Staircase-type magnetic-field dependence of the activation energy of Josephson interlayer vortices inBi2Sr2CaCu2O8

Abstract

The anomalous depinning of Josephson interlayer vortices in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ high-${T}_{c}$ superconductor occurring at nearly field-independent temperature ${T}_{x}=20--40\mathrm{K}$ has been studied by means of ac susceptibility measurements with magnetic field applied parallel to ${\mathrm{CuO}}_{2}$ planes. From the frequency dependence ${T}_{x}{(v}_{m})$ we define the flux-creep activation energy for Josephson vortices ${U}_{\mathrm{JV}}(H)$ which increases with field and shows well-defined plateaus. In contrast, the activation energy of in-plane pancake vortices ${U}_{\mathrm{PV}}(H)$ decreases with field, i.e., demonstrates a qualitatively different behavior.