Chapter 318 Landau-Fermi Liquid Theory

Critical current density is an important parameter for judging the quality of superconducting materials.

Superconductors have three critical conditions, namely critical temperature, critical current density, and critical magnetic field intensity. Only within these three critical conditions can they maintain the superconducting state.

Among them, the critical current density can be said to be the most important data, because it determines the amount of current it can carry.

For example, the critical current density of the most commonly used niobium-titanium alloy is as high as 10^9A/m, which is at least twice that of YBCO. Coupled with its good plasticity and low cost, it is often used in practical applications. , even if yttrium barium copper oxide only needs liquid nitrogen to cool it, because the cost gap makes up for the cost of liquid nitrogen and liquid helium, yttrium barium copper oxide cannot replace the status of niobium titanium alloy.

However, Lin Xiao's experiment now shows that the critical current of this new type of niobium-titanium alloy has increased by tenfold compared with the previous old niobium-titanium alloy.

This kind of performance is absolutely incredible. Who would have thought that they have the same chemical formula, the same molar mass, and the same density. They can be said to be the same material, but in the process of entering the superconducting state, something like this happened!

Chen Xun was very surprised and asked: "Director Lin, why is this? Are the structures after equivalent topological transformation based on the electron topological bonding theory not completely equivalent? Or can it specifically enhance the strength of this material? Critical current density?”

Lin Xiao shook his head: "This is not necessarily true. One thing that can be revealed in the electronic topological bonding theory is that for a compound, there can be many different equivalent topological forms."

"So, for different structures, the current density may increase or decrease."

"After all, superconductivity is the free movement of electrons without resistance inside the material, and the internal structure has a great influence on whether electrons can move freely."

After listening to Lin Xiao's explanation, Chen Xun nodded, but soon had another doubt: "Then how do you know that your YBCO structure has reduced resistance to the free movement of electrons? Electronic topological bonding theory, It shouldn’t be calculated, right?”

If this could be calculated, everyone who studies superconductivity would probably have to call Lin Xiao "Dad".

Because once this can be calculated, Lin Xiao will provide a huge treasure to the superconducting community around the world.

At least hundreds of papers will be published by then.

But obviously, as long as Chen Xun has been studying it, he does not think that the electronic topological bonding theory can calculate such a thing.

After hearing Chen Xun's question, Lin Xiao was silent for a moment, and then said: "Feeling."

"Feeling?" Chen Xun was stunned.

"Well, I feel that this equivalent topological structure will have very little resistance to electrons."

Chen Xun: "...Can you feel this?"

I, a physics professor at Peking University, have little education, so don’t lie to me.

The critical current density of a superconductor, how can you feel whether it has become larger or smaller?

This is simply outrageous!

Lin Xiao shrugged and said: "You can construct a microstructure of yttrium barium copper oxide in your mind, simulate its electrons, and then deduce the effect of electron flow after pressurization, and you can feel it."

Chen Xun: "..."

He tried it silently in his mind, but with more than ten molecules in a yttrium barium copper oxygen unit cell to form a conductor, how about getting a few more unit cells?

As a result, the amount of calculations in the brain increases.

So in the end, Chen Xun chose to give up and complained: "My head is a human brain, not a computer."

Lin Xiao couldn't help shaking his head and laughing. It was obvious that Chen Xun was complaining that his head was like a computer.

But this may really be regarded as a fact. At least, Lin Xiao now feels that his brain is like a computer, and it is also a multi-core and multi-threaded one, because he can already do five things at once.

He even felt that if he trained more, he might be able to do all things at once. Of course, the problem he was thinking about should not be too complicated, but things like building a molecular structure model and simulating the flow of electrons directly in his mind , it is still completely possible.

However, he did not dwell on this issue, and then said: "Okay, actually this is not the key issue."

"Key question?" Chen Xun looked confused.

"Tell me, what determines the critical current density?"

"Maximum carrier throughput."

"Well, BCS theory tells us that the carriers in low-temperature superconductors are Cooper pairs, but what about high-temperature superconductors?"

Chen Xun shook his head, "I can't say for sure."

After all, this is also a point of debate in academic circles.

Lin Xiao smiled mysteriously and did not explain, but said: "I have applied for a patent for this new type of yttrium barium copper oxide first. During this time, I will write a thesis."

Although there were still some doubts, after Lin Xiao said so, Chen Xun nodded and said: "I will go to them to apply."

This kind of superconducting material with an improved critical current density definitely needs to apply for a patent.

Chen Xun took a look at the current that Lin Xiao had tested so far. It had even reached 50,000 amps. This current has reached the level of niobium-titanium alloy.

As long as this is patented, it will definitely make some money in the future. Although it cannot be made into a flexible wire, it can still be used.

As for the paper Lin Xiao was going to write, it was probably about the performance of this new structure, yttrium barium copper oxygen, so Chen Xun didn't ask any more questions and left the laboratory.

Lin Xiao continued to look at the yttrium-barium-copper-oxygen alloy in front of him, thinking in his mind.

He calculated it at the Institute of Physics more than a month ago. The reason why the critical current density of this transformed yttrium barium copper oxide can be increased is because more electron pairs suspected of Cooper pairs appear.

However, he was still not sure whether these electron pairs were real Cooper pairs.

This caused him to start diverging his thinking.

"First of all, why is it possible to superconduct under high pressure?"

On October 15, 2020, several scientists in the United States used extremely sophisticated experimental equipment to achieve superconductivity at 15 degrees Celsius at 267 Gpa.

The superconducting material they used was carbon-containing hydrogen sulfide.

But hydrogen sulfide is a gas at room temperature. How can a gas be tested for superconductivity?

This is because the pressure of 267Gpa is equivalent to exerting a gravity of 26.7 tons on one square millimeter, and then directly compresses the gas into a solid.

There is obviously no difference between turning into a solid and solidifying at low temperature, so there is essentially no difference between high-voltage superconductivity and low-temperature superconductivity.

So Lin Xiao quickly dismissed this idea in his mind.

But then, his eyes suddenly flashed.

"That's right! Landau-Fermi liquid theory!"

"The current academic community is still at a scratch stage in understanding strong correlation physics beyond the framework of Landau-Fermi liquid theory."

"The mechanism of high-temperature superconductivity must be the strong correlation between fermions such as electrons."

"According to the Landau theoretical framework, as long as the perturbation theory of the interacting fermion system converges, its behavior will be qualitatively consistent with that of the non-interacting system..."

"If you substitute the situation of yttrium barium copper oxide..."

In Lin Xiao's brain, countless inspirations emerged like an explosion.

But all these inspirations finally converged on one question.

“Perturbative divergence is not accompanied by spontaneous breaking of symmetry, but forms a highly correlated quantum liquid state—a non-Fermi liquid state.”

"So, what is a non-Fermi liquid?"

Lin Xiao frowned and recalled all the documents and theories he had read.

"The current academic community only clarifies what a Fermi liquid is, but as for what a non-Fermi liquid is, it only explains that any fermion model that is not a Fermi liquid is a non-Fermi liquid. What is it specifically? Still in the clouds and fog..."

Lin Xiao suddenly had a feeling that the mechanism of high-temperature superconductivity was hidden between the similarities and differences between this non-Fermi liquid and the Fermi liquid.

He scratched his head, and that feeling came again.

The more you search for why, the more you get into deeper problems, which often means more and more trouble.

But he had to continue.

He sighed helplessly, and suddenly he asked in his mind: "System, how many truth points are needed to exchange for that room temperature superconductor now?"

System: "We still need 485 truth points."

It is equivalent to saying that after more than a month of hard work, I have only completed 15 truth points.

"Okay, at least the progress is 3%, keep working hard."

As long as he spends another 33.33 multiplied by more than a month, he will have spent 500 truth points in vain.

Well, if you think about it this way, you really haven't lost anything.

After comforting himself, he returned to his office and decided to finish the paper first. As for the Landau-Fermi liquid theory, he should wait until he had published the paper.

…

Just like that, time passed quietly again.

The days have come to the end of December, and the academic world has entered the end of the year. Nothing has changed at all. At most, the vast majority of Westerners have temporarily put down their work at hand and started to celebrate Christmas.

However, two top international journals, "Nature" and "Physical Review Letters", successively published their last articles of the year, and they each had two articles, which caused shock in the superconducting world.

"In the superconducting process of yttrium barium copper oxide, is it still because an electron pair suspected to be a Cooper pair plays a role?"

This theory that shocked superconducting physicists appeared in front of them without any precautions or worries.

Then, these superconducting physicists stopped thinking about taking a break at the end of the year, rushed into the laboratory, and started their experiments.