The interpretation of magnetisation and entropy jumps in the mixed state of high-temperature superconductors

Abstract

In the high-temperature superconductor BSCCO, local measurements of magnetic 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' effects are accounted for, a first-order transition leads to a sharp jump in the global magnetisation only in the case of samples that are significantly 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 Δ M is expected to be twice Δ B/ μ 0. In addition, we emphasise that the Clausius–Clapeyron relationship between magnetisation jump and entropy jump should involve the local H-field, not the B-field or the applied H-field. Re-interpreting some published experimental data, considering 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 than 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 microscopic degrees of freedom. We present and analyse new experimental data on local field jumps and global magnetisation measurements, and find that they agree with the above. We also show that these data are consistent with the boundary region between the liquid and solid phases having a width of around 20 flux line spacings at a field of 10 mT.