Unconventional Sequence of Fractional Quantum Hall States in Suspended Graphene

Graphene provides a unique platform to study many-body correlations due to the relativistic nature of its charge carriers and their fourfold degeneracy. We report local electronic compressibility measurements of a suspended graphene flake performed using a scanning single-electron transistor. Between filling factors $v$ = 0 and 1, our measurements reveal incompressible fractional quantum Hall states at $v$ = 1/3, 2/3, 2/5, 3/5, 3/7, 4/7 and 4/9, which clearly follow the standard composite fermion sequence. In contrast, between $v$ = 1 and 2, incompressible states occur only at $v$ = 4/3, 8/5, 10/7 and 14/9. These fractions correspond to a subset of the composite fermion sequence involving only even numerators, suggesting a robust underlying symmetry. We extract the energy gaps of each fractional quantum Hall state as a function of magnetic field and find that $v$ = 1/3, 2/3, 4/3, and 8/5 are strongest at low field, persisting below 1.5 T. Our results provide insight into the interplay between electronic correlations and SU(4) symmetry in graphene.