Microwave resistivity and quasiparticle transport properties in Re-BCO and MgB2

We present here a wide experimental study of the microwave complex resistivity r(T,H), performed on different superconducting &#64257;lms (Re-BaCuO cuprates and MgB2), with two different, complementary experimental techniques. The &#64257;rst set of measurement is performed using an end-wall cavity technique, which allows us to determine the complex resistivity at &#64257;xed frequency (48 GHz) in moderate applied magnetic &#64257;elds (H < 0.7 T). The second set of data is obtained through a Corbino disc technique, which allows for a continuous frequency spectrum measurement between 5 and 20 GHz. In this latter case, the magnetic &#64257;eld can vary between zero and 12 T. In absence of an applied magnetic &#64257;eld, the real part of the conductivity shows an enhancement below the critical temperature Tc in all superconductor under investigation. The physical origin of this enhancement is still under debate [1], and some indications will be obtained from the observed frequency dependence. On the other hand, the imaginary part of the conductivity shows the expected behavior, allowing for the determination of the temperature dependence of the super&#64258;uid density. Measurements in presence of an applied &#64257;eld show further puzzling characteristics. On general grounds, the variation of the resistivity with the &#64257;eld can be due to both vortex motion and quasiparticle fractional density variation. These two phenomena contribute to both the real and the imaginary part of the resistivity, so that the beahvior of the quantity Dr=r(B)-r(0) can be in general rather complex. The standard approach to this problem [2] is to assume that the quasiparticle contribution can be neglected with respect to vortex motion, at least at low enough temperatures and magnetic &#64257;elds (B<