Interplay of p-d and d-d Charge Transfer Transitions in Rare-Earth Perovskite Manganites

Abstract

We have performed both theoretical and experimental studies of optical response of parent perovskite manganites R MnO3 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 (p-d) and two-center (d-d) 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 MnO 3 (R=La, Pr, Nd, Sm, Eu) was measured by ellipsometric technique at room temperature in the spectral range from 1.0 to 5.0 eV for two light polarizations: E∥c and E⊥c. The comparative analysis of the spectral behavior of ε1 and ε2 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-d CT transitions and intersite d-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 MnO3, it is formed by a superposition of forbidden or weak dipole allowed p-d CT transitions and intersite d-d CT transitions. In fact, the parent perovskite manganites R MnO3 should rather be sorted neither into the CT insulator nor the Mott-Hubbard insulator in the Zaanen, Sawatzky, Allen scheme. © 2010 The American Physical Society.