Control of the electronic state in a series of Pd(dmit)2 salts, a strongly correlated electron system with a quasi-triangular lattice structure

Pressure effects on β′-type Pd(dmit)2 anion radical salts, which belong to a strongly correlated electron system with a quasi-triangular lattice of dimerized Pd(dmit)2 units, were studied with X-ray crystal structure analysis and resistivity measurements. The electronic state of this system is governed by various parameters including the effective on-site Coulomb energy on the dimer (Ueff.), the band width (W), the degree of frustration, and the degree of band-filling. X-ray crystal structure analysis under hydrostatic pressure at low temperatures was performed for the Et2Me2P salt The tight-binding band calculations based on structural data indicate that the application of hydrostatic pressure reduces the electron correlation effect and leads the system from the Mott insulating state to the metallic state accompanied by the superconductivity. Further application of hydrostatic pressure induces another non-metallic behavior. This would originate from a partial nesting of the anisotropic Fermi surface associated with a structural transition which removes the glide plane and the two-fold axis. Based on cation dependence in the pressure effect, we propose a possible interplay of the correlation and the frustration, and discuss the uniaxial strain effect for this unique strongly correlated electron system.