Tag Archives: #physics

Physicists Build A Landau-Ginzburg Theory For Higher Form Symmetries (Quantum Physics)

Physicists like symmetries. Emmy Noether taught us that symmetries result in conserved quantities. For most “ordinary” symmetries, these conserved quantities are (basically) numbers of particles.

For example, if you have a bunch of atoms in a box, the mathematical description of the box has a certain “symmetry” that enforces the dynamical statement that the number of atoms in the box can’t change, and that you can’t lose an atom!

Now, this is fine if you only care about particles. But, many interesting systems have “extended objects” — e.g. strings — which are also conserved.

“My favorite example is ordinary Maxwell electromagnetism, where magnetic field lines are strings that cannot end.”

— Nabil Iqbal, Theoretical Physicist and Associate Professor at Durham University

What is the symmetry principle enforcing the conservation of higher dimensional objects? Nowadays, we call these “higher form symmetries”, and they were explained by Davide Gaiotto and colleagues in their 2014 paper, that influenced Nabil Iqbal’s research greatly.

“The upshot is that, if you ever have extended objects that can’t break or vanish — gauge theory flux tubes, cosmic strings, magnetic field lines — you probably have one of these higher-form symmetries playing an important role.”

— told Nabil Iqbal

The idea behind higher-form symmetries is simple: just as ordinary global symmetries result in conservation laws for particles, theories that are invariant under higher-form global symmetries possess conservation laws for extended objects, such as strings or flux tubes.

“We can now try and use these new symmetries to organize our understanding. In the recent work with John, we build a Landau-Ginzburg theory for such symmetries, where we try and describe the physics close to a point where one of these symmetries is” “about” to break.”

— told Nabil Iqbal

Just as normal Landau-Ginzburg theories describe the condensation of particles, this new framework has to describe the condensation of “strings”. This is complicated but fun, and they tried to get a grasp of it using these new symmetries and principles of effective field theory.

“By the way, these are “not” gauge symmetries; gauge “symmetry” is perhaps a lousy name for something that is not really a symmetry at all. But a lot of gauge theories happen to host extended objects, and so can be nicely understood in this framework.”

If you wanna know more, just check out the video given below, Nabil Iqbal have given some online talks on it.


For more:

Nabil Iqbal, John McGreevy, “Mean string field theory: Landau-Ginzburg theory for 1-form symmetries”, Arxiv, pp. 1-47, 2021.
arXiv:2106.12610


Note for editors of other websites: To reuse this article fully or partially kindly give credit either to Nabil Iqbal / our author/editor S. Aman / provide a link of our article

From Burglar Alarms to Black Hole Detectors (Physics)

Super sensors as possible outputs of a quantum gravity experiment

For decades, physicists have been working on a single theory that encompasses all four major forces in physics. Quantum theory unifies three of these forces, but does not appear to accommodate the fourth, namely gravity. A group of physicists, including Anupam Mazumdar from the University of Groningen, recently proposed an experiment involving observations of two miniscule diamonds in free fall, which could prove whether or not gravity is a quantum phenomenon. This experimental system would also be highly sensitive to even the smallest of disturbances. In a new article, Mazumdar and colleagues show that locating the experiment within a vacuum container removes the noise caused by colliding gas particles in the experiment. Moreover, restricting access to the experimental site takes care of the interference caused by moving masses, ranging from butterflies to passing cars. Furthermore, this sensitivity to moving objects implies that the experimental system could serve as a movement sensor, with applications that include predicting earthquakes by measuring tectonic movements.

Last year, Anupam Mazumdar, a physicist from the University of Groningen, jointly proposed an experiment together with colleagues from the UK that could conclusively prove whether gravity is a quantum phenomenon. This experiment would focus on observing two relatively large, entangled quantum systems in free fall. In a new article, published on 4 June in Physical Review Research, the scientists describe in more detail how two types of noise could be reduced. They suggest that quantum interference could be applied in the production of a sensitive instrument that could detect movements of objects ranging from butterflies to burglars and black holes.

Is gravity a quantum phenomenon? That is one of the major outstanding questions in physics. Last year, together with colleagues, Assistant Professor of Theoretical Physics at the University of Groningen Anupam Mazumdar jointly proposed an experiment that could settle this question. Central to this experiment is a miniscule diamond, just a few nanometres in size, in which one of the carbon atoms has been replaced by a nitrogen atom. According to quantum physics, the extra electron in this atom would either absorb or not absorb the photon energy of a laser.

Diamond

Absorption of the energy would alter the electron’s spin value, a magnetic moment that can be either up or down. ‘Just like Schrödinger’s cat, which is dead and alive at the same time, this electron spin does and does not absorb the photon energy, so its spin is both up and down’, Mazumdar explains. This process results in quantum superposition of the entire diamond. By applying a magnetic field, it is possible to separate the two quantum states. When these quantum states are brought together again by turning off the magnetic field, they will create an interference pattern.

This diamond is small enough to sustain this superposition, but it is also sufficiently large to be affected by the pull of gravity. When two of these diamonds are placed next to each other under conditions of free fall, they only interact via the gravity force between them. The experiment was originally designed to test whether gravity itself is a quantum phenomenon. Simply put, as entanglement is a quantum phenomenon, the entanglement of two objects that interact only through gravity would serve as proof that gravity is a quantum phenomenon.

Collision

Any moving mass will have an effect on this very sensitive quantum system. In their latest paper, Mazumdar and colleagues describe how these disturbances can be reduced. However, it is also apparent that this system could be used to detect moving masses. The first source of noise is the collision of gas with the experimental capsule in free fall. Even the impact of photons can create a disturbance. ‘Our calculations show that these effects are minimized by placing the experimental capsule inside a larger container, which creates a controlled environment’, Mazumdar explains. Inside such an outer container, this noise is negligible at a pressure of 10-6 Pascal, even at room temperature. Requirements for conditions within the experimental capsule are more stringent. Currently, the scientists estimate a required pressure of 10-15 Pascal at around 1 Kelvin. Given the current state of technology, this is not yet feasible, but Mazumdar expects it could well be possible within around 20 years.

Schematic of the proposed experimetn: a beam is split into two (bottom) and subsequently recombined, creating an interference pattern (top). | Illustration Anupam Mazumdar
Schematic of the proposed experimetn: a beam is split into two (bottom) and subsequently recombined, creating an interference pattern (top). | Illustration Anupam Mazumdar

Space debris

Moving objects, even as small as a butterfly, located near the experimental site constitute a second source of noise. Calculations reveal that this noise can also be mitigated relatively easily by limiting access to the experimental site. People should maintain a distance of at least 2 metres from the experimental site, and cars should maintain a minimum distance of 10 metres from the site. Passing planes at a distance of more than 60 metres from the experimental site would not pose a problem. All of these requirements can be accomplished easily.

Once the experiment is up and running, its scope could be extended beyond an investigation of quantum gravity, according to Mazumdar. ‘You could put it in a spacecraft, where it is in free fall all the time. Then, you could use it to detect incoming space debris. By using several systems, it would even be possible to get the trajectory of the debris’. Another option is to place such a system in the Kuiper belt, where it would sense the movement of our solar system in space. ‘And it could detect any nearby black holes’, Mazumdar adds.

Back on Earth, the quantum system would be capable of detecting tectonic movements and perhaps providing early warnings of earthquakes. And, of course, the quantum system’s sensitivity to any movement occurring in proximity to it would make it an ideal, if somewhat complex, movement sensor and burglar alarm. But for now, the focus over the next few decades is on determining whether gravity is a quantum phenomenon.

Featured image: This is Dr Anupam Mazumdar, Professor in Theoretical Physics at the University of Groningen, the Netherlands © University of Groningen


Reference: Marko Toroš, Thomas W. van de Kamp, Ryan J. Marshman, M. S. Kim, Anupam Mazumdar, and Sougato Bose: Relative acceleration noise mitigation for nanocrystal matter-wave interferometry: Applications to entangling masses via quantum gravity. Phys. Rev. Research, 4 June 2021


Provided by University of Groningen

New Evidence for Electron’s Dual Nature Found in a Quantum Spin Liquid (Physics)

Results from a Princeton-led experiment support a controversial theory that the electron is composed of two particles.

A new discovery led by Princeton University could upend our understanding of how electrons behave under extreme conditions in quantum materials. The finding provides experimental evidence that this familiar building block of matter behaves as if it is made of two particles: one particle that gives the electron its negative charge and another that supplies its magnet-like property, known as spin.

“We think this is the first hard evidence of spin-charge separation,” said Nai Phuan Ong, Princeton’s Eugene Higgins Professor of Physics and senior author on the paper published this week in the journal Nature Physics.

The experimental results fulfill a prediction made decades ago to explain one of the most mind-bending states of matter, the quantum spin liquid. In all materials, the spin of an electron can point either up or down. In the familiar magnet, all of the spins uniformly point in one direction throughout the sample when the temperature drops below a critical temperature.

However, in spin liquid materials, the spins are unable to establish a uniform pattern even when cooled very close to absolute zero. Instead, the spins are constantly changing in a tightly coordinated, entangled choreography. The result is one of the most entangled quantum states ever conceived, a state of great interest to researchers in the growing field of quantum computing.

To describe this behavior mathematically, Nobel prize-winning Princeton physicist Philip Anderson (1923-2020), who first predicted the existence of spin liquids in 1973, proposed an explanation: in the quantum regime an electron may be regarded as composed of two particles, one bearing the electron’s negative charge and the other containing its spin. Anderson called the spin-containing particle a spinon.

In this new study, the team searched for signs of the spinon in a spin liquid composed of ruthenium and chlorine atoms. At temperatures a fraction of a Kelvin above absolute zero (or roughly -452 degrees Fahrenheit) and in the presence of a high magnetic field, ruthenium chloride crystals enter the spin liquid state.

Graduate student Peter Czajka and Tong Gao, Ph.D. 2020, connected three highly sensitive thermometers to the crystal sitting in a bath maintained at temperatures close to absolute zero degrees Kelvin. They then applied the magnetic field and a small amount of heat to one crystal edge to measure its thermal conductivity, a quantity that expresses how well it conducts a heat current. If spinons were present, they should appear as an oscillating pattern in a graph of the thermal conductivity versus magnetic field.

The oscillating signal they were searching for was tiny — just a few hundredths of a degree change — so the measurements demanded an extraordinarily precise control of the sample temperature as well as careful calibrations of the thermometers in the strong magnetic field.

The 3D color-plot, a composite of many experiments, shows how the thermal conductivity κxx (vertical axis) varies as a function of the magnetic field B (horizontal axis) and the temperature T (axis into the page). The oscillations provide evidence for spinons. © Peter Czajka, Princeton University

The team used the purest crystals available, ones grown at Oak Ridge National Laboratory (ORNL) under the leadership of David Mandrus, materials science professor at the University of Tennessee-Knoxville, and Stephen Nagler, director of ORNL’s quantum condensed matter division. The ORNL team has extensively studied the quantum spin liquid properties of ruthenium chloride.

In a series of experiments conducted over nearly three years, Czajka and Gao detected temperature oscillations consistent with spinons with increasingly higher resolution, providing evidence that the electron is composed of two particles consistent with Anderson’s prediction.

“People have been searching for this signature for four decades,” Ong said, “If this finding and the spinon interpretation are validated, it would significantly advance the field of quantum spin liquids.”

Czajka and Gao spent last summer confirming the experiments while under COVID restrictions that required them to wear masks and maintain social distancing.

“From the purely experimental side,” Czajka said, “it was exciting to see results that in effect break the rules that you learn in elementary physics classes.”

The experiments were performed in collaboration with Max Hirschberger, Ph.D. 2017 now at the University of Tokyo, Arnab Banerjee at Purdue University and ORNL, David Mandrus and Paula Lempen-Kelley at the University of Tennessee-Knoxville and ORNL, and Jiaqiang Yan and Stephen E. Nagler at ORNL. Funding at Princeton was provided by the Gordon and Betty Moore Foundation, the U.S. Department of Energy and the National Science Foundation. The Gordon and Betty Moore Foundation also supported the crystal growth program at the University of Tennessee.

The study, “Oscillations of the thermal conductivity in the spin-liquid state of α-RuCl3,” by Peter Czajka, Tong Gao, Max Hirschberger, Paula Lampen-Kelley, Arnab Banerjee, Jiaqiang Yan, David G. Mandrus, Stephen E. Nagler and N. P. Ong, was published in the journal Nature Physics online on May 13, 2021. DOI: 10.1038/s41567-021-0124.

Featured image: Researchers at Princeton University conducted experiments on materials known as quantum spin liquids, finding evidence that the electrons in the quantum regime behave as if they are made up of two particles. © Catherine Zandonella, Princeton University


Provided by Princeton University

Enzyme System For The Hydrogen Industry (Physics)

Platinum-free biocatalyst for fuel cells and water electrolysis

An enzyme could make a dream come true for the energy industry: It can efficiently produce hydrogen using electricity and can also generate electricity from hydrogen. The enzyme is protected by embedding it in a polymer. An international research team with significant participation of scientists from Technical University of Munich (TUM) has presented the system in the renowned science journal Nature Catalysis.

Fuel cells turn hydrogen into electricity, while electrolysers use electricity to split water to produce hydrogen. Both need the rare and thus expensive precious metal platinum as a catalyst. Nature has created a different solution: Enzymes, referred to as hydrogenases. They catalyze the conversion of hydrogen very quickly and almost without energy loss.

However, in the past these biocatalysts were not considered suitable for industrial use because of their high sensitivity to oxygen. Now a research team from the Technical University of Munich (TUM), Ruhr-Universität Bochum (RUB), the French National Centre for Scientific Research (CNRS) in Marseille and the Max-Planck Institute for Chemical Energy Conversion has succeeded in embedding the sensitive enzymes in a protective polymer in a way that makes them viable for use in technical hydrogen conversion.

Durability vs. activity

“When the sensitive hydrogenases are embedded in suitable polymers they continue to work for several weeks, even in the presence of oxygen,” says Nicolas Plumeré, Professor for Electrobiotechnology at the TUM Campus Straubing for Biotechnology and Sustainability. “Without this protection they lose their activity within a matter of minutes.”

Embedding the hydrogenases in polymers whose side chains can transfer electrons, referred to as redox polymers, had nevertheless two decisive disadvantages: a high level of resistance countervailed the flow of electrons through the redox polymer. This required the investment of energy which was lost in the form of heat. And the embedded hydrogenases completely lost their ability to generate hydrogen.

Fine tuning potential

With a clever selection of the right polymer side chains, the research team has now succeeded in setting the redox potential of the polymer in such a way that only a small overvoltage is necessary to overcome the resistance.

More detailed investigations then revealed that the potential of the side chains had shifted slightly to positive values due to the embedding in the polymer matrix. In a further attempt they used a side chain with a corresponding negative potential. This trick was the breakthrough: The hydrogenase was now capable of catalyzing the reaction in both directions without energy loss.

Biocatalyst for hydrogen conversion

Utilizing this system the research team then built a fuel cell, in which oxygen is reduced by the enzyme bilirubin oxidase from the bacterium Myrothecium verrucaria, while the hydrogenase embedded in the polymer film oxidizes the hydrogen from the bacterium desulfovibrio desulfuricans, generating electricity in the process.

The cell achieved a value, with an open circuit voltage of 1.16 V, the highest ever measured for a system of this type and close to the thermodynamic maximum. With three milliamperes per square centimeter the cell achieved a very high power density for biological cells at the same time.

The system can also be used for the reverse reaction, producing hydrogen by consuming electrons: The energy conversion efficiency is close to 100 percent, even with power densities of over four milliamperes per square centimeter. 

Blueprint for new biocatalysts

“The reduction in energy loss has two decisive advantages,” says Nicolas Plumeré. “First, it makes the system significantly more efficient; second, the heat generated in a fuel cell stack at high performance levels would pose a problem for biological systems.”

In order to make their system competitive with systems that use platinum-based catalysts, the team’s ongoing research is now focused on improving the stability of the hydrogenases at higher power densities.

Furthermore, the findings can also be transferred to other highly-active but sensitive catalysts for energy conversion and electrosynthesis. Direct objectives here are primarily carbon dioxide-reducing enzymes that can use electricity to produce liquid fuels or intermediate products from carbon dioxide.

Publications:

Reversible H2 oxidation and evolution by hydrogenase embedded in a redox polymer film
Steffen Hardt, Stefanie Stapf, Dawit T. Filmon, James A. Birrell, Olaf Rüdiger, Vincent Fourmond, Christophe Léger & Nicolas Plumeré
Nature Catalysis Vol. 4, 251–258 (2021) – DOI: 10.1038/s41929-021-00586-1
 

More information:

The research was funded by a Starting Grant from the European Research Council (ERC), by the French Center National de la Recherche Scientifique (CNRS) and the Aix Marseille Université, the German Research Foundation (DFG) joint funding with the Agence Nationale de la Recherche, the DFG priority program “Iron – Sulfur for Life” (SPP 1927), the Max Planck Society and in the case of the RESOLV cluster of excellence by the Federal Ministry of Education and Research (BMBF) under the Excellence Strategy of the Federal Government and the German Federal States.

Featured image: Embedded in a protecting polymer, hydrogenases can catalyze the conversion of hydrogen in fuel cells and water electrolysis. Side chains of the polymer enable the electron transfer. Dawit T. Filmon, scientist with Prof. Nicolas Plumere, professorship for bioelectrochemistry at the TUM Campus Straubing for Biotechnology and Sustainability, holds a starting material for these side chains in his hands. © Jan Winter / TUM


Provided by Technical University of Munich

Physicists Extract Proton Mass Radius From Experimental Data (Physics)

Researchers have recently extracted the proton mass radius from the experimental data.

A research group at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) presented an analysis of the proton mass radius in Physical Review D on May 11. The proton mass radius is determined to be 0.67 ± 0.03 femtometers, which is obviously smaller than the charge radius of the proton.

In the Standard Model, the proton is a composite particle made of quarks and gluons and it has a non-zero size. The radius of the proton is a global and fundamental property of the proton. It is related to the color confinement radius — a property governed by quantum chromodynamics (QCD).

The radius of the proton is approximately 100,000 times smaller than that of the atom, and the sizes of the quark and gluon are several orders smaller than the proton radius. Scientists use various distributions to describe the shape of the proton, such as charge distribution and mass distribution.

The charge radius of the proton has been precisely measured by scientists via Lamb shift of the muonic hydrogen or the high energy electron-proton elastic scattering, with the average value of 0.8409± 0.0004 femtometers provided by the Particle Data Group. Nevertheless, knowledge of proton gravitational properties such as proton mass radius has still been very limited.

“According to recent theoretical studies by Dmitri Kharzeev, the proton mass radius is related to the scalar gravitational form factor of the proton,” said Dr. WANG Rong, first author of the paper. By investigating the vector meson photoproduction data for omega, phi and J/psi from the SAPHIR (Spectrometer Arrangement for PHoton Induced Reactions) experiment at Bonn University, the LEPS (Laser Electron Photons) experiment at SPring-8 facility, and the GlueX experiment at Jefferson Lab, the researchers determined the scalar gravitational form factor and the proton mass radius.

Proton mass radius extracted from experimental data. © Physical Review D

Meanwhile, Prof. Dmitri Kharzeev, a theoretical physicist at Stony Brook University, obtained a similar result by using GlueX J/psi data. The proton mass radius was estimated to be 0.55 ± 0.03 femtometers.

“Both results might be the first-ever values of the proton mass radius with experimental evidence,” said WANG. “The determination of the proton mass radius will improve our understanding of the origins of proton mass and the color confinement mechanism of strong interaction.”

A lot of questions still remain. “The smaller mass radius implies that the mass distribution is significantly different from the charge distribution of the proton,” said Prof. CHEN Xurong, a researcher at IMP.

Scientists are now trying to get a clearer picture of the proton mass radius and the proton structure. The GlueX experiment at Jefferson Lab will provide more data in the near future. Even more exciting, future electron-ion colliders both in the United States and in China will provide Upsilon vector meson electroproduction data for researchers to better understand these questions.

This work was supported by the Strategic Priority Research Program of CAS and the National Natural Science Foundation of China.

Featured image: Vector meson near threshold photoproduction process on the proton target. © Kou Wei


Reference: Rong Wang, Wei Kou, Ya-Ping Xie, and Xurong Chen, “Extraction of the proton mass radius from the vector meson photoproductions near thresholds”, Phys. Rev. D 103, L091501 – Published 11 May 2021. DOI: https://doi.org/10.1103/PhysRevD.103.L091501


Provided by Chinese Academy of Sciences

Shaken, Not Stirred: Reshuffling Skyrmions Ultrafast (Physics)

Smaller, faster, more energy-efficient: future requirements to computing and data storage are hard to fulfill and alternative concepts are continuously explored. Small magnetic textures, so-called skyrmions, may become an ingredient in novel memory and logic devices. In order to be considered for technological application, however, fast and energy-efficient control of these nanometer-sized skyrmions is required.

Magnetic skyrmions are particle-like magnetization patches that form as very small swirls in an otherwise uniformly magnetized material. In particular ferromagnetic thin films, skyrmions are stable at room temperature, with diameters down to the ten-nanometer range. It is known that skyrmions can be created and moved by short pulses of electric current. Only recently it was discovered that also short laser pulses are able to create and annihilate skyrmions. In contrast to electric current pulses, laser pulses of sub-picosecond duration can be used, providing a faster and potentially more energy-efficient route to write and delete information encoded by skyrmions. This makes laser skyrmion writing interesting for technological applications, including alternative memory and logic devices.

Scientists of Max Born Institute together with colleagues from Helmholtz-Zentrum Berlin, Massachusetts Institute of Technology and further research institutions now investigated in detail how laser-based creation and annihilation of skyrmions can be controlled to promote application of the process in devices. To image the magnetic skyrmions, the team of researchers used holography-based x-ray microscopy, which can make the tiny magnetization swirls with a diameter of 100 nanometer and less visible. Being able to see the skyrmions, they were able to systematically study how laser pulses with different intensity, applied in the presence of an external magnetic field, can create or delete skyrmions. Two types of material systems, designed to be able to host magnetic skyrmions in the first place, were investigated, both consisting of ultrathin multilayer stacks of  ferromagnetic and paramagnetic materials.

Fig. 1: A single laser pulse of appropriate intensity can create random skyrmion patterns with a density defined by an external magnetic field (thin arrows). This scheme of laser writing of skyrmions may be used as an ultrafast “skyrmion reshuffler” for stochastic computing. The area surrounded by the dashed line marks the field of view of the x-ray microscope used to see the magnetic skyrmions appearing as black dots. The field of view is 1 µm in diameter.

Not surprisingly given the thermal nature of the process, the laser intensity has to be right. However, there is a material-dependent window of laser intensities which allows for the creation of a new skyrmion pattern which is completely independent of the previous magnetic state.  For lower intensities, an existing pattern remains unaltered or is only slightly modified, for much higher intensities, the multilayer structure is damaged. Remarkably, the number of skyrmions created within the laser spot is not influenced by the laser intensity. Instead, the researchers found that the presence of an external magnetic field allows to precisely control the density of skyrmions created. The strength of the external field therefore provides a knob to tune the number of skyrmions created and even allows for annihilation of skyrmions, as the scientists report in the journal Applied Physics Letters.

They demonstrated the controlled creation or annihilation of single skyrmions within the laser spot, as required for applications in data storage where a single bit could then be represented by the presence or absence of a skyrmion. Of interest for potential device application, however, is also the ability to simultaneously generate a particular density of skyrmions in the area illuminated by a single laser pulse. This process could be used as a “skyrmion reshuffler” in stochastic computing. There, numbers are represented as strings of random bits of “0” and “1”, with the probability to encounter “1” encoding the number value. Computations can then be carried out via logic operations between individual bits of different input numbers. While clearly a niche approach compared to the prevalent digital logic, stochastic computing has proven promising for particular problems such as image processing. However, completely randomized bit strings are needed as input signals for correct results of stochastic computing operations. As demonstrated in this work, such randomizing “reshuffling” of skyrmions can be performed optically on a timescale of picoseconds, compatible with state-of-the-art computer clock speed and much faster than in previous concepts based on thermal diffusion operating on the timescale of seconds.

Featured image: The density of skyrmions as a function of the external magnetic field. As the field decreases the skyrmion density increases in a linear manner. The inset images show examples of the skyrmion patterns created by the laser pulse, the field of view is 1.5 µm in diameter. © MBI


Original publication

“Application concepts for ultrafast laserinduced skyrmion creation and annihilation”, Kathinka GerlingerBastian Pfau, Felix Büttner, Michael SchneiderLisa-Marie KernJosefin FuchsDieter Engel, Christian M. Günther, Mantao Huang, Ivan Lemesh, Lucas Caretta, Alexandra Churikova, Piet HessingChristopher KloseChristian Strüber,  Clemens von Korff Schmising, Siying Huang, Angela Wittmann, Kai Litzius, Daniel Metternich, Riccardo Battistelli, Kai Bagschik, Alexandr Sadovnikov, Geoffrey S. D.Beach, and Stefan EisebittAppl. Phys. Lett. 118, 192403 (2021) URL, DOI or PDF


Provided by MBI

Electromagnetic Levitation Whips Nanomaterials into Shape (Physics)

Electromagnetic field directs shape formed by gas phase metal molecules

In order for metal nanomaterials to deliver on their promise to energy and electronics, they need to shape up — literally. 

To deliver reliable mechanical and electric properties, nanomaterials must have consistent, predictable shapes and surfaces, as well as scalable production techniques. UC Riverside engineers are solving this problem by vaporizing metals within a magnetic field to direct the reassembly of metal atoms into predictable shapes. The research is published in The Journal of Physical Chemistry Letters.

Nanomaterials, which are made of particles measuring 1-100 nanometers, are typically created within a liquid matrix, which is expensive for bulk production applications, and in many cases cannot make pure metals, such as aluminum or magnesium. More economical production techniquess typically involve vapor phase approaches to create a cloud of particles condensing from the vapor. These suffer from a lack of control.  

Reza Abbaschian, a distinguished professor of mechanical engineering; and Michael Zachariah, a distinguished professor of chemical and environmental engineering at UC Riverside’s Marlan and Rosemary Bourns College of Engineering; joined forces to create nanomaterials from iron, copper, and nickel in a gas phase. They placed solid metal within a powerful electromagnetic levitation coil to heat the metal beyond its melting point, vaporizing it. The metal droplets levitated in the gas within the coil and moved in directions determined by their inherent reactions to magnetic forces. When the droplets bonded, they did so in an orderly fashion that the researchers learned they could predict based on the type of metal and how and where they applied the magnetic fields.

Iron and nickel nanoparticles formed string-like aggregates while copper nanoparticles formed globular clusters. When deposited on a carbon film, iron and nickel aggregates gave the film a porous surface, while carbon aggregates gave it a more compact, solid surface. The qualities of the materials on the carbon film mirrored at larger scale the properties of each type of nanoparticle.

Because the field can be thought of as an “add-on,” this approach could be applied to any vapor-phase nanoparticle generation source where the structure is important, such as fillers used in polymer composites for magnetic shielding, or to improve electrical or mechanical properties.

“This ‘field directed’ approach enables one to manipulate the assembly process and change the architecture of the resulting particles from high fractal dimension objects to lower dimension string-like structures. The field strength can be used to manipulate the extent of this arrangement,” Zachariah said. 

Abbaschian and Zachariah were joined in the research by Pankaj Ghildiyal, Prithwish Biswas, Steven Herrera, George W. Mulholland, and Yong Yang. The paper, “Magnetic-field directed vapor-phase assembly of low fractal dimension metal nanostructures: experiment and theory,” is available here.

Featured image: Image showing the stringlike particles formed by iron and nickel and the more globular clusters formed by copper. (Abbaschian, Zachariah, et. al. 2021)


Reference: Pankaj Ghildiyal, Prithwish Biswas, Steven Herrera, George W. Mulholland, Yong Yang, Reza Abbaschian, and Michael R. Zachariah, “Magnetic-Field Directed Vapor-Phase Assembly of Low Fractal Dimension Metal Nanostructures: Experiment and Theory”, J. Phys. Chem. Lett. 2021, 12, 16, 4085–4091
Publication Date:April 22, 2021
https://doi.org/10.1021/acs.jpclett.0c03463


Provided by University of California Riverside

Physicists Net Neutron Star Gold from Measurement of Lead (Physics)

Nuclear physicists make new, high-precision measurement of the layer of neutrons that encompass the lead nucleus, revealing new information about neutron stars

Nuclear physicists have made a new, highly accurate measurement of the thickness of the neutron “skin” that encompasses the lead nucleus in experiments conducted at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility and just published in Physical Review Letters. The result, which revealed a neutron skin thickness of .28 millionths of a nanometer, has important implications for the structure and size of neutron stars.

The protons and neutrons that form the nucleus at the heart of every atom in the universe help determine each atom’s identity and properties. Nuclear physicists are studying different nuclei to learn more about how these protons and neutrons act inside the nucleus. The Lead Radius Experiment collaboration, called PREx (after the chemical symbol for lead, Pb), is studying the fine details of how protons and neutrons are distributed in lead nuclei.

“The question is about where the neutrons are in lead. Lead is a heavy nucleus – there’s extra neutrons, but as far as the nuclear force is concerned, an equal mix of protons and neutrons works better,” said Kent Paschke, a professor at the University of Virginia and experiment co-spokesperson.

Paschke explained that light nuclei, those with just a few protons, typically have equal numbers of protons and neutrons inside. As nuclei get heavier, they need more neutrons than protons to remain stable. All stable nuclei that have more than 20 protons have more neutrons than protons. For instance, lead has 82 protons and 126 neutrons. Measuring how these extra neutrons are distributed inside the nucleus is key input for understanding how heavy nuclei are put together.

“The protons in a lead nucleus are in a sphere, and we have found that the neutrons are in a larger sphere around them, and we call that the neutron skin,” said Paschke.

The PREx experiment result, published in Physical Review Letters in 2012, provided the first experimental observation of this neutron skin using electron scattering techniques. Following that result, the collaboration set out to make a more precise measurement of its thickness in PREx-II. The measurement was carried out in the summer of 2019 using the Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility. This experiment, like the first, measured the average size of the lead nucleus in terms of its neutrons.

Neutrons are difficult to measure, because many of the sensitive probes that physicists use to measure subatomic particles rely on measuring the particles’ electric charge through the electromagnetic interaction, one of the four interactions in nature. PREx makes use of a different fundamental force, the weak nuclear force, to study the distribution of neutrons.

“Protons have an electric charge and can be mapped using the electromagnetic force. Neutrons have no electric charge, but compared to protons they have a large weak charge, and so if you use the weak interaction, you can figure out where the neutrons are.” explained Paschke.

In the experiment, a precisely controlled beam of electrons was sent crashing into a thin sheet of cryogenically cooled lead. These electrons were spinning in their direction of motion, like a spiral on a football pass.

Electrons in the beam interacted with the lead target’s protons or neutrons either via the electromagnetic or the weak interaction. While the electromagnetic interaction is mirror-symmetric, the weak interaction is not. That means that the electrons that interacted via electromagnetism did so regardless of the electrons’ spin direction, while the electrons that interacted via the weak interaction preferentially did so more often when the spin was in one direction versus the other.

“Using this asymmetry in the scattering, we can identify the strength of the interaction, and that tells us the size of the volume occupied by neutrons. It tells us where the neutrons are compared to the protons.” said Krishna Kumar, an experiment co-spokesperson and professor at the University of Massachusetts Amherst.

The measurement required a high degree of precision to carry out successfully. Throughout the experimental run, the electron beam spin was flipped from one direction to its opposite 240 times per second, and then the electrons travelled nearly a mile through the CEBAF accelerator before being precisely placed on the target.

“On average over the entire run, we knew where the right- and left-hand beams were, relative to each other, within a width of 10 atoms,” said Kumar.

The electrons that had scattered off lead nuclei while leaving them intact were collected and analyzed. Then, the PREx-II collaboration combined it with the previous 2012 result and precision measurements of the lead nucleus’ proton radius, which is often referred to as its charge radius.

“The charge radius is about 5.5 femtometers. And the neutron distribution is a little larger than that – around 5.8 femtometers, so the neutron skin is .28 femtometers, or about .28 millionths of a nanometer,” Paschke said.

The researchers said that this figure is thicker than some theories had suggested, which has implications for the physical processes in neutron stars and their size.

“This is the most direct observation of the neutron skin. We are finding what we call a stiff equation of state – higher than expected pressure so that it’s difficult to squeeze these neutrons into the nucleus. And so, we’re finding that the density inside the nucleus is a little bit lower than was expected,” said Paschke.

“We need to know the content of the neutron star and the equation of state, and then we can predict the properties of these neutron stars,” Kumar said. “So, what we are contributing to the field with this measurement of the lead nucleus allows you to better extrapolate to the properties of neutron stars.”

The unexpectedly stiff equation of state implied by the PREx result has deep connections to recent observations of colliding neutron stars made by the Nobel Prize-winning Laser Interferometer Gravitational-Wave Observatory, or LIGO, experiment. LIGO is a large-scale physics observatory that was designed to detect gravitational waves.

“As neutron stars start to spiral around each other, they emit gravitational waves that are detected by LIGO. And as they get close in the last fraction of a second, the gravitational pull of one neutron star makes the other neutron star into a teardrop – it actually becomes oblong like an American football. If the neutron skin is larger, then it means a certain shape for the football, and if the neutron skin were smaller, it means a different shape for the football. And the shape of the football is measured by LIGO,” said Kumar. “The LIGO experiment and the PREx experiment did very different things, but they are connected by this fundamental equation – the equation of state of nuclear matter.”

The PREx-II experimental collaboration includes 13 Ph.D. students and seven postdoctoral research associates, as well as more than 70 other scientists from about 30 institutions.

This work was supported by DOE’s Office of Science, the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Italian Istituto Nazionale di Fisica Nucleare (INFN).

Further Reading

Neutron-Rich Matter in Heaven and on Earth: https://physicstoday.scitation.org/doi/10.1063/PT.3.4247

Result Tickler: Lead Nucleus May Bury Positive Side Under Neutral Facade: https://www.jlab.org/news/ontarget/target-july-2011#tickler

PREx-II Experiment Proposal: https://hallaweb.jlab.org/parity/prex/prexII.pdf

It’s Elemental: The Periodic Table of Elements: https://education.jlab.org/itselemental/

Featured image: Jefferson Lab’s Experimental Hall A is one of four nuclear physics research areas in the lab’s Continuous Electron Beam Accelerator Facility. © DOE’s Jefferson Lab


Provided by DOE/JNAF

ATLAS Searches For Pairs of Higgs Bosons in a Rare Particle Decay (Particle Physics)

A fraction of a second after the Big Bang, the Universe experienced a phase transition into a state of minimum energy, where matter particles interacted with the Higgs field to acquire mass. We have been living in this energy state ever since.

In the post-Higgs discovery era, scientists at the Large Hadron Collider (LHC) have been hard at work studying the Higgs boson’s properties. One property that remains to be experimentally verified is whether the Higgs boson can couple to itself (self-coupling). Such an interaction would contribute to the production of a pair of Higgs bosons, and would define the shape of the Higgs potential. If the Higgs boson’s self-coupling differs significantly from the Standard Model prediction, the Universe might be able to transition into a lower energy state where the laws that govern the interactions of matter could take on a very different shape.

Figure 1: Sketch of the HH invariant mass with the two leading contributions from the triangle (left) and box (right) Feynman diagrams. (Image: F. Cairo/ATLAS Collaboration)

The ATLAS Collaboration has released a new result which aims to address this question by searching for pairs of Higgs bosons (HH). This process is incredibly rare in the Standard Model – more than 1000 times rarer than the production of one Higgs boson! Physicists looked for the most common HH production processes that should be present in LHC collisions, both illustrated in the Feynman diagrams in Figure 1. Only the triangle diagram includes the Higgs self-coupling, and it contributes mainly to the production of Higgs pairs at low mass (shown in pink in Figure 1). If new physics is at play, it could change the Higgs self-coupling and ATLAS would see many more pairs of Higgs bosons than expected (i.e. a higher cross section).

The most powerful HH decay channel in the important low-mass region is the two bottom quark plus two photon channel, HH → bbɣɣ. ATLAS physicists developed new analysis techniques to search for this rare process. First, they divided events into low and high mass regions to better target the Higgs self-coupling. Then, they used a multivariate discriminant (Boosted Decision Tree) to separate the events that look like the HH → bbɣɣ process from those that don’t.


The ATLAS Collaboration’s latest result is more than twice as powerful as their previous result in the same channel!


Due to the development of these new analysis techniques, ATLAS’ latest result is more than twice as powerful as the previous ATLAS result in the same channel! Figure 2 shows limits on the HH production cross section (σ) as a function of the ratio of the Higgs self-coupling to its Standard Model value (κλ). The allowed range for the Higgs self-coupling is shown by the intersection of the observed limit with the theoretical prediction, between -1.5 and 6.7 times the Standard Model prediction. Physicists were able to set a limit on the HH production cross section of 4.1 times the Standard Model prediction. Limits are also set on HH production via the decay of a hypothetical new scalar particle.

Figure 2: Limits on the HH production cross section as a function of κλ. The dashed line shows the expected limits, and the solid line shows the observed limits. The theoretical predictions are shown in dark pink, and the Standard Model point is indicated by a star. The allowed range for κλ is given by the intersection between the observed limits and the theory curve. (Image: ATLAS Collaboration/CERN)

Although this result sets the world’s best limits on the size of the Higgs self-coupling, the work is not done. Much more data is needed to precisely measure the Higgs self-coupling and to see whether it agrees with the Standard Model prediction. The High-Luminosity upgrade of the LHC plans to deliver a dataset 20 times larger than the one used here. If HH production behaves as predicted by the Standard Model, it will be observed in this huge dataset – allowing LHC researchers to make a more quantitative statement on the size of the Higgs self-coupling and the nature of the Higgs potential.

Reference: Search for Higgs boson pair production in the two bottom quarks plus two photons final state in pppp collisions at s√=13s=13 TeV with the ATLAS detector


Featured image: Candidate HH → ɣɣbb event in ATLAS data taken in 2017. Charged-particle tracks are shown in green, the two photons are shown as cyan towers and the two b-jets are shown as red cones. (Image: CERN)


Provided by ATLAS