Oolong or take off the "Holy Grail"
Achieving "room temperature superconductivity"? Let the bullets fly a little longer
◎ Chen Xi, reporter of this newspaper, and Shen Wei, intern reporter
In addition to the zero-resistance property, magnetic flux pinning property, superconducting materials also have phase coherent properties. These three main properties make superconductors have many very peculiar and even disruptive applications. At present, superconducting materials have been used in medicine, energy, transportation and other fields, playing an irreplaceable role.
"This conclusion should have been falsified." Wen Haihu, director of the Center for Superconducting Physics and Materials Research at Nanjing University, and his team came to a completely different conclusion after almost replicating the room-temperature superconducting materials studied by Professor Dias of the University of Rochester in the United States and completing measurements. In addition, the relevant experimental results published by many teams in China have also given negative conclusions.
In early March, Dias announced in a presentation at the annual meeting of the American Physical Conference that room temperature superconductivity of 3 K (1°C) had been achieved at 1GPa (about 294,21 standard atmospheres).
Was it an oolong event, or did the Dias team really take off the "holy grail of condensed matter physics"? Everything remains to be verified. But no matter what the results are, it shows that "room temperature superconductivity" comes with a huge "flow", which has attracted countless scientists to work diligently in this field for many years.
So, what exactly is "room temperature superconductivity"? How far is humanity from this goal?
Superconducting properties enable disruptive applications
"As we all know, gold, silver and copper are very good conductor materials, and the resistance is relatively small, but there is still resistance." Ge Junai, a professor at the Institute of Materials Genome Engineering of Shanghai University, introduced that superconducting materials are a class of materials with special electrical and magnetic properties, which exhibit the characteristics of sudden disappearance of resistance and complete magnetic resistance at a certain critical temperature. In theory, superconducting materials can conduct electricity without resistance, heat loss, and attenuation.
"In addition to the zero-resistance property, the magnetic flux pinning property, and the phase coherent property of superconducting materials. These three main properties make superconductors have many very peculiar and even disruptive applications. Wen Haihu introduced.
At present, superconducting materials have been used in medicine, energy, transportation and other fields, playing an irreplaceable role.
For example, Ge Junjiao said that the nuclear magnetic resonance widely used in hospitals generally needs to generate a Tesla-level strong magnetic field, and the resolution of the device is proportional to the size of the magnetic field, and the larger the magnetic field, the easier it is to find some early lesions. The magnetic flux nailing properties of superconducting materials can make them maintain zero resistance characteristics under strong magnetic fields, so very strong magnetic fields can be prepared.
In the operation of the power grid, in order to reduce the loss in transmission, it is often necessary to use ultra-high voltage transmission schemes. However, the construction of ultra-high voltage power grids not only requires high costs, but also causes certain damage to the environment. The use of superconducting cable transmission can transmit the same level of electricity at a voltage of tens of kilovolts. In December 2021, China's first kilometer-level high-temperature superconducting cable independently built was connected to the grid, which took an important step in the large-scale application of superconducting cables.
In addition, the application of superconductivity in energy storage, magnetic levitation and other fields is also developing rapidly. The world's largest international scientific research cooperation project, the International Thermonuclear Experimental Reactor (ITER) Project, is expected to solve the ultimate human demand for clean energy. The ITER device is a superconducting tokamak device that can produce large-scale nuclear fusion reactions, commonly known as the "artificial sun". Among the multiple paths of rapid development of quantum computing in recent years, quantum computers designed based on superconducting Josephson junctions are currently recognized as the most likely scheme to achieve "quantum supremacy".
Why the "holy grail" of room-temperature superconductivity is difficult to pick
"Superconductivity can only be observed at extremely low temperatures or pressures, which means that the superconducting materials used in the experiment cannot be used in long-term, routine applications such as non-destructive power transmission, superconducting maglev high-speed trains and affordable medical imaging equipment." Ji Lu, associate professor of the Department of Electronic Information and Communication at Nankai University, said.
Since the Dutch scientist Camorim discovered the phenomenon of superconductivity in 1911, human beings have not stopped on the road to exploring superconducting materials. For more than 100 years, while trying to explain the mechanism of superconductivity, scientists have been looking for ways to raise the critical temperature of superconducting materials.
The first to appear are low-temperature superconducting materials. Wen Haihu introduced that before 1986, the maximum critical temperature of all superconducting materials discovered by scientists was 23.2K (-249.95 °C). Low-temperature superconducting materials are represented by two types of materials, niobium-titanium superconductors and niobium-tritin compound superconductors, and are currently the most widely used in industry, medical circles, large scientific devices and other fields.
Just when people did not have much hope for high-temperature superconductivity, in 1986, two scientists in Switzerland discovered superconductivity in copper oxide materials, and soon synthesized the superconducting material yttrium barium copper oxygen with a critical temperature greater than 77K (-196 °C). Materials with critical temperatures that break past the McMillan limit of 40K (-233°C) predicted by conventional theory are called high-temperature superconductors.
"It should be noted that 'high temperature' here is only a relative concept. In fact, the critical temperature of high-temperature superconductors that have entered the practical application stage is still more than minus 100 degrees Celsius, far lower than room temperature, and liquid nitrogen is required to operate as a refrigerant. Ge Junjiao emphasized.
Subsequently, a number of new superconducting systems including magnesium diboride and iron-based superconductivity were discovered successively, and attempts were made to increase their superconducting transition temperature through chemical element doping, ionic liquid regulation, charge transfer and other methods. But to date, the record for the critical temperature of superconducting at atmospheric pressure is still held by copper oxide superconductors.
"Although high-temperature superconductors break through liquid nitrogen temperatures, they can be used under liquid nitrogen at a low cost. However, its use is still limited. Jilu added that the cost of refrigeration and some critical parameters of high-temperature superconducting materials limit the development of high-temperature superconductivity.
"Discovering true room-temperature superconductors is the ultimate goal. If room-temperature superconducting materials appear, then the cost of refrigeration will be lower, and room-temperature superconducting materials will be easier to promote. Wen Haihu said.
Ge Junjiao said that room temperature superconductivity is not a new concept, theoretical research has predicted that hydrogen with the smallest atomic mass will exhibit a superconducting critical temperature up to room temperature after being metallized. However, to turn hydrogen into metal, extreme pressure from the outside world is required.
It wasn't until 2017 that metallic hydrogen was first synthesized by Dias and his supervisor under high pressure, but the experiment has not yet been replicated in other labs. The room-temperature superconductivity under high pressure reported by Dias et al. in 2020 was also retracted in September 2022 due to doubtful data reliability and the irreproducibility of the experiment.
"The reason why the research results of Dias's group have once again attracted widespread attention is that the pressure conditions required for room-temperature superconductivity reported this time are much smaller than those required for superconductivity in the previous hydride system, only one hundredth of the previous one." Ge Junjian thought.
How room temperature superconductivity will be achieved in the future
Because Diaz's results reduce the pressure required for experiments, many laboratories around the world are ready to repeat the test.
On March 3, Wen Haihu's team submitted a 15-page research paper with nine authors on the preprint website arXiv, titled "No Superconductivity in Nitrogen-Doped Lutetium Hydride (LuH_9±xN_y) Under Near-Environmental Conditions", which categorically rejected Dias's conclusions.
"Dias's approach to preparing sample materials was almost unfeasible, and we combined our own conditions to synthesize them in a completely new way and obtained lutetium, nitrogen, and hydrogen materials." Wen Haihu said.
After X-ray diffractometer technical examination, the material structure is almost identical to the sample from Dias. Wen Haihu's team then measured the resistance of the material at different pressures below 6,10 atmospheres and found that no superconductivity occurred at temperatures as low as <>K. At the same time, they also made careful magnetization measurements and found that there was no antimagnetic signal required for superconductivity. "These findings are sufficient to show that there is no near-normal room temperature superconductivity in the samples we prepared similar to those prepared by Diaz's team." Wen Haihu said.
Kiru agrees with this conclusion. "Wen Haihu's team placed nitrogen-doped lutetium hydride materials in a pressure environment of 1GPa-6GPa, and did see some changes in resistance data at a temperature of about 300K, but it seems that it should be a change in the structure of the material." This change can be identified as a phase transition, but it should not be a superconducting phase transition, he said. Therefore, there are two explanations for this diametrically opposed result – either Diaz's experimental conditions are more demanding, or Diaz's team's interpretation of the data is clearly wrong.
"In fact, the realization path of room-temperature superconducting materials under atmospheric pressure, in addition to the direction of metal hydrogen and high pressure such as Dias, there are many other methods." Wen Haihu introduced, for example, along the direction of the original copper oxide superconductivity, it is also possible to achieve room temperature superconductivity under normal pressure, which is also the international mainstream direction.
Theoretical results often take a long time to be put into practice. Ge Junjian believes that the road to achieving room-temperature superconductivity is still far away. Just as more than 30 years have passed since the discovery of high-temperature copper oxide superconductors, the most widely used practical superconductors are still low-temperature superconductors of alloys such as niobium-titanium superconductors and niobium-tritin compound superconductors discovered in the 20s of the 50th century.
Wen Haihu said that room temperature superconductivity is a dream goal for humans. At the same time, China is also actively promoting the application process of low-temperature superconducting and high-temperature superconducting materials, and has made more progress. In the application of low-temperature superconducting materials, China has basically reached the international advanced level. In the application of high-temperature superconductivity, companies in Shanghai, Suzhou and other places have been able to prepare yttrium barium copper-oxygen superconductor strips. In addition, Chinese scientists have also made many contributions to the field of iron-based superconductivity research, which is currently the most optimistic in the world.
"If superconducting magnet technology under a stronger magnetic field can be realized, nuclear fusion can be made more likely to occur, and coupled with super accelerators, the cost of controllable nuclear fusion can be greatly reduced, and the application prospects are broad." Wen Haihu said. (Science and Technology Daily)