Angle-resolved photoemission spectroscopy study of BaCo2As2

We use angle-resolved photoemission spectroscopy and full-potential linearized augmented-plane-wave (FP-LAPW) calculations to study the electronic structure of BaCo2As2. The Fermi surface (FS) maps and the corresponding band dispersion data (at 90 and 200 K) reveal a small electron pocket at the center and a large electron pocket at the corner of the Brillouin zone. Therefore the nesting between electron and hole FS pockets is absent in this compound, in contrast to the parent compounds of FeAs-based high-T-c superconductors. The electron pockets at the center of the zone are surrounded by two sets of four smaller electron pockets. The electronic structure at about 500 meV binding energy is very similar to features at the chemical potential in BaFe2As2. This indicates that complete substitution of Co for Fe causes a nearly rigid shift in the chemical potential by adding two electrons per formula unit at higher binding energies. However at lower binding energies similar to 270 meV, the electron pocket at the center of the zone is absent, unlike in the Co-substituted Fe-based materials. This demonstrates that the rigid band picture is valid only at higher binding energies and breaks down closer to the chemical potential in BaCo2As2. We also observed the presence of a flat band near the Fermi energy that may have consequences for transport and thermodynamical properties. The experimental FS topology as well as band dispersion data are in reasonable agreement with the FP-LAPW calculations.