Josephson Phase Coherence and Coupling in High T_c Granular Superconductors

Abstract

In order to understand the superconductive behaviour of the new high T_c superconductors, many experiments have been carried out. Among these, the measurements of a. c. complex susceptibility x = x′ + ix″ as a function of temperature are of great interest^l. The in-phase component x′, corresponding to inductive variations, indicates perfect diamagnetism and flux shielding. The out-of-phase component x″, corresponding to resistive losses, can behave differently depending on which loss mechanism is involved. In the (RE) Ba₂Cu₃O_7−δ oxides, the observed magnetic behaviour has been analyzed by several authors in terms of percolation, junctions, disorder or superconducting inclusions^4,7 with the physical data apparently varying from one sample to another, depending on the synthesis methods and on the different heat treatments used. We have, therefore, performed inductive a. c. complex susceptibility as well as d. c. resistive measurements on sintered bulk and powdered samples of HoBa₂Cu₃O_7−δ and on two types of YBa₂Cu₃O_7−δ samples in order to correlate the resistivity, the morphology and the diamagnetic behaviour in these granular high T_c superconductors. The observed phenomena are directly dependent on the contact resistance between the grains (i. e. the ability of carrying supercurrent) and on the surface to volume ratio of the grains in a given material.