Serway 12

12
Superconductivity
Chapter Outline
12.1 Magnetism in Matter Magnetic Moments of Atoms Magnetization and Magnetic Field Strength Classification of Magnetic Substances Ferromagnetism Paramagnetism Diamagnetism A Brief History of Superconductivity Some Properties of Type I Superconductors Critical Temperature and Critical Magnetic Field Magnetic Properties of Type I Superconductors PenetrationDepth Magnetization Type II Superconductors Other Properties of Superconductors Persistent Currents Coherence Length Flux Quantization Electronic Specific Heat BCS Theory Energy Gap Measurements Single-Particle Tunneling Absorption of Electromagnetic Radiation 12.9 Josephson Tunneling The dc Josephson Effect The ac Josephson Effect Quantum Interference 12.10 High-Temperature SuperconductivityMechanisms of High-Tc Superconductivity 12.11 Applications of Superconductivity (Optional) Summary ESSAY Superconducting Devices, by Clark A. Hamilton 12.6 12.7 12.8

12.2 12.3

12.4 12.5

Most of the material covered in this chapter has to do with the phenomenon
of superconductivity. As we shall see, magnetic fields play an important role in the field of superconductivity, so it is important tounderstand the magnetic properties of materials before discussing the properties of superconductors. All magnetic effects in matter can be explained on the basis of the current loops associated with atomic magnetic dipole moments. These atomic magnetic moments arise both from the orbital motion of the electrons and from an intrinsic property of the electrons known as spin. Our description ofmagnetism in matter is based in part on the experimental fact that the presence of bulk matter generally modifies the magnetic field produced by currents. For ex476

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MAGNETISM IN MATTER

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ample, when a material is placed inside a current-carrying solenoid, the material sets up its own magnetic field, which adds vectorially to the field that was already present. The phenomenon ofsuperconductivity has always been very exciting, both for its fundamental scientific interest and because of its many applications.1 The discovery in the 1980s of high-temperature superconductivity in certain metallic oxides sparked even greater excitement in the scientific and business communities. Many scientists consider this major breakthrough to be as important as the invention of the transistor. Forthis reason, it is important that all students of science and engineering understand the basic electromagnetic properties of superconductors and become aware of the scope of their current applications. Superconductors have many unusual electromagnetic properties, and most applications take advantage of such properties. For example, once a current is produced in a superconducting ring maintained at asufficiently low temperature, that current persists with no measurable decay. The superconducting ring exhibits no electrical resistance to direct currents, no heating, and no losses. In addition to the property of zero resistance, certain superconductors expel applied magnetic fields so that the field is always zero everywhere inside the superconductor. As we shall see, classical physics cannotexplain the behavior and properties of superconductors. In fact, the superconducting state is now known to be a special quantum condensation of electrons. This quantum behavior has been verified through such observations as the quantization of magnetic flux produced by a superconducting ring. In this chapter we also give a brief historical review of superconductivity, beginning with its discovery in1911 and ending with recent developments in high-temperature superconductivity. In describing some of the electromagnetic properties displayed by superconductors, we use simple physical arguments whenever possible. We explain the essential features of the theory of superconductivity with the realization that a detailed study is beyond the scope of this text. Finally, we discuss many of the...