We present here the results from a density dependent study of the activation energy gaps of the fractional quantum Hall effect states at Landau level fillings \nu=8/3 and 7/3 in a series of high quality quantum wells. In the density range from 0.5 x 10^{ 11} to 3 x 10^{ 11} cm^{ -2}, the 7/3 energy gap increases monotonically with increasing density, supporting its ground state being spin polarized. For the 8/3 state, however, its energy gap first decreases with increasing density, almost vanishes at n ~ 0.8 x 10^{ 11} cm^{ -2}, and then turns around and increases with increasing density, clearly demonstrating a spin transition.
Categories: cond-mat.mes-hall cond-mat.str-el