  We report on the observation of the circular transversal terahertz
photoconductivity in monolayer graphene supplied by a back gate. The
photoconductivity response is caused by the free carrier absorption and
reverses its sign upon switching the radiation helicity. The observed dc Hall
effect manifests the time inversion symmetry breaking induced by circularly
polarized terahertz radiation in the absence of a magnetic field. For low gate
voltages, the photosignal is found to be proportional to the radiation
intensity and can be ascribed to the alignment of electron momenta by the
combined action of THz and static electric fields as well as by the dynamic
heating and cooling of the electron gas. Strikingly, at high gate voltages, we
observe that the linear-in-intensity Hall photoconductivity vanishes; the
photoresponse at low intensities becomes superlinear and varies with the square
of the radiation intensity. We attribute this behavior to the interplay of the
second- and fourth-order effects in the radiation electric field which has not
been addressed theoretically so far and requires additional studies.
