Among them, the force on the moving charge in the magnetic field is called Lorentz force, that is, the force of the magnetic field on the moving charge. We all learned the method of left-handed rule in middle school. Put the left palm flat, let the magnetic induction line pass through the palm, and the four fingers indicate the direction of positive charge movement, then the direction in which the thumb is perpendicular to the four fingers is the direction of Lorentz force. But it should be noted that the moving charge is positive, and the thumb direction is the direction of Lorentz force. On the other hand, if the moving charge is negative and the moving direction of the charge is still represented by four fingers, then the opposite direction of the thumb is the Lorentz force direction.
Carrier refers to charged material particles that can move freely, such as electrons and ions. Hall's discovery was later called. Hall effect? This potential difference is also called Hall potential difference.
Simply put, the Hall effect defines the relationship between magnetic field and induced voltage. When a current passes through a conductor in a magnetic field, the magnetic field will exert a transverse force on the electrons in the conductor, resulting in a voltage difference across the conductor.
Although this effect has been known and understood for many years, the sensor based on Hall effect has not been put into practical use until the material technology has made great progress, until there are high-strength permanent magnets and signal conditioning circuits working at small voltage output. According to different designs and configurations, Hall effect sensors can be used as on/off sensors or linear sensors, which are widely used in power systems.
Schematic diagram of hall effect made by Peo
According to the Hall effect, various Hall elements are widely used in precision magnetic measurement, automatic control, communication, computer, aerospace and other industrial departments and national defense fields.
According to the classical Hall effect theory, the Hall resistance RH (RH=U/I=K. B/d= B/nqd) should change continuously with B and decrease with the increase of N (carrier concentration), but when it reaches 1980, the famous physicist Feng? Krizin discovered a new quantum Hall effect from metal oxide semiconductor field effect transistor (MOSFET). He added two electrodes to the silicon MOSFET tube, and then put the silicon MOSFET tube under strong magnetic field and extremely low temperature, and found that a series of plateaus appeared on the curve of Hall resistance changing with gate voltage. Hall resistance Rh=h/(ne2) corresponding to these platforms, where n is a positive integer of 1, 2,3. In other words, these platforms are precisely given, regardless of the changes in materials and device sizes. They are only determined by the basic physical constants H (Planck constant) and E (electron charge).
This is the so-called integer quantum Hall effect, and later scientists also discovered the fractional quantum Hall effect.
Physicists at that time believed that except quarks and other particles, all the elementary particles in the universe were charged by an electron-e (e =1.6? 10- 19 coulombs). And quarks can be carried according to their categories? 1e/3 or? 2e/3 charge. Quarks can only exist in the nucleus, unlike electrons that can flow freely. Therefore, physicists do not expect to see particles or excited States with fractional electron charges like quarks in ordinary condensed systems.
However, in 1982, China scientists Cui Qi and Strom developed a fractional Hall resistance platform in a two-dimensional electronic system. Did you find out at first? And then what? Two platforms. After that, they made purer samples with lower temperature and stronger magnetic field, which were 85mK and 280kG respectively. This is the first time that humans have achieved such a low temperature and such a strong magnetic field in the laboratory (the geomagnetic field is in the order of milligram). This experimental technique is amazing, so they observed richer structures: they also observed richer structures. Their discovery is therefore called the fractional quantum Hall effect.
Feng? Kriging won the Nobel Prize in Physics in 1985, while Cui Qi and Strom won the Nobel Prize in Physics in 1998. In 2005, British scientist Andre? Heim and Constantine? Novoselos. They discovered the semi-integer quantum Hall effect in graphene in 2005 and won the 20 10 Nobel Prize in Physics.
Simply put, the quantum Hall effect generally appears in extreme conditions such as ultra-low temperature and strong magnetic field. Under extreme conditions, the deflection of electrons is no longer the same as in the ordinary Hall effect, but becomes stronger, and the deflection radius becomes very small, just like spinning around a certain point inside the conductor. That is to say, what are some electrons in the middle of the conductor? Locked? If you want to conduct current, you can only reach the edge of the conductor.
The biggest difference between quantum Hall effect and Hall effect is that the response of transverse voltage to magnetic field is obviously different. Transverse resistance is quantified:
20 18 12 18, the latest research achievement "based on the quantum hall effect outside the orbit in cadmium arsenide" was published by the British magazine Nature, which was the first time that Chinese scientists discovered the quantum hall effect in three-dimensional space.
Later, China University of Science and Technology and its cooperative team published a paper in Nature, saying that they verified the three-dimensional quantum Hall effect through experiments and discovered the metal-insulator transition. They found that people can realize metal-insulator transition by controlling temperature and external magnetic field. What can this principle be used for? Quantum magnetic switch? Electronic components such as. The electron mobility in three-dimensional quantum Hall effect materials is very fast, and electrons can be transmitted and responded quickly, so it has application prospects in infrared detection, electron spin devices and so on. Thirdly, the three-dimensional quantum Hall effect can be applied to special carrier transport systems because of its quantized conductivity.
At this time, it is necessary to talk about quantum anomalous Hall effect, because the quantization of Hall effect has two extremely harsh preconditions: first, it needs a strong magnetic field of several hundred thousand gauss, and the strength of the earth's magnetic field is only 0.5 gauss; Second, it needs a temperature close to absolute zero.
In this context, scientists put forward an idea: the Hall phenomenon in ordinary state will be abnormal, so can the quantized Hall phenomenon be abnormal? If so, wouldn't it be possible to solve the prerequisite for applying a strong magnetic field?
In other words, the quantum anomalous Hall effect is produced by the spontaneous magnetization of the material itself independent of the strong magnetic field. Quantum Hall states can be realized in zero magnetic field, and are more easily applied to electronic devices that people need every day. Since 1988, theoretical physicists have put forward various schemes, but the experiment has not progressed.
We can use a simple metaphor to illustrate the relationship between quantum Hall effect and quantum anomalous Hall effect. When we use a computer, we will encounter problems such as computer fever, energy loss and slow speed. This is because under normal circumstances, the electrons in the chip have no specific orbits and collide with each other, resulting in energy loss. Quantum hall effect can make a rule for the movement of electrons, so that they are on their own runway? Go ahead. Walk slowly forward.
However, the quantum Hall effect requires a very strong magnetic field. It is equivalent to adding 10 magnets as big as a computer, which is not only bulky, but also expensive and not suitable for personal computers and portable computers. ? The beauty of quantum anomalous Hall effect is that quantum Hall states can be realized in zero magnetic field without any external magnetic field, and it is easier to be applied to electronic devices that people need every day.
In 2006, the theoretical team led by Professor Zhang Shousheng of Stanford University in the United States successfully predicted the quantum spin Hall effect in two-dimensional topological insulators, and in 2008 pointed out the new direction to realize the quantum anomalous Hall effect in magnetically doped topological insulators. 20 10 China theoretical physicists, Dai et al. cooperated with the professor and proposed that magnetically doped three-dimensional topological insulators may be the best system to realize quantized anomalous hall effect. This scheme has aroused widespread concern in international academic circles. World-class research groups such as Germany, the United States and Japan all searched for quantum anomalous Hall effect in their experiments along this line of thought, but they never made a breakthrough. Therefore, quantum anomalous Hall phenomenon is also called the jewel in the crown of physics research.
It is difficult to realize quantum anomalous Hall effect, which requires accurate material design, preparation and control. Although scientists from all over the world have put forward several different realization methods for many years, the materials and structures needed are very difficult to prepare, so the experiment progress is slow.
In 2009, Xue Qikun and his team also began to tackle the quantum anomalous Hall effect. In the eyes of many people, Xue Qikun is not a genius.
From 65438 to 0963, Xue Qikun was born in a small village in Yimeng Mountain area, Shandong Province, with many brothers and sisters at home. When I was in primary school and middle school, the rural conditions were still relatively backward, and adults were struggling for their livelihood. Xue Qikun didn't dream of becoming a physicist, but he had books to read. Later, the news came that the national college entrance examination was resumed. Xue Qikun felt that this opportunity could not be wasted and began to prepare for the college entrance examination.
1980, 17-year-old Xue Qikun was admitted to the Department of Optics of Shandong University. The reason for choosing the Department of Optics is that the teacher recommended this major. Xue Qikun, who knew nothing about this major, filled in this major. 1984 Xue Qikun, who graduated, began to take part in the postgraduate entrance examination while working. Results I was admitted to the Institute of Physics of Chinese Academy of Sciences for three times. 1990 after graduating from the master's degree, it took another seven years to get a doctorate.
Xue Qikun has a nickname, called? Academician 7- 1 1? . Anyone who knows him well knows that he entered the laboratory at 7: 00 in the morning and worked until 1 1 left at night. This way of working and sleeping has been maintained in Xue Qikun for 20 years. Xue Qikun thinks that since he is not? Genius? , then make one? Stupid person? All right. Do a good job? Stupid person? , not easy.
Since 2009, after nearly five years of research, Xue Qikun team has made unimaginable efforts from the initial success of topological insulator material growth to overcoming many difficulties in the experiment in the later stage. However, the ultimate success of the experiment depends on whether a symbolic experimental data can make the Hall resistance of magnetic topological insulator materials jump to the quantum resistance value of 258 13 ohm under zero magnetic field.
They measured more than 1000 samples. Finally, they grew high-quality chromium-doped (Bi, Sb)2Te3 topological insulator magnetic films by molecular beam epitaxy, and successfully observed the quantum anomalous Hall effect on the extremely low temperature transport measurement device. This is the first time that quantum anomalous Hall effect has been found in the experiment.
In 20 10, the research group completed the measurement of the growth and transportation of thin films with the thickness of 1 nm to 6 nm (one ten thousandth of the thickness of hair), and obtained systematic results, thus making it possible to measure the growth of quasi-two-dimensional ultra-thin films.
In 20 1 1 year, the research group realized the precise regulation of the energy band structure of topological insulators, making their bulk materials become real insulators and removing the influence on transport properties.
At the beginning of 20 12, the research group realized spontaneous long-range ferromagnetism in a quasi-two-dimensional, bulk-insulated topological insulator, and accurately adjusted its electronic structure in situ by applying gate voltage.
20 12, 10 in June, the research group finally found that the abnormal Hall resistance of this material reached the characteristic value h/e2 of quantum Hall effect in a certain range of applied gate voltage. The 25800 ohm world problem is solved.
The research group overcame many difficulties such as thin film growth, magnetic doping, gate voltage control and low temperature transport measurement, and gradually realized the precise regulation of topological insulator electronic structure, long-range ferromagnetic sequence and energy band topological structure, and finally ended the realization of this physical phenomenon.
In the past five years, Xue Qikun team has overcome many difficulties such as thin film growth, magnetic doping, gate voltage control and low-temperature transport measurement, and gradually realized the precise regulation of topological insulator electronic structure, long-range ferromagnetic sequence and energy band topology, and finally ended the realization of this physical phenomenon.
A reviewer of Science magazine said: This work undoubtedly confirms the existence of single-channel edge states with different sources from the ordinary quantum Hall effect. I think this is a very important achievement of condensed matter physics. ? Another commenter said:? This paper ends the exploration of quantum Hall effect without Landau level for many years. This is a landmark article. ?