Dimension dependent energy thresholds for discrete breathers

Discrete breathers are time-periodic, spatially localized solutions of the equations of motion for a system of classical degrees of freedom interacting on a lattice. We study the existence of energy thresholds for discrete breathers, i.e., the question whether, in a certain system, discrete breathers of arbitrarily low energy exist, or a threshold has to be overcome in order to excite a discrete breather. Breather energies are found to have a positive lower bound if the lattice dimension d is greater than or equal to a certain critical value dc, whereas no energy threshold is observed for d < dc. The critical dimension dc is system dependent and can be computed explicitly, taking on values between zero and infinity. Three classes of Hamiltonian systems are distinguished, being characterized by different mechanisms effecting the existence (or non-existence) of an energy threshold. PACS numbers: 45.05.+x, 63.20.Pw, 05.45.-a