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

About the magazine:
Nature

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Caltech claims to have transmitted energy to Earth from satellite

Caltech claims to have transmitted energy to Earth from satellite

Researchers at a US university say they have beamed energy from an orbiting satellite to a detector on Earth, proving that harvesting energy from solar panels in space is technically possible.

The eggheads at the California Institute of Technology, aka Caltech, said they used a satellite launched into orbit in January to demonstrate the ability to wirelessly transmit energy into space and also transmit detectable energy to Earth.

Known as the Space Solar Power Demonstrator (SSPD-1), the satellite is the first technology prototype from Caltech’s Space Solar Power Project (SSPP) to enter orbit.

SSPP was founded about ten years ago, with the aim of collecting solar energy in space and transmitting it to receivers on the ground, such as The register previously reported.

All electromagnetic radiation carries energy, which can be seen in powered radios by the received radio signal itself. The trick is to be able to direct the energy to a desired target instead of radiating it in all directions.

The commercial end of the SSPD-1 satellite is the Microwave Array for Power-transfer Low-orbit Experiment (MAPLE), a series of flexible, lightweight microwave power transmitters driven by custom electronic chips to direct a beam of energy wherever needed .

An interaction of constructive and destructive interference between individual transmitters directs the beam, such that a bank of power systems can shift the focus and direction of the energy it radiates. This is similar to the technique used in military phased-array radar systems to scan the horizon without physically moving the antenna.

In this case, the transmitter array uses precise timing control to focus power to the desired location using the coherent addition of electromagnetic waves, according to Caltech, so that most of the transmitted energy is focused on the desired location of the target. .

However, in this case, the energy didn’t have to travel very far. MAPLE includes two receiver arrays approximately 12 inches apart from the transmitter to receive the energy. When power is received, it turns on some LEDs to demonstrate system operation.

This isn’t much of a test. MAPLE also features a small window on the satellite through which the array can radiate energy. This was used to transmit a test signal to a receiver on the roof of a laboratory on the Caltech campus in Pasadena.

While this provided only a small amount of power, Ali Hajimiri, Bren’s electrical engineering professor and co-director of SSPP, claimed the result for the first time.

“As far as we know, no one has ever demonstrated wireless energy transfer in space even with expensive rigid structures. We are doing it with flexible, lightweight structures and our integrated circuits,” he said in a statement.

“Through the experiments we’ve conducted so far, we’ve received confirmation that MAPLE can successfully beam energy to receivers in space. We’ve also been able to program the array to direct its energy towards Earth, which we’ve detected here at Caltech We’ve obviously had it tested on Earth, but now we know it can survive space travel and operate there.”

The satellite has other experimental hardware besides MAPLE. Deployable on-Orbit ultraLight Composite Experiment (DOLCE) is designed to test the deployment mechanisms of a lightweight, foldable structure to support solar panels, while ALBA is a collection of 32 different types of photovoltaic cells. They are arranged to evaluate which types work best in the space.

Looking to the future, the SSPP project said it aims to deploy a constellation of satellites to harvest sunlight and transmit microwave power wherever it’s needed, including locations that lack reliable access to power. This assumes that the equipment achieves sufficient efficiency to make the effort worthwhile.

“No power transmission infrastructure on the ground will be needed to receive this power. This means we can send power to remote regions and areas devastated by war or natural disaster,” said Hajimiri.

The idea behind solar energy from space is that energy is always available without being subject to day and night cycles, seasons and cloud cover, says the SSPP, potentially producing eight times more energy than the solar panels on the ground.

However, a previous study by the European Space Agency (ESA) of solar harvesting satellites calculated that they would need to be somewhere in the region of a kilometer or more in diameter to transmit about 2 GW of power to the surface which would correspond to the production of a nuclear power plant.

It seems that the SSPP will aim for more modest goals, at least for now.

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