Page 406 - Electrical Properties of Materials

388 Superconductivity

(a)

(b)

Fig. 14.12
(a) The lines of current flow for a
two-dimensional type II
superconductor. The magnetic field is
maximum at the centres of the smaller
vortices and minimum at the centres
of the larger ones. (After Abrikosov,
1957.) (b) Triangular vortex structure
measured on the surface of a
lead–indium rod at 1.1 K (after
Essmann and Träuble, 1967).

The density of superconducting electrons is zero at the maxima of the mag-
netic field. Thus, in a somewhat simplified manner, we may say that there is a
normal region surrounded by a supercurrent vortex. There are lots of vortices;
their distance from each other is about 1 μm. The vortex structure determined
experimentally by Essmann and Träuble is shown in Fig. 14.12(b). It has a
triangular structure, somewhat different from that calculated by Abrikosov.
The preceding treatment of the theories of superconductivity is not a well-
balanced one, neither historically nor as far as their importance is concerned.
A comprehensive review would be far too lengthy, so we have just tried to
follow one line of thought.