Collapse of the fractional quantum Hall effect in a parabolic quantum well

We study, using finite-system calculations, the behavior of the quasiparticle-quasihole energy gap and the collective excitation energy gap of a parabolically confined quasi-two-dimensional electron system subjected to a perpendicular magnetic field. We find the collapse of the energy gap as the areal density of the electron system is increased. Our results, which are in an excellent qualitative agreement with recent experiments, indicate that the collapse of the fractional quantum Hall effect in a parabolic quantum well is due to the electric-subband mixing. The effect of the finite layer thickness is also discussed.