Frequency-dependent thermal response of the charge system and restricted sum rule in La2SrCuO4

By using new and previous measurements of the ab-plane conductivity of La2-xSrxCuO4 (LSCO) it is shown that the spectral weight W obeys the same law W which holds for a conventional metal like gold, below the plasma frequency. However B(Omega), which measures the "thermal response" of the charge system, in LSCO exhibits a peculiarbehavior which points towards correlation effects. In terms of hopping models, B(Omega) is directly related to an energy scale t(T) , smaller by one order of magnitude than the full bandwidth t0.