Chiral phase transition in charge ordered 1T-TiSe2

It was recently discovered that the low temperature, charge ordered phase of 1T-TiSe2 has a chiral character. This unexpected chirality in a system described by only a scalar order parameter could be explained in a model where the inversion symmetry of the lattice is broken by the emergence of relative phase shifts between three charge density wave components. Here, we present experimental verification of the phase transition predicted by that theory, between non-chiral and chiral charge order. Employing both X-ray diffraction and electrical transport measurements, we find that the novel chiral phase transition in 1T-TiSe2 occurs \sim7 K below its main charge ordering transition. We thus observe a hierarchy of charge ordered phases, leading from the high temperature normal state into the low temperature chiral state.