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.

Energy-Dependent Enhancement of the Electron-Coupling Spectrum of the Underdoped Bi2Sr2CaCu2O8+d Superconductor

We have determined the electron-coupling spectrum of superconducting Bi2Sr2CaCu2O8+d from high-resolution angle-resolved photoemission spectra by two deconvolution-free robust methods. As hole concentration decreases, the coupling spectral weight at low energies ~<15 meV shows a twofold and nearly band-independent enhancement, while that around ~65 meV increases moderately, and that in ~>130 meV decreases leading to a crossover of dominant coupling excitation between them. Our results suggest the competition among multiple screening effects, and provide important clues to the source of sufficiently strong low-energy coupling, {lambda}LE ~ 1, in underdoped system.