Weakening of the diamagnetic shielding in FeSe1−xSx at high pressures

The superconducting transition of FeSe1−xSx with three distinct sulfur concentrations x was studied under hydrostatic pressures up to ∼70 kbar via bulk ac susceptibility. The pressure dependence of the superconducting transition temperature (Tc) features a small dome-shaped variation at low pressures for x=0.04 and x=0.12, followed by a more substantial Tc enhancement to a value of around 30 K at moderate pressures. In x=0.21, a similar overall pressure dependence of Tc is observed, except that the small dome at low pressures is flattened. For all three concentrations, a significant weakening of the diamagnetic shielding is observed beyond the pressure around which the maximum Tc of 30 K is reached near the verge of the pressure-induced magnetic phase. This observation points to a strong competition between the magnetic and high-Tc superconducting states at high pressure in this system.