Unexpected Fermi-surface nesting in the pnictide parent compounds BaFe2 As2 and CaFe2 As2 revealed by angle-resolved photoemission spectroscopy

We use angle-resolved photoemission spectroscopy to study the band structure of BaFe2 As2 and CaFe2 As2, two of the parent compounds of the iron arsenic high-temperature superconductors. Our high quality data reveals that although the Fermi surface is strongly three-dimensional, it does indeed have long parallel segments along the kz direction that can lead to the emergence of magnetic order. More interestingly, we find very unusual incommensurate nesting of the Fermi surface in the a-b plane that is present only at low temperatures. We speculate that this is a signature of a failed charge density wave state that was predicted by renormalization-group studies.