OPTICAL-PROPERTIES AND FERMI-SURFACE NESTING IN SUPERCONDUCTING OXIDES

Fermi-surface nesting is found to modify the electron-electron scattering and therefore yields an unusual variation of the optical reflectivity. At long wavelengths a Drude form of the dielectric function is derived with a relaxation rate for a nested Fermi liquid (NFL) that is linear in frequency for omega > T. The corresponding Drude mass is also frequency and temperature dependent. Remarkably good fits to the reflectivity of YBa2Cu3O7, Bi2Sr2CaCu2O8, and La(2-x)Sr(x)CuO4 are achieved using an on-site Coulomb interaction of intermediate strength. The static limit for the NFL conductivity is compatible with the temperature-dependent resistivity of the high-temperature superconductors. Self-energy and vertex corrections yield a long-wavelength susceptibility that is much weaker and different in structure from the response at the nesting wave vector Q, and the distinctions are relevant to the Raman spectrum. In cases of imperfect nesting, a crossover to conventional Fermi-liquid behavior is possible at a temperature T* determined by the quasiparticle orbits. Predictions for the optical response as a function of chemical composition are discussed, with attention to the anomalous resistivity of Nd(2-x)Ce(x)CuO4.