Fermi-surface reconstruction involving two Van Hove singularities across   the antiferromagnetic transition in BaFe2As2
  We report an angle-resolved photoemission study of BaFe2As2, a parent compound of iron-based superconductors. Low-energy tunable excitation photons have allowed the first observation of a saddle-point singularity at the Z point, as well as the Gamma point. With antiferromagnetic ordering, both of these two van Hove singularities come down below the Fermi energy, leading to a topological change in the innermost Fermi surface around the kz axis from cylindrical to tear-shaped, as expected from first-principles calculation. These singularities may provide an additional instability for the Fermi surface of the superconductors derived from BaFe2As2. 
Y. Nakashima, A. Ino, S. Nagato, H. Anzai, H. Iwasawa, Y. Utsumi, H.   Sato, M. Arita, H. Namatame, M. Taniguchi, T. Oguchi,...
cond-mat.supr-con