Nematicity studied with strain-dependent ARPES*

Nematicity is a phenomenon found in an increasing number of strongly correlated materials in close proximity to other quantum phases such as unconventional superconductivity. It is therefore important to understand its microscopic origin. The phase diagram of most iron-based superconductors contains a nematic phase. It breaks rotational symmetry and involves an in-plane anisotropy of lattice, spin and charge degrees of freedom. It is currently debated whether spin or orbital degrees of freedom are the driving force and whether there is a common microscopic mechanism in iron-based superconductors with and without magnetic order. We were able to combine angel-resolved photoemission spectroscopy (ARPES) with in-situ tuneable uniaxial strain and determine the momentum dependence of the nematic order parameter in FeSe and BaFe₂As₂. The order parameter has the same non-trivial symmetry in both compounds suggesting a universal behavior and a common microscopic mechanism.