The cooperation between Chinese and American universities has made a breakthrough in the study of fractional quantum anomalous Hall effect.
Scientists in China have made important progress in the research related to quantum Hall effect.
The research on quantum Hall effect in condensed matter physics has won three Nobel Prizes, one is the discovery of quantum Hall effect in two-dimensional electronic system, and the other is the discovery of fractional quantum Hall effect, both of which are phenomena under strong magnetic field. Another time was a theoretical discovery related to the quantum Hall effect.
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Recently, the team of Li Tingxin and Liu Xiaoxue of Shanghai Jiaotong University discovered and verified the existence of fractional quantum anomalous Hall effect under the condition of zero magnetic field. The results were published in the famous journal Physics Review X in the field of physics research and were highly recommended.
According to Shanghai Jiaotong University, this achievement has made a breakthrough in this field, and the team paved the way for a series of follow-up studies through the original preparation method of new devices. This study opens the door to the study of novel physical properties such as fractional charge excitation and anyon statistics under zero magnetic field, and provides new possibilities and opportunities for topological quantum computing and other research.
Schematic diagram of Mohr superlattice structure formed by two layers of MoTe2 corners, in which red atoms represent Mo and blue atoms represent Te. Three highly symmetrical stacking modes are represented by circles with different colors in the figure.
Past lives of Hall Effect
When Edwin Hall, an American scientist, studied the conductive mechanism of metals in 1879, he found that when a current and a magnetic field perpendicular to the direction of the current were applied to a conductor, the trajectory of electrons in the conductor would be deflected due to Lorentz force, which would generate a voltage across the conductor perpendicular to the direction of the current and the magnetic field, which was called Hall voltage, and this effect was called Hall effect. According to the principle of Hall effect, Hall sensor has been widely used in speed sensing.
At the beginning of 1980, when studying the Hall resistance of two-dimensional electronic system, German physicist K. von Klitzing found that under the extreme conditions of extremely low temperature and strong magnetic field, the Hall resistance of the sample appeared a series of quantized platforms, and the longitudinal resistance would show zero resistance state accordingly. This phenomenon is called integer quantum Hall effect.
Later, theoretical physicists realized that the concept of topology must be introduced into physical research by using the topology theory in mathematics, so as to fully understand the integer quantum Hall effect, thus opening the era of topological physics research. Integer quantum Hall effect has also become the first topological quantum state of matter discovered by human beings.
Generally, electrons in vacuum always have an integer number of charges, so it is impossible to have a fractional charge. Li Tingxin, an associate professor at the School of Physics and Astronomy of Shanghai Jiaotong University, said, for example, that in the fractional quantum Hall effect that was surprisingly discovered later, an electron seemed to be split into three parts, each with a charge of 1/3. It can be said that the fractional quantum Hall effect is a strongly correlated quantum state of matter, which is essentially different from the integer quantum Hall effect. It is a strange quantum fluid and has become one of the important candidates for topological quantum computing.
However, if there is a normal effect, there may be an abnormal effect. In 1988, American theoretical physicist D. Haldane calculated that the integer quantum Hall effect under zero magnetic field could appear, which was later called the quantum anomalous Hall effect. This theoretical work is also one of his representative works that won the 2016 Nobel Prize in Physics. So far, the quantum Hall effect has won three Nobel Prizes.
Direct observation of fractional quantum anomalous Hall effect in rotation angle MoTe2
In recent years, based on the two-dimensional material Moire superlattice system, researchers at home and abroad have carried out a lot of theoretical and experimental research on fractional quantum anomalous Hall effect, and finally focused on the corner MoTe2 system. From April to May this year, the research team of Xiaodong Xu of Washington University and the research team of Kin Fai Mak and Jie Shan of Cornell University in the United States independently reported the signs of fractional quantum anomalous Hall states in corner MoTe2. However, optical measurement methods are generally difficult to give direct and conclusive evidence of the existence of fractional quantum anomalous Hall effect.
Left: The measured Hall resistance of fractional quantum anomalous Hall effect in rotation angle MoTe2 varies with magnetic field;
Right: The change of Hall conductance with the filling factor of Moire superlattice at zero magnetic field limit, and a clear quantized Hall conductance platform of integer (E 2/h) and fractional (E 2/3h) can be seen.
Recently, the teams of Li Tingxin and Liu Xiaoxue of Shanghai Jiaotong University (experiment) cooperated with the team of Zhang Yang of the University of Tennessee (theory), and made a breakthrough in the study of fractional quantum anomalous Hall effect. They designed and fabricated a novel corner MoTe2 Moore superlattice device, and directly observed the conclusive evidence of the existence of fractional quantum anomalous Hall effect by conducting electrical transport experiments, which was one of two independent experimental work in the same period in the world (the other experimental work was completed by Xiaodong Xu research team of the University of Washington, USA).
This work has been highly praised by the reviewers, which is considered as a breakthrough in the field, and the original new device preparation method paves the way for a series of subsequent studies around the fractional quantum anomalous Hall effect. This study opens the door to the study of novel physical properties such as fractional charge excitation and anyon statistics under the condition of zero magnetic field, and provides new possibilities and opportunities for topological quantum computing and other research. The paper was published in Physical Review X 13, 031037 (2023), and was recommended by the editor in the form of Featured in Physics. The review article "news &views" published by Nature holds that two research works provide a smoking-gun signature for the anomalous Hall effect of fractional quantum.