Composite fermions with a spin freedom

We investigate the magnetoresistance of a weakly density modulated high mobility two-dimensional electron system around filling factor View the MathML source. 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 2RC=(λ−1/4)a. Here 2RC is the electron cyclotron diameter at the Fermi-energy and λ an integer. In contrast, we find clear magnetic commensurability features around filling factor View the MathML source with a pronounced minimum for a composite fermion cyclotron radius RC,CF matching 1.25a. This gives direct evidence that the density modulation results in a modulated effective magnetic field for composite fermions. The experimental ρxx-traces around View the MathML source are well described by novel model calculations, based on a semiclassical solution of the Boltzmann equation, taking into account anisotropic scattering.