Condensed Matter Seminars
2009 – 2010

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.

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.

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.

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.

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.

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.

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.

Prof.  Yong Baek Kim

University of Toronto, Canada

 Abstract