Effect of hydrostatic pressure on the structural and magnetic transitions in FeSe

The phase diagram of FeSe is unique among all the iron-based superconductors. At ambient pressure, FeSe undergoes a tetragonal-to-orthorhombic structural phase transition at $T_s$ = 90 K, and becomes superconducting below $T_c$ = 8 K. Unlike other iron-based materials, it does not magnetically order down to the lowest measured temperature ($T$). However, under the application of hydrostatic pressure ($p$), a new magnetic phase is stabilized starting from $\sim$1 GPa. Higher pressure increases $T_c$, whose maximum onset reaches a surprising 37 K at $\sim$7 GPa. We investigate the $p$-$T$ phase diagram using high-quality vapor-grown single crystals, which shows features not seen previously in powder and mixed-phase samples. Specifically, using high-pressure low-temperature diffraction and synchrotron M\"ossbauer we elucidate the effect of pressure - evolution of orthorhombic distortion and emergence of magnetic ordering - in the vicinity of the crossover region of the structural, magnetic and superconducting transitions.\\ Work at Ames Lab. was supported by the DOE, BES, Division of Materials Sciences \& Engineering, under Contract No. DEAC02-07CH11358. This research used resources at Argonne National Lab.