Dichroism in the Parent Ferropnictide BaFe 2 As 2 Across the Nematic Phase Transition

We investigate the anisotropy of the optical response (i.e., dichroism) in the representative ‘parent’ ferropnictide BaFe 2 As 2 . The dichroism is achieved through measurements of the optical reflectivity in a broad spectral range, across the tetragonal-to-orthorhombic and spin-density-wave (SDW) phase transitions, from which we extract the real part of the optical conductivity. The collection of data on the specimen with variable degree of detwinning is made possible via our novel pressure device which allows tuning in situ uniaxial stress. Thedichroism bears testimony to an important electronic polarization, reflecting the impact of the nematic phase and its fluctuations on the electronic structure.