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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 $ν=$ 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 $ν=1/2$, we directly and quantitatively probe the shape of the composite fermions' cyclotron orbit, and therefore their Fermi contour, as a function of magnetic field ($B_{||}$) applied parallel to the sample plane. The composite fermion Fermi contour becomes severely distorted with increasing $B_{||}$ and appears to be elliptical, in sharp contrast to the electron Fermi contour which splits as the system becomes bilayer-like at high $B_{||}$. We present a simple, qualitative model to interpret our findings.