Structure optimization effects on the electronic and vibrational properties of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$

We have studied the effect of structure optimization on the topology of the Fermi surface and on the Raman-active phonons of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8.$ 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 $\sqrt{2}\times \sqrt{2}$ 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 ${\mathrm{CuO}}_2$ 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.