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Investigation of the magnetic dipole field at the atomic scale in quasi-one-dimensional paramagnetic conductor Li$_{0.9}$Mo$_{6}$O$_{17}$

Research paper by Guoqing Wu, Bing Wu, W. G. Clark

Indexed on: 02 Dec '14Published on: 02 Dec '14Published in: Physics - Strongly Correlated Electrons



Abstract

We report magnetic dipole field investigation at the atomic scale in a single crystal of quasi-one-dimensional (Q1D) paramagnetic conductor Li$_{0.9}$Mo$_{6}$O$_{17}$, using a paramagnetic electron model and $^{7}$Li-NMR spectroscopy measurements with an externally applied magnetic field $B_{0}$ = 9 T. We find that the magnetic dipole field component ($B_{||}^{\text{dip}}$) parallel to $B_{0}$ at the Li site from the Mo electrons has no lattice axial symmetry; it is small around the middle between the lattice $c$ and $a$ axes in the $ac$-plane with the minimum at the field orientation angle $\theta$ = +52.5$^{\circ}$, while the $B_{||}^{\text{dip}}$ maximum is at $\theta$ = +142.5$^{\circ}$ when $B_{0}$ is applied perpendicular to $b$ ($B_{0}$ $\perp$ $b$), where $\theta$ = 0$^{\circ}$ represents the direction of $B_{0}$ $\parallel$ $c$. Further estimate indicates that $B_{||}^{\text{dip}}$ has a maximum value of 0.35 G at $B_{0}$ = 9 T, and the Mo ions have a possible effective magnetic dipole moment 0.015 $\mu_{\text{B}}$ per ion, which is significantly smaller than that of a spin 1/2 free electron. By minimizing potential magnetic contributions to the NMR spectrum satellites with the NMR spectroscopy measurements at the direction where the value of the magnetic dipole field is the smallest, the behavior of the independent charge contributions is observed. This work demonstrates that the magnetic dipole field from the Mo electrons is the dominant source of the local magnetic fields at the Li site, and it suggests that the mysterious "metal-insulator" crossover at low temperatures is not a charge effect. The work also reveals valuable local field information for further NMR investigation which is suggested recently [Phys. Rev. B $\bf{85}$, 235128 (2012)] to be key important to the understanding of many mysterious properties of this Q1D material of particular interest.