  Within the rigorous axiomatic framework for the description of quantum
mechanical systems with a large number of degrees of freedom, we construct the
so-called nonequilibrium steady state for the quasifree fermionic system
corresponding to the isotropic XY chain in which a finite sample, subject to a
local gauge breaking anisotropy perturbation, is coupled to two thermal
reservoirs at different temperatures. Using time dependent and stationary
scattering theory, we rigorously prove, from first principles, that the
nonequilibrium system under consideration is thermodynamically nontrivial,
i.e., that its entropy production rate is strictly positive.
