The condensed matter physics research community is in turmoil. In the spring of 2018, a team from MIT in the USA demonstrated that two layers of graphene stacked on top of each other could become superconducting when they are rotated concerning each other at an angle called ” magical. ” In this somewhat special configuration, the electrons pair up to conduct electricity without any resistance.
The novelty: this phenomenon emerges because the superposition of atomic networks generates a “moiré,” when two fabrics are superimposed. Almost two years since this discovery, let’s try to understand in more detail what excites the researchers.
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Graphene, a crystal apart
What do diamond and graphite have in common? Each of these crystals is made up only of carbon atoms. How is it then that the diamond is transparent and does not conduct electricity while the graphite is black and an excellent conductor? To understand it, we must be interested in the microscopic organization of atoms. In diamonds, each carbon atom mobilizes its 4 available electrons to form 4 strong bonds with its neighbors. In graphite, carbon atoms only make three strong bonds with neighbors located in the same plane so that there is one electron per atom available to carry electricity. Each layer of graphite, isolated, is called “graphene.”
Graphene was first isolated 17 years ago from graphite, which earned Kostia Novoselov and Andre Geim the Nobel Prize in 2010. This discovery demonstrated that a two-dimensional crystal could be stable. The free electrons in this crystal are unique: they behave as if they have no mass – just like photons, the particles carrying light. They are also tough to slow down, so graphene is one of the best conductors of electricity known. So it is not the type of atoms that makes the difference between graphene and diamond, but the way they are arranged.
Stack two layers to “stop” the electrons of the graphene.
More recently, researchers have found a way to create new, completely artificial crystals by stacking multiple graphene layers. The technique used is straightforward, at least in theory: recovering the two-dimensional crystal (the graphene layer, therefore) on a polymer film then depositing it on another layer. If the layers are not perfectly aligned, the two layers’ atomic networks’ superposition introduces a new periodicity, which is called a “moiré.”
Who says new periodicity, says crystal of a new type, and new properties. These are determined by the angle of rotation between the layers.
While for large angles between graphene layers, moiré remains just a curiosity, it can strongly slow down unstoppable electrons in graphene when the angle is small. It can even stop them completely when the angle between the layers is close to the so-called “magic” value of 1.1 ° ).
This result may seem incredible: what if we put two highways on top of each other to stop cars that are too fast?
When stopped, the electrons play collectively and are in all their states.
But that’s not the end of the story. Electrons are, in fact, electrically charged particles that follow Coulomb’s law: in 1785, the physicist Charles-Augustin Coulomb explains that charges of the same sign repel each other. Thus, free electrons avoid each other, a bit like a crowd seeking to respect social distancing in the midst of the Covid-19 crisis. If one person moves, the rest of the crowd should adjust their position to maintain a one-meter distance.
This collective behavior is said to be “correlated.” It requires that electrons can move to avoid each other, which is usually possible in metal. But this is not the case in the graphene bilayer rotated by 1.1 °, in which the electrons are forced to interact. To reduce their interactions, they reorganize themselves in new states with particular properties.
Electrons also have a spin, which is an intrinsic property that makes them behave like small magnets. Sometimes it is better to align their spins to minimize their interactions. Thus, the material can become magnetic under the effect of interactions between electrons.
In other situations, electrons may prefer to reorganize and form an insulating state. This type of system is called a Mott insulator, in honor of Nevil Mott, who was the first to understand that unlike usual insulators such as diamond, whose insulating character comes directly from the crystal’s periodicity, this type of material stems rather from the Coulomb repulsion.
A third possibility is a superconducting state, where electric current flows without any resistance.
Depending on the experiment’s conditions, for example, temperature, pressure, and electron density, one of these different states of matter can emerge. These three electronic states have been observed in rotated graphene bilayers: superconductivity, so-called “Mott” insulators, and magnetism. It is this versatility that creates great excitement for the entire scientific community. This system constitutes a new door to answer any questions which remain on this physics of correlated electrons.
And why not other materials than graphene?
Many teams are delving into these questions while diversifying the research subject, which will probably reveal new surprises.
But that’s not all, because, since the discovery of graphene, a whole new family of two-dimensional crystals has been discovered. They are made up of different atoms organized in various ways (e.g., molybdenum disulfide or hexagonal boron nitride ). Their properties are just as diverse as three-dimensional crystals (insulators, semiconductors, conductors, and superconductors). The researchers showed theoretically that the moiré in the superimpositions of these other materials could stop their free electrons just as much as in the bilayers of graphene.
Moreover, correlated states of matter have already been detected in the rotated double bilayers of graphene or the rotated bilayers of other materials (e.g., WSe2 ). Very recently, it was specifically superconductivity that was first observed in another material – graphene tri-layers – by two different groups. By combining materials and their rotation, we can imagine an infinity of new artificial materials with original electronic properties.