Title: Even denominator fractional quantum Hall states at an isospin transition in monolayer graphene,
Abstract: Magnetic fields quench the kinetic energy of two dimensional electrons,
confining them to highly degenerate Landau levels. In the absence of disorder,
the ground state at partial Landau level filling is determined only by Coulomb
interactions, leading to a variety of correlation-driven phenomena. Here, we
realize a quantum Hall analog of the Neél-to-valence bond solid transition
within the spin- and sublattice- degenerate monolayer graphene zero energy
Landau level by experimentally controlling substrate-induced sublattice
symmetry breaking. The transition is marked by unusual isospin transitions in
odd denominator fractional quantum Hall states for filling factors $\nu$ near
charge neutrality, and the unexpected appearance of incompressible even
denominator fractional quantum Hall states at $\nu=\pm1/2$ and $\pm1/4$
associated with pairing between composite fermions on different carbon
sublattices.