Down the rabbit hole: Andreev bound states in Ge-Si core-shell nanowires

This thesis is dedicated to the experimental study of the superconducting proximity effect in Ge-Si core-shell nanowires. The primary objective is to achieve topological superconductivity and investigate signatures of Majorana bound states within these wires. This is of great interests in both fundamental physics and potential quantum applications.
Employing distinct strategies to create superconductor-semiconductor contacts, we discovered the effectiveness of an interlayer approach utilizing a thin Pd layer before the Al layer. This method successfully induced superconductivity while impeding Al diffusion into the Ge-Si core-shell nanowires. Subsequently, an investigation of the proximity-induced gap in nanowires with varying Si shell thicknesses revealed an inversely proportional relationship between shell thickness and induced gap.
To enable a topological transition, the applied Zeeman energy must surpass the proximity-induced gap. Our efforts to enhance the critical field of the Al electrodes involved thinning the Al layer and introducing a sub-monolayer of Pt, elevating the critical field to 3T from the previous 0.36T.
Furthermore, our study uncovered intriguing interplays between superconductivity and energy quantization. Long ballistic channels facilitated the emergence of multiple Andreev bound states within a 2μm length beneath the electrodes, forming a diverse subgap spectrum combined with quantized quantum dot levels. Additionally, we observed a singlet-to-doublet ground state transition under increasing external magnetic fields, denoted by the closure of a characteristic eye-shaped feature in Yu-Shiba-Rusinov (YSR) state bias spectroscopy.
Our subsequent study delves into the anomalous magnetic field dependence of the critical current observed in Ge-Si Josephson devices. Intriguingly, we noted that the critical current increases with an external magnetic field up to a certain value, beyond which it exhibits a monotonic decrease This enhancement was attributed to quasiparticle cooling, supported by both experimental evidence and theoretical models.
Lastly, our exploration of crossed Andreev reflection in a double-nanowire device, although inconclusive in definitive evidence, has underscored Ge-Si core-shell nanowires as a promising platform for investigating intricate interplays between superconductivity and topology.