Magneto-transport measurements in a clean two-dimensional electron system confined to a wide GaAs quantum well reveal that, when the electrons occupy two electric subbands, the sequences of fractional quantum Hall states observed at high fillings (ν > 2) are distinctly different from those of a single-subband system. Notably, when the Fermi energy lies in the ground state Landau level of either of the subbands, no quantum Hall states are seen at the even-denominator ν = 5/2 and 7/2 fillings;

more »

... tead the observed states are at ν = (i + p/(2p ± 1)) where i = 2, 3, and p = 1, 2, 3, and include several new states at ν = 13/5, 17/5, 18/5, and 25/7.