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Finite probability of creation of Cooper pairs in the normal state near the superconducting transition and their interaction with the remnant electrons cause superconducting fluctuation which eventually increase the conductivity in high-temperature superconductor. Based on the theory proposed by Aslamazov and Larkin, fluctuation phenomenon in GdBa 2 Cu 3 O 7 - δ superconductor has been studied close to the superconducting mean field transition temperature (T c mf) in presence of external magnetic field. Weakly first-order transition from normal to superconducting state has been observed as fluctuations near to T c mf (in the critical field region) is beyond 3D-XY-E scaling for both zero and finite fields. Following the novel approach of local pairs model, evolution process of fluctuating Cooper pairs (FCPs) obeying BCS theory from strongly coupled bosons (SCBs) which obey Bose–Einstein Condensation (BEC) theory in the temperature limit T c mf < T < T ∗ ( T ∗ is the temperature below which SCBs start to form) has been studied meticulously. The results indicate that pseudogap parameters Δ ∗ (T) shifted towards BCS phase from BEC state with decreasing temperature after exceeding the maximum critical size of the local pairs (28 Å) in the CuO 2 plane. The values of T pair (at which SCBs initiate to transform into FCPs) obtained from temperature-dependent study of Δ ∗ (T) decreases when magnetic field increases from zero to 4 T. Superconducting gap (Δ (0)) for all magnetic fields calculated from the local pairs model are within the BCS limit but the magnitudes reduce as the field escalates. [ABSTRACT FROM AUTHOR]

Trapped fields of over 20 T are, in principle, achievable in bulk, single-grain high temperature cuprate superconductors. The principle barriers to realizing such performance are, firstly, the large tensile stresses that develop during the magnetization of such trapped-field magnets as a result of the Lorentz force, which lead to brittle fracture of these ceramic-like materials at high fields and, secondly, catastrophic thermal instabilities as a result of flux movement during magnetization. Moreover, for a batch of samples nominally fabricated identically, the statistical nature of the failure mechanism means the best performance (i.e. trapped fields of over 17 T) cannot be attained reliably. The magnetization process, particularly to higher fields, also often damages the samples such that they cannot repeatedly trap high fields following subsequent magnetization. In this study, we report the sequential trapping of magnetic fields of ∼ 17 T, achieving 16.8 T at 26 K initially and 17.6 T at 22.5 K subsequently, in a stack of two Ag-doped GdBa2Cu3O7-δ bulk superconductor composites of diameter 24 mm reinforced with (1) stainless-steel laminations, and (2) shrink-fit stainless steel rings. A trapped field of 17.6 T is, in fact, comparable with the highest trapped fields reported to date for bulk superconducting magnets of any mechanical and chemical composition, and this was achieved using the first composite stack to be fabricated by this technique. These post-melt-processing treatments, which are relatively straightforward to implement, were used to improve both the mechanical properties and the thermal stability of the resultant composite structure, providing what we believe is a promising route to achieving reliably fields of over 20 T. [ABSTRACT FROM AUTHOR]

Large single-grain bulk high-temperature superconducting materials can trap high magnetic fields in comparison with conventional permanent magnets, making them ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems, and electric machines. However, macrosegregation of Y-211 inclusions in melt-processed Y–Ba–Cu–O (YBCO) limits the macroscopic critical current density of such bulk superconductors, and hence, the potential trapped field. A new fabrication technique using graded precursor powders has recently been developed by our research group, which results in a more uniform distribution of Y-211 particles, in order to further improve the superconducting properties and trapped field capability of such materials. In this paper, experimental results on the pulsed field magnetization of a graded single-grain YBCO bulk sample are presented. Pulsed fields of magnitude up to 5.5 T were applied to the sample, at temperatures of 40 and 65 K. The trapped field profiles indicate that magnetic flux enters the sample in a more uniform manner than standard YBCO samples, resulting in a higher overall trapped field and indicating the success of this processing technique to produce a more homogeneous sample. [ABSTRACT FROM PUBLISHER]