Spin polarization of composite fermions: Measurements of the Fermi energy

From the degree of circular polarization of the time-resolved radiative rec ombination of 2D electrons with photoexcited holes bound to accepters we ha ve measured the magnetic field dependencies of the electron spin polarizati on 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 ene rgies 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].