Anisotropic in-plane optical conductivity in detwinned Ba(Fe_1−<i>x</i>Co_<i>x</i>)₂As₂

Abstract

We study the anisotropic in-plane optical conductivity of detwinned\nBa(Fe1-xCox)2As2 single crystals for x=0, 2.5% and 4.5% in a broad energy range\n(3 meV-5 eV) across their structural and magnetic transitions. For temperatures\nbelow the Neel transition, the topology of the reconstructed Fermi surface,\ncombined with the distinct behavior of the scattering rates, determines the\nanisotropy of the low frequency optical response. For the itinerant charge\ncarriers, we are able to disentangle the evolution of the Drude weights and\nscattering rates and to observe their enhancement along the orthorhombic\nantiferromagnetic a-axis with respect to the ferromagnetic b-axis. For\ntemperatures above Ts, uniaxial stress leads to a finite in-plane anisotropy.\nThe anisotropy of the optical conductivity, leading to a significant dichroism,\nextends to high frequencies in the mid- and near-infrared regions. The\ntemperature dependence of the dichroism at all dopings scales with the\nanisotropy ratio of the dc conductivity, suggesting the electronic nature of\nthe structural transition. Our findings bear testimony to a large nematic\nsusceptibility that couples very effectively to the uniaxial lattice strain. In\norder to clarify the subtle interplay of magnetism and Fermi surface topology\nwe compare our results with theoretical calculations obtained from density\nfunctional theory within the full-potential linear augmented plane-wave method.\n