Insights from angle-resolved photoemission spectroscopy on the metallic states of YbB6(001): E(k) dispersion, temporal changes, and spatial variation

We report high-resolution angle-resolved photoelectron spectroscopy (ARPES) results on the (001) cleavage surface of YbB6, a rare-earth compound that has been recently predicted to host surface electronic states with topological character. We observe two types of well-resolved metallic states, whose Fermi contours encircle the time-reversal invariant momenta of the YbB6(001) surface Brillouin zone, and whose full E(k) dispersion relation can be measured wholly unmasked by states from the rest of the electronic structure. Although the two-dimensional character of these metallic states is confirmed by their lack of out-of-plane dispersion, our work reveals two aspects which were not observed in previous experiments. First, these states do not resemble two branches of opposite, linear velocity that cross at a Dirac point, but rather straightforward parabolas that terminate to high binding energy with a clear band bottom. Secondly, these states are sensitive to time-dependent changes of the YbB6 surface under ultrahigh-vacuum conditions. Adding the fact that these data from cleaved YbB6 surfaces also display spatial variations in the electronic structure, it appears there is little in common between the theoretical expectations for an idealized YbB6(001) crystal truncation on the one hand, and these ARPES data from real cleavage surfaces on the other