Geometric Frustration of Colloidal Dimers on a Honeycomb Magnetic Lattice

We study the phase behaviour and the collective dynamics of interacting paramagnetic colloids assembled above a honeycomb lattice of triangular shaped magnetic minima. A frustrated colloidal molecular crystal is realized when filling these potential minima with exactly two particles per pinning site. External in-plane rotating fields are used to anneal the system into different phases, including long range ordered stripes, random fully packed loops, labyrinth and disordered states. At higher amplitude of the annealing field, the dimer lattice displays a two step melting transition where the initially immobile dimers perform first localized rotations and later break up by exchanging particles across consecutive lattice minima.