It is well-known that most materials consume a large amount of energy during the conduction process, while the superconductor has almost no energy loss during transmission and can carry a higher current per square centimeter However, most superconductors are currently only operating at near absolute zero Temperature work.
In 1911, the Dutch physicist Heike Kammerlingh Onnes found that the pure mercury samples lost their resistance at low temperatures of 4.22-4.27 K and then found similar phenomena in some other metals - a phenomenon known as superconductivity. Heike Kammerlingh Onnes won the Nobel Prize in Physics for that year.
Figure 丨 Heike Kammerlingh Onnes
However, physicists have found that the superconducting critical temperatures for large amounts of simple substance and alloy superconductors are low, and that such a low superconducting temperature means that superconducting applications must rely on costly cryogenic liquids such as liquid helium to maintain low temperatures Environment.This led to a sharp increase in the cost of superconducting applications, the cost of maintaining low temperatures even far beyond the value of the material itself.Although even the "high temperature" superconductors only exist at relatively absolute temperature of zero: -140 ° C. That is, Being able to achieve superconducting materials at true room temperature can avoid costly cooling costs and revolutionize the state of the art in energy transfer, medical scanner and transportation.
It is now 107 years since the discovery of superconductivity by Heike Kammerlingh Onnes. People are still exploring ways to achieve superconductivity of materials at low temperatures and use them in life. This goal is also one of the most important missions of the applied physics community. one.
But this goal is getting closer and closer to us. Just on March 5th, "Nature" has published two articles covering important studies from MIT and Harvard University: Just rotate two layers of graphene to a specific ' When the magic angles' are superimposed, they can conduct electrons with zero resistance. This finding is likely to be a very important step in the search for room temperature superconductors for decades.
In addition to publishing related papers, Nature also published an article commenting on this major breakthrough.
It is worth mentioning that, the first author of both papers was 21-year-old MIT doctoral student Cao Yuan.
Figure 丨 Cao Yuan, born in 1996, native of Chengdu, Sichuan Province, was admitted to the 2010 China University Youth Class in 2010, and selected as 'Yan Jici Physics Elite Class', is the 2014 CASU Honorary Award - Guo Moruo scholarship winner. Ph.D. student in the Department of Electrical Engineering and Computer Science at the Polytechnic Institute from Pablo Jarillo-Herrero, Massachusetts Institute of Technology physicist
Pablo Jarillo-Herrero, Associate Professor at the Massachusetts Institute of Technology, condensed matter physicist, received prizes from the Royal Society of Spain Young Investigator Award (2007), the National Science Foundation Award (2008), Alfred Sloan Scholarship (2009), David and Lucy Packard Scholarship (2009) and others
According to the paper, the researchers obtained superconducting properties of the resulting material by superimposing two layers of graphene and shifting the carbon atom pattern by 1.1 °. Although the system still needed to cool to 1.7 degrees above absolute zero, The results show that it may be as conductive as known high-temperature superconductors, which has thrilled physicists.
According to Elena Bascones, a physicist at the Madrid Institute of Materials Science, 'If this finding is confirmed, it may be important to understand HTS', said Robert Laughlin, a Nobel laureate at Stanford University physicist. 'We can look forward to In the coming months there will be crazy experimental activities to fill the missing part of the blueprint. '
One of the highlights of this study is that it means that graphene superconductivity can be used to learn about the mechanism of unconventional superconductivity such as copper oxide superconductivity. However, when Cao Yuan was interviewed by DT Jun, he said that it was not in the near future. Intended to participate directly in the study of copper oxides.
'As we all know, this field has been studied for nearly 30 years and is still continuing. There are a large number of laboratories around the world studying copper oxides. Our laboratory mainly studies two-dimensional materials, the preparation and characterization of two-dimensional materials. There is quite a lot of technology and experience in the field, and there is no experience in researching traditional materials,” said Cao Yuan to DT.
Tumen's three related articles
Why is graphene?
Generally speaking, there are roughly two types of superconductors: conventional superconductors, that is, their activities can be explained by superconducting mainstream theory; unconventional superconductors, which cannot be explained by mainstream theory.
According to a recent study by the MIT team, the superconductivity of graphene belongs to the latter and is similar to that of other unconventional superconductors, copper oxide superconductors.
Here, we have to mention copper oxide superconductors. Such complex copper oxide materials can conduct electricity at absolute 133 degrees Celsius, but the underlying mechanism of copper oxide superconductors remains a mystery. Laughlin said: The surprising implication is that the superconductivity of copper oxide has always been simple, but it is difficult to be correctly understood and calculated.
However, unconventional superconductivity such as copper oxide superconductivity is the most likely to achieve superconductivity at room temperature, has achieved about minus 140 degrees to achieve superconductivity, but the copper oxide superconducting system is very complex, and experimental conditions require Spend a lot of labor and material resources, it is difficult to conduct an effective next study.
Figure 丨 two layers of graphene to 1.1 ° angle of rotation, the resulting material has superconducting properties
Coincidentally, the graphene superconductivity, which has a twist angle between layers, was discovered, at least for the moment it appears to be consistent with the phenomenon of copper oxide superconductivity, and physicists speculate that The mechanism should also be consistent.
Graphene has always been a magical material with surprising properties: This sheet-like material consisting of a single layer of carbon atoms extending in a hexagonal shape is stronger than steel and is more conductive than copper. It is in comparison with other materials. Materials also show superconductivity when they touch, but this behavior can be explained by conventional superconductivity.
Moreover, graphene is a relatively simple material. Scientists have already studied graphene quite well. Many researches on graphene are currently focusing on how to prepare stable and high quality graphene. Therefore, graphene is used to study irregularities. The phenomenon of superconductivity can effectively accelerate the pace of scientists to achieve room temperature superconductivity.
In response, Elena Bascones, a physicist at the Madrid Institute for Materials Science, said graphene-based devices are easier to study than copper oxides and make graphene a useful platform for exploring superconductivity. For example, to explore copper oxide superconductivity , Physicists often need to place the material in an extreme magnetic field. "Tweaking" them to explore different behaviors means a lot of experimentation is going to be done with a lot of data, and with graphene, physicists can The same result is obtained by simply adjusting the electric field.
Figure 丨 Graphene is a layered two-dimensional carbon material of atomic thickness, when two layers of graphene layered at a specific angle, can be used as a superconducting material
'Superconducting magic'
In carrying out the experiment, Cao Yuan and his mentor Pablo Jarillo-Herrero and his team were not trying to explore superconductivity. Instead, they were to explore how the deflection angle of bilayer graphene affects the performance of graphene.
Theoretically, they can only guess that a certain angular shift between layers of two-dimensional material may induce electrons to pass through the material layer and interact in an interesting way, but they do not know exactly what kind of the way.
However, Cao Yuan’s team soon discovered some unexpected behavior of double-layered graphene.
Graphene Graphene
First, the measurement of the conductivity of the graphene and the density of the particles carrying the charge in it shows that the structure has become a Mott insulator (Mott Insulator) - this material has the property of using all its components to conduct electricity, and between particles Interaction will prevent them from flowing.
Next, they used a small electric field to add a small amount of extra charge carriers to the system to make them superconductors. After getting these results, they immediately funded their team. Cao Yuan’s mentor Jarillo-Herrero said, 'We Using different devices to get these results and measuring with collaborators, this is a very confident point for our team.
Then, how does the superconducting effect of bilayer graphene be achieved? Single-layer graphene has a linear energy dispersion characteristic at its charge neutral point. When two aligned graphenes are stacked, the band is caused by the jump between layers. The hybridization results in a change in the low-band structure according to the stacking order (AA or AB stacking).
A new hexagonal moiré pattern consisting of alternating AA and AB stacking regions appears and acts as a lattice modulation if there is an additional twist angle.The superlattice potential folds the band structure into a mini cloth The hybridization effect between the mini Brillouin zone MBZ and the adjacent Dirac cones in the MBZ has an effect on the Fermi velocity at the charge neutral point from a velocity of 106 m / s The typical value is reduced. Different twist angles will determine the different cell structures, ie, determine the hybridization effect between different Dirac cones.
The special angle at which Fermi's speed drops to zero is the 'magic angles', where the first angle is about 1.1°. Near this twist angle, the energy band is close to neutral, and the entire energy band The typical amount of energy in a bandwidth is about 5-10 meV.
Experiments have shown that the flattening of these energy bands results in a large effective mass. And the insulating state can be understood as the result of a contest between the Coulomb energy and the quantum kinetic energy, which brings about the effect of creating an insulating state at the half-fill, and Shows a similar behavior of insulators as Mott. According to the different torsional angles, the required doping concentration to achieve a similar insulator state is different.
As mentioned above, unconventional superconductors (such as copper oxides) are characterized by the existence of an insulating state that is very close to superconductivity. When researchers plotted phase diagrams to describe the electron density of a material as a function of temperature, they discovered that Similar phase diagram results with copper oxide superconductors. In this regard, Jarillo-Herrero said that this provides further evidence that the superconducting mechanism of double-layer graphene and copper oxide may be the same.
Graphene's graphene electronic structure
Finally, although graphene can currently exhibit superconductivity even at very low temperatures, compared to conventional superconductors, superconductivity at the same temperature requires only one-tenth of the electron density of conventional superconducting materials. one.
In addition, the realization of the superconducting properties of the conventional superconductor materials depends on the stable conduction of the paired electrons, and the amount of electrons available in the graphene is small, if the electrons in the graphene can pair the electrons in some way, this shows that The interaction should be much stronger than that of conventional superconductors.
Conductivity doubts
However, on this study, some physicists also expressed different opinions. Kamran Behnia, a physicist at the Institute of Advanced Physics and Chemistry of the Higher Institute of Industry in Paris, said that he still does not believe that Cao Yuan can accurately announce the observation of Mott. The state of the insulator, although the team's findings have shown that graphene is a superconductor, and is probably an unconventional superconductor.
Moreover, physicists have not yet been able to say with certainty that the superconducting mechanism of copper oxide and double-layered graphene superconductors is exactly the same, so what is the significance of this experiment if the last experiments prove that the mechanisms are different reflect?
For this question, Cao Yuan's answer to us is: 'In the paper we compared the transition temperature and carrier concentration in the superconducting state of a rotating bilayer graphene, and found that the superconductor in a rotating bilayer graphene The pairing intensity is even larger than unconventional superconductors such as copper oxides, heavy fermions, etc., and is closer to the BEC-BCS transition line (similar to a very fiery portion of iron-based superconductors in recent years). Therefore, even its superconducting mechanism and copper Unlike oxides, it is theoretically very interesting to study why such superconducting pairs exist in seemingly simple graphene systems.
Robert Laughlin, a physicist at Stanford University and a Nobel laureate, believes that “it is not yet clear whether all the behaviors in copper oxide superconductors will occur in graphene superconductors. Therefore, new related experiments need to be conducted. Recognized by everyone. Physicists have been paralyzed in the dark for 30 years, trying to unravel the secret of copper oxide superconductors. Many of us believe that the light has just been turned on.
(NW, Huang Shan)
Original title: 21-year-old MIT Chinese scientists issued two "Nature" papers: Room temperature superconductivity is expected to achieve a major breakthrough, graphene unveiled 'magic'