SU(3) and SU(4) Singlet Quantum Hall States at ν=2/3.

We report on an exact diagonalization study of fractional quantum Hall states at a filling factor of ν=2/3 in a system with a fourfold degenerate n=0 Landau level and SU(4) symmetric Coulomb interactions. Our investigation reveals previously unidentified SU(3) and SU(4) singlet ground states which appear at a flux quantum shift 2 when a spherical geometry is employed and lie outside the established composite-fermion or multicomponent Halperin state patterns. We evaluate the two-particle correlation functions of these states and discuss quantum phase transitions in graphene between singlet states with a different number of components as the magnetic field strength is increased.