Influence of spin on the activation energies at ν=1/3ν=1/3

The activation energy ΔΔ of the fractional quantum Hall state at constant filling factor ν=13 and also at ν=25 and 23 has been measured as a function of the perpendicular magnetic field B   while modulating the electron density via a top gate. At small magnetic fields we observe a linear increase of ΔΔ with the magnetic field. The slope of ΔΔ vs. B allows us to directly extract the composite fermion g-factor.