  From a combination of high resolution angle-resolved photoemission
spectroscopy and density functional calculations, we show that BaFe2As2
possesses essentially two-dimensional electronic states, with a strong change
of orbital character of two of the Gamma-centered Fermi surfaces as a function
of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level
take on increasingly three-dimensional character. Both the orbital variation
with kz and the more three-dimensional nature of the doped compounds have
important consequences for the nesting conditions and thus possibly also for
the appearance of antiferromagnetic and superconducting phases.
