Category: Physics

by Waseda University

Magnetic monopoles are elementary particles with isolated magnetic charges in three dimensions. In other words, they behave as isolated north or south poles of a magnet. Magnetic monopoles have attracted continuous research interest since physicist Paul Dirac’s first proposal in 1931. However, real magnetic monopoles have not yet been observed and their existence remains an open question. On the other hand, scientists have discovered quasi-particles that mathematically behave as magnetic monopoles in condensed matter systems, resulting in interesting phenomena.

Recently, researchers discovered that a material called manganese germanide (MnGe) has a unique periodic structure, formed by special magnetic configurations called hedgehogs and antihedgehogs, which is called a magnetic hedgehog lattice.

In these special configurations, the magnetic moments point radially outward (hedgehog) or inward (antihedgehog), resembling the spines of a hedgehog. These hedgehogs and antihedgehogs act like magnetic monopoles and antimonopoles, serving as sources or sinks of emergent magnetic fields.

MnGe exhibits what is known as a triple-Q hedgehog lattice. However, recent experiments have shown that the substitution of Ge with Si (MnSi_1-xGe_x) transforms the arrangement into the quadruple-Q hedgehog lattice (4Q-HL).

This new arrangement, also found in the perovskite ferrite SrFeO₃, provides a promising avenue for studying and controlling the properties of hedgehog lattices. Moreover, these magnetic monopoles can also induce electric fields through moving following Maxwell’s laws of electromagnetism. To understand the resulting new physical phenomena, it is essential to study the inherent excitations of hedgehog lattices.

In a recent study, Professor Masahito Mochizuki and Ph.D. course student Rintaro Eto, both from the Department of Applied Physics at Waseda University, theoretically studied the collective excitation modes of 4Q-HLs in MnSi_1-xGe_x and SrFeO₃. Their study was published in the journal Physical Review Letters on 31 May 2024.

“Our research clarified the unknown dynamical nature of emergent magnetic monopoles in magnetic materials for the first time. This can inspire future experiments on hedgehog-hosting materials with applications in electronic devices and for bridging particle physics and condensed-matter physics,” says Mochizuki.

Utilizing the three-dimensional Kondo-lattice model, the researchers reproduced the two distinct 4Q-HLs found in MnSi_1-xGe_x and SrFeO₃ and analyzed their dynamical properties. They discovered that the 4Q-HLs have collective excitation modes associated with the oscillation of Dirac strings.

A Dirac string is a theoretical concept in quantum mechanics which describes a string that connects a magnetic monopole and a magnetic antimonopole, in this case, a hedgehog and an antihedgehog.

The researchers found that the number of these excitation modes depends on the number and configuration of Dirac strings, offering a way to experimentally determine the spatial configuration of hedgehogs and antihedgehogs and their unique topology in real magnets such as MnSi_1-xGe_x and SrFeO₃.

This finding offers insights into the dynamics of hedgehog lattices in other magnets as well. Moreover, the finding enables us to switch on and off the excitation modes through controlling the presence or absence of the Dirac strings with external magnetic field.

Explaining the significance of their results, Eto said, “The collective spin excitation modes revealed in the study are elementary excitations that directly reflect the presence (or absence) of emergent magnetic monopoles. Thus, our findings will be a fundamental guideline for studying more detailed dynamical nature of emergent monopoles in magnetic materials in the future.

“Moreover, they might become the building blocks of novel field-switchable spintronic devices such as nano-sized power generators, light-voltage converters, and light/microwave filters based on emergent electromagnetism.”

These discoveries have the potential to open new research avenues in fundamental physics and lead to the development of new technologies involving emergent magnetic monopoles in magnets.

More information: Rintaro Eto et al, Theory of Collective Excitations in the Quadruple- Q Magnetic Hedgehog Lattices, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.226705. On arXiv: DOI: 10.48550/arxiv.2403.01765

Journal information: Physical Review Letters  , arXiv 

Provided by Waseda University 

by FLEET

A team of Monash researchers have uncovered for the first time the full effects of interactions between exciton-polaritons and their associated dark excitonic reservoir. The study, “Polaronic polariton quasiparticles in a dark excitonic medium,” was published in Physical Review B.

Exciton-polaritons (polaritons, for short) are hybrid mixtures of light and matter that inherit the best properties of both. They form in semiconductors sandwiched between two mirrors, through which a laser is shone.

“Polaritons capture useful properties of both matter and light,” says lead author Kenneth Choo. “Their ‘matter’ part means we can manipulate them through their interactions. Meanwhile, the ‘light’ part makes the polariton almost massless, resulting in the formation of a superfluid condensate that flows effortlessly.

“Put them together, and a new kind of ‘liquid light’ that we can guide and control is formed, potentially the basis for the next generation of computing that uses light to build circuits,” says Kenneth, who is a Ph.D. candidate at Monash.

But with every yang there is a yin, and so it is with the polariton’s dark counterpart—a dark exciton.

These dark excitons are uncoupled to light yet still able to interact with the polaritons.

“Whenever the polaritons are formed in experiments, they tend to be accompanied by a substantial reservoir of these dark excitons which form together with them. The fact that they are dark, however, means it has been difficult to see their true effect,” says corresponding author Prof Meera Parish.

A famous technique dresses up for the occasion

The Monash researchers used a framework known as Fermi liquid theory to explain the effect of the dark reservoir on the bright polaritons. The essential idea is to map the (many, many) possible interactions onto a new ‘quasiparticle’—one with different masses, energies and lifetimes to the original. This allows the many-particle system to be treated as if it were a single particle.

One such quasiparticle is the polaron, which can be thought of as a ‘zone of influence’ around the original particle.

By using this technique and including all possible two-particle interactions, they found that the bright polariton becomes ‘dressed’ by excitations of the dark medium, forming a new quasiparticle called the polaron polariton.

Physically, this shows up as a reduction in the strength of the light-matter coupling as the dark exciton density is increased, an effect which is commonly seen in experiment.

In contrast, the strength of the interactions between the dark reservoir and the polaritons are increased, especially when the spins of the reservoir and polariton are different. This effect can be so strong that an additional quasiparticle is generated—the biexciton polariton—which is associated with the bound state of two opposite-spin excitons.

This phenomenon, known as a Feshbach resonance, was first observed in ultracold atomic gases, and offers the promise of being able to tune the interaction strength to whatever value is desired. It would then be possible to create reservoir ‘hills’ and ‘valleys’ that guide the flow of the polariton condensate, and then change them around at the flick of a (laser) switch.

What’s next?

This study opens up new opportunities in the field of reservoir engineering. Already, there has been experimental progress in using this dark reservoir to trap and focus the bright polaritons, and the study shows how the reservoir can be used to fundamentally change the polaritons themselves.

In addition, this study provides a new way to look at old results—by accounting for the hitherto unknown influence of the reservoir, a more accurate determination of interaction strengths can be obtained.

More information: Kenneth Choo et al, Polaronic polariton quasiparticles in a dark excitonic medium, Physical Review B (2024). DOI: 10.1103/PhysRevB.109.195432. On arXiv: DOI: 10.48550/arxiv.2312.00985

Journal information: Physical Review B  , arXiv 

Provided by FLEET

by Tejasri Gururaj , Phys.org

A new study published in Nature Physics introduces a theory of electron-phonon coupling that is affected by the quantum geometry of the electronic wavefunctions.

The movement of electrons in a lattice and their interactions with the lattice vibrations (or phonons) play a pivotal role in phenomena like superconductivity (resistance-free conductivity).

Electron-phonon coupling (EPC) is the interaction between free electrons and phonons, which are quasiparticles representing the vibrations of a crystal lattice. EPC leads to the formation of Cooper pairs (pairs of electrons), responsible for superconductivity in certain materials.

The new study explores the realm of quantum geometry in materials and how these can contribute to the strength of EPC.

Phys.org spoke to the first author of the study, Dr. Jiabin Yu, Moore Postdoctoral Fellow at Princeton University.

Speaking of the motivation behind the study, Dr. Yu said, “My motivation is to go beyond the common wisdom and find out how the geometric and topological properties of wavefunctions affect interactions in quantum materials. In this work, we focus on EPC, one of the most important interactions in quantum materials.”

Electronic wavefunctions and EPC

A quantum state is described by a wavefunction, a mathematical equation holding all the information about the state. An electronic wavefunction is basically a way to measure the probability of where the electron is located in the lattice (arrangement of atoms in a material).

“In condensed matter physics, people have long used energies to study the behavior of materials. In the last several decades, a paradigm shift led us to understand that the geometric and topological properties of wavefunctions are crucial in understanding and classifying realistic quantum materials,” explained Dr. Yu.

In the context of EPC, the interaction between the two depends on the location of the electron within the crystal lattice. This means that the electronic wavefunction, to some extent, governs which electrons can couple with phonons and impact the conductivity properties of that material.

The researchers in this study wanted to explore the effect of quantum geometry on the EPC in materials.

Quantum geometry

A wavefunction, as mentioned before, describes the state of a quantum particle or system.

These wavefunctions are not always static, and their shape, structure, and distribution can evolve over space and time, just like how a wave in the ocean changes. But unlike waves in the ocean, quantum mechanical wavefunctions follow the laws of quantum mechanics.

Quantum geometry explores this variation of spatial and temporal characteristics of wavefunctions.

“The geometric properties of single-particle wavefunctions are called band geometry or quantum geometry,” explained Dr. Yu.

In condensed matter physics, the band structure of materials describes the energy levels available to electrons in a crystal lattice. Think of them as steps of a ladder, with the energy increasing the higher you go.

Quantum geometry influences the band structure by affecting the spatial extent and shape of electron wavefunctions within the lattice. In simple terms, the distribution of electrons affects the energy structure or layout for electrons in a crystal lattice.

The energy levels in a lattice are crucial as they determine important properties like conductivity. Additionally, the band structure will vary from material to material.

Gaussian approximation and hopping

The researchers built their model by using Gaussian approximation. This method simplifies complex interactions (such as those between electrons and phonons) by approximating the distribution of variables like energies as Gaussian (or normal) distributions.

This makes it easier to handle mathematically and draw conclusions about the influence of quantum geometry on EPC.

“The Gaussian approximation is essentially a way to relate the real-space electron hopping to the momentum-space quantum geometry,” said Dr. Yu.

Electron hopping is a phenomenon in crystal lattices where electrons move from one site to another. For hopping to occur effectively, the wavefunctions of electrons at neighboring sites must overlap, allowing electrons to tunnel through the potential barriers between sites.

The researchers found that the overlapping was affected by the quantum geometry of the electronic wavefunction, thus affecting hopping.

“The EPC often comes from the change of the hopping with respect to the lattice vibrations. So naturally, the EPC should be enhanced by strong quantum geometry,” explained Dr. Yu.

They quantified this by measuring the EPC constant, which tells the strength of the coupling or interaction, using the Gaussian approximation.

To test their theory, they applied it to two materials, graphene and magnesium diboride (MgB₂).

Superconductors and applications

The researchers chose to test their theory on graphene and MgB₂ because both materials have superconducting properties driven by EPC.

They found that for both materials, EPC was strongly influenced by geometric contributions. Specifically, the geometric contributions were measured to be 50% and 90% for graphene and MgB₂, respectively.

They also found the existence of a lower bound or limit for the contributions due to quantum geometry. In simple words, there is a minimum contribution towards the EPC constant due to quantum geometry, and the rest of the contribution is from the energy of the electrons.

Their work suggests that increasing superconducting critical temperature, which is the temperature below which superconductivity is observed, can be done by enhancing EPC.

Certain superconductors like MgB₂ are phonon-mediated, meaning that EPC directly affects their superconducting properties. According to the research, strong quantum geometry implies strong EPC, opening a new route to search for relatively high-temperature superconductors.

“Even if EPC cannot mediate superconductivity alone, it can help cancel part of the repulsive interaction and help generate superconductivity,” added Dr. Yu.

Future work

The theory developed by the researchers has only been tested for certain materials, which means it is not universal. Dr. Yu believes that the next step is to generalize this theory to make it applicable to all materials.

This is especially important for developing and understanding different quantum materials (like topological insulators) that could be affected by quantum geometry.

“Quantum geometry is ubiquitous in quantum materials. Researchers know it should affect many quantum phenomena, but often lack theories that clearly capture this effect. Our work is one step towards such a general theory, but we are still far away from fully understanding it,” concluded Dr. Yu.

More information: Jiabin Yu et al, Non-trivial quantum geometry and the strength of electron–phonon coupling, Nature Physics (2024). DOI: 10.1038/s41567-024-02486-0.

Journal information: Nature Physics 

by University of Michigan

Silicon is one of the most pervasive functional materials of the modern age, underpinning semiconductor technologies ranging from microelectronics to solar cells. Indeed, silicon transistors enable computing applications from cell phones to supercomputers, while silicon photovoltaics are the most widely deployed solar-cell technology to date.

The U.S. Department of Energy (DOE) reports that nearly 50% of new electric generation capacity in 2022 came from solar cells, and according to the International Energy Agency (IEA), silicon has a 95% market share. Yet despite silicon’s undisputed importance to our modern way of life, many open questions remain about its fundamental physical properties.

In semiconductor devices, the functionality of the material comes from the motion and interactions of subatomic particles such as electrons (which have negative charge) and holes (the absence of an electron from an otherwise occupied state that itself behaves like a positively charged particle), which are called carriers as they “carry” electrical charge through the material.

For example, in a solar cell, the material absorbs incoming light, and the absorbed energy is converted into pairs of electrons and holes. These excited electron and holes then move to opposite ends of the solar cell and generate electricity.

Unfortunately, the electrons and holes can also interact in undesirable ways that convert their energy to heat and limit the efficiency of devices. One such loss mechanism occurs when carriers recombine and convert their energy to heat by interacting with a defect in the material. In many cases this defect-mediated recombination can be reduced by improving the quality of the material.

Other interactions, however, are intrinsic to a material and cannot be eliminated even in perfectly pure samples. Auger-Meitner recombination (AMR), historically known also as Auger recombination, is one such interaction. It is named after Lise Meitner and Pierre Auger, two pioneers of nuclear science who independently discovered this effect in atoms.

The new naming convention of the Auger-Meitner effect recognizes the contributions of Lise Meitner, a female Austrian physicist and the eponym of the Meitnerium chemical element, who independently discovered the process a year prior to Pierre Auger.

In the AMR process in semiconductors, one electron and one hole recombine, transferring their energy to a third carrier. The high-energy carrier can then thermalize or leak out of a device, generating heat and reducing the energy-conversion efficiency or reducing the number of available carriers. Unfortunately, despite decades of research, the specific atomistic mechanisms of AMR in silicon have eluded researchers to this date.

With a new implementation of a computational methodology to accurately calculate AMR rates from first principles—that is using only the physical constants of the universe and the atomic number of silicon as input—Dr. Kyle Bushick and Prof. Emmanouil Kioupakis of Materials Science and Engineering at the University of Michigan have provided the first comprehensive characterization of this important recombination process in silicon. This computational approach is key to gaining a full understanding of the AMR mechanism, because it is a process that does not emit light, making it very difficult to study in the lab.

With the aid of supercomputing resources at the National Energy Research Scientific Computing Center (NERSC) of Lawrence Berkeley National Lab, Bushick and Kioupakis were able to carry out the calculations of AMR in silicon, gaining insights to the behavior of the material at an atomic level.

One reason the AMR process in silicon has not been fully understood is that it includes multiple permutations. On one hand, the excited (third) carrier can either be an electron, or a hole, giving rise to the electron-electron-hole (eeh) and hole-hole-electron (hhe) processes, respectively.

Furthermore, AMR can be both direct, where only the three carriers participate, or phonon-assisted, where one of the carriers interacts with the vibrating atoms (phonons) to transfer additional momentum. While experiments can characterize the combined total AMR rate, parsing out the different contributions from these different components can be much harder. However, by using predictive atomistic calculations, each individual component can be directly computed and characterized.

Although past work had investigated the direct process using such calculations, it was clear that the direct process alone didn’t capture the full experimental picture. By overcoming the added complexity of calculating both the direct and phonon-assisted processes at the same level of theory, many of the unanswered questions about AMR in silicon could be addressed. Additionally, achieving such a detailed understanding of the process then opens the door for finding solutions to reduce the impact of AMR on device efficiency.

In their report, published in Physical Review Letters, Bushick and Kioupakis unequivocally elucidate the importance of the phonon-assisted AMR process in silicon.

“We found that the electron-phonon interactions not only account for the entirety of the hhe process, which was hypothesized in previous works but never conclusively demonstrated, but also for a significant portion of the eeh process, a finding that had been a subject of unresolved debate in the literature,” says Bushick, a recently graduated Ph.D. student of Materials Science and Engineering and a DOE Computational Science Graduate Fellow.

Furthermore, they highlight a potential pathway for altering AMR in silicon by applying strain to the material, a conclusion made possible by their newly implemented methodology.

This work provides a hitherto inaccessible fundamental understanding of an important intrinsic loss mechanism in the world’s most important semiconductor. This understanding, which has eluded scientists for decades, can help design better devices with improved performance by reducing the occurrence of the undesirable AMR process.

Emmanouil Kioupakis, Associate Professor of Materials Science and Engineering and Karl F. and Patricia J. Betz Family Faculty Scholar at the University of Michigan notes, “Ultimately, this work paves the way to understand and mitigate losses in silicon devices such as transistors or solar cells. Considering the size of these industries, even small improvements can lead to massive benefits.”

More information: Kyle Bushick et al, Phonon-Assisted Auger-Meitner Recombination in Silicon from First Principles, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.076902

Journal information: Physical Review Letters 

Provided by University of Michigan 

by Dawn Levy, Oak Ridge National Laboratory

Timothy Gray of the Department of Energy’s Oak Ridge National Laboratory led a study that may have revealed an unexpected change in the shape of an atomic nucleus. The surprise finding could affect our understanding of what holds nuclei together, how protons and neutrons interact and how elements form.

“We used radioactive beams of excited sodium-32 nuclei to test our understanding of nuclear shapes far from stability and found an unexpected result that raises questions about how nuclear shapes evolve,” said Gray, a nuclear physicist. The results are published in Physical Review Letters.

The shapes and energies of atomic nuclei can shift over time between different configurations. Typically, nuclei live as quantum entities that have either spherical or deformed shapes. The former look like basketballs, and the latter resemble American footballs.

How shapes and energy levels relate is a major open question for the scientific community. Nuclear structure models have trouble extrapolating to regions with little experimental data.

For some exotic radioactive nuclei, the shapes predicted by traditional models are the opposite of those observed. Radioactive nuclei that were expected to be spherical in their ground states, or lowest-energy configurations, turned out to be deformed.

What can turn a quantum state on its head?

In principle, the energy of an excited deformed state can drop below that of a spherical ground state, making the spherical shape the high-energy one. Unexpectedly, this role reversal appears to be happening for some exotic nuclei when the natural ratio of neutrons to protons becomes unbalanced. Yet, the post-reversal excited spherical states have never been found. It is as though once the ground state becomes deformed, all the excited states do, too.

Many examples exist of nuclei with spherical ground states and deformed excited states. Similarly, plenty of nuclei have deformed ground states and subsequent excited states that are also deformed—sometimes with different amounts or kinds of deformation. However, nuclei with both deformed ground states and spherical excited states are much more elusive.

Using data collected in 2022 from the first experiment at the Facility for Rare Isotope Beams, or FRIB, a DOE Office of Science user facility at Michigan State University, Gray’s team discovered a long-lived excited state of radioactive sodium-32. The newly observed excited state has an unusually long lifetime of 24 microseconds—about a million times longer than a typical nuclear excited state.

Long-lived excited states are called isomers. A long lifetime indicates that something unanticipated is going on. For example, if the excited state is spherical, a difficulty in returning to a deformed ground state could account for its long life.

The study involved 66 participants from 20 universities and national laboratories. Co-principal investigators came from Lawrence Berkeley National Laboratory, Florida State University, Mississippi State University, the University of Tennessee, Knoxville, and ORNL.

The 2022 experiment that generated the data used for the 2023 result employed the FRIB Decay Station initiator, or FDSi, a modular multidetector system that is extremely sensitive to rare isotope decay signatures.

“FDSi’s versatile combination of detectors shows that the long-lived excited state of sodium-32 is delivered within the FRIB beam and that it then decays internally by emitting gamma rays to the ground state of the same nucleus,” said ORNL’s Mitch Allmond, a co-author of the paper who manages the FDSi project.

To stop FRIB’s highly energetic radioactive beam, which travels at about 50% of the speed of light, an implantation detector built by UT Knoxville was positioned at FDSi’s center. North of the beam line was a gamma-ray detector array called DEGAi, comprising 11 germanium clover-style detectors and 15 fast-timing lanthanum bromide detectors. South of the beam line were 88 modules of a detector called NEXTi to measure time of flight of neutrons emitted in radioactive decay.

A beam of excited sodium-32 nuclei stopped in the detector and decayed to the deformed ground state by emitting gamma rays. Analysis of gamma-ray spectra to discern the time difference between beam implantation and gamma-ray emission revealed how long the excited state existed. The new isomer’s 24-microsecond existence was the longest lifetime seen among isomers with 20 to 28 neutrons that decay by gamma-ray emission. Approximately 1.8% of the sodium-32 nuclei were observed to be the new isomer.

“We can come up with two different models that equally well explain the energies and lifetime that we’ve observed in the experiment,” Gray said.

An experiment with higher beam power is needed to determine whether the excited state in sodium-32 is spherical. If it is, then the state would have six quantized units of angular momentum, which is a quality of a nucleus related to its whole-body rotation or the orbital motion of its individual protons and/or neutrons about the center of mass. However, if the excited state in sodium-32 is deformed, then the state would have zero quantized units of angular momentum.

A planned upgrade to FRIB will provide more power, increasing the number of nuclei in the beam. Data from the more intense beam will enable an experiment that distinguishes between the two possibilities.

“We’d characterize correlations between the angles of two gamma rays that are emitted in a cascade,” Gray said. “The two possibilities have very different angular correlations between the gamma rays. If we have enough statistics, we could disentangle the pattern that reveals a clear answer.”

More information: T. J. Gray et al, Microsecond Isomer at the N=20 Island of Shape Inversion Observed at FRIB, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.242501

Journal information: Physical Review Letters 

Provided by Oak Ridge National Laboratory 

by Princeton University

A Princeton University-led team of scientists has imaged the precise microscopic underpinnings responsible for many quantum phases observed in a material known as magic-angle twisted bilayer graphene (MATBG). This remarkable material, which consists of twisted layers of carbon atoms arranged in a two-dimensional hexagonal pattern, has in recent years been at the forefront of research in physics, especially in condensed matter physics.

For the first time, the researchers were able to specifically capture unprecedentedly precise visualizations of the microscopic behavior of interacting electrons that give rise to the insulating quantum phase of MATBG. Additionally, through the use of novel and innovative theoretical techniques, they were able to interpret and understand these behaviors. Their study is published in the journal Nature.

The amazing properties of twisted bilayer graphene were first discovered in 2018 by Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT). They showed that this material can be superconducting, a state in which electrons flow freely without any resistance. This state is vital to many of our everyday electronics, including magnets for MRIs and particle accelerators as well as in the making of quantum bits (called qubits) that are being used to build quantum computers.

Since that discovery, twisted bilayer graphene has demonstrated many novel quantum physical states, such as insulating, magnetic, and superconducting states, all of which are created by complex interactions of electrons. How and why electrons form insulating states in MATBG has been one of the key unsolved puzzles in the field.

The solution to this puzzle would not only unlock our understanding of both the insulator and the proximate superconductor, but also such behavior shared by many unusual superconductors that scientists seek to understand, including the high-temperature cuprate superconductors.

“MATBG shows a lot of interesting physics in a single material platform－much of which remains to be understood,” said Kevin Nuckolls, the co-lead author of the paper, who earned his Ph.D. in 2023 in Princeton’s physics department and is now a postdoctoral fellow at MIT. “This insulating phase, in which electrons are completely blocked from flowing, has been a real mystery.”

To create the desired quantum effects, researchers place two sheets of graphene on top of each other with the top layer angled slightly. This off-kilter position creates a moiré pattern, which resembles and is named after a common French textile design. Importantly, however, the angle at which the top layer of graphene must be positioned is precisely 1.1 degrees. This is the “magic” angle that produces the quantum effect; that is, this angle induces strange, strongly correlated interactions between the electrons in the graphene sheets.

While physicists have been able to demonstrate different quantum phases in this material, such as the zero-resistance superconducting phase and the insulating phase, there has been very little understanding of why these phases occur in MATBG. Indeed, all previous experiments involving MATBG give good demonstrations of what the system is capable of producing, but not why the system is producing these states.

And that “why” became the basis for the current experiment.

“The general idea of this experiment is that we wanted to ask questions about the origins of these quantum phases—to really understand what exactly are the electrons doing on the graphene atomic scale,” said Nuckolls. “Being able to probe the material microscopically, and to take images of its correlated states—to fingerprint them, effectively—gives us the ability to discern very distinctly and precisely the microscopic origins of some of these phases. Our experiment also helps guide theorists in the search for phases that were not predicted.”

The study is the culmination of two years of work and was achieved by a team from Princeton University and the University of California, Berkeley. The scientists harnessed the power of the scanning tunneling microscope (STM) to probe this very minute realm. This tool relies on a technique called “quantum tunneling,” where electrons are funneled between the sharp metallic tip of the microscope and the sample. The microscope uses this tunneling current rather than light to view the world of electrons on the atomic scale. Measurements of these quantum tunneling events are then translated into high resolution, highly sensitive images of materials.

However, the first step—and perhaps the most crucial step in the experiment’s success—was the creation of what the researchers refer to as a “pristine” sample. The surface of carbon atoms that constituted the twisted bilayer graphene sample had to have no flaws or imperfections.

“The technical breakthrough that made this paper happen was our group’s ability to make the samples so pristine in terms of their cleanliness such that these high-resolution images that you see in the paper were possible,” said Ali Yazdani, the Class of 1909 Professor of Physics and Director of the Center for Complex Materials at Princeton University. “In other words, you have to make one hundred thousand atoms without a single flaw or disorder.”

The actual experiment involved placing the graphene sheets in the correct “magic angle,” at 1.1 degrees. The researchers then positioned the sharp, metallic tip of the STM over the graphene sample and measured the quantum mechanical tunneling current as they moved the tip across the sample.

“Electrons at this quantum scale are not only particles, but they are also waves,” said Ryan Lee, a graduate student in the Department of Physics at Princeton and one of the paper’s co-lead authors. “And essentially, we’re imaging wave-like patterns of electrons, where the exact way that they interfere (with each other) is telling us some very specific information about what is giving rise to the underlying electronic states.”

This information allowed the researchers to make some very incisive interpretations about the quantum phases that were produced by the twisted bilayer graphene. Importantly, the researchers used this information to focus on and solve the long-standing puzzle that for many years has challenged researchers working in this field, namely, the quantum insulating phase that occurs when graphene is tuned to its magic angle.

To help understand this from a theoretical viewpoint, the Princeton researchers collaborated with a team from the University of California-Berkeley, led by physicists B. Andrei Bernevig at Princeton and Michael Zaletel at Berkeley. This team developed a novel and innovative theoretical framework called “local order parameter” analysis to interpret the STM images and understand what the electrons were doing—in other words, how they were interacting—in the insulating phase. What they discovered was that the insulating state occurs because of the strong repulsion between the electrons, on the microscopic level.

“In magic-angle twisted bilayer graphene, the challenge was to model the system,” said Tomohiro Soejima, a graduate student and theorist at U.C. Berkeley and one of the paper’s co-lead authors. “There were many competing theories, and no one knew which one was correct. Our experiment of ‘finger-printing’ was really crucial because that way we could pinpoint the actual electronic interactions that give rise to the insulating phase.”

By using this theoretical framework, the researchers were able, for the first time, to make a measurement of the observed wave functions of the electrons. “The experiment introduces a new way of analyzing quantum microscopy,” said Yazdani.

The researchers suggest the technology—both the imagery and the theoretical framework—can be used in the future to analyze and understand many other quantum phases in MATBG, and ultimately, to help comprehend new and unusual material properties that may be useful for next-generation quantum technological applications.

“Our experiment was a wonderful example of how Mother Nature can be so complicated—can be really confusing—until you have the right framework to look at it, and then you say, ‘oh, that’s what’s happening,'” said Yazdani.

More information: Kevin P. Nuckolls et al, Quantum textures of the many-body wavefunctions in magic-angle graphene, Nature (2023). DOI: 10.1038/s41586-023-06226-x

Journal information: Nature 

Provided by Princeton University 

by Liu Jia, Chinese Academy of Sciences

Microplastics—plastics particles smaller than 5 mm in size—have caused an environmental pollution issue that cannot be ignored by our society. Raman spectroscopy technology, with its non-contact, non-destructive and chemical-specific characteristics, has been widely applied in the field of microplastics detection. However, conventional point confocal Raman techniques are limited to single-point detection, impeding the detection speed.

In a study published in Talanta, a research group led by Prof. Li Bei from the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. Wolfgang Langbein from Cardiff University, proposed a novel line-scan Raman micro-spectroscopy technique for rapid identification of micro- and nanoplastics.

Based on the fundamental principles of confocal Raman spectroscopy, the focused excitation spot transforms from a convergent point into a convergent line with diffraction-limited width. The optical setup employs a conjugate imaging design. In the two-dimensional image recorded by the charge-coupled device (CCD), the vertical dimension maps the vertical dimension of the sample along the excitation line, while the spectrum is dispersed along the horizontal dimension. In this way, a single acquisition provides the spectra for all spatial positions along the excitation line.

Researchers developed a confocal line-scan Raman micro-spectroscopy system, established a preprocessing workflow for line-scan Raman spectral data, and applied the factorization into susceptibilities and concentrations (FSC³) algorithm to obtain Raman hyperspectral images. They employed a concave cylindrical lens to generate the excitation line and improved the uniformity of energy distribution using a Powell lens.

Plastic beads of various sizes were used for size and composition identification. The detection of beads with a diameter of 200 nm, which is smaller than the diffraction limit, was realized, demonstrating the exceptional sensitivity of the line-scan Raman spectroscopy system.

Furthermore, four types of plastic powder samples were used for a large-scale area of 1.2 mm in length and 40 μm in height measurement. Impressively, the imaging time is 20 minutes to obtain a 240,000-pixel Raman image. Compared with point confocal Raman imaging, the line-scan confocal Raman technology increases the imaging speed by two orders of magnitude.

Line-scan Raman micro-spectroscopy offers non-destructive analysis with high sensitivity and high-throughput. By employing appropriate sampling techniques such as filtration or sedimentation, environmental samples from various sources, including water, soil and air, are accessible.

More information: Qingyi Wu et al, Rapid identification of micro and nanoplastics by line scan Raman micro-spectroscopy, Talanta (2023). DOI: 10.1016/j.talanta.2023.125067

Provided by Chinese Academy of Sciences

by Brown University

One of the most common and practically useful experiments in all of fluid dynamics involves holding an object in air or submerging it fully underwater, exposing it to a steady flow to measure its resistance in the form of drag. Studies on drag resistance have led to technological advances in airplane and vehicle design and even advanced our understanding of environmental processes.

That’s much tougher these days. As one of the most thoroughly studied aspects in fluid dynamics, it’s become hard to glean or detail new information on the simple physics of drag resistance from these classic experiments. But a team of engineers led by Brown University scientists managed to do so by bringing this problem to the surface—the water surface, that is.

Described in an new paper in Physical Review Fluids, the researchers created a small river-like channel in the lab and lowered spheres—made of different water repellent materials—into the stream until they were almost fully submerged by the flowing water.

The results from the experiment illustrate the fundamental—and sometimes counterintuitive—mechanics of how drag on a partially submerged object may be several times greater than drag on a fully submerged object made of the same material.

For instance, the researchers—led by Brown engineers Robert Hunt and Daniel Harris—found that drag on the spheres increased the moment they touched the water, no matter how water repellent the sphere material was. Each time, the drag increased substantially more than what was expected and continued to increase as the spheres were lowered, beginning only to drop when the spheres were fully beneath the water.

“There’s this intermediate period where the spheres going into the water are creating the biggest disturbances so that the drag is much stronger than if it were way below the surface,” said Harris, an assistant professor in Brown’s School of Engineering. “We knew the drag would go up as the spheres were lowered because they are blocking more of the steady flow, but the surprising thing was how much it goes up. Then as you keep pushing the sphere deeper, the drag goes back down.”

The study shows drag forces on partially submerged objects can be three or four times greater than on fully submerged objects. The largest drag forces, for instance, were measured just prior to the sphere becoming fully submerged, meaning water is flowing all around it but there’s still a small dry spot sticking out at the surface.

“You might expect how much of the sphere is in the water to correspond with how big the drag is,” said Hunt, a postdoctoral researcher in Harris’ lab and the study’s first author. “If so, then you might naively approximate the drag by saying that if the sphere is almost 100% in the water, the drag is going to be almost the same as if it was fully immersed beneath the surface. What we found is the drag can actually can be much larger than that—and not like 50% but more like 300% or 400%.”

The researchers also found that the sphere’s level of water repellency plays a key role in the drag forces it experiences. This is where things get a bit counterintuitive.

The experiment was done with three spheres that are otherwise identical except one was coated with a superhydrophobic material, making it very repellent to water, while the others were made of materials that are increasingly less water repellent.

Running the experiments, the researchers found that the superhydrophobic coating encountered more drag than the other two spheres. It was a surprise because they expected the opposite.

“Superhydrophobic materials are often proposed to reduce drag, but, in our case, we found that superhydrophobic spheres when almost fully immersed have a much larger drag than the sphere made of any other water repellency,” Hunt said. “In trying to decrease the drag, you might actually increase it substantially.”

The paper explains simple physics is the likely cause.

“The water wants nothing to do with this superhydrophobic sphere so it does anything that it can to, sort of, get out of the way of the sphere,” Harris said. “But what happens is much of it piles up in front of it, so there ends up being a wall of water that the sphere is hitting. Intuitively, you would think the water should slip by more freely. Physics actually conspires against that in this scenario.”

The findings from the paper may one day hold implications for designs and structures that operate at an air and water interface, like small autonomous vehicles. For now, the standalone physics of this basic research is interesting enough as studies on partially submerged objects aren’t as currently well characterized or understood in the field.

“We were surprised no one had made these measurements,” Harris said. “It’s such a simple idea but there’s just a lot of rich physics here.”

The researchers chose spheres as the first three-dimensional objects because of how simple their geometry is. They only have one length scale—the radius. The sphere acts as a starting point to be able to strip the physical mechanics down to its most fundamental principles before moving on to more complicated shapes.

“Starting from the simplest point, we look at what are the physics here and then as a next step we begin to apply our knowledge to more realistic structures, whether it’s emulating a biological structure or looking at manmade propulsive structures,” Harris said.

Hunt and fellow lab member Eli Silver designed the flume apparatus for creating the water stream experiment and programmed the motorized lift that lowers the spheres into the water channel. The work started as a collaboration with Yuri Bazilevs, a professor at Brown’s School of Engineering. It also included researchers from the University of Illinois Urbana-Champagne, who performed computer simulations.

More information: Robert Hunt et al, Drag on a partially immersed sphere at the capillary scale, Physical Review Fluids (2023). DOI: 10.1103/PhysRevFluids.8.084003

Provided by Brown University 

by Petra Giegerich, Johannes Gutenberg University Mainz

There has been significant progress in the field of quantum computing. Big global players, such as Google and IBM, are already offering cloud-based quantum computing services. However, quantum computers cannot yet help with problems that occur when standard computers reach the limits of their capacities because the availability of qubits or quantum bits, i.e., the basic units of quantum information, is still insufficient.

One of the reasons for this is that bare qubits are not of immediate use for running a quantum algorithm. While the binary bits of customary computers store information in the form of fixed values of either 0 or 1, qubits can represent 0 and 1 at one and the same time, bringing probability as to their value into play. This is known as quantum superposition.

This makes them very susceptible to external influences, which means that the information they store can readily be lost. In order to ensure that quantum computers supply reliable results, it is necessary to generate a genuine entanglement to join together several physical qubits to form a logical qubit. Should one of these physical qubits fail, the other qubits will retain the information. However, one of the main difficulties preventing the development of functional quantum computers is the large number of physical qubits required.

Advantages of a photon-based approach

Many different concepts are being employed to make quantum computing viable. Large corporations currently rely on superconducting solid-state systems, for example, but these have the disadvantage that they only function at temperatures close to absolute zero. Photonic concepts, on the other hand, work at room temperature.

Single photons usually serve as physical qubits here. These photons, which are, in a sense, tiny particles of light, inherently operate more rapidly than solid-state qubits but, at the same time, are more easily lost. To avoid qubit losses and other errors, it is necessary to couple several single-photon light pulses together to construct a logical qubit—as in the case of the superconductor-based approach.

A qubit with the inherent capacity for error correction

Researchers of the University of Tokyo together with colleagues from Johannes Gutenberg University Mainz (JGU) in Germany and Palacký University Olomouc in the Czech Republic have recently demonstrated a new means of constructing a photonic quantum computer. Rather than using a single photon, the team employed a laser-generated light pulse that can consist of several photons. The research is published in the journal Science.

“Our laser pulse was converted to a quantum optical state that gives us an inherent capacity to correct errors,” stated Professor Peter van Loock of Mainz University. “Although the system consists only of a laser pulse and is thus very small, it can—in principle—eradicate errors immediately.” Thus, there is no need to generate individual photons as qubits via numerous light pulses and then have them interact as logical qubits.

“We need just a single light pulse to obtain a robust logical qubit,” added van Loock. To put it in other words, a physical qubit is already equivalent to a logical qubit in this system—a remarkable and unique concept. However, the logical qubit experimentally produced at the University of Tokyo was not yet of a sufficient quality to provide the necessary level of error tolerance. Nonetheless, the researchers have clearly demonstrated that it is possible to transform non-universally correctable qubits into correctable qubits using the most innovative quantum optical methods.

by Zhang Nannan, Chinese Academy of Sciences

High pressure has revealed surprising physics and created novel states in condensed matter. Exciting examples include near room temperature superconductivity (T_c > 200 K) in high-pressure hydrides such as H₃S and LaH₁₀.

Although the superconducting transition temperature of high-pressure superconductors is constantly increasing, the mechanism of superconductivity at such high pressures remains an open question. Knowledge of the properties and ultrafast dynamics of electrons and quasiparticles in high-pressure quantum states is lacking.

High harmonic generation (HHG) is the up-conversion of laser light to radiation carried at multiples of the laser frequency. HHG in solids originates from the nonlinear driving of electrons within and between electronic bands by strong field light-matter interactions. Therefore, HHG spectroscopy naturally contains fingerprints of intrinsic atomic and electronic properties of materials. There is a great deal of excitement in learning about material properties through this nonlinear, non-perturbative laser-matter interaction.

Using state-of-the-art first-principles time-dependent density-functional theory simulations, Prof. Meng Sheng’s group from the Institute of Physics of the Chinese Academy of Sciences has studied the ultrafast HHG dynamics in the high-pressure superconductor H₃S.

• Electron-phonon coupling reconstruction via HHG spectra. Credit: Institute of Physics
• Band structure reconstruction via HHG spectra. Credit: Institute of Physics
• Electron-phonon coupling reconstruction via HHG spectra. Credit: Institute of Physics
• Band structure reconstruction via HHG spectra. Credit: Institute of Physics

The researchers found that HHG in high-pressure superconductors is strongly dependent on the electronic structures and electron-phonon coupling (EPC). The study, titled “Solid-state high harmonic spectroscopy for all-optical band structure probing of high-pressure quantum states,” is published in PNAS.

Using HHG spectroscopy, they retrieved the band dispersion and EPC, and revealed the significant influence of the many-body EPC on the electron behavior near the Fermi level.

Their results support the phonon-mediated mechanism based on the EPC of high-pressure superconductivity, providing an all-optical approach to probe the band dispersion and EPC of high-pressure quantum states.

More information: Shi-Qi Hu et al, Solid-state high harmonic spectroscopy for all-optical band structure probing of high-pressure quantum states, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2316775121