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

Abstract

The phase transition phenomenon in a semimetallic $1T\ensuremath{-}\mathrm{TiS}{\mathrm{e}}_{2}$ 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 $\mathrm{C}{\mathrm{u}}^{+}$ intercalation. Physical properties measurements and synchrotron x-ray diffraction experiments in $\mathrm{C}{\mathrm{u}}_{x}\mathrm{TiS}{\mathrm{e}}_{2}$ ($x=0--0.35$) 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 $\mathrm{C}{\mathrm{u}}^{+}$ ions between $\mathrm{TiS}{\mathrm{e}}_{2}$ layers at a specific doping amount ($x=1/3$). The behavior of $\mathrm{C}{\mathrm{u}}^{+}$ 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.