Discrete breathers above phonon spectrum

Abstract

It is shown that in some metals (Ni, Nb, Fe, Cu) may exist discrete breathers with frequencies above the top of the phonon spectrum. These excitations are mobile: they may propagate along the crystallographic directions transferring energy of >~ 1 eV over large distances. The discrete breathers with the frequencies above the top of the phonon bands may also exist in covalent crystals (diamond, Si and Ge). It is also found that in monatomic chains and planes (e.g. in graphene), the transverse discrete breathers may be excited above the spectrum of corresponding phonons. Although these vibrations are in resonance with longitudinal (chain) or in-plane (graphene) phonons the lifetime of them may be very long. 1. Introduction It is already a well known fact that in crystal lattices may exist long living anharmonic modes of rather high energy >~ 1 eV. These excitations are called as discrete breathers (DBs), intrinsic localized modes, vibrational solitons, or quodons [2, 4, 7, 11, 13, 14, 16, 17, 29, 31, 32, 34, 42, 43, 46, 47, 48, 51, 52]. In numerical studies of DBs different two-body potential models (Morse, Lennard-Jones, Born-Mayer-Coulomb and other potentials) have been used. All these potentials show strong softening at increasing vibrational amplitudes. Therefore the frequencies of DBs, found in these studies, drop down from the optical band(s) into the phonon gaps, if such gaps exist in the spectrum (see, e.g. Refs. [26, 28, 30]).