VORTEX DYNAMICS STUDIED OVER A WIDE-RANGE OF TIME-SCALE IN BI-2212 CRYSTALS

Three experimental methods are used to study the dissipation in Bi-221 2 single crystals (direct measurements of the electrical resistivity, AC susceptibility and DC magnetic relaxation measurements). This allow s the resistivity to be registered over 14 orders of magnitude. There is no gap in this wide range of measurements and some overlapping of t he ranges obtained by different methods have been obtained. A crossove r in the dissipation has been evidenced by this technique due to the e xtended range of measurement. In the H-T phase diagram, this crossover line does not correspond to the irreversibility line and separates a phase at low temperatures in which the relaxation is slow from a phase at high temperatures in which the relaxation is fast. This transition is limited at high fields by a critical point and at low fields by th e so called ''fish-tail''.