Tuning of superconducting properties with disorder in NbxSn nanocrystalline thin films

TL;DR

This study explores how disorder and stoichiometry influence superconductivity in nanocrystalline NbxSn thin films, revealing a disorder-driven transition to insulating states and dimensional crossovers.

Contribution

It demonstrates the impact of stoichiometry-controlled disorder on superconducting properties and the superconductor-insulator transition in NbxSn nanocrystalline films.

Findings

Superconducting transition temperature decreases with film thickness.

Disorder induces a crossover to insulating behavior around 11 nm thickness.

Superfluid stiffness is significantly suppressed in Sn-rich films.

Abstract

Nanocrystalline superconducting films offer an excellent platform to explore the interplay between disorder, granularity, and dimensionality. In this work, we investigate two series of NbxSn thin films with near-stoichiometric (x =3) and slightly Sn-rich (x =2.5) compositions, deposited on Si (100) substrates via DC magnetron sputtering. Both series exhibit nanocrystalline morphology, with the Sn-rich films displaying smaller grain sizes and a more granular microstructure. A suppression of the superconducting transition temperature (Tc) with decreasing film thickness is observed in both series. Notably, a disorder-driven crossover to an insulating state emerges, occurring at a thickness of approximately 11 nm for the Sn-rich films-about twice that of the stoichiometric films. The estimated disorder parameter (kFl=0.4) in the thinnest films indicates proximity to the Anderson localization regime for these films. Magneto-transport measurements reveal a thickness-driven 3D to 2D crossover, with its onset strongly dependent on film stoichiometry. Furthermore, a pronounced suppression of superfluid stiffness is observed in the Sn-rich films, corroborating the structure-property correlations identified in this study. This work highlights the role of stoichiometry controlled disorder in tuning superconductivity in granular NbxSn thin films.