Pressure dependence of the electronic structure and spin state in Fe1.01Se superconductors probed by x-ray absorption and x-ray emission spectroscopy

Abstract

Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe${}_{1.01}$Se superconductors up to \ensuremath{\sim}66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of \ensuremath{\sim}7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe${}_{1.01}$Se decreases progressively with pressure up to \ensuremath{\sim}10 GPa. A new prepeak at energy of \ensuremath{\sim}7113.7 eV develops for pressure above \ensuremath{\sim}13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe${}_{1.01}$Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe${}_{1.01}$Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe${}_{1.01}$Se. A discontinuous variation of the integrated absolute difference values of the K\ensuremath{\beta} emission line was observed, originating from a phase transition of Fe${}_{1.01}$Se for a pressure >12 GPa. Fe${}_{1.01}$Se shows a small net magnetic moment of Fe${}^{2+}$ at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line K\ensuremath{\beta}\ensuremath{'} was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe${}_{1.01}$Se superconductors. The experimental results provide insight into the spin state of Fe${}_{1.01}$Se superconductors under pressure.