Disproportionation and Electronic Phase Separation in Parent Manganite LaMnO3

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.I thank N. N. Loshkareva, Yu. P. Sukhorukov, K. N. Mikhalev, Yu. B. Kudasov, and V. V. Platonov for stimulating and helpful discussions. The work was supported by RFBR under Grants No. 06-02-17242, No. 07-02-96047, and No. 08-02-00633