Abstract
Abstract We report the complete disappearance of the structural modulation in heavily lead-doped Bi2−xPbxSr2CaCu2O8+δ crystals observed by transmission electron microscopy. Crystals with a nominal lead content of x=0.8, corresponding to an effective lead content of x=0.39, yield the non-modulated phase. The superconducting properties of this modulation-free phase (β phase) have been studied and compared to those of undoped crystals displaying the modulated phase (α phase). Magnetisation measurements reveal that the irreversibility field Hirr(T) and relaxation rates are strongly improved within the β phase. Measurements of the lower critical field, Hc1, show that the anisotropy factor, ϵ, is considerably reduced in the modulation-free crystals. This is the signature of stronger coupling between CuO2 layers which in turn deeply influences the effectiveness of the pinning. These measurements explain the enhanced pinning properties in moderately Pb-doped crystals in which the α phase and β phase coexist. The enhanced pinning is not only due to the α/β interfaces, which act as effective pinning centers: the emergence of modulation-free domains, characterized by a strongly reduced anisotropy, also significantly contribute to this effect.
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