Band Crossing and Novel Low-Energy Behaviour in a Mean Field Theory of a Three-Band Model on a Cu--O lattice

We study correlation effects in a three-band extended Hubbard model of Cu -- O planes within the 1/N mean field approach, in the infinite U limit. We investigate the emerging phase diagram and discuss the low energy scales associated with each region. With increasing direct overlap between oxygen orbitals, $t_{pp} >0$, the solution displays a band crossing which, for an extended range of parameters, lies close to the Fermi level. In turn this leads to the nearly nested character of the Fermi surface and the resulting linear temperature dependence of the quasi-particle relaxation rate for sufficiently large T. We also discuss the effect of band crossing on the optical conductivity and comment on the possible experimental relevance of our findings.Comment: 12 pages, Latex-Revtex, 6 PostScript figures. Submitted to Phys. Rev.

Boundary Energies and the Geometry of Phase Separation in Double--Exchange Magnets

We calculate the energy of a boundary between ferro- and antiferromagnetic regions in a phase separated double-exchange magnet in two and three dimensions. The orientation dependence of this energy can significantly affect the geometry of the phase-separated state in two dimensions, changing the droplet shape and possibly stabilizing a striped arrangement within a certain range of the model parameters. A similar effect, albeit weaker, is also present in three dimensions. As a result, a phase-separated system near the percolation threshold is expected to possess intrinsic hysteretic transport properties, relevant in the context of recent experimental findings.Comment: 6 pages, including 4 figures; expanded versio