Superconducting phase transitions in disordered NbTiN films

TL;DR

This study explores how disorder affects superconducting phase transitions in NbTiN thin films, revealing a crossover from fermionic to bosonic mechanisms and identifying conditions for Bose metal phases.

Contribution

It demonstrates how controlled disorder influences superconducting transitions, highlighting the role of sheet resistance and magnetoresistance features in identifying different phases.

Findings

Disorder induces a crossover from fermionic to bosonic mechanisms.

Moderately disordered films follow conventional superconducting fluctuation theories.

Critically disordered films show features of a possible Bose metal phase.

Abstract

The suppression of superconductivity in disordered systems is a fundamental problem of condensed matter physics. Here we investigate the superconducting niobium-titanium-nitride (Nb_{1-x}Ti_{x}N) thin films grown by atomic layer deposition (ALD) where disorder is controlled by the slight tuning of the ALD process parameters. We observe the smooth crossover from the disorder-driven superconductor-normal metal transition (often reffered to as fermionic mechanism) to the case where bosonic mechanism dominates and increasing disorder leads to formation of metal with Cooper pairing. We show that, in moderately disordered films, the transition to zero-resistance state occurs in a full agreement with the conventional theories of superconducting fluctuations and Berezinskii-Kosterlitz-Thouless transition. However, the critically disordered films violate this accord showing low-temperature features possibly indicating the Bose metal phase. We show that it is the interrelation between film's sheet resistance in the maximum, R_{max}, of the resistive curve R(T) and R_q = h/4e^2 that distinguishes between these two behaviors. We reveal the characteristic features in magnetoresistance of the critically disordered films with R_{max} > R_q