High-Temperature Optical Spectral Weight and Fermi-liquid Renormalization in Bi-Based Cuprate Superconductors

The optical conductivity SIGMA(OMEGA) and the spectral weight W(T) of two superconducting cuprates at optimum doping, Bi2Sr2xLaxCuO6 and Bi2Sr2CaCu2O8, have been first measured up to 500 K. Above 300 K, WðTÞ deviates from the usual T2 behavior in both compounds, even though SIGMA(0) remains larger than the Ioffe-Regel limit. The deviation is surprisingly well described by the T4 term of the Sommerfeld expansion, but its coefficients are enhanced by strong correlation, as shown by the good agreement with dynamical mean field calculations.