  This paper has been withdrawn by the authors due to new theoretical evidence
and experimental proof that the semiconducting bandgap reported in this paper
and ascribed to a surface region is in fact a bulk property of divalent
hexaborides. As reported in J. D. Denlinger et al., cond-mat/0107429, which
supercedes this paper, bulk-sensitive boron K-edge soft x-ray emission provides
a complementary confirmation of the X-point band gap identified by
angle-resolved photoemission.
