Origin of Pressure-induced Superconducting Phase in KxFe2−ySe2 studied by Synchrotron X-ray Diffraction and Spectroscopy

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.

Evidence of Mott physics in iron pnictides from x-ray spectroscopy

“…A possible explanation is that this modulation for the K-doped samples might be ascribed to differences in the dd-interactions between the two types of hole-doping. [23][24][25][26]30,31]. In the supplemental material we include calculations of our HERFD-XANES spectra showing that the A peak has mostly dipole character and that it arises when the four As nearest neighbours conforming the FeAs 4 tetrahedra are included in the structure, which corroborates its origin in the Fe 3d -As 4p mixing.…”

“…In both cases a clear increase of the IAD value with hole-doping is observed, although the trend is somewhat smaller for the K-doped samples. If the FeCrAs spectrum is used as the reference for the IAD analysis as done in [22,23], the lineshape of the difference spectra for both the K-and Cr-doped compounds has the typical reported [22][23][24][25][26] shape showing a bump in the energy position of the Kβ' satellite and the downward-upward feature around the Kβ 1,3 peak. Interestingly, when using the parent compound BaFe 2 As 2 as the reference instead, more structure is revealed.…”

Fluctuating magnetism of Co- and Cu-doped NaFeAs

“…XES is a photon-in photon-out spectroscopic technique that can be used to detect the spin state in materials 24,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] . The incoming photon (hν=7.140 keV) excites an Fe 1s electron to the continuum with the creation of an unstable core-hole filled by the decay of an Fe 3p electron and emission of a photon.…”

Formation of single-phase disordered CsxFe2−ySe2Svitlyk1, Pomjakushina2, Krztoń‐Maziopa3 et al. 2018Phys. Rev. B1030View full textAdd to dashboardCite

“…Upon further pressure increase (P > 11.5 GPa) a new superconducting phase (denoted as SCII) with a Tc reaching 48 K was reported [18]. This result, however, has not yet been experimentally reproduced with independent studies reporting much lower critical temperatures (~5 or ~20 K depending on sample preparation procedures [19,20]).…”

“…Available structural data on the SCII phases of AxFe2-ySe2 are not detailed, and sometimes even contradicting. These phases were reported to preserve tetragonal symmetry [18,19] following a collapse in the crystallographic c direction of the parent phases (tetragonal to collapsed tetragonal phase transition [19]). In addition, a sudden decrease in the Fe-Fe distances was observed during this transformation [32].…”