Spin-reversed excitations in the fractional quantum Hall effect

We have investigated the fractional quantum Hall state at 3/5 and 4/9 fillings of the lowest Landau level. The quasiparticle-quasihole energy gaps for different spin polarizations indicate that at these filling factors, it might be possible to distinguish between the spin reversal of quasiparticles and quasiholes from activation-energy measurements. Novel features of the energy gap, such as its disappearance in a range of magnetic field, explain a similar observation in recent tilted-field experiments in the fractional-quantum-Hall-effect regime.