The composite fermion (CF) model has been strikingly successful in describing many aspects of the fractional quantum Hall effect (FQHE) observed in two-dimensinal electron systems (2DES). In the CF picture, the FQHE is the integer quantum Hall effect of the CFs. In order to assess the effect of an in-plane magnetic field on the CFs we have examined the temperature dependence (40 mK ≤ T ≤ 1 K) of the oscillations in ρxx in a high-mobility GaAs-(Ga,Al)As heterojunction close to Landau level filling factors ν = 1/2 and 3/2 for many different values of θ, the angle between the normal to the 2DES and the magnetic field. The CF energy gaps were evaluated at each angle using a variant of the Lifshitz-Kosevich approach. Close to ν = 1/2. it was found that the CF gaps at each angle could be fitted to within experimental errors using a constant CF effective mass. However, the CF effective mass was found not to follow the θ-dependence expected for a purely 2D system: i.e. the CF energy gap at fixed ν grows markedly with increasing in-plane field. Around ν = 3/2 the situation is more complex, and the oscillations of the energy gaps at ν = 8/5. 7/5 and 4/3 as θ varied were interpreted using a recent model of two independent CF Landau fans separated by the Pauli spin splitting (Du R R, Yeh A S, Stormer H L, Tsui D C, Pfeiffer L N and West K W 1995 Phys. Rev. Lett. 75 3926). However, whilst the model qualitatively predicts some of the behaviour of the ρxx-minima, it is unable to account for the absolute sizes of the energy gaps. In order to reproduce the gaps at ν= 7/5 and 4/3 quantitatively, an angle-dependent CF mass (as observed close to ν= 1/2) is required. The data suggest that the compression of the electronic wave function due to the in-plane field and exchange effects both play a role in determining the size of the CF gaps and cast doubts on the supposed 'universal' behaviour of the CF mass.
ASJC Scopus subject areas