Electronic origin of structural transition in 122 Fe based superconductors

TL;DR

This study demonstrates that the structural transition in 122 Fe-based superconductors is driven by electronic orbital ordering, supported by first-principles calculations and experimental correlations, revealing universal relationships among structural and electronic parameters.

Contribution

The paper provides a quantitative link between orbital order and orthorhombicity in Fe-based superconductors using first-principles simulations and experimental data, establishing the electronic origin of structural transitions.

Findings

Orbital order correlates quantitatively with orthorhombicity across doping and temperature.

Mapping of Fe-Fe distances with band energies reveals electronic-structural relationships.

Fe-As bond distances relate to the density of states at the Fermi level.

Abstract

Direct quantitative correlations between the orbital order and orthorhombicity is achieved in a number of Fe-based superconductors of 122 family. The former (orbital order) is calculated from first principles simulations using experimentally determined doping and temperature dependent structural parameters while the latter (the orthorhombicity) is taken from already established experimental studies; when normalized, both the above quantities quantitatively corresponds to each other in terms of their doping as well as temperature variations. This proves that the structural transition in Fe-based materials is electronic in nature due to orbital ordering. An universal correlations among various structural parameters and electronic structure are also obtained. Most remarkable among them is the mapping of two Fe\--Fe distances in the low temperature orthorhombic phase, with the band energies E$_{d_{xz}}$, E$_{d_{yz}}$ of Fe at the high symmetry points of the Brillouin zone. The fractional co-ordinate $z_{As}$ of $As$ which essentially determines anion height is inversely (directly) proportional to Fe-As bond distances (with exceptions of K doped BaFe$_2$As$_2$) for hole (electron) doped materials as a function of doping. On the other hand, Fe-As bond-distance is found to be inversely (directly) proportional to the density of states at the Fermi level for hole (electron) doped systems. Implications of these results to current issues of Fe based superconductivity are discussed.