  In systems with many local degrees of freedom, high-symmetry points in the
phase diagram can provide an important starting point for the investigation of
their properties throughout the phase diagram. In systems with both spin and
orbital (or valley) degrees of freedom such a starting point gives rise to
SU(4)-symmetric models. Here we consider SU(4)-symmetric "spin" models,
corresponding to Mott phases at half-filling, i.e. the six-dimensional
representation of SU(4). This may be relevant to twisted multilayer graphene.
In particular, we study the SU(4) antiferromagnetic "Heisenberg" model on the
triangular lattice, both in the classical limit and in the quantum regime.
Carrying out a numerical study using the density matrix renormalization group
(DMRG), we argue that the ground state is non-magnetic. We then derive a dimer
expansion of the SU(4) spin model. An exact diagonalization (ED) study of the
effective dimer model suggests that the ground state breaks translation
invariance, forming a valence bond solid (VBS) with a 12-site unit cell.
Finally, we consider the effect of SU(4)-symmetry breaking interactions due to
Hund's coupling, and argue for a possible phase transition between a VBS and a
magnetically ordered state.
