Doped manganites beyond conventional double-exchange model

The problem of adequate electronic model for doped manganites like La1-xSrxMnO3 remains controversial. There are many thermodynamic and local microscopic quantities that cannot be explained by the conventional double-exchange model with dominantly Mn3d location of doped holes. 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 MnO6 centers. In frames of rather conventional cluster approach, crystal field and the ligand field model we address different types of MnO6 centers, different types of d-d, and charge-transfer (CT) transitions. We draw special attention to the so-called CT 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 MnO610-, and hole MnO68- centers with unconventional 6A1g-6T1u and 4A2g-4T2u valent manifolds, respectively. We propose two idealized theoretical models for hole system in doped manganites. The first one implies an overall oxygen localization for the doped holes occupying the non-bonding O2p orbitals. The second assumes a doping induced formation of the electron-hole Bose liquid, or a system of the electron MnO610-, and hole MnO68- centers. In a sense, this scenario resembles a well known disproportionation reaction. In both cases one might expect non-trivial magnetic behavior with strong ferromagnetic fluctuations due to anomalously strong ferromagnetic coupling of non-bonding O2p holes with Mn3d electrons. © 2002 Elsevier Science B.V. All rights reserved.