Does β-PbO harbor topological states?

The electronic properties of β-PbO, have been controversial for several decades. Experiments find behavior ranging from metallic, attributed to oxygen vacancies, to indirect semiconducting for stoichiometric samples with a gap of 0.61 eV. Theory leads to similar ambiguities, and predicts this phase to be metallic (PBE, HSE06) or to possess a small bandgap (HSE06). An area where this inconsistency is amplified, is when a material behavior depends on the electronic structure in the vicinity of the Fermi energy, such as topological states. In our work, we use a self-consistent DFT + U approach and find that stoichiometric β-PbO to be an indirect semiconductor with a band gap of ∼0.8 eV, similar to experiment. The larger bandgap requires at least ∼4% strain, to drive β-PbO into a nodal line semimetallic state, significantly larger strains than reported previously. Moreover, we find that the nodal line semimetallic state is not protected against spin-orbit-coupling. Also, the surface computations do not show any evidence for topologically protected states near the Fermi energy. Therefore, our results strongly suggest that β-PbO is a topologically trivial material, consistent with experiment, but in stark contrast to previous computations. Previously reported topologically protected states in β-PbO are attributed to an inaccurate description of the (bulk) optical properties.