Abstract
We present recent results obtained using angle-resolved photoemission spectroscopy performed on 1T-TiSe2. Emphasis is put on the peculiarity of the bandstructure of TiSe2 compared to other transition metal dichalcogenides, which suggests that this system is an excellent candidate for the realization of the excitonic insulator phase. This exotic phase is discussed in relation to the BCS theory, and its spectroscopic signature is computed via a model adapted to the particular bandstructure of 1T-TiSe2. A comparison between photoemission intensity maps calculated with the spectral function derived for this model and experimental results is shown, giving strong support for the exciton condensate phase as the origin of the charge density wave transition observed in 1T-TiSe2. The temperature-dependent order parameter characterizing the exciton condensate phase is discussed, both on a theoretical and an experimental basis, as well as the chemical potential shift occurring in this system. Finally, the transport properties of 1T-TiSe2 are analyzed in the light of the photoemission results.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. There is a longstanding open question as to whether there is a condensate of electron-hole pairs (known as excitons), as there is for pairs of electrons—Cooper-pairs—in Bardeen–Cooper–Schrieffer (BCS) superconductivity. 1T-TiSe2 exhibits an unusual temperature-dependence for transport experiments, the signature of a phase transition around 200 K and a specific band configuration that has been related to the possible formation of excitons.
Main results. The band structure of the transition metal dichalcogenide (TMDC) 1T-TiSe2 near its Fermi energy (evidenced by photoemission) is such that this system is very susceptible to electronic instabilities, in comparison to other isostructural TMDCs such as 1T-TiS2 or 1T-TiTe2 (see figure). In this paper, we support the exciton condensate phase model as the driving force for the phase transition in 1T-TiSe2, by comparing angle-resolved photoemission spectroscopy data with photoemission intensity maps calculated with the spectral function obtained within this model. The exciton condensate phase model is introduced in comparison to the BCS model. Its order parameter is extracted from photoemission data, as well as a resulting chemical potential shift. Finally, the transport data of 1T-TiSe2 are discussed in the light of these results.
Wider implications. At the moment, no system has been clearly demonstrated to be the realization of an exciton condensate phase. 1T-TiSe2 turns out to be an excellent candidate for such an exotic phase. Direct evidence proving unambiguously the presence of the exciton condensate is still lacking, and the search for such evidence is of great interest.
Figure. Schematic electronic configuration close to the Fermi energy of typical TMDCs. Hole-like and electron-like bands are of chalcogen p character and transition metal d character, respectively.