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Just after the National Day Golden Week, the superconducting research team of the University of Science and Technology of China, in addition to watching the 70th anniversary of the National Day, still works in the laboratory every day. "The next material that can be used to divide the era may be room temperature superconductors." In the eyes of Professor Wu Tao of the University of Science and Technology of China, the superconducting research they are engaged in is full of charm. "If a room-temperature superconductor is found, we can go out and get on a suspended superconducting car, or even charge a mobile phone or a laptop for a few months, and it can be used for several months." It is with such a dream, the University of Science and Technology of China superconducting research The team has been sticking to this field for more than 20 years.
Break through the restricted area of ​​superconducting research
Superconductivity refers to the phenomenon that the resistance of certain materials suddenly disappears when the temperature drops below a certain critical temperature, or below the superconducting transition temperature. In the history of superconducting research, 10 people have won the Nobel Prize five times, and its scientific importance is self-evident.
In 1911, Dutch scientists discovered the superconductivity of mercury under extremely low temperature conditions, opening up a new field of scientific research. In 1986, German scientists and Swiss scientists discovered a copper oxide superconductor with a critical transition temperature of 35K.
What bothers scientists is that the transition temperature of superconductors cannot exceed 40K (approximately minus 233 degrees Celsius). This temperature is also called the McMillan limit temperature.
Can the limit temperature of 40K be broken? Shortly after two European scientists discovered a copper oxide superconductor with copper as a key superconducting element, a research team including Chinese scientists raised the critical transition temperature of the copper oxide superconductor above the temperature range of liquid nitrogen, breaking through McMillan Extreme temperature, making it a high-temperature superconductor.
"The copper oxide high-temperature superconductor family has two main defects. As a cermet material, the processing technology is strict, the overall cost is high, and it is widely used. In addition, copper-based superconductors have not solved the rich physical connotation of the high-temperature superconductivity mechanism." Wu Tao told the Science and Technology Daily reporter that it is imperative to uncover the principle of high-temperature superconductivity and apply it widely to find superconductors with higher critical temperatures.
Because of its magnetic properties, iron-based compounds were once asserted by the international physics community as a restricted zone for exploring high-temperature superconductors.
In March 2008, the Chen Xianhui research group of the Chinese University of Science and Technology and the Wang Nanlin research group of the Institute of Physics of the Chinese Academy of Sciences simultaneously observed the superconducting transition temperatures of 43K and 41K in the iron base, which broke the McMillan limit and proved that the iron-based superconductor is a high-temperature superconductor. Immediately afterwards, the Chinese scientist team not only took the lead in making the transition temperature exceed 50K, and discovered a series of superconductors above 50K, but also created a 55K iron-based superconductor transition temperature record, recognized by the international physics community as the second high-temperature superconducting family .
Looking for higher transition temperature superconducting materials
After breaking through the limits of McMillan, scientists around the world have once again been confused about the exploration of superconducting materials, and it has become increasingly difficult to publish high-temperature superconducting papers in journals with high impact factors.
However, the superconducting team of the University of Science and Technology of China has always adhered to this position, preparing, observing, giving up and restarting countless times ... In order to dispel the fog of superconducting research, they proposed "a new type of two-dimensional layered unconventional superconducting material" This new research direction.
Wu Tao told reporters that since copper oxide and iron-based superconductors are both layered structures, the key structural units carrying superconductivity are the CuO2 surface and FeAs / Se layer, which are called "superconducting primitives". Most superconductors exhibit such structural characteristics.
"These superconducting mechanisms differ from those of conventional superconductors. The mechanism of traditional superconductors is mainly based on the BCS theory of electro-phonon interaction. The superconducting mechanism of two-dimensional layered unconventional superconducting materials is generally considered to be incapable Use BCS theory to explain. "Wu Tao believes that the understanding of the micro-mechanism of copper oxide superconductors and iron-based superconductors will greatly promote the new development of condensed matter physics; The superconductor may become a new growth point for the world's economic and social development like an integrated circuit.
At present, there are two core points of the more recognized superconducting research: the first is new (high transition temperature) unconventional superconducting materials, and the second is the mechanism of high temperature superconducting (and unconventional superconducting).
"We explore the unconventional superconductivity through the research ideas of block layer and heterojunction design based on the construction of a two-dimensional layered superconductor unit based on the universal correlation between two-dimensional structural elements and superconductivity. Wu Tao told reporters that as a major national project, in the implementation of more than two years, two new iron-selenium-based high-temperature superconducting materials have been successfully synthesized by electrochemical intercalation, and these new superconducting materials have found The superconducting pre-pairing phenomenon similar to copper-based high-temperature superconductors also found that the two-dimensional structure has an important influence on the formation of high-temperature superconductors in iron-selenium-based superconductors. "These new findings will provide key experimental evidence for the establishment of a universal high-temperature superconducting mechanism." Wu Tao said that in the new research direction, it may be helpful to discover new practical applications with ultra-high critical fields and critical current densities. Type superconductor.
Explore more suitable superconductors
In fact, superconductivity has begun to enter our lives. If the high-temperature superconducting filter has been used in mobile phones and satellite communications, and the communication quality has been significantly improved; the use of superconducting quantum interference devices (SQUID) in medical equipment has greatly enhanced the accuracy of human heart and brain detection and inspection And sensitivity; the world's first superconducting demonstration substation has also been put into use in China's power grid ...
In Wu Tao's view, their research may have a role in promoting the country's key development in the field of superconducting quantum computing. "It may be applied to the field of new topological quantum computing with low energy consumption and self-correction." Wu Tao said.
At present, superconductors have not yet achieved the wide range of applications expected. The main reason is that there are still two important issues that need to be resolved: one is the economics of superconducting applications; the other is that superconducting materials at room temperature and pressure have not been discovered.
"Therefore, breaking the limitation of the transition temperature of existing superconducting materials and discovering new superconductors with higher transition temperatures have great scientific significance." Wu Tao is full of hope for the future of Chinese superconducting research, "Superconducting research has been rooted in China We hope that through our efforts, we will achieve a major breakthrough in the exploration and mechanism research of new materials for two-dimensional unconventional superconductors and continue to maintain our international leading position in the exploration and related research of high-temperature superconducting materials. "(Reporter Wu Changfeng)
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