Electronic structure of BaFe _2-x Co _x As ₂ revealed by angle-resolved photoemission spectroscopy

We report an angle-resolved photoemission spectroscopy study of BaFe _2-x Co _x As ₂. For x=0, above the structural and magnetic transition temperature (T _s ), the spectral weight near the Fermi level is considerably suppressed around the Γ point where the Fe 3d yz/zx orbital degeneracy is expected. This observation suggests that the Jahn-Teller type instability is playing an important role in the tetragonal phase above T _s . Below T _s, the spectral weight of 0-100 meV is reconstructed to form flat bands at 70-100 meV and Fermi surfaces, consistent with the orbital-dependent excitonic coupling. In the optimally doped and overdoped regimes, the hole pocket around the Γ point and the electron pocket around the M point are apparently nested, indicating that the doping dependence of the superconducting transition temperature cannot be explained by the nesting scenario and that the unusual electron-lattice fluctuation due to the orbital degeneracy is important.