High-field spin resonance of weakly bound electrons in GaAs

Electron spin resonance (ESR) of shallow donor electrons in $n$-type GaAs has been observed by means of direct detection of microwave absorption at magnetic fields of 6–11 T. The ESR structure is smeared out over a magnetic field range of up to $1$ T. The line shape is strongly asymmetric and depends on the magnetic-field sweep direction. These unusual features are assigned to microwave-induced nuclear polarization under ESR conditions, leading to strong effective nuclear fields (Overhauser shift). The ESR curves show a signature of nuclear magnetic resonance if an additional radio-frequency field is applied. The observed ESR line shape is well reproduced by numerical simulation. Furthermore, the Landé $g$ factor of weakly localized electrons in GaAs has been accurately determined $(g=-0.464\pm 0.002$ at $B=0)$.