Observation of anisotropy in the electronic states ofN,N′−dicyanoquinonediimine−Cusalt by x-ray-absorption spectroscopy

Abstract

We have studied the unoccupied electronic states of an organic quasi-one-dimensional conductor partially deuterated ${(\mathrm{D}\mathrm{M}\mathrm{e}\ensuremath{-}\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I}\ensuremath{-}\mathrm{d}}_{7}{)}_{2}\mathrm{Cu}$, where DMe is ${(\mathrm{C}\mathrm{H}}_{3}{)}_{2}$ and DCNQI denotes ${N,N}^{\ensuremath{'}}\ensuremath{-}\mathrm{d}\mathrm{i}\mathrm{c}\mathrm{y}\mathrm{a}\mathrm{n}\mathrm{o}\mathrm{q}\mathrm{u}\mathrm{i}\mathrm{n}\mathrm{o}\mathrm{n}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{i}\mathrm{m}\mathrm{i}\mathrm{n}\mathrm{e}$, by linearly polarized x-ray-absorption spectroscopy in the metallic and insulating phases. The N $1s$ spectra have clearly shown a strong anisotropy in the lowest unoccupied molecular orbital band, indicating the one-dimensional character of the $p\ensuremath{\pi}$ conduction band. From the Cu $2p$ spectra, we have confirmed the mixed valence state of Cu. The polarization dependence of the Cu $2p$ spectra suggests a significant mixture of ${d}_{\mathrm{zx}}$ and ${d}_{\mathrm{yz}}$ character into the predominant ${d}_{\mathrm{xy}}$ character for the Cu $3d$ holes. The Cu $2p$ spectra show a slight difference between the metallic and insulating phases, indicating that the occupation of the Cu $3d$ orbitals increases in going from the insulating to the metallic phase.