We have performed both theoretical and experimental studies of optical response of parent perovskite manganites $R{\text{MnO}}_3$ with a main goal to elucidate nature of clearly visible optical features. Starting with a simple cluster model approach we addressed the both one-center $\left(p\text{−}d\right)$ and two-center $\left(d\text{−}d\right)$ charge transfer (CT) transitions, their polarization properties, the role played by structural parameters, orbital mixing, and spin degree of freedom. Optical complex dielectric function of single-crystalline samples of $R{\text{MnO}}_3$ $\left(R=\text{La},\text{Pr},\text{Nd},\text{Sm},\text{Eu}\right)$ was measured by ellipsometric technique at room temperature in the spectral range from 1.0 to 5.0 eV for two light polarizations: $\mathbf{E}\parallel \mathbf{c}$ and $\mathbf{E}\perp \mathbf{c}$. The comparative analysis of the spectral behavior of ${\epsilon }_1$ and ${\epsilon }_2$ is believed to provide a more reliable assignment of spectral features. We have found an overall agreement between experimental spectra and theoretical predictions based on the theory of one-center $p\text{−}d$ CT transitions and intersite $d\text{−}d$ CT transitions. Our experimental data and theoretical analysis evidence a dual nature of the dielectric gap in nominally stoichiometric matrix of perovskite manganites $R{\text{MnO}}_3$, it is formed by a superposition of forbidden or weak dipole allowed $p\text{−}d$ CT transitions and intersite $d\text{−}d$ CT transitions. In fact, the parent perovskite manganites $R{\text{MnO}}_3$ should rather be sorted neither into the CT insulator nor the Mott-Hubbard insulator in the Zaanen, Sawatzky, Allen scheme.

• Received 28 April 2009

DOI:https://doi.org/10.1103/PhysRevB.82.035106