Enhancement of Tc by point-like disorder and anisotropic gap in FeSe

TL;DR

This study reveals that point-like disorder introduced by electron irradiation in FeSe enhances its superconducting transition temperature and modifies its anisotropic gap, challenging conventional understanding of disorder effects in superconductors.

Contribution

It demonstrates that controlled point-like disorder can increase Tc in FeSe and provides insights into the gap structure and disorder effects in this material.

Findings

Tc increases by 0.4 K after irradiation

Superconducting gap remains anisotropic with no symmetry-imposed nodes

Structural transition temperature decreases after irradiation

Abstract

A highly anisotropic superconducting gap is found in single crystals of FeSe by studying the London penetration depth, $\Delta \lambda$, measured down to 50 mK in samples before and after 2.5 MeV electron irradiation. The gap minimum increases with introduced point - like disorder, indicating the absence of symmetry - imposed nodes. Surprisingly, the superconducting transition temperature, $T_c$, \textit{increases} by 0.4 K from $T_{c0} \approx$ 8.8 K while the structural transition temperature, $T_s$, \textit{decreases} by 0.9 K from $T_{s0} \approx$91.2 K after electron irradiation. We discuss several explanations for the $T_c$ enhancement, and propose that local strengthening of the pair interaction by irradiation-induced Frenkel defects most likely explains the phenomenon.