Origin of the chiral charge density wave in transition-metal dichalcogenide

Chirality refers to a structure that lacks mirror symmetry. It can be observed
in a wide range of platforms, from subatomic particles and molecules to
living organisms. However, the underlying mechanisms that give rise to
chirality in condensed matter systems have been a subject of considerable
interest. Here we elucidate the mechanism of chiral charge density wave
formation in the transition-metal dichalcogenide 1T-TiSe2 . Based on
symmetry analysis, we demonstrate that charge density modulations and
ionic displacements follow distinct irreducible representations of the space
group, despite exhibiting similar wave vectors and a strong coupling. This
charge-lattice symmetry frustration induces lattice distortions that further
break all symmetries that are not common to both sectors. This ultimately
gives rise to chirality. Our theory is verified using Raman spectroscopy and
inelastic X-ray scattering.