First-principles band-structure calculations as a tool for the quantitative interpretation of Raman spectra of high temperature superconductors

We have performed first-principles band structure calculations in order to investigate vibronic and optical properties of YBa₂Cu₃O₇. A formalism describing temperature dependent Raman spectra from such ab-initio results has been applied to the 500 cm^?1 apex oxygen mode and its overtone in good agreement with experimental results. The dynamical matrix of the five A_1g modes established by atomic-force calculations is studied in detail showing rather good agreement with experimental eigen-frequencies and normal coordinates. The effect of isotope substitutions on the phonon frequencies is investigated. We demonstrate that the calculated vibronic properties of high T_c materials are improved by applying a generalized gradient correction scheme for the treatment of exchange and correlation effects instead of the local-density approximation.