Infrared hall effect in high-T_c superconductors: Evidence for non-fermi-liquid hall scattering

Infrared (20–120 and 900–1100cm^-1) Faraday rotation and circular dichroism are measured in high- T_c superconductors using sensitive polarization modulation techniques. Optimally doped YBa₂Cu₃O₇ thin films are studied at temperatures in the range (15< T < 300K) and magnetic fields up to 8 T. At 1000cm^-1 the Hall conductivity σ_xy varies strongly with temperature in contrast to the longitudinal conductivity σ_xy which is nearly independent of temperature. The Hall scattering rate γ_H has a T² temperature dependence but, unlike a Fermi liquid, depends only weakly on frequency. The experiment puts severe constraints on theories of transport in the normal state of high- T_c superconductors.