Novel pressure-induced 2κ_F CDW state in organic low-dimensional compound θ-(BEDT-TTF)₂CsCo(SCN)₄

A band structure calculation based on the X-ray structure analysis reveals that pressure raises quasi one-dimensionality of the Fermi surface of the title compound. A 2κ_F lattice modulation is observed with a wave vector q′2 = (2/3 0 0.29) at 10.7 kbar and room temperature. This is the first observation of pressure-induced CDW. With cooling, the wave vector of the modulation changes from q′₃ to commensurate q₃ = (2/3 0 1/3) below 240 K. On the other hand, at ambient pressure and 20 K where the resistivity begins to increase rapidly, three-dimensional short range ordering occurs for the weak diffuse spot with a wave vector q₂ = (0 0 1/2). This modulation is ascribable to an occurrence of charge ordering possibly caused by the nearest neighbor Coulomb interaction V. A schematic phase diagram for the electronic state is presented.