Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N1: Invited Session: Quantum Computing With Diamond
Sponsoring Units: GQI DCMPChair: Mohammad Hafezi, University of Maryland
Room: Ballroom I
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N1.00001: Entanglement and entanglement storage in dipolar coupled diamond defects Invited Speaker: Joerg Wrachtrup The generation of robust entangled states is one of the key steps in quantum science. Although diamond defects are highly versatile quantum bits mutual entanglement has not been demonstrated so far. The talk will describe the engineering of strongly coupled defect centers as well as their characteristic features. Entanglement generation as well as different means of tomography will be outlined. Correlated photon emission form coupled defect center pairs is analyzed. Robust storage of electron spin entanglement into nuclear spins resulting in entanglement storage lifetime of ms is demonstrated and roads towards efficient generation of strongly coupled defect arrays will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N1.00002: Mobile quantum sensing with spins in optically trapped nanodiamonds Invited Speaker: David D. Awschalom The nitrogen-vacancy (NV) color center in diamond has emerged as a powerful, optically addressable, spin-based probe of electromagnetic fields and temperature. For nanoscale sensing applications, the NV center's atom-like nature enables the close-range interactions necessary for both high spatial resolution and the detection of fields generated by proximal nuclei, electrons, or molecules. Using a custom-designed optical tweezers apparatus, we demonstrate three-dimensional position control of nanodiamonds in solution with simultaneous optical measurement of electron spin resonance (ESR)\footnote{V.R. Horowitz, B.J. Alem\'{a}n, D.J. Christle, A.N. Cleland, and D.D. Awschalom, \textit{Proc. Natl. Acad. Sci. USA}, \textbf{109}, 13493 (2012).}. Despite the motion and random orientation of NV centers suspended in the optical trap, we observe distinct peaks in the ESR spectra from the ground-state spin transitions. Accounting for the random dynamics of the trapped nanodiamonds, we model the ESR spectra observed in an applied magnetic field and estimate the dc magnetic sensitivity based on the ESR line shapes to be $\sim$50 $\mu$T/$\sqrt{Hz}$. We utilize the optically trapped nanodiamonds to characterize the magnetic field generated by current-carrying wires and ferromagnetic structures in microfluidic circuits. These measurements provide a pathway to spin-based sensing in fluidic environments and biophysical systems that are inaccessible to existing scanning probe techniques, such as the interiors of living cells. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N1.00003: Quantum optical networks with diamond nanophotonics Invited Speaker: Nathalie de Leon Scalable quantum optical networks require identical single photons from multiple quantum bits and high collection efficiency of these single photons. Nitrogen vacancy (NV) centers in diamond are a promising candidate for quantum information processing because they have quantum mechanical degrees of freedom that can be addressed optically and, as solid-state structures, can potentially be easily integrated into nanophotonic networks. In particular, they have a zero-phonon line (ZPL), which acts as an atom-like cycling transition that can be used for coherent optical manipulation. However, the ZPL only accounts for 3-5{\%} of the total emission, and it is difficult to generate a high density of NV centers with stable ZPL. I will present progress toward gaining both spectral and spatial control over NV emission by coupling NV centers to nanophotonic devices. In particular, we have fabricated high quality factor (Q), small mode volume (V) photonic crystal cavities directly out of diamond, and have deterministically placed them around stable NV centers to enhance the spontaneous emission at the cavity resonance by a factor of 50-100. This emission is guided efficiently into a single optical mode, enabling integration with other photonic elements, as well as networks of cavities, each with their own optically addressable qubit. These nanophotonic elements in diamond will provide a building block for a variety of applications in quantum information processing, such as entanglement of distant NV centers and single photon transistors. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N1.00004: Single-shot readout of multiple nuclear spin qubits in diamond under ambient conditions Invited Speaker: Vincent Jacques Nuclear spins are attractive candidates for solid-state quantum information storage and processing owing to their extremely long coherence time. However, since this appealing property results from a high level of isolation from the environment, it remains a challenging task to polarize, manipulate and readout with high fidelity individual nuclear spins. A promising approach to overcome this limitation consists in utilizing an ancillary single electronic spin to detect and control remote nuclear spins coupled by hyperfine interaction. In this talk, I will show how the electronic spin of a single Nitrogen-Vacancy (NV) defect in diamond can be used as a robust platform to observe the real-time evolution of surrounding single nuclear spins under ambient conditions. Using a diamond sample with a natural abundance of $^{13}$C isotopes, we first demonstrate high fidelity initialization and single-shot readout of an individual $^{13}$C nuclear spin. By including the intrinsic $^{14}$N nuclear spin of the NV defect in the quantum register, we then report the simultaneous observation of quantum jumps linked to both nuclear spin species, providing an efficient initialization of the two qubits. These results open up new avenues for diamond-based quantum information processing (QIP) including active feedback in quantum error correction protocols and tests of quantum correlations with solid-state single spins at room temperature. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N1.00005: Quantum entanglement between diamond spin qubits separated by 3 meters Invited Speaker: Ronald Hanson Entanglement of spatially separated objects is one of the most intriguing phenomena that can occur in physics. This can lead ``spooky action at a distance'' where measurement of one object instantaneously affects the state of the other object. Besides being of fundamental interest, entanglement is also a valuable resource in quantum information technology enabling secure quantum communication networks and distributed quantum computing. Here we present our most recent results towards the realization of scalable quantum networks with solid-state qubits. We have entangled two spin qubits in diamond, each associated with a nitrogen vacancy center in diamond [1]. The two diamonds reside in separate setups three meters apart from each other. With no direct interaction between the two spins to mediate the entanglement, we make use of a scheme based on quantum measurements: we perform a joint measurement on photons emitted by the NV centers that are entangled with the electron spins. The detection of the photons projects the spins into an entangled state. We verify the generated entanglement by single-shot readout of the spin qubits in different bases and correlating the results. These results open the door to a range of exciting opportunities. For instance, the remote entanglement can be extended to nuclear spins near the NV center. Our recent experiments demonstrate robust methods for initializing, controlling and entangling nuclear spins by using the electron spin as an ancilla [2,3]. Entanglement of remote quantum registers will enable deterministic quantum teleportation, distributed quantum computing tasks and the implementation of an elementary quantum repeater.\\[4pt] [1] H. Bernien et al., in preparation.\\[0pt] [2] T. van der Sar et al., Nature 484, 82 (2012).\\[0pt] [3] W. Pfaff et al., Nature Physics (2012); doi:10.1038/nphys2444. [Preview Abstract] |
Session N2: Invited Session: Electron Matter in FE-Based Superconductors
Sponsoring Units: DCMPChair: Zhi-Xun Shen, Stanford University
Room: Ballroom II
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N2.00001: Pairing mechanism and gap symmetry in Fe-based superconductors with only electron or only hole pockets Invited Speaker: Andrey Chubukov The pairing in moderately doped Fe-pnictides and Fe-chalcogenides is generally understood as being due to magnetically enhanced interaction between hole and electron pockets. Recently, however, superconductivity has been observed in AFe$_{2}$Se$_{2}$ (A $=$ K, Rb, Cs), which contain only electron pockets, and in KFe$_{2}$As$_{2}$, which contains only hole pockets. In the talk, I review different (and sometimes conflicting) scenarios for the pairing in these systems and propose my own. I argue that the pairing condensate in systems with only electron pockets necessary contains not only a conventional intra-pocket component, but also inter-pocket component, made of two fermions belonging to different electron pockets. I analyze the interplay between intra-pocket and inter-pocket pairing depending on the ellipticity of electron pockets and the strength of their hybridization and show that with increasing hybridization the system undergoes a transition from a d-wave state to an s$^{+-}$ state, in which the gap changes sign between hybridized pockets. This s$^{+-}$ state has the full gap and at the same time supports spin resonance, in agreement with the data. Near the boundary between d and s$^{+-}$ states the system develops s$+$id state which breaks time-reversal symmetry. For systems with only hole pockets, I argue for s$^{+-}$ state in which the gap changes sign between hole pockets. I show that this state is qualitatively different from s$^{\mathrm{+-}}$ state when both hole and electron pockets are present. I further show that the transition from one s-wave state to the other involves highly unusual s$+$is state which again breaks time reversal symmetry. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N2.00002: Atomic-scale Visualization of Electronic Nematicity and Cooper Pairing in Iron-based Superconductors Invited Speaker: Milan P. Allan The mechanism of high-temperature superconductivity in the relatively novel iron-based high-T$_c$ superconductors is unresolved, both in terms of how the phases evolve with doping, and in terms of the actual Cooper pairing process. To explore these issues, we used spectroscopic-imaging scanning tunneling microscopy to study the electronic structure of CaFe$_2$As$_2$ in the antiferromagnetic-orthorhombic `parent' state from which the superconductivity emerges. We discovered and visualized the now widely studied electronic `nematicity' of this phase, whose suppression is associated with the emergence of superconductivity (\emph{Science} 327, 181, 2010). As subsequent transport experiments discovered a related anisotropic conductance which increases with dopant concentration, the interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom, and the transport nematicity has become a pivotal focus of research. We find that substituting Co for Fe atoms in underdoped Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ generates a dense population of identical and strongly anisotropic impurity states that are distributed randomly but aligned with the antiferromagnetic $a$-axis. We also demonstrate, by imaging their surrounding interference patterns, that these impurity states scatter quasiparticles and thus influence transport in a highly anisotropic manner (M.P. Allan et al., 2013). Next, we studied the momentum dependence of the energy gaps of iron-based superconductivity, now focusing on LiFeAs. If strong electron-electron interactions mediate the Cooper pairing, then momentum-space anisotropic superconducting energy gaps $\Delta_i(k)$ were predicted by multiple techniques to appear on the different electronic bands $i$. We introduced intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for the determination of anisotropic energy gaps to test these hypotheses and discovered the anisotropy, magnitude, and relative orientations of the energy gaps on multiple bands (\emph{Science} 336, 563 (2012)). Finally, the electron-electron interactions generating Cooper pairing are often conjectured to involve bosonic spin fluctuations generated by interband scattering of electrons. We explore the STM signatures of both the interband scattering and the electron-boson coupling self-energy in LiFeAs, and detect the signatures of the electron-boson coupling (M.P. Allan et al., in preparation). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N2.00003: Spin dynamics in electron and hole-doped iron pnictide superconductors Invited Speaker: Pengcheng Dai |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N2.00004: ARPES studies of the superconducting gap symmetry of Fe-based superconductors Invited Speaker: Pierre Richard The superconducting gap is the fundamental parameter that characterizes the superconducting state, and its symmetry is a direct consequence of the mechanism responsible for Cooper pairing. Here I discuss about angle-resolved photoemission spectroscopy measurements of the superconducting gap in the Fe-based high-temperature superconductors. I show that the superconducting gap is Fermi surface dependent and nodeless with small anisotropy, or more precisely, a function of momentum. I show that while this observation is inconsistent with weak coupling approaches for superconductivity in these materials, it is well supported by strong coupling models and global superconducting gaps. I also stress the importance of scattering and the lifetime of quasiparticles in evaluation the superconducting gap by angle-resolved photoemission spectroscopy and other experimental techniques. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N2.00005: Effects of disordered substitutions and vacancies in Fe based superconductors from first principles Invited Speaker: Tom Berlijn Most Fe pnictide and selenide superconductors are created by chemical substitution which inevitably introduces disorder. The relationship between nominal chemical valence, doping, and quasiparticle spectral weight appears to be quite complex. Using a recently developed Wannier function based first principles method for disordered systems [1], we compute the configuration-averaged spectral function $\langle A(k,\omega)\rangle$ of Fe based superconductors containing disordered substitutions and vacancies. In the transition metal doped Ba(Fe$_{1-x}$M$_x$)$_2$As$_2$ with M=Co/Zn we find[2] a loss of coherent carrier spectral weight. For the case of disordered Fe and K vacancies in K$_{0.8}$Fe$_{1.6}$Se$_2$ we find a disorder induced effective doping to give rise to enlarged electron pockets without adding electrons to the system. For the case of Ru substitutions in Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ we find[4] a cancelation between on- and off-site disorder to give rise to a surprising protection of the Fermi surface.\\[4pt] [1] T. Berlijn, D. Volja and W. Ku, PRL 106, 077005 (2011)\\[0pt] [2] T. Berlijn, C.-H. Lin, W. Garber and W. Ku, PRL 108, 207003 (2012)\\[0pt] [3] T. Berlijn, P. J. Hirschfeld and W. Ku, PRL 109, 147003 (2012)\\[0pt] [4] L. Wang, T. Berlijn, Y. Wang, C.-H. Lin, P. J. Hirschfeld and W. Ku, arXiv:1209.3001 [Preview Abstract] |
Session N3: Invited Session: Physics For Everyone
Sponsoring Units: DMPChair: Laura H. Greene, University of Illinois at Urbana-Champaign
Room: Ballroom III
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N3.00001: New ways to engage the public with quantum physics Invited Speaker: Julien Bobroff We are a few french condensed matter physicists involved in developping new routes to engage the public with our research field. We have worked with designers, graphists, artists, teachers and the public to produce original tools for outreach. In this talk, we will present some of them : \begin{itemize} \item demonstration tools such as a superconducting circus or a levitating Eiffel Tower \item futuristic videos (what life would be if room temperature superconductivity was achieved?) \item graphic animations to make quantum physic simple and appealing \item websites about quantum physics or superconductivity ( www.quantummadesimple.com or www.superconductivity.eu ) \item folding activities for kids to understand orbitals and superconductivity \end{itemize} We will also discuss the engagement of condensed matter physicists in outreach activities. Many of us find it hard to get involved : ``not enough time'', ``my field is to complex to be popularized'', ``not good for my career'', ``science museums do it better''... We will give some insight of how we could overcome this reluctance among our colleagues and get many french researchers involved in engaging the public over the past two years. All our activities and productions can be found in English at the website: www.vulgarisation.fr [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N3.00002: The Physics of NASCAR: Why Going Fast is Harder than You Might Think Invited Speaker: Diandra Leslie-Pelecky NASCAR is unique among major sports in that science, math and engineering are integral to winning. You can't win races without getting the physics right. That constraint provides a novel way to reach the seventy-five million NASCAR fans who desperately want to understand why their driver is (or isn’t winning). Unlike outreach to those already interested in science, using popular culture to reach out requires taking advantage of unexpected events and non-traditional means. Does a loose piece of metal really justify a \$100,000 fine? (NPR didn't think so...) From the science of designing a 900-horsepower, 200 mph aerodynamic bullet to the knowledge and experience required to drive that car, physics is the ultimate arbiter of speed. Moving from simple introductory physics that approximates a race car as a point particle to computational fluid dynamics, you'll learn why driving fast isn’t as easy as you might think. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N3.00003: How the ``Blues'' reveals the intimacy of music and physics Invited Speaker: J. Murray Gibson Little do most people know when they hear blues piano -- and you'll hear some live in this talk -- that physics permeates the style, as it does all of music. Why should you care? By deconstructing blues piano the intimacy of physics, mathematics and music will be revealed in its glory.\footnote{Gibson, J. M. ``The birth of the blues: how physics underlies music,'' Reports on Progress in Physics \textbf{72}, 076001, (2009).} The exercise says something about how the brains of the music composer and of the listener must be intimately linked to the physical principles of acoustics. ~And it provides a great vehicle to explain physical phenomena to non-scientists -- everything from quantum mechanics to protein structure. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N3.00004: How Plastics Work Invited Speaker: Louis Bloomfield We encounter plastics every day, but despite their widespread use, amazing range of properties, and basic scientific underpinnings, most physicists---like most people---know relatively little about plastics. In contrast to hard crystalline and amorphous solids (e.g., metals, salts, ceramics, and glasses), we take plastics for granted, select them carelessly, and examine them more closely only on a need-to-know basis. By ignoring plastics until we need them, however, we risk not knowing what we don't know and using the wrong ones. To repurpose a familiar advertisement, ``there's a plastic for that.'' This talk will review some of the basic physics and science of plastics. It will examine the roles of temperature, order, intermolecular forces, entanglements, and linkages in plastics, and how those issues affect the properties of a given plastic. We'll stop along the way to recognize a few of the more familiar plastics, natural and synthetic, and explain some of their mechanical, chemical, and optical properties. The talk will conclude by explaining the remarkable properties of a plastic that has been largely misunderstood since its discovery 70 years ago: Silly Putty. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N3.00005: Looking at Art in the IR and UV Invited Speaker: Charles Falco Starting with the very earliest cave paintings art has been created to be viewed by the unaided eye and, until very recently, it wasn't even possible to see it at wavelengths outside the visible spectrum. However, it is now possible to view paintings, sculptures, manuscripts, and other cultural artifacts at wavelengths from the x-ray, through the ultraviolet (UV), to well into the infrared (IR). Further, thanks to recent advances in technology, this is becoming possible with hand-held instruments that can be used in locations that were previously inaccessible to anything but laboratory-scale image capture equipment. But, what can be learned from such ``non-visible'' images? In this talk I will briefly describe the characteristics of high resolution UV and IR imaging systems I developed for this purpose by modifying high resolution digital cameras. The sensitivity of the IR camera makes it possible to obtain images of art ``in situ'' with standard museum lighting, resolving features finer than 0.35 mm on a 1.0x0.67 m painting. I also have used both it and the UV camera in remote locations with battery-powered illumination sources. I will illustrate their capabilities with images of various examples of Western, Asian, and Islamic art in museums on three continents, describing how these images have revealed important new information about the working practices of artists as famous as Jan van Eyck. I also will describe what will be possible for this type of work with new capabilities that could be developed within the next few years. [Preview Abstract] |
Session N4: Invited Session: Climate as a Complex Dynamical System
Sponsoring Units: GPCChair: John Wettlaufer, Yale University
Room: Ballroom IV
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N4.00001: Changes in Polar Sea Ice and How They Illustrate the Complex Picture of Global Climate Change Invited Speaker: Claire Parkinson Sea ice spreads over vast areas of the polar oceans, typically covering 17-28 million km2 globally. It is a critical element of the Arctic and Antarctic climate systems, with two of its most important roles being the reflection of solar radiation back to space and the hindering of exchanges of heat, mass, and momentum between the ocean and the atmosphere. Prior to the development of satellite technology, it was not feasible to obtain large-scale data records of the vast expanse of global sea ice. However, with satellites, and especially with multichannel passive-microwave satellite data available since late 1978, we can now monitor both Arctic and Antarctic sea ice coverages on a daily basis, irrespective of sunlight or darkness and under cloudy as well as cloud-free conditions. This has made sea ice one of the best observed climate variables since the late 1970s. The resulting satellite record has revealed many details of the seasonal cycle of the ice cover in both polar regions, considerable inter-annual variability, and long-term trends that show a decrease in the Arctic sea ice and an increase in the Antarctic sea ice since late 1978. The decreases in the Arctic sea ice extents, which have averaged approximately 51,000 km2 per year on a yearly-average basis, were predicted and are tied closely to the warming of the Arctic over the same time period. The increases in the Antarctic sea ice extents, which have averaged approximately 17,000 km2 per year, have come with stark spatial contrasts that suggest the likely impact of changes in atmospheric and/or oceanic circulations. Sea ice decreases in the vicinity of the Antarctic Peninsula, where warming has occurred, have been more than compensated for by increases in the ice cover elsewhere around the continent, especially in the Ross Sea. The patterns are suggestive of increased cyclonic flow centered over the Amundsen Sea, although more research is needed before the changes will be fully understood. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N4.00002: The Solar Climate Link: How Large? How Come? How important? Invited Speaker: Nir Shaviv Solar variations appear to have a significant effect on climate. I will begin by reviewing the evidence pointing to a large solar/climate link and present measurements quantifying it. I will then discuss the atmospheric effects of cosmic rays, which offer the most consistent mechanism linking between solar variations and climate change. I will end by placing the link in context of other climate questions, such as the value of the climate sensitivity and implications to the understanding of 20th and 21st century climate change. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N4.00003: The Atmospheric Chemistry of Climate Change Invited Speaker: Sasha Madronich The chemical composition of the atmosphere regulates the balance between incoming solar short-wave and outgoing terrestrial long-wave radiation, directly \textit{via} absorption and scattering and indirectly \textit{via} modification of clouds. Photo-oxidation reactions remove many chemicals emitted by natural sources, and on geological time scales have prevented runaway growth of infrared-active gases such as methane; however, the same reactions have byproducts (esp. ozone and suspended particles) that affect air quality as well as the radiative forcing of climate. Anthropogenic emissions are now modifying the natural chemical and radiative balances of the atmosphere, but the detailed mechanisms and net effects are still not fully understood. Given the non-linear and coupled nature of the atmospheric chemical system, it is important to realize that future regulations aimed at improving air quality could also influence climate-relevant properties of the atmosphere (and \textit{vice versa}), in ways that may or may not be intended or even beneficial. Careful analyses will be required to distinguish between win-win strategies to address both climate and air quality, and those strategies that penalize one environmental issue to the benefit of the other. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N4.00004: Testing climate models using instrumental and geologic observations Invited Speaker: Peter Huybers |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N4.00005: Climate modeling from first principles: Feasibility and prospects Invited Speaker: William (Bill) Collins |
Session N5: Focus Session: Computational Discovery and Design of New Materials: Electronic properties of 1D and 2D materials
Sponsoring Units: DMP DCOMPChair: Wei Ku, Brookhaven National Laboratory
Room: 301
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N5.00001: The Virial Theorem in Graphene and other Dirac Materials James Stokes, Hari Dahal, Alexander Balatsky, Kevin Bedell The virial theorem is applied to graphene and other Dirac Materials for systems close to the Dirac points where the dispersion relation is linear. From this, we find the exact form for the total energy given by $E = \mathcal{B}$/$r_s$ where $r_s a_0$ is the mean radius of the $d$-dimensional sphere containing one particle, with $a_0$ the Bohr radius, and $\mathcal{B}$ is a constant independent of $r_s$. This result implies that, given a linear dispersion and a Coulombic interaction, there is no Wigner crystalization and that calculating $\mathcal{B}$ or measuring at any value of $r_s$ determines the energy and compressibility for all $r_s$. In addition to the total energy we calculate the exact forms of the chemical potential, pressure and inverse compressibility in arbitrary dimension. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N5.00002: Create Dirac Cones in Your Favorite Materials Chia-Hui Lin, Wei Ku We propose a theoretical recipe to create Dirac cones into anyone's favorite materials. The method allows to tailor anisotropy and quantity of cones in any effective one-band two-dimensional lattice. The validity of our theory is demonstrated with two examples on the square lattice, an ``unlikely'' candidate hosting Dirac cones, and show that a graphene-like low-energy electronic structure can be realized. The proposed recipe can be applied in real materials via introduction of vacancy, substitution or intercalation, and also extended to photonic crystal, molecular array, and cold atoms systems. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N5.00003: Stability of Weyl metals under imuurity scattering Zhoushen Huang, Tanmoy Das, Alexander V. Balatsky, Daniel P. Arovas We investigate the effects of bulk impurities on the electronic spectrum of Weyl semimetals, a recently identified class of Dirac-type materials. Using a $T$-matrix approach, we study resonant scattering due to a localized impurity in tight binding versions of the continuum models recently discussed by Burkov, Hook, and Balents, describing perturbed four-component Dirac fermions in the vicinity of a critical point. The impurity potential is described by a strength $g$ as well as a matrix structure $\Lambda$. Unlike the case in $d$-wave superconductors, where a zero energy resonance can always be induced by varying the impurity scalar and/or magnetic impurity strength, we find that for certain types of impurity ($\Lambda$), the Weyl node is protected, and that a scalar impurity will induce an intragap resonance over a wide range of scattering stength. A general framework is developed to address this question, as well as to determine the dependence of resonance energy on the impurity strength. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N5.00004: A semi-classical analysis of Dirac fermions in 2+1 dimensions Moitri Maiti, R. Shankar We investigate the semiclassical dynamics of massless Dirac fermions in 2+1 dimensions in the presence of external electromagnetic fields. By generalizing the $\alpha$ matrices by two generators of the $SU(2)$ group in the $(2S+1)$ dimensional representation and doing a certain scaling, we formulate a $S\rightarrow\infty$ limit where the orbital and the spinor degrees become classical. We solve for the classical trajectories for a free particle on a cylinder and a particle in a constant magnetic field. We compare the semiclassical spectrum, obtained by Bohr-Sommerfeld quantization with the exact quantum spectrum for low values of $S$. For the free particle, the semiclassical spectrum is exact. For the particle in a constant magnetic field, the semiclassical spectrum reproduces all the qualitative features of the exact quantum spectrum at all $S$. The quantitative fit for $S=1/2$ is reasonably good. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N5.00005: An \textit{Ab Initio Study of the} Interaction between \textit{3d} Transition Metal Atoms and Silicon Carbide Nanotubes Kapil Adhikari, Asok Ray Interaction of \textit{3d-}transition metal atoms with armchair silicon carbide nanotubes (SiCNTs) of chiralities (3,3), (5,5), (7,7), and (9,9) is studied in detail using hybrid density functional PBE0 and an all electron basis set 6-31G**. The results show that the interaction energy between transition metal and SiCNTs depends not only on the number of $d$-electrons but also on the curvature of the nanotubes. Interaction between SiCNTs and transition metals increases with increase in curvature of the nanotubes. To explore the curvature effect in detail, both internal and external adsorption sites were chosen for the functionalization. With the exception for the SiCNTs functionalized by Ni and Zn, all 3d-transition metal-functionalized nanotubes were found to have magnetic ground states. The quenching of magnetism is strongly dependent on the curvature of the nanotubes. Mulliken charge analysis has been performed to study the amount and direction of charge transfer between transition metals and the SiCNTs. SiCNTs doped with transition metals have significantly lower band gaps, in general, than those of bare nanotubes. Transition metal atoms Ni and Zn have the least effect on the band gaps of the SiCNTs. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N5.00006: Interaction of a single Li atom with SiGe(6,6) nanotubes Prabath Wanaguru, Asok K. Ray A study of the interaction between four types of SiGe(6,6) nanotubes\footnote{P. Wanaguru and A. K. Ray, J. Comp. Theo. Nanosci. (in press).} and a Li atom was performed using the cluster approximation. Full geometry and spin optimizations were performed without any symmetry constraints using the hybrid functional B3LYP, an all electron 6-311G**//3-21G* basis set and the GAUSSIAN 09 suite of software. All possible internal and external adsorption sites were considered and it was found that some tubes were deformed as a result of the adsorption process. Among the nanotubes which retained the tubular shape, most preferred site for the external adsorption was quasi on top of Ge site with the highest adsorption energy being 1.639eV. Also, the band gaps of the systems decreased from the values of pristine SiGe nanotube values, the range being 0.880 to 0.958eV. For inside adsorption, most preferred site was the hollow site. Adsorption energies ranged from 1.606 to 1.657eV and band gaps, from 0.777 to 0.807eV. We will present, In detail, adsorption energies, band gaps, density of states, and the bonding nature of Li to the nanotubes. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N5.00007: On the possibility of population inversion in strained silicon nanowires: an atomistic study Daryoush Shiri, Amit Verma, Anant Anantram Density functional theory and Ensemble Monte Carlo studies show the possibility of population inversion in strained silicon nanowires. At room temperature and electric field of 15 KV/cm, a strain induced indirect subband can hold 10 times more electron population compared to the direct subband. The most dominant mechanism which depletes the indirect subband is scattering by longitudinal optical (LO) phonons. At T$=$300K the inter-sub band scattering is almost symmetric with the rate of 10$^{11}$ s$^{-1}$. On the other hand the processes of thermalization to the bottom of the indirect subband (via acoustic phonon emission) and the 2nd order radiative recombination are very slow (10$^{-9}$ sec and 10 sec, respectively). At T$=$77K the LO-phonon absorption rate (indirect to direct subband scattering) drops to 10$^{8}$ s$^{-1}$. This induced asymmetry in scattering leads to the enhanced population difference between indirect and direct subbands even at higher electric fields. The spontaneous emission time is 10$^{-7}$sec and a few seconds for direct and indirect bandgap nanowires, respectively. This study suggests the usability of strained silicon nanowires in nano-lasers. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N5.00008: Vortices in One Dimension: A Soliton Analysis of Gapped Carbon Nanotubes Mark Sweeney, Joel Eaves We study the optical properties of carbon nanotubes using the bosonization technique. The action has a general sine-Gordon form and the fundamental excitations are solitons and antisolitons. The bound soliton-antisoliton of the system is an exciton. Using a mean-field analysis we find bright and dark excitonic energies that are in good agreement with experimental values. Further, the large energy differences between the exitonic spectra and the single particle spectra agrees with perturbative treatments: Bethe-Salpeter excitonic energies compared to Hatree-Fock single particle energies. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N5.00009: Berry phase dependent quantum trajectories of electron-hole pairs in semiconductors under intense terahertz fields Fan Yang, Ren-Bao Liu Quantum evolution of particles under strong fields can be approximated by the quantum trajectories that satisfy the stationary phase condition in the Dirac-Feynmann path integrals. The quantum trajectories are the key concept to understand strong-field optics phenomena, such as high-order harmonic generation (HHG), above-threshold ionization (ATI), and high-order terahertz siedeband generation (HSG) [1]. The HSG in semiconductors may have a wealth of physics due to the possible nontrivial ``vacuum'' states of band materials. We find that in a spin-orbit-coupled semiconductor, the cyclic quantum trajectories of an electron-hole pair under a strong terahertz field accumulates nontrivial Berry phases. We study the monolayer MoS$_2$ as a model system and find that the Berry phases are given by the Faraday rotation angles of the pulse emission from the material under short-pulse excitation. This result demonstrates an interesting Berry phase dependent effect in the extremely nonlinear optics of semiconductors. \\[4pt] [1] B. Zaks, R. B. Liu, and M. S. Sherwin, Nature \textbf{483}, 580 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N5.00010: Transport properties of semi-Dirac Pierre Adroguer Recent theoretical works show the existence of a new type of dispersion relation in both $VO_2/TiO_2$ nanostructures\footnote{V. Pardo and W.E. Pickett, Phys.Rev. Let. 102, 166803 (2009)} and in stressed graphene \footnote{G. Montambaux \emph{et al.}, PRB 80, 153412 (2009)}, where the electrons confined in a plane show a non-relativistic behavior along one direction, and relativistic in the other. This semi-Dirac dispersion $E=\sqrt{(v_F p_x)^2+(p_y^2/2m)^2}$ can be observed in graphene when the Dirac cones of different valleys touch each other because of stress. When stress is increased, a gap is opened, and the graphene turns from a semi-metal to an insulator. We propose to adress this topological phase transition through transport measurements. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N5.00011: Heat transport and correlations in anharmonic oscillator chains, a molecular dynamics study Maxime Gill-Comeau, Laurent J. Lewis It is well known that the anharmonic oscillator chain displays anomalous heat conduction, the most striking feature of which being a thermal conductivity diverging with length as $\kappa \propto L^\alpha$ where $\alpha = 2/5$ or $1/3$. By comparing MD simulations results with an analysis based on the use of the Peierls-Boltzmann equation, we shed light on the mechanisms behind this striking phenomenon in 1D and pseudo-1D systems. The possibility of persistent cross-mode correlations and its consequences were also investigated. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N5.00012: Extended Electronic States above Diskoid Nanostructures Artem Baskin, Petr Kral, Hossein Sadeghpour We demonstrate that charged graphene nanostructures, which can be modeled as charged metallic nanodisks, can support spatially extended electronic states with binding energies of 50-200 meV. In the case of high angular momenta these states can be highly separated from the disk surfaces, in analogy to image states above carbon nanotubes observed experimentally. We present the single-electron and approximate multi-electron wavefunctions. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N5.00013: Low-energy local density of states of the 1D Hubbard model Sebastian Eggert, Stefan Soeffing, Imke Schneider We examine the {local} density of states (DOS) at low energies numerically and analytically for the Hubbard model in one dimension. The eigenstates represent separate spin and charge excitations with a remarkably rich structure of the local DOS in space and energy. The results predict signatures of strongly correlated excitations in the tunneling probability along finite quantum wires, such as carbon nanotubes, atomic chains or semiconductor wires in scanning tunneling spectroscopy (STS) experiments. However, the detailed signatures can only be partly explained by standard Luttinger liquid theory. In particular, we find that the effective boundary exponent can be negative in finite wires, which leads to an increase of the local DOS near the edges in contrast to the established behavior in the thermodynamic limit. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N5.00014: Energy Partitioning of Tunneling Currents into Luttinger Liquids Torsten Karzig, Gil Refael, Leonid I. Glazman, Felix von Oppen Tunneling of electrons of definite chirality into a quantum wire creates counterpropagating excitations, carrying both charge and energy. We find that the partitioning of energy is qualitatively different from that of charge. The partition ratio of energy depends on the excess energy of the tunneling electrons (controlled by the applied bias) and on the interaction strength within the wire (characterized by the Luttinger-liquid parameter $K$), while the partitioning of charge is fully determined by $K$. Moreover, unlike for charge currents, the partitioning of energy current should manifest itself in dc experiments on wires contacted by conventional (Fermi-liquid) leads. [Preview Abstract] |
Session N6: Focus Session: Graphene - Electronic Properties, Gap Formation
Sponsoring Units: DMPChair: Nancy Sandler, Ohio University
Room: 302
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N6.00001: Interaction-induced gapped state in charge neutral bilayer graphene Invited Speaker: Jairo Velasco Jr. Bilayer graphene (BLG) at the charge neutrality point (CNP) possess instability to electronic interactions, and is expected to host a ground state with spontaneously broken symmetries. Within this regime, I will discuss our transport spectroscopy measurements using high quality suspended BLG samples. We observe an insulating state at CNP with a gap $\sim$ 2 meV, which can be closed by finite doping or a perpendicular electric field of either polarity. For magnetic field B \textgreater\ 1T, the gap increases linearly with B. Our work contributes towards understanding the rich interaction-driven physics in BLG. Finally, latest progress on transport spectroscopy measurements of Landau level gaps in these high quality samples will also be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N6.00002: Electron-electron interactions in non-equilibrium bilayer graphene Wei-Zhe Liu, Allan MacDonald, Dimitrie Culcer The charge conductivity of doped bilayer graphene can be understood as a net steady-state pseudospin polarization. Due to the chirality inherent in the Hamiltonian, electron-electron interactions renormalize this polarization even at zero temperature, when the phase space for electron-electron scattering vanishes. Nevertheless, at usual transport densities the electron-electron interaction contribution displays only a weak density dependence and has a negligible effect on the conductivity. This smallness is due to the large value of the interlayer tunneling parameter. Interestingly, the effect of interactions in transport vanishes as the carrier number density tends to zero, in contrast to single-layer graphene and topological insulators. The vanishing is attributed to the fact that the pseudospin winds twice around the Fermi surface. Our study relies on the quantum Liouville equation in the first Born approximation with respect to the scattering potential, with electron-electron interactions taken into account self-consistently in the Hartree-Fock approximation, and screening in the random phase approximation. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N6.00003: Ab initio quasiparticle bandstructure of ABA and ABC-stacked graphene trilayers Marcos Menezes, Rodrigo Capaz, Steven Louie We obtain the quasiparticle band structure of ABA and ABC-stacked graphene trilayers through ab initio density functional theory (DFT) and many-body quasiparticle calculations within the GW approximation. To interpret our results, we fit the DFT and GW $\pi$ bands to a low energy tight-binding model, which is found to reproduce very well the observed features near the K point. The values of the extracted hopping parameters are reported and compared with available theoretical and experimental data. For both stackings, the quasiparticle corrections lead to a renormalization of the Fermi velocity, an effect also observed in previous calculations on monolayer graphene. They also increase the separation between the higher energy bands, which is proportional to the nearest neighbor interlayer hopping parameter $\gamma_1$. Both features are brought to closer agreement with experiment through the quasiparticle corrections. Finally, other effects, such as trigonal warping, electron-hole assymetry and energy gaps are discussed in terms of the associated parameters. This work was supported by the Brazilian funding agencies: CAPES, CNPq, FAPERJ and INCT-Nanomateriais de Carbono. It was also supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N6.00004: Magneto-Coulomb Drag in Double Layer Graphene Wang-Kong Tse, A. H. MacDonald We report on our theoretical investigations on the Coulomb drag in double-layer graphene in strong magnetic fields. Using diagrammatic perturbation theory, we obtain explicit analytical expressions for the nonlinear susceptibility and the drag conductivity. At low temperatures $T$, the drag conductivity behaves as $\mathrm{exp}(-\Delta/T)/T$, where $\Delta$ is the inter-Landau level transition energy nearest to the Fermi level. For full filling at the zeroth Landau level, we find a non-vanishing drag that arises from the intrinsic contribution of filled Landau levels below the Dirac point. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N6.00005: Structural, electronic, and mechanical properties of superlattices of interlayer-bonded domains in twisted bilayer graphene Andre Muniz, Dimitrios Maroudas We present a comprehensive computational analysis of the atomic and electronic structure and mechanical properties of a novel class of carbon nanostructures, formed due to interlayer covalent $sp^{3}$ C-C bonding in twisted bilayer graphene as a result of controlled chemical functionalization (hydrogenation or fluorination). Depending on the twist angle and local stacking of layers, these nanostructures are superlattices of diamond-like nanocrystals or caged fullerene-like configurations embedded within the bilayer. According to density functional theory calculations, the electronic behavior of these $sp^{2}/sp^{3}$ hybrid configurations ranges from semi-metallic, characterized by linear dispersion around the K point in the first Brillouin zone, to semi-conducting/insulating with electronic band gaps ranging from a few meV to $\sim$4 eV; this electronic character depends on the symmetry and periodicity of the superlattices and on the type of chemisorbed species. We have also studied the mechanical response of these superlattices to tensile and shear strain based on molecular dynamics simulations; their interlayer shear modulus increases strongly and their Young's modulus and tensile strength and strain decrease moderately compared to those of pristine bilayer graphene. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N6.00006: Ising Phase in AA Stacked Bilayer Graphene Herbert Fertig, Luis Brey AA stacked bilayer graphene bears a close resemblance to biased, double layer graphene (in which a strong barrier separates the two layers), with layer bonding and anti-bonding states of the AA system playing the roles of layer states in the double layer system. The latter has a U(1) symmetry which can break, to form a condensed exciton groundstate. The AA system however has only Ising symmetry. In this presentation we analyze the possibility that electron-electron interactions break this to open a gap in the energy spectrum. We find that, in the mean field approximation, the ground state has a charge density wave character, with the charge modulation of each layer out of phase. We calculate the gap and the mean field critical temperature as a function of the strength of the Coulomb interaction, taking screening into account self-consistently with the calculation of the gap. We also analyze the transition between ordered and thermally disordered phases based on a continuum model, and find that the transition is controlled by an effective U(1) stiffness. We argue that in the limit of zero layer separation, for which the full U(1) symmetry of the Hamiltonian is restored, the Ising transition continuously goes into a Kosterlitz-Thouless transition. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N6.00007: Electronic Properties of Curved Graphene-Ring Structures Daiara Faria, Andrea Latge, Sergio Ulloa, Nancy Sandler We have undertaken a theoretical investigation of electronic properties of a curved graphene ring in the Dirac approximation making use of elasticity theory. This study is motivated by experimental reports that indicate the existence of gauge-fields in graphene when it is under tension and also by the recent possibility of controlling deformations in its surface in a variety of shapes on different substrates [1]. We discuss how an Aharonov-Bohm field can be used to design new responses obtained by adding real magnetic fluxes and pseudomagnetic fields. We show that the persistent current tends to be inhomogeneous in the same way that the Fermi velocity has a spatial-dependent character[2]. We also discuss the role of strain in the position of the Dirac points that have been the source of recent controversies. [1] T. Georgiou et al., Appl. Phys. Lett. 99, 093103 (2011). [2] F. de Juan et al., PRL 108, 227205 (2012). [3] A. Kitt et al., Phys. Rev. B 85, 115432 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N6.00008: Morphology and electronic transport study of suspended graphene Invited Speaker: Wenzhong Bao This presentation will first describe our recent electrical transport studies of suspended bilayer and trilayer graphene devices at low temperature: Bilayer graphene at the charge neutrality point is unstable to electronic interactions, and expected to host a ground state with spontaneously broken symmetries, whereas in trilayer graphene stacking order provides another important degree of freedom for tuning its electronic properties. For instance, at the Dirac point, Bernal-stacked TLG remains metallic but r-TLG becomes insulating with an intrinsic interaction-driven gap around 6 meV. Our results underscore the rich interaction-induced phenomena in both bilayer and trilayer graphene with different stacking orders, and its potential towards electronic applications. Next we will discuss the manipulation of the morphology of suspended graphene via electrostatic and thermal control: We observe significant deflections of single-, bi-, and trilayer graphene sheets in response to electrostatic force. At low temperature, wide graphene sheets ripple and butterfly features form at its two free edges. These observations have important applications for understanding electrical, mechanical, and thermal properties as well as strain engineering in suspended graphene devices. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N6.00009: Field-induced Energy Gaps in Bilayer Graphene under Shear Seon-Myeong Choi, Young-Woo Son Using the first-principles calculations method, we study the effects of shear on field-induced insulating states of bilayer graphene (BLG). It is shown that the low energy bands near the charge neutral point of BLG change significantly upon application of shear. We also find that the energy gap of BLG under transverse electric field sensitively depend on both direction and amount of shear. Generally, the field-induced energy gap decreases as the sliding increases under shear. For BLG with the specific direction of shear, the shear can quench the energy gap to zero completely even in the presence of electric field thus realizing insulator-to-metal transition just by sliding. We discuss origins of these interesting phenomena and suggest some experimental methods to detect the transition. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N6.00010: Kekule-induced band-gap opening in graphene in contact with ZrO2 Jung Suk Goh, Hyoung Joon Choi We have studied pressure-dependent atomic and electronic structure of graphene in contact with (111) surface of zirconium dioxide (ZrO2) using first-principles calculations. The atomic structures are optimized by relaxation, and we found that the lowest-energy configuration shows a band gap at the Dirac point at ambient pressure and the band gap increases as pressure increases. Our analysis shows that the band-gap opening is due to overlap of wavefunctions, change in potential energy, and in-plane distortion of graphene lattice. This in-plane distortion of graphene is found to be the Kekule distortion, which generates intervalley coupling. As pressure increases, the Kekule distortion in graphene increases and the band gap at the Dirac point is proportional to the size of the distortion. This work was supported by the NRF of Korea (Grant No. 2011-0018306) and KISTI Supercomputing Center (Project No. KSC-2012-C2-14). [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N6.00011: Quantum phase transitions to Kondo states in bilayer graphene Diego Mastrogiuseppe, Arturo Wong, Kevin Ingersent, Nancy Sandler, Sergio Ulloa We study a magnetic impurity intercalated in Bernal-stacked bilayer graphene described by a multiband Anderson Hamiltonian. Through a properly generalized Schrieffer-Wolff transformation, it reduces to a single-channel Kondo model with a strongly energy-dependent exchange coupling. The form of this effective Kondo Hamiltonian suggests the possibility of driving the system through quantum phase transitions via tuning of the chemical potential through doping or electrical means. The microscopic coupling of the impurity to the graphene layers determines symmetries and details of the various phases. We use the numerical renormalization group to accurately access the many-body physics of this system. Our calculations reveal zero-temperature transitions under variation of the band filling and/or the energy of the impurity level between a local-moment phase and a pair of singlet strong-coupling phases. The latter have conventional Kondo, pseudogap Kondo, and local-singlet regimes that can be distinguished through their thermodynamic and spectral properties, as well as their different rates of variation of the Kondo temperature with the chemical potential. [Preview Abstract] |
Session N7: Focus Session: Graphene Devices VIII
Sponsoring Units: DMPChair: Frank Koppens, ICFO Barcelona
Room: 303
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N7.00001: Mechanical resonators based on nanotubes and graphene Invited Speaker: Adrian Bachtold Carbon nanotubes and graphene offer unique scientific and technological opportunities as nanoelectromechanical systems (NEMS). Namely, they have allowed the fabrication of mechanical resonators that can be operable at ultra-high frequencies and that can be employed as ultra-sensitive sensors of mass and charge. In addition, nanotubes and graphene have exceptional electron transport properties, including ballistic conduction over long distances. Coupling the mechanical motion to electron transport in these remarkable materials is thus highly appealing. Here, I will review some of our recent results on nanotube and graphene NEMSs, including the control of the mechanical oscillation using Coulomb blockade and mass sensing at the proton mass level. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N7.00002: Graphene on silicon nitride resonators for optomechanics Roberto De Alba, Vivek Adiga, Isaac Storch, Patrick Yu, Rob Ilic, Robert Barton, Sunwoo Lee, James Hone, Paul McEuen, Harold Craighead, Jeevak Parpia Recently, much work on nanoelectromechanical resonators has focused on high Q systems and on coherent back action used to suppress or enhance device motion. Here we attempt to merge these concepts by studying graphene on silicon nitride bilayer membranes. The high Q's of these hetero-structures, along with the conductivity of graphene, result in both electrostatic and optical tunability of mechanical resonance. By coupling these devices with a movable, highly reflective mirror to form a Fabry-Perot cavity, we are able to modulate resonator frequency and damping through cavity detuning. We thus present evidence of photothermal back action in these devices due to energy absorption from an impinging laser beam. We utilize both optical and electrical read-out schemes to detect device motion, enabling us to compare electrical and optical nonlinearities as a function of cavity detuning and capacitive drive. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N7.00003: Self-sustained graphene mechanical oscillators Changyao Chen, Sunwoo Lee, Vikram Deshpande, Philip Kim, James Hone Graphene poses excellent electrical and mechanical properties, therefore it is the most promising candidate for NanoElectroMechanical Systems. Recent developments of its CVD synthesis and fabrications makes the large scale integration for Radio Frequency (RF) applications possible. In this talk, I will present the structure and characteristics of self-sustained graphene mechanical oscillators, discuss the frequency tuning, and their phase noise performance, as well as low temperature behaviors. The demonstrated voltage controlled oscillators made from graphene pave the pathway for next generation on-chip integration of RF NEMS front-end. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N7.00004: Adiabatic Electron Pumping through Graphene-based Nanoelectromechanical Resonators Caio Lewenkopf, Alexander Croy We theoretically investigate the adiabatic electronic transport through graphene-based nanoelectromechanical resonators. The device is modeled by an effective long-wavelength Hamiltonian (given by the Dirac equation) for the electrons and using the continuum elastic theory for the mechanical motion. One obtains the equations of motion describing the system dynamics employing a non-equilibrium Green's function theory. Due to the mutual coupling between the electronic and mechanical degrees of freedom, both sets of equations have to be solved self-consistently. We present analytical and numerical results of the pumped charge and the mechanical response for a typical resonator setup. We also discuss the role of non-adiabatic corrections and the resulting damping of the mechanical motion. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N7.00005: Fabrication and Detection of Graphene Nano-Mechanical Oscillators Shonali Dhingra, Jen-Feng Hsu, Brian D'Urso Graphene's exceptionally high crystal and electronic quality, combined with being only one-atom thick, make it quite a sought-after material for nano-mechanics, sensing and electronics. We fabricate and characterize Nano-Mechanical Oscillators (NMO) from large-domain single-layer graphene grown with Chemical Vapor Deposition (CVD) on $\sim$2mm thick copper discs. The graphene is transferred from copper using Poly (methyl methacrylate) (PMMA), onto indigenous substrates customized for enhanced graphene adhesion and assistance in its optical detection. It is patterned into devices of different geometrical shapes, such as doubly clamped beams, circular drums and rectangular drums, using deep-UV lithography of PMMA, either before or after transfer. The phase and frequency response of the resonant motion of the NMO is monitored, which is electrically actuated and optically detected using interferometric techniques. These oscillators would be used as building blocks for hybrid quantum systems which couple classical oscillators with a quantum spin system. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N7.00006: Transfer-Free, Wafer-Scale Manufacturing of Graphene-Based Electromechanical Resonant Devices Michael Cullinan, Jason Gorman Nanoelectromechanical (NEMS) resonators offer the potential to extend the limits of force and mass detection due to their small size, high natural frequencies and high Q-factors. Graphene-based NEMS resonators are particularly promising due to their high elastic modulus and atomic thickness. However, widespread use of graphene in such systems is limited by the way in which graphene-based devices are typically fabricated. Most graphene-based NEMS devices are fabricated in a ``one-off'' manner using slow, limited scale methods such as mechanical exfoliation, electron beam lithography, or transfer from copper foils which can't be incorporated into standard micro/nanofabrication lines. This talk will present a method that can be used to manufacture graphene-based NEMS devices at the wafer scale using conventional microfabrication techniques. In this method graphene is grown directly on thin film copper using chemical vapor deposition. The copper film is then patterned and etched to produce graphene-based NEMS resonators. This talk will also address some of the challenges in fabricating a large number of graphene devices at the wafer scale including achieving high uniformity across the wafer, increasing device-to-device repeatability, and producing high device yields. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N7.00007: Manipulating Graphene Alexander Ruyack, Melina Blees, Samantha Roberts, Chris Martin, Arthur Barnard, Paul L. McEuen Graphene is both strong and flexible, making it a promising material for nanoscale hinges and other three-dimensional structures. Using sacrificial layers and surfactants, we are able to demonstrate control over the adhesion of monolayer graphene to a substrate. By patterning gold on the surface of the graphene, we created arrays of rigid pads bridged by graphene strips that can be decoupled from the surface in an aqueous environment. The pads allow us to manipulate the graphene both on and off the surface using lasers or micromanipulators. Our methods yield fundamental material data on graphene such as the macroscopic bending stiffness, and demonstrate the feasibility of a graphene hinge. We are currently exploring the use of magnetic control as a method for applying forces to stretch and fold graphene. We have already created micron-sized permanent magnets made of iron and successfully released them from the substrate, and are now integrating them into graphene devices. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N7.00008: Graphene Membrane Mechanics Qin Zhou, A. Zettl Graphene has extremely low mass density and high mechanical strength, useful qualities for mechanically vibrating systems. Here we report on construction and testing of graphene-based vibrating drumheads. We explore frequency response and damping characteristics, and energy transduction. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N7.00009: Measuring graphene's bending stiffness Melina Blees, Arthur Barnard, Samantha Roberts, Joshua W. Kevek, Alexander Ruyack, Jenna Wardini, Peijie Ong, Aliaksandr Zaretski, Siping Wang, Paul L. McEuen Graphene's unusual combination of in-plane strength and out-of-plane flexibility makes it promising for mechanical applications. A key value is the bending stiffness, which microscopic theories and measurements of phonon modes in graphite put at $\kappa_{\mathrm{0}}=$1.2 eV.$^{\mathrm{1}}$ However, theories of the effects of thermal fluctuations in 2D membranes predict that the bending stiffness at longer length scales could be orders of magnitude higher.$^{\mathrm{2,3}}$ This macroscopic value has not been measured. Here we present the first direct measurement of monolayer graphene's bending stiffness, made by mechanically lifting graphene off a surface in a liquid and observing both motion induced by thermal fluctuations and the deflection caused by gravity's effect on added weights. These experiments reveal a value $\kappa_{\mathrm{eff}}=$12 keV at room temperature --- four orders of magnitude higher than $\kappa_{\mathrm{0}}$. These results closely match theoretical predictions of the effects of thermally-induced fluctuations which effectively thicken the membrane, dramatically increasing its bending stiffness at macroscopic length scales. [1] A. Fasolino et al., Nat. Mater. (2007) [2] D. R. Nelson and L. Peliti, J Physique (1987) [3] F. L. Braghin and N. Hasselmann, Phys Rev B (2010) [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N7.00010: Nonlinear Mechanics of Polycrystalline Two-Dimensional Materials such as Graphene Ryan Cooper, Adam Hurst, Alexandra Hammerberg, Gwan-Hyoung Lee, Christopher Marianetti, Xiaoding Wei, Changgu Lee, Bryan Crawford, James Hone, Jeffrey Kysar Two-dimensional films such as graphene can potentially exist as pristine crystals. These crystals present a unique opportunity to design unique experiments that uncover intrinsic material properties. Recent experimental studies have shown graphene is the strongest material ever measured. An Agilent G200 nanoindenter and Park Systems atomic force microscope are used in this study to make measurements of the mechanical response of graphene and other two-dimensional materials. Chemical vapor deposition is employed to manufacture graphene. The mechanical properties of the chemical vapor deposited graphene is compared to that of pristine graphene. Experiments investigate the elastic response up to the point of fracture. These suspended sheets are probed using atomic force microscopy and nanoindentation. The experimental work is modeled using first-principles density functional theory and finite element analysis. Previous work has shown that density functional theory and finite element analysis accurately predicts the breaking force of graphene and molybdenum disulfide. This work also explores the probability of fracture using a generalized form of the Weibull modulus in finite element analysis. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N7.00011: Electron transport measurement in locally strained graphene Hikari Tomori, Akinobu Kanda, Youiti Ootuka, Hiromasa Karube, Akinobu Kanda Strain engineering is a promising method for controlling electron transport in graphene; Spatial variation of gauge fields produced by non-uniform strain in graphene causes electron scattering, leading to modulation of the electronic state such as band gap formation. We have succeeded in introducing local strain to graphene, by inserting designed dielectric nanostructures between the graphene sheet and its substrate. [1] The transport measurement of strained graphene has revealed that improvement of the mean free path is crucial for clear demonstration of effect of lattice strain on electron transport.\\[4pt] [1] H. Tomori et al., Appl. Phys. Express 4, 075102 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N7.00012: Lubricating graphene with nanometer-thick perfluoropolyethers Lei Li, Andrew Kozbial, Steven Iasella, Alexander Taylor, Zhiting Li, Haitao Liu Due to its excellent optical, electrical and mechanical properties, graphene has found many important applications. Since graphene is atomic thick, the wear resistance is critical to the reliability of graphene-containing devices. In this study, both monolayer and multilayer graphene were coated with nanometer-thick perfluoropolyethers (PFPEs) and the effect of the nanolubricants on the wear and friction was investigated. The coefficient of friction (COF) was measured with a commercial nanotribometer and the wear was characterized with optically microscopy, AFM and Raman microscopy. Coated with PFPEs, monolayer graphene on silicon showed significantly decreased COF. However, the wear resistance was only slightly improved. For multilayer graphene on nickel substrate coated with PFPEs, COF also decreased significantly. Meanwhile, the wear resistance was improved substantially. The results were discussed based on the graphene-substrate adhesion and the thickenss of the graphene. The learning here potentially will lead to the methodology to improve the reliability of graphene-containing devices. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N7.00013: Temperature and size dependent friction of gold nanoislands on graphene Ben D. Dawson, Michael S. Lodge, Zachary Williams, Masa Ishigami Nanoscale motors and machines require the ability to tune frictional properties at the nanoscale. Yet a fundamental understanding of frictional processes of nanoislands still remains unknown. We have performed a quartz crystal microbalance study to investigate the role of temperature and island size on frictional energy dissipation for gold nanoislands on graphene. Significant frictional dissipation is observed even at room temperature, consistent with activated friction on the graphene surface. We will discuss these results and compare them to previously predicted models for thermally activated and size dependent friction. [Preview Abstract] |
Session N8: Transport and Optical Phenomena in Carbon Nanotubes
Sponsoring Units: DMPChair: Dan Prober, Yale University
Room: 307
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N8.00001: Terahertz Detection as a Probe of Luttinger-Liquid Behavior in an Individual Single-Walled Carbon Nanotube Joel D. Chudow, Chris B. McKitterick, Daniel E. Prober, Daniel F. Santavicca, Philip Kim Carbon nanotubes (CNTs) serve as an experimental system for verification of physical models of one-dimensional (1-D) conduction, in particular the Luttinger-liquid theory. We describe measurements of terahertz (THz) absorption in individual single-walled carbon nanotubes and distinguish between two response mechanisms: bolometric detection due to heating a CNT with a temperature-dependent resistance and the response due to non-thermal electrical contact nonlinearities. The effect of the contact nonlinearity is not significantly decreased at THz frequencies, allowing for analysis of the parallel contact capacitance to an individual CNT.[1] We study high-frequency charge excitations in a CNT as a probe of the strength of the electron-electron interactions due to the lack of screening in this 1-D system. This is achieved by exciting terahertz standing wave resonances along the length of a CNT, observed using the nonlinear detection mechanism. We exploit this experimental technique to test predictions of the Luttinger-liquid model. $\backslash \backslash $[4pt] [1] J.D. Chudow, D.F. Santavicca, C.B. McKitterick, D.E. Prober and P. Kim, \textit{Appl. Phys. Lett.} \textbf{100}, 163503 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N8.00002: Determination of absorption cross-section in suspended single-walled carbon nanotubes Xiaoping Hong, Kaihui Liu, Sangkook Choi, Steven Louie, Feng Wang Quantitative determination of optical absorption cross-section at single tube level was performed for over 50 suspended single-walled carbon nanotubes (SWCNTs). The structures of the nanotubes are independently identified by electron diffraction, which allows a chirality-dependent study of the nanotube absorption cross-section. We will discuss the absorption strength as well as the linewidth of the optical resonances in both semiconducting and metallic nanotubes of different diameters. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N8.00003: Triplet-triplet exciton interactions and delayed fluorescence in single-wall carbon nanotubes Tobias Hertel, Florian Spath, Dominik Stich, Hannes Kraus, Andreas Sperlich, Vladimir Dyakonov We present pump-probe-, time-correlated single photon counting and spin-sensitive photoluminescence studies of semiconducting single-wall carbon nanotubes (SWNTs) which unambiguously identify triplet-triplet annihilation as the mechanism underlying a long-lived delayed fluorescence (DF) signal. DF decays with a $t^{-0.9}$ power-law, characteristic of diffusion-limited annihilation reactions in 1-dimensional systems. The experiments allow to determine triplet diffusion constants in SWNTs to be on the order of $1\, {\rm cm^2s^{-1}}$ and the triplet lifetime which is found to be $60\pm30\, {\rm \mu s}$. The experiments indicate that the rate of diffusion-limited photo-reactions, here exemplified by triplet-triplet annihilation, can be reduced by one-dimensional confinement. A comparison of optical transients in aqueous and organic solvent environments also indicates how polaron pair dynamics can be influenced by the environment. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N8.00004: Nonequilibrium Tunneling Spectroscopy of Carbon Nanotubes Nicholas Bronn, Nadya Mason We have used nonequilibrium tunneling spectroscopy to elucidate the nature of electron-electron interactions in carbon nanotubes. Due to their reduced dimensionality, carbon nanotubes are thought to be described by Luttinger liquid theory, where electron-electron interactions play a considerable role. Superconducting tunnel probes are used to measure the electron energy distribution functions, whose shape can be related to electronic energy relaxation and scattering. We measure the dependence of the electron distribution function on nonequilibrium bias, position along the nanotube, and temperature. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N8.00005: Ultrafast Spectral Diffusion of the First Subband Exciton in Single-Wall Carbon Nanotubes Daniel Schilling, Tobias Hertel The width of optical transitions in semiconductors is determined by homogeneous and inhomogeneous contributions. Here, we report on the determination of homogeneous linewidths for the first exciton subband transition and the dynamics of spectral diffusion in single-wall carbon nanotubes (SWNTs) using one- and two-dimensional time resolved spectral hole burning spectroscopy. Our investigation of highly purified semiconducting (6,5)-SWNTs suggests that room temperature homogeneous linewidths are on the order of 4~meV and are rapidly broadened by an ultrafast sub-ps spectral diffusion process. These findings are supported by our off-resonant excitation experiments where we observe sub-ps population transfer reflecting the thermal distribution of energy levels around the first subband exciton transition. The results of temperature-dependent spectral hole burning experiments between 17~K and 293~K suggest that homogeneous linewidths are due to exciton interaction with low energy optical phonons, most likely of the radial breathing mode type. In contrast, we find that inhomogeneous broadening is determined by an electronic degree of freedom such as ultrafast intra-tube exciton diffusion which is characteristic and unique for excitons in these one-dimensional semiconductors. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N8.00006: Free Carrier Dynamics in Photoexcited Semiconducting Carbon Nanotube / C$_{60}$ Planar Heterojunctions Dominick Bindl, Meng-Yin Wu, Andrew Ferguson, Nikos Kopidakis, Jeffrey Blackburn, Michael Arnold Semiconducting single walled carbon nanotubes (s-SWCNTs) have remarkable photophysical properties and are appealing for use as principal absorbers in photovoltaics. We have previously demonstrated the collection of photocurrent from thin s-SWCNT films with efficiencies approaching 100{\%} at C$_{60}$ interfaces. Exploiting this interface in high efficiency photovoltaics requires collecting free carriers from optically dense s-SWCNT/C$_{60}$ films with negligible recombination losses, and therefore, an understanding of free carrier recombination kinetics and mechanisms. Time resolved microwave conductivity (TRMC) is a technique which monitors free carrier generation and decay transients in response to a spectrally tunable pump. Here, we report TRMC studies of free carrier dynamics in s-SWCNT thin films and in heterojunctions with C$_{60}$. We have found that free carrier generation yields increase by nearly an order of magnitude and lifetimes increase up to 850ns following introduction of a C$_{60}$ interface, with free carrier lifetimes and generation yields strongly dependent on excited s-SWCNT diameter. We discuss yields, kinetics, and provide insight into relevant charge transfer and recombination mechanisms. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N8.00007: p-n junction photodetectors based on macroscopic single-walled carbon nanotube films Xiaowei He, S\'ebastien Nanot, Robert H. Hauge, Junichiro Kono Single-Wall carbon nanotubes (SWCNTs) are promising in use of solar technology and photodetection. There have been many reports about photovoltaic effect in nanoelectronic devices based on individual SWCNTs, but they are limited by miniscule absorption. There has been a growing trend for merging SWNTs into mico- and macroscopic devices to provide more practical applications. Here we report p-n junction photodetectors based on macroscopic SWCNTs film. Factors affecting the PV amplitude and response time have been studied, including substrates, doping level. The maximal responsivity $\sim $ 1 V/W was observed with samples on Teflon tapes, while a fast response time $\sim $ 80 $\mu $s was observed with samples on AlN substrates. Hence an optimal combination of photoresponse time and amplitude can be found by choosing proper substrates. We found that the PV amplitude increases nonlinearly with increasing n-doping concentration, indicating the existence of an optimal doping concentration. Finally, we checked photoresponse in a wide wavelength range (360 to 900 nm), and PV was observed throughout, indicating that the device could potential be used as a broadband photodetector. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N8.00008: Energies of higher optical transitions in semiconductor carbon nanotubes Serguei Goupalov We show that short-range electron interactions in semiconductor carbon nanotubes promote inter-subband coupling. This coupling is revealed in a significant alteration of energies of E$_{33}$ and E$_{44}$ optical transitions with respect to the predictions of the non-interacting model. The influence of the short-range electron interactions is traced analytically and numerically, by switching it off entirely or partly while calculating optical absorption spectra. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N8.00009: Effect of Disorder on AC Response of Metallic Carbon Nanotubes Daisuke Hirai, Takahiro Yamamoto, Satoshi Watanabe Metallic carbon nanotubes (M-CNTs) have long coherent lengths. In fact, the Anderson localization has been observed in M-CNTs with defects at room temperature [1]. In considering the AC response, not only the understanding of DC conductance behavior but also that of phase-difference between electric current and bias voltage are important. At present, however, the influence of disorder on the AC response remains unclear. In this study, we calculated the AC response of M-CNTs with disorder based on the nonequilibrium Green's function method combined with nearest-neighbor $\pi $-orbital tight-binding approximation and wide-band-limit approximation. In our simulation, disorder potential is described as $V=\Sigma_{i}V_{i}$, \textit{\textbar V}$_{i}$\textit{\textbar }$\le W$, where $V_{i}$ and $W$ are localized potential at $i$th carbon atom and strength of disorder, respectively. We found that the DC conductance decreases with the CNT length, while the behaviors of phase-difference are drastically different by the disorder strength: for a small disorder the phase-difference always behaves inductive, while for a large disorder the phase-difference transits from inductive response to capacitive one with increase of the CNT length. Moreover, we clarified that inductive-capacitive transition universally occurs at the same value of DC conductance. [1] C. Gomez-Navarro et al., Nature 4, 534 (2005). [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N8.00010: An explicit formula for optical oscillator strength of excitons in semiconducting single-walled carbon nanotubes: family behavior Sangkook Choi, Jack Deslippe, Rodrigo B. Capaz, Steven G. Louie The sensitive structural dependence of the optical properties of single-walled carbon nanotubes (SWCNTs), which are dominated by excitons and tunable by changing diameter and chirality, makes them excellent candidates for optical devices. Because of strong many-electron interaction effects, the detailed dependence of the optical oscillator strength of excitons on nanotube diameter $d$, chiral angle $\theta $, and electronic subband index P (the so called family behavior) however has been unclear. Based on results from an extended Hubbard Hamiltonian with parameters derived from \textit{ab initio} GW-BSE calculations, we have obtained an explicit formula for the family behavior of the oscillator strengths of excitons in semiconducting SWCNTs, incorporating environmental screening. The formula explains well recent measurements, and is expected to be useful in the understanding and design of possible SWCNT optical and optoelectronic devices. 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 and Teragrid. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N8.00011: Theory of coherent phonons in carbon nanotubes and graphene nanoribbons G.D. Sanders, C.J. Stanton, A.R.T. Nugraha, R. Saito We have performed theoretical studies on generating and detecting coherent radial breathing mode (RBM) phonons in single-walled carbon nanotubes and coherent radial breathing like mode (RBLM) phonons in graphene nanoribbons. A microscopic theory incorporating electronic states, phonon modes, optical matrix elements, and electron-phonon interaction matrix elements allows us to calculate the coherent phonon spectrum. The coherent phonon amplitudes satisfy a driven oscillator equation with a driving term that depends on photoexcited carrier density. We study the coherent phonon spectrum for nanotubes of different chirality and for armchair and zigzag graphene nanoribbons. We compare our results with a simpler, effective mass theory where we find reasonable agreement with the main features of our computed coherent phonon spectrum. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N8.00012: Nonlinear motion of cantilevered SWNT and Its Meaning to Phonon Dynamics Heeyuen Koh, James Cannon, Shohei Chiashi, Junichiro Shiomi, Shigeo Maruyama Based on the finding that the lowest frequency mode of cantilevered SWNT is described by the continuum beam theory in frequency domain, we considered its effect of the symmetric structure for the coupling of orthogonal transverse modes to explain the nonlinear motion of free thermal vibration. This nonlinear motion calculated by our molecular dynamics simulation, once regarded as noise, is observed to have the periodic order with duffing and beating, which is dependent on aspect ratio and temperature. It could be dictated by the governing equation from the Green Lagrangian strain tensor. The nonlinear beam equation from strain tensor described the motion well for various models which has different aspect ratio in molecular dynamics simulation. Since this motion is nothing but the interaction between 2nd mode of radial, tangential mode and 1st longitudinal mode, it was found that Green Lagrangian strain tensor is capable to deal such coupling. The free thermal motion of suspended SWNT is also considered without temperature gradient. The Q factor measured by this theoretical analysis will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N8.00013: Heat Pulse Propagation in Carbon Nanotube Peapods Mohamed Osman Earlier studies of heat pulse propagation in single and double wall nanotubes at very low temperatures have shown that the heat pulse generated wave packets that moved at the speed of sound corresponding to LA and TW phonon modes, second sound waves and diffusive components [1,2]. The energy content of LA mode wave packets in SWNT was significantly smaller than the TW mode. The energy of the leading LA mode wavepacket in DWNT had a significant increase in the energy content compared to SWNT LA mode. Additionally, an increase simple strain within the LA mode was higher in DWNT compared to SWNT was also reported in [1]. This has motivated us to examine heat pulse propagation in carbon nanopeapods and the coupling between the (10,10) SWNT nanotube and the C60 fullerenes enclosed. The major coupling frequency between the C60 and the (10,10) occurs at 4.88 THz which correspond to the radial breathing mode frequency. We will discuss these results and report on the major phonon modes involved in heat pulse propagation in the (10,10) SWNT-C60 nanopeapod.\\[4pt] [1] T. Kim and M.A Osman, C. Richards, R. Richards, D. Bahr, Phys. Rev. B 76, 155424 (2007)\\[0pt] [2] M.A. Osman and D. Srivastava, Phys. Rev. B 72, 125413 (2005) [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N8.00014: Microstructural characterization of nanoporous carbon fiber as determined by neutron scattering Lilin He, Yuri Melnichenko, Sofiane Baukhalfa, Gleb Yushin We have applied small angle neutron scattering (SANS) technique to investigate the microstructure of nanoporous carbon fiber. The scattering curves were fitted to various models, which allowed us to estimate the structural parameters (i.e. total radii of gyration of pores as well as cross sectional radius of gyration, physical radius and lengths of cylindrical pores) in the studied samples. Chord length analysis was performed to estimate the average sizes of pores and solid matrix. The information obtained from SANS data is in general agreement with independent measurements of surface area using gas sorption carried out in this study. SANS data obtained from carbons saturated with contrast matching liquid (D$_{\mathrm{2}}$O) indicate that the scattering with power law decay of I(Q) in the low Q domain originates from outer surface of carbon fibers. Lower than anticipated decrease in scattering intensity in the high Q domain suggests that a certain amount of nanopores are not accessible to D$_{\mathrm{2}}$O molecules. The investigation of the isotope effect on the pore filling suggests that the H$_{\mathrm{2}}$O is more penetrated than D$_{\mathrm{2}}$O, which is attributed to the stronger bond network among deuterium atoms than that in hydrogen atoms. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N8.00015: Optical Characterization of Natural Nontoxic Nanomaterials Devulapalli Rao, Chandra Yelleswarapu Synthetic nanomaterials -- carbon nanotubes, semiconductor nanoparticles, nanowires and nanorods, metal clusters in polymer films -- are extensively studied for potential photonic applications. Naturally occurring halloysite nanotubes offer additional advantages of high tensile strength, nontoxcity and biocompatibility. Halloysite is receiving lot of attention for application as low cost nanoscale container for encapsulation of biologically active molecules, drugs, and anticorrosion agents. We studied the optical properties of halloysite nanotube samples of length $\sim$1000 nm with 50 nm external diameter and 15 nm internal diameter. The hollysite sample was provided by Prof. Yuri Lvov, Institute for Micromanufacturing, Louisiana Tech. The sample suspended in water at a concentration 2.5 mg/ml exhibits a broad optical absorption band in the visible region with a peak $\sim$600 nm. Z-scan studies are carried out, with 3 nsec laser pulses of frequency doubled Nd:YAG laser, using 1 mm glass cell containing the sample suspended in acetone at a concentration 0.66 mg/ml. Open aperture z-scan measurements indicate two-photon absorption. Closed aperture z-scan measurements exhibit a positive nonlinear refractive index. Results of photoacoustic z-scan currently in progress will also be presented. [Preview Abstract] |
Session N9: Invited Session: Computational Spectroscopy
Sponsoring Units: DCOMPChair: Giulia Galli, University of California, Davis
Room: 308
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N9.00001: Recent developments in time-dependent density-functional theory within and beyond linear response Invited Speaker: E.K.U. Gross Time-dependent density functional theory (TDDFT) is a popular and rather successful method in the description of photo-absorption spectra of atoms and molecules in the linear response regime. In extended solids, however, a satisfactory description of excitonic effects has become possible only recently with the advent of advanced approximations for the exchange-correlation kernel f$_{\mathrm{xc}}$. One of these advanced approximations is the so-called bootstrap kernel [S. Sharma et al, PRL \textbf{107}, 186401 (2011)]. We shall explore the performance of this kernel in the long-wavelength limit and for finite values of q, looking at electron-loss as well as photo-absorption spectra. We find, in particular, that excitonic effects in LiF and Ar are enhanced for values of q away from the $\Gamma $-point [S. Sharma et al, New J Phys \textbf{14}, 053052 (2012)]. Then we present two recent developments in TDDFT beyond the linear-response regime: (i) By using a geometrical partitioning, we calculate the angle and energy resolved photo-electron spectra of finite systems including multi-photon effects [De Giovannini, et al, A. Rubio, PRA \textbf{86}, 062515 (2012)]. (ii) Finally we show how the dynamics of many-electron systems can be controlled with lasers by marrying TDDFT with optimal control theory [A. Castro et al, PRL \textbf{109}, 153603 (2012)]. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N9.00002: Computational spectroscopy using many-body perturbation theory: Large scale calculations without virtual orbitals Invited Speaker: Dario Rocca An accurate description of electronic excitations is essential to model and understand the properties of several materials of fundamental and technological interest. First principles, many-body techniques based on Green's functions are promising approaches that can provide an accurate description of excited state properties; however their applicability has long been hindered by their numerical complexity. In this talk we will summarize some recent methodological developments based on many-body perturbation theory for the efficient calculation of optical absorption spectra [1], photoemission spectra [2], and multiple exciton generation rates [3]. Several applications to realistic materials will be presented, with emphasis on materials for solar energy applications; these include silicon nanowires and bulk tungsten oxide, that are promising photoelectrode materials in water splitting solar cells, molecules used in organic photovoltaics, and semiconductor nanoparticles with potential use in third generation photovoltaic cells based on multiple exciton generation. Work done in collaboration with Y. Ping, T. A. Pham, M. Voros, D. Lu, H.-V. Nguyen, S. Wippermann, A. Gali, G. T. Zimanyi, and G. Galli.\\[4pt] $^*$Present address\\[4pt] [1] D. Rocca, D. Lu,G. Galli, J. Chem. Phys. 133, 164109 (2010); D. Rocca, Y. Ping, R. Gebauer, G. Galli, Phys. Rev. B 85, 045116 (2012).\\[0pt] [2] H.-V. Nguyen, T.A. Pham, D. Rocca, G. Galli, Phys. Rev. B 85, 081101 (2012).\\[0pt] [3] M. Voros, D. Rocca, G. Galli, G.T. Zimanyi, A. Gali, submitted (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N9.00003: Many-electron interactions and first-principles studies of spectral functions: spin multiplets and plasmon satellites in photoemission spectra Invited Speaker: Johannes Lischner The photoemission spectrum of an interacting system is often simply thought to be qualitatively similar to the corresponding non-interacting system: interactions only cause a shift and a broadening of the quasiparticle peak and result in a transfer of spectral weight into an incoherent background. We discuss two cases where this simple quasiparticle picture of photoemission fails and interactions result in a more drastic, qualitative difference from the non-interacting system. For electronic systems with unfilled shells, the coupling of angular momenta results in a multiplet structure in the photoemission spectrum. We describe how accurate calculations of multiplet splittings are possible within the GW approximation and present results for several magnetic molecules and defects, such as the negatively charged nitrogen-vacancy defect (NV$^{-})$ center in diamond. We also discuss plasmon satellite structures in photoemission spectra. We show for bulk silicon and doped graphene that the \textit{ab initio}GW approximation overestimates the quasiparticle-satellite separation significantly and falsely predicts a plasmaron excitation. By including significant vertex corrections via the \textit{ab initio}GW$+$cumulant approximation, we improve the description of plasmon satellites and find good agreement with experimental photoemission spectra.\\[4pt] Work was done in collaboration with Jack Deslippe, Manish Jain, Derek Vigil-Fowler and Steven G. Louie. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N9.00004: Theory for Time-Domain Photon Spectroscopy Invited Speaker: Thomas Devereaux In this talk I will present some recent work concerning the development of theories for time-domain photon spectroscopies, with a focus on studying non-equilibrium pump-probe dynamics. Studies of several model systems will be presented, including non-equilibrium dynamics across of metal-insulator transition in correlated systems, strong electron-phonon interactions, and spectral properties in a charge density wave state. The similarities and differences between equilibrium dynamics will be highlighted [1]. \\[4pt] [1] Phys. Scripta T151, 014062 (2012); arXiv:1210.3088; arXiv:1207.3835; Phys. Rev. Lett. 109, 176402 (2012); Nature Communications 3, 838 (2012); arXiv:1204.1803 [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N9.00005: Computational Spectroscopy for Nanoscale Photovoltaics Invited Speaker: Marco Bernardi Nanoscale photovoltaic (PV) systems employ nanomaterial interfaces to dissociate bound excitons formed upon sunlight absorption. This mechanism results in a correlated electron, hole, and exciton interface dynamics whose accurate determination is challenging both theoretically and experimentally. In this talk, I will discuss approaches available to compute and combine relevant spectroscopic quantities to predict efficient nanoscale PV systems. Further, I will present our recent work on two novel families of nanoscale PV devices based on: 1) Nanocarbon materials, achieving 1.3\% efficiency, tunable infra-red optical absorption, and superior photostability compared to organic solar cells 2) Two-dimensional monolayer semiconductors such as Graphene-BN and MoS$_2$, capable of absorbing a significant fraction of sunlight within just $\approx10$nm, and showing tunable absorption, band offsets, and power conversion efficiency (PCE).\\[4pt] In closing, I will discuss the errors and necessary accuracy in predicting PCE from first-principles calculations, and propose a suitable figure of merit to quantify absorption solar-matchedness to be used in large-scale searches of nanoscale PV materials. [Preview Abstract] |
Session N10: Invited Session: Smart Magnetic Particles: On-Chip Transport, Assembly and Biomedical Applications
Sponsoring Units: DCMP GMAGChair: Valentyn Novosad, Argonne National Laboratory
Room: 309
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N10.00001: High-speed transport and magneto-mechanical resonant sensing of superparamagnetic microbeads using magnetic domain walls Invited Speaker: Elizabeth Rapoport Surface-functionalized superparamagnetic (SPM) microbeads are of great interest in biomedical research and diagnostic device engineering for tagging, manipulating, and detecting chemical and biological species in a fluid environment [1-5]. Recent work has shown that magnetic domain walls (DWs) can be used to shuttle individual SPM microbeads and magnetically tagged entities across the surface of a chip [1-5]. This talk will describe the dynamics of SPM microbead transport by nanotrack-guided DWs, and show how these coupled dynamics can be exploited for on-chip digital biosensing applications. Using curvilinear magnetic nanotracks, we demonstrate rapid transport of SPM microbeads at speeds approaching 1000 $\mu $m/s [3], and present a mechanism for selective transport at a junction that allows for the design of complex bead routing networks. We further demonstrate that a SPM bead trapped by a DW exhibits a distinct magneto-mechanical resonance that depends on its hydrodynamic characteristics in the host fluid [4, 5], and that this resonance can be used for robust size-based discrimination of commercial microbead populations. By embedding a spin-valve sensor within a DW transport conduit, we show that the resonance can be detected electrically and on-the-fly [5]. Thus, we demonstrate a complete set of essential bead handling functions, including capture, transport, identification, and release, required for an integrated lab-on-a-chip platform.\\[4pt] [1] G. Vieira et al., Phys. Rev. Lett. 103, 128101 (2009).\\[0pt] [2] M. Donolato et al., Lab Chip. 11, 2976--2983 (2011).\\[0pt] [3] E. Rapoport and G.S.D. Beach, Appl. Phys. Lett. 100, 082401 (2012).\\[0pt] [4] E. Rapoport and G.S.D. Beach, J. Appl. Phys. 111, 07B310 (2012).\\[0pt] [5] E. Rapoport, D. Montana, and G.S.D. Beach, Lab Chip. 12, 4433-4440 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N10.00002: Binary Colloidal Superlattices Assembled by Magnetic Fields Invited Speaker: Benjamin Yellen Colloidal particle superlattices represent a fascinating class of complex materials which in many cases have corollary structures at the atomic scale. These complex systems thus not only help elucidate the principles of materials assembly in nature, but further provide design criteria for fabrication of novel materials at the macroscopic scale. Methods for assembling colloidal particle superlattices include controlled drying, ionic interactions, and dipolar interactions. However, a general pathway for producing a wider variety of colloidal crystals remains a fundamental challenge. Here we demonstrate a versatile colloidal assembly system in which the design rules can be tuned to yield over 20 different pre-programmed lattice structures, including kagome, honeycomb, square tiles, as well as a variety of chain and ring configurations. We tune the crystal type by controlling the relative concentrations and interaction strengths between spherical superparamagnetic and diamagnetic particles. An external magnetic field causes like particles to repel and unlike particles to attract. The combination of our experimental observations with potential energy calculations of various lattice structures suggest that the lowest energy lattice configuration is determined by two parameters, namely the dipole moment and relative concentration of each particle type. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N10.00003: Magnetic microstructures for regulating Brownian motion Invited Speaker: Ratnasingham Sooryakumar Nature has proven that it is possible to engineer complex nanoscale machines in the presence of thermal fluctuations. These biological complexes, which harness random thermal energy to provide functionality, yield a framework to develop related artificial, i.e., nonbiological, phenomena and devices. A major challenge to achieving positional control of fluid-borne submicron sized objects is regulating their Brownian fluctuations. In this talk a magnetic-field-based trap that regulates the thermal fluctuations of superparamagnetic beads in suspension will be presented. Local domain-wall fields originating from patterned magnetic wires, whose strength and profile are tuned by weak external fields, enable bead trajectories within the trap to be managed and easily varied between strong confinements and delocalized spatial excursions. Moreover, the frequency spectrum of the trapped bead responds to fields as a power-law function with a tunable, non-integer exponent. When extended to a cluster of particles, the trapping landscape preferentially stabilizes them into formations of 5-fold symmetry, while their Brownian fluctuations result in frequent transitions between different cluster configurations. The quantitative understanding of the Brownian dynamics together with the ability to tune the extent of the fluctuations enables the wire-based platform to serve as a model system to investigate the competition between random and deterministic forces. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N10.00004: Smart Magnetic Materials for Controlling Cell Fate Invited Speaker: Elina Vitol Toxicity of cancer chemotherapy, often resulting in failure of even healthy organs, represents one of the most vivid and still unavoidable outcomes of traditional medical approaches to treating a disease. The lack of specificity remains a fundamental obstacle in performing targeted treatment which should ideally affect only the particular cells in a human body. Nanotechnology has recently enabled the possibility to create materials comparable in sizes with cells and subcellular structures opening the opportunities for affecting intracellular processes on the level unattainable by macroscopic techniques. [1-2] Magnetic nanomaterials are especially promising for applications in life sciences due to their bi-functional behavior. On the one hand side, they are inherently stimuli-responsive and their properties can be controlled and modulated remotely. On the other hand, these materials themselves can be used for applying controlled stimulus to a cell thus changing its function and even inducing cell death [3]. For biological applications, such multifaceted functionality opens the unique opportunity to modulate cell behavior by interfacing it with magnetic material. Historically, chemically synthesized superparamagnetic iron oxide particles have been widely studied for biological applications such as magnetic separation, targeting, MRI contrast enhancement and magnetically induced heating [1,4]. At the same time, there is a growing interest to magnetic materials created by physical fabrication methods which allow for realization of very complex structures in terms of geometry and composition [5]. In this talk, both types of materials will be discussed. Thus, thermo-responsive magnetic micelles were used as nanocontainers for magnetically guided drug delivery and release triggered by heating in the RF frequency a.c. magnetic field. The microfabricated biofunctionalized microdisks targeted to the cancer cells were employed for mechanical stimulation of cell membrane due to oscillation of the disks in the low frequency (10-20 Hz) a.c. magnetic field, resulting in redistribution of free intracellular calcium and subsequent triggering of apoptosis - programmed cell suicide [3,5]. The details of mechanisms by which the cell responds to the stimulus applied by magnetic particles will be discussed.\\[4pt] [1] E. A. Rozhkova, Nanoscale Materials for Tackling Brain Cancer: Recent Progress and Outlook. Advanced Materials, 2011. 23(24): p. H136-H150; [2] E. A. Vitol, Z. Orynbayeva, G. Friedman, Y. Gogotsi, Nanoprobes for intracellular and single cell surface-enhanced Raman spectroscopy, J. Raman Spectrosc., (2012) Accepted, Available online: doi: 10.1002/jrs.3100; [3] D.-H. Kim, E.A. Rozhkova, I.V. Ulasov, S. D. Bader, T. Rajh, M. S. Lesniak, V. Novosad, Biofunctionalized magnetic-vortex microdiscs for targeted cancer-cell destruction. Nature Materials 2009, 9, (2), 165-171; [4] J. Dobson, Remote control of cellular behaviour with magnetic nanoparticles. Nature Nanotechnology, 2008. 3(3): p. 139-143; [5] E. A. Vitol, V. Novosad, E. A. Rozhkova, Microfabricated magnetic structures for future medicine: from sensors to cell actuators, Nanomedicine, 2012 (In press). [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N10.00005: Biomedical Applications of Magnetic Nanoparticles: Delivering Genes and Remote Control of Cells Invited Speaker: Jon Dobson The use of magnetic micro- and nanoparticles for biomedical applications was first proposed in the 1920s as a way to measure the rehological properties of the cell's cytoplasm. Since that time, magnetic micro- and nanoparticle synthesis, coating and bio-functionalization have advanced significantly, as have the applications for these particles. Magnetic micro- and nanoparticles are now used in a variety of biomedical techniques such as targeted drug delivery, MRI contrast enhancement, gene transfection, immno-assay and cell sorting. More recently, magnetic micro- and nanoparticles have been used to investigate and manipulate cellular processes both \textit{in vitro} and \textit{in vivo}. This talk will focus on magnetic nanoparticle targeting to and actuation of cell surface receptors to control cell signaling cascades to control cell behavior. This technology has applications in disease therapy, cell engineering and regenerative medicine. The use of magnetic nanoparticles and oscillating magnet arrays for enhanced gene delivery will also be discussed. [Preview Abstract] |
Session N11: Invited Session: Landmark Reports in Education
Sponsoring Units: FEdChair: Daniel Crowe, Loudon County Publc Schools Academy of Science
Room: 310
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N11.00001: Linking National and International Educational Assessments: NAEP and TIMSS Invited Speaker: Taslima Rahman In an increasingly global economy, comparisons of student achievement in the United States to student achievement in other countries are of interest to the nation. The National Center for Education Statistics (NCES) reports on mathematics and science achievement of 4$^{\mathrm{th}}$- and 8$^{\mathrm{th}}$-grade students for the all U.S. states and 60 countries. However, the reports are based on two separate assessments. Results for the U.S. states are based on the National Assessment~of Educational Progress (NAEP) and results for the other countries are based on the Trends in International Mathematics and Science Study (TIMSS). Further, unlike NAEP, TIMSS does not have an on-going state component. Thus, U.S. states cannot compare performance of their students with those of the students in other countries. To enable such comparisons, NCES launched a NAEP-TIMSS Linking study where the goal is to project TIMSS mathematics and science scores for the students in the 50 states that participated in NAEP. This linking study targeted eighth-grade students. NAEP assessments of mathematics and science were conducted in winter 2011 (January-March) and TIMSS assessments of mathematics and science were conducted in spring 2011 (April-June). Three approaches--- statistical moderation, calibration, and projection---are applied in linking the two scales. In this presentation, discussion will focus on the study design and approaches applied. In addition, results will be shared if released to the public by the NCES before March 2013. Otherwise results of earlier linking study conducted by the American Institutes for Research in 2007 using the statistical moderation technique will be shared. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N11.00002: Linking NAEP to TIMSS Using Statistical Moderation Invited Speaker: Gary Phillips |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N11.00003: The NRC Study of Undergraduate Physics Education: The role, status and outlook for physics education research Invited Speaker: Paula Heron The Board on Physics and Astronomy of the National Academies formed the ``Committee on Undergraduate Physics Education, Research and Implementation'' in 2011 and charged it with producing a report that ``identifies the goals and challenges facing undergraduate physics education and identifies how best practices for undergraduate physics education can be implemented on a widespread and sustained basis.'' (Further information on the committee and its charge can be found at: http://sites.nationalacademies.org/BPA/BPA\textunderscore 059078.) The report is expected to be released in early 2013. This talk will address the committee's process, some of the findings, and their implications for physics education. The role of physics education research in driving innovation will be emphasized. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N11.00004: A Future for Undergraduate Physics Education? Invited Speaker: Donald Langenberg About two years ago, the Board on Physics and Astronomy of the National Research Council created a Committee on Undergraduate Physics Education (UPE), with support from the National Science Foundation. The Committee was given the task to identify ``the goals and challenges facing undergraduate physics education,'' and ``how best practices for undergraduate physics education can be implemented on a widespread and sustained basis.'' The Committee was also asked to ``assess the status of physics education research (PER)'' and to ``discuss how PER can assist in accomplishing the goal of improving undergraduate physics education best practices and education policy.'' This presentation will report the Committee's findings and recommendations, the latter aimed at audiences ranging from individual physics faculty to departmental and university-wide leadership, and professional societies and funding agencies. The Committee's challenge was daunting. We are experiencing revolutionary changes in higher education, driven by new education technologies and demands for broader and deeper STEM education for more students in more fields. Only a relatively small fraction of undergraduates take physics courses. Nevertheless, half a million undergraduates enroll in at least one physics course in every academic year. PER has become a productive research field with the potential for major contributions to the improvement of undergraduate STEM education generally. Yet, in many---probably most--institutions UPE remains persistently traditional. We all have much to do! [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 2:15PM |
N11.00005: Disciplined Based Educational Research -- What is it? What has it done? Where is it going? Invited Speaker: Kenneth Heller The National Research Council of the National Academies of Science has just released its study of Disciplined Based Educational Research (DBER) funded by the National Science Foundation. This two year study attempted to define the emerging field of DBER and investigated its state in the fields of Astronomy, Biology, Chemistry, Engineering, Geosciences, and Physics. This talk will give a brief review of the report, discuss the recommendations, implications for future research, and impact of DBER in improving science and engineering instruction at the undergraduate level. [Preview Abstract] |
Session N12: Focus Session: Thermoelectrics Materials Waste Heat
Sponsoring Units: DMP GERA FIAPChair: Brian Sales, ORNL
Room: 314
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N12.00001: On the thermodynamics of waste heat recovery from internal combustion engine exhaust gas G.P. Meisner The ideal internal combustion (IC) engine (Otto Cycle) efficiency $\eta _{\mathrm{IC}}=$ 1-(1/r)$^{(\gamma -1)}$ is only a function of engine compression ratio r$=$V$_{\mathrm{max}}$/V$_{\mathrm{min}}$ and exhaust gas specific heat ratio $\gamma =$ c$_{\mathrm{P}}$/c$_{\mathrm{V}}$. Typically r$=$ 8, $\gamma =$ 1.4, and $\eta_{\mathrm{IC}}=$ 56{\%}. Unlike the Carnot Cycle where $\eta _{\mathrm{Carnot}}=$ 1-(T$_{\mathrm{C}}$/T$_{\mathrm{H}})$ for a heat engine operating between hot and cold heat reservoirs at T$_{\mathrm{H}}$ and T$_{\mathrm{C}}$, respectively, $\eta_{\mathrm{IC}}$ is not a function of the exhaust gas temperature. Instead, the exhaust gas temperature depends only on the intake gas temperature (ambient), r, $\gamma $, c$_{\mathrm{V}}$, and the combustion energy. The ejected exhaust gas heat is thermally decoupled from the IC engine and conveyed via the exhaust system (manifold, pipe, muffler, etc.) to ambient, and the exhaust system is simply a heat engine that does no useful work. The maximum fraction of fuel energy that can be extracted from the exhaust gas stream as useful work is (1-$\eta_{\mathrm{IC}}) \times \eta_{\mathrm{Carnot}}=$ 32{\%} for T$_{\mathrm{H}}=$ 850 K (exhaust) and T$_{\mathrm{C}}=$ 370 K (coolant). This waste heat can be recovered using a heat engine such as a thermoelectric generator (TEG) with $\eta_{\mathrm{TEG}}$\textgreater\ 0 in the exhaust system. A combined IC engine and TEG system can generate net useful work from the exhaust gas waste heat with efficiency $\eta _{\mathrm{WH}}=$ (1-$\eta_{\mathrm{IC}}) \times \eta _{\mathrm{Carnot}} \times \eta_{\mathrm{TEG}}$, and this will increase the overall fuel efficiency of the total system. Recent improvements in TEGs yield $\eta_{\mathrm{TEG}}$ values approaching 15{\%} giving a potential total waste heat conversion efficiency of $\eta _{\mathrm{WH}}=$ 4.6{\%}, which translates into a fuel economy improvement approaching 5{\%}. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N12.00002: Synthesis of High Performance Thermoelectric Materials Directly from Natural Mineral Xu Lu, Donald Morelli, Yi Xia, Fei Zhou, Vidvuds Ozolins, Hang Chi, Xiaoyuan Zhou, Ctirad Uher We report high performance TE materials synthesized directly from natural mineral. Lattice dynamics and electronic band structure calculations suggest that the compound tetrahedrite (Cu$_{\mathrm{12-x}}$M$_{\mathrm{x}}$Sb$_{4}$S$_{13})$, where M is transition metal such as Zn or Fe, will have low lattice thermal conductivity and good electronic transport properties. We have experimentally investigated the relationship between ZT and x content of different transition metals in synthetic tetrahedrites. We have found that the maximum of ZT value is not sensitive to the value of x but is related to valence band hole filling fraction; high ZT can be maintained over a large range of x. The compositions studied span the range of those of natural mineral tetrahedrite. To demonstrate that the natural mineral itself can be used as a source material, we synthesized samples by mixing natural mineral with synthetic Cu$_{12}$Sb$_{4}$S$_{13}$ by balling milling and hot pressing. The resulting samples were single phase with hole filling fraction in the optimum range and displayed maximum ZT values of unity at 723K. This new synthesis method can directly use natural mineral to produce TE materials in large quantities with little effort. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N12.00003: First-Principles Studies of Earth-Abundant Tetrahedrite Thermoelectrics Yi Xia, Fei Zhou, Vidvuds Ozolins Recent experiments have shown inexpensive and naturally occuring tetrahedrite-based materials that exhibit a thermoelectric figure of merit near unity. These compounds are typically of the form Cu$_{12-x}$M$_{x}$Sb$_{4}$S$_{13}$, where M is a transition metal, such as Zn or Fe, for a wide range of x. Using density-functional theory calculations, the ternary phase diagram and various defect formation energies are calculated. Furthermore, the electronic structure, phonon spectrum and thermoelectric properties are investigated. We observe metallic behavior and strong lattice anharmonicity of stoichiometric Cu$_{12}$Sb$_{4}$S$_{13}$. In addition, doping with transitional metals Zn or Fe increases both resistivity and anharmonicity. The theoretical calculations are in good agreement with experimental measurements. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N12.00004: Configuring pnicogen rings in skutterudites for low phonon conductivity Invited Speaker: Ctirad Uher During the past dozen or so years, skutterudites have attracted much interest as prospective thermoelectric materials for power-generation applications in the temperature range 500K - 850K. Primary interest was focused on filled forms of skutterudites where loosely-bonded filler species resonantly scatter normal phonon modes of the structure thus reducing the lattice thermal conductivity. Using this approach with multiple fillers and incorporating various forms of nanoinclusions, impressive figures of merit ZT = 1.5-1.7 have been reported with n-type filled skutterudites. Since the dominant heat-carrying modes in skutterudites are associated with vibrations of the pnicogen rings, disruptions of the ring structure by substitutional alloying should be a similarly effective approach of lowering the lattice thermal conductivity. In this talk I discuss our recent work exploring alloying configurations of pnicogen rings that yield particularly low values of the thermal conductivity. We found that compensated double-substitution (replacing two Sb atoms with one atom each from the column IV and column VI elements) is a very effective approach. Our ab initio calculations, in combination with a cluster expansion, have allowed us to identify stable alloy configurations on the Sb rings. Subsequent molecular and lattice dynamics simulations on low energy configurations established the range of atomic displacement parameters and values of the thermal conductivity. Theoretical results turned out to be in good agreement with our experimental thermal conductivity values. Combining both approaches of compensated double-substitution and filling of structural cages should be an effective way of further improving the thermoelectric figure of merit of skutterudites. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N12.00005: Vibrational dynamics of filled skutterudites $M_{1-x}$Fe$_4$Sb$_{12}$ ($M$ = Ca, Sr, Ba, and Yb) Andreas Leithe-Jasper, Michael Marek Koza, Hannu Mutka, Walter Schnelle, Helge Rosner, Yuri Grin First-principles density-functional theory and lattice-dynamics calculations were performed to study the vibrational dynamics and related observables of the ternary compounds $M_{1-x}$Fe$_4$Sb$_{12}$ ($A$ = Ca, Sr, Ba, Yb). The calculation results are supported by experimental data, which were obtained from neutron inelastic scattering, neutron-diffraction, and heat-capacity measurements. Within the calculation approach based on the theory of harmonic solids all observables are linked to the phonon density of states $Z(\omega)$. The good agreement with experimental data shows that the vibrational dynamics of the ternary skutterudite structures can be described by a set of normal modes. Features in the experimentally obtained density of states $G(\omega)$ reflecting the variation in properties (mass, ionic radius) of the cations Ca, Sr, Ba, and Yb are reproduced by the calculations. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N12.00006: Cr and Ru substituted defect manganese silicides MnSi$_{\delta }$ ($\delta \sim$ 1.72-1.74) as low thermal conductivity thermoelectrics* Vijayabarathi Ponnambalam, Donald T. Morelli Defect manganese silicides MnSi$_{\delta }$ ($\delta \sim$ 1.72-1.74) belong to a large family of compounds known as Nowotny chimney-ladder (NCL) phases and are closely related to an orthorhombic NCL compound TiSi$_{2}$. One interesting feature is the low lattice thermal conductivity ($\kappa_{L} \sim$ 2.5 W/m K) which may be due to several reasons:$_{\, }$ Since $\delta $ doesn't exceed 1.75 in MnSi$_{\delta }$, a considerable concentration of random vacancies exists on the Si-sublattice and can give rise to a low thermal conductivity. In addition, as synthesized MnSi$_{\delta }$ is a mixture of many phases including Mn$_{4}$Si$_{7}$, Mn$_{11}$Si$_{19}$, Mn$_{15}$Si$_{26}$ and Mn$_{27}$Si$_{47}$ and in all these phases, while a-lattice parameter is closely matched, the c-lattice parameter substantially varies with $\delta $. Such a closely matched a-lattice parameter can cause lattice strain and potentially reduce $\kappa_{L}$. Ru$_{2}$Si$_{3}$ forms solid solutions and Cr can be substituted as much as 20{\%} in MnSi$_{\delta }$. These substitutions can favorably modify the lattice strain and reduce the thermal conductivity further. Hence manganese silicides substituted with small amounts of Cr and Ru have been synthesized. Thermoelectric properties including resistivity, Seebeck and Hall coefficients and thermal conductivity will be studied and presented. *This work was supported as part of the Center for Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001054. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N12.00007: P-type electronic and thermal transport properties of Mg$_{2}$Sn$_{1-x}$Si$_{x}$ Sunphil Kim, Bartlomiej Wiendlocha, Joseph P. Heremans P-type Mg$_{2}$Sn doped with various acceptors$^{(1)}$$^{(2)}$ has been studied as a potential thermoelectric material. Because of its narrow band gap and high lattice thermal conductivity, the zT values of the binary compound are limited: zT$_{max}$ reported is 0.3$^{(3)}$. In this work, we synthesize and characterize p-type-doped Mg$_{2}$Sn$_{1-x}$Si$_{x}$ with various acceptors. Silicon is added in order to widen the band gap and scatter the phonons. The conduction band degeneracy that yields excellent zT in n-type material in the Mg$_{2}$Sn$_{1-x}$Si$_{x}$ alloy system unfortunately does not apply to p-type material. Thermomagnetic and galvanomagnetic properties (electrical resistivity, Seebeck, Hall, and Nernst coefficients) are measured, along with thermal conductivity and band gap measurements. Finally, zT values are reported. (1) H. Y. Chen et al. Journal of Electronic Materials, Vol. 38, No. 7, 2009 (2) S. Choi et al. Journal of Electronic Materials, Vol. 41, No. 6, 2012 (3) H. Y. Chen et al. Phys. Status Solidi A 207, No. 11, 2523-2531 (2010) [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N12.00008: Electronic and thermoelectric properties of CoSbS and FeSbS David Parker, Andrew F. May, Hsin Wang, Michael A. McGuire, Brian C. Sales, David J. Singh We present a combined theoretical and experimental study of the potential thermoelectric performance of three transition metal antimonide sulfides, CoSbS, FeSbS and NiSbS. From theory we find that NiSbS is metallic and hence of little interest regarding thermoelectric performance. CoSbS and FeSbS are both semiconductors with rather heavy valence and conduction bands, whose thermopower can exceed 200 $\mu$V/K at temperatures of 900 K and carrier concentrations of 10$^{21}$cm$^{-3}$, which is similar to the $n$-type high performance thermoelectric filled skutterudites. The experimental results on several non-optimized $n$-type CoSbS samples confirm its semiconducting nature and indicate a potential for good high temperature thermoelectric performance, finding a ZT for two of the samples of 0.35 at 773 K. Substantially higher ZT values may be possible if the lattice thermal conductivity can be reduced by alloying and the effects of extrinsic scattering, which appear to be substantial in the experimental results, are reduced. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N12.00009: Doping studies of alkali-metal rocksalt based I-V-VI$_2$ compounds with intrinsically minimal thermal conductivity Michele Nielsen, Vidvuds Ozolins, Joseph Heremans Past research has shown that rocksalt-based I-V-VI compounds have intrinsically low thermal conductivity as a result of the lone-pair electrons on the group V element. Theoretical calculations have revealed the presence marginally stable acoustic phonons which have extremely large Gr\"{u}neisen parameters. These result in a strong anharmonicity in heat-carrying acoustic phonon branches of select I-V-VI$_{2}$ compounds. Here, we extend this work to the electronic properties of the materials, which all have similar valence band structures. As a result of these two material properties, we are able to explore if the excellent zT observed in AgSbTe$_{2}$ extends to materials with cheaper starting elements and better high-temperature stability. Here we introduce new doping studies in I-V-VI$_{2}$ compounds where the group I element is an alkali-metal atom. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N12.00010: Role of Chemical Doping in Enhancement of Thermoelectric Properties of Ca$_{3}$Co$_{4}$O$_{9}$ Jianming Bai, Tao Wu, Trevor A. Tyson, Haiyan Chen, Kaumudi Pandya, Cherno Jaye, Daniel Fischer Single-phase [Ca$_{2}$CoO$_{3}$][CoO$_{2}$]$_{1.61}$ (Ca$_{3}$Co$_{4}$O$_{9})$ materials doped by transition metals were prepared by solid state reaction followed by annealing under oxygen. The temperature dependent thermoelectric properties, including resistivity ($\rho$), Seebeck coefficient (S) and thermal conductivity ($\kappa )$, were measured. In order to understand the origin of the changes in ZT with doping, local (XAS) and long range (XRD) structural measurements as a function of doping were conducted. The electronic properties were probed by x-ray spectroscopic methods. Identification of the locations of the dopant sites and the impact on ZT will be discussed. This work is supported by DOE Grant DE-FG02-07ER46402. The Physical Properties Measurements System was acquired under NSF MRI Grant DMR-0923032 (ARRA award). [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N12.00011: Thermoelectric Performance of Hole-Doped Cu$_2$O Xin Chen, David Parker, Mao-Hua Du, David J. Singh We present an analysis of the thermopower and related properties of hole-doped Cu$_2$O using first-principles calculations and Boltzmann transport theory. Our results show that hole-doped Cu$_2$O has a high thermopower of above 200 $\mu$V/K with doping levels as high as 5.5 $\times$ 10$^{20}$ cm$^{-3}$ at 500 K, mainly attributed to the heavy valence bands of Cu$_2$O. The current theory suggests that hole-doped Cu$_2$O could be a good thermoelectric material. Future experiments are thus suggested to explore its thermoelectric potential for practical use in cooling and power generation applications. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N12.00012: The Seebeck Coefficient in Oxygen Enriched La$_2$NiO$_4$ Paul Bach, Victor Leboran, Francisco Rivadulla Oxide-based devices show promise for themoelectric applications due to their chemical stability and straightforward fabrication. The La$_2$NiO$_{4+\delta}$ system has been predicted to show an increased thermopower coupled with an increased electrical conductivity around $\delta=0.05$ [Pardo et al. PRB 86, 165114 (2012)] that could lead to a large thermoelectric figure of merit (ZT). We investigate the suitability of lanthanum nickelate as a candidate material for high-ZT devices through a systematic study of oxygenated thin films grown by pulsed laser deposition. We report the electrical conductivity, Seebeck coefficient, and structural morphology of La$_2$NiO$_4$ grown in a range of oxidizing atmospheres and discuss their implications for controlled engineering of thermoelectric properties. We have explored the possibility of gate-tuning these systems in order to fabricate single-oxide based devices. This work was supported by the Ministerio de Ciencia e Innovaci\'on (Spain), grant MAT2010-16157, and the European Research Council, grant ERC-2010-StG 259082 2D THERMS. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N12.00013: Spin-entropy origin and scaling behaviour of thermopower in LaBaCoO system Dewei Zhang, Huaihong Guo, Teng Yang, Zhihe Wang, Zhidong Zhang, Youwei Du We report a detailed investigation of thermopower and magnetic properties for La$_{1-x}$Ba$_x$CoO$_3$. A large negative magnetothempower is found to scale with both magnetic field and temperature, indicating that a spin entropy contribution to thermompower. We have formulated a new and general expression to describe the thermopower from spin entropy with spin-interaction considered. Our formula can fit the scaling behavior quite well and provides a satisfactory description to the observed data. The magnetic results further consolidate our claim. This investigation suggests that spin entropy plays a substantial role in the enhanced thermopower in this cobaltite system. [Preview Abstract] |
Session N13: Focus Session: Topological Materials - Topological Superconductivity
Sponsoring Units: DMPChair: Peter Armitage, Johns Hopkins University
Room: 315
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N13.00001: Zero bias conductance peak anomaly in topological insulator - superconductor junctions Wenqing Dai, Anthony Richardella, Joseph Brom, Joan Redwing, Nitin Samarth, C.X. Liu, Qi Li We have fabricated planar junctions between topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and superconducting Pb with MgO barrier and studied the conductance spectra of the junctions under different temperatures and magnetic fields. Two types of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ thin films, grown by hybrid physical-chemical vapor deposition (HPCVD) and molecular beam epitaxy (MBE), were used. A few nanometers thick MgO layer made by RF sputtering was used as the barrier. We observed a zero bias conductance peak (ZBCP) anomaly in the spectra. The peak width ranges from 1 mV to 17 mV in different samples. The ZBCP height decreases with increasing temperature and disappears when the temperature is above the $T_{c}$ of Pb. The ZBCP is also suppressed by both perpendicular and parallel magnetic fields and vanishes above the critical field of Pb. We will discuss the possible origins of the ZBCP in the junctions. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N13.00002: Superconducting-Tip STM on Cobaltates as a Platform for Exploring Topological Superconductivity Alex W. Contryman, Francis Niestemski, Yulin Chen, Thorsten Hesjedal, Carolina Parra, Suk Bum Chung, Hai-Jun Zhang, Z.X. Shen, Shou-Cheng Zhang, Hari C. Manoharan In recent years, Na$_{x}$CoO$_{2}$ has attracted much attention for its unconventional superconductivity and antiferromagnetic phases. More recently it has been proposed that inducing superconductivity into the stoichiometric compound through the proximity effect could lead to topological superconductivity where Majorana physics might be accessed. We first explore this surface state with standard scanning tunneling spectroscopy and tuning fork-based atomic force microscopy, and then investigate the proximity effect scenario by introducing a superconducting tip to probe the superconductor-vacuum-topological junction. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N13.00003: Dynamics of Josephson vortex interacting with Majorana bound modes in Long Josephson Junctions Yen Lee Loh, Ju H. Kim We investigate the effects of Majorana bound modes on Josephson vortex (i.e., fluxon) dynamics by examining a long Josephson junction deposited on a topological insulator. Majorana bound states are represented as two counterpropagating edge modes along either superconducting side, which couples to the local Josephson phase difference. A fluxon (a 2$\pi$ phase configuration) interacts with Majorana bound states via the Jackiw-Rebbi mechanism [1] as pointed out by Grosfeld and Stern [2]. We find the effective equation of motion for the fluxon by integrating out the Majorana modes. This motion can be described by the double sine-Gordon equation. As a consequence, there may be fractional Shapiro steps in the I-V characteristics. In addition, the fluxon may have internal modes. We study the criteria for these effects to occur and to be detectable. \\[4pt] [1] R. Jackiw and C. Rebbi, Phys. Rev. D \textbf{13}, 3398 (1976).\\[0pt] [2] E. Grosfeld and A. Stern, PNAS \textbf{108}, 11810 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N13.00004: Edge superconducting correlation in attractive-U-Kane-Mele Hubbard model Jie Yuan, Jinhua Gao, Weiqiang Chen, Fei Ye, Yi Zhou, Fuchun Zhang The two-dimensional Kane-Mele model with attractive Hubbard interaction $U$ is studied by using a self-consistent mean-field theory. At $U=0$, the ground state is a topological insulator. At $U$ larger than a critical value $U_c$, the ground state is a bulk superconductor. At $0 [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N13.00005: Strong and weak 2D topological superconductors with spin-orbit coupling Hong Yao, Fan Yang We study pairing symmetries of superconducting states in a centrosymmetric system with quasi-one dimensional bands and spin-orbit coupling. When the spin-orbit coupling is weak, we mainly find even-parity pairing which is topologically trivial. When the spin-orbit coupling is (moderately) strong, the paring is dominantly p-wave, which is an odd-parity pairing. Depending on the interaction parameters, we find two different odd-parity pairing states. One has p+ip pairing with nonzero strong topological invariants, which breaks time reversal symmetry and possesses gapless chiral Majorana modes. The other has p+ip pairing for spin-up electrons but p-ip pairing for spin-down electrons, which preserves time reversal symmetry and hosts nontrivial weak Z$_2$ topological invariants. In the weak topological superconductors, there are gapless modes associated with lattice dislocations. Possible applications to the recent discovered BiS-based superconductors will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N13.00006: Majorana bound states and non-local spin correlations in a quantum wire on a topological superconductor Sho Nakosai, Jan Budich, Yukio Tanaka, Bjoern Trauzettel, Naoto Nagaosa We theoretically study the proximity effect of the one-dimensional quantum wire of usual metal without the spin-orbit interaction on the substrate of unconventional superconductor. Three cases are considered for the substrate, i.e., (i) chiral superconductor in class D with broken time reversal symmetry, and class DIII superconductor (ii) with and (iii) without the nontrivial $Z_2$ number. The Cooper pairs are induced into the wire, resulting effective one dimensional superconducting system. We found the degenerate zero energy Majorana bound states at both ends of the wire for all the cases, unlike single Majorana state in spin-orbit coupled system with $s$-wave superconductor, which might have been experimentally observed. The degenerate Majorana bound states are unstable against the spin-orbit interaction in case (i) while are protected by time reversal symmetry in cases (ii) and (iii). These degenerate Majorana bound states constitute the spin 1/2 degrees of freedom at each end of the wire. It is also shown that the non-locally correlated two spins at the two ends of the wire can be controlled by the gating potential on the wire. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N13.00007: Josephson supercurrent through a topological insulator surface state Invited Speaker: Alexander Brinkman The long-sought yet elusive Majorana fermion is predicted to arise from a combination of a superconductor and a topological insulator. We present direct evidence for a Josephson supercurrent in superconductor (Nb) - topological insulator (Bi$_2$Te$_3$) - superconductor e-beam fabricated junctions by the observation of clear Shapiro steps under microwave irradiation, and a critical current modulation by magnetic field. The dependence of the critical current on temperature and electrode spacing shows that the junctions are in the ballistic limit on a length scale of 100 nm. Shubnikov-de Haas oscillations in magnetic fields up to 30 T reveal a topologically non-trivial two-dimensional surface state. We argue that the ballistic Josephson current is hosted by this surface state despite the fact that the normal state transport is dominated by diffusive bulk conductivity. Nanostructured SQUIDs containing topological Josephson junctions are realized experimentally. Clear critical current modulation of both the junctions and the SQUID with applied magnetic fields have been observed. We show that the SQUIDs have a periodicity in the voltage-flux characteristic of $\Phi_0$ consistent with numerical expectations. We propose several strategies towards realizing a doubled periodicity, belonging to the presence of Majorana fermions. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N13.00008: Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity Victor Galitski, So Takei Motivated by the recent experiment of Wang et al. [Nat. Phys. {\bf 6}, 389 (2010)], who observed a highly unusual transport behavior of ferromagnetic cobalt nanowires proximity-coupled to superconducting electrodes, we study the proximity effect and temperature-dependent transport in such a mesoscopic hybrid structure. It is assumed that the asymmetry in the tunneling barrier gives rise to the Rashba spin-orbit coupling in the barrier that enables induced p-wave superconductivity in the ferromagnet to exist. We first develop a microscopic theory of Andreev scattering at the spin-orbit-coupled interface, derive a set of self-consistent boundary conditions, and find an expression for the p-wave minigap in terms of the microscopic parameters of the contact. Second, we study the temperature dependence of the resistance near the superconducting transition, and we find that it should generally feature a fluctuation-induced peak. The upturn in resistance is related to the suppression of the single-particle density of states due to the formation of fluctuating pairs, whose tunneling is suppressed. In conclusion, we discuss this and related setups involving ferromagnetic nanowires in the context of one-dimensional topological superconductors. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N13.00009: Odd-frequency superconducting pairing in topological insulators Annica Black-Schaffer, Alexander Balatsky We discuss the appearance of odd-frequency spin-triplet $s$-wave superconductivity, first proposed by Berezinskii [J. Exp. Theor. Phys. 20 287 (1974)], on the surface of a topological insulator proximity coupled to a conventional spin-singlet $s$-wave superconductor. Using both analytical and numerical methods, we show that this disorder robust odd-frequency state is present whenever there is an in-surface gradient in the proximity induced gap. Such a gradient exists in both superconductor-normal state junctions as well as when an in-surface supercurrent is present. The time-independent order parameter for the odd-frequency superconductor is proportional to the in-surface gap gradient. The induced odd-frequency component does not produce any low-energy states. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N13.00010: Detection of one-dimensional helical mode in topological insulator nanowire interferometer Seung Sae Hong, Yi Zhang, Judy Cha, Xiao-Liang Qi, Yi Cui In topological insulators (TIs), the spin-momentum locking together with time reversal symmetry (TRS) protects surface electrons from localization, which is the defining signature of TIs and the key property to realize exotic physics and applications. In quasi-one-dimensional (1D) TI nanowires, the surface electrons form 1D quantum modes of different topological natures, allowing us to observe topological protection via quantum interference modulated by magnetic flux[1,2]. We report low-temperature transport of bismuth selenide (Bi2Se3)-Se core-shell nanowire devices in parallel magnetic fields. Magneto-oscillations of different physical origins are studied systematically in ballistic regime and diffusive regime. Especially at strongly disordered limit, we observe a topologically-protected helical 1D mode at half magnetic flux quantum (h/2e). The quantum interference under TRS breaking magnetic field will be discussed as well. \newline [1] J.H. Bardarson, P.W. Brouwer, and J. E. Moore, Phys. Rev. Lett. \textbf{105}, 156803 (2010). \newline [2] Y. Zhang, A. Vishwanath, Phys. Rev. Lett. \textbf{105}, 206601 (2010). [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N13.00011: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N13.00012: Superconducting Proximity Effect in Topological Insulators Andrew Bestwick, Melis Tekant, James Williams, David Goldhaber-Gordon, Kehui Wu, Yongqin Li, James Analytis, Andrew Bleich, Ian Fisher Superconductor-topological insulator interfaces are prime candidates in the search for Majorana fermions in the solid state. We report on recent transport measurements of proximity-induced superconductivity through topological insulators with varying chemical compositions and growth methods. We will discuss the Josephson effect, tunneling spectroscopy, and measurement of normal-state conduction channels as means to detect Majorana states. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N13.00013: Transport study on tunnel junction structures based on In2Se3/Bi2Se3 heterostructures Nikesh Koirala, Matthew Brahlek, Namrata Bansal, SeongShik Oh Bi2Se3 is a 3D Topological Insulator (TI) candidate material with structural similarity to In2Se3, which is a band insulator with large band gap. This compatibility leads to possibility of epitaxial growth of In2Se3/Bi2Se3 heterostructure, which has various application potential. For example, by depositing Superconducting or Ferromagnetic materials on top of this heterostructure, tunnel junctions can be fabricated. We have studied device structures made up of such tunnel junctions. In2Se3 was grown on top of Bi2Se3 using molecular beam epitaxy on Al2O3(0001) substrates. Superconductor (Nb) or Ferromagnet (CoFe, Gd) was then sputtered on top of In2Se3 and photolithography was used to make the tunnel junctions. Transport measurement data obtained from such structures will be presented. [Preview Abstract] |
Session N14: Focus Session: Spin-dependent Tunneling and High Magnetoresistance Devices
Sponsoring Units: DMP FIAP GMAGChair: See-Hun Yang, IBM Almaden Research Center
Room: 316
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N14.00001: Optimizing Co concentration in Co$_{x}$Fe$_{1-x}$/MgO/Co$_{x}$Fe$_{1-x}$ magnetic tunnel junctions to maximize tunneling magnetoresistance Jonathan Trinastic, Yan Wang, Hai-Ping Cheng Fe$_{1-x}$Co$_{x}$ /MgO magnetic tunnel junctions (MTJs) are of great experimental interest due to large differences in conductance between parallel and anti-parallel spin alignment in the electrodes that lead to high tunneling magnetoresistance (TMR). However, the optimal Co concentration in the electrodes that maximizes TMR is still under investigation (Bonell et al 2012,\textit{PRL,}108,176602). Using first-principles calculations, we compare the conductance and TMR of Fe$_{1-x}$Co$_{x}$/MgO junctions using 1) disordered electrodes modeled with the virtual crystal approximation (VCA), and 2) ordered Fe$_{0.75}$Co$_{0.25}$ and Fe$_{0.50}$Co$_{0.50}$ electrodes. For disordered electrodes, we find that the optimal Co concentration varies between 20 and 30 percent and TMR decreases with MgO barrier thickness. For ordered electrodes, pure Co electrodes exhibit the highest TMR for a thin MgO barrier; however, Fe$_{0.75}$Co$_{0.25}$ electrodes demonstrate the highest TMR for a thicker MgO barrier, replicating recent experimental results. In all cases, a decrease in anti-parallel transmission drives the TMR increase. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N14.00002: Large Magnetoresistance of MnBi/Bi/MnBi Spin Valve Nabil Al-Aqtash, Khaldoun Tarawneh, Renat Sabirianov Recently, a large transport spin polarization was demonstrated in MnBi films by Andreev reflection experiments [1]. Furthermore, a large magnetoresistance (MR) of 70{\%} was observed in MnBi junctions at room temperature [2]. Because of this, a spin-valve MnBi/Bi/MnBi device is promising to have large MR that can be controlled by the varying the thickness of Bi spacer. Thin films of Bi show a semimetal-semiconductor transition at reduced thicknesses. Bismuth itself shows a substantial MR and a large mean free path of electron. In this system both the electrodes and the spacer have a hexagonal unit cell. A transport magetoresistance of MnBi/Bi(6 layers)/MnBi film was calculated using density functional theory coupled with nonequilibrium Green's function method as implemented in SIESTA code. The calculations display a high transport spin polarization of MnBi. A transmission MR of the spin valve around 77{\%} is calculated, consistent with the previous experimental observation of a large magnetoresistance in MnBi contacts. Thus, MnBi is promising candidate for high MR devices with tunable spacer properties. [1] P. Kharel, P. Thapa, P. Lukashev, R. F. Sabirianov, E. Y. Tsymbal, D. J. Sellmyer, and B. Nadgorny, Phys. Rev. B 83, 024415 (2011) [2] E. Clifford, M. Venkatesan and J. M. D. Coey, J. Mag. Magn. Mater. 272-276, 1614(2004). [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N14.00003: Self-Organized Defects of Half-Metallic Nanowires in MgO-Based Magnetic Tunnel Junctions Masayoshi Seike, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida The purpose of this study is to examine the possibility of self-organization of defects and defect-induced properties in MgO-based magnetic tunnel junctions (MTJs). Using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, first-principles calculations were performed to estimate the electronic structures and total energies of MgO with various defects. From our thorough evaluation of the calculated results and previously reported experimental data, we propose that self-organized half-metallic nanowires of magnesium vacancies can be formed in MgO-based MTJs. This self-organization may provide the foundation for a comprehensive understanding of the conductivity, tunnel barriers and quantum oscillations of MgO-based MTJs. Further experimental verification is needed before firm conclusions can be drawn.\\[4pt] References:\newline [1] K. Sato et al.: Rev. Mod. Phys. 82, 1633 (2010).\newline [2] M. Seike et al.: Jpn. J. Appl. Phys. 50, 090204 (2011). \newline [3] M. Seike et al.: Physica B 407, 2875 (2012).\newline [4] M. Seike et al.; Jpn. J. Appl. Phys. 51, 050201 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N14.00004: Mechanisms of perpendicular magnetic anisotropy and interlayer exchange coupling in MgO-based tunnel junctions Invited Speaker: Mairbek Chshiev Magnetic tunnel junctions (MTJ) comprising ferromagnetic (FM) electrodes with MgO spacer have been an object of high interest for spintronics due to Bloch states symmetry spin filtering leading to high tunnel magnetoresistance (TMR) [1] and due to observation of antiferromagnetic (AF) coupling between FM electrodes across MgO spacer [2]. This attention have been strongly reinforced in a view of a huge interest in MTJs with perpendicularly magnetized magnetic layers (p-MTJs) originating from large values of interfacial perpendicular magnetic anisotropy (PMA) first observed at Pt\textbar Co\textbar MOx interfaces (M$=$Ta, Mg, Al, Ru\textellipsis ) [3,4] and later reported for Co\textbar MgO [4,5] and CoFeB\textbar MgO p-MTJs [6]. In this talk we will elucidate mechanisms responsible for the PMA from first-principles [7] and report the effect of interfacial oxidation conditions on the PMA in Fe(Co)\textbar MgO p-MTJs. In particular, we found very large PMA values for MTJs with pure interfaces in agreement with recent experiments [4,6]. Furthermore, it will be demonstrated that oxidation conditions strongly affect the PMA which strongly correlates with TMR in agreement with experiments [7,8]. Finally, we will discuss the origin of AF coupling in Co\textbar MgO p-MTJs which oscillates as a function of FM layer thickness in agreement with theories of interlayer exchange coupling in MTJ [5].\\[4pt] [1] W.H. Butler et al, \textit{Phys. Rev. B} 63,054416(2001); \textit{IEEE Trans. Magn. }41,2645(2005).\\[0pt] [2] J. Faure-Vincent et al, \textit{Phys. Rev. Lett.} 89,107206(2002); T. Katayama et al, \textit{Appl. Phys. Lett.} 89,112503(2006); H.-X. Yang et al, \textit{Appl. Phys. Lett.}~ 96,262509(2010).\\[0pt] [3] S. Monso et al, \textit{Appl. Phys. Lett.} 80,4157(2002); B. Rodmacq et al, \textit{J. Appl. Phys.} 93,7513(2003).\\[0pt] [4] L. Nistor et al, \textit{Appl. Phys. Lett.} 94,012512(2009).\\[0pt] [5] L. Nistor et al, \textit{IEEE Trans. Magn.} 45,3472(2009); \textit{Phys. Rev. B} 81,220407(2010).\\[0pt] [6] S. Ikeda et al, \textit{Nature Mat.,} 9,271(2010).\\[0pt] [7] H.-X. Yang et al, \textit{Phys. Rev. B}~84,054401(2011).\\[0pt] [8] L. Nistor et al, \textit{IEEE Trans. Magn.} 46,1412(2010). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N14.00005: Point-contact study of soft magnetic CoSiBFeNb amorphous alloys Heidi Seinige, Cheng Wang, Valerii Tsoi, Maxim Tsoi We study magnetotransport in nanoscale point contacts to soft magnetic CoSiBFeNb ribbons. Such ultrasoft amorphous alloys attracted considerable attention previously because they exhibit Giant Magnetoimpedance (GMI) effect - large variations in the electrical impedance as a function of an external magnetic field [see, for instance, M.-H. Phan, H.-X. Peng, Prog. Mater. Sci. 53, 323 (2008) and references therein]. GMI is attributed to the field-induced variations in alloy permeability and has been established through ac measurements on bulk samples which revealed a strong dependence on ac frequency and amplitude but did not show any variations in dc resistance at all. In our experiments, we use nanocontacts to probe magnetotransport in amorphous CoSiBFeNb at the nanoscale. We use point contacts to inject both ac and dc currents into the alloy ribbons prepared by a melt-spinning technique. Measurements with ac currents revealed GMIs similar to those in macroscopic samples. Interestingly, we also observe a dc magnetoresistance which may be attributed to magnetic domain reorientations in a small contact region. Effects of high dc densities on the magnetoresistance are discussed in terms of spin-transfer torque (STT) effect. We thank A. Serebryakov for providing ribbon samples. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N14.00006: Measurement of the transport spin polarization of Ag/Fe$_3$O$_4$ bilayers using point-contact Andreev reflection Michael Osofsky, Priyanga Jayathilaka, Casey W. Miller The development of point-contact Andreev reflection (PCAR) has provided a relatively simple method for determining the spin polarization of various ferromagnetic materials. This technique utilizes point contact tunneling from a superconducting tip into a ferromagnet (FM) as a probe of the spin-polarization of the FM. Quantitative information can be extracted from the conductance data through a modified Blonder, Tinkham, Klapwijk (BTK) model of supercurrent conversion at a superconductor-metal interface (Andreev reflection) which includes the spin-polarization of the normal metal. The 100{\%} spin polarized oxide, Fe$_{3}$O$_{4}$, which is insulating at low temperature, is of great interest for spintronics applications. In order to use PCAR to measure the spin polarization of this system, it is necessary to provide a conducting layer. In this talk we will describe the results of PCAR measurements of Ag/Fe$_{3}$O$_{4}$ bilayers as a function of Ag thickness. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N14.00007: 3D Effect in Determination of Spin Polarization using Andreev Reflection Spectroscopy Jessica Gifford, Charles Snider, Jonny Martinez, Tingyong Chen Andreev Reflection Spectroscopy (ARS) has been utilized to measure spin polarization of magnetic materials, as well as the superconducting gap of superconductors. These values are extracted by a modified Blonder-Tinkham-Klapwijk (BTK) model or the more recent Chen-Tesanovic-Chien (CTC) model. Both consider the F/S interface as one dimensional (1D). However, a tip may have a point angle with three dimensional (3D) effects. We present both theoretical and experimental studies of the 3D effects in the determination of spin polarization. We have found that for an ideal interface without interfacial scattering (Z), the 3D ARS spectra are the same as 1D spectra. But for non-ideal interfaces the 3D effect can drastically change the conductance spectra depending on the point angle of the tip. The 3D spectra can be well described by the 1D model with a different interfacial scattering factor and a slightly different inelastic scattering factor. The spin polarization and superconducting gap is the same as the1D model, demonstrating that 1D ARS model can be utilized to determine spin polarization as long as Z is not of any concern. Finally, we apply the both the 1D and the 3D models to a set of ARS data and show that the extracted spin polarization value is the same for both models. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N14.00008: Graphene-based magnetic tunnel junctions Invited Speaker: Enrique Cobas Graphene's in-plane transport has been widely researched and has yielded extraordinary carrier mobilities of 10$^{5}$ cm$^{2}$/Vs and spin diffusion lengths of exceeding 100$\mu $m. These properties bode well for graphene in future electronics and spintronics technologies. Its out-of-plane transport has been far less studied, although its parent material, graphite, shows a large conductance anisotropy. Recent calculations [1,2] show graphene's interaction with close-packed ferromagnetic metal surfaces should produce highly spin-polarized transport out-of-plane, an enabling breakthrough for spintronics technology. In this work, we fabricate and measure FM/graphene/FM magnetic tunnel junctions using CVD-grown single-layer graphene. The resulting juctions show non-linear current-voltage characteristics and a very weak temperature dependence consistent with charge tunneling transport. Furthermore, we study spin transport across the junction as a function of bias voltage and temperature. The tunneling magnetoresistance (TMR) peaks at two percent for single-layer graphene junctions and exhibits the expected bias asymmetry and a temperature dependence that fits well with established spin-polarized tunneling models. [3] Results of mutli-layer graphene tunnel junctions will also be discussed.\\[4pt] References:\\[0pt] [1] Karpan et al., Phys. Rev. Lett. 99, 176602, 2007.\\[0pt] [2] Yazyev and Pasquarello, Phys. Rev. B. 80, 035408, 2009.\\[0pt] [3] Cobas et al., Nano Letters 12, 3000, 2012. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N14.00009: Magnetic Tunnel Junctions with a Graphene Tunnel Barrier Wan Li, Daniel Ralph We have fabricated ferromagnet/graphene/ferromagnet (FM/Gr/FM) junctions in which current flows vertically so that the graphene acts as a single-atom-thick barrier. In contrast to previous work, we utilize a fabrication process that avoids oxidation of the magnetic electrodes. We measure typical resistance-area products significantly lower than previously reported. We will present an analysis of whether this difference is due to the absence of a magnetic oxide or to defects in the graphene. We will also discuss the magnetoresistance of these devices, and how both the resistance and the magnetoresistance depend on the quality of the graphene. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N14.00010: Magnetoresistance and negative differential resistance in Ni/Graphene/Ni vertical heterostructures driven by finite bias voltage: A first-principles study Kamal K. Saha, Anders Blom, Kristian S. Thygesen, Branislav K. Nikolic Using the nonequilibrium Green function formalism combined with density functional theory, we study finite-bias quantum transport in Ni/Gr$_n$/Ni vertical heterostructures where $n$ graphene layers are sandwiched between two semi-infinite Ni(111) electrodes. We find that recently predicted pessimistic magnetoresistance of 100\% for $n \ge 5$ junctions at zero bias voltage $V_b \rightarrow 0$, persists up to $V_b \simeq 0.4$ V, which makes such devices promising for spin-torque-based device applications. In addition, for parallel orientations of the Ni magnetizations, the $n=5$ junction exhibits a pronounced negative differential resistance as the bias voltage is increased from $V_b=0$ V to $V_b \simeq 0.5$ V. We confirm that both of these nonequilibrium transport effects hold for different types of bonding of Gr on the Ni(111) surface while maintaining Bernal stacking between individual Gr layers. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N14.00011: Large spin accumulation due to spin-charge coupling across a break-junction Shuhan Chen, Han Zou, Siu-Tat Chui, Yi Ji We investigate large spin signals in break-junction nonlocal spin valves (NLSV). The break-junction is a nanometer-sized vacuum tunneling gap between the spin detector and the nonmagnetic channel, formed by electro-static discharge. The spin signals can be either inverted or non-inverted and the magnitudes are much larger than those of standard NLSV. Spin signals with high percentage values (10{\%} - 0{\%}) have been observed. When the frequency of the a.c. modulation is varied, the absolute magnitudes of signals remain the same although the percentage values change. These observations affirm the nonlocal nature of the measurements and rule out local magnetoresistive effects. Owing to the spin-charge coupling across the break-junction, the spin accumulation in a ferromagnet splits into two terms. One term decays on the charge screening length (0.1 nm) and the other decays on the spin diffusion length (10 nm nm). The magnitude of the former is proportional to the resistance of the junction. Therefore a highly resistive break-junction leads to a large spin accumulation and thereby a large spin signal. The signs of the spin signal are determined by the relationship between spin-dependent conductivities, diffusion constants, and density of states of the ferromagnet. [Preview Abstract] |
Session N15: Focus Session: Frustration in 1D and Spinels
Sponsoring Units: GMAG DMPChair: Craig Fennie, Cornell University
Room: 317
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N15.00001: Block versus Stripy Antiferromagnetism in the Fe-Based Spin-Ladder Materials (Ba,K)Fe$_2$Se$_3$ Wei-Guo Yin, Limin Wang, Wei Ku We present a theoretical study of the novel magnetism in the insulating two-leg spin-ladder material Ba$_{1-x}$K$_x$Fe$_2$Se$_3$, which exhibits a spontaneous formation of block and stripy antiferromagnetic spin orders in the Ba and K end members, respectively, and spin glass behavior in between. The bare spin susceptibility calculated with the first-principles electronic structure is found to remain qualitatively unchanged upon hole doping (substitution of K for Ba), ruling out the simple scenario of Fermi surface nesting. We show that these doping-dependent spin orders can be explained by use of a model of coexisting itinerant and localized electronic states on the Fe atoms, which are coupled by Hund's rule coupling. Our results reveal a strong spin frustration coming from the competing antiferromagnetic superexchange and ferromagnetic double-exchange interactions in this system, and unify its magnetism with that of the iron-based superconductors [1,2]. Work supported by DOE DE-AC02-98CH10886. [1] W.-G. Yin, C.-C. Lee, and W. Ku, Phys. Rev. Lett. \textbf{105}, 107004 (2010). [2] W.-G. Yin, C.-H. Lin, and W. Ku, Phys. Rev. B \textbf{86}, 081106(R) (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N15.00002: Optical Reflection Study of Low-Dimensional Quantum Magnets Judy Cherian, Takahisa Tokumoto, Haidong Zhou, Stephen McGill We performed a linear optical reflection analysis of a low-dimensional, frustrated quantum magnet. Strongly-correlated low-dimensional systems are important for understanding spin-excitations, which form an important class of low-energy phenomena. Of particular interest are how these spin excitations arise and are then tuned by the environment (e.g. temperature, applied magnetic field). The temperature dependence of the reflection spectra from 215 K down to 4 K was measured. Magnetic field dependence of the reflection spectra from 0 T to 35 T was also measured. We will discuss the behavior of the reflection edge with temperature and magnetic field and its correlation with spin excitations. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N15.00003: Non-Fermi liquid $d$-wave metal phase of strongly interacting electrons on the two-leg ladder Ryan V. Mishmash, Hong-Chen Jiang, Matthew S. Block, James R. Garrison, D. N. Sheng, Olexei I. Motrunich, Matthew P. A. Fisher Developing a theoretical framework for conducting electronic fluids qualitatively distinct from those described by Landau's Fermi liquid theory is of central importance to many outstanding problems in condensed matter physics. Perhaps the most important such pursuit is a microscopic characterization of the cuprates, where the so-called ``strange metal'' behavior above $T_c$ near optimal doping is inconsistent with being a traditional Landau Fermi liquid. Indeed, a microscopic theory of such a strange metal quantum phase could possibly shed new light on the interesting low-temperature behavior in the pseudogap and on the $d$-wave superconductor itself. Here, we present a theory for a specific example of a strange metal, which we term the ``$d$-wave metal.'' Using variational wave functions, gauge theoretic arguments, and ultimately large-scale DMRG calculations, we establish compelling evidence that this remarkable quantum phase is the ground state of a reasonable microscopic Hamiltonian: the venerable $t$-$J$ model supplemented with a frustrated electron ring-exchange term, which we study extensively here on the two-leg ladder. These findings constitute one of the first explicit examples of a non-Fermi liquid metal existing as the ground state of a realistic model. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N15.00004: Moving toward two dimensions in a $t$-$J$-$K$ model with frustrating ring exchange: the quest to stabilize a non-Fermi liquid $d$-wave metal phase James R. Garrison, Hong-Chen Jiang, Ryan V. Mishmash, Bryan K. Clark, Olexei I. Motrunich, Matthew P. A. Fisher Recent work (arXiv:1207.6608) has established compelling evidence, on the two-leg ladder, for the existence of a non-Fermi liquid strange metal phase as the ground state of a realistic model Hamiltonian---the $t$-$J$ model supplemented with a frustrating ring-exchange term. Here we present our findings, guided by VMC and DMRG calculations, as we move toward two dimensions in an attempt to fully characterize the phase diagram and to stabilize this ``$d$-wave metal'' phase beyond the two-leg ladder. Ultimately, we are motivated by a desire to understand the strange metal phase in the cuprates, and to determine whether the superconductor and pseudo-gap regimes can potentially be understood as instabilities of the $d$-wave metal phase resulting from this (or a similar) model Hamiltonian. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N15.00005: Nontrivial ferrimagnetism on the low-dimensional quantum spin systems with frustration Tokuro Shimokawa, Hiroki Nakano, Toru Sakai In low-dimensional quantum spin systems with frustration, nontrivial magnetisms often occur due to strong quantum fluctuation. Ferrimagnetism in non-frustrated systems is well-known to occur from the mechanism based on the Marshall-Lieb-Mattis theorem. This type of ferrimagnetism is called ``Lieb-Mattis (LM) type.'' Recently, the occurrence of nontrivial ferrimagnetism has been reported in some one-dimensional Heisenberg spin systems with frustration, in which the continuous change of spontaneous magnetization and the incommensurate modulation in local magnetization are observed. This type is called ``non-Lieb-Mattis (NLM) type.'' In this study, we tackle a problem whether the NLM ferrimagnetism occurs or not in higher dimensional systems. We investigate the S$=$1/2 Heisenberg models on the spatially anisotropic two-dimensional (2D) kagome lattice and on the quasi-one-dimensional (Q1D) kagome strip lattices by the numerical diagonalization and density matrix renormalization group methods. The Q1D models share the same structure in their inner part with the spatially anisotropic 2D kagome lattice; we examine two cases with respect to strip width. We will discuss the relationship between the ground-state properties of the Q1D lattices and those of the 2D lattice. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N15.00006: Theory of the NMR $1/T_1$ relaxation rate in a quantum spin nematic Andrew Smerald, Nic Shannon Recently, it has been proposed that the material LiCuVO$_4$ may realise quantum spin-nematic order when a magnetic field close to saturation is applied [1,2]. Potentially, a bond-centred, 2-sublattice antiferroquadrupole spin-nematic state is stable at low temperature. However, the experimental evidence for this state remains inconclusive. Building on previous work [3], we develop a detailed theory of the NMR $1/T_1$ relaxation rate in spin-nematic states, and apply this to the specific case of LiCuVO$_4$. We show that $1/T_1$ in the proposed spin-nematic state has qualitatively different features to conventionally ordered magnets, and propose this as an unambiguous test of spin-nematic order.\\[4pt] [1] L.E. Svistov et al., JETP Letters {\bf 93}, 21 (2010).\\[0pt] [2] M.E. Zhitomirsky and H. Tsunetsugu, EPL {\bf 92}, 37001 (2010).\\[0pt] [3] A. Smerald and N. Shannon, Phys. Rev. B {\bf 84}, 184437 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N15.00007: Double Magnetic Field-induced Phase Transitions in the Spin-1/2 Alternating Chain System AgVOAsO$_{4}$ Franziska Weickert, Alexander A. Tsirlin, Monika Gamza, Albin Demuer, Alexander Steppke, Ramesh Nath, Helge Rosner The new spin-1/2 compound AgVOAsO4 shows one-dimensional magnetic behavior and a spin gap of about 14 K. The crystal structure of AgVOAsO4 is rather complex with alternating spin chains aligned along the [110] and [110] direction. The experimental magnetic susceptibility yields values of 40 K and 26 K for J1 and J1$'$, respectively. The magnetization curve taken at 1.5 K cannot be fully described by only two coupling constants, which points to sizable inter chain coupling. Furthermore, the magnetization shows the closing of the spin gap at Hc1 =10.5 T and a saturation at Hc2=48.5 T. In the talk, we report the magnetic field - temperature (H-T) phase diagram of AgVOAsO4 measured by specific heat and magnetization experiments. The specific heat taken in high DC fields up to 28 T reveals a distinct double anomaly around 4 K and 2 K. Magnetization experiments follow this double structure down to mk temperatures and reveal a variety of anomalies close to the critical field Hc1 in AgVOAsO4. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N15.00008: Order and excitations near quantum criticality in quasi-1D S$=$1/2 easy-plane antiferromagnet Cs2CoCl4 Gheorghe Pascut, Radu Coldea, Franz Demmel, Zbigniew Tylczynski We explore the magnetic order and spin dynamics in the quasi-one-dimensional spin-1/2 easy-plane anisotropy antiferromagnet Cs2CoCl4 in a magnetic field applied close to the easy-plane which drives a transition from spontaneous long-range magnetic order to a gapped quantum paramagnet. The commensurate antiferromagnetic order observed at low fields is stable over a wide field range but is replaced by an incommensurate magnetic order (spin density wave) just below the transition to paramagnetic. The main result is the observation of the new incommensurate magnetic phase which was not seen experimentally prior to this work and was also not predicted theoretically. Deep in the paramagnetic phase the excitations are sharp, gapped magnons with minima at the incommensurate wavevectors of the magnetic order below BC $=$ 2.36(2) T and the dispersion relations give values for the intra- and inter-chain couplings. In addition to one magnon excitations at high energies we also observe weak magnetic continuum scattering, which becomes stronger upon approaching the critical field from above and is attributed to multi-magnon transverse field scattering processes. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N15.00009: Phase diagram of frustrated ladder and 2D antiferromagnets Alexandros Metavitsiadis, Daniel Sellmann, Sebastian Eggert We investigate the low energy properties of the frustrated two leg diagonal ladder exhibiting both intra- and inter-chain frustration. The renormalization group is used to obtain the phase diagram while varying the microscopic lattice parameters. We particularly emphasize the role of the in-chain marginal operators, which is tuned by the in-chain frustration and can promote a dimer phase in the system. Finally, the physics of the quasi one dimensional diagonal ladder is incorporated into a two dimensional square lattice since the former is used as the primary structure to build up the square lattice. Within the validity of our method, the classical phases---a N\'{e}el antiferromagnet and a collinear antiferromagnet---are predicted. The results are compared to numerical DMRG calculations. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N15.00010: Quantum criticality and fractional charge excitations in itinerant ice-rule systems Masafumi Udagawa, Hiroaki Ishizuka, Yukitoshi Motome ``Ice rule'' is a configurational constraint on Ising-type variables defined on tetrahedron-based lattices, such as a pyrochlore lattice, so that two out of the four sites on a tetrahedron are in the opposite state to the other two. This concept plays an important role in many systems, such as water ice I$_h$, magnetite Fe$_3$O$_4$, and spin ice materials Ho(Dy)$_2$Ti$_2$O$_7$. Under the ice-rule constraint, the ground state is disordered and retains macroscopic degeneracy. Nevertheless, the ice-rule configuration is not completely random but has a peculiar spatial structure with quasi-long-range correlation. It is interesting to ask how itinerant electrons change their properties by coupling to this anomalous spatial structure. To answer this problem, we adopt an extended Falicov-Kimball model as a minimal model, in which itinerant electrons interact with localized charge degrees of freedom under the ice rule. We exactly solve this model on a loop-less variant of the tetrahedron-based lattices, a tetrahedron Husimi cactus and clarify the ground-state phase diagram. The exact solution reveals a quantum critical point separating two insulating phases, where a novel non-Fermi-liquid behavior emerges. We also discuss the nature of fractional excitations breaking the ice-rule manifold. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N15.00011: Doped Mott insulators in (111) bilayers of perovskite transition-metal oxides with the strong spin-orbit coupling Satoshi Okamoto We study the electronic properties of Mott insulators realized in bilayers of perovskite transition-metal oxides grown along the [111] crystallographic axis. The low-energy effective Hamiltonians for such Mott insulators are derived in the presence of the strong spin-orbit coupling. These models are characterized by the antiferromagnetic Kitaev interaction and the antiferromagnetic or ferromagnetic Heisenberg interaction depending on the $d$ orbital occupancy. From exact diagonalization analyses on finite clusters, Kitaev spin liquid phases are shown to be confined in narrow parameter regimes. Slave-boson mean-field analyses indicate the possibility of non-trivial superconducting states induced by carrier doping into the Mott-insulating parent systems. We also discuss the possible experimental realization of these systems in $4d$ and $5d$ transition-metal oxides. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N15.00012: $^{27}$Al-NMR Study of the Spinel Compound CoAl$_{2}$O$_{4}$ Beas Roy, Abhishek Pandey, David C. Johnston, Yuji Furukawa CoAl$_{2}$O$_{4}$, a geometrically frustrated magnet, is believed to be located in the vicinity of a quantum melting point of the AFM ordered state. In CoAl$_{2}$O$_{4}$, magnetic frustration originates from Co$^{2+}(S =$ 3/2) spins on the tetrahedral A-site via non-magnetic Al ions occupying the octahedral B-site. To study the magnetic properties of CoAl$_{2}$O$_{4}$ from a microscopic point of view, we have carried out $^{27}$Al-NMR measurements using a well-characterized powder sample of CoAl$_{2}$O$_{4}$. The temperature dependence of the magnetic susceptibility $\chi $ shows a broad peak around 15 K and does not show any difference in zero-field-cooled and field-cooled measurements. $^{27}$Al-NMR spectra at 9.3 MHz ($H =$ 0.84 T) show seven peaks characterized by quadrupolar splitting with $\nu_{\mathrm{Q}}=$ 0.55 MHz at temperatures above 10 K. Below 10 K, the spectrum broadens suddenly. We also observe a peak of 1/$T_{1}$ of $^{27}$Al at 10 K. These NMR results clearly indicate magnetic ordering at 10 K, although $\chi $ does not exhibit any signature of long-range magnetic ordering. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N15.00013: What controls the sign of exchange-induced phonon splitting in ACr$_2$O$_4$ spinels? Aleksander Wysocki, Turan Birol, Craig J. Fennie The interplay of spin and lattice degrees of freedom can lead to a variety of fundamentally and technologically interesting phenomena. In ACr$_{2}$O$_{4}$ spinels, it has been well established that antiferromagnetic order alone can lower the symmetry of a crystal resulting in a splitting of degenerate phonon frequencies without any structural distortion. A simple model based on nearest neighbor exchange striction has been proposed and confirmed by a novel first-principles approach. Recently however it has been suggested that magnetically induced phonon splitting is universally controlled by the nondominant exchange interaction. In this talk we present our recent first principles study of magnetically induced phonon anisotropy in ACr$_{2}$O$_{4}$ (A$=$Mg, Zn, Cd, Hg) spinels. We demonstrate that the different spin ordering patterns observed in the different spinel compounds can lead to an opposite sign of phonon splitting. This naturally explains the difference in sign experimentally observed for ZnCr$_{2}$O$_{4}$ compared with CdCr$_{2}$O$_{4}$, which have very different magnetic ground states. Additionally, we show that the \textit{ab initio }values for the phonon frequencies can be very well fitted to the previously proposed spin-phonon coupling model including only the nearest neighbor exchange interaction. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N15.00014: Competing Jahn-Teller and spin Jahn-Teller ordering in $A$Cr$_2$O$_4$ spinels Moureen Kemei, Stephanie Moffitt, Matthew Suchomel, Daniel Shoemaker, Ram Seshadri Magnetic ordering is strongly linked to structural distortions in the frustrated antiferromagnets ZnCr$_2$O$_4$ and MgCr$_2$O$_4$. These systems undergo spin Jahn-Teller distortions at the onset of magnetic order. The addition of magnetic $A$ site cations in $A$Cr$_2$O$_4$ spinels can relieve frustration. High-resolution variable-temperature synchrotron powder X-ray diffraction, detailed magnetic studies, and heat capacity measurements show that dilute amounts of Jahn-Teller active Cu$^{2+}$ or Co$^{2+}$ on the $A$ sites of these spinels have different effects on structure but similar effects on magnetism. Partial replacement of $A$ by Cu$^{2+}$ generates Jahn-Teller distortions at temperatures above the endmember Neel temperatures, yet spin interactions remain frustrated to $\sim$ 12 K. This contrasts with Co$^{2+}$ substitution which also maintains frustration, but results in a suppression of spin Jahn-Teller ordering in ZnCr$_2$O$_4$. We report decoupled Jahn-Teller and spin Jahn-Teller ordering in the canonical frustrated systems ZnCr$_2$O$_4$ and MgCr$_2$O$_4$ that is tunable by varying the identity of the magnetic $A$ site substituent. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N15.00015: Volume Sensitivity and Effect of Fluctuations on the Frustrated Magnetism in YMn$_2$ Brian P. Neal, Warren E. Pickett Cubic Laves phase (C15) YMn$_2$ with its highly frustrated pyrochlore type sublattice of Mn sites, is one of a small but growing class of ordered magnets that lie close to a quantum critical point at stoichiometry. Its ground state displays long-spiral helical magnetic order that is highly sensitive to volume, disappearing due the substitution of 3\% Sc for the larger Y atom (chemical pressure), or by application of just 0.4 GPa pressure. The large change of volume (5\%) upon ordering (T$_N$ = 100 K) argues for itinerant magnetism, and in recent years there have been developments in modeling magnetic fluctuations in itinerant magnets near the ordering point. We extend earlier results of Terao and Yamada on the first principles based energetics versus volume, and quantify the sensitivity of the magnetic state to pressure. The effects of fluctuations within an itinerant picture will be discussed. [Preview Abstract] |
Session N16: Focus Session: Molecules on Surfaces
Sponsoring Units: GMAG DMPChair: Sebastian Loth, Center for Free-Electron Laser Science, Hamburg
Room: 318
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N16.00001: Kondo-like Resonances in the high spin MnPc. Atomic and Molecular Theoretical Approach Maria Soriano, David Jacob, Juan Jose Palacios In recent years, Kondo$-$like resonances have been measured by different experimental groups in the 3$\backslash$2 high spin Manganese Phthalocyanine (MnPc) on different kinds of surfaces [1,3]. With the aim to understand these resonances we have performed Dynamical Mean Field Theory calculations based on models extracted from Density Functional Theory calculations and Green's function formalism [4,5]. Two types of models are considered: one based on atomic d orbitals and one based on frontier molecular orbitals which contain the spin of the molecule.\\[4pt] [1] Ying-Shuang Fu et. al. Phys. Rev. Lett. 99, 256601. 2007.\\[0pt] [2] K. J. Franke et. al. Science 332, 940. 2011.\\[0pt] [3] A. Str\'{o}zecka et. al. Phys. Rev. Lett. 109, 147202. 2012.\\[0pt] [4] ANT.G03. www.alacant.dfa.ua.es. \\[0pt] [5] D. Jacob et. al. Phys. Rev. B. 82, 195115. 2010. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N16.00002: DFT$+$U studies of atomic scale magnetism: A curious case study for future spintronic devices Shruba Gangopadhyay, Hossein Hashemi, Barbara Jones Atomic scale magnetism attracts interest due to both its possible application to nanoscale spintroic devices, and due to its inherent interest as a source of basic quantum mechanical interactions. We work together with the local Scanning Tunneling Microscopy (STM) team to match our calculations to experiment, and in the process learn much which can't be measured with the STM. In particular we use DFT$+$U to calculate the properties of magnetic atoms on nanolayers of insulator on top of a metal such as silver. In this talk we report the results of detailed calculations of singles and dimers of Mn on MgO/Ag. As time permits we may include our calculations of other magnetic adatoms for comparison. We find that the local interactions are very different for the three stable binding sites on this surface, both for charge and spin densities. Using on onsite Hubbard U parameter which we determine from first principles, we are able to study the variability of the magnetic moment between the binding sites, as well as determine the lowest energy binding site. The magnetic adatoms affect the surrounding interface layer in unexpected ways. We are able to obtain interesting insights which help us understand how magnetism propagates along surfaces as well as between interfaces. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N16.00003: High magnetic anisotropy of magnetic atoms on thin MgO films on Ag(001) Susanne Baumann, Ileana G. Rau, Christopher P. Lutz, Andreas J. Heinrich High quality thin films of magnesium oxide on silver (MgO/Ag(001)) are obtained by magnesium evaporation in an oxygen atmosphere. MgO is often used as insulating layer in magnetic tunnel junctions. Therefore, the interplay of magnetic atoms with MgO is of interest e.g. to the hard drive industry. We characterize the thin films by a combination of scanning tunneling microscopy (STM) and atomic force microscopy (AFM). In particular, we determine the thickness of the deposited layers by a combined use of the two tools. We find that single transition metal atoms, such as Iron and Cobalt, deposited on the thin oxide film show inelastic tunneling steps at higher voltages compared to other insulating layers. The inelastic tunneling spectroscopy (IETS) is used to detect the discrete spin excitations of these atoms. The measured IETS steps indicate high magnetic anisotropies. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N16.00004: Magnetic excitations of molecular spins on a superconductor Invited Speaker: Katharina J. Franke Single magnetic molecules on a superconducting substrate present a model system to study the influence of a local magnetic moment on the superconducting state at the atomic scale. The magnetic moment of the adsorbate interacts with the Cooper pairs by exchange coupling and tends to break them apart [1]. Signatures of this interaction are localized states in the superconducting gap, which can be probed by tunneling spectroscopy [2,3]. On the other hand, the quasi-free electrons in the substrate screen the localized spin via the Kondo effect. The delicate balance between these phenomena determines the resulting ground state of the system. Using scanning tunneling spectroscopy, we show that the interaction of paramagnetic molecules with a superconducting lead surface is very sensitive to the details of the atomic scale surrounding [4]. Depending on the interaction strength, the magnetic moment is able to perturb the Cooper pairs, or the superconducting state is unaffected.\\[4pt] [1] H. Shiba, Prog. Theor. Phys. 40, 435 (1968)\\[0pt] [2] A. Yazdani, et al. Science 275, 1767 (1997)\\[0pt] [3] S.-H. Ji, et al. Phys. Rev. Lett. 100, 226801 (2008)\\[0pt] [4] K. J. Franke, G. Schulze, J. I. Pascual, Science 332, 940 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N16.00005: Probing superexchange coupling in atomically fabricated d-metal complexes Benjamin Bryant, Anna Spinelli, Marjolein Gerrits, Sander Otte Magnetic coupling between transition metal atoms that are linked through ligand p-orbitals relies on the virtual exchange of electrons between neighboring sites. The characteristics of the resulting superexchange coupling rely on a complex interplay between electron hopping and Coulomb interaction. In this talk I will review recent experiments on individual superexchange coupled d-metal atoms placed inside a covalent surface network. By using low temperature scanning tunneling microscopy, Fe atoms may be positioned in a Cu$_{2}$N lattice with atomic precision, and their quantum-magnetic properties probed. Our experiments reveal novel insights into the resulting p-orbital mediated magnetic coupling, that are of importance in the fields of molecular magnetism and strongly correlated transition metal oxides. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N16.00006: Exploring the magnetic properties of metallophthalocyanines on a thin insulator Ben Warner, Fadi El Hallak, Gabriel Aeppli, Mats Persson, Cyrus F. Hirjibehedin The scaling of electrical components to the atomic-scale limit has led to a great deal of interest in molecular electronics. Further proposals outline the use of magnetic molecules in new applications in information technology and spintronics. Since the electronic and magnetic properties of a molecule can be modified by interactions with the surfaces on which they are deposited, understanding these changes is of significant importance. Here we present studies of metal-doped pthalocyanine (MPc) molecules deposited on the thin insulator copper nitride (Cu$_2$N). FePc molecules have been shown to display a large magnetic anisotropy on copper oxide, which is also a thin insulator [1]. Using STM imaging and theoretical calculations we investigate how the interaction of the surface with the molecule varies and how this can affect the charge transport through the molecules. Through the application of a magnetic field and both elastic and inelastic spectroscopy, we gain access to the magnetic states of the molecule. In addition, by imaging the molecules at different bias voltages, we are able to probe the different molecular orbitals and explore how they are modified by interactions with the surface.\\[4pt] [1] N. Tsukahara et al., Phys. Rev. Lett. 102, 167203 (2009) [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N16.00007: Spin-resolved measurements of single molecular magnets on graphene Jens Brede, Regis Decker, Joerg Schwoebel, Maciej Bazarnik, Roland Wiesendanger The use of magnetic molecules opens a gateway to a flexible design of novel spintronic devices to store, manipulate, and read spin information at the nanoscale. Crucial is the precise knowledge of molecular properties at the interface towards an electrode. Progress in this field relies on resolving and understanding the physics at the relevant interfaces. In particular the role of individual molecular constituents and the impact of the atomic environment on molecular properties determine device relevant parameters, such as conductance and spin polarization. Recently, the incorporation of a graphene sheet to electronically decouple molecules from a ferromagnetic surface has been addressed by surface averaging high-resolution electron energy loss spectroscopy. Here, we applied spin-polarized scanning tunneling microscopy to resolve the physics of the molecule-graphene-ferromagnet interface. The analysis focuses on different phthalocyanine molecules adsorbed on cobalt-intercalated graphene on Ir(111). The phthalocyanine constitutes of an organic macrocyclic ligand and can be functionalized with various metal ions in order to modify, e.g. the molecular spin state. We will discuss the spin-dependent transport from magnetic surfaces through such molecules. In particular, the spin polarization of molecular frontier orbitals is resolved with sub-molecular spatial resolution and the variations in the lifetimes of different orbitals are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N16.00008: A first-principles study of a single-molecule magnet Mn12 adsorbed on Bi(111) Kyungwha Park, Jun-Zhong Wang Recently, elemental Bi and Bi-based alloys have attracted a lot of attention due to unique quantum properties of their surface states induced by strong spin-orbit coupling. A single-molecule magnet Mn12 is known to be a prototype molecular magnet with significant magnetic anisotropy caused by spin-orbit coupling. Despite a great effort to fabricate monolayers of single-molecule magnets on various substrates, there are few studies of single-molecule magnets on strongly spin-orbit coupled substrates. Here we present our theoretical study of electronic and magnetic properties of single-molecule magnets Mn12 adsorbed on a strongly spin-orbit coupled semi-metallic Bi surface without any linker molecules. This work was motivated by a recent low-temperature scanning tunneling microscopy (STM) experiment where individual single-molecule magnets Mn12 were grafted on Bi. We apply density-functional theory (DFT) including on-site Coulomb repulsion U and self-consistent spin-orbit coupling, to two adsorption geometries of Mn12 on Bi. We compare our calculated electronic and magnetic properties of the Mn12 molecule on Bi with those of an isolated Mn12. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N16.00009: STM Studies of Mn$_{12}$-Ph K. Reaves, K. Kim, K. Iwaya, T. Hitosugi, H. Zhao, K.R. Dunbar, H.G. Katzgraber, W. Teizer Mn$_{12}$-Ph displays tunneling of quantized magnetization below 3K. In other Mn$_{12}$ ligand variants this magnetic behavior can alter the electronic behavior of the molecule making them good candidates for a molecular logic gate or q-bit. Mn$_{12}$O$_{12}$(C$_6$H$_5$COO)$_{16}$ (referred to as Mn$_{12}$-Ph) has a Mn$_{12}$ core with 16 Phenyl ligands and is deposited via spray injection onto surfaces of highly oriented pyrolytic graphite (HOPG) and other surfaces. We report Mn$_{12}$-Ph in isolation, resembling single molecules with metallic core atoms and organic outer ligands. The local tunneling current observed within the molecular structure shows a strong bias voltage dependence, which is distinct from that of the surface. Further, evidence of internal inhomogeneity in the local density of states has been observed with high spatial resolution, and this inhomogeneity appears to be due to localized metallic behavior. These results facilitate magneto-metric studies of single molecule magnets in isolation. As compared to bulk crystal studies, our experiments allow the specific investigation of atomic sites in individual molecules. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N16.00010: Magnetic Relaxation in Iron Chains of Phthalocyanine Thin Films Thomas Gredig, Daniel Javier, Mathew Werber, Matthew Byrne Self-assembled iron chains are formed in metallo-organic thin films based on the small iron phthalocyanine molecule. The chains are grown parallel to the substrate and the mean chain length is controlled via deposition parameters from 30 -- 300 nm. The strong intra-chain coupling with weak inter-chain coupling leads to ferromagnetic behavior below the critical temperature. After application of a magnetic saturation field, the remanent magnetic moment is not stable when measured over time scales of 10$^{\mathrm{4}}$ s. The magnetic relaxation can be fit to a stretched exponential function, which yields the mean relaxation time and a stretch exponent. The temperature-dependent peak of the relaxation time occurs at lower temperatures for shorter iron chains that also have smaller coercivities. This means that by templating iron phthalocyanine thin films both magneto-crystalline anisotropy and inter-grain interactions can be selected. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N16.00011: Binding Structures of Diatomic Molecules to Co-Porphyrins on Au(111) Studied by Scanning Tunneling Microscopy Soon-Hyeong Lee, Yun Hee Chang, Howon Kim, Won Jun Jang, Yong-Hyun Kim, Se-Jong Kahng Axial bindings of diatomic molecules to metalloporphyrins involve in the dynamic processes of biological functions such as respiration, neurotransmission, and photosynthesis. The binding reactions are also useful in sensor applications and to control molecular spins in metalloporphyrins for spintronic applications. Here, we present the binding structures of diatomic molecules to surface-supported Co-porphyrins studied using scanning tunneling microscopy. Upon gas exposure, three-lobed structures of Co-porphyrins transformed to bright ring shapes on Au(111), whereas H2-porphyrins of dark rings remained intact. The bright rings are explained by the structures of reaction complexes where a diatomic ligand, tilted away from the axis normal to the porphyrin plane, is under precession. Our results are consistent with previous bulk experiments using X-ray diffraction and nuclear magnetic resonance spectroscopy. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N16.00012: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N16.00013: Molecule-induced Spin Rotation of Photoelectrons from FePc on Fe(110) Andreas Sandin, J.E. (Jack) Rowe, Daniel Dougherty, Elio Vescovo We have studied sub-monolayers to multi-layers of iron phthalocyanine (FePc) adsorbed on $\sim$ 10-20 monolayer epitaxial films on Fe(110) on W(110). We find that the spin-resolved photoemission changes rapidly as a function of coverage and the initial (majority spin axis along [110] rotates by $\sim$ 30 degrees for sub-monolayer coverage and then becomes unpolarized at $\sim$ 1 monolayer (ML). The coverage is determined by work function measurements which show that the initial work function of clean Fe(110) of 5.0 eV decreases monotonically to a value of $\sim$ 3.8 eV at a coverage that we assign as $\sim$ 1 monolayer of FePc. These values were determined from the measurements of the photoelectron spectrum using the low-energy vacuum-level cutoff of a biased sample. Our spin-resolved data for clean Fe(110) show highly spin-polarized photoelectrons from the Fermi energy to values about 3.5 eV below the Fermi energy for an applied B-field along [110] both for majority-spin and minority-spin electrons. The polarization is about 60{\%} at -3.2 eV below E-Fermi. For 0.13 ML adsorbed FePc the spin polarization is somewhat reduced and is rotated from [110] towards [100] in the plane of the sample. We interpret this rotation as due to a strong coupling of the orbital moment of FePc with the conduction electrons of the Fe substrate. [Preview Abstract] |
Session N17: Focus Session: Manganite Dynamics and Structure
Sponsoring Units: DMP GMAGChair: Dmitry Reznik, University of Colorado
Room: 319
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N17.00001: Magnetic structure and dynamics of Rh-doped Sr$_2$IrO$_4$ probed by resonant x-ray scattering J.P. Clancy, H. Gretarsson, Jungho Kim, M.H. Upton, G. Cao, Young-June Kim The physics of 5d iridates has recently attracted considerable attention due to the potential for novel electronic and magnetic ground states driven by strong spin-orbit coupling (SOC). One material which has attracted particular interest is the layered perovskite Sr$_2$IrO$_4$, which has been proposed as the first experimental realization of a spin-orbital Mott insulator with a j$_{eff}$ = 1/2 ground state [1,2]. It has been shown that by substituting Ir$^{4+}$ (5d$^5$) ions for Rh$^{4+}$ (4d$^5$), the strength of the SOC in this system can be tuned through a series of electronic phase transitions [3]. We have performed resonant magnetic x-ray scattering (RMXS) and resonant inelastic x-ray scattering (RIXS) measurements to determine the effect of Rh-doping on the magnetic structure and excitation spectrum of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$. We find that increasing Rh concentration results in (i) suppression of the magnetic transition temperature, (ii) a doping-induced change in magnetic structure, (iii) alteration of the magnon dispersion relation, and (iv) significant reduction of magnon lifetimes. [1] B.J. Kim et al, PRL 101, 076402 (2008). [2] B.J. Kim et al, Science 323, 1329 (2009). [3] T.F. Qi et al, PRB 86, 125105 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N17.00002: Strain control of spin and orbital transitions in La$_2$NiO$_4$ Choong H. Kim, Craig J. Fennie We have studied the electronic and magnetic structure of the layered nickelates, La$_2$NiO$_4$ within density functional theory. We show that biaxial strain induces a high spin to low spin transition, which coincides with a significant change in the $x^2-y$ and $3z^2-r^2$ orbital occupancy. We discuss the role of the on-site Coulomb interaction, the crystal field, and prospects for the strain control of the spin and orbital state. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N17.00003: Magnetic and Orbital Orders Coupled to Negative Thermal Expansion in Mott Insulators, {Ca$_{2}$Ru$_{1-x}$M$_{x}$O$_{4}$} (M = 3d transition metal ion) T.F. Qi, O.B. Korneta, L. Li, Jiangping Hu, S. Parkin, G. Cao {Ca$_{2}$RuO$_{4}$} is a structurally-driven Mott insulator with a metal-insulator transition at $T _{MI}$ = 357K, followed by a well-separated antiferromagnetic order at $T _{N}$ = 110 K. Slightly substituting Ru with a 3d transition metal ion M effectively shifts $T _{MI}$ and induces exotic magnetic behavior below $T _{N}$. Moreover, M doping for Ru produces negative thermal expansion in {Ca$_{2}$Ru$_{1-x}$M$_{x}$O$_{4}$} (M = Cr, Mn, Fe or Cu); the lattice volume expands on cooling with a total volume expansion ratio, $\Delta$V/V, reaching as high as 1\%. The onset of the negative thermal expansion closely tracks $T _{MI}$ and $T _{N}$, sharply contrasting classic negative thermal expansion that shows no relevance to electronic properties. In addition, the observed negative thermal expansion occurs near room temperature and extends over a wide temperature interval [1, 2]. These findings underscores new physics driven by a complex interplay between orbital, spin and lattice degrees of freedom.\\[4pt] [1] T.F. Qi, O.B. Korneta, S. Parkin, L.E. DeLong, P. Schlottmann and G. Cao, Phys. Rev. Lett. 105 177203 (2010)\\[0pt] [2] T. F. Qi, O. B. Korneta, S. Parkin, Jianping Hu and G. Cao, Phys. Rev. B 85 165143 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N17.00004: THz magneto-electric excitations in the chiral compound Ba3NbFe3Si2O14 Sophie de Brion, Laura Chaix, Florence Levy-Bertrand, Virginie Simonet, Rafik Ballou, Benjamin Canals, Pascal Lejay, Jean-Blaise Brubach, Gael Creff, Fabrice Willaert, Pascale Roy, Andres Cano The langasite Ba$_3$NbFe$_3$Si$_2$O$_{14}$ displays a chiral structure and orders magnetically with a Neel temperature $T_N$=27K. We have determined its terahertz (THz) spectrum by means of synchrotron-radiation measurements. Three different types of excitation are present. The first one, at 13cm$^{-1}$, disappears at T$_N$ and is assigned to magnons. The others, at 23cm$^{-1}$ and 29cm$^{-1}$, persist up to four times $T_N$. According to their selection rules, they are interpreted as rotational modes of the lattice whose magneto-electric activity reveals a structural transition into a polar helical state. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N17.00005: Orbital physics in RIXS Krzysztof Wohlfeld, Pasquale Marra, Markus Grueninger, Thorsten Schmitt, Jeroen van den Brink In contrast to magnetism, phenomena associated with the orbital degrees of freedom in transition metal oxides had always been considered to be very difficult to observe. However, recently resonant inelastic x-ray scattering (RIXS) has established itself as a perfect probe of the orbital excitations [1] and orbital order [2] in transition metal oxides. Here we give a brief overview of these recent theoretical and experimental advances which have inter alia led to the observation of the separation of the spin and orbital degree of freedom of an electron~[1, 3].\\[4pt] [1] J. Schlappa, K. Wohlfeld, K. J. Zhou, M. Mourigal, M. W. Haverkort, V. N. Strocov, L. Hozoi, C. Monney, S. Nishimoto, S. Singh, A. Revcolevschi, J.-S. Caux, L. Patthey, H. M. R{\o}nnow, J. van den Brink, T. Schmitt, Nature 485, 82 (2012).\\[0pt] [2] P. Marra, K. Wohlfeld, J. van den Brink, Phys. Rev. Lett. 109, 117401 (2012).\\[0pt] [3] K. Wohlfeld, M. Daghofer, S. Nishimoto, G. Khaliullin, J. van den Brink, Phys. Rev. Lett. 107, 147201 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N17.00006: Magnetism and Magnetic Order in La$_{2}$CuRuO$_{6}$ Matthew Smylie, Xu Luo, Ulrich Welp, Wai-Kwong Kwok, Howard Blackstead, Brendan Benapfl, Paul McGinn Long-range magnetic order has been observed in the insulating double perovskite compound La$_{2}$CuRuO$_{6}$. This monoclinic compound shows a rock salt like ordering of the B sites in the double perovskite A$_{2}$BB'O$_{6}$ lattice. We show that elevated processing temperatures improve the magnetic properties of the material, possibly by reducing the number of antisite defects between the Cu and Ru ions. In polycrystalline samples, microwave resonance and dc SQUID susceptibility measurements indicate a ferrimagnetic or antiferromagnetic ground state at low temperatures (T \textless\ 19 K). Specific heat measurements also show a transition consistent with the magnetization data. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N17.00007: Hourglass dispersion in overdoped single-layered manganites Invited Speaker: Markus Braden Two non-metallic single-layered manganites with a doping level well above half doping, Nd$_{\mathrm{0.33}}$Sr$_{\mathrm{1.67}}$MnO$_{\mathrm{4}}$ and Pr$_{\mathrm{0.33}}$Ca$_{\mathrm{1.67}}$MnO$_{\mathrm{4}}$, exhibit an incommensurate ordering of magnetic, charge and orbital degrees of freedom. Inelastic neutron scattering experiments reveal an hourglass-like excitation spectrum very similar to that seen in various cuprates superconductors, but only for sufficiently short correlation lengths. We find the characteristic features of the hourglass dispersion as the enhanced intensity at the merging of the incommensurate branches, the rotation of intensity maxima at higher energy, and suppression of the outwards-dispersing branches at low energies. The correlation length of the magnetic ordering and the large ratio of intra- to inter-stripe couplings can be identified as the decisive parameters causing the hourglass shape of the spectrum. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N17.00008: Magnetic excitations in a 5d-based double perovskite Ba$_2$FeReO$_6$ Arun Paramekanti, K. Plumb, A. Cook, P. Clancy, A. Kolensikov, Young-June Kim, B.-C. Jeon, T.-W. Noh There is great interest in double perovskite materials, from a fundamental viewpoint of studying correlated electron magnetism as well as spintronics applications. We report theoretical calculations and experimental powder inelastic neutron scattering data on magnetic excitations in the 5d-based double perovskite Ba$_2$FeReO$_6$. We find evidence of multiple spin wave branches consistent with local moment magnetism on Fe sublattice coexisting with highly correlated and spin-orbit coupled local moments on Re. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N17.00009: Effects of Doping and Coulomb Correlations on Tc and Competing Phases in Half-metallic Double Perovskites Daniel Kestner, Onur Erten, Oinam Nganba Meetei, Mohit Randeria, Nandini Trivedi Double perovskites such as $Sr_{2}FeMoO_{6}$ (SFMO) are rare examples of materials with half-metallic ground states and a ferrimagnetic $T_{c}$ much greater than room temperature. We have shown that the electronic and magnetic properties of SFMO are well described by a generalized double exchange model [1] for itinerant electrons from Mo coupled to localized Fe spins. However, the simplest model proves inadequate when SFMO is electron-doped by La-substitution on the Sr sites. Ignoring Coulomb correlations for the itinerant electrons, the ferromagnetism of Fe spins becomes progressively weaker with electron doping, and eventually the model is unstable to a metallic antiferromagnetic ground state. This is in contradiction with experiments [2], which find a ferromagnetic $T_{c}$ increasing with carrier concentration and no evidence for an antiferromagnetic state up to $SrLaFeMoO_{6}$. In this talk we will show that the Hubbard U on the Mo-site is responsible for the observed doping trends. We will show that correlations stabilize the ferromagnetism, with the observed $T_{c}(n)$ behavior, and that the antiferromagnetic metal is not a competitive state for reasonable values of n.\newline[1] O. Erten et al, PRL 107, 257201 (2011)\newline[2] J. Navarro et al, PRB 64, 092411 (2001). [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N17.00010: Predictions for Spin Resolved Spectral Function and Optical Conductivity in Half-metallic Double Perovskites Julia Janczak, Oinam Nganba Meetei, Mohit Randeria, Nandini Trivedi We present the effects of thermal fluctuations and anti-site (AS) disorder on the spin resolved spectral function $A(k,\omega)$ and optical conductivity $\sigma(\omega)$ for half-metallic double perovskite Sr$_2$FeMoO$_6$, which holds great promise in spintronics applications. While both $T\neq 0$ and AS destroy the half metallic state, they produce distinct effects. Increasing $T$ produces smooth broadening in the energy distribution curves of $A(k,\omega)$ while AS produces localized states at specific energies with broad momentum distribution curves for spin up. Our results can be tested directly in spin resolved ARPES experiments. We also calculate $\sigma(\omega)$ by evaluating the Kubo formula in the exact eigenstate basis. We show for $T\neq 0$ the height of the secondary peak in $\sigma(\omega)$, also seen in experiments, tracks the polarization $P$ of conduction electrons, whereas for disordered samples at $T=0$, the weight of the secondary peak indicates the amount of AS. From the spin resolved conductivity, we show that small ($<10\%$) amounts of AS prevalent in real samples has little impact on the spin polarization of the DC current. The features of the optical spectrum provide a relatively simple experimental probe of the polarization and amount of disorder. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N17.00011: Mapping chemical/structural order in double perovskite Sr2-xGdxMnTiO6 by atomic resolution electron microscopy Inmaculada Alvarez, Neven Biskup, Maria Lopez, Mar Garcia-Hernandez, Luisa Veiga, Maria Varela We report on visualizing the chemical and structural order of double perovskite Sr2-xGdxMnTiO6. The antisite disorder of Mn and Ti is detected even at atomic scale at all x, resulting in Mn-rich and Ti-rich regions. For x ?0.75, the majority of manganese ions are in Mn3+ state and are centered in Jahn-Teller distorted MnO6octahedra. The Fourier transformation of atomic resolution images along the [110] zone axis reveals a superstructure that corresponds to the tilting of oxygen octahedra and that doubles the unit cell along [001]c. This superstructure is spatially inhomogeneous and coincides with the regions where B-site ion (Mn/Ti) is displaced along the [110] direction. We discuss these findings in the frame of possible local ferroelectricity and in the light of strong electroresistance observed in Sr1.25Gd0.75MnTiO6. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N17.00012: Structural transition in an unusual $5d$-electron ferromagnetic insulator Zahirul Islam, R.K. Das, J.P.C. Ruff, H. Lee, D.S. Robinson, I.R. Fisher Double-perovskite barium sodium osmate (Ba$_2$NaOsO$_6$) is a ferromagnetic (FM) insulator ($T_c\sim6.8$ K, ordered moment $\sim$0.2 $\mu_B$ per formula unit) with $<110>$ easy axis. We present precision x-ray diffraction studies on single crystals to understand structural symmetry in this rare FM compound. At room temperature there is a subtle splitting of crystal Bragg peaks indicating the global symmetry to be weakly tetragonal. At or slightly above $T_c$, the material becomes orthorhombic. These changes are crucial in lifting the degeneracy of $d$-state manifold and are likely associated with orbital ordering. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N17.00013: Magnetic properties and electronic structure of Cr$_2$(Te$_{\mathrm{1-x}}$W$_{x})$O$_6$ X. Ke, D. Do, H. Zhou, C.R. Dela Cruz, S.D. Mahanti, M. Zhu We report magnetic properties of Cr$_{2}$(Te$_{\mathrm{1-x}}$W$_{x})$O$_{6}$ system combining neutron powder diffraction measurements and first principles electronic structure calculations. Both the end members possess an ordered inverse-trirutile structure, in which there are bilayers of Cr-O separated by a W(Te)-O layer, yet Cr$_{2}$TeO$_{6}$ and Cr$_{2}$WO$_{6}$ display distinct magnetic structures and antiferromagnetic transition temperatures: $T_{\mathrm{N}}$ $\sim$ 92 K for Cr$_{2}$TeO$_{6}$ with antiferromagnetic spin alignment within bilayers, while $T_{\mathrm{N}}$ $\sim$ 45 K for Cr$_{2}$WO$_{6}$ with spins aligned ferromagnetically within the bilayer. Spins belonging to neighboring bilayers are antiferromagnetically coupled for both the compounds. For the mixed system Cr$_{2}$(Te$_{\mathrm{1-x}}$W$_{x})$O$_{6}$, both $T_{\mathrm{N}}$ and sublattice magnetization ($M_{\mathrm{s}}$) reach a minimum ($T_{\mathrm{N}}$ $\sim$ 0 K) for $x$ $\sim$ 0.6, suggesting the existence of a quantum critical point. Electronic structure calculations using \textit{ab initio} density functional theory correctly give the ground state spin configurations for the end compounds (x$=$0,1). We suggest that unoccupied W 5d states play a key role in intra-bilayer ferromagnetic ordering seen in the x$=$1 system. [Preview Abstract] |
Session N18: Focus Session: Spin-Dependent Phenomena in Semiconductors - Spin Orbit and Mesoscopic
Sponsoring Units: GMAG DMP FIAPChair: Jean Heremans, Virginia Polytechnic Institute and State University
Room: 320
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N18.00001: Gate-control of spin polarization waves Luyi Yang, J.D. Koralek, J. Orenstein, D.R. Tibbetts, J.L. Reno, M.P. Lilly We report on control of the persistent spin helix (PSH) in semiconductor quantum wells with tunable spin-orbit (SO) coupling via external gates. The PSH is a collective spin excitation of two-dimensional electron systems that emerges as a new conserved quantity of the SU(2) symmetry. It occurs when the strengths of Rashba and linear Dresselhaus SO coupling are equal. Previously, this effect was demonstrated by a set of samples with different doping asymmetry and well width [1]. Now we fabricate samples with both front and back gates aiming to control Rashba SO coupling continuously and increase the lifetime-enhancement by reducing the symmetry-breaking cubic Dresselhaus term. \\[4pt] [1] J. D. Koralek et al., Emergency of the persistent spin helix in semiconductor quantum wells, Nature 458, 610-613 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N18.00002: Chiral Spin Waves in Fermi Liquids with Spin-Orbit Coupling Ali Ashrafi, Dmitrii Maslov We predict the existence of chiral spin waves--collective modes in a two-dimensional Fermi liquid with the Rashba or Dresselhaus spin-orbit coupling. Starting from the phenomenological Landau theory, we show that the long-wavelength dynamics of magnetization is governed by the Klein-Gordon equations. The standing-wave solutions of these equations describe \lq\lq particles\rq\rq\/ with effective masses, whose magnitudes and {\em signs} depend on the strength of the electron-electron interaction. The spectrum of the spin-chiral modes for arbitrary wavelengths is determined from the Dyson equation for the interaction vertex. We propose to observe spin-chiral modes via microwave absorption of standing waves confined by an in-plane profile of the spin-orbit splitting. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N18.00003: Magnetic control of spin-orbit fields: a first principles study of Fe/GaAs junctions Jaroslav Fabian, Martin Gmitra, Alex Matos-Abiague, Claudia Draxl The possibility to control the spin-orbit fields in semiconductor heterostructures by electric fields has been used to influence the spin dynamics of itinerant electrons. We show that the spin-orbit fields can also be controlled by magnetic fields. On the example of Fe/GaAs junctions we illustrate how the electronic band structure of ferromagnet/semiconductor interfaces, here calculated from first principles for a slab geometry using the FLEUR code, can be mapped to effective spin-orbit field Hamiltonians whose parameters are extracted directly from the band structure, without requiring a priori knowledge of the functional form of the spin-orbit fields, as has been the standard up to now. We show that the spin patterns resulting from the spin-orbit fields change qualitatively as the magnetization orientation of the junction changes in the plane of the interface. The magnetic control of spin-orbit fields is important for transport and optical magnetoanisotropies of ferromagnet/non-magnetic conductor junctions. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N18.00004: Stern-Gerlach effect and spin separation in InGaAs nanstructures Invited Speaker: Makoto Kohda The demonstration of quantized spin splitting by Stern and Gerlach in 1922 is one of the most important experiments in modern physics. We utilized an effective non-uniform magnetic field which originates from Rashba spin orbit interaction (SOI) and demonstrated an experimental manifestation of electronic Stern-Gerlach spin separation in InGaAs based quantum point contacts (QPCs) [1]. Lateral potential confinement in a trench-type QPC creates a spatial modulation of Rashba SOI inducing a spin dependent force Clear conductance plateaus are observed in steps of 2$e^{\mathrm{2}}$/$h $when the strength of Rashba SOI becomes small. However, when the Rashba SOI is enhanced by applying the top gate, a half-integer plateau additionally appears at 0.5(2$e^{2}/h)$, indicating the spin polarized current. We found that the spin polarization of the conduction electrons in this plateau is as high as 70{\%}. Our new approach for generating spin polarization in semiconductor nanostructures provides a way to seamlessly integrate electrical spin generation, manipulation, and detection in a single semiconductor device without the need for either external magnetic fields or magnetic materials. \\[4pt] [1] M. Kohda \textit{et al}. Nature Communications 3, 1082 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N18.00005: Spin blocking effect in symmetric double quantum well due to Rashba spin-orbit coupling Satofumi Souma, Matsuto Ogawa, Yoshiaki Sekine, Atsushi Sawada, Takaaki Koga We report a theoretical study of the spin-dependent electronic current flowing laterally through the In$_{0.53}$Ga$_{0.47}$As/In$_{0.52}$Al$_{0.48}$As double quantum well (DQW) structure, where the values of the Rashba spin-orbit parameter $\alpha_{\rm R}$ are opposite in sign but equal in magnitude between the constituent quantum wells [1]. By tuning the channel length of DQW and the magnitude of the externally applied in-plane magnetic field, one can block the transmission of one spin (e.g., spin-up) component, enabling us to obtain a spin-polarized current. Our experimental progress toward realizing the proposed device is also reported [2]. [1] T. Matsuura, S. Faniel, N. Monta, and T. Koga, Physica E {\bf 42}, 2707 (2010). [2] T. Koga, T. Matsuura, S. Faniel, S. Souma, S. Mineshige, Y. Sekine, and H. Sugiyama, IEICE Trans. Electron. {\bf E95-C}, 770 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N18.00006: Spin-orbit edge states in semiconductor two-dimensional systems L.L. Xu, Shaola Ren, J.J. Heremans, Djordje Minic, C.K. Gaspe, S. Vijeyaragunathan, T.D. Mishima, M.B. Santos The electromagnetic duality between the Aharonov-Casher and the Aharonov-Bohm topological phases can lead to magnetoelectronic edge effects in two-dimensional systems. Based on this duality, we propose and experimentally explore a quantized Hall effect in which magnetization transport may be quantized analogously to charge transport. When the magnetic moment is fully projected, the edge effect is a magnetization dual to the integer quantum Hall effect. An analogy also exists between this dual and the bosonic quantum Hall effect currently under investigation. In experiments we search for edge states induced by the equivalent vector potential from Rashba-type spin-orbit interaction. We use mesoscopic side-gated channel structures on InGaAs/InAlAs heterostructures where backscattering between edge states can experimentally form evidence for edge states. The side-gate voltage varies the effective gauge field and resistance as function of side-gate voltage is measured across the mesoscopic structures at either low applied magnetic field or at fixed magnetic filling factors to obtain states of defined spin (DOE DE-FG02-08ER46532, NSF DMR-0520550). [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N18.00007: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N18.00008: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N18.00009: Antilocalization, quantum coherence and spin coherence in quasi-one-dimensional GaAs/AlGaAs hole wires Shaola Ren, J.J. Heremans, M. Shayegan Antilocalization is measured in quasi-1D (Q1D) lithographic wires fabricated on a GaAs/AlGaAs 2D hole system. Shubnikov-de Haas oscillations show substantial spin-orbit interaction in the asymmetric quantum well. A set of 10 Q1D wires of length 20 $\mu$m and conducting width 300 nm were fabricated. Mobility and density are preserved in the wires, which show predominantly specular boundary scattering, indicating high quality hole wires. Antilocalization is present in both the wires and the unpatterned system, confirming the existence of spin-orbit interaction. The spin and phase coherence times are measured as functions of temperature by fitting the magnetoconductance to antilocalization theory. Q1D antilocalization theory, as used on InSb and InAs wires, does not fit the hole wires well, likely due to a combination of ballistic transport and strong spin-orbit interaction not fully accounted for theoretically. For both wires and unpatterned system the measurements still indicate the spin coherence times and the phase coherence times with the expected temperature dependence. The measurements allow a comparison of the spin coherence times, and of their lengthening under dimensional confinement, with observations on other spin-orbit coupled 2D systems. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N18.00010: The influence of the spin-orbit effect on the 0.7-anomaly: a functional renormalization group approach Olga Goulko, Florian Bauer, Jan Heyder, Jan von Delft In addition to plateaus at integer values of $G_0 = 2e^2/h$, the linear conductance of a quantum point contact shows an anomalous shoulder at around $0.7G_0$ -- the so-called 0.7-anomaly. Although the dependence of the 0.7-anomaly on parameters such as the temperature, the magnetic field, the bias voltage etc. has been widely studied, little is known about the influence of spin-orbit effects. We present a microscopic theory for the 0.7-anomaly, based on a one-dimensional tight binding model with a local on-site interaction, a smooth potential barrier and a homogeneous magnetic Zeeman field. In addition, we introduce Rashba and Dresselhaus terms into the Hamiltonian to capture the effect of spin-orbit coupling. We use a functional renormalization group approach to calculate the influence of interactions on the conductance at zero temperature. In this talk we present our theoretical predictions for the shape of the conductance curve, which depends strongly on the angle of the magnetic field if spin-orbit coupling is present. We also provide a detailed microscopic explanation of how the interplay of the magnetic field, the interaction and the spin-orbit coupling influences the conductance. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N18.00011: Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts J. von Delft, F. Bauer, J. Heyder, E. Schubert, D. Borowski, D. Taubert, B. Bruognolo, D. Schuh, W. Wegscheider, S. Ludwig Despite the simple structure of quantum point contacts, their conductance properties exhibit anomalous features, collectively known as the ``0.7-anomaly'', whose origin is still subject to controversial discussions. We offer a detailed microscopic explanation for the 0.7-anomaly and the zero-bias peak that typically accompanies it: the common origin of both is a smeared van Hove singularity in the local density of states at the bottom of the lowest one-dimensional subband of the point contact, which causes an anomalous enhancement in the Hartree potential barrier, magnetic spin susceptibility and inelastic scattering rate. We present theoretical calculations and experimental results that show good qualitative agreement for the dependence of the conductance on gate voltage, magnetic field, temperature, bias voltage (including the zero-bias peak) and interaction strength. For low field and temperature we predict and observe Fermi-liquid behavior analogous to that known for the Kondo effect in quantum dots. At high energies, however, the analogy between 0.7-anomaly and Kondo effect ceases to be applicable. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N18.00012: Tunable All Electric Spin Polarizer Using A Quantum Point Contact With Two Pairs of In-Plane Side Gates Nikhil Bhandari, James Charles, Maitreya Dutta, Partha Das, Marc Cahay, Richard Newrock, Steven Herbert We report the first experimental investigation of a device consisting of a quantum point contact (QPC) with four gates -- two in-plane side gates in series. The first set of gates (nearest the source contact) is asymmetrically biased to create spin polarization in the channel of the QPC. A symmetric bias is then applied on the second set of side gates (nearest the drain) and varied to tune the location of a conductance anomaly near 0.5 (x2e$^{2}$/h). The experimental results compare well with simulations of the four-gate QPC devices using a Non-Equilibrium Green's Function formalism. The device is shown to be a tunable all-electric spin polarizer. The range of common-mode bias on the first set of gates over which maximum spin polarization can be achieved is much broader for the four-gate structure compared with the case of a QPC with a single pair of in-plane side gates. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N18.00013: ABSTRACT WITHDRAWN |
Session N19: Open Quantum Systems and Decoherence
Sponsoring Units: GQIChair: Daniel Lidar, University of Southern California
Room: 321
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N19.00001: Complementarity of information and the emergence of the classical world Michael Zwolak, Wojciech Zurek We prove an anti-symmetry property relating accessible information about a system through some auxiliary system F and the quantum discord with respect to a complementary system F'. In Quantum Darwinism, where fragments of the environment relay information to observers -- this relation allows us to understand some fundamental properties regarding correlations between a quantum system and its environment. First, it relies on a natural separation of accessible information and quantum information about a system. Under decoherence, this separation shows that accessible information is maximized for the quasi-classical pointer observable. Other observables are accessible only via correlations with the pointer observable. Second, It shows that objective information becomes accessible to many observers only when quantum information is relegated to correlations with the global environment, and, therefore, locally inaccessible. The resulting complementarity explains why, in a quantum Universe, we perceive objective classical reality, and supports Bohr's intuition that quantum phenomena acquire classical reality only when communicated. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N19.00002: Quantum Decoherence with Bath Size: Dynamics, Randomness, and Connectivity Mark Novotny, Fengping Jin, Kristel Michielsen, Seiji Miyashita, Hans De Raedt The decoherence of a quantum system $S$ coupled to a quantum environment $E$ is considered, where $S+E$ is a closed quantum system. For typical states $X$ of the Hilbert space, i.e. for states chosen randomly from the Hilbert space unit hypersphere, we derive a scaling relation for the sum of the off-diagonal elements of the reduced density matrix $\rho_S$ of $S$. This sum is a measure of the decoherence of $S$, and decreases as $D_E^{-\frac{1}{2}}$ as the dimension of the environment Hilbert space $D_E$ increases. We present long-time calculations of the time dependent Schr\"odinger equation (TDSE) of spin $\frac{1}{2}$ particles comprising $S+E$ in order to test this scaling. The Hamiltonian has uniform or random Heisenberg couplings of a spin chain for $S+E$. Factors that affect the approach to the predicted scaling relation for the Heisenberg $d=1$ ring include how quickly and successfully the dynamics drives an initial configuration to an $X$ state, and this depends on the randomness of the coupling strengths in the Hamiltonian and the addition of other connections either within $E$ or between $S$ and $E$. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N19.00003: Open Quantum Walks: Microscopic Derivation and Generalised Master Equation Francesco Petruccione, Ilya Sinayskiy Recently, a formalism for discrete time open quantum walks was introduced [S. Attal et al., J. Stat. Phys., 147 (2012) 832; S. Attal, F. Petruccione, I. Sinayskiy, Phys. Lett. A, 376 (2012) 1545]. This formalism is exclusively based on the non-unitary dynamics induced by the environment. This approach rests upon the implementation of appropriate completely positive maps. Open quantum walks include the classical random walk and through a realization procedure a connection to the Hadamard quantum walk is established. Open quantum walks allow for an unravelling in terms of quantum trajectories. It was shown [I. Sinayskiy and F. Petruccione, QIP 11 (2012) 1301] that open quantum walks can perform universal quantum computation and can be used for quantum state engineering. Here, we present the microscopic derivation of open quantum walks. A walk on a graph is considered and transitions between vertices are mediated by the interaction of the walker with a shared bosonic environment. The reduced dynamics of the walker is shown to be described in terms of a generalised Markovian master equation. The time discretization of the master equation gives raise to an open quantum walk. Based on the class of microscopic models considered here possible physical implementations are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N19.00004: Efficient simulation of stochastically-driven quantum systems Mohan Sarovar, Matthew Grace The simulation of noisy quantum systems is critical for accurate modeling of many experiments, including those implementing quantum information tasks. The expansion of a stochastic equation for the coupled evolution of a quantum system and an Ornstein-Uhlenbeck process into a hierarchy of coupled differential equations is a useful technique that simplifies the simulation of stochastically-driven quantum systems. We expand the applicability of this technique by completely characterizing the class of diffusive Markov processes for which a useful hierarchy of equations can be derived. The expansion of this technique enables the examination of quantum systems driven by non-Gaussian stochastic processes with bounded range. We present an application of this extended technique by simulating Stark-tuned Forster resonance transfer in Rydberg atoms with non-perturbative position fluctuations. \newline \newline 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] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N19.00005: Realizing a lattice-based quantum simulator using circuit quantum electrodynamics Devin Underwood, Will Shanks, Andy Li, James Raftery, Darius Sadri, Jens Koch, Andrew Houck Recent experimental progress in circuit quantum electrodynamics (CQED) has triggered extensive theoretical research on using these systems to implement a CQED lattice-based quantum simulator for non-equilibrium physics. CQED systems are inherently open due to unavoidable photon loss and the ease of replenishing photons through driving. The focus of this research is to experimentally realize proposals focused on building lattice-based simulators, where each lattice site contains a single CQED element. Results will be presented on a kagome lattice of 49 niobium coplanar waveguide resonators, each coupled a single aluminum transmon qubit [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N19.00006: Decoherence and Thermalisation dynamics in many-body systems Derek Lee, Sam Genway, Andrew Ho An isolated quantum system prepared in a pure state will evolve coherently in time. However, local observables of the system can appear thermalised in the sense that the reduced density matrix of a small part of the system approaches the form expected from a thermal Gibbs distribution. This eigenstate thermalisation hypothesis has been demonstrated numerically. We explore the dynamics of how the system approaches this thermalised state. Our previous numerical work on the Hubbard model [Phys. Rev. Lett. 105, 260402 (2010)] has found two dynamical regimes with exponential and Gaussian decay towards the thermal state respectively. We discuss how this can be understood analytically in a generic theory. We will explore the impact of symmetry laws on the dynamics. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N19.00007: Wigner distribution functions for complex dynamical systems: a path integral approach Dries Sels, Wim Magnus, Fons Brosens Starting from Feynman's Lagrangian description of quantum mechanics, we propose a method to construct explicitly the propagator for the Wigner distribution function of a single system. For general quadratic Lagrangians, only the classical phase space trajectory is found to contribute to the propagator. Inspired by Feynman's and Vernon's influence functional theory we extend the method to calculate the propagator for the reduced Wigner function of a system of interest coupled to an external system. Explicit expressions are obtained when the external system consists of a set of independent harmonic oscillators. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N19.00008: Chain representations of Open Quantum Systems and Lieb-Robinson like bounds for the dynamics Mischa Woods This talk is concerned with the mapping of the Hamiltonian of open quantum systems onto chain representations, which forms the basis for a rigorous theory of the interaction of a system with its environment. This mapping progresses as an interaction which gives rise to a sequence of residual spectral densities of the system. The rigorous mathematical properties of this mapping have been unknown so far. Here we develop the theory of secondary measures to derive an analytic, expression for the sequence solely in terms of the initial measure and its associated orthogonal polynomials of the first and second kind. These mappings can be thought of as taking a highly nonlocal Hamiltonian to a local Hamiltonian. In the latter, a Lieb-Robinson like bound for the dynamics of the open quantum system makes sense. We develop analytical bounds on the error to observables of the system as a function of time when the semi-infinite chain in truncated at some finite length. The fact that this is possible shows that there is a finite ``Speed of sound'' in these chain representations. This has many implications of the simulatability of open quantum systems of this type and demonstrates that a truncated chain can faithfully reproduce the dynamics at shorter times. These results make a significant and mathematically rigorous contribution to the understanding of the theory of open quantum systems; and pave the way towards the efficient simulation of these systems, which within the standard methods, is often an intractable problem. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N19.00009: Understanding the role of counter-rotating terms of Rabi Model under dissipation Resul Eryigit, Ferdi Altintas Rabi Hamiltonian is one of the most complete quantum mechanical models that describe the interaction of a qubit with a quantized field which became more relevant with the recent developments in the circuit QED technologies that made possible to obtain strong coupling in the field-qubit interactions. In the dissipative regime, the standart Lindblandian quantum optical master equation with Rabi Hamiltonian leads to unphysical effects such as an increase of total excitation number in the qubit-field system with increasing cavity decay rate. Recently, a new Liouville superoperator describing the loses of the system have been derived [F.Beaudoin, J.M.Gambetta, A.Blais, Phys. Rev. A 84, 043832 (2011)] at the ultrastrong coupling regime. In this study, by using the new dissipators for cavity loses, we have investigated the role of counter-rotating terms on the dynamics of entanglement and quantum discord at ultrastrong coupling regime and provided a comprehensible picture for the role of counter-rotating terms on quantum correlations. Contrary to the standart dissipators case, the steady-state of the system is found to contain non-zero entanglement. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N19.00010: Classical memoryless noise-induced maximally discordant mixed separable steady states Arzu Kurt, Ferdi Altintas, Resul Eryigit Noise is, generally, detrimental to quantum correlations. For some initial states, it has been shown that back-action of the environment or the memory in environment-system interactions can create and/or sustain some of the quantum correlations in the system. In the present study, we have investigated the dynamics of quantum discord and entanglement for two qubits subject to independent global transverse and/or longitudinal memoryless noisy classical fields and have shown that a classical memoryless noise can lead to maximally discordant mixed separable states. Moreover, two independent noises in the system are found to enhance both the steady state randomness and quantum discord in the absence of entanglement for some initial states. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N19.00011: Decoherence effects of a charge detector on a nearby quantum dot David Ruiz-Tijerina, Edson Vernek, George Martins, Sergio Ulloa We study the effects of a charge detector, implemented by a quantum point-contact (QPC), on the Kondo state of an adjacent spin-1/2 quantum dot (QD). The Coulomb interaction between electrons traversing the QPC and those within the QD contribute to charge fluctuations and decoherence of the Kondo state in the QD, which can be detected through conductance measurements. Modeling the QPC as two current leads coupled through a localized level near resonance with the Fermi level of the leads, one can explore different transport regimes of the detector: Coulomb blockade, ballistic resonant-transport, and a Kondo screening state (associated with the ``0.7 anomaly''). Transitions between different states are achieved by tuning the capacitive coupling $u$, or the local gates in the QPC. The transitions are studied using Varma--Yafet variational techniques, providing interesting insights into the different regimes. We employ numerical renormalization-group calculations to accurately evaluate the spectral densities and conductance behavior of the coupled QPC--QD system. We report the dependence of the Kondo temperatures of both subsystems on the capacitive coupling strength $u$, and describe the phases' signatures in the local spectral densities and the conductance profile of the QPC. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N19.00012: Full Counting Statistics of Photons Emitted by a Double Quantum Dot Canran Xu, Maxim Vavilov We analyze the full counting statistics of photons emitted by a double quantum dot (DQD) to a high-quality microwave resonator due to the dipole coupling. We show that at the frequency matching condition $\omega_0=\Delta E/\hbar$ for the energy splitting $\Delta E$ of the DQD and the resonator frequency $\omega_0$, photon statistics exhibits both a sub-Poissonian distribution and anti-bunching. In the ideal case, when the system decoherence stems only from photo-detection process, the photon noise is reduced to nearly one-half of the noise for the Poisson distribution. The photon distribution remains sub-Poissonian even at moderate decoherence in the DQD, but eventually become super-Poissonian in the regime of strong decoherence of the DQD. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N19.00013: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N19.00014: Dephasing by a Zero Temperature Detector and the Friedel Sum Rule Bernd Rosenow, Yuval Gefen Detecting the passage of an interfering particle through one of the interferometer's arms, known as ``which path'' measurement, gives rise to interference visibility degradation (dephasing). Here we consider a detector at {\em equilibrium} [1]. At finite temperature dephasing is caused by thermal fluctuations of the detector. More interestingly, in the zero temperature limit, equilibrium quantum fluctuations of the detector give rise to dephasing of the out-of-equilibrium interferometer. This dephasing is a manifestation of an orthogonality catastrophe which differs qualitatively from Anderson's. Its magnitude is directly related to the Friedel sum rule.\\[4pt] [1] B.~Rosenow and Y.~Gefen, Phys. Rev. Lett. {\bf 108}, 256805 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N19.00015: Quantum dynamics of a spin chain in the presence of engineered collective noise Christopher Zeitler, Laurel E. Anderson, Lorenza Viola, Chandrasekhar Ramanathan We experimentally and theoretically investigate the effect of engineered collective noise on the quantum dynamics of a spin chain evolving under the double-quantum Hamiltonian. This Hamiltonian is related by a similarity transformation to the isotropic XX Hamiltonian, and is experimentally accessible in solid-state NMR using coherent averaging techniques. In the absence of noise, a localized magnetic moment is observed to move down the chain at a constant velocity. We show that this transport is disrupted by the presence of collective z-noise, and that the magnetic moment becomes localized at the initial site as the strength of the noise increases. The relevance to quantum information transport in spin chains is also discussed. [Preview Abstract] |
Session N20: Focus Session: Mesoscopics - Tunneling
Sponsoring Units: DMPChair: Gabriel Ramirez, University of California, San Diego
Room: 322
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N20.00001: Scanning gate spectroscopy of a quantum Hall island near a quantum point contact Benoit Hackens, Frederico Martins, Sebastien Faniel, Vincent Bayot, Bernd Rosenow, Ludovic Desplanque, Xavier Wallart, Marco Pala, Hermann Sellier, Serge Huant We report on low temperature (100 mK) scanning gate experiments performed at high magnetic field (around 10 T) on a mesoscopic device patterned in an InGaAs/InAlAs heterostructure. Magnetotransport measurements yield signatures of ultra-small Quantum Hall Islands (QHI) formed by closed quantum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to locate and probe a single QHI near a quantum point contact. The presence of Coulomb diamonds in the local spectroscopy confirms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias across the device, we uncover the QHI discrete energy spectrum arising from electronic confinement and we extract estimates of the gradient of the confining potential and of the edge state velocity. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N20.00002: Noise Intensity-Intensity Correlations and the Fourth Cumulant of Current Fluctuations Jean-Charles Forgues, Fatou Bintou Sane, Christian Lupien, Bertrand Reulet We report measurements of the correlation between intensities of noise at different frequencies on a tunnel junction under ac excitation. We show that such correlations exist only for certain relations between the excitation frequency and the two detection frequencies, which are similar to three-wave and four-wave mixing in optics, depending on the dc bias of the sample. We demonstrate that the correlation we measure is proportional to the fourth cumulant of current fluctuations. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N20.00003: Optical Spectra of Au Nanoparticle Arrays in Grating Templates on a Silver Mirror Edgar Palacios, Aiqing Chen, Ryan Miller, Eugene dePrince III, Stephen Gray, Elena Shevchenko, Ulrich Welp, Vitalii Vlasko-Vlasov Reflection spectra of close packed spherical gold nanoparticle assemblies in grating templates on a silver film covered with a thin dielectric spacer are studied in a wide range of incidence angles. Wide spectral minina corresponding to the plasmonic eigen-modes of the nanoparticle arrays are observed and compared with spectra of empty gratings. These minima correspond to extended optical bands of the arrays formed due to the strong interactions between localized plasmons modes of nanoparticles, silver film surface plasmons and grating resonances. From the angular variations of the spectra we obtain the dispersion of plasmonic excitations which yield a strong amplificatin of the light intensity in our system. Raman signal enhancement for Benzenethiol molecules in the gaps between nanoparticles is estimated as 3x10$^{10}$. The intense light amplification in a wide spectral range and the large number of regular hot spots makes our structures an advanced platform for optical sensing, solid state lighting, and solar harvesting technologies. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N20.00004: Quantum Mesoscopic Physics of Electrons and Photons Invited Speaker: Eric Akkermans We first review basic notions of coherent quantum transport at the mesoscopic scale for both electronic and photonic systems. We then show that successful descriptions developed for coherent electronic transport (e.g. weak localization and UCF) and thermodynamics (persistent currents), noise and full counting statistics can be extended and applied to the study of Quantum Electrodynamics of quantum conductors and of quantum optics based on photons emitted by such conductors. In this context, we discuss the two following specific problems : (1) Ramsey fringes and time domain interference for particle creation form a quantum vacuum with a specific application to dynamical Coulomb blockade. In that setup, the current noise of a coherent conductor is biased by two successive voltage pulses. An interference pattern between photon assisted processes is observed which is explained by the contribution of several processes to the probability to emit photons after each pulse. Recent experiments in this context will be discussed. (2) Quantum emitter coupled to a fractal environment. A new and unexpected type of oscillatory structures for the probability of spontaneous emission has been obtained which results from the fractal nature of the quantum vacuum. When applied to the case of a tunnel junction as a quantum emitter of photons, the same oscillatory structure arises for the conductance of the tunnel junction. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N20.00005: Squeezing in Photo-assisted Electron Quantum Shot Noise Gabriel Gasse, Bertrand Reulet, Christian Lupien The current/voltage fluctuations generated by a conductor are another point of view of a randomly fluctuating electromagnetic field, i.e. ``white'' light. We demonstrate experimentally that this light is naturally squeezed, i.e. that the noise on one quadrature can go below the vacuum fluctuations, for a tunnel junction at very low temperature irradiated by a microwave. A classical current in a conductor generates a coherent state of light. We show that a quantum current can emit non-classical light. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N20.00006: Electron tunneling in chaotic quantum ring Branislav Vlahovic, Igor Filikhin, Sergei Matinyan Single electron confinement states of two dimensional InAs/GaAs quantum ring (QR) are considered under the effective approach [1]. The symmetry of the QR shape is violated as it is in well-known Bohigas annular billiard [2]. For weak violation of the symmetry, the energy spectrum may be represented by a set of quasi-doublets. We study the correlation between electron localizations and quasi-doublet splitting for complete spectrum. The bands with different ``radial'' quantum numbers are well determined within our calculations. The inter-band tunneling is considered in relation to the chaotic properties of the QR. We propose an alternative interpretation of the experimental data [3] to that made in Ref. [3], where the ``first experimental evidence for chaos-assisted tunneling'' in a microwave annular billiard was reported. We show that this effect can be explained by inter-band tunneling that occurs due to the anti-crossing of the levels having different ``radial'' quantum numbers. [1] I. Filikhin, V. M. Suslov and B. Vlahovic, Phys. Rev. B 73, 205332 (2006). [2] O. Bohigas, D. Boose, R. Egydio de Carvallho, and V. Marvulle, Nucl. Phys. A 560, 197 (1993). [3] C. Dembowski et al., PRL 84 (2000) 867; R. Hofferbert et al., Phys. Rev. E 71, 046201 (2005). [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N20.00007: Emergence of the collective-response of granular solid - liquid mixtures to wave- pulse excitations Hasson M. Tavossi The phenomenon of emergence of new properties observed in the collection of solid particles in liquid, due to pulse-wave excitations, can be found in many macroscopic systems. In this paper the uniform mixtures of solid spherical grains in water were subjected to high intensity, 60-Volts amplitude, pulsed -Ultrasonic waves of 45 kHz peak frequency. The observed response of the mixture was imbedded in the modified transmitted pulse, and could be extracted and compared to that of a reference pulse. Analysis of the results, in the frequency and time domains, includes; Fast Fourier Transform, amplitude and phase changes, and frequency dependent attenuation. The experimental findings and numerical results show that, the response of the mixture can be made independent of the scale, up to relatively small scale. The findings also show that, several collective- response to elastic wave propagation in the crystalline solids at the atomic scale, such as; cut-off frequency, tunneling effect, and absorption and conduction bands, can also have analogous ones in intermediary, and equivalences in these relatively simple mixtures. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N20.00008: Theory of Solvent-Mediated Environmental Effects on Transport in Molecular Junctions Michele Kotiuga, Pierre Darancet, Latha Venkataraman, Jeffrey Neaton Single-molecule junctions, formed with well-defined and robust metal-molecule contacts, can provide an ideal model system to study mechanisms of charge transport at the molecular scale. However, the presence of solvent is often unavoidable, and recent experiments have shown that the junction conductance can be altered by a factor of two depending on the solvent present. Here, we use a first-principles scattering-state approach, based on self-energy corrected density functional theory (DFT), to explore how solvent and coverage impacts the transmission and conductance of bipyridine-Au molecular junctions. We find the conductance can shift by more than a factor of 5 by varying the bipyridine coverage, which is an effect associated with work function shifts that can be understood with a 2D polarizable dipole model fit with DFT values. A generalization of this electrostatic model to include solvent molecules allows us to estimate the work function shift for a mixed molecular coverage based both on the experimental parameters and system thermodynamics. By combining the results of our transmission calculations and the electrostatic model, we can accurately describe the conductance shifts observed experimentally. We acknowledge DOE for support, and NERSC for computational resources. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N20.00009: Correlating Molecular Energy Level Alignment with the Conductance of Single Molecular Junctions Taekyeong Kim, Jonathan R. Widawsky, Pierre Darancet, Michele Kotiuga, Jeffrey B. Neaton, Latha Venkataraman There has been increased interest in understanding electronic and thermoelectric transport in single molecule junctions and metal/organic interfaces. While the ionization potential and electron affinity of molecules can be calculated for molecules in the gas-phase, additional physical effects, such as charge transfer and rearrangement, hybridization, and electrode polarization are expected to alter these electronic energies for molecular junctions. Therefore, it is hard to determine energy level alignments in molecular junctions. Here, we determine the relationship between electronic energy level alignment at a metal-molecule-metal interface and single-molecule junction conductance properties for 4,4'-bipyridine via direct and simultaneous measurement of electrical and thermoelectric currents using a scanning tunneling microscope-based break-junction technique. We measure directly, the position of the lowest unoccupied molecular orbital (LUMO) relative to the Au Fermi level assuming a Lorentzian resonance lineshape. Furthermore, we correlate the energy level alignment and coupling strength between two conductance states through repeated junction elongation and compression. We find that these values are in excellent agreement with our self-energy corrected density functional theory calculations. These results thus provide the first evidence for correlation between energy level alignment and single molecule transport. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N20.00010: Transition from Coulomb Diamonds to Checherboard-like Spectroscopies in a Mesoscopic Quantum Hall Interferometer S. Faniel, F. Martins, V. Bayot, B. Hackens, L. Desplanque, X. Wallart, B. Rosenow, S. Melinte We report low temperature ($\sim$ 100 mK) magnetotransport, scanning gate microscopy and scanning gate spectroscopy measurements in an $\rm In_{0.7}Ga_{0.3}As/In_{0.52}Al_{0.48}As$ quantum point contact (QPC). The magnetoresistance of the QPC shows oscillations in the vicinity of integer quantum Hall states. We attribute these magnetoresistance oscillations to the formation of an electron interferometer around a small, disorder-induced quantum Hall island located within the constriction. The magnetic field $B$ tunes the edge states configuration in the QPC, leading to different signatures in the transport measurements. Interestingly, near the Landau level filling factor $\nu=3$, the spectroscopy measurements performed on the quantum Hall interferometer, as a function of $B$ or scanning gate tip voltage, exhibit a smooth transition from Coulomb diamonds to a checkerboard pattern. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N20.00011: Fluctuations of g factors of discrete levels in ferromagnetic nanoparticles Dragomir Davidovic, Wenchao Jiang, Felipe Tijiwa Birk, Patrick Gartland It has been known that the interplay between electron-electron interactions and spin-orbit scattering can cause a wide distribution of g factors in tunneling spectra of metallic nanoparticles, including g-factors much larger than 2 if electron-electron interactions are strong. Here, we present our studies of single Co nanoparticles in Al/Al$_2$O$_3$/(Co nanoparticles)/Al$_2$O$_3$/Al tunnel junctions using electron tunneling spectroscopy at mK-temperatures. The g factor of discrete energy levels exhibits significant difference between minority-spin and majority-spin levels. We have clearly observed large g factors ($\approx$ 6) in one sample at magnetic field greater than 4T, suggesting $\Delta S=3/2$ in the tunneling transition, $S$ is the magnitude of the spin. We will present the latest results on tunneling junctions containing Ni, Permalloy or Gd nanoparticles, which have weaker magnetic anisotropy fluctuations. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N20.00012: Band edge noise spectroscopy Farkhad Aliev, Juan Pedro Cascales, Frederic Bonell, Stephane Andrieu Although metal/insulator interfaces are expected to play a major role in charge, spin and phonon flow, little is known about the real underlying band structure. The reason is the difficulty in directly obtaining this information from interfaces by the use of a non-invasive physical tool. Here we introduce and demonstrate the feasibility of a conceptually new method that enables us to gather information on the interface electron bands. The low frequency and low temperature noise measurements as a function of applied bias voltage clearly reveal the appearance of the electron band edges at the Fermi level. By analyzing the bias dependence of the normalized 1/f noise (Hooge factor) in Fe1-xVx/MgO/Fe (with 0\textless\ x\textless\ 0.16) epitaxial magnetic tunnel junctions with diminished misfit dislocations, we observe strong anomalies in the 1/f noise at specific voltages where the band edges of the ferromagnetic electrodes which form the tunnel junction are expected to cross the Fermi level. These effects, understood within a simple model of 1/f noise due to localized states near the band edges, open up new perspectives for a reliable ``in situ'' characterization of electron bands in normal metal or spintronic devices. [Preview Abstract] |
Session N21: Focus Session: BiFeO3 and Domain Wall Conductance
Sponsoring Units: DMPChair: Alexei Belik, NIMS Japan
Room: 323
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N21.00001: Current at domain walls, roughly speaking: nanoscales studies of disorder roughening and conduction Invited Speaker: Patrycja Paruch Domain walls in (multi)ferroic materials are the thin elastic interfaces separating regions with different orientations of magnetisation, electric polarisation, or spontaneous strain. Understanding their behaviour, and controlling domain size and stability, is key for their integration into applications, while fundamentally, domain walls provide an excellent model system in which the rich physics of disordered elastic interfaces can be accesses. In addition, domain walls can present novel properties, quite different from those of their parent materials, making them potentially useful as active components in future nano-devices. Here, we present our atomic force microscopy studies of ferroelectric domain walls in epitaxial Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ and BiFeO$_3$ thin films, in which we use piezorespose force microscopy to show unusual domain wall roughening behaviour, with very localised disorder regions in the sample leading to a complex, multi-affine scaling of the domain wall shape [1]. We also show the effects of temperature, environmental conditions, and defects on switching dynamics and domain wall roughness [2]. We combine these observations with parallel conductive-tip atomic force microscopy current measurements, which also show highly localised variations in conduction, and highlight the key role played by oxygen vacancies in the observed domain wall conduction [3]. \\[4pt] [1] Guyonnet et al., PRL 109, 147601 (2012)\\[0pt] [2] Paruch et al, PRB 85, 214115 (2012); Blaser et al, APL. 101, 142906 (2012)\\[0pt] [3] Guyonnet et al., Adv. Mat. 25, 5377 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N21.00002: Nanoscale spatial control of domain wall conductivity in BiFeO$_3$ thin films Brian Smith, Rama Vasudevan, Bouwe Kuiper, Andre ten Elshof, Art Baddorf, Peter Maksymovich, Sergei Kalinin, Mark Hujben, Guus Rijnders, Gertjan Koster Use of ferroelectric domain walls for applications is an attractive prospect. Domain walls can have properties not found in bulk allowing added functionality. The 1D nature of a ferroelectric domain wall could be exploited to create devices with dimensions on the order of a single unit cell. Intensive research on domain wall conductivity in BiFeO$_3$ is ongoing since the first report in 2009 [1]. Here we report on the spatial control of domain wall conductivity in an epitaxial grown BiFeO$_3$ film 25nm thick on self-assembled SrRuO$_3$ nanowires using an ordered mixed terminated DyScO$_3$ substrate as a growth template [2]. The SrRuO$_3$ nanowires (5nm high, 100nm wide separated by 200nm) run across the substrate and are contacted at the sample edge creating alternating insulating/conducting surfaces. Using PFM/cAFM the domains, switching and domain wall conductivity is explored. Domain wall conductivity is only present in over the nanowires. In addition to providing spatial control of the conductivity this result provides evidence that the conduction is confined to a single domain wall throughout the thickness of the film and is not the results of network of interconnected domains.\\[4pt] [1] Seidel J, et. al. Nat. Mat. 2009, 8, 229\\[0pt] [2] Kuiper et al., MRS Communications, Doi:10.1557/mrc.2011.8 [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N21.00003: Domain walls in a perovskite oxide with two primary structural order parameters: first-principles study of BiFeO$_3$ Oswaldo Di\'eguez, Pablo Aguado-Puente, Javier Junquera, Jorge \'I\~niguez We present a first-principles study of ferroelectric domain walls (FE-DWs) in multiferroic BiFeO$_3$ (BFO), a material in which the FE order parameter coexists with anti-ferrodistortive (AFD) modes involving rotations of the O$_6$ octahedra. We find that the energetics of the DWs are dominated by the capability of the domains to match their O$_6$ octahedra rotation patterns at the plane of the wall, so that the distortion of the oxygen groups is minimized. Our results thus indicate that, in essence, it is the discontinuity in the AFD order parameter, and not the change in the electric polarization, what decides which crystallographic planes are most likely to {\em host} BFO's FE-DWs. Such a result clearly suggests that the O$_6$ rotational patterns play a primary role in the FE phase of this compound, in contrast with the usual (implicit) assumption that they are subordinated to the FE order parameter. Interestingly, we find that the structure of BFO at the most stable DWs resembles the atomic arrangements that are characteristic of low-lying (meta)stable phases of the material. Our work thus contributes to shape a coherent picture of the structural variants that BFO can present and the way in which they are related. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N21.00004: Ferroelectric Tunnel Junctions Based on Pseudotetragonal BiFeO$_{3}$ Flavio Y. Bruno, S. Boyn, V. Garcia, S. Fusil, H. Yamada, C. Carretero, C. Deranlot, E. Jacquet, K. Bouzehouane, S. Xavier, J. Grollier, M. Bibes, A. Barthelemy The concept of a ferroelectric tunnel junction (FTJ) was formulated in the early 70s by Esaki et al. It took more than 30 years to realize this idea experimentally in a reliable and reproducible manner[1]. FTJs have shown to be versatile devices and the possibility to use them as memories [2] and memristors [3] have been recently demonstrated on BaTiO$_{3}$ based junctions. With the aim of expanding its functionalities we have realized FTJ with multiferroic pseudotetragonal BiFeO$_{3}$ (T-BFO) tunnel barriers. In order to fabricate junctions we deposited fully epitaxial bilayers consisting of a LaNiO$_{3}$ or doped CaMnO$_{3}$ bottom electrodes and the T-BFO tunnel barriers. On top of this bilayers, Co/Au electrodes as small as 200 nm in diameter were defined by e-beam lithography and lift-off. We have measured ON/OFF ratios as large as 10000 on these junctions, much larger than that observed in FTJs with BaTiO$_{3}$ tunnel barriers. We will show that the resistance of the FTJ in its high, low and intermediate states is related with the polarization state of the barrier as observed by PFM. [1]Nature 460,81(2009). [2]Nat. Nanotech. 7, 101 (2011). [3] Nature Mat. 11, 860 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N21.00005: THz spectroscopy of spin waves in multiferroic BiFeO$_3$ in high magnetic fields Urmas Nagel, T. R\~o\~om, H. Engelkamp, D. Talbayev, H.T. Yi, S.-W. Cheong BiFeO$_3$ is both antiferromagnetic and ferroelectric with high N\`eel and Curie temperatures, about 640\,K and 110\,K, respectively. In low magnetic field Fe$^{3+}$ spins order cycloidally, inducing an additional electric polarization, which interacts with the feeroelctric polarization of the lattice and produces a magneto-electric term in the total energy. We have measured the magnetic field dependence of infrared active magnon modes in an untwinned BiFeO$_3$ single crystal at 4K. The magnon modes soften close to the critical field of about 18.8T along the [001] cubic axis, where the cycloid is destroyed and the low field magnon modes disappear. A new strong mode with linear magmetic field dependence appears above 19T and persists at least up to 31T. The dramatic change of the THz spectrum at 19T allows us to assign all the low field modes as excitations of the cycloid. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N21.00006: Strain dependence of transition temperatures, structural symmetry, and phase coexistence of BiFeO3 within the tetragonal-like structure Wolter Siemons, Christianne Beekman, Gregory MacDougall, Adam Aczel, Michael Biegalski, Jerel Zarestky, Shuhua Liang, Elbio Dagotto, Steve Nagler, Hans Christen We have investigated the influence of strain-imposed in-plane lattice symmetry on the structural and magnetic properties of tetragonal-like BiFeO$_{3}$. We find that an increase in the in-plane distortion results in an increase of the N\'{e}el temperature from 313 K to 324 K for films grown on YAlO$_{3}$ and LaAlO$_{3}$ respectively. The change in magnetic ordering temperature is reproduced in 3D Heisenberg Monte-Carlo simulations. The structural transition temperatures, from M$_{C}$ to M$_{A}$ monoclinic around 100 $^{\circ}$C and to a true tetragonal phase at higher temperature, are also found to depend on strain. Some of the strain is relieved through the creation of an additional polymorph, which causes stripe patterns in the surface morphology. We present how the abundance and shape of these patterns changes with the amount and symmetry of strain. These results show strain cannot be treated as a single scalar number or simply as a direct consequence of the lattice mismatch between the film material and the substrate. Research supported by the U.S. Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division, and performed in part at ORNL's Spallation Neutron Source and Center for Nanophase Materials Sciences (sponsored by DOE-BES). [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N21.00007: Local conductivity in supertetragonal and rhombohedral-like BiFeO$_{3}$ films Saeedeh Farokhipoor, Christianne Beekman, Wolter Siemons, Hans M. Christen, Beatriz Noheda Materials in which structural polymorphs coexist are of great interest in the design of magnetoelectric devices and piezoactuators at the nanoscale. In BiFeO$_{3}$, coexisting polymorphs are stabilized in thin film form by strain resulting from film/substrate lattice mismatch and/or thermal expansion differences. In films on LaAlO$_{3}$ substrates, these polymorphic phases give rise to stripe patterns; they are formed by the coexistence of the highly-strained (T') phase with an intermediary polymorph (S') in samples devoid of the rhombohedral-like relaxed (R') structure. Here, we investigate the local properties of the stripe patterns by piezoresponse force microscopy and conductive atomic force microscopy. This makes it possible to investigate the local conductivity both of specific domains and of different domain walls, and to compare the results to those obtained for R'-BiFeO$_{3}$ films (on SrTiO$_{3}$ substrates). We show that patterns of locally varying polarization and conductivity can be reversibly written and erased at length scales determined by the phase stability of the strain-induced structural polymorphs, and illustrate similarities and differences between R' and T' BiFeO$_{3}$. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N21.00008: Temperature dependent dielectric and ferroelectric studies of BiFeO3 thin film Anand P.S. Gaur, Sujit K. Barik, Ram S. Katiyar Although BiFeO$_{3}$ (BFO) has received a lot of interest due to its good multiferroic properties at room temperature, high leakage current limit its usage for practical applications. Recently, it is found that these properties in thin films can be different due to strain effect induced by substrate, preparation conditions and electrode effects, etc. In this context, we have studied the temperature dependence of polarization and dielectric properties of BFO thin film by varying the bottom electrode thickness and using different electrodes. The strain dependent ferroelectric switching behaviors have also been investigated with a traditional ferroelectric tester and switching spectroscopy piezoresponse force microscopy (SS-PFM), respectively. We used pulsed laser deposition to fabricate thin films of BFO using Si (100) substrate and SrTiO$_{3\, }$(STO) as buffer layer with different bottom electrodes such as SrRuO$_{3\, }$(SRO), LaNiO$_{3\, }$(LNO) and Pt/Si. The thickness of STO layer is kept fixed around 70 nm and the thicknesses of BFO and electrode layer were varied from 70 nm to 200nm. The layers were grown under optimized conditions and polycrystalline nature is found from room temperature XRD. A large enhancement of polarization is found while using LNO electrode and also with reducing the thickness of BFO layer. The remnant polarization and cohesivity also shows large increase with increaisng temperature, although leakage current increases significantly. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N21.00009: Temperature dependence of acoustic and low-energy optic phonons in the multiferroic BiFeO3 studied by inelastic neutron scattering Guangyong Xu, Zhijun Xu, John Shneelock, Peter Gehring, Chris Stock, Masaaki Matsuda, Genda Gu, T. Ito, Jinsheng Wen, R.J. Birgeneau, Stephen Shapiro We report inelastic neutron scattering measurements on the acoustic and low-energy phonons in the multiferroic material BiFeO$_3$. The phonon dispersion in the (200) and (111) zones have been mapped out for temperatures between 300K to 750K. The temperature dependence of the dispersion and phonon intensities will be discussed. Possible connections between the the antiferromagnetic phase transition at 640K and anomalies in the phonon modes are observed. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N21.00010: Phase coexistence, phase transitions, and piezoelectric switching in highly-strained BiFeO$_3$ Hans M. Christen, C. Beekman, W. Siemons, M. Chi, J.Y. Howe, M.D. Biegalski, N. Balke, P. Maksymovych, T.Z. Ward, A.K. Farrar, J.B. Romero, D. Tenne Highly strained ($T'$) BiFeO3 films are investigated as function of temperature by x-ray diffraction in combination with atomic-force, piezo-response force, and transmission electron microscopies. In these films on LaAlO$_3$ substrates, the coexistence of the $T'$ majority phase (c/a $\sim$ 1.25) with an intermediary $S'$ polymorph (c/a $\sim$ 1.09) leads to the formation of stripe patterns in samples where the bulk-like, nearly rhombohedral $R'$ polymorph is absent. While $T'$ films at 300K are monoclinic, our results reveal a true tetragonal high-temperature phase (at T$\geq$700K) for which Raman spectroscopy demonstrates a polar nature. However, piezoelectric switching of the $T'$ phase is possible only in the presence of the $S'$ polymorph. This polymorph, and the stripe patterns that result from its coexistence with the $T'$ form appear after growth upon cooling below $\sim$570K. This shows that the $S'$ polymorph is formed by additional stress resulting from the differences in thermal expansion between film and substrate. These results point to new approaches for tuning functional properties in materials exhibiting strain-induced polymorphic phase transitions. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N21.00011: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N21.00012: Symmetry of Epitaxial BiFeO$_3$ Films in the Ultrathin Regime Yongsoo Yang, Christian Schlep\"{u}tz, Carolina Adamo, Darrell Schlom, Roy Clarke BiFeO$_3$ (BFO) films grown on SrTiO$_3$ (STO) with a SrRuO$_3$ buffer layer exhibit a monoclinic structure at thicknesses greater than 40 nm, but higher structural symmetry can be observed for thinner films [Phys. Rev. B 81, 144115 (2010)]. We report a structural phase transition from monoclinic to tetragonal in ultra-thin BFO films grown directly on (100)-oriented STO. X-ray diffraction measurements of 3-dimensional reciprocal space maps reveal half-integer order peaks due to oxygen octahedral tilting. When the film thickness is decreased below 20 unit cells, the integer-order Bragg peak splitting associated with the presence of multiple domains of the monoclinic phase disappears. Instead, a single peak that is commensurate with the STO substrate lattice appears. The diffraction pattern has four-fold symmetry, ruling out the presence of a single monoclinic domain in favor of a tetragonal film structure. The evolution of the oxygen octahedra tilt pattern inferred from the intensities of half-order peaks suggests that this transition originates from the corner-connectivity of oxygen atoms at the interface between BFO and STO, and also strongly supports this monoclinic to tetragonal transition. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N21.00013: First-Principles Calculation of the Bulk Photovoltaic Effect in Bismuth Ferrite Steve Young, Fan Zheng, Andrew Rappe Bismuth ferrite is a multiferroic material with a large bulk polarization and a band gap in the visible spectrum. Significant anomalous photovoltaic effects have been observed in the material; however, the origins of this effect are unclear. While some investigations indicate that observed photovoltages and photocurrents are due to the bulk photovoltaic effect, in striped polydomain samples there is no evidence of this, and the observed response is attributed to a domain-wall-driven mechanism. We have computed the bulk photovoltaic response from first principles using shift current theory and compared it to the available experimental data, finding good agreement. By accounting for the geometry of the polydomain samples, we are able to explain the lack of observed bulk photovoltaic response. Furthermore, we show that these two mechanisms act antagonistically, suggesting that enhanced efficiency may be found in materials where these two effects interact cooperatively. [Preview Abstract] |
Session N22: Artificially Structured Materials: Growth, Structure, and Related Phenomena
Sponsoring Units: DCMPChair: Connie Li, Naval Research Laboratory
Room: 324
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N22.00001: First-principle Investigation of the Stability and Vibrational Spectrum of MoSx Nanostructures Grown on Cu(111) Talat S. Rahman, Maral Aminpour, Duy Le, Marisol Alc\'antara Ortigoza Recent experiments have successfully synthetized MoS$_{\mathrm{X}}$ nanostructures in a controlled manner by evaporating Mo adatoms on the copper sulfide monolayer that forms on Cu(111) upon sulfur preloading [1,2]. STM observations and total-energy calculations based on density functional theory, including van-der-Waals interactions, have proposed several structures for MoS$_{\mathrm{X}}$/Cu(111). In this study, we investigate the plausibility of those structures and provide elements for further experimental substantiation or refutation. In particular, we perform density-functional-theory calculations (including ab intio van-der-Waals interactions) of vibrational spectrum of the proposed structures to (1) attest their dynamical stability; (2) compare their thermodynamic stability as obtained from the total free energy; and (3) provide the vibrational frequencies that uniquely fingerprint the proposed structures. \\[4pt] [1] Kim et al., Langmuir 27, 11650 (2011)\\[0pt] [2] Dezheng D Sun, Angew Chem. Int. Ed. \textbf{51}, 10284-8 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N22.00002: Vibrational and thermodynamic properties of transition-metal nanoclusters Valeri G. Grigoryan, Michael Springborg The knowledge of the vibrational spectrum of a cluster, which is the fingerprint of its structure, is necessary for the development of thermodynamics of clusters (melting, heat capacity, solid-solid structural transitions) and for the understanding of experimental vibrational spectra. In summary, the full vibrational spectrum of Ni$_N$ and Cu$_N$ nanoclusters with $N$ from $2$ to $150$ atoms has been determined using the analytical expression of the embedded-atom method (EAM) for the {\em force-constant tensor} for the first time. In the determination of the spectra we have employed the global-minimum structures obtained in our previous unbiased EAM studies (see e.g. Physical Review B, 2004; 2006). Furthermore, using those spectra and the superposition approximation, the thermodynamic properties of the clusters have been calculated quantum mechanically, including their heat capacity and solid-solid transition temperatures for several structural changes in the Ni and Cu clusters. Both the vibrational spectrum and the thermodynamic functions show strong cluster-size effects. We emphasized that our approach is general. It is based only on the (common) EAM form of the total energy and applicable to many other many-body potentials. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N22.00003: Material Improvements of ZnCdSe/ZnCdMgSe Heterostructures for Quantum Cascade Laser Applications with Incorporation of Growth Interruptions During MBE Growth Thor Garcia, Joel De Jesus, Arvind Ravikumar, Songwoung Hong, Claire Gmachl, Aidong Shen, Maria Tamargo We report on the growth of ZnCdSe/ZnCdMgSe/InP Quantum Cascade (QC) structures with improved electrical and material properties. Material quality has been previously addressed by limiting the lattice mismatch to within 0.2{\%} of InP. However, the yields of high quality material have remained low and lasing has not been observed. To address the low growth yields we have investigated possible mechanisms for degradation of the material. Growth interruptions during the MBE growth were added to the active core of the QC laser structure. High resolution XRD and PL were used to evaluate the material quality. Fabricated devices with growth interruptions show a dramatic improvement in the electroluminescence spectral properties. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N22.00004: New crystal structures in hexagonal CuInS$_{2}$ nanocrystals Xiao Shen, Emil A. Hern\'andez-Pagan, Wu Zhou, Yevgeniy S. Puzyrev, Juan C. Idrobo, Janet E. Macdonald, Stephen J. Pennycook, Sokrates T. Pantelides CuInS$_{2}$ is one of the best candidate materials for solar energy harvesting. Its nanocrystals with a hexagonal lattice structure that is different from the bulk chalcopyrite phase have been synthesized by many groups. The structure of these CuInS$_{2}$ nanocrystals has been previously identified as the wurtzite structure in which the copper and indium atoms randomly occupy the cation sites. Using first-principles total energy and electronic structure calculations based on density functional theory, UV-vis absorption spectroscopy, X-ray diffraction, and atomic resolution Z-contrast images obtained in an aberration-corrected scanning transmission electron microscope, we show that CuInS$_{2}$ nanocrystals do not form random wurtzite structure. Instead, the CuInS$_{2}$ nanocrystals consist of several wurtzite- related crystal structures with ordered cation sublattices, some of which are reported for the first time here. This work is supported by the NSF TN-SCORE (JEM), by NSF (WZ), by ORNL's Shared Research Equipment User Program (JCI) sponsored by DOE BES, by DOE BES Materials Sciences and Engineering Division (SJP, STP), and used resources of the National Energy Research Scientific Computing Center, supported by the DOE Office of Science under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N22.00005: Measuring the Elastic Modulus of the Grain Boundary Component of Nanocrystalline Copper Guo-Jie Gao, Yunjiang Wang, Shigenobu Ogata In the past twenty years, it has been widely accepted that the Young's modulus of the grain boundary (GB) part of nanocrystalline metals is about $70\%$ of that of the crystalline core component. However, this belief is an assumption based on numerical studies of specific grain boundary like $\Sigma5$ twist boundary where atoms interact with one another via simplified Lennard-Jones potential at $0K$ or experimental studies assuming the GB behaves like amorphous alloys. A thorough investigation driven from completely realistic atomic simulation at finite temperature is still lacking. We reexamine this assumption by measuring the Young's modulus of pure copper (Cu) with grain size ranging from $3$ to $25~nm$ at $300K$ using molecular dynamics (MD) uniaxial tensile tests. We implement a novel Voronoi protocol to build nanocrystalline structures of fully dense pure Cu with well-controlled grain size distribution and Mishin embedded atom model (EAM) potential. We find the following key results concerning the stiffness for nanocrystalline metals at finite temperature: 1) The GB is more thermally sensitive and therefore elastically much softer than the crystalline interior. 2) The Young's modulus of the GB is about $20\%$ or less of that of the grain interior. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N22.00006: Structural and compositional characterization of ``covetics'' a new class of materials containing high C concentration R.A. Isaacs, A. Herzing, D.R. Forrest, A.N. Mansour, M.C. LeMieux, J. Shugart, L. Salamanca-Riba ``Covetics'' are a new class of materials formed by the incorporation of high concentrations ($>6 wt\%$) of nanoscale carbon in a metal matrix. The carbon incorporates into the crystal structure of the host metal and remains dispersed after subsequent melting and re-solidification. The carbon is highly stable in these materials despite the absence of a predicted solid solution at such concentrations in the binary phase diagrams. Covetics have been shown to exhibit enhanced electrical, mechanical and thermal properties when compared with non-covetic metals. We have performed energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), SEM, TEM, STEM/electron energy loss spectroscopy (EELS), AFM, and Raman spectroscopy to investigate the structure of Al, Cu, and Ag covetics. Both bulk samples and thin films are investigated. Carbon was detected in the form of nanoparticles 5 nm - 200 nm in diameter with an interconnecting carbon matrix. The carbon is detectable by EDS and XPS, but not by analytical methods such as LECO and GDMS. Raman indicates a similar signal to that of CNTs in covetics. A detailed investigation of the morphology of the nanocarbon and the structure of several covetics will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N22.00007: Computational Investigations of a Possible New Class of Materials: A Superatom Ionic Solid Karl Sohlberg, Violeta Nasto A ``superatom'' is a cluster of atoms that shows high stability. High stability can arise from the geometric arrangement of the atoms in the cluster. For example, when atoms are close packed, clusters containing an integer number of closed shells of atoms, (i.e. 13, 55, 137... atoms) exhibit enhanced stability and are termed ``magic clusters.'' High stability can also arise from the electronic structure. High symmetry metal clusters that have exactly 8, 20, 40..., valence electrons show enhanced stability. Superatoms can act chemically like a single atom of a different element. We have used electronic structure calculations to test the idea that a new class of materials may be formed based on the periodic arrangement of superatom ions, instead of the typical atomic or polyatomic ions of a conventional ionic solid. A solid is formed based on crystalline packing of anionic (Al@Cu$_{54}^-$) and cationic (Ce@C$_{60}^+$), nearly spherical superatom species that show exceptional stability. According to radius-ratio rules, these ions will favor a CsCl crystal structure with a body-centered (bcc) type of unit cell. Calculations on this material suggest that it is stable, semiconducting and less dense than common metal oxides, but that the metal anion clusters deform within the material. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N22.00008: Electronically Guided Self Assembly within Quantum Corrals Rongxing Cao, Bingfeng Miao, Zhangfeng Zhong, Liang Sun, Biao You, Wei Zhang, Di Wu, An Hu, Samuel Bader, Haifeng Ding A grand challenge of nanoscience is to master the control of structure and properties in order to go beyond present day functionality. The creation of nanostructures via atom manipulation by means of a scanning probe represents one of the great achievements of the nano era. Here we build on this achievement to self-assemble nanostructures within quantum corrals. We constructed circular and triangular Fe quantum corrals on Ag(111) substrate via STM manipulation and studied the quantum confinement of electronic states and the diffusion of Gd atoms inside the corrals. Statistical results reveal the motion of the Gd atoms forming several individual orbits that are closely related to the local density of states. We experimentally demonstrate that different self-organized Gd atomic structures are formed within 30-nm circular and triangular Fe quantum corrals with a step-by-step guiding process. The findings demonstrate that quantum confinement can be used to engineer atomic structures and atom diffusion. And 30-nm resolution can be reached by means of advanced lithography. Adding quantum engineering to augment it opens new possibilities for local functionality design down to the atomic scale. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N22.00009: Simultaneous hypersonic and optical mirrors in nanometric porous silicon multilayers Jesus Manzanares-Martinez, Paola Castro-Garay, Damian Moctezuma-Enriquez, Yohan Jasdid Rodriguez-Viveros We study by theoretical simulations the non-perpendicular propagation of electromagnetic and elastic waves in Porous Silicon Multilayers (PSM). Our work is inspired by recent experimental results where the angular variation of the optical and hypersonic stop bands has been explored in PSM. [L. C. Parsons and G. T. Andrews, J. Appl. Phys. 111, 123521 (2012)] We proceed in three steps. First, we found the conditions to obtain a simultaneous photonic-phononic mirror at normal incidence. Second, we determined the angular variation of the mirrors computing the projected band structure. Finally, we found the conditions to obtain an omnidirectional mirror for hypersonic waves. However, we have found that for the optical case the mirror is limited to an angular cone. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N22.00010: Dynamic Structural Disorder in Supported Nanoparticles F.D. Vila, J.J. Rehr, S.D. Kelly, S.R. Bare Supported Pt based nanoclusters are of wide interest in nano-scale physics and have many industrial applications, yet an understanding of their structure is far from complete. Experimental probes such as x-ray absorption spectroscopy (XAS) only yield globally averaged properties, e.g., mean bond distances and mean-square radial disorder (MSRD), which can give a misleading characterization of such complex systems. To obtain a more detailed picture we have carried out finite temperature DFT/MD simulations\footnote{F. Vila \textit{et al.},\textit{UW preprint} (2012).} of Pt and PtSn nanoclusters up to 600 K (\textit{operando} conditions). These show that the nano-scale structure and charge distribution are inhomogeneous and dynamically fluctuating over several time-scales, ranging from fast (200-400 fs) bond vibrations to slow fluxional bond breaking ($>$10 ps). In particular the anomalous behavior of the MSRD is not static, but rather due to ``dynamic structural disorder'' (DSD) driven by stochastic motion of the center of mass over 1-4 ps time-scales. In addition the nanoclusters exhibit a semi-melted, Sn-rich surface. These findings show that, and how an improved XAS interpretation of supported nano-scale structure must take into account DSD and other structural inhomogeneities. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N22.00011: The Orientation Control of Iodine Molecules inside nano-scale channels Dingdi Wang, Juanmei Hu, Wenhao Guo, Shengwang Du, Z.K. Tang We demonstrate a technique to control the spatial orientation of iodine molecules inside nano-scale channels of an AlPO$_{4}$-11 zeolite crystal. The orientation of iodine molecules can be precisely controlled by the water molecule density inside the channels due to the interaction between iodine and water molecules. Without the presence of water molecules, all the embedded iodine molecules are directed along the direction of nano channels. As increasing the number of water molecules, the iodine molecules gradually ``stand up'' insde nano channels. The experimental results obtained from polarized Raman spectroscopy agree well with the theoretical analysis using molecular dynamics simulation. This technique may be used for engineering molecular orientation in nano-structured devices. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N22.00012: Epitaxial growth of YSi$_{2}$ nanowires the on Si(110) surface Saban Hus, Hanno Weitering High-aspect-ratio YSi$_{2}$ nanowires have been grown epitaxially on the Si (110) surface. In contrast to epitaxial growth on the Si (100) surface, YSi$_{2}$ nanowires on Si (110) grow in a single orientation and show a clear preference of nucleating at terrace edges, thus providing a promising method for fabricating regular nanowire arrays with controlled wire separation. The thinnest YSi$_{2}$ nanowires have a cross section of $\sim$ 0.5 x 2.8 nm$^{2}$ with wire lengths of up to a few hundred nm, while thicker nanowires can grow up to several $\mu$m long. Scanning tunneling spectroscopy measurements on individual nanowires indicate that the nanowires have metallic properties while the surface between the nanowires has a band gap of $\sim$ 1eV. These nanowires thus represent an ideal platform for studies of quasi one-dimensional electrical transport. Such studies are currently underway in our laboratory. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N22.00013: Absence of Dirac Electrons in Silicene on Ag (111) Surfaces Zhi-Xin Guo, Shinnosuke Furuya, Jun-ichi Iwata, Atsushi Oshiyama We report first-principles calculations that clarify stability and electronic structures of silicene on Ag(111) surfaces. We find that several stable structures exist for silicene /Ag(111), exhibiting a variety of surface morphology. We also find that Dirac electrons are absent near Fermi energy in all the stable structures due to buckling of the Si monolayer and mixing between Si and Ag orbitals. We propose that either BN substrate or hydrogen processing of Si surface is a good candidate to preserve Dirac electrons in silicene. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N22.00014: Engineering of the Static Interface Dipole in Metal/Organic Nanohybrid Materials Axel Enders, Donna Kunkel, Justin Nitz, Peter Dowben, Lucie Routaboul, Bernard Doudin, Pierre Braunstein, Scott Simpson, Eva Zurek We studied small molecules with large intrinsic electrical dipole as model system for molecular films adsorption on surfaces for altering the interface dipole screening. More specifically, we investigated the self-assembly and electronic interface properties of zwitterionic molecules of type C6H2(\textellipsis NHR)2(\textellipsis O)2 (R $=$ H, ...), adsorbed on Cu(111), Ag(111), Au(111) surfaces with scanning tunneling microscopy in UHV. These molecules carry positive and negative charges on opposite parts of the molecule, resulting in a large electric dipole of typically 10 Debye. We find that the dipole of the surface-supported molecule is decreased with respect to free species and of order of 1 - 2 Debye, depending on the substrate material. The molecules self-assemble into 2D structures upon adsorption, where the substrate-dependent strength of the dipolar interactions between the adsorbed molecules dictates the network architecture. DFT calculations were performed to analyze adsorption geometry, charge transfer and dipole moment. We will show that zwitterionic molecules provide a unique opportunity to engineer the interface dipole in metal/organic hybrid structures, which ultimately controls the charge injection barrier in devices. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N22.00015: Self-Organized Growth of Single Crystalline Copper Nanobead Strings by Electrodeposition Cong Meng, Ruwen Peng, Mu Wang Here we report a self-organized growth of single-crystalline strings of nano sized copper beads electrodeposited from an ultrathin layer of CuSO4 electrolyte solution without adding any additives. Spontaneous oscillation of voltage/current has been observed when potentiostatic/galvanostatic mode is applied. Scanning electron microscopy indicates that the filaments developed from the cathode are made of smooth copper beads a few hundreds of nanometers in size connected by thin single-crystalline rods. The periodicity along the string may vary from 500nm to one micron, and the spatial periodicity is strict up to hundreds of microns. To pinpoint the growth mechanism, we intentionally terminate the growth at different stage of the spontaneous oscillation of the voltage across the electrodes, and established the relation of the microscopic deposit morphology and the voltage oscillations. A growth mechanism is proposed based on the experimental observations. Structural and luminescent properties of the copper strings have been investigated, and the possible applications of such a unique structure have been discussed. [Preview Abstract] |
Session N23: Fractional Quantum Hall Theory I
Sponsoring Units: FIAPChair: B. Andrei Bernevig, Princeton University
Room: 325
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N23.00001: When is a ``wavefunction'' not a wavefunction?: a quantum-geometric reinterpretation of the Laughlin state F.D.M. Haldane The Laughlin state is the fundamental model for the description of fractional quantum Hall (FQH) fluids and was presented as a ``lowest Landau-level (LLL) Schr\"odinger wavefunction'', i.e., of the form $f(z_1,\ldots ,z_N)\exp -\sum_i z_i^*z_i/2$, where $z_i$ = $(x_i + iy_i)/\surd 2\ell_B$, and $|z_i-z_0|^2$ = constant is the shape of a Landau orbit. Its characterization as a LLL wavefunction was generally accepted without question, and leads to ``explanations'' of its success in terms of judicious placement of its zeroes. However, the Laughlin state also occurs in the n=2 LL, and now has been found in Chern-insulator lattice systems. Numerical studies confirm that (without direct reference to which LL is partially-occupied) its success can be explained solely in terms of the short-range repulsion between the non-commuting guiding centers of Landau orbits. These (as a ``quantum geometry'') do not by themselves have a Schr\"odinger (as opposed to Heisenberg) description. A reexamination shows that the variable ``z'' describes the shape of an emergent geometry of the FQH fluid derived from the Coulomb interaction, not the Landau-orbit shape, and that the holomorphic function is a coherent state representation of a Heisenberg state, not a Schr\"odinger wavefunction. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N23.00002: The correct theory of the quantum Hall effect fractions Keshav Shrivastava The effective charge,e*$=$(1/2)ge, obtained by introducing the symmetric g values, g$=$(2J$+$1)/(2L$+$1) with J given by L and S with both signs for S, and the Bohr magneton, used in the cyclotron frequency leads to factors of the type (1/2)g(n$+$1/2) in the eigen values which give the correct description of the modified Landau levels. The resistivity after introducing the flux quantization is modified by the effective charge which gives the plateaux. The helicity of every electron is defined by the sign of p.s where p is the linear momentum and s is the spin. Hence the $+$s particles move in the direction opposite to those of -s. The principal fractions, two-particle states and resonances explain most of the data. The remaining data is explained by the formation of electron clusters with spin different from 1/2. In this way all of the 101 or more fractions of the experimental data are correctly derived from the theory. The theory does not depend seriously on the dimensionality so it explains the graphite as well as the graphene.\\[4pt] [1] K. N. Shrivastava, AIP Conf. Proc. 1482,335-339(2012); AIP Conf. Proc. 1150, 59-67(2009); International J. Mod. Phys. B 25, 1301-1357(2011).\\[0pt][2] Maher M. A. Ali and K. N. Shrivastava, AIP Conf.Proc. 1482, 43-46(2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N23.00003: Nematic order and a new field theory of the quantum Hall effect Joseph Maciejko, Benjamin Hsu, Yeje Park, Steve Kivelson, Shivaji Sondhi Motivated by recent experimental and theoretical studies of anisotropic versions of the fractional quantum Hall (FQH) effect, we construct an effective field theory for a continuous quantum phase transition between an isotropic FQH state and a nematic FQH state. The theory parallels earlier work on FQH ferromagnets. The $SO(3)$ order parameter $\mathbf{n}$ of the ferromagnet is replaced by the Landau-de Gennes nematic tensor order parameter $Q_{ab}$ which can be mapped to a $SO(2,1)$ Lorentz vector. We construct an analog of the $CP^1$ representation of a ferromagnet in terms of complex $SU(1,1)$ spinors. We identify these vector and spinor order parameters with the unimodular metric and zweibein fields appearing in Haldane's recent geometrical description of the FQH effect, where the metric field $g_{ab}$ is given by the matrix exponential of the nematic order parameter $Q_{ab}$. Our theory predicts that if the gap of the Girvin-MacDonald-Platzman collective mode can be made to collapse at zero momentum in a FQH system, an instability to a FQH nematic state should occur. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N23.00004: Elementary formula for the Hall conductivity of interacting systems Titus Neupert, Luiz Santos, Claudio Chamon, Christopher Mudry We proof a formula for the Hall conductivity of interacting electrons under the assumption that the ground state manifold has finite degeneracy and discrete translation symmetry is neither explicitly nor spontaneously broken. Via an algebraic regularization, our derivation makes use of the noncommutative relations obeyed by the components of the position and density operators in topological band structures. We discuss the implications of our result in the context of fractional Chern insulators. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N23.00005: Fractional Quantum Hall Effect from Phenomenological Bosonization Vladimir Zyuzin In this work we propose a model of the fractional quantum Hall effect within conventional one-dimensional bosonization. It is shown that in this formalism the resulting bosonized fermion operator corresponding to momenta of Landau gauge wave function is effectively two-dimensional. At special filling factors the bulk gets gapped, and the theory is described by a sine-Gordon model. The edges are shown to be gapless, chiral, and carrying a fractional charge. The hierarchy of obtained fractional charges is consistent with existing experiments and theories. It is also possible to draw a connection to composite fermion description and to the Laughlin many-body wave function. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N23.00006: Axial anomaly of Lifshitz fermions with arbitrary anisotropic scaling $z$ in $2n$ spacetime dimensions Xueda Wen We calculate the axial anomaly of a Lifshitz fermion with arbitrary anisotropy scaling exponent $z$ which is coupled to gauge fields in $2n$ spacetime dimensions. We find that the result is identical to the relativistic case. The conclusion is verified with both path integral methods and spectral methods in $2n$ spacetime dimensions. Our work is a generalization of I. Bakas' work (arXiv:1110.1332) which focuses on (3+1) dimensions. In addition, we discuss the application of our conclusion to transport processes in quantum Hall systems as well as Weyl semi-metals. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N23.00007: Metallic phase of the quantum Hall effect in four-dimensional space Jonathan Edge, Jakub Tworzydlo, Carlo Beenakker We study the phase diagram of the quantum Hall effect in four-dimensional (4D) space. Unlike in 2D, in 4D there exists a metallic as well as an insulating phase, depending on the disorder strength. The critical exponent $\nu\approx 1.2$ of the diverging localization length at the quantum Hall insulator-to-metal transition differs from the semiclassical value $\nu=1$ of 4D Anderson transitions in the presence of time-reversal symmetry. Our numerical analysis is based on a mapping of the 4D Hamiltonian onto a 1D dynamical system, providing a route towards the experimental realization of the 4D quantum Hall effect. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N23.00008: Correlations in incompressible quantum liquid states: constructions of electronic trial wavefunctions John Quinn Numerical studies indicate that incompressible quantum Hall states occur when the relation between the single particle angular momentum $l$ and the number $N$ of electrons in the partially filled Landau level is $2l = \nu^{-1}N-c_\nu$. Here, $\nu$ is the filling factor and $c_\nu$ is a ``finite size shift.'' The values of $c_\nu$ found numerically depend on correlations, and for $\nu=p/q\leq 1/2$ are given by $c_\nu = q+1-p$. This finite size shift points the way to constructing electronic trial wavefunctions. A trial wavefunction can always be written $\Psi = FC$, where $F = \prod_{i < j}z_{ij}$ and $C(z_{ij})$ is a symmetric correlation function caused by interactions. For the Moore-Read state, $C_{MR}(z_{ij})$ is a product of $F$ and the antisymmetric Pfaffian. $C_{MR}$ is not the only possible correlation function for this state. Another choice is the quadratic function $C_Q = S \left\{\prod_{i < j\in g_A} \prod_{k < l\in g_B}(z_{ij}z_{kl})^2\right\}$, where $S$ is a symmetrizing operator, and $g_A$ and $g_B$ each contain $N/2$ particles resulting from a partition of N into two sets. For the Jain states (e.g. $\nu=2/5$), different partitioning of $N$ particles into sets of unequal size gives appropriate correlation functions. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N23.00009: Exactly solvable $U(1)\times U(1)$ boson models for integer and fractional quantum Hall insulators in two dimensions Olexei Motrunich, Scott Geraedts We present a solvable boson model with $U(1)\times U(1)$ symmetry in (2+1) dimensions that realizes insulating phases with a quantized Hall conductivity $\sigma_{xy}$. The model is short-ranged, with no topological terms, and can be realized by a local Hamiltonian. For one set of parameters, the model has a non-fractionalized phase with $\sigma_{xy}=2n$ in appropriate units, with $n$ an integer. In this case, the physical origin is dynamical binding between $n$ bosons of one species and a vortex of the other species and condensation of such composites. Other choices for the parameters of the model yield a phase with $\sigma_{xy}=2\frac{c}{d}$, where $c$ and $d$ are mutually prime integers. In this phase, $c$ bosons dynamically bind to $d$ vortices and such objects condense. The are two species of excitations that are bosonic by themselves but carry fractional charge $1/d$ and have mutual statistics $2\pi\frac{b}{d}$, where $b$ is an integer such that $ad-bc=1$, and $a$ is also an integer. The model can be studied using sign-free Monte Carlo. We have performed simulations which include a boundary between a quantum Hall insulator and a trivial insulator, and found gapless edge states on the boundary. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N23.00010: Matrix Product States and Fractional Quantum Hall B. Andrei Bernevig, Benoit Estienne, Nicolas Regnault, Zlatko Papic We present an exact matrix product state expansion (MPS) for a large series of Jack polynomial wavefunctions which serve as Fractional Quantum Hall ground-states of pseudopotential Hamiltonians. Using the basis of descendants in Virasoro and W algebras we build MPS descriptions of the (k,2) Jacks which include the Moore-Read state and the Gaffnian state, as well as MPS representation of the Z$_3$ Read-Rezayi state. We then give a general method for computing MPS representations for other non-abelian states and their quasiholes. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N23.00011: Benchmarking MPS for fractional quantum Hall states Nicolas Regnault, Benoit Estienne, Zlatko Papic, B. Andrei Bernevig We discuss the numerical apsects of the Matrix Product State (MPS) representation for a large series of Fractional Quantum Hall states. We benchmark the MPS for several model states such as the Read-Rezayi series using both overlap, energies, densities and pair correlation functions. We discuss how accurate this description is depending on the geometry (sphere, disk or cylinder). As an application, we use the MPS to compute the size of the quasiholes for the Read-Rezayi series. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N23.00012: Exact Matrix Product States for Quantum Hall Wave Functions Roger Mong, Michael Zaletel We show that the model wave functions used to describe the fractional quantum Hall effect have exact representations as matrix product states (MPS). These MPS can be implemented numerically in the orbital basis of both finite and infinite cylinders, which provides an efficient way of calculating arbitrary observables. We extend this approach to the charged excitations and numerically compute their Berry phases. Finally, we present an algorithm for numerically computing the real-space entanglement spectrum starting from an arbitrary orbital basis MPS, which allows us to study the scaling properties of the real-space entanglement spectra on infinite cylinders. The real-space entanglement spectrum obeys a scaling form dictated by the edge conformal field theory, allowing us to accurately extract the two entanglement velocities of the Moore-Read state. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N23.00013: Long-wavelength corrections to Hall conductivity in fractional quantum Hall fluids Bo Yang, F.D.M. Haldane Recent work by Hoyos and Son, then Bradlyn et al., has investigated the relation between the long-wavelength ($O(q^2)$) corrections to the Hall conductivity $\sigma_H({\mathbf q})$ and the Hall viscosity of quantum Hall states. These works assume the presence of Galilean and rotational invariance. However, these are not generic symmetries of electrons in condensed matter. We identify translation and (2D) inversion symmetry as the only generic symmetries of an ``ideal" quantum Hall liquid, as these are needed to guarantee the absence of any dissipationless ground state current density; then $\sigma_H({\mathbf q})$ = $\sigma_H(-{\mathbf q})$ characterizes the dissipation less current that flows in response to a spatially-non-uniform electric field. We consider the general problem for fractional quantum Hall (FQH) states without Galilean or rotational invariance, when the guiding-center contribution to the Hall viscosity becomes a non-trivial tensor property related to an emergent geometry of the FQH state, (Bo Yang et,al (PRB 85,165318). [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N23.00014: Fractional topological superconductors with fractionalized Majorana fermions Abolhassan Vaezi In Ref[1], I introduced a two dimensional fractional topological superconductor (FTSC) as a strongly correlated topological state which can be achieved by inducing superconductivity into an Abelian fractional quantum Hall (FQH) state, through the proximity effect. When the proximity coupling is weak, the FTSC has the same topological order as its parent state, and thus Abelian. However, upon increasing the proximity coupling, the bulk gap of such an Abelian FTSC closes and reopens resulting in a new topological order: a non-Abelian FTSC. I show that the conformal field theory (CFT) that describes the edge state of non-Abelian FTSC is $U(1)/Z_2$ orbifold theory and use this to write down the ground-state wave-function. Further, I predict FTSC based on Laughlin state at $\nu=1/m$ filling to host vortices with fractionalized Majorana zero modes. These zero modes are non-Abelian quasi-particles which is evident in their quantum dimension of $d_m=\sqrt{2m}$. Using the multi-quasi-particle wave-function based on the edge CFT, I derive the braid matrix for the zero modes. Finally, the potential applications of the non-Abelian FTSCs in the topological quantum computation will be illustrated. [1] A. Vaezi, ArXiv:1204.6245 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N23.00015: Momentum polarization: an entanglement measure of topological spin Xiaoliang Qi Topologically ordered states are states of matter which are distinct from trivial states by topological properties such as ground state degeneracy and quasi-particles carrying fractional quantum numbers and fractional statistics. The topological spin is an important property of a topological quasi-particle, which is the Berry phase obtained in the adiabatic self-rotation of the quasi-particle by $2\pi$. In this paper we propose a new approach to compute the topological spin in candidate systems of two-dimensional topologically ordered states. We identify the topological spin with a new quantity, the momentum polarization defined on the cylinder geometry. We show that the momentum polarization is determined by the quantum entanglement between the two halves of the cylinder, and can be computed from the reduced density matrix. As an example we present numerical results for the honeycomb lattice Kitaev model, which correctly reproduces the expected spin $e^{i\frac{2\pi}{16}}$ of the Ising non-Abelian anyon ($\sigma$ particle). Our result provides a new efficient approach to characterize and identify topological states of matter from finite size numerics. [Preview Abstract] |
Session N24: Electronic Structure Methods II
Sponsoring Units: DCOMPChair: Bin Wang, Vanderbilt University
Room: 326
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N24.00001: Large-scale Bethe-Salpeter equation calculations of core-level x-ray spectra J. J. Rehr, J. Vinson, K. Gilmore Recently an approach has been developed for Bethe-Salpeter equation (BSE) calculations of core-level x-ray spectra, which is implemented in the \textsc{ocean} package \footnote{ J. Vinson, E. L. Shirley, J. J. Rehr, and J. J. Kas, Phys. Rev. B {\bf 83}, 115106 (2011); J. Vinson and J. J. Rehr, Phys. Rev. B (in press , 2012)} which combines plane-wave, pseudopotential DFT electronic structure, PAW transition elements, GW self-energy corrections, and the NIST BSE solver. The method yields both dipole limited and finite momentum transfer spectra. Here we discuss several recent advances which yield a unified treatment of both extended states and atomic multiplet effects. In particular our approach now includes spin-dependent potentials and hole-dependent lifetimes, and gives an improved treatment of L$_{2,3}$ edges, where contributions to spectral weight come from a mix of two distinct core holes. We have also extended the code interface to include pseudopotential wave functions from \textsc{abinit}, \textsc{QuantumEspresso}, or an interpolation based scheme, thus enabling large-scale calculations with unit cells in excess of 2000 \AA$^3$. Applications to water and ice structures are briefly discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N24.00002: Calculation of charge-transfer satellites in x-ray absorption spectroscopy of transition metal oxides E. Klevak, J.J. Kas, J.J. Rehr Charge-transfer (CT) satellites in x-ray absorption spectroscopy (XAS) require treatments of correlation effects beyond the quasi-particle approximation. Here we present an approach for including CT effects in XAS that follows the model of Lee \textit{et al.}\footnote{J.D. Lee, O. Gunnarsson and L. Hedin, Phys. Rev. B \textbf{60}, 8034 (1999)} The approach is based on a three level system coupled to an itinerant state, with parameters obtained from either ab inito calculations or x-ray photoemission spectroscopy. The model yields an approximation to CT satellites in XAS in terms of a convolution of the quasi-particle spectrum with an energy-dependent spectral function that accounts for both localized CT excitations and solid state effects. The approach illustrates the crossover from the sudden to adiabatic approximations. Calculations for transition metal oxides, e.g. NiO and CoO, give reasonable agreement with XAS experiment. Finally, an extension of the present approach to CT satellites in resonant inelastic x-ray spectroscopy is also discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N24.00003: Correlation matrix renormalization approximation for total energy calculations of correlated electron systems Y.X. Yao, C. Liu, J. Liu, W.C. Lu, C.Z. Wang, K.M. Ho The recently introduced correlation matrix renormalization approximation (CMRA) was further developed by adopting a completely factorizable form for the renormalization z-factors, which assumes the validity of the Wick's theorem with respect to Gutzwiller wave function. This approximation (CMR-II) shows better dissociation behavior than the original one (CMR-I) based on the straightforward generalization of the Gutzwiller approximation to two-body interactions. We further improved the performance of CMRA by redefining the z-factors as a function of f(z) in CMR-II, which we call CMR-III. We obtained an analytical expression of f(z) by enforcing the equality in energy functional between CMR-III and full configuration interaction for the benchmark minimal basis H2. We show that CMR-III yields quite good binding energies and dissociation behaviors for various hydrogen clusters with converged basis set. Finally, we apply CMR-III to hydrogen crystal phases and compare the results with quantum Monte Carlo. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N24.00004: Disorder effects in solid state systems beyond a single-site prospective: theories and applications Alberto Marmodoro, Arthur Ernst We review development and applications of improvement attempts upon the original Coherent Potential Approximation for the first-principles treatment of disordered systems. The single-site theory is examined in its basic aspects of analyticity and convergence, and compared with alternative methods for the study of solid state systems where a rigorous application of Bloch's theorem is no longer possible. The aspects of local environment effects, short-range ordering and off-diagonal disorder are considered in different extension proposals, in tight-binding and ab-initio illustrations based on multiple-scattering theory. In this context, results from application of a generalized version of the method are discussed evaluating some effects of disorder in solid state metallic solutions, molar doping materials for fuel cell technology, and magnetic compounds and excitations. Results from alternative methodologies such as supercell or special quasi-random structure approximations are also examined. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N24.00005: First-principles studies of photoelectron spectroscopy of solvated hydronium and hydroxide in water Charles Swartz, Xifan Wu Solvated hydronium (H3O$^+$) and hydroxide (OH$^-$) are important solutions of water defects. In a recent state-of-the-art photoelectron spectroscopy (PES) experiment, the binding energies of these water defects have been measured. Theoretically, we show that the photoelectron spectroscopy can be accurately computed based on GW quasi-particle theories, in which the molecular solvation structures are generated by ab initio molecular dynamics (AIMD). The resulting hydronium and hydroxide binding energies are 10 eV and 19 eV respectively, which are closely consistent with the recent PES experimental values of 9.2 eV and 20 eV. A close inspection reveals that the defect orbitals originate from the 1b$_2$ (1b$_1$) state of H3O$^+$ (OH$^-$) molecules in the gas phase. These orbitals are further strongly distorted by the surrounding water molecules, in which the H3O+ and OH- defects states are clearly localized on the so-called Zundel and Eigen solvation structures respectively. Proton transfers are found to further broaden the PES spectrum, which is more prominent in H3O$^+$ than in OH$^-$ solutions. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N24.00006: Bethe-Salpeter equation calculations of resonant inelastic x-ray scattering at the nitrogen K edge John Vinson, Terrence Jach, Tim Elam, Jonathon Denlinger We present theoretical calculations of resonant inelastic x-ray scattering (RIXS) at the nitrogen K edge of several materials along with direct comparison to experimental results. Our approach is based on a Bethe-Salpeter equation formalism, and our calculations are carried out using an extension of the \textsc{ocean} package,\footnote{J. Vinson, E. L. Shirley, J. J. Rehr, and J. J. Kas, Phys. Rev. B {\bf 83}, 115106 (2011) } including both intermediate and final-state excitonic effects. By building upon a DFT basis we include ground-state effects without system-dependent fitting parameters. We are able to account for the general trends and features seen in experiment. A more \textit{ad hoc} account of other contributions to the measured spectra, primarily phonon coupling, is attempted, but this highlights some current shortcomings limiting fully \textit{ab initio} calculations of the near-edge x-ray spectra of extended systems. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N24.00007: Generalized optimization of Wannier functions Emanuel Lazar, Hyowon Park, Chris Marianetti, Andy Millis Marzari and Vanderbilt introduced and developed a technique for defining and computing ``maximally localized'' Wannier functions to represent localized orbitals in periodic materials [1]. Since then, this method has been heavily used in computational condensed matter physics calculations. The Marzari-Vanderbilt procedure localizes all orbitals in a given energy window. In this talk we present some ongoing work in generalized minimization strategies which can apply different constraints to different subspaces of the manifold (for example, localizing some orbitals more than others). Applications to model systems and more realistic low-dimensional materials are presented.\\[4pt] [1] Marzari et al. Rev. Mod. Phys. 84, 1419 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N24.00008: Improving the Accuracy of Diffusion Monte Carlo: Insights from Calculations of High Pressure Solid-Solid Phase Transitions L. Shulenburger, T. R. Mattsson A challenging application for any electronic structure method is the calculation of solid-solid phase transitions under pressure. Due to stringent requirements on accuracy imposed by the sensitivity of such transitions on small changes in free energy these calculations are exquisitely sensitive to any systematic errors in the calculations. In this talk we will review the present sources of methodological uncertainties in the diffusion quantum Monte Carlo (DMC) technique and study their effects on the calculation of solid-solid phase transitions. Particular attention will be paid to finite size effects and errors arising from the use of pseudopotentials. \\ \\ Sandia National Laboratories is a multiprogram 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 No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N24.00009: Assessing the connection between charge density and local fields Eric Shirley The dielectric screening of a potential disturbance depends on the electronic charge density. Local-field effects, such as those which generate non-zero off-diagonal matrix elements of the dielectric matrix, are related to Fourier components of the charge density. This talk will review the degree to which one can predict such off-diagonal effects based on the charge density alone. This shall be done within an independent-particle approximation and using model dielectric functions so that the results can be compared. We shall sample a range of metals, semiconductors and insulators. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N24.00010: Anomalous Anharmonic Phonons in PbTe Reproduced from First-Principles Calculations Yue Chen, Chris Marianetti PbTe is of great interest due to its thermoelectric properties. Inelastic neutron scattering experiments reveal a signature of strong anharmonicity as evidenced in an anomalous temperature dependence of the phonon spectra. Novel approaches based on first-principles calculations have been developed for computing anharmonic phonons at elevated temperatures in recent years, though these techniques do not include lifetime effects and hence cannot address the anomalies observed in experiment. Here we perform first-principles molecular dynamics which includes the anharmonic terms at lowest order. The temperature dependent phonon spectra is computed and compared to experimental measurements, yielding insight on the origin of the observed anomalies. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N24.00011: Endohedral fullerene as acceptor: A DFT study on charge transfer states of Sc$_3$N@C$_{80}$-porphyrin complex Fatameh Amerikheirabadi, Luis Basurto, Rajendra Zope, Tunna Baruah C$_{60}$ fullerene and its derivatives are the most popular acceptors which are used in molecular/polymeric complexes used in organic photovoltaics. Endohedral fullerenes are shown to produce long lived charge separated states. The Sc$_3$N@C$_{80}$, the third most abundant fullerene after C$_{60}$ and C$_{70}$, has a larger cage with a radius of 4.1 Ang. We have carried out a DFT study on the electronic structure of ground and charge transfer states of a model Sc$_3$N@C$_{80}$-Zn tetraphenyl porphyrin cofacial complex. The C$_{80}$ cage used in our calculations has icosahedral symmetry. We find that the lowest charge transfer state with a hole on the porphyrin and an electron on the Sc$_3$N@C$_{80}$ is at 2.1 eV above the ground state. The calculations show that different orientations of the Sc$_3$N unit to the porphyrin plane do not significantly alter the electronic structure. The electronic structure of the complex and its components along with the exciton binding energies will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N24.00012: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N24.00013: Melting of ice simulated by a multicanonical method combined with a first-principles calculation Yoshihide Yoshimoto Water is a ubiquitous material and is both scientifically and technologically important. For the simulation of water, the most common PBE semi-local exchange correlation (XC) functional has an issue: it gives over-structured liquid compared to the experimental one for a given temperature. On the other hand, the PBE0 hybrid XC functional was claimed to be better for the description of water recently [1,2]. In this study, the melting of ice, one of its most fundamental property, was simulated by a multicanonical method combined with a first-principles calculation [3,4]. Both the PBE XC functional and the PBE0 hybrid XC functional were adopted for the simulation. With accelerated computation of the hybrid functional by GPGPU, it was found that the PBE0 XC hybrid functional gave an improved melting temperature compared to that by PBE [5].\\[4pt] [1] C. Zhang and G. Galli et al., J. Chem. Theory and Comput., 7, 1443 (2011).\\[0pt] [2] B. Santra and M. Scheffler et al., J. Chem. Phys., 131, 124509 (2009).\\[0pt] [3] Y. Yoshimoto, J. Chem. Phys., 125, 184103 (2006).\\[0pt] [4] Y. Yoshimoto, J. Phys. Soc. Jpn., 79, 034602 (2010).\\[0pt] [5] S. Yoo, X.C. Zeng, and S.S. Xantheas, J. Chem. Phys., 130, 221102 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N24.00014: DCA$^+$: Incorporating self-consistently a continuous momentum self-energy in the Dynamical Cluster Approximation Peter Staar, Thomas Maier, Thomas Schulthess The dynamical cluster approximation (DCA) is a systematic extension beyond the single site approximation of dynamical mean field theory (DMFT) to include spatially non-local correlations in quantum many-body simulations using a finite size embedded cluster. In the last decade, the DCA has been very useful in describing and analyzing phase transitions in models of correlated electron systems such as the single-band Hubbard model. In the standard DCA algorithm, the single-particle self-energy is approximated by a step function in momentum space, with constant values in regions about the cluster momenta. As a consequence, results often depend sensitively on the topology and morphology of the chosen cluster and the corresponding cluster momenta. Here, we present an extension to the standard DCA that incorporates a self-energy with smooth, continuous momentum dependence self-consistently in the DCA algorithm. In this new algorithm, the influence of the cluster-geometry is significantly reduced and the self-energy converges much more rapidly as a function of cluster-size. We demonstrate the improved convergence of this algorithm for results of the pseudo-gap temperature $T^*$ and the superconducting temperature $T_c$ versus cluster-size. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N24.00015: Polaron Localization in Conjugated Polymers by Hybrid DFT Methods Nan Shao, Qin Wu Reliable application of density functional theory (DFT) to study the electronic properties of polarons remains controversial. A proper description should exhibit both the formation of a charge-localized electronic state and saturation of the polaron size for increasing oligomer length. The aim of this work is to find a proper hybrid DFT method to study the chain length related electronic properties of charged conjugated polymer system. Using oligopyrrole cations as a test case, global hybrid functionals such as BHandHLYP can show charge localization, but a well-defined polaron size does not emerge when the length of the oligomer is increased; the saturation effect was not predicted correctly. By applying 100{\%} long-range corrected hybrid functionals, LRC-PBE, the saturation of charge distribution has been achieved, implying that the LRC-PBE is a better way to describe the spatial extent of the electronic state of polypyrrole than the conventional hybrid functionals. The tuning of the range parameter and the study of other polymer polaron systems will be discussed. [Preview Abstract] |
Session N25: Focus Session: Modeling of Rare Events II
Sponsoring Units: DCOMPChair: Eric Vanden-Eijnden, New York University
Room: 327
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N25.00001: TBD Invited Speaker: Eric Vanden-Eijnden |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N25.00002: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N25.00003: Temperature and rate sensitivity of melting in Cu Amit Samanta, Tang-Qing Yu, Weinan E The nature of melting of a crystal is a long standing topic of interest in materials science. Using advanced simulation techniques such as finite temperature string method and temperature accelerated molecular dynamics, we trace the minimum free energy path (MFEP) for a transition from solid to liquid phase at different temperatures in copper. Analysis of the configurations along the MFEP reveals that the rate determining transition state and the ensuing melting mechanisms are a function of temperature of the system. Close to equilibrium melting temperatures, the saddle point is determined by the critical size of the liquid nucleus, however, at higher temperatures, we find that the saddle structure consists of defect clusters from which the liquid nucleus is formed. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N25.00004: Efficient minimum mode finding in transition states calculations Weiguo Gao, Jing Leng, Cheng Shang, Zhi-Pan Liu Transition states are fundamental to understanding the reaction dynamics qualitatively in chemical physics. To date various methods of first principle location of the transition states have been developed. In the absence of the knowledge of the final structure, the minimum-mode following method climbs up to a transition state without calculating the Hessian matrix. One weakness of this kind of approaches is that the number of rotations to determine the minimum mode is usually unpredictable. In this work, we propose a locally optimal search direction finding algorithm which is an extension of the traditional conjugate gradient method without additional calculations of the forces. We also show that the translation of forces improves the numerical stability. Experiments for the Baker test system show that the proposed algorithm is much faster than the original dimer conjugate gradient method. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N25.00005: Extension of the string method for saddle points search Weiqing Ren The string method was designed for finding minimum energy paths between two minima of a potential (or free) energy. It evolves a continuous curve in the path space by steepest descent dynamics. In this talk, we discuss how the string method can be modified for saddle point search. Compared to the existing algorithms, the new method has the advantage that the computed saddle points are guaranteed to be directly connected to the minima. We will also discuss how the convergence can be accelerated using an inexact Newton method. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N25.00006: On extreme value statistics of correlated random variables Maxime Clusel, Jean-Yves Fortin The statistics of extreme values of a set on independent and identically distributed random variables is a well established mathematical theory that can be traced back to the late 1920s, with pioneering work by Fisher and Tippett. While efforts have been made to go beyond the uncorrelated case, little is known about the extremes of strongly correlated variables. Notable exceptions are the distribution of extreme eigenvalues of random matrices (Tracy and Widom 1994), the Airy law for one-dimensional random walks (Majumdar and Comtet 2005), and random variables with logarithmic interactions (Fyodorov and Bouchaud 2008). We propose to adapt the equivalence between extremes and sums (Bertin and Clusel 2006) to obtain asymptotic distributions of correlated random variables. We will show how this approach works in the logarithmic case, before extending it to power-law correlations and beyond. We will eventually illustrate these cases with a simple model, a one-dimensional gas of interacting particles. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N25.00007: Study of the diffusion of points defects in crystalline silicon using the kinetic ART method Mickael Trochet, Peter Brommer, Laurent-Karim Beland, Jean-Francois Joly, Normand Mousseau Because of the long-time scale involved, the activated diffusion of point defects is often studied in standard molecular dynamics at high temperatures only, making it more difficult to characterize complex diffusion mechanisms. Here, we turn to the study of point defect diffusion in crystalline silicon using kinetic ART (kART)[1-2], an off-lattice kinetic Monte Carlo method with on-the-fly catalog building based on the activation-relaxation technique (ART nouveau). By generating catalogs of diffusion mechanisms and fully incorporating elastic and off-lattice effects, kART is a unique tool for characterizing this problem. More precisely, using kART with the standard Stillinger-Weber potential we consider the evolution of crystalline cells with 1 to 4 vacancies and 1 to 4 interstitials at various temperatures and to provide a detailed picture of both the atomistic diffusion mechanisms and overall kinetics in addition to identifying special configurations such as a 2-interstitial super-diffuser. \\[4pt] [1] F. El-Mellouhi, N. Mousseau and L.J. Lewis, Phys. Rev. B. 78, 153202 (2008)\\[0pt] [2] L. K. B\'eland, P. Brommer, F. El-Mellouhi, J.-F. Joly and N. Mousseau, Phys. Rev. E 84, 046704 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N25.00008: An iterative action minimizing method for computing optimal paths in stochastic dynamical systems Brandon Lindley, Ira Schwartz We present a numerical method for computing optimal transition pathways and transition rates in systems of stochastic differential equations (SDEs). In particular, we compute the most probable transition path of stochastic equations by minimizing the effective action in a corresponding deterministic Hamiltonian system. The numerical method presented here involves using an iterative scheme for solving a two-point boundary value problem for the Hamiltonian system. We validate our method by applying it to both continuous stochastic systems, such as nonlinear oscillators governed by the Duffing equation, and finite discrete systems, such as epidemic problems, which are governed by a set of master equations. Furthermore, we demonstrate that this method is capable of dealing with stochastic systems of delay differential equations. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N25.00009: The stability of vacancy-like defects in amorphous silicon Jean-Francois Joly, Normand Mousseau The contribution of vacancy-like defects to the relaxation of amorphous silicon (a-Si) has been a matter of debate for a long time. Due to their disordered nature, there is a large number local environments in which such a defect can exists. Previous numerical studies the vacancy in a-Si have been limited to small systems and very short timescales. Here we use kinectic ART (k-ART), an off-lattice kinetic Monte-Carlo simulation method with on-the-fly catalog building [1,2] to study the time evolution of 1000 different single vacancy configurations in a well-relaxed a-Si model. Our results show that most of the vacancies are annihlated quickly. In fact, while 16\% of the 1000 isolated vacancies survive for more than 1 ns of simulated time, 0.043\% remain after 1 ms and only 6 of them survive longer than 0.1 second. Diffusion of the full vacancy is only seen in 19\% of the configurations and diffusion usually leads directly to the annihilation of the defect. The actual annihilation event, in which one of the defective atoms fills the vacancy, is usually similar in all the configurations but local bonding environment heavily influence its activation barrier and relaxation energy. \\[4pt] [1] El-Mellouhi et al,Phys. Rev B. 78, (2008)\\[0pt] [2] Beland et al., Phys. Rev. E. 84, (2011) [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N25.00010: Ga Surface Diffusion on GaAs(001) $\beta$ 2(2 $\times$ 4): An ab initio Local Superbasin Kinetic Monte Carlo Study Yangzheng Lin, Kristen Fichthorn We use first-principles density functional theory to characterize the diffusion of a Ga adatom on GaAs(01) $\beta$ 2(2 $\times$ 4). Beginning with previously identified potential energy minima on this surface, we used the climbing-image nudged elastic band method to identify transition states and delineate diffusion pathways. These studies led to the discovery of eight new binding sites for Ga, which more than doubles the number that had been previously identified. The diffusion pathways for hopping between these minima involve energy barriers that vary significantly in magnitude, such that minima are spatially arranged in groups connected by low barriers and separated from each other by high barriers. Thus, the diffusion process is significantly more complex than was previously believed. To resolve the diffusion, we applied our recently developed local superbasin kinetic Monte Carlo method, which efficiently resolves the long-time dynamics of this complex process. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N25.00011: Physical Point Consequences Alfred Phillips Jr. We have considered a physical point, and accordingly we have made a distinction between a physical derivative and a mathematical derivative. We trace how this consideration impacts spacetime, general relativity (the so-called cosmological constant problem), quantum mechanics, and their one hundred twenty orders-of-magnitude discrepancy in vacuum energy. [Preview Abstract] |
Session N26: Entanglement in Many-Body Systems
Sponsoring Units: GQIChair: Ari Mizel, LPS
Room: 328
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N26.00001: Noise of Quantum Channels can Generate Quantum Entanglement from Classical Correlation Laszlo Gyongyosi, Sandor Imre Transmission of quantum entanglement will play a crucial role in future networks and long-distance quantum communications. Quantum Key Distribution, the working mechanism of quantum repeaters and the various quantum communication protocols are all based on quantum entanglement. To share entanglement between distant points, high fidelity quantum channels are needed. In practice, these communication links are noisy, which makes it impossible or extremely difficult and expensive to distribute entanglement. In this work we first show that quantum entanglement can be generated by a fundamentally new idea, exploiting the most natural effect of the communication channels: the noise itself of the link. We prove that the noise transformation of communication links that are not able to transmit quantum entanglement can be used to generate entanglement from classically correlated, unentangled input. We call this new phenomenon the Correlation Conversion property (CC-property) of communication channels. Our results have serious implications and fundamental consequences for the future of quantum communications, and for the development of global-scale quantum communication networks. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N26.00002: Measuring Entanglement Entropy of a Generic Many-Body System with a Quantum Switch Dmitry Abanin, Eugene Demler Entanglement entropy has become an important theoretical concept in condensed matter physics because it provides a unique tool for characterizing quantum mechanical many-body phases and new kinds of quantum order. However, the experimental measurement of entanglement entropy in a many-body system is widely believed to be unfeasible, owing to the nonlocal character of this quantity. Here, we propose a general method to measure the entanglement entropy. The method is based on a quantum switch (a two-level system) coupled to a composite system consisting of several copies of the original many-body system. The state of the switch controls how different parts of the composite system connect to each other. We show that, by studying the dynamics of the quantum switch only, the R\'enyi entanglement entropy of the many-body system can be extracted. We propose a possible design of the quantum switch, which can be realized in cold atomic systems. Our work provides a route towards testing the scaling of entanglement in critical systems as well as a method for a direct experimental detection of topological order.\\[4pt] [1] D. A. Abanin, E. A. Demler, Phys. Rev. Lett. 109, 020504 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N26.00003: Postion-momentum duality in the entanglement spectrum of free fermions Ching Hua Lee, Xiao-Liang Qi The entanglement spectrum (ES) provides a valuable way of studying the topological properties of a system, i.e. those of exotic phases where no usual topological order parameter exists. In this talk, I shall discuss a framework where the partitionings of various spaces, i.e. real, momentum and spin space are treated on equal footing. This relies on an equivalence of the eigenvalue spectra of certain combinations of projection operators. For instance, the ES remains invariant if we mathematically interchange the real-space projector with the occupied band projector. One can go a step further and conclude that exchanging the physical roles of real-space and momentum space projectors lead to two different systems with identical ES. Such reinterpretations allow one to extend well-known results involving real-space cuts in critical systems to those with simultaneous momentum-space cuts. The results for gapped systems are even more interesting, with the real-space ES of a generic band insulator shown to be identical to that of two different layers or spins in a specific fermi liquid state. This framework also allows one to view the Wannier polarization spectrum as the infinite temperature limit of the ES of a certain system originally defined at zero temperature. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N26.00004: Characterizing disordered fermion systems using the momentum-space entanglement spectrum Ian Mondragon-Shem, Mayukh Khan, Taylor Hughes We show that momentum-space entanglement can reveal the existence of robust extended states in disordered fermions systems. This approach represents a novel alternative to the more conventional position-space entanglement used in condensed matter settings. We illustrate this proposal by using explicit 1D models with spatially correlated disorder that exhibit phases which avoid complete Anderson localization. The momentum space entanglement spectrum clearly reveals the location of delocalized states in the energy spectrum and can be used as a signature of the phase transition between a delocalized and localized phase. We further discuss possible applications to 2D systems that exhibit topological properties which arise from the existence of robust bulk extended states in their energy spectrum. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N26.00005: Geometric entanglement for the toric code, color code and quantum double models Tzu-Chieh Wei, Rom\'an Or\'us, Oliver Buerschaper, Maarten Van den Nest We use the geometric entanglement to characterize ground states in the toric code, color code and quantum double models. We find that the entanglement in all these cases scales with the system size plus a constant term. Such a constant contribution has a topological origin, characterized previously by the entanglement entropy. In particular, the constant term in the color code is twice that in the toric code, a result consistent with a recent study that the color code is equivalent to two copies of the toric code. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N26.00006: Log divergence in finite-size quantum Riemann metric Tiago Souza, Michael Kolodubetz, Anatoli Polkovnikov We study the geometric tensor, an object that describes distances between quantum states within a ground state manifold. Traditionally, it has been studied for changes in external parameters, e.g., magnetic field, at fixed system size. Here, instead, we treat the system size as a tunable parameter, and hence analyze the distance between wave functions at different system sizes. For some simple fermion models, we find that the geometric tensor diverges logarithmically with system size in the thermodynamic limit, similar to the entanglement entropy in a CFT. We discuss similar calculations for the XY model, and comment on the relationship to RG. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N26.00007: Entanglement Entropy of the composite fermion non-Fermi liquid state at $\nu=1/2$ Junping Shao, Eun-Ah Kim There has been much interest in entanglement entropy as a measure to theoretically probe strongly correlated states that do not involve broken symmetries. In particular, one may hope entanglement entropy can offer quantitative characteristic of Non-Fermi liquids which are otherwise defined based on ``what they are not part of.'' Swingle and Senthil [1] conjectured that the entanglement entropy of non-Fermi liquids will be at most of order $L^{d-1}\log{L}$ for a region of linear size $L$. However, to date, there is no explicit calculation of entanglement entropy for non-Fermi liquids (though there are calculations for spin-liquids with spinon fermi surface). Here we perform a Monte Carlo calculation of the entanglement entropy for the best established example of strongly correlated non-Fermi liquid: gapless state at $\nu=1/2$. We use a composite fermion many body wavefunction in a toroidal geometry and use the swap operator to calculate the second Renyi entropy. We discuss the resulting scaling behavior in the context of the Swingle-Senthil conjecture.\\[4pt] [1] B. Swingle and T. Senthil, arXiv:1112.1069. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N26.00008: Renyi Entropy of the Interacting Fermi Liquid Jeremy McMinis, Norm Tubman Entanglement properties, including the Renyi $\alpha$-entropies and scaling laws, are becoming increasingly important in condensed matter physics. In this work we use variational quantum Monte Carlo to compute the Renyi $\alpha$-entropies, their scaling laws, and the relationship between different $\alpha$-entropies for one of the most important phases in condensed matter, the interacting Fermi liquid. Contrary to recent theoretical predictions, we find that interactions increase the prefactor for the $\alpha$-entropy scaling laws for all particle interaction strengths and forms. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N26.00009: Multipartition of Spatially Entangled Systems with Sine Square Deformation Isao Maruyama We propose a method to decouple quantum systems without disturbing the Fermi sea, extending the sine-square deformation (SSD)[1,2] toward more general cases. This multipartition operation opens a way to real-time manipulation for separating the gapless Fermi liquid system spatially into several decoupled systems without losing quantum entanglement among them. As a demonstration of entanglement preservation, by solving the time-dependent Scr\"odinger equation numerically, we show that our method works well in entanglement dynamics of non-interacting tight-binding models on a one dimensional zigzag chain and a two dimensional square lattice. [1] A. Gendiar, et. al., Prog. Theo. Phys. 122. 953 (2009) [2] IM, et.al., Phys. Rev. B. 84. 165132 (2011) and references therein [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N26.00010: Entanglement in fermionic superlattices Raimundo dos Santos, Tiago Mendes-Santos, Thereza Paiva We discuss how entanglement of strongly correlated fermions is influenced by a superlattice structure, by considering a one-dimensional Hubbard superlattice, made up of a repeated pattern of $L_U$ repulsive sites followed by $L_0$ free sites. Lanczos diagonalization of lattices up to 24 sites are used to calculate the von Neumann entropy and the negativity. The breakdown of particle-hole symmetry broadens the maxima of the entropy in the underdoped region, while the entanglement in the overdoped region is crucially influenced by the nature of the magnetic state, with dips at densities corresponding to repulsive layer singlets and to $q=\pi$ (in units of inverse unit cell length, $L_U+L_0$) spin-density waves; at these special densities the system is either a Mott insulator or a `compressible insulator'. We have also found that sites in the repulsive layer (for $L_U\geq2$) are monogamically entangled with each other. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N26.00011: Thermal Reduced Density Matrices in Fermion and Spin Ladder Systems Xiao Chen, Eduardo Fradkin A recent numerical study [1] found that the reduced density matrix of a spin 1/2 system on a two-leg ladder is the same as the spectrum of a spin 1/2 chain at a finite temperature determined by the spin gap of the ladder. We investigate this interesting result by considering two-leg ladders of free fermions and spin systems with a gapped ground state using several controlled approximations. We calculate the entanglement entropy for the cut made between the chains. In the fermionic system we find the explicit form of the reduced density matrix for one of the chains and determine the entanglement spectrum explicitly. In the case of the spin system, we consider both the strong coupling limit by using perturbation theory and weak coupling limit by using replica trick method. The calculation shows that, 1) the Von Neumann entropy equals to the thermal entropy of one chain, 2) the R'enyi entropy is equivalent to the free energy of one chain, and 3) the coupling constant (gap) plays the role of effective temperature. This result can be generalized to other coupled critical systems with a bulk gap. This work was supported in part by the NSF grant DMR-1064319 at the University of Illinois [1] D. Poilblanc, Phys. Rev. Lett. {\bf 105}, 077202 (2010) [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N26.00012: Entanglement measures and the quantum to classical mapping Jesko Sirker A quantum model can be mapped to a classical model in one higher dimension. Here we introduce a finite-temperature correlation measure based on a reduced density matrix $\bar\rho_{\bar A}$ obtained by cutting the classical system along the imaginary time (inverse temperature) axis. We show that the von-Neumann entropy $\bar S_{\rm ent}$ of $\bar\rho_{\bar A}$ shares many properties with the mutual information, yet is based on a simpler geometry and is thus easier to calculate. For one-dimensional quantum systems in the thermodynamic limit we prove that $\bar S_{\rm ent}$ is non-extensive for all temperatures $T$. For the integrable transverse Ising and $XXZ$ models we demonstrate that the entanglement spectra of $\bar\rho_{\bar A}$ in the limit $T\to 0$ are described by free-fermion Hamiltonians and reduce to those of the regular reduced density matrix $\rho_A$---obtained by a spatial instead of an imaginary-time cut---up to degeneracies. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N26.00013: Entanglement Entropy and Spectra of the One-dimensional Kugel-Khomskii Model Rex Lundgren, Victor Chua, Gregory Fiete We study the quantum entanglement of the spin and orbital degrees of freedom in the one-dimensional Kugel-Khomskii model, which includes both gapless and gapped phases, using analytical techniques and exact diagonalization with up to 16 sites. We compute the entanglement entropy, and the entanglement spectra using a variety of partitions or ``cuts'' of the Hilbert space, including two distinct real-space cuts and a momentum-space cut. Our results show the Kugel-Khomski model possesses a number of new features not previously encountered in studies of the entanglement spectra. Notably, we find robust gaps in the entanglement spectra for both gapped and gapless phases with the orbital partition, and show these are not connected to each other. We observe the counting of the low-lying entanglement eigenvalues shows that the ``virtual edge'' picture which equates the low-energy Hamiltonian of a virtual edge, here one gapless leg of a two-leg ladder, to the ``low-energy'' entanglement Hamiltonian breaks down for this model, even though the equivalence has been shown to hold for similar cut in a large class of closely related models. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N26.00014: Understanding the entanglement entropy and spectra of 2D quantum systems through arrays of coupled 1D chains Andrew James, Robert Konik We study the entanglement entropy and spectra of a coupled array of N one dimensional quantum Ising chains in their continuum limit. Employing a DMRG algorithm specifically adapted to the study of coupled, continuum systems, we are able to study large arrays of chains (up to N=200) both in their gapped phase and in the approach to criticality. Away from criticality the entanglement entropy obeys an area law. Close to criticality the entanglement entropy continues to obey the area law but possesses an additive piece scaling as $c_{eff}\log (N)/6$ with $c_{eff} \approx 1$. We also study the entanglement spectra of the coupled chains. Away from criticality in the disordered phase the low lying portion of the entanglement spectra appears similar to that of a single gapped quantum Ising chain. As the critical point is approached the entanglement gap closes. A finite size scaling analysis shows that the entanglement gap and the energy gap vanish at the same value of interchain coupling. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N26.00015: Entanglement spectrum and entangled modes of random XX spin chains Mohammad Pouranvari, Kun Yang We study in this work the ground state entanglement properties of finite XX spin-1/2 chains in with random couplings, using Jordan-Wigner transformation. We divide system into two parts and study reduced density matrixes (RDMs) of its subsystems. Due to the free-fermion nature of the problem, the RDMs take the form of that of a free fermion thermal ensemble. Finding spectrum of the corresponding entanglement Hamiltonian and corresponding eigenvectors, and comparing them with real space renormalization group (RSRG) treatment, we establish the validity of the RSRG approach for entanglement in the limit of strong disorder, but also find its limitations when disorder is weak. In the latter case our work provides a way to visualize the effective spins that form long distance singlet pairs. [Preview Abstract] |
Session N27: Focus Session: Nano/Optomechanics II
Chair: Michael Metcalfe, Booz Allen HamiltonRoom: 329
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N27.00001: On-chip cavity quantum phonodynamics: spin qubits and nano/optomechanics Invited Speaker: Charles Tahan Sound can be just as quantum as light. But our toolbox for single quanta of sound, i.e. phonons, is currently insufficient. Here we propose a new component that enables a chip-based, solid-state analogue of cavity-QED utilizing acoustic phonons instead of photons. We show how long-lived and tunable acceptor impurity states in silicon nanomechanical cavities can play the role of a matter non-linearity for coherent phonons just as, for example, the Josephson qubit plays in circuit-QED. Both strong coupling (number of coherent Rabi oscillations of approximately 100) and strong dispersive coupling (0.1-2 MHz) can be reached in the 1-20 GHz frequency range, making the system compatible with existing high-Q, nanomechanical resonators. We give explicit experimental signatures and measurement protocols of the acceptor-cavity system via a phonon probe. This system enables the control of single phonons and phonon-phonon interactions, dispersive phonon readout of the acceptor qubit, and compatibility with other nano/optomechanical components such as phonon-photon translators. (This work in collaboration with Rusko Ruskov, LPS; work with Oney Soykal, LPS, will also be discussed.) [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N27.00002: Cavity optomechanics in the quantum regime Amir H. Safavi-Naeini, Simon Groeblacher, Jeff Hill, Jasper Chan, Oskar Painter We use coherent laser light to address the mechanical degrees of freedom of engineered nanostructures with record high efficiency. With sufficient cryogenic precooling, the effects of the quantum optical shot-noise coupled onto the mechanics, and its modification by the mechanical susceptibility can be probed. In this talk we present our recent experiments studying the quantum properties of such systems. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N27.00003: Silicon Integrated Cavity Optomechanical Transducer Jie Zou, Houxun Miao, Thomas Michels, Yuxiang Liu, Kartik Srinivasan, Vladimir Aksyuk Cavity optomechanics enables measurements of mechanical motion at the fundamental limits of precision imposed by quantum mechanics. However, the need to align and couple devices to off-chip optical components hinders development, miniaturization and broader application of ultrahigh sensitivity chip-scale optomechanical transducers. Here we demonstrate a fully integrated and optical fiber pigtailed optomechanical transducer with a high Q silicon micro-disk cavity near-field coupled to a nanoscale cantilever. We detect~the motion of the cantilever by measuring the resonant frequency shift of the~whispering~gallery mode of the micro-disk. The sensitivity near the standard quantum limit can be reached with sub-uW optical power. Our on-chip approach combines compactness and stability with great design flexibility: the geometry of the micro-disk and cantilever can be~tailored to~optimize the mechanical/optical Q factors and~tune the mechanical frequency over two orders of magnitudes. Electrical transduction in addition to optical transduction was also demonstrated and both can be used to effectively cool the cantilever. Moreover, cantilevers with sharp tips overhanging the chip edge were fabricated to potentially allow the mechanical cantilever to be coupled to a wide range of off-chip systems, such as spins, DNA, nanostructures and atoms on clean surfaces.~ [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N27.00004: Developement of an optomechanical device for microwave to telecom wavelength quantum state transfer J.M. Fink, A. Pitanti, C.U. Lei, J.T. Hill, A.H. Safavi-Naeini, O. Painter A promising hardware platform for quantum computers is based on solid-state superconducting circuits which offer fast processing times and scalability. Circuit QED systems can however only operate in ultra-cold environments where thermal noise and resistive losses are negligible. We are working on an integrated optomechanical microwave-photonic device which has the potential to efficiently convert microwave excitations to telecom wavelength photons. Such a device would put within reach the realization of hybrid and long distance quantum communication networks. We have designed and fabricated slot mode photonic crystal cavities which share a mechanical mode with the capacitance of a lumped element microwave resonator. A continuously pumped state transfer protocol should enable efficient wavelength conversion even in the absence of strong optomechanical and electromechanical coupling [1] and has recently been demonstrated within the optical domain [2]. We will present our latest progress with the design, fabrication and characterization of our electro-optomechanical wavelength conversion device.\\[4pt] [1] A.~H.~Safavi-Naeini and O.~Painter, New J.~Phys.~13, 013017 (2011)\\[0pt] [2] J.~T.~Hill, A.~H.~Safavi-Naeini, J.~Chan and O.~Painter, arXiv:1206.0704 [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N27.00005: Optomechanical effects of two-level states in electromechanical devices Junho Suh, Aaron Weinstein, Keith Schwab It is now clearly established that the presence of two-level states can act as a power-dependent dielectric and lead to non-linear response of lithographic superconducting circuits. We observe these effects in a parametrically coupled, superconducting electro-mechanical system. In this case, the driven two-level states shift the microwave resonance frequency, and modulate the mechanical resonance through the optical spring effect. When pumping with two tones to realize a back-action evading measurement, these effects produce mechanical frequency modulation at twice the mechanical resonance, leading to a parametric instability for strong drives sufficient to produce a single quadrature measurement near the zero-point level. We also discuss schemes to avoid these effects in future devices. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N27.00006: Diamond mechanical resonators for strain coupling to nitrogen-vacancy centers Preeti Ovartchaiyapong, Laetitia Pascal, Kenneth Lee, Bryan Myers, Ania Bleszynski Jayich The nitrogen-vacancy (NV) center in diamond is promising for applications in quantum information and quantum assisted sensing. We have fabricated NV-containing single-crystal diamond mechanical resonators that exhibit high quality factors in excess of 300,000. These structures provide a highly controlled platform for investigating the effect of strain on the NV. The strain is calculated from the mode shape of a driven resonator and we correlate the strain to the measured energy level shift. Understanding the strain coupling is an important step toward NV center spin manipulation using local strain fields as an alternative to external magnetic and electric fields. Furthermore, such a mechanical-spin interface could enable mechanical control of spin states as well as provide a hybrid approach to a scalable quantum network. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N27.00007: Two-mode back-action-evading measurements in cavity optomechanics Matthew Woolley, Aashish Clerk The field of cavity optomechanics aims to achieve the quantum measurement and control of macroscopic mechanical resonators via coupled, cavity-enhanced electromagnetic fields. Here, we study a system composed of two mechanical oscillators independently coupled to a common electromagnetic cavity mode. By driving the cavity at frequencies both above and below the cavity resonance frequency, with a detuning equal to the average of the two mechanical oscillator frequencies, it is possible to couple a quadrature of the cavity mode to a joint quadrature of the two mechanical modes. This allows a back-action-evading measurement of the joint quadrature of the mechanical oscillators to be performed. If the output of the coupled cavity is continuously monitored, in the regime where the effective joint oscillator frequency greatly exceeds the average damping rate of the mechanical oscillators, it is possible to conditionally generate an all-mechanical, entangled two-mode squeezed state. This conditional entanglement may be verified from the measurement record, and converted to unconditional squeezing via the application of feedback. The same system may be employed for force sensing beyond the standard quantum limit. The experimental prospects for such a system are considered. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N27.00008: Prospects for coupling Surface Acoustic Waves to superconducting qubits Invited Speaker: Martin Gustafsson Recent years have seen great development in the quantum control of mechanical resonators. These usually consist of membranes, cantilevers or suspended beams, whose vibrational modes can be cooled to the quantum ground state. This presentation will focus on a different kind of micromechanical system, where the motion is not confined to a mode with fixed boundaries, but propagates along the surface of a microchip. These modes are known as Surface Acoustic Waves (SAWs), and superficially resemble ripples on water, moving with low loss along the surfaces of solids. On a piezoelectric substrate, electrode gratings known as Interdigital Transducers (IDTs) can be used to convert power between the electric and acoustic domains. Devices based on this effect are of profound technological importance as filters and analog signal processors in the RF domain. In the realm of quantum information processing, SAWs have primarily been used to transport carriers and excitons through piezoelectric semiconductors, in the electric potential wells propagating along with the mechanical wave. Our approach, however, is different in that we aim to explore the mechanical wave itself as a carrier of quantum information. We have previously shown that a single-electron transistor can be used as a local probe for SAWs, with encouraging sensitivity levels. Building on this, we now investigate the prospects for coupling a SAW beam directly to a superconducting qubit. By merging a circuit model for an IDT with a quasi-classical description of a transmon qubit, we estimate that the qubit can couple to an acoustic transmission line with approximately the same strength as to an electrical one. This type of coupling opens for acoustic analogs of recent experiments in microwave quantum optics, including the generation of non-classical acoustic states. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N27.00009: Nanoscale Torsional Optomechanics Paul H. Kim, Callum Doolin, Bradley D. Hauer, Allison J. R. MacDonald, Mark R. Freeman, Paul E. Barclay, John P. Davis Torsional resonators, which can be designed to measure torques with high sensitivity, have been an effective tool to study magnetism, gravity, and various material and optical properties. Taking advantage of improved micro-fabrication techniques, these torque sensors are now pushing the limit in terms of size - scaling all the way down to the nanoscale regime - and therefore must be equipped with sensitive mechanical transduction schemes. Here we present a method for measuring torques as little as 4 $\times 10^{20} N$m, using optomechanics. Recently optomechanics has been revealed as a reliable method for mechanical transduction, with higher sensitivity than previously possible. This sensitivity of the optomechanical system comes from the evanescent coupling between a high quality factor optical resonator and the mechanical device, and is fully integratable on a chip using the silicon-on-insulator platform. We present our first generation torsional optomechanics, using a dimpled optical fiber system for measurement, with a calibrated sensitivity down to 7 fm/$\sqrt{\textrm{Hz}}$. This torsional optomechanical platform will now serve as a basis for further experiments to explore new physics and technology, in particular quantum resonators at low temperatures. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N27.00010: Beating the standard quantum limit for force sensing with a coupled two-mode optomechanical system Xunnong Xu, Jacob M. Taylor The scheme of optomechanical sensing of weak forces with a coupled two-mode cavity is presented. We consider the mirror-in-the-middle setup and use the two coupled cavity modes originated from normal mode splitting as pump and probe to realize force detection. We find that this two-mode model can be reduced to an effective single-mode model, if we drive the pump mode strongly and detect the signal from the weak probe mode. The optimal force detection sensitivity at zero frequency (DC) is calculated and we show that we would be able to beat the standard quantum limit by detuning the cavity far away from resonance. Furthermore, we find that the laser input power requirement will depend linearly on the cavity detuning, if the cavity mode coupling is close to cavity detuning, which is a great advantage over conventional single-mode force sensing scheme where the laser power has a cubic dependence on the cavity detuning. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N27.00011: ABSTRACT WITHDRAWN |
Session N28: Continua, Networks, & Earthquakes
Sponsoring Units: GSNPChair: Oscar Lopez-Pamies, University of Illinois at Urbana-Champaign
Room: 336
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N28.00001: Earthquakes in the Laboratory: Continuum-Granular Interactions Robert Ecke, Drew Geller, Carl Ward, Scott Backhaus Earthquakes in nature feature large tectonic plate motion at large scales of 10-100 km and local properties of the earth on the scale of the rupture width, of the order of meters. Fault gouge often fills the gap between the large slipping plates and may play an important role in the nature and dynamics of earthquake events. We have constructed a laboratory scale experiment that represents a similitude scale model of this general earthquake description. Two photo-elastic plates (50 cm x 25 cm x 1 cm) confine approximately 3000 bi-disperse nylon rods (diameters 0.12 and 0.16 cm, height 1 cm) in a gap of approximately 1 cm. The plates are held rigidly along their outer edges with one held fixed while the other edge is driven at constant speed over a range of about 5 cm. The local stresses exerted on the plates are measured using their photo-elastic response, the local relative motions of the plates, i.e., the local strains, are determined by the relative motion of small ball bearings attached to the top surface, and the configurations of the nylon rods are investigated using particle tracking tools. We find that this system has properties similar to real earthquakes and are exploring these ``lab-quake'' events with the quantitative tools we have developed. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N28.00002: Extreme statistics of avalanches near the depinning transition Michael LeBlanc, Luiza Angheluta, Karin Dahmen, Nigel Goldenfeld Near the depinning transition, motion proceeds by avalanche fluctuations with power law distributed sizes and durations. We derive exact exponents and scaling functions for the statistics of maximum avalanche velocities in a mean field theory of the transition. We find a power law regime in the maximum velocity distribution with an exponent that agrees with the distribution of peak amplitudes observed in acoustic emission experiments of crystal plasticity. Our results should be applicable to the study of a number of systems considered to be in the mean field interface depinning universality class, ranging from magnets to earthquakes. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N28.00003: Acoustic-Friction Networks and the Evolution of Shear Ruptures in Laboratory Earthquakes H.O. Ghaffari, R.P. Young The evolution of shear rupture fronts in laboratory earthquakes are analysed with the corresponding functional networks, constructed over photo-elastic, real-time contacts and acoustic emission friction-patterns. We show that the mesoscopic and transport characteristics of networks follow the same trends for the same type of the shear ruptures in terms of rupture speed, while also comparing the results of four different friction experiments. The classified fronts--obtained from a saw-cut fault and natural faulted Westerly granite - regarding friction network parameters show a clear separation into two groups, indicating two different rupture fronts. With respect to the scaling of local ruptures' durations with the networks' parameters, we show that the gap is related to the possibility of a separation between slow and regular fronts. Based on our results, we develop a statistical based method to model the evolution of functional damage networks while we consider that any rupture flows in a critical plane with two main fixed points. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N28.00004: Forecasting large earthquakes using small-quake correlations Braden Brinkman, Michael LeBlanc, Yehuda Ben-Zion, J.T. Uhl, Karin Dahmen It has long been speculated that periodic stress variations, such as the tides, may trigger earthquakes, and hence tide-earthquake correlations could be used as signals for predicting large earthquakes prior to failure. We developed a simple probabilistic model of earthquake triggering which we used to simulate series of earthquake events in a fault subjected to external periodic stresses of amplitudes and frequencies representative of tidal or seasonal stress variations. By analyzing correlations between small events and periodic stress cycles, we compute the probability that a large event will occur. We find that seasonal stresses are better predictors of impending large earthquakes. In addition, our results also apply to many other sheared frictional stick-slip systems which display small slips, such as rock interfaces or granular matter. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N28.00005: Critical Scaling of Avalanche Dynamics in Sheared Amorphous Solids with Inertia K. Michael Salerno, Craig Maloney, Mark O. Robbins We present results from molecular-dynamics simulations of model disordered solids~under quasi-static, steady-state shear in two and three dimensions.~~Plastic deformation occurs through intermittent ``avalanches'' of local rearrangements. As in other slowly-driven systems from magnets to geologic faults, avalanches exhibit critical scaling behavior. Results for the avalanche statistics, duration and power spectrum are analyzed with finite-size scaling. The exponents describing the power law distribution of avalanches and the relation between their size and duration are independent of dimension, suggesting that mean field behavior extends to two dimensions. In contrast, the scaling exponents depend on the degree of inertia or damping, with distinct universality classes in the underdamped and overdamped limits [1]. The same universality classes are observed with Galilean-invariant and non-Galilean-invariant thermostats, but the crossover between these limits will be contrasted. The implications for different experimental systems will be discussed.~ [1] PRL 109, 105703 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N28.00006: Phase-Field Crystal Models and Elastic Excitations Vili Heinonen, Cristian Achim, Ken Elder, Tapio Ala-Nissil\"a Phase Field Crystal (PFC) models and their amplitude expansions are a novel attempt to bridge the gap between atomistic and continuum models in materials modeling. The studied quantity is the atomic density field that varies in time and space. Not only do these new models allow longer length scales but also longer time scales: many interesting phenomena happen over diffusive time scales that are beyond the reach of classical molecular dynamics or Monte Carlo methods. As with Dynamic Density Field theory a local equilibrium in the system is assumed leading into diffusive dynamics. This implies that the time scale under study is lot slower than time scale of elastic excitations in the system. In other words, we are assuming that phonon modes die out instantaneously. However, it turns out that the system exhibits elastic excitations that have to be relaxed separately. We propose a physical constraint to the time evolution of the density field, which ensures elastic equilibrium at all times. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N28.00007: Strain recovery in dual cross-linked polymer grafted nanoparticle networks Balaji Iyer V S, Victor Yashin, Isaac Salib, Tomasz Kowalewski, Krzystof Matyjaszewski, Anna Balazs Via computational modeling, we investigate the mechanism of strain-recovery in dual cross-linked polymer grafted nanoparticle networks. The individual nanoparticles are composed of a rigid core and a corona of grafted polymers that encompass reactive end groups. With the overlap of the coronas on adjacent particles, the reactive end groups form permanent or labile bonds, and thus form a ``dual cross-linked'' network. We consider the strain recovery of the material after it is allowed to relax from the application of the tensile force. We apply multiple cycles of tension and relaxation and determine how the stress-strain curves change in the course of these repetitive deformations. Notably, the existing labile bonds can break and new bonds can form in the course of deformation. Hence, a damaged material could be ``rejuvenated'' both in terms of the recovery of strain and the number of bonds, if the relaxation occurs over a sufficiently long time. We show that this rejuvenation depends on the fraction of permanent bonds, strength of labile bonds, and maximal strain. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N28.00008: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N28.00009: An analytical method for determining material parameters from inflation tests of thick nonlinear materials Theresa K. Tonge, Thao D. Nguyen The inflation test is a widely used method for applying a biaxial stress state to polymers and biological tissues. The stress response is determined by assuming the inflated specimen can be modeled as a membrane. However, neglecting the effect of bending can generate large errors for thick specimens and in particular for those exhibiting highly nonlinear material behavior. We have developed a novel thin shell method to analytically determine material properties from the inflation test while accounting for bending. The method assumes a linear strain gradient from bending to calculate the in-plane stress resultants from the constitutive relations for the stress response. These stress resultants are fit to the experimentally determined stress resultants calculated from the applied pressure and measured local curvatures. We have applied the method to fit an anisotropic constitutive model to inflation tests of human skin tissue. We have used Finite Element Analysis to validate the method as well as the resulting material parameters for the constitutive model. This thin shell method is sufficiently general to be applied to determine material properties for other thick, nonlinear materials such as aortic valves or gastrointestinal tissues. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N28.00010: Solvent-driven shape-memory effects for amorphous networks Rui Xiao, Thao Nguyen The swelling-induced shape memory behavior in polymers has inspired interest for their implications for biomedical applications. For amorphous polymers, the behavior is caused by a large decrease in the glass transition temperature caused by the absorption of a small amount of solvent. In this work, we present a theoretical model of the effect of low solvent concentration on the glass transition behavior of the materials. Specifically, the presence of solvent increases the configurational entropy; thus altering the temperature-dependence of the molecular mobility. The model was implemented numerically for finite element simulation. The computational model also considers the effect of diffusion process to describe more accurately the time-dependent effects of solvent-induced shape recovery behavior. To validate the model, we performed isothermal uniaxial tension tests on both the dry and fully saturated materials. Shape recovery performance was investigated by observing the shape change of an initially deformed sample in an isothermal water bath by using digital image tracking. Comparison between experimental data and simulations shows good agreement. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N28.00011: Distorted tetrahedral shapes of nematic vesicles Thanh Son Nguyen, Jonathan Selinger In membranes with internal orientational or crystalline order, there is a geometric coupling between 2D internal order and 3D shape. Nonuniformity in internal order tends to induce curvature, and curvature provides an effective potential acting on internal order. For a closed vesicle with nematic liquid-crystalline order, there must be a total topological charge of +2, which normally occurs as four defects of +1/2 each. Previous research has suggested that these four defects form a regular tetrahedron, leading to a tetrahedral shape of the vesicle, which may be useful in colloidal crystals for photonic applications. Here, we develop an explicit model to calculate energies of defect structures in nematic vesicles. When the liquid-crystal interaction energy is a purely 2D intrinsic interaction, we find that the perfect tetrahedral shape is stable only up to a maximum interaction strength (Frank constant), where it changes to an elongated rectangular configuration. When the interaction energy is a 3D extrinsic and intrinsic interaction, the perfect tetrahedral shape is never stable; the vesicle is a distorted tetrahedron for small Frank constant and a highly elongated rectangle for larger Frank constant. These results show the difficulty in designing tetrahedral structures. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N28.00012: Simple model for plastic deformation and slip avalanches in bulk metallic glasses Karin Dahmen, James Antonaglia, Junwei Qiao, Xie Xie, Peter Liaw, Jonathan Uhl Ductile bulk metallic glasses are known to deform under shear in an intermittent way with slip-avalanches detected as acoustic emission and serrations in the stress-strain curves. In many such materials, power laws govern the statistics of these avalanches. A basic micromechanical model for deformation of solids with only one tuning parameter is introduced. The model predicts the observed stress-strain curves, acoustic emissions, related power spectra, and power-law statistics of slip avalanches, including the dependence of the cutoff on experimental parameters with a continuous phase transition from brittle to ductile behavior. Material independent (``universal'') predictions for the power-law exponents and scaling functions are extracted using the mean-field theory and renormalization group tools. The results agree with recent experimental observations on deformed bulk metallic glasses. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N28.00013: Cavitation in Amorphous Solids Michael Falk, Pengfei Guan, Shuo Lu, Michael Spector, Pavan Valavala Molecular dynamics simulations of cavitation in a Zr50Cu50 metallic glass exhibit a waiting time dependent cavitation rate. On short time scales nucleation rates and critical cavity sizes are commensurate with a classical theory of nucleation that accounts for both the plastic dissipation during cavitation and the cavity size dependence of the surface energy. All but one parameter, the Tolman length, can be extracted directly from independent calculations or estimated from physical principles. On longer time scales aging in the form of shear relaxations results in a systematic decrease of cavitation rate. The high cavitation rates that arise due to the suppression of the surface energy in small cavities provide a possible explanation for the quasi-brittle fracture observed in metallic glasses. Analogous simulations of Fe80P20 reveal that segregation of P on the nanoscale leads to qualitatively different behavior that may be attributable to the idiosyncrasies of the interatomic potential. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N28.00014: Cavitation in rubber: An elastic instability or a fracture phenomenon? Oscar Lopez-Pamies In this presentation, I will confront a recently developed theory of cavitation for soft solids to a variety of cavitation experiments with the objective of establishing whether the phenomenon of cavitation is an elastic instability (and hence depends only on the elastic properties of the rubber), or, on the other hand, a fracture process (and hence depends on the fracture properties of the rubber). [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N28.00015: Experimental realization of the zero temperature Random Field Ising Model : the condensation of $^4$He in aerogels Geoffroy Aubry, Laurent Guyon, Mathieu Melich, Panayotis Spathis, Florence Despetis, Pierre-Etienne Wolf Although widely studied, the effect of disorder on a first order phase transition is still highly debated. Numerical simulations of the $T=0$ Random Field Ising Model show that magnetization evolves by avalanches, the average size of which diverges below a critical disorder (Sethna et al., PRL 70 3347 (1993)). Nevertheless, experimental evidence is scarce up to now (Berger et al., PRL 85, 4176 (2000)). In the case of the liquid gas transition in disordered porous media, the same theoretical concepts can be applied (Detcheverry et al., PRE 72 051506 (2005)). We have studied experimentally this phase transition using $^4$He in silica aerogels. Optical and thermodynamical measurements show that the condensation is an out of equilibrium process. We clearly observe two filling regimes separated by a critical temperature $T^*$ : below $T^*$, filling is discontinuous (macro avalanche) whereas above $T^*$ it becomes continuous (micro avalanches). In addition, we have developed a speckle interferometry technique to detect single avalanches. We argue that our results support the disorder induced phase transition. [Preview Abstract] |
Session N29: Granular Packing and Impacting
Sponsoring Units: GSNPChair: Eric Corwin, University of Oregon
Room: 337
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N29.00001: A Dynamically Based Study of Percolation through Spaces between Polyhedral Grains Donald Priour Many porous materials in nature are made up of grains in the form of non-spherical crystallites. Depending on the density of the grains, such systems may admit the flow of fluid through the spaces between the grains on a macroscopic scale (percolation for sufficiently sparsely spaced grains) or prevent fluid flow (percolation is blocked if the grain concentration is high enough that voids between grains are not contiguous). To provide a more realistic treatment of percolation through granular media, we examine systems comprised of randomly placed angular impermeable inclusions (e.g. disks, tetrahedrons, cubes, and octahedrons), and we give a rigorous continuum treatment to the geometry of the grains and the spaces between them. To extrapolate to the bulk limit in the context of a finite-size scaling analysis, we examine multiple systems of different sizes, where disorder averaging mitigates statistical fluctuations unrelated to bulk properties. An order parameter based on root mean square (RMS) excursion of dynamical trajectories is calculated in the context of a large-scale Monte Carlo simulation and used to evaluate the critical concentration $\rho_{c}$ of grains. In addition, critical exponents such as $\nu$ for the correlation length $\xi$ are determined. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N29.00002: Simulation of current-activated pressure-assisted densification Dietrich Wolf, Sebastian Angst, Gabi Schierning Cohesive particles usually form very porous agglomerates. They support loads up to a consolidation pressure, which increases with decreasing particle size. Compaction of nano-powders can therefore be very costly and time consuming. If the particles are electrically conducting, which is the case e.g. for novel nano-structured thermoelectric materials, the technique of current-activated pressure-assisted densification (CAPAD) turns out to have many advantages. Electrical power deposited locally as Joule heat lowers the consolidation pressure such that particles fill nearby pores. This process leads to fast, scalable densification without much coarsening. Simulations are presented which address the influence of correlations on density and conductivity [1]. They also take thermal conductivity and Peltier coefficient into account [2].\\[4pt] [1] S. Hartner et al. in: Nanoparticles from the Gasphase - Formation, Structure, Properties. A. Lorke, M. Winterer, R. Schmechel, Ch. Schulz (eds.) (Springer, Berlin 2012) pp. 231 - 270.\\[0pt] [2] A. Becker et al., Appl. Phys. Lett. 101, 013113 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N29.00003: Fully ordered to disordered granular sphere packings with random deposition Arshad Kudrolli, Andreea Panaitescu Granular packings are typically obtained by pouring grains into a container in a gravitational field as when sugar is poured into a jar, or grains into a silo. We deconstruct this method and study the impact on packing by simply varying the pour rate and energy of particles dropped randomly but spatially uniformly in a large container whose substrate can act as a template. We find that fully disordered packings are observed when large number of particles are added all at once but an ordered fcc crystal is observed when particles are added sequentially at random locations and allowed to come to rest before adding the next layer. By scanning the packings obtained by 3D X-ray tomography, we identify the positions of all the particles and the growth of order and defects. We present an analysis of the structures formed and compare and contrast it with packings obtained using other protocols including by cyclic shear [1]. [1]: ``Nucleation and Crystal Growth in Sheared Granular Sphere Packings," Andreea Panaitescu, K. Anki Reddy, and Arshad Kudrolli, Phys. Rev. Lett. 108, 108001 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N29.00004: Comparison between bridges and force-chains in granular packings Ling Zhang, Shuxiao Cai, Zunpeng Hu, Jie Zhang In dense granular materials, there exist chain-like force networks from which we can obtain much information on the mechanical properties of packings. But it is extremely difficult to characterize these structures, especially in 3D packings. Mehta and her coauthors have proposed theoretically that bridge-like structures can form because of spatial inhomogeneity and large fluctuations, and they conjecture that these mesoscopic geometric structures play the role of force-chains (Mehta et. al. 2004, Pugnaloni et. al. 2001). Some statistical features of bridges have been observed in a recent paper by Mattew C. Jenkins et al (Jenkins et. al. 2011). Despite the success, the lack of independent force network information makes the justification of Mehta et al.'s theoretical conjecture inconclusive. In this study, we focus on the comparison of bridges and force-chains in two different granular packing using photo-elastic granular particles. We have found no clear evidence that there exists a one-to-one mapping between bridges and force-chains. Nonetheless, for systems of different force chain structures, it does seem to show some differences in the respective bridge structures. This seems to suggest that some connection may exist between bridges and force-chains. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N29.00005: Novel spiral-like columnar packings of hard spheres from sequential deposition - a route to new architecture in the scientific world Ho-Kei Chan Recent work [Physical Review E 84, 050302(R) (2011)] shows that the densest columnar packings of identical hard spheres inside a cylinder can be constructed from a sequential deposition of such spheres onto a specially designed template at the cylinder base, if the cylinder-to-sphere diameter ratio D is within [1,2.7013]. In this talk, I will present some novel, non-densest spiral-like structures as discovered from the same deposition algorithm, and will elaborate on how we can manipulate a columnar structure by changing its underlying template. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N29.00006: Cavity method for jammed disordered packings of hard particles at mean-field level Lin Bo, Romain Mari, Chaoming Song, Hernan Makse We apply the cavity method at mean-field level to investigate the problem of random close packings of hard particles. We derive the Belief Propagation equations describing this force/torque balance problem to solve the force distribution and suggest an estimation of the coordination number of the jammed packing. We compare the numerical results with approximate analytical solutions and show the dependence of coordination numbers on particle shapes. The method can be applied to spherical frictionless and frictional particles as well as non-spherical particles to obtain the jamming properties and study the appearance of isostaticity. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N29.00007: Detection of Multidimensional Structures in Granular Materials Danielle Bassett, Karen Daniels, Eli Owens, Mason A. Porter, M. Lisa Manning Granular media display features across a range of spatial scales, from the particle scale to the force-chain scale and the bulk scale. In contrast to particulate and continuum models, network representations facilitate the simultaneous examination of microscopic, mesoscopic, and macroscopic features. We treat granular materials as spatially embedded networks in which the nodes (particles) are connected by weighted edges obtained from contact forces. Using community detection techniques, we identify local 2D geographic domains composed of particles that exert strong forces on one another. We subsequently develop and apply a novel spatial null model constrained by the contact network to extract chain-like structures reminiscent of force chains. We demonstrate that most of these chain-like structures are located close to the center of mass of the 2D geographic domains. However, a minority are located towards the edge of the 2D geographic domains, potentially forming points of instability in granular media. We explore the robustness of these detection techniques to algorithmic degeneracies, to simulation versus experimental data, and to varying pressure states. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N29.00008: Compaction of frictional octahedra N. Nirmal Thyagu, Max Neudecker, Stephan Herminghaus, Matthias Schroeter We perform experiments with frictional polypropylene octahedra to study the packing properties. Starting with the loose packing, compaction of octahedra is done by two types of forcing -- a) tapping and b) shearing. The compaction gives rise to crystallization of octahedra due to heterogenous nucleation from the walls. We obtain the X-ray tomograms of the packing configurations as a function of packing fraction. From the contact geometries we obtain results for the packings such as - pair correlation function, distance to isostaticity, and spatial {\&} angular correlation functions. We contrast these results with a similar study on the simplest platonic solid, the tetrahedron\footnote{Jammed frictional tetrahedra are hyperstatic, M. Neudecker, S. Ulrich, S. Herminghaus, M. Schr\"{o}ter. (arXiv:1202.6272v2)} and the sphere. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N29.00009: Revealing the structure of a granular medium through acoustic measurements Ramon Planet Latorre, S\'ebastien Lherminier, Gilles Simon, Loic Vanel, Osvanny Ramos An array of acoustic sensors records the sound that has travelled across a bi-dimensional granular medium, consisting of photoelastic discs, which are confined between two transparent plates and arranged into different crystalline or disordered structures. The system is compressed along one direction (either force-controlled or displacement-controlled) and can be sheared in the direction perpendicular to the applied force; while the acoustic signals are generated through a well-controlled and local mechanical excitation. The results show power-law regimes in the force vs. sound speed relation, with exponents that are sensitive to the structure of the packing. Small structural changes are also detectable which, in principle, can be used to predict large avalanches during the slow shearing of the system. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N29.00010: The vanishing Janssen effect in a confined 2D granular system compressed by friction Yasin Karim, Eric Corwin As described by H.A. Janssen in 1895, the pressure in a granular packing saturates above a certain filling height, determined by the particle-particle and particle-wall interactions. This effect has been studied extensively for 2D and 3D confined granular packs compressed by gravity. However, many industrially relevant processes involve the horizontal transport of granular materials by conveyor belts. In such a case gravity becomes irrelevant and the system is driven by frictional forces. We study horizontal 2D confined granular packs on a conveyor belt as they are driven into a stationary barrier. We measure the relationship between pressure and filling height and, surprisingly, find no saturation of pressure. Instead, we observe a linear relationship between pressure and filling height irrespective of the particle-wall coefficient of friction demonstrating that the Janssen effect is not relevant for such systems. However, we can recover a Janssen-like saturation if we replace the straight confining walls with a sawtooth pattern on the scale of the particle size. This allows for a mechanical transfer of load onto the walls and can be interpreted in terms of an effective mechanical ``friction.'' [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N29.00011: From Kepler to Ulam: searching for the optimal packing in the space of object shapes Romain Mari, Adrian Baule, Lin Bo, Maximilien Danisch, Hernan Makse The quest for the best packing of particles has been guided by two notorious conjectures. Kepler stated that the optimal sphere packing is the face-centered-cubic lattice, while Ulam conjectured that all convex shapes pack better than spheres. While the former was proved by Hales, there is yet no theoretical framework to predict the density of non-spherical particles. Here, we present a formalism to describe packings of objects of arbitrary shape in random configurations by reducing the particle interactions to simple sets of points, and lines. The framework predicts the optimum packing fraction of a large class of shapes as an analytical continuation from the spherical point, thus paving the way for a proof of Ulam's conjecture. In particular, the formalism predicts that spherocylinders pack better than both spheres and dimers. Ellipsoids and tetrahedra can be studied, highlighting the universality 1of the framework to search for optimal packings. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N29.00012: Forces on Intruders in Granular Media Ibar de la Cruz We measure the forces acting on intruders moving in different directions in a granular medium consisting of mono-disperse spherical glass beads. We present the dependence of the drag force on the intruder's geometry and surface roughness, bead size, dragging speed and immersion depth. We present a model that considers not only the wedge dragged by the intruder but also the pile created as the intruder moves through the granular material to calculate the drag force. We compare our experimental and analytical results. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N29.00013: Granular Dynamics during Impact Kerstin Nordstrom, Emily Lim, Matt Harrington, Wolfgang Losert In this work, we study the impact of a projectile onto a bed of 3 mm grains immersed in an index-matched fluid. Using a laser sheet scanning technique, a high speed camera, and particle tracking, we can measure the trajectory of each grain throughout an impact event. We characterize the bulk and microscopic dynamics within the granular material as a function of initial sample preparation, specifically applying a uniaxial prestrain to the sample. We find that small changes in sample preparation lead to drastic departures from the universal depth scaling seen in previous studies of shallow granular impacts. By examining the nonaffine motion within the system, we propose the effect is due to different loading and buckling of force chains within the system. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N29.00014: What is the granular response to a high-speed impact? Abram Clark, Lou Kondic, Robert Behringer Although many studies of impact on a granular material exist, the connections between the local granular response, the microscopic processes which dissipate kinetic energy, and the intruder dynamics are unclear, largely due to experimental difficulties in obtaining very fast data at the grain scale. We use high-speed imaging (40 kHz) of an intruder striking a quasi-2D system of photoelastic disks, yielding both the intruder dynamics and the force response of individual grains. The frame rates are fast enough to resolve rich acoustic activity on the particle scale. For long time scales, the intruder dynamics are consistent with previously used empirical force laws. However, for short time scales, we observe very large fluctuations in the deceleration, which we connect to the intermittent acoustic activity beneath the intruder as it moves. We show that these intense, intermittent acoustic pulses, which travel much faster than the intruder along networks of grains, are the primary microscopic mechanism of energy loss. These pulses carry energy away into the medium, and they decay roughly exponentially with distance. We examine the statistics of these fluctuations in order to better understand their origin and behavior. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N29.00015: The Jamming transition in photoelastic disks: local perturbations versus diverging responses Corentin Coulais, Antoine Seguin, Olivier Dauchot We investigate the spatial response of the contact network to local perturbations in experiments on horizontal packings of bidisperse photo-elastic soft disks close to jamming. First, an intruder is pulled at constant force through the packing: while the overall contact number remains unchanged, the contact network geometry drastically changes and develops a strong asymmetry between the front and the back of the intruder. Second, an intruder is inflated inside the packing leading to a global increase of the contact number. While particle rearrangements become increasingly large as the unjamming transition is approached, there are only few contact changes in the packing. We discuss these results in the light of a recent work [1] on fluctuations where a similar link between dynamical heterogeneities and contact fluctuation has been reported. \\[4pt] [1] C. Coulais, R. P. Behringer, and O. Dauchot, arXiv eprint: 1202.5687 (2012). [Preview Abstract] |
Session N30: Self-Assembly: Mostly Biopolymers, DNA and Nanoparticles
Sponsoring Units: DCMPChair: Xiangyun Qiu, George Washington University
Room: 338
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N30.00001: Formation of Heterogeneous Toroidal-Spiral Particles -- by Drop Sedimentation and Interaction Ying Liu, Ludwig Nitsche, Richard Gemeinhart, Vishal Sharma, Magdalena Szymusiak, Hao Shen We describe self-assembly of polymeric particles, whereby competitive kinetics of viscous sedimentation, diffusion, and cross-linking yield a controllable toroidal-spiral (TS) structure. Precursor polymeric droplets are splashed through the surface of a less dense, miscible solution, after which viscous forces entrain the surrounding bulk solution into the sedimenting polymer drop to form TS channels. The intricate structure forms because low interfacial tension between the two miscible solutions is dominated by viscous forces. The biocompatible polymer, poly(ethylene glycol) diacrylate (PEG-DA), is used to demonstrate the solidification of the TS shapes at various configurational stages by UV-triggered cross-linking. The dimensions of the channels are controlled by Weber number during impact on the surface, and Reynolds number and viscosity ratio during subsequent sedimentation. Within the critical separation distance, interaction of multiple drops generates similar structure with more flexibility. Furthermore, the understanding of multiple drop interaction is essential for mass production of TS particles by using parallel and sequential arrays of drops. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N30.00002: ReaxFF Reactive Force Field Study of Oriented Attachment of TiO$_{2}$ Nanocrystals in Vacuum and Humid Environments Muralikrishna Raju, Kristen Fichthorn, Adri van Duin We use a ReaxFF reactive force field to study the aggregation of various titanium dioxide (anatase) nanocrystals in vacuum and humid environments. The nanocrystals are in the 2-6nm size range, with shapes dictated by the Wulff construction. In vacuum, the nanocrystals tend to merge along their direction of approach, resulting in a polycrystal. By contrast, in the presence of water vapor, the nanocrystals tend to reorient themselves and aggregate via the oriented attachment mechanism to form a single or twinned crystal. We find that adsorbed water molecules and hydroxyl groups play multiple roles in oriented attachment. As the nanocrystals approach one another closely, adsorbed water molecules and surface hydroxyls prevent their immediate aggregation. These adsorbed species create a hydrogen bonding network, which aligns the nanoparticles in registry. Upon the eventual elimination of these species, the nanoparticles fuse into a single-crystal or twinned aggregate. We observe this aggregation mechanism for anatase(101), anatase(112), and anatase(001) surfaces, as is also seen experimentally. This indicates the important role that solvent plays in nanocrystal aggregation and how solvent can be a powerful tool for directing and controlling nanocrystal growth. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N30.00003: Improving reaction rates by confinement within biocompatible polymers Cecile Malardier-Jugroot, Xia Li, Michael N. Groves, Manish Jugroot The most efficient catalysts have been developed and optimized by living systems. Indeed, in vivo enzyme-catalyzed reactions are several orders of magnitude more efficient than platinum based catalyzed reactions. However, the rate of reaction and equilibrium interactions are considerably reduced when the biological systems are studied in vitro. This phenomenon is largely attributed to the effect of confinement or macromolecular crowding present in the cell. This paper will present the comprehensive characterization of amphiphilic polymeric template with hydrophobic cores inducing 1D and 2D confinement on hydrophobic reactants diffusing within the templates. The paper will show that effect of confinement allows reactions to occur without external factors essential for these reactions to occur in the bulk. The products synthesized in a very controlled environment within amphiphilic polymeric nanotubes and nanosheets are monodispersed at the nanoscale ($\sim$ 2nm). The effect of confinement opens new possibilities for environmentally friendly synthesis of novel nanoscale materials. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N30.00004: Unraveling the Mechanism of Nanotube Formation by Chiral Self-Assembly of Amphiphiles Dganit Danino The self-assembly of \textit{nanotubes} from chiral amphiphiles and peptides is still poorly understood. Here, we present the first complete path to nanotubes by chiral self-assembly studied with C$_{12}$-$\beta_{12}$, a tailored molecule designed to have unique hybrid architecture. Using direct-imaging cryo-transmission electron microscopy (cryo-TEM) we show the time-evolution from micelles to closed nanotubes, passing through several types of 1-dimensional (1-D) intermediates such as elongated fibrils, twisted ribbons, and coiled helical ribbons. Scattering and diffraction techniques confirm that the fundamental unit is a monolayer lamella, with the hydrophobic tails in the gel state and beta-sheet arrangement. The lamellae are held together by a combination of hydrophobic interactions, and 2 sets of hydrogen bonding networks. Our data exclusively indicate that twisted ribbons are the precursors for coiled ribbons, and we show this transition is directly linked to the ribbon width. Furthermore, quantitative analysis shows that neither the ``growing width'' model nor the ``closing pitch'' model accurately describe the process of nanotube formation, and \textit{both} ribbon width and pitch grow with maturation, maintaining a linear growth in their ratio. We also show that chirality is a key requirement for nanotube formation. References: [1] Ziserman L et al., \textit{J Am Chem Soc} \textbf{133(8)}, 2511-2517 (2011) [2] Ziserman L et al., \textit{Phys Rev Lett} \textbf{106,} 238105 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N30.00005: Programmable Mesoscopic Architecture using Directionally-Functionalized Nanoparticles Jonathan Halverson, Alexei Tkachenko Nanoparticles that have been isotropically-functionalized with complementary DNA strands have been shown to self-assemble into a variety of crystalline morphologies. To produce a nanoparticle assembly with a finite size and arbitrary shape, the NPs must be endowed with directional interactions. Directionally-functionalized nanoparticles (dfNPs) can be constructed by grafting ssDNA at specific locations on the particles, and proof-of-principle experiments have successfully demonstrated the self-assembly of such particles. Using these building blocks we have previously demonstrated with numerical simulations that a variety of target mesoscopic structures, each with a programmed local morphology and complex overall shape, can be self-assembled in near perfect yield. Here we present a model to describe the kinetics of assembly of a structure composed on dfNPs. The capability to produce these structures can be utilized in a variety of applications where bottom-up construction of 3D nano-objects with well-defined composition and architecture is required (e.g., nanoplasmonics, nanomedicine, metamaterials). [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N30.00006: Biomimetic DNA emulsions: specific, thermo-reversible and adjustable binding from a liquid-like DNA layer Lea-Laetitia Pontani, Lang Feng, Remi Dreyfus, Nadrian Seeman, Paul Chaikin, Jasna Brujic We develop micron-sized emulsions coated with specific DNA sequences and complementary sticky ends. The emulsions are stabilized with phospholipids on which the DNA strands are grafted through biotin-streptavidin interactions, which allows the DNA to diffuse freely on the surface. We produce two complementary emulsions: one is functionalized with S sticky ends and dyed with red streptavidin, the other displays the complementary S' sticky ends and green streptavidin. Mixing those emulsions reveals specific adhesion between them due to the short-range S-S' hybridization. As expected this interaction is thermo-reversible: the red-green adhesive droplets dissociate upon heating and reassemble after cooling. Here the fluid phospholipids layer also leads to diffusive adhesion patches, which allows the bound droplets to rearrange throughout the packing structure. We quantify the adhesion strength between two droplets and build a theoretical framework that captures the observed trends through parameters such as the size of the droplets, the DNA surface density, the various DNA constructs or the temperature. This colloidal-scale, specific, thermo-reversible biomimetic emulsion offers a new versatile and powerful tool for the development of complex self-assembled materials. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N30.00007: Hybridization dynamics to DNA guided crystallization Ting Li, Rastko Sknepnek, Monica Olvera de la Cruz DNA recognition inspires an elegant protocol to design versatile nanoparticle assemblies. Although great achievements in DNA programmed periodic structures have been obtained, it took over a decade to realize even the basic crystal structures like FCC and BCC in an experiment. We use molecular dynamics simulations to discuss the dynamic aspects of the assembly process and identify ingredients that are key to successfully assemble nanoparticle superlattices through DNA hybridizations. The scale-accurate coarse-grained model [1,2] faithfully captures the relevant dynamics of the DNA hybridization, and is able to recover the in situ formation of all to date experimentally reported binary superlattices (BCC, CsCl, AlB2, Cr3Si and Cs6C60 lattices). We used a multi-scale simulation approach to study the assembly mechanism in systems with up to $10^6$ degrees of freedom and found that the assembly process is enthalpy-driven. Finally, we investigated the optimal strength of DNA linkers, hybridization dynamics, and percentage of hybridizations for different binary systems. Based on these results, we suggest a protocol for future nanomaterial designs with versatile DNA interactions. [1] Knorowski, C., et al. P.R.L. 2011,106,215501; [2] Li, T.I.N.G., et al. Nano Letters 2012,12,2509. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N30.00008: When DNA Meets Depletion Kun-Ta Wu, Lang Feng, Paul Chaikin Depletion is a widely used tool in colloidal particle system for universal attraction. Recently, the rapid development of DNA-coated particles also opens a door to colloidal architecture due to the specificity of DNA hybridization. In our study, we combine these two techniques, depletion and DNA hybridization, in colloidal system and find out that DNA-coated particles in depletion system aggregate faster and have the higher melting temperature than the ones without depletion. We studied quantitatively how the kinetics and thermodynamics of DNA-coated particles are changed with the concentration of depletion and DNA. We also find out that by using the depletion-and-DNA coupled system, particle can form crystals within hours rather than days due to the catalysis effect from depletion. Our study illustrates how DNA and depletion can be used in the same system to create more various and unique systems, which can not be achieved by neither DNA nor depletion along. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N30.00009: Template-mediated catalysis of DNA tiles Corinna Maass, Xiaojin He, Ruojie Sha, Yoel Ohayon, Nadrian Seeman, Paul Chaikin We present a novel mechanism for the selective creation of irreversible bonds between DNA nanotiles in the presence of a DNA template of complementary joined DNA tiles. The hybridisation transition of DNA sticky ends is highly concentration dependent. While immobilised on a template, adjacent DNA tiles are subject to a greatly increased local concentration ($10^{12}$), as compared to free tiles in solution. This reduces the entropy penalty for sticky end hybridisation and shifts the hybridisation transition to higher temperatures. We have developed a tile-template model consisting of two DNA tiles with sticky ends that will, at room temperature, only react when attached to template tiles and which can be bound irreversibly via an UV crosslinkable nucleobase substitute. The selectivity is high and the irreversible crosslinking is enhanced by a factor of roughly 100. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N30.00010: DNA Photo Lithography with Cinnamate-based Photo-Bio-Nano-Glue Lang Feng, Minfeng Li, Joy Romulus, Ruojie Sha, John Royer, Kun-Ta Wu, Qin Xu, Nadrian Seeman, Marcus Weck, Paul Chaikin We present a technique to make patterned functional surfaces, using a cinnamate photo cross-linker and photolithography. We have designed and modified a complementary set of single DNA strands to incorporate a pair of opposing cinnamate molecules. On exposure to 360nm UV, the cinnamate makes a highly specific covalent bond permanently linking only the complementary strands containing the cinnamates. We have studied this specific and efficient crosslinking with cinnamate-containing DNA in solution and on particles. UV addressability allows us to pattern surfaces functionally. The entire surface is coated with a DNA sequence A incorporating cinnamate. DNA strands A'B with one end containing a complementary cinnamated sequence A' attached to another sequence B, are then hybridized to the surface. UV photolithography is used to bind the A'B strand in a specific pattern. The system is heated and the unbound DNA is washed away. The pattern is then observed by thermo-reversibly hybridizing either fluorescently dyed B' strands complementary to B, or colloids coated with B' strands. Our techniques can be used to reversibly and/or permanently bind, via DNA linkers, an assortment of molecules, proteins and nanostructures. Potential applications range from advanced self-assembly, such as templated self-replication schemes recently reported [1], to designed physical and chemical patterns, to high-resolution multi-functional DNA surfaces for genetic detection or DNA computing. [1] Tong, W et al, Nature, 478, 225-228(2011) [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N30.00011: Controlling the temperature-dependent assembly of DNA-coated colloids with toehold exchange William Rogers, Jesse Collins, Vinothan Manoharan DNA is increasingly being used as a tool for directing the self-assembly of particle-based systems. The transient bridging of grafted, complementary DNA strands induces specific, attractive interactions that can direct nanoparticles or colloids to form clusters, ordered crystal lattices, or other interesting structures. In most cases, the DNA-induced binding strength is a monotonic and near exponential function of temperature, resulting in a single, narrow temperature window for equilibrium assembly that may frustrate efforts to make multicomponent or hierarchical structures. Here, we present and quantitatively demonstrate a new approach to controlling the temperature dependence of DNA-induced colloidal interactions using toehold exchange hybridization, a concept borrowed from dynamic DNA nanotechnology. These competitive hybridization pathways allow additional control over the thermodynamics of bridge formation and provide a simple way to engineer novel temperature dependences that need not be exponential or monotonic. This additional functionality will be useful in the rational design of new multicomponent, hierarchical, or reconfigurable self-assembling systems. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N30.00012: Colloidal Clusters via Short, Specific, and Isotropic DNA Interactions Jesse W. Collins, Vinothan N. Manoharan Many of the material systems scientists have successfully described using statistical mechanics have a number of distinct chemical species that does not scale with the total number of particles. Do any different equilibrium phenomena emerge in systems of a much wider variety of chemical species? We investigate the case in which the number of chemical species is equal or very nearly equal the total number of particles. We coat microspheres with short and specific DNA strands, and observe small numbers of these spheres at a time self-assemble using various forms of microscopy, including holography for 3-D particle positions and fluorescence for species identification. We have learned some simple rules that modulate the energy landscape of these particles. The relative chirality of substructures, including pairs of trimers, varies for each local minima on the landscape of small clusters like the ones we observe. If the ground state structure is rigid, the higher energy local minima structures are generally soft. Although our experiments are limited to about 6 particles, ideas from graph theory and statistical mechanics suggest that much larger numbers of short-ranged, specific and chemically isotropic spheres can robustly assemble into rigid ground state clusters as well. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N30.00013: Phases and Dynamics of Self-Assembled DNA Programmed Nanocubes Christopher Knorowski, Alex Travesset Systems of Nanoparticles grafted with complementary DNA strands have been shown to self-assemble into an array of superlattices. In this talk, we extend our previous model [1], which successfully predicted equilibrium phases and dynamics of assembly for spherical Nanoparticles [1,2] without fitting parameters, to the case of nanocubes. We show that the phase diagram consists of bcc and sc lattices, depending on DNA length. The bcc lattices are either rotator and orientational glass or cubatic. For temperatures above the DNA melting temperature, the system is equivalent to f-star polymer systems, and consist of bcc, also with rotator, orientational glass or cubatic orientational order as well as sc. We also provide a characterization of the dynamics, including the role of topological defects in crystal nucleation and growth. \\[4pt] [1] C. Knorowski {\em et al.}, Phys. Rev. Lett. {\bf 106}, 215501 (2011)\\[0pt] [2] C. Knorowski and A. Travesset, Soft Matter. {\bf Advance Article} (2012) DOI: 10.1039/c2sm26832a [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N30.00014: Assembly of tetrahedral gold nanoclusters from binary colloidal mixtures Nicholas B. Schade, Dazhi ``Peter'' Sun, Miranda C. Holmes-Cerfon, Elizabeth R. Chen, Emily W. Gehrels, Jonathan A. Fan, Oleg Gang, Vinothan N. Manoharan We experimentally investigate the structures that form when colloidal gold nanospheres cluster around smaller spheres. We use nanoparticles coated with complementary DNA sequences to assemble the clusters, and we observe them under electron microscopy. Previous experiments using polystyrene microspheres indicate that a 90\% yield of tetrahedral clusters is possible near a critical diameter ratio; random sphere parking serves as a useful model for understanding this phenomenon. Here we examine how this approach can be scaled down by an order of magnitude in size, using gold building blocks. We study how this method can be used to assemble tetrahedral plasmonic resonators in order to create a bulk, isotropic, optical metamaterial. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N30.00015: Field-directed assembly of colloidal ellipsoids Peter J. Beltramo, Eric M. Furst Self-assembly of colloidal building blocks into ordered structures has become a rapidly evolving area of research due to the novel properties (thermal transport, photonic, electromagnetic) imparted by periodicity.\footnote{M. Grzelczak et al. ACS Nano, \textbf{4}, 3591 (2010)} Assembly of anisotropic particles presents numerous challenges, namely kinetic arrest at high particle volume fractions due to glassy dynamics. This prevents the realization of theoretically predicted close-packed phases.\footnote{A. Donev et al. Phys. Rev. Let., \textbf{92}, 255506 (2004); P. Pfleiderer et al. Phys. Rev. E, \textbf{75} 020402 (2007) } In this work, we use AC electric fields to align dilute polystyrene ellipsoidal particles in suspension and a drying front to concentrate the particles into orientationally ordered thin films. Results using several aspect ratio particles are presented. The dilute electrokinetic properties which enable this field-directed assembly are characterized by dielectric spectroscopy and electrophoretic mobility measurements. Light scattering is used to evaluate the frequency and field strength dependence of particle alignment. Finally, the nanomechanical and phononic properties of the films are evaluated by Brillouin light scattering. [Preview Abstract] |
Session N31: Membrane and Membrane Protein Interactions
Sponsoring Units: DPOLYChair: Yossef Elabd, Drexel University
Room: 339
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N31.00001: Investigating the Structural Properties of Integral Membrane Proteins with Pulsed EPR Spectroscopy Invited Speaker: Gary Lorigan Very limited structural and dynamic information on proteins embedded inside a membrane currently exist, because they are difficulty to crystalize. New biophysical/structural biology methods are needed to probe these systems in a lipid bilayer. The Lorigan lab is applying unique hybrid NMR and spin-label EPR spectroscopic techniques to study membrane proteins. Magnetic resonance spectroscopic data of $^{\mathrm{15}}$N-, $^{\mathrm{2}}$H-labeled and/or spin-labeled membrane proteins incorporated into vesicles and bicelles will be presented. State-of-the-art pulsed EPR techniques such as Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy, and Double Electron-Electron Resonance (DEER) spectroscopy will be used. The ESEEM technique can determine short to medium range distances (out to about 8 {\AA}) between a site-specific nitroxide spin label and a nearby NMR-active isotopic labeled residue for a variety of different peptides and proteins which ultimately can be used to determine the difference between an $\alpha $-helical and $\beta $-sheet secondary structure. DEER can be used to measure distances between 2 spin labels out to about 70 {\AA}. We have shown a huge improvement is sensitivity with DEER measurements at Q-band when compared to X-band. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N31.00002: Tocopherol activity correlates with its location in a membrane: A new perspective on the anti-oxidant Vitamin E Drew Marquardt, Justin Williams, Norbert Kucerka, Jeffrey Atkinson, John Katsaras, Stephen Wassall, Thad Harroun There are no proven health benefits to supplementing with Vitamin E, so why do we require it for healthy living? The whole notion that vitamin E is an in-vivo antioxidant is now being seriously questioned. Using neutron diffraction and supporting techniques, we have correlated vitamin E's location in model membranes with its antioxidant activity. experiments were conducted using phosphatidylcholine (PC) bilayers whose fatty acid chains varied in their degree of unsaturation. We observe vitamin E up-right in all lipids examined, with its overall height in the bilayer lipid dependant. Interestingly we observe vitamin E's hydroxyl in the headgroup region of the bilayer for both the fully saturated and poly unsaturated lipids. Vitamin E was most effective at intercepting water borne oxidants than radical initiated within the bilayer core. However for lipids where vitamin E resides slightly lower (glycerol backbone) we observe comparable antioxidant activity against both water borne and hydrocarbon borne oxidants. Thus showing lipid species can modulate the location of vitamin E's activity. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N31.00003: Redistribution of Cholesterol in Model Lipid Membranes in Response to the Membrane-Active Peptide Alamethicin William Heller, Shuo Qian The cellular membrane is a heterogeneous, dynamic mixture of molecules and macromolecules that self-assemble into a tightly-regulated functional unit that provides a semipermeable barrier between the cell and its environment. Among the many compositional differences between mammalian and bacterial cell membranes that impact its physical properties, one key difference is cholesterol content, which is more prevalent in mammals. Cholesterol is an amphiphile that associates with membranes and serves to maintain its fluidity and permeability. Membrane-active peptides, such as the alpha-helical peptide alamethicin, interact with membranes in a concentration- and composition-dependent manner to form transmembrane pores that are responsible for the lytic action of the peptide. Through the use of small-angle neutron scattering and deuterium labeling, it was possible to observe a redistribution of the lipid and cholesterol in unilamellar vesicles in response to the presence of alamethicin at a peptide-to-lipid ratio of 1/200. The results demonstrate that the membrane remodeling powers of alamethicin reach beyond the membrane thinning effect to altering the localization of specific components in the bilayer, complementing the accepted two-state mechanism of pore formation. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N31.00004: Relationship between peptide membrane curvature generation and bactericidal activities Nathan Schmidt, Michelle Lee, David Kuo, Andre Ouellette, Gerard Wong Many amphipathic peptides and amphipathic domains in proteins can restructure biological membranes. Two examples are host defense antimicrobial peptides (AMPs) which disrupt and destabilize the cell membranes of microbes, and apolipoproteins which help stabilize nanoscale lipid aggregates. We use complementary x-ray and bacterial cell assays to elucidate the molecular length scale membrane deformations generated by amphipathic peptides with different structural motifs and relate these deformations to their activities on bacteria. Small angle x-ray scattering is used to study the interactions of model membranes with prototypical AMPs and consensus peptides from the amphipathic domains in apolipoproteins. By characterizing the nanoscale curvature deformations induced by these two distinct classes of membrane restructuring peptides we will discuss the role of amino acid composition on curvature generation. Bactericidal assays are used to access the in vivo activities of different amphipathic peptide motifs in order to understand the relationships between cell viability and membrane curvature generation. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N31.00005: Observing Stepwise Unzipping of Neuronal Snare Protein with Steered Molecular Dynamics Mustafa Tekpinar, Wenjun Zheng Soluble N-ethylmaleimide-sensitive factor Attachment Protein Receptors (SNARE) play a crucial role in membrane fusion. Neuronal SNAREs are made up of four helices: a snaptobrevin, a syntaxin 1, and two SNAP-25 helices. We applied constant velocity pulling forces to C terminal of snaptobrevin in SNARE complex to understand unzipping mechanism of neuronal SNAREs. We successfully observed unzipping of snaptobrevin from the other three helices in two steps: C terminal unzipping and N terminal unzipping. Our results have good agreement with recent optical tweezer experiments that observe this stepwise unzipping. Additionally, our simulations reveal that these two steps differ from each other. We believe that these different mechanisms can help us to understand SNARE mediated membrane fusion process better. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N31.00006: Information processing in the plasma membrane Benjamin Machta The plasma membrane is a 2D liquid where information from the world is received and processed. Motivated by the recent discovery that these membranes seem to be tuned close to a 2D liquid-liquid critical point, we set out to understand the different channels through which membrane bound proteins can communicate with each other. Diffusing proteins can carry out reactions when they come in contact with each other. Near criticality, proteins can also exert long-ranged critical Casimir forces on one another by coupling to the local composition order parameter. By modulating the growth and breakdown of the rigid cytoskeleton, they can direct forces on even more distant regions. In addition, proteins can control the release and production of second messengers that diffuse either through the bulk, or in the plane of the membrane itself. By making simple models for these processes we bound functional measures for them as communication channels. These include information theoretic measures of bandwidth, as well as physical measures of energetic efficiency and speed. Our results will likely prove useful in understanding functional reasons underlying the clustering and collective behavior often seen in experiments. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N31.00007: Yeast mitochondrial fission proteins induce antagonistic Gaussian membrane curvatures to regulate apoptosis Michelle Lee, Ghee Hwee Lai, Nathan Schmidt, Wujing Xian, Gerard C. L. Wong Mitochondria form a dynamic and interconnected network, which disintegrates during apoptosis to generate numerous smaller mitochondrial fragments. This process is at present not well understood. Yeast mitochondrial fission machinery proteins, Dnm1 and Fis1, are believed to regulate programmed cell death in yeast. Yeast Dnm1 has been previously shown to promote mitochondrial fragmentation and degradation characteristic of apoptotic cells, while yeast Fis1 inhibits cell death by limiting the mitochondrial fission induced by Dnm1 [Fannjiang et al, \textit{Genes {\&} Dev.} 2004. 18: 2785-2797]. To better understand the mechanisms of these antagonistic fission proteins, we use synchrotron small angle x-ray scattering (SAXS) to investigate their interaction with model cell membranes. The relationship between each protein, Dnm1 and Fis1, and protein-induced changes in membrane curvature and topology is examined. Through the comparison of the membrane rearrangement and phase behavior induced by each protein, we will discuss their respective roles in the regulation of mitochondrial fission. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N31.00008: On the modeling of endocytosis Tao Zhang, Rastko Sknepnek, Jennifer Schwarz, Mark Bowick Endocytosis is the primary mechanism by which extracellular material enters the cell. During endocytosis, the cell membrane deforms to surround the extracellular material and draw it into the cell, followed by a pinch-off to produce an internal vesicle. Recent experiments on clathrin-mediated endocytosis all agree that the actin cytoskeleton plays a crucial role in the deformation of the cell membrane. The actin cytoskeleton is a crosslinked network of filaments exerting active forces. However, competing ideas remain as to precisely how the actin cytoskeleton organizes itself to help drive the deformation. To begin to resolve this controversy, we mathematically model clathrin-mediated endocytosis using variational methods and Monte Carlo simulations. In particular, we investigate how the deformation of the cell membrane depends on the organization of the actin cytoskeletal network, and its associated active forces, to rule out one or more of the competing ideas. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N31.00009: Structural studies of lipid-protein interactions on cushioned bilayers S.K. Ghosh, M.K. Mukhopadhyay, Y. Ma, I. Lopez, S. Bera, L.B. Lurio, A. Chakrabarti, J.E. Kim, M.K. Sanyal, S.K. Sinha Biological membranes are heterogeneous and dynamical organizations of lipids and proteins, which perform functions fundamental to cell survival. Lipid-protein interactions control these functions by influencing folding and stability of integral or peripheral membrane proteins. Further, the incorporation or adsorption of these proteins into the membrane can in turn influence the lipid bilayer properties. In spite of some progress in understanding this process, a detailed structural analysis is lacking. Towards a better understanding of this interaction, we have performed an advanced interface sensitive scattering experiment using synchrotron x-rays. To accurately mimic the biological membranes with their natural thermal fluctuations and in-plane mobility of lipid molecules, polymer cushioned lipid bilayers have been used. This study shows that the adsorption of peripheral membrane protein\textit{spectrin}depends on the lipid headgroups, exhibiting different types of binding to phosphatidylcholine (PC) and phosphatidylethanolamie (PE). Further, the interaction of \textit{outer membrane protein A (OMP-A)}, an integral membrane protein is sensitive to the thermodynamic phase of the lipids. A detailed physical modeling of the lipid-protein interactions is under way. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N31.00010: Critical cell wall hole size for lysis in Gram-positive bacteria Gabriel Mitchell, Kurt Wiesenfeld, Daniel Nelson, Joshua Weitz Gram-positive bacteria transport molecules necessary for their survival through holes in their cell wall. The holes in cell walls need to be large enough to let critical nutrients pass through. However, the cell wall must also function to prevent the bacteria's membrane from protruding through a large hole into the environment and lysing the cell. As such, we hypothesize that there exists a range of cell wall hole sizes that allow for molecule transport but prevent membrane protrusion. Here we develop and analyze a biophysical theory of the response of a Gram-positive cell's membrane to the formation of a hole in the cell wall. We predict a critical hole size in the range 15-24nm beyond which lysis occurs. To test our theory, we measured hole sizes in \textit{Streptococcus pyogenes} cells undergoing enzymatic lysis via transmission electron microscopy. The measured hole sizes are in strong agreement with our theoretical prediction. Together, the theory and experiments provide a means to quantify the mechanisms of death of Gram-positive cells via enzymatically mediated lysis and provides insight into the range of cell wall hole sizes compatible with bacterial homeostasis. [Preview Abstract] |
Session N32: International Physics Programs and History of Physics
Sponsoring Units: FIP FHPChair: Gloria Lubkin, Physics Today Editor
Room: 340
Wednesday, March 20, 2013 11:15AM - 11:41AM |
N32.00001: Fulbright Opportunities in the Physical Sciences Katrin DeWindt The Fulbright Scholar Program is sponsored by the United States Department of State and is principally funded by taxpayer contributions. Bi-national in nature, it includes academic year opportunities for both American and foreign scholars. More than 800 grants in 125 countries are available each year. The Program supports research, teaching and lecturing opportunities in all academic disciplines, numerous professional fields and the arts. American academics and administrators have multiple opportunities to internationalize their campuses and their discipline points of view. Further, Fulbright not only sends American scholars abroad but also brings scholars to the United States and should be considered a strategic internationalization opportunity both for individuals and for campuses. During the 2013-14 competition cycle there were 33 awards available in physics and astronomy and 175 all discipline awards. The presentation will guide attendees in identifying appropriate opportunities through the Fulbright Scholar Program and will make suggestions as to how to be successful in a proposal. Special attention will be given to opportunities available for specialists in physics. The workshop will also cover non-Core Fulbright Scholar opportunities for physicists and university administrators, including a number of short-term, innovative programs that send an additional 400 scholars from the United States to universities and research institutes abroad to offer expertise on issues of global interest from cutting-edge research to policy, to technical expertise in curriculum development, institutional planning, program assessment, and institutional capacity building. [Preview Abstract] |
Wednesday, March 20, 2013 11:41AM - 12:07PM |
N32.00002: Revisiting the Bohr Atom 100 Years Later Ernst Wall We use a novel electron model wherein the electron is modeled as a point charge behaving as a trapped photon revolving in a Compton wavelength orbit at light speed. The revolving point charge gives rise to spiraling Compton wavelets around the electron, which give rise to de Broglie waves. When applied to the Bohr model, the orbital radius of the electron scales to the first Bohr orbit's radius via the fine structure constant. The orbiting electron's orbital velocity, Vb, scales to that of the electron's charge's internal velocity (the velocity of light, c) via the fine structure constant. The Compton wavelets, if they reflect off the nucleus, have a round trip time just long enough to allow the electron to move one of its diameters in distance in the first Bohr orbit. The ratio of the electron's rotational frequency, fe, to its rotational frequency in the Bohr orbit fb, is fe/fb $=$ 1/$\alpha^{2}$, which is also the number of electron rotations in single orbit. If we scale the electron's rotational energy (h*fe) to that of the orbit using this, the orbital energy value (h*fb) would be 27.2114 eV. However, the virial theorem reduces it to 13.6057, the ground state energy of the first Bohr orbit. Ref: www.tachyonmodel.com. [Preview Abstract] |
Wednesday, March 20, 2013 12:07PM - 12:33PM |
N32.00003: A transformational year in physics: 1932 Charles W. Clark, Joseph Reader On New Year's Day, 1932, the {\em Physical Review} announced Urey's discovery of deuterium by the observation of Balmer emission lines in atomic hydrogen that were at the wavelengths predicted by Bohr's theory for an isotope with a mass twice that of the proton. At the time it was thought that the deuterium nucleus contained two protons and one ``nuclear electron'' confined inside the nucleus by an unknown force. This view quickly changed when {\em Nature} published Chadwick's discovery of the neutron nine weeks later. In June, Heisenberg made the suggestion that the neutron and proton were alternative levels of a quantum two-state system: the isospin concept that guides nuclear theory to this day. In August, Anderson discovered particles with the mass of, and charge opposite to, that of the electron: the first discovery of antimatter. Meantime, Cockroft and Walton effected the first disintegration of nuclei by particles accelerated by high voltages, and Lawrence and Livingston showed that the cyclotron could make high energy particles without high voltages. Six Nobel Prizes are directly traceable to work done within that one year! We review these events and their consequences. This talk is based on an article published in {\em Physics Today}, March 2013. [Preview Abstract] |
Wednesday, March 20, 2013 12:33PM - 12:59PM |
N32.00004: Discovery and development of x-ray diffraction Yeuncheol Jeong, Ming Yin, Timir Datta In 1912 Max Laue at University of Munich reasoned x-rays to be short wavelength electromagnetic waves and figured interference would occur when scattered off crystals. Arnold Sommerfeld, W. Wien, Ewald and others, raised objections to Laue's idea, but soon Walter Friedrich succeeded in recording x-ray interference patterns off copper sulfate crystals. But the Laue-Ewald's 3-dimensional formula predicted excess spots. Fewer spots were observed. William Lawrence Bragg then 22 year old studying at Cambridge University heard the Munich results from father William Henry Brag, physics professor at Univ of Leeds. Lawrence figured the spots are 2-d interference of x-ray wavelets reflecting off successive atomic planes and derived a simple eponymous equation, the Bragg equation d*sin(theta)$=$ n*lamda. 1913 onward the Braggs dominated the crystallography. Max Laue was awarded the physics Nobel in 1914 and the Braggs shared the same in 1915. Starting with Rontgen's first ever prize in 1901, the importance of x-ray techniques is evident from the four out of a total 16 physics Nobels between 1901-1917. We will outline the historical back ground and importance of x-ray diffraction giving rise to techniques that even in 2013, remain work horses in laboratories all over the globe. [Preview Abstract] |
Wednesday, March 20, 2013 12:59PM - 1:25PM |
N32.00005: Latest developments on documentary film ``The State of the Unit: The Kilogram'' Amy Young This presentation shows the recent developments in the documentary film project ``The State of the Unit.'' The film, to be completed Fall 2013, looks at historical and current efforts to define precisely the unit of mass. [Preview Abstract] |
Session N33: Focus Session: Polymers for Energy Storage and Conversion
Sponsoring Units: DPOLYChair: Jodie Lutkenhaus, Texas A&M University
Room: 341
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N33.00001: Power Factor Improvements in PEDOT:PSS Tellurium Nanowire Composites Shannon Yee, Nelson Coates, Jeffrey Urban, Rachel Segalman The thermoelectric properties of a composite consisting of tellurium nanowires in a conducting polymer, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) matrix, is optimized by controlling the shape of the nanowire and doping of the polymeric matrix with polar solvents. The mechanism for an observed improvement in power factor is attributed to the unique conducting nature of PEDOT:PSS, which exhibits a transition from a hopping transport-dominated regime to a diffusive transport-dominated regime upon doping with polar solvents. Near this transition, the electrical conductivity of the composite is improved without significantly reducing the thermopower. Relying on this principle, the power factor optimization for this new thermoelectric material is experimentally carried out and found to exceed 100 $\mu$ W/m-K$^{2}$, which is nearly five orders of magnitude greater than pure PEDOT:PSS. This improvement in power factor suggests a new area of research into polymer based thermoelectric materials where transport interactions between the polymer and an inorganic component can be tuned. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N33.00002: Charge Transport Properties of P3HT-PEO block copolymers that are Electrochemically Oxidized in the Solid-State Shrayesh Patel, Anna Javier, Nitash Balsara We report on the relationship between morphology and electronic/ionic charge transport of Poly(3-hexylthiophene)-$b$-Poly(ethylene oxide) (P3HT-$b$-PEO) and lithium bis-(trifluoromethanesulfonyl) imide (LiTFSI) mixtures. Using ac impedance spectroscopy, we show that P3HT-$b$-PEO/LiTFSI mixtures can conduct electronic and ionic charges simultaneously. The electronic resistance of P3HT-$b$-PEO can be controlled through the electrochemical oxidation of P3HT with LiTFSI. We designed an all solid-state electrochemical cell with three terminals to measure the electronic conductivity of P3HT-$b$-PEO under applied potentials. The addition of a third terminal within the P3HT-$b$-PEO layer allows for the \textit{in-situ} measurement of the electronic conductivity as a function of the P3HT electrochemical oxidation level. Our experimental setup is unique in that electrochemical oxidation occurs in the presence of solid-polymer electrolyte. Previous studies on the electrochemical oxidation of polythiophenes have been done in the presence of a liquid electrolyte. The results of the \textit{in-situ} electronic conductivity measurements as a function of electrochemical doping level and block copolymer composition will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N33.00003: Ion Transport in Amorphous Polymer Electrolytes Katherine P. Barteau, Nathaniel A. Lynd, Glenn H. Fredrickson, Craig J. Hawker, Edward J. Kramer Successful development of lithium polymer batteries has been limited by low ionic conductivities in the polymer electrolyte, especially at low temperatures. In order to generate strategies for improvement of ionic conductivity, we have developed highly-controlled syntheses for a number of well-defined poly(glycidyl ether)s, PGEs, to serve as low temperature polymer electrolytes. The properties of PGEs can be tuned through structure control and functionalization, making them model systems for understanding ion transport and elucidating structure-property relationships. In this work we will discuss the synthesis and characterization of a family of PGEs that exhibit systematic differences in glass transition temperature (Tg), viscosity, oxygen-content, dielectric constant, and ionic conductivity. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N33.00004: Dynamics of a Novel Class of Polymers: Polymerized Sulfur Kevin Masser, Jenny Kim, Vladimir Oleshko, Jared Griebel, Woo Chung, Adam Simmons, Jeff Pyun, Christopher Soles In this study we investigate the dynamics of a new type of polymer, consisting mainly of sulfur. Room-temperature stable polymerized sulfur samples were prepared by crosslinking the well-known living sulfur polymers formed at elevated temperatures by the addition of a crosslinking agent. This reverse vulcanization process was used to create a series of samples with different amounts of crosslinking agent. These polymers show great promise for use in advanced batteries as cathode materials. Each system exhibits a glassy-state beta relaxation, with the intensity of this relaxation proportional to the crosslinking content. A dynamic glass transition is also observed for each system, and the glass transition temperature/segmental relaxation moves to higher temperatures with increased crosslink content as is typically observed in crosslinked systems. As is typical of polymers, ion motion in these systems is closely coupled to the backbone motion of the host polymer. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N33.00005: Investigation of the capacity retention mechanisms in novel composite sulfur copolymer-base cathodes for high-energy density Li-S batteries Vladimir Oleshko, Jenny Kim, Kevin Masser, Steven Hudson, Christopher Soles, Jared Griebel, Woo Jin Chung, Adam Simmonds, Jeffrey Pyun Utilization of the active cathode material in high-energy density Li-S batteries limited by the insulating nature of sulfur and losses in the form of insoluble polysulfides was improved by the use of 1,3-diisopropenylbenzene (DIB) copolymerized with molten sulfur. This approach termed, inverse vulcanization, transforms elemental sulfur into chemically stable processable copolymer forms with tunable thermomechanical properties. According to dielectric spectroscopy and dc conductivity measurements, composite sulfur-DIB copolymer cathodes exhibit a glassy-state beta relaxation related to short sulfur segments or to the DIB cross-linker. High-resolution AEM and FESEM studies down to the atomic scale reveal multiscale 3D-architectures created within the pristine and cycled composite cathodes with various contents of the electroactive copolymers. The morphology, structures, bonding and local compositional distributions of the constituents (sulfur, copolymers, aggregated conductive carbon nanoparticles) as well as extended pore structures and their transformations under cycling have been examined to provide insights into mechanisms of the enhanced capacity retention in the modified Li-S cells. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N33.00006: TiO$_{2}$-SEO Block Copolymer Nanocomposites as Solid-State Electrolytes for Lithium Metal Batteries Inna Gurevitch, Raffaella Buonsanti, Alexander Teran, Jordi Cabana, Nitash Balsara Replacing the liquid electrolyte in lithium batteries by a solid has been a long-standing goal of the battery industry due to the promise of better safety and the potential to produce batteries with higher energy densities. Recently, symmetric polystyrene-block-poly(ethylene oxide) (SEO) copolymers/LiX salt mixtures with high ionic conductivity and high shear modulus were developed as solid electrolytes. For an enhancement in mechanical properties and its effect on the dendrite growth from lithium metal electrodes, we study the effect of adding TiO$_{2}$ nanoparticles to the SEO/LiX mixtures. We find that TiO$_{2}$/SEO/LiX nanocomposite electrolytes have stable performance against the lithium metal electrodes. There appears to be a correlation between the stability of the electrolytes, morphology, and mechanical properties. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N33.00007: High Aspect Ratio Nanofillers for Solid Polymer Electrolytes Lalitha Ganapatibhotla, Janna Maranas In this study, we explore high aspect ratio nanofillers as additives that enhance solid polymer electrolyte (SPE) conductivity at battery working temperatures.SPEs are the key to light-weight and energy-dense lithium ion batteries but suffer from low room temperature ion conductivities.Spherical ceramic fillers are known to improve SPE conductivity and mechanical properties.Our experiments on spherical Al2O3 particle filled SPEs indicate highest conductivity enhancement at eutectic composition and temperature.A new mechanism, via stabilization of alternating layers of PEO and highly conducting PEO6:LiClO4 tunnels at the filler surface, was suggested by us.More such structures would be stabilized at a filler surface with high aspect ratio.Consistent with this hypothesis, $\gamma$-Al2O3 nanowhiskers intensify the effect of $\gamma$-Al2O3 nanoparticles.Increase in conductivity at eutectic composition, and decrease at non-eutectic compositions is more than the nanoparticles.Diameters of the two fillers are similar, but the change in aspect ratio (1to100) improves conductivity by a factor of 5. The influence of morphology and PEO dynamics on conductivity enhancement will be presented.All measurements are performed at a series of Li compositions, temperatures and nanowhisker loadings. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N33.00008: Atomistic Simulations Reveal a Surprising Variety of Morphologies in Precise Ionomers Mark Stevens, Dan Bolintineanu, Amalie Frischknecht Ionomers are being investigated as potential solid electrolytes in battery applications, due to their unique electrical properties. However, the relationships between ionomer chemistry, morphology and ion transport are poorly understood, which has hindered the development of ionomer-based batteries. To this end, we report atomistic molecular dynamics (MD) simulations of a model ionomer (polyethylene-co-acrylic acid) neutralized with different ions at various neutralization levels. The structure factor computed from the simulations is in good agreement with experimental X-ray scattering data. The simulations provide new insight into the shape, size and composition of ionic aggregates. In particular, we observe a wide variety of aggregate morphologies, ranging from small spherical aggregates to string-like shapes and large percolated networks. The unexpected morphologies of these ionic aggregates imply the need for a new interpretation of scattering spectra for these materials. We quantify cation-anion and oxygen-hydrogen association, the two interactions primarily responsible for aggregate formation, and report detailed information pertaining to local structures around cations, which is difficult to obtain experimentally and may have important consequences for ion transport. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N33.00009: Mechanism of Ion Diffusion in Coarse-Grained Ionomer Melts Lisa M. Hall, Mark J. Stevens, Amalie L. Frischknecht Ionomers (polymers with a small amount of charged groups) have been identified as possible single ion conducting battery electrolytes. A barrier their use in such applications is that the strong electrostatic interactions lead to ionic aggregation and can make ion diffusion very slow. In order to understand the physics underlying ionomer dynamics and especially how charge transport occurs, we perform molecular dynamics simulations. Our model has polymers with charged groups either in the backbone or pendant to it, explicit counterions, and long-range Coulomb interactions. Depending on placement, amount, and spacing of the ionic groups, various morphologies of ionic aggregates are formed. We find for all systems, ions can rearrange locally within the ionic aggregates on a relatively short timescale. Ions can move a longer distance when they rearrange collectively on a longer timescale, that is especially long for systems with discrete ionic aggregates. Because of this, a typical ion trajectory shows mostly small movements and rare large, sudden movements. However, these features are not due to `hopping' as typically understood. Instead, nearby aggregates of ions join together, rearrange, and later break apart, during which time some ions are exchanged and appear to have `hopped'. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N33.00010: Coarse-Grained MD Simulation of String-like Aggregates in Single-Ion Conductors Keran Lu, Janna Maranas, Scott Milner Single-ion conductors, or ionomers, have been investigated as a potential polymer electrolyte for advanced batteries. Ionic aggregates are prevalent in ionomers, and their sizes and shapes are not well characterized by experiment. Atomistic molecular dynamics simulations have been used to explore these aggregates, but may not be fully equilibrated because the aggregrates break and join infrequently. We report on an ion-only coarse-grained molecular dynamics simulation of a well-equilibrated ionomer system that reproduces structural features of the parent united atom simulation. Results for radius of gyration, shape anisotropy, and average ion coordination number from our simulation show that ionic aggregates are string-like, with random-walk configurations. An analogy to worm-like micelle equilibrium predicts an exponential length distribution for aggregates, in agreement with simulations. The implications of the size and structure of aggregates on conduction are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N33.00011: Anion Conduction in PEO-Functionalized Polyphosphazene Ionomers Joshua Bartels, Andrew Hess, Harry Allcock, Ralph Colby, James Runt A series of novel polyphosphazene ionomers with short chain poly(ethylene oxide) (PEO) moieties, bound ammonium cations, and free iodide anions were previously synthesized. Ion dynamics during anion conduction of the ionomers were studied by dielectric relaxation spectroscopy (DRS). These polyphosphazenes provide interesting conductive materials to study because of their low glass transition temperature, high segmental mobility, and high ion content. Analysis of DRS results provides static dielectric constant, conducting ion mobility, and conducting ion content for the materials. An increase in the length of the alkyl group extending from the polymer-bound ammonium cation increases conductivity and conducting ion concentration due to new steric interactions weakening ion-ion associations that restrict segmental mobility. By placing ether oxygens in the short alkyl group a large increase in conductivity and a decrease in the glass transition temperature is observed due to strong associations between the cation and ether oxygen lone pairs. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N33.00012: Decoupling ion flux and mechanical strength in polymer battery membranes Derrick Smith, Shan Cheng, Timothy Bunning, Christopher Li While much research has demonstrated repeatable characteristics of electrolyte membranes, the fundamentals behind the interactions during ionic diffusion in solid polymer electrolyte membranes for battery applications are not well understood, specifically the role of nanostructures, which hold the key to improving performance of energy storage devices such as fuel cells and Lithium ion batteries. The challenges in fabricating highly controlled model systems are largely responsible for the interdependent ambiguities between nanostructures and the corresponding ion transport behavior. In this work, Holographic Polymer Electrolyte Membrane (hPEM) volume gratings comprised of alternating layers of cross-linked polymer resin and ionic liquid were fabricated using holographic polymerization with an average d-spacing of 180 nm. These one-dimensional confinement structures were used to quantitatively study the anisotropic ionic conductivity properties, and correlate this behavior to nano-confinement and phase mixing. These membranes provide a platform in decoupling ion flux and bulk mechanical properties for future blend systems for battery applications. These volume gratings also offer an exciting route to fabricate multifunctional gratings for optic and sensing applications. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N33.00013: Structure-Property Relationship of Perfluorinated Sulfonic Acid (PFSA) Membranes Ahmet Kusoglu, Adam Weber Perfluorosulfonic-acid (PFSA) membrane is the most commonly used ionomer in electrochemical energy storage and conversion devices thanks to its remarkable proton conductivity, perm-selectivity, wide electrochemical window, and mechanical stability. Most of these properties are the result of the membrane's phase-separated nanostructure where ions and solvents transport through the hydrated domains while the surrounding hydrophobic PTFE backbone acts as a mechanical support. Thus, it is essential to understand the solvent- and humidity-induced morphological changes and their associated impact on the membrane's properties for optimizing the structure-property relationship desired by the electrochemical devices. In this talk, correlations among the mechanical (e.g., modulus), electrochemical (e.g., ionic conductivity) and nanostructural (e.g., domain spacing) properties during hydration is discussed. Moreover, the impact of thermal history, mechanical reinforcement, and side-chain length on the structure-property correlation is examined. Even though the properties vary for the membranes investigated, similar correlations are found between the degree of hydration, domain spacing, and ionic conductivity. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N33.00014: Hard X-ray tomography as a non-destructive technique to study the growth of lithium dendrites in lithium polymer batteries Katherine Harry, Daniel Hallinan, Dilworth Parkinson, Alastair MacDowell, Nitash Balsara Lithium metal electrodes have the highest energy density of any battery electrode technology and are, therefore, being considered for electric vehicles. However, lithium metal changes its shape under cycling, resulting in the growth of lithium metal dendrites through the electrolyte that eventually short-circuit the cell. While polystyrene-block-poly(ethylene oxide) copolymer electrolytes extend cell life by suppressing dendrite growth, dendrites eventually do grow. We show that hard X-ray microtomography is a non-destructive tool for studying the formation and growth of lithium dendrites at the interface between lithium metal and a block copolymer electrolyte. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N33.00015: A New Mechanical Loading Configuration for Maximizing The Performance of Dielectric Elastomer Generators Samuel Shian, Jiangshui Huang, Zhigang Suo, David Clarke Electrical energy can be generated from mechanical deformations using dielectric elastomers but currently achieved energy densities and conversion efficiencies are still small. In this presentation, we demonstrate that significant improvements, an energy density over 500 mJ/g and up to 10{\%} in efficiency, can be produced using VHB elastomers by altering the mechanical loading geometry. A major limitation is viscous losses in the VHB elastomer indicating that higher efficiencies with other elastomers will be attainable. The basic concept of mechanical energy harvesting with a dielectric elastomer sheet is a straightforward electromechanical cycle leading to a voltage step-up: a sheet is stretched, electrical charge at low voltage is placed on either side using compliant electrodes, the stretch is released causing the sheet's initial thickness and area to be recovered increasing the charge potential which can then be harvested. Integral to maximizing the energy conversion is the amount of mechanical energy that can be stored elastically and the amount of capacitance change in the elastomer sheet during stretching. We show that these factors can be maximized by equi-biaxial loading. Details of our dielectric elastomer generator will be described as well as the procedures we use for quantifying its performance. [Preview Abstract] |
Session N35: Focus Session: Search for New Superconductors II
Sponsoring Units: DMPChair: Ivan Bozovic, Brookhaven National Laboratory
Room: 343
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N35.00001: Charge Density Wave Instability and Soft Phonon in $A$Pt$_3$P ($A$=Ca, Sr, and La) Chao Cao, Hui Chen, Xiaofeng Xu, Jianhui Dai The electronic and phonon properties of the platinum pnictide superconductors $A$Pt$_3$P ($A$=Ca, Sr, and La) were studied using first-principles calculations. The spin-orbit coupling effect is significant in LaPt$_3$P but negligible in CaPt$_3$P and SrPt$_3$P. Moreover, SrPt$_3$P has been demonstrated to exhibit an unexpected weak charge-density-wave (CDW) instability which is neither simply related to the Fermi-surface nesting nor to the momentum-dependent electron-phonon coupling alone. The instability is absent in CaPt$_3$P and can be quickly suppressed by the external pressure, accompanied with decreases in the phonon softening and BCS $T_c$. Our results suggest SrPt$_3$P as a rare example where superconductivity is enhanced by the CDW fluctuations. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N35.00002: Charge density wave fluctuations and possible heavy fermion behavior without magnetism in ThCr$_2$Si$_2$-type KNi$_2$S$_2$ and KNi$_2$Se$_2$ James Neilson, Anna Llobet, Jiajia Wen, Matthew Suchomel, Tyrel McQueen Materials with the ThCr$_2$Si$_2$-type structure host myriad examples of many-body physics, including high-temperature superconductivity and heavy fermion behavior. In these compounds, the emergence of the collective state frequently occurs near a magnetic instability, suggesting that magnetic fluctuations underlie the electronic phenomena. I will provide evidence for similar many-body physics in the structurally related, but non-magnetic compounds, KNi$_2$S$_2$ and KNi$_2$Se$_2$. From the analysis of synchrotron X-ray diffraction and neutron total scattering data, we observe spatially incoherent charge density wave fluctuations that disappear on cooling. Along with the implied and unusual increase in local symmetry, we find that there is negative thermal expansion and enhancement of the electronic band mass below $T\sim15$ K, with superconductivity emerging below 1 K. These findings demonstrate that collective electronic phenomena occurs in ThCr$_2$Si$_2$-type materials without direct proximity to localized magnetism. Furthermore, these results highlight the importance in understanding charge fluctuations and their hybridization in driving the emergence of coherent or many-body electronic states, akin to localized magnetism associated with heavy fermion behavior. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N35.00003: Charge-order fluctuations and electron-phonon coupling in organic superconductors Alberto Girlando, Matteo Masino, Natalia Drichko, Martin Dressel Organic superconductors (SC), like other new classes of SC, are characterized by important electronic correlations. Spin or charge-order (CO) fluctuations have been invoked as mediators in the pairing mechanism, in place of, or in addition to, the conventional phonon mediated pairing. In the phase diagram of BEDT-TTF based 1/4-filled layered charge transfer salts, a CO metal-insulator transition is close to the metal-SC transition, with CO fluctuations in the proximity of the instabilities. We present the characterization of CO fluctuations obtained through optical spectroscopy of SC and non-SC BEDT-TTF salts, with an estimate of the average charge on the molecules and the velocity of charge ``jump'' from one molecule to the other. It turns out that the latter is not connected to the SC. A correlation seems instead to occur between SC and the average charge on the molecules. We shall also discuss other possible signatures of the charge fluctuations in the optical spectra, as well as the connection between CO fluctuations and intra- and inter-molecular electron-phonon coupling. The relevance of these ideas to the recently discovered class of doped acene superconductors will be shortly discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N35.00004: Ferroelectric Soft Phonons, Charge Density Wave Instability, and Strong Electron-Phonon Coupling in BiS$_2$-Layered Superconductors Taner Yildirim Very recently a new family of layered materials, containing BiS$_2$ planes was discovered to be superconducting at temperatures up to T$_c$=10 K, raising questions about the mechanism of superconductivity in these systems. Here, we present state-of-the-art first principles calculations that directly address this question and reveal several surprising findings [1]. The parent compound LaOBiS$_2$ possesses anharmonic ferroelectric soft phonons at the zone center with a rather large polarization of $\approx 10 \mu C/cm^2$, which is comparable to the well-known ferroelectric BiFeO$_3$. Upon electron doping, new unstable phonon branches appear along the entire line $Q=(q,q,0)$, causing Bi/S atoms to order in a one-dimensional charge density wave (CDW). We find that BiS$_2$ is a strong electron-phonon coupled superconductor in the vicinity of competing ferroelectric and CDW phases. Our results suggest new directions to tune the balance between these phases and increase T$_c$ in this new class of materials.\\[4pt] [1] T. Yildirim, arXiv:1210.2418 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N35.00005: Searching for High-T$_{\mathrm{c}}$ Superconductivity in Low-Z, Low-Ne Materials O-Paul Isikaku-Ironkwe, Timothy Haugan, Alex Animalu The discovery in 2001 of HTSC at 39K in MgB$_{2}$, a low-atomic number (Z) and low-valence electron count per atom (Ne) material, strongly suggested that similar materials may exist with comparable or even higher Tcs. Efforts to find MgB$_{2}$-like HTSC materials in binary and ternary systems have not been very successful. Using recently developed material specific formula for Tc, we have extended the computational and experimental search for potential low-Z, low-Ne HTSCs beyond the ternary structure into the 4 and 5-element systems. Exploring the family of low-Z, low-Ne materials represented by Z $=$ 1.333p $+$ 2 where p is an integer, we find in this broad spectrum, hundreds of potential HTSC materials. Here we present some of the combinatorial-computational datasets and preliminary experimental results using a novel non-DFT material-specific characterization dataset (MSCD) method. MSCD promises to accelerate the computational search for new materials, particularly superconductors. The computations and preliminary experimental results suggest that HTSC, comparable to the cuprates may exist in low-Z, low-Ne materials with 4 and 5-element systems. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N35.00006: High-temperature surface superconductivity in rhombohedral graphite Tero Heikkil\"a, Nikolai Kopnin, Mari Ij\"as, Ari Harju, Grigori Volovik We show that rhombohedral graphite may support surface superconductivity with an unusual relation between the BCS coupling constant and the order parameter. This feature results from the properties of the states localized on the graphite surfaces. In a description including only the nearest neighbour coupling of the graphene layers, the surface states are topologically protected and have a flat band dispersion. We show that including higher order couplings destroys this flat band character and leads to a particle-hole symmetry breaking quadratic dispersion with a large effective mass. Employing this dispersion, we then show its effect on superconductivity and find two regimes of parameters, depending on the relation between the strength of the coupling constant and the details of the quadratic dispersion. For low coupling strengths, superconductivity is localized on the surfaces, but the order parameter is exponentially suppressed as in a conventional BCS superconductor, whereas for large coupling strengths we obtain surface superconductivity with a linear relation between the order parameter and the coupling constant. Our results offer an explanation for the recent findings of graphite superconductivity with an unusually high trans [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N35.00007: Phase separation instabilities and pairing in layered BSCCO-like lattice geometries Armen Kocharian, Kun Fang, Gayanath Fernando, Alexander Balatsky, Kalum Palandage The electron spontaneous phase separations accompanied by local inhomogeneities are evaluated by monitoring the charge and spin pairing gaps in the ground state and corresponding crossovers at finite temperatures in various cluster geometries and wide range of inter-site interaction $U$. The effects of the next nearest neighbor hopping on electron instabilities at level crossings in the vicinity of quantum critical points are considered. The calculated energy gap at one hole away from half filling displays universal features consistent with the lattice structure symmetry in non-bipartite geometries. The charge and spin collective excitations in layered pyramidal structures driven by out-of-plane variation of lattice parameters yield intriguing insights into the coherent and incoherent pairings and gap modulations in Bi-like based cuprates, iron pnictides, and other transition metal oxides layered structures. The phase diagrams resemble a number of inhomogeneous, coherent and incoherent nanoscale phases seen in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. The found similarities and differences in the mechanisms of electron pairing, driven by attractive and repulsive electron interaction are analyzed. The phase separation instabilities in related intercalated layered geometries are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N35.00008: Tuning the charge-transfer energy in hole-doped cuprates Chuck-Hou Yee, Gabriel Kotliar Chemical substitution, combined with strain, allows the charge-transfer energy in hole-doped cuprates to be broadly tuned. We theoretically characterize the structural and electronic properties of the family of compounds R$_2$CuO$_2$S$_2$, constructed by sulfur replacement of the apical oxygens and rare earth substitutions in the parent cuprate La$_2$CuO$_4$. Additionally, the enthalpies of formation for possible synthesis pathways are determined. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N35.00009: The effective half-filled band model is inappropriate for the dimerized 2D organic superconductors Niladri Gomes, R. Torsten Clay, Sumit Mazumdar The antiferromagnetism in $\kappa$-(ET)$_{2}$X can be understood within the effective $\frac{1}{2}$-filled band anisotropic triangular lattice Hubbard Hamiltonian for strong anisotropy. DMFT theories have claimed antiferromagnetic-to-superconductor transition within the same model, as the anistropy is reduced. In previous work we have shown the absence of superconductivity within the triangular lattice $\frac{1}{2}$-filled band Hubbard model for any Hubbard $U$ and any anisotropy. Other DMFT approaches theories have claimed superconductivity within the so-called Hubbard-Heisenberg model, which incorporates an additional antiferromagnetic spin-exchange over and above that due to the Hubbard $U$. Very recent work has also claimed a valence-bond solid (VBS) phase within the Hubbard-Heisenberg model, which would seemingly explain the observed VBS phase in EtMe$_{3}$P[Pd(dmit)$_{2}$]$_{2}$. We report exact calculations that show that neither the VBS nor the superconducting phase occur within the Hubbard-Heisenberg model, showing clearly that the effective $\frac{1}{2}$-filled band model is unsuitable for describing the complete phase space of the $\kappa$-(ET)$_{2}$X. Our work raises serious doubts about the DMFT theories of superconductivity of metal intercalated C$_{60}$ and picene. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N35.00010: A possible paired-electron liquid in a $\frac{1}{4}$-filled band model of $\kappa$-(ET)$_2$X R.T. Clay, N. Gomes, S. Mazumdar A minimal model for the $\kappa$-(ET) conducting layers is a $\frac{1}{2}$-filled anisotropic triangular lattice Hubbard model, where a dimer of molecules is replaced with a single effective site. This effective model can explain occurrence of an antiferromagnetic (AFM) phase, but recent results do not find superconductivity in the model. We have shown that in a $\frac{1}{4}$-filled system on a dimerized square lattice, the AFM phase gives way to a Paired Electron Crystal singlet-paired state in the presence of lattice frustration. Here we present results of calculations on the actual $\frac{1}{4}$-filled $\kappa$ lattice rather than the simplified square lattice. We find not only an AFM-to-singlet transition, but show that the singlet phase may be a Paired Electron Liquid state consisting of a superposition of nearest-neighbor singlets. We show that in the excitation spectrum the lowest singlet excited state occurs below the lowest triplet. This may indicate gapless singlet excitations and gapped spin excitations, which would explain the observed heat capacity versus thermoelectric behavior in $\kappa$-(ET)$_2$-Cu$_2$(CN)$_3$ and EtMe$_3$Sb[Pd(dmit)$_2$]$_2$. We further discuss superconducting pair correlation functions. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N35.00011: Crystal structure, electronic properties, and superconductivity mechanism of La-Phenanthrene Shahab Naghavi, Michele Fabrizio, Tao Qin, Erio Tosatti Recently, polycyclic aromatic hydrocarbon (PAH) molecular solids: picene, coronene, dibenzopentacene, phenanthrene among them, have been reported to turn from insulating to metallic and superconducting upon intercalation of electron-donating atoms, such as K, Ba, La. Despite experimental uncertainties, understanding these novel light-element based superconductors is important since both electron phonon coupling and electron electron correlations seem important, as indicated by early theory work. Choosing La-Phenanthrene (La-PA) as our working case, we first search for the theoretical optimal crystal structure and electronic properties by first principles density functional calculations. We single out a stable insulating phase with $P1$ symmetry and, slightly higher in energy, a metastable metallic $P2_{1}$ phase--the same (higher) symmetry of pristine PA, also proposed for La-PA. A tight binding model representing the metallic La-PA electronic structure, its dominant electron phonon coupling with an intermolecular dimerizing mode, and an intramolecular Coulomb $U$ is formulated and discussed. In that model it can be argued that BCS pairing may be essentially unhindered by the Coulomb repulsion. Being symmetry-based, the mechanism could apply to other PAH superconductors as well. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N35.00012: Anisotropic Migdal-Eliashberg theory using Wannier functions Elena Roxana Margine, Feliciano Giustino We combine the fully anisotropic Migdal-Eliashberg theory with electron-phonon interpolation based on maximally-localized Wannier functions, in order to perform reliable and highly accurate calculations of the anisotropic temperature-dependent superconducting gap and critical temperature of conventional superconductors. Compared with the widely used McMillan approximation, our methodology yields a more comprehensive and detailed description of superconducting properties, and is especially relevant for the study of layered or low-dimensional systems as well as systems with complex Fermi surfaces. In order to validate our method, implemented within the EPW package [1,2], we perform calculations on two prototypical superconductors, Pb and MgB$_{2}$, and obtain good agreement with previous studies [3]. [1] F. Giustino, M. L. Cohen, and S. G. Louie, Phys. Rev. B 76, 165108 (2007). [2] J. Noffsinger et. al., Computer Physics Communications 181, 2140 (2010). [3] E. R. Margine and F. Giustino, Phys. Rev. B (submitted). [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N35.00013: Search for chalcogenide based superconductors: Sulfur based solution growth Udhara Kaluarachchi, Sergey Bud'ko, Paul Canfield As part of our effort to develop tools for searching for new chalcogenide based superconductors we are expanding the range of S-based binary melts that we can use for solution growth of single crystals. As a recent example, we have been able to grow single crystals of Rh$_{17}$S$_{15}$ and separate them for excess binary melt via high temperature decanting. In addition to refining the details of the Rh-S binary phase diagram, microscopic, thermodynamic and transport measurement on Rh$_{17}$S$_{15}$ crystals confirm their T$_{c}$ $\sim$ 5.5 K as well as their remarkably large H$_{c2}$(T) behavior. The possible cause of the enhanced H$_{c2}$(T) will be discussed. As time allows we will also review other S-based growths and compounds. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N35.00014: Density Functional Theory studies of Epitaxial Charge Transfer Salts Geoffrey A. Rojas, P. Ganesh, Simon Kelly, Bobby J. Sumpter, John A. Schlueter, Petro Maksymovich Some of the fulvalene-based charge transfer salts (CTS) become superconducting in bulk. The basic physics and ways to control it has been explored by changing the intermolecular spacing using both chemical substitution and pressure, but the fixed stoichiometry limit the occupation of the filled states to what is naturally available. Recent experiments suggest growth of 2D epitaxial layers of CTS allowing stoichiometric and geometric control of the electronic structure, thereby leading to engineered superconducting interfaces. In a combined experiment and theory study, we provide new insight to understand the interplay between structure, stoichiometry and electronic-structure of epitaxially grown (ET)2SF5CH2CF2SO3 salt on Ag(111) surface. Density functional theory studies show that the cohesive energy of the 2D salts are very high, in spite of strong bonding to the underlying Ag surface via Ag-S metallic bonds, and provide a rationale for off-stoichiometric growth with different electronic structures as seen in our experiments, such as 3:1 and a 1:1 cation:anion stoichiometry, necessary for a monolayer coverage~and different from the bulk 2:1 stoichiometry. We also explore the role of van der Waals interactions for structural stability. This research was conducted at the Center for Nanophase Materials Sciences, sponsored at ORNL by the Division of User Facilities, U.S. DOE. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N35.00015: Rational design of novel thallium halide high T$_c$ superconductors from first-principles Zhiping Yin, Gabriel Kotliar Searching for new high-temperature superconductors remains one of the most active research areas in condensed matter physics and material physics. In this talk I will show how we designed, from first principles calculations, a novel family of thallium halide-based compounds as candidates for new high-temperature superconductors. This family together with the celebrated (Ba,K)BiO$_3$ and electron-doped HfNCl families are the ``other high-temperature supercondutors,'' whose superconductivity is mediated by the recently proposed mechanism of non-local correlation-enhanced strong electron-phonon coupling (arXiv:1110.5751). Two prototype compounds namely CsTlF$_3$ and CsTlCl$_3$ are studied with various hole doping levels and volumes. The critical superconducting temperature T$_c$ are predicted to be about 30 K and 20 K with optimal hole doping and volume, respectively. Our procedure of designing this class of superconductors is quite general and can be used to search for other ``other high temperature superconductors.'' [Preview Abstract] |
Session N36: Superconductivity: Mesoscopic Techniques and Applications
Sponsoring Units: DCMPChair: Timir Datta, University of South Carolina
Room: 344
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N36.00001: Cantilever micro-susceptometry of mesoscopic Bi2212 samples Hryhoriy Polshyn, Raffi Budakian, Genda Gu Fluxoid quantization provides a direct means to study phase coherence. In cuprate superconductors, there have been observations which suggest that phase coherent superconducting fluctuations may persist at temperatures significantly above Tc. The focus of this work is to study the vortex states in mesoscopic cuprate superconducting samples to directly probe phase coherence over a wide range of temperatures. We present cantilever torque susceptometry measurements of micron and sub-micron size Bi2212 rings and disks. The high sensitivity of this technique allowed observation of transitions between different fluxoid states of a single ring, and the discrete vortex states of micron size disks. The dependence of magnetic susceptibility on diameter and wall thickness of the ring was investigated. Measurements were made at different values of the in-plane magnetic field, and over a wide range of temperatures. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N36.00002: Measuring superconducting delta-doped strontium titanate bilayers using the scanning SQUID technique Hilary Noad, Katja C. Nowack, Hisashi Inoue, Christopher Bell, Yasuyuki Hikita, Harold Y. Hwang, Kathryn A. Moler Delta-doped strontium titanate is a highly tunable system well-suited for studying two-dimensional superconductivity. Bilayer structures, in particular, offer the possibility of modifying interlayer coupling between sheets of superconducting electrons. We can locally probe superconductivity and magnetism as a function of temperature using scanning SQUID susceptometry. We will discuss prospects for using the scanning SQUID technique to measure unusual effects, such as multi-component superconductivity, that may arise in delta-doped strontium titanate bilayers. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N36.00003: Fabrication of La$_{2-x}$Sr$_{x}$CuO$_{4}$ Superconductor Nanodevices Nicholas Litombe, Anthony Bollinger, Ivan Bozovic, Jenny Hoffman In order to investigate dimension-limited superconductivity in cuprates, we explore methods of nanopatterning La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO). We use high resolution e-beam and photo-lithographic fabrication techniques, coupled with appropriate chemical and physical pattern transfer techniques. In particular, we focus on quasi-1D LSCO nanowire devices where we study random telegraph noise (RTN) signals from possible nematic domain fluctuation dynamics. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N36.00004: Development of a Nb-AlxOy-Nb trilayer process at the University of Washington Andrew Wagner We present progress made at the Washington Micro-Fabrication Facility toward the production of SQUID amplifiers from a $\rm{Nb}-\rm{Al}_x\rm{O}_y-\rm{Nb}$ trilayer process. Details of a simplified trilayer process are presented and the capability to fabricate 3 micrometer Josephson Junctions from the process with controllable critical current densities is demonstrated. We discuss how these results can be applied to the production of SQUID, SLUG or Josephson Parametric amplifiers operating in the microwave band for the Axion Dark Matter eXperiment (ADMX) located at the University of Washington. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N36.00005: Superconducting spin switch with infinite magnetoresistance Bin Li, Niklas Roschewsky, Markus Munzenberg, Marius Eich, Marguerite Epstein-Martin, Jagadeesh S. Moodera Nearly five decades ago de Gennes theoretically predicted that below the superconducting transition temperature $T_{C}$ the resistance in a FI/S/FI (FI-ferromagnetic insulator; S-superconductor) trilayer structure depends on the magnetization direction of the two FI layers [de Gennes, \textit{Phys. Lett.} \textbf{23}, 10 (1966)]. This prediction is experimentally demonstrated here. We present magneto-transport properties in a EuS/Al/EuS structure, showing an infinite magnetoresistance by tuning the internal exchange field at the FI/S interface. The superconducting order parameter was suppressed when the magnetic moment of the two EuS layers aligned in parallel whereas it was least affected when the two EuS layers have their magnetizations in antiparallel alignment: one could tune between the superconducting and normal states by the FI magnetization configuration. Importantly either of these two states could be maintained in zero applied fields, thus creating a nonvolatile two-level memory state. It was also shown that this is entirely an interface proximity effect and could be destroyed by introducing just a monolayer of Al$_{2}$O$_{3}$ barrier in between the interfaces. Furthermore the observed resistance switching field correlated with the surface anisotropy of the EuS layers. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N36.00006: Thin semi-rigid coaxial cables for cryogenics applications Akihiro Kushino, Soichi Kasai We have developed cryogenic coaxial cables for low temperature signal readout from sensitive devices, such as transition edge sensors, superconducting tunnel junctions, and kinetic inductance detectors. In order to reduce heat penetration into cryogenic stages, low thermal conductivity metals were chosen for both center and outer electrical conductors. Various types of coaxial cables, employing stainless-steel, cupro-nickel, brass, beryllium-copper, phosphor-bronze, niobium, and niobium-titanium, were manufactured using drawing dies. Thermal and electrical properties were investigated between 1 and 8 K. Coaxial cables made of copper alloys showed thermal conductance roughly consistent with literature, meanwhile Nb coaxial cable must be affected by the drawing process and thermal conductance was lowered. Attenuation of superconducting Nb and NbTi coaxial cables were observed to be adequately small up to above 10 GHz compared to those of normal conducting coaxial cables, which are subject to the Wiedemann-Franz law. We also measured normal conducting coaxial cables with silver-plated center conductors to improve high frequency performance. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N36.00007: Novel structural transformation in the ultrathin films of cuprates and its influence on electronic and magnetic properties D. Samal, Tan Haiyan, H. Molegraaf, B. Kuiper, W. Siemons, Sara Bals, Jo Verbeeck, Gustaaf Van Tendeloo, Y. Takamura, Elke Arenholz, Catherine Jenkins, G. Rijnders, Gertjan Koster We report on the evidence found for structural transformation in ultrathin films of two cuprate systems viz. SrCuO$_{2}$(SCO) and CuO. In case of SCO ultrathin films, we show a transformation from the bulk planar to chain-like structure, below a critical thickness, due to associated electrostatic instability. Results based on X-ray diffraction, X-ray photoelectron diffraction and scanning transmission electron microscopy reveal an elongation of the unit cell by $\sim$0.5{\AA} along the $c$-axis and the presence of oxygen in the Sr plane for chain like structure. Polarized X-ray absorption spectroscopy reveals a preferential occupation of Cu 3$d_{{3z}^{2}-r^{2}}$ orbital in case of the chain like structure unlike to the planar one. For the case of ultrathin CuO films, we find strain induced structural transformation from monoclinic to tetragonal phase, akin to other 3d transition metal monoxides and reveals relatively higher Neel temperature. Our findings point to a unique structural stabilization process for ultrathin cuprate layers and provide new insight for the experimental realization of novel hybrids to look for enhanced superconducting properties. References: Zhong \textit{et.al,} PRB \textbf{85,} 12411(R) (2012); Siemons \textit{et.al,} Al.PRB \textbf{79},195122 (2009) [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N36.00008: Dynamical I-V Characteristics of SNS Junctions Kevin Spahr, Jonathan Graveline, Christian Lupien, Marco Aprili, Bertrand Reulet We probe the dynamics of a Superconductor /Normal Metal/ Superconductor junction (SNS: Nb / Al above its critical temperature / Nb) by measuring~ its voltage / current characteristics while applying an ac current of frequency in the range 1-200 MHz. We observe a dynamical phase transition as a function of the frequency and amplitude of the ac current. At low frequency there is a continuous change in the dynamical behavior of the junction, replaced an abrupt change and hysteresis at high frequency. The crossover frequency between the two regimes has a strong temperature dependence similar to that of the electron-phonon interaction rate. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N36.00009: Spectroscopy of Andreev Bound States: revealing the hidden side of the Josephson effect \c{C}a\u{g}lar Girit, Landry Bretheau, Hugues Pothier, Daniel Esteve, Cristian Urbina The Josephson effect describes how phase coherence is established between two weakly coupled superconductors. Microscopically, the Josephson current is carried by Cooper pairs, occupying Andreev Bound States, localized at the weak link. Andreev Bound States, which come in particle-hole symmetric pairs, consitute a spin-like degree of freedom. In our experiment, we detect the transition to the \textit{excited} Andreev bound state in a superconducting atomic contact using a Josephson junction as a broadband (5-90 GHz) spectrometer. Not only do we clearly resolve the Andreev transition, but we also identify spectroscopic lines arising from anticrossings with a Josephson plasma mode of the environment. Our results demonstrate the accessibility of a pseudospin degree of freedom in the Josephson effect. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N36.00010: X-ray Structural Studies of Bi2Sr2CaCu2O8$+\delta $ Exfoliated Nanocrystals Andreea Lupascu, Renfei Feng, Luke J. Sandilands, Zixin Nie, Viktoriya Baydina, Genda Gu, Shimpei Ono, Yoichi Ando, Kenneth S. Burch, Young-June Kim Structural studies of nanocrystals produced via mechanical exfoliation are not only essential for examining structure quality or structural changes at reduced-dimensionality, but also for understanding the role of substrates in the exfoliation process. Highly focused, tunable synchrotron X-ray beams enable the use of non-destructive characterization tools to study exfoliated samples on a variety of substrates. We demonstrate that structural and spectroscopic information can be obtained on nanocrystals as thin as 6 nm, by using a combination of micro X-ray fluorescence ($\mu$ XRF), micro X-ray absorption near-edge spectroscopy ($\mu$ XANES), and X-ray microdiffraction ($\mu$ XRD) techniques. $\mu$ XRF is used to locate the sample of desired thickness; $\mu$ XANES and $\mu$ XRD are used to obtain electronic and structural information, respectively. We report a substantial substrate effect for Bi2Sr2CaCu2O8$+\delta $ nanocrystals exfoliated on Si/SiO2 and mica substrates. The ``4.7 b'' structural modulation, characteristic of bulk crystals, vanishes below a thickness of 60 nm on mica, and is drastically suppressed below 60 nm for the Si/SiO2 substrate. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N36.00011: Mechanical detection of single-quantum-level fluxoid relaxation in an Nb micro-ring Jae-Hyuk Choi, Heon-Hwa Choi, Yun-Won Kim, Soon-Gul Lee, Mahn-Soo Choi We developed a highly sensitive static force magnetometry, originally proposed for sub-pico-newton force standard, which enabled the observation of single fluxoids selectively and their dynamics in a superconducting micro-ring. For an Nb ring with inner diameter of 4.0 $\mu$m, the magnetic moment of a single fluxoid quantum was determined as 7.4 pico-emu, corresponding to the static force of 74 femto-newton, in good agreement with a theoretical estimate within 8\%. The magnetic relaxation of moderate number of fluxoids, ranging from 20 to 60, was also measured at temperatures of 4 $\sim$ 6 K and at zero magnetic field. The relaxation results with single-quantum-step feature were analyzed with a theoretical model for thermally activated transition. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N36.00012: Studying phonon and quasiparticle heating effects on SINIS Coolers Thomas Aref, Hung Nguyen, Juha Muhonen, Jukka Pekola A Normal-Insulating-Superconductor (NIS) tunnel junction can function as an electronic cooler. This is typically done in the SINIS configuration where the normal metal island is the object being cooled. By applying proper biasing, the bandgap in the superconductor can be used as an energy filter, allowing hot electrons to escape from the normal island and cold electrons to enter the island from the superconductor. This narrows the Fermi distribution of the electrons on the island, effectively lowering their temperature. By coupling this electronic refrigeration to phonons, the phononic temperature can be reduced as well. These SINIS coolers have potential for replacing other cryogen based refrigeration techniques at low temperatures. One primary aim is to produce an efficient, solid-state, cooling platform that cools small devices from 300 to 100 mK. Our most recent research has helped illuminate various effects that adversely affect the performance of these coolers. Examples of effects probed included geometrical factors, phonon heating, quasiparticle heating, substrate modification, ground planes and direct traps. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N36.00013: Coupling carbon nanotube mechanics to a superconducting circuit B.H. Schneider, S. Etaki, H.S.J. van der Zant, G.A. Steele The quantum behaviour of mechanical resonators is a new and emerging field driven by recent experiments reaching the quantum ground state. The high frequency, small mass, and large quality-factor of carbon nanotube resonators make them attractive for quantum nanomechanical applications. A common element in experiments achieving the resonator ground state is a second quantum system, such as coherent photons or a superconducting device, coupled to the resonators motion. For nanotubes, however, this is a challenge due to their small size. Here, we couple a carbon nanoelectromechanical (NEMS) device to a superconducting circuit. Suspended carbon nanotubes act as both superconducting junctions and moving elements in a Superconducting Quantum Interference Device (SQUID). We observe a strong modulation of the flux through the SQUID from displacements of the nanotube. Incorporating this SQUID into superconducting resonators and qubits should enable the detection and manipulation of nanotube mechanical quantum states at the single-phonon level. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N36.00014: Spin-precession-assisted tunneling in hybrid superconducting point contacts Cecilia Holmqvist, Wolfgang Belzig, Mikael Fogelstrom The charge and spin transport properties of a quantum point contact coupled to a nanomagnet depends strongly on the dynamics of the nanomagnet's spin. We analyze the current-voltage characteristics of a junction coupled to a spin whose dynamics is modeled as Larmor precession brought about by an external magnetic field. The interaction between the spin dynamics and the Josephson effect leads to a rich subgap structure due to spin-precession-assisted multiple Andreev reflections. Additionally, the spin current displays Shapiro-like resonances due to the interplay between the ac Josephson current and the Larmor precession. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N36.00015: Probing the Dynamics of Andreev States in Coherent Normal/Superconducting ring: Evidence for a noisy supercurrent Bastien Dassonneville, Francesca Chiodi, Sophie Gueron, Meydi Ferrier, Helene Bouchiat Most properties of a non superconducting (N) metal connected to two superconductors (an SNS junction) can be seen as resulting from the phase dependent Andreev states (AS) in N. Density of states in N is then drastically changed with the emergence of a small energy gap, the minigap. Whereas AS equilibrium properties are well understood, AS dynamics is a more complex issue [1]. We perform experiments on a phase ($\phi$) biased NS ring coupled to a superconducting resonator. The modification of the resonances ($f$ from 200 MHz up to 14 GHz) yields the complex phase dependent susceptibility $\partial_\phi I_{ring}=\chi(f,\phi)=\chi'+i\chi''$. As expected, we find a non-dissipative $\chi'$ related to the supercurrent flowing through the ring. A more striking finding [2] is the existence of a dissipative $\chi''$ revealing a noisy supercurrent, predicted [3] but never observed before. Moreover, as $f$ increases we show that the main dissipation mechanism changes from population relaxation to microwave-induced transitions across the minigap.\\[4pt] [1] F. Chiodi et al, Sci. Rep, 1 (2011) \newline [2] B. Dassonneville et al, in preparation \newline [3] A. Martin-Rodero et al, PRB, 53 (1996) [Preview Abstract] |
Session N37: Focus Session: Fe-based Superconductors: Spin Fluctuations
Sponsoring Units: DMP DCOMPChair: Guangyong Xu, Brookhaven Natl Lab
Room: 345/346
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N37.00001: Neutron Scattering as a Probe of Fermi Surface Nesting in Iron-Based Superconductors Invited Speaker: Raymond Osborn Superconductivity in the iron-based compounds is induced by suppressing a magnetically ordered phase by doping, pressure, or disorder, so it is no surprise that neutron scattering has had an important role in the field, elucidating both the origin of magnetic fluctuations and their role in the unconventional superconductivity. Our investigations of BaFe$_2$As$_2$ doped with potassium [1], sodium, and phosphor, can be interpreted within the framework of weakly correlated itinerant magnetism, in which Fermi surface nesting between hole pockets at the zone center and electron pockets at the zone boundary is responsible for both the magnetic (SDW) order and the superconductivity. Resonant spin excitations that occur when the superconducting energy gap changes sign on different parts of the Fermi surface were initially observed by inelastic neutron scattering in Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ representing the first phase-sensitive evidence of s$_\pm$-symmetry [2]. We have since shown that the resonance splits into two with hole-doping because of the growing mismatch in the hole and electron Fermi surface volumes, accompanied by a decrease in the binding energy of the resonance and its spectral weight in accordance with RPA theory [3]. A detailed examination of the phase diagram close to the critical phase boundary for SDW order has identified a new phase that is further evidence of the role of Fermi surface nesting in generating magnetic order. \\[4pt] [1] S. Avci, O. Chmaissem, E. Goremychkin, \textit{et al}, Phys Rev B \textbf{83}, 172503 (2011).\\[0pt] [2] A. Christianson, E. Goremychkin, R. Osborn, \textit{et al}, Nature \textbf{456}, 930-932 (2008).\\[0pt] [3] J.-P. Castellan, S. Rosenkranz, E. Goremychkin, \textit{et al}, Phys Rev Lett \textbf{107}, 177003 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N37.00002: Spin fluctuations of BaFe2(As,P)2 studied by neutron scattering Chul-Ho Lee, P. Steffens, N. Qureshi, K. Kihou, M. Nakajima, A. Iyo, H. Eisaki, M. Braden Superconductivity can be induced in parent compounds of iron-based superconductors by several methods: carrier doping, external pressure and chemical pressure. To understand their superconducting mechanism, clarifying what is a common property for achieving high-Tc superconductivity is crucial. To date, studies on spin fluctuations have been mainly performed on carrier doped samples. On the other hand, there are only a few studies on chemical pressurized samples examined by powder samples. In this work, thus, we studied spin fluctuations of P doped BaFe$_2$(As,P)$_2$ ($T_c$ = 29.5K) using single crystal samples. Inelastic neutron scattering measurements were conducted using triple axis spectrometer IN8 of ILL. As results, well-defined commensurate peaks have been observed at (0.5,0.5,$L$), which is consistent with the nesting vector of the Fermi surface. Energy spectrums at $T$ = $T_c$ show $L$ dependence, suggesting a three dimensional character remains even in superconducting BaFe$_2$(As,P)$_2$. Clear spin gap has been observed below $T_c$, whose gap structure depends on $L$. Details will be discussed at the conference. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N37.00003: Role of the spin-orbit coupling in the spin-resonance formation in Fe-based superconductors M.M. Korshunov, Yu.N. Togushova, I. Eremin, P.J. Hirschfeld Determination of the gap symmetry is an important step towards uncovering mechanism of superconductivity in Fe-based materials. One of the key experiments in support of the $s_\pm$ spin-fluctuation-mediated gap was observation of the spin-resonance peak in many pnictides and chalcogenides, see P.J. Hirschfeld et al., Rep.Prog.Phys. 74, 124508 (2011). Recently, in inelastic polarized neutron scattering measurements by Lipscombe et al., Phys.Rev. B 82, 064515 (2010), it was found that the peaks in the transverse and longitudinal components of the spin susceptibility of BaFe$_{1.9}$Ni$_{0.1}$As$_2$ exhibit rather different behavior, and argued that the true spin resonance exists in the transverse channel only. Here, on the basis of the 5-orbital model, we argue that this disparity arises from spin-orbit coupling. It also leads to the relative shift of the two component's resonance frequency with lower frequency one exhibiting larger enhancement. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N37.00004: Compositional dependence of low energy spin fluctuations in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ Gregory Tucker, D.K. Pratt, A. Thaler, N. Ni, K. Marty, A.D. Christianson, M.D. Lumsden, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, A.I. Goldman, R.J. McQueeney The low energy magnetic fluctuation spectrum of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ samples in the range $x=(0.014,0.055)$ were studied in their antiferromagnetic ordered states using inelastic neutron scattering. The parent compound BaFe$_2$As$_2$ exhibits gapped spin-wave excitations at $\mathbf{Q}_{\mathrm{AFM}} = (1, 0, 1)$ with two gap energies [corresponding to in-Fe-plane and out-of-Fe-plane transverse excitations of the ordered moment]. Substitution of Co for Fe via doping acts to increase Landau damping without significantly modifying the parent compound spin-gap structure. For small amounts of Co the two-gap structure is resolvable in our measurements. For larger Co-doping the damping increases sufficiently to obscure the parent compound gap structure and the measured $\mathbf{Q}_{\mathrm{AFM}}$ excitation spectra is best described by a diffusive model. Interestingly, the change in excitation character corresponds in composition with the appearance of superconductivity in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As; these two effects may very well be related. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N37.00005: Spin Excitations in Overdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ A.D. Christianson, S. Calder, J.E. Mitchell, K. Marty, C.H. Wang, M.B. Stone, A.S. Sefat, B.C. Sales, M.D. Lumsden The relationship between spin excitations and unconventional superconductivity has been and continues to be the subject of considerable experimental and theoretical scrutiny. While the underdoped and optimally doped regions of the phase diagrams of unconventional superconductors have been extensively studied there have few studies of the spin excitations in the overdoped region. Here we report an inelastic neutron scattering study of an overdoped sample of the unconventional superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with x=0.15 and T$_c$ = 12 K. At energies below 40 meV the spin excitations are much broader and weaker when compared to samples close to optimal doping. Despite the weakness of the spin excitations a broad spin resonance is still observed at an energy of $\sim$8 meV at the wave vector (0.5 0.5 0). This corresponds to a value of 7.7 K$_B$T$_c$ which is nearly double the value of 4 K$_B$T$_c$ found for many Fe-based superconductors. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N37.00006: Magnetism in parent Fe-chalcogenides: quantum fluctuations select a plaquette order Natalia Perkins, Samuel Ducatman, Andrey Chubukov The analysis of magnetism in parent compounds of iron-based superconductors (FeSCs) is an integral part of the program to understand the origin of superconductivity in these materials. Here we analyze magnetic order in iron-chalcogenide Fe$_{1+y}$Te -- the parent compound of high-temperature superconductor Fe$_{1+y}$Te$_{1-x}$Se$_x$. Neutron scattering experiments show that magnetic order in this material contains components with momentum $Q_1=(\pi/2, \pi/2)$ and $Q_2 =(\pi/2, -\pi/2)$ in Fe-only Brillouin zone. The actual spin order depends on the interplay between these two components. Previous works argued that spin order is a single-$Q$ state (either $Q_1$ or $Q_2$). Such an order breaks rotational $C_4$ symmetry and order spins into a double diagonal stripe. We show that quantum fluctuations actually select another order -- a double $Q$ plaquette state with equal weight of $Q_1$ and $Q_2$ components, which preserves $C_4$ symmetry but breaks $Z_4$ translational symmetry. We argue that the plaquette state is consistent with recent neutron scattering experiments on Fe$_{1+y}$Te. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N37.00007: Unexpected ($\pi,\pi$) order in Fe$_{1.1}$Te David Fobes, Igor Zaliznyak, Zhijun Xu, Genda Gu, John M. Tranquada, Deepak Singh We have studied the evolution of the magnetic and crystal structure in single crystalline Fe$_{1.1}$Te, an iron-rich parent of the chalcogenide superconductor family. While a structural transition to a monoclinic symmetry occurs at $\sim 60$ K, magnetic peaks at $2\pi \cdot (0.48, 0)$ only arise below T$_{N}\approx 58$ K, and can be understood as bicollinear magnetism with discommensuration defects. \footnote{I. Zaliznyak \textit{et al.}, Phys. Rev. B \textbf{85}, 085105 (2012)} Unexpectedly, we have also observed resolution limited peaks at approximately $(\pi, \pi)$, arising at the same temperature T$_{N}$, and exhibiting temperature hysteresis similar to that seen in magnetic susceptibility, perhaps indicating that these peaks are of magnetic origin. Additionally, the position of these peaks is nearly the same as in the parent compounds of the iron pnictide family of superconductors, where magnetic order is simple collinear commensurate antiferromagnetism. The origin of these new peaks near ($\pi, \pi$) and their relationship to the dominant bicollinear magnetic order observed in Fe$_{1.1}$Te presents a puzzle. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N37.00008: Inelastic neutron scattering studies on the incommensurate-to-commensurate transformation of low energy magnetic excitations in $Fe_{1+\delta-y}(Ni/Cu)_{y}Te_{1-x}Se_{x}$ Zhijun Xu, Jinsheng Wen, John Schneeloch, Yang Zhao, Masaaki Matsuda, Wei Ku, Xuerong Liu, Genda Gu, D.-H. Lee, R.J. Birgeneau, J.M. Tranquada, Guangyong Xu We have performed a series of neutron scattering and magnetization measurements on $Fe_{1+\delta-y}(Ni/Cu)_{y}Te_{1-x}Se_{x}$ system to study the interplay between magnetism and superconductivity. Both non-superconducting and superconducting samples with T$_{c}$ 8$\sim$15K are studied. The low energy magnetic excitations of all samples at T$>$$>$T$_{c}$ consist of two incommensurate vertical columns. They change to a distinctly different U-shaped dispersion at T$>$T$_{c}$ for the superconducting samples and the transition temperature depend on the composition.[1] On the other hand, for all non-superconducting samples, there is no clear temperature dependence, and the low energy magnetic excitations remain two columns for temperatures down to 1.5 K. Work is supported by the Office of Basic Energy Sciences, DOE. [1]Zhijun Xu et al., arXiv:1201.4404(accepted by PRL). [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N37.00009: Cu-induced localization in the Fe-based superconductor FeTe$_{0.5}$Se$_{0.5}$ Jinsheng Wen, Zhijun Xu, Cheng Zhang, Masa Matsuda, Oleg Sobolev, Jitae Park, Edith Bourret, Dunghai Lee, Qiang Li, Genda Gu, Guangyong Xu, John Tranquada, Robert Birgeneau We report neutron scattering and resistivity results on the Cu-substitution effects in FeTe$_{0.5}$Se$_{0.5}$ with a $T_c$ of $\sim$15 K. With a 2$\%$ Cu substitution, the $T_c$ is reduced to 8 K, and for Fe$_{0.9}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$, it is not superconducting. In Fe$_{0.9}$Cu$_{0.1}$Te$_{0.5}$Se$_{0.5}$, the low-energy magnetic excitations around the in-plane wave vector (0.5, 0.5) is greatly enhanced. Upon heating, the magnetic scattering is weakened, which is different from the temperature dependences of the Cu-free and 2$\%$ Cu-doped sample. The spectral weight reduction upon warming decreases with increasing energy in the 10$\%$ Cu-doped sample. We take these as evidences that Cu drives the system towards localization, which is confirmed by our resistivity data. These observations probably explain why superconductivity is absent in the Cu-doped BaFe$_2$As$_2$ system and demonstrate the inadequacy of the rigid-band shift model on the substitution effects of the 3$d$ transition metals. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N37.00010: Neutron Scattering in Co Doped NaFeAs Yu Song, Chenglin Zhang, Scott Carr, Zachary Cole, Nicolas Luttrell, Pengcheng Dai In this presentation we will discuss some of our recent neutron scattering work done on Co doped NaFeAs, focusing on the evolution of magnetism and superconductivity with doping. The phase diagram of Co doped NaFeAs is similar to Co,Ni doped BaFe2As2, but also show significant differences in the magnetic response despite the similar Tc in both systems. Thus by comparing these systems, we attempt to find common features for unconventional superconductivity in both systems. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N37.00011: Magnetic excitation in Co-doped NaFeAs studied by neutron scattering Chenglin Zhang, Yu Song, Guotai Tan, Scott Car, Pengcheng Dai Even though NaFeAs ``111'' shares many similarities with BaFe2As2 ``122'' such as magnetic structure and phase diagram with doping , actually they are quite different from many aspects. For one example, the spin resonance is very sharp like delta function and well below 2detal, in sharp contrast with the broaden resonance observed in doped-122 systems. Our result provide a strong piece of evidence to support S$+$\_ and exclude S$++$ pairing symmetry in Fe-based superconductors. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N37.00012: NMR Search for the Spin Nematic State in LaFeAsO Single Crystal Mingxuan Fu, David A. Torchetti, Takashi Imai, Fanlong Ning, Jiaqian Yan, Athena S. Sefat The mechanism underlying high-$T_{c}$ superconductivity in iron-pnictides remains a major puzzle in condensed matter. Earlier NMR measurements provide evidence for a correlation between $T_{c}$ and the enhancement of low frequency spin fluctuations\footnote{F. L. Ning, T. Imai. et al., Phys. Rev. Lett. 104, 037001 (2010).}. However, slowing of spin fluctuations is accompanied by lattice softening, which is a major complication in this scenario. The intermediate temperature range between the tetragonal-orthorhombic structural phase transition at $T_{TO}$ and SDW transition at $T_{SDW}$ may be a realization of spin nematic state\footnote{C. Fang et al., Phys. Rev. B 77, 224509 (2008)}. We report $^{75}$As single crystal NMR study of LaFeAsO \footnote{M. Fu et al., arXiv:1208.5652, to appear in Phys. Rev. Lett.}. We have found that the low frequency spin dynamics exhibits a strong two-fold anisotropy within each orthorhombic domain below $T_{TO}$ This intermediate state then freezes progressively into a static SDW below $T_{SDW}$. Our results reveal the presence of an exotic intermediate spin state below $T_{TO}$ with the signature of spin nematicity. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N37.00013: Two-dimensional magnetic interactions in LaFeAsO Mehmet Ramazanoglu, Jagat Lamsal, Gregory S. Tucker, Stuart Calder, Jiaqiang Yan, Tatiana Guidi, Toby Perring, Thomas A. Lograsso, Andreas Kreyssig, Alan Goldman, Robert J. McQueeney The magnetic excitations in antiferromagnetic (AFM) ordered LaFeAsO (La1111) are mapped out by inelastic neutron scattering (INS) technique using both time-of-flight and triple-axis spectroscopies. The energy dependence of the observed intensity at the AFM ordering wavevector, Q$_{\mathrm{AFM}}$, yields a spin gap of $\sim$11 meV. The independence of the spin gap along the c-direction suggests nearly two-dimensional magnetic interactions. A steep magnetic excitation spectrum is observed for in-plane wavevectors for energy transfers up to 100 meV. The constant energy cuts of these steep excitations are elliptically shaped for the low energy transfers below 50 meV. As energy transfer increases, the elliptical anisotropy develops into a splitting above 50 meV. A phenomenological model based on highly damped diffuse spin dynamics is used to analyze the data. The calculated parameters of anisotropic spin wave velocities, spin gap and the damping values are similar to the previous results in BaFe$_{2}$As$_{2}$ and CaFe$_{2}$As$_{2}$ based materials but La111 has a more two-dimensional character. [Preview Abstract] |
Session N38: Novel Photophysics and Transport in NanoPV I
Sponsoring Units: GERA DPOLY DCOMPChair: Sean Shaheen, University of Denver
Room: 347
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N38.00001: Tuning charge transport in organic devices: From in silico to carbon to device Invited Speaker: Alan Aspuru-Guzik I will describe our work towards first-principles design of organic semiconducting materials. In particular, I will describe our efforts towards the rational design of high hole-mobility organic crystals. I will describe a case study where the in silico prediction of a material led to the synthesis and characterization of it by the Bao and Toney groups. I will also discuss other related research directions. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N38.00002: Conformational Disorder in Energy Transfer: Beyond Forster Theory Tammie Nelson, Sebastian Fernandez-Alberti, Adrian Roitberg, Sergei Tretiak Energy transfer in donor/acceptor chromophore pairs, where the absorption of each species is well separated, can be understood through a F\"{o}rster resonance energy transfer model. The picture is more complex for organic conjugated polymers, where the total absorption spectrum can be described as a sum of the individual contributions from each subunit, whose absorption is not well separated. Although excitations in these systems tend to be well localized, traditional {\it donors} and {\it acceptors} cannot be defined and energy transfer can occur through various pathways. In addition, fast torsional motions between individual monomers can break conjugation and lead to reordering of excited state energy levels. Energy transfer in these systems can be induced by both electronic transitions and by fast torsional fluctuations where both mechanisms occur simultaneously. We use non-adiabatic excited state molecular dynamics (NA-ESMD) to simulate energy transfer between two poly-phenylene vinylene (PPV) segments composed of 3-rings and 4-rings separated by varying distances. The transition density decay represents the transfer rate, and can be decomposed into contributions from various transfer pathways. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N38.00003: Phonon-assisted nonradiative energy transfer in quantum dot-silicon nanostructures Pedro Ludwig Hernandez Martinez, Aydan Yeltik, Burak Guzelturk, Alexander O. Govorov, Hilmi Volkan Demir Silicon is one of the most dominant materials in photovoltaics and understanding the processes of energy transfer is of great importance. In this work, we study the phonon-assisted nonradiative energy transfer (NRET) in quantum dot (QD)-silicon hybrid nanostructures. Here, the NRET dynamics is investigated as a function of temperature for distinct separation thicknesses between the donor QDs and the acceptor silicon plane. We propose a theoretical model based on the phonon-assisted energy transfer process. We estimate the energy transfer rate using the Fermi's Golden rule where the matrix elements are derived for the phonon-assisted energy transfer process. To support our findings the temperature-dependent fluorescence lifetimes in QD-silicon nanostructures are analyzed. The experimental data analyses agree with the resulting theoretical model. The results indicate that phonons play an important role in NRET to Si as an indirect bandgap semiconductor. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N38.00004: Directed Energy Transfer through Size-Gradient Nanocrystal Layers into Si Substrates Michael Nimmo, Louis Caillard, Will deBenedetti, Hue Nguyen, Yves Chabal, Yuri Gartstein, Anton Malko Nanostructured materials attract great interest as candidates for next generation of photoelectronic devices. Presently, the majority of hybrid devices are based on charge transfer in which exciton break-up occurs at the interface between dissimilar materials. Poor interface quality and carrier transport are issues that result in a conversion efficiencies lower than in the inorganic crystalline devices. An alternative approach is based on hybrid structures, which combine strongly absorbing components such as nanocrystal quantum dots (NQDs) and adjacent high-mobility semiconductor layers coupled via proximal energy transfer. Building on our previous work,\footnote{H. M. Nguyen et al., \textit{APL} \textbf{98}, 161904 (2011)} we examine non-radiative energy transfer (NRET) between NQDs grafted on a hydrogenated Si surface via amine modified carboxy-alkyl chain linkers. A macroscopically thick, size--gradient NQD film is prepared on top of crystalline Si layer to explore directed energy tranfer into the substrate. Steady-state and time-resolved photoluminescence studies show effective energy transfer between adjacent layers and into the Si substrate with the transfer efficiency exceeding 90{\%} among layers. This demonstrates the viability of NQD-Si hybrid structures for photovoltaic devices. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N38.00005: Quantum coherence and noise in open quantum systems Invited Speaker: Ahsan Nazir Recent experiments demonstrating signatures of quantum coherence in the excitonic energy transfer dynamics of a variety of systems have sparked renewed interest in the theoretical modelling of energy transfer processes within a dissipative environment. A major challenge remains the development of techniques that allow one to probe the diverse parameter regimes relevant to such systems. Master equation methods provide useful tools with which to efficiently analyse energy transfer dynamics in open quantum systems. However, they are often valid only in rather restrictive parameter regimes, limiting their applicability in the present context. Here, I shall present a versatile variational master equation approach to the non-equilibrium dynamics of dissipative excitonic quantum systems, which allows for the exploration of a wide range of parameter regimes within a single formalism. Derived through the combination of a variationally optimised unitary transformation and the time-local projection operator technique, the master equation can be applied to a range of bath spectral densities, temperatures, and system-bath coupling strengths, and accounts for both non-Markovian and non-equilibrium environmental effects. Applying the formalism in the case of excitonic energy transfer, I shall show that while it correctly reproduces Redfield, polaron, and Foerster dynamics in the appropriate limits, it can also be used in intermediate regimes where none of these theories may be applicable. I shall also discuss the extension of the theory to many-site energy transfer systems Variational master equations thus represent a promising avenue for the exploration of (essentially non-perturbative) dissipative dynamics in a variety of physical systems. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N38.00006: Quantum Relaxation in Singlet Fission Paul Teichen, Joel Eaves Singlet fission is a multielectron process in organic chromophores, where an initially excited singlet state decays into two independent triplets. First observed in organic semiconductors almost 40 years ago, the phenomenon may be a promising route for increasing yields in next-generation photovoltaics. Early theories that ignored quantum coherence between excited states were capable of explaining the fission process on nanosecond timescales, but recent observations of fission on sub picosecond timescales call several tenants of those theories into question. We present a theory of optical dephasing and decoherence in singlet fission, drawing on ideas from quantum information theory to establish conditions for decoherence and disentanglement between the relevant quantum states on the picosecond timescale. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N38.00007: Double Super-Exchange in Silicon Quantum Dots Connected by Short-Bridged Networks Huashan Li, Zhigang Wu, Mark Lusk Silicon quantum dots (QDs) with diameters in the range of 1-2 nm are attractive for photovoltaic applications. They absorb photons more readily, transport excitons with greater efficiency, and show greater promise in multiple-exciton generation and hot carrier collection paradigms. However, their high excitonic binding energy makes it difficult to dissociate excitons into separate charge carriers. One possible remedy is to create dot assemblies in which a second material creates a Type-II heterojunction with the dot so that exciton dissociation occurs locally. This talk will focus on such a Type-II heterojunction paradigm in which QDs are connected via covalently bonded, short-bridge molecules. For such interpenetrating networks of dots and molecules, our first principles computational investigation shows that it is possible to rapidly and efficiently separate electrons to QDs and holes to bridge units. The bridge network serves as an efficient mediator of electron superexchange between QDs while the dots themselves play the complimentary role of efficient hole superexchange mediators. Dissociation, photoluminescence and carrier transport rates will be presented for bridge networks of silicon QDs that exhibit such double superexchange. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N38.00008: Highly Efficient Charge Transfer in Nanocrystalline Si:H Reuben Collins, Matthew Bergren, Brian Simonds, Jeremy Fields, Craig Taylor, Thomas Furtak, Kristin Kiriluk, Guozhen Yue, Baojie Yan, Jeff Yang, Tining Su, Subhendu Guha, Matthew Beard We demonstrate that in films of silicon nanocrystals imbedded in a hydrogenated amorphous silicon matrix, carriers generated in the amorphous region are efficiently transported to the nanocrystals prior to thermalization into band tail states of the amorphous phase. This transfer causes electron paramagnetic resonance and photoluminescence signals from the amorphous phase to be rapidly quenched as the volume fraction of Si nanocrystals exceeds about 30 percent. Ultrafast carrier dynamics, probed using time-resolved terahertz spectroscopy (TRTS), confirm rapid transport between phases before complete relaxation. TRTS results are consistent with a model where electrons excited in the amorphous material are first trapped at interface states at the amorphous/nanocrystal boundary prior to being thermally emitted into the crystalline phase. These results, which indicate nanocrystalline Si:H is effectively a type I bulk heterojunction material, help explain the enhanced photo stability of this material compared to amorphous silicon by itself. They also suggest routes to using similar structures to increase the efficiency of thin film silicon solar cells. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N38.00009: Ultrafast carrier dynamics of CdSe quantum dots prepared by pulse laser deposition for photovoltaic applications Meg Mahat, Baichhabi Yakami, Qilin Qilin Dai, Jinke Tang, Jon Pikal Quantum-dot sensitized solar cells are a promising alternative to existing photovoltaic technology. Over the last decade solution based colloidal quantum dots (QDs) have been extensively studied. Here we have carried out ultrafast transient absorption measurements on CdSe QDs fabricated using pulse laser deposition (PLD) in order to understand the carrier relaxation dynamics in these nanostructures. The differential transmission measurements show that the PLD QDs have a very fast decay process resulting in a recovery time of less than 10 picoseconds. This is in stark contrast to the colloidal QDs that show a decay process of more than 4 nanoseconds. We also find that the fast decay process observed in the PLD QDs is a function of the carriers density generated in CdSe QDs. To understand these carrier relaxation processes and improve the optical properties of the QDs we perform transient absorption measurements on PLD QDs prepared in different media (e.g. water, methanol, ethanol), under different growth conditions, and with and without ligand. We present a comparison study of the carrier relaxation dynamics in these PLD grown QDs to provide insight into the competing relaxation effects and guide their use in Quantum-dot sensitized solar cells. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N38.00010: Spin-Dependent Light-Harvesting in Colloidal Nanocrystals by Controlling Electronic Trap States with Optically Detected Magnetic Resonance K.J. van Schooten, J. Huang, D.V. Talapin, C. Boehme, J.M. Lupton Colloidal synthesis of semiconductor nanocrystals offers high levels of control over both particle size and geometry, leading to the development of novel optoelectronic device architectures (e.g. CdSe/CdS tetrapods). Unfortunately, realization of such devices is forestalled due to the ubiquitous existence of energetic ``trap'' states which compete with quantum-confined band-edge excitonic states and drive down device efficiencies. Although the existence of such states is readily confirmed via observation of single particle photoluminescence blinking and delayed photoluminescence decay dynamics, little detail is actually known as to the characteristics of these trap states due to difficulties in directly accessing them experimentally. We use pulsed optically detected magnetic resonance spectroscopy in order to begin to probe the chemical and electronic nature of these long-lived states, shedding light on their relation to band-edge states. Ultimately, it is found that spin coherence extends up to $T_{2}=328\pm22$~ns at 3.5~K, allowing for the coherent control of light harvesting in heterostructured nano-tetrapods which permits remote readout of spin information. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N38.00011: Charge transfer between a CdSe/CdS quantum rod and an attached ferrocene molecule: a first principle study Kartick Tarafder, Lin-Wang Wang Semiconductor quantum dot (QD) shows interesting opto-electrical properties, very different from bulk semiconductors. However, one major challenge for opto-electrical application is to get the charge carrier out of the QD. One approach is to use an attached molecule to extract the photon generated carrier from the QD. Ferrocene has a potential to change its electron transition level either by adjusting the Ferrocene and Ferrocene$+$ ratio in a solvent, or by adding other functional groups. However, proper understanding of the interactions between QD and molecule is limited, which is extremely useful for further design of such system. One of the main difficult is that there are thousands of atoms contained in the system, a first principle study of which is beyond the limit of existing computational power using direct density functional theory method. In this work we used a novel technique called charge-patching method [1], and combined that with Marcus model to study the electron and hole transfer between ferrocene and CdS/CdSe core-shell quantum dot. This study allows us to gain insights into the molecule dot interactions and underlying photoluminescence quenching process.\\[4pt] [1] L-W Wang, Phys. Rev. B 65, 153410(2002) [Preview Abstract] |
Session N39: Drops, Bubbles, and Interfacial Fluid Mechanics II
Sponsoring Units: DFDChair: Patricia McGuiggan, Johns Hopkins University
Room: 348
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N39.00001: Multimode Multidrop Serial Coalescence Effects during Condensation on Two-Tier Superhydrophobic Surfaces Konrad Rykaczewski, Adam T. Paxton, Sushant Anand, Xuemei Chen, Zuankai Wang, Kripa K. Varanasi Mobile coalescence leading to spontaneous drop motion was initially reported to occur only during water condensation on two-tier superhydrophobic surfaces (SHS), consisting of both nanoscale and microscale topological features. However, subsequent studies have shown that mobile coalescence also occurs on solely nanostructured SHS. Thus, recent focus has been on understanding the condensation process on just nanostructured surfaces rather than on two-tier SHS. Here, we investigate the impact of microscale topography of two-tier SHS on the droplet coalescence dynamics and wetting states during the condensation process. We identify new droplet shedding modes, which consist of serial coalescence events that lead to merging of multiple droplets. The formed drops either depart or remain anchored to the surface. We explain the observed post-merging drop adhesion trends through direct correlation to formation of drops in nanoscale as well as microscale Wenzel and Cassie-Baxter wetting states. We find that optimally designed two-tier SHS, which promote the highest number of departing microdrops, consists of microscale features spaced close enough to enable transition of larger droplets into micro-Cassie state, yet at the same time provide sufficient area in-between the features for occurrence of mobile coalescence. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N39.00002: Elasticity of the contact line for droplets on anisotropic superhydrophobic surfaces Marco Rivetti, Anais Gauthier, Jeremie Teisseire, Etienne Barthel We present an experimental and numerical investigation on the receding of contact line for water droplets on glass superhydrophobic surfaces. In particular, we focus our attention on surfaces textured with anisotropic lattice posts. We measure that the receding contact angle is not affected by the anisotropy of the lattice. This surprising behavior is closely related to the elastic deformations of the contact line which can be by studied by direct observation. We interpret this phenomenon in term of propagation of kink defects along the lattice. We detail the influence of the morphology of the lattice on the propagation of kinks, as well as the importance of the shape of the posts. Three dimensional numerical simulations confirm that kinks are the key ingredient for the comprehension of the receding contact angle. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N39.00003: Dynamics of condensation on lubricant impregnated surfaces Sushant Anand, Adam Paxson, Konrad Rykaczewski, Daniel Beysens, Kripa Varanasi Replacing the filmwise condensation mode with dropwise condensation promises large improvements in heat transfer that will lead to large cost savings in material, water consumption and decreased size of the systems. In this regards, use of superhydrophobic surfaces fabricated by texturing surfaces with nano/microstructures has been shown to lead decrease in contact line pinning of millimetric drops resulting in fast shedding. However, these useful properties are lost during condensation where droplets that nucleate within texture grow by virtue of condensation to large sized droplets while still adhering to the surface. Recently we have shown that liquid impregnated surfaces can overcome many limitations of conventional superhydrophobic surfaces during condensation. Here we discuss aspects related to condensation on lubricant surfaces, such as behavior of growing droplets. We compare the characteristics of droplets condensing on these surfaces with their behavior on conventional un-impregnated superhydrophobic surfaces and show how use of lubricant impregnated surfaces may lead to large enhancement in heat transfer and energy efficiencies. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N39.00004: Contact Angle Hysteresis of Photo-Responsive Materials Samuel Rosenthal, Patricia McGuiggan An atomic force microscope (AFM) is used to measure the meniscus force on individual microspheres coated with photo-responsive materials such as anatase and rutile TiO$_{2}$, azobenzene, and other doped oxides as they contact and are retracted from an air/water interface. By exposing the coated microspheres to UV light, the contact angle change. The change can be detected by measuring the increase in the meniscus force. Exposure to visible, infrared, or far infrared light -- as the specific material requires - reverses the contact angle change. The measured force-distance curves are fitted to macroscopic wetting theory. From these measurements, the contact angle, the contact angle hysteresis, and the position of the contact line pinning were simultaneously determined. This allowed for a quantification of the contact angle changes from photo-switching. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N39.00005: Thermodynamic Model for Contact Angle Hysteresis on Rough Surfaces Rishi Raj, Ryan Enright, Solomon Adera, Evelyn Wang Wettability of solid surfaces can be tuned by introducing roughness. This effect has been explained by Wenzel, whereby texturing increases the degree of hydrophilicity (hydrophobicity) of an intrinsic hydrophilic (hydrophobic) flat surface. However, experimentally observed dynamic contact angles deviate significantly from those predicted by Wenzel equation. In this work, we demonstrate that local contact line distortion and pinning on structured surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions and pinning were visualized and analyzed to develop a thermodynamic model for contact angle hysteresis on rough surfaces. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. The understanding developed in this study offers new insight extending the fundamental understanding of solid-liquid interactions required for the design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N39.00006: Wetting Transition of Water Serah Friedman, Matt Khalil, Peter Taborek Pure liquid water does not wet most solid surfaces. Liquid water on these surfaces beads up and forms droplets with a finite contact angle. General thermodynamic principles suggest that as the temperature approaches the critical point, the contact angle should go to zero, marking the wetting transition. We have made an optical cell which can operate near the critical point of water (Tc$=$373C, Pc$=$217 atm) to study this phenomenon on sapphire, graphite and silicon. We have used two methods to measure the wetting temperature of water on these surfaces. Firstly, we studied a single droplet on a horizontal surface and optically measured the change in contact angle as a function of increasing temperature. Second, we studied the condensation of droplets on a vertical plate as a function of temperature. As the temperature approached the wetting temperature in both cases, the droplets spread and eventually form a smooth film along the surface of the plate. The wetting temperature on sapphire is near 240C and is considerably higher on graphite. Our observed values of Tw are significantly higher than the predictions made by the sharp-kink approximation and recent molecular dynamics simulations. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N39.00007: Moving Water Droplets on Aluminum and Copper Surfaces Using Surface Tension Gradients Muidh Alheshibri, Nathaniel Rogers, Andrew Sommers, Khalid Eid The behavior of water droplets on metal surfaces is very important for many applications, especially in heat exchangers in air conditioning and refrigeration. We use photolithography and/or shadow masks to create alternating hydrophobic/hydrophilic Cu micro-channels on an aluminum surface and to move water droplets on the surface. The contact angle that is formed between water droplets and the surface is clearly asymmetrical due to the different surface properties at the contact line between the droplets and the patterned surface. An HDFT self-assembled mono-layer allows for a large change in the water droplet contact angle on the copper, but seems to have no effect on the aluminum surface. We will show our results on the effect of the surface patterning and surface roughness on water droplet behavior. We also demonstrate that the engineered surface gradients cause water droplets to travel more than 1'' on a horizontal or upward tilted surface. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N39.00008: Atomistic simulations of surfactant adsorption kinetics Eugeniya Iskrenova, Soumya Patnaik Enhancing heat transfer is an important and challenging problem in a variety of industrial and technological applications including aircraft thermal management. Nucleate pool boiling is recognized as one of the most efficient methods to enhance heat transfer. Describing the plethora of multi-physics phenomena involved in nucleate pool boiling requires developing a multiscale model aimed at not only advancing our understanding but also at providing insights into the mechanisms for control and prediction of heat transfer in boiling. Adding surfactants to boiling water has been experimentally observed to enhance or inhibit the heat transfer depending on the surfactant concentration and chemistry. On a molecular level, addition of surfactants leads to the development of dynamic surface tension and changes in interfacial and transfer properties, thus contributing to the complexity of the multiscale model. We present an atomistic modeling study of the interfacial adsorption kinetics of aqueous surfactant systems at a range of concentrations at room and boiling temperature. Large scale classical molecular dynamics simulations were used to study the surfactant kinetics and estimate the adsorption and desorption rates at liquid-solid and liquid-vacuum interfaces. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N39.00009: Small Scale Evaporation Kinetics of a Binary Fluid Mixture Carl Basdeo, Dezhuang Ye, Devendra Kalonia, Tai-Hsi Fan Evaporation induces a concentrating effect in liquid mixtures. The transient process has significant influence on the dynamic behaviors of a complex fluid. To simultaneously investigate the fluid properties and small-scale evaporation kinetics during the transient process, the quartz crystal microbalance is applied to a binary mixture droplet of light alcohols including both a single volatile component (a fast evaporation followed by a slow evaporation) and a mixture of two volatile components with comparable evaporation rates. The density and viscosity stratification are evaluated by the shear wave, and the evaporation kinetics is measured by the resonant signature of the acoustic p-wave. The evaporation flux can be precisely determined by the resonant frequency spikes and the complex impedance. To predict the concentration field, the moving interface, and the precision evaporation kinetics of the mixture, a multiphase model is developed to interpret the complex impedance signals based on the underlying mass and momentum transport phenomena. The experimental method and theoretical model are developed for better characterizing and understanding of the drying process involving liquid mixtures of protein pharmaceuticals. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N39.00010: Red blood cell in simple shear flow Wei Chien, Yayu Hew, Yeng-Long Chen The dynamics of red blood cells (RBC) in blood flow is critical for oxygen transport, and it also influences inflammation (white blood cells), thrombosis (platelets), and circulatory tumor migration. The physical properties of a RBC can be captured by modeling RBC as lipid membrane linked to a cytoskeletal spectrin network that encapsulates cytoplasm rich in hemoglobin, with bi-concave equilibrium shape. Depending on the shear force, RBC elasticity, membrane viscosity, and cytoplasm viscosity, RBC can undergo tumbling, tank-treading, or oscillatory motion. We investigate the dynamic state diagram of RBC in shear and pressure-driven flow using a combined immersed boundary-lattice Boltzmann method with a multi-scale RBC model that accurately captures the experimentally established RBC force-deformation relation. It is found that the tumbling (TU) to tank-treading (TT) transition occurs as shear rate increases for cytoplasm/outer fluid viscosity ratio smaller than 0.67. The TU frequency is found to be half of the TT frequency, in agreement with experiment observations. Larger viscosity ratios lead to the disappearance of stable TT phase and unstable complex dynamics, including the oscillation of the symmetry axis of the bi-concave shape perpendicular to the flow direction. The dependence on RBC bending rigidity, shear modulus, the order of membrane spectrin network and fluid field in the unstable region will also be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N39.00011: Non-laminar motion of biological suspension: an illustration for blood cell passing a 3-micrometer capillary Iat Neng Chan Discovering in video images of blood cell motion, a new concept is developed for cell passing a tight capillary that has a large difference compared to the published simulation results. In video image the deformation of moving blood cell shows abnormal pressure from cell membrane under highly contacted condition with capillary wall. Moreover, when the cell struggles through the narrow capillary the appearance of additional force to assist the cell motion is necessary. In more detail analysis, the flow motion in capillary displaying a non-laminar pattern which is obviously different to that shows in a nearby larger capillary on the same image, can be explained as a non-regular flow described by an equivalent flow companied with sink and source. Using this illustration with the calculated volumes for normal and deformed cells, the flow speed and pressure are derived to compare with the best known results and also to the calculated flow speed from the images. After compared to diffusion effect, the exchange rate of materials in the flow and the efficiency factor to the circulatory system can be estimated. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N39.00012: Stability of a falling viscous sheet Claude Perdigou, Gilles Pfingstag, Basile Audoly, Arezki Boudaoud Falling films can be found in various processes of the food, glass and polymer industry. We study thin viscous films flowing vertically under the action of gravity, when poured from a slit. The lateral sides are unconstrained and the stretching effect of gravity induces a narrowing of the film in the horizontal direction, by Poisson's effect. This leads to compressive stress for some range of parameters, and we study the associated viscous buckling instabilities. A local stability analysis is used to characterized the flow parameters leading to potential instabilities. A global stability analysis is carried out, and an eigenvalue problem is solved numerically. This is implemented using the finite-element method with high order derivatives. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N39.00013: A Different Cone: Bursting Drops in Solids Xuanhe Zhao Drops in fluids tend to be spheres---a shape that minimizes surface energy. In thunderstorm clouds, drops can become unstable and emit thin jets when charged beyond certain limits. The instability of electrified drops in gases and liquids has been widely studied and used in applications including ink-jet printing, electrospinning nano-fibers, microfluidics and electrospray ionization. Here we report a different scenario: drops in solids become unstable and burst under sufficiently high electric fields. We find the instability of drops in solids morphologically resembles that in liquids, but the critical electric field for the instability follows a different scaling due to elasticity of solids. Our observations and theoretical models not only advance the fundamental understanding of electrified drops but also suggest a new failure mechanism of high-energy-density dielectric polymers, which have diverse applications ranging from capacitors for power grids and electric vehicles to muscle-like transducers for soft robots and energy harvesting. [Preview Abstract] |
Session N40: Dipolar Gases and Rydberg Atoms
Sponsoring Units: DAMOPChair: Ryan Wilson, National Institute of Standards and Technology
Room: 349
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N40.00001: Time-reversal-breaking and $d$-wave superfluidity of ultracold dipolar fermions in optical lattices Carlos S{\'a} de Melo, Li Han We describe possible superfluid phases of ultracold dipolar fermions in optical lattices for two-dimensional systems. Considering the many-body screening of dipolar interactions at larger filling factors, we show that several superfluid phases with distinct pairing symmetries naturally emerge in the singlet channel: local $s$-wave $(sl)$, extended $s$-wave $(se)$, $d$-wave $(d)$ or time-reversal-symmetry breaking $(sl + se \pm id)$-wave. The temperature versus filling factor phase diagram indicates that $d$-wave is favored near half-filling, that $(sl + se)$-wave is favored near zero or full filling, and that time-reversal-breaking $(sl + se \pm id)$-wave is favored in between. When a harmonic trap is included a sequence of phases can exist in the cloud depending on the filling factor at the center of the trap. Most notably in the region where the $(sl + se \pm id)$-wave superfluid exists, spontaneous currents are generated, and may be detected using velocity sensitive Bragg spectroscopy. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N40.00002: Dipolar Fermions in Quasi-Two-Dimensional Square Lattice Chen-Yen Lai, Shan-Wen Tsai Motivated by recent experimental realization of quantum degenerate dipolar Fermi gas, we study a system of ultralcold single- and two-species polar fermions in a double layer two-dimensional square lattice. The long-range anisotropic nature of dipole-dipole interaction has shown a rich phase diagram on a two dimensional square lattice*. We investigate how the interlayer coupling affects the monolayer system. Our study focuses on the regime where the fermions are closed to half-filling, which is when lattice effects play an important role. We find several correlated phases by using a functional renormalization group technique, which also provides estimates for the critical temperature of each phase. [*] S. G. Bhongale et. al. arXiv:1209.2671 and Phys. Rev. Lett. 108 145301 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N40.00003: Orbital coupled dipolar fermions in an asymmetric optical ladder Xiaopeng Li, W. Vincent Liu We study a quantum ladder of dipolar atoms/molecules with coupled $s$ and $p$ orbitals. The interaction of such a system can be controlled with dipole moments being aligned by an external field. The two orbital components have distinct hoppings. The tunneling between them is equivalent to a partial Rashba spin-orbital coupling when the orbital space ($s$, $p$) is identified as spanned by pseudo-spin 1/2 states. A rich phase diagram is established. In particular a superconducting phase is found for repulsive fermions and a plaquette phase is found for bosons at 1/4 filling. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N40.00004: Emergence of unconventional spin density waves in dipolar Fermi gases S. G. Bhongale, Ludwig Mathey, Shan-Wen Tsai, Charles W. Clark, Erhai Zhao Motivated by experiments on Fermi gases of dipolar molecules and dysprosium, we study the competing quantum phases of two- component (pseudo-spin 1/2) dipolar fermions on a two-dimensional optical lattice. The anisotropic, long-range dipole-dipole interaction leads to the occurrence of numerous exotic many-body states, e.g. supersolid, nematic, and topological superfluid. Here, using unbiased functional renormalization group approach, we discover that another quantum phase of matter, spin density wave (SDW) with p-wave orbital symmetry, emerges in this system when the dipoles are tilted at intermediate angles with respect to the lattice plane. This phase can be viewed as the particle-hole analogue of p-wave superconductors. We present the phase diagram of the system and show that the order parameter of the unconventional SDW is a vector quantity in spin space, and, moreover, is defined on lattice bonds rather than on lattice sites. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N40.00005: Topological phases in polar-molecule quantum magnets Alexey Gorshkov, Salvatore Manmana, E.M. Stoudenmire, Kaden Hazzard, Ana Maria Rey, Norman Yao, Chris Laumann, Steven Bennett, Andreas Lauchli, Peter Zoller, Jun Ye, Eugene Demler, Mikhail Lukin We show that ultracold polar molecules pinned in an optical lattice and interacting via dipolar interactions can be used to implement a huge variety of exotic quantum magnets. These can be used to realize, for example, fractional Chern insulators, symmetry protected topological phases, the bilinear-biquadratic spin-1 Hamiltonian, and the Kitaev honeycomb model. [References for some of the results: arXiv:1207.4479, arXiv:1210.5518] [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N40.00006: Symmetry Protected Topological Phases in Polar Molecule Spin Ladder Systems S.R. Manmana, E.M. Stoudenmire, K.R.A. Hazzard, A.M. Rey, A.V. Gorshkov We show how to use polar molecules in an optical lattice to engineer quantum spin models with arbitrary spin $S \geq 1/2$ and with interactions featuring a direction-dependent spin anisotropy. This is achieved by encoding the effective spin degrees of freedom in microwave-dressed rotational states of the molecules and by coupling the spins through dipolar interactions. We demonstrate how one of the experimentally most accessible anisotropies stabilizes symmetry protected topological phases in spin ladders. Using the numerically exact density matrix renormalization group method, we find that these phases -- previously studied only in the nearest-neighbor case -- survive in the presence of long-range dipolar interactions. We also show how to use our approach to realize the bilinear-biquadratic spin-1 and the Kitaev honeycomb models. Experimental detection schemes and imperfections are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N40.00007: Realizing Fractional Chern Insulators with Dipolar Spins Norman Yao, Chris Laumann, Andreas Lauchli, Eugene Demler, Jun Ye, Peter Zoller, Mikhail Lukin, Alexey Gorshkov Strongly correlated quantum systems can exhibit exotic behavior that is determined and controlled by topology. Such topological systems are of interest because they constitute fundamentally new states of matter exhibiting fractionalized excitations and robust chiral edge modes. We theoretically predict that the nu = 1/2 fractional Chern insulator, a recently proposed topological state of lattice bosons, arises naturally in a two-dimensional array of driven, dipolar-interacting spins. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N40.00008: Preparation and detection of dipolar fractional Chern insulators Chris Laumann, Norman Yao, Alexey Gorshkov, Mikhail Lukin We describe schemes for preparation and detection of fractional Chern insulators as arise in driven dipolar spin systems. Such topological phases generically compete with superfluid and crystalline orders. We discuss the nature of the phase transitions and describe a dynamical preparation procedure. Prospects for measuring the properties of these topological phases using cold atomic techniques are considered. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N40.00009: Parafermion excitations in superfluid of quasi-molecular chains formed by dipolar molecules or indirect excitons Anatoly Kuklov, Alexei Tsvelik We study a quantum phase transition in a system of dipoles confined in a stack of $N$ identical 1D lattices (tubes) polarized perpendicularly to the lattices. The dipoles may represent polar molecules or indirect excitons. The transition separates two phases; in one of them superfluidity takes place in each individual lattice, in the other (chain superfluid) the order parameter is the product of bosonic operators from all lattices. We argue that in the presence of finite inter-lattice tunneling the transition belongs to the universality class of the $q=N$ two-dimensional classical Potts model. For $N=2,3,4$ the corresponding low energy field theory is the model of Z$_N$ parafermions perturbed by the thermal operator. Results of Monte Carlo simulations are consistent with these predictions. The detection schemes for the chain superfluid of dipolar molecules and indirect excitons are outlined. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N40.00010: Collective excitations of quasi-two-dimensional trapped dipolar fermions Mehrtash Babadi, Eugene Demler We study the collective excitations of quasi-two-dimensional fermions with dipole-dipole interactions in an isotropic harmonic trap by solving the collisional Boltzmann-Vlasov equation. Except for the scaling monopole mode which exhibits a negligible damping, the other collective modes undergo a transition from the collisionless regime to a highly dissipative crossover regime and finally to the hydrodynamic regime upon increasing the dipolar interaction strength. In the 2D limit, we predict the existence of a temperature window within which the characteristics of the collective modes become temperature independent. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N40.00011: Unconventional triplet pairing state in a polarized dipolar Fermi gas Yuki Endo, Daisuke Inotani, Yoji Ohashi We theoretically discuss the possibility of a triplet superfluid state in a polarized dipolar Fermi gas. In this system, it is usually believed that a high-energy cutoff is necessary in solving the superfluid BCS gap equation, reflecting the non-convergent behavior of a dipole-dipole interaction in the high-momentum limit. Because of this, the superfluid theory for a dipolar Fermi gas is believed to need a regularization for the angular-dependent dipole-dipole interaction as in the case of the s-wave interaction. In this talk, we show that such a renormalization is actually unnecessary, when one carefully includes the detailed structure of a dipolar molecule. We present a superfluid theory for a dipolar Fermi gas where the dipole-dipole interaction is only described by the two physical parameters, dipole size and dipole-dipole coupling constant. Using this, we discuss the possibility of a triplet pairing state, as well as superfluid properties, of this system. Since our theory only involves observable physical parameters, it would be useful in quantitatively evaluating superfluid properties of a dipolar Fermi gas, such as the superfluid phase transition temperature. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N40.00012: P-wave superfluid in a quasi-two-dimensional dipolar Bose-Fermi quantum gas mixture Ben Kain, Hong Ling The $p$-wave ($p_{x} + i p_{y})$ superfluid has attracted significant attention in recent years mainly because its vortex core supports a Majorana fermion which, due to its non-Abelian statistics, can be explored for implementing topological quantum computation (TQC). Mixing in bosons may lead to $p$-wave pairing in a Fermi gas. In a dipolar condensate, the dipole-dipole interaction represents a control knob inaccessible to nondipolar Bosons. Thus, mixing dipolar bosons with fermions opens up new possibilities. We consider a mixture of a spin-polarized Fermi gas and a dipolar Bose-Einstein condensate in a quasi-two-dimensional trap setting. We take the Hartree-Fock-Bogoliubov mean-field approach and develop a theory for studying the stability of the mixture and estimating the critical temperature of the $p$-wave superfluid. We use this theory to identify the experimentally accessible parameter space in which the mixture is stable against phase separation and the $p$-wave superfluid pairing can be resonantly enhanced. An enhanced $p$-wave superfluid order parameter can make the fault tolerant TQC less susceptible to thermal fluctuations. This work aims to stimulate experimental activity in creating dipolar Bose-Fermi mixtures. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N40.00013: Superfluidity of atomic Fermi gases with dipolar interactions Yanming Che, Qijin Chen While quantum degenerate dipolar Fermi gases have been made available in experiment, the superfluidity in such Fermi gases has been of very high interest. In this talk, we study the superfluidity and associated BCS-BEC crossover behavior of a two-component atomic Fermi gases in three dimensions in the presence of dipole-dipole interactions, such as polar molecules $^{40}$K$^{87}$Rb and magnetic atoms $^{161}$Dy, using a pairing fluctuation theory. The relative interaction strength can be tuned via the atomic number. Various geometric configurations will be explored. We show that in certain configurations, the superfluidity may disappear altogether for a narrow range of interaction strength, and the Tc curve throughout the BCS-BEC crossover exhibits a reentrant behavior. We argue that such disappearance of the superfluidity is associated with the long range nature of the dipole-dipole interaction. A pseudogap develops naturally as the relative interaction becomes strong. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N40.00014: Superfluid Pairing and Majorana Zero Mode in an Ultracold Rydberg Fermi Gas Bo Xiong, H.H. Jen, Jhih-Shih You, Daw-Wei Wang We systematically calculate the p-wave superfluid phase of spin polarized Fermi gases in a Rydberg state. The mutual interaction between atoms are dressed by external fields and show nonlocal attractive 1/(a$+$r6) interaction. Different from the p-wave pairing phase of regular atoms near p-wave Feshbach resonance, the obtained p-wave superfluid phase can be stable away from three-body collision and has intrinsic nontrivial nodes in the momentum space. The critical temperature and order parameter for various interaction parameters have been calculated analytically and numerically, both in the 2D and 3D free space. When loading into optical lattice, we also show the proximity effect of Tc near half filling. Finally, when considering the harmonic confinement potential, we obtain the gapless Majorana Fermions confined to the boundary via self-consistently solving the DBG equation. We will discuss how to experimentally prepare and measure these Majorana states in Rydberg atoms. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N40.00015: Supersolid phases of two-species Rydberg-dressed Bose-Einstein condensates C.-H. Hsueh, Y.-C. Tsai, M.-S. Chang, W. C. Wu We investigate the supersolid ground states of a binary Rydberg-dressed BEC system. From a many-body perturbation expansion, both intra- and inter-species long-range dressed interactions are derived, which are essential for the study of the ground-state manifold of the binary Rydberg-dressed BEC system. Due to the long-range interactions, five distinct phases of the supersolid ground states are identified, which are experimentally observable. [Preview Abstract] |
Session N41: Hybrid Systems for Quantum Simulation
Sponsoring Units: GSNP DAMOPRoom: 350
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N41.00001: Counting statistics of phase slips in superconducting interferometers Phillip Weinberg, Andrew Murphy, Alex Levchenko, Victor Vakaryuk, Alexey Bezryadin In the superconducting proximity circuits, stochastic switching from the super-current carrying state to dissipative normal state is triggered by the topological fluctuations of the order parameter - phase slips. We study theoretically switching current statistics in a double-nanowire quantum interferometer as a function of the applied magnetic field perpendicular to the plane of the device. This system is a prototype of the double-slit experiment in optics which allows to probe macroscopic coherence of superconducting condensates. Magnetic field induces Meissner currents in the leads that lock superconducting phases across the wires. As a results phase slips that occur in the wires are not independent. We calculate dispersion of the switching current distribution as well as higher moment and find that they oscillate as the function of the field. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N41.00002: Inverse Landau-Zener-Stuckelberg interferometry for the measurement of a resonator's state using a qubit Sergey Shevchenko, Sahel Ashhab, Franco Nori We consider theoretically a superconducting qubit - nanomechanical resonator system, which was realized recently by LaHaye et al. [Nature 459, 960 (2009)]. We formulate and solve the inverse Landau-Zener-Stuckelberg problem, where we assume the driven qubit's state to be known (i.e. measured by some other device) and aim to find the parameters of the qubit's Hamiltonian. In particular, for our system the qubit's bias is defined by the nanomechanical resonator's displacement. This may provide a tool for monitoring the nanomechanical resonator 's position. [S. N. Shevchenko, S. Ashhab, and F. Nori, Phys. Rev. B 85, 094502 (2012).] [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N41.00003: Towards a spin-ensemble quantum memory for superconducting qubits Patrice Bertet, Yuimaru Kubo, Cecile Grezes, Denis Vion, Daniel Esteve, Igor Diniz, Alexia Auffeves, Junichi Isoya, Anais Dreau, Jean-Fran\c{c}ois Roch, Vincent Jacques, Brian Julsgaard, Klaus Moelmer A multi-mode quantum memory able to store coherently large numbers of qubit states is a desirable resource for quantum information. We report progress towards this direction, using an ensemble of electronic spins (NV centers in diamond) coupled to a superconducting transmon qubit via a tunable resonator. We demonstrate the reversible coherent storage and retrieval of a single microwave photon from the qubit into the spin ensemble [1]. In this experiment the storage time was however limited by inhomogeneous broadening of the ensemble of spins. We propose a realistic protocol that should extend the ensemble storage time by several orders of magnitude, based on spin-echo like pulse sequences; first experimental results will be presented. \\[4pt] [1] Y. Kubo et al., PRL 107, 220501 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N41.00004: Superconducting Microstrip Resonator for Spin-Based Quantum Processor Hamid Reza Mohebbi, Olaf Benningshof, Troy Borneman, Ivar Taminiau, Guo-Xing Miao, David G. Cory We report the design and results of a novel superconducting microstrip line resonator for pulsed ESR experiments of thin films.~The resonator generates a homogeneous in-plane microwave magnetic field. This resonator consists of an array of superconducting half-wave microstrip transmission lines~to achieve high-Q resonance. They are driven via an in-phase splitter and so maintain a resonance at one single frequency. In addition the resonator has a relatively small mode. The performance, sensitivity and small mode volume are demonstrated through our observation of strong coupling and ESR spectroscopy. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N41.00005: Systematic studies of optically-trapped dielectric nanospheres Levi Neukirch, Jan Gieseler, Romain Quidant, Lukas Novotny, Nick Vamivakas Mesoscopic resonators have garnered significant interest recently in a number of experiments designed to blur the line between classical and quantum systems. In particular, optically trapped mesoscopic particles offer a distinct advantage over many other systems, as they can be mechanically isolated from the environment. We present results from dynamical studies of micro- and nano-scale dielectric particles suspended in a free-space optical dipole trap. Particle position is monitored via the interference of scattered and unscattered laser light. Of interest are the effects of the trap laser and ambient pressure on the external motion and internal temperature of the particles. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N41.00006: Atomic manipulation for a hybrid system: tapered optical fibers with high transmission and a pyramid MOT J.E. Hoffman, J.A. Grover, M. Hafezi, J.B. Hertzberg, P. Kordell, J. Lee, S. Ravets, U. Chukwu, K.D. Voigt, J.R. Anderson, G. Beadie, F.K. Fatemi, C.J. Lobb, L.A. Orozco, J.M. Taylor, S.L. Rolston, F.C. Wellstood To create a hybrid quantum system, we plan to trap neutral atoms in the evanescent optical field from an optical nanofiber and move them to within a few microns above a SQUID in a dilution refrigerator that operates at 10 mK. A key component in this experiment is a long section (10 cm) of optical fiber with a uniform diameter of about 500 nm, sufficiently small that the light propagates on the surface of the fiber as an evanescent wave. We have produced suitably long nanofibers with carefully tapered sections that allow matching of the optical field in the tapered and untapered sections. We have achieved more than 99.95{\%} transmission of the fundamental mode and good evanescent fields; as well as more than 85{\%} transmission when using higher order modes. A single-beam, magneto-optical trap that uses optical gratings captures and cools atoms to load on the nanofiber to work at cryogenic temperatures. We will present our technique, key results, and progress towards trapping atoms on the fibers. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N41.00007: Development of a hybrid quantum system employing a tunable high-Q superconducting microwave resonator and trapped laser-cooled atoms Jared Hertzberg, K. Voigt, Z. Kim, J. Hoffman, J. Grover, J. Lee, S. Ravets, M. Hafezi, J. Taylor, A. Choudhary, J. Anderson, C. Lobb, L. Orozco, S. Rolston, F. Wellstood We present progress toward a hybrid quantum system in which microwave quanta stored in a superconducting flux qubit are coupled through a magnetic dipole interaction to laser-trapped atoms. In initial experiments, our goal will be to couple a microfabricated superconducting LC resonator to the 6.835 GHz hyperfine splitting in an ensemble of $^{\mathrm{87}}$Rb atoms. By trapping the atoms in the evanescent field of a 500-nm-wide optical fiber, we will seek to place them within 10 micrometers of the chip surface, where they will interact with the near-field of the microwave mode. In previous work we have demonstrated a frequency-tunable superconducting resonator having Q \textgreater 100,000. [1] Here we will describe improvements in the resonator's design to reduce its sensitivity to absorbed photons, as well as the design of components to position the resonator relative to the optical fiber within a dilution refrigerator. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N41.00008: Quantum hybrid platform using electrons and superconducting electronics N. Daniilidis, D. Gorman, L. Tian, H. Haeffner We describe a quantum information processing (QIP) architecture based on single trapped electrons and superconducting electronics. The electron spins function as quantum memory elements, and the electron motion is used to couple the electrons to microwave circuits. To achieve this, we propose a parametric coupling mechanism which utilizes the non-linearity of the electrostatic potential of a sharp electrode placed $10\,\mu$m from a single trapped electron. This mechanism allows parametric coupling rates higher than $350\,$kHz for electrons with trap frequency of $300\,$MHz, coupled to a $7\,$GHz resonant circuit. We discuss state transfer and entangling operations between distant electrons, as well as between electrons and superconducting qubits, e.g. transmon qubits. The coupling to high frequency superconducting electronics enables initialization as well as state read-out of the electron motion. In addition, the electron's $\{ \vert 0\rangle,\,\vert 1\rangle \}$ motional manifold can be mapped onto its spin using a non-linear oscillating magnetic field, completing all requirements for quantum computing with the electron spin. We estimate that all involved operations can be carried out with fidelities on the order of 0.999 enabling fault-tolerant quantum computing. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N41.00009: Discrete Two-Level Systems Coupled to a Tunable High Q Superconducting Microwave Resonator Kristen Voigt, J. Hertzberg, Z. Kim, J. Hoffman, J. Grover, J. Lee, S. Ravets, M. Hafezi, J. Taylor, A. Choudhary, J. Anderson, C. Lobb, L. Orozco, S. Rolston, F. Wellstood We have developed a tunable ``lumped-element" thin-film superconducting Al microwave resonator [1] and used it for measuring two level systems. The device is intended for coupling to the hyperfine splitting of trapped $^{\mathrm{87}}$Rb atoms at 6.83 GHz. By moving a superconducting Al pin towards the inductor of the resonator using a piezo stage, we can tune the resonance over a range of 130 MHz. We measure the system by weakly coupling to an on-chip transmission line. At 12 mK the quality factor is typically 100,000. While holding the tuning pin at a fixed position, we can also apply a dc voltage to the transmission line. We observe small reproducible shifts of the resonance frequency as the voltage is changed. These shifts are more pronounced at lower power, which suggests the effect is attributable to discrete charged two-level systems in the sapphire substrate or surface Al oxide. We discuss our results and the characteristics of the underlying two-level systems. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N41.00010: High cooperativity in coupled microwave resonator - ferromagnetic insulator hybrids Hans Huebl, Christoph Zollitsch, Johannes Lotze, Fredrik Hocke, Moritz Greifenstein, Achim Marx, Rudolf Gross, Sebastian T.B. Gross Solid-state based quantum systems (e.g. single spin systems like NV centers in diamond or phosphor donors in silicon, superconducting qubits, nanomagnets) are building blocks for devices exploiting quantum physics phenomena. With different quantum systems available, schemes allowing to couple them move into focus. In particular, a coupling will enable the exchange of information between dressed states. Here, we report the observation of strong coupling between the exchange-coupled spins in gallium-doped yttrium iron garnet, and a superconducting coplanar microwave resonator made from Nb [1]. The measured coupling rate of 450 MHz is proportional to the square-root of the number of exchange-coupled spins and well exceeds the loss rate of 50 MHz of the spin system. This demonstrates that exchange-coupled systems are suitable for cavity quantum electrodynamics experiments, while allowing high integration densities due to their extraordinary high spin densities. Our results furthermore show that experiments with multiple exchange-coupled spin systems interacting via a single resonator are within reach. [1] H. Huebl, C. Zollitsch, J. Lotze, F. Hocke, M. Greifenstein, A. Marx, R. Gross, S.T.B. Goennenwein, arXiv: 1207.6039 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N41.00011: Interfacing Rydberg atoms with superconducting circuits S. Filipp, T. Thiele, M. Stammeier, A. Wallraff, S.D. Hogan, J.A. Agner, F. Merkt Hybrid quantum system are promising candidates for future quantum computing architectures because they provide the potential to combine the best properties of different physical systems. Here, we bring together Rydberg atoms and microwave photons emanating from a co-planar waveguide with the ultimate goal to interface long-lived Rydberg atoms with well-controllable superconducting qubits. In our cryogenic experiment, helium atoms pass over microwave electrodes hosted on a printed circuit board. By applying resonant microwave pulses, we induce transitions between Rydberg states with principal quantum number n=31-35 and observe coherent Rabi oscillations with typical oscillation periods of about 50ns [1]. From spectral measurements we can characterize the interaction between the atoms and surface fields leading to decoherence. The analysis of the inhomogeneously broadened lineshapes indicates that the stray electric field strength decreases with the inverse square of the atom-surface distance [2]. In experiments in preparation we plan to employ on-chip superconducting resonators to study the strong interaction of Rydberg atoms with few or individual microwave photons.\\[4pt] [1] S.D. Hogan et al., PRL 108, 063004 (2012).\\[0pt] [2] J.D. Carter and J.D.D. Martin, PRA 83, 032902 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N41.00012: Quantum-classical transition of synchronization of two coupled cavities Tony Lee, Michael Cross Synchronization is a phenomenon that appears throughout physics, biology, and chemistry. There has been much work on how synchronization arises in the classical regime. Motivated by current interest in quantum dissipative systems, we investigate whether synchronization can exist in the quantum regime. We consider a pair of cavities with second harmonic generation. In the classical limit, each cavity has a limit cycle solution, in which the photon number oscillates periodically in time. Coupling between the cavities leads to synchronization of the limit cycles. We follow what happens to the synchronization as the system becomes more quantum, by decreasing the photon number. We find that temporal correlations between the cavities survive deep in the quantum limit when there is much less than one photon in each cavity, because classical correlations are replaced by quantum correlations. Our results can be extended to optomechanics and Jaynes-Cummings cavities. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N41.00013: Linear Coupling between Transverse Modes of a Nanomechanical Resonator Patrick Truitt, Jared Hertzberg, Keith Schwab Recently, several groups have identified a linear coupling between different vibrational modes of nanomechanical resonators. We report observations of such a coupling between the two transverse modes of a doubly-clamped Si$_3$N$_4$ resonator with transverse resonance frequencies of 8.4 and 8.7 MHz. The resonator is voltage biased with respect to a nearby gate electrode for capactive readout. Increasing the gate bias introduces an electrostatic contribution to the spring constant of each mode, reducing the frequency gap between the two modes. At degeneracy, we observe an avoided crossing of ~100 kHz. Measurements of the displacement amplitudes and quality factors through degeneracy is consistent with a linear superposition of the two modes. Magnetomotive measurements, which are sensitive to the projection of each mode's displacement onto an applied field, show that the coupled modes remain linearly polarized, with the direction of polarization rotating with increasing gate bias. In an effort to identify the source of the coupling, we constructed a finite element model of the resonator-gate capacitance and find that the observed coupling is an order of magnitude larger than what is expected from electrostatic gradients alone. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N41.00014: Dynamic Simulation of Trapping and Controlled Rotation of a Microscale Rod Driven by Line Optical Tweezers Mahdi Haghshenas-Jaryani, Alan Bowling, Samarendra Mohanty Since the invention of optical tweezers, several biological and engineering applications, especially in micro-nanofluid, have been developed. For example, development of optically driven micromotors, which has an important role in microfluidic applications, has vastly been considered. Despite extensive experimental studies in this field, there is a lack of theoretical work that can verify and analyze these observations. This work develops a dynamic model to simulate trapping and controlled rotation of a microscale rod under influence of the optical trapping forces. The laser beam, used in line optical tweezers with a varying trap's length, was modeled based on a ray-optics approach. Herein, the effects of viscosity of the surrounding fluid (water), gravity, and buoyancy were included in the proposed model. The predicted results are in overall agreement with the experimental observation, which make the theoretical model be a viable tool for investigating the dynamic behavior of small size objects manipulated by optical tweezers in fluid environments. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N41.00015: Ion photon networks for quantum computing and quantum repeaters Susan Clark, David Hayes, David Hucul, I. Volkan Inlek, Christopher Monroe Quantum information based on ion-trap technology is well regarded for its stability, high detection fidelity, and ease of manipulation. Here we demonstrate a proof of principle experiment for scaling this technology to large numbers of ions in separate traps by linking the ions via photons. We give results for entanglement between distant ions via probabilistic photonic gates that is then swapped between ions in the same trap via deterministic Coulombic gates. We report fidelities above 65\% and show encouraging preliminary results for the next stage of experimental improvement. Such a system could be used for quantum computing requiring large numbers of qubits or for quantum repeaters requiring the qubits to be separated by large distances. [Preview Abstract] |
Session N42: Focus Session: Supercooled and Nanoconfined Water I
Sponsoring Units: DCPChair: Valeria Molinero, University of Utah
Room: Hilton Baltimore Holiday Ballroom 3
Wednesday, March 20, 2013 11:15AM - 11:51AM |
N42.00001: Liquid-liquid transition in the ST2 model of water Invited Speaker: Pablo Debenedetti We present clear evidence of the existence of a metastable liquid-liquid phase transition in the ST2 model of water. Using four different techniques (the weighted histogram analysis method with single-particle moves, well-tempered metadynamics with single-particle moves, weighted histograms with parallel tempering and collective particle moves, and conventional molecular dynamics), we calculate the free energy surface over a range of thermodynamic conditions, we perform a finite size scaling analysis for the free energy barrier between the coexisting liquid phases, we demonstrate the attainment of diffusive behavior, and we perform stringent thermodynamic consistency checks. The results provide conclusive evidence of a first-order liquid-liquid transition. We also show that structural equilibration in the sluggish low-density phase is attained over the time scale of our simulations, and that crystallization times are significantly longer than structural equilibration, even under deeply supercooled conditions. We place our results in the context of the theory of metastability. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N42.00002: Entropy-driven liquid-liquid transitions in supercooled water Invited Speaker: Mikhail Anisimov Twenty years ago it was suggested that the anomalous properties of supercooled water may be caused by a critical point that terminates a line of metastable liquid-liquid separation of lower-density and higher-density water. I describe a phenomenological model in which liquid water at low temperatures is viewed as an athermal solution of two hydrogen-bond network structures with different entropies and densities. Alternatively to the lattice-gas/regular solution model, in which fluid phase separation is driven by energy, the phase transition in the athermal two-state water is driven by entropy upon increasing the pressure, while the critical temperature is defined by the reaction equilibrium constant. The order parameter is associated with the entropy, while the ordering field is a combination of temperature and pressure. The model predicts the location of density maxima at the locus of a near-constant fraction of the lower-density structure. Another example of entropy-driven liquid polyamorphism is the transition between the structurally ordered ``Blue Phase III'' and disordered liquid in some chiral materials; this transition is experimentally accessible. I also discuss the application of the two-state model to binary solutions of supercooled water in which liquid-liquid transition may also become accessible to direct observation. Some atomistic ``water-like'' models such as mW, do not show liquid-liquid separation in the metastable liquid domain. However, even without actual liquid-liquid separation, the anomalies observed in MD simulations of mW can be accurately described by the entropy-driven nonideality of two molecular configurations, the same physics that presumably drives the liquid-liquid transition in real water. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N42.00003: Interplay of the Glass Transition and the Liquid-Liquid Phase Transition in Water Invited Speaker: Nicolas Giovambattista Most liquids can form a single glass or amorphous state when cooled sufficiently fast (in order to prevent crystallization). However, there are a few substances that are relevant to scientific and technological applications which can exist in at least two different amorphous states, a property known as polyamorphism. Examples include silicon, silica, and in particular, water. In the case of water, experiments show the existence of a low-density (LDA) and high-density (HDA) amorphous ice that are separated by a dramatic, first-order like phase transition. It has been argued that the LDA-HDA transformation evolves into a first-order liquid-liquid phase transition (LLPT) at temperatures above the glass transition temperature Tg. However, obtaining direct experimental evidence of the LLPT has been challenging since the LLPT occurs at conditions where water rapidly crystallizes. In this talk, I will (i) discuss the general phenomenology of polyamorphism in water and its implications, and (ii) explore the effects of a LLPT on the pressure dependence of Tg(P) for LDA and HDA. Our study is based on computer simulations of two water models -- one with a LLPT (ST2 model), and one without (SPC/E model). In the absence of a LLPT, Tg(P) for all glasses nearly coincide. Instead, when there is a LLPT, different glasses exhibit dramatically different Tg(P) loci which are directly linked with the LLPT. Available experimental data for Tg(P) are only consistent with the scenario that includes a LLPT (ST2 model) and hence, our results support the view that a LLPT may exist for the case of water. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N42.00004: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N42.00005: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N42.00006: The Putative Liquid-Liquid Transition is a Liquid-Solid Transition in Atomistic Models of Water David Chandler, David Limmer Our detailed and controlled studies of free energy surfaces for models of water find no evidence for reversible polyamorphism, and a general theoretical analysis of the phase behavior of cold water in nano pores shows that measured behaviors of these systems reflect surface modulation and dynamics of ice, not a liquid-liquid critical point. A few workers reach different conclusions, reporting evidence of a liquid-liquid critical point in computer simulations of supercooled water. In some cases, it appears that these contrary results are based upon simulation algorithms that are inconsistent with principles of statistical mechanics, such as using barostats that do not reproduce the correct distribution of volume fluctuations. In other cases, the results appear to be associated with difficulty equilibrating the supercooled material and mistaking metastability for coarsening of the ordered ice phase. In this case, sufficient information is available for us to reproduce the contrary results and to establish that they are artifacts of finite time sampling. This finding leads us to the conclusion that two distinct, reversible liquid phases do not exist in models of supercooled water. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N42.00007: Amorphous ices explained in terms of nonequilibrium phase transitions in supercooled water David Limmer, David Chandler We analyze the phase diagram of supercooled water out-of-equilibrium using concepts from space-time thermodynamics and the dynamic facilitation theory of the glass transition, together with molecular dynamics simulations. We find that when water is driven out-of-equilibrium, it can exist in multiple amorphous states. In contrast, we find that when water is at equilibrium, it can exist in only one liquid state. The amorphous non-equilibrium states are solids, distinguished from the liquid by their lack of mobility, and distinguished from each other by their different densities and local structure. This finding explains the experimentally observed polyamorphism of water as a class of nonequilibrium phenomena involving glasses of different densities. While the amorphous solids can be long lived, they are thermodynamically unstable. When allowed to relax to equilibrium, they crystallize with pathways that pass first through liquid state configurations and then to ordered ice. [Preview Abstract] |
Session N43: Focus Session: Protein Misfolding and Aggregation III
Sponsoring Units: DCP DBIOChair: Elsa Yan, Yale University
Room: Hilton Baltimore Holiday Ballroom 2
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N43.00001: Structural Transformation and Aggregation of cc-beta Peptides Into Amyloid Proto-fibrils Yuba Bhandari, Timothy Steckmann, Prem Chapagain, Bernard Gerstman The study of amyloid fibrils has important implications in understanding and treatment of various neurodegenerative diseases such as Alzheimer's and Parkinson's. During the formation of amyloid fibrils, peptide polymers manifest fascinating physical behavior by undergoing complicated structural transformations. We examine the behavior of a small engineered peptide called cc-beta, that was designed to mimic the structural changes of the much larger, naturally occurring amyloid beta proteins. Molecular dynamics (MD) simulations are performed to uncover the underlying physics that is responsible for the large scale structural transformations. By using implicit solvent replica exchange MD simulations, we examined the behavior of 12 peptides, initially arranged in four different cc-beta alpha helix trimers. We observed various intermediate stages of aggregation, as well as an organized proto-fibril beta aggregate. We discuss the time evolution and the various interactions involved in the structural transformation. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N43.00002: Gelation, Phase Behavior and Dynamics of Beta-Lactoglobulin Amyloid Fibrils at Varying Concentrations and Ionic Strengths Raffaele Mezzenga, Sreenath Bolisetty, Ludger Harnau, Jin-Mi Jung We discuss the thermodynamic and dynamic behavior of Beta-lactoglobulin fibrils in a vast region of the concentration-ionic strength phase diagram, by combining static, dynamic and depolarized light scattering (SLS, DLS, DDLS), small angle neutron scattering (SANS), and cryo-TEM. We focus on the region of the phase diagram where ionic strength and concentration changes induce transitions in gelation and lyotropic liquid crystalline behavior. Increase in ionic strength, by NaCl salt, causes the phase transitions from nematic to gel phases. Increase in fibril concentration induces first a phase transition from an isotropic to a nematic phase; further increase induces the formation of a gel phase. SANS and osmotic compressibility calculated by SLS measurements, capture the main features of the IN transition of Beta-lactoglobulin protein fibrils. The form and structure factors measured by scattering experiments are analyzed by polymer reference interaction site model (PRISM). Dynamics of the protein fibrils at different concentrations, measured by polarized and depolarized dynamic light scattering, shows both individual and collective diffusion after the IN transition. cryo-TEM images further demonstrate the alignment of the protein fibrils, quantified by a 2D order parameter. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N43.00003: Amyloid fibril networks nucleated under oscillatory shear Kiersten Batzli, Brian Love The process of amyloid fibril formation is of interest due to the link between these self-aggregating proteins and the progression of neurodegenerative disease. More recently, research has been directed at the exploitation of self-assembly properties of amyloid proteins for use as templates for nanowires and fibrillar networks. Insulin is an ideal protein for these purposes due to the ease of aggregation, as well as the large aspect ratio and high chemical stability of the produced fibrils. Insulin in pH 2 solution quickly forms aggregates in the presence of 65 $^{\circ}$C heat. We have investigated the effect of oscillatory shear on the nucleation and growth of amyloid fibrillar networks using rheology and TEM to characterize the mechanical properties and structure of the network respectively. We contrast networks nucleated under oscillatory shear with networks nucleated in static and agitated conditions, and discuss network properties in the context of use in templating nanostructures. We find that the structural characteristics of the formed networks, including the density of fibrils, are affected by shear during the nucleation phase of amyloid growth. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N43.00004: Amyloid Structure In Vitro and In Vivo Invited Speaker: Robert Tycko Solid state nuclear magnetic resonance (NMR) measurements can provide unique information about the structural properties of proteins in noncrystalline states that are of interest from both the biophysical and the biomedical perspectives. I will discuss recent results from my lab's efforts to characterize the molecular structures of amyloid fibrils, especially the A$\beta $ peptide fibrils that are associated with Alzheimer's disease. From a combination of solid state NMR and electron microscopy measurements, we have developed full structural models for 40-residue wild-type A$\beta $ fibrils that form in vitro and contain parallel $\beta $-sheets with 2-fold and 3-fold overall rotational symmetry. We have recently discovered that the ``Iowa mutant'' (D23N-A$\beta )$ peptide can also form metastable fibrils with a surprising antiparallel $\beta $-sheet structure. And we are in the process of investigating A$\beta $ fibril structures that develop in human brain tissue. In addition to recent results, I will briefly describe recent advances in methodology that contribute to this work. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 1:03PM |
N43.00005: Yeast prion architecture explains how proteins can be genes Invited Speaker: Reed Wickner Prions (infectious proteins) transmit information without an accompanying DNA or RNA. Most yeast prions are self-propagating amyloids that inactivate a normally functional protein. A single protein can become any of several prion variants, with different manifestations due to different amyloid structures. We showed that the yeast prion amyloids of Ure2p, Sup35p and Rnq1p are folded in-register parallel beta sheets using solid state NMR dipolar recoupling experiments, mass-per-filament-length measurements, and filament diameter measurements. The extent of beta sheet structure, measured by chemical shifts in solid-state NMR and acquired protease-resistance on amyloid formation, combined with the measured filament diameters, imply that the beta sheets must be folded along the long axis of the filament. We speculate that prion variants of a single protein sequence differ in the location of these folds. Favorable interactions between identical side chains must hold these structures in-register. The same interactions must guide an unstructured monomer joining the end of a filament to assume the same conformation as molecules already in the filament, with the turns at the same locations. In this way, a protein can template its own conformation, in analogy to the ability of a DNA molecule to template its sequence by specific base-pairing. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N43.00006: Molecular mechanisms for neurodegeneration Invited Speaker: Hilal Lashuel |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N43.00007: ABSTRACT HAS BEEN MOVED TO J45.00003 |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N43.00008: Self-Assembly of Peptides at the Air/Water Interface Mehmet Sayar Peptides are commonly used as building blocks for design and development of novel materials with a variety of application areas ranging from drug design to biotechnology. The precise control of molecular architecture and specific nature of the nonbonded interactions among peptides enable aggregates with well defined structural and functional properties. The interaction of peptides with interfaces leads to dramatic changes in their conformational and aggregation behavior. In this talk, I will discuss our research on the interplay of intermolecular forces and influence of interfaces. In the first part the amphiphilic nature of short peptide oligomers and their behavior at the air/water interface will be discussed. The surface driving force and its decomposition will be analyzed. In the second part aggregation of peptides in bulk water and at an interface will be discussed. Different design features which can be tuned to control aggregation behavior will be analyzed. \\[4pt] [1] O. Engin \& M.S. ``Adsorption, Folding and Packing of an Amphiphilic Peptide at the Air/Water Interface,'' J. Phys. Chem. B 116 (7), 2198-2207 (2012)\\[0pt] [2] O. Engin, A. Villa, M.S. \& H. Berk, ``Driving Forces for Adsorption of Amphiphilic Peptides to Air-Water Interface,'' J. Phys. Chem. B 114, 11093-11101 (2010) [Preview Abstract] |
Session N44: Focus Session: Translocation through Nanopores II
Sponsoring Units: DBIOChair: Gary Slater, University of Ottawa
Room: Hilton Baltimore Holiday Ballroom 1
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N44.00001: How tension propagates for a driven semi-flexible chain while translocating through a nano-pore Ramesh Adhikari, Aniket Bhattacharya Driven translocation of a stiff chain through a nano-pore is studied using Langevin dynamics in two dimension (2D). We observe that for a given chain length $N$ the mean first passage time (MFPT) $\langle \tau \rangle$ increases for a stiffer chain and the translocation exponent $\alpha$ ($\langle \tau \rangle \sim N^\alpha$) satisfies the inequality $2\nu < \alpha < 1+\nu$, where $\nu$ is the equilibrium Flory exponent for a given chain stiffness. We calculate the residence time of the individual monomers and observe that the peak position of the residence time $W(m)$ as a function of the monomer index $m$ shifts at a \textit{lower} $m$-value with \textit{increasing chain stiffness $\kappa_b$}. Finally, we provide qualitative physical explanation for dependence of various quantities on chain stiffness $\kappa_b$ by using ideas from Sakaue's tension propagation(TP) theory [Phys. Rev. E {\bf 76}, 021803 (2007)] and its recent implementation into a Brownian dynamics tension propagation (BDTP) scheme for a finite chain by Ikonen et al. [J. Chem. Phys. {\bf 137}, 085101 (2012); Phys. Rev. E {\bf 85}, 051803 (2012)]for a semi-flexible chain. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N44.00002: Experimental measurements of the rate of capture of synthetic and natural polyelectrolytes by alpha-hemolysin under salt concentration gradients Byoung-jin Jeon, Murugappan Muthukumar We report experimental data on the effect of gradients in salt concentration on the capture rate of synthetic and natural polyelectrolytes by the alpha-hemolysin pore under an electric field. We find that the capture rate is nonmonotonic with the ratio of salt concentration in the trans to that in the cis. We have also determined the extent of the nonmonotonicity at different pH conditions. Our results present challenges for an understanding of the phenomenon. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N44.00003: Linear and ring DNA macromolecules moderately and strongly confined in nanochannels Peter Cifra, Zuzana Benkova, Tomas Bleha Understanding the mechanism of DNA extension in nanochannels is necessary for interpretation of experiments in nanofluidic channel devices that are conducted recently not only with linear but also with ring chains. Except reviewing the situation with linear chains we analyze here the experimental results and simulations for the channel-induced extension (linearization) of ring chains. Results of simulations for confined rings indicate that similar transition between moderate and strong confinement as in the case of linear chains exists also for rings. Due to stronger self-avoidance in confined rings the transition and relative chain extension is shifted in comparison to linear DNA. We suggest that similar relation as used in experiments for the extension of linear chains may be used also for circular DNA. For linear DNA in channel relatively stable distinctive events due to chain backfolding, which complicate chain linearization experiments, are analyzed. The abundance of DNA chains folded at the chain ends and in the chain interior was analyzed as a function of the channel width. Z. Benkova, P.Cifra, Macromolecules 45, 2597-2608 (2012) P. Cifra, T.Bleha, Soft Matter 8, 9022-9028 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N44.00004: Polymer Translocation in a Crowded Environment: Effects due to Obstacle Density and Arrangement Invited Speaker: Hendrick W. de Haan The translocation of a polymer across a membrane through a nanopore has received much attention, primarily due to emerging nanotechnology applications such as DNA sequencing. However, translocation is also a process that is ubiquitous in the natural world with examples including the transport of DNA and proteins across cell walls. Considering this latter motivation, the environment in which translocation occurs is relatively complicated with many intracellular and extracellular inclusions such as the cell organelles, soluble proteins, and components of the cytoskeleton and extracellular matrix. In this talk, we examine translocation in such a crowded environment via computer simulations in which we place immobile, spherical ``obstacles'' on both sides of the membrane. We show that an effective driving force arises i) when the concentration of obstacles across the pore differs and ii) when the arrangement of obstacles across the pore differs. A simple force model is used to estimate the magnitude of these entropic driving forces. Good agreement is found between the results and the simple models. Simulations are also performed with both effects present such that a bias resulting from a lower concentration of obstacles on the $cis$ side of the membrane is opposed by a bias arising from an increased amount of disorder on $trans$. Results from this setup indicate that in a real system where both effects are likely to play a role, it could be difficult to guess even the direction of the intrinsic resulting driving force, let alone the magnitude. We also present results from simulations in which the obstacles are mobile but restricted to different degrees. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N44.00005: Electric field controlled small molecule transport through vertically aligned large diameter multiwalled carbon nanotube forest membrane Purushottam Tiwari, Padmini Krishnakumar, Yesim Darici, Jin He Vertically aligned multi-walled carbon nanotube (MWCNT) forest based porous membranes have been fabricated. The average inner diameter of the CNT is about 7 nm and the length is about 45 $\mu $m. The translocation behaviors of small charged molecules and gold nanoparticles through the CNT membrane under electric field have been investigated. Electrophoresis is found to be the main mechanism for the translocation of small molecules under the applied electric field in the range of 10000 Vm$^{-1}$. The interactions between the molecule and the hydrophobic CNT inner surface play an important role for the transport of small molecules. The chemical modifications at CNT ends can also effectively regulate the transport of molecules. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N44.00006: Electric-field driven translocation of colloidal wild-type and mutant fd viruses through a solid-state nanopore Wang Miao, Liping Liu, Anna Lu, Prerna Sharma, Zvonimir Dogic, Chuong Huynh, Larry Scipioni, Xinsheng Ling Colloidal suspensions of fd viruses are useful model systems for condensed matter physics. Here we explore the transport processes of fd particles in solid-state nanopores. Recently we have observed a nonlinear behavior in the electrophoretic mobility of wild-type fd particles. Here we carried out a comparative study of wild-type and mutant Y21M in their translocation dynamics through a nanopore. This work was supported by NSF-DMR and NSF-MRSEC. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N44.00007: Polymer Translocation Dynamics in the Quasi-Static Limit James Polson Monte Carlo and Langevin dynamics simulations are used to study the dynamics of polymer translocation through a nanopore using a coarse-grained model. We examine the relationship between the translocation free energy barrier and the translocation times through a comparison of the simulation results to predictions using the Fokker-Planck formalism. We illustrate the importance of using free energy profiles obtained from precise numerical calculations rather than those obtained from simple theoretical models. In addition, we determine the parameter regime within which the Focker-Planck approach is valid and beyond which non-equilibrium effects become appreciable. The relevance of these results to recent theoretical and simulation studies of polymer translocation dynamics is discussed. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:39PM |
N44.00008: Nonequilibrium Dynamics of Polymer Translocation Invited Speaker: Takahiro Sakaue When a flexible chain is pulled or sucked, it can initially respond only locally, and sequential nonequilibrium processes with large conformational distortion follow in line with the propagation of tensile force along the chain backbone. This is a generic dynamical response property of polymers, the understanding of which provides us with a viewpoint to capture an essential aspect of the driven translocation process. In the meeting, I will summarize a basic framework to analyze the nonequilibrium dynamics of driven translocation process alongside of recent progresses. \\[4pt] References:\\[0pt] T. Sakaue, Phys. Rev. E, 76, 021803 (2007) ``Nonequilibrium dynamics of polymer translocation and straightening''\\[0pt] T. Sakaue, Phys. Rev. E, 81, 041808 (2010) ``Sucking genes into pores: Insight into driven translocation''\\[0pt] T. Saito and T. Sakaue, Eur. Phys. J. E, 34, 145 (2011) ``Dynamical diagram and scaling in polymer driven translocation''\\[0pt] T. Saito and T. Sakaue, Phys. Rev. E, 85, 061803 (2012) ``Process time distribution of driven polymer transport'' [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N44.00009: Active Control of Protein and Ionic Transport through Semiconducting Conical Nanopores Teena James, Yevgeniy Kalinin, Chih-Chieh Chan, Jatinder Randhawa, Mikhail Gaevski, David Gracias Nanopores with conical geometries have been found to rectify ionic current in electrolytes. While nanopores in semiconducting membranes offer the ability to modulate ionic transport, the fabrication of conical nanopores in silicon has proven challenging. Here, we report the discovery that Au nanoparticle-assisted plasma etching results in the formation of conical etch profiles in Si [1]. We show that this process provides a versatile means to fabricate nanopores on Si substrates with variable pore-diameters and cone-angles. When in contact with aqueous electrolyte solution (pH\textgreater 3), the nanopore was found to exhibit negative surface charge due to de-protonation of the Si-OH surface groups. The rectification ratio of ionic current through the pore was thus found to be variable by altering the pH, owing to the amphoteric nature of Si-OH surface groups (pKa 6.9) and was also dependent on the ionic strengths, agreeing with the theoretical predictions based on Poisson$-$Nernst$-$Planck equation. We demonstrate that these semiconducting conical nanopores can function as ionic switches with high on-off ratios, by varying Si surface charge through voltage gating. Further, we demonstrate voltage gated control over protein translocation through these pores. [1]. Voltage-gated ion transport through semiconducting conical nanopores formed by metal nanoparticle assisted plasma etching, T. James, \textit{et al.} Nano Letters 12, 7, 3437--3442 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N44.00010: Rapid fabrication of sub-5nm solid-state nanopore for low cost biosensing Harold Kwok, Kyle Briggs, Vincent Tabard-Cossa Nanopores-based technologies are emerging as a powerful tool for single molecule analysis. They are also the leading candidates for future generation DNA sequencing platforms. Despite all of these potentials, current solid-state nanopore fabrication techniques, based on focused beams of energetic particles, remains low throughput, complex and expensive. Such drawbacks greatly limit the breadth of applications, and are major barriers to commercialization of any nanopore-based technologies. We have demonstrated a simple, highly scalable and low cost method to fabricate solid-state nanopores. It relies on stressing a thin dielectric membrane with high-electric field while submerged in aqueous salt solution. The technique allows a single sub-5nm nanopore be fabricated within a minute directly in liquids. In addition, a pore can be precisely enlarged by the similar used of high-electric field stressing. We will describe the fabrication method, present our current understanding of the physical mechanism leading to pore formation, and demonstrate its usefulness for single-molecule detection by studying DNA translocation kinetics. The discovery of this new method opens a wide range of possibilities for single-molecule biophysics and commercial sensing applications. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N44.00011: Probing the Influence of Coil Configuration on DNA Translocation Dynamics in Solid-State Nanopores Xu Liu, Karri DiPetrillo, Jason Chan, Lucas Eggers, Angus McMullen, Derek Stein We studied electrophoretic DNA translocations of asymmetric nanopore-cavity structures designed to control the initial configurations of molecules. The structures comprise a thin SiN membrane with a nanopore that leads into a 400 nm-high cavity, which is in turn covered by a 400-nm thick SiN membrane with a circular opening whose diameter ranged from 150\,nm to 1.5\,$\mu$m. These structures maintain a gap between the nanopore and a DNA coil translocating from above, but not one translocating from below. The viscous drag on the DNA segment extending from the coil to the nanopore significantly slowed translocations from above. The mean translocation times for those events were 2.5 times longer than for tranlocations from below when the upper opening of the cavity was only 200\,nm wide. The translocation times converged as the opening was increased to micrometer diameters. This last result can be explained by the DNA coil, whose radius of gyration is $\sim$600\,nm, squeezing into the upper opening by increasing amounts. Our experimental results compare favourably with a quantitative model of DNA translocation speeds, similar to models by Lu \emph{et al.} and by Grosberg, which accounts for the initial configuration of the DNA coil. [Preview Abstract] |
Session N45: Focus Session: Cell Mechanics I
Sponsoring Units: DBIOChair: Jennifer Curtis, Georgia Institute of Technology
Room: Hilton Baltimore Holiday Ballroom 4
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N45.00001: Cellular Tug-of-War: Forces at Work and DNA Stretching in Mitosis Brian Griffin, Maria L. Kilfoil In the microscopic world of the cell dominated by thermal noise, a cell must be able to successfully segregate its DNA with high fidelity in order to pass its genetic information on to its progeny. In this process of mitosis in eukaryotes, driving forces act on the cytoskeleton-based architecture called the mitotic spindle to promote this division. Our preliminary data demonstrates that the dynamics of this process in yeast cells is universal. Moreover, the dynamics suggest an increasing load as the chromosomes are pulled apart. To investigate this, we use three-dimensional imaging to track the dynamics of the poles of this architecture and the points of attachment to chromosomes simultaneously and with high spatial resolution. We analyze the relative motions of chromosomes as they are organized before segregation and as they are pulled apart, using this data to investigate the force-response behavior of this cytoskeleton-chromosome polymer system. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N45.00002: \textit{in vivo} Measurements of Conformational Fluctuations of Chromosomal DNA in \textit{Escherichia Coli} Rudra Kafle, Jens-Christian Meiners The cell is the site of active, motor-driven processes far from thermodynamic equilibrium. Therefore, the intracellular dynamics are complex and subject to a multitude of constraints and forces. We study the conformational fluctuations of chromosomal DNA \textit{in vivo} in live and dead \textit{E. coli} cells by Fluorescence Correlation Spectroscopy (FCS). The fluctuations move the DNA-bound fluorophores stochastically into the diffraction-limited excitation volume of a focused laser beam in a confocal microscope. From the time correlation functions of the fluorescence intensity, we obtain the mean square displacements of the DNA on a time scale from microseconds to seconds. We see a substantial decrease in the power spectral density (PSD) of the displacement fluctuations at frequencies below 10 Hz in the dead cells, compared to the live cells. The larger fluctuations in the living cells may indicate that the fluctuations on this time scale may be driven by active processes involving molecular motors that generate forces by ATP hydrolysis. A small difference in PSD between live and dead cells on shorter time scales suggests that the processes on corresponding short length scales rely primarily on thermally-driven diffusive mechanisms. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N45.00003: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 11:51AM - 12:27PM |
N45.00004: Mediation of cell adhesion by the pericellular matrix Invited Speaker: Jennifer Curtis Cell adhesion requires a close proximity on the nanometer scale between the plasma membrane and the surrounding material (or neighboring cell). Yet, in many classic scenarios where cell adhesion is carefully regulated, including proliferation, migration, embryogenesis and cancer metastasis, the cell's surface is insulated by an invisible but microns thick polymer brush-like structure, called the pericellular matrix. Indeed the presence of the pericellular matrix has been correlated with increased migration and proliferation rates, where disruption of this bound polymer brush interferes with the efficacy of these processes. We present methods to characterize the pericellular matrix distribution, mechanics and mesh size and explore how cells orchestrate adhesion with the help of the pericellular matrix. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N45.00005: Modifications of the structure of the pericellular matrix measured via optical force probe microscopy Louis McLane, Anthony Kramer, Patrick Chang, Jennifer Curtis The pericellular matrix is a large protein and polysaccharide rich polymer layer attached to the surface of many cells, and which often extends several microns out from the cell surface into the surrounding extracellular space. Here we study the intrinsic nature and modifications of the structure of the pericellular coat on rat chondrocytes with the use of optical force probe microscopy. Optical force probe studies allow us to make both dynamic force measurements as well as equilibrium force measurements throughout the coat. These force measurements are used to observe the structural change in the coat with the addition of exogenous aggrecan. Not only does addition of exogenous aggrecan dramatically swell our coat to well over twice in size, our analysis indicates that the addition of exogenous aggrecan decreases the mesh size throughout the coat. We speculate that the added aggrecan binds to available binding sites along the hyaluronan chain, both enlarging the coat's size as well as tightening up the opening within the coat. We further suggest that the available binding sites for the exogenous aggrecan are abundant in the outer edges of the coat, as both the dynamic and equilibrium forces in this region are changed. Here, both force measurements show that forces closest to the cell membrane remain relatively unchanged, while the forces in the outer region of the coat are increased. These results are consistent with those obtained with complementary measurements using quantitative particle exclusion assays. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N45.00006: Quantitative particle exclusion assays of the pericellular coat reveal changing mesh size Patrick Chang, Louis McLane, Nolan Kramer, Jennifer E. Curtis We present a quantitative assay of the pericellular coat, a tethered polymer matrix that decorates the surface of numerous cell types. In these assays, we look at how passivated microspheres of varying diameter penetrate the cell coat. Distinct spatial distributions correspond to different particle sizes. These measurements confirm that the cell coat (on the chondrocyte RCJ-P cell line) has a spatially varying mesh size, in agreement with our independent assays performed with optical force probe microscopy. The data indicate that particles with diameters of 500 nm or greater do not penetrate the inner layer of the matrix, while particles smaller than 500 nm reach different regions, with the smallest reaching the cell surface. In an ongoing effort, we are developing a model for the observed statistical distribution of the beads. These experiments show that accessibility of the cell surface is strongly mediated by the presence of the cell coat, and they have important implications regarding the transport of molecules to the cell surface, protection from bacterial infection, drug delivery, as well as the way the cell interacts and adheres to the surrounding extracellular matrix. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N45.00007: Reversible and irreversible deformations of bacterial cell walls Ariel Amir, Farinaz Babaeipour, David Nelson, Suckjoon Jun Bacterial cell walls determine their shape and hold their large internal pressure. In spite of their biological importance, a full understanding of their structure and mechanics is lacking. Here, we shed new light on the nature of the deformations of bacterial cell walls by showing, theoretically and experimentally, that these can be either elastically (reversibly) or plastically (irreversibly) deformed, depending on the timescales involved. Our data suggests that irreversible bending of the cell wall arises due to an asymmetric insertion of new material, responding to the mechanical stresses. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N45.00008: Mechanical Properties of Primary Cilia Christopher Battle, Christoph F. Schmidt Recent studies have shown that the primary cilium, long thought to be a vestigial cellular appendage with no function, is involved in a multitude of sensory functions. One example, interesting from both a biophysical and medical standpoint, is the primary cilium of kidney epithelial cells, which acts as a mechanosensitive flow sensor. Genetic defects in ciliary function can cause, e.g., polycystic kidney disease (PKD). The material properties of these non-motile, microtubule-based 9$+$0 cilia, and the way they are anchored to the cell cytoskeleton, are important to know if one wants to understand the mechano-electrochemical response of these cells, which is mediated by their cilia. We have probed the mechanical properties, boundary conditions, and dynamics of the cilia of MDCK cells using optical traps and DIC/fluorescence microscopy. We found evidence for both elastic relaxation of the cilia themselves after bending and for compliance in the intracellular anchoring structures. Angular and positional fluctuations of the cilia reflect both thermal excitations and cellular driving forces. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N45.00009: The influence of myosin-generated force to the intracellular microrheology in living cells Ming-Tzo Wei, H. Daniel Ou-Yang The mechanics of cells are governed by cytoskeletal filaments and molecular motors forming a dynamic mechanical entity. A recent experimental study by Mizuno et al. showed local shear modulus of a synthesized cytoskeletal network could increase as a result of myosin-generated internal stresses. To examine whether similar behaviors could take place in living cells we combined active and passive microrheology to measure the myosin-generated fluctuating force and intracellular shear modulus in HeLa cells. While our experiment showed an increase in the fluctuations of the shear modulus with increasing motor forces, the experiment did not find a direct correlation between the mean intracellular shear modulus and the motor-generated fluctuating force. Based on Mizuno et al's assumption shear modulus is increasing as local tensions, the difference between the results obtained by the intracellular behavior and the synthesized cytoskeletal network could be due to the existence of a steady-state intracellular tension that is stronger than the motor-generated fluctuating force. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N45.00010: Cellular pressure and volume regulation and implications for cell mechanics Hongyuan Jiang, Sean Sun In eukaryotic cells, small changes in cell volume can serve as important signals for cell proliferation, death and migration. Volume and shape regulation also directly impacts the mechanics of the cell and multi-cellular tissues. Recent experiments found that during mitosis, eukaryotic cells establish a preferred steady volume and pressure, and the steady volume and pressure can robustly adapt to large osmotic shocks. Here we develop a mathematical model of cellular pressure and volume regulation, incorporating essential elements such as water permeation, mechano-sensitive channels, active ion pumps and active stresses in the actomyosin cortex. The model can fully explain the available experimental data, and predicts the cellular volume and pressure for several models of cell cortical mechanics. Furthermore, we show that when cells are subjected to an externally applied load, such as in an AFM indentation experiment, active regulation of volume and pressure leads to complex cellular response. We found the cell stiffness highly depends on the loading rate, which indicates the transport of water and ions might contribute to the observed viscoelasticity of cells. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N45.00011: Direct mechanical measurements of cytoskeleton-mediated intercellular fluid flow Steven Zehnder, Jolie Breaux, Alison Dunn, Juan Urue\~na, W. Gregory Sawyer, Thomas Angelini Cell behavior in tissues is intimately tied to forces generated by cytoskeletal contractions. Contraction generated tensions are balanced by deformations in the cell's microenvironment, by internal cytoskeletal structures, and by the incompressible cytosolic fluid contained by the cell membrane. However, contraction generated pressures cannot be supported by the cytosol if the cell membrane is adequately permeable. Small, non-selective pores called gap junctions connect cells in a layer, allowing small molecules to pass between cells. The ability of contraction driven fluid movement to transmit forces across gap junctions and the ability of cells to respond to this movement is unexplored. To study the mechanics of intercellular fluid flow, we apply biologically relevant pressures to large regions of cells in a monolayer with a micro-indentation system. We directly measure indentation force and volume as a function of time to determine fluid flow rates and associated stresses between cells. Preliminary results will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N45.00012: Cell stretching in extensional flows for assaying cell mechanics Daniel Gossett, Henry Tse, Oladunni Adeyiga, Otto Yang, Jianyu Rao, Dino Di Carlo There is growing evidence that cell deformability is a useful indicator of cell state and may be a label-free biomarker of metastatic potential, degree of differentiation, and leukocyte activation. In order for deformability measurements to be clinically valuable given the heterogeneity of biological samples, there exists a need for a high-throughput assay of this biophysical property. We developed a robust method for obtaining high-throughput ($>$1,000 cells/sec) single-cell mechanical measurements which employs coupled hydrodynamic lift forces and curvature-induced secondary flows to uniformly position cells in flow, extensional flow stretching, high-speed imaging, and automated image analysis to extract diameter and deformability parameters. Using this method we have assayed numerous in vitro models of cellular transformations and clinical fluids where malignant cells manifest. We found transformations associated with increased motility or invasiveness increased deformability and the presence of large and deformable cells within clinical pleural fluids correlated well with cytological diagnoses of malignancy. This agrees with the hypothesis that cancerous cells are deformable by necessity--to be able to transverse tight endothelial gaps and invade tissues. [Preview Abstract] |
Session N46: Detectors, Sensors, and Transducers
Sponsoring Units: GIMSChair: Brad Ramshaw, Los Alamos National Laboratory
Room: Hilton Baltimore Holiday Ballroom 5
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N46.00001: Sub-diffraction Position Determination with Four Laser Diodes for Tracking/Trapping a Single Molecule James A. Germann, Brian K. Canfield, Jason K. King, Lloyd M. Davis Prolonged observation of single biological molecules by overcoming diffusion can reveal interesting new properties. Observation times may be increased by physically confining a particle, but this often leads to interactions that affect molecular properties. Another way of increasing observation time is to trap a single molecule in solution three-dimensionally. However, optimal trapping of single particles relies on rapid determination of particle position for feedback to counteract Brownian diffusion. In our experiment, a tetrahedral region with foci located at the vertices is created by combining four modulated 635 nm laser diodes with three beam splitters. Fluorescence is measured with a single-photon avalanche diode and separated into bins corresponding to each excitation focus. A maximum-likelihood estimation algorithm is used to determine particle position with sub-diffraction precision in real time. To test the tracking capability of the four-focus setup, fluorescently labeled latex beads were tracked in an aqueous glycerol solution. Two setups, a piezoelectric stage and a three-dimensional electrokinetic trap, are being implemented to maintain a single fluorescent latex bead in the middle of the tetrahedral region. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N46.00002: ac-Calorimetric Measurements of Transverse Thermal Conductivity Hao Zhang, Joseph Brill We are developing an ac-calorimetric technique, heating one surface of a thin sample with oscillating power and measuring the temperature oscillations on the opposite surface, to measure the thermal conductivity of solids. While the temperature oscillations are inversely proportional to the heat capacity at low frequencies, at higher frequencies the response is limited by the transverse thermal diffusivity. Because of the response times of thermometers and the fact that the magnitude of the temperature oscillation varies inversely with frequency, this technique is most useful for materials with low thermal conductivities, such as the interlayer conductivity in layered materials. We will show results on ``standard'' materials (teflon, sapphire) as well as the layered organic semiconductors, rubrene and TIPS-pentacene. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N46.00003: Thermal expansion measurement using optical grating diffraction shifts Tran Vinh Son, Mohamed Touaibia, Alain Hache We demonstrate a novel optical method for accurately measuring thermal expansion in materials. When an optical grating expands or contracts, the Bragg diffraction condition is altered, and the diffracted beams undergo angular shifts. Using a diffracted laser beam, we demonstrate that this effect can be used to measure expansion coefficients as small as 10$^{-6}$ C$^{-1}$. By patterning samples of PMMA and chitosan with grating lines, we measure their thermal expansion coefficients by heating the sample by only a few degrees Celsius. The method can be generalized to opaque materials by texturing the surface and measuring diffraction in reflection. A theory is presented to determine the ideal experimental conditions and the limits of accuracy. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N46.00004: Properties of Holmium Implanted Gold Films and a YSias Absorbers in TES Microcalorimeters for Holmium Neutrino Mass Experiment Krishna Prasai, E. Alaves, D. Bagliani, N. Barradas, M. Biasotti, M. Galeazzi, F. Gatti, P. Manfrinetti, M.R. Gomes, Y. Uprety, S. Yanardag The electron capture decay of Ho-163 can be used for the direct measurement of the electron neutrino mass with Transition Edge Sensor (TES) microcalorimeters. A major requirement for a microcalorimetric holmium experiment is to embed the source in the detector absorber. A logical choice would be to implant the isotope into a regular gold absorber, assuming that it does not change the absorber properties. As an alternate option, since most chemical processes to extract the Ho-163 isotope after fabrication involve yttrium based compounds, it could be possible to use a yttrium compound as absorber, rather than just as an intermediate step. We have studied the properties of gold films implanted with holmium and erbium (which is present due to source manufacturing) and Yttrium silicide (YSi) in the working temperature range of the TES microcalorimeters (90-300 mK).In this paper we present the results of our investigation [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N46.00005: Quasiparticle diffusion in Al film and transmission with an Al/W interface Jeffrey Yen, Paul Brink, Blas Cabrera, Matt Cherry, Matt Pyle, Peter Redl, Astrid Tomada, Betty Young The Cryogenic Dark Matter Search (CDMS) experiment uses both high-purity Si and Ge crystals to directly search for Weakly Interacting Massive Particles (WIMPs). These detectors simultaneously measure the ionization and phonon energy produced by particle interactions. This talk will focus on experiments performed with a separate set of test devices fabricated to study the fundamental physics of the CDMS phonon sensors. In our test experiments, an $^{55}$Fe source was used to excite a NaCl reflector, producing 2.6 keV x-rays that hit our test devices after passing through a collimator. The devices under study consisted of a 250 $\mu$ m wide x 350 $\mu$ m long Al absorber film (300 nm thick) coupled to two 250 $\mu$ m x 250 $\mu$ m (40 nm thick) W transition edge sensors (TESs), one at each end of the Al film. The impinging x-rays break Cooper pairs in the Al film, producing quasiparticles that we detect as they propagate into the W TESs. We studied the diffusion of these quasiparticles, trapping in the Al film, and their transmission probability at the Al/W interfaces. Results from our precision experiments will be presented in this talk. These results are also being used to further optimize the design of SuperCDMS detectors for a proposed 100 kg scale dark matter search. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N46.00006: Depolarization factors in electro-optic crystals and their effects in sensing applications Anthony Garzarella Many applications involving electric field measurements require sensors that are compact and non-intrusive. This is especially true for tests inside small cavities, where conventional antennas and metallic probes are not only too bulky, but will also perturb the very fields they measure. Electro-optic (EO) sensors are ideal in such situations, because they are small and all-dielectric. Despite this, antennas are still predominantly used due to their higher sensitivity (2-3 orders of magnitude). Therefore to make EO sensors viable, sensitivity must be improved. The customary figure of merit (FOM) is the ratio of the EO coefficient to the dielectric constant. LiNbO$_{\mathrm{3}}$ and similar crystals are preferred because of their large FOMs. In these crystals, the EO tensor is such that a transverse configuration must be used where the E-field and laser path are orthogonal. In this report, we demonstrate that sensors based on longitudinal crystals (E-field and laser collinear) can have greater sensitivities, even though their FOMs are substantially lower due to depolarization effects that enhance internal fields. Explicit examples are shown, and the practical limits in making EO sensors more competitive with conventional antennas will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N46.00007: Stress reconfigurable tunable magnetoelectric resonators as magnetic sensors Jillian Kiser, Peter Finkel, Christophe Dolabdjian Magnetoelectric multiferroic materials are extremely attractive due to their potential in sensing, filtering and energy transduction applications. We report a magnetoelastic effect in doubly-clamped ferromagnetic magnetostrictive Metglas resonators, as well as the magnetic field dependence of the resonance frequency as a function of uniaxial stress. Magnetostrictive strain results in a resonance frequency shift when the resonator is exposed to a magnetic field. The resonance frequency can be tracked in real time as a function of magnetic field bias using a feedback loop based on the quadrature of the excited motion. This magnetically reconfigurable resonance response can be used as a simple, tunable, magnetoelectric (ME) magnetic field sensor. The effect of sample pre-tension on the field dependent magnetostrictive constant and the sensor sensitivity is examined, and the resolution of such a sensor is estimated. [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N46.00008: Disruptive Approach Towards 10nm Spatial Resolution In X-PEEM Using Diamondoids Hendrik Ohldag, Hitoshi Ishiwata, Yves Acremann, Olav Hellwig, Peter Schreiner, Nick Melosh, Zhi-Xun Shen Diamondoids are unique molecular nano-materials with diamond structure and fascinating new properties such as negative electron affinity (NEA) and short electron mean free paths. A thin layer of diamondoids deposited on a cathode is able to act as an electron monochromator, reducing the energy spread of photo-emitted electrons from a surface. This property can be applied effectively to improve the spatial resolution in x-ray photoemission electron microscopy (X-PEEM), which is limited by chromatic aberration of the electron optics. In this talk we will present X-PEEM measurements reaching the technological relevant spatial resolution of 10-nm without the need of expensive and complex corrective optics. Our results provide a simple approach to image surface chemical and magnetic information at nanometer scales by employing diamondoid. [1] H. Ishiwata et al. Appl. Phys. Lett. \textbf{101}, 163101 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N46.00009: Energy Analysis in Near Field-Emission SEM Lorenzo Giuseppe De Pietro, Danilo Andra Zanin, Hugo Cabrera, Urs Ramsperger, Danilo Pescia, Mehmet Erbudak In Near Field-Emission Scanning Electron Microscopy (NFESEM) cold field emitted electrons from a sharp polycrystalline W-tip are the source of a primary electron beam. The applied voltage for field-emission accelerates these electrons up to some tens of eV. After having interacted with the sample, secondary and backscattered electrons are detected, while an STM controller is used to scan the tip at a constant average distance ($10$ to $20$ nm) from the sample surface. This technique has been used for topography images on various metals and semiconductors achieving nm lateral resolution. In case of a W(110) surface covered by Fe islands a chemical contrast was observed. We recently added an energy analysis of the electrons used for imaging. The energy distribution of this electrons from the sample shows presence of both secondary and back scattered electrons. The ratio of the two groups of electrons may vary for different distances and energies. In view of including spin polarization analysis, we are currently working to optimize the secondary electron yield. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N46.00010: Scale invariance of a diode-like tunnel junction Hugo Cabrera, Danilo Andrea Zanin, Lorenzo Giuseppe De Pietro, Thomas Michaels, Peter Thalmann, Urs Ramsperger, Alessandro Vindigni, Danilo Pescia In Near Field-Emission SEM (NFESEM), electrostatic considerations favor a diode-like tunnel junction consisting of an atomic-sized source mounted at the apex of a thin wire placed at nanometric distances from a collector. The quantum mechanical tunnel process, instead, can provide a barrier toward miniaturization. In the first place, it deteriorates the generation of electrons by introducing non-linearities within the classically forbidden zone that exponentially increase with decreasing sizes. In addition, in the direct tunnelling regime, i.e. when the distance between emitter and collector $d$ approaches the subnanometer range, a characteristic length appears, making the cross-over from the (almost) scale-invariant electric-field assisted regime to the essentially different STM-regime. We have observed that the experimental data relating the current $I$ to the two experimental variables $V$ (bias voltage between tip and collector) and $d$ can be made (almost) collapse onto a ``scaling curve'' relating $I$ to the single variable $V\cdot d^{-\lambda}$, $\lambda$ being some exponent that depends solely on the geometry of the junction. This scaling property can be used to highlight non-linear aspects of the quantum mechanical tunnelling process. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N46.00011: Particle acceleration on a chip: A laser-driven micro-accelerator for research and industry R.B. Yoder, G. Travish Particle accelerators are conventionally built from radio-frequency metal cavities, but this technology limits the maximum energy available and prevents miniaturization. In the past decade, laser-powered acceleration has been intensively studied as an alternative technology promising much higher accelerating fields in a smaller footprint and taking advantage of recent advances in photonics. Among the more promising approaches are those based on dielectric field-shaping structures. These ``dielectric laser accelerators'' (DLAs) scale with the laser wavelength employed and can be many orders of magnitude smaller than conventional accelerators; DLAs may enable the production of high-intensity, ultra-short relativistic electron bunches in a chip-scale device. When combined with a high-$Z$ target or an optical-period undulator, these systems could produce high-brilliance x-rays from a breadbox-sized device having multiple applications in imaging, medicine, and homeland security. In our research program we have developed one such DLA, the Micro-Accelerator Platform (MAP). We describe the fundamental physics, our fabrication and testing program, and experimental results to date, along with future prospects for MAP-based light-sources and some remaining challenges. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N46.00012: Webcam science -- Can a useful transmission ion microscope be built for less than {\$}1000? Arthur Pallone, Patrick Barnes Scientists and engineers build simple, low-cost, webcam-based instruments for use in many disciplines. Analysis of the optical signal received through the three broadband color filters -- red, green and blue -- form the basis of many of those instruments. The CMOS sensors in webcam pixels also produce signals in response to ionizing radiations -- such as alpha particles from a radioactive source. Simple alpha radiography has been demonstrated with an alpha source and a webcam modified to expose the sensors. The performance of a direct imaging transmission ion microscope built from such a modified webcam and a commercially available polonium-210 antistatic device mounted to an optics rail is analyzed. Potential uses and limitations of the microscope are also discussed. [Preview Abstract] |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N46.00013: Design and implementation of a wireless passive microsensor for methanol detection Diego Sanz, Walter Rosas, Edgar Unigarro, Watson Vargas, Fredy Segura-Quijano Methanol is a public health concern due to its toxicity, characterized by metabolic acidosis and blindness, among others. The third world population affected by the exposure to this compound is increasing, mainly due to the consumption of illicit distilled or adulterated alcoholic beverages. Although methanol is naturally present in some alcoholic drinks, the maximum allowed concentration cannot exceed 10 g of methanol per liter of anhydrous alcohol (0.4\% (v/v) at 40\% of ethanol) according to the general EU limit. A wireless passive microsensor was designed to detect small amounts of methanol at 40\% of alcoholic dissolutions. The sensor consists of a planar inductor in series with an interdigital capacitor that changes its capacitance with the solution's dielectric constant. An antenna is used to readout the real part of the impedance to obtain the resonant frequencies for different amounts of methanol in the solution. The aim of this work was to develop a low cost wireless sensor with the capability to detect concentrations of at least 0.4\% (v/v) of methanol in a 40\% of alcoholic solution. The results obtained show variations of 403 kHz in the resonant frequency for changes of 0.2\% (v/v) on the concentration of methanol in a 40\% alcoholic ethanol-based solution. [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N46.00014: Phase conjugate Sagnac interferometer based on degenerate four-wave mixing using evanescent field Lijuan Gu, Zizhao Gan We propose a phase conjugate Sagnac interferometer based on degenerate four-wave mixing using evanescent field to improve the performance of fiber optic gyroscope. Degenerate four-wave mixing relies on interaction between two pump waves and evanescent fields surrounding the waveguide. By decreasing the radius of the waveguide, we can get sufficient fraction of the evanescent field. Degenerate four-wave mixing process can generate phase conjugated wave of the signal field and they are coherent intrinsically. In a rotational system, the two conjugated waves possess phase difference that is proportional to the rotational velocity of the system. So by measuring the phase difference, we can get the rotational information of the system and this method can avoids noises caused by wave propagation in fiber. [Preview Abstract] |
Session N47: Invited Session: American Science and America's Future
Sponsoring Units: FPSChair: Pushpa Bhat, Fermi National Accelerator Laboratory
Room: Hilton Baltimore Holiday Ballroom 6
Wednesday, March 20, 2013 11:15AM - 11:30AM |
N47.00001: PCAST Report: "Transformation & Opportunity: The Future of the US Research Enterprise" Invited Speaker: Maxine Savitz . [Preview Abstract] |
Wednesday, March 20, 2013 11:30AM - 11:45AM |
N47.00002: American Science and America's Future Invited Speaker: Rush Holt . [Preview Abstract] |
Wednesday, March 20, 2013 11:45AM - 12:00PM |
N47.00003: American Science and America's Future Invited Speaker: Bill Foster . [Preview Abstract] |
Wednesday, March 20, 2013 12:00PM - 12:15PM |
N47.00004: American Science and America's Future Invited Speaker: Neil Gershenfeld . [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:30PM |
N47.00005: American Science and America's Future Invited Speaker: TBD |
Wednesday, March 20, 2013 12:30PM - 1:15PM |
N47.00006: Panel Discussion - American Science and America's Future . [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 2:15PM |
N47.00007: Press Conference |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700