The collapsed tetragonal phase as a strongly covalent and fully nonmagnetic state: persistent magnetism with interlayer As-As bond formation in Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_2$As$_2$

摘要

A well-known feature of CaFe$_{2}$As$_{2}$-based superconductors is thepressure-induced collapsed tetragonal phase that is commonly ascribed to theformation of an interlayer As-As bond. Using detailed X-ray scattering andspectroscopy, we find that Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_{2}$As$_{2}$ doesnot undergo a first-order phase transition and that local Fe moments persistdespite the formation of interlayer As-As bonds. Our density functional theorycalculations reveal that the Fe-As bond geometry is critical for stabilizingmagnetism and that the pressure-induced drop in the $c$ lattice parameterobserved in pure CaFe$_{2}$As$_{2}$ is mostly due to a constriction within theFeAs planes. These phenomena are best understood using an often overlookedexplanation for the equilibrium Fe-As bond geometry, which is set by acompetition between covalent bonding and exchange splitting between stronglyhybridized Fe $3d$ and As $4p$ states. In this framework, the collapsedtetragonal phase emerges when covalent bonding completely wins out overexchange splitting. Thus the collapsed tetragonal phase is properly understoodas a strong, covalent phase that is fully nonmagnetic with the As-As bondforming as a byproduct.