Insights from angle-resolved photoemission spectroscopy on the metallic states ofYbB6(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 ${\mathrm{YbB}}_{6}$, 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 ${\mathrm{YbB}}_{6}(001)$ 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 ${\mathrm{YbB}}_{6}$ surface under ultrahigh-vacuum conditions. Adding the fact that these data from cleaved ${\mathrm{YbB}}_{6}$ surfaces also display spatial variations in the electronic structure, it appears there is little in common between the theoretical expectations for an idealized ${\mathrm{YbB}}_{6}(001)$ crystal truncation on the one hand, and these ARPES data from real cleavage surfaces on the other.