Electronic and magnetic phase diagram of Β-Fe_1.01 Se with superconductivity at 36.7 K underpressure

The discovery of new high-temperature superconductors¹ based on FeAs has led to a new gold rush in high-T_C superconductivity. All of the new superconductors share the same common structural motif of FeAs layers and reach T_C values up to 55 K (ref.2). Recently, superconductivity has been reported in FeSe (ref.3), which has the same iron pnictide layer structure, but without separating layers. Here, we report the magnetic and electronic phase diagram of Β-Fe_1.01 Se as a function of temperature and pressure. The superconducting transition temperature increases from 8.5 to 36.7 K under an applied pressure of 8.9 GPa. It then decreases at higher pressures. A marked change in volume is observed at the same time as T_C rises, owing to a collapse of the separation between the Fe ₂ Se₂ layers. No static magnetic ordering is observed for the whole p-T phase diagram. We also report that at higher pressures (starting around 7 GPa and completed at 38 GPa), Fe_1.01 Se transforms to a hexagonal NiAs-type structure and exhibits non-magnetic behaviour.