The University of Minnesota researchers’ new method of searching for the hypothetical axion involves measuring the “decay” of the particle into two muons—known particles that are essentially the heavier version of the electron—as illustrated in the above image. Credit: Raymond Co, University of Minnesota
One of the most high-profile mysteries in physics today is what scientists refer to as the “Strong CP Problem.” Stemming from the puzzling phenomenon that neutrons do not interact with electric fields despite being made up of quarks—smaller, fundamental particles that carry electric charges—the Strong CP Problem puts into question the Standard Model of physics, or the set of theories scientists have been using to explain the laws of nature for years.
A team led by University of Minnesota Twin Cities theoretical physicists has discovered a new way to search for axions, hypothetical particles that could help solve this mystery. Working in collaboration with experimental researchers at the Fermilab National Accelerator Laboratory, the physicists’ new strategy opens up previously unexplored opportunities to detect axions in particle collider experiments.
The researchers’ paper is published and featured as the Editor’s suggestion in Physical Review Letters.
“As particle physicists, we’re trying to develop our best understanding of nature,” said Zhen Liu, co-author of the paper and an assistant professor in the University of Minnesota School of Physics and Astronomy. “Scientists have been tremendously successful in the past century in finding elementary particles through established theoretical frameworks. So, it’s extremely puzzling why neutrons do not couple to electric fields because in our known theory, we would expect them to. If we do discover the axion, it will be a great advance in our fundamental understanding of the structure of nature.”
One of the primary means for studying subatomic particles, and potentially discovering new ones, is collider experiments. Essentially, scientists force beams of particles to collide—and when they hit each other, the energy they produce creates other particles that pass through a detector, allowing researchers to analyze their properties.
Liu and his team’s proposed method involves measuring the “decay” product—or what happens when an unstable heavy particle transforms into multiple lighter particles—of the hypothetical axion into two muons—known particles that are essentially the heavier version of the electron. By working backward from the muon tracks in the detector to reconstruct such decays, the researchers believe they have a chance to locate the axion and prove its existence.
“With this research, we’re expanding ways we can search for the axion particle,” said Raymond Co, co-author of the paper and a postdoctoral researcher in the University of Minnesota School of Physics and Astronomy and William Fine Theoretical Physics Institute. “People have never used axion decay into muons as a way to search for the axion particle in neutrino or collider experiments before. This research opens up new possibilities to pave the way for future endeavors in our field.”
Liu and Co, along with University of Minnesota physics and astronomy postdoctoral researcher Kun-Feng Lyu and University of California, Berkeley postdoctoral researcher Soubhik Kumar, are behind the theoretical part of the research. They’re a part of the ArgoNeuT collaboration, which brings together theorists and experimentalists from across the country to study particles through experiments at Fermilab.
In this paper, the University of Minnesota-led theoretical team worked with the experimental researchers to perform a search for axions using their new method and existing data from the ArgoNeuT experiment. The researchers plan to use the experimental results to further refine their theoretical calculations of the axion production rate in the future.
More information: R. Acciarri et al, First Constraints on Heavy QCD Axions with a Liquid Argon Time Projection Chamber Using the ArgoNeuT Experiment, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.221802
Three perspectives of the surface on which the electrons move. On the left, the experimental result, in the center and on the right the theoretical modeling. The red and blue colors represent a measure of the speed of the electrons. Both theory and experiment reflect the symmetry of the crystal, very similar to the texture of traditional Japanese “kagome” baskets. Credit: University of Bologna
An international research team has succeeded for the first time in measuring the electron spin in matter—i.e., the curvature of space in which electrons live and move—within “kagome materials,” a new class of quantum materials.
The results obtained—published in Nature Physics—could revolutionize the way quantum materials are studied in the future, opening the door to new developments in quantum technologies, with possible applications in a variety of technological fields, from renewable energy to biomedicine, from electronics to quantum computers.
Success was achieved by an international collaboration of scientists, in which Domenico Di Sante, professor at the Department of Physics and Astronomy “Augusto Righi,” participated for the University of Bologna as part of his Marie Curie BITMAP research project. He was joined by colleagues from CNR-IOM Trieste, Ca’ Foscari University of Venice, University of Milan, University of Würzburg (Germany), University of St. Andrews (UK), Boston College and University of Santa Barbara (U.S.).
Through advanced experimental techniques, using light generated by a particle accelerator, the Synchrotron, and thanks to modern techniques for modeling the behavior of matter, the scholars were able to measure electron spin for the first time, related to the concept of topology.
“If we take two objects such as a football and a doughnut, we notice that their specific shapes determine different topological properties, for example because the doughnut has a hole, while the football does not,” Domenico Di Sante explains. “Similarly, the behavior of electrons in materials is influenced by certain quantum properties that determine their spinning in the matter in which they are found, similar to how the trajectory of light in the universe is modified by the presence of stars, black holes, dark matter, and dark energy, which bend time and space.”
Although this characteristic of electrons has been known for many years, no one had until now been able to measure this “topological spin” directly. To achieve this, the researchers exploited a particular effect known as “circular dichroism“: a special experimental technique that can only be used with a synchrotron source, which exploits the ability of materials to absorb light differently depending on their polarization.
Scholars have especially focused on “kagome materials,” a class of quantum materials that owe their name to their resemblance to the weave of interwoven bamboo threads that make up a traditional Japanese basket (called, indeed, “kagome”). These materials are revolutionizing quantum physics, and the results obtained could help us learn more about their special magnetic, topological, and superconducting properties.
“These important results were possible thanks to a strong synergy between experimental practice and theoretical analysis,” adds Di Sante. “The team’s theoretical researchers employed sophisticated quantum simulations, only possible with the use of powerful supercomputers, and in this way guided their experimental colleagues to the specific area of the material where the circular dichroism effect could be measured.”
More information: Domenico Di Sante et al, Flat band separation and robust spin Berry curvature in bilayer kagome metals, Nature Physics (2023). DOI: 10.1038/s41567-023-02053-z
Experimental setup of the all-fiber Mamyshev oscillator. Credit: Ultrafast Science
High power/energy ultrafast fiber lasers have broadband applications in material processing, medicine, advanced manufacturing and other fields. Compared with solid-state lasers, fiber lasers have the advantages of compact systems, flexibility, good heat dissipation and high beam quality.
However, due to the serious nonlinear effect inside the fiber, the single-pulse energy and average power of the ultrafast fiber lasers, especially those with all-fiber structures, still lag behind that of the solid-state lasers. In recent years, the Mamyshev oscillator has attracted much research attention thanks to its potential in producing high energy ultrafast pulses.
Recently, a research group from National University of Defense Technology reported a Mamyshev oscillator based on all-polarization-maintaining fiber with core/cladding diameter of 10/125 μm, and realized a 153 nJ single pulse energy with a compressed pulse width of 73 fs. Furthermore, through adjusting the cavity parameters, a maximum 5th order harmonic mode-locking operation was obtained with an output average power of 3.4 W and a compressed pulse width of 100 fs.
Recently their work entitled “All-PM Fiber Mamyshev Oscillator Delivers Hundred-Nanojoule and Multi-Watt Sub-100 fs Pulses” was published on Ultrafast Science.
The pulse energy and average power of ultrafast fiber lasers have been greatly improved in recent years based on the Mamyshev oscillator with cascade spectral broadening and offset spectral filtering effect. The continuous light and weak pulses will be blocked in the Mamyshev oscillator. Due to the sufficient spectral broadening, the strong pulse can be survived in the cavity after two filters with different central wavelengths, and the ultrafast laser with large pulse energy can be obtained.
However, most of the previous results adopted spatial structures and introduced a large number of free-space elements for signal collimation and coupling, which render the system cumbersome and susceptible to interference.
“Our aim is to realize high power/energy ultrafast fiber lasers with all-fiber structure,” explained A/Prof. Can Li. In the Ultrafast Science paper, a Mamyshev oscillator based on all-polarization-maintaining fiber with core/cladding diameter of 10/125 μm was reported.
Compared with the conventional Mamyshev oscillators, which generally consist of two stages of gain fiber amplification, there is only a single amplification arm in the proposed laser. A piece of passive fiber was used to broaden the optical spectrum in the other arm, alleviating the nonlinear phase accumulation inside the cavity and making the system more compact in the meantime.
This research group respectively realized the high performance ultrafast fiber lasers with single pulse energy of 153 nJ and average power of 3.4 W, which represent the highest records in pulse energy and average power obtained from an all-fiber ultrafast laser oscillator with picosecond/femtosecond pulse duration.
“By adopting the fiber with larger core diameter and new pulse evolution mechanism that has a higher tolerance of nonlinearity phase accumulation, all-fiber ultrafast lasers with higher power/pulse energy are expectable,” says Professor Pu Zhou.
More information: Tao Wang et al, All-PM Fiber Mamyshev Oscillator Delivers Hundred-Nanojoule and Multi-Watt Sub-100 fs Pulses, Ultrafast Science (2023). DOI: 10.34133/ultrafastscience.0016
Two SuperCDMS detector towers resting inside storage containers within the Low Radon Cleanroom at SNOLAB. Credit: Vijay Iyer/University of Toronto
After years of pioneering work, researchers at the Department of Energy’s SLAC National Accelerator Laboratory have completed the detector towers that will soon sit at the heart of the SuperCDMS SNOLAB dark matter detection experiment.
The team finished building the towers this past September, and SLAC, which leads the SuperCDMS project, sent the first two towers to SNOLAB in Ontario, Canada earlier this month.
SuperCDMS SNOLAB will look for relatively light dark matter particles, between about half the mass of the proton to roughly 10 proton masses, and in that range it will be the world’s most sensitive direct-detection experiment, said Richard Partridge, a senior staff scientist at SLAC and a long-time SuperCDMS researcher. That accomplishment comes down to two things: improvements in the detector design and the location of the experiment itself, Partridge said.
“It’s been a lot of fun,” Partridge said. “We’ve learned a lot of new things, and we’ve built some really interesting technology,” including flexible superconducting cables, electronics systems that function in extreme cold, and improved cryogenics systems—along with advances in shielding the detectors—that have made the detectors and surrounding systems better able to sense passing dark matter than ever before.
The experiment will also benefit from its new location 1.25 miles underground at SNOLAB, where the background of cosmic rays will interfere less with efforts to find a dark matter signal.
“SNOLAB and SuperCDMS are made for each other,” said SNOLAB Executive Director Jodi Cooley. “We are extremely excited about the potential for SuperCDMS to detect dark matter directly and advance our insight into the nature of the universe.”
Vitaly Yakimenko, SLAC’s deputy director for projects and infrastructure and SuperCDMS project director, said researchers look forward to bringing the experiment online. “It’s been 10 years of technological development to build these state-of-the-art detectors,” he said. The team hopes the experiment will capture signs of illusive dark matter particles, but regardless of the outcome, it will establish a path forward for even more sensitive experiments, Yakimenko said. “It’s a major accomplishment.”
JoAnne Hewett, head of SLAC’s Fundamental Physics Directorate and the lab’s chief research officer, said she was pleased by the experiment’s progress. “Understanding dark matter is one of the most important areas of research, both around the world and here at SLAC,” Hewett said. “We’re excited to reach this milestone and to work with our partners to build this cutting-edge experiment.”
Searching deep underground
Scientists know that all the visible matter in the universe—all the dust and planets and stars that we can already detect through telescopes—makes up only about 15% of what’s actually out there. The rest is dark matter, but no one knows exactly what that is. Physicists can tell it’s there through its gravitational pull on ordinary matter, but it is otherwise very hard to detect.
That’s where experiments like SuperCDMS SNOLAB come in. The project is the latest iteration of a series of experiments that use silicon and germanium crystals to try to find dark matter particles. These crystals are cooled to a fraction of a degree above absolute zero—hence the experiments’ name: Cryogenic Dark Matter Search, or CDMS. The hope is that at such low temperatures, researchers could detect passing dark matter particles by the tiny vibrations they create when colliding with the crystals.
Those collisions would also produce pairs of electrons and electron deficiencies, or holes, that move through the crystals, triggering more vibrations and amplifying the dark matter signal. Sophisticated superconducting electronics help detect these signals.
To make that job possible, the experiment will be built and operated at SNOLAB, 6,800 feet underground inside a nickel mine near Sudbury, Ontario. There, the SLAC-built detectors will be shielded from high-energy particles, called cosmic radiation, which can create unwanted background signals.
A long journey
Once the detectors were done, the next step for the SLAC team was getting them to SNOLAB—and here there were tradeoffs. To protect them from cosmic rays, the team wanted to get them to their new underground home as quickly as possible, which might suggest a direct route over the Rocky Mountains or even a flight to Ontario. However, the thinner atmosphere at higher altitudes affords less protection from cosmic rays.
“In principle, we want to keep things as low as possible, but there’s also a cost to the total number of days you’re on the surface,” said Tarek Saab, a University of Florida physicist and SuperCDMS spokesperson. “So, we want a route that gives you the least overall exposure to cosmic rays.”
In the end, the team decided to take a route east to Texas and then north to SNOLAB.
Meanwhile, SNOLAB has been busy getting the SuperCDMS facility ready for the detectors, the first two of which arrived on May 12 and made the trip 6,800 feet underground the following day. The remaining towers will arrive later this year, and initial preparations for the experiment are expected to be complete sometime in 2024, at which point the experimental team can begin taking initial data and working out any kinks that remain in the system. Researchers expect to run the experiment for three to four years before they have enough data to push the limits of what we know about dark matter.
But for now, everyone is looking forward to delivering the detectors, Saab said. “It will be a significant milestone to have the detectors at SNOLAB.”
Artistic rendering of the experimental device with the beam optical photons (red) entering and leaving the electro-optic crystal and resonating within its circular portion as well as the generated microwave photons (blue) leaving the device. Credit: Eli Krantz, Krantz NanoArt
Quantum computers promise to solve challenging tasks in material science and cryptography that will remain out of reach even for the most powerful conventional supercomputers in the future. Yet, this will likely require millions of high-quality qubits due to the required error correction.
Progress in superconducting processors advances quickly with a current qubit count in the few hundreds. The advantages of this technology are the fast computing speed and its compatibility with microchip fabrication, but the need for ultra-cold temperatures ultimately confines the processor in size and prevents any physical access once it is cooled down.
A modular quantum computer with multiple separately cooled processor nodes could solve this. However, single microwave photons—the particles of light that are the native information carriers between superconducting qubits within the processors—are not suitable to be sent through a room temperature environment between the processors. The world at room temperature is bustling with heat, which easily disturbs the microwave photons and their fragile quantum properties like entanglement.
Researchers from the Fink group at the Institute of Science and Technology Austria (ISTA), together with collaborators from TU Wien and the Technical University of Munich, demonstrated an important technological step to overcome these challenges. They entangled low-energy microwave with high-energy optical photons for the very first time.
Such an entangled quantum state of two photons is the foundation to wire up superconducting quantum computers via room temperature links. This has implications not only for scaling up existing quantum hardware but it is also needed to realize interconnects to other quantum computing platforms as well as for novel quantum-enhanced remote sensing applications. Their results have been published in the journal Science.
Cooling away the noise
Rishabh Sahu, a postdoc in the Fink group and one of the first authors of the new study, explains, “One major problem for any qubit is noise. Noise can be thought of as any disturbance to the qubit. One major source of noise is the heat of the material the qubit is based on.”
Heat causes atoms in a material to jostle around rapidly. This is disruptive to quantum properties like entanglement, and as a result, it would make qubits unsuitable for computation. Therefore, to remain functional, a quantum computer must have its qubits isolated from the environment, cooled to extremely low temperatures, and kept within a vacuum to preserve their quantum properties.
They come with a unique variety of properties like entanglement. Entanglement is important for quantum computers because it allows them to do computations in a way that is impossible for non-quantum computers. Credit: Mark Belan/ISTA
For superconducting qubits, this happens in a special cylindrical device that hangs from the ceiling, called a “dilution refrigerator” in which the “quantum” part of the computation takes place. The qubits at its very bottom are cooled down to only a few thousandths of a degree above absolute zero temperature—at about -273 degrees Celsius. Sahu excitedly adds, “This makes these fridges in our labs the coldest locations in the whole universe, even colder than space itself.”
The refrigerator has to continuously cool the qubits but the more qubits and associated control wiring are added, the more heat is generated and the harder it is to keep the quantum computer cool. “The scientific community predicts that at around 1,000 superconducting qubits in a single quantum computer, we reach the limits of cooling,” Sahu cautions. “Just scaling up is not a sustainable solution to construct more powerful quantum computers.”
Fink adds, “Larger machines are in development but each assembly and cooldown then becomes comparable to a rocket launch, where you only find out about problems once the processor is cold and without the ability to intervene and correct such problems.”
Quantum waves
“If a dilution fridge cannot sufficiently cool more than a thousand superconducting qubits at once, we need to link several smaller quantum computers to work together,” Liu Qiu, postdoc in the Fink group and another first author of the new study, explains. “We would need a quantum network.”
Linking together two superconducting quantum computers, each with its own dilution refrigerator, is not as straightforward as connecting them with an electrical cable. The connection needs special consideration to preserve the quantum nature of the qubits.
Superconducting qubits work with tiny electrical currents that move back and forth in a circuit at frequencies about ten billion times per second. They interact using microwave photons—particles of light. Their frequencies are similar to the ones used by cellphones.
The experimental setup with the dilution refrigerator, the superconducting cavity, and the electro-optic crystal splitting and entangling the photons. Credit: Mark Belan/ISTA
The problem is that even a small amount of heat would easily disturb single microwave photons and their quantum properties needed to connect the qubits in two separate quantum computers. When passing through a cable outside the refrigerator, the heat of the environment would render them useless.
“Instead of the noise-prone microwave photons that we need to do the computations within the quantum computer, we want to use optical photons with much higher frequencies similar to visible light to network quantum computers together,” Qiu explains. These optical photons are the same kind sent through optical fibers that deliver high-speed internet to our homes. This technology is well understood and much less susceptible to noise from heat. Qiu adds, “The challenge was how to have the microwave photons interact with the optical photons and how to entangle them.”
Splitting light
In their new study, the researchers used a special electro-optic device: an optical resonator made from a nonlinear crystal, which changes its optical properties in the presence of an electric field. A superconducting cavity houses this crystal and enhances this interaction.
Sahu and Qiu used a laser to send billions of optical photons into the electro-optic crystal for a fraction of a microsecond. In this way, one optical photon splits into a pair of new entangled photons: an optical one with only slightly less energy than the original one and a microwave photon with much lower energy.
“The challenge of this experiment was that the optical photons have about 20,000 times more energy than the microwave photons,” Sahu explains, “and they bring a lot of energy and therefore heat into the device that can then destroy the quantum properties of the microwave photons. We have worked for months tweaking the experiment and getting the right measurements.” To solve this problem, the researchers built a bulkier superconducting device compared to previous attempts. This not only avoids a breakdown of superconductivity, but it also helps to cool the device more effectively and to keep it cold during the short timescales of the optical laser pulses.
“The breakthrough is that the two photons leaving the device—the optical and the microwave photon—are entangled,” Qiu explains. “This has been verified by measuring correlations between the quantum fluctuations of the electromagnetic fields of the two photons that are stronger than can be explained by classical physics.”
“We are now the first to entangle photons of such vastly different energy scales.” Fink says, “This is a key step to creating a quantum network and also useful for other quantum technologies, such as quantum-enhanced sensing.”
The search for exotic Higgs boson decays in future lepton colliders: 1) an electron and a positron from opposing beams collide; 2) the collision produces a high-energy Higgs boson; 3) the boson decays into two exotic particles moving away from the axis of the beams; 4) exotic particles decay into pairs of quark-antiquark, visible to detectors. Credit: IFJ PAN
It may be that the famous Higgs boson, co-responsible for the existence of masses of elementary particles, also interacts with the world of the new physics that has been sought for decades. If this were indeed to be the case, the Higgs should decay in a characteristic way, involving exotic particles. At the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow, it has been shown that if such decays do indeed occur, they will be observable in successors to the LHC currently being designed.
When talking about the “hidden valley,” our first thoughts are of dragons rather than sound science. However, in high-energy physics, this picturesque name is given to certain models that extend the set of currently known elementary particles. In these so-called Hidden Valley models, the particles of our world as described by the Standard Model belong to the low-energy group, while exotic particles are hidden in the high-energy region.
Theoretical considerations suggest then the exotic decay of the famous Higgs boson, something that has not been observed at the LHC accelerator despite many years of searching. However, scientists at the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow argue that Higgs decays into exotic particles should already be perfectly observable in accelerators that are successors to the Large Hadron Collider—if the Hidden Valley models turn out to be consistent with reality.
“In Hidden Valley models we have two groups of particles separated by an energy barrier. The theory is that there could then be exotic massive particles which could cross this barrier under specific circumstances. The particles like Higgs boson or hypothetic Z’ boson would act as communicators between the particles of both worlds. The Higgs boson, one of the most massive particle of the Standard Model, is a very good candidate for such a communicator,” explains Prof. Marcin Kucharczyk (IFJ PAN), lead author of an article in the Journal of High Energy Physics, which presents the latest analyses and simulations concerning the possibility of detecting Higgs boson decays in the future lepton accelerators.
The communicator, after passing into the low energy region, would decay into two rather massive exotic particles. Each of these would, in picoseconds—that is, trillionths of a second—decay into another two particles, with even smaller masses, which would then be within the Standard Model.
So what signs would be expected in the detectors of future accelerators? The Higgs itself would remain unnoticed, as would the two Hidden Valley particles. However, the exotic particles would gradually diverge and eventually decay, generally into quark-antiquark beauty pairs visible in modern detectors as jets of particles shifted from the axis of the lepton beam.
“Observations of Higgs boson decays would therefore consist of searching for the jets of particles produced by quark-antiquark pairs. Their tracks would then have to be retrospectively reconstructed to find the places where exotic particles are likely to have decayed. These places, professionally called decay vertices, should appear in pairs and be characteristically shifted with respect to the axis of the colliding beams in the accelerator. The size of these shifts depends, among other things, on masses and average lifetime of exotic particles appearing during the Higgs decay,” says Mateusz Goncerz, M.Sc. (IFJ PAN), co-author of the paper in question.
The collision energy of protons at the LHC, currently the world’s largest particle accelerator, is up to several teraelectronvolts and is theoretically sufficient to produce Higgs capable of crossing the energy barrier that separates our world from the Hidden Valley. Unfortunately, protons are not elementary particles—they are composed of three valence quarks bound by strong interactions, capable of generating huge numbers of constantly appearing and disappearing virtual particles, including quark-antiquark pairs.
Such a dynamic and complex internal structure produces huge numbers of secondary particles in proton collisions, including many quarks and antiquarks with large masses. They form a background in which it becomes practically impossible to find the particles from the exotic Higgs boson decays that are being sought.
The detection of possible Higgs decays to these states should be radically improved by accelerators being designed as successors to the LHC: the CLIC (Compact Linear Collider) and the FCC (Future Circular Collider). In both devices it will be possible to collide electrons with their anti-material partners, the positrons (with CLIC dedicated to this type of collision, while FCC will also allow collisions of protons and heavy ions).
Electrons and positrons are devoid of internal structure, so the background for exotic Higgs boson decays should be weaker than at the LHC. Only will it be sufficiently so to discern the valuable signal?
In their research, physicists from the IFJ PAN took into account the most important parameters of the CLIC and FCC accelerators and determined the probability of exotic Higgs decays with final states in the form of four beauty quarks and antiquarks. To ensure that the predictions cover a wider group of models, the masses and mean lifetimes of the exotic particles were considered over suitably wide ranges of values.
The conclusions are surprisingly positive: all indications are that, in future electron-positron colliders, the background of exotic Higgs decays could be reduced even radically, by several orders of magnitude, and in some cases could even be considered negligible.
The existence of particle-communicators is not only possible in Hidden Valley models, but also in other extensions of the Standard Model. So if the detectors of future accelerators register a signature corresponding to the Higgs decays analyzed by the Cracow researchers, this will only be the first step on the road to understanding new physics. The next will be to collect a sufficiently large number of events and determine the main decay parameters that can be compared with the predictions of theoretical models of the new physics.
“The main conclusion of our work is therefore purely practical. We are not sure whether the new physics particles involved in Higgs boson decays will belong to the Hidden Valley model we used. However, we have treated this model as representative of many other proposals for new physics and have shown that if, as predicted by the model, the Higgs bosons decay into exotic particles, this phenomenon should be perfectly visible in those electron and positron colliders which are planned to be launched in the near future,” concludes Prof. Kucharczyk.
More information: Marcin Kucharczyk et al, Search for exotic decays of the Higgs boson into long-lived particles with jet pairs in the final state at CLIC, Journal of High Energy Physics (2023). DOI: 10.1007/JHEP03(2023)131
Experimental and simulated (a) 1D- and (b-c) 2D-SAXS patterns of the 12 wt% PBPEO solution by temperature-quenching from disordered states to the crystallization temperatures noted. The left panels of the 2D-SAXS are experimental, and the right panels are simulated patterns. Credit: Soft Matter (2023). DOI: 10.1039/D3SM00199G
When most people think of crystals, they picture suncatchers that act as rainbow prisms or the semi-transparent stones that some believe hold healing powers. However, to scientists and engineers, crystals are a form of materials in which their constituents—atoms, molecules, or nanoparticles—are arranged regularly in space. In other words, crystals are defined by the regular arrangement of their constituents. Common examples are diamonds, table salt, or sugar cubes.
However, in research just published in Soft Matter, a team led by Rensselaer Polytechnic Institute’s Sangwoo Lee, associate professor in the Department of Chemical and Biological Engineering, discovered that crystal structures are not necessarily always regularly arranged. The discovery advances the field of materials science and has unrealized implications for the materials used for semiconductors, solar panels, and electric vehicle technologies.
One of the most common and important classes of crystal structures is the close-packed structures of regular spheres constructed by stacking layers of spheres in a honeycomb arrangement. There are many ways to stack the layers to construct close-packed structures, and how nature selects specific stacking is an important question in materials and physics research. In the close-packing construction, there is a very unusual structure with irregularly spaced constituents known as the random stacking of two-dimensional hexagonal layers (RHCP). This structure was first observed from cobalt metal in 1942, but it has been regarded as a transitional and energetically unpreferred state.
Lee’s research group collected X-ray scattering data from soft model nanoparticles made of polymers and realized that the scattering data contains important results about RHCP but is very complicated. Then, Patrick Underhill, professor in Rensselaer’s Department of Chemical and Biological Engineering, enabled the analysis of the scattering data using the supercomputer system, Artificial Intelligence Multiprocessing Optimized System (AiMOS), at the Center for Computational Innovations.
“What we found is that the RHCP structure is, very likely, a stable structure, and this is the reason that RHCP has been widely observed in many materials and naturally occurring crystal systems,” said Lee. “This finding challenges the classical definition of crystals.”
The study provides insights into the phenomenon known as polytypism, which enables the formation of RHCP and other close-packed structures. A representative material with polytypism is silicon carbide, widely used for high-voltage electronics in electric vehicles and as hard materials for body armor. Lee’s team’s findings indicate that those polytypic materials may have continuous structural transitions, including the non-classical random arrangements with new useful properties.
“The problem of how soft particles pack seems straightforward, but even the most basic questions are challenging to answer,” said Kevin Dorfman of the University of Minnesota-Twin Cities, who is unaffiliated with this research. “This paper provides compelling evidence for a continuous transition between face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices, which implies a stable random hexagonal close-packed phase between them, and thus makes an important breakthrough in materials science.”
“I am particularly pleased with this discovery, which shows the power of advanced computation to make an important breakthrough in materials science by decoding the molecular level structures in soft materials,” said Shekhar Garde, dean of Rensselaer’s School of Engineering. “Lee and Underhill’s work at Rensselaer also promises to open up opportunities for many technological applications for these new materials.”
Lee and Underhill were joined in research by Rensselaer’s Juhong Ahn, Liwen Chen of the University of Shanghai for Science and Technology, and Guillaume Freychet and Mikhail Zhernenkov of Brookhaven National Laboratory.
More information: Juhong Ahn et al, Continuous transition of colloidal crystals through stable random orders, Soft Matter (2023). DOI: 10.1039/D3SM00199G
The prototype metamaterial uses electrical signals transported by these black wires to control both the direction and intensity of energy waves passing through a solid material. Credit: University of Missouri
For more than 10 years, Guoliang Huang, the Huber and Helen Croft Chair in Engineering at the University of Missouri, has been investigating the unconventional properties of “metamaterials”—an artificial material that exhibits properties not commonly found in nature as defined by Newton’s laws of motion—in his long-term pursuit of designing an ideal metamaterial.
Huang’s goal is to help control the “elastic” energy waves traveling through larger structures—such as an aircraft—without light and small “metastructures.”
“For many years I’ve been working on the challenge of how to use mathematical mechanics to solve engineering problems,” Huang said. “Conventional methods have many limitations, including size and weight. So, I’ve been exploring how we can find an alternative solution using a lightweight material that’s small but can still control the low-frequency vibration coming from a larger structure, like an aircraft.”
Now, Huang’s one step closer to his goal. In a new study published in the Proceedings of the National Academy of Sciences, Huang and colleagues have developed a prototype metamaterial that uses electrical signals to control both the direction and intensity of energy waves passing through a solid material.
Potential applications of his innovative design include military and commercial uses, such as controlling radar waves by directing them to scan a specific area for objects or managing vibration created by air turbulence from an aircraft in flight.
“This metamaterial has odd mass density,” Huang said. “So, the force and acceleration are not going in the same direction, thereby providing us with an unconventional way to customize the design of an object’s structural dynamics, or properties to challenge Newton’s second law.”
This is the first physical realization of odd mass density, Huang said.
“For instance, this metamaterial could be beneficial to monitor the health of civil structures such as bridges and pipelines as active transducers by helping identify any potential damage that might be hard to see with the human eye.”
More information: Qian Wu et al, Active metamaterials for realizing odd mass density, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2209829120
(a) Simplified sketch of the experimental setup. (b) Simulated intensity distribution in the focal plane with the phase grating. (c) Photon counts at the AGIPD, measured with the phase grating, averaged over 58 million patterns. This is a flat distribution without any apparent structural information. The mean photon count per pixel per frame was ⟨I⟩=0.0077. Credit: Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.173201
An international team of researchers including scientists from FAU has, for the first time, used X-rays for an imaging technique that exploits a particular quantum characteristic of light. In their article, which has now been published in the journal Physical Review Letters, the researchers detail how this process could be used for imaging non-crystallized macromolecules.
The research team used the extremely short and very intensive X-ray pulses at the X-ray laser European EXFEL in Hamburg in order to generate fluorescence photons that arrived almost simultaneously at the detector—in a time window of less than one femtosecond (one quadrillionth of a second). By calculating the photon-photon correlations in the fluorescence of the illuminated copper atoms, it was possible to create an image of the light source.
On the atomic scale, the structures of materials and macromolecules are usually determined using X-ray crystallography. While this technique relies on coherent X-ray diffraction, the scattering of X-ray light can cause incoherent processes such as fluorescence emissions, which can dominate, even though they do not make a useful contribution to the diffraction measurement. Instead, they add a functionless haze or background to the measurement data.
As long ago as the 1950s, two British astronomers proved that it is possible to gain structural information from such self-luminous light sources, in their case it was the light from stars. Robert Hanbury Brown and Richard Twiss, whose method is known as intensity interferometry, opened a new door to the understanding of light and founded the field of quantum optics.
Recently, scientists from FAU, the Max Planck Institute for the Structure and Dynamics of Matter and the Deutsches Elektronen-Synchrotron (DESY) suggested that intensity interferometry could be adapted for atomic-resolution imaging using X-ray fluorescence. The challenge in extending this idea to X-rays is that the coherence time of the photons, which dictates the time interval available to perform photon–photon correlations, is extremely short. It is determined by the radiative decay time of the excited atom, which is about 0.6 femtoseconds for copper atoms.
Together with scientists from Uppsala University and the European XFEL, the group has now overcome that challenge by using femtosecond-duration XFEL pulses from that facility to initiate X-ray fluorescence photons within the coherence time. The team generated a source consisting of two fluorescing spots in a foil of copper and measured the fluorescence on a million-pixel detector placed eight meters away.
Only about 5,000 photons were detected on each illumination pulse, and the cumulative sum over 58 million pulses produced just a featureless uniform distribution. However, when the researchers instead summed photon-photon correlations across all images from the detector, a striped pattern emerged, which was analyzed like a coherent wave field to reconstruct an image of the fluorescent source, consisting of two well-separated spots of light.
The scientists now hope to combine this new method with conventional X-ray diffraction to image single molecules. Element-specific fluorescent light could expose substructures that are specific to certain atoms and even to certain chemical states. This could contribute to a better understanding of the functions of important enzymes such as those involved in photosynthesis.
More information: Fabian Trost et al, Imaging via Correlation of X-Ray Fluorescence Photons, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.173201
Bulk PEO8k thermodynamics, rheology, and morphology. (A) Differential scanning calorimetry (DSC) heating curve (rate = 10 K/min) showing (apparent) melting at 335 K. The imbibition temperature (T = 330 K) is indicated with an arrow. (B) Storage modulus (green circles), loss modulus (red circles), and viscosity (blue spheres) measured at the imbibition temperature (T = 330 K) measured in rheology. (C) Small-angle x-ray scattering (SAXS) curves of PEO8k obtained at ambient temperature following slow cooling from the melt. The lamellar domain spacing is shown in the inset for different cooling rates, R. a.u., arbitrary units. (D) Spherulitic morphology obtained by polarizing optical microscopy (POM) with crossed polars at ambient temperature following slow cooling from the melt. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adg8865
Semicrystalline polymers are solids that are assumed to flow only above their melting temperature. In a new study published in Science Advances, Chien-Hua Tu and a research team at the Max Planck Institute for Polymer Research in Germany and the University of Ioannina Greece confined crystals within nanoscopic cylindrical pores to show the flowing nature of semicrystalline polymers below their melting point, alongside an intermediate state of viscosity to the melt and crystal states.
The capillary process was strong during the phenomenon and dragged the polymer chains into the pores without melting the crystal. The unexpected improvement in flow facilitated polymer processing conditions applicable to low temperatures, suited for use in organic electronics.
Crystalline state
About 2,500 years ago, the philosopher Heraclitus proposed that “everything flows,” and while perfect crystals at zero temperature do not flow, crystalline materials do flow under specific conditions. For example, existing research from about 100 years ago showed that the flow of cast iron in the form of flowing metal grains surrounded by a thin amorphous layer is analogous to an undercooled liquid.
Using molecular dynamics simulations, researchers have confirmed the ideas to further suggest the significance of the complex grain boundary “fluid” on plastic deformation. For instance, the inner core of the Earth is similarly proposed to retain iron in a crystalline state. Furthermore, the core of planets such as Neptune and Uranus are composed of superionic crystalline water, and flow to generate their magnetic field, which may have ultimately led to our own existence.
Crystalline materials that exhibit fluid-like mobilities are known as “superionics” and are important for energy applications. Semicrystalline polymers are solids that do not flow under normal conditions. In this work, Tu and colleagues showed how even the semicrystalline polymers underwent flow. To examine the phenomenon, they used two semicrystalline polymers; poly (ethylene oxide) and poly(ε-caprolactone) with specific molecular characteristics. The materials scientists developed self-ordered nanoporous alumina templates for the study, based on existing literature protocols.
Imbibition of PEO8k in nanopores revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). (A) Left: SEM image from a fractured surface of anodic aluminum oxide (AAO) having nanopores with a diameter of 400 nm infiltrated with PEO8k at 330 K for 28 days. Right: A zoom-in to the blue rectangular dashed area in left. Blue arrows indicate the meniscus. (B) AFM two-dimensional height image corresponding to the cyan rectangular dashed area of (A, right) (C) AFM two-dimensional image to the same area as (B). (D) A zoom-in AFM two-dimensional image to the meniscus region. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adg8865
Materials characterization
The scientists examined the thermodynamics, structural and rheological properties of bulk polyethylene oxide materials. And the data confirmed the material film on the alumina template to be in a semicrystalline state. The team observed the domain spacing organization of crystalline lamellae with small-angle X-ray scattering. They used polarizing optical microscopy to study the superstructure of bulk polyethylene oxide with a film slowly cooled from melt to ambient temperature. The outcomes indicated a single spherulitic superstructure for polyethylene oxide, while the structural dynamics of poly(ε-caprolactone) synthesized with a catalyst differed.
The research team carried out a 28-day imbibition (uptake of water that leads to swelling of materials) of the two polymer materials within anodic aluminum oxide templates and observed the samples with scanning electron microscopy and atomic force microscopy to characterize them. In contrast to the relatively smooth appearance of polyethylene oxide, the poly(ε-caprolactone) materials showed abundant grain structures due to diverse morphological origins in intracrystalline diffusion. After studying the surface appearances of materials, the researchers performed nano-infrared microscopy to obtain additional images of the surface topography of the two materials. The outcomes clearly showed the semicrystalline nature of polyethylene oxide. They also addressed the possibility of the capillary force in the experimental setup to be sufficiently high to melt the crystals during flow and noted the viscosity of semicrystalline polymers to be reduced during the experiments.
Nano–infrared (IR) reveals the semicrystalline nature of the polymer in the nanopores. (A) Schematic arrangement of the sample for the nano-IR measurements. After scanning the edge of the anodic aluminum oxide (AAO) surface, the atomic force microscopy (AFM) tip is positioned on the polymer or on the AAO surface. Then, the wavelength of the IR laser is tuned. The vibrational amplitude of the AFM tip corresponds to the nano-IR response of the selected position, respectively. (B) Topographic image of the AAO nanoporous sample filled by PEO8k. We selected three positions on the PEO (#1, #2, and #3) and three positions on the AAO (#4, #5, and #6), respectively, and recorded IR spectra. (C) Corresponding IR spectra taken at the marked positions in (B). For positions #1 to #3, absorption peaks at 1061, 1108, and 1149 cm−1 are present (in yellow) that are absent for positions #4 to #6. Peaks at 1061, 1108, and 1149 cm−1 are typical for semicrystalline PEO. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adg8865
Mechanisms of imbibition
The mechanisms of fluid uptake and materials swelling known as imbibition from the semicrystalline state relied on the dynamics of its crystalline and amorphous domains. Four processes acted on the amorphous and crystalline regions; segmental relaxation governed the dynamics in the amorphous domain, whereas three other processes affected the crystalline domain to demonstrate intracrystalline chain diffusion for crystal-mobile polymers such as polyethylene oxide.
Since the imbibition of crystals also involved the diffusion of entire crystallites, Tu and the team examined the influence of the molar mass of the polymers on the process of imbibition. The outcomes showed that the molar mass regulated the imbibition speed.
Molecular mechanism and associated time scales during imbibition of semicrystalline polymers in nanopores.(A and B) Hierarchical structures and associated kinetics pertinent to semicrystalline polymers: (A) Organization of polymer chains into ordered lamellae involves the movement of segments within the crystalline (dc) and amorphous (dα) regions. Four processes are defined as follows: τstem represents the diffusion time of PEO chains in the length scale of the crystalline domain (dc); τlc illustrates the growth time of a unit crystal lamellae; <τc> depicts the switching time of two helical defect jumps within the crystal; τsegmental represents the segmental dynamics within the amorphous regions. l represents the intermolecular distance (l = <a + b>/2; a and b are the crystal unit cell parameters along x and y axes, respectively). (B) Imbibition of polymer chains (τimbibition) proceeds via adsorption with a characteristic time τadsorption. The anchored units on the pore walls are indicated with the red color. (C) Characteristic time scales and their temperature dependence: τimbibition (yellow sphere), τadsorption (red dashed line), τlc (blue spheres), τstem (cyan spheres), <τc > (orange spheres), and τsegmental (green crosses). The original data are provided in table S1. (D) The effect of lamellar orientation with respect to the pore axes on the variable penetration lengths. (E) AFM height (left) and phase (right) images reveal well-oriented crystalline (PCL) lamellae inside nanopores (cyan framed regions). Scale bars, 200 nm. Credit: Science Advances (2023). DOI: 10.1126/sciadv.adg8865
Outlook
In this way, Chien-Hua Tu and colleagues used several imaging methods in materials science, such as scanning electron microscopy, atomic force microscopy, and nano-infrared results to examine how semicrystalline polymers underwent flow within nanopores made of anodic aluminum oxide via capillary action. They measured the viscoelastic behavior of the polymers with a shear rheometer, and the capillary action appeared to drive the polymer adsorption process.
While successful imbibition was a relatively slow process, the capillary force was strong enough to drag polymer crystallites into the nanopores without melting the crystals. The unexpected increase in flow while preserving the polymer crystallites applied to polymer processing at low temperatures. Such phenomenon can lead to cold flow and subsequent bonding of polymers to ceramics or metal under specific conditions to prevent polymer degradation. Such semicrystalline polymers and ferroelectric materials have a variety of applications in organic electronics to affect their electronic and physical properties.
More information: Chien-Hua Tu et al, When crystals flow, Science Advances (2023). DOI: 10.1126/sciadv.adg8865
