Gap discrete breathers in a two-component two-dimensional crystal in thermal equilibrium

Crystals having a gap in the phonon spectrum can maintain gap discrete breathers (DBs), i.e., nonlinear localized oscillatory modes existing in the absence of defects and having a frequency lying in the gap. The lifetime of gap DBs in a two-dimensional perfect crystal of the composition A ₃ B in thermal equilibrium has been studied by the molecular dynamics method. As was shown earlier, the existence of gap DBs in such a crystal is provided by the presence of a wide gap in the phonon spectrum if the component mass ratio m _A /m _B is sufficiently large. For comparison, a crystal with a relatively small ratio m _A /m _B is considered when the gap in the spectrum is absent and the existence of gap DBs is impossible in the case of a weak nonlinearity realized in the considered case. It has been shown that, in contrast to the opposite case, in a crystal maintaining gap DBs, long-lived localized oscillatory modes of large amplitude can emerge, whose concentration and lifetime increase with temperature.