Collapse of the fractional quantum Hall effect in an electron system with large layer thickness

We report the first experimental study of the energy gap for the fractional quantum Hall states at Landau-level filling factors =1/3 and 2/3 in a variable-width electron system realized in a selectively doped, parabolic, AlGaAs quantum well. We observe a dramatic decrease in the measured energy gaps as the electron-layer thickness increases and the electric-subband separation decreases, which we interpret as the collapse of the correlated fractional states.