Nonthermal effects in solids after swift heavy ion impact

This contribution is a brief introduction to nonthermal effects related to modifications of the interatomic potential upon ultrafast excitation of the electronic system of solids, primarily focusing on the swift heavy ion track problem. We clarify the difference between the exchange of the kinetic energy of electrons (and holes) scattering on the lattice (electron-phonon coupling, "thermal effects") and the relaxation of the nonequilibrium potential energy of a solid ("nonthermal effects"). We discuss that at different degrees of electronic excitation, the modification of the interatomic potential may result in various phase transitions without an increase of the atomic temperature, i.e., at room temperature (nonthermal melting, formation of the superionic state), or in atomic acceleration causing "nonthermal heating" of the target atoms. Examples of various theoretically pre-dicted effects are given, supported by known experimental observations.