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According to a recent study, the debate is finally settled: Phonons may possess chirality.


To prove the existence of chiral phonons, the researchers used inelastic X-ray scattering (RIXS). Circularly polarized light shines on quartz. The angular momentum of the photons is transferred to a crystal, causing an in-state revolution of the anions (orange spheres with p orbitals) relative to their neighboring cations (green spheres). Credit: Paul Shearer Institute/Hiroki Ueda and Mahir Dzambigovic

Results published in nature Discordance Resolution: Phonons can be chiral. This basic concept, discovered using circular X-ray light, sees phonons swirling like a corkscrew through quartz.

Throughout nature, and in all scales, you can find examples of copulation — or handedness. Imagine trying to eat a sandwich with two hands that weren’t the same—non-superimposable mirror images—of each other. Consider pharmacological disasters resulting from the administration of a wrong homolog of a drug or, on a subatomic scale, the importance of the concept of equivalence in particle physics. Now, thanks to a new study led by researchers at the Paul Scherrer PSI Institute, we know that phonons can also possess this property.

A phonon is a quasiparticle that describes collective vibrational excitations of atoms in a crystal lattice; Imagine it as the Irish river dance of corns. Physicists predicted that if phonons can show reproductive symmetry, they could have important implications for the fundamental physical properties of materials. With the rapid rise in recent years of research into topological materials that exhibit exotic electronic and magnetic surface properties, interest in chiral phonons has increased. However, experimental evidence for its existence has remained elusive.

What makes the phonons elusive are their dance steps. In the new study, the atomic vibrations dance as the deflection moves forward like a corkscrew. This movement of the key is one of the reasons why there is such a drive to discover this phenomenon. If phonons could spin in this way, like coils of wire forming a solenoid, perhaps they could create a magnetic field in a material.

New direction of the problem

This possibility is what motivated Urs Staub’s group at PSI, who led the study. “Because we are at the crossroads of ultrafast X-ray science and materials research, we can approach the problem from a different angle,” he says. The researchers are interested in manipulating the spiral patterns of materials using chiral light – circularly polarized light.

She was using such light so that the researchers could present their evidence. Using quartz, one of the most well-known minerals whose atoms — silicon and oxygen — form a chiral structure, they showed how circularly polarized light is coupled to chiral phonons. To do this, they used a technique known as inelastic X-ray scattering (RIXS) at the UK’s Diamond Light Source. This has been supplemented by supporting theoretical descriptions of how the process creates and enables the detection of chiral phonons from ensembles at ETH Zurich (Carl Romao and Nicola Spaldin) and MPI Dresden (Jeroen van den Brink).

“You don’t usually work like this in science”

In their experiment, circularly polarized light shines on quartz. Photons of light have angular momentum, which they transfer to the atomic lattice, setting off vibrations in their spiral motion. The direction in which the phonons rotate depends on the intrinsic paradox of the quartz crystal. As the phonons rotate, they release energy in the form of scattered, detectable light.

Imagine that you are standing on a roundabout and throwing a Frisbee. If you throw the Frisbee in the same direction of motion as the spinner, you’ll expect it to sizzle. Throw it the other way and it will spin less, as the angular momentum of the spin and the Frisbee will cancel out. In the same way, when circularly polarized light is twisted in the same way as the phonon that excites it, the signal is enhanced, and chiral phonons can be detected.

A well-planned experiment, accurate theoretical calculations, and then a strange thing happened: almost everything went according to plan. Once they analyzed the results, the difference in response with light reflection off was undeniable.

“The results were convincing almost immediately, especially when we compared the difference with other quartz variants,” recalls PSI scientist and first author of the publication Hiroki Ueda. Sitting in front of his computer analyzing the data, Ueda was the first person to see the results: “I kept checking my analysis codes to make sure they were correct.” Staub asserts, “It’s not normal! It doesn’t usually work like that in science!”

During the search for chiral phonons, there were many false alarms. Will this settle the debate? “Yeah, I think so, that’s the beauty of this work,” thought Staub, whose reviewers shared their opinion nature. “Because it’s simple and beautiful and direct. It’s obvious. It’s very simple, and that’s obviously the spiral motion.”

more information:
Hiroki Ueda et al, Chiral phonons in quartz examined by X-rays, nature (2023). DOI: 10.1038/s41586-023-06016-5

Provided by the Paul Scherrer Institute

the quotePhonons Could Be Chiral: Study Claims to Settle Debate (2023, June 9) Retrieved June 9, 2023 from https://phys.org/news/2023-06-phonons-chiral-debate.html

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