No abstract

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.

In this article we demonstrate the importance of the indirect interaction via the phonon field between pairs of defects in an amorphous material. Using only the approximate value of the lattice coupling constant it is shown that the exchange interaction is large. The most important conclusion that can be drawn from this calculation is that the order of magnitude of the T 2 relaxation time of a spin packet is quite small (10-9 s). It is therefore not possible to interpret the experimental data under the hypothesis 2 T1 = T2 hitherto used; we re-analyse the available results from this new point of view

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,