Thermodynamics and transport properties in the transient regime

Abstract

The nonequilibrium behaviour in the transient regime of metals excited by ultrashort optical pulses is investigated by means of a second order expansion of the Boltzmann equation. By definition, the transition range is located between the time necessary for the establishment of the electron temperature and the time where a description by the standard steady state equations is justified. Relaxation functions are derived for the electrical and thermal currents, and the relaxation times related to them are determined. It is shown that for the electrical transport the relaxation time corre-sponds to Drude's momentum scattering time whereas the corresponding time for the heat flow is identified as the electron temperature relaxation time. Further, expressions for the electrical and thermal conductivity are obtained in the case of a local thermal nonequilibrium between the electron and phonon subsystems in first and second order, respectively. Consequences for the determination of the temperature distributions inside metals are discussed. The solution of the Boltzmann equation is also used for the calculation of the time dependent energy distribution function of the electrons. The results are in good agreement with the experiment.