Optical investigation of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ detwinned by tunable uniaxial applied pressure

The iron-pnictide superconductors are excellent materials where one can study the competition between structural, magnetic and superconducting phases. In the parent compound (i.e. x = 0\%) and in the so called underdoped regime (x$<$6\%) an antiferromagnetic transition occurs at T$_N$ with an almost coincident tetragonal-to-orthorhombic structural distortion at T$_s$ $\geq$ T$_N$. The in-plane anisotropy of the orthorombic phase was found to be masked by the formation of twin domains in these compounds, which can be detwinned by applying uniaxial pressure. Here we report on an optical investigation performed with electromagnetic radiation polarized along the a and b axes of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals , for x=0, 2.5\% and 4.5\%, detwinned by in-situ tunable uniaxial pressure applied across the stuctural and the magnetic transitions. We show in details the experimental setup, i. e. the pressure device used to detwin the samples, and the most remarkable results. In particular we focus on the evolution of the anisotropy in the reflectivity by applying and releasing pressure at different fixed temperatures.