Enhancing Superconductivity by Disorder

TL;DR

This paper investigates two mechanisms by which nonmagnetic disorder can enhance the superconducting transition temperature in both conventional and unconventional superconductors, through density of states enhancements and local density of states modulations.

Contribution

It introduces two novel mechanisms for Tc enhancement via disorder, applicable to multi-band and dense impurity regimes, supported by simplified models.

Findings

Resonant states in off-Fermi-level bands can significantly boost Tc.

Disorder-induced local density of states modulations enhance Tc in short coherence length superconductors.

The mechanisms are relevant to existing multi-band and conventional superconducting materials.

Abstract

We study two mechanisms for enhancing the superconducting transition temperature Tc by nonmagnetic disorder in both conventional (sign-preserving gaps) and unconventional (sign-changing gaps) superconductors (SC). In the first scenario, relevant to multi-band systems in the dilute impurity limit of both conventional and unconventional SC, we demonstrate how favorable density of states enhancements driven by resonant states in off-Fermi-level bands, lead to significant enhancements of Tc in the condensate formed by the near-Fermi-level bands. The second scenario focuses on the dense impurity limit where random disorder-generated local density of states modulations cause a boosted Tc for conventional SC with short coherence lengths. We analyze the basic physics of both mechanisms within simplified models, and discuss the relevance to existing materials.