Anisotropic optical properties of detwinned BaFe\(_{2}\)As\(_{2}\)

Abstract

The optical properties of a large, detwinned single crystal of BaFe\(_2\)As\(_2\) have been examined over a wide frequency range above and below the structural and magnetic transition at \(T_{\rm N}\simeq 138\) K. Above \(T_{\rm N}\) the real part of the optical conductivity and the two infrared-active lattice modes are almost completely isotropic; only the lattice modes show a weak polarization dependence just above \(T_{\rm N}\). For \(T<T_{\rm N}\), the free-carrier response is anisotropic, being larger along the \({a}\) axis than the \({b}\) axis below \(\simeq 30\) meV; above this energy the optical conductivity is dominated by the interband contributions, which appear to be isotropic. The splitting of the low-energy infrared-active mode below \(T_{\rm N}\) is clearly observed, and the polarization modulation of the new modes may be used to estimate that the crystal is \(\simeq 80\)% detwinned. The high-frequency mode, with the threefold increase in strength of the lower branch below \(T_{\rm N}\) and nearly silent upper branch, remains enigmatic.