ESR Detection of optical dynamic nuclear polarization in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells at unity filling factor in the quantum Hall effect

This paper presents a study of the enhancement of the Zeeman energy of two-dimensional (2D) conduction electrons near the $\nu =1\mathrm{}$ filling factor of the quantum Hall effect by optical dynamic nuclear polarization. The change in the Zeeman energy is determined from the Overhauser shift of the transport detected electron spin resonance in GaAs/${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ multiquantum wells. In a separate experiment the NMR signal enhancement factor is obtained by radio frequency detected nuclear magnetic resonance under similar conditions in the same sample. These measurements afford an estimation of the hyperfine coupling constant between the nuclei and 2D conduction electrons.