High resolution angle-resolved photoemission measurements have been carried out on BaFe2As2, a parent compound of the FeAs-based superconductors. In the magnetic ordering state, there is no gap opening observed on the Fermi surface. Instead, dramatic
band structure reorganization occurs across the magnetic transition. The appearance of the singular Fermi spots near (pi,pi) is the most prominent signature of magnetic ordering. These observations provide direct evidence that the magnetic ordering state of BaFe2As2 is distinct from the conventional spin-density-wave state. They reflect the electronic complexity in this multiple-orbital system and necessity in involving the local magnetic moment in describing the underlying electron structure.
The design and performance of the first vacuum ultra-violet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of 6.994 eV and bandwidth of 0.26 meV is achieved from the second harmonic gen
eration using a novel non-linear optical crystal KBe2BO3F2 (KBBF). The new VUV laser-based ARPES system exhibits superior performance, including super-high energy resolution better than 1 meV, high momentum resolution, super-high photon flux and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high temperature superconductor. Issues and further development related to the VUV laser-based photoemission technique are discussed.
