Low-dimensional purple and blue bronzes: Crystal growth, characterization and optical properties

Abstract

The molybdenum oxide bronzes are a family of charge-density-wave (CDW) conductors with distinctive properties originating from their low-dimensional crystal structure and associated Fermi surface instabilities. Single crystals of potassium blue and purple bronzes and sodium purple bronze have been synthesized using a temperature flux technique and XRD measurement was deployed to investigate the formation of the correct phase. Optical spectroscopy and electrical resistivity measurements were carried out on sodium purple bronze and potassium blue bronze. Sodium purple bronze crystals were found to exhibit a clear anisotropy in their natural cleavage (a-b) plane. Electrical resistivity measurements were performed along the different crystallographic directions which displayed two distinct metallic axes perpendicular to each other in the a-b plane. The optical conductivity obtained from Kramers-Kronig analysis of reflectance spectroscopy data was in good agreement with the resistivity measurements. The two in-plane metallic axes undergo CDW transitions at two different temperatures Tp, suggesting a distinction in electronic and phononic subsystems along each axis.

Potassium blue bronze samples exhibit various temperature dependent anomalies in the electrical resistivity measurements, which are associated with CDW instability in this compound. The absolute values of resistivity at room temperature along both in-plane crystallographic axes, were compared to the literature to verify the accuracy of our measurement techniques. The ac-current effects on linear and non-linear regimes of resistivity were closely investigated, and the results support the sliding of the CDW, and the existence of a small-threshold field ET of CDW depinning in this sample. Hysteresis in resistivity, which has been detected in many CDW compounds, has been observed in our measurements and is associated with the instability of the CDW possibly due to the presence of defects.