Optical detection of NMR in high-purity GaAs under optical pumping: Efficient spin-exchange averaging between electronic states

Abstract

In a semiconductor under light excitation, spin exchange between electronic states produces an extremely efficient spin averaging. This is shown theoretically, by a calculation of the corresponding cross section, and experimentally, using the optical detection of nuclear magnetic resonance (NMR) in high-purity gallium arsenide at liquid-helium temperature, excited by circularly polarized light (optical pumping). The feasibility of this detection comes from the large hyperfine nuclear field experienced by the spin-polarized photoelectrons. The change of the direction of this nuclear field in NMR conditions causes a precession of the electronic spins. The resulting electronic depolarization is detected from the polarization of the luminescence light. This holds for electrons trapped on donors. Free electrons experience a very small nuclear field, but are found to be depolarized at resonance, because of their spin-exchange coupling with trapped electrons. Furthermore, the spin exchange is responsible for an amplification and broadening of the optically detected NMR signal. This is shown by a calculation of the depolarization of this system of two electronic states, and is verified by a careful analysis of the observed resonance line. The study of this broadening indicates that the spin-exchange frequency is comparable with the precession frequency in the external field, in agreement with its theoretical estimate. The measured values of the ratios of the hyperfine nuclear fields of the various nuclear isotopic species yield the sharing of the electron between cation and anion sites. We point out that, due to the efficient spin exchange, the various electronic states behave as a single spin state. This basic feature, which has so far been overlooked in semiconductors under light excitation, allows a new insight at the various electronic spin properties, such as spin relaxation and optical detection of electronic resonance.