Two distinctive features of materials known as quantum double-exchange ferromagnets are purely due to quantum spin effects and multiorbital physics, with no need for the lattice vibrations previously invoked to explain them. This theoretical result could lead to new insights into these technologically important materials, as it suggests that some of their properties may arise from interactions hitherto regarded as less important.
Quantum double-exchange ferromagnets have interested scientists since the late 1980s, when physicists led by Albert Fert and Peter Grünberg found that their electrical resistance depends strongly on the magnitude of an external magnetic field. This phenomenon is known as giant magnetoresistance (GMR), and its discovery led to an enormous increase in the storage capacity of modern hard-disk drives, which incorporate GMR structures into their magnetic field sensors. It also led, in 2007, to a Nobel Prize for Fert and Grünberg.
The key question, Herbrych continues, is how Coulomb interactions between many individual electrons lead to the electron spins in these ferromagnets becoming aligned. “Physicists broadly distinguish two mechanisms,” he explains. “For insulating ferromagnets, the Goodenough-Kanamori rules (based on electron shell occupancy and geometrical arguments) can predict spin alignment. For metallic ferromagnets, the double-exchange mechanism is more appropriate.”
In this latter case, Herbrych explains, the electrons’ motion and the alignment of their spins are intrinsically linked, and the electrons often occupy multiple orbitals. This means they need to be modelled in a fundamentally different way.
The approach Herbrych and his colleagues took, which they describe in Rep. Prog. Phys., was conceptually simple, using a basic yet realistic model of interacting electrons to predict the quantum behaviour of electron spins. “In quantum mechanics, ‘simple’ can quickly become complex, however,” Herbrych notes. “Materials in which the double-exchange mechanism dominates typically exhibit multiorbital behaviour, as mentioned. A minimal model must therefore include electron mobility (or ‘itinerancy’), Coulomb interactions and orbital degrees of freedom.”
Two distinctive features
Herbrych and colleagues identified the two-orbital Hubbard-Kanamori model and the Kondo lattice model with interactions as fitting these requirements. They then used these models to explore two distinctive features of quantum double-exchange ferromagnets.
Both features involve magnons, which are collective oscillations of the materials’ spin magnetic moments. In basic “toy” models of ferromagnets, magnons exhibit a well-defined energy-momentum correspondence known as the dispersion relation. Quantum double-exchange ferromagnets, however, experience a phenomenon known as magnon mode softening: at short wavelengths, their magnons become nearly dispersionless, or momentum independent. “This implies that there are fundamental differences between long- and short-distance spin dynamics,” Herbrych says. “Magnons can travel over long distances but appear localized at short scales.”
The second distinctive feature is called magnon damping. This occurs when magnons lose coherence, meaning that the standard picture of spin flips propagating through the material’s lattice breaks down. “It was previously thought that Jahn-Teller phonons (lattice vibrations) were responsible for these features, and that a classical spin model with phonons would do, but our work challenges this view,” says Herbrych. “We show that these phenomena can arise purely from quantum spin effects and multiorbital physics, without requiring lattice vibrations.”
This is, he tells Physics World, “a remarkable result” as it suggests that some experimental features of quantum double-exchange ferromagnets may arise from interactions previously considered secondary.
Limitations and extensions
The researchers’ present work is restricted to one dimension, and they acknowledge that extending it to two or three dimensions will be a challenge. “Still, our approach offers a conceptual framework that can be approximately extended to higher dimensions,” Herbrych says. “The results not only provide insights into the physics of strongly correlated systems, but also into the interplay of competing phases, such as ferromagnetism, orbital order and superconductivity, observed in these materials.”
President Donald Trump’s budget would kill off tax credits for wind and solar, raising costs for new clean energy projects and blowing up valuable investment in those already in the pipeline.
5 June:I am somewhat relieved Professor Born accepted my request for leave at short notice. The hay fever in Göttingen seems worse this year than last when I returned from Copenhagen. Even when not coughing, sneezing or stemming tears from my eyes, I am barely able to string two thoughts together. My thinking jumps from place to place with no sense of continuity, place or direction. I leave for Helgoland immediately.
6 June:The journey has been long and less than pleasant, but I have arrived. Seeing my puffed-up face and eyes swollen shut, the landlady of the guesthouse said, “Oh my, what a state! Who did this to you? I have a quiet room on the second floor where you may recover from your fight. Peace and rest is what you need.” I did not correct her observation for she meant well.
7 June:Sunday has been a day of rest and recovery. This treeless island already offers better relief than my usual attempts at medication. The air is fresh and I am drawn to wander in the sunshine rather than hide from it.
9 June: The sea air has brought with it a new perspective. While we cannot deny that the assortment of observations, equations and ideas we have support a quantum view, it is generous to call their sum a theory. They are parts in loose association. While we can observe the intensity of hydrogen’s spectral lines, we cannot observe all that we believe we need to know in order to explain their intensity. My island perspective, being so close to the stuff of water, is that perhaps it is our belief that is at fault? What if we can let those unobservables remain that way?
10 June:Yes, this thinking has momentum, although I am uncertain where it will lead. Perhaps we must give up the demands of our lingering Newtonian worldview and give ourselves over more fully to the mathematics.
There is a before and an after: we know where the electron is on either side of a transition, and that should be sufficient. We need not trouble ourselves with the story in between – the mathematics is untroubled, it is only our previously held beliefs that cause difficulty!
14 June: I am a little distressed by possible asymmetries in what I have formulated. I am not yet ready to abandon causality and conservation, as Bohr and colleagues so boldly – and unsuccessfully – attempted last year.
15 June: I wandered out in the middle of the night and headed to the south shore where I climbed a rock to sit in thought. I have found no contradiction within this theory or in its relation to other truths – energy is conserved! Within the consistency and coherence of the mathematics, I also see beauty and a wealth of possibility. There is a lingering asymmetry in the operations, but I made peace with that as I watched the sun rise and observed the waves. Wave on wave may be commutative, but wave on shore is not. Such noncommutativity seems also to be the case with the tabular system of numbers I have used.
16 June: I leave for Hamburg. I wish to share these insights with Pauli ahead of my return to Göttingen. Before sharing my insights with Professor Born, I need for Wolfgang to confirm what I have unearthed is not wrong and that this theory is not some sea madness.
Göttingen
5 June: I am somewhat aggrieved that Professor Born did not grant my request for leave. Admittedly, the notice was short, but the hay fever is most wretched. I am barely able to string two thoughts together, let alone a theory for electron transition. The problem of hydrogen’s spectral lines eludes me, as does any coherence during much of the day or night. The lushness of Göttingen’s parks and gardens is a curse in summer. If I am to make progress on this problem of physics, I must first address this problem of my own biology.
6 June: Chemistry is today’s pursuit. I have secured medication in a greater dose than before.
7 June:Empirically, I appear to have determined that a more generous ingestion of cocaine is not the solution to my hay fever problem. I shall instead switch to increasing my intake of aspirin.
11 June: I am feeling most sorry, both for myself and the state of our discipline. It is as though my own ills are entangled with physics as a whole. There is little certainty or clarity, only contradictions and incompleteness. Whether at the scale of the atom or the galaxy, our understanding contradicts our intuition and our progress out of this darkness is pitiful.
Even Professor Einstein’s magnificent general theory of relativity has its difficulties. Without a fix that lacks any theoretical origin, it predicts an expanding universe! There are evensolutions that permitted dark stars whose gravity would be so large that nothing could escape! We are mired in questions and nonsense, all the while I am little more than coughs, sneezes and reddened eyes. What I might generously call my mind is barely deserving of the name.
I am consoled, at least, that in mathematics the story is not the same. Russell and Whitehead have shown that mathematics is complete and consistent – although I know of no one who has managed to read the whole proof. This result offers a firm bedrock I am sure mathematicians will continue to celebrate a hundred years from now.
15 June: I was en route to the department this morning when I entirely lost my bearings after taking a wrong turn from my usual route. Imagine knowing where I was going but not knowing where I was!
Just last week I had the opposite experience. My landlady accosted me just in front of the Friedhofskapelle Stadtfriedhof. I was as surprised to see her as we was to see me. “Good day, Professor Heisenberg.” I long ago stopped reminding her that I was no professor, merely a Privatdozent. She means well. “Where are you heading?” And do you know, I had no idea! How I wish, though, that Born had let me travel to Helgoland.
16 June: As I walk – and sneeze – into the university this morning, I am caused to wonder from where answers to our quantum troubles might emerge. Bohr has great insight, so will it be from Copenhagen that an interpretation will appear? Or perhaps it will from Cambridge — Paul Dirac’s thinking is particularly fresh.
For now, I wish an end to summer and the fog it has brought to my thinking, yet I also wonder whether we are asking more of nature than she is prepared to share with us. Perhaps it is our dearly held beliefs that hold us back. Perhaps nature and mathematics do not share those beliefs. Perhaps. There is an uncertainty within me that I find hard
to articulate.
To hear the author read an extract from the diaries and reflect on the power of “flash fiction”, check out the Physics World Stories podcast.
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As few as five electrons in a semiconductor can exhibit collective behaviour, forming a “Coulomb liquid”, according to researchers in Europe. This extends the study of correlated systems to electron plasmas, and could lead to the study of other exotic phases of matter.
A conventional plasma is a hot, ionized gas of free electrons and positive ions. However, the conduction band of a semiconductor can be considered a one-component plasma. “The effect of the positive charges, as they are locked into the lattice, can be modelled as a uniform background of positive charge,” says team member Vyacheslavs Kashcheyevs of the University of Latvia in Riga. In conventional electronics and semiconductor physics, the conduction band is modelled as a 2D Fermi gas of non-interacting particles, with the Coulomb interaction between the electrons neglected.
The new work focused on electron–electron correlations in the conduction band of gallium arsenide at millikelvin temperatures. The team created a Y-shaped junction. Electrons emitted from a quantum dot were steered through the device by an externally-generated surface acoustic wave (SAW) potential. Part-way through, the path divided, and each electron could either go left or right. The number taking each path was measured by separate quantum dots. The researchers are uncertain, and the model is agnostic, about the extent to which the randomness of left or right arose from quantum mechanics.
When no more than one electron was loaded into each potential minimum, each electron’s choice was random, and the number of electrons counted at each detector after multiple trials could be modelled by a binomial distribution. However, when the researchers tuned the apparatus such that each minimum contained multiple electrons, they found changes in the distribution, with groups of particles less likely to travel to the same detector than would be naïvely expected.
Calculating “cumulants”
The researchers quantified the changes in the distributions using probability theory, calculating “cumulants” of the distributions. “We not only have a cumulant of order two, which would say that two particles are repulsing,” says Hermann Sellier of Institut Néel in Grenoble, France, who led the experimental research. “We have a cumulant of order four for four particles or five for five particles, showing that each particle is talking to all the other particles of the droplet. That’s much stronger and something that has not been measured before.”
This shows, say the researchers, that the 2D electron gas condenses into a strongly correlated Coulomb liquid. This a phase of matter seen in quark–gluon plasma, which is created by the high-energy collision of heavy ions, but never previously identified in electronic matter.
“It’s not like you have atoms which, below a certain temperature, go from the gas phase to the liquid phase because of an attractive interaction,” explains Sellier. “We say that the correlated behaviour is like that of a liquid, but a very special liquid made of repulsive interactions. You push on the right, it pushes on the left.” This is possible only at low temperatures because heating increases the entropy to the point where the correlated state of matter is disfavoured.
The team now wants to look at larger systems approaching the macroscopic limit. They believe similar systems could potentially be used to study many-body physics with other, exotic particles such as anyons – quasiparticles that have properties intermediate to bosons and fermions. Potential technological applications include cold atom quantum simulation.
Considerable interest
Ravi Rau of Louisiana State University in the US says, “It is an interesting method, novel to me, of controlling electron droplets and being able to measure correlations of two, three and up to a maximum of five-particles so far, and addressing the general question of the transition in few-body systems to the statistical limit from explicit dynamics when the number of particles is small”. He adds, “This study, such a system, and the results presented will of course be of considerable interest.”
Rau does however, note that very similar results were achieved in the past in studies of electron collisions with cold atoms and molecules. “[That technique] went under the name of COLTRIMS (cold target recoil-ion momentum spectroscopy) allowed measuring multiple differential cross sections and studying electron–electron correlations in atoms,” he says. “It was the exact analogue of this [work], except that instead of an artificially created and controlled droplet cluster, the electrons were naturally inside the atom.”
The researchers acknowledge the similarity, and thank Rau for bringing the previous work to their attention. However, Kashcheyevs argues that the new work has a generality that allows it to tackle new problems, finding the scaling law that connects the properties of individual electrons to the properties of incompressible Coulomb plasma. “Applying our method at lower temperatures in the future can probe the quantum regime of the phase diagram of this electronic fluid, which is known to support exotic quasiparticles impossible in the 3D vacuum of the Standard Model,” he says.
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New observations support the idea that hot, diffuse threads of gas called cosmic filaments connect clusters of galaxies across the cosmos. That is the conclusion of Konstantinos Migkas at Leiden University and colleagues who say that their study strengthens the idea that much of the normal matter in the universe resides in these structures.
About 5% of the universe’s mass–energy content appears to be baryonic matter – the familiar nuclei and particles that make up atoms and molecules. The rest is believed to be dark energy and dark matter, which are both hypothetical entities. Although they know what baryonic matter is, astronomers have a poor understanding of where much of it is distributed in the universe.
Combining the Standard Model of cosmology with the rigid constraints enforced by observations of cosmic microwave background radiation tells us that structures including stars, black holes, and gas clouds account for around 60% of baryonic matter in the universe. This leaves 40% of baryonic matter unaccounted for.
Previously, cosmologists have argued that this discrepancy could point to a fundamental error in the Standard Model. Recently, however, a growing body of evidence suggests that this matter could be found in vast yet elusive structures, hidden deep within intergalactic space.
On a WHIM
“Large-scale structure simulations of the universe tell us this material should reside within long strings of gas called ‘cosmic filaments’, which connect clusters of galaxies,” Migkas explains. “These missing baryons should be found in the so-called ‘warm-hot intergalactic medium’ (WHIM).”
Despite being extremely sparse, models also predict that the WHIM should be extremely hot – primarily heated by shock waves produced as matter collapses into the large-scale cosmic web, as well as by phenomena including active galactic nuclei and mergers between galaxy clusters. As a result, these cosmic filaments should be emitting a faint yet detectable X-ray signal.
On top of this, the Standard Model places tight theoretical constraints on several physical properties of the WHIM – including its density, temperature, and composition. If X-rays are indeed being emitted by cosmic filaments, these properties should be encoded in their energies, intensities, and frequency spectra – providing astronomers with a clear target in their search for the elusive structures.
These X-ray signals have so far evaded detection because they are extremely faint compared to powerful X-ray signals such as those coming from supermassive black holes
To overcome this, researchers combined data from two of the world’s most advanced X-ray observatories. One is the Suzaku satellite, which was jointly operated by JAXA and NASA and was very good at detecting very faint signals. The other is the ESA’s XMM-Newton, which is very good at observing powerful X-ray signals.
Eliminating black holes
“Combining the two instruments, we carefully and appropriately eliminated the contaminating signal of the black holes throughout our filament,” Migkas explains. “This enabled us to isolate the signal of WHIM and measure its density and temperature for the very first time with such accuracy.”
For an observational target, Migkas’ team searched for cosmic filaments in the Shapley supercluster. This vast structure around 650 million light-years from the Milky Way contains one of the highest concentrations of galaxies in the known universe.
With the combined abilities of Suzaku and XMM-Newton, the researchers detected an X-ray signal indicating the presence of a filament – consistent with predictions of the Standard Model. As they expected, this intergalactic material was extremely hot and sparse: boasting temperatures close to 10 million Kelvin, while containing just around 10 electrons per cubic metre.
“We also found that on average, the filament is around 40 times denser than the average density of the universe – which is pretty empty in general – and around 1000 times less dense than the cores of the four-galaxy cluster it connects,” Migkas describes. Despite having gone undetected so far, this filament also carries a total mass around 10 times that of the Milky Way – making it a vast reservoir of previously hidden matter.
“For the very first time, our work confirms the validity of the predictions of the Standard Model of cosmology regarding the properties of a big part of the missing baryons,” Migkas concludes.
Beyond the technical skills tied to specific aspects of my research, my work involves continuously engaging a balance of creativity, critical thinking and curiosity. Creativity alone isn’t enough – in physics, ideas must ultimately stand up to scrutiny. Something is either right or it isn’t, so the goal is to let your imagination run free, while keeping it anchored to scientific rigour.
This balance becomes especially important when it comes to defining your own research direction. Early in your career, you’re usually handed a problem to work on. But, over time, you have to learn to ask your own questions, and formulating good ones is much harder than it sounds.
In the beginning, most of the ideas you come up with turn out either to be flawed or have already been explored. The alternative is to stay in safe territory and do incremental work, which certainly has its place, but it’s difficult to build a research career on that alone.
What helps is staying curious. Finding a meaningful research question often means diving into unfamiliar literature, following sparks of interest, and carving out time to read and think critically. It also means being open to inspiration from other people’s work, not just from research that overlaps with your own, but potentially from entirely different areas.
To me, one of the most precious traits in research is the ability to keep your curiosity alive
I’ve seen how easy it can be to fall into the trap of only valuing ideas that align with your own. To me, one of the most precious traits in research is the ability to keep your curiosity alive: to remain open to surprise, ready to recognize when you’re wrong, be willing to learn, and to be excited by someone else’s discovery, even when it has nothing to do with your own work.
What do you like best and least about your job?
What I like best is the freedom. I get to choose what my next research project will be about, and sometimes that process starts in the simplest of ways. I see an exciting talk at a conference, become fascinated by a new idea, and find myself reading everything I can about it. I’ll come back, pitch it to a student, and if they’re excited too, we explore it together.
When I start something new, I often feel like an imposter, venturing into foreign territory and trying to operate as if I know my way around, but as time goes on, things start to fall into place. Eventually, you reach the point where you create something new that others in the field may find interesting or inspiring in turn. That moment – when a once-distant topic becomes something you have actually contributed to – is deeply rewarding.
What I like least is answering e-mails. As a student, I couldn’t understand why some professors took ages to reply. Now I do. Some days, my inbox just fills up endlessly, and responding thoughtfully to every message would take the whole day. It’s a balancing act, deciding when to say yes and when to say no, and learning to say no in a considerate and fair way takes time and emotional energy. You want to be generous with your time, especially when someone genuinely needs help, but finding this balance can be exhausting. It’s an important part of the job, but I wish it took up a bit less space.
What do you know today that you wish you’d known at the start of your career?
That everyone feels like an imposter sometimes. When I started out as a student, I looked around and assumed everyone else was an expert, while I was just trying to find my way, painfully aware of how much I didn’t know. Over time, you do gain confidence in certain areas, but research constantly pushes you in new directions. That means learning new things, starting from scratch, and feeling like an imposter all over again.
The first time I heard the term “imposter syndrome”, it felt like a revelation. Just knowing that this feeling had a name, and that others experienced it too, was validating. Does this mean I feel less like an imposter now? Not really. But I’ve come to understand that it’s part of the process. It means I’m still learning, still being challenged, still exploring new directions. And if that feeling never goes away entirely, maybe that’s a good sign.
Scientists have discovered a centrosymmetric crystal that behaves as though it is chiral – absorbing left- and right-handed circularly-polarized light differently. This counterintuitive finding, from researchers at Northwestern University and the University of Wisconsin-Madison in the US, could help in the development of new technologies that control light. Applications include brighter optical displays and improved sensors.
Centrosymmetric crystals are those that look identical when reflected through a central point. Until now, only non-centrosymmetric crystals were thought to exhibit differential absorption of circularly-polarized light, owing to their chirality – a property that describes how an object differs from its mirror image (such as our left and right hands, for example).
In the new work, a team led by chemist Roel Tempelaar studied how a centrosymmetric crystal made from lithium, cobalt and selenium oxide interacts with circularly polarized light, that is, light with an electromagnetic field direction that rotates in a helical or “corkscrew-like” fashion as it propagates through space. Such light is routinely employed to study the conformation of chiral biomolecules, such as proteins, DNA and amino acids, as they absorb left- and right-handed circularly polarized light differently, a phenomenon known as circular dichroism.
The crystal, which has the chemical formula Li2Co3(SeO3)4, was first synthesized in 1999, but has not (to the best of the researchers’ knowledge) been discussed in the literature since.
A photophysical process involving strong chiroptical signals
Tempelaar and colleagues found that the material absorbed circularly polarized light more when the light was polarized in one direction than in the other. This property, they say, stems from a photophysical process involving strong chiroptical signals that invert when the sample is flipped. Such a mechanism is different to conventional chiroptical response to circularly polarized light and has not been seen before in single centrosymmetric crystals.
Not only does the discovery challenge long-held assumptions about crystals and chiroptical responses, it opens up opportunities for engineering new optical materials that control light, says Tempelaar. Potential applications could include brighter optical displays, polarization-dependent optical diodes, chiral lasing, more sensitive sensors and new types of faster, more secure light-based communication.
“Our work has shown that centrosymmetric crystals should not be dismissed when designing materials for circularly polarized light absorption,” Tempelaar tells Physics World. “Indeed, we found such absorption to be remarkably strong for Li2Co3(SeO3)4.”
The researchers say they took on this study after their theoretical calculations revealed that Li2Co3(SeO3)4 should show circular dichroism. They then successfully grew the crystals by mixing cobalt hydroxide, lithium hydroxide monohydrate and selenium dioxide and heating the mixture for five days in an autoclave at about 220 °C.
The “tip of the iceberg”
“This crystal is the first candidate material that we resorted to in order to test our prediction,” says Tempelaar. “The fact that it behaved the way it does could just be a great stroke of luck, but it is more likely that Li2Co3(SeO3)4 is just the tip of the iceberg spanning many centrosymmetric materials for circularly polarized light absorption.”
Some of those compounds may compete with current champion materials for circularly polarized light absorption, through which we can push the boundaries of optical materials engineering, he adds. “Much remains to be discovered, however, and we are eager to progress this research direction further.”
“We are also interested in incorporating such materials into photonic structures such as optical microcavities to amplify their desirable optical properties and yield devices with new functionality,” Tempelaar reveals.
Full details of the study are reported in Science.
Blue Origin launched its third crewed suborbital flight in under three months June 29, flying a group that included a married couple and a lawyer in legal trouble.
An H-2A rocket successfully launched an Earth science satellite June 28 on the final flight of a vehicle that had long been the workhorse for Japanese space access.
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