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The electronic structures, charge and spin dynamics of the cobalt pnictidecompounds ACo2As2 (A=Ba, Sr, Ca) in the paramagnetic state are investigated byusing density functional theory combined with dynamical mean-field theory. Incontrast to their iron counterparts, these cobalt pnictide compounds havethree-dimensional electronic structures and strong ferromagnetic low-energyspin excitations. The Co 3d eg orbitals dominate the electronic states aroundthe Fermi level and have stronger electronic correlation strength than the Co3d t2g orbitals. The overall electronic correlation strength is much weakerthan that in the iron arsenides, however, the most strongly correlated Co 3dx2-y2 orbital, especially in CaCo2As2, has electronic correlation strengthcomparable to Fe 3d t2g orbitals in iron arsenides. ACo2As2 (A=Ba, Sr, Ca)shows similar electronic structures where a conduction band of primarily Co 3dx2-y2 orbital character is close to a Van Hove singularity around theBrillouin-zone corner, which promotes ferromagnetic low-energy spinexcitations. Originated from its increased nearest-neighbor Co-Co distance andsignificantly reduced As height from the Co plane, the strong electroniccorrelation strength and close proximity to the Van Hove singularity of the Co3d x2-y2 orbital in CaCo2As2 is responsible for its unique A-typeantiferromagnetic order observed in experiments. In comparison, despitesubstantial ferromagnetic low-energy spin excitations, BaCo2As2 and SrCo2As2remain paramagnetic down to very low temperature because the Co 3d x2-y2orbital has weaker electronic correlation strength and is further away from theVan Hove singularity.

Electronic structure and spin dynamics of ACo2As2 (A=Ba, Sr, Ca)