Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Committee Chair

Leonid P. Rokhinson

Committee Member 1

David D. Nolte

Committee Member 2

Gabor A. Csathy

Committee Member 3

Yuli Lyanda-Geller

Abstract

Recently, interest in superconductor-semiconductor interfaces was renewed by the search for non-Abelian states. One of the possible platform is proximity induce superconductivity into an 1D semiconductor system with strong spin orbit (SO) interaction, such system is predicted to support Majorana excitation. Another candidate is superconductivity coupled to the edge of fractional quantum Hall state, in such system, higher order of non-Abelian statistics is predicted. With such non-Abelian states, topological quantum computing can be realized. In this thesis, I will discuss the approach made by us to investigate such system.

The thesis will begin with a brief review of superconducting proximity effect in semiconductor, including the study of Andreev reflection, multiple Andreev reflection and current phase relation in a Josepheson junction. The second part of the introduction will focus on quantum Hall effect and fractional quantum Hall effect, the theoretical and experimental study of ν =2/3 edge states, and spin polarizations of the ν =2/3 state.

In chapter 2, I will discuss the suprconducting proximity effect in a two-dimensional electron system in GaAs, where variety of strongly correlated states including fractional quantum Hall effect can be observed. I present the procedure to form transparent superconducting contacts to high mobility two-dimensional electron gas (2DEG) in GaAs using a high critical field superconductor. Induced superconductivity across several microns is demonstrated and supercurrent in a ballistic junction is observed across 0.6 μm of 2DEG. High transparency of contacts are evaluated by measurements of the Andreev reflection at the superconductor/semiconductor interface. We also measured magnetic field dependence of the critical temperature and transport behaviour of a superconductor in the quantum Hall and fractional quantum Hall regimes. The results show modifications of the Hall voltage at certain filling factors.

In chapter 3, I will discuss an experimental realization of helical channels between incompressible spin polarized ν =2/3 and spin unpolarized ν =2/3. Gate control of spin transitions in the ν =2/3 regime allows formation of localized domain walls, which consist of counter-propagating edge states of opposite polarization with fractional charge excitations. These time reversal invariant nature of domain walls can be confirmed by a non-vanishing resistance under different directions of perpendicular magnetic field. Evolution of domain walls with gate voltage and magnetic field will be also discussed. An experimental realization of a helical channel at ν =2/3 allows us to further investigate the nature of FQHE, also enable the building of a potential platform to realize high order non-Abelian exciatations.

In Chapter 4, I present our experimental investigation of S/N/S junctions in Al/InAs/Al system. With the advance of MBE techniques, a thin layer of superconducting Al can be epitaxially grown on top of InAs shallow 2D electron gas, which is a potential candidate for large scale top down approach of a topological quantum computer. Here, we report the fabrication of an InAs-based superconducting quantum intereference devices (SQUID),which consists of two Al/InAs/Al Josepheson junctions (JJs). By using two different top gates, both junctions can be tuned continuously from superconducting regime into insulating regime. From oscillations of critical current with external magnetic field, we can deduce that transport is quasi-balistic in a 150 nm junction.

Download

Share

COinS