PhD Defense: Saulius Vaitiekėnas
From Topological to Anomalous Quantum Phases in Hybrid Nanowires
My thesis deals with quantum phase transitions in hybrid semiconductor-superconductor nanowires. After reviewing some concepts of superconductivity, it introduces Majorana zero modes in condensed matter and their realization in hybrid semiconductor-superconductor nanowires.
We begin the experimental part by showing that the effective g factor of Andreev bound states in half-shell wires is a steplike function of gate voltage, that tunes the charge carrier density and improves the hard induced gap. We observe the closing and reopening of the superconducting gap in the subgap spectrum coincident with the appearance of a zero-bias conductance peak.
The following chapter demonstrates a novel means of creating Majorana zero modes in full-shell wires by using magnetic-flux. Around one applied flux, tunneling spectroscopy reveals a gapped region with a discrete zero-energy state, whereas Coulomb peak spacing shows exponentially decreasing deviation from 1e periodicity with device length.
We continue with a study of magnetic-flux driven reentrant quantum phase transitions between superconducting and metallic phases in full-shell wires. By tuning axial and transverse magnetic fields we control the crossover between the conventional and destructive Little-Parks regimes bridged by an anomalous metal phase.
The thesis is concluded with a presentation of selective area grown hybrid wires as a basis for topological networks. The novel scalable system displays a hard induced gap, unpoisoned 2e-periodic Coulomb blockade, strong spin-orbit coupling and coherence length of several microns.