  Like high Tc cuprates, the newly discovered iron based superconductors lie in
close proximity to a magnetically ordered parent phase. However, while the
magnetic order in parent cuprates is known to derive from a spin-spin local
superexchange interaction, a plethora of experiments including neutron
scattering have so far been unable to conclusively resolve whether a local
moment Heisenberg description applies in parent iron based compounds, or
whether magnetism arises from a collective SDW order instability. These two
alternatives can in principle be distinguished by measuring the low energy
momentum-resolved bulk-representative electronic structure of the magnetically
ordered phase. Using a combination of polarization dependent ARPES and STM, we
have isolated the complete low-lying bulk representative electronic structure
of magnetic SrFe2As2 with d-orbital symmetry specificity for the first time.
Our results show that while multiple bands with different iron d-orbital
character indeed contribute to charge transport, only one pair of bands with
opposite mirror symmetries microscopically exhibit an itinerant SDW instability
with energy scales on the order of 50 meV. The orbital resolved band topology
below T_SDW point uniquely to a nesting driven band hybridization mechanism of
the observed antiferromagnetism in the iron pnictides, and is consistent with
an unusual anisotropic nodal-density-wave state. In addition, these results
place strong constraints on many theories of pnictide superconductivity that
require a strict local moment magnetism starting point.
