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Register a new userCorrelation effect in Sr$_{1-x}$La$_x$RuO$_3$ studied by soft x-ray photoemission spectroscopy
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To clarify how the electronic state of Sr1-xLaxRuO3 evolves with La doping, we conducted photoemission (PES) experiments using soft x-rays. The spectral shape of the Ru 4d derived peak near the Fermi level changes significantly with increasing x. This variation indicates that a spectral weight transfer from the coherent to incoherent component occurs due to an enhancement of the electron correlation effect. Resonant PES experiments at the La 3d_{5/2} edge have confirmed that there is no significant contribution of the La 5d state in the energy range where the spectral weight transfer is observed. Using the dependence of the photoelectron mean free path on the photon energy, we subtracted the surface components from the PES spectra and confirmed that the enhancement of the electron correlation effect with La doping is an intrinsic bulk phenomenon. On the other hand, a large portion of the coherent component remains at the Fermi level up to x = 0.5, reflecting that the Ru 4d state still has itinerant characteristics. Moreover, we found that the PES spectra hardly depend on the temperature and do not exhibit a discernible change with magnetic ordering, suggesting that the temperature variation of the exchange splitting does not follow the prediction of the Stoner theory. The presently obtained experimental results indicate that the electron correlation effect plays an important role in Sr1-xLaxRuO3 and that the Ru 4d electrons possess both local and itinerant characteristics.
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We have investigated the electronic structure of electron-doped Sr$_{2-x}$La$_x$FeMoO$_6$ ($x$=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 ($E_{rm F}$) 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 4$d$ Cooper minimum region compared with the band-structure calculations has shown that the first peak crossing $E_{rm F}$ consists of the (Fe+Mo) $t_{2gdownarrow}$ states (feature A) and the second peak well below $E_{rm F}$ is dominated by the Fe $e_{guparrow}$ states (feature B). Upon La substitution, the feature A moves away from $E_{rm F}$ by $sim$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 $E_{rm F}$. 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 Hunds rule energy stabilization in the 3$d^5$ configuration at the Fe sites in this compound. From an observed band-narrowing, we have also deduced a mass enhancement of $sim$2.5 with respect to the band theory, in good agreement with a specific heat measurement.
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