143 High-temperature Superconductors

Introduction

Some of the most remarkable materials discovered by physicists are superconductors, materials that can conduct electric current with zero electrical resistance below a characteristic temperature known as the critical temperature ([latex]T_c[/latex]). In the superconducting state, electric current can flow indefinitely without losing energy as heat.

This extraordinary property makes superconductors invaluable in technologies that require extremely strong magnetic fields or highly efficient electrical systems. One of the most familiar applications is the magnetic resonance imaging (MRI) scanner, whose powerful magnets are typically made from superconducting coils cooled to cryogenic temperatures.

Superconductors also have potential applications in electrical power transmission, particle accelerators, magnetic levitation (maglev) trains, quantum computing, and highly sensitive scientific instruments. Their ability to eliminate electrical resistance could significantly reduce energy losses in many technologies.

The primary challenge is that most superconductors operate only at very low temperatures. Maintaining these temperatures requires cryogenic cooling, which increases both the complexity and cost of superconducting devices. Consequently, one of the major goals of materials science is to discover superconductors that function at progressively higher temperatures and, ultimately, at room temperature.

Superconductivity

Superconductivity was discovered in 1911 by the Dutch physicist Heike Kamerlingh Onnes. While studying the electrical resistance of mercury cooled with liquid helium, he observed that its electrical resistance suddenly dropped to zero at a temperature of approximately 4.2 K.

The temperature below which a material becomes superconducting is called its critical temperature, denoted by [latex]T_c[/latex>. Above this temperature the material behaves like an ordinary conductor, while below it the electrical resistance becomes immeasurably small.

Superconductors exhibit another remarkable property known as the Meissner effect. When a material enters the superconducting state, it expels magnetic fields from its interior. This behavior distinguishes a superconductor from an ordinary perfect conductor and is responsible for the dramatic magnetic levitation demonstrations often associated with superconductivity.

The discovery of superconductivity revolutionized condensed matter physics and continues to inspire research into new materials with higher critical temperatures and improved performance for medical, scientific, and industrial applications.

High-Temperature Superconductors

For many decades after the discovery of superconductivity, all known superconductors had critical temperatures below about 30 K and required cooling with expensive liquid helium. This limited their widespread use because maintaining such low temperatures is technically challenging and costly.

A major breakthrough occurred in 1986 when researchers discovered a ceramic copper-oxide material that became superconducting at a much higher temperature than previously thought possible. Within only a few years, additional ceramic compounds were found with critical temperatures above 77 K, the boiling point of liquid nitrogen.

This discovery was particularly important because liquid nitrogen is far less expensive and much easier to handle than liquid helium. Cooling superconductors with liquid nitrogen significantly reduces the cost of many superconducting technologies.

Since the late 1980s, scientists have discovered several additional families of superconductors, including iron-based superconductors and other materials with increasingly high critical temperatures. Some materials have exhibited superconductivity at temperatures near or even above room temperature, but only under extremely high pressures that are impractical for most applications. In addition, several highly publicized claims of room-temperature superconductivity have not been independently confirmed.

Although researchers do not yet fully understand the mechanism responsible for high-temperature superconductivity, progress continues in materials science, condensed matter physics, and computational modeling. Discovering a practical room-temperature superconductor remains one of the most important goals in modern physics because it could transform energy transmission, transportation, medical technology, and quantum computing.

Why Do Superconductors Have Zero Resistance?

In an ordinary conductor, such as copper, moving electrons frequently collide with vibrating atoms and other imperfections in the material. These collisions convert electrical energy into thermal energy, producing electrical resistance and heating the conductor.

In a superconductor, a remarkable change occurs below the critical temperature. Instead of moving independently, electrons form weakly bound pairs known as Cooper pairs. These pairs move collectively through the material in a coordinated quantum state that allows them to travel without losing energy in collisions.

This collective behavior explains why a superconductor has essentially zero electrical resistance. Once an electrical current is established in a superconducting loop, it can persist for extremely long periods without an external power source because essentially no electrical energy is lost as heat.

The microscopic theory that explains conventional superconductors was developed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer. Their work, known as BCS theory, showed how interactions between electrons and vibrations of the crystal lattice can produce Cooper pairs.

Although BCS theory successfully explains many conventional superconductors, it does not completely explain the behavior of most high-temperature superconductors. Understanding why certain ceramic materials become superconducting at much higher temperatures remains one of the major unsolved problems in condensed matter physics.

Researchers continue to search for materials that become superconducting at increasingly higher temperatures while also seeking a complete theoretical understanding of these remarkable materials. A practical room-temperature superconductor would revolutionize technologies ranging from electric power transmission and transportation to medical imaging, quantum computing, and scientific research.

Section Summary

• Superconductors are materials that exhibit essentially zero electrical resistance below a characteristic critical temperature ([latex]T_c[/latex]).
• In the superconducting state, electrical current can flow without significant energy loss, making superconductors valuable for applications requiring powerful and efficient electromagnets.
• The Meissner effect causes superconductors to expel magnetic fields from their interior, allowing phenomena such as magnetic levitation.
• High-temperature superconductors have critical temperatures above the boiling point of liquid nitrogen (77 K), making them less expensive to cool than conventional superconductors.
• MRI scanners, particle accelerators, maglev transportation systems, and quantum computing technologies all rely on superconducting materials.
• According to BCS theory, conventional superconductivity occurs when electrons form Cooper pairs that move collectively through a material without electrical resistance.
• The mechanism responsible for most high-temperature superconductors remains an active area of research, and the discovery of a practical room-temperature superconductor would have far-reaching technological applications.

Conceptual Questions

66. What is meant by the critical temperature ([latex]T_c[/latex]) of a superconductor?
67. Why does a superconductor have essentially zero electrical resistance below its critical temperature?
68. How does the Meissner effect distinguish a superconductor from an ordinary conductor?
69. Why are high-temperature superconductors generally less expensive to operate than conventional superconductors?
70. Explain why MRI scanners require superconducting magnets.
71. Why would the discovery of a practical room-temperature superconductor have such a significant technological impact?