Illustrating the nature of ultrafast charge density wave dynamics in TiSe2: Interplay of exciton and electron-phonon coupling

The exact origin of charge density waves (CDW) in TiSe$_2$ has been long debated. Whether the excitonic pairing or the electron-phonon coupling (EPC) dominates the CDW formation remains ambiguous since the periodic lattice distortions (PLD) always accompany the CDW in the ground state. In this work, we conducted a complete separation of the CDW and the PLD in the ultrafast time domain by the real-time time-dependent density functional theory (rt-TDDFT) simulations. Laser pulses reduce the CDW order by introducing homogeneous bonding-antibonding electronic transitions. The reduction of the CDW order initializes the ionic movements against the direction of the original PLD. To reproduce the experimental spectra, the self-amplified EPC-assisted mechanism is essential to drive the subsequent dynamics. Thus, we propose that both the excitonic pairing and the EPC contribute to the CDW formation, albeit they act in different timescales: the CDW formation is initially driven by the excitonic pairing and subsequently enhanced by the EPC.