Abstract
Within the universality class of ferromagnetic vector models with
O(n)
symmetry and purely dissipative dynamics, we study the non-equilibrium critical relaxation
from a magnetized initial state. Transverse correlation and response functions are
exactly computed for Gaussian fluctuations and in the limit of infinite number
n
of components of the order parameter. We find that the fluctuation–dissipation ratios
(FDRs) for longitudinal and transverse modes differ already at the Gaussian level.
In these two exactly solvable cases we completely describe the crossover from
the short-time to the long-time behaviour, corresponding to a disordered and a
magnetized initial condition, respectively. The effects of non-Gaussian fluctuations
on longitudinal and transverse quantities are calculated in the first order in the
-expansion
( = 4−d) and reliable three-dimensional estimates of the two FDRs are obtained.