Effects of the magnetic orderings on the dynamical conductivity: optical investigations of EuFe${}_2$As${}_2$ single crystals

The magnetic, transport and optical properties of EuFe${}_2$As${}_2$ single crystals have been investigated parallel and perpendicular to the $ab$-plane. The anisotropy ${\rho }_c\mathrm{/}{\rho }_{ab}\approx 8$ depends only slightly on temperature. In both orientations, the spin-density wave transition at $T_{\rm SDW}=189$ K shows up as a considerable increase in the dc resistivity. Susceptibility measurements evidence the magnetic order of the Eu${}^{2+}$ moments at $T_N=19$ K with little influence on the electronic transport taking place in the FeAs layers. Polarization-dependent infrared spectroscopy reveals strongly anisotropic optical properties and yields a carrier density of only $4.2\times {10}^{21}{\mathrm{c}\mathrm{m}}^{-3}$ and a bandmass of $m_b=2m_0$. A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below $T_{\mathrm{S}\mathrm{D}\mathrm{W}}$, the spin-density-wave gap develops in the in-plane optical conductivity; no appreciable change is detected for the perpendicular polarization. Modifications in the phonon features are associated with changes of the electronic properties at $T_{\mathrm{S}\mathrm{D}\mathrm{W}}$. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below $T_{\mathrm{S}\mathrm{D}\mathrm{W}}$, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.