Influence of the surface symmetry breaking on the magnetism, collapsing   and three dimensional dispersion of Co pnictides ACo2As2 (A=Ba, Sr, Ca)
  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 of Sr and Ca. These compounds are isostructural to Fe based superconductors, but have one more electron in the Co $3d$ orbitals. Going from Ba to Ca, they become more and more 3D, eventually forming a 'collapsed' tetragonal phase, where the distance between CoAs layers is markedly reduced. The observed periodicity of the 3D electronic structure matches in each case that expected from the distance between the planes in the bulk. However, the electronic structure is better fitted by a calculation corresponding to a slab with 2 CoAs layers than to the bulk structure. We attribute this to subtle modifications of the 2D electronic structure induced by the truncation of the 3D dispersion at the surface in the ARPES measurement. We further study how this affects the electronic properties. We show that, despite this distortion, the electronic structure of CaCo2As2 is essentially that expected for a collapsed phase. Electronic correlations produce a renormalization of the electronic structure by a factor 1.4, which is not affected by the transition to the collapsed state. On the other hand, a small shift of the Fermi level reduces the density of states in the eg bands and suppresses the magnetic transition expected in CaCo2As2. Our study evidences that observing the bulk periodicity is not sufficient to ensure bulk sensitivity. It further gives direct information on the role of 3D interactions, mostly governed by Co-As hybridization, among eg and t2g orbitals. It is also useful to better understand the electronic structure of Fe superconductors and the range of validity of ARPES measurements. 
Joseph Mansart, Patrick Le Fevre, Francois Bertran, Anne Forget,   Dorothee Colson, Veronique Brouet
cond-mat.str-el