In the information age, chips have become the "brain" of processing information. Encapsulating billions of transistors in a chip the size of a fingernail is one of humanity's most complex feats. However, when the amount of data exponentially explodes, it is no longer a "one-trick" to integrate more transistors, and the heating problem of components has become a bottleneck that limits the improvement of computing power. The quantum anomalous Hall effect, on the other hand, offers the possibility of realizing ultra-high-performance electronic devices.
"The era of ultra-massive data will put forward extremely high requirements for the storage and processing of information, and a completely innovative computer is needed to achieve energy-free transportation similar to superconductivity and resistance equal to zero." Xue Qikun, a condensed matter physicist, professor at Tsinghua University, and academician of the Chinese Academy of Sciences, said.
In materials, the movement of electrons is highly disordered. Electrons and lattice vibrations, electrons and impurities, electrons and electrons collide constantly, producing effects such as resistance and heat generation. If a strong magnetic field is applied to a thin film material, it is possible for electrons to immediately "regularize" and move unhindered along the boundary, an interesting phenomenon called the quantum Hall effect. If a special material can be found that has both spontaneous magnetization and electronic state and topology, it is possible to produce a quantum Hall effect without an external magnetic field. This is known as the quantum anomalous Hall effect.
For many years, the quantum anomalous Hall effect has been a legendary "treasure" that has haunted physicists around the world, but no one has been able to prove its real existence.
After several years of exploration, Xue Qikun's team prepared the world's first thin film with novel physical properties with ferromagnetic, insulating and topological properties through molecular beam epitaxy, and found this "treasure" in the laboratory for the first time.
On June 24, Xue Qikun won the 2023 National Highest Science and Technology Award.
Enter the "Arena Without a Track"
In the 1980s, international basic research in the field of quantum materials and states of matter ushered in explosive development. A number of discoveries, such as integer and fractional quantum Hall effects, have opened up a new field of research on topological quantum states of matter and brought new possibilities for the development of low-energy electronic devices.
However, the generation of the quantum Hall effect requires a very strong applied magnetic field, which brings great difficulties to its research and application. Just imagine, if you need to run a chip with a quantum Hall effect, you need to be equipped with a strong magnetic field generator the size of a refrigerator, who can accept it?
In 1988, the 2016 Nobel Laureate in Physics F. D. M. Haldane proposed that there might be a quantum Hall effect system that does not require an external magnetic field. But his assumptions are far from the actual material system. Since then, the discovery of the quantum anomalous Hall effect in real materials has been one of the major scientific goals of condensed matter physics for more than 20 years.
In 2005, the concept of topological insulators was proposed, and scientists believed that it was theoretically possible to achieve the quantum anomalous Hall effect by introducing ferromagnetism into topological insulator films. However, it is extremely difficult to achieve this effect in experiments. Because of the quantization of the anomalous Hall effect, the properties of the material need to meet three very demanding conditions at the same time: first, the band structure of the material must have topological properties, so that it has a conductive one-dimensional edge state, that is, a one-dimensional conductive channel; Second, the material must have a long-range ferromagnetic sequence, so that there is no need for an external magnetic field to have an anomalous Hall effect; Third, the material must be in an insulating state, which does not contribute to conduction, and only one-dimensional edge states participate in conduction. Experimentally, it is difficult to achieve any of the above, and even theoretically, there is great uncertainty about whether all three conditions can be met at the same time. As a result, some have described this enormous challenge for experimental physicists around the world as "an arena without a racetrack".
Xue Qikun in the laboratory. Image source: Tsinghua University
Since 2008, Xue Qikun's team has started to study topological insulators by combining molecular beam epitaxial growth, low-temperature strong magnetic field scanning tunneling microscopy, and angle-resolved photoelectron spectroscopy. For the first time, they established the molecular beam epitaxial growth kinetics of Bi2Te3 family topological insulators, developed a three-temperature method with strict control of material composition, and grew the highest quality topological insulator samples in the world. This method later became an internationally accepted method for the preparation of topological insulator samples.
Subsequently, they used angle-resolved photoelectron spectroscopy for the first time to map the evolution of the electronic band structure of a three-dimensional topological insulator at the two-dimensional limit. This result lays the foundation for a large number of follow-up work based on topological insulator films. They used low-temperature and strong magnetic field scanning tunneling microscopy technology to reveal the unique properties of topological insulator surface states such as topological protection and Landau quantization, which had a great academic impact in the world. This series of efforts and achievements have made Continental one of the world's leading companies in the field of topological insulators.
On this basis, Xue Qikun set his sights on the quantum anomalous Hall effect. "It's a very wonderful physical phenomenon for scientists, and we really want to unravel this mystery and see if it exists. Moreover, with the support of the state, our relevant experimental technology has also reached this level. He said, "It's the right time." ”
In 2009, Xue Qikun led the quantum anomalous Hall effect experiment team into the "arena without a track".
A text message that has been saved for 12 years
"Back then, Professor Xue approached me and several teachers and said that there was a theoretical prediction in the world that we could find the quantum anomalous Hall effect in magnetic topological insulators, and invited us to work together to discover this effect." Wang Yayu, a professor of physics at Tsinghua University, recalls that he was immediately attracted to it.
In 2009, Xue's team and collaborators from Tsinghua University, the Institute of Physics of the Chinese Academy of Sciences, and Stanford University began to conduct experiments on the quantum anomalous Hall effect based on the obtained high-quality topological insulator films.
The process of tackling key problems is extremely difficult, and there are many complex problems such as academic, technical and route. Xue Qikun said that it is not difficult to prepare thin films with a thickness of about 5 nanometers, but the difficulty is to control the doped elements at the atomic scale, and it is even more difficult to understand the internal relationship between structure, materials and physical properties at the electronic level, so as to find the direction for the next experiment.
In the process of continuous exploration, the research team prepared a ternary topological insulator film with precise and controllable composition and thickness, and realized the control of the band structure of the topological surface state and the carrier type and concentration of the film through the proportion of chemical components and the electric field effect of the film. They doped the film with magnetic chromium atoms, in which they established a ferromagnetic order, as well as a magnetization axis perpendicular to the surface of the film. According to statistics, they prepared more than 1,000 samples, and the final material obtained can be ferromagnetic, insulating and topological, which is an ideal material system to realize the quantum anomalous Hall effect. "It takes at least three or four days for each sample to go from growth to measurement." Xue Qikun said, "Everyone has brought their abilities to the extreme, and the efforts they have put in are amazing. ”
In October 2017, Xue Qikun (third from left) discussed the experimental work with students at the State Key Laboratory of Low-Dimensional Quantum Physics of Tsinghua University. Photo by Yuan Jie
At the beginning of 2012, the work hit a bottleneck. "We seem to have met all the required conditions, but the results are far from being successful." He Ke, a member of the team and a professor at the Department of Physics at Tsinghua University, recalled.
Xue Qikun doesn't see this as a failure. "Experimentally, if we don't meet the goal, it means that our academic judgment is not necessarily correct, and this is an opportunity to improve our academic ability. In scientific exploration, it is also a contribution to find out the way that is not working. He said.
With his encouragement and enlightenment, everyone calmed down again and decided to change their thinking, do "subtraction", and eliminate various problems that may exist in the samples one by one.
Xue Qikun's mobile phone still has a text message received on October 12, 2012.
It was a Friday. Chang Cuizu, a member of the project team on duty that night, saw the first signs of the quantum anomalous Hall effect, so he quickly sent a text message to Xue Qikun. "I went home a little early that day, around half past ten, and I received the news as soon as I stopped the car." Xue Qikun remembers this vividly, "I was very excited and relieved at that time. ”
"When we first decided to do this experiment, we were actually mentally prepared, and maybe we found out that the quantum anomalous Hall effect does not exist." Xue Qikun said, "Eventually, we discovered this scientific law in basic research. For scientists, this is a great blessing. ”
Strive to promote the application of results
The quantum anomalous Hall effect is one of the important scientific effects discovered by mainland physicists since the founding of the People's Republic of China, and it is a major contribution made by the Chinese physics community to the development of world physics.
"The production of this achievement should be a reward for the long-term strong support of the country and the people." Xue Qikun said.
After the results were published, they had a huge international academic impact and were highly praised by famous physicists such as Yang Zhenning. In 2014 and 2022, Xue Qikun was invited to give special reports at the Nobel Forum initiated by the Nobel Foundation, and also gave invited reports at the 2014 International Conference on Molecular Beam Epitaxy, the 2014 International Conference on Semiconductor Physics and other most influential international conferences in related fields.
This achievement has also promoted the rapid development of related research. In the following years, researchers from various countries improved the properties of magnetic topological insulator materials, and increased the realization temperature of the quantum anomalous Hall effect from 0.03 Kelvin (K) to more than 1 Kelvin. The national metrology institutions of the United States, Japan, Germany and other countries have carried out research on the resistance quantum standard based on the quantum anomalous Hall effect, and the accuracy of the quantum anomalous Hall resistance has preliminarily met the conditions for applying to the resistance quantum standard. The quantum anomalous Hall effect has also been observed in ultracold atomic systems, intrinsic magnetic topological insulator systems, corner graphene, and corner transition metal sulfide systems. Nowadays, the study of quantum anomalous Hall effect has become one of the fastest-growing research directions in international physics.
Xue Qikun's team has also made new achievements in related research and continues to lead the international academic progress in this direction. In 2018, they and their collaborators discovered for the first time an intrinsic magnetic topological insulator, MnBi2Te4, which has a regular arrangement of magnetic atoms and a large magnetic energy gap, which has the potential to achieve a quantum anomalous Hall effect at higher operating temperatures, allowing it to be used in electronic devices. In recent years, scientists have observed evidence of the existence of quantum anomalous Hall states assisted by magnetic fields at a temperature of 30 K, which further increases the hope of realizing the high-temperature quantum anomalous Hall effect based on this material.
"We also need to raise the observation temperature of the quantum anomalous Hall effect, find cheaper materials, and make it practical as soon as possible. That's what we're working towards. Xue Qikun said.
Strategic scientist who came out of the mountains
Xue Qikun once compared himself to "a small boat sailing out of the Yimeng Mountains".
In 1980, he left his hometown of Mengyin County, Linyi City, Shandong Province, and was admitted to the Department of Optics of Shandong University. He said that he "didn't work very hard and finished his studies in a decent way". In the year he graduated, he was not admitted to graduate school, and after working for two years, he took the exam again and failed again. In 1987, he was admitted to the Institute of Physics of the Chinese Academy of Sciences and began his postgraduate studies. Five years later, with the help of his supervisor Lu Hua, he went to study at the Institute of Metal Materials, Tohoku University, Japan, as a Sino-Japanese joint doctoral student.
Xue Qikun (left) takes a group photo with his university classmates. Photo courtesy of the interviewee
Studying in Japan has had two major influences on Xue Qikun. First, it made him develop the "7-11" work habit. He recalls: "During this period, I was exposed to the world's most advanced experimental technology and the international open environment for the first time. This is a very rare opportunity and I cherish it. Therefore, I arrive at the laboratory at 7 a.m. and leave at 11 p.m. every day, in order to master as many experimental techniques as possible with longer working hours. Second, he gradually developed an extremely rigorous attitude towards experimental scientific research.
In 1994, Xue Qikun returned to China to complete the defense, received a Ph.D. from the Institute of Physics, Chinese Academy of Sciences, and then continued to work at Tohoku University in Japan, and as a visiting scholar, he worked as a postdoctoral fellow in the Department of Physics of North Carolina State University for one year. In 1999, he returned to China and worked as a researcher at the Institute of Physics of the Chinese Academy of Sciences. Since 2005, he has been a professor in the Department of Physics at Tsinghua University.
"In more than 7 years abroad, I have seen a huge gap between the country and the developed countries in terms of scientific research and the living standards of the Chinese people." Xue Qikun said, "This experience has strengthened my belief that I want to do more for the country." I hope that the motherland will become strong in all aspects of science and technology, and that the Chinese will live a happier and more dignified life. ”
Xue Qikun (first from left) took a group photo with his mentor Professor Sakurai during his study in Japan. Photo courtesy of the interviewee
With this belief, Xue Qikun devoted himself to the scientific research of the motherland, arriving at the laboratory at 7 o'clock in the morning and leaving at 11 o'clock in the evening every day. There are often students who want to compete with him to see who can arrive earlier and leave the lab later, but no one can persist for years like him.
Today, he is one of the continent's leading strategic scientists in the field of quantum technology. In addition to the quantum anomalous Hall effect, he also led the team to discover interface-enhanced high-temperature superconductivity, achieving an important breakthrough in the field of high-temperature superconductivity and opening up a new research direction of high-temperature superconductivity in the world.
In terms of talent training and team building, Xue Qikun has also achieved remarkable results. One member of his team has been elected as an academician of the Chinese Academy of Sciences, more than 30 people have been selected into the national talent program, and more than 120 doctoral students and postdoctoral fellows have been trained, establishing a talent team with international standards in the fields of low-dimensional physics and quantum materials in mainland China.
The "small boat" that sailed out of the mountains has now become a "giant ship" galloping in the ocean of science, leading the way at the top of the tide.
Source: Science and Technology Daily