Phonons and Electronic Excitations in the Superconducting Cuprates: Analytical and Numerical Studies Focused on Raman Scattering

Lise Meitner Position M 591 Phonons and Electronic Excitations in the Captures E. SHERMAN 09.10.2000 Despite a great progress achieved in investigation of high-temperature superconductors, understanding of their properties is still far from being reached. Raman scattering of light by phonons and electronic excitations in these compounds is a reliable precise tool for their investigation. To understand the light scattering phenomena and their relation to intrinsic properties of the crystals one should combine advantages of analytical approaches and numerical results based on the first-principles band structure calculations. This combination allows to include effects of realistic band structure, electron-electron, and electron-phonon interactions in the model-based calculations. As a results, the relation between Raman scattering spectra and intrinsic properties of the crystals can be found. The goal of the project is the investigation of this relation. In our researches we are going to concentrate on the following basic problems. Problem 1: Resonant Raman scattering. The energies of photons used in the experiments are close to the energies of the interband electron transitions. This circumstance leads to strongly resonant Raman scattering. In most of Raman experiments polarization of light is parallel to the cuprate planes. An interesting new problem we are going to consider is the resonant Raman response due to low-energy excitations for light polarization perpendicular to the planes. Problem 2: Role of interaction between the quasiparticles for Raman scattering. Investigation of Raman scattering requires the consideration of the interaction of quasiparticles to each other. Calculation of this effect in analytical and numerical form is very complicated. When the interaction is mediated by a boson (a phonon or a magnetic fluctuation), that carries an energy, the numerical evaluation of Raman spectra becomes extremely time consuming. In the case of resonant scattering, the calculations become even more complicated. This problem requires a solution, that should be done within the project. Problem 3: Isotope effect in electron-phonon coupling and Raman spectra. Our calculations showed that phonon eigenvectors are strongly sensitive to the site-selective isotope substitution, where only part of the ions is substituted by another isotope. We are going to examine influence of this kind of substitution on superconductivity and obtain information about the electron-phonon coupling, lattice dynamics, and Raman scattering in these compounds.