Probing the anisotropic properties of low-dimensional purple bronze Na_0.9Mo₆O₁₇ in its natural cleavage plane

The low dimensional molybdenum oxide bronzes exhibit interesting behaviors including superconductivity, charge-density-wave (CDW) states associated with Fermi surface nesting, metal-insulator transitions, and non-linear transport due to sliding of the CDW at low temperature. There is still no firm consensus regarding a theory to explain their unique features. In the purple bronze family A_0.9Mo₆O₁₇ A=Li is quasi 1-D while A=Na,K are reported to be quasi-2D compounds. We have carefully investigated the low-dimensional properties of sodium purple bronze. Optical spectroscopy measurements showed a clear anisotropy in the crystal's natural cleavage (a-b) plane. The ac- and dc-resistivity measured along the different crystallographic directions showed two distinct metallic axes perpendicular to each other in the a-b plane. These two in-plane metallic axes undergo CDW transitions at different temperatures, which suggests a distinction in electronic and phononic subsystems along each axis. By investigating the optical and electrical properties of both in-plane axes, below and above the Peierls transition temperature, we can gain further insight into the nature of the CDW formation in this compound.