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Title:Magnetotransport in superconductor to semiconductor junctions: multiple Andreev reflections and coupling to quantum hall states
Author(s):Law Toner, Stephanie
Director of Research:Eckstein, James N.
Doctoral Committee Chair(s):Mason, Nadya
Doctoral Committee Member(s):Eckstein, James N.; Fradkin, Eduardo H.; Stack, John D.
Department / Program:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
quantum Hall edge modes
multiple Andreev reflection
vortex motion
Abstract:Two-dimensional electron gases (2DEGs) and superconductors share many interesting properties. Both exhibit dissipationless current flow as well as strong electron correlations which can lead to a new ground state charge carrier. In this work, coupling between superconductors and 2DEGs was studied. Using band structure engineering as well as InGaAs and InAs caps, junctions were fabricated in which the superconductor was extremely well-coupled to the semiconductor. The superconducting contacts were separated by a small gap whose length varied from 100nm to 1000nm. For devices capped with InGaAs and a long junction length, a zero bias resistance maximum was seen. In these devices, the maximum increased when the sample was in a quantum Hall state due to the reduction in the number of conduction channels. Samples with a shorter length showed a zero bias resistance minimum when the magnetic field was tuned between quantum Hall states and a maximum otherwise. The minimum is a precursor to a supercurrent. Device were also made with higher transparency InAs caps. One set of devices had an array of small superconducting shorts bridging the gaps. When the sample was in an even filling factor state, a large supercurrent and a sharp transition to the normal state was seen when current was applied in the direction of the edge states. Conversely, when current was applied against the edge states, a smaller supercurrent with a very broad and noisy transition to the normal state was observed. Behavior was reversed when the sample was in an odd filling factor state and when the field was applied in the negative direction. The broad, noisy transition is a result of flux flow behavior. The interaction of the edge state with the sample lowered the barriers to flux motion in one direction, resulting in an asymmetric superconducting flux flow transistor. Devices with InAs caps and no shorts showed peaks in differential resistance due to multiple Andreev reflection. As the magnetic field is increased, the MAR peaks move toward lower voltages and disappear. The movement of the peaks is caused by the decrease of the superconducting gap with field.
Issue Date:2012-05-22
Rights Information:Copyright 2012 Stephanie Law Toner
Date Available in IDEALS:2012-05-22
Date Deposited:2012-05

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