The microwave response of high-T_c superconductors in the vortex state is determined by superfluid and normal fluid together with the motion of flux lines. The mixing of these contributions makes difficult to isolate the contributions due to the charge carriers. At high enough frequencies, the vortex motion reduces to the simple flux flow. Thus, it can be isolated from the oveall response, allowing for the separate determination of the field dependent superfluid fraction n_s(T,B). We have measured the (a,b) plane complex resistivity at 48 GHz in several highly oriented cuprate superconducting thin films, placed in the end-wall configuration in a resonating cavity, for temperatures from 65 K to T_c and magnetic fields up to 0.8 T (applied along the c axis). By preliminarly identificating the vortex contribution, we obtain the vortex viscosity. We compare our results with data from the literature. Having isolated the vortex motion contribution, we determine the field dependence of the superfluid fraction. We find that it follows a sublinear law (n_s(T,B)=n_s(T,0)-A B^1/2), up to temperatures very close to T_c. This behaviour, which is a common feature of all the ReBCO (Re=Y, Sm) and Bi2212 samples investigated, is in agreement with the predictions for a superconductor with lines of nodes in the gap. Moreover, the extension of this field dependence in nearly the full temperature range explored (T>65 K) and up to temperatures close to T_c indicates that such pairing is not smeared out by the high operating temperatures.

We express our thanks to M. Boffa, M. C. Cucolo, A.M. Cucolo (Dip. di Fisica and Unita' INFM, Universita' di Salerno, Italy) and C. Camerlingo (CNR - Istituto di Cibernetica, Napoli, Italy) for samples preparation.

Work on YBCO has been partially supported by Italian MIUR under the FIRB project "Strutture semiconduttore/superconduttore per l'elettronica integrata".

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1. Vortex motion and quasiparticle resistivity in superconductors at microwave frequencies