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Séminaire MCBT

Mardi 23 avril à 11h00,
salle louis Weil, E424

Orateur : Geoffroy Aubry (Institut Néel)
"Experimental realization of the 3D zero temperature Random Field Ising Model : the condensation of $^4$He in aerogels"

Abstract :

Although widely studied, the effect of disorder on a first order phase transition is still highly debated. Numerical simulations of the 3D $T=0$ Random Field Ising Model show 3 features : 1. hysteresis, 2. that magnetization evolves by avalanches, the average size of which diverges below a critical disorder and 3. the return point memory effect (Sethna et al., PRL 70 3347 (1993)). Nevertheless, experimental evidence is scarce up to now (Berger et al., PRL 85, 4176 (2000)), and no experimental system shows these 3 features. We have studied experimentally the liquid gas transition of $^4$He in silica aerogels. Optical and thermodynamical measurements show that the condensation is an out of equilibrium process. We clearly observe two filling regimes separated by a critical temperature $T^*$ : below $T^*$, filling is discontinuous (macro avalanche) whereas above $T^*$ it becomes continuous (micro avalanches), consistent with prediction 2. of the model. Finally, using a speckle interferometry technique, we observe the return point memory effect (prediction 3.), which is a proof that our system is really thermal.

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