Melting of colloidal molecular crystals on triangular lattices

The phase behavior of a two-dimensional colloidal system subject to
a commensurate triangular potential is investigated. We consider
the integer number of colloids in each potential minimum as rigid
composite objects with effective discrete degrees of freedom. It is
shown that there is a rich variety of phases including
"herringbone" and "Japanese 6 in 1" phases. The ensuing phase
diagram and phase transitions are analyzed analytically within
variational mean-field theory and supplemented by Monte Carlo
simulations. Consequences for experiments are discussed.