Temperature dependence of the Hall angle in single-crystal ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We report measurements of the Hall coefficient (${\mathit{R}}_{\mathit{H}}$) and a-b-plane resistivity ρ(T) of cobalt-doped ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Co}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals (0≤x≤0.096) from 20 to 400 K. Co doping gives rise to an unusual downward curvature in ρ(T) while the inverse Hall coefficient [1/${\mathit{R}}_{\mathit{H}}$(T)] also becomes nonlinear. Despite this, Anderson’s formula for the Hall angle ${\mathrm{\theta }}_{\mathit{H}}$, namely, cot${\mathrm{\theta }}_{\mathit{H}}$=${\mathit{AT}}^2$+B continues to remain valid. In contrast to Zn, doping with Co decreases A while B remains constant. A simple model involving a square Fermi surface with rounded corners that could also account for the ${\mathit{AT}}^2$+B law is proposed.