The Science Fiction World of Xueba

In order to clarify the mechanism of superconductors, physicists have proposed a variety of theories, including the London equation proposed in 1935 to describe the relationship between superconducting current and weak magnetic field, and the Pippard equation proposed from 1950 to 1953 to perfect the London equation. Theory, proposed in 1950, is used to describe the relationship between superconducting current and strong magnetic field (close to the critical magnetic field strength) GL theory; proposed in 1957, BCS theory to explain the first type of superconductor from the microscopic mechanism... until now, Scientists have begun to propose a new mechanism to achieve superconductivity through quantum phase transitions: the condensation of topological defects in quantum spin Hall insulators to form superconductors.

Here, the more important ones are GL theory and BCS theory.

GL theory is a phenomenological theory proposed on the basis of Landau's second-order phase transition theory.

The proponents of the theory are Gunzburg and Landau.

The GL theory is based on the following considerations: when the external magnetic field strength is close to the superconductor's adjacent magnetic field strength, the current of the superconductor does not obey the linear law, and the zero-point vibration energy of the superconductor cannot be ignored.

The greatest contribution of GL theory is to predict the existence of the second type of superconductor.

Starting from the GL theory, the concept of surface energy κ can be derived.

When the surface energy of the superconductor κ\u003e1/√2, it is the first type of superconductor; when the surface energy of the superconductor κ\u003c1/√2, it is the second type of superconductor.

The BCS theory is based on the near-free electron model and is established on the premise of weak electron-phonon interaction.

The proponents of the theory are Bardeen (***ardeen), Cooper (), and Schrieffer (J.R.Schrieffer).

BCS theory believes that electrons with opposite spin and momentum in metals can pair to form Cooper pairs, and Cooper pairs can move without loss in the lattice to form superconducting currents.

Simply put, we can compare electrons to a bee with only one wing. Such a bee cannot fly, but when two such bees are combined, they can fly by flapping their wings left and right. up.

The BCS theory explains the reason for the Cooper pair as follows: When electrons move in the lattice, they will attract positive charges on adjacent lattice points, resulting in local distortion of the lattice points, forming a localized high positive charge region. This localized high positive charge region will attract electrons with opposite spins, and combine with the original electrons with a certain binding energy. At very low temperatures, this binding energy may be higher than the vibration energy of the lattice atoms, so that the electron pairs will not exchange energy with the lattice, and there will be no resistance, forming a superconducting current.

The BCS theory well explained the reason for the existence of the first type of superconductor from the microscopic level, and the authors of the theory, Bardeen, Cooper, and Schriever, won the 1972 Nobel Prize in Physics for this reason.

However, the BCS theory cannot explain the reason for the existence of the second type of superconductor, especially the Macmillan limit temperature (the critical transition temperature of a superconductor cannot be higher than 40K) obtained according to the BCS theory, which has already been broken by the second type of superconductor.

Until now, the physics community has not formed a generally accepted mechanism for the formation of superconductivity.

As for the exploration of high-temperature superconductors, academia has made a lot of progress.

In 1986, Müller and Bernoz discovered that a ceramic metal oxide LaBaCuO4 composed of barium, lanthanum, copper, and oxygen has high-temperature superconductivity, and the critical temperature can reach 35K (-240.15°C).

Since ceramic metal oxides are usually insulating substances, this discovery is of great significance, and Müller and Bernoz won the 1987 Nobel Prize in Physics for this reason.

Since then, research on high-temperature superconductivity has developed rapidly.

Driven by scientists from China, the United States and other countries, the record has been continuously refreshed within five years.

And in 1994 set a record of atmospheric pressure 135K,

A new record for the critical temperature of high pressure 164K.

However, copper oxide high-temperature superconducting materials are oxide ceramics, which lack flexibility and ductility, and tend to lose superconductivity and heat up rapidly when carrying high currents. There are many technical difficulties in application.

Moreover, its physical properties are extremely complex and difficult to be explained by existing theoretical frameworks.

In 2008, Japanese scientists discovered the existence of 26K superconductivity in the iron arsenide system. With the efforts of Chinese scientists, the critical temperature of this type of superconducting material quickly exceeded 40K, and even achieved 55K in bulk materials. of superconductivity.

So a new generation of superconductor family iron-based superconductor announced the discovery.

It's just that most of these superconductors contain arsenic or alkali metals, are sensitive to air, and have many limitations in application.

As for the existence of room-temperature superconductors, the academic community generally believes that they exist. Japanese scientists even set the search for superconductors above 400K as their long-term goal.

But it is not an easy task to 100% confirm the existence of a room temperature superconductor.

After all, to judge whether a new material is a superconductor, it must have two characteristics of zero resistance effect and complete diamagnetism at the same time. If the resistance does not drop to zero or the diamagnetism is poor, it cannot be 100% sure that it is a superconductor.

Historically, there have been many "superconductors" dubbed by scientists as suspicious superconductors, or USO for short, because there is no definite evidence, which is comparable to the legendary UFO.

Among these USOs, some claim to have superconductivity at 200K or even 400K, but it has never been proved by more experiments.

Some people even simply carry out academic fraud in order to seek personal benefits.

For example, a German named Jan Hendrik Sean once poured water in 2001, claiming to have discovered high-temperature superconductivity above 52K and a series of other electronic device applications in materials such as C60, and his thesis output The efficiency has reached the speed of one article every eight days.

Ultimately, physicists discovered that almost all of his papers had falsified data.

Sce series journals retracted seven manuscripts in 2002, Nature series magazines retracted eight manuscripts in 2003, and other academic journals also retracted dozens of manuscripts.

Later, his alma mater couldn't stand it anymore and revoked his doctorate. This scandal caused a sensation in the entire academic world, and Sean was also called a big liar in the physics world once in fifty years.

Even so, the enthusiasm for room temperature superconductors in academia has never diminished.

Especially in recent years, new superconductors have been discovered almost every month.

Among them, in 2015, the German scientist A.P.Drozdov discovered that hydrogen sulfide has superconductivity of 203K at 2 million atmospheres, but such harsh conditions can only be completed in the laboratory.

In 2019, the A.P.Drozdov team confirmed that when the pressure is 1 million Earth atmospheres, various hydrogen-rich lanthanide metal hydrides become superconductors at 250K, which is about minus 20 degrees.

It can be said that at the laboratory level, it is only one step away from a real room temperature superconductor.

Another more important discovery is related to the Chinese.

In 2018, the MIT Jarillo Herrero team discovered in experiments that double-layer graphene appeared superconducting when the twist angle was 1.1 degrees and the temperature was 1.7K.

The first author of this paper is an MIT doctoral student, Cao Yuan, a talented young man from China born in 1996. With this discovery, he topped the list of Top Ten Scientific Figures Influencing the World in 2018 by Nature.

The critical temperature of double-layer graphene superconductivity is very low, only 1.7K, which basically has no practical value.

The reason why this discovery is important is that it presents a brand-new physical phenomenon, which is completely different from other superconducting materials, which is of great significance to the explanation of superconducting principles and the search for high-temperature superconducting materials.

It took nearly a week for Pang Xuelin to sort out the current research status of superconductors in the real world, and he came to the conclusion that the symmetry breaking of electromagnetic interaction will inevitably lead to changes in the movement of electron clusters, thereby triggering the phenomenon of superconductivity.

This is a definition of superconductivity in the practical field, and it is also the only consensus on superconductivity in academia.

As for the theoretical explanation, then the Eight Immortals crossed the sea and each showed their magical powers.

...

Sorting out the current research status of superconductor materials is only the first step for Pang Xuelin. Next, Pang Xuelin will study all the relevant technical papers he brought back from the world of rural teachers and the world of dark forests.

In the world of rural teachers, Pang Xuelin obtained the quantum computer technology from the Carbon-Based Life Alliance; in the dark forest world, he obtained the electromagnetic orbital propulsion technology and a complete set of information on aerospace aircraft technology.

Previously, Pang Xuelin had no time to conduct careful research. Therefore, he needed to retreat for a period of time to digest and absorb all these technologies before he could really start the research and development of electromagnetic orbital propulsion technology and aerospace aircraft technology.

Moreover, Pang Xuelin is 80% sure that he can get clues about room-temperature superconductors from these materials.

This morning, as soon as Pang Xuelin arrived at the office, he said to Zuo Yiqiu: "Xiao Zuo, help me to see the schedule for the next week."

"Professor Pang, on October 12, that is, tomorrow morning, you will attend the inauguration ceremony of the first Jinlong battery factory of Jinlong Group. The Pang Xuelin Mathematics Center in Houtianjiang is officially unveiled. You also have to attend. From October 15th to 20th, it is Qiantang The laboratory and the Jiangcheng Institute for Advanced Study will concentrate on the interview time, and more than a hundred scholars from all over the world will come to meet with you..."

Pang Xuelin pondered for a moment, then raised his head and said, "When is the Nobel Prize awarding ceremony?"

"December 10."

Pang Xuelin said: "Help me free up the period from October 21st to December 8th. During this period, I will go to retreat and don't let anyone disturb me."

"Retreat?"

Zuo Yiqiu was slightly taken aback, a little puzzled.

Pang Xuelin said: "I don't want anyone to disturb me when I'm doing research."

"oh."

Zuo Yiqiu's face revealed a look of surprise, and then Zuo Yiqiu said again: "By the way, Professor Pang, the Dr. Cao Yuan you asked me to contact last week, he arrived in Jiangcheng this afternoon to meet you."

"Cao Yuan coming in the afternoon?"

A look of surprise appeared on Pang Xuelin's face.

He approached Cao Yuan, of course, for the research on superconductivity.

If there are really geniuses in this world, then Cao Yuan is undoubtedly one of them.

Even to some extent, Cao Yuan is a true genius compared to Pang Xuelin before he was transformed by the genetic optimizer.

Cao Yuan is a native of Xichuan Province. He has shown extraordinary learning talent since he was a child.

Because of his extraordinary talent, he was picked up by an experimental middle school in Shencheng at the age of eleven.

In the experimental middle school, he completed all the courses of primary school and middle school in only three years.

Three years later, the fourteen-year-old Cao Yuan was admitted to the junior class of the University of Science and Technology of China with a score of 668.

At the undergraduate level, Cao Yuan still had excellent grades and won the Guo Moruo Scholarship of the University of Science and Technology of China.

At the age of eighteen, Cao Yuan graduated with an undergraduate degree and received an offer from MIT.

After entering MIT, Cao Yuan missed the Department of Physics that he wanted to enter, but accidentally entered the Department of Electrical Engineering of MIT. He followed his mentor Herrero to study for a Ph.D., and then conducted research on the relevant properties of double-layer graphene under Herrero.

At the age of 22, Cao Yuan published two articles on graphene superconductivity in the "Nature" magazine as the first author, which aroused widespread attention in the academic circle.

In the same year, Cao Yuan topped the "Nature" 2018 list of top ten influential scientific figures in the world.

Pang Xuelin didn't know much about Cao Yuan's later experience. When he checked the information on superconductivity during this period, he vaguely felt that Cao Yuan's discovery was crucial for him to find a superconducting theory with universal significance.

So he asked someone about Cao Yuan's situation, and only then did he know that Cao Yuan is already an associate professor at the University of Science and Technology of China, and independently leads a team to conduct research on condensed matter physics.

So Pang Xuelin simply asked Zuo Yiqiu to contact Cao Yuan, hoping to meet him.

He did not expect that Cao Yuan would take the initiative to rush over.

At 2:30 in the afternoon, Pang Xuelin met this talented boy who was only two years older than himself in his office.

Cao Yuan is of medium height, thin, with glasses, he looks very energetic.

The two were about the same age, and both belonged to the new forces in the academic world, so they quickly became acquainted.

After exchanging pleasantries for a while, Pang Xuelin said with a smile: "Professor Cao, the reason I invited you over this time is to ask you, are you interested in joining Qiantang Laboratory?"

"Join Qiantang Lab?"

Cao Yuan was taken aback for a moment, and could not help showing a look of embarrassment on his face.

The reason why he took the initiative to rush to Jiangcheng after receiving a call from Zuo Yiqiu was mainly because he was more interested in the large-size high-purity single-layer graphene preparation technology of Pang Xuelin and his team.

He has only returned to China for more than a year. He is currently leading a team at the University of Science and Technology of China to conduct research on graphene superconductivity. There is not a small gap in the graphene of the Nanomaterials Research Center.

He also expected to get some high-purity single-layer graphene from Pang Xuelin, but he didn't expect that Pang Xuelin had the idea of ​​poaching corners.

The temptation to come to work in Qiantang Laboratory is not insignificant.

Especially after Kirton Walker and Pang Xuelin cooperated to get the lithium-air battery, those who want to cooperate with Pang Xuelin in the academic circle are like crucian carp in the river.

If Pang Xuelin offered Cao Yuan an olive branch before returning to China, he might have agreed on the spot.

It’s just that he has just returned to USTC for more than a year now. The benefits and scientific research conditions given to him by USTC are very good, and he was trained by USTC, so he has a lot of affection for his alma mater. Pang Xuelin suddenly threw an olive branch at him. Some hesitation.

Pang Xuelin saw Cao Yuan's state of mind at a glance, and said with a smile: "Professor Cao, I don't mean to let you resign from the University of Science and Technology of China, but that our Qiantang Laboratory and the University of Science and Technology of China will cooperate together to help you build a laboratory. Of course, you also You can be regarded as a member of our Qiantang Laboratory. My only request is that I hope you can help me find possible superconductors in the field of carbon nanomaterials according to the relevant theoretical analysis I gave."

"ah?"

Cao Yuan was slightly taken aback, then nodded quickly and said, "Of course it's no problem! I'll ask the school leader to mention it as soon as I get back. I think our school should be very interested in cooperating with Qiantang Lab."

"That's good!"

Pang Xuelin laughed.

According to his knowledge, Cao Yuan joined Pablo Jarillo-Herrero's team at the Massachusetts Institute of Technology in 2014, and the team had already started to stack and rotate carbon sheets to different angles.

Cao Yuan's main work is to investigate what happens in stacked bilayer graphene if one layer is rotated by an extremely small angle relative to the other.

According to one theory, this twist dramatically changes graphene's behavior, but many physicists are skeptical.

Yuan Cao set out to create this twisted bilayer of graphene at subtle angles, and discovered something bizarre.

Applying a weak electric field to graphene and cooling it to 1.7 degrees above absolute zero turns graphene, which conducts electricity, into an insulator.

Then with just a slight tweak of the electric field, the twisted bilayer graphene becomes a superconductor, allowing electrons to flow with zero resistance.

From what Pang Xuelin learned, in this experiment, Cao Yuan used an original method to tear the single-layer graphene to form a double-layer graphene with the same orientation, and then fine-tuned and calibrated on this basis.

In addition, he adjusted the low-temperature system to achieve a temperature that would make superconductivity more pronounced.

In fact, in the field of condensed matter physics at that time, some teams also noticed that the saddle point of the double-layer graphene system would drop to the vicinity of the Fermi surface when the rotation angle was about 1.2, and the results of tight binding were obviously inconsistent with the experimental results, which also means that the system has strong associated.

But no one thought of cooling the system and transporting it.

It can be seen that Cao Yuan's job was not due to luck, but due to his strength. They did the experiments very carefully and were very clear about their expectations.

Moreover, Cao Yuan's strong hands-on ability has also become the key to this achievement.

This is also the main reason why he was able to become the first author of those two papers.

Such an experimental physicist with excellent talent and strength is exactly the partner Pang Xuelin needs.

Next, Pang Xuelin and Cao Yuan talked about superconducting issues, and reached an agreement on helping him expand the laboratory equipment and related teams. That night, Pang Xuelin also asked Zuo Yiqiu to book a hotel for Cao Yuan. We had a good time talking.

As for the follow-up contact with USTC and the specific content of cooperation, Pang Xuelin left it to his team to handle.

After attending the inauguration ceremony of the first Jinlong battery factory and the unveiling of the Pang Xuelin Mathematics Research Center, Pang Xuelin spent another five days interviewing more than 100 scholars who hoped to join Qiantang Laboratory and Jiangcheng Advanced Research Institute, and then began to enter the closed state .