Moiré are optical effects obtained when repeating patterns are superimposed -
© F. Mesple & V. Renard / UGA, CC BY-NC
Two layers of graphene stacked at a so-called "magic" angle conduct electricity without any resistance, according to a study published by our partner The Conversation.
We can therefore imagine producing, in the near future, an infinity of new artificial materials with original electronic properties.
The analysis of this discovery was carried out by Florie Mesple, doctoral student in condensed matter at the French Atomic Energy and Alternative Energies Commission (CEA) and Vincent Renard, lecturer in physics at the University of Grenoble Alpes (UGA).
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 can become superconducting when they are rotated with respect to 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", as when two fabrics are superimposed.
Almost two years since this discovery;
let's try to understand in more detail what excites the researchers.
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 very good 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 make only three strong bonds with neighbors located in the same plane, so that one electron per atom remains available to carry electricity.
Each layer of graphite, isolated, is called "graphene".
Different carbon crystals are formed depending on the number of bonds between the carbon atoms. In this artistic view the atoms are represented by black balls and the bonds by gray rods © F. Mesple, V. Renard / UGA
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 can be stable and that free electrons in this crystal are quite unique: they behave as if they have no mass - just like photons, the particles carrying light.
In addition, they are very difficult to slow down, so graphene is one of the best known conductors of electricity.
So it's 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 layers of graphene.
The technique used is very simple, 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 superposition of the atomic networks of the two layers introduces a new periodicity, which is called a "moiré".
Researchers have found a method of stacking two-dimensional crystals on top of each other. The image on the right shows a layer of graphene about to be transferred to another from a polymer film © F. Mesple, V. Renard / UGA
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 layers of graphene, moiré remains just a curiosity, it is able to strongly slow the unstoppable electrons of 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 °.
A moiré that appears when two layers of graphene are superimposed with a rotation. It gives rise to new states of matter when the angle of rotation between the layers is close to 1 ° © F. Mesple, V Renard / UGA
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 which 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 distance of one meter.
This collective behavior is said to be “correlated”.
It requires electrons to be able to move to avoid each other, which is usually possible in a 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, a 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 periodicity of the crystal, in this type of material it stems rather from the Coulomb repulsion.
A third possibility is the superconducting state, where electric current flows without any resistance.
Scanning microscope image of a graphene moiré. The image is 19x11 nm² © L. Huder & V. Renard / UGA
Depending on the conditions of the experiment, for example temperature, pressure, 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 many questions which remain on this physics of correlated electrons.
And why not other materials than graphene?
Many teams are delving into these questions, at the same time diversifying the research subject which will probably reveal new surprises.
For example, we have shown that the deformations of one layer relative to the other have a strong impact on this physics.
Our “Physical science” dossier
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 types of atoms organized in various ways (e.g. molybdenum disulfide or hexagonal boron nitride) and their properties are just as diverse as those of three-dimensional crystals (insulators, semiconductors, conductors, superconductors) .
The researchers have theoretically shown that the moiré in the superimpositions of these other materials can stop their free electrons just as much as in the bilayers of graphene.
Correlated states of matter have moreover already been detected in the rotated double bilayers of graphene or in the rotated bilayers of other materials (eg WSe2).
Very recently, it was specifically superconductivity that was first observed in another material - graphene trilayers - by two different groups.
By combining materials and their rotation, we can therefore imagine an infinity of new artificial materials with original electronic properties.
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This analysis was written by Vincent Renard, lecturer in physics at the University of Grenoble Alpes (UGA) and Florie Mesple, doctoral student in condensed matter at the French Atomic Energy and Alternative Energies Commission (CEA).
The original article was published on The Conversation website.
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