Symmetry-unprotected nodes or gap minima in $s_{++}$ state of FeSe single crystal

TL;DR

This study investigates the pairing mechanism in FeSe superconductors, revealing that the material exhibits symmetry-unprotected nodes or gap minima that are sensitive to disorder, supporting an $s_{++}$ pairing state.

Contribution

It provides experimental evidence for the presence of symmetry-unprotected nodes or gap minima in FeSe and how they are affected by disorder, clarifying the nature of its superconducting state.

Findings

Superconductivity in FeSe involves multi-gap structure with nodes or gap minima.

Critical temperature $T_c$ suppression shows a two-step behavior with disorder.

Nodes or gap minima are lifted at higher disorder, indicating an $s_{++}$ pairing state.

Abstract

We report the study on superconducting pairing mechanism of FeSe via the pair-breaking effects induced by H$^+$-irradiation combined with low-temperature specific heat measurements. A multi-gap structure with nodes or gap minima is suggested in a clean FeSe by the specific heat results. The suppression of critical temperature $T_c$ with increasing the defect density manifests a two-step behavior. When the increase in the residual resistivity is small, $\Delta\rho_0$ $<$ $\sim$4.5 $\mu\Omega$cm, $T_c$ is gradually suppressed with increasing the density of scattering centers, suggesting the presence of symmetry-unprotected nodes or gap minima. However, for $\Delta\rho_0$ $>$ $\sim$4.5 $\mu\Omega$cm, $T_c$ is almost independent of the scattering, which indicates that the nodes or gap minima are lifted and the order parameter becomes almost isotropic without sign change. Thus, the superconductivity in FeSe is found to be realized in symmetry-unprotected nodal or highly anisotropic $s_{++}$ state.