Robustness of the Berezinskii-Kosterlitz-Thouless Transition in Ultrathin NbN Films near the Superconductor-Insulator Transition

TL;DR

This study demonstrates that the Berezinskii-Kosterlitz-Thouless transition remains robust in ultrathin NbN films near the superconductor-insulator transition, with vortex-core energy scaling with the superconducting gap rather than superfluid density.

Contribution

It provides new insights into BKT physics in disordered 2D superconductors, contrasting with previous findings in cuprates, and highlights the role of vortex-core energy near the quantum critical point.

Findings

BKT transition persists near the SIT in NbN films.

Vortex-core energy scales with the superconducting gap.

Contrasts with behavior in underdoped cuprates.

Abstract

Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition is investigated by superfluid density measurements for two-dimensional (2D) disordered NbN films with disorder level very close to a superconductor-insulator transition (SIT). Our data show a robust BKT transition even near this 2D disorder-tuned quantum critical point (QCP). This observation is in direct contrast with previous data on deeply underdoped quasi-2D cuprates near the SIT. As our NbN films approach the QCP, the vortex-core energy, an important energy scale in the BKT transition, scales with the superconducting gap, not with the superfluid density, as expected within the standard 2D-XY model description of BKT physics.