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We report swept frequency measurements ͑2-20 GHz͒ of the microwave resistivity in MgB 2 , in the presence of a static magnetic field. Through these data, we experimentally determine the region of the ͕H , T͖ plane in which the band does not significantly contribute to the superfluid density. Within that region, we show that data can be interpreted through standard models for vortex motion and quasiparticle resistivity. We obtain the temperature-dependent band superfluid density and the upper critical field. We find excellent agreement between the dc and microwave estimates of the upper critical field. The temperature-dependent superfluid density agrees with a BCS calculation based on independently obtained data for the large gap. We also measure and discuss the vortex characteristic frequency due to pinning effects.

A general method for the determination of the resistivity tensor from multiterminal measurements is developed, based on the solution of a Laplace equation for the potential inside the sample with suitable boundary conditions. We have performed the calculation in the general case of an arbitrary contact geometry, with finite width of the contacts on the sample surfaces. The precision and the accuracy of the calculation and of the hypothesis of the model have been checked by means of a self-consistent method based on the reconstruction of voltage measurements.

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Measurements of the surface impedance of high critical temperature superconductors (HTS) over a wide frequency range provide important information on the electrodynamic properties of these materials and a severe test for any related theoretical model. We present two experimental techniques for the measurement of the surface impedance of HTS thin films in the microwave range, based on the detection of the reflection coefficient of the sample, connected to a vector network analyzer through a coaxial line. In one case the film forms an electrical short across the terminal section of the coaxial line, according to the so called Corbino geometry. In the other, a small circular gap separates the film from the inner conductor of the cable. In the latter case, the absence of direct electrical contact between the sample and the coaxial core simplifies realization and avoids contact instability always present with large temperature variations. We describe the two necessary steps to extract the film impedance from measured data, namely the study of electromagnetic field propagation in the two structures and the calibration of the coaxial measurement line at cyogenic temperatures. Finally, we present measurements performed on YBa2Cus07_^ thin films with the two techniques.

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