Abstract
An orbital degree of freedom in Mott insulators gives strong impact on their phase transitions induced by the band-filling control or carrier doping. We have investigated the effect of electrostatic carrier doping on the electronic spectra for a layered Mott insulator Sr${}_{2}$MnO${}_{4}$ to reveal orbital-specific optical transitions. Sr${}_{2}$MnO${}_{4}$ is an $n$-type Mott insulator and its conduction band is composed of nearly degenerated ${e}_{g}$ orbitals ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$. The charge modulation spectra for a rectifying Sr${}_{2}$MnO${}_{4}$/Nb-doped SrTiO${}_{3}$ junction clearly revealed an optical transition at 1.7 eV, while a linear absorption spectrum is dominated by a transition at 2.0 eV. These are assigned to the transitions from O 2$p$ to Mn ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ and to ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$, respectively. The accumulated charges with a density as high as 8$\ifmmode\times\else\texttimes\fi{}$10${}^{13}$ cm${}^{\ensuremath{-}2}$ selectively occupy the nearly localized ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ orbitals that hardly contribute to charge transport.
Published Version
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