Showing posts with label Physics. Show all posts
Showing posts with label Physics. Show all posts

Monday, September 20, 2021

Physicists probe light smashups to guide future research

The Compact Muon Solenoid experiment at the
European Organization for Nuclear Research’s
Large Hadron Collider.
Photo courtesy of CERN
Hot on the heels of proving an 87-year-old prediction that matter can be generated directly from light, Rice University physicists and their colleagues have detailed how that process may impact future studies of primordial plasma and physics beyond the Standard Model.

“We are essentially looking at collisions of light,” said Wei Li, an associate professor of physics and astronomy at Rice and co-author of the study published in Physical Review Letters.

Rice physicists teamed with colleagues at Europe’s Large Hadron Collider to study matter-generating collisions of light. Researchers showed the departure angle of debris from the smashups is subtly distorted by quantum interference patterns in the light prior to impact. Illustration by

“We know from Einstein that energy can be converted into mass,” said Li, a particle physicist who collaborates with hundreds of colleagues on experiments at high-energy particle accelerators like the European Organization for Nuclear Research’s Large Hadron Collider (LHC) and Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC).

Accelerators like RHIC and LHC routinely turn energy into matter by accelerating pieces of atoms near the speed of light and smashing them into one another. The 2012 discovery of the Higgs particle at the LHC is a notable example. At the time, the Higgs was the final unobserved particle in the Standard Model, a theory that describes the fundamental forces and building blocks of atoms.

Impressive as it is, physicists know the Standard Model explains only about 4% of the matter and energy in the universe. Li said this week’s study, which was lead-authored by Rice postdoctoral researcher Shuai Yang, has implications for the search for physics beyond the Standard Model.

Wednesday, September 15, 2021

Have we detected dark energy?


A new study, led by researchers at the University of Cambridge and reported in the journal Physical Review D, suggests that some unexplained results from the XENON1T experiment in Italy may have been caused by dark energy, and not the dark matter the experiment was designed to detect.

They constructed a physical model to help explain the results, which may have originated from dark energy particles produced in a region of the Sun with strong magnetic fields, although future experiments will be required to confirm this explanation. The researchers say their study could be an important step toward the direct detection of dark energy.

Everything our eyes can see in the skies and in our everyday world – from tiny moons to massive galaxies, from ants to blue whales – makes up less than five percent of the universe. The rest is dark. About 27% is dark matter – the invisible force holding galaxies and the cosmic web together – while 68% is dark energy, which causes the universe to expand at an accelerated rate.

“Despite both components being invisible, we know a lot more about dark matter, since its existence was suggested as early as the 1920s, while dark energy wasn’t discovered until 1998,” said Dr Sunny Vagnozzi from Cambridge’s Kavli Institute for Cosmology, the paper’s first author. “Large-scale experiments like XENON1T have been designed to directly detect dark matter, by searching for signs of dark matter ‘hitting’ ordinary matter, but dark energy is even more elusive.”

To detect dark energy, scientists generally look for gravitational interactions: the way gravity pulls objects around. And on the largest scales, the gravitational effect of dark energy is repulsive, pulling things away from each other and making the Universe’s expansion accelerate.

About a year ago, the XENON1T experiment reported an unexpected signal, or excess, over the expected background. “These sorts of excesses are often flukes, but once in a while they can also lead to fundamental discoveries,” said Dr Luca Visinelli, a researcher at Frascati National Laboratories in Italy, a co-author of the study. “We explored a model in which this signal could be attributable to dark energy, rather than the dark matter the experiment was originally devised to detect.”

At the time, the most popular explanation for the excess were axions – hypothetical, extremely light particles – produced in the Sun. However, this explanation does not stand up to observations, since the amount of axions that would be required to explain the XENON1T signal would drastically alter the evolution of stars much heavier than the Sun, in conflict with what we observe.

We are far from fully understanding what dark energy is, but most physical models for dark energy would lead to the existence of a so-called fifth force. There are four fundamental forces in the universe, and anything that can’t be explained by one of these forces is sometimes referred to as the result of an unknown fifth force.

However, we know that Einstein’s theory of gravity works extremely well in the local universe. Therefore, any fifth force associated to dark energy is unwanted and must be ‘hidden’ or ‘screened’ when it comes to small scales, and can only operate on the largest scales where Einstein's theory of gravity fails to explain the acceleration of the Universe. To hide the fifth force, many models for dark energy are equipped with so-called screening mechanisms, which dynamically hide the fifth force.

Vagnozzi and his co-authors constructed a physical model, which used a type of screening mechanism known as chameleon screening, to show that dark energy particles produced in the Sun’s strong magnetic fields could explain the XENON1T excess.

“Our chameleon screening shuts down the production of dark energy particles in very dense objects, avoiding the problems faced by solar axions,” said Vagnozzi. “It also allows us to decouple what happens in the local very dense Universe from what happens on the largest scales, where the density is extremely low.”

The researchers used their model to show what would happen in the detector if the dark energy was produced in a particular region of the Sun, called the tachocline, where the magnetic fields are particularly strong.

“It was really surprising that this excess could in principle have been caused by dark energy rather than dark matter,” said Vagnozzi. “When things click together like that, it’s really special.”

Their calculations suggest that experiments like XENON1T, which are designed to detect dark matter, could also be used to detect dark energy. However, the original excess still needs to be convincingly confirmed. “We first need to know that this wasn’t simply a fluke,” said Visinelli. “If XENON1T actually saw something, you’d expect to see a similar excess again in future experiments, but this time with a much stronger signal.”

If the excess was the result of dark energy, upcoming upgrades to the XENON1T experiment, as well as experiments pursuing similar goals such as LUX-Zeplin and PandaX-xT, mean that it could be possible to directly detect dark energy within the next decade.

Source/Credit: University of Cambridge / Sarah Collins


Monday, September 13, 2021

Researchers Create Materials for Shape-Shifting Architecture


Source/Credit: North Carolina State University

Researchers at North Carolina State University have developed materials that can be used to create structures capable of transforming into multiple different architectures. The researchers envision applications ranging from construction to robotics.

“The system we’ve developed was inspired by metamorphosis,” says Jie Yin, corresponding author of a paper on the work and an associate professor of mechanical and aerospace engineering at NC State. “With metamorphosis in nature, animals change their fundamental shape. We’ve created a class of materials that can be used to create structures that change their fundamental architecture.”

Kirigami is a fundamental concept for Yin’s work. Kirigami is a variation of origami that involves cutting and folding paper. But while kirigami traditionally uses two-dimensional materials, Yin applies the same principles to three-dimensional materials.

The metamorphosis system starts with a single unit of 3D kirigami. Each unit can form multiple shapes in itself. But these units are also modular – they can be connected to form increasingly complex structures. Because the individual units themselves can form multiple shapes, and can connect to other units in multiple ways, the overall system is capable of forming a wide variety of architectures.

“Think of what you can build with conventional materials,” Yin says. “Now imagine what you can build when each basic building block is capable of transforming in multiple ways.”

Yin’s lab previously demonstrated a similar concept, in which 3D kirigami units were stacked on each other. In that system, the units could be used to assemble a structure – but the structure could also then be disassembled.

The metamorphosis system involves actually connecting the kirigami units. In other words, once the units are connected to each other they cannot be disconnected. However, the larger structures they create are capable of transforming into multiple, different architectures.

“There are two big differences between our first kirigami system and the metamorphosis system,” Yin explains.

“The first kirigami system involved units that could be assembled into architectures and then disassembled, which is an advantage. However, when the units were assembled, the architecture wouldn’t be capable of transforming. Because the sides of the unit were not rigid and fixed at 90-degree angles, the assembled structure could bend and move – but it could not fundamentally change its geometry.

“The metamorphosis kirigami system does not allow you to disassemble a structure,” Yin says. “And because the sides of each cubic unit are rigid and fixed at 90-degree angles, the assembled structure does not bend or flex very much. However, the finished structure is capable of transforming into different architectures.”

In proof-of-concept testing, the researchers demonstrated that the metamorphosis system was capable of creating many different structures that are capable of bearing significant weight while maintaining their structural integrity.

That structural integrity is important, because Yin thinks construction is one potential application for the metamorphosis system.

“If you scale this approach up, it could be the basis for a new generation of construction materials that can be used to create rapidly deployable structures,” Yin says. “Think of the medical units that have had to be expanded on short notice during the pandemic, or the need for emergency housing shelters in the wake of a disaster.”

The researchers also think the metamorphosis system could be used to create a variety of robotic devices that can transform in order to respond to external stimuli or to perform different functions.

“We also think this system could be used to create a new line of toys – particularly toys that can help people explore some fundamental STEM concepts related to physics and engineering,” Yin says. “We’re open to working with industry collaborators to pursue these and other potential applications for the system.”

The paper, “Metamorphosis of three-dimensional kirigami-inspired reconfigurable and reprogrammable architected matter,” is published in the journal Materials Today Physics. First author of the paper is Yanbin Li, a Ph.D. student at NC State. The work was done with support from the National Science Foundation, under grant 2005374.

Source/Credit: North Carolina State University


Friday, September 10, 2021

Silicon, Subatomic Particles and Possible ‘Fifth Force’


As neutrons pass through a crystal, they create two different standing waves – one along atomic planes and one between them. The interaction of these waves affects the path of the neutron, revealing aspects of the crystal structure.  Credit: NIST
Using a groundbreaking new technique at the National Institute of Standards and Technology (NIST), an international collaboration led by NIST researchers has revealed previously unrecognized properties of technologically crucial silicon crystals and uncovered new information about an important subatomic particle and a long-theorized fifth force of nature.

By aiming subatomic particles known as neutrons at silicon crystals and monitoring the outcome with exquisite sensitivity, the NIST scientists were able to obtain three extraordinary results: the first measurement of a key neutron property in 20 years using a unique method; the highest-precision measurements of the effects of heat-related vibrations in a silicon crystal; and limits on the strength of a possible “fifth force” beyond standard physics theories.

The researchers report their findings in the journal Science.

In a regular crystal such as silicon, there are many parallel sheets of atoms, each of which forms a plane. Probing different planes with neutrons reveals different aspects of the crystal.  Credit: NIST
To obtain information about crystalline materials at the atomic scale, scientists typically aim a beam of
particles (such as X-rays, electrons or neutrons) at the crystal and detect the beam’s angles, intensities and patterns as it passes through or ricochets off planes in the crystal’s lattice-like atomic geometry.

That information is critically important for characterizing the electronic, mechanical and magnetic properties of microchip components and various novel nanomaterials for next-generation applications including quantum computing. A great deal is known already, but continued progress requires increasingly detailed knowledge.

“A vastly improved understanding of the crystal structure of silicon, the ‘universal’ substrate or foundation material on which everything is built, will be crucial in understanding the nature of components operating near the point at which the accuracy of measurements is limited by quantum effects,” said NIST senior project scientist Michael Huber.

Neutrons, Atoms and Angles

Like all quantum objects, neutrons have both point-like particle and wave properties. As a neutron travels through the crystal, it forms standing waves (like a plucked guitar string) both in between and on top of rows or sheets of atoms called Bragg planes. When waves from each of the two routes combine, or “interfere” in the parlance of physics, they create faint patterns called pendellösung oscillations that provide insights into the forces that neutrons experience inside the crystal.

“Imagine two identical guitars,” said Huber. “Pluck them the same way, and as the strings vibrate, drive one down a road with speed bumps — that is, along the planes of atoms in the lattice — and drive the other down a road of the same length without the speed bumps — analogous to moving between the lattice planes. Comparing the sounds from both guitars tells us something about the speed bumps: how big they are, how smooth, and do they have interesting shapes?”

The latest work, which was conducted at the NIST Center for Neutron Research (NCNR) in Gaithersburg, Maryland, in collaboration with researchers from Japan, the U.S. and Canada, resulted in a fourfold improvement in precision measurement of the silicon crystal structure.

Not-Quite-Neutral Neutrons

Each neutron in an atomic nucleus is made up of three elementary particles called quarks. The three quarks’ electrical charge sum to zero, making it electrically neutral. But the distribution of those charges is such that positive charges are more likely to be found in the center of the neutron, and negative charges toward the outside.  Credit: NIST

In one striking result, the scientists measured the electrical “charge radius” of the neutron in a new way with an uncertainty in the radius value competitive with the most-precise prior results using other methods. Neutrons are electrically neutral, as their name suggests. But they are composite objects made up of three elementary charged particles called quarks with different electrical properties that are not exactly uniformly distributed.

As a result, predominantly negative charge from one kind of quark tends to be located toward the outer part of the neutron, whereas net positive charge is located toward the center. The distance between those two concentrations is the “charge radius.” That dimension, important to fundamental physics, has been measured by similar types of experiments whose results differ significantly. The new pendellösung data is unaffected by the factors thought to lead to these discrepancies.

Measuring the pendellösung oscillations in an electrically charged environment provides a unique way to gauge the charge radius. “When the neutron is in the crystal, it is well within the atomic electric cloud,” said NIST’s Benjamin Heacock, the first author on the Science paper.

“In there, because the distances between charges are so small, the interatomic electric fields are enormous, on the order of a hundred million volts per centimeter. Because of that very, very large field, our technique is sensitive to the fact that the neutron behaves like a spherical composite particle with a slightly positive core and a slightly negative surrounding shell.”

Vibrations and Uncertainty

A valuable alternative to neutrons is X-ray scattering. But its accuracy has been limited by atomic motion caused by heat. Thermal vibration causes the distances between crystal planes to keep changing, and thus changes the interference patterns being measured.

The scientists employed neutron pendellösung oscillation measurements to test the values predicted by X-ray scattering models and found that some significantly underestimate the magnitude of the vibration.

The results provide valuable complementary information for both x-ray and neutron scattering. “Neutrons interact almost entirely with the protons and neutrons at the centers, or nuclei, of the atoms,” Huber said, “and x-rays reveal how the electrons are arranged between the nuclei. This complementary knowledge deepens our understanding.

“One reason our measurements are so sensitive is that neutrons penetrate much deeper into the crystal than x-rays – a centimeter or more – and thus measures a much larger assembly of nuclei. We have found evidence that the nuclei and electrons may not vibrate rigidly, as is commonly assumed. That shifts our understanding on the how silicon atoms interact with one another inside a crystal lattice.”

Force Five

The Standard Model is the current, widely accepted theory of how particles and forces interact at the smallest scales. But it’s an incomplete explanation of how nature works, and scientists suspect there is more to the universe than the theory describes.

The Standard Model describes three fundamental forces in nature: electromagnetic, strong and weak. Each force operates through the action of “carrier particles.” For example, the photon is the force carrier for the electromagnetic force. But the Standard Model has yet to incorporate gravity in its description of nature. Furthermore, some experiments and theories suggest the possible presence of a fifth force.

“Generally, if there’s a force carrier, the length scale over which it acts is inversely proportional to its mass,” meaning it can only influence other particles over a limited range, Heacock said. But the photon, which has no mass, can act over an unlimited range. “So, if we can bracket the range over which it might act, we can limit its strength.” The scientists’ results improve constraints on the strength of a potential fifth force by tenfold over a length scale between 0.02 nanometers (nm, billionths of a meter) and 10 nm, giving fifth-force hunters a narrowed range over which to look.

The researchers are already planning more expansive pendellösung measurements using both silicon and germanium. They expect a possible factor of five reduction in their measurement uncertainties, which could produce the most precise measurement of the neutron charge radius to date and further constrain — or discover — a fifth force. They also plan to perform a cryogenic version of the experiment, which would lend insight into how the crystal atoms behave in their so-called “quantum ground state,” which accounts for the fact that quantum objects are never perfectly still, even at temperatures approaching absolute zero.

Source/Credit: National Institute of Standards and Technology


Thursday, September 2, 2021

Discovery paves way for improved quantum devices


Schematic of a superconducting circuit [thin black lines] on a silicon chip [yellow base], being imaged using terahertz scanning near-field microscopy [red beam focused into yellow tip].

Physicists and engineers have found a way to identify and address imperfections in materials for one of the most promising technologies in commercial quantum computing.

The University of Queensland team was able to develop treatments and optimize fabrication protocols in common techniques for building superconducting circuits on silicon chips.

Dr Peter Jacobson, who co-led the research, said the team had identified that imperfections introduced during fabrication reduced the effectiveness of the circuits.

"Superconducting quantum circuits are attracting interest from industry giants such as Google and IBM, but widespread application is hindered by ‘decoherence’, a phenomenon which causes information to be lost,” he said.

“Decoherence is primarily due to interactions between the superconducting circuit and the silicon chip – a physics problem – and to material imperfections introduced during fabrication – an engineering problem.

“So we needed input from physicists and engineers to find a solution.”

 The team used a method called terahertz scanning near-field optical microscopy (THz SNOM) – an atomic force microscope combined with a THz light source and detector.

This provided a combination of high spatial resolution – seeing down to the size of viruses – and local spectroscopic measurements.

Professor Aleksandar Rakić said the technique enabled probing at the nanoscale rather than the macroscale by focusing light onto a metallic tip.  

“This provides new access for us to understand where imperfections are located so we can reduce decoherence and help reduce losses in superconducting quantum devices,” Professor Rakić said.

“We found that commonly used fabrication recipes unintentionally introduce imperfections into the silicon chips, which contribute to decoherence.

“And we also showed that surface treatments reduce these imperfections, which in turn reduces losses in the superconducting quantum circuits.”

Associate Professor Arkady Fedorov said this allowed the team to determine where in the process defects were introduced and optimize fabrication protocols to address them.

“Our method allows the same device to be probed multiple times, in contrast to other   methods that often require the devices to be cut up before being probed,” Dr Fedorov said.

“The team’s results provide a path towards improving superconducting devices for use in quantum computing applications.”

In future, THz SNOM could be used to define new ways to improve the operation of quantum devices and their integration into a viable quantum computer.

The results are published in Applied Physics Letters

News Release
Source/Credit: University of Queensland


Wednesday, September 1, 2021

Physicists find ‘magnon’ origins in 2D magnet


Rice University physicists Pengcheng Dai (left) and Lebing Chen have discovered that unusual magnetic features they previously noticed in 2D chromium triiodide arise from topological features. (Photo by Jeff Fitlow/Rice University)

Rice physicists have confirmed the topological origins of magnons, magnetic features they discovered three years ago in a 2D material that could prove useful for encoding information in the spins of electrons.

The discovery, described in a study published online this week in the American Physical Society journal PRX, provides a new understanding of topology-driven spin excitations in materials known as in 2D van der Waals magnets. The materials are of growing interest for spintronics, a movement in the solid-state electronics community toward technologies that use electron spins to encode information for computation, storage and communications.

Spin is an intrinsic feature of quantum objects and the spins of electrons play a key role in bringing about magnetism.

Rice physicist Pengcheng Dai, co-corresponding author of the PRX study, said inelastic neutron-scattering experiments on the 2D material chromium triiodine confirmed the origin of the topological nature of spin excitations, called magnons, that his group and others discovered in the material in 2018.

The group’s latest experiments at Oak Ridge National Laboratory’s (ORNL) Spallation Neutron Source showed “spin-orbit coupling induces asymmetric interactions between spins” of electrons in chromium triiodine, Dai said. “As a result, the electron spins feel the magnetic field of moving nuclei differently, and this affects their topological excitations.”

In van der Waals materials, atomically thin 2D layers are stacked like pages in a book. The atoms

Graduate student Lebing Chen displays chromium triiodide crystals
 he made in a Rice University laboratory.
(Photo by Jeff Fitlow/Rice University)

within layers are tightly bonded, but the bonds between layers are weak. The materials are useful for exploring unusual electronic and magnetic behaviors. For example, a single 2D sheet of chromium triiodine has the same sort of magnetic order that makes magnetic decals stick to a metal refrigerator. Stacks of three or more 2D layers also have that magnetic order, which physics call ferromagnetic. But two stacked sheets of chromium triiodine have an opposite order called antiferromagnetic.

That strange behavior led Dai and colleagues to study the material. Rice graduate student Lebing Chen, the lead author of this week’s PRX study and of the 2018 study in the same journal, developed methods for making and aligning sheets of chromium triiodide for experiments at ORNL. By bombarding these samples with neutrons and measuring the resulting spin excitations with neutron time-of-flight spectrometry, Chen, Dai and colleagues can discern unknown features and behaviors of the material.

In their previous study, the researchers showed chromium triiodine makes its own magnetic field thanks to magnons that move so fast they feel as if they are moving without resistance. Dai said the latest study explains why a stack of two 2D layers of chromium triiodide has antiferromagnetic order.

“We found evidence of a stacking-dependent magnetic order in the material,” Dai said. Discovering the origins and key features of the state is important because it could exist in other 2D van der Waals magnets.

Additional co-authors include Bin Gao of Rice, Jae-Ho Chung of Korea University, Matthew Stone, Alexander Kolesnikov, Barry Winn, Ovidiu Garlea and Douglas Abernathy of ORNL, and Mathias Augustin and Elton Santos of the University of Edinburgh.

The research was funded by the National Science Foundation (1700081), the Welch Foundation (C-1839), the National Research Foundation of Korea (2020R1A5A1016518, 2020K1A3A7A09077712), the United Kingdom’s Engineering and Physical Research Council and the University of Edinburgh and made use of facilities provided by the United Kingdom’s ARCHER National Supercomputing Service and the Department of Energy’s Office of Science.

News Release
Source/Credit: Rice University / Jade Boyd


Tuesday, August 24, 2021

Emerging from the deep: Stawell’s dark matter lab takes shape

Construction of the Southern Hemisphere’s first dark matter underground physics laboratory is progressing with the concrete slab now in place and the world-class facility on schedule to welcome scientists by Christmas.

The ancillary area where scientists can shower before going into the lab to work.
Image: Stawell Gold Mines

Dr Leonie Walsh, Victoria’s first lead scientist, first woman president of the Australian Innovation Research Group and representative on the Forum of Australian Chief Scientists is interim chair of the company that will operate and manage the Stawell Underground Physics Laboratory (SUPL).

Dr Walsh recently visited the underground laboratory in regional Victorian, seeing first-hand the work underway to ensure that the lab, one kilometer underground,  has an excellent chance of detecting the universe’s elusive dark matter.

“We saw the cavern walls where the lab is being built, being sprayed with a product called Tekflex to reduce the potential for interference from background radon gas in the rock mass, in experiments,” Dr Walsh said. “As an industrial scientist, I have worked across a broad range of industrial sites around the world, but none as unique as SUPL.”

It takes half an hour to journey underground to the site of the lab. Dr Walsh completed the journey after undergoing the strict safety induction and personal protective equipment (PPE) fit-out.

“Researchers will start their day with a 10km drive down a maze of tunnels in protective equipment to the cavernous laboratory, 1100 meters underground to work on their dark matter experiments with equipment designed and built for the purpose of finding dark matter – this thing that makes up 85 per cent of our universe, but which continues to be a mystery,” she said.

“The disused section of Stawell’s gold mine in regional Victoria, has turned out to be the ideal location to progress our understanding of dark matter.”

The 33 meters long, 10 meters wide lab is funded by a $10 million grant from the Federal and State Governments, supported by a $35 million Australian Research Council for the Centre of Excellence for Dark Matter Particle Physics based at the University of Melbourne.

Tom Kelly, the University of Melbourne’s Senior Project Manager, said that major pieces of the plant as well as plumbing, electrical and communications cable and mechanical ductwork and piping are expected to be in place by early October.

“We anticipate the handover to be on time and to commence the installation of experimental equipment before Christmas,” Mr Kelly said.

The five research institutions that will work at Stawell are Melbourne University, Swinburne University of Technology, Adelaide University, the Australian National University and the Australian Nuclear Science and Technology Organization (ANSTO).

ANSTO’s representative on the SUPL company board, Professor Andrew Peele, accompanied Dr Walsh on the inspection and said: “Science goes to extreme lengths to find answers, and in this case, to a very sheltered environment a kilometer underground. It is impressive to see the progress made first-hand and pleasing to see the preparations for the range of activities that will advance our understanding of dark matter.

“ANSTO is delighted to be part of this project and to share our expertise in ultra-sensitive radiation measurement. This is critical to the operation of the instruments that will be housed in SUPL and will also make possible high-precision radiation measurements needed to better understand environmental and other samples.”

Dr Walsh is pictured above at left with Professor Elisabetta Barberio, the Director of the Stawell Underground Physics Laboratory, and board member designate, Professor Andrew Peele, from the Australian Nuclear Science and Technology Organization (ANSTO).

Source / Credit: The University of Melbourne

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