Evidence for a spin transition in the ν=$\frac{2}{5}$ fractional quantum Hall effect

We have studied the effects of hydrostatic pressure on a high-quality two-dimensional electron system under high magnetic field. The ν=2/5 and 3/7 fractional quantum Hall effect (FQHE) states dramatically vanish and then subsequently reappear with increasing pressure. In the high-pressure ν=2/5 state, tilting of the magnetic field away from the normal is found to suppress the FQHE state. These results suggest that the high-pressure ν=2/5 FQHE state is spin unpolarized with the spin transition being driven by reduction of Landé $g$ factor experienced by electrons due to application of pressure.