Non-mean-field-like gap formation in the ultra-nodal pairing state of tetragonal FeSe_1-xS_x

The isovalently substituted FeSe_1-xS_x superconductors have attracted significant interest due to various exotic properties associated with the intertwining of nematicity, magnetism, and unconventional superconductivity [1]. Of particular interest is the abrupt change in the superconducting gap function that occurs at the nematic critical point at x_c ~ 0.17 [2-6], above which the formation of an ultra-nodal pairing state with a putative Bogolubov Fermi surface has been proposed [5-10]. Although the emergence of the Bogoluvov Fermi surface appears to reasonably explain the huge residual density of states observed in the tetragonal FeSe_1-xS_x, the nature of the exotic pairing state remains largely elusive, including the links between the ultra-nodal state and the colossal fluctuation-like behavior reported in the previous experiments [11]. Here, using the high-quality single crystals of FeSe_1-xS_x and the precise measurements of thermodynamic and charge transport properties, we discuss the gap formation of the ultra-nodal pairing state, which deviates from the mean-field-like behavior. In addition, we report the evolution of the superconducting gap in FeSe_1-xS_x under high pressures, where striking enhancement of T_c is found in the non-magnetic tetragonal regime [12]. The results provide fresh insights into our understanding of the ultra-nodal pairing state.

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