Temperature-dependent core-level x-ray photoemission spectroscopy of the organic conductors(MeCl−DCNQI)2Cu,(MeBr−DCNQI)2Cu,and(DI−DCNQI)2Cu

We study the quasi-one-dimensional (1D) organic conductors ${\mathrm{}(\mathrm{M}\mathrm{e}\mathrm{C}\mathrm{l}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu},$ ${\mathrm{}(\mathrm{M}\mathrm{e}\mathrm{B}\mathrm{r}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu},$ and ${\mathrm{}(\mathrm{D}\mathrm{I}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ using x-ray photoemission spectroscopy. We observe negligible change of the $\mathrm{Cu}2p$ core-level spectra of ${\mathrm{}(\mathrm{D}\mathrm{I}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ and ${\mathrm{}(\mathrm{M}\mathrm{e}\mathrm{B}\mathrm{r}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ as a function of temperature, in the metallic phase. In contrast, $\mathrm{Cu}2p$ spectra of ${\mathrm{}(\mathrm{M}\mathrm{e}\mathrm{C}\mathrm{l}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu},$ which has a charge-density-wave/metal-insulator transition at 210 K exhibits clear changes across the transition. The spectral shape change occurring across the transition indicates charge transfer from $\mathrm{Cu}3d$ states to the 1-D DCNQI $p\pi$ conduction band. A line-shape analysis shows that the average valence of Cu ions follows a systematic trend related to the tetrahedral angle α between N-Cu-N, i.e., a more distorted tetrahedral angle corresponds to lesser charge transfer from the Cu ions to the $p\pi$ band. The results also provide direct evidence for coupling of the electronic states involved in charge-ordering with the charge-density-wave formation.