Doped Manganites Beyond Conventional Double-Exchange Model

Abstract

The problem of adequate electronic model for doped manganites like La_{1-x}Sr_{x}MnO_3 remains controversial. In such a situation we argue a necessity to discuss all possible candidate states with different valent structure of manganese and oxygen atoms, as well as different valent states of octahedral MnO_{6} centers. In frames of quantum-chemical approach, crystal field and the ligand field models we address different types of MnO_6 centers, different types of d-d, and charge transfer transitions. We draw special attention to the so-called charge transfer states related to strong intra-center charge fluctuations. As we conjecture, namely these could become active valent states for doped manganites. We discuss some electric and magnetic properties of the electron MnO_{6}^{10-}, and hole MnO_{6}^{8-} centers with unconventional ^{6}A_{1g}-{}^{6}T_{1u} and ^{4}A_{2g}-{}^{4}T_{2u} valent manifolds, respectively. We propose two idealized theoretical models for hole system in doped manganites with an overall oxygen localization for the doped holes occupying the non-bonding O2p orbitals, and a doping induced formation of a system of the electron MnO_{6}^{10-} and hole MnO_{6}^{8-} centers being equivalent to the electron-hole Bose liquid.