Chen Xianhui, academician of the Chinese Academy of Sciences, has been deeply involved in the field of superconductivity for more than 30 years. For a long time, he has been insisting on the exploration of new unconventional superconductors and the research of superconductivity and strong correlation physics, and has made a series of important achievements with international influence in the exploration of unconventional superconductors and functional materials and their physical research, and has discovered a series of new superconductors such as iron-based superconductors and organic superconductors, and has made systematic and innovative achievements, and is one of the scientists with important influence in this field in the world.
In the movie "Avatar", the Hallelujah Mountains suspended in the clouds one after another are amazing, what mysterious power can "hold up" these mountains in the air? Because the mountains contain a magical room-temperature superconducting ore, which holds up Hallelujah Mountains in the air with the help of a strong magnetic field. So, what exactly is a superconducting material, and why does it have such a powerful magnetic levitation force?
What are the properties of superconductivity
Superconductivity is a special physical phenomenon. Ordinary conductors can conduct electricity, but because there is resistance, it will generate heat, and the current loss is relatively large. However, scientists have discovered that at extremely low temperatures, the resistance of ordinary conductors will mysteriously disappear, and not only does it have no loss when conducting electricity, but also has a series of incredible properties, turning it into a magical material - superconductor. So how was superconductivity discovered, and what are its amazing properties?
In 1896, the scientist James Dewar liquefied the air, the liquefaction of the air is mainly the liquefaction of nitrogen, the nitrogen from the gaseous state to the liquid state reached 77K (Kelvin, that is, thermodynamic temperature), Kelvin temperature (K) = Celsius (C) + 273.15, 77K is close to minus 200 degrees Celsius. What is the state of a matter at minus 200 degrees Celsius, this is what we have always wanted to know. After Dewar lowered the temperature to 77 K, he measured the resistance of mercury, but unfortunately he did not see the resistance approaching 0, but had a finite value. Thirteen years later, the Dutch scientist Camerin Annis achieved the liquefaction of the last gas, helium, which reached a liquefaction temperature of 4.2 K. Amazingly, when approaching 4.2 K, the resistance of mercury suddenly disappears. Camerin Onnis keenly felt that this was entering a new physical state, and this physical state he called superconductivity, which gave rise to the concept of superconductivity.
In 1962, Anderson and Ronier made a coil of superconductors to pass an electric current, then cut off the power supply, and the current continued to run in it. The magnetic field generated around it was then measured and it was found that its current did not have any attenuation. This proves that the superconducting resistance is zero, which is the first characteristic of superconductivity. Another property of superconductivity is that it is completely diamagnetic. In 1933, two scientists, Meissner and Ochesenfeld, were studying a superconductor tin (a superconductor with a superconducting temperature of 3.7 K) and found that it was impermeable in a magnetic field. Before the discovery of superconductors, people's understanding of the magnetic field was that any object, including the human body, could be penetrated by the magnetic field, but when the superconductor was in a superconducting state, the magnetic induction intensity in it was always 0. The magnetic field does not penetrate the superconductor, which is its second property. In a scientific sense, complete diamagnetism is a more fundamental physical property of superconductors than zero resistance.
Another miraculous property of superconductivity is the quantization of magnetic flux. The quantization of magnetic flux in superconductors is one of the foundations for the realization of magnetic levitation. To put it simply, although superconductors are completely diamagnetic, when the superconductor has impurities and other defects, and the external magnetic field is large to a certain extent, a small amount of magnetic field will enter the superconductor, appear and be fixed near the defect, like being nailed, this is called "flux pinning". In this state, the superconductor can form a stable interaction with the external magnetic field and levitate. The "pinning effect" plays an important role in the design and application of superconducting magnetic levitation. By controlling the strength of the external magnetic field and the characteristics of the superconducting material, the superconducting magnetic levitation can be made with good stability and anti-disturbance ability, so as to achieve higher performance and more reliable magnetic levitation system.
There are two key factors in superconductivity, the first is that superconductivity occurs, which requires two electrons to form a pair of Cooper pairs, and the second is that there is coherence between Cooper pairs. In 1992, Lee Tsung-dao had a conversation with the famous cartoonist Hua Junwu, and Hua Junwu asked Lee Tsung-dao, "What is the matter with superconductivity that you are talking about?" Lee Tsung-dao said that superconductivity is to be matched and related. Then Hua Junwu painted a picture. When two bees are paired, they fly in the sky, and a single bee climbs on the carbon 60 ball, because it is not superconducting, it is not superfluid (there is no superconducting current), but how does coherence manifest? The wings of the two bees are facing exactly the same direction, indicating that they are in the same phase, so as to achieve coherence. Hua Junwu also wrote a sentence: "The twin knots grow into superconductivity, and the single line encounters resistance." "Accurately depicts such a scene of superconductivity.
What are the magical applications of superconducting materials?
1 kilowatt-hour of electricity can make the kettle work continuously for 1 hour, and the stereo can work continuously for 30 hours. Every kilowatt-hour of electricity in a city is inseparable from cables. However, the cable has resistance, and the power loss is very large in the transmission process, the loss rate is between 5% and 10%, and the increase of the transmission distance will also increase the loss rate. With the advent of the most powerful 35 kV superconducting cable in history, the problem of power line loss has finally been solved, and with the blessing of "superconducting technology", it can achieve almost zero resistance. Compared with the traditional transmission mode, in the case of full load operation, it can successfully transmit 2160.12 amperes of current with 35 kV "small" cable, reaching the transmission capacity of 220 kV "large" cable, and greatly reducing the space required for the construction of high-voltage substations. What are the other amazing applications of superconducting materials?
The three key technologies of today's social development are energy technology, information technology and biotechnology, and the two technologies of transboundary energy and information of superconducting materials have laid the foundation for its wide application prospects. Superconductivity is certainly a strategic technology to replace existing transmission technologies. Superconductivity has two applications: one for strong current applications and the other for weak current applications. The application of strong electricity is related to energy first, transportation and biomedicine. There is also a weak current application, that is, superconducting quantum interferometers or interferometers, qubits, quantum computing, and so on. In short, superconductivity is a strategic science and technology that can bring about profound changes in the fields of energy and information.
The superconducting materials currently in application are divided into two categories: high-temperature superconductivity and low-temperature superconductivity, which must be at extremely low temperatures to achieve superconductivity, with a critical temperature of 25K-30K, that is, minus 248 degrees Celsius to minus 243 degrees Celsius, superconductors below this temperature are low-temperature superconductors, and superconductors above this temperature are high-temperature superconductors. Low-temperature superconductivity has higher requirements for low temperatures and generally needs to work in expensive liquid helium environments, although it has been used in many fields, including magnetic resonance imaging, particle accelerators, maglev trains, etc., but expensive refrigerants limit the scope of applications. In contrast, high-temperature superconductivity only needs to be found in low-cost liquid nitrogen, so the application space is broader.
What surprises will room temperature superconductivity technology bring to the world?
If you take stock of the major events in the field of science and technology that will cause the world to eat melons in 2023, room temperature superconductivity should be on the list. On March 8, 2023, American scientist Dias announced at the American Physical Congress that he had achieved room-temperature superconductivity at 10,000 atmospheres (1Gpa). You must know that in the past five years, scientists have studied superconducting materials under the condition of more than 2 million atmospheres (200GPa), which is equivalent to the pressure on the earth's core, and the difficulty can be imagined. On July 22, 2023, the South Korean room temperature superconductivity team published two papers in a row, claiming to have discovered the room temperature atmospheric pressure superconducting material LK-99 for the "first time", which once again set off a superconducting whirlwind in the world. But just over three months later, events took a turn for the worse. In November, the journal Nature retracted Dias's paper on "room-temperature superconductivity", and immediately after that, the South Korean team's shocking discovery was ultimately unconfirmed. The room-temperature superconductivity drama that has been lively for more than half a year has come to an end for the time being. Why do scientists around the world pay so much attention to room-temperature superconductivity, and what surprises will room-temperature superconductivity technology bring to the world?
The reason why "room-temperature superconductivity" is a breakthrough technological progress is that it is very different from the traditional superconducting phenomenon. Conventional superconducting materials exhibit superconductivity only at extremely low temperatures, which greatly limits the range of applications. For example, copper oxide superconductors can only achieve their superconducting properties at about minus 135 degrees Celsius, and their wide application still requires a lot of research when cooled with liquid nitrogen (the temperature of liquid nitrogen is minus 196 degrees Celsius). Therefore, traditional superconducting materials need to be supported by special refrigeration equipment when they are used. Room-temperature superconductivity, on the other hand, means that superconductivity can be achieved at room temperature, that is, 20 to 30 degrees Celsius, without the need for a special refrigeration system. Room-temperature superconductivity technology, if realized, will bring unprecedented disruptive breakthroughs in improving energy efficiency, accelerating transportation, and achieving higher computing speeds.
Will room-temperature superconductors be discovered? My view is that so far, there is no physical theory that makes room-temperature superconductivity impossible, and that's the first point. The second point is that superconductivity is actually a macroscopic quantum effect, and it will have an energy scale, but in the macroscopic quantum effect, is there any precedent for observing the macroscopic quantum effect at room temperature? Yes! On graphene, we see a macroscopic quantum effect such as the integer quantum Hall effect, and from this point of view, superconductivity is not impossible at room temperature. In addition, from a material point of view, copper-based superconductivity has achieved a temperature of 135 K (minus 138.15 degrees Celsius), and if it doubles in terms of energy scale to 270 K (minus 3.15 degrees Celsius), I think it is also possible in terms of energy scale. But we still have to be down-to-earth and seriously explore, and there are many technical challenges to be done step by step. Why does superconducting technology have fewer application scenarios than we imagined and do not play such a big role? It is because it has many cost problems such as support technology, cooling technology, and mechanical technology. If it rises to room temperature superconductivity, the problem of refrigeration does not need to be considered, so that there must be more application scenarios for it. Room-temperature superconductivity does not need to be cooled, the cost can be greatly reduced, and it can be applied in many scenarios. Superconductivity is not only an energy technology, but also an information technology, which can play a huge role in the field of materials.
From a scientific point of view, superconductivity research has three major tasks: one is to explain the microscopic mechanism of unconventional superconductivity, including copper-oxygen and iron-based high-temperature superconductors, the second is to explore superconductors suitable for applications or high critical temperatures or even room temperatures, and the third is to widely use superconductors. For example, the application of superconducting computer quantum computing, superconducting chips, strong electricity, and strong magnetic fields, scientists in the field of superconductivity generally believe that the solution of these challenges is a Nobel Prize-level work, and the key is the discovery of new superconductor materials. Human civilization can be divided by materials, the ancient period was the Stone Age, then the Bronze Age, after the birth of transistors, now silicon supports information technology. From this perspective, room-temperature superconductors could be a strong candidate for the next technological change. This is why the information about room-temperature superconductivity has attracted great social attention, and room-temperature superconductors can be widely used as energy information materials in scientific research, information computing, communications, biomedicine, electric power, transportation, energy and other fields, and are considered to be key materials that can support the next generation of human civilization. The discovery of room-temperature superconductors and the realization of controlled nuclear fusion will permanently solve the energy problems facing mankind.
Prospects for superconductivity research
One prospect of superconductivity research is to break through the technical bottleneck of semiconductors. With the development of semiconductor technology, integrated circuits have entered the sub-10 nanometer technology, but in fact, there are several bottlenecks in semiconductor technology: one is the speed bottleneck of the computer, the second is the power consumption bottleneck, and the third is the manufacturing bottleneck. What you hear and see now is mainly manufacturing bottlenecks. For example, the problem of the lithography machine of the Dutch company ASML, because we don't have a light source, which is a manufacturing bottleneck. In fact, there is another big bottleneck, which is the bottleneck of power consumption, because the power consumption is too large, and the electricity cost of supercomputer applications is too high and the cost is too high. From the perspective of sustainability, power consumption is an important bottleneck that limits the sustainability of supercomputers. How to solve the problem of power consumption is a problem that the semiconductor industry must solve. At present, all the big data centers in China are basically located in the northwest, which has two advantages: one is the average temperature, the average temperature in the northwest is much lower than that in the east and south, and the other is that the price of energy in the northwest is much lower than that in the developed areas, which is a great advantage. Energy consumption includes two parts, the loss of the computer chip itself in the process of operation will produce heat, how to discharge the heat, heat dissipation is a big problem. As you know, a desktop computer has only one small fan, and if this small fan is broken, then the machine will not be able to run, which is the problem of ambient temperature and working temperature. If superconductivity is used, it itself has less power consumption, heat dissipation may be convenient, or the cost is lower. Now when we talk about artificial intelligence and ChatGPT, we are using machine learning to use a large number of high-power, high-speed supercomputers, but the power consumption is very high. We are eager for a technology that can overcome or improve, and superconductivity is certainly an option in this regard. Superconducting integrated circuits offer significant speed, power, manufacturing, and ecological advantages. In fact, superconducting digital computers have already begun to try in this regard, such as the American Titan. Superconducting integrated circuits are an important direction of post-Moore information technology, and the future energy industry, power transmission, transportation, medical and other fields will undergo earth-shaking changes due to the application of superconducting technology. We look forward to the realization of room-temperature superconductivity to promote the development of human civilization.