The Unprecedented Giant Coupling of Fe-spin State and the As-As Hybridization in Iron-Pnictide

TL;DR

This study reveals a strong coupling between Fe-spin states and As-As hybridization in iron-pnictides, significantly affecting structural and electronic properties, with implications for understanding superconductivity mechanisms.

Contribution

It uncovers the giant influence of Fe-spin states on As-As interactions and structural changes, highlighting the persistent magnetic moments at ambient pressure in Fe-pnictides.

Findings

Strong Fe-spin and As-As hybridization interaction affects structure

Large c-axis reduction linked to Fe-magnetic moment changes

Electron-phonon coupling cannot fully explain observed superconductivity

Abstract

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 c-axis. For CaFe$_2$As$_2$ system, this reduction is as large as 1.4 \AA. Since the large c-reduction has been recently observed only under high-pressure\cite{cTphase}, 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 CaFe$_2$As$_2$ gives a maximum $T_c$ of 0.6 K and can not explain the $\sim12$ K superconductivity observed in some experiments. Implications of these findings on the mechanism of superconductivity in iron-pnictides are discussed.