Superradiant Thomson scattering from graphite in the extreme ultraviolet

We study the Thomson scattering from highly oriented pyrolitic graphite excited by the extreme ultraviolet, coherent pulses of FERMI free electron laser (FEL). An apparent nonlinear behavior is observed and fully described in terms of the coherent nature of both exciting FEL beam and scattered radiation, producing an intensity dependent enhancement of the Thomson scattering cross section. The process closely resembles the Dicke's superradiant phenomenon and also triggers the generation of coherent, low-\textit{q} ($<$ 0.3 \AA$^{-1}$), low energy phonons. The experimental data and analysis provide quantitative information on the sample characteristics, absorption, scattering factor and coherent phonon energies and populations, and open the route for the investigation of the deep nature of complex materials.