Spontaneous orbital anisotropy in the nematic state of FeSe

Nematicity has played a central role in the phenomenology of iron-based superconductors. FeSe is of unique interest due to the absence of a C₄ symmetry-breaking SDW order while preserving robust nematic and superconducting phases. These features have raised intrigue regarding the role of the orbital degrees of freedom and their connection to the pairing mechanism. Here, we report a multi-probe study of the nematic state as a function of in-situ tunable uniaxial strain, using Fe K pre-edge X-ray linear dichroism (XLD) and hard X-ray diffraction measurements combined with simultaneous transport measurement. Together, these probes provide a detailed picture of the anisotropic orbital, transport and structural responses to uniaxial strain in both the high temperature and ordered phases. Strain-detwinning in the nematic phase reveals a sizable ab plane pre-edge XLD which is assigned to orbital anisotropy coupled to the primary nematic order parameter, rather than the secondary lattice orthorhombicity. Strain-induced XLD above Ts is also revealed, exhibiting behavior consistent with a divergent nematic susceptibility that is strongly coupled to an underlying orbital order. Our results shed light on the central role of the Fe 3d orbitals in driving the nematic phase.