Excitons (electron-hole bound states) have been 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 were able to demonstrate that the transition metal dichalcogenide 1T-TiSe₂ contains an exciton condensate. Its experimental signature is a soft electronic collective mode that disperses to zero energy near the charge density wave (CDW) transition temperature, T_c = 190 K, signaling the presence of a macroscopic condensate of electron-hole pairs. It has been known for many years that when TiSe₂ is doped with copper to form Cu_x2TiSe₂, 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 Tc for very low doping values (x < 0.004), but does not soften at any temperature for doping values above x ~ 0.01. Our results indicate that the exciton condensate is rapidly destroyed in Cu_x2TiSe₂ by screening from the additional Cu atoms and that a conventional Peierls phase CDW persists for doping values above x ~ 0.01. This work establishes M-EELS as a versatile technique for studying a new class of macroscopic quantum condensates in condensed matter.<.p>

Department of Physics and Astronomy Condensed Matter Seminar Series

Researchers should cite this work as follows:

• Melinda S. Rak (2019), "Suppression of Exciton Condensation in Copper-Doped TiSe2," https://nanohub.org/resources/30624.

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1. Bose-Einstein condensate
2. condensed matter
3. excitons
4. materials science
5. M-EELS
6. quantum materials