Spin transition in the fractional quantum Hall regime: Effect of extent of the wave function

Using a magnetocapacitance technique, we determine the magnetic field of the spin transition, B*, at filling factor nu=2/3 in the 2D electron system in GaAs/AlGaAs heterojunctions. The field B* is found to decrease appreciably as the wave function extent controlled by back gate voltage is increased. Our calculations show that the contributions to the shift of B* from the change of the Coulomb energy and the g factor change due to nonparabolicity are approximately the same. The observed relative shift of B* is described with no fitting parameters.