Study On The FeSe-based Superconductors Under High Pressure

Pressure is one of the fundamental thermodynamic parameters as temperature.The application of high pressure can effectively shorten the interatomic distances,enhance the orbital overlap intergral and the electron bandwidth,and modify the lattice vibration frequencies.In some cases,high pressure can induced electronic and/or crystal structure phase transitions.Thus,high-pressure techniques have been widely used to explore novel physical phenomena in condensed matter physics.In comparison with the chemical doping,the application of high pressure can be regarded as a clean and pricise tuning knobs in that it does not change the chemical composition of materials,and does not introduce lattice disorder and additional charge carriers.Therefore,high-pressure investigations have been widely performed on the studies of copper-and iron-based high-temperature superconductors.By using the cubic-anvil-cell apparatus that can maintain an excellent hydrostaticity up to 15 GPa,we performed compresenhve high-pressure studies on the bulk FeSe and heavily electron doped FeSe-based superoncondcutors.The main results include:1,Detailed magneto-transport measurements on FeSe single crystal under high pressure enabled us to construct the most comprehensive temperature-pressure phase diagram that show explicitly the competiting nature of nematicity,pressure-induced antiferromagnetic（AFM）order,and superconductivity.The high-T_c superconductivity in FeSe is eventually achieved by suppressing the electronic nematicity and AFM.Especially,the close proximity of high-T_c superconductivity to the AFM order resembles the other iron-based superconducting systems,and the observarion of linear-in-T dependence of normal-state resistivity around the critical pressure where the magnetic order just vanishes implies the important role of critical spin fluctuations in achieving the high-T_c superconductivity.2,We have further performed the Hall effect measurements on FeSe single crystal under high pressures up to 8.8 GPa.Surprisingly,we found that the normal-state Hall coefficeint changes sign from negative to positive above 2 GPa,demonstrating dominant hole carriers in the high-T_c phase of FeSe,in contrast to other FeSe-derived high-T_c materials.In addition,the Hall coefficient is significantly enhanced and the magnetoresistance exhibits anomalous scaling behaviors,evidencing strongly enhanced interband spin fluctuations in the high-T_c phase.Since the first-principles DFT calculations evidenced the presence of both hole and electron Fermi surfaces under pressure,our results in FeSe show great similarities with other FeAs-based high-T_c supercondcutors,constituting a step toward a unified understanding of iron-based superconductivity.3,We performed a detailed high-pressure magneto-transport study on the recently discovered(Li_1-xFe_x)OHFe_1-ySe single crystals,which have high T_c?40 K and share similar Fermi surface topology as A_xFe_2-ySe₂,but are free from the sample complications.We found that the ambient-pressure T_c?40 K is suppressed gradually to below 2 K upon increasing pressure to P_c?5 GPa,above which a second superconducting phase（SC-Ⅱ）with higher T_c emerges and the T_c increases progressively to above 50 K up to 12.5 GPa.Interestingly,our high-precision resistivity data enable us to uncover the sharp transition of the normal state from a Fermi liquid for SC-Ⅰ phase（0<P<5 GPa）to a non-Fermi-liquid for SC-Ⅱ phase（P>5GPa）.In addition,the reemergence of high-T_c SC-Ⅱ phase is found to accompany with a concurrent enhancement of electron carrier density.Our high-pressure structural study based on the synchrotron X-ray diffraction rules out the structural transition below 10 GPa.Instead,the observed SC-Ⅱ phase with enhanced carrier density should be ascribed to an electronic origin presumably associated with a pressure-induced Fermi surface reconstruction.4,High-pressure study on the heavily electron doped Li_0.36（NH₃）_yFe₂Se₂ single crystal also evidenced a SC-Ⅱ phase above 2 GPa.The superconducting transition temperature T_c?44 K at ambient pressure is first suppressed to below 20 K upon increasing pressure to P_c?2 GPa,above which the pressure dependence of T_c（P）reverses and T_c increases steadily to ca.55 K at 11 GPa.The pressurized Li_0.36（NH₃）_yFe₂Se₂ thus exhibits the highest T_c^max?55K among the FeSe-based bulk materials.Hall data confirm that in the emergent SC-Ⅱ phase the dominant electron-type carrier density undergoes a fourfold enhancement and tracks the same trend as T_c（P）.Interesting,we find a nearly parallel scaling behavior between T_c and the inverse Hall coefficient for the SC-Ⅱ phases of both Li_0.36（NH₃）_yFe₂Se₂ and（Li,Fe）OHFeSe.Our work demonstrates that high pressure offers a distinctive means to further raising the maximum T_c of heavily electron doped FeSe-based materials by increasing the effective charge carrier concentration via a plausible Fermi surface reconstruction at P_c.