Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSe(1 − x)Sx

We report muon spin rotation and magnetization measurements under pressure on Fe_1+δSe_1−xS_x with x ≈ 0.11. Above p ≈ 0.6 GPa we find a microscopic coexistence of superconductivity with an extended dome of long range magnetic order that spans a pressure range between previously reported separated magnetic phases. 

The magnetism initially competes on an atomic scale with the coexisting superconductivity leading to a local maximum and minimum of the superconducting T_c(p). The maximum of T_c corresponds to the onset of magnetism while the minimum coincides with the pressure of strongest competition. A shift of the maximum of T_c(p) for a series of single crystals with x up to 0.14 roughly extrapolates to a putative magnetic and superconducting state at ambient pressure for x ≥ 0.2.