Nematic quantum critical points and unconventional superconducting states in Fe(Se/S/Te)*

The interplay among magnetism, electronic nematicity, and superconductivity is the key issue in strongly correlated materials including iron-based, cuprate, and heavy-fermion superconductors. Magnetic fluctuations have been widely discussed as a pairing mechanism of unconventional superconductivity, but recent theory predicts that quantum fluctuations of electronic nematicity, which is characterized by rotational symmetry breaking, may also promote high-temperature superconductivity. FeSe-based superconductors are suitable to study this issue [1] because FeSe exhibits a nonmagnetic nematic order that can be suppressed by S or Te substitution for Se. I will review recent studies of FeSe-based superconductors, showing exotic superconducting states. In FeSe_1-xS_x superconductors, the nematic order can be completely suppressed at x=0.17, above which the superconducting properties change drastically with a significantly reduced critical temperature T_c [2,3]. From recent muon spin rotation (µSR) measurements, we find evidence for a novel ultranodal pair state with broken time-reversal symmetry [4]. In the Te substitution case, however, we find quite different behavior; the suppression of nematic order leads to an enhancement of T_c, which is likely associated with quantum critical fluctuations of nematicity [5-7].

[1] See, for a review, T. Shibauchi, T. Hanaguri, and Y. Matsuda, J. Phys. Soc. Jpn. 89, 102002 (2020). [2] Y. Sato et al., Proc. Natl. Acad. Sci. USA 115, 1227-1231 (2018). [3] T. Hanaguri et al., Sci. Adv. 4, eaar6419 (2018). [4] K. Matsuura et al., preprint (2022). [5] K. Mukasa et al., Nat. Commun. 12, 381 (2021). [6] K. Ishida et al., Proc. Natl. Acad. Sci. USA 119, e2110501119 (2022). [7] K. Mukasa et al., arXiv:2202.11657 (2022).