bei48482_FM

378 Chapter Ten

on the surface of the sample whose magnetic fields exactly cancel the original field inside

it. This Meissner effectwould not occur in an ordinary conductor whose resistance we

can imagine reduced to zero; it is characteristic only of superconductivity, which is evi-

dently a unique state of matter in respects other than ability to conduct electric current.

Type I superconductors exist only in two states, normal and superconducting.

Type II superconductors,which were discovered several decades later and are usu-

ally alloys, have an intermediate state as well. Such materials have two critical mag-

netic fields, Bc 1 and Bc 2 (Fig. 10.52). For an applied magnetic field less than Bc 1 , a type

II superconductor behaves just like its type I counterpart when BBc: it is super-

conducting with no magnetic field in its interior. When BBc 2 , a type II supercon-

ductor exhibits normal behavior, again like a type I superconductor. However, in applied

fields between Bc 1 and Bc 2 , a type II superconductor is in a mixed state in which it

0.10

Superconductor

Ordinary conductor

Bc(0)

Tc

0.08

0.06

0.04

0.02

04 6 8 10

Temperature, K

Critical magnetic field

Bc

, T

2

Figure 10.50Variation of the critical magnetic field Bcwith temperature for lead. Below the curve,

lead is a superconductor; above the curve, it is an ordinary conductor.

Table 10.3 Critical Temperatures and

Critical Magnetic Fields (at T0) of

Some Type I Superconductors

Superconductor Tc, K Bc(0),T

Al 1.18 0.0105

Hg 4.15 0.0411

In 3.41 0.0281

Pb 7.19 0.0803

Sn 3.72 0.0305

Zn 0.85 0.0054

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