Kinetic theory of nonlinear diffusion in a weakly disordered nonlinear Schr\"odinger chain in the regime of homogeneous chaos

Abstract

We study the discrete nonlinear Schr\"oinger equation with weak disorder, focusing on the regime when the nonlinearity is, on the one hand, weak enough for the normal modes of the linear problem to remain well resolved, but on the other, strong enough for the dynamics of the normal mode amplitudes to be chaotic for almost all modes. We show that in this regime and in the limit of high temperature, the macroscopic density $\rho$ satisfies the nonlinear diffusion equation with a density-dependent diffusion coefficient, $D(\rho)=D_0\rho^2$. An explicit expression for $D_0$ is obtained in terms of the eigenfunctions and eigenvalues of the linear problem, which is then evaluated numerically. The role of the second conserved quantity (energy) in the transport is also quantitatively discussed.