Novel Pressure-Induced 2<i><i>k</i>_{<b><i>F</i></b>}</i>CDW State in Organic Low-Dimensional Compound<i>θ</i>-(BEDT-TTF)_<b>2</b>CsCo(SCN)_<b>4</b>

Abstract

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 2 k F lattice modulation is observed with a wave vector q 3 ′ =(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 q 3 =(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 q 2 =(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.