Probing Hybridization of a Single Energy Level Coupled to Superconducting Leads

TL;DR

This paper investigates how a quantum dot coupled to superconducting leads exhibits conductance features near the superconducting gap, emphasizing the role of hybridization and electron cotunneling effects in transport spectroscopy.

Contribution

It provides experimental evidence and theoretical analysis of hybridization effects in superconducting quantum dots, highlighting the significance of electron cotunneling at the gap edge.

Findings

Transport features are accurately described by hybridization with leads.

Divergent density of states at the superconducting gap enhances hybridization.

Electron cotunneling plays a crucial role in spectroscopic measurements.

Abstract

Electron transport through a quantum dot coupled to superconducting leads shows a sharp conductance onset when a quantum dot orbital level crosses the superconducting coherence peak of one lead. We study superconducting single electron transistors in the weak coupling limit by connecting individual gold nanoparticles with aluminum junctions formed by electromigration. We show that the transport features close to the conductance onset threshold can be accurately described by the quantum dot levels' hybridization with the leads, which is strongly enhanced by the divergent density of states at the superconducting gap edge. This highlights the importance of electron cotunneling effects in spectroscopies with superconducting probes.