When analyzing experimental data of the complex microwave resistivity in superconductors in presence of an applied magnetic field, one has to face the problem of correctly taking into account all the relevant contributions. In fact, at nonzero frequencies both the normal and superfluid fractions, as well as moving vortices, contribute to the electrical transport, resulting in intricate expressions for the resistivity. We present here an extended study of microwave resistivity data measured by means of a broadband technique between 2 and 20 GHz and of a resonant system at 50 GHz. Data are collected on several superconducting materials (YBaCuO, SmBaCuO, MgB₂). We discuss the main experimental fingerprints that allow to identify the relevance of the different contributions from the measured microwave response as a function of frequency, temperature and magnetic field. We show that vortex motion and quasiparticle dissipation have different weight and different origins in different materials. In HTCs vortex motion prevails at high fields, while at low fields a significant quasiparticle contribution exists, that we ascribe to the presence of lines of nodes in the gap. In MgB₂ the two contributions are comparable even at high fields. For the latter material, we are able to quantitatively take into account vortex motion and quasiparticle response. In particular, we show that a two fluid model applies well to MgB₂ in a relatively large region of the H - T phase diagram. In this region an excellent agreement is found between derived superfluid parameters (superfluid density, upper critical field), theoretical predictions and independently measured parameter. This result, never obtained on this material by using standard expressions, clearly demonstrates the necessity of taking into account all contributions to the overall microwave resistivity.

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1. Microwave complex resistivity in High-T_c Superconductors in the mixed state