Composite Fermions in a weakly density modulated 2DEG: Directive evidence of a periodic magnetic field around n=1/2

Abstract

We investigate the magnetoresistance of a weakly density modulated high mobility two- dimensional electron system around filling factor . 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=(ë-1/4)a. Here 2RC is the electron cyclotron diameter at the Fermi- energy and ë an integer. In contrast, we find clear magnetic commensurability features around filling factor 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 ñxx-traces around are well described by novel model calculations, based on a semiclassical solution of the Boltzmann equation, taking into account anisotropic scattering.