Macroscopic Diffusive Transport in a Microscopically Integrable Hamiltonian System

We demonstrate that a completely integrable classical mechanical model, namely the lattice Landau-Lifshitz classical spin chain, supports diffusive spin transport with a finite diffusion constant in the easy-axis regime, while in the easy-plane regime it displays ballistic transport in the absence of any known relevant local or quasi-local constant of motion in the symmetry sector of the spin current. This surprising finding should open the way towards analytical computation of diffusion constants for integrable interacting systems and hints on existence of new quasi-local classical conservation laws beyond the standard soliton theory.