Honeycomb lattice type charge density wave associated with interlayer Cu ions ordering in 1T-CuxTiSe2

The phase transition phenomenon in a semimetallic 1T-TiSe2. has attracted attention as an excitonic insulator. However, as the phase transition accompanying superlattice peaks has the q vector connecting the Fermi surfaces of the three-dimensional shape of hole and electron pockets, it also assumes the charge density wave (CDW) state owing to the electron-phonon interaction. To understand the electronic state at the low temperature, control of the chemical potential was attempted by electronic doping through Cu+ intercalation. Physical properties measurements and synchrotron x-ray diffraction experiments in CuxTiSe2 (x = 0-0.35) were performed. The phase transition was determined to occur as a cooperative phenomenon between the honeycomb lattice type CDW corresponding to the nesting vector and the ordered state of the Cu+ ions between TiSe2 layers at a specific doping amount (x = 1/3). The behavior of Cu+ ions in highly doped regions suggests the occurrence of a two-dimensional liquid-solid state transition based on the temperature dependence of the x-ray diffuse scattering.