Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R0: Kavli Foundation Special Session: Forefront Physics for Real World Problems: Energy, Climate, and the Environment
Sponsoring Units: APSChair: Michael Turner, APS President and University of Chicago
Room: Hilton Baltimore Key Ballroom
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R0.00001: The Promise of Photovoltaics Invited Speaker: Steven Chu |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R0.00002: Earth's Climate History from Glaciers and Ice Cores Invited Speaker: Lonnie Thompson Glaciers serve both as recorders and early indicators of climate change. Over the past 35 years our research team has recovered climatic and environmental histories from ice cores drilled in both Polar Regions and from low to mid-latitude, high-elevation ice fields. Those ice core --derived proxy records extending back 25,000 years have made it possible to compare glacial stage conditions in the Tropics with those in the Polar Regions. High-resolution records of $\delta^{\mathrm{18}}$O (in part a temperature proxy) demonstrate that the current warming at high elevations in the mid- to lower latitudes is unprecedented for the last two millennia, although at many sites the early Holocene was warmer than today. Remarkable similarities between changes in the highland and coastal cultures of Peru and regional climate variability, especially precipitation, imply a strong connection between prehistoric human activities and regional climate. Ice cores retrieved from shrinking glaciers around the world confirm their continuous existence for periods ranging from hundreds to thousands of years, suggesting that current climatological conditions in those regions today are different from those under which these ice fields originated and have been sustained. The ongoing widespread melting of high-elevation glaciers and ice caps, particularly in low to middle latitudes, provides strong evidence that a large-scale, pervasive and, in some cases, rapid change in Earth's climate system is underway. Observations of glacier shrinkage during the 20th and 21st century girdle the globe from the South American Andes, the Himalayas, Kilimanjaro (Tanzania, Africa) and glaciers near Puncak Jaya, Indonesia (New Guinea). The history and fate of these ice caps, told through the adventure, beauty and the scientific evidence from some of world's most remote mountain tops, provide a global perspective for contemporary climate. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R0.00003: Physical Controls of the Earth's Climate and Climate change Invited Speaker: Graeme Stephens The Earth's climate system and changes to it are determined by the physical processes that govern the flows of energy to and from the atmosphere and Earth's surface. Although the energy exchanges at the top of the atmosphere are well determined from available satellite measurements, the global character of the energy flows \textit{within} the climate system, and to and from the Earth's surface in particular, are not directly measured and thus are much more uncertain. The surface energy balance is particularly important since geographical variations of its distribution drives ocean circulations, dictates the amount of water evaporated from the Earth's surface, fuels the planetary hydrological cycle and ultimately controls how this hydrological cycle responds to forced climate change. This talk reviews our state of understanding of the physical processes that determine the energy balance, couple to the Earth's water cycle and are responsible for the most important climate feedbacks that dictate the pace of climate change. Challenges in understanding the mechanisms responsible for feedbacks associated with clouds and precipitation, water vapor, snow cover and carbon will be highlighted. The further complexity and uncertainty that aerosols add to the cloud and precipitation feedbacks will also be reviewed. The effects of uncertainties in our understanding of the physical climate system, and feedbacks within it, will be reviewed in the context of climate change projections. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R0.00004: Environmental Forensics: Molecular Insight into Oil Spill Weathering Helps Advance High Magnetic Field FT-ICR Mass Spectrometry Invited Speaker: Amy McKenna The depletion of terrestrial global oil reserves has shifted oil exploration into offshore and ultra-deep water ( \textgreater\ 5000 ft) oil reserves to meet global energy demands. Deep water reservoirs are currently in production in many parts of the world, including the Gulf of Mexico, but production is complicated by the water depth and thick salt caps that challenge reservoir characterization / production. The explosion aboard the \textit{Deepwater Horizon} in April 2010 resulted in an estimated total release of $\sim$5 million barrels (BP claims that they collected $\sim$1M barrels, for a net release of 4 M) of light, sweet crude oil into the Gulf of Mexico and shifted attention toward the environmental risks associated with offshore oil production. The growing emphasis on deep water and ultra-deep water oil production poses a significant environmental threat, and increased regulations require that oil companies minimize environmental impact to prevent oil spills, and mitigate environmental damage when spills occur. Every oil spill is unique. The molecular transformations that occur to petroleum after contact with seawater depend on the physical and chemical properties of the spilled oil, environmental conditions, and deposition environment. Molecular-level knowledge of the composition, distribution, and total mass of released hydrocarbons is essential to disentangle photo- and bio-degradation, source identification, and long-term environmental impact of hydrocarbons released into the environment. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is unsurpassed in its ability to characterize complex mixtures at the level of elemental composition assignment. Only FT-ICR mass spectrometry can routinely achieve the required minimum resolving power necessary to elucidate molecular-level characterization of crude oil. Conversely, the spectral complexity of petroleum facilitates identification of systematic errors in the accumulation, transfer, excitation, and detection events in the FT-ICR experiment. For example, the high density of peaks at each nominal mass unit provides unprecedented insight into how excitation conditions affect ion motion during detection. Aggregated oil (i.e., tar balls, tar mats) that reached the surface exhibits a more than two-fold increase in the total number of detected species, with an increased number of oxygenated species. Principal component analysis (PCA) applied to two possible source oils (contained within the same ship) and weathered samples provide the first application of FT-ICR MS for source identification. Molecular formulae from parent and weathered oil indicate that the lightest petroleum fractions (saturated hydrocarbons) are the most readily oxidized components, and can serve as a template to determine chemical transformations that occur throughout the water column. The ability to differentiate and catalogue compositional changes that occur to oil after its release into the environment relies heavily on gains achieved in nearly all steps in the FT-ICR mass spectral experiment required to accommodate larger ion populations inherent to heavily weathered crude oil. Here, we present the requirement for FT-ICR MS for comprehensive oil spill characterization, and highlight advances made to FT-ICR MS experimental conditions developed from petroleum characterization. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R0.00005: Forefront Research in Batteries for Electric Vehicles Invited Speaker: Stephen Harris |
Session R1: Invited Session: Controllng Magnetism Without Magnetic Fields
Sponsoring Units: DCMP GMAGChair: Ramamoorty Ramesh, University of California at Berkeley
Room: Ballroom I
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R1.00001: Electric Field Control of Magnetization Using Multiferroic BFO Invited Speaker: Sayeef Salahuddin |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R1.00002: Controlling Magnetism by light Invited Speaker: Theo Rasing From the discovery of sub-picosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 femtosecond laser pulse, the manipulation of spins by ultra short laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation and quantum computation. In addition, when the time-scale of the perturbation approaches the characteristic time of the exchange interaction ($\sim$ 10-100 fs), the magnetization dynamics enters a novel, highly non-equilibrium, regime, which was recently demonstrated by both fs optical and X-ray experiments. Theoretically, this field is still in its infancy, using phenomenological descriptions of the none-equilibrium dynamics between electrons, spins and phonons via 2- or 3-temperature models and atomistic spin simulations. A proper description should include the time dependence of the exchange interaction and nucleation phenomena on the nanometer length scale. Such developments need to be supported by experimental investigations of magnetism at its fundamental time and length scales, i.e. with fs time and nanometer spatial resolution. Such studies require the excitation and probing of the spin and angular momentum contributions to the magnetic order at timescales of 10fs and below, a challenge that could be met by the future fs X-ray FEL's but in some cases also with purely optical techniques.\\[4pt] Recent references:\\[0pt] [1] A. Kirilyuk, et al, \textbf{Rev. Mod. Phys. 82}, 2731-2784 (2010)\\[0pt] [2] I. Radu et al, \textbf{Nature 472}, 205 (2011)\\[0pt] [3] J. Mentink et al, \textbf{Phys.Rev.Lett. 108, }057202 (2012)[0pt] [4] T. Ostler et al, \textbf{Nature Comm. 3}, 666 (2012)\\[0pt] [5] A.R. Khorsand et al, \textbf{Phys.Rev.Lett.108}, 127205 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R1.00003: Spin Mechanics in Ferromagnet/Ferroelectric Hybrid Structures Invited Speaker: Sebastian Goennenwein In most ferromagnets, magnetic and elastic degrees of freedom are coupled -- as evident, e.g., from the hum of a transformer. In the ``spin mechanics'' scheme, one intentionally exploits magneto-elastic coupling (inverse magneto-striction) to control the magnetization of ferromagnetic films. On the one hand, I will briefly review spin mechanics in the static limit, taking ferromagnetic nickel thin film/piezoelectric actuator hybrid structures as prototype examples [1]. In these hybrids, the application of an electric field to the actuator results in a uniaxial strain, which is transferred into the Ni film. Due to magneto-elastic coupling, the voltage-controlled strain modifies the magnetic anisotropy and thus induces a magnetization reorientation. This allows for a voltage-controlled, fully reversible magnetization orientation manipulation within a range of approximately 90 degrees at room temperature in these hybrids. On the other hand, I will show that the spin mechanics scheme also is operational at GHz frequencies. In the corresponding experiments, we use surface acoustic waves (SAWs) propagating in Ni/LiNbO$_{\mathrm{3}}$ hybrid devices for the all-elastic excitation and detection of ferromagnetic resonance (FMR). Our SAW magneto-transmission data are consistently described by a modified Landau-Lifshitz-Gilbert approach [2], in which the magnetization precession is not driven by a conventional, external microwave magnetic field, but rather by a purely virtual, internal tickle field stemming from radio-frequency magneto-elastic interactions. This causes a distinct magnetic field orientation dependence of elastically driven FMR, observed in both simulations and experiment. Last but not least, I will address perspectives for spin mechanics experiments, e.g., the study of magnon-phonon coupling, or acoustic spin pumping [3] in normal metal/ferromagnet hybrid structures. \\[4pt] [1] M. Weiler \textit{et al.}, New J. Phys. \textbf{11}, 013021 (2009).\\[0pt] [2] M. Weiler \textit{et al.}, Phys. Rev. Lett. \textbf{106}, 117601 (2011).\\[0pt] [3] M. Weiler \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 176601 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R1.00004: Control of Magnetic Properties Across Metal to Insulator Transitions Invited Speaker: Jose de la Venta Controlling the magnetic properties of ferromagnetic (FM) thin films without magnetic fields is an on-going challenge in condensed matter physics with multiple technological implications. External stimuli and proximity effects are the most used methods to control the magnetic properties. An interesting possibility arises when ferromagnets are in proximity to materials that undergo a metal-insulator (MIT) and structural phase transition (SPT). The stress associated with the structural changes produces a magnetoelastic anisotropy in proximity coupled ferromagnetic films that allows controlling the magnetic properties without magnetic fields. Canonical examples of materials that undergo MIT and SPT are the vanadium oxides (VO$_{2}$ and V$_{2}$O$_{3})$. VO$_{2}$ undergoes a metal/rutile to an insulator/monoclinic phase transition at 340 K. In V$_{2}$O$_{\mathrm{3}}$ the transition at 160 K is from a metallic/rhombohedral to an insulating/ monoclinic phase. We have investigated the magnetic properties of different combinations of ferromagnetic (Ni, Co and Fe) and vanadium oxide thin films. The (0.32{\%}) volume expansion in VO$_{2}$ or the (1.4{\%}) volume decrease in V$_{2}$O$_{3}$ across the MIT produces an interfacial stress in the FM overlayer. We show that the coercivities and magnetizations of the ferromagnetic films grown on vanadium oxides are strongly affected by the phase transition. The changes in coercivity can be as large as 168{\%} and occur in a very narrow temperature interval. These effects can be controlled by the thickness and deposition conditions of the different ferromagnetic films. For VO$_{2}$/Ni bilayers the large change in the coercivity occurring above room temperature opens the possibilities for technological applications. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R1.00005: Controlling Magnetism with electric fields Invited Speaker: Leonid Rokhinson |
Session R2: Invited Session: New Developments in Organic Spintronics
Sponsoring Units: DCMPChair: Markus Wohlgenannt, University of Iowa
Room: Ballroom II
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R2.00001: Electrically detected magnetic resonance in organic light emitting diodes Invited Speaker: Christoph Boehme Due to the built-in weak spin-orbit coupling of carbon based materials, electronic transitions in organic semiconductors are subjected to strong spin-selection rules that are responsible for a number of interesting electron spin- and even nuclear spin-dependent electrical and optical properties of these materials, including device efficiencies of organic light emitting diodes and solar cells or magnetoresistive and magneto-optic effects. In recent years, we have studied how these effects work and how they can be utilized for organic semiconductor device improvement and new device applications. Our focus has been in particular on the effects of spin on $\pi$-conjugated polymer based bipolar injection devices (more commonly known as organic light emitting diodes, OLEDs). In OLEDs, spin-interactions between recombining charge carriers do not only control electroluminescence rates but also the magnetoresistance. We have shown that spin-coherence can be observed through current measurements [1] and that these effects can be utilized for a coherent, pulsed electrically detected magnetic resonance spectroscopy (pEDMR) which enables us to encode the qualitative nature of spin-dependent mechanisms (the polaron pair mechanism [2,3] and the triplet polaron recombination [4]) and the their dynamical nature (spin-relaxation, electronic relaxation, hopping times [5]). The insights gained from these studies have led to the invention of a robust absolute magnetic field sensor based on organic thin film materials with absolute sensitivities of $<$50nT/Hz$^{1/2}$ [6].\\[4pt] [1] D. R. McCamey, et al., Nature Materials, 7, 723 (2008).\\[0pt] [2] D. R. McCamey, et al., Phys. Rev. Lett. 104, 017601 (2010).\\[0pt] [3] S.-Y. Lee, et al., J. Am. Chem. Soc. 133, 072019 (2011).\\[0pt] [4] Baker et al., Phys. Rev. B 84, 165205 (2011).\\[0pt] [5] Baker et al., Phys. Rev. Lett. 108, 267601 (2012). [6] Baker et al., Nature Communications 3, 898 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R2.00002: Direct measurements of spin propagation in organic spin valves by low-energy muon spin rotation Invited Speaker: Alan Drew Organic semiconductors fall into a class of materials that shows significant potential for future applications, but many of the fundamental mechanisms of spin relaxation and transport are not understood. As a result, the field is becoming extremely topical, but there is a need for suitable techniques that can yield information on intrinsic spin dynamics and transport in organic materials. I will present Low Energy Muon Spin Rotation measurements and demonstrate that this technique can directly measure the depth resolved spin polarisation of charge carriers in organic spin injection devices [1]. I will then go on to show that it is possible to separate out the various contributions to spin decoherence, differentiating between interface and bulk effects. By correlating macroscopic measurements with these separated interfacial and bulk effects, I will present evidence that it is possible to engineer interfaces in organic spintronic devices [2]. Finally, I will present some of the latest results on how spin injection and transport depend on bias voltage [3].\\[4pt] [1] A. J. Drew et al., Nature Materials 8, 109 (2009)\\[0pt] [2] L. Schulz et al., Nature Materials 10, 39 (2011)\\[0pt] [3] L. Nuccio et al., in preparation. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R2.00003: Percolative Theory of Organic Magnetoresistance and Fringe-Field Magnetoresistance Invited Speaker: Michael E. Flatt\'e A recently-introduced percolation theory [1,2] for spin transport and magnetoresistance in organic semiconductors describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Increases in the spin-flip rate open up ``spin-blocked'' pathways to become viable conduction channels and hence, as the spin-flip rate changes with magnetic field, produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with measurements of the shape and saturation of measured magnetoresistance curves [3-5]. We find that the threshold hopping distance is analogous to the branching parameter of a phenomenological two-site model [6], and that the distinction between slow and fast hopping is contingent on the threshold hopping distance. Regimes of slow and fast hopping magnetoresistance are uniquely characterized by their line shapes. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory. Extensions to this theory also describe fringe-field magnetoresistance, which is the influence of fringe magnetic fields from a nearby unsaturated magnetic electrode on the conductance of an organic film [7]. This theory agrees with several key features of the experimental fringe-field magnetoresistance, including the applied fields where the magnetoresistance reaches extrema, the applied field range of large magnetoresistance effects from the fringe fields, and the sign of the effect. \\[4pt] All work done in collaboration with N. J. Harmon, and fringe-field magnetoresistance work in collaboration also with F. Maci\`a, F. Wang, M. Wohlgenannt and A. D. Kent. This work was supported by an ARO MURI.\\[4pt] [1] N. J. Harmon and M. E. Flatt\'e, PRL 108, 186602 (2012).\\[0pt] [2] N. J. Harmon and M. E. Flatt\'e, PRB 85, 075204 (2012).\\[0pt] [3] F. L. Bloom et al, PRL 99, 257201 (2007).\\[0pt] [4] T. D. Nguyen et al., Nature Materials 9, 345 (2010)\\[0pt] [5] J. A. Gomez et al., Synth. Met. 160, 317 (2010)\\[0pt] [6] W. Wagemans et al., JAP 103, 07F303 (2008).\\[0pt] [7] F. Wang et al., PRX 2, 021013 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R2.00004: Spin-polarized organic light emitting diode based on a novel bipolar spin-valve Invited Speaker: Tho Nguyen The spin-polarized organic light emitting diode (spin-OLED) has been long sought device within the field of organic spintronics. We designed, fabricated and studied a spin-OLED with ferromagnetic (FM) electrodes that acts as a bipolar organic spin valve (OSV), based on deuterated derivative of poly(phenylene-vinylene) with small hyperfine interaction [1]. In the double-injection limit the device shows $\sim$ 1{\%} spin-valve magneto-electroluminescence (MEL) response that follows the FM electrode coercive fields, which originates from the bipolar spin-polarized space charge limited current [2]. In stark contrast to the response properties of homopolar OSV devices, the MEL response in the double-injection device is practically bias voltage independent, and its temperature dependence follows that of the FM electrode magnetization. Our findings provide a pathway for organic displays controlled by external magnetic fields. \\[4pt] [1] T. D. Nguyen, G. Hukic-Markosian, F. Wang, L. Wojcik, Xiao-Guang Li, E. Ehrenfreund, Z. V. Vardeny, ``Isotope effect in spin response of $\pi $-conjugated polymer films and devices,'' Nature Materials 9, 345-352 (2010)\\[0pt] [2] T. D. Nguyen, E. Ehrenfreund and Z. V. Vardeny, ``Spin-polarized organic light emitting diode based on a novel bipolar spin-valve,'' Science 337, 204 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R2.00005: Spin-orbit coupling in organic spintronics Invited Speaker: Zhi-Gang Yu I will talk about spin-orbit coupling (SOC) in $\pi$-conjugated organicmaterials and its effects on spin characteristics including the spin-relaxation time, spin-diffusion length, and $g$ factor [1]. While $\pi$ electrons are responsible for low-energy electrical and optical processes in $\pi$-conjugated organic solids, $\sigma$ electrons must be explicitly included to properly describe the SOC. The SOC mixes up- and down-spin states and, in the context of spintronics, can be quantified by an admixture parameter in the electron and hole polaron states in $\pi$-conjugated organics. Molecular geometry fluctuations such as ring torsion, which are common in soft organic materials and may depend on sample preparation, are found to have a strong effect on the spin mixing. The SOC-induced spin mixing leads to spin flips as polarons hop from one molecule to another, giving rise to spin relaxation and diffusion. The spin-relaxation rate is found to be proportional to the carrier hopping rate. The spin-diffusion length depends on the spin mixing and hopping distance but is insensitive to the carrier mobility. The SOC influences the $g$ factor of the polaron state and makes it deviate from the free-electron value. The SOC strengths in common organics are quantified based on first-principles calculations and their values in tris-(8-hydroxyquinoline) aluminum (Alq$_3$) and in copper phthalocyanine (CuPc) are particularly strong, due to the orthogonal arrangement of the three ligands in the former and Cu $3d$ orbitals in the latter. The theory quantitatively explains the recent measured spin-diffusion lengths in Alq$_3$ from muon spin rotation and in CuPc from spin-polarized two-photon photoemission. \\[4pt] [1] Z. G. Yu, Phys. Rev. Lett. {\bf 106}, 106602 (2011); Phys. Rev. B {\bf 85}, 115201 (2012). [Preview Abstract] |
Session R3: Invited Session: Nonequilibrium Relaxation and Aging in Materials
Sponsoring Units: GSNP DCMPChair: Uew Tauber, Virginia Polytechnical Institute and State University
Room: Ballroom III
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R3.00001: Nonequilibrium behavior in strongly correlated electron systems Invited Speaker: Dragana Popovi\'c There is growing evidence that nonequilibrium behavior may underlie many complex phenomena exhibited by strongly correlated electronic materials with disorder. A two-dimensional electron system (2DES) in Si metal-oxide-semiconductor field-effect transistors has emerged as an excellent model system for studying glassy or nonequilibrium charge dynamics near the metal-insulator transition (MIT). In particular, studies of both conductance relaxations and noise on disordered samples, using several different experimental protocols, have established that the 2DES in Si exhibits all the main manifestations of glassiness: slow, correlated dynamics, nonexponential relaxations, diverging equilibration time (as temperature $T\rightarrow 0$), aging and memory. The results provide strong evidence that many such universal features are robust manifestations of glassiness, regardless of the dimensionality of the system. In addition, the experiments show that the 2D MIT is closely related to the melting of this Coulomb glass. The observations are consistent with predictions of the theoretical models that describe the MIT as a Mott transition with disorder. Some effects that are unique to Coulomb glasses have also been revealed, which should have important implications for theoretical modeling of the glassy dynamics in a 2DES and other strongly correlated materials. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R3.00002: Universally slow Invited Speaker: Ariel Amir Glassy systems are very common in nature, from disordered electronic and magnetic systems to window glasses and crumpled paper. Among their key properties are slow relaxations to equilibrium without a typical timescale, and dependence of relaxation on the system's age. After reviewing some of these physical systems, I will describe our approach to the problem, and show how it leads to a novel class of aging. The slow relaxations result from a broad distribution of ``relaxation eigenmodes,'' which relates to a particular class of random matrices. I will discuss recent results on the structure and localization properties of these modes, and their implications. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R3.00003: Dynamical symmetries in ageing phenomena Invited Speaker: Malte Henkel Systems undergoing physical ageing can be characterised by (i) undergoing slow relaxation (ii) absence of time-translation-invariance and (iii) dynamical scaling. Specific examples are obtained by quenching many-body systems from a high-temperature initial state to below their critical temperature. Here, we shall consider consequences of an assumed extension of dynamical scaling to a larger group of {\em local scale-transformations}. Explicit scaling forms of two-time responses and correlators are obtained. These will be compared with simulational data in simple magnets, as well as in many-body systems without an equilibrium stationary state, such as critical directed percolation or domain-growth in the Kardar-Parisi-Zhang universality class.\\[4pt] [1] M. Henkel, M. Pleimling, {\it Non-equilibrium phase transitions}, Vol. 2, Springer (Heidelberg 2010) \\[0pt] [2] M. Henkel, J.D. Noh, M. Pleimling, Phys. Rev. {\bf E85}, 030102(R) (2012) {\tt [arxiv:1109.5022]} \\[0pt] [3] M. Henkel, {\tt arxiv:1009.4139} \\[0pt] [4] M. Henkel, S. Stoimenov, Nucl. Phys. {\bf B847} [FS], 612 (2011) {\tt [arxiv:1011.6315]} [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R3.00004: Probing equilibrium by nonequilibrium dynamics: Aging in Co/Cr superlattices Invited Speaker: Christian Binek Magnetic aging phenomena are investigated in a structurally ordered Co/Cr superlattice through measurements of magnetization relaxation, magnetic susceptibility, and hysteresis at various temperatures above and below the onset of collective magnetic order. We take advantage of the fact that controlled growth of magnetic multilayer thin films via molecular beam epitaxy allows tailoring the intra and inter-layer exchange interaction and thus enables tuning of magnetic properties including the spin-fluctuation spectra. Tailored nanoscale periodicity in Co/Cr multilayers creates mesoscopic spatial magnetic correlations with slow relaxation dynamics when quenching the system into a nonequilibrium state. Magnetization relaxation in weakly correlated spin systems depends on the microscopic spin-flip time of about 10 ns and is therefore a fast process. The spin correlations in our Co/Cr superlattice bring the magnetization dynamics to experimentally better accessible time scales of seconds or hours. In contrast to spin-glasses, where slow dynamics due to disorder and frustration is a well-known phenomenon, we tune and increase relaxation times in ordered structures. This is achieved by increasing spin-spin correlation between mesoscopically correlated regions rather than individual atomic spins, a concept with some similarity to block spin renormalization. Magnetization transients are measured after exposing the Co/Cr heterostructure to a magnetic set field for various waiting times. Scaling analysis reveals an asymptotic power-law behavior in accordance with a full aging scenario. The temperature dependence of the relaxation exponent shows pronounced anomalies at the equilibrium phase transitions of the antiferromagnetic superstructure and the ferromagnetic to paramagnetic transition of the Co layers. The latter leaves only weak fingerprints in the equilibrium magnetic behavior but gives rise to a prominent change in nonequilibrium properties. Our findings suggest that scaling analysis of nonequilibrium data can serve as a probe for weak equilibrium phase transitions. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R3.00005: Aging processes in disordered materials: High-$T_c$ superconductors and ferromagnets Invited Speaker: Michel Pleimling Physical aging is generically encountered in systems far from equilibrium that evolve with slow dynamics. Well known examples can be found in structural glasses, spin glasses, magnetic systems, and colloids. Recent years have seen major breakthroughs in our understanding of aging processes in non-disordered systems. Progress in understanding aging in disordered systems has been much slower though. In this talk I discuss non-equilibrium relaxation in two different types of disordered systems: coarsening ferromagnets with disorder, characterized by a crossover from an initial power-law like growth of domains to a slower logarithmic growth regime, and interacting vortex lines in disordered type-II superconductors, where the interplay of vortex-vortex interaction and pinning results in a very rich non-equilibrium behavior. [Preview Abstract] |
Session R4: Topologocal insulators: Nanostructures and Possible Applications: Transport phenomena
Sponsoring Units: DCMPChair: Vidya Madhaven, Boston College
Room: Ballroom IV
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R4.00001: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R4.00002: Thermoelectric power factor of topological insulator Bi$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$Te$_{\mathrm{3-y}}$Se$_{\mathrm{y}}$ Te Chih Hsiung, Ting Yuan Chen, Li Zhao, Yi Hsin Lin, Yang Yuan Chen Topological insulator (TI) is a new quantum material. The surface states of TIs are protected by time-reversal symmetry which allows charge carrier to propagate on the edge of surface conducting channel without scattering. Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ is a well-known TI [1] and thermoelectric material because of its promising thermoelectric performances at room temperature. The conversion efficiency of~thermoelectric material is characterized by the dimensionless figure of merit ZT.~Decades of effort were devoted to ZT optimization either through composition alteration or nanostructure fabrication. In this study, the temperature dependence of resistance of bulk (exfoliated specimen with 140 $\mu $m thickness) shows semiconductor behavior (0.04 $\Omega $ cm at 300 K) without saturating regime in lower temperatures. In contrast, its nanoflake counterpart (100-500 nm) [2] shows a transition from semiconductor to metallic behavior near 100 -- 150 K with decreasing temperature and saturation at 10 K. Surface contribution to the total conductance of exfoliated specimens was acquired through Hall effect measurements in the magnetic field ranging from -9 to 9 Tesla. Surface contribution of BSTS samples increases from 3{\%} to 70{\%} as thickness decreases from 140 to 7~$\mu $m. In this work, we report a systematic study of thermoelectric power factor for various thicknesses of BSTS specimens to examine the thermoelectric power factor of their surfaces.\\[4pt] [1] Zhi Ren et al., Phys. Rev. B 84, 165311 (2011).\\[0pt] [2] Bin Xia et al., e-print arXiv1203.2997 [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R4.00003: Spin-dependent Peltier effect in 3D topological insulators Parijat Sengupta, Tillmann Kubis, Michael Povolotskyi, Gerhard Klimeck The Peltier effect represents the heat carrying capacity of a certain material when current passes through it. When two materials with different Peltier coefficients are placed together, the Peltier effect causes heat to flow either towards or away from the interface between them. This work utilizes the spin-polarized property of 3D topological insulator (TI) surface states to describe the transport of heat through the spin-up and spin-down channels. It has been observed that the spin channels are able to carry heat independently of each other. Spin currents can therefore be employed to supply or extract heat from an interface between materials with spin-dependent Peltier coefficients. The device is composed of a thin film of Bi2Se3 sandwiched between two layers of Bi2Te3. The thin film of Bi2Se3$_{\mathrm{\thinspace }}$serves both as a normal and topological insulator. It is a normal insulator when its surfaces overlap to produce a finite band-gap. Using an external gate, Bi2Se3 film can be again tuned in to a TI. Sufficiently thick Bi2Te3 always retain TI behavior. Spin-dependent Peltier coefficients are obtained and the spin Nernst effect in TIs is shown by controlling the temperature gradient to convert charge current to spin current. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R4.00004: Surface-to-surface scattering in three-dimensional (3D) topological insulator (TI) thin films Gen Yin, Darshana Wickramaratne, Roger Lake When the thickness of a 3D TI material is reduced below approximately 6nm, hybridization of the opposite surfaces states can result in inter-surface tunneling. Due to the rotational symmetry of the thin film, the k-s locking relation on opposite surfaces also has opposite chirality. Thus, in this inter-surface scattering mechanism, back-scattering is allowed without the flip of the spin. This effect breaks the protection of TI surface states against back-scattering. To~investigate the influence of the inter-surface scattering mechanism, we study different near-elastic scattering mechanisms in the surface state transport using Boltzmann transport equations within the relaxation time approximation. The effect of~screened Coulomb~impurities, low-energy acoustic phonons and surface magnetic~impurities on the TI surface states will be discussed. The response of the inter-surface scattering of TI states to various external stimuli such as a Rashba-like splitting and the orientation of the impurity magnetic moments will also be presented. Using our simulation results, we propose possible experimental methods to modulate the back-scatter protection of TI surface states in thin film TI materials. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R4.00005: Realization of Negative Capacitance with Topological Insulator Based MOS Capacitor Hui Yuan, Kai Zhang, Hao Zhu, Haitao Li, Dimitris Ioannou, Helmut Baumgart, Curt Richter, Qiliang Li Negative capacitance is one of way to achieve steep subthreshold slope exceeding its thermal limit in metal-oxide-semiconductor field effect transistor (MOSFET). The common materials under study for negative capacitance are ferroelectric thin films. However, the integration of regular ferroelectric materials (e.g., PZT) into semiconductor based devices is usually difficult due to the high temperature required for crystallization and precise control of oxygen percentage in ferroelectric materials. In this work, we found that negative capacitance can be achieved by introducing a topological insulator interlayer into a conventional MOS capacitor. Three-dimensional topological insulators inherently contain a insulator/semiconductor bulk and a gapless conducting surface. When an electric field is added to topological insulator interlayer, imbalanced charge carriers (electrons and holes) would be generated and then accumulate on either surface of the film, resulting in a temporary residual polarization. As a result, a ferroelectric-like hysteresis and negative capacitance are achieved. We believe this approach will be very attractive to achieve steep subthreshold using negative capacitance. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R4.00006: Transport measurements of the topological surface states in Bi$_2$Te$_3$ nanoribbon field effect devices Luis A. Jauregui, Michael T. Pettes, Li Shi, Leonid P. Rokhinson, Yong P. Chen We have grown nanoribbons (NRs) of Bi$_2$Te$_3$, a prototype topological insulator, by CVD and characterized them by TEM, Raman Spectroscopy and EDS. We fabricate backgated field effect devices where the chemical potential can be tuned and ambipolar field effect has been observed. The as-grown NRs are n-type and the 4-terminal resistance (R4p) versus temperature (T) shows a metallic behavior. Applying a sufficiently negative Vg, the R4p vs T displays an insulating behavior that saturates in a plateau at T $<$ 100K, suggesting a metallic surface conduction dominant at low temperatures. Aharonov-Bohm (AB) oscillations of surface conducting carriers are observed in the magneto-resistance (MR) with a magnetic (B) field parallel to the NR axis. We have also measured the Shubnikov de Haas (SdH) oscillations with the B-field perpendicular to the NR axis at different carrier densities (n). The extrapolated Landau level crossing at 1/B = 0 is $\sim$0.5 and the extracted cyclotron mass from the T-dependence of the SdH oscillations is proportional to $\sqrt{n}$, providing direct evidence of the Dirac fermion nature of the topological surface state. Gate-tunable weak anti-localization is observed and the extracted number of decoupled coherent conduction channels is 2 at the charge neutrality point. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R4.00007: Majorana qubit rotations in microwave cavities Christoph Bruder, Andreas Nunnenkamp, Thomas L. Schmidt Majorana bound states have been proposed as building blocks for qubits on which certain operations can be performed in a topologically protected way using braiding. However, the set of these protected operations is not sufficient to realize universal quantum computing. We show that the electric field in a microwave cavity can induce Rabi oscillations between adjacent Majorana bound states. These oscillations can be used to implement an additional single-qubit gate. Supplemented with one braiding operation, this gate allows to perform arbitrary single-qubit operations. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R4.00008: Electrical transport studies of Topological Insulator Bi$_{2}$Te$_{3}$ Nanotubes Renzhong Du, Weiwei Zhao, Jian Wang, Yuewei Yin, Sining Dong, Xiaoguang Li, Chaoxing Liu, Moses Chan, Qi Li We have studied electrical transport properties of candidate topological insulator Bismuth Telluride (Bi$_{2}$Te$_{3})$ nanotubes. Bi$_{2}$Te$_{3}$ nanotube samples were synthesized by solution phase method, with the outer diameters in the range of 70$\pm $5 nm and inner diameter 50$\pm $5 nm and the length of 3 to 10 um. Platinum contact leads were fabricated on the nanotubes by focusing ion beam assisted deposition. Electrical transport measurements were conducted at low temperatures and high magnetic fields (up to 9T). The nanotubes showed good insulating behavior in comparison with the thin films which are often metallic. Resistance oscillation as a function of magnetic field was observed when the magnetic field is applied parallel to the nanotubes. The periods range from 6000 Oe to 8350 Oe, which correspond to the diameter of 80 to 100 nm according to Aharonov-Bohm oscillation formula. This is close but slightly larger than the outer diameter of the nanotubes. The amplitude of the oscillations decays rapidly as field increases, possibly due to scattering. When the magnetic field was applied perpendicular to the nanotube, no resistance oscillation was observed. The possible origins of the oscillation effect will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R4.00009: Surface state transport in MBE-grown topological insulator (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_3$ thin films and field effect transistors Jifa Tian, Cuizu Chang, Helin Cao, Jiuning Hu, Tai-Lung Wu, Ke He, Xucun Ma, Qikun Xue, Yong Chen Topological insulators feature spin-helical, Dirac fermion surface states, promising potential applications in both nanoelectronics and spintronics. However, experimental identification of a clear transport signal of the surface state conduction is still challenging. Here, we report a systematical study of the gate tunable magneto-transport in MBE grown (Bi$_{1-x}$Sb$_{x})_{2}$Te$_{3}$ (x$=$0.96) thin film on SrTiO$_{3}$ substrate. We observed an ambipolar field effect and a sign change in the Hall resistance as the gate voltage (V$_{g})$ crosses the Dirac point (V$_{D})$. Temperature (T) dependence of the resistance at different V$_{g}$ shows a transition from a metallic to an insulating bulk with 100{\%} surface conduction at low T. Weak antilocalization measurements indicate a $\pi $ Berry phase near V$_{D}$. We also performed spin valve measurements and observed a resistance asymmetry (which reverses with reversing current direction) between the positive and negative in-plane magnetic fields, demonstrating the predicted locking between spin and momentum for the surface state. We also studied the thermal-electric transport, demonstrating a sign change of the thermoelectric power across the V$_{D}$ as the carrier type switches from electron to hole. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R4.00010: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R4.00011: Detection of Majorana Fermions in circuit QED Jerome Bourassa, Clemens Mueller, Alexandre Blais Superconducting quantum circuits, such as the flux and the transmon qubits, have been proposed to measure and control the quantum state of topological qubits based on pairs of Majorana fermions [1-4]. This is possible by making the superconducting qubit transition frequencies sensitive to the fermionic parity representing the topological qubit state. In this talk, we propose to measure the fermionic parity using a flux qubit integrated in a microwave resonator. In this proposal, the flux qubit always remains in its ground state and is used as a passive circuit element which modifies the resonance frequency of the resonator depending on the charge state of a nearby pair of Majorana fermions. Since it is always in its ground state, the requirements on the qubit coherence properties and fabrication parameters are less stringent than in other proposals. \newline [1] F. Hassler et al., New Journ. Phys. 12 125002 (2010) \newline [2] F. Hassler et al., New Journ. Phys. 13 095004 (2011) \newline [3] L. Jiang et al., Phys. Rev. Lett. 106 130504 (2011) \newline [4] P. Bonderson and R. Lutchyn, Phys. Rev. Lett. 106 130505 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R4.00012: Antimony arsenide: Chemical ordering and order-disorder transition in SbAs Daniel Shoemaker, Thomas Chasapis, Dat Do, Melanie Francisco, Duck Young Chung, S. D. Mahanti, Anna Llobet, Mercouri Kanatzidis The A7 structure of the Group V elements can display chemical ordering of Sb and As, which were previously thought to mix randomly. Our structural characterization of the compound SbAs is performed by single-crystal and high-resolution synchrotron x-ray diffraction, and neutron and x-ray pair distribution function analysis. All least-squares refinements indicate ordering of Sb and As, resulting in a GeTe-type structure without inversion symmetry. This lowering of symmetry does not result in any new Bragg reflections, so high-quality scattering data are required. High-temperature diffraction studies reveal an ordering transition around 550 K. Transport and infrared reflectivity measurements, along with first-principles calculations, find that SbAs has a direct band separation larger than that of Sb or As. Because even subtle substitutions in the semimetals, notably Bi$_{1-x}$Sb$_x$, can open semiconducting energy gaps, a further investigation of the interplay between chemical ordering and electronic structure on the A7 lattice is warranted. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R4.00013: Probing Topological Superconductors with Elastic Strain Fields David Schmeltzer, Avadh Saxena We introduce a model for superconductivity in a topological insulator. The response of this system is probed by applying an external stress. We find that the stain field introduces connections in the superconductor and the response of the superconductor is given by the chiral anomaly which is proportional to the disclination density (for s-wave) or dislocations density (for the p-wave) superconductor. This result modifies the sound wave equations. In particular, we find that the core of the disclinations contains the normal matter in a superconductor. The presence of the long-range field induced by the topological response modifies the elastic properties of the solid which can be probed experimentally. The effect of rotating magnetic fields is also considered. Due to the Larmor theorem it is shown that the rotations replace the magnetic field by an effective magnetic field. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R4.00014: High resolution spin- and angle-resolved photoelectron spectroscopy for 3D spin vectorial analysis Taichi Okuda, Koji Miyamoto, Akio Kimura, Hirofumi Namatame, Masaki Taniguchi Spin- and angle-resolved photoelectron spectroscopy (SARPES) is the excellent tool which can directly observe the band structure of crystals with separating spin-up and -down states. Recent findings of new class of materials possessing strong spin orbit interaction such as Rashba spin splitting systems or topological insulators stimulate to develop new SARPES apparatuses and many sophisticated techniques have been reported recently[1-3]. Here we report our newly developed a SARPES apparatus for spin vectorial analysis with high precision at Hiroshima Synchrotron Radiation Center. Highly efficient spin polarimeter utilizing very low energy electron diffraction (VLEED) makes high resolution ($\Delta E<$ 10 meV, $\Delta \theta \sim \pm$ 0.2 $^{\circ}$) compatible with the SARPES measurement[4]. By placing two VLEED spin detectors orthogonally we have realized the polarization measurement of all spin components (x, y and z) with the high resolution. Some examples of the three-dimensional spin observation will be presented. [1] M. Hoesch et al., J. Electron Spectrosc. Relat. Phenom. 124, 263 (2002). [2] T. Okuda, et al., Rev. Sci. Instrum. 79, 123117 (2008). [3] S. Souma, et al., Rev. Sci. Instrum. 81, 095101 (2010). [4] T. Okuda, et al., Rev. Sci. Instrum. 82, 103302 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R4.00015: Transport Measurements on Topological Insulators with Superconductor Electrodes Yang Xu, Tai-Lung Wu, Luis A. Jauregui, Irek Mitkowski, Yong P. Chen Interplay between topological insulators (TIs) and superconductors (SCs) is interesting to study novel physics such as Majorana fermions. Here we report transport measurements on bulk TI interfaced with superconducting electrodes, including indium (In) and niobium (Nb). The TI crystals are high quality $Bi_2Te_3, Bi_2Se_3, Bi_2Te_2Se$ grown by the Bridgman method. Multiple superconducting transitions have been observed in $Bi_2Te_3$/In systems, possibly due to the superconducting alloys formed by In and Bi. Below the superconducting temperature of In (or Nb), the resistance of TI/Sc structure shows a pronounced upturn which may be a probe of spin-polarized surface states in TI and the interplay with SC. [Preview Abstract] |
Session R5: Graphene: Transport and Optical Properties: THz and Plasmons
Sponsoring Units: DCMPChair: Elbert Chia, Nanyang Technological University
Room: 301
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R5.00001: Terahertz conductivity of twisted bilayer graphene Elbert E.M. Chia, Xingquan Zou, Jingzhi Shang, Jianing Leaw, Zhiqiang Luo, Liyan Luo, Siew Ann Cheong, Haibin Su, Jian-Xin Zhu, A.H. Castro Neto, Ting Yu Using terahertz time-domain spectroscopy, the real part of optical conductivity [$\sigma_{1}(\omega)$] of twisted bilayer graphene was obtained at different temperatures (10 -- 300~K) in the frequency range 0.3 -- 3~THz. On top of a Drude-like response, we see a strong and narrow peak in $\sigma_{1} (\omega)$ at $\sim$2.7~THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7~THz to an enhanced density of states at that energy, that is caused by the presence of van Hove singularities arising from a commensurate twisting of the two graphene layers. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R5.00002: Theory of optical responses in the bilayer and trilayer graphene in the quantum Hall regime Takahiro Morimoto, Mikito Koshino, Hideo Aoki In the graphene physics, there are growing interests toward bilayer and trilayer graphene, whose electronic structures are distinct from that of monolayer graphene. It is then interesting to ask how the variety of low-lying electronic structures will affect optical responses, i.e., optical longitudinal and optical Hall conductivities, where the former describes the absorption while the latter the Faraday and Kerr rotations. Thus we study the optical conductivities in bilayer and trilayer graphene systems. We shall show for bilayer graphene that the Lifshitz transition associated with the trigonal warping greatly affects the resonance structures in Faraday rotation not only on low-energy scale where Dirac cones emerges but also in the higher-energy range with parabolic bands as a sequence of satellite resonances. For trilayer graphene, on the other hand, we shall show that the optical conductivities are dominated by the difference in the stacking order. In ABA trilayer, the resonance spectrum is a superposition of effective monolayer and bilayer contributions with band gaps, while ABC trilayer exhibits a distinct spectrum peculiar to the cubic-dispersed bands. In the latter, the trigonal warping effect becomes strong with a larger Lifshitz transition energy ($\sim$10 meV). [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R5.00003: Dynamical Conductivity of AA-Stacked Bilayer Graphene Calvin Tabert, Elisabeth Nicol Motivated by the potential availability of AA-stacked bilayer graphene samples[1,2], we investigate the optical conductivity of this stacking variation[3]. We find the band structure to be made of bonding and antibonding orbitals which are linear at low energy and decoupled for the longitudinal response; this causes the conductivity to behave as the sum of an electron-doped and hole-doped monolayer graphene system. We find a low energy Drude response at charge neutrality and two step features which can be tuned by varying the chemical potential. We find that the interlayer hopping energy plays an important role in determining the onset of these steps. We compute the partial optical sum and find that the Drude weight also depends on the value of chemical potential relative to the interlayer hopping parameter. \\[4pt] [1] J. K. Lee et al. J. Chem. Phys. 129 234709 (2008) [2] W. Norimatsu et al. Phys. Rev. B 81 161410 (2010) [3] C. J. Tabert et al. Phys. Rev. B 86 075439 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R5.00004: Graphene's Dynamic Conductivity in THz Regime Sufei Shi, Tsung-Ta Tang, Bo Zeng, Long Ju, Feng Wang Graphene, a single layer of carbon atoms arranged in honeycomb structure, has linear dispersion relation. The conductivity of graphene in the THz regime is highly tunable due to its gapless dispersion relation, which makes graphene a promising candidate for THz application. Using optical excitation as the pump, we probe graphene with a THz beam and study the THz conductivity in the time domain. This study sheds light on the carrier relaxation in graphene after intense optical excitation and provides information for designing future graphene-based opto-electronic device. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R5.00005: Mid-Infrared Graphene Photoresponse Allen Hsu, Patrick Herring, Yong Cheol Shin, Ki Kang Kim, Jing Kong, Charlie Marcus, Nathaniel Gabor, Tomas Palacios, Pablo Jarillo-Herrero Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since its discovery in 2004. Due to its zero band gap band structure, it has a broad-band optical absorption ranging from the far-infrared all the way to the visible making it potentially useful for infrared photodetectors. Electrostatically gated p-n junctions have demonstrated photocurrents in the near-IR ($\lambda =$ 850nm), primarily due to hot carrier mechanisms. In order to study these mechanisms at longer wavelengths ($\lambda =$ 10 $\mu $m), high quality chemically vapor grown (CVD) graphene is necessary to fabricate electrostatically controlled p-n junctions due to the longer optical length scales. Moreover, at these low energies ($\sim$ 125 meV), optical phonon scattering is suppressed and is predicted to lead to increased carrier lifetimes and enhanced photo-response. Using electrostatic gating, we are able to study the absorption mechanisms in graphene by selecting between conventional photovoltaic effects and photo-thermoelectric effects. Experiments suggest that the photocurrent signal is enhanced by electrostatic gating near the Dirac peak and reduced disorder in the graphene sample. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R5.00006: Ga Nanoparticle/Graphene Platforms: Plasmonic and Charge Transfer Interactions Congwen Yi, Tong-Ho Kim, Yang Yang, Maria Losurdo, April S. Brown Metal nanoparticle (NP) -- graphene multifunctional platforms are of great interest for numerous applications, such as sensing and catalysis, and for fundamental studies on charge transfer and light-matter interactions. To understand platform-photon interactions, it is important to articulate the coupling of photon-based excitations, such as the interaction between plasmons in each of the material components, as well as their charge-based interactions dependent upon the energy alignment at the metal/graphene interface. Herein, we use liquid metal Ga nanoparticles, which can be deposited at 300K on graphene, to explore the surface-enhanced Raman spectroscopy modulation induced by the NPs,. The localized charge transfer between Ga NPs and graphene are investigated, and enhancement of the graphene Raman modes is correlated with metal coverage the transfer of electrons from Ga to graphene creating local regions of enhanced electron concentration which modify the electron-phonon interaction in graphene. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R5.00007: Terahertz and mid-infrared reflectance of epitaxial graphene Cristiane N. Santos, Benoit Hackens, Fr\'ed\'eric Joucken, Robert Sporken, Jessica Campos Delgado, Jean-Pierre Raskin, Domingos De Sousa Meneses, Patrick Echegut Epitaxial graphene grown by thermal decomposition on SiC substrate has been widely investigated as a promising material for electronics and optics. Here, we investigate the infrared (IR) optical properties of few-layer (FL) and multilayer (ML) graphene on the C-terminated face of 6H-SiC substrates [1]. Contrary to IR transmission spectroscopy, which is hampered over a large part of the IR range by the SiC reststrahlen band and multiphonon absorption, IR reflectance gives access to invaluable information from terahertz (THz) to mid-infrared (MIR). Experimental data are well fitted with an explicit model over the entire spectral range using the SiC dielectric function and the graphene optical conductivity, taking into account both intraband and interband transitions. The number of layers extracted from our data in the FL and ML graphene corroborates with the X-ray photoelectron spectroscopy (XPS) measurements. We demonstrate that this consistent and simultaneous analysis leads to precise information on the carrier properties, doping level and the number of layers, even in the case of thick ML (30 layers or more). MIR microscopy was also used to check the sample homogeneity. [1] F. Joucken et al., Phys. Rev. B 85, 161408(R) (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R5.00008: Self-Energy and Excitonic Contributions to the Drude Conductivity of Doped Graphene Felipe Jornada, Steven Louie There has been a growing interest in the far infrared AC conductivity of doped graphene because of possible applications in optoelectronics, but there is still disagreement between recent experiments [1,2] and theories [3] with respect to the Drude weight. In this work we study from an ab-inito GW-BSE perspective the effects of the electron-electron interactions and excitons in the renormalization of the Drude weight. We discuss the role of quasiparticle lifetimes due to electron-electron and electron-phonon interactions, and we determine the AC conductivity in the forbidden region (i.e., for $\omega < 2 E_F$). This work was supported by NSF grant No. DMR10-1006184, U.S. DOE under Contract No. DE-AC02-05CH11231 and the U.S. DOD - Office of Naval Research under RTC Grant No. N00014-09-1-1066. Computational resources have been provided by NERSC. [1] J. Horng et al., PRB 83, 165113 (2011). [2] H. Yan et al., ACS Nano 5, 9854 (2011). [3] S. H. Abedinpour et al., PRB 84, 045429 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R5.00009: Tuning optical conductivity of large-scale CVD graphene by strain engineering Guangxin Ni, Jing Wu, Orhan Kahya, Chee Tat Toh, Jong Hyun Ahn, Vitor M. Pereira, Barbaros \"Ozyilmaz Strain engineering has been widely recognized as an effective way to tailor the electrical properties of graphene. In the optical domain, the strain effect is also predicted to alter the optical conductivity of graphene, making graphene possible for the atomically thin optical elements. However, a direct experimental observation is still missing. Using the nanopillar structure, here we show that optical conductivity of CVD graphene under nonuniform strain exhibits periodic modulation as a function of polarization. The optical absorption can be further modulated via the application of an external uniaxial strain, which is confirmed by Raman spectroscopy as well as AFM images. Our experimental observations are quantitatively interpreted within the Kubo-Greenwood formalism. The manipulation of the optical properties of graphene demonstrated in this study can be effectively utilized in the novel type of optical devices and strain sensor applications. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R5.00010: Tunable magneto-plasmons in graphene: an infrared study Zhiguo Chen, Hugen Yan, Xuesong Li, Wenjuan Zhu, Phaedon Avouris, Fengnian Xia, Zhiqiang Li Plasmons, collective oscillations of electrons, in graphene have attracted much attention due to their important roles in understanding the intriguing physics of graphene and potential applications in optoelectronic devices. Using infrared spectroscopy, we investigated the optical response of the plasmons in micrometer-sized graphene disks in high magnetic fields up to 18 T. Our study shows that the plasmon resonance splits into edge and bulk modes in magnetic fields. Due to the linear band structure of graphene, the splitting exhibits a sensitive doping dependence, which is not observed in conventional two-dimensional electron gas systems. Moreover, the lifetime of the two modes can be dramatically modified by magnetic fields, with the edge plasmons developing increasingly longer lifetimes in high fields. The latter behavior can be understood from the suppression of backscattering at the edges. Our work not only opens an avenue to explore the magneto-plasmons and edge physics in graphene but also supports the great potential of graphene for tunable magneto-optical devices.\\[4pt] [1] Hugen Yan, Zhiqiang Li, Xuesong Li, Wenjuan Zhu, Phaedon Avouris, and Fengnian Xia, Nano letters, 12, 3766 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R5.00011: Long-range plasmon-assisted energy transfer over doped graphene Kirill Velizhanin, Tigran Shahbazyan F\"orster resonance energy transfer (FRET) between spatially separated donor and acceptor fluorophores, such as dye molecules or semiconductors quantum dots, underpins diverse phenomena in physics, chemistry and biology. However, the range of present and potential applications of FRET is limited by its intrinsically short-range nature ($\sim 1/R^6$). We demonstrate that longitudinal plasmons in doped monolayer graphene can mediate highly efficient long-range ($\sim 1/R$) energy transfer between nearby fluorophores, e.g., semiconductor quantum dots. We derive a simple analytical expression for the energy transfer efficiency that incorporates all the essential processes involved. We perform numerical calculations of the transfer efficiency for a pair of PbSe quantum dots near graphene for inter-fluorophore distances of up to 1 $\mu$m and find that the plasmon-assisted long-range energy transfer can be enhanced by up to a factor of $\sim 10^4$ relative to FRET in vacuum. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R5.00012: Graphene multilayers as hyperbolic metamaterials Ashley DaSilva, Allan MacDonald Graphene and multilayer graphene systems show promise for numerous electronic and optical applications in part due to the extraordinary tunability of graphene via gate voltage. We discuss the optical properties of electrically decoupled multilayer graphene systems. These can be described by the reflection and transmission coefficients, which we calculate using a transfer matrix approach. This point of view allows an explicit comparison between graphene multilayers and metal/dielectric multilayer metamaterials. In particular, we will compare multilayer graphene systems to hyperbolic metamaterials which have extreme anisotropy in the effective dielectric constant: $\epsilon_{x}=\epsilon_{y}<0$ and $\epsilon_{z}>0$. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R5.00013: Magnetoplasmons in quasi-neutral epitaxial graphene nanoribbons Jean-Marie Poumirol, Wenlong Yu, Claire Berger, Walter de Heer, Michael Smith, Taisuke Ohta, Wei Pan, Dmitry Smirnov, Zhigang Jiang We report on infrared transmission spectroscopy study of magnetoplasmons in quasi-neutral epitaxial graphene nanoribbon arrays. The energy of the $ L_{0(-1)}\to L_{1(0)}$ inter-Landau level transitions deviates from the characteristic $\sqrt{B}$ dependence observed in two-dimensional graphene. This behavior is explained as a signature of the upper hybrid mode formed between the Landau level transition and the plasmon resonance. Studying the hybrid mode allows us to probe the zero magnetic field plasmon resonance in the interacting regime, when coupling to electron-holes excitations results in strong decay of plasmons. We observe a deviation of the plasmon frequency from the standard $\omega_{pl}\propto q^{1/2}$ dispersion relation, and attribute it to the finite length of the graphene ribbons. [Preview Abstract] |
Session R6: Focus Session: Graphene - Defects, Edges, Theory
Sponsoring Units: DMPChair: Michael Weinert, University of Wisconsin
Room: 302
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R6.00001: Imaging defects on epitaxial graphene/SiC(0001) using non-contact AFM L. Li, Y. Liu, M. Weinert Graphene exhibits linear dispersion at the Dirac point, which leads to novel properties that can be further tailored by the introduction of defects into the honeycomb lattice. In this work, we created defects on epitaxial graphene/SiC(0001) using N and Ar plasma, and studied the atomic structure of the defects using an integrated approach with non-contact atomic force microscopy (AFM) with Q-plus sensor and density functional theory (DFT) calculations. With atomic resolution AFM imaging, straightforward identifications of single- and di-vacancy defects, as well as other more convoluted vacancy complexes can be made. In addition, local contact potentials of these defects are also obtained by frequency shift-bias spectroscopy. These results and comparisons with DFT calculations will be discussed at the meeting. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R6.00002: Graphene nanoflakes with defective edge terminations Igor Romanovsky, Constantine Yannouleas, Uzi Landman Systematic tight-binding investigations of the electronic spectra (as a function of the magnetic field) are presented\footnote{I. Romanovsky, C. Yannouleas, and U. Landman, Phys. Rev. B {\bf 86}, 165440 (2012)} for trigonal graphene nanoflakes with reconstructed zigzag edges, where a succession of pentagons and heptagons, that is 5-7 defects, replaces the hexagons at the zigzag edge. For nanoflakes with such reczag defective edges, emphasis is placed on topological aspects and connections underlying the patterns dominating these spectra. In addition to features that are well known to appear for graphene dots with zigzag edge termination, the electronic spectra of trigonal graphene nanoflakes with reczag edge terminations exhibit unique features. These unique features appear within a stripe of negative energies $E_b < E < 0$ and along a separate regime forming a constant-energy line outside this stripe. The lower bound $(E_b)$ specifying the energy stripe is independent of size. A main finding concerns the limited applicability of the continuous Dirac-Weyl equation, since the latter does not reproduce the special reczag features. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R6.00003: Magnetic-field effects in graphene nanorings: armchair versus zigzag edge terminations Constantine Yannouleas, Igor Romanovsky, Uzi Landman Dirac quasiparticles in narrow graphene nanorings exhibit characteristic differences in their behavior depending on the shape (e.g., trigonal vs. hexagonal) and the type of edge terminations (armchair vs. zigzag). The differences are manifested in the tight-binding single-particle spectra as a function of the magnetic field $B$ and in the patterns of the Aharonov-Bohm oscillations. The symmetry of shape leads to the appearance of three-member (triangles) or six-member (hexagons) braid bands.\footnote{% I. Romanovsky, C. Yannouleas, and U. Landman, Phys. Rev. B {\bf 85}, 165434 (2012)} With the exception of the formation of the braid bands, the characteristic differences maintain in the energy spectra of the continuous Dirac-Weyl equation for a circular ring of finite width. These differences will be further analyzed with the help of a relativistic superlattice model. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R6.00004: Electron transport in graphene monolayers Jun-Qiang Liu, Daniel Valencia We demonstrate electronic transmission of a monolayer can be reduced when covered by a nanoribbon. The transmission reduction occurs at different energies determined by the width of the nanoribbon. We explain the transmission reduction by using of interference between the wavefunctions in the monolayer and the nanoribbon. Furthermore, we show the transmission reduction of a monolayer is combinable and propose a concept of ``combination of control'' for nano-application design. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R6.00005: Unraveling the interlayer-related phonon self-energy renormalization in bilayer graphene Paulo Araujo, Daniela Mafra, Kentaro Sato, Richiiro Saito, Jing Kong, Mildred Dresselhaus In this work, we present a step towards further understanding of the bilayer graphene (2LG) interlayer (IL)-related phonon combination modes and overtones as well as their phonon self-energy renormalizations by using both gate-modulated and laser-energy dependent inelastic scattering spectroscopy. We show that although the IL interactions are weak, their respective phonon renormalization response is significant. Particularly special, the IL interactions are mediated by Van der Waals forces and are fundamental for understanding low-energy phenomena such as transport and infrared optics. Our approach opens up a new route to understanding fundamental properties of IL interactions which can be extended to any graphene-like material, such as MoS$_{2}$, WSe$_{2}$, oxides and hydroxides. Furthermore, we report a previously elusive crossing between IL-related phonon combination modes in 2LG, which might have important technological applications. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R6.00006: Space dependent Fermi velocity in strained graphene Fernando de Juan, Mauricio Sturla, Maria A. H. Vozmediano We investigate some apparent discrepancies between two different models for curved graphene: the one based on tight binding and elasticity theory, and the covariant approach based on quantum field theory in curved space. We demonstrate that strained or corrugated samples will have a space dependent Fermi velocity in either approach that can affect the interpretation of local probes experiments in graphene. We also generalize the tight binding approach to general inhomogeneous strain and find a vector field proportional to the derivative of the strain tensor that has the same form as the spin connection obtained in the covariant approach. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R6.00007: Moir\'{e} minibands in graphene heterojunctions with hexagonal 2D crystals Invited Speaker: Vladimir Falko The transformation of the linear Dirac spectrum of electrons in monolayer graphene and parabolic spectrum in bilayer graphene due to the influence of a tightly bound insulating or semiconducting layer is studied. We present a symmetry-based classification and quantitative analysis of generic miniband structures for electrons in graphene heterojunction with a 2D crystal with the hexagonal Bravais symmetry, such as boron nitride. In particular, we identify conditions at which the first moire miniband is separated from the rest of the spectrum by either one or a group of three isolated mini Dirac points and is not obscured by dispersion surfaces coming from other minibands. In such cases the Hall coefficient exhibits two distinct alternations of its sign as a function of charge carrier density. Then, we study the Hofstadter spectrum of electrons in a magnetic field. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R6.00008: Electric-Field Dependence of the Effective Dieletric Constant in Graphene Materials Elton Santos, Efthimios Kaxiras The dielectric constant of a material is one of the fundamental features used to characterize its electrostatic properties such as capacitance, charge screening, and energy storage capability. Here we address the issue of the effective dielectric constant ($\varepsilon_{G}$) in a few-layer graphene materials (e.g. graphene, MoS$_2$, WS$_2$, etc.) subjected to an external electric field. In particular for graphene, the value of $\varepsilon_{G}$ has attracted interest due to contradictory reports from theoretical and experimental studies. Through extensive first-principles electronic structure calculations, including van der Waals interactions, we show that the graphene dielectric constant depends on the value of the external field ($E_{\rm ext}$): it is nearly constant at $\varepsilon_{G} \sim 3$ for low fields ($E_{\rm ext}<0.1$ V/\AA) but increases at higher fields, reaching $\varepsilon_{G}$=4.5 at $E_{\rm ext} =1.7$ V/\AA. Further increase of $E_{\rm ext}$ drives the system to an unstable state where the layers are decoupled and can be easily separated. Calculations performed for other layered materials follow the same trend. Our results point to a promising way of understanding and controlling the screening properties of few-layer graphene materials via electrical means. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R6.00009: RKKY interaction in monolayer and bilayer graphene: Exact results in terms of the Meijer-G functions Fariborz Parhizgar, Mohammad Sherafati, Reza Asgari, Sashi Satpathy We present the results for the RKKY interaction in monolayer and bilayer graphene in terms of Meijer-G functions for the undoped, doped and biased cases. The results are obtained from the linear-response expression for susceptibility written in terms of the integral over Green's functions and using Dirac bands. The salient features of the large-distance behavior in each case will be discussed. For instance, for doped monolayer graphene, the interaction falls off as $R^{-2}$ and oscillates as the product of two terms, one being a $\{1+ \cos[(K-K').R]\}$-like interference term from both Dirac cones and the second term scaled by Fermi momentum $k_F$. For doped and unbiased bilayer graphene, the interaction decays as $R^{-2}\cos(k_FR)[e^{-k_FR}+\sin(k_FR)]\Phi_{K,K'}$ where $\Phi_{K,K'}$ is a similar Dirac-cone factor. For the gated bilayer graphene, $k_F$ must be replaced by another scaled momentum $k_U$, which depends on Fermi energy, gate voltage and the interlayer hopping energy, allowing the possibility of tuning of the interaction by gate voltage. References: M. Sherafati and S. Satpathy, PRB, 83, 165425 (2011); PRB, 84, 125416 (2011); AIP Conf. Proc. 1461, 24 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R6.00010: Graphene multilayers in the crossed in-plane magnetic and out-of-plane electric fields Sergey Pershoguba, Victor Yakovenko, Yu. Latyshev, A. Orlov, P. Monceau, D. Vignolles We report an experimental study of the out-of-plane differential conductivity $dI/dV$ in graphite mesas as a function of applied out-of-plane voltage $V$ in the in-plane magnetic fields $B_y$ up to 55 T. The spectrum $dI/dV$ vs $V$ has a pronounced peak at the critical voltage $V_0$, which grows linearly with the magnetic field $V_0\propto B_y$. The experimental results are consistent with a theoretical model. The electronic energy spectrum on each graphene layer is given by the two-dimensional (2D) Dirac cone $\varepsilon = v |p|$, where $v$ and $p = (p_x,p_y)$ are the velocity and 2D momentum. As a result of magnetic field $B_y$, the Dirac cones of the consecutive layers are shifted in the momentum space by $\Delta p_x = eB_yd$, where $d$ is a distance between the layers. Whereas electric field $E_z$ shifts the energy by $\Delta \varepsilon = E_z d$. For generic $E_z$ and $B_y$, the wave functions are localized on a finite number of layers in the $z$ direction. However, when the resonant condition $\Delta \varepsilon = v\Delta p_x$ is achieved, i.e. when $E_z = vB_y$, the Dirac cones align, and wave functions become delocalized in the $z$ direction. We believe that the resonant delocalization of the wave functions corresponds to the peak in differential conductance. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R6.00011: Interactions between adsorbates on graphene Dmitry Solenov, Chad Junkermeier, Thomas L. Reinecke, Kirill A. Velizhanin Interactions between adsorbates on a surface of graphene play an important role in many applications. We offer a detailed analysis of interactions between two adsorbed atoms and molecules. We compare the first principles DFT, numerical tight-binding, and analytical functional integral calculations to identify the microscopic nature of the adsorbate-adsorbate interaction and the role of different contributions. The interaction has two distinct regimes: a weak coupling regime, which is akin to RKKY (Ruderman-Kittel-Kasuya-Yosida) magnetic interaction, and a strong coupling regime which is dominated by interaction via a many-body electronic dressing ``cloud'' around each adsorbate. We show that the interplay between these two regimes provides an opportunity to manipulate the magnitude and the structure of the adsorbate-adsorbate interaction (up to complete reversal of sign) via a variety of easily accessible properties, such chemical potential via back-gating, type of an adsorbed atom, electronic configuration of an adsorbed molecule, and strain. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R6.00012: \emph{Ab Initio} Many-body Study of Cobalt Adatoms Adsorbed on Graphene Yudistira Virgus, Wirawan Purwanto, Henry Krakauer, Shiwei Zhang Research interest in the adsorption of transition metal adatoms on graphene has grown rapidly because of their promising use in spintronics. Single Co atoms on graphene have been extensively studied recently, and possible Kondo effects have been considered. However, these calculations show significantly varying results on the bonding nature of Co/graphene system. We use auxiliary-field quantum Monte Carlo (AFQMC) and a size-correction embedding scheme to accurately calculate the binding energy of Co/graphene.\footnote{Y. Virgus, W. Purwanto, H. Krakauer, and S. Zhang, arXiv:1210.6973.} We find that as a function of the distance $h$ between the Co atom and the six-fold hollow site, there are two states that provide binding and exhibit a double-well feature with nearly equal binding energy of $0.4$~eV at $h = 1.51$ and $h = 1.65$~\AA, corresponding to low-spin $^2$Co ($3d^{9}4s^{0}$) and high-spin $^4$Co ($3d^{8}4s^{1}$), respectively. Binding of Co on bilayer graphene is also investigated. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R6.00013: Polarization Waves around Coulomb Impurities in Strained Graphene Valeri Kotov, Anand Sharma, A.H. Castro Neto We study the distribution of polarization charge around external Coulomb centers in graphene. We consider uniaxially strained Graphene so that the Dirac cones are anisotropic and there is a preferred direction on the lattice. Under these conditions we find that the polarization charge exhibits oscillations around the impurity with predominant d-wave symmetry for small anisotropy (strain) and also admixture of g-wave as well as higher waves with increasing anisotropy. The total polarization in the ground state is zero. This rich variety of behavior is in stark contrast to the situation in undeformed graphene, where the polarization charge away from the impurity is identically zero. Thus our results could be used for detection of Coulomb impurity physics even in the case of relatively small charge ions as long as there is sufficient Dirac cone anisotropy. [Preview Abstract] |
Session R7: Si and Other Semiconductors
Sponsoring Units: FIAPChair: Alex Demkov, University of Texas at Austin
Room: 303
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R7.00001: Crystalline phase-stability of tantalum pentoxide Santiago Walton, Antonio Claudio Padilha, Gustavo Dalpian, Jorge Guill\'en Memristive devices are attractive candidates to provide a paradigm change in memory devices fabrication. These new devices would be faster, denser and less power consuming than those available today. However, the mechanism of memristance is not yet well understood. It is believed that a voltage/current-driven phase transition occurs in the material, which leads to significant changes in the device's conductivity. In the particular case of tantalum-oxide-based devices the relevant crystalline phases are still a matter of debate. Some of these phases are not even completely known and there is no agreement about which model best explains the crystallographic results. In this work we have performed ab-initio DFT based calculations to study the structural properties of different phases (and models) of Ta$_2$O$_5$ - the structure which is believed to exist inside Tantalum Oxide based devices. The equations of state for this material were constructed through first principles total energy calculations and we have also calculated the phonon frequencies at $\Gamma$. These results show that the most stable phase of this oxide (B-Ta$_2$O$_5$) is in fact composed of octahedral, instead of pentagonal (as L-Ta$_2$O$_5$) or hexagonal (as $\delta$-Ta$_2$O$_5$) bipyramids. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R7.00002: Dynamical processes in semiconductor nanoclusters Peng Han, Gabriel Bester We study the electronic relaxation processes via electron-phonon interaction in colloidal semiconductor nanoclusters (NCs) using the Liouville-von Neumann equation including a phenomenological Lindblad decay term. The electron-phonon coupling matrix elements used in our study are obtained from frozen-phonon calculations based on \emph{ab initio} density functional theory (DFT). To estimate the phonon lifetime of NCs, which is used in the Lindblad decay term, we perform \emph{ab initio} molecular dynamics simulations of a Si$_{10}$H$_{16}$ cluster and extract the time evolution of the energy of selected vibrational modes from the energy auto-correlation functions. We find vibrational cooling times of around 0.1~ps for high frequency Si-H vibrations, and cooling time of around 1~ps for pure Si modes, which are close to the phonon lifetimes in bulk Si. Analyzing the electronic relaxation processes with the parameters from DFT calculations, we observe a decaying Rabi oscillation with a period of tens of femtoseconds corresponding to the emission/absorption of a phonon. We notice that the Rabi oscillation frequency is proportional to the electron-phonon coupling strength while the decay process is dominated by the phonon lifetime and the energy detuning. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R7.00003: Characterization of Silicon CMOS Quantum Well Field Effect Transistors Clint Naquin, Mark Lee, Hal Edwards, Tathagata Chatterjee Silicon CMOS field effect transistors (FETs) incorporating quantum wells (QWs) are of potential interest as advanced oscillators and sensors. We report on the design and electrical characterization of a set of Si CMOS QW FETs fabricated using industrial 45 nm processing. By using low doped drain and pocket implants, lateral QW potentials between 30 nm to 100 nm in length and approximately 0.1 to 0.5 eV in potential depth have been incorporated into the channel between source and drain. The potential depth can be modulated by the gate voltage. Measurements of drain current as a function of gate voltage for devices from 1.7 K up to room temperature will be reported, with the expectation of observing resonant conductance oscillations from transport through QW levels at temperatures where the QW energy levels are well formed. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R7.00004: Infrared spectroscopy of high purity Si for application in astronomy Berik Uzakbaiuly, Jian Ge, David Tanner High resolution infrared (IR) spectroscopy is an essential tool in astronomical studies and Silicon Immersion Gratings (SIGs) offer 3.4 times gain in spectral resolution over conventional echelle gratings of the same length and blaze angle. SIGs have Si as the host material, relying on its high transparency in much of the infrared region. Si is transparent below the multiphonon absorption for far-infrared ($\sim$20-300 microns) use as well as in the near infrared ($\sim$1-6 microns). We have measured high-purity silicon transmittance from far to near infrared. Narrow lines, due to residual impurities and with interesting temperature dependences, appear in the far IR region. We present the transmittance of high purity bulk Si from the band edge (9000 cm$^{-1})$ to far infrared (20 cm$^{-1})$ using FT-IR spectrometer and modified Perkin Elmer grating spectrometer. Impurities have been identified and their temperature dependence will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R7.00005: Dynamic Formation of NiSi$_2$ in Porous Amorphous Si Nanorods Observed by In Situ TEM Jianguo Fan We investigated the dynamic formation of NiSi$_{2}$ nanocrystals in a porous amorphous silicon nanorod with in situ heating TEM. The nanorod was prepared by sequential electron beam depositions of Si, Ni, and Si at an oblique angle. Due to the nature of atomic shadowing and limited diffusion at low deposition temeperature, the structure was porous and amorphous. Ni diffusion started at 300 $^{\circ}$C and oxides in the porous structure greatly surpressed the formation of early silicide phases such as NiSi, Ni$_{3}$Si$_{2}$, and Ni$_{31}$Si$_{12}$. At 400-500 $^{\circ}$C, NiSi$_{2}$ crystallites formed along the nanorod and were defined by the porous template. These structural evolutions were identified and confirmed by electron diffraction, X-ray analysis, and high-resolution TEM. The formation mechanism and possible applications will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R7.00006: Enrichment and growth of enriched $^{28}$Si films Joshua Pomeroy, Kevin Dwyer In support of quantum information and spintronics efforts, we are producing enriched $^{28}$Si films that are 99.9{\%} $^{28}$Si according to secondary ion mass spectrometry assessment. We use an ionization source to crack and ionize natural abundance silane gas, then extract the ions through a magnetic sector analyzer to isolate the major isotope $^{28}$Si. We have presently demonstrated \textgreater\ 100 nm thick films of silicon and carbon, which was enriched to 99.996{\%} $^{12}$C. With ongoing improvements, we expect to produce $^{28}$Si enriched to better than 99.99{\%} at thicknesses \textgreater\ 1 $\mu $m grown epitaxially on Si(100) substrates. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R7.00007: Atomic and Electronic Processes During the Formation of an Ionic NaCl Monolayer on Si(100) Deng-Sung Lin, Chan-Yuen Chang, Hong-Dao Li, Shiow-Fon Tsay An atomic layer of stoichiometric NaCl was formed on a covalent Si(100) surface after two successive half-reactions at room temperature. The first half-reaction due to Cl$_{\mathrm{2}}$ exposure generates a square array of Cl adatoms with a distance close to that in a NaCl(100) surface plane. By utilizing scanning tunneling microscopy, core-level photoemission spectroscopy and ab initio density functional theory calculations, it was found that progressive deposition of Na in the second-half reaction results in surface-supported Na$_{\mathrm{3}}$Cl clusters, one dimensional cluster chains and (2x2) patches, and eventually turns the Cl-adlayer into a single-terrace, wavy NaCl layer at one monolayer Na coverage. The grown NaCl monolayer rolls over atomic steps like a carpet and covers the entire surface. The atomic and electronic structure of the topmost Si layer underneath the NaCl layer resembles that of the initial silicon surface layer with buckled dimers. Results of the comprehensive investigation together suggest that an ionic NaCl monolayer is very weakly bonded to the covalent substrate and appears nearly free standing. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R7.00008: Electronic structure of the Sr/Si(001) Zintl template from density functional theory and photoemission Hosung Seo, Miri Choi, Richard Hatch, Agham Posadas, Alexander Demkov Since the first demonstration of epitaxial growth of crystalline SrTiO3 on Si(001) by Mckee and co-workers, sub-monolayer Sr on Si(001) has been extensively investigated. Charge transfer induced by half-monolayer of Sr has been shown to be a key element enabling wetting of Si by SrTiO3. However, a detailed understanding of the electronic structure reconstruction is not complete. Such knowledge could be extended and applied to the other epitaxial crystalline oxides on semiconductors. Recently, using in-situ x-ray core-level spectroscopy, we have studied the change in electronic structure of Si(001) induced by sub-monolayer Sr deposition in terms of surface core level shift. One of the interesting features is shift of the Si 2p level toward the higher binding energy by 0.49eV after Sr deposition. In this talk, we present a detailed theoretical investigation of the surface core level shifts in sub-monolayer Sr/Si(001). Using the final state theory, we calculate the bulk 2p binding energy to be increased by 0.42eV when half-monolayer of Sr is deposited in excellent agreement with experiment. We are able to compare the calculated evolution of the surface band structure in sub-monolayer Sr/Si(001) to angle-resolved photoemission spectroscopy (ARPES) data. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R7.00009: Purification of germanium crystal by zone-refining technique Gang Yang, Jayesh Govani, Hao Mei, Guojian Wang, Yutong Guan, Chaoyang Jiang, Dongming Mei Zone refining is a purification technique of metal materials, which was developed at Bell Telephone Laboratories in the early of 1950s. In zone-refining of high-purity germanium crystals, the influential factors include vacuum level, container of germanium ingot, ambient gases, speed of zone travel, the ratio of ingot length to molten zone length, etc. In the present work, we have investigated the influences of the following factors on the purification of germanium crystals: graphite/quartz boats, hydrogen/argon gas, speed of zone travel and the ratio of ingot length to molten zone length. Additionally, we have also analyzed the influences of segregation of three main impurities, such as boron, aluminum and phosphor on the electrical properties of the zone-refined crystals. In this paper, we report the results from the zone-refined germanium ingots produced at the University of South Dakota. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R7.00010: Optical Microscopic and Spectroscopic Study of the High-Purity Germanium (HPGe) Single Crystals Jayesh Govani, Gang Yang, Guojian Wang, Muhammad Khizar, Chaoyang Jiang, Dongming Mei High-purity germanium (HPGe) single crystals are required for the fabrication of radiation detectors. Before grown HPGe crystals can be effectively utilized, they need to be characterized for their purity, identification of impurities and dislocation density. These characterizations help to determine if the grown crystal is qualified for making detector and provide the feedback for crystal growth, so the crystals with the required qualities can be grown consistently. In the present study, we have performed optical microscopic analysis of the grown HPGe crystals. Our experimental results indicated that the crystals exhibit dislocation density in a range of 3000/cm$^2$ to 8000/cm$^2$ demonstrating that the dislocation density is within the required range ($\sim$ 10$^2$ -10$^4$ dislocations/cm$^2)$ to avoid the formation of undesired di-vacancy hydrogen (V$_2$H) complexes. Photo-thermal ionization spectroscopic (PTIS) analysis indicated that aluminum; boron and phosphorus are the dominant impurities in the grown crystals. We also performed the Van-der Pauw hall measurement for the determination of carrier concentration, resistivity and mobility of the charge carrier. In this paper, we show some characterization results from the grown crystals at USD. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R7.00011: Tracing the phosphorus contamination sources and reducing the phosphorus contamination in HPGe crystal growth Guojian Wang, Yutong Guan, Gang Yang, Jayesh Govani, Muhammad Khizar, Hao Mei, Dongming Mei The net impurity concentration and the dislocation density for the grown crystals must be controlled within a narrow range of values to produce crystals acceptable for large-volume coaxial germanium detector fabrication. Phosphorus is the main shallow level donor in high purity germanium crystal. The phosphorus contamination is a disaster for growing p-type high-purity germanium crystal. The phosphorus contamination mainly comes from crucible, insulation, ambient gas or crystal growth chamber. Regrowth method was used to trace the phosphorus contamination sources. The contamination level from sources was discussed in detail in this paper. For different contamination source, targeted approaches were used to reduce the contamination. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R7.00012: Variation of electron-phonon coupling in group IV elemental semiconductors Nandan Tandon, Lisa Pugsley, L. R. Ram-Mohan Electron-phonon (e-ph) coupling determines the transfer of energy from hot electrons to the lattice, resulting in the heating of devices. In the current treatments, the e-ph coupling is determined within the long-wavelength phonon approximation. In this work, we consider the e-ph coupling and its variation over the entire Brillouin zone (BZ). The electronic structure and the full phonon dispersion are evaluated with the phonon dispersion calculated using the density functional perturbation theory (DFPT). The e-ph coupling is evaluated using maximally localized Wannier functions and generalized Fourier interpolation to generate e-ph matrix elements on arbitrary grids. Examples of specific initial electron momentum both in the valence and in the conduction bands are presented, together with the variation of the e-ph coupling over the entire BZ associated with the specific initial carrier momenta. We observe variations of up to about $400$meV in Diamond and $50-100$meV in Silicon and Germanium for the evaluated e-ph matrix element. We comment on the consequence of this variation on the carrier lifetimes in these materials. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R7.00013: Morphology control of WS2 monolayer islands: triangles, stars, and snowflakes Yuanxi Wang, Cheng-Ing Chia, Ana Elias, Nestor Perea-Lopez, A. C. Beltran, A. Berkdemir, Humberto Gutierrez, Florentino Lopez-Urias, Humberto Terrones, Mauricio Terrones, Vincent Crespi Interfaces play an important role in determining the electronic structure and equilibrium morphologies of monolayer nanoclusters. An additional difficulty for polar materials is that a conventional edge energy calculation using a nanoribbon exposes two different types of edge terminations, making the energy of each edge inextricable. Based on density functional theory, we report the energies of different types of edge terminations of monolayer WS$_{2}$ at different experimental environments in terms of varying chemical potentials of the W and S species. The Wulff construction is then applied to show that triangular shapes are most favorable at higher S chemical potential, where bulk sulfur start to become present in the system. Our results are in agreement with recent experiments that triangular islands of WS$_{2}$ are synthesized by CVD method using vaporized sulfur. Stacking energetics and kinetic growth factors will also be discussed to explain the formation of six-pointed star shapes and edge irregularities. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R7.00014: Chemical sensing with ultra-thin MoS2 Adam Friedman, Keith Perkins, Enrique Cobas, Paul Campbell, Glenn Jernigan, Berend Jonker Although the majority of focus and excitement in recent years has been on studying the remarkable properties of single atomic-layer graphene, there exists a whole class of materials called dichalcogenides that are relatively easily fabricated in single-crystal mono- or few-layer format. Graphene, being chemically inert, does not lend itself to chemical sensing applications. However, MoS2, a dichalcogenide of recent interest because of its potential for transistor applications, possesses many advantageous properties for chemical sensing. Two primary examples include a sizable bandgap, which is necessary for fabricating transistors with large on/off current ratios, and a chemically reactive surface, which is necessary for easy surface functionalization. In this talk, we discuss our current research effort on MoS2 chemical sensors. We discuss aspects of transistor device fabrication and chemical sensing experiments. We expose MoS2 chemical sensors to a variety of analytes, finding the best response to triethylamine, a nerve gas by product, and explain our results based on a donor-acceptor model. MoS2 sensors are compared to other similar low-dimensional sensors and found to be of comparable quality. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R7.00015: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 5:30PM - 5:42PM |
R7.00016: Design, fabrication and performance optimization of bi-polar blocking planar HPGe radiation detector Muhammad Khizar, Guojian Wang, Dongming Mei A prototype planar radiation detector is designed, fabricated and characterized using bi-polar contact deposited on high purity single crystal germanium (HPGe). Performances of planar and semi-planar detectors are carried out for their low background counting and high absolute efficiency for high-energy photons applications. For this study, 40mm ? 15mm (diameter to vertical height) p-type HPGe samples with dislocation density EPD \textless 3000 cm-2 are taken from HPGe ingots grown by Czochralski method. After a successful mechanical preparation, and standard cleaning and polishing procedure, samples are chemically etched by using a mixture of highly concentrated acids HF:HNO3 (1:4) in order to remove the surface oxides. A bi-polar blocking layer of amorphous germanium (a-Ge) is deposited on both the samples using low temperature RF sputtering plasma in a pre-mix precursor of H2 (15{\%}) and Ar. For this, an optimized dose of the plasma power and chamber pressure is used for a controlled low temperature. The process was completed with the evaporation of Ohmic contacts using electron beam evaporator. This is worth noticing that special care is introduced during the handling of these samples, especially for the bi-polar blocking and metal contact layers deposition. Finally, the fabricated detectors are characterized at 77K temperature. In this paper, we show the results from the first prototype detector made of home grown crystals at USD. [Preview Abstract] |
Session R8: Correlated Electron Magnetism
Sponsoring Units: GMAGChair: Andre-Marie Tremblay, Universite de Sherbrooke
Room: 307
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R8.00001: Origins of ferromagnetism and antiferromagnetism in Gd$_{5}$Ge$_{4}$ Durga Paudyal, V.K. Pecharsky, K.A. Gschneidner, Jr. The origin of ferromagnetism appearing as a result of a magnetic-field-induced first-order phase transition in Gd$_{5}$Ge$_{4}$ is explored by calculating the total energy, local exchange splitting, density of states, and magnetic moments. The calculations were performed using density functional approaches including the on-site Coulomb interaction parameter. The total energy as a function of shear distortion along the $a$ axis for two different orthorhombic structures is in agreement with experiment, indicating a first-order magnetostructural transition in Gd$_{5}$Ge$_{4}$. The rearrangement of Gd 5$d$ and Ge 4$p$ densities of states, the substantial differences in atom-projected band energies, the exchange splitting, and the magnetic moments calculated with ferromagnetic spin arrangements in the orthorhombic Sm$_{5}$Ge$_{4}$-type and Gd$_{5}$Si$_{4}$-type structures of Gd$_{5}$Ge$_{4}$ help to clarify the differences in the magnetic states of these two structures. Our calculations indicate that the Sm$_{5}$Ge$_{4}$-type structure of Gd$_{5}$Ge$_{4}$ is the structural ground state and that it is antiferromagnetic. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R8.00002: Unconventional metallic magnetism in LaCr$_{\mathrm{1-x}}$V$_{\mathrm{x}}$Sb$_{3}$ system Xiao Lin, Valentin Taufour, Sergey Bud'ko, Paul Canfield Unconventional, low temperature ground states can often be found in the vicinity of a magnetic phase transition that has been continuously tuned to 0 K. As part of our search for exotic superconductivity, we have studied the LaCrSb$_{3}$ system. Whereas magnetization measurements of LaCrSb$_{3}$ under pressure reveal no change of $T_{\mathrm{C}}$ up to $\approx $ 5 GPa, the ferromagnetic transition is gradually suppressed with increasing V substitution. Single crystals of the LaCr$_{\mathrm{1-x}}$V$_{\mathrm{x}}$Sb$_{3}$ series have been characterized by measurements of, temperature dependent magnetic susceptibility, magnetization, electrical resistivity, and specific heat. Determinations of magnetic anisotropies as well as ferromagnetic ordering temperatures have been made. Below $T_{\mathrm{C}}$, spin reorientation has been observed within \textit{bc} plane. A $T$ -- $x$ phase diagram has been assembled to shed light on the magnetism in this system. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R8.00003: Optical investigations on spin density wave instability in SrMnBi$_{2}$ Hyun-Ju Park, Da Woon Joeng, Chang Hee Sohn, Joonbum Park, J.S. Kim, K.W. Kim, S.J. Moon, T.W. Noh We investigated the electronic response of layered transition metal pnictide SrMnBi2 using infrared spectroscopy. SrMnBi2 has a crystal structure similar with that of Fe-based superconductors and shows antiferromagnetic order at high temperature 290 K. We observe that the onset of antiferromagnetic order induces a partial gap formation. Upon entering the antiferromagnetic state, the Drude response is drastically suppressed and the spectral weight is transferred to higher energies. Our results suggest that the antiferromagnetism in SrMnBi2 may be associated with spin-density-wave instability of itinerant carriers. We will discuss possible origins of the density-wave-instability based on the first-principles-calculation results. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R8.00004: Resonant Ultrasound studies of double perovskites A$_{2}$FeReO$_{6}$ (A=Ba, Ca) Ling Li, Jiaqiang Yan, David Mandrus, Veerle Keppens The elastic response as a function of temperature (50-380) K and magnetic field (0-2) T has been studied using Resonant Ultrasound Spectroscopy (RUS) for the polycrystalline double perovskites A$_{2}$FeReO$_{6}$ (A= Ba, Ca). An elastic softening over a wide temperature range is observed below the Curie temperature (T$_{c}$ $\sim$ 305K) of Ba$_{2}$FeReO$_{6}$, which is suppressed upon the application of a magnetic field. For Ca$_{2}$FeReO$_{6}$, both the longitudinal and shear modulus show a step-like softening starting around 140K, indicative of a structural transition. A large change in the magnetoelastic coupling constant is observed at this temperature, suggesting that this transition is strongly coupled to the magnetic properties of this material. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R8.00005: Chemical Doping Induced Ferro- and Antiferro-magnetic States in non-Magnetic Insulating FeGa$_{3 }$ N. Haldolaarachchige, J. Prestigiacomo, Y. Xiong, A. Phelan, J. Chan, D. Sheehy, P. Adams, J. DiTusa, S. Stadler, D. Young A ferromagnetic quantum critical point (FM-QCP) in Ge-doped FeGa$_{3}$ was reported very recently (Umeo \textit{et al}. PRB \textbf{86} (14), 144421, 2012). We have simultaneously observed the FM-QCP in this system. Furthermore, we analyzed the magnetic properties of FeGa$_{3}$ in the context of a unique structural feature, where the four Fe atoms in the unit cell exist as two Fe-Fe dimers (Yin and Picket, PRB \textbf{82} (15), 155202, 2010). We propose a phenomenological model where the extrinsic electrons from the Ge doping creates a mixed valence Fe-dimer with a net effective spin. Such a model provides a novel mechanism for the (FM-QCP) and is consistent with the system's magnetic and thermal properties. In addition to Ge doping, we have investigated effects of Ru/Mn substitution on the Fe site. Ru substitution produces an unexpected ferromagnetic (FM) insulating phase that develops immediately, and it disappears above an intermediate doping level. This behavior agrees well with our model of spin creation on the transition-metal dimers via conduction electrons, and the enhanced insulating behavior in the electrical resistivity suggests the Ru acquires a 2$+$ state. Interestingly, Mn-doped FeGa$_{3}$ shows an apparent antiferromagnetic (AFM) insulating phase, where the magnetic data is consistent with the effective moment coming from Mn(3$+)$. These results provide further evidence of the important role of the Fe-Fe dimer structure in FeGa$_{3\, }$in determining its unique magnetic properties. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R8.00006: Ferromagnetic Fluctuations Enhanced by Mn Doping in Sr$_2$RuO$_4$ John Ortmann, Jin Peng, X. Wu, Zhiqiang Mao Sr$_{2}$RuO$_{4}$ is the first experimentally established example of a spin-triplet superconductor [1]; it has attracted a great deal of interest since its discovery in 1994. Like other unconventional superconductors, the superconductivity of Sr$_{2}$RuO$_{4}$ also occurs in close proximity to magnetic instability. Its normal state is characterized by incommensurate antiferromagnetic (AFM) fluctuations associated with Fermi surface nesting. Moreover, the other ruthenate compounds related to Sr$_{2}$RuO$_{4}$ in the Ruddlesden-Popper series are all magnetic. The Sr-based members Sr$_{3}$Ru$_{2}$O$_{7}$, Sr$_{4}$Ru$_{3}$O$_{10}$ and SrRuO$_{3}$ are either metamagnetic or ferromagnetic (FM), whereas the Ca-based members Ca$_{2}$RuO$_{4}$ and Ca$_{3}$Ru$_{2}$O$_{7}$ are AFM. We have investigated the Mn doping effect in Sr$_{2}$RuO$_{4}$ using floating-zone grown single crystal samples and observed significantly enhanced FM fluctuations in the Mn-doped Sr$_{2}$RuO$_{4}$ samples. The system becomes nearly FM with only a few percent Mn doping. This finding suggests that Sr$_{2}$RuO$_{4}$ involves competing, orbital dependent magnetic fluctuations.\\[4pt] [1] A. P. Mackenzie and Y. Maeno, Rev. Mod. Phys. \textbf{75}, 657 (2003). [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R8.00007: Spin-state controlled electronic and magnetic structures of Sr$_{2-x}$La$_{x}$CoO$_{4}$ Hua Wu Sr$_{2-x}$La$_{x}$CoO$_{4}$ is an interesting group of materials, and they display abundant electronic and magnetic properties. In this work, we studied those properties, using electron-correlation corrected density functional calculations. We find that besides a charge-state variation induced by La doping, a multiple spin-state transition takes place and determines (1) a metal-insulator transition and a ferromagnetic insulating phase for $x$=0.5 [1], (2) a paramagnetic Mott insulating phase with a mixed high-spin and low-spin state for $x$=1, instead of a ferromagnetic half-metallic solution with a homogeneous intermediate-spin state [2], and (3) a charge-ordered highly insulating phase with an active spin-blockade mechanism for $x$=1.5 [3]. [1] H. Wu, Phys. Rev. B 86, 075120 (2012). [2] H. Wu, Phys. Rev. B 81, 115127 (2010). [3] H. Wu and T. Burnus, Phys. Rev. B 80, 081105(R) (2009). [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R8.00008: Doping Induced Itinerant Ferromagnetism in CoAs Chih-Wei Chen, Emilia Morosan The magnetism in $\alpha $-CoAs is dominated by strong spin fluctuations. In this study, we explore the effects of Phosphorus doping in $\alpha $-CoAs. Phosphorus is isovalent with Arsenic, and the resulting doping introduces disorder and chemical pressure. In CoAs$_{\mathrm{1-x}}$P$_{\mathrm{x,}}$ a cross-over from the spin fluctuation-dominated regime to an itinerant ferromagnetic (IFM) state take places around x $=$ 0.04. The IFM state persists up to x $\le $ 0.27. For compositions between x $=$ 0.28 and 0.40, the magnetization data suggests a possible Stoner enhanced state. We acknowledge the support from DOD PECASE. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R8.00009: Magnetic and Orbital Ordering of KCuF$_{3}$ Studied by Soft X-ray Scattering C. H. Lai, W. B. Wu, M. H. Chen, T. C. Hung, C. W. Yuan, D. J. Huang, Y. Murakami The interplay between charge, orbital, and spin degrees of freedom plays an important role in the underlying physics of transition-metal compounds. The charge-transfer insulator KCuF$_{3}$ is an archetype of orbitally ordered materials with large exchange interaction energy. KCuF$_{3}$ has long been known to display quantum one-dimensional antiferromagnetic properties along the $c$-axis originating from the superexchange interaction between the $e_g$ orbitals of Cu$^{2+}$. Due to the large Jahn-Teller distortion in the tetragonal structure, the degeneracy of the two $e_g$ orbitals is lifted and the $e_g$ orbitals form a pattern of orbital ordering . In this talk, we will present our recent measurements of spin and orbital ordering of KCuF$_{3}$ by soft X-ray scattering to address its magnetic transition and the coupling between spin and orbital degrees of freedom. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R8.00010: Controllable chirality-induced geometrical Hall effect in the frustrated strongly-correlated metal UCu$_{5}$ B.G. Ueland, C.F. Miclea, Yasuyuki Kato, O. Ayala-Valenzuela, R.D. McDonald, R. Okazaki, P.H. Tobash, M.A. Torrez, F. Ronning, R. Movshovich, Z. Fisk, E.D. Bauer, Ivar Martin, J.D. Thompson A current of electrons traversing a landscape of localized spins possessing non-coplanar magnetic order gains a geometrical (Berry) phase which can lead to a Hall voltage independent of the spin-orbit coupling within the material--a geometrical Hall effect. In this talk, I will present experimental data and Monte-Carlo simulation results showing that the strongly-correlated metal UCu$_{5}$ possesses an unusually large controllable geometrical Hall effect at $T$ \textless\ 1.2K due to its frustration-induced magnetic order. The magnitude of the Hall response exceeds 20{\%} of the $\nu =$1 quantum Hall effect per atomic layer, which translates into an effective magnetic field of several hundred Tesla acting on the electrons. The existence of such a large geometric Hall response in UCu$_{5}$ opens a new field of inquiry into the importance of the role of frustration in highly-correlated electron materials. \textit{B.G. Ueland et al., Nat. Commun. }\textbf{\textit{3}}\textit{, 1067 (2012).} [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R8.00011: Dynamical spin correlation function in a frustrated two-leg spin-ladder system Takanori Sugimoto, Michiyasu Mori, Takami Tohyama, Sadamichi Maekawa We numerically study the magnetic excitations in a frustrated two-leg spin-ladder system, in which all magnetic exchange interactions, i.e., the nearest-, next-nearest-neighbor sites in the leg direction, and the nearest-neighbor sites in the rung direction, are antiferromagnetic. This is a minimal model describing a low-dimensional quantum spin compound, BiCu$_2$PO$_6$. We calculate a dynamical spin correlation function at zero temperature by using the dynamical density-matrix renormalization-group method in possible magnetic phases, columnar dimer and rung singlet. The columnar dimer phase is characterized by multi-spinon excitations, while the rung singlet phase is dominated by a triplon excitation, which is the triplet excitation in the rung direction. Difference between these two types of excitations appears in the spectral weight, in particular, of the bonding and anti-bonding modes in the rung direction. Therefore, we can distinguish one phase from the other by distribution of the spectral weight. In addition, we examine frustration effect on the bonding mode, so-called bound triplon, with a perturbation theory from the strong coupling limit in the rung direction. Our study is expected to be useful to analyze inelastic neutron scattering data for BiCu$_2$PO$_6$. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R8.00012: Critical magnetic scattering in geometric frastrated multiferroic LuMnO$_3$ Shinichiro Yano, Bing Li, Despina Louca, Yiming Qiu, John Copley The coexistence of competing order parameters in the class of materials referred to as the multiferroics is of great interest. The hexagonal manganites \textit{A}MnO$_3$ (\textit{A} = Y, Lu, Ho and Yb) with the \textit{P6$_3$cm} space group exhibit a ferroelectric transition, at very high temperatures, typically $\sim$ 1000 K, while the antiferromagnetic transition, \textit{T$_N$}, occurs at $\sim$ 100K. Earlier studies on YMnO$_3$ and LuMnO$_3$ using neutron scattering on single crystals showed that diffuse scattering is present around the forbidden nuclear (100) Bragg peak which corresponds to \textit{Q}=1.20 \AA$^{-1}$.Its intensity rises very sharply and drops just around \textit{T$_N$}. We performed inelastic neutron scattering measurement on a powder sample of LuMnO$_3$ form 4 to 250 K using the DCS at NIST. Strong inelastic intensity, not due to magnon excitations, is observed at \textit{Q}=1.32 and 2.50 \AA$^{-1}$. With cooling, the intensity gradually rises and reaches a peak around 100 K. Below, it drops drastically once the system orders. This kind of scattering is due to critical scattering arising from magnetic fluctuations above \textit{T$_N$}. The S(Q, $\omega$) is asymmetric suggesting that the Mn spin correlations are mosmost likely 2-dimensional in nature. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R8.00013: Electron mediated magnetism in two-dimensional spin-ice Jorn W. F. Venderbos, Maria Daghofer, Sanjeev Kumar, Zohar Nussinov, Jeroen van den Brink In this work we study the magnetic phase diagram of classical spins which interact with itinerant electrons on a checkerboard lattice, a lattice that constitutes a two-dimensional equivalent of the three-dimensional spin-ice pyrochlore lattice. We explore both the strong coupling and weak coupling limit and find a rich ground state phase diagram as function of interaction strength and electron doping. The strong coupling limit allows for unbiased Monte Carlo simulations of the classical spins combined with exact diagonalization of the fermionic Hamiltonian. For half filling we find a very robust coplanar orthogonal spin state, the robustness of which originates from the strong geometrical frustration of the checkerboard lattice. In the weak coupling approach this double-Q spin state is a consequence of fermi-surface nesting. The electronic spectrum is this state consists of two Dirac points in full analogy with graphene. For other special electron filling fractions such as $n=p/q=1/4,3/4,3/8,5/8$, we find collinear ``loop'' states, where the spins order in disconnected loops of fixed length $q$. Interestingly, for intermediate fillings the ground state is a mixture of loops of different size, which can be captured by an emergent electromagnetic theory with fractional charge. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R8.00014: Phase diagram and chirality of the spin-1/2 J1-J2 Heisenberg model on the kagome lattice Shoushu Gong, Dongning Sheng We studied the spin-1/2 Heisenberg model on the kagome lattice with nearest (J1) and next-nearest neighbor (J2) interactions by means of the density matrix renormalization group. We set J1 as antiferromagnetic coupling (J1 \textgreater\ 0), and J2 can be either ferromagnetic (J2 \textless\ 0) or antiferromagnetic (J2 \textgreater\ 0). By analyzing the spin-spin correlation function and the bond energy, we find a valence-bond crystal phase for J2 \textless\ -0.1 and a magnetically ordered phase for J2 \textgreater\ 0.2. In the intermediate paramagnetic phase, we investigate the evolution of spin and singlet gaps, topological entanglement entropy, dimer and chirality correlations as a function of the parameter J2. In particular, we investigate the local p6 chiral order parameter proposed recently by measuring the dimer-dimer correlation functions to study the possible reflection symmetry breaking in this spin liquid candidate. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R8.00015: Collinear Magnetic Order in an Isotropic Triangular Antiferromagnet: The Sn/Si(111) Surface System Gang Li, Philipp H\"opfner, J\"org Sch\"afer, Ralph Claessen, Werner Hanke The one-electron spectral function is the key quantity to extract detailed information on the complex spin pattern in a frustrated magnetic system. This is demonstrated here by a detailed comparison of theory, which combines a priori density-functional (LDA) with cluster many-body (LDA + DCA) calculations, with high-precision angle-resolved photoelectron spectroscopy (ARPES). The role model in this work is the isotropic triangular antiferromagnetic Sn/Si(111). Its geometric frustration and strong electronic correlations are shown at low temperatures to combine to an unexpected magnetic, i.e. collinear order, and not the possible spiral ($120^{\circ}$) antiferromagnetic order or a disordered spin-liquid phase. [Preview Abstract] |
Session R9: Invited Session: Advances in Condensed Matter Physics in Latin America
Sponsoring Units: FIPChair: Eugenio Vogel, Universidad de La Frontera
Room: 308
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R9.00001: CLAF: 50 Years of Promotion and Coordination of Physics in Latin America Invited Speaker: Elisa Maria Baggio Saitovitch |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R9.00002: Marshak Lectureship: Women in Physics: Increasing in Number, and What Else? Invited Speaker: Lilia Meza-Montes Latin America is a region with high contrasts. With abundant natural resources and home of several celebrities among the wealthiest in the world, the zone has elevated indexes of poverty. In spite of this, and mostly thanks to the continuing intense efforts of the scientific community, it has been possible to create many excellence research centers. In contrast, illiteracy and lack of access to information and communication technologies are widely spread across our countries. Attitudes toward women have even coined a term, {\it machismo}. The situation of female physicists in this scenario is analyzed. We present a statistical overview of the participation of women as students or researchers in physics and related areas, for countries where data are available. Initiatives and ongoing programs to support and promote participation of women in science are discussed. Beyond statistics, some comments are given, as expressed by colleagues about work environment and gender issues in general, which have been collected through several years of exchanging concerns on the topic. Mexico and Brazil are discussed in more detail. Finally, we propose some joint actions to increase and improve the participation of women in our scientific field, which will give rise to better conditions for us but will also contribute to building a more equitable and developed region. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R9.00003: Brazilian Synchrotron Light Source: current results and future perspectives Invited Speaker: Antonio Jose Roque da Silva The application of synchrotron radiation in a great variety of fields in general, and condensed matter in particular, has increased steadily worldwide. This, to a large extent, is a result of the availability of the much brighter third-generation light sources, which opened up new experimental techniques. Brazil gave an important contribution to science in Latin America through the development of the necessary technology and the construction of the first synchrotron in the southern hemisphere, still the only one in Latin America. The Laborat\'{o}rio Nacional de Luz S\'incrotron -- LNLS, operates this installation as an open facility since 1997, having today more than 1300 users yearly. Despite all this success, the current Brazilian light source is a second-generation machine, with relatively low electron energy, high emittance and few straight sections for insertion devices. LNLS is currently engaged in the design and construction of a new, third-generation synchrotron light source. It is being planned to be a state of the art machine, providing tools for cutting edge research that are non existent today in Brazil. In this talk an overview of the status of the current Brazilian light source will be provided, illustrated with some experimental results from users, as well as the future perspectives of the new synchrotron source. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R9.00004: Physics in Argentina: The Case of Nanoscience and Nanotecnology Invited Speaker: Carlos A. Balseiro Since the creation of the Ministry of Science and Technology in 2008 the science budget has increased and new programs have been launch. After a brief introduction describing general aspects, including the structure of the Ministry and the role of the National Research Council, I will focus on the case of nanoscience and nanotechnology in our country: The main actors and their activities, new programs and facilities, international cooperation and technology oriented projects. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R9.00005: Condensed Matter Physics in Mexico Invited Speaker: Romeo de Coss |
Session R10: Invited Session: New Platforms for Non-Abelian Statistics Majoranas and Beyond
Sponsoring Units: GQI DCMPChair: Kirill Shtengel, University of California, Riverside
Room: 309
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R10.00001: Coulomb-assisted braiding of Majorana fermions in a Josephson junction array Invited Speaker: Carlo Beenakker We show how to exchange (braid) Majorana fermions in a network of superconducting nanowires by control over Coulomb interactions rather than tunneling. Even though Majorana fermions are charge-neutral quasiparticles (equal to their own antiparticle), they have an effective long-range interaction through the even-odd electron number dependence of the superconducting ground state. The flux through a split Josephson junction controls this interaction via the ratio of Josephson and charging energies, with exponential sensitivity. By switching the interaction on and off in neighboring segments of a Josephson junction array, the non-Abelian braiding statistics can be realized without the need to control tunnel couplings by gate electrodes. This is a solution to the problem how to operate on topological qubits when gate voltages are screened by the superconductor. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R10.00002: Zero-bias peaks and splitting in an Al--InAs nanowire topological superconductor as signature of Majorana fermions Invited Speaker: Moty Heiblum Majorana fermions are the only fermionic particles that are expected to be their own antiparticles. While elementary particles of the Majorana type were not identified yet, quasi-particles with Majorana like properties, born from interacting electrons in the solid, were predicted to exist. Here, we present thorough experimental studies, backed by numerical simulations, of a system composed of an aluminum superconductor in proximity to an indium arsenide nanowire, with the latter possessing strong spin-orbit coupling and Zeeman splitting. Induced one-dimensional topological superconductor, supporting Majorana fermions at both ends, is expected to form. We concentrate on the characteristics of a distinct zero bias conductance peak (ZBP) and its splitting in energy - both appearing only with a small magnetic field applied along the wire. The ZBP was found to be robustly tied to the Fermi energy over a wide range of system parameters. While not providing a definite proof of a Majorana state, the presented data and the simulations support its existence. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R10.00003: Exotic non-Abelian anyons from conventional fractional quantum Hall states Invited Speaker: David Clarke Non-Abelian anyons are widely sought after for the exotic fundamental physics they harbor as well as for quantum computing applications. There now exist numerous blueprints for stabilizing the simplest type of non-Abelian anyon, defects binding Majorana fermion zero modes, by judiciously interfacing widely available materials. Following this line of attack, we introduce a device fabricated from conventional fractional quantum Hall states and s-wave superconductors. We show that a new type of zero mode is bound at the interface between the quantum Hall state and the superconductor. These zero mode operators have parafermionic rather than fermionic commutation relations, implying a topologically protected ground state degeneracy larger than that of Majorana fermions. We discuss how these modes might be experimentally identified (and distinguished from Majoranas) using Josephson measurements. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R10.00004: Fractionalizing Majorana Fermions: Non-Abelian Statistics on the Edges of Abelian Quantum Hall States Invited Speaker: Netanel Lindner We study the non-Abelian statistics characterizing systems in which the edges of fractional quantum Hall states are gapped by proximity coupling to superconductors and ferromagnets. We show that as more superconductor-ferromagnet interfaces are introduced, the ground state degeneracy grows with a quantum dimension of a square root of an even integer, corresponding to a new family of non-Abelian anyons. Topologically protected braiding of two anyons can be achieved by a sequence of adiabatic manipulations of the system. We show that the unitary transformations resulting from these braiding operations realize a richer set of representations of the braid group than those realized by non-Abelian anyons based on Majorana fermions. We discuss implications of these braiding operations to topological quantum computation, and consider possible realizations of these ideas in experimentally accessible solid state systems. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R10.00005: Genons, twist defects, and projective non-Abelian statistics Invited Speaker: Maissam Barkeshli An intense focus in the condensed matter community currently is the search for Majorana fermions in solid state systems. Defects which localize Majorana zero modes obey the simplest kind of non-Abelian statistics, and are of interest partially for the goal of achieving topological quantum computing. In this talk, I will present recent advances in our understanding of how to synthesize a much more general class of non-abelian defects using conventional topological states. After discussing the new theoretical foundations, I will present an experimental proposal using only conventional bilayer fractional quantum Hall states and a simple geometry of top and bottom gates. I will also discuss how these ideas can be used to perform universal topological quantum computing (TQC) using non-abelian states that by themselves are not universal for TQC. [Preview Abstract] |
Session R12: Focus Session: Thermoelectrics Phonons and Heat Conduction
Sponsoring Units: DMP GERA FIAPChair: David Cahill, University of Illinois at Urbana-Champaign
Room: 314
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R12.00001: Phonon Thermal Transport in Thermoelectric Materials from First Principles Invited Speaker: David Broido Breakthroughs in nanoscience and materials fabrication technology have led to the creation of materials with very low lattice thermal conductivity [1, 2], a requirement for high thermoelectric efficiency. There is now an unprecedented need for quantitative, predictive theoretical approaches to provide fundamental understanding of lattice thermal transport in thermoelectric materials and insight into the design and development of new materials for enhanced thermoelectric applications. In this talk, I will describe our atomistic first principles approach for calculating lattice thermal conductivity of materials [3-6], which combines a complete solution of the Boltzmann transport equation for phonons with harmonic and anharmonic interatomic forces determined from density functional theory. I will present an overview of this theoretical approach along with some of our recent calculated results for a range of test materials such as Si, Ge and III-V compounds. I will also discuss results for thermoelectric alloys such as Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}}$ and Mg$_{\mathrm{2}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{1-x}}$ and nanoparticle embedded in alloy thermoelectric (NEAT) materials. Finally, I will discuss insights gained from this effort such as the importance of anharmonic coupling of acoustic and optic phonon modes.\\[4pt] [1] Bed Poudel et al., Science 320, 634 (2008);\\[0pt] [2] D. T. Morelli et al., Phys. Rev. Lett. 101, 035901 (2008);\\[0pt] [3] D. A. Broido et al, Appl. Phys. Lett., 91, 231922 (2007);\\[0pt] [4] A. Kundu et al, Phys. Rev. B, 84, 125426 (2011);\\[0pt] [5] L. Lindsay et al., Phys. Rev. Lett. 109, 095901 (2012).\\[0pt] [6] W. Li et al, submitted (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R12.00002: Beyond the constant Lorenz number for separating thermal conductivities of electrons and phonons: A DFT study Mingxing Chen, Raimund Podloucky Lorenz number is an important quantity for separating thermal conductivities of electrons and phonons in the field of thermoelectrics, which is material- and temperature-dependent. Combing DFT calculations with Boltzmann transport equations, we have derived the Lorenz number for realistic compound BaAu$_6$Ge$_{40}$, a good thermoelectric material. It is demonstrated that using the constant Lorenz number of the Wiedemann-Franz law for simple metals leads to strong discrepancies, in particular at higher temperatures. The results suggest that one has to rethink the way of extracting both $\kappa_{el}$ and $\kappa_{ph}$ as usually done based on the measured electrical conductivity. We propose a strategy of correcting the Wiedemann-Franz Lorenz number that subtracts the metallic limiting value by S$^2$ as obtained from Seebeck measurements. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R12.00003: Calculating Lattice Thermal Conductivity via Compressive Sensing Lattice Dynamics Weston Nielson Calculating the lattice contribution to thermal conductivity (TC) is of great importance in a range of materials applications, including thermoelectrics. Common simulation-based methods for calculating the TC typically require either very long simulation times, large system size, or both. These constraints make it difficult or impractical to use DFT-based methods for calculating the TC. Classical molecular dynamics (MD), however, is typically unburdened by these constraints but is instead limited by the accuracy of the interatomic potentials. We have developed a method that uses DTF, combined with compressive sensing, to calculate the higher-order force constants from the theory of lattice dynamics. These force constants are then used to calculate interatomic potentials in a classical MD program. We present our findings from applying this method to a variety of materials. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R12.00004: Phonon Surface Scattering in Monte Carlo Simulations Leon Maurer, Zlatan Aksamija, Edwin Ramayya, Amirhossein Davoody, Irena Knezevic Surface roughness has a significant impact on the thermal conductivity and thermoelectric properties of nanowires. We investigate the effect of surface roughness on thermal transport using a phonon Monte Carlo simulation. In addition to allowing us to simulate a wide range of wire dimensions and surface topographies, Monte Carlo enables us to investigate different models for surface scattering: constant specularity parameters, momentum-dependent specularity parameters, and specular scattering from randomly generated rough surfaces. We investigate the relative merits of different surface scattering models and the limitations on their validity. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R12.00005: First principles and force field calculations of thermal transport in bulk semiconductors and oxides: a comparative study Eamonn Murray, Ivana Savic, Giulia Galli At present, large scale calculations of thermal transport properties of materials are carried using empirical potentials\footnote{See, e.g. Y.He, I.Savic, D.Donadio and G.Galli PCCP 2012 ASAP (DOI: 10.1039/C2CP42394D)}, due to difficulties in scaling ab initio methods to directly compute the thermal conductivity of complex, nanostructured systems. It is therefore important to asses the predictive ability of empirical potentials for representative bulk systems, for which ab initio simulations are possible, and to establish their accuracy in yielding absolute values of computed thermal conductivities ($\kappa$) and trends within given classes of systems. We report on comparisons between thermal conductivities of elemental semiconductors and insulators (Si, C, Ge) and simple oxides (MgO and SiO2) as obtained using the Boltzman Transport equation with first principles, DFT Hamiltonians and Tersoff type empirical potentials. The second and third derivatives of the energy with respect to atomic displacements are obtained by finite difference calculations in supercells in all cases. A detailed discussion of the reasons why these empirical potentials appear to systematically overestimate $\kappa$ will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R12.00006: A Comparative Study of Ab-Initio Thermal Conductivity Approaches: The Case of Cubic Boron Nitride Saikat Mukhopadhyay, Lucas Lindsay, David Broido, Derek Stewart Given its high strength and large thermal conductivity, cubic boron nitride (cBN) provides an important complement to diamond films for heat spreading applications. However, cBN, in contrast to diamond, is a polar material with significant LO-TO splitting in the phonon dispersion. In this talk, we examine the lattice thermal conductivity of cBN using several approaches based on first principles calculations. These approaches include: (1) an analytic modified Callaway-Debye model that relies on parameters from ab-initio harmonic force constants, (2) a fully self-consistent calculation of the thermal conductivity that links an iterative solution of the phonon Boltzmann transport equation (BTE) with harmonic and anharmonic interatomic force constants. The force constants for the BTE are calculated using two approaches: density functional perturbation theory and a real-space supercell approach. We will compare the results from these approaches, highlight the role of normal phonon-phonon scattering, and also examine the impact of optical modes and LO-TO splitting. In addition, we will discuss how isotope scattering affects thermal conductivity and compare this to other boron nitride structures (hexagonal BN, BN sheets and BN nanotubes). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R12.00007: Improved Calculation of Vibrational Mode Lifetimes in Anharmonic Solids Murray Daw, Yang Gao, Doyl Dickel, David Harrison We propose and evaluate a formal foundation for practical calculations of vibrational mode lifetimes in solids. The approach is based on a recursion method analysis of the Liouvillian. From this we derive the lifetime of a vibrational mode in terms of moments of the power spectrum of the Liouvillian as projected onto the relevant subspace of phase space. In practical terms, the moments are evaluated as ensemble averages of well-defined operators, meaning that the entire calculation is to be done with Monte Carlo. These insights should lead to significantly shorter calculations and improved understanding of mode lifetimes and lattice thermal conductivity. Evaluation performed on model systems have been encouraging. [See Dickel \& Daw, Comp Mat Sci, v47 p698 and v49 p445 (2010)]. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R12.00008: Local Distortions in PbTe:Tl Trevor Keiber, Frank Bridges, Brian Sales Lead Telluride (PbTe) is a well characterized thermoelectric material. Tl doping increases the figure of merit with a maximum at 2\% Tl. Recent X-ray diffraction and total neutron scattering experiments suggest Pb moves off-center along the 100 axis as T increases. To investigate the local structure we present an Extended X-ray Absorption Fine Structure (EXAFS) analysis for 0-3\% Tl concentrations at the Tl and Pb L3 edges and at the Te K edge. At 10K the local structure about Pb is well ordered, the Pb-Te (Te-Pb) pair distribution function (PDF) broadens rapidly with T. Attempts to model the increase in $\sigma^{2}$(T) for the Pb-Te pair ($\sigma$ is the width of the PDF) with a 100 Pb off-center displacement, were not successful. However $\sigma^2$(T) for the Pb-Te pair is well described by a correlated Debye model with a low correlated Debye temperature. The Te edge shows increased disorder for the the Te-Te pair and later peaks which may be caused by a structural change around the Te atom. For Tl, the environment is distorted even at 10K within the host material. This indicates a large variation of the Tl-Te bond lengths, presumably as a result of the presence of Tl(+1). We discuss possible models for the disorder about Tl, Pb, and Te in PbTe:Tl. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R12.00009: Anomalous Coherent Oscillations in PbTe from Ultrafast Optical Pump-Probe Measurements Mason Jiang, Paula Giraldo, Ian Fisher, David Reis We report on the observation of anomalous coherent oscillations in single crystals of PbTe from ultrafast optical pump-probe measurements. PbTe is a leading thermoelectric material with an unusually low thermal conductivity, which has recently been attributed to strongly anharmonic phonon interactions. In an attempt to understand in greater detail the nature of these interactions, we perform time-resolved, optical pump-probe measurements on PbTe with femtosecond resolution in a range of temperatures from 77K to room temperature. We see previously unreported, low-frequency reflectivity oscillations that decay on the timescale of a few picoseconds and remain robust through a wide range of temperature variation. This talk will discuss possible origins and explanations for the appearance of these oscillations. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R12.00010: Phonon Dispersions and Relaxation Times in AgSbTe$_2$ and PbTe Olivier Delaire, Jie Ma, Andrew May, Chris Carlton, Michael McGuire, Lindsay VanBebber, Douglas Abernathy, Georg Ehlers, Tao Hong, Ashfia Huq, Wei Tian, Veerle Keppens, Yang Shao-Horn, Brian Sales The thermoelectric material AgSbTe$_{2}$ had attracted much interest due to its high thermoelectric figure-of-merit, and its anomalously low thermal conductivity for a nominally simple rock-salt structure, which is glass-like even in bulk single-crystals. We present results of systematic neutron scattering investigations of the phonon density-of-states, dispersions, and relaxation times in AgSbTe$_{2}$, and contrast these with PbTe. A detailed account of the thermal conductivity is obtained in terms of microscopic phonon mean-free-paths, providing good agreement with bulk transport measurements. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R12.00011: Phonon dynamics in SnTe Chen Li, Olivier Delaire, Xin Chen, David Singh, Andrew May, Jie Ma, Michael McGuire, Georg Ehlers, Andrew Christianson, Ashfia Huq Thermoelectric materials can convert waste heat into electrical energy, and have attracted much attention in recent years for power generation. IV-VI compounds in rock salt structure include some of the most efficient thermoelectric materials and giant phonon anharmonicity is believed to contribute to the low thermal conductivity. In this work, phonon dispersions and linewidths in single-crystalline SnTe were measured at a series of temperatures using time-of-flight and triple-axis neutron spectrometers to study the temperature dependence of the phonon dynamics and phonon anharmonicity. Phonon calculations and molecular dynamics simulations with first-principles methods were used to identify the anomalies in phonon modes and the results were compared to the measurements. Because the phonons involved have an important contribution to the lattice thermal conductivity in this system, the anharmonic coupling is likely to provide a key insight in understanding the surprisingly low thermal conductivity of the rocksalt tellurides in general. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R12.00012: Simulation of Nanostructure and Thermal Conductivity in Binary Alloys Yusuke Konishi, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida, Yoshihiro Asai Thermoelectric materials attract much attention because of concerns about energy conservation. Recently, Sugihara et al. made nanostructures using phase separation of Ni-Cu binary alloy [1]. This structure is about 10nm and has the large Seebeck coefficient. However, the way to make better thermoelectric material is under discussion. For this purpose, we need a large Seebeck coefficient, large electric conductivity, and small phonon thermal conductivity. The goal of this study is finding the condition of making good thermoelectric materials. In our simulation, we made structures in various conditions and evaluated phonon thermal conductivity. First, we simulated quenching binary alloy at high temperature by using Monte Carlo method. The potential between atoms are determined by KKR-CPA method [2]. In this simulation, nanostructures have the size distribution between 1 nm and 50 nm. Next, we simulated phonon conduction by molecular dynamics. Heat baths were placed at both ends and the thermal gradient was made. By calculating energy flux, we determined the value of phonon thermal conductivity. [1] A Sugihara et al., Appl. Phys. Exp. 3, 065204 (2010). [2] H. Akai, J. Phys.: Condens. Matter 1, 211 (1989). [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R12.00013: Landauer approach to thermoelectric transport across grain boundaries in Si Michael Shaughnessy, Doug Medlin, Francois Leonard, Catalin Spataru Thermoelectric transport is strongly influenced by electron and phonon scattering from defects, grain boundaries, and nano structuring. While scattering from point defects is relatively well understood, the impact of the detailed structure of grain boundaries is still poorly understood. We use a Landauer approach based on ab initio Density Functional Theory and classical Molecular Dynamics simulations to compute electron and phonon transport coefficients in the presence of grain boundaries. The approach allows the calculation of all the thermoelectric quantities, including thermal conductivity, electrical conductivity, Seebeck coefficient, and the overall figure of merit, ZT. The method is applied to grain boundaries in Si, focusing on the \textbraceleft 111\textbraceright twin in the high and low density regimes. For ordered arrays of \textbraceleft 111\textbraceright twins in Si a small change in ZT is predicted because of compensating differences between thermal conductivity on the one hand and electrical conductivity and Seebeck coefficient on the other.~ [Preview Abstract] |
Session R13: Focus Session: Topological Materials - Surface Microscopy and Spectroscopy
Sponsoring Units: DMPChair: Alexander Brinkman, University of Twente
Room: 315
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R13.00001: Imaging the Impact of Impurities on Topological Surface States Invited Speaker: Jennifer Hoffman Harnessing the technological potential of the spin-polarized surface states on topological insulators requires a detailed understanding of the impact of nanoscale disorder on those surface states. We employ spectroscopic scanning tunneling microscopy (STM) in the presence of a magnetic field to visualize the impact of intrinsic impurities on topological surface states in Sb and Bi$_2$Se$_3$. We find a variety of impurities with different energy profiles that elastically scatter surface states through dispersive quasiparticle interference (QPI), that inelastically scatter surface states into the bulk, that locally destroy the extended surface state Landau level wavefunctions, or that form local resonant states interacting with the Dirac quasiparticles. By identifying impurities that strongly interact with and limit the mobility of the topological surface states, our impurity studies can directly advise the growth and development of future topological materials.\\[4pt] Measurements carried out by Anjan Soumyanarayanan, Michael Yee, Yang He. Samples grown by Dillon Gardner \& Young Lee; Zahir Salman \& Amit Kanigel; Zhi Ren \& Kouji Segawa \& Yoichi Ando.\\[4pt] Experiments supported by the National Science Foundation, under grant DMR-1106023. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R13.00002: Scanning tunneling microscopy studies of topological crystalline insulators Andras Gyenis, Jungpil Seo, Oliver Jeong, Ilya Drozdov, Stevan Nadj-Perge, Quinn Gibson, Genda Gu, Robert Cava, Ali Yazdani Recent theoretical studies and experimental findings suggest the existence of a new topological phase: topological crystalline insulators (TCI). In contrast to the Z$_{2}$ topological insulators, where the time-reversal symmetry warrants the topological protection of the gapless surface states, in the TCI phase the protection is based on the crystal symmetry. Pb$_{1-x}$Sn$_{x}$Se and Pb$_{1-x}$Sn$_{x}$Te alloys are promising candidates for the TCI state: both of them have the rock-salt crystal structure (at certain doping values) with spatial mirror symmetry, and as a function of doping level the band structure can be changed from normal to inverted bandgap state. We present scanning tunneling microscopy/spectroscopy measurements on these alloys as a function of doping. Similar to previous experiments on spin-orbit coupled topological insulators [1], spectroscopic mapping with the STM can be used to establish the presence of topological properties through examining allowed and disallowed scattering transitions.\\[4pt][1] P. Roushan et al, Nature \textbf{460} 1106 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R13.00003: LT-STM study of Nb islands on Bi2Se3 with W and Nb Tips Rami Dana, Anita Roychowdhury, Ireneusz Miotkowski, Yong P. Chen, Michael Dreyer Proximity effect between an s-wave superconductor (SC) and a topological insulator (TI) are expected to induce px $+$ ipy superconductor like state at the SC-TI interface. The vortex cores of that state are predicted to host Majorana fermions. In this work we study the TI Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ using W and Nb tips at 4.2 K with and without Nb islands. The W tip shows no SC gap on top and around the islands. The Nb tip shows variable SC gaps and a verity of zero bias conductance peaks. The possible sources for these observations and the part of the TI, Nb islands and/or Nb tip will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R13.00004: Scanning Tunneling Microscopy of the Topological Crystalline Insulator SnTe Duming Zhang, Tong Zhang, Jeonghoon Ha, Hongwoo Baek, Young Kuk, Fred Sharifi, Joseph Stroscio Topological insulators are a new state of matter characterized by a bulk insulating gap and gapless surface states protected by time reversal symmetry. This is realized by spin orbit coupling induced band inversion with an odd number of Dirac cones. Recently, the topological classification of states has been extended to a new class of matter called topological crystalline insulators. In contrast to topological insulators, topological crystalline insulators arise from crystal symmetry and are characterized by surface states with an even number of Dirac cones. Here, we report molecular beam epitaxy growth of SnTe thin films, a material recently predicted and experimentally confirmed as a topological crystalline insulator. The film morphology and SnTe (001) surface states were characterized \textit{in-situ} by low temperature scanning tunneling microscopy and spectroscopy will be discussed in relation to the predicated topological properties of this material. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R13.00005: Scanning tunneling microscopy of gate tunable topological insulator Sb2Te3 Tong Zhang, Niv Levy, Jeonghoon Ha, Young Kuk, Joseph Stroscio We achieved gate tunable topological insulator (TI) Sb2Te3 thin films which are suitable for low temperature scanning tunneling microscopy (STM) studies. The film is epitaxially grown on pre-patterned SrTiO3 substrates which are mounted on specially designed sample holders. This allows us to do in-situ gating on epitaxial films without any ex-situ processing of the sample. The tunneling conductance as well as film resistance is investigated as a function of gate voltage (Vg). In a 3 nm thick Sb2Te3 film, a gap opening at the Dirac point due to the coupling of the top and bottom surfaces is observed. More importantly, the gap size is found to be tunable by Vg, a result of the combination of coupling of the surface state bands and electric field effect. We show that our observation can be well described by an effective model of TI thin films and first principle calculations. The reduced surface states gap versus Vg indicates it is possible to create a topological phase transition by apply a strong enough electric field through the film. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R13.00006: Landau quantization and quasiparticle interference of Dirac fermions on a topologically protected Fermi surface Anjan Soumyanarayanan, Michael Yee, Yang He, Dillon Gardner, Young Lee, Jennifer Hoffman The discovery of topological materials hosting spin-polarized Dirac fermion surface states has been driven by the use of surface-sensitive spectroscopic tools. Scanning tunneling microscopy and spectroscopy (STM/STS) can, in principle, access the surface state band structure across a range of energies on the nanometer length scale through a combination of one particle (Landau quantization) and two-particle (quasiparticle scattering) techniques. However, the equivalence of these two STS techniques has yet to be established. Here we report the surprising simultaneous observation of Landau quantization and quasiparticle interference on the Fermi surface of the topological metal Sb(111). We establish the equivalence of the two momentum-resolved STS techniques, and use them to quantitatively reconstruct the multi-component surface state band structure, which would be inaccessible via either of these techniques alone. We further use these techniques to probe the local effects of single atom impurities on the surface states. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R13.00007: First principles transport calculations on topological surface states scattering Ivan Rungger, Awadhesh Narayan, Stefano Sanvito We study the scattering properties of topologically protected states on the Sb(111) and Bi$_2$Se$_3$(111) surfaces by using the ab initio transport code SMEAGOL \footnote{A. R. Rocha, V. M. Garcia-Suarez, S. Bailey, C. Lambert, J. Ferrer, and S. Sanvito, Phys. Rev. B \textbf{73}, 085414 (2006).}. We consider different types of defects, such as adatoms and extended barriers. In the presence of a strong surface perturbation in the form of a step separating surface terraces we obtain standing-wave states resulting from the superposition of spin-polarized surface states. By Fourier analysis, we identify the underlying two dimensional scattering processes and the spin texture \footnote{A. Narayan, I. Rungger, and S. Sanvito, Phys. Rev. B \textbf{86}, 201402(R) (2012).}. We find evidence of resonant transmission across the surface barrier at quantum well state energies and evaluate their lifetimes. Our results for the Sb surface are in agreement with experimental findings \footnote{J. Seo, P. Roushan, H. Beidenkopf, Y. S. Hor, R. J. Cava, and A. Yazdani, Nature (London) 466, 343 (2010).}. We also show that despite the presence of a step edge along a different direction, the surface states exhibit unperturbed transmission around the Fermi energy for states with near to normal incidence [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R13.00008: Electronic structure studies on p-type Pb$_{1-x}$Sn$_x$Te system above and below the band inversion topological transition Nasser Alidoust, Su-Yang Xu, M. Neupane, C. Liu, I. Belopolski, D. Qian, J.D. Denlinger, Y.J. Wang, H. Lin, L.A. Wray, Q. Gibson, R. Sankar, F.C. Chou, R.J. Cava, A. Bansil, M.Z. Hasan We present systematic ARPES studies on p-type Pb$_{1-x}$Sn$_x$Te samples at three different compositions with x = 0.26, 0.5, and 1.0. This material has been predicted as a topological crystalline insulator (TCI) upon band inversion at $x\simeq0.3$. We show that the observed bulk valence band is a single hole-like band in the vicinity of the $\mathrm{\overline{\textit{X}}}$ points of the surface Brillouin zone, and reveal the 3D dispersive nature of the valence band with a clear $k_z$ dispersion. We further show that despite the predicted band inversion and topological phase transition, the observed valence band electronic structure does not exhibit dramatic difference between these samples, demonstrating the critical importance of preparing in-gap or n-type samples for the realization of the TCI phase. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R13.00009: Observing electronic structures on \textit{ex-situ} topological insulator thin films Bo Zhou, S.H. Yao, M.H. Lu, Z.K. Liu, Y.B. Chen, J.G. Analytis, C. Brune, W.H. Dang, S.-K. Mo, Z.-X. Shen, I.R. Fisher, L.W. Molenkamp, H.L. Peng, Z. Hussain, Y.L. Chen Topological insulators represent a new state of quantum matter with insulating bulk but conducting surface states formed by an odd number of Dirac fermions. We present our progress on the study of electronic structures of \textit{ex-situ} grown topological insulator thin films by angle resolved photoemission spectroscopy (ARPES). We successfully obtained the topological band structures, after proper surface cleaning procedures, from HgTe films grown by molecular beam epitaxy and Bi$_2$Te$_3$ nanoplates synthesized by vapor-solid method. This new development will not only enable us to study more topological insulators that cannot be measured by conventional \textit{in-situ} ARPES technique, but also open the door to directly characterize the electronic properties of topological insulators used in functional devices. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 5:18PM |
R13.00010: Spatial fluctuations of helical Dirac fermions on the surface of topological insulators Invited Speaker: Haim Beidenkopf Strong topological insulators are materials that host exotic states on their surfaces due to a topological band inversion in their bulk band structure. These surface states have Dirac dispersion as if they were massless relativistic particles, and are assured to remain metallic by time reversal symmetry. The helical spin texture associated with the Dirac dispersion prohibits backscattering, which we have imaged using scanning tunneling microscopy (STM) and spectroscopic mappings [1,2]. This topological protection can be lifted by time-reversal breaking perturbations that induce a gap at the Dirac point and cant the helical spin texture. Massive Dirac electrons had been visualized by angular resolved photo emission spectroscopy in magnetically doped topological insulators. While we do not identify a gapped spectrum in our STM measurements of similar compounds, we do find a dominating electrostatic response to the charged content of those dopants [3]. In their presence the Dirac spectrum exhibits strong spatial fluctuations. As a result translational invariance is broken over a characteristic length scale and the Dirac-point energy is only locally defined. Possible global manifestations of these local fluctuations will be discussed, as well as alternative avenues for breaking time reversal symmetry while maintaining the integrity of the Dirac spectrum.\\[4pt] [1] P. Roushan, J. Seo, C. V. Parker, Y. S. Hor, D. Hsieh, D. Qian, A. Richardella, M. Z. Hasan, R. J. Cava, A. Yazdani, Nature 460, 1106 (2009).\\[0pt] [2] J. Seo, P. Roushan, H. Beidenkopf, Y. S. Hor, R. J. Cava, A. Yazdani, Nature 466, 343 (2010).\\[0pt] [3] H. Beidenkopf, P. Roushan, J. Seo, L. Gorman, I. Drozdov, Y. S. Hor, R. J. Cava, A. Yazdani, Nat. Phys. 7, 939 (2011). [Preview Abstract] |
Session R14: Focus Session: Magneto-thermal Transport and Spin Current in Insulators
Sponsoring Units: DMP FIAP GMAGChair: Helmut Schulthei, Argonne National Laboratory
Room: 316
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R14.00001: Local spin currents in magnetothermal landscapes M. Weiler, M. Schreier, H. Huebl, M. Althammer, M. Opel, R. Gross, S.T.B. Goennenwein Spin caloritronic effects - such as the spin Seebeck effect - are concerned with the interplay of heat and spin currents and have been experimentally studied using homogeneous thermal gradients to date. However, in order to understand the underlying magnon-phonon interactions that take place on short length scales, a spatially resolved study of spin currents in magnetothermal landscapes~[1] is mandatory. We here use a focussed, scannable laser beam to generate local thermal perturbations in thin film multilayers incorporating the ferromagnetic insulator Y$_3$Fe$_5$O$_{12}$ (YIG). In both, YIG/Pt thin film bilayers and YIG/Au/Pt trilayers, the laser heating results in a difference of the magnon and electron temperatures in the YIG and Pt, respectively, as quantitatively modeled in numerical simulations. In the presence of this temperature difference, we experimentally observe a local in-plane electric field in the YIG/Pt and YIG/Au/Pt samples. This electric field is ascribed to the detection of the local longitudinal spin Seebeck effect via the inverse spin Hall effect in Pt. Our experiments allow to, e.g., electrically image magnetic texture in a magnetic insulator and provide a local, bipolar, magnetically controllable spin current source. [1] M. Weiler et al. PRL 108, 106602 [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R14.00002: Induced Magneto-transport Effects in Non-magnetic Metals on Yttrium Iron Garnet Tao Lin, Chi Tang, Jing Shi Yttrium iron garnet (YIG) was called ``spin Seebeck insulator,'' for it supports heat-generated pure spin currents. Pt thin film, with strong spin-orbit interaction, is used as a spin current generator or detector based on the spin Hall effect or the inverse spin Hall effect. The combination of these two materials plays a very important role in spintronics. A recent magnetotransport study shows strong evidence of a magnetic proximity effect in thin Pt films deposited on YIG. Here, we present a magneto-transport study of several non-magnetic (NM) metal films (e.g. Pt, Pd) on YIG films grown on gadolinium gallium garnet substrates with laser molecular beam epitaxy. The anisotropic magnetoresistance (AMR) and anomalous Hall effect (AHE) reveal clear ferromagnetic characteristics in NM films. The magnitude of the AHE angle $\Theta $ in Pd/YIG structure increases with decreasing temperature, while $\Theta $ in Pt/YIG structure has a sign reversal at an intermediate temperature. Both AMR and AHE have been investigated as the NM film thickness is varied and an optimal effective thickness is identified. The effect of annealing has also been studied and the results are consistent with the observed thickness dependence. In thin NM films, a ln(T) temperature dependence with a resistivity minimum is observed at low temperatures, suggesting that the Kondo effect may be relevant. Detailed discussions about the origin of these effects will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R14.00003: Nonlocal optical generation of spin and charge currents on the surface of magnetic insulators using total absorption and surface plasmons Siu Tat Chui, Z.F. Lin, C.R. Zhang, John Xiao We study the nonlocal spin and charge current generation in a finite metallic element on the surface of magnetic insulators such as yttrium iron garnet due to the absorption of the magnetic surface plasmon (MSP). Whereas a surface plasmon is completely reflected by a metal, an MSP can be absorbed due to the absence of backward states. The injection of MSP generates a voltage in the longitudinal direction parallel to the wave vector, with the voltage proportional to input power. If the metal is a ferromagnet, a spin current can also be induced in the longitudinal direction. Our results provide a way to improve upon integrated circuits of spintronics and spin wave logic devices. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R14.00004: Spin information propagation through metal/magnetic insulator interface Invited Speaker: Shulei Zhang In metal-based spintronics, electron spin current plays pivotal roles in propagating spin information. Here we investigate the propagation of magnon current carried by non-equilibrium magnons, which can also serve as spin carriers in ferromagnet. By exploiting of the semiclassical Boltzmann approach, we explicitly derive the non-equilibrium magnon distribution and magnon current in ferromagnetic insulators [1]. In some limiting cases, we find that magnon density satisfies a diffusion equation, similar to the electron spin diffusion equation. At the interface between a metal layer (ML) and a magnetic insulator layer (MIL), we show that the spin current of the ML and the magnon current of the MIL are mutually transferable. We introduce a concept of spin convertance [1] that quantitatively measures magnon current induced by electron spin accumulation and spin current generated by magnon accumulation at the interface. With the above formalism, we predict some interesting spin transport phenomena for several layered structures with a MIL. In particular, we anticipate a novel electric drag mediated by magnons: an applied electric current in one ML induces an electric field in the other ML separated by a thick MIL. Our theory also provides a new perspective on the longitudinal spin Seebect effect [2] from the point of view of magnon current driven by the thermal gradient across a MIL. We discuss the dependence of these phenomena on temperature, materials properties, and geometric parameters. \\[4pt] [1] S. S.-L. Zhang and S. Zhang, Phys. Rev. Lett. 109, 096603 (2012); S. S.-L. Zhang and S. Zhang, arXiv:1210.2735v2.\\[0pt] [2] K. Uchida et al., Appl. Phys. Lett. 97, 172505 (2010); H. Adachi and S. Maekawa, arXiv:1209.0228v1. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R14.00005: Magnon drag thermopile Invited Speaker: Sergio O. Valenzuela Thermoelectric effects in spintronics are gathering increasing attention as a means of controlling spin information by using heat flow. Thermal magnons (spin-wave quanta) are expected to play a major role, however, the coupling between electrons and magnons in ferromagnetic metals remains poorly understood. We demonstrate a conceptually new device that enables us to gather information on magnon-electron scattering and magnon-drag effects [1]. The device resembles a thermopile formed by a large number of pairs of ferromagnetic wires placed between a hot and a cold source and connected thermally in parallel and electrically in series. By controlling the relative orientation of the magnetization in pairs of wires, the magnon drag can be studied independently of the electron and phonon drag thermoelectric effects. Measurements as a function of temperature reveal the effect on magnon drag following a variation of magnon and phonon populations. These results demonstrate the feasibility of directly converting magnon dynamics of nanomagnets into an electrical signal and could pave the way to novel thermoelectric devices for energy harvesting.\\[4pt] [1] M.V. Costache, G.A. Bridoux, I. Neumann and S.O. Valenzuela, Nature Mater. 11, 199 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R14.00006: Theory of Magnon Drag in Ferromagnetic Bilayers Tianyu Liu, Giovanni Vignale, Michael E. Flatt\'e We introduce and study theoretically a novel drag effect that we expect to occur in ferromagnetic bilayer systems. A steady spin-wave (magnon) spin current propagating in one layer (the active layer) induces an inhomogeneous distribution of magnons in the other layer (the passive layer) through the magnetic dipole-dipole interaction. There are significant differences between this effect and the ordinary and well-studied Coulomb drag in electronic bilayers. First, the particles in questions are bosons, and their number is not conserved (this is at variance with systems of bosonic atoms, where number is conserved). Second, it becomes essential to take into account, besides magnon-magnon scattering, processes in which two magnons in one layer merge to produce a magnon in the other, or a magnon in one layer decays producing two magnons in the other. In analogy to the theory of Coulomb drag we calculate the interlayer transport coefficients (relating, for example, the temperature gradient in one layer to the spin current in the other) for different experimental configurations. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R14.00007: Thermoelectric detection of spin waves Helmut Schultheiss, John E. Pearson, Samuel D. Bader, Axel Hoffmann We report on the thermoelectric detection of spin waves in permalloy stripes via the anomalous Nernst effect\footnote{H. Schultheiss, J.E. Pearson, S.D. Bader, and A. Hoffmann, Phys. Rev. Lett. in press.}. Spin waves are locally excited by a microwave current flowing in a coplanar waveguide placed on top of a permalloy stripe, which acts as a waveguide for spin waves. Electric contacts at the ends of the permalloy stripe measure a dc voltage generated along the stripe. Magnetic field sweeps for different applied microwave frequencies reveal, with remarkable signal-to-noise, an electric voltage signature characteristic of spin-wave excitations. The symmetry of the signal with respect to the applied magnetic field direction indicates that the anomalous Nernst effect is responsible; Seebeck effects, anisotropic magnetoresistance, and voltages due to spin-motive forces are excluded. The dissipation of spin waves causes local heating, that drains into the substrate giving rise to a temperature gradient perpendicular to the sample plane, resulting in the anomalous Nernst voltage. Since this method is solely based on the heat generation inside the magnetic film due to the relaxation of the magnetization it has practically no lower limit for the wavelength of the detected spin waves. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R14.00008: Spin Currents coupling with magnon excitation in Ferromagnetic Insulator Tao Liu, Jiaxi Li, Jianwei Zhang We studied spin currents coupling in two Ferromagnetic/normal metal multilayers which are connected by a Ferromagnetic Insulator(FI) layer(such as YIG). In our modeling, we adopted self-consistent spin dependent Boltzmann equations and magnon Boltzmann equation. When applying an in-plane current in first FM layer, a transverse spin current was generated due to Anomalous Hall effect(AHE), after crossing normal metal layer, this transverse spin current will produce magnon excitation at N/FI interface. With carrying spin information, magnon excitations in FI can eventually excite a new spin current at second F/N interface. Although the FI cannot support any spin current propagation across it, but spin polarization information was passed through FI with propagation of magnon. Finally, the transverse spin current in second FM layer can also generate another in-plane spin current by AHE. Our results showed the spin current in second FM layer can be large as the same order of one in first FM layer at limit case. Through the magnon propagation in FI layer, two spin current circuits were coupled indirectly, i.e.without any charge current exchange. we also showed, when applying a magnetic field on FI layer, spin current in final FM layer can be manipulated by varying magnon excitation. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R14.00009: Effect of magnetism on the vibrational properties of the Ni-Cu alloy: a first-principles study Omar De la Pena-Seaman, Ivan Bustamante-Romero, Rolf Heid, Klaus-Peter Bohnen We have studied the lattice dynamical properties of the Ni$_{1-x}$Cu$_x$ magnetic alloy within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual crystal approximation for modeling the alloy. The system has been investigated for both non-magnetic (NM) and ferromagnetic (FM) phases. The performance of LDA and GGA exchange-correlation functionals on the properties under study was analyzed. The structural optimization for each magnetic phase, NM and FM, in the full range of concentrations ($0\leq x \leq 1$) was performed. By studying the electronic structure and its evolution as a function of $x$, we determined the FM-NM phase transition at $x \approx 0.45$. The calculated full phonon dispersion for NM and FM phases are compared between each other and with experimental data available in the literature at different concentrations. In addition, a detailed analysis of the force constants average coupling was performed, finding a clear signature of the magnetism effects on the vibrational properties for the Ni-Cu alloy. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R14.00010: Thermal Hall effect and Berry curvature of spin waves in magnets Shuichi Murakami, Ryo Matsumoto, Ryuichi Shindou Spin waves (magnons) form band structure similar to electrons, and therefore their geometrical structure in $k$ space can be characterized by Berry curvature. This Berry curvature of spin waves causes various interesting phenomena such as thermal Hall effect [1,2] and topological magnonic crystals [3]. In my presentation, we derive the thermal Hall conductivity for spin waves in generic magnets represented as a bosonic Bogoliubov-de Gennes Hamiltonian. We apply this theory to magnetostatic modes in YIG and evaluate the thermal Hall conductivity for the forward volume-wave mode in YIG. We also discuss the relationship with other previous theories on Hall effect of magnons and other bosons. We also apply our theory to magnets with topological chiral edge modes, and discuss thermal transport for the topological edge modes.\\[4pt] [1] R. Matsumoto, S. Murakami, Phys. Rev. B 84, 184406 (2011).\\[0pt] [2] R. Matsumoto, S. Murakami, Phys. Rev. Lett. 106, 197202 (2011).\\[0pt] [3] R. Shindou et al., arXiv.: 1204.3349. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R14.00011: Molecular dynamics, spin dynamics study of phonon-magnon interactions in BCC iron Dilina Perera, David P. Landau, G. Malcolm Stocks, Don Nicholson, Markus Eisenbach, Junqi Yin By combining an atomistic many-body potential (Finnis-Sinclair) with a classical Heisenberg-like spin Hamiltonian, we perform combined molecular and spin dynamics simulations to investigate phonon-magnon interactions in BCC iron. The coupling between atomic and spin degrees of freedom is established via a distance dependent exchange interaction derived from first principles electronic structure calculations. Coupled equations of motion are integrated using a second order Suzuki-Trotter decomposition of the exponential time evolution operator. To investigate the effect of lattice vibrations on spin wave spectrum, we calculate spin-spin and density-density dynamic structure factors S(q, $\omega$), and compare that to the results obtained from pure spin dynamics simulations performed on a rigid lattice. In the presence of lattice vibrations, we observe an additional peak in the longitudinal spin-spin dynamic structure factor which coincides with the peak position in density-density dynanmic structure factor. [Preview Abstract] |
Session R15: Focus Session: Spin/charge in Frustrated Lattices
Sponsoring Units: GMAG DMPChair: Daniel Arovas, University of California at San Diego
Room: 317
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R15.00001: Spin-charge interplay on frustrated lattices Invited Speaker: Yukitoshi Motome Frustration has gained increasing interest in the study of itinerant electron systems. There, not only spin but also charge degree of freedom of electrons plays a crucial role in the structure of the energetically degenerate manifold, providing a new frontier of the frustration physics. A particular interest is in Kondo-type spin-charge coupled systems, in which itinerant electrons couple with localized moments on a frustrated lattice. In these systems, localized moments act as internal local magnetic fields for itinerant electrons, which significantly affect the electronic and transport properties. On the other hand, the kinetic motion of electrons induces effective magnetic interactions between localized moments, resulting in exotic magnetic correlations and orders. It is highly nontrivial what kind of electronic and magnetic state is realized as a consequence of the spin-charge interplay. In this contribution, we review our recent theoretical and numerical studies of the Kondo-type models on frustrated lattices. We have investigated several types of models, with Heisenberg and Ising spins for localized moments defined on 2D triangular and kagome lattices and 3D pyrochlore lattice. Complementary theoretical techniques have been adopted, such as perturbation, mean-field approximation, variational calculation, exact diagonalization, and Monte Carlo simulation. We found that these models exhibit emergent electronic and magnetic properties, such as a spontaneous spin scalar chiral order and topological Hall effect, non-Kondo resistivity upturn in spin-ice liquid, partial disorder accompanied by charge disproportionation, emergence of Dirac electrons under particular magnetic ordering, quantum anomalous Hall effect in spin scalar chiral liquid, and spin-Hall effect by spontaneous inversion symmetry breaking. These works have been done in collaboration with Y. Akagi, S. Hayami, H. Ishizuka, and M. Udagawa. For references, visit http://www.motome-lab.t.u-tokyo.ac.jp/index-e.html. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R15.00002: Mott Physics at Integer Filling Enforced by Crystalline Symmetries D.P. Arovas, S.A. Parameswaran, Ari M. Turner, Ashvin Vishwanath Insulating states of matter in a crystalline system can be either band insulators or Mott insulators. It is well known that band insulators appear only when the filling (the number of electrons per unit cell and spin projection) is an integer. An insulating phase at fractional filling is a Mott insulator, for which interactions are manifestly required. Here we pose and answer the converse question - at an integer filling is a band insulator always possible? Surprisingly, we find that crystalline symmetries may forbid a band insulator even at certain integer fillings. In these cases, the ground state is either conducting or is a Mott insulator, despite being at integer filling. The lattices on which this occurs have a simple property, they have non-symmorphic space groups. These include lattices with essential glide or screw symmetries, which comprise the majority of three dimensional crystal structures. This is shown to be a consequence of gauge invariance using a flux threading argument, which applies to free and interacting systems alike. For several non-symmorphic lattices we determine the minimum integer filling at which band insulators are possible. This result has several immediate implications for band structures as well as the phases of quantum magnets and bosonic insulat [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R15.00003: Many-Variable Variational Monte Carlo Study of Triangular Hubbard Model Ryui Kaneko, Satoshi Morita, Masatoshi Imada Motivated by the previous numerical studies on the triangular Hubbard model, we study low-energy states of the model at half filling up to 144 sites by using the many-variable variational Monte Carlo method. We consider the variational wave functions with the fermionic singlet-pairing wave functions, with the Gutzwiller-Jastrow factor, and the quantum-number projection to the total spin singlet. We reproduce the metallic state for the small Coulomb interaction, and the antiferromagnetic insulating state with 120$^{\circ}$ spin structure for the large Coulomb interaction. We discuss the energetic and magnetic properties of the intermediate Coulomb interaction region. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R15.00004: Magnetic frustration in itinerant systems: the Kondo polaron problem Leonid Isaev, Cristian Batista, Ilya Vekhter We study the interplay between magnetic frustration and Kondo screening in Kondo lattices by analyzing the $J_1$-$J_2$ antiferromagnetic chain coupled to a conduction band. The system is tuned to the Majumdar-Ghosh point $J_2=J_1/2$ which stabilizes a dimer valence-bond solid at weak Kondo coupling $J_K$. We use an effective low-energy theory to demonstrate that sufficiently large $J_K$ results in a proliferation of ``Kondo polarons'', i.e. Kondo-screened domain-wall excitations of the dimer state, and collapse of the dimer order via a 2nd order quantum phase transition. At the quantum critical point, $J_K=J_K^c$, these polarons become gapless, and we argue that the transition itself belongs to a 2D Ising universality class. For $J_K>J_K^c$ increasing concentration of the polarons leads to a continuous growth of the electron Fermi momentum until all spins are absorbed by the Fermi sea. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R15.00005: Itinerant Kagome Ice: an Anomalous Quantum Hall Liquid Armin Rahmani, Gia-Wei Chern, Ivar Martin, Cristian Batista We show that all magnetic-charge-ordered kagome ice configurations, i.e., a highly disordered energetically stable manifold of Ising spins on the kagome lattice, support a quantized anomalous quantum Hall effect when coupled to itinerant electrons. Despite the strong disorder experienced by the electrons, the Hall effect is robust for almost all canting angles of the Ising spins. Due to the absence of magnetic long-range order, this phase of matter is characterized by the coexistence of a (classical) chiral spin liquid and an anomalous integer quantum Hall one. We further demonstrate that the magnetic monopole defects in this ice-like manifold bind a fluctuating electric dipole. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R15.00006: Exotic correlated electron phases in the Kagome Hubbard model Ronny Thomale We employ different renormalization group (RG) schemes to investigate the Kagome Hubbard model at low, intermediate, and strong coupling. At weak coupling where our RG calculation is asymptotically exact, we develop a new notion of sublattice interference mechanism to describe the Fermi surface instabilities at van Hove filling. For intermediate coupling, we observe an intricate interplay of the Fermi surface topology, sublattice interference, and range of interactions. In particular, we find a charge and spin bond order phase as well as a d+id Pomeranchuk instability. At strong coupling, we employ our recently developed slave particle RG schemes to investigate the J1-J2 Kagome Heisenberg model. We discuss its quantum phase diagram in the light of experiments and evidence from other approaches. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R15.00007: Quantum Fluctuation Effect on a Spin Scalar Chiral Ordering in Frustrated Kondo Lattice System Yutaka Akagi, Masafumi Udagawa, Yukitoshi Motome Recently, noncoplanar spin configurations with spin scalar chirality have drawn considerable attention as an origin of the anomalous Hall effect. As a typical example, a scalar chiral state with noncoplanar four-sublattice magnetic ordering was stabilized through the spin-charge coupling in a Kondo lattice model on a triangular lattice at 1/4 and 3/4 fillings [1,2]. In previous studies however, localized moments are approximated as classical spins. It is interesting to ask how quantum spin fluctuations affect the nontrivial chiral order and electronic state of the system. Here, we examine the effect of quantum fluctuations by the spin-wave approximation with introducing the Holstein-Primakoff transformation to the localized spins. As a result, we find that the four-sublattice order is fragile against quantum fluctuations at 3/4 filling, whereas it remains robust at 1/4 filling. We discuss the magnon excitations in the spin-charge coupled system in details. We also discuss the quantum correction on the thermal Hall effect. [1] Y. Akagi and Y. Motome, J. Phys. Soc. Jpn. {\bf 79}, 083711 (2010). [2] Y. Akagi, M. Udagawa, and Y. Motome, Phys. Rev. Lett. {\bf 108}, 096401 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R15.00008: Featureless and Non-Fractionalized Bose Insulator on the Honeycomb Lattice at 1/2 site-filling Itamar Kimchi, S. A. Parameswaran, Ari Turner, Fa Wang, Ashvin Vishwanath We consider bosons on the Honeycomb lattice at filling one half per site. It is known that free fermions at this filling of the tight binding model cannot form an insulating state while preserving all symmetries, even though there is an integer number of particles per unit cell. We argue, however, that interacting bosons can form an insulating state that preserves all symmetries. We propose a wave function for this state and by a mapping to a classical partition function we compute its properties and demonstrate that the state is insulating, fully symmetric and has no topological order. Our construction suggests that featureless insulators are generically allowed for at a filling of one boson per unit cell on any symmorphic lattice in any dimension. We also discuss related wavefunctions of hard core bosons that model spin 1/2 magnets on this lattice. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R15.00009: Demonstration of a D-metal from a chiral spin liquid Victor Chua, Gregory Fiete We report recent results on a study of a 2D disordered but exactly solvable gapless chiral spin-liquid ground state whose fractionalised quasiparticle excitations are Majorana fermions and are classified as being in the D-class of the Altland-Zirnbauer 10-fold classification scheme [Phys. Rev. B 55, 1142 (1997)]. Transport and quasiparticle localisation properties of this Majorana metal in nanowire configurations are studied and contrasted with the previously predicted D-metal phase of Senthil and Fisher [Phys. Rev. B 61, 9690 (2000)]. The role of Z2 vortices play towards transport properties are also discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R15.00010: Fluctuation Driven Spirals Near Ferromagnetic Quantum Critical Points in Disordered Electron Systems Steven Thomson, Frank Kruger, Andrew G. Green The magnetic properties of itinerant electron systems represent an area of growing experimental and theoretical interest, particularly the peculiar ordered magnetic phases that can occur at low temperatures. It has previously been shown that the quantum order-by-disorder mechanism predicts a spiral magnetic phase in the vicinity of an itinerant ferromagnetic quantum critical point in three spatial dimensions. Here, we present an analytical model of how both charge and spin disorder affect the formation of this spiral magnetic phase at low temperatures, supplemented by numerical evaluation of the fluctuation corrections to the free energy. We show the effect of disorder on the position of the tricritical point and on the stability of the ordered phases. We further discuss the possibility of a helical spin-glass phase and discuss our findings in the context of recent experiments. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R15.00011: The effect of non-magnetic impurities on the motion of a hole in a 2D Ising antiferromagnet Hadi Ebrahimnejad, Mona Berciu A hole in a 2D Ising antiferromagnet was initially believed to be infinitely heavy due to the string of wrongly-oriented spins it creates as it propagates, which trap it near its original location. Trugman showed that, in fact, the hole acquires a finite effective mass due to contributions from so-called {\it Trugman loops} processes, where the hole goes one and a half times around a closed loop and removes the defects it created during the first round, but ends up at a different site. This results in an effective next-nearest-neighbour hoping of the hole which keeps it on the sublattice it was created on. Here we investigate the trapping of such a hole near a single non-magnetic impurity, using a variational calculation of the hole's real-space Green's function. We consider the two cases with the hole and impurity being on the same versus on different sublattices, and contrast the differences between them. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R15.00012: Magnetization and Hall effect measurements on the pyrochlore iridate Nd2Ir2O7 Steven Disseler, Sean Giblin, Chetan Dhital, Kevin Lukas, Stephen Wilson, Michael Graf We present magnetization and Hall effect measurements on the pyrochlore iridate Nd$_2$Ir$_2$O$_7$. Previous muon spin rotation measurements have shown that the system undergoes an unusual transition at T$_M$ $\sim$ 110 K into a magnetic phase lacking long-range order, followed by a transition at T$_{LRO}$ $\sim$ 6 K into a state with long-range magnetic order. We observe a small remnant magnetization when cycling through zero magnetic field at temperatures below T$_M$. Below T$_{LRO}$ an additional hysteresis effect appears at a higher field B$_c$ = 2.8 T, together with the appearance of non-monotonic and hysteretic Hall resistance with a maximum at B$_c$. The dependence on field sweep direction suggests a non-trivial transition in the magnetically ordered state similar to that of spin-ice systems. This work was supported in part by National Science Foundation Materials World Network grant DMR-0710525 and by NSF CAREER award DMR-1056625. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R15.00013: Non-Kramers spin liquids on the pyrochlore lattice Jeffrey G. Rau, Hae Young Kee At low temperatures the pyrochlore iridates Pr$_2$Ir$_2$O$_7$ shows the puzzling combination of an anomalous Hall effect in the absence of experimentally observed magnetic order. The breaking of time-reversal symmetry indicated by the anomalous Hall effect, but without the usual accompanying magnetic order, points to an exotic state, possibly a chiral spin liquid. Considering the most general symmetry allowed model for the Pr non-Kramers doublets, we use the slave-fermion approach to analyze possible spin liquids at the mean field level. A variety of spin liquids can be generated using the projective symmetry group, with novel properties due to the nature of the non-Kramers doublet states. Applications to Pr$_2$Ir$_2$O$_7$ will be discussed. [Preview Abstract] |
Session R16: Focus Session: Magnetic Thin Films
Sponsoring Units: GMAG DMPChair: Roland Kawakami, University of California, Riverside
Room: 318
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R16.00001: Ferroelectric control of magnetocrystalline anisotropy at Co/poly(vinylidene fluoride) interfaces J.P. Velev, P.V. Lukashev, T.R. Paudel, J.M. Lopez-Encarnacion, S. Adenwalla, E.Y. Tsymbal Electric field control of magnetization is one of the promising avenues for achieving high-density energy-efficient magnetic data storage. Ferroelectric materials can be especially useful for that purpose as a source of very large switchable electric fields when interfaced with a ferromagnet. Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), have an additional advantage of being weakly bonded to the ferromagnet, thus minimizing undesirable effects such as interface chemical modification and strain coupling. In this work we use density functional calculations of Co/PVDF heterostructures to demonstrate the effect of ferroelectric polarization of PVDF on the interface magnetocrystalline anisotropy which controls the magnetization orientation. We show that switching of the polarization direction alters the magnetocrystalline anisotropy energy of the adjacent Co layer by about 50{\%}, driven by the modification of the screening charge induced by ferroelectric polarization. The effect is reduced with Co oxidation at the interface due to quenching the interface magnetization. Our results provide a new insight into the mechanism of the magnetoelectric coupling at organic ferroelectric/ferromagnet interfaces and suggest ways to achieve the desired functionality in practice. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R16.00002: The study of perpendicular magnetic anisotropy and Boron diffusion in Ta--CoFeB--MgO--CoFeB--Ta magnetic tunnel junction using polarized neutron reflectometry Valeria Lauter, H. Ambaye, T. Zhu, Y. Yang, R.C. Yu, J.Q. Xiao The current-induced spin transfer torque (STT) plays an important role in spintronic devices. However, the level of current density needed to reorient the magnetization is presently too high for most commercial applications, and reducing the current density is the challenging basis for recent research in spintronics. The magnetic tunnel junction (MTJ) with a perpendicular magnetic anisotropy (PMA) enables a small critical current density for current-induced magnetization switching and provides a pathway for such STT devices. We investigated the origin of PMA in CoFeB sandwiched by MgO and Ti layers using the anomalous Hall effect (AHE) and polarized neutron reflectometry (PNR). It is found that the PMA properties of CoFeB layers deposited above and under MgO layer are different and PNR measurements confirmed that a large PMA in the CoFeB above MgO layer is related to its low magnetization. From PNR experiments, we obtained the details of the magnetic and structural depth profiles inside the film. Using the sensitivity of neutrons to the absorption cross-section of boron, we unambiguously determined the depth profile of the boron distribution and showed that after annealing, most of the boron diffused to form a 2-nm-thick interface layer between the CoFeB and tantalum layers. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R16.00003: Spin-polarized ion scattering spectroscopy study on Si/Fe(100) surfaces Taku Suzuki, Shunichi Hishita We investigated surface magnetic structure in the initial stage of Si deposition on an Fe(100) surface by spin-polarized ion scattering spectroscopy (SP-ISS). [1] We found silicidation at the Si/Fe interface after Si deposition followed by annealing at 823 K. The silicidation occurs by the incorporation of silicon into the Fe substrate via the substitutional site of bcc Fe. After annealing, the incorporated Si atoms are distributed in surface layers several nanometers thick. The SP-ISS analysis revealed that the average magnetic moment of Fe in the silicide surface layer is about 70\% of that of Fe in the Fe(100) surface layer, whereas that of Si is almost zero. These surface magnetic moments are discussed in terms of the local magnetic environment. It is likely that the outermost surface of the silicide layer has an atomic arrangement similar to that of Fe$_3$Si(100) with surface termination by the Fe-Si plane.\\[4pt] [1] T.T.Suzuki and S.Hishita, Appl.Surf.Sci.259(2012)166. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R16.00004: Microscopic Structure of Mn Atom Chains on the Si(001) Surface Investigated by Scanning Tunneling Microscopy A. Fuhrer, F. J. Rue{\ss}, N. Moll, A. Curioni, D. Widmer The Si(001) 2x1 reconstructed surface has the interesting property that many metal atom species form nearly perfect 1D atomic wires oriented perpendicular to the Si dimer rows during deposition at room temperature. These wires are thought to consist of metal dimers located between the dimer rows linking up to form atomic chains. More recent experiments indicated that similar wire formation occurs for Mn which, with its half filled d-shell, has interesting magnetic properties e.g. when used as a dopant in dilute magnetic semiconductors. In our experiments we use scanning tunneling microscopy to study the atomic structure of these Mn-wires in detail and find that it is different from that of the other known metal wires. We show that two distinct types of Mn wires occur, with an asymmetric appearance relative to the underlying Si lattice. While one type of asymmetry can be linked to the buckling of the Si dimers near the Mn-wires the other is found to be intrinsic to the microscopic structure of the Mn-wires. We further compare high-resolution bias-dependent constant current images with images simulated for a Mn trimer wire structure using density functional calculations employing the CPMD code. \\[4pt] A. Fuhrer, F. Rue{\ss}, N. Moll, A. Curioni, D. Widmer, PRL {\bf109}, 146102 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R16.00005: The failure of DFT computations for a stepped-substrate-supported monatomic highly-correlated wire system Nader Zaki, Richard M. Osgood, Andrew J. Millis, Chris A. Marianetti The ab-initio method, density functional theory (DFT), has been immensely successful in its ability to predict physical properties of condensed matter systems. In particular, DFT calculations have proven to be quantitatively accurate in predicting structural properties in a wide range of materials and qualitative failures are rare. Here, however, we show that DFT can fail qualitatively to correctly predict the dimerized structural phase for a recently reported experimentally realized monatomic Co wire system that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate [1]. We attribute this failure to DFT's over-prediction of hybridization of the Co wire with the underlying Cu substrate. We demonstrate that this over-hybridization leads to weakening of the magnetic coupling along the wire, which is responsible for dimerization, while increasing the stiffness of the wire due to strengthening of the non-magnetic elastic term. Additionally, we show that accounting for local interactions via DFT$+U$ also fails at predicting the correct structural phase. [1] N. Zaki et al, arXiv:1208.0612 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R16.00006: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R16.00007: Temperature-dependent proximity magnetism in Pt Weng L. Lim, John C. Owens, Neema Ebrahim-Zadeh, Hilary G. E. Hentschel, Sergei Urazhdin We report the observation of a significant magnetic coupling between two ferromagnets (FM) separated by a thin Pt layer. The coupling remains ferromagnetic regardless of the Pt thickness, and exhibits a strong dependence on temperature $T$. These features of the coupling cannot be explained by the well-known RKKY mechanism of coupling between FM separated by a nonmagnetic spacer. We use a phenomenological model to demonstrate that the observed effects are consistent with the existence of temperature-dependent magnetic ordering induced in Pt in proximity to the interfaces with FM, consistent with a recent report on the ferromagnetic characteristics in Pt films grown on ferromagnetic insulators [1]. The magnetization in Pt decays away from the interfaces with a characteristic length scale that increases with decreasing temperature, and reaches 0.8 nm at $T<25$ K. Our results suggest that, in Pt/FM heterostructures, the magnetic and spin-dependent transport properties of Pt and FM are mutually affected, opening possibilities for engineering of new magnetoelectronic metamaterials. [1] S. Y. Huang et al, Phys. Rev. Lett. 109, 107204 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R16.00008: Tailoring magnetic properties of thin films with quantum well states and external electric field Tamene R. Dasa, Valeriy S. Stepanyuk Dependence of magnetic anisotropy energy (MAE) and spin-polarization of magnetic multilayers on the layer thickness is studied with {\it ab initio} techniques. For thin Fe films adsorbed on a Pt surface a reversal of the MAE (rotation of the easy axis) is observed with changing film thickness. Moreover, our calculation show that capping of magnetic films with Pt in most cases leads to a strong increase of MAE. Both of the later phenomena are traced back to spin-dependent quantum-well states (QWS) in the magnetic thin films. Combining the newly gained understanding with the well-known fact, that quantum well states can be tuned by external electric fields acting on the system, we show that, similar to the case of the quasi 1-D systems [1], the MAE in thin magnetic metallic films can be tailored with external electric field. For example, applying an electric field of $-1~V$/{\AA} to a multilayer of Pt/Co/Pt(001), its MAE can be changed by more than 50\%. To finalize the paper, changes in spin-polarization and the Stark-like shift accompanying exposure of the system to external electric fields are outlined and discussed. \\[4pt] [1] T.~R.~Dasa, P.~A.~Ignatiev, and V.~S.~Stepanyuk, Phys. Rev. B \textbf{85}, 205447 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R16.00009: Giant proximity effect in ferromagnetic bilayers Silvia Ramos, Tim Charlton, Jorge Quintanilla, Andreas Suter, Jagadeesh Moodera, Thomas Prokscha, Zaher Salman, Ted Forgan The proximity effect is a phenomenon where an ordered state leaks from a material into an adjacent one over some finite distance, $\xi$. For superconductors, this distance is $\sim$ the coherence length. Nevertheless much longer-range, ``giant'' proximity effects have been observed in cuprate junctions. This surprising effect can be understood as a consequence of critical opalescence. Since this occurs near all second order phase transitions, giant proximity effects should be very general and, in particular, they should be present in magnetic systems. The ferromagnetic proximity effect has the advantage that its order parameter (magnetization) can be observed directly. We investigate the above phenomenon in Co/EuS bilayer films, where both materials undergo ferromagnetic transitions but at rather different temperatures (bulk $T_C$ of 1400K for Co and 16.6K for EuS). A dramatic increase in the range of the proximity effect is expected near the $T_C$ of EuS. We present the results of our measurements of the magnetization profiles as a function of temperature, carried out using the complementary techniques of low energy muon rotation and polarized neutron reflectivity. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R16.00010: Asymmetric magnetic switching behavior of Py/SmFe/Py exchange spring magnet Jiyeong Gu, Hanming Yuan Magnetic switching behavior of the symmetric exchange spring magnet, Py(Permalloy)/SmFe (or SmCo)/Py thin films, was investigated. Exchange spring magnet shows a unique magnetic hysteresis curve due to the non-collinear magnetization developed by magnetic coupling of the soft and hard magnetic layers. Using Magneto Optical Kerr Effect (MOKE) we could separately measure the magnetic hysteresis loops of the top and the bottom Py layers. We found the magnetic hysteresis loops for the bottom and the top Py layers are different indicating the switching behavior is not symmetric. The coercivity of the bottom Py layer is much smaller than that of the top Py layer. From the comparison of MOKE data to the one measured by Alternating Gradient Magnetometer, we observed that the top Py layer and hard layer switch together representing the top Py layer is strongly coupled to the hard layer and shows a single switching instead of spiral magnetization distribution; while the bottom Py layer shows a non-collinear magnetization behavior as we expect from a typical exchange spring magnet. Thickness of the soft and hard layers was systematically varied to further investigate the asymmetric switching behavior of double exchange spring magnet thin films. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R16.00011: Epitaxial Growth of Heusler Co$_{2}$MnSi Heterostructures: Electronic and Structural Properties Thomas Neulinger, Sahil Patel, Alexander Kozhanov, Brian Schultz, Chris Palmstr{\O}m The Heusler alloy Co$_{2}$MnSi is predicted to be a half-metal, a material that is spin-polarized at the Fermi energy. We have demonstrated growth by molecular beam epitaxy of Co$_{2}$MnSi, Cr/Co$_{2}$MnSi, and a complete Co$_{2}$MnSi/MgO/Co$_{2}$MnSi(001) magnetic tunnel junction on epitaxial GaAs(001) surfaces without air exposure. Epitaxial Cr layers have been used to exchange bias Co$_{2}$MnSi. In-situ electron diffraction and scanning tunneling microscopy, and ex-situ X-ray diffraction techniques are used to characterize the crystal quality. The magnetic properties are investigated using vibrating sample and superconducting quantum interference device magnetometry. We present these results and will compare them with temperature dependent magnetotransport and tunneling spectroscopy measurements, with emphasis on the influence of Co$_{2}$MnSi surface termination. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R16.00012: Magnetostatics and magnetodynamics in Co$_{2}$MnSi on GaAs (001) Michael Pechan, Daniel Stanley, Michael Sinko, Sahil Patel, Alexander Kozhanov, Brian Schultz, Chris Palmstrom We present an investigation of the magnetic properties of Co$_{2}$MnSi films grown by molecular beam epitaxy on lattice matched Sc$_{0.3}$Er$_{0.7}$As films grown on GaAs (001) substrates with various capping layers (Cr, Al, Au). Co$_{2}$MnSi thickness varied from 3 to 21.4 nm. X-ray diffraction analysis confirmed the single crystal nature and crystallographic orientations of the films. Magnetization measurements reveal square loops with low in-plane saturation fields and very narrow (few Oe) coercive fields. An interesting feature of the loops in several of the samples is the presence of a small ($<$10 Oe) exchange-bias field observed at room temperature. Room temperature ferromagnetic resonance (FMR) measurements were carried out at 35 GHz as a function of in-plane angle to quantify the anisotropy in these structures. Resonances follow the typical derivative lineshape with relatively narrow line widths ranging from 30 to 140 Oe, consistent with high quality Heusler alloy film formation. Four-fold anisotropy is clearly observed in all samples confirming the high quality single-crystal nature of the films. A small unidirectional anisotropy associated with the exchange bias mentioned above is also observed. We will also present results on preliminary MJT structures. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R16.00013: topological chiral spin-wave modes in dipolar ferromagnetic thin films Ryuichi Shindou, Ryo Matsumoto, Jun-ichiro Ohe, Shuichi Murakami Magnetic dipole-dipole interaction in ferromagnet plays role of locking a relative angle between the spin space and the orbital space, just in the same way as the relativistic spin-orbit interaction does in ferromagnetic metals, leading to their quantum anomalous Hall effect. Focusing on this similarity, we theoretically design a couple of periodically-structured ferromagnetic thin film models which support unidirectional (chiral) propagations of spin-waves along its sample boundaries in their dipolar regime. Contrary to the Daemon-Eshbach surface mode, the chiral direction and the number of such spin-wave edge modes are determined by so-called topological Chern integer associated with `Bloch wavefunctions' for the volume-type spin wave modes. Namely, even if the direction of the magnetic field is fixed, the chiral direction can be still either left-handed or right-handed, depending on the periodic structuring and the frequency range, which is not the case with the Daemon-Eshbach mode. By introducing proper `atomic orbitals' for the proposed thin film models, we present a simple tight-binding description for the proposed topological chiral edge modes. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R16.00014: Equilibrium properties of Ising metamagnetic films James Mayberry, Michel Pleimling Artificial antiferromagnets have attracted attention lately due to the potential for technological applications. We model these systems as thin Ising metamagnetic films and study their equilibrium properties using Monte Carlo simulations. In variance with previous work but in agreement with the experimental systems, we consider films comprised of ``sets'' of planes, with an antiferromagnetic coupling between sets and a ferromagnetic coupling within sets. This allows us to consider different situations by varying the number of planes in each set. Studying the magnetization density and response functions as a function of temperature and magnetic field, we determine the corresponding phase diagrams. We discuss how a change of the number of planes in each set changes the equilibrium phase diagram. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R16.00015: Characteriziation of $Ni_2MnGa$ Ferromagnetic Shape Memory Alloy nanowires P. Gyawali, Keshab R. Sapkota, B. Dahal, R. Dulal, I. L. Pegg, J. Philip Heusler type $Ni_2MnGa$ ferromagnetic shape memory alloy has been extensively studied in thin films and in bulk. The structural transition to martinsitic phase occurs thermodynamically reversibly within the ferromagnetic region.. For the technological application, magnetic field is used to induce the motion of twin boundaries in martinsite phase. $Ni_2MnGa$ nanowires were grown for the first time using electrospinning method. Structrual characterization were done using XRD and EDX. Nanowires exhibit tetragonal structure with a = b = 5.85 \AA and c/a = 0.96. Magnetic measurements show the pre- martinsite transformation. Curie temperature of nanowires is about 360 K. [Preview Abstract] |
Session R17: Focus Session: Manganite Heterostructures
Sponsoring Units: DMP GMAGChair: Steve May, Drexel University
Room: 319
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R17.00001: Interface effects in oxide heterostructures combining superconductors, ferromagnets and ferroelectrics Invited Speaker: Javier E. Villegas In oxide heterostructures, the interactions at the interfaces often yield novel physical properties, which radically differ from the individual constituents' and provide with new functionalities. Oxide perovskites offer much potential for this, because a variety of isostructural materials exist with very different ground states (superconductors, ferromagnets, ferroelectrics, etc). One interesting possibility is to locally couple one of the heterostructure constituents' sensitivity to an external stimulus (e.g. the electric field for ferroelectrics) to a physical property of the second constituent (e.g. the magnetization in a ferromagnet, or the critical temperature in a superconductor). Such local coupling can be achieved via nanoscale field-effect doping. Through this mechanism, a form of magneto-electric coupling between the local electric polarization in the ferroelectric and the local magnetic induction in the superconductor can be obtained, which allows the electrostatic manipulation of magnetic flux quanta [1]. Another interesting possibility is to intertwine the most distinctive properties from each of the heterostructure constituents. As an example of this, we show how to unite the phase-coherent dissipationless charge transport characteristic of superconductivity and the spin-polarized charge transport characteristic of ferromagnetism [2], which may open the door to novel spintronic devices [3].\\[4pt] [1] A. Crassous, R. Bernard, S. Fusil, K. Bouzehouane, D. Le Bourdais, S. Enouz-Vedrenne, J. Briatico, M. Bibes, A. Barth\'{e}l\'{e}my, and Javier E. Villegas, Phys. Rev. Lett. 107, 247002 (2011).\\[0pt] [2] C. Visani, Z. Sefrioui, J. Tornos, C. Le\'{o}n, J. Briatico, M. Bibes, A. Barth\'{e}l\'{e}my, J. Santamar\'ia and Javier E. Villegas, Nature Physics (2012), doi:10.1038/nphys2318.\\[0pt] [3] M. Eschrig, Phys. Today 64, 43 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R17.00002: Influence of Interface Engineering on the Magnetization in La$_{0.67}$Sr$_{0.33}$MnO$_3$/SrTiO$_3$ Heterostructures S.G.E. te Velthuis, Yaohua Liu, V. Lauter, H. Boschker, G. Koster, G. Rijnders Rich new phenomena have been observed at the interfaces between of complex oxides with different electronic and magnetic properties. In particular electronic reconstruction may occur at epitaxial oxide interfaces because of the broken transitional symmetry, leading to new properties, some of which are in fact less desirable. At the La$_{0.67}$Sr$_{0.33}$MnO$_3$(LSMO) - SrTiO$_3$(STO) interface, it is thought electronic reconstruction, driven by the potential build-up at the interface, results in a degradation of the magnetization of LSMO. To explore this, we have studied LSMO/STO heterostructures with interfaces engineered to avoid this interfacial magnetization suppression [1]. In our case, this engineered interface refers to a La$_{0.33}$Sr$_{0.67}$O monolayer replacing a La$_{0.67}$Sr$_{0.33}$O monolayer at each interface. Depth-dependent magnetization profiles in the heterostructures, determined using polarized neutron reflectometry, show that indeed the interfacial magnetization of LSMO improves with interface engineering. [1] H. Boschker et al., Adv. Funct Mater 22, 2235 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R17.00003: Strain control of electronic structure in La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ Eric Monkman, Carolina Adamo, Daniel Shai, Dawei Shen, John Harter, Charles Brooks, Ilya Elfimov, Richard Hennig, Darrell Schlom, Kyle Shen Introducing biaxial strain into complex oxide thin films by epitaxial growth on lattice mismatched substrates is a powerful approach to engineering electronic and magnetic properties not attainable in bulk materials. Due to the strong many-body interactions characteristic of transition metal oxides, a microscopic understanding of the mechanisms underlying strain-driven phase transitions remains unclear. Here we utilize an integrated oxide molecular-beam epitaxy and angle-resolved photoelectron spectroscopy system to directly measure the electronic structure of colossal magnetoresistive La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ on four substrates, spanning -2.3\% to +1.6\% biaxial strain and two strain driven metal-insulator transitions. Contrary to conventional expectations of a bandwidth driven metal-insulator transition in strongly correlated systems, we find widely dispersive states in both insulating phases with finite weight at the Fermi level under compressive strain and a narrow gap under tensile strain. Our results point to two distinct mechanisms behind the metal-insulator transitions, and highlight the importance of phase coexistence and charge or orbital ordering in oxide thin films. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R17.00004: Tuning out-of-plane strain in epitaxial La[1-x]Sr[x]MnO[3] thin films with noble ion implantation Thomas Zac Ward, Hangwen Guo, Christianne Beekman, Wolter Siemons, Hans Christen, Philip Rack, John Budai, Zheng Gai Strongly correlated materials, such as cuprates, manganites, and heavy-fermions, have a wealth of exotic properties and are often associated with the coexistence of competing nearly degenerate states which couple simultaneously active degrees of freedom---charge, lattice, orbital, and spin states. To understand correlated electronic materials, we must begin to disentangle the underlying correlations and find novel methods to tune individual order parameters to recognize how mesoscopic interactions drive emergent behaviors. In this work, we will discuss recent progress on controlling the strain along the out-of-plane direction in epitaxial [LaSr]MnO3 films through implantation of noble ions. This technique allows for very fine manipulation of the lattice parameter in a manner that effectively gives us a novel means of controlling orbital overlaps without hole/carrier doping the sample. We observe that films can remain epitaxially lattice locked to the substrate while accommodating more than 1{\%} lattice expansion out-of-plane. We will present phase diagrams based on this new type of ``strain doping'' and discuss the implications. Supported by the US DOE Office of Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R17.00005: Characteristics of a Mott field-effect transistor (MottFET) based on La$_{1-x}$Sr$_{x}$MnO$_{3}$ Suyoun Lee, Keundong Lee, Hyojin Gwon, Seung-Hyub Baek, Baeho Park, Jin-Sang Kim Recently, the metal-insulator transition (MIT) phenomenon shown in transition metal oxides has attracted much interest due to its superior characteristics such as fast switching speed ($\sim$ femtoseconds), high on/off ratio, and low power consumption. One example is the MottFET, which utilizes the MIT modulated by electric field through the band-filling in a Mott insulator. In this work, we examined MottFET devices based on La$_{1-x}$Sr$_{x}$MnO$_{3}$(LSMO), which is one of the mostly studied Mott insulators and attractive for the potential application in spintronic devices due to its intriguing properties such as colossal magnetoresistance (CMR) and half-metallicity. For the devices with the composition near the boundary of the metal-insulator transition, we confirmed that the conductivity of the channel could be modulated by a gate electric field of moderate strength. In addition, for the future application in spintronic devices, we investigated the dependence of device characteristics on the magnetic field. As the applied magnetic field increased, we found that the current-voltage characteristic showed anomalous behavior, which might be attributed to the electron-electron interaction, spin ordering, and the magnetic impurities in the channel. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R17.00006: Structure/property relationship for ultrathin films of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ on SrTiO$_{3}$ (001) Zhaoliang Liao, Diogo Duarte dos Reis, Peng Gao, Xiaoqing Pan, Rongying Jin, E. Ward Plummer, Jiandi Zhang Dead layer, the insulating behavior in ultrathin films of metallic oxides, is an intriguing property of TMO films. Is this intrinsic effect caused by dimensionality effect, or by interface, segregation, or stoichiometry? We have studied thickness-dependence of structure/property relationship for thin films of La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) grown with PLD on SrTiO$_{3}$ (001) (STO) by using in-situ characterization such as LEED and STM, and ex-situ transport measurements. By minimizing oxygen deficiency, the thickness of dead layer is found to be as small as 6 u.c., which can be characterized as the intrinsic critical thickness. Our LEED-I(V) structural refinement shows non-monotonic lattice relaxation with thickness. The distortion of the $c$-axis bond length at surface reaches its maximum value for 6 u.c. film being 19\% smaller than the bulk value. Mn is no longer at oxygen octahedron center with Mn-O-Mn bond angles between 167$^{\circ}$ and 176$^{\circ}$ varying with film thickness. Regardless of the thickness, the surface La/Sr-O layer is almost all Sr due to surface segregation. Capping with STO overlayer reduces the critical thickness of dead layer, thus suggesting that LSMO/STO interface enhances the conductivity of LSMO. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R17.00007: Ultrafast conductivity dynamics in epitaxially strained La$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films Jingdi Zhang, Richard Averitt, Xuelian Tan, Wenbin Wu La$_{1-x}$Ca$_{x}$MnO$_{3}$ is a prototype colossal magnetoresistance (CMR) material where the conductivity displays a marked sensitivity to an external magnetic field for reasons that are not fully understood. The underlying rich physics is a result of strong coupling of the spin, lattice, orbital, and charge degrees of freedom. Optical spectroscopy provides experimental access to the underlying interactions in the manganites including spin and orbital ordering and the metal-insulator transition. Ultrafast spectroscopy can dynamically probe photo-induced changes that drive phase transitions. In this work we report on time-resolved terahertz spectroscopic studies of epitaxially strained La$_{1-x}$Ca$_{x}$MnO$_{3}$ thin films. In these films, the strain results in a robust antiferromagnetic insulating phase below 260K. Following 1.5 eV short pulse excitation the THz conductivity reveals a transition to a persistent metallic phase. This response is a result of competition in a dynamic phase fluctuation regime. We will describe, in detail, the observed differences in the conductivity dynamics as a function of lattice strain. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R17.00008: Magnetic behavior of La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ / BaTiO$_{3}$ bilayers John E. Ordonez, Maria E. Gomez, Wilson Lopera, Lorena Marin, Jose A. Pardo, Luis Morellon, Pedro Algarabel, Pedro Prieto We have grown ferroelectric BaTiO$_{3}$(BTO) and ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) onto (001) SrTiO$_{3}$ and Nb:SrTiO$_{3}$ by pulsed laser deposition (PLD) at pure oxygen atmosphere, and a substrate temperature of 820$^{\circ}$ C, seeking for a multiferroic behavior in this structure. From x-ray diffraction (XRD) we found lattice parameter a$_{BTO}$=4.068 {\AA}, and a$_{LCMO}$=3.804 {\AA}, for each individual layer. In the BTO/LCMO bilayer, (002)-Bragg peak for BTO maintain its position whereas (002) LCMO peak shift to lower Bragg angle indicating a strained LCMO film. Magnetization measurements reveal an increase in the Curie temperature from 170 K to 220 K for the bilayer when LCMO (t = 47 nm) is deposited on BTO (t=52 nm) film, while depositing the BTO (50 nm) above LCMO (48 nm) the Curie temperature remains at values close to that obtained for a LCMO single layer ($\sim$175 K), deposited under identical growth parameters [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R17.00009: Canted Antiferromagnetism in Electron-Doped CaMnO$_3$ under Epitaxial Strain Hiromasa Ohnishi, Shoji Ishibashi, Kiyoyuki Terakura CaMnO$_3$ (CMO) is a G-type antiferromagnetic (G-AFM) insulator at low temperature. A small amount of electron doping to CMO induces electronic and magnetic state change to a weak ferromagnetic (FM) metal. The recent experiment in thin-film [1] has revealed that the metallic character by electron-doping is sensitive to the strain exerted by the substrate. In this study, we clarify the electron-doping effect for CMO with the existence of epitaxial strain from substrates, by first-principles electronic structure calculation with noncollinear version of local spin density approximation. We show that a metallic character with a weak FM component is brought by the spin-canting from the G-AFM spin alignment (cG-AFM) by the double exchange effect. The canting angle becomes larger with increase of doping-amount and $c/a$, where $c$ and $a$ represent in-plane and out-of-plane lattice constants, respectively. We also show that a magnetic state change from cG-AFM state to C-AFM one takes place by further enhancement of compressive strain. We analyze our results by comparing with the experimental results.\\[4pt] [1] P.-H. Xiang et al., Adv. Mater. 23, 5822 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R17.00010: Analysis of Stoichiometry Variations in La $_{\mathrm{1-x}}$Ba$_{\mathrm{x}}$MnO $_{\mathrm{y}}$ Thin Films using Laser-Ablation Inductively Plasma Mass Spectrometry and X-ray diffraction E. Kevin Tanyi, Rajeswari Kolagani, Mark Stephen Monk, David Schaefer, Steven Lev Structural, electrical and magnetic properties of thin films of the doped rare earth manganese oxide material s are known to change dramatically by varying the oxygen partial pressure employed during Pulsed Laser Deposition. In contrast to the commonly accepted idea that such variation is solely due to the variation of the oxygen stoichiometry of the films, we find that varying the deposition oxygen partial pressure also results in the variation of the cation stoichiometry at the rare earth site. We also find that in addition to oxygen partial pressure, laser fluence is a determining factor for the stoichiometry. We have analyzed the composition, structure and properties of La $_{\mathrm{1-x}}$ Ba$_{\mathrm{x}}$ MnO$_{\mathrm{y}}$ thin films grown under a range of oxygen pressures. Cation composition is analyzed using the Laser-ablation Inductively Coupled Plasma Mass Spectroscopy technique (LA-ICPMS). LA-ICPMS results, coupled with structural information from 4-circle X-ray diffraction, allows us to delineate oxygen content variations from cation stoichiometry variations. We will correlate the changes in stoichiometry with surface morphology, and electrical and magneto-resistive properties. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R17.00011: Structural, AFM, MFM and magnetic studies of LaMnO$_3$ thin films prepared by atomic layer deposition method Mukesh Chandra Dimri, Himani Khanduri, Sami Vasala, Silver Leinberg, R\"unno L\~ohmus, J\"uri Krustok, Maarit Karppinen, Raivo Stern Structural, microstructural and magnetic properties of the thin films of LaMnO$_3$ (LMO) have been investigated and will be presented. Thin films were deposited by atomic layer deposition (ALD) method on silicon substrates. Effects of various process parameters have been studied on LMO thin films. Single phase perovskite crystal structure was confirmed from the X-ray diffraction and Raman spectra. SEM/AFM studies confirm the uniform and high quality films grown with grains in a range of 20-100 nm, depending on preparation conditions. MFM images measured at low temperature (65K), show different magnetic domains in films annealed in N$_2$ and O$_2$ environments. Stoichiometry, microstructure and magnetic properties of films strongly depend on annealing environments; however there was no change in their crystal structure. Curie temperature in those LMO thin films annealed in N$_2$ and O$_2$ atmospheres were 200 and 250K, respectively. Enhanced Curie temperature from the ideal value ($\sim$140 K) can be related to non-stoichiometry in our LMO films. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R17.00012: The effects of annealing on the infrared and optical properties of La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ films Peng Xu, T.J. Huffman, D.R. Branagan, A.J. Hollingshad, N.E. Penthorn, D.J. Brooker, M.M. Qazilbash, P. Srivastava, T. Goehringer, Grace Yong, V. Smolyaninova, R. Kolagani La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) films grown by pulsed laser deposition on lanthanum aluminate substrates undergo a phase transition from ferromagnetic metallic state to paramagnetic insulating state at T$_{\mathrm{c}}$ of about 350 K. This second-order phase transition proceeds via a phase coexistence regime over an extended temperature range. Annealing affects the strain and oxygen content in films thereby causing significant changes to the magnetic properties, electronic structure, lattice distortion, and possibly the nanoscale properties of coexisting phases. We use ellipsometry and Fourier-transform infrared spectroscopy to investigate the effects of annealing on LSMO films over a broad spectral range from ultraviolet to far infrared. We deduce the Jahn-Teller energy splitting and the Hund's coupling energy from our data on annealed and unannealed films. [Preview Abstract] |
Session R18: Focus Session: Spin-Dependent Phenomena in Semiconductors - GaMnAs
Sponsoring Units: GMAG DMP FIAPChair: John Peters, Northwestern University
Room: 320
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R18.00001: Experimental observations of optical spin-transfer and spin-orbit torques in magnetic semiconductors Invited Speaker: Petr Nemec The spin transfer torque (STT) is a non-relativistic phenomenon where angular momentum of spin polarized carriers electrically injected into a ferromagnet from an external polarizer is transferred to the magnetization [1]. In the absence of an external polarizer a distinct phenomenon can occur in which carriers in a magnet under applied electric field develop a non-equilibrium spin polarization due to the relativistic spin-orbit coupling, resulting in a current induced spin-orbit torque (SOT) [1]. We show, using the experimental data observed in the ferromagnetic semiconductor (Ga,Mn)As, that there exists optical counterparts of STT (OSTT) [2] and SOT (OSOT) [3]. In OSTT a circularly polarized femtosecond pump laser pulse acts as the external polarizer and it induces a coherent magnetization precession due to the angular momentum transfer, in a direct analogy to the current induced STT [2]. The absence of an external polarizer in OSOT corresponds to photo-carrier excitations which are independent of the polarization of the pump laser pulses and the phenomenon relies on spin-orbit coupling of non-equilibrium carriers, as in the case of the current induced SOT [3]. Our work demonstrates the possibility to study the spin-transfer and spin-orbit torques on the sub-picosecond time-scales using the optical pump-and-probe experimental technique.\\[4pt] [1] A. Bratas et al., Nature Materials 11, 372 (2012).\\[0pt] [2] P. Nemec et al., Nature Physics 8, 411 (2012).\\[0pt] [3] N. Tesarova et al., submitted, arXiv: 1207.0307. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R18.00002: Spin-orbit current-induced torques in (Ga,Mn)As Erin K. Vehstedt, Liviu P. Zarbo, Karel Vyborny, Hidekazu Kurebayashi, Pierre Roy, Joerg Wunderlich, Andrew J. Ferguson, Tomas Jungwirth, Jairo Sinova Electrical control of magnetic domains has the potential to overcome key challenges to the development of new non-volatile and down-scalable logic and memory devices. We study the spin-orbit torque induced by an unpolarized electric current in the dilute ferromagnetic semiconductor, (Ga,Mn)As. The current-induced torque (CIT) is modeled as the interaction between the uniform magnetization and an effective magnetic field representing the non-equilibrium carrier spin-polarization. We calculate the current-induced field (CIF) using the Kubo linear-response formalism for a broad range of material parameters. We find that the CIF is composed of a dominant term due to the inverse spin galvanic effect and a small component which is dependent on the relative orientation of the current, magnetization, and crystal axes. In conjunction with experimental studies, we investigate the magnetization dynamics using the phenomenological Landau-Lifschitz-Gilbert equation. The study of (Ga,Mn)As opens the door to a comprehensive theory of CITs in uniform magnetic semiconductors. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R18.00003: Observation of a Photon Echo and Measurement of Interband Dephasing in GaMnAs Kimberley Hall, Murat Yildirim, Sam March, Reuble Mathew, Angela Gamouras, Xinyu Liu, Margaret Dobrowolska, Jacek Furdyna The carrier-mediated ferromagnetism exhibited by III-V diluted magnetic semiconductors (DMS), together with their large magneto-optical response, makes these materials promising for applications in optoelectronics, including integrated optical isolators and ultrafast optically-addressable memory elements. The time scale for decay of coherence in the carrier system (T$_2$) is a key parameter in models of coherent magnetization rotation [1], yet very little is known about the coherent response in DMS. We present results of four-wave mixing measurements of T$_2$ in GaMnAs. We observe a dramatic reduction in the dephasing time with the incorporation of Mn, consistent with earlier experiments in CdMnTe [2]. This fast dephasing process, which leads to an upper bound on T$_2$ of 40 fs for x $\geq$0.014\%, is attributed to spin-flip scattering between the optically excited holes and Mn ions, providing new insight into exchange coupling and nonequilibrium magnetization dynamics in these materials. Direct measurement of the envelope of the four-wave mixing emission reveals a photon echo in GaMnAs, despite the complexity of exchange coupling and defect-induced band tailing.\\[4pt] [1] J. Chovan et al., Phys. Rev. Lett. 96, 057402 (2006).\\[0pt] [2] S. T. Cundiff et al. J. Opt. Soc. Am. B 13, 1263 (1996). [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R18.00004: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R18.00005: Ferromagnetism and infrared electrodynamics of GaMnAs B.C. Chapler, S. Mack, R.C. Myers, K.S. Burch, N. Samarth, D.D. Awschalom, D.N. Basov In this work we experimentally address both the magnetic and the electronic properties of the prototype dilute magnetic semiconductor Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As using infrared (IR) spectroscopy. We first examine the relationship between the carrier density, determined through a sum-rule analysis of our data and additional IR data available in the literature, and the ferromagnetic transition temperature T$_{\mathrm{C}}$. Our analysis supports the conclusion that the Fermi level resides within a Mn-induced IB, and that the location of the Fermi level within the band plays a key role in controlling T$_{\mathrm{C}}$. Additionally, we perform a detailed examination of the spectral features observed in the IR data of our Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As films, and show that these features are also consistent only with a Mn-induced IB scenario. In this latter vein, we will discuss and resolve controversies in the literature related to the peak in a broad mid-IR resonance observed in Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As IR spectra. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R18.00006: Infrared magnetic linear dichroism spectroscopy of (Ga,Mn)As N. Tesarova, J. Subrt, P. Maly, P. Nemec, K. Vyborny, C.T. Ellis, Alok Mukherjee, J. Cerne The sensitivity of magnetic linear dichroism (MLD) to in-plane magnetization makes it well suited to study diluted magnetic semiconductors such as (Ga,Mn)As, where MLD can be used to probe electronic excitations in the material. The band structure supporting these excitations yields rich in-plane magnetization effects, which include anisotropic magnetoresistance and four non-perpendicular, in-plane easy axis orientations. Observation of these effects provides insights into the electronic structure of (Ga,Mn)As. In this work we introduce a new, low-temperature, infrared MLD measurement technique that reduces instrumentation artifacts and enables broadband (0.1 eV \textless E$_{\mathrm{ph\thinspace }}$\textless 2.7 eV) capabilities. Through these MLD measurements we sensitively and systematically probe electronic structure in (Ga,Mn)As samples with Mn concentrations varying from 3{\%}-14{\%}. In general, the data show an MLD enhancement in the visible and infrared regimes, which are indicative of interband transitions between the valence and conduction bands and optical transitions within the valence band, respectively. We find that the behavior of these MLD features with increasing Mn concentration is in reasonable agreement with theoretical predictions. We acknowledge financial support provided by NSF-DMR1006078 and the Faculty of Mathematics and Physics, Charles University in Prague. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R18.00007: Numerical studies of non-Drude ac-conductivity and infrared magneto-optics in (Ga,Mn)As Huawei Gao, Jairo Sinova Optical absorption experiments on (III,Mn)V diluted magnetic semiconductors (DMS's) show that the ac-conductivity has non-Drude behavior at low frequency. STM study show many states deep in the band gap. The numerical simulation of the first problem has been done previously using the effective Hamiltonian model with various treatments of the disorder effects. We re-examine the previous works with a similar numerical method to establish the nature of the transitions in the low to the high-doped regime and also the properties of states in the gap. We use the effective Hamiltonian k.p model to describe the holes introduced by Mn impurities and treat the Mn impurities exactly using the envelope function approximation. We use participation ratios to characterize the localization properties of quasi particle states. This allows us to study the ac-conductivity contributions due to delocalized states to deep in-gap localized states transitions and how the spectral weight is distributed. We will also report on numerical results of the magneto-optical response with this ~treatment of the effect of disorder. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R18.00008: AC-transport measurements of ion beam irradiated GaMnAs semiconductors Elis Sinnecker, Marcelo Sant'Anna, Tatiana Rappoport, Joaquim Mendes, Mauricio Pires, Germano Penello, Deivid Souza, Sergio Mello, Jacek Furdyna, Xinyu Liu GaMnAs is a diluted magnetic semiconductors in which lattice atoms have been partially substituted by magnetic atoms, thus inserting a local magnetic moment into the lattice. Recently it was shown that ion beam irradiation can be an effective tool to modify the magnetic and electronic properties of Ga$_{1-x}$Mn$_x$As thin films [1, 2]. We observed that an increase of the structural disorder by irradiation leads to a systematic decrease on the saturation magnetization. Here, we provide further information on the electronic properties of irradiated samples. Measurements of ac-resistivity, magnetoresistance and Hall resistance were performed from 5K to 300K applying a DC magnetic field up to 7T. The results show an interesting frequency dependence of the ac-transport of measured irradiated samples. For the sake of comparison, data on irradiated non-magnetic semiconductor, grown on the same conditions as Ga1-xMnxAs thin films, are provided.\\[4pt] [1] E. H. C. P. Sinnecker et al., Phys. Rev. B 81, 245203 (2010).\\[0pt] [2] Lin Li et al., J. Phys. D: Appl. Phys. 44, 045001 (2010). [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R18.00009: Andreev Reflection Measurement of Spin Polarization in GaMnAs Khalid Eid, Diana Dahliah, Robert Tolley, Taylor Reid, Xinyu Liu, Jacek Furdyna Current measurement geometries in high-resistivity materials suffer from a large extra resistance that comes from the bulk of the ferromagnet. We use the Circular Transfer Line Method (CTLM) [1-2] to measure the Andreev reflection effect at GaMnAs/superconductor interface and to extract GaMnAs spin polarization. This technique works especially well for high-resistivity films. It has multiple advantages over the point contact and planar geometries, like eliminating the extra resistance contribution from the bulk, producing actual conductance values and not normalized conductance, and eliminating the broadening of the superconducting gap. The effect of the Schottky barrier at the GaMnAs/superconductor interface plays a crucial role and will also be discussed.\\[4pt] [1] K.F. Eid et al, Appl. Phys. Lett 100, 212403 (2012) \\[0pt] [2] K.F. Eid et al, IEEE Trans. Magn. 47, 2636 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R18.00010: Electron diffusivity above and below the Curie temperature of GaMnAs Chris Weber, Kassie Mattia, Eric Kittlaus, Xinyu Liu, Jacek Furdyna Using a transient-grating pump-probe experiment, we measure the diffusion of photoexcited electrons in samples of (Ga,Mn)As with doping levels of 5\%, 6\%, and 7\% Mn. At both 15 K and 80 K the diffusivity increases with density of photoexcited carriers, indicating the degeneracy of both majority holes and minority electrons. We measure electron diffusion in (Ga,Mn)As as rapid as $\sim100$ cm$^2$/s. Converting diffusivity to mobility using the Einstein relation yields $\mu_e\sim 8000$ cm$^2$/Vs, similar to that of GaAs. This high mobility demonstrates that neither the density of states nor the scattering rate of the (Ga,Mn)As conduction band is significantly influenced by Mn doping or by ferromagnetism. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R18.00011: The contribution of critical spin fluctuations to scattering and spin lifetimes in GaMnAs near the ferromagnetic transition Matthew Mower, G. Vignale As GaMnAs transitions between the paramagnetic and ferromagnetic phases, the resistivity exhibits a peak due to enhanced scattering from critical spin fluctuations. Existing work typically focuses on the ferromagnetic side, or to a lesser extent the paramagnetic side, far away from the transition; the effect of strong spin fluctuations near the transition has received little attention. We present a simple model of spin exchange mediated by dynamic spin fluctuations, calculated in the GW approximation. This produces a finite peak in the resistivity that is qualitatively accurate. We then use this model to calculate hole spin lifetimes from the relevant spin relaxation mechanisms. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R18.00012: Anomalous Fermi level behavior in GaMnAs at the onset of ferromagnetism Iriya Muneta, Hiroshi Terada, Shinobu Ohya, Masaaki Tanaka The origin of the ferromagnetism and the metal-insulator transition (MIT) has been a long-debated issue in the prototype ferromagnetic semiconductor GaMnAs. Previously, the valence band (VB) conduction picture has been widely accepted in this material, where the MIT of GaMnAs was understood by the Fermi level crossing over the VB similarly to p-type GaAs doped with non-magnetic acceptors. Here, we carefully analyze the VB structure and the Fermi level position in a series of Ga$_{1-x}$Mn$_x$As from the unexplored insulating region ($x \simeq 0.01$\%) to the metallic region ($x = 3.2\%$) by using resonant tunneling spectroscopy. We find that the Fermi level never crosses over the VB near the MIT: The Fermi level becomes closest to the VB top at $x = 1.0\%$ at the onset of the ferromagnetism, but it moves away from the VB with increasing or decreasing $x$ from 1.0\%. This anomalous behavior of the Fermi level is completely different from that of GaAs doped with other non-magnetic shallow acceptors~[1]. This work was partly supported by Grant-in-Aids for Scientific Research including Specially Promoted Research, Project for Developing Innovation Systems of MEXT, and FIRST Program of JSPS. \\[4pt] [1] I. Muneta, H. Terada, S. Ohya, and M. Tanaka, {\it submitted}; arXiv:1208.0575. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R18.00013: Does the physics of (Ga,Mn)N differ from (GaMn)As qualitatively or quantitatively? Is valance of Mn impurity 2+ or 3+? Ryky Nelson, Tom Berlijn, Wei Ku, Juana Moreno, Mark Jarrell (Ga,Mn)N is a promising material for spintronics due to its potential high currie temperature (Tc) [1]. However, unlike for (Ga,Mn)As, some of the experiments on (Ga,Mn)N are still controversial [2,3] on the intrinsic nature of the magnetism. Furthermore, under debate are the spin and charge state of the disordered Mn impurities in (Ga,Mn)N [4,5] and whether its local moments interact via the same exchange mechanism as in (Ga,Mn)As [6,7]. To address these issues we will present ab-initio-based analyses of disorder and correlation via the recently developed Wannier function based methods [8,9]. [1] T. Dietl et al., PRB 63, 195205 (2001) [2] H. Hori et al., Physica B 324, 142 (2002) [3] S. Dhar et al., APL 82, 2077 (2003) [4] A. Titov et al., PRB 72, 115209 (2005) [5] J. I. Hwang et al., PRB 72, 085216 (2005) [6] T. Dietl et al. Science 287, 1019 (2000) [7] K. Sato et al., RMP 82, 1633 (2010) [8] T. Berlijn et al., PRL 106, 077005 (2011) [9] W.-G. Yin et al., PRB 79, 214512 (2009) [Preview Abstract] |
Session R19: Kondo Screening - Different Aspects
Sponsoring Units: DCMPChair: Piers Coleman, Rutgers University
Room: 321
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R19.00001: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R19.00002: Berezinskii-Kosterlitz-Thouless Transition in Heavy Fermion Superlattices Jian-Huang She, Alexander Balatsky We propose an explanation of the superconducting transitions discovered in the heavy fermion superlattices by Mizukami et al. (Nature Physics 7, 849 (2011)) in terms of Berezinskii-Kosterlitz-Thouless transition. We observe that the effective mass mismatch between the heavy fermion superconductor and the normal metal regions provides an effective barrier that enables quasi 2D superconductivity in such systems. We show that the resistivity data, both with and without magnetic field, are consistent with BKT transition. Furthermore, we study the influence of a nearby magnetic quantum critical point on the vortex system, and find that the vortex core energy can be significantly reduced due to magnetic fluctuations. Further reduction of the gap with decreasing number of layers is understood as a result of pair breaking effect of Yb ions at the interface. Reference: Jian-Huang She, Alexander V. Balatsky, Phys. Rev. Lett. 109, 077002 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R19.00003: Theory for ESR in the heavy fermion system $\beta-$YbAlB$_4$ Aline Ramires, Piers Coleman We propose a theory to explain the unusual temperature dependence of the Electron Spin Resonance (ESR) lines of the critical heavy fermion superconductor $\beta$-YbAlB$_{4}$. This system shows a conduction electron ESR signal at high temperatures, but at low temperatures its g-factor shifts to the f-electron g-factor and it develops strong anisotropy. With our theory we are able to explain this dichotomy based on the fact that the lower crystal field configuration of the local moments in this system is a pure $|\pm 5/2\rangle$. Because of its Ising nature these spins can not be directly probed by ESR, and the f-electron features that appear at low temperatures can be explained by an emergent hybridization model. We can account for the origin of this signal and its main characteristics qualitatively, including g-factor shift and the hyperfine structure with the assumption that the scattering rate is unusually small. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R19.00004: Effects of correlated hybridization in the single-impurity Anderson model Valter L\'Ibero, Rodrigo Veiga The development of new materials often dependents on the theoretical foundations which study the microscopic matter, i.e., the way atoms interact and create distinct configurations. Among the interesting materials, those with partially filled $d$ or $f$ orbitals immersed in nonmagnetic metals have been described by the Anderson model, which takes into account Coulomb correlation ($U$) when a local level (energy $E_d$) is doubled occupied, and an electronic hybridization between local levels and conduction band states. In addition, here we include a correlated hybridization term, which depends on the local-level occupation number involved. This term breaks particle-hole symmetry (even when $U+2E_d=0$), enhances charge fluctuations on local levels and as a consequence strongly modifies the crossover between the Hamiltonian fixed-points, even suppressing one or other. We exemplify these behaviors showing data obtained from the Numerical Renormalization Group (NRG) computation for the impurity temperature-dependent specific heat, entropy and magnetic susceptibility. The interleaving procedure is used to recover the continuum spectrum after the NRG-logarithmic discretization of the conduction band. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R19.00005: Fridel sum rules for one- and two-channel Kondo models and unitarity paradox via bosonization-refermionization approach Maxim Kharitonov, Natan Andrei, Piers Coleman We calculate the single-particle Green's functions and scattering amplitudes of the one-channel and channel-anisotropic two-channel Kondo models at the Toulouse and Emery-Kivelson lines, respectively, where exact solutions via the bosonization-refermionization approach are admitted. We demonstrate that in this approach the Friedel sum rules -- the relations between the trapped spin and ``flavor'' moments and the scattering phase shifts in the Fermi-liquid regime -- arise naturally and elucidate on their subtleties. We also recover the ``unitarity paradox'' [1,2] -- the vanishing of the single-particle scattering amplitude at the channel-symmetric point of the two-channel Kondo model -- stemming from non-Fermi-liquid behavior. We discuss the implications of these results for the development of composite pairing in heavy fermion systems.\\[4pt] [1] A. W. W. Ludwig and I. Affleck, Phys. Rev. Lett. 67, 3160 (1991).\\[0pt] [2] J. M. Maldacena and A. W. W. Ludwig, Nucl. Phys. B. 506, 565 (1997). [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R19.00006: Kondo destruction and superconducting correlations in the two-impurity Bose-Fermi Anderson model Lili Deng, Kevin Ingersent, Jedediah Pixley, Qimiao Si The Bose-Fermi Kondo and Anderson models are among the simplest models for Kondo destruction, the phenomenon believed to underly the anomalous physics of certain heavy-fermion materials near the border of magnetism. With the goal of probing superconductivity near the Kondo-destruction local quantum critical point of the Kondo lattice, here we study the two-impurity Anderson model supplemented both by an inter-impurity exchange of either SU(2) or Ising symmetry and by a linear coupling between the impurity spins and a sub-Ohmic bosonic bath. Using the continuous-time quantum Monte Carlo method and the numerical renormalization group, we elucidate the phase diagram arising from the interplay of Kondo physics, inter-impurity exchange (ferromagnetic or antiferromagnetic), and bosonic decoherence, and demonstrate the existence of a Kondo-destruction quantum critical point in the model. We investigate the properties near this quantum critical point, as well as the effect of a critical suppression of the Kondo effect on superconducting pairing correlations. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R19.00007: Quasiparticle scattering spectroscopy (QPS) of Kondo lattice heavy fermions L.H. Greene, S.M. Narasiwodeyar, P. Banerjee, W.K. Park, E.D. Bauer, P.H. Tobash, R.E. Baumbach, F. Ronning, J.L. Sarrao, J.D. Thompson Point-contact spectroscopy (PCS) is a powerful technique to study electronic properties via measurements of non-linear current-voltage characteristic across a ballistic junction. It has been frequently adopted to investigate novel and/or unconventional superconductors by detecting the energy-dependent Andreev scattering. PCS of non-superconducting materials has been much rarely reported. From our recent studies on heavy fermions [1], we have frequently observed strongly bias-dependent and asymmetric conductance behaviors. Based on a Fano resonance model in a Kondo lattice [2], we attribute them to energy-dependent quasiparticle scattering off hybridized renormalized electronic states, dubbing it QPS. We will present our QPS results on several heavy-fermion systems and discuss QPS as a novel technique to probe the bulk spectroscopic properties of the electronic structure. For instance, it reveals that the hybridization gap in URu$_{2}$Si$_{2}$ opens well above the hidden order transition [1].\\[4pt] [1] W. K. Park \textit{et al}., PRL \textbf{108}, 246403 (2012); \textit{ibid.,} \textbf{100}, 177001 (2008).\\[0pt] [2] M. Maltseva, M. Dzero, P. Coleman, PRL \textbf{103}, 206402 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R19.00008: Quantum Phases of the Shastry-Sutherland Kondo Lattice Jedediah Pixley, Rong Yu, Qimiao Si Motivated by the discovery of the geometrically frustrated heavy fermion metal Yb2Pt2Pb[1], which has a quasi two dimensional Shastry-Sutherland lattice structure, we consider the Heisenberg-Kondo lattice model on a two dimensional Shastry-Sutherland geometry. Using a large-N method, we obtain the phase diagram and, in particular, the quantum transitions between a valence bond solid phase and a heavy Fermi liquid phase. Interestingly, we find intermediate states that break the C4 symmetry. We discuss the implications of our results for the experiments on Yb2Pt2Pb and related 221 materials [1], as well as the possible placement of these systems in a proposed global phase diagram for heavy fermion metals [2]. [1] M. S. Kim and~M. C. Aronson, arXiv:1202.0220 (2012). [2] Q. Si, Phys. Status Solidi B 247, 476-484 (2010). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R19.00009: Kondo hole route to incoherence in the periodic Anderson model Pramod Kumar, N.S. Vidhyadhiraja The interplay of disorder and interactions in strongly correlated electronic systems is a subject of perennial interest. In this work, we have investigated the effect of Kondo-hole type disorder on the dynamics and transport properties of heavy fermion systems. We employ the periodic Anderson model within the framework of coherent potential approximation and dynamical mean field theory. The crossover from lattice coherent behaviour to an incoherent single-impurity behaviour is reflected in all aspects: a highly frequency ($\omega$)-dependent hybridization becomes almost flat, the coherence peak in resistivity (per impurity) gives way to a Hammann form that saturates at low temperature ($T$); the Drude peak and the mid-infrared peak in the optical conductivity vanish almost completely. The zero temperature resistivity can be captured in a closed form expression, and we show how the Nordheim's rule gets strongly modified in these systems. The thermopower exhibits a characteristic peak, which changes sign with increasing disorder, and its location is shown to correspond to the low energy scale of the system ($\omega_L$). In fact, the thermopower appears to be much more sensitive to disorder variations than the resistivity. A comparison to experiments yields quantitative agreement. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R19.00010: Bose-Fermi Kondo model with a local transverse field and its implications for the global phase diagram of heavy fermions Emilian Nica, Qimiao Si, Kevin Ingersent Recent studies of the global phase diagram of quantum critical heavy fermion metals [1] have motivated us to consider the interplay between the quantum fluctuations within the local-moment system and those associated with the Kondo interaction. Towards this goal, we studied a Bose-Fermi Kondo model with Ising anisotropy in the presence of a local transverse field. Using the numerical renormalization group method for co-existing fermionic and bosonic baths [2], we found that tuning the transverse field gives rise to a continuous phase transition between a local moment phase and a Kondo screened phase. We determine the critical fixed point structure by studying the transitions accessed by varying the transverse field for different initial values of the coupling to the dissipative boson bath. Finally, we discuss the implications of these results for the global phase diagram of the Kondo lattice.\\[4pt] [1] Q. Si and F. Steglich, Science 329, 1161 (2010).\\[0pt] [2] M. T. Glossop and K. Ingersent, Phys. Rev. B 75, 104410 (2007) [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R19.00011: Kondo Metal and Ferrimagnetic Insulator on the Triangular Kagome Lattice Yao-Hua Chen, Hong-Shuai Tao, Dao-Xin Yao, Wu-Ming Liu We obtain the rich phase diagrams in the Hubbard model on the triangular kagome lattice as a function of interaction, temperature and asymmetry, by combining the cellular dynamical mean-field theory with the continuous time quantum Monte Carlo method. The phase diagrams show the asymmetry separates the critical points in Mott transition of two sublattices on the triangular kagome lattice and produces two novel phases called plaquette insulator with a clearly visible gap and a gapless Kondo metal. When the Coulomb interaction is stronger than the critical value $U_c$, a short range paramagnetic insulating state emerges before the ferrimagnetic order is formed independent of asymmetry. Furthermore, we discuss how to measure these phases in future experiments. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R19.00012: Doniach Diagram in Disordered Electrons System Hyunyong Lee, Stefan Kettemann We have derived the quantum phase diagram of disordered electron systems with magnetic impurities. The competition between RKKY interaction, $J_{RKKY}$, and Kondo effect gives rise to a rich quantum phase diagram, or Doniach diagram. We present numerical results for disordered 2D electron systems which show that both Kondo temperature, $T_K$ and $J_{RKKY}$ are widely distributed and quantum critical point is extended to a critical region. We find a sharp cutoff in the distribution of their ratio, $J_{RKKY}/T_K$, and from that critical density of magnetic impurity below which Kondo always wins. We find that the spin coupled phase grows at the expense of Kondo phase as increasing disorder.The spin coupled phase shows a succession of 3 phases: 1. a Griffiths phase with anomalous power laws determined by distribution of $J_{RKKY}$, 2. spin glass phase, 3. long range magnetic ordered phase. We report the results on graphene where we find that spin coupled phase is more stable against Kondo screening, but is more easily destroyed by disorder into a paramagnetic phase [1].\\[4pt] [1] H. Lee, S. Kettemann, arXiv:1211.1734(2012) [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R19.00013: Transport properties of a two impurity system: a theoretical approach. Ignacio J. Hamad, Laercio Costa Ribeiro, George Martins, Enrique V. Anda Double magnetic-impurity systems have attracted great attention due to their rich physics and possible technological applications. A system of two interacting Co atoms has been studied in a recent STM experiments (Nature Physics {\bf 7}, 901 (2011)). The precise control of the inter-impurity distance made it possible to explore in detail the transport properties of the system as a function of the impurities' interaction with each other. We explain, for all the parameter range studied, the physics observed in the experiments using a microscopic model, based on the two impurity Anderson model, including a two-path geometry for charge transport. The many-body system is treated in the finite-U Slave Boson Mean Field Approximation. Other results obtained using the Logarithmic Discretization Embedded Cluster Approximation are also discussed. We physically characterize the system and show that, as in the experiments, the features observed in the transport properties depend on the presence of two impurities but also on the existence of two conducting channels for electron transport. In particular, we obtain a splitting in the differential conductance, compatible with the one observed in the experiments, as a result of the superposition of the many-body Kondo states of each impurity. [Preview Abstract] |
Session R20: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Modeling and Electrical Characterization
Sponsoring Units: DMPChair: Blanka Magyari-Kope, Stanford University
Room: 322
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R20.00001: Pseudopotential-based study of electron transport in low-dimensionality nanostructures Invited Speaker: Massimo Fischetti Pseudopotentials-- empirical and \textit{ab initio} -- are now being more commonly used to study not only the atomic and electronic structure of nanometer-scale systems, but also their electronic transport properties. Here we shall give a bird-eye view of the use of density functional theory (DFT) to calibrate empirical pseudopotentials (EPs), of EPs to calculate efficiently the electronic structure of low-dimensionality systems, the most significant electronic scattering processes, and to study semiclassical and quantum electronic transport. Low-dimensionality systems considered here include thin semiconductor layers, graphene, graphene- and silicane-nanoribbons, and silicon nanowires. Regarding graphene, the high electron mobility measured in suspended graphene sheets ($\sim$ 200,000 cm$^{2}$/Vs) is the result of a relatively weak carrier-phonon and the strong dielectric-screening property. However, in practical applications graphene is likely to be supported by an insulating substrate, top-gated, and possibly used in the form of narrow armchair-edge nanoribbons (aGNRs) in order to open a gap. We will discuss several scattering processes which may affect the electron transport properties in these situations. First, we shall present results of the calculation of the intrinsic electron-phonon scattering rates in suspended graphene using empirical pseudopotentials and the rigid-ion approximation, resulting in an electron mobility consistent with the experimental results. We shall then discuss the role of interfacial coupled substrate optical-phonon/graphene-plasmons in depressing the electron mobility in graphene supported by several insulators (SiO$_{2}$, HfO$_{2}$, Al$_{2}$O$_{3}$, and h-BN). We shall also discuss the role of Coulomb scattering with charged defects/impurities in gated graphene sheets and the role of the metal gate in screening this interaction. Finally, we shall review the strong effect of line edge roughness (LER) on electron transport and localization in narrow aGNRs resulting from the ``aromatic'' width dependence of the band-gap of the \textit{sp}$^{2}$-coordinated aGNRs. This will lead us to consider \textit{sp}$^{3}$-coordinate ribbons (silicane) and Si nanowires as possible alternative structures -- less affected by LER scattering -- of interest in nanoelectronics application. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R20.00002: Electron Transport Variability in Armchair Graphene Nanorribbons Shela Aboud, Massimo Fischetti Armchair graphene nanoribbons (AGNR) hold great promise in nano-electronics because of the capability of opening a semiconducting gap in narrow ribbons. However the effective use of AGNR in devices may be limited by structural and chemical modifications from a variety of sources including the support material, edge effects, width variability, and defects which all change the trends in the bandgap scaling. In this work we use density functional theory (DFT) simulations and Empirical Pseudopotentials (EPs) to investigate how structural and chemical variability in the AGNRs influence electron transport through changes in the bandstructure, phonon modes and electron-phonon coupling. Comparisons of the DFT and EPs give the same trend for ribbons with widths of 3N, 3N$+$1 and 3N$+$2 atoms with small differences stemming from the atomic structural relaxation accounted for in the DFT simulations. The dependence of the gap on the ribbon width is attributed to the aromaticity of the graphene that can be understood through the spatial distribution of the Clar resonance structures (Clar sextets) and become more localized because of the formation of the edge states. Chemical functionalization of the edges, defects at the edges and in the bulk of the ribbon, doping, and the type of support material (e.g. h-BN, SiO2, HfO and Al2O3) will all modify the aromaticity of the ribbons. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R20.00003: Photo-induced energy transfer between carbon nanotubes Olena Postupna, Heather Jaeger, Oleg Prezhdo The unique structural, mechanical, and electronic properties of carbon nanotubes (CNTs) have recently been attracting significant attention in academic research and industrial applications. Experimental investigation of the physical properties of CNTs is often hindered by questions that can be answered only with rigorous theoretical approaches, such as ab initio molecular dynamics. Results of time-domain simulations of energy transfer between photo-excited CNTs are reported. Using a system comprised of a pair of CNTs with different chiralities, (6,4) and (8,4), we elucidate the experimental results obtained by Luer at al [1]. Quantification of adiabatic and nonadiabatic contributions to the transfer process clarifies the mechanism of energy transfer. And, the delocalization of the initial exciton is representative of strong donor-acceptor coupling at high energies. Our work contributes to the ever-growing compendium of energy transfer within nanoscale systems and offers valuable insight toward tailoring CNTs for solar energy conversion.\\[4pt] [1] Larry Luer, Jared Crochet, Tobias Hertel, Giulio Cerullo, Gugliermo Lanzani. ACSNano. Vol.4, No. 7, 4265-4273 [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R20.00004: Realization of High-speed Transport in Low Dimensional Disordered Carbon Films Somnath Bhattacharyya, Mikhail Katkov, Dmitry Churochkin, George Chimowa, Ross McIntosh Developing hybrid super-structures including carbon nanostructures for quantum information science is widely sought after and we show a possible route in carbon superlattice structures based on experimental results as well as theoretical analysis which also incorporates high-speed switching capabilities. We propose a theoretical model of disordered carbon superlattice structures where the local density of electronic states is controlled by changing the $sp^{3}-C$ bond alternation as well as the hopping disorder parameter of the $sp^{2}-C$ regions. In addition the incorporation of nitrogen atoms in carbon networks was modeled as a combination of disorders which vary both in correlated and uncorrelated manners. Resonant peaks associated with $C$ and $N$ sites in these structures show a conductance cross-over under variation of the nitrogen concentration in these structures which can explain the observed negative differential resistance in diamond-like carbon superlattices as well as the conductivity cross-over in nano-crystalline diamond films. Detailed analysis of transport measurements over a wide range of temperatures, magnetic fields and also frequency shows an enhanced characteristic length in these systems that supports switching of complex impedance in the range of 50 GHz. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R20.00005: Preparation and electrical transport property study of MoS$_{2}$ single-layer devices on different substrates Zhiyong Wang, Zhisheng Lin, Ray Sachs, Ji Feng, Jing Shi Micro-exfoliated MoS2 flakes on SiO2/Si substrate are identified with optical microscope first and then atomic force microscopy and Raman spectroscopy. Nanodevices are subsequently prepared by E-beam lithography. The as-prepared MoS2 devices are n-type with a high sheet resistance (typically several MOhms). As a gate voltage is applied, a large gate modulation in sheet resistance is observed. At the highest negative gate voltage, the devices remain n-type but the resistance increases by at least 4 orders of magnitude. In the meantime, the current-voltage characteristics turn from linear to non-linear. The field-effect mobility extracted from the gate voltage dependence is about 10 cm$^{2}$/Vs. To study the effect of the dielectric constant, we have developed a transfer technique that transfers entire working devices from SiO2/Si to any substrates. We have successfully applied the technique to graphene and obtained a relatively high yield. We are currently transferring MoS2 devices from SiO2/Si to strontium titanate (STO) substrate which has a much higher dielectric constant (300 at room temperature). Detailed experimental results and discussions will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R20.00006: Electrical characterization of few-layer MoS2 on HfO2 substrate Jatinder Kumar, Hui-Chun Chien, Hsin-Ying Chiu Due to the realization of graphene transistors but without applicable bandgap, the similar layered structure molybdenum disulfide (MoS2) field effect transistors with nonzero bandgap have been demonstrated and reveal promising potential. Previous experiments showed that carrier mobility could be enhanced by depositing hafnium dioxide (HfO2) on top of MoS2 devices, which was possibly attribute to the suppression of Coulomb scattering by high-$\kappa$ environment and surface polar phonon scattering. In our talk, we will present the electrical transport experiments in few layers of MoS2 on HfO2 dielectrics, including the carrier mobility improvement and electrical transport phenomena in high bias region. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R20.00007: Investigation of $E_{2g}^{1}$ and $A_{1g}$ Raman Modes of Few-Layer MoS$_2$ on HfO$_2$ Substrate Hui-Chun Chien, Jatinder Kumar, Hsin-Ying Chiu The recent research work by Radisavljevic \textit{et al.}[1] shows that the mobilities of monolayer MoS$_{2}$ transistors can be improved by employing a thin layer of hafnium oxide as top-gate dielectric. Dielectric screening has been successfully demonstrated to suppress the Coulomb interactions of charged impurities on the substrate. Therefore, we develop an alternative method of building monolayer MoS$_{2}$ transistors on HfO$_{2}$ substrate. Owing to the low contrast of few-layer MoS$_{2}$ flakes on thin HfO$_{2}$ layer, which makes the realization of such device configuration difficult. By utilizing the thickness dependence of in-plane and out-of-plane Raman peaks of MoS$_{2}$ flakes, $E_{2g}^{1} $and$A_{1g} $, respectively, we establish an efficient approach to improve the identification of MoS$_{2}$ layers by Raman spectrum instead of AFM. Our investigation of Raman spectrum of few-layer MoS$_{2}$ on HfO$_{2}$ shows the significant difference from those on SiO$_{2}$. The substrate dependence of Raman spectrum as well as its further application will be discussed in this talk.\\[4pt] [1] Radisavljevic, \textit{et al., Nat. Nanotech}. \textbf{6}, 147 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R20.00008: Liquid-gated ambipolar transistor with ransition-metal dichalcogenides Yijin Zhang, Jianting Ye, Yoshihiro Iwasa Transition-metal dichalcogenides (TMDs) are graphene-like layered materials. In particular, semiconducting group of TMDs are attracting great interests as a post-graphene material since they have a finite band gap which is an important feature for FET applications. We fabricated semiconducting TMD-based FETs using a new type of gate dielectric called electric double layer (EDL). EDL is formed by solid and ions inside liquid at the solid-liquid interface. This nano-scale capacitor provides extremely large charge accumulation capability and realizes high performance FETs and field-effect phase control. We observed ambipolar FET operation of molybdenum disulfide (MoS$_{2})$ for the first time in addition to its well-known n-type operation [1] and field-effect superconducting transition [2]. High performance is not only observed in MoS$_{2}$ but also in other semiconducting TMDs like tungsten diselenide (WSe$_{2})$. The ambipolar operation is also important for applications, for example, light-emitting devices like organic materials. We investigated possibilities of EDL-based optical coupling devices. [1] Y. J. Zhang et al. Nano. Lett. 12, 1136 (2012) [2] J. T. Ye, Y. J. Zhang et al. Science in press [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R20.00009: High on/off ratio field effect transistor based on exfoliated crystalline SnS$_{2}$ nano-membrane Debtanu De, John Manongdo, Sean See, Vincent Zhang, Arnold Guloy, Haibing Peng We report the implementation of field effect transistors based on exfoliated nano-membranes of a layered two-dimensional semiconductor SnS$_{2}$, which exhibit an On/Off ratio exceeding 2x10$^{6}$ and a carrier mobility of $\sim$ 1 cm$^{2}$V$^{-1}$s$^{-1}$. The results demonstrate the great potential of SnS$_{2}$, a layered semiconductor with finite band gap, as the building block for future nanoelectronic applications complementary to graphene-based materials with zero or small band gap. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R20.00010: Topological phase transition in hexagonal boron-nitride bilayers modulated by gate voltage Guojun Jin, Xuechao Zhai We study the gate-voltage modulated electronic properties of hexagonal boron-nitride bilayers with two different stacking structures in the presence of intrinsic and Rashba spin-orbit interactions. Our analytical results show that there are striking cooperation effects arising from the spin-orbit interactions and the interlayer bias voltage. For realizing topological phase transition, in contrast to a gated graphene bilayer for increasing its energy gap, the energy gap of a boron-nitride bilayer is significantly reduced by an applied gate voltage. For the AA stacking-bilayer which has the inversion symmetry, a strong topological phase is found, and there is an interesting reentrant behavior from a normal phase to a topological phase and then to a normal phase again, characterized by the topological index. Therefore, the gate voltage modulated AA-boron nitride bilayer can be taken as a newcomer of the topological insulator family. For the AB stacking-bilayer which is lack of the inversion symmetry, it is always topologically trivial, but exhibits an unusual quantum Hall phase with four degenerate low-energy states localized at a single edge. It is suggested that these theoretical findings could be verified experimentally in the transport properties of boron-nitride bylayers. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R20.00011: Surface-Plasmon Assisted Exciton Transport in 1D Nanostructures Charles Cherqui, David Dunlap, Andrei Piryatinski We consider effect of coupling between exciton propagating in a 1D-nanostructure (e.g., carbon nanotube) and localized surface plasmon modes induced by a metal nanoparticle located in close proximity to the nanostructure. Both regimes of weak and strong exciton-plasmon couplings are taken into account leading to the dressed exciton and plasmon states. In this representation, the dynamics of the dressed excitons is mapped on the impurity scattering problem. The analysis of the scattering matrix indicates that the surface-plasmon modes lead to the exciton intraband scattering and possibility to form localized states within the exciton band gap. Surface plasmon induced exciton radiation pattern and the radiative and non-radiative decay rates are calculated and their dependence on the exciton-plasmon coupling is analyzed. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R20.00012: A novel turbulent state of a dipolar exciton Bose-Einstein condensate German V. Kolmakov, Oleg L. Berman, Roman Ya. Kezerashvili We report the formation of a new state in a non-equilibrium Bose-Einstein condensate (BEC) of dipolar excitons: steady turbulence. Two different systems where the BEC is formed are considered: coupled semiconductor quantum wells and two-layer graphene separated by a semiconducting or dielectric barrier. The non-linear dynamics of the systems are studied by using the generalized Gross-Pitaevskii equation. It is demonstrated that in the BEC a steady turbulent state is formed at high enough pumping rates. This state is characterized by oscillations of the spatial distribution of the excitons and fast redistribution of the energy between the oscillatory modes. The dynamics of the system can be explained in terms of the propagation of the fluxes of two quantities -- the energy and the number of particles. The analysis of these excitonic systems as well as the comparison with an atomic condensed state show that the formation of turbulence is a general effect in the BEC. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R20.00013: Theoretical study of electron transport in DNA Bikan Tan, Miroslav Hodak, Wenchang Lu, Jerry Bernholc Many experiments have observed high conductivity of DNA, but its origin has not yet been satisfactorily explained. In this work, we explore the dynamics of solvated B-DNA sandwiched between metallic nanotubes and connected via alkane linkers. The geometries are relaxed using the CHARMM force field. Conductivities of different snapshots of the system are calculated using the non-equilibrium Green's function method within density-functional theory. Our results show that in certain geometries, the DNA conducts significantly better than in others. For the highest conductivity configuration, a HOMO state extends across DNA's guanine sites to the alkane linkers. In general, we find that the conformational changes strongly affect the energy alignment of HOMO states of the DNA and the linker, and thus have a major effect on the conductivity of the entire system. [Preview Abstract] |
Session R21: Focus Session: Coupling Phenomena in Oxides and Optical and Electronic Properties
Sponsoring Units: DMPChair: Pavlo Zubko, Universite de Geneve
Room: 323
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R21.00001: A Transport Perspective on Local Manipulation of Ferroelectric and Correlated Electron Surfaces Invited Speaker: Petro Maksymovych The majority of transport studies aim to identify intrinsic electronic properties of materials, thus avoiding large electric fields, hysteresis, chemical reactions and hot electrons. In this talk, I will discuss the electron transport signatures of the opposite regime, where a complex oxide surface is subjected to strong local field and/or force gradients. Most notably, we have established an insulator-metal transition within an insulating perovskite oxide controlled solely by ferroelectric switching at the nanoscale [1]. This was the first time metallic conductivity has been found in a ferroelectric, despite a variety of theoretical scenarios dating back to the 70's that hypothesized such a behavior. Equally intriguing is the ability to tune the type and magnitude of metallic conductivity of ferroelectric nanodomains by orders of magnitude using applied electric field. Landau-Ginzburg-Devonshire (LGD) formalism captures the essence of these effects, by describing carrier accumulation or depletion at inclined and charged domain walls. On the other hand, local transport measurements on the surfaces of nominally conducting surfaces (such as manganites and nickel oxide) have induced an insulating state, the effect we refer to as `piezochemistry' and assign to strain-induced redistribution of oxygen vacancies [2]. These coupled transport phenomena in oxides have practical implications, while transport itself appears to be a highly sensitivie probe of ferroic transitions and ionic effects. Experiments were conducted at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. Work was also supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.\\[4pt] [1] P. Maksymovych, A. N. Morozovska, P. Yu, E. A. Eliseev, Y.-H. Chu, R. Ramesh, A. P. Baddorf, S. V. Kalinin, Nano Lett.,12, 209 (2012).\\[0pt] [2] Y. Kim, S. Kelly, E. Strelcov, A. Morozovska, E. Eliseev, S. Jesse, N. Balke, I. Hwang, T. Choi, B. Ho Park, P. Maksymovych, S. V. Kalinin, Submitted (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R21.00002: Interplay of strain and oxygen vacancies in CaMnO$_3$ Ulrich Aschauer, Reto Pfenninger, Sverre M. Selbach, Tor Grande, Nicola A. Spaldin Application of strain through heteroepitaxy has become an established route to engineering novel material properties such as multiferroism in perovskites. First principles calculations have been shown to accurately describe material properties as the in-plane lattice constants are changed by strain, and often indicate that large strain magnitudes ($>$4\%) are required to induce new functionalities. At such large values, however, it is unclear whether strain will be accommodated primarily by changes in intrinsic lattice constants as usually assumed, or by the formation of point defects. Conversely, the use of strain to engineer point-defect concentrations and stoichiometry is largely unexplored. Here we use first-principles calculations to investigate the stability of the $Pnma$ perovskite CaMnO$_3$ under bi-axial strain towards the formation of oxygen vacancies. We discuss the underlying mechanism for strain-vacancy coupling as well as the implications of our results for the growth of highly strained epitaxial films. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R21.00003: Effect of Surface Engineering on Tunneling Across Ferroelectric Thin Films Le Zhang, Shijie Li, Haidong Lu, Alexei Gruverman, Andrei Sokolov For a practical implementation of ferroelectric based devices, it is indispensable to obtain reliable switching characteristic of ME/FE/ME heterostructures. Electrostatic force microscopy shows that polar surfaces are achievable without top ME electrode and ionic adsorbates delivers enough screening ions, it is intriguing enough that top metal electrode appears to be less effective than such screening by adsorbates. This observation emphasizes the importance of surface engineering [1] heterojunctions in order to retain or enhance ferroelectric response. We report on our study of the role of top electrodes in ferroelectric stability and tunneling properties of heterojunctions containing BaTiO$_3$ thin film, grown epitaxially on Ti-terminated SrTiO$_3$ substrate with LaSrMnO$_3$ and SrRuO$_3$ as bottom electrodes by pulsed laser deposition. Epitaxial top electrodes are formed by the same method. Alternatively, Pt and Ni-based metallic films are deposited by electron beam evaporation. PFM response and HRTEM analysis of obtained interface are presented. Transport and magneto-transport measurement are discussed in the frame of ferroelectric polarization induced effects.\\[4pt] [1] H. Lu et all, Adv. Mater., 24, 1209 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R21.00004: Graphene Field Effect Sensors for the Study of Nanoscale Ferroelectric Thin Films Anil Rajapitamahuni, Vijay Raj Singh, Zhiyong Xiao, Xia Hong We have constructed graphene field effect devices as sensors to study the dielectric and pyroelectric properties of nanoscale ferroelectric thin films. Using off$-$axis radio frequency magnetron sputtering, we have grown epitaxial single crystalline Pb(Zr,Ti)O$_{3}$(PZT) and (Ba,Sr)TiO$_{3}$ (BSTO) films of 30$-$100 nm thick on (001) Nb:SrTiO$_{3}$ substrates. X$-$Ray and AFM characterizations show the films have high crystallinity and smooth surface. Piezo-response force microscopy studies show that the as-grown PZT films have uniform polarization pointing towards the substrate. Graphene flakes are mechanically exfoliated on PZT and BSTO thin films and single to few layers are fabricated into field effect devices. We extract the carrier density in graphene from Hall Effect measurements, and use it to probe the polarization change of the ferroelectric gate layer. From the gating efficiency we found the dielectric constant of 100 nm PZT film to be 50. Its pyroelectric coefficient is $\sim$15 nC/cm$^2$K at 300 K and the polarization saturates below 100 K. We have also studied the effect of film thickness on the dielectric and pyroelectric properties of the ferroelectric thin films. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R21.00005: Investigation of laser induced space charge fields in lithium niobate at low temperature with Raman spectroscopy Greg Stone, Volkmar Dierolf We report the measurement of space charges fields generated by a laser beam at low temperatures using Raman spectroscopy. Raman spectra obtained with a focused laser exhibits frequency shifts of certain Raman peaks that appear as a function of time. Analysis of these shifts reveals that they originate from changes in the local electric field that are predominately parallel to the z-axis of the crystal. The magnitude of the frequency shifts and the corresponding maximum space charge field established inside the crystal are dependent on the defect concentration. Above a certain threshold field, the built-up space charge field is drastically reduced by discharges and builds up again afterwards. The changes in the Raman spectrum remain after the laser is turned off but disappear upon heating the sample above 200K. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R21.00006: First-principles Investigations of Fe Impurities in KNbO$_{3}$ Mohua Bhattacharya, Steven Lewis, William Dennis The perovskite based material KNbO$_{3}$ has been studied extensively for its photorefractive properties, where the electro-optic effect combined with photoconductivity changes the local refractive index of the material in response to the incident intensities. The presence of a transition metal impurity like Fe is required for efficient photorefractive performance of this material. To shed light on the physical mechanism of this behavior, we perform first-principles calculations within the density functional theory framework. In this talk, we present the geometric and electronic structures of KNbO$_{3}$:Fe super cells and compare two cases: one in which the Fe$^{3+}$ impurity on the Nb$^{5+}$ site is compensated by an O vacancy in the first coordination shell and one in which the O vacancy is in the second coordination shell. Connections of this work to recent experimental measurements are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R21.00007: Far infrared studies of PrFe$_{3}$(BO$_{3})_{4}$ Kirill Boldyrev, Marina Popova, Taras Stanislavchuk, Andrei Sirenko, Leonard Bezmaternykh We present results on polarized far infrared reflectance, transmittance and ellipsometry measurements of PrFe$_{3}$(BO$_{3})_{4}$ single crystals in a wide temperature range (5 - 300K). Rare-earth iron borates RFe$_{3}$(BO$_{3})_{4}$ undergo an antiferromagnetic phase transition at temperatures below 40 K and all of them demonstrate magnetoelectric and magnetoelastic effects. PrFe$_{3}$(BO$_{3})_{4}$ orders antiferromagnetically at T$_{N}$~$=$~32 K. Pronounced changes in the low-frequency phonon spectra of PrFe$_{3}$(BO$_{3})_{4}$ are observed at T$_{N}$ which points to a significant spin-lattice interaction. Below 90 K, a new feature at 48 cm$^{-1}$ appears in the pi-polarized reflectance spectra. We attribute this feature to a Pr$^{3+}$ crystal-field transition that becomes observable in reflectance due to interaction with a nearby phonon 60cm$^{-1}$. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R21.00008: Band gap hierarchy of single crystal CoFe$_2$O$_4$ thin films from optical absorption spectroscopy Brian Holinsworth, Hunter Sims, Dipanjan Mazumdar, Qi Sun, Mehmet Yurtisigi, Sanjoy Sarker, Arun Gupta, Bill Butler, Janice Musfeldt Thin film materials have a wide variety of applications and also serve as an useful bridge between bulk single crystals and the nanoscale. In this work, we report temperature-dependent optical absorption spectroscopy of single crystal CoFe$_2$O$_4$ thin-films along with complimentary electronic structure analysis. This magnetic insulator has one of the highest Curie temperature among complex oxides and potentially useful in areas such as spintronics. Similar to its Nickel analogue,\footnote{Q.C. Sun, H. Sims, D.Mazumdar, J.X.Ma, B. Holinswoth, K.O'Neal, G.Kim, W.H.Butler, A.Gupta, and J.Musfeldt (accepted to Phys. Rev. B).} our work reveals CoFe$_2$O$_4$ to be an indirect band gap material (1.2 eV) with a direct gap much higher (2.8eV) at 300K. These gap values are robust down to 4.2K. Electronically, both chemical tuning and inversion fraction are found to be important factors in lowering of the band gap compared to NiFe$_2$O$_4$. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R21.00009: Electrocaloric Properties of Epitaxial Strontium Titanate Films Jialan Zhang, Burc Misirlioglu, Pamir Alpay, George Rossetti The pyroelectric and electrocaloric effects in polar dielectric solids result from the coupling between the electrical and thermal properties. Although STO crystals or polycrystalline ceramics remain paraelectric down to 0 K, the ferroelectric phase can be induced by uniaxial stress, an external electrical field, or by doping. Here we develop a nonlinear thermodynamic theory to compute the electrocaloric response of strontium titanate thin films as a function of misfit strain, temperature, electric field strength, and electrode configuration. Our results show that the adiabatic temperature change $\Delta T$ of epitaxial (001) STO films can be controlled by the misfit strain and by varying the thermal and electrical boundary conditions. For films in a capacitor configuration on compressive substrates, the transition between paraelectric and strain-induced ferroelectric tetragonal phases produces a large adiabatic temperature change at room temperature. For films on tensile substrates, the transition between the paraelectric and strain-induced ferroelectric orthorhombic phases can also be accessed using inter-digitated electrodes, and the maximum EC response occurs with a [110] orientation. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R21.00010: Combined Angle-Resolved Photoemission Spectroscopy and Theoretical Study of the Surface Electronic Structure of SrTiO$_{3}$ Richard C. Hatch, Kurt Fredrickson, Chungwei Lin, Miri Choi, Agham B. Posadas, Hosung Seo, Alexander A. Demkov The surface electronic structure of the O 2$p$-derived valence band states of (001)-oriented, TiO$_{2}$-terminated SrTiO$_{3}$ is measured along various crystallographic directions using angle-resolved photoemission spectroscopy (ARPES). A comparison of ARPES spectra to \textit{ab initio}, density functional theory (DFT) band structure calculations as well as the theoretical band structure calculated at the tight binding level are in excellent agreement. ARPES measurements also reveal a mid-gap state located roughly 0.5 eV above the valence band which we interpret as a surface state. This interpretation is supported by DFT calculations of an SrTiO$_{3}$ slab which reveals the existence of a surface state located in the gap roughly 0.5 eV above the projected valence band. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R21.00011: Resonant inelastic soft x-ray scattering as a site-specific probe of electron-phonon coupling in one-dimensional edge-shared cuprates S. Johnston, W.S. Lee, B. Moritz, J. van den Brink, Z.-X. Shen, T. P. Devereaux Resonant inelastic x-ray scattering (RIXS) is a powerful probe for studying excitations in strongly correlated systems. With continued advancements of the technique the overall energy resolution has improved to the point of probing low-energy boson excitations near the elastic line. In this talk we present evidence for coupling to an optical oxygen phonon in the RIXS spectrum at the oxygen K-edge of the quasi-1D edge shared cuprate Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$. This mode is identified as a compressive mode polarized perpendicular to the chain direction, modulating the Cu-O charge transfer energy and setting the size exchange interaction. By comparing to small cluster calculations we extract a sizable electron-phonon coupling strength in a site-resolved manner, implying a strong integration of the lattice degrees of freedom into the electronic structure. [Preview Abstract] |
Session R22: Plasmonics and Optical Interactions in Structured Materials
Sponsoring Units: DCMPChair: Eric Stinaff, Ohio University
Room: 324
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R22.00001: Optimizing coherent Raman scattering with plasmonic nanoparticles Dmitri Voronine, Xia Hua, Alexander Sinyukov, Charles Ballmann, Alexei Sokolov, Marlan Scully Two commonly used techniques that provide species-specific spectroscopic signals in the form of vibrational fingerprints are surface-enhanced Raman scattering (SERS) and coherent anti-Stokes Raman scattering (CARS) spectroscopies. In order to enhance the signal, SERS takes advantage of the electromagnetic near-field enhancement while CARS employs molecular coherence. We have combined these two techniques to achieve best-of-both-worlds maximum signal enhancement by using optimized laser pulse shaping and time-resolved detection. We applied this new time-resolved surface-enhanced coherent anti-Stokes Raman scattering (tr-SECARS) technique to investigate various molecular complexes in a vicinity of gold nanoparticles. While large signal enhancement has previously been achieved in SERS, surfaced-enhanced coherent signals have shown lower values. We investigate the mechanisms of these effects by analyzing the spatial dependence of the coherent Raman spectra for different hot spots in aggregated plasmonic nanoparticles. Understanding coherence effects in surface-enhanced Raman scattering may lead to improved nanoscale sensors. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R22.00002: Complex metallic nanostructures using self-assembled DNA templates for SERS and plasmonic applications Mauricio Pilo-Pais, Anne Watson, Thom LaBean, Gleb Finkelstein We custom-tune the plasmonic resonance of complex metallic nanostructures based on ``DNA origami'' templates ($\sim$90x70nm). Briefly, 5 nm gold nanoparticles are attached at selected places within a DNA-origami ``nano-breadboard'' and later enlarged, and even fused, by electroless deposition of silver. By this method, we are able to control the size and topology, and therefore the plasmonic resonance of the resulting metallic nanostructures. We perform SERS measurements of various Raman molecules (i.e. 4-aminobenzenethiol), which are chosen based on the plasmonic resonance frequency of the structure. The flexibility of the design and multiply parallel nature of the method open the road for designing and fabricating optimum structures for a desired plasmonic application. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R22.00003: Enhanced surface Raman scattering in gold thin films deposited on large array anti-nanoring template Chi Chih Ho, Tze Yang Lee, Wei Li Lee, Fan Gang Tseng To evenly distribute hot spots over large area is an important subject for realistic applications using surface enhanced Raman scattering (SERS) effect. Here, we utilized a monolayer polymer/nanosphere hybrid to prepare a large area and well-ordered anti-nanoring template for gold thin film deposition. The resulting gold nanostructured thin film, which comprises an antidot network with isolated nano-disk (ND) and nanoring (NR) in each antidot, can be employed as an efficient SERS substrate. From finite difference time domain (FDTD) simulation, hot spots occur at the space between isolated ND and NR giving rise to enhanced surface Raman scattering. We fabricated a series of such gold nanostructured thin films with different thickness and geometry. An optimum condition for maximum SERS was obtained in experiment. Detailed size effect on SERS and comparison to FDTD simulation will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R22.00004: Ultraviolet surface-enhanced Raman spectroscopy using aluminum plasmonic gratings Adam T. Roberts, Serkan Butun, Koray Aydin, Henry O. Everitt, Mark Bloemer, Giuseppe D'Aguanno, Nadia Mattiucci Surface-enhanced Raman scattering (SERS) has been widely studied both theoretically and experimentally for chemical and biological sensing, primarily in the visible and near-infrared wavelengths. Although in the ultraviolet (UV) plasmonic behavior is limited by metallic dampening, we have theoretically shown that SERS enhancement factors as large as 10$^5$ can be achieved when the laser is tuned to the plasmonic band edge of an Al metallic grating grown on a sapphire substrate. Using electron beam lithography, aluminum gratings were fabricated whose pitch (150-300 nm), slit widths (64 nm), and thickness (50 nm) were chosen to produce large enhancement factors at wavelengths in the UV. Analytes such as thiophenol were then deposited on the gratings, and UV-SERS spectroscopy was performed to measure the enhancement factors and compare with theoretical estimates. Enhancement factors were measured by comparing the strength of the Raman signal from the grating region with the strength of the Raman signal from adjacent regions without a grating. The dependence of the enhancement factor on laser wavelength relative to the plasmonic band edge for a given grating pitch was explored, as was the effect of using a tapered slit geometry that focuses the local field on the nanoscale. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R22.00005: Surfaced Enhanced Raman Spectroscopy in Nanojunctions with Anomalous Polarization Dependence Joseph B. Herzog, Mark W. Knight, Yajing Li, Kenny Evans, Naomi J. Halas, Douglas Natelson Several papers have been published on surfaced enhanced Raman spectroscopy (SERS) in nanojunctions, and polarization studies have shown that the strongest SERS enhancement is generated when the incident light is polarized so that the electric field is directed across the interelectrode nanogap. This polarization dependence is certainly true for mesoscale structures such as dimers, but this works show that this is not always the case. Here we create nanogaps both by electromigration and a novel ``self-aligned'' process, which can be scaled for mass production. Polarization dependent SERS measurements were performed on these junctions and have determined that transverse polarization of incident light generates the strongest SERS enhancement. Cathodoluminescent experiments as well as finite element method calculations have confirmed these findings and together with the experimental results have determined that the enhancements are due to strong localized hybrid modes in the gap which couple to a resonant transverse plasmon mode. This new finding has increased device sensitivity by an order of magnitude and opens the possibility for improved plasmonically-active optoelectronic devices and other nanophotonic applications. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R22.00006: Surface-enhanced Raman detection of a vibrational Stark effect in C60-containing molecular junctions Yajing Li, Peter Doak, Jeffrey Neaton, Leeor Kronic, Douglas Natelson Understanding the interplay of local electric fields and molecular vibrational degrees of freedom is of considerable interest. One nontrivial consequence of this coupling is the vibrational Stark effect, in which vibrational energies are altered through coupling to externally applied electric fields. We investigate this physics through nanoscale Au bowtie structures functioning as surface enhanced Raman(SERS) substrates. Following electromigration, these metal nanostructures possess nanometer-scale interelectrode gaps that support highly localized surface plasmon resonances, resulting in SERS electromagnetic enhancements sufficient for single-molecule studies. These structures have also proven suitable for simultaneous single-molecule electronic transport experiments, in which we observed the vibrational modes of the molecules shift systematically as a function of applied bias. We will present measurements of the electrically driven vibrational energy shifts of C60 in such junctions and compare those with theoretical expectations obtained from DFT calculations. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R22.00007: Fractal nanostructures with Hilbert curve geometry as a SERS substrate Ilya Grigorenko A new type of substrates for Surface Enhanced Raman Scattering measurements is proposed. The shape of the substrate is based on self-similar fractal space filling curves, which possess properties of both one dimensional and two dimensional geometries. Here I present theoretical studies of the dielectric response of thin film doped semiconductor nanostructures, where conducting electrons are trapped in an effective potential having the geometry of the Hilbert curve. It is found that the system may exhibit the induced charge distributions specific for either two dimensional or one dimensional systems, depending on the excitation frequency. It is also shown that with the increase of the depth of the trapping potential the resonance of the system demonstrates a counter-intuitive shift to lower frequencies. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R22.00008: Exciton-plasmon interaction and photo-injection of plasmonic hot carriers in hybrid nanostructures Alexander Govorov, Hui Zhang, Min Ouyang, Yurii Gun'ko We investigate theoretically the effects of exciton-plasmon interaction and plasmon-assisted carrier injection in a hybrid semiconductor-metal nanostructure under resonant optical excitation. We treat the coupling between the metal and semiconductor nanocrystals using a many-body Fano model and a quantum density-matrix formalism. Hot carriers have a characteristic energy distribution in the plasmon wave function in a metal nanocrystal and participate in tunnel and ballistic injection currents to the neighboring semiconductor nanostructure. The photo-current induced by hot plasmonic electrons in the nanostructure depends on the barrier height, excitation frequency, plasmon energy, relaxation rates, and geometry of a device. The Coulomb exciton-plasmon interaction may also play an essential role in the optical absorption and electron injection. The results obtained in this study can be used to design and describe a variety of plasmonic nanodevices with hot electron injection for photo-catalysis, light-harvesting, and solar cells. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R22.00009: Photoconductance measurments of patterned nanocrystal films on gold nanojunctions Kenneth Evans, Sravani Gullapalli, Michael Wong, Douglas Natelson Large scale production of nanoscale absorbers and emitters based on single, or few, colloidal nanocrystals would be an important advancement for light-based electronics and investigating poorly understood quantum phenomena such as blinking. We present a method for integrating nanocrystals into plasmonically-active gold nanogaps by way of lithographic patterning of nanocrystal films. Initial photoconductance measurements in nanocrystal-based devices are compared with bare gold junctions and the possibility for plasmon-assisted absorption and emission is discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R22.00010: Coherent Oscillations in Spoof-Like Plasmonic Ag deposited by PEALD Ryan Compton, Sharka M. Prokes, Orest J. Glembocki, Jeffrey C. Owrutsky The spoof-like plasmonic properties of Ag thin films produced by plasma enhanced atomic layer deposition (PEALD) were investigated with static and transient spectroscopy. The PEALD process results in a film with cylindrical 2D structures separated by air gaps, giving rise to the plasmonic behavior. Films with thicknesses ranging from 10 to 32 nm were deposited and compared to films of similar thickness produced with traditional e-beam methods. Transmission spectra of the ALD films exhibit a strong surface plasmon resonance (SPR) band at approximately 700 nm, while the e-beam samples were devoid of band structure. The SPR band of the 10 nm ALD sample is blue-shifted (to 550 nm), suggesting morphological differences for the thinnest film. Transient absorption studies with a 400 nm probe revealed electron-phonon coupling times that are similar for both ALD and e-beam films. Transient measurements of the ALD Ag probed near the plasmon band (800 nm), however, feature coherent oscillations attributed to breathing of the cylindrical structures, whereas the e-beam films exhibit no oscillatory behavior. The oscillation period was found to be independent of ALD thickness, except in the 10 nm sample where no oscillations were observed. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R22.00011: Optical analogue of quantum spin and dynamic localization in optical waveguides arrays Kin Chung Au Yeung, Kin Wah Yu We have discovered an optical analogue of quantum spin in optical waveguides arrays. Quantum-optical analogy is recently a hot topic. By using special configuration of optical devices, some optical analogues of quantum systems can be realized. Stefano Longhi and coworkers proposed some classical realization of quantum phenomena like the two-site Fermi-Hubbard system [1] and Rabi oscillation [2]. In this work, we propose an optical waveguides arrays system with evanescent couplings according a symmetrized Kac matrix. The system can mimic the quantum spin under different operators like the rotation operator. Also by adding a suitable time-dependent applied potential to the system, dynamic localization of optical signal can be realized along the signal propagation. The system can be extended to mimic any arbitrary angular momentum by increasing the number of optical waveguides arrays. The occurrences of spin under rotation operator and dynamic localization are simulated by a field-evolution analysis using an input Gaussian beam.\\[4pt] [1] S. Longhi, G. Della Valle, V. Foglietti, arXiv:1111.3460 (November 2011)\\[0pt] [2] Ivan L. Garanovich, Stefano Longhi, Andrey A. Sukhorukov, Yuri S. Kivshar, Physics Reports, Volume 518, Issues 1-2, September 2012, Pages 1-7 [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R22.00012: Comparison of Active and Passive Approaches for Controlling the Near-Field Optical Path of Guided-Light Wave Danhong Huang, Michelle Easter, David Wellems, Henry Mozer, Alexei Maradudin, Godfrey Gumbs, Dave Cardimona Both active and passive approaches are proposed and compared for controlling the optical path of $p$-polarized light wave guided through a surface-patterned metallic structure with sub-wavelength features. For active control, the dynamical role of photo-excited electrons in a slit-embedded atomic system with field-induced transparency (FIT) is demonstrated for modulating transmitted-light intensity in the near-field region. Additionally, the strong coupling between the optical transitions within slit-embedded FIT atoms and the surface-plasmon modes in a metallic slit array is found. For passive control, on the other hand, a geometrical effect is demonstrated for focused transmitted light passing through a Gaussian-shaped metallic lens embedded with an array of slits. This geometrical effect is further accompanied by a swing of the light-focusing pattern in the near-field region as the incident angle is increased, as well as by the reduction of an anomalous light refraction due to higher-order diffraction modes at longer wavelengths and larger incident angles. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R22.00013: Gain/loss induced localization in low-dimensional PT-symmetric models Felix Izrailev, Omar O. Vasquez-Candanedo We show that both loss (absorption) and gain (amplification) can induce the localization of eigenstates in low-dimensional models with PT-symmetric potentials. Main results are obtained for 1D tight-binding models and for bi-layered models widely used in optics. We analyze both closed and open models within a unique approach allowing us to reveal the mechanisms responsible for the onset of localization. Specific attention is paid to the interplay between the localization emerging due to weak disorder and the localization induced by gain and loss. The analytical results are compared with the direct computation of the spectrum and eigenstates (for closed models), as well as of the transport characteristics (for open models). Some of the found effects can be observed experimentally in PT-symmetric photonic devices. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R22.00014: Generalizing speed-of-light limitations to arbitrary passive linear media Aaron Welters, Steven Johnson We prove that well-known speed of light restrictions on electromagnetic energy velocity can be extended to a new level of generality, encompassing even nonlocal chiral media in periodic geometries, while at the same time weakening the underlying assumptions to only passivity and linearity of the medium (along with a transparency window, which ensures well-defined energy propagation). Surprisingly, passivity alone is sufficient to guarantee causality and positivity of the energy density (with no thermodynamic assumptions), in contrast to prior work which typically assumed the latter properties. Moreover, our proof is general enough to include a very broad range of material properties, including anisotropy, bianisotropy (chirality), nonlocality, dispersion, periodicity, and even delta functions or similar generalized functions. The results in this talk are proved using deep results from linear-response theory, harmonic analysis, and functional analysis. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R22.00015: The curvy photonics of squid camouflage Alison Sweeney, Amanda Holt, Morse Daniel, Dariusz Stramski Cephalopods (squids and octopuses) ubiquitously possess reflective structures in their skin composed of ``reflectin'' proteins. Although a few simple laminar, Bragg-stack type optical structures have been known in a handful of common squid species for some time, our extensive survey of optically active tissues of exotic deep-sea species has revealed complex, extended curvatures and topologies in dermal reflectors of these rarely-studied animals. Molecular deep-sequencing has revealed these structures also to be composed of reflectin-like proteins. Here we show a survey of some of these deep-sea reflector structures, and present evidence that each novel structure may be a transform of the radiance in the optical niche in the ocean where each of these species live, such that light reflecting off the sides of these animals in their specific ocean habitat resembles the light that would be transmitted through the animals if they were transparent, from many different viewing angles and possible ocean depths. [Preview Abstract] |
Session R23: Fractional Quantum Hall Theory II
Sponsoring Units: FIAPChair: Michael Manfra, Purdue University
Room: 325
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R23.00001: Interaction induced Landau level mixing in the fractional quantum Hall regime Inti Sodemann, Allan MacDonald We study Landau Level mixing in parabolic bands perturbatively to second order in the ratio of interaction to cyclotron energy, for the lowest ($N=0$) and first excited ($N=1$) Landau levels. The mixing is accounted for by constructing an effective Hamiltonian which includes two body and three body interactions. Our study builds upon two previous treatments~[1,2], using as a stepping stone the observation that the effective Hamiltonian is fully determined by the 2 and 3 body problems. For the $N=0$ problem we provide a compact and transparent derivation of the effective Hamiltonian using first quantization which captures a class of virtual processes omitted in earlier calculations of Landau-level mixing corrected Haldane pseudo-potentials. We will comment on potential application of our results for numerical studies.\\[4pt] [1] G. Murthy and R. Shankar, {\em PRB} {\bf 65}, 245309 (2002)\\[0pt] [2] W. Bishara and C. Nayak, {\em PRB} {\bf 80}, 121302 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R23.00002: Can Ohmic contact in quantum Hall systems be considered a voltage probe? Artur Slobodeniuk, Ivan Levkivskyi, Eugene Sukhorukov Ohmic contacts are crucial elements of mesoscopic systems, which have no clear theoretical description yet. We propose a model of the Ohmic contact with a finite capacitance $C$ attached to a quantum Hall edge channel. It is shown that in contrast to na\"ive expectations the fluctuations of currents originating at such contact have non-equilibrium statistics. Consequently, the Ohmic contact can be considered a ``voltage probe'' only for certain values of the system parameters. In particular, the distribution function of outgoing electrons is close to the equilibrium one if the contact's temperature is much larger than $e^2/2\pi C$. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R23.00003: Hamiltonian Formulation of the Hydrodynamics with Quantum Anomalies Gustavo Monteiro, Alexander Abanov The hydrodynamic limit of a charged massless chiral spinor under the presence of gauge field in 3 dimensions is consider in [1]. For this system, global gauge symmetry is anomalous. In order to satisfy the second law of thermodynamics, charge current and entropy flow have to be corrected. We present a Hamiltonian formulation of the relativistic hydrodynamics which accounts for these new terms; extending the analysis done in [2]. In this formulation, the limit when particles become massless can be performed in a straightforward way and it has the advantage of being the natural framework to quantization. We show that the Poisson's structure of the hydrodynamics of ideal relativistic fluid allows for a one-parameter deformation. The value of the parameter is fixed by quantum anomalies present in the underlying theory. This formulation allows for generalizations to hydrodynamics of systems with additional conserved quantities, and is found to be a higher dimensional analogous to quantum hall effect. [1] D.T. Son and P. Surowka, Phys.Rev.Lett. 103, 191601 (2009). [2] D.D. Holm and B.A. Kupershmidt, Phys Lett. 101A, 23 (1984) [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R23.00004: Quantum Hall viscosity of Hierarchy States Thors Hans Hansson, Mikael Fremling, Juha Suorsa We describe a strategy for calculating the odd, non-dissipative viscosity for hierarchical QH states. Using previously developed techniques for expressing the wave functions on the plane in terms of conformal blocks, we can in simple cases construct the corresponding torus wave functions and show that they have good modular properties. Under certain assumptions, the QH viscosity can be directly extracted from these wave functions, and in the simplest case of the $\nu=2/5$ Jain states, we have verified the result numerically. Our results are consistent with the general formula, given by Read, relating the QH viscosity to the average orbital spin of the electrons. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R23.00005: Extracting net current from an upstream neutral mode in the fractional quantum Hall regime Ron Sabo, Itamar Gurman, Moty Heiblum, Vladimir Umansky, Diana Mahalu Upstream neutral modes, counter propagating to charge modes and carrying energy without net charge, had been predicted to exist in some of the fractional quantum Hall states and were recently observed via noise measurements. Understanding such modes will assist in identifying the wavefunction of these states, as well as shedding light on the role of Coulomb interactions within edge modes. In this work, performed mainly in the 2/3~state, we placed a quantum dot a few micrometers upstream of an ohmic contact, which served as a ``neutral modes source.'' We showed the neutral modes heat the input of the dot, causing a net thermo-electric current to flow through it. Heating of the electrons led to a decay of the neutral mode, manifested in the vanishing of the thermo-electric current at T \textgreater\ 100mK. This setup provides a straightforward method to investigate upstream neutral modes without turning to the more cumbersome noise measurements. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R23.00006: Phase diagram of the composite fermion Wigner crystals Alex Archer, Kwon Park, Jainendra Jain The energies of the Wigner crystal (WC) phase and the fractional quantum Hall (FQH) liquid have been compared in the past at some special filling factors. We deduce in this work the phase diagram of the WC phase as a function of the general filling factor by considering: (i) the WC of electrons; (ii) WCs of composite fermions (CFs) carrying $2p$ vortices; and (iii) FQH states supporting WC of CF quasiparticles or CF quasiholes. In particular, we find that the re-entrant insulating phase between 1/5 and 2/9 is a WC of composite fermions carrying two vortices. To distinguish the CF Wigner crystal from the electron WC, we compute a number of properties, including shear modulus, magnetophonon and magnetoplasmon dispersions, and melting temperatures. The width dependence of the phase diagram is also studied. A technical innovation that makes these comparisons feasible is to model the WC as the thermodynamic limit of the Thomson crystal on the surface of a sphere, which minimizes the Coulomb energy of classical charged particles. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R23.00007: Hierarchy of fractional Chern insulators and competing compressible states Andreas M. L\"auchli, Zhao Liu, Emil J. Bergholtz, Roderich Moessner We study the phase diagram of interacting electrons in a dispersionless Chern band as a function of their filling. We find hierarchy multiplets of incompressible states at fillings $\nu$ = 1/3, 2/5, 3/7, 4/9, 5/9, 4/7, 3/5 as well as $\nu$ = 1/5, 2/7. These are accounted for by an analogy to Haldane pseudopotentials extracted from an analysis of the two-particle problem. Important distinctions to standard fractional quantum Hall physics are striking: absent particle-hole symmetry in a single band, an interaction-induced single-hole dispersion appears, which perturbs and eventually destabilizes incompressible states as $\nu$ increases. For this reason the nature of the state at $\nu$ = 2/3 is hard to pin down, while $\nu$ = 5/7, 4/5 do not seem to be incompressible in our system. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R23.00008: Fractional Chern Insulators beyond Laughlin states Cecile Repellin, Tianhan Liu, B. Andrei Bernevig, Nicolas Regnault We report the first numerical observation of composite fermion (CF) states in fractional Chern insulators (FCI) using exact diagonalization. The ruby lattice Chern insulator model for both fermions and bosons exhibits a clear signature of CF states at filling factors $2/5$ and $3/7$ ($2/3$ and $3/4$ for bosons). The topological properties of these states are studied through several approaches. Quasihole and quasielectron excitations in FCI display similar features as their fractional quantum hall (FQH) counterparts. The entanglement spectrum of FCI groundstates shows an identical fingerprint to its FQH partner. We show that the correspondence between FCI and FQH obeys the emergent symmetry already established, proving the validity of this approach beyond the clustered states. We investigate other Chern insulator models and find similar signatures of CF states. However, some of these systems exhibit strong finite size effects. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R23.00009: Crystal-symmetry preserving Wannier states for fractional Chern insulators Chao-Ming Jian, Xiao-Liang Qi Recently, many numerical evidences of fractional quantum anomalous states (FQAH states), i.e. the fractional quantum Hall states (FQH states) on lattice, when a band with non-zero Chern number (We refer to it as a Chern band) is partially filled. Some trial wavefunction of FQAH states can be obtained by mapping the FQH wavefunctions defined in the continuum onto the lattice through the scheme proposed in Ref. [1] in which the single particle Landau orbits in the Landau levels are mapped to the one dimensional Wannier wavefunctions (which is a plane wave on the other direction) of the Chern bands with Chern number C=1. However, this mapping will generically break the lattice rotational symmetry. In this talk, we shall present a modified scheme to accommodate the mapping with the lattice rotational symmetry. The wavefunctions constructed through this modified scheme should serve as better trial wavefunctions to compare with the numerics. The focus of the talk shall be mainly on the C4 rotational symmetry of square lattices. Related issues on C6 symmetry of honeycomb lattice and higher Chern number bands will be discussed. [1] X.-L. Qi, Phys. Rev. Lett. 107, 126803 (2011) [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R23.00010: Establishing non-Abelian topological order in Gutzwiller projected Chern insulators via Entanglement Entropy and Modular S-matrix Yi Zhang, Ashvin Vishwanath We use entanglement entropy signatures to establish non-Abelian topological order in a new class of ground states, the projected Chern-insulator wave functions. The simplest instance is obtained by Gutzwiller projecting a filled band with Chern number C=2 which may also be viewed as the square of the band insulator Slater determinant. We demonstrate that this wave function is captured by the $SU(2)_2$ Chern Simons theory coupled to fermions. In addition to the expected torus degeneracy and topological entanglement entropy, we also show that the modular S-matrix, extracted from entanglement entropy calculations, provides direct access to the peculiar non-Abelian braiding statistics of Majorana fermions in this state. We also provide microscopic evidence for the generalization (expected from the field theory), that the N$^{\rm th}$ power of a Chern number $C$ Slater determinant realizes the topological order of the $SU(N)_C$ Chern Simons theory coupled to fermions, by studying the $SU(2)_3$ and the $SU(3)_2$ wave functions. An advantage of projected Chern insulator wave functions over lowest Landau level wave functions for the same phases is the relative ease with which physical properties, such as entanglement entropy, can be numerically calculated using Monte Carlo techniques. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R23.00011: Galilean invariance and linear response theory for Fractional Quantum Hall Effect Andrey Gromov, Alexandre Abanov We study a general effective field theory of Galilean invariant two-dimensional charged fluid in external electro-magnetic and gravitational fields. We find that combination of the generalized Galilean [1] and gauge invariance implies nontrivial Ward identities between gravitational and electro-magnetic linear responses in the system. This identity appears to hold in all orders of gradient expansion and it generalizes the relation between Hall viscosity and Hall conductivity recently found by Hoyos and Son. We also check the relation in the case of free electrons with integer filling of Landau levels where corresponding linear responses can be calculated directly. \\[4pt] [1] Carlos Hoyos, Dam Thanh Son ``Hall Viscosity and Electromagnetic Response'' [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R23.00012: Critical behavior of the transport coefficients at the plateau-insulator transition in IQHE Juntao Song, Emil Prodan Using the non-commutative Kubo formula for disordered lattice systems, we mapped the conductivity tensor $\sigma(E_F,T)$ as function of Fermi level $E_F$ and temperature T for the disordered Hofstadter model. Convergence and accuracy tests indicate that the simulations can be used to investigate the critical behavior near the plateau-insulator transition. Our analysis provides the first quantitative theoretical confirmation of the well established experimental facts about the critical behavior: 1) The semicircle law for the components of the conductivity tensor; 2) The existence of the quantized Hall insulator state characterized by zero direct and Hall conductivities, but with Hall resistivity quantized at $h/e^2$; 3) Single scaling behavior with exponents that are consistent with previous studies. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R23.00013: Energy Scales of the Reentrant Integer Quantum Hall States in High Landau Levels Nianpei Deng, John Watson, Michael Manfra, Gabor Csathy The reentrant integer quantum Hall states (RIQHS) have been identified with the electronic bubble phases. These bubble phases are exotic electronic solids similar to the Wigner crystal, but have more than one electron per lattice site. Recently we reported the presence of a peak in the temperature dependent magnetoresistence of the RIQHSs and we have associated this peak with the onset of the RIQHSs. We found that, contrary to the predictions of the bubble theory, the onset temperatures of the RIQHSs in the third Landau level are much higher than those in the second Landau level. We have extended such measurements of the onset temperatures to several high Landau levels. In this talk we will discuss the orbital dependence of the onset temperatures of RIQHSs and we will compare these quantitative results to the predictions of the bubble theory. This work was supported by the DOE BES contract no. DE-SC0006671. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R23.00014: Boundary degeneracy of topological order states Juven Wang, Xiao-Gang Wen It is known that topological ordered states have degenerate ground states on compact space manifold. Its ground state degeneracy on higher genus Riemann surface is encoded by the fusion rules of the fractionalized quasipartcles and the genus number. Here we study topologically ordered states on space manifold with boundary. We find that Bulk-Edge correspondence is not a complete story - edge theory information may not be fully-determined by the bulk theory. Ground state degeneracy of boundary states depends on boundary gapping conditions. Take Abelian topological order as an example, K matrix Chern-Simons theory, the boundary ground state degeneracy counts the number of group elements in a discrete finite quotient group from anyon transport and fusion algebra. We compare this result to Toric code model, Levin-Wen string-net model and flux insertion argument. By glueing the edges of a non-compact manifold to make it compact, we go back to demonstrate bulk ground state degeneracy from edge theory viewpoint, in terms of the Betti number and homology group, such as 2+1 D Chern-Simons or higher dimensional B-F theory. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R23.00015: Monte Carlo Study of a $U(1)\times U(1)$ Loop Model with Modular Invariance Scott Geraedts, Olexei Motrunich We study a $U(1)\times U(1)$ system in (2+1)-dimensions with long-range interactions and mutual statistics. The model has the same form after the application of operations from the modular group, a property which we call modular invariance. Using the modular invariance of the model, we propose a possible phase diagram. We obtain a sign-free reformulation of the model and study it in Monte Carlo. This study confirms our proposed phase diagram. We use the modular invariance to analytically determine the current-current correlation functions and conductivities in all the phases in the diagram, as well as at special ``fixed'' points which are unchanged by an operation from the modular group. We numerically determine the order of the phase transitions, and find segments of second-order transitions. For the statistical interaction parameter $\theta=\pi$, these second-order transitions are evidence of a critical loop phase obtained when both loops are trying to condense simultaneously. We also measure the critical exponents of the second-order transitions. [Preview Abstract] |
Session R24: Focus Session: Recent Developments in Density Functional Theory I
Sponsoring Units: DCOMPChair: David Vanderbilt, Rutgers University
Room: 326
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R24.00001: Density-functional theory: time to move up? Invited Speaker: Nicola Marzari Materials' simulations based on density-functional theory (DFT) have become an extremely powerful and widely used tool for scientific discovery and technological advancement. Still, in the current approximations, they remain an imperfect tool for predicting materials' properties, with open and urgent challenges in the quest towards qualitative and quantitative accuracy. Several of these challenges stem from the remnants of self-interaction in the electronic-structure framework, leading to qualitative failures in describing some of the fundamental processes involved e.g. in energy applications - from charge-transfer excitations to photoemission spectra to the structure and reactivity of transition-metal complexes. I'll discuss these challenges in realistic case studies, and present a brief overview of some of our suggestions for possible solutions - including constrained DFT, DFT + onsite and intersite Hubbard terms, and Koopmans' compliant energy functionals. In particular, I'll highlight how Koopmans' compliant functionals point to a beyond-DFT formulation where both total energies and spectroscopic properties can be accounted for. Such framework will be illustrated with applications to real systems and with simplified models that can be solved exactly. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R24.00002: Self-Consistent Density Functional Including Long-Range van der Waals Interactions Nicola Ferri, Robert A. DiStasio Jr., Roberto Car, Matthias Scheffler, Alexandre Tkatchenko Van der Waals (vdW) interactions are significant for a wide variety of systems, from noble-gas dimers to organic/inorganic interfaces. The long-range vdW energy is a tiny fraction (0.001\%) of the total energy, hence it is typically assumed not to change electronic properties. Although the vdW-DF functional includes the effect of vdW energy on electronic structure [1], the influence of ``true'' long-range vdW interactions is difficult to assess since a significant part of vdW-DF energy arises from short distances. Here, we present a self-consistent (SC) implementation of the long-range Tkatchenko-Scheffler (TS) functional [2], including its extension to surfaces [3]. The analysis of self-consistency for rare-gas dimers allows us to reconcile two different views on vdW interactions: (i) Feynman's view that claims changes in the electron density and (ii) atoms separated by infinite barrier. In agreement with previous work [1], we find negligible contribution from self-consistency in the structure and stability of vdW-bound complexes. However, a closer look at organic/inorganic interfaces reveals notable modification of energy levels when using the SC-TS vdW density functional. [1] Thonhauser et al., PRB (2007). [2] Tkatchenko and Scheffler, PRL (2009). [3] Ruiz et al., PRL (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R24.00003: Efficient Oscillator-Based Approach for Polarizability and van der Waals Interactions Vivekanand Gobre, Robert A. Distasio, Jr., Roberto Car, Matthias Scheffler, Alexandre Tkatchenko The dynamic polarizability measures the response to an applied time-dependent electric field, and its accurate determination is crucial for van der Waals (vdW) interactions and other response properties. First-principles calculations of polarizabilities in principle require a computationally expensive explicit treatment of many-electron excitations, and are only applicable in practice to systems with less than about 100 atoms. In this work, we present an efficient parameter-free approach for calculating accurate frequency dependent polarizabilities for molecules with thousands of atoms, as well as periodic materials. This is achieved by the synergistic coupling of the Tkatchenko-Scheffler method [1], which accurately treats short-range hybridization effects, with the self-consistent screening equation from classical electrodynamics [2,3]. Using only the electron density and free atom reference, we obtain an accuracy of 7\% for both static polarizabilities and vdW coefficients for an extensive database of gas-phase molecules and crystals. We analyze the interplay of hybridization and long-range electrostatic screening effects on the polarizability. [1] Tkatchenko and Scheffler, PRL (2009), [2] Felderhof, Physica (1974), [3] Tkatchenko, DiStasio, Car, and Scheffler, PRL (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R24.00004: Range-separated approach to the RPA correlation applied to van der Waals bond and to diffusion of defects Fabien Bruneval The Random Phase Approximation (RPA) is a promising approximation to the exchange-correlation energy of Density Functional Theory (DFT), since it contains the van der Waals (vdW) interaction and yields a potential with the correct band gap. However, its calculation is computationally very demanding. We apply a range separation concept [1] to RPA and demonstrate how it drastically speeds up the calculations without loss of accuracy. The scheme is succesfully applied to a layered system subjected to weak vdW attraction and to address the controversy of the self-diffusion in silicon [2]. We calculate the formation and migration energies of self-interstitials and vacancies taking into account atomic relaxations. The obtained activation energies deviate significantly from the earlier calculations that were affected by the band gap problem and challenge some of the experimental interpretations [3]: the diffusion of vacancies and interstitials have almost the same activation energy.\\[4pt] [1] J. Toulouse, F. Colonna, and A. Savin, Phys. Rev. A \textbf{70}, 062505 (2004).\\[0pt] [2] F. Bruneval, Phys. Rev. Lett. \textbf{108}, 256403 (2012).\\[0pt] [3] H. Bracht, E. E. Haller, and R. Clark-Phelps, Phys. Rev. Lett. \textbf{81}, 393 (1998). [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R24.00005: Performance of Common Density Functional Methods for the N-Body Interaction Energies of Water Clusters Kenneth Jordan, Fangfang Wang Using an isomer of (H$_2$O)$_{16}$, which has been the subject of several earlier studies, we demonstrate that, in contrast to the commonly held view, the N-body expansion of the interaction energy evaluated at the MP2 level does not converge monotonically with increasing N. Moreover, comparison of the results of HF and MP2 calculations reveals that this unexpected behavior is primarily due to electron correlation effects. The results of various common density functionals are considered, and the implications of our results for various procedures for correcting DFT for dispersion will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R24.00006: Chemi- and Physisorption Together from a Semilocal Density Functional: Graphene on Ni (111) Jianwei Sun, Bing Xiao, Adrienn Ruzsinszky, John Perdew Conventional semilocal approximations of density functional theory at the level of local spin density approximation (LSDA) and generalized gradient approximations (GGA) are thought to lack the ability to describe weak interactions. This is well illustrated by the system of a graphene adsorbed on a Ni (111) surface, in which the graphene can adsorb on the Ni (111) surface chemically or physically at different distances. LSDA, the standard Perdew-Burke-Ernzerhof (PBE) GGA, and its variant designed for solids, PBEsol, miss the physisorption. We show improved descriptions for weak interactions from a newly-developed semilocal meta-GGA (MGGA)---that performs equally well for molecules, surfaces, and solids---by demonstrating its ability to capture both the chemisorption and the physisorption. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R24.00007: A generalized gradient approximation for the Coulomb energy Alberto Vela, Jorge Luis Rosas-Trigueros, Samuel B. Trickey, Jos\'e L. Gazquez In this contribution we generate, implement and fully test expressions for the Coulomb energy without explicit dependence of the electron density at two points in space. These approximate expressions depend solely on the density and its derivatives. The starting point is the implementation and testing of the gradient expansions suggested by Bartolotti and Parr that, to the authors' knowledge, have never been tried in molecules. One of the drawbacks of this approach is that its functional derivative diverges in finite systems. To circumvent this deficiency we will show results for a gradient expansion that incorporates several restrictions, among them to have a finite first functional derivative. Since the functionals are derived imposing some restrictions we call these functionals generalized gradient approximations to the Coulomb energy. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R24.00008: Analysis of the large reduced density gradient limit for the exchange energy Jose Gazquez, Jorge M. del Campo, Juan Pacheco-Kato, Sam Trickey, Alberto Vela Electronic structure calculations have become very important for the analysis, at the microscopic level, of a wide variety of systems in physics, chemistry and biology. The Kohn-Sham version of density functional theory has played a fundamental role in such development. In particular, the generalized gradient approximation (GGA) has proven to be a very useful tool in electronic structure studies of complex systems, because it leads to a reasonable description of many properties, at a moderate computational effort. However, it is desirable to improve beyond the actual limits of accuracy. In this work we will present an analysis of the GGA in the regions of small and large values of the reduced density gradient. Then, taking as starting point the PBE and RPBE functionals, the large reduced density gradient limit will be incorporated, in order to show that it induces small, but subtle changes that lead to a better description of several molecular properties. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R24.00009: Laplacian-based generalized gradient approximations for the exchange energy A.C. Cancio, Chris E. Wagner It is well known that in the gradient expansion approximation to density functional theory (DFT) the gradient and Laplacian of the density make interchangeable contributions to the exchange-correlation (XC) energy. This is an arbitrary ``gauge freedom'' for building DFT models, normally used to eliminate the Laplacian from the generalized gradient approximation (GGA) level of DFT development. We explore the implications of keeping the Laplacian at this level of DFT, in order to develop a model that fits the known behavior of the XC hole, which can only be described as a system average in a conventional GGA. We generate a family of exchange models that obey the same constraints as conventional GGA's, but which in addition have a finite-valued potential at the atomic nucleus unlike GGA's. These are tested against exact densities and exchange potentials for small atoms and finite jellium drops, and for constraints chosen to reproduce the PBEsol and the APBE variants of the GGA. We find that exchange energies of atoms can be reliably reproduced, by breaking the local (but not global) Lieb-Oxford bound. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R24.00010: An Exchange Energy Functional with a Derivative Discontinuity Rickard Armiento, Stephan Kuemmel We explore a way to impose a derivative discontinuity onto a semi-local energy functional in density functional theory, rather than a model potential. The derivative discontinuity is a property of exact exchange that states that the exchange potential may have a uniform discontinuous shift as the particle number passes through an integer. The lack of this property is a known major deficiency of current approximate semi-local exchange functionals. We obtain a closed form expression with a number of attractive properties that can be related to an improved description of charge transfer, overdelocalized orbital states, and band gaps, i.e., deficiencies that are commonly seen in applied use of DFT. Various tests of the construction are presented that clarify the relationship between these issues and the derivative discontinuity. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R24.00011: A Kinetic Energy Functional From the Airy Gas Model Alexander Lindmaa, Ann Mattsson, Rickard Armiento We present a density functional for the kinetic energy derived from the Airy gas model, which is a model system for an edge electron gas. Electronic edges are the regions in a system where the electronic density changes to become exponentially decaying, and the electron physics requires special consideration. The Airy model describes an electron gas around the classical turning point, where the electrons interact with a uniform forcefield, i.e., an effective linear potential along one of the spatial coordinates. A formally exact energy density is derived in terms of Airy functions and is parametrized to behave correctly in the Thomas-Fermi and von Weizs\"acker limits. In contrast to already existing kinetic energy functionals derived from the Airy gas, starting from a closed-form expression yields greater freedom in the choice of parametrization. Comparative tests between our and previous functionals are presented. Improved kinetic energy functionals are highly relevant in the context of orbital-free DFT (OF-DFT) as well as for applications at very high temperature. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R24.00012: Constraint-based, Non-empirical Parameterization of Generalized Gradient Approximation Kinetic Energy Functionals Debajit Chakraborty, Samuel Trickey, Valentin Karasiev Though we have developed constraint-based ``modified conjoint'' generalized gradient approximation forms for the orbital-free Kohn-Sham kinetic energy $T_s[n]$, strategies for parameterizing them without use of small training sets have remained elusive[1]. Here we discuss one possible way to eliminate that empiricism. We take the reparameterized Perdew-Burke-Ernzerhof exchange functional PBEmol [2], which is self-interaction free for the Hydrogen atom density $n_1$. We then constrain the Pauli-term kinetic energy ($T_\theta$ in $T_s=T_W + T_\theta$, with $T_W$ the von Weizs\"acker KE) to cancel the remaining spurious correlation energy $T_\theta[n_1] +E_{c,PBEmol}[n_1] =0$.We bound the functional by $T_W + T_{TF}$, with $T_{TF}$ the Thomas-Fermi KE and retain the original constraint that $T_\theta >0$. We report numerical results and findings for this procedure.\\[4pt] [1] Phys.\ Rev. B \textbf{80}, 245120 (2009);\\[0pt] [2] J.\ Chem.\ Phys.\ \textbf{136}, 104108 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R24.00013: Revised Thomas-Fermi Functional for Singular Potentials James Dufty, Samuel Trickey Approximations to the non-interacting free energy density functional that include the Thomas-Fermi (TF) functional, or a local density approximation, lead to singular densities for singular external potentials (e.g. Coulomb). We address this limitation of the TF approximation by a formal map of the exact functional for a given external potential onto a fictitious TF functional for an effective external potential. The latter functional is found to be a ``regularized'' version of the external singular potential, tempered by convolution with the finite temperature Lindhard response function. The result is a Thomas-Fermi approximation but with the singularity removed. Applications at high and low temperatures are described, including comparison with the Parr-Ghosh cusp-condition procedure for a non-singular TF density at zero temperature. [Preview Abstract] |
Session R25: Focus Session: Computational Studies of Heterostructures
Sponsoring Units: DCOMPChair: Shiwei Zhang, College of William and Mary
Room: 327
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R25.00001: Competition of magnetism and Kondo physics in heterostructures Invited Speaker: Simone Chiesa Heterostructures made of atomically thin strongly correlated materials have been the focus of intense experimental and theoretical study. We report on results obtained using an unbiased numerical technique on a simple model of a metal-magnetic insulator interface: a multilayer system governed by a tight-binding Hamiltonian in which the interaction is nonzero on one set of adjacent planes and zero on another. As the interface hybridization is tuned, magnetic and metallic properties undergo an evolution that reflects the competition between antiferromagnetism and (Kondo) singlet formation, in a scenario similar to that occurring in heavy-fermion materials. Remarkably, for a few-layer system at intermediate hybridization, a Kondo insulating phase results, where magnetic order and conductivity are suppressed in all layers. As more insulating layers are added, magnetic order is restored in all correlated layers except the one at the interface and no evidence of long-range magnetic order induced in the metallic layers is found. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R25.00002: Mechanisms of Electronic Reconstruction at Oxide Interfaces with 001 and 111 Orientation Invited Speaker: Rossitza Pentcheva Remarkably rich electronic behavior has been recently discovered at oxide interfaces ranging from two-dimensional conductivity, superconductivity and magnetism to confinement induced metal-to-insulator transitions. Most of the interest so far has been directed at 001 oriented interfaces as e.g. the ones between the two band insulators LaAlO$_{\mathrm{3}}$ and SrTiO$_{\mathrm{3}}$ or in superlattices containing the correlated metal LaNiO$_{\mathrm{3\thinspace }}$and the band insulator LaAlO$_{\mathrm{3}}$. However, 111 oriented superlattices promise to host even more exotic, possibly topological phases. Despite the difference in stacking with AO and BO$_{\mathrm{2}}$ planes of the perovskite ABO$_{\mathrm{3}}$ structure in 001 oriented superlattices versus AO$_{\mathrm{3}}$ and B layers in the 111 crystallographic direction, analogous effects such as polar discontinuity arise in both cases when the A and B cations are varied across the interface. Based on density functional theory calculations we will compare mechanisms of electronic reconstruction in 001 and 111 oriented superlattices. We will thereby focus on the effect of confinement, band filling, magnetic coupling, structural distortions and substrate strain. Work in collaboration with David Doennig and Warren E. Pickett. Funding by the German Science Foundation, SFB/TR80, is gratefully acknowledged. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R25.00003: Electronic properties of graphene-MoS2 contacts Brandon Cook, Kalman Varga Single layer MoS$_2$ is a two-dimensional semiconductor which has attracted interest due to its electronic and optical properties. However, experimental studies of the material are limited by poor contacts. Graphene, a two-dimensional semimetal, is often touted as an ideal contact material. We investigate graphene-MoS$_2$ contacts with first-principles calculations. The density functional calculations predict the possibility of good charge injection from graphene to the MoS$_2$. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R25.00004: Density of States and Magnetic Correlations at a Metal-Mott Insulator Interface Mi Jiang, George Batrouni, Richard Scalettar The possibility of novel behavior at interfaces between strongly and weakly correlated materials has come under increased study recently. In this paper, we use determinant Quantum Monte Carlo to determine the inter-penetration of metallic and Mott insulator physics across an interface in the two dimensional Hubbard Hamiltonian. We quantify the behavior of the density of states at the Fermi level and the short and long range antiferromagnetism as functions of the distance from the interface and with different interaction strength, temperature and hopping across the interface. Induced metallic behavior into the insulator is evident over several lattice spacings, whereas antiferromagnetic correlations remain small on the metallic side. At large interface hopping, singlets form between the two boundary layers, shielding the two systems from each other. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R25.00005: Numerical simulation study of inhomogeneous metal-semiconductor contact with discrete distribution of varying barrier heights patches Priyanka Kaushal, Subhash Chand The Poisson's equation and the drift diffusion equations were solved by numerical simulation to calculate the potential and electron and hole concentrations inside the bulk semiconductor near the metal-semiconductor contact. The current density was then estimated from the calculated potential and electron-hole concentrations using the continuity equations. The current as a function of bias was calculated by imposing external bias through the boundary condition during the numerical simulation using silicon parameters to obtain the current-voltage characteristics of metal-semiconductor contact. From the simulated current-voltage characteristics the diode parameters were extracted by fitting the current-voltage data into the thermionic emission diffusion current equation. The simulations were performed for the inhomogeneous metal-semiconductor contact having randomly distributed patches of varying barrier heights. The patch size was varied to see its effect of the current-voltage characteristics and the derived apparent barrier parameters. The derived barrier parameters were analyzed to study the effect of inhomogeneities on the current-voltage characteristics on metal-semiconductor contact. The simulations were carried out for discrete distribution of barrier height patches at the metal-semiconductor contact. It is observed that the apparent barrier height of the inhomogeneous contact decreases and ideality factor increases with increasing the deviation of barrier heights in the distribution. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R25.00006: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R25.00007: Plasma instability and wave propagation in gate-controlled semiconductor conduction channels Sergey Rudin, Greg Rupper The plasma wave in the conduction channel of a semiconductor heterostructure high electron mobility transistor is an electron density excitation, possible at frequencies significantly higher than the cut-off frequency in a short channel device. When the electron-electron collision limited mean free path is much smaller than the wavelength of the density variations, the electron gas in the channel can be treated as a two-dimensional fluid. The flow is described by the Navier-Stokes equation and the heat conduction equation. The quality of the plasma resonance is limited by the electron mobility and the viscosity of the electron fluid. We use the hydrodynamic model derived as the balance equations from the quasi-classical Boltzmann equation, starting with a drifted Fermi-Dirac distribution as a zero order term in the expansion of the distribution function in orders of the Knudsen number. The charge flow can become unstable because of plasma wave amplification at the boundaries. The device then can be used as a tunable source of terahertz range radiation. We show that in such configuration the charge flow also develops shock waves due to hydrodynamic nonlinearities. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R25.00008: Investigation of the effect of core/shell interface on exciton binding energy and electron-hole recombination probability in CdSe/ZnS quantum dots Jennifer Elward, Arindam Chakraborty The explicitly correlated configuration interaction (XCCI) method is a variational technique in which an explicitly correlated reference wavefunction is used for performing the CI calculations. This work presents a multi-faceted study of the effect of heterojunction in nanoparticles and detailed analysis of various influential factors. The XCCI method was used for the study and the calculations were performed in three stages. In stage 1, the CdSe core was kept at a fixed size and the ZnS shell thickness was increased. In stage 2, the dot size was kept fixed and volume ratio between the core and the shell was varied. In stage 3, the sharpness of the core/shell interface was investigated by performing calculations on a core/alloy/shell system. Exciton binding energy (EB) and electron-hole recombination probability (eh-RP) were computed and the results were compared with CdSe quantum dots with similar radii. The presence of the heterojunction was found to effect the scaling of EB and eh-RP as a function of dot size. It was also found that EB and eh-RP scale very differently with respect to dot sizes. Expectation value of $r_{\mathrm{eh}}$ and radial 2-particle eh-reduced density matrix were used for analysis of spatial distribution of the quasiparticles in the multilayered qdots. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R25.00009: Effects of Nonlocal Exact Exchange on Electrons in Core/Shell Nanowires Bryan Wong, Andrew Long The unique properties of semiconducting heterostructure nanowires hold great promise for their incorporation in next-generation transistors, circuits, and nanoscale devices. The reduction in dimensionality produced by confining electrons in these heterostructure nanowires results in a dramatic change in their electronic structure, leading to novel properties such as ballistic transport and conductance quantization. In order to understand the formation of electron gases in core-shell nanowires, we developed a new pseudospectral approach for incorporating many-body, nonlocal exact exchange interactions within a self-consistent Schrodinger-Poisson formalism. Our approach is efficiently implemented in the open-source software package PAMELA (Pseudospectral Analysis Method with Exchange {\&} Local Approximations) that can calculate electronic energies, densities, wavefunctions, and band-bending diagrams. Furthermore, in order to present a general-purpose set of tools that both experimentalists and theorists can easily use to predict electron gas formation in core-shell nanowires, we document and provide our efficient and user-friendly PAMELA source code that is freely available at http://alum.mit.edu/www/usagi. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R25.00010: Kondo screening and Magnetism at Interfaces Axel Euverte, George Batrouni, Simone Chiesa, Richard Scalettar As clean heterostructures synthesis and analysis become experimentally accessible, the question of the nature of magnetic and transport properties at correlated interfaces arise. We study a simple Hubbard model of an interface between a metal and an antiferromagnetic insulator using a finite temperature quantum Monte Carlo method. Focusing on the effect of the hybridization at the interface, we show the singlet formation leads in thin systems to an intermediate non-magnetic insulating phase that involves metallic and correlated layers that are not in direct contact with each other. In thicker heterostructures, magnetic proximity effect of correlated layers farther from the interface defeats the formation of that intermediate phase. The large hybridization case is also discussed, showing decoupling of outer layers from the singlet interface. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R25.00011: Engineering three dimensional topological insulator in layered heterostructures T. Das, A.V. Balatsky We show that three dimensional topological insulator can be designed artificially via staking layers of two-dimensional Fermi gases (2DEGs) with finite inter-layer tunneling. The approach is based on stacking bilayers of Rashba-type spin-orbit coupled 2DEG with opposite spin-orbit coupling on opposite planes of bilayers. Spin Orbit interaction locks electronic states with respective spin projections, i.e.$+$/-a(k*s) with `a' is the Rashba-spin-orbit coupling strength, `k' is the momentum, and `s' is Pauli matrices for spin. We find that in the stack of bilayers grown along (001)-direction, a topological phase transition occurs above a critical number of Rashba-bilayers, with formation of a single spin-polarized Dirac cone at the $\backslash $Gamma-momentum . This approach offers a path to design artificial topological insulators in a set up that takes full advantage of atomic layer deposition approach, is free from crystal geometry, and is tunable. Work is supported by US DOE and Nordita. [Preview Abstract] |
Session R26: Foundations of Quantum Theory
Sponsoring Units: GQIChair: Caslav Brukner, University of Austria
Room: 328
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R26.00001: Distinct Quantum States Can Be Compatible with a Single State of Reality Peter Lewis, David Jennings, Jonathan Barrett, Terry Rudolph Perhaps the quantum state represents information available to some agent or experimenter about reality, and not reality directly. This view is attractive because if quantum states represent only information, then wave function collapse is possibly no more mysterious than a Bayesian update of a probability distribution given new data. Several other ``puzzling'' features of quantum theory also follow naturally given this view. In order to explore this idea rigorously, we consider models for quantum systems with probabilities for measurement outcomes determined by some underlying physical state of the system, where the underlying state is not necessarily described by quantum theory. In our model, quantum states correspond to probability distributions over the underlying states so that the Born rule is recovered. More specifically, we consider models for quantum systems where several quantum states are consistent with a single underlying state--i.e., probability distributions for distinct quantum states overlap. Recent work shows that such a model is impossible (e.g. the PBR theorem (Nat. Phys. 8, p.474)). Significantly, our example demonstrates that non-trivial assumptions (beyond those required for a well-defined realistic model) are necessary for the PBR theorem and those like it. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R26.00002: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R26.00003: Our Current Concept of Locality May be Incomplete Armin Nikkhah Shirazi The predictions of Bell's inequalities, and their subsequent experimental verification in the form of correlations between spacelike separated events have led to the prevailing current view that `nature is non-local'. Here we examine the possibility that our current concept of locality may at present not be sufficiently differentiated, and that by using 'nature' synonymously with `spacetime' we may have missed an implication of special relativity which by rendering a more complete conception of locality permits such quantum correlations without either hidden variables or violations of locality. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R26.00004: No Drama Quantum Theory? Andrey Akhmeteli Is it possible to offer a ``no drama'' quantum theory? Something as simple (in principle) as classical electrodynamics - a theory described by a system of partial differential equations (PDE) in 3+1 dimensions, but reproducing unitary evolution of a quantum field theory in the Fock space? The following results suggest an affirmative answer: 1. The scalar field can be algebraically eliminated from scalar electrodynamics; the resulting equations describe independent evolution of the electromagnetic field (EMF). 2. After introduction of a complex 4-potential (producing the same EMF as the standard real 4-potential), the spinor field can be algebraically eliminated from spinor electrodynamics; the resulting equations describe independent evolution of EMF. 3. The resulting theories for EMF can be embedded into quantum field theories. Another fundamental result: in a general case, the Dirac equation is equivalent to a 4th order PDE for just one component, which can be made real by a gauge transform. Issues related to the Bell theorem are discussed. A. Akhmeteli, Int'l Journal of Quantum Information, Vol. 9, Suppl., 17-26 (2011) A. Akhmeteli, Journal of Mathematical Physics, Vol. 52, 082303 (2011) A. Akhmeteli, quant-ph/1111.4630 A. Akhmeteli, J. Phys.: Conf. Ser., Vol. 361, 012037 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R26.00005: A realist, ``local,'' ``hidden variable'' model of quantum mechanics without observers William Sulis The violation of Bell type inequalities hinges upon the non-Kolmogorov nature of quantum probability structures. I show that a Process theory based, game theoretic formulation of quantum mechanics admits non-Kolmogorov probability structures. This formulation is realist, discrete and local at the level of space-time events while having nonlocal properties at the process level. These nonlocal effects respect relativistic constraints. Solutions to the Schrodinger equation arise through sinc interpolation of local samples generated by local path integral calculations based upon local information. Nonrelativistic quantum mechanics emerges in the continuum limit with perfect information transfer. This model avoids Kochen-Specker type restrictions and violates Bell and Leggett-Garg type inequalities. This formulation will be illustrated with a model of the classical two slit experiment. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R26.00006: Quantized Energy Spectrum of a Linear Classical Harmonic Oscillator in Classical Electromagnetic Zero-Point Radiation Wayne Huang, Herman Batelaan Since the early development of Quantum Mechanics, the discrete atomic spectra have been considered as the defining feature of Quantum Mechanics. However, when the classical electromagnetic zero-point radiation is introduced as a modification to Classical Mechanics, our simulation shows that a linear classical harmonic oscillator, when excited by a laser pulse, can exhibit an integer spaced energy spectrum just as its quantum counterpart. This finding may be surprising given the use of a fully classical theory, and it may help us identify the true quantum features in physical systems such as harmonic oscillator and ultimately atoms. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R26.00007: Normalized spacings between zeros of Riemann zeta function follow normalized Maxwell-Boltzmann distribution Siavash Sohrab Through \textit{Planck} relation $\varepsilon =$ h$\nu $ normalized spacings between energy levels of oscillators are related to those between frequencies expressed as \textit{Gauss} clock calculator or \textit{Hensel} p$_{\mathrm{j}}$-adic numbers. Energy-level spacings are related to spacings between ``stationary states'' and through \textit{Euler} golden key to zeros of \textit{Riemann} zeta function. The latter are shown to follow normalized \textit{Maxwell-Boltzmann} (NMB) distribution function, \begin{equation} \rho_{\beta} = (8/\pi_{\beta}) [(2/\sqrt{\pi_{\beta}} )x_{\beta} ]^{2} e^{-[(2/\sqrt{\pi_{\beta}} )x_{\beta}]^{2}} \end{equation} , hence providing physical explanations of \textit{Montgomery-Odlyzko} law and \textit{Hilbert-Polya} conjecture. Position of the critical line is found to coincide with that of stationary states. Normalized spacing between eigenvalues of GUE of an \textit{Adele} space constructed by superposition of infinite NMB distribution functions will coincide with spacing of zeros of \textit{Riemann} zeta function according to the theory of noncommutative geometry of \textit{Connes}. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R26.00008: Shape Invariance in Deformation Quantization Constantin Rasinariu Shape invariance is a powerful solvability condition, that allows for complete knowledge of the energy spectrum, and eigenfunctions of a system. After a short introduction into the deformation quantization formalism, this work explores the implications of the supersymmetric quantum mechanics and shape invariance techniques to the phase space formalism. We show that shape invariance induces a new set of relations between the Wigner functions of the system, that allows for their direct calculation, once we know one of them. The simple harmonic oscillator and the Morse potential are presented as examples. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R26.00009: Entangled states associated with N-qubit GHZ paradoxes Mordecai Waegell, P.K. Aravind Many workers have generalized the original GHZ paradox by replacing the qubits in it by qudits and the three observers by an arbitrary number of observers. We point out that if one stays with qubits but allows an arbitrary number of observers, a large number of paradoxes are possible. Some of the paradoxes come in families that extend upwards to all numbers of qubits. The entangled states connected within these paradoxes come in a wide variety. We survey the different types of entangled states that occur and also discuss some of their applications. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R26.00010: Logical difficulty from combining counterfactuals in the GHZ-Bell theorems Louis Sica Since it depends on predictions of single sets of measurements on three particles, the Greenberger, Horne, Zeilinger (GHZ) theorem eliminates the sampling loophole encountered by the Bell theorem that requires a large number of observations to obtain a relatively small number of useful joint measurements. In evading this problem, the GHZ theorem is believed to have confirmed Bell's historic conclusion that local hidden variables are inconsistent with the results of quantum mechanics. The GHZ theorem depends on predicting the results of sets of measurements of which only one may be performed, i.e., counterfactuals. In the present paper, the non-commutative aspects of these unperformed measurement sequences are critically examined. Three classical examples and the logic of the GHZ construction are analyzed to demonstrate that combined counterfactual results of non-commuting operations may be logically absurd, and in general are logically inconsistent with performed measurement sequences that take non-commutation into account. The Bell theorem is also revisited in the light of this result. It is concluded that negative conclusions regarding local hidden variables do not follow from the GHZ and Bell theorems as historically reasoned. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R26.00011: Observation of a Fast Evolution in a Parity-time-symmetric System Chao Zheng, Liang Hao, Gui Lu Long In the parity-time-symmetric (PT-symmetric) Hamiltonian theory, the optimal evolution time can be reduced drastically and can even be zero. In this letter, we report our experimental simulation of the fast evolution of a PT-symmetric Hamiltonian in a nuclear magnetic resonance quantum system. The experimental results demonstrate that the PT-symmetric Hamiltonian system can indeed evolve much faster than the quantum system, and the evolution time can be arbitrarily close to zero. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R26.00012: Second law of thermodynamics for random walk of quantum particle in a presence of detectors Ivan Sadovskyy, Gordey Lesovik We test H-theorem for a several models of particle random walk. We study interaction with a reservoir/detectors and its influence on entropy and found entropy growing in the time for some models and behaving non-monotonically for the others. We discuss the details of the system-reservoir interaction (such as presence of the interference in the system and number of interactions with detector parts) and their impact on the monotonicity of entropy. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R26.00013: Analogy between optical interferometry and integer factorization inspires novel mathematical results Gabriel Seiden Prime factorization of integers is an outstanding problem in arithmetic with important consequences in a variety of fields, most notably cryptography. We explore the intriguing relationship between prime factorization and optical interferometry with the aim of obtaining novel analytic expressions for number-theoretic functions directly related to prime factorization [1]. [1] G. Seiden, Phys. Rev. A 85, 043842 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R26.00014: Glimpses of the quantal algebra in early papers on quantum mechanics Samir Lipovaca A closer reading of early papers on quantum mechanics reveals that the quantal algebra lies hidden beneath the surface. We will show from the standpoint of the quantal algebra that, in essence, Heisenberg came across the symmetric product of the quantal algebra in his remarkable 1925 paper, Born and Jordan limited a general Hamiltonian function to a linear form of the terms containing the symmetric product of the quantal algebra, and Dirac found that the most general operation d/dv is the Leibnitz identity of the quantal algebra. [Preview Abstract] |
Session R27: Focus Session: Quantum Error Correction and Decoherence Control II
Sponsoring Units: GQIChair: Andrew Landahl, Sandia National Laboratories
Room: 329
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R27.00001: Magic state distillation with low overhead Invited Speaker: Sergey Bravyi Most of error correcting codes used in fault-tolerant quantum computing permit an efficient implementation of high-fidelity encoded Clifford gates and Pauli measurements. On the other hand, implementation of encoded non-Clifford gates such as the $\pi/8$-rotation $T$ usually requires distillation of certain quantum software states known as ``magic states" and substantially increases the space and time overheads. To reduce the distillation overhead we propose a new family of stabilizer codes with an encoding rate $1/3$ that permit a transversal implementation of the $T$-gate on all logical qubits. The new codes are used to construct protocols for distilling high-quality magic states by Clifford group gates and Pauli measurements. The distillation overhead scales as $O(\log^\gamma{(1/\epsilon)})$, where $\epsilon$ is the output accuracy and $\gamma=\log_2{(3)}\approx 1.6$. Our techniques lead to a two-fold overhead reduction for distilling magic states with accuracy $\epsilon \sim 10^{-12}$ compared with the best previously known protocol. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R27.00002: Multilevel distillation of magic states for quantum computing Cody Jones We develop a procedure for distilling magic states used in universal quantum computing which requires substantially fewer resources than prior schemes. Our distillation circuit is based on a family of concatenated quantum codes with a transversal Hadamard operation which can distill the eigenstate of the Hadamard operator. A crucial result of this design is that low-fidelity magic states can be consumed to purify high-fidelity magic states to even higher fidelity, which we call ``multilevel distillation.'' We show numerically that there exist multilevel protocols such that the average number of magic states consumed to distill from error rate $\epsilon_{\mathrm{in}} = 0.01$ to $\epsilon_{\mathrm{out}}$ in the range $10^{-5}$ to $10^{-40}$ is about $14\log_{10}(1/\epsilon_{\mathrm{out}}) - 40$; the efficiency of multilevel distillation dominates all other reported protocols when distilling Hadamard magic states from initial infidelity 0.01 to any final infidelity below $10^{-7}$. These methods are an important advance for magic-state distillation circuits in high-performance quantum computing. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R27.00003: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R27.00004: Direct-to-Toffoli Magic-state Distillation Bryan Eastin In recently proposed quantum computing architectures, approximately 90\% of the required resources are consumed during the distillation of single-qubit magic-states for use in performing Toffoli gates. In this talk I describe how the overhead for magic-state distillation can be reduced by merging distillation with the implementation of Toffoli gates. The resulting routines distill single-qubit magic-states directly to Toffoli ancillae, each of which can be used without further magic to perform a Toffoli gate. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R27.00005: Magic state distillation protocols with noisy Clifford gates Peter Brooks A promising approach to universal fault-tolerant quantum computation is to implement the non-universal group of Clifford gates, and to achieve universality by adding the ability to prepare high-fidelity copies of certain ``magic states''. By applying state distillation protocols, many noisy copies of a magic state ancilla can be purified into a smaller number of clean copies which are arbitrarily close to the perfect state, using only Clifford operations. In practice, the Clifford gates themselves will be noisy, which can limit the efficiency of state distillation and put a floor on the achievable fidelity with the desired state. Recently, a number of new state distillation protocols have been proposed that have the potential to reduce the required resource overhead. I analyze these protocols and explore the tradeoffs between these different approaches to magic state distillation when noisy Clifford gates are taken into account. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R27.00006: Simulating Anyon Interference to Measure the Levin-Wen Plaquette Operator WeiBo Feng, N.E. Bonesteel, David DiVincenzo It may be possible to use the ground states of the Levin-Wen model for Fibonacci anyons as a non-Abelian surface code for fault-tolerant quantum computation [1]. To do this, it will be necessary to repeatedly measure the vertex and plaquette operators of the model to check for errors. Recently, two of us have constructed quantum circuits for performing such measurements [2]. Here we present an alternate measurement scheme based on simulating an interference experiment. This ``experiment'' can be thought of, roughly, as first inserting a pair of Fibonacci anyons with trivial total topological charge onto one edge of a plaquette, ``braiding'' one anyon all the way around the plaquette while the other remains fixed, and then either measuring the total topological charge of the two anyons or manipulating their state in a specific way. We construct explicit quantum circuits which can be used to simulate these processes and show how they can be used to measure the Levin-Wen plaquette operator on a quantum computer.\\[4pt] [1] R. Koenig, G. Kuperberg, and B.W. Reichardt, Ann. Phys. 325, 2707 (2010).\\[0pt] [2] N.E. Bonesteel and D.P. DiVincenzo, Phys. Rev. B 86, 165113 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:42PM |
R27.00007: Optimal control in presence of decoherence and measurement imperfections: Pure state preparation problem Invited Speaker: Alireza Shabani Quantum control is a key component in the mathematical toolbox for designing fault-tolerant quantum processors. It becomes important to find optimal control protocols for realistic experimental conditions. In this talk, I focus on quantum feedback control for preparing pure states as ideal resources for quantum computation and communication. I discuss how the optimal protocols under experimental imperfections can be different from the ones found under theoretical simplifications. The problem of our study is motivated by superconducting circuit QED proposals for quantum computation. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R27.00008: Surface code with decoherence: An analysis of three superconducting architectures Joydip Ghosh, Austin G. Fowler, Michael R. Geller We consider a realistic, multi-parameter error model and develop a methodology to connect logical error rates of a surface code architecture with single qubit coherence time (T1 or T2) for any realistic set of intrinsic parameters, such as state preparation, gate, and readout errors. The amplitude and phase damping are mapped to a diagonal Pauli ``depolarization'' channel via the Pauli twirl approximation. Three existing superconducting architectures are chosen and a numerical Monte Carlo simulation is performed to obtain the logical error rates. A leading order analytic model is also constructed that estimates the scaling behavior of logical error rates below threshold for small distances. Our results suggest that large-scale fault-tolerant quantum computation should be possible with existing state-of-the-art superconducting devices. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R27.00009: Surface Code Threshold in the Presence of Correlated Errors Eduardo Novais, Eduardo Mucciolo We study the fidelity of the surface code in the presence of correlated errors induced by the coupling of physical qubits to a bosonic environment. By mapping the time evolution of the system after one quantum error correction cycle onto a statistical spin model, we show that the existence of an error threshold is related to the appearance of an order-disorder phase transition in the statistical model in the thermodynamic limit. This allows us to relate the error threshold to bath parameters and to the spatial range of the correlated errors. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R27.00010: Surface code fidelity decay in the presence of a bosonic bath Pejman Jouzdani, Eduardo Mucciolo, Eduardo Novais The surface code is a promising quantum computing environment that provides topological protection against errors, ensuring that the distance of the code grows as the physical sizes of the system increases. It has been recently proposed that a surface code in contact with a bosonic bath experiences an effective evolution that induces an constrained Ising-like interaction between qubits. As the coupling to the bosonic bath increases, the system may undergo a transition where the fidelity decays substantially after one quantum error correction cycle even for non-error syndromes. We investigate the manifestation of such a transition by evaluating numerically the fidelity of a surface code qubit system with the proposed Ising interaction. We carry out exact calculations for small systems and perform a finite-size scaling analysis using a cluster mean-field approach. We find a significant change in the fidelity at coupling constant values compatible with the mean-field transition point. Calculations performed with complex coupling constants yield the same behavior for the fidelity. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R27.00011: Fast decoder for local quantum codes using Groebner basis Jeongwan Haah Based on arXiv:1204.1063. A local translation-invariant quantum code has a description in terms of Laurent polynomials. As an application of this observation, we present a fast decoding algorithm for translation-invariant local quantum codes in {\em any} spatial dimensions using the straightforward division algorithm for multivariate polynomials. The running time is $O(n \log n)$ on average, or $O(n^2 \log n)$ on worst cases, where $n$ is the number of physical qubits. The algorithm improves a subroutine of the renormalization-group decoder by Bravyi and Haah (arXiv:1112.3252) in the translation-invariant case. [Preview Abstract] |
Session R28: Interfaces
Sponsoring Units: GSNPChair: Mark Robbins, Johns Hopkins University
Room: 336
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R28.00001: Creep and stress relaxation induced by interface diffusion in metal matrix composites Yinfeng Li, Zhonghua Li An analytical solution is developed to predict the creep rate induced by interface diffusion in unidirectional fiber-reinforced and particle reinforced composites. The driving force for the interface diffusion is the normal stress acting on the interface, which is obtained from rigorous Eshelby inclusion theory. The closed-form solution is an explicit function of the applied stress, volume fraction and radius of the fiber, as well as the modulus ratio between the fiber and the matrix. It is interesting that the solution is formally similar to that of Coble creep in polycrystalline materials. For the application of the present solution in the realistic composites, the scale effect is taken into account by finite element analysis based on a unit cell. Based on the solution, a closed-form solution is also given as a description of stress relaxation induced by interfacial diffusion under constant strain. In addition, the analytical solution for the interface stress presented in this study gives some insight into the relationship between the interface diffusion and interface slip. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R28.00002: De-lamination and Pro-lamination of adhesive films on curved topographies Benny Davidovitch, Evan Hohlfeld Attaching a solid film onto a sphere (or other curved shape) generates elastic stresses in the film. If the spherical substrate is totally rigid, the film will delaminate when its area exceeds a small fraction of the curved substrate. In contrast, if the substrate is very soft (such as a liquid drop), it will deform beneath the film, suppressing stresses and avoiding delamination of the film. Our theoretical analysis predicts that for very thin films, another scenario emerges - the film remains attached, developing tiny wrinkles that allow relaxation of stress without macro-scale deformation of the spherical shape of the substrate. Furthermore - as the film gets thinner, this predicted ``pro-lamination'' effect prevails parameter space, and should be observed for substrates with practically arbitrary stiffness. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R28.00003: Elastomer-glass detachment front observation Jos\'e Bico, Suomi Ponce, Benoit Roman When you peel an elastomeric band from a glass plate, the force needed changes with respect to the peeling angle and it is proportional to the width of the band [1]. As you approach to a zero angle, the process changes abruptly. Here we present the experimental study of a lap-test made on PVS elastomer and glass. We propose a simple imaging technique to observe the detachment front propagation.\\[4pt] [1] K. Kendall, ``Thin-film peeling - the elastic term'' Journal of Physics D : Applied Physics, vol. 8, p. 1449, Sep 1975. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R28.00004: Stick-slip during the peeling of adhesive tape Marie-Julie Dalbe, Stephane Santucci, Loic Vanel, Pierre-Philippe Cortet Using a high-speed camera, we study the instable peeling dynamics of an adhesive tape pulled at an imposed controlled velocity - focusing on the stick-slip regime of the peeling. Thanks to high-resolution fast camera, we can observe directly the peeling point motion and thus quantify the details of the stick and slip phases. To study properly the influence of peeling angle on stick-slip dynamics, we have developed an original experimental set-up where we are able to control the peeling angle while peeling the adhesive from a plane substrate. In this geometry, we extracted the stick and slip periods and studied their evolution with the peeling speed V, the length between the detachment zone and the peeling motor L, and the peeling angle $\theta$. We observe that the stick and slip periods increase non-linearly with L. We report various regimes depending on V, with periods of Stick and Slip either independent or proportional to V. These experiments confirmed that the physics of adhesive peeling is strongly dependent on $\theta$, especially in the Stick-Slip regime. This general feature questions the correct fracture criterion to consider at the peeling point in order to model the Stick-Slip adhesive peeling. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R28.00005: Crack propagation on curved surfaces Melissa Fender, Vinzenz Koning, Vincenzo Vitelli, William T.M. Irvine We investigate the propagation of cracks on curved surfaces. Using a stretched elastic sheet situated at a fluid interface, we generate a surface with spatially varying curvature and observe the trajectory and dynamics of an induced crack. We interpret the results from our experiments using a combination of numerical simulation and analytical considerations. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R28.00006: Static and dynamic friction in sliding colloidal monolayers Andrea Vanossi, Nicola Manini, Erio Tosatti In a recent experimental breakthrough, the controlled sliding of 2D colloidal crystals over perfectly regular, laser generated periodic or quasi-periodic `corrugation` potentials has been realized in Bechinger's group [1]. Based on realistic MD simulations which reproduce the main experimentally observed features, we explore the potential impact of colloid monolayer sliding in nanotribology [2]. The free motion of edge-spawned kinks and antikinks in smooth incommensurate sliding is contrasted with the kink-antikink pair nucleation at the large static friction threshold in the commensurate case. The Aubry pinning/depinning transition is also demonstrated, e.g., as a function of the corrugation amplitude. Simulated sliding data allow the extraction of frictional work directly from particles coordinates and velocities as a function of classic friction parameters, primarily speed, and corrugation strength. Analogies with sliding charge-density waves, driven Josephson systems, sliding of rare gas islands, and other novel features suggest further experiments and insights, which promote colloid sliding to a novel friction study instrument [3]. [1]T. Bohlein et al, Nature Mat. 11, 126 (2012) [2]A. Vanossi et al, PNAS 109, 16429 (2012) [3]A. Vanossi, E. Tosatti, Nature Mat. 11, 97 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R28.00007: Contact mechanics of rough spheres Lars Pastewka, Mark Robbins We use large scale numerical calculations to study the contact mechanics of rough spheres on flat elastic solids. Such geometries are encountered in systems that range from ball bearings to atomic force microscope tips, but the influence of roughness is seldom considered explicitly. Our calculations show that the contact area $A$ grows linearly with load $N$ at small loads and crosses over to Hertzian behavior $A \propto N^{2/3}$ at large loads. The total contact stiffness is defined as $K = dN/dz$ where $z$ is the normal displacement of the sphere. It shows power-law $K \propto N^{\alpha}$ behavior at all loads with an exponent $\alpha $ that is close to the value of $1/3$ expected from Hertzian contact mechanics. The results are discussed in the context of recent theories for flat rough contacts [1] and Greenwood-Williams theory as modified for spherical contacts [2]. \\[4pt] [1] B.N.J. Persson, J. Chem. Phys. 115, 3840 (2001); S. Hyun, L. Pei, J.-F. Molinari, M.O. Robbins, Phys. Rev. E 70, 026117 (2004); S. Akarapu, T. Sharp, M.O. Robbins, Phys. Rev. Lett. 106, 204301 (2011) \\[0pt] [2] K.L. Johnson, Contact Mechanics, Cambridge University Press, 1987 [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R28.00008: Effects of atomic-scale geometry in contact of rough surfaces Tristan A. Sharp, Lars Pastewka, Mark O. Robbins There has been great recent progress in continuum models of the effect of roughness on the area, friction, and stiffness of contacts between two solids. This talk will use molecular dynamics simulations to study how atomic scale features on surfaces can affect contact properties. Beginning from the established case of continuum linear elasticity that gives a linear relationship between real contact area and load, we systematically introduce atomic-scale physics to determine the affects on contact. Replacing an ideal linear isotropic elastic medium with a harmonic atomic lattice produces only small changes in the mechanical response. For more realistic interactions, anharmonicity and plasticity typically increase the contact area. The atomic steps present on rough crystal lattices lead to increased plasticity and change the small scale structure of contacts. Depending on the tendency for the material to yield, the presence of steps can increase or decrease the area of very high pressure, but steps always decrease the area of very low pressures. The large scale structure of the contact is the same for all cases. Application of continuum contact theories to surfaces with atomic-scale features will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R28.00009: Brittle Fracture In Disordered Media: A Unified Theory Ashivni Shekhawat, Stefano Zapperi, James Sethna We present a unified theory of fracture in disordered brittle media that reconciles apparently conflicting results reported in the literature, as well as several experiments on materials ranging from granite to bones. Our renormalization group based approach yields a phase diagram in which the percolation fixed point, expected for infinite disorder, is unstable for finite disorder and flows to a zero-disorder nucleation-type fixed point, thus showing that fracture has mixed first order and continuous character. In a region of intermediate disorder and finite system sizes, we predict a crossover with mean-field avalanche scaling. We discuss intriguing connections to other phenomena where critical scaling is only observed in finite size systems and disappears in the thermodynamic limit. We present a numerical validation of our theoretical results. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R28.00010: Molecular dynamics simulation investigations of atomic-scale wear Yuchong Shao, Michael Falk Frictional running-in and material transfer in wear take place at the micro- and nano-scale but the fundamental physics remain poorly understood. Here we intend to investigate wear and running-in phenomena in silicon based materials, which are widely utilized in micro/nano electromechanical systems(MEMS/NEMS). We use an atomic force microscopy (AFM) model composed of a crystalline silicon tip and substrate coated with native oxide layers. Molecular dynamics simulation has been performed over a range of temperatures, external loads and slip rates. Results show that adhesive wear takes place across the interface in an atom-by-atom fashion which remodels the tip leading to a final steady state. We quantify the rate of material transfer as a function of the coverage of non-bridging oxygen (NBO) atoms, which has a pronounced change of the system's tribological and wear behaviors. A constitutive rate and state model is proposed to predict the evolution of frictional strength and wear. This work is supported by the National Science Foundation under Award No. 0926111. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R28.00011: Elastic instabilities in perfect crystals: from planar dislocation-like modes to diffuse buckling-like modes Akanksha Garg, Asad Hasan, Craig Maloney We perform atomistic computer simulations of a model two dimensional perfect hexagonal crystal subjected to nano-indentation loading. For most crystallographic orientations, we find agreement with previous results for the case where the nearest-neighbor direction was perpendicular to the loading axis (cond-mat/1205.1700). In these orientations, the unstable mode takes the form of a sharply localized pair of atomic planes that slide relative to each other and form what is essentially a dipole of edge dislocations. The pair separation scales with the thickness of the film, $L$, and radius of the nanoindenter, $R$, in a non-trivial way that is independent of crystallographic orientation. For some crystallographic orientations with high surface energy, such as when the nearest-neighbor direction is co-incident with the loading axis, we find a new failure mode that emerges for very flat indenters and competes with the dislocation-like mode. The new diffuse failure mode is reminiscent of a buckling instability with a predominantly transverse character but exhibits both a nontrivial spatial extent and dominant wavelength that both depend on $L$ and $R$. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R28.00012: Defect mechanics in crystalline packings of spherical caps Amir Azadi, Gregory M. Grason Topological defects are ubiquitous in 2D curved crystals. We study the structural features and underlying principals of dislocation mechanics in a crystalline spherical cap. Using nonlinear elasticity, we show that frustration arising from the curvature drives the stability of finite length radial grain boundaries in the ground-state packing. For sufficiently large caps at intermediate Gaussian curvature, linear arrays of dislocations relieve the geometric stresses. The number and length of grain boundaries grows with both the curvature and the size of crystalline patch. We also determine the elastic response of the system subject to radial tension. The interplay between the geometrically induced stresses and the tension leads to inhomogeneous stresses that determines the stability of the grain boundaries. The imposed tension stretching the curved crystal radially destabilizes the curvature-induced compressive zone and decrease the length of the grain boundaries. We characterize the transition from a polycrystalline structure to the perfect packing where all dislocations will be expelled at a critical tension that depends on the system size and the curvature. We find scaling laws for the number and length of minimal configurations of grain boundaries. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R28.00013: Stick-slip nanofriction in cold-ion traps Davide Mandelli, Andrea Vanossi, Erio Tosatti Trapped cold ions are known to form linear or planar zigzag chains, helices or clusters depending on trapping conditions. They may be forced to slide over a laser induced corrugated potential, a mimick of sliding friction [1,2]. We present MD simulations of an incommensurate 101 ions chain sliding subject to an external electric field. As expected with increasing corrugation, we observe the transition from a smooth-sliding, highly lubric regime to a strongly dissipative stick-slip regime. Owing to inhomogeneity the dynamics shows features reminiscent of macroscopic frictional behaviors [3]. While the chain extremities are pinned, the incommensurate central part is initially free to slide. The onset of global sliding is preceded by precursor events consisting of partial slips of chain portions further from the center. We also look for frictional anomalies expected for the chain sliding across the linear-zigzag structural phase transition. Although the chain is too short for a proper critical behavior, the sliding friction displays a frank rise near the transition, due to opening of a new dissipative channel via excitations of transverse modes.\\[4pt] [1] A. Benassi et al, Nature Comm. 2, 236;\\[0pt] [2] T. Pruttivarasin et al, New Jour. of Phys. 13, 075012;\\[0pt] [3] S.M. Rubinstein et al, Nature 4, 1005. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R28.00014: Electronic friction at the atomic scale: Conduction, electrostatic and magnetic effects Jacqueline Krim We have performed a magnetic probe microscopy study of levitation and atomic-scale friction for Fe on YBCO (Tc $=$ 92.5K) in the temperature range 65 - 293 K, to explore electronic contributions to friction at the atomic scale. The samples were prepared with oxygen-depleted surfaces, with thin semiconducting surface layers present atop the bulk. Below Tc, the friction coefficient was observed to be constant at 0.19 and exhibited no correlation with the strength of superconducting levitation forces observed below Tc. The friction coefficient exhibited a change in slope within experimental error of Tc that increased progressively above Tc and reached 0.33 by room temperature. The results were analyzed within the context of underlying atomic-scale electronic and phononic mechanisms that give rise to friction we conclude that contact electrification and static electricity play a significant role above Tc.\\[4pt] [1] I. Altfeder and J. Krim, J. Appl. Phys. (2012) \textbf{111 }(9), art{\#}094916 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R28.00015: A hydrodynamic study of corner flow with leakage to orient dilute suspensions of ellipsoids Jonathan Bauer, Eric Furst The macroscopic characteristics of thin films are related to the microscale arrangement of the underlying particles. Directing the assembly of anisotropic colloids through the use of external fields, such as flow fields, can lead to materials with novel catalytic, transport, and optical properties. Such fields are used to bias particle orientation in solution before deposition onto a solid substrate. Corner flow with leakage, akin to the doctor blade used in the pulp and paper industry, is a solution-based, processing technique that has been used to create nanostructured materials. We present an analysis that describes how dilute suspensions of ellipsoids couple to this field. A Lagrangian and Eulerian perspective is necessary to identify regions with not only a high straining component but also a sufficient time scale for alignment. Trajectories that lie completely within these ``hot spots'' result in a distribution in which greater than 80\% of the particles have an angle less than 20$^{\circ}$ with respect to the flow direction. Our results can be used to describe previously reported trends of particle orientation in literature. Overall, our work gives a broader understanding of some of the difficulties associated with using flow fields to fully align ellipsoids in dilute suspensions [Preview Abstract] |
Session R29: Granular Materials: Phases, Flow, and Rheology
Sponsoring Units: GSNPChair: Mark Shattuck, City College of New York
Room: 337
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R29.00001: Study on a 2D system of granular particles under a cyclic shear Doycho Karagyozov, Abigail Polin, John Royer, Paul Chaikin Recent computer simulations of granular material under cyclic shear revealed an interesting phase diagram with ordered and disordered spatial and/or temporal patterns. The transition from reversible to irreversible dynamics in systems of many interacting particles is of a fundamental importance to many-body physics. To investigate these effects experimentally we built a two dimensional version where mono-disperse and poly-disperse disks are periodically driven in a parallelepiped shear cell. We track the particle dynamics and measure local packing changes as a function of shear amplitude and find diffusive behavior, localization of motion, reversibility, and macroscopic and microscopic limit cycles. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R29.00002: Avalanche Statistics in a Rotating Drum. Aline Hubard, Zhusong Li, Mark Shattuck We perform experiments in a quasi-two dimensional rotating drum. Two glass plates separated by about one particle diameter confine mono-disperse stainless-steel spheres to a cylindrical region. We rotate the system about the cylinder axis, which is perpendicular to gravity. Using high speed video up to 1000 fps we measure the particle positions during very slow rotation in which the flow is dominated by discrete avalanche events. We measure the avalanche size, duration, and time evolution (shape) for up to $10^{5}$avalanches and compare with DEM simulation and a mean field theory that predicts avalanche shape and a power-law distributions of size and duration. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R29.00003: Granular gases of rodlike grains in micro-gravity experiments Kirsten Harth, Kathrin May, Torsten Trittel, Sandra Wegner, Ralf Stannarius Understanding the dynamics of granular materials is relevant both in fundamental physics and from the technological point of view, but many well-known phenomena are still insufficiently understood. Granular gases are dilute ensembles of macroscopic grains, interacting by inelastic collisions. Permanent energy supply is required to maintain dynamic equilibrium. Granular gases of spherical grains have been widely investigated theoretically and in experiments in 2 dimensions. Microgravity is necessary for maintaining such a gas in 3 dimensions (3D). Only dynamics in the Knudsen-regime and clustering instabilities were accessible in previous experiments. Our experiment with rodlike grains offers access to statistical dynamics in the rod-rod collision dominated regime as well as the oppotunity to measure the rotational degrees of freedom of the particles. We present recent results from sounding rocket and drop-tower experiments. Ensembles of rods are confined in a 3D container, monitored by video cameras. Individual rods are tracked in consecutive frames. We analyse spatial and temporal density fluctuations, translational and rotational velocity distributions, the partition of kinetic energy and the influence of different experimental parameters. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R29.00004: Critical Phase Transitions in Vibrated Granular Media Geert Wortel, Olivier Dauchot, Martin van Hecke Granular media, such as sand, jam under low stresses but yield and flow when stressed sufficiently. We present experiments that uncover that weak vibrations qualitatively modify the nature of this yielding transition from 1st to 2nd order: when the vibration strength, which plays a role similar to temperature, is raised sufficiently, the yielding transition becomes continuous. At the critical point, we find diverging fluctuations, growing timescales and the emergence of a length scale: hallmarks of criticality never seen before in sand. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R29.00005: Collision Dynamics of Levitated Granular Clusters Justin Burton, Peter Lu, Sidney Nagel In a granular gas, inelastic collisions cause an initially homogeneous density of particles to evolve into discrete clusters consisting of many particles [1,2]. Further evolution of the system results from the collisions of particles within the clusters and from collisions between the separate clusters. In all of these regimes, however, experimental data is nearly non-existent due to the difficulty of creating a free gas of particles in a terrestrial environment. Here we report experiments of $\sim$ 200 particles moving on a two-dimensional, 90 x 90 cm, anodized aluminum plate. Our particles are composed of solid CO$_2$ disks with diameter $\sim$ 1.0 cm. When placed on a heated flat surface, the disks float on a cushion of sublimated gas, so that they move essentially without friction. The experiment is filmed from above so that particle velocities can be tracked. Our analysis from the collision of two clusters of particles reveals a sharp decrease in the total kinetic energy, which is weakly dependent on the restitution coefficient, and different velocity distributions parallel and perpendicular to the direction of impact.\newline\newline [1] I. Goldhirsch and G. Zanetti, Phys. Rev. Lett. 70, 1619 (1993).\newline [2] S. McNamara and W.R. Young. Phys. Rev. E 50, R28 (1994). [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R29.00006: The relationship between mechanically stable packings of frictional particles and low-dimensional saddle points of frictionless particles Tianqi Shen, Corey O'Hern, Mark Shattuck We perform computational studies of static packings of bidisperse frictionless and frictional disks. We show that there is a one-to-one correspondence between highly probable mechanically stable packings of frictional disks and low-dimensional saddle points for hard frictionless disks. To show this, we enumerate static packings of frictionless disks with one less contact than that required for mechanical stability $N_c = N_c^{\rm iso} - 1$. We find that the collection of these states forms lines in configuration space that emanate from the mechanically stable packings. Saddles with two missing contacts form branches that emanate from the one-missing-contact lines, and so on. For each saddle point, we calculate the minimum static friction coefficient $\mu_{\rm min}$ required to make each one mechanically stable. These studies allow us to calculate the allowed mechanically stable packings of frictional particles using MS packings of frictionless particles as a reference. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R29.00007: Extensional Rheology of Granular Staples Scott Franklin Collections of U-shaped granular materials (e.g. staples) show a surprising resistance to being pulled apart. We conduct extensional stress-strain experiments on staple piles with vary arm/spine (barb) ratio. The elongation is not smooth, with the pile growing in bursts, reminiscent of intruder motion through ordinary and rod-like granular materials. The force-distance curve shows a power-law scaling, consistent with previous intruder experiments. Surprisingly, there is significant plastic creep of the pile as particles rearrange slightly in response to the increasing force. There is a broad distribution of yield forces that does not seem to evolve as the pile lengthens, suggesting that each yield event is independent of the pile's history. The distribution of yield forces can be interpreted in the context of a Weibullian weakest-link theory that predicts the maximum pile strength to decrease sharply with increasing pile length. From this interpretation arise length and force scales that may be used to characterize the sample. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R29.00008: Particle jamming in the gap between a blade and boundary in a granular mixer Carl Wassgren, Shrikant Swaminathan, Jennifer Curtis, Bruno Hancock, Bill Ketterhagen The jamming of particles between the blade of a vertical axis mixer and a cylindrical container wall is examined. A single particle model is developed to understand the factors influencing jamming and experiments are performed to investigate jamming as a function of the mixing blade rotational speed, fill height, and gap width. For the range of angular speeds investigated, the rate at which jamming occurs is independent of the blade speed. The jamming rate is proportional to fill height for level fill heights less than twice the blade height, but remains constant for larger heights. This trend is the result of the blade not being completely covered by the particles for level fill heights less than approximately two blade heights due to the deformation of the surface during operation of the mixer. Jamming is a more complex function of the gap width. For gap widths less than a critical distance, which is a function of the particle-boundary friction coefficient as predicted by the single particle model, no jamming occurs. At the critical width, the rate of jamming increases abruptly to its maximum value. Increasing the gap width further decreases the jamming rate until at a gap width of approximately five particle diameters the jamming rate is zero. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R29.00009: Orientation Effects for Ellipses Flowing in a 2D Hopper Junyao Tang, Robert Behringer Hopper flow of disks has been extensively studied in the past decades. In this work, we investigate how ellipses (aspect ratio = 2) flow in a hopper. This study address the fact that many real-word examples of granular materials have ellipsoidal shapes. We use a quasi-two-dimensional hopper system with photoelastic ellipses so we can obtain stress/force information during the flow. Through synchronized data of particle tracking and stress, we can quantify the orientation of the force networks relative to the orientation of ellipses. The analysis shows that the ellipses which form the force chains have a strong orientation preference, particularly for force chains that form across the opening of the hopper and cause a jam. More generally, the relative orientation of ellipses plays an important role in controlling the flow rheology of ellipses. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R29.00010: Hopper Flow: Experiments and Simulation Zhusong Li, Mark Shattuck Jamming and intermittent granular flow are important problems in industry, and the vertical hopper is a canonical example. Clogging of granular hoppers account for significant losses across many industries. We use realistic DEM simulations of gravity driven flow in a hopper to examine flow and jamming of 2D disks and compare with identical companion experiments. We use experimental data to validate simulation parameters and the form of the inter particle force law. We measure and compare flow rate, emptying times, jamming statistics, and flow fields as a function of opening angle and opening size in both experiment and simulations. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R29.00011: Clogging in hopper flow and the kinetics of jamming Charles Thomas, Douglas Durian Understanding the time evolution of a system from an unjammed to a jammed state is a significant and open problem. The clogging of granular materials during hopper discharge is a quintessential example of a system undergoing such a process. When a hopper has a small opening, grains exit until a stable arch forms at the opening and a jamming front propagates up through the system. Conversely, hoppers with large enough openings do not clog. We define the clogging transition as the boundary in parameter space between those systems which can clog and those which will never clog. We have established experimental techniques for locating the clogging transition and describing the grain-scale behavior in hopper flow. We use these methods to study the approach to the clogging transition for a quasi-2D hopper. By tracking particle positions with a high-speed camera, we measure time-averaged velocity fields as well as velocity fluctuations. We have previously shown that systems which can clog exhibit elevated velocity fluctuations. We currently investigate the correlations between velocity fluctuations throughout the hopper as well as the size of dynamical heterogeneties as further promising grain-scale signatures of the approach to the clogging transition and of the kinetics of jamming. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R29.00012: Jamming to Clogging Transitions for Systems with Obstacle Arrays Charles Reichhardt, Cynthia Reichhardt, Zohar Nussinov Jamming can occur in systems consisting of collections of particles when the response of the system changes from a fluidlike state that can easily flow to a state that acts like a solid. For a loose collection of grains, jamming can occur as a function of density, where the grains readily flow at low densities but with increasing density undergo a transition to a jammed state at point J. Liu and Nagel have proposed that there may be a universal jamming phase diagram as a function of density, load, or temperature that may also include the glass transition. Here we propose that the density of fixed obstacles or quenched disorder can be considered as a new axis for the jamming phase diagram, since the disorder causes the system to jam at densities below point J. For a small number of obstacles, the system exhibits jamming behavior; however, for higher disorder density, there is a crossover to a behavior that we term clogging rather than jamming since the stuck states are highly heterogeneous, fragile, and exhibit memory effects. Our results imply that clogging is a distinct phenomenon from jamming with very different behaviors. These results are of relevance for particle flow in porous media, depinning transitions, and jamming in crowded environments. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R29.00013: Dynamic Jamming in Granular Polymers Lena Lopatina, Cynthia Reichhardt, Charles Reichhardt We present an extensive study of jamming behavior of two-dimensional granular polymers. In previous work, we showed that the nature of the jamming in granular polymer systems has pronounced differences from the jamming behavior observed for bidisperse two-dimensional disk systems at point J [1,2]. We found that the jamming density decreases with increasing length of the granular chain due to the formation of loop structures, in excellent agreement with experiments [3]. Now we present the response of the granular polymers to shear. At low densities, the system unjams independently of boundary conditions or shear rate. At high densities, for a slip wall the system develops plug flow with velocity equal to shear rate, while for a non-slip wall, the system develops a shear band and finite stress. We show that the stress asymptotes to a value that increases with increasing density and decreases with increasing shear rate. The latter is attributed to shear band changes from wide and migrating at low load to very narrow and localized at high load. [1] C. J. Olson Reichhardt and L. M. Lopatina, Science 326 (5951), 374 (2009). [2] L. M. Lopatina, C. J. Olson Reichhardt, and C. Reichhardt, Phys. Rev. E 84, 011303 (2011). [3] L.-N. Zou et al, Science 326 (5951), 408 (2009). [Preview Abstract] |
Session R31: Focus Session: Assembly & Function of Biomimetic & Bioinspired Materials II
Sponsoring Units: DMP DPOLY DBIOChair: Mark Stevens, Sandia National Laboratories
Room: 339
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R31.00001: Self-assembly of elastin-like polypeptides diblocks into micelles of various morphologies Wafa Hassouneh, Ekaterina Zhulina, Michael Rubinstein, Ashutosh Chilkoti Elastin-like polypeptides (ELPs) are a promising class of biopolymers for biomedical applications such as drug delivery. These biopolymers are composed of the pentapeptide repeat VPGXG, where X is any amino acid except proline. ELP diblocks, each block of which contains a different X residue composition, self-assemble into spherical micelles for certain lengths and ratios of hydrophobic and hydrophilic blocks. Our objective is to study morphological transitions, from spherical to cylindrical to lamellar structures, for the ELP diblock system by examining a wider range of diblock ratios and lengths. We employ a model that derives the phase boundaries of spherical-to-cylindrical and cylindrical-to-lamellar by balancing the corona elastic energy, the core elastic energy and the surface tension between the core and corona. Theoretical predictions from the model are compared with experimental results by independently measuring 1) surface tension at the core-corona interface and 2) second virial coefficient of the hydrophilic block monomer-monomer interaction. We report the measurements of these parameters and the initial comparison of experimental and theoretical phase boundaries for the ELP diblock system. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R31.00002: Design of biomimetic super-lubricants by hydrogel-biopolymer aggregates Raymond Seekell, Rachel Dever, Yingxi Zhu Inspired by the superb lubricity of natural synovial fluids for moving articular cartilage joints, we investigate a biomimetic artificial lubricant based on a hydrogel-biopolymer mixture with optimized rheological properties at a microscopic level. Specifically, we examine the structure and rheological relationship of stimuli-responsive poly (N-isopropylacrylamide) (PNIPAM) hydrogel added with hyaluronic acid (HA) to simulate the complexes of HA with a globule protein, lubricin, which are credited as the two key lubricious constituents in natural synovial fluids. By combined microscopic structural characterization and rheology measurement, we tune the rheological and frictional behaviors of HA solutions by optimizing the content of added micron-sized PNIPAM hydrogel particles to form stable PNIPAM-HA network. In a recent work on using zwitterionic hydrogel particles instead of negatively charged PNIPAM, comparable structure and rheological properties of hydrogel-HA aggregates are observed, which may give insight to design new biocompatible lubricants and lubricious coatings for medical ramification. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R31.00003: Forming Self-rotating Pinwheels from Assemblies of Oscillating Gels Debabrata Deb, Pratyush Dayal, Olga Kuksenok, Anna C. Balazs By using computational modeling, we show that millimeter-sized polymer gels undergoing the self-oscillating Belousov-Zhabotinsky (BZ) reaction not only respond to a chemical signal from the surrounding solution, but also emit this signal and thus, multiple neighboring gel pieces can spontaneously self-aggregate into macroscopic objects. We also show that the gels' coordinated motion can be regulated by light, allowing us to achieve selective self-aggregation and control over the shape of the gel aggregates, as well as reconfiguration of the entire structure. We find that the aggregated gel pieces can rotate as a unit. For example, four millimeter-sized gels can associate into a structure that resembles a pinwheel and then undergo spontaneous, autonomous rotation. With eight gel pieces, the system can form two pinwheels, which communicate and coordinate their motion. Notably, this communication can be controlled with light. In particular, light can be used to translate the pinwheels and to control the relative rotation of two such clusters. These findings reveal a new route for creating dynamically reconfigurable materials using self-oscillating BZ gels where reconfiguration is achieved by using auto-chemotatic behavior of the gels, and also applying external light. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R31.00004: Repairable, nanostructured biomimetic hydrogels M. Firestone, S. Brombosz, S. Grubjesic Proteins facilitate many key cellular processes, including signal recognition and energy transduction. The ability to harness this evolutionarily-optimized functionality could lead to the development of protein-based systems useful for advancing alternative energy storage and conversion. The future of protein-based, however, requires the development of materials that will stabilize, order and control the activity of the proteins. Recently we have developed a synthetic approach for the preparation of a durable biomimetic chemical hydrogel that can be reversibly swollen in water. The matrix has proven ideal for the stable encapsulation of both water- and membrane-soluble proteins. The material is composed of an aqueous dispersion of a diacrylate end-derivatized PEO-PPO-PEO macromer, a saturated phospholipid and a zwitterionic co-surfactant that self-assembles into a nanostructured physical gel at room temperature as determined by X-ray scattering. The addition of a water soluble PEGDA co-monomer and photoinitator does not alter the self-assembled structure and UV irradiation serves to crosslink the acrylate end groups on the macromer with the PEGDA forming a network within the aqueous domains as determined by FT-IR. More recently we have begun to incorporate reversible crosslinks employing Diels-Alder chemistry, allowing for the extraction and replacement of inactive proteins. The ability to replenish the materials with active, non-denatured forms of protein is an important step in advancing these materials for use in nanostructured devices [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R31.00005: Morphogenesis in Belousov-Zhabotinsky microdroplets Ning Li, Nathan Tompkins, Camille Girabawe, Irving Epstein, Seth Fraden We present experimental evidence for the six cases Alan Turing predicted using linear stability analysis in his 1952 paper ``The chemical basis of morphogenesis'' in our reaction diffusion system. Our experimental system consists of a microfluidically generated microemulsion consisting of Ru(bipy)3 catalyzed light sensitive BZ aqueous droplets which are diffusively coupled through oil gaps. We observed that some droplets grow and others shrink due to the unequal consumption of chemicals in the droplets which leads to an osmotic pressure change, as Turing predicted in his paper. The initial and boundary conditions of our system were controlled by programmable illumination via the light sensitive catalyst Ru(bipy)3. Simulation and linear stability analysis were performed and compared with the experiments. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R31.00006: Dynamic Elasticity Model of Resilin Biopolymers Xiao Hu, Solomon Duki Resilin proteins are `super elastic rubbers' in the flight and jumping systems of most insects, and can extend and retract millions of times. Natural resilin exhibits high resilience (\textgreater\ 95{\%}) under high-frequency conditions, and could be stretched to over 300{\%} of its original length with a low elastic modulus of 0.1-3 MPa. However, insight into the underlying molecular mechanisms responsible for resilin elasticity remains undefined. We report on the dynamic structure transitions and functions of full length resilin from fruit fly (D. melanogaster CG15920) and its different functional domains. A dynamic computational model is proposed to explain the super elasticity and energy conversion mechanisms of resilin, providing important insight into structure-function relationships for resilins, as well as other elastomeric proteins. A strong beta-turn transition was experimentally identified in the full length resilin and its non-elastic domains (Exon III). Changes in periodic long-range order were demonstrated during this transition, induced either by thermal or mechanical inputs, to confirm the universality of proposed mechanism. Further, this model offers new options for designing protein-based biopolymers with tunable material applications. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R31.00007: Stretching silk-elastin-like peptide polymers induces nucleation of amyloid nanofibers: Mechanistic study using time-lapse lateral force microscopy Nitinun Varongchayakul, Trina Quabili, Sara Johnson, Joonil Seog We studied the nucleation mechanism of silk-elastin-like peptide (SELP) nanofibers using lateral force microscopy. When a single line was repeatedly scanned on SELP coated mica surface, a sudden height increase was observed, indicating that the nucleus of amyloid fiber was formed during lateral scanning. The detailed analysis of frictional force profiles revealed that increase of frictional force was followed by a nucleus formation. The profile of increased frictional force was well fitted with exponential function, suggesting that AFM tip stretches multiple SELP molecules to the scanning direction. The probability of nucleus formation was highly dependent on the maximum level of increased frictional force, implying that the highly stretched SELPs are more likely to form nucleus for nanofiber growth. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:30PM |
R31.00008: Biopolymer networks in cells Invited Speaker: David Weitz This talk will discuss the role of biopolymer networks in cells. We probe their properties through measurements of fluctuating motions of particles within the cell. These motions have many similarities to thermal motion and, in fact, are often misinterpreted in the context of passive microrheology. Here, we demonstrate that the motion is, instead, driven by the presence of molecular motors within the cell, and we show how this motion can be interpreted quantitatively to determine the nature of the fluctuating forces in the cell due to the molecular motors. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R31.00009: Mechanisms and Dynamics of Collagen Assembly Jinhui Tao, Raymond Friddle, Debin Wang, Jim De Yoreo Collagen is the major structural protein of bone, dentine and it template the nucleation of biomineral phases. Both collagen conformation and architecture on substrate are critical for its function. We studied the mechanism of collagen I assembly on mica by in-situ AFM. At acidic condition, assembled architecture evolved from random fibers to co-aligned fibers and finally to bundles as the K$^{+}$ concentration increased from 100 to 300mM. XPS and NEXAFS showed the concentration of K$^{+}$ within the collagen layer increased and the intensity of absorption peak due to $\pi ^{\ast }$(C$=$O) resonance decreased with higher K$^{+\, }$concentration. The magnitude of collagen-mica (C-M) and collagen-collagen (C-C) interactions were measured by dynamic force spectroscopy. The free energy $\Delta $G$_{b}$ for C-M and C-C at 200mM K$^{+\, }$were 13.7kT and 1.4kT, while $\Delta $G$_{b}$ at 300mM K$^{+}$ were 5.7kT and 12.3kT, respectively. The switch from co-aligned fibers to 3D bundles is driven by the reversal in the magnitude of C-C and C-M interactions. Our results indicate K$^{+}$ complex with C$=$O of collagen and its effect on the strength of collagen-collagen bridging is the likely source of architecture control. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R31.00010: Atomistic modeling of bio-based polymeric fibers In-Chul Yeh, B. Christopher Rinderspacher, Jan W. Andzelm, LaShonda T. Cureton, John La Scala We performed molecular dynamics simulations on the amorphous phase of two bio-based polymers, poly (butylene furanamide) and poly (hexamethylene furanamide). Simulations of corresponding petroleum-based polymers, nylon 4, 6 and nylon 6, 6, were also performed. Glass transition temperatures estimated from a series of simulations were in good agreement with experimental measurements. Stress-strain relationships under uniaxial deformation were also analyzed. Bio-based polymers show higher glass transition temperatures and comparable yield points despite having overall weaker hydrogen bonds compared with their counterparts nylons. This result suggests that the furan ring plays an important role in the thermodynamic and mechanical properties of bio-based polymers. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R31.00011: Structural Properties of Silk Electro-Gels A.P. Tabatabai, J.S. Urbach, D.L. Blair, D.L. Kaplan The interest in \textit{Bombyx Mori} silk emerges from its biocompatibility and its structural superiority to synthetic polymers. Our particular interest lies in understanding the capabilities of silk electro-gels because of their reversibility and tunable adhesion. We create an electro-gel by applying a DC electric potential across a reconstituted silk fibroin solution derived directly from \textit{Bombyx Mori} cocoons. This process leads to the intermolecular self-assembly of fibroin proteins into a weak gel. In this talk we will present our results on the effects of applied shear on electro-gels. We quantify the structural properties while dynamically imaging shear induced fiber formation; known as fibrillogenesis. It is observed that the mechanical properties and microstructure of these materials are highly dependent on shear history. We will also discuss the role of surface modification, through micro-patterning, on the observed gel structure. Our results provide an understanding of both the viscoelastiticity and microstucture of reconstituted silks that are being utilized as tissue scaffolds. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R31.00012: Neural Stimulation via Fractal Electrodes Rick Montgomery, William Watterson, Ian Pilgrim, Kurtis Fairley, Darren Johnson, Heiner Linke, Richard Taylor A host of physical phenomena exhibit fractal geometry and benefit from its enhanced properties, which can include large surface area-to-volume ratios and high network connectivity. These properties are exploited in a fractal electrode designed for neural stimulation and recording. Presented are electric field studies of a fractal electrode with an emphasis on applications in retinal implants. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R31.00013: DFT-based prediction of geometric and thermodynamic parameters in the ATP to ADP hydrolysis reaction Mark C. Palenik, Jorge H. Rodriguez Studying covalent (chemical) and noncovalent (physical) mechanisms as well as key structural variations associated with ATP $\rightarrow$ ADP hydrolysis is of interest for understanding a multitude of biophysical and biochemical cellular processes. We have studied geometric variations of the ATP and ADP molecules during their hydrolysis reaction using density functional theory (DFT) with an implicit solvation model. We have computed the change in free energy, $\Delta$G, associated with the hydrolysis reaction and established relationships between key geometric parameters and thermodynamic properties. Our computed values for $\Delta$G were found in good agreement with available experimental data for two different sets of geometric conformations. A link is suggested between these values for $\Delta$G and changes in geometry of the ADP molecule. Of methodological and computational interest, we also determined that, while the conductor-like solvation model in the framework of the polarizable continuum model (C-PCM) was capable of producing biochemically meaningful geometries for ATP and ADP, it also displayed a strong preference for binding between the $H^+$ and $PO_4^{2-}$ ions formed during hydrolysis. [Preview Abstract] |
Session R32: Focus Session: Polymer Liquids and Glasses
Sponsoring Units: DPOLYChair: Rodney Priestley, Princeton University
Room: 340
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R32.00001: Scattering and Physical Aging in High-Free-Volume Polymeric Glasses Amanda G. McDermott, Peter M. Budd, Neil B. McKeown, Coray M. Colina, James Runt Polymers of intrinsic microporosity (PIMs) form glassy, rigid membranes featuring a large concentration of pores smaller than 1 nm, large internal surface area, and high gas permeability and selectivity. Porosity in these materials---equivalent to free volume---arises from an unusual chain structure combining rigid segments with sites of contortion. Like other glasses, PIMs are subject to physical aging, which reduces the permeability of films over time. Although it is possible to derive useful information such as surface areas and pore sizes from the scattering patterns of many porous materials, scattering from PIMs includes some unusual features. A robust interpretation of these features is presented with support from molecular dynamics simulations. The sensitivity of PIM SAXS/WAXS patterns to time, temperature and film thickness is shown to be qualitatively consistent with physical aging. Models for extracting quantitative information about changes in the sizes and volume fraction of pores are also discussed. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R32.00002: Stress Applied during Vitrification Influencing the Subsequent Physical Aging of Polymer Glasses Laura A.G. Gray, Connie B. Roth How stress and mechanical deformation impart mobility to glasses is an active area of study across a range of glassy systems from polymers and small molecules, to colloids and granular materials. Conceptual frameworks such as the jamming phase diagram have been proposed to investigate if stress acts as another independent variable similar to temperature and density (volume fraction). Existing studies have focused primarily on applying stress or strain to a glassy state that has been formed stress free. Here, we investigate the stability of polymer glasses when stress is applied during the formation of the glassy state. We have constructed a jig to apply a known stress to free-standing polymer films during the thermal quench. Ellipsometry is used to measure the physical aging rate of these stress-quenched polystyrene films transferred onto silicon wafers by quantifying the time-dependent decrease in thickness that results from an increase in average density during aging. We observe a transition to a faster aging rate for stresses applied above a critical threshold. We hypothesize that increased stresses may trap the glassy state into higher, less stable potential energy minima resulting in faster aging rates. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R32.00003: Quench, equilibration, and subaging in structural glasses Joerg Rottler, Mya Warren In the glassy state, structural relaxations become increasingly sluggish with the wait time $t_w$ since vitrification. While most theoretical models of aging predict that the relaxation times $t_\alpha$ should increase linearly with the wait time, results from both experiments and simulations are frequently better described by a sublinear scaling: $t_\alpha \sim t_w^\mu$, with an aging exponent $\mu<1$. We show with molecular dynamics simulations of a Lennard-Jones glass former at various temperatures that this apparent ``subaging'' behavior may be explained by crossover effects from the freshly quenched state at short $t_w$, and into the equilibrated state at long $t_w$. Additionally, the aging behavior on the molecular level is quantitatively reproduced by a coarse-grained continuous time random walk description over the entire range of temperatures and wait times. Since this model is formally equivalent to the well known trap model of aging, this suggests that the Lennard-Jones glass belongs to the ``full'' aging class $t_\alpha \sim t_w$. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R32.00004: Dynamics and thermodynamics of polymer glasses Invited Speaker: Daniele Cangialosi The dynamics and thermodynamics of glass-forming systems have been the subject of intense research in the last decades. Among the variety of aspects that have been analyzed, the following can be included: i) the dramatic slowing down of the dynamics when decreasing temperature often described by a Vogel-Fulcher-Tammann (VFT) law; ii) the possible connection between such slowing down and the thermodynamics of the glass-former. These aspects have been deeply investigated above the laboratory glass transition temperature ($T_g$). It has been speculated that mere extrapolation of the dynamics and thermodynamics to low temperatures produces a singularity at a finite temperature. In particular, extrapolating the behavior above $T_g$ to low temperatures would imply that: (i) the relaxation time associated to the glassy dynamics shows a divergence; (ii) the entropy of the glass equals that of the crystal. Experimental as well as theoretical efforts in the sub-$T_g$ regime are required to clarify whether this scenario really exists. Recent experimental studies indicate deviations of the relaxation time from the VFT behavior to a milder temperature dependence [1,2] and several theoretical approaches provide a rationale to such deviations [3-7]. In this contribution the temperature range of dynamics and thermodynamics is extended to temperatures as low as $T_g-$40 K by performing enthalpy recovery experiments on glassy polymers for times up to 10$^7-$10$^8$ seconds. We find a single stage recovery behavior for temperatures larger than about $T_g-$10 K. Interestingly, a double stage recovery is observed for $T <$ $T_g-$ 10 K. In all cases the enthalpy recovered after the two-stage decay approximately equals that extrapolated from the melt. Time-temperature superposition close to each plateau in the enthalpy delivers shift factors containing information on the dynamics below $T_g$. The following scenario emerges analyzing the temperature dependence of the shift factors: i) In both stages of recovery, Arrhenius temperature dependence of the shift factor is observed; ii) The shift factor corresponding to the first stage recovery exhibits relatively low activation energy (several times smaller than that of the $\alpha$ process at $T_g$); iii) The second stage exhibits activation energy similar to that of the polymer $\alpha$ relaxation at $T_g$. These results indicate that divergence of the relaxation time at a finite temperature is likely avoided, whereas the question of a thermodynamic singularity remains open.\\[4pt] [1] P. O'Connell and G. B. McKenna, J. Chem. Phys. {\bf 110}, 11054 (1999).\\[0pt] [2] S. Simon, J. Sobieski, and D. Plazek, Polymer {\bf 42}, 2555 (2001).\\[0pt] [3] I. Avramov and A. Milchev, J. Non-Cryst. Solids {\bf 104}, 253 (1988).\\[0pt] [4] K. S. Schweizer and E. Saltzman, J. Chem. Phys. {\bf 119}, 1181 (2003).\\[0pt] [5] T. Hecksher, A. I. Nielsen, N. B. Olsen, and J. C. Dyre, Nat. Phys. {\bf 4}, 737 (2008).\\[0pt] [6] J. C. Mauro, Y. Yue, A. J. Ellison, P. K. Gupta, and D. C. Allan, Proc. Natl. Acad. Sci. U. S. A. {\bf 106}, 19780 (2009).\\[0pt] [7] Y. S. Elmatad, D. Chandler, and J. P. Garrahan, J. Phys. Chem. B {\bf 113}, 5563 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R32.00005: Molecular mobility measurement during constant strain rate deformation of polymer glasses Benjamin Bending, Kelly Christison, M.D. Ediger We use a dye reorientation method to measure the segmental mobility in poly (methyl methacrylate) during active deformation. During constant strain rate deformation at 14 K below the glass transition we observe that mobility initially increases by up to a factor of 500, as compared to the starting undeformed mobility. After the softening regime the mobility remains constant as the strain is increased. Similar qualitative trends have been seen in simulations by Riggleman et al. and in the model of Govaert et al. Comparison of these simulations and model to our experiment will be the focus of this talk. In our previous studies of poly (methyl methacrylate) and polystyrene glasses deformed with a constant stress protocol (creep), at 16 K below the glass transition of the polymers we have seen a hundred-fold enhancement of mobility. Results from all these systems can be plotted on a master plot of mobility as a function of the local strain rate during creep deformation. We have found that this correlation holds for multiple glassy polymer systems, thermal and temporal histories, and with different deformation protocols. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R32.00006: Origin of mechanical stress from tensile extension of polymer glasses Panpan Lin, Shi-Qing Wang During uniaxial extension, polymer glasses undergo elastic deformation, yielding, strain softening, neck propagation, and ``strain hardening''. Both plasticity and anelasticity emerge under the large deformation, making the origin of the mechanic stress elusive to identify. The present work employs an IR camera to make \textit{in situ} temperature measurements on the extending specimen along with the conventional force measurements. To demonstrate the generality of our findings we studied the ductile polycarbonate as well as two brittle polymers, i.e., PS and PMMA, which can be made ductile by melt extension [1]. We found that the rate of heat generation is only a small fraction of the mechanical power involved in the uniaxial extension of these polymer glasses. Thus, it seems that the origin of the tensile stress is largely intrachain, stemming from straining of the chain network. \\[4pt] [1] Zartman, G. D.; Cheng, S. W.; Li, X.; Lin, F.; Becker, M. L.; Wang, S. Q. \textit{Macromolecules} \textbf{2012}, \textit{45}, 6719-6732. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R32.00007: Observation of yield in a triaxial deformation of a glassy thermoset polymer Grigori Medvedev, Jae-Woo Kim, James Caruthers Yield and post-yield behavior of amorphous polymers in a glassy state have been extensively studied in uniaxial tension and compression. In such geometry, the volume change is relatively small reaching the maximum value of approximately 0.5\% at the yield point. To study the role of the volumetric contribution a different geometry is needed. Here we report on the first observation of yield behavior in the longitudinal deformation, where the volume change is an order of magnitude higher than in the uniaxial test. The experiment is performed sufficiently close to Tg to ensure that yielding occurs before brittle failure. To characterize the evolution of the mobility/relaxation rate induced by deforming the glassy material through yield, a series of stress relaxation experiments are carried out at various pre-yield and post-yield strains. These experiments are done in uniaxial tension, compression and, for the first time in a longitudinal deformation. Implications of the observation of yield in a dilatation dominated deformation in addition to the traditional uniaxial and shear yield for the theories of glassy behavior and the development of constitutive models are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R32.00008: Evidence for non-diverging time-scales in glass-forming liquids Invited Speaker: Gregory McKenna One perceived important signature of the ``ideal'' glass transition and of the complex fluid nature of glass-forming liquids remains the apparent divergence of the dynamics at temperatures above zero Kelvin. Recently, however, this perception has been increasingly challenged both through experiments and in new theories of the dynamics of glass forming systems. In this presentation we summarize some of the prior evidence suggesting that time scales actually do not diverge in glasses that are aged into equilibrium, perhaps 15 K below the conventional glass transition temperature $T_{g}$. We then show new results from an extremely densified glass, 20 Ma old Jamaican amber, in which we were able to obtain the upper bound to the relaxation times through a step-wise temperature scan in which the stress relaxation response of the amber was measured both below and above the fictive temperature $T_{F}. $We find that in the case of the upper bound responses at \textit{T\textgreater T}$_{F}, $there is a strong deviation of the response from the Super-Arrhenius Vogel-Fulcher behavior and this persists to the fictive temperature which is some 33.8 K below $T_{g}$. The results are compared to the parabolic model of Chandler and co-workers and we find the model to be consistent with our results if the value of $T_{x}$ in the model is taken to be the calorimetric glass transition temperature. The significance of the results will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R32.00009: Collective effects on activated segmental relaxation in supercooled polymer melts Stephen Mirigian, Kenneth Schweizer We extend the polymer nonlinear Langevin equation (NLE) theory of activated segmental dynamics in supercooled polymer melts in two new directions. First, a well-defined mapping from real monomers to a freely-jointed chain is formulated that retains information about chain stiffness, monomer volume, and the amplitude of thermal density fluctuations. Second, collective effects beyond the local cage scale are included based on an elastic solid-state perspective in the ``shoving model'' spirit which accounts for longer range contributions to the activation barrier. In contrast to previous phenomenological treatments of this model, we formulate an explicit microscopic picture of the hopping event, and derive, not assume, that the collective barrier is directly related to the elastic shear modulus. Local hopping is thus renormalized by collective motions of the surroundings that are required to physically accommodate it. Using the PRISM theory of structure, and known compressibility and chain statistics information, quantitative applications of the new theory to predict the temperature and chain length dependence of the alpha time, shear modulus, and fragility are carried out for a range of real polymer liquids and compared to experiment. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R32.00010: The Relationship of Dynamical Heterogeneity to the Adam-Gibbs and Random First-Order Transition Theories of Glass Formation Francis Starr, Jack Douglas, Srikanth Sastry We examine measures of dynamical heterogeneity for a bead-spring polymer melt and test how these scales compare with the scales hypothesized by the Adam and Gibbs (AG) and random first-order transition (RFOT) theories. We show that the time scale of the high-mobility clusters and strings is associated with a diffusive time scale, while the low-mobility particles' time scale relates to a structural relaxation time. The difference of the characteristic times naturally explains the decoupling of diffusion and structural relaxation time scales. We examine the appropriateness of identifying the size scales of mobile particle clusters or strings with the size of cooperatively rearranging regions (CRR) in the AG and RFOT theories. We find that the string size appears to be the most consistent measure of CRR for both the AG and RFOT models. Identifying strings or clusters with the``mosaic'' length of the RFOT model relaxes the conventional assumption that the``entropic droplet'' are compact. We also confirm the validity of the entropy formulation of the AG theory, constraining the exponent values of the RFOT theory. This constraint, together with the analysis of size scales, enables us to estimate the characteristic exponents of RFOT. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R32.00011: Enthalpy Recovery of Polystyrene: Is the Liquid Equilibrium Line Reached? Yung P. Koh, Sindee L. Simon Glasses are not in thermodynamic equilibrium below the glass transition temperature (T$_{\mathrm{g}})$, and consequently, their properties such as enthalpy, volume, and mechanical properties evolve toward equilibrium in a process known as structural recovery or physical aging. However, several recent studies have suggested that the equilibrium liquid line is not reached even when properties have ceased to evolve. In this work, we present measurements of the enthalpy recovery of polystyrene at the aging temperature of 15$^{\circ}$C below the nominal T$_{\mathrm{g}}$, for aging times up to 1 year. The results are analyzed in the context of the TNM model of structural recovery. The results show that the equilibrium liquid enthalpy line is indeed reached at temperatures below T$_{\mathrm{g}}$ when enthalpy recovery ceases to evolve. Our results will be discussed and compared to results from works leading to different conclusions. We also use our results to probe the issue of whether or not equilibrium relaxation times diverge from super-Arrhenius behavior below T$_{\mathrm{g}}$. [Preview Abstract] |
Session R33: Focus Session: Organic Electronics and Photonics - Transport in Small Molecules
Sponsoring Units: DMPChair: Jana Zaumseil, Institute of Polymer Materials, Friedrich-Alexander Universitaet Erlangen
Room: 341
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R33.00001: Intrinsic transport and photo-physical properties of high-mobility organic single crystals Invited Speaker: Vitaly Podzorov Small-molecule organic semiconductors form the basis for the emerging field of organic optoelectronics. In order to better understand the intrinsic photo-physical and transport phenomena in this important class of materials, it is necessary to study samples of very high structural order and chemical purity. Such materials exist in the form of molecular single crystals that can be used for fabrication of high-performance prototype devices, such as field-effect transistors, photo-conductors and photo-voltaic cells, in which intrinsic properties of organic semiconductors can be investigated without parasitic effects of disorder (see, e.g., [1,2]). This talk will cover the recent progress in organic single-crystal device electronics. In particular, several phenomena related to the previously discovered long-range triplet exciton diffusion and surface photocurrent generation (see, e.g., [3]) will be discussed.\\[4pt] [1] M. E. Gershenson, V. Podzorov, A. F. Morpurgo, ``\textit{Colloquium}: Electronic Transport in Single-Crystal Organic Transistors,'' invited review, \textbf{\textit{Rev. Mod. Phys.}}\textbf{ 78}, 973 (2006). \\[0pt] [2] V. Podzorov et al., ``Hall effect in the accumulation layers on the surface of organic semiconductors,'' \textbf{\textit{Phys. Rev. Lett.}}\textbf{ 95}, 226601 (2005). \\[0pt] [3] H. Najafov, B. Lee, Q. Zhou, L. C. Feldman and V. Podzorov, ``Observation of long-range exciton diffusion in highly ordered organic semiconductors,'' \textbf{\textit{Nature Mater.}} \textbf{9}, 938 (2010). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R33.00002: Variation in the Single-Molecule Conductance of Oligothiophenes Brian Capozzi, Emma Dell, Kateri DuBay, Jose Moreno, Timothy Berkelbach, David Reichman, Luis Campos, Latha Venkataraman Thiophenes are ubiquitous in organic electronic and photovoltaic applications; yet, they have received minimal attention in single molecule transport studies. Here, we carry out single molecule conductance measurements on a family of methyl sulfide-terminated oligothiophenes using the scanning tunneling microscope based break-junction technique. We find a non-exponential decay in conductance with the number of thiophene units (2 through 6) in the chain, which cannot be explained by a simple tunneling or hopping mechanism. We also find that the oligothiophenes exhibit a rather broad conductance distribution when compared to oligophenyls. Using a combination of experiment and molecular dynamics simulations, we show that this increased breadth is most likely due to different thiophene confomers sampled in the experiments, which do not necessarily maintain conjugation along the backbone. These measurements therefore reinforce the importance of conformation and conjugation effects in thiophene-based organic electronic devices where highly conducting molecular components are required. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R33.00003: X-ray Induced Trap States in the Organic Semiconductor Rubrene Tobias Morf, Tino Zimmerling, Simon Haas, Bertram Batlogg The charge transport in organic semiconductors and thus the device performance is broadly affected by localised electronic states capturing charge carriers. In a controlled irradiation experiment, the formation and microscopic origin of these trap states is studied quantitatively. Rubrene crystals with a low pristine trap density are irradiated with monochromatised CuK$\alpha$ radiation. The spectral density of trap states (DOS) is determined by the well-established SCLC method before and after each exposure step. After irradiation, a well defined additional DOS peak is measured. Its density grows linearly by approximately $10^{17}$ trap states per Joule of absorbed energy. These new states are closely peaked around 0.3\, eV above the HOMO (valence band) mobility level. The results are compared to those of the previous ion-irradiation study. Even though the ionic doses were higher by a factor of $10^{3}$ we find very similar changes in the DOS both with respect to quantity and energy of the trap states. This remarkable result suggests a much higher trap creation efficiency of X-rays as compared to ion radiation. Furthermore, the two different radiation methods seem to cause the same type of microscopic perturbation of the molecular crystal. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R33.00004: Trap effects in the analysis of conducting probe AFM current-voltage relations Kanokkorn Pimcharoen, Danial Olds, Jiebing Sun, Peng Peng Zhang, Phillip Duxbury Current-voltage relations of conducting probe AFM (CP-AFM) measurements demonstrate that trap effects are important in nanostructured P3HT thin films, particularly prior to thermal annealing. In order to analyze these measurements, we have developed fully three dimensional continuum device models incorporating the CP-AFM tip geometry and nanoscale morphology of the films. Results will be presented for a variety of trap systems in three dimensional model morphologies including nanofibrous structures and systems with inhomogeneous trap distributions. The simulation results will be compared with experimental data. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R33.00005: Wave-packet approach to thermal fluctuation effects on charge transport of organic semiconductors Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose Organic materials are formed with weak van der Waals interactions between molecules. For typical organic semiconductors, the transfer integrals are small in the range of 10 - 100meV, which is comparable to the dynamical transfer integral modulations originating from the thermal fluctuations of molecular motions. Therefore the fluctuations provide important contributions to the understanding of the transport mechanism. To investigate such effects, we have developed a methodology to calculate the carrier transport coupled with inter- and the intramolecular vibrations of organic semiconductors based on the time-dependent wave-packet diffusion method [1]. In this methodology, we carry out the quantum-mechanical time-evolution calculations of wave packets and the classical molecular dynamics simulations simultaneously. We evaluate the anisotropic mobility of organic semiconductors, such as pentacene crystals. We also clarify the change of temperature dependence of mobility from the thermal activated behavior to the power law behavior. I will talk about these results in my presentation. [1] H. Ishii, K. Honma, N. Kobayashi, K. Hirose, Phys. Rev. B, 85 (2012) 245206. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R33.00006: Computational Study of Electron-Phonon Coupling in Crystalline Organic Semiconductors Nenad Vukmirovic, Christoph Bruder, Vladimir M. Stojanovic Despite wide interest in organic molecular crystals and the recognition that electron-phonon (e-ph) coupling strength crucially determines the nature of charge carriers in these materials, ab-initio studies of e-ph coupling elements in these materials are still lacking. In this work [1], we calculated the e-ph coupling elements throughout the whole Brillouin zone in crystalline naphthalene using density functional perturbation theory within the generalized gradient approximation. Fourier-Wannier interpolation scheme [2] was then used to obtain the e-ph coupling constants on a fine k-point grid necessary for accurate evaluation of physical properties. Using the obtained e-ph coupling elements, we evaluated the quasiparticle residues for electrons and holes, obtaining the values of 0.74 and 0.78, respectively. These values suggest that e-ph coupling strength is insufficient for formation of small polarons in crystalline naphthalene and other oligoacene semiconductors. [1] N. Vukmirovic, C. Bruder, and V. M. Stojanovic, Phys. Rev. Lett. 109, 126407 (2012). [2] F. Giustino, M. L. Cohen, and S. G. Louie, Phys. Rev. B 76, 165108 (2007). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R33.00007: Van der Waals epitaxy of organic crystal films on hexagonal boron nitride layers for high-quality organic electronics Chul-Ho Lee, Theanne Schiros, Seok Ju Kang, Bumjung Kim, Kevin Yager, Colin Nuckolls, Philip Kim The charge transport in organic field-effect transistors (FETs) is strongly influenced by the dielectric and interface properties because crucial carrier processes including accumulation and transport take place at the interface between dielectric and organic materials. In this sense, hexagonal boron nitride (h-BN), which is a layered van der Waals (vdW) dielectric having atomically flat surface and no surface charge trap states, has great potential for both achieving high-quality organic FETs and investigating the intrinsic carrier transport properties in organic semiconductors. In this talk, we present the direct growth of rubrene crystal films on h-BN layers, demonstrating that there exists vdW epitaxial relation between rubrene and h-BN. Furthermore, charge transport properties in FETs using graphene electrodes will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R33.00008: Voltage dependent capacitance -- a measure of energy level bending in naphthalene-tetra-carboxylic- di-imide based transistors Mathias Nyman, Oskar Sandberg, Josue Martinez Hardigree, Srinivas Kola, Howard Katz, Ronald Osterbacka We demonstrate transient capacitance measurements using charge extraction by a linearly increasing voltage (CELIV) on the small molecule naphthalene-tetra-carboxylic- di-imide (NTCDI) based organic transistors. The OFETs use Aluminum (Al) and Aluminum Oxide (AlO$_{x})$ as bottom gate and dielectric, with gold (Au) source and drain electrodes. The Al/AlO$_{x}$ gate is modified using two different self assembled monolayers, triethoxy(octyl)silane and perfluorooctyltriethoxysilane, in order to tune the turn-on voltage. We have clarified the voltage dependent capacitance in diode structures and found that when the transistor is in the fully on state a charge reservoir is formed at the AlO$_{x}$ interface and a saturation of the steady-state capacitance is seen, equaling the capacitance of the AlO$_{x}$ layer. When the transistor is in the fully off state the steady state capacitance saturates to the capacitance of the semiconductor bulk. We interpret this as a build-up of a charge reservoir in the semi conductor bulk when going from the off to the on state making it possible to charge the AlO$_{x}$ capacitance. By going from the on state towards the off state using a linearly increasing voltage pulse the dynamics of the depletion of the reservoir gives information about the energy level bending in the bulk. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R33.00009: Controlling Leakage Currents in Organic Field-Effect Transistors using Molecular Dipole Monolayers on Nanoscale Oxides Josue F. Martinez Hardigree, Thomas Dawidczyk, Robert Ireland, Gary Johns, Byung-Jun Jung, Nina Markovic, Howard Katz Self-assembled monolayers (SAM) have been explored as easily-processed, ultrathin interfacial layers in organic field-effect transistors (OFETs) for tuning the threshold voltage (Vt). We investigated the influence of Fermi-level pinning of the gate electrode by SAMs on leakage currents in OFETs fabricated on highly-doped n- and p-type Si gates with an intentionally marginal-quality, high leakage 8 nm SiO$_{2}$ dielectric. Two dipolar alkyl SAMs, octyltriethoxysilane (OTS) and its fluorinated analogue (FOTS), were employed under a 40 nm active layer of a naphthalenetetracarboxylic diimide (NTCDI) derivative. Transistors on nSi displayed more positive Vt for OTS ($+$0.23 V) and FOTS ($+$1.09 V) than bare oxide (-0.56 V), while OFETs on pSi showed a lower Vt for OTS ($+$0.26 V) and a higher Vt for FOTS ($+$1.25 V) devices relative to bare oxide ($+$1.15 V). Differences in gate and subthreshold leakage between bare and SAM-treated oxides match the trends in Vt. Scanning Kelvin-probe measurements were consistent with this trend, indicating FOTS made both nSi and pSi oxide surfaces more negative relative to bare oxide, while OTS treatment resulted in more positive surface potentials on pSi and more negative surface potentials on nSi. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R33.00010: Halogenated contorted hexabenzocoronene derivatives for electron transport in thin-film transistors and organic photovoltaics Anna Hiszpanski, Leo Shaw, Matthew Bruzek, Franziska Luettich, Antoine Kahn, John Anthony, Yueh-Lin Loo In investigating electron acceptor substitutes for fullerene derivatives in organic photovoltaic applications, we have modified a semiconductor, contorted hexabenzocoronene (HBC), with halogens to increase its oxidative stability and lower its lower unoccupied molecular orbital energy level relative to vacuum level. We synthesized a series of HBC derivatives with increasing fluorine substitution on the peripheral aromatic rings and elucidated the effect of chemical modification on electronic properties. Though we observe a 57 meV shift in both the highest occupied and lowest unoccupied energy levels of the molecules with each progressive addition of fluorine, none of the fluorinated HBC derivatives demonstrate electron transport in thin-film transistors. By substituting chlorine for four of the peripheral fluorines, however, this mixed-halogenated compound exhibits n-transport characteristics. Unoptimized thin-film transistors comprising 8F-8Cl-HBC have demonstrated electron mobilities as high as 0.01 cm$^{\mathrm{2}}$/Vs, and unoptimized bulk-heterojunction solar cells with poly(3-hexyl thiophene) as the polymer donor have yielded power conversion efficiencies as high as 0.9{\%}. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R33.00011: Tellurium-Organic Thin-Films in Hybrid Electronic Platforms Robert Ireland, Howard Katz Vacuum-deposited tellurium (Te) is considered with semiconducting, insulating, and charged organic films for hybrid electronic platforms. Specifically, polycrystalline films of Te and organic semiconductor (OSC) molecules are combined for the first time in bilayer field-effect transistors (FET). Although Te is not ideal for high dynamic range FETs, it serves as a useful test platform in inorganic-organic heterostructures because of high mobility ($\mu )$, defined composition, and amenable processing methods. Scanning Kelvin probe microscopy directly confirms the interfacial vacuum level offset for different Te-OSC junctions. By implanting electrostatic charges at the dielectric surface we demonstrate that interfacial fields determine the gate voltage range over which Te shows field-effect in heterostructured FETs. FETs are measured under both continuous and pulsed operation. Pulsed gating influences the measured $\mu $ by selectively concentrating charge carriers in semiconductor layers that are farther away from the gate dielectric. FETs comprising various Te-organic junctions gave consistent $\mu $ ranging from 0.001 to above 5 cm$^{2}$ V$^{-1}$ s$^{-1}$, compared to 2.7 cm$^{2}$ V$^{-1}$ s$^{-1}$ for Te deposited on bare silicon dioxide under the same conditions. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R33.00012: Spatial dependence of charge photogeneration and transport in an ordered, phase-separated liquid crystalline organic semiconductor Sanjoy Paul, Suvagata Tripathi, Brett Ellman, Robert Twieg Bulk heterojunction organic photovoltaic cells depend on charge and exciton physics within, and between, small regions of organic semiconductors. To probe the physics of charge generation and transport in these geometries, we have fabricated patterned, phase-separated mixtures of liquid crystalline (LC) organic semiconductors and photopolymerized polymers. To characterize transport in the LC regions, we have developed ``scanning time-of-flight microscopy'' (STOFm), whereby spatially resolved TOF and polarized microscopy data are acquired in parallel. We will discuss the technique, as well as our results on efficiency of charge generation, mobility, and trapping in these confined geometries. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R33.00013: Probing Polaron Dynamics and Transport in Multiporphyrin Conjugated Arrays by EPR and Optical Spectroscopy Paul Angiolillo, Jeff Rawson, Michael Therien The nature of mobile charge carriers and their transport in organic conducting and semiconducting materials is still an area requiring deeper understanding. Unlike in classical metals, charge carriers are not represented well by bare charges but rather as polarons. Hole and electron polarons were chemically generated in a systematic series of meso-to-meso ethyne-bridged (porphinato)zinc arrays (PZn$_{\mathrm{n}})$, spanning a linear dimension of 1.4 nm to 7.5 nm. Determination of the spin distribution through the nuclear hyperfine interaction suggest that both hole and electron polarons are extensively delocalized over the extent of the molecule at 298 K. Low temperature studies at 77 K further reveal that the polaronic states maintain their ability to explore the extent of the molecule. Concomitant optical absorption spectroscopy of the hole polaronic states in these oligomers further supports the delocalized nature of the excitation. Electron spin relaxation in organic materials devoid of heavy atoms is dominated by the nuclear hyperfine interaction. This decreased interaction manifest itself in a simultaneous decrease in the spin lattice relaxation rate (increase in spin lattice relaxation time T$_{\mathrm{1}})$ with oligomer size as determined through progressive microwave saturation with relaxation times on the order of 1$\mu $s at 298 K. These data demonstrate exceptional and unprecedented charge dynamics and polaron delocalization lengths. [Preview Abstract] |
Session R34: Thin Films of Block Copolymers and Hybrid Materials: Hierarchical Structures
Sponsoring Units: DPOLYChair: Fred Phelan, National Institute of Standards and Technology
Room: 342
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R34.00001: Thin Films of Supramolecular Nanocomposites Invited Speaker: Ting Xu Supramolecular nanocomposites, composed of polymers, small molecules and nanoparticles, offer numerous opportunities to achieve nanoparticle assemblies with high spatial precision and to incorporate different built-in functionalities by simply varying building blocks. However, as multi-component systems, building blocks are mixed together without forming covalent bonds. There are different energetic contributions governing their phase behavior in bulk and in thin films. Energetically, these contributions are comparable, typically in the range of a few kcal/mol. This makes it feasible to access a rich library of nanostructured composites and enables one to manipulate dynamic nanoparticle assemblies. However, the rather flat energy landscape also presents challenges to precisely control the assemblies in a predictable manner. Here, we present our recent studies on the phase behavior of supramolecular nanocomposites in thin films. We qualitatively describe the effect of the particle-polymer interaction, the polymer chain conformation, the surface tension of each component and the supramolecular morphology on the nanoparticle assemblies in thin films. These basic studies led to well-defined 3-D nanoparticle assemblies of single type nanoparticle and nanoparticle mixtures in thin films. Furthermore, I will discuss our interesting explorations on the dynamics of nanoparticle assemblies in thin films of supramolecular assemblies. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R34.00002: Hierarchical Structuring in Block Copolymer Nanocomposites through Two Phase Separation Processes Operating on Different Time Scales Roy Shenhar, Elina Ploshnik, Amit Halevi, Meirav Ben-Lulu, Axel H.E. Mueller, Karol M. Langner, Johannes G.E.M. Fraaije, G.J. Agur Sevink The ability to assemble nanoparticles (NPs) hierarchically, with control over their positioning and spacing, is considered an important step toward applications where collective properties are sensitive to the morphology of the NP aggregates. Using block copolymers as matrices for organizing metal and semiconductor NPs through microphase separation leads to hierarchical NP assemblies, but control over NP location within the hosting domains is usually limited to one-dimensional distribution. The presentation will demonstrate by both experimental evidence and mesoscopic simulations that functionalizing the nanoparticles with polymeric ligands that are incompatible with both blocks, but to considerably different extents, leads to hexagonally-packed NP assemblies in every other domain of the copolymer film. Such choice of polymeric components leads to a situation where the microphase separation of the block copolymer precedes the macrophase separation of the NPs from the copolymer. Thus, when the latter finally sets in, it occurs within the confines of the domains hosting the NPs. In the hexagonally-packed arrays formed by this process, the interparticle distances are controlled by the thickness of the nanoparticle coating. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R34.00003: Hierarchical pattern formation through photo-induced disorder in block copolymer/additive composite films Li Yao, James Watkins Segregation strength in hybrid materials can be increased through selective hydrogen bonding between organic or nanoparticle additives and one block of weakly segregated block copolymers to generate well ordered hybrid materials. Here, we report the use of enantiopure tartaric acid as the additive to dramatically improve ordering in poly(ethylene oxide-block-tert-butyl acrylate) (PEO-b-PtBA) copolymers. Phase behavior and morphologies within both bulk and thin films were studied by TEM, AFM and X-ray scattering. Suppression of PEO crystallization by the interaction between tartaric acid and the PEO block enables the formation of well ordered smooth thin films. With the addition of a photo acid generator, photo-induced disorder in PEO-b-PtBA/tartaric acid composite system can be achieved upon UV exposure to deprotect PtBA block to yield poly(acrylic acid) (PAA), which is phase-miscible with PEO. Due to the strong interaction of tartaric acid with both blocks, the system undergoes a disordering transition within seconds during a post-exposure baking. With the assistance of trace-amounts of base quencher, high resolution, hierarchical patterns of sub-micron regions of ordered and disordered domains were achieved in thin films through area-selective UV exposure using a photo-mask. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R34.00004: Hierarchical multiscale patterned flexible PDMS elastomeric film and its ice-retarding properties Ying Chen, Diya Bandyopadhyay, Alamgir Karim Hierarchical structures in nature inspired development of artificial micro-nano structures in recent years, because these structures exhibit unique properties like tunable adhesion and wetting. We demonstrate a simple yet versatile method to fabricate micro-nano surface based on combination of PDMS nano-imprinting and UVO lithography. Nanoscale patterned PDMS is fabricated by imprinting digital recording media discs (CD/DVD) pattern. The micro pattern was then built by selective densification of patterned PDMS by exposing to UVO through a bigger mask like TEM grid or wire mesh. The nano imprinted pattern remains unaffected during the UVO treatment. We observed that tunable hierarchical structures with height up to 900 nm can be created by simply controlling UVO exposure time. This method provides potential applications in various fields such as superhydrophocity, icephobicity, microfludics and solar cell. We demonstrate that these hierarchical surface exhibits improved icephobicity comparing to flat hydrophobic surface. Icephobocity experiments were carried out in a controlled humidity and temperature chamber. Patterned PDMS film coatings were cooled to -10 $^{\mathrm{o}}$C at a relative humidity of 65{\%}. Temporal formation of ice was observed under optical microscopy. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R34.00005: Theory of Hierarchical Morphologies in Binary Blends of AB/CD Diblock Copolymers Ashkan Dehghan, Weiquan Xu, Pingwen Zhang, An-Chang Shi The self-assembled structures formed in binary blends of $AB$/$CD$ diblock copolymers are studied using the real space Self-Consistent Field Theory (SCFT), focusing on the cases with attractive $A/C$ and repulsive $B/D$ interactions. The attractive $A/C$ interaction prevents macroscopic phase separation, whereas the repulsive $B/D$ interaction leads to the formation of complex nanoscopic structures. The combination of these features makes the $AB$/$CD$ blend an ideal model system for the study of hierarchical self-assembly. Our results demonstrate that the $B/D$ separation leads to the emergence of hierarchal alternate lamellar, cylinders and checker board morphologies from the classical lamellar structure. Similar behavior in the cylindrical phase, where an increase in the $BD$ interaction leads to a phase transition from the classical hexagonally packed cylinders to alternating cylinders, has also been predicted. The theoretical predictions are consistent with available experiments and, more importantly, provide an interesting route for the engineering of hierarchically ordered structures using block copolymer blends. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R34.00006: Solution Construction of Multigeometry Nanoparticles and Multicompartment Superstructures from Block Copolymer Mixtures Jiahua Zhu, Shiyi Zhang, Karen Wooley, Darrin Pochan Novel soft objects with both compositional and geometric complexity at nanoscale have been constructed through solution supramolecular assembly from block copolymer mixtures due to their non-ergodic character. The mixture is composed of two block copolymers with distinctive hydrophobic blocks but the same poly(acrylic acid) hydrophilic block. First, multigeometry nanoparticles, due to segregation of unlike block copolymer molecules into multiple subdomains trapped within the same micelle-like structures, have been assembled in tetrahydrofuran/water solution. Through carefully designed molecular architecture, mixing ratio and pathway kinetics, both size and shape of subdomains can be controlled to produce a novel class of multigeometry nanoparticles, including sphere-sphere, sphere-cylinder, cylinder-cylinder, cylinder-disk, and sphere-disk hybrid nanoparticles. Second, hierarchical multicompartment superstructures including particle chains, rings and other nano to micro cluster formations, have been built up from pre-formed multigeometry nanoparticles by taking advantage of their surface anisotropy and the controlled particle-particle association. The interparticle association can be achieved via either covalent or non-covalent bindings due to different post-polymerization chemical modifications with hydroxyethyl acrylate or crown ether functionalities, respectively. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R34.00007: Mixed Solvent Strategy for the Dispersion of PCBM in Block Copolymer Thin Films Abul Huq, Manish Kulkarni, Kevin Yager, Detlef-M. Smilgies, Alamgir Karim In this work a model system of self assembling cylinder forming polystyrene-b-poly(ethylene oxide) (PS-$b$-PEO) block-copolymer (BCP) and photosensitive phenyl-C61-butyric acid methyl ester (PCBM) nanoparticles were utilized to study extent of nanoparticle dispersion into BCP thin films. We studied effects of different solvents and mixture of solvents for casting variable amount of PCBM loaded PS-$b$-PEO films on the final morphology of the films. Atomic force microscope (AFM) as well as transmission electron microscope (TEM) was employed to study the dispersion of PCBM into PS-$b$-PEO matrix. We were able to disperse more than fifty percent PCBM (wt./wt.) in the film, which is higher than the percolation threshold of nanoparticles, without forming PCBM clusters. Grazing incidence small angle scattering (GISAXS) results show that the mixed solvent strategy resulted in change of domain sizes of thin films due to change of effective interaction parameters. It was found by AFM scratch test that the film thickness is highly dependent on the casting solvent mixtures and nanoparticle concentration. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R34.00008: Dynamics of block copolymer / nanoparticle composites Andrei Zvelindovsky, Marco Pinna, Ignacio Pagonabarraga We present results of a large scale coarse grained computer simulation for block copolymer nanoparticle composites, Hybrid Cell Dynamics Simulation. Dynamics of the nanoparticles is found to strongly influence block copolymer nanostructure dynamics and vice versa. Different ratios of nanoparticle diameter and block copolymer domain spacing were investigated. The effect of the external fields such as electric or magnetic fields on the dynamics of the particles was incorporated into the computer model and found to influence block copolymer matrix structure. For example, the nanoparticles can controllably induce phase transitions between different block copolymer morphologies. The simulation results gave insights on underlying physical mechanisms in recent experiments on such systems. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R34.00009: Precise control of magnetic and dielectric nanoparticle placement within block copolymer templates for the fabrication of 3D magneto-dielectric metamaterials Xinyu Wang, Dongpo Song, James Watkins Magneto-dielectric metamaterials fabricated using high permeability (high-$\mu$) nanoparticles (NPs) with precise control over position and orientation could yield superior electromagnetic properties with low loss. In addition, proper tuning of the effective dielectric constant of the host composite could yield more efficient devices with a wider bandwidth. Lin et al. recently reported the use of strong interactions between NPs and one segment of weakly segregated block copolymer (BCP) systems to drive the assembly of well-ordered morphologies while confining the NPs specifically in the desired spherical, cylindrical or lamellar domains. Here we used this approach to assemble high-$\mu$ NPs into well ordered systems. Specifically, FePt nanoparticles functionalized with H-bonding donating ligands were shown to induce strong segregation in weakly segregated BCP systems. In addition, different NP/polymer segment interactions, such as $\pi$-$\pi$ interactions, were introduced to incorporate dielectric NPs in order to tune the effective permittivity of the material. Small-angle X-ray scattering was used to track the morphological evolution of the composite. Transmission electron microscopy was used to investigate the location of the NPs in their respective polymer domains. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R34.00010: Morphological studies on supramolecular hybrids comprising a block copolymer and semiconductor nanoparticles Atsushi Noro, Kota Higuchi, Yoshio Sageshima, Yushu Matsushita Well-ordered periodic nanostructures have been attaining much attention due to their high potential for nano-applications. Nanophase-separated structures of block copolymer/inorganic nanoparticle hybrids are one of good candidates for such applications. Here we report a systematic study on preparation and morphological observation of hybrids composed of a block copolymer and hydroxy-capped cadmium selenide nanoparticles (h-CdSe) via hydrogen bonding. Three polystyrene-$b$-poly(4-vinylpyridine) (PS--P4VP) block copolymers with the same PS chain length but with different P4VP chain length were synthesized for hybrid preparation. Each PS--P4VP was mixed with h-CdSe by varying a weight ratio of PS--P4VP:h-CdSe. A hybrid composed of h-CdSe and PS--P4VP bearing long P4VP blocks represents a single nanophase-separated structure, where domain spacing expansion and morphology transition induced by addition of h-CdSe were observed. On the other hand, macrophase separation accompanied by overflow of h-CdSe from nanophase-separated domains was observed in hybrids which contain PS--P4VP bearing short P4VP blocks. These results are attributed to hydrogen-bonding formation and the stoichiometric balance of functional groups. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R34.00011: Nanoparticle distribution in complex block-copolymer morphologies YongJoo Kim, Hsieh Chen, Alfredo Alexander-Katz We present our work on the distribution of nanoparticles (NPs) having various shapes (sphere, rod or disk) in different types of directed-self-assembled block-copolymer (BCP) morphologies using hybrid particle-field simulations. The BCP patterns are first obtained by modeling a nanoscale template consisting of ordered posts that are attracted to one of the blocks of BCPs. Once a desired pattern is obtained, we run simulations using the pattern as the initial condition while also including nanoparticles with different shapes, sizes and positions. By calculating the mean-field free energy of the entire system, we study the role that chain stretching and nanoparticle shape and size play in the equilibrium location of the NPs in the BCP matrix. Our results can have important implications in directing the self-assembly of multi-component hierarchical materials. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R34.00012: Directed Nanorod Assembly Using Block Copolymer-Based Supramolecules Kari Thorkelsson, Alexander Mastroianni, Peter Ercius, Ting Xu Nanorods display many unique electrical, mechanical, and optical properties unavailable in traditional bulk materials, and are attractive building blocks toward functional materials. The collective properties of anisotropic building blocks often depend strongly on their spatial arrangements, interparticle ordering, and macroscopic alignment. We have systematically investigated the phase behavior of nanocomposites composed of nanorods and block copolymer (BCP)-based supramolecules forming spherical, cylindrical and lamellar morphologies. Initial exploration showed that the nanorods can be readily dispersed in polymeric matrix and the overall morphology of nanorod-containing supramolecular nanocomposite depends on the nanorod-polymer interactions, inter-rod interactions and entropy associated with polymer chain deformation. The energetic contributions from the components of the system can be tailored to disperse nanorods with control over inter-rod ordering and the alignment of nanorods within BCP microdomains.[1] By varying the supramolecular morphology and composition, arrays, sheets, and interconnected networks of nanorods are demonstrated that may prove useful for fabrication of optically and electrically active nanodevices. 1. Thorkelsson, K. et al. Nano letters 2012, 12, 498 [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R34.00013: Morphological Control of Charged Block Copolymer Micelle Complexes in Dilute Aqueous Media Kookheon Char, Misook Lee, Kyung Jee Min, Jinkee Hong Amphiphilic block copolymers dispersed in a selective solvent can be self-assembled into various aggregates such as spherical and cylindrical micelles and bilayer vesicles. Block copolymers typically possess hundred repeat units, leading to kinetically stable or trapped assemblies due to the lack of molecular chain exchange between aggregates in solution; thus, aggregated morphologies are highly path dependent. Here, we demonstrate the amphiphilic block copolymer micelle (BCM) complexes with pH-tunable electrostatic interactions between two differently charged corona blocks in aqueous media. The combination of preformed micellization of each BCM with the dissociation control of the corona blocks provides a distinct assembly pathway. This is to say that the sequential mixing of charged BCMs reveals the effects of both corona complexation (through inter-component interactions) and the manipulation of interfacial curvature between core and corona within a micelle (through the intra-molecular block conformations), resulting in unique complex morphologies such as crystal-like hexagonal prisms, hierarchical spheres, and twisted peapods. [Preview Abstract] |
Session R35: Novel Superconductors I
Sponsoring Units: DCMPChair: James Eckstein, University of Illinois at Urbana-Champaign
Room: 343
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R35.00001: The Introduction of substitutional and non-substitutional dopants into MgB2 in high pressure/Temperature or non-equilibrium regimes Mike Sumption In an attempt to study the effect of doping of MgB2 under conditions leading to efficient doping, we used both an high temperature/high pressure induction furnace to dope into MgB2 bulks at temperatures up to 1600 C and 1500 Psi, and thin film, PLD multilayer and mixed layer film fabrication. The high temperature/high pressure formation was used to explore the solubility at high temperatures of various dopants, and the thin film formation was an attempt to use non-equilibrium conditions to inject dopants more effectively. The dopants used were C, Ti, and Zr. C was seen to reach a maximal level at 4 at{\%} C site substituted into MgB2, as evidenced by EPMA and XRD results. Zr, of interest as a possible Mg site substitution in MgB2 was not seen to enter into the MgB2 phase (instead segregating) in the bulk high temperature/high pressure experiments, but was seen to enter in during PLD, as evidenced by STEM and XRD results. Ti additions were attempted in the high pressures and temperature rig, with some evidence for dopant introduction. Critical field measurements on the Zr doped samples where seen to suppress Bc2 for all except very low levels of Ti addition, presumably associated with the much greater doping efficiency. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R35.00002: The Penetration Depth of MgB$_2$ as measured by DC SQUIDs Daniel Cunnane, Ke Chen, X.X. Xi High-speed superconducting circuits may benefit from the high $T_{c}$ and large superconducting gap of MgB$_{2}$. Nb remains the state of the art for superconducting electronics partly because of its small penetration depth and its isotropic nature. A microscopic theory on the penetration depth of multiband superconductors states that a clean MgB$_{2}$ sample is nearly isotropic while a sample in the dirty limit is anisotropic. We have made and measured DC SQUIDs using MgB$_{2}$ Josephson junctions to determine the inductance of an MgB$_{2}$ microstrip. The penetration depth along the c-axis, $\lambda_{c}$, was calculated using the inductance value and dimensions of the microstrip. We have previously reported the absolute value of the penetration depth of our MgB$_{2}$ films to be around 40 nm. Now we have made devices with film ranging from the clean limit to the dirty limit by adding defects during the deposition. The absolute value of $\lambda_{c}$ at low temperature is compared to the cleanliness of the film. The temperature dependence was also measured which is non-trivial due to the two-gap nature of MgB$_{2}$. These results are compared with theory that confirmed our previous results. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R35.00003: Superconducting properties of aligned flexible networks and yarns of MgB$_2$-CNT nanowires Julia Bykova, M\'{a}rcio Dias Lima, Derrick Tolly, Carter Haines, Austin Howard, Myron Salamon, Ray Baughman, Anvar Zakhidov Magnesium diboride (MgB$_2$) has attracted great interest due to its outstanding superconducting characteristics. Literature reports showed that addition of carbon nanotubes (CNT) to a MgB$_2$ matrix significantly improves its properties: CNTs can carry extremely high currents and also provide electrical and mechanical connection between MgB$_{2}$ grains. Here we present a new method to produce networks of aligned MgB$_{2}$-CNT nanowires which can be spinned into flexible yarns. Free-standing, aligned CNT sheets were used as a starting network. A conformal layer of boron was deposited on CNTs by Laser Assisted Chemical Vapor Deposition. The resultant boron-CNT nanowires (thickness of 70$\pm10$ nm) were exposed to magnesium vapor and were converted into MgB$_{2}$-CNT composites. The MgB$_{2}$-CNT arrays are flexible and can be easily bent and even twisted. Critical temperature reaches 37 K and depends on thickness and crystalline structure of nanowires. Critical current and critical fields were shown to be comparable or even better than standard MgB$_{2}$ wires. We discuss the correlation of observed two step behavior in electric transport curves with interconnects between MgB$_{2}$-CNT nanowires and Josephson junction network formation. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R35.00004: Enhancement of lower critical field in thin MgB$_{2}$ films and MgB$_{2}$/MgO multilayers Teng Tan, Evan Johnson, Narendra Acharya, Michael Hambe, Ke Chen, Alex Krick, Steven May, Xiaoxing Xi Magnesium diboride is a conventional superconductor with a high $T_{c}$ of 39 K, a low residual resistivity of \textless\ 0.1 $\mu \Omega $cm (at 42 K), and higher thermodynamic critical field $H_{c}$ values than Nb. These properties make MgB$_{2}$ a promising superconductor as an alternative to Nb for future SRF cavities. However, the lower critical field $H_{c1}$ of MgB$_{2}$ is low, and vortex dissipation above $H_{c1}$ can lead to degradation of the quality factor and low RF breakdown field. Here, we report an enhancement of $H_{c1}$ in thin MgB$_{2}$ films and MgB$_{2}$/MgO multilayers. The value of $H_{c1}$(5K) is increased from 40 mT in a 300 nm-thick MgB$_{2}$ film to 180 mT when the MgB$_{2}$ layer thickness is 100 nm either in a single-layer film or in a MgB$_{2}$/MgO multilayer with a total MgB$_{2}$ layer thickness of 300 nm. Superconducting MgB$_{2}$ thin films have been coated \textit{in-situ }on the inner wall of a SRF cavity using the hybrid physical chemical vapor deposition (HPCVD) technique. The characterization of the coating will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R35.00005: Thickness dependence of superconducting properties in magnesium diboride thin films Douglas Beringer, Cesar Clavero, Teng Tan, Xiaoxing Xi, Rosa Lukaszew Thin film MgB$_{2}$ is a promising material currently researched for improvements in superconducting radio frequency (SRF) technology and applications. At present, bulk niobium SRF accelerating cavities suffer from a fundamental upper limit in maximally sustained accelerating gradients; however, a scheme involving multi-layered superstructures consisting of superconducting-insulating-superconducting (SIS) layers has been proposed to overcome this fundamental material limit of 50 MV/m. The SIS multi-layer paradigm is reliant upon implementing a thin shielding material with a suitably high Hc1 which may prevent early field penetration in a bulk material layer and consequently delay the high field breakdown. It has been predicted that for thin superconducting films --- thickness less than the London penetration depth ($\sim$ 140 nm in the case of MgB$_{2})$ --- the lower critical field Hc1 will be enhanced with decreasing thickness. Thus, MgB$_{2}$, with a high bulk Hc1 value is a prime candidate for such SIS structures. Here we present our study on the structure, surface morphology and superconducting properties on a series of MgB$_{2}$ thin films and correlate the effects of film thickness and surface morphology on Hc1. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R35.00006: High resolution $^{11}$B NMR of MgB$_2$ using cryogenic magic-angle spinning Raivo Stern, Peter Beckett, Mark S. Denning, Ivo Heinmaa, Mukesh C. Dimri, Edward A. Young, Marina Carravetta Static and magic-angle spinning (MAS) $^{11}$B NMR data at 4.7 T and 8.5 T have been obtained under cryogenic conditions on a diluted sample of magnesium diboride powder in the normal and superconducting state. We demonstrate that MAS NMR is possible on type-II superconductors despite the sample rotation. The data provide accurate information on the magnetic shift variation and longitudinal relaxation data down to a temperature of 8 K, with a resolution improvement over the entire temperature range. The onset of superconductivity is unaffected by the sample rotation, as revealed by a steep variation of the magnetic shift just below the critical temperature. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R35.00007: The full 3D electronic band structure of MgB2 determined by soft x-ray ARPES Yasmine Sassa, Martin Mansson, Bastian M. Wojek, Masaki Kobayashi, Olof Gotberg, Vladimir Strocov, Nikolai Zhigadlo, Oscar Tjernberg, Bertram Batlogg MgB$_{2}$ is a prototypical multi-band multi-gap superconductor with electron-phonon coupling driving T$_{c}$ up to 40~K. Surprisingly, the experimental knowledge of the electronic band structure is rather limited. Here, we present the first results of angle-resolved photoelectron spectroscopy (ARPES) studies on high quality MgB$_{2}$ single crystals, employing photons in the soft x-ray range with variable energy. We have been able to measure the band dispersion not only in the $k_{\rm x}$-$k_{\rm y}$ plane, but also probe in detail the $k_{\rm z}$ dependence and thus, the 3D nature of the bands. Furthermore, we have found the ARPES intensities to be strongly polarization dependent and their analysis provides an excellent agreement with the orbital nature of the electronic states. The calculated electronic band structure captures very well all the features revealed in our experiment. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R35.00008: Fragile Structure Transition in Mo3Sb7 J.-Q. Yan, M.A. McGuire, A.F. May, D.G. Mandrus, B.C. Sales Despite a relatively low superconducting transition temperature T$_{\mathrm{c}} =$ 2.08 K, the Zintl compound Mo$_{3}$Sb$_{7}$ has attracted considerable interest due to the possible involvement of magnetism in superconducting pairing, and promising thermoelectric performance with proper doping. Mo$_{3}$Sb$_{7}$ crystallizes in a Ir$_{3}$Ge$_{7}$-type cubic structure with space group Im3m at room temperature. A structure transition from cubic to tetragonal (I4/mmm) was observed at 53 K and this symmetry lowering is accompanied by the opening of a 120 K spin gap. Here, we will present the growth of Mo$_{3}$Sb$_{7}$ single crystals and our work in exploring the correlation between the low-temperature superconductivity, the structure transition, and the spin gap. The low-temperature superconductivity was observed in both the cubic and tetragonal phases. The structure transition was found to be extremely sensitive to Te or Ru substitution which shifts the Fermi level toward the valence band edge. Work at ORNL was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R35.00009: Crystal growth, complex phase diagram and high pressure studies of layer compound PdBi$_{2}$ Kui Zhao, Xiyu Zhu, Bing Lv, Yuyi Xue, Paul Chu Among the different Pd-Bi Alloys, $\beta $-PdBi$_{2}$, which is crystallized in a layered tetragonal (I4/mmm) structure, has been identified as a superconductor with transition temperature at $\sim$ 5.4K. Band structure calculation indicates that the interlayer Bi-Bi bonds are weak but not negligible, which implies the 3D bonding character of this compound. In order to enhance or weaken the interlayer bonding and ultimately increase the Tc in this system, high pressure measurement, isovalent chemical substitution of Bi with Sb, and chemical intercalation using transition metal Cu and alkali metal Na, are applied to the system. Meanwhile, aliovalent chemical substitution on the Bi site by Pb is also carried out. The magnetic, electrical, and calorimetric properties of these compounds are determined at ambient pressure and compared. The detailed high pressure results and the complete phase diagram of chemical substitution and intercalation will be presented and discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R35.00010: Superconductivity with extremely large upper critical fields in Nb$_{2}$Pd$_{0.81}$S$_{5}$ Qiu Run Zhang, Gang Li, Daniel Rhodes, Andhika Kiswandhi, Tiglet Basara, J Sung, Theo Siegrist, Michelle Johannes, Luis Balicas Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb$_{2}$Pd$_{0.81}$S$_{5}$, with a transition temperature $T_{c} \cong $ 6.6 K. Despite its relatively low $T_{c}$, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. $\mu_{0}H_{c2}$ ($T \to $ 0 K) \textgreater\ 37 T for fields applied along the crystallographic $b$-axis. This value is considerably larger than the value reported for the technologically relevant Nb$_{3}$Sn compound ($\mu_{0}H_{c2}$ $\sim$ 30 T, with $T_{c} =$ 18 K)$^{1,2}$. Its ratio of $\mu_{0}H_{c2}$ ($T$ $\to $ 0 K) to $T_{c}$, is also larger than those of the new Fe based superconductors, e.g. $\beta $-FeSe (20 T/8.7 K)$^{3}$, Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($\sim$ 70 T/28 K)$^{4}$, and even higher than the reported ratio for the Chevrel-phase PbMo$_{6}$S$_{8}$(60T/13.3 K)$^{5}$ compound. For a field applied perpendicularly to the $b$-axis, $\mu_{0}H_{c2}$ shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb$_{2}$Pd$_{0.81}$S$_{5}$ is a multi-band superconductor. This is confirmed by band structure calculations which reveal nearly cylindrical and quasi-one-dimensional Fermi surface sheets having hole and electron character, respectively. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R35.00011: Synthesis, structure, chemical doping and high pressure studies of the SrPt$_{3}$ P with unique structure features BenMaan Jawdat, Bing Lv, Xiyu Zhu, Yuyi Xue, Ching Chu Superconductivity up to 8.4K was reported by Takayama et al.\footnote{T. Takayama, K. Kuwano, D. Hirai, Y. Katsura, A. Yamamoto, and H. Takagi, Phys. Rev. Lett., 108, 237001(2012).} in APt$_{3}$P (A$=$Sr, Ca and La) in 2012 with structural information based only on X-ray powder refinement. The compounds are suggested to crystallize in an antiperovskite-based structure closely related to that of the heavy fermion superconductor CePt$_{3}$Si but are nonpolar unlike CePt$_{3}$Si. Both small single crystals and polycrystalline samples of SrPt$_{3}$P, the compound with the highest T$_{c}$ of this class of materials, are synthesized through solid state reactions. In this presentation, full and detailed structural information will be revealed based on X-ray single crystal analysis. Different chemical doping on different sites and high pressure studies have been carried out on the compound of SrPt$_{3}$P. The results and its implication will be presented and discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R35.00012: Revealing the superconducting state of CaC6 by angle-resolved photoelectron spectrocopy Shuolong Yang, Jonathan Sobota, Chris Howard, Chris Pickard, Makoto Hashimoto, Donghui Lu, Sung-Kwan Mo, Mark Ellerby, Zhi-Xun Shen We studied the electronic band structure of CaC$_{6}$ using angle-resolved photoelectron spectroscopy (ARPES). We were able to make direct connections to the DFT calculation and identify various electron- and hole-pockets both at the $\Gamma$- and K-points. Most importantly, we convincingly observed the interlayer band, which was predicted to be responsible for superconductivity and to display a near-free-electron-like dispersion. The near-circular Fermi surface of the interlayer band is clearly separate from the carbon-derived bands, which enables a pocket-dependent superconducting gap analysis near the $\Gamma$-point. Distinct electron-phonon coupling regimes were observed for the interlayer and the carbon-derived bands using self-energy analysis in agreement with previous studies. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R35.00013: Superconductivity in layered cobaltates: a functional RG treatment Christian Platt, Maximilian Kiesel, Werner Hanke, Ronny Thomale The superconducting state of water-intercalated cobaltates is still poorly understood. Starting with an effective three orbital model which fits the experimentally observed Fermi surface, we apply the functional renormalization group and study the phase diagram of Na$_x$CoO$_2$ as a function of doping. Here, we find ferromagnetic and triplet-pairing tendencies near van-Hove filling as well as $(d+id)$-superconductivity for larger dopings. The calculated gap function in this $(d+id)$-phase reveals a near-nodal behavior, and an increased CoO$_2$ layer distance promotes the ferromagnetic and triplet-pairing channels. Our findings are consistent with recent experimental observations. The cobaltates thus establish a chiral singlet superconductor based on transition metal oxides. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R35.00014: Evidence for phase separation between the co-existing Density Wave and Superconducting orders in (TMTSF)$_2$PF$_6$ Arjun Narayanan, Paul Chaikin Resistance, Thermopower and Angular Dependent Magnetoresistance(AMRO) measurements were used to study the organic conductor (TMTSF)$_{2}$PF$_{6}$ at pressures where co-existence between Superconducting and Spin Density Wave orders occurs. While in other material families such coexistence is poorly understood, in (TMTSF)$_{2}$PF$_{6}$ a clear picture is emerging. Various suggestions had been made regarding the coexistence phase, including homogenous phases showing microscopic coexistence, Soliton walls in Density Waves, and phase separation between normal metal and density wave regions. \textit{We provide strong evidence for the phase separation scenario in (TMTSF)$_2$PF$_6$.} The existence of domains and their pattern of distribution are unambiguously evidenced by thermopower and resistivity anisotropies. The metallic domains are identified as the regular high pressure metal by various unique signatures such as Field Induced Density Waves(FISDW), AMRO and the superconducting Tc. Some surprising details of the evolution of FISDW and AMRO with pressure in the coexistence phase will also be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R35.00015: Electron correlations in C$_{60}$ and aromatic superconductors Yusuke Nomura, Kazuma Nakamura, Ryotaro Arita Recent discovery of superconductivity in fcc/A15 Cs$_3$C$_{60}$ under pressure and in aromatic compounds (ex. alkali-doped picene) has stimulated a renewed interest in molecular superconductors such as K$_3$C$_{60}$ and Rb$_3$C$_{60}$. To clarify the mechanism of the superconductivity, it is essential to understand low-energy electronic structure of these systems. In the present study, we perform a systematic study for understanding the relation between electronic correlation and superconductivity in C$_{60}$ and aromatic compounds [1]. We derived, from first principles, extended Hubbard models for twelve compounds: fcc K$_3$C$_{60}$, Rb$_3$C$_{60}$, Cs$_3$C$_{60}$ (with three different lattice constants), A15 Cs$_3$C$_{60}$ (with four different lattice constants), doped solid picene, coronene, and phenanthrene. We show that these compounds are strongly correlated and have similar energy scales of their bandwidths and interaction parameters. However, they have a different trend in the relation between the strength of the electronic correlation and superconducting-transition temperature. While the C$_{60}$ compounds have a positive correlation, the aromatic compounds exhibit a negative correlation. [1] Y. Nomura, K. Nakamura, and R. Arita, Phys. Rev. B ${\bf 85}$, 155452 (2012). [Preview Abstract] |
Session R36: Superconducting Proximity Effects: Mesoscopic and Related
Sponsoring Units: DCMPChair: Zhiqiang Mao, Tulane University
Room: 344
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R36.00001: Nonlocal correlations in a proximity-coupled normal metal Taewan Noh, Sam Davis, Venkat Chandrasekhar We report evidence of large, nonlocal correlations between two spatially separated normal metals in superconductor/normal-metal (SN) heterostructures, which manifest themselves a nonlocal voltage generated in response to a driving current. Unlike prior experiments in SN heterostructures, the nonlocal correlations are mediated not by a superconductor, but by a proximity-coupled normal metal. The nonlocal correlations extend over relatively long length scales in comparison to the superconduncting case. At very low temperatures, we find a reduction in the nonlocal voltage for small applied currents that cannot be explained by the quasiclassical theory of superconductivity. We believe is a signature of new long-range quantum correlations in the system. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R36.00002: Superconducting proximity effect in MBE grown Nb-InAs junctions Carolyn Kan, Chi Xue, Stephanie Law, James Eckstein Several proposals for the realization of Majorana fermions rely on excellent quality proximity coupling between a superconductor and a high-mobility semiconductor. We examine the long-range proximity coupling between MBE-grown InAs and in situ grown superconducting overlayers by fabricating transport devices, and investigate the effect of substrate choice and growth conditions on the quality of the MBE InAs. GaAs is commonly available as a high quality insulating substrate. Overcoming its lattice mismatch with InAs using GaSb and AlSb layers results in locally smooth terraced surfaces, but global spiral dislocation structures also appear and have a negative impact on the InAs mobility. Growing InAs on homoepitaxial GaSb results in improved morphology and increases the mean free path. We compare the proximity effect in devices made both ways. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R36.00003: Electrostatic Tuning of the Proximity-Induced Exchange Field in EuS/Al Bilayers Tijiang Liu, Joseph Prestigiacomo, Philip Adams We demonstrate that the proximity-induced exchange field, $H_{ex}$ in ferromagnetic/paramagnetic bilayers can be modulated with an electric field. An electrostatic gate arrangement is used to tune the magnitude of $H_{ex}$ in the Al component of EuS/Al bilayers. We produced modulations of $\sim30$ Oe in $H_{ex}$ with the application of perpendicular electric fields of the order of $\pm 10^6$ V/cm. Several possible mechanisms accounting for the electric field's influence on the interfacial coupling between the Al layer and the ferromagnetic insulator EuS will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R36.00004: Unbiased Analysis of Super/Ferro Bilayer Physics Thomas Lemberger, Michael Hinton, Jie Yong, Adam Hauser, Fengyuan Yang, Julia Meyer S/F bilayer physics has been studied for some time now. With a large number of unknown and seemingly-known parameters, some values are traditionally and understandably assumed to be fixed quantities. In particular, the exchange energy, $E_{ex}/k_B$, is believed to be comparable to the Curie temperature. We analyze the data assuming only that the Fermi velocity $v_F$ in F and the density of states, $2N_S(0)$, in S are known. Fitting $T_c$ vs. $d_F$ with the dirty-limit theory, we determine the interface resistance, $R_b$, the ratio $E_{ex}/\rho_F\ell_F$, and the ferromagnetic coherence length $\xi_F$. For physically plausible values of $\rho_F\ell_F$, the dephasing rate of cooper pairs in F is 10 times smaller than expected from the known Curie temperature of F. We propose that dephasing is mitigated by spin-orbit scattering. We also find that the transmission probability for electrons striking the F/S interface is much less than unity. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R36.00005: Exploring the mini-gap state and magnetoresistance in platinum nanowires Daniel Slotcavage, Meenakshi Singh, Thomas Mallouk, Moses Chan Periodic oscillations in differential magnetoresistance and a superconducting mini-gap state were found in single-crystal gold nanowires [Wang et al., PRL 102, 247003 (2009)]. The oscillations were attributed to motion of individual vortices in the nanowire. We have studied proximity-induced superconductivity in polycrystalline platinum nanowires grown using template-based electrodeposition. Systematic studies of the dependence of the mini-gap state on temperature, magnetic field, and sample morphology and geometry were conducted. We found the mini-gap state to persist in polycrystalline samples. The presence of the mini-gap state in polycrystalline samples demonstrates its robustness with respect to sample morphology. On the other hand, the differential magnetoresistance oscillations was not found in these wires. Future work will focus on determining the conditions required for the occurrence of these oscillations. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R36.00006: Unconventional quantum oscillations in mesoscopic rings of spin-triplet superconductor Sr$_2$RuO$_4$ Xinxin Cai, Yiqun Ying, Neal Staley, Yan Xin, David Fobes, Tijiang Liu, Zhiqiang Mao, Ying Liu Spin-triplet superconductor Sr$_2$RuO$_4$ has been found to feature exotic vortex physics including the formation of vortex lattices at low fields and most recently, evidence for half-flux quanta trapped in a doubly connected sample. We carried out the magnetoresistance measurements in mesoscopic ring samples of Sr$_2$RuO$_4$ fabricated on mechanically exfoliated single crystals of Sr$_2$RuO$_4$ by photolithography and focused ion beam. With the magnetic field applied perpendicular to the in-plane direction, thin-wall rings of Sr$_2$RuO$_4$ were found to exhibit a large number of full-flux quantum oscillations with pronounced amplitudes unexpected from the conventional Little-Parks effect. Furthermore, in thick-wall rings, two distinct periods were observed in both resistance and critical current oscillations, which we attribute to the effect of vortices, namely, the ``lock-in" effect of a vortex lattice in Sr$_2$RuO$_4$. No evidence for half-flux-quantum oscillations were identified in any sample measured so far without the presence of an in-plane field. The measurements with an in-plane field are being pursued. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R36.00007: Analytic Description of Superconducting-Ferromagnetic Proximity Systems Thomas E. Baker, Ovidiu E. Icreverzi, Adam K. Moke, Andreas Bill We present the exact analytic solution of the Usadel equations for a proximity system made of a superconductor and a ferromagnet in the wide dirty limit, including spin-flip scattering. The solution was found by mathematical analogy to the Jacobi elliptic function description of a classical mechanics system known as the bead on a hoop [1]. We highlight the parallels between the two systems and present an analysis of the solution with special attention to long rage triplet effects and the inverse Fulde-Ferrell-Larkin-Ovchinnikov state. We determine the Josephson critical current and the variation of the critical temperature with spin-flip scattering.\\[4pt] [1] T.E.~Baker and A.~Bill, {\it Am.~J.~Phys.} {\bf 80}, 506 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R36.00008: Microscopic Study of c-axis Proximity Effect in Cuprate-Manganite Heterostructures H. Zhang, I. Fridman, N. Gauquelin, G.A Botton, J. Y.T. Wei Recent studies have reported long-ranged proximity effect in epitaxial thin-film heterostructures of ferromagnetic manganites and superconducting cuprates, with possible origins in novel spin-triplet correlations [1]. A key evidence for this effect is the suppression of the superconducting $T_c$ observed in multilayer films of La$_{2/3}$Ca$_{1/3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ (LCMO/YBCO). However, scanning tunnelling spectroscopy on $c$-axis LCMO/YBCO bilayers have not seen direct evidence for proximity-induced pairing down to 5nm LCMO thickness [2]. We re-examine the $T_c$ suppression by performing atomically-resolved transmission electron microscopy and resistivity measurements on $c$-axis YBCO/LCMO films grown by pulsed laser deposition, and relating the microstructure in YBCO with the layer thickness and $T_c$. The microscopy revealed double CuO-chain intergrowths forming non-stoichiometric YBCO-247 regions that do not appear in x-ray diffraction, but can be related to the $T_c$ suppression. We attribute these intergrowths to heteroepitaxial strain, by comparing all the lattice parameters and symmetries involved. [1] Z. Sefrioui \emph{et al.}, PRB 67, 214511 (2003); C. Visani \emph{et al}, Nat. Phys. 8, 539 (2012). [2]I. Fridman \emph{et al}, PRB 84, 104522 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R36.00009: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R36.00010: New structural features in solution-derived YBCO nanocomposite films responsible for a successful novel pinning mechanism J. Gazquez, R. Guzman, J. Salafranca, R. Mishra, M. Varela, A. Palau, V. Rouco, M. Coll, A. Llordes, G. Deutscher, X. Obradors, T. Puig The optimization of high temperature superconductors calls for a detailed knowledge about the effects of materials' manipulations on the subnanometer scale, since the subtle interplay of a variety of nanoscale defect structures that pin the magnetic flux lattice will dictate the performance of these materials. The outstanding properties of solution deposited-YBa$_{2}$Cu$_{3}$O$_{7-d}$ nanocomposites arise from the strains associated to the network of YBa$_{2}$Cu$_{4}$O$_{8}$ intergrowths emerging from the spontaneously segregated oxide nanoparticles, and a novel pinning mechanism coupling this lattice strain with superconducting pairing [1]. However, YBa$_{2}$Cu$_{4}$O$_{8}$ intergrowths involve the addition of an extra CuO chain and their ubiquity may lead to an off-stoichiometry that could jeopardize the superconducting properties of the film. Conversely, we will show, by means of aberration corrected scanning transmission microscopy in combination with electron energy loss spectroscopy, how the system balance this deficiency of Cu through new structural features, previously unforeseen, that may constitute new and effective pinning centers and may be responsible for the novel pinning mechanism proposed.\\[4pt] [1] A. Llordes \textit{et al} Nature Mater2012. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R36.00011: Crossover from Peierls distortion to one-dimensional superconductivity in arrays of (5,0) carbon nanotubes Ting Zhang, Ming Yuan Sun, Zhe Wang, Wu Shi, Ping Sheng We consider the electronic instabilities in (3,3)@(8,8) and (5,0)@(15,0) metallic double wall carbon nanotubes. Using 2rd order renormalization group method, we find that in the single wall (3,3) and (5,0) CNTs, the Peierls transition dominates, while if dressed with metallic outer shell, namely the (8,8) CNT and (15,0) CNT to form double wall carbon nanotube system(DWNT), the screening effect greatly reduces the Coulomb interaction of inner tubes, and supercondutivity(SS) instability is identified to be the groud state, although the crossover temperature of which SS response functions take over could be very low. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R36.00012: 1D to 3D Crossover Transition in a System of Weakly Coupled Superconducting Nanowires Qihong Chen, Ming Yuan Sun, Zhi Lin Hou, Ting Zhang, Zhe Wang, Wu Shi, Rolf W. Lortz, Ping Sheng Recent Results have shown the existence of superconductivity in quasi-one-dimensional systems, e.g., the 4{\AA} superconducting carbon nanotubes embedded in the aligned, linear pores of the aluminophosphate-five (AFI) zeolite. In order to understand theoretically the experimental observations on the thermal specific heat and the electrical resistance variation as a function of temperature, we have carried out Monte Carlo simulations on a Ginzburg-Landau (GL) model of Josephson-coupled superconducting nanowires. The results show that the competition between 1D fluctuations and the weak transverse Josephson coupling between the nanowires can give rise to a 1D-3D crossover transition at a temperature $T_{C} $ below the mean field $T_{C}^{O} $ of the wires. The electrical resistance can experience a sharp drop at $T_{C} $, at which point the nanowires become phase coherent. The simulated specific heat exhibits a rounded peak between $T_{C} $and $T_{C}^{O} $, whereas the phase correlation length within the ab plane diverges at $T_{C} $ from above, in a manner that is consistent with the occurrence of a BKT-transition in the ab plane. These Monte Carlo simulated behaviors are in excellent agreement with the experimental data. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R36.00013: Phase slip in large array superconducting anti-dot thin films Wei-Li Lee, Hsiang-Hsi Kung, Ting-Hui Chen, Chia-Tso Hsieh, Chi-Chih Ho, Keng-Hui Lin, Wen-Tau Juan Phase slip is one of the most intriguing phenomena in superconducting nanostructure, which gives rise to a finite resistance below superconducting transition temperature. By using a special technique we developed previously for the preparation of a monolayer polymer/nanosphere hybrid, we fabricated a series of large array niobium antidot thin films with niobium line width ranging from about 36 nm to 90 nm. From the resistance and magnetization measurement, we found that the transition width decreases with increasing magnetic field applied along the normal direction of the antidot thin film, which becomes more significant in samples with smaller niobium line width. We argue that this phenomenon provides an evidence for the existence of thermal activated phase slip effect that was discovered for the first time in superconducting antidot thin film structure. Detailed results and analysis will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R36.00014: p-wave Superconductor in a Mesoscopic Size Grain Sungkit Yip, Bor-Luen Huang Motivated by the claim that Sr2RuO4 is a p-wave superconductor with broken time-reversal symmetry in the bulk, and many recent experimental studies of superconductors in mesoscopic size grains, we study theoretically a two-component p-wave superconductor in confined geometries, considering circular disks and rectangular samples, using both Ginzburg-Landau (GL) and quasiclassical (QC) Green function theories. For GL theory with parameters near the weak-coupling limit, we find that a sufficiently small circular disk remains normal. For zero field and intermediate sizes, a disk with sufficiently smooth boundary is in a time-reversal symmetric state, where the order parameter can be represented by a real vector forming a vortex-like structure. Only for larger grains and at lower temperatures can a broken time-reversal state be recovered. For intermediate sizes but with finite external magnetic field, the system can have possibly re-entrant phase transitions. For rectangular samples with sufficiently large aspect ratios, the superconductor near its transition temperature at zero fields has its order parameter vector parallel to the long side of the sample. Within a critical aspect ratio however, the order parameter vector forms a vortex-like structure, much like for the disk. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R36.00015: Superconductivity and anomalous magnetic phase in LuGe$_2$ single crystals Nakheon Sung, B.K. Cho, Y.J. Jo, S.K. Choi, A.I. Coldea, H. Kim, R. Prozorov LuGe$_2$ single crystals (ZrSi$_2$-type orthorhombic structure, $C_{mcm}$) were synthesized by the high temperature metal flux method. LuGe$_2$ was found to be the type-II superconductivity below superconducting transition temperature, $T_c$ = 2.3 K. An anomalous magnetic and electric phase below $T^*$ = 4 K was found only along $b$-axis in electrical transport measurement, and confirmed additionally by magnetization and heat capacity measurement. From the heat capacity data, LuGe$_2$ was found to be the weak coupling BCS type superconductivity at $T_c$, whereas the anomalous phase above $T_c$ seems to have a close correlation with the superconducting state below $T_c$. Upper critical field, $H_{c2}$($T$), of the superconductivity exhibits significant anisotropy either along $b$-axis or along $a$ (and $c$)-axis. Recently, a possible second superconducting state in superconducting YbSb$_2$ single crystals, which has the same structure as LuGe$_2$, was reported. [1] Thus, we will discuss in detail on the anisotropic superconducting properties and the anomalous phase above $T_c$ in terms of the possible second superconductivity as in YbSb$_2$.\\[4pt] [1] Liang L. Zhao, Stefan Lausberg, H. Kim, M. A. Tanatar, Manuel Brando, R. Prozorov, and E. Morosan, Phys. Rev. B 85, 214526 (2012) [Preview Abstract] |
Session R37: Focus Session: Fe-based Superconductors: Nematicity and Related Phenomena
Sponsoring Units: DMP DCOMPChair: Andrey Chubukov, University of Wisconsin
Room: 345/346
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R37.00001: Effects of competing Neel-type magnetic fluctuations and nematic order on the superconductivity of the iron arsenides Rafael M. Fernandes, Andrew J. Millis In many iron-based superconductors, the presence of two matching electron pockets displaced by $(\pi,\pi)$ gives rise to Neel-type magnetic fluctuations, in addition to the usual stripe-type magnetic fluctuations peaked at $(\pi,0)$ and $(0,\pi)$. Indeed, recent neutron scattering experiments observed both types of fluctuations in certain hole-doped iron pnictides, which intriguingly do not display superconductivity. In this talk, we employ an Eliashberg approach to address the impact of competing $(\pi,\pi)$ and $(\pi,0)$ fluctuations on the superconducting state of the iron arsenides. We show that, surprisingly, even weak short-ranged Neel fluctuations strongly suppress the $s^{\pm}$ state. The main contribution to this suppression comes from a repulsive $s^{\pm}$ interaction induced by the Neel fluctuations, and not from the inelastic scattering pair-breaking that they promote. Upon enhancing the strength of the Neel fluctuations, a d-wave state appears, preceded by either an intermediate $s+id$ state or a non-superconducting region, forming a two-dome structure. We compare our results to experimental findings, and discuss their implications to the optimal $T_c$ of the iron arsenides, arguing that it can be enhanced via a tetragonal-symmetry breaking induced by nematic order. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R37.00002: Divergent nematic charge susceptibility in iron-pnictide Yann Gallais, Ludivine Chauvi\`ere, Yanxing Yang, Marie-Aude M\'easson, Maximilien Cazayous, Alain Sacuto, Doroth\'ee Colson We report doping dependent low energy Electronic Raman Scattering measurements in the normal state of Ba(Fe$_{1-x}$Co$_{x}$As) $_{2}$ stress-free twinned single crystals. The Raman response shows a systematic increase at low energy upon approaching the magneto-structural transition. This quasi-elastic peak displays a distinct symmetry dependence which links it to the nematic charge response in the x$^{2}$-y$^{2}$ symmetry channel, indicating an incipient tetragonal symmetry breaking instability in the charge sector. The extracted static nematic charge response shows Curie - Weiss behavior above the magneto-structural transition with a characteristic temperature which decreases with doping. These results allow us to disentangle the respective roles of spin, charge and lattice degrees of freedom in the mechanism of tetragonal symmetry breaking in iron-pnictides superconductors. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R37.00003: Divergent nematic susceptibility in an iron arsenide superconductor Hsueh-Hui Kuo, Jiun-Haw Chu, James Analytis, Ian Fisher Within the Landau paradigm of continuous phase transitions, ordered states of matter are characterized by a broken symmetry. Although the broken symmetry is usually evident, determining the driving force behind the phase transition can be complicated by coupling between distinct order parameters. We show how measurement of the divergent nematic susceptibility of the iron pnictide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ distinguishes an electronic nematic phase transition from a simple ferroelastic distortion. These measurements also indicate an electronic nematic quantum phase transition near the composition with optimal superconducting transition temperature. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R37.00004: In-plane structural and electronic anisotropy in de-twinned CaFe$_2$As$_2$ compounds Erick Blomberg, M.A. Tanatar, S. Ran, S.L. Bud'ko, P.C. Canfield, R. Prozorov In-plane structural and electronic anisotropy has been studied in a wide range of iron-based superconductors by detwinning via uniaxial stress or strain [1]. In particular, materials based on BaFe$_2$As$_2$ ("112") are among the most studied systems, where different dopants, annealing protocols and different flux growths were extensively explored. However CaFe$_2$As$_2$ remains a much less studied compound and it exhibits properties quite different from Ba-based 122's [2]. Here we report polarized-light microscopy and electric transport measurements of strain-detwinned CaFe$_2$As$_2$ compounds. Our results reveal unusual evolution of the structural, electronic and magnetic properties dependent on annealing, growth from Sn flux vs FeAs flux, and doping, as compared to BaFe$_2$As$_2$. Among the key observations are the differences in twin domain evolution, and a hysteresis in structural and electronic anisotropy upon warming and cooling. This work was supported by the Department of Energy Office of Science, Basic Energy Sciences under Contract No. DE-AC02-O7CH11358.\\[4pt] [1] M. A. Tanatar, E. C. Blomberg, et. al. Phys. Rev. B 81, 184508 (2010).\\[0pt] [2] S. Ran, et. al. Phys. Rev. B 85, 224528 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R37.00005: Chasing the nematic phase in detwinned Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ with optical investigations C. Mirri, S. Bastelberger, A. Dusza, A. Lucarelli, H.-H. Kuo, J.-H. Chu, I.R. Fisher, L. Degiorgi A renewed interest in the study of symmetry-breaking competing states in complex interacting systems followed the discovery of a broken rotational symmetry, due to stripe or nematic order, in the pseudogap phase of the copper oxide superconductors. The most recent playground in which to address the competition between structural, magnetic and superconducting phases is provided by the iron-pnictide superconductors. In these systems, the non-superconducting parent compounds undergo an antiferromagnetic transition into a broken-symmetry ground state at T$_N$, which is always preceded by or coincident with a tetragonal-to-orthorhombic structural distortion at T$_s$. Here, we investigate the optical conductivity with light polarized along the in-plane orthorhombic a- and b-axis of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ for x=0, 2.5\% and 4.5\% (i.e., in the so-called underdoped regime) under $tunable$ uniaxial pressure across their structural and magnetic transitions. We estimate the dichroism, which extends to high frequencies and temperatures. All together, our results on such single domain specimens reveal a nematic susceptibility as well as the electronic nature of the structural transition. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R37.00006: Coupled orthorhombic distortion, antiferromagnetism, and superconductivity in a single twin domain of Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ (x$=$0.047) Qiang Zhang, Wenjie Wang, B. Hansen, N. Ni, S.L. Bud'ko, P.C. Canfield, R.L. McQueeney, D. Vaknin, J.W. Kim The interplay between structure, magnetism, and superconductivity in single crystal Ba(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ (x$=$0.047) has been studied using high-resolution X-ray diffraction by monitoring charge Bragg peaks in each twin domain separately. The emergence of superconducting state is correlated with the suppression of theorthorhombic distortion around $T_{C}$, exhibiting the competition between orthorhombicity and superconductivity. Above $T_{S}$, the Bragg peak widths gradually broaden, possibly induced by orthorhombic (nematic) fluctuations in the paramagnetic tetragonal phase. Upon cooling, anomalies in the peak width are observed at $T_{S}$ and also $T_{N}$ indicative of strong magnetoelastic coupling. Using the capability to study individual twin domains, the peak widths in the \textit{ab}-plane are found to exhibit anisotropic behavior along and perpendicular to the stripe-type AFM wave vector. In contrast, the temperature dependencies of the out-of-plane peak width showan anomaly at $T_{N}$, reflecting the connection between Fe-As distance and Fe local moment. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R37.00007: Nematic transition and hidden quantum critical point in iron-pnictide superconductors Invited Speaker: Shigeru Kasahara A central issue in the physics of iron-based superconductivity concerns the origin of the pairing interaction, in which the importance of the spin and orbital degrees of freedoms has been discussed. Clarifying the anomalies inherent to this system and unveiling their connections to the high-temperature superconductivity are of primary importance. Here, we report our investigations on clean single crystals of BaFe$_2$(As$_{1-x}$P$_x$)$_2$ [1]. The observed quantum critical point (QCP) behaviors as represented by non-Fermi liquid transport [1], effective mass enhancement [2], and a sharp peak in the zero-temperature magnetic penetration depth [3] at a critical doping are discussed. In addition, we discuss the development of electronic nematicity, a unidirectional self organized state which breaks the underlying crystal lattice symmetry. Our highly sensitive magnetic anisotropy measurements, together with high resolution synchrotron X-ray diffraction experiments, indicate that electronic nematicity develops in the normal state, far above the magneto-structural and superconducting transitions, resulting in a new phase diagram of iron-based superconductors. The development of electronic nematicity appears to help the emergence of superconductivity whilst the QCP provides the highest superconducting transition temperature.\\[4pt] [1] S. Kasahara et al., Phys. Rev. B {\bf 81}, 184519 (2010).\\[0pt] [2] H. Shishido et al., Phys. Rev. Lett. {\bf 104}, 057008 (2010).\\[0pt] [3] K. Hashimoto et al., Science {\bf 336}, 1554-1557 (2012).\\[0pt] [4] S. Kasahara, et al., Nature {\bf 486}, 382-385 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R37.00008: Electronic anisotropy in Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$ revealed by ARPES Yoonyoung Koh, Yeongkwan Kim, Wonsig Jung, Manjin Eom, Junsung Kim, Changyoung Kim One of the central issues in field of iron pnitides is the origin of electronic anisotropy observed by in-plane resistivity measurement and STM quasi-particle interference patterns. It is believed that it is related to magnetism and plays an important role in superconductivity in iron pnictides. It was argued that the split bands in ARPES data are from two orthogonal bands with dominant d$_{xz}$ and d$_{yz}$ characters, demonstrating the in-plane electronic anisotropy. It appears to be consistent with anisotropy observed by other probes. We performed temperature dependent ARPES measurements on an iron pnictide system, Ba(Fe$_{1-x}$Ru$_{x})_{2}$As$_{2}$, to experimentally verify existence of electronic anisotropy and compare the results with those of BaFe$_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R37.00009: Elastic softening of the shear modulus in Fe-based superconductors C. Meingast, A. Boehmer, P. Adelmann, R. Fromknecht, P. Schweiss, Th. Wolf, F. Hardy, W. Schranz, M. Reinecker A strong softening of the elastic shear modulus C$_{\mathrm{66}}$ has been observed as one approaches the spin-density-wave (SDW) transition in Ba122 from high temperature [1,2]. A smaller softening is still observed for superconducting Co-doped Ba122 crystals, followed by distinct hardening below T$_{\mathrm{c\thinspace }}$[1,2]. This elastic response has been taken as evidence either for electronic-magnetic nematic fluctuations [1], or as evidence for a structural quantum critical point near optimal doping [2]. Here we study the elastic response of various Fe-based superconductors by a recently developed technique based upon a three-point bending experiment in a high-resolution capacitance dilatometer. We measure the temperature dependence of the Young's modulus, which for thin slabs can be shown to be closely related to C$_{\mathrm{66}}$ for a given orientation. This is confirmed by measurements on Co-doped Ba122, for which we find very similar results as previously reported [1,2]. We will report on new measurements of the Young's modulus on other Fe-based Ba122 systems in order to study the universality of the elastic response at the SDW and superconducting transitions. [1] R. M. Fernandes, et al., Phys. Rev. Lett. 105, 157003 (2010). [2] M. Yoshizawa, et al., J. Phys. Soc. Jpn. 81, 024604 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R37.00010: Magnetic Origin of Electronic Nematicity in NaFeAs (Part I) Carlos J. Arguello, Ethan Rosenthal, Erick Andrade, Rafael Fernandes, Andrew Millis, Changqing Jin, Abhay Pasupathy Several experiments have shown that the parent states of the iron pnictides display electronic nematicity at high temperature, where the electronic states spontaneously break the rotational symmetry of the crystal lattice. A common feature displayed by many pnictide systems is a tetragonal to orthorhombic distortion on cooling down the system below $T_{S}$ and a magnetically ordered phase below $T_{SDW}$. In particular, NaFeAs has a structural to orthorhombic transition ($T_{S}$=54K) and a SDW transition ($T_{SDW}$= 39K). This wide temperature difference between transitions makes it an excellent testing ground for the characterization of the electronic states in each one of these regimes. The electronic states of this material can be directly visualized as a function of temperature using atomic-resolution scanning tunneling microscopy/spectroscopy. Real-space images of the electronic states show domains on the micron scale, with a strong unidirectional character persisting to temperatures well above $T_{S}$. These unidimensional features are found to be localized around defects in the system. We will discuss the details of the energy and temperature dependence of these features in both real space and Fourier space, as well as draw differences with the structurally similar LiFeAs. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R37.00011: Magnetic Origin of Electronic Nematicity in NaFeAs (Part II) Ethan Rosenthal, Carlos Arguello, Erick Andrade, Rafael Fernandes, Andrew Millis, Changqing Jin, Abhay Pasupathy The characterization of possible broken symmetries is essential to understanding high-temperature superconductivity. The electronic states of many iron-based superconductors have been shown to break rotational symmetry, but the origin of this nematicity remains elusive. We use Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) to directly visualize the spatial structure of electronic states in NaFeAs. Intrinsic defects produce unidirectional spectroscopic features that persist to temperatures well above both the spin density wave (SDW) and orthorombic transitions. By comparing our measurements to angle-resolved photoemission spectroscopy (ARPES) data on the same material, we find that these features arise from quasiparticle interference (QPI) in the presence of magnetic order, indicating the primary role of spin interactions in electronic nematicity. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R37.00012: Nematicity driven by hybridization in the iron-based superconductors Valentin Stanev, Peter Littlewood We introduce an effective three-orbital model to study the normal state of the iron-based superconductors. It has both itinerant and localized electrons - the former originate from the d$_{xz}$/d$_{yz}$ iron orbitals, and the latter from the d$_{xy}$ iron orbitals. These distinct degrees of freedom are coupled through hybridization and onsite interactions. On a mean-field level this model has an excitonic instability, driven by the effective delocalization of the d$_{xy}$ electrons. Because of the multiband character of the itinerant Fermi surface the ordered state can spontaneously break the lattice rotation symmetry (and thus is nematic) and generate orbital order. In this scenario the nematic state is induced by the coupling of the d$_{xz}$/d$_{yz}$ with the d$_{xy}$ iron orbitals, rather than the presence of magnetic order, or the proximity to such. We propose this mechanism as an explanation of the tendency towards nematicity observed in several iron-based compounds, and study some of its experimental consequences. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R37.00013: Nematic state of the pnictides induced by the interplay between the spin, orbital, and lattice degrees of freedom Shuhua Liang, Adriana Moreo, Elbio Dagotto The nematic state with orthorhombic distortion observed in several iron based superconductors is stabilized in the undoped three-orbital ($xz$, $yz$, $xy$) spin-fermion model [1] via the addition of lattice degrees of freedom. The Monte Carlo simulations show that the electron-phonon coupling is not sufficient to stabilize the experimentally observed lattice distortion. The nematic phase is induced instead by the spin-lattice coupling. The interplay between the coupling strength of the lattice to the magnetic and charge degrees of freedom determines the separation between the structural and the magnetic transitions. Experimental results for the anisotropic behavior of the resistivity and the orbital spectral weight as a function of the temperature are also reproduced by the numerical simulations.[2] [1] S. Liang, G. Alvarez, C. Sen, A. Moreo, and E. Dagotto, Phys. Rev. Lett. 109 047001 (2012) and references therein. [2] S. Liang, A. Moreo, and E. Dagotto, submitted for publication. [Preview Abstract] |
Session R38: Focus Session: Scalable Technologies for Photovoltaics I
Chair: Sue A. Carter, University of California at Santa CruzRoom: 347
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R38.00001: Increasing the band gap of iron pyrite by alloying with oxygen Matthew Law, Jun Hu, Yanning Zhang, Ruqian Wu Systematic density functional theory studies and model analyses have been used to show that the band gap of iron pyrite (FeS$_{2})$ can be increased from $\sim$ 1.0 to 1.2 -1.3 eV by replacing $\sim$ 10{\%} of the sulfur atoms with oxygen atoms (i.e., $\sim$ 10{\%} O$_{S}$ impurities). O$_{S}$ formation is exothermic, and the oxygen atoms tend to avoid O-O dimerization, which favors the structural stability of homogeneous FeS$_{2-x}$O$_{x}$ alloys and frustrates phase separation into FeS$_{2}$ and iron oxides. With an ideal band gap, absence of O$_{S}$induced gap states, high optical absorptivity, and low electron effective mass, FeS$_{2-x}$O$_{x}$ alloys are promising for the development of pyrite-based heterojunction solar cells that feature large photovoltages and high device efficiencies. Acknowledgement: We thank the NSF SOLAR Program (Award CHE-1035218) and the UCI School of Physical Sciences Center for Solar Energy for support of this work. Calculations were performed on parallel computers at NERSC and at NSF supercomputer centers. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R38.00002: Effect of annealing conditions on the healing of sulfur vacancy in pyrite FeS2(100) surfaces Amanda Weber, Yanning Zhang, Nicholas Berry, Matthew Law, Ruqian Wu Through density functional calculations, we investigated the segregation of a sulfur vacancy from interior sites outward to the FeS$_2$(100) surfaces in different surface conditions in order to provide guidance for the development of iron pyrite in photovoltaics applications. We found that the surfaces with interior S-vacancy are energetically unstable and bulk S-vacancies tend to hop toward the surface, in particular when the surface composition is in the stoichiometric or S-rich side. The segregation process is accompanied by redox reaction near the vacancy site, Fe(2$+) \quad +$ S(1-) $\to $ Fe(3$+) \quad +$ S(2-), and the activation energy decreases near the surface region. We compare the calculated structural, energetic and electronic properties to experimental data, and provide insights for reduction of vacancy density in optimal annealing conditions. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R38.00003: A new paradigm for thin-film solar cells: the case of Earth abundant Cu-N ternary compounds Julien Vidal, Xiuwen Zhang, Stephan Lany, Andriy Zakutayev, David Ginley, Minghui Yang, Amy Allen, Franck DiSalvo The design of thin film solar cells is extremely sensitive to the choice of the material forming the absorbing layer. Indeed, many of the limitations of solar cell devices are either directly linked to the intrinsic properties of the absorber such as in CdTe or design-related indirect consequences of this choice such as for SnS-based devices. Most of the design of current thin film solar cells rely on chalcogenide materials as the absorbing layer. We propose a new paradigm based on Earth abundant Cu-N ternary compounds as the absorbing layer. We will present the theoretical and experimental investigation of the electronic properties of two Cu-N compounds with interesting photovoltaic properties namely CuSrN and CuTaN$_{2}$. We performed state-of-the-art defect calculation and GW-based band structure calculations. CuTaN$_{2}$ was synthesized by ion exchange and its absorption onset was subsequently characterized with diffusive reflectance. While CuSrN displays interesting p-doped capability and defect immunity similar to Cu(In,Ga)Se$_{2}$, CuTaN$_{2}$ presents very strong absorption with a sharp absorption onset in the optimal range for photovoltaic conversion. Finally, we will address potential pitfalls of such absorbers related to stability with respect to O$_{2}$ and H$_{2}$O. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R38.00004: PLD growth of thin film Zinc Phosphide Rajesh Vaddi, Parag Vasekar, Charles Westgate, Bruce White The development of efficient, low cost solar cells to meet society's growing energy needs has triggered tremendous interest in developing photovoltaics formed from earth abundant materials. Zinc phosphide (Zn3P2) is a promising earth abundant absorber layer for photovoltaic energy conversion with a nearly ideal band gap (1.5eV) and a large absorption coefficient of 10$^{4}$/cm. In this work we examine the growth parameters, electrical and optical properties of thin film zinc phosphide produced using pulsed laser deposition (PLD) from a zinc phosphide target at laser fluencies ranging from 1-3 J/cm2. For the laser fluences explored, highly resistive amorphous zinc phosphide thin films were produced with a band gap of approximately 1.7 eV. The thin films could be transformed from amorphous to polycrystalline zinc phosphide by annealing at 400C for 15mins in a N2 atmosphere. High resolution X-ray photoelectron spectroscopy (XPS) is used to examine the binding energies of Zn 2p3/2 and Phosphorous 2p3/2 signals and are in the range of 1021.6 eV and 127.5 eV. Energy Dispersive X-ray Spectroscopy (EDAX) revealed that the Zn3P2 thin films are nearly stoichiometric in composition. Hall mobility in these materials and Zn3P2/ZnS hetrojunction solar cell performance will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R38.00005: The electronic properties of point defects in earth-abundant photovoltaic material Zn$_{3}$P$_{2}$: A hybrid functional method study Wan-Jian Yin, Yanfa Yan Zinc phosphide (Zn$_{3}$P$_{2})$ is an attractive and promising semiconductor for thin-film solar cell application because of its earth abundance and ease of thin-film fabrication. The electronic properties of intrinsic and extrinsic defects in Zn$_{3}$P$_{2}$ are studied by density-functional theory with hybrid functional method. Our results show that undoped Zn$_{3}$P$_{2}$ should be intrinsically $p$-type with Zn vacancies as the responsible shallow acceptors. Na or Cu doping is expected to result in improved $p$-type conductivity as compared to intrinsic Zn$_{3}$P$_{2}$. S or Al doping may lead to weak $n$-type Zn$_{3}$P$_{2}$. Doping of Mg does not produce good $n$-type Zn$_{3}$P$_{2}$, consistent with experimental observations. Contradicting to conventional wisdom, an interstitial P in Zn$_{3}$P$_{2}$ is not a triple-hole acceptor and a P vacancy in Zn$_{3}$P$_{2}$ is not a triple-electron donor. Instead, we find that the interstitial P is actually a single-hole acceptor and the P vacancy is a single-electron donor. The origins of these unusual behaviors are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R38.00006: Local structure of Cu2S/ZnS multi-layer films prepared using ALD Frank Bridges, Leila Jewell, Andrew Short, Glenn Alers, Sue A. Carter We present local structure studies of ZnS, Cu$_2$S, and ZnS/Cu$_2$S composite films, using extended x-ray absorption fine structure (EXAFS) technique. The films were prepared using atomic layer deposition (ALD), which can in principle deposit films layer by layer and hence form mesoscopic structures. ZnS and Cu$_2$S films prepared using ALD are very similar to the bulk material; the main difference is a reduced amplitude for the second neighbor Zn-Zn peak in ZnS, suggesting increased disorder within the film. Relative disorder in the films also increases with decreasing thickness as well as with decreasing deposition temperature. More importantly, multi-layer ZnS/Cu$_2$S films prepared using the same parameters as for individual films do not produce the expected multi-layer for $\sim$1 nm thick layers. If there is some excess Zn, the multi-layer is predominately ZnS and the Cu$_x$S fraction is highly disordered, and may include some ZnS:Cu. In contrast if there is a little Cu excess, the film is nearly all Cu$_2$S and the small Zn fraction is highly disordered ZnS with a shifted Zn-S distance. Consequences for multi-layer formation for solar cell applications will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R38.00007: Enhancement of solar absorption with black Cu2O Nanostructures Hui Xing, John Hatch, Dengxin Ji, Kenneth Kort, Biplob Barman, Yu Tsung Tsai, Yueling Qin, Sarbajit Banerjee, Athos Petrou, Qiaoqiang Gan, Hong Luo, Hao Zeng Cu$_{2}$O is a direct gap semiconductor with a band gap of 2.1 eV. It was considered to be a solar absorber material, while the application is hindered by its large band gap and weak stability. Here we report an electrochemical synthesis of Cu$_{2}$O. By rationally control the synthetic parameters, we achieved two types of Cu$_{2}$O: one of black color and the other ``normal'' red Cu$_{2}$O. Both Cu$_{2}$O films were in cubic phase and their crystal structures are almost identical as seen by X-ray diffraction. This is further corroborated by their nearly identical Raman spectra. The scanning tunneling spectrum (STS) revealed a gap in the red Cu$_{2}$O around 2.1 eV and a significantly lowered gap of $\sim$ 1.7 eV in the black Cu$_{2}$O, indicating that the black color is caused by a change in the electronic structure. The reflectance and transmittance indicated a band gap of $\sim$ 1.7 eV for the black Cu$_{2}$O, with a significantly broadened absorption spectrum. While further effort is needed to understand the mechanism for the lowering of the band gap, we believe that our approach demonstrated means to promote earth abundant and nontoxic materials for potential photovoltaic applications through band gap engineering. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R38.00008: First-principles electronic structure of $\beta $-FeSi$_{2}$ and FeS$_{2}$ surfaces Pengxiang Xu, Timo Schena, Stefan Bl\"{u}gel, Gustav Bihlmayer Applying density functional theory in the framework of the full-potential linearized augmented plane-wave (FLAPW) method [1], we investigated electronic structure of potential future photovoltaic materials, $\beta $-FeSi$_{2}$ and FeS$_{2}$, for selected surface orientations and terminations. The most stable orientations are determined by comparing their surface energy. Detailed electronic structure calculations show that surface states originating from Fe play an important role and might determine photovoltaic properties. Our results show that anti-ferrimagnetic ordering exists for Fe-terminated surface. Furthermore, we also studied how electronic structure and photovoltaic efficiency are affected by the recently observed structural defects such as stacking fault in $\beta $-FeSi$_{2}$. \\[4pt] [1] www.flapw.de [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R38.00009: Photocurrent studies on continuous large area monolayers of WS$_2$ and MoS$_2$ Nestor Perea-Lopez, Ana Laura Elias-Arriaga, Humberto Rodriguez-Gutierrez, Ruitao Lu, Andres Castro, Saikat Talapatra, Sujoy Ghosh, Ayse Berkdemir, Florentino Lopez-Urias, Humberto Terrones, Mauricio Terrones Continuous large area monolayers of WS$_{2}$ and MoS$_{2}$ synthesized by chemical vapor deposition were used as light sensing devices. I-V measurements and photo response measurements were performed on both materials. The photocurrent measurements were carried out from 300 $^{\circ}$K down to 10 $^{\circ}$K using various visible laser wavelengths (405 nm, 488 nm, 514 nm and 667 nm). A resistance decrease was registered on both materials when illuminated with the laser beam, such change was proportional to the laser photon energy and when the laser energy was lower than the band gap of each material, no photo response was observed. The layered materials were structurally characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. Raman spectra confirms the presence of monolayers and UV-visible spectra revealed the resonance peaks at the energies close to the direct band gap predicted for single layers of WS$_{2}$ and MoS$_{2}$ (2.05 eV and 1.85 eV ). Further experiments on time response and continuous spectral response are now underway and will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R38.00010: Printable CIGS thin film solar cells Xiaojuan Fan Among the various thin film solar cells in the market, CuInGaSe thin film solar cells have been considered as the most promising alternatives to crystalline silicon solar cells because of their high photo-electricity conversion efficiency, reliability, and stability. However, many fabrication methods of CIGS thin film are based on vacuum processes such as evaporation and sputtering techniques which are not cost efficient. This work develops a solution method using paste or ink liquid spin-coated on glass that would be competitive to conventional ways in terms of cost effective, non-vacuum needed, and quick processing. A mixture precursor was prepared by dissolving appropriate amounts of composition chemicals. After the mixture solution was cooled, a viscous paste was prepared and ready for spin-coating process. A slight bluish CIG thin film on substrate was then put in a tube furnace with evaporation of metal Se followed by depositing CdS layer and ZnO nanoparticle thin film coating to complete a solar cell fabrication. Structure, absorption spectrum, and photo-electricity conversion efficiency for the as-grown CIGS thin film solar cell are under study. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R38.00011: Crystal structure of Cu$_2$Te predicted within adaptive genetic algorithm Kai-Ming Ho, Manh Cuong Nguyen, Jinho Choi, Cai-Zhuang Wang, Xin Zhao, Zhenyu Zhang Cu$_2$Te is one of the most commonly used conductive back-contacting materials for high-efficiency CdTe-based solar cells. However, the detailed crystal structure of Cu$_2$Te is still undetermined blocking property investigations of the Cu$_2$Te-based solar cell systems. Some models have been proposed but all of them have positive formation energy [1]. We have performed adaptive genetic algorithm crystal structure search to find low energy crystal structures of Cu$_2$Te. We found a new layered-structure edged by Te atoms with negative formation energy from first-principles calculations within local density approximation. This layered structure consists of tilted Cu$_2$Te ribbon arrays. Structural and electronic properties of the newly found Cu$_2$Te structure will be discussed in detail.\\[4pt] [1] J. L. F. Da Silva, S.-H. Wei, J. Zhou, and X. Wu, Appl. Phys. Lett. 91, 091902 (2007) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 5:18PM |
R38.00012: Solution processed solar cells using earth abundant materials for terawatt scale energy production Invited Speaker: David Mitzi |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R38.00013: High photoactivity in ultrathin as-grown hematite films prepared by atomic layer deposition Jeffrey Klug, Nicholas Becker, Shannon Riha, Alex Martinson, Jeffrey Elam, Michael Pellin, Thomas Proslier Nanostructured hematite ($\alpha$-Fe$_2$O$_3$) has been widely studied for use in a variety of thin film applications including solar energy conversion, water oxidation, catalysis, and gas sensing. Among established deposition methods, atomic layer deposition (ALD) is a leading technique for large-scale, controlled synthesis of a wide range of nanostructured materials. In this work, ALD of Fe$_2$O$_3$ is demonstrated using FeCl$_3$ and H$_2$O precursors at growth temperatures between $200-350^{\circ}$C. Self-limiting growth of Fe$_2$O$_3$ is observed with a growth rate of $\sim0.06$ nm/cycle. As-deposited, films are nanocrystalline with low Cl impurities and a mixture of $\alpha$- and $\gamma$-Fe$_2$O$_3$. Post-deposition annealing in O$_2$ leads to phase-pure hematite with increased out-of-plane grain size. Photoelectrochemical measurements under simulated solar illumination reveal high photoactivity toward water oxidation in both as-deposited and post-annealed films. Planar films deposited at low temperature (235$^{\circ}$C) exhibit remarkably high photocurrent densities $\sim0.71$ mA/cm$^{2}$ at 1.53 V vs. the reversible hydrogen electrode (RHE) without further processing. Films annealed in air at 500$^{\circ}$C show current densities of up to 0.84 mA/cm$^{2}$ (1.53V vs. RHE). [Preview Abstract] |
Session R39: Pattern Formation and Nonlinear Dynamics
Sponsoring Units: DFDChair: Gregory Eyink, Johns Hopkins University
Room: 348
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R39.00001: The Peierls Transport Equation --- Revised Yi-Kang Shi, Gregory Eyink In 1929 Peierls derived an equation for the joint probability distribution of all wave amplitudes in classical and quantum anharmonic systems, still widely used in quantum transport theory, plasma physics and weak wave turbulence. For uncorrelated amplitudes, it implies the kinetic equation for the wave spectrum/occupation numbers. This equation was rederived by Brout \& Prigogine (1956), Zaslavskii \& Sagdeev (1967), and recently by Choi et al. (2005) in wave turbulence. We show that these derivations are non-systematic, retaining terms smaller than those neglected. We obtain an equation simpler than Peierls', which still implies the kinetic equation and also a generalized kinetic equation for the distribution of single-mode amplitudes, previously obtained by Choi et al. We show by an H-theorem that the single-mode distributions approach Gaussian, if this equation is valid for all amplitudes. Non-Gaussian statistics can arise if the equation breaks down for large amplitudes/strong nonlinearity. This may explain intermittency observed in laboratory experiments of weak turbulence. Moreover, we show the most general solutions of our revised Peierls equations are statistical ensembles of chaotic solutions of kinetic equations, or ``super-turbulence'', another source of intermittency. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R39.00002: Controlling the position of traveling fronts Jakob L\"{o}ber, Harald Engel, Eckehard Sch\"{o}ll We present a method to control the position as a function of time of a one-dimensional traveling front solution of a one-component reaction-diffusion system according to a specified protocol of movement. Given this protocol, the control function is found as the solution of a perturbatively derived integral equation. Two cases are considered. First, we derive an analytical expression for the space $(x)$ and time $(t)$ dependent control function $f\left(x,t\right)$ valid for arbitrary protocols and arbitrary bistable reaction kinetics. These results for the control agree well with results of an optimal control algorithm. Second, for stationary control the integral equation reduces to a Fredholm integral equation of the first kind. For the Schl\"{o}gl model, we present an analytical solution of the problem to stop a front at a specified position. All analytical results are in good agreement with numerical simulations of the underlying reaction-diffusion equations. Extensions to two spatial dimensions and other equations supporting traveling wave solutions are considered. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R39.00003: Experimental studies of stationary reaction fronts in a chain of vortices with imposed wind Tom Solomon, Carleen Boyer We present experiments that study the behavior of the excitable Belousov-Zhabotinsky (BZ) reaction in a chain of alternating vortices with an imposed uniform wind. Previous experiments\footnote{M.E. Schwartz and T.H. Solomon, Phys. Rev. Lett. {\bf 100}, 028302 (2008).} have shown that fronts in this system are pinned for a wide range of imposed wind speeds, propagating neither forward against the wind nor in the downwind direction. We explain this behavior with a recent theory\footnote{J. Mahoney, D. Bargteil, M. Kingsbury, K. Mitchell and T. Solomon, Europhys. Lett. {\bf 98}, 44005 (2012).} that proposes the existence of {\em burning invariant manifolds} (BIMs) that act as local barriers to front propagation. Fronts are pinned when a BIM or a combination of BIMs spans the width of the vortex chain, blocking the reaction front. We show experimental measurements of the shape of the pinned front for a range of different wind speeds, and compare these shapes to the BIMs calculated theoretically. We also consider the dependence of the front shape on the location of the initial trigger for the front. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R39.00004: Pinning of reaction fronts by burning invariant manifolds in spatially-disordered fluid flows Maya Najarian, Tom Solomon We present experiments on the pinning of reaction fronts in spatially-disordered fluid flows with an imposesd wind. The disordered flow is driven by a magnetohydrodynamic forcing technique, and there is a uniform wind imposed on the flow with the use of a translation stage. Reaction fronts are produced using the excitable Belousov-Zhabotinsky chemical reaction. For a wide range of wind speeds, a complicated stationary front forms, pinned to the underlying vortex flow, neither propagating forward against the wind nor being blown backwards. The shape of the front depends significantly on the magnitude of the imposed wind. We propose that the shape of the stationary front is determined by a collection of overlapping BIMs that act as barriers against forward movement of the reaction front. The location of the BIMs can be predicted by integrating a three-dimensional set of ordinary differential equations\footnote{J. Mahoney, D. Bargteil, M. Kingsbury, K. Mitchell and T. Solomon, Europhys. Lett. {\bf 98}, 44005 (2012).} that describes the dynamics of an element of an evolving reaction front in the fluid flow. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R39.00005: Laboratory Scale Simulating of Strange Spiral Plumes in Fluid with Hight Ptandtl Number Albert Sharifulin, Anatoly Poludnitsin We experimentally investigated the appearance of a plumes from local hot spot and study its interaction with cellular flow in closed cavity filled by silicon oil with Prandtl number $\Pr \approx 2\cdot 10^{3}$. Convective plume generated by a local heat source, located on the top of the small rubber cylinder, which is located in the center of the bottom of the rectangular cell. To simulate the hot-spot green laser has been used. Roll-type large-scale convective flow was generated by heating of the one vertical sides of cavity. Influence of power of hot point on the shape of plume has been investigated. It is shown that the presence of cellular convective motion may lead to the formation of a strange spiral convective plume. This plume looks like Archimedes spiral replaced on vertical plane. Physical mechanism of the formation of strange spiral plume and application of obtained results for mantle convection problems are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R39.00006: Double-diffusive layers adjacent to cold chimney flows during transient mushy-layer growth Jin-Qiang Zhong, Qiwei Xue, John Wettlaufer We examine the cooling effect of chimney flows in the liquid region during transient upward growth of a mushy layer in solidifying aqueous ammonium chloride. Through drainage channels in a mushy layer, cold, relatively fresh fluid is carried into the warm, salt-stratified liquid region. Double-diffusive cells form due to the cooling effect of the chimney flows and evolve into a series of downwelling horizontal layers. Using shadowgraph methods and dyed fluids we demonstrate the vigorous flow circulations and compositional mixing within each layer. Vertical concentration and temperature profiles reveal the double-diffusive staircase structure across the layers. The downward velocity of the layers decreases as they approach to the mush-liquid interface, which is interpreted by a filling-box model representing the momentum and compositional transport of turbulent continuous plumes in a confined region. The present experiment provides insight to evaluate the solute fluxes from growing mushy layers. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R39.00007: Connectivity-disorder effect on collective synchronization Jaegon Um, Hyunsuk Hong, Hyunggyu Park We investigate a system of random frequency oscillators coupled through sparse random networks and explore connectivity-disorder effects on collective synchronization. In particular, we pay attention to how the random quenched disorder in connectivity affects the nature of synchronization transitions. The oscillator frequencies are assigned independently from an unimodal, bimodal, or uniform distribution. Extensive numerical simulations as well as the mean-field analysis have been performed on Erd{\"o}s-R{\'e}nyi random networks. We find that the quenched connectivity disorder invalidates the mean-field prediction of distinctive transition natures depending on frequency distributions in random networks. In fact, the same continuous synchronization transition is found for all types of frequency distributions. The physical origin of this unexpected result is discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R39.00008: Geometry of branching stream networks Hansjorg Seybold, Robert Yi, Alex Petroff, Olivier Devauchelle, Daniel Rothman River networks have been a source of fascination for centuries. Yet, how these networks form and create these geometries remains elusive. Recently we have shown that streams branching in a diffusive field bifurcate at a characteristic angle of $\alpha=2\pi/5=72^\circ$. This result is obtained from Lowner dynamics by combining classical results of groundwater hydrology with the hypothesis that streams grow in the direction of maximal water flux into the channel's tip. Our theoretical results are umambigously consistent with field measurements we conducted in a 100 km$^2$ channel network on the Florida Panhandle. Here we extend our theory to include slope effects and apply our analysis to large drainage basins. We hypothesize that the extension of the network at the tip is driven by a diffusive process leading to a (slope corrected) $2\pi/5$ branching at the leaves of the network. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R39.00009: Absence of power-law scaling in the dendritic crystal growth of ammonium chloride Andrew Dougherty We report measurements of the dendritic crystal growth of NH$_4$Cl from supersaturated aqueous solution at small supersaturations, with a goal of understanding the origin of the sidebranching structure. The early detection of sidebranches requires measurements of small deviations from the smooth steady state shape, but that underlying shape is not precisely known at the intermediate distances relevant for sidebranch measurements. We find that no simple power law describes the average crystal shape, the average sidebranch amplitude, or the average sidebranch envelope. Instead, the effective power law exponents appear to increase steadily as a function of distance from the dendritic tip. Comparisons of the amplitude of sidebranches with that predicted by models of noise-driven sidebranching require careful measurements of materials parameters such as the capillary length. Previous published estimates for this material varied by over a factor of 20. We report new measurements of the capillary length and find $d_0 = 0.224 \pm 0.005\;$nm. Based on those new measurements, we find that the amplitude of the sidebranches in this system is larger than expected from numerical models. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R39.00010: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R39.00011: Multistable dynamics in electroconvecting liquid crystals Zrinka Greguric Ferencek, John Cressman Nonlinear driven system can exhibit a diverse range of dynamics, from highly ordered to chaotic. These systems are ubiquitous, from atmospheric phenomena to brain function. Here we study such dynamics in electroconvecting liquid crystals. There applied electric fields create structured roll-like patterns that support the creation, evolution, and annihilation of defects in the rolls. By using a time scale separation algorithm based on diffusion map delay coordinates we have been able to identify a small number of multistable dynamics in this system. We utilize perturbations to control or steer the system between these different dynamics. We will discuss how this method of identification and interaction can be utilized to better interact with a wide range of dynamic systems. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R39.00012: Coherent Pattern Prediction in Swarms of Delay-Coupled Agents Luis Mier-y-Teran-Romero, Eric Forgoston, Ira Scwartz We consider a general swarm model of self-propelling particles interacting through a pairwise potential in the presence of a fixed communication time delay. Previous work has shown that swarms with communication time delays and noise may display pattern transitions that depend on the size of the coupling amplitude. We extend these results by completely unfolding the bifurcation structure of the mean field approximation. Our analysis reveals a direct correspondence between the different dynamical behaviors found in different regions of the coupling-time delay plane with the different classes of simulated coherent swarm patterns. We derive the spatio-temporal scales of the swarm structures, and also demonstrate how the complicated interplay of coupling strength, time delay, noise intensity, and choice of initial conditions can affect the swarm. In addition, when adding noise to the system, we find that for sufficiently large values of the coupling strength and/or the time delay, there is a noise intensity threshold that forces a transition of the swarm from a misaligned state into an aligned state. We show that this alignment transition exhibits hysteresis when the noise intensity is taken to be time dependent. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R39.00013: Structure-Property Relationships for Branched Worm-Like Micelles Gregory Beaucage, Durgesh Rai Micellar solutions can display a wide range of phase structure as a function of counter ion content, surfactant concentration, and the presence of ternary components. Under some conditions, common to consumer products, extended cylindrical structures that display persistence and other chain features of polymers are produced. These worm-like micelles (WLMs) can form branched structures that dynamically change under shear and even in quiescent conditions. The rheology of these branched WLMs is strongly dependent on migration of the branch points, and the dynamics of branch formation and removal. Persistence and other polymer-based descriptions are also of importance. We have recently developed a scattering model for branched polyolefins and other topologically complex materials that can quantify the branching density, branch length, branch functionality and the hyperbranch (branch-on-branch) content of polymers. This work is being extended to study branching in WLMs in work coupled with Ron Larson at UMich to predict rheological properties. [Preview Abstract] |
Session R40: Focus Session: Growth and Pattern Formation on Surfaces
Sponsoring Units: DMPChair: Zhenyu Zhang, University of Science and Technology of China
Room: 349
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R40.00001: Lateral Charge Transport in Atomically Clean, Ultra-thin Crystalline-Silicon Sheets Weiwei Hu, Shelley Scott, R.B. Jacobson, Donald Savage, Mark Eriksson, Max Lagally In very thin, atomically clean crystalline-Si sheets (``nanomembranes''), the electrical conductance is controlled by the surface. Conductance can occur either through surface transfer doping or directly in the clean-surface electronic bands. The thinner the sheet, the larger should be the contribution of the surface. We have earlier reported [1] conductance measurements on nanomembranes as thin as 77nm, and have shown that not only is the surface antibonding ($\pi^*$) band used to enhance ``bulk'' conduction in the membrane [2], but also the charges are additionally mobile in this band, providing a significant contribution to the overall conductance. We extend these measurements to thinner nanomembranes, between 64nm and 35nm thick, using a back-gated van der Pauw technique in ultra-high vacuum. The sheet conductance is measured after a high-temperature flash to obtain a high-quality Si(2$\times$1) reconstructed surface, and with H adsorbed on the surface. The maximum sheet resistance for a 64nm sample with H deposited in situ is higher than 24 G$\Omega$.\\[4pt] [1] W. Peng, et al., Nature Commun., under review.\\[0pt] [2] Zhang, P. P. et al., Nature 439, 703-706 (2006). [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R40.00002: Electronic properties of epitaxial silicene: a LT-STM/STS study Antoine Fleurence, Chi-Cheng Lee, Taisuke Ozaki, Yukiko Yamada-Takamura, Yasuo Yoshida, Yukio Hasegawa The astonishing properties of silicene, the Si-counterpart of graphene, together with pioneering experimental observations, triggered in the very recent years, an exponentially increasing interest for this atom-thick material, both at fundamental level and for applications in high-speed electronic devices. We demonstrated, that the spontaneous segregation of silicon on (0001) surface of zirconium diboride (ZrB$_2)$ thin films epitaxied on Si(111) wafers gives rise to a wide-scale uniform two-dimensional silicene sheet [1]. The silicene nature of the honeycomb structure imaged by scanning tunneling microscopy is evidenced by the observation of gap-opened $\pi $-electronic bands. The band gap opening is primarily due the specifically imprinted buckling. Here, we present the results of a low-temperature scanning tunneling spectroscopy investigation, which evidences the n-doped nature of silicene. The mapping of the local density of states, together with density functional theory give precious insights into the microscopic origin of the electronic bands of silicene. In particular, it shows the correlation between the degree of \textit{sp}$^{2}$ hybridization of different Si atoms in the internal structure and the character of the electronic bands. \\[4pt] [1] A. Fleurence \textit{et al.}, Phys. Rev. Lett. 108, 245501 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R40.00003: A planar-like silicene on ZrB$_{\mathrm{2}}$(0001) surface revealed by a first-principles study Chi-Cheng Lee, Yukiko Yamada-Takamura, Taisuke Ozaki Given that a free standing planar silicene is unstable [1], it is of great interest to understand the mechanism of stability of any existing planar-like structure that would optimize the understanding of the intrinsic difference from its counterpart, graphene. Recently, silicene was epitaxially grown on the ZrB$_{\mathrm{2}}$(0001) surface and was demonstrated to have ($\sqrt 3 \times \sqrt 3 )$-reconstruction due to irregular buckling [2]. While the deviation from the regularly buckled structure is clearly made by experiment, two possible structures revealed by a first-principles calculation are still in the candidate list, neither one is completely ruled out from the possible groundstate structure. The energetically more favorable one possesses a planar-like structure, with all Si atoms residing in a plane except the one on top of a Zr atom becomes higher. However, this structure is less preferable from available experimental data. By studying the binding energy and electronic band structures of these two structures with and without the substrate, we will explain why such a planar-like structure can gain more energy than the regularly buckled-like phases via the interaction of the ZrB$_{\mathrm{2}}$(0001) surface and why the ground state advocated by density functional theory could become less preferable by experiment. [1] S. Cahangirov \textit{et al}., Phys. Rev. Lett. \textbf{102}, 236804 (2009). [2] A. Fleurence \textit{et al}., Phys. Rev. Lett. \textbf{108}, 245501 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R40.00004: Growth and electronic properties of monolayer and multilayer silicene Baojie Feng, Lan Chen, Kehui Wu Silicene, in which Si atoms replace C atoms in a two-dimensional honeycomb lattice in analogue with graphene, has been experimentally realized recently. In this work we report a systematic study of superstructures formed by sub- monolayer and multiple layer silicon grown on Ag(111), by scanning tunneling microscopy (STM) and spectroscopy (STS). We found that, depending on the substrate temperature and silicon coverage, several monolayer superstructures can form on Ag(111). At proper temperature and Si coverage, monolayer and multilayer silicene films were grown [1]. STS at 4K revealed quasiparticle interference (QPI) patterns suggesting intervalley and intravalley scattering of charge carriers, and a linear energy-momentum dispersion relation and a large Fermi velocity were derived [2]. These results unambiguously prove the existence of Dirac fermions in silicene, and provide a solid basis for further studies on the electronic property and device applications of silicene. [1] Nano Letters 12, 3507 (2012), [2]Physical Review Letters 109, 056804 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R40.00005: Quasiparticle Interference in Silicene on Ag(111) Surface Lan Chen, Kehui Wu Silicene, a single sheet of silicon atoms arranged in a honeycomb lattice analogous to graphene, has been successfully prepared on Ag(111) surface recently. The honeycomb atomic structure of silicene has been confirmed experimentally. However, more important details of the electronic structures of silicene, such as pseudospin or chirality of Dirac Fermions and the shapes of the Dirac cones, still remain illusive. Here we performed scanning tunneling microscopy and spectroscopy to investigate the electronic states of silicene on Ag(111) surface. From the quasiparticle interference (QPI) pattern observed in dI/dV maps, we derived linear energy-momentum dispersion and a large Fermi velocity, which prove the existence of Dirac fermion in silicene. Moreover, through mapping the QPI pattern in q space, we found the Dirac cones of silicene are not circular as in graphene, but significantly warped to hexagon. The theoretical calculations prove that the constant energy contours of Dirac cones of silicene are hexagonal warped due to the unique structure of silicene. Our results pave the way for exploiting anisotropic transport behavior and other exotic quantum effects in silicene. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R40.00006: Nanostressor growth on Silicon Nanomembranes Frank Flack, Benjamin Treml, Donald Savage, Max Lagally Single-crystal semiconductor nanomembranes (NMs) have great potential for microelectronic materials heterointegration. In particular, they allow for the fabrication of custom-strained, dislocation-free growth interfaces. However, thin substrates are extremely compliant and it is, therefore, crucial to understand the added effects of residual processing strain and substrate bonding. We study the strain distributions on silicon NMs transferred to patterned Si substrates such that some NM regions are bonded and others freestanding. As the critical thickness for Stranski-Krastanow growth of quantum dots (QDs) is very strain dependent, we decorate the surface with Ge quantum dots (QDs) and use the resulting distribution as an easily visible indicator of strain. We see dramatic differences between QD distributions on the bound and freestanding regions, and also between the bound regions and the bulk Si substrate, suggesting that the buried interface may influence nanostressor growth. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R40.00007: Investigation of Mn-Co surface alloy on Si(100)-2x1 Gopalakrishnan Ramalingam, Jean-Francois Jacquot, Robert Morel, Matthieu Jamet, Petra Reinke Understanding of magnetic doping of Group-IV semiconductors is critical for the realization of spintronics devices. We present STM investigations of room temperature, sequential and co-deposition of Mn and Co on Si(100)-2x1. Monoatomic Mn-nanowires, which self-assemble on the Si surface, lose their continuity after deposition of 0.04-0.08 ML of Co. This loss in continuity is expressed in the wire length distributions, which are dominated by Mn dimers and ultrashort wires. Protrusions with a height of 0.5-0.8 {\AA} above the surface of the Mn wire are observed, which is evidence for adsorption of Co on wires. The Si defect structure is similar to exclusive Co deposition experiments on Si, and in agreement with the model of subsurface diffusion of Co atoms present in literature. Only 25{\%} of the deposited Co is observed on the surface and the rest are attached to the Mn structures. Wires form even during co-deposition of Mn and Co, indicating stronger Mn-Si and Mn-Mn interaction over Mn-Co interaction. The wire length distribution is dominated by ultrashort wires, similar to sequential deposition. A detailed discussion of the role of Co in breaking up the Mn wires will be presented. SQUID measurements are being performed to study the magnetic properties of Co-Mn-Si structures and will be discussed. DFT calculations for co-deposition of Mn and Co are presented and compared with experimental data. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R40.00008: A Level-Set Approach to Simulate Mound Formation during Epitaxial Growth Christian Ratsch, Joe Papac, Frederic Gibou We have developed an island dynamics model that uses the level-set approach to model epitaxial growth. In recent work we have implemented a numerical scheme to solve the diffusion equation for the adatom concentration with a (mixed) Robin boundary condition. Such a boundary condition properly describes multilayer growth when there is an additional step-edge barrier for atoms to diffuse over a step edge. We will discuss how variations of the boundary condition that correspond to variations of the step edge barrier affect the formation of mounds. We will furthermore discuss how the effect of downward funneling can be implemented within our approach and how it affects the slop of the mounds. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R40.00009: How nucleation can cause stacking faults on the GaAs (111) B surface: A DFT study Joshua Shapiro, Andrew Lin, Diana Huffaker, Christian Ratsch GaAs grows along the [111] direction much faster than other crystal axes at high growth temperature. In this work, we leverage this anisotropy, using catalyst-free selective-area epitaxy, to grow high-aspect ratio nanopillars. However, we find that the resulting crystal structure exhibits a high density of stacking faults that can have detrimental effects on the electronic and optical properties of the material. Each stacking fault is equivalent to a monolayer of wurtzite embedded in an otherwise zinc-blende lattice. The origins of stacking faults are currently under debate, with both thermodynamic equilibrium arguments and nucleation arguments proposed to explain the segments of wurtzite that appear in the primarily zinc-blende crystal. Here we present a density-functional-theory study of nucleation and island growth on the (111)B surface of GaAs that demonstrates how the smallest stable nucleus can transition and stabilize in either a wurtzite or a zinc-blende orientation. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R40.00010: First principles study of Bismuth growth on Nickel (111) surface Qin Gao, Michael Widom A recent experiment (Bollman, et al. 2011) suggests that Bismuth forms flat hexagonally close packed (hcp) films on the Nickel (111) surface, of heights 1, 3, 5 and 7 layers. A quantum size effect (QSE) based on free electrons was proposed in explanation. To test this idea, we calculated the total energy and QSE of Bismuth on Nickel (111) surface using density functional theory. We find the hcp films are destabilized by adding capping atoms which lead to puckering of the hcp layers and covalently bonded structures. Furthermore, we find the rhombohedral films based on the bulk Bi structure are energetically more favorable than the proposed hcp films. These structures also favor odd numbers of layers (a flat wetting layer followed by bulk-like rhombohedral bilayers), but owing to covalent chemical bonding rather than QSE. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R40.00011: Absolute surface energies of polar and non-polar planes in GaN Cyrus E. Dreyer, Anderson Janotti, Chris G. Van de Walle Growth of high quality single crystals and epitaxial layers of GaN is very important for producing optoelectronic devices. First principles calculations can help in determining absolute surface energies, which are key quantities that control crystal-growth rates and fracture toughnesses. By means of hybrid functional calculations, we have determined absolute surface energies for the non-polar \{11-20\} and \{10-10\} and polar (0001) and (000-1) planes in wurtzite GaN. Low energy reconstructions of the bare and hydrogenated surfaces were considered under various conditions chosen to correspond to growth by molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). We find that the non-polar planes are close in energy, and lower in energy than the reconstructed (000-1) polar plane under all conditions considered. The reconstructions of the (0001) plane are lower in energy than the (000-1) plane over the whole range of conditions, and lower in energy than the non-polar reconstructions for high-pressure conditions. From these surface energies, lower bounds on the anisotropic fracture toughness of GaN are determined. Surface energies of polar planes for other III-nitrides will be compared to those of GaN. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R40.00012: DFT studies of the early stages of growth of Nb on MgO(100) Yunsic Shim, Jacques G. Amar Using DFT calculations of binding and adsorption energies for various sizes and shapes of Nb clusters on MgO(100) surfaces, we have examined the effects of cluster shape and a neutral O vacancy on the energies and stability of Nb[100] and [110] island structures. Similarly to other cases of metal adsorbates on MgO(100) surfaces, O-vacancy sites tend to act as nucleation sites for Nb adatoms, while the effect of a nearby O vacancy on the binding energy of a Nb cluster is much weaker. In particular, we find that the binding energy for a Nb monomer at an O site (O-vacancy site) is 1.52 eV (2.2 eV) while the energy barrier for Nb monomer diffusion is 0.58 eV. In addition, although both isolated 4-atom Nb [100] and 5-atom Nb [110] islands are isotropic with a slightly higher binding energy for the [100] island, for larger clusters an anisotropic Nb [110] structure is more stable than a square Nb [100] structure, which is in good agreement with a recent experimental result [1]. \\[4pt] [1] M. Krishnan et al., Phys. Rev. ST. Accel. Beams 15, 032001 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R40.00013: Self Directed Growth of Nanopillar Arrays in Molten Polymer Films: Theory versus Experiment Kevin Fiedler, Sandra Troian It has been known for over a decade that molten polymer films exposed to a large uniform thermal gradient can develop spontaneous arrays of nanopillars. Theoretical predictions based on linear stability theory in the long wavelength approximation suggest that such formations arise from fluctuations either in the electrostatic attraction between the fluid and opposing substrate, acoustic phonon radiation pressure within the film, or thermocapillary forces along the air/polymer interface. Experimental confirmation of the mechanism responsible for such emergent structures requires measurements of the pattern formation process at very early times, a difficult task given that initial thickness fluctuations are of the order of a few nanometers. Here we report results of in-situ microscopy measurements of the dominant wavelength as a function of the applied thermal gradient and initial film thickness. Comparison to all three models indicates closest agreement with the thermocapillary mechanism. However, there remain discrepancies between theory and experiment with regard to the dominant wavelength observed and its corresponding growth rate. We discuss possible origins for the discrepancies, including non-stationary effects and simplified assumptions of the thermocapillary model. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R40.00014: Guided Growth of Millimeter-Long Horizontal Nanowires with Controlled Orientations David Tsivion, Mark Schvartzman, Ronit Popovitz-Biro, Palle von Huth, Ernesto Joselevich The large-scale assembly of nanowires (NWs) on surfaces remains one critical challenge toward their integration into practical devices. We report the vapor-liquid-solid growth of perfectly aligned, millimeter-long horizontal GaN [1] and ZnO$^{\mathrm{\thinspace }}$[2] NWs with controlled orientations on different sapphire planes. The growth directions, crystallographic orientation and faceting of the NWs vary with each surface orientation, as determined by their epitaxial relation with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these NWs display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown NWs. Further control was recently achieved by patterning the catalyst nanoparticles to produce NWs with well-defined locations, orientation and length. This enables the massively parallel integration of NW circuits. The guided growth approach paves the way to highly controlled NW structures with potential applications not available by other means. [1] D. Tsivion, M. Schvartzman, R. Popovitz-Biro,P. von Huth and E. Joselevich, Science \textbf{333}, 1003 (2011). [2] D. Tsivion, M. Schvartzman, R. Popovitz-Biro and E. Joselevich, ACS Nano \textbf{6}, 6433 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R40.00015: Low-energy Alkali Ion Scattering and X-ray Photoelectron Diffraction Studies of the Structure of Pt-Zn/Pt(111) Bimetallic Surfaces Bruce Koel, John Roszell, Eddie Martono, John Vohs Pt-Zn alloys have applications in heterogeneous catalysis, and studies on surfaces of well-defined, ordered Pt-Zn alloys, or intermetallic compounds, clarify the origins of changes that occur in catalysis by the alloy. Many stable intermetallic compounds of Pt and Zn occur in bulk materials, but no long-range ordered surface alloys were formed by depositing Zn on a Pt(111) single-crystal substrate in a search over a considerable range of conditions. These results can be contrasted to those from Pt-Sn, where ordered surface alloys were formed. Zn alloys with Pt upon heating, and XPD and ALISS were used to characterize the Pt-Zn alloy created by annealing one monolayer of Zn on Pt(111) to 650 K. This Pt-Zn/Pt(111) surface alloy had a diffuse (1x1) LEED pattern due to formation of a random, substitutional alloy between Pt and Zn with 0.05-monolayer Zn in the topmost layer. Zn atoms are substitutionally incorporated into Pt lattice positions and alloyed Zn atoms in the surface layer are located coplanar with the surface Pt atoms, without any buckling. TPD shows that both CO and NO chemisorb more weakly on the Pt-Zn alloy than on the clean Pt(111) surface, with NO more strongly affected. [Preview Abstract] |
Wednesday, March 20, 2013 5:30PM - 5:42PM |
R40.00016: Positron states and annihilation characteristics of surface-trapped positrons at the oxidized Cu(110) surface N.G. Fazleev, Antoine Olenga, A.H. Weiss The process by which oxide layers are formed on metal surfaces is still not well understood. In this work we present the results of theoretical studies of positron states and annihilation characteristics of surface-trapped positrons at the oxidized Cu(110) surface. An ab-initio investigation of stability and associated electronic properties of different adsorption phases of oxygen on Cu(110) has been performed on the basis of density functional theory and using DMOl3 code. The changes in the positron work function and the surface dipole moment when oxygen atoms occupy on-surface and sub-surface sites have been attributed to charge redistribution within the first two layers, buckling effects within each layer and interlayer expansion. The computed positron binding energy, positron surface state wave function, and annihilation probabilities of surface trapped positrons with relevant core electrons demonstrate their sensitivity to oxygen coverage, elemental content, atomic structure of the topmost layers of surfaces, and charge transfer effects. Theoretical results are compared with experimental data obtained from studies of oxidized transition metal surfaces using positron annihilation induced Auger electron spectroscopy. [Preview Abstract] |
Session R41: Casimir Forces
Sponsoring Units: DAMOPRoom: 350
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R41.00001: Measurement of the Casimir force between ferromagnetic surfaces Umar Mohideen, Alexandr Banishev, Galina Klimchitskaya, Vladimir Mostepanenko We have measured the Casimir interaction between two ferromagnetic boundary surfaces using the dynamic atomic force microscope in the frequency shift technique. The experimental data are found to be in excellent agreement with the predictions of the Lifshitz theory for magnetic boundary surfaces combined with the plasma model approach for the free electrons in the metal. In an important difference from non-magnetic metals, the Drude description of the free electrons leads to a Casimir force that is less than that from the plasma model approach. Thus the role of hypothetical patch potentials will be opposite to that required for reconciliation of the data with the Drude model. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R41.00002: Validity of effective medium theories in Casimir force calculations Raul Esquivel-Sirvent Effective medium theories have been used extensively to describe the dielectric response of inhomogeneous media. This is media that is composed of a mixture of materials with different dielectric functions. The possibility of using inhomogeneous media to control or tune Casimir forces has been discussed in the literature. In this paper we present results for the Casimir force between two inhomogeneous plates described by different effective medium models. In particular we show how the force depends on the model used. This has implications on the comparison between theoretical and experimental results. Furthermore, we calculate the force between an inhomogeneous sphere like multi layered nano shells and a plane to study the effects of effective models when using the proximity force approximations. The conditions under which effective medium models can be used in the context of the Casimir force are discussed in detail. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R41.00003: Experiments on Sphere Cylinder Geometry Dependence in the Electromagnetic Casimir Effect Shomeek Mukhopadhyay, Ehsan Noruzifar, Jeffrey Wagner, Roya Zandi, Umar Mohideen We report on ongoing experimental investigations on the geometry dependence of the electromagnetic Casimir force in the sphere-cylinder configuration. A gold coated hollow glass sphere which forms one surface is attached to a Silicon AFM cantilever. The cylinder, which is constructed from tapered optical fiber is also gold coated.~ The resonance frequency shift of the cantilever is measured as a function of the sphere-cylinder surface separation.~ The sphere-cylinder electrostatic force is used for alignment of the sphere and the cylinder and also for calibrating the system. The results are compared to numerical simulations in the framework of the Proximity Force Approximation (PFA). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R41.00004: Sum-over-modes approach to the Casimir effect in dissipative systems Francesco Intravaia, Ryan Behunin We show that, within the open-system framework, the sum-over-modes approach \`{a} la Casimir leads to the Lifshitz formula for the Casimir free energy. A general result applicable to arbitrary geometries is obtained through the use of Ford, Lewis, {\&} O'Connell's remarkable formula. Additionally, we address the possibility for obtaining the Casimir energy as a sum over complex ``modes.'' We show in this case that the standard sum-over-modes formula must be suitably generalized to avert unphysical complex energies. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R41.00005: Influence of Casimir-Lifshitz forces on actuation dynamics of MEMS Wijnand Broer, George Palasantzas, Jasper Knoester, Vitaly Svetovoy Electromagnetic fluctuations generate forces between neutral bodies known as Casimir-Lifshitz forces, of which van der Waals forces are special cases, and which can become important in micromechanical systems (MEMS). For surface areas big enough but gaps small enough, the Casimir force can possibly draw and lock MEMS components together, an effect called stiction, causing device malfunction. Alternatively, stiction can also be exploited to add new functionalities to MEMS architecture. Here, using as inputs the measured frequency dependent dielectric response and surface roughness statistics from Atomic Force Microscopy (AFM) images, we perform the first realistic calculation of MEMS actuation. For our analysis the Casimir force is combined with the electrostatic force between rough surfaces to counterbalance the elastic restoring force. It is found that, even though surface roughness has an adverse effect on the availability of (stable) equilibria, it ensures that those stable equilibria can be reached more easily than in the case of flat surfaces. Hence our results can have significant implications on how to design MEM surfaces. The author would like this abstract to appear in a Casimir related session. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R41.00006: Scattering Theory Calculations of Casimir Energies at High Curvature Noah Graham, Thorsten Emig, Aden Forrow, Robert Jaffe, Mehran Kardar, Mohammad Maghrebi, Jamal Rahi, Alex Shpunt Scattering theory provides a powerful tool for capturing the response of an object to electromagnetic charge and field fluctuations. Techniques based on scattering theory have made possible a wide range of new calculations of Casimir energies. In this approach, the Casimir interaction energy for a collection of objects can be expressed in terms of the scattering T-matrices for each object individually, combined with universal translation matrices describing the objects' relative positions and orientations. These translation matrices are derived from an expansion of the free Green's function in an appropriate coordinate system, independent of the details of the objects themselves. This method proves particularly valuable for geometries involving high curvature, such as edges and tips. I will describe this approach in general terms and then give results from several problems to which it has been applied successfully. I will also discuss new developments in scattering theory that have been motivated by these problems. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R41.00007: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R41.00008: Investigation of electrostatic ``patches'' on Au samples: Effects on the Casimir measurements Ricardo Decca, Guillaume Voisin It has been argued by Behunin and co-workers that the measurements done of the Casimir force on Au coated surfaces could suffer from substantial systematic errors arising from the presence of so called electrostatic ``patches'' (i.e. an electrostatic potential distribution on the surface of the Au layer). While these effects can be minimized, in principle they cannot be nullified by the application of uniform potential differences between the investigated surfaces. We present Kelvin probe microscopy studies of Au samples on Si. Au samples (about 200 nm thick) were deposited by thermal evaporation and sputtering. A thin (about 10 nm thick) layer of Cr is used as an adhesion layer. We will discuss the methodology used. We will show that that irrespectively of sample deposition method, there is two characteristic scales for the potential distribution: One, with spatial size of about 100 nm, associated with grain sizes and the other one, typical dimension 1 $\mu$m, most likely associated with unavoidable sample contamination. The effect of this potentials is found to be too small to affect the conclusions found in precision measurements of the Casimir effect. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R41.00009: Casimir effect between Topological Insulators: a proposal for quantum levitation Pablo Rodriguez-Lopez, Adolfo Grushin, Alberto Cortijo In this talk I will study the Casimir interaction between Topological Insulators (TIs). I will start with a brief description of the TIs, to explain what a TI is, and why they are interesting from a Casimir effect point of view. In particular, a three dimensional Topological Insulator is characterized by its topological magnetoelectric coupling $\theta \ne $0. We will discuss the electromagnetic response of the TIs, how a magnetoelectric coupling between TE and TM modes appears in this material and its consequences. We will show how, by tuning the parameter $\theta $ of the TI, we will be able to change the behavior of the Casimir energy between Tis from attraction to repulsion for all distances, and even the appearance of an equilibrium distance in the system. Then TIs can be potentially used to obtain ``quantum levitation'' and to avoid the sticking phenomena in NEMS. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R41.00010: Three-body Casimir effects and repulsion Kimball Milton The Casimir effect arises from quantum fluctuations in the electromagnetic field and results in forces between atoms (van der Waals and Casimir-Polder forces), between atoms and surfaces (Casimir-Polder forces), and between conducting and dielectric surfaces (Casimir-Lifshitz forces). In the past few years, there has been a revolution in our ability to calculate forces between different bodies. Pairwise summation of interatomic forces in general is very inadequate to describe the physics. In particular three-body effects can be large. Two-body forces, for example, between a dielectric sphere and a dielectric plane, can be calculated by a combination of analytic and numerical techniques; non-monotonic effects can occur when three-body interactions are considered. Anisotropic objects with ordinary electrical properties can give rise to repulsive quantum vacuum forces, which might have application in nanotechnology. This talk will focus on the overlap of the three-body force regime and Casimir repulsion, for example, the interaction between an anisotropically polarizable atom and a pair of facing conducting wedges, or two conducting half-planes constituting an aperture. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R41.00011: Electromagnetic fluctuation-induced interactions with metallic gratings Diego Dalvit In this talk I will discuss electromagnetic equilibrium and non-equilibrium fluctuation-induced interactions involving metallic gratings. In particular, I will describe a modal approach [1] to compute Casimir forces between metallic gratings and discuss the description of a recent Casimir force experiment with nanostructures that shows a strong force reduction. I will also discuss the related non-equilibrium problem of nanoscale heat transfer in metallic gratings from a modal approach point of view [2].\\[4pt] [1] {\it Quasi-analytical modal approach for computing Casimir interactions in periodic nanostructures}, F. Intravaia, P.S. Davids, R.S. Decca, V.A Aksyuk, D. Lopez, and D.A.R. Dalvit, Phys. Rev. A 86, 042101 (2012).\\[0pt] [2] {\it Enhanced radiative heat transfer between nanostructured gold plates}, R. Guerout, J. Lussange, F.S.S. Rosa, J.-P. Hugonin, D.A.R. Dalvit, J.-J. Greffet, A. Lambrecht, and S. Reynaud, Phys. Rev. B85, 180301(R) (2012). [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R41.00012: Fundamental limitations on force sensing due to patch potentials Ryan Behunin In this talk I will discuss some of the current methods used for measuring non-Newtonian corrections to gravity at short separation. When polycrystalline metallic test masses are used in these experiments patch potentials may provide a large source of noise. I'll present a simple model to quantify patch effects from which insights may be gained for minimizing deleterious effects on force signal to noise in these experiments. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R41.00013: Measurement of the Casimir force between gold surfaces using a Frequency-Modulation technique Joe Garrett, Jeremy Munday The Casimir force arises from the interactions between fluctuating dipoles in two media separated by a gap. We measure the derivative of the Casimir force with respect to sample separation between a gold-coated sphere and a gold-coated planar substrate using a non-contact Frequency-Modulation (FM) method of Atomic Force Microscopy (AFM) in a thermally controlled environment. The resonant frequency of the cantilever is tracked as the sphere is brought close to the surface. At each distance from the surface, the bias voltage of the sphere is swept, both to measure the distance between the sphere and the plate and to mitigate the effect of any contact potential difference. We will present recently obtained experimental data and discuss the various artifacts associated with Casimir force measurements. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R41.00014: Shape and material effects in Casimir forces Thorsten Emig, Giuseppe Bimonte, Mohammad Maghrebi, Noah Graham, Robert Jaffe, Mehran Kardar Casimir forces depend non-trivially on shape and material properties. Using ideas from electromagnetic scattering theory and conformal mappings, we have derived a number of novel analytical and numerical results for Casimir interactions. We shall give a brief overview of the general approach and present explicit results for some generic examples, including short- and long-distance expansions, interaction of perfect conductors with sharp edges and tips, and exact solutions in two and three dimensions. The predictions are compared to recent experiments. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R41.00015: Heat transfer and non-equilibrium Casimir force in nanostructured surfaces Romain Gu\'erout, Serge Reynaud, Astrid Lambrecht I'll review recent calculations for Casimir interactions between nanostructured surfaces both at thermodynamic equilibrium and out of equilibrium in the framework of the scattering theory. I'll emphasize on the interplay between the thermal Casimir force and the geometry of the surfaces. We predict an enhancement in the heat transfer between metallic gratings due to the appearance of spoof surface plasmons modes. We also show that the thermal component of the Casimir force arise at shorter separation distance in the case of nanostructured surfaces. [Preview Abstract] |
Session R42: Colloids and Interfaces
Sponsoring Units: DCPChair: Jesse Kern, University of Kansas
Room: Hilton Baltimore Holiday Ballroom 3
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R42.00001: Curvature-Induced Potential for Colloidal Particles at an Oil-Water Interface Colm Kelleher, Paul Chaikin At the micrometer scale, surface tension plays a predominant role in the interactions that occur at fluid interfaces. For example, when a spherical colloidal particle is adsorbed onto a curved oil-water interface, the surface must deform in order to satisfy the requirement of constant contact angle. The energy cost of the deformation depends on the local curvature of the interface, and so a particle sitting on an interface of varying curvature will experience a potential which depends on the particle's position on the interface. We present results from an experiment in which a capillary bridge droplet creates an interface of varying Gaussian curvature. The shape of this interface is obtained by using confocal microscopy. One or more spherical microparticles are then introduced to the interface. We demonstrate that a curvature-induced potential exists for a single wetting particle, which attracts the particle to the most highly curved regions. By tracking the motion of the particle in 3D, we are able to calculate the forces acting on the particle. We can then compare these forces to theoretical and numerical predictions based on the shape of the interface. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R42.00002: Self-pinning by colloids confined at a contact line Byung Mook Weon, Jung Ho Je Colloidal particles suspended in a fluid usually inhibit complete wetting of the fluid on a solid surface and cause pinning of the contact line, known as self-pinning. We show differences in spreading and drying behaviors of pure and colloidal droplets using optical and confocal imaging methods. These differences come from spreading inhibition by colloids confined at a contact line. We propose a self-pinning mechanism based on spreading inhibition by colloids. We find a good agreement between the mechanism and the experimental result taken by directly tracking individual colloids near the contact lines of evaporating colloidal droplets. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R42.00003: Holographic Imaging of Interfacial Mobility at Emulsion Interfaces Scott Parker, Melinda Sindoro, Steve Granick The difficulty of achieving nm resolution in the vertical direction has limited prior studies of nanoparticle mobility at the oil-water interface. This can be overcome by techniques of holographic imaging, implemented in this study and applied here to this problem. We have studied both homogeneous and Janus particles with emphasis on what determines the dynamics of surface pinning and desorption. Surprising dependence is found on conditions which govern kinetic depinning and the time scale for desorption. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R42.00004: The Effect of Size, Morphology and Composition on Second Harmonic Light Scattering from Colloidal Particles Grazia Gonella, Hai-Lung Dai Second harmonic light scattering (SHS) is a coherent second-order optical technique that is specifically surface sensitive and can be performed in-situ [1]. It has also been recently shown to be sensitive to size, shape and composition of metallic (Ag) and dielectric (polystyrene) nano and microparticles with or without adsorbed molecular monolayers. An understanding of how the size, shape, composition, structure, charge and surface chemistry influence the nonlinear optical properties makes SHS a versatile in-situ probe of nano- and/or micro-particle whose importance span from plasmonics to biomedicine [2].\\[4pt] [1] S. Roke and G. Gonella. Annu. Rev. Phys. Chem. 2012, 63, 353.\\[0pt] [2] (a) S.-H. Jen, G. Gonella, H.-L. Dai. J. Phys. Chem. A 2009, 113, 4758; (b) S.-H. Jen, H.-L. Dai, G. Gonella. J. Phys. Chem. C 2010, 114, 4302; (c) W. Gan, G. Gonella, M. Zhang, H.-L. Dai. Phys. Rev. B 2011, 84, 121402; (d) G. Gonella, H.-L. Dai. Phys. Rev. B 2011, 84, 121402; (e) G.Gonella, W. Gan, B. Xu, H.-L. Dai. J. Phys. Chem. Lett. 2012, 3, 2877. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R42.00005: Using DNA-directed crystals to template colloidal clusters John Crocker, James McGinley, Talid Sinno DNA is a versatile tool for directing the controlled self-assembly of nanoscopic and microscopic objects. We demonstrate a new, scalable method for producing highly ordered clusters of sub-micron colloidal microspheres at high yield. The basic idea is first to form a binary AB-type crystal using DNA-directed assembly, where a small fraction of the A species, A', contains a unique DNA sequence not present on the other A species. If the DNA domain of the A' and A particles that drive their interaction with the B species are identical, then the A' co-crystallize stiochiometrically as an 'impurity' into a well ordered AB lattice. Once formed, a soluble DNA strand is added to the crystals which binds the unique A' sequence and selectively stabilizes the A'-B bonds. When the crystals are then melted by heating, every A' particle yields a cluster surrounded by its nearest B neighbors. We will discuss the different clusters we have formed using this approach, as well as limits to yield and ordering in the clusters. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R42.00006: Particle-Size Dependency of Single Molecule Properties in Surface-Tethered Particle Systems by Monte Carlo Simulation Ian Hamilton, Marc Robert We consider the behavior of a surface-tethered particle system, comprising a single colloid particle tethered to a flat surface by a single polymer chain. This study is relevant to the interpretation of tethered particle motion experiments, wherein the motion and position of the tethered particle are used as reporters on the conformational properties of the underlying polymer molecule. The dependency of the polymer dimensions on the relative size of the tethered particle at equilibrium is obtained by Monte Carlo simulations with both random walk and self-avoiding walk polymer models. Two local maxima are found in the expansion factors of the polymer tether as a function of particle size, with both models. Comparison of these two models shows that the particle-size effects are separable from expansion by self-excluded volume of the polymer. Furthermore, the non-monotonic behavior persists to very large particle sizes before the expected asymptotic gparallel plate h limit is reached. The maxima are revealed to be due to the rotational entropy of the junction between the polymer and particle. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R42.00007: Liquid-vapor interface in two-dimensional colloid-polymer systems Mariam Nouri, Ryan McGorty, Vinothan Manoharan, Marc Robert The phase diagrams of two-dimensional aqueous colloid-polymer systems are determined experimentally. Mixtures of fluorescent polystyrene spheres and polyacrylamide are confined between a glass slide and a coverslip to construct a two-dimensional system. Liquid--vapor phase coexistence between a colloid-rich phase (colloid liquid) and a polymer-rich phase (colloid vapor) occurs at intermediate polymer concentrations, while vapor--solid phase coexistence between a polymer-rich liquid and a colloid-rich solid is observed at high polymer concentrations. For the interface between the coexisting liquid-vapor phases, the interfacial thickness and tension are measured using image analysis and Fourier analysis of the capillary waves. Close to the critical point, the fluctuations of the inteface become large and can no longer be decomposed into waves. It is also observed that the colloid-rich solid and liquid domains coarsen mainly by Ostwald ripening in a short time and long time regime. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R42.00008: Tunable Soft Structure in Charged Fluids confined by Dielectric Interfaces Jos Zwanikken, Monica Olvera de la Cruz We study the deformation of the local structure in an electrolytic background by micro- and nanoscopic polarizable surfaces, and vice versa, the emergence of induced forces between two surfaces due to the cohesive properties of the background. The range and strength of these forces depend sensitively on the material properties of the charged fluid, and can be varied over decades, offering high tunability and, aided by accurate theory, control in experiments and applications. The attention is directed towards the electrolyte-induced forces between neutral boundaries, to distinguish correlational effects from simple ionic screening. The interplay of thermal motion, short range repulsions, and electrostatic forces is responsible for a typical ordered fluid state, a soft structure, that changes near polarizable interfaces and causes diverse attractions between fluctuation-confining walls that seem well exploited by microbiological systems. We use liquid state theory and classical density functional theory to accurately calculate these interactions and nuance the understanding of double-layer forces, relevant for colloid and emulsion stability, phase-transfer catalysis, and (interface-directed) self-assembly of nanomaterials. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R42.00009: Interfacial free energy calculation of a binary hard-sphere fluid at a hard wall by Gibbs-Cahn Integration Jesse Kern, Brian Laird The interfacial free energy, $\gamma$, of fluids at surfaces is a parameter that is central to a number of technologically important phenomena, such as wetting, nucleation and the stability and self assembly of colloidal particles in solution. In recent years, our group has developed techniques to determine $\gamma$ from atomistic simulation. In this work, we apply one of these methods, Gibbs-Cahn Integration, to determine $\gamma$ for a model two-component (binary) mixture of hard spheres. Molecular dynamics simulation is used to characterize a hard-sphere fluid mixture in a slit-pore confined geometry as packing fraction, mole fraction, and diameter ratio are varied. We find that recent theoretical predictions from the White Bear II classical density functional theory [Roth et al., J. Phys.: Condens. Matter, {\em 18}, 8413, (2006)] agree well with our computational results We also observe that, for this model system, the preferential adsorption of one particle species over the other contributes negligibly to the interfacial free energy at modest diameter ratios. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R42.00010: Understanding the correlation function of the inhomogeneous hard-sphere fluid Jeff Schulte, Patrick Kreitzberg, Chris Haglund, David Roundy We present a new functional for the correlation $g(r)$ at contact for an inhomogeneous distribution of hard spheres. This term is a key input into classical density functional theories developed using Statistical Associating Fluid Theory, a widely used approach for handling complex liquids in which hydrogen bonding plays an important role. We use a thermodynamic approach to derive an exact formula for the correlation expressed as a derivative of the free energy functional. We evaluate this approach using the approximate free energy of the ``White Bear'' version of Fundamental Measure Theory, and test our approach (and two previously published approximations) against correlation values found from Monte Carlo simulations. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R42.00011: Testing ``Soft'' Fundamental Measure Theory for not-so-hard-sphere fluids Eric Krebs, Patrick Kreitzberg, David Roundy A standard approach for modelling in-homogeneous distributions of the hard-sphere fluid is the Fundamental Measure Theory (FMT) formulation of classical density functional theory. Due to the paucity of truly hard spheres in the real world, it seems advisable to consider interactions that are not quite so ``hard,'' a challenge tackled by the ``Soft'' FMT (SFMT) introduced by Schmidt [1]. We apply SFMT to a simple potential describing slightly penetrable spheres, that is, spheres at moderate temperatures---such that the spheres are thermally able to penetrate by a short distance. We compare the predicted equation of state with the result of Monte Carlo simulations, and also compare the free energy and density distribution near a hard wall with simulation.\\[4pt] [1] Schmidt, M. Phys. Rev. E 62(4), 4976 (2000) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R42.00012: Nonlinear functional for solvation in Density Functional Theory Deniz Gunceler, Ravishankar Sundararaman, Kathleen Schwarz, Kendra Letchworth-Weaver, T. A. Arias Density functional calculations of molecules and surfaces in a liquid can accelerate the development of many technologies ranging from solar energy harvesting to lithium batteries. Such studies require the development of robust functionals describing the liquid. Polarizable continuum models (PCM's) have been applied to some solvated systems; but they do not sufficiently capture solvation effects to describe highly polar systems like surfaces of ionic solids. In this work, we present a nonlinear fluid functional within the framework of Joint Density Functional Theory. The fluid is treated not as a linear dielectric, but as a distribution of dipoles that responds to the solute, which we describe starting from the exact free energy functional for point dipoles. We also show PCM's can be recovered as the linear limit of our functional. Our description is of similar computational cost to PCM's, and captures complex solvation effects like dielectric saturation without requiring new fit parameters. For polar and nonpolar molecules, it achieves millihartree level agreement with experimental solvation energies. Furthermore, our functional now makes it possible to investigate chemistry on the surface of lithium battery materials, which PCM's predict to be unstable. [Preview Abstract] |
Session R43: Electron Transfer, Charge Transfer and Transport
Sponsoring Units: DCPChair: Guy Cohen, Columbia University
Room: Hilton Baltimore Holiday Ballroom 2
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R43.00001: Numerically Exact Long Time Magnetization Dynamics Near the Nonequilibrium Kondo Regime Guy Cohen, Emanuel Gull, David Reichman, Andrew Millis, Eran Rabani The dynamical and steady-state spin response of the nonequilibrium Anderson impurity model to magnetic fields, bias voltages, and temperature is investigated by a numerically exact method which allows access to unprecedentedly long times. The method is based on using real, continuous time bold Monte Carlo techniques---quantum Monte Carlo sampling of diagrammatic corrections to a partial re-summation---in order to compute the kernel of a memory function, which is then used to determine the reduced density matrix. The method owes its effectiveness to the fact that the memory kernel is dominated by relatively short-time properties even when the system's dynamics are long-ranged. We make predictions regarding the non-monotonic temperature dependence of the system at high bias voltage and the oscillatory quench dynamics at high magnetic fields. We also discuss extensions of the method to the computation of transport properties and correlation functions, and its suitability as an impurity solver free from the need for analytical continuation in the context of dynamical mean field theory. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R43.00002: Transport and dynamics in multisite subsystems Malay Bandyopadhyay, Manas Kulkarni, Dvira Segal We consider a chain of quantum dots coupled to finite-size reservoirs (prepared out-of-equilibrium) in which each dot is susceptible to decoherence effects or inelastic scattering processes. We observe a ballistic to diffusive crossover in the electronic current. We further investigate the manifestation of this ballistic-diffusive crossover on the dynamics and electron reorganization in the fermionic reservoirs. We find regimes which can be described in a classical framework and regimes whose description is rooted in quantum statistics. Our work can be generalized to understand other multi-site systems and their feedback on the bath degrees of freedom. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R43.00003: Possibilities for Observation of Quantum Transport in (RE)Ba$_{2}$Cu$_{3}$O$_{\mathrm{7-y}}$ Perovskites Paul Grant For y $\approx $ 0, the crystal structure of the ``1-2-3'' family of rare earth copper oxide perovskites displays several curious ``porosity'' features. For example, along the b-axis direction of the region usually termed the ``CuO chains,'' one observes a dramatically wide ``channel'' bounded within a Ba-Cu-O tube. Similar channels can be found in both the b- and a-axes directions contained within RE, Cu and O ion groupings. The cross-sectional area of these channels is roughly that of a single-wall carbon nanotube, suggesting the former may manifest Buettiker-Landauer quantum conductance similar to that observed in the latter. Moreover, by employing various ratios of Pr/Y for the RE component of the host system, the bulk electrical properties of the surrounding host can be tailored from completely insulating to metallic. We test our conjecture predicting ballistic transport down these channels using density functional theory and report our initial findings here along with the likely consequences of paramagnetic spin scattering. We also discuss possible experimental embodiments which could lead to nano-controllable gate structures. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R43.00004: The Information Content of Conductance Histogram Peaks: Transport Mechanisms, Level Alignments, and Coupling Strengths Matthew Reuter, Patrick Williams We develop a theory for describing single channel peaks in conductance histograms by applying probability theory to electron transport. This produces analytical forms for fitting experimental conductance data, where the fitting parameters have physical significance. Depending on the transport mechanism (resonant vs.\ non-resonant tunneling), the peak's line shape contains information on the level alignment of the channel and the channel-electrode coupling(s). Ultimately, this work provides tools for extracting additional information from experimental data, helping us better understand electron transport processes. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R43.00005: Collective Plasmon-Molecule Excitations in Nanojunctions: Quantum Consideration Alexander White, Boris Fainberg, Michael Galperin We use a pseudoparticle nonequilibrium Green function approach to study the coupling between plasmons and molecular excitons in nonequilibrium molecular junctions. This method is shown to be especially convenient for the calculation of the plasmon absorption spectrum of hybrid plasmon-exciton systems where coherent electron and energy transfer processes and strong system interactions play an important role. The formalism treats the intramolecular interactions and plasmon-exciton coupling exactly, going beyond the standard tool of molecular electronics - the nonequilibrium Green function. We demonstrate the sensitivity of the molecule-plasmon Fano resonance to junction bias, Coulomb repulsion, and intramolecular exciton coupling. We also compare our prediction for non-linear optical effects to previous mean-field equilibrium studies of isolated hybrid plasmon-exciton systems, and demonstrate the advantage of our approach. This study opens a way to deal with strongly interacting plasmon-exciton systems in nonequilibrium molecular devices. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R43.00006: Magnetic Field Control of Current through Molecular Ring Junctions Dhurba Rai, Michael Galperin, Oded Hod, Abraham Nitzan We study circular currents driven by voltage bias in molecular wires with ring structures [J. Phys. Chem. C 114, 20583 (2010)]. We revisit magnetic field effects on molecular ring structures presenting conditions under which magnetic field control of molecular ring conduction is realizable. [Phys. Rev. B 85, 155440 (2012)]. We find these conditions to be (a) weak molecule-lead coupling, implying relatively distinct conduction resonances, (b) asymmetric junction structure (e.g., meta or ortho connected benzene ring rather than a para connection), and (c) minimal dephasing (implying low temperature) so as to maintain coherence between multiple pathways of conduction. When these conditions are satisfied, considerable sensitivity to the applied magnetic field normal to the molecular ring plane is found. Although sensitivity to magnetic field is suppressed by dephasing, quantitative estimates indicate that magnetic field control can be observed in suitably constructed molecular ring junctions. We demonstrate control of the spin-flip inelastic electron tunnelling spectroscopy (IETS) signal and discuss spin polarization of total and circular currents in a benzene ring junction with spin impurity [Phys. Rev. B 86, 045420 (2012)]. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R43.00007: Interaction effects in electric transport through self-assembled molecular monolayers Martin Leijnse I will discuss a theoretical study of electric transport in molecular electronic devices based on self-assembled molecular monolayers (or other devices involving a large number of mesoscopic conductors contacted in parallel). In contrast to macroscopic conductors, Coulomb interactions between charge carriers being transported through neighboring molecules within the monolayer are large. I show that such inter-molecular Coulomb interactions not only lowers the conductance level, but lead to a correlated current and give rise to distinct signatures in the current-voltage characteristics. If some molecules fail to bond strongly to both electrodes, interactions can even give rise to negative differential resistance. Knowledge of the effects of Coulomb interactions between different conductors is important not only for the functionality of nanoelectronic devices, but also to isolate the genuine single-device properties, for example when trying to interpret a transport experiment using a molecular monolayer device in terms of single-molecule properties. Reference: Martin Leijnse, arXiv:1210.2843 [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R43.00008: Zero-bias anomaly in thiol-bound molecular junction on Ag(111) Kees Flipse, Erwin Rossen, Jorge Cerda Single molecule transistors are widely regarded as the successor of current silicon-based technology. To investigate the electronic properties of single molecules, they must be connected to the macroscopic world via electrodes. The most used linker group to connect the molecule to the metal leads is a thiol group. One feature that is often observed in these systems is a significant reduction (10-20{\%}) in the conductance in a narrow region around the Fermi-level. While most authors choose to ignore this feature, it is in general attributed to excitations of the metal-sulphur mode and phonon interactions in the metal leads. We will discuss the origin of this zero-bias anomaly (ZBA) by presenting ab-initio calculation results in a Scanning Tunnelling Microscopy (STM) geometry for thiophenol molecules adsorbed on Ag(111), indicating the important role of the inelastic contributions of low energy vibrational modes in charge transport. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R43.00009: Bi-Stable States in Highly Conductive Pyrazine Molacular Junction G.P. Brivio, C. Motta, S. Kanenko, M. Kiguchi Bi-stable molecular junctions are recently deserving attention for their potential in molecular electronics applications. In the present work, we investigate the bi-stable conductance of highly conductive single-molecule pyrazine/Pt junctions. Break-junction measurements show two distinct conductance states of 1.0 G$_{\mathrm{0}}$ and 0.3 G$_{\mathrm{0}}$, G$_{\mathrm{0}}$ being the quantum of conductance. First-principles calculations reveal that the two states could be assigned to different geometrical configurations of pyrazine exhibiting larger and lower coupling with the electrodes, respectively. Inelastic tunneling electron spectroscopy measurements and theoretical analysis of the system vibrations are able to further characterize the configuration dependent conductance of such junctions. We demonstrate that the controlled torsion of the molecule is capable to switch between the two conducting regimes. This process triggered only by mechanical manipulation of the junction is reversible. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R43.00010: Switching of Current in a Molecular Junction Kamal Dhungana, Subhasish Mandal, Ranjit Pati Achieving atomic level control at the metal-molecule interface in a single molecule conductance measurement is a difficult task that hinders the progress in molecular electronics. The lack of atomic level structural information of the interface makes the theoretical interpretation of experimental data much harder. Herein, we create an ensemble of device structure by varying metal-molecule binding sites, the orientation of the molecule at the interface, interface distance, and conformational change within the molecule to study junction dependent conductance behavior in Ruthenium-Bis(terpyridine) molecular wire, which has been fabricated and characterized. An orbital dependent DFT in conjunction with a parameter free, single particle Green's function approach is used to study the I-V characteristics. Our calculation for the weakly-coupled ONTOP junction geometry yields a relatively small (OFF state) current value below a threshold voltage $(V_{th})$. The current value is found to increase at ~ $V_{th}$ and remains flat (ON state) after the threshold value. A similar non-linear I-V curve with a current switching behavior has been reported experimentally. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R43.00011: Effect of electron transfer on direct vs. indirect contact of CdSe quantum dots on TiO$_{2}$ nanoparticles Tess R. Senty, Oshadha Ranasingha, Scott K. Cushing, Congjun Wang, James P. Lewis, Christopher Matranga, Alan D. Bristow CdSe quantum dots (QD) attached to TiO$_{2}$ semiconductors (SC) have been extensively studied over the last decade. They have shown promising results for uses as energy materials including capture of light in solar cells [1] and photocatalytic reduction of CO$_{2}$ [2]. The length of linker molecules between the QD and SC has been shown to decrease the electron transfer (ET) rate exponentially for an increasing linker length [3]. Studies also indicate that this exponential decrease breaks down for direct contact [4] although the exact mechanism is not fully understood. Through visible and NIR transient absorption spectroscopy we directly probe the electron and hole dynamics of CdSe QDs on TiO$_{2}$ nanoparticles comparing intimate contact with mercaptopropionic acid linked QDs. We find that with this direct contact, the ET rate decreases, deviating from previous results. We investigate the mechanisms for this deviation including the effect of oxidation on the QD surface.\\[4pt] [1] Robel, I. et al, JACS, \textbf{128}, 2385 (2006)\\[0pt] [2] Wang, C. et al, J. Phys. Chem. Lett. \textbf{1}, 48 (2010)\\[0pt] [3] Dibbell, R. S. and Watson, D. F., J. Phys. Chem. C, \textbf{113}, 3139 (2009)\\[0pt] [4] Pernik, D. R. et al, J. Phys. Chem. C, \textbf{115}, 13511 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R43.00012: Surface plasmonic responses in semiconductor and metal nanostructures investigated by ultrafast electron diffraction Kiseok Chang, Tzong-Ru T. Han, Fei Yuan, Chong-Yu Ruan Incorporating metallic nanostrcutures in the molecular sensing, nanoelectronics, and catalysis devices has often yielded significantly enhanced efficiency, despite many open questions remain. Using ultrafast electron diffraction as a sensitive contact free voltammetry probe, we find the photoinduced voltages across the heterojunctions consisting of nanostructures and semiconductor or metal surfaces can be highly enhanced when the surface plasmon excitation is used as the ponderomotive drive to induce photocurrent. By using the effective circuit model, and aided by the time domain finite difference method, we are able to describe the observed timescales and spectral responses in the context of dielectric coupling, interfacial charge transfer, and strong proximity-field induced at the interfaces between the nanostrcutures, the substrate, and the surrounding medium, which help understand different origins of the surface plasmon enhancement effect. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R43.00013: Frontier Orbital Energy Change of Poly(3-Hexylthiophene) Oligomers: Effect of Large Amplitude Torsional Motion Ram Bhatta, Mesfin Tsige, David Perry Poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methylester (PCBM) based solar cells remain the most promising organic photovoltaics so far. Despite their promise for organic solar cells, practical application is hindered by low efficiency, associated with poor electron transport from P3HT to PCBM. For isolated P3HT oligomers, we investigate the torsional dependence of electronic properties by performing DFT calculations and extrapolate to the long chain limit. The fully relaxed isolated P3HT oligomers are non-planar with a 47 degree twist angle between each pair of rings and are lower in energy by 0.03 eV per monomer unit than the fully planar oligomers. The non-planarity lowers highest occupied molecular orbital (HOMO) energy by 1 eV and rises lowest unoccupied molecular orbital (LUMO) energy by 0.6 eV compared to the respective orbital energies in a planar P3HT. The shifts in HOMO and LUMO energies increase the band gap from 1.9 eV in planar P3HT up to 3.3 eV when all backbone angles are non-planar and point to a reduced electrical conductivity. The larger band gap in non-planar P3HT accounts for the observed blue shift in the visible P3HT absorption band in P3HT/PCBM mixtures. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R43.00014: Effect of geometrical orientation on the charge transfer energetics of supramolecular (tetraphenyl)-porphyrin/fullerens dyads Marco Olguin, Rajendra Zope, Tunna Baruah We present our study of several low lying charge-transfer (CT) excitation energies for a widely used donor-acceptor system composed of a porphyrin-fullerene pair. The dyad systems consist of C$_{60}$ and C$_{70}$ acceptor systems coupled to tetraphenyl-porphyrin (TPP) and tetraphenyl-(zinc)porphyrin (ZnTPP) donor systems in a co-facial orientation. We find that replacing C$_{60}$ by C$_{70}$ in a given dyad may increase the lowest charge transfer excitation energy by about 0.27 eV, whereas varying the donor in these complexes had marginal effect on the lowest charge transfer excitation energy. Additionally, we examined the effect of geometrical orientation on the CT energy by calculating several CT excited state energies for an end-on orientation of the porphyrin-fullerene dyads. The CT excitation energies are larger for the end-on orientation in comparison to the co-facial orientation by 0.6 eV - 0.75 eV. The difference is attributed to a reduced exciton binding energy in going from the co-facial to the end-on orientation. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R43.00015: Electron Transfer Mechanism in Gold Surface Modified with Self-Assembly Monolayers from First Principles Filipe C. D. A. Lima, Rodrigo M. Iost, Frank N. Crespilho, Mar\'Ilia J. Caldas, Arrigo Calzolari, Helena M. Petrilli We report the investigation of electron tunneling mechanism of peptide ferrocenyl-glycylcystamine self-assembled monolayers (SAMs) onto Au (111) electrode surfaces. Recent experimental investigations showed that electron transfer in peptides can occur across long distances by separating the donor from the acceptor. This mechanism can be further fostered by the presence of electron donor terminations of Fc terminal units on SAMs but the charge transfer mechanism is still not clear. We study the interaction of the peptide ferrocenyl-glycylcystamine on the Au (111) from first principles calculations to evaluate the electron transfer mechanism. For this purpose, we used the Kohn Sham (KS) scheme for the Density Functional Theory (DFT) as implemented in the Quantum-ESPRESSO suit of codes, using Vandebilt ultrasoft pseudopotentials and GGA-PBE exchange correlation functional to evaluate the ground-state atomic and electronic structure of the system. The analysis of KS orbital at the Fermi Energy showed high electronic density localized in Fc molecules and the observation of a minor contribution from the solvent and counter ion. Based on the results, we infer evidences of electron tunneling mechanism from the molecule to the Au(111). [Preview Abstract] |
Session R44: Nucleic Acids: Structure, Function, Protein Interactions
Sponsoring Units: DBIOChair: Xiangyun Qiu, Physics department, George Washington University
Room: Hilton Baltimore Holiday Ballroom 1
Wednesday, March 20, 2013 2:30PM - 2:42PM |
R44.00001: Stretching DNA molecules on a flexible substrate, a polarization-dependent fluorescence microscopy study Ke Zhu, John Mele, Julia Budassi, Jonathon Sokolov DNA molecules absorbed and stretched onto surfaces can be used to analyze DNA structure by imaging fluorescence of labeled hybridization probes or enzymes. A recently proposed method for sequencing by electron microscopy requires either adsorbed single-stranded DNAs or untwisted double-stranded DNA. In this experiment, studies were performed on the adsorption of isolated DNA molecules to a flexible PDMS substrate, which permits continuous stretching, until breakage of the DNA molecules. Lambda and T4 DNAs (48.5 and 165.6 kilobase pairs, respectively) were absorbed onto PDMS out of solution by withdrawing a submerged substrate at a rate of 2mm/s, producing linear molecules deposited on the surface. Incident light polarization was varied and fluorescence emission intensity measured as a function of polarization angle and degree of stretching of the DNA. The stretching and breakage properties of lambda and T-4 DNA on the PDMS substrate were determined. The amount of stretching before breakage occurred was found to be up to 40{\%} relative to the as-deposited length. Supported by NSF-DMR MRSEC program. [Preview Abstract] |
Wednesday, March 20, 2013 2:42PM - 2:54PM |
R44.00002: Controlled enzymatic cutting of DNA molecules adsorbed on surfaces using soft lithography Alyssa Auerbach, Julia Budassi, Emily Shea, Ke Zhu, Jonathan Sokolov The enzyme DNase I was applied to adsorbed and aligned DNA molecules (Lamda, 48.5 kilobase pairs (kbp), and T4, 165.6 kbp), stretched linearly on a surface, by stamping with a polydimethylsiloxane (PDMS) grating. The DNAs were cut by the enzyme into separated, micron-sized segments along the length of the molecules at positions determined by the grating dimensions (3-20 microns). Ozone-treated PDMS stamps were coated with DNase I solutions and placed in contact with surface-adsorbed DNA molecules deposited on a 750 polymethylmethacrylate (PMMA) film spun-cast onto a silicon substrate. The stamps were applied under pressure for times up to 15 minutes at 37 C. The cutting was observed by fluorescence microscopy imaging of DNA labeled with YOYO dye. Cutting was found to be efficient despite the steric hindrance due to surface attachment of the molecules. Methods for detaching and separating the cut segments for sequencing applications will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 2:54PM - 3:06PM |
R44.00003: Localizing the critical point of random RNA secondary structures William Baez, Ralf Bundschuh Previous numerical studies have found that below the denaturation temperature random RNA secondary structures can exist in one of two phases: a strongly disordered, low-temperature glass phase and a weakly disordered, high-temperature molten phase. The probability of two bases pairing in these phases have been shown to scale with the distance between the two bases as -3/2 and -4/3 in the molten and glass phases, respectively. In this study, we characterize the scaling behavior of various sub-strand lengths within the molecule for a range of temperatures both far from and near the critical point. We anticipate that this approach allows to more accurately determine the critical point and to measure the critical exponents of the system right at the phase transition. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R44.00004: Self-assembly of virus particles: The role of genome Gonca Erdemci-Tandogan, Jef Wagner, Rudolf Podgornik, Roya Zandi A virus is an infectious agent that inserts its genetic material into the cell and hijacks the cell's machinery to reproduce. The simplest viruses are made of a protein shell (capsid) that protects its genome (DNA or RNA). Many plant and animal viruses can be assembled spontaneously from a solution of proteins and genetic material in different capsid shapes and sizes. This work focuses on the role of genome in the assembly of spherical RNA viruses. The RNA, a highly flexible polymer, is modeled by mean field approximations. Two RNA models are discussed: (i) A linear polymer model including a pairing affinity between RNA base pairs, and (ii) a branched polymer model. Polymer density and electrostatic potential profiles are obtained, and the relevant free energies are calculated from these profiles. The optimal length of the encapsidated chain is examined as a function of the model parameters. The osmotic pressure of the system is also discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R44.00005: Automated building of three-dimensional RNA structures Yunjie Zhao, Zhou Gong, Yangyu Huang, Yi Xiao, Chen Zeng RNAs have been found to be involved in many biological processes. Difficulties of experimental determination of tertiary structures of RNA limit our understanding of their biological functions. Therefore, some computational methods of building tertiary structures of RNA have been proposed. However, current algorithms of RNA tertiary structure prediction give satisfactory accuracy only for RNA of small size and simple topology, and most are not fully automatic. Here, we present an automated and efficient program, 3dRNA. Since the organization of RNA structure is largely defined by topological constraints in the secondary structure as well as the tertiary contacts, we build the RNA tertiary structure from the smallest secondary elements (SSEs) by using a two-step procedure. We first assemble the SSEs into hairpins or duplexes and then into complete structure since the tertiary structures of hairpins and duplexes usually can be built with a high accuracy. In a benchmark test with known structures, 3dRNA can give predictions with reasonable accuracy for RNAs of larger size and complex topology. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R44.00006: Ion concentration dependent tRNA folding energy landscapes Rongzhong Li, Samuel Cho The RNA folding is highly dependent on the ionic conditions of its environment in the cell because the surrounding ions electrostatically screen the charged phosphates that line the RNA backbone.~Recent studies (Cho, Pincus, and Thirumalai, PNAS, 2007; Biyun, Cho, and Thirumalai, JACS, 2011) demonstrated that the coarse-grained model we use accurately captures the RNA folding mechanisms by incorporating a Debye-Huckel potential to screen the electrostatics. We compare the ion-concentration dependent tRNA folding mechanism to the classical thermodynamic melting profiles of Crothers and co-workers, and we observe excellent agreement. We also supported our findings by performing empirical force field MD simulations with CHARMM and AMBER, and we observe remarkably comparable qualitative similarities between the average base-base distances from simulations and the empirically measured base-stacking potentials from the well-known Turner's Rules.~ [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R44.00007: Determination of counterion distribution around DNA coated nanoparticles (DNA-AuNP) by small angle X-ray scattering (SAXS) Sumit Kewalramani, CheukYui Leung, Jos Zwanikken, Robert Macfarlane, Monica Olvera de la Cruz, Chad Mirkin, Michael Bedzyk The interactions between DNA-Au nanoparticles (AuNP) and the surrounding cationic counterion layer critically determine the melting behavior of DNA duplexes on isolated DNA-AuNP and in crystalline assemblies of DNA-AuNPs. Also, the counterion layer is speculated to cause the enhanced stability of DNA-AuNPs against nuclease degradation, as compared to isolated DNAs. This makes DNA-AuNPs attractive for bio-diagnostic and therapeutic applications. To probe the ion cloud around DNA-AuNPs, we apply the isomorphous heavy ion replacement SAXS approach. Specifically, the SAXS measurements are carried out on DNA-AuNPS dispersed in a series of solutions that contain different monovalent ions (Na$^{\mathrm{+}}$, K$^{\mathrm{+}}$, Rb$^{\mathrm{+}}$ or Cs$^{\mathrm{+}})$. The combined analysis of all four intensity profiles makes it possible to extract, in a model-independent manner, the cation profile contribution $I_{cat}$ ($q)$ from the SAXS intensity that is averaged over the polydispersity of AuNPs. The $I_{cat}$ ($q)$ is found to be consistent with the cation dependent SAXS intensities that are derived on the basis of classical DFT calculations for the counterion distribution around DNA-AuNPs. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R44.00008: Ion Competition in Ordered DNA arrays in the Attractive Regime Xiangyun Qiu, John Giannini, Kurt Andresen Quantitative knowledge of electrostatic interactions is of fundamental importance for many classes of biomolecules and biological processes. Acquiring such knowledge is challenged by inherent complexities such as the long-range nature of electrostatic forces, the non-linear screening of ubiquitous ions, and the involvement of a large number of solvent molecules. Here we report our recent work to address some of the key questions by interrogating electrostatics-governed ordered nucleic acids arrays and bringing together a set of quantitative tools to elucidate the role of each of the electrostatic factors: ion, water, and charged surface. Specifically, we will present measurements and modeling of the spacings between DNA strands and the numbers of interstitial competing ions in the attractive regime. Our results indicate a linear relation between the partition of interstitial ions and the magnitude of inter-DNA attraction, which will be discussed in the context of current theories of electrostatic interactions. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R44.00009: Modeling spatial correlation of DNA deformations: Allosteric effects of DNA protein binding Xinliang Xu, Jianshu Cao We report a study of DNA deformations by a coarse grained mechanical model. Recent single molecule experimental studies show that when DNA molecule is deformed by its binding to a protein, the binding affinity of a second protein at distance $L$ away from the first binding site is altered. To explain this observation, the relaxation of deformation along the DNA chain is examined. Our method predicts a general exponentially decaying behavior for differenct deformation modes. As an example, inter-helical distance deformation is studied in details, and is found to decay at a previously unknown lengthscale of 10 base pairs as a result of the balance between inter and intra DNA strand energy. This lengthscale is in good agreement with the said single molecule experimental observation. This model of local deformation relaxation helps us better understand many important issues in DNA such as the enhanced flexibility of DNA at short lengthscales and DNA repair mechanism inside cells. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R44.00010: Molecular dynamics simulation of DNA base-pair opening by sharp bending Peiwen Cong, Liang Dai, Johan R.C. van der Maarel, Jie Yan Many biological processes require sharp bending of DNA. According to worm-like chain model, the bending energy dominates the free energy cost of those processes containing DNA loops shorter than 40 nm, such as DNA wrapping around histones, Lac repressor looping and virus DNA packaging . However, several recent experimental observations suggest that the WLC model s not applicable under tight bending conditions. In full atom molecular dynamics simulations, a double stranded, 20 base-pairs DNA fragment is forced to bend by an external spring. It is found that one or two AT-rich regions are disrupted for sufficiently small end-to-end distance. The disrupted DNA base-pairs separate and usually stack with the neighbouring base-pairs to form a defect. It is shown that these defects are more bendable than the bending rigidity of the duplex in the regular B-form. The simulation suggests a curvature dependent, non-harmonic bending elasticity of the DNA backbone is necessary to describe the DNA conformation under tight bending conditions. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R44.00011: Looping of anisotropic, short double-stranded DNA Harold Kim, Tung Le Bending of double-stranded DNA (dsDNA) is associated with fundamental biological processes such as genome packaging and gene regulation, and therefore studying sequence-dependent dsDNA bending is a key to understanding biological impact of DNA sequence beyond the genetic code. Average mechanical behavior of long dsDNA is well described by the wormlike chain model, but sequence-dependent anisotropic bendability and bendedness of dsDNA can in principle lead to abnormally high looping probability at short length scales. Here, we measured the looping probability density (J factor) and kinetics of dsDNA as a function of length and curvature using single-molecule FRET (F\"{o}rster Resonance Energy Transfer). For theoretical comparison, we calculated the J-factor using a discrete dinucleotide chain model, and also simulated it by Monte Carlo methods. We provide evidences that even when the intrinsic shape of dsDNA is accounted for, the wormlike chain model fails to describe looping dynamics of dsDNA below 200-bp length scale. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R44.00012: DNA looping by a ligase under nanoconfinement Maedeh Heidarpour-Roushan, Robert Riehn DNA looping is essential for the function and maintenance of genetic information. We have investigated the kinetic evolution of DNA loops (48500 bp) induced by T4 ligase inside a nanofabricated channel system with a channel cross-section of 100x100 nm2, and a few hundred microns channel length. We found that addition of the ligase profoundly alters the behavior of DNA. In particular, ligase acts to stabilize hairpin geometries in which the extended forward and backward arms of the hairpin scan past each other. From the linear density of DNA inside the channel, we deduce that the effective excluded volume vanishes upon addition of T4 ligase and ATP. We conclude that the two strands are effectively stapled together through a large number of weak bonds involving T4 ligase. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R44.00013: Loops determine the mechanical properties of mitotic chromosomes Yang Zhang, Dieter W. Heermann In mitosis, chromosomes undergo a condensation into highly compacted, rod-like objects. Many models have been put forward for the higher-order organization of mitotic chromosomes including radial loop and hierarchical folding models. Additionally, mechanical properties of mitotic chromosomes under different conditions were measured. However, the internal organization of mitotic chromosomes still remains unclear. Here we present a polymer model for mitotic chromosomes and show how chromatin loops play a major role for their mechanical properties. The key assumption of the model is the ability of the chromatin fibre to dynamically form loops with the help of binding proteins. Our results show that looping leads to a tight compaction and significantly increases the bending rigidity of chromosomes. Moreover, our qualitative prediction of the force elongation behaviour is close to experimental findings. This indicates that the internal structure of mitotic chromosomes is based on self-organization of the chromatin fibre. We also demonstrate how number and size of loops have a strong influence on the mechanical properties. We suggest that changes in the mechanical characteristics of chromosomes can be explained by an altered internal loop structure. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R44.00014: New insights into nucleosome unwrapping Razvan Chereji, Alexandre Morozov Eukaryotic genomes are organized into arrays of nucleosomes, in which stretches of 147 base-pairs (bp) of DNA are wrapped around octameric histones. Recently, a new approach for direct mapping of nucleosome centers at bp resolution was developed [Brogaard et al., Nature 486, 496-501 (2012)] and some intriguing results appeared. About 40\% of the inter-dyad distances are smaller than 147 bp, which imply massive nucleosome unwrapping, genome-wide, in vivo. The histogram of the inter-dyad distances presents small oscillations which indicate a step-wise unwrapping of the nucleosomal DNA from the histone. We present a statistical mechanics model for the nucleosome unwrapping, which is able to take into account sequence-dependent binding energies, sequence-independent potential barriers and wells, effective two-body interactions between the nucleosomes, competition between different species, cooperative-binding, and other important factors which dictate the nucleosome distribution along the DNA. We are able to reproduce the distribution of the inter-dyad distances, which cannot be obtained if there is no nucleosome unwrapping. The nucleosome unwrapping model can explain also the variable DNA accessibility and the nucleosome-induced cooperativity, which were observed experimentally. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R44.00015: Mechanisms for enhanced protein dissociation driven by nucleosomes Ralf Bundschuh, Cai Chen When a transcription factor binding site is located within a nucleosome, the DNA in the nucleosome has to unwrap in order for the transcription factor to bind. Thus, it is not surprising that the rate of transcription factor binding is slowed significantly in the presence of a nucleosome. The resulting change in transcription factor binding site occupancy has been known for quite a while as a mechanism for regulation of gene expression via chromatin structure. More surprisingly, recent single molecule experiments have pointed out that not only is the on-rate of transcription factors reduced by the presence of a nucleosome but also is the off-rate increased. There are two possible explanations short of an active role of the nucleosome in pushing the transcription factor off the DNA: (i) the nucleosome can change the equilibrium between binding at the specific binding site and nonspecific binding to the surrounding DNA or (ii) for dimeric transcription factors the nucleosome can change the equilibrium between monomeric and dimeric binding. We explicitly model both scenarios and find that the first mechanism cannot be reconciled with experimental findings. However, we show that the second mechanism can indeed explain increases in off-rate by a factor as high as $100$. [Preview Abstract] |
Session R45: Focus Session: Physics of the Cytoskeleton II
Sponsoring Units: DBIOChair: Timothy Sanchez, Brandeis University
Room: Hilton Baltimore Holiday Ballroom 4
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R45.00001: Biological Physics Dissertation Award Talk - Self-organization in cytoskeletal mixtures: from synthetic cilia to flowing networks Invited Speaker: Tim Sanchez Inspired by biological functions such as ciliary beating and cytoplasmic streaming, we have developed a highly tunable and robust model system from biological components that self-organizes to produce a broad range of far-from-equilibrium materials with remarkable emergent properties. Using only simple components - microtubules, kinesin motor clusters, and a depletion agent that bundles MTs -- we reproduced several essential biological functions, including cilia-like beating, the emergence of metachronal waves in bundle arrays, and internally generated flows in active cytoskeletal gels. The occurrence of these biomimetic functions as self-organized processes provides unique insight into the mechanisms that drive these processes in biology. Beyond these biomimetic behaviors, we have also used the same components to engineer novel active materials which have no biological analogues: active streaming 2D nematics, and finally self-propelled emulsion droplets. These observations exemplify how assemblages of animate microscopic objects exhibit highly sought-after collective and biomimetic properties, challenging us to develop a theoretical framework that would allow for a systematic engineering of their far-from-equilibrium material properties. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R45.00002: Nonlinear force propagation, anisotropic stiffening and non-affine relaxation in a model cytoskeleton Daisuke Mizuno, David Head, Emi Ikebe, Akiko Nakamasu, Suguru Kinoshita, Zhang Peijuan, Shoji Ando Forces are generated heterogeneously in living cells and transmitted through cytoskeletal networks that respond highly non-linearly. Here, we carry out high-bandwidth passive microrheology on vimentin networks reconstituted in vitro, and observe the nonlinear mechanical response due to forces propagating from a local source applied by an optical tweezer. Since the applied force is constant, the gel becomes equilibrated and the fluctuation-dissipation theorem can be employed to deduce the viscoelasticity of the local environment from the thermal fluctuations of colloidal probes. Our experiments unequivocally demonstrate the anisotropic stiffening of the cytoskeletal network behind the applied force, with greater stiffening in the parallel direction. Quantitative agreement with an affine continuum model is obtained, but only for the response at certain frequency $\sim$ 10-1000 Hz which separates the high-frequency power law and low-frequency elastic behavior of the network. We argue that the failure of the model at lower frequencies is due to the presence of non-affinity, and observe that zero-frequency changes in particle separation can be fitted when an independently-measured, empirical nonaffinity factor is applied. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R45.00003: Conformational phases of membrane bound cytoskeletal filaments David A. Quint, Gregory Grason, Ajay Gopinathan Membrane bound cytoskeletal filaments found in living cells are employed to carry out many types of activities including cellular division, rigidity and transport. When these biopolymers are bound to a membrane surface they may take on highly non-trivial conformations as compared to when they are not bound. This leads to the natural question; What are the important interactions which drive these polymers to particular conformations when they are bound to a surface? Assuming that there are binding domains along the polymer which follow a periodic helical structure set by the natural monomeric handedness, these bound conformations must arise from the interplay of the intrinsic monomeric helicity and membrane binding. To probe this question, we study a continuous model of an elastic filament with intrinsic helicity and map out the conformational phases of this filament for various mechanical and structural parameters in our model, such as elastic stiffness and intrinsic twist of the filament. Our model allows us to gain insight into the possible mechanisms which drive real biopolymers such as actin and tubulin in eukaryotes and their prokaryotic cousins MreB and FtsZ to take on their functional conformations within living cells. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R45.00004: Athermal Fluctuations of Probe Particles in Active Cytoskeletal Networks Heev Ayade, Irwin Zaid, Daisuke Mizuno A reconstituted active cytoskeletal networks consisting of an actin filament network coupled to myosins (motor proteins) have been shown to display rich in dynamical and mechanical behaviors that is often in contrast to passive, equilibrium system. The motor proteins, which spontaneously generate forces, kept the active cytoskeletal network out of equilibrium. The athermal fluctuations observed in the network are linked to the active force generation process by motor proteins which give more relevant information including the interaction with the surrounding materials. In prior studies, only the second moment of the athermal fluctuations has been investigated while the full displacement distribution of the athermal fluctuations in active cytoskeleton recently is found to be far from Gauss when observed with video microrheology. Here, we investigated the nonequilibrium statistics and dynamics of the active network by analyzing the athermal fluctuations of different probe sizes embedded in the same active system. The model developed here is based on truncated L\'{e}vy statistics which is generally observed for the force generators whose impact decays as 1/$r^{\mathrm{2}}$. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R45.00005: Spontaneous Motion in Hierarchically Assembled Active Cellular Materials Invited Speaker: Daniel Chen With exquisite precision and reproducibility, cells orchestrate the cooperative action of thousands of nanometer-sized molecular motors to carry out mechanical tasks at much larger length scales, such as cell motility, division and replication. Besides their biological importance, such inherently far-from-equilibrium processes are an inspiration for the development of soft materials with highly sought after biomimetic properties such as autonomous motility and self-healing. I will describe our exploration of such a class of biologically inspired soft active materials. Starting from extensile bundles comprised of microtubules and kinesin, we hierarchically assemble active analogs of polymeric gels, liquid crystals and emulsions. At high enough concentration, microtubule bundles form an active gel network capable of generating internally driven chaotic flows that enhance transport and fluid mixing. When confined to emulsion droplets, these 3D networks buckle onto the water-oil interface forming a dense thin film of bundles exhibiting cascades of collective buckling, fracture, and self-healing driven by internally generated stresses from the kinesin clusters. When compressed against surfaces, this active nematic cortex exerts traction stresses that propel the locomotion of the droplet. Taken together, these observations exemplify how assemblies of animate microscopic objects exhibit collective biomimetic properties that are fundamentally distinct from those found in materials assembled from inanimate building blocks. These assemblies, in turn, enable the generation of a new class of materials that exhibit macroscale flow phenomena emerging from nanoscale components. ~~ [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R45.00006: Mechanical Models of Microtubule Bundle Collapse in Alzheimer's Disease Austin Sendek, Rajiv Singh, Daniel Cox Amyloid-beta aggregates initiate Alzheimer's disease, and downstream trigger degradation of tau proteins that act as microtubule bundle stabilizers and mechanical spacers. Currently it is unclear which of tau cutting by proteases, tau phosphorylation, or tau aggregation are responsible for cytoskeleton degradation., We construct a percolation simulation of the microtubule bundle using~a molecular spring model for the taus and including depletion force attraction between microtubules and membrane/actin cytoskeletal surface tension. The simulation uses a fictive molecular dynamics~to model the motion of the individual microtubules within the bundle as a result of~random tau removal, and calculates the elastic modulus of the bundle as the tau concentration falls. We link the tau removal steps to kinetic tau steps in various models of tau degradation. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R45.00007: Properties of intracellular transport: the role of cytoskeleton topology Nickolay Korabel, Kerwyn C. Huang, Ajay Gopinathan The eukaryotic cytoskeleton is composed of polarized filaments forming a complex, intertwined network. Various motor proteins such as kinesins or myosins convert ATP into mechanical work and are able to walk processively or even diffuse along the cytoskeleton. Large organelles such as vesicles or mitochondria can randomly bind and unbind to one or several motors and their transport in the cell can be described as alternating phases of diffusion in the cytoplasm and phases of directed or diffusive transport along the cytoskeletal network. Intracellular transport has been the focus of extensive studies both experimentally and theoretically. However, the impact of the cytoskeleton network structure on transport properties, which is expected to be significant, is not fully understood. We develop a computational model of intracellular transport, and explore the impact of the cytoskeletal structure on transport properties. We show that transport can be enhanced even by diffusional motion along the cytoskeleton after memory effects due to cytoskeletal structure are taken into account. We also explore the influence of the network structure on the first passage time distributions for a cargo to reach the cell membrane after being exported from the nucleus and for transport from the membrane to the nucleus. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R45.00008: Active Stresses Drive Random Fluctuations in the Cytoplasm of Cells Ming Guo, Allen Ehrlicher, Mikkel Jensen, Jeffrey Moore, Jennifer Lippincott-Schwartz, Fred Mackintosh, David Weitz The cytoplasm of living cells is a highly dynamic environment with continuous intracellular motion that is essential for life. Some intracellular movements appear directional, and are clearly actively transported. However, most intracellular movement appears random in nature. These random movements have often been interpreted as Brownian motion, and have been used to infer cellular mechanics. Here we describe direct quantifications of the random intracellular motion by using sub-micron beads, and independent micromechanical measurements of the local cellular environment using optical tweezers. We demonstrate that the random intracellular motion is driven by active stress fluctuations in a nearly elastic cytoskeletal matrix. The combination of our two measurements allows us to quantify the frequency spectrum of the intracellular forces, and directly shows that non-thermal active stresses dominate thermal forces in the cellular interior at long time scales (t\textgreater 0.1s), which results in the random intracellular motion. By using the photoconvertible fluorescent protein Dendra2, we also show that the movement of very small particles ($\sim$ nm) are also accelerated by active fluctuations. These active-stress driven movements may be an essential part of rapid transport in life. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R45.00009: Microrheology of highly crosslinked microtubule networks is dominated by force-induced crosslinker unbinding Megan Valentine, Yali Yang, Mo Bai, William Klug, Alex Levine We determine the viscoelastic responses of reconstituted networks of microtubules that have been strongly bonded by labile crosslinkers using a magnetic tweezers device to apply localized forces. At short time scales, the networks respond nonlinearly to applied force, with stiffening at small forces, followed by a reduction in the stiffening response at high forces, which we attribute to the force-induced unbinding of crosslinks. At long time scales, force-induced bond unbinding leads to local network rearrangement and significant bead creep. Interestingly, for rigidly crosslinked networks, the material retains its elastic modulus even under conditions of significant plastic flow, suggesting that crosslinker breakage is balanced by the formation of new bonds. To better understand this effect, we developed a finite element model of such a stiff filament network with crosslinkers obeying force-dependent Bell model unbinding dynamics. The coexistence of dissipation, due to bond breakage, and the elastic recovery of the network is possible because each filament has many crosslinkers. Recovery can occur as long as a sufficient number of the original crosslinkers are preserved under the loading period. When these remaining original crosslinkers are broken, plastic flow results. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R45.00010: Microtubules contribute to maintain nucleus shape in epithelial cell monolayer Dominique Tremblay, Lukasz Andrzejewski, Andrew Pelling INTRODUCTION: Tissue strains can result in significant nuclear deformations and may regulate gene expression. However, the precise role of the cytoskeleton in regulating nuclear mechanics remains poorly understood. Here, we investigate the nuclear deformability of Madin-Darky canine kidney cells (MDCK) under various stretching conditions to clarify the role of the microtubules and actin network on the mechanical behavior of the nucleus. METHODS: A custom-built cell-stretching device allowing for real time imaging of MDCK nuclei was used. Cells were seeded on a silicone membrane coated with rat-tail collagen I. A nuclear stain, Hoechst-33342, was used to image nuclei during stretching. We exposed cells to a compressive and non-compressive stretching strain field of 25{\%}. Nocodazole and cytochalasin-D were used to depolymerize the microtubules and actin network. RESULTS: Nuclei in control cells stretched more along their minor axis than major axis with a deformation of 5{\%} and 2{\%} respectively. This anisotropy vanished completely in microtubule-deprived cells and these cells showed a very high nuclear deformability along the minor axis when exposed to a compressive stretching strain field. CONCLUSIONS: The microtubules drive the anisotropic deformability of MDCK nuclei in a monolayer and maintain nuclear shape when exposed to compressive strain. Such intrinsic mechanical behavior indicates that microtubules are essential to maintain nuclear shape and may prevent down regulation of gene expression. [Preview Abstract] |
Session R46: Invited Session: Keithley Session: Enabling Sensitive Measurements Beyond the Standard Quantum Limit
Sponsoring Units: GIMSChair: Eric Hudson, The Pennsylvania State Unversity
Room: Hilton Baltimore Holiday Ballroom 5
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R46.00001: Joseph F. Keithley Award For Advances in Measurement Science Lecture: Squeezing: the future for gravitational wave detectors Invited Speaker: David McClelland |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R46.00002: Joseph F. Keithley Award For Advances in Measurement Science Lecture: Beyond the quantum limit in gravitational wave detection Invited Speaker: Nergis Mavalvala . [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R46.00003: Exploring quantum limits with micro-mechanical membranes Invited Speaker: Cindy Regal The pursuit of increasingly sensitive interferometric measurement of mechanical motion has a rich history. This pursuit has resulted in the development and study of seminal ideas on quantum limits of measurement and beyond. In recent years, an interesting class of devices has been developed in which low-mass, high-frequency, and mechanically isolated objects are well-coupled to optical cavities. The large response of these mechanical objects to applied forces makes them an ideal platform to observe the effects of radiation forces, which are integral to the physics of quantum limits to interferometric measurement. Some of these nanomechanical resonators have been recently cooled with electromagnetic radiation to near their quantum mechanical ground state, illustrating the capacity for harnessing coherent optical forces. In this talk I present our recent work on a silicon nitride (SiN) membrane coupled to an optical cavity in a cryogenic environment. We use cavity coupling to significantly damp and cool membrane motion, and we demonstrate a low-absorption cavity with an efficient readout. Building on these capabilities, we observe the effect of a fluctuating radiation pressure force on the membrane resonator due to optical shot noise. Continued work will focus on further removing effects of classical noise in our devices; this will provide a path to measurement at the standard quantum limit as well as to using our optomechanical interface for applications in quantum information science. In particular, we are working on devices that will connect disparate quantum resources via SiN membrane resonators with hybrid functionalization. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R46.00004: Approaching the Quantum Limits of Displacement Detection Invited Speaker: John Teufel While high quality factor mechanical resonators (such as cantilevers and membranes) are routinely used as exquisite sensors, only recently are these engineered devices encountering the fundamental limits and opportunities afforded by quantum mechanics. The standard quantum limit of displacement detection requires a balance between the measurement imprecision and momentum imparted on the object of interest. One promising measurement scheme for achieving, and possibly surpassing, these quantum limits of measurement is that of cavity optomechanics---an architecture in which a mechanical resonator modulates the frequency of a high frequency electromagnetic resonance. Ideally, the quantized nature of the measurement photons will impart backaction in the form of radiation pressure shot noise, but observation of this quantum effect in macroscopic mechanical resonators has proven experimental difficult due to the relatively weak forces of the light. We realize a microwave cavity ``opto'' -mechanical system by incorporating a freely-suspended membrane in a superconducting microwave resonant circuit, which simultaneously exhibits high quality factor electrical and mechanical modes [1]. The relatively large electomechanical coupling has led to experimental observation of the strong coupling regime [1] as well as sideband cooling of the mechanical mode to its quantum ground state [2]. I will present recent experiments of similar circuits in which the displacement sensitivity goes beyond that at the standard quantum limit by several orders of magnitude. These measurements also clearly show the fundamental trade-off between measurement imprecision and backaction. We observe the radiation pressure shot noise of the microwave photons and show that it can completely overwhelm the classical, thermal motion of the membrane. [1] Teufel et al., Nature 471, 204-208 (2011).\\[4pt] [1] Teufel et al., Nature 475, 359-363 (2011). [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R46.00005: Quantum Non-Demolition Measurements between a Graphene Nanomechanical Resonator and a Diamond Nitrogen-Vacancy Center Invited Speaker: Brian D'Urso A description of the motion of microscopic particles often requires quantum mechanics, but macroscopic objects are typically observed to follow the predictions of classical mechanics. In the transition from microscopic components to a complex macroscopic system, the distinctive features of quantum mechanics can be hidden by thermal excitations and coupling to the environment. In particular, while individual spins are intrinsically quantum objects, nanomechanical resonators are usually observed as classical damped oscillators. With a careful choice of coupling, these two systems can be made to interact such that they perform quantum non-demolition (QND) measurements on each other, enabling a bridge between the quantum and classical worlds. Through this coupling, the nanomechanical resonator provides a classical readout of the spin, while the spin acts as a probe of the discrete quantum states of the resonator. We present a system consisting of a graphene nanoelectromechanical resonator coupled to a single spin through a uniform external magnetic field. The spin originates from a nitrogen-vacancy (NV) center in a diamond nanocrystal, which is positioned on the resonator. The external magnetic field provides quadratic coupling which results in QND measurements between the spin and resonator. The strength of the quadratic coupling is enhanced by utilizing an avoided level crossing of the coupled spin-resonator system. The low mass of a graphene resonator further increases the sensitivity to the force associated with a single spin. NV centers are chosen as the source of a spin due to their exceptional spin state coherence times, large zero-field splitting, and optical addressability. We will present an analysis of the system and report on the status of experimental measurements with graphene-NV center devices. [Preview Abstract] |
Session R47: Invited Session: Simulation of Interfaces in Two-Fluid Flows
Sponsoring Units: DFDChair: Shahriar Afkhami, New Jersey Institue of Technology
Room: Hilton Baltimore Holiday Ballroom 6
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R47.00001: Accelerated boundary integral simulations of particulate and two-phase flows Invited Speaker: Anna-Karin Tornberg In micro-fluidic applications where the scales are small and viscous effects dominant, the Stokes equations are often applicable. The suspension dynamics that is observed already with rigid particles and fibers are very complex also in this Stokesian regime, and surface tension effects are strongly pronounced at interfaces of immiscible fluids. Simulation methods can be developed based on boundary integral equations, which leads to discretizations of the boundaries of the domain only, and hence fewer unknowns compared to a discretization of the PDE. Two main difficulties associated with boundary integral discretizations are to construct accurate quadrature methods for singular and nearly singular integrands, as well as to accelerate the solution of the linear systems, that will have dense system matrices. If these issues are properly addressed, boundary integral based simulations can be both highly accurate and very efficient. We will discuss simulations of periodic suspensions of rigid particles and rigid fibers in 3D, where the simulations are accelerated by a newly developed spectrally accurate FFT based Ewald method, as well as highly accurate simulations of many interacting drops in 2D. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:42PM |
R47.00002: Direct Numerical Simulations of Multiphase Flows Invited Speaker: Gretar Tryggvason Many natural and industrial processes, such as rain and gas exchange between the atmosphere and oceans, boiling heat transfer, atomization and chemical reactions in bubble columns, involve multiphase flows. Often the mixture can be described as a disperse flow where one phase consists of bubbles or drops. Direct numerical simulations (DNS) of disperse flow have recently been used to study the dynamics of multiphase flows with a large number of bubbles and drops, often showing that the collective motion results in relatively simple large-scale structure. Here we review simulations of bubbly flows in vertical channels where the flow direction, as well as the bubble deformability, has profound implications on the flow structure and the total flow rate. Results obtained so far are summarized and open questions identified. The resolution for DNS of multiphase flows is usually determined by a dominant scale, such as the average bubble or drop size, but in many cases much smaller scales are also present. These scales often consist of thin films, threads, or tiny drops appearing during coalescence or breakup, or are due to the presence of additional physical processes that operate on a very different time scale than the fluid flow. The presence of these small-scale features demand excessive resolution for conventional numerical approaches. However, at small flow scales the effects of surface tension are generally strong so the interface geometry is simple and viscous forces dominate the flow and keep it simple also. These are exactly the conditions under which analytical models can be used and we will discuss efforts to combine a semi-analytical description for the small-scale processes with a fully resolved simulation of the rest of the flow. We will, in particular, present an embedded analytical description to capture the mass transfer from bubbles in liquids where the diffusion of mass is much slower than the diffusion of momentum. This results in very thin mass-boundary layers that are difficult to resolve, but the new approach allows us to simulate the mass transfer from many freely evolving bubbles and examine the effect of the interactions of the bubbles with each other and the flow. We will conclude by attempting to summarize the current status of DNS of multiphase flows. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 4:18PM |
R47.00003: Advances and Challenges in Modeling Interfacial Flows Invited Speaker: Marianne Francois Interfacial flows are multi-material flows comprised of two or more immiscible materials demarcated by interfaces. They are encountered in several applications of interest to the Department of Energy. Examples of applications include materials processing (e.g. casting), inertial confinement fusion and solvent extraction. We are interested in the development of accurate numerical methods to simulate with high-fidelity interfacial flows. For such simulation, the position of the interface and interface physics need to be predicted as part of the solution of the flow equations. One of the many techniques is known as the volume tracking method. It is a pure Eulerian method that represents the interface with volume fraction and intrinsically ensures mass conservation. In this talk, I will describe several advances that have been made over the past 25 years and discuss remaining challenges in the context of the volume tracking method. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:54PM |
R47.00004: A moment of fluid method for computing solutions to multiphase/multimaterial flows Invited Speaker: Mark Sussman We combine the multimaterial Moment-of-Fluid (MOF) work of Ahn and Shashkov with the work of Kwatra et al for removing the acoustic time step restriction in order to solve multimaterial flows in which each material might be compressible or incompressible. The mass weights found in the algorithm of Kwatra et al are computed directly from the multimaterial MOF reconstructed interface. We treat the interface(s) between materials as sharp when discretizing the boundary conditions between materials. The combination of the multimaterial MOF reconstruction together with the cell centered formulation devised by Kwatra et al enable us to robustly compute multimaterial flows with large density ratios, stretching and tearing of interfaces and contact line dynamics at the junction of 3 materials with minimal volume fluctuation of each material (if a given material is incompressible). Simulations for multimaterial flows are presented with applications to combustion (atomization and spray) and microfluidics. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:30PM |
R47.00005: Direct numerical simulation of coaxial atomizing jets Invited Speaker: Stephane Zaleski |
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