Multicomponent fractional quantum Hall states with subband and spin degrees of freedom

In two-dimensional electron systems confined to wide GaAs quantum wells we observe a remarkable sequence of transitions of the fractional quantum Hall states in the filling factor range 1<ν<3 near the crossing of two N=0 Landau levels from different subbands and with opposite spins. The transitions attest to the interplay between the spin and subband degrees of freedom and can be explained as pseudospin polarization transitions where the pseudospins are the two crossing levels. The magnetic field positions of the transitions yield a quantitative measure of the transition energies or, equivalently, the composite fermions' discrete energy level separations. Surprisingly, these energies are much larger than those reported for other systems with SU(2) symmetry; moreover, they are larger when the electron system is less spin polarized.