  We report the observation of the circular ac Hall effect where the current is
solely driven by the crossed ac electric and magnetic fields of circularly
polarized radiation. Illuminating an unbiased monolayer sheet of graphene with
circularly polarized terahertz radiation at room temperature generates - under
oblique incidence - an electric current perpendicular to the plane of
incidence, whose sign is reversed by switching the radiation helicity. Alike
the classical dc Hall effect, the voltage is caused by crossed electric and
magnetic fields which are however rotating with the light's frequency.
