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We present a comprehensive study of the low-energy band structure and Fermisurface (FS) topology of $A$Co$_2$As$_2$ ($A=$ Ca, Sr, Ba, Eu) usinghigh-resolution angle-resolved photoemission spectroscopy. The experimental FStopology and band dispersion data are compared with theoretical full-potentiallinearized augmented-plane-wave (FP-LAPW) calculations, which yieldedreasonably good agreement. We demonstrate that the FS maps of $A$Co$_2$As$_2$are significantly different from those of the parent compounds of Fe-basedhigh-temperature superconductors. Further, the FSs of CaCo$_2$As$_2$ do notshow significant changes across its antiferromagnetic transition temperature.The band dispersions extracted in different momentum $(k_{\it x}, k_{\it y})$directions show a small electron pocket at the center and a large electronpocket at the corner of the Brillouin zone (BZ). The absence of the hole FS inthese compounds does not allow nesting between pockets at the Fermi energy({\it E}$_{\rm F}$), which is in contrast to $A$Fe$_2$As$_2$-type parentcompounds of the iron-based superconductors. Interestingly, we find that thehole bands are moved 300--400~meV below $E_{\rm F}$ depending on the $A$element. Moreover, the existence of nearly flat bands in the vicinity of$E_{\rm F}$ are consistent with the large density of states at $E_{\rm F}$.These results are important to understand the physical properties as well asthe possibility of the emergence of superconductivity in related materials.

Electronic structure of $A$Co$_2$As$_2$ ($A=$ Ca, Sr, Ba, Eu) studied using angle-resolved photoemission spectroscopy and theoretical calculations