How will superconducting materials change our world?

Superconductivity phenomenon (Image source: Oak Ridge National Laboratory, USA)

In the science fiction movie "Avatar", the magical room-temperature superconducting ore "Unobtanium" on the planet Pandora is amazing. With its superconducting properties, this ore enables towering "Hallelujah" mountains to float lightly in the air, creating a dreamlike alien wonder.

Stills from the movie Avatar

In the world of science, superconducting materials are like a magical "philosopher's stone". They have "superpowers" that go against common sense. They have zero resistance when electric current passes through them and can repel magnetic fields. These properties make them have unparalleled application potential in many fields, making them a hot topic for scientists to study. Since their discovery, they have attracted many scientists to explore.

What is a superconductor

A superconductor is a material that has the following two properties: when a certain temperature and magnetic field are reached, ① the electrical resistance disappears; and ② the internal magnetic field is completely rejected.

Left: Heike Kamerlingh Onnes first discovered the disappearance of resistivity in mercury at T = 4.21 K; Right: Meissner effect (Image source: Harvard University website)

In 1911, Dutch scientist Heike Kamerlingh-Onnes first discovered that in an extremely low temperature environment, when the temperature dropped to 4.2K, the resistivity of mercury approached zero, confirming for the first time the existence of the superconductivity phenomenon.

In 1933, Meissner and Ochsenfeld discovered that under certain temperature and magnetic field conditions, superconductors can achieve complete repulsion of their internal magnetic flux, a phenomenon later known as the Meissner effect.

Over the next few decades, theorists worked hard to find a microscopic theory of superconductivity. Significant progress was made with the London theory in 1935 and the Ginzburg–Landau theory in 1950. But it was not until 1957, a full 46 years after the initial experimental discovery of superconductivity, that Bardeen, Cooper, and Schrieffer proposed an important microscopic theory of superconductivity that was widely accepted, the famous BCS theory.

The three discoverers of BCS theory were awarded the 1972 Nobel Prize in Physics.

Simply put, when a piece of metal conducts electricity, energy is lost due to particle collisions, and the higher the temperature, the more energy is lost, which means the greater the resistance.

Particle motion in metals

When the temperature drops to a certain level, the thermal motion of the particles can be ignored. At this time, when electrons pass through, they will attract the surrounding atoms and also attract the following electrons, causing the two electrons to gather together to form a Cooper pair.

The force of Cooper pairs is very weak, and thermal motion can easily destroy it. When Cooper pairs are formed, the electrons that were originally two fermions will have the characteristics of bosons, making the electrons in the same state of the lowest energy level. At this time, the electrons can pass through completely without loss, which is the occurrence of superconductivity.

Cooper pair

Of course, the BCS theory is only applicable to the explanation of low-temperature conventional type I superconductors, and the principles of many unconventional superconductors are still unknown to us.

The periodic table, with the critical temperatures of superconducting elements marked.

Low-temperature superconductivity: a veteran pioneer

Low-temperature superconductors generally refer to materials with a critical temperature below 30K, which are cooled by liquid helium (Tc > 4.2 K) to reach a superconducting state. This type of superconductor is also within the scope of what the BCS theory can explain.

Mercury, as the first discovered superconductor, is a typical low-temperature superconducting material, with a superconducting critical temperature of about 4.2K, which is in an extremely low temperature environment close to absolute zero. At such a low temperature, the thermal vibration of mercury atoms is greatly weakened, allowing electrons to pair up smoothly to form Cooper pairs, thereby exhibiting superconducting properties.

In addition to mercury, niobium titanium alloy (NbTi) and niobium tin (Nb₃Sn) are also common low-temperature superconducting materials. With their excellent superconducting properties, they are widely used in many fields such as magnetic resonance imaging (MRI), particle accelerators, and nuclear fusion devices.

High-temperature superconductors: a rising star

With the continuous advancement of science and technology, high-temperature superconducting materials came to the fore in the 1980s.

In 1986, Bednorz and Müller, working at IBM in Switzerland, discovered a new class of superconducting materials, LaBaCuO (30K). The following year, with the discovery of YBa2Cu3O7-x (90K), the liquid nitrogen temperature barrier (77K) was broken.

The economic potential of saving electricity for superconductors above 77K is huge, because this is the boiling point of liquid nitrogen. Although liquid helium can be used to reduce the temperature to 4K to make superconducting materials, the cost per liter is about $5. But if the temperature only needs to be reduced to above 77K, the cost per liter of liquid nitrogen is only about $0.30.

There are currently two types of high-temperature superconducting materials, one is copper oxide and the other is iron arsenic or iron selenide. Simply put, one is called a copper-based superconductor and the other is called an iron-based superconductor.

In October 2024, my country's scientific research team, in collaboration with several foreign research teams, made important progress in the research of nickel-based high-temperature superconductors. This has an important guiding role in the further optimization, design and synthesis of nickel-based high-temperature superconducting materials, and will promote the research progress of nickel-based high-temperature superconductors.

Room-temperature superconductivity: the light of science fiction

As more and more high-temperature superconducting materials are discovered, we can't help but imagine what earth-shaking changes will happen to our lives if room-temperature superconducting materials appear. Although true room-temperature and pressure superconducting materials have not yet been confirmed in the real world, they frequently appear in science fiction works and have become a source of inspiration for people's imagination.

From the perspective of scientific theory, once room-temperature superconductivity becomes a reality, it will trigger a technological revolution that will subvert human society.

In the energy field, superconducting power transmission will completely eliminate the resistance loss in the traditional power transmission process, achieve nearly lossless transmission of electric energy, and allow electricity to be delivered to every corner of the world at extremely low cost, truly opening a new era of unlimited energy.

In the field of transportation, magnetic levitation technology will no longer be limited by high refrigeration costs and complex low-temperature systems. Cars, trains and even airplanes may be able to achieve efficient and high-speed suspension operation with the help of superconducting magnets, and urban traffic congestion may become history.

In the field of computational science, quantum computers based on superconducting materials will be even more powerful. With the ultra-fast computing speed and super-strong stability of superconducting quantum bits, they can easily overcome complex problems that are difficult for existing computers to achieve, bringing leapfrog development to many fields such as artificial intelligence, cryptography, and drug research and development.

Unfortunately, so far, scientists have not yet achieved true room-temperature superconductivity under normal pressure. The room-temperature superconducting materials discovered so far need to be under extremely high pressure conditions to exhibit superconducting properties, which is still a long way from practical application.

For example, in October 2020, the Diaz team in the United States announced that they had achieved superconductivity with a transition temperature of 15 degrees Celsius under 2.67 million atmospheres of pressure. However, due to the extreme experimental conditions and the difficulty of replicating them, the research results were questioned and the relevant papers were withdrawn.

In March 2023, Diaz's team once again announced that they had achieved superconductivity at about 21 degrees Celsius and 10 kilobars (equivalent to about 10,000 atmospheres). Although the required pressure is lower than before, it is still much higher than normal pressure, and the research still needs further verification.

In addition, in July 2023, a South Korean research team announced the discovery of a material called LK-99, claiming that it has superconducting properties at room temperature and pressure. However, the research results of many laboratories around the world on this material failed to confirm its superconductivity, causing doubts about the credibility of the discovery.

Although room-temperature superconductivity is still in the research stage, scientists are continuing to explore new materials and methods to achieve this goal in the future.

END

References:

[1]https://hoffman.physics.harvard.edu/materials/SCintro.php

[2]https://en.wikipedia.org/wiki/Superconductivity#Classification

[3]https://courses.lumenlearning.com/suny-physics/chapter/34-6-high-temperature-superconductors/

[4]https://mp.weixin.qq.com/s/4E6CMB0yxp3EZ5RNpb2o-A

[5]https://www.iop.cas.cn/xwzx/mtsm/202411/t20241107_7435265.html

Author: Yang Yuxin

Planning: Zhang Chao, Li Peiyuan, Yang Liu

Reviewer: Fu Changyi, Associate Professor, Nanjing University of Technology

Chairman of the Science Fiction Committee of Jiangsu Science Writers Association