Dynamics of a Peierls system in a light field

Abstract

Equations describing the temporal dynamics of the order parameter ξ(t) of a metal-semiconductor phase transition and the density n(t) of electron-hole pairs in a Peierls system in a light field are obtained on the basis of the Lagrange equation for the phonon mode and the Liouville equation for the density matrix of the electronic subsystem. The equations obtained are analyzed for a stationary state (with adiabatically slow variation of the light intensity I) and for a transient process near the initial and final states of dynamic equilibrium (with the light field switched on abruptly). It is shown that for adiabatically slow growth of the intensity I up to a certain critical value Ic the band gap of the electronic spectrum decreases but the semiconductor phase of the Peierls system remains stable. For I>Ic the stationary semiconductor state (ξ≠0) becomes unstable. When the light is switched on abruptly, the deviation of the system parameters from the initial values is described by an exponential law with a characteristic reciprocal of the rise time of the process linearly dependent on the irradiation intensity I. As a new position of equilibrium is approached, three qualitatively different regimes of behavior of the order parameter ξ and density n are possible. For low intensities I(I Ic the stationary semiconductor state with ξ≠0 is absent. The experimental data on the irradiation of a vanadium dioxide film with a powerful laser pulse is interpreted on the basis of the theory developed.