Two superconducting states with broken time-reversal symmetry in FeSe1-xSx

TL;DR

This study provides experimental evidence of broken time-reversal symmetry in FeSe$_{1-x}$S$_x$ superconductors, indicating the presence of an ultranodal pair state with Bogoliubov Fermi surfaces, and reveals two distinct superconducting states separated by a nematic critical point.

Contribution

It demonstrates the existence of two different superconducting states with broken TRS in FeSe$_{1-x}$S$_x$, supporting the ultranodal pair state hypothesis and linking nematicity to superconductivity.

Findings

TRS is broken in both nematic and tetragonal phases.

Superfluid density is substantially reduced in the tetragonal phase.

Evidence supports the ultranodal pair state with Bogoliubov Fermi surfaces.

Abstract

Iron-chalcogenide superconductors FeSe$_{1-x}$S$_x$ possess unique electronic properties such as non-magnetic nematic order and its quantum critical point. The nature of superconductivity with such nematicity is important for understanding the mechanism of unconventional superconductivity. A recent theory suggested the possible emergence of a fundamentally new class of superconductivity with the so-called Bogoliubov Fermi surfaces (BFSs) in this system. However, such an {\em ultranodal} pair state requires broken time-reversal symmetry (TRS) in the superconducting state, which has not been observed experimentally. Here we report muon spin relaxation ($\mu$SR) measurements in FeSe$_{1-x}$S$_x$ superconductors for $0\le x \le 0.22$ covering both orthorhombic (nematic) and tetragonal phases. We find that the zero-field muon relaxation rate is enhanced below the superconducting transition temperature $T_{\rm c}$ for all compositions, indicating that the superconducting state breaks TRS both in the nematic and tetragonal phases. Moreover, the transverse-field $\mu$SR measurements reveal that the superfluid density shows an unexpected and substantial reduction in the tetragonal phase ($x>0.17$). This implies that a significant fraction of electrons remain unpaired in the zero-temperature limit, which cannot be explained by the known unconventional superconducting states with point or line nodes. The time-reversal symmetry breaking and the suppressed superfluid density in the tetragonal phase, together with the reported enhanced zero-energy excitations, are consistent with the ultranodal pair state with BFSs. The present results reveal two different superconducting states with broken TRS separated by the nematic critical point in FeSe$_{1-x}$S$_x$, which calls for the theory of microscopic origins that account for the relation between the nematicity and superconductivity.