Origin of vortex melting line in Bi2223/Ag tapes

Abstract

Using AC diamagnetic and transport measurements we have derived a three-dimensional diagram for high-quality Bi2223/Ag tapes. The diagram shows an abrupt decrease of critical current caused by the melting of a vortex glass. The melting line was defined as the contour line for the surface at , where showed the steepest decrease with H and T. This line coincided with the locus of the maximum, at a frequency Hz and an AC field amplitude Oe. The melting line reflects the process of the separation of unit-cell stacks. The most important are the low-number stacks that contain one to three unit cells. These stacks have the lowest Kosterlitz-Thouless transition temperatures and the lowest corresponding vortex melting temperatures. A magnetic field causes the separation of the stacks and reduces in each stack to the melting temperature of the lower stack. At the universal , the vortex melting line is the combination of for different, mainly low-number, stacks.