Spin excitations in the A-type antiferromagnet CaCo$_{2}$As$_{2}$

CaCo$_{2}$As$_{2}$ is an A-type antiferromagnetic (AFM) metal with net ferromagnetic interactions between Co ions within the square-lattice layer and AFM interactions between layers. The material is isostructural to AFe$_{2}$As$_{2}$ (A=Ca, Sr, Ba) based high-temperature superconductors, although the stripe AFM magnetism is manifestly different from the nearly ferromagnetic CaCo$_{2}$As$_{2}$ compound. Surprisingly, AFM stripe spin correlations similar to those found in the iron arsenides are observed in the closely related paramagnetic SrCo$_{2}$As$_{2}$ system. We have studied spin excitations in CaCo$_{2}$As$_{2}$ using inelastic neutron scattering. While we find that low-energy Goldstone modes appear at the A-type AFM wavevector, broad and quasi-one-dimensional spin excitations spanning both A-type and stripe AFM wavevectors persist up to very high energies ($>$ 150 meV) and share many similarities to SrCo$_{2}$As$_{2}$. Combined with the small ordered moment ($<$ 0.5 $\mu_{B}$/Co) of CaCo$_{2}$As$_{2}$, the results suggest that the cobalt arsenides are itinerant magnetic systems where the net magnetic exchange interactions place them close to a quantum critical point separating stripe AFM and ferromagnetic ground states.