NMR studies of the internal electric field in a single crystal of the quasi-one-dimensional conductor Li$_{0.9}$Mo$_{6}$O$_{17}$

The quasi-one-dimensional (Q1D) conductor Li$_{0.9}$Mo$_{6}$O$_{17}$ is of considerable interest because it has a highly conducting phase with properties likely associated with a Luttinger liquid, a poorly understood ``metal-insulator'' crossover at temperature $T_{\mathrm{MI}}$ = 24 K, and a 3D superconducting phase that may involve triplet Cooper pairs at $T_{c}$ = 2.2 K, while the mechanism for many of its properties has been a long mystery and it presents tremendous experimental challenges. We report the $^{7}$Li-NMR measurements of the internal electric field with an externally applied magnetic field $B_{0}$ = 9 - 12 T, and we also show our theoretically calculated result of the electric field based on the structure of the crystal lattice. We find that the $^{7}$Li-NQR frequency ($\nu_{Q}$) has a value of $\sim$ 45 kHz and the electric field gradient (EFG) at the Li site due to the charges of the surrounding Mo conduction electrons has an axial symmetry with the principle axis ($p_{z}$) to be along the lattice $a$-axis. There is no temperature or field dependence for the value of $\nu_{Q}$ or EFG, indicating that the ``metal-insulator'' crossover has a magnetic origin, rather than the charge density wave (CDW) as one of the possible mechanisms previously thought in literature.