Symmetry protected band crossings of spin-orbit coupled surface states of NbGeSb

The electronic structure of materials in the ZrSiS family of nonsymmorphic semimetals hosts, alongside their bulk Dirac line nodes, peculiar surface states created by the breaking of nonsymmorphic symmetry at the surface layer [1-3]. We study the surface electronic structure of NbGeSb (isostructural to ZrSiS) using spin- and angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT). We observe two pairs of surface states, hosting large Rashba-like spin-splittings, split off from the bulk manifold. We find how these two branches intersect each other, forming a fourfold crossing structure along the Brillouin zone boundary. Surprisingly, we find a counter-intuitive hybridisation structure around these crossings, with three remaining protected and just a single hybridised anti-crossing. We demonstrate how a mirror symmetry line provides protection along the Brillouin zone face, opening new routes to realising complex intertwined spin textures of spin-orbit coupled surface states.
[1] Topp et al., PRX 7, 041073 (2017)
[2] Chen et al., PRB 95, 125126 (2017)
[3] Takane et al., PRB 94, 121108 (2016)