Critical state and fluxon pinning in high-T c superconductors

Abstract

The low-field nonresonant microwave absorption in high-T c superconductors results from the interaction of supercurrents with free fluxons and is a sensitive indicator of fluxon pinning. The existence of a critical state for both increasing and decreasing applied magnetic fields accounts for the change of sign of the signal with reversal of field sweep. When the sweep direction is reversed, a field interval 2H c1 = (8π/c) λJ c is required for the establishment of the reverse critical state, where J c is the critical current density and λ is a relaxation length. During this interval fluxons are first pinned and then depinned, causing a minimum in the absorption as has been observed directly in the scanned microwave signal.