Composite fermions in tilted magnetic fields and the effect of the confining potential width on the composite-fermion effective mass

The temperature dependence (40 mK less than or equal to T less than or equal to 1 K) of the oscillations in rho(xx) in a high-mobility GaAs(Ga,Al)As heterojunction close to Landau-level filling factor nu=1/2 has been examined for many different values of theta, the angle between the normal to the sample and the magnetic field. It was found that the energy gaps associated with the fractional quantum Hall effect could be interpreted using the composite-fermion (CF) approach with a fixed CF effective mass mu at each theta. However, mu was found not to follow the theta dependence expected for a purely 2D system; i.e., the CF energy gaps at fixed nu grow markedly with increasing in-plane field. Comparisons with models based on the Fang-Howard variational wave function show that this effect is due to the compression of the electronic wave function caused by the in-plane component of the magnetic field.