  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 theoretically predicted various effects are given, supported
by known experimental observations.
