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Inflatable liquid crystalline elastomers create complex colour-changing displays

Elastomers that undergo large spectral shifts in colour when stretched very little have been developed by researchers in the US and South Korea. The material scientists say that these liquid crystalline elastomers with an unusually large Poisson’s ratio could have a variety of uses, from visual displays to smart windows.

Colour can be produced by the absorbance of light by dyes and pigments. Any wavelengths not absorbed reflect back and create the colour that we perceive. But colour can also be produced by nanoscale structures that scatter and reflect light. Indeed, such structural colours represent some of the brightest colours in the natural world.

There has been a lot of research interest in creating structural colours. As well as having the potential to be brighter, they offer other advantages over dyes and pigments. The nanostructures are more robust and longer lasting, and scientists claim they could be more environmentally friendly. They can also be engineered to scatter non-visible light, with reflection of infrared light having potential applications for passive cooling.

One challenge that material scientists have struggled with, however, is creating structural colouration that can change colour. One way to do this is to mechanically deform the material. The problem with this approach, says Shu Yang, an engineer and materials scientist at the University of Pennsylvania, is that you have to stretch it a lot. To shift from red to blue you would need to stretch the typical liquid crystal elastomers used by at least 40%, and in some cases by as much as 70%, she explains.

Now, Yang and her colleagues have managed to create an elastomer that can colour shift from near-infrared to ultraviolet wavelengths when stretched by less than 20%. They describe their work in Nature Materials.

Pressure-activated displays

To create structural colour, the team used chiral nematic liquid crystalline elastomers. When these elastomers are produced, a chemical dopant encourages the molecules to form helixes. It is these helixes that create the structural colour, with the wavelengths reflected dependent upon their dimensions. If the material is stretched, the helixes compress and the material’s colour changes.

Yang tells Physics World that when one of her colleagues was playing around with the production of these liquid crystalline elastomers to improve the uniformity of the helixes and therefore the colour, they managed to produce a very soft material. It turned out that the elastomers the team created had an unusually large Poisson’s ratio. This means that when you stretch them in one direction, they compress much more than you would expect in the other plane. “That is why we are able to have a very large wavelength change at a small strain,” Yang says.

Next, the researchers used the elastomers to create pneumatically actuated displays. They 3D printed a plastic base containing circular cavities connected by air channels, and then sealed a layer of their new elastomer on the top, creating a series of colour “pixels”. Pumping air into the channels inflates the elastomer membranes, causing them to stretch and changing the colours that they reflect. The colours can be controlled by varying the pressure and the size of the cavities in the 3D-printed base.

Pixelated structural colour platform — Creating structural colour: The pixelated platform includes a base with air channels, a supporting layer and a pneumatically actuated membrane of chiral nematic main-chain liquid crystalline elastomer (MCLCE). (Courtesy: *Nat. Mater.* 10.1038/s41563-021-01075-3)

The researchers demonstrated that an increase in pressure of just 9.6 kPa was enough to switch the wavelengths reflected by the pixels from near-infrared through red, green and blue to ultraviolet. They also showed that by using multiple air channels to activate groups of pixels, or individual pixels, they could create displays such as number countdowns.

Fast-switching structural colour could be used in low power video displays

As well as displays, the researchers suggest the material could be used for colour-changing soft robots and cryptic colouration, such as disruptive imaging and countershading. Yang says that because the material changes colour when subjected to a very small deformation, it could be used to create temperature, pressure or mechanical sensors.

Yang also suggests that the material may be suitable for building smart windows that reflect more infrared light as the ambient temperature rises. “That’s one of the interests, whether we can use the heat induced expansion of the air,” she tells Physics World.

Electric fields cause square droplets and liquid lattices to appear in oil mixtures

When a simple system of two liquids is driven out of equilibrium, it can generate a far more diverse array of structures than previously thought, new experiments have shown. The discovery was made by Jaakko Timonen, Nikos Kyriakopoulos and colleagues at Aalto University in Finland, who identified structures including filaments, lattices, and square-shaped droplets, when applying electric fields to thin sheets of different combinations of conductive, polarizable oils.

If two liquids are brought together, they will usually settle into the steady state of thermodynamic equilibrium. Depending on their miscibility and relative densities, they can form uniform mixtures (such as water and ethanol), separated layers (such as oil floating on top of water) – or emulsions such as mayonnaise. Far more interesting behaviour can emerge if these systems are driven out of thermodynamic equilibrium by the application of external forces such as an electric field.

Non-equilibrium behaviours underly a wide array of fascinating phenomena: from the emergence of unexpected patterns within materials, to collective motions of self-propelling microbes. As a result, they are of immense interest across different numerous fields: from physics to biology. With existing theories, however, it can be incredibly challenging to predict the nature of the complex states that emerge.

Polarized oils

To shed new light on two liquid systems, the Aalto team designed a new set of experiments involving several different oils: each with different densities and electrical conductivities; and which become electrically polarized to different degrees when electric fields are applied.

When a thin sheet of any combination of two of the oils was subjected to an electric field, charges would build up at the interface separating them. Through the effect of electrohydrodynamic shearing, these charges were set into motion, driving the entire system out of equilibrium. The result was a wholly unexpected diversity of patterns and shapes.

At low levels of shearing, the researchers identified symmetry breaking at the interface between the oils, creating 1D patterns of corrugation. When shearing was slightly stronger, topological changes at the interface gave rise to microfilaments that rolled and rotated about their axes, and which could connect to form intricate networks.

Strong shearing

Alternatively, the oils could also form 2D lattices – which are virtually unheard of in liquids. When shearing was at its strongest, the system transitioned into square, hexagonal, and doughnut-shaped droplets, which could actively propel themselves forwards.

Electric fields cause square droplets and liquid lattices to appear in oil mixtures

Timonen’s team hope that their discoveries could soon enable researchers to create temporary droplets, filaments, and lattices with well-defined sizes, which could be turned on and off by an applied voltage. If achieved, this could lead to many exciting applications: including controllable optical devices, and the ability to mimic the dynamics of swimming microbes, such as bacteria and algae.

The research is described in Science Advances.

EEG test could enable early diagnosis of Alzheimer’s disease

A new memory assessment technique could pave the way for earlier diagnosis of Alzheimer’s disease, the underlying cause of around 60% of dementia cases. The approach uses electroencephalography (EEG) to measure brain activity while participants watch flashing images on a computer screen.

Current diagnostic tests for Alzheimer’s disease are not effective during its early stages, meaning that Alzheimer’s is typically only diagnosed late in the disease process. As such, there’s a real need for early diagnosis tools that could enable the timely initiation of lifestyle interventions to slow the rate of cognitive decline. Such a tool could also help drug development, through earlier and more accurate identification of dementia patients in clinical trials.

The research team developing the new EEG-based test, led by cognitive neuroscientist George Stothart from the University of Bath, think that it could lower the age of diagnosis by up to five years in the near future. In the longer-term, the technique – known as Fastball EEG – has potential to improve this further.

EEG works by using multiple electrodes placed on the scalp (in an EEG cap) to record the brain’s electrical activity. Fastball uses a method called fast periodic visual stimulation (FPVS), which measures brain signals as the subject views a series of rapidly presented images, a few of which are repeated at slower intervals. The technique is highly effective at picking up small, subtle changes in brain waves that occur when a person remembers an image.

The big advantage of Fastball EEG is that it is completely passive. The person performing the test is not given any instructions prior to the task, which is important as dementia patients may struggle to follow complex instructions, and is not asked to reflect on, respond to or remember any items. The technique is also low-cost, non-invasive and uses technology that’s already available in hospitals.

“Fastball offers a genuinely novel way of measuring how our brain is functioning,” Stothart explains in a press statement. “The person being assessed doesn’t need to understand the test, or even respond, they simply watch a screen of flashing images and by the way we manipulate the images that appear we can learn an enormous amount about what their brain is, or is not, able to do.”

Fastball trials

The FPVS procedure induces two discrete frequency responses in the EEG, which reflect the participant’s periodic neural responses to the stimuli. The first reflects visual processing at the image presentation frequency. The second mirrors the brain’s response to previously seen images and reflects the patient’s recognition memory. Analysing the EEG spectrum at this second, slower frequency can quantify the patient’s memory response.

Fastball EEG data analysis — Data analysis: George Stothart examines the Fastball EEG data. (Courtesy: Nic Delves Broughton/University of Bath)

In a study reported in Brain, Stothart and collaborators tested Fastball EEG in 20 patients with Alzheimer’s disease, 20 healthy older adults and 20 healthy younger adults. Participants performed the Fastball task, which took less than 3 min, under three conditions: recognition, repetition and control. In the recognition condition, eight images were viewed beforehand, and then repeatedly shown within a stream of unique previously-unseen images. In the repetition condition, the eight images were not seen in advance but were repeated during the Fastball task. In the control condition, subjects only viewed a stream of novel images.

For both the recognition and repetition conditions, Fastball EEG detected significantly impaired recognition memory in Alzheimer’s disease patients compared with healthy older control subjects. There were no differences between the two groups under the control condition, where image recognition was not included. The Fastball test could also discriminate Alzheimer’s disease patients from healthy older adult controls, with an accuracy of 86%. No significant performance differences were seen between older and younger healthy controls.

After the Fastball task, participants completed a forced-choice task, in which they had to identify a previously seen image from two alternatives. Here, the researchers observed little difference between Alzheimer’s disease patients and controls, suggesting that Fastball was more sensitive to memory performance than this behavioural recognition test.

The researchers conclude that this new method for measuring visual recognition memory is sensitive to changes in recognition memory processes in Alzheimer’s disease that would be missed by behavioural testing alone. “The tests we currently use to diagnose Alzheimer’s miss the first 20 years of the disease, which means we are missing huge opportunities to help people,” says Stothart. “Ultimately, the Holy Grail of a tool like this would be a dementia screening tool used in middle age for everyone, regardless of symptoms, in the same way we test for high blood pressure. We are a long way from that, but this is a step towards that goal.”

The team has now extended the approach into earlier stages of dementia, examining subjects with mild cognitive impairment, which for some people (although not all) is a precursor to Alzheimer’s disease. “We are conducting a longitudinal study of patients with mild cognitive impairment using an expanded battery of Fastball tasks designed to capture a range of cognitive functions,” Stothart tells Physics World.

A demon of a puzzle

Across

1 A limo cat, writhing, is concerned with the smallest matters (8)
5 Robotic prostheses created by bishop from charged integrated circuit with sulphur (7)
9 Trendy machismo, in a tizzy, has energetic focus (14)
12 A Copernican revolution? That’s one in the eye, archdeacon (4)
13 A-level physics comes in form that gives Saturn its place (5)
14 In taking the last of the Rolos, craving leads to rush of power (5)
15 Mr Astaire, doing the twist, blows at a high altitude (9)
16 TV’s O’Connor went around coral island to find origins of radiation (6)
18 Wrapped up man was Marie’s relationship to Ève and Irène (5)
19 Partial fan? British lady adores dear Einstein, initially (5)
20 Maxwell’s establishment part of lovemaking scandal (5)
23 Imperial rival encountered the head of Richard (6)
26 Stiff card is material for passing through grating (9)
27 Satyr, aimless, is skilled outside of the sciences (5)
28 Power supply ultimately charges keen jacuzzi yoga team the wrong way (5)
29 Student paper caught in vortex, amazingly (4)
30 Strongmen, ergo a distorted set of radiographs (14)
31 Saturn’s fourth visitor? SI unit gained when gambling palace loses nothing (7)
32 Maltreats, sounds like the young lady is taking them to court, stupidly (8)

Down

2 Physical quantity locates ancient city in clement clime (11)
3 Old US measure of work is a newly formed gamer term (9)
4 Fossil-fuel preserver found in confused eco store (8)
6 Venerated, without origin, acquired charge (7)

7 Nice rum scrambles one type of code (7)
8 Ideal from Carnot is a penny-farthing (5)
10 Not the first law breaker? At the most, a healthy fiend (8,5)
11 Worshipping as a god has steamy action for getting rid of unwanted liquid (13)
17 From Tesla’s birth state, but later on? You, we hear, go to Prague football team with force (11)
21 Entropy always does this! Clothes before you iron them (9)
22 Ordered structures? Weep, heavenly bodies, as right becomes left (8)
24 League tables wrap can in trashy newspapers (7)
25 Sob, love, for short information? Low-temperature provider (7)
27 Keck greeting fractures a halo (5)

Answers will be published in the October issue of Physics World. Please note that this crossword is just for fun; there are no prizes.

Golden eclipse wins Astronomy Photographer of the Year, dancing atoms and molecules, tactile illusions

Chinese photographer Shuchang Dong has beaten thousands of amateur and professional photographers from around the world to win the 2021 Astronomy Photographer of the Year. The award – now in its 13th year – is run by the UK’s Royal Observatory Greenwich in association with insurer Liberty Specialty Markets and BBC Sky at Night Magazine. Dong’s image – The Golden Ring – depicts the annular solar eclipse that occurred on 21 June 2020 and was taken in Ali in Tibet using a Fujifilm XT-4 camera.

As well as winning the £10,000 top prize, Dong’s image will be on display along with other selected pictures at an exhibition at the observatory that opened on 18 September. The competition received over 4500 entries from 75 countries. You can find the other entries here.

From photography to dance, the artist Geraldine Cox has teamed-up with physicists at the UK’s Imperial College and professional dancers to create Elemental Dances, which is a series of six educational videos that explore the nature of atoms and molecules through dance. Aimed at children age 7–11, the videos are 20–30 min long and each one begins with a physicist giving a brief explanation of a physical phenomenon such as Brownian motion. Then the children are encouraged to explore the motion involved in the effect by dancing. Other topics covered include atomic spectra and the vibrations and rotations of a water molecule.

The videos (see trailer above) have already been used in some primary schools and teacher Paul Tyler says, “The content is very well explained and the range of scientists delivering the explanations is brilliant. Dance is such a clever way of letting children explore the different concepts and it is wonderfully delivered by the dancers.”

You can watch all the videos here.

Earlier this year, Cox was a guest on the Physics World Weekly podcast and talked about how she draws on her background in physics to create pieces inspired by the patterns of nature.

Art sometimes involves creating an illusion, be it visual or in sound. But did you know that there are illusions that fool our sense of touch? Researchers in Switzerland have shown that our tactile perception of the frequency at which a surface is vibrating can be altered by changing the amplitude of the vibration.

Vibrational metamerism

Human subjects were asked to compare two vibrations and say which is at the higher frequency. Daniel Huber and colleagues found that their ability to do so was affected by the relative amplitudes of the two vibrations. Indeed, they found that at certain amplitudes, the subjects believed that vibrations at two different frequencies were at the same frequency – an effect called metamerism. The team performed the same experiment with mice as subjects and saw similar results.

Huber and colleagues say that their observations could be useful for creating music for people who have trouble hearing and rely on feeling vibrations. Indeed, the team now plans to work with deaf volunteers and musicians to explore that avenue. Their research is described in Nature Communications.

Amateur astronomers capture flash from asteroid impacting Jupiter

Five amateur astronomers from South America and Europe have captured a burst of light on Jupiter that was the result of an asteroid crashing into the planet’s atmosphere. It is thought that the flash on 13 September – known as a meteor “bolide” – may have been created by a body tens of metres across. It is only the seventh time in history that observers have recorded an impact flash on the gas giant.

José Luis Pereira from Brazil is among those to image the impact. On the evening of the 13 September he was recording videos of the planet with a 275 mm-aperture Newtonian telescope when he spotted a “glow” in one of the captures. “At the time I didn’t give it much importance because I thought it was something related to the capture parameters due to bad weather conditions,” he told Physics World.

I was extremely emotional as it’s something I’ve wanted to discover for many years
José Luis Pereira

Pereira nevertheless ran the data through software that is designed to look for impact flashes on Jupiter, which alerted him to a possible event in one of the videos. Pereira subsequently got in touch with fellow amateur astronomer Marc Delcroix, who leads the development of the DeTeCt program, who was able to confirm his finding. “I was extremely emotional as it’s something I’ve wanted to discover for many years,” says Pereira. “It doesn’t even feel like it’s real.”

Impact zone

Delcroix adds that as well as the five observers who independently discovered the impact, another four also uncovered signs of it in their data after news of the event got out. The detection of the bolide on the Solar System’s largest planet highlights the growing success of regular, international, observing campaigns by amateur astronomers. It also marks the second time that DeTeCt has been involved in identifying a fireball on Jupiter – the other being the discovery, by a US-based astronomer, of an impact flash in 2019.

Delcroix says some 140 observers currently send him their results from using DeTeCt, with the number of users growing appreciably after this latest event. “My hope is to have a more generalised usage of DeTeCt to maximise the number of impacts detected and get a more robust impact frequency estimation on Jupiter,” he says.

As well as aiming to increase the number of astronomers involved in the DeTeCt project, Delcroix hopes that observers will use the software to expand the hunt for meteor fireballs on another of the gas giants, Saturn. If that search proves fruitful, we may soon see – in addition to the seven impact events so far detected on Jupiter – the first discovery of a bolide in the Saturnian skies.

Impact studies can provide important data for researchers trying to understand the nature of the more remote parts of our planetary neighbourhood.

“[Observing impacts] tells us something about the present-day population of small bodies in the outer Solar System, and reveals the dynamic nature of these systems,” says Leigh Fletcher, a planetary scientist at the University of Leicester. “If we see larger bodies impacting, this provides an excellent opportunity to study the atmospheric response to the energy and chemistry of these impactors.”

Ultralow-frequency neuromodulation safely relieves chronic pain

Chronic pain, classified as persistent pain that lasts longer than three to six months, remains an area of considerable unmet medical need. A new treatment that uses electrodes to deliver alternating pulses of ultralow-frequency (ULF) current could help address this need. In a pilot trial, the treatment improved pain ratings by as much as 90% after 15 days of use. Unlike existing clinical neuromodulation techniques, this ULF approach avoids tissue damage and other side effects.

The research team, headed up at Kings College London, used experimental, theoretical and clinical approaches to investigate the potential of the new ULF neuromodulation technique. In research described in Science Translational Medicine, the team examined whether ULF current waveforms can effectively impede the conduction of sensory signals in laboratory rats and reduce chronic pain in patients.

Neuromodulation, the use of electrical currents to block the transmission of pain signals between neurons, has been employed as a non-pharmacological treatment for chronic pain for decades. Existing technologies, however, can be invasive, have limited efficacy and cause side effects.

Spinal cord stimulation, for example, utilizes an implanted device to produce pulsed electrical signals in the region of the spinal cord. The treatment has had limited clinical success, only working for some patients and alleviating pain for short periods of time. It can also cause paresthesia, a sensation of numbness, tingling or burning. Previous research has also explored the application of direct current (DC), which can effectively block the conduction of action potentials (which transmit pain signals), but leads to tissue damage and electrode degradation if maintained.

In this latest work, Stephen McMahon from the Wolfson Centre for Age-Related Diseases and collaborators developed a form of ULF biphasic current. Because the slowly cycling current waveform has a period of more than 10 s, far longer than the millisecond time constant of neuronal action potentials, it effectively mimics DC conditions, but with alternating polarity that avoids potential tissue or electrode damage. Using computational modelling, they determined that the mechanism of immediate conduction block at the plateau phases of the ULF waveform was the same as that produced by DC.

In anesthetized rats, this waveform produced a rapidly developing and completely reversible conduction block in more than 85% of spinal sensory nerve fibres excited by peripheral stimulation. Sustained ULF currents at lower amplitudes led to a slower onset but reversible block. In separate experiments, the researchers verified that ULF currents blocked sensory neuron ectopic activity, an important driver of neuropathic pain, in rats with spinal nerve injury.

Clinical evaluation

The team tested the efficacy of epidural ULF therapy in 20 patients with chronic lower back pain. Eighteen patients received the full treatment during five clinical visits over 15 days, with two withdrawing early due to surgical site infections. The therapy was well tolerated, with none of the patients experiencing muscle weakness or paresthesia.

The researchers assessed pain using the visual analogue scale (VAS), in which patients report their pain level as a distance along a 100 mm line, ranging from no pain at 0 mm to severe pain at the far end. Epidural ULF significantly reduced the patients’ pain ratings, from a mean score of 74.6±1.6 mm at screening, to 15.7±4.1 mm at day seven, and 7.6±3.1 mm at day 15. These changes represent 79% and 90% reduction in pain ratings, respectively. One week after the treatment finished, pain had begun to increase again, ultimately reverting to original values.

Study participants reported that pain started to reduce within 2 to 12 hr of the initial treatment, and increased again within one day of ULF cessation. During the treatment time, the subjects found it easier to take walks and perform other types of physical activity. Seven of the 19 patients who received at least seven days of treatment achieved 100% back pain relief, with an additional 10 achieving at least an 80% improvement, and another achieving 50% pain relief.

Eleven participants also suffered from leg pain, with a VAS score of 69.5±5.6 mm at screening. This score reduced to 5.1±2.0 mm on the final day of treatment. Six patients achieved 100% pain relief, an additional four achieved 80% or greater, and one achieved 50% pain relief.

The researchers conclude that ULF could provide an effective alternative treatment for chronic pain, noting that their results warrant longer studies to define the technique’s long-term safety and further interrogate its mechanisms of action.

How the US keeps its nuclear secrets and what it reveals, double anonymous peer review boosts inclusion

In this episode of the Physics World Weekly podcast the historian Alex Wellerstein explains how the US has kept its nuclear secrets since the Second World War. The author of Restricted Data: the History of Nuclear Secrecy in the United States, he explains how the country has walked the fine line between trying to prevent rogue actors from developing weapons and encouraging the peaceful use of nuclear technologies.

Also in this week’s podcast is Kim Eggleton, who is research integrity and inclusion manager at IOP Publishing. She explains why the scholarly publisher is offering double anonymous peer review on its journals – whereby the identity of the authors is hidden from the reviewers and vice versa.

Alex Wellerstein was interviewed by Physics World’s Margaret Harris, who has reviewed Restricted Data in the September 2021 issue of the magazine.

Car passengers could soon listen to personalized audio using a new acoustic algorithm

Multiple occupants of a car cabin could soon listen to different audio programmes without the need for headphones, thanks to an acoustic algorithm developed by researchers at the carmaker Stellantis and France’s Le Mans University. Lucas Vindrola colleagues designed their system to smartly adapt to changing seat positions, allowing two listeners sitting next to each other to hear entirely different sounds, while maintaining audio quality.

Personalized sound zones (PSZs) have been the subject of research for over 20 years. They allow multiple listeners sharing the same space to hear different sounds, without the need for headphones or insulating panels. The technology works by first filtering input audio signals using a specialized algorithm, then playing them simultaneously through strategically-placed arrays of loudspeakers.

This filtering generates specially tuned acoustic paths called transfer functions, which create specific patterns of constructive and destructive interference. Contained within these patterns are “bright” zones personalized to each listener, where their desired audio programme can be clearly heard; surrounded by “dark” zones, where the intensity of that particular signal is greatly reduced.

Changing acoustic environment

PSZs are of great interest to the auto industry, with the aim of allowing each passenger in a car cabin to listen to different audio programmes. However, this has proven extremely challenging because factors such as seating positions, car temperature, and passenger numbers can vary widely between journeys. Within this constantly changing acoustic environment, it can be incredibly difficult for algorithms to adapt transfer functions accordingly – potentially blurring the boundaries of each passenger’s PSZ.

To address this problem, Vindrola’s team created a new algorithm, capable of adapting a car cabin’s PSZ system when seating positions are changed. Using an array of microphones, their system first monitors the characteristics of the sound emitted by an array of headrest-mounted loudspeakers in real time. From these data, the algorithm adapts the system’s filters whenever seating positions change; adjusting the position of each passenger’s PSZ accordingly.

In their experiments, the team evaluated their algorithm’s performance in a system of two front car seats. The acoustic frequencies they tested were between 100–1000 Hz, which range from bass notes to human speech. Their study focused on several key aspects of improving PSZ technology, including the quality of separation between different zones, the sensitivity of the system to external noise; and the possibility of reducing the number of microphones used to monitor the sound.

Even when one seat was moved forwards by 15 cm, while the other stayed fixed, the algorithm could reliably maintain a 30 dB difference between their two PSZs. This is similar to the difference between whispering, and normal conversation. In the future, the team hopes to extend their algorithm to accommodate frequencies as high as 10 kHz; and may adapt its capabilities to operating in other highly variable acoustic environments, including homes and museums.

The research is described in The Journal of the Acoustical Society of America.

Ferroelectricity goes asymmetric

A team of researchers from the University of North Florida’s Atomic-LEGO laboratory together with researchers at the University of Illinois are the first to have observed asymmetric ferroelectricity in engineered crystals made from oxide heterostructures. The effect could be used to design nanostructured materials with tailored electronic properties.

Ferroelectricity was discovered a hundred years ago and occurs in some naturally occurring crystals. It is now exploited in a wide range of technology applications, including digital information storage and neuromorphic computing.

Ferroelectric materials have permanent electric dipole moments in the same way that their ferromagnetic cousins have permanent magnetic dipoles. The advantage of ferroelectrics is that their dipole moments can be oriented using electric fields, which are much easier to create than the magnetic fields used to manipulate ferromagnetic materials.

Ferroelectrics usually have two stable states with two equal and opposite electric polarizations. A team of researchers led by University of North Florida’s Maitri Warusawithana has now discovered a completely new phenomenon, however, in which there are two unequal stable polarization states. They found this asymmetric ferroelectricity, as they have dubbed it, in engineered crystals made from oxide heterostructures with three atomically-thin molecular components stacked on top of each other.

Breaking inversion symmetry

The researchers observed the new effect by arranging the stacking order of the molecular layers in a way that breaks inversion symmetry. While the high-temperature crystal structure of any naturally occurring ferroelectric preserves inversion symmetry – the coordinates of the crystal lattice are unchanged when reflected over a symmetry centre – the new engineered crystals break inversion symmetry at all temperatures.

“Breaking inversion symmetry is a necessary condition for electronic polarization,” Warusawithana tells Physics World. “We find that these crystals, by-design, are polarized even at high temperatures. As the temperature is lowered, our electrical measurements reveal that they display an unusual bistable response with two unequal polarization states – an asymmetric response governed by their built-in lack of symmetry at high temperatures.”

asymmetric ferroelectricity — Built-in asymmetric distortions in the crystal leads to an asymmetric ferroelectric response. Courtesy: M Warusawithana

Symmetry is a very important concept in physics and is a fundamental way of expressing its laws since it is related to the conservation of quantities such as energy, momentum and charge. Symmetry breaking is equally as important though and can make a system more complex. An everyday example of symmetry breaking is water, which looks the same in all directions in its liquid state but the same in only six directions when it undergoes a phase transition and becomes a snowflake.

In their experiments, Warusawithana and colleagues introduced strain into a “superlattice” made of stacked dielectric and ferroelectric titanate phases of CaTiO₃ (CTO), SrTiO₃ (STO), and BaTiO₃ (BTO). In their bulk form these materials all have different lattice constants (the physical dimensions of unit cells in a crystal lattice) and the sequence in which they are stacked controls the symmetry of the strain field along the stacking direction. This leads to an asymmetric energy versus electrical polarization relation.

A different class of material

The researchers monitored the strain in their superlattice using X-ray diffraction and found that it causes the different phases in the system to be clamped in-plane to the lattice constant of an underlying substrate. This produces changes to the out-of-plane lattice constant of each constituent phase, which is what creates a broken inversion symmetry in the stacking direction, they explain. Indeed, both the degree and direction of the polarization asymmetry in these artificial crystals can be tuned by the stacking architecture of the molecular components.

“The new asymmetric ferroelectricity that we have observed describes a different class of material, which may have unexpected applications,” they say. “We hope this work will trigger further experimental and theoretical investigations that will not only lead to a quantum understanding of this asymmetric state but also harness the potential of symmetry breaking at the atomic scale to obtain nanostructured materials with tailored electronic properties by design.”

The new discovery is detailed in Physical Review B.

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