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

TL;DR

This study investigates the anisotropic optical and electronic properties of EuFe$_2$As$_2$ single crystals, revealing how magnetic orderings influence the spin-density wave transition, carrier dynamics, and phonon features.

Contribution

It provides detailed optical characterization of EuFe$_2$As$_2$, highlighting the anisotropic effects of magnetic ordering on its electronic properties and revealing the interaction between charge carriers and spin fluctuations.

Findings

Spin-density wave transition at 189 K causes resistivity increase.

Eu$^{2+}$ magnetic order at 19 K has minimal impact on FeAs layers.

In-plane optical conductivity shows a SDW gap below 189 K.

Abstract

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 $\rho_c/\rho_{ab}\approx 8$ 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 $4.2\times 10^{21} {\rm cm}^{-3}$ and a bandmass of $m_b=2m_0$. A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below $T_{\rm SDW}$, 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 $T_{\rm SDW}$. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below $T_{\rm SDW}$, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.