Unusual electron-doping effects in Sr₂₋ₓLaₓFeMoO₆ observed by photoemission spectroscopy

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We have investigated the electronic structure of electron-doped Sr₂₋ₓLaₓFeMoO₆ (x=0.0 and 0.2) by photoemission spectroscopy and band-structure calculations within the local-density approximation+U (LDA+U) scheme. A characteristic double-peak feature near the Fermi level (E F) has been observed in the valence-band photoemission spectra of both x=0.0 and 0.2 samples. A photon-energy dependence of the spectra in the Mo 4d Cooper minimum region compared with the band-structure calculations has shown that the first peak crossing E F consists of the (Fe+Mo) t2g states (feature A) and the second peak well below E F is dominated by the Fe eg states (feature B). Upon La substitution, the feature A moves away from E F by 50 meV which is smaller than the prediction of our band theory, 112 meV. In addition, an intensity enhancement of both A and B has been observed, although B is not crossing E F. Those two facts are apparently incompatible with the simple rigid-band shift due to electron doping. We point out that such phenomena can be understood in terms of the strong Hund's rule energy stabilization in the 3d⁵ configuration at the Fe sites in this compound. From an observed band-narrowing, we have also deduced a mass enhancement of 2.5 with respect to the band theory, in good agreement with a specific heat measurement.