\textbf{Wall-like spin excitations in A-type antiferromagnetic CaCo}$_{\mathrm{\mathbf{2}}}$\textbf{As}$_{\mathrm{\mathbf{2}}}$*

The $A$Co$_{\mathrm{2}}$As$_{\mathrm{2\thinspace }}(A =$ Ca, Sr, Ba ) compounds are structurally and chemically similar to $A$Fe$_{\mathrm{2}}$As$_{\mathrm{2\thinspace }}$and possess some interesting similarities and differences in their magnetism. We recently discovered that SrCo$_{\mathrm{2}}$As$_{\mathrm{2}}$ has stripe antiferromagnetic (AFM) spin correlations similar to stripe-ordered $A$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$. On the other hand, CaCo$_{\mathrm{2}}$As$_{\mathrm{2}}$ orders in an A-type AFM structure with ferromagnetic correlation of the spins in the square-lattice Co-layer and AFM correlations between layers. Despite the A-type order, our recent inelastic neutron scattering measurements show that spin excitations in CaCo$_{\mathrm{2}}$As$_{\mathrm{2}}$ are not associated with either the A-type or stripe-type order. Instead, we observe broad excitations that extend longitudinally (along (1,1,0) in reciprocal space), but remain sharply defined in the transverse direction. These excitations seem to be best characterized as a ``wall'' of scattering and suggest that CaCo$_{\mathrm{2}}$As$_{\mathrm{2}}$ has quasi-one-dimensional spin dynamics very different than in $A$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and SrCo$_{\mathrm{2}}$As$_{\mathrm{2}}$.