Experimental evidence for composite fermions

The study of correlated two-dimensional electron systems in the extreme quantum limit has been advanced by development of a field-theoretic model and associated experimental findings. The fundamental theoretical construction transforms the interacting 2D electrons into quasiparticles referred to as composite fermions. The properties of such an interacting quasiparticle system have recently been described, with the main hypothesis that Fermi surfaces should form from the quasiparticle seas at large specific magnetic field values or filling factors. This model serves to provide explanation for an extensive body of experimental data, including description of the fractional quantum Hall effect as the integer quantum Hall effect for the quasiparticles. This review will present the experimental findings which both motivated and subsequently have verified aspects of this model. Following a brief introduction to two-dimensional electron systems, early experimental findings pre-dating and influencing the composite fermion theory are examined. The theoretical construction, and in particular the hypothesis of Fermi surface formation at high magnetic fields, are briefly reviewed with respect to their experimental predictions. Experiments subsequent to the main theoretic developments are then presented. Emphasized are surface acoustic wave experiments which were responsible for measuring the composite fermion Fermi wavevector and mean free path. Also reviewed are antidot and magnetic focusing experiments which demonstrate semiclassical motion of the quasiparticles at the Fermi surface. Experimental determination of the composite fermion effective mass is examined, considering results from measurements in d.c. transport, surface acoustic waves and thermopower. Experimental findings are summarized and potential future studies are discussed.