Molecular conductors based on Pd(dmit)2 and related metal dithiolene complexes —A unique two-band system—

Physical properties and electronic structures of molecular conductors (cation) [Pd(dmit)2]2 (cation = Me4Z, Et2Me2Z; Z = N, P, As, and Sb) are described. The Pd(dmit)2 salts form a unique two-band system. The narrow and two-dimensional HOMO (anti-bonding) band is thought to form the half-filled conduction band. The insulating state under ambient pressure is considered to come from the strong electron-electron correlation. The one-dimensional LUMO (bonding) band is located immediately below the HOMO band. The application of pressure induces an overlap of these two bands and changes the band filling, which brings forth the metallic state. The system under higher pressure, however, shows non-metallic behavior probably due to the one-dimensional instability associated with the LUMO band. The pressure effect strongly depends on the cation size.