Measurements of the reflectivity of a two-dimensional electron gas are used to deduce the polarization of the composite fermion of hole system formed for Landau level occupancies in the regime $1<\mathrm{}\nu <2.\mathrm{}$ The measurements are consistent with the formation of a mixed spin CF system and allow the density of states or ‘polarization’ effective mass of the CF holes to be determined. The mass values at $\nu =3/2\mathrm{}$ are found to be $\sim {1.9m}_e$ for electron densities of $4.4\times {10}^{11}{\mathrm{cm}}^{-2},$ which is significantly larger than those found from measurements of the energy gaps at finite values of effective magnetic field.

• Received 27 November 2001

DOI:https://doi.org/10.1103/PhysRevB.65.161305