A new addition to the high-temperature superconductor family! Why do we keep searching for high-temperature superconductors?

The discovery of high-temperature superconductivity has broken the traditional understanding that superconductivity can only exist at extremely low temperatures, and has promoted the development of cutting-edge fields such as materials science and condensed matter physics. Finding new types of high-temperature superconductors has also been a goal pursued by scientists. Recently, the international academic journal Nature published the latest research results of Chinese scientists discovering another new type of high-temperature superconductor.

Professor Zhao Jun's team from the Department of Physics at Fudan University successfully grew three layers of nickel oxide using high-voltage optical floating zone technology, confirming that nickel oxide has pressure-induced bulk superconductivity, with a superconducting volume fraction of 86%. This achievement provides a new perspective and platform for people to understand the mechanism of high-temperature superconductivity. What is a superconductor? Why do we need to look for new high-temperature superconductor materials? We invited Luo Huiqian, a researcher at the Institute of Physics, Chinese Academy of Sciences , to explain the application charm of high-temperature superconducting materials to the public.

Q: What is a superconductor? What is a high-temperature superconductor?

A: Superconductors refer to a class of materials that have both absolute zero resistance and complete anti-magnetism . Their essence is the macroscopic quantum condensation state of the roving electrons inside the material at a sufficiently low temperature . Most superconducting materials must rely on a low temperature environment to achieve superconductivity, generally below 40K. Currently, only two major types of materials can break through 40K at normal pressure, namely cuprates and iron-based superconductors. Among them, the highest critical temperature of cuprates at normal pressure is 134K, which breaks through the boiling point of liquid nitrogen (> 77 K).

Q: Why are we looking for new high-temperature superconductors?

A: Although there are two families of high-temperature superconductors, and some material systems can be used in the liquid nitrogen temperature range, they all have many bottlenecks in application, such as toxicity, difficulty in preparing pure phase materials, large anisotropy, weak connection of grain boundaries, poor mechanical and mechanical properties, etc. At present, the superconducting materials used on a large scale are mainly conventional low-temperature superconductors such as niobium-titanium alloys and niobium-tin alloys , which must rely on expensive liquid helium refrigeration and have high application costs. Therefore, it is necessary to explore new high-temperature superconducting materials suitable for large-scale applications.

The microscopic mechanisms of copper oxides and iron-based superconductors are not yet understood, and involve cutting-edge scientific issues in condensed matter physics, such as strongly correlated many-body interactions, coexistence of competitive orders of multiple electron states, and the importance of both charge and magnetic interactions. Understanding the mechanism of high-temperature superconductivity is expected to inspire the formation of a new physical theoretical framework or even a completely new research paradigm . However, there are many different opinions on the mechanism of high-temperature superconductivity, and more new high-temperature superconducting materials are urgently needed to verify its universality .

Q: Why is nickel oxide considered one of the important candidate materials for achieving high-temperature superconductivity?

Answer : As early as the 1980s, scientists noticed nickel oxides when searching for superconductivity in oxides, but no superconductivity was found. In 2019, the Hwang team in the United States achieved superconductivity of about 15 K in Nd0.8Sr0.2NiO2 thin film samples. Subsequently, scientists explored superconductivity in structural systems such as La2NiO4, La3Ni2O7, and La4Ni3O10. In July 2023, the team of Professor Wang Meng from the School of Physics of Sun Yat-sen University and his collaborators published a paper in Nature, announcing the discovery of high-pressure induced superconductivity of about 80 K (pressure of 14 GPa) in La3Ni2O7 single crystal samples. However, the superconducting volume fraction of La3Ni2O7 high-pressure superconductivity is not high, and the first reported resistance measurement failed to reach zero resistance, so its superconductivity remains in doubt.

Through the unremitting efforts of Chinese scientists, Yuan Huiqiu's team at Zhejiang University and Cheng Jinguang's team at the Institute of Physics, Chinese Academy of Sciences, achieved the zero resistance state of the material and increased its superconducting volume fraction to more than 40%, confirming superconductivity. At the same time, Wen Haihu's team at Nanjing University, Qi Yanpeng's team at Shanghai University of Science and Technology, and a team of Japanese scientists have successively discovered that the La4Ni3O10 system may also have high-pressure superconductivity, but the volume fractions are very low. In July 2024, Zhao Jun's team at Fudan University achieved superconductivity of 86% volume fraction of the La4Ni3O10 system, confirming the superconductivity of the material. So far, the nickel-based superconductor family includes three members: LaNiO2, La3Ni2O7, and La4Ni3O10 , where La can be replaced by other rare earth metal elements.

Nickel oxide materials have a material structure very similar to that of copper oxides. At the same time, the Ni element is close to Cu and Fe in the periodic table, and the electrons involved in superconductivity are mainly d-orbital electrons. It is generally believed that they have similar electronic states, which means that their superconducting mechanisms have certain commonalities. Although later studies have shown that nickel oxide superconductivity is mainly due to interlayer d-orbital pairing, which is different from the intralayer d-orbital pairing of copper oxides, it has also brought more inspiration to the study of high-temperature superconductivity mechanisms .

Q: What methods do scientists use to search for new high-temperature superconductors?

Answer: There are many ideas to refer to when exploring new high-temperature superconductors.

1. Using ultra-high pressure to generate new material structures that cannot be obtained under normal pressure, materials with more hydrogen may be room-temperature superconductors. This idea has been tried continuously since 2015, and many extremely high-temperature superconductors have indeed been found, such as the 260 K superconductor LaH10.

2. Starting from the existing microscopic mechanism of superconductors, study which interactions help to increase the superconducting temperature, and then redesign and construct new materials to increase the critical temperature with the help of multiple interactions.

3. Break out of the thinking framework of three-dimensional materials and look for room-temperature superconductivity in composite materials or two-dimensional interfaces, or reassemble atomic building blocks in the one-dimensional world.

4. With the powerful computing power of AI, we can train with a huge database of various known superconducting material properties, which can help us predict new superconductors, even room-temperature superconductors, even when the superconducting mechanism is unknown. This depends on the accuracy of the database and the reliability of AI. Currently, scientists have just started their work and have made some progress, but unfortunately, none of them are room-temperature superconductors.

Expert: Luo Huiqian, researcher at the Institute of Physics, Chinese Academy of Sciences

Produced by: Science Popularization China