Unexpected stabilization of a single–q charge density wave in pristine 1T–TiSe₂

The transition metal dichalcogenide 1T–TiSe₂ hosts a triple-q charge density wave (CDW) phase, forming a 2 × 2 × 2 superlattice near T_CDW ~ 200 K. Due to its simple structure, the material has been extensively studied as a model system for CDW physics for decades. However, the origin of its CDW still remains a topic of debate, with proposals including excitonic interactions, electron-phonon coupling, or a combination of both. Adding to the complexity of the CDW landscape, there have been reports of broken threefold rotational symmetry in the CDW phase where the three q-vectors are not equivalent, often referred to as a chiral CDW phase. The direct observation of this phase has been challenging, and the nature of the chiral CDW phase remains elusive.

Here, we present a direct observation of the chiral electronic structure in the CDW phase of 1T–TiSe₂. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal that the three q-vectors in the CDW phase are inequivalent, with distinct spectral weight and onset temperature along different high-symmetry directions in the Brillouin zone. This sequential onset of the CDW order combined with our Landau-Ginzburg theory suggests that the system undergoes a sequential phase transition from P-3m1 to P2 and finally to P1, instead of a single transition to a P-3c1 structure as previously accepted. It also reconciles the apparent discrepancy between previous reports of a simultaneous onset of chirality with the CDW order and a distinct onset temperature.