Quantum oscillations in the parent pnictide BaFe2As2: Itinerant electrons in the reconstructed state

We report quantum-oscillation measurements that enable the direct observation of the Fermi surface of the low-temperature ground state of BaFe2As2. From these measurements we characterize the low-energy excitations, revealing that the Fermi surface is reconstructed in the antiferromagnetic state, but leaving itinerant electrons in its wake. The present measurements are consistent with a conventional band folding picture of the antiferromagnetic ground state, placing important limits on the topology and size of the Fermi surface.