Spin-polarized currents of Dirac fermions at cyclotron resonance

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 ν ~ 7.2 10⁵ 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.