Suppression of the charge density wave instability in R2O2Bi (R = La, Er) due to large spin-orbit coupling

To explore the origin of the suppression of the charge density wave (CDW) in Bi^{2 -} square sheet of R₂O₂Bi (R : rare-earth elements), we have investigated the band structures and phonon dispersions of La₂O₂Bi and Er₂O₂Bi . 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 R₂O₂Bi . The pressure effect has been checked for a similar compound, Er₂O₂Sb , 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 La₂O₂Bi can be explained by the phonon softening anomaly, which is also expected to describe the anomalous resistivity upturn observed in La₂O₂Bi upon cooling.