Nematic-driven anisotropic electronic properties of underdoped detwinned Ba ( Fe 1 − x Co x ) 2 As 2 revealed by optical spectroscopy

We collect optical reflectivity data as a function of temperature across the structural tetragonal-to-orthorhombic phase transition at T-s on Ba(Fe1- Co-x(x))(2)As-2 for x = 0%, 2.5%, and 4.5%, with uniaxial and in situ tunable applied pressure in order to detwin the sample and to exert on it an external symmetry-breaking field. At T < T-s, we discover a remarkable hysteretic optical anisotropy as a function of the applied pressure at energies far away from the Fermi level. Such an anisotropy turns into a reversible linear pressure dependence at T >= T-s. Moreover, the optical anisotropy gets progressively depleted with increasing Co content in the underdoped regime, consistent with the doping dependence of the orthorhombicity but contrary to the nonmonotonic behavior observed for the dc anisotropy.