Ferroaxiality and Nematicity in 1T-TiSe₂ and Cu_xTiSe₂

The charge density wave (CDW) of the transition metal dichalcogenide 1T-TiSe₂ has attracted significant attention since its discovery in 1976. Despite many studies over its nearly 50 year history, questions remain about the CDW’s driving mechanism, its competition with superconductivity in intercalated forms of the compound, and exactly which symmetries it breaks. In past talks, we have highlighted measurements of single-crystal TiSe₂ and Cu_xTiSe₂ which display convincing evidence of electronic nematic behavior via the measurement of the anisotropic (E_g) elastoresistivity coefficient. In this talk, we discuss new evidence for ferroaxiality in the compound, a novel ferroic order which preserves the inversion symmetry of the lattice while breaking all the vertical mirror symmetries. This is contrary to contemporary claims of a chiral charge density wave which is not inversion-symmetric. Measurements of off-diagonal elements of the elastoresistivity tensor, elastocalorimetry, and X-ray diffraction under strain reveal a unified picture of multi-stage symmetry breaking in the Cu_xTiSe₂ family, forming a rich phase diagram which is tunable with both intercalation and strain.