Pressure-induced high-temperature superconductivity retained without pressure in FeSe single crystals

As room temperature superconductivity (RTS) has been reported recently in hydrides at megabar pressures, the grand challenge in superconductivity research and development is no longer restricted to further increasing the superconducting transition temperature under extreme conditions and must now include concentrated efforts to lower, and better yet remove, the applied pressure required. This work addresses directly such a challenge by demonstrating our successful retention of pressure-enhanced and/or -induced superconducting phases and/or semiconducting phases without pressure in single crystals of superconducting FeSe and non-superconducting Cu-doped FeSe. The pressure-quenching technique developed in this work offers the possibility of future practical application and the unraveling of RTS recently detected in hydrides but only under high pressures.