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Among the HTSC cuprates the two layer compound of the bismuth family, i.e. Bi2Sr2CaCu2O8 (Bi-2212), is a prototype for spectroscopic studies due to the relative ease to characterize a stable crystalline surface. A thorough comparison between experimental data and theoretical calculations has been hindered up to now by the complex structure of this compound that makes the ab-initio determination of electronic states a difficult task. Bi-2212 exhibits in fact an incommensurate superstructure in the Bi-O plane, near to a commensurate orthorhombic distorted cell. This distortion involves large atomic displacements that are expected to affect the electronic bands.
A comparison between theory and experiments requires a high level of accuracy in the computation of electronic states and various non-trivial ingredients such as i) a realistic description of the complex crystal geometry and composition of the material, inclusive of surface effects; ii) many body correlations associated to the strong interaction between localized electrons. We have studied the effect of structure optimization and of e-e correlation on the low energy excitations of Bi-2212. Structural effects have been studied within standard Density Functional [1] while the e-e correlation has been included by a many body solution of a multi-band Hubbard Hamiltonian [2] . We have identified the most stable atomic geometry considering both an idealized body centered tetragonal structure, inclusive of surface truncation, and an orthorhombic structure simulating the observed distortions in BiO planes. The optimization of the tetragonal cell leads to small but visible changes in the topology of the Fermi surface, rounding the shape of CuO2 barrels, while the orthorhombic distortion is responsible of the `umklapp' bands that have been observed by angle resolved photoemission spectroscopy. By calculating self-energy corrections associated to e-e correlation we have access to quasi particle spectral functions and to effective masses. We found that e-e correlations do modify the energy dispersion of quasiparticle states near to the Fermi level, especially close to the X and M points, but do not induce sensible changes right at the Fermi level. 1. V. Bellini and F. Manghi, T. Thonhauser, C. Ambrosch-Draxl, Phys. Rev. B 69, 184508 (2004). 2. S. Monastra, F.Manghi and C. Ambrosch-Draxl, Phys. Rev. B 64, 20507 (2001). |