Vortex chains in anisotropic superconductors

Abstract

High-cT superconductors in small magnetic fields directed away from the crystal symmetry axes have been found to exhibit inhomogeneous chains of flux lines (vortices), in contrast to the usual regular triangular flux-line lattice. We review the experimental observations of these chains, and summarize the theoretical background that explains their appearance. We treat separately two classes of chains: those that appear in superconductors with moderate anisotropy due to an attractive part of the interaction between tilted flux lines, and those with high anisotropy where the tilted magnetic flux is created by two independent and perpendicular crossing lattices. In the second case it is the indirect attraction between a flux line along the layers (Josephson vortex) and a flux line perpendicular to the layers (pancake vortex stack) that leads to the formation of chains of the pancake vortex stacks. This complex system contains a rich variety of phenomena, with several different equilibrium phases, and an extraordinary dynamic interplay between the two sets of crossing vortices. We compare the theoretical predictions of these phenomena with the experimental observations made to date. We also contrast the different techniques used to make these observations. While it is clear that this system forms a wonderful playground for probing the formation of structures with competing interactions, we conclude that there are important practical implications of the vortex chains that appear in highly anisotropic superconductors.

Search for broken time-reversal symmetry near the surface of superconducting YBa2Cu3OSaadaoui1, Morris2, Salman3 et al. 2011Phys. Rev. B

“…37 Outside the superconductor, the resulting field inhomogeneity leads to a depth independent broadening, since z ≪ d, consistent with our results (inset of Fig. 3).…”

Vortex molecules in thin films of layered superconductors

“…Such decoration of the JVs by the PVs was clearly observed in experiment (see for example [4] and [14]). The interaction between pancakes and in-plane field in the form of JVs is known to produce zigzag deformation of PV stacks (see Fig.…”

“…In particular, it has been noted that in certain directions it is possible to have the long-range attraction between the tilted vortex lines [2,3]. In bulk anisotropic superconductors this phenomenon is known to result in the formation of vortex chains in the regime of low magnetic fields (see [4] for a review). Indeed, the attraction between two vortices leads to the formation of a vortex pair.…”

Anisotropic superconductors in tilted magnetic fields

“…Some of the above theoretical concepts, in particular, the formation of vortex chains in tilted fields and the attraction of pancake stacks to Josephson strings, were nicely confirmed by experiments. Extensive reviews of vortex behavior in layered high--Tc superconductors can be found in [5,25,26]. However, despite extensive experimental efforts, a larger part of the theoretically predicted vortex configurations in anisotropic superconductors still remains to be confirmed.…”