Pressure effects on the FeSe-based superconductors

Abstract

<p indent="0mm">Among the iron-based superconductors, FeSe and its derived materials have attracted much research interest recently due to its unusual normal- and superconducting-state properties. High-pressure techniques play an important role in unveiling the competing electronic orders and tuning the high-<italic>T</italic>_c superconductivity of FeSe. In this article, we review the recent progress on the pressure effects of FeSe-based superconducting materials by using the cubic anvil cell apparatus that can maintain an excellent hydrostatic pressure condition up to <sc>15 GPa.</sc> For the bulk FeSe, we construct a comprehensive temperature-pressure phase diagram, which uncovers a dome-shaped antiferromagnetic phase boundary and reveals<italic> </italic>the detailed competing relationships between the nematicity, antiferromagnetism, and superconductivity. Our high-pressure Hall results further indicate that the critical antiferromagnetic fluctuations play an important role for achieving the high-<italic>T</italic>_c superconductivity in FeSe under high pressure. For the intercalated FeSe-based high-<italic>T</italic>_c superconductors, we find that the application of high pressure first suppresses the superconducting phase SC-I and then induces a second high-<italic>T</italic>_c superconducting phase SC-II above a critical pressure <italic>P</italic>_c with the optimal <italic>T</italic>_c over <sc>50 K,</sc> resulting in a double-dome shaped phase diagram. In addition, the reemergence of SC-II phase under pressure is found to be accompanied by a concurrent enhancement of electron carrier density. Without structural transition below <sc>10 GPa,</sc> the observed SC-II with enhanced carrier density should be ascribed to an electronic origin presumably associated with pressure-induced Fermi surface reconstruction.