Two-magnons Longitudinal Excitations in the Heisenberg Antiferromagnets

We have studied the microscopic model of longitudinal spin dynamics in a two-sublattice antiferromagnet using the diagrammatic technique for spin operators. This technique gives the possibility to overcome limitations typical for phenomenological approaches. The series of one-loop order diagrams for spin wave propagators are summed up. It results in an expression for the dynamic longitudinal spin susceptibility applicable in all regions of frequency and wave vector space beyond the hydrodynamical and critical regimes. It was shown that, in general, two virtual processes determine the longitudinal excitations spectrum and the longitudinal susceptibility. The obtained results are important for understanding the physics of nonequilibrium magnetic dynamics under the effect of ultrafast laser pulses in antiferromagnetic materials.