Intragranular and intergranular transitions in Y-Ba-Cu-O ceramics

Publisher Website
Google Scholar
Abstract
We describe quantitatively the whole resistive transition (from p ≃ ρ N to ρ = 0) of Y-Ba-Cu-O ceramics as two successive phase transitions : the intragrain superconducting transition at Tcs and the intergrain coherence transition induced by the Josephson coupling between neighbouring grains at Tc < Tcs. The coherence transition is similar to the paramagnetism-ferromagnetism transition in a 3D disordered X-Y ferromagnet. In the paraconductive region T > Tcs, the fluctuations in the amplitude of the superconducting order parameter in the grains lead to an excess conductivity given by the Azlamazov-Larkin and/or the Lawrence-Doniach theories. In the paracoherent region Tc < T < Tcs, phase fluctuations result in critical behaviour of the excess conductivity. Above Tcs, the paraconductivity of samples with different degrees of granularity reflects a layered structure, while near Tc the behaviour of p (T) and V(I) characteristics is always determined by the 3D coherence transition