Investigation of the π plasmon and plasmon-exciton coupling in titanium diselenide (TiSe₂) by momentum-resolved electron energy loss spectroscopy

In this work, π-plasmon dispersion and plasmon-exciton coupling in single-crystal 1T-TiSe₂ along the Γ M direction were investigated by using momentum (q)-resolved electron energy loss spectroscopy (EELS), ω-q map. Both the π plasmon at 7 eV and the π + σ plasmon at ∼19.7 eV were observed in the EELS spectrum. Furthermore, the π plasmon exhibits an unexpected dispersion behavior that transitions from a square root of q dependence (q^1/2) to a quadratic dependence (q²) with increasing q values. A low energy excitation at ∼2.3 eV was also observed, which can be attributed to the plasmon-exciton coupling excitation mode (or plexciton) with a linear positive dispersion turning to negative dispersion by following the band structure along the Γ M direction.