Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D1: New Developments in Quantum Criticality
Sponsoring Units: DCMPChair: Meigan Aronson, Brookhaven National Laboratory
Room: Ballroom A1
Monday, March 21, 2011 2:30PM - 3:06PM |
D1.00001: Quantum critical points and novel phases in heavy fermion metals Invited Speaker: Quantum criticality arises from competing interactions of correlated systems that favor rivaling ground states. It not only influences physical properties over a wide temperature and parameter ranges, but also gives rise to a plethora of new quantum phases. Magnetic heavy fermion metals represent a prototype system in this context, and have in particular provided the setting to study local quantum criticality that involves not only order-parameter fluctuations but also a Kondo breakdown [1]. Surprisingly, recent theoretical and experimental developments have revealed some unusual phases proximate to the heavy-fermion quantum critical points, thereby opening up an entirely new frontier on the relationship between quantum criticality and novel phases [1]. I will summarize the relevant recent experiments [2] and discuss them within the framework of a global phase diagram that was put forward several years ago [3] and has recently been discussed more extensively [4,5]. Our theoretical studies emphasize the interplay between two effects. One is the Kondo screening and its breakdown, and the other is the fluctuations in the quantum magnetism of local moments alone. The insights gained from these studies of the well-defined quantum criticality in heavy fermions may have broader relevance. Such implications will be discussed, in particular on the interplay between metallic antiferromagnetism, electronic localization and unconventional superconductivity. \\[4pt] [1] Q. Si and F. Steglich, Science 329, 1161 (2010).\\[0pt] [2] S. Friedemann et al., Nature Phys. 5, 465 (2009);[0pt] J. Custers et al., PRL 104, 186402 (2010).\\[0pt] [3] Q. Si, Physica B 378, 23 (2006); S. J. Yamamoto and Q. Si, PRL 99, 016401 (2007).\\[0pt] [4] Q. Si, Phys. Status Solidi B247, 631 (2010); S. J. Yamamoto and Q. Si, J. Low Temp. Phys. 161, 233 (2010).\\[0pt] [5] P. Coleman and A. H. Nevidomskyy, J. Low Temp. Phys. 161, 182 (2010). [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D1.00002: Two-dimensional Confinement of Heavy Fermions in Artificial Superlattices Invited Speaker: Low dimensionality and strong electron-electron Coulomb interactions are both key parameters for novel quantum states of condensed matter. A metallic system with the strongest electron correlations is reported in rare-earth and actinide compounds with $f$ electrons, known as heavy-fermion compounds, where the effective mass of the conduction electrons are strikingly enhanced by the electron correlations up to some hundreds times the free electron mass. To date the electronic structure of all heavy-fermion compounds is essentially three-dimensional. We realized experimentally a two-dimensional heavy fermion system, adjusting the dimensionality in a controllable fashion. We grew artificial superlattices of CeIn$_3$($m$)/LaIn$_3 $($n$), in which $m$-layers of heavy-fermion antiferromagnet CeIn$_3$ and $n$-layers of a non-magnetic isostructual compound LaIn$_3$ are stacked alternately, by a molecular beam epitaxy [1]. By reducing the thickness of the CeIn$_3$ layers, the magnetic order was suppressed and the effective electron mass was further enhanced. The N\'eel temperature becomes zero at around $m$ = 2, concomitant with striking deviations from the standard Fermi liquid low-temperature electronic properties. Standard Fermi liquid behaviors are, however, recovered under high magnetic field. These behaviors imply new ``dimensional tuning'' towards a quantum critical point. We also succeeded to fabricate artificial superlattices of a heavy fermion superconductor CeCoIn$_5$ and non-magnetic divalent Yb-compound YbCoIn$_5$. Superconductivity survives even in CeCoIn$_5$(3)/YbCoIn$_5$(5) films, while the thickness of CeCoIn$_5$ layer, 2.3\,nm, is comparable to the $c$-axis coherence length $\xi_{\rm c}$ $\sim$2\,nm. This work has been done in collaboration with Y. Mizukami, S. Yasumoto, M. Shimozawa, H. Kontani, T. Shibauchi, T. Terashima and Y. Matsuda.superconductivity is realized in the artificial superlattices. \\[4pt] [1] H.Shishido $et$ $al$., Science {\bf 327} 980 (2010). [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D1.00003: Universal Signatures of Metamagnetic Quantum Criticality Invited Speaker: The continuous quest for quantum critical materials is inspired by the exotic phases and unusual phenomena that can be observed close to a zero-temperature instability. An appealing realization of such a critical point is found in metamagnetic materials where the magnetization shows a finite step at a certain magnetic field that becomes more pronounced at low temperatures. The most striking advantages of this kind of quantum criticality are that the critical point is i) symmetric in the associated thermodynamic phase diagram and not accompanied by a symmetry-breaking ordered phase and ii) the tuning parameter magnetic field $H$ can be adjusted continuously and makes a very detailed and comprehensive study of this so called quantum critical end-point (QCEP) possible. In the presented talk the qualitative features of a field-driven QCEP are discussed, which result from very basic thermodynamic relations and the two general assumptions that i) the differential magnetic susceptibility diverges at the critical field $H_{c}$ by definition and ii) the QCEP has Ising symmetry. We present real examples of metamagnetic systems, where the characteristics can be found experimentally. Particular emphasis will be placed on the well-known intermetallic material CeRu$_{2}$Si$_{2}$. We argue that a QCEP is approximately realized in this compound and confirm our claims by the combination of new high-resolution thermal expansion, magnetostricion and specific heat results. Very similar behavior was found recently on the prominent material Sr$_{3}$Ru$_{2}$O$_{7}$ whose metamagnetic quantum criticality is masked by the appearance of a phase proposed to be of nematic electronic nature. We believe that our work will facilitate and promote the experimental identification of further metamagnetic systems for quantum criticality in the future. \\[4pt] [1] \textit{Weickert et al}., Phys. Rev. B, \textbf{81}, 134438 (2010). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D1.00004: Quantum criticality and confinement effects in an Ising chain in transverse field Invited Speaker: The Ising chain in transverse field is one of the key paradigms for the theory of continuous zero-temperature quantum phase transitions. We have recently realized this system experimentally by applying strong magnetic fields to the quasi- 1D, low-exchange Ising ferromagnet CoNb2O6 to drive it to its quantum critical point where the spontaneous long-range magnetic order is suppressed by magnetic field [1]. Using high-resolution single-crystal neutron scattering we have probed how the spin dynamics evolves with the applied field and have observed a dramatic change in the character of spin excitations at the quantum critical point, from pairs of domain-wall (kink) quasiparticles in the magnetically-ordered phase, to sharp spin- flip quasiparticles in the paramagnetic phase. The weak, but finite couplings between the chains significantly enrich the physics by stabilizing a complex structure of two-kink bound states due to mean-field confinement effects. In zero field the rich spectrum of bound states can be quantitatitively understood following McCoy and Wu's analytic theory of weak confinement [2]. Just below the critical field the energies of the two lowest bound states approach the ``golden ratio'' as predicted by Zamolodchikov's E8 scaling limit solution of the off-critical Ising model in a weak longitudinal field [3]. \\[4pt] [1] R. Coldea, D.A. Tennant, E.M. Wheeler, E. Wawrzynska, D. Prabhakaran, M. Telling, K. Habicht, P. Smeibidl, K. Kiefer, Science 327, 177 (2010).\\[0pt] [2] B. M. McCoy and T. T. Wu, Phys. Rev. D 18, 1259 (1978).\\[0pt] [3] A.B. Zamolodchikov, Int. J. Mod. Phys. A4, 4235 (1989). [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D1.00005: Strange metals and quantum phase transitions from gauge/gravity duality Invited Speaker: Metallic materials whose thermodynamic and transport properties differ significantly from those predicted by Fermi liquid theory, so-called non-Fermi liquids, include the strange metal phase of cuprate superconductors, and heavy fermion systems near a quantum phase transition. We use gauge/gravity duality to identify a class of non-Fermi liquids. Their low-energy behavior is governed by a nontrivial infrared fixed point which exhibits non-analytic scaling behavior only in the temporal direction. Some representatives of this class have single-particle spectral functions and transport behavior similar to those of the strange metals, with conductivity inversely proportional to the temperature. Such holographic systems may also exhibit novel ``magnetic instabilities'', where the quantum critical behavior near the transition involves a nontrivial interplay between local and bulk physics, with the local physics again described by a similar infrared fixed point. The resulting quantum phase transitions do not obey the standard Landau-Ginsburg-Wilson paradigm and resemble those of the heavy fermion quantum critical points. [Preview Abstract] |
Session D2: Topological Surface States
Sponsoring Units: DCMPChair: Ali Yazdani, Princeton University
Room: Ballroom A2
Monday, March 21, 2011 2:30PM - 3:06PM |
D2.00001: Visualizing surface states of topological insulators using spectroscopic mapping with the scanning tunneling microscope Invited Speaker: In topological insulators, the spin texture of the surface states makes them distinct from conventional two-dimensional electron states, and leads to novel properties for these states. These surface states are expected to be immune to localization and to overcome barriers caused by material imperfections. We have used scanning tunneling microscopy and spectroscopy to study the topological surface states in \textit{Bi}$_{0.9}$\textit{Sb}$_{0.1}$\textit{, Sb, } and \textit{Bi}$_{2}$\textit{Te}$_{3}$. By mapping the interference of the surface states scattering off random alloying disorder in \textit{Bi}$_{0.9}$\textit{Sb}$_{0.1}$, we have demonstrated that despite strong atomic scale disorder, backscattering between states of opposite momentum and opposite spin is absent, resulting from the spin texture [1]. Furthermore, we have measured the transmission and reflection of topological surface states of \textit{Sb} through atomic terraces [2]. In contrast to Schottky surface states of noble metals, these surface states penetrate such barriers with high probability. To examine the possibility of disorder induced localization, we investigated the surface states of Bi$_{2}$Te$_{3}$ in the presence of local defects. In the presence of magnetic dopants, we have observed an interference pattern throughout a broad range of energies, even in the region of linear dispersion near the Dirac point [3]. We discuss the results of a statistical analysis of these patterns which can help to learn about the tendency toward localization for these surface states and how this trend is affected as the energy is tuned to the Dirac point. *Work was done in collaboration with J. Seo, H. Beidenkopf, L. Gorman, Y. S. Hor, C. Parker, D. Hsieh, and A. Richardella, M. Z. Hasan, R. Cava, and A. Yazdani. \\[4pt] [1] P. Roushan \textit{et al.} Nature 460, 1106 (2009). \\[0pt] [2] J. Seo \textit{et al.} Nature 466, 343 (2010). \\[0pt] [3] H. Beidenkopf \textit{et al.} (2010). [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D2.00002: STM and STS studies of electronic states near macroscopic defects in topological insulators Invited Speaker: Bi$_{2}$Te$_{3~}$and Bi$_{2}$Se$_{3~}$have been argued recently to be three-dimensional (3D) topological insulators (TI), exhibiting a bulk gap and a single, non-degenerate Dirac fermion surface band topologically protected by time-reversal symmetry. In this talk we will discuss the physics of topological insulators. We will show that Scanning tunneling spectroscopy (STS) studies on high-quality Bi$_{2}$Te$_{3}$~and Bi$_{2}$Se$_{3}$~crystals exhibit perfect correspondence to ARPES data, hence enabling identification of different regimes measured in the local density of states (LDOS). Unique to Bi$_{2}$Te$_{3}$, we will discuss observations of oscillations of LDOS near a step. Within the main part of the surface band we found that the oscillations are strongly damped, supporting the hypothesis of topological protection. At higher energies, as the surface band becomes concave, oscillations appear which disperse with a particular wave-vector that results from an unconventional hexagonal warping term in the surface-state-band Hamiltonian [1]. For both systems, a bound state was observed in the bulk gap region that runs parallel to the edge of the defect and is bound to it at some characteristic distance. An expression that fits the data, and provides further insight into the general topological properties of the electronic structure of the surface band near strong structural defects, can be obtained using the full three-dimensional Hamiltonian of the system. \\[4pt] [1] Zhanybek Alpichshev, J. G. Analytis, J.-H. Chu, I. R. Fisher, Y. L. Chen, Z. X. Shen, A. Fang, and A. Kapitulnik \textit{Phys. Rev. Lett.} \textbf{104} 016401 (2010) [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D2.00003: Theory of Topological Insulators and Superconductors: Application to Cux-Bi2Se3 Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D2.00004: Investigation and manipulation of the electronic properties of magnetically doped topological insulators Invited Speaker: Topological insulator (TI) is characterized by gapless surface/edge states which are protected by time reversal symmetry (TRS). Magnetic order in or adjacent to a TI can break its TRS, and thus result in various exotic phenomena, e.g. magnetic monopole, quantum anomalous Hall effect, and topological magneto-electric effect. Combining angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, and transport measurement, we have investigated the electronic structures and properties of Bi2Se3 family three dimensional TIs doped with magnetic impurities. Gap opening at the Dirac surface states induced by magnetic impurities has been observed, suggesting the formation of long range magnetic order in the TIs. The Dependences of the gap size on impurity concentration, chemical potential and real space position and the (anomalous) Hall effect of the magnetically doped TIs have been systematically studied, the result of which reveals the nature and mechanism of the magnetic order. The present studies pave the road to the realization of the novel properties predicted in magnet/TI heterostructures. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D2.00005: Theory of surface phenomena in topological insulators Invited Speaker: Recently discovered topological insulators (TIs) are materials with bulk bandgap and robust gapless surface states protected by topological invariants that characterize their bulk band structure. After a brief introduction to the physics of TIs I will describe recent theoretical advances in understanding the behavior of surface electrons in the presence of both magnetic and non-magnetic impurities, surface steps, as well as magnetic and superconducting coating. The key property of the topological surface states -- absence of backscattering from non-magnetic defects -- leads to a number of features that stand in a stark contrast to the physics of ordinary non-topological states. Among these are vastly enhanced transmission through crystal steps, absence of quasiparticle interference patterns caused by non-magnetic impurities and formation of a gap in the presence of magnetic impurities. [Preview Abstract] |
Session D3: Materials for Energy
Sponsoring Units: DMP GERAChair: Robert Nemanich, Arizona State University
Room: Ballroom A3
Monday, March 21, 2011 2:30PM - 3:06PM |
D3.00001: Material tradeoffs in direct thermal to electric energy conversion systems Invited Speaker: Thermoelectric devices allow direct conversion of heat into electricity without any moving parts. However the energy conversion efficiency has been limited due to parasitic Joule heating in the thermoelectric material as well as the heat leakage from the hot to the cold junction mainly through phonons. Using thermionic emission over heterostructures and electron energy filtering, high Seebeck coefficient and high electrical conductivity can be achieved simultaneously. Embedded nanoparticles can also be used to scatter mid and long wavelength phonons and reduce the lattice thermal conductivity with small impact on electrical transport. While the tradeoff in material properties can be reduced with nanoengineered structures, the overall efficiency/cost tradeoff has not been analyzed in detail. In a waste heat recovery system, in addition to the thermoelectric device, the heat sink and the electrical and thermal resistances have to be co-optimized. A recent analytic theory is reviewed which shows the potential of thermoelectric waste heat recovery in a wide range of applications. Co-optimization of the thermoelectric module with the heat sink will permit minimizing the amount of material used in the system and reduce the overall energy payback. Optimization of the thermoelectric system in maximum output power regime, which is important in many practical applications, lead to interesting conclusions about the asymmetric role of thermal resistances with hot and cold reservoirs. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D3.00002: Photophysics of Strongly Confined Multiexcitons from the Perspective of Lasing and Solar Energy Conversion Invited Speaker: Using semiconductor nanocrystals one can produce extremely strong spatial confinement of electronic wave functions not accessible with other types of nanostructures. One consequence of this effect is a significant enhancement in carrier-carrier interactions that lead to a number of novel physical phenomena including ultrafast mutiexciton decay due to Auger recombination and efficient generation of multiple electron-hole pairs by single photons via carrier multiplication. In this talk, I will discuss the implications of ultrafast Auger decay for lasing applications of the nanocrystals and describe several recent approaches developed in our group for resolving this problem by engineering carrier-carrier interactions in various types of heterostructured particles. I will also review the current status of carrier-multiplication research including experimental challenges in studies of this phenomenon, the role of extraneous effects, the competing energy relaxation channels, and applications of carrier multiplication in solar photovoltaics. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D3.00003: Engineering Interfaces for Photovoltaic Energy Conversion Invited Speaker: Dye-sensitized solar cells (DSSCs) and the related quantum dot-sensitized solar cells (QDSSCs) show promise as inexpensive, efficient next-generation photovoltaic technologies. A typical cell design consists of a sensitizer chemisorbed to a nanoporous TiO$_{2}$ substrate; the sensitizer absorbs a photon and an excited electron is injected into the TiO$_{2}$ where it diffuses to the anode. However, many devices suffer from a high rate of electron-hole recombination at the interface between TiO$_{2}$ and the hole conductive material, leading to reduced conversion efficiency. In this work we explore whether a passivating layer at the interface can improve efficiency by acting as a barrier against electron recombination. We have studied both organic and inorganic approaches to modifying the interfacial properties in DSSC and QDSSC devices. In studies of CdS-based QDSSCs, a series of organic self-assembled monolayers were formed at the interface, and their effect on CdS uptake and resulting optoelectronic and device properties was investigated. In DSSCs, nanoscale inorganic dielectric films of different thicknesses were applied to the interface using atomic layer deposition prior to dye absorption. The effect on device performance was measured experimentally and compared with predictions from kinetic models. The results of these investigations will be discussed in the context of the ability of interface engineering to improve photovoltaic energy conversion. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D3.00004: Experimental and Theoretical Studies on Phonon Mean Free Path in Thermoelectric Materials Invited Speaker: Nanostructured thermoelectric materials have shown improved thermoelectric figure of merit due to reduced phonon thermal conductivity. To design nanostructures that effectively scatter phonons via interface and boundary scattering, it is important to know the phonon mean free path of thermoelectric materials in their bulk form. In this talk, we will present recent progress in experimental and theoretical investigation of phonon mean free path in thermoelectric materials. On the experimental side, we extend an optical pump-and-probe technique to measure contributions of phonons with different mean free paths to thermal conductivity via systematically changing the size of the heated regions. On the theoretical side, we apply first-principle calculations to extract anharmonic force constants, and compute the phonon relaxation time due to phonon-phonon scattering. We will present experimental and theoretical results obtained on silicon, half-heuslers, etc, and their implications to thermoelectric materials. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D3.00005: Progress in Materials for Solar Energy Conversion Invited Speaker: This abstract not available. [Preview Abstract] |
Session D4: Simulations Meet Experiments on Ultracold Quantum Gases
Sponsoring Units: DCOMP DAMOPChair: Matthias Troyer, ETH Zurich
Room: Ballroom A4
Monday, March 21, 2011 2:30PM - 3:06PM |
D4.00001: Pairing states of a one-dimensional spin imbalanced Fermi gas accross a Feshbach resonance Invited Speaker: A description of the BCS-BEC crossover in one dimension that properly accounts for the coexistence of fermions and bound pairs can be achieved in the framework of the Bose-Fermi resonance model, in which two fermions in an open channel couple resonantly to a diatomic molecule in the closed channel. In the case of a gas with spin imbalance, pairing correlations consistent with a phase of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type dominate a wide parameter range on the BCS side of the resonance. In the BEC regime, the FFLO correlations are suppressed, leading to a Bose-Fermi mixture consisting of a conventional bosonic superfluid in the molecular channel immersed into a gas of fermions that is either partially or fully polarized. I will present results of a comprehensive numerical study of this model using the density matrix renormalization group method, and determine the dependence of the critical polarization on filling and detuning. [F. Heidrich-Meisner, A.E. Feiguin, U. Schollwoeck, W.Z werger, Phys. Rev. A81, 023629 (2010)] [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D4.00002: Quantum Simulations with Ultracold Bosons in Optical Lattices and Superlattices Invited Speaker: Ultracold quantum gases in optical lattices have opened a new window for understanding strongly correlated many-body systems. They especially allow for ab-initio tests of fundamental condensed matter theories. In the presentation, I will discuss several examples, where static phases and non-equilibrium evolutions of ultracold quantum gases are compared to theoretical simulations. Among the examples that will be discussed are the measurement of the critical temperature for superfluidity in the vicinity of the quantum phase transition from a superfluid to a Mott insulator and the observation of a reentrant phase transition between superfluid and Mott insulating phases in a columnar superlattice. Finally, I will report on experimental and theoretical results that shed light on the question how isolated, strongly interacting quantum systems, can locally appear as if the system has equilibrated globally. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D4.00003: Trapping, cooling and probing fermionic atoms into the Mott and Neel states Invited Speaker: A new form of quantum condensed matter physics has emerged from the study of ultra-cold fermionic atoms in optical lattices. Experiments have recently reached the incompressible Mott regime. Detailed comparison to theory and computational studies at intermediate temperatures have validated the concept of optical lattice emulation of many-body fermionic systems. Cooling these systems deeper into the quantum degenerate regime, and devising new spectroscopic probes to investigate physical issues of interest such as quasiparticle properties, are key challenges in this context. The presentation will be based in part on the following references: L. De Leo, C.Kollath, A.Georges, M.Ferrero and O.Parcollet Phys. Rev. Lett. 101, 210403(2008); J.-S. Bernier et al. Phys. Rev. A 79, 061601 (2009); R. J\"ordens et al. Phys. Rev. Lett. 104, 180401 (2010); J.-S. Bernier et al., Phys. Rev. A 81, 063618 (2010); L. De Leo et al., arXiv:1009.2761 [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D4.00004: Generalized Thermalization in Integrable Systems Invited Speaker: Once only of theoretical interest, integrable models of one-dimensional quantum many-body systems can now be realized with ultracold gases. The possibility of controlling the effective dimensionality and the degree of isolation in the experiments have allowed access to the quasi-1D regime and to the long coherence times necessary to realize integrable models. In general, in integrable quantum systems that are far from equilibrium, observables cannot relax to the usual thermal expectation values. This is because of the constraints imposed by the non-trivial set of conserved quantities that make these systems integrable. Experimentally, relaxation of an observable to a non-thermal expectation value was recently observed in a cold-atom system close to integrability. At integrability, it is natural to describe the observables after relaxation by an updated statistical mechanical ensemble: the generalized Gibbs ensemble (GGE), which is constructed by maximizing the entropy subject to the integrability constraints. In recent studies, the GGE has been found to accurately describe various observables in the steady state of integrable systems, but a microscopic understanding of its origin and applicability remains elusive. In this talk, we review some of the early results on this topic and discuss the justification of the GGE based on a generalized view of the eigenstate thermalization hypothesis, which was originally introduced to explain thermalization in nonintegrable systems. {\bf References:}\\[4pt] [1] M. Rigol, V. Dunjko, V. Yurovsky, and M. Olshanii, Phys. Rev. Lett. {\bf 98}, 050405 (2007).\\[0pt] [2] M. Rigol, A. Muramatsu, and M. Olshanii, Phys. Rev. A {\bf 74}, 053616 (2006).\\[0pt] [3] A. C. Cassidy, C. W. Clark, and M. Rigol, arXiv:1008.4794. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D4.00005: A quantitative analysis of small atomic systems Invited Speaker: Ultracold atoms in an optical lattice provide a unique toolbox for emulating the prototypical models of condensed matter physics. Before the optical lattice system can be trusted as a quantum simulator however, it needs to be validated and benchmarked against known results, for which quantum Monte Carlo simulations are ideally suited. In this talk, an overview of recent numerical studies of ultracold bosonic and fermionic systems in an optical lattice will be given, starting with a full comparison based on experimental time-of-flight images of bosons in an optical lattice and ab-initio simulations. Next, the advantages of single-site resolution detection tools will be highlighted. Finally, the temperature and entropy in present experiments on fermions in an optical lattice will be estimated, and the full thermodynamics on approach to the Neel temperature will be presented. Nearest-neighbor spin-spin correlations are shown to be useful for thermometry. [Preview Abstract] |
Session D5: Industrial Physics Forum: Frontiers in Physics
Sponsoring Units: FIAPChair: Ernesto Marinaro, Hitachi San Jose Research Laboratory
Room: Ballroom C1
Monday, March 21, 2011 2:30PM - 3:06PM |
D5.00001: Controlling how atoms respond to ultra-intense x-ray radiation Invited Speaker: With the advent of the Linac Coherent Light Source, the world's first hard x-ray free electron laser, an era of exploration using ultrafast, ultra-intense x-ray pulses has arrived. One can deposit 100,000 x-ray photons into one square Angstrom within 100 fs, producing an electric field strength that exceeds that binding the electron in a hydrogen atom. How does matter respond under these conditions? Using neon atoms, we investigated the electronic response as the x-ray interaction is tuned from the outer to the inner shell. At photon energies above all inner-shell edges, fully stripped neon is produced via six-photon absorption. The route to bare neon proceeds through photoejection of 1s electrons that produces hollow atoms and an intensity-induced x-ray transparency. X-ray transparency can be induced in all atomic, molecular and condensed matter systems. Going beyond non-resonant x-ray atom interactions, we investigated the atomic response at inner-shell resonances and find evidence for x-ray induced Rabi cycling. These investigations provide a framework for understanding ultra-intense x-ray interactions with matter. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D5.00002: Scanning Tunneling Microscopy of Dirac Fermions at mK Temperatures Invited Speaker: Since the beginning of the last century new frontiers in physics have emerged when advances in instrumentation achieved lower experimental operating temperatures. Notable examples include the discovery of superconductivity and the integer and fractional quantum Hall effects. New experimental techniques are continually adapted in order to meet new experimental challenges. A case in point is scanning tunneling microscopy (STM) which has seen a wealth of new measurements emerge as cryogenic STM instruments have been developed in the last two decades. In this talk I describe the design, development and performance of a scanning probe microscopy facility operating at a base temperature of 10 mK in magnetic fields up to 15 T [1]. The microscope is cooled by a custom designed, fully ultra-high vacuum (UHV) compatible dilution refrigerator (DR) and is capable of in-situ tip and sample exchange. Sub-picometer stability at the tip-sample junction is achieved through three independent vibration isolation stages and careful design of the dilution refrigerator. The system can be connected to, or disconnected from, a network of interconnected auxiliary UHV chambers used for sample and probe tip preparation. Current measurements are focusing on Dirac fermions in graphene and in topological insulators. The history of the fractional quantum Hall states in semiconductor heterostructures suggests that studying graphene at lower temperatures and higher magnetic fields may reveal new quantum phases of matter. Scanning tunneling spectroscopy of graphene at mK temperatures reveals the detailed structure of the degenerate Landau levels in graphene, resolving the full quartet of states corresponding to the lifting of the spin and valley dengeneracies [2]. When the Fermi level lies inside the four-fold Landau manifold, significant electron correlation effects result in enhanced valley splitting and spin splitting. New many-body states are observed at fractional filling factors of 7/2, 9/2, and 11/2. \\[4pt] [1] \textit{A 10 mK Scanning Probe Microscopy Facility}, Y. J. Song, A. F. Otte, V. Shvarts, Z. Zhao, Y. Kuk, S. R. Blankenship, A. Band, F. M. Hess, and J. A. Stroscio, Rev. Sci. Instrum. (in press). \\[0pt] [2] \textit{High Resolution Tunneling Spectroscopy of a Graphene Quartet}, Y. Jae Song, A. F. Otte, Y. Kuk, Y. Hu, D. B. Torrance, P. N. First, W. A. de Heer, H. Min, S. Adam, M. D. Stiles, A. H. MacDonald, and J. A. Stroscio, Nature \textbf{467}, 185 (2010). [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D5.00003: Topological materials and their potential applications Invited Speaker: In this talk I shall give a brief introduction on the physics of the recently discovered topological materials and discuss their potential applications. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D5.00004: The Hottest Liquid on the Planet Invited Speaker: We generally expect high temperature matter to act like a gas. However, nature sometimes holds surprises. Collisions of heavy nuclei at very high energies produce a plasma of quarks and gluons which is a strongly coupled liquid. Its vanishingly small shear viscosity to entropy density ratio means it flows essentially without resistance, making it one of the most ``perfect'' liquids known. Astoundingly, a key tool for theoretical study of the dynamics of this novel liquid arises from the duality of string theory with black holes. I will describe how this liquid is studied, what we've learned about its properties at the Relativistic Heavy Ion Collider in the U.S. and at the Large Hadron Collider in Switzerland, as well as what we haven't figured out yet. I'll also discuss how the quark gluon plasma relates to other strongly coupled systems such as dusty plasmas, cold atomic gases, and strongly correlated condensed matter. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D5.00005: Ultracold polar molecules Invited Speaker: Ultracold quantum gases are model systems for studying many-body quantum physics. For example, superfluidity in ultracold Fermi gases of atoms realizes an electrically neutral analog of superconductivity. Recently, enormous progress has been made toward the goal of creating a new type of quantum gas where the constituent particles are polar molecules rather than atoms. In addition to new internal degrees of freedom of the particles, polar molecules introduce the possibility of long-range dipole-dipole interactions, which make the system fundamentally different from atom gases, which have short-range, or contact, interactions. I will discuss recent experimental work on a trapped gas of ultracold fermionic polar molecules. [Preview Abstract] |
Session D6: Physics of Proteins I: Unifying Principles and Concepts
Sponsoring Units: DBP DPOLY DCMPChair: Robert Austin, Princeton University
Room: Ballroom C2
Monday, March 21, 2011 2:30PM - 3:06PM |
D6.00001: Protein Dynamics Invited Speaker: Proteins combine properties of solids, liquids, and glasses. Schr\"{o}dinger anticipated the main features of biomolecules long ago by stating that they had to be solid-like, but able to assume many different conformations. Indeed proteins can assume a gigantic number of conformational substates with the same primary sequence but different conformations. The different substates are described as craters in a very-high-dimensional energy landscape. The energy landscape is organized in a hierarchy of tiers, craters within craters within craters. Protein motions are pictured as transition between substates - jumps from crater to crater. Initially we assumed that these jumps were controlled by internal barriers between substates, but experiments have shown that nature selected a different approach. Proteins are surrounded by one to two layers of water and are embedded in a bulk solvent. Structural motions of the protein are controlled by the alpha fluctuations in the solvent surrounding the protein. Some internal motions most likely involving side chains are controlled electrostatically by beta fluctuations in the hydration shell. The dynamics of proteins is consequently dominated by the environment (H. Frauenfelder et al. PNAS 106, 5129 (2009). One can speculate that this organization permits exchange of information among biomolecules. The energy landscape is not just organized into two tiers, alpha and beta, but cryogenic experiments have revealed more tiers and protein more properties similar to that of glasses. While proteins function at ambient temperatures, cryogenic studies are necessary to understand the physics relevant for biology. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D6.00002: Engineering electron tunneling in natural and artificial proteins Invited Speaker: Experimental investigation of oxidoreductases has revealed their naturally selected electron tunneling engineering that underlies oxidative and reductive catalysis. This engineering is relatively simple, which allows us to design artificial oxidoreductases from scratch, without the unnecessary complexity found in natural proteins. We have constructed a simple, four $\alpha$-helix protein bundle protein framework that can be manipulated to support a range of cofactor and substrate binding, and redox and light driven actions. For example, by controlling water access and mobility, this framework can support hemoglobin-like oxygen transport without anything resembling a globin fold. The same framework provides a clear path to artificial proteins designed to catalyze single or multi electron tunneling coupled to chemistry. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D6.00003: The Physical Mechanism of Proton Transfer in Proteins Invited Speaker: Proteins are able to perform an enormous variety of functions, while using only a limited number of underlying processes. One of these is proton transfer. The physical mechanism of proton transfer has been extensively studied, using a variety of experimental and computational methods. However, it remains unclear what determines the direction and rate of proton transfer reactions in proteins. We have developed and applied a new approach to this long-standing problem by integrating structural dissection, energy landscape, first principle calculation (quantum theory), and molecular dynamics simulation. Our proof of concept study reveals key structural elements that control the direction and rate of proton transfer in proteins. The results are of predictive power and can be generally applied to different proteins. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D6.00004: Frustration and the Functional and Folding Landscape of Proteins Invited Speaker: The energy landscape for folding is funnel-like and largely correlates topology directly with energetics. Thus many of the ``excited states'' important for function are ensembles of structures in which entropy balances partial unfolding energy costs. I will discuss such spectra for cytochrome c. Another way of achieving low free energy excitations is via frustration which entails deviations from the simple funnel landscapes responsible for setting the overall protein shape. I will discuss interesting examples of the consequences of frustration for binding, allostery and for membrane protein systems. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D6.00005: Dynamics and mechanism of water-protein interactions Invited Speaker: Water-protein interactions are essential to biology and such interactions are not static but dynamic in nature. With femtosecond spectroscopy and site-directed mutagenesis, we have systematically investigated protein surface hydration dynamics and the actual time scales of their fluctuations. These new results are significant to understanding the physics of protein dynamics at the most fundamental level. [Preview Abstract] |
Session D7: From Molecular Control to Spatiotemporal Patterns in Bacteria and Beyond
Sponsoring Units: GSNPChair: Herbert Levine, University of California, San Diego
Room: Ballroom C3
Monday, March 21, 2011 2:30PM - 3:06PM |
D7.00001: Bacterial strategies for chemotaxis response Invited Speaker: Bacteria respond to chemical cues by performing a biased random walk that enables them to migrate towards attractants and away from repellents. Bias is achieved by regulating the duration of the bacterial runs as a function of the environment, inferred from the history of chemoattractant detections experienced by the bacterium. This time-signal is processed using a time convolution function that can be assayed measuring the response of the bacterium to short pulses of chemoattractant. The convolution constitutes an elementary form of memory, which is encoded at the molecular level by the processes of (de-)methylation and (de-)phosphorylation of the underlying biochemical network. While the latter is being characterized in detail, the functional reasons shaping the bacterial chemotactic response are largely unknown. We show that the chemotactic response observed experimentally is the strategy that ensures the highest minimum (MaxiMin) uptake of chemoattractant, in any field thereof. The consequence is that adaptation of the chemotactic bacterial system appears to be evolutionary driven by the need to cope with space-time environmental fluctuations rather than the extension of the dynamic range of response. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D7.00002: Arrested phase separation in reproducing bacteria: a generic route to pattern formation? Invited Speaker: In this talk I will present a generic mechanism by which reproducing microorganisms can form stable patterns. This mechanism is based on the competition between two separate ingredients. First, a diffusivity that depends on the local population density can promote phase separation, generating alternating regions of high and low densities. Then, this is opposed by the logistic law for birth and death of microorganisms which allows only a single uniform density to be stable. The result of this contest is an arrested nonequilibrium phase separation in which dense droplets or rings become separated by less dense regions, with a characteristic steady-state length scale. I will illustrate this mechanism by considering a model of run-and-tumble bacteria, for which a density dependent diffusivity can stem from either a decrease of the swim speed or an increase of the tumbling rate at high density. No chemotaxis is assumed in this model, yet it predicts the formation of patterns strikingly similar to those believed to result from chemotactic behavior. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D7.00003: A tunable sequential and periodic pattern formed by coupling cell motility with density Invited Speaker: The ability of living organisms to form patterns is an untapped resource for synthetic biology. We aim to generate unique patterns by rewiring the genetic circuitry controlling cell motility. Specifically, E. coli cells are programmed to regulate their movement by sensing local cell density. Interesting patterns are formed by newly engineered cells. An engineered low-density mover strain spreads outwards and autonomously forms a sequential and periodic pattern. Moreover, we build a theoretical model that satisfactorily fits our current experimental data, and also predicts some parameters which may significantly affect the pattern formation. The study of this self-organized spatial distribution of cells may help us to probe the principles underlying the formation of natural biological patterns, and to prepare for future engineering of biological structures. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D7.00004: Self-organized biological patterns driven by growth and expansion Invited Speaker: The reaction-diffusion (RD) model, involving the diffusion of two types of morphogens (``activator'' and ``inhibitor'') whose interaction stimulates their own synthesis, is an established paradigm to explain the autonomous generation of space-filling patterns in biology. Starting from random initial perturbations, the RD model typically generates patterns via the development of finite-wavelength dynamical instabilities in confined geometries. In this talk, I will describe examples where elements of the RD model, together with the open, expanding geometries offered by growing biological systems, give rise to novel strategies to generate well-defined patterns in space and time. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D7.00005: Deadly competition between sibling bacterial colonies Invited Speaker: As a result of stress due to nutrient limitation or antibiotics, competing individual bacteria within a single colony may lyse sibling cells to release nutrients (cannibalism) or DNA (fratricide). However, we have recently shown that competition is not limited to individuals, but can occur at the colony level [A. Be'er et al., PNAS 106, 428 (2009); A. Be'er et al., PNAS 107, 6258 (2010).] In response to the presence of an encroaching sibling colony, \textit{Paenibacillus dendritiformis} bacteria secrete a lethal protein, lysing cells at the interface between the colonies. Analysis of the proteins secreted by these competing sibling colonies, combined with a mathematical model, shows how colonies maintain their growth by self-regulating the secretion of two proteins: subtilisin (a well-known growth promoter), and Slf (a previously unknown protein, which is lethal). The results also explain why a single colony is not inhibited by its own secretions. [Preview Abstract] |
Session D8: Spin Currents
Sponsoring Units: GMAGChair: Supriyo Datta, Purdue University
Room: Ballroom C4
Monday, March 21, 2011 2:30PM - 3:06PM |
D8.00001: Spin Currents in Silicon Invited Speaker: I will discuss the results of our recent spin injection experiments using long-distance non-degenerate undoped (and n-type doped) Si devices. We have a unique capability to recover the details of electron transport on a sub-ns timescale through a ``Larmor clock'' transformation of spin precession data, despite using only quasistatic current measurements. I suggest that this is potentially a new tool for probing non-equilibrium phenomena in semiconductors, revealing both intrinsic and extrinsic materials properties through sensitivity to subtleties of the bandstructure and impurity spectrum. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D8.00002: Spin Hall Effects in Ferromagnet-Semiconductor Heterostructures Invited Speaker: The effect of spin-orbit coupling on charge transport has long been studied in the form of the anomalous Hall effect in ferromagnets. Charge current in a ferromagnetic channel is intrinsically spin polarized, and asymmetric transverse scattering of spin-up and spin-down electrons due to spin-orbit coupling leads to charge accumulation on the channel edges. Recent breakthroughs in the ability to inject and detect non-equilibrium spin populations in non-ferromagnetic materials have opened up new avenues to study related phenomena. Of particular interest is the spin Hall effect (SHE) in which an ordinary charge current induces a transverse spin current. The resultant spin accumulation at the channel edges was first detected optically [Y. K. Kato \textit{et al.}, Science 306, 1910 (2004) ; J. Wunderlich\textit{ et al.}, Phys. Rev. Lett. 94, 047204 (2005)]. We report on an all-electrical measurement of the SHE in Fe/$n$-In$_{x}$Ga$_{1-x}$As heterostructures. The edge spin accumulation is detected with spin-sensitive Fe/Schottky tunnel barrier contacts. We investigate the bias and temperature dependence of the SHE and successfully determine the skew and side-jump contributions [E. S. Garlid\textit{ et al.}, Phys. Rev. Lett. 105, 156602 (2010)]. Additionally, we have studied the inverse spin Hall effect (iSHE), in Fe/$n$-GaAs devices. Spin current injected into $n$-GaAs by a biased Fe/Schottky contact results in a spin-dependent Hall voltage. The iSHE signal is an order of magnitude larger than that expected from SHE measurements in the same heterostructure. Temperature dependence, nuclear magnetic resonance, and field cycling measurements show conclusively that the iSHE is coupled to the dynamically polarized nuclear spins. We have therefore discovered a new contribution to spin Hall effects: the hyperfine coupling. Work done in collaboration with E.S. Garlid, Q.O. Hu, C.J. Palmstr{\o}m, and P.A. Crowell. Funding provided by NSF DMR 0804244, ONR MURI, and NSF MRSEC and NNIN programs. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D8.00003: Electrostatically Manipulated Ballistic Spin Currents Invited Speaker: Two decades ago Datta and Das published a remarkable paper [1] concerning spin polarized ballistic electron currents in a semiconductor channel and the Rashba spin orbit interaction. They predicted that the source-drain conductance of a spin-injected Field Effect Transistor (spin FET) would oscillate periodically as a function of monotonically increasing gate voltage. We have observed Datta Das oscillations using spin-FETs with ferromagnetic metal electrodes as source and drain [2]. The channel is composed of a high mobility InAs single quantum well heterostructure with strong spin-orbit interaction. The source-drain length is less than the electron mean free path at T=1.8 K. Using the nonlocal geometry, diffusive carriers are removed at a remote ground and the channel conductance is dominated by a current of spin polarized ballistic electrons. A conductance that oscillates as a function of gate voltage is observed. The oscillation amplitude is calibrated from the lateral spin valve magnetoresistance. The spin-orbit interaction parameter is determined from beats in Shubnikov-de Haas data. Thus, the fit to theory has no adjustable parameters other than a small phase factor. Finally, we compare the temperature dependence of the oscillation amplitude with that of the carrier mean free path. The importance to Spintronics, which proposes the use of both spin and charge as state variables, is the demonstration that carrier spin orientation can be modulated by voltage, a parameter normally associated with charge. \\[4pt] [1] S. Datta and B. Das, Appl. Phys. Lett. v. 56, 665 (1990). \\[0pt] [2] H.C. Koo, J.H. Kwon, J. Eom, J. Chang, S.H. Han and M. Johnson, Science v. 35, 1515 (2009). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D8.00004: Logic Devices Based on Spin Current Invited Speaker: The need to find low power alternatives to digital electronics circuits has led to increasing interest in alternative switching schemes like the magnetic quantum cellular automata that store information in nanomagnets which communicate through their magnetic fields. A recent proposal called all spin logic (ASL) proposes to communicate between nanomagnets using spin currents which are spatially localized and can be conveniently routed. In this talk we present a model for ASL devices that is based on established physics and is benchmarked against available experimental data. We investigate switching energy- delay of ASL devices and provide frameworks that allow simple comparisons with charge based devices like CMOS and can help to determine possible use of ASL in future logic implementation. Expected scaling of switching energy-delay of ASL devices as magnets are downscaled while retaining their stability against thermal fluctuations will be presented. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:30PM |
D8.00005: Optical detection of spin currents Invited Speaker: Extensive efforts are currently being devoted to developing spintronics. Several techniques have been developed to generate pure spin currents in many materials and structures. However, there is still no method available that can be used to directly detect pure spin currents, which carry no net charge current and no net magnetization. Currently, studies of pure spin currents rely on measuring the induced spin accumulation with optical techniques or spin-valve configurations. I will discuss observation of a second-order nonlinear optical effect of pure spin currents that can be used for the non-invasive, non- destructive, and real-time imaging of pure spin currents. This effect is caused by a subtle imbalance of the Faraday rotation of electrons with opposite spin orientations [1]. In our experiment, a transient pure spin current was injected in a GaAs crystal by a quantum interference and control technique using a pair of phase-locked ultrafast laser pulses. Second- harmonic generation of an ultrafast probe pulse with a central wavelength of 1760 nm was observed [2]. We systematically studied the second-harmonic power as a function of the probe delay, probe position, spin current density, and carrier density. All the observations are consistent with a second- order nonlinear optical effect induced by the pure spin current. Since this effect does not rely on optical resonances, it can be used to detect pure spin currents in a wide range of materials with different bandstructures. Furthermore, the control of nonlinear optical properties of materials with pure spin currents may have potential applications in photonics integrated with spintronics. \\[4pt] [1] J. Wang, B. F. Zhu, and R. B. Liu, Phys. Rev. Lett. 104, 256601 (2010).\\[0pt] [2] L. K. Werake and H. Zhao, Nat. Phys. 6, 875 (2010). [Preview Abstract] |
Session D9: Patterns, Nonlinear Dynamics followed by General Fluid Dynamics
Sponsoring Units: DFDChair: Yuri Antipov, Louisiana State University, and Galen Gisler, University of Oslo
Room: D220
Monday, March 21, 2011 2:30PM - 2:42PM |
D9.00001: Rayleigh B\'enard Convection-A Case Study on Pattern Formation Hira Siddiqui, Rudolf Friedrich Spiral turbulence in Rayleigh-Benard convection is studied numerically in the framework of generalized Swift Hohenberg equations. The model equation consist of an order parameter equation for the temperature field coupled to an equation for the mean flow field. In contrast to the earlier work, nonlinearities in the dynamics of the mean flow are retained leading to a two dimensional Navier-Stokes equation coupled to a Swift-Hohenberg equation. We present the numerical investigations of nonlinear effects due to the interaction of nonlinear two dimensional flows and the pattern forming process. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D9.00002: A simple approach to localized convection H. Pleiner, M.G. Clerc, J. Martinez-Mardones, L.M. Perez, D. Laroze Localized structures can be found in many different (dissipative) driven systems [1], an example being stationary and traveling convection structures in the thermal instability of binary fluids. Here, the special localized structure is a convective state between two quiescent, conductive ones, and can been interpreted as a pinning phenomenon close to a stationary sub-critical bifurcation. Generally, localized structures are described by using higher dimensional, complex amplitude or phenomenological prototype (e.g. Swift-Hohenberg) equations or by direct numerical integration of the hydrodynamic equations. Here we show, using the binary mixture convection in porous media as an example, that the analytically derived one-dimensional amplitude equation amended by non-adiabatic (non-resonant) terms important close to convection fronts, well describes localized convection states, in particular the slanted homoclinic bifurcation diagrams.\\[4pt] [1] O. Descalzi, M. Clerc, S. Residori, and G. Assanto (Eds.), Localized States in Physics: Solitons and Patterns, Springer, 2011. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D9.00003: The dynamics of cracks in torn thin sheets Yossi Cohen, Itamar Procaccia The stress field near the tip of a crack due to a mode III shear tearing of a thin plate of elastic material has a universal form but with a non-universal amplitude known as the Stress Intensity Factor. All the non-universal aspects of the stress distribution are collected in the Stress Intensity Factor which depends on everything, including the crack length, the boundary conditions and the history of the loads that drive the crack evolution. Although the equations of elasticity for thin plates are well known, there remains the question of selection of a path for a propagating crack. We invoke a generalization of the principle of local symmetry to provide a criterion for path selection and demonstrate the qualitative agreement of our results with the experimental findings. We also analyze the nature of the singularity at the crack tip with and without the nonlinear elastic contributions. Finally we present an exact analytic results for the stress intensity factor to the linear approximation for the crack developing in thin sheets. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D9.00004: Chaotic Plume-Like Bursts in Rimming Flows Gabriel Seiden, Victor Steinberg We report a detailed experimental investigation of chaotic, plume-like bursts observed in rimming flows of polymer solutions within a partially filled horizontal cylinder. In particular, we investigate the attractive interaction between adjacent plumes and the effect of rotation rate and polymer concentration on the statistics of these unique bursts. A comparison is also made between the Newtonian and non-Newtonian cases. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D9.00005: Cracks formation during blood drop evaporation Benjamin Sobac, David Brutin We firstly presented the pattern formation occurring when drops of whole blood desiccate in a recent publication [1]. The phenomena presented evidence to involve lots of physical field such as surface chemistry, haematolology, fluid mechanics, heat transfer, colloids science{\ldots} All these mechanisms are acting together and produce an axisymetric and reproducible pattern. Dried cellular components are segregated and deposited by a capillary flow. During the evaporation, the system is slowly drying and cracks when stresses are too important leading to the final pattern observed. In this presentation, we will present the mechanisms involved in the formation of crack patterns. The phenomenon presented here with red blood cells as the main colloids involved is very similar to the drying of drop of nanoparticules [2]. We will explain the common point and the differences encountered.\\[4pt] [1] D. Brutin, B. Sobac, B. Loquet and J. Sampol, Pattern formation in drying drops of blood, Journal of Fluid Mechanics, underpress, 2010.\\[0pt] [2] L. Pauchard, B. Abou, K. Sekimoto, Infuence of Mechanical Properties of Nanoparticles on Macrocrack Formation, Langmuir, 25(12), 6672-6677, 2009. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D9.00006: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:42PM - 3:54PM |
D9.00007: Pattern formation and coarsening in crystalline membranes Daniel A. Vega, Aldo D. Pezzutti We study through a Brazovskii-Helfrich Hamiltonian the process of defect formation, annealing and coarsening of two dimensional crystalline membranes. In good agreement with the cosmological model of Kibble and Zurek, proposed to determine the density of topological defects at the onset of a symmetry breaking phase transition, we found that the collision of orientationally uncorrelated domains produces a structure of grains with an average density of topological defects controlled by the temperature of the quench. The strain field of the dislocations and disclinations generated during the phase separation process can induce the buckling of the membrane, slowing down the Lifshitz-Safran mechanism of coarsening observed in flat systems. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D9.00008: Pattern formation in ternary lipid membranes with composition-deformation coupling Matthew Demers, Francisco Solis, Monica Olvera de la Cruz We study patterns formed in three-component lipid membranes, where composition is coupled to shape via differences in spontaneous curvature. The system is examined in the strong segregation regime. System morphology is determined by the competition of bending energy, surface tension, and line tension. We will present the phase behavior as determined by numerical minimization, as well as analytic solutions for select cases. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D9.00009: Supercavitating flow past an elastic curvilinear hydrofoil Yuri Antipov A nonlinear inverse fluid-structure interaction problem is considered. The obstacle is a curvilinear elastic hydrofoil, and the cavity formed behind is modeled according to the single-spiral-vortex model by Tulin. First, the model for a rigid polygonal supercavitating hydrofoil is solved by the method of conformal mappings. The mapping function is expressed through the solutions of two Riemann-Hilbert problems. To identify the vertices of the polygon where the jets break away from the foil, the Brillouin-Villat separation condition is applied. The unknown parameters of the conformal mapping are computed on solving a system of transcendental equations. Next, by increasing the number of vertices of a regular $N$-polygon, the cavitation problem for a circular arc is solved, pressure on the foil is defined, and a boundary- value problem for a thin shell subject to normal loading is stated. The elastic problem is solved exactly for an arc with clamped ends, and the new hydrofoil profile is determined. Finally, a new cavitation problem for the deformed foil is stated and solved. Numerical experiments reveals the presence of two thin partial cavities near the foil ends. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D9.00010: Drag calculations using the inviscid Euler equations alone Galen Gisler Recently Hoffman and Johnson$^{1}$ have proposed a new resolution of d'Alembert's Paradox, the problem that inviscid potential flow predicts zero drag on a body, in contrast to observations. They reject the commonly accepted resolution, that drag results from the very thin viscous boundary layer between the no-slip condition on the surface of the body and the free-flowing fluid. Instead they argue that drag results from turbulence in the body's wake, even if free-slip is assumed. They used a finite-volume code to verify their conclusions. While their calculations look promising, and offer prospects for calculation of rather more complex flows at modest resolution, it is desirable to perform independent verification. I will present independent tests of the Hoffman-Johnson resolution using a finite-volume Euler-equation code, studying the dependence of the inferred drag on meshing style and resolution. \\[4pt] [1] Johan Hoffman and Claes Johnson, J. Math. Fl. Dyn. 12, 321-334 (2010). [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D9.00011: Dynamics of induced dipole ER fluid: a continuous energetic approach Jianwei Zhang, Wenfeng Li, Jiaxi Li We studied dynamics of Electrorheological (ER) fluid by continuum induced dipole fluid method [1]. We found that the velocity profile of ER fluid increases in high shear-rate region and solid particles are separated from colloid in high electric field. These findings demonstrated the breakdown of Bingham fluid model under high shear-rate and high electric field. Our continuum approach describes ER fluid's behaviors under most conditions. We also found that the shift of maximum shear stress under different electric field follows the same trend as that of the maximum static stress. This indicates that the static and dynamic stresses are both dominated by the same energetic process. A connection between micro-particles' structures and macro-dynamic properties under varying conditions is established by our continuum method. Our studies probe the physics of induced dipole ER fluid. \\[4pt] [1] Jianwei Zhang, Xiuqing Gong, Chun Liu, Weijia Wen, and Ping Sheng, Physical Review Letters 101, 194503, 2008. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D9.00012: Velocity fluctuations in steadily sedimenting suspensions K. Vijay Kumar, Sriram Ramaswamy The simplest model of a homogeneous suspension steadily sedimenting under gravity at low Reynolds number indicates that the velocity fluctuations should diverge with the system size. This is, however, not seen in experiments. We improve on a previously described coarse-grained model proposed for this problem by identifying certain crucial missing terms in the equations of motion. These terms are allowed by symmetry considerations and can be generated by a mechanism which is natural in the dynamics of low Reynolds number sedimentation. A dynamical renormalization group calculation of our model leads to the conclusion that these extra terms are always relevant. If these terms are stabilizing, this suggests a natural mechanism for suppressing fluctuations in sedimenting suspensions. We analyze the properties of the critical point where these extra terms vanish. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D9.00013: Stabilization of toroidal droplets using viscoelastic media Ekapop Pairam, Alberto Fernandez- Nieves We inject a viscous liquid through a needle into another rotating viscous liquid to generate toroidal droplets. These droplets are unstable and undergo a transformation into spherical droplets driven by surface tension: They either break ala Rayleigh-Plateau or grow fatter to become a single spherical droplet depending on the aspect ratio of the torus. By replacing the outer phase with a viscoelastic fluid with a non-zero yield stress we can stabilize these and other non-zero genus droplets. We will examine this stabilization mechanism and present criteria to effectively prevent the break-up of these droplets. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D9.00014: Iron chemistry at aqueoues interfaces by near edge X-ray spectroscopy David Vaknin, Wenjie Wang, Alex Travesset, Ivan Kuzmenko Employing synchrotron X-ray absorption near-edge spectroscopy (XANES) combined with X-ray fluorescence (XF) and reflectivity (XR) techniques, we monitor the state of ferrous and ferric iron as it binds to charged carboxylic and phosphate groups. By subphase pH manipulation, arachidic acid and dihexadcyl phosphate monolayers can provide a range of surface charge density from nearly charge-neutral to a fully charged monolayer to which iron ions are attracted from solutions. Analysis of our results from fluorescence show that the driving forces attracting Fe$^{3+}$ and Fe$^{2+}$ to the interface originate from chemical interactions and electrostatic, respectively. XANES shows that the electronic and geometric structure of iron complexes at interface are different from those in the bulk. Moreover, the XANES results demonstrate that valence state and bonding of the interfacially bound Fe$^{3+}$ and Fe$^{2+}$ are practically indistinguishable. This, we argue, is due to the versatility of iron ions in behaving as electron acceptors (Fe$^{3+}$) or as donors (Fe$^{2+}$). [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D9.00015: Interfacial microrheology in viscoelastic membranes Gopal Subedi, Kenneth W. Desmond, Eric R. Weeks Prior studies on interfaces using microrheology have typically been applied to interfaces with only a surface viscosity component and not an elastic one. We are extending the application of interfacial microrheology to viscoelastic lipid monolayers. We use a DPPC and cholesterol lipid monolayer in a Langmuir trough as a model system. The Langmuir trough gives us the flexibility to control the concentration and thus the phase of the monolayer. The microrheology technique allows us to measure the rheology at specific concentrations or in situations as the concentration is changed. The microrheology technique employs video microscopy to record the diffusive motion of micron size spheres placed at the interface. Since the diffusive motion of the microspheres is dominated by the interfacial rheology of the monolayer, the recorded motions of the microspheres are used to infer the rheological properties of the interface. We hope to extend our understanding of viscoelastic interfaces with the study. [Preview Abstract] |
Session D10: SPS Undergraduate Research II
Sponsoring Units: SPSChair: Crystal Bailey, American Physical Society
Room: D221
Monday, March 21, 2011 2:30PM - 2:42PM |
D10.00001: The $^{11}$C Project:Measurement of Root Exudation at Elevated CO2 Levels in Low and High Nutrient Solutions Verida Leandre, Calvin Howell Understanding the plant kingdom's mechanisms of resource management in variable environments is integral to predicting how plants will respond to an increase in atmospheric CO$_{2}$. The goal of this study is to determine the effects of changing nutrient conditions on the root exudation of barley plants at elevated CO$_{2}$ levels. The $^{11}$C group at the Triangle Universities Nuclear Laboratory (TUNL) tags various species of plants with short-lived positron-emitting radioisotopes in order to analyze metabolite transport in response to changes in the environment. $^{11}$C is produced at TUNL using a tandem Van de Graaff particle accelerator, then transported from TUNL to the Duke Univ. Phytotron (100m) where plants are labeled with $^{11}$C in a growth chamber. The chamber allows researchers to control the light intensity, air temperature, humidity and concentration of CO$_{2}$ in the air. The plant absorbs $^{11}$CO$_{2}$ in a leaf that is placed inside a cuvette through which radioactive $^{11}$CO$_{2}$ gas flows. The sugars in the labeling leaf are tagged with $^{11}$C and translocated throughout the plant similar to $^{12}$C. Scintillation detectors are used to track the tagged sugars as they are translocated through the plant and exudated from the root into the nutrient solution or $^{11}$CO$_{2}$ gas is respired by the root. The labeling system, detector arrangement, electronics and data analysis will be described and preliminary results will be presented. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D10.00002: A One Dimensional Stochastic Susceptible-Infectious-Recovered Model and its Variations: a Hamiltonian Approach Ali Hamed, Dia'a Bisharat, Mohamad Amine, Irina Mazilu The spread of an infectious disease is a random process, and a stochastic approach to the problem is justified. The susceptible-infectious-recovered model (SIR) describes the evolution of three types of individuals (in a small community) which undergo an infection and recovery mechanism. The model (and its variations) predicts the number of infected individuals over a certain period of time, gives an estimate of the maximum possible number of infected people, and predicts how long the disease will be threat to the examined community. Using a quantum mechanical approach, we investigate four variations of the original SIR model and compare our analytical findings with the computer simulation results. We also calculate correlations between infected and recovered individuals, and find a good agreement between theory and computer simulations. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D10.00003: Exploring a Parasite-Host Model with Monte Carlo Simulations Nyles Breecher, JiaJia Dong We explore parasite-host interactions, a less investigated subset of the well-established predator-prey model. In particular, it is not well known how the numerous parameters of the system affect its characteristics. Parasite-host systems rely on their spatial interaction, as a parasite must make physical contact with the host to reproduce. Using C++ to program a Monte Carlo simulation, we study how the speed and type of movement of the host affect the spatial and temporal distribution of the parasites. By drawing on mean-field theoretics, we find the exact solution for the parasite distribution with a stationary host at the center and analyze the distributions for a moving host. The findings of the study provide rich behavior of a non-equilibrium system and bring insights to pest-control and, on a larger scale, epidemics spreading. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D10.00004: Optical design of a robotic TV camera probe for minimally invasive abdominal surgery Susanna Todaro, Weiyi He, Dennis Killinger Minimally invasive techniques are a promising new field of surgery; however, they limit the surgeon's access points and maneuverability. In order to increase the number of access points in minimally invasive abdominal surgery, a proposed implantable medical probe braces to the abdominal wall and provides illumination and video signal. The probe is cylindrical, about 25 mm long and 10 mm in diameter. A ring of LEDs on the end of the probe illuminates the tissue, and the resulting image is focused onto an HD video detector. It was necessary to apply beam-shaping reflectors to collimate the light onto a small target area, to avoid illuminating areas not picked up by the video. These reflectors were designed and simulated using the optical ray tracing software TracePro. Two LED chip geometries and three types of reflector geometries were analyzed, and the parameters for each geometry were optimized. For the straight-edged reflectors, the intensity patterns and optimization were compared to experimental results. Although parabolic reflectors produced the best collimation, cone reflectors with a 20-degree half-angle produced significant collimation at a much cheaper price. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D10.00005: Developing a Procedure for the Characterization of Mechanical Properties of Collagen Gels Christopher Chambers, Heather Lovelady, Garrett Matthews The characterization of bulk mechanical properties of type I collagen gels is critical to understanding the role of collagen in the extracellular matrix (ECM), and developing biocompatible devices for use in the human body. Understanding the mechanical properties of the gel state of collagen can lead to the ability to adjust these properties for multiple uses. Here, we examined the Young's modulus of the synthesized gels. This project used a microrheological approach to discover these properties. Gels were first formed using a known process and magnetic microspheres were embedded in the gel prior to formation. An optical microscope was fitted with a magnetic chamber used to drive the embedded beads in two modes, an oscillatory motion and a pulse motion. Tracking software was modified and used to analyze the motion of the beads recorded with a CCD camera on the microscope. These techniques should be sufficient to obtain a reliable value for the Young's modulus of collagen gels, as well as other similar materials. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D10.00006: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:42PM - 3:54PM |
D10.00007: Does Thioflavin-T Detect Oligomers Formed During Amyloid Fibril Assembly Christopher Persichilli, Shannon E. Hill, Jason Mast, Martin Muschol Recent results have shown that oligomeric intermediates of amyloid fibril assembly represent the main toxic species in disorders such as Alzheimer's disease and type II diabetes. Thioflavin-T (ThT) is among the most commonly used indicator dyes for mature amyloid fibrils \textit{in vitro}. We used ThT to monitor amyloid fibril formation of lysozyme (HEWL), and correlated ThT fluorescence to concurrent dynamic light scattering and atomic force microscopy measurements. Specifically, we tested the ability of ThT to discern among oligomer-free \textit{vs.} oligomeric fibril assembly pathways. We found that ThT fluorescence did not detect oligomer growth; however, fluorescence increases did coincide with the formation of monomeric filaments in the oligomer-free assembly pathway. This implies that ThT fluorescence is not generally suitable for the detection of oligomeric intermediates. The results further suggest different internal structures for oligomeric \textit{vs. }monomeric filaments. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D10.00008: The Design and Fabrication of Bismuth Hall Effect Biosensors A.J. Sigillito, M. Rudolph, V. Soghomonian, J.J. Heremans Because of their high sensitivity, accuracy, and low cost, the use of Hall biosensors promises to be an effective diagnostic technique that may aid in the early diagnosis of diseases. In this research, Hall sensors were fabricated from thermally evaporated bismuth thin films. The bismuth films were deposited under high vacuum onto heated Si/SiO$_{2}$ substrates using a two layer deposition technique. The films varied in thickness from 60 nm to 75 nm and were etched into Hall bar geometries using photolithography and wet chemical etching. Magnetoresistance and Hall measurements were taken from 4 K to 300 K. The data indicate that the sensors may be characterized using a two carrier model with high mobility, low density holes and low mobility, high density electrons. Additionally, the sensors were exposed to magnetite nanoparticles and characterized using atomic force microscopy. The results will be reported. This research was funded by the National Science Foundation (NSF Grant DMR-0851662). [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D10.00009: EPR Study of Lithium Borovanadate and Lithium Silicate Glasses Bikesh Dahal, D. Blane Baker, Steve Feller Lithium borovanadate and lithium silicate samples with varying molar ratios were prepared using both roller quenching and plate quenching methods. Electron paramagnetic resonance EPR spectra of those samples show that resolution of the hyperfine structure lines(hfs) depend on their molar ratio. When the molar ratio (K) is less than 0.5 in the borovanadate system, the hyperfine structure lines are well resolved and defined. However, when the molar ratio becomes greater than 0.5; the spectra starts to get less resolved; at molar ratio 0.7 there is no hyperfine resolution. Well resolved samples were modeled by using a modeling program in MATLAB to obtain Hamiltonian parameters. The Hamiltonian parameters that were obtained were g $_{parallel}$, g $_{perpendicular}$, A $_{parallel}$ and A $_{perpendicular}$. The Hamiltonian parameters were calculated to learn about the orientation of V$^{4+}$ ions and electrons in the glass samples. According to our calculation g $_{parallel} \quad <$g$_{perpendicular}<$ g $_{e}$ which suggests that crystal field of the V$^{4+}$ ions has a octahedral site with a tetragonal compression in the glass samples. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D10.00010: EPR study of radical trapping of RAFT polymerization of Multifunctional Acrylates Aayush Regmi, Ashutosh Dahal, D. Blane Baker, John Pojman, Patrick Bunton Electron Paramagnetic Resonance (EPR) was used to monitor radical trapping during Reversible Addition-Fragmentation chain Transfer (RAFT) polymerization of acrylate monomers with different degrees of functionality. Monomers used for the study were 1, 6-Hexanidiol \underline {Di}acrylate (HDODA), Trimethylopropane \underline {Tri}acrylate (TMPTA), and Pentaerythritol \underline {Tetra}crylate (PETA). X-band EPR spectra were obtained for approximately 0.2 g of samples in a 4 mm quartz tube heated at 50$^{0 }$C inside the cavity. The trapped radicals' signals were first observed after the samples were heated for 400-500 minutes. Radical density continued to increase for an additional 180 -190 minutes. EPR spectra of RAFT samples of TMPTA and PETA were compared with subsequent spectra produced by traditional free radical polymerization. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D10.00011: EPR study of Frontally Polymerized Multifunctional Acrylates Ashutosh Dahal, Aayush Regmi, Anna Thoma, Alecia Valencia, Veronika Viner, Rafael Cueto, D. Blane Baker, John Pojman, Patrick Bunton Electron Paramagnetic Resonance(EPR) study of frontally polymerized Trimethylopropane Trimethacrylate(TMPTMA), Trimethylopropane Triacrylate(TMPTA), 1,6-Hexanidiol Diacrylate(HDODA) and Pentaerythritol Tetracrylate(PETA) was done to determine the absolute radical concentration. Higher radical concentrations were found in the frontally polymerized samples compared to the bulk polymerized samples for TMTPMA and PETA. The concentration of radicals was highest in TMPTMA frontal sample at 8.74 X 10$^{-3}$ moles/kg. The lowest measureable concentration was in the HDODA bulk samples at 0.0266 X 10$^{-3}$ moles/kg. For all frontally polymerized samples high radical concentrations were observed at the point of initiation after which the signal intensity decreased to steady state within a few centimeters down the front. An exponential growth in the radical signal was observed in the mixture of TMPTMA and TMPTA when the concentration of the TMPTMA was increased. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D10.00012: First-principles investigation of graphene-metal interfaces Andrew Ross, Lyudmyla Adamska, You Lin, Ivan Oleynik Epitaxial growth of graphene on Ni(111) substrates is one promising method of large-scale, high-quality graphene wafer production, due to the small lattice mismatch between these two materials. We present results of first-principles density functional theory (DFT) investigation of thestructural, electronic, and magnetic properties of graphene/Ni(111) interfaces relevant to experimental studies of graphene growth on nickel substrates. DFT calculations were performed to identify the favored interface geometries and binding sites for different interface configurations. Additional adlayers of Ni and Cu were either adsorbed on top of the graphene/metal interface, or placed between the graphene and substrate to model processes of metal intercalation. It was also found that the interaction between graphene/Ni(111) and the top Cu adlayer is much weaker compared to that for Ni adlayer. The atomic, electronic, and magnetic properties of these interfaces, including induced magnetic moments in graphene/Ni(111) and Cu,Ni/graphene/Ni(111) systems are also discussed. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D10.00013: Growth of Graphene on Metal Substrates Travis Miller, Jayeeta Lahiri, Rafik Addou, Matthias Batzill Graphene, a single layer of graphite, has large potential as an electronic material. For these applications large scale, high quality graphene wafers are required. A promising approach to achieve this is by growth on metal substrates. In this REU project I used Auger electron spectroscopy to study the growth of graphene on Ni(111) and its modification by Cu or Al additions. On pure nickel we found two graphene growth regimes. Below 480 $^{\circ}$C graphene grows by converting a surface carbide phase while above 480 $^{\circ}$C graphene grows on pure nickel. Addition of copper destabilizes the nickel carbide enabling the growth of graphene in the absence of a carbide at lower temperatures. Finally, aluminum intercalation through the Ni- supported graphene layer was investigated in an attempt to form an ordered Ni-Al alloy underneath the graphene. Surprisingly, we found that this intercalation process already occurs at only 100 $^{\circ}$C. Furthermore, the intercalated Al is protected by the graphene against oxidation. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D10.00014: Single-Walled Carbon Nanotubes in Epoxy -- Investigating Behavior under Strain and Alignment Using Fluorescence Spectroscopy Tamika Thomas, Kena Senegal, Sarena Senegal, Paul Withey, Sergei Bachilo, R. Bruce Weisman Single-walled carbon nanotubes (SWCNTs) have been successfully embedded into EPON 862/W epoxy both with and without a surfactant. Applying strain to the nanocomposite permitted the interfacial adhesion between the SWCNT and host to be studied at the single-particle level using near-infrared fluorescence spectroscopy. Load transfer from the host to an embedded CNT is clearly observed as a shift in the nanotube's spectral emission. Loss of adhesion, or slipping, is also detected. Attempts at enhancing CNT-alignment within the nanocomposite will also be discussed. Near-infrared fluorescence imaging and spectroscopy prove to be ideal methods for monitoring the behavior of SWCNTs within nanocomposites, especially at the single-particle level. Much of this work has been carried out by undergraduate physics and chemistry majors. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D10.00015: An Ab-initio Study of Folded Armchair Graphene Nanoribbons Adam Iaizzi, Nam Lee, Lilia Woods We present a first principles approach to the characterization of armchair graphene nanoribbons folded along their long axis using density functional theory, along with ultrasoft psuedopotentials and the local density approximation for the exchange-correlation functional. Based on past studies, we anticipate that folding nanoribbons will produces changes in the band structure, possibly turning normally semiconducting nanoribbons into metallic nanowires. We determine the energy required to produce a number of different folded structures from nanoribbons as well as the energy and band structure as a function of width in single-fold structures. Ribbons as narrow as 13 carbon atoms formed stable folded structures. [Preview Abstract] |
Session D11: Thermal Properties in Semiconductors and Nanostructures
Sponsoring Units: FIAPChair: Hongping Zhao, Lehigh University
Room: D222
Monday, March 21, 2011 2:30PM - 2:42PM |
D11.00001: Thermoelectric Properties of Granular Materials Andreas Glatz, Igor Beloborodov I will present our recent studies of thermoelectric properties of mono-phasic nanocrystalline semiconductors and metals in the weak coupling regime. The focus is in particular on the thermopower and figure of merit for temperatures less than the charging energy. I will show that the dimensionless figure of merit $ZT$, which is a measure for the performance of thermoelectric materials, has a maximum at certain temperatures and grain sizes which can be in the range of technological relevant values $ZT>3$. The talk is based on: Phys. Rev. B {\bf 80}, 245440 (2009) and EuroPhys. Lett. {\bf 87}, 57009 (2009). [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D11.00002: Thermal Conductivity Characteristics of Three-Layer Superlattices Jing Zhang, Hua Tong, Nelson Tansu For thermoelectric applications, the thermal conductivity of the material needs to be reduced as low as possible in order to achieve higher thermoelectric efficiency of devices, as the device efficiency depends on the thermoelectric figure of merit ($Z*T)$. Both theoretical and experimental data show that the cross-plane thermal conductivity of superlattices is much lower than that of the bulk materials The cross-plane thermal conductivity of three-layer superlattices is calculated by a numerical method, which is developed from the lattice dynamical theory. The phonon mean free path is included into the calculation, thus the minimum thermal conductivity occurs at the crossover of the particle-like model and wave-like model of the phonons. The studies focus on the effect of mass ratio, layer thickness, and mean free paths on the minimum thermal conductivity of the three-layer superlattice design. The minimum thermal conductivity of the three-layer superlattice structure is approximately half of that of the conventional two-layer superlattice structure. This finding indicates that the thermoelectric figure of merit for superlattice structure can further be enhanced by 2 times from the use of the three-layer superlattice design. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D11.00003: Lattice thermal conductivity with first-principles anharmonic lattice model Terumasa Tadano, Yoshihiro Gohda, Shinji Tsuneyuki First-principles calculation of lattice thermal conductivity is important to design new devices such as high-efficiency thermoelectric materials. For lattice thermal conductivity calculations of complex materials and nanostructures, non-equilibrium molecular dynamics (NEMD) is more suitable than widely used Boltzmann transport theory. However, a combination of NEMD and FPMD is almost impossible because of its high computational cost, so that NEMD has been performed only with classical model potentials for specific materials. In order to overcome this limitation in materials, we have developed a new methodology for calculating lattice thermal conductivity without relying on any experimental values. In this method, the potential energy of a system is expressed as a many-body anharmonic model, that is, a Taylor expansion of the total energy with respect to displacements of atoms up to 4th order. Parameters of the anharmonic lattice model are determined with Hellmann-Feynman force of FPMD by least-square fitting. We performed thermal conductivity calculations with the anharmonic lattice model combined with NEMD and obtained reasonable agreements with experimental values. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D11.00004: Surface roughness and phonon transport in thin Si nanowires: an atomistic study Jesus Carrete, Luis Javier Gallego, Luis Miguel Varela, Natalio Mingo Good thermal insulation is much harder to achieve than electrical insulation. Thus, the astonishingly low thermal conductivities recently reported on Si nanowires came as a surprise, since the displayed values were an order of magnitude lower than predicted by the diffuse boundary limit of Casimir's theory. Recent theoretical work has employed the Born approximation to predict a very much enhanced boundary scattering rate that would lead to a thermal conductivity well below the Casimir limit. We present a Green's function calculation that answers the question of whether the Casimir limit to the phonon mean free path can be overcome by roughness. Our results show that the mean free path (MFP) and the thermal conductivity of a nanowire are very close to the Casimir limit for shallow disorder, and can only be pushed below it using very deep surface roughness, well beyond previous estimates. We also explore the limits of the Born approximation in this context using vacancies and isotopic impurities as defects. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D11.00005: Quantal Heating of 2D electrons in strong magnetic fields Sergey Vitkalov, Alexey Bykov Usually heating of conducting electrons by $dc$ electric field increases electron temperature and effects weakly the electron transport. In this report we show that the $dc$ heating of 2D electrons with a quantized spectrum is very peculiar and violates strongly the Ohm's Law [1]. The quantal heating establishes nontrivial electron distribution, which has the same broadening or an effective ``temperature'' as the unbiased system. The heating reduces significantly the dissipative electron transport, forcing the quantum conductors into a state with zero differential resistance (ZDR). Furthermore an apparent $dc$ driven metal-insulator transition is found, which correlates with the transition into the ZDR state. This interesting correlation is unexpected and is not understood.\\[4pt] [1] J.-Q. Zhang, S.A. Vitkalov and A.A. Bykov, Phys. Rev. B 80, 045310 (2009); S. A. Vitkalov, International Journal of Modern Physics B, 23, 4727 (2009). [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D11.00006: Light-induced thermodynamic metastability in amorphous silicon Daniel Queen, Julie Karel, Frances Hellman, Qi Wang, Richard Crandall, Eugene Iwaniczko The efficiency of amorphous silicon photovoltaics is limited due to the generation of dangling bond defects upon light soaking which leads to a decrease in their efficiency known as the Staebler-Wronski Effect. These defects act as recombination centers for photoexcited electron-hole pairs and can be reversibly removed by annealing above 150\r{ }C. The electrical properties of these defects are well documented but the mechanism that gives rise to them is still an open question. It is known that hydrogen plays a crucial role in their formation and recovery but it is not clear if hydrogen participates in the defect formation. We present heat capacity data for a-Si:H films grown by the Hot-Wire CVD (HWCVD) technique and a-Si films grown by e-beam evaporationand measured using our MEMS based nanocalorimeter. Both materials have an excess heat capacity observed upon light soaking that is reversibly removed by annealing at 200\r{ }C. This excess is found to be independent of H content in the HWCVD films and is present but at a smaller magnitude in the e-beam evaporated a-Si. The lack of dependence on H content and the presence in the e-beam films indicates the light induced metastability is intrinsic to the amorphous silicon matrix. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D11.00007: Thermodynamic properties of ZrSiO4 polymorphs from DFT based \textit{ab initio} phonon calculations Jincheng Du, Mrunal Chaudhari Zircon and Reidite are the polymorphs of ZrSiO$_{4 }$minerals that are natural hosts of various radioactive elements in the crust of the earth. Its high permittivity also makes it a promising material for the gate dielectric material in metal-oxide semiconductors. Knowledge of the thermodynamic properties at high temperature and high is very important to consider its application as an effective natural storage for the radioactive wastes and high technology ceramics. These properties are thoroughly studied both computationally and experimentally for zircon, while significantly less attention was paid to reidite in the literature. We report studies of thermodynamic properties of Zircon and Reidite from phonon spectra calculations using \textit{ab initio} based periodic density-functional theory (DFT) calculations. Various thermodynamic properties such as free energy, internal energy, entropy, enthalpy, heat capacity and thermal displacement as a function of temperature are calculated. Phoon dispersion curves and density of states are calculated and compared with the experimental data. Calculated bulk properties agree very well with the experimental data in the literature. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D11.00008: Vibrational and thermal properties of ternary semiconductors and their isotopic dependence: chalcopyrite CuGaS2 Aldo Romero, M. Cardona, R. Kremer, R. Lauck, A. Mu\~noz The availability of \textit{ab initio} electronic calculations and the concomitant techniques for deriving the corresponding lattice dynamics have been profusely used in the past decade for calculating thermodynamic and vibrational properties of semiconductors, as well as their dependence on isotopic masses. The latter have been compared with experimental data for elemental and binary semiconductors with different isotopic compositions [1]. Here we present theoretical and experimental data for several vibronic and thermodynamic properties of a canonical ternary semiconductor of the chalcopyrite family: CuGaS2 [2]. Among these properties are the lattice parameters, the phonon dispersion relations and densities of states (projected on the Cu, Ga, and S constituents), the specific heat and the volume expansion coefficient. The calculations were performed with the ABINIT and VASP codes within the LDA approximation for exchange and correlation.\\[4pt] [1] Cardona {\it et al.}, PRB81, 075202 (2010)\\[0pt] [2] Gibin {\it et al.}, Solid State Commun Solid State Commun 133, 569 (2005); Sanati {\it et al.} S.S. Commun 131 229 (2004). [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D11.00009: Temperature dependence of band gap of highly confined CdSe and PbSe nanocrystals Jason Bylsma, Prasenjit Dey, Jason Rejman, Aaron Zaubi, Sarath Witanachchi, Pritish Mukherjee, Denis Karaiskaj We have recorded fluorescence spectra from PbSe and CdSe quantum dots in hexane/toluene respectively between 5K and 300K in order to investigate the temperature dependence of the electronic band gap of these highly confined nanostructures. The band gap for CdSe follows the known blue shift with decreasing temperature (dE/dt = -225 $\mu $eV/K). Olkhovets et. al. first reported a red shift of the band gap energy with decreasing temperature for small (d $<$ 4 nm) PbSe and PbS quantum dots [1]. Such behavior would contradict the expected blue shift of the band gap with decreasing temperature. We have measured the temperature dependence of the band gap of PbSe quantum dots for two different diameters below 4 nm and indeed observe a red shift of the band gap with decreasing temperature (dE/dT = 58 $\mu $eV/K), which is stronger for the smaller size quantum dots (dE/dt = 82 $\mu $eV/K). The origin of this peculiar behavior is not well understood and we are pursuing further theoretical and experimental studies in order to elucidate the mechanism behind it. [1] A. Olkhovets, et. al. Phys. Rev. Lett. 81, 3539 (1998). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D11.00010: Temperature and polarization dependent photoluminescence studies of WO$_{3}$ and WO$_{3-x }$single crystals Prasenjit Dey, Justin Easley, Denis Karaiskaj, Satyen Deb, Ted Ciszek, Daniel Dessau WO$_{3}$ is an important material not only due to its interesting electronic properties but also for applications in electrochromics and energy storage. The mechanism behind the electrochromic effect has been debated for several decades.\footnote{Satyen K. Deb, Solar energy materials and solar cells \textbf{92}, 245 (2008), and the references therein.} We have studied two WO$_{3}$ single crystals, a transparent and a doped WO$_{3-x}$. A photoluminescence center around 865 nm is observed after sub-band gap excitation at 405 nm with relatively higher intensity in the crystal containing oxygen vacancies. The center appears as a broad transition of 35 nm FWHM and does not follow the band gap energy with temperature. However polarization dependent studies reveal at least two polarization dependent component of the center. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D11.00011: Mapping free-carrier diffusion in GaAs with radiative and heat-generating recombination Tim Gfroerer, Ryan Crum, Mark Wanlass We use a tightly focused laser along with optical and thermal imaging to measure the diffusion-driven, free-carrier distribution in a GaAs/GaInP heterostructure. We find that temperature profiles are broader than their luminescence counterparts. This observation is consistent with how the underlying recombination mechanisms depend on carrier density: the rate of heat generation should be approximately proportional to the density of carriers, while the radiative rate should scale with the density squared. We show that the square root of the light signal follows the heat profile, giving consistent, independent measurements of the local carrier density. [Preview Abstract] |
Session D12: Focus Session: Dopants and Defects in Semiconductors: Compound Semiconductors II
Sponsoring Units: DMPChair: Mao-Hua Du, Oak Ridge National Laboratory
Room: D223/224
Monday, March 21, 2011 2:30PM - 2:42PM |
D12.00001: Defect energy distribution in GaN/AlGaN heterostructures grown in Ga-rich and ammonia-rich conditions Tania Roy, Yevgeniy Puzyrev, Enxia Zhang, Daniel Fleetwood, Ronald Schrimpf, Sokrates Pantelides We use low-frequency noise measurements to estimate energy distributions of electrical-stress-induced defects in AlGaN/GaN high electron mobility transistors from 85 K to 450 K. The devices were grown under Ga-rich and ammonia-rich conditions using molecular beam epitaxy. The Ga-rich devices show a positive shift in pinch-off voltage and a decrease in gate leakage current with stress under a gate voltage of -3.6 V and a drain voltage of 20 V. These changes in response are associated with hydrogenated Ga vacancies in AlGaN. The ammonia-rich devices show a negative shift in pinch-off voltage and an increase in gate leakage current under the same stress conditions; these changes in device response are caused by N-antisite defects. The excess drain voltage power spectral density of the low frequency 1/$f$ noise peaks at $\sim $ 100 K in both device types, which corresponds to a trap energy of 0.2 eV. We attribute this to N-vacancy-related defects, which are also observed in proton damage experiments. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D12.00002: Thermal electron capture rate by Fe acceptor in GaN J. Dashdorj, M.E. Zvanut, T. Paskova, K. Udwary Doping GaN with Fe compensates the main residual impurities such as O and Si to produce semi-insulating substrates. Electron paramagnetic resonance measurements were made on GaN grown by hydride vapor phase epitaxy and doped with 1.5x10$^{17}$ to 1.6x10$^{18}$ cm$^{-3}$ Fe. The Fe$^{3+}$ spectra, angular dependence, and concentrations are consistent with literature and secondary ion mass spectroscopy data. During illumination with photon energies greater than 1.2 eV, the Fe$^{3+}$ signal increased in the lowest doped sample, but decreased in the more highly doped samples. One possible interpretation of the results is that the Fe$^{2+/3+}$ and Fe$^{3+/4+}$ levels are about 1.2 eV below the conduction band. Due to our measurement resolution, the spectral separation between the levels cannot be determined. The time-dependence of the Fe$^{3+}$ signal recovery after removal of 2.64 eV was recorded at temperatures between 3.5 and 297 K. Analysis show that capture rate of electrons by Fe$^{3+}$ decreases from 6x10$^{-16}$ to 5x10$^{-17}$ cm$^{3}$/s with an inverse-square-root temperature dependence. The work is supported by the NSF. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D12.00003: Direct atomic imaging of Mn in the GaN growth surface: High-density, Two-dimensional, Striped Superstructures Kangkang Wang, Noboru Takeuchi, Abhijit Chinchore, Wenzhi Lin, Arthur Smith A class of novel well-ordered striped superstructures have been observed by depositing submonolayer Mn onto GaN(0001)-``1$\times$1'' surface. These superstructures consist of stripe domains along [1$\bar{1}$00]$_{GaN}$ with various widths, while scanning tunneling microcopy images resolved a common local $\sqrt{3}$$\times$$\sqrt{3}$-R30$^{\circ}$ structure for the stripes. Combined with first-principles calculations, a new two-dimensional structural model is proposed having a dense Mn$_{x}$Ga$_{1-x}$ surface layer. Mn atomic sites within the GaN surface are directly identified. A spin-induced asymmetry in the Mn electronic structure is revealed in real-space for the narrow stripes. These findings explain the behavior of Mn atoms in the GaN growth surface and herald the development of magnetic nanostructures on GaN surfaces. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D12.00004: Initial Phase of Sub-monolayer Iron Growth on GaN(0001) pseudo-1$\times $1-1+1/12 Surface Studied Using Scanning Tunneling Microscopy and First Principles Theoretical Calculations Wenzhi Lin, Noboru Takeuchi, Kangkang Wang, Abhijit Chinchore, Meng Shi, Arthur Smith, Hamad Albrithen Iron/gallium nitride bi-layer structures have potential use for spintronic applications. Therefore, we have carried out an investigation of the initial phase of sub-monolayer iron growth on GaN(0001) pseudo-1$\times $1-1+1/12 surface. To begin with, we verified an atomically smooth GaN growth surface with the assistance of \textit{in situ} reflection high energy electron diffraction. STM shows smooth terraces separated by single and double height bilayer atomic steps. About 0.4 ML iron was deposited on the smooth GaN, and the subsequent STM images reveal Fe islands with a height of $\sim $ 2 {\AA} growing in a two-dimensional step-flow mode outward from the GaN step edges of the pseudo-1x1-1+1/12 surface. A clear 6 $\times $ 6 structure is observed for the islands. First principles theoretical calculations are being carried out in order to interpret the experimental results. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D12.00005: Atomic displacements in proton-irradiated AlGaN/GaN heterostructures Yevgeniy Puzyrev, Tania Roy, Enxia Zhang, Ronald Schrimpf, Daniel Fleetwood, Sokrates Pantelides We report results of quantum molecular dynamics calculations of atomic recoils in AlGaN and GaN. The recoil energy required to create defects in a perfect AlGaN/GaN lattice is known to be over 40eV. However, drastic changes in atomic configuration occur when defect atom itself recoils with than 10eV. We show that both N antisite defects and N atoms near Ga vacancy require less than 10 eV to introduce N vacancies, divacancies and N interstitials. This phenomenon leads to additional donors that can account for a positive shift in threshold voltage, observed in our electrical measurements in AlGaN/GaN devices irradiated by 1.8 MeV protons.\footnote{T. Roy, et. al.,~\textit{IEEE Trans. Nucl, Sci.}, 2010. accepted} In addition, divacancies and N vacancies have an electron transition level near the Fermi level in AlGaN which also provides explanation for the experimentally observed increase in 1/f noise after proton irradiation.\footnote{T. Roy, et al,~\textit{Microelectron. Reliab.}, 2010, accepted.} [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D12.00006: Nanoscale Potential Fluctuations in (GaMn)AsGaAs Heterostructures: From Individual Ions to Charge Clusters and Electrostatic Quantum Dots Paul Koenraad, Ineke Wijnheijmer, Jens Garleff, Oleg Makarovsky, Laurence Eaves, Richard Campion, Bryan Gallagher During growth of the dilute p-type ferromagnetic semiconductor GaMnAs, interstitial manganese is formed when the Mn concentration exceeds 2{\%}. This interstitial Mn acts as a double donor which compensates the free holes that mediate ferromagnetism. Annealing causes out-diffusion of these interstitials, thereby increasing the Curie temperature. Here, we use cross sectional scanning tunneling microscopy and spectroscopy to visualize the potential landscape which arises due to the clustering of interstitial Mn in annealed p-i-n (GaMn)As-GaAs double barrier heterostructures. We map the local minima in the potential landscape, link them to clusters of individual interstitial Mn ions, and show that the ions are doubly charged. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D12.00007: Controlled layer-by-layer depth-profiling of GaAs(110) using scanning tunneling microscopy David Gohlke, Donghun Lee, Jay Gupta The electronic properties of dopants in semiconductors such as GaAs vary depending on proximity to interfaces. We utilize a low temperature (5K) scanning tunneling microscope to realize a layer-by-layer peeling technique on p-GaAs(110). We apply positive voltage pulses near As vacancies to desorb surface-layer Ga and As atoms. Subsequent motion of the STM tip peels away the first layer from this starting point, fully exposing sections of the subsurface layer. The second and further layers can be readily peeled away by the same technique. This newly created pit allows depth-profiling of subsurface defects with STM. Funded by the Center for Emergent Materials at the Ohio State University, an NSF MRSEC (DMR-0820414). http://www.physics.ohio-state.edu/$\sim$jgupta [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D12.00008: Time-Resolved Far-Infrared Magnetospectroscopy of Electron Relaxation in GaAs S.N. Gilbert, G.L. Carr We report time-resolved magnetospectroscopy results for (S-I) GaAs at T$\sim$10K and fields up to 10T. A pulsed Ti:sapphire laser produces photoelectrons with energy $\sim$10 meV above the conduction band minimum that are subsequently probed by far-infrared transmission spectroscopy. Both free electrons and exciton transitions are observed, including transitions involving Landau levels when the magnetic field is applied. We also observe a time-dependent change in these transitions and discuss a model for the relaxation of a warm (non-equilibrium) distribution of electrons on a $\sim$500 ps time scale. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D12.00009: Tunable control over the ionization state of single Mn acceptors in GaAs with defect-induced band bending Donghun Lee, Jay Gupta The continuous miniaturization of semiconductor devices will ultimately reach a point where control over the properties of single dopants is necessary. Recent STM studies have demonstrated the ability to control the ionization state of single dopants through tip-induced band bending. This change in ionization state appears in STM images as a ring-like feature centered on the dopant, whose diameter depends on voltage, tip-sample distance, and tip termination. Here we demonstrate an additional degree of freedom for controlling the charge state of single Mn acceptors in GaAs by utilizing nearby charged defects which can be positioned with atomic precision. Systematic changes in the ring diameter with the separation between Mn and defect allows us to separately extract contributions from ~defect-induced and tip-induced band bending. These methods provide non-volatile control over the ionization state of single dopants, even in the absence of probe electrodes or STM tip. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D12.00010: Identification of the major cause of endemically poor mobilities in SiC/SiO$_{2}$ structures Xiao Shen, Sokrates T. Pantelides Mobility degradation at semiconductor-dielectric interfaces is generally attributed to defects at the interface or inside the dielectric, as is the case in Si/SiO$_{2}$ structures. In the case of SiC/SiO$_{2}$ structures, a decade of research focused on reducing or passivating interface and oxide defects, but low mobilities have persisted. It is known that during oxidation of Si, Si atoms are emitted into the substrate, but they do not form strongly-bonded complexes and their effects are usually benign. In contrast, during oxidation of SiC, C atoms are emitted into the substrate and they can form strongly-bonded carbon complexes. Here we identify one particular complex that explains a range of experimental defect signatures and electrical measurements. We propose that this complex is a major cause of the poor mobility in SiC/SiO$_{2}$ structures. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D12.00011: Electronic properties of Si-C interfaces Xiang-Guo Li, Hai-Ping Cheng In this work, we report our investigations of interfacial properties of Si-C systems. Electronic properties of Fe-doped carbon on silicon surfaces, Si-Fe-C layered structures and Si-graphene-Si junctions have been studied using first-principles calculations. Charge transfer at the interfaces, densities of states, and magnetization are fully analyzed. These problems are important because recent experiments show that Fe@C-Si materials have giant electro-resistance and magneto-resistance highly sensitive to the external magnetic field. The non-magnetic feature leads to very small magnetic noise. In addition, photovoltaic effects were also observed in some of these systems. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D12.00012: Polariton Formation Enhances Lifetimes of Dense Exciton Gasses in Cuprous Oxide by Suppressing Two-Exciton Decay N. Laszlo Frazer, R.D. Schaller, J.I. Jang, S.E. Mani, J.B. Ketterson Collective excitonic states form at high densities, but in dense gases the long lifetime of excitons in cuprous oxide (Cu$_{2}$O) is compromised by two-exciton annihilation processes. Using the picosecond streak camera spectroscopy facility at the Center for Nanoscale Materials at Argonne National Laboratory we directly measured the decay of orthoexciton-polaritons generated by two photon absorption. The two-body decay lifetime is an order of magnitude longer than for excitons uncoupled to photons. The extended lifetime opens opportunities for experiments that manipulate polariton collective states. Unlike time-averaged measurements of two-body processes, streak camera detection shows decay suppression without effects from production efficiency or additional density dependent processes. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D12.00013: Ultrafast carrier dynamics in Bi2Se3 thin films Keliang He, Liguo Zhu, Chen Xia, Brian Kubera, Jie Shan Bismuth Selenide (Bi2Se3), a group V-VI narrow gap layered semiconductor, is a well-known efficient solid thermoelectric material at room temperature. It has recently also attracted much research attention due to its interesting topological properties. The carrier dynamics and charge transport, electron-phonon coupling, and its role in the transport properties in Bi2Se3 are fundamental issues in understanding its thermoelectric and topological properties. In this work, we employ the optical-pump terahertz-probe technique to study the transient photoconductivity in Bi2Se3 thin crystalline films as a function of the pump-probe delay time and the excitation fluence. The photoconductivity spectrum ranging from 0.3 to 1.9 THz reveals both a Drude and a Lorentz contribution. The former is attributed to a free electron response with a scattering time of 0.7 ps; and the latter, with both its amplitude and peak frequency dependent on the carrier density, arises from a coupled LO phonon-plasmon mode. The nature of this mode and its role in transport will be discussed. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D12.00014: Oxygen in B$_{2}$O$_{3}$ covered Czochralski-grown Ge Toshinori Taishi, Hideaki Ise, Yu Murao, Takayuki Ohsawa, Yuki Tokumoto, Yutaka Ohno, Ichiro Yonenaga Ge has been regained keen interest in applications of Ge for ultra-fast CMOS and PV devices. High quality Ge crystals should be demanded for realization of such devices with higher performances in these circumstances. Oxygen impurity can be expected to enhance thermo-mechanical stability of Ge crystals due to dislocation locking similar to oxygen in Si. For the purpose, we grew oxygen-enriched Ge crystals by the Czochralski method from B$_{2}$O$_{3}$ covered melt added with GeO$_{2}$ powder in a silica crucible. To evaluate precious knowledge oxygen behavior in Ge, local vibrations of oxygen were evaluated by FT-IR spectroscopy. Concentrations of interstitially dissolved oxygen impurity in the crystals were in the range between 8.5 $\times $ 10$^{15}$ and 5.5 $\times $ 10$^{17}$ cm$^{-3}$ determined from the FT-IR absorption at 855 cm$^{-1}$ originating in local vibration of Ge-O$_{i}$-Ge quasi-molecules. Absorption peaks relating to GeO$_{x}$, SiO$_{x}$ and Si-Oi-Si were not detected in the as-grown crystals. By prolonged annealing at 350{\_}C, an absorption peak developed at 780 cm$^{-1}$, indicating formation of oxygen related thermal donors. Such donors disappeared by annealing at 550{\_}C. [Preview Abstract] |
Session D13: Statistical and Nonlinear Physics: General
Sponsoring Units: GSNPChair: James Dufty, University of Florida
Room: D225/226
Monday, March 21, 2011 2:30PM - 2:42PM |
D13.00001: Stability and dynamical properties of Cooper-Shepard-Sodano compactons Andres Cardenas, Bogdan Mihaila, Fred Cooper, Avadh Saxena Extending a Pade approximant method used recently to study the properties of compactons in the Rosenau-Hyman (RH) equation [see B. Mihaila et al. Phys. Rev. E 81, 056708 (2010)], we study the numerical stability of single compactons of the Cooper-Shepard-Sodano (CSS) equation and their pairwise interactions. The CSS equation has a conserved Hamiltonian which has allowed several approaches for studying analytically the nonlinear stability of the solutions. We study three different compacton solutions and find they are numerically stable. Similar to the collisions between RH compactons, the CSS compactons reemerge with the same coherent shape when scattered. The time evolution of the small-amplitude ripple resulting after scattering depends on the values of the parameters characterizing the corresponding CSS equation. The simulation of the CSS compacton scattering requires a much smaller artificial viscosity to obtain numerical stability than in the case of RH compacton propagation. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D13.00002: Pair Correlations for Charges in a Harmonic Trap Jeffrey Wrighton, James Dufty, Hanno K\"{a}hlert, Torben Ott, Patrick Ludwig, Michael Bonitz A classical system of N identical charges in a harmonic trap exhibits both shell structure and orientational ordering due to Coulomb correlations. The shell structure can be reproduced accurately using approximate correlations from the bulk OCP [1]. Here we report additional relationships between correlations in the trap and those for the bulk OCP: 1) pair correlations calculated without reference to their location in the trap agree with those of the bulk OCP, 2) orientational pair correlations among particles within a shell are represented by those of the bulk OCP, when Euclidean distance is replaced by arc length (qualitative agreement using 3D OCP; quantitative agreement using 2D OCP). At stronger coupling, the correlations induce an ordering within the shells (spherical Wigner crystal). It is shown that the orientational correlations for this phase are described by those for the single sphere Thomson problem, i.e. the Thomson sites represent the ``lattice" for the spherical crystal. Finite temperature effects for this phase are described as well. Research supported by DOE award DE-FG02-07ER54946, and by the Deutsche Forschungsgemeinschaft via SFB-TRR24.\\[4pt] [1] J. Wrighton, J. Dufty, H. Kaehlert, and M. Bonitz, Phys. Rev. E \textbf{80}, 038912 (2009); Contrib. Plasma Phys. \textbf{50}, 26-30 (2010). [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D13.00003: Quasi-bound state lifetimes and classical periodic orbits in HOCl Alex Barr, Kyungsun Na, Linda Reichl We use a discrete variable representation together with reaction matrix theory to calculate the quasi-bound states of a Chlorine atom scattering off a diatomic molecule of Hydrogen and Oxygen. The lifetimes of these quasi-bound states are found to vary over six orders of magnitude in a very small energy window. By examining Husimi distributions for various quasi-bound states we show that the longest-lived quasi- bound states are anchored by an island of stability surrounding a stable periodic orbit in the otherwise chaotic classical phase space. This stable periodic orbit, which corresponds to Chlorine rotating around the HO molecule, is responsible for the very long lifetimes of these quasi-bound states. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D13.00004: An Infinite Order Phase Transition Pradeep Kumar, Avinash Khare, Avadh Saxena An infinite order phase transition, in the sense envisioned by Ehrenfest, must show no singularity in any finite (thermal or mechanical) derivative of the free energy. By considering the infinite p limit of a free energy that we have derived for a p-th order phase transition, we can derive a Landau type free energy. We will discuss the properties of the free energy and identify the features essential for a description of an infinite order phase transition. These include a logarithmic interaction between the fields and a novel dependence on spatial gradients. Contrary to popular belief, since some symmetry is broken at each finite p order, we submit that an infinite order phase transition does not exclude a symmetry being broken. Restricting to one dimension, we solve for domain wall solutions. Finally we show the relationship between an infinite order phase transition and Tachyon condensation. They are both analyzed as the infinite p limit of a class of p enumerated field theories. The mathematical difference being that the free energy for infinite order transition belongs to a potential that is negative (inverted) of the action for tachyon condensation. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D13.00005: First Principles Derivation of Fading Models from Wave Chaos Theory Jen-Hao Yeh Wave chaos is the study of solutions to linear wave equations in situations where the ray dynamics recovered in the classical limit is chaotic. Fading is the observation of variations in signal strength measured at a receiver due to time-dependent variations in the propagation or multi-path scattering and interference. A quantitative statistical theory of wave chaos - random matrix theory (RMT) - can be applied to predict statistical properties of many quantities, such as the scattering matrix, of a wave chaotic system. Here we started from the statistical model of the scattering matrix [1] to establish a general fading model that includes Rayleigh fading and then combine the RMT fading model with our random coupling model that takes account system-specific features [2-4] to build a more general fading model that includes Rician fading. In the high loss limit, our model agrees with the Rayleigh/Rice models, however, it shows deviation in the limit of low loss. We have performed experiments [3,4] to verify the RMT fading model. \\[0pt] [1] http://publish.aps.org/search/field/author/Brouwer\_P\_W (P. W. Brouwer) and http://publish.aps.org/search/field/author/Beenakker\_C\_W\_J (C. W. J. Beenakker), Phys. Rev. B 55, 4695 (1997). [2] James A. Hart, \textit{et al.}, Phys. Rev. E~80, 041109 (2009). [3] Jen-Hao Yeh, \textit{et al}.,~Phys. Rev. E 81, 025201(R) (2010). [4] Jen-Hao Yeh, \textit{et al}.,~Phys. Rev. E 82, 041114 (2010). [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D13.00006: Quantum statistical mechanics on infinitely ramified fractals Joe P. Chen I present the thermodynamics of identical particles confined in infinitely ramified, exactly self-similar fractals, such as the Sierpinski carpet (in 2D) and the Menger sponge (in 3D). Recent results from analysis on fractals have established that the heat kernel associated with the Laplacian on such fractals satisfy, in the short-time regime, a scaling relation with exponent $d_{\rm S}/2$ (where $d_{\rm S}$ is the spectral dimension) modulated by log-periodic oscillations. I explain how such a scaling affects the partition function, and the resultant thermodynamics associated with blackbody radiation [1], Casimir effect, and electrons in the fractal box. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D13.00007: Unified Approach to Quantum and Classical Dualities Emilio Cobanera, Gerardo Ortiz, Zohar Nussinov We discuss a new systematic and algebraic approach to searching for dualities in quantum systems. By associating ``bond algebras'' to quantum Hamiltonians we show how dualities can be characterized, recognized as unitary transformations, and mapped to dualities of classical partition functions. Hence our approach unifies classical and quantum dualities and provides a powerful method for determining exact properties of systems of interest. We show how duality transformations can be used always to eliminate gauge symmetries completely, and present a new duality between the Abelian Higgs model and a generalized Kitaev's extended toric code model in {\it three space dimensions} that illustrate this point. We also show new dualities for $Z_p$) gauge models. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D13.00008: Fluctuation-induced forces in strongly anisotropic critical systems M. Burgsm\"uller, H.W. Diehl, M.A. Shpot Strongly anisotropic critical systems have two (or more) correlation lengths $\xi_\alpha$ and $\xi_\beta$ that diverge as nontrivial powers $\xi_\alpha\sim \xi_\beta^\theta\to \infty$ upon approaching criticality. We investigate the effective (Casimir-like) forces that are induced between two confining parallel boundary planes at a distance $L$ by fluctuations in such systems at bulk criticality. Two fundamentally distinct orientations of boundary planes must be distinguished: parallel, for which the planes are parallel to all of the available $1\le m < d$ $\alpha$-directions, and perpendicular, for which they are perpendicular to an $\alpha$-direction, but parallel to all other $\alpha$- and $\beta$-directions. Using a RG approach, we show that universal Casimir amplitudes $\Delta^{BC}_{\|,\perp}$, depending on both the large-scale boundary condition (BC) at both plates and the type of surface plane orientation, can be introduced to characterize the asymptotic $L$-dependence of the critical fluctuation-induced force. This varies as $\mathcal{F}\sim -(\partial/\partial L)$$\,\Delta^{BC}_{\|,\perp}\,L^{-\zeta_{\|,\perp}}$, where the proportionality constant is nonuniversal. To corroborate these findings, $O(n)$ $\phi^4$ models with $m$-axial Lifshitz points are investigated below their upper critical dimension $d=4+m/2$. Explicit one- and two-loop results for $\Delta^{BC}_{\|,\perp}$ are presented for both orientations and periodic or Dirichlet-like boundary conditions, along with large-$n$ results. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D13.00009: Multifractality of instantaneous normal modes at mobility edges Ten-Ming Wu In terms of the multifractal analysis, we investigate the characteristics of instantaneous normal modes (INMs) at mobility edges (MEs) of a simple fluid, where the locations of two MEs in the INM spectrum were identified in a previous work (Phys. Rev. E 79, 041105 (2009)). The mass exponents and the singularity spectrum of the INMs are obtained by both the box-size and system-size scalings under the typical average. The INM eigenvectors at a ME exhibit a multifractal nature and the multifractal INMs at each ME yield the same results in generalized fractal dimensions and singularity spectrum. Our results indicate that the singularity spectrum of the multifractal INMs agrees well with that of the Anderson model at the critical disorder. This good agreement provides a numerical evidence for the universal multifractality at the localization-delocalization transition. For the multifractal INMs, the probability density function and the spatial correlation function of squared vibrational amplitudes are also calculated. The relation between probability density function and singularity spectrum is examined numerically, so are the relations between the critical exponents of the spatial correlation function and the mass exponents of the multifractal INMs. All results will be appeared in Phys. Rev. E. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D13.00010: Experimental study of memory erasure in a double-well potential Yonggun Jun, John Bechhoefer We have experimentally demonstrated memory erasure in a time-dependent, double-well potential using a protocol suggested by Dillenschneider and Lutz [PRL 102, 210601 (2009)]. The protocol implements the erasure of information by removing the potential barrier, skewing the potential to one side, and then raising the barrier back. In this context, erasure means that no matter which well the particle started the cycle in, it ends up in a designated well. We implement the potential by placing an overdamped, charged Brownian particle in a feedback trap that uses electrophoresis to generate an arbitrary virtual two-dimensional potential. In a large system, Landauer's principle gives a lower bound for the heat dissipated in the erasure of a single bit (kT ln2). In a small system such as ours, thermal fluctuations allow for occasional violations. We quantify such violations as a function of barrier size and show that while averages are consistent with Landauer's principle, the tail of the distribution of dissipation per cycle---a fraction of trajectories---violates it. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D13.00011: Cosmology in One Dimension: Fractal Geometry, Power Spectra and Correlation Bruce Miller, Jean-Louis Rouet Concentrations of matter, such as galaxies and galactic clusters, originated as very small density fluctuations in the early universe. The existence of galaxy clusters and super-clusters suggests that a natural scale for the matter distribution may not exist. A point of controversy is whether the distribution is fractal and, if so,over what range of scales. One-dimensional models demonstrate that the important dynamics for cluster formation occur in the position-velocity plane. Here the development of scaling behavior and multifractal geometry is investigated for a family of one-dimensional models for three different, scale-free, initial conditions. A possible physical mechanism for understanding the self-similar evolution is introduced. It is shown that hierarchical cluster formation depends both on the model and the initial power spectrum. Under special circumstances a simple relation between the power spectrum, correlation function, and correlation dimension in the highly nonlinear regime is confirmed. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D13.00012: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 4:54PM - 5:06PM |
D13.00013: Thermodynamics in a complete description of the Landau diamagnetism S. Curilef, F. Olivares, F. Pennini We analyze some consequences that come from semiclassical measures as the Wehrl entropy and the Fisher information for the problem of a particle in a magnetic field starting from a complete description of the Husimi function. We discuss in the most complete form (three dimensions)[1] some results related to measures in contrast with the incomplete form (two dimensions)[2,3]. The formulation in two dimensions is sufficient unto itself to explain the problem whenever the length of the cylindrical geometry of the system is large enough. Our semiclassical description constitutes a useful framework to illustrate problems related to size effects, role of boundaries and other typical anomalies derived from the size of the system, which are refereed to two parameters as area and length and they explicitly appear in the form of the limiting temperature and magnetic field. In addition, we discuss that the zero temperature can be achieved only if the length of the system size is large enough, otherwise physical properties strongly depend on the size of the system. Moreover, from the quantization of the quantum Hall effect, we have obtained a family of quantized Wehrl entropies.\\[4pt] [1] F. Olivares, et al, PRE \textbf{81} 041134 (2010);\\[0pt] [2] D. Herrera, et al, Eur J Phys \textbf{29} 439 (2008);\\[0pt] [3] S, Curilef, et al, PRB \textbf{71} 024420 (2005). [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D13.00014: Newtonian trajectories as a tool for quantum dynamics in an electromagnetic field Fons Brosens, Wim Magnus In previous studies, we showed that the classical equations of motion provide a solution to quantum dynamics, if appropriately incorporated in the Wigner distribution function, exactly reformulated in a type of Boltzmann equation. However, this earlier work was limited to scalar potentials. In the presence of an electromagnetic field, we now show that this description in terms of classical paths remains valid, despite the fact that the definition of the Wigner distribution function is not gauge invariant. Some analytical results are also presented. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D13.00015: The bond problem with an arbitrary percolation radius is solved! Vladimir Udodov, Mariya Bureeva The results of investigations of main characteristics of a one-dimensional percolation theory (percolation threshold, critical exponents of correlation radius and specific heat) are presented for the problem of bonds and sites. It is shown that for a finite-size system the stability condition is fulfilled while the scaling hypothesis is inacceptable for one-dimensional bond problem. The correlation length exponent $\nu $ in a one-dimensional problem of bonds has been found to exceed the values of $\nu $ in the problem of sites for equal-length chains, and, in general, this exponent was found to be extraordinary large compared to the 2-D and 3-D cases for ordinary phase transitions in macrosystems. The scaling hypothesis is inapplicable to random (disordered) one-dimensional nanostructures containing hundreds of structural elements. The results obtained in this work can be used in modeling hopping conduction in semiconductors at low temperatures and polytype transformations in close-packed crystals. For the first time, using the method of computer simulation, we have solved the bond problem for the model of one-dimensional percolation in finite-size systems of tens of nanometers with an arbitrary percolation radius. [Preview Abstract] |
Session D14: Systems Far From Equilibrium
Sponsoring Units: GSNPChair: H. George E. Hentschel, Emory University
Room: D227
Monday, March 21, 2011 2:30PM - 2:42PM |
D14.00001: Fluctuation Relations for Currents Nikolai Sinitsyn, Alexei Akimov, Vladimir Chernyak, Michael Chertkov We consider a non-equilibrium statistical system on a graph or a network. Identical particles are injected, interact with each other, traverse, and leave the graph in a stochastic manner described in terms of Poisson rates, possibly strongly dependent on time and instantaneous occupation numbers at the nodes of the graph. We show that the system demonstrates a profound statistical symmetry, leading to new Fluctuation Relations that originate from the supersymmetry and the principle of the geometric universality of currents rather than from the relations between probabilities of forward and reverse trajectories. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D14.00002: Aging in coarsening ferromagnets with site and bond disorder Hyunhang Park, Michel Pleimling Aging processes during phase ordering are studied in the random-site and random-bond Ising models in two dimensions through Monte-Carlo simulations. The dynamical correlation length $L(t)$ is numerically determined and the behavior of various two-time quantities is investigated. For both models deviations of $L(t)$ from an algebraic growth law $L(t) \sim t^{1/z}$ are observed. Using the correct form of $L(t)$ a simple scaling picture is recovered for the studied disordered ferromagnets in the coarsening regime. Thus various two-time quantities, as for example the autocorrelation function, the space-time correlation function and the time integrated linear response, show a scaling behavior that is fully consistent with simple aging [1]. The similarities and differences between the site-disordered and the bond-disordered models are discussed.\\[4pt] [1] H. Park and M. Pleimling, Phys. Rev. B {\bf 82}, 144406 (2010). [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D14.00003: Nonequilibrium phase transition in a driven Potts model with friction Michel Pleimling, Ferenc Igl\'{o}i, Lo\"Ic Turban We consider magnetic friction between two systems of $q$-state Potts spins which are moving along their boundaries with a relative constant velocity $v$. Due to interaction between the surface spins there is a permanent energy flow and the system is in a steady state which is far from equilibrium. The problem is treated analytically in the limit $v=\infty$ (in one dimension, as well as in two dimensions for large-$q$ values) and for $v$ and $q$ finite by Monte Carlo simulations in two dimensions. Exotic nonequilibrium phase transitions take place, the properties of which depend on the type of phase transition in equilibrium. When this latter transition is of first order, a sequence of second- and first-order nonequilibrium transitions can be observed when the interaction is varied [1].\\[4pt] [1] F. Igl\'{o}i, M. Pleimling, and L. Turban, arXiv:1010.0738. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D14.00004: Aging dynamics for the driven lattice gas George L. Daquila, Uwe C. T\"auber We numerically investigate the two-time behavior of the density-density auto-correlation function in driven lattice gases with particle exclusion and periodic boundary conditions in one, two, and three dimensions using precise Monte Carlo simulations. Starting from strongly correlated initial conditions we investigate the relaxation towards the nonequilibrium steady state. We obtain simple aging scaling behavior in one, two, and three dimensions. The simulation data confirm the density auto-correlation aging exponents determined from simple scaling arguments. For the one-dimensional case we connect with the KPZ surface growth model and establish a relation between the density-density and known height-height auto-correlation aging exponents. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D14.00005: Effect of size distribution on metastability in magnetic nanoparticles Yoh Yamamoto, Kyungwha Park Magnetic nanoparticles that have been synthesized using various methods have size distributions. This results in distributions in the magnetic anisotropy of magnetic nanoparticles. Considering the particle size distributions, we investigate metastability in magnetic nanoparticles at low temperatures. To model this system, we use a spin $S=1$ ferromagnetic Blume-Capel model on a square lattice with periodic boundary conditions. The particle size distribution is incorporated in the model such that the uniaxial magnetic anisotropy parameter has a Gaussian distribution. We perform kinetic Monte Carlo simulations of the Blume-Capel model with the Glauber dynamic to explore magnetization relaxation in the regime where a single droplet of flipped spins forms a critical droplet. We present the lifetime of the metastable state as a function of temperature and standard deviation of the magnetic anisotropy distribution as well as a finite-size effect on the lifetime. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D14.00006: Phase Diagram for a 2-D Two-Temperature Diffusive XY Model Matthew Reichl, Charo Del Genio, Kevin E. Bassler Using Monte Carlo simulations, we determine the phase diagram of a diffusive two-temperature conserved order parameter XY model. When the two temperatures are equal the system becomes the equilibrium XY model with the continuous Kosterlitz-Thouless (KT) vortex-antivortex unbinding phase transition. When the two temperatures are unequal the system is driven by an energy flow from the higher temperature heat-bath to the lower temperature one and reaches a far-from-equilibrium steady state. We show that the nonequilibrium phase diagram contains three phases: A homogenous disordered phase and two phases with long range, spin texture order. Two critical lines, representing continuous phase transitions from a homogenous disordered phase to two phases of long range order, meet at the equilibrium KT point. The shape of the nonequilibrium critical lines as they approach the KT point is described by a crossover exponent $\varphi = 2.52 \pm 0.05$. Finally, we suggest that the transition between the two phases with long-range order is first-order, making the KT-point where all three phases meet a bicritical point. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D14.00007: Avalanches in gauge theories Stefanos Papanikolaou I consider the non-equilibrium behavior of disordered systems which contain a residual gauge symmetry. Remarkably, in this limit each avalanche is a Wilson loop of the associated gauge theory. Such gauge invariant avalanches present interesting critical behavior that we characterize. Also, I show that, when the gauge symmetry is violated, the behavior drastically changes. Finally, the relation of these results to current experimental efforts on spin ice compounds is discussed. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D14.00008: A framework for studying biased stochastic dynamics in continuous space S.M. Ali Tabei, Ye Tian, Martin Tchernookov, Aaron Dinner Typically in the formalism of large deviation functions the biased dynamics are studied in a discrete space. However, in many realistic stochastic systems dynamics take form in a continuous rather than a discrete space. In recent work it was shown that the biased dynamics for continuous-space models can be calculated using transition path sampling: unbiased trajectories were generated by shooting with the original dynamics from an existing path and then accepted or rejected to obtain the biased path ensemble. Here, we instead develop a way to bias continuous-space dynamics directly in the form of a biased Langevin equation. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D14.00009: Measured first-passage-time distributions for a high-dimensional system: noise-induced current switching in semiconductor superlattices Yuriy Bomze, Holger T. Grahn, Rudolf Hey, Stephen W. Teitsworth We report the experimental measurement of first-passage-time distributions associated with current switching in weakly-coupled GaAs/AlAs superlattices, in a regime of nonlinear electronic transport where the static current-voltage ($I - V$) curves exhibit multiple branches and bistability. Precision, high bandwidth current switching data are collected in response to sequential steps in applied voltage to a final voltage $V_f$ near to the voltage $V_{th}$ corresponding to the end of a particular branch. For initial state preparation, a double step procedure is used to insure that the system is close to the true metastable state. For a range of $V_f$ values, switching times reveal large stochastic fluctuations driven by internal shot noise. For smaller times ($<$ 3$\mu$s), the switching time distributions show exponential tails, as expected for activated escape from an initial metastable state. However, at larger times ($>$ 10 $\mu$s), the distributions exhibit power law tails (with exponent ranging from -2 to -1, and dependent on $|V_f -V_{th}|$). Possible sources for the power law decay include collective effects and the presence of multiple escape trajectories. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D14.00010: Non-equilibrium Thermodynamics: Residual Entropy, Internal Variables, Maxwell Relations, and the Prigogine-Defay Ratio Puru Gujrati We extend a recently formulated [Phys. Rev. E 81, 051130 (2010)] non-equilibrium thermodynamic approach to an inhomogeneous system consisting of many smaller subsystems, each in internal equilibrium; their relative motions result in viscous effects. The correct Gibbs free energy of a subsystem contains the temperature and pressure of the medium, making our approach an extension of the classical non-equilibrium thermodynamics due to de Donder. The additivity of entropy requires quasi-independence of subsystems, so that the energy also becomes additive. We use Gibbs' entropy of the isolated system to derive the entropy for the system even when the latter is out of equilibrium. We use this entropy to discuss the residual entropy when the system is confined to one of the components in the phase space. The approach is extended to include internal variables that cannot be controlled by the observer during non-equilibrium evolution. We then identify the form of non-equilibrium Maxwell relations. We apply our formalism to evaluate the Prigogine-Defay ratio in glasses, which is found to be, in general, different from 1 at the apparent glass transition, as is normally seen in experiments. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D14.00011: Generalized Gibbs distribution and energy localization in the semiclassical FPU problem Rafael Hipolito, Ippei Danshita, Vadim Oganesyan, Anatoli Polkovnikov We investigate dynamics of the weakly interacting quantum mechanical Fermi-Pasta-Ulam (qFPU) model in the semiclassical limit below the stochasticity threshold. Within this limit we find that initial quantum fluctuations lead to the damping of FPU oscillations and relaxation of the system to a slowly evolving steady state with energy localized within few momentum modes. We find that in large systems this state can be described by the generalized Gibbs ensemble (GGE), with the Lagrange multipliers being very weak functions of time. This ensembles gives accurate description of the instantaneous correlation functions, both quadratic and quartic. Based on these results we conjecture that GGE generically appears as a prethermalized state in weakly non-integrable systems. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D14.00012: Zero Droplet Stiffness Exponent: Probing Short Range Spin Glasses with Avalanches Induced by Long Range Interactions Gergely Zimanyi, Ferenc Pazmandi We probe the droplet excitations in short range spin glasses by adding a perturbative long range interaction that decays with distance as a power law: $J/r^\sigma $. It is shown that if the power law exponent $\sigma $ is smaller than the spatial dimension $d$, the perturbation induces large scale avalanches which roll until they force the system to develop a pseudo gap in the excitation spectrum of the stabilities. This makes the perturbative long range interactions relevant for $\sigma <\sigma _c =d$. The droplet theory predicts that the critical exponent $\sigma _c $ depends on the droplet stiffness exponent as $\sigma _c =d-\theta .$ Combining these two results leads to a zero stiffness exponent $\theta =0$ in the droplet theory of short range spin glasses. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D14.00013: Introduction of a new thermodynamic property: ``characteristic frequency'' McKendree Pepper, Cristian Bahrim, Rafael Tadmor Fluctuations of thermodynamic properties are observed in the critical region of fluids, multiphase regions, and in systems containing a small number of molecules. We describe the dynamics within the vapor-liquid \textit{interfacial region} (IR) of a monatomic fluid in thermal equilibrium using fundamental principles of mechanics and thermodynamics. Our objective is to provide a new dynamic parameter which characterizes thermodynamic systems fluctuating near equilibrium, such as the IR. We call this new property \textit{``characteristic frequency''}. Our model assumes that the IR is (1) a closed thermodynamic system, (2) has a linear response to a driving force generated by a thermodynamic fluctuation, and (3) has a unique characteristic (resonant) frequency. We find that mild oscillations from equilibrium of a thermodynamic system occur at the most probable speed, and that the amplitude of the oscillations depends solely on the partition functions of the vapor and liquid within the IR. Our conclusion is that fluctuating thermodynamic systems at thermal equilibrium can exhibit oscillations analogous to mechanical systems and manifest a similar resonant response as the classical oscillators near their characteristic frequency. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D14.00014: An effective Fluctuation Theorem in Bidirectional Single-Electron Counting Yasuhiro Utsumi, Dimitry Golubev, Michael Marthaler, Keiji Saito, Toshimasa Fujisawa, Gerd Schoen We investigate the direction-resolved full counting statistics of single-electron tunneling through a double quantum dot system and compare with predictions of the fluctuation theorem (FT) for Markovian stochastic processes. Experimental data obtained for GaAs/GaAlAs heterostructures appear to violate the FT. After analyzing various potential sources for the discrepancy we conclude that the nonequilibrium shot noise of the measurement device influence the tunneling statistics. Taking these modifications into account we show how the FT can be violated due to measurement effects and recovered for fast detection by introducing an ``effective temperature.'' [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D14.00015: Landauer-Buettiker approach to current-induced forces in nanoelectromechanical systems Silvia Viola Kusminskiy, Niels Bode, Reinhold Egger, Felix von Oppen We study current-induced forces in nanoelectromechanical systems with coupling between electronic and mechanical degrees of freedom. We focus on the regime where the mechanical motion is slow and Coulomb blockade effects can be neglected. We derive the current-induced forces both in and out of equilibrium and give the conditions under which these forces can be expressed solely in terms of the S-matrix. We pay particular attention to situations with more than one mechanical degree of freedom which are characterized by several qualitatively new features. We apply our general results to some simple examples. [Preview Abstract] |
Session D15: Electronic Structure I
Sponsoring Units: DCOMPChair: Victor Pardo, University of California, Davis
Room: D171
Monday, March 21, 2011 2:30PM - 2:42PM |
D15.00001: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 2:42PM - 2:54PM |
D15.00002: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 2:54PM - 3:06PM |
D15.00003: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:06PM - 3:18PM |
D15.00004: Unfolding first-principles band structures Wei Ku, T. Berlijn, C.-C. Lee A general method [1] is presented to unfold band structures of first-principles supercell calculations with proper spectral weight, allowing easier visualization of the electronic structure and the degree of broken translational symmetry. The resulting unfolded band structures contain additional rich information from the Kohn-Sham orbitals, and absorb the structure factor that makes them ideal for a direct comparison with angle resolved photoemission spectroscopy experiments. With negligible computational expense via the use of Wannier functions, this simple method has great practical value in the studies of a wide range of materials containing impurities, vacancies, lattice distortions, or spontaneous long-range orders. \\[4pt] [1] Wei Ku, T. Berlijn, and C.-C. Lee, Phys. Rev. Lett. {\bf 104}, 216401 (2010). [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D15.00005: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:30PM - 3:42PM |
D15.00006: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:42PM - 3:54PM |
D15.00007: Finite-basis correction applied to the optimized effective potential within the FLAPW method Christoph Friedrich, Markus Betzinger, Stefan Bl\"ugel The optimized-effective-potential (OEP) method is a special technique to construct local exchange-correlation (xc) potentials from general orbital-dependent xc energy functionals for density-functional theory. Recently, we showed that particular care must be taken to construct local potentials within the all-electron full-potential augmented-plane-wave (FLAPW) approach. In fact, we found that the LAPW basis had to be converged to an accuracy that was far beyond that in calculations using conventional functionals, leading to a very high computational cost. This could be traced back to the convergence behavior of the density response function: only a highly converged basis lends the density enough flexibility to react adequately to changes of the potential. In this work we derive a numerical correction for the response function, which vanishes in the limit of an infinite, complete basis. It is constructed in the atomic spheres from the response of the basis functions themselves to changes of the potential. We show that such a \textit{finite-basis correction} reduces the computational demand of OEP calculations considerably. We also discuss a similar correction scheme for $GW$ calculations. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D15.00008: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 4:06PM - 4:18PM |
D15.00009: ESTEST: A Framework for the Verification and Validation of Electronic Structure Codes Gary Yuan, Francois Gygi ESTEST is a verification and validation (V\&V) framework for electronic structure codes that supports Qbox, Quantum Espresso, ABINIT, the Exciting Code and plans support for many more. We discuss various approaches to the electronic structure V\&V problem implemented in ESTEST, that are related to parsing, formats, data management, search, comparison and analyses. Additionally, an early experiment in the distribution of V\&V ESTEST servers among the electronic structure community will be presented. [1] G. Yuan and F. Gygi, Computational Science and Discovery (2010) (in press). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D15.00010: The random phase approximation and beyond: an assessment for molecular binding energies and reaction barrier heights Xinguo Ren, Patrick Rinke, Matthias Scheffler, Joachim Paier, Andreas Gr{\"u}eneis, Georg Kresse, Gustavo E. Scuseria The random phase approximation (RPA) for the correlation energy has become a promising approach for describing electronic systems in various bonding situations. Recent efforts have focused mainly on correcting the general tendency of RPA to underestimate bond strengths e.g. by adding corrections from second-order screened exchange (SOSEX) [1,2] or single excitations (SE) [3]. In this work, we systematically assess the influence of SOSEX, SE and their combinations on the atomization energies of the G2-I molecular set, as well as the chemical reaction barrier heights of the HTBH38/04 and NHTBH38/04 benchmark sets [4]. We find that RPA+SOSEX+SE based on PBE gives the most balanced description. However, for reaction barrier heights standard RPA based on PBE turns out to be better and is surprisingly accurate. The underlying mechanism governing the performance of RPA and its variants in different circumstances will be analysed. [1] A. Gr\"uneis {\it et al.}, J. Chem. Phys. \textbf{131}, 154115 (2009). [2] J. Paier {\it et al.} J. Chem. Phys. \textbf{132}, 094103 (2010). [3] X. Ren {\it et al.}, arXiv:cond-mat/1011.2724. [4] Y. Zhao {\it et al.} J. Phys. Chem. A \textbf{109}, 2012 (2005) [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D15.00011: Approximating Densities of States with Gaps Roger Haydock, C.M.M. Nex Reconstructing a density of states or similar distribution from moments or continued fractions is an important problem in calculating the electronic and vibrational structure of defective or non-crystalline solids. For single bands a quadratic boundary condition introduced previously [Phys. Rev. B 74, 205121 (2006)] produces results which compare favorably with maximum entropy and even give analytic continuations of Green functions to the unphysical sheet. In this paper, the previous boundary condition is generalized to an energy-independent condition for densities with multiple bands separated by gaps. As an example it is applied to a chain of atoms with s, p, and d bands of different widths with different gaps between them. The results are compared with maximum entropy for different levels of approximation. Generalized hypergeometric functions associated with multiple bands satisfy the new boundary condition exactly. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D15.00012: A New Boundary Condition for Embedding Atoms in Solids G.A. Benesh, Roger Haydock Previously, Haydock and Nex [Phys. Rev. B 82, 205114 (2010)] formulated an approximation for embedding a finite discrete system into an infinite substrate by means of a new boundary condition. This boundary condition requires a maximum breaking of time-reversal symmetry (MBTS) in the sense that probability is carried away from the embedding surface at a maximal rate. The MBTS boundary condition has been useful in discrete systems for constructing accurate densities of states and other distributions from moments or continued fractions. In this work, we generalize the approach to the problem of embedding an atom or a cluster of atoms into an infinite solid. The new, continuous MBTS boundary condition has been applied to model systems and to the embedding of a hydrogen atom. Results are presented and compared with other methods. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D15.00013: Reference Calculation of Temperature-dependent Behavior of Confined Many-electron Systems Frank E. Harris, Travis Sjostrom Confined many-electron systems at finite temperatures present a major challenge to density functional theory. Very little is known about the free energy behavior over the temperature range of interest, for example, in the study of warm dense matter, and as a result, it is difficult to assess the validity of proposed free energy density functionals. We present preliminary results on a comparatively simple but computationally feasible model, namely thermally occupied Hartree-Fock states for eight one-electron atoms in a box. We discuss the main technical task, evaluation of the required matrix elements, and summarize the results thus far obtained. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D15.00014: Finite-temperature Exchange and Correlation Functionals in Self-Consistent Calculations T. Sjostrom, V.V. Karasiev, S.B. Trickey Density functional theory is being used increasingly to investigate systems at substantial electron temperatures (e.g., warm dense matter, order of 1-10 eV or more). A common approach uses a ground-state (zero-temperature) exchange-correlation (XC) functional with thermal occupancy (Fermi distribution) of the Kohn-Sham states. Various finite-temperature extensions for XC free energy (Sommerfeld expansion, RPA, STLS, classical map) have been proposed, however. All have LDA form. We have implemented several in a pseudopotential code (SIESTA), and also extended them to have the PBE-GGA as the zero-temperature limit. We report equation of state calculations for Li from ambient density and temperature through the warm dense matter regime. Nontrivial variation is found. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D15.00015: Finite Temperature Scaling of the Non-interacting Free Energy Density Functional J.W. Dufty, V.V. Karasiev, S.B. Trickey The non-interacting free energy density functional is central to formulation of orbital-free DFT, yet its construction remains a challenge. Here, exact scaling relations and related bounds are obtained for guidance. First, that free energy is expressed as a functional of one-body reduced density operators that deliver the same average number density. For a one-component Fermion system, this functional has a minimum at the Fermi operator whose external potential assures the chosen number density. This is the formal definition of the non-interacting free energy density functional. The associated entropy and internal energy functionals are identified directly. A unitary transformation generating spatial scaling then determines how these functionals change under density scaling. As an application, these scaling laws are used to obtain inequalities and bounds for functionals at different values of the density and temperature. Relationships to similar recent work at finite temperatures, and the extensive prior zero-temperature results are noted. [Preview Abstract] |
Session D16: Magnetic Imaging and Characterization
Sponsoring Units: GMAGChair: Mi-Young Im, Lawrence Berkeley National Laboratory
Room: D173
Monday, March 21, 2011 2:30PM - 2:42PM |
D16.00001: Single Shot Nanoscale Magnetic Imaging at the Linac Coherent Light Source Benny Wu, Tianhan Wang, Catherine Graves, Diling Zhu, William Schlotter, Joshua Turner, Joachim Stohr, Andreas Scherz One of the major challenges of modern magnetism research is the manipulation and control of the magnetization on ultrafast timescales. Using the unprecedented brightness of the Linac Coherent Light Source (LCLS), we have been able to image the nanoscale magnetic worm domain structures in [Co/Pd] multilayer systems with a single x-ray pulse through x-ray Fourier transform holography on the Co L3 absorption edge. We established the threshold fluences for both non-destructive imaging and sample damage. In combination with the femtosecond pulses of LCLS, single shot coherent imaging will enable the observation of nanoscale magnetization dynamics on the sub-picosecond timescale for problems such as ultrafast demagnetization and all-optical magnetization reversal. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D16.00002: Magnetic soft X-ray microscopy at 10nm spatial resolution Peter Fischer, Weilun Chao, Mi-Young Im, Erik Anderson Magnetic soft X-ray microscopy, which combines high spatial and temporal resolution with elemental specificity by utilizing the specific features of X-ray magnetic circular dichroism effects is a unique and powerful analytical technique to image fast spin dynamics of nanoscale magnetism [1]. The spatial resolution is determined by Fresnel zone plate lenses used as diffractive optics. FZPs are fabricated by state-of-the-art lithography techniques and the challenge is to produce a dense, circular line pattern with a high aspect ratio to achieve high efficiency. Using an overlay technique [2-3], which requires high position accuracy of the e-beam writer, FZPs with 12nm outermost zone width could be fabricated. Implementing this optic at BL 6.1.2 at the ALS in Berkeley CA, we have demonstrated that a 10nm line and space test pattern can be clearly resolved. First magnetic images of a PtCo film with a pronounced perpendicular anisotropy will be presented. Further progress to below 10nm can be anticipated in the near future. \\[4pt] [1] P. Fischer, IEEE Transactions on Magnetics, 44(7) 1900 (2008) \\[0pt] [2] W. Chao, et al. Nature 435, 1210 (2005) \\[0pt] [3] W. Chao, et al., Optics Express 17(20) 17669 (2009) [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D16.00003: New Developments in Magnetic Coherent Diffractive Imaging Ashish Tripathi, SangSoo Kim, Sebastian Dietze, Erik Shipton, Eric Fullerton, Oleg Shpyrko, Ian McNulty Magnetism at the nanoscale is central to understanding emergent complexity in transition metal oxides and engineered rare earth-transition metal multilayers, and in designing new magnetic data storage and spintronic technology. We study magnetism at the nanoscale here using coherent x-ray diffractive imaging (CXDI), which is a technique with potentially wavelength-limited spatial resolution that can probe deeply beyond surfaces, and potentially on ultrafast timescales using new x-ray laser sources. We look at the domain evolution vs. applied magnetic field over the whole hysteresis loop of a ferrimagnetic GdFe multilayer film using x-rays resonant at the Gd M5 edge for domain contrast. We explore complimentary and return point memory by imaging the multilayer over a large field of view. We lastly explore experimental and algorithmic improvements in CXDI using dichroism as contrast mechanism, as well as new opportunities for ultra-fast, single-shot imaging using a variation on the CXDI approach. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D16.00004: Design of a Self-Aligned, 300mK-300K Temperature range Magnetic Force Microscope(MFM) with $<$10nm Resolution Ozgur Karci, Ivan Knez, Hilal Atalan, Rui-Rui Du, Ahmet Oral We present the design of a self-aligned MFM, operating from 300mK to 300 K. Unique `Self-Aligned' design uses cantilever alignment chips and eliminates the alignment procedure and sustains the alignment across the full temperature range. The MFM is very compact, 23.6mm OD, and is adopted to fit into Oxford Instruments Heliox TL system. A fiber interferometer with $\sim $12fm/$\surd $Hz noise level is designed and used to detect cantilever deflection. Stick slip coarse approach mechanism is used to bring the sample in to close proximity of the sample. We can also move the sample in XY directions within 3 mm range, while we measure the position with capacitive encoder with 3$\mu $m accuracy. We can also operate the LT-MFM in high magnetic fields. The microscope has been successfully operated between 300mK-300K and we can achieve $<$10nm resolution. MFM images of 394 Gbpsi Harddisk at 1.5-300K and CoPt Multilayers at 350mK will be presented. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D16.00005: Monolithic diamond probes for nanoscale magnetic imaging using single spins in diamond Patrick Maletinsky, Sungkun Hong, Michael Grinolds, Birgit Hausmann, Ron Walsworth, Mikhail Lukin, Marko Loncar, Amir Yacoby Sensitive detection of magnetic fields at the nanoscale is a challenging problem in biological and physical sciences with great relevance to technological applications. Recent experimental demonstrations have shown the outstanding performance of diamond nitrogen-vacancy (NV) centers in magnetic field sensing [1, 2]. Here, we present a robust experimental realization of a scanning NV-magnetometer that exploits the full coherence properties of the NV-center for magnetic imaging. Our apparatus consists of a combined atomic force (AFM) and optical microscope, where the AFM tip is formed by a high purity diamond nanopillar containing a single NV center at its end. This geometry ensures high spatial resolution, long NV coherence times and waveguiding of NV fluorescence through the pillar, which combine to give maximal magnetic field sensitivity. We demonstrate the performance of our nanoscale magnetometer by imaging various magnetic field sources, including few tens of nm wide domains of a magnetic memory.\\[4pt] [1] Nature 455, 648 (2008)\\[0pt] [2] Nature 455, 644 (2008) [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D16.00006: A stroboscopic approach to combining diamond magnetometry with Atomic Force Microscopy Sungkun Hong, Michael Grinolds, Patrick Maletinsky, Mikhail Lukin, Ronald Walsworth, Amir Yacoby Nitrogen-Vacancy (NV) defect centers in diamond have been recently considered as a promising candidate for sensitive magnetic field detection[1, 2] with nanometric spatial resolution. Most applications requiring high spatial resolution necessitate stabilizing the NV center in close proximity to the sample of interest, which can be accomplished using standard Atomic Force Microscopy (AFM) techniques[2]. However, the fluctuations associated with AFM tip oscillation set limits to both spatial resolution and magnetic field sensitivity[2]. Here we demonstrate a stroboscopic approach that locks the magnetic signal acquisition to a particular position of the tip. Our approach allows us to reach the sensitivity and spatial resolution given by the intrinsic properties of NV centers. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D16.00007: High sensitivity SQUID susceptibility measurements B. Kalisky, J.R. Kirtley, L.C. Qian, B.L. Dwyer, K.A. Moler, J. Ngai, Y. Segal, J. Reiner, F. Walker, C. Ahn, A.M. Hamilton, B. Rutt, A.C. Matin, O.M. Auslaender, D.A. Bonn, R. Liang, W.N. Hardy, J.G. Analytis, J.-H. Chu, I.R. Fisher Scanning superconducting quantum interference device (SQUID) sensors have high sensitivity to magnetic flux ($ 10^{-6}\Phi_{0}/\sqrt{Hz} $) and magnetic moment ($\sim$ 100 electron spins) under reasonable scanning conditions. In addition, a single turn field coil co-centered with the SQUID sensing loop provides excitation for simultaneous measurement of low field susceptibility, with sensitivity of $\chi \sim 10^{-6}$ at a spatial resolution of a few microns. I will present our recent measurements on several systems which exhibit weak susceptometry signals: thin film paramagnetic LaNiO3 that are (hopefully) the precursors to engineered superconducting films; individual magnetotactic bacteria, which are used as MRI contrast agents; and twinned high critical temperature cuprate and pnictide superconducting samples that may experience variations in the superfluid density at the twin boundary. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D16.00008: Magnetic Characterization of Individual Magnetotactic Bacteria Lisa Qian, Beena Kalisky, Amanda Hamilton, Bo Dwyer, A.C. Matin, Kathryn Moler Magnetic nanoparticles 5-50nm in size are of wide interest in the biological and medical fields. In particular, magnetotatic bacteria containing chains of nanoscale magnetite particles show potential for MRI contrast agents and targeting tumors. Magnetic characterization is typically done in large ensembles, where variations in shape and structure cannot be determined and interparticle coupling may cause bulk properties from those of isolated particles. We report the detection and magnetic characterization of individual magnetotatic bacteria using a variable temperature scanning SQUID microscope (SSM). SSM is ideal for this challenge due to its high spin sensitivity, $\sim $100 mu{\_}B/sqrt(Hz). AC and DC modes of operation allow for direct probing of susceptibility and magnetic moment. We will also discuss calculation techniques used to obtain values for the magnetic moment, anisotropy energy and magnetosome chain length of individual bacteria. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D16.00009: Magneto-resistance based First Order Reversal Curve (MR-FORC) analysis of MgO based MTJs Joshua Pomeroy, John Read MR-FORC utilizes partial magneto-resistance hysteresis loops to reveal the coercive and interaction field distributions in the free layer of MgO based magnetic tunnel junctions. The interpretation of the FORC diagrams will be discussed with emphasis on the identification of coercive values, the interlayer exchange properties, the magnetic after-effect and a reversible magnetic instability under study. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D16.00010: Stress and Depth Dependence of Stochastic Processes in the Barkhausen Effect David Jiles, Lukasz Mierczak, Eugene Melikhov Magnetic Barkhausen Noise (MBN) consists of discontinuous stochastic changes in flux density caused by sudden irreversible changes in magnetization as the magnetic field H changes continuously. These changes can be detected at the surface by a magnetometer, which in its simplest form can be in the form of voltage pulses caused in a pickup coil. The amplitude of such pulses has been shown to depend on the microstructure and stress in the material. Propagation of Magnetic Barkhausen emissions in magnetic materials is frequency dependent and therefore information from different depths inside the material is contained in the frequency spectrum of the detected Barkhausen signal at the surface. However the depth dependent information, although present, is difficult to extract from the measurements. Despite this extracting the depth dependent information about material conditions, such as variations in microstructure and/or the presence of residual stress is of a great interest. This work presents a new method for extracting this information from measured MBN signals. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D16.00011: Cluster-deposited high-anisotropy magnetic nanoparticles Balamurugan Balasubramanian, Ralph Skomski, Xingzhong Li, David Sellmyer Magnetic nanoparticles of size less than 10 nm with high magnetocrystalline anisotropy are highly desirable to understand the nanoscale effects on their magnetic properties and create building blocks for modern applications such as ultra-high-density recording media and high-performance permanent magnets. In the present study, monodisperse Co-based nanoparticles with an average particle size of 3 -10 nm, such as YCo$_{5}$ and Co$_{1-x}$Pt$_{x}$ (x $<$ 0.2), were produced using an inert-gas-condensation cluster-deposition system and characterized using XRD, TEM, EDX and SQUID magnetometer. These nanoparticles were directly ordered into high-anisotropy crystal structures during the cluster-aggregation process and exhibit high anisotropic constant of order 10$^{7}$ ergs/cm$^{3}$. Size-effects on the structural and magnetic properties of YCo$_{5}$ and Co$_{x}$Pt$_{1-x}$ nanoparticles will be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D16.00012: Improved evaluation of magnetic nanoparticle susceptibility in hyperthermia, spectroscopy, and imaging Yong Wu, Zhen Yao, Timothy Atherton, Lisa Bauer, Mark Griswold, Robert Brown Magnetic nanoparticles are becoming increasingly important for both diagnosis (through applications such as MRI and magnetic particle imaging (MPI), which comes from the nonlinear magnetization of nanoparticles and provides images with both high spatial and temporal resolutions) as well as for therapy (through focal heating). Thus understanding and modeling of the magnetic susceptibility of the nanoparticles is critical. In hyperthermia calculations, a constant chord susceptibility approximation is used to get a lower bound estimate of the power dissipated. In later work, this approximation has been adapted to examine the decay effects due to nanoparticle relaxation under oscillating magnetic fields in the MPI modality. We provide in the present paper both analytical and numerical work to understand where it is, or is not, appropriate to make this approximation. In addition, we provide a more general approach that does not rely on the above approximations, and may provide new insight to manufacture optimal nanoparticles for applications. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D16.00013: The Spin-Lattice Relaxation of Hyperpolarized $^{89}$Y Complexes Ashish Jindal, Lloyd Lumata, Yixun Xing, Matthew Merritt, Piyu Zhao, Craig Malloy, Dean Sherry, Zoltan Kovacs The low sensitivity of NMR can be overcome by dynamic nuclear polarization (DNP). However, a limitation to the use of hyperpolarized materials is the signal decay due to $T_{1}$ relaxation. Among NMR-active nuclei, $^{89}$Y is potentially valuable in medical imaging because in chelated form, pH-sensitive agents can be developed. $^{89}$Y also offers many attractive features -- 100 \% abundance, a 1/2 spin, and a long $T_{1}$, up to 10 min. Yet, developing new $^{89}$Y complexes with even longer $T_{1}$ values is desirable. Designing such complexes relies upon understanding the mechanism(s) responsible for $T_{1}$ relaxation. We report an approach to hyperpolarized $T_{1}$ measurements that enabled an analysis of relaxation mechanisms by selective deuteration of the ligand backbone, the solvent or both. Hyperpolarized $^{89}$Y -- DTPA, DOTA, EDTA, and deuterated EDTA complexes were studied. Results suggest that substitution of low-gamma nuclei on the ligand backbone as opposed to that of the solvent most effectively increase the $^{89}$Y $T_{1}$. These results are encouraging for in vivo applications as the presence of bound water may not dramatically affect the $T_{1}$. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D16.00014: Magnetic Thermal Hysteresis in Dy nanolayers Ajani Ross, Ali Koymen Magnetic thermal hysteresis is observed when the temperature dependent magnetic properties of a material are reliant on the starting point of the measurement. Trilayer samples of pure Dysprosium (Dy) and Gadolinium (Gd) were grown on substrates of glass (Gd$_{n}$/Dy$_{m}$/Gd$_{n})$, n and m constitutes the number of layers. We observed magnetic thermal hysteresis in these thin films at low values of constant external magnetic field strengths. The temperature is swept from 20K to 300K at constant field, then back (300K to 20K) under the same field. In these temperature sweeps differences in magnetic moment were observed near the low end of the temperature range. Experiments are being done, currently, to confirm the existence of alternate helicity (AH-state) and helical (H-state) states in these trilayer films, which are believed to be the cause of the observed thermal hysteresis according to theoretical calculations. In addition, the temperature dependence of entropy change for these samples is calculated. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D16.00015: Chain formation in a magnetic fluid under the influence magnetic fields. Matt Barrett, Andreas Deschner, Jan Embs, An-Chang Shi, Maikel Rheinstadter We studied the aggregation of magnetic particles into simple chainlike structures in a Cobalt-based magnetic fluid, exposed to external magnetic fields [1]. The length of chain segments in very strong magnetic fields of up to 2 T was measured using small angle neutron scattering in-situ. Although it was predicted that the chain length can be described by a Langevin function, leading to chains several hundred particles in length, we observe a maximum correlation length of $\sim $ 650 {\AA}, or 4-5 particles. To gain insight into the molecular mechanisms involved, our experiments were complemented by Monte Carlo simulations. We observed that the chains which formed increased in length as the magnetic field increased until reaching equilibrium at 4 particles, in excellent agreement with our experimental findings. We speculate that the interplay between the entropy and energy of the system combined with the particular properties of the magnetic dipole-dipole interaction ultimately decide the length of the particle chains. We observed attractive or repulsive interaction between chain segments depending on their relative position. [1] \textit{``Chain formation in a magnetic fluid under the influence of magnetic fields'', }M. Barrett, A. Deschner, J.P. Embs, A.-C. Shi, M.C. Rheinst\"{a}dter, submitted to Physical Review Letters. [Preview Abstract] |
Session D17: Focus Session: Bulk Properties of Complex Oxides - Cobaltites
Sponsoring Units: DMP GMAGChair: John Mitchell, Argonne National Laboratory
Room: D174
Monday, March 21, 2011 2:30PM - 3:06PM |
D17.00001: Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La$_{1-x}$Ba$_{x}$CoO$_{3}$ Invited Speaker: The intricate nature of the magnetic ground state near the insulator-metal transition (IMT) in La$_{1-x}$Ba$_{x}$CoO3 was investigated via neutron scattering. For $x$ less than the critical concentration, x$_{c}\sim $0.22, a commensurate antiferromagnetic (AFM) phase initially appears. As $x$ approaches x$_{c}$, the AFM component continuously weakens while ferromagnetic (FM) order sets in the rhombohedral lattice. The two magnetic phases appear to be growing in different domains and have different ordering temperatures, with the FM order parameter setting in first at higher temperatures while the AFM order parameter occurs at lower temperatures. At x$_{c}$, a spin flip to a new FM state occurs while the crystal transforms to an orthorhombic (Pnma) symmetry. The magnetic Pnma phase coincides with the minimum saturation reached in the resistivity. It is proposed that the orbital overlap in the Pnma phase is the most conducive to charge hopping. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D17.00002: Phase Control of Magnetic Order in (Y,Lu)BaCo$_{4}$O$_{7}$ John Mitchell, Hong Zheng, Sevda Avci, Laurent Chapon, Dmitry Khalyavin, Omar Chmaissem, Ashfia Huq The RBaCo$_{4}$O$_{7}$ (R=Ca, Y, Tb-Lu) provides a novel topology for studying geometric frustration, in which face-sharing tetrahedra of magnetic ions link to form trigonal bipyramids on a Kagom\'{e} lattice. Here we describe the structural and magnetic behavior of the Lu member and the solid solution joining Lu to Y as a chemical means to tune between magnetically ordered and disordered ground states. Mean-field models of the generic magnetic phase diagram of RBaCo$_{4}$O$_{7}$ determined recently by our group (D. D. Khalyavin et al. Physical Review B 82, 094401 (2010)) show a variety of magnetic states as a function of two exchange parameters: J$_{ab}$ and J$_{c}$, where J$_{ab}$ links Co ions in the Kagom\'{e} planes and J$_{c}$ links Co ions from the Kagome plane to the interleaving triangular layer. Experimentally, we find that YBaCo$_{4}$O$_{7}$ has a long-range ordered antiferromagnetic ground state, while LuBaCo$_{4}$O$_{7}$ appears to be disordered above 2 K. We use the solid solution to interpolate between these endpoints and discuss these results with respect to the mean-field phase diagram. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D17.00003: Determination of magnetic moments and orbital occupancy in the spin chain compound Ca$_3$Co$_2$O$_6$ Jonathan Duffy, Matthew Butchers, Jonathan Taylor, Stephen Dugdale, Tom Haynes, Stefano Agrestini, Martin Lees The one-dimensional cobaltate Ca$_3$Co$_2$O$_6$ exhibits a number of intriguing phenomena, including several metamagnetic steps as a function of applied magnetic field. Although it has attracted a considerable amount of research, the origin of the magnetism has not yet been fully determined. We report a measurement of the spin density in Ca$_3$Co$_2$O$_6$ using magnetic Compton scattering. The bulk spin moment was determined to be 3.78 $\pm$ 0.05$\mu_B$ at 7 T, confirming the existence of a large unquenched Co orbital moment of 1.4 $\pm$ 0.1 $\mu_B$. In combination with molecular orbital calculations, the results reveal that double occupation of the d$_{x^2-y^2,xy}$ orbital is responsible for the observed large unquenched orbital moment. Fitting the model to the experimental data shows that there is an induced oxygen moment of 0.8 $\pm$ 0.1 $\mu_B$. Unexpectedly, further comparison with KKR-SPA electronic structure calculations strongly indicates the existence of a Fermi surface. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D17.00004: Theory of the carrier concentration-dependent behavior in layered cobaltates Hongtao Li, R. Torsten Clay, Sumit Mazumdar Layered cobaltates -- anhydrous Na$_x$CoO$_2$, Li$_x$CoO$_2$ and the ``misfit'' cobaltates [Bi$_2$A$_2$O$_4$] $\cdot$ [CoO$_2$]$_m$, where A = Ba, Sr or Ca -- have attracted wide attention for their 2D layered structure and metallicity (both reminescent of 2D cuprates), and the tunability of the carrier concentration over a wide range. The Co ions form a 2D triangular lattice, and their formal charge in Na$_x$CoO$_2$ and Li$_x$CoO$_2$ can be tuned from Co$^{3+}$ at $x=1$ to Co$^{4+}$ at $x=0$. Charge carriers in all cases are holes, with the carrier concentration given by the fraction of Co-ions that are in the S = 1/2 Co$^{4+}$ state. Experiments have indicated remarkable carrier concentration dependent magnetic susceptibility and thermoelectric power that remains unexplained to date. Specifically, all three systems show weakly correlated behavior at small nonzero $x$ (large carrier concentration), and strongly correlated behavior at large $x$ (small carrier concentration). In this talk we give clear theoretical explanation of the observed carrier concentration dependence within an $a_{1g}$-only one-band extended Hubbard Hamiltonian. The key to understanding the $x$-dependence is to have realistic finite on-site correlation $U$ and significant intersite Coulomb interaction $V$. We present exact numerical results for triangular lattices upto 20 sites, and make detailed comparisons to experiments. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D17.00005: Probing the Na atomic order in Na$_{x}$CoO$_{2}$, x=0.67 and 0.71 by NMR spectroscopy Ben-Li Young, P.-Y. Chu, J.Y. Juang, G.J. Shu, F.-T. Huang, M.W. Chu, F.C. Chou The sodium cobaltate Na$_{x}$CoO$_{2}$ has a layered structure, consisting of alternating triangular CoO$_{2}$ and Na planes. Evidences of Na atomic ordering have been reported at certain Na contents by different diffraction experiments. The Co magnetism, strongly influenced by the Na ordering, gives a unique phase diagram in Na$_{x}$CoO$_{2}$. In order to investigate the Na ordering and the Co magnetism, we conducted $^{23}$Na and $^{59}$Co NMR experiments in single crystals Na$_{x}$CoO$_{2}$ for x=0.67 and 0.71. We found that Na$_{0.67}$CoO$_{2}$ does not have well-defined Na structural order. However, the oxygen slightly-deficient sample Na$_{0.67}$CoO$_{1.98}$ shows a superstructure, as evidenced by the narrow and well-resolved NMR spectrum. As for Na$_{0.71}$CoO$_{2}$, Na ordering is also observed. We have tried to solve the Na ordering pattern from our NMR spectra. The results will be discussed and be compared with the existing structural models. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D17.00006: Synthesis and anisotropic magnetic and transport properties of cubic SrCoO$_{3}$ single crystal Youwen Long, Yoshio Kaneko, Shintaro Ishiwata, Yasujiro Taguchi, Yoshinori Tokura Solid state oxides containing transition metals with unusually high valence states exhibit interesting physical properties. However, due to the unstableness of these high valence states, high pressure is often needed to stabilize such high valence states. We were successful in growing a large-size SrCoO$_{3}$ single crystal by using high-pressure technique. This material shows good metallic behavior with high ferromagnetic Curie temperature about 305 K, and the easy magnetization axis is $<$111$>$ direction. The spin moment of Co$^{4+}$ ion measured at 2 K and 7 T is about 2.50 $\mu _{B}$, suggesting an intermediate spin configuration as predicted by theoretical calculations. Although SrCoO$_{3}$ has a highly symmetric cubic crystal structure (Pm-3m), it exhibits significant anisotropic magnetoresistance at low temperatures. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D17.00007: The origin of the temperature dependence of the magnetic susceptibility and the large thermoelectric power in metallic layered cobaltites Iv\'an Gonz\'alez, Camilo X. Quintela, Manuel Ba\~nobre-L\'opez, Francisco Rivadulla We perform detailed measurements of the thermoelectric power and the static magnetic susceptibility on metallic Na$_{x}$CoO${_2}$ and Ca$_{3}$Co$_{4}$O$_{9}$, as representatives of layered Co oxides with a triangular Co-lattice. We propose that the observed large thermoelectric power and the Curie-Weiss temperature dependence of the susceptibility have a common origin related to metallic character of these compounds. Thermoelectric power measurements are compared to Boltzmann transport theory calculations. The Curie-Weiss behaviour of the susceptibility is explained within the framework of the self-consistent renormalization theory for spin fluctuations proposed by Moriya for itinerant magnets. Our results clarify the apparent duality in the localised/itinerant behaviour of the electron spin in these systems and provide a unifying view on the physics of metallic layered cobaltites. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D17.00008: TEM imaging and in-situ EELS study of multiple ferroic transitions in LaCoO3 Tiantian Yuan, Robert Klie, Nina Orlovskaya The perovskite oxide LaCoO3 has attracted increasing attention due to its reported room-temperature ferroelastic behavior, and a ferromagnetic transition observed at around 90K in epitaxially strained thin films. To advance our understanding of these nanoscale properties of LaCoO3, a combination of analytical TEM techniques, including the atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy in combination with in-situ cooling experiments have been used to study the relationship of the multiple ferroic transitions in bulk LaCoO3. In particular, we find that the bulk LaCoO3 samples compressed above the coercive stress exhibit ferromagnetic transitions, similar to the ferromagnetic behavior of the epitaxially strained LaCoO3 thin film. While the bulk LaCoO3 samples compressed below the coercive stress do not exhibit any ferromagnetic transitions down to 5K. We will further correlate this ferromagnetic property to the ferroelastic property of LaCoO3, and show how the strain of the LaCoO3 affects the ferromagnetic property of the sample. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D17.00009: Magnetocaloric effect across the coupled structural/magnetocrystalline anisotropy transition in Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ (x=0.3-0.5) N.S. Bingham, M.H. Phan, H. Srikanth, M.A. Torija, C. Leighton Large magnetocaloric effects (MCE) are often observed in materials exhibiting a first order magnetic transition coupled with a crystal structure change. Since the magnetic and structural changes are coupled, it is difficult to decouple the structural entropy contribution from the magnetic entropy contribution to the total MCE. Therefore a clear understanding of the structural entropy change and its field dependence in such materials is lacking. A recent study revealed that Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ (x$>$0.35) undergo a coupled structural/magnetocrystalline anisotropy transition at T$_{A}$, in addition to the paramagnetic-ferromagnetic transition at T$_{C}$. Since the structural change at T$_{A}$ in PSCO is not associated with any magnetic transition, it is an excellent system for studying the structural entropy change and its contribution to the MCE. We report systematic studies of the MCE in Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ (x=0.3, 0.35, 0.4, 0.5) compounds. The results show significant entropy change at T$_{A}$, whose magnitude can be tuned by controlling the magnetocrystalline anisotropy. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D17.00010: Magneto-electronic Phase Separation in Pr$_{1-x}$Ca$_{x}$CoO$_{3-\delta }$: Intrinsic Exchange Spring Magnetism S. El-Khatib, S. Bose, C. He, J. Kuplic, M. Laver, J.A. Borchers , Q. Huang, J.W. Lynn, J.F. Mitchell, C. Leighton We present a neutron diffraction, small-angle scattering, and magnetometry study of the narrow bandwidth perovskite cobaltite Pr$_{1-x}$Ca$_{x}$CoO$_{3}$, demonstrating an unusual form of magneto-electronic phase separation where long-range ordered ferromagnetism coexists spatially with short-range ferromagnetism. The two phases have very different coercivities and, remarkably, are strongly exchange coupled. The electronic phase separation thus leads to spontaneous formation of a hard-soft nanocomposite, exhibiting prototypical exchange-spring behavior in the absence of chemical interfaces. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D17.00011: The nature of magneto-elastic coupling with the isovalent substitution at the B-site in LaCo$_{1-y}$B$_{y}$O$_{3}$ Juan Yu, Despina Louca The influence of magnetic ion doping on the interplay of the lattice~with magnetism in LaCo$_{1-y}$B$_{y}$O$_{3}$ (B = Ni or Fe, y= 0.1, 0.4) has been investigated via neutron scattering techniques. The substitution of~either Ni$^{3+}$ (3d$^{7})$ or Fe$^{3+}$ (3d$^{5})$ does not alter the crystal symmetry~which remains rhombohedral (R-3c) at all temperatures. With doping, the~degree of cooperative octahedral rotations about the (111) axis~increases, but it is only with Ni that such a rotation is accompanied by~a compression along the trigonal axis. The observed crystal distortion~is invoked to break the degeneracy of the magnetic Co$^{3+}$ ions, while maintaining the Co-O bonds at a constant length. The absence of two~distinct types of Co-O bond lengths in the local structure with the~substitution of Fe$^{3+}$ or Ni$^{3+}$ for Co$^{3+}$ (3d$^{6})$ is indicative that, unlike~in the hole doped cobaltites with Ba$^{2+}$ or Sr$^{2+}$ previously studied, the~intermediate spin state of Co is either absent or suppressed. This~leaves us to question the origin of the magnetic interactions, which~most likely arises from a high-spin state of the Co ion. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D17.00012: First Principles Study of Misfit-Layered Calcium Cobaltite Using Fibonacci Approximants Alejandro Rebola, Robert Klie, Serdar Ogut Cobalt oxides have been the focus of many recent studies due to the wide variety of electrical, magnetic, structural and thermoelectrical properties they exhibit. In this talk we present a first-principles study on the misfit-layered Ca$_{3}$Co$_{4}$O$_{9}$. This material can be more accurately described as [Ca$_{2}$CoO$_{3}$][CoO$_{2}$]$_{1.61}$ and consists of two substructures that are incommensurate to each other. Taking into account that the composition ratio is very close to the golden mean (1.6180...), and that this number can be obtained as the limit of the sequence of the ratios of consecutive Fibonacci numbers: 3/2, 5/3, 13/8, {\ldots}, F(n+1)/F(n)..., we model the structure by using supercells of composition [Ca$_{2}$CoO$_{3}$]$_{F(n)}$[CoO$_{2}$]$_{F(n+1) }$. In this way, structural, electronic, transport and lattice properties can be calculated as a function of cell size. We compute the atomic and electronic structures, defect energetics of a series of rational approximants to Ca$_{3}$Co$_{4}$O$_{9 }$within the framework of DFT+U, and examine the convergence of such properties with respect to size, thus allowing us to identify the most realistic and smallest structural model for this misfit-layered compound. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D17.00013: Complex ferrimagnetic state induced by zigzag oxygen-vacancy stripes in Sr$_3$YCo$_4$O$_{10.72}$ D.D. Khalyavin, L.C. Chapon, E. Suard, J.E. Parker, S.P. Thompson, A.A. Yaremchenko, V.V. Kharton The nature of high temperature ferromagnetic behaviour in Sr$_3 $YCo$_4$O$_{10+\delta}$ perovskite has been studied by neutron powder diffraction supplemented with synchrotron X-ray diffraction measurements. The present analysis of the magnetic structure takes into account the complex superstructure formed by oxygen vacancy ordering. These vacancies create zigzag strips in the oxygen-deficient CoO$_{4+\delta}$ layers providing three distinct coordinations for Co ions. The values of the ordered moments were found to be essentially different for the distinct coordinated units and clearly correlate with the coordination number. The symmetry of the superstructure in conjunction with strong antiferromagnetic interactions between neighbour spins results in a net moment whose origin has been the subject of considerable debates. [Preview Abstract] |
Session D18: Focus Session: Low D/Frustrated Magnetism - Kagome Lattices
Sponsoring Units: GMAG DMPChair: Joel Helton, NIST Center for Neutron Research
Room: D172
Monday, March 21, 2011 2:30PM - 2:42PM |
D18.00001: Highly frustrated magnets: a class of emergent gauge systems Michael Lawler Condensed matter exhibit a wide variety of exotic emergent phenomena, such as the topological order in the fractional quantum Hall effect, and the ``cooperative paramagnetic'' response of geometrically frustrated magnets. The classical and quantum dynamics of spins exploring the large configuration space associated with the latter are not well understood analytically. I consider the constrained classical Hamiltonian dynamics of spins exploring such a configuration space as a starting point from which a complete classical and semi-classical description may be reached. The method I employ, introduced by Dirac [1] and now forms the basis of gauge theory, applies to any frustrated system constrained to a continuous set of configurations. Remarkably, in the kagome lattice model I consider as an example, these dynamics are similar to the ``topological'' (Chern-Simons) dynamics of electrons in the fractional quantum Hall effect and have non-locally entangled edge modes as the only degrees of freedom. In principle, these edge states may be found in any kagome-like Heisenberg antiferromagnets such as Herbertsmithite, the Jarosites, SrCr$_{8-x}$G$_{4+x}$O$_{19}$ and Na$_4$Ir$_3$O$_8$. \\[4pt] [1] Dirac, P. A. M. {\it Generalized hamiltonian dynamics}. Can. J. of Math. {\bf 2}, 129-148 (1950) [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D18.00002: Predictions for the ARPES spectral function of kagome antiferromagnetic insulators Sumiran Pujari, Michael J. Lawler There are now a number of spin liquid candidate materials possibly with exotic spin-1/2 ``spinon'' excitations. Motivation by these discoveries, we consider the scaling properties of the hole spectral function for the frustrated Kagome Heisenberg antiferromagnet assuming Dirac Spin Liquid(DSL) ground state proposed for Herbertsmithite $[2]$. We predict a sublinear in energy power law dependence of the ARPES spectral function at certain wave vectors. Using Renormalization group techniques, we show how (gauge) fluctuations of the DSL mean field give an anomalous exponent to spinons$[2]$ and no anomalous exponent to holons thereby leading to the sublinear power law. If this behavior is observed in experiments, they would provide strong evidence for the existence of spinons in highly frustrated magnets.\footnote{Y. Ran et al, Phys. Rev. Lett. 98, 117205 (2007)} [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D18.00003: Exact Chiral Spin Liquid with Stable Spin Fermi Surface on the Kagome Lattice Victor Chua, Hong Yao, Gregory Fiete We study an exactly solvable quantum spin model of Kitaev type on the kagome lattice. We find a rich phase diagram which includes a topological (gapped) chiral spin liquid with gapless chiral edge states, and a gapless chiral spin liquid phase with a spin Fermi surface. The ground state of the current model contains an odd number of electrons per unit cell which qualitatively distinguishes it from previously studied exactly solvable models with a spin Fermi surface. Moreover, we show that the spin Fermi surface is stable against weak perturbations. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D18.00004: Quantum criticality in the kagome staircase system Co$_3$V$_2$O$_8$ in transverse magnetic field K. Fritsch, K.C. Rule, K.A. Ross, Y. Qiu, J.R.D. Copley, K. Kiefer, K. Habicht, H.A. Dabkowska, B.D. Gaulin Co$_3$V$_2$O$_8$ (CVO) belongs to the kagome staircase family of orthorhombic materials in which Ising-like Co$^{2+}$, S=3/2 magnetic moments decorate a stacked and buckled version of the two-dimensional kagome lattice. In zero applied magnetic field, this material displays a complex series of five different magnetically ordered phases below $\sim$11 K which culminate in a simple ferromagnetic state below Tc$\sim$6 K. Previous inelastic neutron scattering work[1] on this quasi-two-dimensional system showed that the exchange interactions within the kagome planes are rather weak (J$\sim$1.25 meV), making this system an ideal candidate for the study of transverse field-induced quantum critical phenomena as have been observed in LiHoF$_4$ or recently in CoNb$_2$O$_6$. We have investigated the phase diagram of CVO with the transverse field applied along the stacking direction using magnetization as well as single crystal neutron scattering techniques. We will discuss how the ground state magnetic structure and spin dynamics of CVO evolve upon tuning the transverse magnetic field through the quantum critical point near Hc$\sim$6 T. [1] M. Ramazanoglu et al., PRB 79, 024417 (2009). [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D18.00005: Herbertsmithite: a slightly less than ideal kagom\'e antiferromagnet Yuan Wan, Zhihao Hao, Oleg Tchernyshyov Herbertsmithite, one of the best realizations of the Heisenberg antiferromagnet on kagom\'e, shows no signs of magnetic order down to the lowest accessible temperatures and likely possesses a quantum-disordered ground state. A recent site-specific X-ray diffraction experiment [1] shows deviations from the ideal model, most notably in the form of excess copper spins residing outside of kagom\'e planes. We study the influence of these impurities on the magnetic properties of herbertsmithite, focusing on locally induced Jahn-Teller distortions in kagom\'e planes and on exchange interactions between the excess spins.\\[4pt][1] Danna E. Freedman \textit{et al}, J. Am. Chem. Soc. {\bf132}, 16185 (2010). [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D18.00006: Magnetic studies of S=1/2 kagom\'{e} lattice single crystals Tianheng Han, Joel Helton, Andrea Prodi, Claudio Mazzoli, Peter Muller, Deepak Singh, Jose Rodriguez, Collin Broholm, Daniel Nocera, Shaoyan Chu, Young Lee The Zn-paratacamite mineral family, Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$Cl$_{2}$, presents a promising system for studies of frustrated magnetism on a S=1/2 kagom\'{e} lattice. Here we report a new synthesis method, by which high quality single crystals of Zn-paratacamite can be produced. The x = 1 mineral herbertsmithite is a spin-liquid candidate. This compound displays a magnetic susceptibility that is anisotropic at high temperatures. A small anisotropy is observed in specific heat measurements with magnetic field applied in-plane and normal-to-plane. Inelastic neutron scattering has been performed and we will discuss the observed structure factor in the context of various theoretical expectations. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D18.00007: Terahertz conductivity of a metal-organic hybrid Kagom\'e lattice: A candidate spin liquid Daniel Pilon, Alex Frenzel, Danna Freedman, Daniel Nocera, Nuh Gedik Recent theoretical studies predict that the optical conductivity of a spin liquid should exhibit power law behavior in frequency at low temperatures. Materials with the Kagom\'e structure are the most promising candidates for observing spin liquid behavior due to their high degree of magnetic frustration. We have measured the optical conductivity of Cu(1, 3-bdc), a spin-1/2 Kagom\'e lattice material, in the range 0.5 - 2 THz. We compare these results to the theoretical predictions and comment on the implications for the existence of a spin liquid state in this material. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D18.00008: Valence-bond crystal in the extended Kagom\'e spin-$1/2$ quantum Heisenberg antiferromagnet: A variational Monte Carlo approach Federico Becca, Yasir Iqbal, Didier Poilblanc The highly-frustrated spin-$1/2$ quantum Heisenberg model with both nearest ($J_1$) and next-nearest ($J_2$) neighbor exchange interactions is revisited by using an extended variational space of projected wave functions that are optimized with state-of-the-art methods. Competition between modulated valence-bond crystals (VBC) proposed in the literature and the Dirac spin liquid (DSL) is investigated. We find that the addition of a {\it small} ferromagnetic next-nearest-neighbor exchange coupling $|J_2|>0.09 J_1$ leads to stabilization of a 36-site unit cell VBC, although the DSL remains a local minimum of the variational parameter landscape. This implies that the VBC is not trivially connected to the DSL: instead it possesses a non-trivial flux pattern and large dimerization. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D18.00009: Magnetization ramp of the Kagome lattice antiferromagnet Toru Sakai, Hiroki Nakano Magnetization process of the S=1/2 isotropic Heisenberg antiferromagnet on the Kagome lattice is studied. Data from numerical-diagonalization method up to 39-spin systems, are reexamined from the viewpoint of the derivative of the magnetization with respect to the magnetic field. We find that the behavior of the derivative around the 1/3 height of the magnetization saturation is quite different from the cases of typical magnetization plateaux. The magnetization process of the Kagome-lattice antiferromagnet reveals a new phenomena, which we call the ``magnetization ramp.'' We also compare it with the 1/3 magnetization plateau of the triangular antiferromagnet. \\[4pt] [1] H. Nakano and T. Sakai: J. Phys. Soc. Jpn. 79 (2010) 053707, arXiv:1004.2528. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:54PM |
D18.00010: Spin dynamics in the hyperkagome compound Gd$_{3}$Ga$_{5}$O$_{12}$ Invited Speaker: We present the first neutron inelastic scattering results on the magnetic state of the frustrated hyperkagome compound $\rm Gd_3Ga_5O_{12}$ (GGG) at low temperatures and in applied magnetic field. Our neutron scattering studies reveal a remarkable range of timescales. Short-range spatial correlations appear static within the instrumental resolution (50~$\mu$eV). Three distinct inelastic modes are found at 0.04(1), 0.12(2) an 0.58(3)~meV at 0.06~K. The application of a magnetic field up to 2.5~tesla reveals disparate behavior of the magnetic excitations. In zero applied field, the lowest and highest energy excitations show spatial dependencies indicative of dimerized short-range antiferromagnetic correlations that survive to high temperatures, comparable to the nearest neighbor exchange interactions. Our results suggest that the ground state of a three dimensional hyperkagome compound differs distinctly from its frustrated counterparts on a pyrochlore lattice and reveal a juxtaposition of cooperative paramagnetism and strong dimerized coupling. These results are surprising since GGG is often classified as a strongly frustrated system with a manifold of connected states for which one would expect a continuum of gapless excitations. \\[4pt] In collaboration with: Pascale Deen, Institut Laue Langevin; G. Balakrishnan, Department of Physics, University of Warwick; B.D. Rainford, Department of Physics and Astronomy, Southampton University; C. Ritter, Institut Laue-Langevin; L. Capogna, Istituto Officina dei Materiali, IOM-CNR; H. Mutka and T. Fennell, Institut Laue-Langevin. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D18.00011: Effects of doping on the geometrically frustrated Heisenberg antiferromagnet Gadolinium Gallium Garnet D.M. Silevitch, M.A. Schmidt, S. Ghosh, G. Aeppli, T.F. Rosenbaum Geometric frustration in the Heisenberg antiferromagnet Gadolinium Gallium Garnet (GGG) gives rise to a set of quantum protectorates where clusters of spins decouple from the overall spin liquid state. At 110 mK, there is a partial transition to an ordered AF state. Here, we examine the effect of lightly doping GGG with Nd ions, which partially alleviates the underlying frustration. We examine the size and binding energy of the spin clusters as a function of doping and temperature, and also characterize the suppression of the Neel temperature as the dopant concentration is increased. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D18.00012: Electron magnetic resonance studies of the Pr$_{3}$Ga$_{5}$SiO$_{14}$ and Nd$_{3}$Ga$_{5}$SiO$_{14}$ kag\'{o}me systems Sanhita Ghosh, Saiti Datta, Haidong Zhou, Michael Hoch, Stephen Hill In recent years, there has been considerable interest in materials exhibiting magnetic frustration due to their novel ground state properties. Pr$_{3}$Ga$_{5}$SiO$_{14}$ (PGS) and Nd$_{3}$Ga$_{5}$SiO$_{14}$ (NGS) have trigonal crystal structures in which the rare earth ions are arranged in corner sharing triangles to form a distorted kagom\'{e} lattice. We report high frequency electron magnetic resonance (EMR) measurements on single crystals of NGS and PGS in order to ascertain the nature of their ground states. Both compounds exhibit extremely rich EMR spectra at low temperatures, with a large number of sharp peaks. For each frequency investigated, the peak positions display a strong, systematic dependence on the temperature. However, the usual paramagnetic resonance frequency/field variation is not observed, with the pattern of peaks varying dramatically from one frequency to the next. We thus conclude that the observed spectra correspond to collective excitations associated with finite size ordered clusters that persist on EMR time scales. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D18.00013: Valence Bond Crystal on the Hyperkagome Antiferromagnet Emil Bergholtz, Andreas Lauchli, Roderich Moessner We describe our recent work that indicates that the ground state of the antiferromagnetic spin-1/2 Heisenberg model on the highly frustrated, three-dimensional, hyper-kagome lattice is a valence bond crystal (VBC). Performing a series expansion around an arbitrary dimer covering on the hyper-kagome we find that a ground state with a huge (72 site) unit cell is selected by the quantum fluctuations. The regularity and favorable energetics of our series expansion establishes the VBC as a serious contender to the earlier spin liquid proposals. We find that the ground state supports many, very low lying, excitations in the singlet sector and that the low energy spinful excitations (spinons and triplons) are effectively confined to various emergent lower-dimensional structures. If applicable to the recently studied sodium iridate compound, Na$_4$Ir$_3$O$_8$, this scenario has interesting observable implications, such as spatially anisotropic neutron scattering spectra and possibly multiple finite temperature signatures in the magnetic specific heat due to a multi-step breaking of discrete symmetries. Most saliently, here---as for several proposed states for analogous kagome and pyrochlore magnets---one might expect a clearly resolved Ising transition at relatively high temperature. \\[4pt] Ref: E.J. Bergholtz, A.M. L\"auchli and R. Moessner, Phys. Rev. Lett., in press (2010) [arXiv:1010.1345] [Preview Abstract] |
Session D19: Low Dimensional Magnetism and Spin Tunneling
Sponsoring Units: GMAGChair: Enrique Del Barco, University of Central Florida
Room: D170
Monday, March 21, 2011 2:30PM - 3:06PM |
D19.00001: GMAG Student Dissertation Award Talk: Effects of Nanoscale Structure on the Magnetism and Transport Properties of Chromium and Chromium-Aluminum Alloys Invited Speaker: Bulk Cr has an incommensurate spin density wave (ISDW) due to nesting of the Fermi surface which is easily disrupted by perturbation. Thus, the properties of Cr are sensitive to small amounts of dopant atoms, application of pressure, etc. which has been well studied in bulk. We have taken advantage of thin film growth techniques to study the effects of nanoscale structure on the properties of Cr and Cr1-xAlx alloys. The first part of my talk will discuss our research on polycrystalline Cr thin films, where variables such as strain and disorder crucially affect the SDW. We find that Cr thin films can be ISDW like in bulk Cr, or transition to commensurate SDW (CSDW) or mixed depending on deposition conditions and the resulting thin film microstructure. The transport properties are also strongly affected, as quasilocal defect states inside the SDW gap cause resonant scattering. This results in anomalous features such as residual resistivity ranging between 3 and 400 $\mu$O -cm and significant resistivity minima at low temperature. Further evidence of quasilocal states inside the SDW gap is seen in the enhanced electronic density of states (DOS) from specific heat measurements of Cr thin films. The second part of my talk will discuss Cr1-xAlx alloys. The addition of Al to Cr causes the ISDW to transition to CSDW for x = 0.03. Cr1-xAlx also exhibits previously unexplained semiconducting behavior for x = 0.15-0.30. I will discuss our ongoing theoretical and experimental research which suggests that a chemically ordered, rhombohedrally distorted Cr3Al structure occurs in nanosized domains and causes a hybridization gap on part of the Fermi surface. The CSDW causes a gap on another part of the Fermi surface, so that the semiconducting behavior can be explained by a combination of structural and magnetic affects. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D19.00002: Suppression of Macrospin Tunneling by Nanomechanical Interference Lorien Hayden, Alexey Kovalev, Gerrit Bauer, Yaroslav Tserkovnyak This research considers the quantum dynamics of a nanomechanical resonator coupled to a macrospin of a magnetic nanoparticle. Suppression of macrospin tunneling by nanomechanical interference is demonstrated. By approximating the macrospin molecule as a two level system, the results are extended to the magnetopolariton splitting between resonantly coupled Fock states in which are observed similar interference patterns. The mentioned interference effects should be observable in a single molecule magnet bridged between two leads. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D19.00003: Spin Tunneling in a Rotating Nanomagnet Michael O'Keeffe, Eugene Chudnovsky We study spin tunneling in a magnetic nanoparticle with biaxial anisotropy that is free to rotate about its anisotropy axis. Exact instanton of the coupled equations of motion is found that connects degenerate classical energy minima. We show that mechanical freedom of the particle renormalizes magnetic anisotropy and increases the tunnel splitting. \\[4pt] M. F. O'Keeffe and E. M. Chudnovsky, cond-mat, arXiv:1011.3134. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D19.00004: Theory of Raman Scattering in One-Dimensional Quantum Magnets Masahiro Sato, Hosho Katsura, Naoto Nagaosa Raman scattering is one of the powerful tools to study the quantum dynamics of the spin systems, and has been studied for a long term. Conventionally, Raman scattering spectra have been interpreted in terms of the two-magnon processes, from which the exchange coupling can be estimated. However, it is known that the magnon is not a good elementary excitation in low- dimensional quantum spin systems, especially in 1D, and the theoretical studies on the Raman processes in 1D have not been developed compared with those for other electromagnetic processes such as NMR and ESR. Here we have developed a theory for Raman scattering of 1D quantum magnets. We show that Raman spectrum can detect some interesting excitations such as spinon pairs, solitons, etc, depending on the additional interactions to the conventional Heisenberg one and polarization direction of external electromagnetic wave. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D19.00005: Surface ferri-magnetism in some antiferromagnetic materials Yongbin Lee, Bruce Harmon In an antiferromagnetic material with inversion symmetry the electronic energy bands are spin degenerate because of time reversal symmetry. However, at the surface the inversion symmetry is broken, which opens the possibility of breaking the spin degeneracy and inducing a significant net moment on the surface. As an example of this spin degeneracy breaking, we discuss the electronic structure of antiferromagnetically ordered BaMn$_{2}$As$_{2}$. Unlike the bulk bands, its surface bands can individually possess a net spin polarization. Also the bulk bands in this material have a gap, however the calculated spin polarized surface states cross the Fermi level. Our calculations show that an applied field perpendicular to the surface and along the spin axis induces a significant net surface magnetization, which does not extend significantly into the bulk. - Work at the Ames Laboratory was supported by the US DOE, Basic Energy Science, under contract No. DE-AC02-07CH11358. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D19.00006: Spin-orbit interactionsand magnetic field in antiferromagnetic triangular molecular magnets J.F. Nossa, M.F. Islam, C.M. Canali, M.R. Pederson Frustrated triangular molecular magnets such as Cu$_3$ are characterized by a doubly generate S=1/2 ground-state (GS) with opposite chirality. Recently it has been proposed theoretically [1] and verified by ab-initio calculations [2] that an external electric field can efficiently couple these two chiral spin states, even in the absence of spin-orbit interaction (SOI). SOIs are nevertheless important, since they introduce a splitting in the GS manifold. In this talk we will discuss different schemes on how to evaluate within spin density functional theory the effect of the SOIs on the chiral states. The connection between SOI and the Dzyalozhinsky-Moriya interaction will be discussed. We will also present calculations of the energy dependence on an external magnetic field, whose presence is important to achieve full control of the spin-electric coupling within the manifold of the GS chiral doublets. \\[4pt] [1] M. Trif et. al. Phys. Rev. B 82, 045429 (2010) and M. Trif et. al. Phys. Rev. Lett. 101, 217201 (2008) \\[0pt] [2] F. Islam et. al. Phys. Rev. B 82, 155446 (2010) [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D19.00007: Real-space imaging of Kondo screening in a two-dimensional Kondo lattice Ying Jiang, Yanning Zhang, Juexian Cao, Ruqian Wu, Wilson Ho Kondo lattice systems exhibit nonuniversal many-body behaviors, mainly resulting from the competition and interplay between onsite Kondo screening and intersite coupling. In reduced spatial dimensions, the many-body correlation effects are expected be to more relevant. We report the realization of a two-dimensional (2D) Kondo lattice formed by self-assembled triplet oxygen molecules on the Au (110)-1$\times $2 reconstructed surface. By mapping the Kondo resonance in the 2D O$_{2}$ lattice with a scanning tunneling microscope, the interplay between the intermolecule coupling and the onsite Kondo effect was manifested as the unexpected coexistence of both local and nonlocal Kondo screening at the atomic level. While the latter provides evidence of the collective deconfinement of magnetization in Au, the former shows local hybridization between the Kondo clouds of nearest-neighbor O$_{2}$ molecules, as revealed by density functional calculations. These findings may assist in our understanding of the unusual electronic properties in various strongly correlated electron systems, such as heavy fermion compounds and Kondo insulators. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D19.00008: Single oxygen and CO molecules on Au(110): what can we learn? Yanning Zhang, Ying Jiang, Wilson Ho, Ruqian Wu Studies of O$_2$ and CO molecules on the reconstructed Au (110) surface are crucial for the understanding of unusually high activity of Au nanostructures that are useful in heterogeneous catalysis. Moreover, the O$_2$/Au system is ideal to probe the Kondo effect using surface science techniques, due to the persisting magnetization of triplet O$_2$ in physisorption systems. In this work, scanning tunneling microscope (STM) measurements and density functional calculations were performed to investigate the adsorption geometries and physical properties of single O$_2$ and CO molecules on Au(110). The calculated atomic structures and vibration frequencies are comparable favorably with our STM experimental results at low temperature, allowing efficient establishment of structural models. Interestingly, the O$_2$ molecule takes a defect site over the Au row, with a tilted geometry. The magnetic moment of O$_2$ is still as large as ~1.9 $\mu_B$, which furthermore induces a pronounced Kondo resonance in a large spatial region. The cloud of Kondo enhancement was found to closely follow the distribution of the calculated spin density at the fermi level, a correlation which is important for the understanding of Kondo effect in molecular systems. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D19.00009: Universal chiral magnetism in low-density 2D Kondo lattice model Dmitry Solenov, Dmitry Mozyrsky, Ivar Martin We demonstrate that (quasi-)two-dimensional systems comprised of localized moments and itinerant electrons form non-coplanar magnetic crystal states when the Kondo coupling energy is smaller than the chemical potential. These states arise for fully isotropic local exchange coupling between electrons and magnetic ions and do not require a spin-orbit (Dzyaloshinskii-Moriya) interaction or magnetic field. We give an analytical argument on instability of simple co-planar states and show that the states with non-zero chirality (degree of non-coplanarity) are energetically favorable. Numerical modeling is performed to estimate the ground state configurations. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D19.00010: Low temperature magnetic dynamics in one-dimensional Co(II) molecular chains A. Amjad, G.M. Espallargas, J.M. Clemente-Juan, R. Klemm, E. del Barco, E. Coronado, M. Evangelisti We present a low-temperature study of one-dimensional Co-based molecular chains, trans-[CoCl$_{2}$(3,5-Br$_{2}$py)$_{2}$]. Ac and dc susceptibility experiments show that the cobalt ions tend to form anisotropic ferromagnetic chains, whose properties are dictated by the thermal excitations of 1D domain walls. The observation of anomalies in the hysteresis loops of the sample on increasing the magnetic field sweep rate reveals interesting dynamical effects at the individual chain level. On decreasing the temperature, no evidence for 3D ordering was observed in specific heat measurements, although the magnetization was strongly irreversible below $\sim $450 mK. Possible sources for this absence of a magnetic phase transition could be the weakness of the interchain interactions, the presence of single-ion anisotropy at skew angles, disordered domains, and lattice defects, etc. These possibilities will be studied both experimentally and theoretically. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D19.00011: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 5:06PM - 5:18PM |
D19.00012: Spatial confinement effect on TbMn$_{2}$O$_{5}$ nanorods Chun Chuen Yang, Jing Huei Wang, Wei Luen Huang, Chang Yu Weng, Cih Lian Hong, Yang Yuan Chen Series of TbMn$_{2}$O$_{5}$ nanorods were fabricated by hydrothermal method with different annealing temperatures. Three samples which width by length equal to 10(4) nm $\times$ 38(14) nm, 25(6) nm $\times$ 64(18) nm, and 101(25) nm $\times$ 216(54) nm are identified by TEM images, x-ray diffraction, and SAED schemes. Furthermore discovery show that the preferred growth direction is along $c$ axis (length). Ac magnetic susceptibility and specific heat measurements revealed incommensurate ($\sim $41 K) and commensurate ($\sim $38 K) Mn antiferromagnetic ordering peaks are only appeared in 101(25) nm $\times$ 216(54) nm sample. In this case, the small size effect resulted entropy difference of Mn magnetic ordering is 27 {\%} less than bulk one. At 5 K, a small hysteresis loop was also observed in the identical sample and indicated the FM domains occurred. No such magnetic and thermal behaviors were found in another two samples. We believe this is attributing to spatial limitation and distortion caused by low surface-volume ratio. The estimated magnetic correlation length of Mn is in between 25 and 64 nm. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D19.00013: Tetrairon(III) Single Molecule Magnet Studied by Scanning Tunneling Microscopy and Spectroscopy Youngtek Oh, Hogyun Jeong, Minjun Lee, Jeonghoon Kwon, Jaejun Yu, Shariful Islam Mamun, Gajendra Gupta, Jinkwon Kim, Young Kuk Tetrairon(III) single-molecule magnet (SMM) on a clean Au(111) has studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to understand quantum mechanical tunneling of magnetization and hysteresis of pure molecular origin. Before the STM studies, elemental analysis, proton nuclear magnetic resonance (NMR) measurement and Energy Dispersive X- ray Spectroscopy (EDS) were carried out to check the robustness of the sample. The STM image of this molecule shows a hexagonal shape, with a phenyl ring at the center and surrounding six dipivaloylmethane ligands. Two peaks are observed at 0.5 eV, 1.5 eV in the STS results, agreeing well with the first principles calculations. Spin-polarized scanning tunneling microscopy (SPSTM) measurements have been performed with a magnetic tip to get the magnetization image of the SMM. We could observe the antiferromagnetic coupling and a centered- triangular topology with six alkoxo bridges inside the molecule while applying external magnetic fields. [Preview Abstract] |
Session D20: Bionanotechnology
Sponsoring Units: FIAPChair: Michael Naughton, Boston College
Room: D168
Monday, March 21, 2011 2:30PM - 2:42PM |
D20.00001: Multifunctional nanoparticles (Au@SiO$_{2}$@Y$_{2}$O$_{3}$:Er$^{3+})$ for biological and photonic application Madhab Pokhrel, Jianhui Yang, Paresh C. Ray, Dhiraj K. Sardar Due to plasmon at the surface, the absorption and scattering of electromagnetic radiation by metal nanoparticles are strongly enough. These properties provide the potential of designing multifunctional nanoparticles which are optically active for simultaneous molecular imaging and photothermal cancer therapy. Gold nanorods with suitable aspect ratios can absorb and scatter strongly in the NIR region. In the present work, we have demonstrated the application of multifunctional nanoparticles (Au@SiO$_{2}$@Y$_{2}$O$_{3}$:Er$^{3+})$ as contrast agents for both molecular imaging and photo thermal therapy. These multifunctional naoparticles has shown the enhancement in Er$^{3+}$fluorescence through plasmon interaction and enhancement in Raman spectrum, which made these nanoparticles potential for biosensor for detecting the biological and chemical molecule. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D20.00002: Fundamental Interactions between Deoxyribonucleic Acid (DNA) Oligomers and Au Nanoparticles: Experimental and Theoretical Studies Molleshree Karna, Radhakrishnan Balu, Mark Griep, Govind Mallick \textbf{E}xperimental and theoretical investigations were performed to understand the nature of fundamental interactions between gold nanoparticles (GNPs) and single stranded DNA (ss-DNA). Atomic force microscopic imaging and UV-Visible spectroscopic measurements revealed binding of NPs with ss-DNA under mildly acidic conditions.. \textit{Ab initio} quantum chemical calculations within the framework of density functional theory provided a possible charge transfer pathway from the DNA base guanine to Au atoms and thus characterizing the interaction as electrostatic. The calculations outline the possible effect of the presence of other bases to guanine mediated charge transfer. Specifically, the presence of an adenine base alters the charge localization at the guanine base and thus prevents charge transfer to NPs. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D20.00003: Modeling the effect of dynamic surfaces on membrane penetration Reid Van Lehn, Alfredo Alexander-Katz The development of nanoscale materials for targeted drug delivery is an important current pursuit in materials science. One task of drug carriers is to release therapeutic agents within cells by bypassing the cell membrane to maximize the effectiveness of their payload and minimize bodily exposure. In this work, we use coarse-grained simulations to study nanoparticles (NPs) grafted with hydrophobic and hydrophilic ligands that rearrange in response to the amphiphilic lipid bilayer. We demonstrate that this dynamic surface permits the NP to spontaneously penetrate to the bilayer midplane when the surface ligands are near an order-disorder transition. We believe that this work will lead to the design of new drug carriers capable of non-specifically accessing cell interiors based solely on their dynamic surface properties. Our work is motivated by existing nanoscale systems such as micelles, or NPs grafted with highly mobile ligands or polymer brushes. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D20.00004: Microsphere whispering gallery optical resonators for biomedical microfluidic devices O. Svitelskiy, A. Darafsheh, D. Sun, V.N. Astratov, M. Sumetsky, A. Lupu, M. Tchernycheva These resonators are potential candidates for broad application range as sensors of various physical quantities, and as key elements for photonic and optomechanical systems. Most of the biomedical applications involve deployment of resonators in fluidic environment. However, closeness of refractive indices of sphere $n_{s}$ and fluid $n_{f}$ obstructs excitation of the resonant modes. Moreover, an attempt to increase $n_{s}$ can deteriorate coupling of light between fiber and sphere. To address these challenges we explore a series of high-Q resonators using a specially developed tapered optical microfiber microfluidic platform. The coupling strength between the cavity and the microfiber taper is shown to depend on the contact position of the microsphere along the taper and on the refractive index contrast between the microsphere and the fluid. We demonstrate that barium titanate glass beads with $n_{s}\sim2$ can be suitable for practical tasks. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D20.00005: Utilizing nonlinear optical properties in nanoparticles for imaging Brian Yust, Neema Razavi, Dhiraj Sardar Optical phase conjugation is a nonlinear effect in which light incident upon a nonlinear medium may be conjugated so that the output signal is in the opposite direction of the input, as seen in four-wave mixing. Recently, we have seen that these nonlinear effects may still be seen in various nanocrystals and nanoparticles. Barium titanate (BaTiO3) is a good candidate for phase conjugation on the nano-scale, because of its large third order susceptibility. BaTiO3 particles of varying size are synthesized through precipitation and hydrothermal methods and analyzed optically and morphologically. The nonlinear absorption, four wave mixing signal in the forward and counter-propagating geometries, and third order susceptibilities are characterized in both the visible and infrared. Possible uses for the unique optical properties of these nanoparticles in imaging, microscopy, and photonics will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D20.00006: Gold Nanostars for Photo-Thermal Ablation of Single Neurons Zurab Kereselidze, Victor Romero, Waskar Egido, Christopher Valdes, Emmanuel Michaelides, Xomalin Peralta, Miguel Jose-Yacaman, Fidel Santamaria Nanoparticle mediated photo-thermal ablation therapy is a technique for removing cells within a tissue with minimal collateral damage. It works by exciting the surface plasmon resonance of metallic nanoparticles so there is an amplification of the absorption of the incident electromagnetic field which is then transformed into heat and results in photo-thermal ablation. Little is known about its effects at the single-cell level. We fabricated various sized and shaped gold nanoparticles, including nanostars, with a surface plasmon mode in the near infrared. Neurons of mouse cerebellar slices internalize bare nanostars during incubation periods of $<$3 hrs. We imaged the slices and excited surface plasmon mode of the nanostars. Our results show that we are capable of destroying individual nanostar containing cells without affecting the neighbors. Other shapes attach to the cell membrane but are not internalized. Therefore nanoparticles can provide a technique for a neuron single-cell photo-thermal without any functionalization. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D20.00007: Biochemically Selective Nanoarrays: From Protein-DNA Interactions to Bio-Inorganic Nanoscale Assembly Justin Abramson, Matteo Palma, Alon Gorodetsky, Colin Nuckolls, Michael Sheetz, Shalom Wind, James Hone The ability to control the arrangement of both biomolecules and bio-inorganic structures on surfaces with nanometer resolution is of great interest in the field of nanoscience and nanotechnology. Nanopatterned arrays of biomolecules can offer unmatched sensitivity in molecular diagnostics. Furthermore, templated assembly of bio-inorganic structures at the nanoscale makes possible interesting quantum optical structures, including switchable photonic cavities. Here we describe different strategies to control the immobilization of single- and double-stranded DNA, as well as quantum dots, on nanopatterned surfaces, with features down to the sub-30nm regime. The bio-functional chemistry allows for the formation of non-sterically hindered DNA nanodomains where the dsDNA attached to the nanodots is accessible and maintains its native conformation, as confirmed by restriction enzymes studies at the single molecule level. We will further highlight the broader utility of such nanopatterned surfaces for the self-organization of quantum dots, demonstrating the ability to both biochemically and covalenty assemble single quantum dots on our nanopatterns. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D20.00008: Nanoscale open-ended coaxial line proximity sensor array for spatio-temporal impedance imaging Jeffrey R. Naughton, Binod Rizal, Michael J. Burns, Gregory McMahon, Stephen Shepard, Michael J. Naughton We describe the development of a dielectric impedance measurement array comprised of open-ended nanoscale coaxial proximity sensors. The device offers the capability of on-chip dielectric impedance tomography for imaging \emph{e.g.} biological cells with $\sim$micron pixel density. Computer simulations of the response of individual pixels and of discrete arrays to changes in dielectric properties of proximate media are presented. Experiments with biological cells on 1st-generation arrays will be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D20.00009: Nonlinear Optical Properties of ZnO for BioimagingCell and Cell Destruction Ben Urban, Samudyatha Chakki, Os Senthilkumar, Kasilingam Senthilkumar, Yasuhisa Fujita, Arup Neogi As of recent years nanotechnology has been at the forefront of scientific research. It promises to have a broad range of applications from turning unhealthy foods into health foods, making computers faster and curing cancer. We present results on using nonlinear optical processes of ZnO nano-crystals to detect, track and destroy cells. By incorporating ZnO into a hydrophobic nano-hydrogel matrix with trace amounts of H$_{2}$O$_{2}$, we can attach antibodies or microRNA for specific cell targeting and, using the heat generating properties of the third order nonlinear process, release H$_{2}$O$_{2}$ in the cell causing instant cell death. Theoretically, with the appropriate sequence for microRNA or the appropriate antibodies, we could target cancer cells in the body and destroy them. This presentation gives our results until now. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D20.00010: Polymeric Nanoelectrodes for Investigating Cellular Adhesion Prem Thapa, Govind Paneru, Bret Flanders Polyethylene dioxythiophene nano-filaments were grown on lithographic electrode arrays by the recently developed directed electrochemical nanowire assembly technique. These filaments are firmly attached to the electrode but are not attached to the glass substrate. Hence, they behave like cantilevered rods (with one free end). Individual cells of the slime mold \textit{Dictystolium discoideum} initiate contact by extending pseudopods to the nanoelectrodes when cultured on the electrode arrays. Scanning electron micrographs of the interfaces show the contact area to be of the order of 0.1 $\mu $m$^{2}$. Confocal images reveal the focal adhesions in the cell-electrode contact region. Deflection of the nanoelectrode by an individual cell can be used to measure the force exerted by the cell. Recent results on this innovative force sensing approach will be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D20.00011: Detecting Lyme disease using antibody-functionalized carbon nanotubes Jennifer Dailey, Mitchell Lerner, Brett Goldsmith, Dustin Brisson, A.T. Charlie Johnson We combine antibodies for Lyme flagellar protein with carbon nanotube transistors to create an electronic sensor capable of definitive detection of Lyme disease. Over 35,000 cases of Lyme disease are reported in the United States each year, of which more than 23 percent are originally misdiagnosed. Rational design of the coupling of the biological system to the electronic system gives us a flexible sensor platform which we can apply to several biological systems. By coupling these antibodies to carbon nanotubes in particular, we allow for fast, sensitive, highly selective, electronic detection. Unlike antibody or biomarker detection, bacterial protein detection leads to positive identification of both early and late stage bacterial infections, and is easily expandable to environmental monitoring. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D20.00012: Fluorescence-Based DNA-Nanotube Platform with Single Molecule Resolution Prakrit Jena, Ankur Jain, Daniel Heller, Markita Landry, Yann Chemla, Michael Strano, Taekjip Ha We have developed an experimental platform to control and modify the DNA on a DNA-Single Walled Nanotube (SWNT) complex for the purpose of detecting labeled and unlabeled protein-DNA interactions via visible fluorescence. By exploiting the distance-dependent photophysical interaction between organic fluorophores and the surface of a SWNT as the sensing mechanism, fluorophore-conjugated DNA-SWNTs are immobilized and observed using single molecule-total internal reflection microscopy. By analyzing the number of molecules, photobleaching steps and the absolute size of the observed DNA-SWNTs, we have confirmed the presence of a duplex, partial duplex and single-strand DNA scaffold on the SWNT surface using both nucleic acids and proteins as probes. Our approach offers multiple experimental schemes to extend the current use of carbon nanotubes for applications involving the interaction with biologically relevant molecules. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D20.00013: Selective Intracellular Activation by Designing pH-Sensitive and Tunable Fluorescent Nanoparticle Kejin Zhou, Yiguang Wang, Xiaonan Huang, Milan Poudel, Gang Huang, Kate Luby-Phelps, Jinming Gao Integration of nanotechnology with molecular biology and medical imaging has propelled the development of various nanoscopic imaging probes and targeted therapeutics. Despite great advances, it remains a formidable challenge to create highly biointeractive nanosystems that can respond to subtle changes in physiological stimuli (e.g. pH, enzymes) to achieve desired biological specificity. Here we report a set of robust, pH-activatable micelle nanoprobes with tunable pH transitions in the physiological range. These nanoprobes have a fast fluorescence response ($<$5 ms), up to 55-fold increase of emission intensity between OFF and ON states, and only require $<$0.25 pH unit for activation (vs. 2 pH unit for small molecular dyes). Nanoprobes with different transition pH can be selectively activated in specific endocytic compartments such as early endosomes or lysosomes. This capability allows for the development of pH-activatable imaging probes or nanocarriers that can target specific subcellular organelles for therapy. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D20.00014: Formation of Lipid Bilayer Membrane including Ion Channels on Graphene Jung Yoon Choi, Kyung Eun Byun, Seon Namgung, Heejun Yang, Jinseong Heo, Hyun-Jong Chung, Sunae Seo, Seunghun Hong Lipid bilayer membrane on a solid electrode has been extensively utilized to study membrane proteins. Recently, graphene has drawn an attention as a transparent and high conductive electrode compatible with biological systems. Herein, we report the successful formation of lipid membrane including ion channels on graphene. In this method, graphene was functionalized by biocompatible molecular layers and utilized as a substrate to support lipid bilayer membrane including ion channels. The functionality of ion channels incorporated in the lipid bilayer membrane was studied via the electrochemical impedance spectroscopy. This lipid membrane-coated graphene structure can be a versatile platform for various applications such as bio-sensing and in vitro drug screening. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D20.00015: Aptamer sandwich-based carbon nanotube sensors for single-carbon-atomicresolution detection of non-polar small molecular species Joohyung Lee, Minjoung Jo, Ji-Young Ahn, Soyoun Kim, Tae Hyun Kim, Dong-ki Lee, Seunghun Hong Portable sensor platforms are crucial for the on-site monitoring of disease-related metabolites, environmental pollutants and food toxicants. However, it is still difficult to build highly-sensitive and selective sensor platforms for small molecular detection. We developed an aptamer sandwich-based carbon nanotube sensor, where aptamers were utilized to capture target molecules as well as to enhance the sensor signals. Using this strategy, we successfully demonstrated the detection of non-polar bisphenol A molecules with a picomolar sensitivity and single-carbon-atomic resolution. Furthermore, by modifying the labeling aptamer with additional biotin, we enhanced the detection limit of our sensors for one hundred times. These results overcome the fundamental limitation of general FET-based sensors and should make a major breakthrough in various applications such as environmental protection and food safety. [Preview Abstract] |
Session D21: Focus Session: Novel Instrumentation & Measurements for Biomedical Research
Sponsoring Units: GIMSChair: Larry Nagahara, National Institutes of Health
Room: D161
Monday, March 21, 2011 2:30PM - 3:06PM |
D21.00001: Physical principles of genomic regulation through cellular nanoscale structure and implications for initiation of carcinogenesis Invited Speaker: Although compelling evidence suggests that cellular nanoarchitecture and nanoscale environment where molecular interactions take place would be expected to significantly affect macromolecular processes, biological ramifications of cellular nanoscale organization have been largely unexplored. This understanding has been hampered in part by the diffraction limited resolution of optical microscopy. The talk will discuss a novel optical microscopy technique, partial wave spectroscopic (PWS) microscopy, that is capable of quantifying statistical properties of cell structure at the nanoscale. Animal and human studies demonstrated that an alteration in the statistical properties of the nanoscale mass density distribution in the cell nucleus (e.g. nuclear nanoarchitecture) is one of the earliest and ubiquitous events in carcinogenesis and precedes any other known morphological changes at larger length scales (e.g. microarchitecture). The talk will also discuss the physical principles of how the alteration in nuclear nanoarchitecture may modulate genomic processes and, in particular, gene transcription.\\[4pt] Work done in collaboration with Hariharan Subramanian, Prabhakar Pradhan, Dhwanil Damania, Lusik Cherkezyan, Yolanda Stypula, Jun Soo Kim, Igal Szleifer, Northwestern University, Evanston, IL, Hemant K. Roy, Northshore University HealthSystems, Evanston, IL [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D21.00002: 3D Cell Culture Imaging with Digital Holographic Microscopy Thomas Dimiduk, Kendra Nyberg, Dariela Almeda, Ekaterina Koshelva, Ryan McGorty, David Kaz, Emily Gardel, Debra Auguste, Vinothan Manoharan Cells in higher organisms naturally exist in a three dimensional (3D) structure, a fact sometimes ignored by in vitro biological research. Confinement to a two dimensional culture imposes significant deviations from the native 3D state. One of the biggest obstacles to wider use of 3D cultures is the difficulty of 3D imaging. The confocal microscope, the dominant 3D imaging instrument, is expensive, bulky, and light-intensive; live cells can be observed for only a short time before they suffer photodamage. We present an alternative 3D imaging techinque, digital holographic microscopy, which can capture 3D information with axial resolution better than $2 \mu m$ in a $100\mu m$ deep volume. Capturing a 3D image requires only a single camera exposure with a sub-millisecond laser pulse, allowing us to image cell cultures using five orders of magnitude less light energy than with confocal. This can be done with hardware costing $\sim\$1000$. We use the instrument to image growth of MCF7 breast cancer cells and p. pastoras yeast. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D21.00003: Carbogen Enhanced Femto Oximetry Breast Cancer Diagnosis Method with High Specificity Bogdan C. Maglich, J.K. Shultis, C.J. Solomon As large malignant tumors are oxygen deficient (hypoxic), cancer could be diagnosed \textit{in vivo} and online, by non-invasive measurement of oxygen difference between tumor and adjacent tissue. Computer simulations of noninvasive diagnosis by Femto Oximetry (FO) of hypoxia in 1 cm tumor in 10 cm breast shows that background $\gamma $'s from non hypoxic tissue will mask hypoxia. To amplify the hypoxic-to-normal O difference, air breathing will be replaced with carbogen (O$_{2}$ 95{\%}, CO$_{2}$ 5{\%}) using vasco-constrictive property whereby carbogen breathing increases O in normal tissue, while not in malignant hypoxic tumors. 90{\%} hypoxia will be detectable by FO with specificity 99{\%}. Our method will be tested on R3230 tumors in Fischer rats at UCI. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D21.00004: Fractal Analysis of Optical Coherence Tomography of Normal and Malignant Breast Tissue Amanda C. Sullivan, John P. Hunt, Amy L. Oldenburg Optical coherence tomography (OCT) provides real-time imaging of tissue several mean free photon paths into tissue by heterodyne detection of backscattered light. OCT can potentially be used to rapidly assess tumor margins during breast cancer resection, however, currently it is difficult to differentiate between normal and malignant tissues with OCT. Because cancer is characterized morphologically by increasing disorder, we investigated the fractal dimension of OCT images of normal and cancerous breast tissue. 3D OCT images of 44 specimens were collected, then tissues were histologically processed to independently determine distinct regions of adipose, stroma and cancer. The fractal dimension of each tissue type was then calculated with a one-dimensional box-counting algorithm applied to the OCT axial scans. We found that the fractal dimensions of stromal tissues were significantly higher than those of cancer (P$<$10$^{-6})$, while those of adipose tissue were significantly lower than those of cancer (P$<$10$^{-4})$. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D21.00005: A new algorithm for detection of apnea in infants in neonatal intensive care units Hoshik Lee, Brooke Vergales, Alix Paget-Brown, Craig Rusin, Randall Moorman, John Kattwinkel, John Delos Apnea is a very common problem for premature infants: apnea of prematurity (AOP) occurs in $>50\%$ of babies whose birth weight is less than 1500 g, and AOP is found in almost all babies who are $< 1000 \, {\rm g}$ at birth. Current respiration detectors often fail to detect apnea, and also give many false alarms. We have created a new algorithm for detection of apnea. Respiration is monitored by continuous measurement of chest impedance (CI). However, the pulsing of the heart also causes fluctuations in CI. We developed a new adaptive filtering system to remove heart activity from CI, thereby giving much more reliable measurements of respiration. The new approach is to rescale the impedance measurement to heartbeat-time, sampling 30 times per interbeat interval. We take the Fourier transform of the rescaled signal, bandstop filter at 1 per beat to remove fluctuations due to heartbeats, and then take the inverse transform. The filtered signal retains all properties except the impedance changes due to cardiac filling and emptying. We convert the variance of CI into an estimated likelihood of apnea. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D21.00006: The Suppression of Dominant Acoustic Frequencies in MRI Xingxian Shou, Robert Brown Patient discomfort and brain imaging distortion are serious MRI acoustic noise problems arising from the rapid switching on and off of gradient coils in the presence of the strong Larmor magnetic field. A study is made of dominant frequencies in the acoustic noise spectrum and, motivated by both spring and string ideas, we propose the cancellation of selected frequencies by appropriate gradient pulse sequence design. From both simulations and experiments, vibrations resulting from an impulsive force associated with a ramping up of a gradient pulse are shown to be cancelled upon the application of another impulsive force coming from the appropriately timed ramping down of that pulse. A method for the suppression of multiple-frequency contributions involving a series of gradient pulses with variable timings is developed and confirmed by experiment. Whether we refer to reduction in terms of dB (about 30-40 dB per peak), or to the verdict of a listener, the conclusion is that a marked reduction in sound can be achieved when at least three of the dominant frequency peaks are suppressed. A variety of pulse profiles and timing combinations can be used to attenuate important contributions to the acoustic spectrum. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D21.00007: Linewidth Narrowing for $^{31}$Phosphorus MRI of Bones Merideth Frey, Sean Barrett Bone is a particularly challenging tissue to study with conventional MRI given the relatively low water density and wider linewidths of its solid components.\footnote{F. W. Wehrli, J. MRI \textbf{25}, 390 (2007); S. Anumula et al., Bone \textbf{42}, 405 (2008); D. Idiyatullin et al., J. Mag Res \textbf{193}, 267 (2008); E.E. Sigmund et al., NMR Biomed \textbf{22}, 436 (2009); Y. Wu et al., J. MRI \textbf{31}, 954 (2010)} Recent fundamental research in quantum computing gave rise to a new NMR pulse sequence that can be used to narrow the broad NMR spectrum of solids.\footnote{Y. Dong et al. Phys. Rev. Lett. \textbf{100}: 247601 (2008); D. Li et al. Phys. Rev. B \textbf{77}: 214306 (2008)} Here we narrow the spectrum of the $^{31}$P in natural bone mineral (by a factor of up to 1600x). This technique offers a new route to do high spatial resolution, 3D $^{31}$P MRI of bone which complements conventional MRI and x-ray based techniques to study bone physiology and structure. Thus far we have used our pulse sequence to do high spatial resolution (sub-250 $\mu$m)$^3$ 3D $^{31}$P MRI of \textit{ex vivo} dry bovine cortical bones, wet procine rib bones, and wet rabbit femoral bones at 4T. We have also explored the use of compressive sampling\footnote{M. Lustig et al., Mag Res Med \textbf{58}, 1182 (2007)} to push imaging time down to less than two hours without distracting artifacts. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D21.00008: Linewidth narrowing for $^{31}$Phosphorus MRI of cell membranes Sean Barrett, Merideth Frey, Joseph Madri, Michael Michaud Most $^{31}$P Magnetic Resonance Spectroscopy studies of tissues try to avoid contamination by a relatively large, but broad, spectral feature attributed to cell membrane phospholipids\footnote{W.J. Thoma et al., J. MR \textbf{61}, 141 (1985); E.J. Murphy et al., MR Med \textbf{12}, 282 (1989); R. McNamara et al., NMR Biomed \textbf{7}, 237 (1994).}. MRI using this broad $^{31}$P membrane spectrum is not even attempted, since the spatial resolution and signal-to-noise would be poor, relative to conventional MRI using the narrow $^{1}$H water spectrum. This long-standing barrier has been overcome by a novel pulse sequence, recently discovered in fundamental quantum computation research\footnote{Y. Dong et al. Phys. Rev. Lett. \textbf{100}, 247601 (2008); D. Li et al. Phys. Rev. B \textbf{77}, 214306 (2008).}, which narrows the broad $^{31}$P spectrum by $\sim$1000$\times$. Applying time-dependent gradients in synch with a repeating pulse block enables a new route to high spatial resolution, 3D $^{31}$P MRI of the soft solid components of cells and tissues. So far, intact and sectioned samples of \textit{ex vivo} fixed mouse organs have been imaged, with (sub-mm)$^{3}$ voxels. Extending the reach of MRI to broad spectra in natural and artificial tissues opens a new window into cells, enabling progress in biomedical research. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D21.00009: Producing $>$60,000-fold room-temperature $^{89}$Y NMR signal enhancement Lloyd Lumata, Ashish Jindal, Matthew Merritt, Craig Malloy, A. Dean Sherry, Zoltan Kovacs $^{89}$Y in chelated form is potentially valuable in medical imaging because its chemical shift is sensitive to local factors in tumors such as pH. However, $^{89}$Y has a low gyromagnetic ratio $\gamma_{n}$ thus its NMR signal is hampered by low thermal polarization. Here we show that we can enhance the room-temperature NMR signal of $^{89}$Y up to 65,000 times the thermal signal, which corresponds to 10 \% nuclear polarization, via fast dissolution dynamic nuclear polarization (DNP). The relatively long spin-lattice relaxation time $T_{1}$ ($\sim$500 s) of $^{89}$Y translates to a long polarization lifetime. The $^{89}$Y NMR enhancement is optimized by varying the glassing matrices and paramagnetic agents as well as doping the samples with a gadolinium relaxation agent. Co-polarization of $^{89}$Y-DOTA with a $^{13}$C sample shows that both nuclear spin species acquire the same spin temperature $T_{s}$, consistent with thermal mixing mechanism of DNP. The high room-temperature NMR signal enhancement places $^{89}$Y, one of the most challenging nuclei to detect by NMR, in the list of viable magnetic resonance imaging (MRI) agents when hyperpolarized under optimized conditions. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D21.00010: Raman spectroscopy as a diagnostic tool to detect head and neck squamous cell carcinoma in archived tissues Suneetha Devpura, Seema Sethi, Jagdish S. Thakur, Vaman M. Naik, Ratna Naik Recently, many spectroscopic techniques are being tried for diagnostic applications. Among them Raman spectroscopy is one of the powerful non-invasive techniques which can differentiate between different biomolecular compositions of tissues on the basis of their vibrational spectra and hence can become an efficient diagnostic tool for detection of cancers. This technique has not yet been explored to study the head and neck squamous cell carcinoma (HNSCC) for archived tissues; here we report its results on HNSCC. Raman spectra were collected from three regions; normal, carcinoma in situ, and carcinoma. The Raman data was analyzed with chemometric methods of principal component analysis (PCA) and discriminant function analysis (DFA). Our preliminary results show that PCA and DFA analysis of Raman spectra can successfully distinguish the pathological states in archived HNSCC tissues. However, large Raman data set from many tissue sections is needed to validate these findings. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D21.00011: Optical measurements of mechanical resonances in biological tissues via magnetic nanoparticle interrogation Vasilica Crecea, Steven Adie, Amy Oldenburg, Renu John, Stephen Boppart We present a real-time phase-resolved optical coherence tomography-based technique that interrogates the mechanical properties of tissue phantoms with different elasticities as well as healthy and cancerous rat tissues, via the interaction of high susceptibility iron oxide nanoparticles that reside inside the samples and an external magnetic field. A chirped magnetic field selects the region of natural resonance in the probed samples as evidenced by scatterer displacements measured with nanometer-level sensitivity. This methodology, entitled magnetomotive optical coherence elastography (MM-OCE), which exploits frequency dependent viscoelastic response in biological media, has potential for detecting tissue pathologies. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D21.00012: Imaging the Vector Magnetic Field of Magnetospirillum Gryphiswaldense by Optically Detected Magnetic Resonance using Nitrogen-Vacancy Centers in Diamond Richelle M. Teeling, Young Woo Jung, Inhee Lee, Justin North, Robin Nakkula, Rohan Adur, Ezekiel Johnston-Halperin, Michael G. Poirier, P. Chris Hammel Nitrogen vacancy centers in diamond are single-spin systems that are stable under ambient conditions with strong optical spin transitions, making them optimal for room-temperature detection of nanoscale magnetic fields using optically detected magnetic resonance (ODMR). We use these ensembles of diamond spins as a scanned probe magnetometer to map the field emitted by Magnetospirillum Gryphiswaldense, in vivo. These bacteria mineralize nanoscale magnetite particles in their internal vesicles. Imaging these living bacteria cells will serve as a strong foundation for the application of our ODMR technique to the medical field, where the bacteria can be used to synthesize functionalized magnetic particles which can be used as biomarkers and targeted drug-delivery systems. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D21.00013: Sub-Nanoparticle Femto Atometry: Measurement of Genome Lengths of Mammalian Tissues Chris Druey, Bogdan C. Maglich, Anna Z. Radovic Measurement of porcine and bovine genome length, giga nucleotide base pairs, Gbp, was made by irradiating each tissue for 30' with neutrons of femto DeBroglie $\lambda \quad \sim $ 10$^{-15}$ m, which, unlike nanoparticles, interact only locally with atomic nuclei in nucleotide. O and C atoms were counted via $\gamma $ rays emitted from (n, n' $\gamma )$ reaction. By irradiating free dA, dC and T nucleotides for 30' we got response constant: (1,450 $\pm $ 44) $\gamma $/O. From 2 measurements we obtained 2.59 $\pm $ .05 and 3.19 $\pm $ .06 Gbp for porcine and bovine, respectively, consistent with 2.7 and 3 Gbp (errors not quoted) obtained by genome sequencing method that took 6 years each. [Preview Abstract] |
Session D22: Metal Insulator Transition in VO2
Sponsoring Units: DCMPChair: Doug Natelson, Rice University
Room: D163
Monday, March 21, 2011 2:30PM - 2:42PM |
D22.00001: The band structure of VO2 measured by angle-resolved photoemission Luca Moreschini, Young Jun Chang, Davide Innocenti, Andrew L. Walter, Young Su Kim, Geoffrey Gaines, Aaron Bostwick, Jonathan Denlinger, Eli Rotenberg The origin of the 340K metal-insulator transition (MIT) in VO2 is still under debate. the main reason is that no direct experimental verifications of the electronic structure of VO2 exist up to this point. The quality of the available single crystals is not sufficient for ARPES measurements, so that photoemission is limited to angle-integrated mode. New opportunities are offered by oxide films, on which data of equal or even higher quality have been reported (Saeki \textit{et al.}, PRB 2009). WIth the \textit{in situ} pulsed-laser-deposition (PLD) system available on beamline 7.0.1 at the Advanced Light Source we have grown VO2(001) films on a TiO2 substrate and measured the Fermi surface of the metallic phase. These results will permit a direct comparison with the existing band calculations and open the way to the study of the MIT as a function, e.g., of film thickness or electron doping with Cr. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D22.00002: Electronic structure of strained vanadium dioxide thin films using soft X-ray spectroscopy Jude Laverock, Louis Piper, Andrew Preston, Bo Chen, James McNulty, Kevin Smith, Salinporn Kittiwatanakul, Jiwei Lu, Stuart Wolf Despite over five decades of intense investigation, the origin of the metal-insulator transition (MIT) in VO$_2$ still presents a challenge to explain. Whether the lattice (Peierls physics) or electron-electron correlations (Mott-Hubbard physics) are responsible for the MIT has been hotly debated; more recently, the general consensus has favored a co-operative description, in which both structural and correlation effects are important and sympathetic to the transition. Key to understanding such a co-operative picture has been the behavior of VO$_2$ under doping and strain. Here, we report recent soft X-ray measurements of strained VO$_2$ thin films grown on TiO$_2$(001) and (110) substrates. We employ X-ray absorption spectroscopy and X-ray emission spectroscopy to probe the changes in both the {\em unoccupied} and {\em occupied} partial density of states across the MIT, observing distinct changes in the V $3d$-O $2p$ hybridization. Additionally, the location in energy of the unoccupied $d_{\parallel}$ state in the insulating phase is found to be dependent on the lattice strain, in agreement with the predictions of recent dynamical mean-field theory calculations. Finally, our results are discussed in the context of the origin of the MIT in VO$_2$. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D22.00003: Scanning Photocurrent Microscopy of VO$_{2}$ Nanobeams Christopher Miller, Mark Triplett, Joel Lammatao, Kevin Wang, Derrick Fu, Junqiao Wu, Dong Yu Vanadium dioxide (VO$_{2}$) is a strongly correlated material that displays a near-room temperature metal-to-insulator transition ($\sim 68^{o}C$). This transition can be explored at the single domain level in single crystalline VO$_{2}$ nanobeams, where the material dimension is smaller than the characteristic domain size. Here we investigate the metal-insulator phase transition and its domain wall physics in single VO$_{2}$ nanobeam devices through scanning photocurrent microscopy. This technique, which measures the photocurrent as a function of the local photo-injection position, allows us to determine the band bending direction and the height of the Schottky barriers at each domain wall. Our results may shed light on the charge dynamics in strongly correlated materials and the metal-insulator phase transition mechanism. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D22.00004: Conductivity anisotropy in strained VO$_{2}$ thin films, probed by THz Time Domain Spectroscopy Mengkun Liu, Elsa Abreu, Jiwei Lu, Kevin West, Salinport Kittiwatanakul, Wenjing Yin, Stuart Wolf, Richard Averitt We used THz time domain spectroscopy to measure the temperature and polarization dependent far-infrared conductivity of high quality strained VO$_{2}$ thin films epitaxially grown on (100) TiO$_{2}$ substrates. A large conductivity anisotropy is observed in the metallic phase of our VO$_{2}$ films with the conductivity along the rutile axis $\sim $30 times larger than the orthogonal direction. The MIT temperature also exhibits anisotropy with a value of 360K along the rutile c axis and 340K along the perpendicular direction. Our results are consistent with substrate induced strain modulation of the energy and bandwidth associated with the vanadium 3d orbitals. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D22.00005: X-ray induced photoconductivity in Vanadium Dioxide samples Sebastian Dietze, Jyoti Mohanty, Moses Marsh, Jong Woo Kim, Kevin West, Ivan K. Schuller, Oleg G. Shpyrko Vanadium Dioxide (VO2) goes through a first-order phase transition at approximately 340K, exhibiting both an insulator to metal transition (IMT) and a structural phase transition (SPT), with a monoclinic (M1) insulating phase at low temperatures and a rutile (R) metallic phase at high temperatures. We show an anomalous behavior of x-ray induced persistent photoconductivity (PPC) well below the temperature induced phase transition in VO2 devices. We present conductivity and X-ray Diffraction (XRD) measurements, revealing a large enhancement of conductivity due to photo-induced carriers. Moreover, with the addition of nominal electric fields, we are able to fully transition into the rutile metallic phase at room temperature. This effect is completely reversible, allowing the monoclinic insulating phase to be recovered via annealing. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D22.00006: Electric field-induced breakdown of the Mott insulating state in V2O3 nanostructures Justin Brockman, Li Gao, Nagaphani Aetukuri, Brian Hughes, Charles Rettner, Mahesh Samant, Kevin Roche, Stuart Parkin The origin of the electric field-induced breakdown of the Mott insulating state in vanadium sesquioxide (V2O3) nanostructures is of considerable interest. We have prepared high quality, epitaxial films of V2O3 on (0001)-oriented sapphire substrates by oxygen plasma-assisted thermal evaporation. Lateral, two-terminal nanostructures were patterned by electron beam lithography. The nanostructures displayed strong metal-to-insulator transitions upon cooling to below $\sim $150K. Modest voltages applied across the devices drive the films into a conducting state. We discuss the role of temperature, applied voltage, device size, and potential Joule heating effects on the switching process, as well as implications for the underlying mechanism involved. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D22.00007: Gatability of vanadium dioxide single crystal nanobeams and hydrogen doping Jiang Wei, Heng Ji, Douglas Natelson Vanadium dioxide is famous for its dramatic metal insulator transition, exhibiting up to 4 or 5 orders magnitude change in conductivity. It is also known to be nongatable, although in the insulating phase it behaves like a semiconductor with 0.5-0.7 eV energy gap. With no sign of gating effects using conventional dielectric materials, such as SiO$_{2}$, Al$_{2}$O$_{3}$ and HfO$_{2}$, ionic liquids were used as the gating medium. Ionic liquids form electric double layers (EDL) and could possibly exert an electric field as high as 10$^{9 }$V/m on the interface of ionic liquid and single-crystal vanadium dioxide nanobeam. No gating effect was observed in the vanadium dioxide device. On the other hand, we found that under positive gate voltage the hydrogen ions originating from trace amounts of water diffuse into the vanadium dioxide crystal, acting as dopants. By controlling the gate voltage and temperature, the insulating phase's conductivity can be reversibly increased up to 2-3 orders magnitude by this process. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D22.00008: Dependence of VO$_{2}$ thin-film metal-insulator transition on its intrinsic impurities Changhong Chen, Yong Zhao, Zhaoyang Fan We present variation in strain, metal-insulator transition
temperature
($T_{MIT})$, activation energy ($\Delta E_{a})$, and charge
carrier type in
the insulating phase of (011) preferred polycrystalline (Poly-) and
multidomain (020) epitaxial (Epi-) VO$_{2}$ films grown at different
temperature ($T_{S})$, to produce variable intrinsic impurities.
Both the
Poly- and Epi-VO$_{2 }$behave $n$-type conductivity when grown at
relative low
$T_{S}$. As $T_{S}$ increases, acceptor density of impurity
increases to alter
conductivity from $n$- to $p$-type in the Poly-VO$_{2}$, while
conductive $n$-type
still keeps in the Epi-VO$_{2}$ with increased donor density.
Moreover, the
strain along monoclinic $a_{m}$ axis dramatically reverses from
tensile to
compressive in both the Poly- (848 K$ |
Monday, March 21, 2011 4:06PM - 4:18PM |
D22.00009: Manipulation of avalanche characteristics in nanoscaled VO$_{2}$ devices Siming Wang, Kevin G. West, Ivan K. Schuller The temperature driven metal insulator transition (MIT) in nanoscaled VO$_{2}$ devices occurs through a series of resistance jumps ranging over two decades in magnitude. A power law distribution of the jump sizes, demonstrates that the transition is caused by avalanches across the percolation transition. We investigate the effect of a DC write current on the intrinsic behavior of the MIT transition in nanoscaled VO$_{2}$ devices. We find an increase in the maximum resistance jump size by as much as a factor of 10x after application of a DC write current at room temperature. Interestingly, we find no significant changes in the exponent of the power law distribution as a function of an applied DC write current. The observations suggest that the DC current changes the intrinsic properties of the VO$_{2}$ thin film and may be related to spatial confinement which leads to an increase in the maximum resistance jump size.\footnote{Hong-Ying Zhai, J.X. Ma, D.T. Gillaspie, X.G. Zhang, T.Z. Ward, E.W. Plummer, and J. Shen, Phys. Rev. Lett. \textbf{97} 167201 (2006).} [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D22.00010: stoichiometry engineering of metal-insulator transition in suspended single crystalline vanadium dioxide nanobeams Shixiong Zhang, In Soo Kim, Lincoln J. Lauhon While the metal-insulator transition (MIT) in VO2 bulk and thin films has been investigated for several decades, recent studies of nanobeams have provided new opportunities to investigate and manipulate the metal-insulator transition and structural domain formation in a correlated manner. We will describe the electrical and structural characterization of suspended single crystal VO2 nanobeams grown/annealed under various conditions. Annealing nanobeams under reducing conditions led to the stabilization of single-crystal rutile nanobeams at room temperature, in some cases suppressing the MIT temperature from 340 K down to below 100 K. Re-annealing under oxidizing conditions led to a recovery of the transition temperature for stoichiometric VO2. Furthermore, growth under oxidizing conditions produced the Mott insulator M2 phase and an intermediate M3. Systematic annealing studies enabled the generation of a pseudo-phase diagram with dimensions of stoichiometry and temperature. The temperature dependence of the electrical resistivity of rutile nanobeams above the transition temperature will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D22.00011: Tungsten as a substitutional dopant and its effect on ultrafast switching of vanadium dioxide Joyeeta Nag, Kannatassen Appavoo, Weidong Luo, Gerd Duscher, Sokrates Pantelides, Richard Haglund VO$_{2}$ undergoes a metal-insulator transition (MIT) at 340K accompanied by a structural change from monoclinic (M1) to tetragonal (R). We have grown W-doped VO$_{2}$ films on glass and epitaxially on sapphire substrates and have characterized them by SEM, white light transmission, RBS, XRD, and Z-STEM. These provide direct experimental evidence that W acts as a substitutional dopant in the VO$_{2}$ lattice in addition to lowering the transition temperature. From GGA+U, DFT-based simulations we have also calculated the formation energy of substitutional W in VO$_{2}$, and relative stability of M1 and R phases before and after doping. Ultrafast pump-probe measurements at 800nm with varying pump fluences show that doped VO$_{2}$ switches at substantially lower fluences than undoped VO$_{2}$, indicating that the W dopant provides additional conduction-band electrons, thus altering the photo-induced dynamics of the phase transition. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D22.00012: Control of the metal-insulator transition in vanadium dioxide nanobeams Jae Hyung Park, Serkan Kasirga, Jiang Wei, Nicholas Noll, Vincent Roma, David H. Cobden Single-crystal nanobeams of vanadium dioxide, which are smaller than the characteristic domain size, exhibit a more reproducible and controllable metal-insulator transition (at around 67 degrees C) than bulk samples. We are exploiting this fact to perform systematic studies of the intrinsic properties of the phases involved, the phase transition, and the interphase wall, as well as to control the transition temperature. For these purposes it is necessary to have high quality crystals and to apply uniform strain. We are therefore investigating and improving the procedure of VO2 small-crystal growth by vapor phase transport, while developing experimental techniques in which thin nanobeams can be suspended across adjustable-widths gaps on silicon structures. The latter will enable application of strain purely along the tetragonal c-axis, to tune the transition, while simultaneously carrying out transport, optical and scattering measurements. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D22.00013: Electrical properties of vanadium dioxide devices for micro-electronic applications making use of metal-insulator phase transitions Koen Martens, Iuliana P. Radu, Sofie Mertens, Christoph Adelmann, Xiaoping Shi, Hilde Tielens, Marc Schaekers, Cedric Huyghebaert, Sven Van Elshocht, Stefan De Gendt, Marc Heyns, Jorge A Kittl In principle the metal-to-insulator transition offers prospects for use in an electronic switch. This study investigates the properties of VO$_{2}$ test devices to evaluate VO$_{2}$'s potential use in micro-electronic applications such as a memory, two-terminal selector or transistor device. Vanadium dioxide thin films were produced by thermal oxidation of vanadium and the physical properties of these layers were investigated. Electrical properties of concentric two-terminal vanadium dioxide structures will be discussed such as current-voltage behavior, switching behavior and contact formation to VO$_{2}$ with different metals and implications such as Fermi-level pinning and Schottky-type behavior for different metals. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D22.00014: Metal-insulator transition mechanism in VO2 under electric bias Yong Zhao, Ji Hao, Changhong Chen, Zhaoyang Fan It is in controversy if metal-insulator phase transition (MIT) of VO$_{2}$ can be triggered by electric field/current. In this work, a series of two terminal devices with different gap length, width, and multiple-channel configurations were fabricated on epitaxially grown VO$_{2}$ thin films, to study its MIT mechanism under the electric bias. Micro-Raman spectroscopy was used to differentiate the rutile metallic phase from the monoclinic insulator phase. Voltage-current measurements indicated that a temperature-dependent critical current density (J$_{c}$) is required to induce MIT. Under the electric bias, the phase transition was observed to be a percolation process until a clear current path (or filament) is formed between the electrodes. Afterwards the pure metallic phase was identified along the current path, while outside of it become pure insulator phase. As current varies, current path width is proportionally changed to keep a constant current density. These observations indicate that a J$_{c}$ is necessary to maintain the metallic phase current path. Contributions of the current effect and Joule heat effect to the phase transition were discussed. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D22.00015: Electric-field-driven phase transition in vanadium dioxide B. Wu, A. Zimmers, H. Aubin, R. Gosh, Y. Liu, R. Lopez In recent years, various strongly correlated materials have shown sharp switching from insulator to metallic state in their I(V) transport curves. Determining if this is purely an out of equilibrium phenomena (due to the strong electric field applied throughout the sample) or simply a Joule heating issue is still an open question. To address this issue, we have first measured local I(V) curves in vanadium dioxide (VO$_2$) Mott insulator at various temperatures using a conducting AFM setup and determined the voltage threshold of the insulator to metal switching. By lifting the tip above the surface ($>$35nm), we have then measured the purely electrostatic force between the tip and sample surface as the voltage between these two was increased. In a very narrow temperature range (below 360K), a tip height range (below 60nm) and a voltage applied range (above 8V), we observed switching in the electrostatic force (telegraphic noise vs. time and vs. voltage). This purely electric field effect shows that the switching phenomenon is still present even without Joule heating in VO$_2$. [Preview Abstract] |
Session D23: Focus Session: Search for New Superconductors I: Exploring Emergent Phases
Sponsoring Units: DMPChair: David Mandrus, Oak Ridge National Laboratory
Room: D165
Monday, March 21, 2011 2:30PM - 2:42PM |
D23.00001: Phase Transitions in CuAs-based K$_{x}$Sr$_{1-x}$Cu$_{4}$As$_{2}$ System Bing Lv, Bernd Lorenz, Melissa Gooch, Feng Chen, Liangzi Deng, Ching-Wu Chu Inspired by the superconductivity with a T$_{c }\sim $ 3.5 K reported by Han et al. in Cu-based LiCu$_{2}$P$_{2}$, the Cu-based compound SrCu$_{4}$As$_{2, }$which has a more complex CuAs layers structure sandwiched by Sr layers, was synthesized and characterized. The magnetoresistance measurements show two anomalies around $\sim $140K and $\sim $60K, respectively, which is also consistent with the results of specific heat measurement. Systematically studies of K-doping into SrCu$_{4}$As$_{2}$ system are carefully investigated at ambient and high pressures and no superconductivity has been found in the K$_{x}$Sr$_{1-x}$Cu$_{4}$As$_{2}$ system down to 2K, although superconductivity might occur at lower temperature in this system. The overall pressure and doping effects on the magnetic and transport properties of the compounds will be presented and discussed. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D23.00002: Synthesis and Characterization of ACu$_{4}$Pn$_{2}$, with A = alkaline and Pn = As and Sb L.Z. Deng, Bing Lv, Y.Y. Xue, C.W. Chu A large number of the Cu-based layer compounds have been known to exist since the 1970's: some display structures similar to the Fe-based pnictide superconductors, such as the 111 and 122 phases. Unfortunately, other than their structures, reports on their physical properties are generally sparse. Only recently observation of superconductivity with a T$_{c }\sim $ 3.5 K was reported by Han et al. in LiCu$_{2}$P$_{2}$ that displays a 122 structure. It was suggested that a more complex layer structure in pnictides may favor higher T$_{c}$ as in the case of cuprates. We have therefore examined systematically ACu$_{4}$Pn$_{2}$ (142), with A = alkaline and Pn = pnictigen which has a more complicated layer substructure than the 1111, 111, 122 or 11 homologues. We have synthesized ACu$_{4}$Pn$_{2}$, with A = Li, Na, K, Rb and Cs and Pn = As and Sb, some of which were made for the first time. The magnetic, electrical, calorimetric and thermal electric properties were determined at ambient pressure and compared. High pressure was also applied to some of the compounds. The results will be presented and discussed. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D23.00003: Superconductivity and Competing Ordered Phase in RuPn (Pn = As, P) Daigorou Hirai, Tomohiro Takayama, Daisuke Hashizume, Ayako Yamamoto, Hidenori Takagi Unconventional superconductivity likely manifests itself when some competing electronic phases are suppressed down to zero temperature such as cuprates and iron-pnictide superconductors. Therefore, the correlated metallic state neighboring a competing electronic ordering can be a promising playground for unconventional superconductivity. Here we report superconductivity emerging adjacent to electronically ordered phases of RuPn (Pn = As, P). We found that RuAs(P) exhibits phase transitions at 240 (265) K, which is discerned as a drop of magnetic susceptibility or a resistivity upturn. Such anomalies can be suppressed by substituting Rh to the Ru site. Accompanied by the disappearance of the electronic order, superconductivity was found to emerge below 1.8 K and 3.8 K for RuAs and RuP, respectively. The superconductivity in Rh substituted RuPn, which neighbors a competing electronic order, might exhibit an exotic pairing state as seen in the unconventional superconductors known to date. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D23.00004: Towards New and Higher Temperature Superconductors via Theory Assisted Synthesis Invited Speaker: We discuss here a new methodology where electronic structure calculations are integrated with the synthesis of new superconducting materials, with the objective to design and realize new lamellar superconductors with high onset temperatures, and to rigorously test the apparent association of high temperature superconductivity with electron delocalization transitions occurring at quantum critical points. Since lamellar superconductors like the cuprates and iron pnictides are comprised of functional layers where superconductivity resides and charge reservoir layers that determine the electron count in the functional layers, we will use realistic electronic structure calculations to assess which transition metal monopnictides are closest to electron delocalization, and hence optimal for superconductivity. Optical conductivity and photoemission measurements will be used to compare the real and calculated electronic structures. We report initial results on electron and hole doped LaMnPO. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D23.00005: Gap physics of the doped semiconductor (Ca,F):LaMnPO J.W. Simonson, M.C. Aronson Single crystals of Ca$_{x}$La$_{1-x}$MnPO and LaMnPO$_{1-y}$F$_{y}$ (x = 0 to 0.50, y = 0 to 0.40) were synthesized to study the effect of Ca and F doping on LaMnPO, which is isostructural with several recently discovered Fe-based superconductors. The inclusion of F into the lattice was confirmed with single crystal XRD, showing a systematic reduction in unit cell volume with dopant content, in agreement with published accounts of similar compounds. Little change from undoped LaMnPO was observed in the resistivity of Ca doped crystals, while doping with successively higher concentrations of F yielded a systematic enhancement of conductivity. Nonetheless, all resistivity measurements were semiconducting, suggesting that E$_{F}$ remains pinned within the gap regardless of dopant concentration. Activated behavior was observed, with activation energies falling below 100 meV, substantially less than the ~1 eV optical gap. At low temperatures, the resistivity of all compositions exhibited temperature dependence in accord with variable range hopping, suggesting that transport is dominated by disordered and localized states at the Fermi level. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D23.00006: Properties of layered iron oxychalcogenides with checkerboard structure Liang L. Zhao, Daniel Ward, Douglas Natelson, Emilia Morosan The layered iron oxychalcogenides La$_2$O$_3$Fe$_2$X$_2$ (X = S, Se) have a unique checkerboard-like Fe$_2$OX$_2$ sublattice. Their Mott insulating behavior makes them promising candidates for novel superconductors. In this talk, we present results on the isostructural A$_2$F$_2$Fe$_2$OS$_2$ (A = Sr, Ba) compounds. Both pure and doped samples are characterized by magnetization, resistivity, heat capacity and Raman spectroscopy measurements. In addition to the previously reported antiferromagnetic transition around T$_N~\approx$ 100 K, we observed another phase transition in the ordered state, as well as a possible structural phase transition near room temperature. A change in the excitation gap at the high temperature (structural) phase transition is indicated by the temperature dependent resistivity. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D23.00007: Development of high-temperature solutions for the crystal growth of calchogenide and pnictide bearing compounds Xiao Lin, Rongwei Hu, Sergey Bud'ko, Paul Canfield With the discovery of superconductivity in the FeAs and FeSe/S based materials and proposed topological insulators in a variety of Te and Se based compounds the need to develop crystal growth techniques that readily incorporate and simultaneously control volatile (and often toxic) elements is of growing importance. In this talk we will review our initial efforts to develop versatile solution growth techniques for single crystal growth of P, As, S, Se and/or Te containing compounds. We will present our results on our use of S-bearing solutions to grow binary and ternary sulphides as well as the development of hybrid solutions the components of which each allow for the incorporation of hard to dissolve elements. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D23.00008: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 4:30PM - 4:42PM |
D23.00009: Bulk superconductivity in intercalated M$_{x}$ZrTe$_{3}$ (M=Cu, Ni) Xiangde Zhu, Hechang Lei, Cedomir Petrovic We report the bulk superconductivity of 3d transition metal (e.g. Cu, Ni) intercalated ZrTe$_{3}$ single crystals, M$_{x}$ZrTe$_{3}$ (M=Cu, Ni). ZrTe3 shows charge density wave (CDW) transition at T$_{CDW}$=63K and the CDW nesting vector q$_{CDW}$=(1/14,0,1/3) (reciprocal space). It is metallic below 300K with an anomaly due to CDW transition and becomes filamentary superconductor below 2 K. The derived superconducting parameters indicate that M$_{x}$ZrTe$_{3}$ are bulk type-II superconductors and can be depicted in the framework of Bardeen-Cooper-Schrieffer (BCS) scenario. Intercalation depresses the CDW transition and enhances the superconductivity. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D23.00010: New layered oxuchalcogenide La$_{2}$O$_{2}$Mn$_{2}$OSe$_{2}$ Hechang Lei, Cedomir Petrovic We report a new La$_{2}$O$_{2}$Mn$_{2}$OSe$_{2}$ and investigate its physical properties. It is a member of Ln$_{2}$O$_{2}$Tm$_{2}$OCh$_{2}$ (Ln= rare earth, Tm=Fe, Co, and Ch=S, Se). The structure of these compounds can be described as an alternate stacking of fluorite type [Ln$_{2}$O$_{2}$]$^{2+}$ or [A$_{2}$F$_{2}$]$^{2+}$ layers and anti-CuO$_{2}$-type [Tm$_{2}$OCh$_{2}$]$^{2-}$ layers (anti-perovskite type). La$_{2}$O$_{2}$Mn$_{2}$OSe$_{2}$ is the first manganese-base compound with anti-CuO$_{2}$-type layers. It is a ferromagnetic (FM) Mott insulator exhibiting several successive magnetic transitions. The magnetic properties are different from other compounds with this structure. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D23.00011: Physical Properties of CaFe$_{4}$As$_{3}$ Single Crystals Amar Karki, Yimin Xiong, Jianneng Li, Shane Stadler, Gregory McCandless, Julia Chan, Rongying Jin New compound CaFe$_{4}$As$_{3}$ crystallizes in an orthorhombic structure with Fe$_{2}$As$_{2}$ layers aligned along $b$ direction but a rectangular cross-section in \textit{ac} plane. The needle-shaped CaFe$_{4}$As$_{3}$ single crystals were grown and are found to undergo two successive phase transitions occurring at T$_{1} \sim $ 90 K and T$_{2} \sim $ 27 K, respectively. At T$_{1}$ the electrical resistivity increases and magnetic susceptibility decreases in both parallel and perpendicular to $b$ directions consistent with the scenario of spin-density-wave formation. At T$_{2}$, resistivity decreases sharply at T$_{2}$ with hysteresis while magnetic susceptibility increases along either $b$ direction or \textit{ac} plane. The underlying physics will be discussed by taking into account other physical properties. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D23.00012: Superconductivity in SnO: a Nonmagnetic Analogue to Fe-based Superconductors Daniel Khomskii, M.K. Forthaus, K. Sengupta, O. Heyer, N.E. Christensen, A. Svane, K. Syassen, T. Lorenz, M.M. Abd-Elmeguid We found that under pressure SnO with $\alpha $-PbO structure, the same structure as in many Fe-based superconductors, e.g. $\beta $-FeSe, undergoes a transition to a superconducting state for p $\ge $ 6 GPa with a maximum T$_{c}$ of 1.4 K at p = 9.3 GPa. The pressure dependence of T$_{c}$ reveals a dome-like shape and superconductivity disappears for p $>$ 16 GPa. It is further shown from band structure calculations that SnO under pressure exhibits a Fermi surface topology similar to that reported for some Fe-based superconductors and that the nesting between the hole and electron pockets correlates with the change of T$_{c}$ as a function of pressure. M.K. Forthaus et al., Phys.Rev.Lett. \textbf{105}, 15701 (2010) [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D23.00013: Development of iron pnictides with very thick perovskite-type blocking layers Hiraku Ogino, Shinya Sato, Naoto Kawaguchi, Yasuaki Shimizu, Kenji Machida, Akiyasu Yamamoto, Kohji Kishio, Jun-ichi Shimoyama Recently we have discovered several new layered iron pnictides having extremely thick blocking layers [1]. These compounds have up to 5 sheets of perovskite-type layers between iron pnictide layers. Similar to LiFeAs, these compounds showed bulk superconductivity without intensive carrier doping, and the Tc of the compounds are 40 K-class. On the other hand, the relationship between Tc and the iron-plane interlayer distance suggested that Tc of the iron based superconductor is basically determined by the local structure of Fe2As2 layer. It is thought that there is still considerable room for the development of new layered iron pnictide compounds, due to the structural and chemical flexibility of the blocking layer. However, the results suggest that optimization of the local structure of Fe2As2 layers and the dimensionality of the crystal structure may not lead to further enhancement of Tc. Their structural features as well as physical properties will be presented.\\[4pt] [1] H. Ogino et al., APL 97 (2010) 072506 [Preview Abstract] |
Session D24: Focus Session: Quantum Transport Simulations and Computational Electronics -- Nanostructures
Sponsoring Units: DCOMPChair: David Ferry, Arizona State University
Room: D167
Monday, March 21, 2011 2:30PM - 2:42PM |
D24.00001: Transient response of a quantum point contact due to the coupling with reservoirs Bozidar Novakovic, Irena Knezevic Transient response is important for better understanding of the DC and AC response of open quantum systems connected to large charge reservoirs. In this study we calculate the transient response of a quantum point contact (QPC) due to its coupling with reservoirs. The QPC, an open system, is modeled by a solution to the coupled, two-dimensional Schr\"{o}dinger and Poisson equations using a discrete subset of the normal modes basis. The normal modes are projected onto the traveling-wave solutions that match the incoming reservoir plane waves. The occupation of the open system states carries the information about the time evolution and is calculated by solving a coarse- grained quantum master equation with suitably defined open system/contact interaction Hamiltonians. The final electronic transient response is obtained by enforcing the current continuity across the open system/contacts boundaries through a time-dependent reservoir drift wavevector. We investigate the transient current response to a voltage step and its dependence on the gate bias and relaxation time in the contacts. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D24.00002: Open boundary-conditions using empirical pseudopotentials in quantum transport Bo Fu, Massimo Fischetti As device dimensions approach the 10 nm length-scale, the study of electronic transport requires the knowledge of an accurate band structure and of transport equations transcending the semiclassical Boltzmann picture. Having as our ultimate goal the study of dissipative quantum transport using the Pauli Master Equation, in this talk we address the numerical issue of how to formulate and implement numerically the open-boundary-condition Schr\"odinger equation within an empirical-pseudopotential full- band framework. Results regarding ballistic transport in Si nanowires will be presented. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D24.00003: Quantum Transport in Crossbar Devices Brandon Cook, Peter Dignard, Kalman Varga Electronic devices with crossbar geometries have recently been fabricated with nanoscale features (Zhong. et al, Science Vol. 302). Consisting of a two dimensional grid of wires, devices have been formed with a variety of components including carbon nanotubes and semiconductor nanowires. These devices are assumed to operate classically, but as the dimensions of the device shrink consideration of quantum effects becomes necessary. We consider a single junction between two wires up to a four by four grid of wires. Through a series of calculations with atomistic first-principles, tight-binding and analytic models of multi-terminal devices we demonstrate the presence of unique behavior, such as interference effects, not present in classical models. It is expected that exploitation of these effects will useful in the creation of circuit components. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D24.00004: Empirical Pseudopotential Approach to Semiclassical and Quantum Electronic Transport in Nanometer-scale Structures Invited Speaker: The study of electronic transport in semiconductor structures requires an accurate knowledge of the kinematics ($i.e$., band structure) and of the dynamics ($i.e$., transport equations and collision processes). As the VLSI technology looks at various sub-10 nm structures as alternatives to the traditional Si CMOS, neither the conventional bulk band structure of the semiconductor nor the semiclassical (Boltzmann) transport equations can be used with confidence to treat the kinematics and dynamics of electronic transport, the former because of size-dependent (quantum confinement) and interface/surface dependent band structure effects; the latter because of the possibility of quantum interference effects at this length scale. Here we will show that empirical pseudopotentials -- obtained from the literature and adjusted to yield correct workfunctions, band-alignment, and strain effects -- can be used to obtain a sufficiently accurate (as compared to first-principle results) band structure of several systems of technological interest ($e.g$., thin Si and Ge layers, III-V hetero-layers, nanowires, graphene nanoribbons and C nanotubes). Using this information, semiclassical transport is studied using a Monte Carlo technique and calculating the scattering rates consistently with the band structure information. In some cases, such as in considering scattering with interface and line-edge roughness, the pseudopotential themselves can be used to obtain accurate scattering potentials. The case of high-field transport in Si inversion layers is discussed, showing how the band-structure near the X symmetry point induces a lower saturated electron velocity. Finally, we discuss the wave equation and open boundary conditions which must be employed to handle ballistic quantum transport accounting for the full band structure. Dissipative transport is discussed in the context of a Master equation approach, illustrating this approach with examples ranging from double-gate FETs to Si nanowires. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D24.00005: Self-Consistent Monte Carlo Study of the Coulomb Interaction under Nano-Scale Device Structures Nobuyuki Sano It has been pointed that the Coulomb interaction between the electrons is expected to be of crucial importance to predict reliable device characteristics. In particular, the device performance is greatly degraded due to the plasmon excitation represented by dynamical potential fluctuations in high-doped source and drain regions by the channel electrons. We employ the self-consistent 3D Monte Carlo (MC) simulations, which could reproduce both the correct mobility under various electron concentrations and the collective plasma waves, to study the physical impact of dynamical potential fluctuations on device performance under the Double-gate MOSFETs. The average force experienced by an electron due to the Coulomb interaction inside the device is evaluated by performing the self-consistent MC simulations and the fixed-potential MC simulations without the Coulomb interaction. Also, the band-tailing associated with the local potential fluctuations in high-doped source region is quantitatively evaluated and it is found that the band-tailing becomes strongly dependent of position in real space even inside the uniform source region. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D24.00006: Oxygen vacancy mediated dielectric breakdown in ultrathin high-k gate dielectric stacks Blanka Magyari-Kope, Yoshio Nishi The reliability of the high-k gate stack becomes a significant challenge with the continuous scaling of the metal-oxide-semiconductor-field-effect-transistors, due to deposition techniques of ultrathin oxides and defects in the gate stack. One of the key problems associated with ultrathin oxide layers is the degradation of the gate oxides under electrical stress, due to traps generated by oxygen vacancies present in these materials. First principles methods based on density functional theory combined with non-equilibrium Green's function calculations are employed to calculate the tunneling current through ultrathin oxide layers of HfO$_{2}$ and SiO$_{2}$ in a gate stack structure with TiN metal electrode. Model systems that incorporate the atomistic description of a conductive filament formation due to ordering of oxygen vacancies in the oxide layers and the oxide-oxide interface of the gate stack were investigated. The microscopic effects of defects ordering on the electronic transport through the gate oxides are analyzed and discussed. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D24.00007: Electron-phonon coupling in semiconductors and their nanostructures: effect on transport properties Jelena Sjakste, Paola Gava, Nathalie Vast, Valeriy Tyuterev Parameter-free description of the electron-phonon coupling is crucial for the simulation of the electron and thermal transport in materials, especially nanostructured ones. Recently, we have developed an \emph{ab initio} approach which allows to calculate the electron-phonon constants and scattering times for collisions of carriers in the conduction band with short-wavelength phonons [1,2]. We will present our results on the electron-short-wavelength phonon interaction in silicon, which enables us, on one hand, to shed new light on the transitions between shallow donor levels in doped Si [2], and, on the other hand, to improve the description of its electronic mobility [3]. Finally, we will discuss the effect of the material nanostructuring on the electron-phonon coupling constants, e.g. in semiconducting superlattices.\\[4pt] [1] J. Sjakste, N. Vast, V. Tyuterev, Phys. Rev. Lett. 99, 236405 (2007).\\[0pt] [2] V. Tyuterev, J. Sjakste, N. Vast, Phys. Rev. B 81, 245212 (2010)\\[0pt] [3] Z. Wang, S. Wang, S. Obukhov, N. Vast, J. Sjakste, V. Tyuterev, N. Mingo, submitted (2010) [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D24.00008: Ballistic Spin Field Effect Transistor Based on Silicon Nanowires Dmitri Osintsev, Viktor Sverdlov, Zlatan Stanojevic, Siegfried Selberherr We investigate the properties of ballistic spin field-effect transistors build on silicon nanowires. An accurate description of the conduction band based on the {\bf k$\cdot$p} model is necessary in thin and narrow silicon nanostructures. The subband effective mass and subband splitting dependence on the nanowire dimensions is analyzed and used in the transport calculations. The spin transistor is formed by sandwiching the nanowire between two ferromagnetic metallic contacts. Delta-function barriers at the interfaces between the contacts and the silicon channel are introduced. The major contribution to the electric field-dependent spin-orbit interaction in confined silicon systems is due to the interface-induced inversion asymmetry which is of the Dresselhaus type [1]. We study the current and conductance through the system for the contacts being in parallel and anti-parallel configurations. Differences between the [100] and [110] orientated structures are investigated in details.\\[4pt] [1] M.O. Nestoklon {\it et al., Phys.Rev.B} {\bf 77}, 155328 (2008); M. Prada {\it et al.}, cond-mat 0908.2417. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D24.00009: Junctionless nanowire field-effect transistor versus inversion mode devices Bart Soree, Anh-Tuan Pham, Dries Sels, An De Keersgieter, Wim Magnus Several years ago, a novel device concept was proposed : the nanowire (NW) iJFET [1]. Today, this device concept is being explored by several research teams [1-3] and is also known as the pinch-off FET (POFET) or junctionless transistor. The most important advantage of the junctionless transistor is the uniform doping throughout source, channel and drain which greatly simplifies its fabrication. We have performed modeling and simulations to compare the performance of the junctionless pinch-off FET with that of inversion mode devices. In order to make the comparison, we address the regime of thick and long nanowires through analytical modeling of the current- voltage characteristics, while for long and thin nanowires we perform dissipative transport modelling to obtain the low-field mobility. Finally, ballistic transport modelling is performed using the sub band decomposition method for ultra- short nanowires. \\[4pt] [1] B. Sor\'ee, et al., Journal of Computational Electronics, vol.7, issue 3, 380-383 2008. \\[0pt] [2] B. Sor\'ee, et al., Nanoelectronics days 2010, Aachen, Germany. \\[0pt] [3] J.-P. Colinge, et al., Nature Nanotechnology 5, 225-229, 2010 [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D24.00010: Gap suppression and delocalization of 1D electron gas driven by a strong AC electric field Kathleen E. Hamilton, Leonid P. Pryadko We will argue that a strong AC electric field can coherently suppress a band gap in a high-mobility one-dimensional wire. At half-filling, the expected effect is delocalization of the carriers, in contrast to Stark localization at low frequencies, or Dynamical Destruction of Tunneling at frequencies exceeding the bandwidth. Another effect of the gap suppression is the doubling of the Bloch oscillations' period. We support these expectations with numerical simulations of the non-linear current response for several model systems driven by a combined high and low-frequency electric field. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D24.00011: Modeling of Phonon-assisted Zener Tunneling in Indirect Semiconductors William Vandenberghe, Bart Sor\'ee, Massimo Fischetti, Wim Magnus, Guido Groeseneken With the scaling in the semiconductor device dimensions, Zener tunneling has become an important source of leakage in conventional MOSFET devices but it could also provide drive current for a novel type of tunnel transistor. A good understanding of the process of Zener tunneling is therefore required and present-day one-dimensional semi-classical models fall short of explaining tunneling in devices with potential profiles with a pronounced two-dimensional shape. We have developed a formalism to calculate the phonon-assisted current under a given three dimensional external potential profile. The current is calculated from the transition probability for an electron to go from the valence to the conduction band. The transition probability is determined from the spectral functions corresponding to the valence and the conduction band. In the presence of a one-dimensional uniform low electric field, the Kane model is recovered. An example of the formalism is given for the case of an abrupt p-n diode and compared with existing semi-classical models. It is seen that the uniform field model is actually better than the WKB model but that none of the semi-classical models give good results at low bias conditions. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D24.00012: Zener Tunneling: Correspondence between Quantum and Semi-Classical Formalisms Kartik Ganapathi, Sayeef Salahuddin The resurgence of interest in band-to-band tunneling has been due to its usefulness in overcoming the 60 mV/decade limit in turn-on characteristics of a MOSFET thereby providing path for lowering the operating power. The expression due to \textit{Kane}, for calculating transmission coefficient and current due to Zener tunneling in a $p-n$ diode, has been extensively used over the years for explaining experimental tunneling characteristics. While this closed form expression relates tunneling probability with simple quantities like bandgap, effective mass, electric field etc., being a formula based on semi-classical approximation, it is valid strictly in the low-field regime. With finite size effects playing significant role in ultra small device dimensions, this approximation breaks down and one needs to have a full quantum mechanical treatment of the tunneling problem with a realistic band-structure. We report a numerical simulation of this problem within the NEGF formalism with a tight-binding Hamiltonian wherein the extent of validity of Kane's formula is examined. We also discuss how Kane's parameters should be altered and interpreted in high field region. The results are compared with experimental data in two different systems -- InAs $p^{+}-n^{+}$ and GaN/AlGaN heterojunction tunnel diodes. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D24.00013: Simulation of nanoscale four-probe resistance measurements under finite bias voltages Asako Terasawa, Keiji Tobimatsu, Tomofumi Tada, Takahiro Yamamoto, Satoshi Watanabe To understand the interesting features in nanoscale four-probe measurements such as the negative value and the oscillation of four-probe resistance [1], we investigate the behavior of nanoscale four-probe resistance theoretically [2-3]. In the present work, we examine the effect of bias voltage on four-probe resistance in nanoscale four-probe systems. For a set bias voltage between current probes, we first estimate the voltage between voltage probes when no current flows between them from the four-probe and two-probe resistances at the zero-bias limit, assuming the linear response. Then we calculate the dependence of currents in the voltage probes on the bias voltage applied to the current probes with applying the voltage thus estimated between the voltage probes. The calculated currents in the voltage probes have nonzero but much smaller values compared with those in current probes, and show the non-linear dependence on the bias voltage. This result indicates assumption of linear response is not valid for the bias voltage of the order of a tenth V, and that currents and voltages should be determined self-consistently to estimate four-probe resistance. [1] B. Gao et al., Phys. Rev. Lett. 95, 196802 (2005). [2] A. Terasawa et al., Phys. Rev. B 79, 195436 (2009). [3] A. Terasawa et. al., New J. Phys. 12, 083017 (2010). [Preview Abstract] |
Session D25: Superconductivity: Vortex Phenomena I
Sponsoring Units: DCMPChair: Carmen Almasan, Kent State University
Room: D166
Monday, March 21, 2011 2:30PM - 2:42PM |
D25.00001: Anomalous field-symmetric Nernst signal in striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ N. Phuan Ong, Lu Li, J. M. Tranquada, Genda Gu Starting at the structural transition temperature $T_{d2}$ = 54 K, the striped cuprate La$_{2-x}$Ba$_x$CuO$_4$ ($x = \frac18$) displays a remarkable cascade of transitions$^1$ at the characteristic temperatures $T_{d2} > T_1^{**} > T_{BKT} >T_c$, before settling down to 3D superconductivity with long-range coherence at $T_c$= 5 K. The Nernst signal $e_N$ and thermopower $S$ have been investigated in detail in these multiple states. As in pure LaSrCuO, the Nernst coefficient $N = lim_{B\to 0} e_N/B$ (initially negative) acquires a positive vortex contribution at 120 K that grows rapidly. However, here, $N$ saturates in the interval $T_{d2}$ (54 K) $\to T_1^{**}$ (34 K). As the vortex liquid becomes increasingly stabilized below $T_1^{**}$, $N$ resumes increasing at an even steeper rate. Surprisingly, below 34 K, $e_N$ acquires a $B$-symmetric component that is very large and oscillatory in $B$. We have excluded $S$ and quasiparticles as the source of the anomalous term. We will discuss various origins including the possibility of vortex formation mechanisms that break time-reversal invariance.\\ 1) J. M. Tranquada \emph{et al.}, Phys. Rev. B 78, 174529 (2008). [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D25.00002: Torque measurements in underdoped Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ single crystals T. Hu, H. Xiao, P. Gyawali, H.H. Wen, C.C. Almasan We report in-plane and out-of-plane angular-dependent torque measurements on underdoped Bi$_2$Sr$_{2-x}$La$_x$CuO$_{6+\delta}$ single crystals both below and above the zero-field superconducting transition temperature $T_{c}(0)$ and in applied magnetic fields $H$ up to 14 T. The out-of-plane torque data show that a diamagnetic signal persists into the normal state, which is consistent with previous reports on Bi$_2$Sr$_2$Ca$_2$CuO$_{8+\delta}$. These data also reveal that the vortex matter in the superconducting state has lower dimensionality compared to the one in the normal state. Furthermore, in-plane torque measurements show that the presence of vortices above $T_c(0)$ destroys the phase coherence of the Cooper pairs. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D25.00003: Generics of the diamagnetism, Nernst signal, and finite size effects in superconductors above the transition temperature S. Weyeneth, T. Schneider Various superconductors exhibit peculiar features above the transition temperature $T_{c}$. In particular the observation of a large Nernst signal $N$ and a remarkable diamagnetism above $T_{c}$ in cuprate and conventional superconductors attracted considerable attention. Noting that in these materials the spatial extent of the homogeneous domains is limited, we explore the relevance of a zero dimensional (0D)-model, neglecting thermal fluctuations, to describe the isothermal magnetization curves $m_{\mathrm{d}}\left(H\right)$ in various superconductors above $T_{c}$. It is shown that for cuprates as well as for Pb nanoparticles, both, the full 0D-model as well as its Gaussian approximation, mimic the essential features of the magnetization curves above $T_{c}$ rather well. Furthermore, the isothermal Nernst signal of a superconducting Nb$_{0.15}$Si$_{0.85}$ film is fully consistent with this scenario. Accordingly, the observed diamagnetism above $T_c$ in Pb nanoparticles, in the cuprates La$_{1.91}$Sr$_{0.09}$CuO$_{4}$ and BiSr$_2$Ca$_2$CuO$_{8-\delta}$, as well as the Nernst signal in Nb$_{0.15}$Si$_{0.85}$ films, are all in excellent agreement with the scaling properties emerging from the here discussed 0D-model. Therefore, singlet Cooper pairs subjected to orbital pair breaking in a 0D-system are the main source of the observed diamagnetism and Nernst signal in an extended temperature window above $T_{c}$. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D25.00004: Effects of Vortex Charge Explored by NMR Spectroscopy in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Andrew Mounce, S. Oh, S. Mukhopadhyay, W.P. Halperin, A.P. Reyes, P.L. Kuhns, K. Fujita, M. Ishikado, S. Uchida We measure nuclear magnetic resonance (NMR) spectra for various dopings of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+y}$ to determine the variations of local magnetic fields generated by the vortex lattice at low temperatures. With increasing the external magnetic field H$_0$, the local field variations decrease to values lower than expected by Ginzburg-Landau calculations for an Abrikosov vortex lattice.\footnote{Brandt, E. H., Phys. Rev. Lett. \textbf{66}, 3213-3216 (1991).} Taking into account charge accumulation in vortex cores, which has been predicted by theory\footnote{Khomskii, D. I. \& Freimuth, A. Phys. Rev. Lett. \textbf{75}, 1384-1386 (1995).} and suggested by experiment,\footnote{Kumagai, \textit{et al.} Phys. Rev. B \textbf{63}, 144502 (2001).} we are able to identify the amount of charge needed to disturb the vortex lattice and decrease local field variations as in the observed spectra. The amount of charge is found to be $\sim$2x$10^{-3}$e and doping dependent in line with theoretical predictions.$^2$ This work is supported by DOE/BES: DE-FG02-05ER46248 and the NHMFL by NSF and the State of Florida. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D25.00005: Vortex dynamics in ferromagnetic / high T$_{c}$ superconducting heterostructures N. Haberkorn, J. Kim, M. Miura, B. Maiorov, P. Dowden, L. Civale In this work we explore the influence of the ferromagnetic landscape generated by magnetic phase separation in manganites on the vortex pinning of HTS/FM heterostructures. We have grown Gd$_{2/3}$Ca$_{1/3}$MnO$_{3 }$(GCMO), Y$_{2/3}$Ca$_{1/3}$MnO$_{3 }$(YCMO) and YBCO films, as well as GCMO/YBCO and YCMO/YBCO bilayers by Pulsed Laser Deposition. GCMO is a ferrimagnetic material with Curie temperature of $\sim $ 80 K and a compensation temperature of 15 K, whereas YCMO is ferromagnetic with Curie temperature $\sim $ 80 K. In both materials the saturation magnetization is smaller than the value expected from ferrimagnetic and ferromagnetic order, suggesting phase separation with small ferromagnetic domains. The magnetic domain size as a function of temperature for the magnetic films was investigated using magnetic force microscopy (MFM). We will present a comparison of the vortex pinning and dynamics in the YBCO single layers and the GCMO/YBCO and YCMO/YBCO bilayers that allows us to identify the temperature and field regimes where magnetic pinning is effective. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D25.00006: Hysteretic magneto-transport of a High-T$_{c}$ superconducting/ferromagnetic multilayer with tunable magnetic domain structure Javier E. Villegas, Cristina Visani, Peter J. Metaxas, Aurelie Collaudin, Baptiste Calvet, Rozenn Bernard, Javier Briatico, Cyrile Deranlot, Karim Bouzehouane The magneto-transport of a hybrid heterostructure combining a YBaCuO$_{7-\delta }$ thin film and a Co/Pt superlattice shows an unusual hysteretic behavior. Depending on the angle between the external applied field and the film plane, and on the magnetic history, either a increase or a decrease of the mixed-state resistance is observed. The combination of magneto-transport, magnetic force microscopy and anomalous Hall effect measurements allows us to correlate these effects to the magnetic domain structures in the Co/Pt superlattice. We unequivocally prove that the hysteretic magneto-transport is induced by the stray magnetic fields from tunable magnetic domain structures, which may induce vortices or produce vortex pinning, leading to the increase/decrease of the mixed-state resistance. Work supported by French ANR ``Superhybrids-II'' and RTRA ``Supraspin'' grants. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D25.00007: Vortex/Domain Coupling in Superconducting Films on Ferromagnetic Substrates with Different Domain Structures Vitalii Vlasko-Vlasov, Ulrich Welp, Daniel Rosenmann, Wai Kwok, Alexander Buzdin, Alexander Melnikov, Lyudmila Uspenskaya, Vincent Fratello In this work we address engineering of magnetic pinning in the superconducting/ferromagnetic hybrids. Using direct magneto-optical observations effects of interactions between superconducting vortices in Nb film and magnetization in domains of substituted iron garnet films are studied. Two garnet films with perpendicular anisotropy but different thickness-to-domain width ratios and one showing the reorientation from the in-plane to the normal easy axis are used as substrates. All three samples demonstrate strong domain/vortex coupling and reveal high pinning by domain walls, which persists up to temperatures close to the superconducting Tc. In turn, vortex motion modifies domain structures of garnets at T$<$Tc resulting in the alignment and strong contraction of wide domains but smaller width changes of narrow domains. A model is proposed explaining the results of our observations. This work was supported by DOE-BES under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D25.00008: Matching field effects in high-$T_{c}$ superconductors with self-assembled columnar defects J.W. Sinclair, Y.L. Zuev, J.R. Thompson, D.K. Christen, C. Cantoni, S.-H. Wee, A. Goyal, C. Varanasi Columnar defects (CDs) in a superconductor provide angularly selective vortex pinning, where the density of CDs provides a natural scale for the magnetic field, the ``matching field.'' While dramatic changes in the critical current density $J_{c}$ might be expected when the vortex density exceeds the CD density, observations of this phenomena has been elusive in systems with chemically produced, self-assembled CDs of BaSnO$_{3}$, BaZrO$_{3}$, SrZrO$_{3}$, etc. Here we describe studies of two RBa$_{2}$Cu$_{3}$O$_{\sim 7}$ systems containing these self-avoiding CDs, using either contact free magnetic or transport measurements. In magnetic measurements on a material with measured areal CD of $\sim $2.5 T, the $J_{c}$ decreased abruptly when the applied field $H$ exceeded this level; this feature was observed over a wide temperature range, from 77 to $\sim $ 40 K. All these features disappeared when the field was tilted away from the CD orientation. Research at ORNL sponsored by US DOE. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D25.00009: Superconducting vortex dynamics on asymmetric arrays with symmetric pinning centers Jose L. Vicent, David Perez de Lara, Alejandro Alija, Elvira M. Gonzalez, Maria Velez, Jose I. Martin Arrays of Ni nanodots embedded in Nb superconducting films have been fabricated by sputtering and electron beam lithography techniques. The arrays are periodic triangular lattices of circular Ni dots arranged in a kagom\'e-like pattern with broken reflection symmetry. DC magnetoresistance shows several fractional matching field minima below the first matching field for vortex motion parallel and perpendicular to the array reflection symmetry. AC magnetoresistance shows reversal ratchet effect when the vortex lattice moves parallel to the array reflection symmetry. These effects could be understood taking into account the vortex lattice density. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D25.00010: Enhanced superconducting vortex pinning in the Corbino geometry Yaniv Rosen, Ivan K. Schuller We probed a dynamic system of superconducting vortices with an artificial pinning landscape in the Corbino geometry. Current was applied from the center of the disc and propagated radially outward to produce a circular force with a strength proportional to 1/r on the vortices. This caused a shearing force on the vortex lattice and was studied with varying current densities and temperature. Matching minima in the magnetoresistance curves were still observed under the Corbino conditions for the square lattice pinning site geometry. Surprisingly the even numbered matching fields show enhanced pinning compared to the odd matching fields. Other interesting temperature and current density dependencies will also be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D25.00011: Proximity Effects and Vortex Dynamics in Nanostructured Superconductors Serena Eley, Nadya Mason We report transport measurements on triangular arrays of proximity-coupled superconducting islands placed on normal-metal substrates. The superconducting islands are well-understood coherent systems with long-range electron interactions, while the intervening normal metal channels introduce known dissipation into the system. We show how by changing the island spacing, we can tune characteristics such as the critical temperature and field. The arrays undergo a Kosterlitz-Thouless vortex-unbinding phase transition at zero-field, and we observe frustration-induced magnetoresistance oscillations at finite fields. We also observe unusual cusp-like behavior in resistance vs temperature at finite magnetic fields [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D25.00012: Vortex-induced dissipation in current-biased superconducting nanowires Lev Bulaevskii, Matthias Graf, Cristian Batista, Vladimir Kogan We study the dissipation due to vortex crossings in thin current-biased superconducting films with thickness on the order of the coherence length, and with width much narrower than the magnetic Pearl length in thin films. We find that for technologically relevant thin and narrow films or nanowires with width much larger than the coherence length, the barrier for phase slips by creation of temporary normal regions across the entire film width is too big. Thus phase slips become highly improbable. Instead, we propose the process of a vortex crossing the strip from one edge to the other, perpendicular to the bias current, as the dominant mechanism for generalized phase slips resulting in detectable voltage pulses. We derive phase-current relations and predict the amplitude and duration of voltage pulses induced by vortex motion due to thermal fluctuations and bias current. The consequnces for the current-voltage characteristics and the fundamental limitations for dark counts in superconducting nanowire single-photon detectors are discussed. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D25.00013: Synthesis and vortex dynamics of high-Tc superconducting nanoribbons with a periodic array of holes Qiong Luo, Michael Latimer, Zhili Xiao, Lei Fang, Wai-Kwong Kwok We fabricated nanoribbons of high-temperature superconductors YBCO-123 and BSCCO-2212. Experimental procedures for growing, manipulating and characterizing the nanoribbons will be presented. Furthermore, we introduced regular arrays of nanoscale holes into these nanoribbons through focused-ion-beam (FIB) milling to study the effects of periodic pinning on vortex dynamics. Resistive measurements reveal vortex matching effect and striking feature in the volage-current behavior associated with various driven regimes of the vortex matter related to vortex dynamics phase transitions [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D25.00014: Strong pinning of vortex lines by nanoparticles Alexei Koshelev, Alejandro Kolton Pinning of vortex lines by array of nanoparticles embedded inside superconductors became the most efficient practical way to achieve high critical currents. In this situation pinning occurs via trapping of the vortex-line segments and the critical current is determined by the typical length of trapped segment. To verify analytical estimates and develop a quantitative description of strong pinning, we use large-scale numerical simulations. We study the dependence of the critical force on the density of pins in the regime of independently pinned lines, statistical properties of trapped lines, and suppression of the apparent critical force by thermal fluctuations. [Preview Abstract] |
Session D26: Focus Session: Iron Based Superconductors -- Electronic Structure
Sponsoring Units: DCOMP DMPChair: Adriana Moreo, University of Tennessee and Oak Ridge National Laboratory
Room: D162/164
Monday, March 21, 2011 2:30PM - 3:06PM |
D26.00001: Confronting LDA+DMFT results with experiments in the iron pnictide families Invited Speaker: The normal state electronic structure of the pnictides is an important challenge to electronic structure theory . Optical conductivity experiments are indicative of electronic electron correlations (with mass renormalizations of the order of three). Neutron scattering experiments have features characteristic of both itinerant and localized electrons. High energy spectroscopies indicate the absence of satellite peaks. In this talk, we will show how LDA+DMFT allows us to reconcile these apparently inconsistent facts, and trace them to the unique chemical ingredients of these compounds: the iron Hunds rule coupling and the hybridization with very broad arsenide bands. Quantitative results for the different iron pnictide families will be presented and the factors that govern the strength of the correlations in this family of compounds will be discussed. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D26.00002: Electronic structure of iron-pnictide superconductors: just scratching the surface Erik van Heumen, J. Vuorinen, K. Koepernik, F. Massee, Y. Huang, J.B. Goedkoop, M. Lindroos, M. Shi, K. Haule, J. van den Brink, M.S. Golden Angle resolved photoemission (ARPES) and scanning tunneling microscopy (STM) are important tools in the study of iron-pnictide high T$_{c}$ superconductors. These techniques are surface sensitive and one has to ensure that the electronic structure probed in an experiment corresponds to the bulk electronic structure. Using a combination of experimental techniques (STM, ARPES, LEED) and theoretical calculations (LEED simulations, DFT), I will show that the surface structure of BaFe$_{2-x}$Co$_{x}$As$_{2}$ is both reconstructed and distorted. LEED data combined with simulations is used to solve the real surface and sub-surface structure. The impact of the surface on the electronic structure is then determined by comparing DFT slab calculations, based on the real surface structure, with ARPES experiments. The presence of surface states gives a natural explanation for the large k-space broadening observed in these materials. Having identified the surface states and bulk bands, I will address the more fundamental questions with regard to the electronic structure and its role in the mechanism of high temperature superconductivity. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D26.00003: The superconducting phase and electronic excitations of (Rb,Cs)Fe$_{2}$As$_{2}$ J. Kanter, Z. Shermadini, R. Khasanov, A. Amato, Z. Bukowski, B. Batlogg We present specific heat, transport and Muon-Spin Rotation ($\mu $SR) results on (Rb,Cs)Fe$_{2}$As$_{2}$. RbFe$_{2}$As$_{2}$ was only recently found to be superconducting below 2.6 K by Bukowski et al. Compared to the related BaFe$_{2}$As$_{2}$ the electron density is lower and no magnetic order is observed. For the superconducting phase the superfluid density was calculated from $\mu $SR data. The temperature dependence of the superfluid density and the magnetic penetration depth is well described by a multi-gap scenario. In addition the electronic contribution the specific heat was studied for different compositions and magnetic fields and reveals a high value for the Sommerfeld coefficient $\gamma $. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D26.00004: Revealing the degree of magnetic frustration in iron pnictides Cheng-Chien Chen, Ryan Applegate, Brian Moritz, Thomas Devereaux, Rajiv Singh Proposed theories for collinear antiferromagnetism in iron pnictides vary greatly in the amount of magnetic frustration and proximity to a quantum critical point. We discuss how imaging the magnetic fields around a non-magnetic impurity can quantify the degree of magnetic frustration and thereby distinguish various theoretical proposals. It is found that in a frustrated J1-J2 model a non-magnetic impurity strongly reduces its neighboring local moments, and overturned dynamical spins appear close to zero energy. In contrast, the spatially anisotropic J1a-J1b-J2 model produces enhanced local moments on sites neighboring the impurity. In both cases, the disturbance in the magnon local density of states exhibits an anisotropic stripe pattern. These predictions can be tested by experiments such NMR or spin-resolved STM measurements. The results can elucidate the role of frustration in antiferromagnets and help narrow the possible models for the iron-pnictide superconductors. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D26.00005: Theory for Magnetism and Triplet Superconductivity in LiFeAs Maria Daghofer, Philip Brydon, Carsten Timm, Jeroen van den Brink Superconducting pnictides are widely found to feature spin-singlet pairing in the vicinity of an antiferromagnetic phase, for which nesting between electron and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by (i) poor nesting properties and (ii) unusually shallow hole pockets. Investigating magnetic and pairing instabilities in an electronic model that incorporates these differences, we find antiferromagnetic order to be absent. Instead we observe almost ferromagnetic fluctuations which drive an instability toward spin-triplet $p$-wave superconductivity.\\ P.M.R. Brydon, M. Daghofer, C. Timm, and J. van den Brink, arXiv:1009.3104 [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D26.00006: Unified Picture for Magnetic Correlations in Iron-Based Superconductors Wei-Guo Yin, Chi-Cheng Lee, Wei Ku The spin-density-wave mechanism due to Fermi surface nesting is widely used to describe magnetism in the parent compounds of iron-based superconductors; however, the field is puzzled by the different magnetic structure of FeTe, which apparently has similar Fermi-surface topology. Here we propose [1] an orbital-degenerate double-exchange model which includes both itinerant electrons and localized spins. The physical properties of the model are governed by the competition between the antiferromagnetic superexchange and Hund's rule coupling. We show that the strength of the effective Hund's rule coupling term strongly depends on the anion height from the iron plane, leading to the collinear (C-type) magnetic order with the ferro-orbital order in iron pnictides and the bicollinear (E-type) magnetic order without orbital ordering in FeTe. This shows that the magnetism in iron-based superconductors can be described in a unified picture and may have a universal origin for all of the materials. Our results reveal the crucial role of Hund's rule coupling for the strongly correlated nature of the system and suggest exploring the interplay of the spin, orbital, charge, and lattice degrees of freedom in promoting high-temperature superconductivity. Work supported by US DOE.\\[0pt] [1] W.-G. Yin, C.-C. Lee, and W. Ku, PRL 105, 107004 (2010). [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D26.00007: Electronic specific heat of the iron chalcogenide superconductor Fe(Te$_{0.55}$Se$_{0.45})$ Jin Hu, Tijiang Liu, Bin Qian, Zhiqiang Mao We report specific heat studies of superconducting Fe(Te$_{0.55}$Se$_{0.45})$[1]. We have obtained the electronic specific heat by subtracting the phonon contribution evaluated from the normalization of the phonon specific heat of a non-superconducting reference sample (Fe$_{0.9}$Cu$_{0.1})$(Te$_{0.55}$Se$_{0.45})$. Our results show that the superconducting ground state is accompanied by unpaired quasiparticles, as in FeAs superconductors, with \textit{$\gamma $}$_{0 }$\textit{$\sim $}2.3 mJ/mol K$^{2}$. The temperature dependence of the electronic specific heat $C_{es}(T)$/$T$ can be well fitted using either a single s-wave gap model with 2\textit{$\Delta $}= 5.2 k$_{B}T_{c}$ or a two-gap model with 2\textit{$\Delta $}$_{1}$/k$_{B}T_{c }$= 5.8 and 2\textit{$\Delta $}$_{2}$/k$_{B}T_{c}$ = 4.0; the two-gap model fitting is slightly better than the single gap fitting. Such large gaps, together with a large specific heat jump $\Delta C(T_{c})$/$T_{c}\sim $57.3 mJ/mol K$^{2}$, suggest a strong-coupling superconducting state. While $C_{es}(T)$/$T$ exhibits isotropic s-wave gap behavior, the magnetic field-induced change in the electronic specific heat $\Delta $\textit{$\gamma $}($H)$ exhibits sublinear field dependence, implying the superconducting pairing in iron chalcogenide superconductors also involves a multiple band effect, as seen in pnictide superconductors. [1] M. H. Fang, H. M. Pham, B. Qian, T. J. Liu, E. K. Vehstedt, Y. Liu, L. Spinu, and Z. Q. Mao, Superconductivity close to magnetic instability in Fe(Se$_{1-x}$Te$_{x})_{0.82}$, Phys. Rev. B 78, 224503 (2008). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D26.00008: Electronic correlations and superconducting response in the optical properties of FeTe$_{\mathbf{0.55}}$Se$_{\mathbf{0.45}}$ C.C. Homes, A. Akrap, J.S. Wen, Z.J. Xu, Z.W. Lin, Q. Li, G.D. Gu The in-plane complex optical properties of the iron-chalcogenide superconductor FeTe$_{0.55}$Se$_{0.45}$ have been determined above and below $T_c = 14$~K. At room temperature the conductivity is described by a weakly-interacting Fermi liquid with $\omega_{p,D}\simeq 7200$~cm$^{-1}$ and $1/\tau_D\simeq 414$~cm$^{-1}$. Below 100~K the conductivity is no longer described by the Drude model. Adopting the generalized Drude model reveals that $1/\tau(\omega) \propto \omega$ in the terahertz region just above $T_c$, signaling the increasingly correlated nature of this material.\footnote{C. C. Homes {\em et al.}, Phys. Rev. B {\bf 81}, 180508(R) (2010).} For $T \ll T_c$ the superconducting plasma frequency $\omega_{p,S} \simeq 3000$~cm$^{-1}$ ($\lambda_{\it eff} \simeq 5300$~\AA ); $\omega_{p,S}^2/\omega_{p,D}^2 \ll 1$ indicating that this material is not in the clean limit. Allowing $\sigma_{dc}\equiv \sigma_1(\omega \rightarrow 0)$, then $\sigma_{dc}(T\simeq T_c) \simeq 3500\pm 400$ $\Omega^{-1}$cm$^{-1}$ and the superfluid density $\rho_{s0} \equiv \omega_{p,S}^2 \simeq 9\pm 1\times 10^6$~cm$^{-2}$ places material close to the scaling line $\rho_{s0}/8 \simeq 8.1\,\sigma_{dc}T_c$ for a BCS dirty-limit superconductor. Below $T_c$ the optical conductivity reveals two energy scales for the superconductivity at $\Delta_1(0) \simeq 2.5$~meV and $\Delta_2(0) \simeq 5.1$~meV, consistent with the $s^\pm$ model. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D26.00009: Microwave surface impedance measurements of LiFeAs and LiFe(As,P) single crystals Y. Imai, H. Takahashi, T. Okada, A. Maeda, K. Kitagawa, K. Matsubayashi, M. Takigawa, Y. Uwatoko, N. Nakai, Y. Nagai, M. Machida We report results of microwave surface impedance measurements in LiFeAs and LiFe(As,P) single crystals [1]. These crystals were grown by self-flux method. The surface impedances of crystals were measured by a cavity perturbation technique. The in-plane penetration depth calculated from the surface reactance shows an exponential temperature dependence at low temperatures in both of LiFeAs and LiFe(As,P). This indicates that these materials do not have any nodes in the superconducting gap. The temperature dependence of the superfluid density indicates that LiFeAs and LiFe(As,P) are multi-gap superconductors with at least two isotropic gaps. In addition, the real part of complex conductivity exhibits an enhancement below $T_{\rm {c}}$, which is different from the so-called coherence peak. This is due to the rapid increase of the relaxation time of the quasiparticle below $T_{\rm {c}}$. We believe that this enhancement is rather common to all superconductors where an inelastic scattering is dominant above $T_{\rm {c}}$, irrespective of the strength of the electron correlation.\\[4pt] $\left[ 1 \right]$ Y. Imai $et$ $al.$, J. Phys. Soc. Jpn, $in$-$press$.(arXiv: 1009.4628.) [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D26.00010: Electromagnetic response of Fe(Se,Te) in the superconducting sate A. Maeda, H. Takahashi, D. Nakamura, T. Akiike, F. Nabeshima, Y. Imai, S. Komiya, I. Tsukada We investigate the electromagnetic response of Fe(Se,Te) from microwave to THz region, with special interests in the superconducting state. Samples were epitaxial thin films or bulk single crystals, depending on the technique of measurements. The results obtained are the followings. (1) Temperature dependence of the penetration depth was almost flat, with a very small contribution of $T^2$, suggesting nodeless gap plus some disorder induced pair breaking. (2) Temperature dependence of superfluid density is rather different from that of, eg, LiFeAs, suggesting weak interband scattering. (3) Quasiparticle (QP) conductivity, $\sigma (\omega,T)$ shows a broad peak below $T_c$. Both of temperature dependence and frequency dependence show that this peak is not the coherence peak in conventional $s$-wave superconductors, but is due to the enhancement of the QP scattering time. These strongly suggest that the symmetry of superconducting wave function is sign-changed s-wave. (4) Sharp peak around $T_c$ was invisible in $\sigma(T)$, suggesting very small superconductivity fluctuation. (5) Conductivity spectra ($\sigma (\omega)$) suggest that the superconducting gap, 2$\Delta$ is 1.2 meV, leading to 2$\Delta/k_BT_c$ is 1.37. We will discuss possible reasons for the small gap value. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D26.00011: Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe$_{1-x}$ Markus Bendele, Rustem Khasanov, Kazimierz Conder, Ekaterina Pomjakushina, Vladimir Pomjakushin, Annette Bussmann-Holder, Hugo Keller The Fe isotope effect (Fe-IE) on the transition temperature Tc and the crystal structure was studied in the Fe chalcogenide superconductor FeSe$_{1-x}$ by means of magnetization and neutron powder diffraction (NPD). The substitution of natural Fe (containing $\simeq92$\% of $^{56}$Fe) by its lighter $^{54}$Fe isotope leads to a shift in T$_{\rm c}$ of 0.22(5) K corresponding to an Fe-IE exponent of $\alpha_{\rm Fe}=0.81(15)$. Simultaneously, a small structural change with isotope substitution is observed by NPD. This may help to clarify the currently controversal results of the Fe-IE. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of $\alpha \sim0.35-0.4$ is observed for at least three different families of Fe-based HTS. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D26.00012: Magnetism of SmFeAsO1-xFx Yoichi Kamihara, Yasuhiro Kobayashi, Shinji Kitao, Yoshitaka Yoda, Makoto Seto, Hideo Hosono Magnetic properties of superconducting SmFeAsO$_{1-x}$F$_{x}$ are demonstrated by $^{57}$Fe Mossbauer spectroscopy (MS) and $^ {149}$Sm Nuclear resonant forward scattering (NRFS). Polycrystalline SmFeAsO$_{1- x}$F$_{x}$ samples were synthesized using two-step solid state reaction described elsewhere. [New J. Phys. 12, 033005 (2010).] Purity of samples was checked by X-ray diffraction patterns using Cu K-alpha radiation. Resistivity and magnetization measurements, as well as by $^{57}$Fe MS and $^ {149}$Sm NRFS spectroscopy, at various temperatures were performed to define superconducting, magnetic ordering temperatures. [Phys. Rev. B 78, 184512 (2008) {\&} J. Phys. Soc. Japan 77, 103706 (2008).] A magnetic phase diagram we have proposed is closer to that by Hess et al [Europhys. Lett. 87, 17005 (2009).]; that is long-range AF ordering of Fe (a static magnetism) does not persist in the superconducting regime. Such a relation between spin dynamics and SC is a common feature among LnFeAsO$_{1-x}$F$_{x}$ (Ln = La, Ce, Pr, Nd, and Sm). Our results indicate that the relation between the static magnetism and $T_{c}$ of LnFeAsO$_{1-x}$F$_{x}$ shows similar topology to that of copper-based high-Tc superconductors. [Phys. Rev. B 42, 7981 (1990).] [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D26.00013: Surface geometric and electronic structures of A(Fe, Co)$_{2}$As$_{2}$ (A=Ba,Ca) Guorong Li, V.B. Nascimento, Xiaobo He, Amar B. Karki, Jiandi Zhang, Rongying Jin, A.S. Sefat, M.A. McGuire, B.C. Sales, D. Mandrus, Ward Plummer We utilize Low Energy Electron Diffraction (LEED) to determine the surface structure combined with real-space scanning tunneling microcopy/spectroscopy (STM/STS), to investigate the local geometric and electronic structures at the (001) surface of the compounds of AFe$_{2}$As$_{2}$ (A= Ba, Ca). In general two competing surface reconstructions are observed with either a 1x2 or a ($\surd $2x$\surd $2)R45$^{\circ}$ (tetragonal notation) structure. The ($\surd $2x$\surd $2)R45$^{\circ}$ structure corresponds to the 1x1 orthorhombic phase. While the ($\surd $2x$\surd $2)R45$^{\circ}$ phase always present for A=Ba, the 1x2 structure dominates for A=Ca. We will discuss the detailed structural change with Co doping, thermal cycling, contamination, electron beam induced damage, and cleaving temperature. Specifically, 1x2 phase is sensitive to the thermal processing, with indications of a temperature dependence phase transition. *Supported by NSF DMR-1002622 [Preview Abstract] |
Session D27: Focus Session: Superconducting Qubits - Gates and Algorithms
Sponsoring Units: GQIChair: Britton Plourde, Syracuse University
Room: C155
Monday, March 21, 2011 2:30PM - 3:06PM |
D27.00001: Scaling Superconducting Qubits with the ResQu Architecture Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D27.00002: Quantum Logic Gates for Coupled Superconducting Resonators Frederick Strauch Superconducting resonators are a promising element for many applications in quantum information processing, such as memory, state transfer, and qubit-qubit coupling. Here I introduce a new application---multi-level quantum logic using superpositions of Fock states. A circuit-QED implementation of single and coupled-resonator gates will be presented and theoretically analyzed. This scheme, using experimentally demonstrated interactions, will be compared with traditional qubit operations. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D27.00003: Two-qubit gates and coupling with low-impedance flux qubits Jerry Chow, Antonio Corcoles, Chad Rigetti, Jim Rozen, George Keefe, Mary-Beth Rothwell, John Rohrs, Mark Borstelmann, David DiVincenzo, Mark Ketchen, Matthias Steffen We experimentally demonstrate the coupling of two low-impedance flux qubits mediated via a transmission line resonator. We explore the viability of experimental coupling protocols which involve selective microwave driving on the qubits independently as well as fast frequency tuning through on-chip flux-bias. Pulse-shaping techniques for single-qubit and two-qubit gates are employed for reducing unwanted leakage and phase errors. A joint readout through the transmission line resonator is used for characterizing single-qubit and two-qubit states. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D27.00004: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:42PM - 3:54PM |
D27.00005: Entangling ISWAP gate using frequency shifted anharmonic qubits Felix Motzoi, Jay Gambetta, Seth Merkel, Amira Eltony, Frank Wilhelm In this talk, we examine the coupling between frequency separated qubits, typical of superconducting implementations. We show how to correct for errors coming from finite turn-on time (corresponding to bringing the qubits into resonance) as well as leakage error (corresponding to exciting population out of the qubit manifold), namely by bringing the qubits in and out of resonance repeatedly to cancel out the unwanted parts of the Hamiltonian. The gates presented are smooth and robust and represent a whole class of analytic and numeric solutions for the evolution of the composite system. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D27.00006: Analytic control methods for high fidelity unitary operations in a weakly nonlinear oscillator Seth Merkel, Jayde Gambetta, Felix Motzoi, Frank Wilhelm In qubits made from a weakly anharmonic oscillator the leading source of error at short gate times is leakage of population out of the two dimensional Hilbert space that forms the qubit. In this talk we explore a general technique based on an adiabatic expansion to find pulse shapes that correct this type of error. This leads to a family of solutions that can be further refined based on what is feasible for a particular application. This set of pulses contains and improves upon the previously developed DRAG solution [F. Motzoi, et. al., Phys. Rev. Lett. 103, 110501 (2009)] and can be further generalized to more complicated systems with additional leakage channels. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D27.00007: CNOT gate for superconducting qubits biased at their symmetry points Sahel Ashhab, Franco Nori, Pieter de Groot, Kees Harmans, Hans Mooij, J\"urgen Lisenfeld, Adrian Lupascu A number of different techniques have been proposed and demonstrated in the past few years for implementing two-qubit gates in a system of two coupled superconducting qubits biased at their symmetry points. Most of these techniques implement the iSWAP gate. I will discuss a new technique that implements the CNOT gate. The two qubits are driven at the frequency of the target qubit, and the amplitudes applied to the two qubits are chosen such that the target qubit undergoes Rabi oscillations for only one of the two possible states of the control qubit. As a result a CNOT gate can be implemented. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D27.00008: Controlled-NOT logic gate based on conditional spectroscopy Michael Geller, Joydip Ghosh A controlled-NOT logic gate based on conditional rotation of a target qubit by applying a microwave pulse of appropriate frequency has been demonstrated experimentally for a pair of superconducting~flux qubits [Plantenberg et. al., Nature 447, 836 (2007)] . Here we discuss a related construction appropriate for coupled phase qubits~or a phase~qubit coupled to a resonator.~Our results show that an intrinsic fidelity of more than 99{\%} is~achievable~in about 45ns. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D27.00009: Quantum Logic Gates for the Rezqu Architecture Joydip Ghosh, Michael Geller A promising quantum computing architecture has been recently proposed by the UCSB superconducting quantum computation group. In this architecture, n phase qubits are capacitively coupled to individual memory resonators as well as a common bus. In this talk we discuss the design of quantum logic gates for this architecture and discuss the intrinsic fidelities. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D27.00010: Idling error and SWAP/MOVE operation in RezQu architecture for phase qubits Andrei Galiautdinov, Alexander Korotkov We analyze several basic operations in the RezQu architecture for superconducting phase qubits recently proposed by John Martinis, concentrating on the idling error, generation of single-excitation states, and the single-excitation transfer (which we call MOVE) between a phase qubit and its memory. We show that the idling error is negligible, being proportional to the sixth power of the coupling strength. We also show that in the rotating wave approximation the MOVE operation, which is simpler than the usual SWAP, can be realized perfectly using a tune/detune pulse with four adjustable parameters. The pulse consists of the front ramp (with proper shaping), a constant near-resonant overshoot, and an arbitrary rear ramp. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D27.00011: Experimental demonstration of quantum algorithms on a 4-qubit/5-resonator quantum microprocessor utilizing superconducting qubits in the RezQu architecture Erik Lucero, Rami Barends, Radoslaw Bialczak, Yu Chen, Julian Kelly, Mike Lenander, Matteo Mariantoni, Anthony Megrant, Aaron O'Connell, Peter O'Malley, Daniel Sank, Amit Vainsencher, Hauhoa Wang, James Wenner, Ted White, Yi Yin, Jian Zhao, Andrew Cleland, John Martinis We present our newly designed and fabricated 4-qubit/5-resonator quantum microprocessor composed of ``off-the-shelf" qubit and resonator components in the RezQu (``rez-(,)kyoo") architecture. The RezQu architecture uses resonators with qubits in the zero state to turn off stray coupling. Each qubit is coupled to a $\lambda /4$ memory resonator and coupling between the qubits is mediated by a common $\lambda /2$ resonator bus. Eight microwave lines drive the individual qubits, memory resonators, and coupling resonator. We demonstrate control over the quantum microprocessor via small scale quantum algorithms that require executing high- fidelity single qubit gates, quantum Fourier transform, Toffoli, CNOT, and other entangling gates. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D27.00012: Efficient Toffoli Gate in Circuit Quantum Electrodynamics Matthew Reed, Leonardo DiCarlo, Luyan Sun, Luigi Frunzio, Robert Schoelkopf The fidelity of quantum gates in circuit quantum electrodynamics is typically limited by qubit decoherence.  As such, significant improvements can be realized by shortening gate duration [1, 2]. The three-qubit Toffoli gate, also called the controlled-controlled NOT, is an important operation in basic quantum error correction.  We report a scheme for a Toffoli gate that exploits interactions with non-computational excited states of transmon qubits which can be executed faster than an equivalent construction using one- and two-qubit gates.  The application of this gate to efficient measurement-free quantum error correction will be discussed. \\[4pt] [1] DiCarlo, et al. Nature 467, 574 (2010). \\[0pt] [2] Chow, et al. Phys. Rev. Lett. 102, 090502 (2009). [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D27.00013: Progress towards a microwave-based high-fidelity Toffoli gate with superconducting qubits Chad Rigetti, Jerry Chow, Antonio Corcoles, Jim Rozen, George Keefe, Mary Beth Rothwell, Jack Rohrs, Mark Borstelmann, David DiVincenzo, Mark Ketchen, Matthias Steffen We describe recent progress at IBM towards a microwave-based implementation of the Toffoli gate using three capacitively shunted flux qubits dispersively coupled to a resonator. We discuss the device architecture and the microwave protocol, along with expected limits to gate fidelity and scaling. [Preview Abstract] |
Session D28: Focus Session: Graphene Growth, Characterization, and Devices: Metal Substrates
Sponsoring Units: DMPChair: Kevin McCarty, Sandia National Laboratory
Room: C156
Monday, March 21, 2011 2:30PM - 2:42PM |
D28.00001: Theory of the Growth of Epitaxial Graphene on Close-Packed Metals Andrew Zangwill, Dimitri Vvedensky We present a simple rate theory of epitaxial graphene growth on close-packed metals. Motivated by recent low-energy electron microscopy experiments [E. Loginova, N.C.Bartelt, P.J. Feibelman, and K.F. McCarty, New Journal of Physics, {\bf 10}, 093026 (2008)], our theory supposes that graphene islands grow predominantly by the addition of five-atom clusters, rather than solely by the capture of diffusing carbon atoms. With suitably chosen kinetic parameters, we find quantitative agreement with (i) the measured time-evolution of the adatom density and (ii) the measured temperature-dependence of the adatom density at the onset of nucleation by assuming that the smallest stable precursor to graphene growth is an immobile island composed of six five-atom clusters. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D28.00002: Graphene Growth on Cu Surface: A Theoretical Study Zhenyu Li, Ping Wu, Wenhua Zhang, Jinlong Yang, J. G. Hou Graphene is an important material with many unique properties and a great application potential. A promising way to produce wafer-size graphene is chemical vapor deposition (CVD) on metal surfaces. To improve sample quality, it is important to understand the atomic details during graphene CVD growth. In this talk, some relevant elementary processes on Cu surface have been studied from first principles. Although diffusion of atomic carbon on Cu (111) surface is almost barrierless, coalescence of carbon atoms on the surface is found to be hampered by an intermediate bridging-metal structure. The fact which makes things more complicated is that thermodynamic analysis indicates that the main species on the Cu surface during graphene growth is not the simplest atomic carbon. Therefore, CxHy species should be explicitly considered for initial stage growth of graphene on Cu surface. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D28.00003: Graphene Made Easy: A Simple Method to Grow Large-Area Single-Layer Graphene on Copper Foils S. Gadipelli, I. Calizo, J. Ford, G. Cheng, A. H. Walker, T. Yildirim In order to realize the remarkable properties of graphene in practical devices, an easy, scalable, and inexpensive synthesis method is necessary. Currently the most promising approach is through chemical vapor deposition (CVD). However, this method requires expensive CVD furnaces and flow controllers, as well as a large amount of explosive gases (H$_{2}$ and CH$_{4})$. Consequently, it is desirable to establish alternative methods to grow large-area, single-layer graphene that are simple and that can be carried out in an ordinary research laboratory. In this talk, we will discuss our systematic study of the parameters that are critical for high-quality, single-layer graphene formation. Our results not only shed light on the graphene growth mechanism, but have also yielded a straightforward synthesis method that requires neither H$_{2}$/CH$_{4}$ nor any special CVD equipment. We have prepared graphene samples at the inch scale that have been characterized by Raman spectroscopy, optical transmittance, and sheet resistance measurements. Our method is simple, safe, and economical and will be of value to both fundamental researchers and nanodevice engineers. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D28.00004: Graphene on metals - structure and properties Invited Speaker: The ``metal route'' to graphene, i.e., the epitaxial growth of graphene on a metal surface by chemical vapor deposition (CVD) of hydrocarbon molecules and the following transfer of the graphene film to an insulating support, has recently made great progress [see, e.g., S. Bae et al., Nature Nanotechnol. 5, 574 (2010)]. However, structurally and from their charge carrier mobilities, metal-grown graphene samples have not yet reached the quality of exfoliated graphene, most likely resulting from uncontrolled processes during the CVD. In order to better understand how graphene interacts with metal surfaces we have performed a series of surface science studies. As experimental techniques we have applied STM, ARPES, LEED, and SXRD, mainly on Ru(0001)/graphene, and we have performed extensive DFT analyses. We find a short metal-graphene separation, a strong deformation of the graphene, a lifting of the Dirac point, and shifts of the electronic bands. The structural and electronic properties evidence surprisingly strong interactions of the graphene with the Ru surface which are probably prototypical for other metals such as Co, Ni, and Pd. A second group of metals, namely Ir, Pt, Cu, Ag, and Au, only show weak interactions. In situ STM experiments at high temperatures (between 380 and 780 C) show that the usual ``carpet mode'' by which graphene grows across steps of the metal surface [P. W. Sutter et al., Nature Mater. 7, 406 (2008)] can lead to defects. However, the growth mode changes at high temperatures and low pressures of the hydrocarbon precursor, partially a result of the relatively high Ru-graphene interactions. They lead to a faceting of the surface, and one can grow extremely large single-crystalline graphene films on single terraces in this way. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D28.00005: Second-layer graphene growth from below on metals Shu Nie, Elena Starodub, Norman Bartelt, Kevin McCarty Once a metal substrate is covered by the first graphene layer, CVD processes slow greatly. However, C dissolved in the metal can still segregate to the surface under the first graphene layer. To determine whether these C atoms nucleate a new layer below or above the first layer, we examine growth on Ir(111), where one-layer graphene has several discrete in-plane orientations relative to substrate directions. LEED reveals that the 1st and 2nd graphene layers are not always rotationally aligned in-plane. This misalignment allows determining which sheets are on the top and the bottom by varying the electron energy and thus the escape depth. We first use LEEM to determine the spatial distribution of rotational domains in a single-layer film. We then cool and observe 2nd layer growth. We find that the top sheet of the bilayer has the exact same domain structure as the initially grown single layer. Thus, new layers are added from below. In this mechanism the nucleation and growth of the 2nd layer strongly depends on the difficulty in debonding the 1st layer from the substrate. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D28.00006: Low Temperature Graphene Growth by Down-Stream Chemical Vapor Deposition Lola Brown, Mark Levendorf, Chad Hunter, Jiwoong Park Integration of high quality graphene directly onto the surface of metal provides a novel way of controlling the functionality of metal surfaces. This can be used to control the chemical and physical surface interactions and enhance energy transfer through the surfaces, thus allowing for new sensors, flexible electronic devices and better electrodes for organic photovoltaics. However, the implementation of a pristine graphene layer in patterned devices is currently limited, due to the high temperature growth ($\sim $1000 C) and contamination of the graphene surfaces during transfer. This work presents graphene grown at low temperatures (below 750 C) using down-stream chemical vapor deposition (DS-CVD), where a metallic growth substrate is positioned down-stream from the CVD furnace ``hot zone''. High quality graphene is produced using long growth times ($\sim $ 60 minutes) and low gas flow rates. We study graphene quality and grain structure using Raman spectroscopy and dark-field transmission electron microscopy (DF-TEM). We demonstrate the strength of this technique by growing graphene on thin and micro patterned copper films, and three dimensional structures. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D28.00007: Evolution of graphene islands growing on Cu foils Joseph Wofford, Shu Nie, Norman Bartelt, Kevin McCarty, Oscar Dubon Using low-energy electron microscopy we investigate, in real time, the growth of graphene monolayers on Cu foils. Graphene islands evolve from an initially compact form into an increasingly ramified, four-lobed shape, reflecting the symmetry of the (100)-textured Cu surface. Diffraction analysis reveals that each lobe is an individual graphene domain, differentiated by a rotation about the film normal, making the islands polycrystalline. An inspection of the morphological evolution of the graphene lobes shows the growth fronts posses an angularly dependent velocity, which is consistent with a growth mode dominated by edge kinetics. The fast growth direction of each lobe tends to align with the $<$001$>$ in-plane directions of the Cu surface but not with a high symmetry direction of the graphene lattice. Finally, the implications of this unexpected growth mechanism on the formation of high-quality graphene films on Cu foils are evaluated. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D28.00008: Characterization of Graphene Films Grown on Cu-Ni Foil by XPS P. Tyagi, R.L. Moore, Z.R. Robinson, C.A. Ventrice, Jr., D.D. Moody, W. Priyantha, R. Droopad, C. Magnuson, D. Munson, S. Chen, R.S. Ruoff Monolayer graphene films can be grown on Cu substrates by the catalytic decomposition of methane molecules. The solubility of carbon in Cu is negligible at the growth temperatures typically used for graphene growth, which results in the formation of films that self-terminate at a monolayer. In an attempt to enhance the catalytic activity of the surface, use of Cu-Ni alloy foils has been investigated. Growth is performed in a CVD system at a temperature of 1000~\r{ }C with pure CH$_{4}$. To determine the graphene coverage and the surface alloy composition during the different phases of growth, XPS measurements have been performed on the Cu-Ni foils before anneal, after anneal in H$_{2}$, and after growth of graphene. Analysis of the C-1s core emission for graphene/Cu is used as reference for monolayer growth. Before anneal, the measurements indicate that the surface is Ni-rich and heavily oxidized. After annealing in H$_{2}$, only a small amount of oxide remained and the Cu:Ni alloy fraction was similar to the bulk. After growth of the graphene overlayers, only trace amounts of oxygen are present, indicating uniform graphene growth. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D28.00009: Effects of heat-treatment and hydrogen adsorption on Graphene grown on Cu foil Jongweon Cho, Li Gao, Jifa Tian, Helin Cao, Qingkai Yu, Jeffrey Guest, Yong Chen, Nathan Guisinger Graphene has recently been a subject of intense research efforts due to its remarkable physical properties as an ideal two-dimensional material. While numerous different methods for graphene synthesis are being explored, CVD-grown graphene on Cu foil presents the possibility of a large-scale and high-quality synthesis of graphene. [1] To improve the quality of graphene films on Cu foil prepared by CVD and better understand its microscopic growth, atomic-scale characterization becomes of great importance. We have investigated the effects of thermal annealing and hydrogen adsorption/desorption on \textit{ex-situ} CVD-grown monolayer graphene on polycrystalline Cu foil at the atomic-scale using ultrahigh vacuum scanning tunneling microscopy, and we will report on these studies. \\[4pt] [1] Li et al, Science \textbf{324}, 1312 (2009). [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D28.00010: Imaging Grains and Grain Boundaries in Single-Layer CVD Graphene P.Y. Huang, A.M. van der Zande, C.S. Ruiz-Vargas, W.S. Whitney, M.P. Levendorf, Y. Zhu, J. Park, P.L. McEuen, D.A. Muller Single-layer graphene can be produced by chemical vapor deposition (CVD) on copper substrates on up to meter scales [1, 2], making their polycrystallinity [3,4] almost unavoidable. By combining aberration-corrected scanning transmission electron microscopy and dark-field transmission electron microscopy, we image graphene grains and grain boundaries across five orders of magnitude. Atomic-resolution images of graphene grain boundaries reveal that different grains stitch together predominantly via pentagon-heptagon pairs. We use diffraction-filtered imaging to map the shape and orientation of several hundred grains and boundaries. These images reveal an intricate patchwork of grains with structural details depending strongly on growth conditions. These imaging techniques will enable studies on the structure, properties, and control of graphene grains and grain boundaries. \\[4pt] [1] X. Li et al., Science 324, 1312 (2009).\\[0pt] [2] S. Bae et al., Nature Nanotechnol. 5, 574 (2010).\\[0pt] [3] J. M. Wofford, et al.,Nano Lett., (2010).\\[0pt] [4] P.Y. Huang et al., arXiv:1009.4714, (2010). [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D28.00011: Analysis of Substrate Grain Size and Orientation on the Growth of Graphene Films Z.R. Robinson, P. Tyagi, T.M. Murray, C.A. Ventrice, Jr., C. Magnuson, D. Munson, S. Chen, R.S. Ruoff Graphene growth on Cu foils by catalytic decomposition of methane forms predominately single layer graphene films due to the low solubility of C in Cu. One of the key issues for the use of CVD graphene in device applications is the influence of defects on the transport properties of the graphene. For instance, growth on foil substrates is expected to result in multi-domain graphene growth because of the presence of randomly oriented grains within the foil. Therefore, the size and orientation of the grains within the metal foil should strongly influence the defect density of the graphene. To study this effect, we have initiated a study of the influence of pre-growth anneal time and H$_{2}$ pressure on the grain size and structure of Cu and Cu-Ni foil substrates. Preliminary measurements of the grain size have been performed with SEM and AFM. These results show a typical lateral dimension of $\sim $100 $\mu $m for an anneal time of 30 min in 40 mTorr of H$_{2}$ at 1000 \r{ }C. Measurements are currently being performed with electron backscatter diffraction to determine the crystallographic orientation within each grain. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D28.00012: Alloy substrates: towards precise control of thickness and quality of multilayer graphene growth Shanshan Chen, Weiwei Cai, Richard D. Piner, Xuesong Li, Yanwu Zhu, Rodney S. Ruoff Graphene has gained a lot of attention due to its remarkable properties, such as high electron hole mobility, high current carrying capability and high mechanical robustness. It has been further demonstrated that the properties of graphene materials depend on the number of graphene layers present. As a result, it is highly desirable to develop reliable synthesis techniques to synthesize few- or multi-layered high quality large area graphene materials. Here we report a facile method to grow few-layer graphene films using an alloy substrate by chemical vapor deposition. The thickness and quality of the graphene and graphite films can be controlled using CVD with methane and hydrogen gas as precursors, by varying the deposition temperature and cooling rate. The optical and electrical properties of the graphene/graphite films were studied as a function of their thickness. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D28.00013: Improving Polycrystalline Copper Surface by Hydrogen Etching for Graphene Growth Merve Arseven, Tayfun Vural, Engin Ozdas Growth of high quality, large scale pattern of graphene is the main important phenomenon to use this material in novel technological applications. CVD methods can provide an effective way to produce graphene, however, require a rigid stable substrate at high temperature processes [1]. Besides, the substrate that can be used in these processes must have low solubility of carbon to obtain mono or few layers of graphene, and be able to provide bigger grains for a large-scale growth [2]. Polycrystalline copper foil is an appropriate candidate to achieve these attributions in case of reducing the pinhole and defect density, and increase the grain size. In this study, we investigate the effect of hydrogen partial pressure, heating rate, annealing temperature and duration on the etching process to optimize the surface. Surface roughness analyses are performed by AFM, and grain size distributions are determined by the analyses of optical microscope images. [1] Sukang Bae \textit{et al}., Nature Nanotechnology \textbf{5}, 574, 2010. [1] Xuesong Li \textit{et al}., Science \textbf{324}, 1312, 2009. [Preview Abstract] |
Session D29: Quantum Computing and Simulation I
Sponsoring Units: GQIChair: Ivan Deutsch, University of New Mexico
Room: C148
Monday, March 21, 2011 2:30PM - 2:42PM |
D29.00001: Scalable Quantum Computing Over the Rainbow Olivier Pfister, Nicolas C. Menicucci, Steven T. Flammia The physical implementation of nontrivial quantum computing is an experimental challenge due to decoherence and the need for scalability. Recently we proved a novel theoretical scheme for realizing a scalable quantum register of very large size, entangled in a cluster state, in the optical frequency comb (OFC) defined by the eigenmodes of a single optical parametric oscillator (OPO). The classical OFC is well known as implemented by the femtosecond, carrier-envelope-phase- and mode-locked lasers which have redefined frequency metrology in recent years. The quantum OFC is a set of harmonic oscillators, or Qmodes, whose amplitude and phase quadratures are continuous variables, the manipulation of which is a mature field for one or two Qmodes. We have shown that the nonlinear optical medium of a single OPO can be engineered, in a sophisticated but already demonstrated manner, so as to entangle in constant time the OPO's OFC into a finitely squeezed, Gaussian cluster state suitable for universal quantum computing over continuous variables. Here we summarize our theoretical result and survey the ongoing experimental efforts in this direction. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D29.00002: Implementing quantum gates through scattering between a static and a flying qubit Guillermo Cordourier-Maruri, Francesco Ciccarello, Yasser Omar, Michelangelo Z, Romeo de Coss, Sougato Bose We investigate whether a two-qubit quantum gate can be implemented in a scattering process involving a flying and a static qubit. We focus on a paradigmatic setup made out of a mobile particle and a quantum impurity, whose respective spin degrees of freedom couple to each other during a one-dimensional scattering process. A condition for the occurrence of quantum gates is derived in terms of spin-dependent transmission coefficients. This can be fulfilled through the insertion of an additional narrow potential barrier. Under resonance conditions this procedure enables a gate only for Heisenberg interactions and fails for an XY interaction. We show the existence of parameter regimes for which gates able to establish a maximum amount of entanglement can be implemented. The gates are found to be robust to variations of the optimal parameters. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D29.00003: The 2D AKLT state is a universal quantum computational resource Tzu-Chieh Wei, Ian Affleck, Robert Raussendorf We demonstrate that the two-dimensional AKLT state on a honeycomb lattice is a universal resource for measurement-based quantum computation. Our argument proceeds by reduction of the AKLT state to a 2D cluster state, which is already known to be universal, and consists of two steps. First, we devise a local POVM by which the AKLT state is mapped to a random 2D graph state. Second, we show by Monte Carlo simulations that the connectivity properties of these random graphs are governed by percolation, and that typical graphs are in the connected phase. The corresponding graph states can then be transformed to 2D cluster states. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D29.00004: Robust benchmarking of quantum processes Easwar Magesan, Jay Gambetta, Joseph Emerson Fault-tolerant threshold theorems show that as long as the noise affecting a quantum system is below some threshold, reliable quantum computation can take place. As a result, methods for noise characterization are of great interest in quantum information theory. Unfortunately, methods for complete noise characterization scale exponentially in the number of qubits comprising the system. This non-scalability highlights the importance of developing mathematical methods for scalable partial characterization of the noise affecting a quantum system. We discuss a randomized benchmarking protocol that provides fitting models for the fidelity decay of the noisy gates used in quantum information processing. We show that when the average variation of the noise is not too large the first order model gives a robust estimate of both the average error affecting the gate set and the gate-dependence of the noise. We also show that the protocol is scalable in the number of qubits comprising the system. The protocol allows the noise to be both time and gate-dependent, and takes into account state preparation and measurement errors. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D29.00005: Universal Quantum Computation within the Bose-Hubbard Model Michael S. Underwood, David L. Feder We present a novel scheme for universal quantum computation based on spinless bosons hopping on a two-dimensional lattice with on-site interactions. Our setup is comprised of a $2\times n$ lattice for an $n$-qubit system; the two rows correspond to the computational basis states, and a boson in each column encodes a qubit. The system is initialized with $n$ bosons occupying the $n$ sites of the $|0\rangle$ row, and the lattice deep enough to prevent tunneling. Arbitrary single-qubit $X$ rotations are implemented by tuning the tunneling strength between the $|0\rangle$ and $|1\rangle$ sites of the appropriate column, and $Z$ rotations by applying a local potential to the $|1\rangle$ site. Entanglement is generated by hopping between the $|1\rangle$ sites of adjacent qubits; by tuning the on-site interaction strength of the bosons, a non-trivial controlled phase is acquired if these two qubits are in the state $|11\rangle$. Because the quantum information is encoded entirely in the lattice positions of the bosons, the encoded qubits are inherently robust against decoherence. An implementation in terms of ultracold atoms in optical lattices is suggested. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D29.00006: Optimal Trajectories for Quantum Adiabatic Factoring Jordan Kyriakidis, Robert Archibald, William Macready We show how a classical multiplication circuit can be expressed as an optimization problem. The circuit can then be effectively run backwards by fixing the output states in the optimization problem and determining the corresponding input states, thereby factoring the output state. This can in turn be expressed as a problem in adiabatic quantum computing. We show by solving a coupled set of Euler-Lagrange equations how (locally) optimal trajectories from initial to final Hamiltonians can be found whose efficacy vastly exceeds that of the usual linear scaling trajectory. Explicit examples will be given for factoring 6-bit integers. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D29.00007: Deterministic Random-Length Computation with Weakly Entangled Cluster States Adam G. D'Souza, David L. Feder Universal quantum computation can be accomplished via single-qubit measurements on a highly entangled resource state, together with classical feedforward of the measurement results. The best-known example of such a resource state is the cluster state, on which judiciously chosen single-qubit measurements can be used to simulate an arbitrary quantum circuit with a number of measurements that is linear in the number of gates. We examine the power of the orbit of the cluster states under GL(2,C), also known as the SLOCC equivalence class of the cluster state, as a resource for deterministic universal computation. We find that, under certain circumstances, these states do indeed constitute resources for such computations, but of random length. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D29.00008: Logical operator tradeoff for local quantum codes Jeongwan Haah, John Preskill We study the structure of logical operators in local $D$-dimensional quantum codes, considering both subsystem codes with geometrically local gauge generators and codes defined by geometrically local commuting projectors. We show that if the code distance is $d$, then any logical operator can be supported on a set of specified geometry containing $\tilde d$ qubits, where $\tilde d d^{1/(D-1)} = O(n)$ and $n$ is the code length. Our results place limitations on partially self-correcting quantum memories, in which at least some logical operators are protected by energy barriers that grow with system size. We also show that two-dimensional codes defined by local commuting projectors admit logical ``string'' operators and are not self correcting. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D29.00009: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 4:18PM - 4:30PM |
D29.00010: Simulating Concordant Computations Bryan Eastin A quantum state is called concordant if it has zero quantum discord with respect to any part. By extension, a concordant computation is one such that the state of the computer, at each time step, is concordant. In this talk, I describe a classical algorithm that, given a product state as input, permits the efficient simulation of any concordant quantum computation having a conventional form and composed of gates acting on two or fewer qubits. This shows that such a quantum computation must generate quantum discord if it is to efficiently solve a problem that requires super-polynomial time classically. While I employ the restriction to two-qubit gates sparingly, a crucial component of the simulation algorithm appears not to be extensible to gates acting on higher-dimensional systems. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D29.00011: Photonic Phase Gate via an Exchange of Fermionic Spin Waves in a Spin Chain Alexey Gorshkov, Johannes Otterbach, Eugene Demler, Michael Fleischhauer, Mikhail Lukin We propose a new protocol for implementing the two-qubit photonic phase gate. In our approach, the $\pi$ phase is acquired by mapping two single photons into atomic excitations with fermionic character and exchanging their positions. The fermionic excitations are realized as spin waves in a spin chain, while photon storage techniques provide the interface between the photons and the spin waves. Possible imperfections and experimental systems suitable for implementing the gate are discussed. [Reference: Phys. Rev. Lett. 105, 060502 (2010)] [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D29.00012: Scalable Neutral Atom Quantum Computer with Interaction on Demand Mikio Nakahara, Elham Hosseini Lapasar, Kenichi Kasamatsu, Tetsuo Ohmi, Yasushi Kondo We propose a scalable neutral atom quantum computer with an on- demand interaction. Artificial lattice of near field optical traps is employed to trap atom qubits. Interactions between atoms can be turned off if the atoms are separated by a high enough potential barrier so that the size of the atomic wave function is much less than the interatomic distance. One-qubit gate operation is implemented by a gate control laser beam which is attached to an individual atom. Two-qubit gate operation between a particular pair of atoms is introduced by leaving these atoms in an optical lattice and making them collide so that a particular two-qubit state acquires a dynamical phase. Our proposal is feasible within existing technology developed in cold atom gas, MEMS, nanolithography, and various areas in optics. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D29.00013: General-Purpose Quantum Simulation with Prethreshold Superconducting Qubits Emily Pritchett, Colin Benjamin, Andrei Galiautdinov, Michael Geller, Andrew Sornborger, Phillip Stancil, John Martinis We introduce a protocol for the fast simulation of $n$-dimensional quantum systems on $n$-qubit quantum computers with tunable couplings. A mapping is given between the control parameters of the quantum computer and the matrix elements of an $n$- dimensional real (but otherwise arbitrary) Hamiltonian that is simulated in the $n$- dimensional {\it single-excitation subspace} of the quantum simulator. A time- dependent energy/time rescaling minimizes the simulation time on hardware having a fixed coherence time. We demonstrate how three tunably coupled superconducting phase qubits can simulate a three-channel molecular collision using this protocol. The method makes a class of general-purpose time-dependent quantum simulation practical with today's sub-thershold-fidelity qubits. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D29.00014: Currently Realizable Quantum Error Detection/Correction Algorithms for Superconducting Qubits Kyle Keane, Alexander N. Korotkov We investigate the efficiency of simple quantum error correction/detection codes for zero-temperature energy relaxation. We show that standard repetitive codes are not effective for error correction of energy relaxation, but can be efficiently used for quantum error detection. Moreover, only two qubits are necessary for this purpose, in contrast to the minimum of three qubits needed for conventional error correction. We propose and analyze specific two-qubit algorithms for superconducting phase qubits, which are currently realizable and can demonstrate quantum error detection; each algorithm can also be used for quantum error correction of a specific known error. In particular, we analyze needed requirements on experimental parameters and calculate the expected fidelities for these experimental protocols. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D29.00015: Interface between Topological and Superconducting Qubits Liang Jiang, Charles Kane, John Preskill We propose and analyze an interface between a topological qubit and a superconducting flux qubit. In our scheme, the interaction between Majorana fermions in a topological insulator is coherently controlled by a superconducting phase that depends on the quantum state of the flux qubit. A controlled phase gate, achieved by pulsing this interaction on and off, can transfer quantum information between the topological qubit and the superconducting qubit. [Preview Abstract] |
Session D30: Graphene: Hydrogenation and Defects
Sponsoring Units: DCMPChair: Taisuke Ohta, Sandia National Laboratories
Room: C147/154
Monday, March 21, 2011 2:30PM - 2:42PM |
D30.00001: Thermal conductivity of partially hydrogenated graphene Jeong Yun Kim, Jeffrey Grossman Graphene superlattices made with partial hydrogenation are of great interest and have been explored recently due to the enhanced tunability of electronic properties as a function of the hydrogenation pattern. However, the thermal transport properties of such materials have received little attention. In this work, we investigate the effects of 2D periodic patterns of hydrogen atoms on the thermal conductivity of partially hydrogenated graphene using classical molecular dynamics simulations and an Einstein relation. Our calculations show that the thermal conductivity of partially hydrogenated graphene varies substantially as a function of hydrogen coverage, periodicity, edge shape, and width of hydrogenated region compared to the bare graphene region. In addition, we show that the use of patterned 2D shapes of hydrogenation on graphene could lead substantially lower thermal conductivities that may be of interest for thermoelectric applications. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D30.00002: Thermoelectric Properties of Hydrogenated Graphene Ruwantha Jayasingha, Kasun Fernando, Christof Keebaugh, Robert Stallard, Gamini Sumanasekera We have studied the temperature dependence of thermopower (S) and 4-probe resistance (R) of large area Graphene subjected to various degree of hydrogenation. Graphene samples with electrical contacts mounted within a quartz reactor was placed inside a custom made inductively coupled plasma coil and hydrogen gas was introduced to a pressure of $\sim $ 10 Torr. Samples were placed well away from the plasma and both S and R were monitored \textit{in --situ} during the hydrogenation. At desired level of hydrogenation the plasma was turned off and the sample was cooled down to $\sim $140 K by lowering the reactor into a liquid nitrogen dewar and both R(T) and S(T) were measured. Both S(T) and R(T) show metal to insulator transition characteristics during the progressive hydrogenation. Both epitaxially grown Graphene on Si-terminated face of SiC and Graphene grown by chemical vapor deposition and transferred on to quartz substrate were studied. The CVD grown sample was found to be p-type under ambient condition but could be tuned to n-type after high temperature annealing at 550 K in a vacuum of 2x10$^{-7 }$Torr. In contrast, epitaxial sample was n-type under ambient conditions. However, the hydrogenation was performed on both samples under degassed conditions. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D30.00003: The Influence of Hydrogenation on the Hall Effect in Exfoliated Mono- and Multi-layer Graphene Y. Mo, J.D. Jones, P.E. Ecton, M. Maneshian, W.D. Hoffman, A.V. Jesseph, N. Shepherd, G.F. Verbeck, J.M. Perez, Z. Ye, G. Zhao Graphene samples exfoliated from highly ordered pyrolytic graphite are deposited using the standard scotch-tape method on 300nm thick SiO$_{2}$ covered and slightly conductive Si substrates. Devices with 4 silver electrode pads on the graphene samples for Hall effect measurements are made with simple evaporation procedures by using transmission electron microscopy grids as masks. At room temperature, we measure the Hall effect of mono- and multi-layer graphene before and after plasma hydrogenation. During plasma hydrogenation, the sample substrates are biased at +150 V to attract electrons in the plasma for hydrogenation and push away ions in the plasma avoiding possible damage to the graphene. We also measure the Hall effect after annealing the samples at 200 $^{\circ}$C and vacuum of 10$^{-6}$ torr for an hour. Micro-Raman is employed to monitor the quality and change of the graphene at each process step. We compare the Hall effect results for pristine, hydrogenated, and annealed mono- and multi- layer graphene samples. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D30.00004: Density of states of a graphene in the presence of strong point defects Bor-Luen Huang, Ming-Che Chang, Chung-Yu Mou The density of states near zero energy in a graphene due to strong point defects with random positions are computed. Instead of focusing on density of states directly, we analyze eigenfunctions of inverse T-matrix in the unitary limit. Based on numerical simulations, we find that the squared magnitudes of eigenfunctions for the inverse T-matrix show random-walk behavior on defect positions. As a result, squared magnitudes of eigenfunctions have equal a priori probabilities, which further implies that the density of states is characterized by the well-known Thomas-Porter type distribution. The numerical findings of Thomas-Porter type distribution is further derived in the saddle-point limit of the corresponding replica field theory of inverse T-matrix. Furthermore, the influences of the Thomas-Porter distribution on magnetic and transport properties of a graphene, due to its divergence near zero energy, are also examined. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D30.00005: Tuning graphene's electronic structure via unbalanced disordered sublattices and defect superlattices Francois Varchon, Aur\'elien Lherbier, Jean-Christophe Charlier Graphene, a single carbon plane arranged on a honeycomb lattice, has received a lot of attention in the last few years due to its very appealing physical properties as the room temperature quantum hall effect, a large coherence length or a high electronic mobility. These basic properties hold a high application potential for graphene in nanoelectronics. Nevertheless the future of this field strongly depends on the possibility to control the electronic properties of this material. On the basis of extensive tight-binding and ab initio calculations, we demonstrate the possibility to tune graphene's electronic structure via realistic atomic defects (epoxide and hydroxyl groups chemisorbed on graphene). For example we report on the bandgap opening in graphene monolayer induced by unbalanced disordered sublattices. Our findings show that the bandgap width depends on the nature, the concentration and the distribution (random, semi-random, periodic) of the impurities. We also perform an indepth study about the special case of periodic distribution of atomic defects. We demonstrate the existence of three different families of defect superlattices which conduct to specific band structures and therefore could lead to different electronic and transport properties [1]. \\[4pt] [1] A. Lherbier, F. Varchon, J.-C. Charlier (in preparation) [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D30.00006: Numerical study of impurity effects in Graphene Zhou Li, Frank Marsiglio, Stepan Grinek, Jie Chen It is known that long-range Coulomb impurities could induce a novel supercritical regime in gapped graphene [1]. For short range impurities, the electron wave function is less localized near the band edge and thus numerical results may depend on the size and boundary conditions of the simulated graphene. For six attractive impurities forming a quantum well with radius=a(a is the distance between two nearest neighbor atoms in graphene), we found that the bound states will not merge into the continuum. The results from a finite size exact diagonalization with open boundary conditions agree well with that from an infinite size study based on Green's functions. Also an efficient numerical approach based on kernel polynomial methods [2] will be adopted to evaluate the Green's function accurately in the regime with strong interference effects and compared to T-matrix results. \\[4pt] [1] V.M. Pereira et.al, Phys. Rev. B, 78, 8, 2008, pp. 085101. \\[0pt] [2] L.Covaci et.al, Phys. Rev. Lett. 105, 167006 (2010) [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D30.00007: Plasma Hydrogenation of n-Layer Graphene J.D. Jones, W.D. Hoffman, A.V. Jesseph, C.J. Morris, G.F. Verbeck, J.M. Perez We propose a new mechanism for the hydrogenation of mono-, bi-, and tri-layer graphene samples using an H$_{2}$ plasma. We find that hydrogenation occurs as a result of electron irradiation of H$_{2}$O adsorbates on the sample rather than H species from within the plasma. We propose that the mechanism is electron-impact fragmentation of the H$_{2}$O adsorbates occurring naturally above and below the sample. The stability of the hydrogenation increases with the incident electron energy, allowing for hydrogenated samples that are stable at temperatures $>$ 200 $^{\circ}$C. We also observe fully hydrogenated bi- and tri-layer graphene, which may be evidence for new materials, diamane and \textit{triamane}. Diamane, a two atom thick layer of hydrogenated diamond, is predicted to have a band gap of 3.12 eV and be stronger than graphane, hydrogenated graphene. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D30.00008: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 4:06PM - 4:18PM |
D30.00009: Oxygen reduction activity of BN decorated bulk defects in graphene Shyam Kattel, Boris Kiefer, Plamen Atanassov We use Density-Functional-Theory to investigate the interaction between O$_{2}$ and H$_{2}$O$_{2}$ with co-doped bulk BN defects in graphene. The results show that the mixed defects are thermodynamically stable in contrast to the nitrogen only defects that need a transition metal for stabilization. The interaction between O$_{2}$ and H$_{2}$O$_{2}$ and the BN defects are found to be very different: O$_{2}$ is adsorbed as a molecule on boron with a bond length increase of $\sim $20{\%}. H$_{2}$O$_{2}$, on the other hand, is predicted to adsorb dissociatively to form B(OH)$_{2}$. The predicted binding energy (BE) of O$_{2}$ is similar to the N only defects. This observation suggests that BN defects promote the reduction of O$_{2}$ to H$_{2}$O$_{2}$. However, we also found that the binding energy per OH is $\sim $75{\%} higher than the corresponding BE for the N only defect. Thus, restoring the catalytic site through OH removal is more difficult as compared to the N only defect. This implies that bulk BN defects are most likely less active than N only defects and edge BN defects which enhance ORR. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D30.00010: Effect of Laser Irradiation on Structural and Electronic Properties of Single-layer and Bi-layer Graphene Pubudu Galwaduge, Joseph Lambert, Roberto Ramos Graphene is a two-dimensional crystal with remarkable electronic properties which have made it a component of interest in fabricating chemical sensors, superconducting devices and room-temperature transistors. Fabrication and metrology techniques typically use energetic beams such as lasers which are likely to induce unintentional changes in graphene. We report results of Time-Resolved Raman Spectroscopy experiments that investigate the effect of low,medium, and high power laser irradiation on the structural and electronic properties of single-layer and bi-layer graphene. We have irradiated graphene using a 514.5nm laser at power levels of 1.8mW, 9mW and 18mW. Changes in electronic and structural properties were observed by observing the time evolution of the Raman D and G bands. Under irradiation at 1.8mW and 9mW, single layer graphene flakes show changes in charge carrier concentration. Under irradiation at 18mW,single layer graphene shows signs of defect formation and breakdown into nano-crystalline graphene. Bi-layer graphene shows no measurable changes in the Raman D and G bands under irradiation at 9mW. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D30.00011: Effect of irradiation by electron-beam and oxygen plasma on graphene studied with Raman spectroscopy and electronic transport Romaneh Jalilian, Isaac Childres, Luis A. Jauregui, Michael Foxe, Jifa Tian, Igor Jovanovic, Yong P. Chen We report a study of the effects of electron-beam irradiation and oxygen plasma etching on graphene and graphene field-effect transistors (GFET). For both types of exposure, Raman spectra show a characteristic evolution with increasing irradiation-induced disorder. Electron-beam exposure causes a down-shifting in the charge-neutral point (CNP), interpreted as due to a hole-doping in the substrate. Oxygen plasma etching causes an up-shifting of the CNP, interpreted as due to hole-doping molecules adsorbed on the plasma-induced defects. Both types of exposure decrease the carrier mobilities and minimum conductivity of graphene. Additionally, weak localization and the quantum Hall effect are characterized in exposed devices. Our findings are valuable for understanding the effects of irradiation damage on graphene and the physics of disordered graphene through artificially generated defects. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D30.00012: Stability and Mobility of Vacancy Defects in Monolayer Graphene Wei Chen, Haiping Lan, Ping Cui, Jinlong Yang, Zhenyu Zhang Using DFTB and first-principle calculations, we study the stability and mobility of vacancy defects in graphene. First, we calculate the formation energy of vacancy defects of varying sizes in different supercells, including its dependence on the Brillouin zone sampling. We find a large difference, of ~1eV, in the formation energy between the value with only Gamma-point sampling and that with more symmetrical k-point sampling in the 3N*3N (N=2,3,4) supercells. This variance is attributed to significant contributions of the electronic states around the Dirac points. We then explore the mobility of the vacancy defects, including single atom vacancy, trivacancy, and tetravacancy. We find that both trivacancy and tetravacancy have relatively small activation energies for migration via a Stone-Wales transformation of the edge atoms. These results will be compared with recent experimental observations. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D30.00013: Controllable defect healing and N-doping of graphene by CO and NO molecules Bin Wang, Sokrates Pantelides Controllable defect healing and N-doping in graphene would be very valuable for potential device applications. Here we report first-principles molecular dynamic simulations that suggest a procedure with fast dynamics and low thermal budget. Vacancies can be healed by sequential exposure to CO and NO molecules. A CO molecule gets adsorbed at a vacancy site and a NO molecule subsequently removes the extra O by forming NO$_{2}$. Controllable N-doping can be achieved by sequential vacancy creation (e.g. by electron beam) and subsequent exposure to NO molecules at room temperature. A combination of CO and NO molecules can potentially provide simultaneous healing and doping at a desirable ratio. The proposed strategy introduces no extra defects and is promising for graphene-based materials in radiation environments. [Preview Abstract] |
Session D31: Focus Session: van der Waals Bonding in Advanced Materials: Applications to Advanced and Functional Materials
Sponsoring Units: DMPChair: Per Hyldgaard, Chalmers University of Technology
Room: C145
Monday, March 21, 2011 2:30PM - 3:06PM |
D31.00001: Rotational and vibrational excitations of van der Waals bonded hydrogen in nanoporous materials: calibrating first-principle calculations with experiments Invited Speaker: The adsorption of H$_2$ within a metal-organic framework is studied via van der Waals density-functional calculations and maximally-localized-Wannier- function analysis. The calculated low-lying vibrational and rotational energy states as well as the adsorption sites are consistent with experiments. The induced dipole due to H$_2$ bond stretching and its quantum mechanic matrix element is found to be accurately given by a first-principles driven approximation. The resulting calculations of IR intensity explain the experimentally mysteriously missing primary line for para hydrogen. The strengths and positions of lines in the complex spectra of rotational-vibrational transitions are in reasonable agreement with experiment, and a selection rule is obtained. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D31.00002: CO2 Binding in Zeolitic Imidazolate Frameworks from First Principles Calculations Keith Ray, David Olmsted, Ning He, Yao Houndonougbo, Brian Laird, Mark Asta Zeolitic Imidazolate Frameworks (ZIFs) are excellent candidate carbon capture materials owing to their high surface area, selectivity, and stability. In this work we use electronic-structure based methods to investigate the binding of CO2 in a set of ZIFs that share the same topology but feature different functionalized linkers [1]. Since a large portion of the CO2 binding comes from van der Waals (vdW) forces, we explore several different schemes for incorporating these contributions into ab-initio density-functional-theory (DFT) including vdW-DFT [2]. The results are combined with those of classical simulation studies to allow comparisons between calculations and experimentally measured values of the heat of adsorption and adsorption isotherms [1]. This research is supported by the Energy Frontier Research Center ``Molecularly Engineered Energy Materials,'' funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001342. \newline [1] W. Morris, B. Leung, H. Furukawa, O. K. Yaghi, N. He, H. Hayashi, Y. Houndonougbo, M. Asta, B. B. Laird, and O. M. Yaghi, J. AM. CHEM. SOC. 132, 11006-11008 (2010) \newline [2] M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Let. 92, 246401 (2004) [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D31.00003: The Importance of van der Waals Interactions in the Stability of the Phases of Mg(BH$_4$)$_2$ Brian Kolb, Andrzej Bil, Aleksey Kolmogorov, Timo Thonhauser As hydrogen gains attention as a potential replacement for fossil fuels, materials to store hydrogen safely and efficiently are becoming increasingly important. Metal borohydrides are attracting much attention for this role and, in particular, Mg(BH\(_4\))\(_2\) is a promising candidate for hydrogen storage because of its relatively high hydrogen content (over 12 wt\%) and the abundance of its constituent elements. This system has been investigated previously, both experimentally and via density functional theory (DFT) studies. These two approaches give conflicting results, however, regarding the identity of the low-temperature ground-state. In this work, we investigate the impact of van der Waals (vdW) interactions on the stability of various phases of Mg(BH\(_4\))\(_2\). vdW interactions are included both through the fully self-consistent vdW-DF approach as well as the semi-empical PBE-D/PBE-D* approach. Our results settle the longstanding discrepancy between theory and experiment, as we find inclusion of vdW interactions stabilizes the experimentally determined ground-state structure at low temperature, relative to those predicted by previous DFT studies. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D31.00004: Influence of van der Waals contact forces on the deformation mechanics of thin flexible membranes assembled from metallic or semiconducting single-wall carbon nanotubes Erik K. Hobbie, John Harris, Swathi Iyer, Ji Yeon Huh, Jeffrey A. Fagan, Steven D. Hudson, Christopher M. Stafford Thin membranes of single-wall carbon nanotubes (SWCNTs) assembled from either metallic or semiconducting SWCNTs are subjected to the compressive strains imposed by a stretched elastic substrate, and the mechanical characteristics of the membranes are inferred from the topography of the wrinkling instability that emerges. By depositing comparable films on quartz, we also use optical (UV-Vis-NIR) absorption spectroscopy to compute the effective London dispersion spectra of the purified materials, and from these we compute the attractive part of the van der Waals potential between nanotubes of identical electronic type as a function of separation and relative orientation. We find significant differences in the strength and shape of the contact potential depending on electronic type, which in turn are evident in the modulus and yield strain measured from the deformation of the films. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D31.00005: Electrostatic Origin of Meandering C60 Chain Formation at ZnPc Interfaces Janice Reutt-Robey, Qiang Liu, Levan Tskipuri, Wei Jin, John Weeks, Daniel Dougherty, Steve Robey We present STM investigations of interface-formation and nanophase separation in binary films of zinc phthalocyanine (ZnPc) and C$_{60}$ on Ag(111) and Au(111) supports. We report ZnPc:C$_{60}$ 1-D and 2-D interfaces with distinctive molecular orientations and unusually low C$_{60}$ packing densities. Meandering C$_{60}$ chains of single-molecular width arise without registration to the underlying ZnPc template, islanding into a disordered chain phase. These structures are reminiscent of dipole fluids (albeit of single molecular widths!) We present detailed measurements and analysis of C$_{60}$ wandering chain formation on ZnPc/Ag (111) and ZnPc/Au (111) substrates. We explore the physical origin of these structures through simulations with a model potential that incorporates short-range C$_{60}$ -- C$_{60}$ attraction and a long-range dipolar repulsion. From simulations of realized structures, we~ estimate the effective dipole needed for chain formation. DFT calculations on the C60/ZnPc/Ag(111) structure support these conclusions and provide more detailed insight on the electrostatic interactions that drive chain formation. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D31.00006: How van der Waals interactions affect alanine-based polypeptides M. Rossi, V. Blum, X. Ren, A. Tkatchenko, M. Scheffler van der Waals interactions play a critical role among the intramolecular interactions that stabilize secondary structure folding motifs in polypeptides. In this work, we quantify its influence \textit{ab initio} for the series of helix-forming alanine based polypeptides Ac-Ala$_n$-LysH$^+$ ($n=$ 4-15). We show that: (i) applying a van der Waals (vdW) correction based on the self-consistent electron density [2] to the PBE functional, a clear $\alpha$-helical conformational preference emerges at $n$=8, in agreement with experiment [1], while a mostly 3$_{10}$ helical structure is preferred at plain PBE; (ii) a numeric atom-centered orbital basis enhanced specifically to converge conformational energy differences from explicitly correlated methods (MP2, EX+cRPA and beyond [3]) gives us benchmark capabilities for treatments that include long-range correlations outrightly; (iii) exploring the free energy surface through \textit{ab initio} dynamics for longer helices ($n$=15) we see a dramatic influence of vdW interactions for high temperature stability and surface explored by these molecules. Our results demonstrate that we are now in a position to quantify vdW contributions accurately, and thus unravel their critical qualitative role in comparison to other contributions (strain, H-bonds) in medium-sized biomolecules. [1] Kohtani and Jarrold, JACS 108, 8454 (2004); [2] Tkatchenko and Scheffler, PRL 102, 073055 (2009); [3] http://www.fhi-berlin.mpg.de/aims [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D31.00007: Dispersion Forces and Self-assembly of Styrene Nanowires on H-Si(100) 2$\times$1 Surface Guo Li, Valentino Cooper, Jun-hyung Cho, Shixuan Du, Hongjun Gao, Zhenyu Zhang We present our first-principles investigation of the influence of dispersion forces (or van der Waals interactions) on the self-assembly of styrene nanowires on the hydrogenated Si(100) 2$\times$1 surface. Using density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations we demonstrate that the dispersion forces enhance the binding between styrene molecules thus allowing us to tune the preferential growth of long wires for the fabrication of desired nanopatterns. Furthermore, this approach is a step towards accurate fully first-principles studies of the effects of dispersion forces on the dynamics at interfaces, and therefore will be invaluable to our understanding of chemical processes such as self-assembly and the catalysis of organic chemical reactions. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D31.00008: Morphological Control in the Synthesis of Silver Nanostructures: Role of Polyvinylpyrrolidone Kristen Fichthorn, Haijun Feng, Rajesh Sathiyanarayanan Solution-phase syntheses are useful for assembling metallic nanostructures with desired morphologies. For example, a wide variety of silver nanostructures have been synthesized in the polyol process [1], including nanowires, nanoplates, cubes, etc. Polyvinylpyrrolidone (PVP) plays a key role in controlling nanostructure morphologies in these fabrication processes. Based on experimental observations, the interaction strength between PVP chains and Ag atoms in different crystallographic facets is expected to vary significantly and this shape selectivity is expected to play a key role in directing the formation of various nanostructures. Using first-principles calculations based on density-functional theory including van der Waals interactions, we compute the interactions of the basic elements of a repeat unit in PVP (2-pyrollidone and ethane) with various crystal faces of Ag. Our results indicate that PVP does exhibit the expected structure sensitivity and that this arises from an interesting balance between van der Waals interactions and direct chemical bonding. We discuss the ramifications of our calculations for the assembly of Ag nanostructures. \\[4pt] [1] B. Wiley et al., Chem. Eur. J. 11, 454 (2005). [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D31.00009: Structure and Formation of Synthetic Hemozoin: Insights from First Principles Calculations Noa Marom, Alexandre Tkatchenko, Sergey Kapishnikov, Leeor Kronik, Leslie Leiserowitz Malaria has reemerged due to parasite resistance to synthetic drugs that act by inhibiting crystallization of the malaria pigment, hemozoin (HZ). Understanding the process of HZ nucleation is therefore vital. The crystal structure of synthetic HZ, $\beta $-hematin ($\beta $H), has recently been determined via x-ray diffraction. We employ van der Waals (vdW) corrected density functional theory to study the $\beta $H crystal and its repeat unit, a heme dimer. We find that vdW interactions play a major role in the binding of the heme dimer and the $\beta $H crystal. Accounting for the $\beta $H periodicity is a must for obtaining the correct geometry of the heme dimer, due to vdW interactions with adjacent dimers. The different isomers of the heme dimer are close in energy, consistent with the observed pseudo-polymorphism. We use these findings to comment on $\beta $H crystallization mechanisms. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D31.00010: Adsorption of methane on Zn(bdc)(ted)0.5 microporous metal-organic framework Vaiva Krungleviciute, Sanhita Pramanik, Aldo Migone, Jing Li Zn(bdc)(ted)0.5 is metal-organic framework crystallized in a tetragonal space group with a 3D porous structure containing intersecting channels of two different sizes. The larger channels are parallel to the c axis and have a cross section 7.5 $\times $ 7.5{\AA}. The smaller channels are along both the a- and b-axes and have a cross section of 4.8 $\times $ 3.2{\AA}. We measured methane adsorption isotherms at several different temperatures between 82 and 102 K. We calculated the effective specific surface area, isosteric heat and binding energy values. Two distinct substeps were observed in the isotherms corresponding to two different adsorption sites. The origin of the substeps will be discussed. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D31.00011: Understanding H$_{2}$-H$_{2}$ interactions in Metal Organic Frameworks (MOFs) with unsaturated metal centers Nour Nijem, Jean F. Veyan, Lingzhu Kong, Yonggang Zhao, Jing Li, David Langreth, Yves J. Chabal Unsaturated Metal Organic Frameworks (MOFs) are particularly interesting due to their high H$_{2}$ uptakes with relatively large isosteric heats of adsorption (Q$_{st }>$8 kJ/mol). This work explores H$_{2}$-H$_{2}$ interactions between adsorbed H$_{2}$ at the different sites in MOF-74 (M$_{2}$(dhtp),dhtp=2,5-dihydroxyterephthalate) and combines IR spectroscopy with vdW-DFT calculations. The adsorption sites in MOF-74 are from highest to lowest binding energies the metal, oxygen, benzene and pore-center sites. The frequency of adsorbed H$_{2}$ at the metal site suffers an additional $\sim $-30 cm$^{-1}$ red shift (for Mg and Zn) and $\sim $-84 cm$^{-1}$ (for Co) when the neighboring oxygen site is occupied. The dipole moment of adsorbed H$_{2}$ is also affected. These interactions extend to the benzene sites for MOF-74-Co. A decrease in dipole moment of H$_{2}$ adsorbed at the metal site is observed with the partial occupation of the benzene sites. However, the complete occupation of the benzene sites induces an additional $\sim $-10 cm$^{-1}$ red shift. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D31.00012: First-Principles Calculations of the Role of Dispersive Interactions in CO$_2$ binding in metal-organic frameworks Roberta Poloni, Joshua Howe, Jeffrey B. Neaton, Giulia Galli, Berend Smit Metal-organic frameworks (MOFs) have attracted much attention over the past 20 years for their possible applications in gas storage. In this study, we provide computational insight into what makes a MOF structure optimum for CO$_2$ capture. We present a density functional theory-based study of the electronic and structural properties of recently synthesized frameworks M'$_3$[(M$_4$Cl)$_3$(BTT)$_8$]$_2$, with M'=extraframework cation and M=Ca [1]. We study the interactions between CO$_2$ and different binding sites, and predict an unexpected favored binding site at the organic linker. We explore how binding energies are affected by the ordering and type of the extraframework cations. Finally, we address the role of dispersion forces by employing a recent non-local van der Waals functional [2], and compare with a DFT+D approach [3].\\[4pt] [1] M. Dinca et al., {\sl J. Am. Chem. Soc.} 128, 16876 (2006)\\[0pt] [2] M. Dion et al., {\sl Phys. Rev. Lett.} 92, 246401 (2004)\\[0pt] [3] A. Tkatchenko et al., {\sl Phys. Rev Lett.} 102, 073005 (2009) [Preview Abstract] |
Session D32: Focus Session: Optical Properties of Nanostructures and Metamaterials III
Sponsoring Units: DMPChair: Kurt Busch, Karlsruhe Institute of Technology
Room: C144
Monday, March 21, 2011 2:30PM - 2:42PM |
D32.00001: Spatial gradient tuning in metamaterials Tom Driscoll, Michael Goldflam, Nan Jokerst, Dimitri Basov, David Smith Gradient Index (GRIN) metamaterials have been used to create devices inspired by, but often surpassing the potential of, conventional GRIN optics. The unit-cell nature of metamaterials presents the opportunity to exert much greater control over spatial gradients than is possible in natural materials. This is true not only during the design phase but also offers the potential for real-time reconfiguration of the metamaterial gradient. This ability fits nicely into the picture of transformation-optics, in which spatial gradients can enable an impressive suite of innovative devices. We discuss methods to exert control over metamaterial response, focusing on our recent demonstrations using Vanadium Dioxide. We give special attention to role of memristance and mem-capacitance observed in Vanadium Dioxide, which simplify the demands of stimuli and addressing, as well as intersecting metamaterials with the field of memory-materials. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D32.00002: Carrier concentration dependence of the tunability of the dipole resonance peak in optically excited metamaterials Ioannis Chatzakis, Liang Luo, Jigang Wang, Nian Hai Shen, Thomas Koschny, Costas Soukoulis Currently, there is strong interest to explore the dynamic control of the electromagnetic properties of metamaterials, which have important implications on their optoelectronic applications. While the design, fabrication and photo-doping of metamaterial/semiconductor structures have been actively pursued, some fundamental issues related to highly photo-excited states, their dynamic tuning and temporal evolution remain open. Using optical-pump terahertz probe spectroscopy, we report on the pump fluence dependence of the electric dipole resonance tunability in metamaterials. We find a previously undiscovered large non-monotonic variation on the strength of the dipole resonance peak with the photo-injected carrier concentration. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D32.00003: Optically-Nonactive Assorted Helices Array with Interchangeable Magnetic/Electric Resonance Mu Wang, Xiang Xiong, Ru-Wen Peng, Xiao-Chun Chen, Dajun Shu, Cheng Sun We report here the designing of optically-nonactive metamaterial by assembling metallic helices with different chirality. With linearly polarized incident light, pure electric or magnetic resonance can be selectively realized, which leads to negative permittivity or negative permeability accordingly. Further, we show that pure electric or magnetic resonance can be interchanged at the same frequency band by merely changing the polarization of incident light for 90 degrees. This design demonstrates a unique approach to construct metamaterial. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:42PM |
D32.00004: Three-dimensional optical metamaterials and nanoantennas: Chirality, Coupling, and Sensing Invited Speaker: This abstract not available. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D32.00005: Temperature-Tunable Transparency Window in Metamaterials Utilizing Superconducting Dark Resonators C. Kurter, A.P. Zhuravel, P. Tassin, T. Koschny, L. Zhang, J. Abrahams, C.L. Bennett, A.V. Ustinov, C.M. Soukoulis, S.M. Anlage We have developed a high quality-factor microwave frequency metamaterial to demonstrate a coherent optical phenomena analogous to electrically induced transparency (EIT). The two-dimensional design employs double planar Nb split rings acting as dark resonators symmetrically placed around a thick Au strip which is a bright resonator [1]. When Nb is in the superconducting state, the significant loss gradient between Nb and Au opens a transparency window along with a strongly enhanced group delay. The data show a systematic evolution with increasing temperature in the superconducting state of Nb, and the features disappear in the resistive state when the loss gradient between the two types of resonators closes. We have observed no RF power dependence of the magnetic response coming out of the EIT configuration, which indicates the process is linear. Laser scanning microscopy images of the RF current distributions in the dark resonators and the other microwave measurements are in good agreement with the simulations run on the same structure. \\[0pt] [1] L. Zhang, \textit{et al}. arXiv:1010.2976 [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D32.00006: Light propagation and Anderson localization in superlattices containing metamaterials: effects of correlated disorder Raimundo Rocha dos Santos, Dmitri Mogilevtsev, Felipe Pinheiro, Solange Cavalcanti, Luiz Oliveira We discuss the effect of correlated disorder on light propagation and Anderson localization in a one-dimensional superlattice made up of air (A) and a dispersive metamaterial (M). Disorder is incorporated by assuming the layer widths to be random variables; however, here we consider the cases of correlated (i.e., the A and M layers with the same width) and completely anti-correlated (the total width of the A and M layers is fixed). We use transfer matrix techniques to obtain the localization length, and compare with the uncorrelated case. We have found that the photonic gaps of the corresponding periodic structure are not completely destroyed in the presence of disorder, giving rise to minima in the localization length. Near a gap, the behavior the localization length depends crucially on the physical origin of the gap (Bragg or non-Bragg gaps). We have found that the asymptotic behavior for the localization length $\xi \propto \lambda^{6}$ for disordered metamaterials is not affected by correlations, and the Brewster anomalies, at which light is delocalized, are also present. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:42PM |
D32.00007: Active Terahertz Metamaterials Invited Speaker: In recent years terahertz technology has become an optimistic candidate for numerous sensing, imaging, and diagnostic applications. Nevertheless, THz technology still suffers from a deficiency in high-power sources, efficient detectors, and other functional devices ubiquitous in neighboring microwave and infrared frequency bands, such as amplifiers, modulators, and switches. One of the greatest obstacles in this progress is the lack of materials that naturally respond well to THz radiation. The potential of metamaterials for THz applications originates from their resonant electromagnetic response, which significantly enhances their interaction with THz radiation. Thus, metamaterials offer a route towards helping to fill the so-called ``THz gap''. Here, we present a series of novel THz metamaterials. Importantly, the critical dependence of the resonant response on the supporting substrate and/or the fabricated structure enables the creation of active THz metamaterial devices. We show that the resonant response can be controlled using optical or electrical excitation and thermal tuning, enabling efficient THz devices which will be of importance for advancing numerous real world THz applications. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D32.00008: Optical properties of chiral metal nanoparticle complexes: Plasmonic chirality and circular dichroism Zhiyuan Fan, Alexander Govorov Plasmonic nanocrystals with chiral geometries are able to create strong circular dichroism (CD) signals in the visible wavelength range. This offers an interesting possibility to design colloidal and other nanostructures with strong optical chirality for applications in biosensors and optoelectronic devices. We present a theoretical study of circular dichroism from chiral metal nanoparticle (NP) complexes. Dipolar Coulomb interactions between NPs are involved as the main mechanism of interaction between spherical NPs in a chiral complex. In our analysis, the CD signal shows strong dependence on geometry and composition of chiral pyramids, tetramers, and helices. The CD spectra have both positive and negative bands. Strongest CD signals were found in helical chains of metal NPs, which resemble helical structures of many biomolecules. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D32.00009: Plasmonic Behavior of Deep Sub-Wavelength Superconducting RF Metamaterials Steven Anlage, Cihan Kurter, Liza Sarytchev, John Abrahams, C. Bennett, Tian Lan, A. P. Zhuravel, A. V. Ustinov We have designed and built ultra-small RF metamaterials with magnetically active spiral elements made of superconducting Nb films [1]. RF transmission measurements on single, 1-D and 2-D arrays of spirals show robust magnetic response when Nb is in the superconducting state [2] at frequencies as low as 14 MHz (corresponding to wavelength $\sim $ 3000 * 'atom' size). Numerical simulations capture the main features of the experimental spectra. The resonant features are tunable via variations in temperature and RF magnetic field [3]. As temperature approaches T$_{c}$, the superconducting kinetic inductance contribution to the total inductance increases, placing this RF metamaterial in the plasmonic limit. We study this approach to the plasmonic limit and compare to the analogous situation of frequency approaching the plasma edge in normal metal metamaterials. \\[4pt] [1] S. M. Anlage, J. Opt. \underline {13}, 024001 (2011). \\[0pt] [2] C. Kurter, \textit{et al}., Appl. Phys. Lett. \underline {96}, 253504 (2010). \\[0pt] [3] C. Kurter, \textit{et al}., IEEE Trans. Appl. Supercond., in press. arXiv:1008.2020. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D32.00010: Invisibility Using Perfect Absorption CNT Carpet at Visible Frequency and Beyond L. Jay Guo, Haofei Shi, Jong Ok, Hyoungwon Baac The concept of invisibility cloak based on transformation optics and metamaterials has tantalized the scientific community. Cloaking of wavelength-size objects were realized at microwave and NIR frequencies. However, the complexity of metamaterials based on the previous principles limits the object to several wavelengths in size. Moreover, cloaking of 3-D objects at visible band demands challenging inhomogeneous 3D nanostructured metamaterials and still unattainable. We propose a perfect absorption ground plane cloak that works at visible range and for large area arbitrarily shaped 3D objects. Such homogeneous perfect absorption carpet is demonstrated by low density carbon nanotube (CNT) forest, which can visually compress arbitrary 3D objects to appear as a 2D perfect absorption sheet. Invisibility was observed by naked eyes for unpolarized light at entire visible band with cloaking area of 10$^{5}$ larger than a wavelength. Such a cloaking approach based on perfect absorption is not restricted to CNT carpet, and can be applied to a broader frequency range from UV to THz and acts as a universal cloak for arbitrarily large objects. In this scheme the deep space is a natural and perfect ``ground plane''. It would only take a ``cloak'' consisting of low density and broadband absorbing particles to render matters and objects totally ``dark'' to our current instruments. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D32.00011: Collective dynamics in optomechanical arrays Florian Marquardt, Georg Heinrich, Max Ludwig, Jiang Qian, Bj\"orn Kubala Photonic crystals can support both localized optical and vibrational modes that couple to each other, leading to a very strong optomechanical interaction. These so-called ``optomechanical crystals'' have been demonstrated experimentally recently. Here we explore the dynamics that results in an array of many coupled optomechanical cells, when these are driven into a regime of self-sustained oscillations. We find synchronization of these oscillations beyond a certain coupling strength. We show that the slow phase dynamics can be efficiently described by an effective Kuramoto model. Other dynamical regimes like chaos will also be accessible in these novel systems. [Preview Abstract] |
Session D33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Multiferroics
Sponsoring Units: DMP DCOMPChair: Alison Hatt, Lawrence Berkeley National Laboratory
Room: C143/149
Monday, March 21, 2011 2:30PM - 2:42PM |
D33.00001: Oxygen rotation driven ferroelectricity enables controllable magnetization-polarization coupling in Ca$_3$Mn$_2$O$_7$ Craig Fennie, Nicole Benedek We show how to achieve the electric field switching of magnetism in a multiferroic with a large polarization by having the ferroelectric state arise from the same lattice instability that modulates the spin system. Oxygen octahedron rotations, ubiquitous in perovskites and related materials, are natural candidates for this lattice instability. First-principles calculations are presented for the layered perovskite Ca$_3$Mn$_2$O$_7$, in which rotations induce both ferroelectricity and weak ferromagnetism. The key point is that this rotation pattern is a combination of two non-polar structural modes with different symmetries. We introduce the term ``hybrid'' improper ferroelectricity to describe this phenomenon. Our results suggest a new strategy in magnetoelectronics, whereby control over magnetism is achieved through functional antiferrodistortive oxygen octahedron rotations.\\ N. A. Benedek and C. J. Fennie, arXiv:1007.1003 (2010). [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D33.00002: The search for multifunctional polar materials Joseph Bennett, Karin Rabe One strategy in the search for new polar semiconducting (and possibily magnetic) materials is to check systems already synthesized and reported as polar in the literature to determine the intrinsic magnitude and switchability of the polarization, the band gap and magnetic properties. In many examples where a polar space group was found, neither polarization or band gap measurements were made because the sample as grown was too conductive. Using a combination of ICSD searching and symmetry analysis, we first identify potentially interesting polar materials and screen out those that are reported to definitely be metallic. We then use first-principles density functional theory (DFT) calculations to investigate the ground state structures of these experimentally synthesized materials for which limited data is available. These calculations will help us to develop criteria for screening candidate systems for polar, magnetic and semiconductive behavior, and broaden the search for new examples of these important functional materials. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D33.00003: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:06PM - 3:18PM |
D33.00004: Higher-order Ginzburg-Landau Model for Multiferroic Hexagonal Manganites Kris Delaney, Sergey Artyukhin, Manfred Fiebig, Nicola Spaldin, Maxim Mostovoy Hexagonal manganites have been studied intensely as some of the few multiferroic materials with relatively high ordering temperatures. The recent experimental discovery of topological defects in the domain structure of YMnO3 has led to renewed interest in these materials [1, 2, 3]. Though a Landau free-energy model has already been parameterized at low order[4], we show the form of the parameterization with higher-order terms, including for the first time an angular dependence to the structural trimerization mode. Analysis of the resulting model explains clearly the origin of the topological defects in the domain structure, provides further theoretical insight into the contentious issue of the nature of the ferroelectric phase transition, and gives theoretical input into understanding the thickness of ferrelectric domain walls. \\[4pt] [1] Choi et al., Nature Mat. 9, 253 (2010)\\[0pt] [2] Mostovoy, Nature Mat. 9, 188 (2010)\\[0pt] [3] Jungk et al., Appl. Phys. Lett. 97, 012904 (2010)\\[0pt] [4] Fennie et al., Phys. Rev. B 72, 100103 (2005) [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D33.00005: Local and Long-Range High Pressure Structure of Orthorhombic REMnO$_{3}$ H. Chen, T. Wu, T. Tyson, R. Tappero, L. Huang, S. Kim, S.-W. Cheong Orthorhombic perovskite REMnO$_{3}$ multiferroic systems were prepared by high pressure synthesis and solid state reaction. High pressure synchrotron x-ray diffraction and x-ray absorption measurements were performed to explore the structural changes. The influence of the pressure on the electrical polarization is discussed. Theoretical simulations are utilized to predict the stable magnetic phases based on the experimental parameters. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D33.00006: On the Nature of the Ferroelectric Transition in Multiferroic Hexagonal REMnO$_{3}$ Trevor Tyson, Tao Wu, Haiyan Chen, Jainming Bai, Sang-Wook Cheong Combined local and long range structural measurements were conducted on REMnO$_{3}$ for temperatures extending significantly above the ferroelectric transition temperature (T$_{FE})$. We find in hexagonal REMnO3 no large atomic (bond distance or thermal factors) or electronic structure changes on crossing T$_{FE}$. The born effective charge tensor is found to be highly anisotropic at the O sites indicating very strong hybridization of the charge. The tensor does not change significantly above T$_{FE}$ revealing no charge redistribution and suggests an unusual transition. This work is supported by DOE Grants DE-FG02-07ER46402 (NJIT) and DE-FG02-07ER46382 (Rutgers University). [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D33.00007: Anomalous Phonon Behavior in Orthorhombic LuMnO3 at Low Temperature Peng Gao, Haiyan Chen, Trevor A. Tyson, Zhenxian Liu, Jianming Bai, Liping Wang, YoungJai Choi, Sang-Wook Cheong We present the pressure dependent phonon spectra of orthorhombic-LuMnO3 which are conducted in the low temperature region (below T$_{N}$ and T$_{L})$. A temperature dependent anomalous phonon coincides with the ferroelectric behavior at low pressure condition. At $\sim $10 GPa, this anomalous phonon exhibits an unusual softening trend which will be suppressed at higher pressure. This work is supported by DOE Grant DE-FG02-07ER46402 (NJIT), by DE-FG02-07ER46402 (Rutgers), by COMPRES (U2A beam line at NSLS), the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR01-35554, U.S. Department of Energy (DOE-BES and NNSA/CDAC) and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886 (use of NSLS at Brookhaven National Laboratory). [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D33.00008: Induction of novel macroscopic properties by local symmetry violations in spin-spiral multiferroics D. Meier, N. Leo, P. Becker, L. Bohaty, R. Ramesh, M. Fiebig Incommensurate (IC) structures are omnipresent in strongly correlated electron systems as high-$T_C$ superconductors, CMR manganites, as well as multiferroics. In each case they are origin of a pronounced symmetry reduction reflecting the complexity of the underlying microscopic interactions. Macroscopically, this can lead to new phases and possibilities to gain control of the host material. Here we report how the IC nature of a spin-spiral multiferroic induces new physical properties by renormalizing the relevant length scales of the system. Local symmetry violations directly manifest in the macroscopic response of the material and co-determine the multiferroic order giving rise to additional domain states. These usually hidden degrees of freedom become visible when non-homogenous fields are applied and condition for instance the second harmonic generation. Our study shows that incommensurabilities play a vital role in the discussion of the physical properties of multiferroics -- they represent a key ingredient for further enhancing the functionality of this class of materials. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D33.00009: Origin of the magnetic-field controlled polarization reversal in multiferroic TbMn$_{2}$O$_{5}$ N. Leo, D. Meier, R.V. Pisarev, S. Park, S.-W. Cheong, M. Fiebig The interplay of multi-dimensional complex magnetic order parameters leads to interesting effects like magnetically induced ferroelectricity. A particular interesting example is TbMn$_{2}$O$_{5}$ because of the associated magnetic-field controllable electric polarization. By optical second harmonic generation we show that the gigantic magnetoelectric effect originates in three independent ferroelectric contributions. Two of these are manganese-generated. The third contribution is related to the magnetism of the Tb$^{3+}$ sublattice and has not been identified so far. It mediates the remarkable magnetic-field induced polarization reversal. This model is verified by experiments on the isostructural YMn$_{2}$O$_{5}$ where Y$^{3+}$ ions are nonmagnetic and only two polarization contributions are present and no magnetoelectric coupling is observed. These results underline the importance of the $3d-4f$-interaction for the intricate magnetoelectric coupling in the class of isostructural RMn$_{2}$O$_{5}$ compounds. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D33.00010: Electronic mechanism for ferroelectricity and strong magneto-electric coupling in charge-ordered multiferroics Gerardo Ortiz, Leonid Isaev, Cristian Batista We study magneto-electric phenomena in multiferroic materials, which exhibit ferroelectricity due to the charge ordering. Using rare-earth iron oxides as an example, we derive an effective model, which takes into account the Coulomb interaction, magnetic superexchange and spin-orbit effects, and is consistent with the recent X-ray absorption spectroscopy measurements in multiferroic ${\rm LuFe_2O_4}$. Then we demonstrate, how the interplay between quantum fluctuations and geometric frustration stabilizes the charge and ferrimagnetic spin orderings. The strong coupling, due to the double-exchange mechanism, between these orders, leads to a large magneto-electric response. Our results provide a complete physical description of the magneto-electric properties of charge-ordered multiferroics. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D33.00011: Colossal Magnetoelectric Effect with Competing Multiferroic and Weak-Ferromagnetic Phases Young Jai Choi, Chenglin Zhang, Nara Lee, Sang-Wook Cheong From our investigation of magnetoelectric properties of Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$, where a multiferroic phase competes with a weak ferromagnetic phase in magnetic fields, we found intriguing hysteretic behaviors of physical properties with variation of temperature and magnetic field. These hysteretic behaviors arise from the kinetic arrest/de-arrest processes of the first order magnetic transition, resulting in freezing or melting of a magnetoelectric glass state with the coexistence of two competing phases. We note that most of large magnetoelectric coupling effects in multiferroics are associated with the large change of polarization with magnetic fields, but the control of ferromagnetic-type magnetization by applying electric fields is most relevant to technological applications, which is scarcely observed. This important issue of mutual controllability is achieved in Eu$_{0.75}$Y$_{0.25}$MnO$_{3}$ utilizing dynamical modulations of the coexistence of two contraindicative phases, highly susceptible to the external perturbations such as electric and magnetic fields. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D33.00012: Pairing and Self-Organization of Vortices and Antivortices in h-YMnO$_{3}$ S.C. Chae, Y. Horibe, D.Y. Jeong, S. Rodan, N. Lee, S.-W. Cheong Fascinating vortices and antivortices with ferroelectric domains were discovered in multiferroic hexagonal YMnO$_{3}$ [1]. Interlocking of ferroelectric and structural antiphase domain walls of h-YMnO$_{3}$ is one of the important ingredients for the topologically-nontrivial domain pattern formation. We have recently investigated the large-scale configuration of vortices and antivortices in h-YMnO$_{3}$ using selective chemical etching. Our results indicate the importance of pairing of vortices and antivortices, and provide valuable insights into understanding the self-organization mechanism of a zoo of vortices and antivortices. Furthermore, we have studied the response of the vortices and antivortices configuration to external stimuli such as external electric fields. \\[4pt] [1] T. Choi et al., Nature Mater. \textbf{9}, 253 (2010). [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D33.00013: Ferroelectricity driven by symmetric exchange striction in orthorhombic HoMnO$_{3}$ Nara Lee, Young Jai Choi, Sang-Wook Cheong Orthorhombic HoMnO$_{3}$ crystallizes in a distorted perovskite structure (space group \textit{Pbnm}). It has been predicted that the spin configuration below the N\'eel temperature corresponds to a collinear E-type antiferromagnetic phase, which accompanies a large ferroelectric polarization originated from local oxygen distortions driven by exchange striction. In order to understand the exact nature of the E-type magnetism-driven ferroelectricity as well as the influence of Ho magnetism on ferroelectricity, we have performed comprehensive measurements of physical properties of the system, including magnetic susceptibility, dielectric constant, ferroelectric polarization and heat capacity with the variation of temperature and magnetic fields. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D33.00014: Landau theory of composite domain walls and vortices in multiferroic hexagonal manganites Sergey Artyukhin, Kris Delaney, Nicola Spaldin, Maxim Mostovoy Multiferroic materials with their coexisting magnetic and ferroelectric orders may find applications in memory devices. In hexagonal manganites, where electric polarization is induced by a periodic lattice distortion, ferroelectric and magnetic domain walls are firmly locked\footnote{M. Fiebig et al., Nature 419, 818 (2002).} even though electric polarization and spin ordering are decoupled in the bulk. Recent measurements showed that electric polarization changes sign at the boundaries of structural domains and revealed the existence of unusual vortices where six structural domains merge and the electric polarization changes sign six times around the defect.\footnote{T. Choi et al., Nature Materials 9, 253 (2010).}$^,$\footnote{M. Mostovoy, Nature Materials 9,188 (2010).} We present a phenomenological theory of coupled lattice, charge and spin degrees of freedom in hexagonal manganites, which we use to calculate how electric polarization, structural distortions and magnetic ordering vary at the domain walls and vortices, and how the shape of these defects changes in an applied electric field. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D33.00015: Analysis of the magnetic structure and spin exchange interactions of multiferroic YBaCuFeO$_{5}$ by first principles DFT calculations Jerry Bettis, Yuemei Zhang, C. Lee, Mike Whangbo In the layered perovskites RBaCuFeO$_{5}$ (R = Y, Lu, Tm), the CuFeO$_{9}$ dumbbells made up of apex-sharing CuO$_{5}$ and FeO$_{5}$ square pyramids share their basal corners to form perovskite layers, and the resulting CuFeO$_{5}$ slabs are stacked along the c-direction. Recently, these compounds were found to exhibit ferroelectric polarization when a modulated magnetic component is superposed on their antiferromagnetic structure. To help understand this finding, we examined the spin exchange interactions between the Fe$^{3+}$ (d$^{5})$ ions, between the Cu$^{2+}$ (d$^{9})$ ions, and between the Fe$^{3+}$ and Cu$^{2+}$ ions on the basis of DFT+U and DFT+U+SOC calculations for YBaCuFeO$_{5}$. The ferroelectric polarization of YBaCuFeO$_{5}$ was also calculated for several modulated magnetic structures that were constructed based on the cone-model. [Preview Abstract] |
Session D34: Focus Session: Interfaces in Complex Oxides - Photo and Electric Field Induced Devices
Sponsoring Units: DMPChair: Peter Abbamonte, University of Illinois at Urbana-Champaign
Room: C141
Monday, March 21, 2011 2:30PM - 3:06PM |
D34.00001: Band profiles of Mott-insulator/band-insulator heterointerfaces revealed by photocurrent and electromodulation spectroscopies Invited Speaker: Heterointerfaces of Mott insulators provide a good laboratory to explore unprecedented electronic states induced by the strong electron correlation. Although a number of intriguing phenomena have been reported so far, their fundamental origins have not been fully addressed yet. This is partly because the interface band profile, which is one of the most basic knowledge to understand the interface electronic states, is still left to be unveiled. In this study, we have investigated in detail the interface band profiles of Mott insulators employing photocurrent and electromodulation spectroscopies as well as the conventional current-voltage and capacitance-voltage characterizations. We chose $p$-type (LaMnO$_{3}$ and La$_{2}$CuO$_{4}$) and $n$-type (SrMnO$_{3}$ and Sm$_{2}$CuO$_{4}$) as the Mott insulators and these are epitaxially connected to Nb doped SrTiO$_{3}$ (electron-doped band insulator). The photocurrent action spectra for these heterojunctions showed negligibly-small band reconstruction as well as the existence of band bending and discontinuity in the Mott insulators, which are of no salient discrepancy with the rigid-band picture valid in the interface of conventional semiconductors~[1]. However, the electromodulation spectra clearly indicate the band reconstruction in the Mott insulators~[2]. The results mean that the rigid-band picture is valid in the low carrier-density regime even in Mott-insulator/band-insulator interfaces, but the intentional charge modulation leads the electron correlation effect in the Mott insulators. This work was done in collaboration with A. Sawa, J. Fujioka, M. Kawasaki and Y. Tokura.\\[4pt] [1]~M. Nakamura~\textit{et al.},~Phys.~Rev.~B~\textbf{82},~201101(R)~(2010)\\[0pt] [2]~M. Nakamura~\textit{et al.},~Phys.~Rev.~B~\textbf{75},~155103~(2007). [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D34.00002: Mechanisms for the enhancement of the lateral photovoltage in perovskite heterostructures Kui-juan Jin, Chen Ge, Huibin Lu, Guozhen Yang The mechanisms for greatly enhanced lateral photovoltaic effect in the perovskite oxide heterostructures are studied by solving \textit{time-dependent} two-dimensional drift-diffusion equations self-consistently. By our calculations, we find that the lateral photovoltage of $p$ type material is larger than that of $n$ type material owing to the larger drift electric field induced in the $p$ type material than that in the $n$ type material. Moreover, the built-in electric field at the interface between the thin film and substrate can also enhance the lateral photovoltage. The above two mechanisms can well explain one-order-of-magnitude enhancement of the LPV in the perovskite heterostructures. In addition, we find that the materials with larger mobility ratio have stronger Dember effect. Such an understanding of the mechanisms for the enhancement of lateral photovoltage in oxide heterostructures should be useful in further designing of the structures of position-sensitive detectors and new THz sources. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D34.00003: Electronic Transport in Ion Gel-Gated Strontium Titanate Men Young Lee, James R. Williams, David Goldhaber-Gordon, Sipei Zhang, C. Daniel Frisbie, Bharat Jalan, Junwoo Son, Susanne Stemmer In recent years much attention has been focused on the structure and properties of two-dimensional electron liquids (2DEL) in complex oxide heterostrucutres and delta- doped layers. We report on the fabrication and measurements of mesoscopic devices of metal oxides, with focus given to an electric field-induced 2DEL at the surface of undoped strontium titanate (STO). We describe the design and fabrication of field- effect structures, gated with an ionic gel, and show the measurements of induced swings of charge carrier density in STO. Other transport properties of the 2DEL are studied by magneto-transport measurements at low temperature. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D34.00004: Origin of Electrical Conduction in Domain Walls of BiFeO${_3}$ Thin Films James Lee, Anoop Damodaran, Lane Martin, Peter Abbamonte, Helen He, Ramamoorthy Ramesh BiFeO${_3}$ thin films grown on DyScO${_3}$ substrates unexpectedly exhibit metallic electrical conduction at ferroelectric (FE) domain walls (DWs). Resonant x-ray scattering near Fe L and O K absorption edges was used to probe the electronic structure of these films. In-plane wavevectors of resonant Fe edge magnetic scattering, and non-resonant Cu K $\alpha $ diffraction peaks near the (0, 0, 1) BiFeO${_3}$ Bragg peak, match the domain period observed by PFM. Fe edge scattering intensifies as the beam energy is tuned to Fe 2p $\to$ ligand-3d transitions. No O K charge scattering is observed. These results suggest that metallic conduction does not arise from charge build- up at the DWs from FE polarization discontinuities, but from the bandgap closing near DWs as the crystal symmetry changes from rhombohedral-like in the domain bulk to higher-symmetries. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D34.00005: Sketched ferroelectric single-electron transistor Guanglei Cheng, Pablo Siles, Feng Bi, Cheng Cen, Daniela Bogorin, Chung Wung Bark, Chad Folkman, Jae-Wan Park, Chang-Beom Eom, Gilberto Medeiros-Ribeiro, Jeremy Levy Oxide heterostructures formed from ultrathin layers of LaAlO$_{3}$ grown on TiO$_{2}$-terminated SrTiO$_{3}$, combined with a reversible nanoscale patterning technique, provide a versatile platform for nanoscale control at the single-electron limit. Here we demonstrate the creation and characterization of ``sketched'' single-electron transistors made from ultrasmall (1-2 nm) quantum dots. Shell filling from N=0 up to N=2 electrons by single-electron tunneling is observed. Resonant tunneling can be controlled in a deterministic and non-volatile fashion by altering the ferroelectric polarization within the SrTiO$_{3}$ tunnel barrier. These single-electron devices may find use as nanoscale hybrid piezoelectric/charge sensors, and as elemental building blocks for solid-state quantum computation and quantum simulation platforms. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D34.00006: Characterizing Interfacial Bipolar Resistive Switches at Low Temperatures Stephen Tsui Bipolar resistive switching has continued to be a topic of interest for many years because of the phenomenon's potential for memory device applications. Typically, a voltage pulse is applied to a metal-oxide sandwich structure, which drives the sample into a nonvolatile high or low resistance state depending upon the pulse polarity. A great deal of research has already been performed on a diverse array of materials with several different characteristics. However, few systematic investigations have been carried out at low temperature, which may have application to ``cryo-memory.'' In this work, we compare the room temperature and low temperature behaviors of switches formed at the interfaces between a silver electrode and CeO$_{2}$, Al$_{2}$O$_{3}$, and Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$, respectively. We investigate the performance of the switching in response to temperature change and characterize the electronic transport at the interfaces in order to identify the dominant physical processes at these various temperatures. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D34.00007: Role of the surface in writing, erasing and maintaining nanostructures at the LaAlO$_{3}$/SrTiO$_{3}$ interface Feng Bi, Daniela F. Bogorin, Cheng Cen, Jeremy Levy, Chung Wung Bark, Jae-Wan Park, Chang-Beom Eom Nanoscale control of the metal-insulator transition in LaAlO$_{3}$/SrTiO$_{3}$ heterostructures can be achieved using local voltages applied by a conducting AFM probe. The mechanism is believed to be governed by a ``water cycle'' in which the surface is locally charged via hydrogen passivation, resulting in high-resolution modulation doping of the LaAlO$_{3}$/SrTiO$_{3}$ interface.\footnote{F. Bi et al., Appl. Phys. Lett.97, 173110 (2010)} A Kelvin probe image method is applied to study how water content in the gas environment influences such charge writing. Persistence tests are performed, in which the long-term behavior is studied by keeping the AFM-written nanostructures (nanowire and sketch FET\footnote{C.Cen et al., Science, 323, 1026 (2009)}) in different ambient environments. The self-erasure process is particularly obvious in moisture environments, but is slowed greatly in dry inert gas and can be even halted under modest vacuum conditions ($\sim$10$^{-3}$ Torr). [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D34.00008: Electric field-tuning of the magneto-transport of superconducting LaAlO$_3$/SrTiO$_3$ interfaces Stefano Gariglio, Nicolas Reyren, Andrea D. Caviglia, Claudia Cancellieri, Toni Schneider, Jean-Marc Triscone LaAlO$_3$/SrTiO$_3$ interfaces display a complex phase diagram that can be explored by an electric field [1,2]. Using transport measurements in magnetic fields for different doping levels, we have characterized the superconducting phase diagram in three dimensions (temperature, electric and magnetic fields). Analyses of the anisotropy for parallel and perpendicular magnetic fields [3] reveal a two-dimensional superconducting state for all doping levels. Magneto-resistances in perpendicular fields present hallmarks of superconductor-insulator and superconductor-metal transitions depending on the doping level. We will discuss scaling analyses of the magnetic field-tuned transitions and the role of fluctuations and disorder in this two-dimensional superconductor. \\[4pt] [1] N. Reyren et al. Science 317, 1196 (2007).\\[0pt] [2] A. Caviglia et al. Nature 456, 624 (2008).\\[0pt] [3] N. Reyren et al. Appl. Phys. Lett. 94, 112506 (2009). [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D34.00009: Cooper Pair Writing at the LaAlO$_{3}$/SrTiO$_{3}$ Interface Cheng Cen, Daniela F. Bogorin, Chung Wung Bark, Chad M. Folkman, Chang-Beom Eom, Jeremy Levy Superconducting semiconductors offer unique ways to exert electrostatic control over macroscopic quantum phases. The recently demonstrated nanoscale control over conductivity at the LaAlO$_{3}$/SrTiO$_{3}$ interface raises the question of whether nanoscale control over superconducting phases can be realized. Here we report low-temperature magnetotransport experiments on structures defined with nanoscale precision at the LaAlO$_{3}$/SrTiO$_{3}$ interface. A quantum phase transition is observed that is associated with the formation of Cooper pairs, but a finite resistance is observed at the lowest temperatures. Higher mobility interfaces exhibit larger Ginsburg-Landau coherence lengths, a stronger suppression of pairing by magnetic field as well as Shubnikov-de Haas oscillations. Cooper pair localization, spin-orbit coupling, and finite-size effects may factor into an explanation for some of the unusual properties observed. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D34.00010: Electric and magnetic field control of superconducting transition at the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface Dmitriy Dikin, Manan Mehta, Venkat Chandrasekhar, Chung Wung Bark, Chad Folkman, Chang-Beom Eom We report on detailed measurements of the normal state-superconducting phase transition of the two-dimensional electron gas that develops at the LAO/STO interface as a function of gate voltage and magnetic field. We will discuss the specifics of the R versus T and the T-H phase diagrams for this superconductor and the potential origin of observed dissipation and hysteretic behavior. These data are analyzed in connection with magnetoresistance and Hall measurements. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D34.00011: Metal-insulator transition at the interface of LaAlO$_3$ /SrTiO$_3$ induced by H$_2$O adsorption Yun Li, Jaejun Yu We investigated the adsorption configurations at various H$_2$O coverages on the AlO$_2$ surface of n-type interface of 3 unit cell layers of LaAlO$_3$ (LAO) overlayer on SrTiO$_3$ (001) (STO) and the effects on the electronic properties at the interface by carrying out density-functional-theory calculations. For 0.25 monolayer (ML) and 0.5 ML coverages of H$_2$O the dissociation processes are barrierless. While for 1 ML coverage the mixing adsorption configuration comprising 0.5 ML molecular and 0.5 ML dissociated H$_2$O is most stable and the dissociation from fully molecular adsorption has to overcome 1 eV barrier. Insulator-metal transition at the n-type interface of (LAO)3/STO occurs as the coverage of dissociated H$_2$O reaches to 0.5ML. Insulator-metal transition at the interface can be realized by two ways: (1) changing H$_2$O coverage of from less than 0.5ML to equal to 0.5ML; (2) fixing H$_2$O coverage at 1ML and converting the adsorption configuration form fully molecular adsorption to mixing (0.5:0.5) adsorption. The second scheme can be utilized to realizing single-electron controlled nanoscale memory and switch. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D34.00012: GHz operation of LaAlO$_{3}$/ SrTiO$_{3}$-based transistor Patrick Irvin, Mengchen Huang, Jeremy Levy, Chung Wung Bark, Chad M. Folkman, Chang-Beom Eom Local modification of the metal-insulator transition of the LaAlO$_{3}$ /SrTiO$_{3}$ interface with a conducting-atomic force microscope (c-AFM) has resulted in a variety of electrical\footnote{C. Cen, S. Thiel, J. Mannhart, and J. Levy, Science \textbf{323}, 1026 (2009).} and photonic\footnote{P. Irvin, Y. Ma, D. F. Bogorin, C. Cen, C. W. Bark, C. M. Folkman, C.-B. Eom, and J. Levy, Nature Photonics advanced online publication, 14 Nov.2010 (DOI 10.1038/nphoton.2010.238)} devices. Using a heterodyne measurement technique, we show that a sketch-based, nanoscale transistor (``SketchFET'') can operate at frequencies in excess of 1 GHz. This demonstration of GHz functionality opens the door for new applications for oxide-based, rewritable nanoscale devices. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D34.00013: Nanoscale control at the LaAlO$_{3}$/SrTiO$_{3}$ Interface grown on LSAT Daniela Bogorin, Cheng Cen, Chung Wung Bark, Chang Beom Eom, Jeremy Levy The two-dimensional electron gas (2DEG) that forms at the interface between two semiconductors or between a semiconductor and oxide is currently the basis for some of the most useful electronic devices. We are able to control the 2DEG interface between LaAlO$_{3}$/SrTiO$_{3}$ with nanoscale precision and create transistors, nanodiodes and other nanostructures. Future scaling of oxide nanoelectronics requires scaling to wafer sizes larger than what can be provided from SrTiO$_{3}$. (LaAlO$_{3})_{0.3}$--(Sr$_{2}$AlTaO$_{3})_{0.7}$ (LSAT) substrates can allow for coherently strained LaAlO$_{3}$/SrTiO$_{3}$ heterostructures to be created. A sharp insulator to metal transition occurs at 8 uc LaAlO$_{3}$ thicknesses, in contrast to what is observed for unstrained SrTiO$_{3}$ substrates. We describe the properties of nanoscale structures created at the 2DEG interface of LaAlO$_{3}$/SrTiO$_{3}$ grown on LSAT wafers and compare them with structures grown on bulk SrTiO$_{3}$ substrates. [Preview Abstract] |
Session D35: Topological Insulators: Theory II
Sponsoring Units: DCMPChair: David Vanderbilt, Rutgers University
Room: C140
Monday, March 21, 2011 2:30PM - 2:42PM |
D35.00001: Antiferromagnetic topological insulators Roger S.K. Mong, Andrew M. Essin, Joel E. Moore We consider antiferromagnets breaking both time-reversal ($\Theta$) and a primitive lattice translational symmetry ($T_{1/2}$) of a crystal but preserving the combination $S = \Theta T_{1/2}$. The $S$ symmetry leads to a $Z_2$ topological classification of insulators, separating the ordinary insulator phase from the ``antiferromagnetic topological insulator'' (AFTI) phase. This state is similar to the ``strong'' topological insulator with time-reversal symmetry, and shares with it such properties as a quantized magnetoelectric effect. However, for certain surfaces the surface states are intrinsically gapped with a half-quantum Hall effect [$\sigma_{xy} = e^2 / (2 h)$], which may aid experimental confirmation of $\theta = \pi$ quantized magnetoelectric coupling. Step edges on such a surface support gapless, chiral quantum wires. In closing we discuss GdBiPt as a possible example of this topological class. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D35.00002: Strong topological insulator phase in cold-atom systems Peter P. Orth, Stephan Rachel, Karyn Le Hur With the recent technological advance of creating (electromagnetic) gauge fields for ultracold atoms, the fascinating prospect of realizing novel topological phases in these systems arises. Specifically, we consider spin-1/2 fermions on a square lattice under the influence of various experimentally feasible gauge fields. In two dimensions and if particles with different spin are exposed to magnetic fields in time-reversed directions, the system displays a quantum spin Hall ground state. We then study the influence of hopping into the third direction (2D-3D crossover), and in the three-dimensional system, we are able to identify a strong topological insulator phase. We further elaborate on the influence of the external trapping potential as well as the unambiguous detection of the topological phases. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D35.00003: Quantum Hall Viscosity and the Torsional Response of Topological Insulators Robert Leigh, Taylor Hughes, Eduardo Fradkin In this talk I will discuss a dissipationless viscosity that has recently appeared in connection with the quantum Hall effect. I will show that this can be connected to the response of time-reversal breaking 2+1-d topological insulators under a mechanical torque. The torque is represented by a coupling of the electronic degrees of freedom to external torsion fields and gives rise to a Chern-Simons-like term commonly seen in gravitational theories in the presence of spacetime torsion. I will discuss possible thought experiments which illustrate the effects and will briefly cover the extension to 3+1-d topological insulators. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D35.00004: Topological $BF$ field theory description of topological insulators Joel E. Moore, Gil Young Cho Topological phases of matter are described universally by topological field theories in the same way that symmetry-breaking phases of matter are described by Landau-Ginzburg field theories. We propose that topological insulators in two and three dimensions are described by a version of abelian $BF$ theory. For the two-dimensional topological insulator or quantum spin Hall state, this description is essentially equivalent to a pair of Chern-Simons theories, consistent with the realization of this phase as paired integer quantum Hall effect states. The $BF$ description can be motivated from the local excitations produced when a $\pi$ flux is threaded through this state. For the three-dimensional topological insulator, the $BF$ description is less obvious but quite versatile: it contains a gapless surface Dirac fermion when time-reversal-symmetry is preserved and yields ``axion electrodynamics'', i.e., an electromagnetic $E \cdot B$ term, when time-reversal symmetry is broken and the surfaces are gapped. Just as changing the coefficients and charges of 2D Chern-Simons theory allows one to obtain fractional quantum Hall states starting from integer states, $BF$ theory could also describe (at a macroscopic level) fractional 3D topological insulators with fractional statistics of point-like and line-like objects. Preprint available at http://arxiv.org/abs/1011.3485. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D35.00005: Detecting 3d Non-Abelian Anyons via Adiabatic Cooling Seiji Yamamoto, Michael Freedman, Kun Yang Majorana fermions lie at the heart of a number of recent developments in condensed matter physics. One important application is the realization of non-abelian statistics and consequently a foundation for topological quantum computation. Theoretical propositions for Majorana systems abound, but experimental detection has proven challenging. Most attempts involve interferometry, but the degeneracy of the anyon state can be leveraged to produce a cooling effect, as previously shown in 2d. We apply this method of anyon detection to the 3d anyon model of Teo and Kane. Like the Fu-Kane model, this involves a hybrid system of topological insulator (TI) and superconductor (SC). The Majorana modes are localized to anisotropic hedgehogs in the order parameter which appear at the TI-SC interface. The effective model bears some resemblance to the non-Abelian Higgs model with scalar coupling as studied, for example, by Jackiw and Rebbi. In order to make concrete estimates relevant to experiments, we use parameters appropriate to Ca doped Bi$_2$Se$_3$ as the topological insulator and Cu doped Bi$_2$Se$_3$ as the superconductor. We find a temperature window in the milli-Kelvin regime where the presence of 3d non-abelian anyons will lead to an observable cooling effect. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D35.00006: Topological invariants of adiabatic cycles of Bloch Hamiltonians Rahul Roy Invariants are constructed for various adiabatic cycles of Bloch Hamiltonians and discuss their physical implications. Many of these cycles lead to a pumping of fermions, but in other cases, the physical implications are more subtle. I also discuss the construction of these invariants for insulators in the various symmetry classes and periodicities in the table of these invariants. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D35.00007: On the stability of surface states in topological insulators Young Hoon Moon, Leonid Isaev, Gerardo Ortiz The existence of robust surface/edge states is arguably a fingerprint of topological insulators. These states are protected against scattering by time-reversal invariant perturbations, and lead to dissipationless transport even at high temperatures. This characteristic behavior is believed to be quite insensitive to the properties of the surface of a particular sample. We investigate the above conjecture by considering the stability of edge states with respect to the {\it time-reversal invariant} surface perturbations in several models of topological insulators. We demonstrate that in certain regimes the surface spectrum is modified quite dramatically. In particular, the number of edge states, which cross the Fermi level inside the bulk band gap, is very sensitive to the properties of the surface. Our results can be of great importance for future transport measurements in topological insulators. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D35.00008: Topological quantization in units of the fine structure constant Joseph Maciejko, Xiao-Liang Qi, H. Dennis Drew, Shou-Cheng Zhang Fundamental topological phenomena in condensed matter physics are associated with a quantized electromagnetic response in units of fundamental constants. Recently, it has been predicted theoretically that the time-reversal invariant topological insulator in three dimensions exhibits a topological magnetoelectric effect quantized in units of the fine structure constant $\alpha=e^2/\hbar c$. In this Letter, we propose an optical experiment to directly measure this topological quantization phenomenon, independent of material details. Our proposal also provides a way to measure the half-quantized Hall conductances on the two surfaces of the topological insulator independently of each other. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D35.00009: Refraction and interference of electrons on the topological insulator surface Ryuji Takahashi, Shuichi Murakami We theoretically study electron transport on the topological insulator surface, in analogy with optics. The surface states are represented by spinors, unlike optics, and therefore different behaviors from those in optics are expected. First, we consider the refraction phenomena at the boundary between the surfaces of two different topological insulators, where the velocities of the surface states are different. We compare its transmission and refraction coefficients with optics. Furthermore, we discuss the case when the velocities of the surface states of the two topological insulators have opposite signs. Second, we study interference phenomena on the surface states. The result shows that if the detector is very far from the scatterer or the slit, the interference is asymptotically similar to ordinary two-dimensional scattering problems. We also study the spin directions of scattered wave due to the surface interference phenomena. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D35.00010: Gauge field fluctuations in three-dimensional topological Mott insulators William Witczak-Krempa, Ting Pong Choy, Yong Baek Kim We discuss the low-energy properties of 3D topological Mott insulators that can be viewed as strong topological insulators of spinons interacting with a 3D gauge field. The low-energy behavior of such systems is dominated by gapless surface spinons (Dirac fermions) coupled to bulk gauge bosons. We find that a dimensional crossover from 3D to 2D in the gauge field fluctuations may occur as the system's thickness and/or temperature is varied. In the thin sample limit, the gauge field fluctuations effectively become 2D and the problem becomes analogous to the standard 2D spinon-gauge field theory. In the 3D limit, the bulk gauge field fluctuations lead to a novel low-energy theory for the coupled system that is more controlled than in the 2D regime. We discuss various experimental signatures such as the heat capacity scaling as T ln(1/T) as well as modified RKKY interactions on the surface. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D35.00011: Symmetry aspects of localized Dirac fermions within topological defects Chi-Ken Lu, Igor Herbut We study the conditions for the existence of zero-energy bound states within topological defects in various insulating and superconducting order parameters for Dirac fermions in graphene and topological insulators. In particular, we discuss several physically relevant realizations of the ``Dirac vortex'' which include the finite chemical potential and Zeeman terms, and the orbital magnetic fields, and present some explicit solutions for the zero-modes. The crucial role in our discussion is assumed by the antilinear symmetry between the positive and negative parts of the energy spectrum. The effects of the orbital symmetry of the defect's underlying order on the zero-modes are also considered. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D35.00012: Massive Dirac Fermion on the Surface of a Magnetically Doped Topological Insulator Yulin Chen, Jiun-Haw Chu, James Analytis, Zhongkai Liu, Kyushiro Igarashi, Hsueh-Hui Kuo, Xiaoliang Qi, Sung-Kwan Mo, Robert Moore, Donghui Lu, Makoto Hashimoto, Takao Sasagawa, Shoucheng Zhang, Ian Fisher, Zahid Hussain, Zhi-Xun Shen Insulating massive Dirac fermion state is a novel state of topological insulators in which the massless surface Dirac fermion becomes massive due to the braking of time reversal symmetry. In this state a gap develops at the Dirac point, with the Fermi energy resides inside both the surface and bulk gaps. By introducing magnetic dopants into three dimensional topological insulator Bi$_{2}$Se$_{3}$ to break the time reversal symmetry, we successfully observed the formation of massive Dirac fermion on the surface, with the Dirac gap magnitude tunable by magnetic dopant concentration. Furthermore, by precise control of simultaneous magnetic and charge doping, we successfully position the Fermi level inside the Dirac gap, thus realizing the much sought after insulating massive Dirac fermion state. This discovery paves the way for realizing striking topological phenomena and testing profound theoretical predictions. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D35.00013: Exotic Effects of Spin-Flip Scattering on Massive Dirac Fermions Shengyuan Yang, Zhenhua Qiao, Yugui Yao, Junren Shi, Qian Niu We investigate the effects of spin-flip scattering on the Hall transport and spectral properties of massive Dirac fermions. We find that in the weak scattering regime, the Berry curvature distribution is dramatically compressed in the electronic energy spectrum, becoming singular at band edges. As a result the Hall conductivity suffers a sudden jump (or drop) of $e^2/2h$ when the Fermi energy sweeps across the band edges, and otherwise is a constant quantized in units of $e^2/2h$. In parallel, spectral properties such as the density of states and spin polarization are also greatly enhanced at band edges. Possible experimental methods to detect these effects are discussed. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D35.00014: Weak indices and dislocations in general topological band structures Ying Ran It has recently been shown that crystalline defects - dislocation lines - in three dimensional topological insulators, can host protected one dimensional modes propagating along their length. We generalize this observation to the case of topological superconductors and other insulators of the Altland Zirnbauer classification, in d=2,3 dimensions. In general, protected dislocation modes are controlled by the topological indices in (d-1) dimensions. This is shown by relating this feature to characteristic properties of surface states of these topological phases. This observation also allows us to constrain these surface states properties, which is illustrated by an addition formula for (d-1) and (d-2) indices of a topological superconductor. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D35.00015: Topological insulator in a non-Abelian lattice model and anyonic fermions in two-body color code model Mehdi Kargarian, Gregory A. Fiete We investigated topological phases in several decorated lattices such as the square- octagon and spin ruby lattices. The underlying models can be potentially simulated in optical lattices or in multi-orbital transition metal oxides. In the square-octagon lattice we apply a set of non-Abelian gauge fields to modulate the hopping between sites. Inversion symmetric fields open a gap and the model realizes topological band insulating phase. If the inversion symmetry is broken, a quantum phase transition between phases with different quantum orders takes place. These phases are characterized by number of Dirac nodes and the associated winding numbers. We also probe the topological phases in the spin ruby lattice with emerging anyonic fermions coupled to nontrivial gauge fields associated with the local symmetry of the model. And we further characterize our results by topological entanglement entropy and entanglement spectrum. M. Kargarian and G. A. Fiete, Phys. Rev. B 82, 085106 (2010). [Preview Abstract] |
Session D36: Photovoltaics: Nanostructured Materials
Sponsoring Units: GERAChair: Steven Konezny, Yale University
Room: C142
Monday, March 21, 2011 2:30PM - 2:42PM |
D36.00001: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 2:42PM - 2:54PM |
D36.00002: Study of photo induced optical transparency of I/I3 redox couple in Dye Sensitized Solar Cells Josef Velten, Julia Bykova, Javier Carretero-Gonzalez, Elizabeth Castillo Martinez, Anvar Zakhidov Dye sensitized solar cells (DSCs) are an alternative to the standard silicon solar cell, consisting of a photoelectrochemical cell that has a light absorbing working electrode, a I-/I3- charge mediator and a counter electrode for reduction of I3- back to I-. Tradtionally, this counter electrode is composed of a few nanometer layer of platinum deposited onto transparent conductive oxide glass. Our work has focused on using structured carbon materials as a counter electrode. Earlier work focused on the use of carbon nanotubes, both single and multiwalled as a replacement for this platinum counter electrode material. Recent work has moved into using 2 dimensional carbon materials, such as graphene flakes and graphene ribbons. With the use of graphene ribbons we have discovered a unique effect that has not been reported for DSCs: under operating condition of AM 1.5 light, the charge mediating electrolyte undergoes a change in its absorption profile. We conclude that this phenomenon must arise from the structure of the graphene ribbons. This process has been demonstrated to be completely reversible, and shows no degradation to the DSC's operation. This presentation will also discuss the application of this phenomenon in the use of inverted DSCs and hybrid tandem cells. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D36.00003: Dye Sensitized Solar Cells with bilayer Multi/Single Carbon Nanotubes Zharkynay Kuanyshbekova, Anvar Zhakhidov In this presentation, we demonstrate the fabrication of dye sensitized solar cells using bilayers of MWCNT coated by SWCNT on top. Each DSC cell uses a typical titania photoelectrode deposited onto a transparent window electrode with photosensitive dye absorbed on the surface of the TiO$_{2. }$Adding SWCNT layer to MWCNT increased short circuit current from 10 to 14 mA/cm$^{2}$. Conventional DSC uses couterelectrode with Pt coated FTO, the Pt layers play role of catalyst for better charge transfer rate from electrolyte. In our bilayer electrode MWCNT sheet play a role a good conductor (similar to FTO), while SWCNT provide better catalytic properties for charge transfer (similar to Pt). The relatively high obtained efficiency of DSS ($\sim $7-8{\%}) cell is determined by the high generated photocurrent, which is comparable to the reference DSS made by same method using the standard PT catalyst. However, the fill-factor of the device is still low ($\sim $0.4-0.5). Therefore further improvement of electrical conductivity of these carbon based electrodes is under investrigation with CNT sheets to achieve high performance device. We acknowledge the help with transparent SWCNTs made in A.Nasibullin of Aolto University (Finland). [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D36.00004: Equivalent Circuit Description of Non-compensated n-p Codoped TiO2 as Intermediate Band Solar Cells(IBSCs)1 Tian-Li Feng, Yi Xia, Guang-Wei Deng, Feng-Cheng Wu, Ping Cui, Haiping Lan, Zhenyu Zhang The novel concept of non-compensated n-p codoping has made it possible to create tunable intermediate bands in the intrinsic band gap of TiO2 [1] as a promising materials for developing IBSCs [2]. Here we investigate the quantum efficiency of such IBSCs with or without current extracted from the intermediate bands (IBs). Using the ideal equivalent circuit model, we find that the maximum efficiency of 57\% in the first case and 53\% in the second are both much higher than the Shockley-Queisser limit. We also obtain various key quantities of the circuits, allowing us to simplify the IBSCs into an ordinary cell with an intrinsic resistance, a useful step in realistic development of TiO2 based solar cells invoking device integration. These equivalent circuit results are also compared with the efficiencies obtained directly from consideration of electron transition between the energy bands, and both approaches reveal the intriguing existence of double peaks in the maximum efficiency as a function of the location of IBs.\\[4pt] [1] Zhu W. G., et al., Phys. Rev. Lett. 103, 226401 (2009).\\[0pt] [2] A. Luque, et al., J. Appl. Phys. 96, 1 (2004). [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D36.00005: Fluctuation-Induced Tunneling Conductivity in TiO$_{2}$ Nanoparticle Thin Films Steven J. Konezny, Christiaan Richter, Robert C. Snoeberger III, Alexander R. Parent, Gary W. Brudvig, Charles A. Schmuttenmaer, Victor S. Batista We integrate temperature-dependent dark DC conductivity measurements and theoretical modeling to elucidate the mechanism of electron transport in nanoporous TiO$_{2}$, a common photoanode material for dye-sensitized solar cells (DSSCs) and solar photocatalysis. We show that fluctuation-induced tunneling conduction through contact junctions between sintered TiO$_{2}$ nanoparticles can account for the temperature dependence over the entire temperature range studied. We find quantitative agreement between experimental and calculated conductivities, which span over four orders of magnitude and change with decreasing temperature from thermally activated to temperature-independent. The reported results suggest that efforts to optimize charge transport in nanoporous TiO$_{2}$ thin films as a means of improving the overall efficiency of DSSCs and solar photocatalysis should focus on fabrication conditions that optimize the properties of the contact junctions between sintered TiO$_{2}$ nanoparticles. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D36.00006: ZnO Nanoparticles and Nanowire Arrays with Liquid Crystals for Photovoltaic Apprications Lourdes Salamanca-Riba, Nicholas Weadock, Luz Martinez-Miranda Liquid crystals are small monodisperse molecules with high mobilities and are easy and cheap to process. In addition, some of their phases exhibit molecular orientation that can provide a path for the electrons, or holes, to move from one electrode to the other. We have mixed a smectic A liquid crystal (8CB) with varying concentrations of ZnO nanoparticles of $\sim $5 nm in diameter and have observed a photovoltaic effect as a function of the concentration of ZnO. The liquid crystal is believed to enhance the alignment of the nanoparticles and aid in the diffusion of electrons through the particles to the collection electrode. We have also made PV cells of ZnO nanowire arrays grown on Au layers on Si substrates. The nanowire arrays are covered with 8CB liquid crystal for hole conduction. We compare the light absorption of the PV cells as a function of wavelength of the light for the ZnO nanoparticle and the ZnO nanowire cells. We present a detailed study of the structure of the two systems. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D36.00007: Intermediate Band Gap Solar Cells: The Effect of Resonant Tunneling on Delocalization Reid William, Doty Mathew, Shilpa Sanwli, Dan Gammon, Allan Bracker Quantum dots (QD's) have many unique properties, including tunable discrete energy levels, that make them suitable for a variety of next generation photovoltaic applications. One application is an intermediate band solar cell (IBSC); in which QD's are incorporated into the bulk material. The QD's are tuned to absorb low energy photons that would otherwise be wasted because their energy is less than the solar cell's bulk band gap. Current theory concludes that identical QD's should be arranged in a superlattice to form a completely delocalized intermediate band maximizing absorption of low energy photons while minimizing the decrease in the efficiency of the bulk material. We use a T-matrix model to assess the feasibility of forming a delocalized band given that real QD ensembles have an inhomogeneous distribution of energy levels. Our results suggest that formation of a band delocalized through a large QD superlattice is challenging; suggesting that the assumptions underlying present IBSC theory require reexamination. We use time-resolved photoluminescence of coupled QD's to probe the effect of delocalized states on the dynamics of absorption, energy transport, and nonradiative relaxation. These results will allow us to reexamine the theoretical assumptions and determine the degree of delocalization necessary to create an efficient quantum dot-based IBSC. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D36.00008: Development of High Efficient Flexible Dye-Sensitized Solar Cells Xiaojuan Fan We are developing a low cost and easy process to fabricate double-layer porous metal oxide thin films on flexible substrates for high performance dye-sensitized solar cells (DSSCs\textbf{). }The research addresses on the formulation of TiO$_{2}$ precursor to create smooth and continuous porous thin films on large size plastic or metal foil substrates enabling excellent adhesion, robust mechanics, and chemical stability. A second layer built on the underline porous nanocrystalline TiO$_{2}$ thin films are primarily used as bedding to receive more organic sensitizers. A variety of blending of polymer and Ti alkoxide precursors at different concentrations has been studied. After depositing the mixture on the substrates such as Al foils, samples are annealed to remove polymer residues leading to a porous nanocrystalline structure. Photo-electricity conversion efficiency of the fabricated solar cells will be tested under one sun illumination. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D36.00009: A novel nano-structured GaAs solar cell Dong Liang, Anjia Gu, Yijie Huo, Jingzhou Yan, Shuang Li, Erik Garnett, Evan Pickett, Yangsen Kang, Meiyueh Tan, Antonio Xavier Cerruto, Jia Zhu, Ching-Mei Hsu, Yan Yao, Majid Riaziat, Yi Cui, James S. Harris In this presentation, we will demonstrate a novel solar cell with nano-structured dense arrays of single crystal GaAs conformally grown on nanopillar templates with wafer-scale uniformity. The template is prepared via plasma enhanced etching with a monolayer of SiO$_{2}$ nanospheres as a mask followed by wet chemical etching. The GaAs p-n junction with an AlGaAs passivation window layer is grown via metal-organic chemical vapor deposition (MOCVD). The rectangular shape of the nano single crystal GaAs reveals anisotropic lateral growth rates of GaAs along (011) and (01\underline {1}) directions, which can be engineered by tuning the AsH$_{3}$ flow and temperature during growth. Optical absorption measurements show the outstanding light trapping properties of the nano-structured cell, which agree with the simulation results. I-V characteristics show an efficiency of 1.67{\%} for the nano GaAs solar cell, which is 15{\%} higher than its planar control cell with the same thickness of 200nm. The efficiency is the highest among all the large area GaAs nanowire core-shell solar cells reported in literature by 2010. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D36.00010: Highly Efficient Dye Sensitized Solar Cells based on Free-Standing Titania Nanotube Chaehyun Kim, Sungjin Kim, Alexander Cartwright, Hao Zeng Dye sensitized solar cells (DSSC) attract great attention due to their respectable efficiency with very low fabrication cost, good performance under diffuse light conditions and ability to be fabricated on flexible substrates. Its main efficiency limiting factor is the random hopping of electrons within the titania nanoparticle network, which causes carrier trapping and recombination. The charge transport and collection can be enhanced by employing ordered nanostructures such as nanowire or nanotube arrays. However, nanowire/nanotube based DSSCs with efficiencies higher than those of conventional DSSCs have yet to be demonstrated. In this work, we report the fabrication of DSSCs using highly crystalline free-standing titania nanotube arrays. The high crystallinity leads to high electron mobility and diffusion length, allowing thick nanotube films to be used for improving the long wavelength light absorption. This greatly enhances the photocurrent and power conversion efficiency as compared to that of nanotube DSSCs in earlier studies. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D36.00011: Nanowire electrodeposition for advanced photovoltaics Erik Menke, Justin Hujdic, Somnath Ghosh According to the Department of Energy's ``Basic Research Needs for Solar Energy Utilization'' report, there are a number of fundamental scientific issues that need to be addressed for nanostructure based solar cells, including: a. Control of nanoarchitecture b. Light harvesting c. Control of charge separation and recombination d. Control of charge carrier transport to the contacts Here, I will describe how lithographically patterned nanowire electrodeposition (LPNE) can address these issues by discussing the synthesis of high-density semiconductor nanowire arrays, as well as their optical and electronic properties. This talk consists of three parts. Part 1 presents a brief overview of how LPNE, essentially the combination of photolithography and electrodeposition, can be used as a general method to prepare high-density nanowire arrays. Part 2 demonstrates this method specifically for CIS/CdS core-shell nanowire arrays by discussing the electrodeposition of the nanowire arrays as well as the physical and chemical properties of the resulting nanowires. Finally, part 3 presents the optoelectronic properties of the resulting nanowire arrays and their potential application as solar cells. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D36.00012: Shedding Light on Solar Cells with Synchrotron Radiation Franz Himpsel, Peter Cook, Phillip Johnson, Xiaosong Liu, Wanli Yang, Angel Rubio, Juan-Maria Garc\'Ia-Lastra, Enrique Ortega, Celia Rogero, Ruben Gonzalez-Moreno, Eneko Azaceta, Ramon Tena-Zaera, Elena Guillen, Juan Anta X-ray absorption and photoelectron spectroscopy with synchrotron radiation are used to systematically determine the energy levels of molecules for dye-sensitized solar cells (including porphyrins and phthalocyanines [1-3]). N 1s absorption spectra combined with theoretical modeling provide the unoccupied molecular orbitals and the charge transfer between the central metal atom and the surrounding N atoms. Metal 2p-to-3d spectra provide the oxidation state of the metal. Fe and Mn, which occur frequently in biological analogs, easily change their oxidation between +3 and +2. Some dyes interact with the electronic states of nano-structured ZnO acceptor electrodes, causing a change in the electronic states of the ZnO or the dye.\\[4pt] [1] P. L. Cook, et al. J. Chem. Phys. 131, 194701 (2009). \newline [2] P. L. Cook, et al. J. Chem. Phys. 131, 214702 (2009). \newline [3] J.M. Garc\'{i}a-Lastra, et al., J. Chem Phys. 133, 151103 (2010). [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D36.00013: Dye Sensitized Solar Cells Using Freestanding TiO2 Nanotube Arrays Xukai Xin, Jun Wang, Lei Zhao, Zhiqun Lin A TiO2 photoanode was prepared by depositing TiO$_{2}$ nanoparticle on the FTO glass followed by placing TiO$_{2}$ nanotube arrays on the top of TiO$_{2}$ nanoparticle film. The resulting TiO$_{2}$ nanotube/nanoparticle photoanode was sensitized with N719 dye after TiCl$_{4}$ treatment and exposure to O$_{2}$ plasma. The resulting dye sensitized solar cell (DSSC) showed that the highest DSSC power conversion efficiency of 8.02{\%} and 7.00{\%} were yielded when a 20 $\mu $m thick TiO$_{2}$ nanoparticle and a 13/7 $\mu $m TiO$_{2}$ nanoparticle/nanotube were used as photoanode, respectively. The I$\sim $V curve analysis suggested that the nanotubes had better electron transport pathway but lower electron generation. Future work will be focused on increasing the dye loading of nanotubes to improve the power conversion efficiency. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D36.00014: Dye-sensitized solar cells employing TiO2 nanotube arrays modified by hydrothermal process Meidan Ye, Changjian Lin, Zhiqun Lin Dye sensitized solar cells (DSSCs) based on TiO2 nanotube photoanode prepared by a facile combination of electrochemical anodization and hydrothermal process exhibited a remarkable performance. Well-ordered and smooth TiO2 nanotube arrays fabricated by a two-step anodic oxidation were subjected to hydrothermal process, thereby creating roughness on the surface of nanotubes and leading to increased dye loading. Subsequently, the resulting nanotubes were used to fabricate DSSC in backside illumination mode, yielding a significantly high power conversion efficiency of 7.12{\%} that was further increased to 7.75{\%} upon oxygen plasma treatment. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D36.00015: Nanostructured Thin Film Solar Cells: A Heterojunction of PbS Colloidal Quantum Dots and TiO2 Nanopillars Ho-Cheol Kim, Illan Kramer, John Bass, Teya Topuria, Leslie Krupp, Philip Rice, Ratan Debnath, Lukasz Brzozowski, Larissa Levina, Edward Sargent Colloidal quantum dot (CQD) has been recognized as a promising solar cell material that offers tunable band gap and inexpensive solution process. Recent report demonstrated the power conversion efficiency (PCE) of above 5{\%} (AM 1.5) using thin films of PbS CQDs and TiO$_{2}$ nanoparticles. This so-called depleted-heterojunction-CQD solar cells have overcome limitations of CQD Schottky devices and promised potential for further improvement of solar cell performance. In this paper, we report the effect of nanostructures of TiO$_{2}$ on the performance of heterojunction CQD solar cells. Well-defined nanopillars of TiO$_{2}$ were prepared on top of F:SnO$_{2}$ substrate using micro-transfer molding technique. TiO$_{2}$ nanopillars of 70 nm in diameter (half-width), 340 nm in height and 275 nm in center-to-center distance were used for subsequent layer-by-layer spin coating of PbS CQD. PCE of $>$5{\%} was measured for the nanopillar solar cells without extensive optimization. Detailed studies on the microstructure of materials, surface properties, optical and electrical properties and optimization will be discussed along with performance of flat TiO$_{2}$-PbS CQD solar cells. [Preview Abstract] |
Session D37: Focus Session: Graphene Structure, Dopants, and Defects: Strain Engineering I
Sponsoring Units: DMPChair: Jeanie Lau, University of California, Riverside
Room: C146
Monday, March 21, 2011 2:30PM - 3:06PM |
D37.00001: Strain Engineering in Graphene Invited Speaker: Graphene is a unique example of a one atom thick metallic membrane. Hence, graphene brings together properties of soft and hard condensed matter systems. The elementary electronic excitations in graphene, the Dirac quasiparticles, couple in a singular way to structural distortions in the form of scalar and vector potentials. Therefore, graphene has an effective electrodynamics where structural deformations couple to the Dirac particles at equal footing to electric and magnetic fields. This so-called strain engineering of the electronic properties of graphene opens doors for a new paradigm in terms of electronic devices, where electronic properties can be manipulated at will using its membrane-like properties. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D37.00002: Optical absorption and giant Faraday rotation in strained graphene Vitor Pereira, Nuno M.R. Peres, Ricardo M. Ribeiro, Antonio Castro Neto Slightly doped or undoped graphene is characterized by an universal optical absorption coefficient of $\pi\alpha$ (nearly 2\%), which is constant in a frequency band spanning the near UV, down to the far IR. Strain-induced anisotropy breaks this universality, while keeping the optical response constant up to energies close to the van-Hove singularity of the spectrum. This allows for the possibility of exploring the photoelasticity of graphene towards the development of atomically thin, broadband optical elements. We show, and analytically quantify, the amount of polarization rotation and dichroism expected for uniaxially strained graphene. The effect can be used to tailor the optical response of graphene or, conversely, to use light to measure the amount and direction of uniaxial strain in graphene for sensing applications. Exposure to an external magnetic field brings about the Faraday effect, which is shown to be extremely large in comparison with conventional materials. Moreover, the sharp enhancement of Faraday rotation and absorption at the field and stain tunable cyclotron frequency opens the possibility of tunable broadband optics in atomically thin transparent membranes. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D37.00003: Electron-electron interactions in strained graphene Anand Sharma, Valeri Kotov We present a theoretical study on the effects of electron-electron interactions under an applied weak uni-axial strain in graphene, described as anisotropic Dirac liquid. We calculate the electron self-energy using perturbation theory in both the Coulomb interactions and strain, and find that near the Dirac point the self-energy exhibits logarithmic singularity structure, similarly to an un-deformed graphene. We present results for the renormalization of the electronic anisotropy by using first the bare Coulomb interaction, as well as the random-phase approximation, which takes into account the anisotropic nature of the vacuum polarization. The mutual interplay of interactions and strain can provide a route towards understanding the role of correlations in graphene, which so far have been quite elusive in the un-deformed case. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D37.00004: Scattering from localized strain profiles in graphene: effects on conductance Matthew Barr, Eric Heller Graphene has attracted significant attention for, amongst other properties, its Dirac-like quasiparticles and long coherence length. In the ballistic regime, we theoretically investigate the scattering properties of localized strain profiles. Manipulating strain in graphene has been proposed as a novel method of shaping graphene devices; modulated hopping parameters effectively introduce vector potentials equivalent to pseudomagnetic fields up to 300T [1]. We determine the localized potential and scattering parameters of several such ``bubbles''; with this information we calculate the effects on conductance in both valleys of introducing one or many such impurities.\\[4pt] [1] N. Levy, S. A. Burke, K. L. Meaker, M. Panlasigui, A. Zettl, F. Guinea, A. H. Castro Neto and M. F. Crommie, Science 30, July 2010 [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D37.00005: Ab-initio study of the Kohn anomalies in strained graphene M.E. Cifuentes-Quintal, R. de Coss, O. de la Pe\~na-Seaman, R. Heid, K.-P. Bohnen Recent experimental studies have show that the electronic and vibrational properties of graphene can be modulated by means of strain. However, there are not studies on strain effects on the Kohn anomalies, which is a principal key to understand the electron phonon coupling in graphene. In this work we have studied the phonon band structure of graphene under biaxial and uniaxial strain using the mixed basis pseudopotential method, within the framework of the density functional perturbation theory. For tensile/compressive biaxial strain, we found an increasing/decreasing behavior on the slop of the phonon frequencies close to Kohn anomalies. Under uniaxial strain, the two highest optical branches show a discontinuity in the frequency derivative at gamma point, instead of only one branch like in the biaxial and unstrained case. The present results suggest that the electron-phonon coupling in graphene can be modulated via strain. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D37.00006: Graphene rubber band: suspended graphene sheets with controlled uniaxial strain Zenghui Wang, David Hutchison, Carlos Ruiz-Vargas, Pinshane Huang, Sunil Bhave, David Muller, Jiwoong Park The recent advances in growth and transfer techniques of CVD graphene have made it an excellent candidate for making electrical and mechanical devices, especially at larger scale than with exfoliated graphene flakes. Nevertheless, the electrical, electromechanical and optomechanical properties of CVD graphene need to be further characterized before one can make full use of this 2D material. Here, we study the properties of CVD graphene under well controlled uniaxial strain. We fabricate devices with adjustable-width gaps actuated by comb drives, and transfer CVD graphene sheets onto these gaps. We study the slipping, straining, and breaking of CVD graphene in real time under TEM, with identification of individual single crystal domains and domain boundaries. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D37.00007: Suspension of Graphene and Bi2Se3 Atomic Membrane Zeng Zhao, Jairo Velasco, Hang Zhang, Fenglin Wang, Zhiyong Wang, Philip Kratz, Lei Jing, Wenzhong Bao, Jing Shi, Jeanie Lau Coupling high quality, suspended atomic membranes to specialized electrodes enables investigation of many novel phenomena, such as spin or Cooper pair transport in these two dimensional systems. However, many electrode materials are not stable in acids that are used to dissolve underlying substrates. Here we present a versatile and powerful multi-level lithographical technique to suspend atomic membranes, which can be applied to the vast majority of substrate, membrane and electrode materials. We also demonstrate, for the first time, fabrication and measurement of a free-standing thin Bi$_{2}$Se$_{3}$ membrane, which has low contact resistance to electrodes and a mobility of $\ga$500 cm$^{2}$/Vs. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D37.00008: In-situ Investigation of Electrical-Mechanical Coupling in graphene-based devices Mingyuan Huang, Tod Pascal, Hyungjun Kim, William Goddard III, Julia Greer Graphene, a truly two-dimensional gapless semiconducting material, recently deemed strongest ever measured, can sustain very high (up to 25{\%}) in-plane tensile elastic strains. Several recent theoretical-only studies on strained graphene predict that strain can shift the Dirac cones, reduce the Fermi velocity, introduce a pseudo-magnetic field, and be used to engineer the electronic structure. However, no direct experiments on electrical measurements of highly strained graphene have yet been reported. Here, we present the results of \textit{in-situ} investigation of electrical-mechanical coupling in graphene-based devices. In our experiment, \textit{in-situ} nanoindentation was performed on suspended graphene transistors to introduce homogeneous tensile strain up to 3{\%}, while electrical measurements were carried out simultaneously. We find that the electrical resistance shows only a marginal change under strain, and the electronic transport measurement confirms that there is no opening of the band gap for graphene under moderate uniform strain. We also report first-principles informed molecular dynamics simulation that lead to Young modulus consistent with our experiments and show no opening of a band gap. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D37.00009: Radio Frequency Electrical Transduction of Graphene Mechanical Resonators Changyao Chen, Vikram Deshpande, Yuehang Xu, Frank DiRenno, Alexander Gondarenko, David Heinz, Shuaimin Liu, Philip Kim, James Hone We report radio frequency (RF) electrical readout of graphene mechanical resonators. The mechanical motion is actuated and detected directly by using a vector network analyzer (VNA), employing a local gate to minimize parasitic capacitance. Resist-free doubly-clamped samples with resonant frequency in MHz range, Q factor $\sim $10,000 at 77 K and signal-to-background ratio of over 20 dB, are demonstrated. In addition to being over two orders of magnitude faster than the electrical RF mixing method, this technique paves the way for use of graphene in RF devices such as filters and oscillators. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D37.00010: Measurement of the shear modulus of single-layer graphene Thomas Metcalf, Xiao Liu, Jeremy Robinson, Keith Perkins, Brian Houston We have measured the shear modulus of large area (2mm $\times$ 5mm), single-layer (90--95\%) polycrystalline graphene sheets and found values consistent with theoretical predictions of $G$=200 GPa. The graphene was grown by chemical vapor deposition onto a copper foil and subsequently transferred onto a mechanical resonator known as a double-paddle oscillator (DPO). DPOs are fabricated from single-crystal, 0.3mm thick silicon wafers, and have a torsional vibratory mode at 5500 Hz which has a very large quality factor, $Q=5\times10^7$, at low ($<10$ K) temperatures, giving the DPO a high sensitivity to a film deposited on its torsional element. Such a film increases the (lumped-element) spring constant of the resonator, and the film's shear modulus can be deduced from the subsequent resonant frequency shift. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D37.00011: Tearing of Graphene Maria Moura, Michael Marder Experiments on free standing graphene can expose the graphene sheets to out-of-plane forces. An example of that is the back-gate voltage experimental setup. Here we show that out-of-plane forces can cause free standing graphene to fracture. This fracture mode is known as tearing mode and is common in materials like paper. We present a numerical study of the propagation of cracks in clamped, free standing graphene as a function of the out-of-plane force. We report a threshold for the graphene fracture energy. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D37.00012: Variable strain in graphene sealed microchambers studied with Raman spectroscopy A.L. Kitt, J.W. Suk, S. Remi, S. Ahmed, R. Piner, K.M. Liechti, R.S. Ruoff, A.K. Swan, B.B. Goldberg Raman measurements are a sensitive tool for evaluating strain in graphene. Graphene sealed cylindrical microchambers provide a unique way of generating strain. Suspended graphene avoids substrate interactions which make it difficult to evaluate the graphene response, e.g., combined graphene-substrate Poisson ratio, or slippage. Additionally, the system provides a wide range of strain states with different lattice symmetries. At a fixed external pressure, the strain state varies radially. The strain is biaxial in the center and changes gradually to only radial strain at the edges. The continuum model is evaluated to find~the radial strain states. Combined with Raman Spectroscopy, several fundamental parameters can be measured. We will discuss the strain and polarization dependent splitting of the G and 2D bands and compare to previous works [1,2]. Furthermore, preliminary measurements of the strain dependence of thermal properties will be discussed. \\[4pt] [1] T. Mohiuddin et al, Phys Rev B 79, 1-8 (2009) \\[0pt] [2] M. Huang, et al, Nano Letters 10, 4074-9 (2010) [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D37.00013: An Effective Tensional Strain View on the Bandgap Tunability of Helical Graphene Nanoribbons with Open and Closed Edges Dong-Bo Zhang, Traian Dumitrica Despite the scientific importance of graphene nanoribbons, little is known about their electronic structure other than in the flat-form presupposition. To quantify the strain stored in helical graphene nanoribbons and fractional carbon nanotubes, we supplement the standard elasticity concepts with an effective tensional strain. Using $\pi$-orbital tight binding and objective molecular dynamics coupled with density functional theory, we develop a unified theory for the electromechanical response in which the consequences of the torsional deformation are taken into account via the effective tensional strain. In spite of the open and closed edges as well as the inverse Poynting effect exhibited by these nanostructures, from the effective strain perspective the twist-induced bandgap modulations appear strikingly similar with those exhibited by carbon nanotubes in tension. Our theory may be useful for designing new electromechanical devices and experiments using carbon nanocomponents, and for establishing edge-chemistry driven nanofabrication principles for helical graphene nanoribbons with tunable bandgaps. [Preview Abstract] |
Session D38: Earle K. Plyler Prize Session II: Spectroscopy
Sponsoring Units: DCPChair: Sunney Xie, Harvard University
Room: A130/131
Monday, March 21, 2011 2:30PM - 3:06PM |
D38.00001: Ultrafast Nonlinear Optical Spectroscopy or where would we be without Shaul Mukamel? Invited Speaker: The development of ultrafast nonlinear optical spectroscopy owes much to the pioneering work of Shaul Mukamel in developing a unifying framework and language with which to understand and relate the content of different types of experiment. The culmination of this work, to date, is in the development of multidimensional optical spectroscopies. In this talk, I will describe recent work in my group on two dimensional electronic spectroscopy of photosynthetic light-harvesting complexes and, if time permits, single walled carbon nanotubes and molecular systems relaxing via conical intersections. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D38.00002: 2D IR Spectroscopy of Protein Conformation, Folding, and Binding Kevin Jones, Andrei Tokmakoff, Ziad Ganim, Joshua Lessing, C. Sam Peng 2D IR spectroscopy is an increasingly powerful tool for investigation of protein structure and dynamics. As an ultrafast spectroscopy, it provides information on protein structure and conformational variation with high time resolution, providing a tool to study the dynamics of folding and binding. Some of the unique characteristics of 2D IR result from the powerful structure based modeling that is available for amide vibrations. This talk will cover recent examples from our group in which different forms of protein 2D IR and computational spectroscopy are used to reveal conformational heterogeneity in peptides, the folding and binding of proteins, and protein-water interactions. When combined with temperature-jump experiments, the formation and interchange of these structures is probed. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D38.00003: Three-dimensional Fourier-transform spectroscopy of potassium vapor Hebin Li, Alan Bristow, Mark Siemens, Galan Moody, Steven Cundiff We have implemented three-dimensional (3D) Fourier-transform spectroscopy to study potassium vapor contained in a $\sim$20 $\mu$m transmission cell with argon buffer gas. The four-wave mixing signal is measured in three time dimensions corresponding to the delays between three $\sim$100 fs, phase-stabilized excitation pulses that are arranged in the box geometry. The emission is detected using a phase-stabilized reference pulse by spectral interferometry, and other time axes are Fourier transformed to construct the 3D spectra. The 3D spectra contain the full information of third-order coherent response of the vapor, yet the contribution from each of the single-quantum excitation pathways is unambiguously isolated. Projecting a 3D spectrum onto a specific two-dimensional (2D) plane retrieves rephasing, non-rephasing, and T-scan 2D spectra, as well as the spectra that are not accessible by conventional 2D scans. The spectral features which overlap in congested 2D spectra can be isolated for studying unique processes represented by a single pathway. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D38.00004: Reflection Geometry Electronic Two-dimensional Fourier Transform Spectroscopy Thomas W. Jarvis, Zheng Sun, Xiaoqin Li, Mikhail Erementchouk, Michael N. Leuenberger Studying dynamics in nanostructures is vital to develop new opto-electronic devices and to understand fundamental processes in the solid state. Electronic Two-dimensional Fourier Transform Spectroscopy (2DFTS) is a powerful technique that coherently probes the nonlinear optical polarization, establishing correlations between absorption and subsequent emission or dispersion. We perform 2DFTS in reflection, a novel experimental geometry that allows us to probe structured materials. The coupling features and dimensionally extended lineshapes revealed by 2DFTS provide a description of decoherence and dephasing processes, coherent and incoherent energy transfer, and relaxation. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 4:18PM |
D38.00005: Ultrafast two dimensional infrared chemical exchange spectroscopy Invited Speaker: The method of ultrafast two dimensional infrared (2D IR) vibrational echo spectroscopy is described. Three ultrashort IR pulses tuned to the frequencies of the vibrational transitions of interest are directed into the sample. The interaction of these pulses with the molecular vibrational oscillators produces a polarization that gives rise to a fourth pulse, the vibrational echo. The vibrational echo pulse is combined with another pulse, the local oscillator, for heterodyne detection of the signal. For fixed time between the second and third pulses, the waiting time, the first pulse is scanned. Two Fourier transforms of the data yield a 2D IR spectrum. The waiting time is increased, and another spectrum is obtained. The change in the 2D IR spectra with increased waiting time provides information on the time evolution of the structure of the molecular system under observation. In a 2D IR chemical exchange experiment, two species A and B, are undergoing chemical exchange. A's are turning into B's, and B's are turning into A's, but the overall concentrations of the species are not changing. The kinetics of the chemical exchange on the ground electronic state under thermal equilibrium conditions can be obtained 2D IR spectroscopy. A vibration that has a different frequency for the two species is monitored. At very short time, there will be two peaks on the diagonal of the 2D IR spectrum, one for A and one for B. As the waiting time is increased, chemical exchange causes off-diagonal peaks to grow in. The time dependence of the growth of these off-diagonal peaks gives the chemical exchange rate. The method is applied to organic solute-solvent complex formation, orientational isomerization about a carbon-carbon single bond, migration of a hydrogen bond from one position on a molecule to another, protein structural substate interconversion, and water hydrogen bond switching between ions and water molecules. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D38.00006: Can the Isomerization of Retinal in Bacteriorhodopsin be Coherently Controlled in Strong Fields? Valentyn Prokhorenko, Alexei Halpin, Philip Johnson, Leonid Brown, Dwayne Miller Conflicting results have been obtained between weak field experiments (one-photon absorption) [1] and strong field recent studies [2] (multi-photon effects). Here we present our strong field experiments performed using linearly-chirped excitation pulses. Contrary to [2], we clearly observe phase-dependent control of photoproduct yield over a wide range of excitation energies. Above the excitation limit of $\sim $200 GW/cm$^{2}$ our results do however come into agreement with [2], but only for a single observation wavelength (650 nm) whereas the transient spectra unambiguously show drastic changes in the protein due to its ionization. At these excitation levels, this deleterious side channel precludes correct determination of the amount of 13-cis isomer. As such, we argue that it is impossible to make assignments of mechanistic details of control at a high field that in effect ``kills'' the protein. [1] V. I. Prokhorenko, A. M. Nagy, S. A. Waschuk, L. S. Brown, R. R. Birge, and R. J. D. Miller, Science 313, 1257- 1261 (2006). [2] A. C. Florean et al., PNAS 106, 10896-10900 (2009). [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D38.00007: Toward Investigating Protein Folding Using the Combination of Computer Simulation and Spectroscopy Wei Zhuang Protein folding is an important problem that is attracting scientists from a wide range of disciplines. One of the major challenges comes from the gap between the experimental and the theoretical studies. We proposed a computational protocol of simulating the T-jump peptide unfolding experiments and the related transient IR and 2DIR spectra based on the Markov State Model (MSM) and Nonlinear Exciton Propagation (NEP) methods. MSMs partition the conformation space into a set of non-overlapping metastable states, and we can calculate spectra signal for each of these states using NEP method. Thus the overall spectroscopic observable for a given system is simply the sum of spectra of different metastable states weighted by their populations. Simulated spectra based on MSM have a much better agreement with the equilibrium experimental 2DIR spectra compared to MD simulations starting from the folded state. MSMs are also capable of simulating the unfolding relaxation dynamics upon the temperature jump. The agreement of the simulation using MSMs and NEP with the experiment provides a justification for our protocol as well as a physical insight underlying the spectroscopic observables. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D38.00008: Supersymmetry and fluctuation relations for currents in closed networks Vladimir Chernyak, Nikolai Sinitsyn The discovery of fluctuation theorems and nonequilibrium work relations has stimulated considerable interest in nonequilibrium statistical mechanics and theory of counting statistics. It is important to obtain exact relations that do not directly rely on the thermodynamic concepts, such as work or entropy, but rather describe unambiguous microscopic characteristics, such as statistics of particle currents in systems driven by time-dependent fields. We identify hidden supersymmetry in evolution, governed by the master equation, that survives on the level of the counting statistics of stochastic particle currents. Supersymmetry connects the evolutions in the spaces of populations (boson component) and empirical currents (fermion component). We present exact relations for statistics of currents in strongly driven mesoscopic stochastic systems. Being reminiscent of known fluctuation theorems, a part of our exact result is not directly related to the condition of microscopic reversibility but rather follows from {\it supersymmetry} of the counting statistics of currents. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D38.00009: Understanding Metal-Adsorbate Binding with Surface-Enhanced Raman Spectroscopy: Theory and Experiment Alexey Zayak, Hyuck Choo, Ying Hu, Jeffrey Bokor, Stefano Cabrini, James Schuck, Jeffrey Neaton Building on recent work [1], we use a combination of density functional theory (DFT) calculations and surface-enhanced Raman spectroscopy (SERS) measurements to explain experimentally observed variations in SERS data of an organic molecule, trans-1,2-two (4-pyridyl) ethylene (BPE). For the BPE on Au surfaces, our DFT calculations provide a quantitative description of chemical enhancement (CE), and elucidate that variations reported in experiments arise from a convolution of two factors: a nonuniform frequency dependent electromagnetic enhancement, and dependence of CE on the sample incubation time. The later reveals aspects of the binding kinetics of BPE to Au surfaces.\\[4pt] [1] A. T. Zayak,et. al., arXiv:1011.1873v1 [Preview Abstract] |
Session D39: Physics of Physiological Systems
Sponsoring Units: DBPChair: Wolfgang Losert, University of Maryland
Room: A124/127
Monday, March 21, 2011 2:30PM - 2:42PM |
D39.00001: Axonal Transport and Morphology: How Myelination gets Nerves into Shape Peter Jung, Peng Zhao, Paula Monsma, Tony Brown The local caliber of mature axons is largely determined by neurofilament (NF) content. The axoskeleton, mainly consisting of NFs, however, is dynamic. NFs are assembled in the cell body and are transported by molecular motors on microtubule tracks along the axon at a slow rate of fractions of mm per day. We combine live cell fluorescent imaging techniques to access NF transport in myelinated and non-myelinated segments of axons with computational modeling of the active NF flow to show that a), myelination locally slows NF transport rates by regulating duty ratios and b), that the predicted increase in axon caliber agrees well with experiments. This study, for the first time, links NF kinetics directly to axonal morphology, providing a novel conceptual framework for the physical understanding of processes leading to the formation of axonal structures such as the ``Nodes of Ranvier'' as well as abnormal axonal swellings associated with neurodegenerative diseases like Amyotrophic lateral sclerosis (ALS). [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D39.00002: Experimental evaluation of biophysical neurite growth models Zachary Wissner-Gross, Mark Scott, David Ku, Priya Ramaswamy, Mehmet Yanik During nervous system development, neurons exhibit complex growth dynamics, as several neurites compete to become each neuron's axon. Numerous mathematical and biophysical models have been proposed to explain this competition, but these models remain experimentally unverified. Large-scale and repeatable measurements of neurite dynamics are difficult to perform, since neurons have varying numbers of neurites, which themselves have complex morphologies. To overcome these challenges using a minimal number of primary neurons, we generated repeatable neuronal morphologies by laser-patterning micron-wide stripes of adhesive proteins on an otherwise highly non-adherent substrate. Upon analyzing thousands of time-lapse measurements, we observed three key neuronal behaviors: total neurite growth accelerated until neurons polarized, immature neurites competed even at very short lengths, and neuronal polarity underwent an apparent phase transition as the neurites grew beyond a critical length. Proposed biophysical neurite growth models agreed only partially with our experimental observations, and simple yet specific modifications significantly improved these models. The protein patterning and high-content analyses presented here could also be employed for studying other structural or biomechanical cellular phenomena. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D39.00003: Deep Brain Stimulation using Magnetic Fields David Jiles, Paul Williams, Lawrence Crowther New applications for transcranial magnetic stimulation are developing rapidly for both diagnostic and therapeutic purposes. Therefore so is the demand for improved performance, particularly in terms of their ability to stimulate deeper regions of the brain and to do so selectively. The coil designs that are used presently are limited in their ability to stimulate the brain at depth and with high spatial focality. Consequently, any improvement in coil performance would have a significant impact in extending the usefulness of TMS in both clinical applications and academic research studies. New and improved coil designs have then been developed, modeled and tested as a result of this work. A large magnetizing coil, 300mm in diameter and compatible with a commercial TMS system has been constructed to determine its feasibility for use as a deep brain stimulator. The results of this work have suggested directions that could be pursued in order to further improve the coil designs. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D39.00004: Growth of Necrotic Cores in Vulnerable Plaque Pak-Wing Fok Plaques are fatty deposits that grow mainly in arteries and develop as a result of a chronic inflammatory response. Plaques are called \textit{vulnerable} when they are prone to mechanical rupture. Vulnerable Plaques (VPs) are characterized by lipid-rich, necrotic cores that are heavily infiltrated with macrophages. The rupture of VPs releases thrombogenic agents into the bloodstream, usually resulting in myocardial infarctions. We propose a quantitative model to predict the development of a plaque's necrotic core. By solving coupled reaction-diffusion equations for macrophages and dead cells, we explore the joint effects of hypoxic cell death and chemo-attraction to Ox-LDL, a molecule that is strongly linked to atherosclerosis. Our model predicts cores that have approximately the right size and shape. Normal mode analysis and subsequent calculation of the smallest eigenvalues allow us to compute the times required for the system to reach its steady state. This study allows us to make quantitative predictions for how quickly vulnerable plaques develop and how their growth depends on system parameters such as chemotactic coefficients and cell death rates. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D39.00005: Understanding cellular architecture in cancer cells Simone Bianco, Chao Tang Understanding the development of cancer is an important goal for today's science. The morphology of cellular organelles, such as the nucleus, the nucleoli and the mitochondria, which is referred to as cellular architecture or cytoarchitecture, is an important indicator of the state of the cell. In particular, there are striking difference between the cellular architecture of a healthy cell versus a cancer cell. In this work we present a dynamical model for the evolution of organelles morphology in cancer cells. Using a dynamical systems approach, we describe the evolution of a cell on its way to cancer as a trajectory in a multidimensional morphology state. The results provided by this work may increase our insight on the mechanism of tumorigenesis and help build new therapeutic strategies. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D39.00006: A two-scale model for correlation between B cell VDJ usage in zebrafish Keyao Pan, Michael Deem The zebrafish (Danio rerio) is one of the model animals for study of immunology. The dynamics of the adaptive immune system in zebrafish is similar to that in higher animals. In this work, we built a two-scale model to simulate the dynamics of B cells in primary and secondary immune reactions in zebrafish and to explain the reported correlation between VDJ usage of B cell repertoires in distinct zebrafish. The first scale of the model consists of a generalized NK model to simulate the B cell maturation process in the 10-day primary immune response. The second scale uses a delay ordinary differential equation system to model the immune responses in the 6-month lifespan of zebrafish. The generalized NK model shows that mature B cells specific to one antigen mostly possess a single VDJ recombination. The probability that mature B cells in two zebrafish have the same VDJ recombination increases with the B cell population size or the B cell selection intensity and decreases with the B cell hypermutation rate. The ODE model shows a distribution of correlation in the VDJ usage of the B cell repertoires in two six-month-old zebrafish that is highly similar to that from experiment. This work presents a simple theory to explain the experimentally observed correlation in VDJ usage of distinct zebrafish B cell repertoires after an immune response. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D39.00007: Modelling Nanoparticle Diffusion into Cancer Tumors Vishwa Priya Podduturi, Pedro Derosa Cancer is one of the major, potentially deadly diseases and has been for years. Non-specific delivery of the drug can damage healthy tissue seriously affecting in many cases the patient's living condition. Nanoparticles are being used for a targeted drug delivery thereby reducing the dose. In addition, metallic nanoparticles are being used in thermal treatment of cancer cells where nanoparticles help concentrate heat in the tumor and away from living tissue. We proposed a model that combines random walk with diffusion principles. The particle drift velocity is taken from the Hagen-Poiseuille equation and the velocity profile of the particle at the pores in the capillary wall is obtained using the Coventorware software. Pressure gradient and concentration gradient through the capillary wall are considered. Simulations are performed in Matlab using the Monte Carlo technique. Number of particles leaving the blood vessel through a pore is obtained as a function of blood pressure, the osmotic pressure, temperature, particle concentration, blood vessel radius, and pore size, and the relative effect of each of the parameters is discussed. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D39.00008: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 4:06PM - 4:18PM |
D39.00009: Hearing and Infinite-Period Bifurcations Seung Ji, Dolores Bozovic, Robijn Bruinsma Auditory and vestibular systems present us with biological sensors that can achieve sub-nanometer sensitivity orders of magnitude in the dynamic range, while operating in a fluid-immersed, room-temperature environment. While the mechanisms behind this extreme sensitivity and robustness of the inner ear have not been fully explained, nonlinear response has been shown to be crucial to its proper function. Recent experiments have recorded innate motility of hair cells of the bullfrog sacculus, under varying degrees of steady-state offset. The bundle deflection was shown to suppress or enhance spontaneous oscillations, and affect the sensitivity of the mechanical response. We will present a theoretical model based on cubic nonlinearity and show that in different parameter regimes, the system can be induced to cross a supercritical Hopf bifurcation, an infinite-period bifurcation, or a multi-critical point. Comparing the numerical simulation to the experiment, we will present evidence that the multi-critical point corresponds most closely to the dynamic state of saccular hair cells. Further, we will discuss the crossing of the bifurcation, and the sensitivity of the phase-locked response in various frequency regimes. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D39.00010: Axonal Transport and Morphogenesis near Retinal Excavation of the eye Yinyun Li, Anthony Brown, Peter Jung Neurofilaments(NFs) represent the main space-filling elements of mature axons. NFs are transported on microtubule (MT) tracks along the axon at a slow rate of $mm/day$ and thus form a dynamic cytoskeleton. During development, the optic nerve forms a sharp increase of caliber at about $150 \mu m$ from the retinal excavation of the eye. Our key hypothesis is a relation between NF kinetics and nerve morphology based on the continuity of the active flow of NFs. We use computational modeling of axonal transport to infer modulation of NF kinetics consistent with the observed increase of nerve caliber. We show that the inferred kinetics is also consistent with reported spatial distribution of NFs and MTs near the retinal excavation. We further show that the predicted time course of development of the observed nerve swelling is consistent with the time course of animal development. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D39.00011: Hydroxyapatite in Physiological Environment Alexander Slepko, Alexander A. Demkov A carbonated form of hydroxyapatite (HA) [Ca$_{10}$(PO$_{4})_{6}$(OH)$_{2}$] is one of the most abundant materials in mammal bone. It crystallizes within the spaces between tropocollagen protein chains in an aqueous solution and strengthens the bone tissue. An emerging application of synthetic HA is bone repair and replacement. Bulk electronic and chemical properties of HA were studied theoretically recently. However, the absorption of H$_{2}$O molecules and amino acids of the tropocollagen chains at HA surfaces remains an area of active research. Using density functional theory we analyze the electronic properties and surface energetics of HA for different orientations and terminations and generate a theoretical surface phase diagram of HA. The reactivity of these surface models is analyzed using the frontier orbital approach. We find two dominant surfaces which are most stable over the widest chemical range. However, we expect them to show little surface reactivity. Using a HA slab with a highly reactive surface we build atomistic models of HA covered with up to one monolayer of water and analyze interactions between this surface and the water molecules. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D39.00012: Excitable signal relay in Dictyostelium discoideum Troy Mestler, David Schwab, Pankaj Mehta, Thomas Gregor The social amoeba D. discoideum transitions when starved from a collection of individual cells into a multicellular spore-complex. During this process, amoebae display several interesting phenomena including intercellular signaling, pattern formation, and cell differentiation. At the heart of these phenomena is the exchange of the signaling molecule cyclic-AMP, which has previously been extensively studied using a variety of indirect methods. Here we employ a sensor that uses a compound fluorescent protein whose emission spectrum changes in the presence of bound cyclic AMP to directly monitor, in real time and in vivo, intracellular cAMP concentrations. We use cells expressing this sensor in microchemostats to study intracellular cAMP concentrations at the single-cell level in response to precise, dynamically-controlled external cAMP stimulation. Specifically, we show that these cells display excitability much like that found in neurons and agree experimentally quite well with a modified FitzHugh-Nagumo dynamical systems model. This single-cell model sets groundwork for a comprehensive multicellular model that promises to explain emergent behavior in D. discoideum. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D39.00013: Modeling Intracellular Oscillations and Polarity Transition in Fission Yeast Tyler Drake, Maitreyi Das, Fulvia Verde, Dimitrios Vavylonis Fission yeast, a pill-shaped model organism, restricts growth to its tips. These cells maintain an asymmetric growth state, growing at only one tip, until they meet length and cell-cycle requirements. With these met, they grow at both. The mechanism of this transition, new-end take-off (NETO), remains unclear. We find that NETO occurs due to long-range competition for fast-diffusing signaling protein Cdc42 between the old and new tips. From experimental results, we suppose that symmetric tips compete for Cdc42, which triggers growth. We describe a symmetric growth model based on competition between tips. This model restricts short cells to monopolar states while allowing longer cells to be bipolar. Autocatalytic Cdc42 recruiting at both cells tips leads to broken symmetry, and the recruiting cuts off as tip Cdc42 levels saturate. Non-linear differential equations describe the model, with stable attractors indicating valid distributions. Linear stability analysis and numerical methods identify stable fixed points over a twofold increase in cell length. The model reproduces qualitative behavior of the organism. We show that observed pole-to-pole Cdc42 oscillations may facilitate the polarity transition and discuss their relationship to the Min system in E. Coli. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D39.00014: Biomineral Structure and Strength of Barnacle Exoskeletons Nathan Swift Studying the construction of organic-inorganic compound structures through biomineralization is potentially very useful. During biomineral formation, organisms restructure naturally occurring minerals in conjunction with their own organically produced minerals to create new structures. While there is extensive knowledge about material properties and structure of the raw minerals themselves, insight into how specific biomineral structures and compounds contribute to an object's mechanical properties is lacking. In this study, the exoskeletons of barnacles from the genus \textit{Balanus} were examined, both for their physical structure (how they're put together) and for their mechanical properties (strength, hardness, and elasticity). Scanning electron microscopy produced close-up, detailed images of the inner shell structure to determine what type of structure barnacles build during exoskeleton formation. Energy dispersive x-ray spectroscopy was used to map the elemental components of the shells. Nanoindentation tested the mechanical properties of these mapped structures to determine how certain characteristics of the exoskeleton contribute to its mechanical properties. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D39.00015: Enhancing mechanical properties of calcite by Mg substitutions: An ab initio study Pavlina Elstnerova, Martin Friak, Tilmann Hickel, Helge Otto Fabritius, Liverios Lymperakis, Michal Petrov, Dierk Raabe, Joerg Neugebauer, Svetoslav Nikolov, Andreas Zigler, Sabine Hild Arthropoda representing a majority of all known animal species are protected by an exoskeleton formed by their cuticle. The cuticle represents a hierarchically structured multifunctional bio-composite based on chitin and proteins. Some groups like Crustacea reinforce the load-bearing parts of their cuticle with calcite. As the calcite sometimes contains Mg it was speculated that Mg may have a stiffening impact on the mechanical properties of the cuticle. We present a theoretical parameter-free quantum-mechanical study of thermodynamic, structural and elastic properties of Mg-substituted calcite. Our results show that substituting Ca by Mg causes an almost linear decrease in the crystal volume with Mg concentration and of substituted crystals. As a consequence the calcite crystals become stiffer giving rise e.g. to substantially increased bulk moduli. [Preview Abstract] |
Session D40: Lipid Bilayers and Biological Membranes: Peptide Interactions
Sponsoring Units: DBPChair: Emmanouil Doxastakis, University of Houston
Room: A122/123
Monday, March 21, 2011 2:30PM - 2:42PM |
D40.00001: Molecular Simulations of Sequence-Specific Association of Transmembrane Proteins in Lipid Bilayers Manolis Doxastakis, Anupam Prakash, Lorant Janosi Association of membrane proteins is central in material and information flow across the cellular membranes. Amino-acid sequence and the membrane environment are two critical factors controlling association, however, quantitative knowledge on such contributions is limited. In this work, we study the dimerization of helices in lipid bilayers using extensive parallel Monte Carlo simulations with recently developed algorithms. The dimerization of Glycophorin A is examined employing a coarse-grain model that retains a level of amino-acid specificity, in three different phospholipid bilayers. Association is driven by a balance of protein-protein and lipid-induced interactions with the latter playing a major role at short separations. Following a different approach, the effect of amino-acid sequence is studied using the four transmembrane domains of the epidermal growth factor receptor family in identical lipid environments. Detailed characterization of dimer formation and estimates of the free energy of association reveal that these helices present significant affinity to self-associate with certain dimers forming non-specific interfaces. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D40.00002: An amino acid composition criterion for membrane active antimicrobials Nathan Schmidt, Ghee Hwee Lai, Abhijit Mishra, Dennis Bong, Paul McCray, Jr., Michael Selsted, Andre Ouellette, Gerard Wong Membrane active antimicrobials (AMPs) are short amphipathic peptides with broad spectrum anti microbial activity. While it is believed that their hydrophobic and cationic moieties are responsible for membrane-based mechanisms of action, membrane disruption by AMPs is manifested in a diversity of outcomes, such as pore formation, blebbing, and budding. This complication, along with others, have made a detailed, molecular understanding of AMPs difficult. We use synchrotron small angle xray scattering to investigate the interaction of model bacterial and eukaryotic cell membranes with archetypes from beta-sheet AMPs (e.g. defensins) and alpha-helical AMPs (e.g. magainins). The relationship between membrane composition and peptide induced changes in membrane curvature and topology is examined. By comparing the membrane rearrangement and phase behavior induced by these different peptides we will discuss the importance of amino acid composition on AMP design. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D40.00003: Interaction between Cell Penetrating pVEC and cell membranes Abhijit Mishra, Ghee Hwee Lai, Nathan Schmidt, Gerard Wong Vascular Endothelial Cadherin (VEC) is a transmembrane-spanning glycoprotein that belongs to the family of cell adhesion molecules and plays an active role in control of vascular permeability and angiogenesis. PVEC, an 18 amino acid domain, has been shown to be able to traverse cell membranes with attached macromolecules. pVEC is an amphiphilic molecule with a high content of basic amino acids resulting in a net positive charge. Electrostatic and hydrophobic interactions can perturb membrane self-assembly and stability and are likely to be responsible for peptide uptake. We use synchrotron x-ray scattering and confocal microscopy to examine the phase behavior of the pVEC lipid system, and its relation to membrane permeation mechanisms. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D40.00004: Interactions between cyclic cell penetrating peptides and lipid membranes Kun Zhao, Tao Liu, Mike Choe, Daniel Kamei, Dehua Pei, Gerard Wong Cyclic peptides exhibit strong enhancement in receptor-binding affinity, specificity, and stability relative to their linear counterparts, partially due to their reduced conformational freedom. In this work, we examine cyclic versions of cell penetrating peptides. Using small-angle x-ray scattering (SAXS) measurements, we show that cyclic polyarginine peptides generate saddle-splay curvature more efficiently than their linear counterparts, We show how this increase in induced saddle splay curvature impinges on the efficiency of cell penetration in a series of giant vesicle and intracellular trafficking experiments. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D40.00005: Interaction of a P. aeruginosa Quorum Sensing Signal with Lipid Membranes Rebecca Morrison, Amelia Hall, Ellen Hutchison, Thuc Nguyen, Benjamin Cooley, Vernita Gordon Bacteria use a signaling and regulatory system called ``quorum sensing'' to alter their gene expressions in response to the concentration of neighboring bacteria and to environmental conditions that make collective activity favorable for bacteria. P. aeruginosa is an opportunistic human pathogen that uses quorum sensing to govern processes such as virulence and biofilm formation. This organism's two main quorum sensing circuits use two different signaling molecules that are amphiphilic and differ primarily in the length of their hydrocarbon side chain and thus in their hydrophobic physical chemistry. How these physical chemistries govern the propagation and spatial localization of signals and thus of quorum sensing is not known. We present preliminary results showing that signals preferentially sequester to amphiphilic lipid membranes, which can act as reservoirs for signal. This is promising for future characterization of how the quorum sensing signals of many bacteria and yeast partition to spatially-differentiated amphiphilic environments, in a host or biofilm. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D40.00006: Criticality in Plasma Membranes Benjamin Machta, Stefanos Papanikolaou, James Sethna, Sarah Veatch We are motivated by recent observations of micron-sized critical fluctuations in the 2d Ising Universality class in plasma membrane vesicles that are isolated from cortical cytoskeleton. We construct a minimal model of the plasma membrane's interaction with intact cytoskeleton which explains why large scale phase separation has not been observed in Vivo. In addition, we use analytical techniques from conformal field theory and numerical simulations to investigate the form of effective forces mediated by the membrane's proximity to criticality. We show that the range of this force is maximized near a critical point and we quantify its usefulness in mediating communication using techniques from information theory. Finally we use theoretical techniques from statistical physics in conjunction with Monte-Carlo simulations to understand how criticality can be used to increase the efficiency of membrane bound receptor mediated signaling. We expect that this sort of analysis will be broadly useful in understanding and quantifying the role of lipid ``rafts'' in a wide variety of membrane bound processes. Generally, we demonstrate that critical fluctuations provide a physical mechanism to organize and spatially segregate membrane components by providing channels for interaction over relatively large distances. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D40.00007: Fluctuation-induced forces between inclusions in a fluid membrane under tension Hsiang-Ku Lin, Roya Zandi, Leonid P. Pryadko We discuss the fluctuation-induced force, a finite-temperature analog of the Casimir force, between two inclusions embedded in a fluid membrane under tension. We suggest a method to calculate this Casimir interaction in the most general case, where membrane fluctuations are governed by the combined action of surface tension, bending modulus, and Gaussian rigidity. We find that the surface tension strongly modifies the power law in the separation dependence of the Casimir interaction. Furthermore, the method allows us to calculate the Casimir force both at short and large separations. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D40.00008: Nanoparticles Induced Microscaled Pore Formation on Supported Lipid Bilayer Benxin Jing, Y. Elaine Zhu Most of recent researches on the cytotoxicity of nanomaterials focused on hydrophilic nanomaterials because of their good dispersion in water, but much less on hydrophobic ones. In this work, we have investigated the effect of semi-hydrophobic nanoparticles (NPs) on the dynamics and morphology of model cell membrane. We have found carboxyl functionalized polystyrene nanoparticles can induce the formation of microscaled pores on neutral supported Egg PC lipid bilayer at the ionic strength range similar to that in the human body with a strong dependence on nanoparticle size and concentration. The hydrophobic interaction between the NP surface and lipid bilayer is accounted for the induced line tension in lipid bilayer; when the tension exceeds a critical value, pores are formed and grow rapidly with dependence on nanoparticle size and ionic strength. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D40.00009: Modeling the Elastic Properties of Lipid Bilayer Membranes Edward Barry, Thomas Gibaud, Mark Zakhary, Zvonimir Dogic Model membranes such as lipid bilayers have been indispensable tools for our understanding of the elastic properties of biological membranes. In this talk, I will introduce a colloidal model for membranes and demonstrate that the physical properties of these colloidal membranes are identical to lipid bilayers. The model system is unique in that the constituent molecules are homogenous and non-amphiphilic, yet their self-assembly into membranes and other hierarchical assemblages, such as a lamellar type phases and chiral ribbons, proceeds spontaneously in solution. Owing to the large size of the constituent molecules, individual molecules can be directly visualized and simultaneous observations at the continuum and molecular lengthscales are used to characterize the behavior of model membranes with unprecedented detail. Moreover, once assembled in solution, molecular interactions can be controlled in situ. In particular, the strength of chiral interactions can be varied, leading to fascinating transitions in behavior that resembles the formation of starfish vesicles. These observations point towards the important role of line tension, and have potential implications for phase separated lipid mixtures or lipid rafts. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D40.00010: Dielectric sensors for measuring membrane potential Kimal Rajapakshe, Asanga Wijesinghe, Jie Fang, William Widger, John Miller Membrane potential in a biological cell depends on the ionic concentration difference between the extracellular and the intracellular medium. Ions close to the membrane show high polarizations under an electric field. Recent theoretical studies have related these polarizations to the alpha ($\alpha )$ dispersions in the impedance spectroscopy of a cell suspension. Therefore these dispersions can be used to measure the membrane potential of a single cell. Here we report the dielectric properties of phosphatidylcholine liposomes and its changes with the membrane potential. Liposomes have been prepared to have a higher concentration of potassium ions (K$^{+})$ inside the membrane compared to external medium. Under valinomycin (K$^{+}$ ionophores) these liposomes generate a negative membrane potential, as verified by fluorescent voltage sensitive dye measurements. Both dielectric and conductivity spectra display low frequency dispersions that are dependent on membrane potential. Possible future applications include noninvasive sensors for in vitro testing of new drugs and other applications. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D40.00011: Laser Transmission Spectroscopy and applications to liposome studies Frank Li, James Marr, Ching-Ting Hwang, Robert Schafer, Zachary Schultz, Steven Ruggiero, Carol Tanner We describe the implementation of precision laser transmission spectroscopy (LTS) for sizing nanoparticles in suspension. Our apparatus incorporates a tunable laser and balanced optical system which measures light transmission over a wide (210 -- 2300 nm) wavelength range with high precision and sensitivity. Spectral inversion was employed to determine both the particle size distribution and absolute density of particles with diameters over a total range of 5 to 3000 nm. LTS has a dynamic range of $\sim$ $10^{3}$ particles/mL to $\sim$ $10^{10}$ particles/mL ($5x10^{-8}$ vol.\% to 0.5 vol.\%). Currently, LTS is being applied as a tool to investigate the behavior of liposomes, dipalmitoylphosphatidycholine (DPPC) and dipalmitoylphosphatidylserine (DPPS), under the presence of fusing and de-aggregating agents. Our measurements indicate a maximum diameter of 400 nm for liposomes suspended in solution after fusion. [Preview Abstract] |
Session D41: LeRoy Apker Prize Session: Clusters and Nanoscale Systems
Sponsoring Units: DCPChair: Jim Lisy, University of Illinois at Urbana-Champaign
Room: A115/117
Monday, March 21, 2011 2:30PM - 3:06PM |
D41.00001: LeRoy Apker Award Talk: Self-Assembly of DNA-Functionalized Nanoparticles Invited Speaker: Nanoparticles tethered with DNA strands can self-assemble into highly organized structures through the bonding of complementary nucleobases. These are promising building blocks for the bottom-up nanotechnology, and computational tools are useful to probe the behaviors of such complex materials. In this talk I will summarize my work on the phase behavior of nanoparticles tethered with a small number of DNA strands, and on the development of theories for the clustering and self-assembly kinetics of a specific case. Due to a separation of repulsion and attraction length scales, these nanoparticles exhibit an interesting hierarchy of phases made up of multiple interpenetrating structures. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D41.00002: DNA-driven assembly of phospholipid bilayer nanodiscs Nienke Geerts, Paul A. Beales, T. Kyle Vanderlick Phospholipid nanodiscs are a rare form of stable lipid self-assembly. The discs are formed by allowing lipids to self-assemble in the presence of membrane scaffold proteins (MSP). Each disc contains two MSP, wrapping around the edge of a leaflet of the bilayer. Although nanodiscs have become an important and versatile tool among model membrane systems to functionally reconstitute membrane proteins, they are yet to be utilized as building blocks in material science. However their highly monodisperse nanoscale structure make them ideal for this purpose. Here we report the first superstructures of nanodiscs self-assembled via membrane anchored single stranded DNA. The discs assemble into columnar stacks with high aspect ratio. The MSPs provide another powerful feature, as the His-tags of the protein can be used to attach the discs to colloids or other molecules of interest. This has strong potential for assembly of nanomaterials with greater degrees of complexity. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D41.00003: Orthogonal DNA-colloid Clusters Jesse W. Collins, Vinothan N. Manoharan We experimentally investigate the self-assembly of colloids labelled with different DNA strands into small clusters. We coat 1 micron diameter spheres with 65 base DNA strands having highly specific ``sticky ends.'' Particles with different surface-bound DNA sequences represent different particle ``types.'' We tune the short-ranged, pairwise interactions between some types to be attractive and interactions between other types to be purely repulsive; in this sense, the interactions are orthogonal. The magnitude of attraction (and repulsion) is constant across various types. We control the number and types of colloids at the single particle level, and distinguish the type of each particle from the types of their binding partners within each cluster. In an example experiment, 2 particles of each of 3 different types explore a volume less than 100 picoliters and assemble into equilibrium configurations. We characterize the structures with a microscope and compare observed averages with statistical mechanical predictions. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D41.00004: Size Characterization of Surfactant and Polymer Coated Gold Nanorods Christopher Grabowski, Paul Luchette, Peter Palffy-Muhoray Polarization-dependent dynamic light scattering was conducted on gold nanorods (Au NRs) coated with CTAB (hexadecyltrimethylammonium bromide) in water and coated with 50k MW PS (polystyrene) in toluene. The autocorrelation function of the scattered light intensity was determined for a series of scattering angles under VH and VV scattering geometries. The data were fit to a model of rotational and translational diffusivities. From this fit, we estimate the effective length (L) and diameter of the coated nanorods in solution. Au NRs coated with 50k PS show greatly reduced rotational diffusion compared to CTAB-coated NRs. Since the rotational diffusion coefficient scales as 1/L$^{3}$, this implies significant extension of the grafted PS chains in toluene. We investigate this phenomenon for PS grafted onto Au nanoparticles and nanorods of varying aspect ratio to determine the impact of surface curvature on polymer layer thickness. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D41.00005: A ligand phase transition on nanorods and its effect on their surface forces Asaph Widmer-Cooper, Phillip Geissler Synthesizing nanometer-scale objects with controlled optical and electronic properties is now a relatively straightforward task, however organizing such objects into extended structures that could revolutionize technology remains a challenge, especially for anisotropic particles. Nanorods behave like liquid crystals in solution and can assemble into structures with the rods oriented perpendicular with respect to a substrate upon drying, assemblies that could potentially be used to print nanostructured solar cells and photoelectrochemical devices. Achieving complete control of this process, however, requires detailed understanding of the rod-rod and rod-surface interactions. Like most nanoparticles, CdS nanorods are passivated with ligands to stabilize them from random aggregation in solution. Using molecular dynamics simulations with explicit ligands and solvent we investigate the structure of phosphonic acid ligands on CdS nanorods as a function of temperature and show that they can undergo an ordering transition close to room temperature. We calculate the potential of mean force between the rods and show that this changes the rod-rod interaction from purely repulsive to attractive. This should have a significant effect on their self-assembly behavior. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D41.00006: Chemical Preparation and Characterization of Elemental Cu, Ni, and Cu/Ni Core/Shell Nanoparticles Laura Higgins, Michael Lattanzi, Brian Kelly, Gerald Poirier, Karl Unruh Elemental Ni, Cu, and Cu core/Ni shell nanoparticles have been prepared in a polyol-type process. The elemental nanoparticles were prepared by dissolving NiCl$_{2}\cdot $6H$_{2}$O and/or CuCl$_{2}\cdot $2H$_{2}$O in ethylene glycol (EG) and heating the solution to reflux prior to the addition of NaOH. The resulting precipitate was then extracted, dried, and characterized by scanning electron microscopy (SEM) with elemental analysis, x-ray diffraction (XRD), and in the case of the elemental Ni nanoparticles, vibrating sample magnetometry (VSM) measurements. The best fit lattice parameters obtained from the elemental Ni and Cu nanoparticles were 0.35289(28) and 0.36171(23) nm, respectively, in good agreement with the corresponding bulk values. On the other hand, the measured saturation magnetization of about 49 emu/g was somewhat smaller than the bulk Ni value. In the case of the Cu/Ni nanoparticles, the best fit lattice parameters for the Ni and Cu components of the core/shell structure were 0.35299(26) and 0.36101(10) nm, indicating the formation of an essentially pure Ni shell and a slight amount of Ni incorporation in the Cu core. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D41.00007: Solid-State Homogenization Reactions in Cu Core/Ni shell Nanoparticles Michael Lattanzi, Laura Higgins, Brian Kelly, Gerald Poirier, Karl Unruh Air stable Cu core/Ni shell nanoparticles have been prepared in a polyol-type process by heating an ethylene glycol (EG) solution containing CuCl$_{2}\cdot $2H$_{2}$O and NiCl$_{2}\cdot $6H$_{2}$O to its boiling temperature, adding an appropriate amount of NaOH, and allowing the reaction to proceed at reflux for 30 minutes prior to cooling. The as-prepared nanoparticles were characterized by scanning electron microscopy (SEM) with elemental mapping, x-ray diffraction (XRD), and vibrating sample magnetometry (VSM) measurements. Chemical composition maps of the particles revealed a well-defined core/shell structure consisting of a Cu core about 100-150 nm in diameter surrounded by a Ni shell about 30-40 nm in thickness. XRD measurements indicated that while the Cu core contained a small amount of incorporated Ni, the shell was essentially pure Ni. The solid-state transformation from the as-prepared core/shell structure to an essentially homogeneous Cu-Ni alloy was studied by high temperature VSM and XRD measurements as a function of annealing temperature and time. These measurements reveal that the core/shell structure remains largely intact to temperatures above 400 \r{ }C and that complete homogenization occurs at temperatures above about 600 \r{ }C. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D41.00008: The Discovery of a New Class of Magnetic Superhalogens P. Jena, M.M. Wu, H. Wang, Y. Ko, Q. Wang, Q. Sun, B. Kiran, A. Kandalam, K. Bowen We report the discovery of a new class of magnetic superhalogens and their unusually stable molecular anions. These are formed when a hot plume of manganese atoms is cooled through collisions with an inert gas in the presence of chlorine atoms. The anions, with a composition of (Mn$_{x}$Cl$_{2x+1})^{-}$ (x = 1, 2, 3. . . ), appear as prominent (magic) peaks in mass spectra. Using calculations based on density functional theory and experiments utilizing anion photoelectron spectroscopy, we traced the origin of their unusual stability to the half-filled d-shell of the Mn atoms in anionic clusters and the large electron affinities of their neutral counterparts. The calculated and measured electron affinities are almost twice as high as that of the chlorine atom. However, unlike conventional superhalogens which are non-magnetic and consist of a single metal atom at the core surrounded by halogen atoms, the superhalogens discovered here are magnetic and have (MnCl$_{2})_{x}$ moiety as a core to which a chlorine atom is attached. In addition, our calculations show that Mn atoms carry large magnetic moments and Mn$_{x}$Cl$_{2x+1}$ superhalogen moieties can serve as building blocks of a new category of salts with magnetic properties. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D41.00009: Dielectron Attachment and Hydrogen Evolution Reaction in Water Clusters Robert Barnett, Rina Giniger, Ori Cheshnovsky, Uzi Landman Binding of excess electrons to nano-size water droplets, with a focus on the hitherto largely unexplored properties of doubly-charged clusters, were investigated experimentally using mass spectrometry and theoretically with large-scale first-principles quantum simulations. Doubly-charged clusters were measured in the range of 83 $\le $ n $\le $ 123, with (H$_{2}$O)$_{n}^{-2 }$ clusters found for 83 $\le $ n $<$ 105, and mass-shifted peaks corresponding to (H$_{2}$O)$_{n-2}$(OH$^{-})_{2}$ detected for n $\ge $ 105. Simulations revealed surface and internal dielectron, e$^{-}_{2}$, localization modes and elucidated the mechanism of the reaction (H$_{2}$O)$_{n}^{-2}\to $ (H$_{2}$O)$_{n-2 }$(OH$^{-})_{2}$ + H$_{2}$ (for n $\ge $ 105), which was found to occur via concerted approach of a pair of protons belonging to two water molecules located in the first shell of the dielectron internal hydration cavity, culminating in hydrogen formation 2H$^{+}$ + e$^{-}_{2 }\to $ H$_{2}$. Instability of the dielectron internal localization impedes the reaction for smaller ( n $<$ 105) clusters. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D41.00010: Investigations into aggregate growth dynamics \textit{via in situ} structural quantification of flame synthesized silica nanoparticle aggregates Durgesh Rai, Gregory Beaucage, Jan Ilavsky, Hendrik Kammler, Sotiris Pratsinis Ramified aggregates are formed in many dynamic processes such as in flames. The structures are disordered and present a challenge to quantification. The topological quantification of such nanostructured materials is important to understand their growth processes. Small-angle X-ray scattering (SAXS) is widely used to characterize such nanoparticle aggregates. Recently, we have developed a method for the quantification of topology in aggregated material using SAXS. This methodology will be used to describe topologies from \textit{in-situ} SAXS studies on flame synthesized silica aggregates on millisecond time scales. This is an important step to facilitate understanding of the growth dynamics and the structural rearrangements that occur during flame synthesis. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D41.00011: Influence of Nanoparticles on Fragility and Collective Particle Motion in Polymer Glass-Formation Jack Douglas, Francis Starr We investigate the impact of nanoparticles (NP) on glass-formation in polymer melts by molecular dynamics simulation. The NP cause significant changes in both fragility and the average length of string-like cooperative motion, where the extent of the effect depends on the NP-polymer interaction and NP concentration. These dynamical changes can be interpreted via the Adam-Gibbs (AG) theory if we assume the strings represent the abstract cooperatively rearranging regions (CRR) of the AG model, whose basic assumptions are reviewed. Molecular additives are also effective at altering the fragility of glass-formation and extent of string-like collective motion so the modulation of fragility and cooperative motion with additives seems to be a general effect. We find that the fragility of glass formation is mainly controlled mainly by the differential change of L with respect to T near the glass transition rather than the actual size L of the collective motion. We also find a near proportionality between m and the glass transition temperature in our nanocomposite system, which greatly simplifies the T dependence of structural. The classical entropy theory of glass-formation is considered as a complementary tool to gain analytic insights into these additive effects on polymer glass formation. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D41.00012: Anion Photoelectron Spectroscopy and First-Principles Study of PbxIny Clusters S. Vincent Ong, Joshua Melko, Ujjwal Gupta, J. Ulises Reveles, Jonathan D'Emidio, Shiv Khanna, A.W. Castleman Anionic and neutral Pb$_{x}$In$_{y}$ clusters containing up to 5 Pb and up to 7 In atoms have been investigated using negative ion photodetachment spectroscopy along with first-principles electronic structure studies within a gradient corrected density functional approach. The stability and electronic properties of these clusters have been characterized through studies of the detachment energies, gaps in the electronic spectrum, variations in binding energy, and nature of the electronic states. Particularly stable clusters have been grouped into two families of stable species. PbIn$_{3}^{-}$, Pb$_{2}$In$_{2}$, and Pb$_{3}$In$_{2}$ exhibit enhanced stability compared to their neighbors and the stability is linked to the aromatic character identified in their molecular orbitals. On the other hand, PbIn$_{5}^{-}$ and Pb$_{2}$In$_{4}$ exhibit enhanced stability associated with filled electronic shells within a confined nearly free electron gas. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D41.00013: Structural Analysis of Bonding in Au-Ge Clusters Danielle McDermott, Kathie Newman The study of Gold-Germanium clusters is important in understanding systems such as gold catalyzed nanowire growth. Of particular concern is the bonding behavior between the two chemical elements, one tending to form metallic bonds, the other covalent. DFT calculations and Conjugate Gradient relaxations were performed on clusters ranging in size from 50 to 150 atoms using the SIESTA code to find the geometries of metastable states. Emphasis has been placed on developing accurate and dependable bases to be used to study nano-sized systems. The binding energy, coordination number, bond lengths and bond angles are studied as a function of the size and composition of Ge-Au clusters. We will discuss a nanoscale ``phase diagram'' for gold and germanium and will also discuss the topology of the bonding network. [Preview Abstract] |
Session D42: Colloids Theory & Computation, Emulsions, and Foams
Sponsoring Units: DFDChair: Eric Weeks, Emory University
Room: A302/303
Monday, March 21, 2011 2:30PM - 2:42PM |
D42.00001: Liquid loss from foams with low water content Michael Conroy, Justin Taylor, John Farley, James Fleming, Ramagopal Ananth The liquid content of a foam can be significantly affected by liquid loss (drainage), a process that occurs both during and after the foam fills a space. We develop a theoretical model to describe liquid loss and evolution of average liquid volume fraction over time for advancing and static foams. We also perform bench-scale drainage experiments on foams with low water content. The theoretical model shows a constant drainage rate during the filling process which decays exponentially after a static column is formed. The measured loss of liquid is found to be in good agreement with the theoretical predictions. We find that drainage is greatly affected by the time scale for filling a space with foam. Significant effects on drainage are also found by varying bubble size, foam column height, and initial liquid content. The study indicates that drainage behavior can substantially deviate from that described by free-drainage theories, which assume that drainage initiates from a foam of static height. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D42.00002: Molecular Dynamics Study of the Foam Stability of a Mixed Surfactant System with and without Calcium Ions Xiaozhen Yang, Wenhong Yang Foam stability performance of a mixture surfactant system with and without calcium ions, including linear alkylbenzene sulfonate (LAS) and sodium dodecyl sulfate (SDS), has been studied by molecular dynamics. Microscopic interaction analysis reveals that the fraction of free calcium ions, $X_{f}$, in film system indicates the extent of the foam stabilities when $X_{f}$ is in different calcium ion zones. In the system without ions, we found the variable of the surfactant tail mass out of water film, $W$, is indicator of foam stability. Performance of the mixture system predicted here was supported by experiments. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D42.00003: Structural Properties of a Sheared Dense Emulsion S.K. Dutta, E.D. Knowlton, D.L. Blair The flow of a compressed emulsion above its yield point can be described by a velocity profile in addition to a rearrangement of individual droplets on top of this time averaged motion. Using a confocal microscope, we have tracked the droplets of an oil-in-water emulsion as they are sheared in a rheometer. When the applied stress is large, the velocity profile shows a nearly affine deformation, while there is strong strain localization close to yield. The crossover between these two behaviors occurs at higher shear rates as the volume fraction of the droplets is increased. At shorter length scales, rearrangement events are heterogeneously distributed, reflecting the disordered packing of the emulsion droplets. This characterization is a step towards linking bulk viscoelastic properties to local structural relaxation as the system leaves the jammed state. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D42.00004: Janus and Gemini Nanoplates Zhengdong Cheng, Andres Mejia, Ya-wen Chang, Peng He, Agustin Diaz, Abraham Clearfield Janus particles were used to make stable Pickering emulsions (emulsions stabilized by particles). Here we demonstrated a novel method to produce high aspect ratio Janus plates with atomic thickness. Gemini plates with only the edges functionalized are also fabricated. These novel nanoplates are observed to have super surface activity. Most importantly, these particles overcome the two \textit{opposite} effects in the stabilization of Pickering emulsions using spherical particles: stabilization requires particles as small as possible; but smaller particles are easy to escape the interface due to Brownian motion since the adsorption energy to the oil-water interface is proportional to the diameter of the spheres. Our nanoplates have a \textit{large} aspect ratio due to the extremely thin thickness, which offers extraordinary stability to the liquid film between two emulsions to prevent coalescence. In the meantime, their large lateral surface area offers strong adsorption energy at the oil-water interface. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D42.00005: Control over the number, size, and type of inner drops inside a double emulsion Laura Adams, Yuanjin Zhao, Anderson Shum, David Weitz The formation of monodisperse double emulsions, drops inside of drops, has revealed a rich range of configurations not possible without the precise control of microfluidics. Yet-to-date, development of double emulsions with a controlled number of two different inner drops has not emerged. Here we demonstrate exquisite control over the number, size and type of inner drops encapsulated inside a double emulsion. These are fabricated using glass capillary devices implemented with a dual bore injection tube. We will show our latest results and discuss the scientific and technological opportunities made possible by these stable binary configurations. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D42.00006: Yielding and Shear Induced Structure Formation in Emulsions with Attractive Interactions Zhen Shao, Ajay Negi, Chinedum Osuji The yielding behavior of colloidal suspensions is a strong function of inter-particle interactions. Recent results [Pham et al. 2006, 2008] indicate that attractive colloidal glasses display a two-step yielding due to inter-particle bond rupture followed by particle cage escape. From this perspective, we examine the yielding behavior of an oil-in-water emulsion system with attractive interactions using dynamic bulk rheology. In strain sweep experiments, after a limited linear regime, the system yields with a pronounced bump in the viscous modulus, a sharp decrease in the elastic modulus and a crossover between the two. The yielding response is marked by bond-breaking at low volume fractions and bond-breaking accompanied by cage escape above a critical concentration. An increase in the complex modulus is observed at yet higher strains ($>$100{\%}), with both the elastic and viscous components showing small frequency dependent peaks. The onset, peak strains and peak stress display different dependences on volume fraction. We speculate that this display is due to the formation of shear induced structures at high strains and advance a simple model for this behavior. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D42.00007: Force Network in a 2D Frictionless Emulsion Model System Kenneth W. Desmond, Pearl Young, Dandan Chen, Eric R. Weeks We confine oil-in-water emulsion droplets between two parallel plates to create a quasi-two-dimensional model system to study the jamming transition. This model system is analogous to granular photoelastic disks with the exception that there is no static friction between our droplets. To study the jamming transition we compress the droplets in small increments and investigate how the force network evolves with increasing area fraction, where the forces are measured using a calibration technique we have developed. The forces in our system are spatial heterogeneous with a probability distribution that is similar to that found for photoelastic disks. We also find that the probability distribution of the forces narrows with area fraction, and that the correlation length of the largest forces is only few particle diameters. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D42.00008: New activated dynamical regimes in dense suspensions of attractive uniaxial colloids Rui Zhang, Kenneth Schweizer Our microscopic theory of cooperative translational-rotational activated glassy dynamics of hard uniaxial particles [PRE,80,011502(2009); JCP,133,104902(2010)] is extended to treat short range attractions. For small aspect ratio dicolloids, a plastic glass (PG) state exists for weak attractions, but is destroyed beyond a critical attraction strength resulting in a new dynamic triple point (fluid, PG, gel), and two novel re-entrant behaviors: PG-fluid-gel, and repulsive glass(RG)-PG-gel. A new mixed ``glass-gel'' state also emerges characterized by center-of-mass and rotational angle localization parameters of intermediate magnitude. At high volume fractions, increasing attraction transforms the RG to an attractive glass (AG) characterized by a dynamic free energy surface with a gel-like localization state but a glass-like saddle point, and a non-monotonic variation of relaxation time and diffusion constant. AG dynamics is of a 2-step nature where physical bonds first break followed by hopping over a glass-like barrier. At high attractions a sharp crossover from a gel to AG with increasing volume fraction is predicted. As the particle aspect ratio grows, the PG state is destroyed, and translational motion becomes increasingly more important for escaping dynamical traps. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D42.00009: A new analysis methodology for the motion of self-propelled particles and its application Young-Moo Byun, Paul Lammert, Vincent Crespi The self-propelled particle (SPP) on the microscale in the solution is a growing field of study, which has a potential to be used for nanomedicine and nanorobots. However, little detailed quantitative analysis on the motion of the SPP has been performed so far because its self-propelled motion is strongly coupled to Brownian motion, which makes the extraction of intrinsic propulsion mechanisms problematic, leading to inconsistent conclusions. Here, we present a novel way to decompose the motion of the SPP into self-propelled and Brownian components; accurate values for self-propulsion speed and diffusion coefficients of the SPP are obtained for the first time. Then, we apply our analysis methodology to ostensible chemotaxis of SPP, and reveal the actual (non-chemotactic) mechanism of the phenomenon, demonstrating that our analysis methodology is a powerful and reliable tool. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D42.00010: Diffraction effects on optical trapping of small particles Rachael Harper, Alex Levine Geometric ray optics is an elegant and computationally efficient means of numerically calculating the forces on particles of arbitrary shape due to their interaction with a beam of light. This method is limited to the regime in which the particle is much larger than the wavelength of light. Ashkin's pioneering work [1] on force exerted by a laser trap on a spherical dielectric particle relies on this geometric optics limit. In current experiments, however, the size of the trapped particles can be comparable to the wavelength of the trapping radiation field. In this talk, we discuss the corrections to ray-tracing-based calculations of the laser trapping forces due to diffraction effects. Specifically, we compare the momentum transfer from a uniform beam of light to hollow dielectric cylindrical shells obtained from two different calculations using: (i) ray-tracing and (ii) the full physical optics formulation. By changing the radii of the inner and outer edges of the hollow cylinder with respect to the wavelength of light we determine the limits of validity of the ray-tracing solution. In the limit in which the radius of the inner cylinder is comparable to the wavelength radiation we show that the corrected momentum transfer is smaller than that predicted by geometric optics. We attribute this result to the reduction in the scattering force on the cylinder due to diffraction effects not accounted for in the geometric optics formalism. [1] A Ashkin, Biophys. J., 61, 569 (1992). [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D42.00011: Capillary Interactions among Spherical Particles at a Curved Liquid Interface Chuan Zeng, Fabian Brau, Benny Davidovitch, Anthony D. Dinsmore Colloidal particles tend to adsorb on liquid interfaces, where in- plane interactions can arise from a variety of mechanisms. We focus on capillary interactions induced by the curvature of the liquid interface, where particles were assumed to have a constant Young-Laplace contact angle at the three-phase contact line. Whereas spherical particles can adsorb on flat or spherical interfaces without deforming the interface, adsorption on a cylindrical interface deforms the interface because of the lack of azimuthal symmetry around the contact line. We present an analytical model of the interfacial shape and energy upon adsorption of a single particle as well as the interaction between two particles. Based on our result on a cylindrical interface, we propose a general formula for the force on a particle on a curved interface. This study provides an important step toward understanding the interactions among interfacial particles when the interface is distorted by an external field. We acknowledge support from the NSF-supported MRSEC on Polymers at UMass (DMR- 0820506) and NSF CBET-0967620. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D42.00012: Slow relaxations of individual colloidal spheres following the breach of a fluid interface Madhav Mani, David M. Kaz, Ryan McGorty, Vinothan N. Manoharan Although the equilibrium state of a micron sized spherical particle at an interface is well understood, the dynamics associated with the approach to equilibrium is not. Recent high-resolution experiments from the Manoharan Lab (Ref: David M. Kaz's Talk) have shown that the dynamics are richer than expected. Subsequent to the initiation of a contact-line at a fluid interface the dynamics towards equilibrium are much slower than predicted by a hydrodynamic theory and the center of mass of the particle appears to follow a logarithmic law in time. We propose the importance of thermally agitated interactions between the contact-line and physical/chemical defects that pin the contact-line locally, thereby leading to an enhancement of the overall dissipation. We deduce that the interface must remain flat during this dynamic process and derive a force-velocity relation, which agrees with both the slow velocities and the logarithmic law. This surprisingly slow approach to equilibrium has significant consequences for processes where interactions between colloids and interfaces are present. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D42.00013: Using Micron-Sized Ellipsoids as a New Tool for Microrheology David Kilgore, Kenneth W. Desmond, Eric R. Weeks It is a well-established principle that the viscosity of a fluid can be calculated by observing the diffusion of microspheres, provided the diameter of the microspheres is known. We are developing a microrheology technique using ellipsoids, where the rheology can be measured without prior knowledge of the length and width of the ellipsoid. The advantage of using ellipsoids is that their asymmetry allows for the diffusion to be decomposed into two translational motions and one rotational motion. For each of these diffusive motions, we can measure a diffusion constant and relate the constant to the three unknowns: the length and width of the ellipsoid, and the viscosity. By measuring the three diffusion constants, we can determine the three unknowns. To verify this technique, we produce ellipsoids in the lab and suspend them in a viscous solution for three-dimensional imaging of the diffusion with a confocal microscope. We are able to get good agreement between the microrheological measurements and macroscopic viscosity measurements. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D42.00014: Inferring elastic properties in colloidal solids: artifacts of a restricted observation window Asad Hasan, Craig Maloney Recently, it has been shown how to extract information about the effective elasticity in colloidal solids, granular packings, {\it etc.}, using two point displacement correlations as obtained in, \emph{e.g.}, optical microscopy experiments or computer simulations. At its core, this technique relies on the observation that, within the harmonic approximation, the Hamiltonian, $H$, is the inverse of the elastic response function, $G$, \emph{defined over the whole domain of the elastic body}. However, most experiments (and even most simulations) have access to G only over some restricted sub-domain of the experimental system. Here, we study restricted observation domains of various size and dimensionality in face centered cubic (fcc) crystals of various size using a pseudo-analytic approach in which $G$ is obtained analytically and is inverted numerically \emph{on a compact sub-domain} to obtain the projected Hamiltonian, $\tilde{H}$. We show that the effective plane-wave energy, $E_k=\langle \psi_k | \tilde{H}| \psi_k \rangle$, for either a [111] or [100] planar subdomain has an unusual dispersion, $E\sim k$, rather than the familiar $E\sim k^2$ and motivate this observation from continuum considerations. We also show how this leads to an anomaly in the density of states of $\tilde{H}$. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D42.00015: Enhancing tracer diffusivity by tuning interparticle interactions and solvation shell structure James Carmer, Gaurav Goel, Tom Truskett Using computer simulations, we explore how tuning the tracer-solvent interactions affects the dynamics of a tracer particle. Optimizing the tracer particle contribution to excess entropy results in significant increases in tracer particle diffusivity. We also observe increases in dynamics while increasing the effective particle size. These changes are investigated at various densities and particle size ratios. [Preview Abstract] |
Session D45: Many Body Physics of Quantum Gases in Reduced Dimension
Sponsoring Units: DAMOPChair: Ana Maria Rey, JILA/University of Colorado
Room: A310
Monday, March 21, 2011 2:30PM - 2:42PM |
D45.00001: Moving Impurities and Spin-Boson Systems in One-Dimensional BECs Thomas Schmidt, Peter Orth, Karyn Le Hur We theoretically investigate the dynamics of two moving impurities immersed in a one-dimensional interacting Bose liquid. Interactions between the two impurities are mediated via excitations in the quantum liquid, and lead to correlations between them. For certain parameter regimes, the system can be mapped onto a spin-boson model, in which the relative momentum of the impurities plays the role of a spin-1/2 or spin-1. We will discuss the implications of the spin-impurity model onto observables of the liquid and impurities. [Preview Abstract] |
Monday, March 21, 2011 2:42PM - 2:54PM |
D45.00002: Exact result for the three-body local correlator in the 1D Bose gas at finite temperature Marton Kormos, Adilet Imambekov The 1D Bose gas with Dirac-delta interaction (Lieb--Liniger model) gives a very good description of cold atomic gases confined in quasi one-dimensional waveguides. While the model is integrable by means of the Bethe Ansatz it can also be regarded as a particular non-relativistic limit of an integrable relativistic quantum field theory, the sinh-Gordon model. This fact can be exploited to calculate form factors and correlation functions for the Bose gas. We derive an exact expression for the finite temperature expectation value of the third power of the density operator $:\rho^3:$, a quantity which is closely related to the three-body losses in cold atom experiments. We achieve this by summing up an infinite integral series obtained using the connection with the sinh-Gordon model. Our method can be generalized to other local correlators. [Preview Abstract] |
Monday, March 21, 2011 2:54PM - 3:06PM |
D45.00003: The density profile of interacting Fermions in a one-dimensional optical trap Sebastian Eggert, Stefan Soeffing The density distribution of the Hubbard model in a one-dimensional external harmonic potential is investigated in order to study the effect of the confining trap. The broadening of the Fermion cloud with increasing interaction is analyzed in detailed, which can be described by a surprisingly simple scaling form. Strong superimposed ``Friedel'' oscillations are always present despite the absence of any hard wall boundaries. The wavelength of the dominant oscillation changes with interaction, which indicates the crossover to a spin-incoherent regime. We present an analytical formula, which describes the behavior of the oscillations very well for all interactions strengths and in return gives some insight for the use of bosonization in a trapping potential. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D45.00004: Pairing correlations in one-dimensional Bose-Fermi mixtures with molecular boundstates Shimul Akhanjee, Masahisa Tsuchiizu, Akira Furusaki We study the ground-state properties of one-dimensional (1D) 3-component mixtures of Tonks bosons having infinite repulsion and nearly free fermionic atoms that can combine to form molecular fermions. Using a bosonization scheme, the form of the interaction is equivalent to the hopping term between weakly coupled spinless Tomonaga Luttinger liquids (TLL). Upon reduction of the energy scale, the 3-component TLL system scales down into a phase with coupled massive modes accompanied by pairing correlations. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D45.00005: Scaling of noise correlations in one-dimensional lattice hard-core boson systems Kai He, Marcos Rigol Noise correlations are studied for systems of hard-core bosons in one-dimensional lattices. We use an exact numerical approach based on the Bose-Fermi mapping and properties of Slater determinants. We focus on the scaling of the noise correlations with system size in superfluid and insulating phases, which are generated in the homogeneous lattice, with period-two superlattices, and with uniformly distributed random diagonal disorder. For the superfluid phases, the leading contribution is shown to exhibit a density independent scaling proportional to the system size, while the first subleading term exhibits a density dependent power-law exponent. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D45.00006: Double occupancy as a probe of the Mott transition for fermions in one-dimensional optical lattices Jorge Quintanilla, Vivaldo L. Campo, Jr, Vito Scarola, Chris Hooley, Klaus Capelle We study theoretically double occupancy D as a probe of the Mott transition for trapped fermions in one-dimensional optical lattices and compare our results to the three-dimensional case. The ground state is described using the Bethe Ansatz in a local density approximation and the behavior at finite temperatures is modelled using a high-temperature series expansion. In addition, we solve analytically the model in the limit in which the interaction energy is the dominant energy scale. We find that enhanced quantum fluctuations in one dimension lead to increased double occupancy in the ground state, even deep in the Mott insulator region of the phase diagram. Similarly, thermal fluctuations lead to high double occupancies at high temperatures. Nevertheless, D is found to be a good indicator of the Mott transition just as in three dimensions. We discuss possible experiments to verify these results and argue that the one-dimensional Hubbard model could be used as a benchmark for quantitative quantum analogue simulations. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D45.00007: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 3:54PM - 4:06PM |
D45.00008: Analysis of the single-particle excitation spectrum of ultracold fermions in 1D optical lattices Atsushi Yamamoto, Susumu Yamada, Masahiko Machida We present single-particle excitation spectra of ultracold fermions in one-dimensional(1D) optical lattices by using dynamical density-matrix renormalization group (DDMRG) method. Our model is described by a Hubbard model with the harmonic trap potential. We find that the spectra show many kinds of intriguing structures owing to the harmonic trap potential and on-site interaction. In an analysis of weakly-interaction regimes, we find that the spectrum structure changes from a typical Hubbard band as obtained from periodical 1D lattice to band branching as increasing the trap potential, and finally, we observe clear discrete bound-state levels. On the other hand, in case of strongly-interacting regimes, we confirm the multiple flat bound-state levels lying above 1D Tomonaga-Luttinger (TL) liquid spectrum on a central Mott-plateau phase surrounded by metallic regions. Furthermore, we also investigate spectral changes as a metallic state partially emerges at the center region and find one-dimensional TL spectrum breakdown with an emergence of a new dispersive band due to the central metal portion. The observed features are closely related with the spectral changes when doping into Mott insulator. We will show the more details of spectra in 1D fermionic optical lattices by comparing non-trapped uniform 1D spectra. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D45.00009: A quantum Monte Carlo study of the two-component mixture of hard-core bosons in one dimension Min-Chul Cha, Jong-Geun Shin, Inho Jeon The two-component mixture of hard-core bosons in one dimension is studied by quantum Monte Carlo simulations. A rich variety of phases exists in the parameter space of the inter-species interaction strengths, the ratio of the hopping amplitudes between two species, and the filling fractions. Physical properties of different phases are investigated by measuring the superfluid stiffness, the counter-flow stiffness, the compressibility, and the structure factor. We examine the nature of some phase transitions between a superfluid and an insulator and the 1st-order transition in the occurrence of phase separations. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D45.00010: Fermionic Cold Atom Systems in Mixed Dimensions Kyle Irwin, Shan-Wen Tsai Cold atom experiments can now realize mixtures of components that move in different dimensions [1]. We investigate such a system with two species of fermions. One species, f-fermions, moves on a two-dimensional square lattice. Another species, c-fermions, is constrained to move on a one-dimensional lattice embedded in the square lattice of f-fermions. We investigate the effective one-dimensional system who's interactions are mediated by the two-dimensional system, and explore effective theories, quantum phases, correlations, and relevant energy scales for various fillings of the mixed dimensional system using a functional renormalization group approach. \\[4pt] [1] G. Lamporesi, J. Catani, G. Barontini, Y. Nishida, M. Inguscio, and F. Minardi, Phys. Rev. Lett. 104, 153202 (2010) [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D45.00011: Long range bosonic correlations in 2D optical lattice traps K.W. Mahmud, E.N. Duchon, Y. Kato, N. Kawashima, R.T. Scalettar, N. Trivedi We use quantum Monte Carlo (QMC) simulations to study the combined effects of harmonic confinement and temperature for bosons in a two dimensional optical lattice. We present the scale invariant, finite temperature, state diagram for the Bose- Hubbard model in terms of experimental parameters - the particle number, confining potential and interaction strength. We examine the correlation decay of the superfluid trapped in annular rings, and find that the width of the superfluid ring determines a distance after which the correlation decays faster than in an equivalent 2D superfluid. At zero temperature, the correlation decay is intermediate between 1D and 2D decay, while at finite temperature, the decay is similar to a 1D decay at a much lower temperature. These provide the strongest evidence for the breakdown of the local density approximation (LDA) in trapped superfluid bosons. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D45.00012: Interference signatures of thermal and quantum phase fluctuations in the two dimensional Bose-Hubbard Model Mason Swanson, Yen Lee Loh, Nandini Trivedi Superfluidity in the Bose-Hubbard model is destroyed by the interplay of thermal and quantum phase fluctuations. In two dimensions, Berezinskii-Kosterlitz-Thouless theory predicts that deep in the superfluid phase quasi-long-range order is destroyed by the proliferation of thermally induced free vortices. As the Mott insulator regime is approached, the effect of quantum phase fluctuations must also be taken into account. By using a (2 + 1)-dimensional XY phase model, we investigate the signatures of thermal and quantum vortices in interference patterns. The possibility of extracting spatial and temporal correlation lengths from such interference images provides a new experimental probe for characterizing the state of ultracold atomic gases in 2D optical lattices. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D45.00013: The Fluctuation-Dissipation Theorem at Low Temperatures in a 2D Optical Lattice Eric Duchon, Yasuyuki Kato, Naoki Kawashima, Nandini Trivedi We calculate local density fluctuations and the local compressibility\footnote{Q. Zhou, et al., {\em Phys. Rev. Lett.} {\bf 103}, 085701 (2009).} of bosons in a two dimensional optical lattice as a function of temperature $T$ and the tuning parameter $U/t$, the on-site boson repulsion strength in units of hopping, using worldline Quantum Monte Carlo. Our numerical results, coupled with the quantum fluctuation-dissipation theorem applied locally, make significantly different predictions for direct simulations of lattice bosons in a harmonic trap versus simulations that treat the trap within a local density approximation, especially at low temperatures. We discuss implications of our results for local thermometry, equilibration and characterization of the quantum critical regime. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D45.00014: The Effects of Disorder on a Quasi-2D System of Ultracold Atoms Matthew Beeler, Matthew Reed, Tao Hong, Steven Rolston An ultra-cold gas of atoms can be used to create many different model Hamiltonians. When tightly confined in one spatial dimension, the gas can become effectively two-dimensional. At low temperature, a quasi-2D Bose gas undergoes a Berezinskii-Kosterlitz-Thouless phase transition to a superfluid, mediated by the binding and unbinding of vortex pairs. As disorder affects vortex transport properties, a slight amount of fine-grain disorder in the potential energy may alter the properties of this phase transition. We will present experimental observations of a 2D Bose gas of rubidium atoms in the presence of disorder created by a laser speckle field. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D45.00015: Atomic Fermi superfluids in a honeycomb optical lattice: Supercurrents and dynamical instabilities Shunji Tsuchiya, Ramachandran Ganesh, Arun Paramekanti Cold Fermi and Bose atoms on a honeycomb lattice have been of great recent interest given the possibility to simulate graphene physics and to realize interesting topological phases of matter.\footnote{A. H. Castro Neto et al., Rev. Mod. Phys. 81, 109 (2009).}$^,$\footnote{C. L. Kane and E. J. Mele, Phys. Rev. Lett, 95, 226801 (2005).}$^,$\footnote{A. Kitaev, Ann. Phys. (N.Y.) 321, 2 (2006).} We study the attractive Hubbard model of fermions on the honeycomb lattice in order to explore the strongly correlated superfluid state in this lattice geometry. We calculate the superfluid order parameter and collective modes in the presence of a superfluid flow in order to investigate the superflow stability. We find that the superfluid order parameter and density fluctuations exhibit nontrivial dependence on the flow, and these collective modes lead to novel dynamical instabilities. [Preview Abstract] |
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