Violation of Kohler's Rule in the Normal-State Magnetoresistance of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{O}_{7-\mathit{\delta }}$ and ${\mathrm{La}}_2{\mathrm{Sr}}_x{\mathrm{CuO}}_4$

The normal-state magnetoresistance (MR) $\Delta \rho /\rho$ of 90-K and 60-K ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ and ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ $(x=0.17\mathrm{})$ has been measured in the longitudinal (field H parallel to current J) and transverse (H $\Vert$ c) geometries, with J in-plane. In all cases, the orbital component of the MR displays a temperature dependence that strongly violates Kohler's rule. We show that the classical orbital MR measures the variance of a local Hall angle $\theta \left(s\right)$ around the Fermi surface. The anomalous transverse MR in these cuprates is closely related to the temperature dependence of the Hall angle.