AC-susceptibility of YBa 2Cu 3O 7−y ceramics in terms of the percolation theory

Abstract

An approach is formulated which permits to analyze AC-susceptibility data at various amplitudes h in terms of the percolation theory. The relative superconducting volume υ( T) and the fraction of superconducting contacts p(T,h) are the main variables of the problem. The whole set of curves σ( T) at various h is situated between two limiting curves which correspond to small enough and large enough amplitudes h. The function υ( T) can be obtained from the second limiting curve. Its derivative is the T c-distribution function with reliably observable width and structure. By changing the oxygen content this distribution function can be smoothly shifted along the T-axis. The function p(T,h) at h≠0 depends on a definite combination of T and h: p= p( γ) where γ= h/ H 0(1− T/ T 0). The parameter p( T, h=0) turned to depend the oxygen content as well. It seems to increase up to p≈1 when T c is near 60 K. This is one of the experimental motivations to extend the meaning of the parameter p so that it would take into account cooperative phenomena such as long-range coherent state along superconducting clusters.