  We propose a theory for the new effects recently observed by Willett et al
[1] in the magnetoresistance of a weakly modulated two dimensional electron gas
near filling factor 1/2. Minima in transverse magnetoresistance and maxima in
longitudinal magnetoresistance at the same magnetic field producing the new
resonance structure are reported. The structure occurs due to geometric
resonance of the composite fermion cyclotron orbits with the modulation period
of the effective magnetic field $ B_{eff} $ due to the applied density
modulation. The transverse minimum occurs due to the inhomogeneity in the field
$ B_{eff} $ in the presence of density modulations, whereas the longitudinal
maximum can arise due to a shape-effect (distortion) of the composite fermion
Fermi surface (CF-FS). Thus the minima and maxima reflect different physical
mechanisms.
  PACS numbers 71.10 Pm, 73.40 Hm, 73.20 Dx {}

Mesoscale and Nanoscale Physics