General decoupling procedure for expectation values of four-operator products in electron-phonon quantum kinetics (Q2855995)

The authors derive generalized decoupling procedures for the expectation values of products of electron and phonon creation and annihilation operators. The derived procedures are valid for arbitrary electron and phonon states. In electron-phonon quantum kinetics this general procedure can be used for decoupling the four-operator products which occur when the hierarchy of infinitely many coupled equations of motion for the operator product expectation values is terminated on the level of the equations for the expectation values of three-operator products. Electron-phonon quantum kinetic effects are important for studying the coherent ultrafast dynamics on time scales shorter than about a pico-second after exciting the electrons of a solid with a short laser pulse of duration smaller than 100 femtoseconds (fs). Examples for such experiments are pulse-excited semiconductors or the fs-demagnetization of ferromagnetic films after exposure to a strong optical laser pulse.