Superconductivity and magnetism of S-doped FeSe with a high <i>T</i>_c (≈25-30K) studied via ⁷⁷Se-NMR measurements under pressure

12%-S doped FeSe system has a high T_c of 25-30 K class under a pressure of 3.0 GPa. We have succeeded in investigating its microscopic properties for the first time via ⁷⁷Se-NMR measurements under pressure. We measured the relaxation time (T₁), the Knight shift, and the AC susceptibilty under pressures up to 3.0 GPa. Unexpectedly, the antiferromagnetic (AFM) fluctuation measured from 1/T₁T at the optimal pressure was strongly suppressed compared to the AFM fluctuation at ambient pressure, despite that the optimal pressure is close to the phase boundary of the AFM phase induced at a high pressure region. In addition, we revealed that the SC phase at an applied field of 6 T exhibited a remarkable double-dome structure in the pressure-temperature phase diagram, unlike the SC phase at zaro field. These phenomena are explained by a pressure-induced Lifshitz transition, a topological changes in Fermi surfaces under pressure. The strong AFM fluctuation at ambient pressure is attributable to a hidden AFM quantum critical point.