Topics in electronic structure and spectroscopy of cuprates

I have applied first-principles calculations to investigate several interrelated problems concerned with the electronic structure and spectroscopy of cuprates. The specific topics addressed in this thesis are as follows.;1. By properly including doping effects beyond rigid band filling, a longstanding problem of the missing Bi-O pocket in the electronic structure of Bi2Sr2CaCu2O8 (Bi2212) is solved. The doping effect is explained in terms of Coulombic effect between layers and is a generic property of all cuprates.;2. A systematic study for Pb/O and rare-earth doping in Bi2212 is carried out to explain the experimental phase diagrams, and a possible new electron doped Bi2212 is predicted.;3. To investigate how the Mott insulators evolve into superconductors with the addition of holes, an analysis of angle-resolved photoemission (ARPES) data of La2-xSr xCuO4 is carried out over a wide doping range of x = 0.03 - 0.30. The spectrum displays the presence of the van Hove singularity (VHS) whose location in energy and three-dimensionality are in accord with the band theory predictions. A nascent metallic state is found in the lightly doped Mott insulator and develops spectral weight as doping increases. This metallic spectrum is 'universal' in the sense that its dispersion depends weakly on doping, in sharp contrast to the common expectation that dispersion is renormalized to zero at half-filling. This finding challenges existing theoretical scenarios for cuprates.;4. Self-consistent mean-field three- and four-band Hubbard models are used to study the Mott gap in electron-doped cuprates. The Hubbard terms are decomposed into a Mott-like term which describes the lifting of Cu bands due to energy cost U and a Slater-like term which describes an additional splitting of Cu bands due to antiferromagnetic (AFM) order. While no set of doping-independent parameters can explain the observed gaps for the entire doping range, the experimental results are consistent with a weakly doping dependent Hubbard U. These parameters enhance Cu character of the bonding band, producing a charge transfer gap dominated by the Slater-like term.;5. The valence bands of Bi2212 extending from about 1 to 7 eV below the Fermi energy (EF) are primarily associated with various Cu d and O p orbitals. Sorting out these bands would provide valuable information on a number of issues relevant to cuprate physics. In particular, the bonding Cu dx2-y2 band has an intimate connection with the true lower Hubbard band (LHB), yet its binding energy has never been experimentally determined. An analysis of the ARPES valence band spectrum of Bi2212 is provided. The local-density approximation (LDA) bands are compared with experiments. While O Sr and OBi bands are in good agreement with LDA, there are disagreements between experiment and LDA associated with bands originating from the CuO2 layers. A necessary correction of the LDA derived TB model is found, and this correction is shown to be related to the Mott physics in such a way that Cu dx2-y2 weight is evenly distributed into bonding and antibonding bands.;6. Scanning tunneling microscopy/spectroscopy (STM/STS) techniques have entered the realm of high-Tc's impressively by offering atomic scale real space resolution and meV resolution in bias voltages. STM/STS spectra, however, represent a complex mapping of electronic states of interest related to the CuO2 planes, since the tunneling current must reach the tip after being filtered through the overlayers (e.g. SrO and BiO in Bi2212). We have developed a material specific theoretical framework for treating the normal as well as the superconducting state where the effect of the tunneling matrix element is included by taking into account various orbitals within a few eV's of the Fermi energy (EF). The tunneling current is evaluated directly including the effect of overlayers. Our computations show the presence of strong matrix element effects, which lead to significant differences between the dI/dV spectra and the local density of states (LDOS) of CuO2 planes. For instance, the dx2-y2 signal is found to be dominated by non-vertical hopping between the CuO2 and BiO layers. A substantial electron-hole anisotropy of the tunneling spectrum, which is in accord with experiments, is naturally explained by the contribution from dz2 and other orbitals below EF.