Graphene has many surprising properties that can be applied in many ways

In 1911, the Dutch physicist Heike Kamerlingh Onnes was surprised to find that when mercury is cooled to near absolute zero (273.15 degrees Celsius), electrons can travel without "blocking." He called this "zero resistance state" "superconductivity."

Since then, physicists have constantly sought to find high-temperature superconducting materials for use in everyday life. However, most materials will only transform to superconductors when they are close to absolute zero. Even the so-called "high-temperature" superconductors are only in relative terms: The current maximum temperature of zero-resistance conduction is about -140oC. If any material can exhibit superconductivity at room temperature, it can revolutionize energy transfer, medical scanners, and transportation.

On March 5, physicists pointed out in two papers published in the journal Nature that when two layers of graphene were twisted together with a “magic angle”, they could conduct electricity under zero resistance. More precisely, physicists stack two layers of graphene, which are only atomically thick, at a special angle when the angle between the carbon atoms is arranged at 1.1 degrees (this angle is called “magic angle”). Will make the material into a superconductor. Although the system still needs to be cooled to 1.7 degrees above absolute zero, the result shows that it may be as conductive as known high-temperature superconductors. Once this result is confirmed, this discovery is crucial for understanding high-temperature superconductivity.

One of the papers. The first authors of both papers were graduate students Yuan Cao (Cao Yuan) who was studying at MIT.

Superconductors can be broadly classified into two types: conventional superconductors that can be explained by mainstream superconductivity theory, and unconventional superconductors that cannot be explained by mainstream theory. The latest research results show that the superconducting behavior of graphene is unconventional and exhibits some similar properties to another unconventional superconductor called copper oxide. This complex copper oxide can conduct electricity above 133 degrees of absolute zero. For more than three decades, although copper oxides have been the focus of physicists in the search for room-temperature superconductors, the mechanism behind them still confuses them.

Stacked graphene systems are relatively simple compared to copper oxides, and physicists have a better understanding of it.

magic

Graphene is a hexagonal flat honeycomb film composed of carbon atoms. It is a two-dimensional material with a thickness of one carbon atom. Since the discovery of graphene, its many outstanding attributes have been impressive: it is stronger than iron, and it is much more conductive than copper. Previously, scientists discovered the superconductivity of graphene, but that occurs only when it comes in contact with other materials, and its superconducting behavior can be explained by conventional superconductivity.

Graphene has many surprising properties that can be applied in many ways.

The physicist Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology did not attempt to study superconductivity at the beginning of the experiment. What they want to explore is how directionality called magic angle affects graphene. According to the theorist's predictions, if atoms in different layers of a two-dimensional material are shifted at a particular angle, they may induce electrons to pass through the lamella and act in an interesting way—but they do not know what exactly would be the way.

In the experimental setup of the double sheets, they immediately saw unexpected behavior. First, in the measurement of the conductivity of graphene and the density of its charged particles, this structure has become a Mott insulator, a material that possesses all the components necessary for the electrical conductivity to occur, but its particles The interaction will prevent the free movement of the electrons and make this impossible. Next, just apply a slight electric field to it to add a little extra charge carriers to the system and it will become a superconductor.

The presence of an insulating state so close to superconductivity is a hallmark of an unconventional superconductor. When the researchers plotted the phase diagram (the electron density on the vertical axis and the temperature on the horizontal axis), they saw a pattern very similar to copper oxide. This further proves that the material may have superconductivity mechanism.

Rotation effects in twisting double-layered graphene: a. When the double-layered graphene is twisted, the upper sheet is rotated so that it cannot align with the lower sheet, allowing the unit cell to expand (red). b. For small-angle rotations, the so-called “moiré pattern” occurs, in which the arrangement of the partial stacks changes periodically.

Finally, although graphene exhibits superconductivity at an extremely low temperature, it only requires one-tenth of the electron density of a conventional superconductor to obtain superconductivity at the same temperature. In conventional superconductors, this phenomenon only occurs when the vibrations allow the electrons to form a pair, and the paired electrons stabilize their path of travel, allowing them to flow under zero resistance. But because there are so few electrons available in graphene, the fact that they can be paired indicates that the interaction in the system is much stronger than what happens in conventional superconductors.

Looking for light in the dark

Physicists have their own opinions about how electrons interact in unconventional superconductors. Robinson said: "One of the bottlenecks in high-temperature superconductors is that, until now, we do not know exactly what is the right pair of electronic bonding."

Bascones said that graphene-based systems are easier to study than copper oxides, so they will be more conducive to the exploration of superconductivity. For example, in order to explore the source of superconductivity in copper oxides, physicists often need to expose the material to an extremely strong magnetic field. In order to explore the different behaviors of copper oxides, the “adjustment” of them means that the research volume of different samples is increasing; and for graphene, physicists only need to simply adjust the electric field to achieve The same effect.

The physicist Kamran Behnia said that although he acknowledged that the MIT team's findings showed that graphene is a superconductor and is likely to be an unusual superconductor, he does not believe that they can safely claim to see the Mott-insulated state. .

Physicists cannot say for sure now that the superconducting mechanism in these two materials is the same. Robert Laughlin, the Nobel laureate, said that it is not yet clear whether all the behavior seen in copper oxide can occur in graphene. However, we have reason to find a reason for celebration in these new experiments that show enough of superconductivity.

To better understand copper oxides, physicists have been groping in the dark for 30 years. The latest discovery may have just lighted a beam of light for the physicists.

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