Nonuniversal power law of the Hall scattering rate in a single-layer cuprate ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_6$

In-plane resistivity, Hall coefficient, and magnetoresistance are measured in a series of high-quality ${\mathrm{Bi}}_2{\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CuO}}_6$ crystals with various carrier concentrations, from underdoped to overdoped. It is found that the temperature dependence of the cotangent of the Hall angle obeys a power law $T^{\alpha }$ with $\alpha$ systematically decreasing with increasing carriers. While the observed power-law behavior supports the scattering-time separation, the universality of the Fermi-liquid-like $T^2$ dependence of the Hall scattering rate is seriously questioned.