Dynamical spin correlations in the Heisenberg ladder under a magnetic field and correlation functions in the SO(5) ladder

The zero-temperature dynamical spin-spin correlation functions are calculat ed for the spin-1/2 two-leg antiferromagnetic Heisenberg ladder in a magnet ic field above the lower critical field H-c1. The dynamical structure facto rs are calculated, which exhibit both massless modes and massive excitation s. These modes appear in different sectors characterized by the parity in t he rung direction and by the momentum in the direction of the chains. The s tructure factors have power-law singularities at the lower edges of their s upport. The results are also applicable to the spin-1 Heisenberg chain. The implications are briefly discussed for the various correlation functions E nd the pi resonance in the SO(5) symmetric ladder model.