Electrodynamic response in the electronic nematic phase of BaFe2As2

We perform, as a function of uniaxial stress, a temperature-dependent optical-reflectivity investigation of the parent Fe-arsenide compound BaFe2As2 over a broad spectral range, from the far infrared up to the ultraviolet, across the coincident structural tetragonal-to-orthorhombic and spin-density-wave ( SDW) phase transitions at T-s,T-N = 135 K. Our results provide knowledge to the complete electrodynamic response of the title compound over a wide energy range as a function of both tunable variables. For temperatures below T-s,T-N, varying the uniaxial stress in situ affects the twin domain population and yields hysteretic behavior of the optical properties as the stress is first increased and then decreased, whereas for temperatures above T-s,T-N the stress-induced optical anisotropy is reversible, as anticipated. In particular, by analyzing the low-frequency infrared response, we obtain detailed insight to the effects determining the intrinsic anisotropy of the (metallic) charge dynamics in the orthorhombic state, and similarly the induced one due to applied uniaxial stress at higher temperatures in the tetragonal phase. The low-frequency optical conductivity thus allows establishing a link to the dc transport properties and reveals that they are determined almost exclusively by changes in the Drude weight, therefore by the anisotropy in the Fermi surface parameters. Finally, we show that the spectral weight distribution in the SDW state occurs for energies below approximately 1 eV, and therefore points towards a correlation mechanism due to Hund's coupling rather than on-site Coulomb interactions.