Experimentally tuning the ground state of BaFe$_{2}$As$_{2}$ by orbital differentiation

The role of structural parameters in layered systems, such as iron pnictides/chalcogenides (Fe-Pn/Ch), cuprates and heavy fermions, has become crucial for the understanding of their properties. In this talk, I will discuss this subject using a combination of macroscopic and microscopic techniques to study Ba$_{1-x}$Eu$_{x}$Fe$_{2-y}M_{y}$As$_{2}$ single crystals ($M =$ Co, Cu, Mn, Ni, and Ru). Interestingly, a close connection arises between the spin-density wave (SDW) phase suppression and local distortions in the structure. Furthermore, these changes are reflected at the Fermi surface by an increase of anisotropy and localization of the Fe $3d$ bands at the FeAs plane. Our results suggest that such increase in the planar ($xy$/$x^{2}-y^{2}$) orbital symmetry seems to be a favorable ingredient for the emergence of superconductivity in this class of materials.