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.Comment: text 9 pages, 5 figure

Antiferromagnetism and Superconductivity in a Model with Extended Pairing Interactions

The competition between antiferromagnetism and the $d+id$ superconducting state is studied in a model with near and next near neighbour interactions in the absence of any on-site repulsion. A mean field study shows that it is possible to have simultaneous occurrence of an antiferromagnetic and a singlet $d+id$ superconducting state in this model. In addition, such a coexistence generates a triplet $d+id$ superconducting order parameter with centre of mass momentum $Q=(\pi, \pi)$ {\it dynamically} having the same orbital symmetry as the singlet superconductor. Inclusion of next nearest neighbour hopping in the band stabilises the $d_{xy}$ superconducting state away from half filling, the topology of the phase diagram, though, remains similar to the near neighbour model. In view of the very recent observation of a broad region of coexistence of antiferrmagnetic and unconventional superconducting states in organic superconductors, the possibility of observation of the triplet state has been outlined.Comment: 12 pages(tex file), 7 figures (ps files