PhD defense: Alisa Danilenko

Nonlocal and local objects: Andreev bound states, and quantum dots as spectroscopic tools

Bringing together superconductor and semiconductor materials allows for the combination of spin-orbit coupling and a high electron g-factor with an induced superconducting pairing, forming a hybrid material. In this thesis, we make use of such hybrids of semiconducting InAs and superconducting Al combined in a two-dimensional heterostructure grown by molecular beam epitaxy. This two-dimensional platform allows us to design an evolving series of advanced devices which enable the detailed study of bound states that emerge when these materials are confined at low temperature, and in the presence of a Zeeman field.

Throughout the thesis, multiple side probes along the length of confined nanowires (NWs) allow for tunneling spectroscopy, enabling the local investigation of the density of states (DOS) at the probe locations. These multiple probes are used to study extended Andreev bound states (ABSs) forming inside the NWs, which appear as subgap resonances which are non-trivially correlated with respect to gate voltage and magnetic field.

Nonlocal conductance was used to determine the charge character of the extended ABSs. The modification of their charge character as a function of electrostatic gate was found to be in agreement with theoretical predicitions for the total Bardeen-Cooper-Schrieffer (BCS) charge of the ABS.

Quantum dots (QDs) were utilized as probes of the ABS properties, starting from a regime in which there was strong hybridization between a QD in a tunnel barrier and an ABS confined in the NW. The effect of hybridization was observed both locally, at the location of the QD, and non-locally at a neighboring tunnel probe. This provided a robust method for confirming ABS extension.

Laterally defined QDs with tunable couplings to both a normal lead and the NW were implemented; multiple designs for such QDs were explored. In a regime of weak coupling to both normal lead and NW, QD levels could be used as spectrometers of the NW DOS. In an applied magnetic field, the QD energy levels became Zeeman split, allowing for spectroscopy of the NW with both spin and charge resolution.

https://ucph-ku.zoom.us/j/66741553109?pwd=K2FGTzZLRVVHS1QvOGZVUll6LzBYdz09

Passcode: 015246