NMR studies ofNbSe3: Electronic structures, static charge-density-wave measurements, and observations of the moving charge-density wave

We describe a series of ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$ NMR studies of the charge-density-wave (CDW) conductor ${\mathrm{NbSe}}_3$. Using an aligned, multicrystalline sample, we have measured the complete shift tensors for the three Nb sites in the normal state. The results confirm a picture in which one chain is essentially insulating, and the other two are conducting. Studies below the 144-K CDW transition give well-resolved CDW-broadened line shapes, which indicate that the CDW is incommensurate with little or no lock-in to the lattice. Further NMR measurements performed with the CDW driven by electrical currents give information about the moving CDW. Motional narrowing of the NMR line indicates that the CDW conduction phenomena are associated with bulk motion of the ${\mathrm{NbSe}}_3$ CDW. On the other hand, the motion is irregular at all voltages, as indicated by the large voltage required for motional narrowing, the short $T_2$, and the behavior of the magnetization under current-induced saturation. Furthermore, NMR spin-echo measurements of the low-voltage dielectric response indicate a spatially nonuniform response of the CDW under its pinning barriers.