Anisotropic charge dynamics in detwinned Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$

We investigate the optical conductivity as a function of temperature with light polarized along the in-plane orthorhombic $a$- and $b$-axes of Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ for $x$=0 and 2.5$\mathrm{\%}$ under uniaxial pressure. The charge dynamics at low frequencies on these detwinned, single domain compounds tracks the anisotropic $dc$ transport properties across their structural and magnetic phase transitions. Our findings allow us to estimate the dichroism, which extends to relatively high frequencies. These results are consistent with a scenario in which orbital order plays a significant role in the tetragonal-to-orthorhombic structural transition.