Low-energy electronic spin excitations between filling factors ν=1 and 12 studied by optically detected nuclear magnetic resonance

We report measurements of the spin relaxation time (T1n) for nuclei in the potential well confining a high-mobility two-dimensional electron system at a single GaAs–GaAlAs heterojunction. At low temperatures nuclear spin relaxation is dominated by electron–nuclear spin scattering: we find that T1n displays sharp maxima at incompressible states throughout the View the MathML source hierarchy of the fractional quantum Hall effect. This behaviour is consistent with the existence of low-energy spin excitations only where the electron system is compressible. Our measurements also provide evidence for a gap in the spin excitation spectrum at View the MathML source.