AC Measurements of Graphene-Superconductor Devices

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Date

2022

Authors

Larson, Trevyn

Advisors

Finkelstein, Gleb

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Abstract

The field of quantum transport studies electron motion at low temperatures in nanos-tructures. Exciting electron phenomenon can be engineered by combining device designs like quantum dots, Josephson junctions, and interferometers with materials which host physics such as various quantum Hall effects and superconductivity. Com- binations of these ingredients can be mixed to design a device which is then cooled down and has its I ́ V curves measured while tuning key physical parameters, such as magnetic field, temperature, and gate electrode voltages. These time independent (DC) measurements can provide a wealth of information, but ultimately they can only access highly averaged physical properties. Fortunately, this is not a fundamental constraint. By measuring the emission of and response to higher frequency signals, we are able to access additional properties of our devices. This dissertation explores two projects related to time oscillating (AC) measure- ments of graphene devices with superconducting contacts. The first project is related to the measurement of “Shapiro steps” in graphene based Josephson junctions. By applying a gigahertz drive to the junction, it becomes possible to probe the dynamics of the phase difference of the junction. The work presented here explores the effects of the RF environment on the Shapiro step pattern, and on a bistability observed in this system. The second project addresses the noise measured downstream of a superconduct- ing contact for a device in the quantum Hall regime. Recent work has observed the coupling of superconductivity to a quantum Hall edge, a promising test-bed for mix- ing superconductivity with topological physics. However, the signal in real devices remains fairly small compared to the ideal limit. Noise measurements should allow us to probe the microscopics in these devices, but we find indications that signals seemingly related to contact heating obscure the desired signal. Additional devices which should show a tunable signal amplitude show only very small signal variation, opening questions about what physical phenomena may be suppressing this noise.

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Condensed matter physics

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Citation

Larson, Trevyn (2022). AC Measurements of Graphene-Superconductor Devices. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/25235.

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