Unraveling the Interplay of Dimensionality, Excitonic Correlations, Electron-Phonon Couplings, and Topological Defects

Despite being a prototypical charge-density-wave compound, the layered transition metal dichalcogenide, 1T-TiSe₂, has been the subject of intense study for its exotic properties both in and out of thermal equilibrium, such as the possibility of excitonic condensation, spontaneous gyrotropic ordering, as well as anomalous light-induced states such as potential energy gap opening and a metastable metallic phase. Central to this plethora of fascinating states in 1T-TiSe₂ is a unique combination of the dimensions of the crystal and the CDW, the strong excitonic correlations, the mode-selective electron-phonon coupling, and the susceptibility to topological defects formation. In this talk, I will discuss how we unravel the interplay of these different aspects via state-of-the-art attosecond transient extreme-ultraviolet (XUV) absorption spectroscopy and mega-electron-volt ultrafast electron diffraction (MeV-UED). I will show how photoexcitation leads to a 3D-to-2D dimension crossover in the CDW order parameter, a process dictated by the excitonic correlations in the system. The excitonic effect is further evidenced in the initial response of selected core-level absorption edges, and these observations help pinpoint the specific role of excitonic correlations in the CDW transition. I will also illustrate the hidden 1D nature of the CDW and its implication on the formation mechanism of domain-wall-like topological defects, which are found to emerge well under one picosecond following photoexcitation. Lastly, I will illustrate how coherent phonon spectroscopy in the XUV regime is a powerful way to uncover band-selective electron-phonon coupling strengths for the unoccupied electronic bands, which are not accessible by other probes but play an essential role in the nonequilibrium dynamics of 1T-TiSe₂ and other strongly-correlated materials.