376 Chapter Ten
10.9 SUPERCONDUCTIVITY
No resistance at all, but only at very low temperatures (so far)Electrical conductors, even the very best, resist to some extent the flow of charge
through them at ordinary temperatures. At very low temperatures, however, most
metals, many alloys, and some chemical compounds all allow current to pass freely
through them. This phenomenon is called superconductivity.
Superconductivity was discovered in 1911 by the Dutch physicist Heike
Kamerlingh Onnes. He found that, down to 4.15 K, the resistance of a mercury sam-
ple decreased with temperature as other metals do (see Fig. 10.18). At Tc4.15 K,
though, the resistance fell sharply to as close to zero as his instruments could measure
(Fig. 10.49). The critical temperatureTcfor other superconducting elements varies
from less than 0.1 K to nearly 10 K. As we shall see later, it is significant that elements
which are ordinarily good conductors, such as copper and silver, do not becomeky(^0) kx
1
(^23)
4
89
10
11 ky
(^0) kx
1
(^23)
4
89
10
ky^11
(^0) kx
1
(^23)
4
23
4
5
Zone boundaries
Vacant energy levels
Fermi level
Occupied
energy levels
(a)
(b)
(c)
Figure 10.48Electron energy contours and Fermi levels in three types of solid: (a) insulator; (b) mono-
valent metal; (c) divalent metal. Energies are in electronvolts.
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