Superconductors: A Breakthrough in Modern Physics

Superconductors are materials that completely lose their electrical resistance when cooled below a critical temperature, known as the superconducting transition temperature (T_c). This phenomenon was first discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes. Normally, materials like copper or silver gradually reduce their resistance as they cool, but superconductors exhibit a sudden drop to zero resistance at T_c. At this point, an electric current can flow indefinitely without energy loss. Superconductors also repel external magnetic fields, a characteristic known as the Meissner effect.

High-Temperature Superconductors

In 1986, scientists discovered that certain ceramic materials, such as copper oxide-based compounds with a perovskite structure, could act as superconductors at temperatures higher than previously thought, often exceeding 90K (-183°C). These are referred to as high-temperature superconductors. This discovery was a major breakthrough and spurred extensive research in chemistry, physics, and materials science, with potential applications in fields like high-speed computing, magnetic levitation trains, and energy-efficient power transmission.

Historical Background

Onnes first observed superconductivity in 1911 by cooling mercury to 4.2K using liquid helium. Since then, over 6,000 superconducting materials have been discovered, each with a unique T_c. Theoretical understanding advanced significantly in 1957 when John Bardeen, Leon Cooper, and Robert Schrieffer proposed the BCS theory, which explained superconductivity in terms of interactions between electrons and lattice vibrations (phonons). According to this theory, electrons form pairs (called Cooper pairs), which move through the lattice without resistance, forming the basis of superconductivity.

Until 1986, the highest T_c recorded was 23.2K in a compound called Nb₃Ge. However, this record was broken when K. A. Müller and J. G. Bednorz synthesized a barium-lanthanum copper oxide compound with a T_c of 35K. Since then, compounds such as YBa₂Cu₃O₇ and HgBa₂Ca₂Cu₃O_8+x have been discovered with even higher critical temperatures, reaching up to 138K.

Structure of High-Temperature Superconductors

High-temperature copper oxide superconductors have a complex crystal structure involving layers of copper-oxygen planes. These planes are essential for superconductivity, and increasing their number can enhance the T_c value up to a certain point. For instance, in YBa₂Cu₃O₇, the molar ratio of yttrium, barium, and copper is 1:2:3, leading to its nickname as the '1-2-3 superconductor.'

Table of High-Temperature Superconductors:
YBa₂Cu₃O₇: T_c = 92K, Structure: Orthorhombic
Bi₂Sr₂CaCu₂O₈: T_c = 85K, Structure: Tetragonal
Tl₂Ba₂Ca₂Cu₃O₁₀: T_c = 125K, Structure: Tetragonal

The Meissner Effect

The Meissner effect is one of the most fascinating properties of superconductors, allowing them to levitate magnets. When a superconductor is cooled below its critical temperature and a magnet is placed near it, the superconductor generates currents on its surface that produce a magnetic field opposite to that of the magnet. This repulsion makes the magnet float above the superconductor. This levitation effect has practical applications in technologies like magnetic levitation (maglev) trains, where the lack of friction allows for extremely efficient and high-speed transportation.

Applications of Superconductors

Superconductors have numerous applications across various industries. Superconducting magnets are crucial in medical imaging technologies like Magnetic Resonance Imaging (MRI) and in highly sensitive measuring devices such as Superconducting Quantum Interference Devices (SQUIDs). Additionally, superconductors are used in particle accelerators to control the paths of charged particles. However, these applications often rely on traditional superconductors, which need cooling with liquid helium to achieve T_c values around 4.2K—a costly process.

The discovery of high-temperature superconductors that operate at more accessible temperatures, such as the boiling point of liquid nitrogen (77K), opens up new possibilities for more practical applications. For example, superconducting power lines could drastically reduce energy loss during long-distance transmission, and current technology even allows for the production of superconducting wires up to 1km in length. Superconductors are also used in microwave filters for cell phone towers and are integral to the development of maglev trains, such as those in operation in Shanghai.