Ballistic effects in a proximity induced superconducting diffusive metal

TL;DR

This study uses low-temperature STM to explore how proximity-induced superconductivity affects the local electronic states in diffusive normal metal structures, revealing subgap peaks and interference effects.

Contribution

It provides experimental evidence of ballistic-like effects and bound states in a diffusive superconductor-normal metal system using STM measurements.

Findings

Observation of subgap peaks in LDOS at low temperatures

Evidence of quasi-particle bound states within normal metal structures

Interference effects explained by refocusing of electronic trajectories

Abstract

Using a Scanning Tunneling Microscope (STM), we investigate the Local Density of States (LDOS) of artificially fabricated normal metal nano-structures in contact with a superconductor. Very low temperature local spectroscopic measurements (100 mK) reveal the presence of well defined subgap peaks at energy |E|<Delta in the LDOS at various positions of the STM tip. Although no clear correlations between the LDOS and the shape of the samples have emerged, some of the peak features suggest they originate from quasi-particle bound states within the normal metal structures (De Gennes St James states). Refocusing of electronic trajectories induced by the granular srtucture of the samples can explain the observation of spatially uncorrelated interference effects in a non-ballistic medium.