Topological Dirac Nodal Lines and Surface Charges in fcc Alkaline Earth Metals

TL;DR

This paper predicts topological nodal lines in fcc alkaline earth metals like Ca, Sr, and Yb, which lead to surface charges and enhanced surface states, with potential implications for surface polarization and Rashba effects.

Contribution

It demonstrates, through ab initio calculations, the existence of topological nodal lines in these metals and explores their effects on surface polarization and Rashba splitting.

Findings

Ca becomes a nodal-line semimetal at high pressure

Nodal lines induce $ ext{π}$ Zak phase and surface polarization charge

Surface states and large surface dipoles are enhanced by nodal lines

Abstract

In nodal-line semimetals, the gaps close along loops in ${\bf k}$ space, which are not at high-symmetry points. Typical mechanisms for the emergence of nodal lines involve mirror symmetry and the $\pi$ Berry phase. Here, we show via ab initio calculations that fcc calcium (Ca), strontium (Sr) and ytterbium (Yb) have topological nodal lines with the $\pi$ Berry phase near the Fermi level, when spin-orbit interaction is neglected. In particular, Ca becomes a nodal-line semimetal at high pressure. Owing to nodal lines, the Zak phase becomes either $\pi$ or 0 depending on the wavavector ${\bf k}$, and the $\pi$ Zak phase leads to surface polarization charge. Carriers eventually screen it, leaving behind large surface dipoles. In materials with nodal lines, both the large surface polarization charge and the emergent drumhead surface states enhance Rashba splitting when heavy adatoms are present, as we have shown to occur in Bi/Sr(111) and in Bi/Ag(111).