Frequency-dependent thermal response of the charge system and restricted sum rules in La2-xSrxCuO4

In La2-xSrxCuO4 ( LSCO) the spectral weight W = integral(0)(Omega)sigma(1)(ab)(omega,T)domega [where sigma(1)(ab)(omega,T) is the ab-plane conductivity] obeys the same law W = W-0 - B(Omega)T-2 as in a conventional metal such as gold, for any Omega up to the plasma edge. However, in LSCO B(Omega) points toward correlation effects and, unlike in gold, is related to an energy scale t(T) much less than t(0) similar to W-0. The Ferrell-Glover-Tinkham sum rule is fulfilled in LSCO superconductors for Omega greater than or similar to 2000 cm(-1).