| Realistic electronic-structure calculations for correlated Mott insulators are notoriously hard. We present a fully ab initio, wavefunction-based quantum chemical scheme that adequately describes strong correlation effects involving Cu 3d and O 2p electrons in both hole and electron doped cuprates. In particular, the O 2p states giving rise to the Zhang-Rice band are explicitly considered. The mixing with "triplet" dz2 hole states at higher binding energy is also accounted for. In addition to that, our scheme incorporates renormalization effects due to nonlocal spin interactions. The resulting quasiparticle dispersion and Fermi-surface (FS) evolution with doping are compared with recent ARPES (angle-resolved photoemission spectroscopy) and SdH (Shubnikov-deHaas) data. Most interestingly, the FS of the hole doped material evolves from small hole pockets in the deeply underdoped region, via one with both hole- and electron-like sheets at slightly higher hole doping, to a large FS consistent with Luttinger's theorem at still higher doping levels. Our ab initio results offer a route toward a resolution of the controversy generated by the apparent inconsistency between the SdH data and the Luttinger sum rule for underdoped cuprates, as well as with recent Hall-effect measurements suggesting an electron-like Hall constant for hole doped cuprates at low T. To our knowledge, other theoretical approaches (density-functional or model-Hamiltonian based) can only partially address these issues. We therefore consider our results as a breakthrough in the field of strong electron correlations in d- and f-metal solid state compounds in general and copper oxide superconductors in particular. We envisage that these methods will find broader application to other correlated materials of great interest in the future. As another example of such a material, we also discuss the electronic structure of the cobalt oxide perovskite LaCoO3. We present results of quantum chemical calculations describing the differe nt charge and spin configurations that can be realized in this system. |
|