Condensed-Matter Physics seminars: Winter 2009

Wednesdays in the Physics Reading Room

Date & timeSpeaker & affiliation Talk title & abstract
Jan 7 4:00pm TBA
Jan 14 4:00pm David Clarke (UCR)
Domain wall motion in thin ferromagnetic strips
Abstract: The rich internal structure of domain walls in ferromagnetic nanostrips greatly affects the motion when an external magnetic field is applied, leading to reduced mobility when the driving force is strong. In particular, in the wider strips used in many experimental studies, the wall is expected to contain a magnetic vortex at its heart. A linearized two-coordinate model, using just the position of the vortex. leads to predictions for the average wall velocity that are in good agreement with numerical simulations when the field is not too high. In higher fields, additional degrees of freedom enter the motion. I will show examples of such modes and discuss the effect that they have on the mobility of the wall.
Jan 21 4:00pm Lloyd Lumata (Florida State and NHMFL)
Spin Dynamics of Density Wave and Frustrated Spin Systems Probed by NMR
Abstract: The quasi-one-dimensional organic conductor (TMTSF)2ClO4 exhibits a series of field-induced spin-density wave (FISDW) phases at low temperature and high magnetic fields. Simultaneous 77Se NMR and electrical transport at magnetic fields up to 30T reveal new information about the electronic structure of this material. On the other hand, 69Ga spin-lattice and spin-spin relaxation rate measurements on the rare-earth kagome system Pr3Ga5SiO4 suggest the presence of short-range nanoscale ordering consistent with neutron scattering results. Detailed field-dependent NMR studies of these two condensed matter systems will be discussed.
Jan 28 4:00pm TBA
Feb 4 4:00pm Cenke Xu (Harvard)
Nematic and Magnetic Orders in Fe-based Superconductors
Abstract: The Fe-based high temperature superconductors (Fe-SC) discovered a year ago has created enormous excitements in condensed matter physics community. In this talk, we will focus on two phenomena shared by all the Fe-SC materials: tetragonal-orthorhombic lattice distortion (LD), and spin density wave (SDW). A unified picture for both effects is proposed, motivated by various evidences from experiments. We will discuss the nature of classical phase transitions of both LD and SDW at finite temperature, affected by the elasticity of the lattice, as well as the quantum phase transitions at zero temperature, driven by doping and pressure. A global phase diagram for LD and SDW will be proposed, with predictions for future experiments. Finally, the quantum nematic transition with a background d-wave superconductor is studied. The strong coupling between nematic order nodal particle will modify many experimental observables.
Feb 10 2:00pm (Tuesday) Ehud Altman (Weizmann Institute)
Quantum steady states and phase transitions in the presence of non equilibrium noise
Abstract: Systems of ultracold atoms, Polar molecules or trapped ions are easily driven out of equilibrium by external (classical) noise sources. I will introduce a new class of quantum dynamical fixed points that can emerge in such systems and discuss the nature of the phase transitions between the different steady states. I will also discuss closely related non-equilibrium transitions in Josephson junctions.
Feb 11 4:00pm Vivek Aji (UCR)
New Route to Superconductivity: Local Criticality and Time Reversal Violation
  Abstract: The discovery of High Temperature superconductors has given rise to a number of new paradigms in Condensed matter physics for the past two decades. Among competing ideas that attempt to explain the phenomena, time reversal violation has gained prominence due to recent experimental observations. In this talk I will present evidence from polarized neutron scattering data that reveal the existence of magnetic order in the pseudo-gapped phase. Quantum disordering of this state results in a quantum critical point which is unconventional. The critical fluctuations are local in space and power law in time. This phase transition is driven by the proliferation of a new topological defect. The coupling of electrons to the quantum fluctuations leads to an attractive interaction in the d-wave channel providing a novel mechanism for superconductivity.
Feb 13 (Friday) 4:00pm Roderich Moessner (MPI Dresden)
Entanglement Skyrmions In Multicomponent Quantum Hall Systems
Abstract: We discuss charged topological spin textures in quantum Hall ferromagnets in which the electrons carry a pseudospin as well as the usual spin degree of freedom, as is the case in bilayer GaAs or monolayer graphene samples. We develop a theory which treats spin and pseudospin on a manifestly equal footing, which may also be of help in visualizing the relevant spin textures. We in particular consider the entanglement of spin and pseudospin in the presence of realistic anisotropies. An entanglement operator is introduced which generates families of degenerate Skyrmions with differing entanglement properties. We propose a local characterization of the latter, and touch on the role entangled Skyrmions play in the nuclear relaxation time of quantum Hall ferromagnets.
Feb 18 4:00pm Maria Vozmediano (ICMM, Madrid))
Coulomb Interactions and Disorder in Undoped Graphene
Abstract: The recent synthesis of a single layer of graphite (graphene) and the experimental observation of some unusual electronic and structural properties has prompted a real revolution in the theory of condensed matter systems. Under a theoretical point of view the fact, confirmed by the experiments, that the low energy elementary excitations are well described by massless Dirac fermions implies a revision of the Landau fermi liquid paradigm. In this talk I will give a general overview of the graphene physics at the neutrality point and address the problem of the Coulomb interactions and disorder. A renormalization group analysis allows to classify graphene as a strange Fermi liquid described by a Lorentz covariant infrared fixed point whose effective coupling constant is the fine structure constant.
Feb 25 4:00pm Jason Alicea (Caltech)
New Puzzles In Bismuth Subjected To Strong Magnetic Fields
Abstract: In sharp contrast to most 3D metals, the carrier density in bulk bismuth is sufficiently small that lowest-Landau-level physics can be explored with laboratory fields. Very recent experiments probing this high-field regime have uncovered a number of surprising features which raise interesting new questions for this material. Specifically, observations of anomalies in the Hall resistance, Nerst effect, and magnetization measurements have led to the speculation that three-dimensional bismuth in the quantum limit hosts novel correlated states, and possibly even electron fractionalization. In this talk I will discuss efforts to shed light on these findings by studying an interacting low-energy theory for bismuth in a magnetic field, describing Dirac-like electrons coupled to holes. I will argue that an anomalous Zeeman effect that arises due to strong spin-orbit coupling, together with various interaction-driven instabilities studied within a functional renormalization group framework, account for some of the puzzling observations. Outstanding questions and interesting future experimental and theoretical directions will also be discussed.
Mar 2 (Monday) 4:00pm Paul Goldbart (UIUC)
Strands of superconductivity at the nanoscale
Abstract: Superconducting circuitry can now be fabricated at the nanoscale by depositing suitable materials on to individual molecules, such as DNA or carbon nanotubes. In this talk I shall examine various themes that arise when superconductivity is explored in this new regime, including thermal barrier crossing and quantum tunneling by the superconducting condensate, as well as the impact of magnetism on nanosuperconductivity. I shall focus on a fascinating class of circuits: nanoscale superconducting quantum interference devices (or n- SQUIDs). After describing how they are made, I shall pay particular attention to the electrical resistance of these devices and, especially, its sensitivity to magnetic fields and patterns of supercurrent. These features hint at possible uses of n-SQUIDs, such as for mapping the quantum phase of superconducting order and testing for superconducting correlations in novel materials and settings.
Mar 4 2:10pm S. Zhao (Penn State & Zhejiang University, Hangzhou)
dc Electric Breakdown in Complex Lead-Based Perovskites: an Investigation of Pb(Mg1/3Nb2/3)O3PbTiO3 Relaxor Ferroelectrics
  Abstract: Complex lead based perovskite compounds and solid solutions have been of great interest due to the large piezoelectric coefficients that are induced on poling these ferroelectric materials, based on which they are extremely important in piezoelectric applications from aerospace, automotive, to civil engineering as transducers, actuators, etc. However, electrochemical processes can give rise to time dependent electric breakdown. Therefore, owing to the practical interest, it is important to develop a better understanding of electric degradation in these materials to improve the reliability and performance of ferroelectric devices based on lead perovskite compounds and solid solutions. In this talk, we shall discuss the dc electric breakdown phenomenon and the nature of electrical changes on a particular lead-based system, <001> oriented Pb(Mg1/3Nb2/3)O3PbTiO3 (PMN-PT) single crystal, which belongs to ferroelectric family and known as relaxor-type ferroelectrics. Highly Accelerated Lifetime Tests (HALT), Thermally Stimulated Depolarization Current (TSDC) and Impedance Spectroscopy (IS) measurements were employed to develop a physical model. It is found that migration of oxygen vacancies with a low activation energy controls the degradation process in this particular lead-based system. Activation energies controlling other conduction species and voltage acceleration factor were estimated, which are of practical importance to the design of reliable piezoelectric materials that could operate under high bias and/or at elevated temperatures for long times.
Mar 4 4:00pm Tami Pereg-Barnea (Caltech)
Quantum Oscillations from Fermi Arcs in the Pseudogap Phase of the High-Tc Cuprates
  Abstract: The problem of high-Tc superconductors continues to provide challenges for the condensed matter community. The key for determining the mechanism for Cooper pairing is inevitably related to the nature of the normal state. On the under doped side of the phase diagram the normal state is the pseudogap (PSG) which displays a wealth of unusual phenomena and will be the focus of this talk. Recent experiments in the mixed state of under doped cuprates revel 1/B oscillations in the Hall resistance and the magnetization. If these oscillations are to be associated with quasiparticle motion along a closed Fermi surface then their frequency is a measure of the Fermi surface size. The observed slow oscillations suggest a tiny Fermi surface which encloses only a few percents of the Brillouin zone. Therefore, the experiments are widely perceived as evidence for the existence of small Fermi pockets. Such a Fermi surface topology is at odds with the existence of a discontinuous Fermi surface, the so called "Fermi arcs" which are observed by ARPES.

In this talk I challenge this interpretation and propose a different origin for the observed oscillations. I adopt a phenomenological picture of the PSG as a phase disordered superconductor with a pairing gap that vanishes on the arcs. The model displays periodicity in the density of states as a function of both the energy and magnetic field. The oscillations result from the appearance of states reminiscent of Andreev bound states.

Mar 6 12:00 noon Ying Ran (UC, Berkeley)
Helical Metal Inside A Topological Band Insulator
Abstract: Topological defects, such as domain walls and vortices, have long fascinated physicists. A novel twist is added in quantum systems like superfluid helium He3, where vortices are associated with low energy excitations in the cores. Similarly, cosmic strings which are vortices of the Higgs field, may be tied to propagating fermion modes. Can analogous phenomena occur in crystalline solids that host a plethora of topological defects? In this talk I will show that indeed dislocation lines are associated with one dimensional fermionic excitations in a `topological insulator', a novel band insulator believed to be realized in the bulk material Bi0.9Sb0.1. In contrast to electrons in a regular quantum wire, these modes are topologically protected, and not scattered by disorder. Since dislocations are ubiquitous in real materials, these excitations could dominate spin and charge transport in topological insulators. Our results provide a novel route to creating a potentially ideal quantum wire in a bulk solid.
Mar 11 4:00pm TBA
Mar 18 4:00pm TBA
(Finals week; APS March Meeting week)
Mar 25 4:00pm TBA
(Spring break)

Seminars for Winter 2009
Seminars for Fall 2008
Seminars for Spring 2008
Seminars for Winter 2008
Seminars for Fall 2007
Seminars for Spring 2007
Seminars for Winter 2007
Seminars for Fall 2006
Seminars for Spring 2006
Seminars for Winter 2006
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Leonid Pryadko <my first name at landau dot ucr dot edu>
Last modified: Fri Feb 27 14:33:24 PST 2009