Weakly nonlinear stochastic CGL equations

Abstract

We consider the linear Schr\"odinger equation under periodic boundary condition, driven by a random force and damped by a quasilinear damping: $$ \frac{d}{dt}u+i\big(-\Delta+V(x)\big) u=\nu \Big(\Delta u-\gr |u|^{2p}u-i\gi |u|^{2q}u \Big) +\sqrt\nu\, \eta(t,x).\qquad (*) $$ The force $\eta$ is white in time and smooth in $x$. We are concerned with the limiting, as $\nu\to0$, behaviour of its solutions on long time-intervals $0\le t\le\nu^{-1}T$, and with behaviour of these solutions under the double limit $t\to\infty$ and $\nu\to0$. We show that these two limiting behaviours may be described in terms of solutions for the {\it system of effective equations for $(*)$} which is a well posed semilinear stochastic heat equation with a non-local nonlinearity and a smooth additive noise, written in Fourier coefficients. The effective equations do not depend on the Hamiltonian part of the perturbation $-i\gi|u|^{2q}u$ (but depend on the dissipative part $-\gr|u|^{2p}u$). If $p$ is an integer, they may be written explicitly.