Category: Physics

Since axions were first predicted by theory nearly half a century ago, researchers have hunted for proof of the elusive particle, which may exist outside the visible universe, in the dark sector. But how does one find particles that can’t be seen?

The first physics results from the Coherent CAPTAIN-Mills experiment at Los Alamos —just described in a publication in the journal Physical Review D—suggest that liquid-argon, accelerator-based experimentation, designed initially to look for similarly hypothetical particles such as sterile neutrinos, may also be an ideal set-up for seeking out stealthy axions.

“The confirmation of dark sector particles would have a profound impact on the understanding of the Standard Model of particle physics, as well as the origin and evolution of the universe,” said physicist Richard Van de Water. “A big focus of the physics community is exploring ways to detect and confirm these particles. The Coherent CAPTAIN-Mills experiment couples existing predictions of dark matter particles such as axions with high-intensity particle accelerators capable of producing this hard-to-find dark matter.”

Demystifying the dark sector

Physics theory suggests that only 5% of the universe is made up of visible matter—atoms that form things we can see, touch and feel—and that the remaining 95% is the combination of matter and energy known as the dark sector. Axions, sterile neutrinos and others may explain and account for all or part of that missing energy density.

The existence of axions could also resolve a longstanding problem in the Standard Model, which outlines the known behavior of the subatomic world. Sometimes referred to as “fossils” of the universe, speculated to originate just a second after the Big Bang, axions could also tell us much about the founding moments of the universe.

The Coherent CAPTAIN-Mills experiment was one of several projects to receive Department of Energy funding for dark sector research in 2019, along with substantial funding from the Laboratory Directed Research and Development program at Los Alamos. A prototype detector dubbed the CCM120 was built and run during the 2019 Los Alamos Neutron Science Center (LANSCE) beam cycle. The Physical Review D publication describes results from the CCM120’s initial engineering run.

“Based on the first run of CAPTAIN-Mills research, the experiment has demonstrated the capability to execute the search for axions,” said Bill Louis, also a physicist on the project at Los Alamos. “We’re realizing that the energy regime provided by the proton beam at LANSCE and the liquid argon detector design offers an unexplored paradigm for axion-like particle research.”

Experiment design

Stationed in the Lujan Center adjacent to LANSCE, the Coherent CAPTAIN-Mills experiment is a 10-ton, supercooled, liquid argon detector. (CAPTAIN stands for Cryogenic Apparatus for Precision Tests of Argon Reactions with Neutrinos.)

High-intensity, 800-megaelectron volt protons generated by the LANSCE accelerator hit a tungsten target in the Lujan Center, then traverse 23 meters through extensive steel and concrete shielding to the detector to interact in the liquid argon.

The prototype detector’s interior walls are lined with 120, eight-inch, sensitive photomultiplier tubes (hence the CCM120 moniker) that detect light flashes—single photons—that result when a regular or dark sector particle jostles an atom in the tank of liquid argon.

A special material coating on the interior walls converts the argon light emission into visible light that can be detected by the photo-multiplier tubes. Fast timing of the detector and beam helps remove the effects of background particles such as beam neutrons, cosmic rays and gamma-rays from radioactive decays.

Pieces of the puzzle

Axions are of great interest because they are “highly motivated”; that is, their existence is strongly implied in theories beyond the Standard Model. Developed over more than 70 years, the Standard Model explains three of the four known fundamental forces—electromagnetism, the weak nuclear force and the strong nuclear force—that govern the behavior of atoms, the building blocks of matter. (The fourth force, gravity, is explained by Einsteinian relativity.) But the model isn’t necessarily complete.

An unresolved problem in Standard Model physics is known as the “strong CP problem,” with “CP” meaning charge-parity symmetry. Essentially, particles and their antiparticle counterparts are acted upon similarly by the laws of physics. Nothing in Standard Model physics mandates that behavior, though, so physicists should see at least occasional violations of that symmetry.

In weak-force interactions, charge-parity symmetry violations do occur. But no similar violations have been observed in strong-force interactions. That puzzling absence of theoretically possible behavior represents a problem for Standard Model theory. What prevents violations of charge-parity symmetry from occurring in strong-force interactions?

Abundant, nearly weightless and electrically neutral, axions may be an important part of the puzzle. The axion earned its moniker in 1978, so-coined by physicist Frank Wilczek after a brand of laundry detergent because such a particle could “clean up” the strong CP problem. Physicists speculate that they are components of a dark matter force that preserves charge-parity symmetry, and that they may couple, or interact with, photons and electrons.

Next steps

If axions do exist, finding them might be a matter of devising the right experimental set-up.

“As a result of this initial run with our CCM120 detector, we have a much better understanding of the signatures connected with axion-like particles coupled to photons and to electrons as they move through liquid argon,” said Louis. “These data give us the insight to upgrade the detector to be more sensitive by an order of magnitude.”

More information: A. A. Aguilar-Arevalo et al, Prospects for detecting axionlike particles at the Coherent CAPTAIN-Mills experiment, Physical Review D (2023). DOI: 10.1103/PhysRevD.107.095036

Journal information: Physical Review D 

Provided by Los Alamos National Laboratory 

New frontiers in the study of the universe—and gravitational waves—have been opened up following a breakthrough by University of the West of Scotland (UWS) researchers.

The groundbreaking development in thin film technology promises to enhance the sensitivity of current and future gravitational wave detectors. Developed by academics at UWS’s Institute of Thin Films, Sensors and Imaging (ITFSI), the innovation could enhance the understanding of the nature of the universe. The research is published in the journal Applied Optics.

Gravitational waves, first predicted by Albert Einstein’s theory of general relativity, are ripples in the fabric of spacetime caused by the most energetic events in the cosmos, such as black hole mergers and neutron star collisions. Detecting and studying these waves provides invaluable insights into the fundamental nature of the universe.

Dr. Carlos Garcia Nuñez, lecturer at UWS’s School of Computing, Engineering and Physical Sciences said, “At the Institute of Thin Films, Sensors and Imaging, we are working hard to push the limits of thin film materials, exploring new techniques to deposit them, controlling their properties in order to match the requirements of current and future sensing technology for the detection of gravitational waves.”

“The development of high reflecting mirrors with low thermal noise opens a wide range of applications, which covers from the detection of gravitational waves from cosmological events, to the development of quantum computers.”

The technique used in this work—originally developed and patented by Professor Des Gibson, Director of UWS’s Institute of Thin Films, Sensors and Imaging—could enable the production of thin films that achieve low levels of “thermal noise.” The reduction of this kind of noise in mirror coatings is essential to increase the sensitivity of current gravitational wave detectors—allowing the detection of a wider range of cosmological events—and could be deployed to enhance other high-precision devices, such as atomic clocks or quantum computers.

Professor Gibson said, “We are thrilled to unveil this cutting-edge thin film technology for gravitational wave detection. This breakthrough represents a significant step forward in our ability to explore the universe and unlock its secrets through the study of gravitational waves. We believe this advancement will accelerate scientific progress in this field and open up new avenues for discovery.”

“UWS’s thin film technology has already undergone extensive testing and validation in collaboration with renowned scientists and research institutions. The results have been met with great enthusiasm, fueling anticipation for its future impact on the field of gravitational wave astronomy. The coating deposition technology is being commercialized by UWS spinout company, Albasense Ltd.”

The development of coatings with low thermal noise will not only make future generation of gravitational wave detectors more precise and sensitive to cosmic events, but will also provide new solutions to atomic clocks and quantum mechanics, both highly relevant for the United Nations’ Sustainable Development Goals 7, 9 and 11.

More information: Carlos Garcia Nuñez et al, Amorphous dielectric optical coatings deposited by plasma ion-assisted electron beam evaporation for gravitational wave detectors, Applied Optics (2023). DOI: 10.1364/AO.477186

Provided by University of the West of Scotland 

A team led by University of Minnesota Twin Cities scientists and engineers discovered a new method for tuning the thermal conductivity of materials to control heat flow “on the fly.” Their tuning range is the highest ever recorded among one-step processes in the field, and will open a door to developing more energy-efficient and durable electronic devices.

The researchers’ paper is published in Nature Communications.

Just as electrical conductivity determines how well a material can transport electricity, thermal conductivity describes how well a material can transport heat. For example, many metals used to make frying pans have a high thermal conductivity so that they can transport heat efficiently to cook food.

Typically, the thermal conductivity of a material is a constant, unchanging value. However, the University of Minnesota team has discovered a simple process to “tune” this value in lanthanum strontium cobaltite, a material often used in fuel cells. Similar to the way a switch controls the flow of electricity to a light bulb, the researchers’ method provides a way to turn heat flow on and off in devices.

“Controlling how well a material can transfer heat is of great importance in daily life and in industry,” said Xiaojia Wang, co-corresponding author of the study and an associate professor in the University of Minnesota Department of Mechanical Engineering. “With this research, we have achieved a record-high tuning of thermal conductivity, showing promise for effective thermal management and energy consumption in the electronic devices people use every day. A well-designed and functioning thermal management system would enable better user experience and make devices more durable.”

Wang’s team worked in tandem with University of Minnesota Distinguished McKnight University Professor Chris Leighton, whose lab specializes in materials synthesis.

Leighton’s team fabricated the lanthanum strontium cobaltite devices using a process called electrolyte gating, in which ions (molecules with an electrical charge) are driven to the surface of the material. This allowed Wang and her research team to manipulate the material by applying a low voltage to it.

“Electrolyte gating is a tremendously powerful technique for controlling the properties of materials, and is well established for voltage-control of electronic, magnetic, and optical behavior,” said Leighton, co-corresponding author of the study and a faculty member in the University of Minnesota Department of Chemical Engineering and Materials Science. “This new work applies this approach in the realm of thermal properties, where voltage-control of physical behavior is less explored. Our results establish low-power, continuously tunable thermal conductivity over an impressive range, opening up some pretty exciting potential device applications.”

“Although it was challenging to measure the thermal conductivity of lanthanum strontium cobaltite films because they are so ultrathin, it was quite exciting when we finally got the experiments to work,” said Yingying Zhang, first author of the paper and a University of Minnesota mechanical engineering Ph.D. alumnus. “This project not only provides a promising example of tuning materials’ thermal conductivity but also demonstrates the powerful approaches we use in our lab to push the experimental limit for challenging measurements.”

More information: Yingying Zhang et al, Wide-range continuous tuning of the thermal conductivity of La0.5Sr0.5CoO3-δ films via room-temperature ion-gel gating, Nature Communications (2023). DOI: 10.1038/s41467-023-38312-z

Journal information: Nature Communications 

Provided by University of Minnesota 

Quantum computing uses the principles of quantum mechanics to encode and elaborate data, meaning that it could one day solve computational problems that are intractable with current computers. While the latter work with bits, which represent either a 0 or a 1, quantum computers use quantum bits, or qubits—the fundamental units of quantum information.

“With applications ranging from drug discovery to optimization and simulations of complex biological systems and materials, quantum computing has the potential to reshape vast areas of science, industry, and society,” says Professor Vincenzo Savona, director of the Center for Quantum Science and Engineering at EPFL.

Unlike classical bits, qubits can exist in a “superposition” of both 0 and 1 states at the same time. This allows quantum computers to explore multiple solutions simultaneously, which could make them significantly faster in certain computational tasks. However, quantum systems are delicate and susceptible to errors caused by interactions with their environment.

“Developing strategies to either protect or qubits from this or to detect and correct errors once they have occurred is crucial for enabling the development of large-scale, fault-tolerant quantum computers,” says Savona. Together with EPFL physicists Luca Gravina, and Fabrizio Minganti, they have made a significant breakthrough by proposing a “critical Schrödinger cat code” for advanced resilience to errors. The study introduces a novel encoding scheme that could revolutionize the reliability of quantum computers.

What is a ‘critical Schrödinger cat code’?

In 1935, physicist Erwin Schrödinger proposed a thought experiment as a critique of the prevailing understanding of quantum mechanics at the time—the Copenhagen interpretation. In Schrödinger’s experiment, a cat is placed in a sealed box with a flask of poison and a radioactive source. If a single atom of the radioactive source decays, the radioactivity is detected by a Geiger counter, which then shatters the flask. The poison is released, killing the cat.

According to the Copenhagen view of quantum mechanics, if the atom is initially in superposition, the cat will inherit the same state and find itself in a superposition of alive and dead. “This state represents exactly the notion of a quantum bit, realized at the macroscopic scale,” says Savona.

In past years, scientists have drawn inspiration by Schrödinger’s cat to build an encoding technique called “Schrödinger’s cat code.” Here, the 0 and 1 states of the qubit are encoded onto two opposite phases of an oscillating electromagnetic field in a resonant cavity, similarly to the dead or alive states of the cat.

“Schrödinger cat codes have been realized in the past using two distinct approaches,” explains Savona. “One leverages anharmonic effects in the cavity, the other relying on carefully engineered cavity losses. In our work, we bridged the two by operating in an intermediate regime, combining the best of both worlds. Although previously believed to be unfruitful, this hybrid regime results in enhanced error suppression capabilities.” The core idea is to operate close to the critical point of a phase transition, which is what the ‘critical’ part of the critical cat code refers to.

The critical cat code has an additional advantage: it exhibits exceptional resistance to errors that result from random frequency shifts, which often pose significant challenges to operations involving multiple qubits. This solves a major problem and paves the way to the realization of devices with several mutually interacting qubits—the minimal requirement for building a quantum computer.

“We are taming the quantum cat,” says Savona. “By operating in a hybrid regime, we have developed a system that surpasses its predecessors, which represents a significant leap forward for cat qubits and quantum computing as a whole. The study is a milestone on the road towards building better quantum computers, and showcases EPFL’s dedication in advancing the field of quantum science and unlocking the true potential of quantum technologies.”

The findings are published in the journal PRX Quantum.

More information: Luca Gravina et al, Critical Schrödinger Cat Qubit, PRX Quantum (2023). DOI: 10.1103/PRXQuantum.4.020337

Journal information: PRX Quantum 

Provided by Ecole Polytechnique Federale de Lausanne 

When we listen to our favorite song, what sounds like a continuous wave of music is actually transmitted as tiny packets of quantum particles called phonons.

The laws of quantum mechanics hold that quantum particles are fundamentally indivisible and therefore cannot be split, but researchers at the Pritzker School of Molecular Engineering (PME) at the University of Chicago are exploring what happens when you try to split a phonon.

In two experiments—the first of their kinds—a team led by Prof. Andrew Cleland used a device called an acoustic beamsplitter to “split” phonons and thereby demonstrate their quantum properties. By showing that the beamsplitter can be used to both induce a special quantum superposition state for one phonon, and further create interference between two phonons, the research team took the first critical steps toward creating a new kind of quantum computer.

The results are published in the journal Science and built on years of breakthrough work on phonons by the team at Pritzker Molecular Engineering.

“Splitting” a phonon into a superposition

In the experiments, researchers used phonons that have roughly a million times higher pitch than can be heard with the human ear. Previously, Cleland and his team figured out how to create and detect single phonons and were the first to entangle two phonons.

To demonstrate these phonons’ quantum capabilities, the team—including Cleland’s graduate student Hong Qiao—created a beamsplitter that can split a beam of sound in half, transmitting half and reflecting the other half back to its source (beamsplitters already exist for light and have been used to demonstrate the quantum capabilities of photons). The whole system, including two qubits to generate and detect phonons, operates at extremely low temperatures and uses individual surface acoustic wave phonons, which travel on the surface of a material, in this case lithium niobate.

However, quantum physics says a single phonon is indivisible. So when the team sent a single phonon to the beamsplitter, instead of splitting, it went into a quantum superposition, a state where the phonon is both reflected and transmitted at the same time. Observing (measuring) the phonon causes this quantum state to collapse into one of the two outputs.

The team found a way to maintain that superposition state by capturing the phonon in two qubits. A qubit is the basic unit of information in quantum computing. Only one qubit actually captures the phonon, but researchers cannot tell which qubit until post-measurement. In other words, the quantum superposition is transferred from the phonon to the two qubits. The researchers measured this two-qubit superposition, yielding “gold standard proof that the beamsplitter is creating a quantum entangled state,” Cleland said.

Showing phonons behave like photons

In the second experiment, the team wanted to show an additional fundamental quantum effect that had first been demonstrated with photons in the 1980s. Now known as the Hong-Ou-Mandel effect, when two identical photons are sent from opposite directions into a beamsplitter at the same time, the superposed outputs interfere so that both photons are always found traveling together, in one or the other output directions.

Importantly, the same happened when the team did the experiment with phonons—the superposed output means that only one of the two detector qubits captures phonons, going one way but not the other. Though the qubits only have the ability to capture a single phonon at a time, not two, the qubit placed in the opposite direction never “hears” a phonon, giving proof that both phonons are going in the same direction. This phenomenon is called two-phonon interference.

Getting phonons into these quantum-entangled state is a much bigger leap than doing so with photons. The phonons used here, though indivisible, still require quadrillions of atoms working together in a quantum mechanical fashion. And if quantum mechanics rules physics at only the tiniest realm, it raises questions of where that realm ends and classical physics begins; this experiment further probes that transition.

“Those atoms all have to behave coherently together to support what quantum mechanics says they should do,” Cleland said. “It’s kind of amazing. The bizarre aspects of quantum mechanics are not limited by size.”

Creating a new linear mechanical quantum computer

The power of quantum computers lies in the “weirdness” of the quantum realm. By harnessing the strange quantum powers of superposition and entanglement, researchers hope to solve previously intractable problems. One approach to doing this is to use photons, in what is called a “linear optical quantum computer.”

A linear mechanical quantum computer—which would use phonons instead of photons—itself could have the ability to compute new kinds of calculations. “The success of the two-phonon interference experiment is the final piece showing that phonons are equivalent to photons,” Cleland said. “The outcome confirms we have the technology we need to build a linear mechanical quantum computer.”

Unlike photon-based linear optical quantum computing, the University of Chicago platform directly integrates phonons with qubits. That means phonons could further be part of a hybrid quantum computer that combines the best of linear quantum computers with the power of qubit-based quantum computers.

The next step is to create a logic gate—an essential part of computing—using phonons, on which Cleland and his team are currently conducting research.

Other authors on the paper include É. Dumur, G. Andersson, H. Yan, M.-H. Chou, J. Grebel, C. R. Conner, Y. J. Joshi, J. M. Miller, R. G. Povey, and X. Wu.

More information: H. Qiao et al, Splitting phonons: Building a platform for linear mechanical quantum computing, Science (2023). DOI: 10.1126/science.adg8715. www.science.org/doi/10.1126/science.adg8715

Journal information: Science 

Provided by University of Chicago 

A group of researchers from Tohoku University has unveiled a new material that exhibits enormous magnetoresistance, paving the way for developments in non-volatile magnetoresistive memory (MRAM).

Details of their unique discovery were published in the Journal of Alloys and Compounds.

Today, the demand for advancements in hardware that can efficiently process large amounts of digital information and in sensors has never been greater, especially with governments deploying technological innovations to achieve smarter societies.

Much of this hardware and sensors rely on MRAM and magnetic sensors, and tunnel magnetoresistive devices make up the majority of such devices.

Tunnel magnetoresistive devices exploit the tunnel magnetoresistance effect to detect and measure magnetic fields. This is tied to the magnetization of ferromagnetic layers in magnetic tunnel junctions. When the magnets are aligned, a low resistance state is observed, and electrons can easily tunnel through the thin insulating barrier between them.

When the magnets are not aligned, the tunneling of electrons becomes less efficient and leads to higher resistance. This change in resistance is expressed as the magnetoresistive ratio, a key figure in determining the efficiency of tunneling magnetoresistive devices. The higher the magnetoresistance ratio, the better the device is.

Current tunnel magnetoresistive devices comprise magnesium oxide and iron-based magnetic alloys, like iron-cobalt. Iron-based alloys have a body-centered cubic crystal structure in ambient conditions and exhibit a huge tunnel magnetoresistance effect in devices with a rock salt-type magnesium oxide.

There have been two notable studies using these iron-based alloys that produced magnetoresistive devices displaying high magnetoresistance ratios. The first in 2004 was by the National Institute of Advanced Industrial Science and Technology in Japan and IBM; and the second came in 2008, when researchers from Tohoku University reported on a magnetoresistance ratio exceeding 600% at room temperature, something that jumped to 1000% with temperatures near zero Kelvin.

Since those breakthroughs, various institutes and companies have invested considerable effort in honing device physics, materials, and processes. Yet aside from iron-based alloys, only some Heusler-type ordered magnetic alloys have displayed such enormous magnetoresistance.

Dr. Tomohiro Ichinose and Professor Shigemi Mizukami from Tohoku University recently began exploring thermodynamically metastable materials to develop a new material capable of demonstrating similar magnetoresistance ratios. To do so, they focused on the strong magnetic properties of cobalt-manganese alloys, which have a body-centered cubic metastable crystal structure.

“Cobalt-manganese alloys have face-centered cubic or hexagonal crystal structures as thermodynamically stable phases. Because this stable phase exhibits weak magnetism, it has never been studied as a practical material for tunnel magnetoresistive devices,” said Mizukami.

Back in 2020, the group reported on a device that used a cobalt-manganese alloy with metastable body-centered cubic crystal structure.

Using data science and/or high-throughput experimental methods, they built upon this discovery, and succeeded in obtaining huge magnetoresistance in devices by adding a small amount of iron to the metastable body-centered cubic cobalt-manganese alloy. The magnetoresistance ratio was 350% at room temperature and also exceeded 1000% at a low temperature. Additionally, the device fabrication employed the sputtering method and a heating process, something compatible with current industries.

“We have produced the third instance of a new magnetic alloy for tunneling magnetoresistive devices showing huge magnetoresistance, and it sets an alternative direction of travel for future improvements,” adds Mizukami.

More information: Tomohiro Ichinose et al, Large tunnel magnetoresistance in magnetic tunnel junctions with magnetic electrodes of metastable body-centered cubic CoMnFe alloys, Journal of Alloys and Compounds (2023). DOI: 10.1016/j.jallcom.2023.170750

Provided by Tohoku University 

The patterns of light hold tremendous promise for a large encoding alphabet in optical communications, but progress is hindered by their susceptibility to distortion, such as in atmospheric turbulence or in bent optical fiber.

Now researchers at the University of the Witwatersrand (Wits) have outlined a new optical communication protocol that exploits spatial patterns of light for multi-dimensional encoding in a manner that does not require the patterns to be recognized, thus overcoming the prior limitation of modal distortion in noisy channels. The result is a new encoding state-of-the-art of over 50 vectorial patterns of light sent virtually noise-free across a turbulent atmosphere, opening a new approach to high-bit-rate optical communication.

Published in Laser & Photonics Reviews, the Wits team from the Structured Light Laboratory in the Wits School of Physics used a new invariant property of vectorial light to encode information. This quantity, which the team call “vectorness,” scales from 0 to 1 and remains unchanged when passing through a noisy channel.

Unlike traditional amplitude modulation which is 0 or 1 (only a two-letter alphabet), the team used the invariance to partition the 0 to 1 vectorness range into more than 50 parts (0, 0.02, 0.04 and so on up to 1) for a 50-letter alphabet. Because the channel over which the information is sent does not distort the vectorness, both sender and received will always agree on the value, hence noise-free information transfer.

The critical hurdle that the team overcame is to use patterns of light in a manner that does not require them to be “recognized,” so that the natural distortion of noisy channels can be ignored. Instead, the invariant quantity just “adds up” light in specialized measurements, revealing a quantity that doesn’t see the distortion at all.

“This is a very exciting advance because we can finally exploit the many patterns of light as an encoding alphabet without worrying about how noisy the channel is,” says Professor Andrew Forbes, from the Wits School of Physics. “In fact, the only limit to how big the alphabet can be is how good the detectors are and not at all influenced by the noise of the channel.”

Lead author and Ph.D. candidate Keshaan Singh adds, “To create and detect the vectorness modulation requires nothing more than conventional communications technology, allowing our modal (pattern) based protocol to be deployed immediately in real-world settings.”

The team have already started demonstrations in optical fiber and in fast links across free-space, and believe that the approach can work in other noisy channels, including underwater.

More information: Keshaan Singh et al, A Robust Basis for Multi‐Bit Optical Communication with Vectorial Light, Laser & Photonics Reviews (2023). DOI: 10.1002/lpor.202200844

Provided by Wits University 

Findings 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 were not enantiomers—non-superimposable mirror images—of each other. Consider the pharmacological disasters caused by administering the wrong drug enantiomer or, at a subatomic scale, the importance of the concept of parity in particle physics. Now, thanks to a new study led by researchers at 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 the 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 on the fundamental physical properties of materials. With the rapid rise in recent years of research into topological materials that exhibit curious electronic and magnetic surface properties, interest in chiral phonons has grown. Yet, experimental proof for their existence has remained elusive.

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

A new slant on the problem

It is this possibility that motivated the group of Urs Staub at PSI, who led the study. “It is because we are at the juncture between ultrafast X-ray science and materials research that we could approach the problem from a different angle,” he says. The researchers are interested in manipulating chiral modes of materials using chiral light—light that is circularly polarized.

It was using such light that the researchers could make their proof. Using quartz, one of the best-known minerals whose atoms—silicon and oxygen—form a chiral structure, they showed how circularly polarized light coupled 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 from groups at the ETH Zurich (Carl Romao and Nicola Spaldin) and MPI Dresden (Jeroen van den Brink).

‘It doesn’t usually work like this in science’

In their experiment, circularly polarized light shines on quartz. The photons of light possess angular momentum, which they transfer to the atomic lattice, launching the vibrations into their corkscrew motion. The direction that the phonons revolve depends on the intrinsic chirality of the quartz crystal. As the phonons revolve, 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 with the same direction of movement as the roundabout, you would expect it to whizz. Throw it the other way and it will spin less, as the angular momentum of the roundabout and the Frisbee will cancel out. In the same way, when the circularly polarized light twists the same way as the phonon it excites, the signal is enhanced, and chiral phonons could be detected.

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

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

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

Journal information: Nature 

Provided by Paul Scherrer Institute 

Scientists from the Department of Energy’s Ames National Laboratory made an intriguing discovery while conducting experiments to characterize magnetism in a material known as a dilute magnetic topological insulator where magnetic defects are introduced. Despite this material’s ferromagnetism, the team discovered strong antiferromagnetic interactions between some pairs of magnetic defects that play a key role in several families of magnetic topological insulators.

Topological insulators (TIs) as their name indicates, are insulators. However, because of their unique electronic band structure, they conduct electricity on the surface under the right conditions. By introducing magnetism, TIs can transmit electrical currents from one point to another without any heat generation or energy loss. This quality means that they have the potential to reduce future energy footprints for computing and electricity transmission.

According to Rob McQueeney, a scientist from Ames Lab and a member of the research team, “Finding topological insulators is not so easy. You have to find this unique situation where the electronic bands are knotted up.” He further explained that applying a magnetic field to a TI turns the surface into a unique two-dimensional insulator, while the very edges of the surface remain metallic.

An important goal is to obtain a ferromagnetic TI. Ferromagnetism is when all of the magnetic moments in the material spontaneously align in the same direction. However, the team also discovered that TIs are susceptible to antiferromagnetic interactions when defects are introduced. Antiferromagnetism is when some of the ions spontaneously align with neighboring ions. The opposing magnetic forces decrease the overall magnetism of the material.

There are two ways that scientists introduce magnetism into a TI. The first is to introduce dilute amounts of magnetic ions, such as manganese-doped bismuth telluride or antimony telluride. The second is to create an intrinsic magnetic TI by inserting a layer of magnetic ions into the material, such as manganese-bismuth-tellurium (MnBi₂Te₄) and manganese-antimony-tellurium (MnSb₂Te₄).

Since the intrinsic magnetic TIs have a full layer of magnetic ions, ideally the magnetism is not randomly distributed the way it is in the first method.

For this project, the team focused on dilute magnetic TIs, which have randomly distributed magnetic defects. “We wanted to understand the magnetic interactions at the most fundamental level. We were doping our sample using small amounts of magnetic ions to try to understand how the magnetic interactions occur,” said Farhan Islam, an Iowa State University graduate student and team member. “So basically, we’re trying to understand how microscopic interactions affect the overall magnetism of the system.”

To conduct their research, the team used a specialized method called neutron scattering. This method involves passing a beam of neutrons (sub-atomic particles with a neutral charge) through a sample of material. Data is collected by noting where and when the neutrons that scattered from the sample hit a detector.

This type of research can only be done a few places in the world. Neutron scattering for this project was conducted at the Spallation Neutron Source, a Department of Energy Office of Science User Facility operated by the Oak Ridge National Laboratory.

One challenge with neutron scattering is its weak signal. The team had concerns about studying dilute magnetism, because of the small overall number of magnetic ions. “I was very skeptical that we would see anything at all,” said McQueeney. “But we did. Actually, what we saw was pretty straightforward to observe, which was surprising.”

The team discovered that despite the overall ferromagnetism of manganese doped antimony telluride (Sb_1.94Mn_0.06Te₃), some isolated pairs of magnetic defects are coupled antiferromagnetically with opposite moment directions. Other magnetic pairs, especially those in different blocks of the layered structure, are ferromagnetically coupled with parallel moments. The competing magnetic forces decrease the overall magnetism of the material.

“The intrinsic magnetic TIs actually have defects in it,” Islam explained. “So for example, manganese can actually go into the sites of antimony where they’re not supposed to be and the way the manganese is going into those sites is random.”

This random manganese site-mixing creates magnetic defects in the intrinsic magnetic TIs. The team found that the same interactions between defects in the dilute materials also occur in the intrinsic materials (i.e. MnSb₂Te₄). The magnetic ground state of the intrinsic magnetic TIs can be either ferromagnetic or antiferromagnetic, and the team now understands how magnetic defects control this behavior.

“We determined the interactions between defects in the dilute case and realized that these interactions are transferable to the intrinsic case,” said McQueeney. “By doing so, we conclude that defects control the magnetic order for both families.”

The study is published in the journal Advanced Materials.

More information: Farhan Islam et al, Role of Magnetic Defects in Tuning Ground States of Magnetic Topological Insulators, Advanced Materials (2023). DOI: 10.1002/adma.202209951

Journal information: Advanced Materials 

Provided by Ames National Laboratory 

We’ve all played at least once with a spring toy, but did you know that light can be shaped like a spring too?

An international team of researchers, led by Marco Piccardo, a former researcher at the Italian Institute of Technology (IIT) and now a Professor in the Physics Department of Técnico Lisboa and Principal Investigator at the Engineering Institute for Microsystems and Nanotechnologies (INESC MN), has harnessed ultrafast optics and structured light to synthesize in the laboratory a new family of spatiotemporal light beams, known as light springs.

The research was done in collaboration between IIT, Politecnico di Milano and Técnico Lisboa. The discovery has a disruptive potential for applications in photonics with complex light, such as time-resolved microscopy (useful, for example, to produce movies depicting the motion of molecules and viruses), laser-plasma acceleration and free-space (e.g., in the atmosphere) optical communications.

The research is published in Nature Photonics.

In ultrafast optics, it is possible to shorten or elongate the duration of extremely short optical pulses—down to a few femtoseconds, or thousandths of billionths of a second—or even produce complex pulses, by means of a technique known as pulse shaping. A central idea to this principle is that short laser pulses are composed of a large range of colors.

Scientists separate a pulse into its constituent colors, which are then separately manipulated and recombined, resulting in a new shape of the laser pulse. While pulse shaping allows to manipulate the time profile of a pulse, there is another set of techniques—known as wavefront shaping—allowing to give light a spatial structure. Designers of light have learnt how to combine these two methods to simultaneously shape light in space and time, bridging ultrafast optics and structured light for entirely new spatiotemporal applications.

A paradigm shift in spatiotemporal light shaping

Reporting now in Nature Photonics, Piccardo and his collaborators have introduced a paradigm shift in spatiotemporal light shaping. Unlike conventional shapers that separate different colors along a colorful strip, now the researchers used a special type of diffraction grating with circular symmetry, to create a round rainbow of colors.

This is an experiment that everyone can try at home: by shining a flashlight on an old CD-ROM and taking a picture with the phone camera, a round rainbow will be captured. Now, replace the flashlight with an ultrashort laser pulse and the CD-ROM with a microstructured diffractive device fabricated in the nanofabrication cleanroom and you are halfway through the experiment. The second part of the experiment consists in using advanced holograms to structure the many colors of light into different optical vortices shaped like a corkscrew.

“This results in a novel family of spatiotemporal light beams, which evolve on an ultrashort femtosecond timescale with a twisted and widely tailorable light structure,” said Marco Piccardo. “It opens unprecedented design capabilities in photonics, with many spectral and structural components to address.”

The broadband nature of these new light beams poses new challenges for their characterization, which the team has overcome by developing a powerful reconstruction technique, called hyperspectral holography, providing the complete tomography of the complex space-time structures.

“Our technique, which combines holography with Fourier transform spectroscopy, enables full characterization of the spatiotemporal profile of complex beams, enabling radically novel applications in the study of light-matter interactions,” said Giulio Cerullo, a Professor in Politecnico di Milano and co-author of the study.

The team showcased the unprecedented control enabled by their space-time shaper by tailoring many properties of the light springs. A beautiful demonstration shows two of such springs dancing together in space and time.

“We have found extremely interesting physics using these beams, which could lead us to a whole new generation of compact accelerators and light sources in plasma. This technique is very exciting because it promises to bring these theoretical concepts to the laboratory and to trigger major advance in laser-plasma physics,” said Jorge Vieira, a Professor in Técnico Lisboa and co-author of the study.

Now that it is finally possible to synthesize these light springs with complete freedom in the laboratory, the next natural step will be to bring them in laser-plasma experiments.

“This is a very challenging goal but the nanophotonic fabrication capabilities of INESC MN in Lisbon and the excellent plasma research groups of Técnico represent an ideal ecosystem to pursue this ambitious research,” said Piccardo. “Combining these advanced space-time beams with intense nonlinear laser-matter interactions could have important fundamental and technological implications. “

More information: Marco Piccardo et al, Broadband control of topological–spectral correlations in space–time beams, Nature Photonics (2023). DOI: 10.1038/s41566-023-01223-y

Journal information: Nature Photonics 

Provided by Técnico Lisboa