Transition in the Correlated 2D Electron System Induced by a Periodic Density Modulation

We have experimentally examined the effects of periodic density modulation on correlations in a 2D electron system with tunable gate structures using dc transport and surface acoustic waves (SAW). Application of a modulation induces a dramatic transition at filling factor $\frac{1}{2}$ where the properties of the composite fermion Fermi surface are replaced by a large effect similar to that displayed by quantum Hall states. This new effect occurs for SAW measurements orthogonal to the modulation wave vector and only for a sufficiently small modulation period. No immediate explanation is assigned to this effect.