Measured with meV-resolution EELS*

The physical mechanism behind the charge density wave (CDW) in 1T-TiSe₂ has been a subject of ongoing debate. Using momentum-resolved electron energy-loss spectroscopy (M-EELS), we observed a soft electronic collective mode in TiSe₂ at the CDW transition temperature, T_CDW = 185 K, demonstrating that the CDW results from exciton condensation. As TiSe₂ is doped with copper to form Cu_xTiSe₂, T_CDW steadily decreases until a doping of x ~ 0.04, above which T_CDW remains approximately constant and a superconducting dome emerges. At low doping values (x ≤ 0.02), the energy of the plasmon at q = 0 decreases as temperature decreases, but for high doping values (x = 0.07, 0.109) the plasmon energy increases as temperature decreases. We also find that for x ~ 0.004, the electronic mode softens partially, but not completely, at T_CDW. For x > 0.01, the electronic mode no longer softens at T_CDW, indicating that copper doping suppresses the exciton condensate in TiSe₂ and that the CDW in Cu_xTiSe₂ becomes a conventional, structural Peierls transition.