Probing photo-induced CDW melting in 1T-TiSe₂using elastic neutron scattering

Quasi-two-dimensional Mott insulating material 1T-TiSe₂ has long been investigated as it exhibits a wide range of exotic phenomena including a charge density wave (CDW), Bose-Einstein exciton condensation, a gyrotropic electronic order, and superconductivity in the vicinity of the CDW phase. It undergoes a structural phase transition at ~200K as it transforms into a commensurate CDW state with a periodic lattice distortion expanding the unit cell into a 2 × 2 × 2 superlattice that lowers the crystal symmetry from an undistorted P-3m1 symmetry to P-3c1 symmetry. Neutron diffraction experiments on 1T-TiSe₂ reveals half-integer superlattice peaks that can be indexed by a 2 x 2 x 2 periodic lattice distortion. Local structure analysis showed splitting of the Ti-Se bonds due to displacements of Ti in the CDW phase which suggested that a Jahn-Teller like mechanism is most likely key to the CDW instability and the Ti atom displacements can be described by a breathing mode model. Studies thus far have focused on the electronic properties, and little is known of the effects on the lattice and phonons in the presence of strong electron-phonon coupling. Given the strong coupling of the CDW to the lattice, we study the photo-excitation of the crystal leading to interesting structural characteristics that have not been directly probed so far. Using elastic and inelastic neutron scattering, we investigate the structural features and dynamics associated with the CDW transition under photo-excitation.