The Interpretation of Magnetisation and Entropy Jumps in the Flux-line Lattice

Abstract

In the HTC superconductor BSCCO, local measurements of magnetic field in the mixed state show sharp changes as a function of applied field or temperature, which have been interpreted as evidence for a first-order flux-lattice melting (or sublimation) transition . The entropy associated with such a transition was calculated from the magnetisation jumps using a Clausius-Clapeyron-type relationship and was found to increase with temperature up to a value of nearly 1.5kB per flux-line per CuO layer near Tc. This is considerably larger than would be expected, even if the fluid phase consisted of completely dissociated `pancake' vortices. We re-examine this calculation and show that the total change in entropy associated with the transition is actually over 4kB per flux-line per CuO layer. However, we also show that the major portion of this entropy can be attributed to the cores of the extra flux lines formed in the transition and that the residue is consistent with that expected for the disorder associated with melting or sublimation, thereby resolving an important controversy.