Cao Yuan, a genius teenager, reposted Nature after three months after sending Science and 95

Just now, the post-95 graphene god Cao Yuan published the latest research results on magic angle graphene in "Nature", ushering in the eighth chapter of his life, "Nature"! And this is only about three months since he last published "Science"
.

Since the discovery of magic angle graphene in 2018, this NS (Nature/Science) madman has published 8 Nature and 1 Science! Among them, only 3 Nature + 1 Science have been published in the first half of 2021 .

The longest publication interval is less than one month, and the shortest is only about one week.

Cao Yuan's eighth chapter "Nature": Discover the rare superconductivity that can exist in a strong magnetic field

On July 21, 2021, Cao Yuan once again published a paper entitled "Pauli-limit violation and re-entrant superconductivity in moiré graphene" in "Nature" as the first author + corresponding author
.

In this study, Cao Yuan and others observed a rare superconductivity in magic-angle twisted trilayer graphene (MATTG) material
.

Studies have found that when θ is equal to a "magic" angle of about 1.
6°, MATTG has zero resistance at low temperatures (as low as 1 Kelvin) and becomes a superconductor

Figure 1.

Magic-angle twisted three-layer graphene, a rare phenomenon of spin triplet superconductivity

Cao Yuan said that the magic-angle three-layer graphene discovered in the study is a very rare superconductor called a "spin triplet state" and is not affected by a strong magnetic field
.

This bizarre superconductor can greatly improve magnetic resonance imaging (MRI) technology

In addition, new evidence of the superconductivity of the spin triplet state in three-layer graphene can also help scientists design more powerful superconductors for practical quantum computing
.

Spin singlet superconductivity VS.
Spin triplet superconductivity

　　The so-called superconductor refers to the ability to conduct electricity efficiently without loss of energy, and transmit current with zero resistance
.

Microscopically speaking, when exposed to an electric current, the electrons in the superconductor will couple in the form of "Cooper pairs" and then move collectively and cooperatively, passing through the material without resistance, so that no energy is lost

　　In most superconductors, these "Cooper pairs" have opposite spins (intrinsic angular momentum).
One electron rotates upward and the other rotates downward.
The total spin is zero.
This configuration is called "spin".
Singlet"
.

These electron pairs are happily accelerated through the superconductor, unless under a high magnetic field, it can move the energy of each electron in the opposite direction, thereby pulling the electron pairs apart

Figure 2.

The spin singlet state of a conventional superconductor and the spin triplet state of magic-angle twisted triplet graphene

　　However, there are also some exotic superconductors that are not affected by magnetic fields, and they are very strong
.

These materials are superconducted by electron pairs with the same spin.

　　Most experimentally known superconductors have spin-single Cooper pairs; these include metals that exhibit traditional superconductivity (such as lead and niobium), and cuprates (layers) that exhibit unconventional superconductivity.
Copper oxide compound)
.

However, there are few reports on unconventional superconductors with spin-triplet states
.

　　incredible! MATTG superconductor survived a strong magnetic field of 10 T

　　At first, the researchers observed superconductivity in the magic angle twisted double-layer graphene, and then quickly followed up the magic angle three-layer graphene test, and found that it is stronger than the double-layer graphene, at a higher Maintain superconductivity at temperature
.
When a moderate magnetic field is applied, the researchers noticed that the three-layer graphene can superconduct with a field strength that destroys the superconductivity of the double-layer graphene
.

　　"We think it's weird," said Jarillo-Herrero, Cao Yuan's mentor
.
Later, Cao Yuan and his mentor Jarillo-Herrero became curious about whether the magic angle three-layer graphene might contain such unusual signs of spin triplet superconductivity
.
Therefore, the researchers stacked three layers of graphene together and rotated the middle layer by 1.
56 degrees relative to the outer layer to make MATTG; then, a large magnet with a magnetic field direction parallel to the material was used to control the magnetic field.
The superconductivity of MATTG was tested under a magnetic field
.

Figure 3.
Superconductivity in MATTG under high planar magnetic field
.

　　Studies have found that when the surrounding magnetic field is continuously increased, the superconductivity of MATTG has been strong until it disappears
.
But what is extremely surprising is that the superconductivity of MATTG actually reappeared under higher field strength and made a comeback! This has never been observed in traditional spin singlet superconductors
.

　　At the same time, Cao Yuan and others also observed that after the "comeback", superconductivity still persists under magnetic field strengths of up to 10 Tesla! You know, this is the maximum field strength that a laboratory magnet can generate
.
According to the Pauli limit (this is a theory that predicts the maximum magnetic field that a material can maintain superconductivity): If an ordinary superconductor is a traditional spin singlet, then the MATTG can withstand approximately three times that of an ordinary superconductor
.

Figure 4.
MATTG superconductivity surpasses Pauli limit

　　Superconductivity makes a comeback after its disappearance? Why?

　　"In a spin singlet superconductor, if you "kill" the superconductor with a magnetic field, it will never come back and disappear forever," said Cao Yuan
.
"And here, it appeared again
.
So this shows that this material is definitely not a spin singlet
.
"

　　The re-emergence of superconductivity in the magic-angle twisted three-layer graphene, coupled with its persistence in a higher than expected magnetic field, ruled out the possibility that this material is an ordinary superconductor
.
This means that it may be a very rare type, that is, the spin triplet, which carries Cooper pairs that pass through the material quickly, and is not affected by a strong magnetic field
.

Figure 5.
Field induced transition between superconducting phases in MATTG
.

　　It is reported that the Cao Yuan team is currently planning to study the material in depth to confirm its exact spin state, hoping to provide more information and scientific basis for the development of the next generation of more powerful MRI machines and the design of more powerful quantum computers
.

　　"Regular quantum computing is very fragile," Jarillo-Herrero said
.
"You look at it, puff, it may disappear
.
About 20 years ago, theorists proposed a kind of topological superconductivity.
If realized in any material, it can [enable] the very robust quantum state responsible for calculations.
Computers, thereby providing unlimited power for computing
.
The key factor in achieving this goal is a certain type of spin triplet superconductor
.
We don’t know whether our type belongs to that type
.
But even if it’s not the case, it can be three-layered Graphene is put together with other materials to design this superconductivity
.
This may be a major breakthrough
.
"

　　references:

　　1.
Cao, Y.
, Park, JM, Watanabe, K.
, Taniguchi, T.
& Jarillo-Herrero, P.
Nature 595, 526–531 (2021).

　　2.
Nature 595, 495-496 (2021).
doi: https://doi.
org/10.
1038/d41586-021-01890-3.