  The optical conductivity and the spectral weight W(T) of two superconducting
cuprates at optimum doping, Bi2Sr2-xLaxCuO6 and Bi2Sr2CaCu2O8, have been first
measured up to 500 K. Above 300 K, W(T) deviates from the usual T2 behavior in
both compounds, even though the zero-frequency extrapolation of the optical
conductivity 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. This renormalization is due to
strong correlation, as shown by the good agreement with dynamical mean field
calculations.
