Abstract
Spatially localized nonlinear oscillations in the form of a discrete breather (DB) is a recently discovered phenomenon widely investigated in different physical systems due to its potential impact on the structure and dynamics of those systems. Our research is focused on the variety of DB in crystals. Depending on the type of the crystal and corresponding phonon spectrum one can get a gap DB or a DB with the frequency above the phonon spectrum. Most of gap DBs previously addressed in the literature were characterized by a soft nonlinearity type with frequencies splitting off from the upper edge of the phonon spectrum gap. In this work we managed to excite a hard nonlinearity type DB with a frequency within the band gap. In order to achieve this goal we have excited four types of delocalized vibrational modes in biatomic crystal with atomic mass difference m1/m2=10 providing the existence of sufficiently wide phonon band gap. Analysis of amplitude-frequency dependences of two of those modes revealed the hard nonlinearity type with the frequency within the spectrum. The fourth mode with moving heavy atoms had the frequency within the band gap growing with the amplitude. Application of localization function with radial symmetry to this delocalized vibrational mode allowed us to obtain a DB with hard nonlinearity type in the band gap. Properties of the DB were analyzed.
Highlights
It is well known that most of condensed matter objects possess crystalline long range order structure with inevitable presence of defects
One of the possible ways of energy localization in small area of the crystal is the excitation of discrete breathers (DB) — nonlinear, spatially localized vibrational modes in defect free lattices that are reported to exist in many areas of modern physics [1 – 3]
A new approach for DB excitation in molecular dynamics simulations has been successfully applied to a two dimensional biatomic lattice of A3B stoichiometry
Summary
It is well known that most of condensed matter objects possess crystalline long range order structure with inevitable presence of defects. Later the possibility of the existence of DB with frequencies above the phonon spectrum of the crystal was confirmed in [10 – 11] for the case of two-dimensional (2D) monoatomic crystal with Morse interatomic potential Another important aspect of DB investigation is the method of their excitation which consists in introduction of initial velocities and displacement to atoms involved in the DB oscillations. Another progressive method of DB excitation in a closely packed atomic raw was application of the ansatz — a set of initial functions determining initial vibration amplitudes, atomic vibration centers shifts and other DB parameters This way is more common and was successfully employed for Morse crystals of different dimensionality [10 – 11] and for several metals [16 – 17].One should recall that possibility of DB excitation and their properties strongly depend on the type of interatomic potential considered in molecular dynamics simulation. In this work we have tried to extend the approach of localization of nonlinear modes to the biatomic Morse crystal with a wide band gap in order to make an attempt to obtain a new type of the DB with hard nonlinearity type and the frequency within the phonon band gap
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