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تسجيل مستخدم جديدHole-doping-induced changes in the electronic structure of La$_{1-x}$Sr$_x$FeO$_3$ : soft x-ray photoemission and absorption study of epitaxial thin films
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We have studied the electronic structure of epitaxially grown thin films of La$_{1-x}$Sr$_x$FeO$_3$ by {it in-situ} photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS) measurements. The Fe 2$p$ and valence-band PES spectra and the O $1s$ XAS spectra of LaFeO$_3$ have been successfully reproduced by configuration-interaction cluster-model calculation and, except for the satellite structure, by band-structure calculation.From the shift of the binding energies of core levels, the chemical potential was found to be shifted downward as $x$ was increased. Among the three peaks in the valence-band spectra of La$_{1-x}$Sr$_x$FeO$_3$, the peak nearest to the Fermi level ($E_F$), due to the ``$e_{g}$ band, was found to move toward $E_F$ and became weaker as $x$ was increased, whereas the intensity of the peak just above $E_F$ in the O $1s$ XAS spectra increased with $x$. The gap or pseudogap at $E_F$ was seen for all values of $x$. These results indicate that changes in the spectral line shape around $E_F$ are dominated by spectral weight transfer from below to above $E_F$ across the gap and are therefore highly non-rigid-band-like.
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We have investigated change in the electronic structures of atomically-controlled La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films as a function of hole-doping level ($x$) in terms of {it in situ} photoemission spectroscopy (PES) and x-ray absorption spectr
oscopy (XAS) measurements. The {it in situ} PES measurements on a well-ordered surface of high-quality epitaxial LSMO thin films enable us to reveal their intrinsic electronic structures, especially the structure near the Fermi level ($E_F$). We have found that overall features of valence band as well as the core levels monotonically shifted toward lower binding energy as $x$ was increased, indicating the rigid-band like behavior of underlying electronic structure of LSMO thin films. The peak nearest to $E_F$ due to the $e_g$ orbital is also found to move toward $E_F$ in a rigid-band manner, while the peak intensity decreases with increasing $x$. The loss of spectral weight with $x$ in the occupied density of states was compensated by simultaneous increment of the shoulder structure in O 1$s$ XAS spectra, suggesting the existence of a pseudogap, that is depression in spectral weight at $E_F$, for all metallic compositions. These results indicate that the simple rigid-band model does not describe the electronic structure near $E_F$ of LSMO and that the spectral weight transfer from below to above $E_F$ across the gap dominates the spectral changes with $x$ in LSMO thin films.
We have performed an angle-resolved photoemission spectroscopy study of La$_{0.6}$Sr$_{0.4}$FeO$_3$ using {it in situ} prepared thin films and determined its band structure. The experimental band dispersions could be well explained by an empirical ba
nd structure assuming the G-type antiferromagnetic state. However, the Fe 3d bands were found to be shifted downward relative to the Fermi level ($E_F$) by $sim 1$ eV compared with the calculation and to form a (pseudo)gap of $sim 1$ eV at $E_F$. We attribute this observation to a strong localization effect of doped holes due to polaron formation.
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. Thi
s 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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