Electronic Phase Separation in Iron Selenide (Li,Fe)OHFeSe Superconductor System
Yiyuan Mao1,2,3†, Jun Li4†, Yulong Huan1,2,3†, Jie Yuan1,3, Zi-an Li1,3, Ke Chai5, Mingwei Ma1,3, Shunli Ni1,2, Jinpeng Tian1,2, Shaobo Liu1,2, Huaxue Zhou1, Fang Zhou1,2, Jianqi Li1,2,3, Guangming Zhang6, Kui Jin1,2,3, Xiaoli Dong1,2,3**, Zhongxian Zhao1,2,3**
1Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190 2University of Chinese Academy of Sciences, Beijing 100049 3Key Laboratory for Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049 4Research Institute of Superconductor Electronic, Nanjing University, Nanjing 210093 5School of Physics, Beijing Institute of Technology, Beijing 100081 6State Key Laboratory of Low-Dimensional Quantum Physics and Department of Physics, Tsinghua University, Beijing 100084
Abstract：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 $\sim$41 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.
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2018, Vol. 35 
