Robust pseudogap across the magnetic field driven superconductor to insulator-like transition in strongly disordered NbN films

TL;DR

This study uses scanning tunneling spectroscopy to explore how a strongly disordered NbN film transitions from superconducting to insulator-like states under magnetic fields, revealing a persistent pseudogap and suggesting a percolative transition mechanism.

Contribution

It provides the first detailed spectroscopic evidence of a robust pseudogap across the superconductor-insulator transition in disordered NbN films, highlighting a percolative transition process.

Findings

Presence of a pseudogap on both sides of the transition

Smooth evolution of the superconducting state across the transition

Superconductor-insulator transition driven by percolation

Abstract

We investigate the magnetic field evolution of the superconducting state in a strongly disordered NbN thin film which exhibits a magnetic field tuned superconductor to insulator-like transition, employing low temperature scanning tunneling spectroscopy (STS). Transport measurements of the sample reveals a characteristic magnetic field, which separates the low field state where resistance decreases with decreasing temperature, i.e. dR/dT > 0 and a high-field state where dR/dT < 0. However, STS imaging of the superconducting state reveals a smooth evolution across this field and the presence of a robust pseudogap on both sides of this characteristic field. Our results suggest that the superconductor-insulator transition might be a percolative transition driven by the shrinking of superconducting fraction with magnetic field.