Effect of defects and further neighbor interactions on a two-dimensional dipolar system on a honeycomb lattice

A theoretical investigation of a planar dipolar system on a honeycomb lattice reveals that the ground-state energy is increased for finite systems over that of an infinite system. The infinite system has a ground-state degeneracy with respect to dipole orientations. This degeneracy is lifted for finite systems. A mean-field calculation including nearest, second, and third neighbors shows that there are three phases, an ordered phase, a paramagnetic one, and a magnetic-field-induced ferromagnetic phase. A defect in the second nearest neighbor has the effect of introducing a finite magnetization at zero field at low temperatures.