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 significant ballistic contribution to heat transfer is observed which prevents the system from showing the features of normal heat conduction, i.e. the existence of finite temperature gradients and the requirement that heat conductivity be an intensive quantity. It is shown that the ballistic energy-transfer part exhibits similarities with solitary pulses as studied by G Leibfried and M Toda independently; they are collisionally stable and the pulse velocity is proportional to the square root of the tranferred energy. The ballistic current may thus be considered as a sequence of Leibfried-Toda (LT) solitons travelling through a dissipative medium. The current decreases 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-like propagation along close-packed directions is highly directional and hardly compatible with the spherical symmetry of a Fourier heat current emanating 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 picoseconds. A simplified formula for the ballistic energy transfer is proposed. Furthermore, 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 conditions to optimize solitary pulse stability are discussed.