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We use angle-resolved photoemission to study the three dimensional (3D)electronic structure of Co pnictides ACo2As2 with A=Ba, Sr, Ca or a mixture ofSr and Ca. These compounds are isostructural to Fe based superconductors, buthave one more electron in the Co $3d$ orbitals. Going from Ba to Ca, theybecome more and more 3D, eventually forming a 'collapsed' tetragonal phase,where the distance between CoAs layers is markedly reduced. The observedperiodicity of the 3D electronic structure matches in each case that expectedfrom the distance between the planes in the bulk. However, the electronicstructure is better fitted by a calculation corresponding to a slab with 2 CoAslayers than to the bulk structure. We attribute this to subtle modifications ofthe 2D electronic structure induced by the truncation of the 3D dispersion atthe surface in the ARPES measurement. We further study how this affects theelectronic properties. We show that, despite this distortion, the electronicstructure of CaCo2As2 is essentially that expected for a collapsed phase.Electronic correlations produce a renormalization of the electronic structureby a factor 1.4, which is not affected by the transition to the collapsedstate. On the other hand, a small shift of the Fermi level reduces the densityof states in the eg bands and suppresses the magnetic transition expected inCaCo2As2. Our study evidences that observing the bulk periodicity is notsufficient to ensure bulk sensitivity. It further gives direct information onthe role of 3D interactions, mostly governed by Co-As hybridization, among egand t2g orbitals. It is also useful to better understand the electronicstructure of Fe superconductors and the range of validity of ARPESmeasurements.

Influence of the surface symmetry breaking on the magnetism, collapsing and three dimensional dispersion of Co pnictides ACo2As2 (A=Ba, Sr, Ca)