Semiconductors

The Physics That Hides in Plain Sight

Daniel Morton — Wed, 22 Apr 2026 15:30:34 +0000

The Physics That Hides in Plain Sight Daniel Morton Wed, 04/22/2026 - 09:30

Daniel Morton

Just published; a Perspective article by RASEI theorists raises new questions on what is hidden by quantum symmetry

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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 by the 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 are not 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 of Alex Zunger in the journal Matter 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 occur has 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 spatially and can be controlled. 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 atoms in 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 symmetry appeared 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 legitimate symmetry 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 team sorts spin effects in magnetic and non-magnetic materials into distinct categories, determined by two key factors: whether the effect is apparent or hidden and 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.

Box 1:

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 overlooked hidden 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. The Perspective 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 allow potential 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.

Box 2:

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, the Zunger group with Emmanuel Rashba 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 LaMnO_3, 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. Predicted hidden spin polarization materials in magnetic AFM include tetragonal Ca₂MnO₄, La₂NiO₄, and MnS₂, and the following tetragonal compounds CoSe₂O₅, Fe₂TeO₆, K₂CoP₂O₇, LiFePO_4, 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 group of 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 effects that researchers are looking for, just in a form that requires a more careful look to find.

The Perspective 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, as the 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, and one 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.

April 2026

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Atomic Musical Chairs: How Tiny Nanocrystals Are Informing the Future of Energy-Efficient Electronics

Daniel Morton — Tue, 17 Mar 2026 19:43:33 +0000

Atomic Musical Chairs: How Tiny Nanocrystals Are Informing the Future of Energy-Efficient Electronics Daniel Morton Tue, 03/17/2026 - 13:43

Daniel Morton

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While most people, when asked about energy innovation, think about some of the "large" technologies, such as wind turbines, long transmission lines, or massive power plants, some of the most important advances in how we use energy are happening at a scale so small that millions of the "machines" involved could fit on the head of a pin.

New research from a team led by RASEI Fellow Gordana Dukovic, working in collaboration with RASEI Fellow Sadegh Yazdi and Prof. Dmitri Talapin from the University of Chicago, reveals new insights on a high-speed game of "atomic musical chairs." This collaboration involved two large teams working together. Researchers from two United States National Science Foundation Science and Technology Centers (STCs) including IMOD and STROBE, employed cutting-edge microscopy techniques to directly visualize, for the first time at this scale, how atoms swap places inside tiny semiconductor nanocrystals, which is a crucial step toward understanding the composition, and ultimately the properties, of these materials.

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NSF STCc

STROBE STC

IMOD STC

Science and Technology Centers are hubs for collaboration, bringing together multidisciplinary researchers from across the United States to solve large, challenging and complex problems. This article describes a space where two of these large networks worked together. STROBE, or Science and Technology Center on Real-Time Functional Imaging pushes the boundaries of microscopy to observe and understand materials at the atomic and nano-scales. IMOD, or The Center for Integration of Modern Optoelectronic Materials on Demand, focuses on making atomically precise semiconductors and integrating them into applications in VR displays, and devices for quantum communication and computing. This team leverages the expertise from both Centers to create new semiconductors and using cutting-edge microscopes to observe and understand them.

Almost all of our electronic devices are built from semiconductors. Whether it is the screen on your smartphone, the components in your car, or the microchips in your computer, these electronics rely on semiconductors. Traditionally, these materials are "grown" through rigid and often expensive processes. Tuning the properties of a semiconductor using this approach is not straightforward. If you want a specific color of light for a display, or a specific energy absorption profile for a solar panel, you often have to start from scratch with an entirely different material.

This is where semiconductor nanocrystals offer remarkable opportunities. The specific size, shape, and composition of these tiny nanocrystals determine the physical and electronic properties of the overall material. A particularly powerful process with such nanocrystals is called cation exchange. Instead of building a new crystal from scratch, you can take an existing one and swap out its internal atomic components to change its properties.

“This is a project that we have been working on for a long time” explains Ben Hammel, a graduate student in the Dukovic Group, and lead author on this research. “We have been looking at these materials from the Talapin Group for a long time”.

This work, just published in ACS Nano, focuses on what are called III–V nanocrystals, which are tiny, four-sided pyramids, or tetrahedrons, named for the groups of the periodic table their constituent elements come from (Group III includes elements like Indium, Gallium, and Aluminum; Group V includes Phosphorus, Arsenic, and Antimony). In this research, the nanocrystals are made of a mixture of Indium, Phosphorus, and Arsenic. To exert more control over the properties of these nanocrystals, the researchers introduced Gallium. Adding Gallium is like tuning a guitar string: it changes the energy of the crystal, influencing how it interacts with light.

“A lot of people have developed ways to make III-V bulk semiconductors, but the real challenge is making them into nanocrystals, where you have more control over their properties, and the Talapin Group have developed a really neat molten salt process to do this” explains Hammel. The molten salt work was published in Science in 2024.

Imagine the inside of one of these tiny crystals as a perfectly ordered lattice of "seats." There are two types of players: Anions (the Phosphorus and Arsenic atoms) and Cations (the Indium atoms). A key observation from the team was that the "house" never moves. The Anions are like the floor and the chairs, they stay perfectly still, maintaining the overall crystal framework. The Cations, on the other hand, are the players sitting in those chairs.

In this work, the nanocrystals were placed into a "hot bath" of molten Gallium salts, essentially starting the music on the game of atomic musical chairs. Previous work had shown that the atoms exchange, but there was not a lot of evidence for how this process worked. “Understanding how this works is very important, and finding out more about the local elemental composition, and how the Gallium atoms move can inform how we design these systems in the future” explains Hammel.

These nanocrystals are only 5 to 10 nanometers wide. A typical human hair is between 80,000 and 100,000 nanometers wide. These crystals are called "nano" for a reason! To observe this game of atomic musical chairs in action, the team used Scanning Transmission Electron Microscopy (STEM), an instrument that uses a focused beam of electrons to probe and image matter at the atomic scale. “Early on there were some signs that there was heterogeneity within the particles, but it was unclear, a big technical challenge we had to overcome was how we can actually measure the Gallium moving through the nanocrystal” said Hammel.

A key challenge they had to figure out was the sensitivity of the nanocrystals to the very tool being used to study them. The electron beam of the STEM, if used at high intensity, can damage the nanocrystals before a useful image can even be collected. To solve this, the team developed an innovative "statistical" imaging approach. Rather than blasting a single crystal with a high dose of electrons to get a sharp image, the researchers instead took many low-dose, and individually blurry, snapshots of hundreds of different crystals at different stages of the molten salt reaction. “We essentially stacked the data on top of each other” describes Hammel, “If I can add together 10 nanocrystals, I can get 10 times the signal”. Adding these kinds of signals together hadn’t been done before with semiconductor nanocrystals. “A lot of this came together from teamwork, I got a lot of really great suggestions from collaborators on how to collect and analyze this information. I used a suite of open source Python tools, which I was a little lost with until I met the researcher who developed them at a conference (Josh Taillon from NIST), who gave me some great suggestions and ideas” said Hammel. Using these advanced computer algorithms, they aligned and stacked hundreds of images on top of each other. Much like a long-exposure photograph of the night sky reveals stars the naked eye cannot see, this averaged stacked image revealed a detailed map of where the Gallium atoms were moving inside the nanocrystals. To the team’s knowledge, this signal-averaging approach for elemental mapping has not previously been applied to semiconductor nanocrystals.

The Gallium atoms rush in to claim “seats”, but not randomly. Gallium grabs the seats near the surface first. Because of the high surface-to-volume ratio of these tiny particles, this surface exchange causes a dramatic and rapid change in overall composition: within the first 15 minutes in the molten salt bath, the outside of the nanocrystals is substantially transformed. However, as the game goes on, it gets progressively harder. The Indium atoms sitting in the seats at the center of the nanocrystal are crowded in, and for a Gallium atom to reach the core, an Indium atom must fight its way out through an increasingly Gallium-rich lattice. This sets up a compositional gradient, essentially a smooth transition from a Gallium-rich exterior to an Indium-rich core, that persists even after 16 hours of reaction.

This new methodology, combining STEM with advanced computational image processing, is sensitive enough to detect and map the movement of atoms through individual nanocrystals. Applying it here directly revealed that the cation exchange process (Indium being replaced by Gallium) creates a graded composition rather than a simple sharp boundary between materials. The team also used computer simulations (finite element analysis in COMSOL) to model this exchange as a diffusion-limited process, finding that the rate of exchange slows dramatically as more Gallium enters the lattice, likely because the smaller Gallium atoms cause the lattice to contract, making it progressively harder for further exchange to occur.

Importantly, the methods developed in this work are broadly applicable and could be used to determine the elemental composition of many other types of nanocrystals that have previously been difficult to study due to their sensitivity to electron beams.

The ability to observe and better understand the cation exchange process in these semiconductor nanocrystals has significant implications for the development of next-generation materials. It has been suggested that graded compositions, like those observed here, could help suppress certain energy-loss processes in semiconductor devices, potentially enabling more efficient lighting and lower-power electronics. Whether these specific nanocrystals deliver on that promise remains an open and exciting research question, but this work provides the observational foundation needed to begin answering it. Additionally, the molten-salt synthesis approach that underpins this research is an active area of development as a potentially more versatile route to III–V semiconductor nanocrystals, materials that have historically been among the most challenging to synthesize with fine compositional control.

By developing new tools to better observe the game of "atomic musical chairs," the researchers are providing the field with insights into how to engineer materials at the atomic scale and revealing that the path from one material to another is more nuanced, and more interesting, than previously understood.

March 2026

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Upward band gap bowing and negative mixing enthalpy in multi-component cubic halide perovskite alloys

Daniel Morton — Wed, 25 Feb 2026 18:14:59 +0000

Upward band gap bowing and negative mixing enthalpy in multi-component cubic halide perovskite alloys Daniel Morton Wed, 02/25/2026 - 11:14

PHYSICAL REVIEW MATERIALS, 2026, 10, 025405

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Tuning Thermal Stability through Dopant Size in Chemically Doped DPP–Thiophene Polymers

Daniel Morton — Fri, 20 Feb 2026 18:17:09 +0000

Tuning Thermal Stability through Dopant Size in Chemically Doped DPP–Thiophene Polymers Daniel Morton Fri, 02/20/2026 - 11:17

CHEMISTRY OF MATERIALS, 2026, 38, 5, 2293-2304

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Structural and Compositional Evolution of Colloidal In1–xGaxP1–yAsy Nanocrystals during Cation Exchange Revealed by Electron Microscopy

Daniel Morton — Fri, 13 Feb 2026 18:11:18 +0000

Structural and Compositional Evolution of Colloidal In1–xGaxP1–yAsy Nanocrystals during Cation Exchange Revealed by Electron Microscopy Daniel Morton Fri, 02/13/2026 - 11:11

ACS NANO, 2026, 20, 7, 5506-5517

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Excitonic frontiers

Daniel Morton — Thu, 29 Jan 2026 17:54:58 +0000

Excitonic frontiers Daniel Morton Thu, 01/29/2026 - 10:54

PHILOSOPHICAL TRANSACTIONS A, 2026, 384, 2313, 20220273

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Influence of Ligand Exchange on Single Particle Properties of Cesium Lead Bromide Quantum Dots

Daniel Morton — Tue, 20 Jan 2026 21:16:48 +0000

Influence of Ligand Exchange on Single Particle Properties of Cesium Lead Bromide Quantum Dots Daniel Morton Tue, 01/20/2026 - 14:16

CHEMISTRY OF MATERIALS, 2026, 38, 3, 1074-1083

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Origins of anisotropic linear magnetoresistance with isotropic mobility in Cd3As2 films on GaAs[110]

Daniel Morton — Fri, 16 Jan 2026 17:20:48 +0000

Origins of anisotropic linear magnetoresistance with isotropic mobility in Cd3As2 films on GaAs[110] Daniel Morton Fri, 01/16/2026 - 10:20

JOURNAL OF APPLIED PHYSICS, 2026, 139, 035102

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Electrochemical quantification of phosphonic acid passivated surface sites of NiOx for perovskite solar cells

Daniel Morton — Mon, 12 Jan 2026 17:26:07 +0000

Electrochemical quantification of phosphonic acid passivated surface sites of NiOx for perovskite solar cells Daniel Morton Mon, 01/12/2026 - 10:26

ENERGY & ENVIRONMENTAL SCIENCE, 2026, 19, 884-895

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Multiscale geometrical and topological learning in the analysis of soft matter collective dynamics

Daniel Morton — Fri, 09 Jan 2026 17:53:07 +0000

Multiscale geometrical and topological learning in the analysis of soft matter collective dynamics Daniel Morton Fri, 01/09/2026 - 10:53

PHYSICAL REVIEW MATERIALS, 2026, 10, 015602

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