Dynamic regimes in MgB2 probed by swept frequency microwave measurements

We report swept frequency measurements (2–20 GHz) of the microwave resistivity in MgB2, in the presence of a static magnetic field. Through these data, we experimentally determine the region of the {H,T} 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.