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

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 $T_{\mathrm{c}}$ 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.

arXiv comment8 pages, 4 figures, typos corrected. Accepted for publication in PNAS

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• Microscopic origin of ultranodal superconducting states in spin-1/2 systems
• Lifting of gap nodes by disorder in ultranodal superconductor candidate FeSe1-xSx
• Classification of multiorbital superconducting state based on augmented multipoles
• Spontaneous magnetic field and disorder effects in BaPtAs_1-x_Sb_x_ with honeycomb network
• Topological in-gap chiral edge states in superconducting Haldane model with spin-orbit coupling
• Quasiparticle interaction originating from Bogoliubov Fermi Surfaces under pressure in 18%-S substituted FeSe studied via NMR
• Evolution and Instability of Bogoliubov Fermi Surfaces under Zeeman Field
• Pair-mixing induced Time-reversal-breaking superconductivity
• Superconductivity near an Ising nematic quantum critical point in two dimensions