Category: Physics

Researchers have uncovered a previously hidden heating process that helps explain how the atmosphere that surrounds the sun called the “solar corona” can be vastly hotter than the solar surface that emits it.

The discovery at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) could improve tackling a range of astrophysical puzzles such as star formation, the origin of large-scale magnetic fields in the universe, and the ability to predict eruptive space weather events that can disrupt cell phone service and black out power grids on Earth. Understanding the heating process also has implications for fusion research.

First clear 3D explanation

“Our direct numerical simulation is the first to provide clear identification of this heating mechanism in 3D space,” said Chuanfei Dong, a physicist at PPPL and Princeton University who unmasked the process by conducting 200 million hours of computer time for the world’s largest simulation of its kind. “Current telescope and spacecraft instruments may not have high enough resolution to identify the process occurring at small scales,” said Dong, who details the breakthrough in the journal Science Advances.

The hidden ingredient is a process called magnetic reconnection that separates and violently reconnects magnetic fields in plasma, the soup of electrons and atomic nuclei that forms the solar atmosphere. Dong’s simulation revealed how rapid reconnection of the magnetic field lines turns the large-scale turbulent energy into small-sale internal energy. As a consequence the turbulent energy is efficiently converted to thermal energy at small scales, thus superheating the corona.

“Think of putting cream in coffee,” Dong said. “The drops of cream soon become whorls and slender curls. Similarly, magnetic fields form thin sheets of electric current that break up due to magnetic reconnection. This process facilitates the energy cascade from large-scale to small-scale, making the process more efficient in the turbulent solar corona than previously thought.”

When the reconnection process is slow while the turbulent cascade is fast, reconnection cannot affect the transfer of energy across scales, he said. But when the reconnection rate becomes fast enough to exceed the traditional cascade rate, reconnection can move the cascade toward small scales more efficiently.

It does this by breaking and rejoining the magnetic field lines to generate chains of small twisted lines called plasmoids. This changes the understanding of the turbulent energy cascade that has been widely accepted for more than half a century, the paper says. The new finding ties the energy transfer rate to how fast the plasmoids grow, enhancing the transfer of energy from large to small scales and strongly heating the corona at these scales.

The new discovery demonstrates a regime with an unprecedentedly large magnetic Reynolds number as in the solar corona. The large number characterizes the new high energy transfer rate of the turbulent cascade. “The higher the magnetic Reynolds number is, the more efficient the reconnection-driven energy transfer is,” said Dong, who is moving to Boston University to take up a faculty position.

200 million hours

“Chuanfei has carried out the world’s largest turbulence simulation of its kind that has taken over 200 million computer CPUs [central processing units] at the NASA Advanced Supercomputing (NAS) facility,” said PPPL physicist Amitava Bhattacharjee, a Princeton professor of astrophysical sciences who supervised the research. “This numerical experiment has produced undisputed evidence for the first time of a theoretically predicted mechanism for a previously undiscovered range of turbulent energy cascade controlled by the growth of the plasmoids.

“His paper in the high-impact journal Science Advances completes the computational program he began with his earlier 2D results published in Physical Review Letters. These papers form a coda to the impressive work that Chuanfei has done as a member of the Princeton Center for Heliophysics, a joint Princeton and PPPL facility. We are grateful for a PPPL LDRD [Laboratory Directed Research & Development] grant that facilitated this work, and to the NASA High-End Computing (HEC) program for its generous allocation of computer time.”

The impact of this finding in astrophysical systems across a range of scales can be explored with current and future spacecraft and telescopes. Unpacking the energy transfer process across scales will be crucial to solving key cosmic mysteries, the paper said.

More information: Chuanfei Dong et al, Reconnection-driven energy cascade in magnetohydrodynamic turbulence, Science Advances (2022). DOI: 10.1126/sciadv.abn7627

Journal information: Physical Review Letters  , Science Advances 

Provided by Princeton Plasma Physics Laboratory 

The breakthrough came in an impossibly small slice of time, less than it takes a beam of light to move an inch. In that tiny moment, nuclear fusion as an energy source went from far-away dream to reality. The world is now grappling with the implications of the historic milestone. For Arthur Pak and the countless other scientists who’ve spent decades getting to this point, the work is just beginning.

Pak and his colleagues at Lawrence Livermore National Laboratory are now faced with a daunting task: Do it again, but better—and bigger.

That means perfecting the use of the world’s largest laser, housed in the lab’s National Ignition Facility that science-fiction fans will recognize from the film “Star Trek: Into Darkness,” when it was used as a set for the warp core of the starship Enterprise. Just after 1 a.m. on Dec. 5, the laser shot 192 beams in three carefully modulated pulses at a cylinder containing a tiny diamond capsule filled with hydrogen, in an attempt to spark the first fusion reaction that produced more energy than it took to create. It succeeded, starting the path toward what scientists hope will someday be a new, carbon-free power source that will allow humans to harness the same source of energy that lights the stars.

Pak, who joined the Lawrence Livermore lab outside San Francisco in 2010, woke at 3 a.m. that day, unable to resist checking the initial results from his San Jose home. He’d tried staying awake for the shot itself, finally giving up as the experiment’s painstaking preparations dragged late into the night. “If you stayed up for every shot, every time for 10 years, you’d go insane,” he said.

For the last several months, it was clear his team was getting close, and in the pre-dawn dark, he checked for a key number that could show whether they’d succeeded—a count of neutrons the blast produced.

“When I saw that number, I was blown away,” he said.

“You can work your whole career and never see this moment. You’re doing it because you believe in the destination, and you like the challenge,” said Pak, leader for diagnostics on the experiment. “When humans come together and work collectively, we can do amazing things.”

The team at Lawrence Livermore—a government-funded research lab—will likely run its next test in February, with several more experiments to come in the months after. The goal will be to keep increasing the amount of energy that’s produced in the reaction. The means more tinkering: Use more laser energy. Fine-tune the laser blast. Generate more X-rays within the target—a key step of the process—using the same amount of energy. Maybe, eventually, upgrade the facility itself, a decision that would require buy-in from the Energy Department and a huge amount of funding.

All of that will take years, if not decades, starting with the Lawrence Livermore lab’s bite-sized experiments that last just nanoseconds.

“We need to figure out: Can we make it simpler? Can we make this process easier and more repeatable? Can we begin to do it more than one time a day?” said Kim Budil, director of the Lawrence Livermore lab. “Each of these is an incredible scientific and engineering challenge for us.”

Most experts forecast that the world is still at least 20 to 30 years away from fusion technology becoming viable on a scale that’s large and affordable enough to produce commercial power. That timeline places fusion beyond the scope of significantly being used to reach the world’s net-zero emissions goals by 2050. In that sense, fusion could be the carbon-free energy source of the future, but not of the current global energy transition that’s faced continuous hurdles.

Fusion has captured the scientific imagination for decades. It’s already used to give modern nuclear weapons their devastating power, but the dream is taming it for civilian energy demand. If it can be brought to scale, it would lead to power plants that supply abundant electricity day and night without emitting greenhouse gases. And unlike the nuclear power of today, sparked through a process called fission, it wouldn’t create long-lived radioactive waste. Entire generations of scientists have pursued it. President Joe Biden’s chief science advisor, Arati Prabhakar, spent a summer working on the lab’s laser-fusion program as a 19-year-old college student in bell bottoms—in 1978.

“This is such a tremendous example of what perseverance can achieve,” she said at a press conference last week. “This is how you do really big, hard things.”

Merging atoms

The successful laser shot produced fusion reactions generating 3.15 megajoules of power, topping the 2.05 megajoules imparted by the laser. It was a major threshold, the first time more energy came out than went in from the laser. But the equation needs to tilt much more in the direction of how much comes out to become commercially viable.

While today’s nuclear power plants employ fission, splitting atoms apart, fusion merges atoms together. Fusion researchers have followed two primary tracks. Lawrence Livermore, using a process called inertial confinement, blasts targets with laser beams, imploding a small amount of hydrogen until it fuses into helium. A commercial plant using this approach would need to repeat the process over and over again, extremely rapidly, to generate enough energy to power the electric grid.

Numerous companies are developing inertial confinement systems, though there are significant differences. Some are looking at different materials for the target, while others use particle accelerators instead of lasers, triggering the fusion reaction by slamming atoms together.

The main competing idea is called magnetic confinement, with systems that create a cloud of plasma, superheated to hundreds of millions of degrees, which can trigger a fusion reaction. Powerful magnets control the plasma and sustain the reaction. This approach has not yet achieved a net-energy gain, and the approach faces challenges including developing better magnets and creating materials that can withstand superhot temperatures and be used for the container to contain the plasma.

About $5 billion in funding has gone into fusion companies to date, with the vast majority aimed at magnetic confinement technologies, according to the Fusion Industry Association trade group.

Inertial confinement may be better suited to proving that fusion can work, said Adam Stein, director of nuclear energy innovation at The Breakthrough Institute, an Oakland, California-based research group. But in the longer run when it comes to commercialization, “plasma magnetic confinement is more likely to succeed,” he said.

‘Be an optimist’

Years were spent refining each part of the process at the Lawrence Livermore lab.

A lot of the success came down to precision. The fuel capsules all contain minute imperfections that can make a significant difference in how the reaction proceeds. So can the frozen hydrogen inside, a mix of the isotopes deuterium and tritium. The team would often produce the hydrogen ice, melt it back down and try again several times before a shot, hoping to get the best possible target and increase the chances of success.

Everyone working on fusion “has to be an optimist,” said Denise Hinkel, a physicist who focuses on improving the predictive ability of the program’s computer simulations and who has worked at Lawerence Livermore for 30 years. “Otherwise, you wouldn’t stay in the field.”

By this summer, the giant laser will be able to deliver about 8% more energy than it did during this month’s shot, according to Jean-Michel Di Nicola, chief engineer for the National Ignition Facility’s laser. Michael Stadermann, the target fabrication program manager, said that the lab is also developing a computer program that can examine the fuel-capsule shells for flaws much faster than humans can. They’re also working with capsule maker on improving the fabrication process.

It’s possible that the Lawrence Livermore breakthrough will remain just a moment of scientific history, and not mark the beginning of a new fusion industry powering the globe. Bridging the gap from experiment to commercialization could take decades, if it happens at all. And magnetic confinement could eventually be the fusion method that wins out, providing the world abundant clean energy. Pak, a soft-spoken man with a swoop of brown hair and a quick wit, said that outcome wouldn’t disappoint him.

“They can learn from us—we can learn from them,” Pak, 40, said. “When I’m an old man, I’m going to be really satisfied with my contributions.”

2022 Bloomberg L.P. Visit bloomberg.com. Distributed by Tribune Content Agency, LLC.

The precise control of micro-mechanical oscillators is fundamental to many contemporary technologies, from sensing and timing to radiofrequency filters in smartphones. Over the past decade, quantum control of mechanical systems has been firmly established with atoms, molecules, and ions in the first wave of development and superconducting circuits in the second quantum revolution.

This has been in particular catalyzed by cavity optomechanics. The field has allowed us to control mesoscopic mechanical objects with electromagnetic radiation pressure force. This has substantially improved our understanding of their quantum nature, which has enabled a host of advances including ground-state cooling, quantum squeezing, and remote entanglement of mechanical oscillators.

Pioneering theoretical studies have predicted that significantly richer physics and novel dynamics can be accessed in optomechanical lattices, including quantum collective dynamics and topological phenomena. But reproducing such devices experimentally under high control, as well as and building optomechanical lattices that can host multiple coupled optical and mechanical degrees of freedom has been a challenge.

Researchers in the group of Tobias J. Kippenberg at EPFL’s School of Basic Sciences have now built the first large-scale and configurable superconducting circuit optomechanical lattice that can overcome the scaling challenges of quantum optomechanical systems. The team realized an optomechanical strained graphene lattice and studied non-trivial topological edge states using novel measurement techniques. This work is now published in Nature.

The key element, which is a part of the single site of the lattice, is a so-called “vacuum-gap drumhead capacitor,” which is made of a thin aluminum film suspended over a trench in a silicon substrate. This constitutes the vibrating part of the device and, at the same time, forms a resonant microwave circuit with a spiral inductor.

“We developed a novel nanofabrication technique for superconducting circuit optomechanical systems with high reproducibility and extremely tight tolerances on the parameters of the individual devices,” says Amir Youssefi, who led the project. “This enables us to engineer the different sites to be virtually identical, like in a natural lattice.”

The graphene lattice is well known to exhibit non-trivial topological properties and localized edge states. The researchers observed such states in what they call an “optomechanical graphene flake,” consisting of twenty-four sites.

“Thanks to the built-in optomechanical toolkit, we were able to directly and non-perturbatively image the collective electromagnetic mode shapes in such lattices,” says Andrea Bancora who contributed to the research. “This is a unique feature of this platform.”

The team’s measurements closely match the theoretical predictions, showing that their new platform is a reliable testbed for studying topological physics in one and two-dimensional lattices.

“By having access to both the energy levels and mode shapes of these collective excitations, we were able to reconstruct the full underlying Hamiltonian of the system, allowing for the full extraction of disorder and coupling strengths in a superconducting lattice for the first time,” says Shingo Kono, another member of the research team.

The demonstration of optomechanical lattices not only provides access to studying many-body physics in such realizations of condensed matter lattice models but will also provide a route towards novel hybrid quantum systems when combined with superconducting qubits.

More information: Tobias Kippenberg, Topological lattices realized in superconducting circuit optomechanics, Nature (2022). DOI: 10.1038/s41586-022-05367-9. www.nature.com/articles/s41586-022-05367-9

Journal information: Nature 

Provided by Ecole Polytechnique Federale de Lausanne 

After over three years of upgrade and maintenance work, the Large Hadron Collider began its third period of operation (Run 3) in July 2022. Since then, the world’s most powerful particle accelerator has been colliding protons at a record-breaking energy of 13.6 TeV. The ATLAS collaboration has just released its first measurements of these record collisions, studying data collected in the first half of August 2022.

The researchers measured the rates of two well-known processes: the production of top-quark pairs and the production of a Z boson, which proceed through strong and electroweak interactions, respectively. The ratio of their cross-sections is sensitive to the inner structure of the proton, and their measurement sets constraints on the relative probabilities that reactions are initiated by quarks and gluons.

These early measurements also validate the functionality of the ATLAS detector and its reconstruction software, which underwent many improvements in preparation for Run 3.

Physicists focused on Z-boson decays to electron and muon pairs, and on top-quark decays to a W boson and a jet—collimated sprays of particles—originating from a bottom quark. The W boson subsequently decays into one electron or muon and an invisible neutrino. As the analysis uses very early Run 3 data, physicists relied on preliminary calibrations of the leptons, jets and luminosity. These were derived promptly after the first data became available.

ATLAS measured a top-quark pair to Z boson production ratio that is consistent with the Standard Model prediction within the current experimental uncertainty of 4.7%.

The calibration and corresponding uncertainties will be improved as more data is processed. Future updates of the calibration will allow researchers to measure the cross sections with greater precision.

To validate their results, physicists performed a series of cross-checks. These included measuring the ratio of the cross section each time the LHC was injected with a new fill of protons for a data-taking run.

More analyses using the Run 3 data will follow, exploiting the unprecedented energies and the increased LHC data set.

Provided by CERN 

We see the world around us because light is being absorbed by specialized cells in our retina. But can vision happen without any absorption at all—without even a single particle of light? Surprisingly, the answer is yes.

Imagine that you have a camera cartridge that might contain a roll of photographic film. The roll is so sensitive that coming into contact with even a single photon would destroy it. With our everyday classical means there is no way there’s no way to know whether there’s film in the cartridge, but in the quantum world it can be done. Anton Zeilinger, one of the winners of the 2022 Nobel Prize in Physics, was the first to experimentally implement the idea of an interaction-free experiment using optics.

Now, in a study exploring the connection between the quantum and classical worlds, Shruti Dogra, John J. McCord, and Gheorghe Sorin Paraoanu of Aalto University have discovered a new and much more effective way to carry out interaction-free experiments. The team used transmon devices—superconducting circuits that are relatively large but still show quantum behavior—to detect the presence of microwave pulses generated by classical instruments. Their research was recently published in Nature Communications.

An experiment with added layer of ‘quantumness’

Although Dogra and Paraoanu were fascinated by the work done by Zeilinger’s research group, their lab is centered around microwaves and superconductors instead of lasers and mirrors. “We had to adapt the concept to the different experimental tools available for superconducting devices. Because of that, we also had to change the standard interaction-free protocol in a crucial way: we added another layer of ‘quantumness’ by using a higher energy level of the transmon. Then, we used the quantum coherence of the resulting three-level system as a resource,” Paraoanu says.

Quantum coherence refers to the possibility that an object can occupy two different states at the same time—something that quantum physics allows for. However, quantum coherence is delicate and easily collapses, so it wasn’t immediately obvious that the new protocol would work. To the team’s pleasant surprise, the first runs of the experiment showed a marked increase in detection efficiency. They went back to the drawing board several times, ran theoretical models confirming their results, and double-checked everything. The effect was definitely there.

“We also demonstrated that even very low-power microwave pulses can be detected efficiently using our protocol,” says Dogra.

The experiment also showed a new way in which quantum devices can achieve results that are impossible for classical devices—a phenomenon known as quantum advantage. Researchers generally believe that achieving quantum advantage will require quantum computers with many qubits, but this experiment demonstrated genuine quantum advantage using a relatively simpler setup.

Potential applications in many types of quantum technology

Interaction-free measurements based on the less effective older methodology have already found applications in specialized processes such as optical imaging, noise-detection, and cryptographic key distribution. The new and improved method could increase the efficiency of these processes dramatically.

“In quantum computing, our method could be applied for diagnosing microwave-photon states in certain memory elements. This can be regarded as a highly efficient way of extracting information without disturbing the functioning of the quantum processor,” Paraoanu says.

The group led by Paraoanu is also exploring other exotic forms of information processing using their new approach, such as counterfactual communication (communication between two parties without any physical particles being transferred) and counterfactual quantum computing (where the result of a computation is obtained without in fact running the computer).

More information: Shruti Dogra et al, Coherent interaction-free detection of microwave pulses with a superconducting circuit, Nature Communications (2022). DOI: 10.1038/s41467-022-35049-z

Journal information: Nature Communications 

Provided by Aalto University 

In recent years, many physicists and material scientists have been studying superconductors, materials that can conduct direct current electricity without energy loss when cooled under a particular temperature. These materials could have numerous valuable applications, for instance generating energy for imaging machines (e.g., MRI scanners), trains, and other technological systems.

Researchers at Fudan University, Shanghai Qi Zhi Institute, Hong Kong University of Science and Technology, and other institutes in China have recently uncovered a new mechanism to generate anisotropically-enhanced in-plane upper critical field in atomically thin centrosymmetric superconductors with topological band inversions. Their paper, published in Nature Physics, specifically demonstrated this mechanism on a thin layer of 2M-WS₂, a material that has recently attracted much research attention.

“In 2020, a paper by our theoretical collaborator Prof. K.T. Law proposed that 2D centrosymmetric superconductors with a topological band inversion, such as 1T′-WTe₂ exhibit a distinct type of superconductivity, called spin-orbit-parity coupled (SOPC) superconductivity,” Enze Zhang, one of the researchers who carried out the study, told Phys.org.

“SOPC is predicted to produce novel superconductivity near the topological band crossing with both largely enhanced and anisotropic spin susceptibility with respect to in-plane magnetic field directions. At that time, we were conducting research on the superconducting properties of atomically thin 2M-WS₂, so after talking with Prof. K.T. Law, we felt that the emergent van der Waals superconductor 2M-WS₂ would most likely be a promising candidate for spin-orbit-parity coupled superconductivity.”

The structure of monolayer 2M-WS₂ is identical to that of 1T′-WTe₂, the material previously investigated by Prof. Law and his team. 2M-WS₂, however, has a unique stacking mode, which distinguishes it from other transition metal dichalcogenides.

The researchers previously found that in its bulk form, this material exhibit a high superconducting transition temperature T_C of 8.8 K. In addition, theoretical calculations suggested that atomically thin layers of 2M-WS₂ hold topological edge states with band inversion.

In their experiments, Zhang and his colleagues measured the in-plane upper critical field at a high magnetic field and confirmed the violation of the Pauli limit law. They also observed a strongly anisotropic two-fold symmetry in the material, in response to the in-plane magnetic field direction.

“Tunneling experiments conducted under high in-plane magnetic fields also showed that the superconducting gap in atomically thin 2M-WS₂ possesses an anisotropic magnetic response along different in-plane magnetic field directions, and it persists much above the Pauli limit,” Zhang explained. “Using self-consistent mean-field calculations, our theoretical collaborators conclude that these unusual behaviors originate from the strong spin-orbit-parity coupling arising from the topological band inversion in 2M-WS₂.”

The researchers’ experiments spanned across several steps. Firstly, the team performed magneto-transport measurements on atomically thin 2M-WS₂ and found that its in-plane upper critical field is not only far beyond the Pauli paramagnetic limit, but also exhibits a strongly anisotropic two-fold symmetry in response to the in-plane magnetic field direction.

Subsequently, they used tunneling spectroscopy to collect measurements under high in-plane magnetic fields. These measurements revealed that the superconducting gap in atomically thin 2M-WS₂ possesses an anisotropic magnetic response along different in-plane magnetic field directions, which persists much above the Pauli limit.

Finally, the researchers performed a series of self-consistent mean-field calculations to better understand the origin of the unusual behaviors they observed in their sample. Based on their results, they concluded that these behaviors originate from the strong spin-orbit-parity coupling arising from the topological band inversion in 2M-WS₂, which effectively pins the spin of states near the topological band crossing and renormalizes the effect of external Zeeman fields anisotropically.

“We uncovered a new mechanism for generating an anisotropically-enhanced in-plane upper critical field in atomically thin centrosymmetric superconductors with topological band inversions, highlighting 2D 2M-WS₂ as a wonderful platform for the study of exotic superconducting phenomena such as higher-order topological superconductivity and further device applications,” Zhang said.

“The novel properties found here are highly nontrivial as they directly reflect a strong SOPC inheriting from the topological band inversion in the normal state of 2M-WS₂, which had been ignored for many years in previous studies of centrosymmetric superconductors.”

In recent years, more research teams worldwide have been exploring the properties and mechanisms of centrosymmetric superconducting transition metal dichalcogenides (TMDs), such as monolayer superconducting 1T′-MoS₂, and 1T′-WTe₂, due to the characteristic co-existence of topological band structure and superconductivity within them.

The recent paper by Zhang and his colleagues could pave the way towards the exploration of large enhanced and strongly anisotropic in-plane upper critical fields, which could further improve the current understanding of these materials’ exotic physics.

“We now plan to explore the usual superconducting properties (such as the in-plane upper critical field and tunneling spectroscopy behavior at high magnetic field) of more atomically thin centrosymmetric superconductors with topological band inversions,” Zhang added.

More information: Enze Zhang et al, Spin–orbit–parity coupled superconductivity in atomically thin 2M-WS2, Nature Physics (2022). DOI: 10.1038/s41567-022-01812-8

Ying-Ming Xie et al, Spin-Orbit-Parity-Coupled Superconductivity in Topological Monolayer WTe2, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.107001

Journal information: Physical Review Letters  , Nature Physics 

© 2022 Science X Network

Luttinger liquids are usually paramagnetic materials exhibiting non-Fermi liquid behavior, such as molybdenum oxides. These “liquids” and their fascinating properties had so far been only observed in 1D and quasi-1D compounds, such as blue bronze A_0.3MoO₃ (A= K, Rb, Tl) and purple bronze Li_0.9Mo₆O₁₇.

Researchers at Tsinghua University, ShanghaiTech University, and other institutes in China recently observed prototypical Luttinger liquid behavior in η-Mo₄O₁₁,a charge-density wave material with a quasi-2D crystal structure. Their findings, published in Nature Physics, could pave the way for the exploration of non-Fermi liquid behavior in other 2D and 3D quantum materials.

“In our previous work, we identified the Luttinger liquid phase in the normal state of blue bronzes, which is not surprising due to its quasi-1D nature,” Lexian Yang and Yulin Chen, two of the researchers who carried out the study, told Phys.org.

“We then noticed that a large family of Molybdenum oxides share common construction unit: Mo-O octahedron chains. But in some of them, such as η-Mo₄O₁₁, quasi-1D chains cross each other and weave into a quasi-2D structure.”

Materials with quasi-2D structures, such as the material examined by Yang, Chen and their colleagues, have attracted considerable research attention, with physicists debating on whether they might preserve some properties of 1D materials, including Luttinger liquid behavior. Initially, the researchers did not expect to observe this behavior, thus they were very surprised when they did.

In their experiments, they used quasi-2D η-Mo₄O₁₁ samples with a layered structure. The advantage of using these samples is that they can be easily cleaved to expose large and flat surfaces, facilitating their examination.

“To protect our samples from contamination, we studied the sample in an ultra-high vacuum environment by exciting electrons inside the crystals using monochromatic light,” Yang and Chen explained. “We then collected these excited electrons, or photoelectrons and analyze their energy and momentum to deduced their initial status back inside the sample.”

To examine their samples, Yang and his colleagues used a spectroscopic technique known as angle-resolved photoemission spectroscopy (ARPES), which allows researchers to directly visualize the electronic structure of materials. This technique can be applied to countless different types of materials, and was previously also used to examine high-temperature superconductors, topological quantum materials, and transition metal dichalcogenides.

“We showed that Luttinger liquid physics, which was previously considered as prototype 1D behavior, can be extended to quasi-2D systems,” Yang and Chen said. “This extension may help us to understand other puzzling non-Fermi liquid behaviors in 2D or even 3D systems. Luttinger liquid behavior is a rare example of an exactly solvable model for interacting systems. Although it has long been regarded as the ‘standard model’ for 1D metals, theorists have proposed that it is related to the non-Fermi liquid behaviors in different systems such as the normal state of high-temperature cuprate superconductors.”

The recent findings gathered by this team of researchers represent a significant step towards achieving a unified understanding of non-Fermi liquid behaviors in 2D and 3D systems. Their work could thus soon inspire new studies exploring Luttinger liquid behavior and other non-Fermi liquid states in other materials.

“Our future research is already underway,” Yang and Chen added. “Our first step will be to explore and find more materials systems (low-dimensional Molybdenum oxides and beyond) featuring presumable Luttinger liquid. Secondly, knowing the common Luttinger liquid behavior in different materials, their similarities and differences will help unveil the physics laws underneath. Thirdly and more interestingly, the interactions between other degrees of freedom and the Luttinger liquid that could lead to long-range ordered states deserve a thorough exploration.”

More information: X. Du et al, Crossed Luttinger liquid hidden in a quasi-two-dimensional material, Nature Physics (2022). DOI: 10.1038/s41567-022-01829-z

L. Kang et al, Band-selective Holstein polaron in Luttinger liquid material A0.3MoO3 (A = K, Rb), Nature Communications (2021). DOI: 10.1038/s41467-021-26078-1

Journal information: Nature Communications  , Nature Physics 

© 2022 Science X Network

Water has puzzled scientists for decades. For the last 30 years or so, they have theorized that when cooled down to a very low temperature like -100C, water might be able to separate into two liquid phases of different densities. Like oil and water, these phases don’t mix and may help explain some of water’s other strange behavior, like how it becomes less dense as it cools.

It’s almost impossible to study this phenomenon in a lab, though, because water crystallizes into ice so quickly at such low temperatures. Now, new research from the Georgia Institute of Technology uses machine learning models to better understand water’s phase changes, opening more avenues for a better theoretical understanding of various substances. With this technique, the researchers found strong computational evidence in support of water’s liquid-liquid transition that can be applied to real-world systems that use water to operate.

“We are doing this with very detailed quantum chemistry calculations that are trying to be as close as possible to the real physics and physical chemistry of water,” said Thomas Gartner, an assistant professor in the School of Chemical and Biomolecular Engineering at Georgia Tech. “This is the first time anyone has been able to study this transition with this level of accuracy.”

The research was presented in the paper, “Liquid-Liquid Transition in Water From First Principles,” in the journal Physical Review Letters, with co-authors from Princeton University.

Simulating Water

To better understand how water interacts, the researchers ran molecular simulations on supercomputers, which Gartner compared to a virtual microscope.

“If you had an infinitely powerful microscope, you could zoom in all the way down to the level of the individual molecules and watch them move and interact in real time,” he said. “This is what we’re doing by creating almost a computational movie.”

The researchers analyzed how the molecules move and characterized the liquid structure at different water temperatures and pressures, mimicking the phase separation between the high and low-density liquids. They collected extensive data—running some simulations for up to a year—and continued to fine-tune their algorithms for more accurate results.

Even a decade ago, running such long and detailed simulations wouldn’t have been possible, but machine learning today offered a shortcut. The researchers used a machine learning algorithm that calculated the energy of how water molecules interact with each other. This model performed the calculation significantly faster than traditional techniques, allowing the simulations to progress much more efficiently.

Machine learning isn’t perfect, so these long simulations also improved the accuracy of the predictions. The researchers were careful to test their predictions with different types of simulation algorithms. If multiple simulations gave similar results, then it validated their accuracy.

“One of the challenges with this work is that there’s not a lot of data that we can compare to because it’s a problem that’s almost impossible to study experimentally,” Gartner said. “We’re really pushing the boundaries here, so that’s another reason why it’s so important that we try to do this using multiple different computational techniques.”

Beyond Water

Some of the conditions the researchers tested were extremes that probably don’t exist on Earth directly, but potentially could be present in various water environments of the solar system, from the oceans of Europa to water in the center of comets. Yet these findings could also help researchers better explain and predict water’s strange and complex physical chemistry, informing water’s use in industrial processes, developing better climate models, and more.

The work is even more generalizable, according to Gartner. Water is a well-studied research area, but this methodology could be expanded to other difficult-to-simulate materials like polymers, or complex phenomena like chemical reactions.

“Water is so central to life and industry, so this particular question of whether water can undergo this phase transition has been a longstanding problem, and if we can move toward an answer, that’s important,” he said. “But now we have this really powerful new computational technique, but we don’t yet know what the boundaries are and there’s a lot of room to move the field forward.”

More information: Thomas E. Gartner et al, Liquid-Liquid Transition in Water from First Principles, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.255702

Journal information: Physical Review Letters 

Provided by Georgia Institute of Technology 

Revealing the impulsive response of the bound electron to an electromagnetic field is of fundamentally importance for understanding various nonlinear processes of matter, and it is one of the ultimate goals of attosecond science.

However, it is still a very challenging task. Recently, a group from Huazhong University of Science and Technology demonstrated a new photoelectron spectroscopy for metrology of the attosecond response of the bound electron exposed to the intense XUV pulses.

In this work, they took advantage of the benefits of the attosecond temporal resolution of tunneling ionization and the subatomic spatial resolution of photoelectron holography. With this scheme, they successfully filmed an attosecond-scale movie of the impulsive response of the bound electron to an intense XUV pulse.

Their work not only revealed how the atomic stabilization is established in the intense laser field, but also established a novel photoelectron spectroscopy to time-resolved imaging of the ultrafast bound-state electron processes in intense laser fields. Extension of this method to more complex molecules is promising, and it will be an exciting aspect in the attosecond science.

The paper is published in the journal Ultrafast Science.

More information: Jintai Liang et al, Direct Visualization of Deforming Atomic Wavefunction in Ultraintense High-Frequency Laser Pulses, Ultrafast Science (2022). DOI: 10.34133/2022/9842716

Provided by Ultrafast Science

The cosmic optical background (COB) is the visible light emitted by all sources outside of the Milky Way. This faint glow of light, which can only be observed using very precise and sophisticated telescopes, could help astrophysics to learn more about the origins of the universe and what lies beyond our galaxy.

Last year, physicists working at different institutes across the United States published the most precise COB measurements collected so far, gathered by the New Horizons spacecraft, an interplanetary space probe launched by NASA over a decade ago. These measurements suggested that the COB is two times brighter than theoretical predictions.

Researchers at Johns Hopkins University have recently carried out a theoretical study exploring the possibility that this observed excess light could be caused by the decay of a hypothesized type of dark matter particles, known as axions. In their paper, published in Physical Review Letters, they showed that axions with masses between 8 and 20 eV could potentially account for the excess COB flux measured by the New Horizons team.

“Marc Postman is a colleague across the street who is an incredible observational cosmologist, and so when his paper with Todd Lauer and the New Horizons team appeared, I noticed it and read it,” Marc Kamionkowski, one of the researchers who carried out the study, told Phys.org.

“The measurement they collected is a great example of cleverly repurposing a powerful astronomical observatory to different ends than those it was designed for. We sent this incredible little spacecraft out toward Pluto years ago, and it did everything it was supposed to, but it had no brakes, and is still speeding further and further from the sun with not much to do. Marc and Todd realized that it could be used to detect—for the very first time—the cosmic background of optical photons from all the unresolved galaxies in the universe, and it did.”

After reading the paper by Lauer and his colleagues, Kamionkowski realized that if the excess they measured was in-fact attributed to the decay of axions, this could be potentially confirmed using available cosmological data. Specifically, this excess would be detected with a high signal-to-noise ratio during SPHEREx, a planned two-year NASA mission that will send a near-infrared space observatory into space to collect new and potentially valuable measurements.

“Our calculations are embarrassingly simple, as they are the types of calculations that we and tons of other people have been doing for years,” Kamionkowski explained. “The idea that two-photon decay of an axion could lead to a cosmic signal was already around when I was a graduate student over 30 years ago. Our work simply involves summing the photons from all those produced by axion decay over time, a simple integral. We had to get some factors of cosmic redshift in there correctly, but that’s a homework problem in a typical cosmology class.”

Overall, the calculations performed by Kamionkowski and his colleagues highlight the possibility of confirming or disproving the connection between axion dark matter decay and the recently observed excess COB using future line-intensity-mapping (LIM) measurements set to be collected by NASA’s SPHEREx satellite.

SPHEREx is expected to launch in 2025, collecting measuring near-infrared signals originating from approximately 450 million galaxies. The researchers at Johns Hopkins have already published a follow-up paper, where they explored the consistency of the axion decay scenario with existing constraints to COB from gamma rays.

“NASA’s Fermi telescope has obtained gamma-ray energy spectra from over 800 blazars, and the highest-energy gamma rays can be attenuated by production of electron-positron pairs via scattering with COB photons,” Kamionkowski added.

“In our new study, we modeled the attenuation expected from this COB scattering and by comparing with Fermi data were able to place an upper limit to the COB background from dark-matter decay, which was still consistent with the COB excess inferred from New Horizons.

“My student Gabriela Sato-Polito has also been working with Dan Grin (Haverford College) looking in deep VLT images of several high-redshift clusters for dark-matter decay lines. These measurements should allow us to probe some, but not all, of the parameter space for dark-matter decays consistent with the New Horizons excess.”

More information: José Luis Bernal et al, Cosmic Optical Background Excess, Dark Matter, and Line-Intensity Mapping, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.231301

Tod R. Lauer et al, New Horizons Observations of the Cosmic Optical Background, The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/abc881

José Luis Bernal et al, Seeking dark matter with γ-ray attenuation, arXiv (2022). DOI: 10.48550/arxiv.2208.13794

Journal information: Physical Review Letters  , Astrophysical Journal  , arXiv