Strong coupling to magnetic fluctuations in the charge dynamics of iron-based superconductors

We present a comprehensive comparison of the infrared charge response of two systems, characteristic of classes of the 122 pnictide $\left({\text{SrFe}}_2{\text{As}}_2\right)$ and 11 chalcogenide $\left({\text{Fe}}_{1.087}\text{Te}\right)$ Fe compounds with magnetically ordered ground states. In the 122 system, the magnetic phase shows a decreased plasma frequency and scattering, and associated appearance of strong midinfrared features. The 11 system, with a different magnetic-ordering pattern, also shows decreased scattering, but an increase in the plasma frequency while no clear midinfrared features appear below the ordering temperature. We suggest how this marked contrast can be understood in terms of the diverse magnetic-ordering patterns of the ground state and conclude that while the high-temperature phases of these systems are similar, magnetic order strongly affects the charge dynamical response. In addition, we propose an optical-absorption mechanism which appears to be consistent with information gained from several different experiments.

Figure 1

(Color online) Low-temperature magnetic structure of (a) ${\text{Fe}}_{1.087}\text{Te}$ and (b) ${\text{SrFe}}_2{\text{As}}_2$. [(c) and (d)] Fermi-surface folding expected from the lowered structural symmetry. Optical conductivity ${\sigma }_1\left(\omega ,T\right)$ of (e) ${\text{Fe}}_{1.087}\text{Te}$ and (f) ${\text{SrFe}}_2{\text{As}}_2$ crossing their magnetostructural phase transitions. Dashed lines indicate expectation from a Dirac cone $G\left(\omega \right)={\sigma }_1\left(\omega \right)d$.

Figure 2

(Color online) Temperature dependence of ${\sigma }_1$ averaged over a narrow band of (a) far-infrared and (b) midinfrared frequencies. (c) Comparison of the high-temperature conductivity and the interband conductivity calculated from LDA (Ref. 19). Horizontal line marks $G=\left(\pi /2\right)e^2/h$. dc resistivity measured by transport (squares) and optically (diamonds) for (d) ${\text{Fe}}_{1.087}\text{Te}$ and (e) ${\text{SrFe}}_2{\text{As}}_2$.

Figure 3

(Color online) Components of fits to the optical conductivity at high and low temperatures for [(a) and (c)] ${\text{Fe}}_{1.087}\text{Te}$ and [(b) and (d)] ${\text{SrFe}}_2{\text{As}}_2$.

Figure 4

(Color online) (a) Separate temperature evolution of narrow and wide Drude contributions to the layer conductance of ${\text{Fe}}_{1.087}\text{Te}$. Comparison of the (b) strengths and (c) widths of the narrow Drude contributions for each of the two systems.

Figure 5

(Color online) Tight-binding model band structure (Ref. 31) for (a) high-temperature unit cell, (c) folded at $Q_{AF}=\left(\pi /2,\pi /2\right)$ and (e) $Q_{AF}=\left(\pi ,0\right)$. (d) and (f), respectively, correspond to (c) and (e) with a small mean-field exchange potential included. Allowed optical transitions are indicated by arrows in (f). (b) shows $ab$-averaged conductivity of a $\text{Kubo}+\text{five}$-orbital tight-binding model (Ref. 32) calculation in the high temperature (dashed) and magnetic phases with 200 meV exchange potential with $\left(\pi ,0\right)$ (solid) and $\left(\pi /2,\pi /2\right)$ (dotted) folding.

Figure 6

(Color online) (a) Optical process resulting in a damped fluctuation. (b) Direct and (c) magnon-assisted indirect SDW gap-crossing excitation. Small diagonal arrows indicate spin direction.