The Physics That Hides in Plain Sight
Just published; a Perspective article by RASEI theorists raises new questions on what is hidden by quantum symmetry
Some of the most interesting actions happening inside a material are the things that, according to the rulebook, shouldn't be happening at all. In the world of quantum physics, that rulebook is written by symmetry, as encoded bythe geometric arrangement of atoms in quantum matter. Essentially, how the atoms are stacked together in a solid. Symmetry sets strict rules about what physical effects are, and arenot permitted. For decades, when experiments on certain materials produced results that symmetry said were impossible, the standard assumption was that something had gone wrong: a flawed measurement, or a contaminated sample. A new framework published by the group ofAlex Zunger in the journal suggests that in many of those cases, nothing had gone wrong at all. The effects were real. Indeed, they were just hidden—permitted by the local symmetry rules operating in small regions, or neighborhoods, not by the material's overall structure. Understanding where and how these hidden effects occurhas practical consequences: the behavior of electrons in magnetic materials underpins technologies from computer hard drives to medical sensors, and knowing the full picture of what electrons can do can save us from discarding potentially critical new materials with hidden technological virtues.
Spin, and why it matters
To understand what this framework is doing, it helps to start with spin itself. Spin is a quantum property of electrons, one that has no obvious everyday analogy, but which causes electrons to behave, in some respects, like tiny magnets with a fixed orientation. In most materials, the spins of individual electrons point in random up or down directions and cancel each other out. But in certain materials, and under certain conditions, spins can be organized spatiallyand can becontrolled.Moreover, even when spins cancel each other out over the global volume of a sample, the local rules operating in smaller regions can have a different spin symmetry, controlling the properties of the sample as a whole.
These unusual spin behaviors control the foundation of a field called quantum spintronics.Spintronics is, broadly, the use of electron spin rather than just electron charge to store, process, and transmit information. The hard drives in most computers already exploit this principle: the read heads that detect stored data work by sensing differences in how electrons with different spin orientations pass through a material. Researchers are working towards spintronic devices that are faster, smaller, and more energy-efficient than what charge-based electronics alone can achieve.
The catch is that developing useful spin behavior out of a material requires the right conditions. This is where symmetry re-enters the picture. The chemical identity and spatial arrangement of atomsin a solid determine its overall properties. Change the atomic arrangement, and you change what spin can do. For this reason, identifying which materials have the right symmetry for a given spin effect has been central to the field. And for a long time, if a material's overall symmetryappeared to rule an effect out, that material was simply set aside.
Walking the streets: a new map of spin physics
The new framework addresses this directly. Rather than treating spin effects as simply present or absent in a given material, it draws a distinction between two types: apparent and hidden effects.
Apparent effects are those that follow directly from a material's overall atomic arrangement. If the global symmetry permits a spin effect, you expect to see it, and you do. Hidden effects are more subtle. They occur in materials where the overall atomic arrangement would, according to the current rulebook, forbid a given behavior, but where smaller, localized regions, or neighborhoods, within the material have their own legitimatesymmetry that permits it. The global picture says no; the local picture says yes. The local picture wins. To comprehensively understand the potential spintronic virtues of a material, we need to also understand the mysteries of the local arrangements and symmetries of the spins.
A good way to think about this is to imagine judging a city's architecture and character purely from a satellite image. At that resolution, everything might look uniform and regular. Walk the streets, and observe the neighborhood at eye level, and an entirely different set of structures and interactions becomes visible. The framework outlined in this Perspective is insisting that materials physics needs to walk the streets, and that a great deal can be missed by staying at altitude.
To organize this, the framework described by the Zunger teamsorts spin effects in magnetic and non-magnetic materialsinto distinct categories, determined by two key factors: whether theeffectis apparent or hiddenand whether the spin effect requires a help from a phenomenon called spin-orbit coupling (SOC)—an interaction emerging from relativistic theory of matter, in which an electron's motion through the electric field of an atomic nucleus influences its spin orientation. Some spin effects depend on this interaction; others do not, and this distinction has meaningful consequences for which materials can host them and how large the effects can be. Check out Box 1 for a deeper dive into these effects.
Apparent spin splitting induced in non-magnetic materials by relativistic SOC: The Rashba and Dresselhaus effect: Across all categories, the framework identifies both an apparent and a hidden version of each effect. The team helps provide understanding around this categorization by providing theoretical physics worked-out examples inspired by real, experimentally studied compounds. For example, in non-magnetic materials, well-known effects called the Rashba and Dresselhaus effects (both involving spin-orbit coupling) producing a separation of electron spin states, have previously overlookedhidden counterparts that can occur in materials whose overall symmetry would appear to rule them out. The framework points to the possibility that there can be materials that violate the nominal conditions for the (apparent) Rashba effect, but a hidden Rashba effect exists. For example, a hidden Rashba effect can show spin polarization even if the global symmetry violates the required broken inversion symmetry, but the structure consists of sectors that are individually non-symmetric. Predicted materials with hidden Rashba spin polarization pointed out by the new framework include tetragonal BaNiS2 and tetragonal LaOBiS2, whereas materials with hidden Dresselhaus spin polarization proposed theoretically exhibits local spin texture (the pattern of spin orientations across the material), but no spin splitting include hexagonal NaCaBi, cubic Si, and cubic Ge.This new perspective legitimizes the search for such materials that violate the (apparent) Rashba conditions yet show a (hidden) Rashba effect.
What can be hidden in magnetic materials?
In magnetic materials, hidden spin effects can arise not from the relativistic effect of spin-orbit coupling, but from the magnetic interactions between atoms. This means they can, in principle, be larger, and occur in materials containing lighter, more abundant elements. In both cases, the street-level view of the material is revealing structures and interactions that the satellite image simply could not see. You can find out more about examples of an apparent and a hidden SOC-independent effect in Box 2.
Controlling the electronics of materials
The practical significance of the framework extends beyond classification. ThePerspective article explores whether hidden and apparent spin effects can be actively controlled, and, in certain materials, the answer is yes. In some antiferromagnetic compounds, switching between hidden and apparent spin states can be achieved using an electric field. This would be enabled if one could design a material that, in addition to (either apparent or hidden) spin-split AFM symmetry can have the added symmetry of polarity (how electrons are arranged across atoms).This will allowpotential applications of the ability to switch spin states using only an electric field.
This is notable for a few reasons. Antiferromagnets carry some practical advantages over the ferromagnets (materials like iron, where all magnetic moments point the same way), that currently dominant magnetic technology. They produce no stray magnetic field, which reduces interference with neighboring components, respond rapidly to switching signals, and are robust against external magnetic disturbances. The ability to toggle spin effects electrically in these materials adds a further tool for device designers to work with.
Apparent, SOC-independent spin splitting in antiferromagnetic materials: Spin configurations consisting of alternation of spin-up layer followed by a spin-down layer are called antiferromagnets.For a long while it was textbook knowledge that electronic states in antiferromagnets would have the same energies for spin-up and spin-down layers (a behavior called “spin degeneracy”) in the absence of SOC.This is because it was assumed that the two atoms with opposite spins will compensate each other, giving rise to spin degeneracy. In 2020, discovered the enabling symmetry conditions for the unusual case where electronic states in an antiferromagnets would have different energies for different spin (“spin-split antiferromagnets”) in the absence of SOC. Since this behavior follows the precise symmetry of the system it constitutes an apparent effect. Theorists soon pointed to real materials that would have such peculiar effects, including orthorhombic LaMnO3, rhombohedral MnTiO3, tetragonal KRu4O8, and tetragonal V2Te2Oand many others.This effect was later dubbed in the literature “altermagnetism” implying another form of magnetism.
Hidden, SOC-independent spin polarization in antiferromagnetic materials: In collinear antiferromagnets (collinear, meaning the psins all point along the same axis), this requires that (i) global system symmetry forbids SOC-independent spin splitting, but the (ii) local sectors break that symmetry. Predictedhidden spin polarization materials in magnetic AFM include tetragonal Ca2MnO4, La2NiO4, and MnS2, and the following tetragonal compounds CoSe2O5, Fe2TeO6, K2CoP2O7, LiFePO4, Sr2IrO4, and SrCo2V2O8.
Finding real materials with previously unsuspected hidden effects
The question is how one can use theoretical physics to search for specific materials with target spintronic properties? The history of material research and condensed matter physics has often proceeded via accidental discovery of materials with interesting physical properties—superconductors and light-emitting semiconductor. Yet, for many applications we know well what type of physical properties we want, but we do not know a material that has those target properties. An interesting advance was worked out in the research groupof Alex Zunger: namely “Inverse Design”, where you find a material that has a specific, desired target property. The obvious obstacle is that there are innumerably many possible atomic structures that could, in principle, be made even from a few elements and we do not know which structure would have the desired target property. It turns out that modern atomic-resolution quantum mechanics (i.e., electronic structure theory) can now be combined with biologically inspired (evolutionary) “Genetic Algorithms” to scan a truly astronomic number of atomic configurations in genomic-like search of the one(s) that have desired, target materials properties. Once the number of configurations with target property is narrowed down to a few, laboratory synthesis becomes viable. Examples of specific compounds, known to exist but not known to be spintronic relevant were predicted theoretically as a result of this work.
A broad implication of this new framework is that the rulebook has been applied too rigidly. By demonstrating that hidden effects are real and systematic rather than accidental, the framework significantly expands the pool of materials worth investigating for spintronic applications. Materials that were previously set aside because their overall symmetry appeared to rule out useful spin behavior may, on closer, street-level, inspection, host exactly the effectsthatresearchers are looking for, just in a form that requires a more careful look to find.
ThePerspective also flags a subtler problem. Some of the theoretical tools routinely used to model materials are themselves guilty of the same“farsightedness” that causes hidden effects to be missed. Certain widely used approximations work at too coarse a resolution to detect local symmetry and therefore fail to predict effects that are genuinely present. Refining the theoretical toolkit is,asthe authors suggest, as important as expanding the materials search.
Taken together, this framework offers a more complete account of what electrons can do inside a solid,andone that takes local structure seriously rather than assuming the view from altitude tells the whole story. The physics was there all along. It just required a closer look to find it.