High-pressure electronic phase diagrams in FeSe$_{1-x}$S$_x$ superconductors

The spin fluctuations are believed to be related to the mechanism of the unconventional superconductors. On the other hand, many recent studies suggest that the nematic order that spontaneously breaks rotational symmetry of the system exists in the Fe-based superconductors and its quantum fluctuations may play an essential role for the superconductivity. However, this remains unclear because the nematic order usually coexists with the magnetic order. To solve this issue, FeSe exhibiting a nonmagnetic nematic order is a key system. Under pressure, this order is suppressed and concurrently magnetic order appears, which competes with high-${\it T}_c$ superconducting phase. In isovalent substitution system FeSe$_{1-x}$S$_x$, we found a nonmagnetic nematic quantum critical point. Here we report our recent high-pressure studies in high-quality single-crystalline FeSe$_{1-x}$S$_x$ up to 8 GPa. We find a systematic change of the pressure phase diagram in FeSe by the S-substitution. Our results imply that the respective role of nematic and magnetic fluctuations can be elucidated from the precise control of pressure and substitution in this system.