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 Bi2- 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.