Measured with M-EELS*

Excitons were predicted to form a Bose condensate, but for many years there was no direct experimental verification of this condensation in a solid. Using momentum-resolved electron energy-loss spectroscopy (M-EELS), we demonstrated the presence of an exciton condensate in 1T-TiSe₂ by observing a soft electronic collective mode at the charge density wave (CDW) transition temperature, T_C = 190 K. As TiSe₂ is doped with copper to form Cu_xTiSe₂, the CDW transition temperature is suppressed and a superconducting dome emerges around x = 0.04. Using M-EELS, we found that the electronic collective mode softens partially near T_C for very low dopings (x < 0.004), but does not soften above x ~ 0.01. Our results indicate that the exciton condensate is rapidly destroyed in Cu_xTiSe₂ by screening from the Cu atoms and that a conventional Peierls CDW persists above x ~ 0.01. We conclude that the excitonic state is not directly related to the superconductivity in Cu_xTiSe₂.