Even-denominator fractional quantum Hall states at an isospin transition in monolayer graphene

In monolayer graphene, the two inequivalent sublattices of carbon atoms combine with the electron spin to give electrons a nearly fourfold degenerate internal isospin. At high magnetic fields, the isospin degeneracy increases the already large intrinsic degeneracy of the two-dimensional Landau levels, making low-disorder graphene systems a versatile platform for studying multicomponent quantum magnetism. Here, we describe magnetocapacitance experiments of ultraclean monolayer graphene devices in which a hexagonal boron nitride substrate breaks the symmetry between carbon sublattices. We observe a phase transition in the isospin system, which is marked by unusual transitions in odd-denominator fractional quantum Hall states for filling factors ν near charge neutrality and by the unexpected appearance of incompressible even-denominator fractional quantum Hall states at ν = ±1/2 and ν = ±1/4. We propose a scenario in which the observed states are multicomponent fractional quantum Hall states incorporating correlations between electrons on different carbon sublattices, associated with a quantum Hall analogue of the Néel-to-valence bond solid transition that occurs at charge neutrality. Quantum Hall states are observed in monolayer graphene at even-denominator fractional filling of the lowest Landau level. This is linked to transitions in the spin and valley structure of the ground state.