PhD defense: Andreas Pöschl

Nonlocal transport signatures of Andreev bound states in superconductor-semiconductor hybrid devices 

The superconductor-semiconductor material platform has become the subject of multiple research directions, with the promise of enabling novel devices that may become useful for quantum information processing. Moreover, certain superconductor-semiconductor combinations may allow us to realize novel quantum states and phases of matter. One example are InAs nanowires proximitized by superconducting Al, which may host a topological superconducting phase.

Novel devices conceived within our research allow us to probe the emerging phase of matter at multiple discrete locations of a gate defined InAs-Al nanowires. In particular, we focus on extended quantum states in these systems, so-called Andreev bound states.  We show the characteristic appearance of these states at both ends of a nanowire in a correlated fashion. The characteristic evolution of these states as a function of gate electric fields and magnetic fields is investigated. Nonlocal conductance measurements are used to assess the charge character of the bound states, which can be a mixture of electron- and hole-like.

Typically, quantum dot resonances form at the nanowire ends where tunnel barriers are formed. We hybridize these quantum dot states with the extended bound state in the nanowire.  The hybridization leads to characteristic signatures that can be observed both at the location of the quantum dot and at the other nanowire end. This is a manifestation of the extended nature of the Andreev bound states.

Concluding, I want to present a device, that uses an array of narrow gates to probe a nanowire at any desired position and tune the nanowire density with a high level of control. This may complete our knowledge of ABS in gate-defined InAs-Al nanowires, and guide the efforts to realize a topological phase of matter.