  We report on the observation of the giant spin-polarized photocurrent in
HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum
emerges. 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 gating. The photocurent data, accompanied
by measurements of radiation transmission as well as Shubnikov-de Haas and
quantum Hall effects, give an evidence that the enhancement of the photocurrent
is caused by cyclotron resonance in a Dirac fermion system. The developed
theory shows that the current is spin polarized and originates from the spin
dependent scattering of charge carriers heated by the radiation.
