Origin of Enhancement of Nematicity Due to Superconductivity in FeSe_1-xS_x

The origin of both superconductivity and nematicity in bulk FeSe is currently under debate. Important clues can be obtained by investigating the interplay between these two orders. Recent thermal expansion measurements on pure and S-doped bulk FeSe have found that in contrast to other iron-based systems, the nematic order parameter is enhanced due to the onset of superconductivity, rather than suppressed as in the canonical Ba-122 system. Here we study a model where electronic nematicity is described by a d-wave Pomeranchuk instability, and superconductivity in the nematic phase is assumed to arise from an anisotropic pairing interaction in the orthorhombic phase. Within mean-field theory, we discuss the question of under which circumstances enhancement of nematic order below the superconducting transition is possible.