Cooled nuclear spin system in a semiconductor probed by transverse spin component in Hanle effect

Optical cooling of nuclear spins in the Knight field of electrons, Be, has been investigated in an Al0.29Ga0.71As alloy at a lattice temperature of TL = 77 K via the transverse electron spin component Sx(B) in the Hanle effect. In contrast to the conventional Sz(B) component, which is collinear with the pumping beam, the Sx(B) component is perpendicular to both the pumping beam and the magnetic field. The Sx(B) curve exhibits a distinct narrow N-shaped component, with a maximum-to-minimum separation of about 10 G, caused by the effective magnetic field of the nuclear spin system, BN. A nuclear spin system temperature of the order of a few hundreds of μK was reached, which is approximately six orders of magnitude lower in absolute value than the lattice temperature TL. A sign reversal of the spin temperature θ and compensation of the Knight field Be were achieved, as evidenced by the N-shaped features of the corresponding Sx(B) curves. Analysis of Sx(B) allows one to determine the sign of the electron g-factor. Thus, the Sx(B) measurement yields more information than the conventional Sz(B) measurement and provides enhanced sensitivity to the nuclear spin polarization.