Non-Equilibrium Steady States and Transport in the Classical Lattice $φ^4$ Theory

We study the classical non-equilibrium statistical mechanics of scalar field theory on the lattice. Steady states are analyzed near and far from equilibrium. The bulk thermal conductivity is computed, including its temperature dependence. We examine the validity of linear response predictions, as well as properties of the non-equilibrium steady state. We find that the linear response theory applies to visibly curved temperature profiles as long as the thermal gradients are not too strong. We also examine the transition from local equilibrium to local non-equilibrium.