The paramagnetic Meissner effect

Sammanfattning: The low field dynamics of some high temperature superconductors have been studied experimentally, using SQUID magnetometry. The research has been focused on the so called Paramagnetic Meissner Effect (PME), a phenomenon seen in some high temperature superconducting cuprates. The signature of this effect is a positive magnetisation when the sample is cooled through its superoconducting transition temperature in sufficiently low magnetic fields. The PME has been explained in terms of spontaneously created magnetic moments, formed in zero magnetic field, a consequence of unconventional symmetry of the superconducting order parameter, which allows the creation of π-junctions. A detailed study of the system Bi2Sr2CaCu2O8 has revealed fundamental characteristics of systems exhibiting PME. It is found that the out-of-phase component of the ac susceptibility is suppressed for small external de fields, for ac field amplitudes in the linear response regime. Furthermore, the spontaneous flux noise also decreases for small superimposed de fields. A good agreement between the two quantities, according to the Fluctuation- Dissipation Theorem, has been found both in zero and small external de fields. The dc magnetisation has been studied as a function of time. It has been shown that the relaxation of the zfc magnetisation displays zero external field magnetic ageing, revealing a collective behaviour of the spontaneous magnetic moments. The collective behaviour points out that intrinsically the ensemble of the spontaneous magnetic moments is disordered and frustrated. The collective behaviour has been discussed in terms of a novel equilibrium thermodynamic phase, the so called "chiral-glass" phase. Dynamic and static scaling analyses of the experimented data have been performed, giving indications, although not unambiguous evidence, of a "chiral-glass" phase at low temperatures.

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