Origin of the resistive anisotropy in the electronic nematic phase of BaFe2As2 revealed by optical spectroscopy
There is a debate going on in the literature as to whether the anisotropy in the dc response of novel iron-pnictide superconductors is due to Fermi surface anisotropy or is induced by impurity scattering. The subject has attracted considerable recent attention, primarily because of the large magnitude of the dc anisotropy, but also because so much can be inferred from the temperature dependence of this quantity. The debate mainly originates from the fact that the strain-induced resistivity anisotropy in the tetragonal state of representative underdoped Fe-arsenide families seems to be independent of disorder over a wide range of defect and impurity concentrations. Nonetheless, STM measurements at very low temperatures reveal extended anisotropic defects (i.e., nematogens), perhaps associated with impurities which locally polarize the electronic structure and cause anisotropic elastic scattering. We perform, as a function of uniaxial stress, an optical-reflectivity investigation of the representative 'parent' ferropnictide BaFe2As2 in a broad spectral range (Fig. 2.17), across the tetragonal-to-orthorhombic phase transition and the onset of the long-range antiferromagnetic order (AFM). The infrared response reveals that the dc transport anisotropy in the orthorhombic AFM state is determined by the interplay between the Drude spectral weight and the scattering rate, but that the dominant effect is clearly associated with the metallic spectral weight.
In the paramagnetic tetragonal phase, though, the dc resistivity anisotropy of strained samples is almost exclusively due to stress-induced changes in the Drude weight rather than in the scattering rate, definitively establishing the anisotropy of the Fermi surface parameters as the primary effect driving the dc transport properties in the electronic nematic state (Fig. 2.18). Therefore, our work now resolves the long-standing debate of fundamental importance; even though impurity scattering can affect the transport anisotropy in subtle ways, it seems to have at best only limited relevance in the electronic nematic phase. Our results put some constraints on future theoretical approaches aimed reproducing the electrodynamic response and its relationship to the dc transport properties of iron-pnictides with respect to their nematic state.
