Preformed Excitons, Orbital Selectivity, and Charge-Density-Wave Order in 1T-TiSe_2

Abstract: Traditional routes to Charge-Density-Wave in transition metal dichalcogenides, relying on Fermi surface nesting or Jahn-Teller instabilities have recently been brought into question. While this calls for exploration of alternative views, paucity of theoretical guidance sustains lively controversy on the origin of, and interplay between CDW and superconductive orders in transition metal dichalcogenides. Here, we explore a preformed excitonic liquid route, heavily supplemented by modern correlated electronic structure calculations, to an excitonic-CDW order in 1T-TiSe$_{2}$. We show that orbital-selective dynamical localisation arising from preformed excitonic liquid correlations is somewhat reminiscent to states proposed for $d$- and $f$-band quantum criticality at the border of magnetism. Excellent quantitative explication of a wide range of spectral and transport responses in both normal and CDW phases provides strong support for our scenario, and suggests that soft excitonic liquid fluctuations mediate superconductivity in a broad class of transition metal dichalcogenides on the border of CDW. This brings the transition metal dichalcogenides closer to the bad actors in $d$- and $f$-band systems, where anomalously soft fluctuations of electronic origin are believed to mediate unconventional superconductivity on the border of magnetism.