September 27, 2012

Professor Shixiong Zhang

Department of Physics, Indiana University

Magneto-transport Studies of 3-dimensional Topological Insulators and Spintronic Nano-devices

The magneto-resistance (MR) of a condensed matter system reveals detailed information about the Fermi surface and various types of interactions, and can sometimes provide great opportunities in magnetic sensor and memory applications. In the first part of my talk, I will present the MR studies of a new class of quantum materials called 3-d topological insulators with unique topological surface states that are protected by time- reversal symmetry. A linear magnetic field dependence of resistance was observed up to 60 Tesla in the epitaxial thin films of topological insulator Bi2Te3 grown by pulsed laser deposition. The correlation between the linear MR and weak anti-localization effect will be discussed. In the second part, I will present my work on the electrical injection and detection of electron spins in quasi-one dimensional silicon nanowires. In particular, I will show the non-local spin valve effect in Co/AlOx/Si nano-devices and discuss the influence of contact geometry on the spin-valve signals.

October 11, 2012

Professor J. A. Sauls

Northwestern University

Chiral Order in an Anisotropic Glass

Recent developments in the fabrication of high porosity, homogeneous anisotropy silica aerogels - a very low density glass - provide new opportunities to investigate the competition between order with broken continuous symmetries and disorder with random field anisotropy. Random field models coupled to broken continuous symmetries have been investigated since the early 1970’s in the context of collective pinning in type II superconductors by Larkin and ferromagnetism in a random-field by Imry and Ma. In this talk I argue that the quantum liquid phases of 3He infused into silica aerogel provide a unique system for studying the struggle between orbital order of Cooper pairs, and disorder characterized by random field anisotropy. This competition leads to remarkable new chiral superfluid phases exhibiting broken time-inversion symmetry, space parity, and as I argue in this talk an new phase that has finite range orientational order in two dimensions, but long range order in a third dimension. This phase is the realization of a biaxial chiral phase with finite range orientational correlations due to the random anisotropy field of the aerogel medium. These conclusions are supported by theoretical analysis of the phase diagram and the NMR spectra for these phases reported by the low-temperature group at Northwestern. I also discuss two different theories of the random anisotropy field in aerogel.

October 18, 2012
Special Time & Place: 4:15 pm, Lecture Room 4

Dr. Kimitoshi Kono

Advanced Sciences Institute, RIKEN

Strong Microwave Excitation and Negative Resistance States
Walter & Christine Heilborn Lecture

Electrons bound to the surface of liquid helium can occupy a discrete set of energy levels, En, each of which has a degree of freedom to move freely parallel to the surface. In the other words, each surface state forms a two-dimensional (2D) sub-band. An electron can be excited from its ground surface state, E1, to an excited state, Ej, by irradiating the surface with microwave radiation at the frequency of (Ej − E1)/h. The first observation of this microwave absorption was achieved by C. C. Grimes at Bell Laboratories, and provided direct evidence for 2D electronic states. Recent renewal of interest in this effect is due to the idea that surface state electrons on liquid He may provide single electron qubits with long coherence times for quantum computation. In the course of development of new experiments at RIKEN, we discovered a number of interesting non-equilibrium phenomena. One of the highlights is the appearance of a zero conductance state in a quantizing magnetic field. This state can be understood in the context of the appearance of negative resistance.

November 1, 2012

Professor Mark Dykman

Michigan State University

Ripplonic Lamb Shift for Electrons on Helium Surface

Electrons on a helium surface make an ideal system for studying many-body phenomena, as it is free from disorder. I will discuss some recently found phenomena related to the electron coupling to capillary waves on the helium surface, ripplons, and to the electron-electron interaction. One of the controversial issues is the role of two-ripplon processes. They lead to energy relaxation and to the Lamb-like shift of the energy levels of quantized electron motion normal to the surface. I will show that the shift is free from the strong divergence that appears if, following conventional wisdom, one disregards the 3D character of the electron motion. It displays a characteristic temperature dependence, which is in quantitative agreement with the experiment. I will also discuss the electron energy relaxation and explain why we expect a long coherence time for qubits based on quantum dots on helium surface. A long-sought effect in 2D electron systems, the onset of intrinsic resonant bistability of the response to a microwave field, will be described. It arises for electrons on helium due to the electron correlations.

November 8, 2012

Professor Takeshi Mizushima

Okayama University & Northwestern University

Symmetry protected topological order and spin susceptibility in superfluid 3He-B

The superfluid 3He-B has been recognized as a concrete example of topological superconductors, where the time-reversal symmetry ensures a nontrivial topological number and the existence of helical Majorana fermions. This may indicate that any time-reversal breaking disturbance wipe out the topological nature. In this seminar, we demonstrate that the B phase under a magnetic field in a particular direction stays topological due to a discrete symmetry, that is, in a symmetry protected topological order. Due to the symmetry protected topological order, helical surface Majorana fermions in the B phase remain gapless and their Ising spin character persists. We unveil that the competition between the Zeeman magnetic field and dipole interaction involves an anomalous quantum phase transition where a topological phase transition takes place together with spontaneous breaking of symmetry. Based on the quasiclassical theory, we illustrate that the phase transition is accompanied by anisotropic quantum criticality of spin susceptibilities on the surface, which is detectable in NMR experiments.

November 15, 2012

Professor Hui Deng

University of Michigan

Macroscopic Quantum States of Matter and Light - Microcavity Polariton Condensation and Beyond

P.W. Anderson once pointed out emergent phenomena in quantum degenerate manybody systems as a new science for the modern era in the hierarchy of sciences.[1] Examples of such systems include superfluids, superconductors, ultra-cold atomic gasses, and most recently -- the light-matter hybrid system of microcavity exciton-polaritons.

I will review in this talk the special properties of polaritons and recent research activities on 2D polariton condensation and lasing.[2] I will then introduce an unconventional hybrid photonic crystal cavity (HPCC) structure, designed to engineer the properties of polaritons, including their polarization, dimensionality, and effective mass. Such an HPCC polariton system provides a sandbox for novel manybody physics.

[1] Anderson, P. W. More is Different. Science 177, 393–396 (1972). [2] Deng, H., Haug, H. & Yamamoto, Y. Exciton-polariton Bose-Einstein condensation. Rev. Mod. Phys. 82, 1489 (2010).

November 16, 2012
Special Time & Place: 11:00 am, F160

Dr. Manuel Houzet

CEA, Grenoble

Topological Josephson junctions out of equilibrium

We study Josephson junctions between superconductors connected through the helical edge states of a two-dimensional topological insulator in the presence of a magnetic barrier. As the equilibrium Andreev bound states of the junction are 4π periodic in the superconducting phase difference, it was speculated that, at finite dc bias voltage, the junction exhibits a fractional Josephson effect with half the Josephson frequency. Using the scattering matrix formalism, we show that his effect is absent in the average current. However, clear signatures can be seen in the finite-frequency current noise. Furthermore, we discuss other manifestations of the Majorana bound states forming at the edges of the superconductors.

November 29, 2012

Professor Anupam Garg

Northwestern University

A new spin on Weyl, Wigner, and Moyal

Weyl ordering of operators and Wigner's quasiprobability distribution function were tied together into a single framework by Moyal, who also showed that the classical dynamical variable corresponding to the commutator of two operators is the Poisson bracket to leading order in h-bar. This formalism is one of the most powerful ways of understanding semiclassics and the relation between quantum and classical mechanics.
All this was done for position and momentum back in 1949. Can we do it for spin also? After all, we often want to think about spin precession, adiabatic passage, etc. using a classical picture. Come to the talk to find out how to give Weyl and Wigner the Moyal treatment.

December 4, 2012

Special Seminar - Tuesday, 4:00 pm, Room F160

Dr. Eric D. Bauer

Los Alamos National Laboratory

Discovery of Heavy Fermion Superconductors PuRhIn5 and PuCoIn5: Probing the Origin of Superconductivity at 18.5 K in PuCoGa5

The discovery of superconductivity in PuCoGa5 with a transition temperature of Tc = 18.5 K has generated renewed interest in Pu-based intermetallic compounds [1-3]. PuCoGa5, and its superconducting cousin PuRhGa5 (Tc = 8.7 K), have the same crystal structure as a growing class of tetragonal CemTnIn2m+3n (T=Co, Rh, Ir) heavy fermion superconductors, suggesting that the structure plays a key role in generating superconductivity in these materials. The Pu 5f electrons in PuCoGa5 appear to be neither fully localized nor fully itinerant; elements of both kinds of behavior manifest themselves in photoemission measurements [4] and in physical properties such as an enhanced electronic specific heat coefficient γ~ 100 mJ/mol-K2 consistent with moderately heavy fermion behavior. While a variety of measurements have firmly established that the CeTIn5 compounds are unconventional d-wave superconductors, most probably mediated by antiferromagnetic spin fluctuations, it is less clear what drives the high transition temperature in PuCoGa5, which is an order of magnitude larger than all other know Ce- or U-based heavy fermion superconductors. In this talk, I will describe the physical properties of two new members of this ``115'' family of superconductors, PuRhIn5 and PuCoIn5, which provide an exciting opportunity to understand the origin of the ``high-Tc'' in PuCoGa5.

[1] J. L. Sarrao et al., Nature 420, 297 (2002).
[2] F. Wastin et al., J. Phys.: Condens. Matter 15 S2279 (2003).
[3] N. J. Curro et al., Nature 434, 622 (2005).
[4] J. J. Joyce et al., Phys. Rev. Lett. 91, 176401 (2003).