  We report high resolution Angle Resolved PhotoElectron Spectroscopy (ARPES)
results on the (001) cleavage surface of YbB$_{6}$, a rare-earth compound which
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 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, two new aspects are revealed in these
experiments. Firstly, these states do not resemble two branches of opposite,
linear velocity that cross at a Dirac point, but rather straightforward
parabolas which terminate to high binding energy with a clear band bottom.
Secondly, these states are sensitive to time-dependent changes of the YbB$_{6}$
surface under ultrahigh vacuum conditions. Adding the fact that these data from
cleaved 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 YbB$_{6}$(001) crystal truncation on the one
hand, and these ARPES data from real cleavage surfaces on the other.

Strongly Correlated Electrons;Materials Science