Magnetic impurities in Mott-Hubbard antiferromagnets

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 nr On impurity sites is different from the number Non host sites, leading to strong magnon damping and singular correction to low-energy magnon modes in two dimensions. 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, with the relative magnon damping decreasing to q(2) for long-wavelength modes. 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. [S0163-1829(98)01317-4].