Condensed Matter Sack Lunch Seminar

semester, 2018

Friday, January 19th 2018
12:20 pm:
Speaker: Evan Moen
Subject: Spin Transport in Superconducting Spin Valves

Friday, January 26th 2018
12:20 pm:
Speaker: Nicholas Lewellyn
Subject: Magnetic Field tuned superconductor-metal transition in InOx

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:
Speaker:  Zedong Yang, University of Minnesota
Subject: Towards spin injection into semiconductor nanowires

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:
Speaker: Xuzhe Ying, University of Minnesota
Subject: Topological Metals

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:
Speaker: Jianlong Fu
Subject: Majorana representations of spin and the Kitaev honeycomb model

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:
Speaker: James Delles, University of Minnesota
Subject: Thermally Activated Hopping over a Barrier in a Mesoscale Permalloy System

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:
Speaker: Various speakers
Subject: APS March Meeting Practice Talks

Friday, March 9th 2018
12:20 pm:
There will be no seminar this week.

Friday, March 23rd 2018
12:20 pm:
Speaker: Daniel Sheehy, LSU
Subject: Interface symmetry and non-helical states in topological insulator-semiconductor heterostructures
Note: this seminar is in addition to the one on Wednesday. Please note change of day, time and room, This week only.

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.

Faculty Host: Rafael Fernandes

Friday, March 30th 2018
12:20 pm:
Speaker: Bo Xiong, University of Minnesota
Subject: Haldane Exclusion Statistics: beyond Fermion and Boson

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:
Speaker: Tianbai Cui, University of Minnesota
Subject: Impact of damping on superconducting gap oscillations induced by Terahertz pulses

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:
Speaker: Zhen Jiang, University of Minnesota
Subject: The temperature and doping dependence of the inverse spin Hall effect in GaAs

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:
Speaker: Hanteng Wang, University of Minnesota
Subject: Interplay between interaction and disorder: Existence of many-body mobility edge in a modified Sachdev-Ye-Kitaev model.

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:
Speaker: Kexin Feng, University of Minnesota
Subject: Signatures of unusual edge physics in the specific heat of Kitaev spin liquids

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:
Speaker: Gino Graziano, University of Minnesota
Subject: Towards Microwave Studies of Josephson Junctions Based on InAs Quantum Wells with an Epitaxial Superconductor

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:
There will be no seminar this week.

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