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 a scalar order parameter could be explained in a model where the emergence of relative phase shifts between three charge density wave components breaks the inversion symmetry of the lattice. Here, we present experimental evidence for the sequence of phase transitions predicted by that theory, going from disorder to nonchiral and finally to chiral charge order. Employing x-ray diffraction, specific heat, and electrical transport measurements, we find that a novel phase transition occurs similar to 7 K below the main charge ordering transition in TiSe2, in agreement with the predicted hierarchy of charge-ordered phases.