The mechanism driving the charge density wave (CDW) transition in ${\mathrm{TiSe}}_2$ has been debated for decades, with proposals ranging from an excitonic insulator to a lattice-driven instability. A central question remains whether the transition involves an opening or enhancement of the bulk band gap. Using high-resolution angle-resolved photoemission spectroscopy, we directly track the temperature evolution of the bulk band edges across the CDW transition at $T_{\text{CDW}}\approx 200\mathrm{K}$. Contrary to the expectation of a gap-opening transition, we find that the size of the fundamental band gap remains constant from the high-temperature normal phase down to 160 K. While the CDW induces clear band-folding signatures and spectral weight redistribution, the underlying band extrema are unperturbed. These results demonstrate that ${\mathrm{TiSe}}_2$ does not undergo a temperature-driven electronic gap opening. Instead, they support a scenario where the transition is governed by a lattice symmetry-breaking reconstruction that folds, but does not gap, the electronic structure of a pre-existing band insulator.

Close

PrevNext

View figure in article

• Charge density waves
• Excitonic insulators
• Transition metal dichalcogenides
• Angle-resolved photoemission spectroscopy