Low frequency collective excitations in the quantum-hall system

We report the observation of low frequency collective excitations of the two-dimensional electron system in a GaAs/GaAlAs heterojunction in a magnetic field. The excitations are present for temperatures less than 2 K and fields corresponding to filling factors between ν = 1 2 and ν = 7. Their dispersion relation is linear in the wave vector k, and shows no evidence of a gap. These modes vanish between the Hall plateaus and elsewhere their propagation velocity follows a H-1 1n(αH) field dependence. The features of these modes are consistent with a propagating perimeter wave in a quantum 2D Coulomb system. © 1987.