Fluctuations and limit of metastability in a periodically driven unstable system

A mechanism for restoring broken symmetry in a system periodically driven above and below its instability point is discussed by studying the homogeneous, n-component, time-dependent Landau-Ginzburg model. We report numerical simulations for the n=1 case and an analytical solution for the spherical limit n. In both cases the results show that fluctuations are enhanced by bringing the system near an unstable state periodically and as a consequence a well-defined shift in the instability point occurs. This shift can alternatively be characterized through the limit of the metastable behavior of the order parameter or by the large increase in the order-parameter fluctuations.