Inversion of meissner effect and granular disorder in Bi2Sr2CaCu2O8+delta superconductors

An explanation for the paramagnetic Meissner effect, which was recently observed in Bi high-temperature superconductors, based on the nature of the granular disorder of the material is developed. This explanation takes into account the appearance of a first-order phase transition (superconductor to superconductor) around randomly distributed defects with localized superconducting states of different orbital momenta (LS(n)-->LS(n+1)), where n is the angular momentum of the state. The transition between superconducting states below the critical temperature in the disordered system is accompanied by jumps of the magnetic moments. When the disordered nature of the defects is taken into account, a state with an average moment greater than zero in a certain domain of the phase space will be formed and the susceptibility M/H in this case will tend toward a finite positive value as H-->0. A comparison between the experimental measurements of a Bi-2212 ceramic sample and the theoretical predictions of the model shows that they are in good agreement.