Retention of Enhanced and Induced Superconductivity at Ambient Pressure Through High-Pressure Quenching

In the past eight years impressively high superconducting critical temperatures (T_cs) have been reported in numerous materials. Among these include instances of anomalously high T_cs that approach, and in some contested reports, meet and exceed room temperature (RT), pushing the field to new heights. Unfortunately, achieving such impressive critical temperatures requires ultra-high external pressures, rendering them unviable for commercial use. Therefore, one of the most significant challenges remaining in the field of superconductivity is to retain the high T_c phases induced by pressure while lowering or removing it completely. We have therefore employed a pressure quenching technique to retain high-pressure-induced/-enhanced superconducting phases in Bi, FeSe, and Cu_xFe_1.01-xSe at ambient pressure. Pressure quenching bismuth at 77 K and 4.2 K from pressures up to 26.6 GPa successfully produced metastable superconducting phases with varying T_cs from ∼ 5 K up to a new record of 9 K. By changing the pressure quenching parameters, different metastable phases could be targeted, namely Bi-III with a T_c around 7 K and Bi-V with a T_c > 8 K. Temporal stability testing and thermal cycling revealed a lower temperature limit below ∼ 60 K and an upper temperature limit of 120 K – 150 K in metastable bismuth. Pressure quenches performed on FeSe and Cu_xFe_1.01-xSe near the superconducting dome resulted in metastable phases with maximum T_cs of 37 K and 25 K, respectively. Thermal cycling of FeSe and Cu_xFe_1.01-xSe showed a similar lower temperature limit for temperatures up to ∼ 120 K and an upper temperature limit around 175 K for Cu_xFe_1.01-xSe. Annealing metastable FeSe to room temperature produced T_cs from 15 K – 24 K. Notably, a non-superconducting hexagonal phase retained in FeSe was slowly annealed to room temperature for a few days resulting in a superconducting phase near the dome peak. Lastly, a temporal stability test of metastable Cu_xFe_1.01-xSe was conducted which showed perfect phase stability for 7 days when kept below 50 K. Overall, these results demonstrate the potential this technique has in targeting desirable superconducting phases induced or enhanced by pressure and retaining them in a metastable state at ambient pressure.