Study on nematic superconductivity in tetragonal Fe(Se, S) using magnetic torque measurements

 The iron-based superconductor FeSe undergoes a nematic transition at 90 K accompanied by a structural transition from orthorhombic to tetragonal structure. Also, this nematic transition temperature is suppressed as the amount of S-substitutions increases in FeSe_1-xS_x, and it becomes zero at x = 0.17 (nematic quantum critical point). Recently, in a SC phase outside this nematic phase (x > 0.17), a large residual quasiparticle density of states appears and anomalously suppressed superfluid density has been found in the zero-temperature limit [1-3]. These are the characteristics of theoretically suggested new gap structure for superconductivity with broken time-reversal symmetry, in which the gap closes in two-dimensional planes (Bogoliubov Fermi surfaces, BFSs) [4].

 Recent angle-resolved photoemission measurements revealed an extended momentum range with no superconducting gap, consistent with the presence of such BFSs [5]. Surprisingly, the gap structure shows two-fold anisotropy instead of four-fold symmetry of the normal-state Fermi surface in the tetragonal phase of FeSe_1-xS_x. Here we investigated the rotational symmetry breaking in the superconducting state of tetragonal FeSe_1-xS_x using magnetic torque measurements. We discuss the possibility of a nematic superconducting state from the obtained results.

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   [5]T. Nagashima et al., https://doi.org/10.21203/rs.3.rs-2224728/v1 (2022).