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 nu = p over 2p+1 for 1/3 = nu = 2/3. The nu-dependence of the gap energy suggests a collapse of the collective excitation gap near nu = 1/2. In a range of filling factors close to nu = 1/2, where the FQH gap is believed to collapse, we observe a novel collective excitation that exists only at temperatures below 200 mK.