插层FeSe高温超导体的高压研究进展

通过对FeSe进行化学插层可以将其超导转变温度（T_c）从约8 K提高到40 K以上，实现高温超导电性.最近，我们对两种插层FeSe高温超导材料（Li_0.84Fe_0.16）OHFe_0.98Se和Li_0.36（NH₃）_yFe₂Se₂开展了高压调控研究，发现压力会首先抑制高温超导相（称为SC-I相），然后在临界压力P_c以上诱导出第二个高温超导相（称为SC-Ⅱ相），呈现出双拱形T-P超导相图.这两个体系的P_c分别约为5和2 GPa，两个体系SC-Ⅱ相的最高T_c分别可以达到约52和55 K，比相应SC-I相的初始T_c提高了10 K.对（Li_0.84Fe_0.16）OHFe_0.98Se的正常态电输运性质分析表明，SC-I和SC-Ⅱ相的正常态分别具有费米液体和非费米液体行为，意味着这两个超导相可能存在显著差异.此外，还发现这两个体系的SC-Ⅱ相的T_c与霍尔系数倒数1/R_H（∝载流子浓度n_e）具有很好的线性依赖关系.对（Li_0.84Fe_0.16）OHFe_0.98Se的高压X射线衍射测量排除了其在10 GPa以内发生结构相变的可能，因此P_c以上SC-Ⅱ相的出现和载流子浓度的增加很可能起源于压力导致的费米面重构.

Effect of high pressure on intercalated FeSe high-Tc superconductors

Among the iron-based superconductors, the structural simplest FeSe and its derived materials have received much attention in recent years due to the great tunability of the superconducting transition temperature (T_c). The relatively low T_c ≈ 8.5 K of FeSe can be raised to over 40 K via the interlayer intercalations such as A_xFe_2-ySe₂ (A=K, Rb, Cs, Tl), Li_x(NH₃)_yFe₂Se₂, and (Li_1-xFe_x)OHFeSe. Although the monolayer FeSe/SrTiO₃ is reported to have a T_c as high as 65 K, none of the T_c values of these FeSe-derived bulk materials has exceeded 50 K at ambient pressure so far. In order to explore other routes to further enhance T_c of FeSe-based materials, we recently performed the detailed high-pressure study of two intercalated FeSe high-T_c superconductors, namely (Li_0.84Fe_0.16)OHFe_0.98Se and Li₀₃₆(NH₃)_yFe₂Se₂, by using a cubic anvil cell apparatus. We find that the applied high pressure first suppresses the superconducting phase (denoted as SC-I) and then induces a second high-T_c superconducting phase (denoted as SC-Ⅱ) above a critical pressure P_c (~5 GPa for (Li_0.84Fe_0.16)OHFe_0.98Se and 2 GPa for Li₀₃₆(NH₃)_yFe₂Se₂). The highest T_c values in the SC-Ⅱ phases of these two compounds can reach~52 K and 55 K, respectively, the latter of which is the highest in the FeSe-based bulk materials, and is very close to the highest T_c of FeAs-based high-T_c superconductors. Our high-precision resistivity data of (Li_0.84Fe_0.16)OHFe_0.98Se also 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 phase under pressure is found to be accompanied with a concurrent enhancement of electron carrier density. Interestingly, we find a nearly parallel scaling behavior between T_c and the inverse Hall coefficient for the SC-Ⅱ phases of both (Li_0.84Fe_0.16)OHFe_0.98Se and Li_0.36(NH₃)_yFe₂Se₂. In the case without structural transition below 10 GPa, the observed enhancement of carrier density in SC-Ⅱ should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction. Our work demonstrates that high pressure offers a distinctive means to further raise the maximum T_c values of intercalated FeSe-based materials.