Masters Defense: Katrine Rasmussen

Magnetoresistance Peak and Commensuration Effects on the Insulating Side of the Field-Driven Superconductor-Insulator Transition.

The superconductor-insulator transition (SIT) in two dimensions (2D) is an archetypal quantum phase transition, which has made it an intensively studied problem. In spite of more than four decades of combined theoretical and experimental efforts, the topic of superconducting transitions in 2D remains a lively field with greatly disputed questions. Two of the most essential controversies regard the origin of an anomalous metallic behavior, which has been found to intervene the direct SIT in a number of experimental studies, and the origin of a peak in the magnetoresistance (MR) on the insulating side of the field-driven SIT. Both of these phenomena are in disagreement with the conventional SIT framework. An obstacle of this field is posed by the great complexity of the systems in which the superconducting transitions are observed, combined with the limited control of experimental parameters in the traditional systems for studying SITs.

In this work, superconducting transitions in 2D, driven by perpendicular magnetic field and dissipation, in the form of reduced carrier density, were investigated in an innovative and greatly unexplored platform, which offers a high degree of control. The system consists of a shallow 2DEG residing in an InAs/InGaAs-hetorostructure, proximitized by a thin layer of epitaxial aluminum, patterned to form a regular array of square islands, of area 250x250 nm2, covered with a global top gate, providing in situ control of the Josephson coupling, EJ , between islands. This platform was found to exhibit both the disputed anomalous metal regime and magnetoresistance peak, along with the well-understood commensuration effects characteristic of artificially patterned systems. Evidently, the system studied here makes for a promising candidate for providing answers to some of the long standing questions regarding SITs in 2D.