Category: Physics

If you know the atoms that compose a particular molecule or solid material, the interactions between those atoms can be determined computationally, by solving quantum mechanical equations—at least, if the molecule is small and simple. However, solving these equations, critical for fields from materials engineering to drug design, requires a prohibitively long computational time for complex molecules and materials.

Now, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and the University of Chicago’s Pritzker School of Molecular Engineering (PME) and Department of Chemistry have explored the possibility of solving these electronic structures using a quantum computer.

The research, which uses a combination of new computational approaches, was published online in the Journal of Chemical Theory and Computation. It was supported by Q-NEXT, a DOE National Quantum Information Science Research Center led by Argonne, and by the Midwest Integrated Center for Computational Materials (MICCoM).

“This is an exciting step toward using quantum computers to tackle challenging problems in computational chemistry,” said Giulia Galli, who led the research with Marco Govoni, a staff scientist at Argonne and member of the UChicago Consortium for Advanced Science and Engineering (CASE).

A computational challenge

Predicting the electronic structure of a material involves solving complex equations that determine how electrons interact, as well as modeling how various possible structures compare to each other in their overall energy levels.

Unlike conventional computers that store information in binary bits, quantum computers use qubits that can exist in superposition of states, letting them solve certain problems more easily and quickly. Computational chemists have debated whether and when quantum computers might eventually be able to tackle the electronic structure problem of complex materials better than conventional computers. However, today’s quantum computers remain relatively small and produce noisy data.

Even with these weaknesses, Galli and her colleagues wondered whether they still could make progress in creating the underlying quantum computational methods required to solve electronic structure problems on quantum computers.

“The question we really wanted to address is what is possible to do with the current state of quantum computers,” Govoni said. “We asked the question: Even if the results of quantum computers are noisy, can they still be useful to solve interesting problems in materials science?”

An iterative process

The researchers designed a hybrid simulation process, using IBM quantum computers. In their approach, a small number of qubits—between four and six—perform part of the calculations, and the results are then further processed using a classical computer.

“We designed an iterative computational process that takes advantages of the strengths of both quantum and conventional computers,” said Benchen Huang, a graduate student in the Galli Group and first author of the new paper.

After several iterations, the simulation process was able to provide the correct electronic structures of several spin defects in solid-state materials. In addition, the team developed a new error mitigation approach to help control for the inherent noise generated by the quantum computer and ensure accuracy of the results.

Hints at the future

For now, the electronic structures solved using the new quantum computational approach could already be solved using a conventional computer. Therefore, the longstanding debate of whether a quantum computer can be superior to a classical one in solving electronic structure problems is not settled yet.

However, the results provided by the new method pave the way for quantum computers to address more complex chemical structures.

“When we scale this up to 100 qubits instead of 4 or 6, we think we might have an advantage over conventional computers,” Huang said. “But only time will tell for sure.”

The research group plans to keep improving and scaling up their approach, as well as using it to solve different types of electronic problems, such as molecules in the presence of solvents, and molecules and materials in excited states.

More information: Benchen Huang et al, Quantum Simulations of Fermionic Hamiltonians with Efficient Encoding and Ansatz Schemes, Journal of Chemical Theory and Computation (2023). DOI: 10.1021/acs.jctc.2c01119

Provided by Argonne National Laboratory 

Developed at Harvard, and successfully tested at Graz University of Technology (TU Graz), a revolutionary new meta-optics for microscopes with extremely high spatial and temporal resolution has proven its functional ability in laboratory tests at the Institute of Experimental Physics at TU Graz.

Microscopes using this kind of lens promise completely new research and development approaches, especially in semiconductor and solar cell technology. The research team from Graz and Boston currently reports on the construction and the successful laboratory experiment with this new meta-optics in the journal Science.

The lens of the microscope has made it possible to use extreme ultraviolet radiation for the first time. Its extremely short wavelength enables it to follow ultra-fast physical processes in the attosecond range. For example, real-time images from the inside of modern transistors or the interaction of molecules and atoms with light. Marcus Ossiander came up with the idea for the novel lens during his research work in Federico Capasso’s group at Harvard University, and since January 2023, the ERC Starting Grant and FWF START Award winner has been conducting research at the Institute of Experimental Physics at TU Graz.

Joint success for Boston and Graz

Attosecond physics uses extreme ultraviolet light. Because this light oscillates quickly and all the materials in the construction kit of optics development are opaque to this light, there have been no usable imaging systems for it until now. Marcus Ossiander remarks, “I asked myself whether the classical principle of optics could not be reversed. Can you use the absence of material in small areas as the basis of an optical element?”

The lens developed at Harvard on the basis of this idea and successfully tested at TU Graz implements this design principle. A precisely calculated arrangement of tiny holes in an extremely thin silicon foil conducts and focuses the incident attosecond light. A remarkable observation of the research team is that these vacuum tunnels transmit more light energy than should be possible due to the hole-covered surface. This means that the innovative meta-optics literally sucks the ultraviolet light into the focal point.

Holes of a few nanometers in diameter

Extremely small and precisely controlled structures are required for this breakthrough. Their production is close to the limits of what is technically feasible today. The technical implementation was achieved by Federico Capasso’s team at Harvard, which is the world leader in this field, after an experimental phase of around two years.

Proof of functionality was achieved in collaboration with TU Graz, where Martin Schultze’s group at the Institute of Experimental Physics is dedicated to the generation and application of ultra-short ultraviolet light flashes. “This is a great success for the cooperation between Boston and Graz. Now we want to use it to study microelectronics, nanoparticles and similar things soon,” explains Marcus Ossiander.

The meta-optics consists of an approximately 200-nanometer-thin film into which tiny hole structures have been etched. The entire lens consists of many hundreds of millions of holes; there are about ten of these structures per micrometer on the membrane. A single hole measures between 20 and 80 nanometers in diameter. For comparison: a human hair is about 60 to 100 micrometers thick, a small virus has a diameter of 15 nanometers. The diameters of the holes vary and decrease from the center of the membrane outwards. Depending on the size of the hole, the incident light radiation there is delayed and thus collapses into a tiny focal point.

Laser meets gas cloud

To measure the new type of lens, Martin Schultze and Hana Hampel from the Institute of Experimental Physics at TU Graz have unique expertise in generating the necessary extreme ultraviolet radiation. “Reliably generating short light pulses with high energy requires precise control of light-controlled atomic processes and very precise optical set-ups. For this project, we have developed a light source that is particularly efficient in generating radiation of the wavelength for which these meta-optics were designed,” says Martin Schultze.

In the experimental set-up in Graz, where a laser was focused into an inert gas jet, the extreme ultraviolet radiation could be generated and concentrated in very short pulses. The effectiveness of the meta-optics was proved by means of this light source which was optimized for attosecond physics.

Next step: A microscope with meta-optics

The development of a microscope that works with this lens is now the next step. The possible applications for the new research field of attosecond microscopy are manifold. Semiconductor and solar cell technology in particular will benefit from the possibility of being able to track the ultrafast movement of charge carriers in space and time for the first time.

In modern transistors and optoelectronic circuits, the relevant processes take place within a few nanometers of spatial expansion and in a time frame of a few attoseconds. The new meta-optics will make it possible to watch these central components of information technology at work and optimize them even further.

More information: Marcus Ossiander et al, Extreme ultraviolet metalens by vacuum guiding, Science (2023). DOI: 10.1126/science.adg6881. www.science.org/doi/10.1126/science.adg6881

Journal information: Science 

Provided by Graz University of Technology 

Diamond material is of great importance for future technologies such as the quantum internet. Special defect centers can be used as quantum bits (qubits) and emit single light particles that are referred to as single photons.

To enable data transmission with feasible communication rates over long distances in a quantum network, all photons must be collected in optical fibers and transmitted without being lost. It must also be ensured that these photons all have the same color, i.e., the same frequency. Fulfilling these requirements has been impossible until now.

Researchers in the “Integrated Quantum Photonics” group led by Prof. Dr. Tim Schröder at Humboldt-Universität zu Berlin have succeeded for the first time worldwide in generating and detecting photons with stable photon frequencies emitted from quantum light sources, or, more precisely, from nitrogen-vacancy defect centers in diamond nanostructures.

This was enabled by carefully choosing the diamond material; sophisticated nanofabrication methods carried out at the Joint Lab Diamond Nanophotonics of the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik; and specific experimental control protocols. By combining these methods, the noise of the electrons, which previously disturbed data transmission, can be significantly reduced, and the photons are emitted at a stable (communication) frequency.

In addition, the Berlin researchers show that the current communication rates between spatially separated quantum systems can prospectively be increased more than 1,000-fold with the help of the developed methods—an important step closer to a future quantum internet.

The scientists have integrated individual qubits into optimized diamond nanostructures. These structures are 1,000 times thinner than a human hair and make it possible to transfer emitted photons in a directed manner into glass fibers.

However, during the fabrication of the nanostructures, the material surface is damaged at the atomic level, and free electrons create uncontrollable noise for the generated light particles. Noise, comparable to an unstable radio frequency, causes fluctuations in the photon frequency, preventing successful quantum operations such as entanglement.

A special feature of the diamond material used is its relatively high density of nitrogen impurity atoms in the crystal lattice. These possibly shield the quantum light source from electron noise at the surface of the nanostructure. “However, the exact physical processes need to be studied in more detail in the future,” explains Laura Orphal-Kobin, who investigates quantum systems together with Prof. Dr. Tim Schröder.

The conclusions drawn from the experimental observations are supported by statistical models and simulations, which Dr. Gregor Pieplow from the same research group is developing and implementing together with the experimental physicists.

The paper is published in the journal Physical Review X.

More information: Laura Orphal-Kobin et al, Optically Coherent Nitrogen-Vacancy Defect Centers in Diamond Nanostructures, Physical Review X (2023). DOI: 10.1103/PhysRevX.13.011042

Journal information: Physical Review X 

Provided by Humboldt-Universität zu Berlin

A study published in Nature Communications highlights the progress made in practical quantum sensing by a team led by academician Guo Guangcan and Prof. Sun Fangwen from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS). The team utilized micro and nano quantum sensing, coupled with local electromagnetic field enhancement at deep sub-wavelength scales, to study the detection of microwave signals and wireless ranging, achieving a positioning accuracy of 10^-4 wavelengths.

Radar positioning technology based on microwave signal measurement is widely used in activities such as automatic driving, intelligent manufacturing, health monitoring, and geological exploration. In this study, the research team combined quantum sensing of solid-state systems with micro/nano resolution and deep subwavelength localization of electromagnetic fields to develop high-sensitivity microwave detection and high-precision microwave positioning technology.

The researchers designed a composite microwave antenna composed of diamond spin quantum sensors and metal nanostructures, which collects and converges microwave signals propagating in free space into nano-space. By probing the solid-state quantum probe state in the local domain, they measured the microwave signals. The method converted the detection of weak signals in free space into the detection of electromagnetic field and solid-state spin interactions at the nanoscale, improving the microwave signal measurement sensitivity of solid-state quantum sensors by 3–4 orders of magnitude.

To further utilize the high sensitivity microwave detection to achieve high-precision microwave localization, the researchers built a microwave interferometry device based on the diamond quantum sensor, and obtained the phase of the reflected microwave signal and the position information of the object through the solid-state spin detection of the interference result between the reflected microwave signal and the reference signal of the object. Based on the coherent interaction between solid-state spin quantum probes and microwave photons multiple times, they achieved quantum-enhanced position measurement with an accuracy of 10 micrometers (about one ten-thousandth of the wavelength).

Compared with traditional radar systems, this quantum measurement method does not require active devices such as amplifiers at the detection end, reducing the impact of electronic noise and other factors on the measurement limit. Subsequent research will allow further improvement of radio localization accuracy, sampling rate, and other indicators based on solid-state spin quantum sensing, and the development of practical solid-state quantum radar localization technology that exceeds the performance level of existing radars.

More information: Xiang-Dong Chen et al, Quantum enhanced radio detection and ranging with solid spins, Nature Communications (2023). DOI: 10.1038/s41467-023-36929-8

Journal information: Nature Communications 

Provided by Chinese Academy of Sciences 

This topic is reviewed by Prof. Ke-Qing Xia (Southern University of Science and Technology, Shenzhen, China) and his collaborators, mainly based on their research work over the past ten years.

Being the last unsolved problem in classical physics, fluid turbulence has attracted much attention from both academic and engineering communities. In contrast to completely disordered systems, one defining feature of turbulent flows is the existence of coherent structures, which are spatial-temporally correlated over a range of scales.

It has long been known that these coherent structures are the primary carriers for mass, momentum, and heat transport in turbulence. However, owing to the inherent characteristics of turbulent flows, such as strong nonlinearity and strong dissipation, how to manipulate coherent structures to control turbulent transport has been a long-standing issue.

In the past decade, Prof. Xia’s team have made significant progresses in this issue. By conducting a series of studies in a canonical thermal turbulence system, namely the turbulent Rayleigh-Bénard convection, they discovered a new mechanism of tuning turbulent heat transport via coherent structure manipulation through simple geometrical confinement.

Under this mechanism, the heat transfer efficiency is controlled by the coherency of thermal structures (characterized by their geometrical properties), rather than the turbulence intensity.

As a result, the heat transport efficiency can be significantly enhanced even the resultant flow is much slower. Very importantly, this mechanism is fundamentally different from the prevalent heat-management approach based on the classical view of wall-bounded turbulence, which usually centers on directly modifying the diffusion-dominant boundary layer to enhance or inhibit turbulent heat transfer.

In the review article, Prof. Xia and his collaborators introduced, and explained in detail, the physical picture behind this newly discovered mechanism, and discussed its potential applications in passive thermal management (such as electronics cooling).

Moreover, by introducing additional examples of thermal turbulence systems that are subject to various dynamical processes (including rotation, double-diffusion, magnetic field, tilting, modification by polymer additive and so on), they further demonstrate how the framework of coherent structure manipulation can be generalized to understand heat transport behaviors in seemingly different turbulence systems in a unified way. This universal mechanism is expected to be realized in other types of turbulent flows.

This review article also covers other important progresses in this research topic and outlines some future directions. These not only provide new understanding for the communities of turbulence research and heat transfer, but also promote the design and development of engineering systems with tunable transport efficiencies.

The work is published in National Science Review.

More information: Ke-Qing Xia et al, Tuning Heat Transport Via Coherent Structure Manipulation: Recent Advances in Thermal Turbulence, National Science Review (2023). DOI: 10.1093/nsr/nwad012

Provided by Science China Press 

An international research team led by Professors Tsuneyuki Ozaki and François Légaré at the Institut national de la recherche scientifique (INRS), has developed a unique method to enhance the power of a laser source emitting extreme ultraviolet light pulses. The underlying mechanism of the newly observed phenomenon involves the unique role of dark-autoionizing states through coupling with other pertinent electronic states.

Thanks to this work, the team will be able to study the ultrafast dynamics of a single dark autoionizing state at the femtosecond timescale, which was previously impossible due to its inability to undergo single-photon emission or absorption, combined with the ultrashort lifetime of these states.

Recently published in the journal Physical Review Letters, their results allow the generation of ultrafast extreme ultraviolet light relevant for advanced ultrafast science applications such as angle-resolved photoemission spectroscopy and photoemission electron microscopy.

This work was done in collaboration with Professor Vasily Strelkov at the Prokhorov General Physics Institute of the Russian Academy of Sciences, Russia, and Research Assistant Professor Muhammad Ashiq Fareed at the University of Nebraska-Lincoln, USA.

Unraveling the mysteries of the dark-autoionizing states

In their laboratories at the Énergie Matériaux Télécommunications Research Centre, Professors Tsuneyuki Ozaki and François Légaré, along with Ph.D. student Mangaljit Singh, have been exploiting special types of electronic states, known as dark-autoionizing states. Their work was accomplished using high-order harmonic generation, an optical phenomenon unconventional to laser physics.

“The newly published results are a step forward not only in understanding the behavior of dark autoionizing states under intense ultrafast laser-matter interactions, but also in bringing intense extreme-ultraviolet laser sources from large-scale synchrotron and free-electron laser facilities to the moderate-sized laser laboratories,” says Ph.D. student Mangaljit Singh, first author of the study.

Many limitations imposed by the fundamentals of laser physics restrict most lasers used in medicine, communications, or industry. Likewise, they tend to operate only in the ultraviolet, visible (from violet to red), or the invisible near and mid-infrared wavelength range. However, many advanced scientific applications require lasers to operate at shorter wavelengths in the extreme ultraviolet range.

The state-of-the-art systems employ commercially available primary laser sources for high-order harmonic generation from noble gases to develop secondary sources of coherent extreme ultraviolet light.

In this study, instead of noble gases, Singh and colleagues used a laser-ablated plume (obtained from the laser ablation of a solid material) for the high-order harmonic generation in sync with the unique response of dark-autoionizing states.

They found that under certain resonance conditions governed by the primary laser parameters and the electronic structure of the atomic and ionic species in the laser-ablated plume, the conversion efficiency, and hence the power of the extreme ultraviolet laser source is enhanced by more than ten times. This implies that the same extreme ultraviolet power can be obtained using a primary laser with power that is one-tenth of the power required for a typical noble gas.

In addition to providing an intense extreme ultraviolet light source, this study also shows for the first time the prospect of studying the dynamics of dark autoionizing states at the femtosecond timescale using the technique of high harmonic spectroscopy. Such dark states could be the basis of several quantum technologies, especially in improving the performance of quantum computation.

More information: Mangaljit Singh et al, Ultrafast Resonant State Formation by the Coupling of Rydberg and Dark Autoionizing States, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.073201

Journal information: Physical Review Letters 

Provided by Institut national de la recherche scientifique – INRS

A collaboration of Australian and European physicists predict that layered electronic 2D semiconductors can host a curious quantum phase of matter called the “supersolid.”

The supersolid is a very counterintuitive phase indeed. It is made up of particles that simultaneously form a rigid crystal and yet at the same time flow without friction since all the particles belong to the same single quantum state.

A solid becomes “super” when its quantum properties match the well-known quantum properties of superconductors. A supersolid simultaneously has two orders, solid and super:

• Solid because of the spatially repeating pattern of particles.
• Super because the particles can flow without resistance.

“Although a supersolid is rigid, it can flow like a liquid without resistance,” explains Lead author Dr. Sara Conti (University of Antwerp).

The study was conducted at UNSW (Australia), University of Antwerp (Belgium) and University of Camerino (Italy) and has been published in Physical Review Letters.

A 50-year journey toward the exotic supersolid

Geoffrey Chester, a Professor at Cornell University, predicted in 1970 that solid helium-4 under pressure should at low temperatures display:

• Crystalline solid order, with each helium atom at a specific point in a regularly ordered lattice and, at the same time,
• Bose-Einstein condensation of the atoms, with every atom in the same single quantum state, so they flow without resistance.

However in the following five decades the Chester supersolid has not been unambiguously detected.

Alternative approaches to forming a supersolid-like state have reported supersolid-like phases in cold-atom systems in optical lattices. These are either clusters of condensates or condensates with varying density determined by the trapping geometries. These supersolid-like phases should be distinguished from the original Chester supersolid in which each single particle is localized in its place in the crystal lattice purely by the forces acting between the particles.

The new Australia-Europe study predicts that such a state could instead be engineered in two-dimensional (2D) electronic materials in a semiconductor structure, fabricated with two conducting layers separated by an insulating barrier of thickness d.

One layer is doped with negatively-charged electrons and the other with positively-charged holes.

The particles forming the supersolid are interlayer excitons, bound states of an electron and hole tied together by their strong electrical attraction. The insulating barrier prevents fast self-annihilation of the exciton bound pairs. Voltages applied to top and bottom metal “gates” tune the average separation r₀ between excitons.

The research team predicts that excitons in this structure will form a supersolid over a wide range of layer separations and average separations between the excitons. The electrical repulsion between the excitons can constrain them into a fixed crystalline lattice.

“A key novelty is that a supersolid phase with Bose-Einstein quantum coherence appears at layer separations much smaller than the separation predicted for the non-super exciton solid that is driven by the same electrical repulsion between excitons,” says co-corresponding author Prof. David Neilson, University of Antwerp.

“In this way, the supersolid pre-empts the non-super exciton solid. At still larger separations, the non-super exciton solid eventually wins, and the quantum coherence collapses.”

“This is an extremely robust state, readily achievable in experimental setups,” adds co-corresponding author Prof. Alex Hamilton (UNSW). “Ironically, the layer separations are relatively large and are easier to fabricate than the extremely small layer separations in such systems that have been the focus of recent experiments aimed at maximizing the interlayer exciton binding energies.”

As for detection, for a superfluid it is well known that this cannot be rotated until it can host a quantum vortex, analogous to a whirlpool. But to form this vortex requires a finite amount of energy, and hence a sufficiently strong rotational force. So up to this point, the measured rotational moment of inertia (the extent to which an object resists rotational acceleration) will remain zero. In the same way, a supersolid can be identified by detecting such an anomaly in its rotational moment of inertia.

The research team has reported the complete phase diagram of this system at low temperatures.

“By changing the layer separation relative to the average exciton spacing, the strength of the exciton-exciton interactions can be tuned to stabilize either the superfluid, or the supersolid, or the normal solid,” says Dr. Sara Conti.

“The existence of a triple point is also particularly intriguing. At this point, the boundaries of supersolid and normal-solid melting, and the supersolid to normal-solid transition, all cross. There should be exciting physics coming from the exotic interfaces separating these domains, for example, Josephson tunneling between supersolid puddles embedded in a normal-background.”

More information: Sara Conti et al, Chester Supersolid of Spatially Indirect Excitons in Double-Layer Semiconductor Heterostructures, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.057001

Journal information: Physical Review Letters 

Provided by FLEET

National University of Singapore (NUS) physicists have demonstrated a new way of controlling Rashba interactions in oxide systems. Tuning and controlling Rashba interactions is a particularly promising technology, as it can potentially be integrated directly into functional logic and memory devices.

Scientists and engineers need devices that can process information efficiently with ultra-low power consumption. Recently, there have been new developments in logic and memory devices that use the spin of electrons, in addition to their electronic charge, to store and process information. To accomplish this, the device architecture needs a strong interaction between the spin of electrons and their orbital moments.

This coupling, known as Rashba effect, allows for easy manipulation of spin currents and can lead to lower energy consumption. In particular, it can facilitate voltage-driven magnetization switching for logic and memory computing with reduced energy consumption, as proposed by Intel. However, to be useful in this regard, this effect must be significantly large even at zero applied voltages, and this has been difficult to realize in traditional semiconducting and metallic systems.

The research team led by Professor Ariando from the Department of Physics, NUS, demonstrated that lattice polarization and the trapping of interfacial charge carriers can result in a large Rashba spin-orbit effect at the oxide material interface in the absence of an external bias. Their findings have now been published in Physical Review Letters.

Oxide systems have great potential for exploiting the Rashba effect because their multiple degrees of freedom (charge, spin, orbital, and lattice) are entangled with one another. Under Prof. Ariando’s guidance and with the support of the research team, Dr. Ganesh Ji Omar, the lead author of the paper, came up with a new and different methodology for controlling the Rashba effect in oxide material systems.

In their experiments, the researchers developed two-dimensional electron gas (2DEG) at LaAlO₃-SrTiO₃ interfaces and buffered by a LaFeO₃ carrier modulating layer. All individual oxide layers during the fabrication process were controlled in real time and the resulting structures were validated using high-resolution electron microscopy. This heterostructure led to significant enhancement of Rashba effect at the correlated LaAlO₃-SrTiO₃ interfaces even when no external voltage was applied. This remarkable enhancement effect was explained using a framework of “orbital hybridization of interfacial electronic wave functions” at oxide interfaces (see figure above). The theoretical research team further validated this unconventional Rashba effect using density functional theory.

Prof. Ariando said, “This work highlights the fundamental role of lattice polarization in achieving the observed enhancement of Rashba spin-orbit coupling and is particularly promising for efficient spin-to-charge conversion. Moreover, it can lead to the discovery of various other exotic properties, such as spiral magnetism, topological superconductivity, and intrinsic spin Hall effect.”

More information: G. J. Omar et al, Experimental Evidence of t2g Electron-Gas Rashba Interaction Induced by Asymmetric Orbital Hybridization, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.187203

Journal information: Physical Review Letters 

Provided by National University of Singapore 

Microelectronics forms the foundation of much modern technology today, including smartphones, laptops and even supercomputers. It is based on the ability to allow and stop the flow of electrons through a material. Spin electronics, or spintronics, is a spinoff. It is based on the spin of electrons, and the fact that the electron spin along with the electric charge creates a magnetic field.

“This property could be exploited for building blocks in future computer memory storage, brain-like and other novel computing systems, and high-efficiency microelectronics,” said Charudatta Phatak, group leader in the Materials Science division at the U.S. Department of Energy’s (DOE) Argonne National Laboratory.

A team including researchers at Argonne and the National High Magnetic Field Laboratory (MagLab) discovered surprising properties in a magnetic material of iron, germanium and tellurium. This material is in the form of a thin sheet that is only a few to 10 atoms in thickness. It is called a 2D ferromagnet.

The team discovered that two kinds of magnetic fields can coexist in this ultrathin material. Scientists call them merons and skyrmions. They are like miniature swirling storm systems dotting the flat landscape of the ferromagnet. But they differ in their size and swirling behavior.

Known and studied for about 15 years, skyrmions are about 100 nanometers in size—approximately the same as a single virus molecule—and their magnetic fields flow in complicated patterns, resembling those of the strands of a knot in a rope. Only recently discovered, merons are roughly the same size and have magnetic fields that swirl around like whirlpools.

“Both skyrmions and merons are very stable because like firmly tied knots, they are difficult to untangle,” said Luis Balicas, who holds a joint appointment at MagLab and Florida State University. “This stability along with their magnetic properties makes them attractive as carriers of information.”

The team is the first to observe both of these magnetic textures in a thin film at the same time at low temperature, from minus 280 to minus 155 degrees Fahrenheit. Also, merons remained present up to room temperature, an important consideration to exploit them in practical devices. In the past, they had only been observed at much lower temperature in different materials.

The team also showed that skyrmions and merons are detectable from their effect on an applied current, by measuring the voltage. This feature means they are adaptable to the binary code used in all digital computers. This code consists of combinations of 1 and 0. In a spintronic device, a 1 would be indicated by an electrical signal detecting a skyrmion or meron. The absence of an electrical signal would then convey a 0.

Detecting and characterizing the different magnetic textures in a film fewer than ten atoms thick required a special scientific tool. Argonne physicist Yue Li led that challenging task using an instrument called a Lorentz transmission electron microscope (TEM). This microscope includes aberration correction technology to improve its resolution. This TEM can visualize the magnetization of materials at the nanoscale under different magnetic fields over a wide temperature range, a unique capability available at Argonne. The range extends from minus 280 Fahrenheit to room temperature.

The team performed additional magnetic and other imaging at Argonne’s Center for Nanoscale Materials, a DOE Office of Science user facility.

“Much more basic research is needed to fully understand the behavior of skyrmions and merons under different conditions, and how to employ them in coding information,” Balicas said. “Many seemingly science fiction schemes are out there. We cannot predict the future, but it seems likely that one or more might come to fruition.”

The research was published in Advanced Materials.

More information: Brian W. Casas et al, Coexistence of Merons with Skyrmions in the Centrosymmetric Van Der Waals Ferromagnet Fe_5–x GeTe₂, Advanced Materials (2023). DOI: 10.1002/adma.202212087

Journal information: Advanced Materials 

Provided by Argonne National Laboratory 

Researchers from the Institute of Physics (IOP) of the Chinese Academy of Sciences, collaborating with international colleagues, have presented an overview of recent progress in emerging moiré photonics and optoelectronics. It was published in Science on March 30.

Moiré superlattices are artificial quantum materials formed by vertically stacking two or more two-dimensional (2D) layered materials with a slight lattice mismatch and/or a small rotational twist. They introduce a potential landscape of much larger length scale than the crystal periodicity of the constituent 2D layers, providing a novel paradigm for engineering band structures and hence a plethora of exotic quantum phenomena.

For instance, the moiré potential landscape can fold the electronic band structure into a mini-Brillouin zone, resulting in the formation of flat bands and a rich phase diagram of strongly correlated and topological states, such as superconductivity, orbital magnetism, Wigner crystal states, Chern insulator states and quantum simulators.

When moiré superlattices couple with light, they open up unprecedented possibilities for catching the first glimpse of many emergent photonic and optoelectronic phenomena. For example, moiré superlattices offer a powerful strategy for engineering excitonic quasiparticles in both real and momentum spaces, giving rise to quantum-dot-like moiré excitons and Bragg-umklapp moiré excitons, respectively.

Triggered by the breakthrough of moiré excitons, a plethora of fascinating photonic and optoelectronic properties have been witnessed in moiré superlattices over the past few years with unprecedented speed, including but not limited to moiré excitons/polaritons, resonantly hybridized excitons, reconstructed collective excitations, strong mid-/far-infrared photoresponses, terahertz single-photon detection, and symmetry-breaking optoelectronics.

The new degree of freedom afforded by using moiré superlattices provides new paradigms for engineering light-matter interactions for numerous applications, such as versatile quantum light sources, ultralow-threshold broadband excitonic lasing and intelligent infrared sensors.

The researchers also discussed future opportunities and research directions in this field, such as developing advanced techniques for probing emergent photonics and optoelectronics in an individual moiré supercell, exploring new ferroelectric, magnetic, and multiferroic moiré systems, and using external degrees of freedom to engineer moiré properties, thus resulting in exciting physics and potential technological innovations.

“The dizzying pace of recent achievements suggests that we are just starting down the path of exploring moiré photonics and optoelectronics,” said Prof. Du Luojun from IOP, first author of the study. Du also noted that the future progression of the field will undoubtedly bring more surprises and further transform the landscape of basic scientific research and technological innovation in physics, materials science, optical quantum technologies, energy harvesting, information and beyond. Overall, the era of moiré photonics and optoelectronics is coming.

More information: Luojun Du et al, Moiré photonics and optoelectronics, Science (2023). DOI: 10.1126/science.adg0014. www.science.org/doi/10.1126/science.adg0014

Journal information: Science 

Provided by Chinese Academy of Sciences