Synchrotron X-ray absorption and Fe-57 Mossbauer spectroscopy studies of electronic structure of cobalt-doped SmFeAsO superconductors

摘要

We report on the electronic structure of cobalt-doped oxypnictide SmFe1-xCoxAsO superconductor crystals (x=0.1-0.25) by synchrotron X-ray absorption spectroscopy (XAS) and Fe-57 Mossbauer spectroscopy. The Mossbauer spectra of the superconductor samples show only paramagnetic singlet patterns indicating the absence of hyperfine magnetic field, and the central shift values, 0.41 mm/s referring to alpha-Fe, reveal coexistence of high- and low-spin Fe2+ in this system. The Fe L-2,L-3-edge and O K-edge XAS spectra for all the samples are found to be similar, though the peaks have different intensity. This indicates that the hybridization of Fe with As becomes weakened as Co content increases from 0.10 to 0.25, leading to decrease of the transition temperature as obtained by the resistance-temperature measurement. The Sm M-4,M-5-edge XAS spectra exhibit two main peaks without any shoulders, but with an abnormal shift as increasing Co concentration, revealing evident influence of the magnetic moment of cobalt on the moment of Sm ions.