Abstract
In order to elucidate pressure-induced second superconducting phase (SC-II) in A (x) Fe(2-y)Se(2) (A = K, Rb, Cs, and Tl) having an intrinsic phase separation, we perform a detailed high-pressure magnetotransport study on the isoelectronic, phase-pure (Li(1-x)Fe (x) )OHFe(1-y)Se single crystals. Here we show that its ambient-pressure superconducting phase (SC-I) with a critical temperature T(c) ≈ 40 K is suppressed gradually to below 2 K and an SC-II phase emerges above P(c) ≈ 5 GPa with T(c) increasing progressively to above 50 K up to 12.5 GPa. Our high-precision resistivity data uncover a sharp transition of the normal state from Fermi liquid for SC-I to non-Fermi liquid for SC-II phase. In addition, the reemergence of high-T(c) SC-II is found to accompany with a concurrent enhancement of electron carrier density. Without structural transition below 10 GPa, the observed SC-II with enhanced carrier density should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction.