Unconventional Sequence of Fractional Quantum Hall States in Suspended Graphene

TL;DR

This study reveals an unconventional sequence of fractional quantum Hall states in suspended graphene, highlighting unique many-body effects and symmetries in the material's electronic properties under strong magnetic fields.

Contribution

It demonstrates the observation of fractional quantum Hall states following a subset of the composite fermion sequence with even numerators, revealing new symmetry considerations in graphene.

Findings

Incompressible states follow a subset of the composite fermion sequence.

Unusual fractional states suggest a robust underlying symmetry.

Energy gaps are measured as a function of magnetic field.

Abstract

Interactions among electrons can give rise to striking collective phenomena when the kinetic energy of charge carriers is suppressed. One example is the fractional quantum Hall effect, in which correlations between electrons moving in two dimensions under the influence of a strong magnetic field generate excitations with fractional charge. Graphene provides a platform to study unique many-body effects due to its massless chiral charge carriers and the fourfold degeneracy that arises from their spin and valley degrees of freedom. Here we report local electronic compressibility measurements of a suspended graphene flake performed using a scanning single-electron transistor. Between Landau level filling v = 0 and 1, we observe incompressible fractional quantum Hall states that follow the standard composite fermion sequence v = p/(2p \pm 1) for all integer p \leq 4. In contrast, incompressible behavior occurs only at v = 4/3, 8/5, 10/7 and 14/9 between v = 1 and 2. These fractions correspond to a subset of the standard composite fermion sequence involving only even numerators, suggesting a robust underlying symmetry. We extract the energy gaps associated with each fractional quantum Hall state as a function of magnetic field. The states at v = 1/3, 2/3, 4/3 and 8/5 are the strongest at low field, and persist below 1.5 T. The unusual sequence of incompressible states provides insight into the interplay between electronic correlations and SU(4) symmetry in graphene.