In order to elucidate the mechanism of superconductors, physicists have proposed a variety of theories, including the London equation proposed in 1935 to describe the relationship between superconducting currents and weak magnetic fields, the Pippard theory proposed in 1950 and 1953 to refine the London equations, and the GL theory proposed in 1950 to describe the relationship between superconducting currents and strong magnetic fields (near-critical magnetic field strengths); In 1957, the BCS theory was proposed to explain the first type of superconductors from a microscopic mechanism...... Until now, scientists have begun to propose a new mechanism for superconductivity through quantum phase transitions: that is, the topological defects of quantum spin Hall insulators are condensed to form superconductors.

Among them, the more important ones are GL theory and BCS theory.

GL theory is a phenomenological theory based on Landau's second-order phase transition theory.

The theories were proposed by Kingzburg and Landau.

GL theory is based on the following considerations: when the external magnetic field strength is close to the near magnetic field strength of the superconductor, the current of the superconductor does not obey the linear law, and the zero vibration energy of the superconductor is not negligible.

The greatest contribution of GL theory is the prediction of the existence of a second type of superconductor.

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

When the surface energy of a superconductor is >1/√2, it is a first-class superconductor. When the surface energy of a superconductor is < 1/√2, it is a type II superconductor.

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

The theories were proposed by Bardeen (***ardeen), Cooper (. Cooper), Schriever (. Schrieffer）。

The BCS theory suggests that electrons with opposite spins and momentum in metals can pair up to form Cooper pairs, which can move losslessly in the crystal lattice to form superconducting currents.

To put it simply, we can compare the electron to a small bee with only one wing, such a small bee cannot fly, but two such small bees are combined, and the wings are agitated on the left and right, and they can fly.

The BCS theory explains the cause of Cooper's pairs as follows: when electrons move in the lattice, they attract positive charges on neighboring lattices, resulting in local distortion of the lattice points and the formation of a localized region of high positive charge. This localized region of high positive charge attracts electrons with opposite spins and pairs with the original electrons with a certain binding energy. At very low temperatures, this binding energy may be higher than the energy of the lattice atoms vibrating, 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 is a good microscopic explanation for the existence of the first type of superconductors, for which Bardeen, Cooper, and Schriever, the proposers of the theory, were awarded the 1972 Nobel Prize in Physics.

However, the BCS theory cannot explain the reason for the existence of the second type of superconductors, especially the McMillan limit temperature (the critical transition temperature of a superconductor cannot be higher than 40 K) derived from the BCS theory, which has long been broken by the second type of superconductors.

Until now, the physics community has not developed a universally recognized superconductivity formation mechanism.

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

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

Since ceramic-based metal oxides are usually insulating substances, this discovery was so significant that Müller and Bernoz were awarded the 1987 Nobel Prize in Physics.

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

Driven by scientists from China and the United States, the record has been broken in five years.

In 1994, it set a new critical temperature record of 135K at atmospheric pressure and 164K at high pressure.

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

Moreover, its physical properties are so complex that they are difficult to explain by existing theoretical frameworks.

In 2008, Japanese scientists discovered the existence of 26K superconductivity in the iron arsenide system, and with the efforts of Chinese scientists, the critical temperature of such superconducting materials soon exceeded 40K, and even achieved superconductivity of 55K in bulk materials.

As a result, a new generation of superconductivity family, iron-based superconductivity, was discovered.

However, most of these superconductors contain arsenic or alkali metals, which are sensitive to air, and there are also many limitations in their applications.

As for the existence of room-temperature superconductors, it is generally believed in the academic circles at present, and Japanese scientists have even taken the search for superconductors above 400K as their long-term goal.

However, it is not easy to confirm the existence of a room-temperature superconductor 100% accurately.

After all, in order to judge whether a new material is a superconductor, it must have both zero resistance effect and complete diamagnetism, and the resistance does not drop to zero or the diamagnetism is very poor, and it cannot be 100% concluded that it is superconductive.

Historically, there have been a number of "superconductors" that have been jokingly called suspicious superconductors by scientists because there is no definite evidence, referred to as USO, and the legendary UFO.

Some of these USOs claim to be superconductive at 200K or even 400K, but have never been proven by more experiments.

There are even some people who simply engage in academic fraud for personal gain.

For example, a German named Jane Hendrick Sean once flooded in 2001, claiming to have discovered high-temperature superconductivity of more than 52K in materials such as C60 and a series of other electronic device applications, and his paper output efficiency reached the rate of one paper every eight days.

Eventually, physicists discovered that almost all of his papers were falsified.

In 2002, seven articles were retracted in the Science series of journals, eight articles were retracted in the Nature series in 2003, and dozens of articles were also retracted in other academic journals.

Later, his alma mater couldn't stand it anymore and revoked his doctorate, a scandal that caused a sensation in the entire academic community, and Sean was also called a big liar in the physics world once in 50 years.

Despite this, the academic community has continued to be enthusiastic about room-temperature superconductors.

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

One of the more important ones is the German scientist in 2015. Drozdov discovered that hydrogen sulfide has superconductivity of 203K at 2 million atmospheres, but such harsh conditions can only be achieved in the laboratory.

In 2019, . Drozdov's team also confirmed that at a pressure of 1 million Earth atmospheres, various hydrogen-rich lanthanide metal hydrides become superconductors at about 250 K, or minus 20 degrees Celsius.

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

Another important finding is related to the Chinese.

In 2018, the team of Jarillo Herrero at the Massachusetts Institute of Technology found in experiments that the superconductivity of bilayer graphene appeared at a torsion angle of 1.1 degrees and a temperature of 1.7 K.

The paper's first author is Cao Yuan, a 1996-born Chinese genius from MIT Ph.D. student, who topped Nature's 2018 list of the top 10 influential scientists in the world.

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

This discovery is important because it presents a completely new physical phenomenon that is completely different from other superconducting materials, which is of great significance for the explanation of the principle of superconductivity 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 came to the conclusion that the symmetry 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 superconductors in the academic community.

As for the theoretical explanation, then the eight immortals crossed the sea to show their own magical powers.

……

Sorting out the current research status of superconductor materials is only the first step for Pang Xuelin, and 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 quantum computer technology derived from the Carbon-Based Life Alliance; In the world of the Dark Forest, he obtained a complete set of information on electromagnetic orbital propulsion technology and aerospace aircraft technology.

Previously, Pang Xuelin had not had time to conduct careful research, so he needed to retreat for a period of time and digest and absorb all these technologies before he could really start deploying the research and development of electromagnetic orbital propulsion technology and aerospace aircraft technology.

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

On the morning of this day, as soon as Pang Xuelin arrived at the office, he said to Zuo Yiqiu: "Xiao Zuo, help me look at the itinerary for the coming week." ”

"Professor Pang, on October 12th, that is, tomorrow morning, you will attend the inauguration ceremony of the first Golden Dragon battery factory of Jinlong Group, and the official unveiling of the Tianjiang Dapang Xuelin Mathematics Center, you also have to participate, October 15th to 20th, is the centralized interview time of Qiantang Laboratory and Jiangcheng Institute for Advanced Study, when there will be more than 100 scholars from all over the world to meet with you......"

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

"December 10th."

Pang Xuelin said: "Help me vacate the period from October 21 to December 8, during which I will retreat and don't let anyone disturb me." ”

"Retreat?"

Zuo Yiqiu was slightly stunned, a little puzzled.

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

"Oh."

Zuo Yiqiu's face showed a sudden look, 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 to meet you in the afternoon. ”

"Cao Yuan is here in the afternoon?"

Pang Xuelin's face showed a look of surprise.

He looked for Cao Yuan, naturally for the research of superconductivity.

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

Even to a certain extent, compared to Pang Xuelin before the genetic optimizer, Cao Yuan is a genius in the true sense.

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

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

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

Three years later, 14-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 from his bachelor's degree and received an offer from the Massachusetts Institute of Technology.

After entering the Massachusetts Institute of Technology, although Cao Yuan missed out on the physics department he wanted to enter, he accidentally entered the Department of Electrical Engineering of the Massachusetts Institute of Technology, studied for a doctorate with his supervisor Herrero, and then conducted research on the related properties of bilayer graphene under HERRERO.

At the age of 22, Cao Yuan published two articles on graphene superconductivity in the journal Nature as the first author, which attracted wide attention from the academic community.

In the same year, Cao Yuan topped Nature's 2018 list of the top 10 scientific figures influencing the world.

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

Therefore, he asked someone to ask Cao Yuan about the situation, and only then did he know that Cao Yuan was already an associate professor at the University of Science and Technology of China, and he independently led 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 didn't expect that Cao Yuan would take the initiative to rush over.

At half past two in the afternoon, Pang Xuelin saw this talented boy who was only two years older than himself in the office.

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

The two are of the same age, and they both belong to the new force in the academic world, so they quickly became acquainted.

After greeting for a while, Pang Xuelin smiled and said, "Professor Cao, the reason why I invited you here this time is to ask you, are you interested in joining the Qiantang Laboratory?" ”

"Join the Qiantang Laboratory?"

Cao Yuan was slightly stunned, and his face couldn't help but show a look of embarrassment.

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

He has only returned to China for more than a year, and is currently leading a team to engage in graphene superconductivity research at the University of Science and Technology of China, but the preparation of graphene at the University of Science and Technology of China uses CVD epitaxial growth equipment, whether it is purity or defects, there is a big gap with the graphene of the Carbon Nanomaterials Research Center of Jiangda University.

He also counted on getting some high-purity single-layer graphene from Pang Xuelin to go back, but he didn't expect Pang Xuelin to start the idea of poaching.

The temptation to work in the Qiantang laboratory is not small.

Especially after Corton Walker and Pang Xuelin cooperated to get a lithium-air battery, the people in academia who want to cooperate with Pang Xuelin are like carp crossing the river.

If Cao Yuan had sent him an olive branch before returning to China, maybe he would have agreed on the spot.

It's just that now he has just returned to the University of Science and Technology of China for more than a year, and the welfare and scientific research conditions given to him by the University of Science and Technology of China are very good, and he is trained by the University of Science and Technology of China, and he has a lot of feelings for his alma mater.

Pang Xuelin saw Cao Yuan's mentality at a glance, and said with a smile: "Professor Cao, I don't mean to let you resign from USTC, but our Qiantang Laboratory and USTC will work together to help you build a laboratory, of course, you are also 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 have given. ”

"Huh?"

Cao Yuan was slightly stunned, and hurriedly nodded: "Of course it's no problem!" I mentioned it to the school leaders as soon as I went back, and I think our school should be very interested in cooperating with the Qiantang Laboratory. ”

"That's good!"

Pang Xuelin laughed.

As far as he knows, Cao Yuan joined Pablo Jarillo-Herrero's team at MIT in 2014, which was already experimenting with stacking and rotating layers of carbon sheets at different angles.

Cao Yuan's main work is to investigate what happens when one layer of stacked bilayer graphene is rotated at a very small angle relative to the other.

One theory predicts that this distortion would dramatically alter the behavior of graphene, but many physicists are skeptical.

Cao Yuan was determined to create this double-layer graphene distorted at subtle angles, and discovered some bizarre phenomena.

When a weak electric field is applied to graphene and cooled to 1.7 degrees above absolute zero, the graphene, which conducts electricity, becomes an insulator.

Then, with a slight adjustment of the electric field, the twisted double-layer graphene becomes a superconductor that allows electrons to flow with zero resistance.

Judging from what Pang Xuelin learned, Cao Yuan used an original method to tear apart the single-layer graphene to form a double-layer graphene in the same direction, and fine-tuned and calibrated it on this basis.

In addition, he adjusted the cryogenic system to achieve a temperature that would make the superconducting state more significant.

In fact, at that time, some teams in the condensed matter physics community also noticed that the saddle point of the double-layer graphene system would drop to near the Fermi surface at an angle of about 1.2, and the results of tight binding were obviously inconsistent with the experimental results, which also meant that there was a strong correlation between the system.

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

It can be seen that Cao Yuan's job is not due to luck, but strength, they are very meticulous in their experiments, and they are very clear about their expectations.

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

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

Such an experimental physicist with excellent talent and strength is exactly the object that Pang Xuelin needs to work with.

Next, Pang Xuelin and Cao Yuan chatted about superconductivity again, and reached an agreement on helping him expand laboratory equipment and related teams.

As for some follow-up contacts with USTC and specific cooperation contents, Pang Xuelin handed it over to his team to deal with.

After attending the inauguration ceremony of the first Golden Dragon Battery Factory and the unveiling of the Pang Xuelin Mathematical Research Center, Pang Xuelin spent another five days interviewing more than 100 scholars who wanted to join the Qiantang Laboratory and the Jiangcheng Institute for Advanced Study, and then began to enter a state of retreat.

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