High-Tc superconductivity up to 55 K under high pressure in the heavily electron doped Lix(NH3)yFe2Se2 single crystal

TL;DR

This study reveals that applying high pressure to heavily electron-doped Lix(NH3)yFe2Se2 single crystals induces a second high-temperature superconducting phase with Tc reaching 55 K, linked to increased electron carrier density and potential Fermi surface changes.

Contribution

It demonstrates a pressure-induced second superconducting phase with higher Tc in Lix(NH3)yFe2Se2, highlighting pressure as a tool to enhance Tc via carrier concentration modifications.

Findings

Tc drops below 20 K at 2 GPa, then rises to 55 K at 11 GPa.

Electron carrier density increases fourfold in the SC-II phase.

Tc correlates with inverse Hall coefficient, indicating carrier-driven Tc enhancement.

Abstract

We report a high-pressure study on the heavily electron doped Lix(NH3)yFe2Se2 single crystal by using the cubic anvil cell apparatus. The superconducting transition temperature Tc = 44 K at ambient pressure is first suppressed to below 20 K upon increasing pressure to Pc = 2 GPa, above which the pressure dependence of Tc(P) reverses and Tc increases steadily to ca. 55 K at 11 GPa. These results thus evidenced a pressure-induced second high-Tc superconducting (SC-II) phase in Lix(NH3)yFe2Se2 with the highest Tcmax = 55K among the FeSe-based bulk materials. Hall data confirm that in the emergent SC-II phase the dominant electron-type carrier density undergoes a fourfold enhancement and tracks the same trend as Tc(P). Interesting, we find a nearly parallel scaling behavior between Tc and the inverse Hall coefficient for the SC-II phases of both Lix(NH3)yFe2Se2 and (Li,Fe)OHFeSe. The present work demonstrates that high pressure offers a distinctive means to further raising the maximum Tc of heavily electron doped FeSe-based materials by increasing the effective charge carrier concentration via a plausible Fermi surface reconstruction at Pc.