Disproportionation and Electronic Phase Separation in Parent Manganite LaMnO3

Abstract

Nominally pure undoped parent manganite LaMnO3 exhibits a puzzling behavior inconsistent with a simple picture of an A -type antiferromagnetic insulator (A-AFI) with a cooperative Jahn-Teller ordering. We do assign its anomalous properties to charge transfer (CT) instabilities and competition between insulating A-AFI phase and metalliclike dynamically disproportionated phase formally separated by a first-order phase transition at Tdisp = TJT ≈ 750 K. The unconventional high-temperature phase is addressed to be a specific electron-hole (EH) Bose liquid (EHBL) rather than a simple "chemically" disproportionated La (Mn2+Mn4+)O3 phase. The phase does nucleate as a result of the CT instability and evolves from the self-trapped CT excitons or specific EH dimers, which seem to be a precursor of both insulating and metalliclike ferromagnetic phases observed in manganites. We arrive at highly frustrated system of triplet (eg2)3A 2g bosons moving in a lattice formed by hole Mn4+ centers. Starting with different experimental data we have reproduced a typical temperature dependence of the volume fraction of high-temperature mixed-valence EHBL phase. We argue that a slight nonisovalent substitution, photoirradiation, external pressure, or magnetic field gives rise to an electronic phase separation with a nucleation or an overgrowth of EH droplets. Such a scenario provides a comprehensive explanation of numerous puzzling properties observed in parent and nonisovalently doped manganite LaMnO3 including an intriguing manifestation of superconducting fluctuations. © 2009 The American Physical Society.