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 p\ensuremath{\pi}-d mixing band structures (${\mathit{R}}_{1}$,${\mathit{R}}_{2\mathrm{\ensuremath{-}}}$N,N'-dicyanoquinonediimine${)}_{2}$Cu [(${\mathit{R}}_{1}$,${\mathit{R}}_{2\mathrm{\ensuremath{-}}}$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{\ensuremath{-}}\mathit{x}}$${\mathrm{MeBr}}_{\mathit{x}\mathrm{\ensuremath{-}}}$DCNQI${)}_{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}$) in (DMe-DCNQI${)}_{2}$Cu was confirmed by ir experiments performed on neutral DMe-DCNQI crystals and (DMe-DCNQI${)}_{2}$M (M=Li, Ba, Cu).