We investigate theoretically the dynamics of nonequilibrium phonons arising in metal films under laser pulse excitation and the propagation process of phonons in a one-dimensional chain. Firstly, based on the generalized formalism of Boltzmann equation of nonequilibrium Green's function developed by Kadanoff and Baym, we derive the Boltzmann equation for phonons. The expressions for the phonon decay rates in various interaction processes are obtained from the equation. Numerical calculations are performed for copper and aluminum by using experimental parameters. The results show that, in our case, the thermalization of nonequilibrium phonons may delay in time to the interaction process of electron-phonon. These calculations are used to describe quantitatively the dynamical process of nonequilibrium phonons in metal films excited by laser pulse. Parts of our calculations are compared with experimental results and found to be in agreement. Then, according to the Heisenberg equation of motion, the equation of motion are obtained for the creation operator of a single collective mode which describes the superposition of phonons. Under certain conditions, an envelop solitary wave solution is derived from the equation. Thus, for the first time, we show theoretically that the interaction of phonons in a one-dimensional chain can lead to a solitary phonon packet propagation which is similar to the phenomenon of self-induced transparency in nonlinear optics. The conclusion can be used to explain the anomalously slow decay of the high-frequency phonon component observed in some experiments.
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