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The Solid State 379

(a) T > Tc (b) T < Tc

B B

Figure 10.51The Meissner effect. (a) An applied magnetic field can exist inside a superconductor at tem-

peratures above its critical temperature Tc. (b) When the superconductor is then cooled below Tc, sur-

face currents appear whose effect is to expel the magnetic field from the interior of the superconductor.

Tc

Bc 1

Bc 2 (0)

Bc 1 (0)

Critical magnetic field

Temperature

Bc 2

Figure 10.52Variation of the critical magnetic fields Bc 1 and Bc 2 with temperature for a type II su-

perconductor. For magnetic fields between Bc 1 and Bc 2 the material is in a mixed state in which it is

superconducting but a magnetic field can exist in its interior.

contains some magnetic flux but is superconducting. The stronger the external field,

the more flux penetrates the material, up to the higher critical field Bc 2.

A type II superconductor behaves as though it consists of filaments of normal and

of superconducting matter mixed together. A magnetic field can exist in the normal

filaments, while the superconducting filaments are diamagnetic and resistanceless like

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