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Helicity-dependent photocurrents in graphene layers excited by mid-infrared radiation of a CO$_2$-laser

We report the study of the helicity driven photocurrents in graphene excited by mid-infrared light of a CO$_2$-laser. Illuminating an unbiased monolayer sheet of graphene with circularly polarized radiation generates -- under oblique incidence -- an electric current perpendicular to the plane of incidence, whose sign is reversed by switching the radiation helicity. We show that the current is caused by the interplay of the circular $ac$ Hall effect and the circular photogalvanic effect. Studying the frequency dependence of the current in graphene layers grown on the SiC substrate we observe that the current exhibits a resonance at frequencies matching the longitudinal optical phonon in SiC.