Giant Overhauser shift of conduction-electron spin resonance due to optical polarization of nuclei in semiconductors

The hyperfine interaction with optically oriented electrons creates a nuclear polarization in $p$-type GaSb at 1.7 K, which is evidenced on the shift $B_N$ of the optically detected conduction-electron spin resonance (CESR). The experiments are performed in low external magnetic fields, where the nuclei are in a spin-temperature regime. In some samples the optically created nuclear polarization is so large that the shift $B_N$ of the CESR due to the polarized nuclei is larger than the external magnetic field. An electron-nuclear-double-resonance experiment has confirmed the nuclear origin of the observed effects. The existence of a spin temperature explains the experimental variation of $B_N$ with the electronic resonance frequency and the mean value of the electronic spin. We qualitatively interpret the variation of $B_N$ with the doping levels by assuming that the diffusion sphere around each donor contains the polarized nuclei. We evidence the importance of the quadrupolar interaction of nuclei to the impurity electric field gradients.