Credit: Visual China

BEIJING, August 29 (TMTPost) – In 1908, Professor Heike Kamerlingh Onnes at Leiden University in the Netherlands first liquefied helium in the laboratory, refreshing the record of artificial low temperature to -269℃ (4.2 Kelvin).

In 1991, Onnes measured the electrical conductivity of pure metals (mercury, later tin and lead) at extremely low temperatures, and discovered the famous "superconducting" phenomenon.

Public information shows that superconductivity refers to the phenomenon where the resistance of a material becomes zero below a certain temperature (or as small as 10-25 ohm·square millimeter/meter based on current observations). This temperature is called the superconducting transition temperature. The characteristics of superconductivity are zero resistance and complete diamagnetism, so superconducting materials are revolutionary.

Over the past 112 years, scientists have been continuously searching for superconducting materials with higher critical temperatures. At the same time, controversies, denials, and affirmations in the academic community have been popping up. After multiple rounds of scientific breakthroughs, the academic community finally completed the establishment of the theory of superconductivity and the discovery of high-temperature superconducting materials. So far, 10 scientists have obtained the Nobel Prize through superconductivity research. Nowadays, in-depth exploration of room temperature superconductors has been thrust into the spotlight.

Recently, the winners of the 2023 Future Science Prize were announced.

Zhao Zhongxian and Chen Xianhui, two key figures in the field of superconductivity in China, received the Future Science Prize in physical sciences for their significant breakthroughs in the discovery of high-temperature superconducting materials and systematic advancements in elevating the transition temperature.

Chen’s research group first raised the superconducting transition temperature above the McMillan limit, proving that iron-based superconductors are indeed unconventional high-temperature superconductors. This research has promoted the development of high-temperature superconductivity.

TMTPost had a dialogue with Chen, who is a professor in the Department of Physics at the University of Science and Technology of China, and an academician of the Chinese Academy of Sciences. He not only talked about his past research process, the applications and research directions of superconductivity, but also responded to the current hot topic of room temperature superconductivity research.

The road of superconductivity technology: tracing back to 37 years ago

"Without high-temperature superconductivity, I wouldn't be here today talking to you all. The breakthrough of high-temperature superconductivity in 1986 was an opportunity for me to go to where I am now," Chen said. He, 60, mentioned that he was able to enter the field of superconductivity technology because he happened to witness the breakthrough in the field of high-temperature superconductivity in 1986.

At the age of 23 then, Chen was a graduate student at Hangzhou University (now Zhejiang University)’s master program and was about to study in the laboratory of Qian Yitai and Chen Zuyao in the Department of Applied Chemistry at the University of Science and Technology of China.

In January 1986, German scientist Johannes Georg Bednorz and Swiss scientist Karl Müller discovered high-temperature superconductivity in ceramics, thus opening the era of copper-based high-temperature superconductors. This also prompted the scientific community to explore and discover high-temperature superconductivity technology.

In 1992, Chen obtained his Ph.D. degree in Condensed Matter Physics from the University of Science and Technology of China, and stayed at the university for work. After that, he worked as a Humboldt scholar at the Karlsruhe Research Center in Germany and the Max Planck Institute for Solid State Physics in Stuttgart. He also worked as a visiting professor at Japan Advanced Institute of Science and Technology and the Texas Center for Superconductivity in the U.S.

In 1998, Chen was engaged as a professor in the Department of Physics at the University of Science and Technology of China. His main research interests are the exploration of new unconventional superconductors and the study of superconductivity and strongly correlated physics. In addition, he also serves as a director of the Chinese Physical Society, the head of the Low Temperature Physics Professional Committee of the Chinese Physical Society, and a member of the Academic Committee of the Superconductivity National Key Laboratory.

In February 2008, Japanese scientists discovered a new type of superconducting material with a critical temperature of 26K (minus 247.15 degrees Celsius), which attracted Chen’s attention. He led his graduate students to work day and night for more than a month, and his group became the first to achieve a critical temperature of 43K (minus 230.15 degrees Celsius) in an iron-based compound superconductor, breaking the 40K McMillan limit.

On March 25, 2008, the paper on this achievement was published in the journal Nature and became one of the top five most influential and most cited papers in the world in 2008.

"Before 2008, there was only one type of unconventional high-temperature superconductor, which is copper oxide superconductor. However, in the 22 years, the mechanism and other related scientific issues of copper oxide superconductors could not be clearly explained. If more types of high-temperature superconductors appear, then by discovering their commonalities, we can better understand unconventional superconductors, the type that BCS theory cannot explain," said Chen.

The BCS theory was proposed by American physicists John Bardeen, Leon Cooper, and John Robert Schrieffer in 1957. They believed that in conventional superconductors, two electrons, originally negatively charged and repelling each other, form "Cooper pairs" by indirectly attracting each other through the vibrations (quantum of energy of this vibration is called a phonon) generated by the atomic lattice. Under the quantum coherence effect, these "Cooper pairs" can move without loss in the lattice, forming a collective superconductivity. At the same time, the internal magnetic induction intensity of the superconductor is zero, which is complete diamagnetism or Meissner effect.

After 37 years of exploration step by step, Chen has now become a key figure driving the development in the global superconductivity field.

Science can only be explored

"Superconductivity is indeed a big strategic technological field." Chen said.

According to him, the current development of superconductivity technology mainly focuses on three categories: energy, information, and biotechnology. Superconducting materials can support both energy technology and information technology, thus having a wide range of applications, such as nuclear magnetic resonance and superconducting maglev trains. In science, the temperature of controlled nuclear fusion is above hundreds of millions of degrees, and there is no material that can constrain it, so superconductivity is mainly used. Superconductivity is also needed for electron acceleration and control in the Tokamak fusion experiment accelerator.

In terms of talent cultivation mechanism, Chen said that talent cultivation cannot be rushed and needs to go through a process, especially considering the problem of cross-disciplinary knowledge cultivation.

He called for a transformation of China's education and talent cultivation mode from the "knowledge imparting" model to the "ability cultivation" model, in the context where artificial intelligence, machine learning, and big data models represented by ChatGPT are being widely applied and will continue to be applied.

In fact, in recent years, scientists in the field of superconductivity have started to challenge the "holy grail" of physics - the search for room-temperature superconducting materials.

Room-temperature superconductivity refers to the phenomenon of superconductivity occurring at room temperature or higher temperatures under normal pressure or near-normal pressure conditions. Superconducting materials have two characteristics - zero resistance and complete diamagnetism, with zero resistance meaning that current can pass through superconductors without energy loss.

When asked about the recent hot topic of room-temperature superconductivity research, Chen did not directly express his opinion, but he emphasized that scientific matters can only be explored and must be approached rigorously.

So far, we have not yet seen a practical application of "room-temperature superconductivity" technology. From a structural point of view, it is only about using "room-temperature superconductivity" to contemplate scientific problems in the case of hydride compounds and extremely high pressures, while other systems are left for scientists to freely explore.

"If (room-temperature superconductivity) is true, it would indeed be a remarkable advancement for humanity, because it is room-temperature superconductivity at such a high temperature (approximately 127 degrees Celsius). The changes that room-temperature superconductivity will bring to people's lives will be earth-shattering. By then, we can ride on floating superconductive cars when we go out, and even a single charge of our phones and laptops can last for months." Chen added.

Besides, in the field of superconductivity, is it theory that first achieves a breakthrough and then guides experimental directions, or is it the discovery of more groundbreaking superconducting materials that stimulates theoretical advancements? Regarding this, Chen believes that the two are complementary, but so far, in terms of superconductivity, it is still difficult for theory to guide experiments.

In the dialogue with TMTPost, Chen mentioned that in the past 40 years, we have witnessed the development of China's superconductivity research from trailing to keeping pace, and then leading in many aspects. He believes that in the future, China will discover new superconducting materials with significant impacts and even explore room-temperature superconductors.