bei48482_FM

The Solid State 377

superconducting when cooled. The highest critical temperatures, as much as 134 K,

are found in certain ceramic materials.

Does a superconductor actually have zero resistance or just very little? To find out,

currents have been set up in superconducting wire loops and the resulting magnetic

fields monitored, sometimes for years. No decrease in such currents has ever been

found: superconductors do have no resistance at all.

Magnetic Effects

The presence of a magnetic field causes the critical temperature of type I super-

conductorsto decrease in the manner shown in Fig. 10.50. If the magnetic field

exceeds a certain critical value Bc, which depends on the material and its tempera-

ture, its superconductivity disappears altogether. Such materials are superconduc-

tors only for values of Tand Bbelow their respective curves and are normal con-

ductors for values of Tand Babove these curves. The critical field Bcwould be a

maximum at 0 K.

Table 10.3 gives critical temperatures and critical magnetic fields Bc(0) extrapolated

to 0 K for several type I superconductors. The critical fields are all quite low, less than

0.1 T, so type I superconductors cannot be used for the coils of strong electromagnets.

Superconductors are perfectly diamagnetic—no magnetic field can exist inside them

under any circumstances. If we put a sample of a superconductor in a magnetic field

weaker than the critical field and then reduce the temperature below Tc, the field is ex-

pelled from the interior of the sample (Fig. 10.51). What happens is that currents appear

4.0

Tc

4.1 4.2 4.3 4.4

Temperature, K

Resistance

Figure 10.49Resistance of a mercury sample at low temperature. Below the critical temperature of

Tc4.15 K mercury is a superconductor with zero electrical resistance.

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