Pancake vortices

I describe the magnetic-field and current-density distributions generated by two-dimensional (2D) pancake vortices in infinite, semi-infinite, and finite-thickness stacks of Josephson-decoupled superconducting layers. Arrays of such vortices have been used to model the magnetic structure in highly anisotropic layered cuprate high-temperature superconductors. I show how the electromagnetic forces between pancake vortices can be calculatated, and I briefly discuss the effects of interlayer Josephson coupling.Comment: 16 pages, 4 figures, accepted in J. Supercond. for the Special Issue celebrating Michael Tinkham's 75th birthday, typos [notably in Eq. (67)] correcte

Field and current distributions and ac losses in a bifilar stack of superconducting strips

In this paper I first analytically calculate the magnetic-field and sheet-current distributions generated in an infinite stack of thin superconducting strips of thickness d, width 2a >> d, and arbitrary separation D when adjacent strips carry net current of magnitude I in opposite directions. Each strip is assumed to have uniform critical current density Jc, critical sheet-current density Kc = Jc d, and critical current Ic = 2a Kc, and the distribution of the current density within each strip is assumed to obey critical-state theory. I then derive expressions for the ac losses due to magnetic-flux penetration both from the strip edges and from the top and bottom of each strip, and I express the results in terms of integrals involving the perpendicular and parallel components of the magnetic field. After numerically evaluating the ac losses for typical dimensions, I present analytic expressions from which the losses can be estimated.Comment: 8 pages, 9 figure

Self-fields in thin superconducting tapes: implications to the thickness effect in coated conductors

Most applications of superconductors, such as power transmission lines, motors, generators, and transformers, require long cables through which large currents circulate. Impressive progress has recently been achieved in the current-carrying capability in conductors based on high-temperature superconductors. Coated conductors are likely the best examples, consisting of very good quality thin layers of YBCO superconductor grown on top of a metallic tape with some intermediate layers. However, there is an important problem for achieving large currents: a large decrease in transport critical-current density Jc when increasing film thickness has been observed in coated conductors made by all available techniques. Here, we theoretically explain the nature and the ubiquitous presence of this so-called thickness effect by analyzing the self-field created by the transport currents in the superconductor, assuming a realistic field-dependent Jc. This knowledge can help finding new ways to improve transport current in thick superconducting films.Comment: 7 pages, 3 figure

Self-field effects upon the critical current density of flat superconducting strips

We develop a general theory to account self-consistently for self-field effects upon the average transport critical current density Jc of a flat type-II superconducting strip in the mixed state when the bulk pinning is characterized by a field-dependent depinning critical current density Jp(B), where B is the local magnetic flux density. We first consider the possibility of both bulk and edge-pinning contributions but conclude that bulk pinning dominates over geometrical edge-barrier effects in state-of-the-art YBCO films and prototype second-generation coated conductors. We apply our theory using the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba) as a function of a perpendicular applied magnetic induction Ba and show how Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux density zero at the strip's edge. However, Jc(Ba) is suppressed relative to JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference

Flux domes in superconducting films without edges

Domelike magnetic-flux-density distributions previously have been observed experimentally and analyzed theoretically in superconducting films with edges, such as in strips and thin plates. Such flux domes have been explained as arising from a combination of strong geometric barriers and weak bulk pinning. In this paper we predict that, even in films with bulk pinning, flux domes also occur when vortices and antivortices are produced far from the film edges underneath current-carrying wires, coils, or permanent magnets placed above the film. Vortex-antivortex pairs penetrating through the film are generated when the magnetic field parallel to the surface exceeds H_{c1}+K_c, where H_{c1} is the lower critical field and K_c = j_c d is the critical sheet-current density (the product of the bulk critical current density j_c and the film thickness d). The vortices and antivortices move in opposite directions to locations where they join others to create separated vortex and antivortex flux domes. We consider a simple arrangement of a pair of current-carrying wires carrying current I_0 in opposite directions and calculate the magnetic-field and current-density distributions as a function of I_0 both in the bulk-pinning-free case (K_c = 0) and in the presence of bulk pinning, characterized by a field-independent critical sheet-current density (K_c > 0).Comment: 15 pages, 23 figure