Novel electronic states in the (DCNQI)2Cu system

A series of molecular conductors (R1,R2-DCNQI)2Cu exhibit unique physical properties associated with the interaction between the pπ organic one-dimensional band and the d-orbital of Cu atom. The DCNQI-Cu salts are classified into three types: type I salt shows a metallic behavior down to low temperature, type II salt shows a metal-insulator (M-I) transition, and type III salt shows a M-I-M (reentrant) transition. All the types of transitions are first-order. In addition, the DI-system (R1=R2=I) shows another metallic phase accompanied by the small resistivity anomaly in the higher-pressure region. There are two important parameters that govern the electronic states of this system. The one is the amount of charge transfer from Cu to DCNQI which is quite sensitive to pressure or substituent (R1,R2). The other parameter is an inter-chain LUMO···LUMO interaction which plays an important role in the iodine-containing system. This interaction significantly affects the electronic structure. A complicated P-T phase diagram of the DI-system could be explained by these two parameters.