Superconductor

A material that exhibits superconductivity under certain conditions.

Superconductor (English name: superconductor), also known as superconducting material, refers to a conductor with zero resistance at a certain temperature. In experiments, if the measured value of the conductor resistance is lower than 10-25Ω, the resistance can be considered as zero.

Superconductors not only have the property of zero resistance, another important feature is complete diamagnetism.
Humans first discovered superconductors in 1911. In this year, Dutch scientist Heike Kamerlingh Onnes and others discovered that mercury’s resistance disappeared at extremely low temperatures and it was in a superconducting state. Since then, the research on superconductors has become more and more in-depth. On the one hand, a variety of superconducting materials with practical potential have been discovered. On the other hand, the research on the superconducting mechanism has also made some progress.

Superconductors have been used in a series of experimental applications, and have certain military and commercial applications, and can be used as defect materials for photonic crystals in the field of communication.

Background

The discovery of superconductors is inseparable from the study of low temperatures. In the 18th century, due to the limitations of cryogenic technology, it was believed that there were “permanent gases” that could not be liquefied, such as hydrogen, helium, etc. In 1898, British physicist Dewar made liquid hydrogen. In 1908, Professor Camerin Onnes of the Leiden Low Temperature Laboratory of Leiden University in the Netherlands successfully liquefied helium, the last “permanent gas”, and obtained 1.15~4.25 by reducing the vapor pressure of liquid helium. K low temperature. [2] The breakthrough in low temperature research laid the foundation for the discovery of superconductors.

At the end of the 19th century and the beginning of the 20th century, there were different opinions about the change of the resistance of metals near absolute zero. One view is that the electrical resistance of pure metals should decrease with decreasing temperature and disappear at absolute zero. Another point of view, represented by William Thomson (Baron Kelvin), believes that as the temperature decreases, the resistance of the metal will become infinite due to the condensation of electrons on the metal atoms after reaching a minimum value.

In February 1911, Catherine Onnes, who had mastered liquid helium and cryogenic technology, discovered that below 4.3K, the resistance of platinum remained a constant instead of increasing after passing through a minimum value. Therefore, Catherine Onnes believed that the resistance of pure platinum should disappear at the temperature of liquid helium. In order to verify this conjecture, Camerin Onnes chose mercury, which is easier to purify, as the experimental object. First, Catherine Onnes cooled the mercury to minus 40°C to solidify the mercury into a line; then used liquid helium to lower the temperature to around 4.2K, and applied a voltage across the mercury line; when the temperature was slightly lower than 4.2K K, the resistance of mercury suddenly disappears, showing a superconducting state.

Basic Features

Superconductors have three fundamental properties: complete conductivity, complete diamagnetism, and flux quantization.
fully conductive

Complete conductivity, also known as zero resistance effect, refers to the phenomenon that the resistance suddenly disappears when the temperature drops below a certain temperature.

Complete conductivity is suitable for direct current. When a superconductor is exposed to alternating current or an alternating magnetic field, there will be AC loss, and the higher the frequency, the greater the loss. [1] AC loss is an important problem to be solved in the practical application of superconductors. Macroscopically, AC loss is caused by the difference between the induced electric field and the induced current density generated inside the superconducting material; microscopically, AC loss is caused by the quantized magnetic Caused by viscous motion of the through-wire. AC loss is an important parameter to characterize the performance of superconducting materials. If the AC loss can be reduced, the cooling cost of the superconducting device can be reduced and the operation stability can be improved.

Completely Diamagnetic

Complete diamagnetism is also known as the Meissner effect. “Diamagnetism” means that when the magnetic field strength is lower than the critical value, the magnetic field lines cannot pass through the superconductor, and the internal magnetic field of the room superconductor is zero. “Complete” means that the temperature is reduced to reach the superconducting state. The order of the two operations of applying a magnetic field can be reversed. The reason for complete diamagnetism is that a lossless diamagnetic superconducting current can be generated on the surface of the superconductor, and the magnetic field generated by this current cancels the magnetic field inside the superconductor.

The zero-resistance properties of superconductors are well known, but superconductors are not the same as ideal conductors. Starting from electromagnetic theory, the following conclusions can be deduced: if the ideal conductor is cooled to a low temperature first, and then placed in a magnetic field, the internal magnetic field of the ideal conductor is zero; but if the ideal conductor is first placed in a magnetic field, and then cooled to a low temperature, the ideal The magnetic field inside a conductor is not zero. For superconductors, the two operations of lowering the temperature to reach the superconducting state and applying a magnetic field, regardless of the order, the internal magnetic field of the superconductor is always zero, which is the core of complete diamagnetism and the key to distinguish superconductors from ideal conductors.

Flux Quantization

Flux quantization, also known as the Josephson effect, refers to the phenomenon that when the insulating layer between two superconductors is as thin as the atomic size, electron pairs can pass through the insulating layer to generate tunnel current, that is, in the superconductor (superconductor) – insulator (insulator) – Superconductor (superconductor) structure can produce superconducting current.

The Josephson effect is divided into DC Josephson effect and AC Josephson effect. The DC Josephson effect means that electron pairs can form a superconducting current through an insulating layer. The AC Josephson effect means that when the applied DC voltage reaches a certain level, in addition to the DC superconducting current, there is also an AC current. If the superconductor is placed in a magnetic field, the magnetic field penetrates into the insulating layer, and the maximum superconducting current of the superconducting junction increases with the external current. The size of the magnetic field changes regularly.

Critical Parameters

A superconductor has three critical parameters: critical transition temperature Tc, critical magnetic field strength Hc, and critical current density Jc. Superconductivity is only exhibited when the superconductor is within the three critical conditions simultaneously.

(1) Critical transition temperature Tc: When the temperature is lower than the critical transition temperature Tc, the material is in a superconducting state; exceeding the critical transition temperature Tc, the superconductor returns to a normal state from a superconducting state.

(2) Critical magnetic field strength Hc: When the external magnetic field strength exceeds the critical magnetic field strength Hc, the superconductor returns to a normal state from a superconductor. The critical magnetic field strength Hc is related to temperature, and the relationship is as follows:

(3) Critical current density Jc: When the current density passing through the superconductor exceeds the critical current density Jc, the superconductor returns to a normal state from a superconductor. The critical current density Jc is related to temperature and magnetic field strength.

Theoretical Explanation

In order to clarify the mechanism of superconductors, scientists have proposed a variety of theories, including: the London equation proposed in 1935 to describe the relationship between superconducting current and weak magnetic field; the Pippard theory proposed in 1950~1953 to perfect the London equation; GL (Ginzburg-Landau) theory proposed in 1950 to describe the relationship between superconducting current and strong magnetic field (close to the critical magnetic field strength); proposed in 1957 to explain the first type of superconductor BCS (Bardeen-Cooper -Schrieffer) theory, etc. [4] [5] The more important theories include BCS theory and GL theory.

BCS theory

The BCS theory is based on the near-free electron model and is established on the premise of weak electron-phonon interaction. The proponents of the theory are J.Bardeen, L.V.Cooper, and J.R.Schrieffer.

BCS theory believes that electrons with opposite spin and momentum in metals can pair to form Cooper pairs, and Cooper pairs can move without loss in the lattice to form superconducting currents. The BCS theory explains the reason for the Cooper pair as follows: When electrons move in the lattice, they will attract positive charges on adjacent lattice points, resulting in local distortion of the lattice points, forming a localized high positive charge area. This localized high positive charge region will attract electrons with opposite spins, and combine with the original electrons with a certain binding energy. At very low temperatures, this binding energy may be higher than the vibration energy of the lattice atoms, so that the electron pairs will not exchange energy with the lattice, and there will be no resistance, forming a superconducting current.

The BCS theory well explained the reason for the existence of the first type of superconductor from the microscopic level, and the authors of the theory, Bardeen, Cooper, and Schriever, won the 1972 Nobel Prize in Physics for this reason. However, the BCS theory cannot explain the reason for the existence of the second type of superconductor, especially the Macmillan limit temperature (the critical transition temperature of a superconductor cannot be higher than 40K) obtained according to the BCS theory, which has already been broken by the second type of superconductor.

GL theory

GL theory is a phenomenological theory proposed on the basis of Landau’s second-order phase transition theory. The proponents of the theory are Ginzburg and Landau.

The GL theory is based on the following considerations: when the external magnetic field strength is close to the superconductor’s adjacent magnetic field strength, the current of the superconductor does not obey the linear law, and the zero-point vibration energy of the superconductor cannot be ignored.

The greatest contribution of GL theory is to predict the existence of the second type of superconductor. Starting from the GL theory, the concept of surface energy κ can be derived. When the surface energy of the superconductor is κ, it is the first type of superconductor; when the surface energy of the superconductor is κ, it is the second type of superconductor.

Classification

Superconductors can be classified in the following ways:

(1) According to the response of the material to the magnetic field: the first type of superconductor and the second type of superconductor. From the perspective of macroscopic physical properties, the first type of superconductor has only a single critical magnetic field strength; the second type of superconductor has two critical magnetic field strength values. Between the two critical values, the material allows part of the magnetic field to penetrate the material. From a theoretical point of view, as stated in the GL theory in the “Theoretical Explanation” above, the parameter κ is the criterion for dividing the two types of superconductors.

Among the elemental superconductors that have been discovered, the first type superconductors account for the majority, and only vanadium, niobium, and technetium belong to the second type superconductors; but many alloy superconductors and compound superconductors belong to the second type superconductors. [6]

(2) According to the explanatory theory: traditional superconductors (can be explained by BCS theory or its inferences) and non-traditional superconductors (cannot be explained by BCS theory).

(3) According to the critical temperature: high temperature superconductor and low temperature superconductor. High-temperature superconductors generally refer to superconductors whose critical temperature is higher than the temperature of liquid nitrogen (greater than 77K), and low-temperature superconductors generally refer to superconductors whose critical temperature is lower than the temperature of liquid nitrogen (less than 77K).

(4) According to the type of material: elemental superconductors (such as lead and mercury), alloy superconductors (such as niobium-titanium alloy), oxide superconductors (such as yttrium barium copper oxide), organic superconductors (such as carbon nanotubes).

History

early 20th century

In 1911, the Dutch scientist Kamerin Onnes cooled mercury with liquid helium. When the temperature dropped to 4.2K (-268.95°C), the resistance of mercury disappeared completely. Kamerin called this phenomenon superconductivity. Kammering received the Nobel Prize in 1913 for this.

In 1933, two scientists, Meissner and Oxenfeld, discovered the complete diamagnetism of superconductors, which was later called the “Meissner effect”.

From March 16, 1954 to September 5, 1956, in order to prove that the resistance of superconductors is zero, scientists put a lead ring into a space with a temperature lower than Tc=7.2K, and used electromagnetic induction to make it An induced current is induced in the ring. The current has not decayed in two and a half years, which means that there is no loss of electric energy in the ring. When the temperature rises above Tc, the ring changes from a superconducting state to a normal state, and the resistance of the material increases suddenly. The current disappears immediately, which is the famous Onnes persistent current experiment.

In 1962, Josephson, a graduate student at the University of Cambridge, predicted theoretically that electrons could pass through a thin insulating layer between two superconductors. In less than a year, Anderson, Rowell and others confirmed Josephson’s prediction experimentally. . This important discovery provides evidence for the movement of electron pairs in superconductors, and enables a deeper understanding of the nature of superconductivity. The Josephson effect forms the basis for the detection of weak electromagnetic signals and other electronics applications.

1970s

In 1973, a superconducting alloy – niobium-germanium alloy was discovered, and its critical superconducting temperature was 23.2K (-249.95°C), and this record has been maintained for nearly 13 years.

In 1979, on the Miyazaki Line, Japan’s experimental railway, a superconducting train successfully carried out a manned feasibility test with a speed of 517 kilometers per hour.

1980s

In 1980, Denmark’s Bechgaard and others synthesized the first organic superconductor (TMTSF) 2PF6.

In 1986, Müller and Bernoz discovered that a ceramic metal oxide LaBaCuO4 composed of barium, lanthanum, copper, and oxygen has high-temperature superconductivity, and the critical temperature can reach 35K (-240.15°C). Since ceramic metal oxides are usually insulating substances, this discovery is of great significance, and Müller and Bernoz won the 1987 Nobel Prize in Physics for this reason. Since then, research on high-temperature superconductivity has developed rapidly.

In 1986, Bell Laboratories in the United States developed a superconducting material with a critical superconducting temperature of 40K (-235.15°C), breaking the “temperature barrier” (40K) of liquid hydrogen.

In 1987, Chinese-American scientist Zhu Jingwu, a professor at the University of Houston, and Chinese scientist Zhao Zhongxian successively developed yttrium-barium-copper-oxygen materials, and the critical superconducting temperature was raised to above 90K (-185.15°C), breaking the “temperature barrier” of liquid nitrogen. (77K).

At the end of 1987, it was discovered that the critical temperature of thallium-barium-calcium-copper-oxygen materials reached 125K (-150.15°C). In just over a year from 1986 to 1987, the critical superconducting temperature increased by nearly 100K.

In 1988, Japan’s Hitachi found that the critical temperature of mercury-based superconducting materials reached 135K, and under high pressure conditions, its critical temperature will reach 164K.

1990s

In March 1991, Japan’s Sumitomo Electric Industries Corporation demonstrated the world’s first superconducting magnet.

In October 1991, the Japan Institute of Atomic Energy and Toshiba jointly developed a superconducting coil for nuclear fusion reactors made of niobium and tin compounds. The current density of the coil reaches 40 amperes per square millimeter, which is more than three times that of the past.

In 1992, a superconducting super collider super-large facility based on giant superconducting magnets was built and put into use in Texas, USA, at a cost of more than 8.2 billion US dollars.

On January 27, 1992, the first ship using superconducting magnetic fluid propulsion, “Yamato” 1, built by the Japan Ship and Ocean Foundation, was launched in Kobe, Japan for sea trials.

In 1996, the European cable giant Pirelli Cable Company, the American Superconductor Corporation and the Electric Power Research Institute in San Francisco jointly made the first underground high-temperature superconducting power transmission cable. It is made of liquid nitrogen empty tube made of superconducting material.

In 1999, Bernhard et al. from the Max Planck Institute in Germany discovered that the ruthenium-copper compound RuSr2GdCu2O8-δ has both superconductivity and ferromagnetic order, its superconducting critical temperature is 15~40K, and its ferromagnetic transition temperature is 133~ 136K. [10] Because the compound has both superconductivity and ferromagnetic order, it has great application potential in computer data storage.

early 21st century

On January 29, 2004, a joint research team composed of scientists from the American Institute of Standards and Technology and the University of Colorado proposed a new form of matter – fermionic condensate (fermionic condensate), and predicted that it would help humans make the following A generation of superconductors.

In 2006, Professor Hideo Hosono of Tokyo Institute of Technology synthesized LaFeOP, a compound with iron as the superconducting body, and pioneered the research on iron-based superconductors.
In September 2012, the University of Leipzig in Germany discovered that graphite particles can exhibit superconductivity at room temperature.

Application

The applications of superconductors can be divided into three categories: strong current applications, weak current applications and diamagnetic applications. Strong current applications are high current applications, including superconducting power generation, transmission and energy storage; weak current applications are electronics applications, including superconducting computers, superconducting antennas, superconducting microwave devices, etc.; antimagnetic applications mainly include maglev trains and thermonuclear Fusion reactors, etc.

Strong current application

Superconducting generator: Superconducting generator has two meanings. One meaning is to replace the copper winding of an ordinary generator with a superconductor winding to increase the current density and magnetic field strength, which has the advantages of large power generation capacity, small size, light weight, small reactance, and high efficiency. [15] Another meaning refers to the superconducting magnetic fluid generator. The magnetic fluid generator has the advantages of high efficiency and large power generation capacity, but the traditional magnet will generate a lot of loss during the power generation process, while the superconducting magnet itself loses Small, can make up for this deficiency.

Superconducting power transmission: Superconducting wires and superconducting transformers made of superconducting materials can transmit electricity to users with almost no loss. According to statistics, about 15% of the power loss is on the transmission line when copper or aluminum wires are used for power transmission. In China alone, the annual power loss reaches more than 100 billion kWh. If it is changed to superconducting power transmission, the saved electric energy is equivalent to building dozens of large power plants.

Weak current application

Superconducting computer: High-speed computers require dense arrangement of components and connection lines on integrated circuit chips, but densely arranged circuits will generate a lot of heat during operation, and heat dissipation is a difficult problem faced by VLSI. VLSI in superconducting computers, the interconnection lines between its components are made of superconducting devices with close to zero resistance and ultra-micro heating, so there is no heat dissipation problem, and the computing speed of the computer is greatly improved. In addition, scientists are studying semiconductors and superconductors to make transistors, or even completely superconductors to make transistors.

Anti-magnetic application

Superconducting maglev train: Using the diamagnetism of superconducting materials, place superconducting materials on top of a permanent magnet. Since the magnetic force lines of the magnet cannot pass through the superconductor, a repulsive force will be generated between the magnet and the superconductor, making the superconductor suspend above the magnet. . Using this magnetic levitation effect can make high-speed superconducting magnetic levitation train.

Nuclear fusion reactor “magnetic enclosure”: During nuclear fusion reactions, the internal temperature is as high as 100 million to 200 million degrees Celsius, and no conventional materials can contain these substances. The strong magnetic field generated by the superconductor can be used as a “magnetic enclosure” to surround and confine the ultra-high temperature plasma in the thermonuclear reactor, and then release it slowly, thus making controlled nuclear fusion energy a promising new energy source in the 21st century.