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 orde r to investigate vibronic and optical properties of YBa2Cu3O7. A forma lism describing temperature dependent Raman spectra from such ab-initi o results has been applied to the 500 cm(-1) apex oxygen mode and its overtone in good agreement with experimental results. The dynamical ma trix of the five A(1g) modes established by atomic-force calculations is studied in detail showing rather good agreement with experimental e igenfrequencies and normal coordinates. The effect of isotope substitu tions on the phonon frequencies is investigated. We demonstrate that t he calculated vibronic properties of high T-c materials are improved b y applying a generalized gradient correction scheme for the treatment of exchange and correlation effects instead of the local-density appro ximation.