Controlling Electron-Phonon Coupling with Ultrashort Core-Resonant X-ray Pulses

Ultrafast control strategies aim to steer materials into metastable states unreachable at equilibrium. In optics, this is pursued either by nonlinear phononics or by displacive electronic excitation, which launches coherent lattice motion through electron-phonon coupling. Extending this scheme to the X-ray domain promises achieving element-selectivity in the phonon generation process by tuning the pump pulses on-resonance with core electronic excitations.

In this seminar, I will present our recent implementation of core-resonant impulsive vibrational spectroscopy in trigonal tellurium performed at the FERMI free-electron laser in Trieste. By tuning the pump photon energy either off- or on-resonant with the tellurium N_4,5-edge, we demonstrate a resonant enhancement of the coherent phonon amplitude by more than an order of magnitude. Ab initio simulations show an excellent agreement with our experiment, underscoring the key contribution of band-specific electron phonon coupling terms and establishing a quantitative link between core excitation and lattice driving.

I will then outline our latest results in quantum materials, where switching the core-resonant excitation channel functionally modulates the phononic response. These findings chart a path to element-selective coherent control of lattice dynamics with ultrafast X-rays and open opportunities to manipulate symmetry and functionality in complex systems.