Xue Qikun, academician of the Chinese Academy of Sciences and president of Southern University of Science and Technology, spoke at the Li Zhengdao Institute of Shanghai Jiao Tong University through video on the 28th, "Special congratulations to Professor Xiao Li and Professor Xiao Liu." Exactly 10 years ago, it was the experimental team led by him that first observed the "integer quantum anomalous Hall effect", which also won the first prize of the National Natural Science Award in 2018.
Li Suo's "Tianwen" long scroll
Before the 2023 Nobel Prize season, in the face of many academicians from this school and other universities such as Xue Qikun, Xie Xincheng, Feng Donglai and Zhang Jie, the team of Li Tingxin and Liu Xiaoxue, a young teacher couple in their 30s at Shanghai Jiaotong University, released the major results of the "fractional quantum anomalous Hall effect" in Li Institute that afternoon. Among them, Liu Xiaoxue, a "Li Zhengdao scholar" born in 1990, is also a new mother with a baby of less than 100 days, mainly responsible for the design and preparation of experimental devices.
Li Tingxin, Liu Xiaoxue and his wife
Their "compounding" of the two Nobel Prize discoveries, carrying out experimental physics research, highly praised by the top international reviewers, considered to be a breakthrough in the field, and the original new device preparation method paved the way for a series of subsequent research. It is reported that this research opens the door to the study of novel physical properties such as fractional charge excitation and arbitrary substatistics under zero magnetic field conditions, and provides new possibilities and opportunities for topological quantum computing and other research.
[Conclusive evidence was found, it took 144 years]
In fact, from the Hall effect to the quantum Hall effect to the quantum anomalous Hall effect, their theoretical predictions and experimental evidence took 144 years to find the exact answer. The relevant paper led by China was first published in Physical Review X on the 27th and was recommended by the editor; A review article published on the same day in Nature is also a "highlight report", arguing that the two research efforts provide definitive evidence for the fractional quantum anomalous Hall effect.
When American scientist Edwin Hall studied the conductive mechanism of metals in 1879, he found that applying an electric current to a conductor and a magnetic field perpendicular to the direction of the current produced a "Hall voltage", an effect known as the Hall effect. Hall sensors made according to the Hall effect principle have been widely used in speed sensing and other aspects.
In early 1980, German physicist K. When von Klitzing studied the Hall resistance of two-dimensional electronic systems, he found that under extreme conditions of extremely low temperature and strong magnetic field, a series of quantized platforms appeared in the Hall resistance of the sample, and the longitudinal resistance would show a corresponding zero resistance state. This phenomenon is known as the integer quantum Hall effect.
Later, theoretical physicists realized that it was necessary to use topological theory in mathematics to introduce the concept of topology into physical research in order to fully understand the integer quantum Hall effect, which opened the era of topological physics research. The integer quantum Hall effect has also become the first topological quantum state of matter discovered by mankind.
In general, electrons in a vacuum always have an integer number of charges, and it is impossible to have a fractional charge; Li Tingxin, an associate professor at Jiaotong University's School of Physics and Astronomy, said, for example, in the fractional quantum Hall effect, which was later surprisingly discovered, an electron can split into three parts, each carrying a 1/3 charge. It can be said that the fractional quantum Hall effect is a strongly correlated quantum state of matter that is essentially different from the integer quantum Hall effect, which is a singular quantum fluid and has become one of the important candidates for topological quantum computing.
However, if there are normal effects, there may be abnormal effects. American theoretical physicist D. Haldane calculated in 1988 that the integer quantum Hall effect at zero magnetic field could occur, which came to be known as the quantum anomalous Hall effect. This theoretical work is also one of his representative works to win the 2016 Nobel Prize in Physics. So far, the quantum Hall effect has won the Nobel Prize three times.
Li Tingxin made a report
【"Superposition" Nobel Prize two-dimensional materials, special "magic angle" observation]
It is expected and regrettable that the quantum anomalous Hall effect has not been experimentally realized for more than 20 years. It was not until 2013 that the experimental team led by mainland scientist Academician Xue Qikun first confirmed it in magnetically doped topological insulator films.
So, can the fractional quantum Hall effect under zero magnetic field conditions, that is, the fractional quantum anomalous Hall effect, really be realized? In 2011, five independently published theoretical research works affirmed the possibility of fractional quantum anomalous Hall effect from different angles. However, no material system has been found that can achieve this quantum anomalous Hall effect, until the emergence of the two-dimensional material "Mohr superlattice" brings new opportunities to solve this problem.
In this regard, Li Tingxin told the Jiefang Daily Shangguan News reporter that the most representative two-dimensional material with a layered structure is graphite, which is composed of layers of carbon atoms. In recent years, two British scientists obtained single-atom-layer graphene for the first time, winning the 2010 Nobel Prize in Physics.
More interesting than the Bino Prize found that when two layers of graphene are put together, but not aligned, and turned away at a specific angle, a new periodic structure appears, called "magic angle graphene", which can also be said to be the Moiré superlattice. The Mohr superlattice system with the "magic angle" exhibits many properties that ordinary graphene does not exist in a zero magnetic field. Among them, this includes the observation of the integer quantum anomalous Hall effect.
Through the design and preparation of the public platform for micro-nano processing of Jiaotong University, they locked the experimental system of the semiconductor material, molybdenum telluride MoTe2, based on the two-dimensional layered material Mohr superlattice system. Liu Xiaoxue introduced that their upper and lower layers show an angle of about 4 degrees, and the thickness of one layer is only one hundred thousandth of a piece of paper. Therefore, the experiments of Li Tingxin and Liu Xiaoxue's team cooperated with the theoretical "superposition" of Zhang Yang's team at the University of Tennessee in the United States, and made this breakthrough in the study of fractional quantum anomalous Hall effect.
International journal pages
On this new corner material's Mohr superlattice device, they directly observed conclusive evidence of the existence of the fractional quantum anomalous Hall effect by carrying out electrical transport experiments. The work became one of two independent experiments in the same period internationally, the other completed by the Xiaodong Xu research team at the University of Washington, and was also published internationally within the same week.
Image source: Photo by Xu Ruizhe Image source: Photo by Xu Ruizhe
题图说明：李政道研究所 Source: Author: Xu Ruizhe