重电子掺杂FeSe基超导体压力诱导的超导II相研究

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.

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。

通过高压调控以及化学掺杂，在重电子掺杂FeSe基超导体、重费米子超导体CeCu2(Si1-xGex)2以及FeAs基超导体LaFeAsO1-x(H/F)x中均发现了两个超导相，而在非常规超导体系中出现两个超导相往往暗示该体系蕴涵着丰富的物理内容。本项目的研究目标就是在此基础上进一步探索并理解重电子掺杂FeSe基超导体出现的超导II相，并以此为指导进一步理解非常规超导体系的非常规超导相图。在本项目的资助下取得了如下进展：（1）利用高压调控的手段，测量了(CTA)0.3FeSe单晶与Lix(NH2)yFeSe多晶的交流磁化率以及FeSe0.92S0.08单晶的磁电输运，进一步在多个FeSe基超导体中发现压力诱导的超导II相；（2）拓展了高压下的测量技术，在8GPa的静水压综合极端条件下完成了对FeSe超导体的原位高压与门电压联合调控测量，并申请相关专利一项；（3）通过高压电阻率、磁化率以及原位高压弹性X射线共振测量，建立了CsV3Sb5与RbV3Sb5的温度-压力相图，阐释了电荷密度波(CDW)与超导(SC)之间的非常规竞争关系，其中，CDW在高压下单调降低，但是超导相出现了非单调的双拱形相图，揭示了交织电子序之间的竞争与协作关系。以上研究结果为进一步探索高压调控与化学掺杂诱导的超导II相以及其与多种电子序之间的关联提供了重要参考价值。