Possible Dimensional Crossover in Li$_{0.9}$Mo$_6$O$_{17}$ observed through Thermal Expansion Experiments

During recent years much attention has been given to Li$_{0.9}$Mo$_6$O$_{17}$ which is believed to be the best-known example of a Luttinger-liquid (LL). In order to study the thermal expansion behavior of this compound prepared single crystals via flux growth using Li$_2$MoO$_4$, MoO$_2$ and MoO$_3$. The crystals were mechanically, chemically separated and characterized by x-ray and Laue diffraction, and electrical resistance. Thermal expansion measurements performed with a high-resolution quartz dilatometer revealed high anisotropy. Dominant a-axis thermal expansion reduces the separation of the conducting zigzag chains enhancing the interchain coupling. This is argued to destabilize the LL leading to a crossover from metallic to insulating-like behavior near 28 K and a repulsion-induced CDW. Competing electron-electron interactions support superconductivity at 1.9 K.