Hybridization of CaFe1−xCoxAsF (x = 0, 0.06, 0.11, 0.144) studied by x-ray absorption spectroscopy and ultraviolet photoemission spectroscopy

Abstract

The parent compound CaFeAsF undergoes a structural phase transition from tetragonal to orthorhombic at T ∼ 121 K without superconductivity. The superconductivity emerged when the Fe sites are partially substituted by Co with a doping level of above 0.06 and the superconducting transition temperature is raised to 22 K in CaFe0.89Co0.11AsF. In this paper, the electronic structures of CaFe1−xCoxAsF (x = 0, 0.06, 0.11, 0.144) samples were studied by x-ray absorption spectroscopy (XAS). With the doping of Co, the Fe L3 XAS spectra shift to lower photon energy. Combined with the valence band spectra study, it is indicated that the strength of the hybridization between Fe 3d and As 4p is enhanced together with the increment of the superconducting transition temperature, after Co doping. The results suggest the enhancement of the hybridization between Fe 3d and As 4p plays an important role in the evolution of the superconductivity in Fe-based superconductors.