Statistical solutions to the Schr\"odinger map equation in 1D, via the randomly forced Landau-Lifschitz-Gilbert equation

Abstract

We prove the existence of statistically stationary solutions to the Schr\"odinger map equation on a one-dimensional domain, with null Neumann boundary conditions. We deal directly with the equation in its real-valued formulation, without using any transform. To approximate the Schr\"odinger map equation, we employ the stochastic Landau-Lifschitz-Gilbert equation. By a limiting procedure \`a la Kuksin, we establish existence of a random initial datum, whose distribution is preserved under the dynamics of the deterministic equation. Among other properties, the corresponding statistically stationary solution is proved to exhibit non-trivial dynamics in space and time and to be genuinely random. With an analogous argument, we prove the existence of stationary solutions to a stochastic Schr\"odinger map equation. We discuss the relationship between the statistically stationary solutions to the Schr\"odinger map equation, the binormal curvature flow and the cubic non-linear Schr\"odinger equation. Additionally, we prove the existence of statistically stationary solutions to the binormal curvature flow.