Studies on phonon instability and charge density wave transition in layered systems including pnictogen

Abstract

In this thesis, we have investigated the charge density wave (CDW) transition in R2O2Bi and R2O2Sb (R = La, Er) by using ab initio density functional theory (DFT). To explore the electronic and structural instabilities, we have calculated susceptibility and the phonon dispersion using frozen phonon method. The spin-orbit coupling (SOC) was also considered to investigate its SOC effect on the CDW of R2O2Bi. We have also studied the suppression of the CDW by the chemical pressure variation in R2O2Sb.

To investigate the structural and electronic properties of R2O2Sb, total energy band structures, susceptibility and phonon dispersion are obtained for La2O2Sb and Er2O2Sb. We have found the electronic instability within Sb square sheet through three dominant Fermi surface nesting vectors and peaks of susceptibility at q1 =0.25a*, q2 = 0.5a*+0.25b*, and q3 = 0.25a*+0.25b*. Also, we showed that phonon softening instabilities occur at the same q vectors. These results suggest that the lattice instability in Sb layers is driven by the CDW instability through the electron-phonon coupling. We also found that the most stable structure of R2O2Sb is a herringbone-type arrangement of Sb dimers, which is the linearly superposed mode of two degenerate normal modes at q3. In addition, the CDW-induced structure shows the insulating behavior, which explain existing experiments.

To study the the origin of the stable Bi2- square sheets in R2O2Bi (R = La, Er), the spin-orbit coupling (SOC), total energy band structures, susceptibility and phonon dispersion are investigated. We have found that the large SOC of Bi atoms and the chemical pressure effect of rare-earth atoms reduce the Fermi surface nesting, which triggers the the suppression of the CDW in R2O2Bi.

To see the pressure effect,we have performed the phonon dispersion of Er2O2Sb, which shows that the Sb layers are distorted from herring-bone type, to 1D-ladder type, and to square sheet under volume contraction. In addition, we showed that the upturn of the resistivity in La2O2Bi with decreasing temperature can be explained by the phonon-softening anomaly.