Coupled normal heat and matter transport in a simple model system

We introduce the first simple mechanical system that shows fully realistic transport behavior while still being exactly solvable at the level of equil ibrium statistical mechanics. The system is a Lorentz gas with fixed freely rotating circular scatterers which scatter point particles via perfectly r ough collisions. Upon imposing either a temperature gradient and/or a chemi cal potential gradient, a stationary state is attained for which local ther mal equilibrium holds. Transport in this system is normal in the sense that the transport coefficients which characterize the flow of heal and matter are finite in the thermodynamic limit. Moreover, the two hows are nontrivia lly coupled, satisfying Onsager's reciprocity relations.