Suppression of the exciton Bose condensate in TiSe2Cux by metallic screening

The transition metal dichalcogenide semimetal 1T-TiSe2 exhibits a Bose condensate of excitons traditionally referred to as an “excitonic insulator” (though the material is not technically insulating) and highlighted as “excitonium” in the popular press. Using momentum-resolved electron energy-loss spectroscopy (M-EELS) experiments with meV energy resolution, we showed the experimental signature of this phase is a soft plasmon whose frequency falls to zero at the Bose condensation temperature. Here, we analyze the stability of this phase against electron doping by performing M-EELS experiments in intercalated TiSe2Cux. We find that the excitonic state is quickly suppressed, the soft plasmon effect disappearing for x > 1% which coincides with the semimetal-metal transition (suitably defined). Surprisingly, we find that the periodic lattice distortion accompanying the excitonic transition persists even when the condensate has vanished, evolving into a trivial structural phase transition. Our study shows that exciton condensation is quickly destabilized when the Coulomb interaction is suppressed by metallic screening.

Speaker: Peter Abbamonte, Univ of Illinois