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Ultra-fast magnetic switching with potential to transform fiber optical communications

Researchers at CRANN and the School of Physics at Trinity College Dublin have discovered that a new material can act as a super-fast magnetic switch. When struck by successive ultra-short laser pulses it exhibits “toggle switching” that could increase the capacity of the global fibre optic cable network by an order of magnitude.

Expanding the capacity of the internet

Switching between two states — 0 and 1 — is the basis of digital technology and the backbone of the internet. The vast majority of all the data we download is stored magnetically in huge data centres across the world, linked by a network of optical fibres.

Obstacles to further progress with the internet are three-fold, specifically the speed and energy consumption of the semiconducting or magnetic switches that process and store our data and the capacity of the fibre optic network to handle it.

The new discovery of ultra-fast toggle switching using laser light on mirror-like films of an alloy of manganese, ruthenium and gallium known as MRG could help with all three problems.

Not only does light offer a great advantage when it comes to speed but magnetic switches need no power to maintain their state. More importantly, they now offer the prospect of rapid time-domain multiplexing of the existing fibre network, which could enable it to handle ten times as much data.

The science behind magnetic switching

Working in the photonics laboratory at CRANN, Trinity’s nanoscience research centre, Dr Chandrima Banerjee and Dr Jean Besbas used ultra-fast laser pulses lasting just a hundred femtoseconds (one ten thousand billionth of a second) to switch the magnetisation of thin films of MRG back and forth. The direction of magnetisation can point either in or out of the film.

With every successive laser pulse, it abruptly flips its direction. Each pulse is thought to momentarily heat the electrons in MRG by about 1,000 degrees, which leads to a flip of its magnetisation. The discovery of ultra-fast toggle switching of MRG has just been published in leading international journal, Nature Communications.

Dr Karsten Rode, Senior Research Fellow in the ‘Magnetism and Spin Electronics Group’ in Trinity’s School of Physics, suggests that the discovery just marks the beginning of an exciting new research direction. Dr Rode said:

“We have a lot of work to do to fully understand the behaviour of the atoms and electrons in a solid that is far from equilibrium on a femtosecond timescale. In particular, how can magnetism change so quickly while obeying the fundamental law of physics that says that angular momentum must be conserved?

“In the spirit of our spintronics team, we will now gather data from new pulsed-laser experiments on MRG, and other materials, to better understand these dynamics and link the ultra-fast optical response with electronic transport. We plan experiments with ultra-fast electronic pulses to test the hypothesis that the origin of the toggle switching is purely thermal.”

Next year Chandrima will continue her work at the University of Haifa, Israel, with a group who can generate even shorter laser pulses. The Trinity researchers, led by Karsten, plan a new joint project with collaborators in the Netherlands, France, Norway and Switzerland, aimed at proving the concept of ultra-fast, time-domain multiplexing of fibre-optic channels.

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Revised code could help improve efficiency of fusion experiments

An international team of researchers led by the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) has upgraded a key computer code for calculating forces acting on magnetically confined plasma in fusion energy experiments. The upgrade will be part of a suite of computational tools that will allow scientists to further improve the design of breakfast-cruller-shaped facilities known as stellarators. Together, the three codes in the suite could help scientists bring efficient fusion reactors closer to reality.

The revised software lets researchers more easily determine the boundary of plasma in stellarators. When used in concert with two other codes, the code could help find a stellarator configuration that improves the performance of the design. The two complementary codes determine the optimal location for the plasma in a stellarator vacuum chamber to maximize the efficiency of the fusion reactions, and determine the shape that the external electromagnets must have to hold the plasma in the proper position.

The revised software, called the “free-boundary stepped-pressure equilibrium code (SPEC),” is one of a set of tools scientists can use to tweak the performance of plasma to more easily create fusion energy. “We want to optimize both the plasma position and the magnetic coils to balance the force that makes the plasma expand while holding it in place,” said Stuart Hudson, physicist, deputy head of the Theory Department at PPPL and lead author of the paper reporting the results in Plasma Physics and Controlled Fusion.

“That way we can create a stable plasma whose particles are more likely to fuse. The updated SPEC code enables us to know where the plasma will be for a given set of magnetic coils.”

Fusion combines light elements in the form of plasma — the hot, charged state of matter composed of free electrons and atomic nuclei — and in the process generates massive amounts of energy in the sun and stars. Scientists are seeking to replicate fusion in devices on Earth for a virtually inexhaustible supply of safe and clean power to generate electricity.

Plasma stability is crucial for fusion. If plasma bounces around inside a stellarator, it can escape, cool, and tamp down the fusion reactions, in effect quenching the fusion fire. An earlier version of the code, also developed by Hudson, could only calculate how forces were affecting a plasma if the researchers already knew the plasma’s location. Researchers, however, typically don’t have that information. “That’s one of the problems with plasmas,” Hudson said. “They move all over the place.”

The new version of the SPEC code helps solve the problem by allowing researchers to calculate the plasma’s boundary without knowing its position beforehand. Used in coordination with a coil-design code called FOCUS and an optimization code called STELLOPT — both of which were also developed at PPPL — SPEC lets physicists simultaneously ensure that the plasma will have the best fusion performance and the magnets will not be too complicated to build. “There’s no point optimizing the shape of the plasma and then later finding out that the magnets would be incredibly difficult to construct,” Hudson said.

One challenge that Hudson and colleagues faced was verifying that each step of the code upgrade was done correctly. Their slow-and-steady approach was crucial to making sure that the code makes accurate calculations. “Let’s say you are designing a component that will go on a rocket to the moon,” Hudson said. “It’s very important that that part works. So you test and test and test.”

Updating any computer code calls for a number of interlocking steps:

  • First, scientists must translate a set of mathematical equations describing the plasma into a programming language that a computer can understand;
  • Next, scientists must determine the mathematical steps needed to solve the equations;
  • Finally, the scientists must verify that the code produces correct results, either by comparing the results with those produced by a code that has already been verified or using the code to solve simple equations whose answers are easy to check.

Hudson and colleagues performed the calculations with widely different methods. They used pencil and paper to determine the equations and solution steps, and powerful PPPL computers to verify the results. “We demonstrated that the code works,” Hudson said. “Now it can be used to study current experiments and design new ones.”

Collaborators on the paper include researchers at the Max Planck Institute for Plasma Physics, the Australian National University, and the Swiss École Polytechnique Fédérale de Lausanne. The research was supported by the DOE’s Office of Science (Fusion Energy Sciences), the Euratom research and training program, the Australian Research Council, and the Simons Foundation.

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When Dirac meets frustrated magnetism

The fields of condensed matter physics and material science are intimately linked because new physics is often discovered in materials with special arrangements of atoms. Crystals, which have repeating units of atoms in space, can have special patterns which result in exotic physical properties. Particularly exciting are materials which host multiple types of exotic properties because they give scientists the opportunity to study how those properties interact with and influence each other. The combinations can give rise to unexpected phenomena and fuel years of basic and technological research.

In a new study published in Science Advances this week, an international team of scientists from the USA, Columbia, Czech Republic, England, and led by Dr. Mazhar N. Ali at the Max Planck Institute of Microstructure Physics in Germany, has shown that a new material, KV3Sb5, has a never-seen-before combination of properties that results in one of the largest anomalous Hall effects (AHEs) ever observed; 15,500 siemens per centimeter at 2 Kelvin.

Discovered in the lab of co-author Prof. Tyrel McQueen at Johns Hopkins University, KV3Sb5 combines four properties into one material: Dirac physics, metallic frustrated magnetism, 2D exfoliability (like graphene), and chemical stability.

Dirac physics, in this context, relates to the fact that the electrons in KV3Sb5 aren’t just your normal run-of-the-mill electrons; they are moving extremely fast with very low effective mass. This means that they are acting “light-like”; their velocities are becoming comparable to the speed of light and they are behaving as though they have only a small fraction of the mass which they should have. This results in the material being highly metallic and was first shown in graphene about 15 years ago.

The “frustrated magnetism” arises when the magnetic moments in a material (imagine little bar magnets which try to turn each other and line up North to South when you bring them together) are arranged in special geometries, like triangular nets. This scenario can make it hard for the bar magnets to line up in way that they all cancel each other out and are stable. Materials exhibiting this property are rare, especially metallic ones. Most frustrated magnet materials are electrical insulators, meaning that their electrons are immobile. “Metallic frustrated magnets have been highly sought after for several decades. They have been predicted to house unconventional superconductivity, Majorana fermions, be useful for quantum computing, and more,” commented Dr. Ali.

Structurally, KV3Sb5 has a 2D, layered structure where triangular vanadium and antimony layers loosely stack on top of potassium layers. This allowed the authors to simply use tape to peel off a few layers (a.k.a. flakes) at a time. “This was very important because it allowed us to use electron-beam lithography (like photo-lithography which is used to make computer chips, but using electrons rather than photons) to make tiny devices out of the flakes and measure properties which people can’t easily measure in bulk.” remarked lead author Shuo-Ying Yang, from the Max Planck Institute of Microstructure Physics. “We were excited to find that the flakes were quite stable to the fabrication process, which makes it relatively easy to work with and explore lots of properties.”

Armed with this combination of properties, the team first chose to look for an anomalous Hall effect (AHE) in the material. This phenomenon is where electrons in a material with an applied electric field (but no magnetic field) can get deflected by 90 degrees by various mechanisms. “It had been theorized that metals with triangular spin arrangements could host a significant extrinsic effect, so it was a good place to start,” noted Yang. Using angle resolved photoelectron spectroscopy, microdevice fabrication, and a low temperature electronic property measurement system, Shuo-Ying and co-lead author Yaojia Wang (Max Planck Institute of Microstructure Physics) were able to observe one of the largest AHE’s ever seen.

The AHE can be broken into two general categories: intrinsic and extrinsic. “The intrinsic mechanism is like if a football player made a pass to their teammate by bending the ball, or electron, around some defenders (without it colliding with them),” explained Ali. “Extrinsic is like the ball bouncing off of a defender, or magnetic scattering center, and going to the side after the collision. Many extrinsically dominated materials have a random arrangement of defenders on the field, or magnetic scattering centers randomly diluted throughout the crystal. KV3Sb5 is special in that it has groups of 3 magnetic scattering centers arranged in a triangular net. In this scenario, the ball scatters off of the cluster of defenders, rather than a single one, and is more likely to go to the side than if just one was in the way.” This is essentially the theorized spin-cluster skew scattering AHE mechanism which was demonstrated by the authors in this material. “However the condition with which the incoming ball hits the cluster seems to matter; you or I kicking the ball isn’t the same as if, say, Christiano Ronaldo kicked the ball,” added Ali. “When Ronaldo kicks it, it is moving way faster and bounces off of the cluster with way more velocity, moving to the side faster than if just any average person had kicked it. This is, loosely speaking, the difference between the Dirac quasiparticles (Ronaldo) in this material vs normal electrons (average person) and is related to why we see such a large AHE,” Ali laughingly explained.

These results may also help scientists identify other materials with this combination of ingredients. “Importantly, the same physics governing this AHE could also drive a very large spin Hall effect (SHE) — where instead of generating an orthogonal charge current, an orthogonal spin current is generated,” remarked Wang. “This is important for next-generation computing technologies based on an electron’s spin rather than its charge.”

“This is a new playground material for us: metallic Dirac physics, frustrated magnetism, exfoliatable, and chemically stable all in one. There is a lot of opportunity to explore fun, weird phenomena, like unconventional superconductivity and more,” said Ali, excitedly.

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Scientists achieve major breakthrough in preserving integrity of sound waves

In a breakthrough for physics and engineering, researchers from the Photonics Initiative at the Advanced Science Research Center at The Graduate Center, CUNY (CUNY ASRC) and from Georgia Tech have presented the first demonstration of topological order based on time modulations. This advancement allows the researchers to propagate sound waves along the boundaries of topological metamaterials without the risk of waves traveling backwards or being thwarted by material defects.

The new findings, which appear in the journal Science Advances, will pave the way for cheaper, lighter devices that use less battery power, and which can function in harsh or hazardous environments. Andrea Alù, founding director of the CUNY ASRC Photonics Initiative and Professor of Physics at The Graduate Center, CUNY, and postdoctoral research associate Xiang Ni were authors on the paper, together with Amir Ardabi and Michael Leamy from Georgia Tech.

The field of topology examines properties of an object that are not affected by continuous deformations. In a topological insulator, electrical currents can flow along the object’s boundaries, and this flow is resistant to being interrupted by the object’s imperfections. Recent progress in the field of metamaterials has extended these features to control the propagation of sound and light following similar principles.

In particular, previous work from the labs of Alù and City College of New York Physics Professor Alexander Khanikaev used geometrical asymmetries to create topological order in 3D-printed acoustic metamaterials. In these objects, sound waves were shown to be confined to travel along the object’s edges and around sharp corners, but with a significant drawback: These waves weren’t fully constrained — they could travel either forward or backward with the same properties. This effect inherently limited the overall robustness of this approach to topological order for sound. Certain types of disorder or imperfections would indeed reflect backwards the sound propagating along the boundaries of the object.

This latest experiment overcomes this challenge, showing that time-reversal symmetry breaking, rather than geometrical asymmetries, can be also used to induce topological order. Using this method, sound propagation becomes truly unidirectional, and strongly robust to disorder and imperfections

“The result is a breakthrough for topological physics, as we have been able to show topological order emerging from time variations, which is different, and more advantageous, than the large body of work on topological acoustics based on geometrical asymmetries,” Alù said. “Previous approaches inherently required the presence of a backward channel through which sound could be reflected, which inherently limited their topological protection. With time modulations we can suppress backward propagation and provide strong topological protection.”

The researchers designed a device made of an array of circular piezoelectric resonators arranged in repeating hexagons, like a honeycomb lattice, and bonded to a thin disk of polylactic acid. They then connected this to external circuits, which provide a time-modulated signal that breaks time-reversal symmetry.

As a bonus, their design allows for programmability. This means they can guide waves along a variety of different reconfigurable paths, with minimal loss. Ultrasound imaging, sonar, and electronic systems that use surface acoustic wave technology could all benefit from this advance, Alù said.

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Excitation of robust materials

In physics, they are currently the subject of intensive research; in electronics, they could enable completely new functions. So-called topological materials are characterised by special electronic properties, which are also very robust against external perturbations. This material group also includes tungsten ditelluride. In this material, such a topologically protected state can be “broken up” using special laser pulses within a few trillionths of a second (“picoseconds”) and thus change its properties. This could be a key requirement for realising extremely fast, optoelectronic switches. For the first time physicists at Kiel University (CAU), in cooperation with researchers at the Max Planck Institute for Chemical Physics of Solids (MPI-CPfS) in Dresden, Tsinghua University in Beijing and Shanghai Tech University, have been able to observe changes to the electronic properties of this material in experiments in real-time. Using laser pulses, they put the atoms in a sample of tungsten ditelluride into a state of controlled excitation, and were able to follow the resulting changes in the electronic properties “live” with high-precision measurements. They published their results recently in the scientific journal Nature Communications.

“If these laser-induced changes can be reversed again, we essentially have a switch that can be activated optically, and which can change between different electronic states,” explained Michael Bauer, professor of solid state physics at the CAU. Such a switching process has already been predicted by another study, in which researchers from the USA were recently able to directly observe the atomic movements in tungsten ditelluride. In their study, the physicists from the Institute of Experimental and Applied Physics at the CAU now focused on the behaviour of the electrons, and how the electronic properties in the same material can be altered using laser pulses.

Weyl semimetals with unusual electronic properties

“Some of the electrons in tungsten ditelluride are highly mobile, so they are excellent information carriers for electronic applications. This is due to the fact that they behave like so-called Weyl fermions,” said doctoral researcher Petra Hein to explain the unusual properties of the material, also known as a Weyl semimetal. Weyl fermions are massless particles with special properties and have previously only been observed indirectly as “quasi-particles” in solids like tungsten ditelluride. “For the first time, we were now able to make the changes in the areas of the electronic structure visible, in which these Weyl properties are exhibited.”

Excitations of the material changes its electronic properties

To capture the barely-visible changes in the electronic properties a highly-sensitive experimental design, extremely precise measurements and an extensive analysis of the data obtained were required. During the past years the Kiel research team was able to develop such an experiment with the necessary long-term stability. With the generated laser pulses they put the atoms inside a sample of tungsten ditelluride into a state of vibrational excitation. Different overlapping vibrational excitations arose, which in turn changed the electronic properties of the material. “One of these atomic vibrations was known to change the electronic Weyl properties. We wanted to find out exactly what this change looks like,” said Hein to describe one of the key goals of the study.

Series of snapshots shows how properties change

In order to observe this specific process, the research team irradiated the material with a second laser pulse after a few picoseconds. This released electrons from the sample, which allowed drawing conclusions about the electronic structure of the material — the method is known as “time-resolved photoelectron spectroscopy.” “Due to the short exposure time of only 0.1 picoseconds, we get a snapshot of the electronic state of the material. We can combine many of these individual images into a film and thereby observe how the material reacts to the excitation by the first laser pulse,” said Dr Stephan Jauernik to explain the measurement method.

Recording a single data set on the extremely short modification process typically took one week. The Kiel research team evaluated a large number of such data sets using a newly developed analytical approach and was thus able to visualize the changes in the electronic Weyl properties of tungsten ditelluride.

Extremely short switching processes conceivable

“Our results demonstrate the sensitive and highly-selective interplay between the vibrations of the atoms of the solid and the unusual electronic properties of tungsten ditelluride,” summarised Bauer. Follow-up research aims at investigating whether such electronic switching processes can be triggered even faster — directly by the irradiating laser pulse — as has already been theoretically predicted for other topological materials.

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High-order synthetic dimensions in waveguide photonic lattices

In physics, a very intuitive way of describing the evolution of a system proceeds via the specification of functions of the spatiotemporal coordinates. Yet, there often exist other degrees of freedom in terms of which the physical entities pertaining to a variety of structures can be seen to evolve and that are not amenable to a description via spatial coordinates.

Yet, there often exist other degrees of freedom in terms of which the physical entities pertaining to a variety of structures can be seen to evolve and that are not amenable to a description via spatial coordinates.

This is precisely the idea of synthetic dimensions: coexisting frameworks in which a wavefunction, defined in specific degrees of freedom, takes another form that “lives” in a domain with much higher dimensions than what the structures’ (apparent) geometry would suggest. This approach is rather appealing as it can be used to access and probe dimensions beyond our 3-dimensional world, e.g. 5-dimensional or 8-dimensional, etc.

In our recent work we have shown that a multitude of high-dimensional synthetic lattices naturally emerge in (abstract) photon-number space when a multiport photonic lattice is excited by N indistinguishable photons. More precisely, the Fock-representation of N-photon states in systems composed of M evanescently coupled single-mode waveguides yields to a new layer of abstraction, where the associated states can be visualized as the energy levels of a synthetic atom. In full analogy with ordinary atoms, such synthetic atoms feature allowed and disallowed transitions between its energy levels.

These concepts have far-reaching implications as they open a route to the simultaneous realization of, in principle, an infinite number of lattices and graphs with different numbers of nodes and many dimensions. This possibility is rather appealing for realizing parallel quantum random walks where the corresponding walkers can perform different numbers of steps on different, planar and nonplanar, multidimensional graphs that depend on the number of photons involved in each process. These quantum walks can be implemented, for instance, by exciting a simple four-waveguide system with a standard quantum light source comprising infinite coherent superpositions of states, e.g. a coherent state . Similarly, the symmetric excitation of a two-waveguide system with identical photons, when properly viewed in abstract space, feature the phenomena of discrete diffraction and Bloch oscillations.

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Towards lasers powerful enough to investigate a new kind of physics

In a paper that made the cover of the journal Applied Physics Letters, an international team of researchers has demonstrated an innovative technique for increasing the intensity of lasers. This approach, based on the compression of light pulses, would make it possible to reach a threshold intensity for a new type of physics that has never been explored before: quantum electrodynamics phenomena.

Researchers Jean-Claude Kieffer of the Institut national de la recherche scientifique (INRS), E. A. Khazanov of the Institute of Applied Physics of the Russian Academy of Sciences and in France Gérard Mourou, Professor Emeritus of the Ecole Polytechnique, who was awarded the Nobel Prize in Physics in 2018, have chosen another direction to achieve a power of around 10^23 Watts (W). Rather than increasing the energy of the laser, they decrease the pulse duration to only a few femtoseconds. This would keep the system within a reasonable size and keep operating costs down.

To generate the shortest possible pulse, the researchers are exploiting the effects of non-linear optics. “A laser beam is sent through an extremely thin and perfectly homogeneous glass plate. The particular behaviour of the wave inside this solid medium broadens the spectrum and allows for a shorter pulse when it is recompressed at the exit of the plate,” explains Jean-Claude Kieffer, co-author of the study published online on 15 June 2020 in the journal Applied Physics Letters.

Installed in the Advanced Laser Light Source (ALLS) facility at INRS, the researchers limited themselves to an energy of 3 joules for a 10-femtosecond pulse, or 300 terawatts (1012W). They plan to repeat the experiment with an energy of 13 joules over 5 femtoseconds, or an intensity of 3 petawatts (1015 W). “We would be among the first in the world to achieve this level of power with a laser that has such short pulses,” says Professor Kieffer.

“If we achieve very short pulses, we enter relativistic problem classes. This is an extremely interesting direction that has the potential to take the scientific community to new horizons,” says Professor Kieffer. “It was a very nice piece of work solidifying the paramount potential of this technique,” concludes Gérard Mourou.

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Achievement isn’t why more men are majoring in physics, engineering and computer science

While some STEM majors have a one-to-one male-to-female ratio, physics, engineering and computer science (PECS) majors consistently have some of the largest gender imbalances among U.S. college majors — with about four men to every woman in the major. In a new study published today in the peer-reviewed research journal, Science, NYU researchers find that this disparity is not caused by higher math or science achievement among men. On the contrary, the scholars found that men with very low high-school GPAs in math and science and very low SAT math scores were choosing these math-intensive majors just as often as women with much higher math and science achievement.

“Physics, engineering and computer science fields are differentially attracting and retaining lower-achieving males, resulting in women being underrepresented in these majors but having higher demonstrated STEM competence and academic achievement,” said Joseph R. Cimpian, lead researcher and associate professor of economics and education policy at NYU Steinhardt.

Cimpian and his colleagues analyzed data from almost 6,000 U.S. high school students over seven years — from the start of high school into the students’ junior year of college. When the researchers ranked students by their high-school math and science achievement, they noticed that male students in the 1st percentile were majoring in PECS at the same rate as females in the 80th percentile, demonstrating a stark contrast between the high academic achievement of the female students majoring in PECS compared to their male peers.

The researchers also reviewed the data for students who did not intend to major in PECS fields, but later decided to. They found that the lowest achieving male student was as least as likely to join one of these majors as the highest achieving female student.

The rich dataset the researchers used was collected by the U.S. Department of Education, and it contained measures of many factors previously linked to the gender gap in STEM. The NYU team tested whether an extensive set of factors could explain the gender gap equally well among high, average, and low achieving students. While the gender gap in PECS among the highest achievers could be explained by other factors in the data, such as a student’s prior career aspirations and confidence in their science abilities, these same factors could not explain the higher rates of low-achieving men in these fields.

This new work suggests that interventions to improve gender equity need to become more nuanced with respect to student achievement.

“Our results suggest that boosting STEM confidence and earlier career aspirations might raise the numbers of high-achieving women in PECS, but the same kinds of interventions are less likely to work for average and lower achieving girls, and that something beyond all these student factors is drawing low-achieving men to these fields,” said Cimpian.

“This new evidence, combined with emerging literature on male-favoring cultures that deter women in PECS, suggests that efforts to dismantle barriers to women in these fields would raise overall quality of students,” continued Cimpian.

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Reaction microscope ‘X-rays’ individual molecules

“The smaller the particle, the bigger the hammer.” This rule from particle physics, which looks inside the interior of atomic nuclei using gigantic accelerators, also applies to this research. In order to “X-ray” a two-atom molecule such as oxygen, an extremely powerful and ultra-short X-ray pulse is required. This was provided by the European XFEL which started operations in 2017 and is one of the the strongest X-ray source in the world

In order to expose individual molecules, a new X-ray technique is also needed: with the aid of the extremely powerful laser pulse the molecule is quickly robbed of two firmly bound electrons. This leads to the creation of two positively charged ions that fly apart from each other abruptly due to the electrical repulsion. Simultaneously, the fact that electrons also behave like waves is used to advantage. “You can think of it like a sonar,” explains project manager Professor Till Jahnke from the Institute for Nuclear Physics. “The electron wave is scattered by the molecular structure during the explosion, and we recorded the resulting diffraction pattern. We were therefore able to essentially X-ray the molecule from within, and observe it in several steps during its break-up.”

For this technique, known as “electron diffraction imaging,” physicists at the Institute for Nuclear Physics spent several years further developing the COLTRIMS technique, which was conceived there (and is often referred to as a “reaction microscope”). Under the supervision of Dr Markus Schöffler, a corresponding apparatus was modified for the requirements of the European XFEL in advance, and designed and realised in the course of a doctoral thesis by Gregor Kastirke. No simple task, as Till Jahnke observes: “If I had to design a spaceship in order to safely fly to the moon and back, I would definitely want Gregor in my team. I am very impressed by what he accomplished here.”

The result, which was published in the current issue of the journal Physical Review X, provides the first evidence that this experimental method works. In the future, photochemical reactions of individual molecules can be studied using these images with their high temporal resolution. For example, it should be possible to observe the reaction of a medium-sized molecule to UV rays in real time. In addition, these are the first measurement results to be published since the start of operations of the Small Quantum Systems (SQS) experiment station at the European XFEL at the end of 2018.

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Physics principle explains order and disorder of swarms

Current experiments support the controversial hypothesis that a well-known concept in physics — a “critical point” — is behind the striking behaviour of collective animal systems. Physicists from the Cluster of Excellence “Centre for the Advanced Study of Collective Behaviour” at the University of Konstanz showed that light-controlled microswimming particles can be made to organize into different collective states such as swarms and swirls. By studying the particles fluctuating between these states, they provide evidence for critical behaviour — and support for a physical principle underlying the complex behaviour of collectives. The research results were published in the scientific journal Nature Communications.

Animal groups exhibit the seemingly contradictory characteristics of being both robust and flexible. Imagine a school of fish: hundreds of individuals in perfect order and alignment can suddenly transition to a convulsing tornado dodging an attack. Animal groups benefit if they can strike this delicate balance between being stable in the face of “noise” like eddies or gusts of wind, yet responsive to important changes like the approach of a predator.

Critical transition

How they achieve this is not yet understood. But in recent years, a possible explanation has emerged: criticality. In physics, criticality describes systems in which a transition between states — such as gas to liquid — occurs at a critical point. Criticality has been argued to provide biological systems with the necessary balance between robustness and flexibility. “The combination of stability and high responsiveness is exactly what characterizes a critical point,” says the study’s lead author Clemens Bechinger, Principal Investigator in the Centre for the Advanced Study of Collective Behaviour and Professor in the Department of Physics at the University of Konstanz, “and so it made sense to test if this could explain some of the patterns we see in collective behaviour.”

The hypothesis that collective states are hovering near critical points has been studied in the past largely through numerical simulations. In the new study published in Nature Communications, Bechinger and his colleagues have given rare experimental support to the mathematical prediction. “By demonstrating a close link between collectivity and critical behaviour, our findings not only add to our general understanding of collective states but also suggest that general physical concepts may apply to living systems,” says Bechinger.

Experimental evidence

In experiments, the researchers used glass beads coated on one side by a carbon cap and placed in a viscous liquid. When illuminated by light, they swim much like bacteria, but with an important difference: every aspect of how the particles interact with others — from how the individuals move to how many neighbours can be seen — can be controlled. These microswimming particles allow the researchers to eschew the challenges of working with living systems in which rules of interaction cannot be easily controlled. “We design the rules in the computer, put them in an experiment, and watch the result of the interaction game,” says Bechinger.

But to ensure that the physical system bore a resemblance to living systems, the researchers designed interactions that mirrored the behaviour of animals. For example, they controlled the direction that individuals moved in relation to their neighbours: particles were programmed either to swim straight towards others in the main group or to deviate away from them. Depending on this angle of movement, the particles organized into either swirls or disordered swarms. And incrementally adjusting this value elicited rapid transitions between a swirl and a disordered but still cohesive swarm. “What we observed is that the system can make sudden transitions from one state to the other, which demonstrates the flexibility needed to react to an external perturbation like a predator,” says Bechinger, “and provides clear evidence for a critical behaviour.”

“Similar behaviour to animal groups and neural systems”

This result is “key to understanding how animal collectives have evolved,” says Professor Iain Couzin, co-speaker of the Centre for the Advanced Study of Collective Behaviour and Director of the Department of Collective Behavior at the Konstanz Max Planck Institute of Animal Behavior. Although not involved with the study, Couzin has worked for decades to decipher how grouping may enhance sensing capabilities in animal collectives.

Says Couzin: “The particles in this study behave in a very similar way to what we see in animal groups, and even neural systems. We know that individuals in collectives benefit from being more responsive, but the big challenge in biology has been testing if criticality is what allows the individual to spontaneously become much more sensitive to their environment. This study has confirmed this can occur just via spontaneous emergent physical properties. Through very simple interactions they have shown that you can tune a physical system to a collective state — criticality — of balance between order and disorder.”

Application areas

By demonstrating the existence of a link between collectivity and critical behaviour in living systems, this study also hints at how the intelligence of collectives can be engineered into physical systems. Beyond just simple particles, the finding could assist with designing efficient strategies of autonomous microrobotics devices with on-board control units. “Similar to their living counterparts, these miniature agents should be able to spontaneously adapt to changing conditions and even cope with unforeseen situations which might be accomplished by their operation near a critical point,” says Bechinger.

Key facts:

  • Physicists from the University of Konstanz show a link between collective behaviour and a concept in physics known as criticality.
  • Through experiments using tiny glass particles, they create collective states of swarms and swirls.
  • Showing that the particles can make sudden transitions from one state to the other provides clear evidence for a critical behaviour
  • Original publication: Bäuerle, T., Löffler, R.C. & Bechinger, C. Formation of stable and responsive collective states in suspensions of active colloids. Nat Commun 11, 2547 (2020). https://doi.org/10.1038/s41467-020-16161-4
  • Authors include Tobias Bäuerle (lead author) and Robert Löffler, both doctoral students at the University of Konstanz. Senior author Clemens Bechinger is Professor of Physics at the University of Konstanz
  • Clemens Bechinger is also part of the University of Konstanz’s Cluster of Excellence “Centre for the Advanced Study of Collective Behaviour,” which has been funded in the Excellence Strategy of the German Federal and State Governments since 2019.
  • The research was supported by an ERC Advanced Grant ASCIR and the Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy — EXC 2117 — 42203798.
  • campus.kn is the University of Konstanz’s online magazine. We use multimedia approaches to provide insights into our research and science, study and teaching as well as life on campus.

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Materials provided by University of Konstanz. Original written by Carla Avolio. Note: Content may be edited for style and length.

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