Two-band effects in transport properties of MgB2

We present resistivity and thermal conductivity measurements on bulk samples, prepared either by a standard method or by a one-step technique. The latter samples, due to their high density and purity, show residual resistivity values as low as 0.5 microohm cm and thermal conductivity values as high as 215 W mK-1, higher than the single-crystal values. The thermal and electrical data of all the samples are analysed in the framework of the Bloch-Gruneisen equation, giving reliable parameter values. In particular, the temperature resistivity coefficient, obtained both from resistivity and thermal conductivity, comes out ten times larger in the dirty samples than in the clean samples. This result supports the hypothesis of Mazin et al (2002 Preprint cond-mat/0204013) that pi and sigma bands conduct in parallel, with prevailing pi conduction in clean samples and sigma conduction in dirty samples.