Geometric Resonance Of Four-Flux Composite Fermions

Two-dimensional interacting electrons exposed to strong perpendicular magnetic fields generate emergent, exotic quasiparticles phenomenologically distinct from electrons. Specifically, electrons bind with an even number of flux quanta, and transform into composite fermions (CFs). Besides providing an intuitive explanation for the fractional quantum Hall states, CFs also possess Fermi-liquid-like properties, including a well-defined Fermi sea, at and near even-denominator Landau-level filling factors such as nu = 1/2 or 1/4. Here, we directly probe the Fermi sea of the rarely studied four-flux CFs near nu = 1/4 via geometric resonance experiments. The data reveal some unique characteristics. Unlike in the case of two-flux CFs, the magnetic field positions of the geometric resonance resistance minima for nu < 1/4 and nu > 1/4 are symmetric with respect to the position of nu = 1/4. However, when an in-plane magnetic field is applied, the minima positions become asymmetric, implying a mysterious asymmetry in the CF Fermi sea anisotropy for nu < 1/4 and nu > 1/4. This asymmetry, which is in stark contrast to the two-flux CFs, suggests that the four-flux CFs on the two sides of nu = 1/4 have very different effective masses, possibly because of the proximity of the Wigner crystal formation at small nu.