We report on the observation of the giant photocurrents in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. At an exciting QW of 6.6 nm width by terahertz (THz) radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (−0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. The photocurrent data, accompanied by measurements of radiation transmission as well as Shubnikov-de Haas and quantum Hall effects, prove that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows us to obtain the effective electron velocity v ≈ 7.2 × 10 5 m/s. We develop a microscopic theory of the effect and show that the inherent spin-dependent asymmetry of light-matter coupling in the system of Dirac fermions causes the electric current to flow.

We report on the observation of magneto-oscillations of terahertz radiation induced photocurrent in HgTe/HgCdTe quantum wells of different widths, which are characterized by a Dirac-like, inverted, and normal parabolic band structure. The photocurrent data are accompanied by measurements of photoresistance (photoconductivity), radiation transmission, as well as magnetotransport. We develop a microscopic model of a cyclotron-resonance assisted photogalvanic effect, which describes main experimental findings. We demonstrate that the quantum oscillations of the photocurrent are caused by the crossing of Fermi level by Landau levels resulting in the oscillations of spin polarization and electron mobilities in spin subbands. Theory explains a photocurrent direction reversal with the variation of magnetic field observed in experiment. We describe the photoconductivity oscillations related with the thermal suppression of the Shubnikov-de Haas effect.

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We report on the observation of the giant photocurrent in HgTe/HgCdTe quantum wells (QW) of critical thickness at which a Dirac spectrum emerges 1 , 2 . Exciting QW of 6.6 nm width by terahertz (THz) radiation and sweeping the magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (-0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. We show that the photocurrent is caused by cyclotron resonance (CR) in a Dirac fermion system, which allows us to obtain the electron velocity v -7.2 10 5 m/s. We develop a microscopic theory of the effect and show that the inherent spin dependent asymmetry of the Dirac fermion scattering in QWs causes the electric current to flow.