Mardi 24 mai 2022 à 15 h.
Sorbonne Université – INSP – 4 place Jussieu – 75005 Paris – Barre 22-32, 2e étage, pièce 201
Thomas Jaouen – Institut de Physique de Rennes
Layered metallic transition-metal dichalcogenides (TMDs) form the ideal platform for studying charge density wave (CDW) instabilities at the quasi-two-dimensional (2D) limit as well as their interplay with Mott states and superconductivity. The 2 x 2 x 2 commensurate CDW that occurs at ~200 K in the prominent TMD 1T-TiSe2 is the representative case of a many-body state with a strong entanglement of the electronic and structural parts of the order parameter. Thereby, the origin of the CDW still remains debated after many decades within either the purely electronic excitonic insulator scenario or the Peierls mechanism of electron-phonon coupling. Since few years a cooperative exciton-phonon mechanism has emerged as the most likely explanation of the CDW phenomenology in 1T-TiSe2. Yet, despite the increasing number of theoretical approaches of such a scenario, experimental studies going beyond putting excitons and phonons back to back are scarce and mainly focus on the ultrafast dynamics of the CDW state and its out-of-equilibrium physics. After a short introduction on the long-standing story of 1T-TiSe2, we will present in this seminar, an angle-resolved photoemission spectroscopy study of electron-doped 1T-TiSe2 surfaces as brought by low-temperature alkali metal atoms adsorption. We will show that such a thermodynamic phase control approach allows to induce a surface electron accumulation layer that triggers a pronounced band bending supporting a two-dimensional electron gas (2DEG) coexisting with the surface-confined CDW. The additional carrier-density control of the Coulomb interaction between electrons and holes in the surface quantum well (QW) allows us to selectively quench the excitonic correlations and to unveil the pure electron-phonon coupled CDW of 1T-TiSe2. Our study not only provides compelling evidences for a combined exciton-phonon CDW mechanism in 1T-TiSe2 in which electron-hole correlations boost a primarily structural instability, but also highlights how the electronic band structures of excitonic-related materials are affected by electrical tuning as reached by alkali-metal dosing.