Spin polarization of composite fermions: Measurements of the Fermi energy

From the degree of circular polarization of the time-resolved radiative recombination of 2D electrons with photoexcited holes bound to accepters we have measured the magnetic field dependencies of the electron spin polarization for various fractional (nu = 2/3, 3/5, 4/7, 2/5, 3/7, 4/9, 8/5, 4/3, and 7/5) and composite fermions (nu = 1/2, 1/4, and 3/2) states. The Fermi energies of these composite fermion states are measured for the first time and the corresponding value of the composite fermion density of states mass at nu = 1/2 is found to be about 4 times heavier than the previously reported values of the "activation" mass. [S0031-9007(99)09059-6].