Optical Anisotropy in the Electronic Nematic Phase of FeSe

FeSe undergoes a structural tetragonal-to-orthorhombic transition below 90 K, without any subsequent onset of magnetic ordering. FeSe thus provides an opportunity to address the impact of nematicity on its intrinsic physical properties without the limitations of the reconstruction of the Fermi surface due to the SDW collective state in the orthorhombic phase, typical for several other iron-based superconductors. We describe results of reflectivity measurements over a broad spectral range that probe the optical response to variable uniaxial stress, detwinning the specimen and acting as an external symmetry breaking field, and as a function of temperature across the structural transition [1]. We extract the optical conductivity through Kramers-Kronig transformation. Our data reveal an astonishing anisotropy of the optical response in the mid-infrared-to-visible spectral range, which bears testimony of an important polarization of the underlying electronic structure in agreement with ARPES results. Our optical results at energies close to the Fermi level furthermore emphasize scenarios based on scattering by anisotropic spin-fluctuation, shedding new light on the origin of nematicity in FeSe.
[1] M. Chinotti et al., Phys. Rev. B, 96, 121112(R) (2017).