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Abstract
Pressure dependence of the electronic and crystal structures of KxFe2−ySe2, which has pressure-induced two superconducting domes of SC I and SC II, was investigated by x-ray emission spectroscopy and diffraction. X-ray diffraction data show that compressibility along the c-axis changes around 12 GPa, where a new superconducting phase of SC II appears. This suggests a possible tetragonal to collapsed tetragonal phase transition. X-ray emission spectroscopy data also shows the change in the electronic structure around 12 GPa. These results can be explained by the scenario that the two SC domes under pressure originate from the change of Fermi surface topology. Our results here show the pronounced increase of the density of states near the Fermi surface under pressure with a structural phase transition, which can help address our fundamental understanding for the appearance of the SC II phase.
Highlights
Since the discovery of high-temperature superconductivity in F-doped LaFeAsO in 20081, many iron-based superconductors with different crystal structures have been synthesized and are still hot topics in condensed matter physics
In this paper we report a systematic study of KxFe2−ySe2 with x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) under pressure
The XRD and XES studies under pressure have been performed for the 122*system, which have pressure-induced two superconducting domes
Summary
Since the discovery of high-temperature superconductivity in F-doped LaFeAsO in 20081, many iron-based superconductors with different crystal structures have been synthesized and are still hot topics in condensed matter physics. The electron-doping causes a Fe-deficiency of the FeSe layer to keep the charge valance, and this system is called 122*phase These new iron-defected systems (122*family) have attracted many interests because of the following several unique features, which are very different from other iron-based superconductors. A theoretical study of the SC I and SC II phases in the 122*system suggested that superconducting symmetry is d-wave without Γ-point hole pocket at SC I and s±-pairing at SC II21 In these systems, since no experimental study of the electronic structure, and the crystal structure under pressure has been reported so far, the issue of the appearance of SC II dome under pressure remains unclear. The Fe Kβ XES shows a pressure-induced change in the electronic structure at the transition pressure
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