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 polariz ation for the fractional states v = 1/3 and v = 2/3 displays activated beha vior. This study enables the first measurement of the fractional quantum Ha ll spin-flip gaps. They are found to be systematically larger in comparison with the gaps simultaneously measured in transport. For y = 1/3 and v = 1/ 2, these spin-flip gaps allow the determination of the composite fermion in teraction energy. This energy is investigated as a function of the finite w idth of the 2D channel.