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Superconductivity is a physical phenomenon, which means that the resistance of some materials suddenly disappears at low temperature, showing the characteristics of zero resistance and complete diamagnetism. Superconductivity was first discovered by Dutch scientist Onness in 1911, when he cooled mercury to 4.2 K and found that its resistance dropped to zero. Later, people have found many other superconducting materials, such as lead, tin, niobium and so on.

Superconductivity has two important characteristics: zero resistance and complete diamagnetism. Zero resistance means that superconductors can transmit large currents without loss and generate a strong magnetic field around them. Complete diamagnetism means that the superconductor can repel the external magnetic field and keep the internal magnetic flux unchanged. These two characteristics make superconductors have a wide application prospect in many fields, such as nuclear magnetic resonance imaging, high-energy physics experiments, nuclear fusion devices, energy storage systems, power transmission and so on.

So why do some materials become superconductors at low temperatures? What is the physical mechanism behind this?

Conventional superconductors refer to those materials that produce superconductivity through electron-phonon interaction at low temperatures, such as metals and alloys. The theoretical basis of conventional superconductors is the BCS theory, which was proposed by Bardeen, Cooper and Shriver in 1957.

According to BCS theory, in conventional superconductors, two electrons which are negatively charged and repel each other at low temperature establish indirect attraction by affecting the vibration produced by the atomic lattice (the energy quantum of this vibration is called phonon), so that the two pairs form a "Cooper pair". Under the quantum coherence effect, these "Cooper pairs" can move without loss in the lattice, forming the overall superconductivity.

BCS theory also gives a method for calculating the critical temperature, critical magnetic field, energy gap and other physical quantities of conventional superconductors, which are in good agreement with the experimental results. BCS theory made a great contribution to the explanation and development of conventional superconductivity, and its three founders won the Nobel Prize in Physics in 1972.

The principle of high temperature superconductivity refers to those materials that still exhibit superconductivity at temperatures above 77K (liquid nitrogen temperature), such as copper oxides and iron-based compounds. The discovery of high temperature superconductors broke the critical temperature limit of conventional superconductors predicted by BCS theory, and aroused great shock and widespread concern in the field of physics.

The most important feature of high temperature superconductors is that they have a layered structure, which contains transition metal oxide planes such as copper oxide plane or ferrous arsenic surface. Scientists generally believe that these planes are the key to producing high temperature superconductivity because they provide a large enough density of states and strong electron-electron interaction.

However, the mechanism of electron pairing in high temperature superconductors is still unclear. The electron-phonon interaction proposed in BCS theory is obviously not applicable here because the phonon energy is too low to explain the critical temperature above 100K. Therefore, many other possible pairing mechanisms have been proposed, such as electron-spin wave interaction, electron-polaron interaction, electron-charge wave interaction and so on. However, there is no unified and complete theory to explain all kinds of strange phenomena observed in high temperature superconductors.

High temperature superconductors not only have theoretical challenge and attraction, but also have great application potential. Because they can work in low-temperature media such as liquid nitrogen, they can greatly reduce refrigeration costs and expand the application of superconducting technology in energy, transportation, medical, communications and other fields.

Applications of high temperature superconductors the applications of high temperature superconductors are usually divided into two categories: strong electricity applications and weak electricity applications.

High power application refers to the use of the zero resistance and high current density characteristics of high temperature superconducting materials to realize low-loss, high-efficiency and high-capacity transmission and distribution systems, such as superconducting transformers, superconducting generators, superconducting energy accumulators, superconducting current limiters, superconducting transmission lines and so on. These applications can improve the stability and reliability of the power grid, reduce line loss and carbon emissions, and save energy and land resources.

Weak current application refers to the use of the quantum effect and magnetic flux pinning characteristics of high temperature superconducting materials to achieve high sensitivity, high precision, high speed measurement and signal processing systems, such as superconducting magnets, superconducting quantum interference devices (SQUID), superconducting filters, superconducting logic devices and so on. These applications can be widely used in scientific research, medical diagnosis, communication technology, radar detection and other fields.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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