Abstract
Abstract We report the complete disappearance of the structural modulation in heavily lead-doped Bi 2− x Pb x Sr 2 CaCu 2 O 8+ 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 H irr ( T ) and relaxation rates are strongly improved within the β phase. Measurements of the lower critical field, H c1 , show that the anisotropy factor, ϵ , is considerably reduced in the modulation-free crystals. This is the signature of stronger coupling between CuO 2 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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