Electronic Phase Separation in Iron Selenide (Li,Fe)OHFeSe Superconductor System

The phenomenon of phase separation into antiferromagnetic (AFM) and superconducting (SC) or normal-state regions has great implication for the origin of high-temperature (high-T_\rm c) superconductivity. However, the occurrence of an intrinsic antiferromagnetism above the T_\rm c of (Li,Fe)OHFeSe superconductor is questioned. Here we report a systematic study on a series of (Li,Fe)OHFeSe single crystal samples with T_\rm c up to \sim41 K. We observe an evident drop in the static magnetization at T_\rm afm \sim 125 K, in some of the SC (T_\rm c \lesssim 38 K, cell parameter c \lesssim 9.27 Å) and non-SC samples. We verify that this AFM signal is intrinsic to (Li,Fe)OHFeSe. Thus, our observations indicate mesoscopic-to-macroscopic coexistence of an AFM state with the normal (below T_\rm afm) or SC (below T_\rm c) state in (Li,Fe)OHFeSe. We explain such coexistence by electronic phase separation, similar to that in high-T_\rm c cuprates and iron arsenides. However, such an AFM signal can be absent in some other samples of (Li,Fe)OHFeSe, particularly it is never observed in the SC samples of T_\rm c \gtrsim 38 K, owing to a spatial scale of the phase separation too small for the macroscopic magnetic probe. For this case, we propose a microscopic electronic phase separation. The occurrence of two-dimensional AFM spin fluctuations below nearly the same temperature as T_\rm afm, reported previously for a (Li,Fe)OHFeSe (T_\rm c \sim 42 K) single crystal, suggests that the microscopic static phase separation reaches vanishing point in high-T_\rm c (Li,Fe)OHFeSe. A complete phase diagram is thus established. Our study provides key information of the underlying physics for high-T_\rm c superconductivity.