Strong Coupling of the Fe-Spin State and the As-As Hybridization in Iron-Pnictide Superconductors from First-Principle Calculations

From first-principles calculations, we unravel surprisingly strong interactions between arsenic ions in iron pnictides, the strength of which is controlled by the Fe-spin state. Reducing the Fe-magnetic moment weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant reduction in the axis. For the system, this reduction is as large as 1.4 Å. Since the large reduction has been recently observed only under high pressure, our results suggest that the iron magnetic moment should be present in Fe-pnictides at all times at ambient pressure. Finally, the conventional electron-phonon coupling in the collapsed phase of gives a maximum of 0.6 K and cannot explain the superconductivity observed in some experiments. Implications of these findings on the mechanism of superconductivity in iron pnictides are discussed.

Figure 1

Total energy as the Ca122 tetragonal phase goes to collapsed-tetragonal phase without any energy barrier during nonspin polarized structural optimization. The insets show the initial ($T$-Phase) and final (cT-phase) Ca122 structures with relevant bond-distances (in Å) and angles (in degrees).

Figure 2

The projected electronic density of states (PDOS) for As $p$-orbital (top-left) and Fe-d orbital (bottom left) for $T$- and cT-phases. The right two panels show the band structure along $\Gamma -M-\Gamma$. The As-As $p_z$-bonding orbital is indicated by the arrow on the top left panel, and it is shown in Fig. 3.

Figure 3

Contour plot of the As-As $p_z$-hybridization orbital for cT-phase (left) and $T$-phase (right), respectively. Note that the As-As hybridization present in both phases is much more significant in the cT-phase. There is also clear hybridization between intra-As atoms below and above the Fe-plane in the $T$-phase (right).

Figure 4

Various structural parameters as a function of Fe-moment from fixed-spin calculations with ferromagnetic iron spin configuration.

Figure 5

Phonon density of states (DOS), Eliashberg function [${\alpha }^{2}F(\omega )$] and the frequency-dependent e-ph coupling $\lambda (\omega )$ (dashed line) for ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ in the cT-phase.