Electron energy-loss spectroscopy and ab initio electronic structure of the LaOFeP superconductor

The electronic band structures of the LaOFeP superconductor have been calculated theoretically by the first-principles method and measured experimentally by electron energy loss spectroscopy (EELS). The calculations indicate that the Fe atom in a LaOFeP crystal shows a weak magnetic moment (0.14 mu(B)/atom) and does not form a long-range magnetic ordering. Band structure, Fermi surfaces, and fluorine-doping effects are also analyzed based on the data of the density functional theory. The fine structures of the EELS data have been carefully examined in both the low loss energy region (<60 eV) and the core losses region (O K, Fe L-2,L-3, and La M-4,M-5). A slight bump edge at similar to 44 eV shows notable orientation dependence: it can be observed in the low loss EELS spectra with q parallel to c but becomes almost invisible in the q perpendicular to c spectra. Annealing experiments indicate that low oxygen pressure favors the appearance of superconductivity in LaOFeP: this fact is also confirmed by the changes of Fe L-2,L-3 and O K excitation edges in the experimental EELS data.