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La condensation de l'hélium 4 dans les aérogels : une réalisation expérimentale du RFIM athermique

Abstract : Up to now, the effect of disorder on critical phenomena has been mainly studied theoretically. The motivation of these studies is to understand to what extent the difference between the behavior of the real systems and the ideal theoretical models is due to disorder. This thesis addresses this problem experimentally through the condensation of helium 4 in silica aerogels. This system provides the opportunity to tune the disorder strength. More precisely, we show that this phenomenon is an experimental realization of the athermal Random Field Ising Model (RFIM) studied by Sethna et al. (Phys. Rev. Lett., 70, 3347 (1993)). To this end, we observe two properties of this model, in agreement with numerical simulations done by Detcheverry et al. on a lattice gas model (Phys. Rev. E, 72, 051506 (2005)): the existence of disorder driven critical points and the Return Point Memory (RPM). The change of shape of the adsorption isotherms measured by light scattering is interpreted as the signature of the disorder driven critical points. In order to detect the avalanches predicted by the Detcheverry et al.'s model, we have developed a technique based on the great sensitivity of the speckle patterns to the local microscopic liquid-gas domains. Although no single avalanche could be clearly resolved, this method allowed the first observation of microscopic RPM in an experimental system, showing the athermal character of the system. These are the first measurements to exhibit the athermal RFIM behavior in a non-magnetic system.
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Geoffroy Aubry. La condensation de l'hélium 4 dans les aérogels : une réalisation expérimentale du RFIM athermique. Systèmes désordonnés et réseaux de neurones [cond-mat.dis-nn]. Université de Grenoble, 2013. Français. ⟨NNT : 2013GRENY075⟩. ⟨tel-01223439⟩

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