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

TL;DR

This study investigates how parallel magnetic fields distort the Fermi contour of composite fermions in GaAs quantum wells, revealing significant anisotropy and elliptical deformation, contrasting with electron Fermi contour behavior.

Contribution

It provides the first direct, quantitative measurement of the shape of composite fermion Fermi contours under parallel magnetic fields in GaAs systems.

Findings

Composite fermion Fermi contour becomes elliptical with increasing parallel magnetic field.

Electron Fermi contour splits into a bilayer-like shape at high parallel magnetic fields.

The observed Fermi contour distortion is qualitatively explained by a simple model.

Abstract

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 ($B_{||}$) 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.