Inelastic light scattering by collective excitations in the fractional quantum Hall regime

We report an inelastic light scattering study of long wavelength collective gap excitations of fractional quantum Hall (FQH) states at ν=p/(2p+1) for View the MathML source. The ν-dependence of the gap energy suggests a collapse of the collective excitation gap near View the MathML source. In a range of filling factors close to View the MathML source, where the FQH gap is believed to collapse, we observe a collective excitation mode that exists only at temperatures below 150 mK.