↩ Accueil
❌

Vue normale

Reçu avant avant-hier

Highest-resolution images ever taken of a single atom reveal new kind of vibrations

✇Physics World
Par :Isabelle Dumé
25 août 2025 à 16:00

Researchers in the US have directly imaged a class of extremely low-energy atomic vibrations called moiré phasons for the first time. In doing so, they proved that these vibrations are not just a theoretical concept, but are in fact the main way that atoms vibrate in certain twisted two-dimensional materials. Such vibrations may play a critical role in heat and charge transport and how quantum phases behave in these materials.

“Phasons had only been predicted by theory until now, and no one had ever directly observed them, or even thought that this was possible,” explains Yichao Zhang of the University of Maryland, who co-led the effort with Pinshane Huang of the University of Illinois at Urbana-Champaign. “Our work opens up an entirely new way of understanding lattice vibrations in 2D quantum materials.”

A second class of moiré phonons

When two sheets of a 2D materials are placed on top of each other and slightly twisted, their atoms form a moiré pattern, or superlattice. This superlattice contains quasi-periodic regions of rotationally aligned regions (denoted AA or AB) separated by a network of stacking faults called solitons.

Materials of this type are also known to possess distinctive vibrational modes known as moiré phonons, which arise from vibrations of the material’s crystal lattice. These modes vary with the twist angle between layers and can change the physical properties of the materials.

In addition to moiré phonons, two-dimensional moiré materials are also predicted to host a second class of vibrational mode known as phasons. However, these phasons had never been directly observed experimentally until now.

Imaging phasons at the picometre scale

In the new work, which is published in Science, the researchers used a powerful microscopy technique called electron ptychography that enabled them to image samples with spatial resolutions as fine as 15 picometres (1 pm = 10-12 m). At this level of precision, explains Zhang, subtle changes in thermally driven atomic vibrations can be detected by analysing the shape and size of individual atoms. “This meant we could map how atoms vibrate across different stacking regions of the moiré superlattice,” she says. “What we found was striking: the vibrations weren’t uniform – atoms showed larger amplitudes in AA-stacked regions and highly anisotropic behaviour at soliton boundaries. These patterns align precisely with theoretical predictions for moiré phasons.”

Coloured dots showing thermal vibrations in a single atom
Good vibrations: The experiment measured thermal vibrations in a single atom. (Courtesy: Yichao Zhang et al.)

Zhang has been studying phonons using electron microscopy for years, but limitations on imaging resolutions had largely restricted her previous studies to nanometre (10-9 m) scales. She recently realized that electron ptychography would resolve atomic vibrations with much higher precision, and therefore detect moiré phasons varying across picometre scales.

She and her colleagues chose to study twisted 2D materials because they can support many exotic electronic phenomena, including superconductivity and correlated insulated states. However, the role of lattice dynamics, including the behaviour of phasons in these structures, remains poorly understood. “The problem,” she explains, “is that phasons are both extremely low in energy and spatially non-uniform, making them undetectable by most experimental techniques. To overcome this, we had to push electron ptychography to its limits and validate our observations through careful modelling and simulations.”

This work opens new possibilities for understanding (and eventually controlling) how vibrations behave in complex 2D systems, she tells Physics World. “Phasons can affect how heat flows, how electrons move, and even how new phases of matter emerge. If we can harness these vibrations, we could design materials with programmable thermal and electronic properties, which would be important for future low-power electronics, quantum computing and nanoscale sensors.”

More broadly, electron ptychography provides a powerful new tool for exploring lattice dynamics in a wide range of advanced materials. The team is now using electron ptychography to study how defects, strain and interfaces affect phason behaviour. These imperfections are common in many real-world materials and devices and can cause their performance to deteriorate significantly. “Ultimately, we hope to capture how phasons respond to external stimuli, like how they evolve with change in temperature or applied fields,” Zhang reveals. “That could give us an even deeper understanding of how they interact with electrons, excitons or other collective excitations in quantum materials.”

The post Highest-resolution images ever taken of a single atom reveal new kind of vibrations appeared first on Physics World.

The Butler-Volmer equation revisited: effect of metal work function

✇Physics World
Par :hayley.liddle@ioppublishing.org
6 août 2025 à 15:22

bv graphic main image

The Butler-Volmer equation is commonly the standard model of electrochemical kinetics.  Typically, the effects of applied voltage on the free energies of activation of the forward and backward reactions are analyzed and used to derive a current-voltage relationship. Traditionally, specific properties of the electrode metal were not considered in this derivation and consequently the resulting expression contained no information on the variation of exchange current density with electrode-material-specific parameters such as work function Φ. In recent papers1,2, Buckley and Leddy revisited the classical derivation of the Butler-Volmer equation to include the effect of the electrode metal.  We considered in detail the complementary relationship of the chemical potential of electrons μe and the Galvani potential φ and so derived expressions for the current-voltage relationship and the exchange current density that include μe The exchange current density j0 appears as an exponential function of Δμe.  Making the approximation Δμe ≈ —FΔΦ yields a linear relationship between ln j0 and Φ. This linear increase in ln j0 with Φ had long been reported3 but had not been explained.  In this webinar, these recent modifications of the Butler-Volmer equation and their consequences will be discussed.

1 K S R Dadallagei, D L Parr IV, J R Coduto, A Lazicki, S DeBie, C D Haas and J Leddy,  J. Electrochem. Soc, 170, 086508 (2023)

2 D N Buckley and J Leddy,  J. Electrochem. Soc, 171, 116503 (2024)

3 S Trasatti,  J. Electroanal. Chem., 39, 163—184 (1972)

D Noel Buckley
D Noel Buckley

D Noel Buckley is professor of physics emeritus at the University of Limerick, Ireland and adjunct professor of chemical and biomolecular engineering at Case Western Reserve University.   He is a fellow and past-president of ECS and has served as an editor of both the Journal of the Electrochemical Society and Electrochemical and Solid State Letters. He has over 50 years of research experience on a range of topics.  His PhD research on oxygen electrochemistry at University College Cork, Ireland was followed by postdoctoral research on high-temperature corrosion at the University of Pennsylvania.  From 1979 to 1996, he worked at Bell Laboratories (Murray Hill, NJ), initially on lithium batteries but principally on III-V semiconductors for electronics and photonics. His research at the University of Limerick has been on semiconductor electrochemistry, stress in electrodeposited nanofilms and electrochemical energy storage, principally vanadium flow batteries in collaboration with Bob Savinell’s group at Case. His recent interest in the theory of electron transfer kinetics arose from collaboration with Johna Leddy at the University of Iowa. He has taught courses in scientific writing since 2006 at the University of Limerick and short courses at several ECS Meetings. He is a recipient of the Heinz Gerischer Award and the ECS Electronics and Photonics Division Award. Recently, he led Poetry Evenings at ECS Meetings in Gothenburg and Montreal.

ECS logo

The post The Butler-Volmer equation revisited: effect of metal work function appeared first on Physics World.

Feynman diagrams provide insight into quasiparticles in solids

✇Physics World
Par :No Author
1 août 2025 à 13:08
Artist's impression of a polaron
Illustration of a polaron The bright sphere is the electron, which is distorting the surrounding lattice. The wavy lines are high-order Feynman diagrams for the electron–phonon interaction. (Courtesy: Ella Maru Studio)

Electron–phonon interactions in a material have been modelled by combining billions of Feynman diagrams. Using a modified form of the Monte Carlo method, Marco Bernardi and colleagues at the California Institute of Technology predicted the behaviour of polarons in certain materials without racking up significant computational costs.

Phonons are quantized collective vibrations of the atoms or molecules in a lattice. When an electron moves through certain solids, it can interact with phonons. This electromagnetic interaction creates a particle-like excitation that comprises a propagating electron surrounded by a cloud of phonons. This quasiparticle excitation is called a polaron.

By lowering the electron’s mobility, while increasing its effective mass, polarons can have a substantial impact on the electronic properties of a variety of materials – including semiconductors and high-temperature superconductors.

However, physicists have struggled to model polarons and it would be extremely helpful for them to represent polarons using Feynman diagrams. These are a mainstay of particle physics, which are used to calculate the probabilities of certain particle interactions taking place. This has been challenging because polarons emerge from a superposition of infinitely many higher-order interactions between electrons and phonons. With each successive order, the complexity of these interactions steadily increases – along with the computational power required to represent them with Feynman diagrams.

Higher-order trouble

Unlike some other interactions, each higher order becomes more and more important in representing the polaron as accurately as possible. As a result, calculations cannot be simplified using standard perturbation theory – where only the first few orders of interaction are required to closely approximate the overall process.

“If you can calculate the lowest order, it’s very likely that you cannot do the second order, and the third order will just be impossible,” Bernardi explains. “The computational cost typically scales prohibitively with interaction order. There are too many diagrams to compute, and the higher-order diagrams are too computationally expensive. It’s basically a nightmare in terms of scaling.”

Bernardi’s team – which also included Yao Luo and Jinsoo Park  – approached the problem with the Monte Carlo method. This involves taking repeated random samples within a space of all possible events contributing to a process, then adding them together. It allows researchers to build up a close approximation of the process, without accounting for every possibility.

The team generated a series of Feynman diagrams spanning the full range of possible electron–phonon interactions. Then, they combined the diagrams to gain precise descriptions of the dynamic and ground-state properties of polarons in real materials.

Statistical noise

One issue with a fully-random Monte Carlo approach is the sign problem, which arises from statistical noise that can emerge as electrons scatter between different energy bands during electron–phonon interactions. Since different bands can contribute positively or negatively to the interaction probabilities represented by Feynman diagrams, these contributions can cancel each other out when added together.

To avoid this, Bernardi’s team adapted the Monte Carlo method to evaluate each band contribution in a structured, non-random way – preventing sign cancellations. In addition, the researchers applied a matrix compression approach. This vastly reduced the size and complexity of the electron–phonon interaction data, without sacrificing accuracy. Altogether, this enabled them to generate billions of diagrams without significant computational costs.

“The clever diagram sampling, sign problem removal, and electron–phonon matrix compression are the three key pieces of the puzzle that have enabled this paradigm shift in the polaron problem,” Bernardi explains.

The trio hopes that its technique will help us understand polaron behaviours. “The method we developed could also help study strong interactions between light and matter, or even provide the blueprint to efficiently add up Feynman diagrams in entirely different physical theories,” Bernardi says. In turn, it could help to provide deeper insights into a variety of effects where polarons contribute – including electrical transport, spectroscopy, and superconductivity.

The research is described in Nature Physics.

The post Feynman diagrams provide insight into quasiparticles in solids appeared first on Physics World.

Hints of a 3D quantum spin liquid revealed by neutron scattering

✇Physics World
Par :No Author
24 juillet 2025 à 12:52

New experimental evidence for a quantum spin liquid – a material with spins that remain in constant fluctuation at extremely low temperatures – has been unveiled by an international team of scientists. The researchers used neutron scattering to reveal photon-like collective spin excitations in a crystal of cerium zirconate.

When most magnetic materials are cooled to nearly absolute zero, their spin magnetic moments will align into an ordered pattern to minimize the system’s energy. Yet in 1973, the future Nobel laureate Philip Anderson proposed an alternative class of magnetic materials in which this low temperature order does not emerge.

Anderson considered the spins of atoms that interact with each other in an antiferromagnetic way. This is when the spin of each atom seeks to point in the opposite direction of its nearest neighbours. If the spins in a lattice are able to adopt this orientation, the lowest energy state is an ordered antiferromagnet with zero overall magnetism.

Geometrical frustration

In a tetrahedral lattice, however, the geometrical arrangement of nearest neighbours means that it is impossible for spins to arrange themselves in this way. This is called frustration, and the result is a material with multiple low-energy spin configurations, which are disordered.

So far, this behaviour has been observed in materials called spin ices – where one of the many possible spin configurations is frozen into place at ultralow temperatures. However, Anderson envisioned that a related class of materials could exist in a more exotic phase that constantly fluctuates between different, equal-energy states, all the way down to absolute zero.

Called quantum spin liquids (QSLs), such materials have evaded experimental confirmation – until now. “They behave like a liquid form of magnetism – without any fixed ordering,” explains team member Silke Bühler-Paschen at Austria’s Vienna University of Technology. “That’s exactly why a real breakthrough in this area has remained elusive for decades.” “We studied cerium zirconate, which forms a three-dimensional network of spins and shows no magnetic ordering, even at temperatures as low as 20 mK.”. This material was chosen because it has a pyrochlore lattice, which is based on corner-sharing tetrahedra.

Collective magnetic excitations

The team looked for collective magnetic excitations that are predicted to exist in QSLs. These excitations are expected to have linear energy–momentum relationships, which is similar to how conventional photons propagate. As a result, these particle-like excitations are called emergent photons.

The team used polarized neutron scattering experiments to search for evidence of emergent photons. When neutrons strike a sample, they can exchange energy and momentum with the lattice. This exchange can involve magnetic excitations in the material and the team used scattering experiments to map-out the energy and momenta of these excitations at temperatures in the 33–50 mK range.

“For the first time, we were able to detect signals that strongly indicate a 3D quantum spin liquid – particularly, the presence of so-called emergent photons,” Bühler-Paschen says. “The discovery of these emergent photons in cerium zirconate is a very strong indication that we have indeed found a QSL.”

As well as providing evidence for Anderson’s idea, the research pave the way for the further exploration of other potential QSLs and their applications. “We plan to conduct further experiments, but from our perspective, cerium zirconate is currently the most convincing candidate for a quantum spin liquid,” Bühler-Paschen says.

The research could have important implications for our understanding of high-temperature superconductivity. In his initial theory, Anderson predicted that QSLs could be precursors to high-temperature superconductors.

The research is described in Nature Physics.

The post Hints of a 3D quantum spin liquid revealed by neutron scattering appeared first on Physics World.

❌