THE FISHTAIL EFFECT IN DIFFERENT TL BASED SINGLE-CRYSTALS - A POSSIBLE INTERPLAY WITH THE ELECTRONIC ANISOTROPY

The fishtail effect has been intensively investigated in single crysta ls of six Tl based compounds: Tl2Ba2CaCu2O8, Tl2Ba2Ca2Cu3O10, TlBa2CaC u2O7, TlBa2Ca2Cu3O9, (Tl, Bi)Sr2CaCu2O7 and (Tl, Pb)Sr2CaCu2O7. The lo cation in the H-T plane of the line of the second peak has been determ ined in each case. This location depends strongly on the nature of the compound (structure and cationic composition) whereas it is essential ly sample independent. No influence of the method of preparation, of t he size of the sample and of annealing treatments (oxygen non-stoichio metry) has been observed. Relaxation studies have demonstrated that th e appearance of the second peak cannot be accounted for simply by a de crease of the relaxation rate. In particular, the occurrence around th e second peak of a crossover between two regimes of collective creep h as been discarded. Otherwise, the transient increase of pinning effici ency which characterizes the fishtail effect could be induced by a sof tening of the vortex lattice allowing a better adaptation to the struc ture of defects to occur. Several scenario's can be invoked such as a 3D-2D transition or a 3D melting. On the basis of three of these model s, values of the electronic anisotropy for each compound have been cal culated.