Low field electron-nuclear spin coupling in gallium arsenide under optical pumping conditions

Abstract

In a semiconductor, absorption of circularly polarized light (optical pumping) leads to spin-oriented photoelectrons. In this situation, the nuclei of the crystal are dynamically polarized through their hyperfine interaction with the electronic spins. Consequently the electrons experience a hyperfine magnetic field due to the oriented nuclei which may reach several kilogauss. When this large nuclear field is driven obliquely with respect to the direction of the exciting light, the precession of the electronic spins around the nuclear field leads to a decrease of the electronic polarization along the light excitation: it is a nuclear Hanle effect. This work is an experimental and theoretical study of these effects in weak external magnetic fields, of the order of the local field which characterizes the nuclear spin-spin interactions (a few gauss). Large nuclear fields are obtained at 77°K in strongly doped and compensated $p$-type GaAs samples. We present a model which includes the different effects of the hyperfine coupling when there is a nuclear spin temperature among all the nuclei of the sample: Dynamic polarization, nuclear field, but also, existence of an electronic field acting on the nuclei. We show that a small external field is able to drive the large nuclear field acting on the electrons; consequently the electronic polarization is very sensitive to external fields which are too small to have a direct effect on the electronic spin motion. We study experimentally the variation of the electronic polarization with the direction and magnitude of a small external magnetic field, by measuring the circular polarization of the luminescence light. The experimental results are in quantitative agreement with the theoretical predictions. The usual Hanle Lorentzian depolarization curve is strongly modified in low fields and $W$-like singularities appear around zero field. The experimental values of the average electronic and nuclear fields are in reasonable agreement with theoretical evaluations. These nuclear effects may strongly alter the measurement of the Hanle linewidth in standard optical pumping experiments.