The observation of charge stripe order in the doped nickelate and cuprate materials has motivated much theoretical effort to understand the underlying mechanism of the stripe phase. Numerical studies of the Hubbard model show two possibilities: (i) stripe order arises from a tendency toward phase separation and its competition with the long-range Coulomb interaction or (ii) stripe order inherently arises as a compromise between itinerancy and magnetic interactions. Here we determine the restricted phase diagram of the two-dimensional Falicov-Kimball model and see that it displays rich behavior illustrating both possibilities in different regions.
The electronic and magnetic properties of ferromagnetic doped manganites are investigated by means of model tight-binding and self-interaction corrected local spin density ͑SIC-LSD͒ approximation calculations. It is found that the surface alone by breaking the cubic symmetry induces a difference in the occupation of the two e g orbitals at the surface. We found surface localization of one orbital and hence a change in the Mn valency from four in the bulk to three at the subsurface. Different surface or disordered interface induced localization of the orbitals are considered too with respect to the nature and the strength of the local orbital ordering and magnetic exchange coupling between the surface/interface and the bulklike region. We predict that better tunneling can be achieved in tunnel barriers that favor slightly the occupancy of the Mn e g͑3z 2 −r 2 ͒ at the interface.
Self-interaction corrected local spin density approximation calculations were performed for La (1−x) SrxMnO3 (LSMO) (0.0 < x < 0.5). The influence and inter-relationship of Sr doping, magnetic structure, O displacements and phase segregation on the Mn charge state were studied. A half-metallic state was obtained for LSMO with manganese configuration Mn 3+ , whilst Mn 4+ gave rise to a metallic state with a negligible spin polarisation at the Fermi level. Elongating the MnO6 octahedron led to a static mixed valence Mn 3+ /Mn 4+ configuration. In the mixed valence state the total energy was minimized by an ordered array of Mn 4+ and Mn 3+ MnO2 planes which showed charge ordered stripes.
The first-principles SIC-LSD theory is utilized to study electronic, magnetic and orbital phenomena in LaMnO3. The correct ground state is found, which is antiferro orbitally ordered with the spin magnetic moments antiferromagnetically aligned. Jahn-Teller energies are found to be the largest energy scale. In addition it is the Jahn-Teller interaction which is the dominant effect in realising orbital order, and the electronic effects alone do not suffice.
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