Direct evidence of uneven dxz and dyz orbital occupation in the superconducting state of iron pnictide

The origin of nematicity and its relation to superconductivity in iron pnictide high-temperature superconductors remains unclear. One of its possible sources is the uneven occupation of Fe ${d}_{xz}$ and ${d}_{yz}$ orbitals. Here, by using x-ray linear dichroism technique, we show that such an imbalance is present in $\mathrm{Eu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x})}_{2}{\mathrm{As}}_{2}$ single crystals deep in the tetragonal phase and also in the superconducting state, where we find that the ${d}_{xz}$ orbital has a higher occupation as indicated by an x-ray linear dichroism magnitude of 1.5%. Our result shows the importance of orbital polarization in the theoretical description of nematicity and superconductivity, particularly for determination of the superconducting gap symmetry, which is affected by orbital fluctuations.