C-13 NMR-STUDY OF THE METAL-INSULATOR-TRANSITION IN (DME-DCNQI)(2)CU SYSTEMS WITH PARTIAL DEUTERATION

The electronic states of-a series of (DMe-DCNQI)(2)Cu systems with non deuterated and partially deuterated methyl groups in a DMe-DCNQI molec ule have been investigated.by C-13 NMR measurements at the cyano carbo n sites. The Knight shift of the nondeuterated specimen, which is meta llic in the;whole temperature range, is not scaled-to the total spin s usceptibility, demonstrating that the metallic state has several elect ronic bands with different local spin susceptibility due to the pi-d h ybridization. In the insulating phases of the partially deuterated spe cimens, C-13 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 Cu rie-Weiss temperature dependence. The former line comes from the cyano group coordinated to the nonmagnetic Cu+ ion, while the latter comes from the cyano group coordinated to the magnetic Cu2+ ion. The-sign of the shift and intensity analysis provide an evidence that, in the ins ulating phase, spin is localized exclusively at one-third of the Cu si tes without any population on the DCNQI molecule. This is considered a s a manifestation of strong electron correlation in the Cu sites. The low-temperature reentrant metallic state in the partially deuterated s ystem 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 a nd insulator-metal reentrant transitions, showing that the transitions are of the first order. [S0163-1829(98)01628-2].