« fall 2018 - spring 2019 - summer 2019 »

This week | Next week | This semester | All future | Print view

This week | Next week | This semester | All future | Print view

Friday, January 19th 2018

12:20 pm:

Friday, January 26th 2018

12:20 pm:

Quantum phase transitions are an interesting topic in condensed matter physics that has been studied extensively. One example of a quantum phase transition is the superconductor-insulator transition in InOx. These transitions are typically studied using scaling analysis. In the case of a field tuned transition this starts with measuring various resistance versus field isotherms. The field at which the isotherms cross is taken as the critical field and used in the scaling analysis and to find the critical exponents. However, in some systems, such as one of our InOx thin films, there is not a single crossing, instead the crossing is smeared out resulting in a temperature dependent critical field and critical exponents. Detailed transport measurements of the superconductor-metal transition in this sample suggest that this temperature dependent behavior may be the result of a quantum Griffiths singularity. Similar behavior how been observed by other groups in different materials that also undergo superconductor-metal transitions.

Friday, February 2nd 2018

12:20 pm:

InSb nanowires have been established as a versatile materials platform for realizing Majorana bound states and for investigating spin-orbital interaction (SOI) and large effective g-factor. By coupling InSb nanowires to ferromagnetic (FM) contacts, a spin-polarized current can be injected into the nanowires and subsequently controlled by electric fields. We will report on our progress fabricating nanowire devices with FM electrodes in local spin-valve geometry, as well as discussing our low-temperature transport experiments on these devices.

Friday, February 9th 2018

12:20 pm:

I will discuss how to obtain a gapless phase from a topological gapped phase by breaking both time reversal and inversion symmetry. The specific model would be 2D p+ip chiral superconductor. 2D chiral superconductor belongs to symmetry class D with particle-hole symmetry and could support chiral Majorana. A supercurrent could break inversion symmetry and make system gapless. I'm going to discuss that particle-hole symmetry guarantees that there is a metallic phase which is distinct from trivial metal.

Friday, February 16th 2018

12:20 pm:

Quantum spin liquid has been studied intensively in recent years. Among various types of spin liquid, the Kitaev model is unique because it is exactly solvable. Kitaev’s original solution is based on Majorana representation of spin. In this talk, I will discuss three types of Majorana representation of spin and argue that they can be linked together by Dirac spinor representation of SO(4) group. Then, I will discuss an alternative solution of the Kitaev model using another Majorana representation and talk about aspects of the solution.

Friday, February 23rd 2018

12:20 pm:

Thermally activated hopping between energy minima in a double well system is expected to follow an Arrhenius Law. Experiments have shown that the rate of switching between two wells is proportional to the Boltzmann factor but little work has been done to probe the nature of the characteristic dwell time. A square, permalloy, mesoscale dot with an applied magnetic field can be used to create a double well system to explore the characteristic dwell time. I will show that the characteristic dwell time has an exponential dependence on the height of the barrier. There is a significant quantitative disagreement between accepted models of the dwell time and our results.

Friday, March 2nd 2018

12:20 pm:

Friday, March 9th 2018

12:20 pm:

Friday, March 23rd 2018

12:20 pm:

Topological insulators (TIs) possess topologically-protected helical

surface states that are immune to non-magnetic disorder and described

by a relativistic "Dirac" Hamiltonian. Similar states are expected

at the interface between a TI and any topologically trivial material.

I will discuss recent work showing that the interface states at the

boundary between a topologically-trivial semiconductor (SE) and a TI

possess a much richer phenomenology compared to the helical surface

states, including complex spin textures and broken helicity.

Friday, March 30th 2018

12:20 pm:

Haldane exclusion statistics is one of the many attempts to interpolate between Fermi and Bose cases. However, the distribution function of Haldane statistics is still an open question, parallel to the well-known Fermi and Bose distribution function. I’ll discuss the negative weight problem of Yongshi Wu’s distribution function of Haldane statistics, Murthy and Shankar’s solution and our improvement.

Friday, April 6th 2018

12:20 pm:

We investigate the interplay between gap oscillations and damping in the dynamics of superconductors taken out of equilibrium by strong optical pulses with sub-gap Terahertz frequencies. A semi-phenomenological formalism is developed in which damping is incorporated by relaxation and dephasing processes within the electronic subsystem, which correspond to T1 and T2 times in the standard pseudospin language for BCS superconductors. Comparing with data on NbN that we report here, we argue that the superconducting dynamics in the picosecond time scale, after the pump is turned off, is governed by the T2 process.

Friday, April 13th 2018

12:20 pm:

The study of spintronics includes 3 tasks: the generation, manipulation and detection of spin polarization. The inverse spin Hall effect(ISHE), which converts pure spin currents to electric currents, is an important way to detect spin accumulation. Although it has been used widely, its origins are not necessarily understood. For example, the ISHE in GaAs has been found to be surprisingly large in n-GaAs(doped with Si), and at low temperatures it is very sensitive to hyperfine fields. In this talk, I will present my measurements on devices with channels of varying doping at different temperatures. The goal of this experiment is to study the ISHE in a regime where the complications of hyperfine interactions are absent.

Friday, April 20th 2018

12:20 pm:

Sachdev-Ye-Kitaev (SYK) model is a strongly correlated disordered model which exhibits many interesting properties, such as non-Fermi liquid behavior, AdS/CFT correspondence, and maximally chaoticity. In this talk, I will introduce a modified SYK model to investigate the interplay between interaction and disorder numerically. By adding constrain in coupling constant of SYK, a many-body "mobility edge" in the spectrum can be found by checking the nearest neighbor spacing distribution. When choosing a specific basis of many-body Hilbert space, one can find localized and delocalized wavefunction in Fock space. A discussion of connection and difference between chaoticity and locality of the states in this model will also be present.

Friday, April 27th 2018

12:20 pm:

Some new candidate Kitaev spin liquid material has recently been found in the experiment. Qualitative signatures of spin liquids are important to identify them. We try to explore the features of unusual edge physics in the specific heat. We find that, the fluxe excitation has some edge effects in specific heat. We also show that there is zero-temperature degenerate entropy which indicates the existence of gapless edge mode.

Friday, May 4th 2018

12:20 pm:

Two-dimensional electron gases (2DEGs) with strong spin-orbit coupling are expected to host topological states in the presence of superconductivity and broken time-reversal symmetry. Josephson junctions based on such 2DEGs are predicted to exhibit an unconventional current-phase relation, characterized by a component at twice the Josephson frequency due to the presence of gapless topological Andreev bound states. In order to study the current-phase relation of these bound states, progress towards characterization of the microwave response of Josephson junctions based on InAs quantum wells proximitized with epitaxial Al.

Friday, May 11th 2018

12:20 pm:

Friday, September 7th 2018

12:20 pm:

Friday, September 14th 2018

12:20 pm:

Friday, September 21st 2018

12:20 pm:

Friday, September 28th 2018

12:20 pm:

Friday, October 5th 2018

12:20 pm:

Recent experiments have described the measurement of a near-quantized thermal Hall conductance in α-
-RuCl_3
as direct evidence for the propagation of a chiral Majorana mode in this system and consequently as smoking gun evidence for an intermediate chiral spin liquid phase in this system. In practice, however, contacts are applied to the lattice, and one must ask in detail whether the experimental setup can directly, or even at all, probe the Majorana current. Moreover, the large experimental longitudinal thermal conductivity, attributed to phonons (in the chiral phase, κ_{xx}^{spins} = 0-
) begs for an investigation of the spin-lattice coupling, and what its role is on the quantization of the thermal Hall conductivity. We show that due to the bulk and edge mixing of energy propagation, the temperature gradient of the bulk phonons develops a transverse component which may contribute to a quantized effective thermal Hall conductance. We also discuss the situations where the quantization breaks down and predict notable experiments that test it

Friday, October 12th 2018

12:20 pm:

Friday, October 19th 2018

12:20 pm:

Friday, October 26th 2018

12:20 pm:

Friday, November 2nd 2018

12:20 pm:

Friday, November 9th 2018

Friday, November 16th 2018

Friday, November 30th 2018

Friday, December 7th 2018

Friday, December 14th 2018

The weekly calendar is also available via subscription to the physics-announce mailing list, and by RSS feed.