Anisotropic temperature-dependent resistivity of Cd, Zn, and Mg

The electrical resistivity observed along and perpendicular to the hexagonal axis has been measured in monocrystals of Cd, Zn, and Mg over the range $\ensuremath{\approx}(3\ensuremath{-}300)$\ifmmode^\circ\else\textdegree\fi{}K [$\ensuremath{\approx}(3\ensuremath{-}590)$\ifmmode^\circ\else\textdegree\fi{}K for Cd]. The temperature dependence of the anisoptropy parameter ($\ensuremath{\alpha}=\frac{{\ensuremath{\rho}}_{\ensuremath{\parallel}}}{{\ensuremath{\rho}}_{\ensuremath{\perp}}}$) has been examined in the light of the Case-Gueths simplified model of conductivity in an anisotropic metal. In the case of Cd there is good quantitative agreement with the predictions of the model, while in Zn there is at least good qualitative agreement with them. But for Mg (which alone has $\ensuremath{\alpha}<1$) quite the wrong behavior of $\ensuremath{\alpha}(T)$ is observed. It is suggested that the discrepancy reflects the oversimplified view of the Fermi surface which is used, particularly since it neglects the conductivity in the basal plane arising from the third-band "needles"---which of course are absent in Cd and genuinely negligibly small in Zn.