Chiral-Nematic Lattice Instability in Charge Density Wave of 1T-TiSe2.

Chiral charge-density waves (CDWs) are a rare form of electronic order with handedness, enabling novel couplings among light, lattice, and topology in quantum materials. Yet the microscopic origin of chirality has remained elusive, even in the prototypical chiral CDW system 1T-TiSe2. Using ab initio calculations, we investigate the structural stability and electronic structure of competing CDW models in 1T-TiSe2. We show that a chiral monoclinic ( C 2 ) structure is stabilized by the cooperative condensation of multiple phonon modes and reproduces the chiral topography and energy gap observed experimentally. We further find an exceptionally soft potential landscape around this energy-minimized structure, along the direction that removes the remaining rotational symmetry, allowing the lattice to fluctuate through fully asymmetric configurations while its average structure remains chiral. This dynamical chiral CDW naturally reconciles the chiral domains seen by time-averaged topographic probes with the complete loss of symmetry detected by bulk spectroscopy, and clarifies how chiral CDWs emerge from the interplay of lattice instability and fluctuation.