Dual character of magnetism in ${\text{EuFe}}_2{\text{As}}_2$: Optical spectroscopic and density-functional calculation study

We investigate the electronic structure of ${\text{EuFe}}_2{\text{As}}_2$ using optical spectroscopy and density-functional calculations. At low temperature we observe the evolution of two gaplike features, one having a weak-coupling mean-field behavior and another with strongly nonmean-field behavior. Using band-structure calculations, we identify the former with a spin-Peierls-type partial gap in $d_{yz}$ bands and the latter with the transition across the large exchange gap in $d_{xz}/d_{xy}$ bands. Our results demonstrate that the antiferromagnetism in the ferropnictides is neither fully local nor fully itinerant but contains elements of both.

Figure 1

(Color online) Temperature-dependent in-plane $\left({\rho }_{ab}\right)$ and out-of-plane $\left({\rho }_c\right)$ resistivities of ${\text{EuFe}}_2{\text{As}}_2$ single crystal.

Figure 2

(Color online) In-plane $\sigma \left(\omega \right)$ of ${\text{EuFe}}_2{\text{As}}_2$. The dotted lines under the experimental data represent the contributions of the Drude, peak $\alpha$, peak $\beta$, and a high-energy interband optical transition in the Drude-Lorentz oscillator fit at 20 K.

Figure 3

(Color online) Drude-Lorentz oscillator model fit of $\sigma \left(\omega \right)$ at (a) 300 K, (b) 200 K, (c)180 K, and (d) 150 K.

Figure 4

(Color online) $T$-dependent changes in (a) the energy values of the peaks $\alpha$ and $\beta$ and (b) the SW of the peaks $\alpha$ and $\beta$, as well as the SW of the two Drude peaks. The dashed lines show the $T$ dependences of the peak energies and SW according to the weak-coupling mean-field BCS theory for the peaks $\alpha$ and $\beta$.

Figure 5

(Color online) (a) Band structure of the PM tetragonal state. (b) Band structures of the AFM orthorhombic state. The contributions of the $xy$, $yz$, and $xz$ orbital states are depicted by the open squares, circles, and triangles. The size of the symbol indicates the fraction of the orbital character of bands. The symmetry points are $\Gamma =\left(0,0,0\right)$, $H=\left(2\pi ,0,0\right)$, $N=\left(\pi ,\pi ,0\right)$, and $P=\left(\pi /2,\pi /2,\pi /2\right)$ in the PM Brillouin zone for (a) and AFM Brillouin zone for (b)–(f). Note that the AFM Brillouin zone is rotated by $\pi /4$ relative to that of the tetragonal state and is downfolded. (c) The contribution of Fe-1 $d_{xz}$ spin-up character. (d) The contribution of Fe-1 $d_{xz}$ spin-down character. (e) The Fe-2 $d_{xy}$ spin-down character. Here the Fe-2 atom is adjacent to Fe-1 in the direction of the ordering vector. (f) The Fe-1 $d_{yz}$ spin-up character. (g) Calculated in-plane $\sigma \left(\omega \right)$ for the PM and AFM states. As no correlation effects are included in the calculation, the calculated bandwidths are larger by about a factor of 3 compared to the ARPES data (Ref. 34). Therefore, the frequency dependence of the calculated $\sigma \left(\omega \right)$ should be scaled down by the same factor in comparing with experimental $\sigma \left(\omega \right)$.