The superconducting transition temperature, T$_{ C}$, the residual resistivity $\rho_{ 0}$ and the slope of resistivity curve at high temperature, d$\rho$/dT, have been measured in a series of MgB$_{ 2}$ samples that have been chemically substituted to varying degree with Li or Cu at the Mg-site and by Li or Cu at the Mg-site along with C substitution at the B-site. DC resistivity and ac susceptibility measurements were employed to extract the above parameters. T$_{ C}$ versus the electron count (estimated from simple chemical valence count arguments) shows a universal behaviour, with T$_{ C}$ being constant at the MgB$_{ 2}$ value for electron counts lower than in MgB$_{ 2}$ but rapidly decreasing for larger electron counts. The temperature dependence of resistivity in the normal state fits to the Bloch- Gruneisen formula, from which the Debye temperature, $\theta_{ D}$, and the $\rho_{ 0}$ are extracted. $\theta_{ D}$ variation with T$_{ C}$ is not systematic, whereas $\rho_{ 0}$ versus T$_{ C}$ shows a systematic variation that depends on the type of the chemical substituent. This dependence has a signature of the nature of the intraband/interband scattering affected by the chemical substitutions. d$\rho$/dT increases with C substitution, but decreases with Li and Cu substitution, implying that C substitution leads to the domination of conductivity by the $\sigma$ band, while in the Li/Cu substituted samples the $\pi$ band dominates conduction.
Categories: cond-mat.supr-con