Abstract
We prepared two superconducting phases, which are called ‘‘low- phase’’ and ‘‘high- phase’’ of showing ’s of 35 and 44 K, respectively, at ambient pressure, and studied the superconducting behavior and structure of each phase under pressure. The of the 35 K at ambient pressure rapidly decreases with increasing pressure up to 10 GPa, and it remains unchanged up to 22 GPa. Finally, superconductivity was not observed down to 1.4 K at 29 GPa, i.e., . The of the 44 K phase also shows a monotonic decrease up to 15 GPa and it weakly decreases up to 25 GPa. These behaviors suggest no pressure-driven high- phase (called ‘‘SC-II’’) between 0 and 25 GPa for the low- and high- phases of , differing from the behavior of , which has a pressure-driven high- phase (SC-II) in addition to the superconducting phase (SC-I) observed at ambient and low pressures. The phase diagram for both low- and high- phases shows that the can be linearly scaled with (or FeSe plane spacing), where is a lattice constant. The reason why a pressure-driven high- phase (SC-II) was found for neither low- nor high- phases of is fully discussed, suggesting a critical value as the key to forming the pressure-driven high- phase (SC-II). Finally, the precise phase diagram is depicted using the data obtained thus far from FeSe codoped with a metal and or amine, indicating two distinct lines below .
- Received 11 November 2017
- Revised 23 January 2018
DOI:https://doi.org/10.1103/PhysRevB.97.094505
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