This week

We study how slowly varying perturbations of a steady nonequilibrium process affect the excess of observables such as heat flux, leading to a definition of nonequilibrium heat capacity. For cyclic transformations, these excesses can be described geometrically through a Berry phase, with the Berry potential and curvature characterizing the response. The mathematical framework is the Poisson equation LV+f=0, where (L) is the backward generator of an irreducible finite Markov jump process and (f) is a centered state function. The solution (V) determines the excess associated with (f).