Revealing the role of electrons and phonons in the ultrafast recovery of charge density wave correlations in 1T−TiSe2

Using time- and angle-resolved photoemission spectroscopy with selective near- and midinfrared photon excitations, we investigate the femtosecond dynamics of the charge density wave (CDW) phase in $1T\text{−}{\mathrm{TiSe}}_2$ as well as the dynamics of CDW fluctuations at 240 K. In the CDW phase, we observe the coherent oscillation of the CDW amplitude mode. At 240 K, we single out an ultrafast component in the recovery of the CDW correlations, which we explain as the manifestation of electron-hole correlations. Our momentum-resolved study of femtosecond electron dynamics supports a mechanism for the CDW phase resulting from the cooperation between the interband Coulomb interaction, the mechanism of excitonic insulator phase formation, and electron-phonon coupling.