Observation of multiband effects in the microwave complex conductivity of pure and Al-doped MgB 2 samples

There is presently a general agreement that the simple inter-metallic compound MgB 2 is a conventional, phonon mediated, superconductor, and that the anomalies in its behaviour can be consistently explained by the existence of two different gaps. We present a study of the complex conductivity as a function of temperature in pure and Al-doped MgB2 pellets by using a dielectrically loaded resonant cavity at 19 GHz.