Abstract
The dynamical properties of the finite-size magnetization M in the critical region T ⩽ TKTB of the planar rotor model on a L×L square lattice are analyzed by means of microcanonical simulations. The behavior of the q = 0 structure factor at high frequencies is consistent with field-theoretical results, but new additional features occur at lower frequencies. The motion of M determines a region of spectral lines and the presence of a central peak, which we attribute to phase diffusion. Near TKTB the diffusion constant scales with system size as D ∼ L−1.6(3).