Break-up of long-range coherence due to phase fluctuations in ultrathin superconducting NbN films

TL;DR

This study investigates how phase fluctuations cause the loss of long-range coherence in ultrathin NbN films, revealing a transition from conventional superconductivity to a phase-fluctuation-driven insulator as thickness decreases.

Contribution

It provides experimental evidence linking phase fluctuations to the superconductor-insulator transition in ultrathin NbN films using scanning tunneling spectroscopy.

Findings

Thicker films show homogeneous BCS-like superconductivity with vortices.

Thinner films exhibit reduced coherence peaks and spatial inhomogeneities.

At the thinnest limit, coherence peaks vanish and vortices are absent, indicating loss of phase coherence.

Abstract

Using scanning tunneling spectroscopy (STS), we address the problem of the superconductor-insulator phase transition (SIT) in homogeneously disordered ultrathin (2-15 nm) films of NbN. Samples thicker than 8 nm, for which the Ioffe-Regel parameter $k_F l \geq 5.6$, manifest a conventional superconductivity : A spatially homogeneous BCS-like gap, vanishing at the critical temperature, and a vortex lattice in magnetic field. Upon thickness reduction, however, while $k_F l$ lowers, the STS revealed striking deviations from the BCS scenario, among which a progressive decrease of the coherence peak height and spatial inhomogeneities. The thinnest film (2.16 nm), while not being exactly at the SIT ($T_C \approx 0.4 T_{C-bulk}$), showed astonishingly vanishing coherence peaks and the absence of vortices. In the quasi-2D limit, such clear signatures of the loss of long-range phase coherence strongly suggest that, at the SIT the superconductivity is destroyed by phase fluctuations.