PHENOMENOLOGICAL FRAMEWORK FOR FLUCTUATIONS AROUND STEADY-STATE

A phenomenological framework to describe fluctuations around steady st ates is formulated. The framework is illustrated for a magnetic system maintained at a nonequilibrium steady state by an oscillating magneti c field, modeled at the mesoscopic level by a Langevin dynamics. The l arge deviation formalism along the time axis is employed to construct a generalized entropy to describe the fluctuations in the steady state for time averaged observables (state variables). We propose a phenome nological postulate that the fluctuations about the steady stare can b e obtained from the response of the state variables to ''thermodynamic conjugate forces'' (fluctuation-response relation), as in the ordinar y thermodynamic fluctuation theory. An experimentally realizable metho d to study the linear response about the steady state against state va riable perturbations is proposed, and illustrated for the driven magne tic system. The notion of a proper state space to describe nonequilibr ium steady states is discussed, and to this end, we introduce a dissip ation variable to extend the state space for our model system. In the extended state space, we elucidate and study various stability and Max well-type relations that follow from our local phenomenological (therm odynamic) frame-work. Some relevant issues regarding a more general th ermodynamic framework are also discussed.