Stability of charge density waves in the rare earth polytellurides.

Abstract

This work explores the stability and description of charge density waves (CDWs) in the layered rare earth polychalcogenides $RX\sb{n}$ (R = rare earth; X = S, Se, Te; n = 2, 2.5, 3). Two somewhat different CDW descriptions have evolved in the literature. In the first, a low-dimensional compound is susceptible to the CDW due to a combination of phonon softening and Fermi surface nesting, which can lead to a long-wavelength, incommensurate distortion with metallic states remaining at the Fermi level. A more localized viewpoint regards the CDW as a commensurate distortion in which covalent bonding leads to lowered atomic coordination and insulating behavior. We investigate the applicability of these two descriptions to planar square chalcogen lattices, experimentally realized in the $RX\sb{n}$. Magnetic susceptibility measurements of Sm and La polytellurides indicate trivalent Sm, with no local moment character in LaTe$\sb3$. Electrical resistivity measurements reveal a very large electrical anisotropy for Sm and La polytellurides. RTe$\sb3$ and $R\sb2$Te$\sb5$ are metallic, while SmTe$\sb{1.9}$ is an insulator. Through x-ray and electron diffraction we find that the RTe$\sb3$ host incommensurate small amplitude distortions. The distortion wavevectors are very close to the maximal Fermi surface nesting wavevectors determined by extended Huckel tight binding band calculations on two-dimensional square Te lattices. Although band energy calculations predict the CDW to be destabilized under pressure, we find the distortion persists across the rare earth series. Finally we compare the incommensurate CDWs of RTe$\sb3$ to the commensurate distortions reported previously for rare earth diselenides. We show that the theoretical square sheet Fermi surface consistently explains the CDWs of both families, under the variation of bandfilling. However, this information does not allow us to determine the importance of phonon softening (stabilizing the commensurate distortion) relative to Fermi surface nesting. To address this question we investigated LaTe$\sb{2-x}$Sb$\sb{x}$ for $0\leq x\leq1$. We show that this family of Sb doped compounds exhibits CDWs driven by Fermi surface nesting under the continuous variation of bandfilling.