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 acceptors we have measured the magnetic field dependencies of the electron spin polarization for various fractional ( $\nu =2/3\mathrm{}$, $3/5$, $4/7$, $2/5$, $3/7$, $4/9$, $8/5$, $4/3$, and $7/5$) and composite fermions ( $\nu =1/2\mathrm{}$, $1/4$, 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 $\nu =1/2\mathrm{}$ is found to be about 4 times heavier than the previously reported values of the “activation” mass.