Structural and optical investigations of the iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ under high pressure

摘要

Iron-chalcogenide superconductor Fe$_{1.03}$Se$_{0.5}$Te$_{0.5}$ has beeninvestigated under high pressure using synchrotron based x-ray diffraction andmid-infrared reflectance measurements at room temperature. Pressure dependenceof the superconducting transition temperature (T$_c$) of the same sample hasbeen determined by temperature-dependent resistance measurements up to 10 GPa.Although the high pressure orthorhombic phase ($\textit{Pbnm}$) starts emergingat 4 GPa, structural transition becomes clearly observable above 10 GPa. Astrong correlation is observed between the Fe(Se,Te)$_{4}$ tetrahedraldeformation in the tetragonal phase ($\textit{P4/nmm}$) and the sharp rise ofT$_c$ up to $\sim$4 GPa, above which T$_c$ is found to be almost pressureindependent at least up to 10 GPa. A subtle structural modification of thetetragonal phase is noticed above 10 GPa, suggesting a structural transitionwith possible Fe$^{2+}$ spin-state transition. The evolution with pressure ofthe optical conductivity shows that the Drude term increases systematicallywith pressure up to 5.4 GPa, indicating the evolution of the tetragonal phasetowards a conventional metallic state. At further higher pressures the Drudeterm reduces drastically implying a poor metallic character of the highpressure orthorhombic phase. Our results suggest that occurrence of large onsetT$_c$ above 4 GPa is due to a systematic increase of high pressure orthorhombicphase fraction.