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Abstract: Electronic instabilities are at the origin of phenomena as diverse as the formation of spin and charge density waves (SDW and CDW) or superconductivity, the interplay of which is among the most intriguing open questions of modern solid state physics. Besides determining the ground state of the quantum system in which they occur, electronic instabilities also fundamentally affect its excitation spectrum. The focus of my talk will be on the novel iron-pnictide superconductors, which set the stage for the discussion of relevant issues in ongoing solid state physics. I will first present our thorough optical investigation of the charge dynamics in Ba(CoxFe1-x)2As2 compounds for Co-doping ranging between 0 and 18%, from where we extract the degree of electronic correlations over the entire phase diagram. Furthermore, a renewed interest in the study of symmetry-breaking competing states in complex interacting systems followed the discovery of a broken rotational symmetry, due to stripe or nematic order, in the pseudogap phase of the copper oxide superconductors. In this context, we also investigate the optical conductivity with light polarized along the in-plane orthorhombic a- and b-axes of Ba(CoxFe1-x)2As2 for x = 0 and 2.5% under uniaxial pressure across their structural and magnetic transitions. Our findings allow us to estimate the dichroism and reveal a substantial nematic susceptibility as well as demonstrate the electronic nature of the structural transition.

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