Plasmons are truly peculiar quasiparticles. They arise from quantized oscillations in plasma, the fourth and most common state of matter. Just like light is an electromagnetic oscillation with its own particle, the photon, other oscillations can create something similar. One of the weirdest plasmons of all is without doubt Pines’ Demon, and the fact that it sounds like a cryptid only helps its infamous status. But after decades, it has finally been observed.
So what has this plasmon done to deserve the name of “demon”? In 1956, theoretical physicist David Pines worked out that under the right conditions, electrons in a solid could behave quite unlike themselves. Electrons have a small but well determined mass and they are electrically charged. But they can interact to create a quasiparticle that has no mass, no electric charge, and does not interact with light. Basically, this quasiparticle would be there but we wouldn’t be able to see it. Sounds fairly demonic to us.
The discovery of Pines’ Demon was actually quite serendipitous. The team was studying the metal strontium ruthenate because it has similarities to high-temperature superconductors. We do not exactly know how superconductivity arises, and this is why there was so much interest in LK-99 as a possible room-temperature superconductor. Demons have been suggested to be playing a role, but their elusiveness did not make for a worthwhile hunt.
The team studied the electronic properties of the sample by shooting electrons at high-quality samples of the metal. This is not a standard technique but it allowed them to study plasmons that form in the metal. They were surprised to see that one of them appeared to have no mass at all.
“At first, we had no idea what it was. Demons are not in the mainstream. The possibility came up early on, and we basically laughed it off. But, as we started ruling things out, we started to suspect that we had really found the demon,” lead author Dr Ali Husain, now a research scientist at Quantinuum, said in a statement.
To confirm that they were indeed seeing a demon, the researchers needed to analyze the material’s properties in detail. Demons don’t emerge just anywhere.
“Pines’ prediction of demons necessitates rather specific conditions, and it was not clear to anyone whether strontium ruthenate should have a demon at all,” added co-author Dr Edwin Huang, from the University of Illinois Urbana-Champaign. “We had to perform a microscopic calculation to clarify what was going on. When we did this, we found a particle consisting of two electron bands oscillating out-of-phase with nearly equal magnitude, just like Pines described.”
The team argue that it was no accident that they found this serendipitously. They were trying to test a not-widely-studied material with a non-standard technique. And they found something different.
“It speaks to the importance of just measuring stuff,” explained Professor Peter Abbamonte, also at the University of Illinois Urbana-Champaign. “Most big discoveries are not planned. You go look somewhere new and see what’s there.”
The study is published in Nature.
Source Link: "Demon" Quasiparticle Finally Observed After Decades Of Predictions