Evidence for a spin transition in the ν=2/3 fractional quantum Hall effect

Magnetotransport measurements on a low-density two-dimensional electron system have revealed a re-entrant dependence of the activation energy on magnetic field for the fractional quantum Hall state at 2/3 filling of the lowest Landau level. The data are consistent with a change in the spin structure of the ground state at 2/3 filling, but do not provide a simple picture of the quasiparticle excitation process. As yet we find no similar effect for the ν=2/5 state.