Weak-links criterion for pnictide and cuprate superconductors

Abstract

Weak-links are defects that limit dissipation-free transport current flow in superconductors. Grain boundaries, nano- and micro-cracks, and planar precipitations of secondary phases are typical examples of weak-links in practical superconductors. There is an unanswered practical question: is the critical current for a given superconductor limited by weak-links, or does wire fabrication provide a weak-link-free superconductor? In this paper, we answer this question for layered quasi-two-dimensional (quasi-2D) superconductors, namely pnictides and cuprates. Our approach is based on the fact that the self-field critical current density in weak-link-free superconductors is J c(sf, T) = A/λ 3(T), where λ(T) is the London penetration depth and A is the relevant fundamental constant. Taking into account that the transition temperature, T c, in layered quasi-2D superconductors is limited by the phase fluctuation temperature, T fluc = B/λ 2(0) ≥ 1.2•T c, where B is the relevant fundamental constant, then the substitution of λ(0) deduced from the measured J c(sf, T) gives a tool to compare the deduced T fluc and experimentally measured T c. This provides a simple criterion to reveal the presence or absence of weak-links which has been proven by an analysis of self-field critical currents in a variety of high-temperature superconductors, ranging from atomically thin FeSe up to commercially available tapes of RBa2Cu3O7.