Composite fermions in a weakly density modulated 2DEG:: direct evidence of a periodic magnetic field around ν=1/2

We investigate the magnetoresistance of a weakly density modulated high mobility two-dimensional electron system around filling factor v = 1/2. The density modulation with periods a between 300 nm and 500 nm was generated by in situ interferometric illumination. At low B we found commensurability oscillations of the magnetoresistance characteristic for transport in weak electrostatic potentials with minima positions on the magnetic field axis given by 2R(C) = (lambda - 1/4)a. Here 2R(C) is the electron cyclotron diameter at the Fermi-energy and lambda an integer. In contrast, we find clear magnetic commensurability features around filling factor v = 1/2 with a pronounced minimum for a composite fermion cyclotron radius R-C,R-CF matching 1.25a. This gives direct evidence that the density modulation results in a modulated effective magnetic field for composite fermions. The experimental rho(xx)-traces around v = 1/2 are well described by novel model calculations, based on a semiclassical solution of the Boltzmann equation, taking into account anisotropic scattering. (C) 2000 Elsevier Science B.V. All rights reserved.