Thermal conductivity of Al-doped MgB2:: Impurity scattering and the validity of the Wiedemann-Franz law

We report data on the thermal conductivity kappa(T,H) along the basal plane of the hexagonal crystal structure of superconducting Mg1-yAlyB2 with y=0.02 and 0.07 at temperatures between 0.5 and 50 K and in external magnetic fields between 0 and 70 kOe. The substitution of Al for Mg leads to a substantial reduction of the heat transport via electronic quasiparticles. The analysis of the kappa(T,H) data implies that the Al impurities provoke an enhancement of the intraband scattering rate, almost equal in magnitude for both the sigma and the pi bands of electronic excitations. This is in contrast with conclusions drawn from analogous data sets for material in which carbon replaces boron and where mainly the intraband scattering rate of the sigma band is enhanced. Our complete data set, including additional results of measurements of the low-temperature thermal conductivity of pure MgB2, confirms the validity of the Wiedemann-Franz law for both pure and doped MgB2.