PhD Defense: Jose Manuel Chavez-Garcia

Andreev Interferometers based on Quantum Point Contacts

In solid-state physics, there are largely two types of materials that are used to engineer quantum circuits: superconductors and semiconductors. The properties of superconductors are largely set by the energy gap of their particular material and by the fact it is fundamentally a macroscopic quantum phenomenon. That is to say, its quantum wave-function is a composite of many of its charge carriers - Cooper pairs. On the other hand, semiconductors exhibit a wider range of behaviors set by a plethora of factors including specific material composition, band structure, device geometry, etc. In this thesis, we combine both platforms to study their macroscopic and microscopic behavior.

The primary tool that we will use to study the macroscopic properties will be the DC SQUID - superconducting quantum interference device. By threading magnetic flux through a Josephson interferometer, one can change the relative phase of the superconducting condensate. Here, we use planar junctions whose supercurrent is mediated by Andreev reflection. Fundamentally different than in tunnel junctions with an oxide barrier, their transport is set by the transparencies of the semiconductor wedged between the two superconducting condensates.

We model the behavior of high channel transparencies and their signatures in transport. Using the superconducting diode effect, recently introduced in the field, we observe the diode efficiency approaching 40% - the maximum expected from highly transparent single channels.
We also introduce another metric, which we dub ∆Φ, to quantify how symmetric transport is in both sides of the interferometer. In the second part, we study the microscopic properties of these
Andreev junctions. We begin in the highly conductive regime and validate the canonical characteristic of a Josephson junction - a zero voltage superconducting state - with a four terminal measurement. We then focus on the few-channel regime and validate ballistic transport by observing the quantization of normal conductance across several devices. We study its differential conductance/resistance with respect to both measured voltage and current bias. Moreover, we observe a strong non-monotonic correlation between zero-bias and normal state conductance.