How it works Researchers use twisted surfaces to manipulate mechanical waves, enabling new technologies for imaging, electronics and sensors. (Courtesy: A Alù)
By simply placing two identical elastic metasurfaces atop each other and then rotating them relative to each other, the topology of the elastic waves dispersing through the resulting stacked structure can be changed – from elliptic to hyperbolic. This new control technique, from physicists at the CUNY Advanced Science Research Center in the US, works over a broad frequency range and has been dubbed “twistelastics”. It could allow for advanced reconfigurable phononic devices with potential applications in microelectronics, ultrasound sensing and microfluidics.
The researchers, led by Andrea Alù, say they were inspired by the recent advances in “twistronics” and its “profound impact” on electronic and photonic systems. “Our goal in this work was to explore whether similar twist-induced topological phenomena could be harnessed in elastodynamics in which phonons (vibrations of the crystal lattice) play a central role,” says Alù.
In twistelastics, the rotations between layers of identical, elastic engineered surfaces are used to manipulate how mechanical waves travel through the materials. The new approach, say the CUNY researchers, allows them to reconfigure the behaviour of these waves and precisely control them. “This opens the door to new technologies for sensing, communication and signal processing,” says Alù.
From elliptic to hyperbolic
In their work, the researchers used computer simulations to design metasurfaces patterned with micron-sized pillars. When they stacked one such metasurface atop the other and rotated them at different angles, the resulting combined structure changed the way phonons spread. Indeed, their dispersion topology went from elliptic to hyperbolic.
At a specific rotation angle, known as the “magic angle” (just like in twistronics), the waves become highly focused and begin to travel in one direction. This effect could allow for more efficient signal processing, says Alù, with the signals being easier to control over a wide range of frequencies.
The new twistelastic platform offers broadband, reconfigurable, and robust control over phonon propagation,” he tells Physics World. “This may be highly useful for a wide range of application areas, including surface acoustic wave (SAW) technologies, ultrasound imaging and sensing, microfluidic particle manipulation and on-chip phononic signal processing.
New frontiers
Since the twist-induced transitions are topologically protected, again like in twistronics, the system is resilient to fabrication imperfections, meaning it can be miniaturized and integrated into real-world devices, he adds. “We are part of an exciting science and technology centre called ‘New Frontiers of Sound’, of which I am one of the leaders. The goal of this ambitious centre is to develop new acoustic platforms for the above applications enabling disruptive advances for these technologies.”
Looking ahead, the researchers say they are looking into miniaturizing their metasurface design for integration into microelectromechanical systems (MEMS). They will also be studying multi-layer twistelastic architectures to improve how they can control wave propagation and investigating active tuning mechanisms, such as electromechanical actuation, to dynamically control twist angles. “Adding piezoelectric phenomena for further control and coupling to the electromagnetic waves,” is also on the agenda says Alù.
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Crystal structure In the new high-Tc superconductor, lanthanum and scandium atoms constitute the MgB2-type sublattice, while the surrounding hydrogen atoms form two types of cage-like configurations. (Courtesy: Guangtao Liu, Jilin University)
Researchers in China claim to have made the first ever room-temperature superconductor by compressing an alloy of lanthanum-scandium (La-Sc) and the hydrogen-rich material ammonia borane (NH3BH3) together at pressures of 250–260 GPa, observing superconductivity with a maximum onset temperature of 298 K. While these high pressures are akin to those at the centre of the Earth, the work marks a milestone in the field of superconductivity, they say.
Superconductors conduct electricity without resistance and many materials do this when cooled below a certain transition temperature, Tc. In most cases this temperature is very low – for example, solid mercury, the first superconductor to be discovered, has a Tc of 4.2 K. Researchers have therefore been looking for superconductors that operate at higher temperatures – perhaps even at room temperature. Such materials could revolutionize a host of application areas, including increasing the efficiency of electrical generators and transmission lines through lossless electricity transmission. They would also greatly simplify technologies such as MRI, for instance, that rely on the generation or detection of magnetic fields.
Researchers made considerable progress towards this goal in the 1980s and 1990s with the discovery of the “high-temperature” copper oxide superconductors, which have Tc values between 30 and 133 K. Fast-forward to 2015 and the maximum known critical temperature rose even higher thanks to the discovery of a sulphide material, H3S, that has a Tc of 203 K when compressed to pressures of 150 GPa.
This result sparked much interest in solid materials containing hydrogen atoms bonded to other elements and in 2019, the record was broken again, this time by lanthanum decahydride (LaH10), which was found to have a Tc of 250–260 K, albeit again at very high pressures. Then in 2021, researchers observed high-temperature superconductivity in the cerium hydrides, CeH9 and CeH10, which are remarkable because they are stable and boast high-temperature superconductivity at lower pressures (about 80 GPa, or 0.8 million atmospheres) than the other so-called “superhydrides”.
Ternary hydrides
In recent years, researchers have started turning their attention to ternary hydrides – substances that comprise three different atomic species rather than just two. Compared with binary hydrides, ternary hydrides are more structurally complex, which may allow them to have higher Tc values. Indeed, Li2MgH16 has been predicted to exhibit “hot” superconductivity with a Tc of 351–473 K under multimegabar pressures and several other high-Tc hydrides, including MBxHy, MBeH8 and Mg2IrH6-7, have been predicted to be stable under comparatively lower pressures.
In the new work, a team led by physicist Yanming Ma of Jilin University, studied LaSc2H24 – a compound that’s made by doping Sc into the well-known La-H binary system. Ma and colleagues had already predicted in theory – using the crystal structure prediction (CALYPSO) method – that this ternary material should feature a hexagonal P6/mmm symmetry. Introducing Sc into the La-H results in the formation of two novel interlinked H24 and H30 hydrogen clathrate “cages” with the H24 surrounding Sc and the H30 surrounding La.
The researchers predicted that these two novel hydrogen frameworks should produce an exceptionally large hydrogen-derived density of states at the Fermi level (the highest energy level that electrons can occupy in a solid at a temperature of absolute zero), as well as enhancing coupling between electrons and phonons (vibrations of the crystal lattice) in the material, leading to an exceptionally high Tc of up to 316 K at high pressure.
To characterize their material, the researchers placed it in a diamond-anvil cell, a device that generates extreme pressures as it squeezes the sample between two tiny, gem-grade crystals of diamond (one of the hardest substances known) while heating it with a laser. In situ X-ray diffraction experiments revealed that the compound crystallizes into a hexagonal structure, in excellent agreement with the predicted P6/mmm LaSc2H24 structure.
A key piece of experimental evidence for superconductivity in the La-Sc-H ternary system, says co-author Guangtao Liu, came from measurements that repeatedly demonstrated the onset of zero electrical resistance below the Tc.
Another significant proof, Liu adds, is that the Tc decreases monotonically with the application of an external magnetic field in a number of independently synthesized samples. “This behaviour is consistent with the conventional theory of superconductivity since an external magnetic field disrupts Cooper pairs – the charge carriers responsible for the zero-resistance state – thereby suppressing superconductivity.”
“These two main observations demonstrate the superconductivity in our synthesized La-Sc-H compound,” he tells Physics World.
Difficult experiments
The experiments were not easy, Liu recalls. The first six months of attempting to synthesize LaSc2H24 below 200 GPa yielded no obvious Tc enhancement. “We then tried higher pressure and above 250 GPa, we had to manually deposit three precursor layers and ensure that four electrodes (for subsequent conductance measurements) were properly connected to the alloy in an extremely small sample chamber, just 10 to 15 µm in size,” he says. “This required hundreds of painstaking repetitions.”
And that was not all: to synthesize the LaSc2H24, the researchers had to prepare the correct molar ratios of a precursor alloy. The Sc and La elements cannot form a solid solution because of their different atomic radii, so using a normal melting method makes it hard to control this ratio. “After about a year of continuous investigations, we finally used the magnetron sputtering method to obtain films of LaSc2H24 with the molar ratios we wanted,” Liu explains. “During the entire process, most of our experiments failed and we ended up damaging at least 70 pairs of diamonds.”
Sven Friedemann of the University of Bristol, who was not involved in this work, says that the study is “an important step forward” for the field of superconductivity with a new record transition temperature of 295 K. “The new measurements show zero resistance (within resolution) and suppression in magnetic fields, thus strongly suggesting superconductivity,” he comments. “It will be exciting to see future work probing other signatures of superconductivity. The X-ray diffraction measurements could be more comprehensive and leave some room for uncertainty to whether it is indeed the claimed LaSc2H24 structure giving rise to the superconductivity.”
Ma and colleagues say they will continue to study the properties of this compound – and in particular, verify the isotope effect (a signature of conventional superconductors) or measure the superconducting critical current. “We will also try to directly detect the Meissner effect – a key goal for high-temperature superhydride superconductors in general,” says Ma. “Guided by rapidly advancing theoretical predictions, we will also synthesize new multinary superhydrides to achieve better superconducting properties under much lower pressures.”
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International research collaborations will be increasingly led by scientists in China over the coming decade. That is according to a new study by researchers at the University of Chicago, which finds that the power balance in international science has shifted markedly away from the US and towards China over the last 25 years (Proc. Natl. Acad. Sci. 122 e2414893122).
To explore China’s role in global science, the team used a machine-learning model to predict the lead researchers of almost six million scientific papers that involved international collaboration listed by online bibliographic catalogue OpenAlex. The model was trained on author data from 80 000 papers published in high-profile journals that routinely detail author contributions, including team leadership.
The study found that between 2010 and 2012 there were only 4429 scientists from China who were likely to have led China-US collaborations. By 2023, this number had risen to 12714, meaning that the proportion of team leaders affiliated with Chinese institutions had risen from 30% to 45%.
Key areas
If this trend continues, China will hit “leadership parity” with the US in chemistry, materials science and computer science by 2028, with maths, physics and engineering being level by 2031. The analysis also suggests that China will achieve leadership parity with the US in eight “critical technology” areas by 2030, including AI, semiconductors, communications, energy and high-performance computing.
For China-UK partnerships, the model found that equality had already been reached in 2019, while EU and China leadership roles will be on par this year or next. The authors also found that China has been actively training scientists in nations in the “Belt and Road Initiative” which seeks to connect China closer to the world through investments and infrastructure projects.
This, the researchers warn, limits the ability to isolate science done in China. Instead, they suggest that it could inspire a different course of action, with the US and other countries expanding their engagement with the developing world to train a global workforce and accelerate scientific advancements beneficial to their economies.
Connexion
