In nodal-line semimetals, the gaps close along loops in >k space, which are not at high-symmetry points. Typical mechanisms for the emergence of nodal lines involve mirror symmetry and the π Berry phase. Here we show via ab initio calculations that fcc calcium (Ca), strontium (Sr) and ytterbium (Yb) have topological nodal lines with the π 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 π or 0, depending on the wavevector >k, and the π 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).
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机译：在节点线半金属中，间隙沿> k 空间中的环闭合，该环不在高对称点处。结线出现的典型机制包括镜像对称和πBerry相。在这里，我们通过从头算得出的结果表明，当忽略自旋轨道相互作用时，fcc钙（Ca），锶（Sr）和（Yb）的拓扑结线具有费米能级附近的πBerry相。特别地，Ca在高压下变成节点线半金属。由于结线，Zak相变为π或0，具体取决于波矢> k ，并且πZak相导致表面极化电荷。运营商最终对其进行了筛选，留下了巨大的表面偶极子。在具有结线的材料中，当存在重原子时，大的表面极化电荷和新兴的鼓面表面状态都会增强Rashba分裂，正如我们已经证明的那样，这会在Bi / Sr（111）和Bi / Ag（111）中发生。
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