Signatures of spin in the ν=1/3 fractional quantum Hall effect

The activation gap Delta of the fractional quantum Hall state at constant filling nu=1/3 is measured in a wide range of perpendicular magnetic field B. Despite the full spin polarization of the incompressible ground state, we observe a sharp crossover between a low-field linear dependence of Delta on B and a Coulomblike square root behavior at large B. From the global gap reduction we get information about the mobility edges in the fractional quantum Hall regime.