重电子掺杂FeSe基高温超导体(Li1-xFex)OHFeSe在压力调控下出现了第二个具有更高超导转变温度的超导SC-II相，其正常态的电输运行为从超导SC-I相的费米液体向SC-II相的非费米液体转变，而且同时伴随电子型载流子浓度的显著提高，高压结构分析表明在整个压力范围内没有发生结构相变，因此，超导SC-II相的出现很可能与压力诱导的费米面重构有重要联系。本项目计划合成系列FeSe基超导体单晶样品，利用高压低温的实验手段进行综合物性调控，包括压力下电阻率、磁阻、霍尔效应以及交流磁化率的实验技术手段，探究压力诱导的超导SC-II相是否为FeSe基超导体所拥有的共同属性，并且通过霍尔效应定性分析其费米面的拓扑结构，揭示出FeSe基超导体决定高温超导SC-II相出现的关键因素与超导SC-II相和费米面拓扑结构变化的关联。

The second superconducting (SC-II) phase with a higher superconducting transition temperature appears in heavy-electron doped FeSe-based superconductor (Li1-xFex)OHFeSe under high pressure, and its normal state is changing from Fermi liquid in superconducting SC-I phase to non-Fermi liquid in SC-II phase with the electron carrier concentration increasing significantly at the same time. The high-pressure structure analysis shows that no structure transition occurs in the whole pressure range. Therefore, the appearance of superconducting SC-II phase is probably related to the pressure-induced Fermi surface reconstruction. This project plans to synthesize a series of FeSe-based superconductor single crystal samples, and we will use the high pressure and low temperature conditions to preform comprehensive physical property measurements, including resistivity, magnetoresistance, Hall effect and AC susceptibility under pressure, to explore whether pressure-induced SC-II phase is the common feature of FeSe-based superconductors. We also employ Hall effect measurements to qualitatively analyze its Fermi surface topological structure. Finally, we want to reveal the key factors for determining the appearance of SC-II phase in FeSe-based high temperature superconductors and the relationship between SC-II phase and changes of the Fermi surface topological structure.