Simulation of energy transport in crystal with NaCl structure assisted by discrete breathers

Abstract

Discrete breather (DB) is a spatially localized vibrational mode of large amplitude with vibration frequency outside the phonon band of the crystal. DB frequency can leave phonon spectrum due to the anharmonicity of interatomic bonds owing to the fact that the frequency of a nonlinear oscillator is amplitude dependent. In the case of soft (hard) anharmonicity the nonlinear oscillator frequency decreases (increases) with amplitude. Crystals having a gap in the phonon spectrum can, in principle, support the so-called gap DBs, i.e., DBs with frequencies within the gap. The alkali halide NaI crystal possesses a wide gap in the phonon spectrum and the existence of gap DBs in this crystal has been shown by Kiselev and Sievers with the use of the molecular dynamics method. Later on, several experimental works have been undertaken to support the results of the numerical study and also the possibility of energy exchange between two closely positioned DBs was shown by atomistic simulations. In the present study the energy exchange between DBs in larger clusters is simulated by molecular dynamics. It is shown that the energy initially given to the DB cluster stays in the localized form for a long time (hundreds of DB oscillation periods) even though the energy can travel from one lattice site to another and even polarization of DBs can change. These results contribute to our better understanding of the mechanism of energy localization and transport in crystals.