Fermi Contour Anisotropy of GaAs Electron-Flux Composite Fermions in Parallel Magnetic Fields*

In high-quality two-dimensional electrons confined to GaAs quantum wells, near Landau level filling factors $\nu =$1/2 and 1/4, we observe signatures of the commensurability of the electron-flux composite fermion cyclotron orbits with a unidirectional periodic density modulation. Focusing on the data near $\nu =$1/2, we directly and quantitatively probe the shape of the composite fermions' cyclotron orbit, and therefore their Fermi contour, as a function of magnetic field (B$_{\mathrm{\vert \vert }})$ applied parallel to the sample plane. The composite fermion Fermi contour becomes severely distorted with increasing B$_{\mathrm{\vert \vert }}$ and appears to be elliptical, in sharp contrast to the electron Fermi contour which splits as the system becomes bilayer-like at high B$_{\mathrm{\vert \vert }}$. We present a simple, qualitative model to interpret our findings.