When a material orders nematically, it breaks rotational symmetry without breaking translational symmetry. It is an electronically driven order, but the coupling of lattice, spin, and charge degrees of freedom allow us to observe its signature in all three channels. Importantly, nematicity is found in an increasing number of quantum materials in close proximity to unconventional superconductivity. It is speculated that nematic fluctuations enhance the superconducting transition temperature. Therefore, it is important to understand the microscopic origin of nematicity.
Iron-based superconductors are a prototype material class to study nematic order. A key experimental signature of nematicity is an anisotropic resistivity. However, it is unclear how changes in the band dispersion, the scattering rate, and quasiparticle coherence contribute to this anisotropy. To obtain a microscopic understanding, we combine angel-resolved photoemission spectroscopy (ARPES) with in-situ tunable uniaxial strain in order to gain access to the orbital contribution of the nematic susceptibility. I will present our results on the strain-induced band splitting and anisotropic quasiparticle coherence in two members of the iron-based superconductors: BaFe2As2 and FeSe. Our results emphasize the importance of electronic correlations for a description of nematic order.