Mixed valency of Cu, electron-mass enhancement, and three-dimensional arrangement of magnetic sites in the organic conductors (R1,R2-N,N’-dicyanoquinonediimine)2Cu (where R1,R2=CH3,CH3O,Cl,Br)

Abstract

The unique molecular conductors with pπ-d mixing band structures (${\mathit{R}}_1$,${\mathit{R}}_{2\mathrm{-}}$N,N’-dicyanoquinonediimine${)}_2$Cu [(${\mathit{R}}_1$,${\mathit{R}}_{2\mathrm{-}}$DCNQI${)}_2$Cu] (${\mathit{R}}_1$,${\mathit{R}}_2$=${\mathrm{CH}}_3$,${\mathrm{CH}}_3$O,Cl,Br) were examined. General features of the phase diagram of the DCNQI-Cu system were depicted. A region that is related to the existence of anomalously heavy-metal electrons has been found. The ${\mathit{T}}^2$ dependence of the low-temperature resistivity of the alloyed system (${\mathrm{DMe}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{MeBr}}_{\mathit{x}\mathrm{-}}$DCNQI${)}_2$Cu (where Me=${\mathrm{CH}}_3$) suggests a large enhancement of the electron mass at the critical situation where the system begins to exhibit a characteristic metal instability. The mixed valency of Cu (${\mathrm{Cu}}^{+1.3\mathrm{}}$) in (DMe-DCNQI${)}_2$Cu was confirmed by ir experiments performed on neutral DMe-DCNQI crystals and (DMe-DCNQI${)}_2$M (M=Li, Ba, Cu).