Ultrafast dynamics of strongly-coupled charge-density-wave systems

This thesis is devoted to the study of the non-equilibrium dynamics of two charge-density wave (CDW) materials in which the onset of the phase transition is determined by the presence of a peak in the momentum-dependent electron-phonon coupling (EPC). The existence of a strong EPC heavily affects the properties of the resulting CDW phase, determining in particular the nature of the thermal disordering. Given the strong interplay between the structural and the electronic degrees of freedom that characterizes the CDW systems, their out-of-equilibrium response has been investigated by combining time-resolved reflectivity and Time- and Angle-Resolved Photoelectron Spectroscopy (TR-ARPES) experiments. The combination of these techniques allows to access both the electronic band structure and the collective excitation of the CDW ground state. The first system investigated is the transition-metal dichalcogenide (TMDC) compound VTe2. Time-resolved reflectivity experiments have allowed to unveil the presence of two amplitude modes (AMs) of the low-temperature CDW phase. By implementing a double pump excitation scheme, we demonstrated that, despite being linked to the same electronic order parameter, these two collective excitations are independent. The study of the light-induced quench of the electronic order has revealed that the closure of the CDW gap is not driven by the two AMs of the system, but is instead dominated by an incoherent process. By applying a three-temperature model (3TM), we demonstrated that the quench of the CDW gap arises as a consequence of the excitation of a subset of strongly-coupled phonon modes which determines a loss of the long-range CDW order. These results therefore suggest that the phase transition is driven by non-CDW phonons that interact with the CDW order, thus highlighting the role played by the phonon-phonon interactions. The second system investigated is the kagome compound ScV6Sn6. Systematic time resolved reflectivity experiments, corroborated by DFT simulations, have allowed to unveil the AM of the system and to highlight the marked resilience of this compound to a near-infrared excitation. Furthermore, by applying a lattice strain to ScV6Sn6 crystals, we demonstrated that an elongation of the c-axis determines a reduction of the temperature-dependent softening of the CDW amplitude mode. DFT simulations have shown that this reduction of the AM softening arises from an enhancement of the CDW phase, that in the strained compound is characterized by a larger energy gap and larger lattice displacements. Our findings suggest that the light-induced phase transition in the strongly-coupled CDW materials VTe2 and ScV6Sn6 arises only thanks to increased lattice fluctuations, and thus, its understanding requires the modeling of the electron-phonon and phonon-phonon interactions. Due to the particular nature of the phase transition, the light-induced switching between the CDW and the normal phase turns out to be slower with respect to the one observed in compounds in which the transition is determined by purely electronic phenomena.