Correlation between the residual resistance ratio and magnetoresistance inMgB2

The resistivity and magnetoresistance in the normal state for bulk and thin-film ${\mathrm{MgB}}_2$ with different nominal compositions have been studied systematically. These samples show different temperature dependences of normal-state resistivity and residual resistance ratios although their superconducting transition temperatures are nearly the same, except for the thin-film sample. The correlation between the residual resistance ratio (RRR) and the power-law dependence of the low-temperature resistivity, $\rho$ vs $T^c,$ indicates that the electron-phonon interaction is important. It is found that the magnetoresistance (MR) in the normal state scales well with the RRR, $a_0\left(\mathrm{MR}\right)\propto (\mathrm{RRR}{)}^{2.2\pm 0.1}$ at 50 K. This accounts for the large difference in magnetoresistance reported by various groups, due to different defect scatterings in the samples.