Search

Abstract: It is shown that As vacancies in the LaFeAsO superconductor induce a strong enhancement of the paramagnetic response both in the normal and the superconducting state. Although As vacancies themselves are non-magnetic defects, they are able to form localized magnetic moments due to strong covalent bonding with neighboring Fe ions. The microscopic theory of magnetic moment formation in semi-metallic state of non-stoichiometric LaFeAsO is constructed in a framework of appropriately modified Anderson–Wolff model. Experimental manifestations of enhanced paramagnetism and the influence of magnetic fluctuations on superconducting pairing are briefly discussed. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]