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We use scanning superconducting quantum interference device (SQUID) microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: They tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in twinned samples in pnictides and other superconductors. [ABSTRACT FROM AUTHOR]

A study of the DC I-V curves of BSCCO tapes exposed to AC perpendicular and parallel magnetic fields is presented. AC magnetic fields with amplitudes up to 700 G and frequencies from 1 to 430 Hz have been applied perpendicular and parallel to tapes carrying DC currents at T = 77 K. Both field orientations result in a significant DC electric field increase, in comparison to applying DC magnetic fields of the same intensity. However, the observed I-V curves behavior is different for the perpendicular and parallel cases. In the perpendicular case, we distinguish between two frequency regimes: for frequencies above 40 Hz, the observed I-V curves obey the empirical power law dependence with current density, J c, and power index, n, that strongly depend on the field amplitude. For frequencies below 20 Hz, the observed I-V curves deviate from power law description. In the parallel field case, the I-V curve can be regarded as a superposition of an additional DC voltage over the no field I-V curve. This additional voltage increases with increasing current, peaks near the critical current and decreases thereafter. The results are discussed assuming a narrowing down of the DC current path under the application of a perpendicular AC field. Shaking of the pinned vortices under the application of a parallel AC field is assumed for explaining the results in this case. [ABSTRACT FROM AUTHOR]

Presented here are measurements at 77 K of I–V curves of reinforced BSCCO tapes in a perpendicular field regime of 0–700 G, and I–V curves of Ricor-made pancakes in self-field. The difference between the I–V characteristics of tapes and pancakes is discussed and explained by traveling across field different I–V curves of the tapes. We estimate the critical current of coils, based on the magnetic field dependence of the critical current in BSCCO tapes. The current distribution in the tape is assumed to be homogeneous, or non-homogeneous distribution opposing the local critical current density resulting from the magnetic field distribution in the tape. Applying our computational methods to the measured pancakes, results in a 90% (homogeneous current distribution) and 98% match (field-dependent current distribution) with the measured pancake’s critical current. [Copyright &y& Elsevier]