Negative deviations from Matthiessen's rule for SrRuO₃ and CaRuO₃

We have measured the change in the resistivity of thin films of SrRuO₃ and CaRuO₃ upon introducing point defects by electron irradiation at low temperatures, and we find significant negative deviations from Matthiessen's rule. For a fixed irradiation dose, the induced change in resistivity decreases with increasing temperature. Moreover, for a fixed temperature, the increase in resistivity with irradiation is found to be sublinear. We suggest that the observed behavior is due to the marked anisotropic scattering of the electrons together with their relatively short mean free path (both characteristic of many metallic oxides including cuprates) which amplify effects related to the Pippard ineffectiveness condition.