WINTER QUARTER 2012
National Institute of Standards
The simple harmonic oscillator is a fundamental building block of everyday physics, from playground swings to the state of light. However, energy dissipates in many of these systems in ways that can be well described by extending these oscillator-like systems through coupling to other oscillators. This kind of coupling can be intuitively understood from a classical standpoint, but it also forms the basis for the most basic linear optical element-- the beamsplitter. In this talk, I'll show how simple superconducting devices (SQUIDs and resonators) can be used to create this interaction with microwave light, coupling microwave Fock states between harmonic oscillator modes at different frequencies combining elements of both cavity-QED and linear quantum optics. This generalized beam-splitter process, although simple, can be used to swap unknown quantum states and even build up straightforward interferometers (like 'real' beam splitters) albeit in a more abstract frequency space. I will discuss the experimental verification of this process so far, as well as briefly discuss how systems like this might be extended to address other problems more traditionally addressed in quantum optics.
University of Massachusetts at Amherst
Superconductivity is a state where electric charge flows without resistance. In Type-I and Type-II superconductors the charge flow patterns are dramatically different. Type I was discovered a century ago. In response to a weak magnetic field it creates a supercurrent near its surface and expels the applied magnetic field from its interior. Type II superconductivity was experimentally discovered by Shubnikov in 1937. Applied magnetic field can gradually penetrate this type of superconductors. In mid-1950, A.A.Abrikosov explained type-II behavior as a formation of a regular lattice of quantum vortices. In this talk, I will argue that some of the newly discovered materials can have a different kind of superconductivity which breaks the type-I/type-II dichotomy and can form new phases in applied magnetic field.
George Mason University
At the interface between a three-dimensional topological insulator (TI) and an s-wave superconductor (S) forms a remarkable two-dimensional superconductor. It resembles the p_x+ip_y superconductor, e.g., it can host Majorana fermions at vortex cores. After reviewing the basic idea of the original work of Fu and Kane [Phys. Rev. Lett. 100, 096407 (2008)], I will present our microscopic, self-consistent theory for the proximity effect near the TI-S interface. Recent experimental puzzles on TI-S proximity structures will be surveyed to argue that the proximity effect involving helical Dirac fermions is yet to be fully understood.
Argonne National Laboratory
The coherent transport of spin information is one of the great challenges in condensed matter physics and is of fundamental importance for the development of spintronic devices. Spin waves carry angular momentum and can be utilized to transport spin information over distances much larger than the spin diffusion length in metals. Recent experiments showing that spin waves can be manipulated via spin currents and vice versa due to spin torque, spin pumping, spin Hall and spin Seebeck effects have drawn great attention to the transport properties of spin waves. Fundamental topics are spin-wave propagation characteristics in microstructures with reduced dimensionality, realization of spin-wave transport in two-dimensional waveguides, including directional changes along the spin-wave propagation path, and the effect of nonlinear damping mechanisms when spin waves are spatially confined in microstructures. We use phase- and time-resolved Brillouin light scattering microscopy to address these topics in micron-sized spin-wave conduits made from permalloy. These experiments allow us to develop a simple model for calculating dispersion relations in spin-wave conduits. This model can be applied to understand how spin waves are transported in conduits with broken translation symmetry and how nonlinear damping via four-magnon-scattering is enhanced due to spatial confinement.
References:
Phys. Rev. Lett. 100, 047204 (2008),
Appl. Phys. Lett. 95 182508 (2009),
Appl. Phys. Lett. 99, 162505 (2011).
University of Chicago
In this talk, I will discuss some phenomenological models of gap function relevant for different families of iron based superconductors. These models are based on microscopic calculations with spin-fluctuation pairing mechanism. I will also discuss recent penetration depth measurements and thermal conductivity measurements in BaFe2As2 based superconductors. These experiments put a strong constraint on the structure of order parameters in these materials. I will show that these experimental observations can be explained by including impurity scattering and DFT calculated Fermi surface with the proposed model of order parameters.
Université de Sherbrooke
Coupling of superconducting qubits to quantized microwave fields stored in electrical circuits has opened new possibilities for quantum optics and quantum information processing in solid-state devices. With the steady improvements of the coherence time of superconducting qubits and with the large qubit-field coupling that can be achieved, these on-chip realizations of cavity QED, also known as circuit QED, can reach new parameter regimes currently unexplored in traditional quantum optics. In this talk, I will present an overview of circuit QED explaining how the quantum nature of microwave fields can be preserved for long times in specially designed electrical circuits and how this field can be strongly coupled to artificial on-chip atoms. I will then present recent results on using a superconducting qubit to probe the rich physics of a nonlinear resonator. I will explain how spectroscopy of the qubit can reveal information about bifurcation and measurement back-action. This talk is aimed at non-specialists.
Northwestern University
Semiconductors with anisotropic electron and hole dispersions are shown to function as anisotropic two-band transverse thermoelectrics (A2T), whereby longitudinal electrical currents drive transverse Peltier heat flow with no external magnetic field. Equations for thermoelectric transport under a two-band electron-hole model yield the optimal orientation for the current and transverse heat flow to achieve maximum transverse figure of merit Z⊥T. The type II broken-gap InAs/GaSb superlattice (T2SL) is shown to have the appropriate A2T band structure, and band gaps of order the thermal energy kT are calculated to give competitive Z⊥T. Preliminary geometries are proposed for maximal thermal cooling in nanostructures.