Breathing subsonic crowdion in Morse lattices

Based on molecular dynamics simulations, we discuss dynamics of nonlinear localized excitations in two-dimensional crystal with Morse interatomic interactions and without on-site potential. It is shown that supersonic solitons (SupS) or supersonic crowdions (SupC) can be excited by kicking one atom with initial velocity along a close-packed atomic row. The difference between SupS and SupC is that the former is excited with the kicking velocity insufficient for Frenkel pair (vacancy and interstitial atom) creation, while the latter one is formed with initial momentum sufficient for creation of two such defects and while propagating, SupC carries a mass of one atom. It is shown that, in a range of kicking velocity, SupC soon after its formation transforms into a pair of excitations, one is SupS and another one is subsonic crowdion bearing internal vibrational mode. The latter one is called here breathing subsonic crowdion (BSubC). BSubC is localized in the close-packed atomic row on a dozen of atoms that vibrate out-of-phase with the nearest neighbors along the row, and it carries one atom. To the best of our knowledge such excitation has not been reported before. We offer a physically motivated ansatz to set initial conditions for excitation of BSubC in molecular dynamics simulations. With this ansatz BSubC was successfully excited also in three-dimensional fcc Morse lattice. Our results contribute to a deeper understanding of nonlinear excitations in crystals.