Double exchange and orbital correlations in electron-doped manganites

A double-exchange model for degenerate e(g) orbitals with intra- and inter-orbital interactions has been studied for the electron-doped manganites A(1-x)B(x)MnO(3) (x > 0.5). We show that such a model reproduces the observed phase diagram and orbital ordering in the intermediate-bandwidth regime and the Jahn-Teller effect, considered to be crucial for the region x < 0.5, does not play a major role in this region. Brink and Khomskii have already pointed this out and stressed the relevance of the anistropic hopping across the degenerate e(g) orbitals in the infinite Hund's coupling limit. From a more realistic calculation with finite Hund's coupling, we show that inclusion of interactions stabilizes the C-phase, the antiferromagnetic metallic A-phase moves closer to x = 0.5 while the ferromagnetic phase shrinks. This is in agreement with the recent observations of Kajimoto et al. and Akimoto et al.