THE DEPENDENCE OF THE COMPOSITE FERMION EFFECTIVE-MASS ON CARRIER DENSITY AND ZEEMAN ENERGY

Measurements of the temperature-dependent resistivity of high-mobility GaAs/GaAlAs heterojunctions are used to measure the effective mass of Composite Fermions (CF). The CF effective mass is found to increase a pproximately linearly with the effective field B up to effective fiel ds of 14 T. Data from all fractions around nu = 1/2 are unified by the single parameter B for samples studied over a wide range of temperat ure. The energy gap is found to increase as root B at high fields. Hy drostatic pressure is used to reduce the value of the electron g-facto r, and this is shown to have a large effect on the relative strengths of different fractions. By 13.4 kbar, where the Zeeman energy is only 1/4 of its value at 0 bar, fractions with odd numerators are found to be strongly suppressed, and new features with even numerators appear. The energy gaps measured for 5/3 as a function of carrier density and pressure are consistent with a g-factor equal to the bulk value enhanc ed by a factor of two due to exchange interactions.