Microwave properties of YBa2Cu3O7-d films: magnetic-field-dependent order parameter fluctuations

We present measurements of the microwave resistivity in YBa2Cu3O7-δ films as a function of the temperature and magnetic field at 48 GHz above the critical temperature Tc . Even in moderate fields (up to 0.7 T) a noticeable magnetodissipation is observed; we attribute this to order parameter fluctuations. We find that Gaussian fluctuations are not consistent with the observed behaviour. Starting from the fact that the measurements taken in zero magnetic field can be quantitatively described in terms of the three-dimensional anisotropic renormalized fluctuations theory, we phenomenologically extend the theory by assuming that (to the first order in the magnetic field) the effects of renormalization reduce to the introduction of a temperature-dependent field scale. In this way we obtain a quantitative description of the experimental field-dependent microwave resistivity.