Spin dynamics and antiferromagnetic short-range order in the two-dimensional Heisenberg model

We investigate the spin dynamics in the presence of short-range order in th e square lattice S=1/2 Heisenberg antiferromagnet by a spin-rotation-invari ant Green's-function theory for the dynamic spin susceptibility. The self-e nergy is calculated in the lowest-order Born approximation using the result s of a sum-rule-conserving mean-field approximation. In the spin-wave regio n, where the damping of magnons is found to be small compared with their en ergy, the dynamic structure factor is obtained in reasonable agreement with Monte Carlo data. Moreover, the structure factor yields an indication of t he crossover to the relaxation region. [S0163-1829(99)00809-7].