Strain-Dependent Magnetoresistance of Potassium

The transverse magnetoresistance of single-crystal potassium has been measured by the helicon method, a probeless technique. The magnetoresistance was found to vary linearly with field in the region ${\omega }_c\tau \gg 1$. This result is in substantial agreement with previous dc measurements. Standard theory cannot explain a linear magnetoresistance. This inability is especially interesting because potassium is thought to be the simplest metal. It was found that Kohler's rule was not obeyed and that strain was an important parameter in the deviation from Kohler's rule. Carefully prepared single crystals of potassium had 40% of the magnetoresistance observed in polycrystalline samples. Cold working increased the magnetoresistance by as much as a factor of 5, and annealing reduced the magnetoresistance. In low-strain single crystals the magnetoresistance was anisotropic; the smallest values were observed for the field parallel to [110] and [123]. The field dependence of the Hall coefficient was also measured. The Hall coefficient of potassium decreased by several percent in the region ${\omega }_c\tau \gg 1$ where standard theory predicts field independence. New theoretical investigations of high-field transport properties and the Fermi surface of potassium will be necessary to explain these observations.