内容来源：量子前哨（ID：Qforepost）
Text丨Langweixian typesetting丨Peixian
In-depth good article: 2000 words丨8 minutes to read
On June 24, the 2023 National Science and Technology Award was announced in Beijing, and Xue Qikun, a 61-year-old scientist in the field of condensed matter physics and academician of Tsinghua University, won the high honor of China's scientific and technological community.
Based on the scientific research achievement of the first observation of the "quantum anomalous Hall effect", Academician Xue Qikun has won two of the highest awards in the field that were awarded to Chinese physicists for the first time, and is also respectfully called "the closest physicist to the Nobel Prize".
So the question is, what exactly is the "quantum anomalous Hall effect"? What are the challenges in observing this effect? What is the practical value of this phenomenon? Let's talk about that today.
In 1879, Edwin Herbert Hall, who was stumbling on his doctoral dissertation, stumbled upon an interesting phenomenon.
Edwin Herbert Hall
Hall is a Ph.D. student at Johns Hopkins University, where he studied Maxwell's electromagnetism, in which he fixed a piece of gold leaf vertically and applied a direct current to the left and right ends, and then applied a magnetic field perpendicular to the surface of the gold leaf.
At this time, the upper and lower ends of the gold leaf were measured, and it was found that there was a voltage generated, called Hall voltage, and this phenomenon was called the Hall effect, and Hall successfully submitted his graduation thesis with this discovery.
The Hall effect is an important transport phenomenon for conductors and semiconductor materials, is the basis for many electronic components, including cell phones, rocket thrusters, and is widely used to detect concentrations or magnetic fields or currents around charge carriers, and has been used for resistance calibration worldwide since 1990.
Two years later, in 1881, while studying the Hall effect of magnetic metals, Hall made another discovery: the Hall effect could also be observed without an external magnetic field. That is, the magnetic field is not a necessary condition for the Hall effect.
This Hall effect in a zero magnetic field is known as the "anomalous Hall effect", and for substances with natural magnetism, such as iron, no external magnetic field is required to achieve the Hall effect.
At this time, Hall would never have imagined that his discovery would have a new breakthrough 100 years later.
In 1980, when the German physicist von Klitsin was studying a metal-oxide semiconductor field-effect transistor under a high magnetic field, he accidentally discovered that the Hall resistance was precisely quantized in the two-dimensional electronic gas in it.
Von Klitsin
In other words, there is a quantum mechanical version of the Hall effect. The quantum Hall effect can only be observed under extreme conditions of low temperature and strong magnetic field, when the Hall resistance is no longer linear with the magnetic field, which indicates that quantum mechanical phenomena can be manifested at the control scale.
The quantum Hall effect is a collective term that includes the integer quantum Hall effect and the fractional quantum Hall effect, and the discoverers have won the Nobel Prize:
In 1985, German physicist von Klitsin was awarded the Nobel Prize for his discovery of the Hall effect of integer quants;
In 1988, Chinese-American physicist Cui Qi, German physicist Stermer of Bell Labs, and Laughlin of Lawrence Livermore National Laboratory in the United States won the Nobel Prize for their discovery of the fractional quantum Hall effect.
Von Klitsin's Nobel Prize certificate
One thing to say, when it comes to practical applications, the quantum Hall effect has a clear limitation: it needs to be externally strengthened magnetic fields. The quantum Hall effect requires the help of a very strong magnetic field, but the magnetic field required to generate it is not only expensive, but also very large, which adds a lot of resistance to the popularization of applications.
The quantum anomalous Hall effect can just break this limitation.
In the quantum anomalous Hall effect, "anomalous" refers to the fact that in some special materials, because the material itself has a strong internal magnetic field, there is no need to apply an external magnetic field to produce the quantum Hall effect.
It may seem easy to describe it this way, but in fact, the quantum anomalous Hall effect is the ultimate problem, and the materials that make it possible require very demanding properties.
The biggest challenge of the quantum anomalous Hall effect is to produce magnetic, topological and insulating films, and these three properties require the thickness of the film to be interrelated and difficult to describe accurately by functions, like a black box, making it difficult to determine the thickness of the film, just like "a person is required to run as fast as Bolt, but also be very strong and have the skills of a gymnast".
Topological insulators are a fascinating and interesting quantum phenomenon like a ceramic bowl coated with an extremely thin (about 1 nanometer) film of conductive gold. Interestingly, this film cannot be removed, and the miraculous thing is that even if you forcibly remove it with a spatula, it will immediately and spontaneously produce a new film of gold, and unless the material is completely broken down into atoms, this layer of gold film attached to the surface will never disappear.
The magnetic topological insulator is even more magical, by introducing magnetism into the material, it will automatically remove most of the gold film of the ceramic bowl, leaving only the edge part, but the gold film on the edge cannot be removed.
The material samples used by Academician Xue Qikun's team for experiments are paved with atoms layer by layer, with a thickness of only 5 nanometers, about 1/100,000 of a human hair, and the difficulty of preparation can be imagined. In the past four years, the number of samples prepared has reached more than 1,000, countless improvements, innovations, and profound control of materials with the bright "eye" of scanning tunneling microscope, and finally at the end of 2012, the Cr-doped (Bi,Sb)2Te3 magnetic topological insulator film prepared by Academician Xue Qikun's team has beautifully proved the real existence of this wonderful quantum state with experiments for the first time in the world.
Academician Xue Qikun supervises the students of the research team
The quantum anomalous Hall effect is expected to promote the development of low-energy transistors and electronic devices, and overcome a series of problems caused by the current computer heat generation and energy consumption. With this scientific research achievement, Academician Xue Qikun has won two highest awards in the field that are both awarded to Chinese physicists for the first time: the 2020 International Prize for Low-Temperature Physics - Fritz London Prize, and the 2024 International Highest Award for Condensed Matter Physics - Oliver Buckley Prize.
Seongshik Oh, a professor in the Department of Physics and Astronomy at The State University of New Jersey, pointed out in an article published in the journal Science that "it [the quantum anomalous Hall effect] has finally allowed people to play the trio of the quantum Hall effect in its entirety."
In the history of quantum Hall effect research, there have been four Nobel laureates in physics, and their work is the quantum Hall effect that requires a magnetic field, and the quantum anomalous Hall effect is the only quantum Hall effect that does not require a magnetic field, and it is also the last important member of the quantum Hall effect family. and the quantum anomalous Hall effect.
Every scientific breakthrough in the quantum Hall effect is a new Nobel Prize seed, so the 61-year-old academician Xue Qikun, who lowered the average age of the winner of the country's highest science and technology award with his strength, is also respectfully called "the closest physicist to the Nobel Prize".
But the world of scientists is pure, nothing is more important than love, for Academician Xue Qikun, the strong sense of happiness as a scientist also comes from the sincere love for the motherland and science.
"Everybody has to have a believing. When you have faith, no matter how big the difficulties are and what kind of tests you are undergoing, you will move forward and enjoy it because of your firm faith. ”
Resources:
1. Quantum Science and Technology: Open Class for Leading Cadres
2. "The Reader II."