Berry Phase and Anomalous Transport of the Composite Fermions at the Half-Filled Landau Level*

Recent developments in the theory of composite fermions (CFs) have led to a prediction of a π Berry phase for the CFs circling around the Fermi surface at half-filling ν=1/2 (where ν is the Landau level filling factor). In this talk, we provide the first experimental evidence for the detection of the Berry phase of CFs in the fractional quantum Hall effect by studying their density-dependent magnetotransport in heterojunction insulated-gate field-effect transistors (HIGFETs), in which the electron density can be tuned over a large range. Our measurements of the Shubnikov-de Haas oscillations of CFs as a function carrier density at a fixed magnetic field provide a strong support for an existence of a π Berry phase at ν = 1/2. We also discover that the conductivity of composite fermions at ν = 1/2 displays an anomalous linear density dependence, whose origin remains mysterious yet tantalizing.