Position of antiferromagnetic excitonic insulators. It is a magnetic state whose existence was predicted long ago. It was in the 1960s when scientists began to think about how the laws of quantum mechanics affect the electronic properties of materials. However, no one was able to see it yet. Till date. Because physicists at Brookhaven National Laboratory (USA) just announced it. He discovered an antiferromagnetic excitonic insulator. And it deserves some clarification.

From insulation to exciton via antiferromagnetism

Let’s start with the simplest. Period “Insulator”. It is a word of our daily life. We know soundproof insulators that block out noise. Thermal insulators that protect against hot and cold. But here, of course, it is a question of electrical insulation. of a material that does not conduct electricity. The concept is familiar to us. However, it may also be that it has little meaning from a physical point of view. In fact, what makes an insulator not conduct electricity is that the electrons in it are incapable of motion. They are always in their lowest state of energy. Like dropped on the ground.

This is a small reminder, let’s see the next term, “provocative”, It is related to exciton. And an excitation is what physicists call a quasiparticle. It may appear under certain circumstances. Those that would allow electrons to move and interact strongly with each other. Then they form bound states. Between them or with “holes”. Those that the electrons themselves leave behind when they change their state or energy level.

In the first case, as in the case of electron-electron interaction, the exciton appears under the influence of magnetic attraction forces strong enough to overcome the natural repulsion between two negatively charged particles. In the case of electron–hole interactions, it is the energy difference between the ground state of the electron and the higher level – what physicists call the band gap – that must be overcome.

Before knowing and understanding what an antiferromagnetic excitonic insulator corresponds to, it remains for us to remember what antiferromagnetism is. It is a magnetic property of some materials for which the nuclear magnetic moments line up in parallel. Understand that atoms’ electrons move side by side and align one up and one down. Thus these magnetic moments tend to cancel out by 2. So that there is no longer any magnetism on the scale of the material. Magnetization is zero.

The interest of antiferromagnetic materials – chromium, for example, at room temperature, or ceramics based on transition metals (titanium, iron, cobalt, nickel, copper, etc.) or oxygen – is that they allow them to switch quickly between different can be made for. states. And that they are resistant to loss of information due to interference from external magnetic fields. What makes them interesting for applications in the field of communication technologies.

© Brookhaven National Laboratory

Physicists used X-rays to measure how spins (blue arrows) oscillate when rotated. A behavior that occurs because the amount of electric charge (represented by the yellow disc) at each site can also vary. It is thanks to these variations that this new behavior can be identified.

The first antiferromagnetic excitonic insulator

It is the advanced technologies available today that have allowed physicists at Brookhaven Lab to explore the special conditions under which the antiferromagnetic excitonic insulator state can emerge. He worked on the oxides of strontium and iridium (Sr.)₃IR₂hey₇) At high temperatures, this material appears as weakly insulating. And the researchers ran it through X-rays from an advanced photon source to track and measure the magnetic interactions and the associated energy costs of moving electrons as temperatures drop.

They were thus able to note that the energy gap was reduced. Until observed, at about 285 Kelvin – or about 12 °C – the electrons that begin to jump between the magnetic layers of the material immediately pair with the holes they left behind. As a result of triggering the antiferromagnetic alignment of the spins of adjacent electrons.

And as the connection created by the attraction between electrons and holes restores more energy than an electron jumping over the band gap in this case, all electrons want to be part of it. Electrons also want to save energy. The material then finds itself in a new configuration, in which the electron spins are ordered in an antiferromagnetic pattern and bound electron-hole pairs that “lock” it into an insulating state. This is exactly what the model describes the concept of antiferromagnetic excitonic insulators.

Now it remains for researchers to understand a little more about the link that unites spin and charge in these materials. To imagine what new technologies he could benefit from.

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