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

Effects of Fe substitution on the electronic, transport, and magnetic properties of ZnGa2_2O4_4: A systematic ab-initio study

We present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide band gap spinel ZnGa$_2$O$_4$. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined.Comment: 9 pages, 8 figures; to be published in Phys. Rev.

Magnetic and orbital order in overdoped bilayer manganites

The magnetic and orbital orders for the bilayer manganites in the doping region $0.5 < x <1.0$ have been investigated from a model that incorporates the two $e_g$ orbitals at each Mn site, the inter-orbital Coulomb interaction and lattice distortions. The usual double exchange operates via the $e_g$ orbitals. It is shown that such a model reproduces much of the phase diagram recently obtained for the bilayer systems in this range of doping. The C-type phase with ($\pi,0,\pi$) spin order seen by Ling et al. appears as a natural consequence of the layered geometry and is stabilised by the static distortions of the system. The orbital order is shown to drive the magnetic order while the anisotropic hopping across the $e_g$ orbitals, layered nature of the underlying structure and associated static distortions largely determine the orbital arrangements.Comment: 8 pages, 5 figure