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 $T_{\mathrm{N}}\simeq 138$ K. Above $T_{\mathrm{N}}$ the real part of the optical conductivity and the two infrared-active lattice modes are almost completely isotropic; the lattice modes show a weak polarization dependence just above $T_{\mathrm{N}}$. For $T<T_{\mathrm{N}}$, the optical conductivity due to 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 $T_{\mathrm{N}}$ is clearly observed, and the polarization modulation of the new modes may be used to estimate that the crystal is $\simeq 70$% detwinned. The high-frequency mode, with a threefold increase in strength of the lower branch below $T_{\mathrm{N}}$ and nearly silent upper branch, remains enigmatic.

8 pages with 5 figures and 1 table; expanded supplementary material