
Thursday, March 21, 2013 11:15AM - 11:51AM |
U26.00001: Single-atom spin qubits in silicon Invited Speaker: Andrew Dzurak Spin qubits in silicon are excellent candidates for scalable quantum information processing (QIP) due to their long coherence times and the enormous investment in silicon MOS technology. Here I discuss qubits based upon single phosphorus (P) dopant atoms in Si [1]. Projective readout of such qubits had proved challenging until single-shot measurement of a single donor electron spin was demonstrated [2] using a silicon single electron transistor (Si-SET) and the process of spin-to-charge conversion. The measurement gave readout fidelities \textgreater\ 90{\%} and spin lifetimes T$_{\mathrm{1e}}$ \textgreater\ 6 s [2], opening the path to demonstration of electron and nuclear spin qubits in silicon. Integrating an on-chip microwave transmission line enabled single-electron spin resonance (ESR) of the P donor electron. We used this to demonstrate Rabi oscillations of the electron spin qubit, while a Hahn echo sequence revealed electron spin coherence times T$_{\mathrm{2e}}$ \textgreater\ 0.2 ms [3]. This time is expected to become much longer in isotopically enriched $^{28}$Si devices. We also achieved single-shot readout of the $^{31}$P nuclear spin (with fidelity \textgreater\ 99.6{\%}) by monitoring the two hyperfine-split ESR lines of the P donor system. By applying (local) NMR pulses we demonstrated coherent control of the nuclear spin qubit, giving a coherence time T$_{\mathrm{2n}}$ \textgreater\ 60 ms. \\[4pt] [1] B.E. Kane, \textit{Nature} \textbf{393}, 133 (1998). \newline [2] A. Morello et al., \textit{Nature} \textbf{467}, 687 (2010). \newline [3] J.J. Pla et al., \textit{Nature} \textbf{489}, 541 (2012). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U26.00002: Measurements of spin life time of an antimony-bound electron in silicon T.M. Lu, N.C. Bishop, L.A. Tracy, R. Blume-kohout, T. Pluym, J.R. Wendt, J. Dominguez, M.P. Lilly, M.S. Carroll We report our measurements of spin life time of an antimony-bound electron in silicon. The device is a double-top-gated silicon quantum dot with antimony atoms implanted near the quantum dot region. A donor charge transition is identified by observing a charge offset in the transport characteristics of the quantum dot. The tunnel rates on/off the donor are first characterized and a three-level pulse sequence is then used to measure the spin populations at different load-and-wait times in the presence of a fixed magnetic field. The spin life time is extracted from the exponential time dependence of the spin populations. A spin life time of 1.27 seconds is observed at B $=$ 3.25 T. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U26.00003: Shuttling electrons on and off As donor atoms in silicon A.M. Tyryshkin, S.A. Lyon, C.C. Lo, R. Lo Nardo, J.J.L. Morton, S. Simmons, C.D. Weis, T. Schenkel, J. Bokor, J. Meijer, D. Rogalla Hybrid quantum devices where electron spins are used for state initialization, fast manipulation, long range entanglement and detection, while nuclear spins are used for long term storage promise revolutionary advantages. Here we report our first experiments using a silicon-based device that utilizes electron and nuclear spins of arsenic donors. The device is a large-area, parallel-plate capacitor fabricated on a silicon-on-insulator (SOI) wafer where the SOI layer is implanted with arsenic donors, and a back gate is formed in the silicon below the buried oxide by a high-energy boron implantation. The electrons can be controllably stripped from the donors and then reintroduced to the ionized donors by applying appropriate gate voltages. We use ensemble ESR experiments (X-band, magnetic field of 0.35 T) to track the occupancy of the donors during these operations. Pulsed ESR is used to characterize the spin state of the donor electrons and the effect of applied electric fields below the ionization threshold. The spin state of the arsenic nuclei, and the effect of electron removal and reintroduction on the nuclear state is expected to be observable in pulsed ENDOR experiments. The work is funded by LPS and NSF-MWN. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U26.00004: Electronic structure of sub-surface Boron acceptors in silicon for potential qubits Rajib Rahman, Jan Mol, Gerhard Klimeck, Sven Rogge Single acceptors in silicon are investigated as potential qubits. Due to the p-type nature of the valence band (VB), the acceptor states are less susceptible to the hyperfine interaction of the neighboring nuclear spins. The presence of a stronger spin-orbit coupling in the VB also enables the possibility of an all-electric qubit control. Whereas donor qubits exhibit exchange oscillation with separation distance due to conduction band valleys, Boron acceptors are expected to have smoother exchange curves. We investigate the electronic structure of single Boron acceptors in silicon in the presence of electric field, strain, magnetic field, and interfaces. Bulk Boron acceptors have a four-fold degenerate ground state 45 meV above the VB with angular momentum states of 3/2 and 1/2. An interface splits this manifold into Kramer's doublets. Application of E and B fields allow several possibilities for forming a two-level qubit driven by an ac electric field. We compare calculations from atomistic tight-binding theory to scanning tunneling microscope (STM) measurements and k.p calculations. The tight-binding method captures additional wavefunction symmetries due to the crystal that help to explain the STM measurements. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U26.00005: Interface-split Kramers doublets for acceptor-based qubits in silicon Jan Mol, Joseph Salfi, Rajib Rahman, Sven Rogge Single dopants in silicon form a particular attractive platform for hosting spin quantum bits (qubits). The effective spin-3/2 states of acceptor-bound holes in silicon can be used to store bits of quantum information for several $\mu$s. Strong coupling of spin and momentum in the silicon valence band allows for rapid electrical manipulation of the hole spin. Acceptors in silicon have a four-fold degenerate ground-state, reflecting character of the top of the valence band. Symmetry breaking, by an electric field, strain or confinement, lifts this degeneracy, resulting in two Kramers doublets. The states within these isolated Kramers doublets are protected against decoherence by time reversal symmetry and form the working levels of a hole spin qubit. Here we investigate the effect of the presence of an interface on the ground-state energy splitting of individual sub-surface acceptors, as a function of dopant depth, by means of low temperature scanning tunneling spectroscopy. The depth of individual acceptors is determined by probing the Coulomb potential of the ionized acceptor nuclei. Resonant tunneling through the localized acceptor states provides a direct measure of the excited state spectrum of single dopants. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U26.00006: Towards isolating a single impurity-bound hole Russell Barbour, Todd Karin, Kai-Mei Fu, Yoshiro Hirayama, Arne Ludwig, Andreas Wieck Single acceptor-bound holes embedded in III-V semiconductor quantum wells could provide an ideal qubit system for scalable quantum information processing and quantum computation. This system combines strong homogenous optical transitions and millisecond long spin coherence times in a fabrication ready material (GaAs). However, single acceptor-bound excitons (A$^{0}$X) have yet to be optically isolated even in the purest bulk GaAs samples. This is primarily due to the high acceptor density (10$^{14}$ cm$^{-3}$) and exceptional optical homogeneity. We propose using stimulated emission depletion microscopy (STED) to increase our optical resolution far beyond the diffraction limit in order to spatially isolate a single acceptor-bound exciton. We report the first demonstration of stimulated emission of acceptor-bound excitons at 4.2K. We resonantly excite the A$^{0}$1s-A$^{0}$X transition and apply a second laser with high power (P$=$10mW) resonant with the 2s two-hole transition (THT). We observe a 30 percent reduction in the 1s PL intensity when the STED laser is resonant with the THT's. We will present our two-laser spectroscopy work that explores this coherent system and discuss our progress towards isolating a single acceptor-bound exciton using STED microscopy. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U26.00007: Ultrafast coherent optical control of a single diamond spin L.C. Bassett, F.J. Heremans, D.D. Awschalom, G. Burkard As an optically addressable solid-state electronic spin, the nitrogen-vacancy (NV) center in diamond has great promise for applications in quantum information science and metrology. At temperatures below $\approx 10$ K, the NV center's optical fine structure facilitates coherent coupling between the electronic spin and light, providing the means for all-optical spin control and other applications in quantum optics. Here, using ultrafast optical pump-probe techniques, we investigate the interplay of orbital, vibrational, and spin dynamics on timescales ranging from femtoseconds to nanoseconds. These techniques provide a flexible and powerful probe of orbital dynamics in the NV center's optically excited state, and enable optical spin control with sub-picosecond resolution. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U26.00008: All-optical quantum dynamical control of an NV center spin in diamond B.B. Buckley, C.G. Yale, D.J. Christle, F.J. Heremans, L.C. Bassett, D.D. Awschalom, G. Burkard The nitrogen-vacancy (NV) center in diamond has emerged as a promising optically addressable qubit candidate, but optical methods are usually used only for spin initialization and readout through the defect's spin-dependent intersystem crossing (ISC) transition. Quantum dynamical control typically requires the application of microwave magnetic fields, limiting possible applications. Here, we demonstrate an all-optical method for unitary, arbitrary-axis spin control of single NV spins below 10 K based on stimulated Raman transitions\footnote{C. G. Yale*, B. B. Buckley*, D. J. Christle, G. Burkard, F. J. Heremans, L. C. Bassett, and D. D. Awschalom (submitted)}. Using our recently-demonstrated arbitrary-basis spin initialization and readout, we perform time-domain spin coherence measurements on single NV center spins solely with optical pulses. These techniques enable individual addressing of proximal NV center spins and could be used to probe other previously-inaccessible defect spin systems without ISC spin addressability. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U26.00009: Experimental control of a nuclear spin quantum register in diamond with decoherence-protected gates Tim Hugo Taminiau, Toeno van der Sar, V. V. Dobrovitski, Ronald Hanson Nuclear spins are one of the most promising candidates for long-lived quantum bits that store and process quantum information. Individual nuclear spins in diamond have been addressed using the nearby electron spin of a nitrogen vacancy center. However, the relatively fast decoherence of the electron spin limits coherent control to the nearest, strongly coupled, nuclear spins. Here, we employ decoherence-protected gates [1] to access individual spins embedded in a bath of nuclear spins that are weakly coupled to an electron spin [2]. We demonstrate the initialization, control and readout of the nuclear spins and discuss our recent progress in implementing two-qubit entangling operations between nuclear spins. These results greatly extend the number of available quantum bits in diamond and provide a way towards tomography with single nuclear spin sensitivity even in decohering environments. [1] T. van der Sar et al., Nature 484, 82 (2012). [2] T. H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U26.00010: Entanglement by measurement and Bell inequality violation with spins in diamond Wolfgang Pfaff, Tim Taminiau, Lucio Robledo, Hannes Bernien, Matthew Markham, Daniel Twitchen, Ronald Hanson Single spins in diamond have emerged as a promising platform for quantum information processing in the solid state. In particular, individual nuclear spins coupled to nitrogen-vacancy (NV) centers have been recognized as excellent candidates for solid state qubits, because they combine outstanding stability, excellent control by spin resonance techniques, and high-fidelity optical initialization and readout provided by the NV center. Here we report the achievement of a milestone towards quantum computation with spins: The creation of high quality quantum entanglement between two nuclear spins in diamond. Such entanglement is an important resource for quantum computation and lies at the heart of many key quantum protocols, such as teleportation and error correction. We show that we can produce entangled states of high fidelity using a projective quantum measurement. Our technique is non-destructive, and thus leaves the quantum information that is required for further computation unharmed. This enables us to demonstrate the violation of Bell's inequality for the first time with spins in the solid state. Reference: Pfaff et al., Nature Physics, doi:10.1038/nphys2444 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U26.00011: Pulsed ESR of photo-polarized NV centers in diamond at X-band magnetic fields Brendon Rose, Alexei Tyryshkin, Stephen Lyon, Christoph Weis, Thomas Schenkel Recently nitrogen-vacancy (NV) color centers in diamond have become the focus of many studies aimed towards their use as quantum bits (qubits) in quantum computing applications and as precision magnetic field sensors in scanned imaging applications. The NVs have a ground triplet state (S$=$1) with ZFS of 2.88 GHz. It has been previously shown that optical excitation, when shining green light at low magnetic fields (below 100 G), polarizes spins preferentially into the T$_{\mathrm{0}}$ state. Here we will report an X-band pulsed ESR measurement and demonstrate that the optical spin polarization is more complex at higher magnetic fields (3400 G) and can lead to preferential spin polarization into T$_{\mathrm{+}}$ and T$_{\mathrm{-}}$ states, instead of T$_{\mathrm{0}}$. This effect can be understood from a simple one electron spin Hamiltonian and depends mainly on the relative orientation of the ZFS and external magnetic field. In addition, we observe strong ESEEM effects originating from the central nitrogen nucleus which are most prominent when measuring the T$_{\mathrm{0}}$ to T$_{\mathrm{-}}$ transition and when the field is along the ZFS. From the orientation dependence of ESEEM we are able to accurately determine the nitrogen hyperfine and nuclear quadrupole tensors. Spin coherence of 0.8 ms is seen at 10 K, limited by 1 percent of magnetic $^{\mathrm{13}}$C nuclei in our natural diamond sample. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U26.00012: Polytype control of spin qubits in silicon carbide A.L. Falk, B.B. Buckley, G. Calusine, W.F. Koehl, A. Politi, D.D. Awschalom, V.V. Dobrovitski, C.A. Zorman, P. X.-L. Feng The search for coherently addressable spin states in technologically important materials is a promising direction for solid-state quantum information science. Silicon carbide, a particularly suitable target, is not a single material but a collection of about 250 known polytypes, each with its own set of physical properties and technological applications. We show that in spite of these differences, the 4H-, 6H-, and 3C-SiC polytypes all exhibit optically addressable spins with long coherence times [1]. These results include room temperature spins in all three polytypes and suggest a new method for tuning quantum states using crystal polymorphism. Long spin coherence times allow us to use double electron-electron resonance to measure magnetic dipole interactions between spin ensembles in inequivalent lattice sites of the same crystal. Since such inequivalent spin have distinct optical and spin transition energies, these interactions could lead to dipole-coupled networks of separately addressable spins.\\[4pt] [1] A. Falk et al., submitted [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U26.00013: Defects as qubits in 3C and 4H polymorphs of SiC Luke Gordon, Audrius Alkauskas, WIlliam F. Koehl, Anderson Janotti, David D. Awschalom, Chris G. Van de Walle Using hybrid functional calculations we study defects in SiC that can serve as qubits for quantum computing. We investigate the divacancy in 4H- and 3C-SiC and the N-V center in 3C-SiC, in which the N impurity replacing a C atom is sitting next to a Si vacancy. The calculated excitation and emission energies of the divacancy in 4H-SiC are in excellent agreement with the available experimental data. Most importantly, we predict that the neutral divacancy and the negatively charged NV center in 3C-SiC have all the required characteristics to serve as qubits; in addition, both defects are stable in n-type 3C-SiC, which is in principle easy to fabricate. We calculate luminescence lineshapes and Huang-Rhys factors for these defects in 4H and 3C-SiC, and compare with experimental photoluminescence spectra. [Preview Abstract] |
Session U27: Quantum Entanglement: Theory and Experiment
Sponsoring Units: GQIChair: Andrews Doherty, University of Sydney
Room: 329
Thursday, March 21, 2013 11:15AM - 11:27AM |
U27.00001: Positivity of Partial Transpose and Separability of Dicke state mixtures Elie Wolfe, Susanne Yelin We study mixtures of permutation symmetric (Dicke) states, with a special focus on superradiance time evolution. For such systems we develop necessary separability criteria for general N-qubit systems based on the condition of Positive Partial Transpose. We also compose sufficient separability criteria for the specific cases of two and three qubits. Comparing the criteria we prove that, for Dicke state mixtures, the PPT test is always sufficient to imply full separability. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U27.00002: Polynomial invariants to quantify Four-body Correlations Santosh Shelly Sharma, Naresh Kumar Sharma Local unitary invariance and notion of negativity fonts are used as the principle tools to construct four qubit polynomial invariants of degree 8, 12, and 24. Determinants of negativity fonts are linked to matrices obtained from state operator through selective partial transposition. Our general aim is to construct the polynomial invariants that quantify entanglement due to $K-$body correlations in an $N$-qubit ($N?K$) pure state. This is done by constructing N-qubit invariants from multivariate forms with ($K-1$)-qubit invariants as coefficients. In particular, the invariant that quantifies entanglement due to $N$-body correlations is obtained from a biform having as coefficients the $N-1$ qubit invariants. A polynomial invariant that is non-zero on four qubit pure states with four-body correlations and zero on all other states, is identified. Classification of four qubit states into seven major classes, using criterion based on the nature of correlations, is discussed. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U27.00003: Numerical Calculations of the Three Tangle for Mixed States Samuel Rodriques, Peter Love We present a steepest descent convex roof optimization algorithm, using the Cayley parametrization of the unitary group, which can be used to calculate the convex roof of any entanglement monotone on mixed states. We use the algorithm to calculate the three tangle on a set of states for which the tangle is known analytically, and show that our results are in good agreement with the analytical calculations. We then randomly generate a set of full-rank three qubit states, of varied mixedness and tangle, calculate the tangle on these states using our convex roof algorithm, and also calculate the lower bound on the three-tangle which has been provided by Eltschka and Siewert[1]. We thus provide a profile of the strength of the Eltschka-Siewert bound, as a function of mixedness and tangle. [1] ``Optimal Witnesses for Three Qubit Entanglement from Greenberger-Horne-Zeilinger Symmetry,'' Eltschka, C. and Siewert, J., forthcoming.~arXiv: 1204.5451 [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U27.00004: Quantum steering ellipsoids: The way to represent two qubits Sania Jevtic, Matthew Pusey, David Jennings, Terry Rudolph A single qubit state is faithfully represented as a vector in the Bloch sphere. A two qubit state may be faithfully represented as two vectors and a quantum steering ellipsoid (QSE) in the Bloch sphere. When Alice and Bob share a pair of qubits, the QSE is the geometric set of states that Bob can steer Alice's qubit to when he implements all possible measurements on his qubit. We argue that the QSE is the way one should visualise a two qubit state and show how the correlative properties of the state manifest themselves in this paradigm, in particular we give simple conditions for when the state is entangled, or has discord. We will also present novel features of the two qubit state that are revealed by the QSE formalism, and show that a state corresponding to an ellipsoid with non-zero volume contains a new type of correlation. Such a state is a useful resource in a game where Bob succeeds if he can steer Alice's qubit to three states with linearly independent Bloch vectors. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U27.00005: Quantum Discord Bounds the Amount of Distributed Entanglement Marco Piani, Tan Kok Chuan, Jean Maillard, Kavan Modi, Tomasz Paterek, Mauro Paternostro The ability to distribute quantum entanglement is a prerequisite for many fundamental tests of quantum theory and numerous quantum information protocols. Two distant parties can increase the amount of entanglement between them by means of quantum communication encoded in a carrier that is sent from one party to the other. Intriguingly, entanglement can be increased even when the exchanged carrier is not entangled with the parties. However, in light of the defining property of entanglement stating that it cannot increase under classical communication, the carrier must be quantum. Here we show that, in general, the increase of relative entropy of entanglement between two remote parties is bounded by the amount of nonclassical correlations of the carrier with the parties as quantified by the relative entropy of discord. We study implications of this bound, provide new examples of entanglement distribution via unentangled states, and put further limits on this phenomenon. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U27.00006: Topological Classification of Types of Quantum Discord Evolutions Nga Nguyen, Robert Joynt Quantum discord is a type of quantum correlation that has recently attracted extensive attention. One question that is of experimental importance is how quantum correlations such as entanglement and discord are erased by external noise. A general classification of time evolution is seen to depend essentially on the understanding of the topology of the set $C$ of concordant (zero-discord) states. In the 2-qubit case, we show that $C$ is a 9-dimensional simply-connected manifold with boundary that can be embedded in the 15-dimensional space of 2-qubit density matrices. This yields 6 topologically distinct categories for the joint time evolution of entanglement and discord that exhaust all possibilities. We show that these 6 categories can be obtained in one physical model using independent or correlated random telegraph noise sources in the Markovian regime. Transition between these categories is of topological nature and is governed by changing physical parameters or initial conditions. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U27.00007: Mutual Preservation of Entanglement Andrzej Veitia We study a generalized double Jaynes-Cummings (JC) model where two entangled pairs of two-level atoms interact indirectly. We focus on the case where the cavities and the entangled pairs are uncorrelated. We show that there exist initial states of the qubit system so that two entangled pairs are available at all times. In particular, the minimum entanglement in the pairs as a function of the initial state is studied. Finally, we extend our findings to a model consisting of multi-mode atom-cavity interactions. We use a non-Markovian quantum state diffusion (QSD) equation to obtain the steady-state density matrix for the qubits. We show that the multi-mode model also displays dynamical preservation of entanglement. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U27.00008: Quantum geometry and entanglement in the Rabi model Justin Wilson, Victor Galitski In composite systems, entanglement can be useful for control since one system's properties become fundamentally linked with another system's properties. One way of measuring entanglement is with a quantity called I-concurrence, a generalization of concurrence to systems that have more states than a qubit. We show that I-concurrence can be rewritten in terms of quantum geometric quantities. In particular, we show a dependence on the Hilbert-Schmidt distance measure on the Hilbert space of one of the subsystems. Using this quantity and the recently exactly solved Rabi model, we calculate the entanglement between eigenstates in the Rabi model. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U27.00009: Classical Analogs of Quantum Entanglement Brian La Cour Quantum computing algorithms rely upon entanglement and context-based measurements, properties that are well exhibited by atomic or photonic systems. In some cases, these properties can be mimicked by cleverly contrived classical systems. We present a notional scheme for such classical analogs and compare their predictions to those of an associated quantum system. Entanglement is verified operationally using quantum tomography, wherein the quantum mixed state is inferred from measurements on a complete orthonormal set of Hermitian observables. Using the Peres-Horodecki criterion for separability, we examine the partial transpose of the estimated density matrix to establish a necessary, and in some cases sufficient, condition for entanglement. Through the use of Monte Carlo simulations, we find that certain classical systems do indeed exhibit a measurably significant level of entanglement. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U27.00010: Robust distant-entanglement generation using coherent multiphoton scattering Ching-Kit Chan, L. J. Sham The generation and controllability of entanglement between distant quantum states have been the heart of quantum computation and quantum information processing. Existing schemes for solid state qubit entanglement are based on the single-photon spectroscopy that has the merit of a high fidelity entanglement creation, but with a very limited efficiency. This severely restricts the scalability for a qubit network system. Here, we describe a new distant entanglement protocol using coherent multiphoton scattering. The scheme makes use of the postselection of large and distinguishable photon signals, and has both a high success probability and a high entanglement fidelity. Our result shows that the entanglement generation is robust against photon fluctuations, and has an average entanglement duration within the decoherence time in various qubit systems, based on existing experimental parameters. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U27.00011: Optical control of entangled states in semiconductor quantum wells Mario Borunda, Esa Rasanen, Thomas Blasi, Eric Heller The ability to coherently control arbitrary two-electron states, and to maximize the entanglement, opens up further perspectives in solid-state quantum information. In this talk, we present theory and calculations for coherent high-fidelity quantum control of many-particle states in semiconductor quantum wells. We have shown that coupling a two-electron double quantum dot to a terahertz optical source enables targeted excitations that are one to two orders of magnitude faster and significantly more accurate than those obtained with electric gates. The optical fields subject to realistic physical constraints are obtained through quantum optimal control theory that is applied in conjunction with the numerically exact solution of the time-dependent Schrodinger equation. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U27.00012: Persistent Quantum Beats and Long-Distance Entanglement from Waveguide-Mediated Interactions Huaixiu Zheng, Harold U. Baranger We study photon-photon correlations and entanglement generation in a one-dimensional waveguide coupled to two qubits with an arbitrary spatial separation [1]. Such a system can be realized by coupling a 1D open transmission line to superconducting qubits. To treat the combination of nonlinear elements and 1D continuum, we develop a novel Green function method. The vacuum-mediated qubit-qubit interactions cause quantum beats to appear in the second-order correlation function. We go beyond the Markovian regime and observe that such quantum beats persist much longer than the qubit lifetime. A high degree of long-distance entanglement can be generated, increasing the potential of waveguide-QED systems for scalable quantum networking. [1] H. Zheng, and H. U. Baranger, arXiv:1206.4442 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U27.00013: PPLN Device Characterization and Novel Entanglement Schemes Sean Krupa, Eric Stinaff, David Nippa, Lee Oesterling Bright sources of entangled photons are of great interest in the quantum information community, and the non-linear optical process of Spontaneous Parametric Downconversion (SPDC) is a well-known means to create entangled photons. Additionally, periodic polling has emerged as a viable choice for quasi-phase matching the downconverted photons rendering them useful for experimentation. Periodically Poled Lithium Niobate (PPLN) is among the best choices for these materials as it optically robust, temperature tunable, and commercially available. The addition of waveguide structures in PPLN devices not only increase its viability as a source of entangled photons but can also become an integral part of the entanglement schemes as well. Thorough characterization of PPLN devices is essential for the optimization of SPDC and their use to create entangled states. We will report characterization results for wave-guided PPLN devices including: waveguide geometry, fiber coupling efficiency, polling period details, and downconversion efficiency. Of particular interest is our device's ability to be used for novel entanglement states involving one or more waveguides. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U27.00014: Measurement of the joint spectrum of entangled photons using rotary dispersion Daniel Jones, Todd Pittman We report a new method of observing the spectral entanglement of photons generated in spontaneous parametric down-conversion (PDC). In contrast to previous methods based on spatial or temporal dispersion, our method is based on rotary dispersion and polarization measurements. Our experiment utilizes a variation of the S\'{e}narmont compensator in order to rotate the polarization state of the entangled signal and idler photons. By passing the photons through several stages of these ``rotators,'' we essentially create a Lyot filter in which we can directly correlate an analyzer measurement after the rotators with a specific wavelength, within a resolution defined by the theory. This method is fundamentally different than previous experiments to measure the joint spectrum of PDC photons because of the periodicity of using analyzers as the measurement devices. The periodicity of the analyzers causes a trade-off between the resolution of the device and the maximum bandwidth of the entangled photons that can be measured. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U27.00015: Realistic loophole-free Bell test with atom-photon entanglement Colin Teo, Mateus Ara\'{u}jo, Marco Quintino, Ji\v{r}\'{I} Min\'{a}\v{r}, Daniel Cavalcanti, Valerio Scarani, Marcelo Terra Cunha, Marcelo Santos The establishment of nonlocal correlations, guaranteed through the violation of a Bell inequality, is not only important from a fundamental point of view, but constitutes the basis for device-independent quantum information technologies. Although several nonlocality tests have been performed so far, all of them suffered from either the locality or the detection loopholes. Recent studies have suggested that the use of atom-photon entanglement can lead to Bell inequality violations with moderate transmission and detection efficiencies. In this paper we propose an experimental setup realizing a simple atom-photon entangled state that, under realistic experimental parameters available to date, achieves a significant violation of the Clauser-Horn-Shimony-Holt Bell inequality. Most importantly, the violation remains when considering typical detection efficiencies and losses due to required propagation distances. [Preview Abstract] |
Session U28: Focus Session: Tunable Materials
Sponsoring Units: GSNPChair: Jongmin Shim, University at Buffalo, The State University of New York
Room: 336
Thursday, March 21, 2013 11:15AM - 11:27AM |
U28.00001: Bukliball and Beyond: 3-D Soft Auxetic Metamaterials Jongmin Shim, Sahab Babaee, James C. Weaver, Nikita Patel, Elizabeth R. Chen, Katia Bertoldi We present a new class of 3-D soft metamaterials whose microstructure can be dramatically changed in response to mechanical loading. Patterned spherical shells, the Buckliballs (PNAS 109(16):5978) which undergo undergoing a buckling-induced structural transformation under pressure, are employed as building blocks, and are assembled to construct 3-D super-structures. We present procedures to guide the selection of both the building blocks and their arrangement, and design materials with tunable 3-D auxetic behavior that exploit buckling as the actuation mechanism. The validity of the proposed material design is demonstrated through both experiments and finite element simulations. This pattern transformation induced by a mechanical instability opens the possibility for fabrication of 3-D auxetic materials/structures over a wide range of length scales. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U28.00002: Grayscale gel lithography: From umlti-strips to responsive origami Myunghwan Byun, Ryan Hayward, Christian Santangelo Non-uniform swelling of hydrogel sheets with two-dimensional (2D) patterns of crosslink density has the potential to yield a rich array of three-dimensional (3D) structures, yet many of the design rules remain poorly understood. Here, we study the geometrically simple case of ``multi-strips'', consisting of alternating parallel strips of high and low crosslink density. These materials are patterned using sequential UV exposure of a photo-crosslinkable polymer film through two photomasks. We show that these materials deform by rolling around the axis perpendicular to the interface between the regions, with a characteristic dimension that depends on the strip width and sheet thickness. However, beyond a critical minimum strip width, the material remains flat, instead forming an anisotropically swelled state that provides fruitful information on the contrast in modulus between the two regions. We also consider the deformation of sheets patterned with multiple regions that define geometrically incompatible rolling axis. Finally, we discuss the formation of hinges based on symmetric tri-strips that can be used to defined fold patterns, yielding responsive gel origami structures. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U28.00003: Materials with Tailored Thermal Expansion Coefficient Katia Bertoldi, Jia Liu, Sicong Shan, Sung Hoon Kang Designing materials with tailored coefficient of thermal expansion (CTE) has applications in a number of fields, including biomedical and mechanical engineering and solar energy. It is particularly important to combine a desired (usually low) CTE with mechanical robustness. Most of previous work has been focused on designing low-CTE materials by modifying compounds at the chemical level. It is also possible to design materials with tailored CTE by using specific topologies of different materials to achieve overall properties outside the range of the constituent materials. Here, we exploit buckling in laminated periodic structures to design materials whose coefficient of thermal expansion can be tuned (from positive to negative) by varying the unit cell geometry. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U28.00004: Buckligami: Actuation of soft structures through mechanical instabilities Arnaud Lazarus, Pedro Reis We present a novel mechanism for actuating soft structures, that is triggered through buckling. Our elastomeric samples are rapid-prototyped using digital fabrication and comprise of a cylindrical shell patterned with an array of voids, each of which is covered by a thin membrane. Decreasing the internal pressure of the structure induces local buckling of the ligaments of the pattern, resulting in controllable folding of the global structure. Using rigid inclusions to plug the voids in specific geometric arrangements allows us to excite a variety of different fundamental motions of the cylindrical shell, including flexure and twist. We refer to this new mechanism of buckling-induced folding as ``buckligami.'' Given that geometry, elasticity and buckling are the underlying ingredients of this local folding mechanism, the global actuation is scalable, reversible and repeatable. Characterization and rationalization of our experiments provide crucial fundamental understanding to aid the design of new scale-independent actuators, with potential implications in the field of soft robotics. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:39PM |
U28.00005: Shaping and morphing three dimensional structures using thin film stress Invited Speaker: David Gracias The spatial patterning and stimuli-responsive manipulation of mechanical stresses within thin films can be used to self-assemble static and reconfigurable materials and devices. I will discuss the utilization of stresses associated with the minimization of surface tension, the relaxation of polycrystalline films, and the differential cross-linking of polymers and hydrogels to realize assembly and reversible actuation of functional structures of importance in electronics, optics and medicine. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U28.00006: Tunable phononic crystals through dielectric elastomers David Henann, Katia Bertoldi Phononic crystals are periodic materials that display phononic band gaps -- frequency ranges in which elastic waves are prohibited. Through deformation of the periodic structure the frequency ranges of band gaps may be adjusted or new band gaps may be created. Phononic materials made from elastomers enable large reversible deformation and, as a result, significant tunability of the phononic properties. Dielectric elastomers may be used in phononic crystals, in which deformation is actuated through the application of an electrical voltage, opening the door for easily tunable phononic crystals. In order to realize these exciting capabilities, robust simulation and design tools are needed. We have developed finite-element technology to address this problem and have applied these tools to designing phononic crystals with band gaps tuned through the application of voltage. The key ingredients of our finite-element tools are (i) the incorporation of electro-mechanical coupling, (ii) large-deformation capability, and (iii) an accounting for inertial effects. We present an application of our simulation capability to the design of a phononic crystal consisting of a square array of circular-cross-section threads embedded in a dielectric elastomeric matrix. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U28.00007: Tunable Mechanical Response in Biholar Elastic Media Bastiaan Florijn, Henk Imthorn, Martin van Hecke We probe the mechanics of 2D and 3D elastic media that are structured with arrays of holes of two different sizes. Hole size ratio plays a crucial role for the mechanical response - allowing to tune the Poisson ratio and qualitative response of the material under uniaxial loading. Biaxial and triaxial loading of these biholar structures leads to a wealth of new phenomena, including mechanically switchable hysteresis and memory effects. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U28.00008: Coupling geometrical frustration with mechanical instabilities to design surfaces with three dynamically changing states Sung Kang, Sicong Shan, Katia Bertoldi The interplay between mechanical instabilities and non-linear deformation in soft, porous structures give us the exciting opportunities to design materials that can suddenly change from one shape to another in response to an external stimulus. Based on this approach, there have been an increasing number of studies demonstrating reversible pattern formation between two states. Inspired by triple-shape-memory polymers [1], here we show a new mechanism to generate three-state ordered pattern formation using athermal process by exploiting buckling and geometrical frustration of cellular structures. Our new approach allows dynamical switching among three successive states simply by varying the external stimuli. Moreover, our scale-independent mechanism based on geometry and mechanical instability can provide a unique opportunity for studying dynamics of complex pattern formation with tunable surface properties. Reference: [1] I. Belin, S. Kelch, R. Langer, and A. Lendlein, Proc. Natl. Acad. Sci. USA, 103, 18043-18047 (2006). [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U28.00009: Shape transformations in liquid crystal elastomers with complex microstructure Vianney Gimenez-Pinto, Jonathan Selinger, Robin Selinger Recent experimental and theoretical studies have reported thermal-induced shape transformations in nematic liquid crystal elastomer (LCE) sheets with a complex director field. Director twist across the film thickness induces formation of twisted or curled structures whose chiral sense switches with temperature [1]. Using finite element simulations, we explore more complex director geometries that produce a variety of different actuation behaviors. We explore films containing a $+$1 topological defect with radial or azimuthal director alignment; and stripes and checkerboard patterns of twisted domains. We compare our results with recent experimental studies by D. Broer and coworkers and theoretical work by Modes and Warner. These results demonstrate the potential for application of LCE materials as mechanical actuators. [1] Y. Sawa, F. Ye, K. Urayama,~ T. Takigawa, V. Gimenez-Pinto, R. L. B. Selinger, and J. V. Selinger, PNAS 108, 6364 (2011). [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U28.00010: Soft 3-D Phononic Crystals: Design and engineering of the band-gap and propagation directionality Pai Wang, Sahab Babaee, Jongmin Shim, Katia Bertoldi We present a new class of 3-D bi-continuous soft phononic crystals. Different solid-fluid inter-penetrating periodic micro-structures are proposed for the geometric configurations. Buckling and large deformation of the meta-material is intentionally exploited as a novel and very simple approach to tune and transform the phononic band gaps as well as the preferential propagation directions of acoustic and elastic waves. The nonlinear effects of both geometry and material behavior during the deformation are investigated. The dispersion relations of deformed phononic crystals are calculated by using frequency domain numerical simulations on the unit cell of spatial periodicity. The characteristics of soft phononic crystals are demonstrated with tunable band-gaps, adjustable directionality and adaptive refractive index. This study provides us with a deeper understanding of the design parameters and engineering guidelines for various potential applications, including sound filters in noise-cancelling devices, wave guides, acoustic imaging equipment and vibration isolators. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U28.00011: Multifunctional Applications of Nanostructured Mechanical Metamaterials Lifeng Wang Mechanical metamaterials have been shown to possess extraordinary properties, and thus have been of great interest to mathematicians, physical scientists, material scientists, and biologists. A large part of the study of materials science is to obtain new structure-property-function relationships needed for achieving optimized mechanical properties. Here, we demonstrate the potential to design and fabricate periodically ordered structures. These structures are shown to have a unique combination of stiffness, strength, and energy absorption, as well as damage tolerance. The results provide guidelines to advance the digital design (materials by design) and manufacturing concepts (advanced manufacturing) into the realm of engineered materials with desired properties and further to create multifunctional materials. For example, the periodic nature of the structures enables mechanically tunable band gap (phononic or photonic) materials, and tunable sensors in tissue engineering. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U28.00012: Fluid-structure Interactions for the Design of Adaptive Acoustic Metamaterials Filippo Casadei, Katia Bertoldi The present research focuses on the analysis of fluid-structure interactions as a new paradigm for the design of adaptive phononic crystals and acoustic metamaterials. Whereas in conventional design procedures couplings between structures and fluids represent a source of concern due to the possible onset of catastrophic instabilities, in this research such interactions are exploited as the enabling mechanism for mechanical adaptation. Analytical and numerical models illustrate how such interactions can be exploited for the design of periodic structures with wave propagation properties that can be controlled by the surrounding fluid environment. Analysis of the dispersion relations computed for one-dimensional phononic crystals and acoustic metamaterials show that the location of frequency bandgaps is directly correlated to the conditions of the external fluid flow. Direct simulations of assemblies of finite size and preliminary experimental results are presented to further illustrate the concept. [Preview Abstract] |
Session U29: Focus Session: Jamming: Marginal Solids II
Sponsoring Units: GSNPChair: Joshua Dijksman, Duke University
Room: 337
Thursday, March 21, 2013 11:15AM - 11:51AM |
U29.00001: Shear jamming in granular materials Invited Speaker: Jie Zhang For frictionless particles with purely repulsive interactions, there is a critical packing fraction $\phi_J$ below which no jammed states exist. Frictional granular particles in the regime of $\phi < \phi_J$ act differently under shear: early experiments by Zhang \& Behringer at Duke University show jammed states can be created by the application of shear stress. Compared to the states above $\phi_J$, the shear-jammed states (SJS) are mechanically more fragile, but they can resist shear. Formation of these states requires the anisotropic contact network as a backbone and these new states must be incorporated into a more general jamming picture (Bi et al Nature 2011). If time permits, I will present some new results from recent experiments at SJTU aimed towards understanding the more detailed nature of SJS and the transition from unjammed states to SJS. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U29.00002: Compression and Shear Driven Jamming of Frictionless U-Shaped Particles in Two Dimensions Theodore Marschall, Andrew Loheac, Scott Franklin, Stephen Teitel We simulate a system of soft, frictionless, U-shaped particles (staples), under both isotropic compression and uniform shear flow in two dimensions. The shape of the particles allows them to interlock, causing a geometry induced particle cohesion. We investigate the jamming transition of this system as the packing fraction is increased, in an effort to learn whether such geometric cohesion in novel shaped frictionless particles can produce effects similar to what is found for frictional smooth disks. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U29.00003: Shear Reversibility in Model Granular Systems Carl Schreck, Rob Hoy, Mark Shattuck, Corey O'Hern Athermal particulate systems such as foams and granular media are out-of-thermal equilibrium and therefore must be externally driven using shear or vibration to explore different configurations. Of particular interest is being able to predict and control the structural and mechanical properties of athermal systems as a function of the driving mechanism. In this work, we show numerically how particle collisions in cyclically sheared hard sphere systems can lead to microreversibility. We map out the steady-state ``phase diagram'' as a function of packing fraction ($\phi$) and strain amplitude ($\gamma_{max}$), and identify ``point-reversible'' states at low $\phi$ and $\gamma_{max}$ in which particles do not collide over the course of a shear cycle, and ``loop-reversible'' states at intermediate $\phi$ and $\gamma_{max}$ in which particles undergo numerous collisions but return to their initial positions at the end of each shear cycle. Loop-reversiblity is a novel form of self organization that gives rise to non-fluctuating dynamical states over a broad range of packing fractions from contact percolation to jamming, i.e. $\phi_P=0.55$ to $\phi_J=0.84$ in two dimensions. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U29.00004: Stress dynamics of a 2D dense granular system near shear jamming Jie Ren, Joshua Dijksman, Robert Behringer We study the dynamics of pressure and shear stress in a frictional 2D dense granular system using a novel apparatus that can provide fixed-volume shear without generating inhomogeneities. Under increasing shear strain, the system's pressure shows a strong increase with strain, characterized by a ``Reynolds coefficient,'' $R = d^2 P / d \gamma ^2$. R depends only on packing fraction $\phi$, and shows a strong increase as $\phi$ approaches $\phi_J$ from below. In the meantime, the system's shear stress shows a non-monotonic behavior with increasing strain. It first increases with strain as the system is in ``fragile'' states and builds up long force chains along the compression direction. After a certain amount of strain, force chains along the dilation direction starts to build up, and the system transfers into a ``shear-jammed'' state and the shear stress starts to decrease with strain. Under oscillatory shear, both pressure and shear stress show limit-cycle behavior and reach steady states after many cycles. However, the limit cycles of pressure and shear stress are very different: the pressure exhibits a hysteresis-free parabolic curve, while the shear stress exhibits a strongly hysteretic loop. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U29.00005: Shear jamming in a two dimensional granular system without basal friction Hu Zheng, Joshua Dijksman, Robert Behringer Two dimensional granular systems are an important tool to explore the dynamics of granular materials. However, traditional experimental methods could not avoid the effects of friction between particles and the base on which they rest. Here, we develop a novel apparatus which allows us to tune the basal friction of the particles. We do so by submersing the particles in a density matched liquid, thus removing the normal force, hence the friction, between the particles and base. We use this technique to investigate the effect of shear jamming found by Bi et. al. (2011) by probing the overall shear stress, particle motion and the photoelastic response of the particles under simple shear. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U29.00006: Bagnold and linear scalings in shearing simulations of massive particles Daniel V{\aa}gberg, Peter Olsson, S. Teitel We consider the rheology of massive bidisperse soft-core discs in two dimensions driven by a constant shear rate $\dot\gamma$ at zero temperature. We study how the behavior depends on the details of the dynamics, by investigating three different models for the energy dissipation. In these models the dissipation from two colliding particles are proportional to (1) the total velocity difference, (2) the normal component of the velocity difference, (3) the tangential component of the velocity difference, respectively. It turns out that these seemingly minor differences have major implications for the scaling of the pressure $p$ with respect to $\dot\gamma$. The system can exhibit linear scaling, $p\sim\dot\gamma$, or Bagnold scaling, $p\sim\dot\gamma^2$, depending on the details of the dissipation used. It is found that the onset of linear scaling is related to the appearance of force chains spanning the system. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U29.00007: Relaxation time, viscosity and scaling at densities below jamming Peter Olsson We simulate soft-core bidisperse frictionless disks in two dimensions with overdamped dynamics at zero temperature and densities below jamming. We first prepare configurations by shearing at several constant shear rates $\dot\gamma$. These configurations are then used as starting points for simulations \emph{without} shearing that relax the system to zero energy. From these simulations we determine both the relaxation time, $\tau$, and the average path length traversed by the particles to reach the zero energy state. We find that $\tau$ diverges algebraically as a function of density, $\tau \sim (\phi_J-\phi)^{-\beta}$, if $\dot\gamma$ in the preparatory simulations is sufficiently small. We further find that the shear viscosity $\eta$ can be formally related to $\tau$, and that this gives a way to understand the origin of corrections to scaling in the scaling analysis of $\eta$[1]. The presence of the exponent $\beta+y$, where $y\approx 1.1$, in the scaling of the deviations from the $\dot\gamma\to0$ limit, $\eta(\phi,\dot\gamma)/ \eta(\phi,\dot\gamma\to0) = f((\phi_J-\phi)^{-(\beta+y)}\dot\gamma)$ [1], is also given an intuitive interpretation.\newline [1] P. Olsson and S. Teitel, Phys.\ Rev.\ E \textbf{83}, 030302(R), 2011. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U29.00008: Shear shocks in fragile matter Vincenzo Vitelli, Stephan Ulrich, Nitin Upadhyaya Random media, like polymer networks, covalent network glasses, or grains under pressure can be viewed as elastic networks composed of springs and balls. The shear moduli of these types of materials typically vanish as the network connectivity $z$ approaches a critical value. In this talk, I show that shear strains propagate as diffusive fronts, whose {\bf width diverges} and whose {\bf transverse speed of sound vanishes}, as the transition is approached. Consequently, in this regime, linear theory breaks down, giving rise to {\bf nonlinear transverse waves}. Comparison of the analytical front profile to molecular dynamics simulations allows the extraction of the material constants of the network. Interestingly, even an {\bf undamped network} yields a {\bf diverging effective viscosity} caused by leaking of energy into non-affine degrees of freedom. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U29.00009: Soft particle packings near jamming: correlations in static structure Kamran Karimi, Craig Maloney We extend our previous results report on 2D simulations of soft harmonic packings at various area fractions $\varphi $ above the jamming point $\varphi_{\mathrm{c}}$. We employ several statistical analyses to determine whether one or more characteristic lengths can be associated with either the quenched stress field in the packing or the structure of local elastic moduli. First, we define a locally anisotropic variant of the standard two-point correlation function. This anisotropic correlation function follows a power law even in globally isotropic stress states with a $\varphi $ independent exponent and no discernible cutoff within the statistically accessible regime. Secondly, we define a coarse-grained stress field on a scale R. The average anisotropic component and the fluctuations in the trace can both be collapsed onto similar master curves after rescaling R by a characteristic length scale $\xi $. $\xi $ accelerates as $\varphi $ approaches $\varphi_{\mathrm{c}}$, consistent with a divergence at $\varphi_{\mathrm{c}}$. Surprisingly, a similar analysis on the local coarse-grained elastic modulus tensor shows a non-trivial power-law scaling behavior as a function of the coarse-graining size yet no characteristic $\xi $ as exhibited by the stress. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U29.00010: Elastic modulus of solid-like microsphere heaps Carlos Ortiz, Karen Daniels, Robert Riehn We study the elastic modulus of heaps of repulsive microspheres to gain insight into the nature of the rigidity of the material. The heaps are initially created by flowing a colloidal microsphere suspension towards a flat-topped ridge placed within a quasi two-dimensional microfluidic channel. The suspension flow-rate determines the heap size via the angle of repose. Using fluorescence video microscopy, we measure the fluorescent heap size until it reaches steady state. We directly visualize the elastic recoil of these steady state heaps in response to controlled changes in the fluid flow rate. We change the flow rate by an amount $\Delta v$ in a step-like fashion, and measure the amplitude of the bulk heap deformation $\Delta A$. We investigate both compressions and decompressions of varying amplitudes with respect to the steady state. Three deformation regimes are observed. No deformations are observed below a critical perturbation magnitude $\Delta v_c$. Above $\Delta v_c$, deformation amplitudes are linear with $\Delta v$. However, for large perturbations, nonlinear deformation amplitudes are observed, and their relationship is asymmetric with respect to compression and decompression. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U29.00011: Investigating the stability of jammed systems with respect to generalized boundary deformations Samuel Schoenholz, Carl Goodrich, Oleg Kogan, Andrea Liu, Sidney Nagel At zero temperature and applied stress, amorphous packings of spheres exhibit a jamming transition as a function of packing fraction. Above the jamming transition, systems of repulsive spheres have a nonzero bulk moduli. However, some jammed states prepared with periodic boundary conditions are unstable to shear. These instabilities motivate several questions: How does the fraction of systems that exhibit instabilities scale with packing fraction and system size? Are there other classes of boundary deformations with respect to which jammed packings could be unstable, and if so, how can they be explored? We answer these questions by considering each finite packing with periodic boundary conditions in $d$ dimensions as the basis of an infinite hypercubic lattice. We study the properties of modes that do not respect the periodicity of the initial system and thereby characterize the linear response to a large class of boundary deformations. In this way we systematically explore the effects of system size and packing fraction on stability with respect to these boundary deformations, and show that our results can be understood in terms of competition between plane waves and anomalous vibrational modes associated with the jamming transition. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U29.00012: Strong reduction of the rigidity of repulsive contact systems at vanishingly low temperatures Hajime Yoshino, Satoshi Okamura Contrarily to ordinary solids, the amorphous solid states of repulsive contact systems such as colloids and emulsions may not be regarded simply as harmonic states \footnote{C. F. Schreck,T. Bertrand, C. S. O'Hern, and M. D. Shattuck, Phys. Rev. Lett. 107, 078301 (2011).}. We studied the rigidity, i.~e. the shear-modulus of such a class of systems at vanishingly low but finite temperatures using the cloned liquid approach \footnote{H. Yoshino and M. M\'{e}zard, Phys. Rev. Lett. {\bf 105}, 015504 (2010), H. Yoshino, J. Chem. Phys. {\bf 136}, 214108 (2012), H. Yoshino, arXiv:1210.6826 (2012).} and molecular dynamic simulations. Our result implies breakdown of the commutation of the thermodynamic limit $N \to \infty$ and zero temperature limit $T \to 0$ for the response to shear: we found the rigidity in the limit $T \to 0$ is significantly smaller and exhibit a different scaling compared with that at $T=0$. Interestingly the rigidity in the limit $T \to 0$ exhibits the same scaling as the pressure, as observed experimentally in emulsions\footnote{T. G. Mason, J. Bibette and D. A. Weitz, Phys Rev. Lett. {\bf 75}, 2051 (1995)}. Detailed numerical examination suggests that the strong stress relaxation is due to contact opening events activated at vanishingly small temperatures. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U29.00013: Mechanical instability at finite temperature Xiaoming Mao, Carlos I. Mendoza, Anton Souslov, Tom C. Lubensky Rigidity transitions have been well studied in a wide range of athermal systems such as jammed packings and diluted lattices, in which the balance between the number of degrees of freedom and constraints generally determines the onset of mechanical instability, as predicted by Maxwell. The effects of thermal fluctuations on these transitions, however, have not yet been systematically studied. Characterizing rigidity transitions at finite temperature is very important to the understanding of fundamental problems such as the relation between the glass transition and jamming. We report an analytic study of a finite-temperature rigidity transition in the square lattice. At zero temperature, this lattice exhibits a continuous transition between the square phase and a phase composed of rhombic cells as the nonlinear potential connecting next-nearest-neighbors vary. At nonzero-temperature, diverging vibrational entropy associated with the floppy modes play a very important role in selecting the phase and determining the order of the transition. We calculate the phase diagram of this system and identify interesting behaviors such as negative thermal expansion. [Preview Abstract] |
Session U30: Glassy Materials: Colloids, Traffic, Disordered Crystals, Etc.
Sponsoring Units: DCMPChair: Peter Schall, University of Amsterdam
Room: 338
Thursday, March 21, 2013 11:15AM - 11:27AM |
U30.00001: Criticality in dynamic arrest: Correspondence between glasses and traffic Daniel Miedema, Astrid De Wijn, Bernard Nienhuis, Peter Schall Dynamic arrest is a general phenomenon across a wide range of dynamic systems including glasses, traffic flow, and dynamics in cells, but the universality of dynamic arrest phenomena remains unclear. We connect the emergence of traffic jams in traffic flow to the dynamic slow down in glasses. A direct correspondence is established by identifying a simple traffic model as a kinetically constrained model. In kinetically constrained models, the formation of glass becomes a (singular) phase transition in the zero temperature limit. Similarly, using the Nagel-Schreckenberg model to simulate traffic flow, we show that the emergence of jammed traffic acquires the signature of a sharp transition in the deterministic limit, corresponding to overcautious driving. We identify a true dynamical critical point marking the onset of coexistence between free flowing and jammed traffic, and demonstrate its analogy to the kinetically constrained glass models. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U30.00002: Glass-like dynamics of a structural colloidal crystal in a disordered potential landscape Kevin Aptowicz, Tim Still, Matthew Gratale, Ye Xu, Arjun Yodh Disordered solids exhibit a boson peak at low frequencies, where many more modes appear than is expected for sound modes behavior. The origin of the boson peak remains unclear, although two explanations have risen to the forefront: (i) the boson peak is composed of quasi-localized modes arising from peculiarities of the interatomic forces in amorphous materials and (ii) the boson peak is the amorphous equivalent of the Van Hove singularity in crystalline systems. We experimentally explore these two possible explanations by studying a quasi-two-dimensional colloidal structural crystal residing in a disordered potential landscape. The potential landscape is generated by non-uniform heating of the sample. Thermophoretic effects lead to a heterogeneous force distribution that is tunable with temperature. With this experimental geometry, we explore the evolution of the density of vibrational states as a function of the strength of the disorder potential landscape. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U30.00003: Correlations Between Structure, Vibrational Modes and Collective Motion in Dense Attractive 2D Colloidal Packings Matthew Lohr, Tim Still, Kevin Aptowicz, Ye Xu, Matthew Gratale, 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 (\textgreater 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. Despite similarities in local structure, we find several consistent differences in the dynamic and vibrational properties of these two extreme systems. Specifically, we observe a drastic change of the presence of low-frequency modes between the two states. These modes appear to be coupled to collective motion of large groups of particles. By investigating the correlation between these collective motions and local packing structures, we gain further insight into the origins of dynamic heterogeneity in disordered systems. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U30.00004: Resolving structural modifications of colloidal glasses by combining x-ray scattering and rheology Dmitry Denisov, Triet Dang, Bernd Struth, Gerard Wegdam, Peter Schall Glasses have liquid-like structure, but exhibit solid-like properties. A central question concerns the relation between the structure and mechanical properties of glasses, but structural changes remain difficult to resolve. We use a novel combination of rheology and x-ray scattering to resolve structural changes in colloidal glasses and link them directly to their mechanical behavior. By combining stress and structure factor measurements, we resolve shear induced changes in the nearest neighbor configuration as a function of applied stress, allowing us to elucidate the structural origin of the genuine shear banding transition of glasses. Our results reveal a coupling between structural parameters and the applied shear that underlies this instability: the non-monotonic behavior of the flow curve is directly mirrored in simple structural measures such as the position, the width, and the height of the nearest neighbor peak of the structure factor. Besides small changes in the nearest neighbor distances, our results underscore the importance of anisotropy in the structure of out of-equilibrium systems, in agreement with structure analysis of jammed and unjammed granular packings. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U30.00005: Free energy transition of sheared colloidal glasses Minh Triet Dang, Rojman Zargar, Daniel Bonn, Peter Schall Glasses have liquid-like structure, but solid-like properties. An important question concerns the relation between the macroscopic flow behavior and the microscopic structure. However, for atomic glasses, microscopic configurations are prohibitively difficult to visualize due to the small molecular length scales. Here, we use a colloidal glass to directly visualize and analyze particle configurations of quiescent and sheared colloidal glasses. We determine the free volumes of the particles, and relate this free volume distribution directly to the free energy of the glass. This allows us to obtain novel insight into the relation between rigidity/flow and changes in the amorphous structure. We identify a clear change in the free energy at the transition from homogenous to inhomogenous flow. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U30.00006: Phonon Dispersion and Elastic Properties of Two-Dimensional Soft Particle Colloidal Crystals and Glasses Tim Still, Ke Chen, Peter J. Yunker, Carl P. Goodrich, Samuel Schoenholz, Andrea J. Liu, A.G. Yodh We investigate phonon dispersion relations and associated mechanical properties of two-dimensional colloidal glasses and crystals composed of soft, thermoresponsive microgel particles whose temperature-sensitive size facilitates in-situ variation of particle packing fraction. The phonon modes were measured using particle tracking and displacement covariance matrix techniques. Measurements of the hexagonal crystal served to check our methodology and, as expected, the observed phonon dispersion was largely in agreement with theoretical expectations. Measurements of phonon dispersion in the glassy colloids, as a function of packing fraction above the jamming transition, permitted study of the scaling of bulk and shear moduli as a function of packing fraction. We performed numerical simulations and were able to recover the experimental findings. Moreover, the obtained shear moduli are in good agreement with rheological measurements. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U30.00007: Aggregation, Gelation and Glass Transition in Mixed Suspension of Polystyrene Microsphere and Poly(N-isopropyl-acrylamide) Microgel Guangcui Yuan, Chuanzhuang Zhao, Charles C. Han Poly(N-isopropylacrylamide) microgel is adsorbable to the polystyrene microsphere surface. The saturated adsorption concentration of microgel ($\Phi *_{\mathrm{MG}})$ is in a linear relationship with the given concentration of microsphere ($\Phi_{\mathrm{MS}})$. Depending on the concentration of microgel ($\Phi_{\mathrm{MG}})$ added into the suspension microspheres, the microgel can induce bridging ($\Phi _{\mathrm{MG}}$ \textless $\Phi *_{\mathrm{MG}})$, stabilizing ($\Phi _{\mathrm{MG}} = \Phi *_{\mathrm{MG}})$ and depletion ($\Phi _{\mathrm{MG}}$ \textgreater $\Phi *_{\mathrm{MG}})$ effect. With combination of various $\Phi_{\mathrm{MS}}$ and $\Phi _{\mathrm{MG}}$/$\Phi *_{\mathrm{MG}}$, different structures including stable solution, bridging and depletion cluster, bridging and depletion gel, attractive glass and repulsive glass, were obtained. The transitions between these states were investigated by rheology and microscopy. Two-step yielding behavior was observed in attractive glass, which was contributed from bridging bonds of microgels and caging effect of dense microspheres. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U30.00008: Dynamics of concentrated dicolloid particles Mark M. Panczyk, Norman J. Wagner, Eric M. Furst Nonspherical colloidal particles exhibit a variety of equilibrium structures, including colloidal crystals. However, with increasing concentration, particle dynamics in these suspensions slow, and the creation of equilibrium close-packed structures may be ultimately inhibited by the presence of a glass transition. For dicolloid particles, dimer particles with asymmetric or symmetric lobes, suspension dynamics have been studied using Stokesian dynamics simulations [1] and mode-coupling theory [2], and the glass transitions have been determined using rheology [3]. In this study, the dynamics of polystyrene dicolloids in water are measured by diffusing wave spectroscopy (DWS) at particle concentrations between 1 and 60 volume percent. Relaxation times of the dicolloid particle suspensions are determined as a function of particle concentration and shape. Strong particle localization occurs at the highest concentrations. The localization lengths measured by DWS are compared to their mode coupling theory predictions.\\[4pt] [1] Kumar A, Higdon JJL\textit{. J. Fluid. Mech}. \textbf{2011}, \textit{675, 297-335.}\\[0pt] [2] Zhang R, Schweizer KS. \textit{J. Chem. Phys. }\textbf{\textit{2010}}\textit{, 133 104902.}\\[0pt] [3] Kramb R.C. et al., \textit{J. Phys.: Condens. Matter. }\textbf{\textit{2011, }}\textit{23, 035102} [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U30.00009: On the Absence of Red Structural Color in Colloidal Glasses Sofia Magkiriadou, Jin-Gyu Park, Young-Seok Kim, Gi-Ra Yi, Vinothan N. Manoharan When a colloidal glass is illuminated, the short-ranged spatial correlations between neighboring particles can give rise to constructive interference for a particular wavelength. Unlike the structural colors arising from Bragg diffraction in colloidal crystals, the colors of these colloidal glasses are independent of angle due to the disordered, isotropic microstructure. We therefore call them ``photonic glasses.'' A similar coloration mechanism is found in the feathers of certain birds. However, there are few examples of red photonic glasses either in nature or in colloidal systems. Using scattering theory, we show that the absence of red photonic glasses can be explained by the wavelength-dependence of the single-particle scattering cross-section, which can override the interference condition set by the structure. We propose ways to overcome this obstacle, and we report on experimental methods to make non-iridescent, structural red color. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U30.00010: Structure/dynamics coupling in suspensions of microgel particles on their approach to the glass A. Fernandez-Nieves, J. Clara-Rahola, P.N. Segre, A.B. South, L.A. Lyon We measure the structure factor, S(q), and the q-dependent diffusion coefficient, D(q), of dense suspensions of pNIPAm microgel particles. We do this at different temperatures, and hence different swelling degrees, at constant generalized volume fraction, and find dramatic changes in behavior. While for certain temperatures, 1/D(q) follows the behavior of S(q), at other temperatures the behavior of these two quantities completely decouples. Interestingly, this behavior correlates with fragility: Structure/dynamics decoupling is observed for suspensions resembling strong glass formation. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U30.00011: Evolution of dynamical facilitation approaching the glass transition Raphael Candelier, Asaph Widmer-Cooper, David Reichman, Giulio Biroli, Olivier Dauchot We investigate the relaxation dynamics of simulated dense bidimensional supercooled liquids composed of softly interacting particles. We show that the long time scale dynamical heterogeneities result from the aggregation of several elementary relaxation events, themselves formed by collective leaps. By varying the temperature, we show that for low temperatures there is a growing excess of probability to find cage jumps that are close both in space and time, and that the network of spatio-temporal facilitation evolves towards a collection of clearly defined large events. We discuss these observations and specifically the relative importance of facilitation when approaching the glass transition. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U30.00012: Aging in dense colloids through the growth and breakup of strongly correlated clusters Skanda Vivek, Stefan Boettcher, Paolo Sibani Colloidal systems exhibit glassy behaviour under the right physical conditions that can be observed through mean square displacements in experiments. Our phenomenological model of aging in colloids is based on the growth and breakup of strongly correlated clusters, which introduces dynamical heterogeneity in the system.\footnote{Boettcher \& Sibani, J.Phys.CM \textbf{23}, 065103 (2011)} Particles move and associate into clusters that can break up with a probability that decreases with cluster size. Different colloidal density regimes correspond to different probabilities. The mean square displacements measured in this system for a low density colloid shows a linear increase in time and shows a linear increase in log-time for high densities, which matches experimental data. The cluster breakup rate was measured to be uniform in time for low densities and $\propto 1/t$ in the aging regime, which provides a clock for the slowing down of the dynamics. Measurements of the four-point susceptibility $\chi_4$ show a peak indicating the response to a growing lengthscale that satisfies a scaling relation with sample age, $t_w$. For larger $t_w$, $\chi_4$ peaks higher, and decays more slowly with time, which we hypothesize is due to the dominance of relatively stable large clusters. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U30.00013: Pinning Susceptibility at the Jamming Transition Amy Graves, Elliot Padgett, Carl Goodrich, Andrea Liu Jamming in the presence of fixed or pinned obstacles, representing quenched disorder, is a situation of both practical and theoretical interest. We study the jamming of soft, bidisperse discs in which a subset of discs are pinned while the remaining particles equilibrate around them at a given volume fraction. The obstacles provide a supporting structure for the jammed configuration which not only lowers the jamming threshold, $\phi_J$, but affects the coordination number and other parameters of interest as the critical point is approached. In the limit of low obstacle density, one can calculate a pinning susceptibility $\chi_P$, analogous to the magnetic susceptibility, with obstacle density playing the role of the magnetic field. The pinning susceptibility is thus expected to diverge in the thermodynamic limit as $\chi_P \propto |\phi-\phi_J|^{-\gamma_P}$. Finite-size scaling calculations allow us to confirm this and calculate the critical exponent, $\gamma_P$. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U30.00014: Probing the depinning transition: contrasting lattice and continuum models Yan-Jiun Chen, Stefano Zapperi, James P. Sethna Models of depinning are used to study a wide variety of disordered systems where there are interfaces with jerky motion, including magnetic domain wall motion, fluid imbibition, and superconductor vortex lines. Analytic results from field theories are written in continuous time and space coordinates; but efficient algorithms are often done with cellular automata (CA). The equivalence of CA rules with the continuum models were justified by the appearance of a cusp in the disorder correlator after a finite-number of RG steps, especially for avalanche behavior that involve many degrees of freedom. However, in between this abrupt behavior, there exist slower dynamics where the avalanche almost stops, involving fewer degrees of freedom, and these regions may alter the scaling, as seen in recent studies of plastic deformation in crystals and crackling noise in glasses. Also, in our simulations, we find that discretization may introduce unwanted effects or relevant perturbations, such as a broken rotational symmetry. We compare and contrast results of the spatial and temporal structure of depinning from lattice and continuum simulations, and also provide complete functional forms to describe crossovers between different model classes. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U30.00015: Dynamical Instabilities of a Brownian Particle in Weak Adhesion. Deepak Kumar, Shankar Ghosh, Shobo Bhattacharya Dynamical processes involved in weak adhesion are explored through a single cycle of an optically trapped Brownian colloidal silica particle detaching from, and reattaching to, a glass substrate immersed in a fluid in the presence of an externally applied force. Micro-rheology, video-microscopy and Nyquist noise measurements reveal both stochastic and deterministic dynamics of the process. When analyzed in terms of the viscoelastic response of the stress coupling medium between the objects, the unsticking instability shows remarkable similarities with yielding and fracture-mechanics of macro-scale solids. The resticking dynamics demonstrates stochastic instabilities through a spatio-temporally punctuated descent of the particle down an energy landscape with a hierarchy of metastable minima. [Preview Abstract] |
Session U31: Focus Session: Assembly & Function of Biomimetic & Bioinspired Materials III
Sponsoring Units: DMP DPOLY DBIOChair: Shengfeng Cheng, Sandia National Laboratories
Room: 339
Thursday, March 21, 2013 11:15AM - 11:27AM |
U31.00001: Multiscale self-assembly of DNA-functionalized nanoparticles and cationic phospholipids Sunita Srivastava, Dmytro Nykypanchuk, Oleg Gang Cationic phospholipids (CLs) when mixed with oppositely charged biomolecules exhibit rich structural diversity including lamellar, inverted hexagonal, honeycomb and rectangular columnar phases. Our study explores how CLs can be used to control the organization of nanoparticles (NP) and their ligands on molecular and nano scales by tuning lipid composition. We utilized a synchrotron-based x-ray scattering to probe in-situ electrostatic assembly of double stranded (ds) DNA-functionalized nanoparticles with cationic phospholipids. The assembly of the DNA-NP and CLs is driven by attraction between negatively charged ds-DNA and positively charged CLs. We investigated the role of DNA length, lipid charge density and charge ratio on structural behavior of the assembly. Interplay of electrostatic interaction and curvature effects results in hierarchical organizations in which DNA-NP and CLs exhibit lamellar and hexagonal phases at different length scales. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U31.00002: Revealing Structural Transformations during Crystallization of DNA-Nanoparticle Assemblies Yugang Zhang, Fang Lu, Daniel van der Lelie, Oleg Gang Nanoparticle assembly via sequence-specific DNA recognition emerges as a powerful strategy for the fabrication of nanoparticle (NP)-based crystalline materials. Generally, a delicate thermal annealing is essential for the crystallization of NPs from kinetically trapped disordered states. Due to the complex coupling between interactions, entropic and chain effects in these systems, the crystallization pathway remains an intricate and open question. Herein, we present an experimental study of the crystallization process for DNA-directed nanoparticle assembly systems using synchrotron-based small angle x-ray scattering (SAXS). We demonstrated the effects of two crystallization-dominant factors, namely, temperature and volume fraction, on the structural transformation and order development. By combining a single component and binary systems we uncovered the evolution of global and local particle arrangements, such as correlation length, compositional disorder and coordination number, during the phase transformation. Research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U31.00003: Modeling Lattice Structures of DNA-Coated Nanoparticles with Tetrahedral Linkers Joshua Neitzel, Oleg Gang, Francis Starr Much attention has recently focused on using DNA as a linking agent to engineer nanoparticle (NP) lattices with specific geometries. There has been success generating a broad range of crystal symmetries, but the formation of a tetrahedral or diamond lattice has been particularly challenging. We use molecular simulations to examine a combination of NP uniformly coated with DNA that connect via linking units that incorporate tetrahedral structure. We test the stability of spherical NP-DNA complexes with tetrahedral linkers in a 1:1 ratio, which allow for a variety of lattices, including a diamond structure. Previously postulated interpenetrating diamond lattices are also possible. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U31.00004: Directed assembly of hierarchical light-harvesting complexes using virus capsid scaffolds and DNA origami tiles Debin Wang, Stacy Capehart, Suchetan Pal, Minghui Liu, Jolene Lau, Hao Yan, Matthew Francis, Jim DeYoreo Directed assembly of nanostructures with molecular precision is of great importance to develop an insightful understanding of assembly pathways and dynamics as well as to derive new functionalities. In this work, we explore the use of virus capsids and DNA origami tiles as 3D scaffolds and 2D templates for directed assembly of light-harvesting molecules and plasmonic gold nanoparticles to achieve tunable photoemission. Bacteriophage MS2 virus capsids with well-defined spherical macromolecular structures are genetically modified to provide predictable steric arrangements of light-harvesting molecules. DNA origami tiles act as programmable planar templates to provide higher-order organization of oligonucleotide-functionalized light-harvesting capsids and plasmonic gold nanoparticles. The direct observation of distance dependent photoluminescence emission is carried out by our correlative approach combining atomic force microscopy and confocal fluorescence microscopy, which is in good agreement with our numerical simulation and theoretical calculation. This work will facilitate the construction of multicomponent biological-metal hybrid plasmonic nanostructures for nanophotonics and biosensing applications. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:39PM |
U31.00005: Shape Remodeling Assemblies in Biologically Inspired Materials Invited Speaker: Cyrus Safinya Much of our research is inspired by, and directed at, understanding the formation of novel structures (both relatively static and highly dynamic) with distinct shapes and morphologies observed in charged biological systems. The structures, in turn, often correlate to specific functions. For example, charged nanoscale tubules and rods and their assemblies are of interest in a range of applications, including as templates for hierarchical nanostructures, encapsulation systems, and biosensors. A series of studies will be described on charged biological assemblies exhibiting ``molecularly-triggered'' dynamical shape changes. In particular, we will focus on protein and lipid based nanotubule formation through small molecule stimuli-induced shape remodeling events. The systems include invertible protein nanotubes from two-state tubulin-protein building blocks and lipid nanotubes and nanorods from curvature stabilizing lipids (mimicking membrane curvature generating proteins). [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U31.00006: Visualizing DNA Nanoparticle Motion under Graphene Liquid Cell TEM Qian Chen, Jessica Smith, Jungwon Park, Somin Lee, Alex Zettl, Paul Alivisatos We think of a simple colloidal nanocrystal as one type of artificial atoms. They mutually interact, cluster into artificial molecules, and further arrange into macroscopically functional artificial solids. The ``atomic'' resolution dynamics of this bottom-up strategy in materials design is studied here in a system of artificial molecules composed of DNA and nanoparticle. The observation of dynamics in their liquid environment is recently enabled by graphene liquid cell transmission electron microscopy (TEM). In comparison to conventional TEM, wherein the assembled 3D artificial structures are dried out during sample preparation and thus are collapsed, this graphene liquid cell introduces a special local liquid structure that retains the conformations as well as the dynamics of the assemblies. In situ imaging of correlated motions of DNA and nanoparticle provides insights into the design principles of artificial nanocrystal molecules and solids linked by DNA. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U31.00007: Controlling Assembly and Crystallization of S-layers on Diblock Copolymer Patterns Ilja Gunkel, Magal{\'I} Lingenfelder, Bart Stel, Xiaodan Gu, Thomas Russell, James DeYoreo Block copolymers (BCPs) self-assemble into arrays of nanoscopic morphologies, including lamellar, cylindrical, and spherical microdomains, that serve as ideal templates for the fabrication of nanostructured materials. The size of the microdomains is a function of the polymer size so tuning the copolymer's molecular weight allows for a precise control over the dimension of the BCP morphologies. Moreover, the heterogeneous chemical nature of BCPs allows them to be used as templates for well-defined protein adsorption. Here, we used nanoscopic BCP patterns as templates to study the assembly of S-layer proteins SbpA from Lysinibacillus sphaericus (ATCC 4525) by in-situ Atomic Force Microscopy (AFM). The templates were formed by polystyrene-b-poly(ethylene oxide) BCPs of various molecular weights after spin coating on solid surfaces and subsequent controlled solvent-vapor annealing. Our results show that by controlling the chemical contrast in templates of different geometry and periodicity, protein assemblies could be directed exclusively to the hydrophobic domains of the template. More importantly, our high-resolution AFM measurements indicate that the proteins crystallized in their native lattice while following the structure of the underlying template by preferential adsorption. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U31.00008: Structural control in model microtubule self-assembly Shengfeng Cheng, Mark Stevens Being able to control the structure formed in self-assembly is the goal of many nanoscience studies. Here we explore various ways to control the structure of self-assembled tubules. We have previously developed a model wedge-shaped monomer that can self-assemble into tubule structures. We now add chirality and a lock-and-key mechanism to the model to enhance structural control of the self-assembly. Previously, we found that helical tubes are frequently formed despite the fact that chiral symmetry is not present in the monomer. We now identify the physical origin of helicity as the large overlap in the energy distributions between nonhelical and helical tubes. The helical tubes typically undergo a twist deformation that lowers the energy substantially. We find that a modification of the location of binding sites on the bottom and top surfaces of the wedge into a lock-and-key configuration leads to a better control of the helicity and twist deformation of the assembled tubes. Better control occurs when the interaction strength between the vertical binding sites is stronger than that between the lateral ones. We can also control the pitch of the helicity by adjusting the location of binding sites on the lateral surfaces of the monomer. Our results shed new light on the structure of in vitro microtubules formed with various numbers of protofilaments of tubulins, which also exhibit twisted structures when the number is different from 13. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U31.00009: Self-folding polyhedra and analogies to biomolecular assembly Shivendra Pandey, Govind Menon, David Gracias We detail model studies aimed at uncovering design principles that govern the self-assembly of polyhedral structures from two-dimensional precursors using surface tension forces. For a given polyhedron, there are a very large number of two-dimensional precursor nets that can be utilized, and remarkably many of these will self-assemble but with varying yields. We uncovered design rules that suggest striking analogies to biomolecular assembly such as observed in proteins and viruses. For example our studies revealed that the compactness of two-dimensional nets determines the yield of self-folding polyhedra and that certain intermediates and pathways were preferred. Consequently, a search algorithm was implemented to screen the large numbers of nets (e.g. 2.3 million for the truncated octahedron) and find high-yielding precursors. This assembly process represents a model system that can be utilized to design and then visualize self-assembly processes. The model system, design rules and findings will be discussed. References: S. Pandey, M. Ewing, A. Kunas, N. Nguyen, D. H. Gracias and G. Menon, Algorithmic design of self-folding polyhedra, \textit{PNAS }108, 50, 19885-19890 (2011). [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U31.00010: Functional quantum dot-protein nano bio-assembly for superior light harvesting applications Evren Mutlugun, Urartu Ozgur Safak Seker, Pedro Ludwig Hernandez- Martinez, Vijay Kumar Sharma, Vladimir Lesnyak, Nikolai Gaponik, Alexander Eychmuller, Hilmi Volkan Demir The formation of functional bio-assemblies is crucial for the advanced biophotonic applications. In this work, we formed a nano bio-assembly, consisting of green fluorescent protein (GFP) and inorganic quantum dots (QDs), to employ as an excitonic biofunctional composite to use for light harvesting and biosensing applications. Using QDs as donor molecules with the acceptor GFP in the formed bio-assembly, we observed up-to 15-fold enhancement on the GFP emission, mediated by the strong nonradiative energy transfer. The lifetime modifications of the donor-acceptor pair were studied as a function of the number of proteins per quantum dot, and in good agreement with the proposed theoretical model based on the excitonic interaction among the species. Apart from the light harvesting system, a biosensing medium was also established, facilitated by the enzymatic activity destructing the light harvesting complex. The energy transferring QD-GFP complex was controllably modified by the addition of trypsin, by destroying the bond in between the QD-GFP complex, as verified by the observation of lifetime modifications. In summary, we developed functional excitonic nano-bio-assemblies, which we believe will open up new possibilities for advanced biophotonic applications. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U31.00011: Controlling size and patchiness of soft nanoparticles via kinetically arrested co-assembly of block copolymers Jose Santos, Margarita Herrera-Alonso Engineering patchy particles from block copolymers provides an effective route for the preparation of nanoparticles with surface heterogeneity and unique properties. In the current work, co-assembly of block copolymers amphiphiles with distinct macromolecular architectures under kinetically arrested conditions was used to control the size and patchiness of polymeric nanoparticles. The block copolymer mixture is composed of linear and linear-dendritic polymeric amphiphiles, the later of which provides pre-assembled ``patches'' with well-controlled dimensions and chemical functionality. Parameters including but not limited to the molecular diffusity of the amphiphiles and the kinetics of self-assembly were found to play an important role on the control of the particle size and formation of the patches. The patchy particles are stable for several months and its stability against protein/blood plasma solutions can be tuned. We will also discuss the use of these constructs to probe nanoparticle-cell interactions. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U31.00012: Structural color of butterflies: The case of Papilio butterflies Beom-Jin Yoon, Jung Ok Park, Mohan Srinivasarao The term ``structural color'' is often used to describe color produced by a material possessing periodic variations in refractive index, which is commonly observed in many species of butterflies. Papilio butterflies commonly have multilayered bowl structures on their wing scales but the resulting colorations are different each other. Papilio ulysses has blue colored wing and Papilio palinurus shows green coloration on its wing, while Papilio blumei has green coloration on the wing scales but display a blue colored tail. We investigated the structures of the scale on the wings of Papilio butterflies using focused ion beam milling and analyzed the structural origin of the structural color from each Papilio butterfly. The coloration mechanism was attributed to the combination of the multilayer reflection from different feature size coupled with additive color mixing. [Preview Abstract] |
Session U32: Charged Polymers and Ionic Liquids
Sponsoring Units: DPOLYChair: Mu Ping Nieh, University of Connecticut
Room: 340
Thursday, March 21, 2013 11:15AM - 11:27AM |
U32.00001: An Optimized Solvation Theory for Charged Macromolecules Immersed in Aqueous Electrolyte Solutions Zaven Ovanesyan, Bharat Medasani, Marcelo Marucho In this talk, we introduce an accurate solvation model based on integral equation theory to study highly interacting charged systems. This approach is able to account for strong ion screening effects on charged macromolecules where conventional approaches may be inappropriate. A detailed knowledge of the structural arrangement of ions and solvent molecules in the vicinity of macromolecules is of crucial importance to get a microscopic understanding of these polyelectrolyte systems. We present the results obtained for ion-sphere density profiles, integrated charge and mean-electrostatic potential. These calculations are generated at low computational cost without losing important structural features of these strongly interacting charged systems. The results predict charge inversion and are in good agreement with Monte Carlo simulations. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U32.00002: Effect of Ion Content on Conductivity and Morphology of Single-Ion Conducting Ionomers Jing-Han Helen Wang, Ralph H. Colby Ionomers based on short poly(ethylene oxide) side chains and sodium sulfonated styrene are synthesized by reversible addition fragmentation chain transfer (RAFT) polymerization, to systematically study the effect of ion content and counterion species on ionic conductivity. Glass transition temperature increases gradually as ions are incorporated at low ion content then sharply as the ion content reaches 1:4 ions to ether oxygen (EO) ratio. Dielectric relaxation spectroscopy is used to measure the conductivity, dielectric constant and segmental relaxations in these ionomers. The ionomer with 1:80 ions to EO ratio shows highest room temperature conductivity that results from the best combination of number density of simultaneously conducting ions and their mobility, assessed by an electrode polarization model. The micro-phase separation that is anticipated in the ionomers with higher ion contents is probed by x-ray scattering. Sodium counterions are mostly trapped in ionic aggregates while larger counterions, such as tetramethylammonium, exhibit higher conductivity and conducting ion concentration. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U32.00003: Charge regulation and local dielectric function in planar polyelectrolyte brushes Rajeev Kumar, Bobby Sumpter, S. Michael Kilbey II Understanding the effect of inhomogeneity on the charge regulation and dielectric properties, and how it depends on the conformational characteristics of the macromolecules is a long-standing problem. In order to address this problem, we have developed a field-theory (J. Chem. Phys. 136, 234901 (2012)) to study charge regulation and local dielectric function in planar polyelectrolyte brushes. The theory is used to study a polyacid brush in equilibrium with a bulk solution containing monovalent salt ions, solvent molecules, and pH controlling acid. In particular, we focus on the effects of the concentration of added salt and pH of the bulk in determining the local charge and dielectric function. Our theoretical investigations reveal that the dipole moment of the ion-pairs formed as a result of counterion adsorption on the chain backbones play a key role in affecting the local dielectric function. Furthermore, an increase in the bulk salt concentration is shown to increase the local charge inside the brush region. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U32.00004: Resolving the Difference in Electric Potential within a Charged Macromolecule Shuangjiang Luo, Jingfa Yang, Jiang Zhao The difference of the electric potential between the middle and end of polystyrene sulfonate (PSS-) chain is discovered experimentally. Using a pH-responsive fluorophore attached to these two locations on the PSS- chain, the local pH value was determined by single molecule fluorescence technique: photon counting histogram (PCH). By the observation of a very high accumulation of proton (2-3 orders of magnitude in concentration) at the vicinity of the PSS- as the result of the electrostatic attraction between the charged chain and protons, the electric potential of the PSS- chain is determined. A higher extent of counterion adsorption is discovered at the middle of the PSS- chain than the chain end. The entropy effect of the counterion adsorption is also discovered - upon the dilution of protons, previously adsorbed counterions are detached from the chain. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U32.00005: Effects of the dielectric inhomogeneity in polyelectrolyte solution Issei Nakamura, Zhen-Gang Wang We study the effects of dielectric inhomogeneity on the statistical properties of polyelectrolyte in solution, developing a new lattice Monte Carlo method based on the bond fluctuation model with a local algorithm for computing the electrostatic interactions. Our theory accounts for the difference in the dielectric properties between the polymer backbone and the solvent. Taking the coil-globule transition of a single polyelectrolyte in solvent as an example, we show that the chain conformation and the degree of counterion condensation are substantially affected by the electrostatic response of the polymer backbone. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U32.00006: Polyelectrolyte solutions in solvents of extremely high dielectric constant Thomas Seery, Sergey Fillipov, Jiri Panek, Peter Cernoch, Petr Stepanek The physics of polyelectrolyte solutions are of great importance in understanding various processes in nature but they pose a challenge due to their complex behavior. For strong electrolytes discussed here the fraction of the condensed counterions depends on the charge density of polyion, i.e., 1-1/z$\lambda $ where z is the valence of the counterions, and $\lambda $ is the reduced coupling constant defined by .$\lambda =l_{B} /a$Here $a$ is the distance between ions on the polyion and $l_{B}$ is the Bjerrum length $l_{B} =\frac{e^{2}}{4\pi \varepsilon_{0} \varepsilon kT}$ where $e$ is the elementary charge, $\varepsilon $ the dielectric constant of the solvent, $k$~the Boltzmann constant and $T$ absolute temperature. The Bjerrum length is the distance between charged species (counterions, co-ions or charged monomers) when the electrostatic energy between them is equal to the thermal energy k$T$. We exploit the strong temperature dependence of dielectric constant of N-methylformamide to vary the Bjerrum length in a solution of polyelectrolytes (sodium polystyrene sulfonate) and to thus investigate the dynamic properties of salt-free solutions over a broad temperature range, from $+$54 to --58$^{o}$C. Fast and slow diffusion processes are observed. The ratio of diffusion coefficients, $D_{s}$/$D_{f}$ , increases and the ratio of amplitudes $A_{s}$/$A_{f}$ decreases, both by a factor of about two in this temperature range corresponding to the expected temperature variation of the Bjerrum length. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U32.00007: Highly-correlated charges in polyelectrolyte gels Charles Sing, Johannes Zwanikken, Monica Olvera de la Cruz Polyelectrolyte gels are ubiquitous in polymer physics due to their attractive combination of structural and chemical features that permit the realization of ``environmentally responsive'' systems. The conventional conceptual picture of the volume response of these systems is based on a competition between osmotic and elastic effects. We elaborate on this fundamental understanding by including ion correlations through the use of liquid-state integral equation theory. This allows for a statistical mechanical representation of the state of the system that not only surpasses traditional Poisson-Boltzmann theories but also renders structural features in a highly accurate fashion. In particular, the local ion structure is elucidated, allowing for detailed articulation of charge inversion and condensation effects in the context of gel swelling. The inclusion of correlations has a number of ramifications that become apparent, with enhanced gel collapse and excluded volume competitions that give rise to novel and ion-dependent reentrant swelling effects. We expect this rigorous theory to prove instructive in understanding any number of gelated structures, such as chromosomes or designed synthetic materials for drug delivery. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U32.00008: Ionic Association States in Polyester Copolymer Ionomers Hanqing Masser, Shichen Dou, Ralph Colby, Paul Painter, James Runt A series of random copolyester ionomers were previously synthesized from poly(ethylene oxide) (PEO600) and poly(tetramethylene oxide) (PTMO650) oligomers, separated by the lithium or sodium salt of a sulfonated phthalate. PEO exhibits better solvating ability, while PTMO based ionomers have somewhat lower T$_{\mathrm{g}}$. By changing the ratio of PEO/PTMO, the polymer's ability to solvate ions at the same ion content was varied, in order to explore the trade-off between ion solvation and lower T$_{\mathrm{g}}$. Ionomers with different PEO/PTMO ratios were investigated by FTIR spectroscopy. The results show a systematic change in the ion association states and ion aggregation geometries with PEO/PTMO ratio and temperature. Ionomers with sodium cations have more ion pairs compared to the Lithium ionomers at the same PEO/PTMO ratio, which correspond to the higher dielectric constants in the sodium ionomers. These findings agree with previous X-ray scattering and dielectric relaxation spectroscopy results that the system microphase separates into PEO-rich and a PTMO-rich microphases and the majority of the cations reside in the PEO-rich microphase. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U32.00009: Controlling self-assembly and transport properties of ionomer thin films Miguel Modestino, Rachel Segalman Electrochemically active materials, such as ionomer composites, allow for both ionic and electrical conduction. Commonly, these materials involve inorganic electrocatalytic particles surrounded by ionomer thin films. This work presents insights in the effects of confinement and wetting interactions in the self-assembly and transport properties of perflourosulfonic acid ionomers thin films. Using in situ grazing-incidence X-ray scattering (GISAXS), we demonstrate that interfacial interactions and thin-film confinement can significantly affect phase separation, domain orientation and dynamics of ionomer films during water uptake. Thin-films casted on hydrophobic substrates result in parallel orientation of ionomer domains, while films prepared on SiO$_{2}$ surfaces result in isotropic orientation of these domains. These morphological characteristics, translate directly into effects on their macroscopic swelling behavior, where parallel orientation of ionomer domains limits the maximum water uptake of films. Furthermore, confinement to thickness below 10 nm hinders microphase separation of the material and results in high levels water uptake. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U32.00010: Effect of Morphology on Ion Transport in Polymerized Ionic Liquid Block Copolymers Jae-Hong Choi, Yuesheng Ye, Yossef Elabd, Karen Winey We investigate the impact of morphology on ion transport in single-ion conductor polymerized ionic liquid (PIL) diblock copolymers. The morphology for two types of PIL block copolymers with different degrees of miscibility between blocks was studied using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). For poly(methyl methacrylate-b-1-[(2-methacryloyloxy)ethyl]-3-butylimidazolium-bis(trifluoromethylsulfonyl)imide) (MMA-b-MEBIm-TFSI) PIL diblock copolymers, the partial miscibility between the MEBIm-TFSI and MMA blocks resulted in a weakly microphase-separated morphology without long-range order. In poly(styrene-b-1-[(2-acryloyloxy)ethyl]-3-butylimidazolium-TFSI) (S-b-AEBIm-TFSI) PIL block copolymers, a variety of self-assembled nanostructures including hexagonally packed cylinders, lamellae, and coexisting lamellae and network morphologies were observed by varying PIL composition. A comparison of ionic conductivity between PMMA- and PS-based PIL block copolymers suggests that strong microphase separation with well-defined structures can improve ionic conductivity. The local ion concentration and connectivity of the conducting microdomains also play an important role in ion conduction in these PIL block copolymers. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U32.00011: Diffusion of polyelectrolyte chains within layer-by-layer films: a combined FRAP and neutron reflectometry study Viktar Selin, Li Xu, John F. Ankner, Svetlana A. Sukhishvili We report a comparative study of the diffusion of polyelectrolyte chains of various types and various molecular weights within polyelectrolyte layer-by-layer (LbL) films. To that end, we used a combination of fluorescence recovery after photobleaching (FRAP) and neutron reflectometry (NR) to probe chain diffusion in directions parallel and perpendicular to the substrate, respectively. LbL films were assembled using poly(methacrylic acid) (PMAA) as a polyanion and poly-2-(dimethylamino)ethyl methacrylate (PDMA) or quaternized PDMA (QPDMA) as a polycation. Fluorescently labeled and/or deuterated PMAA chains were incorporated within films as marker layers in FRAP and NR experiments, respectively. We found that in solutions of 0.2-0.6 M NaCl, chain diffusion was enhanced, with significantly faster chain motion in the direction parallel to the substrate. We will also discuss the effects of pH, salt concentration and polyelectrolyte type and molecular weight on mobility of polyelectrolyte chains within LbL films. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U32.00012: Origins of Symmetry in Polymer Ionic Liquid Phase Diagrams Jane Lipson, Ronald White Recent experimental work [Lee et al. \textit{Macromolecules} \textbf{45}, 3627 (2012)] reveals rather symmetric looking coexistence curves for poly(ethylene oxide) in [EMIM][BF$_{4}$]. This is in marked contrast to solutions involving non-ionic solvents, which show a characteristic and strong asymmetry, correlated with the molecular weight disparity between the two components. Using our simple theoretical approach we show that the special character of this systems derives from two thermodynamically-based properties. First, we find that the ionic solvent has a considerably stronger cohesive energy densities than non-ionic counterparts. In addition, we propose that aggregation in the ionic liquid has a significant impact on the entropy of mixing, typically a strong driving force for miscibility in polymer solutions. In this talk we explain how each of these features serves to drive the critical composition to the middle of the phase diagram. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U32.00013: Ordered and Disordered Polymerized Ionic Liquid Block Copolymers: Morphology and Ionic Conductivity Sharon Wang, Yuesheng Ye, Yossef Elabd, Karen Winey We systematically studied the influence of temperature and relative humidity on morphology and ionic conductivity in polymerized ionic liquid block copolymers (PIL BCP). Poly(methyl methacrylate-$b$-1-[2-(methacryloyloxy)ethyl]-3-butylimidazolium-X$^{\mathrm{-}})$ block copolymers (X$^{\mathrm{-}} \quad =$ OH$^{\mathrm{-}}$, Br$^{\mathrm{-}})$ were characterized by SAXS, dynamical mechanical analysis, and electrochemical impedance spectroscopy. At 25 $^{\mathrm{^{\circ}}}$C, weak microphase separation was observed for the PIL BCP with $\phi _{\mathrm{PIL}} \quad =$ 0.38 and X$^{\mathrm{-}} \quad =$ OH$^{\mathrm{-}}$. Upon increasing the relative humidity to 90{\%}, this polymer exhibited an order-disorder transition (ODT). The ODT was further studied in the PIL BCPs with X$^{\mathrm{-}} \quad =$ OH$^{\mathrm{-}}$ and 0.11 \textless $\phi _{\mathrm{PIL}}$ \textless 0.38 over a range of temperatures and {\%}RH. In contrast, the PIL BCP with $\phi_{\mathrm{PIL}} \quad =$ 0.38 and X$^{\mathrm{-}} \quad =$ Br$^{\mathrm{-}}$ formed strongly microphase separated lamellae at all investigated T and {\%}RH. At elevated temperature and 90 {\%}RH, ionic conductivities of 30 and 6 mS/cm were observed for $\phi _{\mathrm{PIL}} \quad =$ 0.38 and X$^{\mathrm{-}} \quad =$ OH$^{\mathrm{-}}$ and Br$^{\mathrm{-}}$, respectively, surpassing the conductivities of the corresponding PIL homopolymer. By selecting the counterion and relative humidity, we significantly impact the morphology and ionic conductivity of these PIL block copolymers. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U32.00014: Morphology, Modulus, and Ionic Conductivity of a Triblock Terpolymer/Ionic Liquid Electrolyte Membrane Lucas D. McIntosh, Timothy P. Lodge A key challenge in designing solid polymer electrolytes is increasing bulk mechanical properties such as stiffness, without sacrificing ionic conductivity. Previous work has focused on diblock copolymers, where one block is a stiff, glassy insulator and the other is a flexible ion conductor. Disadvantages of these systems include difficulty in achieving network morphologies, which minimize dead-ends for ion transport, and the necessity to operate below both the \textit{T}$_{g}$ of the glassy block and the order-disorder temperature. We have investigated the triblock terpolymer poly[isoprene-\textit{b}-(styrene-\textit{co}-norbornenylethyl styrene)-\textit{b}-ethylene oxide] because it self-assembles into a triply-continuous network structure. SAXS and TEM revealed the bulk morphology of INSO to be disordered but strongly correlated after solvent casting from dichloromethane. This apparent disordered network structure was retained after chemical crosslinking and addition of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide. Impedance spectroscopy confirmed the expected conductivity for ions confined to continuous PEO channels. The mechanical response before and after crosslinking showed an increase in the material modulus. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U32.00015: Decoupling of charge transport from structural dynamics in protic ionic liquids Joshua Sangoro, Alexei Sokolov, Friedrich Kremer, Marian Paluch Broadband dielectric spectroscopy, differential scanning calorimetry and rheology are employed to investigate charge transport and dynamics in protic and aprotic ionic liquids. While the structural $\alpha$-relaxation rates and the characteristic charge diffusion rates coincide for aprotic ionic liquids, the latter is found to be more than 100 times for the protic ionic liquids studied. Moreover, the analysis of protic ionic liquids revealed a decoupling of temperature dependence of ionic transport from that of structural relaxation with the degree of decoupling increasing with fragility of the liquid. The potential technological impact of these results will be discussed. [Preview Abstract] |
Session U33: Focus Session: Organic Electronics and Photonics - Organic Photovoltaics I - Theory and Processing
Sponsoring Units: DMPChair: Michael Chabinyc, University of California at Santa Barbara
Room: 341
Thursday, March 21, 2013 11:15AM - 11:51AM |
U33.00001: David Adler Lectureship Award in the Field of Materials Physics Lecture Invited Speaker: Jean-Luc Bredas We first review the current state-of-the-art in the field of organic electronics and then focus on organic solar cells, which we define as solid-state cells in which the semiconducting materials between the electrodes are organic, be them polymers, oligomers, or small molecules. We describe the optical and electronic processes that take place in such cells and turn our attention briefly to: (i) optical absorption and exciton formation; (ii) exciton migration to the electron donor -- electron acceptor interface; (iii) exciton dissociation into charge carriers, resulting in the appearance of holes in the donor component and electrons in the acceptor component; (iv) charge carrier mobility; and (v) charge collection at the electrodes [1-3]. In the second part of the presentation, we underline the complexity of the processes taking place at the nanoscale at the donor/acceptor interfaces and highlight the molecular understanding that comes from a computational approach combining electronic-structure theory calculations, molecular mechanics / molecular dynamics simulations, and Monte Carlo simulations [4-6].\\[4pt] References:\\[0pt] [1] B. Kippelen and J.L. Bredas , Energy {\&} Environmental Science \underline {2}, 251 (2009).\\[0pt] [2] J.L. Bredas, J. Norton, J. Cornil, and V. Coropceanu, Accounts of Chemical Research \underline {42}, 1691 (2009).\\[0pt] [3] Y. Zhou \textit{et al.}, Science \underline {336}, 327 (2012).\\[0pt] [4] N.C. Miller \textit{et al.}, Advanced Materials, 2012 (DOI: 10.1002/adma.201202293).\\[0pt] [5] N.C. Miller \textit{et al.}, Advanced Energy Materials, 2012 (DOI: 10.1002/aenm.201200392).\\[0pt] [6] Y.T. Fu, C. Risko, and J.L. Bredas, Advanced Materials, 2012 (DOI: 10.1002/adma.2012 03412). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U33.00002: An ab initio approach to organic photovoltaics Vincent Gosselin, Nicolas B\'erub\'e, Josiane Gaudreau, Michel C\^ot\'e Within the recent years, we have witnessed continual improvements in the Power Conversion Efficiencies (PCE) of organic photovoltaic devices. These improvements have been achieved by the discovery of new polymers which are being syntesised and their performance assessed experimentally. Scharber has introduced a simple model which determines the desired properties of polymers in order to achieve high PCE. An appealing alternative to the lengthy process of polymer synthesis consists in using ab initio calculations in order to predict the electronic structure of polymer candidates and evaluate the relevant properties in the determination of their PCE. In this work, Density Functional Theory (DFT) is being used to compute the optical band gap and HOMO / LUMO levels which, in conjunction with Scharber's model, allows to predict the efficiency of various polymer - fullerene blends. In order to assess the quality of such calculations and the validity of the model, we first compare the predictions with experimental device performances. We find that the model offers an indication as to what one should expect in terms of the maximum efficiency attainable experimentally. Lastly, we present new unsynthesised polymers which have shown promising results within this framework. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U33.00003: First principles modeling of donor materials for organic solar cells: where theory complements experiment Andriy Zhugayevych, Sergei Tretiak, Guillermo Bazan We discuss the predictive power and accuracy of first principles modeling of small-molecule crystalline donors for organic solar cells. First of all, in order to understand where the theory can help us in improving the performance of photovoltaic devices, we clarify what factors constituting power conversion efficiency needed to be improved. We argue these are short circuit current and fill factor, rather than bandgap and open circuit voltage. This implies that the optimization of intramolecular properties (e.g. HOMO/LUMO), which is best suitable for theoretical search, will not give the anticipated gain in efficiency. The intermolecular properties are amenable to first principles modeling on a single-crystallite scale and we discuss some challenges in this avenue. As an example of how theory can provide design rules for architecturing small-molecule crystals we analyze the dependence of charge carrier mobility on the intermolecular geometry of a pi-stack. In the other case study we show that changes in device performance due to small changes in chemical composition can be well tracked by the theory. Finally, we analyze the performance of commonly used density functionals for typical molecular systems used in organic electronics (oligomers, polymers, dimers, crystals). [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U33.00004: A Dynamic Monte Carlo Model with an Improved Charge Injection Mechanism for the Photocurrent Generation of Organic Solar Cells Dylan Kipp, Venkat Ganesan Previous dynamic Monte Carlo studies have made great strides in connecting organic solar cell device microstructure to final properties. One challenge still remaining is to capture the full illuminated and dark current-voltage curves and their dependencies on the charge injection mechanism. By modifying the injection mechanism of previous algorithms, we have developed an improved model for the simulation of photocurrent generation in organic solar cells. We include and utilize an injection rate prefactor to control the portion of dark current attributed to each of 4 kinds of charge injection. By shifting the dark current between electrode-polymer pairs, the injection timescales are aligned even when modeling ohmic contacts. Using our model, we are able to generate charge density and potential profiles that better agree with theory and better reproduce experimental results as compared to previous dynamic Monte Carlo methods. We are able to demonstrate how charge accumulation and band bending effects the shape and placement of the various current-voltage regimes. Finally, we are able to demonstrate how various parameters influence the current-voltage characteristics. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U33.00005: Computational materials design for bulk heterojunction solar cells Xi Lin, Yongwoo Shin The adapted Su-Schrieffer-Heeger Hamiltonian is further developed in this work to predict the optical bandgaps of more than 200 different $\pi $-conjugated systems. Insights on the structure-property relationship of these $\pi $-conjugated systems lead to guiding rules for new photovoltaic materials design. A copolymer of parallel and perpendicular benzodithiophenes, differing only in sulfur atom locations, is proposed as a candidate to achieve the optimal 1.2 eV donor optical gap for organic photovoltaics. The charge transfer mechanisms and the exciton and charge carrier mobilities are computed and compared for various bulk-heterojunction structures to improve the overall power convention efficiency. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U33.00006: New way of polymer design for organic solar cells using the quinoid structure Nicolas Berube, Josiane Gaudreau, Michel Cote Research in organic photovoltaic applications are receiving a great interest as they offer an environmentally clean and low-cost solution to the world's rising energy needs. Controlling the device's active polymer optical bandgap is an important step that affects its absorption of the solar spectrum, and ultimately, its power conversion efficiency. The use of fused heterocycles that favors the polymer's quinoid structure has been a known method to lower the bandgap, for example, with isothianapthene, but there is a lack of quantifiable data on this effect. Density functional theory (DFT) calculations were done on over 60 polymers with bandgaps between 0.5 eV and 4 eV. They clearly show that low bandgaps are observed in copolymers that carefully stands between their quinoid and aromatic structures. Such balance can be obtained by mixing monomer units with quinoid characteristics with aromatic ones. Time-dependant DFT results also links low bandgaps with lower reorganization energy, which means that polymers with this structural form could possess higher charge mobilities. This link between the geometrical structure and the bandgap is compatible with a vast variety of polymers and is more convincing than the commonly used donor-acceptor method of polymer design. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:27PM |
U33.00007: Morphology-property insights into high-performance organic photovoltaics Invited Speaker: Seth Darling Organic solar cells have attracted increasing attention as potential low-cost alternatives to traditional inorganic photovoltaic (PV) technologies. Additional advantages of OPVs include the use of earth-abundant materials, mechanical flexibility, light weight, rapid energy payback time, and the option for tunable coloring for aesthetic architectural installation. Key to their low-cost is solution-based high-throughput processing. Power conversion efficiency (PCE) of organic photovoltaics (OPVs) has steadily improved, with PTB series polymers exhibiting some of the highest PCEs. Using a suite of advanced characterization techniques, it is possible to decipher the morphology of OPV active layers across length scales from the molecular to the mesoscopic. Correlating these structural features with optoelectronic function leads to morphology-performance relationship insights, which in turn can be utilized as the foundation for a rational design of improved performance in OPV devices. Initial results from this methodology are encouraging, suggesting a viable alternative to the traditional Edisonian approach to device performance improvement. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U33.00008: The impact of miscibility on organic solar cell performance and stability Brian A. Collins, John R. Tumbleston, Jon A. Bartelt, Michael D. McGehee, Christopher R. McNeill, Harald Ade The recent demonstration of molecular miscibility/solubility between polymers and fullerenes [1] has revealed a much more complex picture of nanostructure, charge dynamics, and device stability -- aspects that are all entangled. Here we show that miscibility is important in several ways that depends on the particular material blend. For example, recent absolute measurements on domain size and composition [2] have revealed nanostructure in PTB7:PC$_{71}$BM blends that is controlled by miscibility and that well-mixed regions likely hinder charge separation in this system. On the other hand, PBDTTPD:PC$_{61}$BM blends rely on high levels of mixing for electron percolation [3]. Such evidence leads to a complex interplay between charge separation, electron trapping, and percolation. Miscibility, a thermodynamic parameter, can, furthermore, determine the thermal stability of device active layers, which we show varies widely between materials systems. This suggests tailoring of the molecular interactions between donor and acceptor materials in solar cells may be the key to high-performing, highly stable and, therefore, economically viable organic electronics technologies. [1] B. A. Collins et al., J Phys. Chem. Lett. 1, 3160, (2010). [2] B. A. Collins et al., Adv. Energy Materials DOI: 10.1002/aenm.201200377 [3] J. A. Bartelt et al., Adv. Energy Materials DOI: 10.1002/aenm.201200637 [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U33.00009: An Alternative Processing Strategy for Polymer-Fullerene Organic Photovoltaic Devices Using Supercritical Carbon Dioxide Jojo Amonoo, Emmanouil Glynos, Chelsea Chen, Anton Li, Bong-Gi Kim, Jinsang Kim, Peter Green Bulk heterojunction thin film polymer solar cells based on poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester (PC$_{61}$BM) donor/acceptor blends have received extensive attention in recent years. Well-established processing protocols, such as heating to elevated temperatures, have been employed to obtain optimum three-dimensional nano-scale morphologies critical for enhanced device performance. We show for the first time that supercritical carbon dioxide (scCO$_{2})$ processing provides a viable alternative strategy to achieve same or better power conversion efficiencies and short circuit currents compared to high temperature thermal annealing. Furthermore, energy-filtered transmission electron microscopy, and electron energy loss spectroscopy studies show that the same nano-scale morphologies are achieved using scCO$_{2}$, at an optimized temperature and pressure as those achieved using thermal annealing. Photoconductive atomic force microscopy revealed that the higher efficiency devices possessed larger fractions of photoactive regions throughout the active layer. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U33.00010: Optimization of low band gap polymer photovoltaics through structure modification Feng Liu, Yu Gu, Alejandro Briseno, Thomas Russell, Cheng Wang In BHJ-type solar cells, the ability to control and optimize the active layer morphology is a critical issue to improve device efficiency, and this is usually achieved by optimizing the processing conditions, eg. using varied annealing procedures and choosing the right solvent additive. In this work, we shown that device performance of DPP based low band gap polymers should be optimized both in processing and structural optimization approach. Without the use of chemical additive in blended thin film preparation, large size-scaled phase separation, up to several hundred of nanometers exist. This morphology is due to the surface aggregation of phenyl-C71-butyric acid methyl ester (PCBM), which forms large oval structures and then buried by a polymer-PCBM mixture thin film. In this process, the miscibility of polymer matrix plays an important role. While using chemical additive processing method can tune the general morphology to a more fibril network texture, fine-tuning of fibril dimensions and domain size needs delicate chemical structure modification. Through this modification, a 30{\%} device performance enhancement was observed, which mostly came from an enhancement of short circuit current, thus strongly related to the morphological details. Besides conventional morphology characterizations, an initiative effort of understanding the domain interface structure was also carried out by using polarized soft x-ray scattering, in which we observed polymer crystal orientation plays an important role. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U33.00011: Relating Organic Solar Cell Fabrication Methods to Internal Electronic Properties Using Impedance Spectroscopy James Basham, David Gundlach, Thomas Jackson We report on the use of impedance spectroscopy to quantify the effect of processing on an array of important OPV device metrics. Interestingly, extract modeled mobilities over the range of 2x10$^{\mathrm{-3}}$ to 1x10$^{\mathrm{-2}}$ cm$^{\mathrm{2}}$/Vs by changing the spinning recipe. We find fast carrier relaxation times of 1x10$^{\mathrm{-4}}$ s for 3{\%} efficiency cells vs 3x10$^{\mathrm{-6}}$ s for a 1.8{\%} efficiency cell, possibly demonstrating reduced recombination in more efficient devices. Devices made via slowly dried films exhibit repressed recombination compared to quickly dried films. Measurements are taken across a bias range of -1 to 1 volt with illumination intensities spanning .001 to 3 suns, in order to test under conditions which are most relevant to real device operation. Impedance spectra are analyzed through the use of a 5 element compact model based upon the work of Bisquert et al [1,2]. We report an array of device metrics measured via impedance spectroscopy including shunt resistance, effective carrier lifetime, mobility, and capacitance for P3HT:PCBM devices with efficiencies of 3.5{\%} to \textless 1{\%}, fabricated via several common recipes, in an effort to elucidate the varied and complex interplay between processing and device physics, and the overall effect on solar cell efficiency. [1] Fabregat-Santaigo, F., Garcia-Belmonte, G., Mora-Sero, I., and Bisquert, J. Phys. Chem. Chem. Phys., 2011, 13, 9083--9118 [2] Garcia-Belmonte, G.,Boix, P.P., Bisquert, J., Sessolo, M., and Bolink, H.J. Solar Energy Materials {\&} Solar Cells 94(2010)366--375 [Preview Abstract] |
Session U34: Thin Films, Surfaces and Interfaces I
Sponsoring Units: DPOLYChair: Mesfin Tsige, The University of Akron
Room: 342
Thursday, March 21, 2013 11:15AM - 11:27AM |
U34.00001: Comparison of experimental and computational estimation of non-freezing interfacial molecules Rahmi Ozisik, Nihat Baysal, Deniz Rende, Samuel Amanuel Recently, we have estimated that about 2.14 +/- 0.14 nm of interfacial cyclohexane molecules do not participate in phase transition. This estimation was determined from calorimetric measurements of physically confined cyclohexane in silica nanopores. In agreement with previous work, melting and freezing temperatures of the confined cyclohexane were lower than that of the bulk cyclohexane, and the apparent heat of fusion changed with silica pore size. Correcting for the layers of molecules at the interface that do not participate in the phase transition keeps the heat of fusion independent of the confined size scale. In the current study, we used molecular dynamics simulations to investigate the behavior of the cyclohexane molecules at the interface and compared their behavior to those in the bulk (away from the interface). [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U34.00002: A molecular view of latex-water interfaces Zifeng Li, Kristen Fichthorn, Scott Milner, Fang Yuan, Ronald Larson Latex paints and coatings are colloidal suspensions, in which amorphous polymer particles are dispersed in an aqueous phase. The polymer-water interface plays a key role in the stability and rheology of the suspension. To obtain a molecular level view of this interface, atomistic simulations were performed for a slab of poly(methyl methacrylate)-poly(butyl acrylate) random copolymer in water, focusing on polymer and water density profiles, the hydrogen bonding of water with polymer carbonyl groups, and surface tension. The carbonyl groups at the interface were found to orient significantly towards water. We also calculated the temperature dependence of the surface tension between the polymer/water and polymer/ vacuum interfaces, including tail corrections for cut-off dispersion interactions, and we predict the contact angle of a water droplet at room temperature. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U34.00003: Pathways and Time Scales for Water Movement to a Metal/Polymer Interface Hyungjin Lee, Bulent Akgun, Jim Browning, Mark Foster The movement of water underneath polymer films is important in the process of corrosion of metals protected by polymer coatings. Here the ingress of water to such an interface is studied with Neutron Reflectometry (NR), which allows the measurements to be done in situ. We have shown that water incursion along the interface between an epoxy hybrid coating and aluminum is fast compared to incursion through the face of the coating. With a more highly crosslinked coating, after a small amount of water has entered along the interface, water incursion slows dramatically. When the sample is once again in a dry environment, swelling of the coating caused by water is not fully reversible, but is reduced slowly over a period of a month. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U34.00004: Desorption Kinetics of Water from Poly (methyl methacrylate) Films and other Polymer Films Carolina Ilie, Thorin Kane, Ross Netusil, Anastasia Yorke We present herein the water desorption from the dipole oriented poly (methyl methacrylate) PMMA. Water desorption from PMMA presents the ``ice species'' at 150 K and a bulk peak at about 280 K. We note that the desorption peak temperature does not vary greatly with increasing coverage. The energy of desorption is obtained by employing the Arrhenius and Polany-Wigner equations. The comparison with previous thermal desorption spectra of water from two ferroelectric polymers is also discussed. [1] Dowben, P.A., Rosa, Luis G., Ilie, C.C., Zeitschrift f\"{u}r Physikalische Chemie 222 (2008) 755-778. [2] Ilie, C.C., Rosa, L.G., Poulsen, M., Takacs, J., Integrated Ferroelectrics (2011) 125:1, 98-103. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U34.00005: A Molecular Dynamics Simulation Study on the Wetting Behavior of Water on Oxidized and Non-Oxidized atactic Polystyrene Surface Selemon Bekele, Mesfin Tsige All-Atomistic Molecular dynamics simulations have been carried out to study the wetting of oxidized and non-oxidized atactic polystyrene (aPS) thin films by water droplets. The dependence of the contact angle on droplet size has been studied using spherical and hemispherical water droplets of varying sizes. The effect of oxidation of the aPS surface on the contact angle has been studied as a function of oxygen concentration. Oxidation of the aPs has been achieved by randomly replacing the ortho and/or meta hydrogen on the phenyl rings within 1 nm of the aPS surface by oxygen until the desired concentration of oxygen on the surface is reached. The simulated contact angle is found to decrease monotonically with oxygen concentration consistent with recent experimental results. We will present results on the variation of water contact angle with oxygen concentration on the aPS surface. In addition, the effect of oxidization on the roughness of the polystyrene surface, the ordering of the phenyl rings and the water molecules and the number of hydrogen bonding between water molecules and the polystrene at the interface have been investigated and will be presented. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U34.00006: Wetting of star-shaped macromolecules Emmanouil Glynos, Bradley Frieberg, Georgios Sakellariou, Peter Green We~show that the equilibrium contact angles and line tensions of macroscopic droplets of star-shaped polystyrene (PS) macromolecules of functionality, f, and degree of polymerization per arm, Narm, on oxidized silicon substrates, may be as much as one and two orders of magnitude, respectively, smaller than their linear analogs, depending on f and~Narm. The dewetting characteristics of the linear and star polymers also differ. Thin film of LPS and SPS dewet SiOx~substrates due to destabilizing long-range intermolecular forces.~~However, while macroscopic droplets surrounded by droplets of nanoscale dimensions characterize the late-stage dewetting morphology of the LPS system, the macroscopic droplets of the SPS molecules reside on a stable layer of molecules adsorbed to the substrate. The thickness of the adsorbed layer depends on both f and Narm. We provide evidence that the wetting/dewetting characteristics of the SPS macromolecules are largely determined by the competition between interfacially attractive conformational entropic effects and steric repulsion effects, for molecules of sufficiently large f and small Narm. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U34.00007: The Role of Acid-Base Interactions in Controlling Interfacial Segregation in Polymer Blends He Zhu, Shishir Prasad, Anish Kurian, Ila Badge, Ali Dhinojwala We have studied segregation of polymethylmethacrylate (PMMA)/polystyrene (PS) blends next to solid surfaces using interface sensitive infrared-visible sum frequency generation (SFG) spectroscopy. We have monitored the SFG spectra as a function of blend compositions and used the shift in the surface hydroxyl peak, due to acid-base interactions, to determine the concentration of PMMA groups next to the sapphire substrate. A quantitative connection between the extent of interfacial segregation and the strength of the acid-base interactions will be discussed. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U34.00008: Measurement of monolayer viscosity using non-contact microrehology Alex Levine, Arthur Evans, Roie Shlomovitz, Thomas Boatwright, Michael Dennin Microrheological studies of phospholipid monolayers, bilayers, and other surfactant monolayer systems present a particularly useful avenue for studying the flow properties of fragile, complex fluid systems. Unfortunately, in some cases microscopic particle tracking methods disagree with macroscopic flow methods by several orders of magnitude. This ``missing modulus'' problem has been speculated to originate in the heterogeneity of the monolayer under study, as well as the unknown boundary conditions and uncertainty in particle position intrinsically associated with coupling the tracer bead to the monolayer. In this talk we discuss an alternative method for performing microrheology experiments, where the tracer bead is submerged a known depth beneath the monolayer. Using both theory and experiment, we demonstrate that despite the weaker coupling between the tracer and the monolayer, the well-characterized hydrodynamics between the bulk sub-phase and the surface allows for the calculation of particle response functions and recovery of the ``missing modulus'' for several model monolayer systems. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U34.00009: Evidence of Phase Separation during Vapor Deposition Polymerization Ran Tao, Mitchell Anthamatten Initiated chemical vapor deposition (iCVD) is a solventless, free radical technique predominately used to deposit homogeneous films of linear and crosslinked polymers directly from gas phase feeds. We are developing multicomponent iCVD techniques to induce phase separation during film growth. Small molecule porogens and crosslinkers are introduced into the iCVD process during film growth of poly(glycidyl methacrylate). Analogous to well established polymerization induced phase separation (PIPS) processes, porogens, such as dimethyl phthalate, are well mixed at the growing gas-film interface but are immiscible with high molecular weight polymer. Polymerization, crosslinking and PIPS are intended to occur simultaneously on the substrate, resulting in a vitrified microstructure. A series of films were grown by varying deposition rate, porogen type, and reagent flowrates. Deposited films were studied by electron microscopy and spectroscopic techniques. Experiments are compared to Cahn-Hilliard theory predictions that relate the length and time scale of the phase separation to the polymer-porogen interaction energy, the rate of polymerization and the species mobility. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U34.00010: Surface phase separation between polythylene oxide of different molecular weight Rui Chen, Jingfa Yang, Jiang Zhao In-plane phase separation of polyelethylene oxide of different molecular weight has been observed. A systematic investigation on a broad range of Mw show that the process is originated from the conformatinal entropy for a polymer confined on a surface (thin film). [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U34.00011: Understanding diblock copolymer colloidal particle anisotropy Debra Audus, Se Gyu Jang, Daniel Krogstad, Alexandre Cameron, Sang-Woo Kim, Kris Delaney, Su-Mi Hur, Edward Kramer, Craig Hawker, Glenn Fredrickson Colloidal particles are formed by emulsifying a mixture of PS-$b$-P2VP, nanoparticles and chloroform in water with surfactant and then evaporating the chloroform. With the addition of a sufficient number of nanoparticles, the colloids form prolate ellipsoids with lamellae oriented along the major axis. These colloidal particles are of interest for potential applications such as photonic materials and drug delivery. In order to explain the colloidal particle anisotropy and its dependence on colloidal particle size, a theoretical model that balances internal and external surface tension was developed. Agreement between the model and experimental results suggests that thermodynamic factors control the particle anisotropy. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U34.00012: Multiblock copolymer adsorption on a hydrophobic surface: A Monte Carlo simulation study Max Kolb, Virginie Hugouvieux Dilute solutions of long multiblock copolymers with alternating hydrophilic and hydrophobic segments in contact with a hydrophobic surface have been investigated by Monte Carlo simulation in order to characterize the structure of the adsorption layer. Its properties are determined as a function of the bulk hydrophobicity, the surface hydrophobicity and the monomer concentration. The influence of the copolymer length and its block structure is also investigated. Interesting features appear close to the bulk critical micelle concentration: surface micelles, a secondary surface layer of bulk micelles, depletion effects. Depending on the interaction strengths the surface layer consists of individually adsorbed hydrophobic segments or of surface micelles, at equilibrium with bulk micelles, as found in a previous study of the bulk properties [1]. At higher surface coverage the surface micelles form a regularly spaced layer of hydrophilically connected micellar cores. For sufficiently long copolymers a layer of bulk micelles is hydrophilically attached to the layer of surface micelles. \\[4pt] [1] Hugouvieux, V. et al., Soft Matter 7, 2580 (2011) [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U34.00013: Capillary Levelling of Stepped Polymer Films - A Nanofluidic Probe of the Slip Boundary Condition Oliver Baeumchen, Joshua D. McGraw, Thomas Salez, Michael Benzaquen, Paul Fowler, Elie Raphael, Kari Dalnoki-Veress For flows on small length scales, the hydrodynamic boundary condition of a liquid at a solid surface plays an enormous role. In recent years much has been learned about this slip boundary condition from flows that are driven by internal, capillary, forces such as dewetting of thin liquid films. For the case of dewetting, holes in the film grow, driven by exposing the underlying substrate. Here, we present the opposite approach: We show that the capillary levelling of initially curved surfaces, in our case stepped polymer films, is sensitive to the nano-rheological properties of the liquid and the dependence on the slip boundary condition at the buried liquid/substrate interface. A thin film model which includes the slip boundary condition enables us to quantify the boundary condition at the buried interface and the dependence of slip on the molecular weight of the polymers used. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U34.00014: Relaxation of non-equilibrium entanglement networks in thin polymer films Paul Fowler, Joshua McGraw, Melissa Ferrari, Kari Dalnoki-Veress It is well established that polymer films, prepared by spincoating, inherit non-equilibrium chain conformations which can affect macroscopic film properties. Here we present the results of crazing measurements that elucidate the non-equilibirum chain configurations in spin-cast films. Furthermore, we find that the entanglement network equilibrates on a time scale comparable to one reptation time. In a second set of experiments, we confine polymers to films with thickness comparable to the molecular size. By stacking two such films at room temperature, a glassy bilayer film with a buried entropic interface is created. According to Silberberg's reflection principle, such an interface has an entropic cost associated with the restricted configurations of molecules that cannot cross the mid-plane of the bilayer. In the melt, the interface heals as chains from the two layers mix and entangle with one another. Crazing measurements reveal that it takes less than one bulk reptation time for a bilayer to become indistinguishable from a single film. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U34.00015: Ion Dispositions in Polyelectrolyte Multilayer Films David Hoagland, Zhaohui Su, Xingjie Zan, Tian Wang Polyelectrolyte multilayers (PEMs) fabricated from sodium chloride-containing solutions of poly(diallyldimethylammonium chloride) (PDDA) and poly(styrene sulfonate) (PSS) were examined by various techniques to determine the dispositions of polyelectrolytes and counterions across the PEM thickness. The key technique was dry film QCM, which quantified incremental mass depositions during PEM assembly. Counterion dispositions depended strongly on salt concentration, and three salt regimes were identified: zero to near zero salt ([NaCl] less than 0.1M), low salt ([NaCl] between 0.1M and 0.75M), and high salt ([NaCl] greater than 0.5M]). The first two are associated with linear PEM growth while the latter is associated with exponential PEM growth. At zero salt, no counterions are present in the PEM bulk (middle), while at low salt, an excess of PDDA charge across the bulk coincides with an excess of counteranions. Differently, at high salt, deposited PSS permeates the PEM bulk, conveying an excess of countercations. At all salt concentrations, the PEM surface charge alternates according to the capping polyelectrolyte's identity. Accumulations of small ions in the bulk can be ascribed to as yet poorly understood property asymmetries between the two deposited polyelectrolytes. [Preview Abstract] |
Session U35: Focus Session: Search for New Superconductors III
Sponsoring Units: DMPChair: Jochen Mannhart, Max-Planck Institute for Solid State Physics, Stuttgart, Germany
Room: 343
Thursday, March 21, 2013 11:15AM - 11:51AM |
U35.00001: Designing heterostructures -- a route towards new superconductors Invited Speaker: Thilo Kopp By now it has become technologically feasible to grow controllably transition metal oxides layer by layer. In effect, the achieved progress allows to design heterostructures with optimized electronic properties. The talk will specifically address scenarios for interface superconductivity and the possibility to raise the transition temperature of bulk superconductors by layer design. Heterostructures offer a complexity beyond that of bulk materials. The nature of the superconducting states formed in layered materials and at interfaces is a fascinating topic of recent research which will be in the focus of this presentation. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U35.00002: Reversible Superconductivity in Electrochromic Indium-Tin Oxide Films Ali Aliev, Miron Salamon Transparent conductive indium tin oxide (ITO) thin films, electrochemically intercalated with sodium or other cations, show tunable superconducting transitions with a maximum $T_{\mathrm{c}}$ at 5 K. The transition temperature and the density of states, $D(E_{F})$ (extracted from the measured Pauli susceptibility $\chi^{\mathrm{p}})$ exhibit the same dome shaped behavior as a function of electron density. Optimally intercalated samples have an upper critical field $\approx $ 4 T and $\Delta $/$k_{\mathrm{B}}T_{\mathrm{c}} \approx $ 2.0. Accompanying the development of superconductivity, the films show a reversible electrochromic change from transparent to colored and are partially transparent (orange) at the peak of the superconducting dome. This reversible intercalation of alkali and alkali earth ions into thin ITO films opens new opportunities for tunable, optically transparent superconductors. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U35.00003: Molecule/Surface Interactions and the Control of Electronic Structure In Epitaxial Charge Transfer Salts Geoffrey Rojas, P. Ganesh, Simon Kelly, Bobby Sumpter, John Schlueter, Petro Maksymovych The two-dimensionality of the fulvalene-based superconducting charge transfer salts has lead to an increasing interest in the epitaxial growth and local probe analysis of monolayer CTS films. Curiously, these studies have shown remarkable differences in both the electronic structure and topography of the monolayers grown on metals, suggesting that the organic/metal interactions introduced by epitaxial growth strongly influence the resulting structures. Through recent experiments on monolayer films of the CTS (ET)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ and the bare fulvalene ET grown on Ag(111), we illustrate what effect the metal-molecule interaction has on the electronic structure and 2D charge transport of epitaxial CTS and how this differs from the bare fulvalene. Through a comparative analysis of the differences in stoichiometry and topography of these and heretofore published systems, the relative roles of ionic bonding, surface chemisorption, and hybridization for the preparation of this and future compounds are explored. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U35.00004: Search for Very High-T$_{\mathrm{c}}$ Superconductivity in Modified Compositions of Strontium Ruthenates Armen Gulian, Vahan Nikoghosyan In 2004-2007 we discovered unusual properties in laser-processed crystals of strontium ruthenates (including resistive and magnetic transitions) pointing towards superconductivity at 200K and higher [1]. Being interested in understanding and reproducing their properties we explored their composition further. We obtained, via Auger-analysis, the presence of sulfur in the explored sample. The appearance of iron-based superconductors further enhanced our interest, since compositionally our materials turned out to be close to some of these new materials. If our reported observations [1] have been caused by superconductivity that may mean that one can get T$_{\mathrm{c}}$ as high as 200-250K or even higher with these materials at proper processing. We undertook systematic research of ceramic materials Sr$_{\mathrm{2}}$RuO$_{\mathrm{4}}$ with sulfur and other dopants. Data on resistive, magnetic and other physical properties, as well as preparation techniques are reported. [1] A.M. Gulian, V.R. Nikoghosyan, Unusual properties of laser-processed strontium ruthenates, in: T. Frias, V. Maestas (Eds.), Bulk Materials: Research, Technology and Applications, Nova Science Publishers, Inc., NY, 2010, Ch. 9 (see also arXiv: cond-mat/0509313 and cond-mat/0705.0641). [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U35.00005: Structural instability and superconductivity in (Ir,Pt)Te2: an optical spectroscopic study A.F. Fang, G. Xu, T. Dong, P. Zheng, N.L. Wang Ir$_{1-x}$Pt$_x$Te$_2$ is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe$_2$ in an effort to elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the energy levels of Te (p$_x$, p$_y$) and Te p$_z$ bands. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U35.00006: Superconductivity in electron-doped \emph{Ln}OBiS$_2$ Compounds Duygu Yazici, Kevin Huang, Ben White, Sooyoung Jang, Alan Chang, Aaron Friedman, Brian Maple We present observations of superconductivity in electron-doped \emph{Ln}OBiS$_2$ compounds (\emph{Ln} = La, Ce, Pr, Nd, Yb). Polycrystalline samples were synthesized by a two step solid-state reaction and characterized by x-ray diffraction. The parent compounds, \emph{Ln}OBiS$_2$, exhibit a non-metallic ground state. Superconductivity with $T_c$ in the range 1.9 K - 5.4 K was induced by electron doping these compounds via the substitution of F for O. Prior to the onset of superconductivity, the electrical resistivity of the electron-doped \emph{Ln}OBiS$_2$ compounds exhibit semiconductor like behavior, similar to the behavior observed in the parent compounds. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U35.00007: Characterization of superconductivity in electron-doped \textit{Ln}OBiS$_2$ compounds with specific heat measurements Benjamin White, Duygu Yazici, Kevin Huang, Alan Chang, Aaron Friedman, M. Brian Maple Superconductivity has been reported recently in Bi$_4$O$_4$S$_3$ and electron-doped \textit{Ln}OBiS$_2$ compounds with \textit{Ln} = La, Ce, Pr, Nd, Yb. These materials share a similar crystal structure composed of superconducting BiS$_2$ layers, which are separated by oxide blocking layers. Early studies have concentrated primarily on the electrical transport properties and magnetic susceptibility measurements of these systems. We present results from specific heat measurements, which were performed in order to study and characterize the superconducting and normal-state properties of several electron-doped \textit{Ln}OBiS$_2$ systems. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U35.00008: Crystal growth of Pt-doped IrTe$_{2}$ Sunseng Pyon, Kazutaka Kudo, Minoru Nohara IrTe$_{2}$, a layered compound with a triangular iridium lattice, exhibits a structural phase transition at approximately 250 K. Electric resistivity and magnetic susceptibility exhibit anomalies at the transition with hysteresis [1]. Charge-orbital density wave or orbitally induced Peierls effect, a crystal field effect are suggested as candidates of the origin of the transition [2-4]. On the other hand, superconducting phase emerges when the structural phase transition is suppressed by chemical substitution or intercalation [2,5]. Analysis of physical property using single crystal should be helpful to clarifying the relation between the ground states of IrTe$_{2}$ and superconductivity. Recently, Fang \textit{et al}. reported the growth of single crystal of parent compound [4]. However, single crystal of superconducting sample had not been reported yet. For these reason, we studied superconductivity and the structural transition in platinum doped IrTe$_{2}$ single crystals. We successfully synthesized several composition of the Ir$_{1-x}$Pt$_{x}$Te$_{2}$ single crystal by flux method. From magnetization and transport measurement, we confirm the suppression of structural phase transition and emergence of superconductivity. Detail of the experiment will be discussed.\\[4pt] [1] N. Matsumoto \textit{et al}., J. Low Temp. Phys. \textbf{117} (1999) 1129.\\[0pt] [2] J. J. Yang \textit{et al}., Phys. Rev. Lett. \textbf{108} (2012) 116402.\\[0pt] [3] D. Ootsuki \textit{et al}., Phys. Rev. B. \textbf{86} (2012) 014519.\\[0pt] [4] A. F. Fang \textit{et al}., arXiv:1203.4061 (2012).\\[0pt] [5] S. Pyon \textit{et al}., J. Phys. Soc. Jpn. \textbf{81}, 053701 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U35.00009: Enhanced Upper Critical Fields in a New Quasi-one-dimensional Superconductor Nb$_2$Pd$_{x}$Se$_5$ Seunghyun Khim, Bumsung Lee, Ki-Young Choi, Byung-Gu Jeon, Eun Sang Choi, Kee Hoon Kim We report a discovery of superconductivity with $T_{\mathrm{c}} =$ 5.5 K in Nb$_2$Pd$_{x}$Se$_5$ in which one-dimensional (1D) Nb-Se chains exist along the $b$-direction and each conducting chain is hybridized to form the conducting \textit{bc}* planes. Magnetic susceptibility and heat capacity data in both single- and poly-crystals constitute evidences of bulk superconductivity and BCS-type pairing mechanism. The zero temperature upper critical fields, $H_{\mathrm{c2}}$(0), of a single crystal are found to be 10.5, 35 and 22 T for $a$', $b$ and $c$* directions respectively. $H_{\mathrm{c2}}$(0) is clearly much larger than the expected Pauli limiting field 1.84$T_{\mathrm{c}} \approx $ 9 T along the $b$ and $c$*-direction. We will discuss the possible explanations of such enhancement of $H_{\mathrm{c2}}$ via suppression the Pauli limiting effect, based on the large spin-orbit scattering and the quasi-1D nature of electronic structure in analogy to an organic superconductor (TMTSF)$_{2}X$ ($X =$ PF$_6$, ClO$_4)$ and a purple bronze Li$_{0.9}$Mo$_6$O$_{17}$. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U35.00010: Quantum Oscillations and Superconductivity in Subband Quantized SrTiO$_3$ Bilayer Delta-Doped Structures Hisashi Inoue, Minu Kim, Christopher Bell, Yasuyuki Hikita, Harold Hwang SrTiO$_3$ delta-doped structures show two-dimensional (2D) Shubnikov de-Haas oscillations (SdH) and 2D superconductivity (SC) [1]. Lightly doped systems, with clear SdH signals are ideal to study the link between 2D single electron states and SC [2]. The subbands (SB) should strongly influence SC: their splitting is larger than the superconducting gap. However, the similar spatial extent of the SB in single delta-layers prohibits the detection of SB modulated SC.
Growing two delta-layers (DL) in parallel with varying interlayer (IL) thickness $d$, we can spatially separate the SB and identify their contributions to SC and SdH. For small $d$, all SB spread over the DL and the IL. For larger $d$ only lower SB are confined around the DL. From the angular-dependence of the main SdH frequency we find a 2D to three-dimensional crossover for $\sim 60 |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U35.00011: Search for new phases in the Praseodymium-Silicon system Jose De La Venta, Ali C. Basaran, Ted Grant, J. Gallardo-Amores, J.G. Ramirez, M.R. Suchomel, M.A. Alario-Franco, Zachary Fisk, Ivan K. Schuller We searched for new superconducting and magnetic phases in the Pr-Si system using high-pressure high-temperature and conventional arc melting syntheses. High pressure synthesis is a unique technique which allows incorporation of elements into compounds which otherwise cannot be synthesized at ambient pressure Both high and low Si concentration areas of the phase diagram were explored. To investigate the high Si concentration compounds, PrSi$_2$ with an excess of Si was subjected to HP-HT synthesis. To explore the high Pr concentration binary compound Pr$_5$Si$_3$, we have synthesized undoped Pr$_5$Si$_3$ as well as different samples doped with C or B. High resolution X-ray powder diffraction, Magnetic Field Modulated Microwave Spectroscopy and magnetic characterization found that the addition of C gave rise to multiple previously-unknown ferromagnetic phases. Furthermore, X-ray refinement of the undoped samples confirmed the existence of the so far unconfirmed Pr$_3$Si$_2$ phase. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U35.00012: Superconductivity in Bundles of Double-Wall Carbon Nanotubes Zhe Wang, Wu Shi, Qiucen Zhang, Yuan Zheng, Chao Ieong, Mingquan He, Rolf Lortz, Yuan Cai, Ning Wang, Ting Zhang, Haijing Zhang, Zikang Tang, Ping Sheng, Hiroyuki Muramatsu, Yoong Ahm Kim, Morinobu Endo, Paulo T. Araujo, Mildred S. Dresselhaus We will present electrical and thermal specific heat measurements that show superconductivity in double-wall carbon nanotube (DWCNT) bundles. Clear evidence, comprising a resistance drop as a function of temperature, magnetoresistance and differential resistance signature of the supercurrent, suggest an intrinsic superconducting transition below 6.8 K for one particular sample. Additional electrical data not only confirm the existence of superconductivity, but also indicate the Tc distribution that can arise from the diversity in the diameter and chirality of the DWCNTs. A broad superconducting anomaly is observed in the specific heat of a bulk DWCNT sample, which yields a Tc distribution that correlates well with the range of the distribution obtained from the electrical data. As quasi one dimensionality of the DWCNTs dictates the existence of electronic density of state peaks, confirmation of superconductivity in this material system opens the exciting possibility of tuning the Tc through the application of a gate voltage. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U35.00013: Microwave absorption across phase transitions Juan Gabriel Ramirez, Ali Basaran, J. de la Venta, Juan Pereiro, I.K. Schuller Magnetic Field Modulated Microwave Spectroscopy (MFMMS) is a high-sensitivity technique capable of detecting superconducting phases in volumes as small as 10$^{-11}$ cm$^{3}$ even in discontinuous samples. This method measures the temperature dependence of the reflected microwave power from a sample in an oscillating magnetic field. The signature of superconductivity appears as a peak in the reflected microwave power at the transition temperature. However, the absorption mechanism is still unclear. We present an exhaustive number of measurements of known superconductors as well as other materials that undergo phase transitions to test different microwave absorption mechanisms. MFMMS measurements in micro-patterned superconducting structures were performed in order to determine the detection limit of the superconducting volume. [Preview Abstract] |
Session U36: Electron Phonon Superconductivity and Isotope Effect
Sponsoring Units: DCMPChair: Roxanna Margine, Binghamton University - SUNY
Room: 344
Thursday, March 21, 2013 11:15AM - 11:27AM |
U36.00001: Fermiology and Superconductivity of LaNiGa$_2$ David J. Singh LaNiGa$_2$ has been identified as a possible centrosymmetric triplet superconductor based on observations of time reversal symmetry breaking in $\mu$SR experiments. Here we report calculations of the Fermiology and related properties. In spite of the seemingly layered crystal structure, the Fermi surface has several large sheets and is only moderately anisotropic, so that the material is best described as a three dimensional metal. These include sections that are open in the in-plane direction as well as a section that approaches the zone center. The density of states is high and primarily derived from Ga $p$ states, which hybridize with Ni $d$ states. Comparing with experimental specific heat data, we infer a superconducting $\lambda \le$ 0.55, which implies that this is a weak to intermediate coupling material. Ni occurs in a nominal $d^{10}$ configuration in this material, which places the compound far from magnetism. The implication is that this is most likely a standard electron phonon mediated s-wave superconductor. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U36.00002: Electron-phonon coupling in potassium-doped superconducting picene Michele Casula, Matteo Calandra, Francesco Mauri We explore the properties of electron-phonon couplings in K$_3$Picene, in the framework of density functional theory (DFT). By exploiting the maximally localized Wannier function formalism, we identify the contribution of the intra- and intermolecular phonon vibrations and the role of local and non-local electronic states in determining the electron-phonon coupling. Despite the molecular nature of the crystal, we find that the purely molecular contributions account for only 20{\%} of the total electron-phonon interaction $\lambda $. In particular, the Holstein-like contribution to $\lambda $ are four times smaller than those computed for an isolated neutral molecule, as they are strongly screened by the metallic bands of the doped crystal. The major contribution (80{\%}) to $\lambda $ in K$_3$Picene comes from non-local couplings due to phonon modulated hoppings. We show that the crystal geometry together with the molecular picene structure leads to a strong 1D spatial anisotropy of the non-local couplings. Finally, we propose a lattice model of the electron-phonon couplings in K3Picene that gives 90{\%} of the $\lambda $ obtained in first principles calculations [1]. \\[4pt] [1] M. Casula, M. Calandra and F. Mauri, PRL 107, 137006 (2011), PRB 86, 075445 (2012) [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U36.00003: First-principles study of cobalt pnictide SrCo$_{2}$N$_{2}$ Andrew O'Hara, Alexander Demkov With the recent discovery of high temperature superconductivity in BaFe$_{2}$As$_{2}$, there has been renewed interest in other members of the AT$_{2}$X$_{2}$ family (A $=$ alkaline earth element or lanthanide, T $=$ transition metal, X $=$ an element of groups IIIB-VIB) and in particle isovalent members of the 122 family. In this work, we describe a hypothetical cobalt pnictide, SrCo2N2, using density functional theory (DFT) in the local density approximation (LDA) with a Hubbard U correction. In this work, we determine both the lattice and chemical stability of SrCo$_{2}$N$_{2}$ as well as explore how the substitutions affect the electronic and magnetic properties in comparison to BaFe$_{2}$As$_{2}$ and 122 rare-earth cobalt phosphides. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U36.00004: Dynamical Jahn-Teller effect in Cs$_{3}$C$_{60}$ superconductors Liviu Chibotaru, Naoya Iwahara The Cs$_{3}$C$_{60}$ shows a superconducting critical temperature of 38K which is one of the highest among phonon-mediated superconductors. Recent infrared spectroscopy data of insulating Cs$_{3}$C$_{60}$ apparently support the presence of Jahn-Teller dynamics in this fulleride [1]. To check this possibility, we have performed the DFT calculations of vibronic constants and multiplet splitting parameters, and have calculated from the first principles the spectrum of low-lying vibronic states on C$_{60}^{3-}$ sites by diagonalizing the full vibronic Hamiltonian in a large vibrational basis. The splitting of the t$_{\mathrm{1u}}^{3}$ shell into degenerate multiplets and their vibronic mixing has been fully taken into account, as well as the effect of the environment on the local vibrations. The results show that in the insulating phase an unhindered dynamical Jahn-Teller effect takes place at each C$_{60}$ site. Using Gutzwiller approach in combination with LDA band structure, we demonstrate that the Jahn-Teller instability also persists in the metallic phase for a wide range of values of intrasite repulsion energy ($U)$.\\[4pt] [1] G. Klupp, P. Matus, K. Kamaras, A.Y. Ganin, A. McLennan, M.J. Rosseinsky, Y. Takabayashi, M.T. McDonald, K. Prassides, \textit{Nat. Comm}. \textbf{2012}, \textit{3}, 912. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U36.00005: Repulsive interaction helps superconductivity in fullerides Satoshi Yamazaki, Yoshio Kuramoto Alkali metal (A) doped fullerides (A$_{3}$C$_{60})$ show not only superconductivity (SC) with high transition temperature Tc up to about 40K, but also antiferromagnetism (AF) with A$=$Cs. In view of nearby presence of the AF state, the Coulomb repulsion should play a significant role in the SC state. However, various experimental evidences point to a fully symmetric s-wave SC state being realized. In the conventional theory, the s-wave state is unfavorable in the presence of Coulomb repulsion. Then the fundamental question remains why the Tc in fullerides is so high. As a step toward the complete understanding, we study a purely repulsive interaction model with the characteristic band structure derived by degenerate molecular orbitals in fullerides. We calculate SC coupling constants for various symmetries of SC pairs by using the second order perturbation theory We find that even with the repulsive interaction model, the s-wave pair can be formed. With the electron-phonon interaction combined, it is likely that the s-wave pair becomes the most stable. According to our result, we propose that the cooperation between Coulomb repulsion and electron-phonon interaction is responsible for the high Tc. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U36.00006: Phonon Vibrations and Superconductivity of a Bi-based Superconductor Jooseop Lee, Matthew Stone, Taner Yildrim, Ashfia Huq, Georg Ehlers, Yoshikazu Mizuguchi, Seunghun Lee Elastic and Inelastic neutron scattering experiments have been carried out on polycrystalline samples of the newly discovered layered superconductor LaO0.5F0.5BiS2, and its nonsuperconducting parent compound LaOBiS2 to determine their crystal structures and lattice vibrational modes. The Bragg peaks from the superconducting sample shows large broadening in width in the powder diffraction pattern. For the lattice vibrations, significant difference was observed upon F doping. Using the density functional perturbation theory, we identified all phonon modes, and show the major change in the phonon spectrum comes mainly from the change in the Oxygen mode. Even though strong electron phonon coupling constant was estimated, no significant difference in the phonon spectrum from BiS2 superconducting layer was found above and below Tc. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U36.00007: Intercalant dependent electronic structure studies on alkali metal intercalated graphite compounds Wonshik Kyung, Yeongkwan Kim, Garam Han, Choonshik Leem, Chul Kim, Yeongwook Kim, Junsung Kim, Changyoung Kim We present electronic structure study results on various alkali-metal intercalated graphite compounds (GIC) using angle-resolved photoemission spectroscopy(ARPES). There are two competing theories on where the superconductivity occurs in this material; intercalant metal or charge doped graphene layer. To elucidate this issue, we measured electron doping amount of each alkali-metal intercalated GICs. In addition, there are some mysterious problems that can't be explained with current theories. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U36.00008: Strong Variation of Density of States and Anomalous Isotope Effect in Low and High Tc Superconductors Guang-Lin Zhao In this work, first-principles density functional theory (DFT) calculations of electronic structures are integrated into the fundamental formalism of many-body physics for superconductivity in Zr, Nb$_{3}$Sn, and YBa2Cu3O7. It is shown that the electronic structures of the transition metals and compounds such as Zr, Nb$_{3}$Sn, and YBa2Cu3O7 are very complex. The electron densities of states around their Fermi levels possess sharp variations that have a large contribution to the anomalous isotope effect in these superconductors. The work was funded in part by NSF LASIGMA Project (Award No. EPS-1003897, NSF92010-15-RII-SUBR), AFOSR (FA9550-09-1-0367), and NSF project CBET-0754821. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U36.00009: Charge density wave transport in heterogeneously doped NbSe$_{3}$ single crystals by masked ion (B$^{+}$, Li$^{+})$ implantation Kalyan Sasmal, Asanga Wijesinghe, Dharshana Wijesundera, Zhongjia Tang, Arnold Guloy, Wei-Kan Chu, John H. Miller Charge-density wave is competing electron spectrum instability of superconductivity (SC). CDW transport vs. SC with boundary between CDWs and SC are well known examples of correlated transport of macroscopic numbers of electrons.CDW superconductors include layered dichalcogenides, NbSe$_{3}$. On selective area medium energy ion (B$^{+}$, Li$^{+})$ implantation was used to create irradiated/unmodified/irradiated CDW heterostructures with well-defined interfaces on single NbSe$_{3}$ crystal. The effects of impurities go beyond simply increasing CDW pinning (J.P.McCarten.et.al J.Phys.IV France 9,1999).The dV/dI vs. bias at several temperatures, and zero-bias resistance vs. temperature, where two voltage contacts straddle the interface (near the boundary between B$^{+}$/ Li$^{+}$-implanted and unimplanted regions) are well studied. Injected charge B$^{+}$/Li$^{+}$ is a non-isoelectronic impurity. The results of ongoing investigations of similar studies of boron- and lithium-doped NbSe$_{3}$ will be discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U36.00010: A Time-Domain Susceptibility Model for a BCS Superconductor in FDTD Calculations G.L. Carr, Xiaoxiang Xi We have developed a simple time-domain electric susceptibility model for a BCS type superconductor, valid for the spectral range spanning the optical energy gap frequency $\hbar\omega$$\sim$2$\Delta$ and $\it{T}\ll\it{T}_C$. The expression can be used in Finite Difference Time Domain (FDTD) calculations for propagating electromagnetic waves through systems containing superconductor materials, including meta-materials. Since the energy gap appears explicitly, it can be varied as a function of time to describe non-linear and non-equilibrium effects as observed in microwave experiments. We use the expression in a FDTD calculation for the transmission through and reflection from a thin film of NbN on a substrate, and compare with both conventional frequency domain calculations as well as actual experimental results. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U36.00011: Influence of the interplay between de Gennes boundary conditions and cubicity of Ginzburg-Landau equation on the properties of superconductors Oleg Olendski Solutions of the Ginzburg-Landau (GL) equation for the film subjected to the de Gennes boundary conditions (BCs) with extrapolation length $\Lambda$ are analyzed with emphasis on the interaction between $\Lambda$ and the coefficient $\beta$ of the cubic GL term and its influence on the temperature $T$ of the strip. Very substantial role is played also by the carrier density $n_s$. Physical interpretation is based on the $n_s$-dependent effective potential $V_{eff}({\bf r})$ created by the nonlinear term and its influence on the lowest eigenvalue of the corresponding Schr\"{o}dinger equation. For the large cubicities, the temperature $T$ becomes $\Lambda$ independent linearly decreasing function of the growing $\beta$ since in this limit the BCs can not alter very strong $V_{eff}$. The temperature increase produced in the linear GL regime by the negative de Gennes distance is wiped out by the growing cubicity. In this case, the decreasing $T$ passes through its bulk value $T_c$ at the unique density $n_s^{(0)}$ only, and the corresponding $\Lambda_{T=T_c}$ is an analytical function of $\beta$. For the large cubicities, the concentration $n_s^{(0)}$ transforms into the density of the bulk sample. Other analytical asymptotics are analyzed too. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U36.00012: Numerical study of the magnetic and pair binding properties in aromatic hydrocarbon superconductors Zhongbing Huang, Chao Zhang, Haiqing Lin We performed a systematic numerical study of the magnetic and pair binding properties in recently discovered aromatic hydrocarbon superconductors, by using exact diagonalization and quantum Monte Carlo methods. The $\pi $-electrons on the carbon atoms of a single molecule are modelled by the one-orbital Hubbard model, which takes into account the energy difference between carbon atoms with and without hydrogen bonds. Our results show that the spin polarized ground state is realized for charged molecules in the physical parameter region. This provides a reasonable explanation of local spins observed in experiments. In alkali-metal-doped picene and phenanthrene, the pairing binding energy is always negative for different electron doping densities, suggesting that electron correlation has no contribution to the formation of Cooper pairs. However, a positive pair binding energy for the charged dibenzopentacene molecule with one or three added electrons indicates that electron correlation may produce an effective attraction between electrons. [Preview Abstract] |
Session U37: Focus Session: Fe-based Superconductors: Tunneling Spectroscopy
Sponsoring Units: DMP DCOMPChair: Laura Greene, University of Illinois
Room: 345/346
Thursday, March 21, 2013 11:15AM - 11:27AM |
U37.00001: STM on LiFeAs - Momentum Resolved Superconducting Gap Structure, Electron-Boson Interactions and Charge Susceptibilities in a Prototypical Iron-Based Superconductor A.W. Rost, M.P. Allan, T.-M. Chuang, F. Massee, K. Lee, M. Fischer, Y. Xie, K. Kihou, C.-H. Lee, A. Iyo, H. Eisaki, A.P. Mackenzie, E.-A. Kim, D.J. Scalapino, J.C. Davis Tunneling spectroscopy on strong coupling superconductors has been one of the key experiments confirming the phonon-mediated mechanism of superconductivity. In the last two decades it has become possible using STM to access this information in real space with atomic resolution. One of the most important aspects of these developments is the ability to extract momentum space resolved information from Fourier-Transform STM measurements. Here we will demonstrate using our recent data on LiFeAs how this technique allows access to a range of fundamental properties of the electronic excitation spectrum. In particular I will show that it is now in principle possible to access momentum space resolved information not only on the superconducting gap structure but also on quantities such as electron-boson interactions and geometric information on `nesting' vectors giving rise to peaks in the charge susceptibility. The resulting `fingerprint' of the mechanism driving superconductivity goes well beyond the information obtained in traditional tunneling experiments and has the potential of being a key experimental tool in the study of the mechanism of unconventional superconductors. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U37.00002: Tunneling Spectroscopy in Iron Pnictides to Track Orbital Splitting and Spin Density Waves Nachum Plonka, Alexander Kemper, Thomas Devereaux, Siegfried Graser, Arno Kampf In iron-based superconductors, nematicity has been reported in transport measurements and a broad range of spectroscopies, including angle-resolved photoemission, neutron scattering, and scanning tunneling spectroscopy (STS). Several theories have attributed these observed anisotropies of broken tetragonal symmetry to either pure spin physics or unequal occupation of the iron d-electron orbitals, referred to as orbital ordering. We use realistic multi-orbital tight-binding Hamiltonians and T-matrix formalism to explore the effects of non-magnetic impurities in an orbitally split and spin density wave (SDW) state. In each of these, the local density of states around the impurity in both position space and Fourier-transformed quasiparticle interference (QPI) have very specific signatures that may be observable in STS. These allow one to identify and track the evolution of orbital splitting and SDW gaps in regimes that have not previously been explored. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U37.00003: Electronic Inhomogeneity and Vortex Disorder in Superconducting Sr$_{0.75}$K$_{0.25}$Fe$_{2}$As$_{2}$ Can-Li Song, Yi Yin, Martin Zech, Tess Williams, Michael Yee, Gen-Fu Chen, Jian-Lin Luo, Nan-Lin Wang, Eric W. Hudson, Jennifer E. Hoffman We characterize the surface structure, superconducting, and vortex properties in the hole-doped superconductor Sr$_{0.75}$K$_{0.25}$Fe$_{2}$As$_{2}$ (underdoped, $T_{c}=$32 K) by scanning tunneling microscopy. A 1 $\times$ 2 surface reconstruction and inhomogeneous superconducting gap with clear coherence peaks are universally found on the dominant Sr/K-terminated surfaces. Rarer patches of As termination show no reconstruction and no gap. The superconducting gap energy $\Delta $ anti-correlates with both the zero bias conductance and coherence peak strength with a characteristic length scale of $\sim$ 3 nm. Isotropic single-quantum vortices with short-range hexagonal order are imaged at 9 T magnetic field. By fitting the vortex-induced subgap density of states, the coherence length $\xi \sim$ 2.8 nm is found to be comparable to the length scale of $\Delta $ variations. We suggest that the vortices are strongly pinned by nanoscale electronic inhomogeneity arising from K clustering. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U37.00004: Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As Peng Cai, Xiaodong Zhou, Wei Ruan, Aifeng Wang, Xianhui Chen, Dung-Hai Lee, Yayu Wang Although the origin of high Tc superconductivity in the iron pnictides is still under debate, it is widely believed that magnetic interactions or fluctuations play an important role in triggering Cooper pairing. Because of the relevance of magnetism to pairing, the question of whether long range spin magnetic order can coexist with superconductivity microscopically has attracted strong interests. The available experimental methods used to answer this question are either bulk probes or local ones without control of probing position, thus the answers range from mutual exclusion to homogeneous coexistence. In this talk we present STM studies of the local electronic structures of an underdoped NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As near the spin density wave (SDW) and superconducting (SC) phase boundary. Spatially resolved spectroscopy directly reveal both the SDW and SC gap features at the same atomic location, providing compelling evidence for the microscopic coexistence of the two phases. The strengths of the SDW and SC features are shown to anti correlate with each other, indicating the competition of the two orders. The microscopic coexistence clearly indicates that Cooper pairing occurs when portions of the Fermi surface are already gapped by the SDW order. [1]P. Cai, et al., arxiv:1208.3842(2012) [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U37.00005: Conductance spectra of the Fe111 compounds in the normal and superconducting states Hamood Arham, W.K. Park, L.H. Greene, D.Y. Chung, D. Bugaris, M.G. Kanatzidis We use quasiparticle scattering spectroscopy (QPS), also known as point contact spectroscopy, to study Co doped NaFeAs. A conductance enhancement is observed in the normal state of NaFeAs with an onset temperature $\sim$ 95 K. Our previous work on the electron and hole doped Fe122 compounds revealed that a conductance enhancement in the normal state is only observed for those compounds that have an in-plane resistive anisotropy. This enhancement is caused by the non-Fermi liquid behavior of these compounds due to orbital fluctuations. (Arham et al. PRB 85, 214515 (2012); Lee et al. arXiv:1110.5917). Our initial results indicate that the same conditions hold true for the Fe111 compounds as well. QPS is effective in detecting strong electron correlations (hybridization gap, Fano resonance, orbital fluctuations) in the normal state of a variety of strongly correlated electron systems that exhibit the ubiquitous `domed' phase diagram. The need for some kind of a microscopic theory that explains how QPS detects strong electron correlations will be discussed. This work is supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the US DOE, Office of Science, Award No. DE-AC0298CH1088. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U37.00006: Doping dependence of the gap of cobalt doped BaFe$_{2}$As$_{2}$ from Point Contact Spectroscopy John Timmerwilke, Brendan Faeth, J.S. Kim, G.R. Stewart, Amlan Biswas Point-contact spectroscopy (PCS) is a unique method which has been used for investigating the gap/s of various superconductors including the iron based superconductors. PCS measurements are capable of systematically identifying the size and number of gaps in a superconductor, certain features of various gap symmetries and gap anisotropy. We have performed a-b plane point contact measurements on single crystal Ba(Fe$_{1-x}$Co$_{x)2}$As$_{2}$ samples in the under, optimal, and over-doped cases. Previously we had shown clear evidence of two full gaps in the optimally-doped case. The under and over-doped crystals do not show such definitive evidence of two gaps. The changes in anisotropy and weight of the gaps for these dopings will be presented. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U37.00007: Similarities in the Tunneling Spectral Dip in FeAs-based and Cuprate Superconductors John Zasadzinski, Liam Coffey, Omid Ahmadi, Ken Gray, David Hinks Recent STS measurements on LiFeAs revealed an above-gap spectral dip feature in the superconducting state that diminished in size with increasing T and disappeared at Tc. We argue that such a feature mimics conventional strong coupling effects and bears a striking resemblance to dip features found in cuprates such as Bi2212. In all cases, the estimated boson energy, $\Omega$, lies within the superconducting gap, 2$\Delta $, suggesting a spin exciton, and is $\sim$ 5k$_{\mathrm{B}}$Tc, consistent with the resonance mode found in neutron scattering. The doping dependence of the dip in Bi2212 break junctions is reviewed and it is shown that fits of the tunneling data can be achieved using an Eliashberg formalism. The electron-boson spectral function is dominated by a sharp peak at $\Omega $. These results indicate that the two classes of superconductors have a similar pairing interaction of electrons coupled to a spin fluctuation spectrum renormalized by superconductivity. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U37.00008: Temperature-concentration phase diagram and multigap superconductivity revealed by soft point-contact spectroscopy in (Ca $_{\mathrm{1-x}}$ La$_{\mathrm{x}})_{10}$(Pt$_3$As$_8)$(Fe$_2$As$_2)_5$ Ni Ni, Eunsung Park, Warren E. Straszheim, Xin Lu, Darrick J. Williams, Makariy A. Tanatar, Ruslan Prozorov, Eric D. Bauer, Filip Ronning, Joe D. Thompson, Robert J. Cava Sizable single crystals of the superconducting iron-pnictide system (Ca $_{\mathrm{1-x}}$ La$_{\mathrm{x}})_{10}$(Pt$_{3}$As$_{\mathrm{8}})$(Fe$_{2}$As$_{2})_{5}$ (x$=$0 to 0.182) have been grown and characterized by X-ray, microscopic, resistivity, Hall coefficient, susceptibility and specific heat measurements. Features in magnetic susceptibility, specific heat and two kinks in the derivative of the electrical resistivity around 100 K in the x$=$0 compound support the existence of decoupled structural and magnetic phase transitions. With La doping, the structural/magnetic phase transitions are suppressed and a dome of superconductivity with a maximal T$_{\mathrm{c}}$ up to 23 K is observed in the temperature-concentration phase diagram. Soft point-contact spectroscopy was performed on the optimally doped sample of x$=$0.145. By fitting the multigap Blonder-Tinkham-Klapwijk(BTK) model to the data, three gaps with $\Delta _{1}=$1 meV, $\Delta_{2}=$8 meV and $\Delta _{3}=$27 meV are revealed. Acknowledgement: Work at Los Alamos was performed under the auspices of the US DOE. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U37.00009: Surface investigation of Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$ by scanning tunneling microscopy Dennis Huang, Ilija Zeljkovic, Can-Li Song, Bing Lv, Ching-Wu Chu, Jennifer E. Hoffman Rare-earth-doped CaFe$_{2}$As$_{2}$ exhibits small volume-fraction superconductivity up to 49 K of unknown origin [1,2]. We use scanning tunneling microscopy to locally probe possible sources of this phase in Ca$_{1-x}$Pr$_{x}$Fe$_{2}$As$_{2}$. We encounter three kinds of surface morphologies and infer their chemical identities with local work function measurements. We also image Pr$^{3+}$ dopants as positive-energy resonances in tunneling conductance and examine their relationship with an observed inhomogeneous spectral gap. [1] B. Lv, L. Denga, M. Goocha, F. Weia, Y. Suna, J. K. Meena, Y.-Y. Xuea, B. Lorenza, and C.-W. Chu, Proc. Nat. Acad. Sci. 108, 15705 (2011). [2] S. R. Saha, N. P. Butch, T. Drye, J. Magill, S. Ziemak, K. Kirshenbaum, P. Y. Zavalij, J. W. Lynn, and J. Paglione, Phys. Rev. B 85, 024525 (2012) [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U37.00010: Identification of surface terminations of iron pnictides with low-temperature STM/STS Jihui Wang, Ang Li, Jihua Ma, Zheng Wu, Jiaxin Yin, Bing Lv, C.W. Chu, A. Sefat, M. McGuire, B. Sales, D. Mandrus, Chenglin Zhang, Pengcheng Dai, Rongying Jin, Jiandi Zhang, E.W. Plummer, Genfu Chen, Hong Ding, Shuheng H. Pan The alkaline-earth metal iron pnictide superconductor AEFe2As2 (AE$=$Ca, Sr, Ba) have been studied extensively with modern surface techniques, such as scanning tunneling microscopy/spectroscopy (STM/STS) and Angle Resolved Photoemission Spectroscopy (ARPES). Yet the surface termination upon cleaving is still controversial. Hence, the interpretation of those results of STM/STS and reconcile with results of other surface techniques tend to be challenging. We have performed a systematic low-temperature STM/STS study on a series of (Ca,Na)Fe2As2, (Ba,K)Fe2As2, Ba(Fe,Co)2As2, and BaFe2(As,P)2. We found that, with cryogenic cleaving method, all three crystalline atomic layers can be revealed and identified. We will discuss their identities and their implications. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U37.00011: Observation of orbital governed surface selection of superconducting gap in iron Pnictides with low temperature STM/S Jiaxin Yin, Ang Li, Zheng Wu, Jihui Wang, Jian Li, Chin-Sen Ting, Chenglin Zhang, Pengcheng Dai, ChangQing Jin, Hong Ding, Shuheng H. Pan The strong anisotropy of orbitals plays important roles in strongly correlated electron systems. For iron pnictides, due to their layered structure, overlaping of iron 3d with arsenic 4p orbitals is essential in the pairing mechanism. To reveal such physics, Ba(K)Fe2As2 and LiFeAs are the ideal candidates owing to their integrity in the Fe-As layer. We have used low temperature scanning tunneling microscopy/spectroscopy (STM/STS) to investigate the orbital physics in Ba0.6K0.4Fe2As2 and LiFeAs at atomic level. By comparing the STM/S results on these two materials and referring to the results of angle resolved photoemission spectroscopy (ARPES), we found the phenomenon of surface dependent selection of superconducting gaps. We discuss the implications of these observations with the orbital physics in these materials. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U37.00012: (1x2) Surface Reconstruction for Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_2$As$_2$: Spin-Charge-Lattice-Coupling Guorong Li, Liangbo Liang, V.B. Nascimento, Xiaobo He, A.B. Karki, Yimin Xiong, Vincent Meunier, Rongying Jin, Jiandi Zhang, E.W. Plummer Low energy electron diffraction (LEED) and density functional theory (DFT) have been utilized to investigate the surface structure for the stripe 1x2 phase of Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ iron pnictides, for x $=$ 0 and x $=$ 0.075. Quantitative structural analysis of LEED-I(V) using the fractional spots of the 1x2 phase on both parent and doped samples gives a similar surface structure with a termination layer of half Ca atoms. The surface Ca layer has a large inward relaxation about 0.5 Angstrom and the underneath As-Fe-As layer displays a buckling distortion of about 0.07 Angstrom. DFT calculations show significant charge rearrangements at the surface, which is driven by spin charge coupling, verified by freezing the structure and reducing the magnetic moment to zero. The role of spin-charge coupling in determining the surface reconstruction will be elucidated by self-consistent calculations of the structure as a function of the magnetic moment. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U37.00013: Surface structure and electronic properties in Ca$_{10}$(Pt$_{4}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ Jisun Kim, Guorong Li, Amar Karki, Jiandi Zhang, Rongying Jin, E.W. Plummer Among Iron-based superconductors, a new family of Ca$_{10}$(Pt$_{n}$As$_{8}$)(Fe$_{2}$As$_{2}$)$_{5}$ with n= 3 (``10-3-8'') or n=4 (``10-4-8'') is unique owing to the existence of Pt$_{n}$As$_{8}$ layer. This sets them with different electronic properties than the rest of Iron-based superconductors. By cleaving 10-4-8 single crystals (T$_{c}$ $\sim$ 34 K) in the ultra-high vacuum, we are able to observe three surfaces: Ca layer, FeAs layer, and Pt$_{4}$As$_{8}$ layer. Scanning tunneling microscope (STM) reveals both the topology and electronic density of individual layers. We discuss the implications of our results with the combination of bulk electronic properties. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U37.00014: Multi-band model analysis of transport properties of Ba(FeAs)$_2$ Huynh Khuong, Yoichi Tanabe, Takahiro Urata, Satoshi Heguri, Takanori Kida, Masayuki Hagiwara, Katsumi Tanigaki In iron pnitides, unique energetic band topology and interband antiferromagnetic scatterings are the main sources of rich physics, including multiband superconductivity and Dirac cones quantum states [1, 2]. Despite its importance, the band structure of iron pnictides is not fully understood, especially in terms of transport phenomena. In this meeting, we present that the tranport properties of Ba(FeAs)$_2$, a typical iron pnictide compound, are strongly affected by the shape of Fermi surfaces and the high mobility ($\mu$) in the Dirac cones. From magnetic-field ($B$) dependencies of the conductivity tensor under $B < 50$ T, we successfully extracted a spectrum of carrier number as a function of $\mu$. Whereas the hole side of the spectra is purely characterized by parabolic hole pockets, the electron side shows interesting effects originating from partly concave Fermi pockets as well as the very high $\mu$ (50,000 cm$^2$V$^{-1}$s$^{-1}$) of the Dirac carriers. Our observations are also in a good agreement with the first principles band calculations and experimental spectroscopic observations on its Fermi surface [3, 4]. [1] K. Kuroki et al, PRL 101 (2008) [2] Ran et al, PRB 79 (2009) [3] Yin et al, Nat.Phys. 7 (2011) [4] T. Shimojima, PRL 104 (2010) [Preview Abstract] |
Session U38: Novel Photophysics and Transport in NanoPV II
Sponsoring Units: GERA DPOLY DCOMPChair: Richard Wiener, Research Corporaton
Room: 347
Thursday, March 21, 2013 11:15AM - 11:27AM |
U38.00001: Hybrid passivated colloidal quantum dot solids for photovoltaics Susanna M. Thon, Alexander H. Ip, Sjoerd Hoogland, Oleksandr Voznyy, David Zhitomirsky, Ratan Debnath, Larissa Levina, Lisa R. Rollny, Graham H. Carey, Armin Fischer, Kyle W. Kemp, Illan J. Kramer, Zhijun Ning, Andr\'{e} J. Labelle, Kang Wei Chou, Aram Amassian, Edward H. Sargent Colloidal quantum dot (CQD) films are an attractive photovoltaic material due to their large-area-compatible solution processing and bandgap tuning through the quantum size effect. However, the large internal surface areas make CQD films prone to high trap state densities, leading to recombination of charge carriers. We quantify the density of midgap trap states in PbS CQD solids and show that the current photovoltaic performance is limited by these states. We develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger standard organic ligands, and combine this with an organic crosslinking strategy to form the film. We use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0\%, which is a record for a CQD photovoltaic device. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U38.00002: Elimination of deep surface traps in charged colloidal PbS and CdSe quantum dots Oleksandr Voznyy, Susanna Thon, Alex Ip, Edward Sargent Colloidal quantum dots (CQDs) offer a promising path towards high efficiency, scalable, solution and room processed photovoltaics and electronics. Their promise is curtailed today by difficulty of doping, inefficient transport, nonradiative recombination, and blinking, all generally attributed to electronic trap formation. Using first-principles simulations on off-stoichiometric colloidal quantum dots, we show that preparing a CQD free of traps is possible. However, self-compensating defects can form deep electronic trap states in response to charging or doping even in the most idealized CQDs. Surface traps arise from atomic dimers whose energy levels reside within the bandgap. The same states can also form upon photoexcitation, providing an atomistic mechanism for blinking. We show that avoiding the trap formation upon doping is possible by incorporation of select cations on the surface which shift the dimer energy levels above the quantum-confined bandedge. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U38.00003: High pressure core structures of Si nanoparticles for solar energy conversion S. Wippermann, M. Voros, D. Rocca, A. Gali, G. Zimanyi, G. Galli Multiple exciton generation (MEG) in semiconductor nanoparticles (NPs) is a promising path towards surpassing the Shockley-Queisser limit in solar energy conversion efficiency. Recent studies demonstrate MEG to be more efficient in NPs than in the bulk, including Si [1]. However, the increased efficiency is observed only on a relative energy scale in units of the gap: quantum confinement (QC) effects believed to be responsible for efficient MEG in NPs, also increase their optical gap, swiftly shifting the MEG threshold beyond the solar spectrum. Device applications require NPs with low gaps despite the QC enhanced Coulomb interaction. We propose that Si NPs with a core structure resembling that of high pressure Si phases, especially Si-III/BC8, exhibit significantly lower optical absorption thresholds than Si-I NPs, while retaining efficient MEG. The existence of such NPs was recently demonstrated [2]. Our predictions [3] are based on density functional and many body perturbation theory calculations of the electronic, optical and impact ionization properties of hydrogenated Si NPs with high pressure core structures. [1] M. Beard, JPCL 2, 1282 (2011); [2] M. Smith et al., JAP 110, 053524 (2011); [3] S. Wippermann et al. (submitted); [4] M. Voros et al. (submitted) [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U38.00004: Exploring the Influence of the Chemical Passivation on Electron Relaxation in Silicon Quantum Dots Using First-Principles Surface Hopping Methods Yosuke Kanai, Kyle Reeves, Andre Schleife The generation of hot carriers in nano-materials is an exciting phenomenon that could potentially increase the efficiency of photovoltaic and photo-electrochemical cells significantly. The electron relaxation dynamics of a system is related to both the electronic and phononic contributions. Given that both of these contributions are ultimately derived from the electronic structure of a system, chemical substitutions may play a significant role in augmenting and controlling the electron relaxation dynamics in nano-materials. With greater insight into the phenomenon from the first-principles theory, engineering new nano-materials with novel opto-electronic properties via a chemical functionalization of its surface becomes a more realistic avenue. A first-principles surface hopping approach based on density functional theory calculations is used to elucidate the relaxation dynamics in silicon quantum-dots. We explore how varying the passivating species on silicon quantum dots influences the electron relaxation dynamics in the system. The two systems considered here are hydrogen-passivated and fluorine-passivated silicon quantum dots. We present a detailed analysis of the electron relaxation dynamics in these nano-materials. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U38.00005: Germanium nanoparticles for solar energy conversion M\'arton V\"or\"os, Stefan Wippermann, Dario Rocca, Giulia Galli, Adam Gali, Gergely Zimanyi We propose a strategy to enhance the efficiency of solar energy conversion by elemental germanium, by using Multiple Exciton Generation (MEG) in Ge nanoparticles with a ST12 core structure. The latter is the structure of a high pressure phase of solid Ge. MEG is more efficient in bulk Ge in the diamond phase than in several other semiconductors, e.g.\ Si. In principle it may be further improved at the nanoscale, due to an increased effective Coulomb interaction. However the electronic energy gap of semiconducting nanoparticles may be too large compared to the visible solar spectrum and their density of states (DOS) too low for efficient solar energy conversion. Using ab initio calculations we found that ST12 Ge nanoparticles of $\sim$1-2~nm exhibit high impact ionization rates and thus presumably efficient MEG, as well as a gap of $\sim$2~eV and a sizable DOS in the low energy part of the spectrum. Therefore these nanoparticles appear to be promising materials for solar energy conversion exploiting MEG. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U38.00006: Temperature-Dependent Electron Transport in Si and Ge Nanoparticle Photovoltaics Derek Padilla, Carena Church, Elayaraja Muthuswamy, Susan Kauzlarich, Sue Carter We have studied both Si and Ge nanoparticle-based photovoltaic (PV) devices fabricated in a layered structure via spin-coating of the colloidal Si or Ge solution. With the low toxicity and high abundance of these group IV elements, combined with the relatively low costs of manufacturing via solution deposition, large-scale device processing offers high dollar-per-Watt opportunities as efficiencies continue to improve. To that end, we previously reported temperature effects of solution-processed PbS quantum dot (QD) PVs, wherein the capping ligand's thermal properties were shown to have strong effects on device performance. Here we show similar ligand effects in group IV QD devices. Current-voltage (I-V) measurements at temperatures from 100 to 360 K under dark conditions were fit to the ideal diode equation revealing the electron transport mechanism, with fit parameters matching transport models. The illuminated I-V data provide insight into each device's built-in potential, carrier mobility, and activation energy. In addition, modulating the illumination intensity gives the ideality factors of the solar cells. We show how these variations with temperature and light-intensity can be used to increase device performance for future studies. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U38.00007: Carrier Multiplication Effects Between Interacting Nanocrystals for Solar Cell Applications Ivan Marri, Marco Govoni, Stefano Ossicini Carrier multiplication is a carrier relaxation process that results in the generation of multiple electron-hole pairs after absorption of a single photon. Such effect can potentially increase power conversion efficiency in solar cells by minimizing effects induced by thermalization loss processes.The possibility of increasing carrier multiplication efficiency by exploiting nanocrystals interplay have been recently demonstrated in both PbSe \footnote{A. Aerts, et al., Nano Lett. 11 4485 (2011)} and Silicon \footnote{D. Timmerman, et al., Nat. Photon. 2, 105 (2008)} \footnote{M.T. Trinh, et al., Nature Photon. 6, 316 (2012)} strongly coupled nanocrystals. In this talk we will analyze the role played by nanocrystal-nanocrystal interaction on carrier multiplication dynamics considering a system of interacting silicon nanoparticles. Using first-principles calculations, quantum cutting energy-transfer processes will be quantified and a new carrier multiplication effect, defined by us Coulomb driven Charge Transfer, will be introduced. Conditions that maximize effects induced by nanocrystals interplay on Carrier Multiplication dynamics will be pointed out and the role played by wavefunctions delocalization will be clarified \footnote{M. Govoni, et al., Nature Photon. 6, 672 (2012)}. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U38.00008: Monte Carlo modeling of charge transport in nanocrystalline PbSe films Ian Carbone, Gergely Zimanyi, Sue Carter The electronic properties of three-dimensional nanocrystalline (NC) PbSe materials are of particular interest for next generation solar energy conversion technologies. With size-tunable optical and electronic properties, solution processing, and multiple exciton generation, these materials could represent an exciting new class of cost-effective and efficient solar cells. Two models, a multiple trapping random walk(MTRW) and a hopping model, were developed to simulate electron and hole transport in films of PbSe nanoparticles crosslinked with ethane dithiol ligands. This Monte Carlo code could easily be adapted to model solar cell current-voltage characteristics and variety of experimental conditions and device structures. In both simulations, films are represented by a regular cubic lattice, transport is carried out as a series of hopping events between neighboring nanocrystals, and electrons occupy energetic states determined by the particle size of the PbSe nanocrystals. We find that the hopping model represents a simpler parameter set and a better match to experimental measurements. This presentation will discuss the two transport mechanisms and the effects of particle size, energetic disorder, and coulomb blockade effects on electron and hole mobilities. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U38.00009: 3D engineering of potential profile by charged quantum dots for effective photovoltaic conversion Andrei Sergeev, Nizami Vagidov, Vladimir Mitin, Kimberly Sablon Charging of quantum dots (QDs) is an effective tool for managing of potential profile at micro- and nanoscales. Without radiation, QDs are charged as electrons from the dopants fill QDs. Filling of QDs under solar radiation is determined by the condition of equality of electron and hole capture rates. Because of strong difference in effective masses of electrons and holes, an electron level spacing in QDs substantially exceeds a level spacing for holes. Therefore, QDs play a role of deep traps for electrons, but they are just shallow traps for holes. The holes trapped in QDs may be excited by thermal phonons, while excitation of localized electrons requires IR radiation. Therefore, n-doping of QD structures is strongly preferable for photovoltaic applications. Optimized selective n-doping of QD medium provides micro- and nanoscale potential profiles favorable for effective photovoltaic conversion. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U38.00010: Toward an Impurity Band PV: Dynamics of Carriers Generated via Sub-band gap Photons Joseph Sullivan, Christie Simmons, Austin Akey, Michael Aziz, Tonio Buonassisi Intermediate band solar cells are a pathway to cells that surpass the Shockley-Queisser limit by enabling the utilization of sub-band gap photons. A proposed method for fabricating an intermediate band material is to use impurities that introduce electronic levels within the band gap. At sufficiently high dopant concentrations, band formation may lead to a suppression of Shockley-Reed-Hall recombination, an idea known as ``lifetime recovery'' [1]. We investigate a proposed intermediate band material, silicon hyper-doped with sulfur. This material system exhibits strong sub-band gap optical absorption and metallic conductivity at sufficiently high sulfur concentrations [2], which makes it a strong candidate for an impurity-band material. We employ low-temperature photoconductivity using sub-band gap light to estimate the trapping rate of electrons in the conduction band. We vary the sulfur concentration near the critical value for the metal-insulator transition to test the idea of ``lifetime recovery'' in the S:Si system.\\[4pt] [1] A. Luque and A. Mart\'i, Adv. Mater. 22, 160 (2010).\\[0pt] [2] M. T. Winkler et.al. Phys. Rev. Lett. 106, 178701 (2011) [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U38.00011: Intermediate Band Performance of GaSb Type-II Quantum Dots Located in n-Doped Region of GaAs Solar Cells Ara Kechiantz, Andrei Afanasev The intermediate band (IB) electronic states assist sub-bandgap photons in generation of additional photocurrent in single-junction solar cells. Such non-linear effect of resonant two-photon absorption of concentrated sunlight attracts much attention because it promises up to 63{\%} conversion efficiency in IB solar cells. The main obstacle to achieving high performance is involvement of IB-states in electron-hole recombination that is drastically increasing the dark current and reducing the open circuit voltage of IB solar cells. The IB-states can be composed of quantum dots (QDs). Concentration of sunlight limits recombination through type-II QD IB-states located outside of the depletion region. In this work we model GaAs solar cell with strained GaSb type-II QDs separated from the depletion region. The focus is on type-II QDs located in n-doped region of p-n-junction. Our calculation shows that photovoltaic performance can be essentially improved by concentration of sunlight, and that this improvement is highly sensitive to the doping of materials and the shape of potential barriers surrounding type-II QDs. For instance, strained GaSb type-II QDs may increase the performance of GaAs solar cell by 20{\%} compared to the reference GaAs solar cell without QDs. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U38.00012: Single Element n-p Co-doped Wide Band-gap Semiconductors as Candidate Materials for Intermediate-Band Solar Cells Guangfen Wu, Chunlei Yang, Guohua Zhong, Xudong Xiao, Zhenyu Zhang Non-compensated n-p codoping by different element combinations has proven to be an effective approach to introduce intermediate bands in wide band-gap semiconductors. In this approach, the electrostatic attraction within an n-p dopant pair helps to enhance both the thermodynamic and kinetic solubilities of the dopants. Here we present a conceptually new and appealing approach to achieve non-compensated n-p codoping by substitutionally occupying the anionic and cationic sites in the host materials with a single element. The validity of this approach is demonstrated using first-principles calculations, showing that half filled energy bands are created within the forbidden gaps of the semiconductors because of the non-compensated nature of the codpants. Moreover, the electrostatic attraction between the neighboring dopant pairs enhances their thermodynamic and kinetic solubilities in the host semiconductors. Efforts on experimental confirmation of the single element n-p co-doping concept will also be discussed. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U38.00013: Optical conductivity of GaP alloys studied by hybrid-density functional theory Yoshihiro Gohda, Shinji Tsuneyuki Highly-mismatched semiconductor alloys are promising to produce multiple gaps utilizing wider frequency range of the solar spectrum. Quantitative first-principles calculations of the optical conductivity, which is of importance to access the performance of solar cells, are out of reach for the standard generalized gradient approximation (GGA) in density functional theory (DFT) due to well-known underestimation of the band gap. To overcome this problem, hybrid-DFT scheme is quite useful, which incorporates nonlocality of the exchange interaction reducing the self-interaction error in the GGA. In this work, highly-mismatched GaP alloys are studied as candidates for intermediate-band solar cells, where the optical conductivity is calculated on the basis of hybrid-DFT combined with time-dependent perturbation theory. Thanks to the practical computational costs of hybrid-DFT compared with the GW approximation, structures with realistic dopant concentrations are handled with 216-site supercells. Ideal composition of alloys in the sense of active optical transition energies and the formation energy are compared, where calculated results propose that the optimal doping condition is Mg-O co-doping [Y. Gohda and S. Tsuneyuki, Appl. Phys. Lett., in press.] [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U38.00014: Study of vertical correlation in type-II ZnCdTe/ZnCdSe submonolayer quantum dots for efficient intermediate band solar cells. Siddharth Dhomkar, Igor Kuskovsky, Uttam Manna, Ismail Noyan, Maria Tamargo Intermediate band solar cells (IBSCs), having an intermediate band (IB) of states within the bandgap of the host semiconductor that enhances the light absorption without reducing the open circuit voltage, are substantially more efficient than single-gap devices. The IB, in principle, can be fabricated using quantum dots (QDs) embedded in the host semiconductor; however, there are many growth and material issues related to fabricating practical devices. We tackle some of these problems by growing the type-II ZnCdTe/ZnCdSe submonolayer QD system that lack the wetting layer. We present results of high resolution x-ray diffraction based reciprocal space map studies, complemented by photoluminescence, showing that this material system is an excellent candidate for IBSCs. Specifically, we found that the sample with larger Te fractions has larger QDs with increased vertical correlation. The vertical correlation is particularly important to have sufficient overlap of the hole wavefunctions, to facilitate the IB formation in this material system. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U38.00015: Multiple Exciton Generation in Colloidal Si Nanocrystals at the Energy-Conservation-Limit M. Sagar Dodderi, Jihua Yang, Uwe Kortshagen, Erin Whitney, Octavi Semonin, Arthur Nozik, Mathew C. Beard Silicon covers more than 90{\%} of photovoltaic cell production and is the 2$^{\mathrm{nd}}$ most Earth-abundant element. In a Bulk Silicon solar cell about half of the total absorbed energy is lost as heat, following the detailed balance Shockley-Queisser (SQ) analysis. Generating multiple excitons (MEG) in quantum confined Nanocrystals per absorbed high energy photon is a route to circumvent some of the heat losses and thereby enhance photoconversion efficiency. However, to utilize the absorbed excess energy for MEG and to break the SQ limit it is desirable to establish MEG threshold as close as possible to 2 x E$_{\mathrm{g}}$. Using femtosecond transient absorption spectroscopy, we demonstrate for the first time the generation of multiple excitons \textit{right at the energy-conservation-limit (2 x E}$_{g})$ in colloidal Si nanocrystals. The observed `near hard MEG-onset' is independent of the size of the nanocrystals studied (2.8 nm and 3.5 nm dots). Unlike Lead chalcogenides, the effect of photocharging on MEG yield is not observed in Si nanocrystals even at moderate pump-photon fluences ($\sim$ 10 nJ), much higher than the fluence typically used to measure MEG (\textless\ 1nJ). The efficient MEG and the observation of `near hard MEG-onset' at 2 x E$_{\mathrm{g}}$ in an indirect band gap semiconductor is extremely promising and has strong implications for third generation photovoltaics and is expected to enhance photoconversion efficiencies. [Preview Abstract] |
Session U39: Metals: Alloys and Actinide Compounds
Sponsoring Units: DCMPChair: Lin-Lin Wang, Ames Laboratory
Room: 348
Thursday, March 21, 2013 11:15AM - 11:27AM |
U39.00001: Statistical Mechanics of Nanoscale Metallic Materials Based on Thermodynamic Availability Robert Cammarata When characterizing the equilibrium behavior of small metallic systems, capillary effects can strongly influence the thermal behavior and need to be taken into account in a complete thermodynamic analysis. Although a variety of approaches have been offered to incorporate these effects, they sometimes invoke certain intensive thermodynamic quantities (e.g., chemical potentials) that are not well-defined when dealing with a physically and/or chemically inhomogeneous interfacial region.It has been proposed that many of these difficulties can be resolved by employing the thermodynamic availability function rather than the conventional free energy potentials [R.C. Cammarata, Phil Mag. 88, 927 (2008); R.C. Cammarata, Sol. State Phys. 61, 1 (2009)]. When applied to statistical mechanical calculations, capillary effects on nanoscale system behavior can be obtained in a natural and rigorous way. This procedure will be briefly reviewed and then applied to nanoscale metallic fluid and solid systems. Important issues contrasting the thermodynamic differences between fluid and solid surfaces and how they need to be included in order to obtain physically meaningful results will be discussed. Applications to gas adsorption and nucleation will be presented. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U39.00002: Grain Rotation and Growth in Nanocrystalline Silver and Silver/Copper Alloys Michael Chandross, Shengfeng Cheng Grain rotation and growth play important roles in nanotribology and the plastic deformation of nanocrystalline metals and alloys. It is difficult, however, to study these processes with full atomistic detail experimentally. We used molecular dynamics simulations to investigate the grain rotation, coalescence, and growth in pure silver and silver/copper alloys after imposing various modes of deformation, including stretch, compression, and shear. Our results show that the degree of grain rotation and growth in pure silver depends on the state of stress in the sample and is most significant under shear deformation, where very large grains are observed after substantial shear. However, in silver/copper alloys, almost no grain growth was found even under strong shear. The presence of atoms with different lattice constants in alloys stabilizes the grain boundaries and makes grain coalescence less energetically favorable. The implications of these results on nanotribology of pure metals and alloys are discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U39.00003: Temperature-driven Phase Transformation in Y$_3$Co: Neutron Scattering and DFT Studies A. Podlesnyak, G. Ehlers, H. Cao, M. Matsuda, M. Frontzek, O. Zaharko, V.A. Kazantsev, A.F. Gubkin, N.V. Baranov The effects of a crystal structure deformation due to subtle atomic displacements have attracted much attention because they can result in colossal changes of the electronic and magnetic properties of solids. The R$_3$Co binary intermetallic systems exhibit a number of complicated phenomena, including field-induced magnetic phase transitions (R=Er, Ho, Tb), giant magnetoresistance (R=Dy), a substantial magnetocaloric effect (R=Gd) and superconductivity (R=La). Contrary to previous studies that defined the ground state crystal structure of the entire R$_3$Co series as orthorhombic Pnma, we find that Y$_3$Co undergoes a structural phase transition upon cooling around Tc~160K. Density functional theory calculations reveal a dynamical instability of the Pnma structure of Y$_3$Co. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\xi)$ acoustic phonon mode below the critical temperature Tc. We suggest that some other members of the R$_3$Co series (or even all of them) have ground state crystal symmetry lower than reported Pnma. This raises a question about the true magnetic structures and hence the influence of magnetic properties of the entire R$_3$Co series. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U39.00004: Optical Absorption Spectrum of Gold from First Principles Jamal Mustafa, Emmanouil Kioupakis, Steven Louie Phonon-assisted optical absorption is an important optical process in metals for photons in the visible part of the spectrum. Developments in first-principles computational methods have enabled the calculation of phonon-mediated optical absorption spectra of materials. The use of Maximally Localized Wannier Functions enables the interpolation of the GW quasiparticle band structure, along with the optical and electron-phonon coupling matrix elements, to very fine meshes in the Brillouin zone, which are needed for the calculation of the phonon-assisted absorption coefficient. We present calculations on gold that include the quasiparticle band structure and lifetimes, phonon dispersion, Wannier functions, and the phonon-assisted absorption spectrum. Since indirect absorption is a second-order process, the lifetime of the virtual intermediate state is of central importance. The results are compared to experimentally determined optical constants. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U39.00005: Less than perfect C$_{\mathrm{2v}}$ symmetry: loss of mirror plane symmetry in angle-resolved photoemission Thomas Scott, Keisuke Fukutani, Hirokazu Hayashi, Tula Paudel, Eike Schwier, Taiki Horike, Yorito Nagata, Jian Jiang, Hideaki Iwasawa, Kenya Shimada, Evgeny Tsymbal, Yaroslav Losovyj, Peter Dowben The effects of lack of in-plane $C_{2}$ invariance of the crystal on the angle-resolved photoemission spectra are investigated for Mo(112). The results indicate that, for Mo(112), the absence of $C_{2}$ symmetry gives rise to noticeable asymmetry in the ARPES band mapping along the \textless\ 1 1 -1\textgreater\ direction. The apparent differences in the experimental band structure in $+$k versus --k wave vectors can be understood quantitatively in terms of the asymmetries in the electronic bulk band structure, photoelectron diffraction as well as the initial state contribution to the photoemission matrix elements. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U39.00006: Prediction of dislocation junction strength in hexagonal close-packed crystals Chi-Chin Wu, Peter Chung Determination of dislocation junction strengths in \textit{hcp} crystals is important in order to understand and control the fundamental mechanisms in plastic deformation in new lightweight metals and to reduce the density of deleterious dislocations in wide band-gap wurtzite semiconductors. The many factors that may be involved, such as combinations of available slip systems, native material properties, and local morphology due to growth conditions, make systematic investigations via combinatorial experimental approaches challenging. Utilizing discrete dislocation (DD) simulations, we determine yield surfaces comprised by loci of critical stresses required to unzip junctions. Then, using a comparative study of different binary junctions formed by non-coplanar dislocations using different pairs of Burgers vectors on different intersecting planes in Mg and Be crystals, we find that the shape and orientation of yield surfaces are most sensitive to the planes on which the junction forms but independent of the elastic properties. The latter only appears to affect the size of yield surface which is consistent with known behavior in fcc crystals. This work particularly detects similarities and differences in dislocation junctions in hcp crystals. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U39.00007: Phonon Engineering in Metals from First Principles Nicholas Lanzillo, J. Thomas, E.B. Watson, M. Washington, Saroj K. Nayak The electron-phonon interaction in metallic systems controls the electronic transport properties, including both electrical and thermal resistivity. The effect of compressive strain on the electron-phonon interaction in metals is investigated using first-principles density functional theory, and we propose various ways to ``engineer'' this interaction for various technological applications. In particular, we show that by applying compressive strain on the FCC crystals of Al, Cu, Ag and Au, the net electron-phonon scattering rate decreases and likewise the electrical resistivity decreases with increasing pressure. This trend is corroborated by experimental measurements of the resistance of a 0.5 mm diameter high-purity Al wire pressurized up to 2 GPa in a solid-media pressure apparatus at room temperature. The rate of the decrease in electrical resistivity as a function of pressure as determined by experiment is matched by the rate predicted by theory. Our simulations show that Al nanowires have the same response to strain as the bulk crystal; the net electron-phonon scattering can be reduced through compressive strain. Modifying the electron-phonon interaction in metallic structures shows great promise for future nano-electronic devices. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U39.00008: Density-functional study of U-TRU-Zr and U-TRU-Mo alloys Alexander Landa, Per Soderlind, Patrice Turchi The U-Zr and U-Mo alloys proved to be very promising fuels for liquid metal fast breeder reactors. The optimal composition of these alloys is determined from the condition that the fuel could remain stable in the bcc phase ($\gamma $-U) in the temperature range of stability of $\alpha $-U phase. In other words, both Zr and Mo play a role of ``$\gamma $-stabilizers'' helping to keep U in the metastable bcc phase upon cooling. In the present study we perform KKR-ASA-CPA and EMTO-CPA calculations of the ground state properties of $\gamma $-U-Zr and $\gamma $-U-Mo alloys and compare their heats of formation with CALPHAD assessments. Though the U-Zr and U-Mo alloys can be used as nuclear fuels, a fast rector operation on a closed fuel cycle will, due to the nuclear reactions, contain significant amount of TRU elements (Np, Pu, and Am). Above mentioned density-functional theory techniques are extended to study ground-state properties of the bcc-based X-Zr and X-Mo (X $=$ Np, Pu, Am) solid solutions. We discuss how the heat of formation correlates with the charge transfer between the alloy components, and how magnetism influences the deviation from Vegard's law for the equilibrium atomic volume. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Work at LLNL was funded by the Laboratory Directed Research and Development Program under project tracking code 12-SI-008. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U39.00009: Thermal properties of UO$_{2}$ single crystal K. Gofryk, S. Du, A.D. Andersson, C.R. Stanek, R. Schulze, D. Safarik, B. Mihaila, J.C. Lashley, J.L. Smith For decades UO$_{2}$ has been the most widely studied actinide oxide because of its technological importance as fuel material for nuclear reactors. Therefore there is a large interest in understanding its thermal, transport and thermodynamic properties. We present recent experimental results for the thermal conductivity and thermal expansion of high quality UO$_{2}$ single crystal, obtained for different crystallographic directions, and compare with results of molecular dynamics simulations. We will discuss the implications of this study. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U39.00010: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U39.00011: Imaging electronic hot spots in the spectral function of the actinide UCoGa$_5$ Matthias J. Graf, Tanmoy Das, Tomasz Durakiewicz, Jian-Xin Zhu, John J. Joyce, John L. Sarrao We performed self-consistent GW-like calculations within the intermediate Coulomb-U coupling regime to investigate dynamic correlation effects in the intermetallic actinide UCoGa$_5$. This material is often used to contrast anomalous behavior in other U-115 and Pu-115 compounds, because it is presumed to be a conventional Fermi liquid that resembles a ``vegetable.'' First-principles electronic structure calculations were used as input, combined with the spin-fluctuation exchange approximation, to compute self-consistently the many-body self-energy responsible for dynamic correlation effects. We validated theory by angle-resolved photoemission spectroscopy (ARPES). The occurrence of electronic hot spots in the spectral function, accompanied by kinks and abrupt breaks in the slope of the quasiparticle dispersion were detected both at low (130 meV) and high (1 eV) binding energies below the Fermi energy. In conclusion, we found that dynamic correlation anomalies are adequately described by coupling between itinerant fermions and spin fluctuations arising from the particle-hole continuum of the spin-orbit-split 5f states of uranium. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U39.00012: Probing the f-state configuration of $\alpha$U and URu$_2$Si$_2$ with RXES Scott Medling, Corwin H. Booth, Ryan Baumbach, Eric D. Bauer We directly probed the electronic configuration of several uranium compounds using Resonant X-ray Emission Spectroscopy (RXES). Previous investigations by several groups into the magnetic properties of uranium compounds (such as URu$_2$Si$_2$) suggested that some are multiconfigurational. RXES is particularly useful for probing the configurations because measuring the energies of both the incident and scattered photons reveals information about both the empty and occupied electronic states. We collected data for several uranium samples ($\alpha$U, UO$_2$, and URu$_2$Si$_2$) which indicate that in some of these compounds the uranium is multiconfigurational, with a mixture of f$^1$,f$^2$, and f$^3$ occupancies. The degree of intermediate valence that this implies will be related to electronic and magnetic properties of the compound. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U39.00013: Towards a Density Functional Theory Exchange-Correlation Functional able to describe localization/delocalization Ann E. Mattsson, John M. Wills The inability to computationally describe the physics governing the properties of actinides and their alloys is the poster child of failure of existing Density Functional Theory exchange-correlation functionals. The intricate competition between localization and delocalization of the electrons, present in these materials, exposes the limitations of functionals only designed to properly describe one or the other situation. We will discuss the manifestation of this competition in real materials and propositions on how to construct a functional able to accurately describe properties of these materials. I addition we will discuss both the importance of using the Dirac equation to describe the relativistic effects in these materials, and the connection to the physics of transition metal oxides. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U39.00014: Actinide electronic structure based on the Dirac equation and density functional theory John M. Wills, Ann E. Mattsson Density functional theory (DFT) provides a formally predictive basis for predicting the structural properties of actinides. Although available approximations to the exchange/correlation functional provide accurate predictions for many materials, they fail qualitatively and sometimes quantitatively when applied to actinides. Major contributors to this deficiency are an inadequate treatment of confinement physics and an incomplete treatment of relativity in the underlying equations. The development of a functional correctly incorporating confinement physics with a proper treatment of relativity would provide definitive, internally consistent predictions of actinide properties. To enable the development of such a functional and quantify the predictions of currently available functionals, we have developed an efficient first-principles electronic structure method based on the Dirac equation. Results are compared with current methods, and the implications for relativistic density functionals discussed. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Session U40: Non-equilibrium Cold Atom Systems
Sponsoring Units: DAMOPChair: Marcos Rigol, Pennsylvania State University
Room: 349
Thursday, March 21, 2013 11:15AM - 11:27AM |
U40.00001: Emergence of an effective thermal correlation length in the course of prethermalization Remi Geiger, Maximilian Kuhnert, Tim Langen, Michael Gring, Bernhard Rauer, Takuya Kitagawa, Eugene Demler, David Adu-Smith, Joerg Schmiedmayer Understanding non-equilibrium processes in many-body quantum systems is an important unsolved problem in many areas of physics. Here, we study the relaxation dynamics of a coherently split one-dimensional Bose gas by measuring the full probability distribution functions of matter-wave interference. After splitting, the system rapidly relaxes to a thermal-like quasi-steady state retaining partial information about the initial conditions. We observe this state to be independent on the initial temperature before splitting and associate the relaxation dynamics with prethermalization. Observing the system on different length scales allows us to probe the dynamics of excitations on different energy scales, revealing two distinct length-scale dependent regimes of relaxation. We measure the crossover length-scale separating these two regimes and identify it with the prethermalized phase-correlation length of the system. Our work provides a direct vizualization of prethermalization and multimode dynamics in a one-dimensional many-body quantum system. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U40.00002: Quasi-universal transient behavior of a nonequilibrium Mott insulator driven by an electric field Karlis Mikelsons, Jim Freericks, H.R. Krishnamurthy We use a self-consistent strong-coupling expansion for the self-energy (perturbation theory in the hopping) to describe the nonequilibrium dynamics of strongly correlated lattice fermions. We study the three-dimensional homogeneous Fermi-Hubbard model driven by an external electric field showing that the damping of the ensuing Bloch oscillations depends on the direction of the field, and that for a broad range of field strengths, a long-lived transient prethermalized state emerges. This long-lived transient regime implies that thermal equilibrium may be out of reach of the time scales accessible in present cold atom experiments, but shows that an interesting new quasi-universal transient state exists in nonequilibrium governed by a thermalized kinetic energy, but not a thermalized potential energy. In addition, when the field strength is equal in magnitude to the interaction between atoms, the system undergoes a rapid thermalization, characterized by a different quasi-universal behavior of the current and spectral function for different values of the hopping. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U40.00003: Quench Dynamics in the Presence of a Bath Adam Rancon, Andreas Glatz, Igor Aranson, Kathy Levin Feshbach resonance are now widely used to tune the interaction strength in cold atoms. This allows one to experimentally study the out-of-equilibrium dynamics of a quench associated with instantaneously changing the strength of the interactions between fermionic and bosonic atoms. Previous theoretical studies based on standard time dependent Bogoliubov or BCS theory (for bosons and fermions) have not included the presence of a thermal bath. This bath is essential for ultimate equilibration. In this talk we show how to include the bath following a Leggett-Caldeira type approach. We point out some of the important differences in the quench dynamics between bosonic and fermionic superfluids. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U40.00004: Many-body analysis of a quasi-disordered integrable lattice system after a quench Lea Santos, Marcos Rigol It has been recently argued that the transition between a delocalized and a localized regime in a quasi-disordered integrable lattice system affects the dynamics and description of one-body observables after relaxation following a quench [1]. Specifically, the generalized Gibbs ensemble description was found to be applicable in the delocalized phase, but to break down in the localized phase. Here we present a many-body analysis of those quenches. We discuss how the expectation values of one-body observables in the many-body eigenstates behave in both regimes, and provide a microscopic understanding of the results in Ref. [1]. \\ Ref. [1]: C. Gramsch and M. Rigol, Phys. Rev. A (in press); arXiv:1206.3570. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U40.00005: When Is a Bath a Bath? Relaxation Dynamics and Thermalization in a Fermionic Chain Nicholas Sedlmayr, Jie Ren, Florian Gebhard, Jesko Sirker We study thermalization in a one-dimensional quantum system consisting of a non-interacting fermionic chain with each site of the chain coupled to an additional bath site. Using a time-dependent density matrix renormalization group algorithm we investigate the time evolution of observables in the chain after a quantum quench. For a weakly interacting bath and low densities we show that the dynamics can be quantitatively described by a system of coupled equations of motion. For higher densities our numerical results show equilibration for local observables and a thermalization to the canonical ensemble independent of the initial state. In particular, we find a Fermi momentum distribution in the chain in equilibrium in spite of the seemingly oversimplified bath in our model. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U40.00006: Quench dynamics in the one-dimensional sine-Gordon model: Quantum kinetic equation approach Marco Tavora, Aditi Mitra We study dynamics after a quantum quench in the one-dimensional sine-Gordon model in its gapless phase. We construct the Dyson equation to leading (quadratic) order in the cosine potential and show that the resulting quantum kinetic equation is atypical in that it involves multi-particle scattering processes. We also show that using an effective action, which generates the Dyson equation by a variational principle, the conserved stress-momentum tensor can be constructed. We solve the dynamics numerically by making a quasi-classical approximation that makes the quantum kinetic equation local in time while retaining the multi-particle nature of the scattering processes. We find that the boson distribution function reaches a steady-state characterized by an effective temperature in the long-wavelength limit. We present an analytic argument for the value of the effective temperature and the time-scales to reach this steady-state. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U40.00007: Thermalization in isolated quantum many-body systems and dependence on initial states Eduardo Torres-Herrera, Lea Santos We study the viability of thermalization in isolated quantum many-body systems described by one-dimensional Heisenberg spin-1/2 models. We show that the onset of thermal equilibrium depends on the interplay between initial states, observables and regimes. Our numerical studies are based on the spectrum analysis of the system and on its long-time evolution after a quench. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U40.00008: Initial-state dependence of the quench dynamics in integrable quantum systems at finite temperature Kai He, Marcos Rigol We study properties of isolated integrable quantum systems after a sudden quench starting from thermal states. We show that, even if the system is initially in equilibrium at finite temperature, the diagonal entropy after a quench remains a fraction of the entropy in the generalized ensembles introduced to describe integrable systems after relaxation. The latter is also, in general, different from the entropy in thermal equilibrium. Furthermore, we examine the difference between the distribution of conserved quantities in the thermal and generalized ensembles after a quench and show that they are also, in general, different from each other. This explains why these systems fail to thermalize in the usual sense. A finite size scaling analysis is presented for each quantity, which allows us making predictions for thermodynamically large lattice sizes. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U40.00009: Non-Equilbrium Behavior and Thermalization in 1D Bose Gases Robert Konik, Jean-Sebastien Caux Using a new numerical renormalization group based on exploiting an underlying exactly solvable nonrelativistic theory, we study the equilibrium properties and out-of-equilibrium dynamics of interacting many-body quantum systems. Focusing on the example of the Lieb-Liniger model we study quantum quenches with a focus on protocols in which the gas is released from a parabolic trap. Our method allows one not only to accurately describe the equilibrium state of the gas in the trap, but also to track the post-quench dynamics all the way to infinite time. Exploiting integrability, we are also able to exhibit a general protocol for the explicit construction of the generalized Gibbs ensemble, which is a candidate to govern the equilibriation of the trapped gas after its release. This construction does not rely on the underlying Hamiltonian being quadratic and works for arbitrary initial conditions. By comparing the predictions of equilibration from this ensemble against the long time dynamics observed in our method, we find that it is considerably more accurate than the effective grand canonical ensemble. See J.S. Caux and R. M. Konik, PRL 109, 175301 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U40.00010: How Long Does it Take for a Non-Equilibrium System to Reach a Quasi-Thermal State? Herbert F. Fotso, Karlis Mikelsons, James K. Freericks We study the relaxation of an interacting system driven out of equilibrium by a constant electric field using Non-Equilibrium Dynamical Mean Field Theory. We use on the one hand a DMFT method which solves the steady state problem directly in frequency space, and on the other hand, a DMFT method that follows the transient time evolution of the system on the Keldysh contour. The system is described by the Falicov Kimball model which we follow across the metal - insulator transition. We find that the retarded Green's function quickly approaches that of the steady state while the lesser Green's function and, as a result the distribution function, slowly approach that of a steady state with an increased temperature due to the additional energy transferred to the system by the electric field. Analyses of this type can help understand the results of some experiments involving ultracold atomic gases. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U40.00011: Probing thermalization and dephasing using the Kibble-Zurek mechanism Michael Kolodrubetz, Bryan Clark, Anushya Chandran, Shivaji Sondhi, David Huse The Kibble-Zurek mechanism was introduced to describe defect creation after ramping through critical points. Recent work has extended this concept to a full non-equilibrium scaling theory, described by the same low-energy critical exponents as in equilibrium. In this talk, I will discuss applying Kibble-Zurek analysis and its extensions to probe open questions in non-equilibrium dynamics, specifically working to understand thermalization or -- in the case of integrable systems -- dephasing to a generalized Gibbs ensemble. The major advantage of investigating these questions within the Kibble-Zurek scaling regime is that the results are universal in the renormalization group sense, i.e., insensitive to microscopic details that often confound analyses of thermalization. I will describe both analytical and numerical (TEBD) approaches to address the problem, with an emphasis on understanding the long-time behavior after a slow ramps and small quenches. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U40.00012: Dynamics of Large Quantum Systems: Equilibration, Thermalization and Interactions Dvira Segal, Manas Kulkarni, Kunal Tiwari The question of how/whether large quantum systems equilibrate and/or thermalize when prepared in an out-of-equilibrium state has been of enormous interest given recent experimental progress. We address this question in fermionic [1,2] and bosonic [3] systems, by following the dynamics of the full density matrix. We particularly study the case of two large-twin systems connected by a weak link (a quantum impurity), and we show that the total system equilibrates and thermalizes when the weak link is susceptible to incoherent and inelastic processes. We thus provide an experimentally feasible prescription for equilibrating and thermalizing large finite quantum systems. Our calculations are based on extending methods originally developed to treat subsystem dynamics (such as impurity), namely, the quantum Langevin equation method, the well known fermionic trace formula, and an iterative path integral approach. We also explore the role of interactions. While the fermionic system [1,2] shares many common features with the bosonic analog [3], we will describe certain crucial differences that arise as a result of different statistics.\\[4pt] [1] M. Kulkarni, K. L. Tiwari, D. Segal, arXiv:1206.2408\\[0pt] [2] M. Kulkarni, K. L. Tiwari, D. Segal, arXiv:1208.5725\\[0pt] [3] M. Kulkarni and D. Segal (in preparation) [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U40.00013: Quench Dynamics of the Interacting Bose Gas in one Dimension Natan Andrei, Deepak Iyer We obtain an exact expression for the time evolution of the interacting Bose gas following a quench from a generic initial state using the Yudson representation for integrable systems. We study the evolution of the density and noise correlation for a small number of bosons and their asymptotic for any number. We show that for any value of the coupling, as long as it is repulsive, the system asymptotes towards a strongly repulsive gas, while for any value of an attractive coupling long time behavior is dominated by the maximal bound state. This occurs independently of the initial state and can be viewed as an emerging ``dynamic universality''. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U40.00014: Quantum Quenches of Ultracold Atoms in the Presence of Synthetic Gauge Fields Matthew Killi, Stefan Trotzky, Arun Paramekanti Motivated by the experimental realization of synthetic gauge fields for ultracold atoms in optical lattices, we consider quantum quenches in such gauge field backgrounds. We show that the density dynamics following sudden anisotropic quenches can be used as a probe of equilibrium mass currents of atoms. We show, using diverse examples of Bose superfluids and normal Fermi fluids, that bulk equilibrium currents produced by the background gauge fields can be uncovered using this method. Such quenches are also shown to provide an effective route to probing the edge currents in topological states such as quantum Hall or quantum spin Hall insulators. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U40.00015: Thermalization Processes in Quantum Mechanics Van Ngo, Stephan Haas In quantum mechanics, the emergence of thermalization processes from unitary evolution has remained one of the greatest challenges. The two outstanding theories of this issue by Srednicki and Tasaki cannot address the concepts of temperature, heat, and work. Here, we present a theory using multiple quenches to examine the thermalization processes to advance thermodynamics concepts. To perform multiple quenches, one can vary one single control parameter ($\lambda )$ in a series of time evolutions, which create a set of density operators. The average of these density operators results into a diagonal operator with probability distribution function that can describe the emerging ensembles. Measuring probability distribution functions of key physical observables, temperature, equal to the derivative of energy with respect to entropy, can be easily measured. Therefore, simulations via multiple quenches can mimic dynamics in open quantum systems with much cheaper computational cost. They allow (1) tuning of temperature and entropy via $\lambda $, (2) measuring work distribution functions from distributions of a reaction coordinate, and (3) computing free-energy changes via Jarzynski's Equality. We hope that this approach can provide a new foundation and open up new directions for studying control of quantum systems. [Preview Abstract] |
Session U41: Quantum Simulation in Hybrid Systems (and Nano/Optomechanics IV)
Sponsoring Units: GQI DAMOPChair: Aashish Clerk, McGill University
Room: 350
Thursday, March 21, 2013 11:15AM - 11:27AM |
U41.00001: Quantum Dynamics of Photon Condensates Peter Kirton, Jonathan Keeling Recent experiments have, for the first time, been able to observe the Bose condensation of a gas of weakly interacting photons. We develop a full out-of-equilibrium quantum mechanical treatment of the dynamics of this system. Our model consists of a series of photon modes coupled to the background dye molecules which we simply treat as two-level systems in which each level is separated into a ladder of rovivibration states. We find that the behavior of the photon field is very much like that of a two-level laser in which there is an asymmetry between the effective pump and decay rates induced by the rovivibrational states of the dye. This motivates us to use techniques based on those for the inversionless two-level laser. We are able to calculate the coherence properties of the photons as well as giving insights into the thermalization processes which equilibrate the populations of the various photon modes. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U41.00002: Excitations of a driven condensate in a cavity: dynamics of the roton-like mode Baris Oztop, Manas Kulkarni, Hakan Tureci Recent experiments have demonstrated the superfluid-supersolid quantum phase transition (PT) of an optically driven Bose-Einstein condensate (BEC), via the observation of a roton-like softening of a mode in the Bogoliubov excitation spectrum [1,2]. This phenomenon is usually studied within two-mode approximation for the BEC which results in Dicke-like effective model. In this system, the long-range interactions between the atoms are mediated by cavity photons and the strength of the interactions is controlled by pump power. In this work, we investigate the effect of including the full spectrum of atomic modes. We find a finite lifetime for the roton-like mode below the threshold that is strongly pump-dependent. The corresponding decay rate and critical exponents for the PT are calculated.\\[4pt] [1] K. Baumann, C. Guerlin, F. Brennecke and T. Esslinger, Nature, 464, 1301 (2010).\\[0pt] [2] R. Mottl, F. Brenneck, K. Baumann, R. Landig, T. Donner and T. Esslinger, Science, 336, 1570 (2012). [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U41.00003: Quantum optomechanics in the strong-driving, strong-coupling regime Marc-Antoine Lemonde, Wei Chen, Aashish Clerk There is considerable interest in trying to develop quantum optomechanical systems where the coupling is appreciable at the level of a single photon and single phonon. Theoretically, such strongly-coupled optomechanical systems have been largely studied using a polaron transformation in the regime of very weak optical driving. We present here a theoretical approach based on the Keldysh technique that describes single-photon strong coupling physics in an optomechanical system in the presence of a strong optical drive. We show that strong driving can be used to dramatically enhance the effects of the single-photon nonlinearity, leading to striking modifications to the usual linearized optomechanical theory. We discuss the resulting strong modifications of the optomechanically-induced transparency (OMIT) spectrum, a quantity easily accessible in experiment. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U41.00004: Quantum many body systems with qubits and phonons in the solid state \"O.O. Soykal, Charles Tahan We previously proposed a nano-mechanical system where phonons trapped in an acoustic cavity can strongly hybridize with impurity qubit states in silicon (forming a so-called cavity-phoniton). Here, we extend the idea to the quantum many-body limit by investigating the physics of phonon-tunnel-coupled arrays of such components. The silicon qubit cavity phoniton system potentially offers advantages in this regime over purely optomechanical systems where the optomechanical coupling is still quite small. First, single phonons in a crystal can have large effective de Broglie wavelengths (microns). Second, as we have previously shown, qubit-phonon coupling can be quite large, easily allowing the system to enter the strong coupling regime and enabling phonon-blockade. Such arrays can be fabricated in semiconductor heterostructures or in on-chip, optomechanical crystals. We calculate the parameter regime where the Mott-Superfluid quantum phase transition occurs in realizable devices. We also demonstrate the emergence of super-splitting, phonon anti-bunching, and phonon blockade through the non-equilibrium density matrix master equation approach in few cavity systems. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U41.00005: Quantum Dynamics of Optomechanical Arrays Max Ludwig, Florian Marquardt Optomechanical system are typically composed of a single mechanical and a single optical mode interacting via radiation pressure. In this talk, we will introduce an array of optomechanical cells, and discuss our theoretical results on the nonlinear quantum dynamics of such a setup. In particular, we have discovered a phase transition between incoherent mechanical oscillations and a collective phase-coherent mechanical state. We describe how quantum fluctuations drive this transition at low temperatures. We will also discuss the prospects of observing these non-equilibrium dynamics in an experimental implementation based on currently available setups. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U41.00006: Signatures of nonlinear optomechanics and engineering of nonclassical mechanical steady states Kjetil Borkje Motivated by recent improvements in coupling strength between light and mechanical motion, we study the strong coupling regime of cavity optomechanics theoretically. We focus on the regime where the optomechanical coupling rate is still small compared to the mechanical resonance frequency, but where the mechanically induced Kerr nonlinearity is significant. The response of the system to an optical drive is characterized. The average photon number in the cavity as a function of drive detuning can feature several peaks due to multi-photon transitions. Furthermore, we show that by optically driving the system at multiple frequencies, multi-photon transitions can facilitate the engineering of nonclassical steady states of the mechanical oscillator. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U41.00007: Nonlinear Quantum Relaxation and Generation of Non-classical States in Duffing Oscillators Aurora Voje, Alexander Croy, Andreas Isacsson The dissipative quantum dynamics of an anharmonic oscillator is theoretically investigated in the context of carbon-based nano-mechanical systems. In the short-time limit, it is known that macroscopic superposition states appear for such oscillators\footnote{A.~Voje, J.~M.~Kinaret, and A.~Isacsson, Phys.~Rev.~{\bf B85}, 205415 (2012).}. Linear and non-linear dissipation leads to decoherence of such non-classical states in the long-time limit. However, as a result of two-vibron losses at zero temperature, the quantum oscillator eventually evolves into a non-classical stationary state -- a qubit-like state. The relaxation of the qubit due to thermal excitations and one-vibron losses is numerically and analytically studied. The possibility of verifying the occurrence of the qubit is discussed and signatures of the non-classicality arising in a ring-down setup are presented. Additionally, the generation of entanglement between two coupled oscillators in presence of strong two-vibron losses is discussed. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U41.00008: Parametric feedback squeezing of an opto-electromechanical device below 3dB Menno Poot, Hong Tang Parametric squeezing can reduce the uncertainty in one quadrature of the position of a mechanical resonator, even below the standard quantum limit, and it can improve measurement sensitivity. Here we demonstrate squeezing of the thermal motion of a 570 kHz opto-electromechanical resonator made out of high-stress SiN by modulating its spring constant at twice the resonance frequency. Parametric and direct actuation are achieved by applying a.c. voltages between strongly coupled electrodes on the resonator and a fixed one. It is well know that using this method the motion of one quadrature cannot be decreased more than 3 dB below the undriven case before instabilities kick in. However, by measuring the phase-space trajectory of the resonator and adjusting the phase of the parametric drive in real-time we achieve a stationary reduction in both quadratures that is far beyond this limit. Finally, due to the strong coupling between the drive electrodes, the nonlinearity of the resonator can be tuned all the way from a stiffening spring to a softening one. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U41.00009: Non-classical correlations of scattered photons in a one-dimensional waveguide with multiple atoms Dibyendu Roy We study the scaling of photon-photon correlations mediated by resonant interactions of photons with atoms in a one-dimensional photonic waveguide. Recently a new theoretical approach based on the Bethe-ansatz technique has been developed to study transport in an open quantum impurity. Here we generalize the approach to study multiple atoms. We derive the exact solution of single and two-photon scattering states, and the corresponding photon transmission through the atomic ensemble. We show how various two-photon nonlinear effects, such as spatial attraction and repulsion between photons as well as background fluorescence can be tuned by changing the number of atoms and the coupling between atoms (controlled by the separation). Finally we propose a simple scheme for nonreciprocal optical transmission in the waveguide by placing different atoms. Our fully quantum-mechanical approach provides a better understanding of cascaded optical nonlinearity at the microscopic level. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U41.00010: Recent theoretical advances on superradiant phase transitions Alexandre Baksic, Pierre Nataf, Cristiano Ciuti The Dicke model describing a single-mode boson field coupled to two-level systems is an important paradigm in quantum optics. In particular, the physics of ``superradiant phase transitions'' in the ultrastrong coupling regime is the subject of a vigorous research activity in both cavity and circuit QED. Recently, we explored the rich physics of two interesting generalizations of the Dicke model: (i) A model describing the coupling of a boson mode to two independent chains A and B of two-level systems [1], where chain A is coupled to one quadrature of the boson field and chain B to the orthogonal quadrature. This original model leads to a quantum phase transition with a double symmetry breaking and a fourfold ground state degeneracy. (ii) A generalized Dicke model with three-level systems [2,3] including the diamagnetic term. In contrast to the case of two-level atoms for which no-go theorems exist, in the case of three-level system we prove that the Thomas-Reich-Kuhn sum rule does not always prevent a superradiant phase transition.\\[4pt] [1] P. Nataf, A. Baksic and C. Ciuti, Phys. Rev. A {\bf 86}, 013832 (2012).\\[0pt] [2] C. Ciuti and P. Nataf, Phys. Rev. Lett. {\bf 109}, 179301 (2012).\\[0pt] [3] A. Baksic, P. Nataf, and C. Ciuti, arXiv:1206.3213 (2012). [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U41.00011: Light-induced phase transition in a quantum spin chain: Breakdown of the Haldane phase by circularly polarized laser Shintaro Takayoshi, Hideo Aoki, Takashi Oka We theoretically propose a new category of non-equilibrium phase transitions in quantum spin systems that can be induced by the magnetic component of strong lasers. As an example, we consider a Haldane chain with single ion anisotropy radiated by circularly polarized light. We study the spin dynamics by combining the numerical infinite time-evolving block decimation method and an analytical calculation via the Floquet theory, and demonstrate that the laser can magnetize even an antiferromagnet quantum mechanically. It is also shown that the string order is broken by the magnetization, which indicates that a photo-induced breakdown of the Haldane phase has occurred. This phenomenon can be realized using strong THz lasers. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U41.00012: Testing Kibble-Zurek mechanism in ion traps Ramil Nigmatullin, Adolfo del Campo, Gabriele de Chiara, Giovanna Morigi, Martin Plenio, Alex Retzker A quench through a critical point of a second order phase transition results in the formation of topological defects in the system. Kibble-Zurek (KZ) theory predicts the scaling of a number of defects as a function of quench rate. This scaling depends on the critical exponents of the phase transition, and hence the study of the defect density reveals something about the nature of phase transition itself. There are a number of proposals to test KZ theory experimentally. In this talk, we discuss the possibility of studying defect formation in ion traps. A linear ion chain confined in a Paul trap undergoes a continuous phase transition to a zigzag chain when the confining potential is lowered. If the chain is in a ring trap then the zigzag chain can be in a helical configuration with a nonzero winding number. Using molecular dynamics simulations we show that the scaling of the average winding number of the resulting helical chain is consistent with KZ theory. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U41.00013: Space-Time Crystals of Trapped Ions Tongcang Li, Zhe-Xuan Gong, Zhang-Qi Yin, H. T. Quan, Xiaobo Yin, Peng Zhang, L.-M. Duan, Xiang Zhang Spontaneous symmetry breaking can lead to the formation of time crystals, as well as spatial crystals. Here we propose a space-time crystal of trapped ions and a method to realize it experimentally by confining ions in a ring-shaped trapping potential with a static magnetic field. The ions spontaneously form a spatial ring crystal due to Coulomb repulsion. This ion crystal can rotate persistently at the lowest quantum energy state in magnetic fields with fractional fluxes. The persistent rotation of trapped ions produces the temporal order, leading to the formation of a space-time crystal. We show that these spacetime crystals are robust for direct experimental observation. We also study the effects of finite temperatures on the persistent rotation. The proposed space-time crystals of trapped ions provide a new dimension for exploring many-body physics and emerging properties of matter. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U41.00014: Electromagnetic induced transparency and slow light in strongly correlated atomic gases Hsiang-Hua Jen, Bo Xiong, Ite A. Yu, Daw-Wei Wang We develop the quantum theory for the electromagnetic induced transparency (EIT) and slow light property in ultracold Bose and Fermi gases. It shows a very different property from the classical theory which assumes frozen atomic motion. For example, the speed of light inside the atomic gases can be changed dramatically near the Bose-Einstein condensation temperature, while the presence of the Fermi sea can destroy the EIT effect even at zero temperature. This quantum EIT property is mostly manifested in the counter-propagating excitation schemes in either the low-lying Rydberg transition or in D2 transition with a very weak coupling field.\\ Using linear response theory, we further derive an exact and universal form for the EIT spectrum, which applies even in strongly correlated systems of ultracold atoms. We find that the spectrum is closely related to the single particle Green's function, which is not easily observable in most experimental technique. As an example, we show results of 1D Luttinger liquid, Mott-insulator state, and BCS pairing phase, and compare to the results of standard classical theory. Our theory therefore paves the way to measure strongly correlated physics of ultracold atoms via the state-of-art manipulation of light propagation inside the quantum gases. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U41.00015: Orbital Angular Momentum as Manifestation of Photonic Zitterbewegung Basil Davis The phenomenon of photonic orbital angular momentum has received considerable attention since its theoretical prediction by Allen et al in 1992. It has been established theoretically and experimentally that laser beams with a Laguerre Gaussian profile possess angular momentum in addition to their intrinsic spin angular momentum. A parallel development has been the renewed interest in zitterbewegung, first predicted for relativistic electrons by Schrodinger. It is now known that zitterebewegung is a property of all particles, regardless of spin, charge or rest mass, since it is basically a quantum mechanical phenomenon. Recently there has arisen an interest in photonic zitterbewegung. This paper shows that photonic orbital angular momentum is one experimentally observable manifestation of photonic zitterbewegung. [Preview Abstract] |
Session U42: Focus Session: Supercooled and Nanoconfined Water III
Sponsoring Units: DCPChair: Anders Nilsson, SLAC
Room: Hilton Baltimore Holiday Ballroom 3
Thursday, March 21, 2013 11:15AM - 11:51AM |
U42.00001: Phase transitions of liquid water at nanoscale Invited Speaker: Christiane Alba-Simionesco The behaviour of fluids confined within nanometric pores significantly differs from that of the bulk. The effect of confinement, surface forces, and reduced dimension is to shift the phase transitions of the confined fluid (condensation, freezing and crystallisation). By postponing or avoiding the inconvenient crystallization process it is often suggested that confinement allows a deeper penetration into the supercooled regime and helps in the understanding the glass formation; in the case of water, confinement might helps to extend the liquid state into the so-called ``no-man's land.'' However below confining conditions of about 10$\sigma$, $\sigma$ being the size of the molecule, water or van der Waals liquids are strongly perturbed by the presence of a surface. Thus a question always remains whether the confined liquid, water or any other fluid, is an extension of the ``bulk'' supercooled regime or refers to specific behavior controlled by external parameters such as the size and the surface interactions imposed to the system. Despite the obvious fundamental interest in understanding bulk water, this situation corresponds to most of the cases in biological and geological systems and deserves particular attention per se. However a prerequisite is to understand and quantify how pores are filled and how much; so we studied the processes of entrance and saturation to a pore (adsorption, imbibition and intrusion) in connection with the structure and the local dynamics of liquid water. Then, we will present new experimental results on the thermodynamic, structural and vibrational properties of water confined within nanometric pores (size of a few molecular diameters). [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U42.00002: How does confinement affect the structure and dynamics of water and other liquids? Anatoli Milischuk, Branka Ladanyi We studied the effects of confinement on static and dynamical properties of liquids including water, acetonitrile, and benzene in amorphous silica nanopores in equilibrium with the bulk liquid at ambient conditions. The model pores are approximately cylindrical, with diameters ranging from 20 to 40 \AA. The filled pores are prepared using grand canonical Monte Carlo simulation and molecular dynamics simulation is used to calculate liquid structure and dynamics. Our studies of dynamics included translational mean squared displacements, orientational time correlations, and survival probabilities in interfacial shells. We also studied polarizability anisotropy time correlations that are related to experimentally observed optical Kerr effect response functions. We found that there is layering and preferential orientational ordering of solvent molecules in the interfacial region. Molecular translational and rotational mobility is reduced in the layers near the interface. Confinement leads to slowdown of the polarizability anisotropy relaxation in agreement with experimental findings. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U42.00003: Strongly Anisotropic Dielectric Response of Confined Water Cui Zhang, Francois Gygi, Giulia Galli We carried out atomistic simulations of water within hydrophobic surfaces, which revealed remarkable modifications of the dynamics and dielectric relaxation of the liquid under confinement. We found that dipolar fluctuations are modified by the presence of surfaces up to strikingly large distances, i.e., tens of nanometers. Fluctuations are suppressed by approximately an order of magnitude in the z direction, perpendicular to the interface, and are enhanced in the x-y plane, giving rise to strong anisotropies in the components of the dielectric response. Our findings are consistent with recent terahertz and ultrafast infrared pump-probe spectroscopy experiments. Work supported by DOE-CMSN DE-SC0005180. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U42.00004: Dynamics of the fast component of nano-confined water under electric field Souleymane Diallo, Eugene Mamontov, Andrey Podlesnyak, Georg Ehlers We have investigated the diffusion of water molecules confined in the pores of folded silica materials (FSM), by means of quasielastic neutron scattering in the time range of 1 picosecond and 65 picoseconds. The measurements were performed on the direct geometry time-of-flight instrument CNCS at the Spallation Neutron Source, for temperatures between 220 K and 245 K, and at two electric field values, 0kV/mm and 2kV/mm. The goal was to investigate the effects of moderate electric field on the previously observed fast component of nano-confined water. In contrast to our earlier observation on the slow dynamics (at longer times) [1], the present results indicate a less drastic effect of applied electric field on the fast dynamics.\\[4pt] [1] S.O. Diallo, E. Mamontov, S. Inagaki, Y. Fukushima, and N. Wada, ``Enhanced Translational Dynamics of Water under Electric Field'' Phys. Rev. E 86, 021506 (2012). [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U42.00005: The structure of water in bulk and in confinement by total neutron and x-ray scattering Invited Speaker: Alan Soper In the past decade or so there has been a significant worldwide effort to try to obtain a consistent set of radial distribution functions for water. Exactly how those distribution functions should be interpreted in terms of the local order in water remains a somewhat open question -- whether for instance they imply water has a degree of heterogeneity in its local structure or whether it is in fact a uniform fluid with normal statistical fluctuations in density and structure. However combining a number of different x-ray and neutron data sets together is now indicating a rather consistent view of the local distribution functions in water. This consistency is achieved partly as a result of different researchers applying state-of-the-art data analysis methods to their data, both neutron and x-ray, but partly also by the application of computer simulation methods of structure refinement which help to eliminate some of the artifacts that can be introduced by uncertainties in that data analysis. The situation as regards confined water is much less clear. However it is possible to investigate water near a surface using radiation total scattering methods in the case where the pores which contain the water, whether sheet-like, cylindrical, or spherical, have a regular arrangement in the material. This is because the Bragg peaks arising from that regular arrangement are strongly affected depending on how the fluid is distributed within the pore. This talk will focus on the MCM41 silicas which have cylindrical pores on a hexagonal lattice. Combining the scattering data with computer structure refinement in the same way that is done for the bulk liquid is leading to unprecedented insight into how water is organized near the silicate surface. This work is aimed at clarifying the underlying processes that may have lead to recent observations of fragile to strong transitions in these materials. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U42.00006: Fluctuating confinement of water in aqueous organic nanodroplets Gerald Wilemski, Fawaz Hrahsheh Supercooled and nano-confined water occurs frequently as nanometer-sized aqueous-organic aerosol droplets that are ubiquitous in the atmosphere and in many industrial processes. Nanodroplet structure is important because it influences droplet growth and evaporation rates, heterogeneous reaction rates, and radiative properties. We use classical molecular dynamic simulations to study the structure of ternary water-butanol-nonane nanodroplets for several temperatures and droplet sizes. We study the effects of butanol on the wetting of the water/butanol core-shell droplet by the nonane lens. At low concentrations, butanol acts as a surfactant to significantly enhance the wetability of the water droplet by nonane. At 250 K, with sufficient butanol and nonane, perfect wetting (thin film formation by nonane) occurs. Perfect wetting also occurs at higher temperatures, 270 K to 300 K, but this wetting state is progressively destabilized at higher temperature. All of the nanodroplets studied undergo distinct transitions between partial dewetting and perfect wetting states due to isothermal fluctuations in the local distribution of butanol on the surface of the water core. These fluctuations favor the wetted state at lower temperatures and the dewetted state at higher temperatures. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U42.00007: The Effect of Contact Angle on the Depletion Layer when Water Meets a Hydrophobic Surface Adele Poynor By definition hydrophobic substances hate water. Water placed on a hydrophobic surface will form a drop in order to minimize its contact area. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low-density depletion layer forms near the surface. We investigate the effect of contact angle on depletion layer formation using the surface sensitive technique of Surface Plasmon Resonance. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U42.00008: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U42.00009: The dynamical relaxation: a key to understand Water Anomalies. Results from bulk and confined water Invited Speaker: Francesco Mallamace The anomalous behavior of thermodynamic response functions is an unsolved problem in the physics of water. The mechanism that causes the apparently indefinite increase in the heat capacity, the compressibility, and the coefficient of thermal expansion, inside the supercooled regime, is unknown. We explore this problem by analyzing both new and old experimental data coming out from the power spectrum S(Q,), on bulk and confined water at ambient pressure. On decreasing the temperature, we find that the liquid undergoes a structural transformation with the onset of an extended hydrogen bond network.Such a structure is at the basis of the marked viscoelastic behavior observed as a well defined frequency and wave vector dependence of the water sound velocity, and thus of the water response functions. All these observed properties appear consistent with the water polymorphism. We stress that, under these conditions, the thermal response functions and their corresponding fluctuations remain finite at ambient pressure. From the observation that the water density maximum dominating the system thermodynamics under ambient conditions is strongly P-dependent and disappears at a crossover pressure (P$_{cross}$ 1.8kbar) we have studied such a variable in a wide area of the T-P phase diagram. On these basis we have considered new and old data of both the isothermal compressibility $_{T}$(T,P) and the coefficient of thermal expansion $_{P}$(T,P). In the first case the main observation is that $_{T}$(T) shows a minimum located at the same temperature (T$_{MC}$ 315$\pm$5K) for all the studied pressures. As in the $_{T}$(T) case, also the behavior of $_{P}$ is surprising: all the $_{P}$(T) curves measured at different P cross at T$_{MC}$; specifically, the experimental data show a ``singular and universal expansivity point'' at T$_{MC}$ 315K and $_{P}$(T$_{MC}$) 0.44 10$^{-3}$K$^{-1}$. Moreover, on the contrary of other water singularities we stress that such temperature has a precise thermodynamical consistence lying in the relationship connecting the two studied response functions. [Preview Abstract] |
Session U43: Molecules, Clusters, and Complexes
Sponsoring Units: DCPChair: Jeff Cina, University of Oregon
Room: Hilton Baltimore Holiday Ballroom 2
Thursday, March 21, 2013 11:15AM - 11:27AM |
U43.00001: Terahertz Spectroscopy of Water Vapors, Chemical Vapors and Ionized Air Benjamin Graber, Rongjia Tao, Dong Ho Wu In the past, a few research groups have demonstrated that terahertz spectroscopy could be a useful tool for the identification of chemicals. However most of those demonstrations have been done with solid-phase or liquid-phase chemicals. There are little demonstrations for the detection and identification of chemicals in the gas-phase, as it is very difficult in part due to the presence of water-absorption lines in the terahertz frequency range. As the water absorption lines predominate in the 0.1 - 2THz spectral range, and can interfere with already weak terahertz signatures generated by chemical vapors, it is often very hard to obtain meaningful terahertz spectrum of chemical vapor. Regardless we recently have been able to obtain some terahertz spectra of chemical vapors and ionized air produced by several different ionization sources, including corona discharge and nuclear isotopes. Throughout data analysis we learned that water molecules, nitrogen and oxygen molecules play very important roles in these terahertz spectra. In this presentation we will discuss our experiments and the roles of these molecules. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U43.00002: Radiative electron attachment to molecules of astrophysical interest. Benchmark study of CN$^-$ Viatcheslav Kokoouline, Nicolas Douguet, Samantha Fonseca dos Santos, Olivier Dulieu, Maurice Raoult, Ann Orel We have developed a first-principles approach to study the process of radiative electron attachment (REA) to linear molecules of astrophysical interest Mol $+e^-\to$ Mol$^- + \hbar\omega$. (Mol$^-$ = C$_n$H$^-$, C$_n$N- ). The approach is based on accurate ab initio calculations of electronic bound and continuum states of the negative ion. The electronic continuum states are obtained with the complex-Kohn variational method. The benchmark calculation for the formation of the simplest observed ion, CN-, by REA has produced a low rate coefficient, $5\times 10^{-17}$cm$^3/$s at 30 K. We will present also a preliminary result for the C$_4$H$^-$ formation by REA. For this molecule, the REA rate coefficient is expected is larger by about a factor of 10 due to a larger transition dipole moment. This study suggests that the negative molecular ions, recently observed in the interstellar medium, can hardly be formed by the process of radiative electron attachment. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U43.00003: Stability and Meta-stability of Clusters in a Reactive Atmosphere: Theoretical Evidence for Unexpected Stoichiometries of Mg$_M$O$_x$ Saswata Bhattacharya, Sergey V. Levchenko, Luca M. Ghiringhelli, Matthias Scheffler Applying genetic algorithm and replica exchange molecular dynamics in a cascade approach we calculate structure and composition of Mg$_M$O$_x$ clusters at realistic temperatures and oxygen pressures. The cascade starts with force field and goes up to density functional theory with exact exchange plus correlation in the random phase approximation\footnote{X. Ren, P. Rinke, C. Joas, and M. Scheffler, Invited Review: Random-phase approximation and its applications in computational chemistry and materials science. J. Mater. Sci. {\bf 47}, 21 (2012).}. The stable compositions are identified using \textit{ab initio} atomistic thermodynamics. We find that at realistic environmental conditions small clusters ($M$ = 1-5) are in thermodynamic equilibrium when $x > M$. Non-stoichiometric clusters are found to have in general higher spin multiplicity than stoichiometric ones. This suggests a possibility of tuning magnetic properties by changing environmental conditions. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U43.00004: First Observation of the $2^1\Pi$ state of NaH Chin-Chun Tsai, Hsien-Yu Huang, Tsai-Lien Lu, Thou-Jen Whang The upper levels (to the last bound vibrational level) of NaH $2^1\Pi$ state have been observed for the first time by using pulsed optical-optical double resonance fluorescence depletion spectroscopy. About 20 rovibrational energy levels, $\textit{v}$=2-5 and $\textit{J}$=1-9, were assigned to this electronic state by comparing the successive rotational spectra through selected intermediate levels of the $A^1\Sigma^+$ state. A decreased background fluorescence on the recorded spectra near the atomic asymptotic of Na(3$\textit{d}$)+H(1$\textit{s}$) indicates that the dissociation limit of $2^1\Pi$ state is approaching. Compared with the eigenvalues solved from the potential of Aymer's $\textit{ab}$ initial calculations, the vibrational quantum numbers were assigned. Un-observed lower levels ($\textit{v}$=0 and 1) are due to the lack of Franck-Condon factor under accessible intermediate levels of the $A^1\Sigma^+$ state. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U43.00005: Probing the Electronic Structure of Small Metal-Nitride Clusters using Anion Photoelectron Spectroscopy Cuneyt Berkdemir, K.D. Dasitha Gunaratne, Shibo Cheng, A.W. Castleman, Jr. Gas-phase spectroscopic studies have greatly enhanced our understanding of the electronic structure and chemical bonding in metal-nitrides and oxides as well as metal-halides. While photoelectron spectroscopy of negatively charged clusters is a useful technique, spectroscopic investigations concerning metal-nitrides are still scarce. To gain insights into the electronic structures of select metal-nitrides, we have investigated the structures, ground electronic states and electron affinities of niobium and tantalum mono and dinitrides by obtaining their electron binding energies and photoelectron angular distributions via a Wiley-McLaren time-of-flight mass spectrometer coupled with a velocity map imaging apparatus. The metal-nitride anions are formed by laser ablation of niobium and tantalum metal rods with a buffer gas consisting of N2 in excess argon. The formation of anionic NbNx and TaNx (x=1,2) species have been confirmed via isotopic distributions of the respective molecules. DFT calculations are performed to predict the structures, vibrational frequencies and electron affinities of the observed anions and their neutral counterparts. As an analogy, we compared the electronic properties of NbN/ZrO and TaN/WC diatomics because they have the same number of valence electron. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U43.00006: Ab Initio Study of KCl and NaCl Clusters Clifton Brownrigg, Ajit Hira, Jose Pacheco, Justin Salazar We continue our interest in the theoretical study of molecular clusters to examine the chemical properties of small K$_{\mathrm{n}}$Cl$_{\mathrm{n}}$ and Na$_{\mathrm{n}}$Cl$_{\mathrm{n}}$ clusters (n $=$ 2 - 15). The potentially important role of these molecular species in biochemical and medicinal processes is well known. This work applies the hybrid ab initio methods of quantum chemistry to derive the different alkali-halide (M$_{\mathrm{n}}$H$_{\mathrm{n}})$ geometries. Of particular interest is the competition between hexagonal ring geometries and rock salt structures. Electronic energies, rotational constants, dipole moments, and vibrational frequencies for these geometries are calculated. Magic numbers for cluster stability are identified and are related to the property of cluster compactness. Mapping of the singlet, triplet, and quintet, potential energy surfaces is performed. Calculations have been performed to examine the interactions of these clusters with some atoms and molecules of biological interest, including O, O2, and Fe. The potential for design of new medicinal drugs is explored. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U43.00007: Structural and thermodynamic properties of Au$_{2-58}$ clusters Yi Dong, Michael Springborg, Ingolf Warnke The geometries and electronic properties of the isolated neutral Au$_{2-58}$ are studied theoretically using a parametrized density-functional tight-binding method combined with genetic algorithms. Various descriptors are used in analyzing the structural and electronic properties. In addition, the temperature dependence of the vibrational heat capacities of the optimized clusters will be presented, which allow to study the low temperature properties of the clusters. We find that the vibrational heat capacity of the Au clusters is strongly size dependent in particular at low temperatures. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U43.00008: Quantum Theoretical Study of Palladium and Silver Clusters Ajit Hira, Justin Salazar, Jose Pacheco We continue our interest on the chemisorption of different atomic and molecular species on small clusters of metallic elements, by examining the interactions of H, O and F atoms with Pd$_{\mathrm{n}}$ and Ag$_{\mathrm{n}}$ clusters (n $=$ 2 thru 12). Transition-metal clusters can be useful for the study of quantum size effects and for formation of metallic states, and are ideal candidates for catalytic processes. Hybrid ab initio methods of quantum chemistry (particularly the DFT-B3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bond-lengths, ionization potentials, electron affinities and HOMO-LUMO gaps for the clusters of the two different metals. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E) sites, hollow sites (H) site and on-top (T) sites. Effects of crystal symmetries corresponding to the bulk structures for the two metals will also be investigated. The implications for the molecular dissociation of the H$_2$ and O$_2$ species will be considered. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U43.00009: First principles NEXAFS simulations of N-donor Uranyl complexes C.D. Pemmaraju, R. Duan, R. Copping, B. Jeon, S.J. Teat, M. Janousch, T. Tyliszczak, A. Canning, N. Gr{\O}nbech-Jensen, D.K. Shuh, D. Prendergast The synthesis and study of soft-donor uranyl complexes can provide new insights into the coordination chemistry of non-aqueous [UO]2$^{+}$ Recently, the tunable N-donor ligand 2,6-Bis(2-benzimidazyl)pyridine (BBP) was employed to produce novel uranyl complexes in which the [UO]2$^{+}$ cation is ligated by anionic and covalent groups with discrete chemical differences. In this work we investigate the electronic structure of the three such uranyl-BBP complexes via near-edge X-ray absorption fine structure (NEXAFS) experiments and simulations using the eXcited electron and Core-Hole (XCH) approach [1]. The evolution of the structural as well as electronic properties across the three complexes is studied systematically. Computed N K-edge and O K-edge NEXAFS spectra are compared with experiment and spectral features assigned to specific electronic transitions in these complexes. Studying the variations in spectral features arising from N K-edge absorption provides a clear picture of ligand-uranyl bonding in these systems. References: [1] D. Prendergast and G. Galli, X-ray absorption spectra of water from first-principles calculations, Phys. Rev. Lett., 215502 (2006). [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U43.00010: Electronic structure and charge transfer states of a multichromophoric heptad Luis Basurto, Rajendra Zope, Tunna Baruah A multichromophoric Heptad molecule containing Zn-tetraphenyl porphyrin, BDPY dye, bisphenyl anthracene, and C$_{60}$ attached to a hexaphenyl -benzene core was synthesized by Gust et al. (J. Phys. Chem. B, 113, 7147 (2009)). The snowflake like molecule behaves like an antenna capturing photons at different wavelengths and transferring the energy to the porphyrin. We present a DFT based study on the ground state of the complex and also on the lowest two charge transfer (CT) states of the complex carried out using a perturbative delta-SCF method. The calculations, done using a mixed all-electron and pseudo-potential approach, show that the ionization potential of porphyrin and the electron affinity of C$_{60}$ in the complex changes significantly from isolated molecules. Our calculated value of the lowest CT state is within 0.2 eV of the experimental estimate. This CT state contains a hole on porphyrin HOMO and a particle on the C$_{60}$ LUMO. A comparison of the energetics with experiment indicates that the process probably involves excitation from the HOMO-1 of porphyrin to the porphyrin LUMO followed by electron transfer and hole bubbling up resulting in a CT state with the hole on porphyrin HOMO and particle on C$_{60}$ LUMO. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U43.00011: The effect of structural conformations and solvent effects in a light-harvesting Carotenoid-diaryl-Porphyrin-C$_{60}$ (CPC$_{60}$) molecular triad on the charge transfer excitation energies Tunna Baruah, Marco Olguin, Rajendra Zope We present a detailed study of charge transfer (CT) excited states for a large number of structural conformations in a light-harvesting Carotenoid-diaryl-Porphyrin-C$_{60}$ (CPC$_{60}$) molecular triad. The molecular triad undergoes a photinduced charge transfer state exhibiting a large excited state dipole moment, making it suitable for application as a molecular-scale optoelectronic device. One important consideration is that the conformational flexibility of the CPC$_{60}$ triad impacts its dynamics in solvents. Since many experimental photochemical measurements for the traid are made in solution, studying the effect of conformational changes on the CT energy furthers the understanding of its photoconversion properties. We have calculated a few low lying CT excited state energies for a series of triad conformers, where the conformers were generated by incrementally scanning a 360 degree torsional (dihedral) twist at the C$_{60}$-porhyrin linkage and the porphyrin-cartotenoid linkage. The CT excitation energy was calculated at each 45 degree dihedral increment. Addtionally, several different CPC$_{60}$ conformations were taken from molecular dynamics simulations of the triad in water and other solvents of varying polarity. Our calculations show that structural change [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U43.00012: Chemical Nonlinearities and Radical Pair Lifetime Estimation Gregory Robinson Much attention has recently developed around chemical reactions that depend on applied static magnetic fields as weak as earth's. This interest is largely motivated by experiments that implicate the role of spin-selective radical pair recombination in biological magnetic sensing. Existing literature uses a straightforward calculation to approximate the expected lifetime of coherent radical pairs as a function of the minimum RF amplitude that is observed to disrupt magnetic navigation, apparently by decohering the radical pair via electronic Zeeman excitations. But we show that chemical nonlinearities can preclude direct computation of coherent pair lifetime without considering the cellular signalling mechanisms involved, and discuss whether it can explain the surprising fragility of some animals' compass sense. In particular, we demonstrate that an autocatalytic cycle can introduce threshold effects on the disruption sensitivity to applied oscillatory magnetic fields. We will show examples in the mean-field limit and consider the consequences of noise and fluctuations in the Freidlin-Wentzell picture of perturbed dynamical systems. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U43.00013: Analysis of direct and indirect phonon-mediated bond excitation in the explosive RDX Brent Kraczek, Peter W. Chung Understanding detonation pathways is essential to controlling the sensitivity of high energy explosives. Central to these pathways is initiation, the initial chemical reactions that lead to detonation. Phonons play an active role in initiation caused by compressive wave energy, such as those caused by shock loading, by converting the wave energy to thermal energy that causes bond-breaking. In the conventional model for phonon-mediated initiation energy follows an indirect route, in which the wave energy excites low-frequency phonons which in turn excite higher-frequency vibrons that break the key initial bonds in the chemical decomposition pathways. Using lattice dynamics calculations of $\alpha$-RDX (the crystalline $\alpha$-phase of cyclotrimethylene trinitramine), we find that a direct route of energy transfer is more likely. We have calculated the total energy available to different phonon modes and the fractions of the mode energies that go into the bonds of the material. This enabled approximation of the maximum and minimum energy exciting the bonds due to different phonon modes throughout thermal relaxation. We find that low-frequency modes provide significantly more energy than high-frequency modes to the key bonds, implying that the direct pathway is responsible. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U43.00014: The first-principles study on the electronic and optical properties of (Ga$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})$(N$_{\mathrm{1-x}}$O$_{\mathrm{x}})$ from many-body perturbation theory Hiroki Kawai, Giacomo Giorgi, Maurizia Palummo, Koichi Yamashita Gallium zinc oxynitride (Ga$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})$(N$_{\mathrm{1-x}}$O$_{\mathrm{x}})$ is one of the promising candidates as overall water-splitting photocatalyst under visible light. In 2005, the high photocatalytic activity was reported on the GaN-rich alloys$^{\mathrm{[1]}}$ and nowadays, the ZnO-rich ones with the higher visible-light absorption were also synthesized by some groups$^{\mathrm{[2,\thinspace 3]}}$. Thus the further improvement of the photocatalytic water splitting is being expected. In spite of such a huge potential of this material, the origin of the visible-light absorption is not well understood. The first-principles methods based on many-body perturbation theory (MBPT), GW approximation and Bethe-Salpether equation, combining with density functional theory, enable us to do reliable analysis of the electronic and optical properties. On this meeting, we will discuss the origin of visible-light absorption of (Ga$_{\mathrm{1-x}}$Zn$_{\mathrm{x}})$(N$_{\mathrm{1-x}}$O$_{\mathrm{x}})$ by MBPT results focusing on the non-isovalent character. [1]K. Maeda. et al. \textit{J.Am.Chem.Soc}. 127, 8286 (2005), [2]H. Chen. et al. \textit{J.Phys.Chem.C}, 114, 1809 (2010), [3]K. Lee. et al. \textit{Nano Lett}, 12, 3268 (2012) [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U43.00015: ABSTRACT WITHDRAWN |
Session U44: Focus Session: Physics of Single-Cell Heterogeneity
Sponsoring Units: DBIOChair: Wolfgang Losert, University of Maryland College Park
Room: Hilton Baltimore Holiday Ballroom 1
Thursday, March 21, 2013 11:15AM - 11:27AM |
U44.00001: Single-molecule RNA observation in vivo reveals dynamics of co-transcriptional splicing M.L. Ferguson, A. Coulon, V. de Turris, M. Palangat, C.C. Chow, R.H. Singer, D.R. Larson The synthesis of pre-mRNA and the splicing of that pre-mRNA to form completed transcripts requires coordination between two large multi-subunit complexes (the transcription elongation complex and the spliceosome). How this coordination occurs in vivo is unknown. Here we report the first experimental observation of transcription and splicing occurring at the same gene in living cells. By utilizing the PP7/MS2 fluorescent RNA reporter system, we can directly observe two distinct regions of the nascent RNA, allowing us to measure the rise and fall time of the intron and exon of a reporter gene stably integrated into a human cell line. The reporter gene consists of a beta globin gene where we have inserted a 24 RNA hairpin cassette into the intron/exon. Upon synthesis, the RNA hairpins are tightly bound by fluorescently-labeled PP7/MS2 bacteriophage coat proteins. After gene induction, a single locus of active transcription in the nucleus shows fluorescence intensity changes characteristic of the synthesis and excision of the intron/exon. Using fluctuation analysis, we determine the elongation rate to be 1.5 kb/min. From the temporal cross correlation function, we determine that splicing of this gene must be co-transcriptional with a splicing time of $\sim$100 seconds before termination and a $\sim$200 second pause at termination. We propose that dual-color RNA imaging may be extended to investigate other mechanisms of transcription, gene regulation, and RNA processing. [Preview Abstract] |
Thursday, March 21, 2013 11:27AM - 11:39AM |
U44.00002: Cellular volume is a global controller of mRNA abundance Olivia Padovan-Merhar, Arjun Raj Many researchers have observed large variability in the numbers of RNA and protein molecules from cell to cell, a phenomenon thought to result from random bursts of transcription. These findings hold even for genes involved in core cellular processes, raising questions as to how cells can function in the presence of such molecular noise. However, biochemical processes typically depend on concentrations of cellular constituents rather than absolute numbers, so we use RNA fluorescence in situ hybridization to measure mRNA counts and cellular volume in single cells. We find that while both mRNA numbers and volume vary widely between cells, mRNA density does not. Thus, for many genes, mRNA abundance is precisely controlled to match the volume of the cell, as though the genes know how big the cell is. We measure transcription on a global and single-gene scale, and find that transcriptional activity scales with volume, suggesting that density is regulated at a transcriptional level. We present a mathematical model explaining which transcriptional bursting parameters account for the presence or lack of density conservation. Our findings suggest that global properties of RNA dynamics require a reassessment of our understanding of cellular heterogeneity and stochastic gene expression. [Preview Abstract] |
Thursday, March 21, 2013 11:39AM - 11:51AM |
U44.00003: Protocols for discriminating sources of intrinsic noise in gene expression Niraj Kumar, Rahul Kulkarni The intrinsic stochasticity of gene expression leads to heterogeneity of protein levels across a population of cells. Different molecular mechanisms have been proposed that contribute to this variability in protein levels. Among these are Poissonian fluctuations of mRNAs, promoter fluctuations based on a random telegraph process, and general waiting-time distributions (``gestation'') for the arrival of mRNAs. Given these different sources, an important problem in the field is the development of protocols for discriminating the dominant molecular mechanisms giving rise to the observed noise. Considering the ``burst'' limit (for which mRNA lifetimes are much shorter than protein lifetimes) we develop protocols for discriminating the sources of intrinsic noise based on accessible experimental measurements. Computational validation of these protocols indicates that they could lead to promising experimental approaches for discriminating the sources of intrinsic noise in gene expression. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U44.00004: Relating Single Cell Heterogeneity To Genotype During Cancer Progression Invited Speaker: Satwik Rajaram Progression of normal cells towards cancer is driven by a series of genetic changes. Traditional population-averaged measurements have found that cell signalling activities are increasingly altered during this progression. Despite the fact that cancer cells are known to be highly heterogeneous, the response of individual pathways to specific genetic changes remains poorly characterized at a single cell level. Do signalling alterations in a pathway reflect a shift of the whole population, or changes to specific subpopulations? Are alterations to pathways independent, or are cells with alterations in one pathway more likely to be abnormal in another due to crosstalk? We are building a computational framework that analyzes immunofluorescence microscopy images of cells to identify alterations in individual pathways at a single-cell level. A primary novelty of our approach is a ``change of basis'' that allows us to understand signalling in cancer cells in terms of the much better understood patterns of signalling in normal cells. This allows us to model heterogeneous populations of cancer cells as a mixture of distinct subpopulations, each with a specific combination of signalling pathways altered beyond the normal baseline. We used this framework to analyze human bronchial epithelial cell lines containing a series of genetic modifications commonly seen in lung cancer. We confirmed expected trends (such as a population-wide epithelial mesenchymal transition following the last of our series of modifications) and are presently studying the relation between the mutational profiles of cancer cells and pathway crosstalk. Our framework will help establish a more natural basis for future investigations into the phenotype-genotype relationship in heterogeneous populations. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U44.00005: Mapping chromatin modifications in nanochannels Shuang Fang Lim, Alena Karpusenko, Robert Riehn DNA and chromatin are elongated to a fixed fraction of their contour length when introduced into quasi-1d nanochannels. Because single molecules are analyzed, their hold great potential for the analysis for the genetic analysis of material from single cells. In this study, we have reconstituted chromatin with histones from a variety of sources, and mapped the modification profile of the chromatin. We monitored methylation and acetylation patterns of the histone tail protein residues using fluorescently labelled antibodies. Using those, we distinguished chromatin reconstituted from chicken erythrocytes, calf thymus, and HeLa cells. We discuss prospects for profiling histone modifications for whole chromosomes from single cells. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U44.00006: What is Growth? Concurrent determination of a bacterial population's many shades of growth Guillaume Lambert, Edo Kussell One of the most exciting developments in the study of the physics of microbial life is the ability to precisely monitor stochastic variations of gene expression in individual cells. A fundamental question is whether these variations improve the long-term ability of a population to adapt to new environments. While variations in gene expression in bacteria are easily measured through the use of reporter systems such as green fluorescent proteins and its variants, precise determination of a cell's growth rate, and how it is influenced by its immediate environment, remains challenging. Here, we show that many conflicting and ambiguous definitions of bacterial growth can actually be used interchangeably in E. coli. Indeed, by monitoring small populations of E. coli bacteria inside a microfluidic device, we show that seemingly independent measurements of growth (elongation rate and the average division time, for instance) agree very precisely with one another. We combine these definitions with the population's length and age distribution to very precisely quantify the influence of temperature variations on a population's growth rate. We conclude by using coalescence theory to describe the evolution of a population's genetic structure over time. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U44.00007: Noise in Exponential Growth Srividya Iyer-Biswas, Charles Wright, Jon Henry, Stas Burov, Yihan Lin, Sean Crosson, Aaron Dinner, Norbert Scherer The interplay between growth and division of cells is has been studied in the context of exponential growth of bacterial cells (in suitable conditions) for decades. However, bulk culture studies obscure phenomena that manifest in single cells over many generations. We introduce a unique technology combining microfluidics, single-cell imaging, and quantitative analysis. This enables us to track the growth of single Caulobacter crescentus stalked cells over hundreds of generations. The statistics that we extract indicate a size thresholding mechanism for cell division and a non-trivial scaling collapse of division time distributions at different temperatures. In this talk I shall discuss these observations and a stochastic model of growth and division that captures all our observations with no free parameters. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U44.00008: From Molecules to Cells to Organisms: Understanding Health and Disease with Multidimensional Single-Cell Methods Invited Speaker: Juli\'an Candia The multidimensional nature of many single-cell measurements (e.g. multiple markers measured simultaneously using Fluorescence-Activated Cell Sorting (FACS) technologies) offers unprecedented opportunities to unravel emergent phenomena that are governed by the cooperative action of multiple elements across different scales, from molecules and proteins to cells and organisms. We will discuss an integrated analysis framework to investigate multicolor FACS data from different perspectives: Singular Value Decomposition to achieve an effective dimensional reduction in the data representation, machine learning techniques to separate different patient classes and improve diagnosis, as well as a novel cell-similarity network analysis method to identify cell subpopulations in an unbiased manner. Besides FACS data, this framework is versatile: in this vein, we will demonstrate an application to the multidimensional single-cell shape analysis of healthy and prematurely aged cells. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U44.00009: Stochastic Cell Fate Progression in Embryonic Stem Cells Ling-Nan Zou, Adele Doyle, Sumin Jang, Sharad Ramanathan Studies on the directed differentiation of embryonic stem (ES) cells suggest that some early developmental decisions may be stochastic in nature. To identify the sources of this stochasticity, we analyzed the heterogeneous expression of key transcription factors in single ES cells as they adopt distinct germ layer fates. We find that under sufficiently stringent signaling conditions, the choice of lineage is unambiguous. ES cells flow into differentiated fates via diverging paths, defined by sequences of transitional states that exhibit characteristic co-expression of multiple transcription factors. These transitional states have distinct responses to morphogenic stimuli; by sequential exposure to multiple signaling conditions, ES cells are steered towards specific fates. However, the rate at which cells travel down a developmental path is stochastic: cells exposed to the same signaling condition for the same amount of time can populate different states along the same path. The heterogeneity of cell states seen in our experiments therefore does not reflect the stochastic selection of germ layer fates, but the stochastic rate of progression along a chosen developmental path. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U44.00010: Exact protein distributions for stochastic models of gene expression Rahul Kulkarni, Hodjat Pendar, Thierry Platini Stochasticity in gene expression gives rise to variations in protein levels across a population of genetically identical cells. Such fluctuations can drive phenotypic variation in clonal populations, hence there is considerable interest in quantifying noise in gene expression using stochastic models. However, obtaining exact analytical results for protein distributions has been an intractable task for all but the simplest models. We develop a novel mapping that significantly simplifies the analysis of stochastic models of gene expression. Using this mapping, we derive exact analytical results for steady-state and time-dependent protein distributions for the basic 2-stage model of gene expression. Considering extensions of the basic model, we obtain exact protein steady-state distributions for models that include the effects of post-transcriptional and post-translational regulation. The approach developed in this work is widely applicable and can contribute to a quantitative understanding of stochasticity in gene expression and its regulation. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U44.00011: An Experimental Determination of Static Magnetic Fields Induced Noise in Living Systems Megan Brady, Craig Laramee Living systems are constantly exposed to static magnetic fields (SMFs) from both natural and man-made sources. Exposures vary in dose and duration ranging from geomagnetic ($\sim$50$\mu$T) to residential and industrial ($\sim$10s of mT) fields. Efforts to characterize responses to SMFs have yielded conflicting results, showing a dependence on experimental variables used. Here we argue that low to moderate SMF exposure is a sub-threshold perturbation operating below thermal noise, and assays that evaluate statistical characteristics of a single cell may identify responses not consistently found by population averaging approaches. Recent studies of gene expression show that it is a stochastic process capable of producing bursting dynamics. Moreover, theoretical and experimental methods have also been developed to allow quantitative estimates of the associated biophysical parameters. These developments provide a new way to assess responses of living systems to SMFs. In this work, we report on our efforts to use single molecule fluorescence \textit{in situ} hybridization to assess responses of NIH-3T3 cells to SMF exposure at flux densities ranging from 1 to 440 mT for 48 hours. Results will contribute to determining mechanisms by which SMF exposure influences gene expression. [Preview Abstract] |
Session U45: Focus Session: Cell Mechanics II
Sponsoring Units: DBIOChair: Helim Aranda-Espinoza, University of Maryland
Room: Hilton Baltimore Holiday Ballroom 4
Thursday, March 21, 2013 11:15AM - 11:51AM |
U45.00001: Regulation of Cellular Tension in Adherent Cells Invited Speaker: Patrick Oakes Cells generate stress on their surrounding extracellular matrix (ECM) via myosin II motor generated forces which are transmitted through the actin cytoskeleton. The mechanisms in the cell which regulate the magnitude and spatial distribution of these stresses, however, remain unknown. Consistent with previous reports, we find that the total magnitude of traction force exerted on the ECM scales with cell size. Such scaling is observed across numerous cell types and reflects an inherent cellular tension determined by the level of myosin II activity. Surprisingly, while stiffness modulates the cellular spread area, we find this scaling relationship to be independent of ECM stiffness. To identify the biophysical mechanisms regulating the generation of tension, we utilize micro-patterning to isolate cell spread area from cell geometry and to spatially control the distribution of stress on the ECM. We find that traction stress magnitude is dependent on the local curvature of the cell. Changes in cell geometry result in a redistribution of local stresses, but little change in the total stress applied to the ECM. Finally, for a constant geometry, we find that both the total stress and the average stress exerted on the ECM increase with cell area. Together these data suggest that the cell can be modeled as a uniformly contracting mesh, where the magnitude of tension is regulated by the cell spread area, and the distribution of tension is regulated by local geometry. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U45.00002: Mechanical Coupling of Smooth Muscle Cells Using Microengineered Substrates and Local Stimulation Craig Copeland, David Hunter, Leslie Tung, Christopher Chen, Daniel Reich Mechanical stresses directly affect many cellular processes, including signal transduction, growth, differentiation, and survival. Cells can themselves generate such stresses by activating myosin to contract the actin cytoskeleton, which in turn can regulate both cell-substrate and cell-cell interactions. We are studying mechanical forces at cell-cell and cell-substrate interactions using arrays of selectively patterned flexible PDMS microposts combined with the ability to apply local chemical stimulation. Micropipette ``spritzing'', a laminar flow technique, uses glass micropipettes mounted on a microscope stage to deliver drugs to controlled regions within a cellular construct while cell traction forces are recorded via the micropost array. The pipettes are controlled by micromanipulators allowing for rapid and precise movement across the array and the ability to treat multiple constructs within a sample. This technique allows for observing the propagation of a chemically induced mechanical stimulus through cell-cell and cell-substrate interactions. We have used this system to administer the acto-myosin inhibitors Blebbistatin and Y-27632 to single cells and observed the subsequent decrease in cell traction forces. Experiments using trypsin-EDTA have shown this system to be capable of single cell manipulation through removal of one cell within a pair configuration while leaving the other cell unaffected. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U45.00003: Contractile Film Model for Polymorphism in Adherent Cells Shiladitya Banerjee, Luca Giomi The optimal shapes attained by contractile cells on elastic substrates are determined by the crosstalk between intracellular forces and extracellular forces of adhesion. We model an adherent stationary cell as a contractile film bounded by an elastic cortex and connected to the substrate via elastic links. When the adhesion sites are continuously distributed, optimal cell shape is constrained by the adhesion geometry, with a spread area sensitively dependent on the substrate stiffness and contractile tension. For discrete adhesion sites, equilibrium cell shape is convex at weak contractility, while developing local concavities at intermediate values of contractility. Increasing contractility beyond a critical value, controlled by substrate stiffness, cell contour undergoes a discontinuous transition to a star-shaped configuration with cusps and protrusions, accompanied by a region of bistability and hysteresis. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U45.00004: Unidirectional Contact guidance via surface nanotopography Wolfgang Losert, Xiaoyu Sun, Meghan Driscoll, Can Guven, John Fourkas Unidirectional cell migration plays a key role in many critical physiological processes. Guidance of cells in a preferred direction has been explored in the context of chemotaxis and durotaxis. However, a stable field of gradient within a dynamic range needs to be maintained to achieve persistent unidirectional guidance. Hence the spatial extent of gradient sensing is limited. Contact guidance on the other hand can be achieved on surfaces with large spatial extent without changes in guidance efficiency. However, contact guidance is generally bidirectional. Here we demonstrate that unidirectional guidance efficiency is achievable by nanofabrication of asymmetrically shaped surfaces. We analyze cell velocity and orientation, as well as the dynamic changes in cell shape in response to surface topography. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U45.00005: Biphasic cell responses on laterally mobile films Andreas Kourouklis, Ronald Lerum, Harry Bermudez The engineering of polymer surfaces or matrices that are capable of controlling cell adhesion has been widely explored. In nearly all of these works, the polymer chains (and ligands) are chemically attached to the underlying substrate, and therefore these systems are inherently static. By contrast, cellular environments such as the extracellular matrix (ECM) are dynamic and remodeled by biochemical reactions and biophysical forces. Borrowing this concept from Nature, we created polymer films by an interfacial self-assembly process, whereby individual chains can exhibit lateral mobility (in-plane diffusive motion). NIH 3T3 fibroblasts seeded on such RGD-presenting polymer films show biphasic responses in spreading and adhesion strength to lateral mobility, with a minimal response for intermediate mobility values. Futhermore, preliminary immuno-staining experiments reveal that the total area of focal adhesions demonstrates a similar biphasic trend to the cellular-scale behaviors. In contrast, actin filaments or stress fibers appear to be unaffected by the substrate lateral mobility. These results show that lateral mobility is an important, although not fully explored aspect of mechano-sensing by cells, and can potentially give new perspectives on cell-ECM interactions. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U45.00006: Remote, In-Plane Mechanosensing by Cells on Thin Floating Collagen Matrices Hamid Mohammadi, Paul Janmey, Christopher McCulloch The mechanical properties of the extracellular matrix impact many cellular functions but little is known about the contribution of matrix deformations to cellular mechanosensing that extends beyond the immediate cell-matrix interface. We examined remote mechanosensing by developing a cell culture model that employs collagen gels circumferentially supported by nylon mesh frames that float on culture medium. This approach obviates mechanical interference from the underlying rigid foundation of tissue culture plastic and enables assessment of remote, in-plane mechanosensing. With this model we found that 3T3 cells rapidly formed cellular processes whose lengths and number per cell depended on the frame opening size. When the opening sizes were increased (from 200 $\mu$m to 1700 $\mu$m widths) mean cell extension length, mean number of extensions per cell, and the sum of cell extension lengths significantly decreased (40-60{\%}; p \textless\ 0.0001). In grids of 200 $\mu$m and 500 $\mu$m widths, cells sensed the presence of nylon frames because cell-generated deformation fields extended to the grid boundaries while this did not occur in grids of 1700 $\mu$m width. This new model demonstrates the ability of cells to sense remotely, variations of matrix stiffness in the absence of a rigid underlying substrate. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U45.00007: The Crawling Cell as a Brownian Inchworm Moumita Das, J.M. Schwarz Cell migration is integral to several physiological processes such as immune response, wound healing, tissue formation, fertilization etc. Previous studies, both theoretical and experimental, have attempted to model different aspects of cell migration, including adhesion, protrusion and retraction at the level of single cells, and collective motion at the multicellular level. The entire motility process of a single cell and its ability to navigate a landscape containing obstacles is, however, not well understood. We attempt to address this issue by modeling a single moving cell as a Brownian inchworm composed of two beads attached by a spring that can sense and respond to the mechanical properties and architecture of its environment. The elastic interaction between inchworm and the substrate is modeled by molecular clutches. We study the dynamics of this inchworm in a corrugated potential. In particular we focus on the interplay between confinement and adhesion in the motility of this inchworm. This model may provide important insights on cell movement through a biological maze of other cellular and extracellular structures. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U45.00008: Frequency- dependent cell responses to an electromagnetic stimulus Toloo Taghian, Abdul Sheikh, Daria Narmoneva, Andrei Kogan External electric field (EF) acting on cells in the ionic environment can trigger a variety of mechanical and chemical cell responses that regulate cell functions, such as adhesion, migration and cell signaling; thus manipulation of EF can be used in therapeutic applications. To optimize this process, realistic studies of EF interaction with cells are essential. We have developed a combined theoretical-experimental approach to study cell response to the external EF in the native configuration. The cell is modeled as a membrane-enclosed hemisphere which is cultured on a substrate and is surrounded by electrolyte. Maxwell's equations are solved numerically (ANSYS-HFSS) to obtain 3D EF distribution inside and near the cell subjected to an external EF. Theoretical results indicate that the cell response is frequency dependent, where at low frequency EF is excluded from the cell interior while EF penetration into the cell increases for higher frequencies. In both regimes the spatial distribution and strength of induced EF in membrane varies with frequency. Experimental results are consistent with theoretical predictions and show frequency-dependent cell response, including both membrane-initiated and intracellular pathway activation and growth factor release. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U45.00009: Measuring and modeling cellular contact guidance through dynamic sensing of nanotopography Can Guven, Meghan Driscoll, Xiaoyu Sun, John Fourkas, Wolfgang Losert We investigate the shape dynamics of the amoeba Dictyostelium discoideum on nanotopographical gratings. Multiple studies have previously implicated the patterning of focal adhesion complexes (FACs) in contact guidance. However, we observe significant contact guidance of Dictyostelium along ridge-shaped nano- and microtopographic surface features, even though Dictyostelium lacks FACs. We measure the surface contact guidance efficiency, which we calculate from the statistics of cell orientations, as a function of the distance between parallel ridges. Ridges with a spacing of about 1.5 $\mu$m lead to the greatest contact guidance efficiency. We previously observed that Dictyostelium cells exhibit oscillatory shape dynamics. Therefore, we model contact guidance as a resonance between the cell oscillations and the nanogratings. In particular, we model cells as stochastic cellular harmonic oscillators that couple to the periodicity of the ridges. The spatial and temporal scales of the oscillations that best couple to the surface are consistent with those of protrusive dynamics. Our results suggest that the coupling of protrusive dynamics, which are governed by actin dynamics, to surface topography is one possible mechanism for contact guidance. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U45.00010: Interaction of mechanical and electrical oscillations and sensitivity in a model of sensory hair cell Rami M. Amro, Alexander B. Neiman Sensory hair cells are the first stage in conveying the mechanical stimuli into the electrical signals in auditory and vestibular organs of vertebrates. Experiments showed that hair cells rely on active processes in hair bundles to achieve high selective sensitivity, e.g. due to myosin molecular motors inside stereocilia. In lower vertebrates these active processes result in spontaneous oscillations of hair bundles which can be accompanied by oscillations of the cells' membrane potentials. We use modeling to study how the dynamics of both the membrane potential and the hair bundle interact to produce coherent self-sustained oscillations and how this interaction contributes to the cell's sensitivity to external mechanical perturbations. The model incorporates a mechanical stochastic hair bundle system coupled to a Hodgkin-Huxley type system for the membrane potential. We show that oscillatory regimes result in enhanced sensitivity and selectivity to harmonic stimuli. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U45.00011: Polyacrylamide scaffolds for studying cellular response to substrate stiffness in three dimensions Keng-hui Lin Recent developments in two-dimensional (2D) culture substrates with tunable stiffness and patterned adhesion ligands have demonstrated that biochemical and mechanical cues regulate the biological functions of living cells. We have extended these cell culture platforms into three dimensions (3D), as in complex biological systems, by producing highly ordered scaffolds of polyacrylamide coated with extracellular matrix proteins. We characterized parameters for the scaffold fabrication. We then grew individual fibroblasts in the identical pores of our scaffolds, examing cellular morphological, cytoskeletal, and adhesion properties. We have observed rich variety of morphologies and anchoring strategies assumed by cells growing on our tunable 3D polyacrylamide scaffolds to demonstrate the richness of cell-mciroenvironment interactions when cell adhesions are not confined to 2D surfaces. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U45.00012: Cell migration under ultrasound irradiations in micrometer scale Shinya Murakami, Yo Otsuka, Yusuke Oshima, Atsuhiko Hikita, Toshiyuki Mitsui Cell movements, migration play an important role in many physiological processes including cell proliferation and differentiation. C2C12, a line of mouse myoblasts is known to differentiate into osteoblast under appropriate conditions. Therefore, C2C12 cells can be chosen for the differentiation studies. However, the movement of the C2C12's has not been fully investigated although the movements may provide a better understanding of the healing processes of bone repair, regeneration and differentiation. In addition, low intensity ultrasound has been thought and used to promote bone fracture healing although the microscopic mechanism of this healing is not well understood. As a first step, we have investigated single cell migration of C2C12 under optical microscopy with and without ultrasound irradiations. The ultrasound is irradiated from an apex of a sharp needle. The frequency is 1.5 MHz and the power intensity is near 24 mW/cm$^2$. These values were similar to the ultrasound treatment values. In this conference, we will show the influence of the ultrasound irradiation on the cell movement by plotting the mean squared displacement and the velocity autocorrelation function as a function of time. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U45.00013: Scaffold-independent Patterning of Cells using Magnetic Nanoparticles Suvojit Ghosh, Moanaro Biswas, Subbiah Elankumaran, Ishwar Puri Spatial patterning of cells in vitro relies on direct contact of cells on to solid surfaces. Scaffold independent patterning of cells has never been achieved so far. Patterning of cells has wide applications including stem cell biology, tissue architecture and regenerative medicine besides fundamental biology. Magnetized cells in a suspension can be manipulated using an externally applied magnetic field enabling directed patterning. We magnetized mammalian cells by internalization of superparamagnetic nanoparticles coated with bovine serum albumin (BSA). A magnetic field is then used to arrange cells in a desired pattern on a substrate or in suspension. The control strategy is derived from the self-assembly of magnetic colloids in a liquid considering magnetostatic interactions. The range of achievable structural features promise novel experimental methods investigating the influence of tissue shape and size on cell population dynamics wherein Fickian diffusion of autocrine growth signals are known to play a significant role. By eliminating the need for a scaffold, intercellular adhesion mechanics and the effects of temporally regulated signals can be investigated. The findings can be applied to novel tissue engineering methods. [Preview Abstract] |
Session U46: Focus Session: Advances in Scanned Probe Microscopy 2: High Frequencies and Optical Techniques
Sponsoring Units: GIMSChair: Robert McMichael, NIST
Room: Hilton Baltimore Holiday Ballroom 5
Thursday, March 21, 2013 11:15AM - 11:51AM |
U46.00001: Edge mode imaging in magnetic nanodisks using ferromagnetic resonance force microscopy Invited Speaker: Feng Guo Edge modes are trapped spin wave modes that can form at film edges. The spontaneous localization of edge modes makes them fine probes of edge properties and test objects for magnetic resonance imaging. We use ferromagnetic resonance force microscopy (FMRFM) to study the edge modes in magnetic nanodisks with an improved resolution of less than 100 nm. In this presentation we will describe imaging and spectroscopy of the normal modes in Permalloy disks, manipulation of edge modes to characterize the disk edges, and the disk-diameter dependence of the spectrum. Micromagnetic modeling of a 500 nm diameter, 25 nm thick disk predicts a main mode that is nearly uniform across the sample and three edge modes with higher resonance fields. The spectra measured with various tip positions are consistent with the modeling results. Besides the broad center mode, three distinct edge modes are observed and appear when the tip is near the disk edge. However, in contrast to the symmetric edge behavior predicted by the modeling, the measured left and right edge modes are detected at different resonance fields, suggesting inhomogeneity of the edge properties. By rotating the applied field, we are able to move the localized edge mode along the edge of a single structure and thus probe the inhomogeneity in edge properties. The fundamental edge mode with the highest resonance field is most sensitive to the edge inhomogeneity while the center mode is relatively isotropic. The disk size dependence of the edge mode is also investigated for disk diameters ranging from 100 nm to 750 nm. The number of trapped edge modes reduces with decreasing disk size in agreement with micromagnetic modeling. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U46.00002: Magnetic imaging with shallow spins in nitrogen delta-doped diamond Bryan A. Myers, Jens Boss, Kenichi Ohno, Preeti Ovartchaiyapong, David D. Awschalom, Ania C. Bleszynski Jayich Nitrogen-vacancy (NV) electronic spins in diamond are atomic-size sensors of magnetism at the nanoscale. Shallow NVs with long spin coherence times ($T_2$) are desirable for ultrasensitive magnetometry. However, $T_2$ tends to decrease for shallow NVs, which couple most strongly to external spins. To optimize magnetic sensitivity, it was recently shown that delta-doping nitrogen during chemical vapor deposition of single-crystal diamond (SCD) can produce films with a $<5$ nm thick layer of NVs that retain long $T_2$ [1]. Here, using a magnetic field gradient produced by a scanning probe, we investigate optically-detected magnetic resonance measurement protocols to simultaneously determine the relative and absolute depths of the NVs in SCD films containing multiple doped layers separated by a few nm. A consistent comparison of NV properties, such as $T_2$, versus depth is important for engineering spin placement. Furthermore, this magnetic field gradient technique enables sub-diffraction imaging of NV centers, which itself will be explored for high resolution NV-based magnetometry. [1] K. Ohno et al., Appl. Phys. Lett. 101, 082413 (2012). [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U46.00003: Nanoscale Fourier-transform magnetic resonance imaging John Nichol, Tyler Naibert, William Rose, Eric Hemesath, Lincoln Lauhon, Raffi Budakian Magnetic resonance force microscopy is a promising technique for nanoscale magnetic resonance imaging, but the detection sensitivity must still be improved to reach the single proton level. Multiplexed imaging schemes, such as Fourier encoding, are used in clinical magnetic resonance imaging for sensitivity enhancement. Here, we report a method for Fourier encoding nanoscale samples, where statistical fluctuations dominate the spin polarization. The protocol uses periodic encoding pulses to create correlations in the spin fluctuations. We demonstrate this technique using a silicon nanowire mechanical oscillator as a force sensor to image $^{1}$H spins in a polystyrene sample. The sample is encoded using pulsed magnetic field gradients generated by a nanoscale current-carrying wire. We reconstruct a 2-dimensional projection of the proton density in the sample with 10~nm resolution. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U46.00004: On infrared and terahertz imaging of surface plasmons in high-Tc superconductors H.T. Stinson, Z. Fei, A.S. Rodin, A.S. McLeod, M.M. Fogler, D.N. Basov Recent scattering-mode scanning near-field optical microscopy (s-SNOM) experiments have imaged surface plasmons in graphene at infrared frequencies.\footnote{Z. Fei et al., Nature, \textbf{487}, 82 (2012).} The scanning probe launches surface plasmons and detects their standing-wave interference pattern upon reflection from the sample edge. The surface plasmon dispersion relation directly relates the standing wave fringe separation and amplitude decay to the optical constants of the sample. We have modeled surface plasmon s-SNOM imaging for high-Tc superconductor (HTSC) thin films. Our results indicate that surface plasmons can be imaged in HTSCs at frequencies near or below the superconducting gap. This would allow for a direct measurement of HTSC optical constants below the gap. For known HTSCs such as YBCO, this is in the far-IR or terahertz range. Our simulations show that this method can also distinguish between superconducting and normal states at the nanoscale. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U46.00005: Quantifying the Stochastic Dynamics of the Elastic Probe used in Cavity Optomechanical Force Micropscopy Stephen Epstein, Mark Paul Atomic force microscopy has revolutionized surface science and is now as essential tool for micro and nanoscale studies in science and engineering. Cavity optomechanical force microscopy consists of an atomic force microscopy probe that is placed in close proximity to a microfabricated optical cavity. The interaction between the probe and the optical cavity is used to quanitfy the probe dynamics. Cavity optomechaincal force microscopy extends conventional atomic force microscopy by being more sensitive with increased frequency resolution. In many situations of interest the probe operates while immersed in a viscous fluid which can strongly affect the probe dynamics. In this talk we quantify the stochastic dynamics of the elastic probe when driven by Brownian motion where the dominant source of dissipation is the surrounding viscous fluid. We use deterministic finite-element numerical simulations with the fluctuation-dissipation theorem to quantify the stochastic dynamics of the probe for the precise conditions and geometries used in current experiments. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U46.00006: Enhanced Electroluminescence from A Nanocavity Due to Dynamical Coupling of Plasmonic and Molecular Emissions Xiaoguang Li, Gong Chen, Zhenchao Dong, Jian Shen, Zhenyu Zhang We investigate the electroluminescence from a nanocavity formed by a luminescent molecule within the tip-substrate junction of a scanning tunneling microscope. The light emissions from the molecular luminescence and plasmonic radiation are evaluated using respectively a density matrix approach and classical electromagnetic theory. The molecular luminescence is described in two different components: the radiation associated with the excited states effectively pumped by the tunneling electrons and the spontaneous emission enhanced by the plasmonic field. In particular, by explicitly treating the near field of the plasmons, we explore in detail the dynamical coupling between the plasmonic and molecular emissions, and identify conditions for enhanced electroluminescence. We discuss these results in comparison with experiments. [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U46.00007: Measurement of optical force in plasmonic resonant cavities using dynamic mode AFM Dongshi Guan, Zhihong Hang, Zsolt Marcet, Hui Liu, Ivan Kravchenko, Cheting Chan, Hobun Chan, Penger Tong We report an experimental study of the optical force induced by a plasmonic resonance mode in metallic cavities using dynamic mode atomic force microscopy (AFM). The plasmonic cavity is made of a (upper) gold coated glass sphere and a (lower) quartz substrate patterned with an array of gold disks, whose diameter $d$ varies from 250 to 750 nm. The gold coated sphere is glued to an AFM cantilever, by which we measure the optical force acted on the sphere using AFM and phase-sensitive lock-in amplifier. With this technique the sensitivity of the force measurement is significantly increased to $\sim$0.1 pN, which may have many applications in precise force measurement. The measured optical force is found to have a strong resonance dependence on the cavity separation $r$, as well as the diameter of gold disk $d$. The conventional optical force obtained in the far-field ($r$$>$3$\mu$m) for different values of $d$ agrees well with the measured transmission. In the near-field ($r$$<$0.5$\mu$m), resonance is excited in the plasmonic cavity and the induced force by an infrared laser is found to be increased by an order of magnitude compared with the photon pressure generated by the same laser light. *Work supported by the Research Grants Council of Hong Kong SAR. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U46.00008: Nano-FTIR: infrared spectroscopic chemical identification of materials at the nanoscale Florian Huth, Alexander Govyadinov, Sergiu Amarie, Wiwat Nuansing, Fritz Keilmann, Rainer Hillenbrand Recently, we applied the principles of FTIR to scattering-type Scanning Near-field Optical Microscopy (s-SNOM). s-SNOM employs an externally illuminated sharp metallic tip to create a nanoscale hot-spot at its apex which greatly enhances the near-field interaction between the probing tip and the sample. The light backscattered from the tip transmits the information about this near-field interaction to the far zone where the FTIR spectra can be recorded. The result is a novel nano-FTIR technique, which is capable to perform near-field spectroscopy and imaging with nanoscale resolution. Here we demonstrate nano-FTIR with a coherent-continuum infrared light source. We show that the method can be used to determine the fingerprint IR absorption spectrum of organic samples with a spatial resolution of 20 nm. Corroborated by theory, the nano-FTIR absorption spectra correlate well with conventional FTIR absorption spectra, as experimentally demonstrated with PMMA samples. Nano-FTIR can thus make use of standard infrared databases of molecular vibrations to identify organic materials in ultra-small quantity and at ultrahigh spatial resolution. [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U46.00009: Broadband vibrational nano-spectroscopy with a synchrotron infrared source Hans A. Bechtel, Robert L. Olmon, Eric A. Muller, Benjamin Pollard, Markus B. Raschke, Michael C. Martin Scattering-scanning near-field optical microscopy (s-SNOM) is capable of providing chemical contrast with deep sub-wavelength spatial resolution of a few 10's of nanometers. Unfortunately, the wide applicability of the technique has been hindered by the lack of suitable broadly-tunable or broadband IR sources that can provide the necessary high spectral irradiance. Here, we demonstrate broadband, Fourier-transform infrared spectroscopic s-SNOM using infrared synchrotron radiation from the Advanced Light Source (ALS). We show near-field spectra spanning the full mid-infrared, including the fingerprint absorption region (700 cm$^{-1}$ --- 4000 cm$^{-1}$) and spectroscopic multi-modal imaging in combination with laser-based IR sources. We discuss the potential of the approach for a wide range of soft and hard matter nanoscale spectroscopic applications. [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U46.00010: The Lightning Rod Model: a Genesis for Quantitative Near-Field Spectroscopy Alexander McLeod, Gregory Andreev, Gerardo Dominguez, Mark Thiemens, Michael Fogler, D.N. Basov Near-field infrared spectroscopy has the proven ability to resolve optical contrasts in materials at deeply sub-wavelength scales across a broad range of infrared frequencies. In principle, the technique enables sub-diffractional optical identification of chemical compositions within nanostructured and naturally heterogeneous samples. However current models of probe-sample optical interaction, while qualitatively descriptive, cannot quantitatively explain infrared near-field spectra, especially for strongly resonant sample materials. We present a new first-principles model of near-field interaction, and demonstrate its superb agreement with infrared near-field spectra measured for thin films of silicon dioxide and the strongly phonon-resonant material silicon carbide. Using this model we reveal the role of probe geometry and surface mode dispersion in shaping the measured near-field spectrum, establishing its quantitative relationship with the dielectric properties of the sample. This treatment offers a route to the quantitative determination of optical constants at the nano-scale. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U46.00011: Interferometric Scanning Microwave Microscope for Nanotechnology Application Nicolas Clement, Thomas Dargent, Hassan Tanbakuchi, Katsuhiko Nishiguchi, Ragavendran Sivakumarasamy, Fei Wang, Akira Fujiwara, Damien Ducatteau, Gilles Dambrine, Dominique Vuillaume, Bernard Legrand, Didier Th\'eron Scanning probe microscopes (SPMs) allow scientists to image, characterize and even manipulate material structures at exceedingly small scales including features of atomic dimensions. Although most microelectronics devices operate at high frequency, SPMs have mainly been used with electrical excitation at DC (Conducting Atomic Force Microscope) or kHz (Electric Force Microscope, Kelvin Force Microscope). The main reason is that at GHz frequency, nanoscale objects are far from the standard impedance of 50ohms and almost all the signal is reflected. Here we show, using an interferometer to enable extraction and amplification of the signal of interest, that Scanning Microwave Microscopes (SMM) are ideal tools for tiny capacitances imaging. We demonstrate applications in several fields of nanotechnology with capacitance evaluation down to aF of nanoscale integrated capacitors, biased nanotransistors, molecular junctions and biomolecule flow in a nanofluidic channel. The frequency range of excitation varied from 2 GHz to 20 GHz. With a finite element analysis, we discuss the limits of such microscope. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U46.00012: Highly enhanced green emission of ZnO via plasmonic resonance of a tungsten tip Huiqi Gong, Xiaodong Guo, Li Dong, Nan Xie, Shichao Yan, Xinyan Shan, Yang Guo, Jimin Zhao, Qian Sun, Xinghua Lu We present a systematic investigation of the photoluminescence of a single crystal ZnO with the aid of a metallic tungsten tip in a pulse laser assisted scanning tunneling microscope. When excited with 740nm laser pulses and as the tip approaches ZnO surface up to the tunneling region ($\sim$ 1nm), an enhancement in green emission (centered at 560nm), up to a factor of 70, is observed. The photoluminescence is a two-photon excitation process, which is evident by the observation of the second-harmonic peak of excitation light and the up-converted luminescence. By measuring the green emission intensity as a function of incidence power, wavelength, and tip-sample distance, we illustrate the critical role of plasmonic resonance of the tungsten tip for the enhanced green emission. The observed broad plasmonic response (680nm to 1080nm) implies possible applications in designing novel solar cells with the aid of tungsten plasmon. [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U46.00013: Single \& Multiprobe Apertureless Thermal Imaging of Electromagnetic Excitation Over A Wide Range of Wavelengths Rimma Dekhter, Aaron Lewis, Sophia Kokotov, Patricia Hamra, Boaz Fleischman, Hesham Taha Near-field optical effects have generally been detected using photodetectors. There are no reports on the use of the temperature changes caused by electromagnetic radiation using thermal sensing probes for scanned probe microscopy. In this paper we apply our development of such probes to monitor the effects of electromagnetic radiation at a number of different wavelengths using the heating caused in a sample by specific wavelengths and their propagation. The paper will catalogue effects over a wide spectrum of wavelengths from the near to mid infrared. The thermal sensing probes are based on glass nanopipettes that have metal wires that make a contact at the very tip of a tapered glass structure. These probes are cantilevered and use normal force tuning fork methodology to bring them either into contact or near-contact since this feedback method has no jump to contact instability associated with it. Data will be shown that defines the resolution of such thermal sensing to at least the 32 nm level. In addition the probes have the important attribute of having a highly exposed tip that allows for either optical sensing methodologies with a lens either from directly above or below or heat sensing with a single or additional probe in a multiprobe scanning probe system. [Preview Abstract] |
Session U47: Invited Session: Controlling Biological Networks
Sponsoring Units: DBIO GSNPChair: Albert-Laszlo Barabasi, Northeastern University/Harvard Medical School
Room: Hilton Baltimore Holiday Ballroom 6
Thursday, March 21, 2013 11:15AM - 11:51AM |
U47.00001: Hard limits on control in fluctuating systems Invited Speaker: Johan Paulson All intracellular processes involve components present in low numbers, creating spontaneous fluctuations that in turn can enslave the components present in high numbers. The mechanisms are often complex, with reaction rates that depend nonlinearly on concentrations, indirect feedback loops, and distributed delays. Most systems are also sparsely characterized, with a few steps known in detail but many important interactions not even identified. I will present exact analytical mathematical frameworks for deriving limits on behavior in such systems, for example showing how hard it is to tightly control processes that involve bursts, delays, or finite signaling rates - regardless of the nature of the downstream chemical networks. I will also discuss various ways of designing experiments to rigorously exploit conditional independences in fluctuations to infer underlying mechanisms, without having to guess the nature of feedback loops or interacting processes. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:27PM |
U47.00002: Controllability and observability of biological systems Invited Speaker: Yang-Yu Liu The ultimate proof of our understanding of complex biological systems is reflected in our ability to control them. Although control theory offers mathematical tools for steering engineered systems towards a desired state, a framework to control complex biological systems is lacking. In this talk I will show that many dynamic properties of complex biological systems can be quantitatively studied, via a combination of tools from control theory, network science and statistical physics. In particular, I will focus on two dual concepts, i.e. controllability and observability, of general complex biological systems. Controllability concerns our ability to drive the system from any initial state to any final state within finite time, while observability concerns the possibility to deduce the system's internal state from observing its input-output behavior. I will show that by exploring the underlying network structure of complex biological systems one can determine the driver (or sensor) nodes that with time-dependent inputs (or measurements) will enable us to fully control (or observe) the whole system. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 1:03PM |
U47.00003: Epigenetics and Why Biological Networks are More Controllable than Expected Invited Speaker: Adilson Motter A fundamental property of networks is that perturbations to one node can affect other nodes, potentially causing the entire system to change behavior or fail. In this talk, I will show that it is possible to exploit this same principle to control network behavior. This approach takes advantage of the nonlinear dynamics inherent to real networks, and allows bringing the system to a desired target state even when this state is not directly accessible or the linear counterpart is not controllable. Applications show that this framework permits both reprogramming a network to a desired task as well as rescuing networks from the brink of failure, which I will illustrate through various biological problems. I will also briefly review the progress our group has made over the past 5 years on related control of complex networks in non-biological domains. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:39PM |
U47.00004: Control of cancer-related signal transduction networks Invited Speaker: Reka Albert Intra-cellular signaling networks are crucial to the maintenance of cellular homeostasis and for cell behavior (growth, survival, apoptosis, movement). Mutations or alterations in the expression of elements of cellular signaling networks can lead to incorrect behavioral decisions that could result in tumor development and/or the promotion of cell migration and metastasis. Thus, mitigation of the cascading effects of such dysregulations is an important control objective. My group at Penn State is collaborating with wet-bench biologists to develop and validate predictive models of various biological systems. Over the years we found that discrete dynamic modeling is very useful in molding qualitative interaction information into a predictive model. We recently demonstrated the effectiveness of network-based targeted manipulations on mitigating the disease T cell large granular lymphocyte (T-LGL) leukemia. The root of this disease is the abnormal survival of T cells which, after successfully fighting an infection, should undergo programmed cell death. We synthesized the relevant network of within-T-cell interactions from the literature, integrated it with qualitative knowledge of the dysregulated (abnormal) states of several network components, and formulated a Boolean dynamic model. The model indicated that the system possesses a steady state corresponding to the normal cell death state and a T-LGL steady state corresponding to the abnormal survival state. For each node, we evaluated the restorative manipulation consisting of maintaining the node in the state that is the opposite of its T-LGL state, e.g. knocking it out if it is overexpressed in the T-LGL state. We found that such control of any of 15 nodes led to the disappearance of the T-LGL steady state, leaving cell death as the only potential outcome from any initial condition. In four additional cases the probability of reaching the T-LGL state decreased dramatically, thus these nodes are also possible control targets. Our collaborators validated two of these predicted control mechanisms experimentally. Our work suggests that external control of a single node can be a fruitful therapeutic strategy. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 2:15PM |
U47.00005: Controllability of Complex Systems Invited Speaker: Jean-Jacques Slotine We review recent work on controllability of complex systems. We also discuss the interplay of our results with questions of synchronization, and point out key directions of future research.\\[4pt] Work done in collaboration with Yang-Yu Liu, Center for Complex Network Research and Departments of Physics, Computer Science and Biology, Northeastern University and Center for Cancer Systems Biology, Dana-Farber Cancer Institute; and Albert-L\'aszl\'o Barab\'asi, Center for Complex Network Research and Departments of Physics, Computer Science and Biology, Northeastern University; Center for Cancer Systems Biology, Dana-Farber Cancer Institute; and Department of Medicine, Brigham and Women's Hospital, Harvard Medical School. [Preview Abstract] |
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