Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Q1: Gapless Spin Liquids
Sponsoring Units: DCMPChair: Claudio Castelnovo, University of Oxford
Room: Ballroom A1
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q1.00001: Thermal-transport Studies of Quantum Spin Liquids Invited Speaker: Quantum spins, coupling antiferromagnetically on a 2D triangular lattice, cannot simultaneously satisfy all interactions. This frustrated situation is expected to give rise to mysterious fluid-like states of spins without long- range order, so called quantum spin liquid (QSL). The ground state of QSL and its exotic phenomena, such as fractionalized excitation with an artificial gauge field, have been extensively discussed for decades, yet to be identified by lack of any real materials. This is why the recent discoveries of materials possessing an ideal 2D triangular lattice have spurred a great deal of interest. To understand the nature of QSL, knowledge of the low-lying excitation, particularly the presence/absence of an excitation gap, is of primary importance. We employ thermal transport measurements on newly discovered QSL candidates, $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3 $ and EtMe$_3$Sb[Pd(dmit)$_2$]$_2$, and report that the two organic insulators possess different QSLs characterized by different elementary excitations. In $\kappa$-(BEDT-TTF)$_2 $Cu$_2$(CN)$_3$ [1], heat transport is thermally activated in low temperatures, suggesting presence of a spin gap in this QSL. In stark contrast, in EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ [2], a sizable temperature-linear term of thermal conductivity is clearly resolved in the zero-temperature limit, showing gapless excitation with long mean free path ($\sim$1,000 lattice distances), analogous to excitations near the Fermi surface in normal metals. These results are consistent with theoretical suggestions including 2D gapless spinons with a Fermi surface. This work was done in collaboration with N. Nakata, Y. Senshu, M. Nagata, Y. Kasahara, S. Fujimoto, T. Shibauchi, Y. Matsuda, T. Sasaki, N. Yoneyama, N. Kobayashi, H. M. Yamamoto and R. Kato. \\[4pt] [1] M. Yamashita \textit{et al.}, Nature Physics \textbf{5}, 44- 47 (2009). \newline [2] M. Yamashita \textit{et al.}, Science \textbf{328}, 1246 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q1.00002: Spin liquids, spin-orbit coupling, and band topology Invited Speaker: Much of the search for quantum spin liquids has focused on spin-rotationally invariant model Hamiltonians, appropriate for weakly spin-orbit coupled materials. With strong spin-orbit interactions, distinct theoretical approaches are required, and novel spin liquid states may occur. Some such candidate states will be discussed, along with the mechanisms leading to them. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q1.00003: Time-reversal symmetry breaking and spontaneous Hall effect without magnetic dipole order in Pr$_2$Ir$_2$O$_7$ Invited Speaker: An electric current flowing through a conductor in a magnetic field produces a transverse voltage drop known as the Hall effect. In the absence of the field, this effect also appears in ferromagnets in a plane normal to its spontaneous magnetization vector owing to the spin-orbit coupling. Generally, it may also detect a nontrivial order parameter breaking the time-reversal symmetry on a macroscopic scale, for example, scalar spin chirality. In this talk, we present our recent results in the study of the frustrated magnetism and Hall transport of the metallic pyrochlore magnet Pr$_2$Ir$_2$O$_7$.\footnote{S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. v. Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan, \textit{Phys. Rev. Lett.} \textbf{96}, 087204 (2006).}$^,$\footnote {Y. Machida, S. Nakatsuji, Y. Maeno, T. Tayama, T. Sakakibara, and S. Onoda, \textit{Phys. Rev. Lett.} \textbf{98}, 057203 (2007).} Strikingly, a spontaneous Hall effect is observed in the absence of both an external magnetic field and conventional magnetic long-range order.\footnote{Y. Machida, S. Nakatsuji, S. Onoda, T. Tayama, and T. Sakakibara, \textit{Nature} \textbf{463}, 210 (2010).} This strongly suggests the existence of a chiral spin liquid, a spin-liquid phase breaking the time-reversal symmetry. Both our measurements indicate that spin-ice correlations in the liquid phase lead to a non-coplanar spin texture forming a uniform but hidden order parameter: the spin chirality. Interesting phenomena seen under high field will also be discussed. This is the work performed in collaboration with Y. Machida, Y. Ohta, T. Sakakibara, T. Tayama, Y. Uwatoko (ISSP, Univ. of Tokyo), S. Onoda (Riken, Tokyo), L. Balicas (NHMFL), D. E. MacLaughlin (UC, Riverside) and C. Broholm (JHU). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q1.00004: Fractional spin textures in the frustrated magnet SCGO Invited Speaker: Spin liquids are remarkable states of matter that do not order magnetically even at very low temperatures and show collective phenomena like emergent gauge fields and topological order. Impurities can potentially reveal the underlying correlations in such states that appear deceptively featureless in their ground state properties. We consider the archetypal frustrated antiferromagnet SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$ (SCGO) in which Ga ions act as non-magnetic impurities in the magnetic lattice composed of Cr$^{3+}$ S=3/2 spins for disordered $p<1$ samples. We demonstrate that a spin in direct proximity to a pair of vacancies is cloaked by a spatially extended spin texture that encodes the correlations of the parent spin liquid. In this spin liquid regime, our analytic theory predicts that the combined object has a magnetic response identical to a classical spin of length S/2=3/4, which dominates over the small intrinsic susceptibility of the pure system. We calculate the full texture on the lattice in the spin liquid regime and check that it agrees well with Monte-Carlo simulations. This fractional-spin texture leaves an unmistakable imprint on the measured $^{71}$Ga nuclear magnetic resonance (NMR) lineshapes, which we compute using Monte-Carlo simulations and compare with experimental data. We also study the long-ranged interactions between these spin textures at low temperatures to gain a better understanding of the case of finite dilution in the parent spin liquid. [Preview Abstract] |
Session Q3: The Kavli Foundation Special Symposium: Superconductivity Centennial: Future Research Opportunities
Sponsoring Units: DCMP DMPChair: Zachary Fisk, University of California, Irvine
Room: Ballroom A3
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q3.00001: Superconductivity: A Continuous Surprise or ``How I Learned to Love the Surprises'' Invited Speaker: When I was invited to give this talk, I thought I would talk about new materials, or new opportunities, or the electron-phonon interaction, or non electron phonon interactions. Then when I looked at the speakers in this symposium and the subjects that they were scheduled to talk about, I realized that they were better equipped to talk individually about each of these subjects than I was, and it was my role to introduce the subject and give a perspective on the future of research and applications in superconductivity. My experience over the past decades has been that it is very dangerous to make predictions in this field. There are many examples of our distinguished colleagues who have engaged in this practice only to be proven wrong by surprise after surprise Fortunately, this has not harmed their reputations or their courage to continue to make predictions. In this talk I will reflect on my personal experience over several decades of research in superconductivity and how my own thinking has changed. Hopefully, I will \begin{enumerate} \item Transmit a flavor of this field and stimulate other younger investigators to be adventuresome \item Prepare the audience for the talks that follow \item Motivate the following speakers to reveal their own predictions and surprises. \end{enumerate} [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q3.00002: Research opportunities in new superconducting materials Invited Speaker: Since the discovery of superconductivity 100 years ago, new superconducting materials have rarely successfully been designed, with almost every new superconductor being discovered serendipitously. Through the years we have developed a variety of guidelines based on observations, but some of these guidelines remain disparate - such as, reduced dimensionality seems to give rise to higher Tc but isotropic materials would be better for applications. It is encouraging to note that this is an area where physicists, chemists, and material scientists seamlessly work together without boundaries, and ideas between groups are exchanged freely. Just as the theory of superconductivity and its development has had a major impact on how we do theoretical physics, our approach, with consilience, in searching for ``better'' superconductors may change the way we do experimental physics. I will present some of our guidelines and how our approach will help to provide exciting new research opportunities in superconducting materials. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q3.00003: Iron-based superconductors and relevant materials:progress and opportunity Invited Speaker: Iron, a representative magnetic element, was believed to the last constituent for emergence of superconductivity because long range magnetic ordering competes with the formation of Cooper pair requisite for superconductivity. However, once LaFeAs(O,F) with Tc=26K was discovered, many iron-pnictide (chalcogenide) superconducting materials have been found and the maximum Tc reached 56K, which is next to the high Tc cuprates exceeding MgB$_{2}$. I think there are two significances in discovery of iron-based superconductors. First, we realized that magnetic element is not a hateful enemy but a powerful friend to realize high Tc superconductors. Second it provides a large opportunity to find new high Tc materials because there exist several hundreds of layered compounds containing square lattice of transition metal cations taking tetrahedral coordination with non-oxide anions. We expect materials with higher Tc and/or novel class of superconductors would be hidden among these. To our interest, the crystal structure of 122 is the same as that of a representative heavy fermion superconductor CeCu$_{2}$T$_{2}$(T=Si,Ge). One may expect some clue to bridge these two superconducting systems would be found. What we have not to forget is a historical fact that most of ground-breaking materials including high Tc superconductors have been discovered by serendipity in the course of concentrated exploration effort. I am anticipating new material functions would be discovered as a result of concentrated material exploration with a help of theoretical modeling and advanced characterization. Iron is the most important element led to leap of civilization. I hope iron would serve as the same role in the history of superconductivity. \textit{Strike while the iron is hot}. I think this saying is still true for superconductivity research. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q3.00004: Exploring electron-phonon interactions in superconductors Invited Speaker: Superconductors can be roughly assigned to two classes. The first class contains materials in which the electronic pairing induced by electron-phonon interactions is the fundamental mechanism giving rise to the superconductivity. The BCS theory, together with its extensions, explains the properties of these superconductors extremely well . The second class is all other superconductors including the cuprate, Fe based, and heavy Fermion superconductors. Again, electronic pairing appears to be evident, but there is no consensus on the correct underlying theory of the superconducting mechanism at this time. I will discuss calculations for materials in the former class and describe the progress made in explaining and predicting their superconducting properties. I will emphasize calculations of the transition temperature and discuss some suggestions for raising the maximum transition temperature for materials in this class. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q3.00005: Electronic Pairing Interactions Invited Speaker: The heavy fermion, actinide, cuprate, iron-pnictide/chalcogenide and Bechgaard organic salts form a class of superconducting materials which are believed to share an electronic pairing mechanism. While the early electronic pairing interactions which were suggested involved charge fluctuations, it appears that for these materials it is the spin (and orbital) fluctuations that play a central role. Here I will discuss some of what is known about the electronic pairing interaction in this class of materials and conclude with some questions for future research. [Preview Abstract] |
Session Q4: Macromolecular Crowding Effects in the Cytoplasm
Sponsoring Units: DPOLY DBPChair: Arun Yetheraj, University of Wisconsin
Room: Ballroom A4
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q4.00001: Formation of protein-complexes in crowded environments: from in vitro to in vivo Invited Speaker: Rates of protein interactions are one to five orders of magnitude slower than the theoretically calculated collision rate of spheres of the same size. The rates can be increased by favorable electrostatic forces between the two proteins. Recent studies have established that the association reaction proceeds through transient complexes, which may be specific or diffusive in nature. To bring binding studies closer to the in vivo environment, we investigated the role of crowding on binding. For crowding we added various polymers to the solution, including Dextran and PEGs of different molecular weights. While crowding enhances oligomerization and polymerization of macromolecules, it has only a small effect on the binding rates and affinities of transient protein-protein interactions. We suggest that the limited effect of crowders, which is much bellow the expected from the increased viscosity of the solutions, is a result of the occluded volume effect in high crowder concentrations. Direct measurements of the stability of the encounter complex shows that crowders slow both k1 and k-1, resulting in an increased half-life of the encounter complex. High crowder concentrations also slow k2, suggesting an increased size of the encounter region. These results fit double-mutant cycle measurements on the activated complex, which suggest an increased size of the fruitful encounter region. These results are in line with the suggested occluded volume effect of crowders. We contrasted these with the effect of crowding on the weak binding pair CyPET-YPET. On this pair, aggregation, and not enhanced dimerization, was detected in PEG solutions. The results suggest that typical crowding agents have only a small effect on specific protein-protein dimerization reactions while promoting aggregation. To further validate these results, we performed real time binding assays in living cells, showing that even in the crowded cellular environment binding can be fast and specific. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q4.00002: Crowding effects on protein association Invited Speaker: The cell cytoplasm is a dense environment where the presence of inert cosolutes can significantly alter the rates of protein folding and protein association reactions. These crowding effects can either increase or decrease the rates of association reactions (or protein folding) depending on the nature of the crowding agents and the type of reaction. Our work aims to obtain a quantitative understanding of crowding effects. We present the first kinetic study of the effect of hard sphere crowding agents on protein association reactions where reactants and crowding agents are both hard spheres. If every collision results in a reaction, crowding always decreases the reaction rate but if the probability of a reaction is low then crowding increases the reaction rate. We find that the thermodynamics of crowding are relatively insensitive to interactions between the crowding agents suggesting that the hard sphere model of crowding agents has a surprisingly large regime of validity, and should be sufficient for a qualitative understanding of the thermodynamics of crowding effects. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q4.00003: Protein structure, stability and folding in the cell -- \textit{in silico} biophysical approaches Invited Speaker: How the crowded environment inside a cell affects the structural conformation of a protein with aspherical shape is a vital question because the geometry of proteins and protein-protein complexes are far from globules in vivo. Here we address this question by combining computational and experimental studies of several aspherical proteins (calmodulin, VlsE, and phosphoglycerate kinase) under crowded, cell-like conditions. The results show that macromolecular crowding affects protein folding dynamics, structures and functions. Our work demonstrates the malleability of ``native'' proteins and implies that crowding-induced shape changes may be important for protein function and malfunction in vivo. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q4.00004: Phosphoglycerate kinase in crowded and cellular environments Invited Speaker: We developed the temperature-jump fluorescence microscope to spatio-temporally resolve fast biomolecular kinetics and stability inside a single mammalian cell. We measured the reversible fast folding kinetics as well as folding thermodynamics of a fluorescent phosphoglycerate kinase construct in a bone marrow cell with subcellular resolution. The same instrument was also used to perform the comparative in vitro measurement in dilute buffer and crowded environments. Investigating an ensemble of cells, each cell has its own unique kinetic signature that can differ substantially from the in vitro result. Variations in the cytoplasmic environment are significant modulators of the protein energy landscape. We quantitate these variations with a statistical analysis of multiple cells and compare folding dynamics on the nm length scale with $\mu $m length scale diffusion processes. Cytoplasmic energy landscape modulation may be a candidate for non-genetic regulation of proteins but also challenges protein homeostasis. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q4.00005: Crowding and hydrodynamic interactions likely dominate in vivo macromolecular motion Invited Speaker: To begin to elucidate the principles of intermolecular dynamics in the crowded environment of cells, employing brownian dynamics (BD) simulations, we examined possible mechanism(s) responsible for the great reduction in diffusion constants of macromolecules in vivo from that at infinite dilution. In an Escherichia coli cytoplasm model comprised of 15 different macromolecule types at physiological concentrations, BD simulations of molecular-shaped and equivalent sphere representations were performed with a soft repulsive potential. At cellular concentrations, the calculated diffusion constant of GFP is much larger than experiment, with no significant shape dependence. Next, using the equivalent sphere system, hydrodynamic interactions (HI) were considered. Without adjustable parameters, the in vivo experimental GFP diffusion constant was reproduced. Finally, the effects of nonspecific attractive interactions were examined. The reduction in diffusivity is very sensitive to macromolecular radius with the motion of the largest macromolecules dramatically slowed down; this is not seen if HI dominate. In addition, long-lived clusters involving the largest macromolecules form if attractions dominate, whereas HI give rise to significant, size independent intermolecular dynamic correlations. These qualitative differences provide a testable means of differentiating the importance of HI vs. nonspecific attractive interactions on macromolecular motion in cells. [Preview Abstract] |
Session Q5: The Physics of Confronting Weapons of Mass Destruction: Chemical, Biological and Nuclear
Sponsoring Units: FIAPChair: Jay Davis, Hertz Foundation
Room: Ballroom C1
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q5.00001: Physics in the Confrontation of Nuclear Weapons Invited Speaker: Had the detonations on 9/11 involved nuclear explosives rather than jet fuel the number of deaths and the costs would have been multiplied by 100 or 1,000. This talk will briefly describe the nuclear threat and then focus on the technologies, both extant and evolving, for the detection and interdiction of clandestine trafficking of nuclear weapons and nuclear and radiological material. The methods vary from passive detection of heat, gamma radiation, neutrons, or other signatures from nuclear material, through radiological approaches to examine contents of vehicles and cargo containers, to active interrogation concepts that are under development. All of these methods have major physics components ranging from simple gamma ray detection as learned in a senior undergraduate lab to the latest ideas in muon production and acceleration. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q5.00002: Discrimination and classification of bio-aerosol particles using optical spectroscopy and scattering Invited Speaker: For more than a decade now, there has been significant emphasis for development of sensors of agent aerosols, especially for biological warfare (BW) agents. During this period, the Naval Research Laboratory (NRL) and other labs have explored the application of optical and spectroscopic methods relevant to biological composition discrimination to aerosol particle characterization. I will first briefly attempt to establish the connection between sensor performance metrics which are statistically determined, and aerosol particle measurements through the use of computational models, and also describe the challenge of ambient background characterization that would be needed to establish more reliable and deterministic sensor performance predictions. Greater attention will then be devoted to a discussion of basic particle properties and their measurement. The NRL effort has adopted an approach based on direct measurements on individual particles, principally of elastic scatter and laser-induced fluorescence (LIF), rather than populations of particles. The development of a LIF instrument using two sequential excitation wavelengths to detect fluorescence in discrete spectral bands will be described. Using this instrument, spectral characteristics of particles from a variety of biological materials including BW agent surrogates, as well as other ``calibration'' particles and some known ambient air constituents will be discussed in terms of the dependence of optical signatures on aerosol particle composition, size and incident laser fluence. Comparison of scattering and emission measurements from particles composed of widely different taxa, as well as from similar species under different growth conditions highlight the difficulties of establishing ground truth for complex biological material compositions. One aspect that is anticipated to provide greater insight to this type of particle classification capability is the development of a fundamental computational model of fluorescent emission for a particle of known composition but arbitrary size and shape. Finally if time permits, I will review the recent development and use of a 40 MHz mode-locked 524 nm laser source to evaluate the utility of sub-picosecond excitation of fluorescence with 2-photon absorption in biological aerosols. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q5.00003: Minimizing the bioterrorist threat: Fear, fancy, folly, and physics Invited Speaker: There can be little doubt that a bioterrorist attack represents one of the most significant dangers facing the Nation today. It is cheap, relatively easy to use, and can produce huge casualties and significant financial losses. Because of its apparent simplicity, there has been a great amount of attention directed towards developing means for early detection of an attack in progress. Most developmental funding for protective strategies comes from the Federal government: DoD is focused on protecting the military (personnel and facilities) whereas DoHS is most concerned with civilian response (police, triage, epidemiology, cure). Associated with such activities are some pretty amazing ideas both concerning the threat and means to detect its presence. These include the belief that certain bioterrorist attacks could equal the consequences of a nuclear weapon, that crop dusters with the proper agents could wipe out a city, that if it ``glows'' (i. e. is fluorescent) it must be an agent, or even that gravitational forces are actually far greater than believed. From the early warning side of things, the huge costs associated with a false positive call have resulted in the generally recognized need to identify before any alarm is triggered. Some consequences of this strategy have been the development of so-called smart chips and even handheld mass spectrometers! A brief review of some of these devices and the physics required for their success is discussed. An approach based upon some simple features of inverse scattering theory is proposed whereby identification may be unnecessary. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q5.00004: Physics and National Security: The Spectrum of Opportunities Invited Speaker: Physicists reflexively tend to approach their role in addressing national security problems by reaching for technologies and inventing devices or systems. While this is appropriate, the space for contribution is much larger. The application of technology is frequently constrained by doctrinal, operational, financial, and logistic constraints, not to mention those of security. The speaker, who has played a role in technology development, field operations, advisory processes, and policy considerations will discuss the role physicists can have in definition of the threat space, creation and assessment of technologies needed, and helping with response and recovery. In particular, he will address the opportunities and risks for both academic and industrial physicists in trying to assist in these matters, i.e., when in career to address such problems, what are the obstacles or consequences, and when do you know you have contributed? [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q5.00005: Detecting Chemical Weapons: Threats, Requirements, Solutions, and Future Challenges Invited Speaker: Although chemicals have been reportedly used as weapons for thousands of years, it was not until 1915 at Ypres, France that an industrial chemical, chlorine, was used in World War I as an offensive weapon in significant quantity, causing mass casualties. From that point until today the development, detection, production and protection from chemical weapons has be an organized endeavor of many of the world's armed forces and in more recent times, non-governmental terrorist organizations. The number of Chemical Warfare Agents (CWAs) has steadily increased as research into more toxic substances continued for most of the 20$^{\rm th}$ century. Today there are over 70 substances including harassing agents like tear gas, incapacitating agents, and lethal agents like blister, blood, chocking, and nerve agents. The requirements for detecting chemical weapons vary depending on the context in which they are encountered and the concept of operation of the organization deploying the detection equipment. The US DoD, for example, has as a requirement, that US forces be able to continue their mission, even in the event of a chemical attack. This places stringent requirements on detection equipment. It must be lightweight ($<$2 lbs), detect a large array of chemical warfare agents and toxic industrial chemicals, detect and warn at concentration levels and time duration to prevent acute health effects, meet military ruggedness specifications and work over a wide range of temperature and humidity, and have a very high probability of detection with a similarly low probability of false positives. The current technology of choice to meet these stringent requirements is Ion Mobility Spectrometry. Many technologies are capable of detecting chemicals at the trace levels required and have been extensively developed for this application, including, but not limited to: mass spectroscopy, IR spectroscopy, RAMAN spectroscopy, MEMs micro-cantilever sensors, surface acoustic wave sensors, differential mobility spectrometry, and amplifying fluorescence polymers. In the future the requirements for detection equipment will continue to become even more stringent. The continuing increase in the sheer number of threats that will need to be detected, the development of binary agents requiring that even the precursor chemicals be detected, the development of new types of agents unlike any of the current chemistries, and the expansion of the list of toxic industrial chemical will require new techniques with higher specificity and more sensitivity. [Preview Abstract] |
Session Q6: Hydrogen Storage Materials
Sponsoring Units: DCOMPChair: Mei-Yin Chou, Georgia Institute of Technology
Room: Ballroom C2
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q6.00001: First-Principles Prediction of Crystal Structures, Reaction Pathways, and Intermediate Products in Hydrogen Storage Reactions Invited Speaker: Practical hydrogen storage for mobile applications requires materials that exhibit high hydrogen densities, low decomposition temperatures, and fast kinetics for absorption and desorption. Unfortunately, no reversible materials are currently known that possess all of these attributes. Here we present an overview of our recent efforts aimed at developing a first-principles computational approach to the discovery of novel hydrogen storage materials. We have developed computational tools which enable accurate prediction of decomposition thermodynamics, crystal structures for unknown hydrides, and thermodynamically preferred decomposition pathways. We present examples that illustrate each of these three capabilities. Specifically, we focus on recent work on crystal structure and dehydriding reactions of (i) borohydride materials, such as Ca(BH$_{4})_{2}$ and Mg(BH$_{4})_{2}$, (ii) amidoboranes and their decomposition products, and (iii) mixtures of complex hydrides. \\[4pt] [1] J. Yang, A. Sudik, C. Wolverton, and D. J. Siegel, Chem. Soc. Rev. \textbf{39}, 656 (2010). \\[0pt] [2] D. E. Farrell, D. Shin, and C. Wolverton, Phys. Rev. B \textbf{80}, 224201 (2009). \\[0pt] [3] C. Weidenthaler, A. Pommerin, M. Felderhoff, W. Sun, C. Wolverton, B. Bogdanovic, and F. Schuth, J. Amer. Chem. Soc. \textbf{131}, 16735 (2009). \\[0pt] [4] A. R. Akbarzadeh, C. Wolverton, and V. Ozolins, Phys. Rev. B \textbf{79}, 184102 (2009). \\[0pt] [5] Y. S. Lee, Y. Kim, Y. W. Cho, D. Shapiro, C. Wolverton, and V. Ozolins, Phys. Rev. B \textbf{79}, 104107 (2009). \\[0pt] [6] V. Ozolins, E. H. Majzoub, and C. Wolverton, J. Amer. Chem. Soc. \textbf{131}, 230 (2009). \\[0pt] [7] V. Ozolins, E. H. Majzoub, and C. Wolverton, Phys. Rev. Lett. \textbf{100}, 135501 (2008). \\[0pt] [8] Y. Zhang, E. Majzoub, V. Ozolins, and C. Wolverton, Phys. Rev. B 82, 174107 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q6.00002: Towards predictor based design of thermodynamic and kinetic properties of complex materials for hydrogen storage Invited Speaker: A calculational approach for the design of new complex materials for hydrogen storage with favorable thermodynamic stability and enhanced diffusion kinetics is presented. By combining density functional theory (DFT) calculations on stable crystal structures and local coordination models with database methods, we perform large-scale screening studies to determine a number of potential alloys/mixtures with favorable thermodynamic stabilities and identify simple descriptors for subsequent materials prediction. Predictors for the kinetic properties of the materials are derived from combining materials screening with path techniques and harmonic transition state theory (TST) to indentify materials parameters, e.g. the hydrogen binding energy, which correlate with the macroscopic diffusion rates. These predictors are then used to design new alloy/mixture compositions and ratios to favor structures with optimal diffusion kinetics. We present results from binary and ternary alkali-transition metal borohydrides and Perovskite based hydrogen permeable membranes, as well as results from studies of binary and mixed metal ammines. Results from the modeling of pathways and rates of dynamical processes involved in the ab-/desorption mechanisms will also be presented and compared to quasi elastic neutron scattering data. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q6.00003: Theory of molecular hydrogen sorption for hydrogen storage Invited Speaker: Molecular hydrogen (H$_{2})$ sorption has the advantage of fast kinetics and high reversibility. However, the binding strength is often too weak to be operative at near room temperatures. Research into such hydrogen sorption materials has branched into the study of pure van der Waals (vdW) physisorption and that of weak chemisorption (known to exist in the so-called Kubas complexes). In either case, however, theoretical tools to describe such weak interactions are underdeveloped with error bars that often exceed the strength of the interaction itself. We have used quantum-chemistry (QC) based approaches to benchmark the various available DFT methods for four classes of weak chemisorption systems [Sun et al., Phys. Rev. B \textbf{82}, 073401 (2010)]. These involve complexes containing Li, Ca, Sc, and Ti with increased strength of H$_{2}$ binding from predominantly vdW to mostly Kubas-like. The study reveals that most of the DFT functionals within the generalized gradient approximation underestimate the binding energy, oppose to overestimating it. The functionals that are easy to use yet yielding results reasonably close to those of accurate QC are the PBE and PW91. I will also discuss the effort of implementing vdW interaction into the currently available density functional methods [Sun, J. Chem. Phys. \textbf{129}, 154102 (2008)]. The rationale is that while the true vdW is an electron-electron correlation, a DFT plus classical dispersion approach may be too simple and unnecessary within the DFT. A local pseudopotential approach has been developed to account for the core part of the polarizability of the elements. Applications to a number of benchmark systems yield good agreement with QC calculations. The application of this method and the QC methods to vdW hydrogen binding will also be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q6.00004: Point-defect-mediated dehydrogenation of alane Invited Speaker: For the engineering of better hydrogen storage materials a systematic understanding of their hydrogen sorption kinetics is crucial. Theoretical studies on metal hydrides have indicated that in many cases point defects control mass transport and hence hydrogen uptake and release. Manipulating point-defect concentrations thus allows control over hydrogen sorption kinetics, opening up new engineering strategies. However, in some cases the relevance of kinetic limitations due to point defects is still under debate; kinetic inhibition of hydrogen sorption has also been attributed to surface effects, e.g. oxide layers or low recombination rates. We present a systematic analysis of the dehydrogenation kinetics of alane (AlH3), one of the prime candidate materials for hydrogen storage. Using hybrid-density functional calculations we determine the concentrations and mobilities of point defects and their complexes. Kinetic Monte Carlo simulations are used to describe the full dehydrogenation reaction. We show that under dehydrogenation conditions charged hydrogen vacancy defects form in the crystal, which have a strong tendency towards clustering. The vacancy clusters denote local nuclei of Al phase, and the growth of these nuclei eventually drives the AlH3/Al transformation. However, the low concentration of vacancy defects limits the transport of hydrogen across the bulk, and hence acts as the rate-limiting part of the process. The dehydrogenation is therefore essentially inactive at room temperature, explaining why AlH3 is metastable for years, even though it is thermodynamically unstable. Our derived activation energy and dehydrogenation curves are in excellent agreement with the experimental data, providing evidence for the relevance of bulk point-defect kinetics. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q6.00005: Kinetics of hydrogen transport in metal hydrides, crystalline alloys, and amorphous metals Invited Speaker: The diffusion of hydrogen is critical in the kinetics of hydrogen uptake and release in metal hydrides and in membrane-based approaches to hydrogen purification. First principles calculations have become a valuable counterpart to experimental methods to study hydrogen diffusion. Examples will be presented of using first principles calculations to understand hydrogen diffusion in a diverse range of solid materials, including metal hydrides in their bulk state and near interfaces, crystalline alloys for membrane applications, and amorphous metals for membrane applications. [Preview Abstract] |
Session Q7: System Biology III: The Physics of Evolution
Sponsoring Units: DBP DCPChair: Wouter Hoff, Oklahoma State University
Room: Ballroom C3
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q7.00001: Does Tumor Development Follow a Programmed Path? Invited Speaker: The initiation and progression of a tumor is a complex process, resembling the growth of a embryo in terms of the stages of development and increasing differentiation and somatic evolution of constituent cells in the community of cells that constitute the tumor. Typically we view cancer cells as rogue individuals violating the rules of the games played within an organism, but I would suggest that what we see is a programmed and algorithmic process. I will then question If tumor progression is dominated by the random acquisition of successive survival traits, or by a systematic and sequential unpacking of ``weapons'' from a pre-adapted ``toolkit'' of genetic and epigenetic potentialities? Can we then address this hypothesis by data mining solid tumors layer by layer? [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q7.00002: Adaptation Driven by Spatial Heterogeneities Invited Speaker: Biological evolution and ecology are intimately linked, because the reproductive success or ``fitness'' of an organism depends crucially on its ecosystem. Yet, most models of evolution (or population genetics) consider homogeneous, fixed-size populations subjected to a constant selection pressure. To move one step beyond such ``mean field'' descriptions, we discuss stochastic models of evolution driven by spatial heterogeneity. We imagine a population whose range is limited by a spatially varying environmental parameter, such as a temperature or the concentration of an antibiotic drug. Individuals in the population replicate, die and migrate stochastically. Also, by mutation, they can adapt to the environmental stress and expand their range. This way, adaptation and niche expansion go hand in hand. This mode of evolution is qualitatively different from the usual notion of a population climbing a fitness gradient. We analytically calculate the rate of adaptation by solving a first passage time problem. Interestingly, the joint effects of reproduction, death, mutation and migration result in two distinct parameter regimes depending on the relative time scales of mutation and migration. We argue that the proposed scenario may be relevant for the rapid evolution of antibiotic resistance. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q7.00003: Experimental Ignition of Evolution on Fitness Landscapes Invited Speaker: Microbiologist are starting to revise the single cell centered view of evolution to a multicellular view, considering it at entire population scale, and even whole ecosystem. Indeed, as Darwin recognized on the Galapagos Island, evolution of a community of bacteria is strongly influenced by the global spatial stress and depends of the neighboring communities. This collective dynamical process can be studied using micro-nanofabricated landscape to create stressed conditions. Our microfluidic device consists of interconnected chambers in 2D hexagonal geometries. The design of our ecology enables to combine gradients of antibiotic and nutrient, thus generating population gradient and motion of bacteria across them. We study here evolution of resistance to the antibiotic ciprofloxacin in highly-stressed conditions. Non-random mutations are induced in the collectivity to develop resistance to the antibiotic. Channels between microhabitats allow motion of bacteria between different islands, and once evolution is ignited in a local metapopulation, a very fast spread through the collectivity happens. In such environments, evolution is observed in typical time scales of few hours. Knowing the complexity of natural world, we believe that our approach provide a model to understand the rapid emergence of resistance to antibiotic and its spread in the entire population. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q7.00004: Towards a Quantitative Endogenous Network Theory of Cancer Genesis and Progression: beyond ``cancer as diseases of genome'' Invited Speaker: There has been a tremendous progress in cancer research. However, it appears the current dominant cancer research framework of regarding cancer as diseases of genome leads impasse. Naturally questions have been asked that whether it is possible to develop alternative frameworks such that they can connect both to mutations and other genetic/genomic effects and to environmental factors. Furthermore, such framework can be made quantitative and with predictions experimentally testable. In this talk, I will present a positive answer to this calling. I will explain on our construction of endogenous network theory based on molecular-cellular agencies as dynamical variable. Such cancer theory explicitly demonstrates a profound connection to many fundamental concepts in physics, as such stochastic non-equilibrium processes, ``energy'' landscape, metastability, etc. It suggests that neneath cancer's daunting complexity may lie a simplicity that gives grounds for hope. The rationales behind such theory, its predictions, and its initial experimental verifications will be presented. \\[4pt] References:\\[0pt] [1] Cancer as Robust Intrinsic State of Endogenous Molecular-Cellular Network Shaped by Evolution. P. Ao, D. Galas, L. Hood, X.-M. Zhu, Medical Hypotheses \textbf{70} (2008) 678-684. http://dx.doi.org/10.1016/j.mehy.2007.03.043\\[0pt] [2] Towards Predictive Stochastic Dynamical Modeling of Cancer Genesis and Progression. P. Ao, D. Galas, L. Hood, L.Yin, X.M.Zhu. Interdiscip Sci Comput Life Sci (2010) 2: 140-144 DOI: 10.1007/s12539-010-0072-3\\[0pt] [3] Orders of Magnitude Change in Phenotype Rate Caused by Mutations. P. Ao, Cellular Oncology (2007) \textbf{29}: 67-69. http://arxiv.org/PS\_cache/arxiv/pdf/0704/0704.0429v1.pdf [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q7.00005: Understanding the distribution of fitness effects of mutations by a biophysical-organismal approach Invited Speaker: The distribution of fitness effects of mutations is central to many questions in evolutionary biology. However, it remains poorly understood, primarily due to the fact that a fundamental connection that exists between the fitness of organisms and molecular properties of proteins encoded by their genomes is largely overlooked by traditional research approaches. Past efforts to breach this gap followed the ``evolution first'' paradigm, whereby populations were subjected to selection under certain conditions, and mutations which emerged in adapted populations were analyzed using genomic approaches. The results obtained in the framework of this approach, while often useful, are not easily interpretable because mutations get fixed due to a convolution of multiple causes. We have undertaken a conceptually opposite strategy: Mutations with known biophysical and biochemical effects on E. coli's essential proteins (based on computational analysis and in vitro measurements) were introduced into the organism's chromosome and the resulted fitness effects were monitored. Studying the distribution of fitness effects of such fully controlled replacements revealed a very complex fitness landscape, where impact of the microscopic properties of the mutated proteins (folding, stability, and function) is modulated on a macroscopic, whole genome level. Furthermore, the magnitude of the cellular response to the introduced mutations seems to depend on the thermodynamic status of the mutant. [Preview Abstract] |
Session Q8: New Developments in Organic Spintronics
Sponsoring Units: GMAGChair: Valy Vardeny, University of Utah
Room: Ballroom C4
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q8.00001: Spin Injection/detection using organic-based magnetic semiconductor Invited Speaker: This abstract not available. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q8.00002: Magnetic Field Effect in Organic Devices: the Role of Hyperfine, Exchange and Spin Orbit Interactions Invited Speaker: Recently we have observed a novel phenomenon in both magneto-electroluminescence (MEL) and magneto-conductance (MC) in a variety of organic light emitting diodes that consists of a sign reversal at very small magnetic fields ($B\le $1 mT), dubbed hereafter ultra-small magnetic field effect (USMFE) [1]. Similar response has been obtained in MC($B)$ of unipolar organic diodes [2]. As $B$ is reduced below the zero crossing field, the magnitude of the obtained MEL and MC increases to a maximum value at $B=B_{m}$, before diminishing at $B$=0. We found that $B_{m}$ is isotope dependent: it is lower when the protons in the organic material are replaced by deuterons having a smaller nuclear magnetic moment and reduced hyperfine interaction (HFI), and is higher when the $^{12}$C atoms (nuclear spin I=0, no HFI) are replaced by $^{13}$C atoms (I=$\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, with finite HFI). We also found that $B_{m}$ scales with the half width at half maximum, $\Delta $B, of the high field response. From the MEL($B)$ and MC($B)$ responses, the marked isotope effect, and voltage and temperature dependencies we explain the USMFE as well as the width $\Delta $B, as due to loosely coupled pairs of polarons (either with same or opposite charges) of which spins are intermixed via the HFI [1,2]. The model captures the sign reversal and its dependence on the HFI strength. The role of the HFI anisotropy, exchange interaction between the polaron pair spins, and spin orbit interaction effect on the USMFE will be discussed. *Supported by the Israel Science Foundation grant 745/08, and NSF grant DMR 08-03325. **In collaboration with T. D. Nguyen, B. R. Gautam, and Z. V. Vardeny, University of Utah. \\[4pt] [1] T. D. Nguyen, G. Hukic-Markosian, F. Wang, L. Wojcik, X-G. Li, E. Ehrenfreund, Z. V. Vardeny, \textit{Nature Materials} \textbf{9}, 345 (2010). \\[0pt] [2] T. D. Nguyen, B. R. Gautam, E. Ehrenfreund, Z. V. Vardeny, Phys. Rev. Lett. \textbf{105}, 166804 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q8.00003: Molcular spntronics: tailoring spin polarization with molecules Invited Speaker: Organic/molecular spintronics is a rising research field at the frontier between spintronics and organic chemistry. Organic molecules and semiconductors were first seen as promising for spintronics devices due to the expected long spin lifetime. An exciting challenge is now to find opportunities arising from chemistry to develop new spintronics functionalities that were unavailable with inorganic materials. Here one can hope to control the spin dependent transport by using the chemical versatility brought by molecules and molecular engineering. Starting from the use of Alq3 and Phthalocyanine molecules we will show how the ferromagnetic metal/molecule hybridization can strongly influence the interfacial spin properties: from spin polarization enhancement to its sign control. CNRS/Thales team: C. Barraud, P. Seneor, R. Mattana, S. Tatay, K. Bouzehouane, S. Fusil, C. Deranlot, F. Petroff, A. Fert in collaboration with ISMN, Bologna, Italy \& IPCMS, Strasbourg, France [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q8.00004: Spin relaxation in organic semiconductors Invited Speaker: Intriguing magnetic field effects in organic semiconductor devices have been reported: anomalous magnetoresistance in organic spin valves and large effects of small magnetic fields on the current and luminescence of organic light-emitting diodes. Influences of isotopic substitution on these effects points at the role of hyperfine coupling. We performed studies of spin relaxation in organic semiconductors based on (i) coherent spin precession of the electron spin in an effective magnetic field consisting of a random hyperfine field and an applied magnetic field and (ii) incoherent hopping of charges. These ingredients are incorporated in a stochastic Liouville equation for the dynamics of the spin density matrix of single charges as well as pairs of charges. For single charges we find a spin diffusion length that depends on the magnetic field, explaining anomalous magnetoresistance in organic spin valves. For pairs of charges we show that the magnetic field influences formation of singlet bipolarons, in the case of like charges, and singlet and triplet excitons, in the case of opposite charges. We can reproduce different line shapes of reported magnetic field effects, including recently found effects at ultra-small fields. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 2:15PM |
Q8.00005: Coherent spin spectroscopy in organic thin film semiconductor devices Invited Speaker: With the emergence of organic spintronics and renewed interest in magnetoresistive effects, there is much need to illuminate the properties of spins in molecular electronic materials. Examples include spin-relaxation times, spectral diffusion times, spin dephasing times and spin interactions. In this presentation, an overview is given about the concepts of pulsed, electrically and optically detected magnetic resonance spectroscopy as techniques to manipulate and observe and thus characterize these fundamental properties of electron and nuclear spins in organic semiconductors [1]. By coherent (pulsed) magnetic resonant perturbation of spin states one may cause the spins to coherently propagate in a defined manner [2]. Spin-dependent charge carrier-transport or -recombination allow the observation of this coherent spin motion through electrical or optical measurements in working devices, such as organic light-emitting diodes. The ubiquitous presence of hydrogen nuclei gives rise to strong hyperfine interactions, which appear to provide the basis for many of the magnetoresistive effects observed in these materials. Since hyperfine coupling influences resonantly driven quantum spin beating in electrically or optically detectable electron-hole pairs, an extraordinarily sensitive probe for hyperfine fields in such pairs is given [3]. This allows scrutinizing the various existing models for these electronic processes. Qualitative as much as quantitative insights are gained into some of the physical intricacies of organic semiconductor device fabrication such as the influence of contact materials on spin-orbit coupling. \\[4pt] [1] D. R. McCamey, et al. Nature Mat. 7, 723, (2008). \\[0pt] [2] C. Boehme et al. Phys. Stat. Sol B. 246, 11-12, 2750 (2009).\\[0pt] [3] D. R. McCamey, et al. Phys. Rev. Lett. 104, 017601 (2010). [Preview Abstract] |
Session Q9: Fluid Dynamics at Interfaces
Sponsoring Units: DFDChair: Takeru Yano Osaka University, and Angbo Fang, Hong King University of Science and Technology
Room: D220
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q9.00001: Slosh dynamics and rebound suppression of a partially filled sphere Taylor Killian, Robert Klaus, Tadd Truscott We introduce a study on the slosh dynamics of a partially filled elastic sphere. Currently the physical design of fluid-filled containers utilizes clever construction and machinery to mitigate sloshing motions. There are numerous cases that have been observed but we focus on the impact of a sphere under free fall with an initially undisturbed free surface. The study focuses on measurement and simulation of the force distribution between the fluid and the sphere through the use of high-speed imaging and finite element analysis. Using the cavity shape data, a potential flow numerical model is developed that predicts the unsteady forces. Our hypothesis is that the sphere's movements can be counteracted or cancelled by the exchange of energy between the sphere and the fluid. Forces are modulated by the formation of a parabolic cavity in the fluid, formed after the first impact. The second impact results in a collapse of this cavity forming a powerful jet which effectively dampens the motion of the sphere. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q9.00002: Shrinking Instabilities of Toroidal Liquid Droplets in The Stokes Flow Regime Zhenwei Yao, Mark Bowick We analyze the stability and dynamics of toroidal liquid droplets. In addition to the Rayleigh instabilities akin to those of a cylindrical droplet there is a shrinking instability that is unique to the topology of the torus and dominates in the limit that the aspect ratio is near one (fat tori). We first find an analytic expression for the pressure distribution inside the droplet. We then determine the velocity field in the bulk fluid, in the Stokes flow regime, by solving the biharmonic equation for the stream function. The flow pattern in the external fluid is analyzed qualitatively by exploiting symmetries. This elucidates the detailed nature of the shrinking mode and the swelling of the cross-section following from incompressibility. Finally the shrinking rate of fat toroidal droplets is derived by energy conservation. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q9.00003: Normal elasticity of liquid bridge by atomic force microscope Bongsu Kim, Wonho Jhe The quartz tuning-fork based atomic force microscope (QTF-AFM) has previousely been established as a suitable measurement technique for investigating liquid bridges. By operating a QTF-AFM in the non-contact tapping mode, we are able to measure the normal elasticity of liquid bridges that are formed via capillary condensation or that result from an adsorbed liquid layer. Elasticity, a property typically associated with solids, is studied here for the case of the nano-scale water bridge. We present results that add to our understanding of the origin of the elasticity in nano liquid bridges. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q9.00004: Droplet impact and the dynamics of rapidly moving contact lines Shmuel M. Rubinstein, John M. Kolinski, Shreyas Mandre, Lakshminarayanan Mahadevan, David A. Weitz When a~liquid drop approaches a flat solid surface, the air~beneath it is compressed,~flattening the bottom of the drop and~forcing initial contact to occur in a~ring-shape, trapping a pocket~of air in its center as two wetting fronts~rapidly expand both~outward and inwards to completely wet the surface. We combine total internal~reflection (TIR) microscopy with a~novel virtual frame technique~(VFT) to directly observe the sub-micron length~scales above a~solid surface as the drop approaches, impacts and then spreads~over it. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q9.00005: Interfacial Effects on Droplet Dynamics in Poiseuille Flow Jonathan Schwalbe, Kendra Erk, Jeffrey Martin, Petia Vlahovska, Steven Hudson Interfacial rheology governs many properties of emulsions, and here we report theory and experiments that account for and measure surface viscous and elastic forces. For the theoretical portion, Stokes flow is assumed in bulk phases and a jump in hydrodynamic stress at the interface is balanced by Marangoni and surface viscous forces according to the Boussinesq-Scriven constitutive law. Our model employs linear equation of state for the surfactant. Our analysis predicts slip, cross-stream migration and droplet-circulation velocities for a spherical drop in plane Poiseuille flow. These results and the corresponding interfacial parameters are separable: e.g., cross- stream migration occurs only if surfactant is present; slip velocity depends on viscosity contrast and dilatational Boussinesq number, but not shear Boussinesq number. Drop dynamics in plan Poiseuille flow are measured experimentally using microfluidics, particle velocimetry, and shape analysis. Several types of surfactant-stabilized aqueous drops in oil are examined and the interfacial properties depend on interfacial composition. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q9.00006: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q9.00007: Nonequilibrium molecular dynamics of vapor--liquid interface Takeru Yano Evaporation and condensation at a vapor--liquid interface are studied by moderately large-scale nonequilibrium molecular dynamics simulations for a vapor--liquid two phase system composed of about 0.3 million Lennard-Jones molecules. Constant evaporation and condensation are realized by driving two vapor regions on the either side of a planar liquid film, and thereby the simulation is free from artificial controls of molecular motions in the liquid film and in the neighborhood of the interfaces. This enables us to evaluate the mass, momentum, and energy fluxes across the system, which are relevant to the velocity distribution of molecules leaving the interface at the vapor--liquid nonequilibrium states. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q9.00008: Evaporation out of a 2D model soil Bertrand Selva, Remi Dreyfus Our goal is to improve our understanding of water transport in the soil-plant-atmosphere continuum. For this purpose, we focus on water losses due to evaporation at the soil surface. Such losses are known to be important at places where plants do not entirely cover the surface. Our model soil is a 2D porous medium with controlled wettability and humidity. It has been reported that evaporation is characterized by three stages: a first stage with a strong and constant evaporation flux, a second stage where mass transfer is dominated by diffusion mechanisms, and a third stage that occurs when the medium is almost empty. Here we focus on the first two stages and the transition between them which occurs when an intermediate unsaturated zone has reached its maximum width. This width strongly depends on the wettability distribution of the porous medium. In our experiments, we have explored a regime where gravity effects and capillary forces have similar contributions. In this particular regime we found that the first stage is characterized by a continuously decreasing evaporation flux and the second stage by usual diffusion transfer mechanisms. In order to understand this behavior, we have developed a model which allows us to predict the transition between the two stages and which is in agreement with the decreasing values of the first stage evaporation flux. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q9.00009: Effects of Sub-Phase Thickness on Interfacial Microrheology Paul Christophel Martin, Kenneth W. Desmond, Eric R. Weeks The interface between two fluids is known to have a rheological response. In our work, we study human serum albumin protein molecules (HSA) at an air-water interface. Prior experimental work showed that the ratio of the surface viscosity to the sub-phase ``bulk'' viscosity influences the rheology of the HSA interface. Recent theoretical work has shown that the thickness of the sub-phase h can also influence the rheology of the interface. The finite thickness of the sub-phase only becomes important once h is on the order of the ratio of the surface viscosity to the sub-phase ``bulk'' viscosity, which is on the order of 100 microns for an HSA-air-water interface. To characterize the interfacial rheology, we suspend tracer particles at the interface, measure their correlated motions, and investigate how the results depend on h for water layers O(100 microns) thick. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q9.00010: Probing single- and multi-phase flow at the pore level Sujit Datta, Amber Krummel, David Weitz We use a new experimental technique to study 3D flow behavior in a porous medium in situ with high spatiotemporal resolution. At the multi-pore level, we probe the fluid configurations resulting from two-phase flow conditions imposed upon the system and correlate these to bulk flow measurements. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q9.00011: Viscous flow and heat transfer in channels with structured walls Vladimir Ajaev, Steffen Hardt, Peter Stephan We develop a mathematical model of pressure-driven flow in channels with walls structured by arrays of parallel grooves filled with air or gas. Motivated by cooling applications, we study heat transfer from a heater embedded in the wall to the liquid. The flow in the liquid is described using a Stokes flow model, and thermocapillary effects due to presence of the liquid-gas interface segments in the grooves are also taken into account. The rate of heat transfer is determined by a competition of two physical effects: the insulating effect of the gas in the grooves, due to small thermal conductivity of the gas phase, and the reduction of the effective slip length at the channel wall due to the presence of the liquid-gas interface segments in the grooves. Criteria for heat transfer enhancement are formulated for different parameters of the structuring. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q9.00012: Utilizing an Automated Home-Built Surface Plasmon Resonance Apparatus to Investigate How Water Interacts with a Hydrophobic Surface Adele Poynor By definition hydrophobic substances hate water. Water placed on a hydrophobic surface will form a drop in order to minimize its contact area. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low- density region forms near the surface. We have employed an automated home-built Surface Plasmon Resonance (SPR) apparatus to investigate this boundary. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q9.00013: Quantification of slip at a liquid-solid interface -- a novel approach Aleks Ponjavic, Mourad Chennaoui, Janet Wong Much effort has been spent recently on experimentally proving the existence of interfacial slip of a Newtonian fluid. A constant limitation is the proximity to the surface at which the velocity of a fluid can be measured. A new technique is developed to maximise this proximity. The objective is to acquire velocity measurements of a fluid as close as possible to the liquid-solid interface while still using a direct method of observation. To ensure proximity to the surface the technique of photobleaching is adopted. Dye-doped water is pumped through a microfluidic channel. A short, intense pulse from a laser causes dye within the focal volume to bleach, creating a spot. The geometry of this spot evolves depending on the velocity profile of the fluid. By fitting the evolution of the spot with a Poiseuille velocity profile with slip the slip length is extracted. The hydrophobicity of the channel is varied by flowing silane through the channel prior to measurement, forming a self-assembled monolayer. Effects of shear rate and wettability on interfacial slip length are investigated. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q9.00014: Effects of Interfacial Translation-rotation Coupling for Confined Ferrofluids Angbo Fang Ferrofluids have wide applications ranging from semiconductor fabrications to biomedical processes. The hydrodynamic spin diffusion theory for ferrofluids has been successful in explaining many experimental data, but it suffers from some fatal flaws. For example, it fails to predict the incorrect flow direction for a ferrofluid confined in a concentric cylinder channel in the presence of a rotating magnetic field. In this work we develop a method to establish the general hydrodynamic boundary conditions (BCs) for micro-polar fluids such as ferrofluids. Through a dynamic generalization of the mesoscopic diffuse interface model, we are able to obtain the surface dissipation functional, in which the interfacial translation-rotation coupling plays a significant role. The generalized hydrodynamic BCs can be obtained straightforwardly by using Onsager's variational approach. The resulted velocity profile and other quantities compares well with the experimental data, strikingly different from traditional theories. The methodology can be applied to study the hydrodynamic behavior of other structured fluids in confined channels or multi-phase flows. [Preview Abstract] |
Session Q10: Chemisorption and Surface Reactions
Sponsoring Units: DCMPChair: Yevgeniy Puzyrev, Vanderbilt University
Room: D221
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q10.00001: Catalytic Reactions of DNT and TNT Molecules on Porphyrin Complexes Keith Warnick, Bin Wang, Sokrates Pantelides Reactions of molecules with substrates can be used to identify them, as in sensor applications. Here we examine reactions of DNT and TNT molecules on porphyrin and metal-porphyrin via first-principles DFT calculations. We find that the oxidation of DNT by O$_{2}$ using Fe-porphyrin as a catalyst is exothermic. The affinity of O$_{2}$ to Fe-porphyrin weakens the O$_{2}$ intramolecular bond, which lowers the oxidation reaction barrier is lowered by $\sim$1 eV. Substrate effects on this process are accounted for. One way to use this selective oxidation reaction for DNT/TNT sensor applications is to exploit the metal-semiconductor transition in thin-film VO$_{2}$ to detect the energy deposited by the exothermic reaction between the adsorbed molecules. This work was supported in part by DTRA grant HDTRA1-10-0047. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q10.00002: Reactivity of TiO$_{2}$ Rutile and Anatase Surfaces toward Nitroaromatics Shao-Chun Li, Ulrike Diebold The Au-TiO$_{2}$ system is a promising catalyst for the synthesis of nitro-aromatic compounds. The adsorption of azobenzene (C$_{6}$H$_{5}$N-NH$_{5}$C$_{6})$ and aniline (C$_{6}$H$_{5}$NH$_{2})$ on two single-crystalline TiO$_{2}$ surfaces, anatase (101) and rutile (110), has been investigated with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoemission spectroscopy (XPS), and synchrotron Ultraviolet photoemission (UPS). While azobenzene adsorbs as an intact molecule at low coverages, ordered overlayers of phenyl imide (C$_{6}$H$_{5}$N) form at saturation coverage, indicating that TiO$_{2}$ surfaces cleave the N-N bond even without the presence of Au. The same superstructures, p(1 $\times $ 2) on anatase and c(2 $\times $ 2) on rutile and the same electronic structures, form upon adsorption of aniline, suggesting the formation of the same, or a very similar, reaction intermediate. These results suggest that the main role of the supported Au in catalytic aniline $\leftrightarrow $ azobenzene conversion is the activation of O$_{2}$/H$_{2}$ for de/hydrogenation reactions. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q10.00003: The role of subsurface oxygen in the selectivity enhancement of ethylene epoxidation on Ag-Cu Catalysts Ngoc Linh Nguyen, Stefano de Gironcoli, Simone Piccinin The role of subsurface oxygen on the Ag-Cu alloy catalysts for the ethylene epoxidation reaction has been studied by means of first principles Density Functional Theory (DFT) calculations. We find that in presence of oxygen and ethylene reactants, the subsurface oxygen adsorption is energetically favorable on fcc sites under the thin oxide-like CuO layer formed at the catalyst surface. On this substrate the reaction proceeds via the formation of a common oxametallacycle precursor. The calculated activation energies show favorable energetics for the pathway leading to the formation of the desired product, ethylene oxide, with respect to the one leading to the formation of the undesired product, acetaldehyde, while the opposite order is obtained on pure Ag catalyst. These findings provide an understanding, at the atomistic level, of the selectivity enhancement of Ag-Cu alloy with respect to pure Ag catalysts. Furthermore, we find that under temperature and partial pressure conditions close to the experimental ones, the ethylenedioxy intermediate is present on the phase diagram of Ag-Cu (111) surface. Our calculations indicate, however, that the formation of this structure could poison the catalyst surface. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q10.00004: Surface defects activate new reaction paths: formation of formate during methanol oxidation on defective Ru(0001) I. Palacio, O. Rodr\'Iguez de la Fuente An optimum understanding of the existing molecular mechanisms taking place while reactions occur on surfaces, should preferably be based on a correct identification of the intermediate species and the reaction paths, so to avoid trial-and-error approaches. Otherwise, a good control of the chemical activity is not easily attainable. We have adsorbed methanol on Ru(0001), with surfaces having a variable density of defects. In this way, with Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED) and Infrared Reflection-Absorption Spectroscopy (IRAS) we have identified reaction paths in the methanol/Ru(0001) system. While the sole methanol adsorption leads to its complete dehydrogenation towards CO, we show that oxygen coadsorption stabilizes intermediate products, namely methoxy (CH$_{3}$O), formaldehyde (CH$_{2}$O) and formyl (CHO). We show as well that a new reaction path appears just on the defective surface: the formation of formate (HCOO). The presence of the defects (mainly steps) catalyzes the oxidation of formaldehyde to formate. This particular case shows how surface defects profoundly affect the catalytic activity, opening new reaction channels which are not available when the density of defects is low. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q10.00005: Dynamics of Low-Energy Electron Induced Reactions in Condensed Michael Boyer, ChanMyaeMyae Soe, Kristal Chamberlain, Yomay Shyur, Christopher Arumainayagam We present insights into the dynamics of low-energy electron-induced reactions in thin films of methanol (CH$_{3}$OH). Low-energy electrons in matter can initiate chemical reactions though electron impact ionization of a molecule, electron impact excitation of a molecule, or through dissociation of a transient negative ion formed by electron attachment to a molecule. Our studies focus on the dynamics by which low-energy electron interaction with condensed methanol initiates chemical reactions which lead to the formation of methoxymethanol (CH$_{3}$OCH$_{2}$OH) and ethylene glycol (HOCH$_{2}$CH$_{2}$OH). The results of our post-irradiation temperature programmed desorption experiments indicate that both products can form from irradiating methanol with electrons at subionization energies. In addition, we find evidence that dissociative electron attachment plays a role in the formation of methoxymethanol but not in ethylene glycol. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q10.00006: X-ray Induced Reorganization/Polymerization of Resorcinol on the TiO$_{2}$ Rutile (110) Surface Vinod Kumar Paliwal, Shao-Chun Li, Ulrike Diebold The room-temperature adsorption of resorcinol (1, 3 benzenediol, C$_{6}$H$_{4}$(OH)$_{2})$ on the (110) surface of rutile TiO$_{2}$ was investigated with STM and x-ray photoemission (XPS). The saturation coverage of resorcinol is smaller as compared to catechol (1,2 benzenediol) with a C1s/Ti2p$_{3/2}$ ratio of $\sim $ 7.3{\%} and 12{\%}, respectively. This indicates that resorcinol occupies on average more than two Ti sites on the surface. STM suggests that resorcinol molecules are mobile at lower coverage, whereas a weakly-ordered overlayer with a periodicity of 3 unit-cells along [001] is observed at higher coverages. Interestingly, exposure of resorcinol-saturated TiO$_{2}$ surface to an XPS Mg-K$\alpha $ beam (1253.6 eV) induces a reorganization of adsorbed resorcinol molecules. STM shows well-resolved double chains that run across [001]-oriented rows of TiO$_{2}$(110) surface. These results suggest that irradiation induces a polymerization reaction of adsorbed resorcinol molecules, where neighboring aromatic rings are arranged in a zig-zag configuration. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q10.00007: Propane-1,3-diol Adsorption and Dissociation on Rutile TiO$_{2}$(110): A Scanning Tunneling Microscopy Study Zhenrong Zhang, Xiao Lin, Bruce Kay, Zdenek Dohn\'alek Titanium Dioxide (TiO$_{2})$ has attracted great attention in the past decades due to its importance in heterogeneous catalysis. Here the adsorption and dissociation of Propane-1,3-diol molecules on partially reduced rutile TiO$_{2}$(110) surfaces are studied via variable temperature scanning tunneling microscopy (VT-STM). STM images of TiO$_{2}$(110) surfaces obtained before and after \textit{in-situ} doses of melecules at room temperature show that the molecules preferentially bind in bridge-bonded oxygen vancanies (BBO$_{V}$'s) \textit{via} one O-H bond scission. The dynamics of Propane-1,3-diol molecules motion has been investigated at room and elevated temperatures. Propane-1,3-diol molecules swing on TiO$_{2}$ surface with one end (-CH$_{2}$-O$^{-})$ anchored on vacancies. Strong interaction of the other end (-O-H) with Ti$_{5c}$ reduces the swing rate when compared with octanol. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q10.00008: First-Principles Studies of Electric Field Effects in Heterogeneous Catalysis: NH$_{3 }$on Ru(0001) Aaron Sisto, Alexey Zayak, Jeffrey Neaton The catalytic dissociation of NH$_{3}$ has been the focus of recent studies due to the prospect of efficient hydrogen storage and generation. The effects of a static electric field on the surface electronic structure and energy barriers of reactions are examined using density functional theory calculations with gradient corrections. It is found that the interaction strength between the adsorbate and surface can be tuned based on the magnitude and polarity of the field, as evidenced by a field-induced shift of the d-electron band. Correspondingly, energy barriers along minimum energy pathways for desorption and dissociation reactions are significantly affected by the change in substrate-adsorbate interaction. It is concluded that the application of an electric field enhances the catalytic performance of Ru through increased activity and selectivity of NH$_{3}$ dissociation. We acknowledge support from DOE, DOE CSGF Fellowship. Computational resources provided by NERSC. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q10.00009: Adsorption and dissociation of molecular oxygen on $\alpha $-Pu (020) surface: A density functional study Jianguang Wang, Asok Ray Molecular and dissociative oxygen adsorption on the (020) surface of $\alpha $-Pu have been studied using the full-potential linearized augmented-plane-wave plus local orbitals (FP-LAPW+lo) basis method. Four adsorption sites and three approaches of O$_{2}$ molecule have been considered. Adsorption energies have been optimized according to the distance of the adsorbates from the Pu surface as well as the oxygen dimer bond length. Dissociative adsorption is found for two horizontal approaches (O$_{2}$ is parallel to the surface and parallel/perpendicular to a lattice vector) and to be more energetically favorable at the scalar and ``fully'' relativistic (NSOC vs. SOC) levels of theory. Hor2 approach on the top site was the preferred adsorption site among all cases studied here. Molecular adsorption occurs at the Vert (O$_{2}$ is vertical to the surface) approach. The work functions, net spin magnetic moments, and charge transfer are also calculated. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q10.00010: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q10.00011: Structure and Adsorption on Hydrated Alumina Surfaces Vincenzo Lordi, Patrick Huang, Eric Schwegler Understanding the mechanisms of adsorption of chemical agents on environmental materials under different atmospheric conditions is important for applications in environmental remediation, industrial catalysis, and protection against chemical warfare. In this work, we study molecular adsorption of the chemical agent simulant dimethyl-methylphoshonate (DMMP) on various alumina surfaces, using density functional theory-based molecular dynamics simulations. Both alpha and gamma alumina surfaces of different orientations (and thus surface terminations/reconstructions) are studied, under both wet and dry conditions. Adsorption from the gas phase onto dry and hydroxylated surfaces is compared to adsorption from an aqueous layer in the limit of a fully bulk-like liquid water layer. Interfacial structure and dynamics are directly compared to previous synchrotron X-ray scattering and sum-frequency vibrational spectroscopy experiments, from which specific contributions of different surface functional groups are identified and resolved. Differences in site reactivity on the various surfaces are also compared. Prepared by LLNL under Contract DE-AC52-07NA27344. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q10.00012: Ag/Al$_2$O$_3$/FeAl(110): Electronic structure and NO$_2$ adsorption Matthew Patterson, Orhan Kizilkaya, Richard Kurtz, Phillip Sprunger Ag/Al$_2$O$_3$ systems are widely studied as catalysts in the selective catalytic reduction of NO$_x$ with hydrocarbons. The exact nature of the active sites and the role of the Ag clusters in such reactions is still not fully understood. In this study, we characterize thermally evaporated Ag clusters on the ultrathin alumina film produced by oxidizing FeAl(110). ARPES demonstrates the evolution of Ag cluster electronic structure and morphology with increasing Ag coverage. Changes in electronic binding energy distinguish charged from metallic clusters. Vibrational EELS of NO$_2$ adsorbed on the Ag/Al$_2$O$_3$/FeAl(110) system elucidates the nature of the NO$_x$ binding site and the changes in the surface chemistry both as a function of Ag cluster size and NO$_x$ adsorption temperature. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q10.00013: Neutron and Thermodynamic Studies of Hydrogen on Pd Decorated Metal Oxides Paige Landry, A. Ramirez-Cuesta, E. Cruz Silvia, B. Sumpter, J.Z. Larese We report our investigations of thermodynamic, inelastic and quasielastic neutron scattering (INS and QENS) studies of H2 adsorbed on bare and Pd decorated metal oxide (MO) surfaces, specifically ZnO, SBA-15 silica, and alumina. Guided by our volumetric adsorption measurements, we used INS and QENS to probe the dynamics of the adsorbed hydrogen molecules. These measurements provide insight into how the microscopic behavior of hydrogen is changed when it is confined at interfaces or interacts with a Pd catalyst. Using INS, the motion of the adsorbed hydrogen are examined as a function of surface adsorbate composition. For rotational motion we use the ortho-to-para transition as a guide and find that the rotational barrier for H2 adsorbed on some of these MO surfaces shift to lower energy (relative to bulk H2). For comparison, the hydrogen adsorption and microscopic behavior when the MO are decorated with 1{\%} Pd metal will be discussed. Evidence for the presence of adsorbed H2, Zn hydroxide and the potential role of spillover will be discussed. This work was partially supported by the U.S. DOE, BES under contract No. DE-AC05-00OR22725 with ORNL managed and operated by UT-Battelle, LLC, the NSF under grant DMR-0412231 and a grant from the University of Tennessee, JINS. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q10.00014: Atomistic Mechanism of Surface Oxide Formation on Pt(111) Zhengzheng Chen, Chao Wu, William Schneider A detailed understanding of the interaction of oxygen with Pt surfaces is essential for understanding its catalytic activity and deactivation in oxidizing environments. Here we analyze the transition between metallic and oxidized Pt surfaces. Using first-principles calculations, we characterize the chain-like oxide reconstruction on the Pt(111) surface associated with O coverage $>$50\%. We describe the sensitivity of the reconstruction energy to the occupancy of adjacent fcc and hcp sites and present a phenomenological model that relates the reconstruction to the balance between elastic strain energy and screening of O-O repulsions. Core level shift calculations indicate the reconstruction generates two O states with different binding energies and reactivity. Finally, we analyze the thermodynamic stability and equilibrium states of the reconstruction a cluster expansion model. The results are important in developing models of oxidation catalysis on Pt (111) surface. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q10.00015: Tuning Semiconductor Band Edge Energies via Surface Ligand Passivation Shenyuan Yang, David Prendergast, Jeffrey Neaton Semiconductor band gaps and band edge energies are key parameters that can dictate the efficiency of photocatalysis in solar energy conversion applications. CdSe is a representative semiconducting system with an ideal band gap for solar photon absorption, but with band edge energies that are not positioned for efficient water splitting. Using first-principles calculations within density functional theory, we present a study of the electronic structure of passivated CdSe surfaces and nanostructures, exploring the ability to tune band edge energies in this system via chemisorbed ligands. We predict substantial shifts in band edge energies that are electrostatic in origin, and due to the induced dipole at the CdSe-ligand interface and the intrinsic dipole of the ligand. We further show that, by changing the size and orientation of the ligand's intrinsic dipole moment via novel functionalization strategies, we can control the magnitude and direction of the shifts of CdSe energy levels. The effect of ligands on energy levels of two-dimensional CdSe surfaces and nanocrystal surfaces are thoroughly discussed. [Preview Abstract] |
Session Q11: Fractional Quantum Hall Effect II
Sponsoring Units: FIAPChair: Albert Chang, Duke University
Room: D222
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q11.00001: Unexpected Roles for Spin Degrees of Freedom in Competing Phases of the Second Landau Level Trevor D. Rhone, J. Yan, U. Wurstbauer, Y. Gallais, A. Pinczuk, L. Pfeiffer, K. West Competing liquid and solid ground states as well as intriguing quantum Hall fluids such as that at $\nu $=5/2 create great current interest in the N=1 Landau level. The spin degrees of freedom in quantum phases of the 2nd Landau level is probed by resonant light scattering. The long wavelength spin wave mode, which monitors the degree of spin polarization, is at the Zeeman energy in the spin polarized state at $\nu $=3. At lower filling factors the intensity of the Zeeman mode collapses indicating loss of spin polarization. At filling factors slightly lower the intensity of the spin wave attenuates and a broad continuum of low-lying excitations emerges - sharp and broad modes coexist. While the coexistence of spectral features has not been explained, the observation could manifest the presence of mixed quantum phases and some loss of spin polarization. A continuum of low-lying excitations emerges that dominates near $\nu $=8/3 and $\nu $=5/2. Resonant Rayleigh scattering reveals that the quantum fluids away from $\nu $=3 break up into robust domains. It is conceivable that these domains could comprise both spin polarized and depolarized quantum fluids. While the state at $\nu $=5/2 is considered to be polarized, these results reveal unprecedented roles for spin. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q11.00002: Electron teleportation via Majorana Bound States in a Mesoscopic Superconductor Liang Fu Majorana fermions are non-Abelian anyons in 5/2 fractional quantum Hall states and superconductors, which can store quantum information in an inherently nonlocal way. We describe a phase-coherent electron transport phenomena through two spatially separated Majorana bound states in a mesoscopic superconductor. This striking nonlocal effect arises from the interplay between topological order, superconducting order parameter and mesoscopic effects. We discuss its implications for experimental detection of Majorana fermions and topological quantum computation. Ref: Liang Fu, Phys. Rev. Lett. 104, 056402 [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q11.00003: Entanglement Entropy as a Function of the Aspect Ratio in the First and Second Landau Level Barry Friedman, Curtis Balusek, Darwin Luna Entanglement entropy as a function of aspect ratio has been studied by direct diagonalization in the first and second Landau levels. The torus geometry is used and spin polarized electrons interact via long range Coulomb interaction . As previously noted by Haque et al. (N J Phys 12, 2010 075004), in the first Landau level there is very smooth behavior as a function of aspect ratio making it possible to obtain the topological entanglement entropy. In the second Landau level, the entanglement entropy is much less regular, with possible signatures of quantum phase transitions. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q11.00004: The phase diagram and particle-hole asymmetry of the reentrant integer quantum Hall states of the second Landau level A. Kumar, M.J. Manfra, L.N. Pfeiffer, K. W. West, G.A. Csathy The second Landau level of a two-dimensional electron gas reveals a rich set of competing ground states. Besides an increasing number of fractional quantum Hall states, there are also eight reentrant integer quantum Hall states observed. These reentrant integer states are currently not understood, although they are believed to be collective insulators akin to the field induced Wigner solid with one or more electrons per site. These states are strongly affected by tilt in magnetic field and carrier density but surprisingly there is very limited data on their temperature dependence. We present a detailed study of the melting of the reentrant integer quantum Hall states of the second Landau level from which we extract the phase diagram in the temperature versus filling factor plane. We find that the melting temperatures of the various reentrant integer states violate the particle-hole symmetry. We also report that as the temperature is lowered the magnetoresistance deviates from an activated dependence. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q11.00005: The even denominator fractional quantum Hall states at large Landau level mixing Nodar Samkharadze, Michael Manfra, Gabor Csathy, Loren Pfeiffer, Ken West We present a study of the energy gaps of the even denominator fractional quantum Hall states of the second Landau level in a two-dimensional electron gas with a record low density of n = 8.2x10$^{10}$ cm$^{-2}$. These measurements are motivated by the expectation that Landau level mixing present in samples of low densities breaks the degeneracy of the Pfaffian and its particle-hole conjugate anti-Pfaffian. Cooling the electron gas in our Helium-3 immersion cell to 5mK reveals at filling factor 5/2 a fully quantized Hall plateau and a vanishingly small magnetoresistance. Because of the low density of our sample, the 5/2 fractional state is observed at the highest degree of Landau level mixing reported to date. We have measured the energy gaps of the 5/2 and 7/2 fractional quantum Hall states. The intrinsic gap deduced in the limit of no disorder will be compared to previously reported values for samples with higher densities. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q11.00006: Nonconventional odd denominator fractional quantum Hall states in the second Landau level Gabor Csathy, Ashwani Kumar, Michael Manfra, Loren Pfeiffer, Ken West The odd denominator fractional quantum Hall states in the second Landau level of a two-dimensional electron gas are believed to be different from those of the lowest Landau level. While at first sight these states could be part of the composite fermion hierarchy, several recent theoretical works suggest that some might be supporting generalized Pfaffian-like correlations. Recent progress in cooling electrons allowed us to observe a new fractional quantum Hall state at the filling factor 2+6/13. By assuming that the effective mass of the composite fermions does not explicitly depend on the Landau level index we find that energy gaps of the prominent 2+1/3 and 2+2/3 states are consistent with the values predicted by the free composite fermion model. However, the weaker 2+2/5 and 2+6/13 states deviate significantly from the prediction of this model. This deviation constitutes a first demonstration of the nonconventional nature of the latter two odd denominator fractional quantum Hall states. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q11.00007: Tunneling experiments in the lowest Landau level C. Dillard, Xi Lin, M.A. Kastner, L.N. Pfeiffer, K.W. West Recently, a quasiparticle-tunneling experiment on the 5/2 state [1] led to the unintentional discovery of a process we term annealing. In this experiment top gates are used to bring counter-propagating edge states close enough together for tunneling to occur. By keeping the quantum point contact (QPC) top gates energized for a few days at 4 Kelvin, one can create equal electron densities in the QPC region and the bulk of a GaAs heterostructure. This is a great advantage for studying quasiparticle tunneling in QPCs. Conditions under which annealing has proved effective are presented. In addition, in order to better understand and control quasiparticle tunneling in QPCs, further tunneling experiments have been performed in the lowest Landau level. \\[4pt] [1] Iuliana P. Radu, J. B. Miller, C. M. Marcus, M. A. Kastner, L. N. Pfeiffer, and K. W. West, Science 320, 899 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q11.00008: Haldane Statistics in the Finite Size Entanglement Spectra of Laughlin States Maria Hermanns, Anushya Chandran, Nicolas Regnault, Bogdan Andrei Bernevig We conjecture that the counting of the levels in the orbital entanglement spectra (OES) of finite-sized Laughlin Fractional Quantum Hall (FQH) droplets at filling 1/m is described by the Haldane statistics of particles in a box of finite size. This principle explains the observed deviations of the OES counting from the edge-mode conformal field theory counting and directly provides us with a topological number of the FQH states inaccessible in the thermodynamic limit- the boson compactification radius. It also suggests that the entanglement gap in the Coulomb spectrum in the conformal limit protects a universal quantity- the statistics of the state. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q11.00009: Polarized Fractional Quantum Hall States at 1/3 and 5/2 Filling: a Density-Matrix Renormalization Group Calculation Jize Zhao, Donna Sheng, F. Duncan M. Haldane In this talk, the density-matrix renormalization group method is employed to investigate the fractional quantum Hall effect (FQHE) at filling numbers $\nu=1/3$ and $5/2$. We present benchmark results for both filling numbers for larger system sizes to show the accuracy as well as the capacity of our numerical algorithm. Furthermore, we demonstrate that by keeping a large number of states, one can also obtain reliable entanglement spectrum at $\nu=5/2$, which characterizes the topological properties of FQHE states. Based on a finite-size scaling analysis, we also confirm that the entanglement gap defined by Li and Haldane for $\nu=5/2$ state with Coulomb interaction remains finite in the thermodynamic limit. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q11.00010: Paired composite fermion wavefunctions for excitations at 5/2 Sreejith Ganesh Jaya, Csaba Toke, Arkadiusz Wojs, Jainendra Jain The Pfaffian wave function, which is thought to be relevant for the ground state at filling fraction $\frac{5}{2}$, represents a paired state of composite fermions. It can be expressed as an antisymmetrized bilayer (331) wave function. This formulation can be extended to construct wave functions for neutral as well as charged excitations of the Pfaffian. The space spanned by the quasihole excitations exactly matches that of the previously known quasihole wave functions. By comparison to exact results with up to 14 particles, we find that our neutral excitations and also the quasiparticle excitations describe well the actual excitations of the model three body interaction for which the Pfaffian ground state wave function is exact. The relevance to the solutions of the second Landau level Coulomb interaction is less conclusive. Also, the counting of states on the quasihole and quasiparticle sides is significantly different. Relation of our wave functions to other ansatz wave functions in the literature will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q11.00011: Suppression of Interlayer Phase Coherence by Gauge Fluctuations in Bilayer Composite Fermi Liquids Robert Cipri, Yafis Barlas, N.E. Bonesteel The $\nu$ =1/2+1/2 bilayer quantum Hall system exhibits at least two phases as a function of layer spacing, $d$. For $d/l \gg 1$, ($l$ is magnetic length), the system decouples into two $\nu = 1/2$ composite fermion (CF) liquids. For $d/l$ sufficiently small, the system enters an incompressible bilayer quantum Hall state. Recently, Alicea et al. [1] have proposed a state which might exist for intermediate layer spacing ($d \sim l$). In this ``interlayer phase coherent" state, CFs tunnel coherently between layers forming well-defined bonding and antibonding Fermi seas, though there is no actual tunneling of physical electrons. Here we show that scattering from gauge fields in the CF liquids leads to strong layer-dependent fluctuations in the Aharonov-Bohm phases seen by CFs which suppress interlayer phase coherence. This suppression appears as a singular contribution to the correlation energy which inhibits any T=0 phase transition into an interlayer phase coherent state, and drives any such transition first order. Work supported by US DOE.\\[4pt] [1] J. Alicea, O.I. Motrunich, G. Refael, M.P.A. Fisher, PRL 103, 256403 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q11.00012: Pinning mode of 2D electron system with short-range alloy disorder B.H. Moon, B.A. Magill, L.W. Engel, D.C. Tsui, L.N. Pfeiffer, K.W. West At the low Landau filling ($\nu )$ termination of the fractional quantum Hall effect (FQHE) series, a two-dimensional electron system (2DES) becomes an insulator, which is identified in sufficiently low-disorder samples as a form of pinned Wigner solid. The microwave conductivity spectrum of such a solid shows a striking resonance, which is understood as a pinning mode, in which pieces of solid oscillate within the disorder potential. We report on the observation of the pinning mode of a 2DES that resides within Al$_{x}$Ga$_{1-x}$As with x=0.85{\%}. For a carrier density of n= 8.7 x10$^{10}$ /cm$^{2}$, a resonance with a peak frequency (f$_{pk})$ of about 5 GHz appears as $\nu $ goes below the 2/3 FQHE. A local minimum in resonance amplitude vs. $\nu $ occurs around $\nu $ =1/2. We will discuss the contribution of the alloy disorder to f$_{pk}$. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q11.00013: Properties of the Composite Fermion Wigner Crystal Alex Archer, Jainendra Jain In two dimensional electron systems at small filling factor the ground state is a Wigner crystal. Wigner crystals can also be observed for systems near integer fillings, where electrons or holes in the partially filled Landau Level form a Wigner crystal. Recent experimental evidence (PRL 105, 126803 (2010)) suggests that a Wigner crystal of composite fermions forms near the filling factor of $v=\frac{1}{3}$. Motivated by these results, we calculate the shear modulus of the composite fermion Wigner crystal in the vicinity of several fillings of the form $v=\frac{1}{3},\frac{2}{5},\frac{3}{7}$, following the procedure of Maki-Zotos, using the effective two-body real space interactions between composite fermions calculated by Lee, Scarola, and Jain. We discuss the differences from the electron Wigner crystal, and also the experimental implications of our results. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q11.00014: Fabry-Perot Interferometry in the Integer and Fractional Quantum Hall Regimes Douglas McClure, Willy Chang, Angela Kou, Charles Marcus, Loren Pfeiffer, Ken West We present measurements of electronic Fabry-Perot interferometers in the integer and fractional quantum Hall regimes. Two classes of resistance oscillations may be seen as a function of magnetic field and gate voltage, as we have previously reported. In small interferometers in the integer regime, oscillations of the type associated with Coulomb interaction are ubiquitous, while those consistent with single-particle Aharonov-Bohm interference are seen to co-exist in some configurations. The amplitude scaling of both types with temperature and device size is consistent with a theoretical model. Oscillations are further observed in the fractional quantum Hall regime. Here the dependence of the period on the filling factors in the constrictions and bulk of the interferometer can shed light on the effective charge of the interfering quasiparticles, but care is needed to distinguish these oscillations from those associated with integer quantum Hall states. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q11.00015: Topological screening and interference of fractionally charged quasi-particles Ivan Levkivskyi, Juerg Froehlich, Eugene Sukhorukov Interference of fractionally charged quasi-particles is expected to lead to Aharonov-Bohm oscillations with periods larger than the flux quantum $\Phi_0$. However, according to the Byers-Yang theorem, observables of an electronic system are invariant under insertion of a quantum of singular flux. We resolve this paradox by considering a {\em microscopic} model of an electronic interferometer made from quantum Hall edges at filling factor $\nu=1/m$. An approximate ground state of such an interferometer is described by a Laughlin type wave function, and low-energy excitations are incompressible deformations of this state. We construct a low-energy effective theory by projecting the state space onto the space of such deformations. Amplitudes of quasi-particle tunneling in this theory are found to be insensitive to the singular flux. This behavior is a consequence of {\em topological screening} of the flux by the quantum Hall liquid. We describe strong coupling of the edges to Ohmic contacts and the resulting quasi-particle current through the interferometer with the help of a master equation. As a function of the singular magnetic flux, the current oscillates with the period $\Phi_0$. These oscillations are suppressed with increasing system size. When the magnetic flux is varied with a modulation gate, current oscillations have the quasi-particle period $m\Phi_0$ and survive in the thermodynamic limit. [Preview Abstract] |
Session Q12: Focus Session: Dopants and Defects in Semiconductors: Conducting Oxides
Sponsoring Units: DMPChair: Joel Ager, Lawrence Berkeley National Laboratory
Room: D223/224
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q12.00001: Transparent Conductors: Understanding and Optimization Invited Speaker: The unique combination of two mutually exclusive properties -- optical transparency and electrical conductivity -- is known to be a prerogative of only a few oxides of post-transition metals, namely, In$_2$O$_3$, ZnO, CdO and SnO$_2$. Advances in theoretical understanding of the underlying physical phenomena in conventional transparent conducting oxides (TCOs) and rapid development of the technologies for which TCO is a vital component, stimulate further research aimed at (i) broadening the range of the electronic and optical properties of application-specific transparent conductive materials in a controllable way; (ii) improving the functional capabilities and efficiency of TCOs in a device; and (iii) designing novel materials as a viable, inexpensive alternative to conventional TCOs. Here, we employ first-principles density-functional approach to investigate the structural, electronic and optical properties of several classes of transparent conductors including conventional single-cation main-group oxides, multi-component binary and ternary oxides, as well as several non-oxide materials. Systematic comparative investigations allow us to determine the role of the crystal structure, chemical composition and carrier generation mechanisms on the resulting optical and electronic properties and predict ways to optimize the properties. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q12.00002: Origins of superior symmetrical doping ability of monoclinic BiVO4 Yanfa Yan, Wan-Jian Yin, Su-Huai Wei, Mowafak Al-Jassim, John Turner Application of semiconductors for functional devices depends critically on their dopability. However, there are strong doping bottlenecks for wide-band-gap semiconductors -- symmetrical doping is usually difficult, which severely restrict their potential applications. Here, we report superior symmetrical doping properties, i.e., $n$-type and $p$-type, of monoclinic BiVO$_{4}$ by first-principles density-functional theory calculation. Our results reveal that without external doping, BiVO$_{4}$ with moderate $n$-type and $p$-type conductivities can be obtained. However, doping of Sr, Ca, Na, and K atoms under oxygen-rich growth conditions can lead to outstanding $p$-type conductivity, whereas doping of Mo and W under oxygen-poor growth conditions can result in excellent $n$-type conductivity. We find that Bi 6s state is responsible for the good p-type doping and the presence of V 3d state is responsible for the good n-type doping. Furthermore, the Bi $6s$ and V 3$d$ states are also responsible for producing very dispersive valence and conduction band edges, leading to small electron and hole effective masses. The superior symmetrical doping properties and high carrier mobility make BiVO$_{4}$ a promising candidate for electronic and optoelectronic device applications. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q12.00003: Donors and H impurities in SnO$_{2}$ studied by IR spectroscopy Figen Bekisli, Michael Stavola, W. Beall Fowler, Lynn Boatner, Erik Spahr, Gunter Luepke Theory predicts that SnO$_{2}$ is an attractive wide band gap candidate for achieving p-type conductivity. Interstitial H and H$_{O}$ are suggested to be shallow donors in SnO$_{2}$ [1-3]. We have studied the properties of H in SnO$_{2}$ single crystals. An O-H line is observed at 3261 cm$^{-1}$ that is polarized perpendicular to the c direction along with weaker features at 3258 and 3272 cm$^{-1}$. When D is introduced into SnO$_{2}$ by annealing in a D$_{2}$ ambient at 700\r{ }C, a variety of new O-H and O-D lines is produced along with the low-frequency absorption that is characteristic of free carriers. To probe the relationship between H and the free carriers it introduces, we have examined the thermal stabilities of the O-H and O-D lines and their relationship to the thermal stability of the free-carrier absorption. \\[4pt] [1] A.K. Singh \textit{et al}., Phys. Rev. Lett. \textbf{101}, 055502 (2008). \\[0pt] [2] J.B. Varley \textit{et al.,} Phys. Rev. B \textbf{79}, 245206 (2009). \\[0pt] [3] W.M. Hlaing Oo \textit{et al.}, Phys. Rev. B \textbf{82}, 193201 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q12.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q12.00005: Asymmetric Cation stoichiometry in Spinels : Site occupancy in Co$_{2}$ZnO$_{4}$ and Rh$_{2}$ZnO$_{4}$ Tula Paudel, S. Lany, A. Zunger, A. Sigdel, A. Zakutayev, J. Perkins, D. Ginley, J. Bettinger, Y. Shi, M. Toney, A. Nagaraja, N. Perry, T. Mason Cations A and B in A$_{2}$BX$_{4}$ spinels normally appear in precise 2:1 Daltonian ratio only at low temperature. At finite temperature, they become either A-rich or B-rich, which control dopability of the compound. We survey the experimentally observed stoichiometry asymmetries and describe the first principles framework for calculating these. The results of the calculations compare well with the phase boundary determined from XRD and the site occupancy measured by anomalous-XRD on Co$_{2}$ZnO$_{4}$ and Rh$_{2}$ZnO$_{4}$ samples grown in thermodynamic equilibrium. Good comparison between theory and experiment allows us predict the co-existence line in composition range form first principle for other spinels, which in turn can be extended to predict the nature of electrical conductivity of a compound, while designing the material with the desired properties via principle of inverse design. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q12.00006: Dopants and defects in conductive oxide spinels Andriy Zakutayev, John Perkins, Phillip Parilla, Tula Paudel, Staphan Lany, David Ginely, Alex Zunger We will discuss the effects of extrinsic and intrinsic imperfections (dopants and defects) in a group of conductive oxide materials related to Co3O4. Co3O4 is a spinel with Co2+ and Co3+ on tetrahedral and octahedral sites, respectively. Doping of Co3O4 with Zn and Ni represent two limiting cases: Zn2+ ions have a preference to occupy tetrahedral (Co2+) sites and are predicted to be unable to dope effectively; Ni2+ ions have a preference to occupy octahedral (Co3+) sites, so these atoms are expected to be efficient dopants. We found that substitution of Co3O4 spinel with up to 33 percent of Zn and Ni results in formation of ZnCo2O4 normal spinel and NiCo2O4 inverse spinel, and causes 100-fold and 1000-fold increases in conductivity, respectively, matching the predicted trend. Increase in Zn and Ni concentraion up to 40 percent cause phase separation of ZnO and NiO and leveling out of the conductivity. The conductivity decreases sharply above 50-60 percent Zn and Ni substitution level. Small differences with the theoretical predictions may be explained by non-equilibrium character of the thin film deposition process. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q12.00007: Possible \textit{n}-type carrier producers in In$_2$O$_3$(ZnO)$_k$ homologous compounds Haowei Peng, Jung-Hwan Song, Arthur J. Freeman In$_2$O$_3$(ZnO)$_k$ (${\rm k = integers}$) homologous compounds are promising intrinsic \textit{n}-type transparent conducting semiconductors.\footnote{T. Moriga, et.al., J. Am. Ceram. Soc. \textbf{81}, 1310 (1998).} To find out the carrier producers, we investigated the energetics and thermodynamic properties of \textit{n}-type defects and their complexes in In$_2$O$_3$(ZnO)$_k$, with the k=3 phase as prototype, using the first-princiles density functional method. We calculated the defect formation energies and defect transition energy levels of oxygen vacancies (V$_{\rm O}$), substitutional indium on zinc sites (In$_{\rm Zn}$), zinc and indium interstitials (Zn$_i$ and In$_i$) on different atomic sites, and also some V$_O$--In$_{\rm Zn}$ and V$_{\rm O}$--Zn$_i$ defect complexes. We find, under the experimental growth condition of O-poor and $T=1300$$^{\circ}$C, that V$_{\rm O}$, In$_{\rm Zn}$, and V$_{\rm O}$-In$_{\rm Zn}$ complexes have much lower formation energies than the others, among which V$_{\rm O}$ will stay in the neutral charged state and the latter two are the most possible \textit{n}-type carrier producers. The V$_{\rm O}$-In$_{\rm Zn}$ complex tends to form between V$_{\rm O}$ and In$_{\rm Zn}$ in the same atomic layer; thus its distribution should be affected by the site-preference of V$_{\rm O}$. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q12.00008: Protonic motion in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ thin films and its implications on resistance change properties Mihir Tendulkar, Nicholas Breznay, Yoshio Nishi Thin films of Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO) exhibit resistance-change properties that are of acute interest for next-generation memory solutions. Recent work has demonstrated that oxidation / reduction of a reactive electrode is critical to the switching process, suggesting that interface engineering will solve the reliability issue. We show that an overlooked contributor to the process is hydrogen, which dopes the bulk film. Activated conduction and loss tangent measurements are correlated with FTIR spectra to demonstrate protonic motion through the repeated breaking and reforming of --OH bonds. SIMS and Hall measurements are presented in conjunction with UV-Vis spectroscopy to show that hydrogen also alters the electronic structure of the PCMO film. The implications of these effects on forming and switching are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q12.00009: Dynamics of Interstitial H in TiO$_{2}$ W.B. Fowler, A. Murphy, M. Stavola H transport in rutile TiO$_{2}$ is important because of the low energy barriers within the open lattice in the c-direction [1]. As part of a study of the dynamics of interstitial H, potential energy functions for the vibration of hydrogen between two cross-channel O were generated. Double-well functions were modeled using experimental data [2] for the fundamental stretching vibrational frequencies of the three isotopes of H along with theoretical information obtained from quantum-mechanical calculations using [3] CRYSTAL06. These functions were then used to predict where the first overtone may lie, and its relative transition probability. The unexpectedly large anharmonicity observed for the OH vibration is correlated with the hydrogen-bond nature of the O-H ----O potential.\\[4pt] [1] O. W. Johnson et al., J. Appl. Phys. 46, 1026 (1975); J. B. Bates et al., Phys. Rev. B 19, 4130 (1979); E. J. Spahr et al., Phys. Rev. Lett. 104, 205901 (2010).\\[0pt] [2] J. B. Bates and R. A. Perkins, Phys. Rev. B 16, 3713 (1977).\\[0pt] [3] R. Dovesi et al., Crystal06 User's Manual, Univ. of Torino, Torino, 2006. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q12.00010: Interstitial and substitutional Zr in SrTiO3 John Jaffe, Renee Van Ginhoven, Weilin Jiang We investigate Zr in SrTiO$_{3}$ (STO) as an electronic dopant and as a model for nuclear waste forms in which radioactive Sr decays to Y and then to stable Zr through beta emission. Density functional theory (DFT) within the supercell model is used to predict the thermodynamic stability and electronic states of interstitial and Sr- or Ti-substituted Zr atoms in the STO lattice. Native point defects such as vacancies and antisites are also considered. When Zr replaces Sr, its most stable configuration is to simply occupy the Sr site (instead of, for example, replacing a Ti and displacing the Ti to the Sr site.) For Zr added to the lattice, its most stable configuration is to replace a Ti, making a Zr$_{Ti}$ impurity plus a Ti interstitial (as opposed to the Zr just remaining as an interstitial atom.) Zr$_{Sr}$ is predicted to be a double electron donor, Zr$_{Ti}$ is electrically inactive and interstitial Zr and Ti are predicted to be quadruple donors, with all donor levels in the conduction band. Zr$_{Sr}$ and the tetravalent interstitials are all predicted to increase the crystal volume, and the interstitials also are predicted to lead to a tetragonal distortion of the lattice. Experiments with injection of Zr atoms into STO qualitatively confirm these predictions of crystal structural changes. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q12.00011: ABSTRACT WITHDRAWN |
Session Q13: Glassy Systems and Jamming I
Sponsoring Units: GSNP DFDChair: Corey O'Hern, Yale University
Room: D225/226
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q13.00001: The Influence of Boundary Roughness on the Dynamics of Confined Colloidal Suspensions near the Glass Transition Daniel J. Real, Kazem V. Edmond, Eric R. Weeks We study the relationship between boundary conditions and particle motion in confined, concentrated colloidal suspensions. Studies of polymer fluids in confinement have shown that changes in mobility are strongly dependent upon the polymer/surface interaction. We model this interaction by observing the effects of textured surfaces on colloidal particle mobility in confined dense suspensions (near the glass transition). We use high-speed confocal microscopy to directly image and track the colloidal particles in thin, wedge-shaped sample chambers made from textured glass. We texture the glass in a controlled, reproducible manner by spincoating and sintering colloidal suspensions onto glass slides. We expect the texturing to frustrate the formation of layers seen in smooth-walled confinement, resulting in decreased translational diffusion as compared to the smooth wall case. By studying these dynamics we gain a better understanding of the glass transition and its dependence on interfacial dynamics versus finite size effects. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q13.00002: Structural Rearrangements in Confined Colloidal Liquids under Oscillatory Shear Prasad Sarangapani, Andrew Schofield, Y. Elaine Zhu We have investigated the dynamics of confined suspensions under oscillatory shear using a micron-gap rheometer interfaced with confocal microscopy. Our system consists of sterically stabilized poly-(methyl methacrylate) (PMMA) particles suspended in density and refractive index matched solvents at particle volume fractions, \textit{$\phi $ }= 0.40-0.43, confined between two solid surfaces with gaps ranging from $\sim $10-30 particle layers. Above a threshold strain of $\sim $6{\%} where an applied deformation is sufficient to induce plastic behavior, we find that structural rearrangements are highly anisotropic. Non-affine motion, determined by subtracting the globally uniform strain from the bare particle coordinates, reveals that particles move as cooperatively rearranging groups with a preferred orientation transverse to the flow direction. Measures which probe cooperative dynamics all reveal a strong amplitude, thickness, and directional dependence on the characteristic sizes of cooperatively rearranging regions. Interestingly, we find that medium range orientational order has a significant influence on shear-induced dynamics, particularly the shapes of rearranging regions. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q13.00003: Application of Edwards' statistical mechanics to polydisperse and high-dimensional jammed sphere packings Maximilien Danisch, Yuliang Jin, Hernan Makse, Patrick Charbonneau, Sam Meyer, Chaoming Song, Francesco Zamponi The Edward's statistical mechanics of jammed sphere packings [Song et al., Nature (London) 453, 629 (2008)] is generalized to different systems: polydisperse sphere packings in three dimensions, and high-dimensional monodisperse sphere packings. The theory predicts the density of random close packing and random loose packing of polydisperse systems for a given distribution of particle size and describes packings for any interparticle friction coefficient. In the high-dimensional limit, an asymptotic solution of the self-consistent relation is obtained by saddle-point evaluation and checked numerically. The resulting random close packing density scaling is consistent with that of other approaches, such as replica theory and density-functional theory. The theory could serve as a starting point to solve more difficult problems: such as predicting the optimal density of non-spherical packings, and understanding the higher-order correlations present in amorphous jammed packings. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q13.00004: Cyclic simple shear in a two-dimensional granular system Jie Ren, Joshua Dijksman, Robert Behringer We study the evolution of a 2D granular system consisting of frictional photo-elastic disks under large numbers of small-amplitude cyclic shear cycles. We are particularly interested in the reversibility of the system under cyclic shear. The experiments are carried out on a specially designed apparatus which can create quasi-static, nearly uniform simple shear. By using photo-elastic particles and a fluorescent labelling technique, we obtain information about displacement, rotation and contact forces for each particle following each small strain. We also obtain the system-level behaviour over many shear cycles. To better understand the nature of jamming, we have carried out shearing runs that explore various initial states which are initially unjammed, isotropically jammed or anisotropically jammed, and we compare the results for different initial states. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q13.00005: Microscopic Dynamics of Quasi-2D Dense Colloidal Gels Matthew Lohr, Arjun Yodh In this work, we investigate the microscopic dynamics of quasi-2D dense attractive colloidal systems. We confine bidisperse polystyrene spheres between glass coverslips in a suspension of water and 2,6-lutidine; as we increase the temperature of the sample into a critical regime, lutidine wets the colloids, creating a strong attractive interaction ( $>$ 4kT). We specifically study suspensions in the ``dense gel'' regime, i.e., at a volume fraction high enough that the attractive particles form a spanning cluster, yet just low enough that there exists some structural heterogeneity larger than the individual particle size. We track the particle locations via bright-field video microscopy and analyze the dynamics of the system in order to compare them to lower-volume-fraction gel states and higher-volume-fraction glassy states. In doing so, we pinpoint the similarities and differences in the mechanisms for dynamic arrest in low-density colloidal gels and high-density colloidal glasses. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q13.00006: Structural Correlations in Glass-Forming Hard Spheres Fluids Patrick Charbonneau, Benoit Charbonneau Recent studies have detected the presence of a growing static length scale associated with the glassy dynamical slowdown. Yet no fully satisfying microscopic description of such a length scale has yet been formulated. We critically evaluate the hypothesis that correlated structural defects could underlie the growing relaxation time in deeply supersaturated fluid. Though a clear structural signature of a developing order in these systems is found, the resulting defect geometry does not lead quite match the Frank- Kasper defect scenario. The dimensionally generalizable nature of the defects, however, make them promising options for defining static observables. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q13.00007: Study of experimental protocols for producing random close packed colloids Kelsey Hattam, Eric R. Weeks A collection of spheres can be packed tightly into an amorphous state known as ``random close packing.'' In our experiment, colloidal particles are allowed to slowly sediment forming a random close packed state. By adjusting the solvent's density we finely control the rate at which the sedimentation occurs. We then use confocal microscopy to image the sample. By imaging overlapping regions we determine the positions of hundreds of thousands of particles. From this data, we measure the distribution of Voronoi volumes and the contact number distribution, and examine how these distributions depend on the sedimentation rate. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q13.00008: Rotational and Translational Phonon Modes in Glasses Composed of Ellipsoidal Particles Peter J. Yunker, Ke Chen, Zexin Zhang, Wouter G. Ellenbroek, Andrea J. Liu, Arjun G. Yodh The effects of particle shape on the vibrational properties of colloidal glasses are studied experimentally. `Ellipsoidal glasses' are created by stretching polystyrene spheres to different aspect ratios and suspending the resulting ellipsoidal particles at high packing fraction. By measuring displacement correlations between particles, we extract vibrational properties of the ellipsoidal glass. Low frequency modes in glasses composed of ellipsoidal particles with major/minor axis aspect ratios $\sim$1.1 are observed to have predominantly rotational character. By contrast, low frequency modes in glasses of ellipsoidal particles with larger aspect ratios ($\sim$3.0) exhibit a mix of rotational and translational character. All glass samples were characterized by a distribution of particles with different aspect ratios. Interestingly, even within the same sample it was found that small- aspect-ratio particles participate relatively more in rotational modes, while large-aspect-ratio particles tend to participate relatively more in translational modes. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q13.00009: Dynamical heterogeneities and fluctuations of the time variables in structural glasses Karina E. Avila, Horacio E. Castillo, Azita Parsaeian The existence of dynamical heterogeneities in disordered materials is considered now as a crucial element in explaining many observed features of their dynamical behavior. In this work, we investigate a possible hypothesis for their origin, which assumes that they emerge from soft (Goldstone) modes associated with a broken continuous symmetry under time reparametrizations. To test this hypothesis, we construct coarse grained observables from data obtained in simulations of four models of structural glasses. The fluctuations of these observables are decomposed into transverse components associated with the postulated time-fluctuation soft modes and a longitudinal component unrelated to them. We find that as temperature is lowered and timescales are increased, the time reparametrization fluctuations become increasingly dominant and their correlation volumes grow together with the correlation volumes of the dynamical heterogeneities, while the correlation volumes for longitudinal fluctuations remain small. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q13.00010: Experimental observation of deformation and structural defects in hard-sphere colloid glasses Katharine Jensen, Nobutomo Nakamura, David Weitz, Frans Spaepen We performed experiments on a 1.55-$\mu$m-diameter monodisperse, hard-sphere colloid glass under simple shear at various strain rates, while simultaneously tracking real-time individual positions of roughly 100,000 particles by confocal microscopy. We probe the elastic, anelastic, and plastic responses of the system to applied strain, with particular focus on identifying the local mechanisms of deformation. In plastic deformation, we observe thermally activated rearrangements of groups of particles, the nature and concentration of which are correlated with local parameters such as strain, Voronoi volume, and free volume. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q13.00011: Aging dynamics of a colloidal glass - time resolved viscoelastic properties and the role of flow history Chinedum Osuji, Ajay Negi Many colloidal suspensions are inherently out of equilibrium and display a slow evolution of their dynamics over time. However, many features of the glass transition as encountered in polymer and molecular glasses are not conserved. This phenomenon is still not completely understood and little is known of the connection between flow history, as a determinant of the initial system state, and subsequent aging dynamics. Further, the changes in the energy landscape during aging can be understood from the frequency and strain dependence of the shear modulus but the non-stationary nature of these systems frustrates investigation of their instantaneous underlying properties. Here we discuss the use of stress jump experiments that investigate the role of flow history on aging, and the systematic reconstruction of the frequency and strain dependence as a function of age for a repulsive colloidal glass undergoing structural arrest and aging. We uncover a connection between the aging behavior and the rate of flow cessation that is additionally reflected in the dynamics of residual stress relaxation. Strikingly, the frequency dependence at fixed times can be rescaled onto a master curve, implying a simple connection between the aging of the system and the change in the frequency dependent modulus. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q13.00012: Jamming, Clogging, and Fragility in Frictionless Disk Systems with Quenched Disorder Charles Reichhardt, Evan Groopman, Zohar Nussinov, Cynthia Olson Reichhardt We consider a two-dimensional simulation model of binary frictionless disks which have a well defined jamming density of $\phi_{j} \approx 0.84$ in the absence of quenched disorder. When quenched disorder is added in the form of impenetrable immobile disks, the jamming density is reduced. As the density of the quenched disorder sites increases, we observe a crossover from a jamming transition to a clogging transition. The clogged state is defined as a highly heterogeneous granular packing that resists shear along one direction and that is composed of a combination of high density patches at the clean jamming density and very low density patches or voids. These clogged states are fragile in the sense that they are only clogged in the direction of an externally applied drive. After a clogged state has formed, if a new drive is applied in a different direction the disks can flow freely for a period of time before reorganizing into a new clogged state. In contrast, jammed systems are jammed in all directions simultaneously. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q13.00013: Coupling Between Translational and Orientational Order in Fiber Suspensions Alexandre Franceschini, Emmanouela Filippidi, Elisabeth Guazzelli, David Pine Suspensions of non-Brownian fibers under a small oscillatory shear flow find a random but completely reversible state, called ``random organization'': at each period, the non-hydrodynamic interactions modify both the orientation and positions of fibers, until a reversible configuration is found. As observed in sphere suspensions, there is a nonequilibrium absorbing phase transition when the strain is increased above a concentration-dependant threshold. The transient time, during which the activity decays algebraically, has a diverging duration; critical exponents are consistent with Manna universality class. Above the threshold, fibers get progressively aligned towards the vorticity and a reversible steady state is eventually found for a range of strain. This behavior is specific to fiber suspensions. We study whether or not these oriented reversible states are critical states. We experimentally evaluate the angles distribution of fibers in both vertical and horizontal planes and discuss the relation between these distributions and the existence of a reversible state. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q13.00014: Force correlations near point J in a lattice model of jamming Scott Milner, Jillian Newhall We have constructed a lattice model of a jammed system in $d=2$ dimensions near the isostatic point (Point J). Adapting the Tighe model, we represent a jammed pack of particles as a regular hexagonal array, with repulsive forces between nearest neighbors. We generate near-isostatic jammed configurations by carrying out a Monte Carlo simulation with Tighe ``wheel moves'', which rearrange forces locally while preserving force balance on every particle. (Wheel moves correspond to a small dilation of a given particle.) The MC simulation is progressively biased towards the creation of ``missing contacts'', bonds which bear zero force. We reveal long-range correlations in the force network near Point J by determining for each particle the smallest ``collective move'' --- a set of wheel moves that taken together dilates the given particle, while preserving the existing missing contacts. The size of these collective moves diverges as Point J is approached. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q13.00015: Time and volume fraction dependence of dynamic heterogeneity in a glass-forming binary hard-sphere mixture Elijah Flenner, Grzegorz Szamel We examined dynamic heterogeneity in a glass-forming binary hard-sphere mixture for volume fractions up to and including the so-called mode-coupling transition. We calculated the dynamic susceptibility $\chi_4(t)$, the four-point structure factor $S_4(q;t)$ and the dynamic correlation length $\xi(t)$. We find that the correlation length increases with time as $\xi(t) \sim \ln(t)$ and is independent of $\phi$ for times approximately between the $\beta$ and $\alpha$ relaxation time. The dynamic length plateaus at a $\phi$ dependent value $\xi_{\mathrm{max}}(\phi)$. We find that $\xi_{\mathrm{max}}(\phi)$ is proportional to the dynamic length at the $\alpha$ relaxation time, $\xi(\tau_\alpha)$. Finally, while for a limited range of volume fractions $\xi(\tau_\alpha) \sim \tau_\alpha^{1/z}$ with $1/z \approx 0.2$, we find that $\xi(\tau_\alpha) \sim \ln(\tau_\alpha)$ describes our data well for all $\phi$. [Preview Abstract] |
Session Q14: Focus Session: Extreme Mechanics: Elasticity and Deformation I
Sponsoring Units: GSNPChair: Katia Bertoldi, Harvard University
Room: D227
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q14.00001: Geometric Nonlinear Computation of Thin Rods and Shells Invited Speaker: We develop simple, fast numerical codes for the dynamics of thin elastic rods and shells, by exploiting the connection between physics, geometry, and computation. By building a discrete mechanical picture from the ground up, mimicking the axioms, structures, and symmetries of the smooth setting, we produce numerical codes that not only are consistent in a classical sense, but also reproduce qualitative, characteristic behavior of a physical system----such as exact preservation of conservation laws----even for very coarse discretizations. As two recent examples, we present discrete computational models of elastic rods and shells, with straightforward extensions to the viscous setting. Even at coarse discretizations, the resulting simulations capture characteristic geometric instabilities. The numerical codes we describe are used in experimental mechanics, cinema, and consumer software products. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q14.00002: Wrinkling of an Annulus Kamil Toga, Benoit Roman, Jose Bico, Thomas Russell, Narayanan Menon We report on an experiment in which we study the wrinkling of an annular elastic film subject to different radial tensions at the inner and outer diameter. The annuli were made from polystyrene films of thickness ranging from 62 to 180 nm, and floated on water. They were then transferred onto a Langmuir-Blodgett trough filled with acidic aqueous subphase. The surface tension on the inside of the annulus is held fixed, while the surface tension outside the annulus is continuously varied by compressing an insoluble surfactant. When the differential tension is increased beyond a threshold value, radial wrinkles form in the interior of the annulus and extend outwards. We studied the length of wrinkles formed as a function of the differential tension produced by the surfactant, and for a range of film thickness. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q14.00003: Dancing Discs: Bending and Twisting of Soft Materials by Anisotropic Swelling Douglas Holmes, Matthieu Roch\'e, Tarun Sinha, Howard Stone Soft materials, e.g. biological tissues and gels, undergo morphological changes, motion, and instabilities when subjected to external stimuli. Tissues can exhibit residual internal stresses induced by growth, and generate elastic deformations to move in response to light or touch, curl articular cartilage, aid in seed dispersal, and actuate hygromorphs, such as pine cones. Understanding the dynamics of such osmotically driven movements, in the influence of geometry and boundary conditions, is crucial to the controlled deformation of soft materials. We examine how thin elastic plates undergo rapid bending and buckling instabilities after anisotropic exposure to a favorable solvent that swells the network. An unconstrained beam bends along its length, while a circular disc bends and buckles with multiple curvatures. In the case of a disc, a large-amplitude transverse travelling wave rotates azimuthally around the disc. Theoretical interpretations inspired by the complementary thermal expansion problem of transient shape changes triggered by time-dependent heating are presented and allow collapse of time-dependent data on universal curves. Understanding the dynamics of strain-driven shape changes provides new insight into natural systems and control of advanced functional materials. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q14.00004: Sinusoidal to helical buckling of a thin rod under a cylindrical constraint James Miller, Arnaud Lazarus, Nathan Wicks, Jahir Pabon, Pedro Reis We investigate the buckling and post-buckling behavior of a thin, elastic rod loaded under cylindrical constraint. Our desktop experiments consist of compressing a hyper-elastic rod inside a transparent acrylic pipe with a motorized linear actuator. Under imposed displacement, the initially straight rod first buckles into a sinusoidal mode and eventually undergoes a secondary instability into helical buckling. This buckling and post-buckling behavior is found to be highly dependent on the systems' geometry, namely the rod length and the aspect ratio of the rod to pipe diameter. We quantify the wavelength and pitch of the period patterns through direct digital imaging and record the reaction forces at both end of the pipe. The observed behavior is rationalized through scaling arguments. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q14.00005: Shape evolution of a thin loop sedimenting in a viscous fluid James Hanna, Christian Santangelo We consider the non-local elastic problem of a closed thin filament settling under gravity in a fluid at zero Reynolds number. The filament is modeled as an inextensible chain, with no bending or twist rigidity. Although the equations admit rigid motions of the chain, there are no stable trajectories. We explore whether a stable envelope may exist around a recirculating blob and tail arrangement. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q14.00006: 3D micro-modeling of wrinkling phenomena Damien Eggenspieler, Gozde Ince, Karen Gleason, Mary Boyce Wrinkles, formed by the buckling of stiff layers adhering to soft substrates, are commonplace in nature. From wrinkles on smiling or aging faces to the wrinkled shape of pumpkins or the wrinkled electrospun nano-fibers due to the radial evaporation of the solvent used in the processing of these fibers, wrinkles have been found ranging from the nano- to the macroscopic scales. More recently, studies have shown that this buckling phenomenon can be directed via a selective stiffening of either ones of the layers composing this composite system. We are introducing a 3D numerical model for the buckling of a shell on a soft layer. The selective stiffening of the shell can reproduce the ``stiffness patterning'' obtained experimentally by UV-Ozone treatment of a soft PDMS substrate through a photomask. This model can predict the final shape of the surface of this composite system for periodic photomasks and might be used in the design of specific micro-topographies. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q14.00007: Coiling Spaghetti: Deposition of a Thin Rod onto a Moving Substrate Pedro Reis, Jungseock Joo, Josephine Mannent, Joel Marthelot, Danny Kaufman, Eitan Grinspun We investigate the oscillatory coiling patterns obtained when a thin elastic rod is deposited onto a moving solid boundary (conveyor belt). Through a combination of well controlled desktop experiments and numerics, we explore the phase diagram of this coiling process and identify the underlying physical ingredients. Our novel numerical method implements a discrete notion of bending and twist based on ideas ported from differential geometry, and exhibits excellent performance and robustness. This enables us to carry out predictive direct simulations of the large deformations of the thin elastic rod interacting with the moving substrate, that are in excellent agreement with our experiments. Applications of this coiling process range from the coiling of nanotubes to the laying down of transoceanic cable and pipelines in the ocean bed. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q14.00008: Geometry-ruled deformation of thin elastic shells Arnaud Lazarus, Pedro Miguel Reis We study the mechanical response of thin elastic shells subject to point or plate load and in different mechanical environments (with or without an in-out pressure difference). The geometry and material properties of the ellipsoidal shells used in our experiments can be accurately controlled using digital fabrication techniques. The linear and nonlinear mechanical response of the shells is quantified through load-displacement compression tests and the post-buckling patterns are analyzed using digital imaging. In the linear regime, we explore the geometry-induced rigidity of shells with different shapes. In the nonlinear regime, we focus on the formation of structures with localized curvature, which we denote by s-cones (shell-cones) and examine their mechanical and morphological properties. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q14.00009: The Buckliball: Pressure Induced Pattern Transformation of a Structured Elastic Shell Jongmin Shim, Claude Perdigou, Elizabeth R. Chen, Katia Bertoldi, Pedro Reis We report an experimental and computational study of a patterned elastic shell which, under pressure loading, undergoes a transformation in its structural configuration. The geometry of the ball comprises of an elastomeric spherical shell patterned with a regular array of circular holes. These voids are covered with a thin membrane, thereby making the ball air tight. Upon reduction of the internal pressure, the thin membranes first invert their curvatures inward. Consequently, beyond the critical pressure, the thin ligaments between the holes buckle leading to a cooperative buckling cascade of the skeleton of the ball. During this process, the initially circular holes evolve into an elliptical shape, and eventually become fully closed. This pattern transformation is induced by mechanical instability that opens the possibility for reversible encapsulation, over a wide range of length scales. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q14.00010: Polymer Thin Film Buckling: Wrinkling and Strain Localizations Yuri Ebata, Andrew B. Croll, Alfred J. Crosby Out of plane deformations of thin films are observed in everyday life, e.g. wrinkled aging human skin or folded fabrics. Recently, these deformations are being pursued for fabricating unique patterned surfaces. In this study, the transition from wrinkling, a low-strain buckling behavior, to localized deformations such as fold and delamination, is investigated for polystyrene films with thickness ranging from 5nm to 180nm. The thin films are attached to a uniaxially strained polydimethysiloxane substrate and the strain is released incrementally to apply increasing compressive strain to the attached film. The wavelength and the amplitude of local out-of-plane deformation are measured as global compression is increased to distinguish between wrinkling, folding, and delamination. The transition from wrinkling to strain localizing events is observed by tracking the statistics of amplitude distribution sampled across a large lateral area. A critical strain map is constructed to denote the strain regimes at which wrinkle, fold, and delamination occur. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q14.00011: Statistical Mechanics of Pressurized Shells Jayson Paulose, Gerrit Vliegenthart, Gerhard Gompper, David Nelson It is well known that thermal fluctuations strongly modify the large length scale elastic behavior of flat solid membranes. A thin spherical shell may be considered a solid membrane with a uniform nonzero curvature. This curvature couples the in-plane stretching modes with the out-of-plane undulation modes, giving rise to qualitative differences in the fluctuations of spherical shells compared to flat membranes. In addition, a shell can support a pressure difference between its interior and exterior. We study the statistical mechanics of deformations of a spherical shell using perturbation theory and Monte Carlo simulations, explicitly including the effects of curvature and pressure. Thermal corrections to the predictions of classical shell theory for point indentation and pressure-induced buckling experiments on microscale shells diverge as the ratio of shell radius to thickness tends to infinity. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q14.00012: Complex Morphogenesis from Elastic Instability of Thin Sheets Pascal Damman Thin sheets are mechanically unstable to boundary or substrate-induced compressive loads. Moderate compression results in regular wrinkling while further confinement can lead to crumpling. In this communication, we will first show the emergence of a new morphological instability triggered by a period-doubling bifurcation observed for large compression ratio. A periodic self-organized focalization of the deformation energy is observed provided a symmetry breaking, induced by the elastic foundation, occurs. This effect will be explained by considering geometrical nonlinearities leading to a Euler-Lagrange equation similar to the equation of a parametric resonance in nonlinear oscillator. In the second part, we will show that thin sheets, from suspended graphene to ordinary hanging curtains, under boundary confinement spontaneously generate a universal self-similar cascade of wrinkled patterns. We develop a formalism based on \emph{wrinklons}, a localized transition zone in the merging of two wrinkles, as building-blocks to describe the cascade morphology. These physical models based on elasticity and geometry constitutes a new theoretical toolkit to understand the morphology of various confined systems, such as coated materials or living tissues. Moreover, it also opens the way to new kind of microfabrication design of multiperiodic or chaotic (aperiodic) surface topography via self-organization. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q14.00013: Stability of a drop-strip system Marco Rivetti, S\'ebastien Neukirch, Arnaud Antkowiak When a flexible material is placed in contact with a liquid-air interface, capillary forces may cause deformations and large displacements in the structure. Such kind of elastocapillary interactions play a crucial role in many technological applications, like deflection of nanotubes carpets or microscale self-assembly. We study the problem of a drop deposited on a thin and narrow strip. Using a simplified 2D model including surface tension interactions, elastic and gravitational energies, we are able to predict the shape of the equilibrium solutions, as well as the appearance of instability in the system. Theoretical predictions are confronted to experiments and a good agreement is obtained. [Preview Abstract] |
Session Q15: Focus Session: Spins in Semiconductors - Quantum Dots and Nuclear Spins
Sponsoring Units: DMP GMAG FIAPChair: Ezekiel Johnston-Halperin, Ohio State University
Room: D171
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q15.00001: Protecting the electron spin coherence in a quantum dot with inhomogeneously polarized nuclear spins via dynamic nuclear polarization Wenxian Zhang, Jun Zhuang, J.Q. You An electron spin in a quantum dot is decohered by its surrounding nuclear spins via hyperfine coupling. During the dynamic nuclear polarization process, when the nuclear spins are polarized inhomogeneously by repeatedly injected polarized electron spins, a fully polarized nuclear spin core is formed. As a consequence, the polarized nuclear spin core can be harnessed to protect the electron spin coherence. In this way, we find that the electron spin coherence time can be extended tens times with the total nuclear polarization as low as 20\%, in contrast to the usual requirement of 90\% polarization, in a quantum dot with 256 nuclear spins. The total nuclear polarization goes even lower for larger quantum dots. The effect of the dipolar interaction between nuclear spins is also discussed. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q15.00002: Electrically detected nuclear magnetic resonance in GaAs/AlGaAs-based quantum point contacts Zachary Keane, Matthew Godfrey, Adam Burke, Jason Chen, Sebastian Fricke, Oleh Klochan, Adam Micolich, Harvey Beere, Dave Ritchie, Kirill Trunov, Dirk Reuter, Andreas Wieck, Alex Hamilton Nuclear magnetic resonance (NMR) is a well-known technique with widespread applications in physics, chemistry and medicine. Conventional NMR studies use inductive coils to detect the magnetic field produced by precessing nuclear spins; this approach requires on the order of $10^{12}$ spins for detection. Recently, resistive detection of NMR through the hyperfine interaction has been demonstrated with electrons in mesoscopic 2- and 1-dimensional devices based on high-quality GaAs/AlGaAs heterostructures. These studies are typically sensitive to $10^8$ spins, enabling NMR on much smaller sample volumes. Holes are predicted to have much weaker nuclear spin coupling than electrons, which could be relevant to the emerging fields of spintronics and quantum information processing. We present a preliminary comparison between the magnitude of the NMR signal in electron and hole quantum point contacts. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q15.00003: Enhancing 29Si Dynamic Nuclear Polarization Through Microwave Frequency Modulation Maja Cassidy, Men Young Lee, Charles Marcus We demonstrate up to a four-fold enhancement in the dynamic nuclear polarization (DNP) of silicon particles by applying an a.c. modulation to the microwave frequency used for irradiation of the electron spin system. The DNP enhancement is studied at temperatures ranging from 2-20 K and across a range of microwave powers. The total nuclear polarization is found to increase with decreasing temperature and increasing microwave power however, surprisingly, the polarization enhancement increases as the temperature is increased. The DNP enhancement is seen to increase with polarization time and is highest in spin-diffusion regime of polarization. By varying the frequency and amplitude of the applied modulation, dynamics of the electron spin system are probed. We find that the highest polarization enhancements are achieved with the frequency is modulated at a rate much greater than the electron spin lattice relaxation rate, where higher order electron spin processes can contribute to the polarization process. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q15.00004: Dynamic Nuclear Polarization in Double Quantum Dots Invited Speaker: We theoretically investigate the controlled dynamic polarization of lattice nuclear spins in GaAs double quantum dots containing two electrons.\footnote{M. Gullans, et. al., Phys. Rev. Lett. 104, 226807 (2010).} Three regimes of long-term dynamics are identified, including the build up of a large difference in the Overhauser fields across the dots, the saturation of the nuclear polarization process associated with formation of so-called ``dark states,'' and the elimination of the difference field. We show that in the case of unequal dots, build up of difference fields generally accompanies the nuclear polarization process, whereas for nearly identical dots, build up of difference fields competes with polarization saturation in dark states. The elimination of the difference field does not, in general, correspond to a stable steady state of the polarization process. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q15.00005: Spin-dependent tunneling into an empty lateral quantum dot Peter Stano, Philippe Jacquod In a recent experiment [Phys. Rev. B {\bf 78}, 041306(R) (2008)] Amasha {\it et al.} reported a strong spin dependence of the rate for electrons to tunnel into an empty quantum dot in a Zeeman field. Such dependence is intriguing, as one expects tunneling rates to depend on the orbital structure of the wavefunction, over which a Zeeman field has no effect. In search for an explanation, we find two mechanisms leading to a spin-dependent tunneling rate. The first originates from different electronic $g$-factors in the lead and in the dot, and favors the tunneling into the spin ground (excited) state when the $g$-factor magnitude is larger (smaller) in the lead. The second is triggered by spin-orbit interactions via the opening of off-diagonal spin-tunneling channels. It systematically favors the spin excited state. Numerically modeling the experimental setup, we find that in GaAs the spin-orbit interaction is unable to explain the experimental results, as it leads to no more than a $\sim$10\% discrepancy in the spin up vs spin down tunneling rates. We conjecture that the significantly larger discrepancy observed experimentally originates from the enhancement of the $g$-factor in the laterally confined lead. Reference: P. Stano and Ph. Jacquod, Phys. Rev. {\bf B} 82, 125309 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q15.00006: Theory of spin blockade in a triple quantum dots Chang-Yu Hsieh, Yun-Pil Shim, Pawel Hawrylak We present a theory of electronic properties and spin blockade in a linear triple quantum dots. We use micoroscopic LCHO-CI and double-band Hubbard model to analyze the electronic and spin properties of a triple quantum dots near a symmetrical quadruple point involving the (1,1,1) configuration which is essential for implementing quantum information processing with electron spin. We calculate spectral functions and relate them via the rate equation, including coupling with a phonon bath, to current as a function of applied bias. We show that the spin blockade in a triple quantum dots can serve as a spectroscopic tool to distinguish spin polarized states from spin depolarized states. We also show that a spin blockade is developed only at high bias when an onsite triplet state on the edge quantum dot connected to the source lead becomes accessible in the transport window. In contradiction to the case of double quantum dot molecule, the onsite triplet is not only essential for lifting spin blockade but also important for building up spin polarisation and spin blockade in the system. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q15.00007: Spin-polarized current generation in multiterminal quantum dot in Kondo regime Mikio Eto, Tomohiro Yokoyama We theoretically study the generation of spin-polarized current in a quantum dot with strong spin-orbit interaction, such as InAs quantum dot. As a minimal model, we consider two energy levels in a quantum dot, which is connected to $N$ leads through tunnel barriers. When an unpolarized current is injected from a lead, spin-polarized currents are ejected to other leads in the case of $N \ge 3$. First, we show that the spin polarization of the output currents is markedly enhanced by resonant tunneling, around current peaks of Coulomb oscillation, when the level spacing in the dot is smaller than the level broadening. Next, we examine the many-body resonance induced by the Kondo effect in the Coulomb blockade regime. A large spin current is created in the presence of the SU(4) Kondo effect when the level spacing is less than the Kondo temperature.\footnote{M.\ Eto and T.\ Yokoyama, J.\ Phys.\ Soc.\ Jpn., in press; arXiv:1010.5956.} [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q15.00008: Spin blockade in the optical response of a charged quantum dot Eleftheria Kavousanaki, Guido Burkard We theoretically model the population dynamics in a semiconductor quantum dot charged with a single electron in an optical pump-probe setup when the two lowest quantum dot levels are photoexcited. We calculate the differential transmission spectrum as a function of the time delay between the two circularly polarized optical pulses by using a density matrix formalism and treating intraband relaxation with the Lindblad equation. Taking into account both spin conserving and spin-flip relaxation processes we investigate the possibility for spin-dependent blocking of intraband relaxation due to the presence of the ground state electron for zero and finite magnetic fields. We show that the differential transmission spectrum is initially dominated by the fast spin-conserving mechanism before the slower spin-flip processes start to contribute at longer time scales. As a consequence of spin conservation for short time scales, we find a spin blockade effect in the optical recombination process. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q15.00009: Simulating electron spin entanglement in a double quantum dot M.A. Rodriguez-Moreno, A.D. Hernandez de la Luz, Lilia Meza-Montes One of the biggest advantages of having a working quantum-computing device when compared with a classical one, is the exponential speedup of calculations. This exponential increase is based on the ability of a quantum system to create and operate on entangled states. In order to study theoretically the entanglement between two electron spins, we simulate the dynamics of two electron spins in an electrostatically-defined double quantum dot with a finite barrier height between the dots. Electrons are initially confined to separated quantum dots. Barrier height is varied and the spin entanglement as a function of this variation is investigated. The evolution of the system is simulated by using a numerical approach for solving the time-dependent Schr\"{o}dinger equation for two particles. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q15.00010: Exchange coupling between hole qubits and between electron qubits in quantum dot molecules Alex Greilich, Stefan C. Badescu, Danny Kim, Allan S. Bracker, Daniel Gammon The exchange interaction between electron spins has been a paradigm for solid-state implementation of quantum gates. Holes are receiving an increasing attention for their reduced hyperfine coupling as compared to electrons in III-V semiconductors. Besides the isotropic exchange, both electrons and holes couple through spin-nonconserving interactions. Here we present detailed experimental evidence of these interactions for electrons and for holes in stacked InAs/GaAs quantum dots, achieved through electrical and magnetic fields that induce energy level resonances. Particularly large spin-mixing effects are found for holes, which involve their multi-band structure. We provide a theoretical understanding of the essential mixing mechanisms involved, tracing them down to system asymmetries and inhomogeneities. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q15.00011: Chiral spin currents and spectroscopically addressable single merons in quantum dots Catherine Stevenson, Jordan Kyriakidis We provide unambiguous theoretical evidence for the formation of correlation-induced isolated merons in rotationally-symmetric quantum dots beyond the lowest-Landau-level approximation. For experimentally accessible system parameters, unbound merons condense in the ground state at magnetic fields as low as B* = 0.3 T and for as few as N = 3 confined fermions. The four-fold degenerate ground-state at B* corresponds to four orthogonal merons characterised by their winding number $\pm$1 and topological charge $\pm$1. This degeneracy is completely lifted by the Rashba and Dresselhaus spin-orbit interactions, yielding spectroscopic accessability to individual merons. We further derive a closed-form expression for the topological chirality in the form of a chiral spin current and use it to both characterise our states and predict the existence of other topological textures in other regions of phase space. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q15.00012: Magnetoconductance of a Single-Electron Transistor in the Kondo Regime Tai-Min Liu, Bryan Hemingway, Andrei Kogan, Steven Herbert, Michael Melloch, Theo A. Costi We have measured the zero-bias conductance of a Single-Electron Transistor (SET) in the Kondo regime as a function of temperature, $T,$ and magnetic field, $B,$ oriented parallel to the plane of the device. Our SETs are fabricated on a GaAs/AlGaAs heterostructure with electron sheet density $4.8\times 10^{11}$ cm$^{-2}$ and mobility $5\times 10^5$ cm$^2$V$^{-1}$s$^{-1}$. Scaled plots of both the $T$ and $B$-dependent data show universal behavior. At moderate and high $B$, the magnetoconductance data show good agreement with renormalization group calculations in the spin-1/2 Kondo regime. At very low $B$, we observe a non-monotonic behavior: as $B$ increases, the conductance initially increases and only starts to decrease at a finite $B$. A possible explanation of this effect due to the presence of multiple orbital dot levels with similar energies will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q15.00013: Measuring mesoscopic spin currents by spin-to-charge conversion Philippe Jacquod, Peter Stano A number of theoretical investigations show that spin currents can be magneto-electrically generated by passing electric currents through spin-orbit coupled quantum dots. Measuring these currents has however not been achieved to date. In this talk, we present a theoretical proposal for measuring such mesoscopic spin currents with a voltage probe connected to the quantum dot via a single channel quantum point contact. We demonstrate that a spin current flowing through the quantum point contact results in an odd dependence of the charge current $I_{\rm qpc}$ on an externally applied Zeeman field, while this response is even in the absence of the spin current. The magnitude of the spin current is proportional to the zero field derivative of $I_{\rm qpc}$, with a constant of proportionality depending weakly on the geometry of the point contact. Numerical estimates suggest that in this way, mesoscopic spin currents can successfully be measured in GaAs quantum dots. [Preview Abstract] |
Session Q16: Electronic Structure III
Sponsoring Units: DCOMPChair: Pierre Darancet, Lawrence Berkeley National Laboratory
Room: D173
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q16.00001: Oxides -- a challenge for (theoretical) spectroscopy P. Rinke, H. Jiang, M. Scheffler, A. Greuling, M. Rohlfing, A. Janotti, E. Kioupakis, C. G. Van de Walle Oxides are of tremendous technological importance, yet challenging materials to characterize. In many cases the agreement between experimental and theoretical spectroscopy observed for other material classes has not been attained. We use rutile TiO$_2$ as an example to illustrate some of the problems. Many-body perturbation theory in the $G_0W_0$ approach based on density-functional theory in the local-density approximation gives a fundamental band gap of 3.3~eV in seemingly good agreement with the 3.3$\pm$0.5~eV measured in direct and inverse photoemission [1]. However, the lowest exciton computed in Bethe-Salpeter calculations for the optical spectrum is found at an energy of 3.21~eV, while optical experiments only give 3.03~eV [2]. Polaronic effects, i.e. the renormalization of the band edges due to electron-phonon coupling, reduce the band gap, but it remains a challenge to include the ionic contribution to the dielectric function, which can be substantial in oxides, in the $G_0W_0$ calculations and to incorporate both effects consistently into Bethe-Salpeter calculations. Another aspect to consider is the role of electron correlations. [1] Y. Tezuka \textit{et al.}, J.\ Phys.\ Soc.\ Jpn.\textbf{63}, 347 (1994). [2] J. Pascual \textit{et al.}, Phys.\ Rev.\ B \textbf{18}, 5606 (1978). [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q16.00002: Calculated Electronic Properties of Rutile TiO$_{2}$ and Cubic SrTiO$_{3 }$ Chinedu Ekuma, Diola Bagayoko We present preliminary, calculated, electronic properties of rutile titanium dioxide (TiO$_{2})$ and of cubic strontium titanate (SrTiO$_{3})$. Our computations employed local density approximation (LDA) and generalized gradient Approximation (GGA) potentials for TiO$_{2}$ and SrTiO$_{3}$, respectively. We implemented the linear combination of atomic orbitals (LCAO) within the framework of the Bagayoko, Zhao, and Williams (BZW) method. In doing so, we solved, self-consistently, both the Kohn-Sham equation and the equation giving the ground state charge density in terms of the wave functions of the occupied states. Our preliminary findings indicate that TiO$_{2}$ has an indirect band gap of 2.95 eV, from $\Gamma $ to R. The direct gap at $\Gamma $ is 0.10 eV larger. The indirect band gap of SrTiO$_{3}$, from L to $\Gamma $ or X, is 3.05 eV. \\[4pt] Work funded in part by the Louisiana Optical Network Initiative (LONI), the National Science Foundation (NSF) and the Louisiana Board of Regents [Award Nos. EPS-1003897 and NSF (2010-15)-RII-SUBR], and and Ebonyi State, Federal Republic of Nigeria [Award No: EBSG/SSB/FSA/040/VOL. VIII/039] [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q16.00003: The confinement error corrections for the exchange energy in transition metal oxides Feng Hao, Rickard Armiento, Ann E. Mattsson We present some recent advances towards a straightforward scheme to correct for the confinement errors of the exchange energy of the transition metal oxides (TMO). This approach includes two steps: (i) identifying the spatial regions where the confinement errors exist, using local density and kinetic energy density information, and (ii) mapping these spatial regions to harmonic-oscillator (HO) models [1], and quantifying and correcting the relative confinement errors based on the model system. The scheme has been applied to calculations with several local and semi-local functionals, and a trend of improvement for the equilibrium structure is obtained after applying these confinement error corrections. Sandia is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.\\[4pt] [1] Hao et al, PRB {\bf 82}, 115103 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q16.00004: First principles study of electronic and structural properties of CuO Burak Himmetoglu, Matteo Cococcioni The ground state of CuO is particularly challenging to study with DFT-based computational techniques even below its Neel temperature. This situation is due to the inability of most approximate DFT energy functionals to describe electronic regimes hat are dominated by many-body effects. In this study, we show how a description of the ground state of this material in better agreement with observations can be obtained using extended Hubbard-based corrective energy functionals (DFT+U and DFT+U+V). In particular we uncover an orbitally ordered insulating ground state for the cubic phase of CuO (that was expected, but never reported before) whose appearance is determined by a fine interplay between correlation effects and magnetic interactions. Starting from this ground state we also study the tetrahedral distortion of the unit cell (recently reported in experiments), characterizing the reorganization of the electronic states and identifying all the equilibrium structures. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q16.00005: Transition metal dioxides: a case for the intersite term in Hubbard-model functionals Heather Kulik, Nicola Marzari Triatomic transition-metal oxides in the ``inserted dioxide'' (O-M-O) structure represent one of the simplest examples of systems that undergo qualitative geometrical changes via subtle electronic-structure modulation. We discuss three transition-metal dioxide molecules (MO$_2$ where M = Mn, Fe, or Co), for which equilibrium structural (eg bent or linear geometry) and electronic (eg spin or symmetry) properties have been challenging to assign both theoretically and experimentally. Augmenting a standard density-functional theory (DFT) approach with a Hubbard term (DFT+$U$) occasionally over-localizes the $3d$ manifold, leading to incorrect bond elongation and, in turn, poor equilibrium geometries for MO$_2$ molecules, while preserving good spin-state splittings. We recover a proper description of both geometry and energetics for these molecules through either calculating DFT+$U$ relaxations at fixed M-O bond lengths or by inclusion of an inter-site interaction term $V$ that favors M($3d$)-O($2p$) interactions. In the latter case, both $U$ and $V$ are calculated fully from first-principles and are not fitting parameters. Finally, we present an approach that more accurately determines the Hubbard $U$ over a coordinate in which the character of bonding varies. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q16.00006: On the origins of the deficiencies of density functional theory exchange-correlation functionals for transition metal oxides Ann E. Mattsson, Rickard Armiento, Feng Hao The transition metal oxides (TMO) are a class of compounds that are difficult to treat in density functional theory (DFT) with simple local and semi-local functionals. Especially for CuO, they failed to give the correct equilibrium monoclinic structure. The major source of the deficiency is attributed to the imperfect cancellation of the electronic self-interaction (SI) in the approximated exchange energy. Previous studies [1] show that a large part of the SI error is connected to the confinement error that can be modeled by harmonic-oscillator (HO) systems. We discuss recent advances towards a simple methodology to quantify the confinement errors in real TMO systems. Our results show that these confinement errors may account for the deficiencies of DFT functionals in obtaining the correct equilibrium structure of the TMO. Sandia is a multiprogram laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.\\[4pt] [1] Hao et al, PRB {\bf 82}, 115103 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q16.00007: Ab initio calculation of the orbital magnetization by Wannier interpolation Graham Lopez, David Vanderbilt, Ivo Souza, Timo Thonhauser We present an analytic, first-principles scheme to efficiently calculate exactly the spontaneous orbital magnetization of ferromagnetic crystals [1,2]. This is in contrast to the standard method of integrating inside muffin-tin spheres which, while a good approximation in practice, is still an approximation. The method uses Wannier interpolation to perform the necessary Brillouin-zone integrals in a similar way as was done previously for the anomalous Hall conductivity [3]. The method has been implemented to work with a plane-wave density-functional code, and calculations were done on iron, cobalt and nickel. We compare our calculations of the orbital magnetization in these systems to recent ab initio and experimental results and find good agreement with both [4]. \\[4pt] [1] T. Thonhauser et al., Phys. Rev. Lett. {\bf 95}, 137205 (2005).\newline [2] D. Xiao et al., Phys. Rev. Lett. {\bf 95}, 137204 (2005).\newline [3] X. Wang et al., Phys. Rev. B, {\bf 74}, 195118 (2006).\newline [4] D. Ceresoli et al., Phys. Rev. B {\bf 81}, 060409(R) (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q16.00008: Origin for the disorder-induced quantum criticality in NbFe$_2$ Aftab Alam, Duane Johnson Using KKR-CPA ab-initio electronic-structure method founded on an optimal site-centered basis-set, we investigate the key features giving rise to the quantum critical transitions observed in NbFe$_2$ upon doping with 1.75{\%} Nb [1]. These phase transitions involve lowest-energy excitations at/near the Fermi surface. In particular, it is suggested to arise due to an accidental unconventional band critical point (uBCP) with vanishing quasi-particle velocity [2]. Moving off-stoichiometry by increasing Nb, or reducing electrons (e/a), we find the Fermi level E$_f$ increases (rather than decreases based only on band-filling) and meet the uBCP to produce excitations driving the anomalies. We detail the concentration-dependence electronic dispersion, density of state, E$_f$ shift, and energies for NbFe$_2$, and why disorder increase the E$_f$ with electron loss. At stoichiometry all our results agree with those from full potential calculations, including itinerant magnetism.\\[4pt] [1] D. Moroni-Klementowicz et al., Phys. Rev B 79, 224410 (2009).\\[0pt] [2] Brian Neal, Erik R. Ylviskar, and Warren E. Pickett, private communication (2009). [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q16.00009: Electronic structures of superionic conductor Li$_3$N Masaru Aoki, Yoshiyuki Ode, Kazuo Tsumuraya Lithium nitride is a superionic conductor with high Li conductivity. The compound has been studied extensively because of its potential utility as electrolyte in solid-state batteries. Though the mobility of the cations within the crystalline solid is high comparable to that of molten salts, the mechanism of the high mobility of the cations remains unsolved. To clarify the origin of the mobility we investigate the electronic states of the Li cations in the Li$_3$N crystal with the first principles electronic structure analysis, focusing a correlation between the cations and the ionicities of the constituent atoms. We have found the existence of the covalent bonding between the Li atoms in the Li$_3$N crystal in spite of the ionized states of the constituent atoms. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q16.00010: Why are the copper cations superionic in the $\alpha $-CuI crystal? Kazuo Tsumuraya, Takamitsu Ohtsuka, Hidekazu Tomono The mechanism of the superionic conduction is an unresolved issue in the solid-state physics. The cations are mobile species in $\alpha $-CuI and $\alpha $-AgI crystals. In these conductors, the constituent atoms are ionized. The clarification of the mechanism of the high mobility of the cations needs to investigate the electronic structures in the $\alpha $-CuI crystal. We obtain the dynamically-averaged local (DAL) positions of the mobile copper cations in the crystal from the pair distribution function and the angle distribution functions, which we calculate from the first principles molecular dynamics simulations at 700 K\@. The positions predict the existence of a correlation among the cations in the $\alpha $-CuI. The static electronic structure analysis, of the DAL structure, allows us to clarify the correlation. The correlation enables us to clarify the mechanism of the migration and the difference in the electronic structures between the conductors and the ionic crystals. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q16.00011: First-principles investigation of band offsets and dielectric properties of Silicon-Silicon Nitride interfaces Tuan Anh Pham, Tianshu Li, Francois Gygi, Giulia Galli Silicon Nitride (Si3N4) is a possible candidate material to replace or be alloyed with SiO2 to form high-K dielectric films on Si substrates, so as to help prevent leakage currents in modern CMOS transistors. Building on our previous work on dielectric properties of crystalline and amorphous Si3N4 slabs [1], we present an analysis of the band offsets and dielectric properties of crystalline-Si/amorphous Si3N4 interfaces based on first principles calculations. We discuss shortcomings of the conventional bulk-plus line up approach in band offset calculations for systems with an amorphous component, and we present the results of band offsets obtained from calculations of local density of states. Finally, we describe the role of bonding configurations in determining band edges and dielectric constants at the interface.\\[4pt] [1] T. Anh Pham et al., Appl. Phys. Lett., 96, 062902 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q16.00012: STEM-EELS calculations including both fine structure and diffraction M.P. Prange, M.P. Oxley, S.J. Pennycook, S.T. Pantelides Electron energy loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) probes electronic excitations with high spatial and energy resolution. Interpretation of the spectra requires accurate treatment of both the diffraction of the electron probe and the electronic excitation of the sample. We present a theory of core loss STEM-EELS based on a detailed calculation of the mixed dynamic form factor (MDFF) using DFT which informs a Bloch wave treatment of the probe/sample interaction. No dipole approximation is made. The probe diffraction is computed using the Bloch wave method which includes the microscope geometry, multiple elastic, and thermal diffuse scattering. We illustrate the method with calculations of complex oxide materials. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q16.00013: Extended Pi-Sigma tilde orbital model for CO adsorption on Pt and Ru Thomas Mion, Nicholas Dimakis, Faisal Alamgir, Cherno Jaye, Daniel Fischer, Paul McGinn, James Cooper, Steve Greenbaum, Eugene Smotkin Several discrepencies between the predicted Blyholder-type adsobtion models and experimental, as well as DFT calculated infared spectra have been addressed for atop CO on Pt in contrast to Ru. This model correlates increased Near Edge X-Ray Absorbtion Fine Structure intensity as the result of a sub-eV downshift from CO on Ru compared to CO on Pt thereby forming a weaker C-O bond. The model accounts for the hybrid orbitals electron transfer between the CO - metal bonds while taking in to consideration the orbital polarization within the CO itself. The charge redistribution of the s-tilde orbitals and reduced charge donation from CO to the surface results in a weaker internal CO bond upon Ru relative to Pt. The extended Pi-Sigma model explains why atop C-O stretching frequencies do not correlate with carbon p-type vacancies. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q16.00014: Chirality and Electronic Structure of the Thiolate-Protected Au$_{38}$ Nanocluster Olga Lopez-Acevedo, Hironori Tsunoyama, Tatsuya Tsukuda, Hannu H\"akkinen, Christine M. Aikens Our joint computational and experimental investigation of the structural properties of the Au$_{38}$(SR)$_{24}$ gold protected nanocluster will be presented [1]. We have identified a new low-energy, chiral, D$_3$ symmetric structure that yields an excellent match between computed and measured powder XRD function. We have characterized the electronic shell structure of this nanocluster in terms of a particle-in-a-cylinder model. The CD response in the low-energy region (below 2.2 eV) of the new structure is very similar to the one reported several years ago from experiments for Au$_{38}$(SG)$_{24}$. The mechanism of the chiral response for low excitation energies is related to the chiral arrangement of the gold-thiolate ligand shell around the bi-icosahedral Au$_{23}$ core. The determination of the total structure of Au$_{38}$(SC$_2$H$_4$Ph)$_{24}$ nanoparticles by single crystal X-ray crystallography confirmed our results [2]. \\[4pt] [1] O. Lopez-Acevedo et al J. Am. Chem. Soc., 2010, 132 (23)\\[0pt] [2] Qian et al J. Am. Chem. Soc., 2010, 132 (24) [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q16.00015: Pseudospectral Calculation of Helium Wave Functions, Expectation Values, and Oscillator Strength Paul Grabowski, David Chernoff We extend the pseudospectral method from the solution of Schr\"{o}dinger's equation for two-electron atom S states to arbitrary angular momentum states. We evaluate the oscillator strength for the helium $1^1$S $\to$ $2^1$P transition. The result, 0.27616499(27), compares favorably to the best determination in the literature. The length, velocity, and acceleration expressions all have roughly the same accuracy in a pseudospectral treatment. We evaluate leading order finite-nuclear-mass and relativistic corrections for the helium ground state. The pseudospectral method achieves near state-of-the-art accuracy without requiring the implementation of any special-purpose numerical treatments. All the relevant quantities tested converge exponentially with increasing resolution and at roughly the same rate. Quantum mechanical matrix elements are directly and reliably calculable with pseudospectral methods. A general prescription is given for choosing coordinates and subdomains to achieve exponential convergence when two-particle Coulomb singularities are present. [Preview Abstract] |
Session Q17: Focus Session: Bulk Properties of Complex Oxides - Ruthenates
Sponsoring Units: DMP GMAGChair: Ward Plummer, Louisiana State University
Room: D174
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q17.00001: High-pressure Synthesis and Magnetic Properties of 4d and 5d Transition-metal Oxides Invited Speaker: The pressure effect on synthesis of oxides with perovskite ABO$_{3}$ and perovskite-related structures has become more clear in recent years. The geometric tolerance factor t $\equiv $ (A-O)/$\surd $2(B-O) measures the structural stability. High-pressure synthesis enlarges the range of the t factor where the perovskite structure can be stabilized. For the ABO$_{3}$ compounds with t $>$ 1, high pressure reduces the t factor since the A-O bond is more compressible than the B-O bond. Therefore, perovskite would be the high-pressure phase for ambient-pressure polytype structures. However, the bonding compressibility argument is no longer valid for the ABO$_{3}$ with t $<$ 1. A dt/dP $>$ 0 is normally obtained for t less than but very close to 1$, i.e.$ the orthorhombic distortion becomes smaller under pressure. For those highly distorted perovskites with t factor far less than one, pressure enlarges further the orthorhombic distortion and eventually leads to a phase transition to the post-perovskite phase. As for PbRuO$_{3}$, high pressure prefers the small-volume perovskite phase relative to a competitive pyrochlore phase Pb$_{2}$Ru$_{2}$O$_{7}$. Understanding the pressure effect and the new capacity provided by a Walker-type multianvil press enabled us to expand the perovskite family and to obtain new phases of 4d and 5d oxides. Studies of these new 4d and 5d oxides allow us not only to address long-standing problems, but also to explore exotic physical properties. (1) In the perovskite ARuO$_{3}$ (A= alkaline earth), we have completed the phase diagram from A= Ca to Sr and to Ba and also accounted for the A-cation size-variance effect. A systematic study of the Curie temperature T$_{c}$ and the critical behavior as a function of the average A-site size and the size variance as well as external high pressures reveals explicitly the crucial role of the lattice strain on the ferromagnetism. The mean-field critical behaviour near T$_{c}$ found previously in SrRuO$_{3}$ is not typical of these perovskite ruthenates. T$_{c}$ is completely suppressed by Pb doping in Sr$_{1-x}$Pb$_{x}$RuO$_{3}$ not due to the steric effect, but to the orbital hybridization between Pb$^{2+}$ 6s and Ru$^{4+}$ 4d. As the end member, metallic PbRuO$_{3}$ undergoes a first-order phase transition to a metallic \textit{Imma} phase at T$_{t} \approx $ 90 K. (2) A new polytype phase 5H has been synthesized under a narrow pressure range, which fits the structural sequence along with other polytypes 9R, 6H and 3C of BaIrO$_{3}$. The ground states of these BaIrO$_{3}$ polytypes evolve from a ferromagnetic insulator with T$_{c }\approx $ 180 K in the 9R phase to a ferromagnetic metal with T$_{c }\approx $ 50 K in the 5H phase, and finally to an exchange-enhanced paramagnetic metal in the 6H phase, which may be close to a quantum critical point. (3) In the Ca$_{1-x}$Sr$_{x}$IrO$_{3}$ system, high pressure stabilizes the post-perovskite structure on the Ca side (x $<$ 0.3), but favors the perovskite structure on the Sr side (x $>$ 0.6). Refs. J.-G. Cheng, \textit{et al.} PRB \textbf{80}, 104430(2009); \textbf{80}, 174426 (2009); \textbf{81}, 134412(2010); JACS \textbf{131}, 7461(2009). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q17.00002: Spectroscopic Imaging - Scanning Tunneling Microscopy studies of the Nematic Metamagnet Sr$_3$Ru$_2$O$_7$ M.P. Allan, T.-M. Chuang, Y. Xie, A.W. Rost, R.S. Perry, J.-F. Mercure, A. Gibbs, A.P. Mackenzie, J.C. Davis The metamagnetic perovskite Sr$_3$Ru$_2$O$_7$ can be tuned towards a putative quantum critical point in an external magnetic field, and in ultrapure samples, an electronic nematic forms in a small region of the phase diagram around this putative quantum critical point. Much insight about these phenomena in Sr$_3$Ru$_2$O$_7$ come from a wealth of high-quality thermodynamic experiments but little is known about the microscopic electronic origin of criticality and nematicity. We recently re-engineered our SI-STM to achieve sub-Kelvin temperatures and magnetic fields up to 9T, and are now imaging the local density of states in different regions of the Sr$_3$Ru$_2$O$_7$ phase diagram, including within the nematic phase. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q17.00003: Entropy Accumulation near Itinerant Magnetic Quantum Critical Points Jianda Wu, Lijun Zhu, Qimiao Si, Andreas Rost, Andy Mackenzie Quantum critical point (QCP) occurs at a continuous phase transition at zero temperature. It follows from general hyperscaling argument that, near a QCP, the Gr\"uneisen ratio (ratio of thermal expansion coefficient to specific heat) diverges and entropy accumulates [1]. The enhanced entropy has been observed near the field-induced metamagnetic QCP in Sr3Ru2O7[2]. Here we present a detailed theoretical study of entropy across itinerant-magnetic QCPs, with a focus on the ferromagnetic cases. We propose a regularization scheme for the effect of a dangerously irrelevant quartic coupling on the free energy [3], and calculate the entropy using this scheme. While the entropy accumulation near the QCP basically follows the hyperscaling arguments, the correction to scaling is sizeable especially for the two-dimensional case. We compare the theoretical results with the experimental data for Sr3Ru2O7 [2], providing an entropic characterization of the degree to which the metamagnetic QCP in this system is described by the itinerant-magnetic quantum criticality. \\[4pt] [1] L. Zhu et al, PRL 91, 066404 (2003).\\[0pt] [2] A.W. Rost et al, Science 325, 1360 (2009).\\[0pt] [3] J. Wu, L. Zhu, and Q. Si, arXiv:1010.4593, to appear in J. Phys.: Conf. Series. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q17.00004: Thermodynamics of the critical fluctuations and nematic phase formation of Sr$_3$Ru$_2$O$_7$ Andreas W. Rost, Robin S. Perry, Jean-Francois Mercure, Santiago A. Grigera, Andrew P. Mackenzie The itinerant metamagnet Sr$_3$Ru$_2$O$_7$ has motivated a wide range of experimental and theoretical work in recent years because of the discovery of an unusual low temperature phase which is forming in the vicinity of a proposed quantum critical point. The transport properties of this phase which exhibit strong electron-nematic-like behaviour [1] have led to a range of theoretical proposals for the underlying physics [2]. A major challenge both experimentally and theoretically is the investigation of the thermodynamic properties of both this unusual phase and the fluctuations associated with the quantum critical point. Here I will report on recent thermodynamic measurements. I will concentrate on new specific heat measurements investigating the nature of the critical fluctuations of the system as well as the low energy excitations of the novel phase. \\[4pt] [1] R.A. Borzi, S.A. Grigera, J. Farrell, R.S. Perry, S. Lister, S.L. Lee, D.A. Tennant, Y. Maeno \& A.P. Mackenzie, Science 315, 214 (2007). \newline [2] For a recent review, see E. Fradkin, S. A. Kivelson, M. J. Lawler, J. P. Eisenstein \& A. P. Mackenzie, Annual Review of Condensed Matter Physics 1, 153 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q17.00005: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q17.00006: Magnetic and structural transitions tuned by non-magnetic Ti doping in Ca3Ru2O7 Jin Peng, Gaochao Wang, Leonard Spinu, Xianglin Ke, Tao Hong, Zhiqiang Mao We report the effect of Ti doping on the structural and magnetic transitions in the bilayer Ruthenate Ca3Ru2O7. Ca3Ru2O7 orders antiferromagnetically at 56 K followed by a simultaneous structural and metal-insulator transition at 48 K [1]. Ti doping in Ca3Ru2O7 causes dramatic changes in both antiferromagnetic and structural transitions. With the Ti doping concentration above 5\%, both transitions move to much higher temperature and merge, e.g. TN~85 K for 5\% Ti, 113 K for 10\% Ti. For the latter sample, the structural parameters change much more remarkably through the transition compared to Ca3Ru2O7 [2]. Such structural and magnetic transitions tuned by Ti-doping highlight the strong spin-lattice coupling in Ca3Ru2O7. Neutron scattering measurement on these samples will also be discussed. \\[4pt] [1] Cao G et al; PRB 78, 1751 (1997)\\[0pt] [2] Yoshida Y et al; PRB 72, 054412 (2005) [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q17.00007: Element-resolved electronic and magnetic properties of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ V.B. Nascimento, J.W. Freeland, Biao Hu, R. Jin, E.W. Plummer Bulk Sr$_{3}$Ru$_{2}$O$_{7}$ is a metal with short-range antiferromagnetic correlation developed at low temperatures. In the Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ series, the partial substitution of Ru by Mn changes both electronic and magnetic correlations driven by the modification of lattice degree of freedom. We have employed polarized x-rays to perform an element-resolved study of electronic and magnetic properties of this system with 0 $<$ x $\le $ 0.5. Our results indicate that at low doping Mn goes in as an electron acceptor (i.e. Mn 3+) which effectively dopes holes into the Ru system in a systematic way with increasing x. Using x-ray magnetic circular dichroism we have extracted the Mn and Ru contributions to the total magnetic moment, which will be connected to measurements of the total moment. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q17.00008: Metastable Magnetic states in Ca$_{3}$Ru$_{2}$O$_{7}$ D. Fobes, J. Peng, Z.Q. Mao We have performed systematic in-plane angle dependent c-axis transverse magnetotransport measurements on the double layered ruthenate Ca$_{3}$Ru$_{2}$O$_{7}$ throughout a broad field and temperature range. Our results reveal the magnetic states for $H\|b$ to be significantly more complex than for $H\|a$. When magnetic field is applied along the b-axis we probe several metastable magnetic states in close proximity to phase boundaries of long-range ordered antiferromagnetic (AFM) states previously revealed by neutron scattering, i.e. AFM states with magnetic moments oriented along the b-axis (AFM-b) and a-axis (AFM-a); canted AFM state (CAFM) (Wei Bao \textit{et al.}, Phys. Rev. Lett. \textbf{100}, 247203 (2006)). These metastable states are characterized by magnetoresistivity anisotropy distinct from that seen in the AFM-a, AFM-b, or CAFM phases, and switch either to a weakly ferromagnetic or AFM-b state when the magnetic field is rotated toward the \textit{a}-axis. Additionally, our results highlight the complex nature of the spin-charge coupling in Ca$_{3}$Ru$_{2}$O$_{7}$. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q17.00009: Interplay of oxygen octahedral rotations and electronic instabilities in strontium ruthenate Ruddlesden-Poppers from first principles Johannes Voss, Craig J. Fennie The Ruddlesden-Popper ruthenates Sr$_{n+1}$Ru$_n$O$_{3n+1}$ display a broad range of electronic phases including $p$-wave superconductivity, electronic nematicity, and ferromagnetism. Elucidating the role of the number of perovskite blocks, $n$, in the realization of these differently ordered electronic states remains a challenge. Additionally dramatic experimental advances now enable the atomic scale growth of these complex oxide thin films on a variety of substrates coherently, allowing for the application of tunable epitaxial strain and subsequently the ability to control structural distortions such as oxygen octahedral rotations. Here we investigate from first principles the effect of oxygen octahedral rotations on the electronic structure of Sr$_2$RuO$_4$ and Sr$_3$Ru$_2$O$_7$. We discuss possible implications for the physics of the bulk systems and point towards new effects in thin films. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q17.00010: Ca$_{2}$Ru$_{1-x}$Cr$_{x}$O$_{4 }$(0 $<$ x $<$ 0.13): Negative volume thermal expansion via orbital and magnetic orders T.F. Qi, O.B. Korneta, M. Ge, L.E. De Long, G. Cao, S. Parkin, P. Schlottmann Ca$_{2}$RuO$_{4}$ undergoes a metal-insulator transition at T$_{MI}$= 357 K, followed by a well-separated transition to antiferromagnetic order at T$_{N }$= 110 K. Dilute Cr doping for Ru reduces the temperature of the orthorhombic distortion at T$_{MI}$ and induces ferromagnetic behavior at T$_{C}$. The lattice volume V of Ca$_{2}$Ru$_{1-x}$Cr$_{x}$O$_{4}$ (0 $<$ x $<$ 0.13) abruptly expands with cooling at both T$_{MI}$ and T$_{C}$, giving rise to a total volume expansion $\Delta $V/V $\approx $ 1 {\%}, which sharply contrasts the smooth temperature dependence of the few known examples of negative volume thermal expansion driven by anharmonic phonon modes. In addition, the near absence of volume thermal expansion between T$_{C}$ and T$_{MI}$ represents an Invar effect. The two phase transitions, which surprisingly mimic the classic freezing transition of water, suggest an exotic ground state driven by an extraordinary coupling between spin, orbit and lattice degrees of freedom. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q17.00011: Correlation between Structural and Magnetic Properties in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ Single Crystals Biao Hu, Gregory T. McCandless, O.V. Garlea, S. Stadler, E.W. Plummer, R. Jin We have studied the Mn-doping (x) dependence of structural and magnetic properties in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7 }$. The system remains tetragonal as determined by single-crystal X-ray diffraction with the lattice parameters $a$ and $c$ varying with x. Correspondingly, the value of Jahn-Teller distortion ($\Delta _{JT})$ of (Ru,Mn)O$_{6}$ octahedron decreases with increasing x with $\Delta _{JT }$= 1.0 for x$\sim $0.5. At the same doping level, we note the sign change of Curie-Weiss temperature $\Theta _{CW}$ derived from high-temperature magnetic susceptibility. The correlation between structural and magnetic properties will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q17.00012: Managing magnetization and antiferromagnetic coupling in epitaxially grown magnetic oxide heterostructures of (Ga,Fe)2O3 and SrRuO3 Jihye Lee, William Jo, Christian Meny , Francois Roulland, Nathalie Viart We have grown b-axis oriented epitaxial (Ga,Fe)2O3 (GFO) thin films on (111) oriented SrRuO3 (SRO) by pulsed laser deposition to know spin interaction in multilayer system. The easy axis of magnetization of the GFO is located on the plane of the thin films. On the other hand, SRO has unique anisotropic properties on various crystallographic directions in their structure. Magnetic properties of the films were measured as a function of temperature and external magnetic field by a superconducting quantum interference magnetometer. Curie temperature of SRO and GFO was measured at 150K and 370K, respectively. According to the direction of external magnetic field, the magnetic moment value of the GFO/SRO heterostructures show different behavior due to antiferromagnetic coupling. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q17.00013: ARPES Studies of the Evolution of the Ruthenate Family with Dimensionality John Harter, Dawei Shen, Carolina Adamo, Charles Brooks, Daniel Shai, Eric Monkman, Darrell Schlom, Kyle Shen The Ruddlesden-Popper homologous series of ruthenates exhibits a wide range of remarkable electronic phenomena coupled to dimensionality, from spin-triplet superconductivity in quasi-two-dimensional single-layer Sr$_{2}$RuO$_{4}$, to metamagnetism and nematicity in bilayer Sr$_{3}$Ru$_{2}$O$_{7}$, to ferromagnetism in the fully three-dimensional pseudocubic end-member SrRuO$_{3}$. We report high resolution angle-resolved photoemission spectroscopy measurements of the electronic structure of ruthenate films grown by molecular beam epitaxy, with particular interest in the evolution of the quasiparticle interactions with increasing dimensionality. We start by investigating the anisotropic renormalization of quasiparticles in SrRuO$_{3}$ by strong electron-boson coupling, and examine changes in the Fermi surface and associated quasiparticles as a function of temperature through the Fermi-liquid to non-Fermi-liquid crossover and above the ferromagnetic Curie temperature. We also investigate the strain dependence of the low-energy electronic structure and quasiparticle interactions of Sr$_{3}$Ru$_{2}$O$_{7}$. [Preview Abstract] |
Session Q18: Focus Session: Magnetic Oxide Thin Films - Multiferroic Thin Films
Sponsoring Units: DMP GMAGChair: Lane Martin, University of Illinois at Urbana-Champaign
Room: D172
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q18.00001: Enhanced magnetoelectric effects via strain engineering and structural softness Invited Speaker: After describing a general theory of the magnetoelectric response, I will argue that inducing {\em structural softness} -- i.e., tuning a material so that it takes a small amount of energy to distort its structure -- constitutes a general and robust strategy to obtain very large effects. Further, I will argue that this design strategy will be effective at room temperature, and will not affect other desirable properties of the materials (i.e., their insulating character). I will illustrate this possibility with first-principles results for thin films of room-temperature multiferroic BiFeO$_3$, where the structural softness is induced by epitaxial strain. I will also present results for BiFeO$_3$-based solid solutions, discussing several alternative mechanisms by which their electromechanical and magnetoelectric responses can be enhanced. Finally, I will discuss the prospect of inducing in BiFeO$_3$ a so-called {\em morphotropic phase boundary}, where the material is expected to display very large functional responses in analogy to what occurs in strong piezoelectric PbZr$_{1-x}$Ti$_{x}$O$_{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q18.00002: Electrically Controllable Magnetism in Strained BiFeO$_3$ Thin Films Qing He, W. Luo, R. Ramesh, J.-C. Yang, Y.-H. Chu, A. Scholl multiferroic BiFeO$_3$ (BFO) thin films epitaxial strain can lead to the formation of a mixed phase system -- highly distorted rhombohedral (R') and distorted tetragonal (super-tetragonal) (T) phases. Interestingly, this R' phase has been observed to be with enhanced magnetization compare to bulk BFO. Then, in order to investigate the origin of the magnetism in R' phase, synchrotron x-ray absorption, and x-ray (magnetic) circular/linear dichroism have been employed with assistance of spectra simulation and the ferroelectric, antiferromagnetic and ferromagnetic properties of this magnetic R' films have been clearly identified. Surprisingly, enhanced magnetization emerges in (001) plane as soon as a critical DC field is applied to the film in $<001>$ direction. The key is that the movement of Fe$^{3+}$ can be controlled by external electric field, which magnifies the effect of Dzyaloshinsky-Moriya interaction to the system, and enlarges the canting magnetic moment of Fe spins. Finally, the direction of the local magnetic moment can be deterministically by external electric field will be demonstrated. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q18.00003: Origin of Reversible Electric Exchange Bias Modulation in a Multiferroic Field Effect Device Stephen Wu, Shane Cybart, Pu Yu, R. Ramesh, R.C. Dynes We report the fabrication and characterization of two different oxide heterostructure based electric field effect devices: the multiferroic/ferromagnet, BiFeO$_{3 }$(BFO) / La$_{0.7}$Sr$_{0.3}$MnO$_{3 }$(LSMO) and the ferroelectric/ferromagnet, Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$(PZT)/LSMO. By switching FE polarization of BFO in the multiferroic device we observe a change in conductivity in the channel of 50{\%}, and a 55{\%} change in magnetic coercivity at 5.5 K. Furthermore, we can reversibly switch between two distinct exchange bias states corresponding to the different FE polarizations without additional field cooling. The difference in exchange bias between the two states is approximately 20mT. We further characterize the device by performing Hall Effect and temperature dependent exchange bias modulation measurements. Finally, we compare this device to a similarly fabricated PZT/LSMO field effect device. We observe no exchange bias and significantly smaller coercivity. No change in magnetic coercivity is observed when ferroelectric polarization is switched in PZT. Models based on these results will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q18.00004: Electrical Transport Measurements of a Manganite Multiferroic Field Effect Device James Parker, Shane Cybart, Stephen Wu, Pu Yu, R. Ramesh, R.C. Dynes We report electrical transport measurements of multiferroic/ferromagnet, BiFeO3 (BFO) / La0.7Sr0.3MnO3 (LSMO), electric field effect devices. The antiferromagnetic (AFM) ordering of the BFO dielectric layer is coupled to the ferromagnetic (FM) ordering of the LSMO channel layer and is observed as exchange bias --a shift of the LSMO magnetic hysteresis curve along the applied field axis. We will present the temperature dependence of this exchange bias between 2K and 100K. Furthermore, we also investigate the exchange bias with respect to multiple gating variables, including channel current and gate pulsing patterns. We have observed that the current in the channel during gating plays an important role in setting the exchange bias. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q18.00005: Magnetic Structure of Engineered Multiferroic Thin Films Invited Speaker: The intriguing properties of multiferroics, i.e., materials exhibiting the coexistence of magnetism and ferroelectricity, have stimulated intense research interest in recent years. From the viewpoint of practical applications, one needs to exploit the thin film architectures of multiferroic materials. However, fewer studies have addressed the magnetic structures of multiferroic thin films. I will present recent experimental works on two multiferroic films, BiFeO$_{3}$ and EuTiO$_{3}$, physical properties of which can be engineered via the epitaxial growth on appropriate substrates. Neutron diffraction studies on BiFeO$_{3}$ films deposited on vicinal SrTiO$_{3}$ substrates show that the magnetic structure of these films is closely correlated with the ferroelectric states that depend on the chosen substrate miscut [1]. Epitaxial EuTiO$_{3}$ grown on DyScO$_{3}$ susbtrate, which is a paraelectric antiferromagnet in its bulk form, is strain-tuned into multiferroics, displaying both ferroelectric and ferromagnetic characters [2]. The relationship between the strong magnetic anisotropy and the film microstructure will be discussed. \\[4pt] [1] X. Ke, P. P. Zhang, S. Baek, J. Zarestky, W. Tian, and C. B. Eom, Phys. Rev. B \textbf{82}, 134448 (2010). \\[0pt] [2] J.~H. Lee, L. Fang, E. Vlahos, X. Ke, Y.W. Jung \textit{et al}., Nature \textbf{466}, 954 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q18.00006: Electric-field control of spin waves at room temperature in multiferroic BiFeO$_{3}$ Maximilien Cazayous, Pauline Rovillain, Yann Gallais, Alain Sacuto, Marie-Aude Measson, Rogerio de Sousa, Dorothee Colson, Anne Forget, Manuel Bibes, Agnes Barthelemy A particularly exciting prospect in the field of spintronics is to use the wave like excitations of a magnetic material as a means to transmit and process information. This technology named magnonics relies on the control of spin waves. The key goal of magnonics is to read/write non-volatile spin information with minimal energy consumption. Multiferroic materials have at least two coupled magnetic and ferroelectric orders leading to electrical control of magnetic effects and vice-versa. Multiferroic materials are thus potentially interesting as a medium for spin-wave-based information processing. Here we show that the spin wave excitations in BiFeO$_{3}$, a room temperature multiferroic can be controlled by an electric field at low power and in a non-volatile way. The present experiment clearly demonstrates spin waves can be tuned over 30{\%} of their frequencies, several orders of magnitude larger than with previous methods. The switch and the control of the polarization is used to manage this tuning. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q18.00007: Electric-field control of spin waves in multiferroic BiFeO3: Theory Rog\'{e}rio de Sousa, P. Rovillain, Y. Gallais, A. Sacuto, M.A. M\'{e}asson, D. Colson, A. Forget, M. Bibes, A. Barth\'{e}l\'{e}my, M. Cazayous Our recent experiment [1] demonstrated gigantic (30\%) electric-field tuning of magnon frequencies in multiferroic BiFeO3. We demonstrate that the origin of this effect is related to two linear magnetoelectric interactions that couple the component of electric field perpendicular to the ferroelectric vector to a quadratic form of the N\'{e}el vector. We calculate the magnon spectra due to each of these interactions and show that only one of them is consistent with experimental data. At high electric fields, this interaction induces a phase transition to a homogeneous state, and the multi-magnon spectra will fuse into two magnon frequencies. We discuss the possible microscopic mechanisms responsible for this novel interaction and the prospect for applications in magnonics.\\[4pt] [1] P. Rovillain, {\it et al.}, Nature Materials advance online publication Nov. 14 2010 (DOI 10.1038/nmat2899), http://dx.doi.org/10.1038/nmat2899 [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q18.00008: Ultrafast dynamics in multiferroic BiFeO$_{3}$ Yu-Miin Sheu, Rohit Prasankumar, Antoinette Taylor We report the ultrafast time-resolved optical measurements of multiferroic BiFeO$_{3, }$which exhibits both magnetic and ferroelectric ordering at room temperature. The coupling between these two orders makes it an attractive material for potential data-storage devices. However, a detailed understanding of this coupling is still under debate. Ultrafast optical spectroscopy can potentially shed light on magnetoelectric coupling in BiFeO$_{3}$ by unraveling the different contributions in the time domain. Here, we use degenerate 400 nm pump-probe spectroscopy to excite and probe a BiFeO$_{3}$ thin film above its bandgap. The measured relaxation consists of a fast decay ($\sim $1 ps) followed by a slow recovery ($\sim $150 ps). We attribute the fast component to the recovery of photoexcited carriers. The slow recovery may be due to spin-lattice relaxation. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q18.00009: Substrate induced strain effects on the multiferroism of BiMnO$_{3}$ thin films Hyoung Jeen Jeen, Patrick Mickel, A.F. Hebard, Amlan Biswas, Valentin Craciun BiMnO$_{3}$ is a single phase multiferroic material, which shows ferroelectricity and ferromagnetism at low temperature. However, it is difficult to grow BiMnO$_{3}$ either in bulk or thin film form, since it is metastable with substantial desorption of Bi ions at high growth temperature. Here we have used SrTiO$_{3}$ and SrLaGaO$_{4}$ substrates, which provide different degrees of compressive strain, to stabilize the BiMnO$_{3}$ phase and have introduced fast quenching after deposition in oxygen atmosphere to suppress re-evaporation of Bi-ions but retain film crystallinity. Surface morphology indicates island growth mode. X-ray diffraction (XRD) shows that the BiMnO$_{3}$ [111] is parallel with [001] SrTiO$_{3}$ and [001] SrLaGaO$_{4}$. XRD results confirm that the BiMnO$_{3}$ films on SrTiO$_{3 }$substrates are epitaxial, with in-plane alignment. The films have a magnetization of 1 $\mu _{B}$/Mn at 5 T and 10 K. We also observed ferroelectricity in our BiMnO$_{3}$ films. Based on these results, we will discuss the substrate induced strain effects on the multiferroic properties of BiMnO$_{3}$ thin films. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q18.00010: Room-Temperature Multiferroic Properties of Bismuth Manganite Thin Film W.C. Kuo, C.Y. Kuo, H.J. Liu, H.J. Lin, Y.H. Chu, Y.C. Chen, T.M. Uen, J.Y. Juang In multiferroic materials, low temperature multiferroic properties in perovskite type BiMnO$_{3}$ has been demonstrated. In this work, through epitaxial strains exerted by LaAlO$_{3}$ single crystal substrate, we successfully demonstrate the growth the c-axis oriented new phase in BMO thin film. After the deposition, HRXRD has been used to characterize the lattice structure, which show new fascinating phase that are different from the papers early reported. We further revealed manganese valence through the X-ray Magnetic Linear Dichroism measurement and exhibit the mixed +3 and +4 valence of manganese. With LaNiO$_{3}$ bottom electrode, room-temperature ferroelectricity is demonstrated by piezoelectric force microscopy, which revealed the reversible ferroelastic switching through the external electrical bias. M-H curves are measured by SQUID magnetometer as a function of temperatures. Ferromagnetic behaviors have been probed from room temperature (300K) to low temperature (10K), furthermore, ferromagnetic phenomenon has been observed at 300K. In our preliminary results, room temperature ferroelectric and ferromagnetic properties could be coexistent in single phase material through epitaxial strain, thus provide a modeling system to study the multiferroic material and a powerful candidate for the next- generation electronic devices. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q18.00011: Magnetic field dependence of the spin wave excitations in Sr$_2$FeSi$_2$O$_7$ Kazuki Iida, Jooseop Lee, Naoyuki Katayama, Sungdae Ji, Seunghun Lee, Duc Le, Sung Chang, Taehwan Jang, Yoonhee Jeong, Sangwook Cheong Without field, the multiferroic Sr$_2$FeSi$_2$O$_2$ orders below 4.7 K into a simple square-lattice antiferromagnetic collinear structure. Application of a magnetic field at low temperatures induces four different magnetic phases with spontaneous electric polarization. We report inelastic neutron scattering measurements on a single crystal of Sr$_2$FeSi$_2 $O$_7$ under magnetic fields to investigate how the magnetic fluctuations change with field. In zero field, no dispersion was observed in $L$-direction, indicating that Sr$_2$FeSi$_2 $O$_7$ is a two dimensional magnet. The dispersion relation along ($H$ 0 0.5) shows a Goldstone mode arising from the magnetic Bragg position at (1 0 0.5) with a periodicity of $2 \times2\pi/a$, suggesting that the strongest magnetic interaction is between the nearest neighbor Fe$^{2+}$ ions. Under the field, the magnetic fluctuations become more complex than the simple splitting of the doubly degenerate Goldstone mode into two gapped modes. [Preview Abstract] |
Session Q19: Ising, Spin Glass, Frustrated Magnets
Sponsoring Units: GMAGChair: Daniel Stein, New York University
Room: D170
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q19.00001: Thermodynamics of the two-dimensional random-bond Ising model Creighton K. Thomas, Helmut G. Katzgraber The two-dimensional Ising spin glass possesses the disorder and frustration necessary to describe the rich behavior found in glassy materials with complex free-energy landscapes. Recently-developed exact algorithms for this model with arbitrary quenched bond disorder have allowed for equilibrium simulations of systems far larger than those accessible by other methods. We use a Pfaffian technique to measure thermodynamic quantities such as the specific heat and the domain-wall free energy to characterize the phase transitions in this model as either temperature or disorder strength is varied. We also present precision measurements on the disorder-temperature phase diagram of this model, including a detailed study of the reentrance that has been seen for bimodal disorder. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q19.00002: Competing Low-Temperature Phases in a Dilute Ising Magnet M.A. Schmidt, D.M. Silevitch, T.F. Rosenbaum, G. Aeppli LiHo(x)Y(1-x)F4 serves as a physical manifestation of the Ising model in transverse field with controllable disorder. At dilute Ho3+ dipole concentration, the combination of ferromagnetic and antiferromagnetic couplings via the spatial anisotropy of the dipolar coupling, disorder, and random internal fields combine to produce a variety of possible ground states. We show for x = 0.045 the ability to choose between spin liquid and spin glass behavior with proper thermal preparation. We present both linear and nonlinear magnetic susceptibility data as well as magnetic pump/probe techniques to quantify the stability of the liquid, and to probe the coupling between the spin states and the nuclear spin bath. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q19.00003: The $p,q$-binomial distribution applied to the Ising model Per H{\aa}kan Lundow, Anders Rosengren Monte Carlo simulations have shown that the $p,q$-binomial distribution closely fits the magnetisation distribution for the $d$-dimensional Ising model at all temperatures when $d>4$. It also fits well for some temperatures near $T_c$ for $d=2,3$ and especially so for $d=4$. At high and low temperatures, away from $T_c$, the $p,q$-distribution always fits extremely well. However, it appears very difficult to determine how the parameters $p$ and $q$ depend of the temperature. From high and low temperature series expansions we can get partial results on their temperature dependence. Near $T_c$ for $d=5$ we have approximately that $p=1-0.0736/L^5$ and $q=1-9.87/L^5$ whereas for $d<5$ the linear coefficient of $q$ grows logarithmically. We show numerically how the parameters behave near $T_c$ with increasing $d$. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q19.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q19.00005: An ab initio study of radiation damage effects on the magnetic structure of bulk Iron Yang Wang, G. Malcolm Stocks, Roger Stoller, Don Nicholson, Aurelian Rusanu, Markus Eisenbach A fundamental understanding of radiation damage effects in solids is of great importance in assisting the development of structural materials with improved mechanical properties for nuclear energy applications. In this presentation, we discuss our recent theoretical investigation on the magnetic structure evolution in bulk Fe after an energetic particle has disturbed the lattice by a displacement cascade. We applied a linear scaling ab initio method to the study of magnetic moment distributions in a low energy cascade for a series of time steps. The primary damage state and the evolution of the defects were simulated using molecular dynamics with a Finnis-Sinclair interatomic potential. We will show the statistics of the magnetic moments in the sample and discuss its relationship with the atomic volume distribution. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q19.00006: Chaos, broken hyperscaling, and nonuniversality in a spin glass A. Alan Middleton, Creighton Thomas, David Huse Recently extended precise numerical methods and newly modified scaling arguments allow for a coherent picture of the glassy state in a two-dimensional spin glass to be assembled. This glassy state, where the correlation length is larger than the system size, is characterized by ``chaos,'' the extreme sensitivity of the state to temperature. This chaos is shown to lead to a breakdown of hyperscaling in spin glasses. The length scale at which entropy becomes important is found to depend on the type of randomness, so that though there is a type of universality, the critical exponents depend on the distribution of disorder. The numerical simulations use multiprecision arithmetic to exactly compute the partition function in samples of sizes up to $L^2=512^2$ down to temperatures of less than $J/20$, where the typical strength of the disorder is $J$. These results can be used in support of studies of the non-equilibrium behavior of glassy models. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q19.00007: Disordered Pinned Anyons in Two Dimensions Chris Laumann, David Huse, Andreas Ludwig, Gil Refael, Simon Trebst, Matthias Troyer We consider the effect of disorder on the behavior of pinned anyons in two spatial dimensions. Within an approximate numerical strong disorder renormalization group (SDRG) treatment, we find that both Fibonacci and Majorana anyons exhibit flows back to weaker disorder rather than toward infinite randomness phases such as those they exhibit in d=1. Restricting to the technically simpler Majorana anyons, we map out the effects of sign and coupling strength disorder on the proposed translation invariant topological liquid found in the absence of disorder. In disordered Hall bars, the nature of this descendant phase, and in particular its localization properties, may be relevant to the interpretation of transport and non-Abelian interferometry. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q19.00008: Effective potential study of the Diluted Antiferromagnet in a Field David Yllanes, L.A. Fernandez, V. Martin-Mayor We present a numerical study of the three-dimensional Diluted Antiferromagnet in a Field (DAFF), one of the experimental realizations of the Random Field Ising Model. We work in a constrained ensemble (tethered ensemble) where the Helmholtz effective potential is featured, rather than the free energy. Our method cures the problem of a strong violation of self-averaging, thus allowing us to compute the correlation length for systems sizes up to $L=32$. This quantity, when measured in units of the lattice size, is independent of the system size at the critical point, a strong indication of a second-order phase transition. This scale invariance allows us to apply finite-size scaling in the form of Nightingale's phenomenological renormalization. We obtain accurate estimates of the critical exponents. Since our method reconstructs the effective potential, we can also compute accurately the hyperscaling violation exponent. We perform as well an investigation of the geometrical properties of the instanton-like configurations, namely, the minimal cost configurations joining the two ordered phases. This study sheds light on previous claims of a first-order phase transition in this system. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q19.00009: Quantum Fidelity Susceptibilities of the Anisotropic Triangular Antiferromagnet: Conjugate Field Fidelity Susceptibilities Mischa Thesberg, Erik S. Sorensen The Heisenberg model of the Anisotropic Triangular Antiferromagnet(HATM) has seen a surge of interest owing to its relation to Cesium Copper Chloride, an inorganic salt with a potential spin-liquid phase. In this talk a new approach to quantum fidelity susceptibilities will be introduced and used to explore the phase diagram of the HATM. These fidelity susceptibilities are computable via exact diagonalization techniques and can be coupled to specific order parameters. We present results from such calculations shedding new light on the phase diagram of the Anisotropic Triangular Antiferromagnet. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q19.00010: Stabilization of surface spin glass behavior in core/shell-Fe67Co33/CoFe2O4 nanoparticles Ghulam Jaffari, Syed Ali, Syed Hasanain, Gernot G\"untherodt, Syed Shah Magnetic properties of Co33Fe67--CoFe2O4 (core-shell) nanoparticles are presented. Both dc magnetization and ac susceptibility measurements indicate a spin glass (SG) like transition occurring at $T$F $\sim $175 K. The SG nature of the transition is also confirmed by the field dependence of the freezing temperature $T$F(H) following the well known Almeida--Thouless line, $\delta T$F$\sim $H$^{2/3}$. Additionally, the particles exhibit a large exchange bias ($H$EB $\sim $ 1357 Oe) arising from the core-shell (ferromagnetic-SG) coupling. The unusually high SG transition temperature and large exchange bias effects are attributed to a combination of several factors including the thickness of the amorphous oxide shell and large values of the exchange and anisotropy constants associated with the CoFe2O4 shell. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q19.00011: Critical Behavior of L\'evy Spin Glasses Juan Carlos Andresen, Helmut G. Katzgraber Universality, one of the foundations of the theory of critical phenomena, is well established for many problems in statistical physics. However, there is still debate if changing the disorder between the spins-spin interactions in spin glasses can influence the universality class of the system. This apparent violation of universal behavior can be attributed to the numerical complexity of these systems which limits simulations to small systems sizes, typically paired with strong corrections to scaling. Although it is well established that universality is not violated for nearest-neighbor spin glasses with compact disorder distributions (e.g., Gaussian and bimodal), some studies suggest that this might not be the case when the disorder distributions are broad, as in the case of the L\'evy distribution. Using large-scale Monte Carlo simulations that combine parallel tempering with specialized cluster moves, as well as innovative scaling techniques, we show that L\'evy spin glasses do obey universality for the system sizes studied. Furthermore, we probe recent analytical predictions made for the critical temperature of L\'evy spin glasses as a function of the disorder distribution width. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q19.00012: Spin glasses on scale-free networks: Simple models to describe opinion formation? Helmut G. Katzgraber, Creighton K. Thomas We study the critical behavior of Ising spin glasses on scale-free networks using large-scale Monte Carlo simulations. Our results show that when the exponent that describes the decay of the interaction degree in the scale-free graph is strictly larger than 3 the system undergoes a finite-temperature spin-glass transition. However, when the exponent is equal to or less than 3, the spin-glass phase is stable for all temperatures. This robustness to local (temperature) perturbations and global biases (field) is compared to experimental data from social networks. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q19.00013: Using Azimuthal Hysteresis for Determining the Anti-ferromagnet Moment Density at the Spin Glass Interface: The case of BFO Khalid Ashraf, Sayeef Salahuddin We report a systematic procedure for extracting the anisotropies, exchange energies and the surface anti-ferromagnet (AFM) moment of AFM-ferromagnet (FM) systems that show spin glass (SG) behavior. In any SG system, the hysteresis characteristics at a critical angle combined with the azimuthal hysteresis properties give important information about the surface AFM moment density and the coupling energy. Using this scheme we report the interface magnetic energy parameters of the epitaxial BFO-FM system. We find a single value for the interface coupling energy that reproduces both the exchange bias and the enhancement. Our extracted surface AFM moment density is of the order of the FM moment density that is independent of the FM material used. The high moment density on the BFO surface indicates a significant magnetic property modification at the BFO-FM interface. The implication of the presence of this high AFM moment is discussed in the context of achieving deterministic electric field driven magnetic moment switching. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q19.00014: Study of the Gr\"{u}neisen Parameters at a Field-induced Quantum Critical Point in NiCl2-SC(NH2)2 Franziska Weickert, Robert Kuechler, Alexander Steppke, Luis Pedrero Ojeda, Michael Nicklas, Manuel Brando, Frank Steglich, Vivien Zapf, Marcelo Jaime, A. Paduan-Filho NiCl$_{2}$-SC(NH$_{2})_{2}$, also known as DTN, is a quantum paramagnet, where the Ni$^{2+}$ single ion anisotropy $D$ = 8.9K opens an energy gap between the $S_{z}$ = 0 ground state and the $S_{z}=\pm $1 exited state. In this material an XY-antiferromagnetic ordered state is induced at low temperatures by applying magnetic fields between $H_{c1} \quad \approx $ 2T and $H_{c2}$ = 10.5T. At the phase boundaries critical exponents consistent with Bose-Einstein condensation of magnons are found. Here we present investigations of quantum criticality close to $H_{c1}$ by thermal expansion, magnetization and specific heat measurements. Our data reveal a divergency for T$\to $ 0 of the thermal and magnetic Gr\"{u}neisen parameters as expected for a quantum critical point of a diluted Bose gas. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q19.00015: Characteristic time scales and overlap distributions in replica exchange Monte Carlo simulations of spin glasses Burcu Yucesoy, Jon Machta, Helmut G. Katzgraber We present a large-scale numerical study using replica exchange Monte Carlo (parallel tempering) of time scales of the three- dimensional Ising spin glass. We measure the integrated and exponential autocorrelation times for several observables, as well as the round-trip times for different disorder realizations in order to investigate the relationship between the characteristic time scales of a disorder realization and its overlap distribution. [Preview Abstract] |
Session Q21: THz and Impedance Spectroscopy
Sponsoring Units: GIMSChair: Rachael Floyd, Janis Research Company
Room: D161
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q21.00001: Bulk focused ion beam fabrication of nanoelectromechanical systems Wayne Hiebert, Doug Vick, Vince Sauer, Alastair Fraser, Oleksiy Svitelskiy, Mark Freeman Focused ion beam (FIB) nanomilling of NEMS devices out of bulk material will be presented. Ion impingement from multiple directions allows sculpting with considerable 3-dimensional control of device shape, including tapering and notching. Finite element modeling of device frequencies agrees with interferometric measurements, including for the effect of a localized notch. The measurements are sensitive enough to determine the thermomechanical noise floor of a bulk FIBed NEMS device with displacement sensitivity of 166 fm per root Hz, limited only by a combination of optical shot noise and detector dark current. We envision that bulk FIB fabrication will be useful for NEMS prototyping, milling of tough-to- machine materials, and generalized nanostructure fabrication with 3-dimensional shape control. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q21.00002: Terahertz spectroscopy of ionized air and explosive vapor Benjamin Graber, Rongjia Tao, Dong Ho Wu In the past it has been demonstrated that terahertz spectroscopy could identify various chemical agents and explosives in solid and liquid phases. However peaks and dips in the terahertz spectra obtained from solid and liquid phases are not sharp and often ambiguous or ill-defined, as the interferences among the molecules in the solid or liquid obscure the molecule's characteristic resonances. Hence there has been considerable interest in obtaining terahertz spectrum of gas phase. Recently we have increased terahertz output power of our terahertz spectrometer, and measured terahertz spectra of gases, including water vapor, and ionized air produced by various ionization sources as well as explosive vapors. Our experiments revealed: (1) our terahertz spectrum of water vapor was highly consistent with other published data, (2) the spectra of ionized air produced by corona discharge and nuclear isotopes including Am-241, Bi-207, Ba-133, Co-60, Na-22 and Cs-137 were all different, and the characteristic spectrum changes largely depending on the type of ionization source, and (3) terahertz spectra of explosive vapor taken from TNT, PETN and RDX which were dissolved in acetonitrile or water exhibit very sharp resonance peaks and dips. We will present details of our experimental results. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q21.00003: Characterization of high power near THz radiation from CMOS circuits using a Michelson Interferometer Daniel J. Arenas, Dongha Shim, Dimitrios Koukis, Eunyoung Seok, David B. Tanner, Kenneth K. O Recently, a high frequency SiGe BiCMOS Colpitts oscillator circuit was reported capable of emitting a second, third and fourth harmonic signal at 295, 442 and 589 GHz, respectively. The operating frequencies of the circuit and the emission powers were characterized using a Fourier transform interferometric spectrometer. The results show that this optical technique is an efficient way to characterize high-frequency circuits. The power emitted from the circuit at each frequency was also compared to that emitted from conventional blackbody sources. The results show that the high power emission of these circuits makes them ideal candidates for future spectroscopic applications. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q21.00004: CNT Quantum dots as Terahertz detectors Mohamed Rinzan, Greg Jenkins, Dennis Drew, Serhii Shafranjuk, Paola Barbara We study Carbon Nanotube (CNT) quantum dots as detectors of THz radiation via photon assisted single electron tunneling. Although successful detection was recently demonstrated [1], the coupling between the CNT and THz radiations was very weak. Here, we implement a novel device design where the radiation is effectively coupled to the CNT quantum dot through broad band on-chip antennas. We show that the enhanced coupling yields a highly sensitive broad band Terahertz sensor. \\[4pt] [1] Y. Kawano, S. Toyokawa, T. Uchida and K. Ishibashi, THz photon assisted tunneling in carbon-nanotube quantum dots, Journal of Applied Physics 103, 034307 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q21.00005: Highly absorbing metal nanolaminates for THz bi-material detectors Dragoslav Grbovic, Fabio Alves, Apostolos Karamitros, Gamani Karunasiri Interest in THz-ray sensing has significantly increased in recent years. It has been demonstrated that bi-material MEMS detectors show a great potential to be used for THz imaging. Our work aims to identify metal nanolaminates to improve the detector absorption in the range of interest. Using a finite element simulation tools we demonstrate that Chromium and Nickel films can absorb up to 50 and 35\%, respectively, between 1 and 5 THz, depending on the thickness of the layer. Different thickness of Cr and Ni layers were deposited on Si substrate using e-beam evaporation and the wafers were characterized using a FTIR expanded to THz range. The experimental results show excellent match with the simulations. Further analysis shows that by decreasing the surface filling fill-factor of Ni, it is possible to increase absorption closeup to the values obtained for the Cr films indicating that much lower stress Ni films can be used in bi-material MEMS detectors with absorption comparable with Cr films. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q21.00006: Nanocoax arrays via NIL for high resolution sensing applications Binod Rizal, Patrick Jamieson, Svet Simidjiyski, Huaizhou Zhao, Dong Cai, Stephen Shepard, Thomas C. Chiles, Michael J. Naughton We have used nanoimprint lithography to fabricate nanocoax array-based chemical sensors, starting from SU-8 polymer replicas of silicon nanopillars. Nanocoaxes are formed by metalizing the polymer pillars, followed by oxide dielectric coating and outer metal deposition, and a polymer filling for stabilization. Chemical mechanical polishing and reactive ion etching were then used to open the ends of the coaxes and form coaxial cavity (with a nanoporous component) structures, respectively. Adsorption of water and organic solvent molecules into the coax annuli caused significant changes to the complex impedance of the coaxial capacitor array ($\Delta $C/C $>$ 100{\%} for 50{\%} relative humidity air). Impedance measurements with such coaxial nanocavity arrays thus provide highly sensitive and selective information for molecular detection, with ultimate sensitivity below 1 ppb, or $\sim $1 $\mu $g/m$^{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q21.00007: A high performance humidity sensor based on dielectric detection with a novel coaxial nanostructure Dong Cai, Huaizhou Zhao, Binod Rizal, Timothy Kirkpatrick, Zhifeng Ren, Michael J. Naughton, Thomas C. Chiles High throughput coaxial nanocavity arrays are developed by overlaying porous Al$_{2}$O$_{3}$ and Al layers on vertically aligned carbon nanotube arrays. The porosity of Al$_{2}$O$_{3}$ was electrochemically characterized. The dielectric properties of the nanocoax structure were measured by impedance spectroscopy, from 10 mHz to 1 MHz. The capacitance of the sensor responded to humidity applied to the chip, $i.e.$ soaking the array with water increased the capacitance by 130{\%}. The detection mechanism was established for sensing changes to the dielectric constant due to adsorbed moisture in the porous Al$_{2}$O$_{3}$ coax annulus, with theoretical calculations based on~the Clausius-Mossotti equation~in agreement with the measurements.~ Highly sensitive humidity detection was demonstrated by applying relative humidity between 0.1{\%} and 100{\%}, with a power-law response, \textit{RH}$\sim x^{\alpha }$. This nanocoaxial structure thus offers the possibility of unprecedented performance of porous Al$_{2}$O$_{3}$-mediated capacitancer sensing for humidity detection. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q21.00008: Capacitance response of porous and cavitized nanocoax arrays to various gases Patrick Jamieson, Binod Rizal, Svet Simidjiyski, Huaizhou Zhao, Dong Cai, Mark Hasenauer, Michelle Archibald, Stephen Shepard, Gregory McMahon, Michael J. Burns, Thomas C. Chiles, Michael J. Naughton Arrays of nanoscale coaxial electrodes with hollow or porous annuli offer the potential of highly sensitive detection and identification of gases and molecules. We report on the response of a porous and a partially hollow (cavitized) array to the introduction of various vaporized laboratory solvents. The response is measured as the capacitance and loss changes due to the introduction of molecules into the annuli, associated with the dielectric constant of the solvents, as well as the quantity and pressure. A monotonic dependence on concentration in dry nitrogen was observed. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q21.00009: Damping effects of capacitive comb fingers on biomimetic MEMS directional microphone John Roth, Michael Touse, Jose Sinibaldi, Gamani Karunasiri MEMS directional sound sensors that use two coupled wings moving in air are subjected to viscous damping. The amplitude of oscillation of the sensors is read out by measuring the capacitance of interdigitated comb fingers along the edges of the wings. In this presentation, effects of damping on MEMS sensors with and without comb fingers will be described. It was found that the sensors with comb fingers have a significantly larger damping indicating that the longer perimeter due to combs is responsible for the observed increase [1]. However, the increase in damping reduces the quality factor which improves the response time of the device. \\[4pt] [1] W. Zhang and K. Turner, \textit{Sensors \& Actuators: A.} \textbf{134} p.594 (2007). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q21.00010: Attofarad capacitance measurement on organic thin films using Scanning Microwave Microscopy Shijie Wu, Jing-Jiang Yu Scanning microwave microscopy (SMM) is a recent development in SPM technique that combines the lateral resolution of AFM and the measurement precision of microwave analysis. It consists of an AFM interfaced with a vector network analyzer (VAN). In the reflection mode (S11 measurement), the measured complex reflection coefficient of the microwave from the contact point directly correlates to the impedance of the sample under test. The maximum sensitivity of the measurement is obtained at the resonance where the impedance of the sample under test matches the characteristic impedance. Since the measured load impedance is largely determined by the impedance of the sample under test, SMM can be used to measure the capacitances over dielectric thin films. In this presentation, we report the calibration of SMM using a capacitance standard developed by NIST. Then SMM is used to measure the minute capacitance difference between decanethiol and octadecanthiol SAM layers. The coexistence of two different SAMs on the same substrate with a well-known height difference of 0.88 nm is achieved via an AFM-based nanolithography method known as Nanografting. The measured capacitance difference is about 24 attofarads under the condition that the effective tip/sample contact area was estimated to be about 60nm in diameter. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q21.00011: Intermodulation Spectral Analysis and The Intermodulation Lockin David Haviland, Erik Th\"olen, Daniel Platz, Daniel Forchheimer, Carsten Hutter High quality factor oscillators are very useful for sensitive measurement. A weak perturbation to the oscillator gives a large change of response near resonance, which is typically analyzed to first order as change in the linear response (e.g. shift of resonance frequency). In many cases the measurement can be greatly enhanced by detecting higher order nonlinear response. With a single drive frequency, high order non-linearity gives response at high frequency harmonics, which are filtered out by the high $Q$ oscillator. With two drive frequencies, high-order nonlinear response can be crowded near resonance by intermodulation, or frequency mixing. The intermodulation spectrum near resonance is highly correlated and from its analysis one can reconstruct high-order non-linearity\footnote{C. Hutter et al. Phys. Rev. Lett. {\bf 104}, 050801 (2010)} without high frequency spectral data. We developed a general-purpose lockin measurement instrument and software analysis algorithms for preforming this type of measurement. The instrument drives a system with two pure tones while simultaneously measuring both quadratures of response at 32 intermodulation product frequencies.\footnote{E. A. Th\"olen et al. submitted to RSI, arXiv:1008.2722} [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q21.00012: A Study of Ionic Transport Through Randomly-Aligned Silica Nanospring Using Electrochemical Impedance Spectroscopy Yukta P. Timalsina, Joshua Branen, Eric Aston, Ken Noren, David N. McIlroy A study of ionic transport through randomly aligned (silica) nanospring (RANS) using electrochemical impedance spectroscopy is presented. The device used for this study is a parallel plate capacitor consisting of two conducting surfaces with RANS as the dielectric spacer layer. The device response is evaluated with test solutions consisting of sodium chloride in a phosphate buffer. The experimental impedance data is analyzed using a model equivalent resistor-inductor-capacitor (RLC) circuit. The solution resistance through RANS and electric double layer formed at solution- electrode interface are elements of equivalent circuit that are more responsive and are more likely to be affected by changes of ionic concentrations. From our analysis we have determined that an electric double layer forms at the solution- RANS interface, which acts as a barrier to diffusion of ions from the solution into the RANS, and vice versa. We have also determined that ion diffusion is impeded by the RANS, as illustrated by changes in the resistance of the element of the equivalent circuit that corresponds to diffusion of ions through the RANS. The linear response of the RANS-based device below 10 kHz is potentially useful for many sensing applications. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q21.00013: Gas Sensing Properties of Hybrid SnO$_{2}$/Carbon Nanotubes Azlin Biaggi-Labiosa, Laura J. Evans, Jennifer C. Xu, Gary W. Hunter, Gordon Berger, Francisco Sola Chemical sensors involving nanostructured materials can be developed into sensor systems with unique properties and improved performance. One approach is to combine different nanomaterials in order to form hybrid structures with properties different than that of the constituent materials. Hybrid nanostructures consisting of tin oxide (SnO$_{2})$ nanocrystals distributed on the surface of multiwalled carbon nanotubes (MWCNTs) and singlewalled carbon nanotubes (SWCNTs) were fabricated and incorporated on a sensor platform in a controlled and efficient manner with a novel approach that combines dielectrophoresis with standard microprocessing techniques. Current vs. voltage and current vs. temperature curves were taken at different concentrations of hydrogen (H$_{2})$, hydrocarbons and nitrogen oxides (NO$_{x})$ at various operating temperatures for the hybrid nanostructures and were compared with their counterparts without SnO$_{2}$ nanocrystals. The tests showed that the hybrid nanostructures exhibit room temperature sensing capability when exposed to low concentration gases in contrast to the high operating temperature typically required for SnO$_{2}$ nanocrystals alone. High resolution electron microscopy and electron energy-loss spectroscopy will also be presented. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q21.00014: Development of a Tunnel Diode Resonator technique for magnetic measurements in Electrostatic Levitation chamber N.S. Spyrison, P. Prommapan, H. Kim, J. Maloney, G.E. Rustan, A. Kreyssig, A.I. Goldman, R. Prozorov The incorporation of the Tunnel Diode Resonator (TDR) technique into an ElectroStatic Levitation (ESL) apparatus was explored. The TDR technique is known to operate and behave well at low temperatures with careful attention to coil-sample positioning in a dark, shielded environment. With these specifications a frequency resolution of $10^{-9}$ in a few seconds counting time can be achieved.\footnote{C. V. Degrift, ``Tunnel diode oscillator for 0.001 ppm measurements at low temperatures,'' Rev. Sci. Instrum. \textbf{46}, 599 (1975).} Complications arise when this technique is applied in the ESL chamber where a sample of molten metal is levitating less then 10 mm from the coil in a large electrostatic field. We have tested a variety of coils unconventional to TDR; including Helmholtz pairs and Archimedean spiral coils. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q21.00015: Microwave impedance imaging on semiconductor memory devices Worasom Kundhikanjana, Keji Lai, Yongliang Yang, Michael Kelly, Zhi-Xun Shen Microwave impedance microscopy (MIM) maps out the real and imaginary components of the tip-sample impedance, from which the local conductivity and dielectric constant distribution can be derived. The stray field contribution is minimized in our shielded cantilever design, enabling quantitative analysis of nano-materials and device structures. We demonstrate here that the MIM can spatially resolve the conductivity variation in a dynamic random access memory (DRAM) sample. With DC or low-frequency AC bias applied to the tip, contrast between n-doped and p-doped regions in the dC/dV images is observed, and p-n junctions are highlighted in the dR/dV images. The results can be directly compared with data taken by scanning capacitance microscope (SCM), which uses unshielded cantilevers and resonant electronics, and the MIM reveals more information of the local dopant concentration than SCM. [Preview Abstract] |
Session Q22: Metal-Insulator Phase Transitions II
Sponsoring Units: DCMPChair: Cengiz Sen, University of Tennessee
Room: D163
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q22.00001: Fast pulsed measurements of the electric-field-driven metal-insulator transition in magnetite J. Spencer Morris, R.G. Sumesh Sofin, Igor V. Shvets, Douglas Natelson Magnetite, Fe3O4, is an example of a strongly correlated material in which strong electron-electron interactions lead to unusual magneto-electronic properties. ~In particular, it undergoes a first-order phase transition on cooling through TV$\sim $122K in bulk, in which a structural transition is accompanied by a significant drop in electrical conductivity. ~Recent electronic transport measurements have shown an electric-field driven breakdown of the insulating state in large aspect-ratio nanogaps fabricated on magnetite thin-films. ~The mechanism of this breakdown is of great interest in understanding the Verwey transition, and probing the intrinsic speed of the breakdown may further constrain possible mechanisms. ~We investigate the kinetics of this nonequilibrium transition by employing a high-speed pulse generator to apply voltages approaching the nanosecond time scale that exceed the critical switching value, and measuring the transmitted pulse via a high-speed oscilloscope. ~A significant change in transmission is observed for pulses that exceed the critical amplitude necessary to initiate the transition. ~Our initial results include an evaluation of the material response as a function of temperature and amplitude of the applied voltage. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q22.00002: Magnetic field dependence of the nonequilibrium metal-insulator transition in magnetite nanostructures Douglas Natelson, Alexandra A. Fursina, R. G. S. Sofin, Igor V. Shvets At low temperatures magnetite undergoes a Verwey transition from a comparatively conducting state to a strongly correlated, ordered, more insulating state, the detailed nature of which remains under active debate. Recent experiments using nanostructures based on epitaxial magnetite films have shown that an applied dc electric field can lead to a nonequilibrium transition out of the insulating state. The kinetics of this nonequilibrium transition are nontrivial, with switching taking place over a distribution of applied voltages in a particular device at a given temperature below the Verwey transition. An externally applied magnetic field is observed to alter the kinetics of the nonequilibrium transition as the magnetization of the magnetite film is coerced out of plane. We present this data and discuss what it implies about the nature of the ordered, insulating ground state. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q22.00003: Mesoscopic transport in ultrathin films of La$_{0.67}$Ca$_{0.33}$MnO$_3$ C. Beekman, J. Zaanen, J. Aarts We investigate the electrical transport in mesoscopic structures of La$_{0.67}$Ca$_{0.33}$MnO$_3$ in the regime of the metal-insulator transition by fabricating microbridges from strained and unstrained thin films. We measure current-voltage characteristics as function of temperature and in high magnetic fields and with varying film thickness. For strained films, in warming from the metallic to the insulating state, we find non-linear effects in the steep part of the transition characterized by a differential resistance with a strong peak around zero applied current, and saturating at higher currents after resistance drops up to 60 $\%$. We propose that this nonlinear behavior is associated with melting of the insulating state by injecting charge carriers, signalling the occurrence of an intervening phase which involves the formation of short range polaron correlations. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q22.00004: In-situ TEM Analysis and Transport in Manganites La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3}$ Exhibiting Phase Separation below Metal-Insulator Transition V. Volkov, J. He, T. Osaka, Y. Zhu, S. Chaudhuri, R. Budhani Epitaxial films of doped La$_{5/8-y}$Pr$_{y}$Ca$_{3/8}$MnO$_{3}$ (LPCMO: y = 0.275-0.375) manganites were examined by \textit{in-situ} Lorentz microscopy and other TEM methods below the metal-insulator transition point T$_{MI}\sim $164 K. Such films are known for colossal magneto-resistance effect (CMR). Clear evidences were obtained for mesoscale two-phase separation process involving antiferromagnetic charge-ordered (AFM/CO) and ferromagnetic (FM) phases, coexisting below T$_{MI}$ in LPCMO films. The first-order CO-FM phase transition is accompanied by partial magnetic melting of the CO phase at CO/FM interfaces thereby creating charge-disordered spin-glass metastates. In contrast, FM phase shows specific ``zig-zag'' magnetic domains coupled with dense (101) crystal twins. This allows refining relations for charge-orbital and spin-ordering vectors in films. Transport resistance data show that T$_{MI}$ point is decreased with Pr$_{y}$ growth in LPCMO. On cooling films below T$_{MI}$ their resistance drops by several orders in magnitude. The observed M-I transition shows striking linear relation for log-conductance curve versus FM fraction measured by TEM, which does not follow typical percolation equations, suggesting that percolation transport model in manganites needs further revisions. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q22.00005: First Order CMR Transitions and Spin-Charge Dynamics Above the Curie Temperature in the Two-Orbital Model for Manganites Cengiz Sen, Gonzalo Alvarez, Elbio Dagotto We study the two $e_g$-orbital model including Jahn-Teller lattice distortions and the superexchange interaction using exact diagonalization Monte Carlo techniques at various dopings, $x$. We report the presence of first order CMR transitions at the Curie temperature ($T_{\rm C}$) for doping $x=0.25$ for the clean system and with weak disorder [1], in qualitative agreement with several experiments. We also discuss spin and charge dynamics as a function of Monte Carlo time above $T_{\rm C}$, addressing the properties that characterize the exotic CMR state. It is observed that in CMR regimes there are various quasidegenerate spin states that may play an important role in this phenomenon [2].\\[4pt] [1] C. Sen {\it et al.}, Phys. Rev. Lett. {\bf 105}, 097203 (2010).\\[0pt] [2] Shuhua Liang {\it et al.}, preprint, submitted to PRB. See also Hotta {\it et al.}, Phys. Rev. Lett. {\bf 86}, 4922 (2001). [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q22.00006: Spin frustration effect near the Mott transition in the pyrochlore-type molybdates Satoshi Iguchi, Yuta Kumano, Koji Oishi, Yoshinori Tokura Spin frustration and nontrivial spin structures by antiferromagnetic spins on a frustrated lattice have been widely studied such as a spin ice system with the pyrochlore structure Ho$_{2}$Ti$_{2}$O$_{7}$. However, spin frustration effects on conduction electrons have been less studied so far. Here, we have investigated spin frustration effects near the insulator-metal (Mott) transition in pyrochlore molybdates, where a paramagnetic diffuse metal state with antiferromagnetic spins is characteristic. Hole carriers were introduced by doping of Cd ions into the spin glass (Mott) insulator Y$_{2}$Mo$_{2}$O$_{7}$. The insulator to metal transition occurs at around $x = 0.1$ in (Y$_{1-x}$Cd$_{x}$)$_{2}$Mo$_{2}$O$_{7}$ with magnetically spin glass ground state. With increasing in the hole concentration, the spin glass transition disappears at around $x = 0.30$ and the resistivity shows almost no temperature dependence. Such a paramagnetic diffusive metallic character has been widely observed in R$_{2}$Mo$_{2}$O$_{7}$ (R = rare-earth ion) under high pressures. We have also measured the heat capacity in the system and found the anomalous enhancement of effective electron mass at around the transition from the spin glass metal to the paramagnetic metal phase. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q22.00007: Mott transition in multi-orbital Hubbard models for iron pnictides Rong Yu, Qimiao Si The bad-metal behavior of the iron pnictides has motivated a theoretical description in terms of a proximity to Mott localization. Since the parent compounds of the iron pnictides contain an even number of 3d-electrons per Fe, it is important to determine whether a Mott transition robustly exists and the nature of the possible Mott insulating phases. We address these issues in multi-orbital Hubbard models for the parent iron pnictides using a slave-spin approach. We show a metal-to-Mott-insulator transition in $xz$ and $yz$ orbitals generally exists in these models [1]. The nature of the metal-to-insulator transition may be strongly affected by the Hund's couplings. For certain values of Hund's couplings, we show there is a orbitally selective metal-to-insulator transition: the transition to a Mott insulator in the $xz$ and $yz$ orbitals takes place at the same critical coupling as the transition to either an orbitally polarized insulator or a band insulator in the other orbitals. Implications for the electronic and magnetic properties of the iron pnictides are discussed. \\[4pt] [1] R. Yu and Q. Si, arXiv:1006.2337. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q22.00008: Transport and spectra in the half--filled Hubbard model Himadri Barman, Vidhyadhiraja Sudhindra We study the issues of scaling and universality in spectral and transport properties of the infinite dimensional particle--hole symmetric Hubbard model within dynamical mean field theory. We have used and reformulated the iterated perturbation theory approach to avoid problems such as analytic continuation of Matsubara frequency quantities or calculating multi-dimensional integrals, while taking full account of the very sharp structures in the Green's functions that arise close to the Mott transitions. We find a ``coherence peak'' in the dc resistivity of the metallic regime, which appears to be a universal feature occurring at a temperature roughly equal to the low energy scale of the system and agrees qualitatively well with the pressure dependent dc resistivity experiments on Selenium doped NiS$_2$. Resistivity hysteresis across the Mott transition is found and a direct comparison of the thermal hysteresis observed in V$_2$O$_3$ with our theoretical results yields a value of the hopping integral, which we find to be in the range estimated through first-principle methods. Finally, a systematic study of optical conductivity is carried out and the changes in absorption as a result of varying interaction strength and temperature are identified. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q22.00009: Spectral function near the Mott transition in the one-dimensional Hubbard model Masanori Kohno Spectral properties near the Mott transition are investigated in the one-dimensional Hubbard model. The single-particle spectral function is calculated using the dynamical density-matrix renormalization group method, and the dominant modes are identified using the Bethe ansatz. Characteristic features near the Mott transition, such as the pseudogap, hole-pocket behavior, spectral-weight transfer, and upper Hubbard band, are explained in a unified manner in terms of spinons, holons, antiholons, and doublons. From the insulating side, the Mott transition is characterized by the emergence of a gapless mode whose dispersion relation extends up to the order of hopping (spin exchange) in the weak (strong) interaction regime caused by infinitesimal doping. From the metallic side, the transition is characterized as a loss of charge character from the mode having both spin and charge characters, while the spin excitation remains gapless and dispersing. These features cannot be explained in either the rigid-band picture or the Fermi liquid theory. I expect that generic features near Mott transitions can be deduced from the present results. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q22.00010: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q22.00011: Localized states in Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl probed by photoluminescence Natalia Drichko, Rudi Hackl, John Schlueter We present the luminescence spectra of a low-temperature Mott insulator (T$_c$=35 K) $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_{2}$]Cl and a metal with a superconductor transition $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Br in the temperature range between 300 and 10 K. In the Mott insulating state we observe an appearance of a luminescence at 1.95 eV, which corresponds to a LUMO-HOMO transition in a BEDT-TTF molecule. This luminescence is quenched both in the higher-temperature semiconducting state of $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Cl and metallic $\kappa$-(BE\-DT\--TTF)$_2$\-Cu\-[N\-(CN)$_{2}$]Br. To our knowledge, it is the first observation of luminescence driven by electronic correlations. This observation gives an evidence of the local character of BEDT-TTF energy levels in the Mott insulating state, in contrast to the band-character in the metallic and semiconducting states. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q22.00012: Quantum electric dipoles in spin liquid dimer Mott insulator Chisa Hotta We present an effective dipolar-spin model based on the strong coupling analysis, which may explain the possible origin of the ``spin liquid Mott insulator'' of the organic triangular lattice system, kappa-ET$_{2}$Cu$_{2}$ (CN)$_{ 3}$, whose gapless spin liquid state had been discussed in the context of geometrical frustration of exchange coupling, $J$, between spins on dimer orbitals. We find out that another degrees of freedom within the insulator, quantum electric-dipoles on dimers, interacts with each other by the Coulomb interaction and brings about a significant suppression of$ J$ through the dipolar-spin coupling. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q22.00013: Numerical study on the ionic Hubbard model in one and two dimensions Ji-Woo Lee, Yong Chul Lee, Soo Hyun Cho We investigate the quantum phase transition of an ionic Hubbard model in one and two dimensions. There are three parameters in the ionic Hubbard model, one is the hopping term $t$, the other is the Coulomb term $U$ between local spin-up and spin-down electrons, and another is the band energy $\Delta$ which corresponds to the difference in local chemical potentials of bipartite lattice. Via exact diagonalization and quantum Monte Carlo simulations, we obtained the phase boundary of Mott insulator, metal, and band insulator. We measure the ground state energy and the energy gap between the ground-state and the first excited-state energy, and also measure the order parameters such as Drude weight and double occupancy in the three phases to understand the nature of three phases. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q22.00014: Spectral properties of the one-dimensional Hubbard model: cluster dynamical mean-field approaches Ara Go, Gun Sang Jeon We investigate static and dynamic properties of the one-dimensional Hubbard model using cluster extensions of the dynamical mean-field theory. It is shown that the two different extensions, the cellular dynamical mean-field theory and the dynamic cluster approximation, yield the ground-state properties which are qualitatively in good agreement with each other. We compare the results with the Bethe ansatz results to check the accuracy of the calculation with finite sizes of clusters. We also analyze the spectral properties of the model with the focus on the spin-charge separation and discuss the dependency on the cluster size in the two approaches. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q22.00015: Temperature Effects on Metal-Insulator Transitions in the Ionic Hubbard Model Aaram Kim, Gun Sang Jeon The ionic Hubbard model is known to exhibit interesting transitions between metallic and insulating phases at zero temperature. We investigate finite-temperature phase transitions in the model. For an extensive finite-temperature study, we apply a dynamical mean- field theory with continuous-time quantum Monte Carlo method employed as an impurity solver. We examine how the transitions at zero temperature evolve as the temperature increases. We also discuss the effects of temperature on the nature of transitions and other properties of the system. [Preview Abstract] |
Session Q23: Focus Session: Iron Based Superconductors -- Fermi Topology
Sponsoring Units: DMP DCOMPChair: Mark Lumsden, Oak Ridge National Laboratory
Room: D165
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q23.00001: Ab initio study of de Haas van Alphen effect in BaRh$_{2}$P$_{2}$ and BaIr$_{2}$P$_{2}$ Simon Blackburn, Michel Cote, Bobby Prevost, Andrea Bianchi, Marek Bartkowiak, Beate Bergk, Oleg Ignatchik, Jochen Wosnitza, Gabriel Seyfarth, Cigdem Capan, Zachary Fisk The de Haas-van Alphen (dHvA) effect is a powerful probe of the Fermi surface (FS) of a metal. Since it measures the area of a cross-section of the Fermi surface, a theoretical description of this surface complements well these experiments. However, a very accurate description of the FS is required from the ab initio calculations in order to calculate the dHvA frequencies. This is achieved using maximally localized Wannier functions (MLWF) (Marzari {\&} Vanderbilt, \textit{Phys. Rev. B}, 56, 12847)to interpolate the Hamiltonian on a dense k-point grid. In this work, we present a dHvA study of BaRh$_{2}$P$_{2}$ and of its isovalent material BaIr$_{2}$P$_{2}$, both structurally analog to the iron pnictide BaFe$_{2}$As$_{2}$. We also present results concerning LaFe$_{2}$P$_{2}$ and CeFe$_{2}$P$_{2}$ which are also related to BaFe$_{2}$As$_{2}$ by a rigid electronic band shift to account for the difference in the number of electrons. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q23.00002: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q23.00003: Transport properties of 3D extended s-wave states appropriate for iron-based superconductors Vivek Mishra, Siegfried Graser, Peter Hirschfeld The Fermi surfaces of Fe-pnictide superconductors are fairly two-dimensional (2D), and it has thus come as a surprise that recent penetration depth and thermal conductivity measurements on some systems have reported $c$-axis transport at low temperatures in the superconducting state comparable to or even larger than that in the $ab$ plane. These results should provide important information on both the Fermi surface and the superconducting state. Here we consider the theory of the superfluid density and thermal conductivity in models of extended-$s$ wave superconducting states expected to be appropriate for Fe-pnictide systems. We include both intra- and interband disorder and consider a range of different Fermi surfaces where gap nodes might exist. We show that qualitative fits can be obtained to match recent experiments on Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, and discuss their implications. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q23.00004: Fermi Surface of the Pnictide Superconductor LaRu$_2$P$_2$ studied by quantum oscillations Philip Moll, Fedor Balakirev, Ross McDonald, Janusz Karpinski, Zbigniew Bukowski, Peter Blaha, Karlheinz Schwarz, Bertram Batlogg LaRu$_2$P$_2$ is a stochiometric pnictide superconductor (T$_c$ $\sim$ 4.1K) and crystallizes in the ThCr$_2$Si$_2$ structure (the ``122'' pnictide family). We have mapped out its Fermi surface via the deHaas-vanAlphen effect in pulsed magnetic fields up to 60T (LANL/NHMFL). Pronounced oscillations were observed in the magnetic torque measured with a microcantilever setup. Two features are particularly noteworthy: The oscillations can be followed to surprisingly high temperatures beyond 20K, and the main frequency component at $\theta$ = 20$^{\circ}$ ($\theta$ = 0$^{\circ}$ at HIIc) is at 349T ($\alpha$-peak), significantly lower than in the related compounds LaFe$_2$P$_2$. A second frequency originating from a larger Fermi surface cross-section at 1921 T ($\beta$-peak) is identified. The temperature dependence of the amplitudes is well described by the Lifshitz- Kosevich formalism and gives low effective masses m*/m = 0.80 ($\alpha$ sheet) and 1.09 ($\beta$ sheet). Therefore, most ``122'' metals appear to have similarly low effective masses. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q23.00005: Campbell penetration depth in stoichiometric LiFeAs - evidence for static fishtail effect Plengchart Prommapan, Hyunsoo Kim, Makariy A. Tanatar, Ruslan Prozorov, Bumsung Lee, Seunghyun Khim, Kee Hoon Kim The ``fishtail'' or second magnetization peak is one of the most intriguing properties of high$-T_c$ cuprate superconductors. Now it has also been observed in iron-based materials and has been associated with weak collective pinning. To understand whether the fishtail effect has dynamic (due to field-dependent magnetic relaxation) or static behavior (due to actual non-monotonic field dependence of the true critical current) one needs to measure the clean system, which are rare in pnictide superconductors. A stoichiometric LiFeAs is one of the cleanest of the pnictides with RRR=65. We measured the Campbell penetration depth using a 10 MHz tunnel-diode resonator in DC magnetic fields of up to 9 T. As opposed to the ``apparent'' current density, estimated from the magnetization relaxed over tens of seconds, the Campbell penetration depth depends on the curvature of the pinning potential sampled at time intervals of 0.1 $\mu$sec, thus allowing one to estimate the unrelaxed, ``true'' $j_c(T,B)$. The obtained $j_c(T,B)$ shows a non-monotonic trend with a second peak shifting toward lower fields at higher temperatures implying a static origin of the fishtail effect in LiFeAs. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q23.00006: Measuring the Absolute Penetration Depth in Superconducting Materials. Nicholai Salovich, Russell Giannetta, Ruslan Prozorov, Paul Canfield, Sergey Bud'ko The absolute penetration depths of a variety of superconductors were measured using a method involving a high precision tunnel diode oscillator and an Aluminum film coating method [1]. Variations of the Al film geometry (thickness, coverage area, etc) and microstructure (grain size, RRR, etc) were used to test the reliability and versatility of the coating technique. A variety of supplemental techniques (dual beam SEM, EBS, AFM, XRD, etc) were used to independently characterize the films and control their quality. Special emphasis was placed on measurements of cobalt doped iron pnictide samples given the well established quality of such samples now available. Work at UIUC supported by NSF DMR 10-05708, and Center for Emergent Superconductivity USDOE Award No. DE-AC02-98CH10886. Work at the Ames Laboratory was supported by the division of Materials Science and Engineering, Basic Energy Sciences, Department of Energy (US DOE), under Contract No. DEAC02-07Ch11358. \\[4pt] [1] R. Prozorov, et al, Appl. Phys. Lett 77,1202 (2000) [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q23.00007: dHvA studies of the Fermi topology of Iron-based Superconductors and Metals Invited Speaker: Quantum oscillations studies on various non-magnetic iron pnictides reveal a Fermi surface in broad agreement with the details of the band structure calculations and moderate enhancement of the electronic correlations [1,2,3,4]. Whether or not the nesting of the electron and hole bands are essential for explaining the superconducting behaviour in these materials is still under debate but it is becoming clear that structural alteration have a significant effect in determining their electronic properties. In this talk I will present quantum oscillations studies in materials in which the Fermi surface suffers major topological changes. I will discuss the effect of isoelectronic substitution and doping on the Fermi surface and the quasiparticle masses and their relevance for understanding the complex physics of these materials. This work is in collaboration with groups at Bristol University, Stanford University and Kyoto University [1,2,3,4] and experiments were performed at high magnetic field facilities in Tallahassee, Nijmegen and Toulouse. \\[4pt] [1] A. I. Coldea et al., Phys. Rev. Lett. 101, 216402 (2008); \\[0pt] [2] A. I. Coldea, et al., Phys. Rev. Lett. 103, 026404 (2009). \\[0pt] [3] J. G. Analytis, et al., Phys. Rev. Lett.103 076401 (2009). \\[0pt] [4] H. Shishido, et al., Phys. Rev. Lett.104, 057008 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q23.00008: London penetration depth measurements in Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ single crystals Ryan Gordon, Makariy Tanatar, Alexander Thaler, Ruslan Prozorov The variation of the in-plane London penetration depth with temperature, $\Delta\lambda_{ab}(T)$, has been measured in Ba (Fe$_{1-x}$Ru$_x$)$_2$As$_2$ single crystals by using a tunnel diode resonator (TDR). The crystals were grown out of self- flux, with superconductivity stabilized for $x>0.2$ and the maximum $T_c\approx16$ K corresponding to $x\approx0.29$. The substitution of Ru for Fe in this compound is particularly interesting because it is isovalent, so that no additional charge carriers are added by the Ru. The low-temperature penetration depth has been found to exhibit a power law dependence, $\Delta\lambda_{ab}(T)\propto T^n$, with an $x$- dependent exponent, $n$. Using the penetration depth data, the superfluid density has been constructed and compared to known theoretical models for different superconducting pairing symmetries and impurity scattering limits. These results will be compared to previous measurements on different iron-based superconductors. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q23.00009: Superfluid Density Measurements of Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2}$ Films near Optimal Doping Jie Yong, Sanghan Lee, J. Jiang, C.W. Park, J.D. Weiss, E.E. Hellstorm, D.C. Larbalestier, C.B. Eom, T.R. Lemberger We report the first direct, low-frequency measurements of superfluid density, n$_{s}$(T) $\propto \quad \lambda ^{-2}$(T), in Ba(Co$_{x}$Fe$_{1-x})_{2}$As$_{2}$ thin films, near optimal doping. 100 nm thick films are fabricated by Pulsed Laser Deposition (PLD) in high vacuum and SrTiO$_{3}$ is used as template to match Ba layer in ba-122. Temperature dependence of superfluid density is measured by our two-coil mutual inductance apparatus down to 1.3K. The magnetic penetration depth, $\lambda $, at T $\approx $ 0 is 350 to 430 nm. The T-dependence of $\lambda ^{-2}$ is well characterized by a small s-wave gap, 2$\Delta $(0)/k$_{B}$T$_{c}$ = 2.2 $\pm $ 0.1. In detail, $\lambda $ has power-law behavior at low T: $\lambda $(T)/$\lambda $(0) -- 1 = 0.60*(T/T$_{c})^{2.5\pm 0.1}$. A tail of superfluid density near T$_{c}$ is the only possible indication of a bigger gap. The small gap, together with power-law behavior at low-T, suggests strong intraband scattering on the larger-gap Fermi surface plus significant interband scattering between large-gap and small-gap Fermi surfaces. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q23.00010: Linear magnetoresistance in the underdoped iron pnictide Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Jiun-Haw Chu, Hsueh-Hui Kuo, Scott Riggs, James Analytis, Ian Fisher BaFe$_2$As$_2$ suffers an antiferromagnetic transition which has been described in terms of a nodal spin density wave. The material exhibits a striking linear magnetoresistance in the low temperature antiferromagnetic state, possibly related to the unique character of the reconstructed Fermi surface. Here we present data showing the evolution of the magnetoresistance as a function of both composition and temperature for the specific case of Ba(Fe$_{1- x}$Co$_x$)$_2$As$_2$, revealing a correlation with other transport properties, including the in-plane resistivity anisotropy. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q23.00011: Hall effect study of iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) T.J. Liu, J. Hu, B. Qian, Z.Q. Mao Our previous work reveals three composition regions with distinct physical properties in the phase diagram of Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) (Liu \textit{et al.}, Nature Materials \textbf{9}, 719 (2010)). Region I $(0 \leq x \leq 0.09)$ exhibits long range $(\pi, 0)$ antiferromagnetic (AFM) order, while Region II $(0.09 < x < 0.29)$ displays short range $(\pi, 0)$ magnetic correlations and is characterized by a weakly localized electronic state. Only in Region III $(x \geq 0.29)$ do we find evidence of bulk superconductivity. In this talk, we will report Hall effect studies of this system. In the AFM state of Region I, we find that the inverse Hall angle (IHA) exhibits a quadratic temperature dependence, consistent with the Fermi liquid behavior probed by resistivity and specific heat measurements. In the weakly localized state of Region II, however, the IHA shows a linear temperature dependence, implying that the quasiparticle scattering rate in this region changes significantly compared with the AFM phase. We will discuss how quasiparticle scattering is associated with $(\pi, 0)$ magnetic fluctuations. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q23.00012: Upper Critical Field and the Fulde-Ferrel-Larkin-Ovchinnikov Transition in Multiband Superconductors Alex Gurevich The effect of orbital and Zeeman pairbreaking on the upper critical field $H_{c2}$ and the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) transition in clean Fe-based superconductors is addressed using a multiband BCS theory. It is shown that the crystalline anisotropy and the $s^\pm$ pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase the orbitally-limited $H_{c2}(T)$ and facilitate the FFLO transition. Small pockets of the Fermi surface emerging upon doping can trigger the FFLO transition even for moderate values of the Maki parameter in the main bands. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q23.00013: Upper critical field study of a LiFeAs single crystal grown by Sn flux Seunghyun Khim, Bumsung Lee, Jae Wook Kim, Eun Sang Choi, G. R. Stewart, Kee Hoon Kim Temperaturedependence of the upper critical fields $H_{c2}(T)$ was investigated in a LiFeAs single crystal by measuring resistivity at a fixed magnetic field up to 36 T. $H_{c2}^{ab}$(0) and $H_{c2}^{ab}$ (0) values are obtained as 30 and 17.2 T respectively. $H_{c2}^{ab}$(0) is lower than expected one from the orbital limiting field $H_{c2}^{orb}$(0) because of the presence of a moderate Pauli limiting effect; upon fitting $H_{c2}^{ab}(T)$ with the WHH formula, the Maki parameter \textit{$\alpha $} = 0.65. For $H_{c2}^{c }$(T), rather a linearly increasing behavior of $H_{c2}(T)$ could be explained by a two-band model in a dirty limit. The anisotropy $H_{c2}^{ab}$ /$ H_{c2}^{c}$ ($T)$ is $\sim $2.3 near $T_{c }$and decreases with temperature being lowered to reaches $\sim $1.3 at $T$ = 0. We also compare $H_{c2}$ (T) of this 111 system with those of other Fe-based superconductors and conclude that the moderateness of the spin-paramagnetic effect is related with rather a weak slope of $H_{c2}(T)$ near $T_{c}$, which is inversely proportional to the Fermi velocity and mean free path. Thus, the comparison of the slope of $H_{c2}$ supports that LiFeAs is located in a rather clean limit among the Fe-based superconductors. [Preview Abstract] |
Session Q24: Focus Session: Multiscale Modeling: Heterogeneous Systems and Interfaces
Sponsoring Units: DCOMP DMPChair: Srinivasan Srivilliputhur, University of North Texas
Room: D167
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q24.00001: Kinetic Monte Carlo with fields: diffusion in heterogeneous systems Invited Speaker: It is commonly perceived that to achieve breakthrough scientific discoveries in the 21$^{st}$ century an integration of world leading experimental capabilities with theory, computational modeling and high performance computer simulations is necessary. Lying between the atomic and the macro scales, the meso scale is crucial for advancing materials research. Deterministic methods result computationally too heavy to cover length and time scales relevant for this scale. Therefore, stochastic approaches are one of the options of choice. In this talk I will describe recent progress in efficient parallelization schemes for Metropolis and kinetic Monte Carlo [1-2], and the combination of these ideas into a new hybrid Molecular Dynamics-kinetic Monte Carlo algorithm developed to study the basic mechanisms taking place in diffusion in concentrated alloys under the action of chemical and stress fields, incorporating in this way the actual driving force emerging from chemical potential gradients. Applications are shown on precipitation and segregation in nanostructured materials. Work in collaboration with E. Martinez, LANL, and with B. Sadigh, P. Erhart and A. Stukowsky, LLNL. Supported by the Center for Materials at Irradiation and Mechanical Extremes, an Energy Frontier Research Center funded by the U.S. Department of Energy (Award {\#} 2008LANL1026) at Los Alamos National Laboratory \\[4pt] [1] B. Sadigh et al. to be published \newline [2] E. Martinez et al. J. Comp. Phys. 227 (2008) 3804-3823 [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q24.00002: On the atomic-scale design of metal-metal heterointerfaces Invited Speaker: I will describe a multiscale modeling effort to understand and control the properties of heterointerfaces in metal-metal nanocomposites, using their effect on radiation response as an example. For selected model interfaces, atomistic simulations are used to characterize interface structure and to determine the mechanisms of interface-point defects interactions, including trapping, diffusion, and defect reactions. This information is then incorporated into mesoscale dislocation-based and continuum approaches to investigate the steady-state interface response to radiation-induced defect fluxes. With insights gained from studying this ``forward'' problem of predicting radiation response of selected model interfaces, one may attempt to solve the ``inverse'' problem of determining what interfaces will yield desired radiation response. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q24.00003: Atomistic Mechanism of Kinking in the Vapor-Liquid-Solid Growth of Silicon Nanowires Seunghwa Ryu, Wei Cai Understanding the atomistic growth mechanism of semiconductor nanowires from the catalytic droplet is important for better control of the shape and orientation of nanowires deposited through the Vapor-Liquid-Solid (VLS) process. Kinking is a frequently observed event, in which the nanowire suddenly changes the growth orientation. This behavior is usually undesirable, but can also be explored to grow nanowires of complex shapes if it can be controlled. Unfortunately, the atomistic origin of kinking is not well understood. We employ advanced sampling methods to compute the probability of the orientation change during VLS growth. Several growth directions and nanowire diameters are simulated at 1000 K. The simulation uses a recently developed Au-Si inter-atomic potential fitted to the experimental binary phase diagram. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q24.00004: Prediction of the anisotropic properties of energetic materials at elevated pressures and temperatures Oscar Ojeda, Tahir Cagin Localization of strain and changes under extreme conditions in energetic materials (EM) can cause runaway reactions and unexpected initiation. A clear understanding of the mechanical properties is a perquisite in understanding the interplay between mechanical, chemical and thermodynamic properties that relate sensitivity and EM's before they undergo initiation. We have conducted first principles ground state studies, complemented by atomistic calculations at elevated temperatures and pressures, for energetic commonly used secondary EM's with varying sensitivities. Chemical information found from ab intio methods, and from compression at elevated temperatures show that external conditions relevant to impact and shock behavior can have different effects on the studied systems. These range from changes in local conformation, changes in the hydrogen-bonding network, and more drastically to a full crystallographic transition in which the symmetry of the system undergoes a transformation. Due to the chemical, mechanical and thermodynamic level information that provides, multiscale modeling methods, can then be applied to the understanding of other type of systems and give a clearer understanding of the molecular processes that undergo energetic materials, prior to initiation. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q24.00005: Migration energies of native defects and fission products in uranium dioxide Alexander Thompson, Chris Wolverton Despite the importance of fission products like Xe in nuclear fuels, the mechanism of how these atoms diffuse in the lattice is not known. In an effort to identify this mechanism, we have used density functional theory as well as a variety of different classical potentials for to study the migration energies of a variety of atomic steps in UO2, with and without Xe impurities and native defects. We find that the classical potential of Basak gives results which compare favorably with density functional theory for the diffusion of a Schottky defect cluster. We observe a new path for xenon-tetravacancy (a UO2 Schottky defect plus an additional U vacancy) motion using molecular dynamics. This path has a lower energy barrier than previously reported xenon-tetravacancy paths. We examine the possibility of a uranium vacancy dissociating from the xenon-tetravacancy cluster and find that large barriers for this dissociation. We also calculate xenon-double Schottky defect migration and find it has a slightly larger barrier than xenon-tetravacancy motion with the oxygen vacancies being weakly bound to the defect. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q24.00006: Interface Mediated Nucleation and Growth of Dislocations in fcc-bcc nanocomposite Ruifeng Zhang, Jian Wang, Irene J. Beyerlein, Timothy C. Germann Heterophase interfaces play a crucial role in determining material strength for nanostructured materials because they can block, store, nucleate, and remove dislocations, the essential defects that enable plastic deformation. Much recent theoretical and experimental effort has been conducted on nanostructured Cu-Nb multilayer composites that exhibited extraordinarily high strength, ductility, and resistance to radiation and mechanical loading. In decreasing layer thicknesses to the order of a few tens of nanometers or less, the deformation behavior of such composites is mainly controlled by the Cu/Nb interface. In this work, we focus on the cooperative mechanisms of dislocation nucleation and growth from Cu/Nb interfaces, and their interaction with interface. Two types of experimentally observed Cu/Nb incoherent interfaces are comparatively studied. We found that the preferred dislocation nucleation sites are closely related to atomic interface structure, which in turn, depend on the orientation relationship. The activation stress and energies for an isolated Shockley dislocation loop of different sizes from specific interface sites depend strongly on dislocation size, atomic interface pattern, and loading conditions. Such findings provide important insight into the mechanical response of a wide range of fcc/bcc metallic nanocomposites via atomic interface design. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q24.00007: Atomic and Surface Interactions of Electrode Metals with a p-Type Organometallic Conductor Bhaskar Chilukuri, Thomas Cundari A computational study of the interaction of high and low work function electrode metal atoms (M' = Al, Au, Cu, La, Ni, Pd, Pt, Ru, Ni) used in electronic devices with cyclo-[Au($\mu $-Pz)]$_{3}$ trimer (T) (Pz = pyrazolate ligand), a p-type organometallic semiconductor is presented. Metal (M'$_{M})$ and ligand (M'$_{L})$ sites of the gold trimer are investigated as the possible sites of deposition for the metal atoms. Examination of metal binding, geometric and electronic properties suggest that these metal-based, p-type conductors will form stable interfaces with good electron transfer with typical source/drain electrode metals. Encouraged by the molecular simulation results, we performed periodic interface calculations of metal (001) and (111) surfaces with a monolayer of cyclo-[Au($\mu $-Pz)]$_{3}$ trimer using a plane-wave DFT approach. Structural and electronic properties of metal-trimer interfaces and implications for interface stability and electron transfer will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q24.00008: Structure of charge trapping in cerium-doped aluminophosphate and phosphosilicate glasses: combining molecular dynamics simulations and \textit{ab initio} DFT calculations Leopold Kokou, Yun Li, Jincheng Du Cerium doping glasses find wide applications in optical and photonic devices. Both Ce$^{3+}$ and Ce$^{4+}$ can be present in oxide glasses, and their ratio depends on the glass composition, heat history and melting environment. In either oxidation state, the environments of cerium ions are important to the optical absorption and emission properties. In this paper, we present classical molecular dynamic simulations of cerium-containing aluminosilicate and phosphosilicate glasses using newly developed potential models containing cerium ions. The local environments around Ce$^{3+}$ and Ce$^{4+}$ are studied, and the bond length and coordination of cerium ions are determined. Small samples of the glasses are simulated using MD and then further relaxed with Density Functional Theory (DFT) calculations. Comparison of the structure of glasses from MD and after DFT relaxation is made, and the two are found to be in reasonable agreement. It is found that Ce$^{3+}$ has a longer bond distance and higher coordination number of oxygen. Most interestingly, cerium ions are found to be preferentially coordinated by phosphorus ions in the second coordination shell in the glasses. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q24.00009: Surface Structure and Work Function of ZnO Based on First Principle DFT Calculations Yun Li, JinCheng Du Zinc Oxide is a well known n-type wide band gap semiconductor material and remains actively as a strategic material for various photonic applications. The fabricate ZnO, is effectively used as a sensor in various applications, Because of its high infrared reflectance and high visible transmittance. Due to that fact, its electron property plays vital role and attract our attention. Via simulation method, their electron properties were studied through density function theory. Based on first principle theory, their structures with distinct cleaved planes were obtained and completed relaxed in DFT based methods. Depending on cleaved planes, there were Oxygen or Zinc atoms terminated along (001) direction and both of them locating on the cleaved surface (110). Work function and other electron properties will be discussed in detail for all of them and compared with the experimental values, the difference and prediction will be made. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q24.00010: Coupling Fluctuating Hydrodynamics with Molecular Dynamics at the Nanoscale Nikolaos Voulgarakis, Jhih-Wei Chu Hydrodynamic fluctuations and solvation interactions are essential driving forces of transport phenomena in the micrometer to nanometer regime, including inter- and intra-cellular flows and flows in nanofabricated devices. Although all-atom molecular dynamics (MD) simulations can be used to model molecular fluids, the accessible time- and length-scales are severely limited. Since most of computational cost for MD simulations comes from the represetion of solvent molecules, a possible solution to this limitation is to model fluids with fluctuating hydrodynamics (FHD). While this approach reduces the computational time of MD simulations by three orders of magnitude, an accurate protocol to couple FHD with MD is still necessary. In this work we present a new methodology that couples FHD with MD by allowing the fluctuating fields to directly interact with particles through repulsive, attractive, and dissipating/fluctuating forces without introducing new degrees of freedom or boundary conditions. Numerical results show that solvation energy and diffusion dynamics are correctly described within our framework. Simulations on the collapse of two hydrophobic particles are also presented. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q24.00011: Multiscale Modeling of Solutions Olayinka Olatunji-Ojo, Sandra Boetcher, Thomas Cundari The~sequestration of carbon dioxide is one proposed solution to alleviate the~growing problem of increased atmospheric CO$_{2}$ concentration, and its resulting effect on global climate. However, the efficacy of such methods has yet to be demonstrated. Improved CO$_{2}$ sequestration methods are needed and this can be achieved through a better understanding of the chemical and physical consequences of CO$_{2}$ encapsulation through multiscale modeling. Multiscale modeling is an effective tool for combining different methods thereby creating an efficient way of modeling diverse chemical and physical phenomena. The goal of this research is to model carbon dioxide interactions in solutions from the quantum to continuum level. This is achieved through a combination of DFT calculations, molecular modeling (mesoscale) and computational fluid dynamics (continuum) simulations on CO$_{2}$ + H$_{2}$O. Interaction energies and interatomic distances are obtained from DFT calculations, which are used to derive a Lennard-Jones potential, from which one may obtain continuum properties such as viscosity via reverse non-equilibrium molecular dynamics (RNEMD) simulations. [Preview Abstract] |
Session Q25: Superconductivity: Transport Properties
Sponsoring Units: DCMPChair: Wai-Kwong Kwok, Argonne National Laboratory
Room: D166
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q25.00001: Networks of ultra-small MoGe nanowires: fabrication and properties Zhili Xiao, Michael Latimer, Qiong Luo, Wai-Kwong Kwok By developing a template-based method we were able to fabricate networks of MoGe nanowires with widths and thicknesses of few nanometers. Resistive measurements reveal magnetoresistance oscillations with a field period up to 2 Tesla. Detailed information on sample fabrication will be presented. Possible mechanisms on the observed magnetoresistance oscillations will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q25.00002: Magnetization-dependent resistance of double ferromagnet-superconductor junctions Kuei Sun, M. Colci, D.J. Van Harlingen, Nayana Shah, Smitha Vishveshwara Studies of the crossed Andreev reflection (CAR) process in double ferromagnet-superconductor junctions have attracted a lot of attention as way of realizing solid-state entanglement. Here, we perform a theoretical analyses of such a system motivated by our surprising experimental findings that the resistance in the antiparallel alignment of the magnetization of ferromagnets is larger than that in the parallel state. We model the system using an extended Blonder-Tinkham-Klapwijk (BTK) treatment with spin-dependent interfacial barriers associated with the magnetization. We compute scattering amplitudes of CAR and other possible processes as well as the resistance as a function of interfacial parameters. Our results reveal significantly altered physics due to the magnetization-dependent scattering, such as a sign change in the relative resistance between the parallel and antiparallel cases. We can model the positive relative resistance corresponding to our experimental findings as well as the negative results observed in other experiments, both within sufficiently large parameter regions. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q25.00003: Measurements of the critical current of small Sr$_2$RuO$_4$ crystals Yiqun Ying, Neal Staley, Ying Liu, Yan Xin, David Fobes, Tijiang Liu, Zhiqiang Mao We report critical current measurements of chiral $p$-wave superconductor Sr$_2$RuO$_4$. Because of the strong anisotropy possessed by Sr$_2$RuO$_4$, vortex lines along the in-plane direction are expected to be pinned more strongly than those along the $c$ axis, resulting in anisotropic critical currents. We prepared small single crystals of Sr$_2$RuO$_4$ with a typical size of 50$\mu$m$\times$ 10$\mu$m$\times$1$\mu$m by mechanical exfoliation and characterized them by Raman spectroscopy and high-resolution transmission electron microscopy, showing that they were either pure Sr$_2$RuO$_4$ or eutectic phase containing one or more Ru microdomains. Four-point or Hall probes were prepared on the small crystals by photo lithography. While samples of pure Sr$_2$RuO$_4$ exhibited a typical transition temperature ($T_c$) of 1.2K, slightly lower than the optimal bulk $T_c$, 1.5K, those with Ru microdomains showed multiple resistive transitions with the highest $T_c$ around 2K. The critical current and critical field phase diagrams were determined for these small crystals. Surprisingly, the in-plane critical current density, measured for the first time, was found to be significantly larger than that along $c$-axis of the bulk. The physical implications of these observations will be discussed. Supported by DOE. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q25.00004: Angle-dependent transport behavior near the magnetic-field tuned superconductor-insulator transition Min-Soo Kim, Tai-Lung Wu, L.W. Engel, G. Sambandamurthy Thin films of superconducting, amorphous indium oxide were driven insulating by the application of magnetic field and their transport behavior at different magnetic field values are studied. Well below the critical field of the transition, the current-voltage characteristics follow a power law V$\propto$ I$^{p}$, where p depends on the magnetic field. The dependence of the power p on magnetic field and the angle between the sample plane and the magnetic field direction will be presented. In particular, we find two distinct magnetic field values, well above the critical field, where the sample resistance is independent of the angle. Implications of these results in improving our current understanding of the transition will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q25.00005: $T_c$ Enhancement in Electron-Doped Cuprate Heterostructures P.L. Bach, K. Jin, X.H. Zhang, R.L. Greene, U. Grupel, E. Zohar, E. Diamant, Y. Dagan, S. Smadici, P. Abbamonte Multilayer thin films of La$_{2-x}$Ce$_x$CuO$_4$ (LCCO) and Pr$_{2-x}$Ce$_x$CuO$_4$ (PCCO) were fabricated as superlattices of different dopings. Pairing over-doped and under-doped (or un-doped) layers is found to increase $T_c$ significantly above that of the single-phase films corresponding to the under- or over-doped layers. We report transport measurements on these mulitlayer films and discuss possible mechanisms for the $T_c$ enhancement. This work was supported by the US-Israel Binational Science Foundation Grant \#2006385 and the Center for Nanophysics and Advanced Materials (CNAM). [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q25.00006: Electronic Transport and Superconductivity in Bi Confined in a 200nm Opal Host Michael Nieskoski, Ryan Johnson, Steven Disseler, Michael Graf, Tito Huber, Austin Howard, Anvar Zakhidov While bulk bismuth at ambient pressure is not a superconductor, changes in morphology are known to induce superconductivity in Bi at low temperatures. We present a study of bismuth nanoparticle arrays fabricated by confining bismuth into a porous opal host consisting of close-packed 200 nm silica spheres. Electrical transport was studied down to temperatures of 0.3K and magnetic fields up to 2T. We find the onset of superconductivity at 4.4K (confirmed by AC magnetic susceptibility) and global superconductivity at a temperature of 1.3K. This two step transition is typical for granular superconductors. Measurements of the critical temperature in magnetic field show that the higher temperature transition consists of at least two transitions. The low temperature upper critical field was calculated to be approximately 0.82T. These results are discussed in terms of the morphologies, namely amorphous and granular structure, that are known to make Bi a superconductor, and the three characteristic sizes for the nanoparticles in that has been used to describe superconductivity in lead imbedded in these host materials [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q25.00007: Electrostatic Modification of Properties of Ultrathin YBCO Films using an Electronic Double Layer Transistor Xiang Leng, Allen Goldman We have modified the transport properties of ultrathin films of YBCO using an electronic double layer transistor configuration employing the ionic liquid DEME-TFSI [1]. The films were prepared on STO substrates using high pressure oxygen sputtering. The electronic double layer configuration permits extraordinarily large transfers either involving the accumulation or depletion of carriers, employing relatively low gate voltages. Thus far the transition temperature of a 10 unit cell thick film has been shifted by as much as 30K, and the insulating state has been induced in a 7 unit cell thick film. The latest results will be reported on the use of this technique as an alternative to chemical doping. This work was supported by the National Science Foundation under grant NSF/DMR-0709584. \\[4pt] [1] J.T. Ye et al., Nature Materials 9, 125(2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q25.00008: Phase-fluctuating superconductivity in overdoped La$_{2-x}$Sr$_x$CuO$_4$ P.M.C. Rourke, I. Mouzopoulou, X. Xu, C. Panagopoulos, Y. Wang, B. Vignolle, C. Proust, E.V. Kurganova, U. Zeitler, Y. Tanabe, T. Adachi, Y. Koike, N.E. Hussey In underdoped cuprates, an energy gap (pseudogap), appears in the electronic density of states well before superconductivity develops. Similarities between the pseudogap and superconducting gap have led to the idea that the pseudogap is a precursor superconducting state in which the superconducting order parameter is finite but the phase fluctuates. However, this picture of precursor pairing has been challenged by measurements indicating that the pseudogap itself closes at a critical doping concentration just beyond optimal doping. By tracking the restoration of the normal state magnetoresistance in overdoped La$_{2-x}$Sr$_x$CuO$_4$, we show that the phase fluctuation regime remains broad across the entire superconducting composition range, in contrast to the evolution of the pseudogap. The universal low phase stiffness is shown to be correlated with a low superfluid density, a characteristic of both underdoped and overdoped cuprates. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q25.00009: Voltage Oscillations in Silver Doped YBa$_{2}$Cu$_{3}$O$_{7-x}$ Atilgan Altinkok, Murat Olutas, Kivilcim Kilic, Atilla Kilic Nonlinear transport phenomena and time effects were investigated by the time evolution of sample voltage ($V \quad - \quad t$ curves) on long time scales in Ag-doped YBa$_{2}$Cu$_{3}$O$_{7-x}$ sample (YBCO/Ag). We also investigated influence of bidirectional square wave current with various periods ($P)$ and dc currents ($I)$ on the evolution of $V \quad - \quad t$ curves in YBCO/Ag sample material at different temperatures ($T)$ and external magnetic ($H)$ fields. It was observed that a non-linear response seen in $V \quad - \quad t$ curves to bidirectional square wave (BSW) current with sufficiently short periods or sufficiently low amplitude reflects itself as regular sinusoidal- type voltage oscillations. The observed oscillating mode was correlated to the dynamic competition between pinning and depinning. Further, the similarity between the flux dynamics and the charge density waves is considered as a possible explanation of voltage oscillations in YBCO/Ag. Detailed analysis of $V \quad -$ $t$ curves and voltage oscillations reveals that adding of Ag causes degradation in both intergranular and surface pinning of YBCO material. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q25.00010: Andreev and Single-Particle Tunneling Spectra of Underdoped Cuprate Superconductors Kun Huang, Kai-Yu Yang, Wei-Qiang Chen, T.M. Rice, Fu-Chun Zhang We study tunneling spectroscopy between a normal metal and an underdoped cuprate superconductor modeled by a phenomenological theory in which the pseudogap is a precursor to the undoped Mott insulator. In the low barrier tunneling limit, the spectra are enhanced by Andreev reflection only within a voltage region of the small superconducting energy gap. In the high barrier tunneling limit, the spectra show a large energy pseudogap associated with single particle tunneling. Our theory semi- quantitatively describes the two gap behavior observed in tunneling experiments. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q25.00011: Doping evolution of nodal quasiparticles in the cuprate superconductor YBCO via low-temperature thermal conductivity Samuel Rene de Cotret, J.-Ph. Reid, N. Doiron-Leyraud, L. Taillefer, B.J. Ramshaw, R. Liang, D.A. Bonn, W.N. Hardy The thermal conductivity of the cuprate superconductor YBa$_2$Cu$_3$O$_y$ was measured at temperatures down to $T \sim 50$~mK in magnetic fields up to $H=15$~T on high-quality single crystals with a hole doping ranging from $p = 0.08$ to $p = 0.18$. The residual linear term at $T \to 0$, a direct measure of the nodal quasiparticle velocities [1], is tracked as a function of doping, and compared to recent, high-resolution ARPES measurements of the Fermi velocity and gap magnitude as a function of doping, in the related cuprate superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ [2]. \\[4pt] [1] D.G. Hawthorn {\it et al.}, Phys. Rev. B {\bf 75}, 104518 (2007). \\[0pt] [2] I.M. Vishik {\it et al.}, Phys. Rev. Lett. {\bf 104}, 207002 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q25.00012: Correlation Strength, Optical Conductivity and d-d excitons in high $T_c$ cuprates Andrew Millis, Xin Wang, Luca deMedici A single site dynamical mean field analysis is presented of models of high $T_c$ copper-oxide superconductors, including oxygen orbitals and both $x^2-y^2$ and $3z^2-r^2$ $Cu$ d-orbitals. The optical conductivity, doping dependent effective mass and the $e_g$ portion of the d-d exciton spectrum are determined. The details of the oxygen-oxygen hopping are shown to be unimportant. A general connection between $d$ valence and the metal/charge transfer insulator phase boundary is outlined. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q25.00013: Determining Transport Parameters for Superconductor/Normal Metal Point Contacts in an Applied Magnetic Field from Conductance versus Field Data at Fixed Temperature Paul J. Dolan, Jr., Charles W. Smith Superconductor/normal metal point contact transport data often consists of normalized conductance as a function of reduced temperature, from which the elastic scattering parameter and the inelastic scattering parameter for the contact can be determined, in addition to other features of interest. We show a strategy for determining these parameters from normalized conductance as a function of reduced applied magnetic field, at fixed temperature, even when conductance versus variable temperature data is absent. This analysis strategy will be demonstrated for several point contacts, over a wide range of parameter values. [Preview Abstract] |
Session Q26: Focus Session: Iron Based Superconductors -- Optics, Heat Capacity, Thermopower
Sponsoring Units: DMP DCOMPChair: Hong Ding, Institute of Physics, Chinese Academy of Sciences
Room: D162/164
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q26.00001: Optical Investigation of the Charge Dynamics in Ba(Co$_x$Fe$_{1-x}$)$_2$As$_2$ Invited Speaker: We report on a thorough optical investigation over a broad spectral range and as a function of temperature of the charge dynamics in Ba(Co$_x$Fe$_{1-x}$)$_2$As$_2$ compounds for Co-doping ranging between 0 and 18\%. For the parent compound as well as for $x$=0.025 we observe the opening of a pseudogap, due to the spin-density-wave phase transition and inducing a reshuffling of spectral weight from low to high frequencies. For compounds with 0.051$\le x \le$ 0.11 we detect the superconducting gap, while at $x$=0.18 the material stays metallic at all temperatures. We describe the effective metallic contribution to the optical conductivity with two Drude terms, representing the coherent components, and extract the respective scattering rates. Finally through spectral weight arguments, we give clear-cut evidence for moderate electronic correlations for 0$\le x \le$0.061, which then crossover to values appropriate for a regime of weak interacting and nearly-free electron metals for $x\ge$0.11. We also investigate the optical conductivity with light polarized along the in-plane orthorhombic $a$- and $b$-axes of Ba(Co$_x$Fe$_{1-x}$)$_2$As$_2$ for $x$=0 and 2.5$\%$ under uniaxial pressure across their structural and magnetic transitions. The charge dynamics at low frequencies and temperatures on these detwinned, single domain samples reveals an enhancement of both the scattering rate and Drude weight of the charge carriers along the antiferromagnetic $a$-axis with respect to the ferromagnetic $b$-axis. Our findings also allow us to estimate the dichroism, which extends to high frequencies. These results demonstrate the electronic nature of the structural transition found in underdoped Fe-pnictides. Co-authors: A. Dusza, A. Lucarelli, F. Pfuner, J.-H. Chu, I.R. Fischer. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q26.00002: In-plane and c-axis optical spectroscopy study on 122 Fe-pnictides Nan Lin Wang I present the in-plane and the c-axis optical spectroscopy investigations on 122 Fe-pnictides. For the parent compound BaFe$_2$As$_2$, the in-plane measurement revealed two different energy gaps in the SDW state, whereas for the c-axis polarized measurement only the energy gap at smaller energy scale could be clearly observed. We suggest different driving mechanisms for the formation of the two energy gaps. The large energy gap is caused by the nesting between disconnected 2D cylinder-like electron and hole Fermi surfaces. It is the main driving force for the SDW instability. The small energy gap is the one formed on the 3D Fermi surface due to the presence of reduced magnetic Brillouin zone which crosses the 3D Fermi surface. It is the consequence of the establishment of the magnetic order. For the doped superconducting 122 samples, the in-plane optical measurement revealed a formation of full superconducting energy gap, whereas the c-axis optical measurement indicated a large residual quasiparticle population down to very low temperature. Those quasiparticles contribute specifically to the c-axis transport. We suggest that there exist horizontal nodes in the superconducting gap in regions of the 3D Fermi surface that contribute dominantly to the c-axis optical conductivity. Work done with Z. G. Chen, W. Z. Hu, B. Cheng, G. Li, J. Dong, T. Dong, R. H. Yuan, P. Zheng, G. F. Chen, J. L. Luo, Z. Fang, X. Dai, C. L. Zhang and P. Dai. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q26.00003: Optical signature of sub-gap absorption in the superconducting state of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ Urmas Nagel, T. R\~o\~om, R.P.S.M. Lobo, Y.M. Dai, J. Carbotte, T. Timusk, D. Colson The optical conductivity of Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ shows a clear signature of the superconducting gap, but a simple $s$-wave description fails in accounting for the low frequency response. This task is achieved by introducing an extra Drude peak in the superconducting state representing sub-gap absorption, other than thermally broken pairs. This extra peak and the coexisting $s$-wave response respect the total sum rule indicating a common origin for the carriers. We discuss the possible origins for this absorption as (i) quasiparticles due to pair-breaking from interband impurity scattering in a two band $s_{\pm}$ gap symmetry model, which includes (ii) the possible existence of impurity levels within an isotropic gap model; or (iii) an indication that one of the bands in is highly anisotropic. The results are published in Phys. Rev B vol 82, 100506(R) (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q26.00004: Femtosecond low-energy dynamics of superconducting and spin-density wave gaps in pnictides K.W. Kim, A. Pashkin, M. Beyer, H. Sch\"{a}fer, M. Porer, T. Wolf, C. Bernhard, J. Demsar, R. Huber, A. Leitenstorfer Magnetism and superconductivity (SC) in pnictides as well as a possible link between them are subjects of intense studies. The infrared spectral regime plays a pivotal role en route to a microscopic understanding since it provides direct access to the fundamental low-energy excitations, such as spin-density waves (SDW) and SC-induced energy gaps. We investigate Ba(Fe,Co)2As2 by combining ellipsometry and ultrabroadband terahertz (THz) pump-probe experiments. Following a femtosecond near-infrared excitation, the spectral hallmark of SDW located in the 10 - 30 THz window disappears with a characteristic saturation fluence of $\Phi_{s} \approx$ 50 J/cm$^{2}$ and recovers fast ($\tau <$ 1 ps), while the SC gaps below 3 THz are fully closed at a much smaller fluence $\Phi_{s} \approx$ 3 J/cm$^{2}$ and exhibit a slower relaxation behavior ($\tau >$ 10 ps). The distinct spectral, temporal and saturation behavior provide a unique environment to monitor the interplay of the two order parameters. Furthermore we observe coherent oscillation at 5.5 THz which corresponds to an Arsenic vibration. Our results may add new aspects toward an understanding of interactions between fundamental excitations in pnictides. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q26.00005: Optical properties of BaFe$_{\mathbf{1.85}}$Co$_{\mathbf{0.15}}$As$_{\mathbf 2}$ J.J. Tu, L.J. LI, G.H. Cao, Z.A. Xu, C.C. Homes The detailed in-plane optical properties of the electron-doped iron-arsenic superconductor BaFe$_{1.85}$Co$_{0.15}$As$_2$ have been determined over a wide frequency range above and below $T_c = 25$~K. Despite being a multiband system, the normal state reveals that a single (electron) band dominates the low-frequency conductivity, which can be modeled by a single Drude component with plasma frequency $\omega_{p,D} \simeq 7840$~cm$^{-1}$ and scattering rate $1/\tau_D \simeq 126$~cm$^{-1}$, determined just above $T_c$. For $T \ll T_c$ the superconducting plasma frequency is $\omega_{p,S} \simeq 5200$~cm$^{-1}$ ($\lambda_{\it eff} \simeq 3000$~\AA ), indicating that less than half the free carriers in the normal state have collapsed into the condensate, suggesting that this material is not in the clean limit. There are two energy scales for the superconductivity, $\Delta_1(0) = 3.1\pm 0.2$~meV and $\Delta_2(0) = 7.4\pm 0.3$~meV. This corresponds to either the gapping of the electron and hole pockets, respectively, or an anisitropic {\em s}-wave gap on the electron pocket; both views are consistent with the $s^\pm$ model.\footnote{J. J. Tu {\em et al.}, Phys. Rev. B {\bf 82}, 174509 (2010).} [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q26.00006: Raman investigation of the magneto-structural transition in electron doped Ba(FeAs)$_2$ Yann Gallais, Ludivine Chauviere, Maximilien Cazayous, Marie-Aude Measson, Alain Sacuto, Dorothee Colson, Anne Forget We report a doping dependent Raman scattering study of the magneto-structural transition in Co doped Ba(FeAs)2. Several zone centered phonons display significant anomalies at the tetragonal to orthorhombic transition. In particular, the doubly degenerate in-plane Eg phonon shows an enhanced splitting in the ortho phase. The splitting weakens considerably with doping and gives evidence for strong spin-phonon coupling in iron-pnictides. The electronic Raman continuum displays a systematic upturn at low energy around the magneto-structural transition. This quasi-elastic scattering is similar to magnetic energy fluctuations usually observed in magnetic insulators. Interestingly significant fluctuations are observed at low temperature even for x=0.065 doping, where the Neel temperature goes to zero and optimal Tc is reached. At high energy and low doping, the electronic Raman continuum displays clear signatures Fermi surface reconstruction due to the opening of the spin density wave gap at the magnetic transition. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q26.00007: Effect of annealing on the gap structure of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$: low temperature specific heat studies K. Gofryk, F. Ronning, E.D. Bauer, J.D. Thompson, A.B. Vorontsov, I. Vekhter, A.S. Sefat, T. Imai We report on the effect of annealing on the temperature and field dependencies of the low temperature specific heat of the electron-doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ for under- (x = 0.045), optimal- (x = 0.08) and over-doped (x = 0.105 and 0.14) regimes. We observed that annealing significantly improves some superconducting characteristics in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. It considerably increases $T_{c}$, decreases $\gamma_{0}$ in the superconducting state and suppresses the Schottky-like contribution at very low temperatures. The improved sample quality allows for a better identification of the superconducting gap structure of these materials. We examine the effects of doping and annealing within a self-consistent framework for an extended s-wave pairing scenario. At optimal doping our data indicates the sample is fully gapped, while for both under- and over-doped samples significant low-energy excitations remain, possibly consistent with a nodal structure. The difference of sample quality offers a natural explanation for the variation in low temperature power laws observed by many techniques. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q26.00008: The low-temperature specific heat of Co-doped BaFe$_{2}$As$_{2}$ Jiunn-Yuan Lin We have measured the low-temperature specific heat of Ba(Fe$_{1- x}$Co$_{x}$)$_{2}$As$_{2}$ ($x$=0,0.08,0.2) single crystals. The electronic specific heat of Ba(Fe$_{0.92}$Co$_{0.08}$)$_{2} $As$_{2}$ in the superconductiong state with $T_{c}$=21 K is revealed. A $T^{2}$ term was observed at low temperatures, providing the evidence of nodes in the gap. Furthermore, the data suggest a multi-gap feature for Ba(Fe$_{0.92}$Co$_{0.08}$) $_{2}$As$_{2}$. The mixed state data will also be reported. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q26.00009: The Electronic Specific Heat of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ from 2K to 380K James Storey, John Loram, John Cooper, Zbigniew Bukowski, Janusz Karpinski Using a unique differential technique, we have measured the specific heat capacity of polycrystalline Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ with x = 0, 0.1, 0.2, 0.3, 0.5 0.9 and 1.0, between 2K and 380K and in magnetic fields (H) from 0 -- 13T. We determine the electronic specific heat coefficient $\gamma $ ($\equiv $ C$_{el}$/T) over the entire range of T, H and x and compare it with the magnetic susceptibility of the seven samples. We show that our results are consistent with single crystal studies but give further interesting information. For x $<$ 0.3, $\gamma $ is progressively reduced at low T by a SDW gap, but is only weakly doping and T-dependent above the structural/magnetic transition. For x = 0.3 the normal state $\gamma _{n}$ is constant up to 380K, but as x increases from 0.3 to 1.0, $\gamma _{n}$ becomes increasingly T-dependent, increasing by a factor two at low-T and decreasing by a factor 1.5 at 380K for x = 1. We consider possible explanations for this striking T-dependence in terms of a sharp peak in the electronic density of states, a strongly x- and T-dependent effective mass enhancement, or low energy magnetic excitations. The H-dependent measurements allow us to extract the critical fields, superfluid density and coherence length as functions of doping and temperature. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q26.00010: Specific Heat to 35 T in P-doped and Co-doped BaFe$_{2}$As$_{2}$: Evidence for Nodes or Not? G.R. Stewart, J.S. Kim, P.J. Hirschfeld, F. Ronning, K. Gofryk, A.S. Sefat, S. Kasahara, T. Shibauchi, T. Terashima, Y. Matsuda We have measured the low temperature specific heat of annealed single crystal Ba(Fe$_{0.955}$Co$_{0.045})_{2}$As$_{2}$, unannealed single crystal BaFe$_{2}$(As$_{0.7}$P$_{0.3})_{2}$, and other BaFe$_{2}$As$_{2}$ derivatives in fields to 35 T. We report contrasting behavior, with the underdoped Co sample exhibiting behavior (specific heat $\gamma \quad \sim $ H$^{0.7})$ essentially up to H$_{c2}$ similar to the Volovik effect prediction ($\gamma \quad \sim $ H**0.5) for nodal behavior for fields H $<$ 0.1H$_{c2}$. In contrast, $\gamma $ up to 35 T (2/3 of H$_{c2})$ in BaFe$_{2}$(As$_{0.7}$P$_{0.3})_{2}$ exhibits linear with field dependence, consistent with fully gapped behavior but inconsistent with indications of nodal behavior from other measurements. Possible explanations, and up-to-date measurements will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q26.00011: Thermoelectric power of Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$: possible changes in Fermi surface with and without changes in electorn count Halyna Hodovanets, Eundeok Mun, Alex N. Thaler, Sergey L. Bud'ko, Paul C. Canfield Temperature-dependent, in-plane, thermoelectric power (TEP) data are presented for Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ ($0\le x \le 0.36$) single crystals. The previously outlined by resistivity and susceptibility $x - T$ phase diagram for this system is confirmed. The analysis of TEP evolution with Ru-doping suggests two concentrations, $ x\sim 7\%$ and $x\sim 30\%$ of Ru-doping levels, near which significant changes in the electronic structure, correlations and/or scattering occur. These results are compared with an extended set of TEP data for the electron doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($0.13\le x \le 0.42$) single crystals. An analysis of TEP data for Co-doping in the overdoped region suggests two more concentrations, $x\sim 11\%$ and $x\sim 22\%$, in addition to x$\sim 2\%$ previously reported, where Lifshitz transition might occur. These data for Co-doping were recently confirmed by ARPES measurements. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q26.00012: Nernst and Seebeck coefficients of the iron-pncitide superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ Xigang Luo, H. Shakeripour, J. Chang, F. Laliberte, J. -Ph. Reid, N. Doiron-Leyraud, L. Taillefer, M.A. Tanatar, R. Prozorov, H.Q. Luo, Z.S. Wang, H.-H. Wen The Nernst and Seebeck coefficients of the iron-pnictide superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ were measured in single crystals for K concentrations ranging from the parent compound at x=0 to the optimally-doped superconductor at x=0.40, where Tc=38 K. Both coefficients show sharp anomalies at T$_{N}$, the onset temperature for antiferromagnetic order. This allows us to track the doping dependence of T$_{N}$ and hence to map out the T-x phase diagram of Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_ {2}$. The reconstruction of the Fermi surface by the antiferromagnetic order causes a huge enhancement of the quasiparticle Nernst signal, suggesting that carrier density and Fermi temperature are dramatically reduced in the magnetic phase. The Nernst signal due to superconducting fluctuations is small by comparison, and it remains detectable up to a temperature approximately 15{\%} above T$_{c}$, in the optimally-doped sample. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q26.00013: Coherence peak and pair-breaking effects in the ac conductivity of BaFe$_{2-2x}$Co$_{2x}$As$_2$ epitaxial thin films N.P. Armitage, Rolando Vald\'es Aguilar, L.S. Bilbro, S. Lee, C.W. Bark, C.B. Eom We report a study of high quality pnictide superconductor BaFe$_{1.84}$Co$_{0.16}$As$_2$ epitaxial thin films using time-domain THz spectroscopy. Near T$_c$ we find evidence for a coherence peak and qualitative agreement with the weak-coupling Mattis-Bardeen form of the conductivity. At low temperature, we find that the real part of the THz conductivity is not fully suppressed and $\sigma_2$ is significantly smaller than the Matthis-Bardeen expectation. The temperature dependence of the penetration depth $\lambda$ follows a power law with an unusually high exponent of 3.1. We interpret these results as consistent with impurity scattering induced pair-breaking. Taken together our results are strong support for an extended s$\pm$ symmetry order parameter. [Preview Abstract] |
Session Q27: Focus Session: Semiconductor Qubits- Quantum Control
Sponsoring Units: GQIChair: Sergey Frolov, Delft University of Technology, Netherlands
Room: C155
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q27.00001: Control and Manipulation of Two-Electron Spin Qubits in GaAs Quantum Dots Invited Speaker: We have developed means to both couple and decouple a two electron spin qubit from its environment. Using dynamic nuclear polarization we are able to suppress fluctuations in the nuclear environment and prolong T2* by nearly an order of magnitude reaching 150 nano seconds. Our polarization scheme employs a quantum feedback mechanism that directly conditions the rate at which the qubit polarizes its nuclear environment on a quantum limited measurement of the hyperfine field seen by the same qubit. In addition, the stabilized state of the nuclear environment allows us to perform controlled X rotations and thereby demonstrate full control over the entire Bloch sphere as well as full quantum state tomography. Using dynamic decoupling of the two electron spin qubit from its environment we are able to prolong T2 by nearly three orders of magnitude reaching nearly 300 micro seconds. Our results indicate that gate fidelities of up to 99.99{\%} are within reach despite the fluctuating nuclear environment. Moreover, the demonstrated ultra long coherence time allows for more than 10$^5$ coherent gate operations which exceed the estimated threshold for quantum error corrections by a substantial margin. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q27.00002: Quantum gates for the singlet-triplet $\mathrm{T}_+$ qubit Hugo Ribeiro, J.R. Petta, Guido Burkard We theoretically show that hyperfine interactions can be harnessed for quantum gate operations in GaAs semiconductor quantum dots [1]. In the presence of an external magnetic field $B$, which splits the triplet states, the hyperfine interaction results in an avoided crossing between the spin singlet $\textrm{S}$ and spin triplet $\textrm{T}_{+}$, which form the basis of a new type of spin qubit. Coherent quantum control for this qubit is achieved through Landau-Zener-St\"uckelberg transitions at the S-T$_{+}$ avoided crossing [2]. A set of suitable transitions allows to build any single qubit gates on timescales shorter than the decoherence time $T_2^* \sim 16\mathrm{ns}$ [1]. We also show how to build a conditional two-qubit gate by capacitively coupling two S-T$_{+}$ qubits. \\[4pt] [1] H. Ribeiro, J. R. Petta, and G. Burkard, Phys. Rev. B 82, 115445 (2010). \newline [2] H. Ribeiro and G. Burkard, Phys. Rev. Lett. 102, 216802 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q27.00003: Coherent spin manipulation with a triple quantum dot Ghislain Granger, Louis Gaudreau, Alicia Kam, Sergei Studenikin, Piotr Zawadzki, Geof Aers, Michel Pioro-Ladri\`ere, Zbigniew Wasilewski, Andrew Sachrajda Recently, Landau-Zener-Stuckelberg (LZS) oscillations have been demonstrated in a double quantum dot device [1]. In this talk we demonstrate LZS oscillations in a triple quantum dot environment. Our triple quantum dot design allows us to tune to either the charge or spin qubit regimes. Using a pulsing technique in the spin qubit regime, we create a superposition of triple quantum dot states, allow for phase accumulation, and interfere. We demonstrate coherent LZS oscillations with three spins across the triple quantum dot structure. We investigate their dependence on pulse rise time, separation time, energy detuning, and magnetic field. \\[4pt] [1] J. R. Petta et al., Science 327, 669 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q27.00004: Novel Coherent Spin Oscillations in a Triple Quantum Dot Circuit Andrew Sachrajda, Ghislain Granger, Louis Gaudreau, Alicia Kam, Sergei Studenikin, Piotr Zawadzki, Geof Aers, Michel Pioro-ladriere, Zbig Wasilewski We have demonstrated Landau-Zener-Stuckelberg oscillations in a triple quantum dot circuit related to pairs of triple quantum dot states. Different initialization schemes and pulse shapes involving all three dots will be discussed. However, the complexity of a triple quantum dot system suggests that in general coherent behaviour can be expected from interplays between various combinations of states. Here we demonstrate both experimentally and theoretically in a triple quantum circuit containing three spins, a coherent interplay between two coexisting qubits as a function of pulse amplitude and rise time. To further clarify the behaviour within the system we also observe and study coherent oscillations after a fourth spin has been added to the system in one of the relevant dots. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q27.00005: A Single Electron Charge Qubit in the Strong Driving Limit J. Stehlik, Y. Dovzhenko, J. R. Petta, H. Lu, A. C. Gossard The dynamics of strongly driven two-level systems in the presence of dissipation have been thoroughly studied using theoretical models.$[1]$ We use a model system, a GaAs double quantum dot (DQD) containing a single electron, to experimentally explore the strong-driving regime. We measured the transport through the DQD as a function of detuning and applied microwave power and compare with the Tien-Gordon model. In contrast with previous experiments, we directly access the occupation of the DQD using a quantum point contact charge sensor. In the high frequency regime ($\hbar \omega_{driving} \gg \Delta$, where $\Delta$ is the tunnel coupling) we observe up to 9-photon transitions and clear Bessel function behavior of the DQD occupation with applied microwave power. We also studied the intermediate frequency regime, observing 18-photon transitions. The data are modeled using the time-dependent Schrodinger equation.$[2]$ By comparing the data with the simulations, we estimate $T_1\sim 15$ ns and $T_2\sim3$ ns. \\ \noindent $[1]$ A. J. Leggett \textit{et al.}, Rev. Mod. Phys. \textbf {59}, 1 (1987).\\ \noindent $[2]$ S. N. Shevchenko, S. Ashhab, F. Nori, Phys. Rep. {\bf492}, 1 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q27.00006: Non-adiabatic Quantum Control of a Semiconductor Charge Qubit Yuliya Dovzhenko, Jiri Stehlik, Karl Petersson, Jason Petta, Hong Lu, Arthur Gossard A GaAs double quantum dot is configured in the single-electron regime and operated as a charge qubit. The two basis states correspond to the electron being in either the left or the right dot. Non-adiabatic voltage pulses are applied to the depletion gates to drive coherent rotations, and the double dot occupation is read out using a nearby quantum point contact charge sensor. In contrast with previous work, where a single non-adiabatic pulse was applied for quantum control, we apply multiple pulses working towards a charge echo.[1,2] Data for $\frac{\pi}{2}$ - $\tau$ - $\frac{\pi}{2}$ and the $\frac{\pi}{2}$ - $\tau_1$ - $\pi$ - $\tau_2$ - $\frac{\pi}{2}$ ``charge echo" pulse sequences are obtained and compared with numerical simulations of the charge qubit evolution. \noindent References:\\ \noindent [1] K. D. Petersson \textit{et al.}, Phys. Rev. Lett. (in press).\\ \noindent [2] Y. Nakamura \textit{et al.}, Phys. Rev. Lett. {\bf88}, 047901 (2002).\\ [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q27.00007: Extended coherence of exchange operations in double quantum dot spin qubits using Hahn echo Michael Shulman, Hendrik Bluhm, Oliver Dial, Vladimir Umansky, Amir Yacoby Semiconductor spin qubits are promising candidates for quantum computation because of their long coherence times and potential for scalability. The exchange interaction is a powerful resource in these qubits, as it can drive single qubit rotations and inter-qubit entanglement. However, spin qubits driven by exchange become sensitive to charge noise, which in free induction decay experiments has lead to dephasing after a few coherent exchange oscillations. We perform a Hahn echo measurement in two-electron spin qubits in GaAs quantum dots. The $\pi$-pulse is applied by means of a stabilized nuclear gradient in the quantum dots. We find an exponential dephasing with a time constant of up to 10$\mu$s, which is more than an order of magnitude larger than $T_{2}^{*}$, and corresponds to 500 coherent exchange operations within $T_{2}$. This increase in $T_{2}$ is expected to allow for several cPHASE operations between two charge coupled two-electron qubits within $T_{2}$. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q27.00008: Two-qubit operations of two-electron spin qubits in GaAs quantum dots Hendrik Bluhm, Michael Shulman, Oliver E. Dial, Vladimir Umansky, Amir Yacoby The realization of two-qubit entangling gates is one of the most important milestones for the development of quantum-dot based electron spin qubits. Our measurements and simulations of the coupling strength and the relevant coherence time indicate very favorable prospects for the realization of such gates using the Coulomb interaction between adjacent spin qubits. This operation can be protected against dephasing due to low frequency electric noise by simultaneously applying a $\pi$-pulse to both qubits, which is essential to achieve the required coherence time. We report the experimental realization of this echo operation in a two-qubit device, conditional evolution of one qubit depending on the charge state of the neighboring double dot, and further progress toward two-qubit entanglement. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q27.00009: Spin Manipulation in InAs Nanowire Double Quantum Dots M.D. Schroer, M. Jung, K.D. Petersson, C.M. Quintana, J.R. Petta Recently, much effort has been devoted to the development of physical qubits for integration into quantum computers. Qubits allowing control with electric fields are attractive, as ac magnetic fields are more difficult to generate and localize on the nanoscale. The material properties of InAs allow efficient driving of electron spin resonance via the spin--orbit interaction. Our work has focused on developing quantum dots in InAs nanowires as fully characterized and controllable qubits. We have optimized our nanowire growth to eliminate the presence of planar defects, which impede the predictable formation of quantum dots. Using a bottom--gated architecture [1], we demonstrate tunable InAs nanowire double quantum dots, with the occupation controllable to the last electron. Pauli blockade is observed in the two-electron regime, demonstrating spin-dependent transport. We are able to drive single spin rotations by applying microwaves to one of the local metallic gates; from the electron spin resonance condition we extract a g--factor of $\sim$9. Finally, we demonstrate full electrical control of the two-electron system and characterize gate fidelities.\\[4pt] [1] S. Nadj-Perge {\it et al.}, arXiv:1011.0064v1 [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q27.00010: On-demand single-electron transfer between distant quantum dots with nanosecond pulses of surface acoustic waves R.P.G. McNeil, M. Kataoka, C.J.B. Ford, C.H.W. Barnes, J.P. Griffiths, G.A.C. Jones, I. Farrer, D.A. Ritchie Quantum dots (QDs) provide a useful system for manipulating and storing quantum information. Methods for moving quantum information (spin) between processor and storage, or to a region of holes for conversion to photon qubits, will be required. Tunnelling of electrons over long distances between QDs is not viable. We show controlled long-range transfer of single electrons between QDs through a depleted 1D channel using pulses of surface acoustic waves (SAWs). In our device, two QDs are connected by a 4$\mu$m channel with QD occupancy monitored by 1D charge detectors. Electrons may be trapped and raised above the Fermi energy by stepping gate voltages. Having set the first QD to be `full' and the other QD `empty', a short SAW pulse is sent to transfer the electron to the opposite QD. This bi-directional process may be repeated over 100 times with the same electron. SAW power and pulse-width dependences suggest that transfer is achieved during the first few SAW cycles allowing sub-20ns pulses to be used. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q27.00011: A proposed all-electrical spin qubit CNOT gate robust against charge noise Sankar Das Sarma, Jason Kestner We shall propose an alternative to the Loss-DiVincenzo implementation of the CNOT gate in a quantum dot spin qubit system. Our all-electrical proposal has the advantage of being robust against uncertainties and fluctuations in the tunnel coupling, barrier gate voltage pulse area, and interwell detuning which typically arise due to charge noise. The core idea is to introduce an auxiliary dot and use an analog to the stimulated Raman adiabatic passage (STIRAP) pulse sequence in three-level atomic systems, often referred to in the context of electron transport in quantum dot systems as CTAP (Coherent Tunneling by Adiabatic Passage). Spin-dependent tunneling opens the possibility of performing entangling two-qubit gates by this method. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q27.00012: Sensitivity to electronics error in coupled double quantum dot qubits Erik Nielsen, Richard Muller, Malcolm Carroll Reducing the effects of electronics control error in double quantum dot (DQD) quantum bits (qubit) is a central challenge to the creation of a solid-state quantum computing architecture. We investigate a system of capacitively coupled DQDs which implement a variant of the controlled phase gate when using each DQD as a singlet-triplet qubit. We identify regimes in which the gate action is more robust to sources of noise such as error around the applied bias point due to electronics or charge noise. Energy spectra are found using a configuration interaction (CI) method that accurately captures the (2,0) configuration of the DQD system, which is important for operating in these potentially low-noise regimes. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin company, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q27.00013: Gate Capacitance Reproducibility and Modeling in Silicon Double Quantum Dots Ted Thorbeck, Akira Fujiwara, Neil Zimmerman For many applications the ability to design quantum dots with a specific set of gate capacitances and then rely on the reproducibility of those capacitances is crucial. For quantum computing, the ability to design our gate capacitances would help in reaching the few electron regime and in coupling multiple devices. For other applications the ability to design our gate capacitances would enable higher temperature operation. Our double quantum dots are formed by electrostatic gates on a silicon nanowire. We have measured 20 similar devices with 8 different sets of lithographic parameters. We will report on the reproducibility of the gate capacitances. For example, the range of capacitances is typically within 10{\%} of the average. We will also compare our measured capacitances to simulations based on lithographic parameter. This simulation could then be used to design new devices. [Preview Abstract] |
Session Q28: Focus Session: Carbon Nanotubes and Related Materials: Devices II
Sponsoring Units: DMPChair: David Tomanek, Michigan State University
Room: C156
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q28.00001: Carbon nanotube based NEMS actuators and sensors Michael Forney, Jordan Poler Single-walled carbon nanotubes (SWNTs) have been widely studied due to superior mechanical and electrical properties. We have grown vertically aligned SWNTs (VA-SWNTs) onto microcantilever (MC) arrays, which provides an architecture for novel actuators and sensors. Raman spectroscopy confirms that the CVD-grown nanotubes are SWNTs and SEM confirms aligned growth. As an actuator, this hybrid MC/VA-SWNT system can be electrostatically modulated. SWNTs are excellent electron acceptors, so we can charge up the VA-SWNT array by applying a voltage. The electrostatic repulsion among the charged SWNTs provides a surface stress that induces MC deflection. Simulation results show that a few electrons per SWNT are needed for measureable deflections, and experimental actuators are being characterized by SEM, Raman, and an AFM optical lever system. The applied voltage is sinusoidally modulated, and deflection is measured with a lock-in amplifier. These actuators could be used for nano-manipulation, release of drugs from a capsule, or nano-valves. As a sensor, this MC/VA-SWNT system offers an improved sensitivity for chemical and bio-sensing compared to surface functionalized MC-based sensors. Those sensors only have a 2D sensing surface, but a MC/VA-SWNT system has significantly more sensing surface because the VA-SWNTs extend microns off the MC surface. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q28.00002: High yield assembly and electron transport investigation of semiconducting-rich local-gated single-walled carbon nanotube field effect transistors Kristy Kormondy, Paul Stokes, Saiful Khondaker Single-walled carbon nanotubes (SWNTs) are ideal for use in nanoelectronic devices because of their high current density, mobility and subthreshold slope. Using individual local gates and scaling the gate oxide has shown faster switching behavior and lower power consumption. However, assembly methods must be developed to reproducibly align all-semiconducting SWNTs at specific locations with individually addressable gates for future integrated circuits. We show high yield assembly of local-gated semiconducting SWNTs assembled via AC-dielectrophoresis (DEP). Detailed electron transport investigations on the devices show that 98{\%} display good FET behavior, with an average threshold voltage of 1V and subthreshold swing as low as 120 mV/dec. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q28.00003: Measurement of quantum capacitance in individual semiconducting single-walled Yanfei Yang, Georgy Fedorov, Serhii Shafranjuk, Paola Barbara The capacitance of a carbon nanotube consists of its geometrical capacitance and its quantum capacitance. The latter is determined by the electronic density of states of the nanotube and the electron interactions, therefore it is a tool for probing fundamental electronic properties in carbon nanotubes, as well as an important parameter to design carbon nanotube electronic devices. The quantum capacitance of a carbon nanotube was first measured by using a capacitance bridge at 77K [1]. Here we extract the quantum capacitance of a semiconducting single-walled carbon nanotube in two one-dimensional subbands from electronic transport measurements at 4.2 K. We compare our results to other experiments and predictions from theoretical models.\\[4pt] [1] S. Ilani, L. A. K. Donev, M. Kindermann, and P. L. McEuen, Nature Physics, 2, 687, (2006). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q28.00004: Electronic transport in intermediate sized carbon nanotubes Markus Ahlskog, Davie Mtsuko, Antti Juutilainen We have measured low temperature transport properties of multiwalled carbon nanotubes (MWNT) of different diameters in the range 2-10 nm [1]. In nearly all samples the gate dependent conductance exhibits a gap whose size increases with decreasing tube diameter and increasing electrode separation. This so called transport gap is attributed, based on the experimental findings, on a combination of localization effects and narrow diameter induced gaps in the electronic band structure. \\[4pt] [1] M. Ahlskog, O. Herranen, A. Johansson, J. Lepp\"{a}niemi, and D. Mtsuko, Phys. Rev. B \textbf{79}, 155408 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q28.00005: Resistance of individual long suspended carbon nanotubes with known atomic structures Mitsuhide Takekoshi, Vikram Deshpande, Yuhei Miyauchi, Zhengyi Zhang, Chenguang Lu, Tony Heinz, James Hone, Philip Kim We present electrical transport measurement on long individually suspended carbon nanotubes. Single walled carbon nanotubes (SWNTs) are grown by a chemical vapor deposition method across a slit made on silicon oxide/silicon substrate with pre-patterned platinum electrodes. Rayleigh spectroscopy allows us to determine atomic structure indices of individual SWNTs that connect the electrodes across the slit. We investigate the temperature dependent resistance of metallic SWNTs. The relation between electron-phonon interaction in SWNTs in the connection of the atomic structure will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q28.00006: Electrical Resistance of Double-Wall Carbon Nanotubes with Determined Chiral Indices Letian Lin, Taoran Cui, Lu-Chang Qin, Sean Washburn The properties of carbon nanotubes (CNT), especially single-wall nanotubes (SWNT) and double-wall nanotubes (DWNT), are profoundly sensitive to the atomic structure described by its chirality. CNTs connected to sub-micron electrodes were suspended for transmission electron microscope (TEM) study. We determined the chiral indices of each individual CNT via its nano beam electron diffraction patterns and measured its electrical resistance by the four-probe method at room temperature. We studied the factor of different combinations of semiconducting/metallic shells on the electrical characterizations of DWNTs. The electrical properties were compared between DWNTs and SWNTs and the result show that the electrical transport of a DWNT is dominated by the chiral indices of outer shell. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q28.00007: Carbon nanotubes for interconnects in integrated circuits Invited Speaker: Carbon nanotubes are one of the materials that may be used for advanced interconnects beyond the 16nm node thanks to there extreme resistance to electro migration and to bottom up approach which allow to grow them in tiny holes with very high aspect ratio. The resistance of a via with area A and height h filled with CNT is expressed by $R_{via} =\frac{rq+rsh+rc}{Ad_t }$ where rq, rs, rc are respectively the 6.5k$\Omega $ quantum resistance, the scattering resistance and the contact resistances of one tube. To be competitive with copper via resistance, a large density d$_{t}$ of carbon walls have to be paralleled. Following ITRS needs a density of 2 or 3 10$^{13 }$cm$^{-2}$ conducting CNT walls have to be obtained. This optimum wall density requests the growth of highly packed few nanometre diameter CNTs. Such density has been the main bottleneck for the development of CNT interconnects. Recently ultra high density integration scheme have been demonstrated and for the first time wall density close to the requested one have been integrated in devices. Such density comes from the development on conductive substrates of a CNT growth mode normally used to obtain forests of small tube diameter on insulating substrate like alumina. With this mode, CNTs are grown with base growth mode which is the mode requested for SWCNT or DWCNT thus by continuity it will be possible to increase the density still further by increasing the density of catalyst particles. Our bottom metal of choice is AlCu with iron as catalyst. With this system tube contact resistance between 10$^{4}$ to 10$^{6}$ Ohm have been measured on blanket AlCu substrates. This resistance must be decreased by one or two order of magnitude while increasing further CNT density. In this paper we will present our last integration developments and the role of plasma pre-treatment of the iron aluminium interface in order to decrease the contact resistance. We will show that the bottom profile of via has a major impact on the quality of CNT growing in the holes and discuss future evolutions of this technology. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q28.00008: Wide Contact Structures for Low-noise Nano-channel Devices based on Carbon Nanotube Network Hyungwoo Lee, Minbaek Lee, Seon Namgung, Seunghun Hong We developed a wide-contact structure for low-noise devices based on carbon nanotube (CNT) networks. This wide-contact CNT network-based device has a dumbbell-shaped channel which is comprised of a narrow channel region and wide CNT/electrode contact regions. We showed that the wide-contact structure reduced 1/f noise which originated from CNT/electrode contact regions. We also systematically analyzed the noise characteristics of the structured CNT networks and established an empirical formula that can describe the noise behavior of CNT network-based devices including the effect of contact regions and CNT alignment. Interestingly, our noise analysis revealed that the noise amplitude of aligned CNT networks behaves quite differently compared with that of randomly-oriented CNT networks. These results would be an important guideline in designing low-noise nanoscale devices based on CNT networks for various applications such as a highly sensitive low-noise sensor. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q28.00009: Physics of aligned arrays of single-walled NTs: From transistor to diode applications Slava V. Rotkin, John A. Rogers NTs have been originally proposed as a 1D high mobility semiconductor material for field-effect transistors (FET). This format is though appeared to be less practical due to low values of the currents through a single NT channel. On contrary, NT massive parallel arrays have already found implementation in flexible and RF electronics. Can we think of NT arrays being another semiconductor thin film materials? Where does the conventional knowledge apply for NT parallel array devices? This talk discusses specialized aspects of physics of electronic and optoelectronic device prototypes and presents recent results for NT FETs and LEDs (light-emitting diode) in parallel array geometries. Cross-talk between individual NTs in the array allows to beat the statistical ``noise'' in the device properties which appears due to randomized NT distribution in the array. Although, taking this into account, device-level characteristics should be used with a care to extract a single NT physical parameters. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q28.00010: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q28.00011: Macroelectronic Integrated Circuits Using High-Performance Separated Carbon Nanotube Thin-Film Transistors Chuan Wang, Jialu Zhang, Chongwu Zhou Macroelectronic integrated circuits are widely used in applications such as flat panel display, transparent electronics, as well as flexible and stretchable electronics. However, the challenge is to find the channel material that can simultaneously offer low temperature processing, high mobility, transparency and flexibility. Here in this paper, we report the application of high-performance separated nanotube thin-film transistors (TFTs) for macroelectronic integrated circuits. We have systematically investigated the performance of TFTs using separated nanotubes with 95{\%} and 98{\%} semiconducting nanotubes, and high mobility transistors have been achieved. In addition, we observed that while 95{\%} semiconducting nanotubes are ideal for applications requiring high mobility (up to 67 cm$^{2}$V$^{-1}$s$^{-1})$ such as analog and radio-frequency applications, 98{\%} semiconducting nanotubes are ideal for applications requiring high on/off ratios ($>$10$^{4}$ with channel length down to 4 $\mu $m). Furthermore, integrated logic gates such as inverter, NAND and NOR have been designed and demonstrated using 98{\%} semiconducting nanotube devices, and symmetric input/output behaviour is achieved, which is crucial for the cascading of multiple stages of logic blocks and larger scale integration. Our approach can serve as the critical foundation for future nanotube-based thin-film macroelectronics. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q28.00012: Anisotropic electronic transport in highly aligned carbon nanotube films Sebastien Nanot, Xuan Wang, Junichiro Kono, Yanhua Dai, Rui-Rui Du, Cary Pint, Robert H. Hauge Electronic transport in carbon nanotube (CNT) networks has recently attracted much renewed interest due to the numerous advancements in controlling, sorting, and aligning CNTs. Understanding the roles of intra-tube and inter-tube transport in these systems is fundamentally important both from basic and applied points of view. We have studied samples of ultra-long and highly-aligned CNTs grown by CVD and laid down on Si/SiO2 substrates. We designed and fabricated a novel device structure in which we can separately study intra-tube and inter-tube transport. In the intra-tube configuration, ends of ultra-long CNTs were contacted and the current parallel to the alignment direction was measured, whereas, in the inter-tube configuration, transport perpendicular to the alignment direction was probed. We studied the magnetic field and temperature dependence of the resistance between 0.3 K and 300 K, revealing an interesting evolution of transport regimes as for the localization of charge carriers. Preliminary results of photoconductivity measurements will also be presented. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q28.00013: Air-Stable Conversion of Separated Carbon Nanotube Thin-Film Transistors from P-type to N-type Using Atomic Layer Deposition of High-$\kappa $ Oxide and Its Application in CMOS Logic Circuits Jialu Zhang, Chuan Wang, Yue Fu, Yuchi Che, Chongwu Zhou Pre-separated, high purity semiconducting carbon nanotubes hold great potential for thin-film transistors (TFTs) and integrated circuit applications. One of the main challenges it still faces is the fabrication of air-stable N-type nanotube TFTs with industry compatible techniques. Here in this paper, we report a novel and highly reliable method of converting the P-type TFTs using pre-separated semiconducting nanotubes into air-stable N-type transistors by adding a high-$\kappa $ oxide passivation layer using atomic layer deposition (ALD). The N-type devices exhibit symmetric electrical performance compared with the P-type devices in terms of on-current, on/off ratio and mobility. Various factors affecting the conversion process including ALD temperature, metal contact material, channel length, have also been systematically studied. A complementary metal-oxide-semiconductor (CMOS) inverter with rail-to-rail output, symmetric input/output behavior and large noise margin has been further demonstrated. The excellent performance gives us the feasibility of cascading multiple stages of logic blocks and larger scale integration. Our approach can serve as the critical foundation for future nanotube-based thin-film macroelectronics. [Preview Abstract] |
Session Q29: Focus Session: Quantum Information for Quantum Foundations - Experiments and Tests
Sponsoring Units: GQIChair: Christoph Schaeff, University of Vienna
Room: C148
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q29.00001: Foundational aspects of energy-time entanglement Jan-{\AA}ke Larsson This presentation will discuss whether energy-time entanglement is a properly Quantum Information representation, by considering its relation to Einstein-Podolsky-Rosen (EPR) elements of reality. The interferometric experiment proposed by J.\ D. Franson in 1989 provides the background, and the main issue here is whether a Local Realist model can give the Quantum-Mechanical predictions for this setup. The Franson interferometer gives the same interference pattern as the usual Bell experiment (modulo postselection). Even so, depending on the precise requirements made on the Local Realist model, this can imply a) no violation, b) smaller violation than usual, or c) full violation of the appropriate statistical bound. The discussion will include the nature of the requirements, the motivation for making them, and their effect. The alternatives include using a) only the measurement outcomes as EPR elements of reality, b) the emission time as EPR element of reality, and c) path realism. These subtle requirements need to be taken into account when designing and setting up future experiments of this kind, intended to test Local Realism, or indeed to do Quantum Information Processing. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q29.00002: Large violation of Bell's inequalities using both counting and homodyne measurements Valerio Scarani, Daniel Cavalcanti, Nicolas Brunner, Paul Skrzypczyk, Alejo Salles So far, all the optical demonstrations of violations of Bell's inequalities have involved discrete degrees of freedom (e.g. polarization, time-bins) and are plagued by the detection-efficiency loophole. Continuous degrees of freedom would be a very interesting alternative because of the efficiency of the homodyne measurement; but the feasible schemes proposed so far reach very weak violations. We show that large violations for easily-prepared states can be achieved if both photon counting and homodyne detections are used. Our simple scheme may lead to the first violation of Bell's inequalities with continuous variables and pave the way for a loophole-free Bell test. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q29.00003: A non-local quantum eraser X. Ma, J. Kofler, A. Qarry, N. Tetik, T. Scheidl, R. Ursin, S. Ramelow, L. Ratschbacher, T. Herbst, A. Fedrizzi, T. Jennewein, A. Zeilinger The complementarity behavior of quantum systems is strikingly illustrated by the quantum eraser, where one can actively choose whether or not to erase which-path information of one particle by performing suitable measurements on another particle entangled with it [1-2]. Quantum mechanics predicts that this choice can be arbitrarily delayed and spatially separated from interference [1-3]. We report the first quantum eraser experiment performed under Einstein locality, i.e. under relativistic space-like separation. We employ the hybrid entanglement between path and polarization of photon pairs and distribute the photons over an optical fibre link of 55 m and, in another experiment, over a free-space link of 144 km. A complementarity inequality is measured and well fits the predictions of quantum mechanics. Our experiment represents a conclusive demonstration of the quantum eraser concept. \\[0pt] [1] M. O. Scully, K. Dr\"{u}hl, Phys. Rev. A 25, 2208 (1982). [2] J. A. Wheeler, in Quantum Theory and Measurement (1984). [3] V. Jacques, \textit{et al.}, Science 315, 966 (2007). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q29.00004: On the Experimental Violation of Mermin's High-Spin Bell Inequalities in the Schwinger Representation Ruffin Evans, Olivier Pfister Since Bell's original paper in 1964, a wide variety of experimental tests have overwhelmingly supported the completeness of quantum mechanics over local hidden-variable theories. However, relatively little effort has focused on systems of spins larger than $\frac{1}{2}$; generalizing Bell's result to higher dimensions is difficult, and the experiments needed to test these high-spin Bell inequalities are exacting. New advances in high efficiency photon-number-resolving detectors suggest that experimental tests of these inequalities should be possible in the Schwinger representation, using the continuous-variable entangled (two-mode squeezed) fields produced by an optical parametric oscillator below threshold. In this paper, we explore the realistic experimental implementation of this proposal to violate Mermin's high-spin inequalities. We demonstrate that violation for spin values greater than 1 should be attainable under a range of feasible experimental conditions that include finite squeezing and nonideal detection efficiency. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q29.00005: Surface based detection schemes for molecular interferometry experiments - implications and possible applications Thomas Juffmann, Adriana Milic, Michael Muellneritsch, Markus Arndt Surface based detection schemes for molecular interferometry experiments [1] might be crucial in the search for the quantum properties of larger and larger objects [2] since they provide single particle sensitivity. Here we report on molecular interferograms of different biomolecules imaged using fluorescence microscopy. Being able to watch the build-up of an interferogram live and in situ reveals the matter-wave behavior of these complex molecules in an unprecedented way. We examine several problems encountered due to van-der-Waals forces between the molecules and the diffraction grating and discuss possible ways to circumvent these. Especially the advent of ultra-thin (1-100 atomic layers) diffraction masks might path the way towards molecular holography. We also discuss other possible applications such as coherent molecular microscopy.\\[4pt] [1] T. Juffmann, S. Truppe, P. Geyer, A.G. Major, S. Deachapunya, H. Ulbricht, M. Arndt, Phys. Rev. Lett. 103, 263601 (2009).\\[0pt] [2] T. Juffmann, S. Nimmrichter, M. Arndt, H. Gleiter, K.Hornberger, in print, Foundations of Physics. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q29.00006: Matter wave interferometry with large and complex molecules Stefan Gerlich, Sandra Eibenberger, Mathias Tomandl, Jens T\"{u}xen, Marcel Mayor, Markus Arndt Matter wave interferometry with molecules of increasing size, mass and complexity explores the frontiers of quantum mechanics and it is a promising tool for determining molecular properties with high precision. The quantum wave nature of organic molecules is used in a Kapitza-Dirac-Talbot-Lau interferometer to generate a set of high-contrast interference fringes that are highly sensitive to external forces. This is exploited to access thermally averaged internal molecular properties, such as optical and static polarizabilities, static and thermally activated electric dipole moments, information about conformational differences and state changes, optical absorption spectra and more. The information about the internal states can be extracted through conservative interactions, i.e. allowing the persistence of full quantum delocalization in position space. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q29.00007: Violation of local realism with freedom of choice Johannes Kofler, Thomas Scheidl, Rupert Ursin, Sven Ramelow, Xiao-Song Ma, Thomas Herbst, Lothar Ratschbacher, Alessandro Fedrizzi, Nathan Langford, Thomas Jennewein, Anton Zeilinger Bell's theorem shows that local realistic theories place strong restrictions on observable correlations between different systems, giving rise to Bell's inequality which can be violated in experiments using entangled quantum states. Bell's theorem is based on the assumptions of realism, locality, and the freedom to choose between measurement settings. In experimental tests, ``loopholes'' arise which allow observed violations to still be explained by local realistic theories. Violating Bell's inequality while simultaneously closing all such loopholes is one of the most significant still open challenges in fundamental physics today. We present an experiment that violates Bell's inequality while simultaneously closing the locality loophole and addressing the freedom-of-choice loophole, also closing the latter within a reasonable set of assumptions. Reference: T. Scheidl et al., Proc. Natl. Acad. Sci. USA 107, 19708 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q29.00008: Experimental non-classicality of an indivisible system Radek Lapkiewicz, Peizhe Li, Christoph Schaeff, Nathan Langford, Sven Ramelow, Marcin Wiesniak, Anton Zeilinger In Quantum Mechanics (QM) not all properties can be simultaneously well defined. An important question is whether a joint probability distribution can describe the outcomes of all possible measurements, allowing a quantum system to be mimicked by classical means. Klyachko et al. [PRL 101, 020403 (2008)] derived an inequality which allowed us to answer this question experimentally. The inequality involves only five measurements and QM predicts its violation for single spin-1 particles. This is the simplest system where such a contradiction is possible. It is also indivisible and as such cannot contain entanglement. In our experiment with single photons distributed among three modes (isomorphic to stationary spin-1 particles) we obtained a value of -3.893(9), which lies more than 90 standard deviations below the ``classical'' bound of -3. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q29.00009: Testing spontaneous localization with ultra-massive cluster interferometry Stefan Nimmrichter, Klaus Hornberger, Markus Arndt Understanding the transition from the microscopic domain of quantum mechanics to our everyday classical world is still an open problem in modern physics. Collapse models are a possible way to resolve this issue by introducing mechanisms which break the quantum superposition principle above a certain mass and time scale. One of the best studied models is the theory of continuous spontaneous localization (CSL) by Ghirardi, Pearle and Rimini [1]. We show that it should be possible to test the predictions of the CSL model in the new matter-wave interferometer for heavy metal clusters that is currently built in Vienna. Extending the original Talbot-Lau setup for biomolecules, the new scheme will operate in the time-domain using three pulsed standing-wave gratings of UV laser light. We argue that this should enable us to see single-particle interference in an unprecedented mass range from $10^5$ up to even $10^8$ atomic mass units. Recent estimates of the strength of the CSL effect by Adler and Bassi [2,3] suggest that a breakdown of the quantum superposition principle would occur in precisely this mass regime.\\[4pt] [1] Phys. Rev. A 42, 78 (1990)\\[0pt] [2] J. Phys. A 40, 2935 (2007)\\[0pt] [3] arxiv eprint 1011.3767v1 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q29.00010: Hardy's paradox and a violation of a state-independent Bell inequality in time Alessandro Fedrizzi, Marcelo P. Almeida, Matthew A. Broome, Andrew G. White, Marco Barbieri Tests such as Bell's inequality and Hardy's paradox highlight the differences between local realistic theories and quantum predictions for measurement probabilities and correlations between distant particles. Transposing these tests to the temporal domain, i.e. making two measurements on the one quantum particle at different times, yield Hardy and Bell tests mathematically identical to their spatial counterparts, but give very different physical results. Here, we use a photonic entangling gate to implement non-destructive temporal measurements on a quantum system. We measure a much stronger form of Hardy's paradox and demonstrate violation of a Bell inequality in time independent of the quantum state, including for fully-mixed states. Our work yields interesting fundamental insights and opens up a path to more efficient quantum information processing protocols based on temporal quantum correlations. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q29.00011: Experimental Violation of Two-Party Leggett-Garg Inequalities with Semi-weak Measurements Justin Dressel, Curtis Broadbent, John Howell, Andrew Jordan We generalize the derivation of Leggett-Garg inequalities to systematically treat a larger class of experimental situations by allowing multi-particle correlations, invasive detection, and ambiguous detector results. Furthermore, we show how many such inequalities may be tested simultaneously with a single setup. As a proof of principle, we violate several such two-particle inequalities with data obtained from a polarization-entangled biphoton state and a semi-weak polarization measurement based on Fresnel reflection. We also point out a non-trivial connection between specific two-party Leggett-Garg inequality violations and convex sums of strange weak values. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q29.00012: Causality, Bell's theorem, and Ontic Definiteness Joe Henson Bell's theorem shows that the reasonable relativistic causal principle known as ``local causality'' is not compatible with the predictions of quantum mechanics. It is not possible maintain a satisfying causal principle of this type while dropping any of the better-known assumptions of Bell's theorem. However, another assumption of Bell's theorem is the use of classical logic. One part of this assumption is the principle of \textit{ontic definiteness}, that is, that it must in principle be possible to assign definite truth values to all propositions treated in the theory. Once the logical setting is clarified somewhat, it can be seen that rejecting this principle does not in any way undermine the type of causal principle used by Bell. Without ontic definiteness, the deterministic causal condition known as Einstein Locality succeeds in banning superluminal influence (including signalling) whilst allowing correlations that violate Bell's inequalities. Objections to altering logic, and the consequences for operational and realistic viewpoints, are also addressed. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q29.00013: On dipole anisotropy in spatial distribution of Plank's constant values Simon Berkovich The work relates to the remarkable fact discovered by John Webb et al. of angular variations of the fine structure constant $\alpha $ = e$^{2}$/hc. We elaborate on this fact using our model of quantum mechanics (see [1] and references within). The peculiarity of quantum behavior stems from interactive holography appearing on top of the cellular automaton mechanism of the Universe. Nonlocality comes naturally from sliced holographic processing. As to the anisotropy of $\alpha $, its is due to variations of h caused by different undulation control patterns in different positions with respect to the source of the holographic reference beam. The angular divergences in $\alpha $ are determined by the eccentric placement of the Solar system with respect to this reference holographic beam. This eccentricity factor imposes dipole structuring on several types of astrophysical observations. So, following [1], small opposite changes in h with respect to the eccentricity displacement of the Solar system could be anticipated. Before we have shown that the same eccentricity factor leads to the appearance of the ``axis-of-evil'' in CMB. Further, the recently discovered anisotropy in high-energy cosmic rays should be also determined by the eccentricity factor, i.e. it should adhere to the same dipole. [1] S. Berkovich, ''A Comprehensive Explanation of Quantum Mechanics'', http://www.bestthinking.com/topics/science/physics/ quantum{\_}physics/a-comprehensive-explanation-of-quantum-mechanics [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q29.00014: Scaling of quantum Zeno dynamics in thermodynamic systems Wing Chi Yu, Li-Gang Wang, Shi-Jian Gu Quantum Zeno effect (QZE) refers to the inhabitation of the unitary time evolution of a quantum system by repeated frequent measurements. It has been studied intensively within the content of quantum optics in recent decades. Among those analyses, the systems under consideration are only of a few levels. Little attention of QZE in thermodynamic systems has been paid so far. In this presentation, we will investigate the QZE in thermodynamic systems from the viewpoint of condensed matter physics. We take the one-dimensional transverse-field Ising model and the Lipkin-Meshkov-Glick (LMG) model as examples to illustrate analytically the criteria, in terms of the size dependence of the leading term of the survival probability in the short-time limit, for observing the QZE. Our analysis shows that in order to observe the QZE in the Ising model, the frequency of the projective measurement should be of comparable order to that of the system sizes. The same criterion also holds in the symmetry broken phase of the LMG model. However, in the polarized phase of the LMG model, the leading term of the survival probability is independent of the system size and the QZE can be easily observed. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q29.00015: Decoherence Free Neutron Interferometry Dmitry A. Pushin, David G. Cory, Michael G. Huber, Mohamed Abutaleb, Muhammad Arif, Charles W. Clark A neutron interferometer (NI) is a unique example of the macroscopic quantum coherence and has been used to test fundamental principles of quantum mechanics. In practice, neutron interferometers are not widely used because of their extreme sensitivity to environmental noise which is in part due to the slow velocity (relative to light) of the neutron. We show that a neutron interferometer design can benefit from concepts of quantum information processing. We have machined a Decoherence Free (DF) neutron interferometer designed using a quantum error correction code,\footnote{D. A. Pushin, M. Arif, and D. G. Cory, Phys. Rev. A (http://pra.aps.org/abstract/PRA/v79/i5/e053635) 79, 053635 (2009)} and have shown it to be much less sensitive to mechanical vibrations than is the standard Mach-Zehnder (MZ) interferometer. Both the MZ and DF geometries are incorporated in one crystal, which allows direct comparisons to be made. We believe that our results and related quantum information approaches, such as ``the power of one qubit,''\footnote{E. Knill and R. Laflamme, Phys. Rev. Lett. (http://prl.aps.org/abstract/PRL/v81/i25/p5672\_1) 81, 5672 (1998)} will enable a new series of compact neutron interferometers that can be tailored to specific applications in soft condensed matter and spintronics. [Preview Abstract] |
Session Q30: Graphene: Electron-Electron Interactions
Sponsoring Units: DCMPChair: Bruno Uchoa, University of Illinois at Urbana-Champaign
Room: C147/154
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q30.00001: The effective fine-structure constant of freestanding graphene measured in graphite Yu Gan, James Reed, Bruno Uchoa, Young-Il Joe, Diego Casa, Eduardo Fradkin, Peter Abbamonte Electrons in graphene behave like Dirac fermions, permitting phenomena from high- energy physics to be studied in a solid-state setting. A key question is whether or not these fermions are critically influenced by Coulomb correlations. We performed inelastic x-ray scattering experiments on crystals of graphite and applied reconstruction algorithms to image the dynamical screening of charge in a freestanding graphene sheet. We found that the polarizability of the Dirac fermions is amplified by excitonic effects, improving screening of interactions between quasiparticles. The strength of interactions is characterized by a scale-dependent, effective fine-structure constant, $\alpha^{\ast}_{g}(\mathbf{k},\omega)$, the value of which approaches $1/7$ at low energy and large distances. This value is substantially smaller than the nominal $\alpha_g=2.2$, suggesting that, on the whole, graphene is more weakly interacting than previously believed. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q30.00002: Excitonic Gap from Long-Range Coulomb Interaction in Graphene Jose Gonzalez We apply renormalization group methods to analyze the development of an excitonic gap in the theory of Dirac fermions in graphene with long-range Coulomb interaction. In the large-N approximation, we show that the chiral symmetry is only broken below a critical number of two-component Dirac fermions $N_c = 32/\pi^2$, that is precisely half the value found in quantum electrodynamics. Adopting otherwise a ladder approximation, we give evidence of the existence of a critical coupling at which the order parameter of the transition to the gapped phase diverges. This result supports that the opening of an excitonic gap may be driven by a sufficiently strong Coulomb interaction, despite the divergence of the Fermi velocity at low energies in the Dirac theory of graphene. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q30.00003: Manifestation of explicit and spontaneous chiral symmetry breaking in graphene Sung-Hoon Lee, Hyun-Jong Chung, Jinseong Heo, Heejun Yang, Sunae Seo Using first-principles calculations of graphene having high- symmetry distortion or defects, we investigate the chiral symmetry breaking in graphene as the source of gap opening. We identify that the gap opening by the chiral symmetry breaking in the honeycomb lattice is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in a linear lattice, the elemental 1D Dirac lattice, and find that the chiral symmetry breaking manifests itself in graphene by the formation of an internal structure of the lattice, which represents the intrinsic internal structure of massive Dirac fermions. We then show that the gap opening of many of previously reported structures of gapped graphene occurs by explicit breaking of the chiral symmetry, rather than by quantum confinement effects or others, and also show that spontaneous chiral symmetry breaking takes place via electron- phonon coupling at certain quasi-1D graphene structures and at 2D graphene under strain. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q30.00004: Theory of Kekule superconductor on graphene's honeycomb lattice Bitan Roy, Igor Herbut A spatially non-uniform superconducting state is proposed as a variational ground state on honeycomb lattice, with the chemical potential close to and right at the Dirac point, when the nearest-neighbor attraction is the dominant component of the interaction. This state spontaneously breaks the translational invariance of the underlying lattice into the Kekule pattern of superconducting bond order parameters. Otherwise it is fully gapped, spin triplet, and odd under the exchange of two sublattices. Symmetries of the ground state for a range of nearest-neighbor interaction, the topological excitations of the Kekule superconductor, and its competition with other superconducting orders proposed in literature will also be discussed. \\[4pt] B. Roy and I. F. Herbut, Phys. Rev. B 82, 035429 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q30.00005: Pairing in graphene: A Monte Carlo study Tianxing Ma, Zhongbing Huang, Feiming Hu, Hai-Qing Lin To address the issue of possibility of inducing superconductivity in graphene, we study the behavior of pairing correlation in the extended repulsive Hubbard model on a honeycomb lattice within both determinant quantum Monte Carlo and constrained path Monte Carlo method. We find that the system shows an antiferromagnetic correlation below Van Hove fillings. In the filling range of $<$$n$$>$=1.00 $\sim$ 1.20, pairing with $d+id$ symmetry is dominant over pairing with extent $s$ symmetry, especially at low temperatures. The $d+id$-wave pairing susceptibility is enhanced as the electron filling increases, while the effective pairing interaction is suppressed. The summation of pairing correlation for long-range part is enhanced as the repulsion increases, however, for various lattice sizes and interactions, we find that the long-range part of $d+id$-wave pairing correlations both vanishes. Our results suggest that there maybe no superconductivity in pure and low doped graphene. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q30.00006: Interacting fermions on the honeycomb bilayer: From weak to strong coupling Oskar Vafek Many-body instabilities of the half-filled honeycomb bilayer are studied using weak-coupling renormalization group (RG) as well as strong-coupling expansion [1,2]. For spinless fermions, there are 4 independent four-fermion contact couplings. Generally, we find runaway RG flows which we associate with ordering tendencies. The broken symmetry state is typically a gapped insulator with either broken inversion or broken time-reversal symmetry, with a quantized anomalous Hall effect. Additionally, a tight-binding model with nearest-neighbor hopping and nearest-neighbor repulsion is studied in weak and strong couplings and in each regime a gapped phase with inversion symmetry breaking is found. In the strong-coupling limit, the ground-state wave function is constructed for vanishing in-plane hopping but finite interplane hopping, which explicitly displays the broken inversion symmetry and a finite difference between the number of particles on the two layers. In the spin-1/2 case we use Fierz identities to show that there are 9 independent four-fermion contact couplings[2]. The 9 RG equations in this case reduce to the 3 found in Ref.[1] under the assumptions stated in Ref.[1]. They are further used to show that, just as in strong coupling, the most dominant weak-coupling instability of the repulsive Hubbard model (at half filling) is an antiferromagnet. [1] O. Vafek and K. Yang, PRB {\bf 81}, 041401 (2010). [2] O. Vafek, PRB {\bf 82}, 205106 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q30.00007: Compressibility Instability of Interacting Electrons in Bilayer Graphene Xin-Zhong Yan, C.S. Ting Using the self-consistent Hartree-Fock approximation, we study the compressibility instability of the interacting electrons in bilayer grapheme at finite temperature. The chemical potential and the compressibility of the electrons can be significantly altered by an energy gap (tunable by external gate voltages) between the valence and conduction bands. For zero gap case, we show that the homogeneous system is stable. When the gap is finite, the compressibility of the electron system becomes negative at low carrier doping concentrations and low temperature. We also present the phase diagram distinguishing the stable and unstable regions of a typically gapped system in terms of temperature and doping. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q30.00008: Limits to universal conductance fluctuations of massless Dirac fermions Mario Borunda, Jesse Berezovsky, Robert Westervelt, Eric Heller We study conductance fluctuations (CFs) and the sensitivity of the conductance to the motion of a single scatterer in two-dimensional massless Dirac systems. Our extensive numerical study finds limits to the predicted universal value of CFs. We find that CFs are suppressed for ballistic systems near the Dirac point and approach the universal value at sufficiently strong disorder. The conductance of massless Dirac fermions is sensitive to the motion of a single scatterer. CFs of order $e^2/h$ result from the motion of a single impurity by a distance comparable to the Fermi wavelength. This result applies to graphene systems with a broad range of impurity strength and concentration while the dependence on the Fermi wavelength can be explored via gate voltages. Our prediction can be tested by comparing graphene samples with varying amounts of disorder and can be used to understand interference effects in graphene mesoscopic devices. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q30.00009: Interplay between curvature and in-plane magnetic field in bilayer graphene Avadh Saxena, Yogesh Joglekar For a two-dimensional electron gas (2DEG) in a uniform magnetic field, the effect of the in-plane component on the orbital motion of carriers is ignored because ``it can be gauged away.'' However, the effect of such a field on a massive quantum particle confined to a curved surface has been only recently explored [1]. We obtain the single-particle spectra for such a particle on a sphere, a cylinder, and a torus in the presence of a constant magnetic field. In addition to the geometric potential $V_G$ that arises due to the confinement on a curved surface, we find that in-plane field leads to energy shifts $\Delta E\propto V_G(R/l_B)^4$ where $R$ is the radius of curvature of the surface, and $l_B$ is the magnetic length for the in-plane field. With bilayer graphene as a model for massive quantum particle on a curved surface, we estimate the energy shift for a cylindrical geometry, and show that it is significant for typical experimental parameters. \\[4pt] [1] G. Ferrari and G. Cuoghi, Phys. Rev. Lett. {\bf 100}, 230403 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q30.00010: Plasma Instability in Graphene Bilayers Antonios Balassis, Godfrey Gumbs The problem of plasma instability in a pair of coupled semiconductor layers when a dc current is passed through one of the layers has been vigorously investigated over the years. This may be carried out by solving for the real and imaginary parts of the frequency in the polarization function making the dielectric function vanish. We analyze the conditions for plasma instability in a graphene bilayer for various chemical potentials (doping) as well as layer separation. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q30.00011: Tight-binding theory of the spin-orbit coupling in graphene structures Sergej Konschuh, Martin Gmitra, Jaroslav Fabian Spin-orbit coupling changes qualitatively the electronic band structure of graphene. Most important, the coupling induces spectral gaps at the K(K') points. Earlier theories estimated the \emph{intrinsic} gap of $1$ $\mu$eV for the single layer and several meVs for bi- and tri-layer graphene, based on $\sigma$-$\pi$ coupling. Our first-principles calculations give the value of 24 $\mu$eV for all these systems, due to the presence of the orbitals of the $d$ symmetry in the Bloch states of the $\pi$ bands. A realistic multiband tight-binding model is presented to explain the effects the $d$ orbitals play in the spin-orbit coupling of graphene and derive an effective single-orbital next-nearest-neighbor hopping model that accounts for the spin-orbit effects. We also study the \emph{extrinsic} spin-orbit coupling, due to an applied transverse electric field. In a single layer the \emph{extrinsic} effect is dominated by the $\pi$-$\sigma$ hybridization. In contrast, in the multi-layer structures the \emph{extrinsic} spin-orbit band splittings come from an interplay of the $d$-orbitals, the inter-layer hopping, and the electrostatic potential from the applied field. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q30.00012: Spin-orbit coupling in bi-layer and tri-layer graphene in transverse electric field: first-principles calculations Martin Gmitra, Sergej Konschuh, Jaroslav Fabian Few-layer graphene structures may be potentially useful for optical and transport applications, due to the possibility of electrical control of the band gaps. Here we investigate the spin-orbit coupling of bilayer and tri-layer graphene around the Fermi level. We show, by performing first-principles full potential linearized augmented plane waves calculations that the spin-orbit physics in these structures derives essentially from monolayer graphene. In particular, the spin splitting of the bands is due to the spin-orbit coupling of the d-orbitals. These give a splitting of the order of 24$\,{\rm \mu eV}$ at the K point, as in graphene. Breaking the spatial inversion symmetry by a transverse electric field does not change this (intrinsic) picture, unlike what we know from graphene. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q30.00013: Dynamical Jahn-Teller Effect at a Vacancy Center in Graphene Sashi Satpathy, Mohammad Sherafati, Birabar Nanda, Zoran Popovic We study the substitutional vacancy center in graphene from density-functional LAPW calculations and show that it is magnetic and at the same time forms a dynamical Jahn-Teller center. A net magnetic moment of $2 \mu_B$ is found, which is explained in terms of the occupation of the $sp^2\sigma$ dangling bond state and the zero-mode state derived from the $\pi$ bands. The adiabatic potential surface resulting from the $ E \otimes e$ vibronic coupling was computed and subsequently the Schr\"odinger equation was solved for the nuclear motion of the carbon atoms. Our calculations show the tunneling splitting $3 \Gamma$ to be about 80 cm$^{-1}$, which is substantially larger than the typical strain fields, leading to a dynamical Jahn-Teller effect (JTE). This explains the puzzling behavior of why in the STM measurements a symmetric carbon triangle is observed around the vacancy, while at the same time we predict the splitting of the vacancy-induced electron states by the static JTE {\it in spite of} the triangular symmetry. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q30.00014: Plasma Excitations in Graphene: Their Spectral Intensity and Temperature Dependence in Magnetic Field Jhao-Ying Wu, Szu-Chao Chen, Godfrey Gumbs, Ming-Fa Lin We calculated the dielectric function, the loss function, the magnetoplasmon dispersion relation and the temperature-induced transitions for graphene in a uniform perpendicular magnetic field. The calculations were performed using the Peierls tight-binding model to obtain the energy band structure and the random-phase approximation to determine the collective plasma excitation spectrum. The single-particle and collective excitations have been precisely identified based on the resonant peaks in the loss function. The critical wave vector at which plasmon damping takes place is clearly established. This critical wave vector depends on the magnetic field strength as well as the levels between which the transition takes place. The temperature effects were also investigated. At finite temperature, there are plasma resonances induced by the Fermi distribution function. Whether such plasmons exist is mainly determined by the field strength, temperature, and momentum. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q30.00015: Field Modulation on the Electronic Structure for the Bilayer and Trilayer Graphene Bi-Ru Wu The electronic band gap plays a central role in modern device physics and a tunable band gap provides great flexibility in device design. I present the investigation of electric filed effect on the electronic structure of the bilayer and trilayer graphene. The hexagonal and Bernal type structures are studied for the bilayer and trilayer graphene, additionally, the rhombohedral type is also take into account for the trilayer one. It is found the band gaps of the Bernal type bilayer graphene and the Rhombohedral type trilayer graphene are tunable by a perpendicular electric field. The symmetry of the graphene plays a crucial role in the field modulation. The perpendicular electric field opens the band gap of the Bernal type bilayer graphene and the Rhombohedral type trilayer graphene by breaking the symmetry in z-direction. [Preview Abstract] |
Session Q31: Focus Session: Materials at High Pressure V: Structure Prediction and Complex Materials
Sponsoring Units: DMP GSCCM DCOMPChair: Salah Boulfelfel, SUNY Stony Brook
Room: C145
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q31.00001: Crystal structure prediction using evolutionary algorithms: how to predict large and complex systems Invited Speaker: Evolutionary crystal structure prediction proved to be a powerful approach in discovering new materials. Algorithm USPEX allows one to predict the most stable crystal structure for a given compound without requiring any experimental input. However, certain limitations are encountered for systems with a large number of degrees of freedom and complex energy landscapes. We explore the nature of these limitations and address them with a number of newly developed tools. For large systems a major problem is the lack of diversity. It is countered with modified variation operators that favor atoms with higher local order and a special initialization procedure for the first generation. For complex energy landscapes, the key problem is the possible existence of several energy funnels. To address this problem, we develop an algorithm incorporating the ideas of abstract ``distance'' between structures using the so called ``fingerprint function.'' We will compare the efficiency of the old and new algorithm USPEX for different systems and show that the range of application for algorithm is increased. Some systems, where old algorithm couldn't find a solution are now solvable with the new algorithm. And the speed of finding the solutions for systems with the complicated energy landscape is substantially increased. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q31.00002: Prediction of complex high-pressure M-B crystal structures with an evolutionary algorithm Aleksey Kolmogorov, Sheena Shah, Roxana Margine We have carried out an ab initio ground state search in two binary metal-boron systems using an evolutionary algorithm [1] and identified remarkably complex configurations stabilized at high pressures [2,3]. An alkali-earth metal boride is shown to undergo a structural transformation from a semiconducting to a metallic state while a new semiconducting transition metal boride is stabilized at a composition known to have only metallic ground states. For the proposed candidate materials we calculate the electron-phonon coupling and demonstrate their potential to be phonon-mediated superconductors. \\[4pt] [1] A.N. Kolmogorov, http://maise-guide.org (MAISE) \\[0pt] [2] A. N. Kolmogorov, S. Shah, E. R. Margine, A. F. Bialon, T. Hammerschmidt, R. Drautz, Phys. Rev. Lett. 105, 217003 (2010). \\[0pt] [3] A. F. Bialon, T. Hammerschmidt, R. Drautz, S. Shah, E. R. Margine, A. N. Kolmogorov (submitted) [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q31.00003: Structure prediction for molecular crystals using evolutionary algorithms: methodology and applications Qiang Zhu Evolutionary crystal structure prediction proved to be a powerful approach in determining the atomic crystal structure of materials. Here, we present a specifically designed algorithm for the prediction of the structure of molecular crystals. The main feature of this new approach is that each molecule is treated as a whole body, which drastically reduces the search space and improves the efficiency, but necessitates the introduction of new variation operators described here. We illustrate the efficiency of this approach by a search for ice (H2O) structures at zero pressure and temperature, which easily finds the structures of ice Ih and Ic, as well as the thermodynamically stable at these conditions ice XI. We successfully apply this method to finding the hitherto unknown structures of plastic phases of methane at high pressure. These structures are distinguished by an icosahedral packing of the molecules, and are likely candidate solutions for methane A and B. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q31.00004: Burnett-Cattaneo Continuum Theory for Shock Waves B.L. Holian, M. Mareschal, R. Ravelo We model strong shockwave propagation, both in the ideal gas and in the dense Lennard-Jones fluid, using a refinement of earlier work \footnote{B.L. Holian, M. Mareschal, and R. Ravelo, J. Chem. Phys. {\bf 133}, 114502 (2010)}, which accounts for the cold compression by a nonlinear, Burnett-like, strain-rate dependence of the thermal conductivity, and relaxation of temperature components on the hot, compressed side of the shock front. The relaxation of the disequilibrium among the three components of the kinetic temperature, namely, the difference between the temperature in the direction of a planar shock wave and those in the transverse directions, particularly in the region near the shock front, is accomplished by a rigorous application of the Cattaneo-Maxwell relaxation equation to a reference state, namely, the steady shockwave solution of linear Navier-Stokes-Fourier theory, along with the nonlinear Burnett heat-flux term. Our new continuum theory is in nearly quantitative agreement with non-equilibrium molecular-dynamics simulations under strong shockwave conditions. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q31.00005: High-pressure behavior of a novel, nitrogen-rich energetic material R. Stewart McWilliams, Jennifer Ciezak-Jenkins, Yasmin Kadry, Vitaly Prakapenka, Mohammad Mahmood, Alexander Goncharov Energetic materials are of great interest in energy and defense applications. In the search for new energetic materials with improved properties, such as reduced environmental impact, a crystalline solid Triaminoguanidinium 1-methyl-5-nitriminotetrazolate (TAG), C$_{3}$H$_{12}$N$_{12}$O$_{2}$, has recently been synthesized (Klap\"{o}tke et. al. 2008). We have studied the properties of TAG under static compression, and under reaction initiation at high pressure, using Raman and IR spectroscopy and x-ray diffraction. TAG appears to remain a stable, crystalline solid up to at least 35 GPa at room temperature. Laser initiation at 10-15 GPa reveals a rapid self-propagating reaction (deflagration) that consumes the sample, similar to other energetic materials such as nitromethane. Post-initiation products include crystalline molecular nitrogen (delta-phase), and nitrogen crystallites with regular defects. The formation of bulk molecular nitrogen during deflagration - in both phase segregated and impurity-hosting forms - distinguishes TAG from other known energetic materials, and suggests a pathway for the generation of novel phases from element-enriched energetic substances. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q31.00006: High pressure--high temperature studies of ammonia Julius Ojwang, Ryan McWilliams, Alexander Goncharov Raman scattering measurements and x-ray diffraction of ammonia have been made under simultaneous conditions of high temperature and high static pressure in the laser heated diamond anvil cell. The experimental results on phase transitions with pressure increase at room temperature are found to be in accord with previous studies [1]. Pressure was increased up to 52 GPa and temperature ramped up to 2000 K. On increasing temperature at high pressure, strong changes in the ammonia Raman spectra are observed, which could be associated with melting. On melting, ammonia undergoes partial decomposition into nitrogen and hydrogen. We also observed the appearance of new N-H stretch bands at high temperatures which may be related to the formation of new bonds. When quenched back to room temperature the starting phase of solid ammonia is recovered. The shift in frequencies of the vibron bands of nitrogen with pressure shows that it is phase segregated from ammonia. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q31.00007: Atomic structure and elastic properties at high pressure of aluminum oxynitride in cubic phase I.G. Batyrev, J.W. McCauley, B.M. Rice, G.A. Gazonas, A.R. Oganov The atomic structure and elastic properties of aluminum oxynitride spinel (AlON) at high pressure (up to 40 GPa) have been calculated from first principles. We have assumed an ``ideal'' stoichiometry of cubic AlON with 35.7 mole {\%} AlN using the constant anion model. The elastic constants were calculated from independent strains that were applied to a unit cell, parameterizing the total energy as a function of the strain and from a stress-strain relationship. At ambient conditions a clustered distribution of N atoms has $\sim $ 1 eV per 55 atoms higher total energy than for a random distribution and slightly, but systematically lower elastic constants. The pressure dependence of C$_{11}$, C$_{12}$ and C$_{44}$ for random and cluster distributions of N atoms was calculated in the range of 0-40 GPa by performing six finite distortions of the lattice and deriving the elastic constants from the strain-stress relationship. The calculated values of dC$_{11}$/dP are in the range of 4.0-6.2 and for dC$_{44}$/dP $\sim $0.8-1.5. The estimates are in reasonable agreement with experimental measurements of polycrystalline AlON. The minimum energy structure of AlON was found using the evolutionary algorithm USPEX (Oganov {\&} Glass, 2006) [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q31.00008: In Situ Neutron and Synchrotron X-ray Diffraction Studies of Jarosite at High-Temperature High-Pressure Conditions H. Xu, Y. Zhao, D. Hickmott, J. Zhang, S. Vogel, L. Daemen, M. Hartl Jarosite (KFe$_{3}$(SO$_{4})_{2}$(OH)$_{6})$ occurs in acid mine drainage and epithermal environments and hot springs associated with volcanic activity. Jarosite is also of industrial interest as an iron-impurity extractor from zinc sulfide ores. In 2004, jarosite was detected by the Mars Exploration Rover M\"{o}ssbauer spectrometer, which has been interpreted as a strong evidence for the existence of water (and possibly life) on ancient Mars. This discovery has spurred considerable interests in stability and structural behavior of jarosite and related phases at various temperature, pressure, and aqueous conditions. In this work, we have investigated the crystal structure and phase stability of jarosite at temperatures up to 900 K and/or pressures up to 9 GPa using \textit{in situ} neutron and synchrotron X-ray diffraction. To avoid the large incoherent scattering of neutrons by hydrogen, a deuterated sample was synthesized and characterized. Rietveld analysis of the obtained diffraction data allowed determination of unit-cell parameters, atomic positions and atomic displacement parameters as a function of temperature and pressure. In addition, the coefficients of thermal expansion, bulk moduli and pressure-temperature stability regions of jarosite were determined. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q31.00009: Testing the minimum thermal conductivity model for amorphous polymers using high pressure Wen-Pin Hsieh, Mark Losego, Paul Braun, Sergei Shenogin, Pawel Keblinski, David Cahill Pressure dependence of thermal conductivity provides a critical test of the validity of the model of the minimum thermal conductivity for describing heat transport by molecular vibrations of an amorphous polymer. We measure the pressure dependence of the thermal conductivity of poly(methyl methacrylate) (PMMA) brushes grafted from SiC substrates using a combination of time-domain thermoreflectance and SiC anvil cell techniques. We also determine the pressure dependence of the thermal conductivity from a computational model of amorphous polystyrene. In both cases, thermal conductivity as a function of pressure is accurately predicted by the minimum thermal conductivity model via the pressure dependence of the elastic constants and density. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q31.00010: The Influence of Crystal Structure on the Thermal Expansion Behavior of GeZn$_{2}$O$_{4}$ Michael Crawford, R.L. Harlow, W.E. Guise, R.A. Fisher, W. Woerner, J.B. Parise, Q. Huang, J.W. Lynn, R. Stevens, B. Woodfield, J. Boerio-Goates, J. Lashley, O. Gourdon, A. Huq, J. Hormadaly, P.L. Lee, Y. Zhang GeZn$_{2}$O$_{4}$ synthesized at ambient pressure adopts the rhombohedral phenacite crystal structure, whereas cubic or tetragonal inverse spinel phases are formed at high pressures. We have measured the thermal expansion for all three forms of GeZn$_{2}$O$_{4}$ at temperatures from 10 K to 400 K (or higher) using synchrotron x-ray powder diffraction. The phenacite form exhibits negative thermal expansion below 300 K, changing to positive thermal expansion above that temperature. In contrast to this behavior, the cubic and tetragonal inverse spinel phases exhibit positive thermal expansion below room temperature. Characterization of these materials using x-ray and neutron diffraction, as well as heat capacity and Raman spectroscopy, will be described. Possible structural reasons for the different thermal expansion behaviors of the phenacite and spinel forms of GeZn$_{2}$O$_{4}$ will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q31.00011: First-principles calculation of Ca2RuO4 at high pressure Nobumi Miyawaki, Tatsuya Shishidou, Tamio Oguchi It has been observed that the layered perovskite antiferromagnetic insulator Ca$_2$RuO$_4$ reveals a phase transition into a ferromagnetic metal at 0.5GPa [1]. This insulator-metal transition is accompanied by a structural change with tilt and rotation of RuO$_6$ octahedron within the space group $Pbca$. Above about 9GPa, another transition from the ferromagnetic to superconducting phase has been recently reported [2]. The transition includes a structure change from $Pbca$ to $Bbcm$. In this study, a first-principles calculation is performed to study the electronic structure of Ca$_2$RuO$_4$, especially focusing on the changes of Ru 4$d$ states, with pressure. As the pressure is increased, calculated ferromagnetic spin moment of Ru is gradually decreased in $Pbca$ owing to the widening of Ru 4$d$ band. It is interesting that a ferromagnetic solution still exists in $Bbcm$. Similar structural changes (the tilt and rotation of RuO$_6$ octahedron) take place in Ca$_{2-x}$Sr$_x$RuO$_4$, where orbital hybridization with spin-orbit coupling (SOC) is crucial [3]. We also investigated effects of SOC, with the result that those appear even in the electronic structure of Ca$_2$RuO$_4$. Calculation results optimizing the structure will be also discussed. [1] F. Nakamura, et al., Phys. Rev. B {\bf 65}, 220402(R) (2002). [2] P. L. Alireza, $et$ $al$.: J. Phys.: Condens. Matter {\bf 22}, 052202 (2010). [3] T. Oguchi, J. Phys. Soc. Jpn. {\bf 78}, 044702 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q31.00012: Effect of high pressure on transport and structural properties of topological insulator Bi$_2$Se$_3$ J.J. Hamlin, J.R. Jeffries, N.P. Butch, P. Syers, D. A. Zocco, S.T. Weir, Y.K. Vohra, J. Paglione, M.B. Maple We report a series of electrical resistivity, magnetotransport, and xray diffraction measurements on the topological insulator Bi$_2$Se$_3$ under pressures as high as 34 GPa. The results demonstrate that applied pressure can be used to controllably tune the transport properties without chemical substitution. [Preview Abstract] |
Session Q32: Focus Session: Optical Properties of Semiconductor and Metal Nanostructures
Sponsoring Units: DMPChair: Rainer Hillenbrand, CIC nanoGUNE, Spain
Room: C144
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q32.00001: Application of modified phonon confinement model in Raman characterization of Ge nanowires K. Roodenko, I.A. Goldthorpe, P.C. McIntyre, Y.J. Chabal Raman spectroscopy is an attractive tool for characterization of low-dimensional materials, such as carbon nanotubes, graphene sheets or semiconductor nanowires. Phonon confinement model [1,2] was proposed to interpret Raman signal obtained from low-dimensional materials. Due to the finite-size of the nanostructures, the fundamental q$\sim $0 Raman selection rule is relaxed, allowing the contribution from phonons away from the Brillouin-zone center. In this contribution we address several unresolved issues, such as the factors within the confinement function, incorporation of crystallographic orientation, and the interplay between the temperature and the nanostructure size [3]. Application of the modified model to the interpretation of Raman signal from Ge nanowires will be discussed.\\[4pt] [1] H. Richter, et al., Solid State Commun. 39, 625 (1981).\\[0pt] [2] I. H. Campbell et al., Solid State Commun. 58, 739 (1986).\\[0pt] [3] K. Roodenko et al., Phys. Rev. B 82, 115210 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q32.00002: Laser-Induced, Local Oxidation of Copper Nanoparticle Films During Raman Measurements Angela R. Hight Walker, Guangjun Cheng, Irene Calizo The optical properties of gold and silver nanoparticles and their films have been thoroughly investigated as surface enhanced Raman scattering (SERS) substrates and chemical reaction promoters. Similar to gold and silver nanoparticles, copper nanoparticles exhibit distinct plasmon absorptions in the visible region. The work on copper nanoparticles and their films is limited due to their oxidization in air. However, their high reactivity actually provides an opportunity to exploit the laser-induced thermal effect and chemical reactions of these nanoparticles. Here, we present our investigation of the local oxidation of a copper nanoparticle film induced by a visible laser source during Raman spectroscopic measurements. The copper nanoparticle film is prepared by drop-casting chemically synthesized copper colloid onto silicon oxide/silicon substrate. The local oxidation induced by visible lasers in Raman spectroscopy is monitored with the distinct scattering peaks for copper oxides. Optical microscopy and scanning electron microscopy have been used to characterize the laser-induced morphological changes in the film. The results of this oxidation process with different excitation wavelengths and different laser powers will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q32.00003: Observation of UV Surface-Enhanced Raman Spectra using Ga Nanoparticles Yang Yang, John Callahan, Kevin Lantz, John Foreman, Pae Wu, Tong-Ho King, April Brown, Henry Everitt Ultraviolet (UV) surface enhanced Raman spectra (SERS) are observed for the first time using gallium nanoparticles (Ga NPs). Ga NP ensembles were synthesized on sapphire substrates at room temperature by molecular beam epitaxy. In situ spectroscopic ellipsometry was used to tune the UV local surface plasmon resonance (LSPR) wavelengths of the Ga NP ensembles during deposition. Three samples were prepared with LSPR wavelengths of 325, 295, and 260nm. UV Raman spectra using a 325nm HeCd laser were collected from fixed thicknesses of cresyl violet, poly(3-hexylthiopene), or MEH-CN-PPV that were spin cast onto these three samples, each of which had a NP-free region. A sample!`\={ }s enhancement was measured by comparing selected Raman signal intensities from the NP-covered and bare surfaces. Enhancements were found to decrease with increasing detuning between the laser and LSPR wavelengths. Similar behavior was observed from Ga NPs after 3 months of exposure to air, demonstrating the resilience of Ga NPs to oxidation. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q32.00004: Electronic origin of photoluminescence from Si nanocrystal embedded in amorphous SiO2 matrix Tianshu Li, Francois Gygi, Giulia Galli Through combining classical molecular dynamics and {\em ab initio} calculation, we have created composite models of Si nano crystal embedded in SiO$_2$ amorphous matrices, with the sizes of Si nanocrystals ranging from 1.3 nm $\sim$ 1.9 nm. Electronic structure calculations showed that the band gap of composite structure increases as the size of Si nanocrystal reduces, however the increase of gap is mainly attributed to the {\em lowering of valence band edge}, with conduction band edge virtually unchanged. It was also found that while the wavefucntions from conduction band edges are extended over the entire Si nanocrystal, those from the valence band edges are mainly distributed near the nanocrystal/matrix interface. Further analysis identified that the valence band edges are dominated by the local distortion of nanocrystal from diamond cubic structure, which increases as both approaching the surface of Si nanocrystal, and decreasing the size of Si nanocrystal. This finding suggests that the local strain induced by surrounding amorphous SiO$_2$ matrix may play a key role in the photoluminescence of Si nanocrystal/SiO$_2$ amorphous matrix composite structures. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q32.00005: Enhanced luminescence in terbium-cerium co-doped tin oxide quantum dots Christie Larochelle, Kelly McCutcheon, Rebecca Sobel SnO$_2$ quantum dots doped with Tb$^{3+}$ exhibit strong luminescence from the Tb$^{3+}$ dopants due to efficient energy transfer from the SnO$_2$ donors to the Tb$^{3+}$ acceptor ions. We report results from a study of the effect of co-doping the SnO$_2$ dots with both Tb$^{3+}$ and Ce$^{3+}$ on the photoluminescence properties of the samples. The dots were synthesized using a sol-gel technique and the Ce$^{3+}$/Tb$^{3+}$ ratio was varied while keeping the total doping level at 1wt$\%$. X-ray diffraction and TEM results confirm the presence of nanocrystals of less than 10 nm in diameter. Photoluminescence results indicate that the Tb$^{3+}$ ions are incorporated in a crystalline environment and that co-doping with Ce$^{3+}$ enhances the energy transfer efficiency and therefore the intensity of the Tb$^{3+}$ luminescence. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q32.00006: Stabilization of fluorescent silver clusters by RNA homopolymers and their DNA analogs: C,G vs A,T(U) Dichotomy Danielle Schultz, Elisabeth Gwinn We show that single-stranded RNA stabilizes fluorescent silver nanoclusters (Ag:RNAs) in aqueous solution, analogous to previously studied Ag:DNAs. To determine whether the different canonical nucleosides play similar roles in stabilizing fluorescent silver species in RNA and DNA hosts, we compare RNA homopolymers of rA,rC,rG and rU to their DNA counterparts, and observe the same base-dependent dichotomy: visible- to IR-emitting silver complexes are stabilized by C and G homopolymers, but not by A or T(U) homopolymers at neutral pH. Shifts in emission wavelengths between Ag:RNA and Ag:DNA analogs show that both base and sugar influence populations of fluorescent species. The data indicate a minimum binding-pocket size of roughly five C or G bases for fluorescent species. These findings open the scope of silver cluster fluorophores to the diversely structured and functional arena of RNA and have implications for rational designs of nucleic acid hosts. Supported by NSF CHE-0848375. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q32.00007: Electronic and Optical Excitations in Perylene Diimide Derived Dye Molecules from First Principles Kopinjol Baishya, Serdar Ogut, Ersen Mete, Oguz Gulseren, Sinasi Ellialtioglu Halogenated perylene diimide dyes, such as Br-PDI (Br$_2$C$_{24} $H$_8$N$_2$O$_4$) and their glycine (BrGly) and aspartine (BrAsp) derivatives are known to absorb and emit light in the visible range with high quantum yields, and have good heat and chemical stability. As such, they are promising alternatives to the expensive (Ru-based) metal-driven dye sensitizers for solar cell applications. In this talk, we present results for the electronic structures, quasiparicle gaps, and the absorption spectra of PDI-derived dye molecules BrPDI, BrGly, and BrAsp, computed within the time-dependent density functional theory as well as many body perturbation techniques such as the GW method and the solution of the Bethe-Salpeter equation. In addition to discussing our results for bare molecules, we also present our preliminary studies for the change in their electronic and optical properties when they are attached to stoichiometric and reduced rutile TiO$_2$ (110) surfaces. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q32.00008: Enhanced light emission via plasmonic and non-plasmonic effects in metal ion-implanted Silicon Akhilesh Singh, Karol Gryczynski, Arkadii Krokhin, Floyd McDaniel, Arup Neogi We have observed enhanced photoluminescence from metal implanted nanoscale Silicon light emitters. Low energy (30 keV) Au and Ag metal ions were implanted in crystalline silicon to achieve non-plasmonic and plasmonic enhancement of light emission over a broad spectral range. The emission in the UV region can be significantly enhanced by the surface plasmon (SP) induced radiative recombination process. The recombination of carriers in Si bound exciton is also influenced by transverse optical phonon due to the polarization of the surface of bound exciton complex. The recombination life time of the electron-hole pair as estimated from the time resolved PL measurement changes from $\sim $ 2 ns to 400 ps in the presence of Ag ion induced SP polaritons. The non-resonant emission can be enhanced by electrostatic-image charge effects. The emission in the visible (570 nm) and UV (370 nm) wavelength range can also be significantly enhanced by electrostatic image charge effects induced by Au nanoparticles [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q32.00009: Exciton-plasmon and spin-plasmon interactions in hybrid semiconductor-metal nanostructures Alexander Govorov Coulomb and electromagnetic interactions between excitons and plasmons in nanocrystals cause several effects: energy transfer between nanoparticles, plasmon enhancement, Lamb shifts of exciton lines, Fano interference. In a complex composed of semiconductor quantum dot and metal nanoparticle, plasmons interact with spin-polarized excitons. This interaction leads to the formation of coupled spin-plasmon excitations and to spin-dependent Fano resonances. If an exciton-plasmon system includes chiral elements (chiral molecules or nanocrystals), the exciton-plasmon interaction is able to create new plasmonic lines in circular dichroism spectra. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q32.00010: Generalized Ellipsometry on Ferromagnetic Sculptured Thin Films. Daniel Schmidt, Tino Hofmann, Kah Mok, Heidemarie Schmidt, Ralf Skomski, Eva Schubert, Mathias Schubert We present and discuss generalized ellipsometry and generalized vector-magneto-optic ellipsometry investigations on cobalt nanostructured thin films with slanted, highly-spatially coherent, columnar arrangement. The samples were prepared by glancing angle deposition. The thin films are highly transparent and reveal strong form-induced birefringence. We observe giant Kerr rotation in the visible spectral region, tunable by choice of the nanostructure geometry. Spatial magnetization orientation hysteresis and magnetization magnitude hysteresis properties are studied using a 3-dimensional Helmholtz coil arrangement allowing for arbitrary magnetic field direction at the sample position for field strengths up to 0.4 Tesla. Analysis of data obtained within this novel vector-magneto-optic setup reveals magnetization anisotropy of the Co slanted nanocolumns supported by mean-field theory modeling. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q32.00011: Coupled Quantum Dot-Metal Nanoparticle Systems: Hybrid Behavior and Excitation Transport Ryan Artuso, Garnett Bryant Transmission of information between qubits for quantum communication and quantum computing will require directed nanoscale transmission where the quality of the information can be maintained. One paradigm proposes to achieve directed nanoscale transmission by coupling qubits, for example in quantum dots, to plasmonic nanoantennas or nanoguides made from metallic nanowires and nanoparticles. We study theoretically the response of hybrid nanostructure molecules consisting of multiple semiconductor quantum dots (SQD), and metal nanoparticles (MNP) subject to an applied optical field. We consider the situation where the SQDs interact directly without an MNP and the case in which the interaction is mediated by a MNP. We find modifications to the previously predicted SQD-MNP hybrid response. We also find a new regime of behavior in which breaking the SQD-SQD identical particle symmetry causes the system to no longer reach a steady state and instead oscillate at a beating frequency. Lastly, we identify the effects that MNP size and shape, and the placement of the SQDs have on the SQD-SQD interaction. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q32.00012: Plasmon Behavior in Gold Nano-dot Media N. Limberopoulos, J. Derov, A. Drehman, E. Crisman We present correlations between the plasmon resonance of a gold nano-dots medium suspended in Al$_{2}$O$_{3}$ and the plasmon resonance of a single layer film of solid gold. We also present the effective medium properties of multi-layered, dot-film media. The gold dots were fabricated on Al$_{2}$O$_{3}$ substrates by sputter-depositing the gold and then annealing the resultant films. The median dot size ranged from 70 to 250 nm depending on the processing. Successive Au-dot layers were made by depositing Al$_{2}$O$_{3}$ over the previous dot pattern followed by depositing another gold film, followed by re-annealing. We used attenuated-total-reflection to couple transverse-magnetic optical waves to the plasmon resonance and controlled the degree of coupling by varying the spacing between the dot medium and the coupling prism. The plasmon resonances for the dot media had asymmetric line shapes compared to that for the continuous gold film. We were able to show correlations between that asymmetry and the spatial parameters of the dot/Al$_{2}$O$_{3}$ media construction. Effective media parameters and the dispersion characteristics for nano-dot/Al$_{2}$O$_{3}$ bi-layer were determined. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q32.00013: Surface Plasmon Generation by Excitons in Carbon Nanotubes Igor Bondarev, Todor Antonijevic Optical properties of semiconducting carbon nanotubes (CNs) originate from excitons and may be tuned by either electrostatic doping [1], or via the quantum confined Stark effect (QCSE) by means of an electrostatic field applied perpendicular to the CN axis[2]. In both cases exciton properties are mediated by surface plasmon excitations [2,3]. We have shown recently that the QCSE allows one to control the exciton-interband-plasmon coupling in individual CNs and their optical absorption, accordingly [2]. Here, we extend our studies to demonstrate the possibility of low-energy localized surface plasmon generation by optically excited excitons in small-diameter ($\sim $1nm) CNs. The stimulated character of such an energy transfer causes the buildup of the macroscopic population numbers of coherent localized surface plasmons and, as a consequence, high-intensity coherent optical-frequency fields localized at nanoscale, which can be used for various applications, such as near-field nonlinear-optical probing, sensing, or materials nanoscale modification. [1] M.Steiner, et al., NL9,3477. [2] I.V.Bondarev, et al., PRB80,085407. [3] C.D.Spataru and F.Leonard, PRL104,177402. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q32.00014: Nanocrystal optoelectronic devices by plasmon-based optical trapping Kenneth Evans, Daniel Ward, Gautam Kini, Michael Wong, Douglas Natelson Optical trapping is an important tool for studying and manipulating nanoscale objects. In conventional laser trapping, the trapping volume is diffraction limited. Recent experiments have shown that subwavelength control of nanoparticles can be achieved by using plasmonic nanostructures, rather than using the laser directly, to generate the electric fields necessary for trapping. We present a numerical model describing the trapping forces on an individual semiconducting nanocrystal in a nanoscale metallic junction, and discuss initial experimental results. Calculations of the fields are performed in COMSOL, a commercial finite element solver package, and the trapping forces are computed using the full Maxwell stress tensor formalism. We propose the use of plasmonic optical trapping in this geometry as a method to fabricate electrically driven, single nanocrystal light-emitting devices. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q32.00015: The Morphology and Evolution of Bipyramidal Gold Nanoparticles for Plasmon-assisted Nanosheet Biosensor Nicholas Geitner, Amos Deopke, Melodie Fickenscher, Jan Yarrison-Rice, William Heineman, Howard Jackson, Leigh Smith We examine the growth and evolution bipyramidal gold nanoparticles. These particles are then characterized based on their longitudinal LSPR peak and their physical dimensions. Bipyramidal particles are grown using a seed-mediated growth process, and variations in the particles are produced by varying silver nitrate concentration and growth time. While each growth's physical dimensions were well defined and consistent with previous results, two different distinct modes of temporal evolution are observed after the primary growth period. We also observe a distinct linear relationship between tip radius of curvature and wavelength of longitudinal LSPR peak, in agreement with numerical calculations. These particles are to be functionalized and dispersed onto CdS nanosheets for biosensor applications. [Preview Abstract] |
Session Q33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Interfaces and Optical Properties
Sponsoring Units: DMP DCOMPChair: Venkatraman Gopalan, Pennsylvania State University
Room: C143/149
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q33.00001: Engineering exotic phenomena at oxide interfaces Invited Speaker: Complex transition metal oxides form an important class of compounds, exhibiting a wide variety of functional properties exploited in many applications. Thanks to advances in deposition techniques, these oxides can also nowadays be combined in heterostructures, with a structural quality comparable to what is achieved for conventional semiconductors, and the appearance of new phenomena at the interfaces where oxides with different properties meet brought recently the field to an entirely new level. Such phenomena include, for instance, the appearance of a two- dimensional electron gas at the interface between insulator oxides, the possibility of unexpected coupling between structural instabilities at some interfaces yielding unusual functional properties or the tunneling through ferroelectric and multiferroic barriers. Concentrating on few selected examples, I will illustrate how first-principles calculations can efficiently help the experimentalists to characterize the interfaces between complex transition metal oxides and sometimes guide them toward the design of new interesting heterostructures with exotic properties. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q33.00002: Harnessing competition in artificially layered ferroelectric superlattices to engineer enhanced piezoelectrics Matthew Dawber, Benedikt Ziegler, Sara Callori, John Sinsheimer, Valentino Cooper, Tahir Yusufaly, Karin M. Rabe, Premala Chandra First principles calculations by Cooper and Rabe (V. R. Cooper and K.M. Rabe, Phys. Rev. B 79, 180101 (R) (2009)), predicted that in PbTiO$_{3}$/BaTiO$_{3}$ superlattices an enhancement of the d$_{33}$ piezoelectric coefficient could be achieved at a particular ratio of the thickness of the constituent layers. We have fabricated high quality artificially layered PbTiO$_{3}$/BaTiO$_{3}$ superlattices on SrTiO$_{3}$ substrates (with SrRuO$_{3}$ bottom electrodes) using an off-axis RF magnetron sputtering technique, allowing us to perform x-ray diffraction, electrical measurements and atomic force microscopy on this system. The experimental results confirm the prediction from first principles calculations, and we apply a Landau theory model as a useful bridge between the first principles predictions and experimental results at elevated temperature. In this work we have demonstrated that by finely balancing competing material properties in artificial heterostructures, desirable properties that exceed those of the parent compounds can be achieved. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q33.00003: Ferroelectrical and Dielectric Properties of BaTiO$_{3}$/Ba$_{(1-x)}$Sr$_{x}$TiO$_{3}$ Superlattices Nora Ortega, Ashok Kumar, Ram S. Katiyar Artificially designed superlattices (SL) composed of alternate layers of BaTiO$_{3}$ (BT) and SrTiO$_{3}$ (ST) have attracted interests due to the possibility of producing superior and new functional properties, which are attractive for device applications. We have fabricated SL of BT/Ba$_{(1-x)}$Sr$_{x}$TiO$_{3}$ (BST) with x = 0, 0.3, 0.4, 0.5, 0.6, 0.7, 1, utilizing multi-target by pulsed laser deposition technique. The modulation period ($\Lambda )$ in all SL was $\Lambda $= 80 {\AA} and the total thickness of each SL films were 600 nm. The x-ray diffraction revealed well oriented (00l) perovskite structure and the so-called satellite peaks. The polarized Raman spectra showed the substantial transformation of the ferroelectric E(1TO) soft mode, depending of the ratio of Ba/Sr in BST layer. The dielectric constant of SL showed linear frequency dispersion above of 20 kHz, and their values are in the range of 400 to 900 at 1 kHz, while the tangent loss values were below to 0.1 at 1 kHz. Well defined ferroelectric loop was observed in all the SL at different frequencies (1 kHz-10 kHz), with remanent polarization (2P$_{r})$ 10 $\mu $C/cm$^{2}$. Improvement in saturation in the ferroelectric loop was observed with increase of Ba composition in BST layer. All these superlattices show very low leakage current far above its coercive field. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q33.00004: Artificially layered PbTiO$_{3}$/CaTiO$_{3}$ superlattices John Sinsheimer, Youcef Benkara, Jonathan Daley, Sara Callori, Matthew Dawber It has been shown that in artificially layered PbTiO$_{3}$/SrTiO$_{3}$ superlattices, a form of improper ferroelectricity occurs where the rotations of the oxygen octahedra at the interfaces couple with the polar mode and increase the ferroelectric polarization of the material when the layers are very thin. PbTiO$_{3}$/CaTiO$_{3}$ superlattices grown on SrTiO$_{3}$ substrates are also highly likely to display this kind of behavior, as the CaTiO$_{3}$ ground state is dominated by rotational distortions. This system should also play host to a competition between in-plane ferroelectricity (as CaTiO$_{3}$ is subjected to a large tensile strain when grown on SrTiO$_{3}$) and out-of-plane ferroelectricity (the usual result when in PbTiO$_{3}$ is grown on SrTiO$_{3}$). Using off-axis RF magnetron sputtering, we have produced high quality superlattices of PbTiO$_{3}$/CaTiO$_{3}$ with various layer thicknesses on SrTiO$_{3}$ substrates with SrRuO$_{3}$ bottom electrodes. The samples were analyzed using x-ray diffraction, electrical measurements, and atomic force microscopy. Our experimental results reveal a fascinating transition region at certain ratios of the relative layer thicknesses. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q33.00005: Polarization switching and dielectric properties of ferroelectric bilayers Mandana Meisami Azad, Daniel Tinberg, Donald Walko, Susan Trolier-Mckinstry, Alexei Grigoriev In this work, we analyze polarization switching and dielectric properties of ferroelectric multilayer thin films of lead zirconate titanate. The interlayer coupling and polarization dynamics of ferroelectric multilayers are largely unknown. The studies of multilayers present a significant interest due to both fundamental understanding of interlayer interactions and practical applications of ferroelectrics in nanoelectronics and nanoelectromechanical systems. It is predicted that unusual switching characteristics and domain configurations such as the antiparallel alignment of the spontaneous polarization in adjacent layers can be observed in these materials. Using electrical measurements and time-resolved x-ray microdiffraction we analyzed physical properties of $PbZr_{0.8} Ti_{0.2}O_{3}$/$PbZr_{0.6}Ti_{0.4}O_{3}$. Strong nonlinearities in piezoelectric and dielectric responses of the bilayer to applied electric fields, which were observed in our experiments, can be explained by unusual polarization domain dynamics and interface charging effects. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q33.00006: Metallic oxides as dielectrics in artificially layered ferroelectric superlattices Sara Callori, Dong Su, John Sinsheimer, Matthew Dawber Artificially structured oxides provide many opportunities to develop systems with novel and tunable properties. SrRuO$_3$ has a metal-insulator transition as a function of thickness, which suggested to us the idea that we could use extremely thin layers (less than 3 unit cells) of SrRuO$_{3}$ as a novel dielectric component within an artificially layered superlattice system. We have created high quality PbTiO$_3$/SrRuO$_3$ superlattices by using an off-axis RF magnetron sputtering technique. The samples were characterized by x-ray diffraction, atomic force microscopy, transmission electron microscopy, and electrical measurements. When the PbTiO$_3$ layers are above a certain critical thickness, significant out-of-plane ferroelectricity develops in the system and the overall material has a semiconducting character. In this talk we will present a detailed experimental investigation of the behavior of ferroelectric polarization and domain size as the relative thicknesses of the superlattice layers are varied. Our work serves as a demonstration that a new set of materials, metallic oxides, can be considered for inclusion as novel dielectric layers in ferroelectric superlattices. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q33.00007: Stabilizing ferroelectric polarization of ultrathin BaTiO3 films through interface engineering Xiaohui Liu, Yong Wang, Pavel Lukashev, J.D. Burton, Evgeny Tsymbal Ferroelectric tunnel junctions have recently attracted considerable interest due to their potential for device applications [1]. The main challenge for the implementation of these devices is to stabilize ferroelectricity in nanometer- thick films where depolarizing fields and interface effects play an important role. Here, we report results of first- principles calculations of ferroelectric polarization in epitaxial SrRuO3/BaTiO3/SrRuO3 junctions. We show that the ferroelectric polarization is very sensitive to the surface termination of the electrodes and film thickness. In particular, we find that the presence of RuO2/BaO interface is detrimental to ferroelectricity due to the pinning of polar displacements in BaTiO3 in the direction away from the interface making the polarization of ultra-thin films non- switchable. We find that ferroelectricity can be stabilized by adding a thin layer of SrTiO3 at this interface. A phenomenological model is developed to explain the correlation between ferroelectric properties and junction geometry. \\[4pt] [1] E.Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q33.00008: Inverse layer capacitance in perovskite oxide superlattices Xifan Wu, Massimiliano Stengel, David Vanderbilt Ferroelectricity is one of the most important functionalities that can be tuned in perovskite oxide superlattices. At fixed displacement field $D$, the overall polar instability can be accessed by the inverse of the capacitance per basal area as $C^{-1}=\partial V / \partial D$, where $V$ is the potential drop across the supercell.\footnote{M. Stengel, D. Vanderbilt, and N.A. Spaldin, Nature Mater. {\bf 8}, 392 (2009).} Here we propose that $C^{-1}$ can be further rigorously decomposed into contributions from individual AO or BO$_2$ layers, giving an {\it layer inverse capacitance} defined as $c_j^{-1}=\epsilon_0^{-1}(h_j+D\partial h_j/\partial D - \partial p_j/\partial D)$, where $h_j$ and $p_j$ are the layer height and Wannier-based layer polarization\footnote{X. Wu, O. Di\'{e}huez, K.M. Rabe and D. Vanderbilt, Phys. Rev. Lett. {\bf 97}, 107602 (2006).} of layer $j$, respectively. We compute the $c_j^{-1}$ in several typical multicomponent perovskite superlattices such as CaTiO$_3$/BaTiO$_3$ and PbTiO$_3$/SrTiO$_3$, and demonstrate that they satisfy a {\it locality} principle: their behavior depends mainly on the local chemical environment (i.e., the identities of neighboring layers). Thus, we show that the $c_j^{-1}$ can provide an insightful {\it local} analysis of the ferroelectric tendency at interfaces in functional oxide superlattices. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q33.00009: Light-Driven Ferroelectric Polarization Dynamics Probed with Time-Resolved X-ray Scattering D. Daranciang, M. Highland, H. Wen, N. Brandt, H. Hwang, J. Larsson, K. Sokolowski-Tinten, D. Reis, K. Nelson, P. Fuoss, G.B. Stephenson, A.M. Lindenberg We report femtosecond resolution time-resolved x-ray scattering measurements of dynamical changes in the polarization of PbTiO3 (PTO) nanolayers on SrTiO3 (STO) and DyScO3 (DSO) substrates under 400 nm, 40 fs optical excitation. For PTO on STO, an optically-induced polarization enhancement occurs on picosecond timescales that can be associated with a carrier-induced screening of the depolarization field. For PTO on DSO, qualitatively different effects are observed, indicating that the light initially couples to c-domains. We also observe optically-driven ferroelectric to paraelectric phase transitions (and vice versa) near the Curie temperature. The optical response of PTO on STO in the monodomain phase is consistent with a bulk photovoltaic effect. Optical excitation in the stripe phase at 515 C drives strains of order 1 percent, with an associated non-thermal disordering of the stripe domains. For PTO on DSO, temperature-dependent in-plane and out-of-plane structural dynamics are simultaneously captured, allowing the complex coupling between a- and c-domain motions to be mapped out. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q33.00010: Optical Properties of Epitaxial Sr-Ti-O Compounds from First Principles Robert Berger, Jeffrey Neaton SrTiO$_{3}$ is a representative of the property-rich perovskite family, and a material whose ability to convert solar photons to H$_{2}$ fuel would be more efficient if its wide optical bandgap (3.25 eV) better matched the solar spectrum. The Sr- and Ti-based Ruddlesden-Popper (RP) phases, Sr$_{n+1}$Ti$_{n}$O$_{3n+1}$, are structural modifications of SrTiO$_{3}$ with potentially useful electronic properties. While bulk growth is limited to $n<4$ and $n=\infty$ (SrTiO$_{3}$), thin films of larger finite $n$ structures have been grown epitaxially. In optical experiments, bandgaps of these films decrease monotonically with increasing $n$.\footnote{C.-H. Lee et al., to be published.} In density functional theory (DFT), however, the $n=\infty$ gap is larger than those for finite $n>3$. This disagreement could stem from limitations in both experiment and theory. We explore this issue in depth using DFT, many-body perturbation theory, and tight-binding techniques. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q33.00011: Spectroscopic investigation on the electronic structure of a 5$d$ band insulator SrHfO$_{3}$ Yunsang Lee, Y.K. Seo, D.J. Lee, H.J. Noh We investigated the high-energy electronic structure of a 5d perovskite SrHfO$_{3}$. By using optical spectroscopy and O 1$s$ x-ray absorption spectroscopy, the values of electronic structure parameters are estimated properly. In particular, the crystal field splitting energy, which is closely associated with the $p-d$ hybridization strength, is as high as 5 eV, and the Sr 4$d$ bands appear to be strongly mixed with the Hf 5$d$ bands. Moreover, the emission spectra with a 325 nm light excitation exhibit a sizable strength near 500 nm at low temperatures due to oxygen defects. These findings in SrHfO$_{3}$ are compared with electronic properties of similar compounds, 3$d$ SrTiO$_{3}$ and 4$d$ SrZrO$_{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q33.00012: Precise measurements of index of refraction at Brewster angle Wei-Tai Hsu, Christian Bahrim A simple and accurate method is proposed for finding the index of refraction of solid and liquid dielectrics using the polarization of light reflected by a dielectric surface near the Brewster angle. The method allows measuring the Brewster angle with a precision better than 0.01 degrees and the index of refraction with a precision of 0.0001 by running a parabolic fit of the parallel component of the reflectance normalized to the total reflectance in a narrow region of about 15 degrees around the Brewster angle [1]. Our measurement is about 100 times better than other existing methods. The best precision in our measurements is achieved when a computer-based filtering procedure of the experimental reflectance is used during the data acquisition [2]. Our apparatus allows measuring small variation of the index of refraction, such as due to the change in temperature or the interference with another E-field.\\[4pt] [1] Bahrim C and Hsu Wei-Tai, 2009 \textit{Am. J. Phys.} \textbf{77} (4) 337-343; [2] Hsu Wei-Tai and Bahrim C, 2009 \textit{Eur. J. Phys.} \textbf{30} 1325-1336. [Preview Abstract] |
Session Q34: Focus Session: Interfaces in Complex Oxides - Polar Interfaces
Sponsoring Units: DMPChair: Evgeny Tsybal, University of Nebraska
Room: C141
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q34.00001: Effect of polar interfaces on thin-film ferroelectricity Yong Wang, Manish Niranjan, Karolina Janicka, Julian Velev, Mikhail Zhuravlev, Sitaram Jaswal, Evgeny Tsymbal Based on first-principles and model calculations we investigate the effect of polar interfaces on the ferroelectric stability of thin-film ferroelectrics [1]. We consider Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and SrRuO3/LaO/BaTiO3/LaO heterostructures as representative systems, where a LaO monolayer at the interface with a TiO2-terminated BaTiO3 produces a polar interface. The polar interfaces create an intrinsic electric field which produces electric polarization in BaTiO3 directed into the interior of the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces that is non-switchable and thus detrimental to ferroelectricity. The effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric field that penetrates deeper into the ferroelectric. The predicted mechanism for a ferroelectric dead layer at the interface controls the critical thickness for ferroelectricity in systems with polar interfaces. [1] Y. Wang et al., Phys. Rev. B. 82, 094114 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q34.00002: Lattice screening of the polar catastrophe in KNbO$_{3}$/BaTiO$_{3}$ interfaces Pablo Garcia-Fernandez, Pablo Aguado-Puente, Javier Junquera The discovery of a bidimensional electron gas (2DEG) in the interface between two insulating lattices like LaAlO$_3$ and SrTiO$_3$ has triggered much interest around bidimensional conductivity in these heterostructures. In this work we study polar interfaces between KNbO$_{3}$ (KNO) and BaTiO$_{3}$ (BTO), equivalent from the layer by layer charge point of view to the LAO/STO. In particular we focus on: (a) the possibility of formation of a 2DEG, (b) its interaction with the ferroelectric distortions of these materials, and (c) the effect of external electric fields. For this, we have performed Density Functional Theory calculations for a KNO($m$)/BTO(2$m$)/KNO($m$) slab ($m$= width in unit cells) with different kinds of interfaces ($n$ or $p$). We find that a 2DEG is formed only in the unrelaxed configuration, where there is no rumpling between the atoms of a given layer. However, when geometry is relaxed, KNbO$_{3}$ polarizes and the 2DEG is effectively screened. This effect is robust even under application of electric fields of moderate size. Finally, we find that an easily-rotated [110] in-plane polarization, driven by electrostatic effects, appears in the vicinity of the KO/TiO$_2$-type interface even thought the system is under in-plane compressive strain. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q34.00003: Evolution of the Band Alignment at Polar Oxide Interfaces J.D. Burton, Evgeny Tsymbal The next generation of electronic devices and systems are envisioned to exploit the multifunctional properties of complex oxide interfaces. Fundamental to this endeavor is an understanding of the electronic band alignment across such interfaces. Engineering this band alignment in all-oxide systems by properly preparing the interfaces is highly desirable. Here we explore an all-oxide metal-insulator interface between, SrTiO$_{3}$ (STO), and La$_{1-x}A_{x}$MnO$_{3}$ (LAMO), where $A$ is a divalent cation [1]. The doping level of the manganite, $x$, offers a parameter which can be varied to engineer the band alignment. We use first-principles density-functional calculations to determine the evolution of the band alignment at La$_{0.7}A_{0.3}$MnO$_{3}\vert $La$_{1-x}A_{x}$O$\vert $TiO$_{2}\vert $SrTiO$_{3}$(001) heterointerfaces as the interfacial composition, La$_{1-x}A_{x}$, is varied. The position of the valence band maximum (VBM) with respect to the Fermi level increases linearly with interfacial composition $x$ due to the linear dependence of the screened electrostatic interface dipole on the interfacial ionic charge. The importance of the polar nature of LAMO and its background dielectric properties will be discussed. Our results are agreement with recent experimental data reported by Hikita \textit{et al} [2].\\[0pt] [1] J. D. Burton and E. Y. Tsymbal, Phys. Rev. B 82, 161407 (2010).\\[0pt] [2] Y. Hikita\textit{ et al.}, Phys. Rev. B 79, 073101 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q34.00004: Polarization discontinuities and compensation mechanisms at oxide-oxide interfaces Massimiliano Stengel, David Vanderbilt Polar interfaces between insulating perovskite materials have been the subject of special attention in the past few years, following the discovery of two-dimensional conductivity in LaAlO$_3$/SrTiO$_3$. In this talk I will introduce the problem by using general concepts of macroscopic electrostatics, in the framework of the modern theory of polarization [1]. Based on these ideas, I will show how we can understand the origin and the spatial distribution of the metallic electron gas in terms of few basic ingredients, which can be readily extracted from bulk calculations of SrTiO$_3$. These results provide an unified view over the factors determining the confinement and decay of the compensating free charge within arbitrary electrical boundary conditions.\\[4pt] [1] M. Stengel and D. Vanderbilt, Phys. Rev. B 80, 241103(R) (2009). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q34.00005: First-principles study of the band offset at the anatase TiO$_{2}$/SrTiO$_{3}$ (001) interface Alexander Demkov, Hosung Seo, Chandrima Mitra Band offsets at the interface of two oxides have recently attracted considerable attention in the context of the high-k dielectric gate stack in field effect transistors. The problem is also important in oxide heteroepitaxy. Recently, Chambers et al. reported that using the x-ray photoelectron spectroscopy no valence band offset is found between anatase TiO$_{2}$ (001) and SrTiO$_{3}$ (001). In this talk we describe the electronic structure at the TiO$_{2}$/SrTiO$_{3}$ (001) interface. We calculate the valence band offset to be 0.94 eV in the Schottky limit and 0.76 eV when the oxides are brought in contact, in apparent contradiction with experiment. A careful analysis of the electronic structure evolution from the bulk region of SrTiO$_{3}$ to the interface and through anatase all the way to the surface allows us to clarify the experimental results. We compare local density approximation results to those obtained with the GW method. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q34.00006: Many-body effects on the capacitance of multilayers made from strongly correlated materials Simon Hale, Jim Freericks Recent work by Kopp and Mannhart on novel electronic systems formed at oxide interfaces has shown interesting effects on the capacitances of these devices. In our work, we identify effects on the capacitance that stem from many-body physics and from other factors. In order to do so, we employ inhomogeneous dynamical mean- field theory to calculate the capacitance for multilayered nanostructures. These multilayered nanostructures are composed of semi-infinite metallic leads coupled via a strongly correlated dielectric barrier region. The barrier region can be adjusted from a metallic regime to a Mott insulator through adjusting the interaction strength. We are able to vary the barrier thickness allowing comparison to the expected geometric capacitance. We also examine the effects of varying the temperature, potential difference, screening length, chemical potential, and electron filling. We set up a system that depletes the charge from the barrier so that the capacitance approaches the geometric capacitance, allowing us to study nonlinear effects. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 1:03PM |
Q34.00007: Electronic Reconstruction at oxide interfaces: from an electron-hole bilayer to a spin-polarized 2DEG Invited Speaker: The origin of conductivity at the interface between the band insulators LaAlO$_{3}$ and SrTiO$_{3}$ has been subject of continued interest and debate. Density functional theory calculations can provide not only insight into the underlying mechanisms but also allow one to identify further parameters to tune the electronic reconstruction in this system in view of device applications. In particular, the potential build up induced by the polarity of the LaAlO$_{3}$ film is found to be counteracted by a strong lattice polarization in the LaAlO$_{3}$ film. The latter allows the system to remain insulating for the first several LaAlO$_{3}$ layers before a crossover to an electronic reconstruction takes place at around 4-5 monolayers (ML). We demonstrate that, owing to a dispersive surface state, an additional SrTiO$_{3}$ capping layer can trigger the insulator-to-metal transition already at two ML of LaAlO$_{3}$. As a result, two spatially separated sheets of carriers emerge: electrons at the interface and holes at the surface that are only 1 nm apart and can be used to study excitonic phenomena. Furthermore, we explore the effect of a metallic contact layer on top of the LaAlO$_{3}$ film which not only eliminates the potential build up but turns out to be a promising way to enhance the carrier concentration and possibly to realize a spin-polarized electron gas at the interface. Work in collaboration with W.E. Pickett, K. Otte, V. Ruiz L\'{o}pez. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q34.00008: Electronic properties of the conducting interface in LaAlO$_3$/SrTiO$_3$ heterostructures - the view from x-ray spectroscopies M. Sing, G. Berner, A. Muller, J. Walde, F. Pfaff, R. Claessen, H. Hollmark, L.-C. Duda, S. Paetel, C. Richter, J. Mannhart, S. Thiess, W. Drube, S.A. Pauli, C.W. Schneider, P.R. Willmott Novel phases with often unexpected electronic and magnetic properties may form at the interfaces of epitaxial heterostructures composed out of complex oxides. A case in point is LaAlO$_3$ (LAO) on TiO$_2$-terminated SrTiO$_3$ (STO), for which a conducting interface has been found if the LAO thickness exceeds 3 unit cells. Although there is growing evidence that the origin is intrinsic and involves a transfer of charge to the interface to compensate the electric potential due to the polar nature of LAO there are a number of open issues. These refer, e.g., to the LAO thickness dependence of the charge carrier concentration, the potential gradient and the role of defects in LAO, the band alignment and bending at the interface, the lateral mobility or confinement of the interface charge carriers etc. We address these questions from the viewpoint of high-energy spectroscopies, i.e. hard x-ray photoelectron spectroscopy and resonant inelastic soft x-ray scattering, both of which we applied for the first time to these materials. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q34.00009: Internal electric field in SrTiO$_3$/LaAlO$_3$ heterostructures probed with hard x-ray photoemission spectroscopy Erik Slooten, Z. Zhong, H. Molegraaf, S. de Jong, F. Massee, E. van Heumen, M. Gorgoi, G. Rijnders, D. Blank, M. Huijben, P. Kelly, M. S. Golden The origin of the conducting layer at the interface between insulating SrTiO$_3$ (STO) and LaAlO$_3$ (LAO) is still widely debated. The alternatingly charged layers within the LAO blocks give rise to an internal electric field, which at some point has to be screened. This built-in potential is predicted to close the LAO bandgap at a critical thickness of 4 layers of LAO. Using hard x-ray photoemission spectroscopy we study the core levels of these systems as a function of the LAO layer thickness. By measuring the La 4d and Al 2s core levels with respect to the Sr 3d core level we carefully determine the core level shifts for samples with 2 to 6 layers of LAO. Although the observed shifts are an order of magnitude smaller than predicted, we do find an interesting increase of the core level shifts for samples with more than 4 layers. We perform DFT slab calculations to show that oxygen vacancies can significantly reduce the potential build-up. Our results suggest that in real materials the electronic reconstruction is pre-empted by other effects of which oxygen vacancies are one possibility. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q34.00010: Interfacial Electronic Properties of LaAlO$_{3}$/SrTiO$_{3}$ superlattice Probed with Hard X-ray Photoemission Di-Jing Huang, Ying-Yi Chu, Jan-Chi Yang, Ku-Ding Tsuei, Wen-Chung Liu, Yen-Fa Liao, Ying-Hao Chu, Jonas Weinen, Stefano Agrestini, Valerio Oliana, Hao Tjeng, C.T. Huang Unexpected electronic and magnetic properties at interfaces between distinct transition-metal oxides have received much attention recently. The electronic phase at the interface of a heterostructure often differs from those of the sandwiching bulks. For instance, the interface between two band insulators LaAlO$_{3}$ and SrTiO$_{3}$ exhibits rich interfacial phases, drastically different from their original bulks. Here we will present measurements of hard X-ray photoemission and x-ray absorption on a LaAlO$_{3}$/SrTiO$_{3}$ superlattice to address the issue of interfacial electron at interfaces. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q34.00011: ARPES studies in LaTiO3/SrTiO3 heterostructures Young Jun Chang, Luca Moreschini, Yong Su Kim, Andrew L. Walter, Davide Innocenti, Aaron Bostwick, Geoffrey A. Gaines, Karsten Horn, Eli Rotenberg Electronic band structure of LaTiO3/SrTiO3 heterostructures was studied using angle resolved photoemission spectroscopy (ARPES). LaTiO3 films, grown by in situ pulsed laser deposition (PLD) on beamline 7.0.1 at Advanced Light Source, exhibit a thickness-dependent phase transition from the correlated metallic interface to Mott insulator. We observed the quasi-particle peak at the Fermi level explaining the metallic interface for ultrathin LaTiO3, and the band gap opened for thick LaTiO3 similar to the bulk. We compare the thickness dependent electronic structure with theoretically calculated phase diagram (S. Okamoto and A. J. Millis, Nature 428, 630 (2004)). [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q34.00012: Direct Spectroscopic Evidence of Charge Reversal at PZT/LSMO Heterointerface Chung-Lin Wu, Pei-Wei Lee, Yi-Chun Chen, Lo Yueh Chang, Chia-Hao Chen, Chen-Wei Liang, Pu Yu, Qing He, Ramamoorthy Ramesh, Ying-Hao Chu At the heterointerface of a top ferroelectric Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ (PZT) ultrathin film and a bottom La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) electrode, we used continuous synchrotron radiation photoelectron spectroscopy (SR-PES) to probe \textit{in situ} and demonstrated that the interfacial charges are reversible and their affected valence-band barrier height becomes modulated on switching the polarization in the top layer. By monitoring the core-level shifting of the buried LSMO layer under continuous illumination of synchrotron radiation, we directly observed a temporal screening of polarization induced by the photon-generated carriers in the top PZT layer. This dynamic characterization of the core-level shifting of the buried layer demonstrates an effective method to probe the electric conduction and ferroelectric polarization of an ultra-thin ferroelectric oxide thin film. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q34.00013: Scanning tunneling microscopy investigation of the electronic structure across LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces Ya-Ping Chiu, Bo-Chao Huang, Jan-Chi Yang, Ying-Hao Chu Atomically controlled polarity discontinuities induced unusual charge states have been found in the model interface between two insulating perovskite oxides, LaAlO$_{3}$ and SrTiO$_{3}$. In this work, by using cross-sectional scanning tunneling microscopy, local and direct evidence of nontrival local structural and electronic information across the heterointerfaces are investigated. A combination of scanning tunneling spectroscopy analysis with atomic resolution across the LaAlO$_{3}$/SrTiO$_{3}$ heterointerface reveals how the oppositely charged atomic planes undergo electronic reconstructions and introduce a built-in electric field across the polar LaAlO$_{3}$ thin films grown on SrTiO$_{3}$ substrates. Further analysis of the related electronic and geometrical properties not only realizes the properties both in bands of electron and hole characters across the LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces but also helps to elucidate the mechanism of the interface conductivity. [Preview Abstract] |
Session Q35: Topological Insulators: Interactions
Sponsoring Units: DCMPChair: Christopher Varney, Georgetown University / University of Maryland
Room: C140
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q35.00001: Fragile Mott Insulators Steven Kivelson, Hong Yao We prove that there exists a class of crystalline insulators, which we call ``fragile Mott insulators'' which are not adiabatically connected to any sort of band insulator provided time-reversal and certain point-group symmetries are respected, but which are otherwise unspectacular in that they exhibit no topological order nor any form of fractionalized quasiparticles. Different fragile Mott insulators are characterized by different nontrivial one-dimensional representations of the crystal point group. We illustrate this new type of insulators with two examples: the d-Mott insulator discovered in the checkerboard Hubbard model at half-filling and the Affleck-Kennedy-Lieb-Tasaki insulator on the square lattice. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q35.00002: Correlation effects in quantum spin Hall states: a Quantum Monte Carlo study Thomas C. Lang, Martin Hohenadler, Fakher F. Assaad We consider a quantum spin hall insulator as realized by the Kane-Mele model with spin orbit coupling $\lambda$ supplemented by a Hubbard $U$ term. On the basis of projective auxiliary field quantum Monte Carlo simulations on lattice sizes up to $ 12 \times 12 $, we map out the magnetic phase diagram. Beyond a critical value of $U> U_c$ the quantum spin Hall insulating state is unstable towards magnetic ordering. At $U < U_c$ we study the spin, charge and single particle dyanmics of the helical edge state by retaining the Hubbard interactions only on the edge of a ribbon. As $U_c$ is approached we observe a substantial depletion of low-lying spectral weight in the dynamical charge structure factor, and a robust signature of the helical edge state in the single particle spectral function. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q35.00003: Quantum Monte Carlo simulations on interaction effects in the 2D Kane-Mele-Hubbard model Dong Zheng, Congjun Wu, Guang-Ming Zhang Interaction effects in topological insulators remain an open question. We have proved that the determinant quantum Monte-Carlo simulation on the two dimensional Kane-Mele model augmented by the Hubbard interaction is free of the sign-problem. Consequentially, the interplay between band topology and strong interaction can be studied at a high numeric precision. The process how the topological band insulator evolves into the antiferromagnetic Mott insulator as increasing interaction strength is studied by calculating both the bulk and edge electronic properties. The possibility of an exotic topological Mott-insulator is examined. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q35.00004: Interactions and doping effects in a topological insulator Stephan Rachel, Karyn Le Hur We investigate the effect of repulsive and attractive onsite interactions on a Quantum Spin Hall Insulator (QSHI). For repulsive interactions, we show that the topological phase is stable up to quite large interactions $U\sim t$ before the system reaches a magnetically ordered phase\,[1]. For attractive interactions, we discuss superconductivity in a doped QSHI and compare it with a doped trivial band insulator. We also consider the effect of spin orbit coupling to zero--mode bound states at vortex cores.\\[10pt] [1] S.Rachel and K.Le Hur, Phys.\,Rev.\,B 82, 075106 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q35.00005: Mott Physics at the Boundaries of Topological Insulators Amal Medhi, Pramod Kumar Verma, Vijay Shenoy, H. R. Krishnamurthy We address how the nature of linearly dispersing edge states of a topological insulating solid evolves with increasing electron-electron correlation engendered by a Hubbard like on-site repulsion. We report studies on strips (2D) and slabs (3D) of varying widths and thicknesses of topological insulators described by model Hamiltonians using an inhomogeneous slave rotor mean-field theory. Motivated by these studies, we construct variational wavefunctions with approriate Gutzwiller-Jastrow correlations and study them using the Monte-Carlo method. These studies reveal the width/thickness dependence of the critical on-site repulsion that obtains an edge Mott insulating state, and uncover the mechanism of the Mott transition in such systems. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q35.00006: Interaction and distortion driven topological phases in multi-band lattices Jun Wen, Mehdi Kargarian, Gregory Fiete In this work we investigate the phase diagram of $5d$ transition metal oxides on the pyrochlore lattice. In particular, the competition between Coulomb interaction, spin-orbit coupling and distortion are discussed. Spin-orbit coupling entangles the spin and $t_{2g}$ orbitals giving rise to doublet $j=1/2$ and quadruplet $j=3/2$ states. While most pervious works discussed the doublet manifold, we focus on the quadruplet manifold which is relevant for several perovskites. Coulomb interaction is taken into account using the slave-rotor mean field theory and we obtain a phase diagram for this model, which includes exotic phases. We extend the model by including lattice distortion which further splits the quadruplet $j=3/2$ manifold. Under a variety of distortions a topological phase is stabilized, and we discuss how the overall phase diagram is altered with lattice distortions. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q35.00007: Magnetic responce in the quantized spin Hall system with electron correlation Jun Goryo, Nobuki Maeda We investigate the magnetic response in the quantized spin Hall (SH) phase of layered-honeycomb lattice system with intrinsic spin-orbit coupling $\lambda_{\rm SO}$ and on-site Hubbard $U$. The response is characterized by a parameter $g= 4 U a^2 d / 3$, where $a$ and $d$ are the lattice constant and interlayer distance, respectively. When $g< (\sigma_{xy}^{s2} \mu)^{-1}$, where $\sigma_{xy}^{s}$ is the quantized spin Hall conductivity and $\mu$ is the magnetic permeability, the magnetic field inside the sample oscillates spatially. The oscillation vanishes in the non-interacting limit $U \rightarrow 0$. When $g > (\sigma_{xy}^{s2} \mu)^{-1}$, the system shows perfect diamagnetism, i.e., the Meissner effect occurs. We find that superlattice structure with large $a$ is favorable to see these phenomena. We also point out that, as a result of Zeeman coupling, the topologically-protected helical edge states shows weak diamagnetism which is independent of $g$. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q35.00008: Electrostatic Effects in Topological Insulators Dimitris Galanakis, Tudor Stanescu We study electrostatic effects in topological insulators generated by non-uniform charge distributions and by external electric fields. The system is modeled using a tight-binding model and the Coulomb interaction is included at a mean-field level within a self-consistent calculation. The self-consistent charge profiles are calculated numerically for both insulating and low density metallic systems. Using this approach, we investigate the bending of the bulk bands due to the presence of surface states and of charged surface impurities and the effect of applying gate voltages to topological insulator films of variable thickness. Our results shed new light on the potential differences between surface- and bulk-sensitive measurements of topological insulators. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q35.00009: Coulomb drag between helical edge states Vladimir Zyuzin, Gregory Fiete We theoretically investigate the Coulomb drag between the edge states of two quantum spin Hall systems. Using an interacting theory of the one-dimensional helical edge modes, we show that the drag vanishes at second order in the inter-edge interaction, where it is typically finite in other systems, due to the absence of backscattering within the edges. However, in the presence of a small external magnetic field, the drag is finite and scales as the fourth power of the magnetic field, a behavior that sharply distinguishes it from other systems. We obtain the temperature dependence of the drag for regimes of both linear and quadratic edge dispersion in the presence of a finite field. This work was financially supported by ARO under Grant No. W911NF-09-1-0527. V. A. Zyuzin and G. A. Fiete, Phys. Rev. B 82, 113305 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q35.00010: Effect of electron-electron interaction on surface transport in three-dimensional topological insulators Hridis Pal, Dmitrii Maslov We study the effect of electron-electron interaction on the temperature dependence of surface charge transport in three dimensional topological insulators. In conventional two dimensional materials at small temperatures, the presence or absence of $T^2$ dependence in the resistivity is found to depend on the Fermi surface geometry- whether it is concave or convex and whether it is simply connected or multiply connected. In the recently discovered three-dimensional topological insulators such as Bi$_2$Te$_3$, Bi$_2$Se$_3$, and Sb$_2$Te$_3$ the Fermi surface of the two dimensional surface states, owing to the underlying lattice symmetry, changes curvature from convex to concave as a function of energy. The contribution from electron-electron interaction is therefore expected to affect the resistivity in these materials which we investigate in this study. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q35.00011: Cooper Pair Injection into Topological Insulators Koji Sato We theoretically study tunneling of Cooper pairs (CP's) from a superconductor spanning a two-dimensional topological insulator strip into its helical edge states. The coherent low-energy electron-pair tunneling sets off positive nonlocal current cross-correlations along the edges, which reflect an interplay of two quantum-entanglement mechanisms. First of all, superconducting spin pairing dictates a CP partitioning into the helical edge liquids, which transport electrons in the opposite directions for opposite spin orientations. Luttinger-liquid (LL) correlations for the electron-density fluctuations are, furthermore, forcing paired electrons to enter into opposite insulator-strip edges, revealing CP spin entanglement in the inter-edge current correlations. At the same time, the LL behavior, in the absence of Fermi-liquid leads, fractionalizes electrons injected at a given edge into counter-propagating charge pulses carrying definite fractions of the elementary electron charge. The superconductivity as well as LL correlations thus introduce positive current cross-correlations, which reveal a wealth of information about both subsystems. Sato, Loss, Tserkovnyak, arXiv:1003.4316v1. To be published in Physical Review Letters [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q35.00012: Instabilities of quadratic band crossing points Stefan Uebelacker, Carsten Honerkamp The variation of the orbital composition of bands around band crossing points near the Fermi level can generate interesting effects. In particular, rather simple interactions can give rise to the spontaneous formation of topological insulating phases (S. Raghu et al., Phys. Rev. Lett. 100, 156401 (2008)). In contrast with Dirac points, quadratic band crossing points offer the advantage of a nonzero density of states at the crossing point, and instabilities occur already at small interaction strengths. Here, we present results of functional renormalization group calculations for models with a quadratic band crossing point and discuss the possibilities for nontrivial insulating phases induced by local interactions. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q35.00013: Fractionalization and topological point defects in the charge-ordered kagome lattice Andreas Ruegg, Gregory A. Fiete The charge-ordered state on the kagome lattice shows some features which are closely related to two-dimensional topological insulators. This motivated us to study a two-dimensional system of spin-polarized fermions on the kagome lattice at filling fraction $f=1/3$ interacting through a nearest-neighbor interaction $V$ using the unrestricted mean-field approach. Above a critical interaction strength $V_c$, a charge-density wave is stabilized. We find that topological point defects in the charge order bind a fractional charge. The value of the bound charge is 1/2 as long as an effective sublattice symmetry is preserved but changes continuously with the strength of the symmetry-breaking field. Moreover, we compute the confinement potential between two fractionally charged defects and argue that the polaron state, formed upon doping the charge-density wave, can be viewed as a bound state of two defects. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q35.00014: Stability of spontaneous quantum Hall state in the Triangular Kondo-lattice model Yasuyuki Kato, Ivar Martin, Cristian Batista We study the behavior of the quarter-filled Kondo lattice model on a triangular lattice by combining a zero-temperature variational approach and finite-temperature Monte-Carlo simulations. For intermediate coupling between itinerant electrons and classical moments ${\bf S}_j$, we find a thermodynamic phase transition into an exotic spin ordering with uniform scalar spin chirality and $\langle {\bf S}_j \rangle=0$. The state exhibits spontaneous quantum Hall effect. We also study how its properties are affected by the application of an external magnetic field. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q35.00015: Quantum corrections to conductivity in topological insulator thin films: Weak localization and electron-electron interaction Ashley DaSilva, Jian Wang, Cui-Zu Chang, Ke He, Xu-Cun Ma, Qi-Kun Xue, Jainendra Jain, Nitin Samarth, Moses Chan We study quantum corrections to transport in topological insulator candidate Bi$_2$Se$_3$, with and without doping with Pb. We study thin films with the expectation that the topological surface states will have substantial contribution to transport. Our observations are not consistent with the theory of diffusive transport of noninteracting electrons, because while the temperature dependence is consistent with weak localization, the magnetoresistance is positive, suggestive of weak anti-localization. We show that the theory including quantum corrections from both electron-electron interaction and disorder is qualitatively correct in all magnetic field directions that we have studied. We mention the implications of our results to the possibility of conducting surface states. [Preview Abstract] |
Session Q36: Focus Session: Graphene Structure, Dopants, and Defects: Nanoparticles
Sponsoring Units: DMPChair: Joshua Robinson, Penn State University
Room: C142
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q36.00001: Manipulating graphene mobility and charge neutral point with ligand-bound nanoparticles as charge reservoir Deqi Wang, Xinfei Liu, Le He, Yadong Yin, Di Wu, Jing Shi In this work, we first demonstrate a significant enhancement in carrier mobility in SiO2-supported graphene decorated with a layer of ligand-bound nano-particles (NPs) such as iron oxide, titanium dioxide, or cadmium selenide acting as a charge reservoir. By transferring charges between graphene and the NP reservoir through the molecules, we show a remarkable reversible tunability in mobility (4,000 -- 19,000 cm2/Vs) in the same device, which unambiguously proves that the charged impurity scattering is the prevailing mechanism for graphene mobility. In addition, the charge neutral point can also be independently tuned over a wide gate voltage range. Finally, we study the thermopower of graphene sample with different mobility. By properly taking account of the high temperature effects, we obtain good agreement between the Boltzmann transport theory and our experimental data. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q36.00002: Imaging grain boundaries in monolayer graphene by transmission electron microscopy Kwanpyo Kim, Zonghoon Lee, William Regan, C. Kisielowski, M. Crommie, A. Zettl Using transmission electron microscopy (TEM), we investigate the structure of grain boundaries in large-area monolayer polycrystalline graphene sheets at micron and atomic length scales. At micron scale, grain boundary mapping is performed by electron diffraction and dark field imaging techniques. The atomic scale imaging by an aberration-corrected ultra-high resolution TEM reveals an alternating pentagon-heptagon structure along the high-angle tilt grain boundary. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q36.00003: Electromagnetic interaction of graphene with nanoparticles: applications to nanoscale imaging spectroscopy and plasmonics L.M. Zhang, A.H. Castro Neto, Michael Fogler Interaction of graphene-covered substrate with a nearby nanoscale particle is studied theoretically. Graphene is shown to induce broadening and frequiency shifts of electromagnetic resonaces (cavity modes) localized near the particle. The effect is strongly enhanced for substrates that possess narrow surface polariton excitations. In turn, the coupling to polaritons modifies the spectrum of graphene plasmons. The theory is applied to model scanning near-field optical microscopy (SNOM) experiments where the role of nanoparticle is played by the sharp tip of the scanned probe. The origin of the extraordinary fine spatial resolution of SNOM is explained and proposals for detecting the novel modes by SNOM in the infrared and THz domains are outlined. Also discussed are other applications, including infrared and Raman scattering from graphene covered by a layer of colloidal nanoparticles. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q36.00004: Interlayer Physics in Few Layer Graphenes Invited Speaker: Few layer graphenes (FLG's) represent a family of materials with physical properties distinct from those of single layer graphene and bulk graphite. Their electronic behavior is determined by the nature of electronic motion between layers and by the interactions of electrons in different layers. This talk reviews our experimental and theoretical work studying aspects of nanoparticle growth on FLG's that are determined by this interlayer physics. We observe and analyze: (1) a systematic film thickness dependence of the surface potential for FLG's deposited on SiO$_{2}$ substrates, (2) a related thickness dependence of the sizes of gold nanoparticles that nucleate on the exposed surface of FLG's and (3) a shape instability for growing nanoparticles formed from low workfunction metals adsorbed on FLG's. Finally we discuss some novel aspects of the interlayer electronic motion that are controlled by the rotational registry of neighboring layers. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q36.00005: Energetics and Electronic Structure of Transition Metal Adatoms and Clusters on Graphene Christopher Porter, David Stroud Using density functional theory (DFT), we calculate both the atomic arrangement and electronic structure of transition metal (TM) adatoms, and clusters of adatoms, on graphene. We use a periodic arrangement of unit cells which typically include about 64 C atoms. For Fe on graphene, we have found that the stable position of the adatom is above the center of a hexagon of C and that most of the relaxation in the graphene occurs in the six C atoms closest to the adatom. We use DFT to map out a potential energy surface for Fe adatoms on graphene at any point in the unit cell, allowing an estimate of the energy barrier for an adatom to hop from one energy minimum to another. We also calculate the lowest energy configurations of pairs and larger clusters of TM adatoms on graphene. Finally, we have calculated the electronic structure and density of states associated with the adatoms and clusters on graphene, and have extended these calculations to spin- dependent properties, using a spin density functional approach. These results should be relevant to electronic and spin transport properties of graphene, both of which are expected to be strongly influenced by TM adatom impurities. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q36.00006: Doping efficiencies and physisorption of small molecules on graphene Alexander Samuels, David Carey Ab initio calculations have been employed to study the doping efficiencies of NO$_{2}$, NO and NH$_{3}$ on graphene. We have used both the local density approximation (LDA) and the generalised gradient approximation (GGA) to obtain the molecular binding energies and have employed the Hirshfield charge transfer method to calculate the charge transfer. Spin polarised calculations were employed for the open shell molecules (NO and NO$_{2})$ and we explored the effects of different adsorption sites and orientations. It was found that for all orientations of the molecule and using both LDA and GGA functionals that the adsorption of NO$_{2}$ results in p type doping of graphene with 0.06 e transferred to the molecule. For NO, LDA calculations show a p type behaviour with 0.03 e transferred per molecule but both n and p type doping of 0.003 -- 0.004 e/molecules is calculated using a GGA functional. Finally for NH$_{3}$ both donor and acceptor behaviour (0.03 -- 0.05 e/molecule) is calculated. In all cases the origin of the doping is related to the relative position of the HOMO and LUMO molecular orbitals with respect to the graphene Dirac point and low energy density of states. The effect of molecular adsorption on electron scattering is also discussed. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q36.00007: Surface Functionalization of Graphene Field Effect Transistors with Polyhistidine-Tagged Proteins Ye Lu, Joseph Mitala, Jong-Hsien Lim, Mitchell Lerner, Zhengtang Luo, Nicholas Kybert, Brett Goldsmith, Bohdana Discher, A.T. Charlie Johnson We have developed a facile and reliable method to covalently functionalize the surface of graphene field effect transistors (FETs) with polyhistidine-tagged proteins We demonstrated success of chemical functionalization by both atomic force microscopy (AFM) and Raman spectroscopy. Additionally, we characterized the electronic properties of graphene FETs at successive functionalization stages. The specificity enabled by such functionalization, along with the two dimensional nature and intrinsic high sensitivity of graphene, facilitates the emergence of graphene as a promising candidate in surface biochemistry research as well as graphene-based biosensor applications. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q36.00008: First principles simulations of molecules and nanostructures subjected to ion irradiation Kalman Varga, Sergiy Bubin, Bin Wang, Sokrates Pantelides In the framework of real-time real-space time-dependent density functional theory complemented with classical molecular dynamics for ions, we have studied the behavior of small molecules and nanostructure fragments, such as graphene sheets, irradiated by charged energetic particles. In particular, we have investigated the importance of electronic excitations and examined the regime when bond breaking (or defect formation) occurs. Based on the microscopic description of these processes, several quantities that are of interest for ion beam physics have been determined, such as the amount of energy transferred to the target system and the distribution of this energy between electronic excitations and vibrational motion. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q36.00009: Point and Line Defect-mediated Binding of Metal Nanoparticles to Graphene Ioanna Fampiou, Ashwin Ramasubramaniam The synthesis of well dispersed, size-controlled metal nanoclusters on carbon supports is highly desirable since such clusters have been shown to possess enhanced catalytic activity and selectivity in a variety of chemical reactions. However, metal clusters interact rather weakly with defect-free carbon supports and can coarsen over time leading to loss of surface area and thence catalytic activity. Defects in carbon supports play an important role in enhancing metal-carbon bonding, thereby reducing the propensity for cluster coalescence. Using a combination of density functional theory and empirical potential simulations, we examine the interaction of metal Pt clusters with point (vacancies, holes) and line defects (dislocations, grain boundaries) in graphene. We compare and contrast the binding energies and diffusivities of clusters bound at defects and on pristine graphene. Our results suggest possible avenues for controlling the dispersion of Pt catalyst clusters on carbon supports via defect engineering. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q36.00010: Molecular Mechanics on Graphene Surface and its Detection Kabeer Jasuja, Nihar Mohanty, Vikas Berry In this talk, we demonstrate that the light-induced reversible mechanical motion of an azo-molecule-tethered on graphene can be sensitively detected electronically by motion-induced molecular-gating of graphene (without external gate). The \textit{in-situ} mechanical actuation of the azo-molecule is shown to redistribute the fermionic density \textit{via} due to the change in the proximity of electron-rich benzene moiety of the azo molecule. The results demonstrate that the ultra-sensitive platform offered by graphene makes it possible to electrically detect molecular-scale mechanics. We envision that this research will enable development of next-generation graphene based actuating systems with applications including FETs, optoelectronic-switches and nano-pistons. [Preview Abstract] |
Session Q37: Focus Session: Graphene Structure, Dopants, and Defects: Transport II
Sponsoring Units: DMPChair: Shaffique Adam, National Institute of Standards and Technology, Gaithersburg
Room: C146
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q37.00001: Quantum motion of electrons and holes in the random puddle landscape of graphene Invited Speaker: The transport properties of graphene, especially close to the Dirac point, have puzzled physicists since its discovery in 2004. Only recently a fairly complete understanding of transport in graphene has emerged [1]. The interplay of disorder, gapless nature of the dispersion, and chirality of the quasiparticles induces the anomalous transport properties of graphene close to the Dirac point. In particular, in presence of long-range disorder the carrier density landscape close to the Dirac point breaks up in electron-hole puddles. In this highly inhomogeneous density landscape the standard theoretical approaches to transport are not valid. I will present a transport theory for graphene, and bilayer graphene, that is able to properly take into account the strong disorder-induced density inhomogeneities. The theory has three main features: {\em 1)} it treats disorder microscopically and can therefore take into account its long-range nature, {\em 2)} it provides a fully quantum mechanical analysis of transport, {\em 3)} it is able to model experimentally relevant sizes. In particular the theory presented can be used to calculate the transport properties in the crossover regime, particularly relevant for graphene, between the ballistic and the diffusive regime. I will present results for single layer graphene and bilayer graphene. In addition I will discuss the transport properties of disordered graphene p-n-p junctions for which the semiclassical approaches are inadequate and the full quantum transport analysis is necessary. \\[4pt] [1] S. Das Sarma, S. Adam, E. H. Hwang, E. Rossi, {\it Electronic transport in two dimensional graphene}, arXiv:1003.4731 (2010), to be published in Rev. Mod. Phys. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q37.00002: Interplay between density inhomogeneity and temperature in graphene transport Qiuzi Li, Euyheon Hwang, Sankar Das Sarma Motivated by recent experimental measurements of the temperature-dependent resistivity in graphene, we study the transport properties in monolayer graphene in the presence of electron-hole puddles induced by charged impurities in the environment. We explain the apparent insulating behavior of temperature-dependent conductivity observed in low mobility samples using an analytic statistical theory, which takes into account the non-mean-field nature of transport in the highly inhomogeneous density and potential landscape. In particular, the existence of puddles allows local activation of carriers in low density samples, mimicking an insulating temperature dependence in graphene conductivity. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q37.00003: Electron scattering in graphene by a correlated charged impurities Michael Fuhrer, Jun Yan, Jianhao Chen, Shudong Xiao We study charge transport in graphene with correlated charged impurities. Potassium is deposited on graphene in ultra-high vacuum at temperatures below 20 K, and the conductivity of graphene is measured as a function of carrier density in situ. Upon heating, the potassium ions order due to repulsive interactions, resulting in significant improvement of device mobility due to decrease of long range scattering. The charge density dependence of the conductivity becomes increasingly non-linear with increase of annealing temperature of the potassium/graphene. We find the experimental carrier-density-dependent conductivity in good agreement with a model of correlated charged impurities including a Gaussian-broadened structure factor at a finite wavevector corresponding to the potassium lattice. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q37.00004: Tunneling between two independently contacted graphene layers Christopher Corbet, Seyoung Kim, David C. Dillen, Babak Fallah, Michael Ramon, Emanuel Tutuc, Sanjay Banerjee We study the tunneling between two overlapped, independently contacted graphene monolayers. We use micromechanical exfoliation to deposit graphene monolayers on separate substrates. Using electron beam lithography (EBL) patterning and etching we isolate the two monolayers and remove the multilayer graphene in their close proximity. Once patterned, one monolayer was removed from the substrate and manually aligned to the other monolayer with an overlap region of a few square micrometers. EBL and metal deposition were used to define hall bars on the two separate monolayers. This design allows the extraction of each sheet's mobility and density using standard four-point resistance measurements. Using a finite element model, we calculate the current flow in each layer, as well as in between the two layers. The tunneling resistance is modeled as a contact resistance between the two graphene layers in this overlap region. We extract an upper limit for the specific tunneling resistance between the two graphene layers of 1.4E-4 Ohms*cm$^{2}$. We discuss the current density and potential dependence on the shape of the overlap region. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q37.00005: Electron Hole Asymmetry in Graphene Coupled to an SiO2 Substrate Robert Higginbotham, Nan Sun, Gerald Arnold, Steven Ruggiero The conductance of graphene generally exhibits an asymmetry in the electron and hole branches. We propose a contribution to this asymmetry that is based upon the coupling between the graphene and an SiO2 substrate. Treating the coupling in the tight-binding approximation, we calculate an exact Green's function for the coupled graphene/SiO2 system. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q37.00006: Electron-hole interference in graphene Atikur Rahman, Janice Wynn Guikema, Soo Hyung Lee, Nina Markovic The crystal symmetry of graphene gives rise to massless Dirac low-energy quasiparticles which are described by a two-component spinor that has contributions from two interpenetrating sublattices. As a result, the electron and hole states are interconnected, in sharp contrast to conventional semiconductors. Through the Aharonov-Bohm effect, we demonstrate that the electrons and holes in graphene exhibit quantum interference with each other. Our device is made of a graphene ring in contact with gold leads. A top gate on one arm of the ring independently controls the carrier type and concentration in that arm, while the back gate acts on both arms. We observe clear Aharonov-Bohm oscillations (overall visibility $\sim$10\%) in the magnetoresistance when the charge carriers are holes in one arm and electrons in the other arm. This indicates phase coherence between the electrons and holes in the two arms of the interferometer. Phase coherence is further substantiated by our observations of $T^{-1/2}$ temperature dependence of the oscillation amplitude. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q37.00007: Measurements of the energy gap in biased bilayer graphene Conor Puls, Ying Liu The application of bilayer graphene in logic-based electronics necessitates the demonstration of reliable bandgap opening, a matter complicated by charge inhomogeneity and midgap states due to local impurities and other disorder. We use dual-gated field effect transistor (FET) and planar tunnel junction devices prepared on mechanically exfoliated bilayer graphene flakes to probe the temperature dependent resistivity and density of states near the charge neutrality point. In both devices, the Fermi level and theoretical bandgap width are simultaneously controlled with a perpendicular displacement field. We report that at high displacement fields and with the Fermi level at the charge neutrality point, the temperature dependence of the resistivity follows a simple thermal activation across a gap width of up to 110 meV at high temperatures. Low temperature transport is dominated by hopping channels whose presence also increase conductivity at high temperatures, reducing the achievable $\sigma _{ON}$/$\sigma _{OFF}$ ratio, a value of great interest for FET devices. We explore the role of charged impurities found in the deposited dielectric in limiting FET performance in this respect. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q37.00008: A Unified Description of the DC Conductivity of Monolayer and Bilayer Graphene Based on Resonant Scatterers Aires Ferreira, J. Viana-Gomes, Johan Nilsson, Eduardo R. Mucciolo, Nuno M.R. Peres, Antonio H. Castro Neto We show that a coherent picture for the dc conductivity of monolayer and bilayer graphene emerges from considering that strong short-range potentials are the main source of scattering in these two systems. The origin of the strong short range potentials may lie in adsorbed hydrocarbons at the surface of graphene. The equivalence between results based on the partial wave description of scattering, the Lippmann-Schwinger equation, and the T-matrix approach is established. Scattering due to resonant impurities close to the neutrality point is investigated via a numerical computation of the Kubo formula using a kernel polynomial method. We find that realistic adsorbates originate impurity bands in monolayer and bilayer graphene close to the Dirac point. In the midgap region, a plateau of minimum conductivity of about $e^2/h$ (per layer) is induced by the resonant disorder. In bilayer graphene, a large adsorbate concentration can develop an energy gap between midgap states and high energy states. As a consequence, the conductivity plateau is supressed near the edges and a ``conductivity gap'' takes place. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q37.00009: Dynamic Screening and Spectral Functions in Bilayer Graphene Rajdeep Sensarma, Euyheon Hwang, Sankar Das Sarma We study the dynamic screening of Coulomb interactions in a bilayer graphene system within Random phase approximation. We derive an analytic expression for the dielectric function of the system and study the dispersion and damping of low energy plasmon modes. The quadratic dispersion and chirality of bilayer graphene systems lead to a plasmon dispersion which is distinct both from 2D electron gas and monolayer graphene plasmons. We also look at the effects of dynamic screening on the single particle spectral function of the system. We determine the quasiparticle weight, the effective mass and the damping of quasiparticles, which give a complete description of the low energy spectral function of the system.The compressibility of the system is also obtained from the self-energy renormalization of the chemical potential. We find that the presence of the second band leads to a well screened effective interaction, leading to much smaller renormalization of the Fermi liquid parameters in comparison to a 2D electron gas. However, the dynamic nature of the screening is very important in obtaining the single particle properties of this system. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q37.00010: Electron transport properties of bilayer graphene Kostyantyn Borysenko$^1$, Jeffrey Mullen$^2$, Xiaodong Li$^1$, Yuriy Semenov$^1$, John Zavada$^1$, Marco Buongiorno Nardelli$^{2,3}$, Ki Wook Kim$^1$ We investigate the role of different phonon scattering mechanisms in determining the electron transport properties of bilayer graphene (BLG). The ever-present electron-phonon interaction imposes the limitations on transport characteristics of any device and thus, must be always taken into account. However, in a realistic device setup, when BLG is laid (or epitaxially grown) on the top of a substrate, extrinsic scattering mechanisms (due to charged impurities, surface polar phonons, etc.) will dominate. The electron coupling with surface polar phonons of the substrate is always present and this scattering mechanism can be dominant. Using first principles approach (density functional perturbation theory) we calculate the electron-phonon matrix elements of BLG and estimate the intrinsic electron scattering rates. We show that the transport properties of the free-standing BLG resemble those of the bulk graphite. Using the Monte Carlo simulation we estimate the low-field mobility and saturation velocity of the free-standing BLG, as well as BLG on various substrates (SiC, SiO$_2$, HfO$_2$). [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q37.00011: Dual Gating of Suspended Graphene Devices via Contactless Gates Jairo Velasco Jr., Lei Jing, Gang Liu, Philip Kratz, Yongjin Lee, Wenzhong Bao, Jeanie Lau Monolayer and Bilayer graphene devices with local electrostatic gates present a rich platform for both academic and application driven inquiry. Realization of the veselago lensing effect and band gap engineering are a few of the most dazzling and promising physical phenomena that these systems are predicted to host. However, a major roadblock in this quest is the strict requirement of exceedingly clean samples. We have developed a method to fabricate suspended top gates above a freestanding graphene flake to address this challenge. Using this technique we demonstrate dual gating of a suspended graphene flake. We will discuss the latest experimental progress towards the electrical transport of such a device in the zero-magnetic field regime, as well as in the quantum Hall regime. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q37.00012: Current Annealing~of Suspended Graphene Atomic Membranes Fenglin Wang, Jairo Velasco Jr., Zeng Zhao, Hang Zhang, Philip Kratz, Lei Jing, Wenzhong Bao, Chunning Lau Using a multi-level lithographical technique, we are able to suspend graphene membranes coupled to vast majority of electrode materials. The device's mobility is significantly improved upon current annealing. By combining transport measurement with in-situ SEM imaging, we are able to monitor morphological changes in graphene and correlate with its current-voltage characteristics, thus optimizing the current annealing process. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q37.00013: Pulsed current-voltage measurements of GFETs Inanc Meric, Cory Dean, Andrea Young, Philip Kim, Kenneth Shepard Pulsed current-voltage measurements are used to measure high-bias characteristics of graphene field-effect transistors (GFET). In contrast to standard DC measurements, current saturation for channel lengths as small as 100 nm is observed when measured by this method. Our results indicate that hot carrier injection into traps in the gate oxide masks saturating characteristics in standard DC measurements. Devices exhibit constant transconductance and output conductance with scaling channel length, despite a variation in low field mobility, supporting a velocity saturation model due to optical phonon scattering. [Preview Abstract] |
Session Q38: Focus Session: Organic Electronics and Photonics -- Morphology in polymer-based solar cells
Sponsoring Units: DMP DPOLY GERAChair: Garry Rumbles, National Renewable Energy Laboratory
Room: A130/131
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q38.00001: Structural measurements of polymer-fullerene blend films for organic photovoltaics Invited Speaker: Organic photovoltaic (OPV) technology has the potential to greatly lower the cost of solar cell fabrication by enabling ink-based deposition of active layers. In bulk heterojunction (BHJ) OPV devices, the power conversion efficiency critically depends on the distribution of the polymer absorber and the fullerene electron acceptor (e.g., the blend morphology). I will describe measurement methods to probe the structure of OPV devices, with a focus on the morphology of the BHJ layer. For example, the vertical distribution of absorber and electron acceptor in BHJ films follows segregation behavior similar to that of miscible polymer blends. The top (air) interface becomes rich in the polymer absorber, whereas the bottom interface composition depends on the substrate surface energy. Thin film transistors fabricated from BHJs can therefore exhibit ambipolar or hole-only transport depending on the dielectric, because of different interfacial segregation. We extend these results to practical photovoltaic devices by comparing BHJs cast upon hole transport layers that have similar work functions but different surface energies. This study includes the application of variable angle spectroscopic ellipsometry (VASE) to BHJ films, and emphasizes the importance of absorber anisotropy and vertical heterogeneity in the optical model. Additional results will describe the nanometer-scale structure in the BHJ interior. The application of solid-state nuclear magnetic resonance (SS-NMR) can reveal details about the segregation of absorber and acceptor in a BHJ film. Nanoscale BHJ morphology information can also be collected using tomographic transmission electron microscopy (TEM). Together these measurements allow us to reveal a detailed picture of BHJ morphology, explain how the morphology originates from materials and processing choices, and relate the morphology to device performance and stability. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q38.00002: Tuning Rod-Rod Interactions in Poly(3-alkylthiophene) Derivatives Bryan Boudouris, Victor Ho, Rachel Segalman Poly(3-alkylthiophene) (P3AT) derivatives are used commonly in polymer semiconducting applications. However, during the coating of P3AT thin films strong intermolecular interactions generally lead to the formation of semiconducting fibers. This prevents the formation of long-range ordered domains and complicates analysis of structure-property relationships in P3AT-containing devices (e.g., organic photovoltaic cells). Here, we show rod-rod interactions can be controlled by rational polythiophene side chain design. The effects of side chain passivation are evidenced by a depressed melting temperature and the presence of a liquid crystalline region. We show also that while the rod-rod interactions are lowered significantly in a polythiophene derivative with a branched side chain relative to straight chain P3ATs, the optoelectronic properties remain approximately constant. Importantly, this reduced melting temperature allows for the real-time evolution of a P3AT crystal structure at room temperature to be monitored on an experimentally convenient time scale. These structural data correlate well with field-effect charge carrier mobility measurements and provide a path for studying the mechanism of ordering in plastic electronics. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q38.00003: Polymer/Polymer Heterojunctions for Ambipolar Charge Transport in Organic Electronics Felix Kim, Selvam Subramaniyan, Samson Jenekhe Understanding of charge transport in polymer semiconductor heterojunctions is of basic interest in developing high-performance organic optoelectronic devices based on multicomponent polymer semiconductors. We report ambipolar charge transport in thin films of layered heterojunctions and bulk heterojunctions of solution-processable unipolar polymer semiconductors. Selective solubility of the polymer semiconductors, poly(thiazolothiazole)s and ladder-type poly(benzobisimidazo-benzophenanthroline), in organic and acidic solvents enabled the sequential deposition or blending of the polymer semiconductors. Charge carrier mobilities of 0.001-0.01 cm2/Vs were observed for both electrons and holes in the polymer/polymer heterojunction field-effect transistors. Thin film deposition and processing with various solvents are effective to improve charge-carrier mobilities by a factor of 100-1000. We have investigated the effects of the processing methods on morphology, and photophysical and charge transport properties of the polymer semiconductor heterojunctions. Integrated circuits and solar cells based on the polymer semiconductor heterojunctions are also demonstrated. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q38.00004: The role of donor-acceptor intermixing in the performance of polymer-polymer OPVs Eleni Pavlopoulou, Stephanie Lee, Chang Su Kim, Yueh-Lin Loo, Zhihua Chen, Antonio Facchetti, Michael F. Toney We investigated the effect of donor-acceptor intermixing in bulk-heterojunction active layers on device performance of polymer-polymer organic photovoltaics (OPVs). Poly(3-hexylthiophene) (P3HT) was blended with poly{\{}[N,N'-bis(2-octyldodecyl) -naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene){\}} (PNDI) and P3HT/PNDI films were spin-cast from dichrolobenzene, a good solvent for PNDI; chlorobenzene, a good solvent for P3HT; and xylene, a bad solvent for both. The short-circuit current densities and device efficiencies vary with casting solvent quality; devices with active layers cast from xylene exhibit the highest efficiencies while those cast from dichlorobenzene the lowest. Grazing Incidence X-ray Diffraction show that intermixing on a molecular scale increases with decreasing dissolution of the polymers in the parent solutions. Accordingly, increasing intermixing enhances device efficiencies. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q38.00005: Phase purity in organic solar cells Brian Collins, Eliot Gann, Lewis Guignard, Xiaoxi He, Christopher McNeill, Harald Ade To date, the device function of organic bulk heterojunction solar cells has been commonly interpreted to arise from two interpenetrating, phase-separated donor and acceptor materials with charge separation of excitons occurring at discrete interfaces. However, little attention has been paid to phase purity and the consequences of a mixed phase on the operation of devices. To probe this possibility and its implications, the miscibility of common fullerenes in (3-hexylthiophene) (P3HT) and a number of new low bandgap polymers including PCDTBT have been measured directly via x-ray absorption spectroscopy in a scanning transmission x-ray microscope on films brought to thermodynamic equilibrium. A mixed amorphous phase is always observed, along with a fullerene-rich phase and possibly a pure crystalline polymer phase if the polymer is able to crystallize. For example, grazing incidence x-ray scattering shows no intercalation of fullerenes into P3HT crystallites, while amorphous portions of the polymer contain $\sim $20 wt.{\%} of the fullerene. In fact, all systems tested to date have failed to exhibt a pure amorphous polymer phase, suggesting that the device paradigm of pure phases and discrete interfaces requires modification. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q38.00006: Diffusion rates and crystallization of phenyl-C61-butyric acid methyl ester in poly(3-hexylthiophene) L. Guignard, B. Collins, J. Seok, H. Ade Bulk heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are an important model system for studying organic solar cell operation. Recent experiments reveal that PCBM is partially miscible in the amorphous regions of P3HT [1-3], implying that P3HT:PCBM devices have three phases. The miscibility depends on temperature and regioregularity of the P3HT. To better understand the influence of P3HT regioregularity and molecular weight on the the P3HT:PCBM system, diffusion rates of PCBM are determined as a function of regioregularity of P3HT and temperature with visible light microscopy by analyzing the growth of the PCBM depletion region near PCBM crystals or agglomerates. Lower diffusion constants are found for less crystalline regiorandom P3HT than highly regioregular P3HT. The shape and growth behavior of PCBM crystal or agglomerate is also found to vary greatly. \\[4pt] [1] B. Watts\textit{ et al.}, Macromolecules \textbf{42}, 8392 (2009) \\[0pt] [2] B. A. Collins\textit{ et al.}, J. Phys. Chem. Lett \textbf{1}, 3160 (2010) \\[0pt] [3] J. W. Kiel\textit{ et al.}, Phys. Rev. Lett. \textbf{105}, 168701 (2010) and N. D. Treat\textit{ et al.}, Adv Funct Mater [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q38.00007: Structural evolution in polythiophene-fullerene mixtures Enrique Gomez, Derek Kozub, Kiarash Vakhshouri The morphology of organic semiconductor mixtures employed as the active layer of organic solar cells is a result of the complex interplay between the crystallinity of the constituents and the chemical incompatibility. Given that device performance can depend critically on the morphology of the active layer, efforts aimed identifying at the critical parameters for the structure formation process are important for the development of high-performance devices. We demonstrate that polythiophene-fullerene mixtures are partially miscible and that the crystallization of the electron donor drives the characteristic length scales of the structure. By modeling fullerene as a solvent for polythiophene, we have estimated the Flory-Huggins interaction parameter from measurements of the melting point depression of polythiophene. The miscibility between poly(3-hexylthiophene) (P3HT) and fullerene at P3HT volume fractions greater than 0.4 leads to a severe suppression of the crystallization of fullerene. Our efforts have enabled us to develop a hypothesis for the structure formation process in polythiophene/fullerene mixtures. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q38.00008: Miscibility Study of PCBM/P3EHT Organic Photovoltaics via Small Angle Neutron Scattering Wen Yin, Bryan McCulloch, Rachel Segalman, Mark Dadmun Organic photovoltaics (OPV) attracted considerable interest as lightweight, inexpensive, and easily processable replacement of inorganic photovoltaics. Current results indicate that the morphology of these photovoltaic materials is essential to their solar energy conversion efficiency but a detailed and fundamental understanding is absent. In this paper, the miscibility and structure of P3EHT/PCBM composites with varying PCBM loading level are investigated via small angle neutron scattering (SANS). With P3EHT having a melting temperature below 100\r{ }C, SANS experiments of the blends are conducted above the melting point to unequivocally determine the miscibility of PCBM and P3EHT without the added complexity of polymer crystals. Our SANS results show that blends with 20 and 50 wt{\%} PCBM exhibit dramatically larger scattering at low-Q regime relative to 10 and 15wt{\%} PCBM samples. This result implies that the miscibility limit of PCBM and P3EHT lies between 15:85 and 20:80. Further analysis is underway to correlate these results to OPV efficiency. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q38.00009: Characterization and Improvement on the Morphology in Polymer-Based Solar Cells Hao Shen, Wenluan Zhang, Brett Guralnick, Michael Mackay, Brian Kirby, Charles Majkrzak Polymer-based solar cells are promising for their cost-effective solar energy, yet this technology is still far from practical application owing to its low energy conversion efficiency. It has been known that the morphology in the active layer, or the nano-scaled intermixing between the polymer and fullerene derivative, is critical to the device performance. We have quantitatively measured the morphology in one of the most-studied polymer-based solar cells consisting of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), by means of neutron and x-ray scattering techniques. In particular, the effects of thermal and co-solvent-assisted annealing on the PCBM cluster formation and vertical distribution are characterized. Basing on the observations, we are proposing a new design of solar cell architecture to approach a more controlled morphology in the active layer, by utilizing a thermodynamically-driven assembly of fullerenes onto the surface of silica microspheres. This presentation will focus on its application in the P3HT:PCBM system. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q38.00010: Investigation of fullerene concentration profile in polymer based solar cell by using magnetic contrast neutron reflectivity Wenluan Zhang, Brian Kirby, Michael Mackay Poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b] thiophene) (pBTTT) has recently caused great interest as the electron donor in organic photovoltaics. It was demonstrated that fullerene molecules intercalate between side-chains of this semiconducting polymer creating a stable crystalline structure, so, a large concentration of fullerene must be used relative to the polymer to promote phase separated electron conductive pathways. We used state-of-the-art neutron reflectivity methods, with the application of magnetic contrast variation, to investigate the concentration profile of [6,6]-phenyl-C$_{71}$- butyric acid methyl ester(PC$_{71}$BM) in order to understand the internal structure within the active layer. The PC$_{71}$BM concentration profile greatly depends on the weight ratio of polymer to fullerene. XRD and other data are also used to show the morphology change of active layer. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q38.00011: Correlation of Structure and Roughness with Fabrication Conditions of P3HT-PCBM Bilayer Interfaces with X-Ray Reflectometry Stuart Kirschner, Ming-Ling Yeh, Nathaniel Smith, Howard Katz, Daniel Reich Organic semiconductors, including poly(3-hexylthiophene) (P3HT) and polymer-phenyl-C61-butyric acid methyl ester (PCBM), are considered as promising materials for applications such as photovoltaics, transistors, sensors, thermoelectrics, optoelectronics, and magnetoelectronics. In many cases, the interface plays a crucial role in device performance and in determining the origins of many effects. In this research, neat bilayers of P3HT-PCBM, and PCBM blended with polystyrenes, were studied with X-ray reflectometry (XRR), atomic force microscopy, and ultraviolet-visible spectroscopy. A polymer with a high atomic number element was included to improve the scattering length density contrast, and provided improved XRR resolution. A mobility of order 10$^{-4}$ cm$^{2}$/V*s was maintained. The effect of different annealing, solvent, spin coating, and other fabrication conditions, was explored. Applicability of XRR to study interface characteristics, in these systems, will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q38.00012: Polymer Solar Cells, Deconstructed Yueh-Lin Loo, Jongbok Kim, He Wang, Stephanie Lee, Zelei Guan, Antoine Kahn Soft-contact lamination and delamination has enabled us to construct polymer solar cells for testing, and deconstruct them subsequently for structural characterization of the active layers and electronic characterization of relevant charge transfer interfaces. We have thus been able characterize buried active layers and interfaces that are otherwise inaccessible. Structural characterization post-device fabrication and testing reveals simultaneous crystallization of the polymer donor and the electron acceptor in the once-buried bulk-heterojunction active layers to be responsible for photocurrent enhancement in these devices. Electronic characterization of the active layer-bottom electrode interface reveals an electronic band gap of 1.5 eV, attributable to the difference between the ionization potential of the polymer donor and the electron affinity of the electron acceptor. This value is significantly larger than the band gap predicted by examining the energy levels of the individual constituents, likely due to the presence of interfacial dipoles when the polymer donor and the electron acceptor are intimately mixed. [Preview Abstract] |
Session Q39: Information Processing in Biological Systems
Sponsoring Units: GSNP DBPChair: Madan Rao, Raman Research Institute
Room: A124/127
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q39.00001: TBD Invited Speaker: This abstract not available. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q39.00002: Adaptive processing of natural signals in the fly peripheral visual system Liming Zhou, Rob de Ruyter van Steveninck For modest light intensity variations, fly photoreceptors and their postsynaptic targets, the Large Monopolar Cells (LMCs) behave approximately linearly. In this linear and stationary regime, signal transmission is described by a combination of impulse response and noise autocorrelation function. But natural visual signals often show fast and large intensity variations, and cells adapt to cope with such strong variations. As a result, responses to small contrast perturbations are still linear, but the system is no longer stationary. We study signal transfer under these conditions by measuring responses to small pseudorandom contrast perturbations that ride on large cyclically repeated intensity fluctuations. Those measurements allow us to describe signal transmission by matrices representing nonstationary analogs of the impulse response and the noise autocorrelation function. This description makes it possible to quantify information transmission as the system is continuously adapting to large intensity fluctuations, and to study trade off in adaptation and reliable information transmission in a natural context. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q39.00003: Optogenetic dissection of neural circuit underlying locomotory decision-making in Caenorhabditis Elegans Askin Kocabas, Zengcai Guo, Sharad Ramanathan Despite the knowledge of the physical connectivity of the entire nervous system of C.elegans, we know little about how neuronal dynamics results in decision-making. Detailed understanding of functional and dynamic relations of the neural circuitry requires spatiotemporal control of the neuronal activity. Recent discoveries of light gated ion channels have allowed temporal optical control of neural activity. However, excitation of a specific neuron from among many expressing the channel has been a challenge. By combining optogenetic tools, micro mirror array technology and fast real time image processing, we have developed a technique to activate specific single or multiple neurons in an intact crawling animal while tracking its behavior. Using this setup we traced the neural pathway controlling the gradual turning of the animal during the locomotion. We found that the activity of a specific neuronal circuit that receives inputs from sensory neurons is coordinated with head movement. This coordination allows the animal to turn left or right based on the variation of sensory stimulus during head movement. By directly modulating the activity of the neural circuit, we can force the animal to turn in a specific direction independent of sensory stimuli. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q39.00004: Physical limits to concentration sensing in biochemical signaling Nicholas Licata, Sima Setayeshgar In many biological systems, signals are carried by changes in the concentration of diffusable molecules which are transduced by receptors. It has been demonstrated experimentally that many signaling systems, from regulation of gene expression during development to bacterial chemotaxis, operate with remarkable sensitivity as indicated by a reliable response to small fractional changes in concentration. This sensitivity has contributions from an irreducible noise arising from the inherent random nature of the diffusing input signal, as well as from the chemical measurement process. By explicitly evaluating these theoretically derived contributions for the experimentally well- characterized bacterial chemotaxis network and motor response, we show that they are comparable to within factors of order unity, consistent with the observation that the measurement error approaches the physical lower limit set by diffusion. We extend our analysis to a class of ligand-gated ion channels, demonstrating the generality of this result where accuracy is especially important for the signaling system. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q39.00005: Spatial phase patterns in locally coupled Kuramoto oscillators with repulsive interactions Zahera Jabeen, Michael Giver, Dapeng Bi, Bulbul Chakraborty Recent experiments in microfluidic arrays of interacting Belousov-Zhabotinsky droplets, which belong to the class of active emulsions, show a rich variety of spatial patterns [J. Phys. Chem. Lett. 1, 1241-1246 (2010)]. The predominant coupling between these droplets is inhibitory. Motivated by this experimental system, we study repulsively coupled Kuramoto oscillators with nearest neighbor interactions on a triangular lattice in two dimensions. We show that the geometry of the lattice constrains the phase difference between two neighboring oscillators to $2\pi/3$. We report the existence of domains with either clockwise or anticlockwise helicity, leading to defects in the lattice. We study the time dependence of these domains and show that at large coupling strengths the domains freeze due to frequency synchronization. A variant of this model, in which amplitude variations are introduced by an additional Ising-like coupling between the oscillators, explores the strong coupling limit phenomenon in the experimental system. We discuss these results in the context of the experiments. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q39.00006: Monte-Carlo Study of Axonal Transport in a Neuron Uttam Shrestha, Clare Yu, Zhiyuan Jia, Robert Erickson, Steven Gross A living cell has an infrastructure much like that of a city. A key component is the transportation system that consists of roads (filaments) and molecular motors (proteins) that haul cargo along these roads. We will present a Monte Carlo simulation of intracellular transport inside an axon in which motor proteins carry cargos along microtubules and are able to switch from one microtubule to another. The breakdown of intracellular transport in neurons has been associated with neurodegenerative diseases such as Alzheimer's, Lou Gehig's disease (ALS), and Huntingdon's disease. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q39.00007: Phase and frequency entrainment in locally coupled phase oscillators with repulsive interactions Michael Giver, Zahera Jabeen, Bulbul Chakraborty Recent experiments in one and two-dimensional microfluidic arrays of droplets containing Belousov - Zhabotinsky reactants show a rich variety of spatial patterns [J. Phys. Chem. Lett. 1, 1241-1246 (2010)]. These experiments provide the first steps towards creating easily reproducible model active emulsion systems. Motivated by this experimental system, we study repulsively coupled Kuramoto oscillators with nearest neighbor interactions on a linear chain as well as a ring in one dimension. We show using linear stability analysis as well as numerical study, that the stable phase patterns depend on the geometry of the lattice and that a transition to the ordered state does not exist in the thermodynamic limit. We will also present results comparing our Kuramoto model with finite element simulations of the Brusselator model in geometries similar to those of the experiment. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q39.00008: Simulation studies of creeping flexible nematogens: flocking and rectifying barriers Adam Nicholas, Robin Selinger Recent simulation studies of active nematics have focused on rigid rods or swimming bacteria undergoing collisions via simple rules. Here we present a more physically detailed model of self-propelled creeping flexible nematogens. Each segmented ``worm'' is represented as nine interaction sites connected by springs. The springs' equilibrium length is modulated, causing each worm to elongate and contract periodically. Each worm alternately grips the substrate at its leading or trailing end, producing creeping locomotion. Inter-worm interactions are described via the Weeks-Chandler-Anderson potential between nearby interaction sites. Random forces and damping are also added. For worms that reverse crawling direction at random intervals, we observe a homogeneous nematic phase, and study its behavior in the presence of a rectifying barrier. For worms that move only in a single direction, we observe flocking behavior characterized by evolving stripes of densely crowded particles interspersed with low-density regions. We compare these results with relevant experiments and related theory/simulation approaches. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q39.00009: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q39.00010: Glass-like dynamics in collective cell migration Thomas Angelini, David Weitz The collective movement of tissue cells is essential to fundamental biological processes in both health and disease, and occurs throughout embryonic development, during wound healing, and in cancerous tumor invasion. Most knowledge of cell migration, however, comes from single cell studies. Single cells migrate by executing cyclic processes of extension, adhesion, and retraction, during which the cell body fluctuates dramatically and the cell changes direction erratically. These sub-cellular motions must be coupled between neighbors in confluent layers, yet the influence of this coupling on collective migration is not known. In this talk we present a study of motion in confluent epithelial cell sheets. We measure collective migration and sub-cellular motions, covering a broad range of length-scales, time-scales, and cell densities. We find that that collective cell migration exhibits many behaviors characteristic of classical supercooled particulate fluids, including growing dynamic heterogeneities in the migration velocity field, non-Arrhenius relaxation behavior, and peaks in the density of states analogous to the Boson peak. These results provide a suggestive analogy between collective cell motion and the dynamics of supercooled fluids approaching a glass transition. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q39.00011: Active Chemical Thermodynamics promoted by activity of cortical actin Bhaswati Bhattacharya, Abhishek Chaudhuri, Kripa Gowrishankar, Madan Rao The spatial distribution and dynamics of formation and breakup of the nanoclusters of cell surface proteins is controlled by the active remodeling dynamics of the underlying cortical actin. To explain these observations, we have proposed a novel mechanism of nanoclustering, involving the transient binding to and advection along constitutively occuring ``asters'' of cortical actin. We study the consequences of such active actin-based clustering, in the context of chemical reactions involving conformational changes of cell surface proteins. We find that the active remodeling of cortical actin, can give rise to a dramatic increase in efficiency and extent of conformational spread, even at low levels of expression at the cell surface. We define a activity temperature ($\tau_a$) arising due to actin activities which can be used to describe chemical thermodynamics of the system. We plot TTT (time-temparature-transformation) curves and compute the Arrhenius factors which depend on $\tau_a$. With this, the active asters can be treated as enzymes whose enzymatic reaction rate can be related to the activity. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q39.00012: Brown movement in complex asymmetric periodic potential under the influence of colored noise Mikhail Sviridov, Sergey Guz The idea of the molecular motor in an asymmetric periodic potential is a well-known problem. The motion of a Brownian particle is often studied when the system is subjected to the action of white noise. In practical situations noise is colored (``red'') random process. The red noise is the Ornstein-Uhlenbeck process. In this work we consider noise when the spectral density of the external noise is equal to zero on the zeroth frequency. In our previous works such a noise is been called as ``green'' noise. For the analytical study of green noise action, we use an approach based on a Krylov-Bogoliubov averaging method which is modified to study the action of noise with arbitrary intensity. A certain effective potential can be built which determines the basic features of the system dynamics. Further, we compare two numerical cases. The first one is the time-derivative of the Ornstain-Uhlenbeck process (green noise). The complex potentials when the system does not work as a molecular motor in the case of red noise, i.e. the average motion of the particle does not exhibit a drift in a given direction. If green noise operates on the same system, it turn out the effective molecular motor. We demonstrate this fact by a histograms for realizations of these processes. [Preview Abstract] |
Session Q40: Theoretical Methods and Algorithms for Chemical Physics
Sponsoring Units: DCPChair: Donald Truhlar, University of Minnesota
Room: A122/123
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q40.00001: Primordial Particles; Collisions of Inelastic Particles George Sagi Three-dimensional matter is not defined by Euclidian or Cartesian geometries. Newton's and Einstein's laws are related to the motions of elastic masses. The study of collisions of inelastic particles opens up new vistas in physics. The present article reveals how such particles create clusters composed of various numbers of particles. The Probability of each formation, duplets, triplets, etc. can be calculated. The particles are held together by a binding force, and depending upon the angles of collisions they may also rotate around their center of geometry. Because of these unique properties such inelastic particles are referred to as primordial particles, Pp. When a given density of Pp per cubic space is given, then random collisions create a field. The calculation of the properties of such primordial field is very complex and beyond the present study. However, the angles of collisions are infinite in principle, but the probabilities of various cluster sizes are quantum dependent. Consequently, field calculations will require new complex mathematical methods to be discovered yet. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q40.00002: Aggregation of Thermal Particles in Simulation Iat Neng Chan Based on the Schrodinger Equation, energy levels are evaluated for charged particle or atom surrounded by few atoms imitated to atomic cavity situations under multipole or Lennard-Jones interactions. To examine the states of corresponding eigenvalues, the associated wave functions from simulation are plotted in three-dimension to elucidate the space distribution of particles. In cases for testing on effect of different adjacent atomic structures, concentration region of distribution is revealed from a series of results. The range of localization shown also is affected by the type and strength of interactions between particles and atoms, besides the number and position of surrounding atoms. The thermal effect considered in the computation is modeled by average over results from random fluctuation of atom positions for a given heating grade. Moreover, analysis with fuzzy conditions is applied to reduce the complicated and time-consumption approach, also for the training in science education. Even the investigation is limited and tentative, qualitative studies on different parameters and structures can provide the influence of factors and approximate information to compare with the experience evidences. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q40.00003: Many Body Density Matrix Theory C.J. Tymczak One fundamental limitation of quantum chemical methods is the accuracy of the approximate many-body theoretical framework. Accurate many-body formalisms for quantum chemical methods do exist, but these methods are computationally very expensive. Methods also exist that are much less computationally expensive such as Hatree-Fock, Density Functional and the Hybrid Functional theories, but at a reduced representation of the exact many-body ground state. This severely limits either the system size that can be addressed accurately, or the accuracy of the representation. What is needed is a method that represents the many-body ground states accurately, but with a low computational cost. Recently, a method for determining the response, to any order of the perturbation, within the density matrix formalism has been discovered. This method opens up the possibility of computing the variational many-body ground states to unprecedented accuracy within a simplified computational approach. We report on the theoretical development of this methodology, which we refer to as Many Body Density Matrix Theory. This theory has many significant advantages over existing methods. One, its computational cost is equivalent to Hartree-Fock or Density Functional theory. Two it is a variational upper bound to the exact many-body ground state energy. Three, like Hartree-Fock, it has no self-interaction. And four, it is size extensive. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q40.00004: Probing the Surface-to-Bulk Transition: A Closed-Form, Constant-Scaling Algorithm for Computing Subsurface Green Functions Matthew Reuter, Tamar Seideman, Mark Ratner A closed-form algorithm for computing subsurface Green functions---the blocks of a material's Green function between the surface and the bulk---is presented, where we assume the system satisfies a common principal layer approximation. By exploiting the block tridiagonal and nearly block Toeplitz structure of the Hamiltonian and overlap matrices, this method scales independently of the system size (constant scaling), allowing studies of large systems. As a proof-of-concept example, we investigate the decay of surface effects in an armchair graphene nanoribbon, demonstrating the persistence of surface effects hundreds of atomic layers ($\sim$0.5 $\mu$m) away from a surface. We finally compare the surface-to-bulk transitions of finite and semi-infinite systems, finding that finite systems exhibit amplified surface effects. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q40.00005: Projecting the phase-space trajectory of multidimensional non-equilibrium systems onto a discrete set of states: a Projective Dynamics approach Katja Schaefer, M.A. Novotny Phase-space trajectories, which are either continuous or possess small discontinuities, can be projected onto a discrete set of states with nearest neighbor coupling. The pointwise projection leads for non- equilibrium system to a non-Markovian process, even if the dynamics of the original system is Markovian. However, using time-averaged transition-rates a Markov process can be obtained, which has the same overall properties as the original dynamics of the system. The projected process defines a new dynamics, which only in the limit $t\rightarrow \infty$ obtains the property on the time-scale of the averaging procedure. We demonstrate the Projective Dynamics method in theory and applications to absorption processes, which in general are not describable through equilibrium or steady-state models. We show the discrete set of states $\{\zeta_k\}$ can be chosen arbitrarily (with slight restrictions) for all systems. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q40.00006: Finding lowest saddle point Qing Lu, Minghai Li, Akihiro Kushima, Xi Lin A history-penalized basin filling algorithm is presented in this work which identifies the lowest saddle point starting from any given initial state on any given potential energy hypersurface. The natural analogy of this algorithm is filling a barrel with water; by monitoring the location where leakage occurs one identifies the lowest opening on the wall of the barrel. The successful implementation of this algorithm relies on insightful choices of the penalty function, penalty function combination, and peak refinement. Several types of penalty functions are implemented to study two classical systems, the ad-cluster surface diffusion and supercooled binary Lennard-Jones liquid, and one quantum system of the topological soliton migration. The most efficient penalty function is found to be a triangle penalty function with uniform forces and large 3N+1-dimensional volume. The combination of penalty functions dramatically improves the computational efficiency. The lowest saddle point can be precisely located by the basin filling algorithm coupled with a few standard peak-refinement methods. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q40.00007: The Reaction of Carbon Dioxide with Water Clusters: an Ab Initio Metadynamics Study Gregoire Gallet, Fabio Pietrucci, Wanda Andreoni Simulations are often invoked as aid to understand and optimize carbon capture and sequestration processes. The hydration of carbon dioxide (CO$_{2})$ offers an excellent test case for assessing the reliability of computational schemes. We present a density-functional-theory study of the reaction of CO$_{2}$ with water clusters. The first step was to validate DFT results in different approximations of the exchange and correlation functional with respect to quantum chemical methods for the structure, binding energies and vibrational frequencies of several isomers. Next, simulations of the reactions leading to the formation of carbonic acid were performed using metadynamics as accelerating procedure. This method allows us both to identify the reaction mechanisms and to obtain an estimate of the free energy barriers via the reconstruction of the free energy profiles. Comparisons were drawn with previous static calculations of the barriers. As reference, a similar calculation in liquid water will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q40.00008: Triplet Pairing and Odd-Electron Densities in Constrained-Pairing Mean-Field Theory Jason K. Ellis, Carlos A. Jimenez-Hoyos, Gustavo E. Scuseria Describing strong (also known as static or non-dynamical) correlation caused by degenerate or nearly degenerate orbitals near the Fermi level remains a theoretical challenge, particularly in molecular systems. Constrained-pairing mean-field theory (CPMFT) has been quite successful capturing the effects of static correlation in bond formation and breaking in closed- shell molecular systems. This method uses singlet electron entanglement to model static correlation at \textit{mean field} cost. The present work extends the previous formalism to include triplet pairing, allowing a description of same-spin correlation and open-shell species. Additionally, a spin-orbital extension of the ``odd-electron'' formalism of Yamaguchi and co-workers is presented as a method for understanding triplet radical character in molecules. Results from representative systems are presented. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q40.00009: Dynamical Mean-Field Theory for Quantum Chemistry Nan Lin, Chris Marianetti, Andrew Millis, David Reichman The dynamical mean-field concept of approximating an unsolvable many-body problem in terms of the solution of an auxiliary quantum impurity problem, introduced to study bulk materials with a continuous energy spectrum, is here extended to molecules, i.e. finite systems with a discrete energy spectrum. Application to chains and small clusters of hydrogen atoms yields ground state energies which are competitive with leading quantum chemical approaches at intermediate and large interatomic distances, and provides good approximations to the excitation spectrum. The method is a promising approach to the strong correlation problems of quantum chemistry. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q40.00010: Constrained Active Space Unrestricted Mean-Field Approaches for Controlling Spin-Contamination Takashi Tsuchimochi, Gustavo E. Scuseria We have recently shown that unrestricted Hartree-Fock (UHF) variationally reduces to high-spin restricted open-shell Hartree-Fock when constraints are imposed on the eigenvalues of the spin density matrix [T. Tsuchimochi and G. E. Scuseria, J. Chem. Phys. {\bf 133}, 141102 (2010)]. We here generalize these ideas and propose to control spin-contamination in UHF by releasing the constraints in an active space while imposing them elsewhere. If the active space is properly chosen, our constrained UHF (CUHF) method greatly benefits from a controlled broken-symmetry effect while avoiding the massive spin contamination arising in the traditional UHF. We apply L$\rm{\ddot{o}}$wdin's projection operator to CUHF and obtain multireference wave functions with moderate computational cost. We report results on singlet-triplet energy gaps to show that our constrained scheme outperforms fully unrestricted methods. This constrained approach can be readily used in Kohn-Sham (KS) density functional theory with similar favorable effects, provided that spin-contamination is given by the KS orbitals as in UHF. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q40.00011: Nuclear quantum effects using selective mode excitation in water Sriram Ganeshan, Marivi Fernandez-Serra Recently, Ceriotti et. al. [1] introduced a comprehensive framework to use a custom-tailored Langevin equation with correlated-noise in the context of MD simulations. One of the interesting applications of these thermostats is that, such a framework can be used to selectively excite normal modes whose frequency falls within a prescribed, narrow range [2]. The general idea of this work is to understand whether, the selective excitation of modes in some systems like water is enough to reproduce the necessary nuclear quantum effects at a given temperature. Ceriotti et. al has also implemented their colored noise thermostat (Langevin) to the PIMD of TIP4P/F model [3]. In this work we study how the TIP4P/f responds to the selective mode excitation using the delta-thermostats. We apply this delta thermostat to the molecular dynamics of TIP4P/F [4] water force field, a model explicitly fitted with the lack of zero point ionic vibrations. TIP4P/F provides us an ideal platform to study the effect of selective mode excitation on water. We address the question of whether selective mode excitations are enough to generate the nuclear quantum effects in water. This work will also provide a way to identify the dominant modes for which the quantum effects are important. [1] Chem. Theory Comput.6, 1170 (2010) [2] Proc. Comp. Sci. 1, 1601 (2010), [3] J. Chem. Phys. 131, 024501 (2009), [4] J. Chem. Phys 133, 124104 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q40.00012: \textit{Ab Initio} Composite Methods Angela Wilson, Wanyi Jiang, Gbenga Oyedepo, Marie Laury In this brief presentation, we highlight recent developments of the \textit{ab initio} composite method, the correlation consistent Composite Approach (ccCA). Recent work has enabled ccCA to be utilized for 3d transition metals, as well as for species for which a multireference wavefunction is required. We overview the development, as well as applications of the method to the prediction of spectroscopic and thermodynamic properties of molecules. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q40.00013: Distortion of scanning-tunnelling-spectroscopy images of isolated molecules induced by electron correlation Massimo Rontani, Dimitrios Toroz, Stefano Corni Scanning tunnelling spectroscopy (STS) visualizes electron states in both extended systems and nano-objects, as quantum dots and molecules. Whereas bulk quantum states are insensitive to electron number fluctuations, an energy gap opens each time a new electron is injected by the STS tip into a sufficiently small system. This gap originates from the interaction of the next incoming electron with the others already present in the system. In this Coulomb blockade regime a fundamental question is whether the wave function of the ``quasi-particle'' added to the system -imaged by the STS tip- is sensitive to electron-electron interaction. Here we show that the STS images of single planar molecules with metal centres predicted by ab initio many-body calculations differ qualitatively from their uncorrelated counterparts. We find in the maps resolved at the Fermi energy that correlation significantly removes spectral weight from the metal atom, as well as the overall weight is remarkably reduced. This change may be measured and compared with STS images of molecules without the metal center, whose many-body and uncorrelated versions are alike. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q40.00014: Maximizing the hyperpolarizability poorly determines the potential Rolfe Petschek, Timothy Atherton, Joseph Lesnefsky, Greg Wiggers Increasing the non-linear response of materials to an electric field, characterized by quantities such as the first hyperpolarizibility $\beta$, is a mattter of importance for applications. We optimized the zero frequency $\beta$ of a one-dimensional potential well containing a single electron by freely adjusting the shape of that potential. It is shown that with careful optimization the maximum hyperpolarizability converges quickly with increasing numbers of parameters in the potential to approximately 0.708951 of the proven upper bound. The Hessian of $\beta$ at the maximum makes it clear that there is a very wide range of nearby, nearly optimal potentials: with several measures of differences between potentials, this Hessian has only two large eigenvalues with the others diminishing quickly. The optimum potentials are substantially different and more affected by small eigenvectors than the wavefunctions. Thus, wavefunctions are superior for describing the conditions that optimize the hyperpolarizability. Prospects for a concise description of the two important constraints on near-optimum potentials and wavefunctions are discussed. [Preview Abstract] |
Session Q41: Focus Session: Fundamental Issues in Interfacial Charge Transport for Energy Applications II
Sponsoring Units: DCPChair: Victor Batista, Yale University
Room: A115/117
Wednesday, March 23, 2011 11:15AM - 11:51AM |
Q41.00001: First principles simulations of materials and processes in photo- and electro-catalysis Invited Speaker: I shall discuss applications of electronic structure calculations and molecular dynamics simulations to understand materials properties and reaction mechanisms in photo- and electro-catalysis. Examples will include studies of the interface between water and titanium dioxide (TiO$_{2})$, a widely used photocatalyst capable of splitting water in O$_{2}$ + H$_{2}$, and the cycle of H$_{2}$ production from water by the active site of an enzyme of hydrogen-producing bacteria, the di-iron hydrogenase, linked to a pyrite electrode. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q41.00002: Theoretical studies on a new pattern of laser-driven systems: towards elucidation of direct photo-injection in dye-sensitized solar cells Kenji Mishima, Koichi Yamashita We theoretically and numerically investigated a new type of analytically solvable laser-driven systems inspired by electron-injection dynamics in dye-sensitized solar cells. The simple analytical expressions were found to be useful for understanding the difference between dye excitation and direct photo-injection occurring between dye molecule and semiconductor nanoparticles. More importantly, we propose a method for discriminating experimentally dye excitation and direct photo-injection by using time-dependent fluorescence. We found that dye excitation shows no significant quantum beat whereas the direct photo-injection shows a significant quantum beat. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q41.00003: Investigation of the Potential Difference between C60 and TiOPc on Ag(111) by Local Probe Techniques Kristen Burson, Yinying Wei, William Cullen, Janice Reutt-Robey One challenge for increasing efficiency of organic photovoltaics is to understand the barrier to exciton separation that exists at the interface between organic molecules. Here we report a local probe measurement of the potential barrier at the interface between submonolayer C60, a good electron acceptor, and honeycomb phase TiOPc, an organic with high hole mobility, on Ag(111). We employ UHV AFM (atomic force microscopy) and KPFM (Kelvin probe force microscopy) to obtain simultaneous images of the potential and topographic landscapes. This technique allows for high spatial resolution of both the potential and the topography. In addition to reporting the work function difference between C60 and TiOPc, we investigate the work function for C60 on Ag(111). [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:51PM |
Q41.00004: Ultrafast proton coupled charge transfer dynamics in photocatalysis Invited Speaker: In this talk I will present our experimental and theoretical studies on the nature of electron and hole acceptor states and their dynamics for protic solvent molecule (H$_{2}$O, CH$_{3}$OH) covered TiO$_{2}$ surfaces. Electron-hole pair generation by band gap excitation can introduce charges into protic solvent/TiO$_{2}$ interface, which can drive photocatalytic processes. By time resolved two-photon photoemission and DFT electronic structure calculations we identify the partially solvated or ``wet'' electron accepter states, and their proton-coupled electron transfer (PCET) dynamics. Because holes are through to be the primary reagents for photocatalysis on TiO$_{2}$, we also explore possible hole driven PCET dynamic pathways. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q41.00005: First-principles study on Ru(4,4',4''-tricarboxy-2,2':6',2''-terpyridine)(NCS)$_3$ sensitizer on TiO$_2$ anatase(101) surface: Adsorbed structures and electronic states for dye-sensitized solar cells Keitaro Sodeyama, Masato Sumita, Yoshitaka Tateyama Dye-sensitized solar cells are expected as a cost effective solar-to-electricity energy conversion devices. The efficiency of the power conversion is greater than 10\% when Ru(II) polypyridyl sensitizers are used. For further improvement of the efficiency, we need to understand the adsorbed structures at atomistic level in detail. In this study, we investigated the adsorbed structures of Ru(4,4',4''-tricarboxy-2,2':6',2''-terpyridine)(NCS)$_3$ sensitizer on TiO$_2$ anatase(101) surface. For four possible adsorbed structures (two candidates have one adsorbed carboxyl group(one-leg) and the others have two adsorbed groups(two-leg)), we found the adsorption energies are quite similar within 0.4 eV. This is attributed to the presence of the hydrogen bond between the hydrogen of carboxyl group and the oxygen of the surface in the one-leg structure. We also calculated the excited states of the four structures of the sensitizer by TDDFT and found that the UV spectrum shift depending on the structure differences. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q41.00006: Reverse-engineering the atomic-scale structure of the TiO2/N3 interface in dye-sensitized solar cells using O1s core-level shifts Christopher Patrick, Feliciano Giustino Dye-sensitized solar cells employing mesoporous titania films sensitized with ruthenium-based dyes have shown consistently good performance over the past two decades. Understanding the process of charge injection in these devices requires accurate atomistic models of the interface between the light-absorbing dye and the semiconducting substrate. Despite considerable efforts devoted to the experimental and theoretical investigation of such interfaces, their atomistic nature remains controversial. In this work we pursue a novel computational approach to the study of the semiconductor/dye interface which does not rely on the calculated adsorption energies. In our approach we reverse-engineer photoemission data through the first-principles calculation of O1s core-level spectra for a number of candidate interface models. Our calculations allow us to discard some of the adsorption geometries previously proposed and point to an interface model which reconciles conflicting assignments based either on photoemission or infrared data. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:51PM |
Q41.00007: Studies of Interfacial Electronic Processes in Nanoporous TiO2 Thin-Films Invited Speaker: Metal-oxide nanoparticles sensitized to visible light by covalent attachment of molecular adsorbates have attracted considerable attention in recent years due their central role in technologies for solar energy conversion, including dye-sensitized solar cells (DSSCs) and solar photocatalysis. However, the mechanisms of interfacial electron transfer and subsequent electron transport induced by photoexcitation of the molecular adsorbates remain only partially understood. We report recent progress in studies of nanoporous TiO2 thin-films functionalized with molecular adsorbates, with emphasis on interfacial electron injection, molecular rectification and the mechanism of electron transport through sintered TiO2 nanoparticles in thin-films relevant to DSSCs. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q41.00008: Efficient adsorbate transport by electron wind: The role of resonant photoexcitation Kirill Velizhanin, Dmitry Solenov We study the surface electromigration force acting on an organic molecule at a conducting (metal) surface. The dominant contribution to the force comes from the scattering of metallic electrons off the molecule, as they tunnel to and from nearby molecular orbitals. When metal carries non-zero current, the net force is directed with the current flow. This force, however, is often too small for efficient transport of adsorbed molecules and only reveals itself through a contribution to the metal resistivity. We show that surface-molecule electron wind force can be substantially enhanced and controlled by exploiting appropriate resonances between molecular and metallic states activated by coherent light. This effect opens a path to new surface-molecule functionality, including high resolution spatially controlled force patterns, controlled molecule motion, etc. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q41.00009: Real-time observation of bond-by-bond interface formation during oxidation of H-terminated (111)Si by second-harmonic generation Bilal Gokce, Eric J. Adles, David E. Aspnes, Kenan Gundogdu Structure of solids is typically determined at the atomic level by techniques such as X-ray and electron diffraction, which are sensitive to positions of atomic nuclei. However, structure is determined by bonds between atoms, which are difficult to measure directly. We combine second-harmonic generation and the bond-charge model of nonlinear optics to probe, in real time, the dynamics of bond-by-bond chemical changes during the oxidation of H-terminated (111)Si, a surface that has been well characterized by static methods. Oxidation is activated by applied macroscopic strain, and exhibits anisotropic kinetics with one of the three equivalent back- bonds of on-axis samples reacting differently from the other two. This also leads to transient changes in bond directions.~Strain is known to increase oxidation rate of Si for thermal oxides, however its affects at the microscopic scale has not been studied at the bond level. By comparing results for surfaces strained in different directions, we show that in-plane control of surface chemistry is possible. The use of nonlinear optics as a bond-specific characterization tool is readily adaptable for studying structural and chemical dynamics in many other condensed-matter systems. [Preview Abstract] |
Session Q42: Polymer Composites
Sponsoring Units: DPOLYChair: Tirtha Chatterjee, The Dow Chemical Company
Room: A302/303
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q42.00001: Dispersion and composite processing of polymer coated graphene Sriya Das, Ahmed Wajid, John Shelburne, Abel Cortinas, Micah Green Liquid phase exfoliation and dispersion of graphene, i.e. single layer graphite, is a critical challenge for bulk processing of graphene into advanced materials and devices. We demonstrate a suite of techniques for dispersing pristine graphene using polymer coatings for the purpose of liquid-phase nanocomposite processing. First, we illustrate a unique in situ polymerization technique to develop localized polymer coatings on the surface of dispersed pristine graphene sheets in solution. These polymer coatings do not disrupt the pristine structure or superior properties of the graphene sheets; instead, these coatings allow for stable, aggregation resistant graphene dispersions, as characterized through rheology, SEM, and AFM. We also demonstrate that certain polymers naturally wrap and stabilize pristine graphene in various organic solvents. We use this technique to prepare epoxy and PVA nanocomposites loaded with polymer-wrapped graphene as filler. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q42.00002: Fabrication of Graphene Oxide/Polypropylene Nanocomposites and Their Electrical Conductivity Study Jinyong Dong Graphene oxide (GO) /polypropylene nanocomposites were fabricated via in situ polymerizing propylene monomer over a GO that had been treated with a Grignard reagent and TiCl$_{4}$ successively when GO was not only catalytically activated but also largely reduced to an almost O- free state. The polymerization led to the in situ formation of the PP matrix, which was synchronized by the nanoscale exfoliation of the reduced GO as well as its gradual dispersion. Morphological examination of the ultimate GO/PP nanocomposites by TEM and SEM techniques revealed effective dispersion in nanoscale of GO in PP matrix. High electrical conductivity was discovered with thus prepared GO/PP nanocomposites, e.g. at a GO loading of 4.9 wt{\%}, $\sigma _{c}$ was measured at 0.3 S$\cdot$m$^{-1}$ [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q42.00003: Controlling nanorod self-assembly in polymer thin film composites Miguel Modestino, Jeffrey Urban, Rachel Segalman Semiconducting nanorods are of particular interest for use in polymer composites due to their anisotropic physical properties; however such properties can only be harnessed in systems with orientational order. Here, we demonstrate control over nanorod self-assembly in solution which leads to arrays of vertically aligned nanorods in polymer thin films over large areas ($>$1 cm$^{2})$. Transmission electron microscopy and X-ray scattering techniques were used to determine the structure of composites and probe the nanorod self-assembly mechanism. This work demonstrates that strong interactions between alkane-covered colloidal nanorods can enable the formation of hexagonally packed arrays of nanorods in a wide range of polymer matrices. Kinetic effects during the casting process are shown to affect the final morphology of the composites, leading to reduced array sizes for systems with increasing polymer molecular weight and nanorod concentration. The results presented show that thin film confinement as well as surface segregation of the nanorod arrays enhance the orientational order of nanorods in composites. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q42.00004: Multi-Walled Carbon Nanotube Network Formation in Extruded High Density Polyethylene/MWNT Composites Frank Yepez Castillo, Brian P. Grady, Daniel E. Resasco Multi-walled carbon nanotube (MWNT) / high density polyethylene (HDPE) composites with varying amounts of carbon nanotubes were investigated and the effect of MWNT weight fraction on their electrical conductivity, crystallinity and mechanical properties is presented here. Samples were prepared by melt dilution of a HDPE masterbatch containing 20 wt{\%} MWNT with varying amounts of neat HDPE. Conductivity measurements on compression molded samples showed that electrical percolation occurs at 4.5 wt{\%} MWNTs. The effect of processing conditions on the formation of a MWNT network in extruded samples was assessed by the addition of a low-shear annealing zone (shear rate 1-10 s$^{-1})$ before final extrusion through a die. The time in the low shear zone was varied from almost zero to 90 sec. Extruded samples above the compression-molded percolation threshold were tested, and electrical conductivity did not develop. However, a significant increase in electrical conductivity was observed in these samples when annealed for 5 minutes at the same temperature. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q42.00005: Effect of Nanowire Size Dispersity on the Electrical Conductivity in Polymer Nanocomposites Rose Mutiso, Michelle Sherrott, Ju Li, Karen Winey In this simulation study, we model the percolation threshold and electrical conductivity of three-dimensional networks containing finite, conductive cylinders with experimentally typical (Gaussian) and engineered (Bidisperse) distributions in their length and/or diameter. We have previously used this approach to explore the effects of cylinder orientation and aspect ratio. Preliminary results suggest that narrow Gaussian distributions do not affect the threshold concentration or electrical conductivity significantly in both isotropic and oriented networks, while the addition of a small fraction of longer rods in a bidisperse system can improve the electrical properties considerably. Additionally, polydispersity in the filler length has a more pronounced effect on the electrical percolation behavior than that in filler diameter. This implies that the separate effects of length and diameter should be decoupled from the overall filler aspect ratio when probing the effects of size dispersity in conducting polymer nanocomposites with elongated fillers. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q42.00006: Simulating the Effect of Flame Retardant Materials on Heat Diffusion in Polymers Joseph Ortiz, Arpon Raksit, Dilip Gersappe Many commonly used polymers have low ignition temperatures, presenting the dangers of combustion and thermal degradation. Simulating the effect of flame retardants on the spread of heat throughout a polymer may provide a better understanding on how to effectively manipulate and make use of flame retardant materials. Using the lattice Boltzmann method, a simulation of heat diffusion from a heat source to sink was implemented in three dimensions. The polymer and flame retardant material were incorporated into the system by implementing ignition within the particles of the polymer and by adding heat absorbing microscale filler particles within the polymer matrix, while allowing for reduced-rate heat transfer between interspecies particles. Flame retardant particles were given various volume fractions and morphologies in order to simulate the addition of a variety of particles such as carbon nanotubes. By manipulating the flame retardant particles' ability to absorb heat, and their efficiency in removing heat from the system, different degrees of polymer heat transport were simulated while polymer systems ranged from single polymer systems to multi-component blends. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q42.00007: Diffusion of small molecules in polymer nanocomposites: relationship between local free volume dynamics and penetrant diffusivity Victor Pryamitsyn, Venkat Ganesan Polymer membranes are widely used as barrier or gas/vapors separation materials. Recent experiments have demonstrated that the barrier properties of the polymer nanocomposites (PNC) dramatically different from pure polymer. Usually such properties are quantified by the permeability $P$ of the material to a penetrant which consists of two contributions: the penetrant solubility $S$ and diffusivity $D$: $P=S~D$ In present work we only discuss term $D$. We use the Bond Fluctuation Model, which allows us to model the diffusivity of the penetrant, the dynamics of the polymer and the dynamics of the polymer free volume in a single framework. We modeled PNC's at different particle load and the penetrant size and found that addition of nanoparticles increseses the penetrant diffusivity and selectivity to the penetrant size. This increase is attributed to the free volume increase and the acceleration of the free volume relaxation in PNC relatively to the pure polymer. We have compared the penetrant diffusivity in a rubbery and glassy PNC's and found than the effect of the PNC load on diffusivity and selectivity is much stronger for the glassy system which is due to rubbery system $D$ is controlled by the rate of matrix free volume relaxation and in glassy regime it is controlled by the static free volume percolation, which is more sensitive to the PNC load. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q42.00008: Nanoparticle Size Dependence of a Polymer's Mechanical Properties Joseph Moll, Shushan Gong, Sanat Kumar, Ralph Colby Nanoparticle size critically affects the properties of polymer nanocomposites. We use a silica/poly(2-vinyl) pyridine (P2VP) polymer nanoparticle composite to investigate these effects by varying the nanoparticle sizes from 2nm to a micron. Since silica and P2VP are miscible, we obtain uniform nanoparticle spatial distribution in all cases. Rheology is employed to measure the macroscopic mechanical properties. X-ray photon correlation spectroscopy is used to probe nanoparticle dynamics. We rationalize our search for an optimal nanoparticle size (with regards to composite mechanical properties) by using thermogravimetric analysis to determine particle bound layer thickness as a function of particle size. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q42.00009: Preparation and Characterization of Electrospun Poly(Methyl Methacrylate)-QDs NanoComposite Fibers Suying Wei, Jayanthi Sampathi, Dan Rutman, Ashwini Kucknoor, Zhanhu Guo In this talk, we describe the simple electrospinning method to fabricate PMMA-CdSe/ZnS Quantum Dots (QDs) nanocomposite fibers followed by property analysis using a variety of techniques. The parameters that affect the electrospinning process including concentration, feed rate, applied voltage and working distance between the needle tip and the fiber collector are investigated and optimized to acquire uniform and defect-free polymer nanocomposite fibers. The surface morphology of the fiber was characterized by scanning electron microscopy, while the fluorescence emission characteristics were analyzed with fluorescence microscopy. In addition, the PMMA-QDs nanocomposite is thermally more stable than the pristine PMMA fibers as determined by the thermal-gravimetric analysis technique. The glass transition temperature and the melting temperature of the polymer are also altered due to the incorporation of QDs. This can be attributed to the interaction between the included QDs and the polymer structure, as disclosed by the surface analysis techniques attenuated total reflectance-infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS).It showed new vibration bands in the composite fiber in the ATR-IR spectra while the binding energy of both C1s and O1s shifted in their corresponding high-resolution XPS spectra. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q42.00010: Improvement of the dispersion of silica nanoparticles in PMMA Kerem Goren, Osman B. Okan, Limeng Chen, Linda S. Schadler, Rahmi Ozisik Creating well-dispersed polymer nanocomposites is an important part of controlling composite properties. Nanoparticles have been shown to demonstrate quite beneficial electrical and thermo-mechanical properties when they are added to polymers. In the current study, the effects of foaming on de-aggregation of nanoparticles in silica/PMMA nanocomposites are investigated. It was found that the saturation of polymer nanocomposite with supercritical carbon dioxide and subsequent rapid de- pressurization is successful in improving the dispersion of nanoparticles in polymer matrix. In addition, by varying saturation pressure, the degree of dispersion improvement can be controlled. Controlled saturation pressure experiments demonstrated that a decrease in saturation pressure led to decreased improvement of nanoparticle dispersion in polymer matrix. By monitoring the inter-nanoparticle distance using transmission micrographs, a quantitative comparison via radial distribution function (RDF) was constructed for before and after each saturation pressure. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q42.00011: Dispersion of Magnetic Brushes in Polymer Melts Yang Jiao, Pinar Akcora It is now known that polymer grafted amphiphilic spherical silica nanoparticles can self-assemble into anisotropic nanostructures. In this study, we will show how dipolar interactions can affect the self-assembly mechanism of magnetic nanoparticles in polymer composite melts. Hydrophobic iron oxide nanoparticles of 6nm in size are synthesized and then decorated with poly(styrene) by reversible addition fragmentation chain transfer (RAFT) polymerization at various grafting densities and brush lengths. Dispersion of these magnetic brushes are examined in poly(styrene) matrices in TEM. Structures obtained from the balance of attractive dipolar interactions and repulsive forces between polymer chains are investigated. The influence of grafting densities and grafted chain lengths on the dispersion of magnetic nanoparticles and formation of the ``equilibrium'' structures will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q42.00012: Modeling nanoparticle aggregation in nanocomposites Tanya Chantawansri, Lee Trask, Eric Cochran, Jan Andzelm A hybrid self-consistent field theory model (H-SCFT) was utilized to model the morphology of nanocomposities composed of cylinder forming ABA triblock copolymer and large nanoparticles (radius on the order of 10 nm). In this system, the size of the nanoparticles is comparable to the cylindrical domains such that nanoparticle segregation into this otherwise compatible phase would cause a significant loss in conformational entropy. To reduce this loss, the nanoparticles could instead macrophase separate out to form aggregates. To capture this particle aggregation in the H-SCFT model, we incorporated a Lennard-Jones potential into the framework. The incorporation of this interaction into the model can significantly alter the observed phase morphology since aggregation can prevent the nanoparticles from swelling and distorting the compatible block copolymer domain. We will demonstrate how the morphology of this nanocomposite varies as a function nanoparticle volume fraction and functionalization. Results will be compared to experimental findings when available. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q42.00013: Effect of Silicon Dioxide Nanoparticles on the Morphology and Interphase Structure of Electrospun PET Nanofibers Qian Ma, Bin Mao, Peggy Cebe Poly(ethylene terephthalate), PET, nanofibers containing silicon dioxide nanoparticles were electrospun from solutions in hexafluoro-2-propanol. Various fill fractions of silicon dioxide nanoparticles in PET were used, ranging from 0-2.0{\%} by weight. The morphologies of both the electrospun (ES) nanofibers and the SiO2 powders were investigated by scanning and transmission electron microscopies. The phase structure of the non-woven, nanofibrous composite mats was investigated with differential scanning calorimetry and real-time wide-angle X-ray scattering. The amount of immobilized layer, the rigid amorphous fraction (RAF), was obtained based on measurement of the specific reversing heat capacity for both as-spun amorphous and isothermally crystallized PET/silica nanocomposite fibers. For the first time, existence of rigid amorphous phase in the absence of crystallinity was verified for electrospun nanocomposite fibers, and two locations of the rigid amorphous fraction are proposed. The effect of interaction between the filler and polymer matrix on the mechanical properties of single fiber is also investigated using atomic force microscopy. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q42.00014: Flexible Polymer Nanocomposite Hydrogen Sensors by Solution Processes Howard Wang, Yayong Liu, Liwei Huang, Kaikun Yang, Lianfeng Zou, Cheol Park Using solution processes such as flow coating and inkjet printing, flexible hydrogen sensors arrays have been fabricated on thin polymer nanocomposite films containing dispersed palladium nanoparticles (Pd-NPs). Composite films were annealed at temperatures from 150 $^{\circ}$C to 200 $^{\circ}$C to allow Pd-NPs to sinter and form a conductive network. An optimal processing temperature is found to yield the most sensitive sensors due to a good balance between the electrical resistance and connectivity of the Pd-NP network. As-fabricated hydrogen sensors can detect a hydrogen level of ca. 200 ppm or lower with a response time of less than 1 second upon the exposure to hydrogen gas, and a recovery time of \textit{ca.} 5 min upon the removal of hydrogen. The sensitivity, repeatability and linearity of sensor arrays are shown to depend on the processing history, the morphology of sensing films, and the geometry of sensor layout. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q42.00015: Thermal Properties of PEO-anatase nanocomposites Mircea Chipara, He Huang, Karen Lozano, David Hui, Xu Hailan, Rafael Villegas, Thomas Mion Polyethylene-oxide (PEO) - anatase composites containing various amounts of anatase ranging between 0 and 20 {\%} wt. have been prepared. The as obtained samples have been investigated by Thermogravimetric Analysis at different heating rates ranging from 5 to 40 K/min. This study was focused on the effect of nanofillers on the activation energy and overall reaction order as well as on the temperature at which the mass loss rate is maxim. The first derivative of the as obtained thermogram has been fitted by using an extended Wigner-Breit-Fano function. The effect of the concentration of anatase nanoparticles on the parameters of the Wigner-Breit-Fano are discussed in detail. Non-isothermal differential scanning calorimetry measurements have been performed in order to determine the effect of anatase nanoparticles on the melting and crystallization of PEO. Isothermal crystallization at various temperatures ranging from 303 to 320 K have been also performed. The study aimed to correlate isothermal crystallization data with non-isothermal results and to determine the effect of anatase nanoparticles on the melting and crystallization of PEO. The crystallization process has been investigated within Avrami and Ozawa approaches. [Preview Abstract] |
Session Q43: Focus Session: Translocation through Nanopores I
Sponsoring Units: DPOLY DBPChair: Aniket Bhattacharya, University of Central Florida
Room: A306/307
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q43.00001: A Coupled-Dynamics Model for Polymer Translocation Timo Ikonen, Tapio Ala-Nissila, Aniket Bhattacharya, Wokyung Sung We study a coarse-grained model of driven translocation of biopolymers, which comprises coupled equations of motion for the translocation coordinate $s$ and the spatial coordinates for the first and the last bead of the translocating chain. We use Langevin dynamics simulations to solve the equations of motion and to study the dynamics of translocation through a nanopore, including the residence time distribution of the individual monomers and the average translocation time. In addition, we consider the time evolution of the spatial coordinates of the first and last bead, which underline the asymmetry of the dynamical chain conformations on the $cis$ and $trans$ sides. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q43.00002: A new approach to polymer translocation Johan Dubbeldam, Vakhtang Rostiashvili, Andrey Milchev, Thomas Vilgis Polymer translocation is ubiquitous in nature. It plays a role in phenomena like virus infections and in trafficking of proteins through pores in a cell membrane. Many theoretical models have been developed to explain scaling properties of simple polymer chains through tiny nanopores. This has not resolved the controversies in this field, however. In this paper we employ novel methods to shed light on the results that were obtained using the different models that are in use today. We use, for example fractional Brownian motion to explain the scaling of the variance in the translocation length with time and find good agreement between simulation results and theoretical predictions. An extension of the theory to nanopores with more complex geometries are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q43.00003: Stepping dsDNA through a solid-state nanopore one basepair at a time Anthony Ho, Jeffrey Comer, Aleksei Aksimentiev Solid-state nanopores hold great promise for single-molecule detection and manipulation, including low-cost, high-speed DNA sequencing. In a typical experiment, single molecules of DNA are driven through a nanopore by applying an electric potential difference across the membrane. As DNA passes through the pore, it modulates the ionic current, which potentially can be used to determine the DNA sequence. However, the typical rate of DNA transport in experiment is too high for detection of DNA sequences by ionic current measurement. It has been shown that it is possible to slow and weakly trap dsDNA in solid-state nanopores with diameters smaller than that of dsDNA [Nanotechnology 21:395501]. Using all-atom molecular dynamics simulations, we demonstrate that such pores can be used not only to trap but also to displace dsDNA in discrete steps using nanosecond-long pulses of electric field. Specifically, we have identified the pore geometry and pulse profiles that impel dsDNA by one basepair when the pulse is on and retain dsDNA in the same position when the pulse is off. Such nanopore traps may offer new means for manipulating single molecules in biophysics experiments. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:27PM |
Q43.00004: Universal and Non-Universal Translocation Dynamics of Coarse-Grained Biopolymers Invited Speaker: I will discuss recent progress on the dynamical scaling of coarse-grained models of (bio)polymers under spontaneous and forced translocation. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q43.00005: DNA translocation through grapheme nanopores Christopher A. Merchant, Ken Healy, Meni Wanunu, Vishva Ray, Neil Peterman, John Bartel, Michael D. Fischbein, Kim Venta, Zhengtang Luo, A.T. Charlie Johnson, Marija Drndic We report on DNA translocations through nanopores created in graphene membranes. Devices consist of nanometer-thick graphene membranes with electron-beam sculpted nanopores. Due to the thin nature of the graphene membranes, we observe larger blocked currents than for traditional solid-state nanopores. Unlike traditional solid-state nanopore materials that are insulating, graphene is an excellent electrical conductor. Use of graphene as a membrane material opens the door to a new class of nanopore devices in which electronic sensing and control are performed directly at the pore. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q43.00006: Characterization of DNA Translocation and Detection in Functionalized Nanopores Yaling Liu, Abhijit Ramachandran, Qingjiang Guo Functionalized nanopores have been used in selective detective of DNA. While the interaction between a bare nanopore and a DNA has been analyzed extensively, little is know for that of a functionalized nanopore. This work focuses on studying the DNA translocation dynamics and mechanism of DNA sequencing in a functionalized nanopore through a coarse-grained molecular dynamics model. Physical properties of chemically modified nanopores, i.e., the effective pore diameter under different bias voltages are characterized. The DNA translocation dynamics under different nanopore coatings and different bias voltages are studied. The simulation results reveal that molecular selective translocation larges lies on the flexibility and orientation of the coating molecules and their interaction with the translocating DNA. This research supports rational designs of DNA transportation- and manipulation-based diagnostic systems. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q43.00007: Direct observation of DNA motions into solid state nanopore under applied electrical potentials on conductive surface Yoshitaka Hayashi, Genki Ando, Ichiro Idutsu, Toshiyuki Mitsui Solid state nanopore is one of emerging methods for rapid single DNA molecule detection because the translocation of the DNA though nanopore produces ionic current changes. One of issues in this method is clogging long DNA molecules. Once DNA molecules clogged, the molecules are rarely removed by varying or switching the polarity of applied bias voltages across the nanopore. We develop a modified nanopore by 50nm Au coating on top of the nanopore surface to be able to remove the clogged DNA molecules during the DNA translocation experiment. Fluorescence microscopy was implemented for observation of stained DNA molecules. The nanopores with diameters near 100 nm can be used initially. DNA translocation rates changes dramatically by tuning the applied electrical potentials on surface higher or lower than the potentials across the nanopore. Furthermore, the Au potentials modifies IV characteristic of the ionic current across the nanopore which is similar to the gate voltages controlling the SD current in FET. We will discuss the influence of surface potential on DNA motion and translocation and clogged DNA molecules. Finally, we will present the recent results of DNA translocation into the SiN-Au-SiO2 nanopore and discuss the effect of applied voltages on Au. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q43.00008: Polymer Translocation through a Nanopore in the Presence of a Viscosity Gradient Hendrick W. de Haan, Gary W. Slater Of interest for both biological and technological applications, the translocation of a polymer across a membrane through a nanopore has been studied via simulations under a great variety of conditions. In this work, results will be presented from Langevin Dynamics (LD) simulations of polymer translocation where the viscosity on the $cis$ side of the membrane is different from the viscosity on the $trans$ side - a scenario both applicable to biological instances of translocation and replicable with artificial nanopores. Starting with the polymer halfway through the pore, the establishment of a preferential direction for large viscosity differences is observed. To investigate the origin of this effect, a simple model of the system as a 1D biased random walker in a viscosity gradient is explored by Monte Carlo and LD simulations. Good agreement between the simple model and the full polymer simulations for both the preferential direction and mean first passage time indicate that the effects that a viscosity difference across the membrane may have on translocation arise in the general case of a particle at a viscosity interface. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q43.00009: Ionic Coulomb Blockade in Nanopores Matt Krems, Massimiliano Di Ventra Understanding the dynamics of ions in nanopores is essential for potential applications in molecule detection, DNA sequencing, and other technologies [1]. We show both analytically and by means of molecular dynamics simulations that ion-ion interactions in nanopores leads to the phenomenon of ionic Coulomb blockade, namely the build-up of ions inside a nanopore with specific capacitance impeding the flow of additional ions due to Coulomb repulsion. This is the classical counterpart of electronic Coulomb blockade in mesoscopic systems. We discuss the analogies and differences with the electronic case as well as experimental situations in which this phenomenon could be detected. \\[4pt] [1] M. Zwolak, M. Di Ventra, Physical Approaches to DNA sequencing and Detection, Rev. Mod. Phys. 80, 141 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q43.00010: Modeling polyelectrolyte translocation through protein channels Jyoti Mahalik, Jing Hua, Yanbo Wang, Murugappan Muthukumar We will present results from Brownian Dynamics simulations of translocation of polyelectrolyte chains through alpha-hemolysin and MspA protein channels. Comparisons will be made between these two pores in terms of the various characteristics of translocation events. Specifically, we will discuss the distribution functions of blocked ionic current and translocation time. The critical roles played by the charge decorations and the geometries of these two protein pores will be presented. The sequence of the polymer will also be addressed. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q43.00011: Polyelectrolyte translocation through a spherical cavity with tunable charge Alexander Eliseev, Murugappan Muthukumar We will present theoretical results on the free energy barrier for a translocating polyelectrolyte through a charge-decorated hole from a confining spherical cavity. Our results are based on self-consistent-field theory for the combined system of polyelectrolyte chain, counterions, electrolyte ions, and the dielectric mismatch between the cavity and the enclosing space. The effects of degree of ionization of the polymer and the net charge of the hole on the translocation barrier will be presented. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q43.00012: Polymer translocation facilitated by Chaperones Aniket Bhattacharya, Tapio Ala-Nissila, Wokyung Sung We study translocation of biopolymers through a nanopore in a membrane facilitated by attractive binding particles (Chaperones) using Langevin dynamics simulation. Specifically we study how the density and attractive strength of these bindings particles affect the chain conformations at the $trans$ side and mean first passage time (MFPT). We also consider model larger chaperone that can bind reversibly on the multiple units of the translocating chain. Finally, we consider translocation of heteropolymers and how a specific sequence affect the translocation process. We discuss relevance of our studies in biological translocation processes.\\ $^1$R. Zandi, D. Reguera, J. Rudnick and W.~M. Gelbart, Proc. Natl. Acad. Sci. USA {\bf 100} 8649 (2003).\\ $^2$W. Sung and P.~J. Park, Phys. Rev. Lett. {\bf 77}, 783 (1996).\\ [Preview Abstract] |
Session Q44: Focus Session: Dynamics of Polymers-Phenomena due to Confinement
Sponsoring Units: DPOLYChair: Rob Riggleman, University of California, Santa Barbara
Room: A309
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q44.00001: The effect of pattern dimensions on the thermal decay of polymer patterns created by nanoimprint lithography Kenneth Kearns, H.W. Ro, Heather J. Patrick, Thomas A. Germer, Christopher Soles Spectroscopic ellipsometry, combined with rigorous coupled wave modeling, is used to characterize the thermal decay of polymeric patterns prepared by nanoimprint lithography. When the residual layer is on the order of 10 nm, the pattern decay kinetics of patterns with a 420 nm periodicity near their glass transition temperatures are nearly an order of magnitude slower than patterns sitting on a thick residual layer. Pattern decay is not observed when the periodicity increased to 800 nm for the 10 nm residual layers. Polystyrene, poly(methyl methacrylate), and poly(4-t-butyl styrene) all show this behavior suggesting that changes in entanglement density are not important. The difference in the radius of curvature for the two different pattern periodicities is the likely origin for the pattern decay. The sensitivity of the technique to thin residual layers and nanoscale patterns is enhanced with an optical cavity of SiO$_{2}$ between the polymer and Si substrate. The SiO$_{2}$ layer enhances the changes in the ellipsometric parameters alpha and beta, which are related to psi and delta. The model dependent scatterometry data is corroborated by atomic force microscopy. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q44.00002: Capillary levelling as a probe of rheology in polymer thin films Joshua D. McGraw, Nick M. Jago, Kari Dalnoki-Veress While measuring the rheology of bulk polymer systems is routine, when the size of a system becomes comparable to the molecular size, flow properties are poorly understood and hard to measure. Here, we present the results of experiments that are easily performed and can probe the rheological properties of polymer films that are mere tens of nanometres in thickness. We prepare glassy bilayer polymer films with height profiles well approximated by a step function. Upon annealing above the glass transition, broadening of the height profiles due to gradients in the Laplace pressure is observed. By validating the technique as a probe of the rheology with a range of molecular weights, we will show that this robust technique can be used to investigate the effects of confinement and interfaces on the rheology of ultrathin polymer films. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q44.00003: Direct evidence of heterogeneous dynamics within ultrathin polystyrene melt films Tad Koga, Naisheng Jiang, Peter Gin, Maya Endoh, Suresh Narayanan, Larry Lurio, Sunil Sinha We report heterogeneous dynamics associated with cooperative motions of polymer chains within single polystyrene (PS) films at temperatures far above its glass transition temperature. The technique used was a marker x-ray photon correlation spectroscopy technique using ``dilute'' gold nanoparticles embedded in PS films in conjunction with resonance-enhanced x-rays scattering which intensifies the probing electrical field in the regions of interest within the films. We found that as the thickness decreased below around 60 nm, the diffusive motions of the markers were significantly suppressed both at the free surface and the center of the film relative to those for the thicker films ($>$100 nm thickness). It is attributed to the long-range effects on the polymer dynamics induced by an immobile layer at the substrate interface. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q44.00004: Anisotropic Dependence of Capillary Dynamics of Confined Polymer Liquid Films Yeling Dai, Oleg Shpyrko, Kyle Alvine, Suresh Narayanan, Alec Sandy We experimentally investigate the effect of highly anisotropic confinement on the capillary dynamics of polymer liquid films. Polystyrene films confined laterally within line-space silicon grating patterns of varying channel width represent a highly anisotropic liquid. The capillary fluctuation modes of such system can be expected to persist along the direction of the channels, while fluctuations perpendicular to the channels are likely to be suppressed. We utilized X-ray Photon Correlation Spectroscopy (XPCS) to access this capillary wave dynamics. In addition to the channel-width dependence of the capillary relaxation times, we also observe the anisotropic dependence of the capillary wave fluctuations on the confined polymer surface. I will discuss how XPCS can access the directional dependence of capillary dynamics and comment on the role played by interfacial pinning in suppressing capillary fluctuations. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q44.00005: Surface Dynamics of Macrocyclic Polystyrene Films Shih-fan Wang, Mark D. Foster, Zhang Jiang, Suresh Narayanan Thermally stimulated fluctuations of the surface of a melt of macrocyclic polymers have been studied for the first time. The surface fluctuations of macrocyclic polystyrene (cPS) of 2k, 7k, 17k, and 37k molecular weight (M) were probed using x-ray photon correlation spectroscopy (XPCS), a recently-developed technique that has already been applied to study surfaces of linear PS melts. The surface fluctuations for the cPS films are slower than those of linear chain analogs for all M. However, the glass transition temperatures (Tg) of the cyclic chains are higher than those of the linear analogs, with the discrepancy decreasing with increasing M. A continuum hydrodynamic theory of thermally stimulated capillary waves with a nonslip boundary condition is adequate to rationalize the behavior of the cPS films. When results from cPS of different M are plotted as a function of T/Tg the data nearly collapse to a single curve, indicating that Tg is a key parameter for the surface dynamics of macrocyclics in the temperature range studied. Acknowledgements: NSF CBET 0730692 [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q44.00006: Surface dynamics of micellar diblock copolymer films Sanghoon Song, Wonsuk Cha, Hyunjung Kim, Zhang Jiang, Suresh Narayanan We studied the structure and surface dynamics of poly(styrene)-b-poly(dimethylsiloxane) (PS-b-PDMS) diblock copolymer films with micellar PDMS surrounded by PS shells. By `in-situ' high resolution synchrotron x-ray reflectivity and diffuse scattering, we obtained exact thickness, electron density and surface tension. A segregation layer near the top surface was appeared with increasing temperature Surface dynamics were measured as a function of film thickness and temperature by x-ray photon correlation spectroscopy. The best fit to relaxation time constants as a function of in-plane wavevectors were analyzed with a theory based on capillary waves with hydrodynamics with bilayer model Finally the viscosities for the top segregated layer as well as for the bottom layer are obtained at given temperatures [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q44.00007: Polymer Transport Near Rough Surfaces Moses Bloom, Jonathan Whitmer, Erik Luijten The rheology of dilute polymer solutions under confinement is important in biology, medicine, microfluidic device design, synthetic polymer processing, and even geologic porous media. However, the solution's specific interactions with the confining surface are poorly understood. This situation is exacerbated for composite nanoparticles, such as polymer/metallic hybrids. Using multi-particle collision dynamics, we find a rich array of transport regimes depending on small-scale surface roughness and the specific surface/solute interactions. These factors couple to hydrodynamic conditions, including flow strength and confinement geometry in unexpected ways. Our findings may be relevant to transport phenomena in certain rough-walled capillaries, such as the distribution of various nanoconjugates in vivo. [Preview Abstract] |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q44.00008: Molecular dynamics simulation study of slip flows over surfaces with periodic and random anisotropic textures Nikolai Priezjev The influence of surface patterns on slip flow of a Lennard- Jones fluid is investigated using molecular dynamics simulations. We consider a situation when the typical pattern size is smaller than the channel width. First, anisotropic slip lengths are reported at low shear rates for flows over periodic stripes of different wettability when the shear flow direction is misaligned with respect to the stripe orientation. The results of MD simulations are compared with continuum predictions. Second, in case of random chemical patterning, the slip length depends sensitively on the total area of wetting texture. Finally, we found that at sufficiently high shear rates the slip length is anisotropic even for atomically flat crystalline surfaces; and, in particular, the slip length is enhanced when the shear flow is oriented along the crystallographic axis of the wall lattice. The simulation results indicate that the onset of the nonlinear regime between the slip length and shear rate is determined by the diffusion of fluid monomers within the first layer. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q44.00009: Glass Transition Dynamics and Surface Layer Mobility in Unentangled Polystyrene Films Zhaohui Yang, Yoshihisa Fujii, F.K. Lee, C.H. Lam, Dongdong Peng, Ophelia Tsui Most polymers solidify into a glassy amorphous state, accompanied by a rapid increase in the viscosity when cooled below the glass transition temperature (Tg). There has been an ongoing debate on whether the Tg changes with decreasing polymer film thickness and the origin of the changes. We measured the viscosity of unentangled, short-chain polystyrene (2.4kg/mol) films on silicon at different temperatures and found that the transition temperature for the viscosity decreases with decreasing films thickness, in agreement with the changes in the Tg of the films observed by thermal expansion measurements. By applying the hydrodynamics equations to the films, we are able to explain the data fully by assuming that a highly mobile layer is present within the top 2.3 nm thick region of the films and follows an Arrhenius dynamics while the remaining of the films is bulk-like. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:39PM |
Q44.00010: Fingering Instability of Debonding Soft Elastic Adhesives Invited Speaker: We study the crack-front fingering instability of an elastic adhesive tape that is peeled off a solid substrate. Our analysis is based on an energy approach using fracture mechanics and scaling laws and provides simple physical explanations for (i) the fact that the wavelength depends only on the thickness of the adhesive film and (ii) the threshold of the instability, and (iii) additionally estimates the characteristic size of the fingers. The scaling laws for these three observables are in agreement with existing experimental data. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q44.00011: Effect of molecular weight on surface mobility of polystyrene films Dongdong Peng, Zhaohui Yang, Ophelia Tsui There have been mounting experimental results showing that a two-layer model is appropriate for describing the dynamics of polymer films. The model postulates that a surface mobile layer exists at the free surface and can modify the dynamics of the entire film. In a recent study, we measured the viscosity of unentangled, short-chain polystyrene (Mw=2.4kg/mol) films supported by silicon at different temperatures including the bulk Tg, and found that the data could be fully explained by assuming a surface mobile layer with a constant thickness exists and sits atop a bulk-like layer. In this talk, I will report the result we obtained by measuring the viscosity of polystyrene films with a wide range of molecular weights from 6.4 to 2316 kg/mol supported by silicon. Our result shows that the same two-layer model is applicable in describing the data if the mobility of the surface layer assumes a molecular weight dependence that differs from either the Rouse or Reptation model. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q44.00012: Scaling of confined and interacting comb polymers Catherine Yeh, Philip Pincus We study the scaling of polymer chains grafted to a line, i.e. a 1-D brush or comb polymer, on a repulsive plane in good solvent using classical molecular dynamics. The grafting density is large enough to cause chain stretching. The confined comb geometry is motivated by intermediate filaments where the unstructured monomer c-termini form annular rings that can be modeled as a confined comb bent into a ring. We find that the scaling of brush size as a function of the number of monomers per chain is the same for a comb with and without confinement by a repulsive plane. We also consider the transition of a line of parallel interacting combs to the planar brush geometry as they are compressed from isolated combs; we present results for the dependence of brush height on the distance between combs. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q44.00013: Geometry and molecular architecture effects in nanobubble inflation measurements Shanhong Xu, Sylvie Castagnet, Gregory McKenna Confinement effects on the mechanical properties of ultrathin polymer films were investigated by a bubble inflation technique developed in our lab. Prior studies of ultrathin films of poly(vinyl acetate) (PVAc) and linear polystyrene (PS) were performed on circular bubbles of different diameters. Here the creep behaviors of ultrathin films of linear PS were investigated on rectangular bubbles. The modulus of the thin film rectangular bubbles was analyzed by approximation methods. The inflation of rectangular bubbles was simulated by finite element analysis (FEA). The mechanical properties of the thin films with the same thickness for circular and rectangular bubbles are compared and we find that the rubbery plateau compliance is geometry independent. We also investigated the creep behaviors of ultrathin films of 3-arm star PS on circular bubbles. We find the rubbery plateau compliance is molecular architecture independent. [Preview Abstract] |
Session Q45: Non-equilibrium Physics with Cold Quantum Gases II
Sponsoring Units: DAMOPChair: Eite Tiesinga, National Institute of Standards and Technology
Room: A310
Wednesday, March 23, 2011 11:15AM - 11:27AM |
Q45.00001: Quenched dynamics in interacting one-dimensional systems: Appearance of current carrying steady states from initial domain wall density profiles Jarrett Lancaster, Emanuel Gull, Aditi Mitra Dynamics arising after an interaction quench in the quantum sine-Gordon model is studied for the case of a system initially prepared in a spatially inhomogeneous domain wall state. The time-evolution of the density, current and equal time correlation functions are studied using the truncated Wigner approximation (TWA) to which quantum corrections are added in order to set the limits on its validity. For weak to moderate strengths of the back-scattering interaction, the domain wall is found to spread out ballistically with the system within the light cone reaching a non-equilibrium steady-state characterized by a net current flow. A steady state current is also found to exist for a quench at the exactly solvable Luther-Emery point. The magnitude of the current decreases with increasing strength of the back-scattering interaction. The two-point correlation function of the variable canonically conjugate to the density reaches a steady state which is spatially oscillating at a wavelength which is inversely related to the current. [Preview Abstract] |
Wednesday, March 23, 2011 11:27AM - 11:39AM |
Q45.00002: Quantum quenches and off-equilibrium dynamical transition in the infinite dimensional Bose Hubbard model Giulio Biroli, Bruno Sciolla We study the off-equilibrium dynamics of the infinite dimensional Bose Hubbard Model after a quantum quench. The dynamics can be analyzed exactly by mapping it to an effective Newtonian evolution. For integer filling, we find a dynamical transition separating regimes of small and large quantum quenches starting from the superfluid state. This transition is very similar to the one found for the fermionic Hubbard model by mean field approximations. [Preview Abstract] |
Wednesday, March 23, 2011 11:39AM - 11:51AM |
Q45.00003: Optimal control for unitary preparation of many-body states: application to Luttinger liquids Armin Rahmani, Claudio Chamon Many-body ground states of local Hamiltonians can be prepared via unitary evolution in cold atomic systems. Given the initial state and a fixed time for the evolution, how close can we get to a desired ground state if we can tune the Hamiltonian in time? Here we study this optimal control problem focusing on Luttinger liquids with tunable interactions. We show that the optimal protocol can be obtained using the simulated annealing method. Rather surprisingly, we find that in the Luttinger liquid case the interaction strength in the optimal protocol can have a \textit{non-monotonic} time-dependence. We find a marked difference in the behavior of the system when the ratio $\tau/L$ of the preparation time to the system size exceeds a critical value around 1/8. In this regime, the optimal protocol can prepare the states with almost perfect accuracy. Finally, we argue that the time-scale of the optimal evolution defines a dynamical measure of distance between quantum states. [Preview Abstract] |
Wednesday, March 23, 2011 11:51AM - 12:03PM |
Q45.00004: Dissipative Transport of Trapped Bose-Einstein Condensates through Disorder Satyan Bhongale, Paata Kakashvili, Carlos Bolech, Han Pu After almost half a century since the work of Anderson [Phys. Rev. {\bf 109}, 1492 (1958)], at present there is no well established theoretical framework for understanding the dynamics of interacting particles in the presence of disorder. Here, we address this problem for interacting bosons near $T=0$, a situation that has been realized in trapped atomic experiments with an optical speckle disorder. We develop a theoretical model for understanding the hydrodynamic transport of \emph{finite-size} Bose-Einstein condensates through disorder potentials. The goal has been to set up a simple model that will retain all the richness of the system, yet provide analytic expressions, allowing deeper insight into the physical mechanism. Comparison of our theoretical predictions with the experimental data on large-amplitude dipole oscillations of a condensate in an optical-speckle disorder shows striking agreement. We are able to quantify various dissipative regimes of slow and fast damping. Our calculations provide a clear evidence of reduction in disorder strength due to interactions. The analytic treatment presented here allows us to predict the power law governing the interaction dependance of damping. The corresponding exponents are found to depend sensitively on the dimensionality and are in excellent agreement with experimental observations. [Preview Abstract] |
Wednesday, March 23, 2011 12:03PM - 12:15PM |
Q45.00005: Mesoscopic Transport of Ultracold Atoms in Optical Lattices Martin Bruderer, Wolfgang Belzig Transport of quantum gases is attracting considerable attention, both on a theoretical and experimental level, in part because ultracold atoms confined to optical lattices can be coherently manipulated and detected on microscopic scales. In particular, substantial technological progress has opened the way for a bottom-up approach to mesoscopic transport in optical lattices, in which case the coherence in certain parts of the system is deliberately destroyed. We show based on a specific setup, namely two incoherent atomic reservoirs connected by a short optical lattice, that mesoscopic phenomena such as, e.g., phonon assisted transport, coherent suppression of tunneling and non-adiabatic quantum pumping can be realized with ultracold atoms. For our analysis in the tight-binding regime we use the non-equilibrium Green's functions formalism extended to include the time dependence of the reservoirs. [Preview Abstract] |
Wednesday, March 23, 2011 12:15PM - 12:27PM |
Q45.00006: Detecting Paired and Counterflow superfluidity via dipole ocsillations Anzi Hu, Ludwig Mathey, Ippei Danshita, Carl Williams, Charles Clark We study the dynamic response of the paired superfluid (PSF) and counterflow superfluid (CFSF) states in a binary mixture of ultra-cold bosonic atoms following an abrupt displacement of the trapping potential. In the PSF and CFSF states, the pairing and anti-pairing orders lead to novel transport properties and distinctive dynamic responses to the abrupt displacement. The findings provide a clear experimental procedure to detect these orders and give an intuitive insight into the dynamics of paired and counterflow superfluidity. [Preview Abstract] |
Wednesday, March 23, 2011 12:27PM - 12:39PM |
Q45.00007: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 12:39PM - 12:51PM |
Q45.00008: Adiabatic Quantum Transport of Bosonic Atoms in Double Well Optical Lattices Yinyin Qian, Chuanwei Zhang Quantum charge pump, where the amount of pumped charges is controlled precisely through the quantized adiabatic charge transport in periodic crystals, has many important applications in electronics. The quantum pump of cold neutral atoms may play a similar significant role in atomtronics. Neutral atoms can be bosons, and their transport properties can be very different from electrons (fermions). We study the adiabatic quantum transport of bosonic atoms in double well optical lattices where the lattice parameters are adiabatically and periodically tuned. The effects of the interaction between atoms on the transport properties are characterized. In the strong interacting regime, the bosonic atoms behave similarly as fermions with quantized atom transport. In the weak interacting regime, the atom transport depends strongly on the paths in the lattice parameter space and the quantized transport may be destroyed. The effects of harmonic traps and disorder potentials are also studied. The investigation is based on the numerical simulation of the exact quantum dynamics of cold atoms in double well optical lattices using the time evolving block decimation algorithm. [Preview Abstract] |
Wednesday, March 23, 2011 12:51PM - 1:03PM |
Q45.00009: Schwinger-Keldysh approach to the Bose-Hubbard model with time varying hopping Malcolm P. Kennett, Denis Dalidovich Cold bosonic atoms confined in an optical lattice potential give a realization of the Bose Hubbard model, and it is possible to study the phase transition between a superfluid and a Mott insulator as the depth of the optical lattice is varied. We study the real time dynamics of the Bose Hubbard model at zero and finite temperature in the presence of time-dependent hopping using the Schwinger-Keldysh technique. Using a strong-coupling approach, we determine the effective action in the vicinity of the zero-temperature transition between superfluid and Mott insulating phases. We then study the solutions of the resulting saddle-point dynamical equations as the hopping is varied to sweep across the phase transition from the superfluid to insulating phase. [Preview Abstract] |
Wednesday, March 23, 2011 1:03PM - 1:15PM |
Q45.00010: Optimal loading for a Tonks-Girardeau gas Claudia De Grandi, Anatoli Polkovnikov We analyze the process of loading a one-dimensional system of hard-core bosons, i.e. a Tonks-Girardeau gas, into a commensurate optical lattice. We consider different loading protocols (e.g. linear, quadratic or sudden ramp in time, or cyclic loading). We discuss possible ways of optimization to minimize the heating and the excitations rate of the system due to the loading process. Combining analytical and numerical methods we analyze the problem under experimentally realistic conditions and we compare the results with earlier scaling predictions. [Preview Abstract] |
Wednesday, March 23, 2011 1:15PM - 1:27PM |
Q45.00011: Weakly interacting bosons in a periodic optical lattice Qinqin Lu, Kelly R. Patton, Daniel E. Sheehy We study an interacting boson gas in a periodic optical potential, with the goal of understanding the properties of such a gas away from the Mott insulating regime at large optical lattice depth. In particular, we analyze the density dependence of the transition temperature as a function of optical lattice depth and the response to a dynamical modulation of the optical lattice. [Preview Abstract] |
Wednesday, March 23, 2011 1:27PM - 1:39PM |
Q45.00012: Pairsuperfluid in Dynamically Constraint Bose-Hubbard Models Lars Bonnes, Stefan Wessel We consider ultra-cold atoms loaded into a two-dimensional optical lattice with strong three-body losses, i.e. three bosons sharing one lattice site scatter inelastically and dissipate from the system. This process dynamically stabilizes a three-body on-site repulsion in analogy to the quantum Zeno effect. The system studied here is described by a Bose-Hubbard model on a square lattice with on- site attraction. The maximal number of particles per lattice site is restricted to two in order to take the three-body repulsion into account. Field theoretical considerations and numerical simulations using Matrix Product States in one dimension suggest the existence of a dimer superfluid phase for small tunneling rates that is effectively described by the condensation of boson pairs and the absence of an atomic condensate. In this work we explore the ground state and finite-temperature phase diagram for our model using large-scale quantum Monte- Carlo simulations. Our main emphasis is the detection of the dimer superfluid phase and we address the issue of extrapolating our finite-temperature data to the thermodyanmic limit at $T = 0$. Furthermore, we explore the possibility of adding an explicit dimer hopping term that drastically changes the behavior of our system. [Preview Abstract] |
Wednesday, March 23, 2011 1:39PM - 1:51PM |
Q45.00013: Beyond mean-field dynamics in open Bose-Hubbard chains Holger Hennig, Dirk Witthaut, Friederike Trimborn, Georgios Kordas, Theo Geisel, Sandro Wimberger We investigate the effects of phase noise and particle loss on the dynamics of a Bose-Einstein condensate in an optical lattice. Starting from the many-body master equation, we discuss the applicability of generalized mean-field approximations in the presence of dissipation and methods to simulate quantum effects beyond mean-field by including higher-order correlation functions. It is shown that localized particle dissipation leads to surprising dynamics, as it can suppress decay and restore the coherence of a Bose-Einstein condensate. These effects can be applied to engineer coherent structures such as stable discrete breathers and dark solitons. [Preview Abstract] |
Wednesday, March 23, 2011 1:51PM - 2:03PM |
Q45.00014: Strong local-field effect on dynamics of a dilute atomic cloud irradiated by two counterpropagating optical fields: beyond standard optical lattices Guangjiong Dong, Jiang Zhu, Weiping Zhang, Mikhail Shneider We study a recent experiment (K. Li et al., Phys. Rev. Lett. 101, 250401 (2008)) on diffracting a Bose-Einstein condensate by two counterpropagating optical fields. Including the local field effect, we explain asymmetric momentum distribution and self-imaging of the BEC in a self-consistent way, and find that the self-imaging is not dependent on the intensity difference of the two optical fields, but on the light-condensate interaction time. We show further that the local field effect leads to deformation of an optical lattice, and thus is essential for getting better quantitative analysis of other current optical lattice experiments of cold atoms. Moreover, intensity imbalance of the two optical fields could be applied as a new means to tailor both cold atom dynamics and optical propagation. [Preview Abstract] |
Wednesday, March 23, 2011 2:03PM - 2:15PM |
Q45.00015: Quenched dynamics in a spin-1/2 chain prepared in a sharp domain wall state Lea Santos, Aditi Mitra, Emil Prodan Using exact diagonalization and Expokit, we study the time evolution of current, magnetization and correlation functions in an isolated spin-1/2 chain initially prepared in a domain wall state. The domain wall consists of spins pointing up in the first half of the chain and down in the other half. Integrable and nonintegrable regimes are reached by adjusting the parameters of the Hamiltonian, which allows for the comparison of behaviors in both limits. In a chain with nearest- neighbor couplings, chaos is induced by adding on-site disorder or by adding next-nearest-neighbor couplings. The magnitude of the current decreases with interaction for the clean integrable system and for the chaotic disordered case. For the chaotic clean system with next-nearest-neighbor couplings, a non- monotonic behavior in the current is found as the interaction strength is increased. [Preview Abstract] |
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