Upper critical field and superconductor-metal transition in ultrathin niobium films

TL;DR

This study explores how ultrathin niobium films transition from superconducting to metallic states, revealing differences in critical fields and fluctuation effects between amorphous and polycrystalline structures, with implications for understanding inhomogeneity.

Contribution

It provides a comparative analysis of superconducting properties in amorphous versus polycrystalline ultrathin Nb films, highlighting distinct limiting mechanisms and fluctuation phenomena.

Findings

Upper critical field is orbitally limited in polycrystalline films.

Paramagnetically limited in amorphous films.

Presence of fluctuating Cooper pairs above $H_{c2}$.

Abstract

Recent studies suggests that in disordered ultrathin films superconducting (SC) state may be intrinsically inhomogeneous. Here we investigate the nature of SC state in ultrathin Nb films, of thickness $d$ ranging from 1.2 nm to 20 nm, which undergo a transition from amorphous to polycrystalline structure at the thickness $d \simeq 3.3$ nm. We show that the properties of SC state are very different in polycrystalline and amorphous films. The upper critical field ($H_{c2}$) is orbitally limited in the first case, and paramagnetically limited in the latter. The magnetic field induced superconductor-metal transition is observed, with the critical field approximately constant or decreasing as a power-law with the film conductance in polycrystalline or amorphous films, respectively. The scaling analysis indicates distinct scaling exponents in these two types of films. Negative contribution of the SC fluctuations to conductivity exists above $H_{c2}$, particularly pronounced in amorphous films, signaling the presence of fluctuating Cooper pairs. These observations suggest the development of local inhomogeneities in the amorphous films, in the form of proximity-coupled SC islands. An usual evolution of SC correlations on cooling is observed in amorphous films, likely related to the effect of quantum fluctuations on the proximity-induced phase coherence.