Spontaneous formation of Kagom\'e lattice in two-dimensional Rydberg atoms

Two-dimensional Rydberg atoms are modeled at low temperatures by means of the classical Monte Carlo method. The Coulomb repulsion of charged ions competing with the repulsive van der Waals long-range tail is modeled by a number of interaction potentials. We find that under specific conditions the usual triangular crystal becomes unstable with respect to more exotic lattices such as Kagom\'e, flower, molecular crystal and rectangular chain packings. Ground-state configurations are obtained by means of the annealing procedure and their stability is additionally studied by the normal modes analysis. While commonly the square lattice is mechanically unstable due to softening of the shear modulus, we were able to find a specific set of parameters for which the square lattice can be made stable.