Ultrafast charge carrier and exciton dynamics in an excitonic insulator probed by time-resolved photoemission spectroscopy

Charge density waves in two-dimensional transition metal dichalcogenides

“…This aligns with theoretical expectations that the exciton binding energy increases in 2D monolayers and is highly sensitive to the dielectric environment [133][134][135]. The experimental enhancement of the CDW instability with lower dimensions and reduced screening lends an interesting implication for an excitonic condensation scenario as the mechanism driving the CDW in 1T-TiSe 2 [61,120,121].…”

“…However, the origin of the CDW state in 1T-TiSe 2 is still under debate. While it is generally considered a prime candidate for an excitonic insulator with finite momentum transfer [4,32,61,[119][120][121], a clear-cut consensus has not been reached since the resultant CDW phase and PLD is hard to discern between the electronically driven excitonic insulator and the conventional CDW. The successful epitaxial growth of ML 1T-TiSe 2 has allowed the control of the dimensionality of this material [122][123][124][125], which inspires new experimental and theoretical investigations on the origin of the CDW transition [126][127][128][129].…”

“…The excitonic insulator is an intriguing condensed phase of matter where electron-hole pairs condense into a coherent macroscopic quantum state, analogous to BCS superconductivity [31,32]. However, an unequivocal material realization of excitonic insulators has remained elusive despite intense theoretical and experimental efforts on several candidate materials [119][120][121][217][218][219][220][221][222][223]. The materials family 1T-TiSe 2 , TiTe 2 , and ZrTe 2 share an ideal electron band structure that is known to be advantageous to host an exciton condensate state [31,32], with differences in the size of small band gap or band overlap.…”

Disentangling lattice and electronic instabilities in the excitonic insulator candidate Ta$_2$NiSe$_5$ by nonequilibrium spectroscopy

“…To investigate the origin of the SI transition in TNS, pump-probe spectroscopy is a promising technique because it can in principle, resolve the origin of the insulating gap using the inherently different timescales of the electronic and the lattice degrees of freedom [28][29][30]. For TNS, previous pump-probe experiments focused on the photoexcited dynamics of the insulating gap using time-resolved reflectivity [31][32][33][34][35] and Angle-Resolved Photo-Emission Spectroscopy (ARPES) [36][37][38][39][40][41][42][43]. However, these measurements have yielded conflicting results, and how the nonequilibrium dynamics of the insulating gap correlates with the monoclinic distortion has not been clarified.…”

“…We now discuss the possible origins of the observed metastable state. Within an excitonic insulator picture, the metastable state can be ascribed to the photoexcited carriers, which screen the Coulomb interaction and prevent the reformation of excitons as proposed in [36,43]. Another possibility is a structural change, distinct from the monoclinic one, which would significantly weaken the electronic instability responsible for the insulating gap.…”