PULSED MAGNETIC FIELD MEASUREMENTS OF THE COMPOSITE FERMION EFFECTIVE MASS

D.R. Leadley1, M. Van der Burght1, R.J. Nicholas1, J.J. Harris2 and C.T. Foxon3

  1. Department of Physics, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU
  2. Department of Electronic Enginering, University College, London
  3. Department of Physics, Nottingham University, University Park, Nottingham

The Fractional Quantum Hall Effect (FQHE) may be neatly described by considering the motion of composite Fermions (CFs), formed by attaching pairs of flux quanta to each electron. At filling factor 1/2 these CFs see zero magnetic field. All the observed FQHE features may be interpreted as the Integer QHE of the CFs in an effective magnetic field B*, without reference to the strong underlying electron-electron interactions.

Treating the FQHE in \rhoxx as Shubnikov-de~Haas oscillations, we previously obtained values for the CF effective mass M* in very high mobility, low density, GaAs-GaAlAs heterojunctions [1]. We have now extended this work using pulsed magnetic fields and find the same approx. linear increase with B* seen at low fields. A 3He cryostat giving 500mK in 50T, allowed us to use a large range of T/B and obtain accurate mass values. For a sample with electron density of 3.4 x1015m-2, \nu=1/3 occurs at 42T corresponding to an effective field of 14T. In this case M*=1.5me, enormously enhanced over the band edge value of 0.07me. The effective cyclotron gap between CF Landau levels determines the FQHE strength and for this 1/3 fraction is 12.5K, allowing it to be seen at 4K! Since M* increases with B*, the energy gap saturates at high fields, as found in activation measurements. We will discus how to visualise CFs and what the effective mass means.

[1] D.R.~Leadley et al. Phys. Rev. Lett. 72, 1906 (1994)

Presented at: Institute of Physics, Condensed Matter and Materials Physics Conference, Warwick 1994