Optical Investigation of Spin-Wave Excitations in Fractional Quantum Hall States and of Interaction between Composite Fermions

We demonstrate that the temperature dependence of the electron spin polarization for the fractional states $\nu =1\mathrm{}/3$ and $\nu =2\mathrm{}/3$ displays activated behavior. This study enables the first measurement of the fractional quantum Hall spin-flip gaps. They are found to be systematically larger in comparison with the gaps simultaneously measured in transport. For $\nu =1\mathrm{}/3$ and $\nu =1\mathrm{}/2$, these spin-flip gaps allow the determination of the composite fermion interaction energy. This energy is investigated as a function of the finite width of the 2D channel.