  The electronic structures, charge and spin dynamics of the cobalt pnictide
compounds ACo2As2 (A=Ba, Sr, Ca) in the paramagnetic state are investigated by
using density functional theory combined with dynamical mean-field theory. In
contrast to their iron counterparts, these cobalt pnictide compounds have
three-dimensional electronic structures and strong ferromagnetic low-energy
spin excitations. The Co 3d eg orbitals dominate the electronic states around
the Fermi level and have stronger electronic correlation strength than the Co
3d t2g orbitals. The overall electronic correlation strength is much weaker
than that in the iron arsenides, however, the most strongly correlated Co 3d
x2-y2 orbital, especially in CaCo2As2, has electronic correlation strength
comparable to Fe 3d t2g orbitals in iron arsenides. ACo2As2 (A=Ba, Sr, Ca)
shows similar electronic structures where a conduction band of primarily Co 3d
x2-y2 orbital character is close to a Van Hove singularity around the
Brillouin-zone corner, which promotes ferromagnetic low-energy spin
excitations. Originated from its increased nearest-neighbor Co-Co distance and
significantly reduced As height from the Co plane, the strong electronic
correlation strength and close proximity to the Van Hove singularity of the Co
3d x2-y2 orbital in CaCo2As2 is responsible for its unique A-type
antiferromagnetic order observed in experiments. In comparison, despite
substantial ferromagnetic low-energy spin excitations, BaCo2As2 and SrCo2As2
remain paramagnetic down to very low temperature because the Co 3d x2-y2
orbital has weaker electronic correlation strength and is further away from the
Van Hove singularity.
