Coherence transition in granular YBa 2 Cu 3 O 7-δ , YBa 2 Cu 2.95 Zn 0.05 O 7-δ , and YBa 1.75 Sr 0.25 Cu 3 O 7-δ superconductors

We have studied experimentally the electrical magneto-conductivity near the superconducting transition of YBa 2 Cu 3 O 7-δ , YBa 2 (Cu 2.95 Zn 0.05 )O 7- δ and Y(Ba 1.75 Sr 0.25 )Cu 3 O 7-δ polycrystalline samples. The measurements were performed in magnetic fields ranging from 0 to 400 Oe applied parallel to the current orientation. The results show that the resistive transition of these systems proceeds in two stages. The pairing transition occurs at the bulk critical temperature T c , where superconductivity is stabilized within small and homogeneous regions of the sample generically called grains. The regime of approach to the zero resistance state reveals the occurrence of a coherence transition at a lower temperature T c0 . This transition is related to the connective nature of the granular samples and is controlled by fluctuations of the order-parameter phase of individual grains. Our experiments show that the Zn-doping, besides depressing the pairing critical temperature, strongly enlarges the temperature range dominated by effects related to the coherence transition. The substitution of Ba by Sr causes only a small reduction of T c , but also enhances significantly the effects related to the grain coupling phenomenology. In general, our results indicate that these impurity substitutions in YBa 2 Cu 3 O 7-δ produce or magnify the granularity at a microscopic level, enhancing the effects of phase fluctuations in the conductivity near the transition. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007