Phonons may be chiral: Study claims to settle debate

Phonons may be chiral: Study claims to settle debate

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To demonstrate the existence of chiral phonons, the researchers used X-ray resonant inelastic scattering (RIXS). Circularly polarized light shines on the quartz. The angular momentum of the photons is transferred to a crystal, in this case causing anions (orange spheres with p-orbitals) to revolution with respect to neighboring cations (green spheres). Credits: Paul Scherrer Institute / Hiroki Ueda and Mahir Dzambegovic

Results published in Nature settle the dispute: phonons can be chiral. This fundamental concept, discovered using circular X-ray light, sees phonons twisting like a corkscrew through quartz.

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

A phonon is a quasiparticle that describes the collective vibrational excitations of atoms in a crystal lattice; imagine it as the Irish Riverdance of the atoms. Physicists have predicted that if phonons can demonstrate chirality they could have important implications for the fundamental physical properties of materials. With the rapid increase in recent years of research into topological materials exhibiting curious electronic and magnetic surface properties, interest in chiral phonons has grown. However, experimental proof of their existence has remained elusive.

What makes phonons chiral is their dance steps. In the new study, atomic vibrations dance in a twist that moves forward like a corkscrew. This corkscrew movement is one of the reasons there has been such a push to discover the phenomenon. If phonons can spin like this, like the coil of wire that forms a solenoid, perhaps they could create a magnetic field in a material.

A new perspective on the problem

It is this possibility that motivated Urs Staub’s group at PSI, which led the study. “It’s because we’re at the junction of ultrafast X-ray science and materials research that we might be approaching the problem from a different perspective,” he says. Researchers are interested in manipulating the chiral modes of materials using circularly polarized chiral light.

He was using such light that the researchers could make their own test. Using quartz, one of the best-known minerals whose atoms silicon and oxygen form a chiral structure, they showed how circularly polarized light couples to chiral phonons. To do this, they used a technique known as resonant inelastic X-ray scattering (RIXS) at the Diamond Light Source in the UK. This was complemented with supporting theoretical descriptions of how the process would create and enable the detection of chiral phonons by groups from ETH Zurich (Carl Romao and Nicola Spaldin) and MPI Dresden (Jeroen van den Brink).

“It doesn’t usually work like that in science”

In their experiment, circularly polarized light shines on the quartz. Photons of light possess angular momentum, which they transfer to the atomic lattice, launching the vibrations in their corkscrew motion. The direction in which the phonons spin depends on the intrinsic chirality of the quartz crystal. As phonons spin, they release energy in the form of scattered light, which can be detected.

Imagine standing on a roundabout and throwing a Frisbee. If you throw the Frisbee in the same direction of movement as the roundabout, you would expect it to zip. Throw it the other way and it will spin less, as the angular momentum of the roundabout and the Frisbee cancel each other out. Similarly, when circularly polarized light twists in the same way as the phonon it excites, the signal is enhanced and chiral phonons can be detected.

A well-planned experiment, accurate theoretical calculations and then something strange happened: almost everything went according to plan. As soon as they analyzed the results, the difference in the response to the light chirality flip was undeniable.

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

While searching for chiral phonons, there were several false alarms. Will this settle the debate? “Yes, I think so, that’s the beauty of this work,” believes Staub, whose opinion was shared by reviewers of Nature. “Because it’s simple and beautiful and straightforward. It’s obvious. It’s so simple, it’s obvious that this is chiral motion.”

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

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