Numerical evidence for Goldstone-mode effects in the three-dimensional O(4)-model

We investigate the three-dimensional O(4)-model on 24(3)-96(3) lattices as a function of the magnetic field H. Below the critical point, in the broken phase, we confirm explicitly the H-1/2 dependence of the magnetization and the corresponding H-1/2 divergence of the longitudinal susceptibility, whi ch are predicted from the existence of Goldstone modes. At the critical poi nt the magnetization follows the expected finite-size-scaling behavior, wit h critical exponent delta = 4.87(1).