Lattice distortions in the gyrotropic semimetal 1T-TiSe2

1T-TiSe2 exhibits a circular photogalvanic effect, which implies the absence of any mirror planes, inversion centers, and roto-inversion axes. It is believed that a formation of a chiral charge density wave order underlies this effect. However, no corresponding lattice distortions have been found to date, leading to apparent violation of the Neumann's principle, which states that the symmetry of any physical property must include the symmetry elements of the point group of the lattice. A necessary condition to realize a chiral lattice is the condensation of phonon modes at both L and M points in the Brillouin zone. In this talk, we will discuss on our measurements of the phonon modes using Raman spectroscopy and inelastic x-ray scattering. Our results show that while there is no condensation of the M phonon mode, breaking of the three-fold roto-inversion is implied by the lifting of degeneracy of Eg phonon modes. We analyze possible lattice structures consistent with these observations.