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. 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 MnO 6 centers. In frames of rather conventional cluster approach, crystal field and the ligand field model we address different types of MnO 6 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 MnO 6 10−, and hole MnO 6 8− centers with unconventional 6 A 1 g – 6 T 1 u and 4 A 2 g – 4 T 2 u 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 MnO 6 10−, and hole MnO 6 8− 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.