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Relationship between pinning mechanism and excess conductivity analysis of x wt% C4H6O5 (x=0.0, 4.0, and 6.0)-added bulk MgB2

2021    Cakir, Bakiye; Koparan, Ezgi Taylan; Savaskan, Burcu

The influence of x wt% C4H6O5 (malic acid) (x = 0.0, 4.0, and 6.0) addition on excess conductivity in bulk MgB2 superconductors were fabricated by two-step solid-state reaction method. Resistivity measurements were carried out as a function of temperature starting from the room temperature at 0 T magnetic field. The electrical resistivity measurements did not show any significant change on T-c with the increasing addition ratio. Estimated alpha is a coefficient related with the intrinsic electronic interaction was found approximately the same value for added sample but pure sample was found to be lower; indicating increasing electronic interactions with addition. For the analysis of the excess conductivity, Aslamazov and Larkin (AL) model was used and the impacts of C4H6O5 additions on the excess conductivity of MgB2 have been investigated. The samples indicated the existence of three distinct fluctuation regimes above and near to T-c. The samples with x wt% malic acid additions (x = 4.0 and 6.0) showed 2D character such as exhibited by the HTSs, while the pure MgB2 sample exhibited the 3D nature. While the T-LD was not observed for pure sample, it was observed for malic acid-added samples. Also, the temperature value increased leading to growing pinning capability with the increasing addition ratio. Flux pinning mechanisms in MgB2 superconductor were analyzed and discussed in detail. Normal point pinning and surface pinning of flux pinning mechanisms were found to be effective in MgB2 superconductors. In addition, while all samples exhibited normal point pinning in the low magnetic field region, it was observed that the pinning mechanism shifted to the surface pinning with the increasing addition rate in the high magnetic field region. In the malic acid-added samples in which the agglomeration occurred, a dominant 2D fluctuation was observed while the 1D fluctuation was observed in the pure sample. Because the agglomeration causes existing of effective pinning centers, 2D fluctuations dominate in the malic acid-added samples.