Double exchange and orbital correlations in electron-doped manganites

Abstract

A double-exchange model for degenerate eg orbitals with intra-and inter-orbital interactions has been studied for theelectron-doped manganitesA 1 − x B x MnO3 (x > 0.5). We show that such a model reproduces the observed phasediagram and orbital ordering in the intermediate-bandwidth regimeand the Jahn-Teller effect, considered to be crucial for theregion x < 0.5, does not play a major role in this region. Brinkand Khomskii have already pointed this out and stressed therelevance of the anistropic hopping across the degenerate egorbitals in the infinite Hund's coupling limit. From a morerealistic calculation with finite Hund's coupling, we show thatinclusion of interactions stabilizes the C-phase, theantiferromagnetic metallic A-phase moves closer to x = 0.5 whilethe ferromagnetic phase shrinks. This is in agreement with therecent observations of Kajimoto et al.and Akimoto et al.