# Geometric Resonance of Four-Flux Composite Fermions

## TL;DR

This study investigates four-flux composite fermions near filling factor 1/4 using geometric resonance, revealing unique symmetry properties and anisotropies that differ from two-flux CFs, and suggesting different effective masses on either side of 1/4.

## Contribution

First direct experimental probing of the Fermi sea of four-flux composite fermions near ν=1/4, uncovering novel symmetry and anisotropy features.

## Key findings

- Symmetric geometric resonance minima around ν=1/4 without in-plane magnetic field.
- Asymmetry in minima positions when an in-plane magnetic field is applied.
- Indication of different effective masses for four-flux CFs on either side of ν=1/4.

## Abstract

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 $\nu$.