Ballistic energy transfer in dielectric Ar crystals

The results of a molecular dynamics study of the supersonic propagation of femtosecond-energy pulses in a three-dimensional dielectric Ar crystal are presented. Within the first few picoseconds following pulse excitation, a s ignificant ballistic contribution to heat transfer is observed which preven ts the system from showing the features of normal heat conduction, i.e. the existence of finite temperature gradients and the requirement that heat co nductivity be an intensive quantity. It is shown that the ballistic energy- transfer part exhibits similarities with solitary pulses as studied by G Le ibfried and M Toda independently; they are collisionally stable and the pul se velocity is proportional to the square root of the tranferred energy. Th e ballistic current may thus be considered as a sequence of Leibfried-Toda (LT) solitons travelling through a dissipative medium. The current decrease s with the lattice temperature and with the distance from the heat source. It may, however, contribute to heat transfer even at distances roughly 150 lattice constants away from the excitation site. The ballistic, soliton-lik e propagation along close-packed directions is highly directional and hardl y compatible with the spherical symmetry of a Fourier heat current emanatin g from a point heat source. Radial and lateral anisotropy of the ballistic heat current is shown to be present during a time span of several picosecon ds. A simplified formula for the ballistic energy transfer is proposed. Fur thermore, we have proven that coherent many-atom excitation can be devised in such a way that the lifetime of the LT solitons is enhanced. The conditi ons to optimize solitary pulse stability are discussed.