## Fluctuation-Dissipation Relations Out of Equilibrium

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Members of our group working in this topic: J. M. R. Parrondo and L. Dinis

Fluctuation-dissipation relations (FDR) have been a crucial tool in Statistical Physics. They relate the response of a system slightly removed from equilibrium to equilibrium properties and allow to connect transport coefficients to microscopic properties [1]. In the recent years, there has been a growing interest in generalizing FDR to arbitrarily far from equilibrium systems [2], where classical FDR do not in general apply.

Our group has recently contributed to this field showing, in collaboration with J. Prost and J-F. Joanny, that FDR are valid out of equilibrium provided the correct conjugate observables are used [3]. This new theorem has attracted considerable attention due to its general hypothesis (it can be applied to any Markovian system). We are currently working on its generalization to non-Markovian systems and its application to biophysical systems like Brownian motors or active gels.

[1] R. Kubo, Fluctuation-Dissipation Theorem. Reports on Progress in Physics **29**, 255 (1966).

[2] U.M.B. Marconi, A. Puglisi, L. Rondoni, and A. Vulpiani, Fluctuation-Dissipation: Response Theory in Statistical Physics. Physics Reports-Review Section of Physics Letters **461**, 111-195 (2008).

[3] J. Prost, J.F. Joanny, and J.M.R. Parrondo, Generalized Fluctuation-Dissipation Theorem for Steady-State Systems. Physical Review Letters **103**, 090601 (2009).