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Dielectric constant measurements have been performed in the ferroelectric lowtemperature phase of a KTa0.973Nb0.027O3 crystal. It is found that aging at a fixed temperature T pl induces a memory effect: the real part ε and the imaginary part ε of the complex dielectric constant ε around T pl remain weaker than they would be without aging, as long as the temperature stays below T pl ; on the contrary, a sojourn of the sample at temperatures higher than T pl rapidly erases the dips on ε and ε . Similarities with recent results obtained on two spin-glasses are emphasized and discussed.

Isothermal kinetics of the orientational l'ergodicit4, Ceci montre que l'espace des phases est repr4sentable par un paysage compliqu4 oh des val14es mutuellement inaccessibles sont s4par4es par de trAs hautes barriAres. Des exp4riences de cycles en temp4rature en apportent une vision plus d4tail14e : elles s'expliquent en effet dons le cadre d'une organisation h14rarchique d4pendaut de la temp4rature. 1. constraints are easily taken into account by means of the relevant thermodynamic potential,

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