DRASTIC ENHANCEMENT OF COMPOSITE FERMION MASS NEAR LANDAU-LEVEL FILLING NU=1/2

We have determined the effective mass of composite fermions in the vicinity of half Landau level filling and observe a mass enhancement by as much as 40% as the filling factor ν approaches ν=12. These measurements provide the first experimental data for the energetics of this novel fermion system as ν→12. The apparently divergent particle mass indicates that the system at exactly ν=12 is not an ordinary Fermi liquid. © 1994 The American Physical Society.