β-FeSe is the simplest iron-based superconductor but with diverse properties. It starts to superconduct at 8 K at normal pressure but its critical temperature (T) is dramatically increased to 38 K under pressure, by means of intercalation, or after quenching at high pressures. The combination of both intercalation and pressure results in re-emerging superconductivity at 48 K.
In 2013 it was reported that a single atomic layer of FeSe epitaxially grown on SrTiO is superconductive with a then-record transition temperature for iron-based superconductors of 70 K. This discovery has attracted significant attention and in 2014 a superconducting transition temperature of over 100K was reported for this system.