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C-axis current flow in high temperature superconductor (HTS) tape-shaped wires arises in configurations where the local wire axis is not perpendicular to the local magnetic field, such as in power cables with helically wound HTS tapes. The c-axis critical current density Jcc has been recently found to be orders of magnitude lower than the ab-plane critical current density Jcab. Here we report on Jcc (77 K, sf) values of various YBa2Cu3O7-based (YBCO) tapes with different microstructures. Our results show that the value of Jcc (77 K, sf) decreases significantly with increasing concentration of ab-plane stacking faults in YBCO thin films and that the critical current anisotropy γ = Jcab/Jcc can reach values as high as 2070, implying that in the highest-anisotropy tape, ∼20% of the tape width carries c-axis current in a helically wound power cable. [ABSTRACT FROM AUTHOR]

We report the direct imaging of a novel modulated flux striped domain phase in a nearly twin-free YBCO crystal. These domains arise from instabilities in the vortex structure within a narrow region of tilted magnetic fields at small angles from the in-plane direction. By comparing the experimental and theoretically derived vortex phase diagrams we infer that the stripe domains emerge from a first-order phase transition of the vortex structure. The size of domains containing vortices of certain orientations is controlled by the balance between the vortex stray field energy and the positive energy of the domain boundaries. Our results confirm the existence of the kinked vortex chain phase in an anisotropic high temperature superconductor and reveal a sharp transition in the state of this phase resulting in regular vortex domains. [ABSTRACT FROM AUTHOR]

An expression P = 2RcVL rmsHa rms was theoretically derived for the ac power losses in a circular superconducting disk in a perpendicular ac applied magnetic field when the losses are measured by a planar circular pickup coil surrounding the disk. Here, Rc is the radius of the pickup coil, which is coplanar with the disk, and VL rms and Ha rms are rms values of the loss voltage and applied magnetic field. This method of measuring losses has an advantage over a susceptibility technique, since the coil need not be calibrated. However, this expression is valid only for large values of Rc relative to the radius R of the disk, since the dipole approximation was used in the derivation. We experimentally evaluated the minimum value of the diameter 2Rc for which this method provided approximately correct losses. A YBa2Cu3O7 disk with diameter 2R ≈ 5.3 mm was used for the measurements. The electronically determined apparent losses at all field values decreased with increasing coil radius, and asymptotically approached a constant value for Rc/R >m ∼3. [ABSTRACT FROM AUTHOR]