Evidence for pseudo–Jahn-Teller distortions in the charge density wave phase of 1T−TiSe2

The charge density wave (CDW) instability in $1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$ is revisited to investigate its electronic and structural origins using angle resolved photoemission spectroscopy (ARPES), neutron diffraction, and density functional theory (DFT) calculations. The evidence for Jahn-Teller (JT)-like distortions in the CDW state is provided from the local structure analysis that shows splitting of the Ti-Se bonds into short and long. The magnitude of the split is inconsistent with the commonly accepted Di Salvo model for the structural distortions in the CDW phase. From the APRES data, it is deduced that a nontrivial temperature-dependent energy shift of the Se $4p$ valence bands occurs, but is absent for the Ti $3d$ conduction bands. Collectively, these observations suggest that a JT-like mechanism is most likely central to the CDW instability in $1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$.