@phdthesis{14547,
  abstract     = {Superconductor-semiconductor heterostructures currently capture a significant amount of research interest and they serve as the physical platform in many proposals towards topological quantum computation.
Despite being under extensive investigations, historically using transport techniques, the basic properties of the interface between the superconductor and the semiconductor remain to be understood.

In this thesis, two separate studies on the Al-InAs heterostructures are reported with the first focusing on the physics of the material motivated by the emergence of a new phase, the Bogoliubov-Fermi surface. 
The second focuses on a technological application, a gate-tunable Josephson parametric amplifier.

In the first study, we investigate the hypothesized unconventional nature of the induced superconductivity at the interface between the Al thin film and the InAs quantum well.
We embed a two-dimensional Al-InAs hybrid system in a resonant microwave circuit allowing measurements of change in inductance.
The behaviour of the resonance in a range of temperature and in-plane magnetic field has been studied and compared with the theory of conventional s-wave superconductor and a two-component theory that includes both contribution of the $s$-wave pairing in Al and the intraband $p \pm ip$ pairing in InAs.
Measuring the temperature dependence of resonant frequency, no discrepancy is found between data and the conventional theory.
We observe the breakdown of superconductivity due to an applied magnetic field which contradicts the conventional theory.
In contrast, the data can be captured quantitatively by fitting to a two-component model.
We find the evidence of the intraband $p \pm ip$ pairing in the InAs and the emergence of the Bogoliubov-Fermi surfaces due to magnetic field with the characteristic value $B^* = 0.33~\mathrm{T}$.
From the fits, the sheet resistance of Al, the carrier density and mobility in InAs are determined.
By systematically studying the anisotropy of the circuit response, we find weak anisotropy for $B < B^*$ and increasingly strong anisotropy for $B > B^*$ resulting in a pronounced two-lobe structure in polar plot of frequency versus field angle.
Strong resemblance between the field dependence of dissipation and superfluid density hints at a hidden signature of the Bogoliubov-Fermi surface that is burried in the dissipation data.

In the second study, we realize a parametric amplifier with a Josephson field effect transistor as the active element.
The device's modest construction consists of a gated SNS weak link embedded at the center of a coplanar waveguide resonator.
By applying a gate voltage, the resonant frequency is field-effect tunable over a range of 2 GHz.
Modelling the JoFET minimally as a parallel RL circuit, the dissipation introduced by the JoFET can be quantitatively related to the gate voltage.
We observed gate-tunable Kerr nonlinearity qualitatively in line with expectation.
The JoFET amplifier has 20 dB of gain, 4 MHz of instantaneous bandwidth, and a 1dB compression point of -125.5 dBm when operated at a fixed resonant frequency.
In general, the signal-to-noise ratio is improved by 5-7 dB when the JoFET amplifier is activated compared.
The noise of the measurement chain and insertion loss of relevant circuit elements are calibrated to determine the expected and the real noise performance of the JoFET amplifier.
As a quantification of the noise performance, the measured total input-referred noise of the JoFET amplifier is in good agreement with the estimated expectation which takes device loss into account.
We found that the noise performance of the device reported in this document approaches one photon of total input-referred added noise which is the quantum limit imposed in nondegenerate parametric amplifier.},
  author       = {Phan, Duc T},
  issn         = {2663 - 337X},
  keywords     = {superconductor-semiconductor, superconductivity, Al, InAs, p-wave, superconductivity, JPA, microwave},
  pages        = {80},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{Resonant microwave spectroscopy of Al-InAs}},
  doi          = {10.15479/14547},
  year         = {2023},
}

@phdthesis{10663,
  abstract     = {The superconducting state of matter enables one to observe quantum effects on the macroscopic scale and hosts many fascinating phenomena. Topological defects of the superconducting order parameter, such as vortices and fluxoid states in multiply connected structures, are often the key ingredients of these phenomena. This dissertation describes a new mode of magnetic force microscopy (Φ0-MFM) for investigating vortex and fluxoid sates in mesoscopic superconducting (SC) structures. The technique relies on the magneto-mechanical coupling of a MFM cantilever to the motion of fluxons. The novelty of the technique is that a magnetic particle attached to the cantilever is used not only to sense the state of a SC structure, but also as a primary source of the inhomogeneous magnetic field which induces that state. Φ0-MFM enables us to map the transitions between tip-induced states during a scan: at the positions of the tip, where the two lowest energy states become degenerate, small oscillations of the tip drive the transitions between these states, which causes a significant shift in the resonant frequency and dissipation of the cantilever. For narrow-wall aluminum rings, the mapped fluxoid transitions form concentric contours on a scan. We show that the changes in the cantilever resonant frequency and dissipation are well-described by a stochastic resonance (SR) of cantilever-driven thermally activated phase slips (TAPS). The SR model allows us to experimentally determine the rate of TAPS and compare it to the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory for TAPS in 1D superconducting structures. Further, we use the SR model to qualitatively study the effects of a locally applied magnetic field on the phase slip rate in rings containing constrictions. The states with multiple vortices or winding numbers could be useful for the development of novel superconducting devices, or the study of vortex interactions and interference effects. Using Φ0-MFM allows us to induce, probe and control fluxoid states in thin wall structures comprised of multiple loops. We show that Φ0-MFM images of the fluxoid transitions allow us to identify the underlying states and to investigate their energetics and dynamics even in complicated structures.},
  author       = {Polshyn, Hryhoriy},
  keywords     = {physics, superconductivity, magnetic force microscopy, phase slips},
  pages        = {103},
  publisher    = {University of Illinois at Urbana-Champaign},
  title        = {{Magnetic force microscopy studies of mesoscopic superconducting structures}},
  year         = {2017},
}