Abstract
Arsenic vacancies in LaFeAsO-derived superconductors are nominally non-magnetic defects. However, we find from a microscopic theory in terms of an appropriately modified Anderson-Wolff model that in their vicinity local magnetic moments form. They can arise because removing an arsenic atom breaks four strong, covalent bonds with the neighboring iron atoms. The moments emerging around an arsenic vacancy orient ferromagnetically and cause a substantial enhancement of the paramagnetic susceptibility in both the normal and superconducting state. The qualitative model description is supported by first principles band structure calculations of the As-vacancy related defect spectrum within a larger supercell.
Highlights
Arsenic vacancies in LaFeAsO-derived superconductors are nominally non-magnetic defects
Certain features of strong correlation generated by the on-site Coulomb repulsion and local exchange (Hund’s rule coupling) in the Fe sublattice are suggested theoretically[8,9] for strongly hole-doped Fe pnictides such as KFe2As2 and possibly evidenced experimentally[10,11], it is generally believed that at least for optimally electron doped counter parts the basic features of electron-hole excitation spectra in these distinctly anisotropic materials may be revealed within a self-consistent band theory description
The magnetic response of iron pnictides substitutionally doped with non-magnetic impurities on the iron sublattice[12] and structural defects such as As-vacancies[13] demonstrate distinct features of localized magnetic moments arising on the background of the itinerant character of pristine materials
Summary
Various band calculations[3,17,18,19,20,21] reveal the structure of valence and conduction bands which reflects the structure of covalent bonds in the As-Fe planes. Fe-related 3d orbitals form the top of the valence bands and the bottom of conduction bands, so that the d-partial waves form the hole and electron pockets of the Fermi surface. Dyz and dxy form the electron pockets around the M points of the Brillouin zone. As-related p-partial waves contribute mainly to bonding (occupied) and antibonding (empty) bands well below and well above the d-waves around the Fermi level εF. Such disposition reflects an essential role of dp covalent bonds in formation of the band spectra
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