Physikalisches Kolloquium: Moving vibrational solitons of high frequency in solids

Abstract

Nonlinear dynamics of solids is of great interest for material science. Here we present a MD method of calculations of atomic motion in crystals which includes: 1) long-range linear interatomic forces described by means of phonon Green functions and 2) short-range nonlinear (anharmonic) forces considered explicitly. The method allows one to take into account the effects of the macroscopic field on vibrational solitons in insulators. We used this method for calculations of vibrational solitons in ionic crystals. In these materials, as a rule, the frequencies of vibrational solitons fall into the gaps of the phonon spectrum. However, in metals Ni, Nb, Cu, Fe and probably some others, due to Friedel oscillations of atomic forces caused by free electrons, the attraction forces are remarkably increased at intermediate distances. As a result the mentioned frequencies are found above the phonon spectrum. It has been shown that these vibrational solitons are highly mobile and can efficiently transfer a concentrated vibrational energy of the order of few eV to large distances along crystallographic directions. Semiconductors (Ge and Si) are another type of crystals in which high-frequency vibrational solitons are numerically found. Here the reason of high frequency is covalence: fast switching-off of the overlapping integrals of valence electrons with increasing the distance results in the increasing of attraction forces between the atoms and in this way causes appearance of vibrational solitons with the frequencies above the phonon spectrum.