Suppression of the charge density wave instability inR2O2Bi(R= La, Er) due to large spin-orbit coupling

Abstract

To explore the origin of the suppression of the charge density wave (CDW) in ${\mathrm{Bi}}^{2\ensuremath{-}}$ square sheet of ${R}_{2}{\mathrm{O}}_{2}\mathrm{Bi}$ ($R$: rare-earth elements), we have investigated the band structures and phonon dispersions of ${\mathrm{La}}_{2}{\mathrm{O}}_{2}\mathrm{Bi}$ and ${\mathrm{Er}}_{2}{\mathrm{O}}_{2}\mathrm{Bi}$. We have found that the large spin-orbit coupling of Bi atoms together with the chemical pressure effect reduces the Fermi surface nesting, which results in the suppression of the CDW instability in ${R}_{2}{\mathrm{O}}_{2}\mathrm{Bi}$. The pressure effect has been checked for a similar compound, ${\mathrm{Er}}_{2}{\mathrm{O}}_{2}\mathrm{Sb}$, which shows a series of structural distortions in the Sb layer under volume contraction, from herringbone type, to 1D-ladder type, and to square sheet. Furthermore, we show that the observed local disorder in the Bi square sheet of ${\mathrm{La}}_{2}{\mathrm{O}}_{2}\mathrm{Bi}$ can be explained by the phonon softening anomaly, which is also expected to describe the anomalous resistivity upturn observed in ${\mathrm{La}}_{2}{\mathrm{O}}_{2}\mathrm{Bi}$ upon cooling.