Retention of metastable superconductivity in FeSe up to 24 K at ambient by pressure quenching*

High pressure has been shown to be an effective route to achieving high temperature superconductivity. This is exemplified by the current record high T_cs at 164 K in the cuprate HgBa₂Ca₂Cu₃O_8+δ under ~31 GPa and at 287 K in C-S-H under ~ 267 GPa. These impressive record high T_cs are significant not only scientifically, but also technologically. Should these superconducting (sc) states under high pressures be retained at ambient, their impacts on technology would be profound and limited only by the imagination. We have chosen the sc FeSe and non-sc Cu-doped FeSe as model compounds to test this possibility. We have succeeded in retaining the high-pressure sc phase with a T_c up to 24 K upon the complete removal of pressure at 77 K following a specific thermodynamic path. A high-pressure non-sc phase of FeSe was also retained and proved to be stable up to room temperature. Detailed results will be presented and discussed.