Ferroelectric instability and topological crystalline insulating nature in PbPo

Abstract

We have investigated the lattice instability and the topological property of the electronic structure in PbPo in comparison with other IV-VI semiconductors, SnTe and PbTe. In the conventional exchange-correlation schemes of the density functional theory, the fcc structure of PbPo tends to be unstable in the presence of spin-orbit coupling (SOC) under [111] distortion so as to have a ferroelectric instability. This feature is revealed in the calculated phonon dispersion of PbPo by the phonon softening instability at k = Gamma. But, in the modified Becke-Johnson (mBJ) potential scheme, we have shown that the tendency for SOC-driven ferroelectric instability is suppressed, and the fcc structure becomes stabilized. We have demonstrated that PbPo is a semiconductor having a band inversion at k = L, which leads to a topological crystalline insulator (TCI) phase of PbPo, and the TCI and the ferroelectric states coexist for moderate [111] distortions.