Structural dynamics of two-dimensional charge-density waves in CeTe ₃ investigated by ultrafast electron crystallography

The electron-phonon mechanism that gives rise to various charge-ordered systems is often controversial because of the cooperative nature of the transformation, and because the structural aspect of the transformation is generally poorly understood. Using femtosecond electron crystallography, we reveal a two-step (-400 fs and 3.3 ps) suppression of the structural order parameter of a two-dimensional charge-density wave (CDW) that clearly decouples from its electronic counterpart following fs optical quenching. Through atomic fluctuational analysis on Bragg reflections and satellite features, we identify important momentum-dependent electron-phonon couplings appearing on both time scales that may be related to interactions between the unidirectional CDW collective modes, the lattice phonons, and the perturbed electronic subsystem. We show that the characteristic time scales of these couplings and relative fluctuational amplitudes as characterized by fs crystallography jointly determine the cooperativity between the electronic and structural subsystems, and from this it is possible to elucidate the underlying mechanism of the charge-ordered system.