Magnetic impurities in Mott-Hubbard antiferromagnets

TL;DR

This paper develops a formalism to analyze magnetic impurities in Mott-Hubbard antiferromagnets, revealing how impurity characteristics influence magnon behavior and magnetic properties.

Contribution

It introduces a multi-orbital representation to study impurity effects, highlighting the impact of orbital differences on magnon scattering and magnetic excitations.

Findings

Impurity scattering of magnons is strong when impurity and host orbitals differ.

Magnon mode softening enhances thermal magnon excitation, lowering Neel temperature.

Weak impurity scattering occurs when impurity and host orbitals are the same.

Abstract

A formalism is developed to treat magnetic impurities in a Mott-Hubbard antiferromagnetic insulator within a representation involving multiple orbitals per site. Impurity scattering of magnons is found to be strong when the number of orbitals N' on impurity sites is different from the number N on host sites. The impurity-scattering-induced softening of magnon modes leads to enhancement in thermal excitation of magnons, and hence to a lowering of the Neel temperature in layered or three dimensional systems. Weak impurity scattering of magnons is obtained in the case N'=N, where the impurity is represented in terms of modified hopping strength, and a momentum-independent, multiplicative renormalization of magnon energies is obtained. Split-off magnon modes are obtained when the impurity-host coupling is stronger, and implications are discussed for two-magnon Raman scattering. The mapping between antiferromagnets and superconductors is utilized to contrast formation of impurity-induced states.