Structure optimization effects on the electronic and vibrational properties of Bi2Sr2CaCu2O8

We have studied the effect of structure optimization on the topology of the Fermi surface and on the Raman-active phonons of Bi2Sr2CaCu2O8. By total energy and force minimizations within the density-functional theory we have identified the most stable atomic geometry considering both an idealized body-centered tetragonal structure, inclusive of surface truncation, and a root2xroot2 orthorhombic cell simulating the observed distortions in the 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 the CuO2 barrels, while the orthorhombic distortion is responsible for the umklapp bands that have been observed by angle-resolved photoemission spectroscopy. The latter also gives rise to Raman-active vibrations not permitted in the tetragonal cell and strongly influences the attribution of the phonon peaks measured by experiments.