Fluctuation-dissipation ratio of a spin glass in the aging regime
Abstract
Almost half a century ago, derivation of the fluctuation dissipation theorem (FDT) which links the response function of a system to its time autocorrelation function, made it possible to work out dynamics from the knowledge of statistical properties at equilibrium. Nevertheless, this progress was limited by severe restrictions. FDT applies only to ergodic systems at equilibrium. It is a direct consequence of the ergodic property that, for a given observable, time averages are equivalent to thermodynamic ensemble averages, or, physically, that the system is invariant by time translation. Yet, such systems represent a very limited part of natural objects, and there is now a growing interest on non-ergodic systems and on the related challenging problem of the existence of fluctuation dissipation (FD) relations valid in off-equilibrium situations . Recent theoretical studies predicted a generalization of FDT, which has to be tested experimentally. Here we show the first experimental determination of the time autocorrelation of magnetization in the non-stationary regime of a spin glass. Our findings are compared with the theoretical consequences of the "weak ergodicity breaking" property of complex systems.Keywords
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