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$_2$

K. Zhao; J. K. Glasbrenner; H. Gretarsson; D. Schmitz; J. Bednarcik; M. Etter; J. P. Sun; R. S. Manna; A. Al-Zein; S. Lafuerza; W. Scherer; J. G. Cheng; P. Gegenwart

doi:10.1103/physrevb.97.020510

arXiv · 1702.02398

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$_2$

K. Zhao·J. K. Glasbrenner·H. Gretarsson·D. Schmitz·J. Bednarcik·M. Etter·J. P. Sun·R. S. Manna·A. Al-Zein·S. Lafuerza·W. Scherer·J. G. Cheng·P. Gegenwart

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Abstract

A well-known feature of CaFe$_{2}$As$_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As-As bond. Using detailed X-ray scattering and spectroscopy, we find that Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_{2}$As$_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As-As bonds. Our density functional theory calculations reveal that the Fe-As bond geometry is critical for stabilizing magnetism and that the pressure-induced drop in the $c$ lattice parameter observed in pure CaFe$_{2}$As$_{2}$ is mostly due to a constriction within the FeAs planes. These phenomena are best understood using an often overlooked explanation for the equilibrium Fe-As bond geometry, which is set by a competition between covalent bonding and exchange splitting between strongly hybridized Fe $3d$ and As $4p$ states. In this framework, the collapsed tetragonal phase emerges when covalent bonding completely wins out over exchange splitting. Thus the collapsed tetragonal phase is properly understood as a strong, covalent phase that is fully nonmagnetic with the As-As bond forming as a byproduct.

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K. Zhao, J. K. Glasbrenner, H. Gretarsson, D. Schmitz, J. Bednarcik, M. Etter, J. P. Sun, R. S. Manna, A. Al-Zein, S. Lafuerza, W. Scherer, J. G. Cheng, P. Gegenwart. 2017-02-08. 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$_2$. https://doi.org/10.1103/physrevb.97.020510

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