Dichalcogenides of transition metals such as NbSe2, superconductor/insulator superlattices, and high Tc oxide superconductors all have a layered structure. Most of these systems are Type II superconductors with relatively large K values. One of the fascinating characteristics of the layered superconductors is their strongly anisotropic magnetic properties. Usually, the coherence length perpendicular to the layer plane (ξ⊥ is much smaller than that parallel to the layer (ξ∥). The anisotropy can be characterized by an anisotropy ratio, γ, defined as γ = ξ∥ξ⊥. Other length scales of interest are the penetration length, λ, and the scale of intrinsic inhomogeneities.

Depending on the relative size of ξ⊥ and the layer spacing, s, we may identify three different regimes for the vortex structure in the layered, high-K superconductors: (1) If ξ⊥(T) > > s, then the layered structure is largely irrelevant and the superconductor may be regarded as threedimensional: anisotropic, but uniform. Since the coherence length diverges as Tc is approached, this regime will always occur sufficiently close to Tc. The vortex structure can be described using the London or G–L theories by introducing an anisotropic mass tensor as discussed in Sec. 11. (2) With decreasing temperature, we may have a regime where ξ⊥(T) < < s (especially in high Tc oxides). If both regimes can be entered by sweeping the temperature a 3D–2D crossover will occur at some temperature.