Quantized intrinsically localized modes of a 3D lattice

Intrinsically Localized Modes are anharmonic oscillations found in one-dimensional systems, and occur relatively infrequently in classical higher dimensional lattices. However, when ILMs appear in simulations of classical lattices, their energies are too high for them to be seen in thermal equilibrium. We investigate quantized ILMs in a three-dimensional lattice using the Ladder Approximation, and find that ILMs occur preferentially for centre of mass momenta at which the van-Hove singularities in the two-phonon density of states coalesce. For interactions larger than a critical value, the ILMs form above the top of the two-phonon continuum, but fall into the continuum as q? is shifted away from the optimal value. This indicates that ILM excitations may be more ubiquitous in 3D lattices than previously expected. Furthermore, we find that the ILMs have intrinsic spins of either S = 0 or S = 2 and have internal structures associated with their spatial symmetry.