Rice University
Department of Physics & Astronomy

# Condensed Matter Seminars 2006 – 2007

Where: HZ 116
When: Mondays at 4:00 p.m.

 September 07, 2006 Probing Hybridization through Optics Dr. Ken Burch Center for Integrated Nanotechnologies Los Alamos National Laboratory In this talk I detail recent optical spectroscopic studies of as grown and annealed thin films of Ga1-xMnxAs across its phase diagram. We find that the spectra significantly differ from what is expected and has been observed in semiconductors doped with non-magnetic impurities. These studies indicate that the Insulator to Metal transition in this magnetic semiconductor is significantly different from most other doped semiconductors, indicating an important role for magnetism. I will then switch to discussing magnetic impurties in the lattice via our work on Yb14MnSb11. We found that this system is the first underscreened d-electron Kondo lattice. Our optical studies were the first to demonstrate the heavy electron nature of the carriers and the interestingly interplay between the Kondo and Jahn-Teller interactions in this system. If time allows, I will also discuss recent optical pump-probe measurements on this material that indicate the unconventional nature of the Ferromagnetic state. September 25, 2006 Anisotropy in 2D electronic quantum Hall systems Professor Orion Ciftja Prairie View A&M University Strongly correlated electronic systems in two-dimensions (2D) have constantly been a source of new discoveries. Novel phenomena such as the integer and fractional quantum Hall effect have emerged when such systems have been subjected to very strong perpendicular magnetic fields. A recent experimental discovery in the quantum Hall regime has been the observation of very strong magneto-transport anisotropies at certain low values of magnetic field below a critical temperature of about 100 mK. While the origin of such anisotropy is yet unknown, we explain the emergence of such anisotropies in terms of electronic liquid crystalline states with broken rotational symmetry. We investigate the stability of liquid crystalline phases with nematic order at fractional fillings of the valence Landau level. Monte Carlo results indicate that while isotropic states are stable in the lowest Landau level, there are regions of instability towards liquid crystalline states in higher Landau levels. A possible connection of the recently discovered magneto-transport anisotropy in low magnetic fields and these liquid crystalline states is discussed. October 02, 2006 Field-Induced Superconductivity in an Organic Insulator Professor Kevin Storr Prairie View A&M University Organic conductors are excellent candidates for high magnetic field studies because of their anisotropy, instabilities and interactions at low temperatures.  l-(BETS)2FeCl4 a quasi two-dimensional organic conductor undergoes an antiferromagnetic-insulator and metallic phase before displaying strong evidence for Field Induced Superconductivity  (FISC) which persists in fields up to 40 tesla after which it is re-entrant.  By gradual replacement of the Fe in the FeCl4 anion, the superconducting phase moves towards lower fields with a zero field  superconductor resulting with total substitution in the form of l-(BETS)2GaCl4.  We explain the continuance of the FISC in terms of the Jaccarino-Peter effect which requires that the applied magnetic field be compensated by the exchange field of the aligned Fe3+ ions. October 30, 2006 Magnet on Magnet: the Exchange Bias Effect in Co/CoO Nanoparticles Professor Meigan Aronson Univeristy of Michigan When a ferromagnet is in close proximity to an antiferromagnet, the direct exchange interaction  between the moments in each can create a unidirectional anisotropy which impedes the reversal of the ferromagnetic moment. It is thought that this is a promising way to stabilize the moment for the smallest nanoparticles, but so far understanding the fundamental mechanism has been obscured by lack of information and control of the antiferromagnetic-ferromagnetic interfaces.  We present here a neutron scattering study of the internal magnetic structure and dynamics of Co/CoO core-shell nanoparticles. Transmission electron microscopy measurements show that the Co/CoO interface is both highly directional and virtually epitaxial. Neutron diffraction measurements show that antiferromagnetic order occurs below a Neel temperature much suppressed from the bulk CoO value. Further, the magnetic structure involves the spatial segregation of oxygen interstitials near the interface, inducing a large uncompensated moment. Normal CoO antiferromagnetism is found in the outer regions of the CoO shell. Our results suggest that the interface layer enhances the ferromagnetic-antiferromagnetic coupling, and is responsible for the large exchange bias observed in our samples. We have also performed inelastic neutron scattering measurements which show that the interface has a dual role, providing an unusual braking mechanism which slows the reorientations of the ferromagnetic core at the onset of the exchange bias. November 06, 2006 Anisotropic thermodynamics of unconventional superconductors Professor Ilya Vekhter Louisiana State Univeristy Determination of the shape of the energy gap in unconventional superconductors is of significant importance since it is believed to be related to the pairing mechanism. A candidate technique for finding the positions of zeroes (nodes) of the gap is via measurements of the anisotropy of thermal and transport properties under a rotated magnetic field. This anisotropy was predicted and found experimentally in the specific heat and thermal conductivity in a number of materials. However, limitations of the original semiclassical theory made unambigious interpretation of experiments not easy or impossible. After giving the background to the problem, I will present the results of a microscopic fully self-consistent calculation of the anisotropy of the specific heat and thermal conductivity in anisotropic superconductors across the T-H phase diagram. I will explain the physical origin for the unexpected inversion of the anisotropy. For heavy fermion CeCoIn5 I will show that the results are in a semi-quantitative agreement with experiment and resolve the controversy between the existing specific heat and thermal conductivity measurements. November 13, 2006 Quantum Electro-Mechanics Professor Keith Schwab Cornell University The technology is at hand to produce and measure mechanical structures at quantum mechanical limits.  We have  demonstrated the closest approach to the Heisenberg Uncertainty Principle and the quantum ground state for a radio-frequency nanomechanical structure, achieved by coupling to a single electron transistor.  Recently, we have measured the quantum noise of the transistor and discovered that cooling of the mechanical mode is possible by an electronic process analogous to laser cooling.  Future experiments will pursue more sophisticated situations by integrating quantum coherent electronic elements (qubits) and will probe the production and detection of the true quantum states of the resonator, including squeezed, superposition, and entangled states. December 04, 2006 Magnetic field asymmetry and interactions in single-walled carbon nanotubes Professor Dave Cobden University of Washington The length scales and scattering processes in the one-dimensional electron system in single-walled carbon nanotubes remain only partially understood. Measuring the magnetoresistance, in both linear and nonlinear response, is away to investigate these processes.  In disordered nanotubes with ballistic paths much shorter than the length, we observe magnetoresistance in the metallic regime which at low temperatures resembles the universal fluctuations and weak localization seen in higher dimensional metals.  A parabolic magnetoresistance persists at room temperature, indicating a significant role for phase coherence and/or interactions at high temperatures.  While the linear resistance of a two-terminal sample must be an even function of magnetic field B by Onsager's principle, the nonlinear resistance need not be.  Importantly, the B-asymmetric nonlinear terms can in principle be used to infer the strength of electron-electron interactions in the sample.  We have therefore also measured in detail the lowest order B-asymmetric current contributions, with a focus on the B-linear term.  This has apparently not been done before in any system. Consistent with general theory, at high temperatures the term is small and has a constant sign independent of Fermi energy.  At low temperatures it grows and develops mesoscopic fluctuations.  Although these result imply that interactions are involved in the transport, calculations specific to nanotubes will be needed in order to extract interaction parameters. December 11, 2006 From noisy quantum dots to quantum-critical heavy fermions: recent developments of the numerical renormalization group method Dr. Matthew Glossop University of Florida I will discuss recent progress in describing Bose-Fermi quantum impurity models using the numerical renormalization group approach. In particular I will focus on results obtained for the Bose-Fermi Kondo model (BFKM), describing a local magnetic moment coupled to a conduction band and a dissipative bosonic bath. The model is of current interest in connection with noisy quantum dot systems and non-Fermi-liquid behavior observed in quantum critical heavy-fermion materials. The latter systems are described by the Kondo lattice model, which maps onto a self-consistent BFKM within the extended dynamical mean-field theory (EDMFT) framework.  I will discuss our recent NRG-EDMFT solution of the anisotropic Kondo lattice model, addressing in particular the topical issue of local quantum criticality. February 05, 2007 Universal phase diagram of interacting quantum liquids near the unitarity limit Dr. Predrag Nikolic Harvard University We consider several models of particles with short-range attractive interactions whose universal properties are controlled by an unstable renormalization-group fixed point at zero density and temperature. The fixed point corresponds to the Feshbach resonance, and relevant perturbations are the detuning of the resonance, and parameters that control the particle densities. Some critical exponents are determined exactly, and scaling functions are expressed as expansions about two or four spatial dimensions. The existence of a renormalization-group fixed point implies a universal phase diagram as a function of density, temperature, population imbalance, and detuning. We study this phase diagram in the context of BEC-BCS crossover of s-wave paired fermions.  We develop a 1/N expansion, based upon models with Sp(2N) symmetry, and use it to systematically analyze the universal properties of interacting  fermions near the unitarity limit. This approach overcomes several limitations of the expansions about two and four dimensions, and allows a well controlled exploration of the full phase diagram of imbalanced fermion populations in the experimentally relevant three-dimensional space. February 09, 2007 Density Wave Formation in Graphene Facilitated by in-plane Magnetic Field Dr. Sebastian Reyes Brookhaven National Lab We suggest that by applying a magnetic field lying in the plane of graphene layer one may facilitate an excitonic condensation of electron-hole pairs with opposite spins. The provided calculations yield a conservative estimate for the transition temperature Tc~0.1B. Below Tc the system is in insulating state with critical spin fluctuations. February 19, 2007 POWER-LAWS IN ONE-DIMENSIONAL TRANSPORT: Luttinger Liquid or Disorder? Prof. Michael Fogler University of California San Diego When the conductance of a 1D wire shows a power-law dependence on temperature T and voltage V, it is often attributed to Luttinger liquid or related interaction effects. For this explanation to hold the power-law exponents in T and V must be equal. This condition is systematically violated in long wires (typically, longer than 10 micron), where the thermal exponent is much larger than the voltage one. To shed light on the physics involved we study a theoretical model of a long 1D wire with a finite density of strong random impurities that convert it into a chain of weakly-coupled quantum dots. Electron transport in such a system is shown to exhibit a rich dependence on V and T due to the interplay of sequential and co-tunneling. Remarkably, we indeed find a broad parameter range where the conductance exhibits an algebraic dependence on T and V with unequal exponents, in agreement with the experiments. At much lower temperatures the conductance eventually crosses over to the stretched exponential laws typical of the variable-range hopping. Reference: M. M. Fogler, S. V. Malinin, T. Nattermann, "Coulomb blockade and transport in a chain of one-dimensional quantum dots," Phys. Rev. Lett. 97, 096601 (2006). February 27, 2007 Transport in Nanofabricated Electronic Devices Dr. Gavin Scott Two separate nanoscale electronic device projects will be discussed, addressing issues of fabrication and transport in the few-electron regime. The first topic focuses on the construction and characterization of single molecule transistors (SMTs) using the recently synthesized Borromean Ring complex.  While stability and reliability have been pervasive issues in the study of SMTs, a comprehensive understanding of  the primary physical influences enables us to ascertain new means of exerting further control in these systems.  For example, I will introduce a simple means of enhancing rectification properties in SMT devices.  The second topic concerns our effort to fabricate and characterize electrostatically defined lateral quantum dots on  modulation doped Si/SiGe heterostructures for future quantum computation applications.  Towards attaining the goal of developing Si/SiGe based nanostructures with a level of control near what has been achieved with GaAs/AlGaAs, I will detail our progress overcoming fabrication obstacles related to the use of surface gates.  Proposed, near-future experiments for both of these projects are directed along strikingly similar paths, the success of which may underscore their respective viabilities. March 19, 2007 Orbital-free Density Functionals for Materials Simulations Prof. Samuel B. Trickey University of Florida Prediction of the structure and properties of a single complicated material is a demanding task. The task is more difficult in the presence of solvents, and especially so in combination with imposed mechanical stress. Such "chemo-mechanical" systems require quantum mechanical treatment of critical reactive zones, but cannot be facilitated by either localization (as in molecular calculations at high accuracy) or translational periodicity (as in perfect crystals). In view of the successes of density functional theory, especially continued improvement of approximate exchange-correlation functionals, a reasonable goal is to move the nuclei in the material (or a sub-system) via molecular dynamics (MD) on the ground state potential surface (Born-Oppenheimer approximation).  Despite advances (pseudo-potentials, order-$N$ methods), solution of the Kohn-Sham equations is too slow computationally to be a fully viable approach. An alternative,Car-Parrinello dynamics, does not guarantee motion on the B-O surface. Another approach, orbital-free DFT, therefore is appealing.  But that approach has a long history of difficulties in constructing reliable approximations to the Kohn-Sham kinetic energy functional.  To be useful for MD, such functionals must be local ({\it i.e.} one-point), which makes the task harder.  However, we have made progress by requiring only that the approximate functional deliver high-quality forces, by exploiting the conjointness'' hypothesis of Lee, Lee, and Parr, and by parameterizing to simple molecules (e.g., SiO).  This talk will survey the fundamentals, discuss our approach, report a range of test results, and discuss a graded-sequence-of-approximations procedure for making relatively inaccurate functionals useful. April 16, 2007 TBA Prof. Ron Naaman Department of Chemical Physics Weizmann Institute April 26, 2007 ULTRA SLOW MOTION OF AN INTERFACE PINNED BY IMPURITIES Prof. Alberto Rosso Massachusetts Institute of Technology Many experimental systems can be modeled as interfaces pulled by a force on a disordered landscape. Prominent examples are the magnetic domain walls continuously pushed by magnetic fields while our computer hard disks work, or similar walls in ferroelectric systems and crack propagation in solids. In all these systems the physical properties are drastically modified by the existence of impurities in the material on which the interface pins. Understanding these properties is thus a considerable challenge, both from a fundamental and a practical point of view. Although the problem is simple to model, it is extremely complicated to solve. Indeed, while for large forces the interface simply slides, for small forces it is pinned by the impurities. Motion is still possible thanks to thermal kicks, but becomes exceedingly slow. The relevance for applications of this regime has motivated a continuous research activity during the last two decades. In spite of these efforts, the slow motion of the interface has remained numerically inaccessible. We have developed a new technique that allows to overcome this problem and to get the important physical quantities characterizing this ultra-slow motion. Our work thus opens the way for a better understanding of the motion of a wide class of interfaces. May 21, 2007 Theory of d-wave Fermi surface symmetry breaking Dr. Hiroyuki Yamase Max Planck Institute for Solid State Physics The Fermi surface usually respects the point-group symmetry of the underlying lattice. However, recently it was shown [1,2] that such symmetry of the Fermi surface can be broken spontaneously by electron correlation effects (Pomeranchuk instability). In this talk, after reviewing a possible relevance to actual materials, we analyze a pure forward scattering model, which exhibits spontaneous Fermi surface symmetry breaking. We clarify the phase diagram including a possible quantum critical point, and universal quantities charactering the phase diagram. [1] H. Yamase and H. Kohno, JPSJ 69 , 332 (2000); 69, 2151 (2000). [2] C.J. Halboth and W. Metzner, PRL 85, 5162 (2000).

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