13CNMR study of the metal-insulator transition in(DMe−DCNQI)2Cusystems with partial deuteration

The electronic states of a series of ${\mathrm{}(\mathrm{D}\mathrm{M}\mathrm{e}-\mathrm{D}\mathrm{C}\mathrm{N}\mathrm{Q}\mathrm{I})\mathrm{}}_2\mathrm{Cu}$ systems with nondeuterated and partially deuterated methyl groups in a DMe-DCNQI molecule have been investigated by ${}^{13}\mathrm{C}$ NMR measurements at the cyano carbon sites. The Knight shift of the nondeuterated specimen, which is metallic in the whole temperature range, is not scaled to the total spin susceptibility, demonstrating that the metallic state has several electronic bands with different local spin susceptibility due to the $\pi -d$ hybridization. In the insulating phases of the partially deuterated specimens, ${}^{13}\mathrm{C}$ NMR spectra are split into two lines, one of which has a nearly zero shift and the other has a large positive shift with the Curie-Weiss temperature dependence. The former line comes from the cyano group coordinated to the nonmagnetic ${\mathrm{Cu}}^{+}$ ion, while the latter comes from the cyano group coordinated to the magnetic ${\mathrm{Cu}}^{2+}$ ion. The sign of the shift and intensity analysis provide an evidence that, in the insulating phase, spin is localized exclusively at one-third of the Cu sites without any population on the DCNQI molecule. This is considered as a manifestation of strong electron correlation in the Cu sites. The low-temperature reentrant metallic state in the partially deuterated system is found to be just the same as the stable metallic state in the nondeuterated system through the shift and relaxation rate behaviors. No fluctuations are observed in the vicinity of the metal-insulator and insulator-metal reentrant transitions, showing that the transitions are of the first order.