OBSERVATION OF COLLECTIVE EXCITATIONS IN THE FRACTIONAL QUANTUM HALL-EFFECT

A long wavelength, low-energy excitation of the fractional quantum Hal l state at v=1/3 has been observed by inelastic light scattering. The mode appears as a very sharp peak with marked temperature and magnetic field dependence. Its energy is consistent with theoretical predictio ns for the collective gap excitations of the incompressible quantum fl uid. Spectra interpreted as q=0 collective spin-wave excitations also display the strong dependence on field and temperature associated with the fractional quantum Hall state.