Symmetry breaking and circular photogalvanic effect in epitaxial <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Cd</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>Hg</mml:mi><mml:mrow><mml:mn>1</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:mi>Te</mml:mi></mml:mrow></mml:math> films
S. Hubmann, G. V. Budkin, M. Otteneder, D. But, D. Sacré, I. Yahniuk, K. Diendorfer, V. V. Bel'kov, D. A. Kozlov, N. N. Mikhailov, S. A. Dvoretsky, V. S. Varavin, V. G. Remesnik, S. A. Tarasenko, W. Knap, S. D. Ganichev
Abstract
We report on the observation of symmetry breaking and the circular photogalvanic effect in ${\mathrm{Cd}}_{x}{\mathrm{Hg}}_{1\ensuremath{-}x}\mathrm{Te}$ 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 noninverted band structure. We suggest that strain is a reason for 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.