  We study a bilayer GaAs hole system that hosts two distinct many-body phases
at low temperatures and high perpendicular magnetic fields. The higher-density
(top) layer develops a Fermi sea of composite fermions (CFs) in its half-filled
lowest Landau level, while the lower-density (bottom) layer forms a Wigner
crystal (WC) as its filling becomes very small. Owing to the inter-layer
interaction, the CFs in the top-layer feel the periodic Coulomb potential of
the WC in the bottom-layer. We measure the magnetoresistance of the top-layer
while changing the bottom-layer density. As the WC layer density increases, the
resistance peaks separating the adjacent fractional quantum Hall states in the
top-layer change nonmonotonically and attain maximum values when the cyclotron
orbit of the CFs encloses one WC lattice point. These features disappear at T =
275 mK when the WC melts. The observation of such geometric resonance features
is unprecedented and surprising as it implies that the CFs retain a
well-defined cyclotron orbit and Fermi wave vector even deep in the fractional
quantum Hall regime, far from half-filling.
