NUCLEAR-SPIN RELAXATION IN ALGAAS GAAS HETEROSTRUCTURES OBSERVED VIA OPTICALLY DETECTED MAGNETIC-RESONANCE (ODMR) EXPERIMENTS/

The resonant field at which an electron spin resonance (ESR) occurs ma y be shifted by an effective magnetic field, which is due to spin pola rized nuclei. This shift, known as Overhausershift, is caused by the f ield of all polarized nuclei without respect to their isotopic number. Irradiation of the nuclear magnetic resonance (NMR) frequency of some of the isotopes during the ESR experiment allows to measure the nucle ar field of only one kind. The time dependent evolution of the Overhau sershift is according to the nuclear spin relaxation time. In undoped semiconductors e.g. in AlGaAs/GaAs heterostructures, typical time cons tants are determined to be between 10 minutes and one hour; irradiatio n with visible light which changes the number of free carriers affects the relaxation process. We further show that the ratio of the relaxat ion times of the two Ga-isotopes Ga-69 and Ga-71 behaves as a quadrupo lar relaxation rather than relaxation via Fermi contact interaction wi th free carriers. This result is supported by optically detected NMR m easurements, which exhibit a quadrupolar splitting of the NMR resonanc e line. (C) 1997 Elsevier Science Ltd.