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 crystals of six Tl based compounds: Tl2Ba2CaCu2O8, Tl2Ba2Ca2Cu3O10, TlBa2CaCu2O7, TlBa2Ca2Cu3O9, (Tl, Bi)Sr2CaCu2O7 and (Tl, Pb)Sr2CaCu2O7. The location in the H-T plane of the line of the second peak has been determined in each case. This location depends strongly on the nature of the compound (structure and cationic composition) whereas it is essentially sample independent. No influence of the method of preparation, of the size of the sample and of annealing treatments (oxygen non-stoichiometry) has been observed. Relaxation studies have demonstrated that the appearance of the second peak cannot be accounted for simply by a decrease of the relaxation rate. In particular, the occurrence around the second peak of a crossover between two regimes of collective creep has been discarded. Otherwise, the transient increase of pinning efficiency which characterizes the fishtail effect could be induced by a softening of the vortex lattice allowing a better adaptation to the structure 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 models, values of the electronic anisotropy for each compound have been calculated.