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

摘要

Occurrence of the Berezinskii-Kosterlitz-Thouless (BKT) transition isinvestigated by superfluid density measurements for two-dimensional (2D)disordered NbN films with disorder level very close to asuperconductor-insulator transition (SIT). Our data show a robust BKTtransition even near this 2D disorder-tuned quantum critical point (QCP). Thisobservation is in direct contrast with previous data on deeply underdopedquasi-2D cuprates near the SIT. As our NbN films approach the QCP, thevortex-core energy, an important energy scale in the BKT transition, scaleswith the superconducting gap, not with the superfluid density, as expectedwithin the standard 2D-XY model description of BKT physics.