Rice University
Department of Physics & Astronomy

Condensed Matter Seminars
2009 – 2010

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

September 14, 2009
Nonlinear Wave-Packet Dynamics in a Disordered Medium
Prof.  Alexander Finkelstein & Dr. Georg Schwiete
Texas A&M University
 Recent experiments on pulse propagation in a nonlinear and disordered medium, realized in (i) photonic crystals and (ii) cold atomic gases, opened a new theoretical problem that has neither been discussed in the context of non-linear physics, nor in condensed matter physics. In this talk I will explain why despite the apparent success of the experiments that aimed at the visualization of Anderson localization the essential physics of nonlinear pulse propagation in disordered media still remains to be observed. We have formulated an effective theory of pulse propagation in a nonlinear and disordered medium in terms of a nonlinear diffusion equation. Despite its simplicity this equation describes novel phenomena which we refer to as ?locked explosion? and ?diffusive? collapse.
September 28, 2009
Electron transport through a single-molecule magnet from first principles
Prof.  Kyungwha Park
Virginia Tech University
 Recently, there have been a great amount of experimental efforts to build and characterize nanoscale single-molecule magnets deposited on surfaces or bridged between electrodes aiming at applications for information storage devices or quantum computing materials. Single-molecule magnets showed magnetic quantum tunneling and quantum interference, and may exhibit an intriguing effect of the coupling between spin and charge degrees of freedom on transport. Reported transport measurements through the prototype single-molecule magnet Mn12 demand theoretical inputs on the roles of the interfaces and molecular geometries on the transport, and on whether electronic and magnetic properties are maintained in low-dimensional structures. To provide such theoretical inputs, large-scale simulations at the atomistic level are required. We simulate semi-infinite electrodes and different molecular geometries and interfaces which would mimic experimental set-ups. Then we compute transport properties through the single-molecule magnet Mn12, using the non-equilibrium Green's function method in conjunction with density-functional theory. We discuss the coupling between the Mn12 and the electrodes, as well as charge distribution of conduction electrons over the Mn12 depending on molecular geometries and interfaces. We also present a possibility of using the Mn12 as a spin filter at low bias voltages.
October 5, 2009
Direct observation of spin-charge separation and interaction effects in GaAs quantum wires by momentum-conserved tunneling
Prof.  Chris Ford
University of Cambridge and University of Birmingham
 Coulomb interactions have been predicted to have a profound effect on the behaviour of electrons in one dimension. We have fabricated a 1D system in which we observe spin-charge separation in momentum-conserved tunneling from an array of 1D wires into a 2D electron gas, and also a power-law suppression of tunneling into the wires. These are as predicted for a Tomonaga-Luttinger Liquid (TLL), the simplest analytic model of an interacting 1D system. The use of an array of wires averages out impurity effects and allows the lowest 1D subband to be probed with precise control of electron density. We observe spin-charge separation in the dispersion relation of the 1D wires, mapped by varying the in-plane magnetic field and the dc-bias. We find that the separation persists beyond the regime of the TLL approximation. Furthermore, the measured 1D-2D tunneling current is suppressed at zero dc bias in the presence of a magnetic field, confirming that interactions are important in the 1D wires. This suppression has been measured as a function of temperature and source-drain voltage. These both have similar power-law dependences, as predicted by the TLL model.
October 21, 2009
Lattice distortion and magnetic quantum phase transition in CeFeAs1-xPxO
Dr.  Clarina R. dela Cruz
University of Tennessee, Knoxville
 With the advent of Fe-based superconductivity initially discovered in the prototypical electron doped Fe-pnictide LaFeAsOxF1-x, came a surge of renewed interest in high temperature superconductivity. Neutron scattering was used to show the antiferromagnetic (AFM) order in the parent compounds of the iron arsenide superconductors. The discovery of the ubiquitous magnetic order, and its competition with superconductivity, across the various families brings attention to understanding the interplay between magnetism and high-transition temperature (high-Tc) superconductivity in these materials. A feature of the parent compounds is the structural distortion that occurs in the vicinity of the onset of long range magnetic order of the Fe-spins. In the RFeAsO(R=rare earth) family, the magneto-structural transition is suppressed in favor of superconductivity upon doping charge carriers into the system, which alters the system electronically and crystallographically as well. To understand the lattice effect on the suppression of the AFM ground state itself, it is important to isoelectronically tune the crystal lattice structure without the influence on charge carrier doping and superconductivity. Here we use neutron powder diffraction to show that replacing the larger arsenic with smaller phosphorus in CeFeAs1-xPxO simultaneously suppresses the AFM order and orthorhombic distortion near x = 0.4, providing evidence for a magnetic quantum phase transition. Furthermore, we find that the pnictogen height in these iron arsenides is an important controlling parameter for their electronic and magnetic properties, and may play an important role in electron pairing and superconductivity.
October 22, 2009
From Black Holes to Strange Metals: Many-Body Physics Through A Gravitational Lens
Prof.  Hong Liu
Massachusetts Institute of Technology
 In the last ten years string theory has revealed a surprising and deep connection between gravity and many-body physics. Puzzling issues in quantum gravity can now be formulated as questions in a many-body system without gravity, where conventional quantum mechanics applies. Alternatively, difficult questions in some strongly coupled many-body systems can be answered by simple calculations in classical gravity. The physical intuition behind this connection will be briefly described, as well as new insights into the quantum nature of black holes that have been obtained. I will then focus on some recent work where the connection has been used to find universality classes of non-Fermi liquids, with a possible application to the strange metal phase of the cuprate high temperature superconductors.
November 3, 2009
Interplay between Antiferromagnetic Quantum Critical Point and Selective Mott Transition in Pure and Doped YbRh2Si2
Prof.  Frank Steglich
Max-Planck Institute for Chemical Physics of Solids, Dresden, Germany
 In the heavy-fermion metal YbRh2Si2 a quantum critical point (QCP) has been established by driving a continuous antiferromagnetic (AF) phase transition from TN ~70 mK at B = 0 to TN = 0 via application of a tiny magnetic field Bc (?c) ~ 60 mT. New results on the Hall coefficient, magnetic Gr¨¹neisen ratio and thermoelectric power support the conclusion drawn from earlier studies that this AF QCP coincides with a Kondo-breakdown QCP or Mott transition, selective to the Yb3+ - 4f states. In a recent investigation, (positive and negative) chemical pressure was applied to YbRh2Si2 to explore the evolution of its B-T phase diagram under changes of the unit-cell volume: Clear signatures of the selective Mott transition were observed within the magnetically ordered phase under volume compression (i.e., Co substitution for Rh). Here, the AF QCP appears to be of the conventional (3D SDW) type. Under slight volume expansion (doping with 2.5 at % Ir) the AF instability and the selective Mott transition were found to still coincide at Bc (?c) ~ 40 mT. For 6 at% Ir doping, however, AF order appears to be largely suppressed (TN < 20 mK), while the Kondo-breakdown QCP remains virtually unchanged. For this composition, a new type of low-T spin-liquid phase shows up in a finite range of magnetic fields. Further ongoing studies concerning the interplay between the AF QCP and the selective Mott transition in this material will also be briefly mentioned. In collaboration with: M. Brando, S. Friedemann, P. Gegenwart, C. Geibel, S. Hartmann, S. Kirchner, C. Krellner, M. Nicklas, N. Oeschler, Q. Si, O. Stockert, Y. Tokiwa, T. Westerkamp and S. Wirth.
November 9, 2009
Emergent Supersymmetry and String in Condensed Matter Systems
Prof.  Sung-Sik Lee
McMaster University, Canada
 Quantum field theories arise as low energy effective descriptions for gapless states in condensed matter systems. Although strongly coupled quantum field theories are rather common, currently there is no systematic way of understanding those theories. In this talk, I will discuss about two condensed matter systems where non-perturbative tools may shed some light on the strongly coupled low energy physics. In the first part, I will talk about a 2+1 dimensional lattice model where emergent superconformal symmetry enables one to understand a strongly interacting critical point non-perturbatively. In the second part, a 2+1 dimensional non-Fermi liquid state will be discussed where a matrix/string theory emerges in the low energy limit.
November 19, 2009
From Frustration to Correlation via Fluctuation
Prof.  Yong Baek Kim
University of Toronto, Canada
 Competing interactions between electrons or spins on geometrically frustrated lattices may not be satisfied simultaneously. The resulting frustration often leads to macroscopically degenerate classical ground states. We take an example of the frustrated magnets, namely interacting local moments on geometrically frustrated lattices, and discuss how thermal/quantum fluctuations on the degenerate classical ground states lift the degeneracy. Relieving the frustration via such fluctuations leads to various conventional and exotic quantum ground states with characteristic correlations. Applications to the experiments on newly discovered frustrated quantum magnets are discussed.
January 12, 2010
1D Bose Gas as the Non-Relativistic Limit of the sinh-Gordon Model
Dr.  Marton Kormos
SISSA, Italy
 Due to recent experimental achievements with trapped ultracold atoms, the properties of the 1D non-relativistic Bose gas are of great interest. In many experimental setups the behavior of the particles is very well described by the Lieb-Liniger model which in many aspects can be regarded as a theoretical benchmark in the research of integrable models. In spite of its integrability, calculating correlation functions in the model is notoriously difficult. In the talk I will propose a novel approach to compute expectation values and other physical quantities in the Lieb-Liniger model. The method is based on the fact that in the repulsive case the S-matrix, the Lagrangian and the operators can be obtained from a certain non-relativistic limit of the sinh-Gordon model. This observation allows us to compute expectation values in the Lieb-Liniger system both at zero and finite temperature.
January 25, 2010
What Do We Learn from the Thermoelectric Transport in Graphene?
Prof.  Jing Shi
University of California, Riverside
 Graphene has attracted much attention in the condensed matter physics and materials science communities due to its exotic electron excitation spectrum and unique physical properties. Electrical transport has already revealed a great deal of interesting physical properties such as the vanishing cyclotron mass and quantum anomaly at the Dirac point. In this talk, I will present our recent thermoelectric transport property study on graphene in both classical and the quantum transport regimes. The thermoelectric coefficients such as the Seebeck and Nernst coefficients probe the energy derivatives of the electrical conductivities (both longitudinal and the Hall); therefore, they show more pronounced anomalous behaviors near the Dirac point. In addition to the unique band structures, the nature of the impurities may also be inferred from the temperature dependence of these thermoelectric transport coefficients.
January 26, 2010
Non-Equilibrium Steady State Charge Transport in an Interacting Open Quantum System
Dr.  Dibyendu Roy
University of California, San Diego
 I will discuss how Lippmann-Schwinger scattering theory can be employed to calculate non-equilibrium steady state current through an open quantum dot with local electron-electron interaction. The two-particle current is evaluated exactly while we use perturbation theory to calculate the current when the leads are Fermi liquids at different chemical potentials. Finally I will tell some applications of this technique to demonstrate different interesting phenomena like two-particle resonance, current asymmetry, and spin-filtering in different dot models.
February 5, 2010
The Phase Diagrams of Strongly Interacting Ultracold Atoms
Dr.  Charles Mathy
Princeton University
 The experimental realization of model condensed matter systems in ultracold atoms allows for a complete exploration of a variety of phase diagrams, as one is able to tune such parameters as mass, polarization and interaction strength, with a flexibility unmatched in any other experiments. In this talk I will discuss two ultracold atomic systems : two-component fermions in an optical lattice, and imbalanced fermi gases. The interest in these systems stems from their relation to important problems in condensed matter physics, and their experimental accessibility. I will discuss what has already been achieved in these systems, what still remains to be done, and what the main experimental and theoretical challenges are to that end.
February 8, 2010
Spin-Triplet Superconductivity in Co-Based Josephson Junctions
Prof.  Norman Birge
Michigan State University
 Superconducting/Ferromagnetic (S/F) hybrid systems exhibit a number of interesting properties due to the interplay between the competing symmetries of their order parameters. With conventional spin-singlet superconductors, the proximity effect in S/F systems decays over an extremely short length scale in the ferromagnet due to the large exchange splitting between the spin-up and spin-down electron bands. In S/F/S Josephson junctions, the critical current oscillates and decays rapidly as a function of the ferromagnetic layer thickness. If there were spin-triplet superconducting correlations present, however, then both the proximity and Josephson effects would persist over much longer distances. Such correlations have been predicted to occur in S/F systems with certain forms of magnetic inhomogeneity near the S/F interface. Moreover, these correlations exhibit a strange symmetry never before observed: they are odd in frequency or time. In this talk I will discuss our efforts to produce and measure these elusive spin-triplet correlations in S/F/S Josephson junctions, culminating in our very recent success.
February 15, 2010
Spin-Triplet Superconductivity in Co-Based Josephson Junctions
Dr.  Junliang Song
University of British Columbia, Canada
 Spin correlations and coherent dynamics in a spinor BEC are usually driven by mean field energies, either due to scattering between atoms or due to coupling to external fields; coherent quantum dynamics have been observed in various cold atom experiments. In this talk, I will report our studies on some beyond-mean-field phenomena that are driven by quantum fluctuations in hyperfine spin-two ultracold atoms. It is shown that zero point quantum fluctuations of collective spin coordinates can completely lift the accidental continuous degeneracy that is found in mean field analysis of quantum spin nematic phases of hyperfine spin-two cold atoms. Distinct spin nematic states with higher symmetries are selected by quantum fluctuations. It also shown that a new type of coherent spin dynamics can be driven purely by quantum fluctuations. Unlike the usual mean-field coherent dynamics, quantum fluctuation-controlled spin dynamics are very sensitive to variation of quantum fluctuations. They have peculiar dependence of Zeeman field and potential depths in optical lattices.
February 22, 2010
Scratching the Surface
Prof.  Michael Fuhrer
University of Maryland
 Graphene, a single atom-thick plane of graphite, has recently been isolated and studied experimentally. In this two-dimensional hexagonal lattice of carbon atoms, the electrons obey the Dirac equation for massless particles, complete with a two-component spinor degree of freedom that mimics the spin of a relativistic particle. Graphene is also composed entirely of surface atoms, making the techniques of surface science useful in studying its properties. In this talk, I will first discuss the electronic structure of graphene, and its implications for electronic properties. I will then discuss experiments which combine ultra-high vacuum (UHV) surface science with electronic transport measurements. Surface science techniques can be used to controllably modify graphene's properties: potassium atoms can be deposited to form charged impurity scatterers; ice can be deposited to modify the dielectric environment of graphene and tune the electron-electron interaction strength; and ion irradiation can be used to create atomic vacancies which act as Kondo impurities, and at high densities induce strong localization. Graphene may also be used as a sensitive detector to observe chemical reactions occurring on its surface at concentrations below 1/1000th of a monolayer.
April 2, 2010
Unusual Magneto-Electric Phenomena in Topological Insulators
Prof.  Marcel Franz
University of British Columbia, Canada
 Recently discovered topological insulators (TIs) are materials with bulk bandgap and robust gapless surface states protected by topological invariants that characterize their bulk band structure. After a brief introduction to the physics of TIs I will describe two new effects that have been predicted to occur in these materials: the Witten effect and the `wormhole' effect. According to the first a unit magnetic monopole inserted into a TI binds precisely quantized fractional electric charge -e/2. According to the second an infinitely thin solenoid carrying half of the magnetic flux quantum inserted into a TI carries one-dimensional topologically protected gapless fermionic modes. Both effects depend solely on the topological invariants and are thus universally present in all topological insulators. I will discuss broad physical significance of these findings as well as possibilities for experimental observation of closely related phenomena.
April 12, 2010
Magnetism and Pairing Symmetry in the Iron-Based Superconductors
Prof.  Jiangping Hu
Purdue University
 I discuss the existence of strikingly identical paradigms applicable to both cuprates and iron-based superconductors in understanding magnetism, superconductivity and the interplay between the two. The magnetic states and transitions in iron- based superconductors are well described by a J1-J2-Jz magnetic exchange model where J1, J2 and Jz are nearest neighbour, next nearest neighbour and inter-layer couplings respectively. Differing from the t-J model for cuprates where d-wave pairing symmetry is favored, the magnetic exchange in the iron based superconductors leads to a new prediction that an unconventional s-wave coskxcosky pairing dominates. I emphasize that the superconducting gaps in different Fermi pockets are determined by a single energy scale parameter which is a distinctive prediction that differs from the weak coupling theories. I will show recent experimental results that support the model and it¡¯s predictions.
April 26, 2010
FRUSTRATION BY DESIGN: Artificial Frustrated Magnets
Prof.  Peter Schiffer
Penn. State University
 Our group has developed and studied 'artificial frustrated magnets', model systems based on geometrically frustrated magnetic materials. Artificial frustrated magnets consist of arrays of lithographically fabricated single-domain ferromagnetic islands, arranged in different geometries such that the magnetostatic interactions between the island moments are frustrated. Magnetic force microscopy imaging of these arrays allows us to study the accommodation of frustration through the local correlations between the moments as a function of both the strength of the interactions and the geometry of the frustration. The results closely mimic those of the "spin ice" materials, and allow a detailed analysis of the local correlations in two dimensions. We have also used these arrays to analyze the process of demagnetization, which is necessary to access low energy collective states in our arrays and in many other magnetic systems. Our results shed light on the nature of magnetism in patterned arrays and provide a rich arena in which to study the physics of frustration. References: R. F. Wang et al. 439, 303 (2006); C. Nisoli et al. Physical Review Letters 98, 217203 (2007); X. Ke et al. Physical Review Letters 101, 037205 (2008) and Applied Physics Letters 93 252504 (2008); Li et al. Physical Review B 81, 092406 (2010).
May 3, 2010
Topological Transitions in Dissipative Quantum Transport
Dr.  Mark Rudner
Harvard University
 We investigate quantum transport in a family of one-dimensional models where a particle can decay whenever it visits sites on one of two sublattices. The corresponding non-Hermitian tight-binding problem exhibits distinct topological phases, characterized by a winding number defined in terms of the Bloch eigenstates in the Brillouin zone. We find that the mean displacement of a particle initially localized on one of the nondecaying sites can be expressed in terms of the winding number, and is therefore quantized as an integer, changing from zero to one at the critical point. This distinctive and robust feature can be used as an experimental test for quantum behavior in multilevel systems such as Josephson arrays, and holds additional implications for photon and nuclear spin pumping.
May 4, 2010
HOLOGRAPHIC MODELS OF SYMMETRY BREAKING AND QUANTUM PHASE TRANSITIONS
Dr.  Nabil Iqbal
MIT
 Certain string-theory inspired ideas allow us to understand strongly coupled field theories by a mapping to a weakly coupled gravity theory in one higher dimension. Recently these techniques have been turned to problems relevant to condensed matter theory. I will review these ideas and discuss a holographic model realizing an "antiferromagnetic" phase in which a global symmetry is broken by the strongly interacting dynamics of a finite charge density. The transition can be driven to zero temperature, when it becomes a quantum phase transition of the Berezinskii-Kosterlitz-Thouless type, and I will explain how this fact can be related to the near-horizon geometry of a zero-temperature black hole.
June 10, 2010
EVIDENCE FOR NON-FERMI LIQUID PHASE IN Ge-SUBSTITUTED YbRh2Si2
Prof.  Silke Bühler-Paschen
Technische Universität Wien
 The canonical view of heavy fermion quantum criticality assumes a single quantum critical point separating the paramagnet from the antiferromagnet. Recent experiments on quantum critical Yb-based heavy fermion compounds including Yb(Rh{0.94}Ir{0.06}2Si2, YbAgGe, and ß-YbAlB4 have tentatively observed the presence of non-Fermi liquid behavior over a finite zero-temperature region of the magnetic field- or pressure-tuned phase diagram, rather than at a single quantum critical point. Our detailed susceptibility and transport measurements show that the ``classic' quantum critical system, Ge-substituted YbRh2Si2, also displays a finite zero-temperature region of non-Fermi liquid behavior. I shall discuss possible interpretations of these results, advancing arguments that the non-Fermi liquid phase in this material is not a disorder-smeared quantum critical point, but a new class of metallic phase.

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