Magnetoresistivity as a probe of disorder in the pi and sigma bands of MgB2

In this paper we present normal-state magnetoresistivity data of magnesium diboride epitaxial thin films with different levels of disorder, measured at 42 K in magnetic fields up to 45 T. Disorder was introduced in a controlled way either by means of neutron irradiation or by carbon doping. From a quantitative analysis of the magnetoresistivity curves with the magnetic field either parallel or perpendicular to the plane of the film, we extract the ratio of the scattering times in pi and sigma bands. We demonstrate that the undoped unirradiated thin film has pi scattering times smaller than sigma ones, i.e., a shorter mean free path for pi charge carriers; upon irradiation, both bands become increasingly more disordered; eventually the highly irradiated sample (neutron fluence 7.7 10^17 cm^-2) and the C-doped sample have comparable scattering times in the two types of bands. This description of the effect of disorder in the two kinds of bands on transport is consistent with the residual resistivity values and with the temperature dependence of the resistivity.