Copper valence fluctuation in the organic conductor (dimethyl-N,N’-dicyanoquinonediimine)2Cu studied by x-ray photoemission spectroscopy

We have studied the electronic structure of the organic conductor (DMe-DCNQI${)}_2$Cu, where DMe-DCNQI is dimethyl-N,N’-dicyanoquinonediimine, by x-ray photoemission spectroscopy in order to elucidate the origin of the metal-insulator transitions and the anomalous magnetic properties of (DCNQI${)}_2$Cu salts. The line-shape analysis of the Cu 2p core-level spectrum shows a ${\mathrm{Cu}}^{+\mathrm{-}}$ ${\mathrm{Cu}}^{2+}$ mixed valency; the ratio of ${\mathrm{Cu}}^{+}$ to ${\mathrm{Cu}}^{2+}$ is found to be nearly 2:1. This suggests that Cu is in a valence-fluctuating state in the metallic (DMe-DCNQI${)}_2$Cu and that a ${\mathrm{Cu}}^{+\mathrm{-}}$ ${\mathrm{Cu}}^{2+}$ charge ordering coupled with cooperative lattice distortion takes place below the metal-insulator transition temperature in other (DCNQI${)}_2$Cu salts.