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