Current distribution across type II superconducting films: a new vortex-free critical state

Imaging of Strong Nanoscale Vortex Pinning in GdBaCuO High-Temperature Superconducting Tapes

The high critical current density of second-generation high-temperature superconducting (2G-HTS) tapes is the result of the systematic optimisation of the pinning landscape for superconducting vortices through careful engineering of the size and density of defects and non-superconducting second phases. Here, we use scanning Hall probe microscopy to conduct a vortex-resolved study of commercial GdBaCuO tapes in low fields for the first time and complement this work with "local" magnetisation and transport measurements. Magnetic imaging reveals highly disordered vortex patterns reflecting the presence of strong pinning from a dense distribution of nanoscale Gd2O3 second-phase inclusions in the superconducting film. However, we find that the measured vortex profiles are unexpectedly broad, with full-width-half-maxima typically of 6 μm, and exhibit almost no temperature dependence in the range 10-85 K. Since the lateral displacements of pinned vortex cores are not expected to exceed the superconducting layer thickness, this suggests that the observed broadening is caused by the disruption of the circulating supercurrents due to the high density of nanoscale pinning sites. Deviations of our local magnetisation data from an accepted 2D Bean critical state model also indicate that critical state profiles relax quite rapidly by flux creep. Our measurements provide important information about the role second-phase defects play in enhancing the critical current in these tapes and demonstrate the power of magnetic imaging as a complementary tool in the optimisation of vortex pinning phenomena in 2G-HTS tapes.

The onset of dissipation in high-temperature superconductors: magnetic hysteresis and field dependence

Recently, we showed that the self-field transport critical current, I_c(sf), of a superconducting wire can be defined in a more fundamental way than the conventional (and arbitrary) electric field criterion, E_c = 1 μV/cm. We defined I_c(sf) as the threshold current, I_c,B, at which the perpendicular component of the local magnetic flux density, B_⊥, measured at any point on the surface of a high-temperature superconducting tape abruptly crosses over from a non-linear to a linear dependence with increasing transport current. This effect results from the current distribution across the tape width progressively transitioning from non-uniform to uniform. The completion of this progressive transition was found to be singular. It coincides with the first discernible onset of dissipation and immediately precedes the formation of a measureable electric field. Here, we show that the same I_c,B definition of critical currents applies in the presence of an external applied magnetic field, B_a. In all experimental data presented here I_c,B is found to be significantly (10–30%) lower than I_c,E determined by the common electric field criterion of E_c = 1 µV/cm, and E_c to be up to 50 times lower at I_c,B than at I_c,E.