• Accepted Paper

Ferroaxial and nematic transitions in the charge density wave phase of 1T-TiSe2

Sarah P. Edwards, Elliott Rosenberg, Ilaria Maccari, Jiaqin Wen, Chaowei Hu, Xiaodong Xu, Jong-Woo Kim, Philip J. Ryan, Rafael M. Fernandes, Fernando de Juan, Maria N. Gastiasoro, and Jiun-Haw Chu

Phys. Rev. X - Accepted 20 October, 2026

DOI: https://doi.org/10.1103/x2k1-7dst

Abstract

Charge density waves (CDWs) with multi-component order parameters can break unexpected symmetries through the interplay of nearly degenerate instabilities. In the widely investigated material 1T-TiSe2, whether the observed CDW spontaneously breaks mirror and inversion symmetries to form a chiral state has remained controversial, with previous experiments reporting conflicting results. Here, we resolve this controversy by identifying the bulk broken symmetry as ferroaxial, corresponding to the breaking of vertical mirrors while preserving inversion symmetry. Symmetry-resolved elastoresistivity reveals the spontaneous emergence of off-diagonal coefficients that are forbidden in both the high-temperature phase and the widely accepted triple-Q CDW state, providing direct evidence that the CDW breaks mirror symmetries while preserving inversion below TCDW ~200 K. The diagonal coefficients reveal a divergent nematic susceptibility only at a much lower temperature (~165 K), establishing that nematic order is not responsible for the mirror symmetry breaking near TCDW. Simultaneous elastocaloric measurements resolve two successive transitions: a primary CDW onset and a secondary transition approximately 7 K lower, consistent with the condensation of a secondary CDW mode required to produce ferroaxial order. Together, our data identify a hierarchy of symmetry-breaking transitions: the primary CDW is followed by a ferroaxial instability, which in turn acts as the parent state for a secondary nematic instability. Our findings demonstrate that the proposed “chiral” CDW in 1T-TiSe2 is actually a centrosymmetric ferroaxial state, reconciling previous surface-sensitive observations with bulk symmetry constraints.

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