Coherent phonon generation mechanism in solids

We investigate the driving force for coherent phonons by employing a nonlinear two-pump/probe technique. In accordance with the displacive excitation mechanism we demonstrate that the electronic excitation is the driving force for the coherent phonons in semimetals. This is accomplished by analyzing the relation between the nonlinearities in the electronic and phononic contributions. On the other hand, no dependence of the coherent phonon dynamics on the electronic excitation was observed in high temperature superconductors (HTSC), although displacive excitation has been invoked for these materials. We discuss the possible implications of this observation on the microscopic nature of the coherent phonons in the HTSC.