What Is the Quantum Hall Effect?

The Hall effect is a type of electromagnetic effect. This phenomenon was discovered by the American physicist Hall (EHHall, 1855-1938) in 1879 while studying the conductive mechanism of metals. ^[1] When a current passes through a semiconductor perpendicular to an external magnetic field, the carriers are deflected, and an additional electric field is generated perpendicular to the direction of the current and the magnetic field, which causes a potential difference between the two ends of the semiconductor. This phenomenon is the Hall effect. The potential difference is also called the Hall potential difference. The Hall effect is judged using the left-hand rule.

Hall Effect

Hall effect ^[2]

The application of a magnetic field perpendicular to the direction of the current on the semiconductor will cause the electrons in the semiconductor to interact with

When a carrier in a solid material moves in an external magnetic field, the trajectory shifts due to the Lorentz force, and charges accumulate on both sides of the material, forming an electric field perpendicular to the direction of the current. The Lorentz force on the son is balanced with the repulsive force of the electric field, thereby establishing a stable

The Hall effect is particularly important in applied technology.

About 100 years after the Hall effect was discovered, German physicists

1. The mechanism of the integer quantum Hall effect has been basically clear, and some scientists, such as von Klitzing and VJ Goldman of the State University of New York at Stony Brook, are still doing some research on fractional quantum effects. Some theorists have pointed out that certain platforms in the fractional quantum Hall effect can constitute non-Abelian states, which can become the basis for building topological quantum computers.

   The quantum Hall effect in graphene is very different from the general quantum Hall behavior, and it is called the Anomalous Quantum Hall Effect.

   In addition, Hirsh and Zhang Shousheng proposed the concept of the spin quantum Hall effect, and experiments related to it are attracting more and more attention.

   Chinese scientists discover quantum anomalous Hall effect

   Science published an article online announcing for the first time that a team led by Chinese scientists has experimentally discovered the quantum anomalous Hall effect. This discovery may have a significant impact on information technology progress.

   The discovery was led by Xue Qikun, a professor of Tsinghua University and an academician of the Chinese Academy of Sciences (former teacher of the School of Physics and Engineering of Qufu Normal University). A team of researchers from Tsinghua University, the Institute of Physics of the Chinese Academy of Sciences and Stanford University completed the research in 4 years. 133 years after the discovery of the anomalous Hall effect by American physicist Hall in 1880, the quantization of the anomalous hall effect was finally achieved. This discovery is a major breakthrough in related fields and an important scientific discovery in the world's basic research field. .

   The American scientist Hall discovered the Hall effect and the anomalous Hall effect in 1879 and 1880, respectively. In 1980, German scientist von Klitzing discovered the integer quantum Hall effect, and in 1982, American scientists Cui Qi and Stemmer discovered the fractional quantum Hall effect. These two results obtained Nobel physics in 1985 and 1998, respectively. prize.

   The joint research team composed of researchers from the Institute of Physics of the Chinese Academy of Sciences and the Department of Physics of Tsinghua University, after years of unremitting exploration and hard research, successfully achieved the "quantum abnormal Hall effect". This is an important scientific breakthrough in this field in the world. The whole process of the physical effect from theoretical research to experimental observation is independently completed by Chinese scientists.

   The quantum Hall effect is one of the most important and basic quantum effects in the entire field of condensed matter physics. It is a typical macroscopic quantum effect and a perfect embodiment of the quantum behavior of the microelectronic world on a macroscopic scale. In 1980, German scientist Klaus von Klitzing discovered the "Integer Quantum Hall Effect" and won the Nobel Prize in Physics in 1985. In 1982, Chinese-American physicist Daniel CheeTsui, American physicist Horst L. Stormer and others discovered the "fractional quantum Hall effect". B. Laughlin) gave a theoretical explanation, and the three of them won the 1998 Nobel Prize in Physics. In the quantum Hall effect family, the effect that has not yet been discovered is the "quantum anomalous Hall effect"-a quantum Hall effect that does not require an external magnetic field.

   The "quantum anomalous Hall effect" has been a very difficult and major challenge in this field for many years. It has a completely different physical nature from the known quantum Hall effect and is a brand new quantum effect. At the same time, its implementation is also more Difficult, requires accurate material design, preparation and regulation. In 1988, the American physicist F. Duncan M. Haldane proposed that there might be a quantum Hall effect that does not require an external magnetic field. Physical approach.

   In 2010, the team led by Fang Zhong and Dai Xi from the Institute of Physics of the Chinese Academy of Sciences and Professor Zhang Shousheng made a breakthrough in theory and material design. They proposed that Bi2Te3, Bi2Se3, and Sb2Te3 topological insulators doped with Cr or Fe magnetic ions. With the special V.Vleck ferromagnetic exchange mechanism, it can form a stable ferromagnetic insulator and is the best system to realize the quantum anomalous Hall effect [Science, 329, 61 (2010)]. Their calculations indicate that the magnetic topological insulator multilayer film is in a "quantum anomalous Hall effect" state with a certain thickness and magnetic exchange strength. The breakthrough in this theory and material design has aroused widespread international interest, and many of the world's top laboratories are scrambling to invest in this competition, looking for the quantum anomalous Hall effect along this line of thought.

   The realization of the "quantum anomalous Hall effect" in magnetically doped topological insulator materials places extremely high requirements on material growth and transport measurement: the material must have ferromagnetic long-range order; the ferromagnetic exchange effect must be strong enough to Causes energy band reversal, which results in a topologically non-mediocre band structure; at the same time, the carrier concentration in the body must be as low as possible. The team consisting of He Ke, Lu Li, Ma Xucun, Wang Lili, Fang Zhong, and Dai Xi from the Institute of Physics of the Chinese Academy of Sciences and Xue Qikun, Zhang Shoucheng, Wang Yayu, Chen Xi, and Jia Jinfeng from the Department of Physics of Tsinghua University worked together to solve the problem The competition showed strong strength. They have overcome many obstacles such as thin film growth, magnetic doping, gate voltage control, and low temperature transport measurement. They have realized the precise control of the topological insulator's electronic structure, long-range ferromagnetic sequence, and band topology, using molecular beams. The epitaxial method has grown a high-quality Cr-doped (Bi, Sb) 2Te3 topological insulator magnetic film, and successfully observed the "quantum anomalous Hall effect" on a very low temperature transport measurement device. The results were published online in Science on March 14, 2013. Tsinghua University and the Institute of Physics, Chinese Academy of Sciences are the co-first authors.

   The achievement of this achievement is a successful example of Chinese scientists' long-term accumulation, collaborative innovation, and collective research. In the early stage, the team members have made a series of progress in the research of topological insulators. The research results have been selected as the top ten scientific advances in China in 2010 and the top ten scientific and technological progress in Chinese universities. The team members also won the 2011 "Seek Outstanding Scientist Award" , "Seek Outstanding Scientific and Technological Achievement Collective Award" and "Chinese Academy of Sciences Outstanding Scientific and Technological Achievement Award", and 2012 "Global Chinese Physics Society Asian Achievement Award", "Chen Jiageng Science Award" and other honors. This work was supported by the Chinese Academy of Sciences, the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Ministry of Education.

   What the quantum anomalous Hall effect will bring us

   The discoveries related to the quantum Hall effect have repeatedly won academic awards because the Hall effect is particularly important in applied technology. Many electronic devices commonly used in human daily life come from the Hall effect. The only Hall devices widely used in automobiles include: signal sensors, speed sensors in ABS systems, automobile speedometers and odometers, liquid physical quantity detectors, various Current detection and working status diagnosis of electric loads, engine speed and crank angle sensors.

   The quantum anomalous Hall effect discovered by Chinese scientists this time also has extremely high application prospects. The generation of the quantum Hall effect requires a very strong magnetic field, so it has not been widely used in personal computers and portable computers until now. Because the required magnetic field is not only expensive, but also as large as a wardrobe. The anomalous Hall effect is completely different from the ordinary Hall effect in essence, because there is no deflection of the motion orbit caused by the Lorentz force of the external magnetic field on the electron. of.

   Now that Chinese scientists have experimentally realized the quantum Hall effect in a zero magnetic field, it is possible to use its non-dissipative edge states to develop a new generation of low-energy transistors and electronics, thereby solving the computer heating problem and the bottleneck of Moore's Law problem. These effects may play a special role in future electronic devices: high-speed electronic devices with low energy consumption, such as chips with very low energy consumption, can be prepared without high magnetic fields, which may lead to the birth of highly fault-tolerant full-topology quantum computers-which means Personal computers may be replaced in the future. ^[5]