Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session A1: Invited Session: Spin Caloritronics
Sponsoring Units: DCMP GMAGChair: Kai Liu, University of California, Davis
Room: Ballroom I
Monday, March 18, 2013 8:00AM - 8:36AM |
A1.00001: Longitudinal Spin Seebeck Effect Invited Speaker: Eiji Saitoh The spin Seebeck effect (SSE) refers to the generation of a spin voltage as a result of a temperature gradient in magnetic materials [1-7]. Here, a spin voltage is a potential for electron spins to drive a nonequilibrium spin current; when a conductor is attached to a magnet with a finite spin voltage, it induces a spin injection into the conductor. The SSE is of crucial importance in spintronics and spin caloritronics, since it enables simple and versatile generation of a spin current from heat. The simplest and most straightforward setup of the SSE is the longitudinal configuration [4], in which a spin current flowing parallel to a temperature gradient is measured via the inverse spin Hall effect (ISHE). The longitudinal SSE device consists of a ferromagnetic or ferrimagnetic insulator (FI, e.g. YIG) covered with a paramagnetic metal (PM, e.g. Pt) film. When a temperature gradient is applied perpendicular to the FI/PM interface, an ISHE-induced voltage is generated in the PM layer. In this talk, we report the observation of the longitudinal SSE in various FI/PM systems and provide evidence that the longitudinal SSE is free from thermoelectric artefact [7], i.e., the anomalous Nernst effect caused by extrinsic magnetic proximity [8]. Then, we discuss the longitudinal SSE from an application point of view [6]. We thank E. Saitoh, S. Maekawa, G. E. W. Bauer, X.-F. Jin, H. Adachi, D. Hou, D. Tian, T. Kikkawa, A. Kirihara, and M. Ishida for their support and valuable discussions. \\[4pt] [1] K. Uchida et al., Nature 455, 778 (2008).\\[0pt] [2] K. Uchida et al., Nature Mater. 9, 894 (2010).\\[0pt] [3] C. M. Jaworski et al., Nature Mater. 9, 898 (2010).\\[0pt] [4] K. Uchida et al., Appl. Phys. Lett. 97, 172505 (2010).\\[0pt] [5] K. Uchida et al., Nature Mater. 10, 737 (2011).\\[0pt] [6] A. Kirihara et al., Nature Mater. 11, 686 (2012).\\[0pt] [7] T. Kikkawa et al., arXiv:1211.0139 (2012). \\[0pt] [8] S. Y. Huang et al., Phys. Rev. Lett. 109, 107204 (2012). [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A1.00002: Transport Magnetic Proximity Effects in Platinum Invited Speaker: Ssu-Yen Huang Platinum (Pt) metal, being non-magnetic and having a strong spin-orbit coupling interaction, has been central in detecting pure spin current and establishing most of the recent spin-based phenomena. Thus, it is important to ascertain the transport and magnetic characteristics of thin Pt films in contact with a ferromagnet. In this work, we use both electric and thermal means to conclusively show the transport magnetic proximity effects (MPE) of thin Pt film in contact with ferromagnetic insulator YIG. At thicknesses comparable to, and less than, the spin diffusion length, the strong ferromagnetic characteristics in Pt films on YIG are indistinguishable from those of ferromagnetic permalloy on YIG. [1] The MPE occurs at the interface and decreases exponentially away from the interface, concentrating in only a few monolayers. As a result, the pure spin current detected by a thin Pt is tainted with a spin polarized current. The pure spin current phenomena, such as the inverse spin Hall effect and the spin Seebeck effect, have been contaminated with the anomalous Hall effect and the anomalous Nernst effect respectively. These results raise serious questions about the suitability, and the validity, of using Pt in establishing pure spin current phenomena; on the other hand, a much stronger spin-based effect can be induced by the MPE at the interface. This research is in collaboration with X. Fin, Y. P. Chen, J. Wu, and J. Q. Xiao (University of Delaware), T. Y. Chen (Arizona State University) and D. Qu, W. G. Wang, and C. L. Chien (The Johns Hopkins University).\\[4pt] [1] S. Y. Huang \textit{et al.,} Phys. Rev. Letts. \textbf{109}, 107204 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A1.00003: Observation of the planar Nernst effect in Permalloy and Nickel Thin Films with In-plane Thermal Gradients Invited Speaker: Barry Zink The reliable generation of pure spin currents is an important ingredient in future spintronic circuits that may offer lower power consumption and greater processing capabilities than current technology. Over the past few years some groups have reported that such a spin current can be generated simply by applying a thermal gradient to a ferromagnetic material. This effect, called the spin Seebeck effect (SSE), has generated tremendous interest in the interaction of heat, charge and spin in ferromagnetic systems. In this talk we will present our own recent measurements of thermoelectric and thermomagnetic effects in thin film metallic ferromagnets. These are enabled by a micromachined thermal isolation platform that removes potentially confounding effects introduced in such measurements by the presence of a highly thermally conductive bulk substrate. One of the main results is the observation of a transverse thermopower, called the planar Nernst effect (PNE), that is caused by spin-dependent scattering. This PNE should therefore be present in any attempted measurement of the SSE in a metal system where spin-dependent scattering of electrons occurs. Furthermore our ``zero substrate" experiment shows no signal with the expected symmetry of the SSE, suggesting that the presence of the substrate is required to cause such a signal. Further experiments are required to determine if a pure spin current is actually involved in the generation of the signal associated with the SSE in ferromagnetic metal films. This work was performed in collaboration with A. D. Avery, and M. R. Pufall. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A1.00004: Non-universal shot noise in quasiequilibrium spin valves Invited Speaker: Tero Heikkila Shot noise can be used as a diagnostic tool characterizing mesoscopic wires, especially the inelastic scattering in them. This characterization is based on the fact that in the absence of inelastic scattering that carries the energy away from the system, disordered wires are described by a universal Fano factor defined as the ratio of the noise power and the average current. In particular, the value of this Fano factor is invariant even for wires with non-uniform conductivity. We show that this universality breaks down in spin valves with strong electron-electron scattering. The reason for this breakdown is that the inter-spin energy relaxation due to electron-electron scattering in the absence of inter-spin charge relaxation breaks the Wiedemann-Franz relation between charge and heat conductivity. In particular, we predict that the Fano factor gets strongly suppressed for the antiparallel configuration of magnetizations.\\[4pt] T.T. Heikkil\"a and K.E. Nagaev, arXiv:1302.1372 [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A1.00005: Charge Voltages from Magnetization Dynamics Invited Speaker: Axel Hoffmann The main challenge of spin caloritronics is to establish a connection between heat currents and spin currents. Towards this end, spin Hall effects have become very important, since they allow to convert a pure spin current into a transverse charge voltage. I will show how these spin Hall effects can be characterized with great accuracy using spin pumping, where the excitation of ferromagnetic resonance generates a pure spin current in an adjacent non-magnetic conductor.\footnote{O.~Mosendz, V.~Vlaminck, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.\ B {\bf 82}, 214403 (2010); O.~Mosendz, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.\ Lett.\ {\bf 104}, 046601 (2010).} The change in the line-width of the ferromagnetic resonance determines the spin-mixing conductance and thus after proper calibration of the {\em rf} magnetic fields and the concomitant opening angles of the magnetization precession, allows to determine the magnitude of the spin current. The charge current generated from inverse spin Hall effect is measured through the associated electrical voltage and the ration of spin and charge current directly determines the spin Hall angle. Furthermore I will present an alternative approach for converting magnetization dynamics into measurable charge voltages. Namely, the dissipation of magnetization dynamics in thin films generally also results in a temperature gradient perpendicular to the film, since the supporting substrate acts as a heat sink. This in turn can generate a transverse voltage through the anomalous Nernst effect. Interestingly this allows to detect spin waves with very good signal to noise\footnote{H.~Schultheiss, J.~E.~Pearson, S.~D.~Bader, and A.~Hoffmann, Phys.\ Rev.\ Lett.\ (in press).} and unlike optical or inductive detection techniques there is practically no lower limit for the wavelength of the detected spin waves. [Preview Abstract] |
Session A2: Invited Session: Novel Superconductivity in FE Selenide Superconductors
Sponsoring Units: DCMPChair: Douglas Scalapino, University of California at Santa Barbara
Room: Ballroom II
Monday, March 18, 2013 8:00AM - 8:36AM |
A2.00001: Ab initio calculations and crystal symmetry considerations for novel FeSe-based superconductors Invited Speaker: Igor Mazin Density functional calculations disagree with the ARPES measurements on both K$_{0.3}$Fe$_2$Se$_2$ superconducting phase and FeSe/SrTiO$_3$ monolayers. Yet they can still be dramatically useful for the reason that they respect full crystallographic symmetry and take good account of electron-ion interaction. Using just symmetry analysis, it is shown that nodeless d-wave superconductivity is not an option in these systems, and a microscopic framework is derived that leads to a novel s-wave sign-reversal state, qualitatively different from the already familiar s$_\pm$ state in pnictides and bulk binary selenides. Regarding the FeSe monolayer, bonding and charge transfer between the film and the substrate is analyzed and it is shown that the former is weak and the latter negligible, which sets important restrictions on possible mechanisms of doping and superconductivity in these monolayers. In particular, the role of the so-called ``Se etching,'' necessary for superconductivity in FeSe monolayers, is analyzed in terms of electronic structure and bonding with the substrate. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A2.00002: Scanning tunneling microscopy study on superconductivity of FeSe thin films Invited Speaker: Xucun Ma Searching for superconducting materials with high transition temperature (T$_{C})$ is one of the most exciting and challenging fields in physics and materials science. By using MBE technique, we are able to prepare stoichiometric and superconducting FeSe single crystalline films on various substrates, which enables us investigate superconductivity mechanism of FeSe thin films in well-controlled way [1-3]. Most importantly, by using low temperature scanning tunneling spectroscopy, a superconductive gap as large as 20 meV and the vortex state under high magnetic field are revealed in the single unit-cell thick FeSe films on SrTiO$_{3}$(001) substrate [4]. Such a high Tc superconductor is further confirmed by recent transport measurement. The study not only demonstrates a powerful way for finding new superconductors and for raising T$_{C}$, but also provides a well-defined platform for systematic study of the mechanism of unconventional superconductivity by using different superconducting materials and substrates. The study is collaborated with Professor Qi-Kun Xue, Department of Physics, Tsinghua University, China.\\[4pt] References:\\[0pt] [1] C. L. Song, Y. L.Wang, P. Cheng, Y. P. Jiang, W. Li, T. Zhang, Z. Li, K. He, L. L. Wang, J. F. Jia, H. H, Hung, C. J. Wu, X. C. Ma, X. Chen, and Q. K. Xue, \textbf{Science 332}, 1410 (2011).\\[0pt] [2] C. L. Song, Y. L. Wang, Y. P. Jiang, Z. Li, L. L. Wang, K. He, X. Chen, X. C. Ma, and Q. K. Xue, \textbf{Phys. Rev. B} \textbf{84}, 020503 (2011).\\[0pt] [3] C. L. Song, Y. L. Wang, Y. P. Jiang, L. L. Wang, Ke He, Xi Chen, J. E. Hoffman, X. C. Ma, and Q. K. Xue, \textbf{Phys. Rev. Lett. 109}, 137004 (2012).\\[0pt] [4] Wang Qingyan, Li Zhi, Zhang Wenhao, Zhang Zuocheng, Zhang Jinsong, Li Wei, Ding Hao, Ou Yunbo, Deng Peng, Chang Kai, Wen Jing, Song Canli, He Ke, Ji Shuaihua, Jia Jinfeng, Wang Yayu, Wang Lili, Chen Xi, Ma Xucun, and Xue Qikun, \textbf{Chin. Phys. Lett. 29}, 037402 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A2.00003: Phase Diagram and High Temperature Superconductivity at 65K in the Single-Layer FeSe Films Revealed by ARPES Invited Speaker: Shaolong He The discovery of the iron-based superconductors in 2008 not only provides another venue to understand the origin of high-Tc superconductivity but also a new playground to explore novel superconductors with higher superconducting transition temperature. The latest report of possible high temperature superconductivity in the single-layer FeSe films grown on SrTiO3 substrate is both surprising and interesting [1]. In this talk, we report the electronic structure and phase diagram of the single-layer FeSe films by angle-resolved photoemission spectroscopy (ARPES) [2,3]. Our high-resolution ARPES results show that it has a simple Fermi surface topology consisting only of electron pockets near the zone corner without indication of any Fermi surface around the zone center. In addition, our observation of large and nearly isotropic superconducting gap in this strictly two-dimensional system rules out existence of node in the superconducting gap. We also established a phase diagram in this single-layer FeSe films by an annealing procedure to tune the charge carrier concentration over a wide range. By optimizing the annealing process, we observed evidence of a record high Tc of $\sim$ 65K in the single-layer FeSe films. The wide tunability of the system across different phases, and its high-Tc, make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications.\\[4pt] [1] Q.Y. Wang et al., Chin. Phys. Lett. 29, 037402 (2012)\\[0pt] [2] D. F. Liu et al., Nature Communications. 3, 931 (2012)\\[0pt] [3] S. L. He et al., arXiv:1207.6823v1, (2012) [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A2.00004: Pairing and Fermiology in iron-chalcogenide superconductors Invited Speaker: Dung-Hai Lee ``Stripe''-type magnetic fluctuations has been postulated as the trigger of Cooper pairing in iron-based superconductors. In iron pnictides the matching of the peak magnetic fluctuations wavevector and the Fermiology lands support to the above idea. However recent ARPES results on high T$_{\mathrm{c}}$ A$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{\mathrm{2}}$ and FeSe/SrTiO$_{\mathrm{3}}$ and neutron results on A$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{\mathrm{2}}$ pose challenges to the above picture. In this talk we will take a fresh new look at the purported pairing mechanism of iron-based superconductors. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A2.00005: S$_{4}$ Symmetric Microscopic Model for Iron Based Superconductors Invited Speaker: Jiangping Hu How are cuprates and iron-based high temperature superconductors correlated? What is the common mechanism behind two different families of iron-based superconductors, iron-pnicitides and iron-chalcogenides? These two questions are two major challenges in the today's field of high temperature superconductors. In this talk, we will show when the lattice symmetry, the \textbf{S}$_{\mathbf{4}}$ symmetry, of the building block, the tri-layer structure of FeAs or FeSe, is properly considered, the low energy physics of iron-based superconductors is governed by a two-orbital Hamiltonian near half filling that can be divided two weakly coupled one-orbital model. We will discuss the microscopic origin and some unique properties of the model, including magnetism and pairing symmetry. The model provides a unified understanding of iron pnictides and iron chalcogenides, and suggests that cuprates and iron-based superconductors share an identical high-T$_{c}$~superconducting mechanism. The model leads to a natural classification of pairing symmetry according to \textbf{S}$_{\mathbf{4}}$ symmetry. When the pairing is driven by antiferromagnetic exchange couplings, there are two different s-wave states. One s-wave is the well-studied s$\pm$ pairing which is in the A phase of \textbf{S}$_{\mathbf{4}}$ symmetry ( even under \textbf{S}$_{\mathbf{4}}$ symmetry operation), and the other is a new type of extended s-wave pairing which is in the B phase of \textbf{S}$_{\mathbf{4}}$ symmetry ( odd under\textbf{S}$_{\mathbf{4}}$ symmetry operation). The superconductivity order in the B phase are characterized by opposite signs between up and bottom As(Se) layers in the trilayer Fe-As(Se) structure. The 122 Iron-chalcogenides and the single layer FeSe are most likely in the B-phase. We believe that the model establishes a new foundation for exploring novel properties of iron based superconductors. \\[4pt] References: J.P. Hu and Ningning Hao, Phys. Rev. X, 021009 (2012); T. X. Ma, H.Q. Lin and J.P. Hu, Arxiv:1206.6277 (2012); N.N. Hao, Y.P. Wang and J.P. Hu, Arxiv: 1207.6798 (2012); J.P. Hu, and NingNing Hao, unpublished; J.P. Hu, Arxiv: 1208.6201 (2012) [Preview Abstract] |
Session A3: Invited Session: Second Landau Level: Quantum Phases
Sponsoring Units: DCMPChair: Loren Pfeiffer, Princeton University
Room: Ballroom III
Monday, March 18, 2013 8:00AM - 8:36AM |
A3.00001: Evidence of low-lying gapped excitations in the $\nu =$5/2 quantum fluid Invited Speaker: Ursula Wurstbauer The competition between quantum phases that dictate the physics in the second Landau level (SLL) results in striking phenomena. A highly fascinating state is the even denominator fractional quantum Hall (FQHE) state at filling $\nu =$5/2 that is widely believed to support non-Abelian quasi-particle excitations. Our work explores the low-lying neutral excitation modes in the SLL by resonant inelastic light scattering measurements. At 5/2 the spectra revealed a band of gapped modes with peak intensity at energy of 0.07meV. These modes are interpreted as a roton minimum in the wave vector dispersion of spin-conserving excitations. The intensity of the roton band significantly diminishes by increasing the temperature to 250mK and it fully collapses for T\textgreater\ 250mK. This temperature dependence is consistent with activated magneto-transport of the incompressible quantum fluid at 5/2. A long wavelength spin wave mode (SW) is seen at the bare Zeeman energy, indicating that there is non-zero spin-polarization. Both, roton and SW modes appear only in a very narrow filling factor range of less than $\nu $\textless\ 5/2 $\pm$ 0.01. A gapless continuum of low-lying excitations emerges at filling factors slightly away from 5/2. This demonstrates a transition from an incompressible quantum Hall fluid at exactly $\nu =$5/2 to compressible states at very close filling factors. This work is in collaboration with A. Pinczuk, A. Levy, K. West, L. Pfeiffer, S. Mondal, J. Watson and M. Manfra. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A3.00002: Spin transition in the second Landau level Invited Speaker: W. Pan The fractional quantum Hall effect (FQHE) states in the second Landau level have attracted growing interest and intensive theoretical and experimental investigations due to their being non-Abelian states. Recently, we systematically examined the spin polarization of the 8/3 and 12/5 states in a series of high quality two dimensional electron systems. Evidence of spin transition was observed for both states, suggesting a more complicated nature of the FQHE ground states in the second Landau level. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A3.00003: Quantum Hall Interferometry and Detection of Anyonic Braiding Statistics Invited Speaker: Steven Simon In two spatial dimensions quantum mechanical particles are not limited to being bosons or fermions as they are in three dimensions, but can be particles known as anyons. Such anyons come in two major varieties --- Abelian and non-Abelian --- both of which were long ago predicted to be realized in certain Fractional Quantum Hall (FQH) systems. However, experimental demonstration of anyonic braiding properties has remained elusive and very controversial. New results by Willett et. al. and Kang et. al. have been interpreted as evidence of anyonic braiding in Fabry Perot interferometers in the second Landau level. In this talk, I will discuss my current understanding of these works based on a number of recent publications.\\[4pt] References:\\[0pt] [1] arXiv:1112.3400, Braiding of Abelian and Non-Abelian Anyons in the Fractional Quantum Hall Effect, Sanghun An, P. Jiang, H. Choi, W. Kang, S. H. Simon, L. N. Pfeiffer, K. W. West, K. W. Baldwin\\[0pt] [2] Physical Review B 85, 201302, 2012, Telegraph noise and the Fabry-Perot quantum Hall interferometer, Bernd Rosenow, Steven H. Simon [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A3.00004: Quantifying disorder and its impact in the 2$^{\mathrm{nd}}$ Landau level Invited Speaker: Michael Manfra Strong electronic correlations are evident in the 2$^{\mathrm{nd}}$ Landau level (LL) of an ultra-high quality GaAs/AlGaAs two-dimensional electron gas (2DEG). The exotic fractional quantum Hall states at filling factors $\nu =$5/2 and 12/5, as well as the reentrant integer quantum Hall states flanking half-filling, are a few examples presently under intense investigation. While it is generally accepted that samples must be of extremely low disorder to exhibit correlations in the 2$^{\mathrm{nd}}$ LL, our understanding of how to quantify residual disorder and its impact on the states of interest remains primitive. In this talk we will critically examine how disorder is quantified for the 2DEG both at zero magnetic field and in the quantum Hall regime, and then compare the results of this analysis with measurements of the excitation gap at $\nu =$5/2 in samples in which disorder is varied in a well-defined manner. Our results highlight the very different impact on the excitation gap generated by different types of disorder and the limitations of presently employed characterization methods. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A3.00005: Role of inter-composite fermion interactions in fractional quantum Hall effect Invited Speaker: Jainendra Jain One of the important challenges in the field of the fractional quantum Hall effect (FQHE) is to understand the nature of states that cannot be explained as integer quantum Hall effect of weakly interacting composite fermions; such states include FQHE at 5/2, 7/3, 8/3, 12/5, 13/5, 19/8 in the second Landau level (of GaAs) and 4/11, 5/13, 3/8(?) in the lowest Landau level. I will report on results [1,2] that demonstrate that multipartite wave functions of composite fermions provide an excellent account of the low energy physics of the model 3-body and 4-body interaction Hamiltonians that have the Pfaffian and the Read-Rezayi wave functions as their exact ground states. The relevance of these wave functions to the Coulomb solutions at 5/2 and 13/5 is investigated through an adiabatic scheme connecting the two models. In particular, the multipartite wave functions are shown to shed light on the structure of the neutral and charged excitations and of unpaired composite fermions [1]. We find that the residual interaction between composite fermions also has substantial effect at certain other fractions; it is predicted to produce a paired FQHE of the anti-Pfaffian kind at 3/8 [3], and unconventional trion excitations at 7/3 [4].\\[4pt] [1] G. J. Sreejith, C. Toke, A. Wojs and J.K. Jain, PRL 107, 136802 (2011).\\[0pt] [2] G. J. Sreejith, Y.-H. Wu, A. Wojs and J.K. Jain, unpublished.\\[0pt] [3] S. Mukherjee, S. S. Mandal, A. Wojs, J.K. Jain, unpublished.\\[0pt] [4] A. C. Balram, Y.-H. Wu, G. J. Sreejith, A. Wojs, J. K. Jain, unpublished. [Preview Abstract] |
Session A4: Industrial Physics Forum: Frontiers in Physics
Chair: David Seiler, National Institute of Standards and TechnologyRoom: Ballroom IV
Monday, March 18, 2013 8:00AM - 8:36AM |
A4.00001: Has the Higgs Boson Been Discovered? Latest Results from the ATLAS Experiment at the LHC Invited Speaker: Michael Tuts The Large Hadron Collider (LHC) at CERN has had a very successful first data taking run that ended in December 2012. The ATLAS detector has collected about 25 fb$^{-1}$ of integrated luminosity of proton-proton collisions at a center-of-mass energy of 8 TeV. The LHC is the highest energy particle accelerator in the world. The highest profile result has been the announcement of the discovery of a new particle at a mass of about 125 GeV/c$^{2}$, consistent with the long sought Higgs Boson. I will start by describing why there is such excitement about this discovery and why it is not just ``another'' new particle -- if it is indeed the Higgs Boson, it is not only the last missing piece of the Standard Model, but represents a fundamentally new type of particle. The discovery comes after almost two decades of construction of the particle accelerator (LHC) and some of the most complex scientific instruments ever built. I will focus on and describe the ATLAS detector and how it works. The ATLAS experiment, together with CMS experiment, announced the discovery of this new particle in July 2012. I will present the latest results on the properties of this new ``Higgs-like'' particle and the prospects for future discoveries at the LHC. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A4.00002: Entangled magnetism: synthesis, detection, and potential applications Invited Speaker: Collin Broholm |
Monday, March 18, 2013 9:12AM - 9:48AM |
A4.00003: Quantum Computing Invited Speaker: Matthias Steffen Quantum mechanics plays a crucial role in many day-to-day products, and has been successfully used to explain a wide variety of observations in Physics. While some quantum effects such as tunneling limit the degree to which modern CMOS devices can be scaled to ever reducing dimensions, others may potentially be exploited to build an entirely new computing architecture: The quantum computer. In this talk I will review several basic concepts of a quantum computer. Why quantum computing and how do we do it? What is the status of several (but not all) approaches towards building a quantum computer, including IBM's approach using superconducting qubits? And what will it take to build a functional machine? The promise is that a quantum computer could solve certain interesting computational problems such as factoring using exponentially fewer computational steps than classical systems. Although the most sophisticated modern quantum computing experiments to date do not outperform simple classical computations, it is increasingly becoming clear that small scale demonstrations with as many as 100 qubits are beginning to be within reach over the next several years. Such a demonstration would undoubtedly be a thrilling feat, and usher in a new era of controllably testing quantum mechanics or quantum computing aspects. At the minimum, future demonstrations will shed much light on what lies ahead. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A4.00004: Catching the Light: the Giant Magellan Telescope Invited Speaker: Daniel Fabricant There has been an explosion of theoretical work outlining how the first galaxies might have formed 13 billion years ago. The Giant Magellan Telescope (GMT) is to be the first of three extremely large ground-based telescopes capable of confronting theory with detailed observations of primordial galaxies. With a collecting area approaching 400 square meters and adaptive optics to remove the image blurring of the earth's turbulent atmosphere, the GMT offers a huge leap in sensitivity over the largest existing telescopes. Building a high-performance telescope of this scale relies on recent technical advances in optics and electronics. I describe the major technical challenges addressed in the GMT's design, and offer a glimpse of ground-based astronomy with extremely large telescopes a decade hence. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A4.00005: Frontiers of the Physics of Carbon Nanotubes Invited Speaker: Mildred Dresselhaus Carbon nanotubes entered the scene of materials physics about 20 years ago, exhibiting unusual structures and properties stemming from their strong sp2 carbon bonds, their lower mass density, their very large length-to-diameter ratio, and their ability to be either semiconducting or metallic depending on their tube diameter and the orientation of their in-plane hexagons relative to their tube axis. You might ask what potential applications could be envisioned for such unusual nano structures, and what practical application have in fact been realized to date. This will be the focus of my presentation. [Preview Abstract] |
Session A5: Focus Session: Van der Waals Bonding in Advanced Materials - Materials Behavior
Sponsoring Units: DMPChair: Roberto Car, Princeton University
Room: 301
Monday, March 18, 2013 8:00AM - 8:12AM |
A5.00001: Beyond RPA correlation energies: Evaluation of model exchange-correlation kernels Deyu Lu The adiabatic-connection fluctuation-dissipation theorem (ACFDT) has drawn considerable attention in describing van der Waals (vdW) dispersion interactions. Under the random phase approximation (RPA), the EXX/RPA method yields the correct asymptotic behavior at large distances. However, for many advanced materials, e.g., organic/inorganic interfaces and molecular crystals, it is important to capture the short-range dispersion interaction within several angstrom. Because RPA pair distribution function is incorrect at short distances, the contribution of the exchange-correlation kernel has to be included properly. In this work, we implemented several model exchange-correlation kernels in the framework of ACFDT. Special attention was paid to develop non-local kernels suitable for inhomogeneous electronic systems. The performance of the exchange-correlation kernels were evaluated for both bulk and molecular systems. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A5.00002: Long-range van der Waals interaction between nanoclusters Jianmin Tao, John Perdew, Adrienn Ruzsinszky van der Waals (vdW) interaction is an important long-range correlation that affects many properties of materials. However, this effect cannot be accurately accounted for by first-principles calculations, due to computational challenges. Recently, we have developed a model for the vdW coefficients between quasispherical clusters such as fullerenes, sodium and silicon clusters. Our study shows that the widely-used atom-pairwise interaction picture surprisingly breaks down. A quick remedy of this problem leads to a counterintuitive scaling law of the vdW coefficients for caged molecules and clusters. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A5.00003: Binding and Diffusion of Lithium in Graphite: Quantum Monte-Carlo benchmarks and validation of van der Waals density functional methods Paul Kent, Panchapakesan Ganesh, Mina Yoon, Jeongnim Kim, Fernando Reboredo Benchmark diffusion quantum monte-carlo (DMC) studies of the adsorption and diffusion of atomic lithium in graphite are compared with density functional theory (DFT) calculations using several van der Waals methods. The charge transfer is captured adequately with conventional local density functionals. At fixed geometries, these yield surprisingly accurate energetics. In unconstrained geometries, van der Waals corrections are required to correctly reproduce graphite and lithium binding. We find that the empirical method of Grimme et al. only gives correct diffusion barriers when the Li polarizability is reduced to nearly zero, consistent with the charge transfer in the solid-state environment. The Tkatchenko-Scheffler scheme captures the polarizability reduction, yielding accurate results at low computational cost. The self-consistent vdw-DF2 functional yields the best overall results but at increased cost. Slight differences in barrier heights remain with all the DFT approaches compared to the DMC. These results establish a hierarchy of modeling approaches for the lithium-carbon system. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A5.00004: Probing van der Waals Forces at the Single-Molecule Level Invited Speaker: Latha Venkataraman Single molecule junctions represent an attractive platform to understand and control functionality of materials and devices at the nanoscale. While their electronic transport properties have received tremendous attention thus far, measurements of mechanics are new and allow for a more complete understanding of the structure-function relationship of these atomic scale devices. Here we report simultaneous measurement of force and electrical conductance across Au-Bipyridine-Au junctions using a conducting atomic force microscope (AFM). We show that these junctions have two distinct structures each with a characteristic conductance. Using statistically relevant analysis, these two structures are found to have very different mechanical properties. Specifically, we find that the higher conductance junctions have a significantly larger rupture force and stiffness than the lower conducting junctions. They also have a larger rupture force than Au single-atom point contacts, suggesting multiple points of contact. Using density functional theory simulations we show that van der Waals (vdW) interactions between the pyridine ring and Au electrodes plays a key role in the junction mechanics. These measurements thus provide a quantitative characterization of vdW interactions at metal/organic interfaces at the single-molecule level~[1].\\[4pt] [1] Aradhya, S. V., Frei, M., Hybertsen, M. S. {\&} Venkataraman, L., Nature Materials, 11, 872-876 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A5.00005: Liquid water from first principles: The importance of exact exchange, dispersion interactions, and nuclear quantum effects Robert DiStasio, Zhaofeng Li, Biswajit Santra, Xifan Wu, Roberto Car Quantitative agreement between theory and experiment on the structure of liquid water at ambient conditions has been quite difficult to achieve to date. In this work, we report that highly accurate {\it ab initio} molecular dynamics simulations of liquid water that account for exact exchange (via the hybrid PBE0 functional [PRB {\bf 79}, 085102 (2009)]), dispersion interactions [PRL {\bf 102}, 073005 (2009)], and nuclear quantum effects (presently approximated by a 30K increase in the simulation temperature) result in excellent agreement with experiments [PRL {\bf 101}, 065502 (2008)]. The importance of each of these effects in the theoretical prediction of the structure of liquid water will be demonstrated by a detailed comparative analysis of the predicted and experimental oxygen-oxygen radial distribution functions. In addition, we will discuss the connection between the experimentally observed scattering intensity, I(k), and the final radial distribution function, g(r), via the structure and form factors. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A5.00006: Spatially Resolved Raman Spectroscopy of Single- and Few-Layered WS$_2$ Ayse Berkdemir, Humberto R. Gutierrez, Andres R. Botello-Mendez, Nestor Perea-Lopez, Ana L. El\'Ias, Cheng-Ing Chia, Bei Wang, Vincent H. Crespi, Florentino Lopez-Urias, Jean-Christophe Charlier, Humberto Terrones, Mauricio Terrones We systematically investigated the Raman scattering of single- and few-layered WS$_{2}$ as a function of the number of S-W-S layers and the excitation laser wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A$_{1g}(\Gamma )$ phonon mode monotonically decreases with the number of layers, while the E$^{1}_{2g}(\Gamma )$ frequency increases. For single-layer WS$_{2}$, 514.5 nm excitation generates a second-order Raman resonance for the longitudinal acoustic mode at the M point. This 2LA(M) resonance results from a double-resonant Raman coupling between the electronic band structure and lattice vibrations, an effect not previously seen in any single-layered metal dichalcogenide. We performed ab initio calculations to determine the electronic and phonon band structures of single-layer and bulk WS$_{2}$, these results were used to compute the reduced intensity of the 2LA mode from the fourth order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS$_{2}$ islands. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A5.00007: Extraordinary room-temperature photoluminescence in WS$_{2}$ monolayers Humberto Rodriguez Gutierrez, Nestor Perea-L\'opez, Ana Laura El\'Ias, Ayse Berkdemir, Bei Wang, Ruitao Lv, Florentino L\'opez-Ur\'Ias, Vincent Crespi, Humberto Terrones, Mauricio Terrones Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from their bulk layered counterpart. Here we describe the direct synthesis of WS$_{2}$ monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S-W-S layers is also reported. The PL becomes weaker with the increase of S-W-S layers number due to a transition from direct (in a monolayer) to indirect band gap (in multilayers). The edges of WS$_{2}$ monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than the platelets center. The structure and composition of the platelet edges appear to be critical for the PL enhancement effect. These novel 2D nanoscale light sources could find diverse applications including the fabrication of flexible/transparent/low-energy optoelectronic devices [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A5.00008: How van der Waals Interactions Influence Cohesive Properties of Non-Metallic Solids Guo-Xu Zhang, Anthony M. Reilly, Alexandre Tkatchenko, Matthias Scheffler Standard semilocal and hybrid density functionals are widely used for studying cohesive properties of covalent, metallic, and ionic materials. Only recently it has been recognized that long-range van der Waals (vdW) interactions, that are missing in all semilocal and hybrid functionals, are important for an accurate description of cohesion in solids. Here we construct a database of 64 solids where reference cohesive properties are obtained from a critical revision of the available experimental data. All-electron DFT calculations with explicit treatment of zero-point vibrations for all cohesive properties are performed using the LDA, PBE, and the empirical meta-GGA M06-L [1] functionals. For 23 semiconductors, we carry out PBE and M06-L calculations with the inclusion of screened long-range vdW energy [2]. We find that PBE is the most systematic from the three employed functionals, and its accuracy is improved by a factor of two after the inclusion of vdW interactions. The LDA functional considerably overbinds for all the studied solids. The M06-L functional describes middle-range correlation better for certain semiconductors and ionic crystals, but fails for heavier semiconductors and metals.\\[4pt] [1] Zhao and Truhlar, JCP (2006).\\[0pt] [2] Tkatchenko, DiStasio, Car, Scheffler, PRL (2012). [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A5.00009: Analyzing the vdW-DF description of binding mechanisms: Comparison of C60 and benzene adsorption on graphene Kristian Berland, Per Hyldgaard There has been several efforts to improve the accuracy of the description of sparse matter problems like molecular adsorption on surfaces using non-local correlation functionals. We have explored the vdW-DF~[PRL 92, 246401 (2004)] interaction at different length scales, density regimes, density gradients, and for different system. As test calculations, we compare the potential energy curves of benzene and C60 on graphene and related systems like boron nitride because these reveal the role of geometry and band gap on the functional components. Our analysis is facilitated by explicit control of cutoff parameters in our real-space evaluation of the non-local correlation. We find that vdW-DF is very sensitive to the low density regions, but more so for the original version than in the newer one, vdW-DF2 [PRB 82, 081101 (2010)]. Our results also illustrate that a transferable account of many different geometries requires an accurate account of all length scales involved in the problem. These results are discussed in light of the functional form of vdW-DF. We also show how functional choices greatly affects corrugation. Finally, we examine the role of induced dipoles on the adsorption. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A5.00010: Bi-layer excitons in two-dimensional layered materials Mahesh Neupane, Gen Yin, Darshana Wickramaratne, Roger Lake Following the prediction of exciton condensation in closely spaced two-dimensional electron-hole bilayer systems [1], there has been a sustained theoretical and experimental investigation of this condensation phase in coupled quantum well material systems. The electron-hole pairs are bound by the interlayer Coulomb interaction, which is tuned by electrostatic gating of the charge density [2]. The magnitude of this interaction is determined by the binding energy between the electron and the hole. Improvements in the exciton binding energy can be achieved by an appropriate choice of materials. The family of van der Walle materials is considered in this study, and the effect of material choice and insulating layer thickness on the excitonic properties will be discussed and compared to experimental investigations using traditional GaAs-AlGaAs coupled quantum wells.\\[4pt] [1] Y. Lozovik and V. Yudson, JETP Letters, vol. 22, 1975\\[0pt] [2] J. Shumway and M.J. Gilbert, Phys. Rev. B., vol. 85, no. 3, 2012 [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A5.00011: Effects of interatomic potentials on mechanical deformation of glasses Wei-Ren Chen, Takuya Iwashita, Takeshi Egami Apparently glasses behave like an elastic solid, which shows a linear relationship between stress and strain in mechanical deformation. However the understanding of the mechanical response of glasses remains elusive because of structural disorder. Mechanical deformation of monatomic model glasses was studied using athermal quasi-static shear (AQS) simulation and with three different potentials. As the interatomic potentials we employed the 12-6 Lennard-Jones (LJ) potential, modified Johnson (mJ) potential, and Dzugutov (Dz) potential, respectively. For mJ and Dz glasses the shear modulus keeps constant below a critical strain, below which it decreases rapidly or discontinuously with strain. Such changes in shear modulus were mostly related to the change in local topology of atomic connectivity or anelasticity. In contrast LJ glass shows a gradual decrease in shear modulus in a continuous manner. The results indicated that the difference arises from the nature of the potentials if the topology of atomic connectivity can be clearly defined. [Preview Abstract] |
Session A6: Topological Insulators: Novel States in Topological Insulators
Sponsoring Units: DCMPChair: Tudor Stanescu, West Virginia University
Room: 302
Monday, March 18, 2013 8:00AM - 8:12AM |
A6.00001: Noncommutative Magneto-Electric Responses of Topological Insulators Bryan Leung, Emil Prodan Topological magneto-electric response, constructed on a Brillouin torus, defines a $Z_2$ invariant and classifies topological phases. In the presence of disorder or B field, the notion of Brillouin torus is destroyed. This problem can be overcome by using noncommutative geometry. Starting from a generic 3D lattice model, we derive the magneto-electric response on a noncommutative Brillouin torus. Our result is a noncommutative topological formula. We show that its topological stability requires only mobility gap, therefore the robustness extends to strong disorder regime. Our formula doesn't involve gauge or twisted boundary condition, hence can be computational effective. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A6.00002: Heavy Adatoms on Magnetic Surfaces: A Search for Chern Insulators Kevin Garrity, David Vanderbilt The theoretical possibility of a quantum anomalous Hall (QAH) insulator, or Chern insulator, has been known for many years, and several strategies for achieving this topological phase have been proposed. However, no unambiguous experimental realization is yet in hand. In this work, we propose a new QAH search strategy and verify its viability with first-principles calculations. We propose constructing a QAH insulator by depositing a layer of adatoms with large spin-orbit coupling (e.g., Pb, Bi) on the surface of a magnetic insulator. We argue that such systems will typically have surface bands with non-zero Chern numbers, so that if metallic, they will typically have a large anomalous Hall conductivity. Thus, the search for Chern insulators reduces to looking for examples exhibiting a global gap across the entire BZ. Using first-principles calculations, we search through many examples of heavy elements on MnTe, MnSe, and EuS surfaces. Our search reveals several Chern insulators with band gaps of up to 0.14 eV, which may be promising targets for future experimental investigations. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A6.00003: Topology, Delocalization via Average Symmetry and the Symplectic Anderson Transition Charles Kane, Liang Fu A field theory of the Anderson transition in two dimensional disordered systems with spin-orbit interactions and time-reversal symmetry is developed, in which localization is driven by the proliferation of vortex-like topological defects. The sign of the vortex fugacity determines the $Z_2$ topological class of the localized phase. There are two distinct fixed points with the same critical exponents, corresponding to transitions from a metal to an insulator and a topological insulator respectively. The critical conductivity and correlation length exponent of these transitions are computed in a $N=1-\epsilon$ expansion in the number of replicas, where for small $\epsilon$ the critical points are perturbatively connected to the Kosterlitz Thouless critical point. Delocalized states, which arise at the surface of weak topological insulators and topological crystalline insulators, occur because vortex proliferation is forbidden due to the presence of symmetries that are violated by disorder, but are restored by disorder averaging. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A6.00004: Floquet Majorana Fermions for Topological Qubits D.E. Liu, A. Levchenko, H.U. Baranger We develop an approach to realizing a topological phase transition and non-Abelian statistics with dynamically induced Floquet Majorana Fermions (FMFs). When the periodic driving potential does not break fermion parity conservation, FMFs can encode quantum information. Quasi-energy analysis shows that a stable FMF zero mode and two other satellite modes exist in a wide parameter space with large quasi-energy gaps, which prevents transitions to other Floquet states under adiabatic driving. We also show that in the asymptotic limit FMFs preserve non-Abelian statistics and, thus, behave like their equilibrium counterparts. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A6.00005: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 9:00AM - 9:12AM |
A6.00006: Emergent Supersymmetry in Topological Superconductors Tarun Grover, Donna Sheng, Ashvin Vishwanath In the absence of interactions, topological superconductors (TSC) host helical Majorana fermion edge states protected by time reversal symmetry. Increasing interactions can lead to spontaneous magnetic order at the boundary. We show that the associated quantum phase transition, if continuous, has emergent space-time supersymmetry at low energies. The magnetic order parameter field is identified as the superpartner of the Majorana fermions. These results are obtained using field theoretical arguments and are verified by numerical DMRG solution of a lattice model that mimics the physics of the phase transition. The emergent supersymmetry, accessed by tuning a single parameter, has striking consequences such as an exact relation between the correlation functions of fermions and those of the order parameter. Similar results are argued to hold for the 2+1 dimensional boundary of a bulk topological superconductor. Generalization to topological insulator surfaces will be mentioned. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A6.00007: Dynamical manipulation of 2D topological insulator edge states for Majorana fermion braiding Shu-Ping Lee, Jason Alicea Edge states of 2D topological insulators such as HgTe provide a promising platform for realizing Majorana modes. Networks required for braiding Majoranas along the edge channels can be obtained by adjoining HgTe quantum wells to form corner junctions. Physically cutting quantum wells for this purpose, however, presents technical challenges. Here we propose a more accessible means of forming networks that relies on dynamically manipulating the location of edge states inside of a \emph{single} HgTe sheet. In particular, we show that edge states can effectively be dragged into the system's interior by gating a region near the edge into a metallic regime and then removing the resulting gapless carriers via proximity-induced superconductivity. This method allows one to construct rather general quasi-1D networks along which Majorana modes can be shuttled and exchanged by electrostatic means. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A6.00008: Topological Excitonic Superfluids in Three Dimensions Matthew Gilbert, Ewelina Hankiewicz, Youngseok Kim We study the equilibrium and non-equilibrium properties of topological dipolar intersurface exciton condensates within time-reversal invariant topological insulators in three spatial dimensions without a magnetic field. We elucidate that, in order to correctly identify the proper pairing symmetry within the condensate order parameter, the full three-dimensional Hamiltonian must be considered. As a corollary, we demonstrate that only particles with similar chirality play a significant role in condensate formation. Furthermore, we find that the intersurface exciton condensation is not suppressed by the interconnection of surfaces in three-dimensional topological insulators as the intersurface polarizability vanishes in the condensed phase. This eliminates the surface current flow leaving only intersurface current flow through the bulk. We conclude by illustrating how the excitonic superfluidity may be identified through an examination of the terminal currents above and below the condensate critical current. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A6.00009: Quantum interference and quantum oscillation on the surface of mirror symmetric topological insulators Chen Fang, Aris Alexandradinata, Matthew Gilbert, Su-Yang Xu, Zahid Hasan, Andrei Bernevig We first study the quasiparticle interference (QPI) of the surface states in crystalline topological insulators which possess mirror symmetry and time-reversal symmetry, by analyzing the Fourier transformed local density of states (FT-LDOS), $\rho(bq,\omega)$ around a single static impurity on the surface. The topological characters of the surface states of these new materials lead to QPI patterns distinct from those of 2D metals and of surface states on 3D time-reversal TI's. We apply the general analysis to the QPI on the $\langle{001}\rangle$-surface of Pb$_{1-x}$Sn$_x$Te and predict all vanishing singularities in $\rho(bq,\omega)$. We also demonstrate that QPI can also be used to probe the Lifshitz transition of the surface states observed in recent ARPES experiment. We next study the Shubnikov de Hass oscillation of these surface states and show that the oscillation has a single period before the Lifshitz transition and two distinct periods after the transition. Adding in-plane magnetic field before the Lifshitz transition leads to splitting of the period into two close periods and a beating thereof. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A6.00010: Chern and Majorana Modes in QuasiCrystals Indubala Satija, Gerardo Naumis The topology of quasicrystals is found to have a novel manifestation in the spatial profile of band edge states as topological invariants transform peaks into doublets of size equals the Chern number. The Chern-dressed peaks form a self-similar pattern encoding topological fingerprints at all length scales. For quasicrystals exhibiting localized states, fluctuations about exponentially localized zero modes describe the onset to topological transition where Majorana modes delocalize. These exotic modes can be captured in their entirety using $U(1)$ symmetry breaking perturbation that supports both the Chern and the Majorana modes. Here topological transition is accompanied by localization as edge-localized modes move to the interior, loosing topological protection. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A6.00011: Vortex zero modes, chiral anomaly and effective field theory for odd parity topological superconductor in three dimensional Dirac materials Bitan Roy, Pallab Goswami The low energy quasiparticle dispersion of various narrow gap and gapless semiconductors are respectively described by three dimensional massive and massless Dirac fermions. The three dimensional Dirac spinor structure admits an interesting time-reversal invariant, odd parity and Lorentz pseudo-scalar topological superconducting state. In this talk we demonstrate the existence of fermion zero energy states in the vortex core of this odd parity topological superconductor under generic conditions. Guided by the existence of the zero modes and its intimate connection with the chiral anomaly and the index theorem, we derive an effective topological field theory for such a superconducting state. We also discuss the experimental consequences of the zero modes and the topological field theory for the low temperature unconventional superconducting states of copper intercalated bismuth selenide, and indium doped tin telluride. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A6.00012: Ab initio study of topological insulating property in the heterojunction of Bi (111) bilayer and Bi$_{2}$Te$_{2}$Se Kyung-Hwan Jin, Seung-Hoon Jhi Study of topological insulator (TI) is showing rapid progress in both theory and experiment, particularly on three dimensional materials. Examples of two dimensional TI (quantum spin Hall) materials are, on the other hand, comparatively less common. As such, theoretical predictions of single Bi (111) bilayers as TI draw great attention from experiment. A recent report of ARPES measurements claims verification of the 2D TI property of Bi bilayer, but analysis leaves much room for further clarification. Because Bi (111) bilayers grown on 3D TI substrates such as Bi$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{2}$Se; understanding the details of the interface between the Bi bilayer and 3D TI substrates is essentially required. We study the electronic structures of Bi bilayer-Bi$_{2}$Te$_{2}$Se heterojunction from first-principles calculations. We find a substantial shift of Dirac cone from the TI substrates into Bi bilayer that thus becomes metallic on TI substrates. It is shown that the origin of this change is the inversion-symmetry breaking in Bi bilayer due to TI substrate. By comparing our calculations of Bi bilayer nanoribbons on Bi$_{2}$Te$_{2}$Se with STM/STS measurements, we successfully resolve and locate the edge states of a single Bi bilayer and confirm its 2D TI character. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A6.00013: Topological indices, defects, and Majorana fermions in chiral superconductors Daichi Asahi, Naoto Nagaosa We study theoretically the role of topological invariants to protect the Majorana fermions in a model of two-dimensional (2D) chiral superconductors which belong to class D of the topological periodic table. A rich phase diagram is revealed. Each phase is characterized by the topological invariants for 2D (Z) and 1D (Z2), which lead to the Majorana fermion at the edge dislocation and the core of the vortex. The topological invariants are determined by the hopping integrals along x and y directions. Interference of the Majorana fermions originating from the different topological invariants is studied. The interaction between zero-energy states at edge dislocations and at vortex cores eliminates the zero-energy states when they coexist at the same position. The stability of the Majorana fermion with respect to the interlayer coupling, i.e., in 3D, is also examined. We found that the zero-energy state survive a finite hopping integral along the z-direction unless the energy gap closes. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A6.00014: On the possibility of the fractional ac Josephson effect and doubled Shapiro steps in non-topological Josephson junctions Jay Sau, Erez Berg, Bertrand Halperin Topological superconductors supporting Majorana Fermions with non-abelian statistics are presently a subject of intense theoretical and experimental effort. It has been proposed that the observation of a half-frequency or a fractional Josephson effect is a more reliable test for topological superconductivity than the search for end zero modes. In fact, the fractional Josephson effect has been observed for the semiconductor nanowire system in the form of doubled Shapiro states. Here we consider the possibility of seeing such a fractional ac Josephson and doubled Shapiro steps from a superconducting nanowire in the non-topological phase. Using a semiclassical treatment we find that both the fractional ac Josephson effect and the doubled Shapiro step can, in principle, occur in the non-topological phase because of non-dynamical Landau Zener processes associated with the Andreev bound state spectrum of the junction. Therefore, while the observation of doubled Shapiro steps can be taken as indicative of a topological phase, it may not be a smoking gun signature for topological superconductivity and Majorana fermions. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A6.00015: Majorana Flat Bands and Uni-directional Majorana Edge States in Gapless Topological Superconductors Kam Tuen Law, Chris L.M. Wong, Jie Liu, Patrick A. Lee In this work, we show that an in-plane magnetic field can drive a fully gapped $p \pm i p$ topological superconductor into a gapless phase which supports Majorana flat bands (MFBs). Unlike previous examples, the MFBs in the gapless regime are protected from disorder by a chiral symmetry. In addition, novel uni-directional Majorana edge states (MESs) which propagate in the same direction on opposite edges appear when the chiral symmetry is broken by Rashba terms. Unlike the usual chiral or helical edge states, uni-directional MESs appear only in systems with a gapless bulk. We show that the MFBs and the uni-directional MESs induce nearly quantized zero bias conductance in tunneling experiments. [Preview Abstract] |
Session A7: Focus Session: Graphene Devices I
Sponsoring Units: DMPChair: K.C. Fong, Caltech
Room: 303
Monday, March 18, 2013 8:00AM - 8:12AM |
A7.00001: Graphene-based Hall Sensors for direct magnetic imaging by using Scanning Hall Probe Microscope Selda Sonusen, Seda Aksoy, Munir Dede, Ahmet Oral Graphene has been attracting great interest due to its unique electronic and mechanical properties for both fundamental and experimental studies since 2004. Graphene is a promising material for many applications in high speed electronic and spintronic devices as well as sensors. Its high mobility makes graphene a good candidate for magnetic imaging in Scanning Hall Probe Microscope (SHPM). Hall probes are used to scan the magnetic samples to image magnetic domains in SHPM. In this work, single layer graphene produced by chemical vapor deposition technique is used to fabricate Hall sensors by optical and the e-beam lithography with sizes from 500 nm to a few micrometers. The Hall crosses are characterized by Raman mapping to make sure that they are made of a single layer graphene. The Graphene Hall Sensors noise spectra is measured as a function of different bias currents and carrier concentrations at 300 K, 77 K and 4.24K. The imaging performance of the Hall sensor will be demonstrated at different temperatures by imaging a garnet crystal using a Low Temperature Scanning Hall Probe Microscope (LT-SHPM). [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A7.00002: Molecular Dynamics Studies of Graphene Nanobubbles Zenan Qi, Harold Park, Vitor M. Pereira, Antonio H. Castro-Neto, David K. Campbell We apply classical molecular dynamics to study pressure-induced deformations and the resulting pseudomagnetic (PSM) fields for monolayer graphene nanobubbles (NBs) of various geometries. We obtain the PSM field distributions for triangular, square, rectangular, hexagonal, and circular graphene NBs and find that in most cases the PSM fields near the tops of the NBs are smaller than around the NB edges. For circular NBs of diameter smaller than about 2nm, we find that the PSM field contribution from bending and curvature becomes comparable to from the traditional in-plane component of the gauge field. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A7.00003: Spatial manipulation of massless Dirac fermions in ballistic graphene devices Piranavan Kumaravadivel, Xu Du Pseudo-spin conservation of the chiral quasi-particles in graphene, governed by the Dirac-Weyl equation, has resulted in interesting study of transport phenomena such as their selective transmission across potential barriers. Utilizing these properties to spatially manipulate the electrons in graphene devices require ballistic samples with well-defined, sharp junctions. We report our current work on the fabrication and characterization of such ballistic devices that will enable us to guide and control electron flow in 2D. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A7.00004: Graphene as an etch mask for silicon Aniruddh Rangarajan, Joshua Wood, Justin Koepke, Joseph Lyding We are using graphene as a hard etch mask for silicon. The error introduced by its edges is hypothesized to be far less compared to innate issues of photolithography (e.g. undercut, sidewall hardening). This presents the possibility of making a highly precise etch mask. We lithographically pattern a graphene layer transferred to a Si(100) surface and fluorinate the sample to demonstrate the selective etching on exposed regions. The graphene layer becomes fluorinated, but shields the silicon underneath. The Si(100) with selective graphene coating was subjected to isotropic etching by xenon difluoride (at 1.0 Torr, and N$_{2}$ at 35.0 Torr) for 180 s to remove approximately 190 nm of silicon. Raman spectroscopy confirms the onset of sp$^{3}$ hybridization of carbon atoms in the hexagonal lattice, brought on by covalent C--F bonding. Along with the possibility of producing highly precise silicon structures, the monolayer mask has added advantages, such as not requiring as many processing steps as the conventional method involving photoresist. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A7.00005: Low-Damage Sputter Deposition on Graphene Ching-Tzu Chen, Emanuele Casu, Marcin Gajek, Simone Raoux Despite its versatility and prevalence in the microelectronics industry, sputter deposition has seen very limited applications for graphene-based electronics. We have systematically investigated the sputtering induced graphene defects and identified the reflected high-energy neutrals of the sputtering gas as the primary cause of damage. In this talk, we introduce a novel sputtering technique that is shown to dramatically reduce bombardment of the fast neutrals and improve the structural integrity of the underlying graphene layer. We also demonstrate that sputter deposition and in-situ oxidation of 1 nm Al film at elevated temperatures yields homogeneous, fully covered oxide films with r.m.s. roughness much less than 1 monolayer, which shows the potential of using such technique for gate oxides, tunnel barriers, and multilayer fabrication in a wide range of graphene devices. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A7.00006: Enhanced Breakdown Reliability and Spatial Uniformity of Atomic Layer Deposited High-k Gate Dielectrics on Graphene via Organic Seeding Layers Vinod Sangwan, Deep Jariwala, Stephen Filippone, Hunter Karmel, James Johns, Justice Alaboson, Tobin Marks, Lincoln Lauhon, Mark Hersam Ultra-thin high-$\kappa $ top-gate dielectrics are essential for high-speed graphene-based nanoelectronic circuits. Motivated by the need for high reliability and spatial uniformity, we report here the first statistical analysis of the breakdown characteristics of dielectrics grown on graphene. Based on these measurements, a rational approach is devised that simultaneously optimizes the gate capacitance and the key parameters of large-area uniformity and dielectric strength. In particular, vertically heterogeneous oxide stacks grown \textit{via} atomic-layer deposition (ALD) seeded by a molecularly thin perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) organic monolayer result in improved reliability (Weibull shape parameter $\beta $ \textgreater\ 25) compared to the control dielectric directly grown on graphene without PTCDA ($\beta $ \textless\ 1). The optimized sample also showed a large breakdown strength (Weibull scale parameter, E$_{\mathrm{BD}}$ \textgreater\ 7 MV/cm) that is comparable to that of the control dielectric grown on Si substrates. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A7.00007: Gate-defined Quantum Confinement in Suspended Bilayer Graphene Invited Speaker: Monica Allen Quantum confined devices in carbon-based materials offer unique possibilities for applications ranging from quantum computation to sensing. In particular, nanostructured carbon is a promising candidate for spin-based quantum computation due to the ability to suppress hyperfine coupling to nuclear spins, a dominant source of spin decoherence. Yet graphene lacks an intrinsic bandgap, which poses a serious challenge for the creation of such devices. We present a novel approach to quantum confinement utilizing tunnel barriers defined by local electric fields that break sublattice symmetry in suspended bilayer graphene. This technique electrostatically confines charges via band structure control, thereby eliminating the edge and substrate disorder that hinders on-chip etched nanostructures to date. We report clean single electron tunneling through gate-defined quantum dots in two regimes: at zero magnetic field using the energy gap induced by a perpendicular electric field and at finite magnetic fields using Landau level confinement. The observed Coulomb blockade periodicity agrees with electrostatic simulations based on local top-gate geometry, a direct demonstration of local control over the band structure of graphene. This technology integrates quantum confinement with pristine device quality and access to vibrational modes, enabling wide applications from electromechanical sensors to quantum bits. More broadly, the ability to externally tailor the graphene bandgap over nanometer scales opens a new unexplored avenue for creating quantum devices. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A7.00008: Band gap estimation in bilayer graphene through quantum capacitance measurement Kosuke Nagashio, Tomonori Nishimura, Akira Toriumi The estimation of the quantum capacitance ($C_{Q})$ through the capacitance measurement provides the direct information on Density of states (\textit{DOS}) in graphene since the energy cost to induce carriers is introduced as $C_{Q}=e^{2}$\textit{DOS} in series with the geometrical capacitance ($C_{ox})$ in the equivalent circuit (1/$C \quad =$ 1/$C_{ox} \quad +$ 1/$C_{Q})$. For bilayer graphene with Y$_{\mathrm{2}}$O$_{\mathrm{3}}$ topgate structure, the band gap opening was qualitatively observed in DOS - energy relation estimated from $C_{Q}$ under the large displacement. The band gap determined by C$_{\mathrm{Q}}$ was larger than the transport gap determined by variable-range hopping in gap states on IV measurement since carriers which respond to the alternating voltage are not required to transport throughout the device. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A7.00009: Local Spectroscopy of the Electrically Tunable Band Gap in Trilayer Graphene Matthew Yankowitz, Fenglin Wang, Chun Ning Lau, Brian LeRoy Trilayer graphene exhibits two natural stacking orders (Bernal and rhombohedral), and the electronic properties differ substantially between the two. While Bernal-stacked trilayer graphene is a semimetal with an electrically tunable band overlap, rhombohedrally stacked trilayer graphene has an electrically tunable band gap. We have performed low-temperature ultra-high vacuum STM measurements of both stacking orders. In Bernal-stacked trilayer graphene, we observe metallic behavior for all energies and electric fields probed. In rhombohedrally stacked trilayer graphene, we measure an electric field tunable band gap whose magnitude is well described by theoretical predictions. Furthermore, we explore the microscopic nature of the band gap by probing spatial variations throughout the sample. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A7.00010: Dual-dated suspended double-layer graphene devices Fenglin Wang, Jhao-Wun Huang, Yongjin Lee, Lei Jing, Kevin Myhro, Jairo Velasco Jr., Hang Zhang, Chunning Lau Using a transfer technique, we are able to align two graphene sheets together and fabricate a dual-gated suspended double-layer system via a delicate multi-step fabrication process. This experimental system includes variety of parameters, which provides potential for the discovery of new physics, such as the coupling and decoupling between two graphene sheets. We will present and discuss latest experimental results. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A7.00011: Electron-state engineering of bilayer graphene by sandwiching ionic molecules Nguyen Thanh Cuong, Minoru Otani, Susumu Okada Graphene has stimulated intense interest not only in the field of the low-dimensional sciences but also in the electronic device engineering because of its unique structural and electronic properties. In particular, they are regarded as a candidate material for the next-generation semiconductor devices. However, graphene is a metal with a pair of liner dispersion bands at the Fermi level, so that they are not utilized for the logic circuit. Therefore, it is important to tune the electronic structure and to get a semiconducting graphene. In this work, we demonstrate the possibility of controlling the band-gap and carrier type of bilayer graphene by using ionic molecules based on the first-principles total-energy calculations. Our calculations suggest that bilayer graphene sandwiched by a pair of cation and anion molecules is a semiconductor with a moderate energy gap of 0.26 eV that is attributable to the strong local dipole field induced by the ionic molecules. Furthermore, we also show that the carrier type of semiconducting bilayer graphene is controllable,i.e. intrinsic, p-type, or n-type semiconductors, by tuning anion-cation pair. [1]. \\[4pt] [1] N. T. Cuong, M. Otani, and S. Okada, Appl. Phys. Lett. (2012), in press. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A7.00012: Towards \emph{in situ} correlation of atomic structure and device functionality in graphene-based devices S. M. Hollen, Nancy Santagata, Justin Young, Jay Gupta, Ezekiel Johnston-Halperin The use of scanned probe microscopy to study atomic-scale phenomena is well established; however, there is a gap in our understanding of how atomistic studies connect to macroscopic measurements such as electron and spin transport. This gap is of particular importance to 2D materials such as graphene, germanane and MoS$_2$ due to the extreme sensitivity to defects, adatoms, and local electronic inhomogeneities. We present work towards the integration of low temperature scanning tunneling microscopy (STM) with \emph{in situ} transport measurements on nanoscale graphene devices in ultrahigh vacuum. Challenges we are addressing include (i) fabricating devices small enough that a sufficient fraction of the surface can be modified within the small scan range typical of STM, (ii) design of alignment electrodes for locating the devices with STM, and (iii) modification of the STM hardware to integrate electron transport measurements. We anticipate that being able to correlate device transport measurements with atomic scale characterization and modification of the graphene surface will allow us to address the importance of the local environment to device functionality. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A7.00013: In-situ Fabrication and Electronic Characterization of Junction-confined Single Layer Graphene Nanoribbons Zhengqing John Qi, Julio Rodriguez-Manzo, Sung Ju Hong, Marija Drndic, A.T. Charlie Johnson We report electronic measurements on high quality single layer junction-confined graphene nanoribbons fabricated in a transmission electron microscope (TEM). In this work, a process is demonstrated for the fabrication and confirmation of pristine single layer graphene nanoribbons using high vacuum current annealing and precision nano-sculpting, both conducted within the vacuum chamber of a TEM. Briefly, CVD-grown graphene is patterned into a freely-suspended nanoribbon connected to large area contacts. The sample is then mounted on a TEM holder with electrical feedthroughs to allow for simultaneous imaging and in-situ electrical transport measurements within the TEM. A focused electron beam is used to progressively narrow the ribbon, providing a platform to controllably sculpt and define the device geometry while characterizing its electrical properties. In-situ electrical measurements and TEM imaging with sub-nm resolution revealed the dependence of the nanoribbon resistance as a function of width in the range 17 -- 280 nm. Monolayer graphene were found to sustain current densities in excess of 5 x 10$^9$ A/cm$^2$, orders of magnitude higher than copper while the conductance varied approximately as w$^{0.75}$, where w is the ribbon width in nanometers. These results demonstrates graphene's potential as a next generation, high performance interconnects material with the ability to reach single-digit technology nodes at the level of a single atomic layer. [Preview Abstract] |
Session A8: Graphene: Quantum Hall Effect
Sponsoring Units: DCMPChair: Herb Fertig, Indiana University
Room: 307
Monday, March 18, 2013 8:00AM - 8:12AM |
A8.00001: Four-flux fractional quantum Hall states in suspended graphene Andrei Levin, Benjamin Feldman, Benjamin Krauss, Jurgen Smet, Amir Yacoby The interactions between charge carriers in ultra-clean graphene subject to a perpendicular magnetic field can drive the system to condense into one of a set of incompressible fractional quantum Hall (FQH) states. We use a scanning single-electron transistor to measure the local electronic compressibility of suspended graphene. In addition to observing incompressible behavior at fractional filling factors in the two-flux composite fermion sequence, we also observe FQH states arising from four-flux composite fermions, including states at filling factors $\nu =$ 1/5, 2/7, 2/9, 3/11, 5/7 and 6/5. We measure the energy gaps of these states as a function of magnetic field; most display approximately linear scaling. Interestingly, several four-flux FQH states are conspicuously absent near filling factors $\nu =$ 1 and 2, despite the robust appearance of their counterparts near $\nu =$ 0. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A8.00002: Measurements of Chiral Heat Current in Graphene in the Quantum Hall Regime Seung-Geol Nam, E.H. Hwang, Hu-Jong Lee Heat transport measurements can offer a new window to probe the low-energy physics in quantum-Hall systems, which cannot be provided by the electronic transport measurements. In this presentation, we report chiral heat transport measurements in monolayer graphene in the integer quantum Hall regime. We inject charge carriers at a higher temperature than the system bulk and measure the thermoelectric voltage corresponding to the local electron temperature at a distance from the injection point. We find that in graphene heat transport at the edge in the quantum Hall regime is chiral and its direction is dependent on both the carrier type and the magnetic field direction. Measured thermoelectric signals in unipolar regions can be understood by the Mott relation, but a severe deviation of the signals from the Mott relation is found at a p-n junction. Thermoelectric signal decays with distance from the heater and saturates with increasing heating power even though it increases linearly at low powers, which indicates that a part of heat is transferred out of the edge current. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A8.00003: Drude weight, cyclotron resonance, and the Dicke model of graphene cavity QED Marco Polini, Luca Chirolli, Vittorio Giovannetti, Allan MacDonald The unique optoelectronic properties of graphene make this two-dimensional (2D) material an ideal platform for fundamental studies of cavity quantum electrodynamics (QED) in the strong-coupling regime. The celebrated Dicke model of cavity QED can be approximately realized in this material when the cyclotron transition of its 2D massless Dirac fermion carriers is nearly resonant with a cavity photon mode. In this talk I will discuss the theory of strong matter-photon coupling in this circumstance, emphasizing the essential role of a dynamically generated matter energy term that is quadratic in the photon field and absent in graphene's low-energy Dirac model. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A8.00004: Theory of unconventional quantum Hall effect in strained graphene Zi-Xiang Hu, Bitan Roy, Kun Yang We show graphene discerns an unconventional sequence of quantized Hall conductivity, when subject to both magnetic fields (B) and strain through both theoretical arguments and numerical calculations. The strain produces time-reversal symmetric pseudo/axial magnetic fields (b). The single electron spectrum is composed of two inter-penetrating sets of Landau levels (LLs), located at $\pm\sqrt{2n|b \pm B|}$, n = 0, 1, 2,.... For $b > B$, these two sets of LLs have opposite chiralities, resulting in oscillating Hall conductivity between 0 and $\mp 2e^2/h$ in electron and hole doped system, respectively, as the chemical potential deviates from the neutrality point, but remains in its vicinity. The electron electron interactions stabilizes various correlated ground states, e.g., spin-polarized, quantum spin-Hall insulators at and near the neutrality point, and possibly anomalous Hall insulating phase at incommensurate filling. Such broken symmetry ground states have similarities as well as significant differences from there counterparts in the absence of strain. For realistic strength of magnetic fields and interactions, we present scaling of interaction induced gap for various Hall states within the zeroth Landau level. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A8.00005: Valley-kink in Bilayer Graphene at $\nu =$0: A Charge Density Signature for Quantum Hall Ferromagnetism Chia-Wei Huang, Efrat Shimshoni, Herbert Fertig We investigate the interaction-induced valley textured domain walls in bilayer graphene at the $\nu =$0 quantum Hall state, subject to a kink-like perpendicular electric field. Such a state can be realized in a double-gated suspended sample, where the electric field changes sign across a line in the middle of the sample. Using the Hartree-Fock approximation, we find that the Coulomb interaction opens a gap between the two lowest-lying states near the Fermi level, and yields a smooth valley texture throughout the domain walls. Moreover, our results suggest possibilities to visualize the resulting texture via measuring the charge density difference between the two graphene layers, which is predicted to exhibit a charge density wave. The width of the smooth texture and the resulting pattern can be tuned by the interplay between the magnetic field and gate electric fields. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A8.00006: Cyclotron-resonance-induced photovoltaic effect in high-mobility graphene in the quantum Hall regime Satoru Masubuchi, Masahiro Onuki, Miho Arai, Kenji Watanabe, Takashi Taniguchi, Tomoki Machida We have investigated the infrared photoinduced voltage $\Delta V$ in high-mobility graphene on hexagonal boron nitride in the quantum Hall regime. We observed $\Delta V$ of up to several $\mu$V at $\nu=\pm 2$ quantum Hall states under the cyclotron resonance conditions. The dependence of $\Delta V$ on the bias current indicates that $\Delta V$ signals derive from the photovoltaic effect rather than the bolometric effect. The dependence of $\Delta V$ on magnetic field direction and measurement geometry suggest the edge channel transport as an origin of photovoltaic effect. $\Delta V$ signals were robust up to $T=180$ K, indicating that $\Delta V$ signals can be used for developing novel terahertz photodetectors operating at high temperatures. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A8.00007: Landau-level mixing in the fractional quantum Hall effect in graphene Michael Peterson, Chetan Nayak We study the effects of Landau level mixing on the fractional quantum Hall effect in graphene. Landau level mixing in graphene is especially important since the ratio of the Coulomb energy to the cyclotron energy is independent of magnetic field and of order one. In particular, we derive an effective Hamiltonian that fully incorporates Landau level mixing by renormalizing the two-body Coulomb potential (renormalizing the Haldane pseudopotentials) and inducing particle-hole symmetry breaking three-body terms, cf. Bishara and Nayak, Phys. Rev. B 80, 121302(R) (2009). As opposed to the FQHE in GaAs semiconductor devices, graphene has no finite-thickness corrections since the two-dimensional graphene sheet is atomically thin and the Dirac nature of the electrons in graphene forces the particle-hole symmetry breaking three-body terms to exactly vanish in the lowest Landau level. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A8.00008: Self-similar occurrence of massless Dirac particles in graphene under magnetic field Jun-Won Rhim, Kwon Park Intricate interplay between the periodicity of the lattice structure and that of the cyclotron motion gives rise to a well-known self-similar fractal structure of the Hofstadter butterfly for an electron moving in lattice under magnetic field. Evolving from the $n=0$ Landau level, the central band of the Hofstadter butterfly is especially interesting since it may hold a key to the mysteries of the fractional quantum Hall effect in graphene. In this paper, we develop an effective Hamiltonian method that can be used to provide an accurate analytic description of the central Hofstadter band in the weak-field regime. One of the most important discoveries obtained in this work is that massless Dirac particles always exist inside the central Hofstadter band no matter how small the magnetic flux may become. In other words, with its bandwidth broadened by the lattice effect, the $n=0$ Landau level contains massless Dirac particles within itself. In fact, by carefully analyzing the self-similar recursive pattern of the central Hofstadter band, we conclude that massless Dirac particles should occur under arbitrary magnetic field. As a corollary, the central Hofstadter band also contains a self-similar structure of recursive Landau levels associated with such massless Dirac particles. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A8.00009: Observation of the Hofstadter butterfly in graphene on boron nitride Patrick Maher, Cory Dean, Carlos Forsythe, Lei Wang, Fereshte Ghahari, Pilkyung Moon, Mikito Koshino, Kenji Watanabe, Takashi Taniguchi, Ken Shepard, James Hone, Philip Kim In 1976, Douglas Hofstadter considered the general problem of 2D electrons subject to both a magnetic field and a periodic potential. His solution predicted a remarkably complex energy spectrum exhibiting self-similar fractal structure, termed the Hofstadter Butterfly. Experimental exploration of this problem has been limited by the difficulty of fabricating a system with a lattice constant on the order of the magnetic length. It has recently been shown that single layer graphene on hexagonal-BN develops a Moir\'{e} pattern with a length of up to 15 nm when the rotational angle between the two lattices approaches zero. We present data demonstrating that for bilayer graphene on hexagonal boron nitride, the effect of the modulation potential associated with the Moir\'{e} pattern is large enough to be observable by standard transport. Under large magnetic fields, additional gaps appear within the usual bilayer quantum Hall spectrum, consistent with calculations of the Hofstadter spectrum. We present the first direct experimental evidence of the longstanding theoretical prediction that the gaps arising from the superlattice are characterized by two integer quantum numbers. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A8.00010: Symmetry Breaking in Hofstadter's Butterfly in graphene Carlos Forsythe, Cory Dean, Lei Wang, Patrick Maher, Fereshte Ghahari, Pilkyung Moon, Mikito Koshino, Takashi Taniguchi, Kenji Watanabe, Ken Shepard, Jim Hone, Philip Kim We will present magnetotransport measurements in hBN encapsulated bilayer graphene devices where one of hBN substrates provides a weak modulation of lattice potential. Under a strong magnetic field, interplay between periodic electric potential and quantizing magnetic field lead to a fractal energy spectrum known as Hofstadter's butterfly. In graphene, while spin and layer symmetry breakings are expected in dual gated devices under large magnetic fields, valley symmetry breaking in the Hofstadter regime is not so easily understood. We will present the observance of these measured gaps along with a discussion of symmetry breaking in our BLG-hBN devices. Further quantitative analysis of these breakings will be presented through the temperature dependence of quantized conductance at these gaps. Through careful modulation of temperature and electron density, we have extracted a range of activation energies associated with symmetry breakings. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A8.00011: Andreev Reflections and Superconducting Proximity Effect in lateral BN/Graphene/NbSe$_{2}$ Heterostructures in the Integer Quantum Hall Regime Dmitri K. Efetov, Clevin Handschin, Cory Dean, Lei Wang, Philip Kim Inducing Superconductivity (SC) via proximity effect into the topological edge states of a 2D conductor in the Quantum Hall Regime (QHE) has been a long standing proposition which has recently reinvigorated attention. Here the combination of SC and QHE has a wide range of predictions such as the appearance of additional edge-states in the integer QHE. With the recent development of high mobility graphene on h-BN with an extremely low onset of the QHE (0.5T) and its high compatibility with various superconductors the road to test these predictions is now open. In this study we present lateral magneto-transport and electronic spectroscopy measurements of BN/graphene/NbSe2 heterostructures. We find that the NbSe2/graphene superconductor-normal metal interface (SN) has a very high transparency with extremely low electrical resistances of R$\sim$100Ohm and gives rise to Andreev reflections and a strong SC proximity effect in graphene below the critical SC transition temperature Tc $\sim$ 7.2K. The high mobility of the graphene on h-BN and the relatively high SC upper critical magnetic field of NbSe2 Hc2 $\sim$ 5T allow for a wide magnetic field range of 1-5T in which the SC and the QHE coexist. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A8.00012: Edge magnetoplasmons in graphene: determination of carrier drift velocity in Quantum Hall regime Ivana Petkovic, F.I.B. Williams, Keyan Bennaceur, Fabien Portier, Patrice Roche, D.C. Glattli Edge Magneto-Plasmons (EMP) are gapless quasi 1D elementary excitations which are split off from the bulk magneto-plasmon modes by the sample boundary, and are a tool of choice to investigate the structure of the edge of a 2D electron gas. We give a first experimental demonstration of their presence in graphene in the quantum Hall regime and use our results to evaluate the carrier drift velocity along the edge [1]. The group velocity of these modes is a sum of the Hall conductivity contribution and the carrier drift velocity at the edge. In graphene, due to its particular dynamics and an abrupt edge, the drift velocity is expected to be of the order of the Fermi velocity, thus becoming experimentally accessible. We show EMP to exist by timing the travel of narrow wave-packets on picosecond time scales around exfoliated samples. They show chiral propagation with low attenuation at a velocity which is quantized on Hall plateaus. We extract the carrier drift contribution and find it to be slightly less than the Fermi velocity, as expected for an abrupt edge. We also extract the spatial spread of edge accumulated charge and find it to be narrower than for soft edge systems.\\[4pt] [1] I. Petkovic, F.I.B. Williams, K. Bennaceur, F. Portier, P. Roche and D.C. Glattli, Phys. Rev. Lett.(2012). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A8.00013: Quantum spin Hall effect in the graphene zero energy Landau level - Part I Andrea F. Young, Javier D. Sanchez-Yamagishi, Ben Hunt, Pablo Jarillo-Herrero, Ray C. Ashoori, Takashi Taniguchi, Kenji Watanabe Shortly after the experimental discovery of graphene, it was predicted that Zeeman splitting of the graphene zero energy Landau level results in a quantum spin Hall phase, characterized by counterpropagating spin-filtered edge states. However, experimental realization of this state has been obscured by the existence of competing Coulomb interaction-driven insulating phases. We address this problem by fabricating monolayer graphene devices in which the Coulomb interaction is heavily screened by a proximal graphite gate. Despite the reduction in the strength of intralayer interactions, the resulting high mobility samples show all the usual signatures of Coulomb-driven symmetry breaking in high magnetic fields, with a strong insulating state developing at charge neutrality at fields of $\sim$1 Tesla. Unlike in conventional samples, however, we observe a continuous transition from this insulating state to a conducting state of order e$^2$/h as a function of in-plane field. Simultaneous high-sensitivity capacitance measurements reveal that the sample bulk remains gapped throughout the transition. The observation of finite conduction in the presence of a bulk insulator strongly implies that transport occurs via the edge states characteristic of the quantum spin Hall state. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A8.00014: Quantum spin Hall effect in the graphene zero energy Landau level - Part II Javier D. Sanchez-Yamagishi, Andrea F. Young, Benjamin Hunt, Kenji Watanabe, Takashi Taniguchi, Ray C. Ashoori, Pablo Jarillo-Herrero Zeeman splitting of graphene's zeroth Landau level has been predicted to lead to a quantum spin Hall effect, but a competing interaction-driven insulating state has hampered previous attempts to drive the graphene into this regime. By using a proximal graphite gate to screen Coulomb interactions in the graphene, we are able to reduce the strength of this competing insulating state and observe a continuous transition to a conductive state as a function of in-plane field. We study this transition simultaneously in capacitance and transport, and find that despite conduction increasing by many orders of magnitude with in-plane field the bulk remains gapped throughout the transition. These observations indicate the continuous closing of a transport gap along the edge of the sample, with resulting counter-propagating edge states that are characteristic of the quantum spin Hall effect. We discuss the behavior of this transition across multiple samples with various levels of Coulomb screening, and present nonlocal multiterminal transport measurements designed to probe the nature of backscattering within the edge states. We also comment on the implications of our work for the rest of the graphene phase diagram at high magnetic fields. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A8.00015: Role of the charge inhomogeneity on the breakdown of the quantum Hall effect in narrow single layer graphene devices Cenk Yanik, Ismet Kaya The breakdown of the quantum Hall effect, which is observed as an abrupt escalation in the longitudinal resistance with an associated loss in the quantization of Hall voltage is the major obstacle against improving the resistance standard which is currently based on this effect. Graphene is inherently a 2D material and has an unusual band structure that allows the quantization of the Hall resistance even at room temperature. These unique properties of graphene make it a good candidate as a high precision metrological characterization tool for the quantum Hall resistance. The uncertainty in the quantum Hall resistance in graphene has been rapidly improving recently and graphene samples have already been shown to reach the precision of the current best 2DEG samples. In this talk, experimental results on the breakdown of the quantum Hall effect in graphene on SiOx is presented. In narrow graphene samples of 1 micrometer width, the charge inhomogeneity is quite prominent and strongly affects the nondissipative transport in the quantum Hall regime. It is observed that in such samples the quantization of the Hall resistance can retain at high current densities in the excess of 1 A/m even in the presence of dissipative potential along the longitudinal probes. [Preview Abstract] |
Session A9: Invited Session: Teaching Physics and Other STEM Subjects in an Urban Environment
Sponsoring Units: FEdChair: Mel Sabella, Chicago State University
Room: 308
Monday, March 18, 2013 8:00AM - 8:36AM |
A9.00001: Away from the ivory tower: Real challenges teaching high school physics in an urban environment Invited Speaker: Richard Steinberg For more than 20 years, I have been a physicist and a science educator, primarily at the college level. My research is on understanding and improving the learning of science, from elementary school science through quantum physics. Since 1999 I have been Professor in the School of Education and the Department of Physics and Program Director of Science Education at City College of New York. In that time I have had the privilege of working with hundreds of K-12 students, with over a thousand science teachers in and around New York City, and with even more college science students who are graduates of the city school system. To improve my ability to work with all these groups, I spent my sabbatical as a full time high school physics teacher in a public high school in New York City. For me, it was where the rubber meets the road. In this presentation, I will share experiences as an instructor and researcher from the perspectives of college physics instructor, science teacher educator, and high school teacher. With few exceptions, teachers are taught physics one way, are taught to teach it another, are put in a system where neither approach works, and have their students assessed in a way that promotes instructional strategies at odds with how students learn. I will share both challenges I encountered and what I learned about what works in this environment. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A9.00002: Universities Reaching Outwards: Science Education Partnerships with Urban School Systems Invited Speaker: Cody Sandifer The goals of this talk are to: (1) describe how universities, physics departments, and individual faculty can partner with urban school systems to benefit K-16 students, teacher education programs, and university instructors, (2) summarize research on effective university-school system education partnerships, and (3) offer advice and share lessons learned so that university partners can avoid common pitfalls and maximize the potential for collaborative success. Possible areas of university-school collaboration include resident teachers, curricular review, early teaching experiences, demo sharing sessions, ongoing professional development, on- and off-campus science outreach, RET programs, science education resource centers, and others. University-school educational partnerships offer numerous benefits but can be challenging to implement and maintain. Research shows that most successful partnerships possess the following characteristics: mutual self-interest, participant commitment, mutual trust and respect, shared decision-making, information sharing, and ongoing evaluation. K-16 course and curriculum redesign is a specific issue that has its own unique set of contextual factors that impact the project's chance at success, including available materials, administrative support, formative assessments, pilot-testing and instructor feedback, and ongoing professional development. I have learned a number of lessons in own science education collaborations with the Baltimore City Public School System, which is an urban school system with 200 schools, 84,000 students, and 10,700 teachers and administrators. These lessons pertain to: communication, administrative power, and the structure of the school system; relevant contextual factors in the university and K-12 schools; and good old-fashioned common sense.\footnote{Common sense is encouraged, but not required, to attend the invited talk.} Specific advice on K-16 science education partnerships will be provided to help universities increase student and instructor satisfaction with their physics and teacher education programs, maintain a positive and mutually beneficial relationship with local schools, and improve science education at all levels of instruction. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A9.00003: Meeting Urban Science Students Where They Are: Perspectives from Two Physics Teachers and Four Schools Invited Speaker: Rosalind Echols The phrase `urban education' tends to be used in ways that suggest we see urban education (and urban students) as a monolithic construct. Often, `urban' indexes children of color, with low levels of academic readiness from low socio-economic status communities in crowded, under-resourced classrooms taught by poorly prepared and/or poorly motivated teachers. While teachers and students in urban schools do face challenges that those in more suburban or rural areas may not, we argue that the differences across urban school contexts, even within the same city, outweigh the similarities. Furthermore, these differences have profound implications for the kind of work urban science teachers must do and the support they need from the science and science education research communities. In this talk, two high school physics teachers with experience in four radically different urban teaching contexts discuss the differences across schools that shape their teaching practice and their students' learning. Against this backdrop, we'll address the most common `misconceptions' about inquiry science teaching in urban schools that we've encountered among scientists, science education researchers and teacher educators. The presentation will conclude with our synthesis of how scientists and science education researchers can best support urban science teachers and students. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A9.00004: Engineering Education in K-12 Schools Invited Speaker: Anne Spence Engineers rely on physicists as well as other scientists and mathematicians to explain the world in which we live. Engineers take this knowledge of the world and use it to create the world that never was. The teaching of physics and other sciences as well as mathematics is critical to maintaining our national workforce. Science and mathematics education are inherently different, however, from engineering education. Engineering educators seek to enable students to develop the habits of mind critical for innovation. Through understanding of the engineering design process and how it differs from the scientific method, students can apply problem and project based learning to solve the challenges facing society today. In this talk, I will discuss the elements critical to a solid K-12 engineering education that integrates science and mathematics to solve challenges throughout the world. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A9.00005: Preparing teachers for ambitious \textit{and}Â culturally responsive science teaching Invited Speaker: Gale Seiler Communities, schools and classrooms across North America are becoming more ethnically, racially, and linguistically diverse, particularly in urban areas. Against this backdrop, underrepresentation of certain groups in science continues. Much attention has been devoted to multicultural education and the preparation of teachers for student diversity. In science education, much research has focused on classrooms as cultural spaces and the need for teachers to value and build upon students' everyday science knowledge and ways of sense-making. However it remains unclear how best to prepare science teachers for this kind of culturally responsive teaching. In attempting to envision how to prepare science teachers with cross-cultural competency, we can draw from a parallel line of research on preparing teachers for \textit{ambitious science instruction}. In ambitious science instruction, students solve authentic problems and generate evidence and models to develop explanations of scientific phenomenon, an approach that necessitates great attention to students' thinking and sense-making, thus making it applicable to cultural relevance aims. In addition, this line of research on teacher preparation has developed specific tools and engages teachers in cycles of reflection and rehearsal as they develop instructional skills. While not addressing cross-cultural teaching specifically, this research provides insights into specific ways through which to prepare teachers for culturally responsive practices. In my presentation, I will report on efforts to join these two areas of research, that is, to combine ideas about multicultural science teacher preparation with what has been learned about how to develop ambitious science instruction. This research suggests a new model for urban science teacher preparation---one that focuses on developing specific teaching practices that elicit and build on student thinking, and doing so through cycles of individual and collective planning, rehearsal, review, and reflection. In this way, a defined set of science-specific, ambitious \textit{and} culturally responsive instructional practices can be articulated and taught during science teacher preparation. [Preview Abstract] |
Session A10: Invited Session: Hard and Soft Materials Modeling, Simulations and Big Data
Sponsoring Units: DCOMPChair: Priya Vashishta, University of Southern California
Room: 309
Monday, March 18, 2013 8:00AM - 8:36AM |
A10.00001: First-principles modeling of hard and soft matter Invited Speaker: Roberto Car Electronic and atomistic processes are key to bio-inspired functional materials and nanocatalysts for energy applications. This talk will review recent simulation studies and discuss the challenges that first-principles quantum mechanical approaches face when addressing these issues. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A10.00002: First Principles based methods and applications for realistic simulations on complex soft materials to develop new materials for energy, health, and environmental sustainability Invited Speaker: William Goddard For soft materials applications it is essential to obtain accurate descriptions of the weak (London dispersion, electrostatic) interactions between nonbond units, to include interactions with and stabilization by solvent, and to obtain accurate free energies and entropic changes during chemical, physical, and thermal processing. We will describe some of the advances being made in first principles based methods for treating soft materials with applications selected from new organic electrodes and electrolytes for batteries and fuel cells, forward osmosis for water cleanup, extended matter stable at ambient conditions, and drugs for modulating activation of GCPR membrane proteins, [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A10.00003: Nanoparticles in Complex Fluids, at Interfaces, in Polymers: Topology Matters Invited Speaker: Gary S. Grest One versatile way to control the assembly and integration of nanoparticles is to tether organic molecules with specific functionalized groups to their surface. The tethers modify both inter particle interactions and their interaction with their surroundings, without disrupting the nanoparticles' unique properties. While it is often assumed that uniformly coating spherical nanoparticles with short organic ligands will lead to symmetric hybrids, using explicit-atom molecular dynamics simulations of model nanoparticles, we discovered that the hybrids exhibit a large variety of non-symmetric coatings, driving new pathways to control assemblies. These configurations of the coating stem from the high curvatures of small particles and comparable size tethers. In solution geometric factors dictate the symmetry of the hybrid and its stability, where the chain end-group coding and the solvent play only a secondary role. At water-vapor interface the anisotropic nanoparticle coatings seen in bulk solvents are enhanced. The coatings become significantly asymmetric and assume distinctive orientation with respect to the liquid interface. The asymmetry and degree of orientation depend strongly on the free volume provided by the geometry and the end group, as well as the solvent properties. At an interface asymmetric hybrids align with the surface to minimize free energy. These asymmetric coatings and oriented hybrids are expected to drive new self-assemblies symmetries in the bulk and at surfaces. 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] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A10.00004: Translocation of Small Interfering RNA and Cholesterol Molecules in Biomembranes Invited Speaker: Rajiv Kalia This presentation will focus on all-atom molecular dynamics (MD) simulation studies of (1) structural and mechanical barriers to translocation of small interfering RNA (siRNA) across a phospholipid bilayer, and (2) flip-flop dynamics of cholesterol (CHOL) molecules across a phospholipid bilayer. In the first case, we find that the siRNA induces a liquid-to-gel phase transformation. In the gel phase we find large compressive lateral stresses in the hydrocarbon chains of lipid molecules, which present a considerable barrier to siRNA passage across the bilayer. In the second case, we study spontaneous CHOL inter-leaflet transport (flip-flop), the effect of this process on mechanical stresses across the bilayer, and the role of CHOL in inducing molecular order in bilayer leaflets. The simulation was run for 15 microseconds and we found 24 CHOL flip-flop events over that duration. On average, a CHOL molecule migrates across the lipid bilayer in about 73 ns after a flip-flop event is triggered. We have calculated diffusion maps and determined free energy surfaces and flip-flop mechanisms for CHOL molecules. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A10.00005: Multiscale modeling of nanostructures for electronic and energy-related applications Invited Speaker: Efthimios Kaxiras The optimization of materials properties for opto-electronic and energy-related applications is a crucial component in the design of new devices. To this end, multiscale modeling of nanostructures is essential in understanding and predicting materials properties ranging from optical response to the mechanical failure. We present a number of examples where multiscale modeling has yielded useful information concerning the optimal choices for nanostructured device elements. These include the excitation and charge transfer processes in hybrid photovoltaic devices, the tuning of optical and electrical properties of layered materials like graphene and transition-metal-dichalcogenides, and the mechanical response and deformation of silicon-based high-energy-density electrodes. [Preview Abstract] |
Session A11: Invited Session: Directed Assembly of Hybrid Materials
Sponsoring Units: DPOLYChair: Richard Vaia, Air Force Research Laboratory
Room: 310
Monday, March 18, 2013 8:00AM - 8:36AM |
A11.00001: Engineered Self-Assembly of Plasmonic Nanomaterials Invited Speaker: Andrea Tao A critical need in nanotechnology is the development of new tools and methods to organize, connect, and integrate solid-state nanocomponents. Self-assembly -- where components spontaneously organize themselves -- can be carried out on a massively parallel scale to construct large-scale architectures using solid-state nanocrystal building blocks. I will present our recent work on the synthesis and self-assembly of nanocrystals for plasmonics, where light is propagated, manipulated, and confined by solid-state components that are smaller than the wavelength of light itself. We show the organization of polymer-grafted metal nanocrystals into hierarchical nanojunction arrays that possess intense ``hot spots'' due to electromagnetic field localization. We also show that doped semiconductor nanocrystals can serve as a new class of plasmonic building blocks, where shape and carrier density can be actively tuned to engineer plasmon resonances. These examples demonstrate that nanocrystals possess unique electromagnetic properties that rival top-down structures, and the potential of self-assembly for fabricating designer plasmonic materials. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A11.00002: Polymer Functionalized Nanoparticles in Polymer Nanocomposites Invited Speaker: Arthi Jayaraman Significant interest has grown around the ability to control spatial arrangement of nanoparticles in a polymer nanocomposite to engineer materials with target properties. Past work has shown that one could achieve controlled assembly of nanoparticles in the polymer matrix by functionalizing nanoparticle surfaces with homopolymers. This talk will focus on our recent work using Polymer Reference Interaction Site Model (PRISM) theory and Monte Carlo simulations and GPU-based molecular dynamics simulations to specifically understand how heterogeneity in the polymer functionalization in the form of a) copolymers with varying monomer chemistry and ~monomer sequence, and b) polydispersity in homopolymer grafts can tune effective interactions between functionalized nanoparticles, and the assembly of functionalized nanoparticles. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A11.00003: Gels from soft hairy nanoparticles in polymeric matrices Invited Speaker: Dimitris Vlassopoulos Hairy particles represent a huge class of soft colloids with tunable interactions and properties. Advances in synthetic chemistry have enabled obtaining well-characterized such systems for specific needs. In this talk we present two model hairy soft particles with diameters of the order of tens of nanometers, star polymers and polymerically grafted spherical particles. In particular, we discuss design strategies for dispersing them in polymeric matrices and eventually creating and breaking gels. Control parameters are the matrix molar mass, the grafting density (or functionality) and the size of the grafts (or arms). The linear viscoelastic properties and slow time evolution of the gels are examined in view of the existing knowledge from colloidal gels consisting of micron-sized particles, and compared. In the case of stars we start from a concentrated glassy suspension in molecular solvent and add homopolymer at increasing concentration, and as a result of the induced osmotic pressure the stars shrink and a depletion gel is formed. For the grafted colloidal particles, they are added at low concentration to a polymer matrix, and it has been shown that under certain conditions the anisotropy of interactions gives rise to network formation. We then focus on the nonlinear rheological response and in particular the effect of shear flow in inducing a solid to liquid transition. Our studies show that the yielding process is gradual and shares many common features with that of flocculated colloidal suspensions, irrespectively of the shape of the building block of the gel. Whereas shear can melt such a gel, it cannot break it into its constituent blocks and hence fully disperse the hairy nanoparticles. On the other hand, the hairy particles are intrinsically hybrid. We show how this important feature is reflected on the heating of the gels. In that case, the mismatch of thermal expansion coefficients of core and shell appears to play a role on the particle response as it imposes and internal strain on the particle, which in turn changes the shell conformation and under some conditions can lead to thermal melting of the gel. These alternative avenues for manipulating the gel-to-liquid transition have potential implications in directing the properties of hairy nanoparticles and their assemblies in viscoelastic matrices. Parts of this work reflect collaboration with D. Truzzolillo (FORTH), J. F. Moll and S. K.Kumar (Columbia). R. H. Colby (Penn State), M. Gauthier (Waterloo) and B. C. Benicewicz (Univ. South Carolina). [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A11.00004: Canopy Dynamics in Nanoscale Ionic Materials Probed by NMR Invited Speaker: Peter Mirau Nanoscale ionic materials (NIMs) are hybrids prepared from ionically functionalized nanoparticles (NP) neutralized by oligomeric polymer counter-ions. NIMs are designed to behave as liquids under ambient conditions in the absence of solvent and have no volatile organic content, making them useful for a number of applications. We have used NMR relaxation and pulse-field gradient NMR to probe local and collective canopy dynamics in NIMs based on silica nanoparticles (NP), fullerols and proteins in order to understand the relationship between the core and canopy structure and the bulk properties. The NMR studies show that the canopy dynamics depend on the degree of neutralization, the canopy radius of gyration and molecular crowding at the ionically modified NP surface. The viscosity in NIMs can be directly controlled with the addition of ions that enhance the exchange rate for polymers at the NP surface. These results show that NIMs for many applications can be prepared by controlling the dynamics of the NP interface. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 11:00AM |
A11.00005: Colloidal Crystallization in Confinement: Icosahedral Symmetry {\&} Plastic-Crystal Transitions Invited Speaker: Alfons van Blaaderen The crystallization of spherical nanoparticles in liquid droplets results under certain conditions in crystals with an icosahedral shape. Experiments with larger spherical colloids and computers simulations of hard particles demonstrate that such crystal shapes do not rely on energetic arguments, but already result from confinement and entropy alone. Experiments on rod shaped fluorescent colloidal particles that are monodisperse enough to form nematic and smectic liquid crystal phases under conditions where the double layer thickness is small compared to the diameter of the rods show that if the double layer thickness is significantly larger than the rod length plastic crystals with a body centered crystal structure are formed. In such crystals there is three dimensional positional order, but no orientational order of the rods. These plastic crystals under strong planar confinement show intriguing phase behavior where plastic crystal and full crystal phases alternate as a function of the separation between the confining plates. [Preview Abstract] |
Session A12: Focus Session: Complex Oxide Interfaces - Nickelates
Sponsoring Units: DMPChair: Prasana Balachandran, Drexel University
Room: 314
Monday, March 18, 2013 8:00AM - 8:12AM |
A12.00001: Designing a Spin-one Mott Insulator: Complete Charge Transfer in Nickelate-Titanate Heterostructures Hanghui Chen, Chris Marianetti, Andrew Millis $Ab$ $initio$ calculations are performed to show that complete charge transfer may occur from the TiO$_2$ to the NiO$_2$ layers in (LaTiO$_3$)$_1$/(LaNiO$_3$)$_1$ superlattices. Although the two component materials are an $S = 1/2$ Mott insulator and a weakly correlated paramagnetic metal, strong correlation effects on Ni $d$ states can render the superlattice an unusual $S = 1$ charge transfer insulator, with the Ti-$d$ band empty, the Ni in the $d^8$ state and the oxygen bands filled. The charge transfer gap is set by the Ti/Ni $d$ level splitting. Magnetic, photoemission and x-ray scattering experiments are suggested to test the theory. The results show that heterostructuring can lead to very high levels of electron doping of oxides. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A12.00002: Two dimensional Mott physics in the rare earth nickelates Ankit Disa, Divine Kumah, Joseph Ngai, Jarrett Moyer, Fred Walker, Charles Ahn The strong electron correlations inherent in the rare-earth nickelate system (RNiO$_{3}$) lead to a metal-insulator transition, the temperature of which can be tuned by changing the rare-earth ion, R. Bulk LaNiO$_{3}$ is metallic at all temperatures, and NdNiO$_{3}$ undergoes a metal-insulator transition at 150 K. However, reducing the thickness of both LaNiO$_{3}$ and NdNiO$_{3}$ strongly affects the transport behavior, where LaNiO$_{3}$ undergoes a thickness-driven metal-insulator transition below $\sim$4 unit cells. Here, we identify the physics of this transition and demonstrate two-dimensional metallic behavior in thin films. We show that by direct chemical doping of LaNiO$_{3}$ thin films we can restore metallic behavior and tune the conductivity. We apply the same technique to thin films of NdNiO$_{3}$ and control the metal-insulator transition temperature. Finally, combining artificial confinement and doping, we observe metallicity in nickelate layers as thin as two unit cells. The effects of both structural and charge-carrier modifications on the transport properties of the thin films will be discussed. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A12.00003: Epitaxial growth of (111)-oriented LaAlO$_3$/LaNiO$_3$ ultra-thin superlattices S. Middey, D. Meyers, M. Kareev, E.J. Moon, B.A. Gray, J.W. Freeland, J. Chakhalian The epitaxial stabilization of a single layer or superlattice structures composed of complex oxide materials on polar (111) surfaces is severely burdened by reconstructions at the interface, that commonly arise to neutralize the polarity. We report on the synthesis of high quality LaNiO$_3$/mLaAlO$_3$ pseudo cubic (111) superlattices on polar (111)-oriented LaAlO$_3$, the proposed complex oxide candidate for a topological insulating behavior. Comprehensive X-Ray diffraction measurements, RHEED, and element specific resonant X-ray absorption spectroscopy affirm their high structural and chemical quality. The study offers an opportunity to fabricate interesting interface and topology controlled (111) oriented superlattices based on ortho-nickelates. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A12.00004: Topological phases in complex oxide interfaces and heterostructures Invited Speaker: Gregory A. Fiete In this talk we highlight recent theoretical work from our group aimed at identifying complex oxide interfaces and heterostructures that are expected to support topological phases, namely the $Z_2$ time-reversal invariant topological insulator and the zero magnetic field Chern insulator, or quantum anomalous Hall state. We focus on two particular systems: (1) Perovskites of the form ABO$_3$ and (2) Pyrochlores of the form A$_2$B$_2$O$_7$ where A is usually a rare earth element and B is a transition metal element. One of our main results is that thin film growth along the [111] direction is favorable for the realization of topological phases in experiment. We lay out the most important film properties that appear to favor topological phases and discuss the different physics associated with realizing topological phases in 3d, 4d, and the heaviest 5d-based transition metal oxide systems. Key open questions and experimental challenges are presented, as well as the potential advantages that oxide systems offer over the Bi-based topological insulator material class in device applications. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A12.00005: Controlling the metal insulator transition using the ferroelectric field effect in rare earth nickelates Matthew Marshall, Ankit Disa, Divine Kumah, Hanghui Chen, Sohrab Ismail-Beigi, Fred Walker, Charles Ahn A ferroelectric field effect transistor (FE-FET) modulates conductivity in a non-volatile manner by electrostatically accumulating and depleting charge carriers at the interface between a conducting channel and ferroelectric gate. The rare earth nickelate LaNiO$_{3}$ is metallic in bulk, while other rare earth nickelates, such as NdNiO$_{3}$, exhibit metal-insulator transitions and anti-ferromagnetic behavior in the bulk. Here, we show that by coupling the ferroelectric polarization of Pb$_{0.8}$Zr$_{0.2}$TiO$_{3}$ (PZT) to the carriers in a nickelate, we can dynamically induce a metal- insulator transition in ultra-thin films of LaNiO3, and induce large changes in the MIT transition temperature in NdNiO3. Density functional theory is used to determine changes in the physical and electronic Ni-O-Ni bond angle of the nickelate at the interface between PZT and LaNiO3. The effect of the ferroelectric polarization is to decrease the Ni-O-Ni bond angle from 180 degrees and increase the carrier effective mass. Related to this change in electronic structure, we observe a change in resistivity of approximately 80{\%} at room temperature for an ultra-thin 3 unit cell thick film of LaNiO$_{3}$. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A12.00006: Atomic-scale structure and composition mapping in nickelate heterostructures Divine Kumah, Ankit Disa, Joseph Ngai, Hanghui Chen, Sohrab Ismail-Beigi, Charles Ahn, Fred Walker Strongly correlated electronic systems display a wide range of interesting properties, including ferroelectricity, superconductivity, metal-insulator transitions, and novel magnetic phenomena. The electrical and magnetic properties of thin film heterostructures based on these systems are directly linked to their atomic scale structure and composition. This link is important for the rare earth nickelates, which exhibit first-order metal-insulator transitions, antiferromagnetism, and charge ordering. At the interfaces present in these systems, structural coupling can lead to new effects. We use a synchrotron-based resonant anomalous x-ray scattering method to elucidate the physical and electronic structure at complex oxide heterointerfaces of nickelates grown using molecular beam epitaxy. Temperature dependent resonant x-ray studies in doped 8 unit-cell thick NdNiO$_3$ films reveal subtle changes in atomic structure and Ni charge disproportionation at the metal-insulator transition. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 10:12AM |
A12.00007: Ultrafast phase control in complex oxide heterostructures Invited Speaker: Andrea Caviglia Complex oxide heterostructures have emerged as multifunctional materials of striking flexibility, in which unconventional electronic phases can be realised by engineering the strain field across interfaces. This same mechanical coupling is also expected to be effective on the ultrafast timescale, and could be exploited for the dynamic control of materials properties. Here, we demonstrate that a large-amplitude mid-infrared field, made resonant with a stretching mode of the substrate, can switch the electronic properties of a thin film across an interface. Exploiting dynamic vibrational propagation between different components of a heterostructure, insulating antiferromagnetic NdNiO$_3$ is driven through a prompt, five-order-of-magnitude increase of the electrical conductivity, with resonant frequency and susceptibility that is controlled by choice of the substrate material. Vibrational phase control, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A12.00008: Investigation of Nonlinear Differential Conductance in NdNiO$_{3}$ Thin Films Will Hardy, Heng Ji, Junwoo Son, Susanne Stemmer, Douglas Natelson We will report on recent investigations of the voltage and temperature dependence of the nonlinear differential conductance, dI/dV, of the insulating state in thin films of NdNiO$_{3}$. This compound exhibits a metal-insulator transition near 100 K [1] between a high temperature paramagnetic metal and a low temperature charge-transfer insulator. These investigations are motivated by previous observations in Fe$_{3}$O$_{4}$, a strongly correlated material that undergoes the Verwey transition at a similar temperature scale, in which hysteretic, voltage-driven breakdown of the insulating state has been reported [2]. We examine the evolution of the nonlinear conductance, as well as its dependence on the device geometry, in planar devices at temperatures near the transition.\\[4pt] [1] Son, Junwoo, Bharat Jalan, Adam P. Kajdos, Leon Balents, S. James Allen, and Susanne Stemmer. ``Probing the Metal-Insulator Transition of NdNiO$_{3}$ by Electrostatic Doping.'' App. Phys. Lett. 99, 192107 (2011).\\[0pt] [2] Fursina, A.A., R.G.S. Sofin, I.V. Shvets, and D. Natelson. ``The Origin of Hysteresis in Resistive Switching in Magnetite is Joule Heating.'' Phys. Rev. B 79, 245131 (2009). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A12.00009: Strain mediated suppression of the metal-insulator and anti-ferromagnetic transition in EuNiO3 thin films Derek Meyers, Srimanta Middey, Mikhail Kareev, Benjamin Gray, John Freeland, Jak Chakhalian Ultrathin epitaxial films of EuNiO$_3$ were grown on a series of substrates traversing highly compressive (-2.4\%) to highly tensile (2.5\%) lattice mismatch. X-ray absorption spectroscopy measurements revealed a strong multiplet splitting in the tensile samples that progressively weakens with increasing compressive strain. Transport measurements further collaborated these findings, showing a successively (from tensile to compressive) lower resistance and a complete suppression of the metal-insulator transition at -2.4\% lattice mismatch. The derivative of the transport showed a strong downturn around the bulk Neel temperature, which was also suppressed with compressive strain. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A12.00010: Composition spread studies of Nd$_{1-x}$La$_{x}$NiO$_{3}$ combinatorial thin films Richard Suchoski, Kui Jin, Shintaro Yasui, Richard Greene, Ichiro Takeuchi Rare earth nickelates have attracted a great deal of attention in recent years due to a host of interesting features, one being a transition from paramagnetic metal to antiferromagnetic insulator through distortions from the ideal perovskite unit cell. This metal-to-insulator transition (MIT) can be manipulated by modifying variables such as temperature, rare earth ion size, oxygen content, or stress from lattice-mismatched epitaxial thin film growth. Research on this family has been extensive, though there still exists an absence of thin film studies focusing on intermediate compositions. We have fabricated epitaxial thin film composition spreads of Nd$_{1-x}$La$_{x}$NiO$_{3}$ grown via combinatorial PLD to investigate these transitional compositions. While our films exhibit a smooth composition progression, we observe a composition threshold where orthorhombic NdNiO$_{3}$ transforms to rhombohedral LaNiO$_{3}$, correlating with disappearance of the MIT, and displays a non-Vegard evolution of the film's in-plane lattice constant in HRXRD and Raman scattering data of the A$_{1g}$ rotational mode. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A12.00011: First-principles study of the properties of oxygen-deficient LaNiO$_{3-x}$ structures Andrei Malashevich, Sohrab Ismail-Beigi There has been a great deal of recent interest and activity regarding rare earth nickelates in bulk form, as superlattices, and as thin films. The parent nickelate in these cases is typically LaNiO$_3$, which in bulk form is a paramagnetic metal. In addition, due to its relatively good lattice match to other perovskites, it also serves as an electrode in functional oxide film devices. However, it is known that the conductivity of LaNiO$_3$ in any form is strongly affected by the presence of oxygen vacancies. Here, we present a first-principles study of a variety of oxygen-deficient LaNiO$_{3-x}$ structures. We describe our theoretical results for the atomic-scale geometry and energetics of the vacancies (formation and aggregation), their mobilities, and their electronic structure. [Preview Abstract] |
Session A13: Focus Session: Topological Materials - Magnetic Topological Insulators
Sponsoring Units: DMPChair: Michael Fhurer, Monash University
Room: 315
Monday, March 18, 2013 8:00AM - 8:12AM |
A13.00001: Ab initio study of topological surface states of Sb (111) surface with magnetic impurities Jinhee Han, Hyungjun Lee, Hyoung Joon Choi We study effects of magnetic impurities on topological surface state of Sb (111) surface by using an ab-initio pseudopotential density-functional method. We have implemented the spin-orbit interaction into the SIESTA code in a form of additional fully non-local projectors. To calculate surface band structures, we use a slab of Sb using a 4x4 supercell containing 20 atomic layers. In particular, we compare Fe impurities with Mn impurities, whose atoms have larger magnetic moments, and compare interstitial impurities with substitutional impurities for each atom. To understand the impurity effects on the topological surface states, we simulate ARPES spectra and calculate projected density of states of impurity near Fermi level. This work was supported by NRF of KOREA (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2012-C2-14). [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A13.00002: Manipulating Surface-induced Ferromagnetism in Modulation-doped Topological Insulators Xufeng Kou, Liang He, Murong Lang, Yabin Fan, Ying Jiang, Yong Wang, Faxian Xiu, Kang Wang The manipulation of topological surface states is a key to realize applicable devices of topological insulators. In addition to the direct engineering of time-reversal-symmetry protected surface states, recent work suggests that various physical responses can be obtained from surface helical states by integrating additional ferromagnetism or superconductivity to the original topological order. Here, we report the coexistence and tunability of bulk carrier density-independent and surface-mediated electrically controllable ferromagnetisms in modulation-doped Crx(BiySb1-y)2Te3 epitaxial thin films. We demonstrate for the first time a dramatic enhancement of surface-induced magnetization on TI / Cr-TI bilayer devices. The surface magneto-electric effects can be either enhanced significantly or completely switched-off, by tuning the separation of the surface from the magnetic impurities. The electric-field-modulated ferromagnetism in our modulation-doped TI hetero-structures is fundamentally important for the realization of the quantum anomalous Hall Effect as well as the axion electromagnetic dynamics, and thus provides a new approach for spintronics applications. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A13.00003: Effect of Magnetic Doping on Electrical and Thermal Conductivities and Seebeck Coefficient of Suspended Bismuth Telluride Nanoplates Insun Jo, Michael Thompson Pettes, Zhen Yao, Li Shi Bismuth telluride has been investigated intensively as a model system for topological insulators. In this work, we have studied electrical and thermal transport properties of suspended bismuth telluride nanoplates grown by the vapor-solid method. The thin crystals were transferred onto micro-fabricated suspended structures with built-in electrodes and thermometers, which allowed us to measure electrical ($\sigma )$ and thermal ($\kappa )$ conductivities as well as the Seebeck coefficient ($S)$. The through-etched hole in the devices enabled us to evaporate Cr layers on both surfaces of the crystal. After H$_{2}$ annealing at 500 K, we measured enhanced $\sigma $, $\kappa $, and $S$ values by 40, 10, and 20{\%}, respectively. In comparison, H$_{2}$ annealing without Cr evaporation resulted in 10, 10, and -8{\%} changes of $\sigma $, $\kappa $, and $S$ values, respectively. The effect of magnetic doping by Cr will be discussed. Additionally, magneto-transport measurements were performed on the samples to resolve the transport properties of the surface states. We observed a pronounced weak anti-localization feature in undoped samples. Changes in this feature after Cr doping will be presented. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A13.00004: Electrical control of the ferromagnetism in Sb$_{\mathrm{2-x}}$Cr$_{\mathrm{x}}$Te$_3$ magnetic topological insulators Zuocheng Zhang, Xiao Feng, Minghua Guo, Cuicu Chang, Jinsong Zhang, Kang Li, Lili Wang, Xi Chen, Ke He, Qikun Xue, Xucun Ma, Yayu Wang The spin helical Dirac fermions living on the surface of three-dimensional topological insulators (TIs) provide a platform for exploring the coupling between the charge and spin degrees of freedom. In particular, breaking the time reversal symmetry in TIs is expected to create exotic topological magnetoelectric effects. To realize these phenomena and apply them in TI-based spintronic devices, it is desirable to achieve in situ manipulation of the magnetism in TIs via an electrical field. In this talk we present the fabrication and transport studies of Cr doped Sb$_2$Te$_3$ magnetic TI thin films. By applying a gate voltage in a field effect transistor device, we can control the coercive force and Curie temperature. The ferromagnetic order is found to be enhanced when more hole-type carriers are injected into the sample. This trend suggests the itinerant bulk holes in TIs can mediate ferromagnetic ordering of local moments in a similar manner as that in the diluted magnetic semiconductors. The electrical control of the ferromagnetism in TIs demonstrated here paves the road for realizing the TI-based devices. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A13.00005: Ferromagnetism in Mn-doped Bi$_2$Te$_3$ Thin Films by Molecular Beam Epitaxy Joon Sue Lee, Anthony Richardella, David W. Rench, Duming Zhang, Nitin Samarth We demonstrate the ferromagnetic properties of Mn-doped thin films of the topological insulator Bi$_2$Te$_3$ grown by molecular beam epitaxy. Films with Mn concentrations up to 10\% and thickness up to 60 nm were studied. The electrical transport measurements reveal a strong anomalous Hall effect (AHE) with a coercive field of 3000 Oe at 500 mK. The onset (10 - 16 K) of the AHE is at about the same temperature with the Tc obtained by the superconducting quantum interference device (SQUID) measurements. The magneto-conductivity shows hysteresis and a crossover from weak antilocalization to weak localization when going below Tc. The carrier type and the carrier concentration are modified by varying the Mn doping and the film thickness. Most of films are n-type, but some films thicker than 50 nm at a certain Mn concentration are p-type. Shifts in x-ray diffraction indicate that the n-type films have Mn atoms between quintuple layers, but the p-type films are substitutional. Funded by ONR and DARPA. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A13.00006: Metallic states in Topological Insulators with Magnetic Impurities Leonardo Abdalla, Adalberto Fazzio, Tome Schmidt, Roberto Miwa Topological insulators are characterized by an insulating bulk, and an odd number of Dirac cones in the surface. Their existence are due a band inversion in the bulk phase created by a strong spin orbit coupling. Those metallic states have their spin polarization locked in a plane giving rise to a chiral spin texture, similar to the quantum spin hall effect. Such spin helicity suppresses backscattering processes. Based on first principles calculations, we performed a systematic study of transition metal (TM) impurities (Co, Mn, Ni, Cr and Fe) lying on the topmost layers of the $Bi_2Se_3$ topological insulator. Based upon formation energy results, by considering a number of plausible configurations, we find an energetic preference for the TMs occupying the topmost Bi substitutional site, and the subsurface interstitial sites neighboring Bi atoms. Our simulated scanning tunneling images (STM) show that there is local perturbation on the electronic structure of the surface. Further electronic band structure calculations indicate that (for some systems) the topologically protected surface metallic bands are suppressed, opening a band gap. In those systems the time reversal symmetry has been broken due to the formation net magnetic moment aligned perpendicularly to the surface plane. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A13.00007: Electrical transport studies in the topological insulator Bi$_{2}$Se$_{3}$ with exchange induced ferromagnetism Peng Wei, Ferhat Katmis, Badih Assaf, Donald Heiman, Jagadeesh Moodera The proximity-induced ferromagnetic order in topological insulator (TI)/ferromagnetic insulator (FI) heterostructures induces ferromagnetism in TI, which breaks local time reversal symmetry that can lead to many exotic properties, such as image magnetic monopole, topological magneto-electric effects, etc.[1] We achieved this novel ferromagnetic order in a TI Bi$_{2}$Se$_{3}$ through Bi$_{2}$Se$_{3}$/EuS bi-layer structures. Electric transport studies show a dramatic suppression of the weak anti-localization (WAL) effect in Bi$_{2}$Se$_{3}$/EuS compared to controlled Bi$_{2}$Se$_{3}$ samples. In contrast to the case of surface doping a TI with magnetic atoms (i.e. Fe), here the WAL cannot be quenched even with a full coverage EuS capping layer, which points that its origin can be the opening of a surface gap rather than a reduction of the magnetic scattering length. The results are analyzed with a theoretical model providing a value for the induced surface exchange gap. Other experimental results, such as the anomalous Hall effect that support the proximity induced ferromagnetism in Bi$_{2}$Se$_{3}$ will be discussed.\\[4pt] [1] Qi, X.-L. {\&} Zhang, S.-C., Rev Mod Phys 83, 1057-1110, (2011). [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A13.00008: Magnetotransport in topological insulator-ferromagnetic insulator heterostructure devices Abhinav Kandala, Anthony Richardella, David Rench, Duming Zhang, Thomas Flanagan, Nitin Samarth Topological surface states modified by the presence of magnetism are predicted to play host to a number of exotic phenomena and are of great fundamental as well as applied interest. Interfacing topological insulators with magnetic insulators offers a unique opportunity to access these effects by transport, without affecting the bulk band structure. We demonstrate the integration of MBE grown thin films of Bi$_2$Se$_3$ with the insulating ferromagnet GdN. SQUID measurements of the hetero-structure reveal an in-plane easy axis with a ferromagnetic Curie temperature $T_c$ $\sim$ 13 K. The fabrication of hall devices with bare and GdN-capped channels enables direct comparison of magneto-transport properties. While the bare channel displays conventional weak anti-localization (WAL), the capped channel reveals a weakened WAL and a superimposed negative magnetoresistance (MR) associated with weak localization. These observations are discussed in the context of gap-opening in the Dirac surface state. Finally, we discuss the observation of hysteresis in the MR of the capped channel below 2 K. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A13.00009: Dirac Surface State of Metamagnetic Topological Insulators Y.S. Hor, S.H. Lee, J.E. Medvedeva, M. Iavarone, U. Chatterjee, W. Ratcliff We report the observation of metamagnetism in iron-doped bismuth selenide topological insulators. The structural, magnetic, and transport properties of the materials were investigated both computationally and experimentally. First-principles density functional calculations are employed to determine the most favorable site location of the iron atoms in the bismuth selenide lattice and to analyze the magnetic properties of the resulting structures. Magnetization measurements showed the system is anisotropic with a magnetic phase transition at $\sim$ 100 K. However, this magnetic-doped topological insulator did not show an opening of a surface gap in ARPES data at temperatures below the transition temperature. This is due to the antiferromagnetic ground state of the system. With an applied magnetic field greater than 300 Oe, the system becomes ferromagnetic. In addition, Shubnikov-de Haas oscillations were observed in the longitudinal resistivity measurements under the applied magnetic fields up to 9 T. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A13.00010: Experimental Realizations of Magnetic Topological Insulator and Topological Crystalline Insulator Invited Speaker: Suyang Xu Over the past few years the experimental research on three-dimensional topological insulators have emerged as one of the most rapidly developing fields in condensed matter physics. In this talk, we report on two new developments in the field: The first part is on the dynamic interplay between ferromagnetism and the Z$_2$ topological insulator state (leading to a magnetic topological insulator). We present our spin-resolved photoemission and magnetic dichroic experiments on MBE grown films where a hedgehog-like spin texture is revealed on the magnetically ordered surface of Mn-Bi$_2$Se$_3$ revealing a Berry's phase gradient in energy-momentum space of the crystal. A chemically/electrically tunable Berry's phase switch is further demonstrated via the tuning of the spin groundstate in Mn-Bi$_2$Se$_3$ revealed in our data (Nature Physics 8, 616 (2012)). The second part of this talk describes our experimental observation of a new topological phase of matter, namely a topological crystalline insulator where space group symmetries replace the role of time-reversal symmetry in an otherwise Z$_2$ topological insulator predicted in theory. We experimentally investigate the possibility of a mirror symmetry protected topological phase transition in the Pb$_{1-x}$Sn$_x$Te alloy system, which has long been known to contain an even number of band inversions based on band theory. Our experimental results show that at a composition below the theoretically predicted band inversion, the system is fully gapped, whereas in the band-inverted regime, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order (Nature Communications 3, 1192 (2012)) distinct from that observed in Z$_2$ topological insulators. We discuss future experimental possibilities opened up by these new developments in topological insulators research. This work is in collaboration with M. Neupane, C. Liu, N. Alidoust, I. Belopolski, D. Qian, D.M. Zhang, A. Richardella, A. Marcinkova, Q. Gibson, R. Sankar, Y.J. Wang, T. Chang, H. Jeng, H. Lin, L.A. Wray, J.D. Denlinger, M. Leandersson, T. Balasubramanian, J. S\'anchez-Barriga, O. Rader, G. Landolt, B. Slomski, J.H. Dil, F.C. Chou, E. Morosan, N. Samarth, R.J. Cava and M.Z. Hasan. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A13.00011: Efficient Generation of Spin Current and Spin Transfer Torque by the Topological Insulator Bismuth Selenide Alex Mellnik, Jennifer Grab, Peter Mintun, Joon Sue Lee, Anthony Richardella, Nitin Samarth, Daniel Ralph We study the use of topological insulators as a source of spin current for applying spin transfer torque to a ferromagnet. We fabricate bismuth selenide / permalloy bilayers and use the spin-torque FMR technique to make quantitative measurements of the torque applied to the magnetic permalloy layer resulting from an in-plane current. Despite the fact that only a small fraction of the current flows in the bismuth selenide, we still observe large spin torque effects. There is a component of torque in the sample plane with the symmetry expected from the spin Hall effect, with a strength corresponding to a spin Hall angle greater than 1, larger than measured for any other material. There is also an additional out-of-plane, field-like torque several times larger than expected from the Oersted field. We will discuss the dependence of these effects on layer thickness, and attempt to distinguish whether they result from bulk or surface-state effects. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A13.00012: Surface state driven spin-torque in topological-insulator / ferromagnetic-metal bilayers Mark H. Fischer, Abolhassan Vaezi, Aurelien Manchon, Eun-Ah Kim A hallmark of surface states in strong 3D topological insulators (TI) is the helical spin texture. While there have been proposals on exploiting this spin texture for spintronics applications, they focused on TI/ferromagnetic-insulator (FI) structures predicting field-like torque due to spin accumulation. Motivated by recent spin-torque experiments on Pt/ferromagnetic-metal(FM) structures, we consider a TI/FM bilayer, where the magnetic moment as well as the current driven through the system are in plane. While existing TIs have a conducting bulk, recent transport experiments showed that the main contribution to the current in Bi$_2$Se$_3$ thin films comes from two distinct surface states: the topological Dirac surface state and an additional 2D electron gas with Rashba spin-orbit coupling. We thus consider spin torque in the TI/FM structure due to these two surface states. We find that each surface state leads to out-of-plane (field-like) torque due to current driven spin accumulation. Moreover, we find an in-plane torque due to spin diffusion into the FM, an effect absent in TI/FI structures. Interestingly, the two surface states contribute with opposite sign to the spin density. This allows for the experimental identification of the dominant state based on its sign. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A13.00013: Topological Magnetic Heterostructures of Epitaxial Bi$_{2}$Se$_{3}$ on Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As Anthony Richardella, Joon Sue Lee, David W. Rench, Robbie D. Fraleigh, Nitin Samarth Topological Insulators (TI) are characterized by conducting surface states with a Dirac-like dispersion protected by time reversal symmetry. A magnetic perturbation that breaks this symmetry, such as placing a ferromagnet in proximity with a TI, can lead to a wide range of unusual effects such as a half integer quantum Hall conductance, magnetic monopoles, or an inverse spin-galvanic effect, among others. Such structures are challenging to create however due to the difficulty in finding insulating magnetic materials that are compatible with topological materials. We demonstrate one approach to this, the epitaxial growth of Bi$_{2}$Se$_{3}$ on the ferromagnetic semiconductor Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As. We discuss the growth and characterization of these heterostructures, where the Mn concentration of the GaMnAs can be tuned from a highly resistive state near the metal-insulator transition, up to a highly doped semiconductor with a T$_{\mathrm{C}}$ well in excess of 100 K. This allows the study of a wide range of regimes and interactions between the two layers. As GaMnAs is a prototypical material for the demonstration of many spintronic devices, and has a highly tunable anisotropy, this opens up the possibility of an exciting range of hybrid spintronic/Topological Insulator structures. Funded by ONR and DARPA. [Preview Abstract] |
Session A14: Focus Session: Perovskite Cobaltite & Titanate Heterostructures
Sponsoring Units: DMP GMAGChair: Chris Leighton, University of Minnesota
Room: 316
Monday, March 18, 2013 8:00AM - 8:12AM |
A14.00001: Orientation and Strain Dependence of the Magnetic Phase Separation at Perovskite Cobaltite Interfaces S. Bose, M. Sharma, M.A. Torija, J. Gazquez, M. Varela, H. Ambaye, R. Goyette, V. Lauter, M.R. Fitzsimmons, J. Schmitt, C. Leighton We recently showed that the degraded magnetic and electronic properties in very thin STO(001)/La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CoO$_{3}$ films is due to a form of magnetic phase separation. This is primarily due to the strain driven accumulation of O vacancies near the interface. In this work we demonstrate how this understanding allows us to engineer these interfacial properties via crystallographic orientation and strain control. Using PNR, magnetometry and transport, we show how this degradation can be significantly mitigated by using LAO(001) and STO(110) substrates cf. STO(001). PNR on 400{\AA} x$=$0.28 films reveals an interfacial layer with suppressed magnetism on all three substrates. However, while this layer is 150{\AA} on STO(001), it extends at most to 30{\AA} on LAO(001) and STO(110). Transport measurements on x$=$0.5 films show that at a thickness of $\sim$ 55{\AA}, films on STO(110) and LAO(001) exhibit AMR whereas films on STO(001) are dominated by inter-cluster GMR. Finally, thickness dependent magnetometry shows that the magnetic order deteriorates more quickly on STO(001) than on LAO(001) and STO(110). Our work thus opens up a possible new route to tailor interfacial magneto-electronic properties in oxide heterostructures. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A14.00002: Synthesis of Epitaxially Strained Brownmillerite Strontium Cobaltate (SrCoO$_{2.5})$ Eric Jin, Matthew Marshall, Joseph Ngai, Charles Ahn, Fred Walker Strontium cobaltate (SrCoO$_{3})$ is a perovskite oxide predicted to have metal-insulator transitions and magnetic phases induced by epitaxial strain. The related brownmillerite phase SrCoO$_{2.5}$ has a similar structure, but contains alternating planes of oxygen vacancies in the octahedral oxygen cages of the perovskite structure. We demonstrate epitaxial growth of SrCoO$_{2.5}$ on both SrTiO$_{3}$ and LaAlO$_{3}$ substrates by molecular beam epitaxy using RF oxygen plasma. X-ray diffraction measurements show finite thickness oscillations that are characteristic of smooth films, and half-order diffraction peaks that are representative of the brownmillerite phase. We observe a single tetragonal domain when the film is deposited on SrTiO$_{3}$ with the planes of oxygen vacancies parallel to the interface. When grown on LaAlO$_{3}$, the film contains multiple orthorhombic domains. We conclude that the observed domain structures for SrCoO$_{3}$ grown on SrTiO$_{3}$ and LaAlO$_{3}$ are due to ordering of the oxygen vacancies to reduce strain. We will also present strategies to increase the oxygen content to that of SrCoO$_{3}$. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A14.00003: Magnetic Structure and Phase Separation in Epitaxial SrCoO$_{\mathrm{x}}$ Thin Films F.J. Rueckert, C. Abughayada, S.A. Sabok, F. He, H. Mohottala, J.I. Budnick, W.A. Hines, B. Dabrowski, B.O. Wells Bulk SrCoO$_{\mathrm{x}}$ separates into three distinct ferromagnetic phases as the oxygen content is increased from x $=$ 2.75 to 3.0, corresponding to T$_{\mathrm{C}} =$ 165 K (SrCoO$_{2.75})$, T$_{\mathrm{C}} =$ 220 K (SrCoO$_{2.88})$, and T$_{\mathrm{C}}$ $=$ 280 K (SrCoO$_{3.0})$. Over this composition, the lattice evolves smoothly and remains a single crystallographic phase. Using pulsed laser deposition and electrochemical oxidation, we have prepared epitaxial films of SrCoO$_{\mathrm{x}}$ of varying thickness and orientation on SiTiO$_{3}$ substrates. While in polycrystalline samples intermediate oxygen concentrations show a two-phase magnetic behavior, 100nm thick (0 0 1) films remain single phase but still favor the same ferromagnetic transitions. Thicker, 150 nm (1 1 1) films also order at comparable T$_{\mathrm{C}}$'s, but again show two-phase behavior during deoxidation. Resonant x-ray diffraction on these samples reveals both commensurate and incommensurate ordering dependent on the oxidation state. This implies a charge or orbital ordering which may be influenced by finite size effects. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A14.00004: Oxygen vacancy ordering in transition-metal-oxide LaCoO$_{3}$ films Neven Biskup, Juan Salafranca, Virat Mehta, Yuri Suzuki, Stephen Pennycook, Sokrates Pantelides, Maria Varela Oxygen vacancies in complex oxides affect the structure and the electronic and magnetic properties. Here we use atomically-resolved Z-contrast imaging, electron-energy-loss spectroscopy and densityfunctional calculations to demonstrate that ordered oxygen vacancies may act as the controlling degree of freedom for the structural, electronic, and magnetic properties of LaCoO$_{3}$ thin films. We find that epitaxial strain is released through the formation of O vacancy superlattices. The O vacancies donate excess electrons to the Co $d$-states, resulting in ferromagnetic ordering. The appearance of Peierls-like minigaps followed by strain relaxation triggers a nonlinear rupture of the energy bands, which explains the observed insulating behavior. We conclude that oxygen vacancy ordering constitutes a degree of freedom that can be used to engineer novel behavior in complex-oxide films. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A14.00005: Local atomic and electronic structure of LaCoO3/SrTiO3 thin films by HAADF STEM and EELS Albina Borisevich, Jae Hyuck Jang, Young-Min Kim, Liang Qiao, Michael Biegalski For perovskite films with several competing functionalities, magnetic and electronic properties can be affected both by structural order parameters and chemical factors. For example, in LaCoO3 (LCO) thin films, magnetic and transport properties are strongly dependent on strain state and oxygen content. For this study, LCO thin films were deposited by pulsed laser deposition method with different thicknesses (2, 5, 15 unit cell and 20 nm thickness) on SrTiO3 substrate. X-ray photoelectron spectroscopy studies of the grown films have demonstrated that Co 3p edges shift up to 2 eV for 15 u.c. and 20 nm films, indicating possible presence of 2D electron gas. The structure of the 5 u.c and 15 u.c LCO films was examined. Atomic position mapping from STEM HAADF and BF images can reveal lattice parameter and octahedral tilt behavior with atomic resolution. BF STEM imaging showed that octahedral tilts were active in the 15 u.c. film but not in the 5 u.c. film. A complex pattern of O K fine structure evolution at the interface was observed; results of the deconvolution of different contributions to this behavior using advanced simulations, as well as data on oxygen vacancy mapping, will be presented. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A14.00006: Dimensional control of cobalt spin state in oxide superlattices Da Woon Jeong, W.S. Choi, S. Okamoto, C.H. Sohn, H.J. Park, J.-Y. Kim, H.N. Lee, K.W. Kim, S.J. Moon, T.W. Noh Perovskite cobalt oxide is a very intriguing system with various spin states owing to the delicate balance between crystal field splitting and Hund exchange energy. In this talk, we show that its spin state can be altered through dimensional control, enabled by digital synthesis of perovskite cobalt oxide superlattices. We employed a few unit cells of LaCoO$_{\mathrm{3}}$ as an active magnetic layer, separated by LaAlO$_{\mathrm{3}}$ spacer layer. High quality [(LaCoO$_{\mathrm{3}})n$(LaAlO$_{\mathrm{3}})n$]8 ($n \quad =$ 2, 6, and 10) superlattices were fabricated using pulsed laser epitaxy. Spectroscopic tools including x-ray absorption spectroscopy and optical spectroscopy revealed clear evolution of the electronic structure and resultant spin state by changing dimensionality. Specifically, the spin state changed from a high to a low spin state with a larger optical band gap, as the dimension reduced from 3D to 2D. Dynamic mean field calculation supported the critical role of dimensionality on the spin state and electronic structure of LaCoO$_{\mathrm{3}}$. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A14.00007: Emergent phenomena and magnetism in high-density electron gases in SrTiO$_3$ Invited Speaker: Susanne Stemmer GdTiO$_3$/SrTiO$_3$ interfaces grown by molecular beam epitaxy exhibit mobile carrier densities that are remarkably well predicted by the electrostatic requirements of the compensation of the polar discontinuity at the interface. Carrier densities are $\sim$3x10$^{14}$~cm$^{-2}$, or $\sim$0.5 electron per surface unit cell. By sandwiching few-unit-cell-thick SrTiO$_3$ layers between GdTiO$_3$, carrier concentrations in the SrTiO$_3$ approach densities under which on-site Coulomb interactions may appear. By changing the width of the quantum well, the 3D electron density can be varied, which allows for a systematic study of interaction effects. In this presentation, we discuss evidence for short-range Coulomb interactions, and associated phenomena, in ultrathin, confined the SrTiO$_3$ quantum wells containing extreme charge densities. We show that narrow SrTiO$_3$ quantum wells exhibit ferromagnetism at low temperatures, as evidenced by a hysteresis in the magnetoresistance. The Curie temperature scales with the thickness of the SrTiO$_3$ quantum well. We discuss evidence for on-site Mott-Hubbard-type correlation physics in the temperature-dependent transport in metallic quantum wells. With increasing 3D carrier densities we observe a correlation-induced mass enhancement, followed by a transition to a correlated insulator at the highest 3D densities. We also discuss the role of disorder in the insulating state. This work was done in collaboration with Pouya Moetakef, Clayton A. Jackson, Leon Balents, Jim Allen, Jimmy Williams and David Goldhaber-Gordon. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A14.00008: Insulating room temperature ferromagnetic SrTiO$_3$ Agham Posadas, Chandrima Mitra, Chungwei Lin, Ajit Dhamdere, David Smith, Maxim Tsoi, Alex Demkov We report the epitaxial growth of ferromagnetic insulating material based on SrTiO$_{3}$ using molecular beam epitaxy (MBE). SrTi$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$O$_{3-\delta}$ films (x $=$ 0.1 to 0.5) were grown on Si(100) substrates via a buffer layer of four unit cells of undoped SrTiO$_{3}$. The crystalline structure was characterized by reflection high energy electron diffraction, x-ray diffraction, and cross-section transmission electron microscopy. Robust room-temperature ferromagnetism is confirmed in samples with composition 30-40{\%} Co. We also performed \textit{in situ }x-ray photoelectron spectroscopy of the Sr, Co, Ti, and O core levels to determine stoichiometry and cobalt oxidation state. In all single phase samples, an oxygen vacancy concentration of approximately equal to the amount of Co substitution was measured (compensated doping). In order to elucidate the origin of ferromagnetism, we also performed first-principles calculations of SrTiO$_{3}$ simultaneously doped with Co and an oxygen vacancy. We find that such a configuration at concentrations of $\sim$ 25{\%} can result in a ferromagnetic insulating state with high spin Co$^{2+}$. The ability to integrate an insulating ferromagnet on silicon in epitaxial form may potentially be useful for spin filtering and spin wave applications in the field of spintronics. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A14.00009: Linear magnetoresistance in non-degenerately doped SrTiO$_{3}$ Anand Bhattacharya I will present transport measurements on non-degenerately doped $n-$SrTiO$_{3}$ single crystals. The samples were doped by annealing at high temperatures in vacuum. The resistance decreases monotonically down to the lowest temperatures, for carrier densities as low as 3.85 x 10$^{15}$/cm$^{3}$. The magnetoresistance (MR) is found to be positive and linear at high fields, with R(9 T)/R(0 T) \textgreater\ 28 at 2 K for the lowest doping levels measured. The magnitude of the MR decreases with increasing temperature, and with increased doping. I will discuss the data in light of various mechanisms for linear magnetoresistance in the context of $n-$SrTiO$_{3}$. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A14.00010: The role of interface magnetic centers on the spin lifetime measured in doped SrTiO3 using Hanle technique Wei Han, Xin Jiang, Adam Kajdos, See-Hun Yang, Susanne Stemmer, Stuart Parkin Recently, the two dimensional electron gas that is formed at the surface of strontium titanate, SrTiO3 (STO), has attracted considerable attention, both concerning its origin as well as the many phenomena that it apparently displays: these include, gate tunable metallicity and superconductivity, and magnetic effects including Kondo scattering. Here, we report electrical injection and detection of spin currents in Nb doped STO substrates and La doped STO thin films using the Hanle technique and CoFe / MgO tunnel spin injectors. The spin lifetimes measured are on the order of 100 ps and vary little with temperature for temperatures varying from 10 K to 300 K, whereas the mobility of the STO has very strong temperature dependence. This suggests that the spin lifetime is limited by spin-dependent scattering at the MgO/STO interface, perhaps related to the formation of Ti3$+$ or other magnetic centers. Of considerable interest is that the spin lifetime decreases systematically with increasing dopant concentration, indicating that the number of magnetic centers at the interface increases with increasing dopant concentration. These results reveal a severe limitation of the Hanle technique for measuring spin lifetimes within the interior of the subject material. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A14.00011: Magnetocapacitance in surface-reconstructed LaMnO$_{3}$/SrTiO$_{3}$ multilayers Rainer Schmidt, Javier Garcia-Barriocanal, Norbert Nemes, Mar Garcia-Hernandez, Carlos Leon, Jacobo Santamaria We report on large magnetocapacitance (MC) effects in LaMnO$_{3}$/SrTiO$_{3}$ multilayer structures. Frequency, temperature and magnetic field dependent dielectric spectroscopy was employed using in-plane and out-of-plane measurement set-ups to investigate multilayers of LaMnO$_{3}$ (15 u.c.) and SrTiO$_{3}$ (2 u.c.) with a repetition rate of 8: (LMO 15/STO 2)$_{8}$. Such multilayer structures have been identified previously to display an electron transfer across the epitaxial interface from LMO to STO, orbital reconstruction and a considerable Ti$^{3+}$ magnetic moment near the LMO/STO interface [Garcia-Barriocanal et al. Adv. Mater. 22 (2010) p.627]. We demonstrate moderate in-plane magnetocapacitance (MC) of up to -5 {\%} associated with an intrinsic magneto-electric coupling (MEC) effect originating from magnetic STO layers. Massive out-of-plane MC of up to -52 {\%} was ascribed to current path changes due to magnetically active STO pin-holes and current path meandering. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A14.00012: Enhanced Magnetism in SrRuO$_{3}$ Thin Film by SrTiO$_{3}$ Capping Monolayers Sean Thomas, Bouwe Kuiper, Jeff Botimer, Elliot Persico, Gertjan Koster, Jing Xia Substrate induced mechanical strain is known to reduce the magnetism in itinerant ferromagnetic SrRuO$_{3}$ thin films. Here we show that monolayers of SrTiO$_{3}$ epitaxial capping film can be used to enhance the magnetism in ultra-thin SrRuO$_{3}$ films. For a device with a 6 monolayer thick SrRuO$_{3}$ film, a 2 monolayer thick SrTiO$_{3}$ capping layer can boost SrRuO$_{3}$'s magnetic Curie temperature by 15 Kelvin. Unlike thick substrates, the monolayers-thick SrTiO$_{3}$ capping layer can be patterned using standard lithography methods for making complex oxide electronic devices. We demonstrate a SrRuO$_{3}$ film device with regions of different Curie temperatures by patterning the SrTiO$_{3}$ capping layer. [Preview Abstract] |
Session A15: Focus Session: Exchange Bias and Magnetic Interfaces
Sponsoring Units: GMAG DMPChair: Igor Roshchin, Texas A&M University
Room: 317
Monday, March 18, 2013 8:00AM - 8:12AM |
A15.00001: Influence of magnetic annealing and interdiffusion on the exchange bias of CoFe/IrMn Waldemar Macedo, Luis Fernandez-Outon, Mario Araujo Filho, Raphael Araujo, Jose Ardisson Magnetic annealing is broadly used to set exchange bias (EB). The EB field depends on the magnetic field and the temperature at which the F/AF exchange interaction is set. Atomic interdiffusion is also expected to have strong influence on EB. For systems containing IrMn, different results have been reported regarding the effect of setting EB between 200 and 400 $^{\circ}$C. We study the effect of atomic interdiffusion on the exchange bias of polycrystalline IrMn/($^{57}$Fe$+$CoFe) multilayers due to the magnetic annealing between 225 and 500 $^{\circ}$C. The samples have been prepared by magnetron sputtering, and $^{57}$Fe probe layers (10 {\AA} thick) were grown at the F/AF interface, and 1 nm and 2 nm above it, inside the CoFe layer. Depth-resolved $^{57}$Fe conversion electron M\"{o}ssbauer spectroscopy (CEMS) was used to quantify atomic interdiffusion, and vibrating sample magnetometry was used to monitor the variation of exchange bias and magnetisation. We found that interface sharpness is only affected above $\sim$350 $^{\circ}$C. Three different stages for the setting of exchange bias can be inferred from our results. At temperatures \textless\ 350 $^{\circ}$C, no interdiffusion is observed and the F/AF exchange coupling establishes partial spin alignment of interfacial and bulk AF spins. At intermediate setting temperatures (350-450 $^{\circ}$C) interfacial spin order is dominant over chemical intermixing effects, and both exchange field and coercivity increase up to 450 $^{\circ}$C. Above 450 $^{\circ}$C, severe chemical intermixing reduces significantly ($\sim$50{\%}) the F/AF coupling. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A15.00002: Controlling Exchange Bias in FeMn with Cu Igor V. Roshchin, Dogan Kaya, Pavel N. Lapa, Priyanga Jayathilaka, Hillary Kirby, Casey W. Miller One of the puzzles of exchange bias (EB) that remains unsolved is the origin and role of uncompensated magnetization (UM) in the antiferromagnet (AF). We offer a way of controlling the \textit{intrinsic} EB in FeMn by growing it in contact with Cu. The multilayers of Ta(5~nm)/[Cu(5~nm)/FeMn(\textbf{\textit{t}})]$_{\mathrm{10}}$/Ta(5~nm) with 5~nm \textless \textbf{\textit{t}} \textless 15~nm are deposited by RF and DC magnetron sputtering on top of Si/SiO$_{\mathrm{2}}$. The hysteresis loops at 10 K for field-cooled Cu/FeMn multilayers are EB-shifted, while the samples without Cu exhibit no EB. Unlike the ``classical'' EB observed at the interface of AF-ferromagnet (FM) bilayer systems, this EB is ``intrinsic'' to this system with no separate FM layer. The exchange bias field, $H_{\mathrm{E}}$ scales with the inverse thickness of FeMn. This fits Malozemoff's model,\footnote{ A. P. Malozemoff, Phys. Rev. B \textbf{35}, 3679 (1987), \textbf{37}, 7673 (1988).} where the thickness of the FM is replaced with the thickness of FeMn, which supports that the role of the FM is played by the UM which scales with the thickness of the FeMn film. Coercivity ($H_{\mathrm{C}})$ and $H_{\mathrm{E}}$ dependences on the FeMn thickness and temperature are similar to those for Cu/FeMn/Co samples.$^{\mathrm{\thinspace }}$\footnote{ B. T. Bolon, \textit{et al.}, J. Magn. Magn. Mat. \textbf{309}, 54 (2007).} This suggests that the \textit{intrinsic} EB in Cu/FeMn may be determining the EB in AF-FM samples. The role of Cu in the intrinsic EB in FeMn will be discussed. Work is supported by TAMU-CONACyT Collaborative Research Program, and by NSF (at USF). [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A15.00003: Exchange Bias in Ferromagnetic/Antiferromagnetic/Ferromagnetic Co/FeF$_{2}$/Co Trilayers Trent Johnson, David Lederman We have measured the magnetic properties of Co(20 nm)/FeF$_{2}$(2 nm)/Co(5 nm) trilayers grown on Al$_{\mathrm{2}}$O$_{3}$ substrates via e-beam evaporation. The layers were polycrystalline and the samples were capped with 5 nm of Pd to avoid oxidation. The sample surface was very smooth, as indicated by AFM images where the underlying substrate's atomic terraces were visible, while the interface roughness parameters were on the order of 1 nm determined from x-ray reflectivity. After field-cooling to below the N\'{e}el temperature of FeF$_{2}$ in either 1 kOe and 5 kOe, magnetic hysteresis loops were measured as a function of temperature. We found that both layers have a negative exchange bias, with the exchange bias of the thinner layer larger than that of the thicker layer. In addition, the coercivity below the blocking temperature T$_{\mathrm{B}}$ of the thinner layer was significantly larger than that of the thick layer, even though the coercivity of the two layers is the same for T\textgreater\ T$_{\mathrm{B}}$. The drastic difference in coercivities for T\textless\ T$_{\mathrm{B}}$ illustrates the importance of the interface magnetic order on the reversal mechanism of the ferromagnet. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A15.00004: Probing boundary magnetization through exchange bias in heterostructures with competing anisotropy Yi Wang, Christian Binek Cr$_{2}$O$_{3}$ (chromia) is a magnetoelectric antiferromagnet with a bulk T$_{\mathrm{N}}$ of 307 K. It has been utilized for electrically controlled exchange bias (EB) by taking advantage of voltage-controllable boundary magnetization (BM) occurring as a generic property in magnetoelectric single domain antiferromagnets.\footnote{Xi He, et al., Nature Mater.\textbf{9}, 579-585 (2010)} In the perpendicular Cr$_{2}$O$_{3}$(0001)/CoPd EB system the EB-field shows an order parameter type T-dependence close to T$_{\mathrm{N}}$ reflecting the T-dependence of the BM. At about 150 K a decrease of the EB-field sets in with decreasing temperature suggesting canting of the BM. To evidence this mechanism we use EB as a probe. Specifically, we investigate EB in Permalloy(5nm)/Cr$_{2}$O$_{3}$ (0001)(100nm) with Permalloy and chromia having competing anisotropies. We measure easy axis magnetic hysteresis loops via longitudinal magneto-optical Kerr effect for various temperatures after perpendicular and in-plane magnetic field-cooling. The T-dependence of the EB field supports the canting mechanism. In addition to the all thin film EB system, we explore a Permalloy(10nm)/Cr$_{2}$O$_{3}$(0001 single crystal) heterostructure where magnetoelectric annealing allows selecting Cr$_{2}$O$_{3}$ single domain states. Here the effect of T-dependent canting of the BM is compared with findings in the complementary perpendicular EB system. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A15.00005: Positive exchange bias in thin film multilayers produced with nano-oxide layer Byong Sun Chun, Mohamed Abid, Han-Chun Wu, In Chang Chu, Chanyong Hwang We report a positive exchange bias in thin film multilayers produced with nano-oxide layer.The positive exchange bias, obtained for our system results from an antiferromagnetic coupling between the ferromagnetic CoFe and the antiferromagnetic CoO layers, which spontaneously from on top of the nano-oxide layer. The shift in the hysteresis loop along the direction of the cooling field and the change in the sign of exchange bias are evidence of antiferromagnetic interfacial exchange coupling between the CoO and CoFe layers. Our calculation indicates that uncompensated oxygen moments in the nano-oxide layer results in antiferromagnetic interfacial exchange coupling between the CoO and CoFe layers. One of the interesting features observed with our system in that it displays the positive exchange bias even above the bulk Neel temperature of CoO [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A15.00006: Isothermal electric field-tuning of Exchange bias training in Cr$_{2}$O$_{3}$/PdCo Will Echtenkamp, Christian Binek Voltage-controlled exchange bias (EB) is investigated in a Cr$_{2}$O$_{3}$/PdCo EB heterosystem where a ferromagnetic and perpendicular anisotropic Pd/Co multilayer has been deposited on a (0001) Cr$_{2}$O$_{3}$ (chromia) single crystal. The EB of the system arises from chromia's electrically controllable boundary magnetization (BM) which is switched isothermally and at room temperature by magnetoelectric means [1]. The BM couples to the bulk AF order parameter and follows the latter during switching. In the work reported here, we electrically and isothermally tune chromia into distinct AF multi-domain states. As a result, exchange bias training, which originates from triggered rearrangements of the AF domain state of the pinning system during consecutively cycled hysteresis loops, can be tuned in a controlled manner between zero and sizable effects. We quantify the training effect through best fits of our Landau-Khalatnikov analytic expression [2] to the EB vs loop number. The electric field dependence of the fitting parameters is interpreted in terms of the hysteretic E-field dependence of the AF order parameter.\\[4pt] [1] Xi He, et al., Nature Mater.9, 579--585 (2010).\\[0pt] [2] Ch. Binek, Phys. Rev. B. 70, 014421 (2004). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A15.00007: Depth profiling of interfacial spin complexities in magnetic heterostructures Invited Speaker: Sujoy Roy Attentively restrained interfaces or superlattices between two materials can lead to emergent functionalities not shown by either constituents in their bulk form. Direct quantitative investigation of spatio-temporal correlations of magnetic and electronic properties of such interfaces is crucial in controlling and tailoring the close proximity of competing energy landscape that naturally exist in these systems. Due to the smallness and buried nature of the magnetization, characterizing these materials at the appropriate length scale is of considerable challenge. In this talk I will give examples from a variety of systems where the unique sensitivity of soft x-ray beams in reflection geometry have been exploited to obtain a quantitative description of the complex magneto-chemical depth profile across the interface between ferromagnetic (F) and antiferromagnetic (AF) thin films. In exchanges bias Co/FeF$_{\mathrm{2}}$ heterostructures we have found antiferromagnetic coupling across the interface with the net magnetization having a twisted ``fan-like'' structure near the F/AF interface. For Py/CoO we observed a redox reaction driven novel interfacial layer that has magnetic properties very different from bulk. We found that 10{\%} of the net spins in this layer get pinned antiparallel to the cooling magnetic field at low temperatures. In complex oxide BiFeO$_{\mathrm{3}}$-La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ we have obtained direct experimental evidence of transitory layers, ionic rearrangements and depleted magnetism at the BiFeO$_{\mathrm{3}}$-La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ interface. Our examples show that interface-selective probing of magnetism in thin film heterostructures can provide vital understanding needed for rational design of future nanoelectronic devices. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A15.00008: Temperature Dependence of Current Induced Magnetic Domain Wall Motion in a Multilayered Co/Ni Nanowire with MgO Cap Kohei Ueda, Ryo Hiramatsu, KabJin Kim, Daichi Chiba, Takahiro Moriyama, Hironobu Tanigawa, Eiji Kariyada, Tetsuhiro Suzuki, Yoshinobu Nakatani, Teruo Ono Current-induced magnetic domain wall motion (CIDWM) has been investigated not only for the fundamental physics but also for its potential application for nonvolatile magnetic random access memory. Our group reported that adiabatic spin transfer torque (STT) dominates the DW motion in nanowires made of a perpendicularly magnetized Co/Ni multilayer with symmetrical top and bottom non-magnetic layers (Ta/Pt and Pt/Ta). Recently, new aspect of the DW motion was reported that DW moves against electron flow direction in asymmetric AlO/Co/Pt system, which is in contrast to STT theory that predicts the DW motion along electron flow direction. We found, in a nanowire made of an Co/Ni multilayer with asymmetric top (MgO) and bottom (Pt/Ta) layers, that the DW moves against electron flow direction as reported in AlO/Co/Pt system. We also investigated the temperature dependences of the threshold current density for DW displacement (Jth). It was found that Jth increases with decreasing device temperature whereas it is almost independent of temperature in a symmetric Co/Ni system, suggesting that the observed DW motion was not simply dominated by the adiabatic STT brought by the electron flow in the Co/Ni multilayer. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A15.00009: Electrical Probing of Magnetic Phase Transition and Domain Wall Motion in Single-Crystalline Mn$_5$Ge$_3$ Nanowire Jianshi Tang, Chiu-Yen Wang, Kang L. Wang, Lih-Juann Chen We studied the magnetic phase transition and domain wall motion in single-crystalline Mn$_{5}$Ge$_{3}$ nanowires fabricated by thermally germaniding Ge nanowires with Mn contacts. The R-T curve showed a clear slope change near 300 K accompanied by a magnetic phase transition from ferro- to para-magnetism. Near this phase transition, the critical behavior was characterized by a power-law relation with a critical exponent of about 0.07. Besides, a cusp revealed in the dR/dT curve at about 67 K was attributed to a possible magnetic transition between non-collinear and collinear ferromagnetic states. Furthermore, temperature-dependent magneto-transport measurements demonstrated a hysteretic, symmetric and stepwise axial magnetoresistance. The interesting features of abrupt jumps indicated the presence of multiple domain walls in the Mn$_{5}$Ge$_{3}$ nanowire and the annihilation of domain walls driven by the magnetic field. The fitting on the temperature-dependent depinning fields yielded an energy barrier of 0.166 eV based on the Kurkijarvi model describing the domain wall depinning as thermally assisted escape from a single energy barrier. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A15.00010: Topological classification of domain walls in a cylindrical nanowire Se Kwon Kim, Christopher Mogni, Oleg Tchernyshyov We classify possible configurations of domain walls in a cylindrical nanowire [1-3] using topology. Dipolar interactions induce effective shape anisotropy so that magnetization tends to be tangential to the surface locally and is parallel to the axis of the wire in the ground states. Topological defects in the bulk are Bloch points with integer skyrmion numbers [second homotopy group $\pi_2(S^2)$]. The surface anisotropy gives rise to surface defects (boojums) with integer vorticity [first homotopy group $\pi_1(S^1)$] and half-integer skyrmion number [relative second homotopy group $\pi_2(S^2,S^1)$]. These defects are weakly bound by the easy-axis anisotropy into composite domain walls. Thus transformations and mergers of domain walls are constrained by the topological conservation laws. Long-lived textures left behind after annihilation of domain walls are classified by the third homotopy group $\pi_3(S^2)$.\\[4pt] [1] R. Hertel, Physica B \textbf{343}, 206 (2004).\\[0pt] [2] R. Wieser, U. Nowak, and K. Usadel, Phys. Rev. B \textbf{69}, 1 (2004).\\[0pt] [3] N. Cooper, Phys. Rev. Lett. \textbf{82}, 1554 (1999). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A15.00011: Domain wall remote pinning in magnetic nano wires Dan Read, Jorge Miguel, Francesco Maccherozzi, Stuart Cavill, Sarnjeet Dhesi In the current race for information storage media with ever increasing density the position of magnetic domain walls, the region in a magnetic system where the local magnetization continually rotates its direction between adjacent magnetic domains, is one of the most promising routes for future storage media devices. Information storage requires ultrafast read-out and writing operations, but domain walls need to be pinned so that the information is safely stored in the long term. Here we investigate the use of remote magnetostatic charges to trap domain walls. By using X-ray photoelectron emission microscopy we have followed the position of domain walls of opposite charge being pinned or repelled by pinning potentials of increasing strength. Micromagnetic simulations show an excellent agreement with the experimental results. We demonstrate the attractive or repulsive character of the interaction between domain wall and trap depending upon the sign of their magnetic charges. These quasi-static experiments are the antecedent to ultrafast time-resolved XMCD-PEEM experiments where the spin-transfer torque effect will be studied dynamically by applying picosecond-long current pulses across the magnetic nanowire. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A15.00012: ABSTRACT WITHDRAWN |
Session A16: Focus Session: Spin-Dependent Physics in Organic Compounds
Sponsoring Units: GMAG DMPChair: Christoph Boehme, University of Utah
Room: 318
Monday, March 18, 2013 8:00AM - 8:36AM |
A16.00001: Competing Mechanisms in Organic Magnetoresistance Invited Speaker: Bert Koopmans A surprisingly large ``organic magnetoresistance'' (OMAR) has been found in both polymers and small molecule organic semiconductors at relatively small applied magnetic fields ($\sim$ 5 mT) and at room temperature. Unlike spin-injection devices, where the occurrence of a finite \textit{spin polarization} of the current is essential for measuring a finite magnetoresistance, OMAR is generally considered to be due to \textit{spin correlations} between spin carrying particles in the organic material. Although the microscopic mechanisms of hyperfine field induced spin mixing are relatively well understood, it is still intensively debated which particles are involved and how they can affect the current in such a drastic manner. In this presentation recent developments and new insights as to the underlying physics are discussed. Quantitative models will be introduced, based on different pairs of particles and mechanisms, and giving rise to effects at a variety of field scales. It will be discussed how specific device physics causes a non-trivial relation between microscopic spin-dependent reactions and macroscopic device behaviour. Finally, it will be shown how comprehensive studies on especially engineered organic systems, including polymer-fullerene blends and molecular doping, can be used to pinpoint the relevance of different mechanisms in the complementary regimes. The experimentally observed linewidth, sign and amplitude of both ``high-field'' (\textgreater 100 mT) and ``low-field'' ($\sim$ 5 mT) effects, as well as their bias voltage dependence display very pronounced features as a function of fullerene doping. They provide unique fingerprints for which mechanism is of relevance. After careful analysis, this allows for identification of three earlier proposed mechanisms, involving exciton-charge, electron-hole and bipolaron (polarons of like charge) reactions. Present activities are aiming at using this insight for tailoring OMAR response by design. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A16.00002: Recent Advance in Organic Spintronics and Magnetic Field Effect Invited Speaker: Z. Valy Vardeny In this talk several important advances in the field of Organic Spintronics and magnetic field effect (MFE) of organic films and optoelectronic devices that have occurred during the past two years from the Utah group will be surveyed and discussed. (i) Organic Spintronics: We demonstrated spin organic light emitting diode (spin-OLED) using two FM injecting electrodes, where the electroluminescence depends on the mutual orientation of the electrode magnetization directions [1]. This development has opened up research studies into organic spin-valves (OSV) in the space-charge limited current regime. (ii) Magnetic field effect: We demonstrated that the photoinduced absorption spectrum in organic \textit{films} (where current is not involved) show pronounced MFE [2]. This unravels the underlying mechanism of the MFE in organic devices, to be more in agreement with the field of MFE in Biochemistry. (iii) Spin effects in organic optoelectronic devices: We demonstrated that certain spin 1/2 radical additives to donor-acceptor blends substantially enhance the power conversion efficiency of organic photovoltaic (OPV) solar cells [3]. This effect shows that studies of spin response and MFE in OPV devices are promising.\\[4pt] In collaboration with T. Nguyen, E. Ehrenfreund, B. Gautam, Y. Zhang and T. Basel.\\[4pt] [1] Nguyen \textit{et al}., \textit{Science} 337, 204 (2012);\\[0pt] [2] Gautam \textit{et al}. PRB 85, 205207 (2012);\\[0pt] [3] Zhang \textit{et al}. \textit{Nature Commun}. 3, 1043 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A16.00003: Spin-dependent charge carrier recombination in PCBM Hiroki Morishita, William J. Baker, David P. Waters, Rachel Baarda, John M. Lupton, Christoph Boehme We present room temperature pulsed electrically detected magnetic resonance (pEDMR) measurements on [6,6]-phenyl-C$_{61}$-butyric acid methyl ester (PCBM) (electron acceptor) thin film unipolar and bipolar devices. Our study aimed at identifying the dominating spin-dependent transport and recombination processes therein. Experimentally, the devices were operated under a constant positive bias, and the resultant transient current response was then monitored after the application of a short resonant microwave pulse excitation. The measurements did not reveal any observable signal for unipolar electron devices which suggests that spin-dependent transport mechanisms are not dominant in PCBM. However, under bipolar injection, at least two pronounced spin-dependent signals were detected whose magnitudes increased as the devices degraded upon exposure to air. Electrical detection of spin-Rabi beat oscillation revealed that one of these two signals is due to weakly coupled pairs of spins with s$=$1/2. We therefore attribute this signal to electron-hole recombination. This observation shows that while PCBM is a poor hole conductor, hole injection can be significant. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A16.00004: Robust Absolute Magnetometry with Organic Thin-Film Devices David P. Waters, William J. Baker, Kapil Ambal, Rachel Baarda, Hiroki Morishita, Kipp van Schooten, Dane R. McCamey, John M. Lupton, Christoph Boehme Magnetometers based on organic thin film materials have attracted considerable interest in recent years as they can be manufactured at very low cost and on flexible substrates. In spite of these advantages, the technological relevance of such magnetoresistive sensors is limited due to their narrow magnetic field ranges ($\sim$30mT) and the continuous calibration required to compensate temperature fluctuations and materials degradation. Conversely, magnetic resonance based sensors, which utilize fundamental physical relationships for extremely precise measurements of fields, are usually large and expensive. This presentation will discuss an organic magnetic resonance based magnetometer [1], employing spin-dependent electronic transitions in an organic diode, which combines the low-cost thin-film fabrication and integration properties of organic electronics with the precision of a magnetic resonance based sensor.\\[4pt] [1] Baker et al., Nature Commun. 3, 898 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A16.00005: Organic magnetoresistance near saturation: mesoscopic effects in small devices Robert Roundy, Zeev Vardeny, Mikhail Raikh In organic light emitting diodes with small area the current may be dominated by a finite number, $N$ of sites in which the electron-hole recombination occurs. As a result, averaging over the hyperfine magnetic fields, ${\mathbf b}_h$, that are generated in these sites by the environment nuclei is incomplete. This creates a random ({\em mesoscopic}) current component, $\delta I({\mathbf B})$, at field ${\mathbf B}$ having relative magnitude $\sim N^{-1/2}$. We demonstrate that mesoscopic fluctuations develop at fields $|{\mathbf B}| \gg |{\mathbf b}_h|$, where the average magnetoresistance is near saturation. These fluctuations originate from the slow beating between $S$ and $T_{0}$ states of the recombining $e$-$h$ spin pair-partners. We identify the most relevant processes responsible for the current fluctuations as due to anomalously slow beatings that develop in sparse $e$-$h$ polaron pairs at sites for which the ${\mathbf b}_h$ projections on the external field direction almost coincide. To find the characteristic period $\Delta {\mathbf B}$ of the fluctuations, we calculate the correlator $K({\mathbf B}, \Delta {\mathbf B}) = \left<\delta I\left( {\mathbf B} - \Delta {\mathbf B} \right) \delta I\left( {\mathbf B} + \Delta {\mathbf B} \right)\right>$. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A16.00006: Photocontrolled spin polarization at hybrid organic-ferromagnetic interfaces Yan Wang, Hai-Ping Cheng We report a first-principles study of magnetic properties at an organic-ferromagnetic interface by placing light-switchable azobenzene molecules on a Fe/W(110) surface. Our calculations clearly demonstrate that the magnetic properties of the hybrid interface, such as the local magnetic moment and spin polarization, change significantly as the azobenzene molecule switches reversibly from the trans to the cis form. The molecule-surface interaction, which determines the feasibility of photo-switching of the azobenzene on the surface, can be altered by chemical functionalization of the molecule. Specifically, we find that substitution of the H atoms with electronegative F atoms substantially reduces the binding energies of the molecule on the Fe surface. This study suggests a new way to manipulate magnetism by application of light at organic-ferromagnetic hybrid interfaces. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A16.00007: The Effects of Fringe Fields on Organic Magnetoresistance Nicholas Harmon, Ferran Maci\`a, Fujian Wang, Markus Wohlgenannt, Andrew Kent, Michael Flatt\'e The importance of random hyperfine fields is now widely acknowledged as a vital ingredient for the phenomena of organic magnetoresistance (OMAR). Recent experiments (Phys. Rev. X 2 021013 (2012)) have shown that another type of random field - fringe fields due to a nearby ferromagnet - can also dramatically affect magnetoconductivity. A theoretical analysis of the fringe field OMAR is challenging due to the different properties of the fringe fields when compared to the hyperfine fields. For instance, the range of fringe field strengths is 1-2 orders of magnitude larger than that of the hyperfine couplings. The correlation length between fringe fields is also larger by the same degree. We use a recent theory of OMAR that is well-suited to numerically calculate the magnetoresistance with both hyperfine and fringe fields present. We find agreement with key features of experimental fringe-field magnetoresistance dependences on applied magnetic field, including the field values of extrema of the magnetoresistance, the region of large magnetoresistance effects from the fringe fields, and the sign of the effect. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A16.00008: Using photoexcited triplet states to probe small-molecule endohedral fullerenes by ESR Vasileia Filidou, Salvatore Mamone, Alessandro Bagno, Federico Rastrelli, Yasujiro Murata, Koichi Komatsu, Xuegong Lei, Yongjun Li, Nicholas J. Turro, Malcolm H. Levitt, John J.L. Morton Ortho to para conversion of molecular hydrogen $\mbox{H}_{2} $ can be catalyzed by the use of a coupled paramagnet such as a fullerene in its triplet state. The recently synthesized endohedral fullerenes $\mbox{H}_{2} @\mbox{C}_{60} $ and $\mbox{H}_{2} @\mbox{C}_{70} $ were photoexcited to their long lived triplet state (S= 1) and probed by electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) spectroscopic techniques. With these techniques we characterized both spin systems by extracting the hyperfine interaction the kinetic parameters of the triplet state and the spin relaxation times. The observed variations of the linewidths and the lineshape are discussed in the context of a dynamic Jahn-Teller effect. Irradiation of the $\mbox{H}_{2} @\mbox{C}_{70} $ at different temperatures reveals that the fullerene triplet state can serve as a spin catalyst for ortho to para interconversion while for the triplet $\mbox{H}_{2} @\mbox{C}_{60} $ no appreciable interconversion is observed [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A16.00009: Electron Spin Relaxation Dynamics in Single-Walled Carbon Nanotubes William Rice, Ralph Weber, Pavel Nikolaev, Sivaram Arepalli, Vladimir Burka, Ah-Lim Tsai, Junichiro Kono We have measured temperature-dependent electron spin resonance (ESR) in an ensemble of single-walled carbon nanotubes. From the linewidths of these traces, we clearly observe that the spin-spin dephasing time, $T_{2}$, decreases by over a factor of two when temperature, $T$, is lowered from 300 K to 3 K, a phenomenon we attribute to motional narrowing. We fit the temperature dependence of $T_{2}$ with a hopping model and obtain a spin hopping frequency of 285 GHz. At selected temperatures below 100 K, we performed microwave power-dependent scans to investigate the saturation behavior of the ESR signal. A homogenously broadened two-level model fit the saturation data well, which allowed us to extract the spin-lattice relaxation times, $T_{1}$, for the investigated temperature range. We observed that the spin-lattice relaxation rate,1/$T_{1}$, is proportional to $T$ from 100 K to 3 K, suggesting that the relaxation occurs via phonon emission. Last, we show that the Dysonian lineshape asymmetry, which is roughly proportional to the conductivity, follows a three-dimensional variable-range hopping behavior from 3 K to 20 K, from which we estimate a spin hopping localization length of 100 nm. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A16.00010: Mechanical read out of a single electron spin in a carbon nanotube Guido Burkard, Heng Wang, Philipp Struck The spin of a single electron in a suspended carbon nanotube can be read out by using its coupling to the nano-mechanical motion of the nanotube. To show this, we consider a single electron confined within a quantum dot formed by the suspended carbon nanotube. The spin-orbit interaction induces a coupling between the spin and one of the bending modes of the suspended part of the nanotube [1]. We simulate the response of the system to the external driving with a Jaynes-Cummings model by solving the quantum master equation. Using parameters comparable to those used in recent experiments, we show how information of the spin state of the system can be acquired by measuring its mechanical motion [2]. The mechanical motion can be detected by observing the current through a nearby charge detector. \\[4pt] [1] A. Palyi, P.R. Struck, M. Rudner, K. Flensberg, G. Burkard, Phys. Rev. Lett. 108, 206811 (2012).\\[0pt] [2] H. Wang, P. R. Struck, G. Burkard, manuscript in preparation. [Preview Abstract] |
Session A17: Focus Session: Multiferroic Skyrmions
Sponsoring Units: DMP GMAGChair: Nicholas Butch, Lawrence Livermore National Laboratory
Room: 319
Monday, March 18, 2013 8:00AM - 8:12AM |
A17.00001: Simulation on doping dependent phase transition in MnSi by Monte Carlo method Jhih-An Yang, Dmitry Reznik Recently, the skyrmion lattice has been found in the A phase of the itinerant helimagnet MnSi by small angle neutron scattering and the magnetic analogue of blue phases has been reported to explain a number of puzzling features of MnSi. Here we use a different approach based on Monte Carlo methods, showing the thermal behavior around transition temperatures in doped systems under the simplest Dzyaloshinsky-Moriya nearest-neighbor interactions. Interestingly, the transition temperature decreases with increasing doping concentrations, which is consistent with the experimental observations. We also show how the topological order parameter changes with temperature and its relation with the specific heat and thermal fluctuations. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A17.00002: Observation of coherent helimagnons in the Skyrmionic helimagnets Jake Koralek, Dennis Meier, James Hinton, Andreas Bauer, Sid Parameswaran, Ashvin Vishwanath, Christian Pfleiderer, Roomamoorthy Ramesh, Bob Schoenlein, Joe Orenstein In MnSi and Fe$_{1-x}$Co$_{x}$Si the interplay between the spin-orbit and exchange interactions leads to a variety of helical magnetically ordered states. Perhaps the most interesting of these is the Skyrmion lattice phase in which the spins form topologically-stabilized vortices which decouple from the host lattice to form their own lattice structure. We use pump-probe reflectivity and Kerr rotation to study the dynamics in these materials, observing coherent collective excitations unique to helimagnets known as helimagnons. Monitoring helimagnon decay in the time-domain directly yields the Gilbert damping parameter in these systems. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A17.00003: The Chiral Hall Effect in the Presence of Impurities: a Study on Mn$_{1-x}$Fe$_x$Si Benjamin Chapman, Thomas Wolf, Minhyea Lee Recently much attention has been paid to the itinerant ferromagnet MnSi. This is due largely to the emergence of an exotic topological object--the so-called skyrmion, which forms a lattice near $T_c$ at ambient pressure. Past efforts to understand this configuration have observed its response to various perturbations, including temperature gradients, strong electric fields, and hydrostatic pressure. Here, we present Hall effect measurements on single crystals of Fe doped MnSi at ambient pressure, exploring how impurities interact with electronic charge and the long range magnetic order in this peculiar magnetic phase. In pure MnSi, the chiral Hall signal below $T_c$ is significantly enhanced as $T_c$ is suppressed by application of pressure. With chemical doping, though, the chiral Hall signal is substantially weakened, appearing in a narrower window of temperature and magnetic field relative to pure MnSi under pressure with comparable $T_c$. Interestingly, however, and in contrast to hall data taken under pressure, the chiral contribution in iron doped MnSi is found to have opposite sign as the anomalous Hall effect. We will discuss the implications of this Hall effect result and compare it to measurements on pure MnSi under pressure. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A17.00004: Observation of Skyrmions in a Multiferroic Material Invited Speaker: Shinichiro Seki Magnetic skyrmion is a topologically stable particle-like object, which appears as nanometer-scale vortex-like spin texture in a chiral-lattice magnet [1]. In metallic materials (MnSi, FeGe, Fe$_{1-x}$Co$_x$Si etc), electrons moving through skyrmion spin texture gain a nontrivial quantum Berry phase, which provides topological force to the underlying spin texture and enables the current-induced manipulation of magnetic skyrmion [2]. Such electric controllability, in addition to the particle-like nature, is a promising advantage for potential spintronic device applications. Recently, we newly discovered that skyrmions appear also in an insulating chiral-lattice magnet Cu$_2$OSeO$_3$ [3,4]. We find that the skyrmions in insulator can magnetically induce electric polarization through the relativistic spin-orbit interaction, which implies possible manipulation of the skyrmion by external electric field without loss of joule heating [5]. The present finding of multiferroic skyrmion may pave a new route toward the engineering of novel magnetoelectric devices with high energy efficiency. In this talk, the latest experimental and theoretical results on the dynamical aspect of magnetoelectric skyrmions will also be discussed.\\[4pt] [1] S. M{\"u}hlbauer et al., Science {\bf 323}, 915 (2009).\\[0pt] [2] F. Jonietz et al., Science {\bf 330}, 1648 (2010).\\[0pt] [3] S. Seki et al., Science {\bf 336}, 198 (2012).\\[0pt] [4] S. Seki et al., Phys. Rev. B {\bf 85}, 220406(R) (2012).\\[0pt] [5] S. Seki et al., Phys. Rev. B {\bf 86}, 060403(R) (2012). [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A17.00005: Inertia and chiral edge modes of a skyrmion magnetic bubble Imam Makhfudz, Benjamin Krueger, Oleg Tchernyshyov Dynamics of topological defects is a topic of longstanding interest in magnetism. The attention to it stems from rich basic physics as well as from its connection to technological applications. The dynamics of a vortex in a thin-film ferromagnet resembles the motion of a charged massless particle in a uniform magnetic field. Similar dynamics is expected for other magnetic textures with a nonzero skyrmion number. However, recent numerical simulations revealed that skyrmion magnetic bubbles show significant deviations from this model. In this talk we present the derivation of the correct dynamical model of a skyrmion magnetic bubble. We first introduce our model phenomenologically and then derive it from the standard theory of a thin-film ferromagnet. This allows us to characterize not only the center-of-mass motion of the bubble but also the dynamics of its shape within the same framework.We show that a skyrmion bubble possesses inertia and derive its mass from the standard theory of a thin-film ferromagnet. Besides center-of-mass motion, other low energy modes are waves on the edge of the bubble traveling with different speeds in opposite directions. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A17.00006: Universal current-velocity relation of skyrmion motion in chiral magnets Junichi Iwasaki, Masahito Mochizuki, Naoto Nagaosa Current-driven motion of the magnetic domain wall requires large critical current density $j_c \sim 10^9-10^{12}$ A/m$^2$, at which the joule heating is a serious problem. The skyrmions recently discovered in chiral magnets, on the other hand, have much smaller critical current of $j_c \sim 10^5-10^6$ A/m$^2$. We present a numerical simulation of the Landau-Lifshitz-Gilbert equation, which reveals a remarkably robust and universal current-velocity relation of the slyrmion motion driven by the spin transfer torque unaffected by either impurities or nonadiabatic effect in sharp contrast to the case of domain wall or spin helix (HL). Simulation results are analyzed using a theory based on Thiele's equation, and it is concluded that this surprising behavior is due to the Magnus force and flexible shape-deformation of individual skyrmions and skyrmion crystal (SkX), which enable them to avoid pinning centers and then weaken the net pinning force. Dynamical deformation of SkX leads to the fluctuation of Bragg peak with large amplitude, which can be detected by the recent neutron-scattering experiment. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A17.00007: Cooperative organization of local and itinerant moments in antiferromagnetic GdSi Yejun Feng, Jasper van Wezel, J.W. Kim, Y. Ren, P.B. Littlewood, B. Mihaila, R.K. Schulze, J.-Q. Yan, Jiyang Wang, Nayoon Woo, A. Palmer, D.M. Silevitch, T.F. Rosenbaum With strong correlations and reduced dimensionality, spin and charge instabilities emerge in a broad range of materials. Direct magnetic exchange, interactions mediated by the conduction electrons, and coupling to the lattice are all familiar drivers of density waves. In materials which have significant localized and itinerant spins, it is not obvious which will induce order. We combine transport, magnetic diffraction, and photoemission studies with band structure calculations to elucidate the nature of successive antiferromagnetic transitions in GdSi. GdSi has both sizable local moments and a partially-nested Fermi surface of itinerant spins, without confounding contributions from orbital effects. We propose a new route to incommensurate order, based on a cooperative feedback mechanism between localized Gd 4f and itinerant Gd 5d electronic spins. The nested Fermi surface of the itinerant electrons induces a strong interaction between local moments at the nesting vector, while the presence of ordered local moments in turn provides the necessary coupling strength for a spin density wave to form among the itinerant electrons. This mechanism echoes the cooperative interactions between itinerant electrons and localized ionic cores in charge density wave materials, and should be germane across a spectrum of transition metal and rare earth intermetallic compounds. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A17.00008: Magnetic Phase Diagram of the Binary Intermetallic GdSi D.M. Silevitch, Yejun Feng, J.-Q. Yan, Jiyang Wang, Nayoon Woo, T.F. Rosenbaum The magnetic phase diagram of the binary intermetallic GdSi is investigated via magnetotransport and magnetization measurements along all three principal crystal axes. At zero applied field, two distinct phase transitions are observed at 53 and 54.7 K, corresponding to a spin flip and Neel transition, respectively. An additional ordered antiferromagnetic state appears for magnetic fields applied in the a-c plane, transitioning to a ferromagnetic ground state at $H\sim20$ T. Although Gd ions are well characterized by local spin-only moments, and the magnetic anisotropy is small in this system, the additional antiferromagnetic ordering transition is observed to be considerably softer along c than along a. The interplay between this complex magnetic phase diagram, the band structure, and quantum effects will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A17.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 10:12AM - 10:24AM |
A17.00010: Near room temperature helical magnetism in Fe(1-x)Cr(x)Ge Yuen Yiu, Nirmal Ghimire, Michael McGuire, Ashfia Huq, David Mandrus, Stephen Nagler Helical magnetic structures in chiral metallic magnets have attracted much interest recently because of the observation of complex spin textures, for example the skyrmion lattice.~ FeGe is a known B20 spiral ferromagnet that orders at the relatively high temperature of 280K with a helical modulation period of 700 angstroms, which propagates along either the [100] or [111] directions depending on temperature. ~Here we report a study on the evolution of helimagnetism as a function of Cr doping on a series of Fe(1-x)Cr(x)Ge samples with x $=$ 0.03 to 1. Magnetic susceptibility measurements have shown that the ferromagnetic-like transition associated with helimagnetism in FeGe is suppressed around x $=$ 0.4.~ [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A17.00011: Ferromagnetic ordering and halfmetallic state in a shandite: Co$_3$Sn$_2$S$_2$ Walter Schnelle, Andreas Leithe-Jasper, Helge Rosner, Richard Weihrich The rapid advance in spintronics challenges an improved understanding of the underlying microscopic properties. Here, we present a joint experimental and theoretical study of Co$_3$Sn$_2$S$_2$ (shandite) and related compounds. From magnetic susceptibility, specific heat and magneto-transport measurements on a shandite single crystal sample we find a phase transition to a ferromagnetic metallic state at 177\,K with a saturation moment of 0.92 $\mu_B$/f.u. Full potential electronic structure calculations within the local spin density approximation result in a halfmetallic ferromagnetic groundstate with a moment of 1 $\mu_B$/f.u.\ and a tiny gap in the minority spin channel. The calculated structure optimization and structure variations show that the size of the gap is rather sensitive to the lattice geometry. Possiblities to stabilize the halfmetallic ferromagnetic behavior by various substitutions have been studied theoretically and will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A17.00012: Thermodynamic and anisotropic properties of single crystalline RCo$_{2}$Ge$_{2}$ (R = Y, La-Nd, Sm-Tm) Tai Kong, Malinda Buffon, Xiao Lin, Alex Thaler, Charles Cunningham, Sergey Bud'ko, Paul Canfield Single crystals of RCo$_{2}$Ge$_{2}$ (R = Y, La-Nd, Sm-Tm) were grown using a self-flux method and were characterized from 1.8-300 K by heat capacity, magnetization and in-plane resistivity measurements. Anisotropic metamagnetism was studied at 1.8 K up to 9 T. Due to a strong crystal electric field (CEF) effect, the magnetic ordering temperatures of the heavy rare earth members do not follow the de Gennes scaling, but rather a CEF modified trend. The RCo$_{2}$Ge$_{2}$ series offers an opportunity to study different types of magnetic anisotropy ranging from Heisenberg-like GdCo$_{2}$Ge$_{2}$ to Ising-like TbCo$_{2}$Ge$_{2}$. Correlation between the local moments and conduction electrons as well as the influence of interplay between CEF effect and long-range indirect exchange interaction (RKKY type) will also be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A17.00013: Resistivity and anisotropic magnetization of single crystalline $R$Ni$_{\mathrm{1-x}}$Bi$_{\mathrm{2\pm y}}$ (R $=$ La--Nd, Sm, Gd--Dy) X. Lin, Warren Straszheim, Sergey Bud'ko, Paul Canfield We present a detailed study of $R$Ni$_{\mathrm{1-x}}$Bi$_{\mathrm{2\pm y}}$ (R $=$ La--Nd, Sm, Gd--Dy) single crystals by measurements of temperature dependent magnetic susceptibility, magnetization, and electrical resistivity. The isostructural compounds \textit{RT}Bi$_{2}$ and \textit{RT}Sb$_{\mathrm{2}}$ are known to have widths of formation, except for $T =$ Ag. The $R$Ni$_{\mathrm{1-x}}$Bi$_{\mathrm{2\pm y}}$ series forms with partial Ni occupancy as well as a variable Bi occupancy. For R $=$ Ce--Nd, Gd--Dy, the $R$Ni$_{\mathrm{1-x}}$Bi$_{\mathrm{2\pm y}}$ compounds show local-moment-like behavior and order antiferromagnetically at low temperatures. Determination of anisotropies as well as antiferromagnetic ordering temperatures for $R$Ni$_{\mathrm{1-x}}$Bi$_{\mathrm{2\pm y}}$ (R $=$ Ce--Nd, Sm, Gd--Dy) have been made. Although crystalline samples from this family exhibit minority, second phase, superconductivity at low temperatures associated with Ni-Bi and Bi contamination, no evidence of bulk superconductivity has been observed. [Preview Abstract] |
Session A18: Focus Session: Spin-transfer Torque: Devices and Dynamics
Sponsoring Units: DMP FIAP GMAGChair: Dan Ralph, Cornell University
Room: 320
Monday, March 18, 2013 8:00AM - 8:12AM |
A18.00001: Spin transfer torque in ferroelectric tunnel junctions Arthur Useinov, Aurelien Manchon The worldwide interest for spintronics grows up every year, magnetic oscillators and resistance switchers became an important part of electronics with promising applications such as tunable microwave radiation, magnetic memory cells, magnetic field sensors, etc. A non-equilibrium spin-dependent transport in magnetic tunnel junctions comprising a ferroelectric barrier was studied. The exact solutions of the free electron Schr\"odinger equation for electron tunneling in the presence of interfacial screening are obtained by Bessel and Airy functions. As a result, bias-dependence of the tunneling magneto- and electro-resistance are obtained. The barrier asymmetry induced by the ferroelectric polarization produces strong modifications compared to regular tunnel junctions in the bias-dependence of the transport properties. Furthermore, manipulating the electric polarity of the barrier provides a way to control the magnitude and sign of the spin transfer torque. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A18.00002: Spin-transfer torque at finite bias from density functional theory and non-equilibrium Green's functions Stefano Sanvito, Maria Stamenova, Igor Popov, Ivan Rungger The spin-transfer torque (STT) exerted on a magnetic layer by a spin-polarized current represents a powerful handle to manipulate the magnetization. This can make magnetic random access memories a reality. We have now implemented STTs in the electron transport code Smeagol (www.smeagol.tcd.ie), which combines density functional theory with the non-equilibrium transport formalism. In particular we are able to compute the STT both in the linear response limit and at finite bias, and for magnets with an arbitrary complex electronic structure, including spin-orbit interaction. Examples will be provided for both magnetic tunnel junctions and spin-polarized scanning tunnel microscopy of magnetic ions on non-magnetic surfaces. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A18.00003: Current-Induced Spin Wave Instability Scott Bender, Yaroslav Tserkovnyak, Arne Brataas Current in conducting ferromagnets imparts angular momentum to the magnetic texture. Above a critical current, an instability is reached wherein this angular momentum transfer is able to overcome intrinsic damping, and spin waves begin to grow exponentially in time. We examine the conditions required to observe this instability for bulk and surface spin waves in different dimensions, and investigate the subsequent spin wave turbulence engendered by nonlinear terms in the Hamiltonian that couple different modes. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A18.00004: Magnetic droplets in nano-contact spin-torque oscillators with perpendicular magnetic anisotropy Invited Speaker: Johan {\AA}kerman The theoretical prediction, by Ivanov and Kosevich [1], of ``magnon drop'' solitons in thin films with perpendicular magnetic anisotropy (PMA) and zero damping, dates back to the 1970s. More recently, Hoefer, Silva and Keller [2], demonstrated analytically and numerically that related ``magnetic droplet'' solitons should be possible to excite in nano-contact spin-torque oscillators (NC-STOs) based on PMA materials, where spin transfer torque locally realizes the zero-damping condition required in [1]. In my talk, I will present the first experimental demonstration of such magnetic droplets, realized using 50-100 nm diameter nano-contacts (NCs) fabricated on top of orthogonal GMR stacks of Co8/Cu/Co0.3[Ni0.8/Co0.4]x4 (thicknesses in nm). The nucleation of a magnetic droplet manifests itself as a dramatic 10 GHz drop in microwave signal frequency at a drive-current dependent critical perpendicular field of the order of 0.5 - 1 T. The drop in frequency is accompanied by a simultaneous sharp resistance increase of the device and a sign change of its magnetoresistance, directly indicating the existence of a reversed magnetization in a region of the [Co/Ni] free layer underneath the NC. As predicted by numerical simulations the droplet exhibits rich magnetodynamic properties, experimentally observed as auto-modulation at approximately 1 GHz and sometimes sidebands at 1/2 and 3/2 of the fundamental droplet frequency. The 1 GHz modulation can be shown numerically to be related to the drift instability of the droplet [2], albeit with enough restoring force to make the droplet perform a periodic motion instead of leaving the NC region. The sidebands at 1/2 and 3/2 the droplet frequency are related to eigenmodes of the droplet perimeter. Magnetic droplet nucleation is found to be robust and reproducible over a wide number of NC-STOs with different NC sizes, making this new nanomagnetic object as fundamental and potentially useful to nanomagnetism as e.g. domain walls and vortices.\\[4pt] [1] B. A. Ivanov and A. M. Koseich, Zh. Eksp. Teor. Fiz. 72, 2000 (1977)\\[0pt] [2] M. A. Hoefer, T. J. Silva, and M. W. Keller, Phys. Rev. B 82, 054432 (2010) [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A18.00005: Precessional magnetization reversal in magnetic tunnel junctions with a perpendicular polarizer Invited Speaker: Huanlong Liu The interaction between the spins of itinerant electrons and the magnetization of ferromagnetic materials is of great interest both for fundamental physics and applications. While a ferromagnetic layer can polarize the spin of electrons passing through it, a spin-polarized current also changes the magnetization of the ferromagnet via a spin-transfer torque (STT). Here we present an orthogonal spin transfer device [1] with an in-plane magnetized free layer (FL) and a perpendicularly magnetized spin polarizing layer, separated by a thin copper spacer. The initial STT acting on the in-plane FL is perpendicular to the plane due to the spin polarization from the polarizer. For large torques, the FL magnetization will be tilted out of its easy plane, which creates a demagnetization field on the order of tens to hundreds of millitesla. The FL magnetization will then precess about the demagnetization field. The FL in our device forms a magnetic tunnel junction with an in-plane magnetized reference layer (RL), which is used to read out the state of the free layer. The resistance of the device then depends on the relative orientation between the magnetizations of the FL and the RL. We experimentally demonstrated fast switching of the FL magnetization, switching for pulses less than 500 ps in duration [2]. We also conducted subthreshold single-shot time-resolved resistance measurements that probe the FL magnetization reversal mechanisms on time scales in which thermal fluctuations can play an important role. We identify the antiparallel (AP) and parallel (P) states and the transition between these two states during a pulse from single-shot oscilloscope traces. We find that there is a strong asymmetry between the AP to P and P to AP transitions under the same pulse conditions$^{\mathrm{\thinspace }}$[3]. The different switching processes can be explained by the strength of the perpendicular spin torque, which depends on the pulse current through the device and is initially larger in the P state than in the AP state. Spin torques from the RL also influence both the switching process and the switching probability. Our results illustrate new ways to control the magnetization of a nanomagnet on short time scales and optimize device operation.\\[4pt] [1] A. D. Kent, B. Ozyilmaz, and E. del Barco, Appl. Phys. Lett. \textbf{84}, 3897 (2004).\\[0pt] [2] H. Liu, D. Bedau, D. Backes, J. A. Katine, J. Langer, and A. D. Kent, Appl. Phys. Lett. \textbf{97}, 242510 (2010).\\[0pt] [3] H. Liu, D. Bedau, D. Backes, J. A. Katine, and A. D. Kent, Appl. Phys. Lett. \textbf{101}, 032403 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A18.00006: Energy landscape for switching in spin-valve nanopillars with perpendicular magnetic anisotropy Daniel B. Gopman, Daniel Bedau, Jordan Katine, Eric E. Fullerton, Stephane Mangin, A.D. Kent Recent experiments have established that thermally activated switching in perpendicularly magnetized spin-valve (SV) nanopillars larger than about 40 nm in diameter is dominated by sub-volume nucleation and domain wall propagation. Despite this complex behavior, room temperature measurements of the switching field distributions indicate thermal activation over a single energy barrier [1]. To better understand the magnetization reversal process, we conducted temperature dependent studies of the switching statistics in nanopillars in which we stabilize non-uniform magnetization states formed by a sub-volume nucleation event. We present results on Co|Ni free layers in SV nanopillars, which include a perpendicularly magnetized fixed layer. Here we measure the distribution of switching events as a function of temperature from 20 K to 300 K. The temperature dependence of both nucleation and propagation distributions is consistent with a thermal activation model, with distinct field-dependent barrier heights for each stage in the reversal process. This is evidence of an energy landscape for switching, which should be relevant for understanding the switching of SV devices even at temperatures that no longer show metastable non-uniform states. [1] Appl. Phys. Lett. 100, 062404 (2012) [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A18.00007: Thermally-assisted magnetization reversal in nanomagnets with spin-transfer torque: diffusive energy space dynamics Daniele Pinna, A.D. Kent, D.L. Stein A direct current applied to a nanomagnet produces a spin-transfer torque that drives the magnetization out of equilibrium.\footnote{J. C. Slonczewski, JMMM. 159, L1 (1996); L. Berger, Phys. Rev. B 54, 9353(1996).} In this talk, scalings between switching time and current for a macrospin under the effects of both spin-torque and thermal noise are explored analytically by focusing on its diffusive energy space dynamics. The procedure allows us to characterize the full dynamics with a one dimensional stochastic differential equation.\footnote{D. Pinna, D. L. Stein, A. D. Kent, arXiv:1210.7675, arXiv:1205.6509 (2012).} We establish the limits of this reduction and elucidate the nature of the limit cycle stabilities observed in nanomagnet reversal experiments. We further proceed to show that the thermally activated dynamics in the presence of a tilt between easy and spin-polarization axes differ only by a rescaling of the threshold current as long as easy, hard and spin-polarization axes all lie in the same plane. Our analytics are verified by employing modern GPU computational techniques to massively parallelize the Langevin equations and probing the long time switching behavior. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A18.00008: Electron tunneling induced nonequilibrium magnetization noise in single Co nanoparticles Wenchao Jiang, Felipe Tijiwa Birk, Dragomir Davidovic We have studied magnetic hysteresis loops of single Co nanoparticles in Al/Al$_2$O$_3$/(Co nanoparticles)/Al$_2$O$_3$/Al tunnel junctions using electron tunneling measurement at mK-temperatures. The magnetic switching field decreases and its distribution broadens versus tunneling current while the current does not heat the environment. The finding indicates that the magnetic switching field can be interpreted as a thermometer of the nonequilibrium magnetization noise. We present a phenomenological model that incorporates magnetic anisotropy fluctuations among discrete levels, to explain the noise properties. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A18.00009: X-ray imaging of magnetic normal modes driven by spin transfer torque in magnetic nanopillar devices Lin Xue, Yong-Tao Cui, R.A. Buhrman, D.C. Ralph, Tolek Tyliszczak, Mi-Young Im, Peter Fischer We have used time-resolved x-ray microscopy to image the fundamental dynamical modes that are driven by spin transfer torque in magnetic devices. We apply a continuous microwave current to exert an oscillating spin torque in a nanopillar structure. By varying frequency and the applied magnetic field, this spin torque selectively excites different individual magnetic normal modes, which are then imaged by x-ray pulses synchronized to the microwave current. We obtain images with 70 ps time resolution and 25 nm spatial resolution. Our results identify modes having different spatial distributions of amplitude and phase, which can be explained by the combined effects of spin transfer torque and the Oersted field. We will discuss the implications of our results for understanding spin-torque-driven magnetic dynamics. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A18.00010: Spin Dynamics and Resonant Inelastic X-ray Scattering in Chromium with Commensurate Spin-Density Wave Order Koudai Sugimoto, Zhi Li, Eiji Kaneshita, Kenji Tsutsui, Takami Tohyama After the discovery of iron-pnictide superconductors, the spin dynamics of itinerant antiferromagnetic systems with multi-orbital has attracted much attention. In order to elucidate such spin dynamics, we focus on a similar system, chromium, which is known to show a spin density wave (SDW), and theoretically investigate dynamical spin susceptibilities and $L_3$-edge resonant inelastic X-ray scattering (RIXS) spectra [1]. We use multi-band Hubbard model composed of $3d$ and $4s$ orbitals. After the SDW mean-field approximation, we obtain the dynamical spin susceptibilities and RIXS spectra by employing random phase approximation. In our calculation, we assume the perfect commensurate SDW state. We find a collective spin-wave excitation undamped up to $\sim 0.6$~eV. Above the energy, excitation overlaps individual particle-hole excitations as expected. In RIXS spectra, particle-hole excitations with various orbital channels show a large spectral weight, masking the spectra of the spin collective mode. However, it may be possible to detect the spin-wave excitation in RIXS experiments in the future if resolution is high enough. [1] K. Sugimoto, Z. Li, E. Kaneshita, K. Tsutsui, and T. Tohyama, arXiv:1211.1598 [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A18.00011: Spin Transfer torques in Antiferromagnets Hamed Saidaoui, Xavier Waintal, Aurelien Manchon Spin Transfer Torque (STT) has attracted tremendously growing interest in the past two decades. Consisting on the transfer of spin angular momentum of a spin polarized current to local magnetic moments, the STT gives rise to a complex dynamics of the magnetization. Depending on the the structure, the STT shows a dominated In plane component for spin valves [1], whereas both components coexist for magnetic tunneling junctions (MTJ) [2]. For latter case the symmetry of the structure is considered to be decisive in identifying the nature and behavior of the torque [3]. In the present study we are interested in magnetic structures where we substitute either one or both of the magnetic layers by antiferromagnets (AF). We use Non-equilibrium Green's function formalism applied on a tight-binding model to investigate the nature of the spin torque. We notice the presence of two types of torque exerted on (AF), a torque which tends to rotate the order parameter and another one that competes with the exchange interaction. We conclude by comparison with previous works [4-5].\\[4pt] [1] Xia, K., Kelly, P. J., Bauer, G. E. W., Brataas, A. {\&} Turek, Phys. Rev. B 65, 220401 (2002). [2] Sankey, J. C. et al. Nature Phys. 4, 67--71 (2008). [3] A. Kalitsov. et al. and W. H. Butler, Phys. Rev. B 79, 174416 (2009). [4] A. S. N\'u\~nez , R. A. Duine,~Paul Haney, and A. H. MacDonald, Phys. Rev. B 73, 214426 (2006). [5] R. A. Duine et al., Phys. Rev. B 75, 014433 (2007). [Preview Abstract] |
Session A19: Metal-Insulator Transitions: Iridiates and Heterostructures - Experiment & Theory
Sponsoring Units: GMAGChair: Dennis Drew, University of Maryland
Room: 321
Monday, March 18, 2013 8:00AM - 8:12AM |
A19.00001: Microscopic Evidence for Slater-Type Metal-Insulator Transition in Sr$_{2}$IrO$_{4}$ Minghu Pan, Qing Li, G.-X. Cao, Satoshi Okamoto, G. Zheng, Wenzhi Lin, Brian C. Sales, J.Y. Yi, J.-Q. Yan, R. Arita, J. Kunes, M. Imada, D. Mandrus The interplay between spin-orbit coupling, bandwidth and on-site coulomb repulsion in layered 5d transition metal oxides (TMO) acquired much interest recently. In Sr$_{2}$IrO$_{4}$, the interplay opens a gap near the Fermi energy and stabilizes a J$_{\mathrm{eff}}=$1/2 spin-orbital entangled insulating state at low temperatures. However, whether this metal-insulating transition (MIT) is Mott-type (electronic-correlation driven) or slater-type (magnetic order driven) is still under hot debate. In this presentation, we give, for the first time, the atomic resolved structure of Sr$_{2}$IrO$_{4}$ surface in real space by using scanning tunneling microscopy. Tunneling spectroscopic results illustrate the gap opening of Sr$_{2}$IrO$_{4}$ at low temperatures with the gap size of 250 mV, indicating the metal to insulator transition. More importantly, the pair of peaks around gap in spectra suggests the quasi-particle coherent excitation, implying the Slater-type insulating state. This is further confirmed by temperature dependent measurements and density functional theory calculations. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A19.00002: Magnetotransport properties of Sr$_2$IrO$_4$ thin films modulated by epitaxial strain Ludi Miao, Dae Ho Kim, Zhiqiang Mao Sr$_{2}$IrO$_{4}$ (SIO) has attracted much attention due to its $J_{\mathrm{eff}}=$1/2 Mott state induced by relativistic spin-orbit coupling [1]. In 3$d$/4$d$ transition metal oxides, exotic phenomena, such as high-$T_{\mathrm{C}}$ superconductivity and colossal magnetoresistance, occur when a Mott insulting state is suppressed by charge carrier doping or band width tuning. Whether the Mott state in SIO can be tuned to new exotic states is an interesting question under active investigation. We have grown epitaxial SIO films on the substrates of SrTiO$_{3}$(STO) and NdGaO$_{3}$ (NGO) using a pulsed laser deposition method and investigated the strain effect on the properties of SIO. The SIO/STO film exhibits a tetragonal structure, while the SIO/NGO film displays a orthorhombic structure due to the NGO's orthorhombic nature. Although both types of films show insulating properties, their magnetic properties appear to be distinct: the SIO/STO film shows negative magnetoresistance (MR) with negligible anisotropy, whereas the SIO/NGO film exhibits positive MR with two-fold anisotropy. Such differences in magnetotransport imply the strong coupling between the lattice, spin and orbital degrees of freedom in SIO.\\[4pt] [1] B.J. Kim \textit{et al}., Phys. Rev. Lett. \textbf{101}, 076402 (08). [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A19.00003: Magneto-transport of filling controlled Mott insulator, Sr$_2$IrO$_4$ Jayakanth Ravichandran, Dmitri Efetov, Claudy Rayan Serrao, Di Yi, Ramamoorthy Ramesh, Philip Kim Sr$_2$IrO$_4$ (SIO) is shown to be a special Mott insulator with the ground state stabilized by a combination of electron correlation and spin-orbit coupling. [1] Both structurally and electronically, electron doped SIO shows characteristics comparable to hole doped La$_2$CuO$_4$, one of the parent compounds of the high-T$_{\mathrm{c}}$ cuprates. [2] This leads us to a natural question of whether doped SIO can turn into a metal and eventually a superconductor. Sustained efforts of chemically doping SIO, [3] without altering its band structure significantly, have been severely hampered due to solubility limitations. In this work, we perform a combination of chemical and electrostatic doping of SIO, to explore the possibility of achieving a robust metallic state. We show that undoped SIO shows ambipolar characteristics and there is significant gating action even after heavy alloying of SIO with La. In depth magneto-transport measurements such as Hall effect and magnetoresistance also provide us a deeper understanding of electronic structure and transport in this exotic Mott insulator. \\[4pt] [1] B. J. Kim et. al., PRL (2008).\\[0pt] [2] F. Wang and T. Senthil, PRL (2011).\\[0pt] [3] Y. Klein and I. Terasaki, J. Phys. : CM (2008). [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A19.00004: Visualizing the gap closure by Rh dopant in SOC induced Mott insulator Sr$_{2}$IrO$_{4}$ with Scanning Tunneling Microscopy Jixia Dai, Eduardo Calleja, Kyle McElroy, Tongfei Qi, Gang Cao Sr$_{2}$IrO$_{4}$ is a novel J$_{\mathrm{eff}}=$1/2 Mott insulator with characteristics of 5d electrons. The strong spin orbit coupling (SOC) in the 5d orbitals of iridium plays an important role in the insulating nature of the parent compound, while replacing Ir$^{4+}$ with the isoelectronic Rh$^{4+}$ is able to drive the system to a metallic regime. We use variable temperature Scanning Tunneling Microscope to study both the insulating parent compound and the Rh doped Sr$_{2}$Ir$_{\mathrm{1-x}}$Rh$_{\mathrm{x}}$O$_{4}$, with x$=$0.04, 0.11. By doing differential conductance measurement, we were able to observe an insulating gap both in the parent compound and the low doping areas of the x$=$0.04 and 0.11 samples. We also observed that in the doped samples, local gaps varies largely at the atomic length scale. By correlating the locations of Rh dopant and the size of local gaps, we found that Rh doping will decrease the insulating gap size which is in accordance with the metallic behavior observed by transport measurements. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A19.00005: Heterostructuring iridate-based spin-orbit Mott materials Jian Liu, Di Yi, Claudy R. Serrao, Jiun-Haw Chu, S. Suresha, Ashvin Vishwanath, Elke Arenholz, Xavi Marti, Ramamoorthy Ramesh Mott materials with strong spin-orbital coupling (SOC) have emerged as a new playground for searching quantum many-body phases with exotic electronic and magnetic properties. Numerous attentions have been paid to 5d transition metal oxides due to the intriguing opportunities to obtaining novel topological insulators, superconductivity, Weyl semimetals, quantum spin liquid, and so on. While realizing these fascinating phenomena would lead to a new generation of electronic and spintronic devices, the rich physics derived from the cooperation of strong correlation and SOC remains to be explored. Here we present our investigation on using epitaxy to control perovskite-based strontium iridates, a prototype of 5d complex oxides, as ultrathin films and heterostructures. We utilize epitaxial stabilization, strain, confinement and interfacial coupling to tune the competing interactions and the multiple degrees of freedom. The combination of these various controls offers a unique pathway to novel phase behaviors and innovative functions. Our experimental findings derived from transport, magnetometry and advanced resonant x-ray spectroscopy, including linear and circular dichroism, will be discussed. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A19.00006: Probing spatial evolution of local density of states in Sr3Ir2O7 Yoshinori Okada, Daniel Walkup, Wenwen Zhou, Tay-Rong Chang, Hsin Lin, Sovit Khadka, Chetan Dhital, Horng-Tay Jeng, Arun Bansil, Ziqiang Wang, Stephen Wilson, Vidya Madhavan Amongst the iridate families, the Ruddlesden-Popper series (Sr$_{n+1}$Ir$_{n}$O$_{3n+1})$ goes through a transition from insulator to metal with increasing $n$. Within this series the $n=$2 compound Sr$_{3}$Ir$_{2}$O$_{7}$ (Ir327) occupies a unique place, straddling a well-defined insulator ($n=$1) on one side and a metal ($n=$infinity) on the other, placing Ir327 in close proximity to a delicate and interesting transition point. In this study, we probe the spatial evolution of the local density of states (LDOS) of Ir327 by means of scanning tunneling spectroscopy. In the parent Ir327 compound, we find local regions of metallic density of states that exist within an underlying insulating electronic structure. Based on the experimental data, we discuss the mechanism of how this metallic LDOS evolves from the intrinsically gapped electronic structure in terms of spin-orbit and Coulomb interactions. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A19.00007: Strongly spin-orbit coupled spin-3/2 model for $5d^1$ $AB_2O_4$ spinels Yi-Ping Huang, Gang Chen, Michael Hermele Research on 5d transition metal oxides has been more and more active recently. Unlike in 3d transition metals, the strong spin orbit interaction cannot be treated as a perturbation. The competition between correlation, spin orbit coupling and the kinetic energy of 5d electrons makes the problem nontrivial. We model the $AB_2O_4$ spinel structure with single d electron on atom B as a Hubbard type model. By treating the hopping term perturbatively under large spin orbit coupling we derive an effective spin 3/2 model which is not Heisenberg-like. We further investigate the possible phase diagram of the effective spin 3/2 model. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A19.00008: Unusual magnetic phases in the strong interaction limit of two-dimensional topological band insulators in transition metal oxides Mehdi Kargarian, Abdollah Langari, Gregory A. Fiete The expected phenomenology of non-interacting topological band insulators (TBI) is now largely theoretically understood. However, the fate of TBIs in the presence of interactions remains an active area of research with novel, interaction-driven topological states possible, as well as new exotic magnetic states. In this work we study the magnetic phases of an exchange Hamiltonian arising in the strong interaction limit of a Hubbard model on the honeycomb lattice whose non-interacting limit is a two-dimensional TBI recently proposed for the layered heavy transition metal oxide compound, (Li,Na)$_2$IrO$_3$. By a combination of analytical methods and exact diagonalization studies on finite size clusters, we map out the magnetic phase diagram of the model. We find that strong spin-orbit coupling can lead to a phase transition from an antiferromagnetic Ne\'el state to a spiral or stripy ordered state. We also discuss the conditions under which a quantum spin liquid may appear in our model, and we compare our results with the different but related Kitaev-Heisenberg-$J_2$-$J_3$ model which has recently been studied in a similar context. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A19.00009: Time-reversal symmetry breaking and anomalous Hall effect in heavy fermion metals Wenxin Ding, Qimiao Si Motivated by recent experimental evidence for a possible chiral spin liquid phase in the metallic pyrochlore heavy fermion iridates (Pr$_2$ Ir$_2$ O$_7$) [Phys.Rev.Lett, {\bf 96}, 087204 (2006), Phys.Rev.Lett {\bf 98}, 057203 (2007), Nature {\bf 463}, 210 (2010), Phys.Rev.Lett, {\bf 106}, 217204 (2011)], we study the effect of Kondo coupling on a time-reversal symmetry breaking state of the $J_1-J_2$ model on square lattices. We use a slave fermion representation for the $f$-moments which are coupled to conduction electrons, and study the mean field solution in the large-N limit. We calculate the ground state energies of various feasible states, and map out the mean field phase diagram by energetic consideration. As the probe for time-reversal symmetry breaking, we calculate the anomalous Hall response for the chiral phase. Finally we discuss the implications of our results on the pyrochlore heavy-fermion iridates. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A19.00010: The low-energy magnetic excitations of a three-band Hubbard model with a strong spin-orbit coupling for 5d transition metal oxide Sr$_2$IrO$_4$ Tomonori Shirakawa, Hiroshi Watanabe, Seiji Yunoki 5$d$ transition metal oxides in a layered perovskite structure such as Sr$_2$IrO$_4$ have attracted much attention because of their unique properties caused by a strong relativistic spin-orbit coupling of 5$d$ transition element. Recent experiments on Sr$_2$IrO$_4$ have revealed that the low-energy magnetic excitations can be described by an ``isospin''-1/2 Heisenberg model with an effective exchange interaction as large as $\sim$ 60-100 meV. Motivated by these experiments, we study theoretically the ground state magnetic structure and the low-energy magnetic excitations for Sr$_2$IrO$_4$ using a three-band Hubbard model with the spin-orbit coupling. Our results demonstrate that the low-energy magnetic excitations are well described by an effective antiferromagnetic Heisenberg model composed of a local Kramers doublet. The estimated value of the effective exchange interaction is as large as 79 meV, which is in good quantitative agreement with the experiments. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A19.00011: First-order metal-insulator transitions in vanadates from first principles Anil Kumar, Karin Rabe Materials that exhibit first-order metal-insulator transitions, with the accompanying abrupt change in the conductivity, have potential applications as switches in future electronic devices. Identification of materials and exploration of the atomic-scale mechanisms for switching between the two electronic states is a focus of current research. In this work, we search for first-order metal-insulator transitions in transition metal compounds, with a particular focus on d$^1$ and d$^2$ systems, by using first principles calculations to screen for an alternative low-energy state having not only a electronic character opposite to that of the ground state, but a distinct structure and/or magnetic ordering which would permit switching by an applied field or stress. We will present the results of our investigation of the perovskite compounds SrVO$_3$, LaVO$_3$, CaVO$_3$, YVO$_3$, LaTiO$_3$ and related layered phase, including superlattices and Ruddlesden-Popper phases. While the pure compounds do not satisfy the search criteria, the layered phases show promising results. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A19.00012: Real-time dynamics in electron-lattice coupled system: Numerical study on an extended double-exchange model Wataru Koshibae, Nobuo Furukawa, Naoto Nagaosa We have developed a new theoretical method to study the photo-induced insulator-to-metal (IM) transition in strongly correlated electron systems [PRL \textbf{103}, 266402 ('09); EPL \textbf{94}, 27003 ('11).]. In the manganese oxides, it has been observed that the photo-induced dynamics with several tens of THz in frequency can drive IM transition [Nature Materials \textbf{6}, 643 ('07).]. The excitation energy with several tens of THz in frequency is fairly lower than the insulating energy gap of the electronic state. In this study, we introduce an extended double exchange model where the conduction electron couples with the orbital-ordering field and lattice distortion, and numerically examine the lattice vibration induced IM transition in the electron-lattice coupled system. To simplify the numerical calculation, the electronic states are restricted in the Hilbert space for perfect ferromagnetic states involving the ground state. In the numerical simulation, we find that the low frequency vibration of Jahn-Teller distortion can change the orbital-ordering pattern and trigger the IM transition. A threshold behavior of the lattice-vibration induced IM transition and the electron-hole excitation by continuous forced lattice-vibration are also examined. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A19.00013: Photo-doped carrier dynamics in Mott insulatoring systems Eiki Iyoda, Sumio Ishihara Electron/hole doping in Mott insulators, for example two-dimensional cuprates, has been well investigated in relation to high-Tc superconductivity. Especially related to photo-doping, many experiments on photo-induced phase transition in strongly correlated systems have been made. In the usual photo-doping setup, the system is excited with fs-laser pulse and generated electron-hole pairs affect properties of materials. Recently, another type of photo-doped experiment with heterostructure has been made, and hole or electron carriers are dynamically injected through the heterostructure. In this theoretical study, we examine photo-doped carrier dynamics in the t-J model with dynamically doped holes. We formulate dynamics of the carriers by non-equilibrium Green functions. We take an initial state of holes and decompose the non-equilibrium Green's function into a series of equilibrium Green's functions by using Wick's theorem. The effect of the initial distribution appears from the higher terms in the series. We treat magnons with the self-consistent Born approximation. The non-equilibrium Green function derived in this way shows double time dependence. We will present physical quantities in transient process, for example, one-particle excitation spectra for holes. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A19.00014: Tailoring spin-orbit Mott insulators via designed superlattices Vijay Shankar V, Jobu Matsuno, Tomohiro Takayama, M. Ahzan Zeb, Hae-Young Kee, Hidenori Takagi The layered perovskite iridates Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ show a transition from a magnetic insulating to a semi-metallic state as the number of layers $n$ is increased. This behaviour is intimately related to the interplay between spin-orbit coupling, electronic correlations and dimensionality. In this talk, we will show that the fabricated superlattices [(SrIrO$_3$)/SrTiO$_3$], provide new insight into this behaviour. Theoretical calculations using density functional and tight binding approaches will be presented to support our results. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A19.00015: Tuning the conductivity of LaMnO$_{3}$/SrTiO$_{3}$ superlattices by stacking Yanpeng Yao, Karin Rabe First-principles density-functional-theory calculations have been applied to study the structure and electronic properties of ultrathin LaMnO$_{3}$/SrTiO$_{3}$ superlattices. We predict that upon the change of stacking, antiferromagnetic LaMnO$_{3}$/SrTiO$_{3}$ superlattices can be tuned from non-conducting insulator to conducting metal. The corresponding microscopic structure change in the superlattices is also analyzed. We find that the metal-insulator transition is accompanied by a corresponding reduction/disappearance of the Jahn-Teller (JT) distortion in the LaMnO$_{3}$ layer. The findings of this work illustrate the role of the JT distortion in the conductivity of transition-metal perovskites, and also suggest a new method for tuning metal-insulator transitions for functional device design. [Preview Abstract] |
Session A20: Focus Session: Metamaterials
Sponsoring Units: DMPChair: John Pendry, Imperial College London
Room: 322
Monday, March 18, 2013 8:00AM - 8:12AM |
A20.00001: Symmetry Breaking and Optical Negative Index of Closed Nanorings Boubacar Kante, Yong-Shik Park, Kevin O'Brien, Daniel Shuldman, Norberto Daniel Lanzillotti-Kimura, Zi Jing Wong, Xiaobo Yin, Xiang Zhang We report the first experimental demonstration of broadband negative-index metamaterial made solely of closed metallic nanorings. Using symmetry breaking that negatively couples the discrete nanorings, we measured negative phase delay in our composite chess metamaterial. Our approach open avenues towards topological nanophotonics with on demand linear and non-linear responses. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A20.00002: A subwavelength magnetic metamolecule Farbod Shafiei, Francesco Monticone, Khai Le, Xing-Xiang Liu, Thomas Hartsfield, Andrea Alu, Xiaoqin Li The weakness of magnetism at optical frequencies in nature has led to intense effort to create artificial magnetism, which is at the basis of anomalous refractive properties and other exciting optical phenomena. Plasmonic nanoclusters have been shown to exhibit strong magnetic response because magnetic effects are indistinguishable from spatial dispersion of permittivity at optical frequencies. In a different context, plasmonic Fano resonances have raised great interest, particularly for use in sensing applications that benefit from sharp spectral features and extreme field localization. So far, optical Fano resonances have been based on purely electric effects. In this work, we use an atomic force microscope to assemble a four-particle nanoring consisting of Au nanoparticles of approximately 100 nm in diameter and to actively modify its configuration until we observed the desired spectral response in the total scattering cross section, namely the first magnetic-based optical Fano resonance in a subwavelength metamolecule. Support from ARO, AFOSR, NSF, and ONR are gratefully acknowledged. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A20.00003: Bridging the Gap Towards the Monolithically Integrated Selective Polarizer: A Dynamic Metamaterial Polarization Grid Corey Shemelya, Nicole Pfiester, Ganesh Balakrishnan, Thomas Vandervelde The use of active metamaterials in devices has gained much attention recently based on their scalabilty, tunability, and the ability to turn them on or off. This work describes the use of metamaterial patterning to create a dynamic polarizer for monolithic integration on photodetectors. The design was tuned for wavelengths in the mid-infrared transmission window for the use of sensing man-made objects. Samples were fabricated using Si doped GaAs epitaxially grown on a c-plane sapphire substrate. Gold metamaterials were patterned and deposited along with ohmic and schottky contacts using physical vapor deposition. The results are compared to similar metamaterials on various substrate materials: double-sided polished (DSP) intrinsic GaAs, DSP n-type GaAs, N$+$ GaSb, and p-type Si. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A20.00004: Geometry Induced Optimization of Energy Consumption in an Ultrafast Metamaterial Modulator Atish Agarwala, Keshav Dani We investigate the energy consumption per bit of an all-optical ultrafast metmaterial modulator via improvements in the geometric design of the device. The device is a 100nm thick tri-layered Ag-Si-Ag fishnet structure metamaterial with a negative index resonance in the NIR. Previously, the device has been shown to be capable of terabit per second all-optical modulation requiring 3nJ/bit of energy. In this talk, we study different device geometries including stacked fishnet structures and variations in sidewall angles in order to reduce the energy consumption required to switch a single bit. Our simulations indicate an optimized structure that allows us to reduce our energy requirement to only 25pJ/bit for a device integrated with an optical fiber. Such improvements in energy consumption are essential for future practical devices allowing for terabit per second all-optical communication. They also provide insight into future energy-efficient metamaterial photonic devices. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A20.00005: Impact of patterned anti-reflection coating on the performance of Broadband Blackbody Absorber Based on Dielectric-Thin Metal Film Multilayers Shyhauh Guo, Andrei Sushkov, Dennis Drew, Raymond Phaneuf We present results from measurements on double period structures of alternating dielectric and thin metal layer coated with micro-patterned anti-reflection layer to improve absorption in mid-infrared range. We examine the effect on performance of patterns' period and the correlation with the effective medium theory. We find that the numerical results agree with the measured absorption spectra. We also investigate the limit of pattern feature size to achieve performance suggested by effective medium theory. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A20.00006: Solving the inverse problem of metamaterials with permittivity measurement Hon Ping Lee, Ka Shing Hui, Kin Wah Yu We have developed a new strategy for the reconstruction of volume fraction distribution of metallic inclusion in a graded composite from the measured electric permittivity data. Some of the techniques by Milton\footnote{R. C. McPhedran, G. W. Milton,Applied Physics A , Volume 26, Issue 4, pp 207-220 (December 1981)} and McPhedran\footnote{R. C. McPhedran and D. R. McKenzie, Appl. Phys. A 29, 19-27 (1982)} on homogenous two phase composites, together with Bergman-Milton representation, electromagnetic representation of effective permittivity and optimization method are used, and the strategy consist of the following two parts: reconstructing the effective permittivity in spectral space with Bergman representation by minimizing the cost function, and obtaining the volume fraction distribution by a contact of Bergman representation and electromagnetic representation of effective permittivity. Demonstration of the strategy is carried out by typical monotonically decreasing graded profile. The study could be extended to arbitrary profiles. The results obtained are useful for solving various inverse problems for the reconstruction of the structures of composites. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A20.00007: Theory of Spatial Optical Solitons in Metallic Nanowire Materials Invited Speaker: Mario Silveirinha Arrays of metallic nanowires stand as one of the most exciting structures in the metamaterial realm due to their applications in the electromagnetic field manipulation and transport in the nanoscale. Nanowire materials can also lead to interesting physics in the context nonlinear optics, and in particular previous works have shown that stable subwavelength solitons can be formed in arrays of metallic nanowires embedded in a Kerr-type material. Such solitary waves can have an important impact in nanophotonics and in the realization of ultra-compact devices. Thus, it would be highly interesting to characterize them using an effective medium approach, because this can highlight the relevant physical processes and simplify the numerical modeling. In this talk, we derive an effective medium theory that describes the dynamics of the macroscopic electromagnetic fields in a nanowire array embedded in a Kerr-type dielectric. We apply such a theory to the characterization of optical solitons, and unravel the physical mechanisms that enable the formation of stable subwavelength solitary waves in nanowire arrays. It is shown that because of the exotic hyperbolic dispersion of the photonic states in a nanowire material, the effective medium behaves as a self-focusing material when the nanowires are embedded in a self-defocusing dielectric host. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A20.00008: Development of Metamaterial Structures by Laser Direct-Write Alberto Pique, Nicholas Charipar, Heungsoo Kim, Matthew Kirleis, Andrew Smith The use of metamaterials structures has been the subject of extensive discussions given their wide range of applications. However, a large fraction of the work available to date has been limited to simulations and proof-of-principle demonstrations. One reason for the limited success inserting these structures into functioning systems and real-world applications is the high level of complexity involved in their fabrication. Direct-write processes are ideally suited for the fabrication of arbitrary periodic and aperiodic structures found in most metamaterial and plasmonic designs. For these applications, laser-based processes offer numerous advantages since they can be applied to virtually any surface over a wide range of scales. Furthermore, laser direct-write or LDW allows the precise deposition and/or removal of material thus enabling the fabrication of novel metamaterial designs. This presentation will show examples of metamaterial and plasmonic structures developed at the Naval Research Lab using LDW, and discuss the benefits of laser processing for these applications. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A20.00009: Design of Tunable Superconducting Metamaterials Melissa Trepanier, Daimeng Zhang, Steven Anlage Our goal is to create a superconducting metamaterial utilizing deep sub-wavelength meta-atoms with a quickly-tunable index of refraction. To accomplish this we will combine two different materials: an array of rf SQUIDs (with tunable effective permeability) and an array of thin wires interrupted by Josephson junctions (with tunable effective permittivity). These materials have been designed to maximize tunablility in the range easily measured via X-band, Ku-band, and K-band waveguides. Various sizes of rf SQUIDs were designed to be non-hysteretic, be sufficiently insensitive to noise, and to have resonant frequencies ranging from 6.5 - 22 GHz. The wire array was designed so that the inductance of the Josephson junctions can completely cancel the geometric and kinetic inductance of the wires, giving rise to strong tunability. We will present the design considerations and simulation results for this new class of metamaterials. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A20.00010: Measurement of a SQUID metamaterial Daimeng Zhang, Melissa Trepanier, Steven Anlage We report experimental results on a new type of superconducting metamaterial consisting of arrays of RF SQUIDs operating in the microwave frequency range with tunable properties (S parameters, effective permeability, effective permittivity, etc.). DC magnetic field is applied to bias the sample and to vary the Josephson inductance, thus tuning the resonant frequency over a multi-GHz range. The experiment is done in a magnetic-shielded cryostat where we examine the temperature, RF-field, and DC-field dependence of S parameters of this superconducting metamaterial. We also perform a cryogenic calibration to eliminate the effects of transmission lines on our results. From the calibrated S-matrix of this metamaterial, we are able to extract the effective permeability and its response to various stimuli. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A20.00011: Experimental demonstration of a broadband array of invisibility cloaks in the visible frequency range Vera Smolyaninova, Igor Smolyaninov, Kurt Ermer Since the first experimental demonstration in the microwave and visible ranges, invisibility cloaks stimulated considerable progress in the fields of metamaterials and transformation optics. Arrays of invisibility cloaks may find important applications in low-interference communication, noninvasive probing, sensing and communication networks, etc. We report on the first experimental realization of such an array of broadband invisibility cloaks, which operates in the visible frequency range. Wavelength and angular dependencies of the cloak array performance will be demonstrated. Potential biochemical sensing applications will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A20.00012: Experimental demonstration of birefrigent broadband transformation Luneburg lenses Henry Ermer, Vera Smolyaninova, Alex Piazza, Todd Adams, David Schaefer, Igor Smolyaninov Transformation optics (TO) has recently become a useful methodology in the design of unusual optical devices, such as novel metamaterial lenses and invisibility cloaks. Here we report the first experimental realization of birefrigent TO designs of Luneburg lens based on lithographically defined metal/dielectric waveguides. Adiabatic variations of the waveguide shape enable control of the effective refractive indices experienced by the TE and TM modes propagating inside the waveguides. We have studied wavelength and polarization dependent performance of the resulting birefrigent TO devices. These novel optical devices considerably extend our ability to control light on submicrometer scales. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A20.00013: Electromagnetic ``black holes'' in hyperbolic metamaterials Igor Smolyaninov We demonstrate that spatial variations of the dielectric tensor components in a hyperbolic metamaterial may lead to formation of electromagnetic ``black holes'' inside this metamaterial. Similar to real black holes, horizon area of the electromagnetic ``black holes'' is quantized in units of the effective ``Planck scale'' squared. Potential experimental realizations of such electromagnetic ``black holes'' will be considered. For example, this situation may be realized in a hyperbolic metamaterial in which the dielectric component exhibits critical opalescence. [Preview Abstract] |
Session A21: Focus Session: New Ferroelectrics and Ferroelectric Mechanisms
Sponsoring Units: DMPChair: Eric Bousquet, Universite de Liege
Room: 323
Monday, March 18, 2013 8:00AM - 8:12AM |
A21.00001: Effects of Manganese Addition on Ferroelectric Properties of BaTiO$_3$ : Ab initio Study Ivan Naumov, R.E. Cohen As it is well known, Mn is intensively used to improve the electromechanical properties of perovskite oxides like BaTiO$_{3}$, PbTiO$_{3}$ or PbZr$_{\mathrm{x}}$T$_{\mathrm{1-x}}$O$_{3}$. Despite the interest in Mn as a dopant, it is currently poorly understood from the fundamental point of view. Here, we present the results of our ab initio study aimed to elucidate the role of Mn defects and associated with them vacancies on the electronic, atomic and ferroelectric properties of BaTiO$_{3}$. Namely, we discuss the equilibrium geometry and electronic properties of the Mn ions occupying A or B sites and their valence or oxidation states in the presence or absence of an compensated oxygen vacancy. A special attention is given to the formation of dipole moments \textbf{P}$_{\mathrm{d}}$ associated with the dopants and to the interaction between \textbf{P}$_{\mathrm{d}}$ and spontaneous polarization \textbf{P}$_{\mathrm{s}}$. This work is supported by the US Office of Naval Research. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A21.00002: Understanding the role of A-site and B-site cations on piezoelectric instability in lead--free (1-x) BaTiO$_3$ -- xA(Cu$_{1/3}$Nb$_{2/3})$O$_3$ (A $=$ Sr, Ca, Ba) solid solutions Deepam Maurya, Yuan Zhou, Shashank Priya This study provides fundamental understanding of the enhanced piezoelectric instability in lead-free piezoelectric (1-x) BaTiO$_3$-xA(Cu$_{1/3}$Nb$_{2/3})$O$_3$(A: Sr, Ba and Ca and x $=$ 0.0-0.03) solid solutions. These compositions were found to exhibit large longitudinal piezoelectric constant ($d_{33})$ of $\sim$330 pC/N and electromechanical planar coupling constant (\textit{kp}) $\sim$ 46{\%} at room temperature. The X-ray diffraction coupled with atomic pair distribution functions (\textit{PDF})s indicated increase in local polarization. Raman scattering and electron paramagnetic resonance (EPR) analysis revealed that substitutions on A and B-site both substantially perturbed the local octahedral dynamics and resulted in localized nano polar regions with lower symmetry. The presence of nano domains and local structural distortions smears the Curie peak resulting in diffuse order-disorder type phase transitions. The effect of these distortions on the variations in physical property was modeled and analyzed within the context of nanodomains and phase transitions. *spriya@vt.edu [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A21.00003: First-principles-based modeling of epitaxial-strain-induced ferroelectricity in CaTiO$_3$ Qibin Zhou, Karin Rabe Epitaxial strain can be used to induce phase transitions from the bulk phase to non-bulk phases in thin films and superlattices. In CaTiO$_3$, it has been previously shown that tensile epitaxial strain induces a transition from the nonpolar bulk orthorhombic phase to a ferroelectric phase. In this study, our first-principles computations revealed that compressive strain also induces ferroelectric phases, one of which has unexpected in-plane polarization. To construct a parametrized energy function that reproduces the properties of CaTiO$_3$ for epitaxial strain over a wide range, I developed an approach in which the parameters in a symmetry expansion are determined by a combination of curve-fitting and constraints to computed first-principles results. This energy function allows the analysis of the competition between the oxygen-octahedron-rotation distortion and the polar mode. The use of this function in modeling the structures and properties of superlattices containing CaTiO$_3$, and in constructing effective Hamiltonian for large scale studies, will be discussed. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A21.00004: Proper, improper and hybrid improper ferroelectricity in oxide perovkites and related compounds Invited Speaker: Philippe Ghosez Ferroelectricity in oxide perovskites and related compounds has been a topic of intensive research for more than 60 years. Recently, the coupling of the ferroelectric mode with other structural distortions has attracted an increasing interest since it offers promising and still widely unexplored possibilities to couple ferroelectricity with other functional properties and even to produce unusual phenomena. In this context, the trilinear coupling between ferroelectric and oxygen rotational modes in naturally occuring and artificial layered perovskites appeared as a practical way to produce unusual dielectric properties or achieve enhanced magneto-electric coupling. Here, I will first briefly reintroduce the concepts of proper, improper and hybrid improper ferroelectricity, highlighting how to rationalize better the concept of improper ferroelectricity. I will contrast the intrinsic behavior of these three classes of compounds in ferroelectric capacitors. Taking then the prototypical example of BiFeO3/LaFeO3 superlattices, I will illustrate how hybrid improper ferroelectricity and trilinear mode coupling is a promising route to potentially achieve electric switching of the magnetization. Finally, considering the case of PbTiO3/SrTiO3 superlattices, I will discuss how to access from first-principles the phase-transition sequence and finite temperature properties of complex systems combining various structural instabilities, which still remains a challenging issue. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A21.00005: Nanoscale design routes to polar oxides Joshua Young, James Rondinelli Many useful material properties, such as ferroelectricity, arise because of inversion symmetry breaking in a material's ground state. Understanding how to purposefully lift spacial parity operations is critical to engineering compounds with `acentric' properties. Using first-principles density functional calculations, we describe the crystal-chemistry criteria necessary to design artificial nanoscale oxides that display spontaneous polarizations using non-polar metal-oxygen polyhedra. By controlling the flavor of A-site cation ordering in AA$^\prime$B$_2$O$_6$ perovskites, we show that spontaneous electric polarizations comparable in magnitude to conventional ferroelectrics are attainable. We conclude by explaining how the criteria can be extended to other material classes to realize polar oxides by design. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A21.00006: Turning ABO$_3$ antiferroelectrics into ferroelectrics: Design rules for practical rotation-driven ferroelectricity in double perovskites and Ruddlesden-Popper compounds Andrew T. Mulder, Nicole A. Benedek, James M. Rondinelli, Craig J. Fennie The discovery of octahedral rotation-induced ferroelectricity has provided a new avenue to realize novel materials to explore the interplay of the electrical polarization and correlated phenomena such as magnetism. Design rules recently established suggest ferroelectricity will exist in layered AA$^{'}$B$_2$O$_6$ perovskites when at least one of the ABO$_3$ constituents forms in the nonpolar Pnma structure. As the majority of perovskites form in Pnma, these rules are widely accessible to many chemistries and therefore have the potential to lead to new classes of multifunctional materials. This recent advance however does not directly address the question of whether or not this polar state is a functional ferroelectric or simply a pyroelectric. In this talk we derive from first principles a chemically and physically intuitive model, based only on the properties of the ABO$_3$ constituents, to guide the realization of both large polarizations and small ferroelectric switching barriers. We show how this model follows from a complex interplay of octahedral rotations, antiferroelectric lattice distortions inherent in every Pnma material, and A-site cation ordering. Finally we demonstrate its applicability not only to the double perovskites but also to Ruddlesden-Popper compounds. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A21.00007: First-principles calculations of epitaxially strained PbZrO$_3$: Coexistence of antiferroelectricity and ferroelectricity Sebastian E. Reyes-Lillo, Karin M. Rabe The antiferroelectric (AFE) - ferroelectric (FE) field-induced transition has important applications in energy-storage capacitors and piezoelectric devices. PbZrO$_3$ is the best known AFE material. Polycrystalline and single crystals PbZrO$_3$ posses a stable AFE ground state below 505 K. In thin films, experimental results show coexistence of antiferroelectricity and ferroelectricity at room and low temperatures. First-principles calculations of epitaxially strained PbZrO$_3$ are carried out to give further evidence of this coexistence and to study the polarization switching path. The space groups of the AFE and FE structures are identified together with their important structural and electrical features. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A21.00008: New classes of piezoelectrics, ferroelectrics, and antiferroelectrics by first-principles high-throughput materials design Invited Speaker: Joseph Bennett Functional materials, such as piezoelectrics, ferroelectrics, and antiferroelectrics, exhibit large changes with applied fields and stresses. This behavior enables their incorporation into a wide variety of devices in technological fields such as energy conversion/storage and information processing/storage. Discovery of functional materials with improved performance or even new types of responses is thus not only a scientific challenge, but can have major impacts on society. In this talk I will review our efforts to uncover new families of functional materials using a combined crystallographic database/high-throughput first-principles approach. I will describe our work on the design and discovery of thousands of new functional materials, specifically the LiAlSi family as piezoelectrics, the LiGaGe family as ferroelectrics, and the MgSrSi family as antiferroelectrics. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A21.00009: Prediction of ferroelectric order in PbCrO$_3$ Martin Schlipf, Marjana Le\v{z}ai\'{c} In this contribution, we employ density-functional theory (DFT) to analyze the properties of PbCrO$_3$. Experimental observations indicate that PbCrO$_3$ exhibits a semiconducting ground state and crystallizes in a perfect cubic perovskite structure. However, symmetry considerations show that these two properties conflict with each other and as a consequence prior DFT calculations obtained a metallic ground state. Investigating tiltings and Jahn-Teller distortions of the oxygen octahedra with a DFT+$U$ approach, we find a semiconducting ground state in which a polar shift of the ions is energetically favorable. Depending on the size of the Hubbard $U$ parameter, we obtain either a structure with a $P4bm$ or one with a $P4_2mc$ space group. In the $P4bm$ structure, the mechanism driving the polar displacement is analogous to PbVO$_3$. The $P4_2mc$ structure is characterized by a displacive ferroelectic order caused by empty $sp$ orbitals. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A21.00010: Why isn't CsSnF$_3$ ferroelectric? Eva H. Smith, Nicole A. Benedek, Craig J. Fennie Complex fluorides are an interesting class of materials to explore for new ferroelectrics and multiferroics. The elucidation of design rules for new ferroelectric fluorides is challenging because polar fluorides tend to form in structures with a large number of atoms in the unit cell and the ferroelectricity is almost always of the geometric type. In this talk we will discuss our recent attempt to rationally design new polar fluorides from first principles. By exploring the relative stability of the subgroups of the perovskite manifold (using the phonons of the 5-atom cubic structure as a guide), we reveal the origin of ferroelectricity in R3c CsPbF$_3$, the only known polar ABF$_3$ compound. Comparison with CsSrF$_3$, which has a similar tolerance factor but no lone pair cation, reveals that the interplay between lone-pair localization and rotations stabilizes the rotation pattern most compatible with ferroelectricity, $i.e.$, a$^-$a$^-$a$^-$, rather than the more common a$^-$a$^-$c$^+$. Next we replace Pb$^{2+}$ with another lone pair cation, Sn$^{2+}$. Within a perovskite manifold of states CsSnF$_3$ is ferroelectric, yet synthesis of this compound by our experimental collaborators shows that not only isn't it ferroelectric, it isn't even a perovskite. Why? [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A21.00011: Why are there so few perovskite ferroelectrics? Nicole Benedek, Craig Fennie Nearly all cubic ABO$_3$ perovskites are unstable to energy-lowering structural distortions, the most common being those that give rise to ferroelectricity (usually an off-centering of the B-site cation) and tilts or rotations of the BO$_6$ octahedra. Whereas there are many perovskites that are either ferroelectric or have rotated octahedra, there are very few perovskites that are both ferroelectric and have rotated octahedra. This observation has lead to the widespread assumption that rotations suppress ferroelectricity and vice versa. Using first-principles density functional theory calculations, in combination with crystal chemistry and symmetry principles, we show that rotations do not always suppress ferroelectricity. In fact, the most fertile place to search for new ferroelectrics may be the place that has thus far been considered the least likely to contain them: materials that are expected to have large rotations. We will show why and how ferroelectricity is suppressed in the most common space group adopted by perovskites (Pnma) and explain how we can use this knowledge to design new ferroelectrics and functional materials. [Preview Abstract] |
Session A22: Organic Conductors & Other Correlated Electron Systems
Sponsoring Units: DMPChair: Claude Bourbonnais, Universite de Sherbrooke
Room: 324
Monday, March 18, 2013 8:00AM - 8:12AM |
A22.00001: Optical evidence of competitive nature between charge-order and dimer-Mott insulators Ryuji Okazaki, Yukio Yasui, Ichiro Terasaki, Yuka Ikemoto, Taro Moriwaki, Takahisa Shikama, Hatsumi Mori, Kazuyuki Takahashi, Hideki Nakaya, Takahiko Sasaki A family of two-dimensional (2D) quarter-filled organic materials exhibits various intriguing electronic and magnetic states. These salts are essentially metallic due to the partially-filled band, however, several materials show the correlated insulating states such as charge-order and dimer-Mott insulators owing to strong correlation effects coupled with their unique internal degrees of freedom. In this talk, we show a competitive nature between charge-order and dimer-Mott insulating phases in the 2D quarter-filled organic salt $\beta$-(meso-DMBEDT-TTF)$_2$PF$_6$ through the optical conductivity measurements. This material has been known to exhibit charge ordering below $T_c$ = 70 K. We find optical evidence of a dimer-Mott insulating phase above $T_c$, indicating that the transition in this material is a transition from dimer-Mott to charge-order insulator. Below $T_c$, the optical peak feature of dimer-Mott insulator is significantly suppressed by the formation of charge order, implying a competition of these two insulators in this system. Furthermore our infrared imaging spectroscopy reveals a spatially inhomogeneous electronic state far below $T_c$, which is attributed to the competition between charge-order and dimer-Mott insulators. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A22.00002: Superconductivity and polar charge fluctuation in low dimensional organic salts Sumio Ishihara, Akihiko Sekine, Joji Nasu Organic conductors are one of the families in which exotic superconductivities have been examined intensively. Recently, dielectric anomaly is reported in one of the $\kappa $-type BEDT-TTF salts, $\kappa $-(BEDT-TTF)$_{\mathrm{2}}$Cu$_{\mathrm{2}}$(CN)$_{\mathrm{3}}$. A dielectric anomaly is observed in the temperature dependence of the dielectric constant around 30K. These experimental results trigger reinvestigations of the electronic structure in the dimer-Mott insulating systems, and a mechanism of the superconductivity. Superconductivity and polar charge fluctuation are studied in an organic conductor where the dimer-molecule degree of freedom exists. The two-types of the extended Hubbard models, where the intra- and inter-dimer Coulomb interactions are taken into account, are analyzed by the random-phase approximation and the fluctuation-exchange approximation. The superconductivity appears in a vicinity of the charge-density wave (CDW) phase where the electronic distributions are polarized inside dimers. The extended s-wave type paring is favored and is cooperative with the d$_{\mathrm{xy}}$-type paring due to the spin fluctuation. This superconductivity is compared with that realized near the CDW phase where the charge is not polarized inside dimers. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A22.00003: Collective charge excitation in low dimensional organic salts Makoto Naka, Sumio Ishihara Electronic ferroelectricity is known as phenomena where electric polarization is attributed to the charge order without inversion symmetry. This is seen in some transition metal oxides, e.g. LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$, and charge transfer salts. Quasi 2-dimesional organic salt kappa-(ET)$_{\mathrm{2}}$Cu$_{\mathrm{2}}$(CN)$_{\mathrm{3\thinspace }}$is one of the electronic ferroelectricities. Two ET molecules construct a dimer and are arranged on a triangular lattice. Recently, it is reported that a dielectric anomaly is experimentally observed around 30K. An origin of this dielectric anomaly is thought to be an ?electronic? dipole generated by a localized hole in one side of the ET molecules in dimers. Motivated by the experimental results, we study charge dynamics in dimer-Mott insulating system with internal charge degree of freedom in a dimer. We adopt the three kinds of models, extended Hubbard model, V-t model and its effective pseudo-spin model. We analyze these models by utilizing the exact diagonalization method and spin wave approximation, and focus on the collective charge excitation. In the ground state, paraelectric dimer-Mott phase and ferroelectric charge ordered phase compete with each other. We find the low-energy intra-dimer charge excitations which show a strong light polarization dependence. The collective excitation mode which is observable by light being parallel to the electric polarization shows a softening and a remarkable frequency dispersion around the phase boundary. This collective charge excitation of the ?electronic? dipole explains the recently observed peak structure in optical conductivity for the THz region. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A22.00004: Microscopic investigation of Fabre charge transfer salts as function of temperature and pressure Helene Feldner, Anthony Jacko, Eva Rose, Martin Dressel, Roser Valenti, Harald O. Jeschke The Fabre charge transfer salts are quasi-1D materials with a rich temperature and pressure phase diagram. We use literature as well as newly obtained crystal structures to sample many temperatures and pressures (both chemical and physical). We find that general trends in their electronic properties can be connected to their phase diagram. Finally, we analyze the importance of correlations in these systems using an extended Hubbard model, parametrized using DFT Wannier orbital overlaps. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A22.00005: LDA+DMFT investigation of the organic charge transfer salt $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl Johannes Ferber, Kateryna Foyevtsova, Harald O. Jeschke, Roser Valenti We combine density functional theory with dynamical mean field theory for the study of organic molecular crystals using a new scheme to construct molecular Wannier functions. We calculate spectral and optical properties for the strongly correlated material $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl. The new method allows us to analyze the contributions of intradimer and interdimer contributions to the optical conductivity on the same footing. We find in agreement with experiment that strong correlations lead to a Hubbard peak in the optical conductivity. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A22.00006: Charge Induced Spin Polarization in Thiophene Oligomers Avadh Saxena, Dong Hou, Junjie Qiu, Shijie Xie Charge induced spin polarization in organic small molecules is a key factor for spin transport and magnetic effects in related organic devices. We study the spin polarization in charged thiophene oligomer molecules by calculating the magnetic moment with density functional theory (DFT). We find that the emergence and variation of the net magnetic moment is related to both the amount of charge injected and the polymerization of the oligomer, i.e. the number of monomer units. Combined with model analysis, we conclude that the strong electron-electron (e-e) interaction and electron-lattice (e-l) interaction in organic materials are responsible for charge induced spin polarization in organic oligomers. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A22.00007: Inter-chain transport in the quasi-one-dimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$ Joshua Cohn, Benjamin D. White, Carlos A.M. dos Santos, John J. Neumeier We report measurements of electrical resistivity ($\rho$) and thermoelectric power ($S$) transverse to the conducting chains (crystallographic \textit{c} axis) on single crystals of the quasi-one-dimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$. While $\rho_c(T)$ exhibits metallic behavior at $T\leq T_{max}\sim 270$~K, it decreases with increasing $T$ above this temperature similar to the behavior of $\rho$ transverse to the conducting planes in a variety of two-dimensional metals.\footnote{See, e.g., D. B. Gutman and D. L. Maslov, Phys. Rev. Lett. {\bf 99}, 196602 (2007).} We discuss the corresponding thermopower, $S_c$, which is relatively $T$-independent and a modest 30 $\mu$V/K at low $T$, increases sharply with increasing $T$ near $T_{max}$, and exceeds 200 $\mu$V/K at $T>400$~K. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A22.00008: Nanoscale interplay of inhomogeneity and electron interactions in the quasi one-dimensional purple bronze Li$_{0.9}$Mo$_6$O$_{17}$ Jung Hoon Liu, Anjan Soumyanarayanan, Michael Yee, Yang He, Martha Greenblatt, Nigel Hussey, Jennifer Hoffman The marked deviation from Fermi liquid behavior for the quasi one-dimensional (1D) purple bronze, Li$_{0.9}$Mo$_6$O$_{17}$ (LPB), has been observed by both bulk transport and surface sensitive spectroscopic probes, and has generated much theoretical interest. Here we report on spectroscopic scanning tunneling microscopy (STM) studies of 1D `chains' on the surface of LPB in the presence of a magnetic field. While we can consistently identify high-energy features in the tunneling density of states corresponding to the bulk band structure, we find that the Coulomb suppression of tunneling around the Fermi energy is inhomogeneous on the nanometer length scale. We discuss the inhomogeneity in the context of the 1D `chains', and its implications on other measurements. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A22.00009: Direct observation of electronic nematicity in charge and orbital ordered La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ J. Tao, K. Sun, J.M. Zuo, Y. Zhu Nematic and smectic states have been demonstrated to be very important in understanding high-T$_{\mathrm{c}}$ superconductivity. Here we report similar observations of electronic nematicity in doped manganites. Both the electron diffraction results and HRTEM images obtained from single crystal domain of La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ clearly show a C4 to C2 symmetry broken in charge ordered (CO) and orbital ordered superstructures at intermediate temperature range. The electronic nematicity persists in the crystal until long-range CO forms as a stripe phase at lower temperatures upon cooling. During warming process, we observed topological defects in the charge ordering superstructures, indicating that the melting of the CO superstructure is defect mediated. Theoretical simulations will also be provided for better interpretation of the phenomenon. Research at Brookhaven National Laboratory was sponsored by the US Department of Energy (DOE)/Basic Energy Sciences, Materials Sciences and Engineering Division under Contract DE-AC02-98CH10886. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A22.00010: Checkerboard to Stripe Charge Ordering Transition in TbBaFe$_2$O$_5$ Daniel Pratt, Sung Chang, Wei Tian, Alexey Taskin, Yoichi Ando, Jerel Zarestky, Andreas Kreyssig, Alan Goldman, Robert McQueeney A combined neutron and x-ray diffraction study of TbBaFe$_2$O$_5$ reveals a rare checkerboard to charge ordering transition. TbBaFe$_2$O$_5$ is a mixed valent compound where Fe$^{2+}$/Fe$^{3+}$ ions are known to arrange into a stripe charge-ordered state below $T_{V} =$ 291 K, that consists of alternating Fe$^{2+}$/Fe$^{3+}$ stripes in the basal plane running along the \textbf{\textit{b}} direction. Our measurements reveal that the stripe charge-ordering is preceded by a checkerboard charge-ordered phase between $T_{V}$\textit{ \textless\ T} \textit{\textless\ T}* $=$ 308 K. The checkerboard ordering is stabilized by inter-site coulomb interactions which give way to a stripe state stabilized by orbital ordering. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A22.00011: Novel electronic transition in layered IrTe$_{2}$ Yoon Seok Oh, J.J. Yang, Y. Horibe, S.-W. Cheong Layered chalcogenides such as 1T-TaS$_{2}$, 1T-TiSe$_{2}$, Bi$_{2}$Se$_{3}$, and MoS$_{2}$ exhibit rich low-dimensional physical properties such as superconductivity, topological insulator, charge density waves (CDW), and field-effect-transistor with high mobility. IrTe$_{2}$ forms in the layered CdI$_{2}$ structure, and exhibits diamagnetism and superlattice modulations below $\sim$260 K. In addition, superconductivity appears when the $\sim$260 K transition is fully suppressed by, for example, chemical doping. The origin of the $\sim$260 K transition in IrTe$_{2}$ has been controversial. It was claimed to be a structural transition, which suppresses electronic conduction. It was also reported that Fermi surface instability drives the transition - $i.e.$ it is charge density wave-type. In this talk, we present our comprehensive studies on electron diffraction and transport experiments under chemical/hydrostatic pressure to unveil the origin of the novel electronic transition in IrTe$_{2}$. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A22.00012: Superstructure and its domain structure in layered IrTe$_2$ at low temperatures Y. Horibe, F.T. Huang, J.J. Yang, Y.S. Oh, Y.J. Choi, A. Hogan, S.-W. Cheong IrTe$_2$, forming in layered CdI$_2$ structure, exhibits a unique phase transition accompanied by the appearance of diamagnetism and a sharp increase of electrical resistivity. This transition has been discussed to be due to a charge-density-wave formation related to the Fermi surface nesting [1]. In this talk, we will report the three dimensional superstructure below the transition temperature, obtained from electron diffraction patterns using low-temperature transmission electron microscopy. The superstructure is characterized by the presence of the superlattice reflections with the modulation wave vector q$=$ 1/5 [101]. Its domain structure and chemical doping effects on the modulation wave vector will also be discussed.\\[4pt] [1] J. J. Yang et al., PRL 108, 116402 (2012). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A22.00013: Resonant Ultrasound Studies of Mo$_{3}$Sb$_{7}$ Lindsay VanBebber, Jiaqiang Yan, David Mandrus, Brian C. Sales, Veerle Keppens The elastic behavior of a series of Mo$_{3-x}$M$_{x}$Sb$_{7-y}$X$_{y}$ (M= Cr, Ru, X= Te) single crystals is examined with resonant ultrasound spectroscopy (RUS) as a function of temperature (300 K - 5 K). The elastic response of the parent compound Mo$_{3}$Sb$_{7}$ reveals a transition at around 53K, evidenced by a dramatic softening in the shear modulus c=(c$_{11}$-c$_{12}$)/2. This softening is associated with a cubic-to-tetragonal structural transition as well as a spin gap formation. The transition temperature is lowered by a few degrees upon doping with Cr. Doping with Ru and Te is known to suppress the structural transition, but the softening in the shear modulus suggests that a structural instability remains present in these compounds. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A22.00014: Electronic thermoelectric power factor and metal-insulator transition in FeSb2 Cedomir Petrovic, Qing Jie, Rongwei Hu, Emil Bozin, Anna Llobet, Igor Zaliznyak, Qiang Li We show that synthesis-induced metal-insulator transition (MIT) for electronic transport along the orthorhombic c axis of FeSb2 single crystals has greatly enhanced electrical conductivity while keeping the thermopower at a relatively high level. By this means, the thermoelectric power factor is enhanced to a new do a new record high S2$\sigma \sim $8000 $\mu $WK$-$2 cm$-$1 at 28 K. We find that the large thermopower in FeSb2 can be rationalized within the correlated electron model with two bands having large quasiparaticle disparity, whereas MIT is induced by subtle structural differences. The results in this work testify that correlated electrons can produce extreme power factor values. [Preview Abstract] |
Session A23: Focus Session: Dopants and Defects in Semiconductors I
Sponsoring Units: DMPChair: Marek Skowronski, Carnagie Melon University
Room: 325
Monday, March 18, 2013 8:00AM - 8:36AM |
A23.00001: Defects in Carbon-Based Materials Invited Speaker: Gerd Duscher Two distinctly different carbon based semiconducting materials were investigated as to how point defects can influence the electric properties. SiC is a high power electronic material with high bulk mobility. The interface between SiC and SiO$_2$ is generally considered to be the cause for the reduced mobility of SiC devices compared to bulk SiC. We investigated this interface with atomic resolution Z-contrast and electron energy-loss spectroscopy. We come to the conclusion that the previously observed interface layer is due to the miscut and does not exhibit any stoichiometric change. The structure of the interface which is limiting the device performance is caused by the steps and facets at the interface introduced by the miscut. We observed a high number of carbon in the oxide right next to the interface. Aberration corrected transmission electron microscopy enabled the investigation of the atomic structure of this highly stepped interface and the impact of geometry and chemistry on the electronic properties of this material. Graphene is an emerging electronic material also with high mobility. We investigated the defects and dopants in graphene were investigated. We observed point and extended defects in this 2D material. Due to the clear observation of all atoms involved, this material can serve as a model material to study point defects directly. We observe a electronegativity doping of substitutional Si. We observed a remarkable resistance to oxidation of a variety of point defects of elements that readily oxidize in normal circumstances. Boron and nitrogen doped graphene was investigated and the exact nature of the dopant sites and interactions will be shown. Generally speaking modern electron microscopy can directly visualize the full atomic structures in geometrically simple materials like graphene. The knowledge of point defects can be the basis to understand the electronic property structure relationship of structurally complex materials like SiC. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A23.00002: Characterization of the oxide-semiconductor transition layer in NO, P, and N-plasma passivated 4H-SiC/SiO$_2$ structures using transmission electron microscopy Joshua Taillon, Joonhyuk Yang, Claude Ahyi, John Williams, John Rozen, Leonard Feldman, Tsvetanka Zheleva, Aivars Lelis, Lourdes Salamanca-Riba The 4H-SiC/SiO$_2$ interface in MOSFET devices contains a high density of electrically active traps. Recent work has revealed an inverse relationship between the SiC-SiO$_2$ transition layer width and FET channel mobility. Interfacial N and P, introduced by nitric oxide (NO) anneals, nitrogen plasma (N2P), or phosphosilicate glass (PSG) passivations improve carrier mobility, but a relationship to transition layer width is lacking. We present a characterization of the SiC/SiO$_2$ transition layer as a function of NO anneal time using high resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM), and electron energy-loss spectroscopy (EELS). The transition layer was measured with HRTEM and HAADF-STEM and characterized by the evolution of the C/Si and O/Si composition ratios and the Si-\textit{L}$_{2,3}$ edge in the EEL spectra across the interface. We show an inverse relationship of NO anneal time and transition layer width, which correlates with improved channel mobility, increased N interfacial density, and reduced interface trap density. No excess C was noted at the interface. NO annealed samples are compared to N2P and PSG passivations. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A23.00003: Study of surface potential variation in p-/n-type 4H-SiC using scanning kelvin probe microscopy Jung-Joon Ahn, Lin You, Liangchun Yu, Sang-Mo Koo, Joseph Kopanski We report surface potential images of p-n junctions in 4H-SiC measured using scanning kelvin probe microscopy (SKPM) and relate them to the local dopant concentration. SKPM has been demonstrated on various semiconductor materials to examine crystalline defects and doping profiles. SKPM measured surface potential depends on the local dopant concentration and clearly differentiates between n-type and p-type materials. As opposed to scanning capacitance microscopy, which requires a good quality surface insulating layer, SKPM requires a clean surface and the lack of a screening oxide might result in higher spatial resolution. For the measurement, partially de-processed SiC high power LMOSFETS were used. The p-n junctions were formed from 4H-SiC wafers having a p-epilayer on p-substrate that was ion-implanted with nitrogen and annealed to build a shallow n-type region. The samples were observed in plan-view and in cross-section. Amplitude modulated, double pass SKPM was implemented with a commercial AFM. We conducted a detailed study of various data acquisition parameters and it seems that the lateral resolution of the potential difference can be enhanced by applying higher ac modulation amplitude and small tip-sample scanning height. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A23.00004: Near-infrared luminescent cubic silicon carbide nanocrystals for in vivo biomarker applications: an ab initio Study Adam Gali, Viktor Z\'olyomi, B\'alint Somogyi Small molecule-sized fluorescent emitters are needed as probes to image and track the locations of targeted nano-sized objects with minimal perturbation, and are much sought-after to probe biomolecules in living cells. For in vivo biological imaging, fluorescent biomarkers have to meet the following stringent requirements: (i) they should be non-toxic and bioinert, (ii) their hydrodynamical size should be sufficiently small for clearance, (iii) they should be photo-stable. Furthermore, it is highly desirable that (iv) they have intense, stable emission in the near-infrared range, and (v) they can be produced in relatively large amount for biological studies. Here we report time-density functional calculations on SiC-based QDs in the aspect of in vivo biological imaging applications. We find that Si-vacancy, divacancy, as well as single metal dopants such as Vanadium (V), Molybdenum (Mo) and Tungsten (W) in molecule-sized (1-2~nm) SiC QDs emit light efficiently in the near-infrared range. Furthermore, their emission wavelength varies on the size of host SiC QDs at less extent than that of pristine SiC QDs, thus sharper emission spectrum is expected even in a disperse size distribution of these QDs. These fluorescent SiC QDs are paramagnetic in the ground state. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A23.00005: The Search for Sub-Bandgap Optoelectronic Response in Silicon Hyperdoped with Gold Jonathan Mailoa, Austin Akey, Jay Mathews, David Hutchinson, Christie Simmons, Joseph Sullivan, Mark Winkler, Dan Recht, Peter Persans, Jeffrey Warrender, Michael Aziz, Tonio Buonassisi Deep-level dopants have been long known as the lifetime-killer in microelectronic devices. Nevertheless, it has been shown that deep-level donor can facilitate strong absorption of light with energy below the semiconductor bandgap. Due to this strong sub-bandgap absorption, it is possible to engineer silicon devices exhibiting sub-bandgap optoelectronic response, such as silicon-based infrared photodetectors and intermediate-band solar cells. In this work, we show the optoelectronic response of silicon doped with a gold concentration surpassing the equilibrium solubility limit (gold-hyperdoped silicon, Au:Si). We fabricated Au:Si by ion implantation followed by nanosecond pulse laser melting, achieving a gold dopant concentration of over 10$^{19}$ cm$^{-3}$. UV-VIS spectrophotometry was performed to measure sub-bandgap light absorption in the Au:Si layer. Our samples with the highest gold concentration have 10-15{\%} absorption of sub-bandgap light. We will present and discuss the sub-bandgap optoelectronic response of this gold-doped silicon. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A23.00006: Recombination lifetimes in laser hyperdoped Si layers measured via microwave photoconductive decay Jay Mathews, David Hutchinson, Ryan McAvoy, Mark Winkler, Daniel Recht, Austin Akey, Jonathan Mailoa, Michael Aziz, Tonio Buonassisi, Peter Persans, Jeffrey Warrender Silicon hyperdoped with impurities via ion implantation followed by pulsed laser melting has attracted much attention lately due to potential for forming an intermediate band. Such materials have shown significant optical absorption well below the band gap of Si and are being explored for applications in photovoltaics and infrared detection. However, while optical absorption can be increased, high dopant concentration generally leads to a substantial decrease in recombination lifetime, which can detrimentally affect the performance of detectors and solar cells. In this work, we use microwave photoconductive decay ($\mu $-PCD) to explore the transient behavior of Si hyperdoped with S at various levels. Excitation is achieved via a pulsed Nd:YAG laser at 355 nm (FWHM $\sim$ 5 ns), ensuring that carriers are generated only in the hyperdoped region. Decay times were found to decrease monotonically with increasing S concentration, and the highest concentrations do not show measureable photoconductivity, which could indicate unacceptably low lifetimes. Additional $\mu $-PCD measurements are presented on Si hyperdoped with Au, which are promising despite the fact that Au is typically a ``lifetime killer,'' as well as Si hyperdoped with Ti, which has been previously shown to exhibit lifetime recovery. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A23.00007: Insulator-to-metal transition with deep-level impurities in silicon achieved by compensated hyperdoping Christie Simmons, Austin Akey, Mark Winkler, Jacob Krich, Joseph Sullivan, Daniel Recht, Michael Aziz, Tonio Buonassisi Hyperdoping (achieved via nanosecond pulsed laser melting and rapid resolidification) allows the substitutional incorporation of impurities at concentrations orders of magnitude beyond the equilibrium solubility limit. This technique opens the door for studying the insulator-to-metal transition (IMT) in silicon doped with impurities for which the critical concentration necessary to drive the transition is inaccessible by conventional doping techniques; specifically, impurities that introduce deep, highly localized states. IMTs have already been observed for silicon hyperdoped with sulfur and with selenium. It may be possible to use these deep impurities to create an intermediate band semiconductor in which there is a delocalized band of impurity states isolated within the conventional band gap. We will discuss the possible nature of these IMTs (impurity band merging with the conduction band vs. closing of the Hubbard gap), and we will present further observations of a metal-to-insulator transition in highly compensated sulfur-doped samples. Sulfur is a double donor in silicon, and by adding varying concentrations of boron, a shallow acceptor, we demonstrate a tunable depletion of the impurity band as evidenced by the materials' optoelectronic properties. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A23.00008: Optoelectronic Characterization of Impurity Supersaturated Silicon Junctions David Hutchinson, Joseph Sullivan, Jay Mathews, Daniel Recht, Aurore J. Said, David J. Lombardo, Christie Simmons, Tonio Buonassisi, Jeffrey M. Warrender, Michael J. Aziz, Peter D. Persans Intermediate band semiconductors have been proposed as a path to high efficiency photovoltaics. Silicon doped to high levels with impurities such as S, Se, Au, and Ti which can produce deep levels, may fulfill this promise. We report here on the optoelectronic properties of diode structures prepared by implantation of 10$^{15}$ to 10$^{16}$ impurity atoms/cm2 into a p-type or n-type wafers, followed by nanosecond pulsed laser melting and resolidification. Experimental results from wavelength and temperature dependent diode response, spatial quantum efficiency mapping, intensity dependent efficiency, and current-voltage characterization will be reported. Current-voltage measurements under photoexcitation yield information on the built in voltage and absorption mechanisms. Most devices show maximum quantum efficiency for excitation wavelengths between 900 and 1000 nm. The drop in quantum efficiency for short wavelengths can yield the minority carrier diffusion length in the hyperdoped material. Long wavelength response elucidates photocarrier excitation mechanisms. The fundamental properties of the junction and the supersaturated material will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A23.00009: Cross-spectrum noise spectroscopy for characterization of deep-levels in nanoscale devices Deepak Sharma, Sergiy Krylyuk, Abhishek Motayed, Qiliang Li, Albert Davydov Applications of traditional methods to study deep-levels, such as deep-level transient spectroscopy, or photo-induced current transient spectroscopy, often become impractical for nanoscale devices. In low frequency noise spectroscopy, the accurate measurements of the noise signal in low-current nanowire devices are extremely challenging because the device noise, which is proportional to the dc current, becomes comparable with the measurement setup noise. To overcome these issues, we have implemented a LFN measurement method based on dual-channel cross-spectrum analysis technique, which reduced the power spectral density (PSD) by three orders of magnitude by reducing the parasitic background 1/f noise, enabling high sensitivity measurements. The method was applied to probe deep-levels in n- and p-type Si nanowires grown by Ni and Au catalysts. Temperature-dependent noise measurement clearly showed Lorentzian peaks due to the generation-recombination (G-R) process via the deep levels introduced by Ni and Au atoms diffused into the Si nanowires during the growth. Important parameters such as trap energies and concentrations of the deep levels, minority carrier life times, hole and electron capture cross sections were calculated for both Ni and Au deep-levels. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A23.00010: Defect engineering of complex semiconductor alloys: Cu$_{\mathrm{2-2x}}$M$_{\mathrm{x}}$O$_{\mathrm{1-y}}$X$_{\mathrm{y}}$ Stephan Lany, Vladan Stevanovic The electrical properties of semiconductors are generally controlled via doping, i.e., the incorporation of dilute concentrations of aliovalent impurity atoms, whereas the band structure properties (gap, effective masses, optical properties) are manipulated by alloying, i.e., the incorporation of much larger amounts of isovalent elements. Theoretical approaches usually address either doping or alloying, but rarely both problems at the same time. By combining defect supercell calculations, GW quasi-particle energy calculation, and thermodynamic modeling, we study the range of electrical and band structure properties accessible by alloying aliovalent cations (M $=$ Mg, Zn, Cd) and isovalent anions (X $=$ S, Se) in Cu$_{\mathrm{2}}$O. In order to extend dilute defect models to higher concentrations, we take into account the association/dissociation of defect pairs and complexes, as well as the composition dependence of the band gap and the band edge energies. Considering a composition window for the Cu$_{\mathrm{2-2x}}$M$_{\mathrm{x}}$O$_{\mathrm{1-y}}$X$_{\mathrm{y}}$ alloys of 0 $\le $ (x,y) $\le $ 0.2, we predict a wide range of possible band gaps from 1.7 to 2.6 eV, and net doping concentrations between p $=$ 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$ and n $=$ 10$^{\mathrm{17}}$cm$^{\mathrm{-3}}$, notably achieving type conversion from p- to n-type at Zn or Cd compositions around x $=$ 0.1. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A23.00011: Ab-Initio Study of Defect Physics for Layered LaCuChO and BaCuChF (Ch=\{S,Se,Te\}) Structures Jason Vielma, David H. Foster, Guenter Schneider Layered oxychalcogenides LnCuChO (Ln = \{La,Pr,Nd\}, Ch = \{S,Se,Te\}) and isostructural layered fluorochalcogenides BaCuChF have drawn much interest in recent years as p-type wide bandgap semiconductors with applications in transparent electronics and photovoltaics. Previous experimental and computational studies concluded for both LaCuChO, with a bandgaps between 2.4-3.1 eV, and BaCuChF, with optical bandgaps between 2.8-3.5 eV, that p-type conductivity is primarily due to copper vacancies. We report a comparative {\it{ab-initio}} computational study of the defect physics for both families of materials. Point defects and defect complexes are taken into account and previously omitted corrections have been included.\footnote{A. Zakutayev, J. Tate, G. Schneider. \emph{Phys. Rev. B.} \textbf{82}, 195204, (2010)}$^,$\footnote{H. Hiramatsu, T. Kamiya, T. Tohei, E. Ikenaga, T. Mizoguchi, Y. Ikuhara, K. Kobayashi, H. Hosono. \emph{J. Am. Chem. Soc.} \textbf{132}, 15060, (2010)} Accurate chemical potential stability diagrams and formation energies are calculated using the GGA+U method and fitted elemental-phase reference energies.\footnote{V. Stevanovic, S. Lany, X. Zhang, A. Zunger. \emph{Phys. Rev. B.} \textbf{85}, 115104, (2012)} [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A23.00012: X-ray absorption spectroscopy to investigate the doping mechanism in amorphous Cu$_2$ZnSnS$_4$ thin films Sin Cheng Siah, Rupak Chakraborty, Peter Erslev, Glenn Teeter, Chenjun Sun, Tonio Buonassisi Recently, Teeter \textit{et al.} at NREL have discovered that Cu$_{2}$ZnSnS$_{4}$ thin films, of interest for photovoltaics, are amorphous (a-CZTS) when grown at room temperature and the film resistivity can be tuned over a wide range by controlling the Cu:Sn ratio. Tetrahedrally-coordinated amorphous semiconductors belong to an interesting class of compounds that are predicted to have the ability of being doped both $p$- and $n$-type. The four-fold coordination plays a critical role in unpinning the Fermi level to allow effective control over doping levels in a disordered structure. We performed extended X-ray absorption fine structure spectroscopy at the $K$-edges of Cu, Zn and Sn to determine the extent of structural disorder and tetrahedral coordination in a-CZTS films grown with varying Cu:Sn content. All films exhibit a high degree of structural disorder beyond the cations' first coordination shell. Both Cu and Zn atoms have high degree of tetrahedral coordination with respect to S atoms while the average coordination number of Sn decreases with increasing Sn content, indicative of either the favorable formation of sulfur vacancies around Sn atoms or the presence of Sn-related secondary phase. We combine these results with conductivity measurements to understand the relationship between the structural and electrical properties of this new material. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A23.00013: Phonon-induced spin-spin interactions in diamond nanostructures: application to spin squeezing Steven Bennett, Norman Yao, Johannes Otterbach, Peter Zoller, Peter Rabl, Mikhail Lukin We propose a novel mechanism for long-range spin-spin interactions in diamond nanostructures. The interactions are mediated by the coupling of electronic spins, associated with nitrogen vacancy centers, to the vibrational mode of a diamond mechanical nanoresonator. This results in phonon-mediated effective spin-spin interactions that can be used to generate squeezed states of a spin ensemble. We develop an approach combining spin echo techniques and coherent mechanical driving to suppress spin dephasing and relaxation, and find that substantial squeezing is possible under realistic experimental conditions. Our results have implications for spin-ensemble magnetometry, as well as phonon-mediated quantum information processing with spin qubits. [Preview Abstract] |
Session A24: Novel Technologies and Algorithms
Sponsoring Units: DCOMPChair: Brandon Cook, Oak Ridge National Laboratory
Room: 326
Monday, March 18, 2013 8:00AM - 8:12AM |
A24.00001: Cubic-scaling algorithm and self-consistent mean field for the random-phase approximation with second-order screened exchange Jonathan Moussa The random-phase approximation including second-order screened exchange (RPA+SOSEX) is an accurate model of electron correlation energy with two caveats. Its accuracy depends on an arbitrary mean field choice and its scaling of $\mathcal{O}$($n^5$) operations and $\mathcal{O}$($n^3$) memory for $n$ electrons cannot compete with the $\mathcal{O}$($n^3$) operations and $\mathcal{O}$($n^2$) memory scaling of density functional theory (DFT). We rederive RPA+SOSEX as an approximation of the Brueckner doubles coupled-cluster (BCCD) equations, which produces a self-consistent mean field and other model corrections. In addition, we present a new algorithm for RPA+SOSEX that matches the scaling of DFT. We verify the accuracy of the new model on H$_2$ dissociation and the uniform electron gas and verify the reduced scaling of the new algorithm on H$_n$ rings. \\ \\ This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. 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] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A24.00002: Large-Scale Hybrid-DFT First-Principles Molecular Dynamics William Dawson, Francois Gygi The recursive subspace bisection algorithm[1] is used to accelerate the computation of the Hartree-Fock exchange operator in hybrid-DFT, First-Principles Molecular Dynamics (FPMD) simulations. This approach provides a set of maximally localized orbitals in domains of variable size and allows for a reduction of the number of computed exchange integrals with controlled accuracy. It does not require a priori assumptions about the localization of orbitals in limited domains and can be used with both occupied and empty orbitals, thus enabling computations of the HOMO-LUMO gap during hybrid-DFT FPMD simulations. We discuss algorithmic improvements of the method and demonstrate its use in hybrid-DFT FPMD simulations of water, solvated ions, and a liquid-solid interface in which maximally localized orbitals show a wide range of localization properties. [1] F.Gygi, Phys. Rev. Lett. 102, 166406 (2009). [2] F. Gygi and I. Duchemin, JCTC (submitted). [3] http://eslab.ucdavis.edu/software/qbox/ . [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A24.00003: One-shot calculation of the Electronic Structure across the Metal Insulator Transitions in V$_{2}$O$_{3}$ by Hybrid Density Functional John Robertson, Yuzheng Guo We present the first calculation of the electronic structure of V$_{2}$O$_{3}$ in its different phases using the screened exchange (sX) hybrid functional [1]. The sX functional reproduces the observed band gaps, magnetic moments and photoemission spectra of the corundum paramagnetic metal (PM) phase, the monoclinic anti-ferromagnetic insulating (AFI) phase, and the corundum Cr-doped paramagnetic insulating (PI) phase. The PI phase has a 0.15eV band gap in good agreement with experiment. Using the generalised Kohn-Sham nature of the hybrid functional, a fully relaxed supercell model of the Cr-doped V$_{2}$O$_{3}$ PI phase is calculated, and it shows that the local strain field around Cr atoms is the driving force for the PI-PM transition. This illustrates that hybrid functionals that fix the exchange interaction can give a good, one-shot description of single particle spectra, and are efficient enough compared to DMFT or GW to treat the complex electron-lattice interactions that occur in the more interesting systems.\\[4pt] [1] S J Clark, J Robertson, Phys Rev B 82 085208 (2010) [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A24.00004: Ab initio calculations of non-radiative carrier trapping due to deep impurity levels Lin-Wang Wang, Lin Shi Non-radiative carrier decay due to deep impurity levels in semiconductors is an important process which affects the efficiencies of devices from solar cells to light emitting diode. This process is due to multiple phonon emission. Despite of the fact the analytical formalisms have been derived long time ago, so far there is no direct ab initio calculations due to the high cost of calculating all the electron-phonon coupling constants. Here we introduce an algorithm which calculates all the electron-phonon coupling constants at once, hence allows the ab initio calculations of such processes. Another approximation is introduced to calculate the phonon modes of a given impurity system. We use a Zn$_{\mathrm{Ga}}$-V$_{\mathrm{N}}$ paired defect in GaN as an example to study this process. We found that while most of the promoting phonon modes (used to promote the transition with the electron-phonon coupling) come from the optical modes, the accepting phonon modes (used to satisfy the energy conservation) come mostly from the acoustic phonons. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A24.00005: Characterizing Oxidation State using Bader Analysis, Maximally Localized Wannier Functions and Atomic Orbitals Projection Kyle Reeves, Yosuke Kanai The concept of oxidation state of atoms in molecules and materials is widely used to predict and understand chemical and physical properties. This concept is perhaps driven more empirically than by any rigorous criteria differentiating one oxidation state from another. Within the oxidation state framework, an integer number of electrons is assigned to the nuclei within a system. In practice, a distribution of electron density makes it difficult to quantify such discrete assignments without some ambiguities. We explore three different charge analysis approaches in density functional theory calculations for addressing the oxidation state of important organometallic molecules [Ru(bpy)$_{\mathrm{3}}$]$^{\mathrm{2+}}$ and [Ru(bpy)$_{\mathrm{3}}$]$^{\mathrm{3+}}$, which are widely used for solar energy conversion applications. Bader charge analysis, Wannier function analysis, and atomic orbital projection are employed in this work. Given the highly-localized nature of the d-electrons of the ruthenium atom, the charge analysis methods are also compared with Hubbard-U correction. We also discuss how the solvation by water molecules influences the oxidation state characterization for these organometallic complexes. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A24.00006: Revised Basin Hopping Monte Carlo Algorithm Applied for Nanoparticles Juarez L. F. Da Silva, Gustavo G. Rondina The Basin Hopping Monte Carlo (BHMC) algorithm has been very successful in obtaining the atomic structure of nanoparticles (NPs), however, its application for unbiased randomly initialized NPs have been restricted to few hundreds atoms employing empirical pair-potentials (EPP) and for small clusters employing first-principles interacting potentials based on density functional theory (DFT). In this talk, we will present our suggestions for bringing improvements to the the BHMC algorithm, which successfully extend its application for relatively large systems employing EPP and DFT potenticals. Using our implementation from scratch, we have found all the reported putative minimum energy configurations for Lennard-Jones and Sutton-Chen EPPs (N = 2 - 147, 200, 250, 300, ..., 1000). We addressed also binary systems described by the Lennard-Jones or Sutton-Chen empirical potentials, and excellent results have been obtained. Finally, our revised BHMC implementation was combined with DFT potentials (FHI-AIMS), which was employed to study the atomic structure of Al clusters from 2 - 55 atoms in the neutral and charged states. Thus, our results indicate the our suggestions provide an important contribution to improve the quality of the BHMC results employing EPP or DFT potentials. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A24.00007: Generic parallel Wang-Landau sampling for complex systems Ying Wai Li, Thomas Vogel, David P. Landau, Thomas W\"ust We introduce a parallel realization for Wang-Landau sampling in Monte Carlo simulations based on a replica-exchange framework. The key idea is to split the entire energy range of the system under consideration into several smaller, overlapping sub intervals. The survey of configurational phase space can then be distributed over multiple processors, with exchanges of random walkers taking place in the overlapping energy windows. To demonstrate the robustness and advantages of this parallel scheme for the simulations of complex systems, we have applied it to protein adsorption problems using the HP lattice protein model\footnote{K. A. Dill, Biochemistry \textbf{24}, 1501 (1985).}. The method gives significant speed-up and achieves strong scaling on small computer architectures like multi-core processors, with a possible improvement in accuracy. We believe that it could be potentially beneficial for large-scale petaflop machines. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A24.00008: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 9:36AM - 9:48AM |
A24.00009: A Scientific Cloud Computing Platform for Condensed Matter Physics K. Jorissen, W. Johnson, F. D. Vila, J. J. Rehr Scientific Cloud Computing (SCC) makes possible calculations with high performance computational tools, without the need to purchase or maintain sophisticated hardware and software. We have recently developed an interface dubbed SC2IT [1] that controls on-demand virtual Linux clusters within the Amazon EC2 cloud platform [2]. Using this interface we have developed a more advanced, user-friendly SCC Platform configured especially for condensed matter calculations. This platform contains a GUI, based on a new Java version of SC2IT, that permits calculations of various materials properties. The cloud platform includes Virtual Machines preconfigured for parallel calculations and several precompiled and optimized materials science codes for electronic structure and x-ray and electron spectroscopy. Consequently this SCC makes state-of-the-art condensed matter calculations easy to access for general users. Proof-of-principle performance benchmarks [1] show excellent parallelization and communication performance. [1] K. Jorissen, F.D. Vila, and J.J. Rehr, Comp. Phys. Comm. 183 1911 (2012) [2] http://aws.amazon.com and http://www.feffproject.org/scc [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A24.00010: Anderson Localization: Dynamical Cluster Approximation - Typical Medium Theory Perspective Chinedu Ekuma, Ziyang Meng, Hanna Terletska, Juana Moreno, Mark Jarrell, Vladimir Dobrosavljevic Mean field theories like the coherent potential approximation (CPA) and its cluster extensions, including the dynamical cluster approximation (DCA), fail to describe the Anderson localization transition in disordered systems. This failure is intrinsic to these theories as the algebraically averaged quantities used in them always favor the metallic state, and hence cannot describe the localization transition. Here we extend the Typical Medium Theory (TMT), which replaces the average quantities with their corresponding typical (geometrically averaged) equivalents, to its cluster form such that non-local correlations can be incorporated systematically. We apply our method to study the localization phenomena in various dimensions. Such an approach opens a new avenue to study localization effect both in model and in real materials. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A24.00011: Gradient-Stable Linear Time Steps for Phase Field Models Benjamin Vollmayr-Lee Phase field models, which are nonlinear partial-differential equations, are a widely used for modeling the dynamics and equilibrium properties of materials. Unfortunately, time marching the equations of motion by explicit methods is usually numerically unstable unless the size of the time step is kept below a lattice-dependent threshold. Consequently, the amount of numerical computation is determined by avoidance of the instability rather than by the natural time scale of the dynamics. This can be a severe overhead. In contrast, a gradient stable method ensures a decreasing free energy, consistent with the relaxational dynamics of the continuous time model. Eyre's theorem proved that gradient stable schemes are possible, and Eyre presented a framework for constructing gradient-stable, semi-implicit time steps for a given phase-field model. Here I present a new theorem that provides a broader class of gradient-stable steps, in particular ones in which the implicit part of the equation is linear. This enables use of fast Fourier transforms to solve for the updated field, providing a considerable advantage in speed and simplicity. Examples will be presented for the Allen-Cahn and Cahn-Hilliard equations, an Ehrlich-Schwoebel-type interface growth model, and block copolymers. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A24.00012: Total energy and force calculations for correlated materials Ivan Leonov, Vladimir I. Anisimov, Dieter Vollhardt We present a computational scheme for the investigation of complex materials with strongly interacting electrons which is able to treat atomic displacements, and hence structural relaxation, caused by electronic correlations. It combines \textit{ab initio} band structure and dynamical mean-field theory and is implemented with the linear response formalism regarding atomic displacements. We employ this approach to compute the equilibrium crystal structure and phase stability of a couple of correlated electron materials, such as elemental hydrogen, SrVO$_3$, and KCuF$_3$. Our results show an overall good agreement between the total energy and force computations of the equilibrium atomic position for these materials. The approach presented here allows one to study the structural properties of materials with strongly correlated electrons such as lattice instabilities observed at correlation induced metal-insulator phase transitions from first principles. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A24.00013: Self-consistent implementation of the vector disordered local moment method for magnetic alloys and its applications to magnetic thermodynamics Kirill Belashchenko, Bhalchandra Pujari, Paul Larson, Vladimir Antropov, Mark van Schilfgaarde We describe an implementation of the coherent potential approximation within the LMTO formalism, which combines chemical and magnetic disorder treated within the vector disordered local moment model. It allows for arbitrary degree of magnetic order for Heisenberg spins specified by axially symmetric spin distribution functions. The atomic charges and potentials are determined self-consistently, and the transverse constraining fields are included as required by density functional theory. Total energies and spectral functions are available, and the spin distribution functions can be used as variational parameters to determine the magnetic state at the given temperature by minimizing the free energy. The performance of this method is illustrated using several examples. The predictions of the Curie temperatures by different approximations for several materials (such as Fe, Co, Gd, FePt, FePd, CoPt) are compared, including the effect of the constraining fields. We also discuss competing magnetic interactions in the Fe$_{1-x}$Mn$_x$Pt alloy, which is known from experiment to present five magnetic phases, including two noncollinear ones. We construct the magnetic phase diagram using the variational minimization of the free energy and obtain the correct sequence of phases. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A24.00014: Large-scale Atomic Effective Pseudopotential Method for the Electronic Structure of Semiconductor Nanostructures Gabriel Bester, R. Cardenas, F. Zirkelbach, P.-Y. Prodhomme, P. Han, R. Cherian In the {\em Large-scale Atomic Effective Pseudopotential Method}, the Schr\"odinger equation of an electronic system is solved within an effective single-particle approach. {\em Atomic Effective Pseudopotentials} are utilized, which are derived from screened local effective crystal potentials obtained from self-consistent density functional theory calculations on elongated and slightly deformed bulk structures. A self-consistency cycle is not required, which reduces the computational effort. Furthermore, iterative solvers can be used to focus only on a few eigenstates of interest, e.g., states in the vicinity of the band gap of a semiconductor. Hence, this approach is particularly well suited for first-principles investigations of the electronic structure of nanostructures consisting of up to ten thousands of atoms, when the knowledge of the total energy of the system is not required. The treatment includes semi-local pseudopotentials (Kleinman Bylander separable form in real space) as well as the spin-orbit interaction. The obtained single-particle wavefunctions are then used to treat excited state properties by means of a configuration interaction approach. We will illustrate the capabilities of the method on some selected semiconductor nanostructures. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A24.00015: Fidelity susceptibility of one-dimensional models with twisted boundary conditions Diptiman Sen, Manisha Thakurathi, Amit Dutta It is well-known that the ground state fidelity of a quantum many-body system can be used to detect its quantum critical points (QCPs). If $g$ denotes the parameter in the Hamiltonian with respect to which the fidelity is computed, we find that for one-dimensional models with a large but finite size, the fidelity susceptibility $\chi_F$ can detect a QCP provided that the correlation length exponent satisfies $\nu < 2$. We then show that $\chi_F$ can be used to locate a QCP even if $\nu \ge 2$ if we introduce boundary conditions labeled by a twist angle $N\theta$, where $N$ is the system size. If the QCP lies at $g=0$, we find that if $N$ is kept constant, $\chi_F$ has a scaling form given by $\chi_F \sim \theta^{-2/\nu} f(g/\theta^{1/\nu})$ if $\theta \ll 2\pi/N$. We illustrate this in a tight-binding model of fermions with a spatially varying chemical potential with amplitude $h$ and period $2q$ in which $\nu = q$. Finally we show that when $q$ is very large, the model has two QCPs at $h=\pm 2$ which cannot be detected by studying the energy spectrum but are clearly detected by $\chi_F$. The peak value and width of $\chi_F$ scale as non-trivial powers of $q$ at these QCPs. We argue that these QCPs mark a transition between extended and localized states at the Fermi energy. [Preview Abstract] |
Session A25: Superconducting Qubits: Magnetic Flux and Vortex Noise on Qubits and Resonators, Quasiparticles, and Qubit-Resonator Designs
Sponsoring Units: GQIChair: Britton Plourde, Syracuse University
Room: 327
Monday, March 18, 2013 8:00AM - 8:12AM |
A25.00001: Simulations of Noise in the 2D XY Spin Model Chuntai Shi, Sungho Han, Clare C. Yu Experiments implicate spins on the surface of metals as the source of flux and inductance noise in SQUIDs. There is experimental evidence that interactions between these surface spins cannot be ignored. As a candidate model of the surface spins, we present Monte Carlo simulations of the classical 2D XY spin model on a square lattice. We investigate the magnetization noise as a function of frequency and temperature. Finite size effects are considered through studies of different size systems. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A25.00002: Surface spins in superconducting qubits: noise of noise and noise of inductance Alexander Shnirman, Pablo Schad, Boris Narozhny, Gerd Schoen In the last several years a growing bulk of experimental evidence has emerged explaining the 1/f magnetic flux noise in superconducting circuits, e.g., qubits, by a very high density of paramagnetic impurities on the surfaces or interfaces of the superconducting metal. A theoretical picture of this phenomenon is still missing. Here we study a model of weakly interacting dissipative spins or spin clusters with the aim to determine their noise properties. In particular we compare the noise of noise (second spectrum) with the noise of the magnetic susceptibility measured as noise of inductance. Both of these were recently studied in experiments. We argue that the noise of noise is dominated by a simple gaussian background, whereas the noise of susceptibility can provide a hint about the microscopic nature of the spins. In particular we discuss the influence of the spin-spin interactions on the susceptibility noise. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A25.00003: Experimental results on decoherence and readout of coupled superconducting flux qubits in a circuit-QED setup Jean-Luc Orgiazzi, David Layden, Ryan Marchildon, Mustafa Bal, Chunqing Deng, Florian Ong, Adrian Lupascu We present the results of experiments with two superconducting flux qubits coupled to a high-quality factor aluminum coplanar waveguide resonator. The flux qubits have a loop area of $\sim$ 24 $\mu$m$^2$. The coupling to the resonator is implemented using the inductance of a shared line. The qubits are independently controlled via on-chip fast flux bias lines. Readout is performed by homodyne detection at large resonator driving power. Readout contrast exceeds 70\% for each qubit. We observed long relaxation times, approaching 10 microseconds. The coherence time at the symmetry point exceeds 1 microsecond. Away from the symmetry point, decoherence is due to $1/f$ flux noise, with a measured density of $2.6 \times 10^{-6}$ $\Phi_0$ $/\sqrt{\mathrm{Hz}}$ at 1 Hz. We discuss the implementation of a two-qubit controlled-NOT gate using the selective darkening technique [1]. [1] P. C. de Groot, J. Lisenfeld, R. N. Schouten, S. Ashhab, A. Lupascu, C. J. P. M. Harmans, and J. E. Mooij. Nat. Phys., 6(10):763-766, October 2010. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A25.00004: Ultrasensitive detection of magnetic field using a single artificial atom Mustafa Bal, Chunqing Deng, Jean-Luc Orgiazzi, Florian Ong, Adrian Lupascu We employ a single artificial atom to implement ultrasensitive magnetic field detection. The artificial atom is a persistent current qubit with a size in the micron range, which couples very strongly to magnetic field, with an equivalent magnetic moment of $3.8\times 10^5$ Bohr magnetons. Sensitive detection is realized by employing the field-dependent coherent evolution of the artificial atom and high-fidelity quantum measurement, in a way similar to atomic magnetometry. Using an operation mode based on spin-echo manipulation and qubit reset by energy relaxation, we demonstrate a magnetic field detection sensitivity of $7.5\, pT/\sqrt{Hz}$ for an AC field at $10 MHz$. The sensitivity is further improved if the reset step is eliminated and the correlation of consecutive projective measurements is used instead, reaching $3.3\, pT/\sqrt{Hz}$. The intrinsic sensitivity of this method to AC fields at frequencies in the $100\, kHz - 10\, MHz$ range compares favourably with DC-SQUIDs and atomic magnetometers of equivalent spatial resolution. More than an order of magnitude increase in sensitivity is possible using feasible improvements of qubit design and readout. This result illustrates the potential of artificial quantum systems for sensitive detection and related applications. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A25.00005: Magnetic Field Effects on High Quality Factor Superconducting Coplanar Resonators Anthony Megrant, Charles Neill, Rami Barends, Yu Chen, Ben Chiaro, Julian Kelly, Matteo Mariantoni, Josh Mutus, Peter O'Malley, Daniel Sank, Amit Vainsencher, James Wenner, Ted White, David Low, Shinobu Ohya, Christopher Palmstrom, John Martinis, Andrew Cleland Superconducting coplanar waveguide resonators have proven to be invaluable tools in studying some of the same decoherence mechanisms as those found in superconducting qubits. Prior improvements in fabrication led to resonator internal quality factors (Qi's) in excess of 10 million at high power, enabling us to sensitively probe environmental effects on the resonance frequency and Qi. We have found these resonators to be very susceptible to applied and stray magnetic fields, with measurable changes in the resonator's Qi and resonance frequency from fields as small as a few milligauss. I will present more recent measurements of resonators in magnetic fields. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A25.00006: Effects of vortices trapped in superconducting microwave resonators Ibrahim Nsanzineza, B.L.T. Plourde The microwave response of superconductors can be influenced by the presence of vortices and the dynamics they exhibit at high frequencies. We present measurements of vortices trapped in coplanar waveguide superconducting resonators fabricated from thin aluminum films, a common material for superconducting qubit circuits. In particular, by adjusting the geometry of our resonators we are able to trap only a few vortices in certain regions of the resonators. We perform field-cooled measurements to study the dependence of the microwave vortex response on magnetic field and frequency for various resonator modes. In most cases, the addition of vortices results in a downward shift in resonant frequency and a reduction in the quality factor. However, under certain circumstances, the presence of trapped vortices can actually lead to an enhancement of the resonator quality factor. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A25.00007: Large kinetic inductance microwave resonators in magnetic field Martin Weides, Philipp Mayer, Fengbin Song, Sebastian Probst, Hannes Rotzinger, Alexey Ustinov Superconducting resonators of high quality factors are of great interest for photon detection and quantum computation. Conventionally, they operate in or close to the magnetic vacuum. However, for some circuits -for instance resonators coupled to spin ensemble crystals or Majorana fermions- the magnetic field is not negligible and the resonator's field robustness has to be well engineered. The magnetic field dependencies of resonance frequency and quality factor are of considerable interest to improve resonant quantum devices. In this presentation we will discuss thin film titanium nitride resonators operating in a homogeneous magnetic field. Titanium nitride has remarkably high internal microwave quality factors down to single photon levels, and a significant kinetic inductance contribution for thin film resonators. The microwave scattering data of frequency multiplexed resonators was taken in a Helium-3 refrigerator over a large range of photon number levels, temperatures, and magnetic fields. The resonators exhibit strong magnetic hysteresis effects in frequency and quality factor. The magnetic memory -caused by a spatial distribution of trapped vortices- is related to the resonator geometry. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A25.00008: Flux-dependent loss in aluminum nanobridge SQUID resonators E. M. Levenson-Falk, R. Vijay, I. Siddiqi Unlike traditional tunnel junctions, nanobridge Josephson junctions have weaker nonlinearity, higher transmittivity, and relatively few conduction channels. These parameters carry with them their own intrinsic loss mechanisms. In particular, quasiparticle trapping has been recently shown [1] to be prevalent in quantum point contact junctions operating in a similar parameter regime. We investigate losses in resonant circuits comprised of nanobridge SQUIDs. We observe an increase in loss and an anomalous frequency shift as the SQUIDs are flux-biased, which we speculate to be the result of quasiparticle trapping in a phase-biased nanobridge. We present detailed measurements of this effect, and discuss efforts towards eliminating it. [1] Bretheau et al., PRL 106, 257003 (2011) [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A25.00009: Random frequency modulation of a superconducting qubit Matti Silveri, Jian Li, Karthikeyan Sampath, Juha-Matti Pirkkalainen, Antti Veps\"{a}l\"{a}inen, Wei-Cheng Chien, Jani Tuorila, Mika Sillanp\"{a}\"{a}, Pertti Hakonen, Erkki Thuneberg, Gheorghe Paraoanu Superconducting circuits with Josephson junctions are a promising platform not only for developing quantum technologies, but, importantly, also for the study of effects that typically occur in complex condensed-matter systems. Here, we employ a transmon qubit to conduct an analog simulation of motional averaging, a phenomenon initially observed in nuclear magnetic resonance spectroscopy. To realize this effect, the flux bias of the transmon is modulated by a controllable pseudo-random telegraph noise, which results in stochastic jumping of the energy separation (frequency) between two discrete values. This can also be seen as a simulated fast-fluctuation environment under direct experimental control. Additionally, we discuss the population dynamics using an analytical master equation, and apply the motional averaging analysis on phenomena where the fluctuation of the energy is due to quasiparticles or to photon shot noise. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A25.00010: Driven Dynamics and Rotary Echo of a Qubit Tunably Coupled to a Harmonic Oscillator William Oliver, Simon Gustavsson, Jonas Bylander, Fei Yan, Pol Forn-Diaz, Vlad Bolkhovsky, Danielle Braje, George Fitch, Khalil Harrabi, Donna Lennon, Jovi Miloshi, Peter Murphy, Rick Slattery, Steven Spector, Ben Turek, Terry Weir, Paul Welander, Fumiki Yoshihara, David Cory, Yasunobu Nakamura, Terry Orlando We have investigated the driven dynamics of a superconducting flux qubit that is tunably coupled to a microwave resonator. We find that the qubit experiences an oscillating field mediated by off-resonant driving of the resonator, leading to strong modifications of the qubit Rabi frequency. This opens an additional noise channel, and we find that low-frequency noise in the coupling parameter causes a reduction of the coherence time during driven evolution. The noise can be mitigated with the rotary-echo pulse sequence, which, for driven systems, is analogous to the Hahn-echo sequence. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A25.00011: Effect of environmental coupling on tunneling of quasiparticles in Josephson junctions Mohammad Ansari, Frank Wilhelm, Urbasi Sinha, Aninda Sinha We study quasiparticle tunneling in Josephson tunnel junctions embedded in an electromagnetic environment. We identify tunneling processes that transfer electrical charge and couple to the environment in a way similar to that of normal electrons, and processes that mix electrons and holes and are thus creating charge superpositions. The latter are sensitive for the phase difference between the superconductor and are thus limited by phase diffusion even at zero temperature. We show that that term is suppressed in many environments, thus leading to lower quasiparticle decay rates and thus better qubit coherence than previously expected. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A25.00012: Real-time measurement of quasiparticle tunneling in a single-junction transmon qubit using feedback Diego Rist\`e, Niels Bultink, Marijn Tiggelman, Raymond Schouten, Konrad Lehnert, Leonardo DiCarlo With coherence times of superconducting qubits now exceeding $100~\mathrm{\mu s}$, the contribution of quasiparticle (QP) tunneling to qubit relaxation and dephasing becomes potentially relevant. We report the real-time measurement of QP tunneling across the single junction of a 3D transmon qubit. We integrate recent developments in projective qubit readout with $99\%$ fidelity and feedback-based reset to transform the qubit into a charge-parity detector with $6~\mathrm{\mu s}$ resolution. We detect a symmetric random telegraph signal matching a QP tunneling time of $0.8~\mathrm{ms}$. By measuring the correlation function of charge parity conditioned on specific initial and final qubit states, we determine that most QP tunneling does not induce qubit transitions, in contradiction with recent theory [1]. We extract a QP-induced qubit relaxation time $T_1^{\mathrm{qp}} \sim 3~\mathrm{ms}$, decidedly not limiting the measured $T_1 = 0.14~\mathrm{ms}$.\\[4pt] [1] G. Catelani et al., Phys. Rev. B 84, 064517 (2011). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A25.00013: Interfacing Superconducting Qubits and Resonator Qudit Frederick Strauch, Xiaoting Wang, Kurt Jacobs We consider methods to transfer multi-qubit states into the higher-dimensional state space of a superconducting resonator, acting as a qudit. Several methods are proposed, using different combinations of resonant, dispersive, and auxiliary interactions. The complexity of such schemes are explored using analytical and numerically optimized control sequences. Extension to resonator measurement and qudit logic will be also be described. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A25.00014: Frequency multiplexed dispersive readout of transmon qubits with the UCSB paramp Daniel Sank, R. Barends, J. Bochmann, B. Campbell, Y. Chen, B. Chiaro, E. Jeffrey, J. Kelly, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, P. O'Malley, S. Ohya, P. Roushan, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis Our new Xmon qubit shows good coherence, controllability and simplified coupling to other circuit elements, making it a good candidate for a large scale quantum computer. Like all qubits, it requires high fidelity readout. To this end we have developed a new parametric amplifier circuit. Simplified input coupling of the amplifier allows straightforward interfacing with our frequency multiplexed dispersive readout circuitry. The amplifier features five different modes of pump power delivery, some of which allow us to reduce the microwave component count in our readout chain. We characterize our readout system using each of these modes of operation, as well as multi qubit readout. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A25.00015: Engineering and control of coupled superconducting qubits arrays for quantum simulation E. Henry, A. Schmidt, O. Viehmann, I. Siddiqi Superconducting qubit technology allows for engineering experimentally accessible, macroscopic quantum systems to arbitrary specifications within a large parameter space. By coupling multiple superconducting qubits in a periodic array, it is possible to fabricate physical objects which mimic the properties of naturally occurring systems not readily accessible to measurement or parameter variation, or theoretical systems not occurring in nature. We discuss design, fabrication, and measurement of a physical realization of the quantum Ising model in zero and one dimension. This is accomplished using a chain of identical transmon qubits acting as artificial spins whose interaction is dominated by nearest neighbor coupling. Control and readout of the system is accomplished by coupling only one of the artificial spins to a microwave resonator in a circuit QED architecture. [Preview Abstract] |
Session A26: Focus Session: Semiconductor Qubits - Optical Hybridization
Sponsoring Units: GQIChair: Thaddeus Ladd, HRL Laboratories, LLC
Room: 328
Monday, March 18, 2013 8:00AM - 8:36AM |
A26.00001: Observation of quantum dot spin-photon entanglement Invited Speaker: Atac Imamoglu Entanglement plays a central role in the burgeoning field of quantum information processing. A possible route towards a scalable architecture is provided by the concept of distributed quantum computation, based on small-scale few-qubit quantum processor nodes interconnected by single photon pulses. Generation of quantum correlated spin-photon pairs is a key step in such an approach. In this talk, we report the observation of quantum entanglement between a semiconductor quantum dot spin and the color of a propagating optical photon. The demonstration of entanglement relies on the use of fast single-photon detection which allows us to project the photon into a superposition of its two frequency components. Our results extend the previous demonstrations of single-spin photon entanglement in trapped ions, neutral atoms and nitrogen vacancy centers to the domain of artificial atoms in semiconductor nano-structures that allow for on-chip integration of electronic and photonic elements. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A26.00002: Entanglement between a single quantum spin and a photon through ultrafast frequency downconversion to telecom wavelengths Kristiaan De Greve, Leo Yu, Peter McMahon, Jason Pelc, Chandra Natarajan, Na Young Kim, Eisuke Abe, Sebastian Maier, Christian Schneider, Martin Kamp, Sven Hoefling, Robert Hadfield, Alfred Forchel, Martin Fejer, Yoshihisa Yamamoto We demonstrate high-fidelity entanglement between a single InAs quantum dot electron spin, and the polarization of a spontaneously emitted single photon. With a magnetic field in Voigt geometry, the quantum dot's excited (trion) states are connected to the spin states in a lambda-configuration. We use these lambda-systems for all-optical spin manipulation, and spontaneous emission from one of the trion states gives rise to entanglement between both the polarization and color of the photon, as well as the spin state. Leakage of which-path information through e.g. the color of the photon obscures the spin-photon-polarization entanglement, which we overcome by a quantum erasure procedure. By time-resolved frequency conversion to a low-fiber-loss wavelength (1560 nm), we measure the photon arrival time with sub-10 ps resolution. Such ultrafast detection is inherently broadband, and incapable of distinguishing between the respective colors of the decay paths, providing the necessary quantum erasure. The conversion to 1560 nm also provides a means to extend the distance over which spin-photon entanglement can be maintained. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A26.00003: Tomography of a high-fidelity spin-photon entangled state Peter McMahon, Kristiaan De Greve, Leo Yu, Jason Pelc, Chandra Natarajan, Na Young Kim, Eisuke Abe, Sebastian Maier, Christian Schneider, Martin Kamp, Sven Hoefling, Robert Hadfield, Alfred Forchel, M.M. Fejer, Yoshihisa Yamamoto The generation of entanglement between a quantum memory and a flying qubit is an important step towards building a quantum repeater node. Entanglement between a photon and a matter qubit has been demonstrated in several systems, including neutral atoms, trapped ions, NV centers and quantum dots. Quantum dots have a natural advantage that their radiative lifetimes are short, and therefore the rate of entanglement generation can be much faster than in other systems. We have recently demonstrated entanglement between an electron spin in a quantum dot, and the polarization of an emitted photon. In addition, the photon is converted to the low-loss 1550 nm band, which is important for implementing long-distance quantum communication systems. In this talk, I will present the reconstruction of the full density matrix of the entangled spin-photon state that we produce. We calculate the fidelity of the state from the density matrix, and conclude that it is $>90\%$. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A26.00004: Ultrafast downconversion quantum interface for a single quantum dot spin and 1550-nm single-photon channel L. Yu, J.S. Pelc, K. De Greve, P.L. McMahon, M.M. Fejer, Y. Yamamoto, S. Maier, C. Schneider, M. Kamp, S. Hofling, A. Forchel, C.M. Natarajan, R.H. Hadfield Long-distance quantum communication networks require appropriate interfaces between matter qubit-based nodes and low-loss photonic quantum channels. Quantum frequency conversion (QFC), whereby a photonic qubit's carrier frequency is translated while maintaining its quantum state, is well-suited to the task. Quantum dots have been studied extensively as potential quantum network nodes, but they do not emit indistinguishable single photons at telecomm wavelengths. We report an ultrafast, low-noise downconversion quantum interface, in which 910-nm single photons from a quantum dot are downconverted to the 1.5-$\mu$m lowest-loss telecom band, showing near-perfect preservation of antibunched photon statistics. Moreover, the resulting time resolution could also improve photon indistinguishability. Together with the III-V semiconductor quantum dot spin system, this ultrafast downconversion quantum interface provides new possibility to realize long-distance quantum communication networks. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A26.00005: Observation of quantum entanglement between a photon and a single electron spin confined to an InAs quantum dot John Schaibley, Alex Burgers, Greg McCracken, Luming Duan, Paul Berman, Duncan Steel, Allan Bracker, Daniel Gammon, Lu Sham A single electron spin confined to a single InAs quantum dot (QD) can serve as a qubit for quantum information processing. By utilizing the QD's optically excited trion states in the presence of an externally applied magnetic field, the QD spin can be rapidly initialized, manipulated and read out. A key resource for quantum information is the ability to entangle distinct QD spins. One approach relies on intermediate spin-photon entanglement to mediate the entanglement between distant QD spin qubits. We report a demonstration of quantum entanglement between a photon's polarization state and the spin state of a single electron confined to a single QD. Here, the photon is spontaneously emitted from one of the QD's trion states. The emitted photon's polarization along the detection axis is entangled with the resulting spin state of the QD. By performing projective measurements on the photon's polarization state and correlating these measurements with the state of the QD spin in two different bases, we obtain a lower bound on the entanglement fidelity of 0.59 (after background correction). The fidelity bound is limited almost entirely by the timing resolution of our single photon detector. The spin-photon entanglement generation rate is $3 \times 10^{3}$ s$^{-1}$. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A26.00006: A Spin Qubit Coupled to a Photonic Crystal Cavity Timothy Sweeney, Samuel Carter, Mijin Kim, Chul Soo Kim, Dmitry Solenov, Sophia Economou, Thomas Reineke, Lily Yang, Allan Bracker, Daniel Gammon The development of a scalable light-matter quantum interface is an important goal of quantum information research. Photonic crystal (PC) membranes provide an architecture in which the interaction of photons with an optically active matter qubit can be controlled through the introduction of optical cavities and waveguides. Charge neutral quantum dots are commonly integrated into PC architectures and are useful for sources and switches, but do not demonstrate long-lived coherences. A charged quantum dot in a PC environment could lead to a spin-photon quantum interface, where it is the long-lived spin of the electron, not the exciton that serves as a qubit. We demonstrate optical spin initialization and coherent control of an electron in a quantum dot that is embedded in and coupled to a 2D PC membrane cavity. The PC membrane is incorporated into an asymmetric NIP diode that allows for charging of an InAs quantum dot via an applied bias. Resonant laser spectroscopy performed in a transverse magnetic field enables the optical measurement and initialization of the electron spin. Furthermore, with the introduction of detuned control pulses, we perform coherent rotations of the electron spin state. These studies demonstrate several essential accomplishments toward a spin-photon interface. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A26.00007: Quantum Dots in H1 Photonic Crystal Microcavities for Quantum Information Jenna Hagemeier, Cristian Bonato, Tuan-Anh Truong, Hyochul Kim, Morten Bakker, Gareth J. Beirne, Martin P. van Exter, Pierre Petroff, Dirk Bouwmeester Coupling semiconductor quantum dots to optical microcavities is a promising technique for implementing quantum information processing protocols in the solid-state. By placing one or more emitters in a cavity, it is possible to create an efficient source of single photons or to explore collective interactions of few-emitter systems. Our devices consist of two layers of quantum dots, embedded in the cavity region of H1 photonic crystal microcavities. One of the quantum dot layers can be frequency-tuned deterministically, allowing two resonant quantum dots to be coupled to a single cavity mode. Because good mode-matching between the cavity mode and the input/output channel is necessary for many applications, we optimize the far-field profiles of our H1 cavities and demonstrate strong enhancement of the external mode matching properties. We will discuss our far-field optimization results as well as our ongoing work to study interactions of multiple emitters in a cavity. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A26.00008: Dynamical effects of Stark-shifted quantum dots strongly coupled to photonic crystal cavities Kaushik Roy Choudhury, Ranojoy Bose, Edo Waks Single semiconductor quantum-dots (QDs) strongly coupled to photonic crystal cavities are a strong candidate for single photon generation, ultra-fast all optical switching and quantum information processing. Recent experiments on coupled-cavity quantum dot systems show possible manipulation of emission wavelength of the dot through optical Stark effect. Interesting dynamical features arise when the Stark pulse duration is comparable to QD-cavity interaction time. Here, we present a theoretical treatment of these dynamical effects and investigate dynamical emission spectrum, energy transfer and single photon generation. We study these effects through numerical solution of the full master equation. We demonstrate that dynamic Stark effects can be used to generate ultra-fast indistinguishable single photons using rapid Stark tuning of the quantum dot. The theoretical limit for the speed is shown to be faster than adiabatic rapid passage technique used for microwave photon generation in circuit QED. A systematic study of role of device parameters such as pulse-shape, dot-cavity coupling and incoherent losses on the efficiency and speed of single photon generation is also presented for possible experimental realization. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A26.00009: Optical tuning of single quantum dots coupled to photonic crystal molecules using the optical Stark effect Ranojoy Bose, Kaushik Roy, Tao Cai, Glenn S. Solomon, Edo Waks The interaction of semiconductor quantum dots (QD) with photonic crystal resonator systems provides a highly integrated, solid-state platform for studies in ultra-low energy nonlinear optics and quantum optical phenomena. Here, we present a method to tune a semiconductor quantum dot (QD) all-optically into resonance with a cavity mode using the non-resonant optical Stark effect. We use a system comprised of two evanescently coupled photonic crystal cavities containing a single QD in one of the cavities. One mode of the coupled cavity system is used to generate a cavity-enhanced optical Stark shift, enabling the QD to be resonantly tuned to the other cavity mode. We show that the optical tuning of the QD results in a large radiative enhancement of the QD photon emission via the Purcell effect. We will further discuss dynamic experiments in the system using a Stark laser that has a time-duration on the order of the system decay rates. We will show that under this scenario, the cavity-QD spectrum provides a rich array of information on the system dynamics. The experiments are promising for a variety of applications in highly-efficient single photon generation, cavity quantum electrodynamics, ultra-fast optical switching, and classical and quantum information processing. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A26.00010: Controlling interactions between coupled photonic crystal cavities using photochromic tuning Tao Cai, Ranojoy Bose, Glenn Solomon, Edo Waks Strongly coupled photonic crystal (PhC) resonator systems provide a promising platform for studying cavity quantum electrodynamics (QED) using semiconductor quantum dots (QDs). These device structures enable important applications such as photon blockade, quantum simulation, quantum-optical Josephson interferometer, and quantum phase transition of light. Many of these applications require the ability to accurately tune the resonant frequencies of individual cavities in the array, which provides a method to control their coupling interactions. This tuning method must be sufficiently local to address individual cavities spaced by less than 1 micron spatial separation. Here, we present a method for controlling the coupling interaction of photonic crystal cavity arrays by using a local and reversible photochromic tuning technique. By locally altering the refractive index of the photochromic material all-optically, the coupling interaction between two cavity modes could be modified over a tuning range as large as 700 GHz. By using this technique, we demonstrate the ability to couple photonic crystal cavities with a normal mode splitting of only 31.50 GHz. We further demonstrate that this tuning method can be extended to control the coupling interaction in larger cavity arrays. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A26.00011: A qubit-photon controlled-NOT gate using a quantum dot strongly coupled to a cavity Hyochul Kim, Ranojoy Bose, Thomas Shen, Glenn Solomon, Edo Waks Strong interactions between matter quantum bits (qubits) and photons play an essential role in quantum information. Quantum dots (QDs) provide a promising implementation of a matter qubit that can be strongly coupled to optical nanocavities, providing a direct light-matter interface. We use this light-matter interface to demonstrate a picosecond timescale controlled NOT logic gate between a QD and a photon, which is a fundamental building block for complex quantum logic. Coherent control of the QD qubit state by optical pulses results in a modification of cavity reflectivity, enabling a conditional bit-flip on the polarization state of a photon incident on the cavity. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A26.00012: All-optical, arbitrary-basis initialization and readout of a diamond spin qubit C.G. Yale, B.B. Buckley, D.J. Christle, F.J. Heremans, L.C. Bassett, D.D. Awschalom, G. Burkard The nitrogen-vacancy (NV) center in diamond is a promising spin qubit candidate, in large part due to its optical addressability via a spin-selective intersystem crossing. Here we demonstrate a general all-optical technique to initialize and readout the NV spin state along an arbitrarily-chosen basis using coherent light fields\footnote{C. G. Yale*, B. B. Buckley*, D. J. Christle, G. Burkard, F. J. Heremans, L. C. Bassett, and D. D. Awschalom (submitted)} below 10 K, which negates the need for this special addressability. By tuning the NV center's excited-state structure to a lambda ($\Lambda$) configuration with a magnetic field, we use coherent population trapping (CPT) to initialize its spin into any desired superposition. We investigate the CPT time dynamics and use quantum state tomography to characterize the resultant state. We also demonstrate spin-state readout along an arbitrarily-chosen basis by measuring photoluminescence emitted during the transient period of the CPT interaction. Since these techniques do not rely on the intersystem crossing, they provide a pathway for all-optical control of other potential defect spin qubits, which may lack the NV center's unique structure. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A26.00013: Defect qubit-nanophotonic structures based on silicon carbide G. Calusine, A. Politi, D.D. Awschalom Defect qubits in silicon carbide (SiC) have recently emerged as a promising alternative to the nitrogen vacancy center in diamond for applications in solid state quantum information technologies\footnote{W. F. Koehl, B.B Buckley, F.J Heremans, G. Calusine, and D.D. Awschalom, \textit{Nature} \textbf{479}, 84-87 (2011)}. One common polytype of SiC, commonly referred to as 3C, is commercially available as a high quality single crystal epitaxial film grown on silicon substrates. We demonstrate that various techniques used to create, polarize, manipulate, and measure nitrogen vacancy centers can be similarly applied to defect spin qubits in 3C silicon carbide, even up to room temperature\footnote{A. L. Falk, B.B. Buckley, G. Calusine, W.F. Koehl, V.V. Dobrovitski, A. Politi, and D.D. Awschalom, (submitted)}. Additionally, we exploit 3C SiC's availability as a heteroepitaxial layer on silicon to incorporate these defect qubits into nanophotonic devices. We present the results of simulations and measurements on nano-fabricated optical devices incorporating defect qubits. These results demonstrate a promising route towards silicon carbide based hybrid light-matter quantum systems. [Preview Abstract] |
Session A27: Focus Session: Adiabatic Quantum Computing I
Sponsoring Units: GQIChair: Sergio Boixo, Information Sciences Institute, University of Southern California
Room: 329
Monday, March 18, 2013 8:00AM - 8:36AM |
A27.00001: Adiabatic Quantum Computation with Neutral Atoms Invited Speaker: Grant Biedermann We are implementing a new platform for adiabatic quantum computation (AQC)\footnote{ E. Farhi, et al. Science \textbf{292}, 472 (2000)} based on trapped neutral atoms whose coupling is mediated by the dipole-dipole interactions of Rydberg states. Ground state cesium atoms are dressed by laser fields in a manner conditional on the Rydberg blockade mechanism,\footnote{S. Rolston, et al. Phys. Rev. A, \textbf{82}, 033412 (2010)}$^,$\footnote{T. Keating, et al. arXiv:1209.4112 (2012)} thereby providing the requisite entangling interactions. As a benchmark we study a Quadratic Unconstrained Binary Optimization (QUBO) problem whose solution is found in the ground state spin configuration of an Ising-like model.\\[4pt] In collaboration with Lambert Parazzoli, Sandia National Laboratories; Aaron Hankin, Center for Quantum Information and Control (CQuIC), University of New Mexico; James Chin-Wen Chou, Yuan-Yu Jau, Peter Schwindt, Cort Johnson, and George Burns, Sandia National Laboratories; Tyler Keating, Krittika Goyal, and Ivan Deutsch, Center for Quantum Information and Control (CQuIC), University of New Mexico; and Andrew Landahl, Sandia National Laboratories. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A27.00002: On scalable, universal adiabatic quantum computation Ari Mizel We investigate scalable, universal adiabatic quantum computation. We exhibit a specific Hamiltonian of local one- and two-body interactions for which the ground state (a) yields the correct answer with high probability and (b) is provably fault-tolerant against local excitations. The effects of finite temperature are discussed. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A27.00003: Ground State Spin Logic James Whitfield, Mauro Faccin, Jacob Biamonte Designing and optimizing cost functions and energy landscapes is a problem encountered in many fields of science and engineering. These landscapes and cost functions can be embedded and annealed in experimentally controllable spin Hamiltonians. Using an approach based on group theory and symmetries, we examine the embedding of Boolean logic gates into the ground-state subspace of such spin systems. We describe parameterized families of diagonal Hamiltonians and symmetry operations which preserve the ground-state subspace encoding the truth tables of Boolean formulas. The ground-state embeddings of adder circuits are used to illustrate how gates are combined and simplified using symmetry. Our work is relevant for experimental demonstrations of ground-state embeddings found in both classical optimization as well as adiabatic quantum optimization. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:36AM |
A27.00004: Experimental signatures of quantum annealing Invited Speaker: Sergio Boixo Quantum annealing is a general strategy for solving optimization problems with the aid of quantum adiabatic evolution. How effective is rapid decoherence in precluding quantum effects in a quantum annealing experiment, and will engineered quantum annealing devices effectively perform classical thermalization when coupled to a decohering thermal environment? Using the D-Wave machine, we report experimental results for a simple problem which takes advantage of the fact that for quantum annealing the measurement statistics are determined by the energy spectrum along the quantum evolution, while in classical thermalization they are determined by the spectrum of the final Hamiltonian only. We establish an experimental signature which is consistent with quantum annealing, and at the same time inconsistent with classical thermalization, in spite of a decoherence timescale which is orders of magnitude shorter than the adiabatic evolution time. For larger and more difficult problems, we compare the measurements statistics of the D-Wave machine to large-scale numerical simulations of simulated annealing and simulated quantum annealing, implemented through classical and quantum Monte Carlo simulations. For our test cases the statistics of the machine are - within calibration uncertainties - indistinguishable from a simulated quantum annealer with suitably chosen parameters, but significantly different from a classical annealer. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A27.00005: Benchmarking the D-Wave adiabatic quantum optimizer via 2D-Ising spin glasses Zhihui Wang, Sergio Boixo, Tameem Albash, Daniel Lidar We present results on benchmarking the D-Wave One quantum optimizer chip using random 2D Ising spin problems. Finding the ground state of the 2D Ising model with randomly assigned local fields and couplings is NP-hard. The chip attempts to find the ground state via quantum annealing, interpolating between a transverse field and the final Ising Hamiltonian. The experimentally obtained final states are checked against exact results and the performance of the chip is characterized by the probability of finding the ground state and the estimated annealing time for finding the ground state with high probability. By analyzing results for 8 to 108 spins, the scaling of the estimated annealing time as a function of the number of spins is compared with the computation time required by classical solvers. The correlation between classical and quantum hardness is also studied. Furthermore, we analyze the correlation between the experimental success probability and the minimum energy gap during the quantum annealing, as well as the interplay between the adiabatic condition and thermalization. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A27.00006: Using coupling strength to tell apart experimental quantum annealing and classical thermalization models Milad Marvian, Sergio Boixo, Tameem Albash, Daniel Lidar Working with a two-qubit Ising Hamiltonian as the target Hamiltonian of quantum annealing implemented on a D-Wave One chip, we study how the qubit-qubit coupling strength affects the probability of finding the ground state. We solve the same problem analytically and numerically using classical thermalization models, and discuss conditions under which the classical prediction for the ground state probability, as a function of coupling strength, differs from the experimental results. For certain reasonable noise models this allows us to tell apart quantum annealing and classical thermalization. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A27.00007: Computational performance and scaling of adiabatic quantum annealing processors Troels Frimodt R{\O}nnow, Sergei Isakov, Dave Wecker, Sergio Boixo, Matthias Troyer We characterise the recent 128 qubit quantum annealing processor, D-Wave One, through investigation of hardness and scaling of ``time-to-solution'' for several thousand realisations of $\pm J$ spin glass problems, ranging from 8 to 108 qubits in size. We compare statistics of the results to classical- and simulated quantum annealing. Within the processors noise and calibration uncertainties, we find that the results generated by the D-Wave One are statistically indistinguishable from results generated by a simulated quantum annealer while significantly different from those of a classical annealer. An intriguing feature is strong bimodal separation of the instances into two categories: hard and easy. This feature is not observed for the classical annealer. Based on the similarities between the simulated quantum annealer and D-Wave One, we make predictions for the 512 qubit processor, D-Wave Two. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A27.00008: Error correction in adiabatic quantum computation Kevin Young, Mohan Sarovar, Robin Blume-Kohout In conventional quantum computing models (e.g. the circuit-model) it is well understood that error suppression techniques by themselves are insufficient for fault-tolerant quantum computing. From a thermodynamic perspective this is because error suppression alone does not provide a mechanism to remove the entropy generated by errors from the encoded system . Since the thermodynamic argument is independent of the computational model it is expected that error suppression alone is insufficient for fault-tolerant quantum computing in the adiabatic quantum computing (AQC) model also. In this talk we provide a scheme for performing error correction for AQC and discuss the differences between our method and those used in quantum circuit model implementations. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A27.00009: Experimental Quantum Error Correction Kristen Pudenz, Daniel Lidar We demonstrate an experimentally implemented quantum error correcting code (QECC) in an adiabatic quantum computation (AQC) setting. In AQC, the computation proceeds by slowly changing the controls of the system to move from an initial Hamiltonian with an easily prepared ground state to a final Hamiltonian whose ground state embodies the solution to the problem. Our QECC is a repetition code in the computational basis, and encodes the final Hamiltonian of the computation. In this way, we provide an energy penalty for excursions outside the codespace which increases as the AQC progresses. We supplement this with classical decoding of the results at the end of the computation, so that the computation may finish in a state other than the ground state and still solve the problem, as long as it stays within the low-lying spectrum of decodable states. We will show experimental results demonstrating that AQCs encoded with our QECC exhibit better success rates than both unencoded and classically encoded versions. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A27.00010: Adiabatic quantum computational properties of Hopf link Omar Shehab Topological quantum computation has recently become an active field of research with a promise of tackling decoherence. Another track of research effort has presented adiabatic quantum computation as a candidate for implementing quantum computers with presently available technologies. We investigate the potential of combining the strengths of both regime. This report conducts adiabatic evolution on low dimensional topological constructs. We study the properties of a Hopf link related to adiabatic quantum computation. The graph and Seifert surface for the link are calculated. The Ising model representing the Hopf link is then derived from the surface. The Edwards-Anderson Hamiltonian is also solved for the Ising model. The associated eigenfunction and eigenvalues are then used to investigate computational problems which can be represented by the ground state of the adiabatic Hamiltonian. We also consider a type II Reidemeister move on the link. The graph and Seifert surface are calculated for the new link. Then the Edwards-Anderson Hamiltonian is solved for the associated Ising model. The constraints of adiabatic evolution are calculated for both cases. Finally, computational problems are investigated which can be represented by the ground state of the adiabatic Hamiltonian. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A27.00011: Symmetry and Controllability for Quantum Spin Networks Xiaoting Wang, Sophie Schirmer, Daniel Burgarth, Peter Pemberton-Ross, Kurt Jacobs Symmetry is found to be an important tool to study the controllability problems in quantum control. Based on quantum spin networks subject to control of a single node by a local potential (Z-control), we have considered the relation of symmetriy and subspace controllability. Focusing on the single excitation subspace it is shown that for single-node Z-controls external symmetries are characterized by eigenstates of the system Hamiltonian that have zero overlap with the control node, and there are no internal symmetries. For uniformly coupled XXZ chains a characterization of all possible symmetries is derived from Bethe ansatz. Moreover, for uniform Heisenberg and XX chains, basic number theory can be used to prove that the lack of symmetry is equivalent to subspace controllability. On the other hand, symmetries in the Hamiltonian can be classified into two types: the internal and the external symmetries. Based on the external symmetries, we can rigorously prove the subspace controllability in each of the invariant subspaces for both XXZ and XYZ chains, but not for XX or Ising chains. All these results are useful to design the appropriate control strategy when implementing QIP in real physical systems. [Preview Abstract] |
Session A28: Focus Session: Statistical Physics of Active Systems Away from Detailed Balance
Chair: Aparna Baskaran, Brandeis UniversityRoom: 336
Monday, March 18, 2013 8:00AM - 8:36AM |
A28.00001: Motility-Induced Phase Separation in Active Matter: a generic formalism for active brownian particles and run-and-tumble particles Invited Speaker: Julien Tailleur In this talk I will show that several classes of active particles admit an identical coarse-grained description in terms of fluctuating hydrodynamic fields. This equivalence holds as long as the microscopic parameters (e.g. swim speed $v$, diffusivity or tumbling rate), that may be spatially varying, depend on the local density $\rho$ of particles but not on their orientation. This equivalence can thus extend to interacting particles and shows that motility-induced phase separation is generic in these systems: a steeply enough decreasing $v(\rho)$ generates phase separation in dimensions d=1,2,3. I will discuss the consequences of this phenomenon for pattern formation in bacterial colony and effective temperatures in Active Matter. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A28.00002: Defect Interactions in Active Nematics Xu Ma, Mark J. Bowick, Luca Giomi, M. Cristina Marchetti Topological defects play prominent roles in passive nematic systems, but defect-antidefect pairs ultimately attract and annihilate in a finite time as the system coarsens and approaches its uniform ground state. The situation changes in active systems, which generate energy at the level of the microscopic constituents. We discuss analytic and numerical studies of two-dimensional active nematics focusing on the ability of activity to generate both defect production and annihilation and to stabilize defect-antidefect pairs at arbitrarily long times. In particular we analyze the dynamics of defect pair annihilation as a function of activity and friction and compare to experimental systems consisting of active bundled microtubule suspensions. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A28.00003: Structure and Dynamics of a Phase-Separating Active Colloidal Fluid Gabriel Redner, Michael Hagan, Aparna Baskaran We examine a minimal model for an active colloidal fluid in the form of self-propelled Brownian spheres that interact purely through excluded volume. Despite the absence of an aligning interaction, this system shows the signature behaviors of an active fluid, including anomalous number fluctuations and phase separation. Using simulations and analytic modeling, we quantify the phase diagram and separation kinetics. We show that this nonequilibrium active system undergoes an analog of an equilibrium continuous phase transition, with a critical point and a binodal beneath which the system separates into dense and dilute phases whose concentrations depend only on activity. The dense phase is a unique material that we call an active solid, which exhibits the structural signatures of a crystalline solid near the crystal-hexatic transition point, and anomalous dynamics including superdiffusive motion on intermediate timescales. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A28.00004: Thermally Active Colloids Jack Cohen, Ramin Golestanian We present a model of thermally active colloids that can propel and act as heat sources through absorption of light. We study the resulting dynamics of a system of many of these interacting colloids. The interplay between light absorption, long range fields and diffusion leads to novel collective dynamics. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A28.00005: Active Colloids, a new building block for smarter materials Invited Speaker: Jeremie Palacci Self-propelled micro-particles are intrinsically out-of-equilibrium. This renders their physics far richer than that of passive colloids while relaxing some thermodynamical constraints and give rise to the emergence of complex phenomena e.g. collective behavior, swarming\textellipsis I will present a new form of self-assembly originating from non-equilibrium driving forces. When activated by light, a set of new self-propelled particles spontaneously assemble into \textit{living crystals} which form, break, explode and reform somewhere else. We will show that this complex dynamics originates in the competition between self-propulsion of the particles attractive interactions induced respectively by osmotic and phoretic effects. The particles can moreover be steered by an external magnetic field. Light activated and steerable self propelled particles new perspectives in the design and the properties of smarter materials. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A28.00006: Asymmetric gears in a bacterial bath: Crossover between equilibrium and active motion Ayhan Duzgun, Jonathan Selinger A fundamental distinction between active matter and equilibrium systems is that active matter is not governed by the conventional laws of thermodynamics. As a specific example, recent experiments have put asymmetric gears into a ``bacterial bath,'' in which bacteria consume food, propel themselves forward, collide into the gears, and induce asymmetric rotation, thus converting chemical energy into mechanical work (Sokolov et al, 2010). By comparison, the same gears would not rotate in a thermal bath, because the second law of thermodynamics prohibits converting equilibrium thermal energy into mechanical work. This experiment leads to the basic question of what makes the difference between self-propelled motion and equilibrium thermal motion. To address this question, we perform simulations of a gear in a bacterial bath, following the approach of Angelani et al (2009); these simulations confirm that bacterial motion leads asymmetric rotation. We then modify the equations of motion, interpolating between bacteria and equilibrium Brownian particles, and determine the motion of the gear. These results help to identify what features of active bacterial motion are necessary to violate the laws of thermodynamics and generate rotation, and how these features can be controlled. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A28.00007: Microfluidic ratchets: from bacterial separation to sperm guidance Carlos Condat, Ivan Berdakin, Veronica Marconi, Alejandro Guidobaldi, Laura Giojalas, Alejandro Silhanek, Yogesh Jeyaram, Victor Moshchalkov, Lyn Venken, Jozef Vanderleyden It has been shown that a suitably built asymmetric microdevice can be used to separate and select self-propelled microorganisms. The efficiency of this rectification effect depends on the detailed dynamics of the individual microorganism. In the case of run-and-tumble bacteria we show that the distribution of run lengths and the partial preservation of run orientation memory through a tumble are important factors when computing the rectification efficiency. In addition, we show that this ratchet effect can be used to separate or concentrate sperm cells. Using a simple phenomenological model we optimize the geometry of the confining habitat in order to accumulate the cells. Both swimming strategy and swimmer size should be taken into account to optimize the design of a micro-patterned architecture for a device that can be used for effective physical bacterial separation or sperm guidance. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A28.00008: Establishing the Turing mechanism using synthetic cells Camille Girabawe, Nathan Tompkins, Ning Li, G. Bard Ermentrout, Irving R. Epstein, Seth Fraden In 1952 Alan Turing published his seminal pape~\textit{The Chemical Basis of Morphogenesi~}in which he described a basis for physical morphogenesis due solely to a reaction-diffusion system. His mechanism has been tested extensively but remains controversial and not fully demonstrated for cellular systems. Now 60 years after its debu, we describe an experimental system that demonstrates all six of his phenomenological predictions with additional support that these observations are due specifically to the Turing mechanism itsel. Further we demonstrate a nonlinear phenomena in the same system that was not predicted by Turing and which is not explained by a linear solution analysis of the governing system equations. Finally we also demonstrate that this system undergoes chemical and physical morphogenesis as Turing suggeste. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A28.00009: Do Ion Channels Spin? Robert Shaw Ionic current flowing through a membrane pore with a helical architecture may impart considerable torque to the pore structure itself. If the channel protein is free to rotate, it will spin at significant speeds. Order of magnitude estimates of possible rotation rates are presented, as well as a few arguments why such motion could improve ion transport. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A28.00010: Interactions Between Flocks and Obstacles Pearson Miller, Nicholas Ouellette The collective behavior of interacting active particles has generated considerable interest in recent years. Many models for such behavior have been proposed, ranging from simple systems of discrete particles with ad hoc interaction rules to continuum models with assumed interaction potentials to complex, bio-inspired models of collective animal motion. But in almost all cases, the resulting emergent behavior is studied in isolated systems far from boundaries. In contrast, we present results from a computational study of a simple discrete flocking model in the presence of obstacles. We consider both the behavior of the system in restricted domains bounded by solid walls and the scattering of developed flocks off of stationary targets, and discuss discuss the relationship of our results to liquid and granular systems. [Preview Abstract] |
Session A29: Three Dimensional Topological Insulators: Chalcogenides
Sponsoring Units: DCMPChair: Cihan Kurter, University of Illinois
Room: 337
Monday, March 18, 2013 8:00AM - 8:12AM |
A29.00001: Superconductivity in a topological insulator Sb$_2$Te$_3$ Lukas Zhao, Haiming Deng, Milan Begliarbekov, Inna Korzhovska, Zhiyi Chen, Jeffrey Secor, Lia Krusin-Elbaum We report an observation of superconductivity in a topological material Sb$_2$Te$_3$ synthesized under modest pressure ($\sim$ 5.5~MPA ) that has the zero-field superconducting transition temperature $T_c = 8.3$~K -- the highest among any topological systems reported thus far. High resolution TEM and XRD Rietveld refinement analysis of the superconducting crystals show that while there is a 0.2\% elongation of the lattice parameter in the \textit{c}-direction, the rhombohedral van der Walls unit cell structure is preserved. The upper critical field $H_{c2}$ anisotropy is surprisingly small, only $\sim 1.5$, much smaller than the crystalline anisotropy of $\sim 8$. This anisotropy appears consistent with the paramagnetically limited critical field, given the reported large value ($\sim 10$) of the \textit{g}-factor. The diamagnetic state of this new superconductor is also unusual, since even in the normal state the system supports large orbital currents. We will discuss our observations in the context of topological superconductivity and Dirac energy-momentum dispersion of the surface states. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A29.00002: Bi$_{1-x}$Sb$_{x}$(110): A non-closed packed surface of a topological insulator Lucas Barreto, Wendell Simoes e Silva, Malthe Stensgaard, S{\O}ren Ulstrup, Marco Bianchi, Xie-Gang Zhu, Matteo Michiardi, Maciej Dendzik, Philip Hofmann Topological insulators are characterised by an insulating bulk band structure, but topological considerations require their surfaces to support gap-less, metallic states. Meanwhile, many examples of such materials have been predicted and found experimentally, but experimental effort has concentrated on the closed-packed (111) surface of these materials. Thus, the theoretical picture of an insulating bulk embedded in a metallic surface from all sides of a crystal still needs to be confirmed. Here we present angle-resolved photoemission spectroscopy results from the (110) surface of the topological insulator Bi$_{1-x}$Sb$_{x}$ ($x \approx 0.15$). The observed band structure and Fermi contour are in excellent agreement with theoretical predictions and slightly different from the electronic structure of the parent surface Bi(110), in particular around the $X_1$ time-reversal invariant momentum. We argue that the preparation of surfaces different from (111) opens the possibility to tailor the detailed electronic structure and properties of the topological surface states. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A29.00003: Mass acquisition of Dirac fermions in the presence of magnetic doping in the topological insulator Sb$_{2}$Te$_{3}$ Yeping Jiang, Zhi Li, Canli Song, Ke He, Lili Wang, Xi Chen, Xucun Ma, Qikun Xue The nontrivial bulk band topology and time reversal symmetry yield gapless surface states in three dimensional topological insulators. The gapless nature of surface states in strong topological insulator is predicted to be violated by time-reversal-symmetry breaking perturbations, which opens back-scattering channels between Kramers pairs and induces a massive gap near the Dirac point of surface states. Such a massive Dirac fermion system gives rise to an unconventional magnetoelectric response relating to many exotic phenomena such as half-quantized anomalous Hall effect, topological quantized magnetoelectric effect and even the magnetic monopole. Here we introduce time-reversal-symmetry breaking by doping Cr atoms into the topmost quintuple layer or into the bulk of Sb$_{2}$Te$_{3}$ thin films. We demonstrate for the first time by Landau level spectroscopy the deviation of zero modes, which indicates the acquirement of a mass term in the presence of surface or bulk magnetic doping. We also show that the magnitude of the mass term in the surface states depends on both the Cr doping level and the magnetic field, offering a new way of measuring the doping- and field-dependence of local magnetization of dopants. Our observation suggests Cr-doped Sb$_{2}$Te$_{3}$ is a promising candidate for realization of proposed novel magnetoelectric effects. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A29.00004: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 8:48AM - 9:00AM |
A29.00005: Possible topological insulating state in bismuth doped with arsenic: magneto-optical study G.M. Foster, S.V. Dordevic, N. Stojilovic, M.V. Nikolic, S.S. Vujatovic, Z.Z. Djuric, P.M. Nikolic, Z. Chen, Z.Q. Li Bismuth and its alloys with antimony have attracted attention in recent years due to possible realization of topological insulating state. In this study we have used infrared and magneto-optical spectroscopies to probe the electrodynamic response of bismuth doped with 1.0 $\%$ of arsenic. The spectra will be presented for temperatures down to 5 K, and in magnetic fields as high as 18 Tesla. The results reveal strong magneto-optical activity, especially around the plasma minimum in reflectance. These findings will be compared and contrasted with magneto-optical results on topological insulator Bi$_{1-x}$Sb$_x$. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A29.00006: Exotic magnetic properties of diluted magnetic binary chalcogenides Maia G. Vergniory, Xabier Zubizarreta, Mikhail M. Otrokov, Igor V. Maznichenko, J\"urgen Henk, Evgueni V. Chulkov, Arthur Ernst Using first-principles Green function approach we studied electronic and magnetic properties of diluted magnetic binary chalcogenides A$_2$B$_3$, doped with transition metals substituing the A element. The electronic structure of the impurities in the chalcogenides is mainly featured by the crystal field splitting. We found that two main mechanisms are responsible for long-range magnetic order in these materials: hole mediated magnetism within the layer of A atoms and indirect interaction between magnetic moments via a B atom. We also estimated Curie temperature of these systems, which was found in good agreement with the available experimental data. Our results shed light on the understanding of magnetic interaction and control in toplogical insulators. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A29.00007: Magneto-transport properties of the ternary topological insulator (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$ in the presence of electrostatic gating and magnetic impurity Liuqi Yu, Jorge Barreda, Longqian Hu, P. Xiong, Tong Guan, Xiaoyue He, K. Wu, Y. Li A three-dimensional topological insulator, (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$, is used to characterize the electronic properties of the spin helical conducting surface state. Epitaxial films are grown via MBE on (111) SrTiO$_{3}$ substrate, which serves as the gate dielectric. Magnetoresistance (MR) and Hall effect measurements have been performed at various back gate voltages. Ambipolar field effect has been observed, enabling effective tuning of the Fermi level across the band gap. Weak antilocalization effect is identified and used to differentiate the surface state. The Hikami-Larkin-Nagaoka (HLN) equation is used to analyze the MR data and the results show the top and bottom surfaces become decoupled when the Fermi level is in the bulk band gap. We also examine the effects of paramagnetic impurity (MI), which introduces time reversal symmetry breaking scattering, on the TI surface states. Taking advantage of the unique capability of \textit{in situ} deposition in a customized dilution refrigerator, paramagnetic Cr atoms were incrementally quench-condensed onto the sample surface and transport measurements were performed at each MI density. The procedure eliminates any sample-to-sample variation and complications from air exposure. Pronounced changes in the weak antilocalization effect and the sample carrier density with increasing MI concentration were observed. Possible origins of these observations will be discussed. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A29.00008: Visualizing Landau levels of Dirac electrons in Bi$_{2}$Te$_{3}$ in a one dimensional potential Daniel Walkup, Yoshinori Okada, Wenwen Zhou, Chetan Dhital, Ying Ran, Ziqiang Wang, Stephen Wilson, Vidya Madhavan When a magnetic field is applied to a solid, the electrons fall into discrete, highly degenerate Landau levels. In each Landau level the wavefunction has a certain characteristic spread, which increases with the energy (index) of the level. This has important physical consequences especially in the presence of spatial inhomogeneity. Using scanning tunneling spectroscopy, we have examined the Dirac electrons on Bi$_{2}$Te$_{3}$ under a magnetic field and subject to a smooth one-dimensional periodic potential. We find that the lowest Landau levels track the potential variation, but the higher levels are more homogeneous. Through a calculation of the Landau level wavefunctions, we form a coherent picture of how their spread interacts with the potential landscape, explaining the experimental data. Our findings have important implications for transport and magneto-resistance measurements in Dirac materials with engineered potential landscapes. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A29.00009: Topological States Ruled by Stacking Faults in Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ Leandro Seixas, Leonardo Abdalla, Tome Schmidt, Adalberto Fazzio, Roberto Miwa Extended defects like stacking faults (SF) can originate topologically protected metallic states in bulk topological insulators (TI). These induced topological states are a response to the weakening of the inter-layer van der Waals interactions due to the SF defect. In TI thin films the degeneracy of Dirac bands of opposite surfaces can be lifted upon the formation of SF defects. Such slab asymmetry can promote a net spin current, absent of backscattering processes, in thin film made of TIs. These results have been obtained by fully relativistic first principles calculations. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A29.00010: Proximity effect in MBE grown bismuth chalcogenide thin films Brian Mulcahy, Mao Zheng, Can Zhang, Allison Dove, Zachary R. Yoscovits, Gustaf Olson, James N. Eckstein Topological insulators (TIs) comprise a new state of matter which provides access to novel physics. Of the set of materials that have exhibited spectroscopic evidence of topologically protected surface states, bismuth chalcogenide systems have garnered particular interest due to their relatively large nominal bulk band gap and single Dirac cone near the Fermi surface. We are studying the superconducting proximity effect in MBE grown thin films of Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and ternary compounds. After \textit{in situ} deposition of a low temperature superconductor, the films are patterned into devices containing a matrix of superconducting islands of tunable size and density on top of the TI layer. We discuss growth optimization, device processing, the role of the superconductor-TI interface, and proximity effect transport results. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A29.00011: Topological phase transition induced by atomic displacements in PbS and PbTe Jinwoong Kim, Seung-Hoon Jhi Discovery of 3D topological insulator initiates exploration of finding new materials having topological insulating phase or mechanisms for topological phase transitions. Introducing interactions or strains into non-interacting electron systems, for example, can produce non-trivial topological phases in them otherwise having trivial band insulating phase at equilibrium conditions. Using first-principles methods, we study emerging topological phases in band insulating PbS and PbTe, which are induced by selective atomic displacements. Phonon modes corresponding to the displacements are identified and conditions of inducing the topological phase transition are suggested. We show that surface states develop flickering Dirac cones at band-inversion k-points upon dynamic atomic displacements with sufficient amplitude. Our results demonstrate that elementary excitation modes like phonon can induce topological phases in trivial band insulators. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A29.00012: Optical selection rules for electron-hole pair excitation in 3D topological insulators Hari Paudel, Michael Leuenberger Experiments using ARPES, which is based on the photoelectric effect, have shown that the surface states in 3D topological insulators (TI) are helical. Here we consider Weyl interface fermions due to band inversion in narrow-bandgap semiconductors, such as $Pb_{1-x} Sn_{x} Te$ and $Bi_{1-x} Sb_{x}$. We determine the optical selection rules of electron-hole pair (EHP) excitation by means of the solutions of the 3D Dirac equation. While EHPs in graphene are generated through intraband transitions, we show that in 3D TI they are generated through both intraband and interband transitions. For their analysis, we calculate explicitly the electric dipole matrix elements by means of bandstructure calculations for $Pb_{1-x} Sn_{x} Te.$ We will introduce a spin helicity operator in 3D TI. Our results are crucial for future opto-spintronic devices based on 3D TI. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A29.00013: Disorder tuned anomalous Hall effect in thin films of Cr doped topological insulators Zhiyi Chen, Lukas Zhao, Inna Korzhovska, Haiming Deng, Simone Raoux, Jean Jordan, Lia Krusin The anomalous Hall effect (AHE) -- an appearance of a voltage transverse to the electric current in the absence of an external magnetic field -- is a process that arises from the spin-orbit coupling between current and magnetic moments that has been fundamentally linked to the topological nature of the Hall current. Recent first-principle calculations predict that when topological insulators (TIs) are doped with transition metal ions, such as Cr or Fe, a novel \emph{magnetically ordered} insulating state will form -- a state that in thin samples may support a \textit{quantized} anomalous Hall conductance. Here we report an observation of AHE in \textit{rf} sputtered thin Cr doped films of Bi$_2$Te$_3$. The anomalous Hall resistivity $\rho_{xy}$ scales with the longitudinal resistivity squared, $\rho_{xx}^2$, and a distinct ferromagnetic hysteretic response (loops) at temperatures below 10 K with coercive fields of the order of 0.5 T is observed. In as-deposited films the resistivity is below the resistivity quantum $h/e^2$. Using 2.5 MeV electron beam irradiation with varying fluence we can tune the resistivity upward by orders of magnitude. A large effect of controlled quenched point disorder on the quantization of AHE in Bi$_2$Te$_3$ will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A29.00014: Imaging single-atom impurities in topological materials Michael Yee, Anjan Soumyanarayanan, Yang He, D. Gardner, Y.S. Lee, Z. Salman, A. Kanigel, Y. Ando, J.E. Hoffman We use low temperature spectroscopic scanning tunneling microscopy to study topological materials in which the surface states are protected by time reversal symmetry. We image the local density of states around a variety of single-atom impurities in the presence of a magnetic field. On a subset of these impurities, we observe broad peaks in the local density of states at energies around the Dirac point. Furthermore, we use Landau level spectroscopy and quasiparticle scattering to discuss the interplay between impurities and the surface states. [Preview Abstract] |
Session A30: Colloids: Diffusion and Transport
Sponsoring Units: DCMPChair: Kazem Edmond, New York University
Room: 338
Monday, March 18, 2013 8:00AM - 8:12AM |
A30.00001: Long range transport of colloids in aqueous solutions Daniel Florea, Sami Musa, Jacques M.R.J. Huyghe, Hans M. Wyss Colloids in aqueous suspensions can experience strong, extremely long range repulsive forces near interfaces such as biological tissues, gels, ion exchange resins or metals. As a result exclusion zones extending over several millimeters can be formed. While this phenomenon has been previously described, a physical understanding of this process is still lacking. This exclusion zone formation is puzzling because the typical forces acting on colloidal particles are limited to much shorter distances and external fields that could drive the particles are absent. Here we study the exclusion zone formation in detail by following the time and distance-dependent forces acting on the particles. We present a simple model that accounts for our experimental data and directly links the exclusion zone formation to an already known physical transport phenomenon. We show that the effect can be tuned by changing the zeta potential of the particles or by varying the species present in the aqueous solution. We thus provide a direct physical explanation for the intriguing exclusion zone formation and we illustrate how this effect can be exploited in a range of industrial applications. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A30.00002: Transport of charged colloidal particles in a nonpolar solvent in response to an electric field Tina Lin, Thomas Kodger, David Weitz In nonpolar solvents, particle charging is often controlled through the addition of suitable surfactants, which form charge-stabilizing reverse micelles. By combining microfluidics and confocal microscopy, we directly visualize the dynamics of charged colloidal particles in a nonpolar solvent with reverse micelles in response to an external electric field; this enables us to probe the internal electric field as well as the charging properties of the particle solution. We discover some surprising particle behavior: despite a constant applied electric field, particle transport through the fluid is nonlinear and the apparent particle mobility decays in time; subsequently, the charged particles appear to diffuse freely within the bulk solution. We characterize this behavior and find that the charged reverse micelles play a significant role. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A30.00003: Revisiting Taylor Dispersion: Differential enhancement of rotational and translational diffusion under oscillatory shear Brian Leahy, Desmond Ong, Xiang Cheng, Itai Cohen The idea of Taylor dispersion - enhancement of translational diffusion under shear - has found applications in fields from pharmacology to chemical engineering. Here, in a combination of experiment and simulations, we study the translational and rotational diffusion of colloidal dimers under triangle-wave oscillatory shear. We find that the rotational diffusion is enhanced, in addition to the enhanced translational diffusion. This ``rotational Taylor dispersion'' depends strongly on the strain rate (Peclet number), aspect ratio, and the shear strain, in contradistinction to translational Taylor dispersion in a shear flow, which depends only weakly on strain rate and aspect ratio. This separate tunability of translations and orientations promises important applications in mixing and self-assembly of solutions of anisometric colloids. We discuss the corresponding effect on the structure and rheology of denser suspensions of rod-like particles. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A30.00004: Hydrodynamic Behavior of Colloidal Nanorods and Characterization of Length Distributions Carlos Silvera Batista, Constantine Khripin, Xiaomin Tu, Ming Zheng, Jeffrey Fagan Single-walled carbon nanotubes (SWCNTs) are 1D, cylindrical, structures of carbon with long persistence lengths and consistent diameters. In this talk, I will discuss the use of doubly sorted SWCNTs (by buoyancy and length), which are effectively colloidal rods, to explore experimentally the effectiveness of theoretical approximations for the hydrodynamic drag of a freely rotating rod.~ The objective of this work is to establish and validate the use of Analytical Ultracentrifugation (AUC) as a technique to measure the length distribution of rodlike colloidal particles including SWCNT dispersions. This is particularly necessary for applications of nanotube dispersions, as the transport, optical, and thermal properties, as well as the toxicity of SWCNTs have all been demonstrated to depend on the length. Contrary to AFM, the technique most commonly used to measure length distributions, AUC is able to measure the whole population of particles as they exist in liquid phase. I will present measurements and analysis of SWCNT samples with narrow distributions in length, diameter and buoyancy as measured through AUC and compare them against independent measurements conducted with AFM. ~Using this data, the validity of hydrodynamic theory for this application is verified. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A30.00005: Clustering of Attractive Colloids in Flow Ming Han, Jonathan K. Whitmer, Erik Luijten The behavior of colloidal suspensions under flow is important for numerous applications, including direct-write techniques employing ``colloidal ink.'' Here we investigate the behavior of colloids flowing through narrow channels. When colloidal particles experience sufficiently strong attractive interactions, cluster formation and ultimately gelation may result. We employ computer simulations to investigate how the size and structure of these clusters, as well as their distribution in the flow, is influenced by various experimental variables, including flow velocity, attraction strength, fluid viscosity, and channel diameter. These simulations incorporate explicit hydrodynamics through the multiparticle collision dynamics (MPC) algorithm. Particular attention is paid to the role of channel boundaries and to the dimensionless parameters characterizing the suspension. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A30.00006: Measurements of anisotropic Brownian motion of colloidal clusters Jerome Fung, Thomas G. Dimiduk, Rebecca W. Perry, Vinothan N. Manoharan Nonspherical colloidal particles can exhibit anisotropic Brownian motion characterized by different translational and rotational diffusion constants about different particle axes. We discuss measurements of anisotropic translational and rotational diffusion constants in triangular colloidal clusters made from three micron-sized colloidal spheres. We use digital holographic microscopy (DHM) and electromagnetic scattering solutions to image the three-dimensional Brownian motion of isolated clusters. We track the cluster centers of mass with $\sim$20 nm precision and the cluster orientations with an angular resolution of $\sim$0.1 radians. We also use DHM to measure the diffusion of colloidal spheres bound to the surface of an emulsion droplet and show that the sphere behavior differs significantly from diffusion on planar surfaces at long time scales. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A30.00007: Determination of the hydrodynamic friction matrix for various anisotropic particles Daniela Kraft, Raphael Wittkowksi, Hartmut L\"owen, David Pine The relationship between the shape of a colloidal particle and its Brownian motion can be captured by the hydrodynamic friction matrix. It fully describes the translational and rotational diffusion along the particle's main axes as well as the coupling between rotational and translational diffusion. We observed a wide variety of anisotropic colloidal particles with confocal microscopy and calculated the hydrodynamic friction matrix from the particle trajectories. We find that symmetries in the particle shape are reflected in the entries of the friction matrix. We compare our experimentally obtained results with numerical simulations and theoretical predictions. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A30.00008: Enhanced Diffusion in Quasi-Two-Dimensional Suspensions Adar Sonn, Haim Diamant, Yael Roichman We study the Brownian motion of quasi-two-dimensional suspensions of micron-sized particles parallel to a single wall. The dynamics of a suspension near a single wall has two characteristics; the self diffusivity is smaller than in unconfined suspensions, and the hydrodynamic interactions between particles decay with inter-particle distance $r$, as $1/r^3$. We track the motion of silica beads that sediment to the sample floor due to their high density. Screened Coulomb interactions between the bottom glass wall and the heavily charged surface of the beads maintain the beads floating a few hundred nanometers above the wall. We follow the change in the self diffusivity and hydrodynamic interactions as a function of particle area fraction in the sedimented monolayer, $\phi$. As expected, the self diffusion decreases as $\phi$ increases; however, at large $\phi$, we observed an increase in self diffusivity. We also observe strongly correlated motion between particles separated by a distance much larger than their distance from the wall. This long-range hydrodynamic coupling has non-trivial dependence on particles' density. Some possible explanations for these observations will be discussed. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A30.00009: Vibrational properties of dense colloidal suspensions with short-range interparticle attraction Martin Iwanicki, Ke Chen, Arjun G. Yodh, Piotr Habdas We investigate vibrational properties of dense colloidal suspensions with short-range attractive particle interactions. Preliminary results show that the so-called boson peak in the attractive glass density of states is weaker than in comparable repulsively-interacting disordered suspensions. Interestingly, the position of the peak shifts to higher frequencies with increasing interparticle attraction strength. The participation ratio, which measures the degree of spatial localization, also shifts to higher frequencies with increasing interparticle attraction. Interestingly, characteristics of quasi-localized modes do not seem to depend on the attraction strength between particles. The observations are consistent with studies in hard-sphere colloidal suspensions where the boson peak frequency decreased with increasing volume fraction, and was understood in the jamming framework. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A30.00010: Dynamics of Repulsing Charged Particles: a Fluorescence Cross-Correlation Spectroscopy Study Jingfa Yang, Ligang Feng, Jiang Zhao, Andeas Best, Hans-Jurgen Butt, Kaloian Koynov Electrostatic interaction controls the stability of charged colloidal particles dispensed in an aqueous solution. In our study, we measured the interaction between charged polystyrene particles by fluorescence cross-correlation spectroscopy (FCCS). Negative correlation function was observed for these repulsing particles and a detailed analysis by Brownian dynamics simulation provided a few important factors of the system: the correlation length at which the interaction dominates and the cage effect in the diffusion of the particles. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A30.00011: Order Preservation Between Brownian Particles Modeled By Langevin Dynamics William Maulbetsch, William Poole, Joseph Bush, Derek Stein We studied the dynamics of two overdamped Brownian particles in an elongational force gradient following their release from some initial separation. Using a modified one-dimensional Langevin equation, we computed the probability that the particles maintain their order as a function of time. The probability approaches unity when the work required to bring the particles together against the force gradient greatly exceeds the thermal energy, $k_BT$. The time window within which the particles are most likely to reverse their order is given by the time to diffuse the initial separation. We apply our theoretical model to the dynamics of DNA monomers approaching the vertex of the Taylor cone in an electrospray ionization mass spectrometer. The likelihood of preserving the sequential order is estimated to be 95\% when the neighboring monomers of a stretched polymer are cleaved within 10 nm of the vertex. The implications of these results to a DNA sequencing strategy will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A30.00012: Eliminating cracking during drying Qiu Jin, Peng Tan, Andrew B. Schofield, Lei Xu When colloidal suspensions dry, stresses build up and cracks often occur - a phenomenon undesirable for important industries such as paint and ceramics. We demonstrate that the two viscoelastic moduli, $G'$ and $G''$, determine the cracking behavior. By adding emulsion droplets into colloidal suspensions, we systematically decrease the storage modulus, $G'$, and increase the importance of the loss modulus, $G''$, and effectively decrease the amount of cracks. At a critical droplet concentration, cracking disappears completely. Furthermore,adding droplets also varies the speed of air invasion and provides a powerful method to adjust drying rate. With the effective control over cracking and drying rate, our experiment may find important applications in many drying and cracking related industrial processes. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A30.00013: Aging in Colloidal Glasses: a comparison between micro and macrorheology Xiaojun Di, Xiaoguang Peng, Gregory McKenna The analogy between colloidal dynamics and the dynamics of molecular glasses remains an important area of study. Of particular interest to our team is the aging responses of the two systems. We have been investigating the dynamics of colloidal systems composed of thermosensitive particles that change diameter upon change of temperature and comparing the behavior to what is expected in molecular glass-formers. In particular, we have found that concentration jumps in these systems mimic three important behaviors of molecular glasses: the intrinsic isotherm, the asymmetry of approach, and memory effect. In our early work, we were able to show, using multispeckle diffusing wave spectroscopy, that although the three signatures are observed in the concentration jump conditions, they are not identical to the observations in molecular glasses. In the present work, in order to get better resolution for the temperature dependent properties, we are employing PNIPAAM/PS particles with core-shell structure to lessen the temperature sensitivity of the system. A series of different particles with different PNIPAAM fractions (different thermal sensitivity) is being investigated and a comparison of the aging between the microrheology and the macrorheology will be made. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A30.00014: When Colloids Can Deform Jie Zhang, Changqian Yu, Sung Chul Bae, Steve Granick Most colloidal systems that have been explored so far are hard-spheres, which limits their phase behavior and other physical properties to be not so rich as atomic and molecular systems. Here we present a new class of soft and deformable microgel colloidal particles with thermo-sensitivity and ability to display autonomous oscillation when driven by special fuels. The deformability, size changes and structure formation of micron-sized poly(NiPAM) particles and dumbbells of polystyrene-poly(NiPAM) interpenetrating networks can be imaged in situ and analyzed. Other mechanical and other physical properties attributable to deformability can be measured. [Preview Abstract] |
Session A31: Polymer Membranes for Clean Energy and Water I
Sponsoring Units: DPOLY GERAChair: Xinran Zhang, Georgetown University
Room: 339
Monday, March 18, 2013 8:00AM - 8:12AM |
A31.00001: Conductivity Scaling Relationships for Nanostructured Block Copolymer/Ionic Liquid Membranes Megan Hoarfrost, Rachel Segalman Nanostructured membranes containing structural and ion-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled with high thermal stability. To optimize the properties of such membranes, it is essential to understand scaling relationships between composition, structure, temperature, and ionic conductivity. The conductivity behaviors of mixtures of two block copolymer chemistries with two different ionic liquids have been investigated. The conductivities of all the mixtures are described by a single expression, which combines the Vogel-Tamman-Fulcher (VTF) equation with percolation theory. The VTF equation takes into account the effect of the glass transition temperature of the conducting phase on the temperature dependence of conductivity, while percolation theory reflects the power law dependence of conductivity on the overall volume fraction of ionic liquid in the membrane. The dominance of the overall volume fraction of ionic liquid in determining conductivity indicates that there is incredible flexibility in designing highly conductive block copolymer/ionic liquid membranes. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A31.00002: Ionic Block Copolymers for Anion Exchange Membranes Tsung-Han Tsai, Dan Herbst, Guinevere A. Giffin, Vito Di Noto, Tom Witten, E. Bryan Coughlin Anion exchange membrane (AEM) fuel cells have regained interest because it allows the use of non-noble metal catalysts. Until now, most of the studies on AEM were based on random polyelectrolytes. In this work, Poly(vinylbenzyltrimethylammonium bromide)-b- (methylbutylene) ([PVBTMA][Br]-b-PMB) was studied by SAXS, TEM and dielectric spectroscopy to understand the fundamental structure-conductivity relationship of ion transport mechanisms within well-ordered block copolymers. The ionic conductivity and the formation of order structure were dependent on the casting solvent. Higher ion exchange capacity (IEC) of the membranes showed higher conductivity at as IEC values below 1.8mmol/g, as above this, the ionic conductivity decreases due to more water uptake leading to dilution of charge density. The humidity dependence of morphology exhibited the shifting of d-spacing to higher value and the alteration in higher characteristic peak of SAXS plot as the humidity increase from the dry to wet state. This phenomenon can be further explained by a newly developed polymer brush theory. Three ionic conduction pathways with different conduction mechanism within the membranes can be confirmed by broadband electric spectroscopy. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A31.00003: Anion Exchange Membranes Based on Reactive Block Copolymers Rick Beyer, Samuel Price, Aaron Jackson, Xiaoming Ren, Deryn Chu, Yuesheng Ye, Yossef Elabd The unmet needs for polymeric AEMs include high hydroxide conductivity, chemical stability under strongly basic conditions, and sufficient mechanical properties to withstand the temperature and humidity fluctuations in a fuel cell. This presentation will include our most recent findings from an effort to develop cation-containing polymers based on phosphonium and ammonium derivatives of styrene using co-polymerization of reactive, ion-containing block copolymers with a small molecule ``matrix'' monomer. By creating polymer membranes with co-continuous cation-containing domains in a cross-linked matrix, we hope to demonstrate high conductivity simultaneously with the robust mechanical properties required in the fuel cell environment. Morphological data from SAXS and TEM, mechanical property measurements, in- and through-plane charge transport measurements, and the results of fuel cell testing will be presented. It was found that the surface transport characteristics of these materials differ from the through-plane properties, that chemical crosslinks may not produce membranes with the required toughness, and that a polymerization technique that is highly sensitive to reaction kinetics is not ideal for the production of AEMs. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A31.00004: Morphology and Proton Transport in Sulfonated Block Copolymer and Mesoporous Polymer Electrolyte Membranes Chelsea Chen, David Wong, Keith Beers, Nitash Balsara In an effort to understand the fundamentals of proton transport in polymer electrolyte membranes (PEMs), we have developed a series of poly(styrene-b--ethylene-b--styrene) (SES) membranes. The SES membranes were subsequently sulfonated to yield proton conducting S-SES membranes. We examine the effects of sulfonation level, temperature and thermal history on the morphology of S-SES membranes in both dry and hydrated states. The effects of these parameters on water uptake and proton transport characteristics of the membranes are also examined. Furthermore, building upon the strategy we deployed in sulfonating the SES membranes, we fabricated mesoporous S-SES membranes, with pores lined up with the proton conducting channels. These membranes have three distinct phases: structural block, proton-conducting block, and void. We examine the effects of pore size, domain structure and sulfonation level on water uptake and proton conductivity of the mesoporous PEMs at different temperatures. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A31.00005: Characterization of Hybrid Polyhedral Oligomeric Silsesquioxane (POSS)-Polybenzimidazole (PBI)-Phosphoric Acid (PA) Materials Intended for Proton Exchange Membranes (PEM) Robert Bubeck, Edmund Stark, Berryinne Decker, Claire Hartmann-Thompson Isophthalic acid and 3,3'-diaminobenzidine (DAB) were polymerized in the presence of polyphosphoric acid (PPA) and various additives, degree of polymerization was monitored by viscosity and torque change measurements, and membranes were prepared by casting the reaction solution and allowing PPA to hydrolyze to PA under ambient conditions. As a function of relative humidity, the membranes were characterized for (1) acid content, (2) in-plane conductivity and (3) complex shear modulus G* obtained via oscillatory parallel plate dynamic mechanical spectroscopy. The addition of sulfonated octaphenyl polyhedral oligomeric silsesquixane (S-POSS) to $m$-polybenzimidazole (PBI)-phosphoric acid (PA) membranes resulted in increased in-plane proton conductivity at high temperatures (120-150 $^{\circ}$C) and increased G* relative to a $m$-PBI control membrane and to $m$-PBI control membranes carrying comparable weight loadings of non-proton conducting octaphenyl-POSS nanoadditive or silica. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A31.00006: Molecular Dynamics Simulation of Polysulfone-Based Anion Exchange Membrane Fuel Cell Seung Soon Jang, Kyung Won Han, Ji Il Choi In this study, we investigate the nanophase-segregated structures and transport properties of quaternary ammonium grafted polysulfone membranes using molecular dynamics simulation method. For this, we develop a new force field from a reference density functional theory modeling with B3LYP and 6-31G** in order to describe the hydroxide anion. The bond stretching force constant is determined to reproduce the quantum mechanical vibrational frequency. The atomic charges are determined by Mulliken population analysis. Through the annealing procedure, the nanophase-segregated structure is developed as a function of water contents such as 10 and 20 wt {\%}. The extent of nanophase-segregation is evaluated by the structure factor analysis, which can be compared with the experimental small angle scattering data. Once the equilibrium structures are obtained, we run long MD simulations to analyze the diffusion of water and hydroxide using the mean-square displacement analysis with an assumption of Gaussian diffusion. The nanophase-segregated structures and the transport properties will be compared to the proton exchange membrane consisting of the same polymer backbone except for the acidic functional group. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A31.00007: Predicting inhomogeneous water absorption in an ionic diblock polymer membrane Daniel Herbst, Thomas Witten Fuel cells convert fuel directly into electrical power. Their performance depends on a permeable (yet strong) membrane to allow ion conduction (while preventing combustion). Anion-exchange membrane fuel-cells are especially economical to produce, but technological hurdles currently limit durability and $\mathrm{OH}^{-}$ conductivity of the membrane. One solution to these problems is a diblock morphology. Layers of stiff hydrophobic polymer provide structure, while interspersed layers of polyelectrolyte provide avenues for conduction. Previously, little was known about the structure within the conducting layer. We adapted Scheutjens-Fleer polymer-brush theory to a lamellar geometry. The calculation tells where the polyelectrolytes congregate within a lamella, and hence how conduction occurs. This talk focuses on a new diblock material, PMB-PVBTMA. We show how the features of the material determine the intra-lamellar structure. We conclude that at low humidity, the bulkiness of PVBTMA causes it to adopt a near-uniform distribution within the conducting block. At high humidity, however, a phase separation may induce abrupt water channels. Understanding the architecture within the conducting layer will help guide research into better anion-exchange membranes materials. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A31.00008: Swelling of ultrathin crosslinked polyamide water purification membranes Edwin Chan, Christopher Stafford Polyamide (PA) ultrathin films represent the state-of-the-art nanofiltration and reverse osmosis membranes used in water desalination. The performance of these materials, such as permselectivity, is intimately linked with extent of swelling of the PA network. Thus, quantifying their swelling behavior would be a useful and simple route to understanding the specific network structural parameters that control membrane performance. In this work, we measure the swelling behavior of PA ultrathin films using X-ray reflectivity as a function of water hydration. By applying the Flory-Rehner theory used to describe the swelling behavior of polymer networks, we quantify the PA network properties including Flory interaction parameter and the monomer units between crosslinks. Finally, we demonstrate application of this measurement approach for characterizing the network properties of different types of PA ultrathin films relevant to water purification and discuss the relationship between network and transport properties. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A31.00009: Molecular Dynamics Simulations of a Single Chain Pentablock Ionomer in Dilute Solutions Dipak Aryal, Dvora Perahia, Gary S. Grest Co-polymers are in the core of many applications such as fuel cells, batteries and purification membranes that require transport across membranes. The challenge remains however that under the condition that transport is optimized, the stability of the membranes is compromised. To surmount this challenge, co-polymers with blocks targeting specific roles have been designed. Using molecular dynamics simulations we have studies the structure and dynamics of ionic single chain pentablock copolymer \textit{(A-B-C-B-A)} containing randomly sulfonated polystyrene in the center, tethered to poly-ethylene-r-propylene end-capped by poly$-t-$butyl styrene. The ionic block facilitates transport while the A and B componenet are incorporated for mechanical stability. The conformation and dynamics of single pentablock ionomer of molecular weight M$_{\mathrm{w}} \quad =$ 50,000g/mol in an implicit poor solvent with dielectric constant of 1 and 77$.$7, water, and mixture (1:1) of cyclohexane and n-heptane at 300K and 500K will be presented. The effect of solvents on conformation of a single molecule of pentablock was determined and compared with experiment, providing a stepping stone to the understanding phase behavior of this polymer. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A31.00010: Gas Permeation through Polystyrene-Poly(ethylene oxide) Block Copolymers Daniel Hallinan Jr., Matteo Minelli, Marco Giacinti-Baschetti, Nitash Balsara Lithium air batteries are a potential technology for affordable energy storage. They consist of a lithium metal anode and a porous air cathode separated by a solid polymer electrolyte membrane, such as PEO/LiTFSI (PEO $=$ poly(ethylene oxide), LiTFSI $=$ lithium bis-trifluoromethane sulfonimide). For extended operation of such a battery, the polymer electrolyte must conduct lithium ions while blocking electrons and gases present in air. In order to maintain a pressure difference the membrane must be mechanically robust, which can be achieved by incorporating the PEO into a block copolymer with a glassy block such as PS (PS $=$ polystyrene). To protect the lithium electrode, the membrane must have low permeability to gases in air such as CO$_{\mathrm{2}}$, N$_{\mathrm{2}}$, and O$_{\mathrm{2}}$. We have therefore studied the permeation of pure gases through a PS-PEO block copolymer. A high molecular weight, symmetric block copolymer with a lamellar morphology was used to cast free-standing membranes. Gas permeability was measured through these membranes with a standard, pressure-based technique. A model was developed to account for transport through the polymer membrane consisting of semi-crystalline PEO lamellae and amorphous PS lamellae. PEO crystallinity was extracted from the permeation model and compares well with values from differential scanning calorimetry measurements. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A31.00011: Nanoporous thin films from nanophase-separated hybrids of block copolymer/metal salt Yoshio Sageshima, Atsushi Noro, Yushu Matsushita Block copolymers self-assemble into periodic nanostructures, i.e. nanophase-separated structures, which can be scaffolds for nano-applications such as nanoporous membranes, nanolithographic masks, photonic crystals, etc. In this study, we report facile preparation to achieve nanoporous thin films from nanophase-separated hybrids comprising polystyrene-$b$-poly(4-vinylpyridine) (PS-P4VP, $M_{\mathrm{n}}=$54k, PDI$=$1.13, $f_{\mathrm{s}}=$0.61) and water-soluble iron(III) chloride (FeCl$_{\mathrm{3}})$, where FeCl$_{\mathrm{3}}$ are incorporated into a P4VP phase via metal-to-ligand coordination. To obtain a nanoporous film, firstly a hybrid thin film was prepared by microtoming. Then, the film was immersed into water to remove metal salts, this simple procedure can produce nanoporous thin film. Morphological observations were conducted by using transmission electron microscopy (TEM). Ordered cylindrical nanopores were observed in the thin film of the water-immersed hybrid, which originally presents cylindrical nanodomains. The nanoporous film was modified by loading another metal salt, samarium(III) nitrate, into nanopores via coordination between the metal salt and P4VP tethered to the pore walls. The structure of the sample after modification was evaluated by TEM and an energy dispersive X-ray spectroscopy. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A31.00012: Highly-Ordered Thin Films from Photocleavable Block Copolymers Weiyin Gu, Hui Zhao, E. Bryan Coughlin, Patrick Theato, Thomas Russell A robust route for the preparation of nanoscopic dot/line patterns with long range lateral order from poly(styrene-block-ethylene oxide) (PS-b-PEO) with an o-nitrobenzyl ester junction (PS-h$\nu $-PEO) is demonstrated. Solvent annealing condition is optimized to achieve the highly ordered cylindrical block copolymer (BCP) microdomains oriented normal or parallel to the silicon substrates. Following a very mild UV exposure and successive washing with methanol, PS-hv-PEO thin films were transformed into highly ordered porous or trench templates. Afterwards the pores or trenches were either filled with PDMS by spin-coating or exposed to direct metal deposition of Au. After a plasma etching or lift-off process to remove the polymer templates, highly ordered arrays of silica or Au nanopatterns were obtained. This represents the first template application example from highly ordered nanoporous thin films derived from block copolymers featuring a photocleavable junction. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A31.00013: Understanding the Internal Structure of Layered Organic Compounds deposited on mineral surface using Neutron Reflectivity Haile Ambaye, Sindhu Jagadamma, Loukas Petridis, Melanie Mayes, Valeria Lauter Organic carbon (OC) stabilization in soils plays a significant role in the global C cycle, therefore the understanding of the structure and function of the OC-soil mineral interface is of high importance. To study the internal structure, films with different combination of simple OC compounds, natural organic matter (NOM), Bi-layers of SA (Stearic Acid) on Glucose and NOM/Hydrophilic-NOM/Hydrophobic-NOM were deposited onto sapphire using spin coating. The phobic and phylic fractions of the NOM are operationally separated by exchange resins. We obtained detailed structural depth profile of the films using the depth-sensitive technique of the neutron reflectometry. The neutron reflectivity data were collected at the MAGICS Reflectometer at Spallation Neutron Source at the ORNL. Self-assembled ordering of SA in a repeating bi-layer structure was observed when it was deposited on NOM, phylic-NOM and Glucose. However, when SA was added to phobic-NOM no ordering of SA was detected. The formation of distinct, immiscible layers is due to insolubility of SA with NOM/Hydrophilic-NOM and Glucose. Our results reveal that the OC-mineral interface form complex layering and that the sequence of the layering depends on the compounds. [Preview Abstract] |
Session A32: Focus Session: Crystallization and Directed Assembly of Multicomponent Systems
Sponsoring Units: DPOLYChair: Christopher Li, Drexel University
Room: 340
Monday, March 18, 2013 8:00AM - 8:36AM |
A32.00001: Kinetically Trapped Morphologies in Organic Photovoltaics Invited Speaker: Thomas Russell Controlling the morphology in the active layer of organic photovoltaic (OPV) devise is key in optimizing the performance. To this end, bicontinuous morphologies with characteristic length scales of several tens of nanometers of the electron and hole conducting materials, where the order and orientation of both components are optimized to absorb light over the broadest possible range of the visible spectrum and to transport holes and electrons, after exciton dissociation, Yet, these morphologies are trapped in morphologies that are far removed from equilibrium where multiple kinetic processes, including ordering, phase separation, and the segregation of components to interfaces are arrested as solvent, co-solvents and additives are removed during the preparation of the active layer. Time resolved hard x-ray scattering, resonance soft x-ray scattering and high resolution transmission electron microscopy, along with mobility and transport measurements, have been used to understand the parameters that lead to the development of and can be used to control the morphology of the active layers. In addition, by using ternary mixtures of two polymers active in different parts of the solar spectrum along with an electron transporting material, like PCBM, morphologies can be developed to further enhance the efficiency of these devices. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A32.00002: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 8:48AM - 9:00AM |
A32.00003: Polythiophene-CdSe Nanorod Assembly Using Electric Field Sirinya Chantarak, Todd Emrick, Thomas P. Russell We report controlled solvent evaporation and electric-field assisted vertical alignment of CdSe nanorods (NRs) in a poly(3-hexylthiophene) (P3HT) matrix over large micron areas. NRs of well-defined sizes were synthesized to optimize the geometries of devices made from these nanorods. Regioregular P3HT chains and oligothiophene were functionalized with ligating end-groups to provide contact to the NRs. Hexagonal arrays of these nanocomposites were characterized by transmission electron microscopy (TEM). [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A32.00004: SANS and SAXS Studies of DNA-Templated Silver Nanoclusters Hongyu Guo, Sunil K. Sinha, Jaswinder Sharma, Jennifer S. Martinez, Andrew P. Shreve Due to the high affinity of silver cations (Ag+) for DNA bases, following reduction of the Ag+, silver atoms may form short oligonucleotide-encapsulated Ag nanoclusters ($<$1 nm) without the formation of large particles. Such DNA-templated silver nanoclusters have received significant attention as potential fluorescent labels due to their useful properties, including high molar absorptivities, good quantum yields and photostability, and small size. It is thus of great interest to find out the configuration of the Ag nanoclusters which associate with the DNA strands. We have conducted Small Angle Neutron Scattering (SANS) and X-ray Scattering (SAXS) experiments to investigate the formation of the Nanoclusters. By comparing SANS and SAXS data from conjugated samples, pure DNA and DNA/Ag complex, we can characterize the size and position of the Ag clusters along the DNA strand. The time evolution of the DNA/Ag complex can also be studied and can be understood as due to silver oxidation, reduction, or regrouping. We find that the formation and aging of the Ag Nanoclusters are also strongly dependent on the DNA template sequence. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A32.00005: Polymer Crystallization at Curved Liquid-Liquid Interface Christopher Li, Wenda Wang, Hao Qi, Ziyin Huang Curved space is incommensurate with typical ordered structures with three-dimensional (3D) translational symmetry. However, upon assembly, soft matter, including colloids, amphiphiles, and block copolymers (BCPs), often forms structures depicting curved surface/interface. Examples include liposomes, colloidosomes, spherical micelles, worm-like micelles, and vesicles (also known as polymersomes). For crystalline BCPs, crystallization oftentimes overwrites curved geometries since the latter is incommensurate with crystalline order. On the other hand, twisted and curved crystals are often observed in crystalline polymers. Various mechanisms have been proposed for these non-flat crystalline morphologies. In this presentation, we will demonstrate that curved liquid/liquid (L/L) interface can guide polymer single crystal growth. The crystal morphology is strongly dependent on the nucleation mechanism. A myriad of controlled curved single crystals can be readily obtained. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A32.00006: Dynamic Temperature Gradient Effects on Directed Self Assembly of Thin Films of Block Copolymer/Au Nanoparticle Multicomponent Systems Ren Zhang, Gurpreet Singh, Alei Dang, Michael Bockstaller, Alamgir Karim The influence of temperature and Au nanoparticle (NP) concentration on the morphology and properties of poly(styrene-b-methylmethacrylate) (PS-PMMA) block copolymer (BCP) thin films (thickness 80100nm) were investigated. The Au core was grafted with thiol-terminated polystyrene to ensure the preferential interaction to the PS domains. The concentration of Au NPs was varied between 0-10{\%} with respect to PS-PMMA by weight. To induce microphase separation, both static oven annealing and a dynamic thermal field termed cold zone annealing (CZA) were performed. At low temperature annealing (\textless\ 150$^{\circ}$C), horizontal cylindrical morphologies were observed, while at high temperature annealing (150-210$^{\circ}$C), an orientation transition of cylindrical microdomains from vertical to horizontal were observed with increasing Au NPs concentration coupled with an increase in reflective index. The morphology transition is attributed to the decreased thermal conductivity caused by the increasing heterogeneity and growing number of scattering centers. Additionally, we demonstrate unidirectional alignment of BCP/Au NP domains by a novel modification of the CZA method. The dispersion of Au NPs was investigated via TEM and AFM. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A32.00007: A Study on the Packing and Phase Separation of Dissimilar Nanoparticles Xiaobo Shen, Dong Wang, Dhandapani Venkataraman, Tadafumi Adschiri, Ken Nakajima, Thomas Russell To develop a novel approach for the preparation of organic photovoltaic active layer using binary functional nanoparticle assemblies, the nature and characteristics of the interactions and packing between dissimilar nanoparticles must be understood. Here, polymer-based, namely polystyrene (PS), and inorganic-based, namely zinc oxide (ZnO) and titanium oxide (TiO2), nanoparticles are prepared by miniemulsion and hydrothermal reaction methods, respectively. Different functionalities on the particle surface are imparted by further functionalization. The binary assembly of the dissimilar particles is carried out in a variety of ways including solution mixing, non-solvent precipitation, thermal- and solvent annealing, etc. and characterized by Force Volume-AFM (FV-AFM), SEM, TEM and GISAXS techniques. The resulting packing and segregation of the dissimilar particles are shown to be effectively dependent on the molecular weight, inter-particle interactions, particle aspect ratios and sizes, etc. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A32.00008: Self-Assembly of Giant Molecular Shape Amphiphiles based on Polystyrene Tethered Hydrophilic POSS/C$_{60}$ Nanoparticles Xinfei Yu, I-Fan Hsieh, Kan Yue, Wen-Bin Zhang, Stephen Cheng Giant molecular shape amphiphiles (GMSAs) are molecules with two blocks which have different chemical properties and shapes. These molecules are precisely synthesized by controlled/living polymerization and ``click'' chemistry. Self-assembly behaviors of GMSAs are explored in solution, bulk, and thin film states. Micelles (spheres, cylinders, and vesicles) are obtained in the solutions, which are controlled by molecular topology, polymer length, and solvent properties. Nanophase separated structures at 10 nm scale are obtained in the bulk state, which are dependent on volume fractions of each block as well as molecular topology. The nanophase separated structures of GMSAs in the bulk state imply their potential applications in thin-film nano-patterning. Compared with traditional block copolymers, the shape-persistent feature of the molecular nanoparticles might help to reduce the line-edge roughness. Shape and interactions are conclude as two important factors to determine the self-assembly of these molecules. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A32.00009: Self-assembly of ABA amphiphilic block copolymers and its metastable behavior Wei Jiang Amphiphilic triblock copolymer can self-assemble into a vast variety of micelles in selective solvents. We investigated, both theoretically and experimentally, the kinetics of the vesicle formation of ABA amphiphilic triblock copolymers in a selective solvent by cooling the system from an initially homogeneous state at different rates. It was found that the pathway of spontaneous vesicle formation depended on the cooling rate. This road path difference for vesicle formation can be attributed to the existence of many metastable states in the system. Moreover, it was found that in uniform shear flow, the size distribution of the vesicles was much narrower than that in nonuniform shear flow and the uniformity of the vesicles increased with increasing shear rate. The results show that the metastable states in the system can be modulated and the morphological polydispersity of amphiphilic ABA triblock copolymer vesicles can be controlled by shear flow. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A32.00010: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 10:24AM - 10:36AM |
A32.00011: Phase Separation in a Dynamically Asymmetric Polymer Blend: a Stepwise Growth Mechanism Charles Han, Weichao Shi Phase separation dynamics of a polymer blend can be mediated under competition between thermodynamic perturbation and asymmetric viscoelasticity due the contrast in the glass transition temperatures of the two polymer components. The viscous fluidic and soft elastic properties will meet in the phase separation dynamics in this study. Between the two cases, we further revealed a stepwise concentration growth phenomenon, which consists of two individual growths and a ``frozen'' period in between. This stepwise growth should be a general mechanism for asymmetric polymer blends. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A32.00012: Rigid Amorphous Fraction in PLA Electrospun Fibers Peggy Cebe, Qian Ma, Erika Simona Cozza, Marek Pyda, Bin Mao, Yazhe Zhu, Orietta Monticelli Electrospun fibers of poly(lactic acid) (PLA) were formed by adopting a high-speed rotating wheel as the counter-electrode. The molecular orientation, crystallization mechanism, and phase structure and transitions of the aligned ES fibers were investigated. Using thermal analysis and wide angle X-ray scattering (WAXS), we evaluated the confinement that exists in as-spun amorphous, and heat-treated semicrystalline, fibers. Differential scanning calorimetry confirmed the existence of a constrained amorphous phase in as-spun aligned fibers, without the presence of crystals or fillers to serve as fixed physical constraints. Using WAXS, for the first time the mesophase fraction, consisting of oriented amorphous PLA chains, was quantitatively characterized in nanofibers. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A32.00013: Diameter-Dependent Modulus and Melting Behavior in Electrospun Semicrystalline Polymer Fibers Ying Liu, Shuang Chen, Eyal Zussman, Chad Korach, Wei Zhao, Yichen Guo, Miriam Rafailovich Confinement of the semicrystalline polymers, poly-(ethylene-co-vinyl acetate) (PEVA) and low-density polyethylene (LDPE), produced by electrospinning has been observed to produce fibers with large protrusions, which have not been previously observed in fibers of comparable diameters produced by other methods. SAXS spectra confirmed the crystalline structure and determined that the lamellar spacing was almost unchanged from the bulk. Measurement of the mechanical properties of these fibers, by both shear modulation force microscopy (SMFM) and atomic force acoustic microscopy (AFAM), indicates that the modulii of these fibers increases with decreasing diameter, with the onset at $\sim $10 $\mu $m, which is an order of magnitude larger than previously reported. Melting point measurements indicate a decrease of more than 7{\%} in Tm/T$_{\mathrm{0}}$ (where Tm is the melting point of semicrystalline polymer fibers and T$_{\mathrm{0}}$ is the melting point of the bulk polymer) for fibers ranging from 4 to 10 $\mu $m in diameter. The functional form of the decrease followed a universal curve for PEVA, when scaled with T$_{\mathrm{0}}$. [Preview Abstract] |
Session A33: Focus Session: Dielectric and Ferroelectric Polymers for Electrical Applications: Dielectrics
Sponsoring Units: DPOLY DMPChair: Lei Zhu, Case Western Reserve University
Room: 341
Monday, March 18, 2013 8:00AM - 8:36AM |
A33.00001: Imaging the Effect of Electrical Breakdown in Multilayer Polymer Capacitor Films Invited Speaker: Mason Wolak Multilayer polymer films show great promise as the dielectric material in high energy density capacitors. Such films show enhancement in both dielectric strength ($E_{\mathrm{B}})$ and energy density ($U_{\mathrm{d}})$ relative to monolithic films of either source polymer. Composites are typically comprised of alternating layers of a high $E_{\mathrm{B}}$ polymer and a high permittivity polymer. Here, we discuss a multilayer system based on polycarbonate (PC) interleaved with polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP). The dielectric properties of the PC/PVDF-HFP films are influenced by both composition and individual layer thickness. Optimized films show $E_{\mathrm{B}}=$750 kV/mm and $U_{\mathrm{d}}=$13 J/cm$^{3}$. Further enhancements in $E_{\mathrm{B}}$ and $U_{\mathrm{d}}$ are expected through optimization of the component polymers, composition, and layer structure. To guide next generation design, it is important to understand the breakdown mechanism, as it directly influences $E_{\mathrm{B}}$. To elucidate the role of the layer structure during electrical breakdown, we use a tandem focused ion beam (FIB) / scanning electron microscope (SEM) imaging technique. The technique allows us to image the internal layer structure of both `as fabricated' control films, and those subjected to high electric fields. It is therefore a powerful tool to assess film quality and analyze failure mechanisms. Specifically, the FIB is used to mill site-specific holes in a film and the resulting cross-sections are imaged via SEM. Individual layers are easily resolved down to 50 nm. For films subjected to electrical breakdown, the location and propagation of damage is tracked with sequential FIB milling and SEM imaging. Spatially resolved FIB/SEM imaging allows preparation of quasi-3D maps displaying the evolution of internal voids in areas adjacent to the breakdown location (pinhole of d $=$ 30-80 microns). A majority of the voids are localized at the interfaces between layers and may propagate as far as 30-50 microns from the pinhole. The data suggest that the enhancement in dielectric strength arises from a barrier effect, whereby the propagation of an electrical breakdown in the direction of the applied field is impeded by the layer interfaces. We will also discuss recent TEM imaging results that are used to characterize the interfacial length scale and chemical makeup, factors that may influence breakdown. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A33.00002: Accelerating Dielectrics Design Using Thinking Machines Invited Speaker: R. Ramprasad High energy density capacitors are required for several pulsed power and energy storage applications, including food preservation, nuclear test simulations, electric propulsion of ships and hybrid electric vehicles. The maximum electrostatic energy that can be stored in a capacitor dielectric is proportional to its dielectric constant and the square of its breakdown field. The current standard material for capacitive energy storage is polypropylene which has a large breakdown field but low dielectric constant. We are involved in a search for new classes of polymers superior to polypropylene using first principles computations combined with statistical and machine learning methods. Essential to this search are schemes to efficiently compute the dielectric constant of polymers and the intrinsic dielectric breakdown field, as well as methods to determine the stable structures of new classes of polymers and strategies to efficiently navigate through the polymer chemical space offered by the periodic table. These methodologies have been combined with statistical learning paradigms in order to make property predictions rapidly, and promising classes of polymeric systems for energy storage applications have been identified. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A33.00003: Dielectric Properties of Poly(carbonate) Containing Oxide Nanoparticles Steve Greenbaum, John Fontanella, John Bendler, Charles Edmondson, Mary Wintersgill, David Boyles, Tsvetanka Filipova, Mark Westgate, Armando Rua, Xavier Bogle Nanocomposite of poly(carbonate) (PC) and nanoscopic BaTiO$_{3}$ have been studied. The complex relative permittivity, $\varepsilon $*$= \varepsilon $'-j$\varepsilon $'', at audio frequencies from 5K to about 500K and the room temperature breakdown strength have been determined. In addition, SEM, DSC and TGA studies have been carried out as well as variable temperature and pressure proton NMR relaxation measurements. $\varepsilon '$ is 11 for PC containing 59 wt{\%} of 50-70 nm diameter BaTiO$_{3}$ and $\varepsilon '$ vs. nanoparticle content for the untreated nanoparticles is larger than would be expected on the basis of a recently proposed modified Hanai equation. In addition, the breakdown strength is low and decreases as nanoparticle content increases. Higher breakdown strength is observed when using surface treated nanoparticles. The gamma relaxation (200K and 1000 Hz) does not change as nanoparticle content increases to 59 wt-{\%}. Also, a low temperature relaxation region (in the vicinity of 20K) is found in the heat-treated nanocomposites, which is associated with the nanoparticles themselves. Next, the breakdown strength increases as BaTiO$_{3}$ nanoparticle size increases from 50 nm to 500 nm. Finally, data for PC containing SrTiO$_{3}$, BaZrO$_{3}$, ZrO$_{2}$, TiO$_{2}$ or SiO$_{2}$ may be presented. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A33.00004: Theoretical considerations in the design of polymer dielectrics Philip Taylor, Gavin Brown, Jiayuan Miao, Elshad Allahyarov An ideal dielectric is one that reversibly stores a large amount of energy when exposed to a modest electric field. We have used theory and molecular dynamics simulation as an aid to the development of polymeric materials with favorable properties for energy storage with low dielectric losses. Because the stored energy in a capacitor resides mostly in the energy of distortion of the molecular bonds within the material, it is necessary to optimize the size of the deformable polar units. We achieve this by modeling some of the copolymers of polyvinylidene fluoride, and identifying the preferred density and nature of the cross-linking that pins certain regions of the polymer chains to prevent their rotation when exposed to fields. We then relate this to the electrostatic interactions within chains and between chains in order to take account of the depolarizing fields. We find the optimal length of chain between pinning points to be a function of the applied field strength, and to vary from about ten monomer units at the highest of fields to over a hundred monomers at very weak fields. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A33.00005: Properties of Poly(carbonate) Containing Oxide Nanoparticles Joseph Lomax, John Bendler, John Fontanella, Charles Edmondson, Mary Wintersgill, Mark Westgate Nanocomposites composed of poly(carbonate) (PC) and oxide nanoparticles have been studied. For BaTiO$_{\mathrm{3}}$ both as-received and surface-treated (3-aminopropyl-trimethoxysilane) nanoparticles were utilized. The complex relative permittivity, $\varepsilon $*$= \varepsilon $'-j$\varepsilon $'', at audio frequencies from 5K to about 500K and the room temperature breakdown strength have been determined. Also, SEM, DSC and TGA studies have been carried out. $\varepsilon '$ is 11 for PC containing 59 wt-{\%} of untreated 50-70 nm diameter BaTiO$_{\mathrm{3}}$ and $\varepsilon '$ vs. nanoparticle content is larger than would be expected on the basis of the modified Hanai equation. Also, the breakdown strength is low and decreases as nanoparticle content increases. However, $\varepsilon '$ is low and the breakdown strength is high for PC containing the surface-treated nanoparticles. The gamma relaxation (200K and 1000 Hz) does not change as nanoparticle content increases to 59 wt-{\%}. Also, a low temperature relaxation region (in the vicinity of 20K) associated with the nanoparticles is found in the nanocomposites. Next, the breakdown strength increases as BaTiO$_{\mathrm{3\thinspace }}$nanoparticle size increases from 50 nm to 500 nm. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A33.00006: Morphology of candidate high dielectric constant polymers Daniel W. Sinkovits, Manish Agarwal, Mayank Misra, Sanat Kumar We perform all-atom molecular dynamics simulations of polymers which have been identified as promising candidates for high dielectric constant capacitor applications by single-chain density functional theory calculations. These include both organic polymers and those with SnF$_2$ substitutions. We determine the large-scale morphology of these polymers using both $NPT$ molecular dynamics simulations and a multistep thermodynamic integration technique. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A33.00007: Aromatic Polythiourea Dielectrics with High Energy Density, High Breakdown Strength, and Low Dielectric Loss Shan Wu, Quinn Burlingame, Minren Lin, Qiming Zhang There is an increasing demand on dielectric materials with high electric energy density and low loss for a broad range of applications in modern electronics and electrical power systems such as hybrid electric vehicles (HEV), medical defibrillators, filters, and switched-mode power supplies. One major challenge in developing dielectric polymers is how to achieve high energy density U$_{\mathrm{e}}$ while maintaining low dielectric loss, even at very high-applied electric fields. Here we show that amorphous polar-polymers with very low impurity concentration can be promising for realizing such a dielectric polymer. Polar-polymer with high dipole moment and weak dipole coupling can provide relatively high dielectric constant for high U$_{\mathrm{e}}$, eliminate polarization and conduction losses due to weak dipolar coupling and strong polar-scattering to charge carriers. Indeed, an aromatic polythiourea thin film can maintain low loss to high fields (\textgreater 1 GV/m) with a high U$_{\mathrm{e}}$ ($\sim$ 24 J/cm$^{\mathrm{3}})$, which is very attractive for energy storage capacitors. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A33.00008: Exploration of the Chemical Space of Group 4 Polymer Dielectrics Chenchen Wang, Ghanshyam Pilania, Rampi Ramprasad The current standards for capacitive energy storage applications are polypropylene (PP) and polyethylene (PE) which have large band gap and high breakdown strength, but a small dielectric constant. The envisaged next generation dielectric should provide high dielectric constant, while still preserving the insulating characteristics of PP and PE. To meet these growing needs, we use high throughput density functional theory (DFT) calculations in combination with machine learning (ML) methods to identify classes of polymers with large dielectric constant and band gap. In our work, we consider various possible local chemical modifications to polyethylene (PE). To be specific, we allow the -CH$_{2}$- unit in the PE backbone segment to be replaced by -SiF$_{2}$-, -SiCl$_{2}$-, -GeF$_{2}$-, -GeCl$_{2}$-, -SnF$_{2}$-, or -SnCl$_{2}$- units in a systematic manner. High throughput methods were used first to accurately determine the dielectric constant and band gap of the chemically modified PE chains for a set of limited compositions and configurations. ML methods were then used to predict the properties of systems spanning a much larger part of the configurational and compositional space. A set of most promising PE modifications (with simultaneously large dielectric constant and band gap) is identified using this strategy. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A33.00009: General methodology for creating improved polymeric dielectrics Mayank Misra, Manish Agarwal, Daniel Sinkovits, Sanat Kumar We use molecular dynamics and density functional theory to show that the addition of a small number of polar -OH groups to an apolar, hydrocarbon polymer increases the dielectric constant by a factor of 2, but without substantially increasing the dielectric loss. While these results, which are in good agreement with experiments, point to a specific route to creating improved capacitors, more generally, these results suggest that improved polymeric based dielectric materials can be designed by incorporating polar groups on the chain, but only those whose relaxations can be substantially slowed due to cooperative effects, e.g., through long-lived hydrogen bonds. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A33.00010: Large, Uni-directional Actuation In Dielectric Elastomers Achieved By Fiber Stiffening Jiangshui Huang, David Clarke, Zhigang Suo Cylindrical actuators are made with dielectric elastomer sheets stiffened with fibers in the hoop direction. When a voltage is applied through the thickness of the sheets, large actuation strains are achievable in the axial direction, with or without pre-straining and mechanical loading. For example, actuation strains of 35.8{\%} for a cylinder with a prestrain of 40{\%}, and 28.6{\%} for a cylinder without pre-strain have been achieved without any optimization. Furthermore, the actuation strain is independent of the aspect ratio of the cylinder, so that both large strains and large displacements are readily actuated by using long cylinders. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A33.00011: Dielectric Performance of Matrix Free, Hairy Nanoparticle Films Christopher Grabowski, Elizabeth Opsitnick, Hilmar Koerner, Michael Durstock, Richard Vaia Addressing the increasing electrical energy storage and power delivery needs of industry has driven development of novel insulating materials. The voltage breakdown characteristics of two-component polymer nanocomposites (PNCs) -- nanoparticles dispersed in a polymer matrix -- have been previously explored. Control of morphology and dispersion is challenging, however, due to aggregation at high inorganic fractions (\textgreater\ 5{\%} v/v). To fully establish the potential of these nanostructure hybrid materials, we examine the dielectric performance of matrix free, hairy nanoparticle films. These single-component PNCs are comprised of silica nanoparticles with a polystyrene corona such that coronas of adjacent nanoparticles interpenetrate and entangle. Grafting the polymer directly to the nanoparticle provides certain benefits, including more uniform/predictable film morphologies and higher achievable nanoparticle loading. Energy storage capabilities will be assessed from dielectric experimental methods, which include measuring the characteristic dielectric film strength and dielectric permittivity for varying volume fractions of silica. [Preview Abstract] |
Session A34: Focus Session: Dynamics of Glassy Polymers Under Nanoscale Confinment: Glass Transition
Sponsoring Units: DPOLYChair: Robert Riggleman, University of Pennsylvania
Room: 342
Monday, March 18, 2013 8:00AM - 8:36AM |
A34.00001: Calorimetry of Polymer Nanoparticles Invited Speaker: Rodney Priestley Significant understanding regarding the dynamics of glassy polymers geometrically confined to the nanoscale has been obtained by investigating thin films. While thin films are an attractive model system to investigate the influence of confinement on material properties, measurements on other geometries is important from both scientific and technological viewpoints. Investigating glassy dynamics of polymer nanoparticles is useful for exploring the influence of geometry on the behavior of confined polymer, and thus, to gain insight into the generality of size-effects on material properties irrespective of the confining shape. Here, we use calorimetry to measure the glassy dynamics (e.g., glass transition temperature, fragility and structural relaxation) of polymers confined to the nanosphere geometry. We illustrate how nanoscale confinement can significantly alter the glassy dynamics of polymer nanoparticles. Our results suggest that interfaces are a key factor in modifying the glassy dynamics of confined polymer, irrespective of geometry. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A34.00002: Viscoelasticity of Ultra Thin Films Probed via Temperature-Controlled Quartz Crystal Microbalance With Dissipation Jodie Lutkenhaus, Joe Puhr, Ajay Vidyasagar Temperature-controlled quartz crystal microbalance with dissipation (QCM-D) is a powerful technique for probing glass transitions in ultra thin films via changes in viscoelasticity. QCM-D has the added benefit of monitoring such changes as a function of overtone, which allows for one to probe transitions at different locations vertically throughout the film. Here, we present a general approach towards discerning glass transitions in layer-by-layer (LbL) assemblies, which are formed via the alternate adsorption of oppositely charged polyelectrolytes. LbL assemblies consisting of strong polyelectrolytes or of weak polyelectrolytes are presented. A glass transition was only observed in the presence of water, which extensively plasticized the film and facilitates the breaking and reformation of ion pairs. Early results of glass transitions in homopolymers such as polystyrene are also presented. A strong dependence of glass transition temperature on overtone number was observed, suggesting a range of changes in viscoelasticity with respect to temperature and distance from the substrate. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A34.00003: Making the Tg-Confinement Effect Disappear in Thin Polystyrene Films: Good Physics vs. Inappropriate Analysis John Torkelson, Lawrence Chen The Tg-confinement effect in polymers was first characterized in supported polystyrene (PS) films by Keddie et al. in 1994. Since then, many researchers have shown that (pseudo-)thermodynamic Tg measurements of supported PS films taken on cooling consistently yield the same qualitative results, with a decrease from bulk Tg beginning at 40-60 nm thickness and becoming very strong below 20 nm thickness. Some quantitative differences have been noted between studies, which may be ascribed to measurement method or the analysis employed. In 2004, we showed that the Tg-confinement effect in PS may be suppressed by adding several wt{\%} of small-molecule diluents such as dioctyl phthalate. Recently, Kremer and co-workers (Macromolecules 2010, 43, 9937) reported that there was no Tg-confinement in supported PS films based on an analysis of the second derivative of ellipsometry data and use of a ninth order polynomial fit. Here, we demonstrate a new method for suppressing the Tg-confinement effect. In particular, PS made by emulsion polymerization yields no Tg-confinement effect as measured by ellipsometry or fluorescence, while PS made by anionic or conventional free radical polymerization yield strong Tg-confinement effects. The difference is hypothesized to result from surfactant in the emulsion polymerized PS. We also show that the absence of the Tg-confinement effect reported by Kremer is due to inappropriate analysis of ellipsometry data and that correct analysis yields Tg-confinement effects. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A34.00004: Reduced Calorimetric Tg in Confined Thin Polymer Films with Controlled Interface Gi Xue, Jiao Chen, Dongshan Zhou When most prior studies on thin polymer film have shown that glass transition temperature (Tg) decreases under nano-confinement, differential alternating current (AC) chip calorimetric method shows little dependence of Tg on thickness for supported film. To reveal this contradiction, we have manipulated a free-interface by spin-coating polystyrene with an immiscible surfactant [tetraoctylammonium bromide (TOAB)], which had a melting point lower than Tg of polystyrene. When the sample was heated during AC chip measurement, TOAB molecules assembled on the interface and became a mobile layer. As a result, Tg was reduced for ultra thin polymer film. Moreover, stacked free-standing polymer films also show Tg dependence on thickness. The releasing of interface stresses caused by spin-coating is the major reason for reduction of calorimetric Tg. These data unambiguously show that thickness dependence of Tg is an intrinsic property of thin polymer film confined by geometry and dimensions. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A34.00005: The Calorimetric Glass Transition of Polystyrene Ultrathin Films Siyang Gao, Yung P. Koh, Sindee S. Simon The glass transition temperature (Tg) for nanoconfined materials have been widely studied since the early 1990s. For supported polystyrene ultrathin films, Tg differs from bulk value. Recent work has attributed nanoconstrained Tg effects to artifact. In this study, we attempted to resolve this controversy and measure Tg for single polystyrene ultrathin films using Flash DSC. Films have been prepared in two ways: spincast films placed on a layer of inert oil or grease and films directly spincast on the back of the calorimetric chip. For the films on oil or on grease, the 160 nm thick films show no Tg depression. On the other hand, thinner films on oil and on grease show a Tg depression which decreases with increasing cooling rate. The depression reverts to the bulk values over the course of a day at 160 $^{\circ}$C due to dewetting and thickening. For directly spincast films, no Tg depression is observed, consistent with results from other nanocalorimetry work. Our results are consistent with literature results that Tg decreases with decreasing substrate surface energy, and they also demonstrate that the Tg depression observed is not due to degradation or to plasticization effects. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A34.00006: Local Variation of Fragility and Glass Transition Temperature of Ultra-thin Supported Polymer Films Paul Hanakata, Jack Douglas, Francis Starr Extensive studies have shown that the properties of ultra-thin polymer films can differ significantly from the bulk. The effect of the film thickness $h$ on the glass transition temperature has been widely examined, but this does not account for the fragility of glass-formation, which quantifies how rapidly relaxation time varies with temperature $T$. Accordingly, we simulate polymer films of a bead-spring model on a smooth or rough surface and determine both $T_g$ and fragility, both as function of $h$ and film depth. We find that the commonly invoked free-volume layer model does not describe our results. In addition, as opposed to the bulk, we find that $T_g$ and fragility do not generally vary proportionally. Therefore, determination of fragility is essential for the characterization of dynamic changes in film. Finally, we relate these changes of fragility to changes in the cooperative monomer dynamics. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A34.00007: Fragility of an Isochorically Confined Polymer Glass Chuan Zhang, Yunlong Guo, Rodney Priestley When polymers are confined to the nanometer length-scale, the glass transition temperature ($T_{g})$ and its associated dynamics can deviate substantially from the bulk. As confined polymers continue to play an important role as enabling materials in technologies ranging from drug delivery to plastic electronics, a greater understanding of size effects on glass transition dynamics is warranted. Here, we present the effect of isochoric confinement on the dynamic fragility of a polymeric glass-former, $i.e.$, polystyrene (PS). Utilizing silica-capped PS nanospheres as a model system, the fictive temperature ($T_{f})$ and the isochoric heat capacity ($C_{v})$ are measured as a function of diameter \textit{via} differential scanning calorimetry (DSC). By examining $T_{f}$ as a function of cooling rate for each sample, the isochoric fragility ($m_{v})$ is obtained, which decreases significantly as the diameter of the nanospheres is reduced from 462 nm to 129 nm. Hence, the contribution of thermal effects on structural relaxation is reduced with isochoric confinement for PS geometrically restricted by silica. Furthermore, we explore the impact of chemical structure, \textit{via} PS derivatives, on the observed confinement effect on the dynamic fragility. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A34.00008: Effects of aging on glass-forming polymers Amit Shavit, Robert Riggleman Despite nearly twenty years of active research, the effects of nanoscale confinement on the properties of glass forming polymers remain poorly understood. Furthermore, molecular simulations have so far only played a limited role in our understanding of these confinement effects, which are important for applications in both membrane separation and semiconductor manufacturing. We have used molecular dynamics simulations to investigate the effects of aging on bulk and free-standing thin-film glass-forming polymers. We demonstrate that in the vicinity of the bulk glass transition temperature, there are regimes where the free surface is in equilibrium while the center of the film exhibits bulk-like aging. We compare our results with those published from recent experiments, and we provide a microscopic picture on the differences in physical aging in bulk and free-standing polymer films. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A34.00009: Physical Aging of Thin Polystyrene Films Quenched and Measured Free-Standing Justin Pye, Connie Roth High molecular weight (MW) free-standing polymer films exhibit unusual and yet unexplained nanoconfinement effects. We have recently demonstrated that such ultrathin, high MW free-standing polystyrene (PS) films show two reduced glass transition temperatures (Tgs) which can be separated by more than 60 K, indicating that two separate mechanisms act simultaneously to propagate enhanced mobility into the film from the free surface. These studies indicate that the majority of the film transitions to a glass at the upper Tg leaving only a small fraction of the material mobile to much lower temperatures. In an effort to gain insight into the properties of these films between the two reduced Tgs, we aim to measure the physical aging characteristics at temperatures both above and below the lower transition temperature. To this end, we have developed a method using ellipsometry to measure the physical aging rate of thin free-standing PS films that remain free-standing after being thermally quenched. Measurements on thicker free-standing films, greater than 500 nm, supported by rigid, circular sample holders show no thickness dependence to the aging rate, consistent with the thickness independent stress applied to these films by the thermal expansion mismatch between film and holder. Measurements on thinner films will also be presented. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A34.00010: Glass transition temperatures in nanoscale equilibrated polystyrene droplets Chad Daley, James Forrest Measurements of thin film glass transition temperature (Tg) in thin polymer films are only made possible through the metastability of the film with respect to dewetting. Even in the melt state, such samples are not in thermal equilibrium, and resulting Tg values may not be conclusive. In this talk we discuss recent measurements of Tg for equilibrium polystyrene droplets on silicon substrates as measured through their thermal expansion with true non-contact atomic force microscopy. These measurements show promise to not only definitively address the continuing controversy surrounding thin film Tg measurements, but are also readily applied to study non-polymeric glass formers. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A34.00011: Confinement effects on the glass transition of nanolayered polymers David Simmons, Ryan Lang, Mark Mackura Despite numerous studies of glass transition confinement effects in liquids confined in freestanding films, on rigid substrates, and in pores, many outstanding questions remain regarding the origin, nature, and magnitude of these effects. In recent years, studies have demonstrated that these effects are also present in materials under soft confinement, including in internally nanostructured polymers such as nanolayered polymers and block copolymers. This latter class of materials offers a new platform for exploration of confinement effects in the absence of issues surrounding substrate selection and preparation. In this talk, we describe the results of coarse-grained molecular dynamics simulations probing the glass-formation behavior of nanolayered polymers, with a focus on the role of `cooperatively rearranging regions' in nanoconfinement effects in these systems. Furthermore, we discuss the role of miscibility and bulk T$_{g}$ of the nanolayered polymers in determining the magnitude and direction of changes to T$_{g}$ and mobility of polymers under this form of `soft' nanoconfinement. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A34.00012: Dynamic Cluster Size Effects on the Glass Transition of Thin Films Richard Wool During cooling from the melt of amorphous materials, it has been shown experimentally that dynamic rigid clusters form in equilibrium with the liquid and their relaxation behavior determines the kinetic nature of T$_{\mathrm{g}}$ [Stanzione et al, J. Non Cryst Solids 357(2): 311-319 2011]. The fractal clusters of size R $\sim$ 5-60 nm (polystyrene) have relaxation times $\tau $ $\sim$ R$^{1.8}$ (solid-to-liquid). They are analogous to sub critical size embryos during crystallization as the amorphous material tries to crystallize due to the strong intermolecular forces at T \textless\ T$_{\mathrm{m}}$ ; they are not related to density fluctuations or surface capillary waves. In free-standing thin films of thickness h, several important events occur: (a) The large clusters with R \textgreater\ h are excluded and the thin films have an average faster relaxation time compared to the bulk; consequently T$_{\mathrm{g}}$ decreases as h decreases. (b) The segmental dynamics at the 1 nm scale are largely not affected by nanoconfinement since T$_{\mathrm{g}}$ is determined only by the cluster dynamics with R $\gg$ 1 nm. (c) The mobile layer on the surface of free standing films is due to the presence of smaller clusters on the surface which will disappear with increasing rate of testing. (d) With adhesion to a solid substrate, the surface mobile layer disappears as the surface clusters size grow and the change in T$_{\mathrm{g}}$ is suppressed. (e) Physical aging is controlled by the relaxation of the rigid fractal clusters and in thin films, physical aging will occur more rapidly compared to the bulk. (f) The large effect of molecular weight M on T$_{\mathrm{g}}$ appears to be related to the effect on the cluster size distribution giving smaller clusters and faster relation times with increasing M. These results are in accord with the Twinkling Fractal theory of the glass transition. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A34.00013: How does Tg reduction affect the chain mobility in confined PS films? Bulent Akgun, Michael Dimitriou, Sushil K. Satija It is well established that the glass transition temperature (Tg) of supported polystyrene (PS) thin films decrease with decreasing film thickness. This Tg reduction due to the free surface effect is associated with enhanced mobility. However, the correlation between the enhanced mobility and Tg reduction has not been studied yet. To understand the effect of Tg reduction on the vertical mobility of PS chains across the interfaces we have investigated the interdiffusion between PS and deuterated PS (dPS) films in bilayer and trilayer geometries using neutron reflectivity (NR). Bilayer films of 42 nm thick dPS bottom layer and 20 nm thick PS top layer are created in such a way to mimic the films where large Tg reductions has been demonstrated by recent fluorescence measurements. Trilayer films were created using the same bottom layer but floating a 10 nm thick PS middle layer and 10 nm thick dPS top layer to compare the mobilities at the interfaces between the top/middle and middle/bottom layers. NR results showed that there is almost no mixing between the layers up to 90-95 C for both bilayer and trilayer films which is not consistent with large Tg reductions observed in the literature. Our results also indicate no difference in the mobility of PS chains at the top/middle and middle/bottom interfaces in the trilayer film which argues against the enhanced mobility reported in the literature for the top 10 nm of PS thin films. Diffusion of PS chains across the interface gets faster as the MW decreases. [Preview Abstract] |
Session A35: Superconductivity: Tunneling Phenomena
Sponsoring Units: DCMPChair: John Zasadzinski, Illinois Institute of Technology
Room: 343
Monday, March 18, 2013 8:00AM - 8:12AM |
A35.00001: Scanning Tunneling Microscopy of Fe Impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$ Michael Boyer, Brian Koopman, Ling Fu, W.D. Wise, Kamalesh Chatterjee, Genda Gu, E.W. Hudson We utilize scanning tunneling microscopy measurements to probe the effects of intentionally doped magnetic Fe impurities in the high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$. Our spectroscopy measurements indicate an absence of particle-hole symmetry in impurity affected regions. In addition, we find evidence that the Fe impurities which substitute for Cu atoms in the CuO$_{2}$ plane are shifted from their expected locations. Both of these findings are in contrast to previous STM measurements on magnetic Ni impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+d}$ which find spectra which are overall particle-hole symmetric and centered at Cu sites.[1] Interpretations of our measurements may help us understand on a local scale why introduced Fe impurities are more detrimental to superconductivity than Ni impurities as determined by bulk measurements.[2] [1] E.W. Hudson et al., Nature 411, 920 -- 924 (2001). [2] T.D. Hien et al., J. Magn. Magn. Mater. 262, 508 -- 513 (2003). [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A35.00002: Can STM detect nematic ordering in underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{\mathrm{8+x}}$ or other correlated systems? Eduardo da Silva Neto, Pegor Aynajian, Shimpei Ono, Ryan Baumbach, Eric Bauer, John Mydosh, Ali Yazdani Electronic nematic phases, where, for example, the electronic states undergo a spontaneous four-fold (C$_{4})$ to two-fold (C$_{2})$ symmetry breaking, have recently gained vast interest as a possible candidate for various hidden order states in several correlated electron systems such as cuprates, pnictides, and heavy fermions. Such states are difficult to detect using non-local probes because of possible twin domain structures in macroscopic samples. STM spectroscopy has been proposed as a possible approach to detect such nematic orders, with several recent experiments reporting signals in the cuprates and iron-based superconductors. We specifically investigate the situation in which STM topographic data shows C$_{4}$ symmetry while energy-resolved spectroscopic maps signal C$_{2}$ symmetry. We find that such behavior can in fact occur for asymmetric tip geometries and discuss both model calculations and experimental results that provide evidence for this false nematic signature. We discuss possible future STM experiments that could unambiguously detect electronic nematic order. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A35.00003: STM investigation of incipient order in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Pegor Aynajian, Eduardo H. da Silva Neto, Shimpei Ono, Jinsheng Wen, Zhijun Xu, Genda Gu, Ali Yazdani We investigate the spatial and momentum structure of electronic excitations in underdoped samples of the high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ using spectroscopic mapping with the scanning tunneling microscope. A defining feature of the electronic states in these samples is a strong Cu-O bond oriented modulation of the local density of states (Q*). Characterizing Q* as a function of temperature and doping we have established that it appears at the onset of the pseudogap phase at T*, above the regime attributed to fluctuating superconductivity [1]. Model calculations that include both the effects of impurity-induced quasiparticle scattering and incipient order reproduce the energy-dispersion of the measured Q* below and above Tc near optimal doping --- where incipient order effects are weak [2]. To extend our understanding to the underdoped samples, we have carried out new high-resolution spectroscopic mapping measurements as a function of doping which more clearly identify the low-energy signatures of the incipient order. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A35.00004: Studies of Magnetic Impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Eduardo Calleja, Jixia Dai, Genda Gu, Kyle McElroy Impurities in high temperatures superconductors, studied with spectroscopic imaging scanning tunneling spectroscopy (SI-STS) have served as a valuable tool to investigate the electronic structure of these materials (E.W. Hudson \textit{et al}., Nature \textbf{411}, 920 (2001), S.H.Pan \textit{et al}., Nature \textbf{403},746 (2000)). These experiments revealed the appearance of a quasi-localized bound state near the impurity site whose structure is sensitive to the superconducting gap symmetry and the band structure and originates from the charge scattering nature of these impurities. We studied the effects of Fe impurities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ and discovered that the impurities have a different behavior than those previously observed. In particular the quasi bound state near the impurity seems to be behaving as that predicted for a magnetic impurity. The superconducting gap and local electronic density of states was studied in the vicinity of the impurities using SI-STS and will be presented. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A35.00005: Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures Holger Motzkau, Thorsten Jacobs, Sven-Olof Katterwe, Andreas Rydh, Vladimir M. Krasnov We study resistive switching phenomena in small $\mathrm{Bi_2Sr_2CaCu_2O}_{8+x}$ (Bi-2212) mesa structures. Applying a significantly large bias voltage or short current pulses, we are able to controllably and reversibly manipulate the normal state resistance and doping state of the same single crystal from an underdoped to the overdoped state without changing its chemical composition. We employ this effect for an analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. It is observed that such a physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping the c-axis critical current rapidly increases, while the critical temperature, the superconducting gap and the pseudogap decrease, indicating the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even in voltage contribution, attributed to a charge transfer effect, and (ii) odd in voltage contribution, attributed to reordering of oxygen impurities. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A35.00006: Tunneling Spectroscopy of Heavily Underdoped Bi2212 Films Nickolas Groll, Chaoyue Cao, Mike Hinton, Thomas Lemberger, Thomas Proslier, John Zasadzinski SIS break junctions exhibiting quasiparticle and Josephson tunneling were obtained on heavily underdoped Bi2212 films grown by sputtering, with Tc values as low as 5K. Well defined, but extraordinarily large gap values ($\Delta$) were reproducibly observed, even as the Josephons $I_{c}R_{n}$ $\ll$ $\Delta$. The largest values of $\Delta$ were close to $J$, the antiferromagnetic exchange energy. When the new data are combined with previous break junction data on Bi2212 crystals an abrupt change of slope of $\Delta$ vs. doping is found, suggesting a second, magnetic energy scale is being observed in the underdoped region. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A35.00007: Imaging chemical disorder in cuprates using scanning tunneling microscopy Ilija Zeljkovic, Dennis Huang, Can-Li Song, Tay-Rong Chang, Horng-Tay Jeng, Zhijun Xu, Jinsheng Wen, Genda Gu, Jouko Nieminen, Arun Bansil, Robert Markiewicz, Jennifer Hoffman High-T$_{c}$ cuprate superconductors are chemically, electronically and structurally inhomogeneous at the nanoscale. Although a body of theoretical work has predicted that local and global superconductivity may be dramatically impacted by particular dopant configurations, the exact positions of dopants introduced into cuprates to induce superconductivity are generally unknown. Here we use scanning tunneling microscopy to reveal the intra-unit-cell location of two different types of oxygen dopants in Bi$_{2+y}$Sr$_{2-y}$CaCu$_{2}$O$_{8+x}$. Furthermore, we show the relationship between these interstitial oxygen dopants, oxygen vacancies, and a global structural buckling known as the supermodulation. We compare our findings to theoretical simulations. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A35.00008: Cryomagnetic STM spectroscopy study of multiband pairing in layered superconductors Igor Fridman, Vladimir Lukic, Christian Kloc, Cedomir Petrovic, Pengcheng Dai, J.Y.T. Wei Cooper pairing in layered superconductors can involve multiple bands and give rise to complex gap structures in momentum space. Using scanning tunneling microscopy (STM) with a magnetic field applied parallel to the $ab$-plane, we investigate multiband pairing under diamagnetically-induced superfluid momentum. STM spectroscopy and conductance imaging were performed down to 300 mK and up to 9 T, on single-crystals of the Nb-chalcogenide $2H$-NbSe$_2$ and the Fe-pnictides LiFeAs and electron-doped BaFe$_2$As$_2$. Spectroscopy data taken on $2H$-NbSe$_2$ at 300 mK showed a distinctly two-sloped field evolution of the zero-bias conductance, consistent with Doppler-induced depairing on parts of the Fermi surface [1]. Spatial conductance maps revealed stripe patterns that originate from in-plane vortices whose cores are buried in the bulk [2] and which undergo a transition as pairing on one of the bands is suppressed. Our results demonstrate a general method for probing multiband superconductors, especially ones whose band structures host coexisting orders and also play a direct role in the pairing mechanism. \\[4pt] [1] I. Fridman \emph{et al.}, arXiv:1110.6490 (2011) \\[0pt] [2] I. Fridman \emph{et al.}, Appl. Phys. Lett. \textbf{99}, 192505 (2011) [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A35.00009: Doping-dependent vortex-state scanning tunneling spectroscopic (STS) studies of cuprate superconductors C.-C. Chen, M. L. Teague, Z.-J. Feng, R.T.-P. Wu, N.-C. Yeh We report STS studies of YBa2Cu3O7$-\delta $(Y-123) and Ca-doped Y-123 superconductors as a function of magnetic field ($H)$ and hole doping level ($p)$. Our studies suggest that the origin of the pseudogap (PG) is associated with competing orders (COs), and that the occurrence (absence) of PG above the superconducting (SC) transition $T_{c}$ is associated with a CO energy $\Delta_{CO}$ larger (smaller) than the SC gap $\Delta_{SC}$. We derive $\Delta_{SC}$ and $\Delta_{CO}$ by two approaches. For zero-field STS we apply Green function techniques to fit the ``peak'' features for $\Delta_{SC}$ and the ``kink'' features for $\Delta_{eff} \quad \equiv $ [($\Delta_{SC})^{2}+(\Delta_{CO})^{2}$]$^{1/2}$. For $H$ \textgreater\ 0 we analyze the PG features in the intra-vortex STS for $\Delta_{CO}$ and the peak features in the inter-vortex STS for $\Delta _{SC}$. Both approaches yield consistent results. For optimally and underdoped Y-123, we find that $\Delta_{SC}$ \textless\ $\Delta_{CO}$ with dominant $d_{x^{2}-y^{2}}$-wave pairing, and that $\Delta_{SC}$ decreases with decreasing $p$ while $\Delta_{CO}$ increases. Both $\Delta _{SC}$ and $\Delta_{CO}$ exhibit long-range spatial homogeneity. For Ca-doped Y-123, the substitution of Y by Ca contributes to excess holes and disorder. For $p$ \textgreater\ 0.16, both $\Delta_{SC}$ and $\Delta_{CO}$ decrease with increasing $p$, $\Delta_{CO}$ \textless\ $\Delta_{SC}$ for $p$ \textgreater\ 0.23, and the pairing symmetry becomes ($d_{x^{2}-y^{2}}+s$) with increasing $s$-wave component, implying the diminishing Mott nature in overdoped cuprates. This work was supported by NSF through IQIM at Caltech. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A35.00010: Spatial Complexity Due to Locally Oriented Charge Modulations in a Cuprate Superconductor Erica Carlson, Elizabeth Main, Benjamin Phillabaum, Hiroshi Ikuta, Karin Dahmen, Eric Hudson, Jennifer Hoffman Surface probes such as scanning tunneling microscopy (STM) have detected complex electronic patterns at the nanoscale in many high temperature superconductors. We use scanning tunneling microscopy to image the local orientation of the static charge modulations in Bi$_{2-y}$Pb$_y$Sr$_{2-z}$LazCuO$_{6+x}$, for samples spanning a wide range of doping. For each sample, we compute the universal cluster properties arising from the locally $x$-oriented and locally $y$-oriented clusters in order to identify the fundamental physics controlling the complex pattern formation. By comparing these quantitative measures to known universality classes for rotational symmetry breaking, we find that the charge modulations are not confined only to the surface, but they also extend throughout the bulk of the material. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A35.00011: Tunneling Spectroscopy of SRF Cavity Grade Niobium Chaoyue Cao, Nick Groll, Thomas Proslier, John Zasadzinski Mechanical contact tunneling measurements are presented on high purity Nb pieces from the starting plate for superconducting radio frequency (SRF) cavity construction as well as from hot spot and cold spot regions of a tested cavity. A varying scattering rate, gamma, is found which broadens the BCS density of states. Detailed fits using Shiba theory indicate that this scattering may be due to magnetic pairbreaking. Hot spot samples reveal a zero bias conductance peak that splits in magnetic field and can be fit using Appelbaum-Anderson theory of spin flip scattering. Together these measurements indicate that the native oxide of Nb can contain varying amounts of localized magnetic moment defects, possibly due to oxygen vacancies in niobium pentoxide. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A35.00012: Andreev Reflection Spectra of $d$-wave Superconductors Charles Snider, Jessica Gifford, Jonny Martinez, Tingyong Chen At a normal metal/superconductor interface Andreev reflection occurs, which can be utilized to measure spin polarization of the normal metal and also the superconducting gap of the superconductor. An $s$-wave superconductor has an isotropic gap and for an unpolarized current the Andreev reflection spectrum within the gap is twice that of outside the gap. A fully spin polarized current suppresses the Andreev reflection therefore causes zero conductance within the gap. The scenario is quite different in a $d$-wave superconductor because the order parameter has anisotropy and phase. In this work, we calculate Andreev Reelection spectra of an interface between a normal metal and a $d$-wave superconductor for a current with any polarization, based on the recent Chen-Tesanovic-Chien (CTC) model. It is shown that the point angle of the interface can drastically change the Andreev spectra and a zero bias anomaly (ZBA) is observed in the tunneling regime only if the point angle is large. The spin polarization can also drastically affect the spectra and can completely suppress the ZBA. Our calculation shows that one can use both the spin polarization and the point angle to verify the ZBA in unconventional superconductors. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A35.00013: Direct Probe of Interplay between Local Structure and Superconductivity in FeTe0.55Se0.45 Wenzhi Lin, Qing Li, Brian Sales, Stephen Jesse, Athena Safa-Sefat, Sergei Kalinin, Minghu Pan We explore the interplay between local crystallographic structure, composition and local electronic and superconductive properties. Direct structural analysis of scanning tunneling microscopy (STM) data allows local lattice distortions and structural defects across a FeTe0.55Se0.45 surface to be explored on a single unit-cell level. Concurrent superconducting gap (SG) mapping reveals suppression of the SG at well-defined structural defects, identified as a local structural distortion (Guinier-Preston zone). The strong structural distortion is related to the vanishing of the superconducting state. This study provides insight into the origins of superconductivity in iron chalcogenides by providing an example of atomic-level studies of the structure-property relationship. Research was supported (WL, BCS, AS, SVK) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. This research was conducted (MP, QL) at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. (Wenzhi Lin and Qing Li, these authors contributed equally to this work) [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A35.00014: ABSTRACT WITHDRAWN |
Session A36: Theory and Computation of Novel Superconductivity
Sponsoring Units: DMPChair: Roser Valenti, Universitaet Frankfurt
Room: 344
Monday, March 18, 2013 8:00AM - 8:12AM |
A36.00001: Superconductivity at the onset of spin-density-wave order in a metal Yuxuan Wang, Andrey Chubukov We revisit the issue of superconductivity at the quantum-critical point between a 2D paramagnet and a spin-density-wave (SDW) metal with ordering momentum $(\pi,\pi)$. This problem is highly non-trivial because the system at criticality displays a non-Fermi liquid behavior and because the effective coupling constant $\lambda$ for the pairing is generally of order one, even when the actual interaction is smaller than fermionic bandwidth. Previous study [M. A. Metlitski, S. Sachdev, Phys.Rev.B 82, 075128 (2010)] has found that the leading renormalization of the pairing vertex contains $\log^2$, like in color superconductivity. We analyze the full gap equation and argue that summing up $\log^2$ term does not lead to a pairing instability. Yet, superconductivity has no threshold, even if $\lambda$ is set to be small: the subleading $\log$ terms give rise to BCS-like $T_c \propto e^{-1/\lambda}$. We argue that the analogy with BCS is not accidental as superconductivity at a QCP is a Fermi liquid phenomenon -- it comes from fermions which retain Fermi liquid behavior at criticality. We compute $T_c$ for the actual $\lambda$ and find consistency with the numerical results. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A36.00002: Coexistence of Antiferromagnetism and Superconductivity in Bilayer Cuprates and Iron Arsenides Takami Tohyama, Hiroyuki Yoshizumi, Yasunori Matsui, Takao Morinari The coexistence of antiferromagnetism (AFM) and superconductivity (SC) is one of important issues in strongly correlated electron systems. One example is seen in multilayered cuprate superconductors, and another one is in iron-arsenide superconductors. In cuprates, motivated by the recent experiment reporting the enhancement of AFM order below the SC transition temperature, we study the proximity effect of the AFM correlation in a bilayer system and also examine the possibility of the coexistence. We present the result of mean-field theory that is consistent with the experiment and supports the proximity-effect picture [1]. In iron arsenides, we study possible coexistence of AFM with Dirac dispersions and SC with the same and different phase of pairing potential, based on the knowledge of the cuprates. [1] Y. Yoshizumi, T. Morinari, and T. Tohyama, Phys. Rev. B $\mathbf{85}$, 184523 (2012). [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A36.00003: Robust nodal $d$-wave spectrum in simulations of strongly fluctuating competing order in underdoped cuprates William Atkinson, J. David Bazak, Brian Andersen While many experiments suggest that the pseudogap in cuprate superconductors originates from some nonsuperconducting broken-symmetry phase, clear spectral signatures of such a phase have not been observed in angle resolved photoemission experiments. We report on numerical simulations of the spectral function, in which competing superconducting and nonsuperconducting phases experience strong thermal fluctuations. In our work, we consider the competition between $d$-wave superconductivity and a low temperature spin density wave (SDW) phase that is widely observed in underdoped cuprates. Because of this competition, our simulations sample highly inhomogeneous states that are far from the mean-field saddle point configurations. We find that the computed spectral function is, in many cases, almost indistinguishable from that of the pure $d$-wave superconductor, and that there is no sign of the Fermi surface reconstruction generically expected for SDW phases. We argue that this work explains the absence of any clear experimental signature of such a reconstruction. We find that signatures of the fluctuating competing order can be found mainly in a splitting of the antinodal band and, for strong magnetic order, in small induced nodal gaps similar to those found in recent experiments [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A36.00004: Theory of nonequilibrium superconductivity in cuprates Takashi Oka, Ville Pietil\"a Recently, nonequilibrium properties of Hi Tc superconductors are attracting much interest. This is because new experimental methods such as time resolved ARPES has been applied to cuprates and succeeded in observing the dynamics of photo-excited quasiparticles as well as the temporal evolution of the d-wave superconducting order parameter (e.g., [1]). One can also realize nonequilibrium states in interfaces between cuprates and metal electrodes and control the superconducting order by changing the applied bias [2]. In order to study the dynamics of superconductivity in strongly correlated systems, we developed a novel numerical method by combining the quantum kinetic equation with the fluctuation exchange approximation (FLEX, self-consistent T-matrix approximation) [3]. This method enables us to study the interplay between pair mediating fluctuations, e.g., antiferromagnetic and charge fluctuations, and the dynamics of quasiparticles and superconducting order parameter. In the presentation, we explain the physical insights we obtain by applying this method to nonequilibrium dynamics in d-wave superconductors.\\[4pt] [1] C. L. Smallwood, et al., Science 336, 1137 (2012).\\[0pt] [2] T. Oka, and H. Aoki, Phys. Rev. B 82, 064516 (2010).\\[0pt] [3] T. Oka, and V. Pietilä, in progress. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A36.00005: Pair density wave superconducting state in a Nematic Liquid Crystal Phase Rodrigo Soto Garrido, Eduardo Fradkin We consider the problem of the superconducting states that arise in a fermionic system in a nematic-like $l=2$ state in the spin-triplet channel. This nematic state is invariant under a $\pi/2$ rotation followed by a spin flip. Under these circumstances the only infinitesimal superconducting instability is in the p-wave channel. However, close enough to the nematic transition both a uniform d-wave superconducting state and a pair density wave (PDW) state (also with d-wave symmetry) can arise. We compute the phase diagram and study the competition between an uniform (BCS type) superconducting state, the PDW state and the non-superconducting state. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A36.00006: Incommensurate Nematic Charge Order in the Three Band Model for Cuprate Superconductors Sinan Bulut, William A. Atkinson, Arno Kampf Recent experimental evidence for charge order in cuprates is a possible source of anomalous electronic properties in the underdoped regime. Intra-unit cell charge ordering tendencies point to electronic nematic order involving oxygen orbitals. In this context we investigate charge instabilities in the Emery model. The charge susceptibilities reveal three different kinds of nematic order. The first is an intra-unit cell ($q=0$) nematic order. The second and the third are incommensurate charge orders with wavevectors that are either uniaxial or oriented along the Brillouin zone diagonal. The two latter charge patterns correspond to a spatially modulated nematic phase. The selection of the leading instability depends on the filling, the interaction parameters, and details of the band structure. For these candidate charge orderings we discuss their possible relevance for the charge ordering signatures in X-ray and STM experiments. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A36.00007: Orbital Nematic Instability in Two-Orbital Hubbard Model: A Renormalization-Group Study Masahisa Tsuchiizu, Seiichiro Onari, Hiroshi Kontani Motivated by the nematic electronic fluid phase in Sr$_3$Ru$_2$O$_7$, we analyze the ($d_{xz}$, $d_{yz}$)-orbital Hubbard model by the one-loop renormalization-group method [1]. We find that, in the weak-interaction case, the $q=0$ component of the orbital susceptibility $\chi^{\mathrm{q}}(q)$ is critically enhanced by the Aslamazov-Larkin (AL) type vertex correction due to the superconducting fluctuations. In the strong-interaction case, we also find the development of $\chi^{\mathrm{q}}(q)$ driven by the AL-type vertex correction due to spin fluctuations, consistently with the perturbation analysis [2]. Thus the strong orbital nematic fluctuation, i.e., orbital Pomeranchuk instability, emerges near the magnetic or superconducting quantum criticality. This mechanism of orbital nematic order presents a natural explanation for the nematic order in Sr$_3$Ru$_2$O$_7$, and is expected to be realized in various multiorbital systems, such as Fe-based superconductors [3]. \\ \noindent [1] M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3664. \\ \noindent [2] Y. Ohno, M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3629. \\ \noindent [3] S. Onari and H. Kontani, Phys. Rev. Lett. \textbf{109}, 137001 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A36.00008: Superconductivity in CuCl/Si superlattices: excitonic pairing? S.H. Rhim, Rolando Saniz, Michael Weinert, A.J. Freeman Two-dimensional (2D) hetero-bonded semiconductor interfaces have been suggested as candidate geometries where excitonic superconductivity \footnote{V.L. Ginzburg, Sov. Phys. JETP {\bf 20},1549 (1965)} -- and the greatly enhanced where $T_C$ compared to phonon mechanisms mediation -- can be realized. Among experimental efforts, epitaxially grown CuCl on Si (111) has reportedly exhibited excitonic superconductivity at 60$\sim$150 K. Our first-principles calculations confirm 2D metallicity at the interfaces due to charge transfer by valence mismatch. \footnote{S.H. Rhim {\em et al.}, Phys. Rev. B {\bf 76}, 184505 (2007).} The excitonic mechanism is investigated by calculating the kernel function, $K(\omega)$, for the average of the electronic contributions to the effective interaction.\footnote{Zakharov {\em et al.}, J.Phys.Condes.Matter {\bf 9} 8501 (1997)} The attractive interaction found in the CuCl/Si superlattice indicates the feasibility of excitonic pairing for a certain frequency range. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A36.00009: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 9:48AM - 10:00AM |
A36.00010: Real-Space Holon Pairing in Underdoped Cuprates Timothy Lovorn, Sanjoy Sarker We examine the behavior of a recently developed model for underdoped cuprates [1] in the fluctuation regime above $T_c$. It is characterized by a spin gap and describes sublattice preserving hopping by holons and holon pairs, accompanied by a backflow of spin singlets. The singlets form a short-range valence-bond state which is continuously connected to the correct spin state at half filling. The theory, thus constrained, leads to the correct phase diagram and also explains the two-dimensionality of the metallic states. Superconductivity is due to pair hopping, as holons form real-space pairs at low densities and undergo a Bose-Einstein condensation below $T_c$. The pairs exist up to a temperature $T_p > T_c$, which is consistent with the observed Nernst effect and diamagnetism above $T_c$. The pair spectrum is calculated by identifying poles of the pair Green's function. Here we show that the specific heat of this system is in qualitative agreement with recent measurements [2].\\[4pt] [1] S. K. Sarker and T. Lovorn, Phys. Rev. B {\bf 82}, 014504 (2010); ibid {\bf 85}, 144502 (2012)\\[0pt] [2] H.-H. Wen \textit{et al.}, Phys. Rev. Lett. {\bf 103}, 067002 (2009) [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A36.00011: Analyses of High-Temperature Superconductivity in Doped Hubbard Model --High-Precision Variational Monte Carlo Study-- Takahiro Misawa, Masatoshi Imada Two-dimensional Hubbard model, which only includes the on-site Coulomb interaction $U$ and the nearest hopping $t$, is one of the simplest models proposed for describing the high-T$_{\rm c}$ superconductivity. Although numerically unbiased methods such as auxiliary-field quantum Monte Carlo(QMC)[1] and Gaussian-basis QMC [2] do not find an indication for the superconductivity for intermediate coupling region($U/t<8$), several approaches such as the variational Monte Carlo(VMC) method[3,4] suggest that the $d$-wave superconductivity appears in the doped Hubbard model. To quantitatively resolve the origin of the controversy and to reveal the superconducting mechanism, by using a high-precision VMC[5], we present results which successfully reproduces the results of previous unbiased calculations[1,2], while finds the superconductivity in a strong coupling region. We focus on the relation of the superconductivity to proximity of phase separation with charge fluctuations as its mechanism. [1]N.Furukawa and M.Imada, J. Phys. Soc. Jpn. 61, 3331 (1992). [2]T.Aimi and M.Imada, J. Phys. Soc. Jpn. 76, 113708 (2007). [3]H.Yokoyama $et$ $al$. J. Phys. Soc. Jpn. 73, 1119(2004). [4]D.Baeriswyl $et$ $al$. New J. Phys. 11 075010 (2009). [5]D.Tahara and M.Imada,J. Phys. Soc. Jpn. 77,114701(2008). [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A36.00012: Superconductivity in two-leg ladder iron selenides Weicheng Lv, Elbio Dagotto, George Martins Recently, evidence of superconductivity has been discovered in the single-layer potassium-doped iron selenide that consists of weakly coupled two-leg iron ladders (Wei Li {\it et al.}, arXiv:1210.4619). Using a self-consistent mean-field approximation, we analyze the pairing symmetry and structure of the multi-orbital $t$-$J$ model defined in these two-leg ladder systems. Similar to the case of the iron pnictides, a modified $s$-wave pairing state is stabilized by the next-nearest-neighbor superexchange $J_2$. The presence of competing states will be discussed. Our result demonstrates the potential importance of the local magnetic couplings in iron-based superconductors. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A36.00013: Magnetic States of the Two-Leg Ladder Iron Selenides Qinlong Luo, Andrew Nicholson, Julian Rincon, Shuhua Liang, Adriana Moreo, Elbio Dagotto, Jose Riera, Gonzalo Alvarez, Limin Wang, Wei Ku Neutron scattering experiments have unveiled a dominant spin arrangement in the two-leg ladder selenide compound BaFe$_2$Se$_3$, involving ferromagnetically ordered 2$\times$2 iron-superblocks, that are antiferromagnetically coupled among them (the ``block-AFM'' state). Our numerical study of the electronic five-orbital Hubbard model, within the Hartree-Fock approximation and using first principles techniques for the hopping amplitudes, has shown that the exotic block-AFM state is indeed stable at realistic electronic densities $n \sim 6.0$. Another state with wavevector $(\pi,0)$ becomes stable in other portions of the phase diagrams, including $n \sim 5.5$, as found experimentally in KFe$_2$Se$_3$. In addition, our study unveils several competing magnetic phases that could be experimentally stabilized varying either $n$ chemically or the electronic bandwidth by pressure. Similar results were obtained using two-orbital models, studied here via Lanczos and DMRG techniques [1]. [1] Qinlong Luo, et al, arXiv: 1205.3239, and references therein. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A36.00014: Second Corollary to the Five Principles of Photoemission Via Dipolon Theory of High Temperature Superconductivity Ram R. Sharma Recently, we presented theory of dipolon-phonon interaction to explan the isotope shift in HTSC. Also we deduced five principles with one corollary [1] of photoemission (PE) from the dipolon theory [2,3] which not only explained the peak-dip-hump phenomenon [4] and low energy kink in quasiparticle energy dispersion (QED) but also predicted two more high energy kinks [1,4] that have now been observed. Here we present second corollary to the five principles of PE which states: ``As one changes dipolon density of states by changing or creating interactions with the factors such as doping, occupation number of ions, vacancies, defects, impurities, phononS with and without different isotope exchange, lattice structure, lattice distortion etc. there appear corresponding changes (shifts) in PE spectra, $T_C$, QED and the kink structure (predictably, one may observe the apparent isotope shift negative as well as zero or positive depending on the simultaneous action of the other factors).''\\[4pt] [1] R. R. Sharma, ``Dipolon Theory..'', in ``.. Cuprates'', Ed. K. N. Courtlandt, P. 81-100, Nova Sc, Pub., New York, 2009.\\[0pt] [2] R. R. Sharma, Phy. Rev. {\bf B 63}, 054506 (2001).\\[0pt] [3] R. R. Sharma, Physica {\bf C 439}, 47 (2006).\\[0pt] [4] R. R. Sharma, Physica {\bf C 468}, 190 (2008) [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A36.00015: Different roles of Zn$^{2+}$ and Li$^{+}$ impurities in the CuO2 plane in undoped cuprate compounds Jiawei Mei A planar Mott insulator with easy plane Neel order can be mapped unto a Gutzwiller projected topological insulator model. Under the assumption that the projection operator can be permuted, Zn$^{2+}$ and Li$^{+}$ impurities can be represented as vacancies introducing a zero mode, which has a local spin moment for Zn$^{2+}$ and a charged hole for Li$^{+}$, respectively. While the local spin moment for Zn$^{2+}$ is screened by the long-range spin correlations, the active charge degree of freedom for Li$^{+}$ impurity twists the spin background. This proposal explains the very different roles of the Zn$^{2+}$ and Li$^{+}$ impurities in the CuO2 plane in the undoped cuprate compounds. [Preview Abstract] |
Session A37: Focus Session: Fe-based Superconductors: Coexistence with Magnetism
Sponsoring Units: DMP DCOMPChair: Ulrich Welp, Argonne Natl Lab
Room: 345/346
Monday, March 18, 2013 8:00AM - 8:36AM |
A37.00001: Various forms of coexistence of superconductivity and magnetism in iron-pnictide superconductors: a NMR study Invited Speaker: Julien Bobroff Our NMR studies of iron pnictides allowed us to discover various forms of coexistence between superconductivity and magnetism. In Co-doped BaFe2As2, superconductivity and incommensurate antiferromagnetism coexist at the atomic level in an homogeneous state. In contrast, Ru isovalent doping leads to a disorderd situation where superconducting clusters appear in an antiferromagnetic background. Finally, in the 245 iron-selenide RbFeSe, antiferromagnetism and superconductivity separate in alternate layers of nanometer thickness. But in all these componds, the superconducting state remains similar in terms of local susceptibility and carrier doping. It looks as if, for superconductivity to appear, frozen Fe magnetic moments need to be small enough or far enough in distance, whatever the cause. Y. Texier et al., PRL 108, 237002 (2012); Y. Laplace et al., PRB Rapid Com 86, 020510(R) (2012); Y. Laplace, PRB Rapid Com 80, 140501 (2009) [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A37.00002: Nuclear magnetic resonance studies of coexisting antiferromagnetism and superconductivity in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Adam Dioguardi, John Crocker, Abigail Shockley, Nicholas apRoberts-Warren, Ching Lin, Kent Shirer, David Nisson, Alex Thaler, Paul Canfield, Nicholas Curro We present $^{75}$As nuclear magnetic resonance (NMR) spectra and spin lattice relaxation data from Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for $x=$ 0.05757, 0.05898, and 0.06163, with $T_C=$ 21.5 K, 22 K, and 22.7 K respectively. The spectra become broadened below the antiferromagnetic (AFM) phase transition. Spin lattice relaxation was measured by inversion recovery at the central line with $H \perp \hat{c}$ down to 4.5 K into the coexistence region. As temperature is decreased toward the AFM phase transition, the exponential inversion recovery curve begins to deviate from the theoretical prediction. The curves were fit to a stretched exponential to characterize this deviation as a function of temperature. This behavior persists into the coexistence region and may be related to nematic fluctuations. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A37.00003: Novel Phase Separation and Magnetic Volume Tuning in Underdoped NaFe$_{1-x}$Co$_x$As ($x \simeq 0.01$) Long Ma, J. Dai, X.R. Lu, Guotai Tan, Yu Song, Pengcheng Dai, C.L. Zhang, B. Normand, Weiqiang Yu NaFeAs is a quasi-2D pnictide parent compound with a weak magnetic moment and separate structural and antiferromagnetic transitions. Because Co doping leads to a superconductor with $T_c \simeq 20$ K at a very low optimal doping of $x = 0.02$, NaFe$_{1-x}$Co$_x$As is uniquely suited to sensitive studies of the cohabitation and competition between magnetism and superconductivity. Using NMR as a local probe of both antiferromagnetic order and superconductivity, we have compared Knight shifts and relaxation rates on the Na, As, and Co nuclei. Above $T_c$, we find weak doping inhomogeneity, in the form of residual paramagnetic regions with differing $T_N$ values, and a strongly field-controlled magnetic volume. Below $T_c$, we observe a strong competition between antiferromagnetism and superconductivity, in which the temperature is the dominant control parameter, suppressing the magnetic volume fraction very significantly in favor of the superconducting one, while the external field suppresses $T_c$. Our results suggest both a microscale phase separation in real space and in reciprocal space a competition between two order parameters requiring the same electrons on the quasi-2D Fermi surface. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A37.00004: $^{75}$As and $^{23}$Na NMR study in optimally Co doped NaFe$_{0.975}$Co$_{0.025}$As Sangwon Oh, A.M. Mounce, Jeongseop A. Lee, W.P. Halperin, C. L. Zhang, Pengcheng Dai, A.P. Reyes, P.L. Kuhns The normal and superconducting state of NaFe$_{0.975}$Co$_{0.025}$As, single crystals with exceptionally narrow $^{75}$As and $^{23}$Na NMR spectra, were investigated in external magnetic fields from 6.4 T to 24 T. The Knight shift ($^{75}K$) shows an almost linear decrease in the normal state on cooling and a sharp transition to spin-singlet superconductivity below $T_c$. A temperature independent $^{75}K$ at low temperature, below 0.4 $T_c$, indicates that there are no gap nodes. The penetration depth, $\lambda_{ab}$, was found to be 456 $\pm$ 7 nm at zero temperature, after convoluting the normal state spectrum with the vortex field distribution expected from Ginzburg-Landau theory. The spin lattice relaxation rate, $1/T_1$, shows a T$^{3}$ behavior in the superconducting state at low field which becomes T$^{1.5}$ at higher fields. Additionally, the average of $1/T_1$ over the vortex unit cell at T = 4 K is linear in H$^2$. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A37.00005: Structural and magnetic phase transitions in TbRuAsO and DyRuAsO Michael McGuire, Andrew May, Ovidiu Garlea, Brian Sales The compounds \textit{Ln}RuAsO (\textit{Ln} $=$ lanthanide) are isoelectronic, isostructural, 4$d$ transition metal analogues of the parent phases of 1111-type iron superconductors, but display markedly different behaviors. Recent results from crystallographic and physical properties measurements on TbRuAsO and DyRuAsO reveal particularly unusual properties in these materials. Analysis of low temperature x-ray and neutron powder diffraction data indicate a symmetry-lowering crystallographic phase transition in DyRuAsO at 25 K, and ordering of rare-earth magnetic moments at 7.0 and 10.5 K for TbRuAsO and DyRuAsO, respectively. The structural distortion observed in DyRuAsO (to space group P\textit{mmn}) is different than the well-known distortion that occurs in \textit{Ln}FeAsO. In addition, the findings indicate some coupling between the magnetism and the lattice, and hints of Ru magnetism are observed. A response to the structural transition is apparent in the magnetic susceptibility, and the associated heat capacity anomaly responds strongly to a magnetic field. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A37.00006: Structural and magnetic properties of Ba$_{1-x}$Na$_x$Fe$_2$As$_2$ Omar Chmaissem, S. Avci, R. Osborn, S. Rosenkranz, H. Claus, D.Y. Chung, M. Kanatzidis, D.D. Khalyavin, P. Manuel Iron pnictides have attracted significant intrigue because of their astonishing superconducting properties in a large number of materials that support chemical substitutions at literally every site. Of particular interest is AFe2As2 (A$=$ Ba, Sr, Ca) in which hole or electron-doping is achieved by chemical substitution of alkaline or transition metal elements at the Ba and Fe sites, respectively. Nominally isovalent P substitutions for As have also been achieved producing a phase diagram remarkably similar to the electron- or hole-doped diagrams. A universal picture has emerged: a spin density wave region is stable at low substitution levels in which the Fe magnetic moments are aligned antiferromagnetically along the a- and c-axes and ferromagnetically in the direction of the b-axis. With increased substitution, the magnetic structure progressively loses strength to a point where it's suppressed in favor of superconductivity usually extending over a broad substitution range to form a superconducting dome. In these 122 systems, samples with compositions in the crossover region have been demonstrated by neutron diffraction and other techniques to allow the microscopic coexistence of both magnetism and superconductivity. I will present insights on the structural and magnetic properties of the Ba$_{1-x}$Na$_{x}$Fe$_{2}$As$_{2}$ system and discuss the results in a general context. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A37.00007: Synthesis, structure and magnetic properties of BaFe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_{2}$ as determined by elastic and inelastic neutron scattering Keith M. Taddei, J.M. Allred, R. Osborn, S. Rosenkranz, D. Bugaris, H. Claus, M. Kanatzidis, S. Avci, C. de la Cruz, O. Chmaissem Unconventional superconductivity and microscopic phase coexistence have been demonstrated in a narrow compositional region of the122 system between two competing spin density wave and superconducting order parameters. Quantum critical fluctuations induced by the suppression of the antiferromagnetic order have been proposed to mediate pairing in analogy with the role played by phonons in conventional cuprates. Establishing unambiguous conclusions concerning the pairing mechanism has proven difficult in the pnictides due to the complexity of the electronic structures. Recent reports have shown that isovalent P substitution for As in BaFe$_{2}$As$_{2}$ suppresses the structural and magnetic transitions and lead to superconductivity similar to hole or electron doping. From the chemical point-of-view, there is no net change in the electron-to-hole ratio in this charge compensated system. I will briefly discuss synthesis details of high quality pnictides and BaFe$_{2}$(As$_{\mathrm{1-x}}$P$_{\mathrm{x}}$)$_{2}$ samples and present structural results obtained by neutron diffraction. Inelastic neutron measurements will also be discussed. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A37.00008: The phase diagram of BaFe$_2$(As$_{\mathrm{1-x}}$P$_{x})_2$ as determined by neutron diffraction Jared Allred, Keith Taddei, Daniel Bugaris, Sevda Avci, Omar Chmaissem, Clarina Dela Cruz, Duck Young Chung, Mercouri Kanatzidis, Stephan Rosenkranz, Ray Osborn The iron-arsenides are a now famous family of high-$T_{\mathrm{c}}$ superconductors where the superconducting state is stabilized by suppressing a magnetic ground state in a parent compound. The phenomenon is quite robust, and BaFe$_{2}$As$_{2}$, for example, can be made superconducting either by applying pressure or by electron, hole, or isovalent doping. The isovalently doped BaFe$_{2}$(As$_{\mathrm{1-x}}$P$_{x})_{2}$ materials are particularly interesting because it is not obvious what is driving the suppression of the SDW and enhancing $T_{\mathrm{c}}$. The driving force has been variously ascribed to chemical pressure, changes in polarity of the Fe-(As,P) bond, and other even more subtle chemical effects. Moreover, reports on various general features in the iron-arsenide phase diagram---such as short-range nematic order and the separation of the N\'{e}el transition ($T_{\mathrm{N}})$ and the structural transition ($T_{\mathrm{s}})$---remain contradictory and underexplored. We have undertaken a detailed neutron diffraction study of the phase diagram in order to clarify some of the ambiguities. We find that $T_{\mathrm{s}} =$ $T_{\mathrm{N}}$ and that the superconducting dome rises more sharply than for the aliovalently doped materials. Moreover, the $T$ dependence of the structural and magnetic order parameters and a discontinuous increase in $c$/$a$ below $T_{\mathrm{N}}$ suggest a first order phase transition. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A37.00009: Magnetic neutron diffraction study of BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$ critical exponents through the tricritical doping D.M. Pajerowski, C.R. Rotundu, J.W. Lynn, R.J. Birgeneau We present temperature dependent magnetic neutron diffraction measurements of BaFe$_{2(1-x)}$Co$_{2x}$As$_{2}$ for x~=~0.039, 0.022, and 0.021 as-grown single crystals. We are motivated to investigate the magnetic tricritical point in the (x,T) plane near x$^{m}_{tr}$$\approx$0.022,[1] as well as to systematically probe the character of the magnetic phase transition across a range of dopings. All samples show long range antiferromagnetic order that may be described near the transition by $I\propto(1-T/T_N )^{2\beta}$ with $\beta$~=~0.291 for x~=~0.039, $\beta$~=~0.208 for x~=~0.022, and $\beta$~=~0.198 for x~=~0.021, showing a monotonic increase from the parent BaFe$_{2}$As$_{2}$ compound, $\beta$~=~0.103.[2] We will discuss the results based on theoretical predictions for the behavior of the order parameter in the vicinity of a tricritical point.\\[4pt] [1] M.G. Kim, R.M. Fernandes, A. Kreyssig, J.W. Kim, A. Thaler, S.L. Bud'ko, P.C. Canfield, R.J. McQueeney, J. Schmalian, and A.I. Goldman, Phys. Rev. B 83, 134522 (2011). \newline [2] S.D. Wilson, Z. Yamani, C.R. Rotundu, B. Freelon, E. Bourret-Courchesne, and R.J. Birgeneau, Phys. Rev. B 79, 184519 (2009). [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A37.00010: First order quantum phase transition under the superconducting dome of Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ T. Hu, H. Xiao, Y.P. Singh, D.J. Haney, X.Y. Huang, M. Dzero, H.H. Wen, C.C. Almasan We present the results of magnetoresistivity and magnetization measurements performed under pressure (P) on single crystals of Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ (x $=$ 0, 0.042, 0.06, 0.08). Our results show that the antiferromagnetic phase macroscopically coexists with the superconducting phase and can be induced by the magnetic field locally nucleated in the vortex core for the x $=$ 0.06 sample. In addition, the diamagnetic signal of the x $=$ 0.06 sample shows a huge jump around P $=$ 0.5~GPa, where the superconducting transition temperature displays a maximum. This suggests that a first order antiferromagnetic quantum phase transition (QPT) is present inside the superconducting dome, and that the superconductivity in this system is closely related to this QPT. A magnetic tricritical point is observed inside the superconducting dome, and no quantum critical point is expected in zero magnetic field. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A37.00011: Intermediate orthorhombic phases in Ba-122 Iron Arsenides J.P.C. Ruff, Z. Islam, R.K. Das, H.-H. Kuo, I.R. Fisher Despite widespread interest, there are details of the tetragonal-orthorhombic structural phase transition in the iron arsenide superconductors that remain controversial. We have revisited the transition in three characteristic compositions of the canonical ``122'' family Ba(Fe/Co)$_2$(As/P)$_2$ using single crystal synchrotron x-ray diffraction. In the parent compound, we confirm previous observations of a sequence of structural transitions which are closely spaced in temperature, and uncover pronounced magnetoelastic effects in the intermediate orthorhombic phase. Modification of the structural transitions by doping is observed to differ significantly depending on whether the dopant is Co or P. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A37.00012: Effects of the adjacent antiferromagnetic layer on superconductivity for the case of K$_{y}$Fe$_{2-x}$Se$_{2}$ Shin-Ming Huang, Chung-Yu Mou, Ting-Kuo Lee A mesoscopic phase separation of superconductivity and antiferromagnetism has been recently reported as a prominence in ternary iron selenides. The iron vacancy is free in the superconducting (SC) segment, but clusters and forms order in the antiferromagnetic (AFM) segment. In this report we use a two-orbital model of one AFM layer coupled with another vacancy-free layer for superconductivity and study the effects of the interlayer coupling and the AFM order on SC instability. The SC instability is evaluated by solving the Bethe-Salpeter equation within a local pairing model. Since two individual layers have different Fermi surface (FS) structures, when coupled the FS topography will change depending on the interlayer coupling and the AFM order. We demonstrate that the superconductivity is more stable when FS sheets are disconnected. Interlayer coupling will deteriorate superconductivity and its effect becomes weak when the AFM moment is saturated. Due to lack of reflection symmetry, the SC gap is highly anisotropic and the presence of accidental nodes on disconnected FS sheets of d-wave superconductivity is possible. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A37.00013: Resonance peak of neutron scattering in iron-based superconductors Seiichiro Onari, Yusuke Ohno, Masahisa Tsuchiizu, Hiroshi Kontani Recently, nematic electronic states had been discovered in various strongly correlated metals such as iron-based superconductors, Sr$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ and heavy fermions. These phenomena originate from the electron-electron correlation, since the lattice distortions are very small. Interestingly, many of these materials exhibit unconventional superconductivity, suggesting that the fluctuations of the nematic order parameter would cause the superconductivity. The origin of the nematic states had been unsolved since they cannot be explained by the mean-field approximation. Here, we study this issue beyond the mean-field approximation. We calculate the vertex correction (VC) for the irreducible susceptibility in various multiorbital Hubbard models, and derive the spin and orbital fluctuations self-consistently [1,2]. Near the magnetic quantum critical point, it is found that strong ferro- and antiferro-orbital fluctuations are induced by the VC in both iron-based superconductors and Sr$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$. The divergence of the ferro-orbital fluctuations presents the orbital nematic state in these materials. [1] S. Onari and H. Kontani, Phys. Rev. Lett. 109, 137001 (2012). [2] Y. Ohno, M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3629. [Preview Abstract] |
Session A38: Focus Session: Instrumentation and Measurement Science for a Sustainable Energy Future
Sponsoring Units: GIMSChair: Eric Palm, National High Magnetic Field Laboratory, Tallahassee
Room: 347
Monday, March 18, 2013 8:00AM - 8:36AM |
A38.00001: Our Sustainable Earth Invited Speaker: Raymond L. Orbach Recent evidence demonstrates that the Earth has been warming monotonically since 1980. Transient to equilibrium temperature changes take centuries to develop, as the upper levels of the ocean are slow to respond to atmospheric temperature changes. Atmospheric CO$_{\mathrm{2}}$ concentrations, from ice core and observatory measurements, display consistent increases from historical averages, beginning in about 1880. They can be associated with the use of coal ecause of the spread of the industrial revolution from Great Britain to the European continent and beyond. The climactic consequence of this human-dominated increase in atmospheric CO$_{\mathrm{2}}$ has been suggested to define a geologic epoch, termed the ``Anthropocene.'' This could be a short term, relatively minor change in global climate, or an extreme deviation that lasts for thousands of years. In order to stabilize global temperatures, sharp reductions in CO$_{\mathrm{2}}$ emissions are required: an 80{\%} reduction beginning in 2050. U.S. emissions have declined sharply recently because of market conditions leading to the substitution of natural gas for coal for electricity generation. Whether this is the best use for this resource may be questioned, but it nevertheless reduces CO$_{\mathrm{2}}$ production by 67{\%} from a coal-fired power plant, well on the way to the 80{\%} reduction required for global temperature stabilization. Current methods for CO$_{\mathrm{2}}$ capture and storage are not cost effective, and have been slow (if not absent) to introduce at scale. This paper describes research into some potentially economically feasible approaches: cost-effective capture and storage of CO$_{\mathrm{2}}$ from injection of flue gas into subterranean methane-saturated aquifers at the surface; fuels from sunlight without CO$_{\mathrm{2}}$ production; and large-scale electrical energy storage for intermittent (and even constant) electricity generating sources. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A38.00002: High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt Invited Speaker: Piotr Zelenay With the growing awareness that the use of platinum needs to either be greatly reduced or completely eliminated from the polymer electrolyte fuel cell (PEFC), non-precious metal catalysts for oxygen reduction reaction (ORR) have received lots of attention in recent years as a possible replacement of Pt and its alloys at the fuel cell cathode. A successful cathode catalyst must combine high ORR activity with good long-term stability -- a major challenge in the strongly acidic environment of the PEFC cathode. In response to the possibly greatest challenge of the PEFC technology, we have developed a family of non-precious metal ORR catalysts capable of minimizing the performance gap to platinum-based catalysts at a cost sustainable for high-power fuel cell applications, possibly including the automotive power plant. The approach utilizes polyaniline (PANI) as a precursor of a carbon-nitrogen template for high-temperature synthesis of catalysts in the presence of transition metals (Fe and/or Co). The most active materials in the group allow for the ORR to occur within ca. 60 mV of the potential delivered by a state-of-the-art carbon-supported Pt catalyst. A distinctive combination of (i) high ORR activity, (ii) unique performance stability for non-precious metal catalysts (more than 700 hours at a fuel cell voltage of 0.4 V), and (iii) excellent four-electron selectivity (H$_{\mathrm{2}}$O$_{\mathrm{2}}$ yield less than 1.0{\%}), make the leading catalyst in this group, PANI-FeCo(3:1), the best overall non-precious metal ORR catalyst studied to date. More recently, we have also focused on better understanding of the active ORR site via the use of advanced surface characterization techniques, such as nuclear resonance vibrational spectroscopy (NRVS), Monte Carlo pre-screening of possible active sites and more advanced DFT modeling of the most likely active-site structures. Combination of the experiment and theory is expected to aide in the rational design of the future ORR catalysts. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A38.00003: Modulated photocurrent spectroscopy of thin film solar cells Behrang Hamadani, John Roller, Panagiotis Kounavis, Nikolai Zhitenev, David Gundlach We used the modulated photocurrent spectroscopy technique based on sinusoidal excitation of high-powered LEDs to investigate the dynamic response of charge carrier transport in thin film solar cells based on CdTe. The impact of light bias, voltage bias and the temperature over a broad excitation frequency bandwidth were studied. The observed features of the data, including a photocurrent phase-lead and a phase-lag over different regions of the frequency spectrum, were explored in the context of an equivalent circuit model. Comparisons between the model's predicted performance and the measured data suggest that charge carrier recombination at the cell's back metal/semiconductor contact is the main source of photocurrent loss in the cells that were investigated by our group. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A38.00004: Ultrasensitive spectroscopy of ultrasmall quantum dots for energy conversion and lighting applications Lloyd Davis, Noah Orfield, Sandra Rosenthal Quantum dots typically have narrow spectra with a peak that tunes with their size but ultrasmall semiconductor nanocrystals of diameters less than a few nanometers have size-independent spectra and many other strikingly different properties. One especially interesting feature is that ultrasmall CdSe nanocrystals emit an almost pure white-light spectrum, which has great potential for solid-state lighting that yields excellent color rendering. To gain understanding of the photophysical properties and mechanisms for broadband emission, we have constructed a modular fluorescence microscope for ultrasensitive spectroscopy of individual nanoparticles. Using 400-nm wide-field excitation from a frequency-doubled Ti-Sapphire laser and a high-efficiency electron-multiplying CCD, we observe that single CdSe nanocrystals exhibit blinking and abrupt photobleaching, often after detection of only a few hundred photons. Moreover, spectrally dispersed imaging shows that each particle emits the entire broadband spectrum. We discuss mechanisms for homogeneous broadband emission and ongoing experiments in which the instrument is configured for scanning, confocal, two-channel, time-resolved single photon counting for studies of photon antibunching, emission lifetimes, and correlations between spectral regions. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A38.00005: Measuring Building Insulation Beth Parks Currently, the only way for homeowners to learn about the effectiveness of their home insulation is to hire an energy auditor. This difficulty deters homeowners from taking action to improve energy efficiency. In principle, measuring the temperature difference between a wall surface and the interior of a home is sufficient to determine the wall insulation, but in practice, temperature cycles from the heating system make a single measurement unreliable. I will describe a simple and inexpensive thermocouple-based device to measure this temperature difference and report results obtained by monitoring this temperature difference over multiple heating cycles in a range of buildings. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A38.00006: Optimized Electronic Transport Measurements in Titanium Oxide Jeffrey Lindemuth Titanium Oxide is a material with applications in thermal electric and solar cell applications. Measurement of electronic transport properties by standard methods, for instance Hall effect are made difficult by the low mobility of the material and coupled with the thermal electric properties of the material. Measurements of the resistivity and Hall effect are optimized to reduce the thermal electric effects on the measurement. The Hall measurement is further optimized, by use of AC field Hall method, to obtain reliable mobility values and carrier type determination. Optimization of the measurement includes noise reduction and repeatability of the measurement. Both constant temperature and room temperature measurements are used in the optimization method. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A38.00007: Neutron scattering studies of glassy Li$^{+}$ superionics Tom Heitmann, Leo Zella, Ali Zaidi, Munesh Rathore, Anshuman Dalvi, Saibal Mitra Two distinct neutron scattering techniques were implemented in the study of glassy superionic materials composed of a complex network of their interconnected sub-units: Li$_2$O, NH$_4$H$_2$PO$_2$, and Li$_2$SO$_4$. The use of disordered materials underlies an effort to promote Li$^{+}$ mobility, while suppressing e$^{-}$ conductivity, which makes them good candidates for use as electrolytes in lithium ion batteries. We present triple-axis spectrometer results of energy resolved vs. energy integrated neutron scattering that indicate the presence of a broad range of dynamic processes in the materials, rather than well-defined excitations. Additionally, we report on neutron diffraction data that demonstrates the formation of crystallites within the material upon annealing up to 450 $^{\circ}$C. Such crystallites hinder the performance of the materials as electrolytes, which is evident in thin film devices where heating is unavoidable during fabrication. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A38.00008: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 10:24AM - 10:36AM |
A38.00009: Lithiation of UHV-prepared CoO Conversion Battery Materials Studied by XPS and TEM Ryan Thorpe, Sylvie Rangan, Robert Bartynski, Mahsa Sina, Frederic Cosandey Lithium-ion conversion batteries can store 2-3 times more energy than intercalation batteries by fully reducing their constituent divalent or trivalent transition metal compounds during discharge. A prototypical conversion compound is CoO, which follows the reaction 2Li$^{+}$ + 2e$^{-}$ + Co$^{(2+)}$O $\rightarrow$ 2Li$_{2}$O + Co$^{(0)}$ upon discharge. However, the cycling stability of conversion electrodes is poor, and capacity losses have prevented their implementation. To study the electronic and morphological changes that occur during the conversion reaction, we have grown 5 nm polycrystalline and epitaxial CoO films and exposed them to atomic Li in UHV to simulate cell discharge. Using XPS to monitor the valence state of Co and film stoichiometry, we find that at 25$^{\circ}$C this reaction is inhibited by the formation of a Li$_{2}$O$_{2}$ overlayer, which is a kinetic barrier for Li diffusion. This is alleviated by heating the film to 150$^{\circ}$C, thereby enhancing Li diffusivity through the overlayer and enabling complete reduction of the film. Epitaxial films are reduced with less Li than is required by polycrystalline films, suggesting the presence of channels through which Li is able to diffuse. In both cases, no cobalt phases other than CoO and Co are observed. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A38.00010: In situ Measurements of the Solid Electrolyte Interphase in Li-Ion Batteries Using Neutron Reflectometry Joseph Dura, Jeanette Owejan, Steven DeCaluwe, Jon Owejan The huge advantages of Li-ion batteries, i.e. high energy density and specific power are due not only to the low mass of Li, but also a direct result of the high operating voltage provided by the large electrochemical potential of Li. However, these advantages come at a cost, as all known electrolytes are unstable at these potentials. Li-ion batteries are only made possible by the solid electrolyte interphase, SEI, a passivation layer that forms from the decomposition products of certain electrolytes. Ideally the SEI offers sufficient electronic resistance when it has grown thick enough to stop additional electrolyte decomposition. However, slow continued SEI growth leads to capacity fade and increased cell resistance. Despite the SEI's critical significance, currently structural characterization is incomplete because of the reactive and delicate nature of the SEI and the electrolyte system in which it forms. Here we present the first in situ neutron reflectometry measurements of the SEI layer as function of potential in a working lithium half-cell. The SEI layer after 10 and 20 CV cycles is 4.0 and 4.5 nm, respectively, growing to 8.9 nm after a series of potentiostatic holds that approximates a charge/discharge cycle. Specified data sets show uniform mixing of SEI components. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A38.00011: Low temperature MRFM probe development and initial characterization of organic solar cells Mark Monti, Dimitri Alexson, Doran Smith We report on the construction of a Magnetic Resonance Force Microscope (MRFM) for organic solar cell characterization. Organic bulk-heterojunction solar cells (OSCs) consist of a blend of two organic semiconductors- an electron donating polymer and an electron accepting fullerene. The efficiency of blended OSCs is highly dependent on the phase separation between the donor and acceptor materials. MRFM offers a unique toolset to study OSCs with the potential to gain insight into the morphology of the buried heterostructure on an actual device. The MRFM probe will operate at 4K and up to 9T using force gradient detection of magnetic resonance via an ultra sensitive single crystal silicon cantilever. We plan on performing NMR spectroscopy on OSCs using a shuttling technique whereby the sample is shuttled far from the gradient magnetic particle during the encoding portion of the NMR RF pulses. We will present on the status of the probe development and on our initial experiments on organic solar cells. [Preview Abstract] |
Session A39: Focus Session: Materials in Extremes: Theory and Simulations
Sponsoring Units: GSCCM DCOMP DMPChair: Aleksey Kolmogorov, Binghamton University-SUNY
Room: 348
Monday, March 18, 2013 8:00AM - 8:36AM |
A39.00001: Aluminum/water reactions under extreme conditions Invited Speaker: Joseph Hooper We discuss mechanisms that may control the reaction of aluminum and water under extreme conditions. We are particularly interested in the high-temperature, high-strain regime where the native oxide layer is destroyed and fresh aluminum is initially in direct contact with liquid or supercritical water. Disparate experimental data over the years have suggested rapid oxidation of aluminum is possible in such situations, but no coherent picture has emerged as to the basic oxidation mechanism or the physical processes that govern the extent of reaction. We present theoretical and computational analysis of traditional metal/water reaction mechanisms that treat diffusion through a dynamic oxide layer or reaction limited by surface kinetics. Diffusion through a fresh solid oxide layer is shown to be far too slow to have any effect on the millisecond timescale (even at high temperatures). Quantum molecular dynamics simulations of liquid Al and water surface reactions show rapid water decomposition at the interface, catalyzed by adjacent water molecules in a Grotthus-like relay mechanism. The surface reaction barriers are far too low for this to be rate-limiting in any way. With these straightforward mechanisms ruled out, we investigate two more complex possibilities for the rate-limiting factor; first, we explore the possibility that newly formed oxide remains a metastable liquid well below its freezing point, allowing for diffusion-limited reactions through the oxide shell but on a much faster timescale. The extent of reaction would then be controlled by the solidification kinetics of alumina. Second, we discuss preliminary analysis on surface erosion and turbulent mixing, which may play a prominent role during hypervelocity penetration of solid aluminum projectiles into water. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A39.00002: Development of a Reactive Force Field for Shock-Induced Chemistry in Ti/B Nanocomposite Jason Quenneville A ReaxFF reactive force field is under development for describing the physics and chemistry of Ti/B mixtures under shock compression. In this presentation, we will summarize the parameterization of the force field for the reactants and the most stable product of the reaction, TiB$_{2}$ in the P6/mmm space group. We will describe the behavior of crystalline TiB$_{2}$ under uniaxial and hydrostatic compression and the structure of the crystal with varying void densities as calculated with periodic DFT. In addition, we will compare the results obtained for these properties and others ($e.g.$, lattice constants, elastic constants, bulk modulus) with the newly developed ReaxFF force field. The force field developed in this work for TiB$_{2}$ is combined with Ti and B ReaxFF force fields developed previously to yield a force field suitable for describing shock-induced reactions of Ti and B. Preliminary molecular dynamics studies will also be detailed. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A39.00003: Rotational defects and plastic deformation in molecular crystal RDX Anirban Pal, Catalin Picu Defects in molecular crystals differ in many aspects from their atomic counterparts. Molecules in the crystal lattice can undergo conformational changes or twist and rotate into various configurations during deformation. These processes play an important role in the mechanics at a larger scale by controlling critical parameters like dislocation mobility. We present a computational study of such processes in cyclo-trimethylene-trinitramine (RDX), an energetic molecular crystal. Conformational changes, rotational defects and their role in the deformation mechanics of RDX is investigated using molecular dynamics simulations. Structure and mobility of dislocations are also presented and role of conformational and rotational defects in dislocation mobility is discussed. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A39.00004: Electronic stopping power from ab-initio Ehrenfest molecular dynamics Andre Schleife, Yosuke Kanai, Alfredo Correa Many materials are exposed to particle radiation: Metal walls of nuclear reactors in fission systems are subject to ion bombardment. Solar cells and semiconductor components in satellites are damaged by ions from cosmic rays. In order to achieve high radiation tolerance, it is essential to comprehend the interaction of fast projectiles with the ionic and electronic system of the target at a fundamental level. Based on the real-time propagation of time-dependent Kohn-Sham equations we developed a highly parallel plane-wave implementation of non-adiabatic Ehrenfest molecular dynamics, overcoming the adiabatic Born-Oppenheimer approximation. Thanks to the excellent scalability of our explicit integration scheme on supercomputers, it allows for the parameter-free computation of electronic stopping with hundreds of atoms in the calculation. We summarize our approach with some attention to important computational details. The influence of different charge states of H, He, and Li projectiles penetrating an Al target will be outlined. While we find good agreement with experiment up to the maximum of electronic stopping, deviations for high velocities are discussed in the light of the theoretical framework and off-channeling effects. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A39.00005: Simulations of nonequilibrium warm dense gold produced by ultrafast heating B. Holst, V. Recoules, M. Torrent, Z. Chen, V. Sametoglu, Y.Y. Tsui, S.E. Kirkwood, M. Reid, S. Mazevet, A. Ng The interaction of femtosecond laser pulses with metals produces nonequilibrium states consisting of hot electrons and cold ions. These can last for many picoseconds before relaxing to a thermodynamic equilibrium. Recent experiments using a chirped pulse probe technique provided AC conductivity data of gold at a sufficient time resolution to observe this relaxation process. We developed an ab-initio model that characterizes thermodynamic properties of warm dense matter states in nonequilibrium. Our theoretical scheme combines a standard two temperature model with temperature dependent material parameters and an energy transfer rate that are obtained by means of ab-initio simulations. This enables us to give a prediction for the temperature evolution during the relaxation process. Additionally, we derive the AC conductivity of the nonequilibrium states from our simulations using the Kubo-Greenwood formula. It is used to test our model against measurements. We observe agreement with experiment using an energy relaxation rate, that is smaller than predicted, giving us reason to revisit its determination. We can furthermore provide thermodynamical and structural data of nonequilibrium warm dense gold which are not accessible in experiment. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A39.00006: Finite-temperature orbital-free GGA molecular dynamics for warm dense hydrogen Valentin Karasiev, T. Sjostrom, S.B. Trickey The computational description of warm dense matter (WDM) by means of a combination of the Kohn-Sham (KS) finite-temperature density functional theory (DFT) for the electrons and classical molecular dynamics (MD) for the ions becomes an intractable task at high $T$ (typically a few hundred kK). Finite-temperature orbital free DFT (OF-DFT) is a less expensive alternative. Only two non-interacting free-energy functionals for OF-DFT had been published and used until recently: the finite-temperature Thomas-Fermi (ftTF) model (Feynman {\it et al.}, 1949) and ftTF with second-order gradient corrections (ftSGA) (Perrot, 1979). Here we report first results of OF-DFT MD simulations for warm dense H with a pair of newly developed ftGGA free energy functionals [1] for the non-interacting kinetic energy and entropy. The equation of state from these new functionals shows much better agreement with the reference KS MD results than results from the ftTF and ftSGA models. Other issues, {\it e.g.} convergence of the OF self-consistent procedure, also will be discussed.\\[4pt] [1]. V.V.\ Karasiev, T.\ Sjostrom and S.B.\ Trickey, Phys.\ Rev.\ B {\bf 86}, 115101 (2012). [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A39.00007: Temperature Dependence of the Kinetic Energy of the Correlated Electron Plasma by Restricted Path-Integral Molecular Dynamics Keith Runge, Pierre Deymier Recent progress in orbital-free Density Functional Theory (OF-DFT), particularly with regard to temperature dependent functionals, has promise for the simulation of warm dense matter (WDM) systems. WDM includes systems with densities of an order of magnitude beyond ambient or more and temperatures measured in kilokelvin. A challenge for the development of temperature dependent OF-DFT functionals is the lack of benchmark information with temperature and pressure dependence on simple models under WDM conditions. We present an approach to fill this critical gap using the restricted path-integral molecular dynamics (rPIMD) method. Electrons are described as harmonic necklaces within the discrete path integral representation while quantum exchange takes the form of cross linking between electron necklaces. A molecular dynamics algorithm is used to sample phase space and the fermion sign problem is addressed by restricting the density matrix to positive values. The temperature dependence of kinetic energies for the strongly coupled electron plasma is presented for a number of Wigner-Seitz radii in terms of a fourth order Sommerfeld expansion. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A39.00008: Path Integral Simulations of Heavy, Warm Dense Matter Kevin Driver, Burkhard Militzer We develop an all-electron path integral Monte Carlo (PIMC) method for warm dense matter composed of elements with core electrons. For several second- and third-row elements, PIMC pressures, internal energies, and pair-correlation functions compare well with density functional theory molecular dynamics (DFT-MD) at low temperatures and enable the construction of coherent equations of state over a wide range of temperatures and densities. Details of the method and results will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A39.00009: Atomistic and first principles studies of Si nanoparticles under pressure Maria Chan, Daniel Hannah, Richard Schaller In this talk, we will discuss the structural, optical and electronic properties of silicon nanoparticles under high pressure, obtained using a combination of classical molecular dynamics and first principles density functional theory calculations. The results will be corroborated with experimental findings. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A39.00010: Anharmonic Phonons in Complex Systems: Application to MgSiO3-Perovskite Dong-Bo Zhang, Tao Sun, Renata Wentzcovitch We propose a strategy to capture phonon frequency renormalization due to phonon-phonon interactions included in molecular dynamics simulations (self-consistent phonons). This strategy is effective irrespective of crystal structure complexity and facilitates the Fourier interpolation of anharmonic frequencies throughout the Brillouin zone. Calculation of anharmonic frequency shifts in MgSiO$_{3}$-perovskite validates the method by reproducing well irregular thermal shifts measured by Raman spectroscopy at ambient conditions. \textit{Research supported by NSF/EAR} [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A39.00011: Phase stability in pulsar and magnetar crusts Tyler Engstrom, Vincent Crespi, Benjamin Owen, James Brannick, Xiaozhe Hu The outermost several hundred meters of a neutron star crust is similar to a white dwarf interior, consisting of nuclei screened by a relativistic, degenerate electron gas. Free neutrons don't appear until a density of $4\times10^{11}$ g/cc. Below a depth of several tens of meters, corresponding to $10^6$-$10^8$ g/cc, the nuclei are thought to crystallize. Unlike white dwarfs, most observed neutron stars have enormous magnetic fields. On the surface of a typical pulsar, the field is $\sim 10^{12}$ gauss, while for magnetars it is several orders of magnitude stronger. Sub-surface fields are likely to be of a similar or greater strength. Quantum ab-initio methods for this regime are still in a state of infancy. In this talk we describe a solution of the nonlinear Thomas-Fermi PDE for completely degenerate, super-strongly magnetized electrons, using a domain decomposition technique with boundary conditions appropriate to close-packed lattices of nuclei. Excited Landau levels are included in the model. Our numerical method makes use of Hypre multigrid-preconditioned solvers. Equation of state and phase diagram calculations will be presented, and implications for astrophysical observations discussed. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A39.00012: Magnetic Evolution of the \textless 100\textgreater\ Interstitial Loop Formation Process in bcc Iron Haixuan Xu, Roger Stoller, G. Malcolm Stocks Interstitial loops are a signature of radiation damage in materials and are only observed in systems far from equilibrium state due to their high formation energies (approximately 4eV). Unlike other bcc metals, in which the interstitial loops are almost exclusively $\frac{1}{2}$ \textless 111\textgreater\ type, two types of loops, \textless 100\textgreater and $\frac{1}{2}$ \textless 111\textgreater\, are identified in bcc iron. Although $\frac{1}{2}$ \textless 111\textgreater\ loops can be formed directly by atomic displacment cascades, the mechanism of \textless 100\textgreater\ loop formation had remained undetermined since they were observed fifty years ago. Recently, the formation mechanism has been discovered using self-evolving atomistic kinetic Monte Carlo (SEAKMC) simulations. Here we describe the influence of magnetism in the corresponding loop formation process using the \textit{ab initio} locally self-consistent multiple-scattering (LSMS) method. Significant magnetic moment changes during the loop formation process are observed and their effect on the loop stability are evaluated. In addition, the effects of \textless 100\textgreater\ loop formation on the microstructural evolution and material properties will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A39.00013: Using numeric simulations to inform experimental data analysis: A new method to account for characteristic bending under dynamic loading and release Scott Alexander, Justin Brown Dynamic high pressure experiments are often subject to unwanted wave interactions such as reflections from window interfaces or free surfaces where there is an impedance mismatch. In ramp loading experiments or under shock loading of materials resulting in a complex wave structure, these wave interactions can result in changes to the observed wave speeds. This effect, known as characteristic bending, can lead to significant errors in the measured material properties if not properly accounted for. Several approaches exist to correct for characteristic bending, however, they are limited to a one-to-one material response. New methodology has been developed based on control system theory to correct for characteristic bending without this limitation. By comparing simulated \textit{in-situ} and window (or free surface) data, a transfer function is defined which captures the effects due to wave interactions. Application of this function to the experimental data results in \textit{in-situ} profiles free from perturbations due to wave interactions. Experimental data, both with and without strong characteristic bending present, will be presented to illustrate the utility of this new approach. [Preview Abstract] |
Session A40: Focus Session: Collective Diffusion and Self Organization
Sponsoring Units: DMPChair: Michael S. Altman, Hong Kong University of Science and Technology
Room: 349
Monday, March 18, 2013 8:00AM - 8:12AM |
A40.00001: Evidence for Collective Motion in LEEM Measurements of Metals on Semiconductors Shirley Chiang, Yu Sato, Jason Giacomo, Cory Mullet, Marshall van Zijll, Bret Stenger, Dylan Lovinger We review evidence for collective motion from LEEM measurements of three metal on semiconductor systems: Pb/Ge(111), Au/Ge(111), and Ag/Ge(110). Pb/Ge(111) shows a novel phase separation with fluctuating domains of $\sim$ 100nm size which spontaneously switch back and forth from the (1x1) to $\beta $ phase in the region of the phase diagram where the two phases coexist. This striking mechanism occurs because nm-scale domains can have thermally-induced density fluctuations comparable to the density difference between the two phases (PRL, \underline {99}, 096103 (2007)). Au/Ge(111) also shows evidence for fluctuating domains between the ($\surd $3x$\surd $3)R30 and (1x1) phases, both for small domains of 100nm diameter, and at the edges of large domains on a sample with low step density. LEEM movies also show ``hopping'' of large islands (tens to hundreds of nm in diameter) of Au on Ge(111). Self-assembly of large one dimensional (1D) islands (1-10 $\mu $m x 70-140 nm, $\sim$ 7nm high, for 7ML) along the [1,-1,0] direction occurs for Ag grown from 300-530 $^{\circ}$C on Ge(110). During the growth process, such 1D islands have been observed to collapse into other islands and defects in \textless\ 1sec. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A40.00002: Super-diffusive Motion of the Pb Wetting Layer on the Si(111) Surface M.S. Altman, K.L. Man, M.M.T. Loy, M.C. Tringides An unusual mass tansport behavior has been observed in the dense Pb wetting layer on the Si(111) surface. Mass transport is studied by observing non-equilibrium coverage profile evolution with low energy electron microscopy and micro-low energy electron diffraction (m-LEED). The strong sensitivity of diffraction features to Pb coverage in this system allows the Pb coverage profile to be determined precisely with high spatial resolution using m-LEED. Equilibration of an initial coverage step profile produced by laser induced thermal desorption proceeds by the exchange of mass between two steep coverage gradients that travel in opposite directions with invariant shapes. The coverage profile between these two moving edges unexpectedly exhibits a concave shape that apparently contradicts local mass conservation given by the continuity equation. The equilibration time is independent of Pb coverage above a critical coverage, 1.24 ML, but diverges sharply below. The observed spatio-temporal characteristics and lower cutoff for fast dynamics may signal a very unusual coverage dependence of diffusion or might suggest an exceptional collective super-diffusive mechanism by which diffusion is not driven by the local coverage gradient in the usual way. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A40.00003: Incorporation-limited growth of molecular film: Zn-phthalocyanine on Ag(100) Jerzy Sadowski, Abdullah Al-Mahboob Metal phthalocyanines draw considerable attention due to their potential for energy and environmental applications. However, the control of film fabrication, necessary for achieving optimized performance of organic devices, remains a challenge. Here, we present results of low-energy electron microscopy (LEEM) studies of incorporation-limited growth mechanism of zinc-phthalocyanine (ZnPc) thin films on Ag(100) substrate. We found that when ZnPc is deposited on a substrate kept at lower temperature (RT to 430K), the resulting film has a double domain epitaxial structure, resembling that of bulk ZnPc, with domain sizes in the sub-micrometer range. At above 440K, the film ordering changes into a 5x5 commensurate structure, having only a single crystalline orientation. In situ LEEM of the nucleation and growth of the film allowed us to find a crossover of equilibrium concentration of diffusing molecules versus temperature for both adsorption phases, and thus to tune the type of nucleation. We determined that at favorable growth conditions, the 5x5 phase undergoes a thermodynamic phase transition into the bulk-like structure and we were able to obtain ZnPc bulk-like films with unprecedented quality, with single crystalline domain sizes in the range of millimeters. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A40.00004: Modeling Patterning of Heteroepitaxial Overlayers from Nano to Micron Scales Invited Speaker: Tapio Ala-Nissila Thin heteroepitaxial overlayers have been proposed as templates to generate stable, self-organized nanostructures at large length scales, with a variety of important technological applications. However, modeling strain-driven self-organization is a formidable challenge due to a large span of length and time scales involved. In this talk, I will present a method for predicting the patterning of ultrathin films on micron length scales with atomic resolution [K.R. Elder {\it et al.}, Phys. Rev. Lett. {\bf 108}, 226102 (2012)]. It is based on the Phase-Field Crystal model, which allows one to reach diffusive time scales for relaxation of the system. We make quantitative predictions for the type of superstructures (stripes, honeycomb, triangular) and length scales of pattern formation of both compressively strained and tensile overlayers on metal-metal systems, including Cu on Ru(0001), Cu on Pd(111), and Ag on Cu(111). Our findings are in excellent agreement with previous experiments and call for future experimental investigations of such systems. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A40.00005: Dewetting of Epitaxial Silver Film on Silicon by Thermal Annealing Charlotte E. Sanders, Gary L. Kellogg, C.-K. Shih It has been shown that noble metals can grow epitaxially on semiconducting and insulating substrates, despite being a non-wetting system: low temperature deposition followed by room temperature annealing leads to atomically flat film morphology. However, the resulting metastable films are vulnerable to dewetting, which has limited their utility for applications under ambient conditions. The physics of this dewetting is of great interest but little explored. We report on an investigation of the dewetting of epitaxial Ag(111) films on Si(111) and (100). Low energy electron microscopy (LEEM) shows intriguing evolution in film morphology and crystallinity, even at temperatures below 100$^{\mathrm{o}}$C. On the basis of these findings, we can begin to draw compelling inferences about film-substrate interaction and the kinetics of dewetting. Financial support is from NSF, DGE-0549417 and DMR-0906025. This work was performed, in part, at the Center for Integrated Nanotechnologies, User Facility operated for the U.S. DOE Office of Science. Sandia National Lab is managed and operated by Sandia Corp., a subsidiary of Lockheed Martin Corp., for the U.S. DOE's National Nuclear Security Administration under DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A40.00006: Dewetting of nanometer-sized thin films on a solid substrate: A large-scale simulation study Trung Nguyen, Miguel Fuentes-Cabrera, Jason Fowlkes, Javier Diez, Alejandro Gonzalez, Lou Kondic, W. Michael Brown, Philip Rack Directing the assembly of nanoparticles into ordered arrays using interfacial instability has been of practical interest. Recent experimental and theoretical studies have revealed the role of the Raleigh-Plateau instability in determining the breakup process of fluidic thin films deposited on a solid substrate. Using all-atom models, we investigate the dynamic behavior of nanometer-sized thin rings as a function of initial geometry in the presence of Raleigh-Plateau-type instability and inward pressure due to initial azimuthal curvature. We consider systems at close-to-experiment scales consisting of hundreds of thousands to millions of atoms using LAMMPS, a massively parallel molecular dynamics package, with GPU acceleration. The simulation results are shown to be consistent with continuum modeling calculations in predicting the fastest growth mode and breakup times, both of which are important to the evolution of the thin films. Our study serves to stimulate future investigations connecting experimental and theoretical findings towards fabricating ordered arrays of nanoparticles. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A40.00007: Fe on graphene: interaction, growth morphology, and thermal stability Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Hai-Qing Lin, Kai-Ming Ho, Michael Tringides The nucleation and growth of Fe on graphene is highly unusual. Constantly increasing in island density with coverage is observed by experiment which indicates the presence of strong adatom predominantly repulsive interactions. We study Fe adatoms interactions on graphene by first-principles calculations and showed that the interactions between Fe adatoms consist of a short-range attraction and long-range repulsions. By investigating the adsorption energies and diffusion barriers for Fe adatoms on graphene, we also predict that Fe on graphene exhibit a three-dimensional growth mode. Fe nanostructures on graphene are also shown be stable against aggregation. The predictions from first-principles calculations are consistent with experimental observations. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A40.00008: Distribution of Steps with Finite-Range Interactions: Analytic Approximations and Numerical Results Diego Luis Gonz{\'a}lez, Diego Felipe Jaramillo, Gabriel T{\'e}llez, T.L. Einstein While most Monte Carlo simulations assume only nearest-neighbor steps interact elastically, most analytic frameworks (especially the generalized Wigner distribution) posit that each step elastically repels all others. In addition to the elastic repulsions, we allow for possible surface-state-mediated interactions. We investigate analytically and numerically how next-nearest neighbor (NNN) interactions and, more generally, interactions out to q'th nearest neighbor alter the form of the terrace-width distribution and of pair correlation functions (i.e.\ the sum over n'th neighbor distribution functions, which we investigated recently.\footnote{D.L. Gonz{\'a}lez, A. Pimpinelli, \& TLE, Phys. Rev. E 88, 011151 (2012)} For physically plausible interactions, we find modest changes when NNN interactions are included and generally negligible changes when more distant interactions are allowed. We discuss methods for extracting from simulated experimental data the characteristic scale-setting terms in assumed potential forms. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A40.00009: Developments in Characterizing Capture Zone Distributions in Island Growth T.L. Einstein, Alberto Pimpinelli, Diego Luis Gonz{\'a}lez, Rajesh Sathiyanarayanan The utility of using the distribution of capture zones (CZD) to characterize epitaxial growth continues to mount. For non-Poisson deposition (i.e.\ when island nucleation is not fully random) the areas of these Voronoi cells (proximity polygons) can be well described by the generalized Wigner distribution (GWD), particularly in the central region around the mean area. We discuss several recent applications to experimental systems, showing how this perspective leads to insights about the critical nucleus size. In contrast, several studies have shown that the GWD may not describe the numerical data from painstaking simulations in both tails. We discuss some refinements that have been proposed. Finally, we comment on applications to social phenomena such as area distributions of secondary administrative units (like counties) and of Voronoi cells around Metro stops. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A40.00010: Magicity of Transition Metal Nanoclusters Based on Generalized Wulff Construction Shunfang Li, Xingju Zhao, X.S. Xu, Y.F. Gao, Zhenyu Zhang Nanoclusters with extra stability at certain cluster sizes are known as magic clusters which may exhibit exotic properties. Two dominant mechanisms have been invoked to define the magicity: electronic shell closure for simple and noble metal clusters, and atomic shell closure for rare-gas and other clusters. The latter mechanism is inherently rooted in the classic Wulff construction, which stipulates that the preferred structure of a cluster should minimize its total surface energy, resulting in close-shelled icosahedronal transition metal (TM) clusters with magic sizes of 13, 55, and 147. Here we use TM clusters around 55 as examples to demonstrate that the Wulff construction must be generalized to also include the contribution of edge atoms. Specifically, a majority of the TM$_{55}$ clusters are found to be fcc or hcp crystal fragments with much fewer edge atoms than the icosahedrons, and the magic number is shifted to its nearby even numbers. The generalized Wullf construction established here should be instrumental in fabricating nanoclusers with desirable functionalities. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A40.00011: Magicity of Ag nanoclusters on Si(111)-(7$\times$7) by atomic manipulation Fangfei Ming, Guohua Zhong, Kedong Wang, Zhenyu Zhang, Xudong Xiao Nanoclusters with extra stability at certain cluster sizes are known as magic clusters, whose magicity depends sensitively on the environments. Using scanning tunneling microscopy and first-principles calculations, we explore the dynamics and magicity of Agn (n$=$1-26) clusters constructed atom-by-atom on a Si(111)-(7$\times$7) surface. By measuring the thermal stability of clusters of increasing size, a set of magic clusters are distinctly established, which in return helps to reveal the preferred growth sequence towards geometrically close-shelled Ag10 and Ag25 clusters with extra inertness. We further use a probing atom to demonstrate that the adatom-cluster interaction is highly anisotropic, preserving the attractive nature of an Ag-Ag bond at short distances, but becoming repulsive at large distances mediated by the substrate. These innovative findings of fundamental importance are also expected to be significant in surface catalytic reactions and related technological areas. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A40.00012: Atomic Force Microscopy of Vertically Stacked Focused-Ion-Beam Induced Quantum Dots Marta Luengo-Kovac, Timothy Saucer, Andrew Martin, Joanna Millunchick, Vanessa Sih Control over the positioning of semiconductor quantum dots (QDs) could facilitate the coupling of QDs to photonic crystal cavities and has applications in the development of high-efficiency solar cells. QDs grown through self-assembly nucleate at random spatial locations. However, a focused ion beam (FIB) can be used to create preferential sites for QD nucleation, and this pattern can be transferred to subsequent layers of QDs, either due to strain or residual effects of the templating. Multilayer QD stacks can therefore maintain the lateral pattern of the initial layer while separating QDs from material damage induced by the patterning. Multilayer QD structures were grown on FIB-patterned GaAs(001) substrates with 10 nm thick GaAs spacers between the layers. The substrates were patterned with sixteen square arrays of holes with spacings of 0.25, 0.5, 1.0, and 2.0 $\mu$m each at FIB dwell times of 1.0, 3.0, 6.0, and 9.0 ms. We report on the effects of multilayer QD growth on the initial layers through atomic force microscope (AFM) imaging of single, two-, and three-layer FIB-templated QD samples. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A40.00013: Kinetic Monte-Carlo Simulation of Substrate Vacancy Diffusion in C$_{60}$ on Ag(111) Joseph Dulny III, Sangzi Liang, John Groh, Jorge Sofo, Renee Diehl Recently, clean Ag(111) surfaces with monolayer C$_{60}$ adsorbates have been studied with scanning tunneling microscopy and low energy electron diffraction. These studies revealed that the C$_{60}$ forms a commensurate $(2\sqrt{3}\times2\sqrt{3})R30^\circ$ phase on the Ag(111) substrate and when observed with STM, the C$_{60}$ molecules appear either ``bright'' or ``dim.'' LEED studies showed that these two species of C$_{60}$ are a result of the C$_{60}$ taking two different orientations on the Ag substrate, one of which only occurs when the C$_{60}$ is located over an Ag lattice vacancy. STM also shows the bright and dim C$_{60}$ molecules change location over time. This ``flipping'' behavior implies that vacancy diffusion in the Ag lattice is taking place. Here, using the kinetic Monte-Carlo algorithm, we model the diffusion of vacancies in the Ag lattice. Data collected from simulations is compared to experimental data on the flipping rate of the C$_{60}$ vs. temperature and the bright/dim C$_{60}$ ratio vs. temperature. Our model tells us that intralayer vacancy diffusion is taking place and that adsorption of C$_{60}$ on Ag(111) results in vacancy creation in the Ag(111) surface. Additional density functional theory calculations support the conclusions of the model. [Preview Abstract] |
Session A41: Localization, Cooling, Trapping and Clocks
Sponsoring Units: DAMOPChair: Matthew Beeler, University of Maryland
Room: 350
Monday, March 18, 2013 8:00AM - 8:12AM |
A41.00001: Theory of interaction-induced localization for mobile impurities Jian Li, Jin An, Chin-Sen Ting A phenomenological model is proposed for the interaction-induced localization of mobile impurities in the cold atomic systems. The fundamental properties of the transition between the extended and localized impurity state in one, two and three dimension are investigated with this model. We find that the transition is continuous in one and two dimension while discontinuous in three dimension. We show that the dynamics of single localized impurity is described by a soliton and predict the formation of bipolaron and Wigner lattices for many fermionic impurities. Our theory explains the essential features from specific models in a unified picture and can be used to realize several exotic phenomena with ultracold impurity atoms. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A41.00002: Single parameter scaling for 1d systems with scale-free long-range correlated disordered potentials Nancy Sandler, Greg Petersen Disordered optical lattices have renewed the interest in localization physics under power-law long-range correlated disorder potentials. For these systems, insight can be gained by combining numerical data and analytic expressions based on scaling laws. Thus, the absence of a transition in short-range correlated disordered systems can been proved by verifying the validity of the single parameter scaling (SPS) hypothesis for the distribution function of the dimensionless conductance. In this talk we discuss this hypothesis for a system with scale-free long-range correlated disorder potentials of the form $\sim 1/r^{\alpha}$ as a function of the correlation exponent $\alpha$. We present results for the $1^{st}$ (the $\beta$-function) and $2^{nd}$ (variance) cumulants of the distribution function, and show a violation of SPS at an energy scale $E_{SPS}$, that scales with an $\alpha$-renormalized disorder strength. Calculations for the localization length reveals the existence of a crossover scale $E_{cross}$ between two regions as correlations increase. An increased number of more extended-like states appear near the band-center while states near the band edges experience reduced localization lengths. We confirm previously predicted scaling behavior near the band edge and center. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A41.00003: Anderson localization of pairs in bichromatic optical lattices Giuliano Orso, Gabriel Dufour We investigate the formation of bound states made of two interacting atoms moving in a one dimensional quasi-periodic optical lattice. We derive the quantum phase diagram for Anderson localization of both attractively and repulsively bound pairs. We calculate the pair binding energy and show analytically that its behavior as a function of the interaction strength depends crucially on the nature -extended, multifractal, localized- of the single-particle atomic states. Experimental implications of our results are discussed. Reference: Phys. Rev. Lett. 109, 155306 (2012) [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A41.00004: Many-body energy localization transition in periodically driven system Luca D'Alessio, Anatoli Polkovnikov According to the second law of thermodynamics the total entropy and energy of a system is increased during almost any dynamical process. Notable exceptions are known in noninteracting systems of particles moving in periodic potentials. Here the phenomenon of dynamical localization can prevent heating beyond certain threshold. However, it was believed that driven ergodic systems will always heat without bound. Here, on the contrary, we report strong evidence of dynamical localization transition in periodically driven ergodic systems in the thermodynamic limit. This phenomenon is reminiscent of many-body localization in energy space. We report numerical evidence based on exact diagonalization of small spin chains and theoretical arguments based on the Magnus expansion. Our findings are valid for both classical and quantum systems. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A41.00005: Many-Body Localization in a Quasiperiodic System Shankar Iyer, Gil Refael, Vadim Oganesyan, David Huse Recent theoretical and numerical evidence suggests that localization can survive the introduction of interactions in disordered many-body systems, giving rise to a so-called many-body localization transition. This dynamical phase transition is relevant to questions of thermalization in quantum systems. It separates a many-body localized phase, in which localization prevents thermalization, from an ``ergodic'' phase in which the usual assumptions of quantum statistical mechanics hold. Here, we present numerical evidence that many-body localization also occurs in models that omit true disorder in favor of a quasiperiodic potential. In one dimension, these systems already have a single-particle localization transition, and we show that this transition becomes a many-body localization transition upon the introduction of interactions. These issues are increasingly experimentally relevant, because quasiperiodic potentials have been used in place of true disorder in recent experiments with cold atoms and with photonic waveguides. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A41.00006: Dynamic Localization of Interacting Particles in an Anharmonic Potential Mark Herrera, Thomas Antonsen, Edward Ott, Shmuel Fishman We investigate the effect of anharmonicity and interactions on the dynamics of an initially Gaussian wavepacket in a weakly anharmonic potential. We note that depending on the strength and sign of interactions and anharmonicity, the quantum state can be either localized or delocalized in the potential. We formulate a classical model of this phenomenon and compare it to quantum simulations done for a self consistent potential given by the Gross-Pitaevskii Equation. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A41.00007: Anderson Localization of a non-interacting Bose-Einstein condensate with effective spin-orbit interaction in a quasiperiodic optical lattice Lu Zhou, Han Pu, Weiping Zhang We theoretically investigate the localization properties of a noninteracting atomic Bose-Einstein condensate moving in a one-dimensional quasiperiodic optical lattice potential in the tight-binding regime. The atoms are subject to effective spin-orbit coupling induced by external laser fields. We present the phase diagram in the parameter space of the disorder strength and those related to the effective spin-orbit coupling. The phase diagram are verified via multifractal analysis of the atomic wavefunctions. We found that spin-orbit coupling can lead to the spectra mixing (coexistence of extended and localized states) and the appearance of mobility edges. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A41.00008: Laser Controlled Rotational Cooling in Na$_2$ Based on Exceptional Points Adam Wearne, Viatcheslav Kokoouline, Osman Atabek, Roland Lefebvre In this study, we describe a computational simulation of the interaction of diatomic molecule with an applied laser field. It is known that for certain laser wavelengths and intensities, the wave functions and eigenenergies of two states become degenerate. Such locations in the laser parameter space are known as ``exceptional points.'' By applying a laser pulse which encircles one or more exceptional points in the parametric plane of wave length versus intensity, one can bring an ensemble of diatomic molecule into a pre-selected rovibrational state after the laser pulse is over. During this process, a fraction of the molecules dissociate, and those which remain, are brought to the chosen rovibrational state. Although this scheme can be applied more generally, here we use Na$_2$ as an illustrative example. We examine the locations in the parameteric space of exceptional points, which lead to the exchange of rotational states, and how the shape of laser pulse in the parametric plane affects the ``purification'' of the chosen rovibrational state and the dissociation of other states. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A41.00009: Dynamic dimer formation between superionic fluorines in CaF$_{2}$ Masashi Saito, Tomofumi Tasaka, Kazuo Tsumuraya Recently we have elucidated the formation of the dynamic dimers in the conductor $\alpha $-CuI through the analyses of the correlation peaks of the partial pair-distribution functions and the partial angle distribution functions with the first principles molecular dynamics (MD) method.(J. Phys. Soc. Jpn. 81,055603(2012).) The present study investigate the formation of the dynamic dimers and the migration paths of the dimers in the conductor CaF$_{2}$ with the MD method. The fluorines form the dynamic 32$f$-8$c$ dimers with the coordinate (x,x,x) x=0.300. These incommensurate dimers allow to decrease the migration barriers of the fluorines. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A41.00010: Laser cooling of Iron atoms Thierry Bastin, Nicolas Huet, Stephanie Krins We report on the first laser cooling of Iron atoms. Our laser cooling setup makes use of 2 UV laser radiation sent colinearly in a 0.8~m Zeeman slower. One laser is meant for optical pumping of the Iron atoms from the ground state to the lowest energy metastable state. The second laser cools down the atoms using a quasi-perfect closed transition from the optical pumped metastable state. The velocity distribution at the exit of the Zeeman slower is obtained from a probe laser crossing the atom beam at an angle of 50 degrees. The fluorescence light is detected using a photomultiplier tube coupled with a boxcar analyzer. The Iron atom beam is produced with a commercial effusion cell working at around 1950 K. Our laser radiations are stabilized using standard saturated-absorption signals in both an Iron hollow cathode absorption cell and an Iodine cell. We will present our experimental setup, as well as the first evidences of cooled down Iron atoms at the exit of the Zeeman slower. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A41.00011: Ytterbium in quantum gases and atomic clocks: van der Waals interactions and blackbody shifts S. G. Porsev, M. S. Safronova, Charles W. Clark We evaluated the $C_6$ coefficients of Yb-Yb and Yb-alkali/group II van der Waals interactions with 2\% uncertainty. The only existing results for such quantities are for the Yb-Yb dimer. Our value, $C_6 = 1929(39)$ a.u., is in excellent agreement with the recent experimental determination of 1932(35) a.u. [M. Kitagawa, {\em et al., Phys. Rev. A } {\bf 77}, 012719 (2008)]. We have also developed a new approach for the calculation of the dynamic correction to the blackbody radiation shift. We have calculated this quantity for the Yb $6s^2$ $^1{\rm S}_0 - 6s6p$ $^3{\rm P}^{\circ}_0$ clock transition with 3.5\% uncertainty. This reduces the fractional uncertainty due to the blackbody radiation shift in the Yb optical clock at 300 K to the $10^{-18}$ level. For further details, see http://arxiv.org/abs/1208.1456 [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A41.00012: Blackbody radiation shift in the Sr optical atomic clock M.S. Safronova, S.G. Porsev, U.I. Safronova, M.G. Kozlov, Charles W. Clark We evaluated the static and dynamic polarizabilities of the $5s^2$ $^1$S$_0$ and $5s5p$ $^3{\rm P}^{\circ}_0$ states of Sr using the high-precision relativistic configuration interaction + all-order method. Our calculation explains the discrepancy between the recent experimental $5s^2$ $^1$S$_0$ - $5s5p$ $^3{\rm P}^{\circ}_0$ dc Stark shift measurement = 247.374(7) a.u. [T. Middelmann, S. Falke, C. Lisdat and U. Sterr, arXiv:1208.2848 (2012)] and the earlier theoretical result of 261(4) a.u. [S. G. Porsev and A. Derevianko, {\em Phys. Rev. A} {\bf 74}, 020502(R) (2006)]. Our present value of 247.5 a.u. is in excellent agreement with the experimental result. We also evaluated the dynamic correction to the BBR shift with 1\% uncertainty; -0.1492(16) Hz. The dynamic correction to the BBR shift is unusually large in the case of Sr (7\%) and it enters significantly into the uncertainty budget of the Sr optical lattice clock. We suggest future experiments that could further reduce the present uncertainties. For further information, see http://arxiv.org/abs/1210.7272 [Preview Abstract] |
Session A42: Integer Quantum Hall Effect
Sponsoring Units: FIAPChair: James Tse, University of Texas
Room: Hilton Baltimore Holiday Ballroom 3
Monday, March 18, 2013 8:00AM - 8:12AM |
A42.00001: Study of the correlation between microwave reflection and microwave-induced magnetoresistance oscillations in the GaAs/AlGaAs two dimensional electron system Tianyu Ye, R.G. Mani, W. Wegscheider High frequency microwave illumination produces oscillatory magneto-resistance in the high mobility two dimensional electron systems (2DES) at liquid helium temperatures, in a perpendicular magnetic field. Present theories for this phenomenon include the displacement model and the inelastic model, which have hardly perfectly simulated- or predicted- experimental results such as, for example, the linear microwave polarization dependence of this effect [1]. Besides the usual direct electrical measurement on the 2DES samples, we have examined the microwave reflection to remotely sense the electron-transport in the 2DES, in order to better understand the physical contributions. Here, we compare the concurrently observed direct transport and remotely sensed reflection from the high mobility GaAs/AlGaAs samples under various microwave illumination conditions. Correlated changes between the two types of measurements are reported.\\[4pt] [1] R. G. Mani et al., Phys. Rev. B 84, 085308 (2011); A. N. Ramanayaka et al., Phys. Rev. B 85, 205315 (2012); and references therein. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A42.00002: Study of the phase-shift in the linear-polarization-angle-dependence of the microwave radiation-induced magnetoresistance oscillations in the GaAs/AlGaAs system Han-Chun Liu, Tianyu Ye, R. G. Mani, W. Wegscheider Microwave- and terahertz-induced magneto-resistance oscillations (MTIMRO) in the two-dimensional electron system have been a topic of interest since the observation of photo-excited zero-resistance states in the GaAs/AlGaAs system. Some theoretical developments in this area have been based on the premise of linear-polarization-insensitivity of MTIMRO. Recent studies using new experimental methods have shown, however, a strong linear polarization sensitivity of MTIMRO.[1,2] In addition, Ramanayaka \textit{et al.}[2] have observed that the phase shift $\theta_{0}$, which is a parameter in a fitting formula to sinusoidal variation of diagonal resistance, R$_{\mathrm{xx}}$, with polarization angle, $\theta $, as R$_{\mathrm{xx}}(\theta )=$A$\pm $Ccos$^{\mathrm{2}}(\theta $-$\theta_{0})$, depends upon radiation frequency $f$, magnetic field $B$, and sign of $B$.[2] Here, we investigate the dependence of the phase shift $\theta_{0}$ in the linear-polarization-angle-dependence upon the above-mentioned experimental variables. In particular, we examine the relationship between $f$ and $\theta _{0}$. The results will be compared with theory.[3,4] [1] R. G. Mani \textit{et al.}, Phys. Rev. B 84, 085308 (2011). [2] A. N. Ramanayaka \textit{et al.}, Phys. Rev. B 85, 205315 (2012). [3] J. Inarrea, Appl. Phys. Lett. 100, 242103 (2012). [4] X. L. Lei and S. Y. Liu, Phys. Rev. B 86, 205303 (2012). Work has been supported by DOE DE-SC0001762. [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A42.00003: Nonlinear conductance of highly mobile 2D electrons in Corbino geometry Sean Byrnes, Scott Dietrich, Sergey Vitkalov, D. V. Dmitriev, I. V Marchishin, A. A. Bykov Current induced oscillations of differential conductivity of two-dimension electrons, placed in quantizing magnetic fields, are observed in GaAs quantum wells in Corbino geometry. The conductance oscillations are described by Zener tunneling between Landau orbits in the absence of the Hall electric field[1]. An electronic state with zero-differential conductance is found in nonlinear response to an electric field E applied to two dimensional Corbino discs of highly mobile carriers. The state occurs above a critical electric field $E>E_{th} $ at low temperatures and is accompanied by an abrupt dip in the differential conductance. The proposed model consider local instability of the electric field E as the origin of the observed phenomenon. [1] A.A Bykov, D.V. Dmitriev, I.V.Marchishin, S.Byrnes, S.A.Vitkalov, Appl. Phys. Lett.$\mbox{100}$, 251602 (2012) [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A42.00004: Quantum oscillations of nonlinear response in electron systems with variable density Scott Dietrich, Sean Byrnes, Sergey Vitkalov, D.V. Dmitriev, A.V. Goran, A.A. Bykov Oscillations of dissipative resistance of two-dimensional electrons in GaAs quantum wells are observed in response to an electric current and a strong magnetic field applied perpendicular to the two-dimensional systems. The period of the current-induced oscillations does not depend on the magnetic field and temperature. At a fixed current the oscillations are periodic in inverse magnetic fields with a period that does not depend on dc bias. Oscillations were also studied in GaAs quantum wells with variable two dimensional electron density. At a fixed magnetic field the period of the current induced oscillations depends linearly on the electron density. Both results corroborate the recently proposed model that considers the DC bias-induced spatial re-population of Landau levels as the origin of the resistance oscillations. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A42.00005: Inter-subband resistance oscillations in crossed electric and magnetic fields Sergey Vitkalov, Scott Dietrich, Sean Byrnes, A.V. Goran, A.A. Bykov Quantum oscillations of nonlinear resistance are investigated in response to electric current and magnetic field applied perpendicular to single GaAs quantum wells with two populated sub-bands. At small magnetic fields current-induced oscillations appear as Landau-Zener transitions between Landau levels inside the lowest sub-band. The period of these oscillations is proportional to the magnetic field. At high magnetic fields, a different kind of quantum oscillations emerges with a period that is independent of the magnetic field. At a fixed current the oscillations are periodic in inverse magnetic field with a period that is independent of the dc bias. The proposed model considers these oscillations as a result of spatial variations of the energy separation between two sub-bands induced by the electric current (Scott Dietrich, Sean Byrnes, Sergey Vitkalov, A. V. Goran, and A. A. Bykov Phys. Rev. B~86, 075471). [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A42.00006: Nonlinear transport in two-dimensional electron systems with separated Landau levels Maxim Khodas, Michael Zudov, Loren Pfeiffer, Kenneth West The resistivity of a high mobility two-dimensional electron gas subject to a weak perpendicular magnetic field and low temperatures is strongly non-linear. This nonlinearity becomes more pronounced when the Landau level width becomes smaller than the cyclotron energy; at very small dc electric fields the differential resistivity becomes strongly suppressed and can even approach zero. Using the quantum kinetics approach we calculate the characteristic current responsible for the suppression and compare the results to the experimental data obtained in a high mobility 2DES at low temperatures. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A42.00007: Microwave-induced resistance oscillations at low temperatures Peter Martin, Michael Zudov, John Watson, Michael Manfra, John Reno, Loren Pfeiffer, Kenneth West At low temperatures, the amplitude of microwave-induced resistance oscillations in two dimensional electron systems is predicted to scale as $1/T^2$. In contrast to this prediction, our experiments shows that the amplitude tends to saturate at low temperatures, even in the regime of very low microwave intensities. In this talk we will discuss radiation-induced heating as a possible source of the observed saturation and ways to estimate actual temperature of irradiated 2D electrons. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A42.00008: Anomalies in nonlinear transport of two-dimensional electron gas Quentin Ebner, Michael Zudov, Loren Pfeiffer, Kenneth West When a dc current is passed through a high-mobility two-dimensional electron system subject to a weak magnetic field, its differential resistivity exhibits periodic oscillations as a function of applied current. The waveform of these oscillations, known as Hall field-induced resistance oscillations, is well established both experimentally and theoretically. In this talk we will present experimental data which show dramatic deviations of the oscillation waveform from the theoretically predicted. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A42.00009: Microwave-induced resistance oscillations in tilted magnetic fields Alex Bogan, Sergei Studenikin, Andy Sachrajda, Anthony Hatke, Michael Zudov, Loren Pfeiffer, Kenneth West We have studied the effect of an in-plane magnetic field on microwave-induced resistance oscillations in a high mobility two-dimensional electron system. We have found that the oscillation amplitude decays exponentially with an in-plane component of the magnetic field. While these findings cannot be accounted for by existing theories, our analysis suggests that the decay can be explained by a quadratic-in-parallel-field correction to the quantum scattering rate. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A42.00010: Effective mass from microwave photoresistance in high-mobility 2D electron systems Michael Zudov, Anthony Hatke, John Watson, Michael Manfra, Loren Pfeiffer, Kenneth West We have performed microwave photoresistance measurements in high mobility GaAs/AlGaAs quantum wells and investigated the value of the effective mass. Surprisingly, the effective mass, obtained from the period of microwave-induced resistance oscillations, is found to be considerably lower than the band mass in GaAs. This finding provides evidence for electron-electron interactions which can be probed by microwave photoresistance in very high Landau levels. In contrast, the measured magneto-plasmon dispersion revealed an effective mass which is close to the band mass, in accord with previous studies. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A42.00011: Pinning modes of solid phases in wide quantum wells near $\nu =$ 1 Anthony Hatke, Brenden Magill, Yang Liu, Lloyd Engel, Mansour Shayegan, Loren Pfeiffer, Ken West, Kurt Baldwin Near filling factor $\nu =$ 1 the microwave spectra of sufficiently low disorder two-dimensional electron systems (2DESs) exhibit a resonance [1], understood as a Wigner solid pinning mode, in which quasiparticles or quasiholes oscillate about their pinned positions. For 2DESs in a wide quantum well of thickness 54 nm and density n$=$2.4 x 10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$, we find that the resonance frequency, f$_{\mathrm{pk}}$, is comparatively enhanced for $\nu $\textless 0.88, and interpret this as a phase transition between Wigner solids, as shown by the reentrant integer quantum Hall effect (RIQHE) recently observed in wide wells under similar conditions [2]. As n is increased by gating, the transition to enhanced f$_{\mathrm{pk}}$ moves closer to $\nu =$ 1, similar to the RIQHE in [2]. [1] Chen et al., Phys. Rev. Lett. \textbf{91},016801 (2003). [2] Liu et al., Phys. Rev. Lett. \textbf{109}, 036801 (2012). [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A42.00012: Effects of short-ranged disorder on pinning modes of 2D electron system near $\nu =$1 B.H. Moon, B.A. Magill, L.W. Engel, D.C. Tsui, L.N. Pfeiffer, K.W. West We performed microwave measurements on 2-D electron systems (2DES) in heterostructures of Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As/ Al$_{0.3}$Ga$_{0.7}$As in which the 2DES resides in dilute Al alloy, with x$=$0.21, 0.33 and 0.85{\%}. The dilute Al atoms are randomly distributed [1]. Around Landau filling $\nu =$1,the samples exhibit microwave resonances which differ from pinning mode resonances observed [2] near $\nu =$1 in unalloyed samples. The alloyed samples have larger resonance frequencies, and resonances that exist over wider ranges of $\nu $, extending to \textbar $\nu $-1\textbar $\sim$ 0.2. Also in the disordered samples, the resonances are not quasiparticle-quasihole symmetric around $\nu =$1, and there is strong frequency and $\nu $ dependence in the spectra away from rational fractional $\nu $, down to 0.6. \\[4pt] [1] W. Li \textit{et al.}, \textit{Appl.Phys. Lett. }\textbf{83}, 2832 (2003).\\[0pt] [2] Y. P. Chen et al., Phys. Rev. Lett. \textbf{91},016801 (2003). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A42.00013: Probing the lowest Landau level energy of the light hole subband in wide quantum wells Sukret Hasdemir, Yang Liu, Mansour Shayegan, Roland Winkler, Loren Pfeiffer, Ken West, Kirk Baldwin In two-dimensional hole systems (2DHSs) with finite thickness, the degeneracies of the heavy hole (HH) and light hole (LH) states are lifted. The HH-LH anti-crossing and mixing lead to non-parabolic 2D dispersion relations, especially for the LH subbands, invalidating the simple effective-mass approximation for 2DHSs. We study the magneto-resistance of 2DHSs confined to symmetric, wide GaAs quantum wells, where the second subband (LH1) is occupied. From the magnetic field ($B$) and densities where the lowest Landau level of the LH1 subband crosses the Landau levels of HH1 subband, we can extrapolate the cyclotron energy of the LH1 subband, which decreases with increasing $B$ at low fields, and increases at high field. This peculiar behavior reveals that the cyclotron mass for the LH1 subband is negative at small magnetic fields, but becomes positive at high magnetic field, consistent with theoretical simulations. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A42.00014: Determining the Location of Charged Background Impurities in High Mobility AlGaAs/GaAs 2DEG Structures Jerry Lee, Kirk Baldwin, Ken West, Loren Pfeiffer The two main Coulomb scattering contributions to the scattering rate 1/$\tau $ in the modulation-doped AlGaAs/GaAs two-dimensional electron gas (2DEG) system are scattering from unintentional background charged impurities present in the GaAs and AlGaAs materials, and scattering by the intentional dopants in the doped layer. Theoretical studies [1] indicate that for structures dominated by scattering from unintentional background charged impurities in the conducting channel, a carrier mobility $\mu $ versus 2DEG density n relationship of $\mu \approx $ n$^{\mathrm{0.8}}$ is anticipated. On the other hand, in structures where the dominant scattering mechanism is due to charged impurities or dopants in the nearby AlGaAs barriers, a relationship of $\mu \approx $ n$^{\mathrm{1.8}}$ is expected. Using high-mobility heterostructure insulated-gate field-effect transistors (HIGFETs) fabricated by molecular beam epitaxy (MBE) and lithography, we demonstrate a technique for determining the location of the mobility limiting charged impurities. We intentionally introduce charged impurities into either the barrier or the quantum well of our HIGFETs and measure the slope of $\mu $ versus n. We find a dependence of $\mu \approx $ n$^{\mathrm{0.7}}$ when the dopants are inserted into the quantum well. In contrast, we measure a dependence of $\mu \approx $ n$^{\mathrm{1.8}}$ when impurities are introduced into the barrier. Our results are in excellent agreement with theoretical predictions and pave the way towards utilizing these relationships to diagnose the exact location of impurities in high-mobility structures for FQHE applications. [1] A. Gold, Appl. Phys. Lett. \textbf{54}, 2100 (1989). [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A42.00015: Monitoring Excitations of the N$=$1 Landau Level by Optical Emission at mK Temperatures Antonio Levy, Ursula Wurstbauer, Dov Fields, Aron Pinczuk, John Watson, Sumit Mondal, Michael J. Manfra, Ken W. West, Loren N. Pfeiffer Optical emission experiments have proven to be powerful contactless probe of collective states of electrons in the second (N$=$1) Landau Level (LL) [1,2]. We report the emission spectrum from optical recombination in the N$=$0 and N$=$1 LL's the second LL. The 2DEG is confined in ultra-high-mobility GaAs quantum well structures. Optical emission red-shifted from the main luminescence of the N$=$0 and N$=$1 LL are interpreted as shakeup processes of quasiparticles in the N$=$1 LL. Results of two samples with different carrier densities measured in the temperature range of 42mK$\le $T$\le $650mK will be compared. The experimental observations will be discussed taking into account the striking quantum phases dominating the second LL. [1] Manfra, M. J. et al. Phys. Rev. B 57, R9467 (1998) [2] Gravier, L. et al. Phys. Rev. Lett. 80, 3344 (1998). [Preview Abstract] |
Session A43: Focus Session: Multiscale modeling--Coarse-graining in Space and Time I
Sponsoring Units: DCPChair: William Noid, Pennsylvania State University
Room: Hilton Baltimore Holiday Ballroom 2
Monday, March 18, 2013 8:00AM - 8:36AM |
A43.00001: The Theory of Ultra Coarse-graining Invited Speaker: Gregory Voth Coarse-grained (CG) models provide a computationally efficient means to study biomolecular and other soft matter processes involving large numbers of atoms correlated over distance scales of many covalent bond lengths and long time scales. Variational methods based on information from simulations of finer-grained (e.g., all-atom) models, for example the multiscale coarse-graining (MS-CG) and relative entropy minimization methods, provide attractive tools for the systematic development of CG models. However, these methods have important drawbacks when used in the ``ultra coarse-grained'' (UCG) regime, e.g., at a resolution level coarser or much coarser than one amino acid residue per effective CG particle in proteins. This is due to the possible existece of multiple metastable states ``within'' the CG sites for a given UCG model configuration. In this talk I will describe systematic variational UCG methods specifically designed to CG entire protein domains and subdomains into single effective CG particles. This is accomplished by augmenting existing effective particle CG schemes to allow for discrete state transitions and configuration-dependent resolution. Additionally, certain conclusions of this work connect back to single-state force matching and open up new avenues for method development in that area. These results provide a formal statistical mechanical basis for UCG methods related to force matching and relative entropy CG methods and suggest practical algorithms for constructing optimal approximate UCG models from fine-grained simulation data. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 9:12AM |
A43.00002: Coarse-graining with information theory and the relative entropy Invited Speaker: M. Scott Shell There remain many both fundamental and practical/methodological questions regarding how coarse-grained models should be developed. Are there theoretically intuitive and numerically robust strategies for turning small-scale all-atom simulations into coarse models suitable for large-scale modeling? How can we identify what atomic details are unnecessary and can be discarded? Are there systematic ways to detect emergent physics? Here we discuss a fundamentally new approach to this problem. We propose that a natural way of viewing the coarse-graining problem is in terms of information theory. A quantity called the relative entropy measures the information lost upon coarse graining and hence the (inverse) fitness of a particular coarse-grained model. Minimization of the relative entropy thus provides a sort-of universal variational principle for coarse-graining, and a way to ``automatically'' discover and generate coarse models of many systems. We show that this new approach enables us to develop very simple but surprisingly accurate models of water, hydrophobic interactions, self-assembling peptides, and proteins that enable new physical insights as well as simulations of large-scale interactions. We discuss both theoretical and numerical aspects of this approach, in particular highlighting a new coarse-graining algorithm that efficiently optimizes coarse-grained models with even thousands of free parameters. We also discuss how the relative entropy approach suggests novel strategies for predicting the errors of coarse models, for identifying relevant degrees of freedom to retain, and for understanding the relationships among other coarse-graining methodologies. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A43.00003: A Top Down Approach to Multiscale Modeling of Structured Materials Invited Speaker: Juan de Pablo There is considerable interest in developing multiscale modeling approaches capable of describing both the structure and dynamics of inhomogeneous materials having characteristic features ranging from a few to a several hundred nanometers. Examples include block polymers, which exhibit an array of ordered morphologies with characteristic dimensions in the tens of nanometers, or liquid crystalline materials, where ordered domains and defects can also span tens of nanometers. This presentation will describe a relatively new class of particle-based methods that rely on established continuum models to describe thermodynamic properties, but that adopt a molecular representation to describe molecular structure and mesoscale morphology. While these methods can describe mesoscale structure quantitatively, they can also be augmented to describe the dynamics of complex fluids, including entangled polymers, composites, and nanoparticle dispersed in structured fluids, over dynamic ranges that in some cases span multiple orders of magnitude. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A43.00004: Coarse graining approach to First principles modeling of structural materials Khorgolkhuu Odbadrakh, Don Nicholson, Aurelian Rusanu, German Samolyuk, Yang Wang, Roger Stoller, Xiaoguang Zhang, George Stocks Classical Molecular Dynamic (MD) simulations characterizing extended defects typically require millions of atoms. First principles calculations employed to understand these defect systems at an electronic level cannot, and should not deal with such large numbers of atoms. We present an efficient coarse graining (CG) approach to calculate local electronic properties of large MD-generated structures from the first principles. We used the Locally Self-consistent Multiple Scattering (LSMS) method for two types of iron defect structures 1) screw-dislocation dipoles and 2) radiation cascades. The multiple scattering equations are solved at fewer sites using the CG. The atomic positions were determined by MD with an embedded atom force field. The local moments in the neighborhood of the defect cores are calculated with first-principles based on full local structure information, while atoms in the rest of the system are modeled by representative atoms with approximated properties. This CG approach reduces computational costs significantly and makes large-scale structures amenable to first principles study. Work is sponsored by the USDoE, Office of Basic Energy Sciences, ``Center for Defect Physics,'' an Energy Frontier Research Center. This research used resources of the Oak Ridge Leadership Computing Facility at the ORNL, which is supported by the Office of Science of the USDoE under Contract No. DE-AC05-00OR22725. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A43.00005: Construction of interaction models of dissipative particle dynamics by coarse-graining Lennard-Jones fluids: Evaluation of non-Markovian formulation Yuta Yoshimoto, Toshiki Mima, Akinori Fukushima, Ikuya Kinefuchi, Takashi Tokumasu, Shu Takagi, Yoichiro Matsumoto The application of molecular dynamics (MD) simulation to mesoscale (10-100 nm) flow analysis is computationally expensive at present. Dissipative particle dynamics (DPD) simulation is a powerful candidate for the alternative method because the DPD interaction, which has a soft potential between mesoscopic particles, enables larger space and longer time simulation. In the present study, we develop the method of bottom-up construction of non-Markovian DPD (NMDPD) models by means of MD simulations. We focus on the center of mass of the cluster containing Lennard-Jones particles, and extract the effective forces exerted on the clusters. Moreover, we sample the spectra of fluctuating forces acted on the clusters in the MD system, and find that the white noise used in the conventional DPD simulations should be replaced by colored noise. In order to reproduce the spectra, finite impulse response filters are employed in NMDPD simulations. Finally we evaluate the NMDPD models by comparing the simulation results with the MD counterparts. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A43.00006: Solvation free energies of aqueous mixtures in a ``truly" open boundary simulation Debashish Mukherji, Kurt Kremer (Bio)macromolecular solvation in water cosolvent mixtures are dictated by the preferential interaction of cosolvents with the proteins. The numerical studies in the field are limited to the closed boundary schemes, which, however, suffers from severe system size effects. More specifically, when the conformational transitions are intimately linked to the large concentration fluctuations, the excess of cosolvents near a protein lead to depletion elsewhere in a small-sized closed boundary setup. This disturbs solvent equilibrium within the bulk solution. Therefore, by combining the adaptive resolution scheme (AdResS) with a metropolis particle exchange criterion, we propose a ``truly'' open boundary method that heals the particle depletion in a closed boundary setup. In AdResS, an all-atom region, containing protein, is coupled to a coarse-grained (CG) reservoir. Particle exchange is performed in the CG region, which otherwise would be impossible in an all-atom setup of dense fluids. We calculate solvation free energies within the all-atom region using Kirkwood-Buff theory. Our method produces well converged solvation energies that are impossible in a brute force all-atom MD of small system sizes. We will discuss two cases of triglycine in aqueous urea and PNIPAm in aqueous methanol. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A43.00007: Coarse-Grained Modeling of Mixtures of Charged Macroions Jun Kyung Chung, Alan R. Denton In suspensions of charged macroions, such as charge-stabilized colloids and polyelectrolyte microgels, the electrostatic interactions between macroions are relatively easily controlled by changing the sizes and charges of the macroions, as well as the concentration of salt. This tunability of interactions can be exploited to stabilize various structures that self-assemble under appropriate conditions. In this talk, a statistical mechanical coarse-graining approach to modeling effective electrostatic interactions in mixtures of charged spherical macroions will be discussed. Taking effective interactions as input, we perform molecular dynamics simulations to calculate pair distribution functions of binary mixtures of charged colloids. For highly charged macroions, incorporating charge renormalization is found to be important. Using thermodynamic perturbation theory, we also analyze phase behavior and explore the possibility of a demixing instability as a function of size and charge asymmetry. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A43.00008: Coarse-Grained Modeling of Colloid-Nanoparticle Mixtures Alan R. Denton, Jun Kyung Chung Colloid-nanoparticle mixtures have attracted much recent attention for their rich phase behavior. The potential to independently vary size and charge ratios greatly expands the possibilities for tuning interparticle interactions and stabilizing unusual phases. Experiments have begun to explore the self-assembly and stability of colloid-nanoparticle mixtures, which are characterized by extreme size and charge asymmetries. In modeling such complex soft materials, coarse-grained methods often prove essential to surmount computational challenges posed by multiple length and time scales. We describe a hierarchical approach to modeling effective interactions in ultra-polydisperse mixtures. Using a sequential coarse-graining procedure, we show that a mixture of charged colloids and nanoparticles can be mapped onto a one-component model of pseudo-colloids interacting via a Yukawa effective pair potential and a one-body volume energy, which contributes to the free energy of the system. Nanoparticles are found to enhance electrostatic screening and to modify the volume energy. Taking the effective interactions as input to simulations and perturbation theory, we calculate structural properties and explore phase stability of highly asymmetric charged colloid-nanoparticle mixtures. [Preview Abstract] |
Session A44: Focus Session: Population and Evolutionary Dynamics I
Sponsoring Units: DBIO GSNPChair: Michel Pleimling, Virginia Tech
Room: Hilton Baltimore Holiday Ballroom 1
Monday, March 18, 2013 8:00AM - 8:36AM |
A44.00001: Evolutionary dynamics in finite populations Invited Speaker: Christoph Hauert Traditionally, evolutionary dynamics has been studied based on infinite populations and deterministic frameworks such as the replicator equation. Only more recently the focus has shifted to the stochastic dynamics arising in finite populations. Over the past years new concepts have been developed to describe such dynamics and has lead to interesting results that arise from the stochastic, microscopic updates, which drive the evolutionary process. Here we discuss a transparent link between the dynamics in finite and infinite populations. The focus on microscopic processes reveals interesting insights into (sometimes implicit) assumptions in terms of biological interactions that provide the basis for deterministic frameworks and the replicator equation in particular. More specifically, we demonstrate that stochastic differential equations can provide an efficient approach to model evolutionary dynamics in finite populations and we use the rock-scissors-paper game with mutations as an example. For sufficiently large populations the agreement with individual based simulations is excellent, with the interesting caveat that mutation events may not be too rare. In the absence of mutations, the excellent agreement extends to small population sizes. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A44.00002: Neutral species domination on different lattices for the symmetric stochastic cyclic competition of four species Ben Intoy, Sven Dorosz, Michel Pleimling Although the mean-field solution for four species in cyclic competition is generally in good agreement with stochastic results, it fails to describe the extinction and absorbing states that finite size systems inevitably fall into. We study the effects of dimension, lattice type, and swapping rate between particles on the time it takes for the system to go into a static absorbing state, which consists of a neutral species pair. Lattice types discussed are the well mixed environment, the one-dimensional chain, the Sierpinski triangle, and the two-dimensional square lattice. Data presented were acquired with simulations that have around the order of a thousand lattice sites or less, to capture finite size effects. The formation of domains composed of neutral species yields long lived states which promote coexistence. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A44.00003: Biodiversity and co-existence of competing species Ahmed Roman, Debanjan Dasgupta, Michel Pleimling Understanding why and how species co-exist is a necessary step to the program of manipulating multispecies environments in order to preserve the biodiversity of the environment of interest. To this end we consider a generalization of the cyclic competition of species model. We show that our model enjoys a $Z_n$ symmetry which is explained via a simple graph theoretic technique. This symmetry gives rise to pattern formation and cluster coarsening of the species. We show that biodiversity is achievable in the mean field limit provided that the species in the clusters have reaction rates which correspond to non-trivial equilibria. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A44.00004: Population oscillations in stochastic Lotka--Volterra models: field theory and perturbational analysis Uwe C. T\"auber Field theory tools are applied to analytically study fluctuation and correlation effects in spatially extended stochastic predator-prey systems. In the mean-field rate equation approximation, the classic Lotka--Volterra model is characterized by neutral cycles in phase space, describing undamped oscillations for both predator and prey populations. In contrast, Monte Carlo simulations for stochastic two-species predator-prey reaction systems on regular lattices display complex spatio-temporal structures associated with persistent erratic population oscillations. The Doi--Peliti path integral representation of the master equation for stochastic particle interaction models is utilized to arrive at a field theory action for spatial Lotka--Volterra models in the continuum limit. In the species coexistence phase, a perturbation expansion with respect to the nonlinear predation rate is employed to demonstrate that spatial degrees of freedom and stochastic noise induce instabilities toward structure formation, and to compute the fluctuation corrections for the oscillation frequency and diffusion coefficient. The drastic downward renormalization of the frequency and the enhanced diffusivity are in excellent qualitative agreement with Monte Carlo simulation data. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A44.00005: Patterns and Oscillations in Reaction-Diffusion Systems with Intrinsic Fluctuations Michael Giver, Daniel Goldstein, Bulbul Chakraborty Intrinsic or demographic noise has been shown to play an important role in the dynamics of a variety of systems including predator-prey populations, biochemical reactions within cells, and oscillatory chemical reaction systems, and is known to give rise to oscillations and pattern formation well outside the parameter range predicted by standard mean-field analysis. Initially motivated by an experimental model of cells and tissues where the cells are represented by chemical reagents isolated in emulsion droplets, we study the stochastic Brusselator, a simple activator-inhibitor chemical reaction model. Our work extends the results of recent studies on the zero and one dimensional systems with the ultimate goals of understanding the role of noise in spatially structured systems and engineering novel patterns and attractors induced by fluctuations. In the zero dimensional system, we observe a noise induced switching between small and large amplitude oscillations when a separation of time scales is present, while the spatially extended system displays a similar switching between a stationary Turing pattern and uniform oscillations. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A44.00006: Flow-driven instabilities during aggregation and pattern formation of Dictyostelium Discoideum: Experiments and modeling Azam Gholami, Oliver Steinbock, Vladimir Zykov, Eberhard Bodenschatz We report the first experimental verification of the Differential Flow Induced Chemical Instability (DIFICI) in a signaling chemotactic biological population, where a differential flow induces traveling waves in the signaling pattern. The traveling wave speed was observed to be proportional to the flow velocity while the wave period was 7 min, which is comparable to that of starved Dictyostelium cells. Analysis and numerical simulations of the Goldbeter model show that the resulting DIFICI wave patterns appear in the oscillatory regime. In the experiments, we observe that the DIFICI wave pattern disappears after 4-5 h of starvation. We extrapolated the Goldbeter model to the experimental situation. This suggests that the dynamics switches from the oscillatory to the excitable regime as the DIFICI waves disappear in the experiment. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A44.00007: Statistical Thermodynamics of Populations Themis Matsoukas Suppose a population of $M$ individuals forms $N$ groups such that group $i$ contains $n_i$ individuals. Form all possible partitions of $M$ into $N$ and select distributions from this ensemble with selection bias $W[\{n_i\}]$, where $W$ is a functional of distribution $\{n_i\}$. We develop the thermodynamics of this ensemble and its most probable distribution for arbitrary bias $W$. We obtain the temperature of the ensemble and its relationship to the microcanonical and canonical partition functions; and (ii) show that, depending on the bias functional $W$, the population may exhibit the equivalent of a phase transition, manifested as the coexistence of two distinct subpopulations in equilibrium with each other. We apply this theory to binary clustering with special interest in conditions that result in the emergence of a single dominant group that overtakes all smaller coexisting groups when the number of groups $N$ is decreased. We show the emergence of the dominant group represents a formal phase transition that is governed by the maximization of the free energy of the ensemble. We provide closed analytical solutions for the special case that the merging probability between two groups is proportional to the product of the number of members in each group. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:24AM |
A44.00008: Predictability of evolution in complex fitness landscapes Invited Speaker: Joachim Krug Evolutionary adaptations arise from an intricate interplay of deterministic selective forces and random reproductive or mutational events, and the relative roles of these two types of influences is the subject of a long-standing controversy. In general, the predictability of adaptive trajectories is governed by the genetic constraints imposed by the structure of the underlying fitness landscape as well as by the supply rate and effect size of beneficial mutations. On the level of single mutational steps, evolutionary predictability depends primarily on the distribution of fitness effects, with heavy-tailed distributions giving rise to highly predictable behavior [1]. The genetic constraints imposed by the fitness landscape can be quantified through the statistical properties of accessible mutational pathways along which fitness increases monotonically. I will report on recent progress in the understanding of evolutionary accessibility in model landscapes and compare the predictions of the models to empirical data [2,3]. Finally, I will describe extensive Wright-Fisher-type simulations of asexual adaptation on an empirical fitness landscape [4]. By quantifying predictability through the entropies of the distributions of evolutionary trajectories and endpoints we show that, contrary to common wisdom, the predictability of evolution depends non-monotonically on population size. \\[4pt] [1] M.F. Schenk, I.G. Szendro, J. Krug and J.A.G.M. de Visser, PLoS Genetics 8, e1002783 (2012).\\[0pt] [2] J. Franke, A. Kl\"ozer, J.A.G.M. de Visser and J. Krug, PLoS Computational Biology 7, e1002134 (2011).\\[0pt] [3] J. Franke and J. Krug, Journal of Statistical Physics 148, 705 (2012).\\[0pt] [4] I.G. Szendro, J. Franke, J.A.G.M. de Visser and J. Krug (under review). [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A44.00009: A Condition for Cooperation in a Game on Complex Networks Tomohiko Konno We study a condition of favoring cooperation in a Prisoner's Dilemma game on complex networks. There are two kinds of players: cooperators and defectors. Cooperators pay a benefit $b$ to their neighbors at a cost $c$, whereas defectors only receive a benefit. The game is a death-birth process with weak selection. Although it has been widely thought that $b/c>\langle k \rangle$ is a condition of favoring cooperation [2], we find that $b/c>\langle k_\textrm{nn} \rangle$ is the condition. We also show that among three representative networks, namely, regular, random, and scale-free, a regular network favors cooperation the most, whereas a scale-free network favors cooperation the least. In an ideal scale-free network, cooperation is never realized. Whether or not the scale-free network and network heterogeneity favor cooperation depends on the details of the game, although it is occasionally believed that these favor cooperation irrespective of the game structure.\\[4pt] [1] T.K, A condition for cooperation in a game on complex networks, Journal of Theoretical Biology 269, Issue 1, Pages 224-233, (2011)\\[0pt] [2] H. Ohtsuki, C. Hauert, E. Lieberman, M. A. Nowak, A simple rule for the evolution of cooperation on graphs and social networks, Nature 441 (7092) (2006) [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A44.00010: Evolution of regulatory complexes: a many-body system Armita Nouemohammad, Michael Laessig In eukaryotes, many genes have complex regulatory input, which is encoded by multiple transcription factor binding sites linked to a common function. Interactions between transcription factors and site complexes on DNA control the production of protein in cells. Here, we present a quantitative evolutionary analysis of binding site complexes in yeast. We show that these complexes have a joint binding phenotype, which is under substantial stabilizing selection and is well conserved within Saccharomyces paradoxus populations and between three species of Saccharomyces. At the same time, individual low-affinity sites evolve near-neutrally and show considerable affinity variation even within one population. Thus, functionality of and selection on regulatory complexes emerge from the entire cloud of sites, but cannot be pinned down to individual sites. Our method is based on a biophysical model, which determines site occupancies and establishes a joint affinity phenotype for binding site complexes. We infer a fitness landscape depending on this phenotype using yeast whole-genome polymorphism data and a new method of quantitative trait analysis. Our fitness landscape predicts the amount of binding phenotype conservation, as well as ubiquitous compensatory changes between sites in the cloud. Our results open a new avenue to understand the regulatory ``grammar'' of eukaryotic genomes based on quantitative evolution models. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A44.00011: Inference of fitness from genealogical trees Marija Vucelja, Adel Dayarian, Boris Shraiman Natural populations are fitness diverse and can have numerous genes under selection. The genealogical trees, that one obtains by sampling, often bear hallmarks of selection, such multiple mergers, asymmetric tree branches and long terminal branches (the trees are squished towards the root). These are qualitative differences compared to trees in the absence of selection. We propose a theoretical model that links the morphology of a tree with the fitness of the leaves. We obtain multipoint correlation functions of the fitness along the tree. In this way we are able extract some quantitative information about the strength of selection from data-reconstructed trees. The extensions of this approach can potentially be useful for inferring relative fitness of sequenced genomes of tumors and for predicting viral outbreaks. [Preview Abstract] |
Session A45: Focus Session: Structure and Dynamics of Biomembranes I
Sponsoring Units: DBIO DPOLYChair: Fredrick Heberle, Oak Ridge National Laboratory
Room: Hilton Baltimore Holiday Ballroom 4
Monday, March 18, 2013 8:00AM - 8:36AM |
A45.00001: Molecular simulation studies of edges in bilayers and bicelles Invited Speaker: James Kindt The instability of the free edge of a lipid bilayer can be quantified by a line tension, or excess free energy per unit length of the edge. Atomistic simulations of bilayer ribbons composed of a series of lipids with varying tail lengths and degrees of saturation have been performed to determine line tensions, with the goal of relating edge stability to structural and elastic properties of the bilayer. Line tensions are relevant to the mechanical stability of bilayer membranes, and can be reduced or eliminated by the inclusion of edge-stabilizing molecules (edge-actants) to the bilayer system. Mixtures of long- and short-tailed phospholipids are known to form aggregates known as ``bicelles'' that contain bilayers with stable edges. Simulations of ``bicelle'' mixtures using coarse-grained and atomistic lipid models have been performed to study the partitioning of short-tailed lipids towards the edge and the flexibility of the stabilized edge. Input from these simulations has been used in the development of simple thermodynamic models to rationalize some aspects of bicelle aggregate morphology. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A45.00002: Shear-induced alignment of ``bicellar'' phospholipid membranes Mu-Ping Nieh, Ming Li, Norbert Kucerka ``Bicellar'' phospholipid mixtures, composed of two types of lipids (i.e., long-chain and short-chain lipids), self-assemble into a magnetically alignable bilayered structure. As a result, the model membranes have been used as membrane substrates for the structural study on membrane-associated proteins in many nuclear magnetic resonance experiments. In this presentation, I will demonstrate the shear-alignability of the bicellar model membranes through an in-situ neutron diffraction study under shear flows, the important controlling parameters and their applications and biological implications. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A45.00003: Estimation of Structural Properties Of The Thermally Fluctuated Membrane Based on The Small-Angle Neutron Scattering Data Takumi Hawa, Victor Lee SANS (Small-angle neutron scattering) and SAXS (Small-angle X-ray scattering) experiments are one of the most important laboratory techniques to determine nanoscale structure of biological and nanotechnology-related systems. These experimental techniques provide extensive information due to the sensitivity of about 1-1000 nm and 2-25 nm length scales for SANS and SAXS, respectively. Recently, the author and his collaborator, Dr. Nagao, studied swollen lamellar structure systems consisting of nonionic surfactant, water, and oil using SANS/NSE (Neutron Spin Echo) and MD (molecular dynamics) simulation. They proposed a new experimental technique to measure the thickness fluctuations of surfactant layers and verified their approach using MD simulations. In this talk we derive a simple mathematical model to estimate the thickness of the membrane as well as the amplitude and the wavelength of the surfactant layers in the membrane. The model is developed based on the harmonic motion of the surfactant layers. We consider both bending and thickness fluctuation motions of the membrane. The membrane thickness estimated from the proposed approach showed an excellent agreement with the SANS experimental results available in the literatures. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A45.00004: Lipid bilayer dynamics: Effects of segregation between DMPC and DSPC Michihiro Nagao, Paul Butler, Andrea Woodka, Rana Ashkar Dynamics in lipid bilayers are believed to play a key role in membrane stabilization. During the past decade, neutron spin echo (NSE) has been used to study the bending elastic behavior of large unilamellar vesicles (radius of around 50 nm). These results reveal that above Tm, where the lipid tails display liquid ordering, the bending modulus is on the order of 10 kT. Below Tm, the value increases by more than an order of magnitude. Recently NSE revealed thickness fluctuations of lipid bilayers above Tm, while none are discernable below Tm. The estimated amplitude of the observed membrane thickness fluctuations is approximately 4 angstroms and the time scale of the motion is on the order of 100 ns. In the present research, structure and dynamics of mixed lipid between dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) were investigated using small-angle neutron scattering (SANS) and NSE. DSPC has a higher Tm than DMPC. The mixed lipid systems show segregation between domains in the temperature range between Tm of DMPC and DSPC. The SANS and NSE measurements were performed with changing temperature from above to below the Tm of DSPC. The result indicates a slow down of thickness fluctuations once the segregation takes place. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A45.00005: X-ray reflectivity study of a DPPC floating bilayer: Effect of Ca$^{2+}$ ions and temperature Sambhunath Bera, Sajal Ghosh, Yicong Ma, Curt DeCaro, Zhang Jiang, Laurence Lurio, Sunil Sinha We have used a floating bilayer of DPPC (1,2- dipalmitoyl-sn-glycero-3-phosphocholine) to examine the effects on Ca$^{++}$ ion concentration on membrane fluctuations. The density profile of the double bilayer system normal to the support was measured via x-ray specular reflectivity. We find an increase in membrane spacing with ion concentration which we attribute to Ca$^{++}$ ions preferentially binding to the head group of the bilayer. We also find an increase in interfacial roughness which we attribute to the Ca$^{++}$ ions causing a reduction in the layer's bending modulus. These effects are studied as a function of temperature up to the temperature at which the bilayer if found to unbind from the support. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A45.00006: Deposition of Homogeneous Single-supported DMPG Lipid Membranes onto a Silica Substrate for Quasielastic Neutron Scattering Experiments Andrew Miskowiec, Mia Brown, Jason Cooley, Renee Jiji, Haskell Taub, Justin Grayer, Gavin King, Helmut Kaiser, Flemming Hansen, Madhusudan Tyagi We report deposition of single bilayers of dimyristoyl-phosphoglycerol (DMPG) lipid onto a SiO$_{2}$-coated Si(100) substrate. These anionic membranes have large-scale homogeneity comparable to that achieved with single-supported uncharged DMPC membranes used for neutron scattering studies.\footnote{M. Bai, \textit{et al.} Europhys. Lett. \textbf{98}, 48006 (2012).} Optimum deposition conditions were found by systematically varying the lipid concentration and both the monovalent and divalent buffer salt concentrations. Plausible mechanisms for the bilayer stability will be discussed. In addition, we report Atomic Force Microscopy measurements of the membrane thickness as a function of temperature near the gel-to-fluid phase transition. We also report initial results of elastic neutron intensity scans vs. temperature taken on the backscattering spectrometer at NIST that probe the freezing of the membrane-associated water for comparison with results obtained with the neutral membrane DMPC.\footnote{Ibid.} [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A45.00007: Mobility of water and selected atoms in DMPG lipid bilayer membranes F.Y. Hansen, A. Roennest, G.H. Peters, H. Taub, A. Miskowiec Molecular dynamics simulations have been used to study the structure and mobility of water and selected atoms in dimyristoyl-phosphoglycerol (DMPG) lipids forming a fully hydrated free standing bilayer membrane at 310 K. The effect of the anionic headgroup in DMPG on structure and dynamics has been studied by comparison with simulation\footnote{F.Y. Hansen {\em et al.}, J. Chem. Phys., in press.} and experimental\footnote{M. Bai {\em et al.}, Europhys. Lett. {\bf 98}, 48006 (2012).} results for bilayer membranes of dimyristoyl-phosphorylcholine (DMPC) lipids, which have a neutral head group and the same aliphatic tails. The membrane is found to be in the fluid phase with monovalent sodium counter ions and in the gel phase with divalent calcium counter ions as evidenced by an area/lipid change and the NMR order parameter. The simulation results are compared with preliminary neutron scattering results. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A45.00008: High-speed Membrane Imaging with Digital Holography Thomas Dimiduk, Amy Chen, Laura Arriaga, Vinothan Manoharan Lipid membranes can change on timescales faster than traditional three dimensional imaging tools can follow. Digital holography offers a the potential to observe observe membranes in 3D at 1000 Hz or greater to resolve dynamics down to thermal fluctuations. This works because holography encodes 3D information into a single 2D image, allowing imaging limited only by camera speed. However, precise quantitative interpretation of holograms has proved challenging for samples of any complexity. To address this limitation, I am developing methods based on the discrete dipole approximation and a new mathematical approach to solving inverse problems. I will present these methods and preliminary measurements of membrane dynamics using holography. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A45.00009: Compositional interface dynamics within symmetric and asymmetric planar lipid bilayer membranes Tao Han, Mikko Haataja Compositional domains within multicomponent lipid bilayer membranes are believed to facilitate many important cellular processes. In this work, we will first develop a general model of planar lipid bilayer membrane within a phase field framework, which includes not only advective and diffusive lipid transport mechanism, but also incorporates an asymmetry between the lipid compositions and thermodynamic behavior between the two leaflets, as well as an intermonolayer thermodynamic coupling and friction effects. Then, we will derive the general equations that describe the dynamics of compositional domains within planar membranes with asymmetry in leaflet properties and in the presence of a thermodynamic coupling between the leaflets. These equations are then employed to develop analytical solutions to the dynamics of the recurrence of registration for circular domains in the case of weak coupling. The validity of the analytical solutions is established by a direct comparison between the predicted dynamics and those obtained from numerical simulations of the the phase-field model. [Preview Abstract] |
Monday, March 18, 2013 10:12AM - 10:24AM |
A45.00010: The effects of cholesterol concentration in lipid packing and domain registration in ternary mixture lipid multilayer Yicong Ma, Sajal Ghosh, Laura Connelly, Ratneshwar Lal, Sunil Sinha The effects of cholesterol in membrane rafts formation remain a mystery even until today. In our study of model membrane multilayer systems consisting of DPPC/DOPC/Cholesterol, we have characterized the morphology changes using AFM and optical microscopy, and the bilayer electron density profile using X-ray reflectivity, as a function of cholesterol concentration. In this presentation, we shall discuss how the cholesterol concentration affects the lipid packing within the bilayer, as well as the interlayer coupling of phase separated domains. X-ray scattering, AFM and optical microscopy which look at different length scales would constitute a complete picture. Our results may shed new light on the understanding of the role of cholesterol in raft formation in biological membranes. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A45.00011: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 10:36AM - 10:48AM |
A45.00012: The Effects of Ca$^{2+}$ on the Dynamics of PIP$_2$ containing Lipid Bilayers Ian McCabe, Martin Forstner Changes of intercellular Ca$^{2+}$ concentrations are one of the most ubiquitous signaling events that accompany or precede large scale cellular responses. We are in particular interested in the direct modulation of phosphatidylinositol 4,5-bisphosphate (PIP$_2$) organization in the membrane due to Ca $^{+2}$. At physiological conditions, PIP$_2$'s headgroup is multiply negatively charged (\textgreater\ 3 effective charges) and interacts with the cationic Ca$^{2+}$. By coordinating several PIP$_2$ head-groups, calcium ions can induce condensation and aggregation of PIP$_2$. A series of experiments were conducted on supported lipid bilayers containing physiological quantities of PIP$_2$. Fluorescence correlation spectroscopy (FCS) was used to study the response of the PIP$_2$ to changes in the concentration of Ca$^{2+}$ ions. As Ca$^{2+}$ concentration increases, the FCS indicates that PIP$_2$ goes from a freely diffusing single species to a multiple species system. The diffusion rates of the additional species decrease with increasing [Ca$^{2+}$], thus indicating increasing aggregate sizes with increasing, but physiological relevant Ca$^{2+}$ concentrations. An intriguing effect was observed at very low Ca$^{2+}$ levels. The diffusion rate was consistently measured to increase upon addition of small concentrations of Ca$^{2+}$ before decreasing as the concentrations increased. A series of polymer cushioned bilayers were used to attempt to gain greater insight into the nature of the membrane/support interaction and the nature of this effect. [Preview Abstract] |
Session A46: SPS Undergraduate I
Sponsoring Units: SPSChair: Toni Sauncy, American Institute of Physics
Room: Hilton Baltimore Holiday Ballroom 5
Monday, March 18, 2013 8:00AM - 8:12AM |
A46.00001: Electron microscopy of sillenites Craig Scurti, Nicolas Auvray, Michael Lufaso, Hideo Kohno, Daniel Arenas In this undergraduate project, the student performed transmission and scanning electron microscopy measurements on two sillenite compounds: Bi$_{12}$SiO$_{20}$ and Bi$_{25}$InO$_{39}$. To our knowledge, the electron diffraction patterns of sillenites have not been reported in the literature before. Our preliminary results show that both the tetravalent and trivalent compound have the sillenite structure. Using concepts from undergraduate solid state physics, the student will explain how the electron diffraction patterns were analyzed. [Preview Abstract] |
Monday, March 18, 2013 8:12AM - 8:24AM |
A46.00002: Inverted Pyramid Texturing of Si by Single Exposure Three-beam Interference Lithography B. Summers, M. Langhoff, K. Ghosh Increasing energy demands combined with environmental concerns prompts the need for cost-efficient solar cells. One way in which this can be achieved is by etching an inverted nano-pyramid texture into the silicon substrate thereby reducing the requisite amount of material. This is due to the ability of altering the pyramid size such that it corresponds to specific wavelengths, which results in higher light trapping efficiency. These inverted pyramids can be fabricated using three-beam lithography to create the desired hole/dot photoresist pattern in order to etch the substrate. The process can be done as a single exposure by aligning two dielectric mirrors and the sample at specific angles with respect to one another and the incoming laser beam. Using this method, nanostructures of Si and wide bandgap oxide semiconductors such as ZnO and NiO will be fabricated. Detailed results will be discussed in this presentation. This work is partially supported by National Science Foundation (DMR- 0907037). [Preview Abstract] |
Monday, March 18, 2013 8:24AM - 8:36AM |
A46.00003: Effects of Sputtering Energy on Surface Defect Formation on Ge(110) Samantha MacIntyre, Marshall van Zijll, Bret Stenger, Michael Norton, Noelle Oguri, Shirley Chiang Pyramid-shaped defects were observed in STM images to form on clean Ge(110) surfaces as a result of argon ion sputtering. By periodically imaging the samples after various numbers of sputtering and annealing cycles, we systematically studied the formation of these defects as a function of the Ar$^{\mathrm{+}}$ ion sputtering energy. Although the number and size of pyramids increased with sputtering energy from 100 to 200eV, the sample sputtered with 300eV ions showed a very flat surface with very few pyramids. The sample sputtered with 400eV ions appears to have mountain ranges of highly stepped regions with numerous pyramids on the edges, separated by flat valleys of reconstructed c(8x10) surface. Many pyramids are capped by a cluster of atoms, probably carbon, which may have served as the nucleation site. To explain the dependence of defect formation on sputtering energy, we present a mechanism involving competition between uncovering parts of new pyramids and breaking down older pyramids. Using different sputtering energies for controlled defect formation could be an effective tool for controlling island growth at defects on substrates. [Preview Abstract] |
Monday, March 18, 2013 8:36AM - 8:48AM |
A46.00004: Studies of electron spin in GaAs quantum dots Daniel Craft, John Colton, Tyler Park, Phil White We have studied electron spins in GaAs quantum dots with a pump-probe technique that normally yields the T1 spin lifetime, the time required for initially polarized electrons to relax and randomize. Using a circularly polarized laser tuned to the wavelength response of the quantum dot we can ``pump'' the spins into alignment. After aligning the spins we can detect them using a second, linearly polarized ``probe'' laser. By changing the delay between the two lasers we can trace out the spin response over time. In contrast with other samples (bulk GaAs and a GaAs quantum well), where the spin response decayed exponentially with time, initial data on the quantum dots has shown an unexpected, oscillating behavior which dies out on the order of 700 ns, independent of both temperature and magnetic field. [Preview Abstract] |
Monday, March 18, 2013 8:48AM - 9:00AM |
A46.00005: Surface plasmon enhanced F\"{o}rster resonance energy transfer in fluorescent molecules using metal wire gratings Zach Wetzel, Jennifer Steele Forster resonance energy transfer (FRET) is a powerful tool used to study spatial relationships in biological systems. FRET relies on a nonradiative energy transfer between a donor (D) and acceptor (A) fluorophore. The D-A pair must be located within their Forster radius for an efficient transfer of energy. Surface plasmon (SP) excitations increase the emission of fluorescent molecules by two mechanisms. SPs excited at the fluorophore absorption wavelength increase the excitation rate of the fluorophores. SP modes at the fluorophore emission wavelength provide an additional decay channel for the fluorophores to return to the ground state, increasing the quantum yield and the photostability of the fluorophore. In this study, metal wire gratings were chosen because gratings support SP resonances over a wide wavelength range, allowing overlap for both absorption and emission wavelengths. This research seeks to develop methods for using metal grating SPs to increase the Forster radius for D-A pairs. For this project, gold gratings with a period of 500 nm were fabricated using a nanotransfer printing method. Fluorescence was measured as a function of angle to determine the enhancement. These outcomes will increase the number of physical systems that can utilize FRET. [Preview Abstract] |
Monday, March 18, 2013 9:00AM - 9:12AM |
A46.00006: Remotely Tunable Nonlinear Metamaterial at Microwave Frequency Shelby Lee, Sinhara Silva, Jiangfeng Zhou We demonstrate a remotely tunable metamaterial at microwave frequency. The metamaterial consists of a two-gap split ring resonator with varactor diodes integrated in to one of the gaps. By varying a microwave pump signal remotely, the capacitance of the varactor diodes can be controlled. Thus we can tune the working frequency of the metamaterial. Our metamaterials enable an easily-applicable approach to realize tunable frequency without an external bias circuit compared to other tunable metamaterials. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:24AM |
A46.00007: Determination of the surface spin-polarization of perovskite oxides using point-contact Andreev reflection spectroscopy Everett Grimley, Amlan Biswas Materials with surface spin-polarization are invaluable for incorporation into devices that utilize spin-polarized currents. Point-contact Andreev reflection spectroscopy is currently one of the few techniques capable of direct measurement of surface spin-polarization. Niobium wire was electrochemically etched in a potassium hydroxide solution to form sharp tips which were used to form point-contacts with perovskite oxides in single crystal and thin film forms. Surface spin-polarization values were determined at 4.2 K for several materials including La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, which is a material with purported 100{\%} spin polarization. The results show that surface spin polarization of perovskites is smaller than theoretically predicted. [Preview Abstract] |
Monday, March 18, 2013 9:24AM - 9:36AM |
A46.00008: Spin Propagation Through Antiferromagnetic Bulk Structure in Exchange Biased Magnetic Trilayers Michael Crumrine, Hillary Kirby, Casey Miller When an exchange bias is induced in materials with a ferromagnetic (FM) -- antiferromagnetic (AF) interface, the interfacial coupling between the antiferromagnet and FM manifests itself as a shift in the magnetic hysteresis loop. It has been an unresolved issue as to the role the bulk spin of the antiferromagnet plays in exchange bias and whether or not exchange bias is entirely an interfacial effect. We fabricated several FM/AF/FM trilayer structures of Py(100{\AA})/FeMn(x)/Ni$_{69}$Cu$_{31}$(200{\AA}) with varying antiferromagnet thicknesses and used a field cool procedure to induce an exchange bias. A Magneto-Optical Kerr Effect magnetometer was used to investigate the propagation of spin information through the antiferromagnet by examining the hysteresis loops at different angles of applied field with respect to the magnetization. It was observed that there was no induced exchange bias in the NiCu probe layer for any of the antiferromagnet thicknesses, and we conclude that the patterning of the antiferromagnetic layer transmits no spin information for thicknesses greater than 100{\AA}. [Preview Abstract] |
Monday, March 18, 2013 9:36AM - 9:48AM |
A46.00009: Systematic Investigation of Magnetostriction in Composite Magnetorheological Elastomers: the Effect of Particle Shape, Alignment, and Volume Fraction Christopher Kassner, William Rieger, Paris Von Lockette, Samuel Lofland We have completed a study of the magnetoelastic properties of several types of magnetorheological elastomers (MREs), composites consisting of magnetic particles cured in an elastic matrix. We have made a number of samples with different particle arrangements (pseudo-random and aligned), volume fraction, and particle shape (rods, spheres, and disks) and measured the field dependent strain in order to determine the magnetostriction. We found that the magnetostriction in these samples is highly dependent on the sample particle shape (aspect ratio) and volume fraction and ordering to a lesser extent. While much of the past work has focused on spherical particles, our results indicate that both rods and disks can yield enhanced results. We discuss our findings in terms of magnetic energy of the particles and elastic energy of the matrix. We then consider the issue of optimization. This work was supported in part by NSF Grant CMMI - 0927326. [Preview Abstract] |
Monday, March 18, 2013 9:48AM - 10:00AM |
A46.00010: Magnetic-Field Dependence of the Spinon Velocity in the $S=$1/2 Linear-Chain Heisenberg Antiferromagnet Copper Pyrazine Dinitrate K.E. Marino, C.P. Aoyama, M.M. Turnbull, C.P. Landee, Y. Takano We have measured the specific heat of fully deuterated copper pyrazine dinitrate (CuPzN), a spin-1/2 antiferromagnetic chain compound, at temperatures down to 0.12 K in magnetic fields up to 14 T. This was done to reduce nuclear heat contributions by using deuterated CuPzN and to better define the magnetic heat capacity by taking measurements beyond the saturation field. The results are in good agreement with previous data taken by Hammar \textit{et al.} in fields up to 9 T. The spinon velocity obtained from the specific heat is compared to theoretical predictions as a function of magnetic field. [Preview Abstract] |
Monday, March 18, 2013 10:00AM - 10:12AM |
A46.00011: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 10:12AM - 10:24AM |
A46.00012: Bulk Growth of YBa$_2$Cu$_3$O$_{7-\delta}$ Superconductors with Enhanced Flux Pinning Jodi-Ann McLean, Matthew C. Sullivan, Janet Hunting We present our work on the bulk growth of YBa$_2$Cu$_3$O$_{7-\delta}$ (Y-123) superconductors with enhanced flux pinning abilities grown using the melt textured growth method. Polycrystalline precursor materials of superconducting Y-123 and insulating Y$_2$BaCuO$_5$ (Y-211) are synthesized by sintering commercially available Y$_2$O$_3$, CuO, and BaCO$_3$. This process is repeated multiple times to improve the purity and crystal structure of the precursors. In order to make a superconductor with enhanced flux-pinning, it is necessary to add insulating Y-211 impurities to act as pinning centers to the bulk Y-123 superconductor, heat the mixture to temperatures that liquefy the superconducting phase, then cool the mixture slowly to crystallize the superconducting phase. Afterwards we anneal the enhanced flux-pinning superconductor in oxygen to restore oxygen content that was removed during the firing process. We present data on the crystal structure of the precursor materials (Y-123 and Y-211)) and the superconducting transition temperature of the precursor Y-123. In addition, we present data on the transition temperatures and the flux pinning forces of the enhanced flux-pinning superconductors. [Preview Abstract] |
Monday, March 18, 2013 10:24AM - 10:36AM |
A46.00013: Growth and Characterization of Na-doped KFeAs Zachary Sims, Guotai Tan, Scott Carr, Chenglin Zhang, Pengcheng Dai We grew mulitple dopings of Na-doped KFeAs, with a goal of observing an upward shift in the Tc from the KFeAs parent compound and a sharpening of the transistion phase curve. Using a VSM and PPMS to charecterize the magnetic transport, resistivity, and heat capacity, we have come to a conclusion on the sucess of Na-doping into the KFeAs family of FeAs superconductors. [Preview Abstract] |
Monday, March 18, 2013 10:36AM - 10:48AM |
A46.00014: Synthesis and Characterization of Ytterbium-filled CoGe$_{1.5}$Se$_{1.5}$ compositions Walter Hill, Yongkwan Dong, George S. Nolas Polycrystalline skutterudite-related compounds with nominal composition YbCo$_{4}$Ge$_{\mathrm{6+x}}$Se$_{\mathrm{6-x}}$(0 \textless x \textless 1) were prepared by melting of the constituent elements followed by annealing, and subsequent hot-pressing for densification. Structural and phase characterized was achieved by X-ray diffraction and electron microscopy. The crystal structure of skutterudites allows for voids within the crystal lattice that can be filled by ``guest atoms'' such as ytterbium. It is well known that this guest-atom-filling of the voids can result in significant phonon scattering, although these materials possess relatively good electrical properties, and are therefore thought of as PGEC (Phonon Glass Electron Crystal) materials. The goal of this research was to synthesize these skutterudite-related compounds and examine their thermoelectric properties. Their composition and properties will be discussed. [Preview Abstract] |
Monday, March 18, 2013 10:48AM - 11:00AM |
A46.00015: Designing Drops, Loops, and Hills: The Physics behind Roller Coaster Design Katharyn Christiana, Carolina Ilie Almost everyone has seen a roller coaster at one time in their life. They range in type from old wooden coasters from decades passes to modern machines made of steel that allow you to stand up while riding. The basic physics behind these machines is relatively simple, but in the modern world we strive to design bigger and better machines that push the human body and the laws of physics to their limits. But how do the designers of these rides maintain the balance between making riders feel like they're on the brink of death while keeping them completely safe? The answer can be found in basic physics and mechanical engineering. This is a part of the honors thesis that focuses on the mechanical principles applied in roller coaster design. The theoretical part of the thesis will be complemented by a full small scale ride design. [Preview Abstract] |
Session A47: Invited Session: Excitable Dynamics in Biological Systems
Sponsoring Units: DBIOChair: Pankaj Mehta, Boston University
Room: Hilton Baltimore Holiday Ballroom 6
Monday, March 18, 2013 8:00AM - 8:36AM |
A47.00001: Cell signaling at the single-cell level Invited Speaker: Michael Elowitz |
Monday, March 18, 2013 8:36AM - 9:12AM |
A47.00002: Excitability in Dictyostelium development Invited Speaker: David Schwab Discovering how populations of cells reliably develop into complex multi-cellular structures is a key challenge in modern developmental biology. This requires an understanding of how networks at the single-cell level, when combined with intercellular signaling and environmental cues, give rise to the collective behaviors observed in cellular populations. I will present work in collaboration with the Gregor lab, showing that the signal-relay response of starved cells of the amoebae Dictyostelium discoideum can be well modeled as an excitable system. This is in contrast to existing models of the network that postulate a feed-forward cascade. I then extend the signal-relay model to describe how spatial gradient sensing may be achieved via excitability. One potential advantage of relying on feedback for gradient sensing is in preventing ``cheaters'' that do not produce signals from taking over the population. I then combine these models of single-cell signaling and chemotaxis to perform large-scale agent-based simulations of aggregating populations. This allows direct study of how variations in single-cell dynamics modify population behavior. In order to further test this model, I use the results of a screen for mutant cell lines that exhibit altered collective patterns. Finally, I use an existing FRET movie database of starved cell populations at varying cell densities and dilution rates to study heterogeneity in repeated spatio-temporal activity patterns. [Preview Abstract] |
Monday, March 18, 2013 9:12AM - 9:48AM |
A47.00003: Excitability in neural coding Invited Speaker: Adrienne Fairhall |
Monday, March 18, 2013 9:48AM - 10:24AM |
A47.00004: Action Potential Initiation in the Hodgkin-Huxley Model Invited Speaker: Michael Brenner |
Monday, March 18, 2013 10:24AM - 11:00AM |
A47.00005: Cell motility: Combining experiments with modeling Invited Speaker: Wouter-Jan Rappel Cell migration and motility is a pervasive process in many biology systems. It involves intra-cellular signal transduction pathways that eventually lead to membrane extension and contraction. Here we describe our efforts to combine quantitative experiments with theoretical and computational modeling to gain fundamental insights into eukaryotic cell motion. In particular, we will focus on the amoeboid motion of Dictyostelium discoideum cells. [Preview Abstract] |
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