THE INTERPRETATION OF MAGNETIZATION AND ENTROPY JUMPS IN THE MIXED-STATE OF HIGH-TEMPERATURE SUPERCONDUCTORS

In the high-temperature superconductor BSCCO, local measurements of ma gnetic field at the surface of a crystal in the mixed state show sharp changes as a function of applied field or temperature. These 'jumps' have been interpreted as signs of a first-order flux lattice melting ( or sublimation) transition. We show that if 'intermediate state' effec ts are accounted for, a first-order transition leads to a sharp jump i n the global magnetisation only in the case of samples that are signif icantly non-ellipsoidal in shape. We also investigate the relationship between a jump in magnetisation, M, and the associated change in the B-field immediately above the crystal surface and show that Delta M is expected to be twice Delta B/mu(0). In addition, we emphasise that th e Clausius-Clapeyron relationship between magnetisation jump and entro py jump should involve the local H-field, not the B-field or the appli ed H-field. Re-interpreting some published experimental data, consider ing these factors, leads to the conclusion that the entropy change can be as much as 4.0 k(B) per flux line per layer, compared with less th an 2 k(B) previously reported for this data, and that a similar factor should be applied to other measurements, where the entropy change can now be as high as 14 k(B) per flux line per layer. We show that part of this entropy can be attributed to the cores of the extra flux lines introduced into the sample by the transition, and that a considerable amount of the remainder may be associated with changes in the microsc opic degrees of freedom. We present and analyse new experimental data on local field jumps and global magnetisation measurements, and find t hat they agree with the above. We also show that these data are consis tent with the boundary region between the liquid and solid phases havi ng a width of around 20 flux line spacings at a field of 10 mT. (C) 19 98 Elsevier Science B.V. All rights reserved.