Transport Studies on Gated Three-Terminal Josephson Junctions

Abstract

This thesis covers experimentally the properties of gate-tunable three-terminal Josephsonjunctions based on a superconductor-proximitized two-dimensional semiconductor. Specifically, InAs quantum wells with an epitaxially grown aluminum capping layer. Multiterminal Josephson junctions have recently attracted experimental work following several theoretical predictions of emergent topological physics. Namely, that topologically protected states may manifest themselves in the generalized multiterminal Andreev bound state spectrum of such devices, as a function of applied voltages and relative phase between the superconducting leads. The theoretical efforts focus on the case of multiple superconducting leads in contact with a small central scattering region characterized by a scattering matrix. Detection of the topological Andreev bound states via quantized transconductance is predicted to be possible at low bias when there are few conductance modes between each terminal, and for a subset of possible scattering matrices. In this work, three-terminal Josephson devices are fabricated, and subsequently measured at millikelvin temperatures. The electronic transport characteristics of a top-gated Y-shaped three-terminal Josephson junction are measured, as well as their gate dependencies. These behaviors are explained by combining qualitative, analytical, and simulation based approaches. Further, three-terminal Josephson junctions with closely-spaced independent gates are studied. The effect of asymmetric gating by these independent gates is elaborated and supported by simulations. These devices also show access to a few-conductance-mode regime via quantum point contact-like gating, moving closer to the strict requirements proposed by topological multiterminal Josephson junction theory. Lastly, a gate-tunable superconducting Josephson diode effect is measured and described in a three-terminal Josephson junction. It is demonstrated that this effect is fundamental to multiterminal Josephson devices, and in principle independent of the material used.