Residual and intrinsic surface resistance of YBa2Cu3O7-δ

A simple model is proposed which describes. correctly the essential baits of the microwave surface resistance R-s of YBa2Cu3O7-delta above and below the transition temperature T-c(-). One of the key features is an electron scattering rate, which is the sum of a temperature- (T-) independent term and one that obeys the Gruneisen T dependence, and is carried over unchanged from the normal to the superconducting state. Furthermore, the T dependence of the quasiparticles is related to the empirical penetration depth, lambda(T). We find that R-s(T) depends linearly on T at low temperatures, which is a consequence of a predominantly constant elastic electron scattering rate and a linear T dependent [lambda(T)](-2) in the ab plane as T-->0 K. We confirm previous experiments indicating that a maximum and a minimum of R-s(T) between absolute zero and T-c is the signature of a clean specimen, corresponding to a small residual resistivity. Increasing the residual resistivity suppresses the peak of R-s(T) and lowers the surface resistance. The real part of the conductivity sigma' of a relatively clean specimen shows a distinct peak below T, which is caused by the Gruneisen T dependent intrinsic resistivity, together with a gradual freezing out of the quasiparticles, and a remaining residual resistivity as T-->0 K. The slopes of R-s(T) and of sigma'(T) are discontinuous at T-c. Increasing the frequency and/or the residual resistivity decreases the peak of sigma' and shifts it to higher temperatures. The slope of R-s(2)(T) just above T-c is linear and proportional to the Debye temperature. [S0163-1829(98)05938-4].