Incoherence-to-coherence crossover observed in charge-density-wave material 1T-TiSe.

Analogous to the condensation of Cooper pairs in superconductors, the Bose-Einstein condensation (BEC) of electron-hole pairs in semiconductors and semimetals leads to an emergence of an exotic ground state - the excitonic insulator state. In this paper, we study the electronic structure of 1T-TiSe utilizing angle-resolved photoemission spectroscopy and alkali-metal deposition. Alkali-metal adatoms are deposited in-situ on the sample surface, doping the system with electrons. The conduction bands of 1T-TiSe are thereby pushed down below the Fermi energy, which enables us to characterize its temperature dependence with precision. We found that the formation of the charge density wave (CDW) in 1T-TiSe at ~ 205 K is accompanied by a significant increase of the band gap, supporting the existence of excitonic pairing in the CDW state of 1T-TiSe. More importantly, by analyzing the linewidth of the single-particle excitation spectrum, we unveiled an incoherence-to-coherence crossover at 165 K, which could be attributed to a possible exciton condensation that occurs beneath the CDW transition in 1T-TiSe. Our results not only explain the exotic transport properties of 1T-TiSe, but also highlight the possible existence of an excitonic condensate in this semiconducting material.