  The pressure-induced reemergence of the second high-Tc superconducting phase
(SC-II) in the alkali-metal intercalated AxFe2-ySe2 (A = K, Rb, Cs, Tl) remains
an enigma and proper characterizations on the superconducting- and normal-state
properties of the SC-II phase were hampered by the intrinsic inhomogeneity and
phase separation. To elucidate this intriguing problem, we performed a detailed
high-pressure magnetotransport study on the recently discovered
(Li1-xFex)OHFe1-ySe single crystals, which have high Tc~40 K and share similar
Fermi surface topology as AxFe2-ySe2, but are free from the sample
complications. We found that the ambient-pressure Tc~41 K is suppressed
gradually to below 2 K upon increasing pressure to Pc ~5 GPa, above which a
SC-II phase with higher Tc emerges and the Tc increases progressively to above
50 K up to 12.5 GPa. Interestingly, our high-precision resistivity data enable
us to uncover the sharp transition of the normal state from a Fermi liquid for
SC-I phase (0 < P < 5 GPa) to a non-Fermi-liquid for SC-II phase (P > 5GPa). In
addition, the reemergence of high-Tc SC-II phase is found to accompany with a
concurrent enhancement of electron carrier density. Since high-pressure
structural study based on the synchrotron X-ray diffraction rules out the
structural transition below 10 GPa, the observed SC-II phase with enhanced
carrier density should be ascribed to an electronic origin associated with a
pressure-induced Fermi surface reconstruction.
