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 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.Comment: 6 pages; To appear in Physical Review

On evaluation of transverse spin fluctuations in quantum magnets

A numerical method is described for evaluating transverse spin correlations in the random phase approximation. Quantum, spin-fluctuation corrections to sublattice magnetization are evaluated for the half-filled Hubbard antiferromagnet in two and three dimensions in the whole U/t range. Extension to the case of defects in the Hubbard AF is discussed, and the vacancy-induced enhancement in transverse spin fluctuation is obtained in the infinite-U limit for the spin vacancy problem in two dimensions, for vacancy concentration up to the percolation threshold. For low-U impurities, the overall spin fluctuation correction is found to be strongly suppressed, although surprisingly spin fluctuations are locally enhanced at the low-U sites.Comment: 7 pages, 6 figure