  We report on the observation of symmetry breaking and the circular
photogalvanic effect in Cd$_x$Hg$_{1-x}$Te alloys. We demonstrate that
irradiation of bulk epitaxial films with circularly polarized terahertz
radiation leads to the circular photogalvanic effect (CPGE) yielding a
photocurrent whose direction reverses upon switching the photon helicity. This
effect is forbidden in bulk zinc-blende crystals by symmetry arguments,
therefore, its observation indicates either the symmetry reduction of bulk
material or that the photocurrent is excited in the topological surface states
formed in a material with low Cadmium concentration. We show that the bulk
states play a crucial role because the CPGE was also clearly detected in
samples with non-inverted band structure. We suggest that strain is a reason of
the symmetry reduction. We develop a theory of the CPGE showing that the
photocurrent results from the quantum interference of different pathways
contributing to the free-carrier absorption (Drude-like) of monochromatic
radiation.
