What is superconductivity? Believe me, you will definitely understand it in this article

Superconductivity is a "star" in the physics world, and every new development can bring a wave of traffic. Last July, a Korean research team claimed to have successfully synthesized the world's first room-temperature and normal-pressure superconductor. This blockbuster news instantly attracted global attention. What secrets does this highly anticipated "top stream" in the physics world hide?

What is superconductivity?

Everyday materials can be classified as insulators, semiconductors, and conductors based on their conductivity. There is a special kind of conductor that is called a "superconductor" when it is in a "superconducting state." The "superconducting state" describes a state in which certain materials, under certain low temperature conditions, have their electrical resistance reduced to zero and exhibit complete anti-magnetism at the same time.

Superconductor Image Source: Rochelle University

This phenomenon is the first macroscopic quantum effect discovered by humans. It is also a unique landscape full of mystery and beauty in the field of physics.

How was superconductivity discovered?

The superconductivity phenomenon was first discovered by Dutch physicist Heike Kamerlingh Onnes in 1911. In the laboratory, he successfully prepared extremely pure mercury and cooled it to near absolute zero (-273.15℃); when he measured the resistance of mercury, he was surprised to find that when the temperature dropped below 4.2K (-268.95℃), the resistance of mercury suddenly disappeared, showing almost perfect conductivity.

This was a revolutionary discovery. He named this phenomenon "superconductivity" and was awarded the 1913 Nobel Prize in Physics for it.

Dutch physicist Heike Kameling Onnes

In January 1986, Swiss physicist Karl Alexander Müller and his German collaborator Johannes Georg Bednorz announced that they had discovered a type of copper oxide superconductor with a critical temperature as high as 30K (-243.15℃). The discovery of this high-temperature superconductivity broke the people's understanding that superconductivity can only exist at extremely low temperatures, and they won the 1987 Nobel Prize in Physics. Since then, scientists have also discovered a variety of high-temperature superconductors with critical temperatures higher than the liquid nitrogen temperature range.

Swiss physicist Carl Alexander Mueller

Johannes Georg Bednorz, German physicist

Image source: Internet

Why does superconductivity occur?

In order to delve deeper into the nature of superconductivity, we first need to understand why ordinary conductors produce resistance:

PART 0 1

Resistance of a common conductor

Imagine a sunny day, with butterflies happily flapping their wings and flying towards the blooming sunflowers. In the mysterious microscopic world, the electrons that shuttle freely inside the conductor are like butterflies. Once attracted by the electric field, they will move in a specific direction, that is, the direction of the positive pole of the power source.

Although the naughty butterflies (free electrons) have the ability to fly freely, they will always encounter obstacles from unpleasant spiders (central atoms) on their journey to the beautiful sunflowers (positive pole of the power supply); in order to break free from the restraints of these spiders (central atoms), the butterflies will fight hard.

Similarly, the central atoms distributed around the free electrons in a conductor act like spiders; when the free electrons try to move in a particular direction, they inevitably collide with the surrounding central atoms, and this collision causes the movement of the free electrons to be hindered.

Despite all the obstacles, the butterflies (free electrons) never waver in their desire to fly to the sunflower (positive pole of the power source). Similarly, the free electrons always stick to their determination to move toward the positive pole of the power source and keep moving.

Finally, the butterfly (free electron) flew firmly towards the blooming sunflower in its heart (the positive pole of the power supply); this process is just like the free electron breaking free from the constraints of the central atom and successfully reaching its final destination - the positive pole of the power supply.

During the movement of the free electrons and their collisions with the central atom, energy is transferred from the free electrons to the central atom, which then releases this energy into the surrounding environment in the form of heat. From a macroscopic perspective, this is why electrical current generates resistance when it flows through a conventional conductor.

PART 0 2

Zero resistance in superconductors

As mentioned above, superconductors have two remarkable properties: one of which is the zero resistance state. So, compared with the resistance state of conventional conductors, how does the zero resistance state of superconductors occur?

Imagine that when the ambient temperature drops to -196℃, the butterflies (free electrons) that used to be light and free and dancing seem to feel an unprecedented challenge. In order to survive in this harsh low-temperature environment, they no longer fly alone as usual, but choose a new survival strategy - hugging each other (Cooper pairs).

The butterflies (Cooper pairs) hugged each other tightly, as if they had formed a small community of life. In this way, they jointly resisted the cold outside, kept each other warm, and overcame difficulties together. What is amazing is that the "butterfly cp" perfectly avoided all obstacles and finally reached the sunflower in the distance (the positive pole of the power supply).

In the microscopic world of superconductors, free electrons will also exhibit similar behaviors in such a low-temperature environment; they will pair up in pairs to form Cooper pairs, just like butterflies embracing each other, and then move together toward the positive pole of the power source; thus achieving the unimpeded flow of current in the superconductor, that is, the zero resistance state.

In a superconductor, when Cooper pairs are formed and start to move, there is no energy exchange between them and the central atom, that is, there is no transfer and release of energy; this special state, at the macroscopic level, is the fundamental reason why superconductors can exhibit zero resistance.

PART 0 3

Complete diamagnetism of superconductors

Another notable property of superconductors is that they are completely diamagnetic, a phenomenon often referred to as the "Meissner effect."

Specifically, at room temperature, the magnetic field lines can easily penetrate the superconductor; however, once the superconductor is cooled below the superconducting phase transition temperature, it seems to have a magical power inside, which almost completely cancels out the magnetic field and makes the magnetic field lines unable to penetrate the superconductor. This repulsion of the magnetic field is so strong that the superconductor can float above the magnet, showing its unique anti-magnetism.

The principle of the "Meissner effect" is that when a superconductor enters a superconducting state and is acted upon by an external magnetic field, currents are generated on the surface of the superconductor. The magnetic fields generated by these currents and the effects of the external magnetic field cancel each other out, forming a special equilibrium state that reduces the magnetic induction intensity inside the superconductor to almost zero. This phenomenon is a manifestation of the unique physical properties of superconductors.

What is the use of superconductivity?

The wide application of superconducting technology has penetrated into many fields, demonstrating its huge potential and value.

Medical imaging field

Superconducting magnets play a vital role. They can generate strong and stable magnetic fields, which is particularly important in medical diagnostic technologies such as magnetic resonance imaging (MRI). Through the application of superconducting magnets, the resolution and signal-to-noise ratio of imaging can be significantly improved, thus providing doctors with more accurate and reliable basis for disease diagnosis.

Superconducting MRI system

Power system field

Superconducting power technology has shown great potential with its lossless characteristics. The application of superconducting transformers, superconducting energy storage, superconducting current limiters, and superconducting cables in power systems can significantly improve the operating efficiency and stability of traditional power systems and effectively reduce energy waste and environmental pollution.

In addition, superconducting cables can also be used to connect power sources and loads, such as wind power and solar power generation facilities, providing strong support for the large-scale application of renewable energy.

The world's first 35 kV kilometer-level superconducting cable is put into operation in Shanghai

Construction of large scientific facilities

Superconducting magnets play an indispensable role. They can be used to manufacture cutting-edge equipment such as large particle accelerators, "artificial sun" all-superconducting tokamak nuclear fusion devices, and synchrotron radiation sources. Through the operation and use of these large scientific devices, scientists can deeply explore the microstructure and basic laws of matter, and also provide solid support for the research and development of new materials and new energy.

Large scientific device - "artificial sun" all-superconducting tokamak (EAST)

Quantum computing field

In the development of quantum bits and quantum computers, superconductors are used to manufacture quantum bits (qubits), the basic building blocks of quantum computers. Quantum computers use the principles of quantum mechanics to demonstrate computing speeds and capabilities far exceeding those of traditional computers, and can solve a series of problems that are difficult to solve with traditional methods, such as cryptography, optimization problems, and artificial intelligence.

Today, my country's third-generation independent superconducting quantum computer "Benyuan Wukong" has been successfully realized and has completed more than 178,000 computing tasks.

Superconducting Quantum Computing Cloud Platform

END

author:

Wang Yinshun, Executive Director of Beijing Refrigeration Society, Professor of the School of Electrical and Electronic Engineering, North China Electric Power University

He Ye, Shi Yanchen, members of Beijing Refrigeration Society, PhD students at the School of Electrical and Electronic Engineering, North China Electric Power University

Editor: Dong Xiaoxian