Anisotropic renormalized fluctuations in the microwave resistivity in YBa2Cu3O7-delta

We discuss the excess conductivity above T-c due to renormalized order-para meter fluctuations in YBa2Cu3O7-delta (YBCO) at microwave frequencies. We c alculate the effects of the uniaxial anisotropy on the renormalized fluctua tions in the Hartree approximation, extending the isotropic theory develope d by Dorsey [Phys. Rev. B 43, 7575 (1991)]. Measurements of the real part o f the microwave resistivity at 24 and 48 GHz and of the de resistivity are performed on different YBCO films. The onset of the superconducting transit ion and the deviation from the linear temperature behavior above T-c can be fully accounted for by the extended theory. According to the theoretical c alculation here presented, a departure from Gaussian toward renormalized fl uctuations is observed. Very consistent values of the fundamental parameter s (critical temperature, coherence lengths, penetration depth) of the super conducting state are obtained. [S0163-1829(98)01342-3].