Dimensional crossover and incipient quantum size effects in superconducting niobium nanofilms

TL;DR

This study systematically investigates superconducting properties of niobium nanofilms across various thicknesses, revealing a 3D-to-2D crossover, quantum size effects, and proximity effects that enhance critical parameters near the BCS-BEC crossover.

Contribution

It provides new insights into the interplay of quantum-size, proximity, and strong-coupling effects in niobium nanofilms, especially near the BCS-BEC crossover regime.

Findings

Maximum critical current density at ~25 nm thickness

Enhanced magnetic penetration depth in the thinnest films

Doubling of critical magnetic field near 20 nm thickness

Abstract

Superconducting and normal state properties of sputtered Niobium nanofilms have been systematically investigated, as a function of film thickness in a d=9-90 nm range, on different substrates. The width of the superconducting-to-normal transition for all films remained in few tens of mK, thus remarkably narrow, confirming their high quality. We found that the superconducting critical current density exhibits a pronounced maximum, three times larger than its bulk value, for film thickness around 25 nm, marking the 3D-to-2D crossover. The extracted magnetic penetration depth shows a sizeable enhancement for the thinnest films, aside the usual demagnetization effects. Additional amplification effects of the superconducting properties have been obtained in the case of sapphire substrates or squeezing the lateral size of the nanofilms. For thickness close to 20 nm we also measured a doubled perpendicular critical magnetic field compared to its saturation value for d>33 nm, indicating shortening of the correlation length and the formation of small Cooper pairs in the condensate. Our data analysis evidences an exciting interplay between quantum-size and proximity effects together with strong-coupling effects and importance of disorder in the thinnest films, locating the ones with optimally enhanced critical properties close to the BCS-BEC crossover regime.