  We examine theoretically the effects caused by a periodic external potential
on the correlated motion of a two-dimensional electron system under strong
magnetic fields corresponding to a filling factor 1/2. To describe the
resulting complex dynamics, we adopt a composite fermion approach and we
determine in a two loop approximation the density-response function
$K_{00}(\bfq,\omega)$ and the compressibility. We show explicitly that the
long-wavelength limit of $K_{00}(\bfq,\omega)$ exhibits substantial anisotropic
behavior induced by the modulation, and that the system tends to be
incompressible in a direction orthogonal to the modulation as opposed to its
response along the modulation.

Strongly Correlated Electrons