In High Temperature Superconductors

The anisotropy in HTSs is reflected in a special structure of magnetic vortices in them. They are described as a pancake-shaped structures residing in CuO2 layers, with weak connection between the pancake vortices in the neighboring CuO2 layers (Fig. 3) [64, 65]. The higher the anisotropy of the superconductor, the weaker this connection. Because of a slight anisotropy in the ab-plane, the shape of the pancakes is elliptical. The vortex core is of the size of £a and £b in the direction of a- and b-axes, respectively. The screening currents circulate the core over the length scale of Aa and Ab in the direction of b- and a-axes, respectively.

The density of the vortices depends on magnetic field, because each of the vortices carries one magnetic flux quantum i>0. At elevated fields, the density of the vortices is high and they closely approach one another. Because of this, there is a strong interaction between the pancake vortices in

Figure 3. Schematic drawing of pancake vortices in high-temperature superconductors. The nonsuperconducting core (solid ellipse) of the size of coherence length £ is surrounded by screening currents (open ellipse) that decay with the distance from the core, with a characteristic decay length A. The diagram is not to scale, because £ ^ A for high-temperature superconductors. The planes represent the CuO2 layers. There is only weak interaction between the pancake vortices in neighboring layers and therefore there is almost no correlation of the distribution of pancake vortices in different layers. The shaded area is a columnar defect, which pins down the pancake vortices in different CuO2 layers along a straight line.

Figure 3. Schematic drawing of pancake vortices in high-temperature superconductors. The nonsuperconducting core (solid ellipse) of the size of coherence length £ is surrounded by screening currents (open ellipse) that decay with the distance from the core, with a characteristic decay length A. The diagram is not to scale, because £ ^ A for high-temperature superconductors. The planes represent the CuO2 layers. There is only weak interaction between the pancake vortices in neighboring layers and therefore there is almost no correlation of the distribution of pancake vortices in different layers. The shaded area is a columnar defect, which pins down the pancake vortices in different CuO2 layers along a straight line.

each CuO2 layer in high magnetic fields. In contrast to this, magnetic field strongly suppresses the interaction between the pancake vortices in neighboring layers [65].

This makes it very difficult to suppress the vortex motion. If there is no strong interaction between the vortices, pinning of one of them will not prevent the others from moving. Introducing the pinning center for each of the vortices is not a viable solution, because the large density of defects in the crystal would destroy the superconductivity in it. The only feasible way is to strengthen the interaction between the pancake vortices. By pinning one pancake vortex in an array of strongly interacting vortices, the unpinned vortices will be tied to the pinning center via this interaction.

Was this article helpful?

0 0

Post a comment