Discontinuous Change of Metal-Insulator Transition in the Alloyedθ-system:θ-(BEDT-TTF)2(Rb1-xCsx)Zn(SCN)4

Abstract

The behaviors of the metal-insulator (MI) transitions for θ-(BEDT-TTF) 2 (Rb 1- x Cs x )Zn(SCN) 4 [ x =0.00, 0.07, 0.25, 0.34, 0.47, 0.61, 1.00] were investigated by transport and magnetic measurements. Although the unit cell volume ( V ) increases and the dihedral angle between the donor columns (θ) decreases in proportion to the Cs content ( x ), the MI transition temperature does not change continuously. For x ≤0.07, the MI transitions occur around 200 K with a large hysteresis due to the structural modulation coupled with charge ordering. Magnetic behaviors in slow and rapid cooling runs are distinctly different. These transitions ( x ≤0.07) are called the Rb-type. The MI transition temperature at x =0.25 drops to 35 K abruptly and the transition temperature decreases slowly with increasing x further. The transport and magnetic behaviors show a reduced hysteresis and no hysteresis is observed at x =1. These features characterize the Cs-type transition (0.25≤ x ). These results indicate that the mechanism of the MI transitions for the isostructural M= Rb and Cs of θ-(BEDT-TTF) 2 MZn(SCN) 4 is different from each other, influenced by the structural modulation feature.