Microwave complex resistivity in YBCO films above T-c: effects of fluctuations

We present measurements of the (a,b) plane complex resistivity at 24 GHz in nearly-optimally doped YBCO films grown by de sputtering with T-c similar or equal to 90 K. The attention is focused above T-c, where we discuss the data in terms of 3D, anisotropic fluctuations. We find that at microwave frequencies simple gaussian fluctuations are unable to describe our data both well above and in the vicinity of the critical temperature. The discrepancy well above T-c is recovered by including a short-wavelength cutoff` in the frequency-dependent fluctuational conductivity. In order to follow the slowing down of the fluctuations at T-c, we treat the region close to T-c including the quartic term of the order parameter in the Ginzburg-Landau functional within the Hartree approximation. We then find excellent agreement with our data from high temperature (T = 1.3 T-c) down to T-c.