The 2023 National Science and Technology Award Conference was held in Beijing on June 24, 2024. The 2023 State Preeminent Science and Technology Award, China’s top scientific honor, was granted to Academician Qi-Kun Xue for his outstanding contribution to scientific and technological innovation. Moreover, Qi-Kun Xue is the first Chinese national to win the Fritz London Memorial Prize (2022) and the first Chinese national to win the Oliver E. Buckley Condensed Matter Physics Prize (2024).

Qi-Kun Xue is a professor at Tsinghua’s Department of Physics, president of the Southern University of Science and Technology, and a member of the Chinese Academy of Sciences. He is dedicated to research in the fields of scanning tunneling microscopy, molecular beam epitaxy, topological quantum matter, and high-temperature superconductivity. In the past 20 years, Xue and his research team have accomplished several important original works in condensed matter physics, including experimental observation of the quantum anomalous Hall effect (QAHE) and interfacial enhanced high-temperature superconductivity using heterojunction systems. Xue’s work on QAHE will be highlighted below.

The discoveries of integer and fractional quantum Hall effects in two-dimensional electron systems were awarded the Nobel Prizes in Physics in 1985 and 1998, respectively. The observation of the half-integer quantum Hall effect in graphene played a key role in the Nobel Prize in Physics awarded for the discovery of graphene. All these quantum Hall effects can only be observed under a strong magnetic field. The quantum anomalous Hall effect is a quantum Hall effect that uses a different mechanism that can occur without an external magnetic field. The effect is not only the key to practical applications of quantum Hall states but also the basis for many other novel quantum phenomena. Although the first theoretical proposal for a quantum Hall effect without a magnetic field appeared as early as 1988, little experimental progress was made in the following two decades. After 2005, theoretic studies on topological insulators (TIs) and the QAHE in TI-based materials provided a new approach to realize the effect. The experimental discovery of the QAHE in real materials thus became a major scientific goal of condensed matter physics during those years.

Aiming to experimentally realize the QAHE, Qi-Kun Xue’s research team systematically studied molecular beam epitaxy (MBE) growth and engineering of magnetically doped TI films. They obtained a series of cutting-edge experimental results, culminating in the first experimental observation of the QAHE in 2012.

1. With their MBE technique, Xue’s team fabricated uniformly Cr-doped (Bi,Sb)2Te3 TI films and realized a ferromagnetic insulator phase in them. They developed the techniques of precisely controlling their carrier density and band structure and they were the first in the world to experimentally observe the QAHE. Their result has been repeated by several research groups in the field.

2. Xue’s team discovered topology-induced magnetic quantum phase transition in magnetically doped TI films. The finding unveiled the crucial role of topological electronic states in the magnetic coupling mechanism and magneto-transport properties of magnetically doped TIs.

3. Xue’s team first established the MBE growth kinetics of Bi2Te3 family TIs and developed the standard method of preparing high-quality Bi2Te3 family TI films. They were the first to experimentally map the evolution of the electronic structure of a three-dimensional TI in the two-dimensional limit and demonstrated forbidden back-scattering and Landau quantization of topological surface states in the films.

The discovery of the QAHE represents a milestone breakthrough in condensed matter physics, and also has promoted the rapid development of research on quantum anomalous Hall effect. In the following years, researchers have improved the material properties of magnetic topological insulators, and the temperature for realizing the QAHE has been raised from 0.03 K to more than 1 K. Quantum anomalous Hall resistors have been achieved with an accuracy of 10-8, preliminarily satisfying the conditions applied to the resistance quantum standard. National metrology institutions in the United States, Japan, Germany, and other countries have carried out research on resistance quantum standards based on QAHE. Quantum anomalous Hall states have also been observed in ultra-cold atomic systems, angular graphene, and angular transition metal sulfide systems. The fractional QAHE has also recently been experimentally realized. The research on QAHE has become one of the fastest developing research directions in international physics.

Qi-Kun Xue has made multiple outstanding contributions to the study of QAHE. We congratulate him for receiving China’s top scientific honor.