Disorder-robust high-field superconducting phase of FeSe single crystals

TL;DR

This study investigates the high-field superconducting phase of FeSe single crystals, revealing it to be robust against disorder and unlikely to be a conventional FFLO state, challenging previous interpretations.

Contribution

The paper provides experimental evidence that the high-field SC phase in FeSe is disorder-robust, suggesting it is not a conventional FFLO state, which is a novel insight.

Findings

High-field SC phase observed in all crystals regardless of disorder level

The phase is robust against disorder, contradicting FFLO expectations

The high-field SC phase is not a conventional FFLO state

Abstract

When exposed to high magnetic fields, certain materials manifest an exotic superconducting (SC) phase that has attracted considerable attention. A proposed explanation for the origin of the high-field SC phase is the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. This state is characterized by inhomogeneous superconductivity, where the Cooper pairs have finite center-of-mass momenta. Recently, the high-field SC phase was observed in FeSe, and it was deemed to originate from the FFLO state. Here, we synthesize FeSe single crystals with different levels of disorder. The level of disorder is expressed by the ratio of the mean free path to the coherence length and ranges between 35 and 1.2. The upper critical field \textit{B}$_{\rm{c}2}$ was obtained by both resistivity and magnetic torque measurements over a wide range of temperatures, which went as low as $\sim$0.5 K, and magnetic fields, which went up to $\sim$38 T along the \textit{c} axis and in the \textit{ab} plane. In the high-field region parallel to the \textit{ab} plane, an unusual SC phase was confirmed in all the crystals, and the phase was found to be robust against disorder. This result suggests that the high-field SC phase in FeSe is not a conventional FFLO state.