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  • richardmitnick 1:39 pm on August 3, 2021 Permalink | Reply
    Tags: "Newly discovered planetary nebulae could improve cosmic distance measurements", A planetary nebula occurs at the end of a star’s red giant phase when helium has been exhausted and can no-longer be fused to create carbon and oxygen., , , , DELF: differential emission-line filter, , physicsworld.com (UK), PNLF could help soon help astronomers solve one of cosmology’s most troubling puzzles: the Hubble Tension., PNLF: planetary nebula luminosity function, , The Hubble tension: mismatch between the local rate of expansion using standard candles and the value that is calculated using the lambda-cold-dark-matter (ΛCDM) model of cosmology.   

    From physicsworld.com (UK) : “Newly discovered planetary nebulae could improve cosmic distance measurements” 

    From physicsworld.com (UK)

    02 Aug 2021
    Hamish Johnston

    1
    Eye on the universe: the planetary nebula NGC 7294, or the “Helix Nebula”, is nearby in the Milky Way. However, astronomers have managed to observe similar objects at much greater distances. (Courtesy: National Aeronautics Space Agency (US), National Optical Astronomy Observatory (US), European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU), the Hubble Helix Nebula Team, M Meixner (Space Telescope Science Institute (US)), and TA Rector (National Radio Astronomy Observatory (US))

    Planetary nebulae as far away as 40 Mpc (about 130 million light–years) have been observed by astronomers for the first time. The objects had been too distant to see until an international team of astronomers used a new filter on data from the Multi-Unit Spectroscopic Explorer (MUSE) instrument – which operates on European Space Agency’s Very Large Telescope (VLT).

    The team was led by Martin Roth at the Leibniz Institute for Astrophysics [Leibniz-Institut für Astrophysik] (DE) in Potsdam, Germany and it applied a differential emission-line filter (DELF) to archival data collected by MUSE. This revealed 15 planetary nebulae that were previously too faint to be seen. The technique could prove to be a useful tool for studying the cosmos and could even help resolve a mystery about the expansion of the universe.

    “Planetary nebulae are like Swiss Army Knives for extragalactic study,” says team member Robin Ciardullo at Penn State University(US). “You can use them to learn about stellar dynamics, dark matter, stellar evolution, galactic chemical evolution, the history of galaxy clusters and, of course, measure extragalactic distances.”

    Shedding layers

    A planetary nebula occurs at the end of a star’s red giant phase when helium has been exhausted and can no-longer be fused to create carbon and oxygen. At this point, a star that began life at less than 8 solar-masses will shed its outer layers leaving behind a stellar core surrounded by a cloud of material. Eventually, the star will become a white dwarf.

    Light emitted by the star will ionize atoms in the cloud, freeing electrons that can then collide with electrons still bound in atoms, kicking them up to higher energy states. When these bound electrons decay to lower energy states they emit light at very specific wavelengths.

    “Now that its outside is gone, the star is very hot. The high-energy light from the star slams into the material that just came off the star and lights it up,” explains Ciardullo. “We see what the star ejected into space — the ‘planetary nebula’. It is a terrible name since it has nothing at all to do with planets!”

    Standard candles

    It is the uniformity of the light emitted by planetary nebulae that make them excellent yardsticks for the measurement of extragalactic distances. Such yardsticks are known as standard candles because they have known luminosities and therefore their distances can be inferred from how bright they appear in the sky.

    Standard Candles to measure age and distance of the universe

    Along with George Jacoby and Holland Ford, Ciardullo introduced the planetary nebula luminosity function (PNLF) in 1989 and it has been used as a distance indicator for galaxies up to around 15 Mpc ever since.

    Using planetary nebulae to look further than about 15 Mpc had not been possible because the luminosity of objects drops as the square of the distance, making more distant objects significantly fainter and therefore harder to observe. Now, however, the application of DELF and the power of MUSE/ VLT has extended this limit considerably allowing precision distance measurements to be made for galaxies up to around 40 Mpc.

    Bigger and better

    “By the turn of the century, we had explored planetary nebulae in nearby galaxies and had pushed the PNLF distance measurement technique about as far into the universe as we could,” says Ciardullo. “Now telescopes are bigger, and the instrumentation is better. Observations that were extremely difficult 20 years ago are trivial, and we can extend the technique out to much larger distances”

    With this boost, PNLF could help soon help astronomers solve one of cosmology’s most troubling puzzles. This is the apparent mismatch between the local rate of expansion of the universe that astronomers observe using standard candles and the value that is calculated using the lambda-cold-dark-matter (ΛCDM) model of cosmology. Dubbed the Hubble Tension, resolving this discrepancy could point to new physics beyond the ΛCDM.

    Lamda Cold Dark Matter Accerated Expansion of The universe http scinotions.com the-cosmic-inflation-suggests-the-existence-of-parallel-universes. Credit: Alex Mittelmann.

    The idea is to use planetary nebulae as a complementary distance measurement to other standard candles.

    “One way to address [the Hubble tension] is to double-check the local measurements with other precision ways of measuring distances,” explains Ciardullo. “Until now, planetary nebulae were not bright enough to observe deep enough into the universe to test this tension. Our [research] shows that with MUSE and the VLT, we can get to these necessary distances.”

    The observations are described The Astrophysical Journal.

    See the full article here .


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    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 4:39 pm on July 22, 2021 Permalink | Reply
    Tags: "Cosmic challenge-protecting supercomputers from an extraterrestrial threat", , “Cosmic stress-tests” on electronic components., “Exascale” computing at risk, Cosmic rays can alter data in supercomputers., Fast neutrons from cosmic-ray showers can cause significant errors in supercomputers., In 2002 when LANL installed ASCI Q-then the second fastest supercomputer in the world-initially it couldn’t run for more than an hour without crashing due to errors., Looking even further into the future where computing is likely to be quantum cosmic rays may pose an even bigger challenge., physicsworld.com (UK), SEU: single-event upset, Supercomputers at risk, The continued increase in the scale of supercomputers is likely to exacerbate the problem in the next decade.   

    From physicsworld.com (UK) : “Cosmic challenge-protecting supercomputers from an extraterrestrial threat” 

    From physicsworld.com (UK)

    17 Jul 2021
    Rachel Brazil
    rachelbrazil@hotmail.com

    Fast neutrons from cosmic-ray showers can cause significant errors in supercomputers. But by measuring the scale of the problem, physicists hope not only to make such devices less prone to cosmic corruption but also protect everything from self-driving cars to quantum computers.

    1
    Courtesy: iStock/Vladimir_Timofeev

    In 2013 a gamer by the name “DOTA_Teabag” was playing Nintendo’s Super Mario 64 and suddenly encountered an “impossible” glitch – Mario was teleported into the air, saving crucial time and providing an advantage in the game. The incident – which was recorded on the livestreaming platform Twitch – caught the attention of another prominent gamer “pannenkoek12”, who was determined to explain what had happened, even offering a $1000 reward to anyone who could replicate the glitch. Users tried in vain to recreate the scenario, but no-one was able to emulate that particular cosmic leap. Eight years later, “pannenkoek12” concluded that the boost likely occurred due to a flip of one specific bit in the byte that defines the player’s height at a precise moment in the game – and the source of that flipping was most likely an ionizing particle from outer space.

    The impact of cosmic radiation is not always as trivial as determining who wins a Super Mario game, or as positive in its outcome. On 7 October 2008 a Qantas flight en route from Singapore to Australia, travelling at 11,300 mph suddenly pitched down with 12 passengers seriously injured as a result. Investigators determined that the problem was due to a “single-event upset” (SEU) causing incorrect data to reach the electronic flight instrument system. The culprit, again, was most likely cosmic radiation. An SEU bit flip was also held responsible for errors in an electronic voting machine in Belgium in 2003 that added 4096 extra votes to one candidate.

    Cosmic rays can also alter data in supercomputers, which often causes them to crash. It’s a growing concern, especially as this year could see the first “exascale” computer – able to calculate more than 10^18 operations per second. How such machines will hold up to the increased threat of data corruption from cosmic rays is far from clear. As transistors get smaller, the energy needed to flip a bit decreases; and as the overall surface area of the computer increases, the chance of data corruption also goes up.

    Fortunately, those who work in the small but crucial field of computer resilience take these threats seriously. “We are like the canary in the coal mine, we’re out in front, studying what is happening,” says Nathan DeBardeleben, senior research scientist at DOE’s Los Alamos National Laboratory (US). At the lab’s Neutron Science Centre (US), he carries out “cosmic stress-tests” on electronic components, exposing them to a beam of neutrons to simulate the effect of cosmic rays.

    While not all computer errors are caused by cosmic rays (temperature, age and manufacturing errors can all cause problems too), the role they play has been apparent since the first supercomputers in the 1970s. The Cray-1, designed by Seymour Roger Cray, was tested at Los Alamos (perhaps a mistake given that its high altitude, 2300 m above sea level, makes it even more vulnerable to cosmic rays).

    Cray was initially reluctant to include error-detecting mechanisms, but eventually did so, adding what became known as parity memory – where an additional “parity” bit is added to a given set of bits. This records whether the sum of all the bits is odd or even. Any single bit corruption will therefore show up as a mismatch. Cray-1 recorded some 152 parity errors in its first six months (IEEE Trans. Nucl. Sci. 10.1109/TNS.2010.2083687). As supercomputers developed, problems caused by cosmic rays did not disappear. Indeed, in 2002 when Los Alamos installed ASCI Q-then the second fastest supercomputer in the world-initially it couldn’t run for more than an hour without crashing due to errors.

    2
    Credit: https://www.researchgate.net

    The problem only eased when staff added metal side panels to the servers, allowing it to run for six hours.

    Cosmic chaos

    Cosmic rays originate from the Sun or cataclysmic events such as supernovae in our galaxy or beyond. They are largely made up of high-energy protons and helium nuclei, which move through space at nearly the speed of light. When they strike the Earth’s atmosphere they create a secondary shower of particles, including neutrons, muons, pions and alpha particles. “The ones that survive down to ground level are the neutrons, and largely they are fast neutrons,” explains instrument scientist Christopher Frost, who runs the ChipIR beamline at the Rutherford Appleton Laboratory(UK)

    STFC Rutherford Appleton Laboratory at Harwell in Oxfordshire, UK.

    It was set up in 2009 to specifically study the effects of irradiating microelectronics with atmospheric-like neutrons.

    Millions of these neutrons strike us each second, but only occasionally do they flip a computer memory bit. When a neutron interacts with the semiconductor material, it deposits charge, which can change the binary state of the bit. “It doesn’t cause any physical damage, your hardware is not broken; it’s transient in nature, just like a blip,” explains DeBardeleben. When this happens, the results can be completely unobserved or can be catastrophic – the outcome is purely coincidental.

    3
    Rapid-testing single-event effects Schematic of the components needed to produce the ChipIr atmospheric neutron beam. It has been built at the ISIS spallation source at the Rutherford Appleton Laboratory, UK, in collaboration with the CNR Italian National Research Council [Consiglio Nazionale delle Ricerche] (IT) and is used to study the effects of irradiation on microelectronics, and to rapidly test the effects of “single event upsets” caused by high-energy neutrons. (Courtesy: Science & Technology Facilities Council (UK)/CNR).

    Computer scientist Leonardo Bautista-Gomez, from the Barcelona Supercomputing Center [Centro Nacional de Supercomputación](ES), compares these errors to the mutations radiation causes to human DNA. “Depending on where the mutation happens, these can create cancer or not, and it’s very similar in computer code.” Back at the Rutherford lab, Frost – working with computer scientist Paolo Rech from the Institute of Informatics of the Federal University of Rio Grande do Sul [Universidade Federal do Rio Grande do Sul](BR) – has also been studying an additional source of complications, in the form of lower energy neutrons. Known as thermal neutrons, these have nine orders of magnitude less energy than those coming directly from cosmic rays. Thermal neutrons can be particularly problematic when they collide with boron-10, which is found in many semiconductor chips. The boron-10 nucleus captures a neutron, decaying to lithium and emitting an alpha particle.

    Frost and Rech tested six commercially available devices, run under normal operating conditions and found they were all impacted by thermal neutrons (The Journal of Supercomputing). “In principle, you can use extremely pure boron-11” to be rid of the problem, says Rech, but he adds that this increases the cost of production. Today, even supercomputers use commercial off-the-shelf components, which are likely to suffer from thermal neutron damage. Although cosmic rays are everywhere, thermal neutron formation is sensitive to the environment of the device. “Things containing hydrogen [like water], or things made from concrete, slow down fast neutrons to thermal ones,” explains Frost. The researchers even found the weather affected thermal neutron production, with levels doubling on rainy days.

    Preventative measures

    While the probability of errors is still relatively low, certain critical systems employ redundancy measures – essentially doubling or tripling each bit, so errors can be immediately detected. “You see this particularly in spacecraft and satellites, which are not allowed to fail,” says DeBardeleben. But these failsafes would be prohibitively expensive to replicate for supercomputers, which often run programmes lasting for months. The option of stopping the neutrons reaching these machines altogether is also impractical – it takes three metres of concrete to block cosmic rays – though DeBardeleben adds that “we have looked at putting data centres deep underground”.

    Today’s supercomputers do run more sophisticated versions of parity memory, known as error-correcting code (ECC). “About 12% of the size of the data [being written] is used for error-correcting codes,” adds Bautista-Gomez. Another important innovation for supercomputers has been “checkpointing” – the process of regularly saving data mid-calculation, so that if errors cause a crash, the calculation can be picked up from the last checkpoint. The question is how often to do this? Checkpointing too frequently costs a lot in terms of time and energy; but not often enough and you risk losing months of work, when it comes to larger applications. “There is a sweet spot where you find the optimal frequency,” says Bautista-Gomez.

    The fear of the system crashing and a loss of data is only half the problem. What has started to concern Bautista-Gomez and others is the risk of undetected or silent errors – ones that do not cause a crash, and so are not caught. The ECC can generally detect single or double bit flips, says Bautista-Gomez, but “beyond that, if you have a cosmic ray that changes three bits in the memory cell, then the codes that we use today will most likely be unable to detect it”.

    Until recently, there was little direct evidence of such silent data-corruption in supercomputers, except what Bautista-Gomez describes as “weird things that we don’t know how to explain”. In 2016, together with computer scientist Simon McIntosh-Smith from the University of Bristol (UK), he decided to hunt for these errors using specially designed memory-scanning software to analyse a cluster of 1000 computer nodes (data points) without any ECC. Over a year they detected 55,000 memory errors. “We observed many single-bit errors, which was expected. We also observed multiple double-digit errors, as well as several multi-bit errors that, even if we had ECC, we wouldn’t have been seen,” recalls Bautista-Gomez (SC ‘16: Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis 10.1109/SC.2016.54).

    Accelerated testing

    The increasing use of commercial graphics processing units (GPUs) in high-performance computing, is another problem that worries Rech. These specialized electronic circuits have been designed to rapidly process and create images. As recently as 10 years ago they were only used for gaming, and so weren’t considered for testing says Rech. But now these same low-power, high-efficiency devices are being used in supercomputers and in self-driving cars, so “you’re moving into areas where its failure actually becomes critical” adds Frost.

    Rech, using Frost’s ChipIR beamline, devised a method to test the failure rate of GPUs produced by companies like Nvidia and AMD that are used in driverless cars. They have been doing this sort of testing for the last decade and have devised methods to expose devices to high levels of neutron irradiation while running an application with an expected outcome. In the case of driverless car systems, they would essentially show the device pre-recorded videos to see how well it responded to what they call “pedestrian incidents” – whether or not it could recognize a person.

    Of course, in these experiments the neutron exposure is much higher than that produced by cosmic rays. In fact, it’s roughly 1.5 billion times what you would get at ground level, which is about 13 neutrons cm^–2 hr^–1. “So that enables us to do this accelerated testing, as if the device is in a real environment for hundreds of thousands of years,” explains Frost. Their experiments try to replicate 100 errors per hour and, from the known neutron flux, can calculate what error rate this would represent in the real world. Their conclusion: an average GPU would experience one error every 3.2 years.

    This seems low, but as Frost points out, “If you deploy them in large numbers, for example in supercomputers, there may be several thousand or if you deploy them in a safety-critical system, then they’re effectively not good enough.” At this error rate a supercomputer with 1800 devices would experience an error every 15 hours. When it comes to cars, with roughly 268 million cars in the EU and about roughly 4% – or 10 million cars – on the road at any given time, there would be 380 errors per hour, which is a concern.

    Large scale

    The continued increase in the scale of supercomputers is likely to exacerbate the problem in the next decade. “It’s all an issue of scale,” says DeBardeleben, adding that while the first supercomputer Cray-1 “was as big as a couple of rooms…our server computers today are the size of a football field”. Rech, Bautista-Gomez and many others are working on additional error-checking methods that can be deployed as supercomputers grow. For self-driving cars, Rech has started to analyse where the critical faults arise within GPU chips that could cause accidents, with a view to error correcting only these elements.

    Another method used to check the accuracy of supercomputer simulations is to use physics itself. “In most scientific applications you have some constants, for example, the total energy [of a system] should be constant,” explains Bautista-Gomez. So every now and then, we check the application to see whether the system is losing energy or gaining energy. “And if that happens, then there is a clear indication that something is going wrong.”

    Both Rech and Bautista-Gomez are making use of artificial intelligence (AI), creating systems that can learn to detect errors. Rech has been working with hardware companies to redesign the software used in object detection in autonomous vehicles, so that it can compare consecutive images and do its own “sense check”. So far, this method has picked up 90% of errors (IEEE 25th International Symposium on On-Line Testing and Robust System Design 10.1109/IOLTS.2019.8854431). Bautista-Gomez is also developing machine-learning strategies to constantly analyse data outputs in real-time. “For example, if you’re doing a climate simulation, this machine-learning [system] could be analysing the pressure and temperature of the simulation all the time. By looking at this data it will learn the normal variations, and when you have a corruption of data that causes a big change, it can signal something is wrong.” Such systems are not yet commonly used, but Bautista-Gomez expects they will be needed in the future.

    Quantum conundrum

    Looking even further into the future where computing is likely to be quantum cosmic rays may pose an even bigger challenge. The basic unit of quantum information – the qubit – is able to exist in three states, 0, 1 and a mixed state that enables parallel computation and the ability to handle calculations too complex for even today’s supercomputers. It’s still early days in their development, but IBM announced it plans to launch the 127-qubit IBM Quantum Eagle processor sometime this year.

    For quantum computers to function, the qubits must be coherent – that means they act together with other bits in a quantum state. Today the longest period of coherence for a quantum computer is around 200 microseconds. But, says neutrino physicist Joe Formaggio at the Massachusetts Institute of Technology (US), “No matter where you are in the world, or how you construct your qubit [and] how careful you are in your set up, everybody seems to be petering out in terms of how long they can last.” William Oliver, part of the Engineering Quantum Systems Group at MIT, believes that radiation from cosmic rays is one of the problems, and with Formaggio’s help he decided to test their impact.

    Formaggio and Oliver designed an experiment using radioactive copper foil, producing the isotope copper-64, which decays with a half-life of just over 12 hours. They placed it in the low-temperature 3He/4He dilution refrigerator with Oliver’s superconducting qubits. “At first he would turn on his apparatus and nothing worked,” describes Formaggio, “but then after a few days, they started to be able to lock in [to quantum coherence] because the radioactivity was going down. We did this for several weeks and we could watch the qubit slowly get back to baseline.” The researchers also demonstrated the effect by creating a massive two-tonne wall of lead bricks, which they raised and lowered to shield the qubits every 10 minutes, and saw the cycling of the qubits’ stability.

    From these experiments they have predicted that without interventions, cosmic and other ambient radiation will limit qubit coherence to a maximum of 4 milliseconds [Nature]. As current coherence times are still lower than this limit, the issue is not yet a major problem. But Formaggio says as coherence times increase, radiation effects will become more significant. “We are maybe two years away from hitting this obstacle.”

    Of course, as with supercomputers, the quantum-computing community is working to find a way around this problem. Google has suggested adding aluminium film islands to its 53-qubit Sycamore quantum processor. The qubits are made from granular aluminium, a superconducting material containing a mixture of nanoscale aluminium grains and amorphous aluminium oxide. They sit on a silicon substrate and when this is hit by radiation, photons exchange between the qubit and substrate, leading to decoherence. The hope is that aluminium islands would preferentially trap any photons produced [Applied Physics Letters].

    Another solution Google has proposed is a specific quantum error-correction code called “surface code”. Google has developed a chessboard arrangement of qubits, with “white squares” representing data qubits that perform operations and “black squares” detecting errors in neighbouring qubits. The arrangement avoids decoherence by relying on the quantum entanglement of the squares.

    In the next few years, the challenge is to further improve the resilience of our current supercomputer technologies. It’s possible that errors caused by cosmic rays could become an impediment to faster supercomputers, even if the size of components continues to drop. “If technologies don’t improve, there certainly are limits,” says DeBardeleben. But he thinks it’s likely new error-correcting methods will provide solutions: “I wouldn’t bet against the community finding ways out of this.” Frost agrees: “We’re not pessimistic at all; we can find solutions to these problems.”

    See the full article here .


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    Please help promote STEM in your local schools.

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    PhysicsWorld(UK) is a publication of the Institute of Physics(UK). The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
  • richardmitnick 10:55 am on June 27, 2021 Permalink | Reply
    Tags: "‘Slidetronics’ makes its debut", , “Slidetronics” occurs when adjacent atomic layers in the material slide across each other., , , physicsworld.com (UK), Researchers have discovered a novel way of switching the polarization of an ultrathin ferroelectric material., Slidetronics is key for modern technologies such as hard disk drives that store and retrieve large volumes of information., The researchers found that scanning an electrically-biased tip across the surface makes the domain walls in the material slide across each other.   

    From physicsworld.com (UK) : “‘Slidetronics’ makes its debut” 

    From physicsworld.com (UK)

    27 Jun 2021
    Isabelle Dumé

    1
    Adjacent atomic layers slide across each other. Courtesy: M Ben Shalom.

    Researchers have discovered a novel way of switching the polarization of an ultrathin ferroelectric material. The mechanism, dubbed “slidetronics” because it occurs when adjacent atomic layers in the material slide across each other, could be an efficient alternative way of controlling tiny electronic devices.

    Being able to switch electrical polarization over small areas is key for modern technologies such as hard disk drives that store and retrieve large volumes of information. The dimensions of individually polarizable domains (that is, regions with a fixed polarization) within the silicon-based devices commonly used for information storage has dramatically decreased in recent years, going from roughly 100 nm thick to the atomic scale.

    The main challenge to making these structures even tinier is overcoming long-range interactions between neighbouring domains, which tend to cause the polarization of individual domains to align. Surface effects also become more important as domain sizes become smaller because the surface-to-volume ratio increases.

    Breaking the symmetry

    To address these difficulties, researchers have begun to explore alternatives to silicon in the form of two-dimensional materials such as hexagonal boron nitride (h-BN) and the transition metal dichalcogenides (TMDs). These materials, which can be made just one atom thick but remain crystalline with a well-defined lattice and symmetry, are made up of stacked layers held together by weak van der Waals (vdW) interactions. However, polarization in naturally-grown h-BN and TMDs is limited because it is energetically favourable for these materials to adopt a so-called “centrosymmetric” vdW structure that looks the same when the crystal is flipped.

    Researchers led by Moshe Ben Shalom at Israel’s Tel Aviv University אוּנִיבֶרְסִיטַת תֵּל אָבִיב (IL) have now broken this undesirable symmetry by controlling the angle, or twist, between two stacked h-BN layers. In the process, they discovered a multitude of permanent and switchable polarizations – oriented perpendicular to the material’s surface – at the interface between the layers.

    “The stacking arrangement that breaks the symmetry and hosts polarization is one out of five possible configurations in bilayer h-BN,” Ben Shalom explains. “We divided these into two groups: ‘antiparallel’ and ‘parallel’ twist orientations.”

    2
    A Moiré pattern of triangle domains. Courtesy: M. Ben Shalom.

    “A beautiful ‘Moiré’ pattern”

    In the experiments, team member Maayan Vizner Stern used sticky tape to separate individual atomic layers from a multilayer bulk crystal of h-BN and transfer them onto a flat surface. (Andre Geim and colleagues at Manchester University (UK) used a similar technique to isolate layers of graphene from bulk graphite in 2004.) Stern then picked up the layers using a microscope slide covered with a soft, transparent polymer and placed one crystalline layer on top of another such that the lattices of both were oriented parallel to each other in an AB stack.

    “The artificially stacked parallel structure we made results in a tiny interlayer shift with only half of the nitrogen or boron atoms eclipsed and is not symmetric when flipped,” Ben Shalom tells Physics World. “What is more, when we scanned the local surface potential with the tip of an atomic force microscope, we observed a beautiful ‘Moiré’ pattern of triangle domains in which the (out-of-plane) polarization flips.”

    Domain wall sliding

    Most importantly, the researchers found that scanning an electrically-biased tip across the surface makes the domain walls in the material slide across each other. This sliding allows the polarization orientation to be switched locally, as desired.

    Thanks to extensive numerical calculations by team member Wei Cao, the researchers were able to follow how the charge on the different lattice sites in h-BN reorders due to the broken symmetry. Led by another member of the collaboration, Eran Sela, they used this information to build an intuitive model that explains the phenomenon. According to Ben Shalom, it turns out that there is a competition between the Coulomb attraction and the vdW forces in the pairs of fully-eclipsed atoms versus the separated pairs. This mechanism could be used to predict similar polarization behaviour in other hexagonal diatomic crystals.

    Ben Shalom observes that the presence of such a stable polarization in a two-atom-thin system could be very useful for efforts to miniaturize non-volatile electronics devices. At the atomic scale, the electrons can efficiently quantum-tunnel across the two layers, and this tunnelling mechanism can be used to rapidly read and write the polarization. Looking longer term, he suggests that the lateral mechanical sliding and perpendicular polarization switching mechanisms observed in this study, which is detailed in Science, may even have applications beyond what we can predict today.

    See the full article here .


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    Please help promote STEM in your local schools.

    a href=”http://www.stemedcoalition.org/”>Stem Education Coalition

    PhysicsWorld(UK) is a publication of the Institute of Physics(UK). The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
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  • richardmitnick 5:42 pm on June 17, 2021 Permalink | Reply
    Tags: "‘Talking’ quantum dots could be used as qubits", Atomic-scale computer simulations show how quantum dots “talk” to each other., In their study the team successfully modelled the behaviour of the quantum dots at the femtosecond scale., More complex behaviours can occur when two or more quantum dots are close enough together to interact with each other., physicsworld.com (UK), , Quantum dots are tiny pieces of semiconductor crystal contain thousands of atoms., With further improvements to the model the use of quantum dots could be expanded to include a diverse array of real-world applications.   

    From physicsworld.com (UK) : “‘Talking’ quantum dots could be used as qubits” 

    From physicsworld.com (UK)

    16 Jun 2021
    Sam Jarman

    1
    It’s good to talk: illustration showing two quantum dots “communicating” with each other by exchanging light. (Courtesy: Helmholtz-Zentrum Berlin (HZB) (DE))

    New atomic-scale computer simulations of how quantum dots “talk” to each other could lead to a wide range of practical applications ranging from quantum computing to green energy.

    The research was done by Pascal Krause and Annika Bande at the Helmholtz Centre for Materials and Energy in Germany and Jean Christophe Tremblay at National Centre for Scientific Research [Centre national de la recherche scientifique, [CNRS] (FR) and the University of Lorraine [Université de Lorraine](FR), who modelled the absorption, exchange, and storage of energy within pairs of quantum dots. With further improvements to the model the use of quantum dots could be expanded to include a diverse array of real-world applications.

    Quantum dots are tiny pieces of semiconductor crystal contain thousands of atoms. The dots are quantum systems that behave much like atoms, having electron energy levels that can absorb and emit light at discrete wavelengths. For example, when illuminated with ultraviolet light a quantum dot can be excited to a higher energy state. When it drops back down to its ground state, it can emit a visible photon – allowing quantum dots to produce glow with vivid colours.

    More complex behaviours can occur when two or more quantum dots are close enough together to interact with each other. For example, interactions can stabilize excitons, which are quasiparticles that comprise an electron and a hole and are created when electrons are excited. Long-lasting excitons can have applications ranging from photocatalysis to quantum computing.

    Sheer complexity

    So far, computer simulations of quantum dot interactions have been limited by their sheer complexity. Since the processes involve thousands of atoms, each hosting multiple electrons, the characteristics of exciton formation and recombination cannot be fully captured by even the most advanced supercomputers. Now, Krause, Bande and Tremblay have approximated the process through simulations of scaled-down quantum dots, each containing just hundreds of atoms.

    In their study the trio successfully modelled the behaviour of the quantum dots at the femtosecond scale. Their simulations revealed how the quantum dot pairs absorb, exchange, and store light energy. They also found how excitons can be stabilized by applying a sequence of ultraviolet and infrared pulses to quantum dots. While an initial ultraviolet pulse can generate an exciton in one quantum dot, a subsequent infrared pulse can shift the exciton to a nearby quantum dot – where the energy it contains can be stored.

    The team simulated interactions between three pairs of germanium/silicon quantum dots, which had different shapes and sizes. They now plan to create more realistic simulations that will allow them to model how environmental factors such as temperature can affect interactions. Through further improvements, their results could lead to a wide range of applications for quantum dots including quantum bits (qubits) that can reliably store and read out quantum information and photocatalysts that absorb sunlight, facilitating reactions that produce hydrogen gas as a carbon-free fuel source.

    The research is described in the Journal of Physical Chemistry A.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    a href=”http://www.stemedcoalition.org/”>Stem Education Coalition

    PhysicsWorld(UK) is a publication of the Institute of Physics(UK). The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
  • richardmitnick 10:40 am on June 13, 2021 Permalink | Reply
    Tags: "Exotic quantum state could make smallest-ever laser", A microcavity acts like a cage for light., , , Bose-Einstein condensate (BEC), Exciton-polaritons form when excited electrons in solids couple strongly with photons., Momentum-resolved micro-photoluminescence spectroscopy, Optoelectronic circuits operate using light instead of electric current., , , physicsworld.com (UK),   

    From physicsworld.com (UK) : “Exotic quantum state could make smallest-ever laser” 

    From physicsworld.com (UK)

    13 Jun 2021
    Isabelle Dumé

    1
    A Bose-Einstein condensate of exciton-polaritons. Credit: Johannes Michl.

    Physicists have taken a step towards realizing the smallest-ever solid-state laser by generating an exotic quantum state known as a Bose-Einstein condensate (BEC) in quasiparticles consisting of both matter and light. Although the effect has so far only been observed at ultracold temperatures in atomically thin crystals of molybdenum diselenide (MoSe2), it might also be produced at room temperature in other materials.

    When particles are cooled down to temperatures just above absolute zero, they form a BEC – a state of matter in which all the particles occupy the same quantum state and thus act in unison, like a superfluid. A BEC made up of tens of thousands of particles therefore behaves as if it were just one single giant quantum particle.

    A BEC of exciton-polaritons

    An international team of researchers led by Carlos Anton-Solanas and Christian Schneider from the Carl von Ossietzky University of Oldenburg [Carl von Ossietzky Universität Oldenburg] (DE), Germany; Sven Höfling of the Julius Maximilian University of Würzburg [Julius-Maximilians-Universität Würzburg] (DE), Germany; Sefaattin Tongay at Arizona State University (US); and Alexey Kavokin of Westlake University [西湖大学] (CN) in China, has now generated a BEC from quasiparticles known as exciton-polaritons in atomically thin crystals. These quasiparticles form when excited electrons in solids couple strongly with photons.

    “Devices that can control these novel light-matter states hold the promise of a technological leap in comparison with current electronic circuits,” explains Anton-Solanas, who is in the quantum materials group at Oldenburg’s Institute of Physics. “Such optoelectronic circuits, which operate using light instead of electric current, could be better and faster at processing information than today’s processors.”

    Anton-Solanas, Schneider and colleagues studied crystals of MoSe2 that were just a single atomic layer thick. MoSe2belongs to a family of materials known as transition-metal dichalcogenides (TMDCs). In their bulk form, these materials act as indirect band-gap semiconductors, but when scaled down to a monolayer thickness, they behave as direct band-gap semiconductors, capable of efficiently absorbing and emitting light.

    In their experiments, the researchers assembled sheets of MoSe2 less than a nanometre thick and sandwiched them between alternating layers of silicon dioxide and titanium dioxide (SiO2/TiO2), which reflect light like a mirror. The resulting structure is known as a microcavity and acts like a cage for light. “It’s like trapping the light-emitting material in a room filled with mirrors and mirrors only,” Tongay tells Physics World. “The light gets reflected these mirrors and is absorbed by the material back and forth.”

    Sudden increase in light emission

    The team cooled the system to 4 K and stimulated it with short pulses of laser light to produce excitons in the MoSe2. These excitons then coupled with light in the microcavity to generate the exciton-polaritons. Using a technique called momentum-resolved micro-photoluminescence spectroscopy, the researchers observed a sudden increase in the light emission from the sample above a certain threshold laser intensity. The researchers say that this, together with the build-up of interference fringes from the polariton light emission, indicates that a BEC had been created from exciton-polaritons.

    “In theory, this phenomenon could be used to construct coherent light sources based on just a single layer of atoms,” says Anton-Solanas. “This would mean we had created the smallest possible solid-state laser.”

    The researchers, who report their work in Nature Materials, are confident that the effect could also be produced at room temperature in other exitonic materials, such as tungsten-based TMDCs or organic halide-based materials. This means it could be exploited in practical applications. To this end, they are now looking for condensation signatures by studying the interference properties of tungsten diselenide polaritons at room temperature.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    a href=”http://www.stemedcoalition.org/”>Stem Education Coalition

    PhysicsWorld(UK) is a publication of the Institute of Physics(UK). The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
  • richardmitnick 4:55 pm on May 3, 2021 Permalink | Reply
    Tags: "Shaped light waves penetrate further into photonic crystals", , , , , physicsworld.com (UK)   

    From physicsworld.com (UK) : “Shaped light waves penetrate further into photonic crystals” 

    From physicsworld.com (UK)

    03 May 2021
    Isabelle Dumé

    1
    Light propagation inside a photonic crystal with shaped and unshaped incident light waves. Courtesy: R Uppu.

    An international team of researchers has succeeded in steering light waves deep into “forbidden” regions of photonic crystals by manipulating the shape of the waves. The technique, which was developed by scientists at the University of Twente [ Universiteit Twente] (NL), the University of Iowa (US) and the University of Copenhagen [Københavns Universitet](DK), takes advantage of nanoscale channels created naturally when the crystals are fabricated, and could find use in a host of optoelectronics applications.

    Photonic crystals are made by etching patterned nanopores into a substrate such as a silicon wafer. These patterned structures are specially designed to make the crystal’s refractive index vary periodically on the length scale of visible light. This periodic variation, in turn, produces a photonic “band gap” that affects how photons propagate through the crystal – similar to the way a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands.

    The presence of this band gap means that only light within certain wavelength ranges can pass through the crystal. Outside these ranges, the light is reflected due to an effect called Bragg interference. The prohibition on light travel at forbidden wavelengths is so restrictive that if a quantum dot that emits light at one of these wavelengths is placed inside the crystal, it stops emitting the forbidden colour of light.

    “Out of control”

    Photonic crystals were discovered 30 years ago, and they are now routinely integrated into devices such as light sources, lasers, efficient solar cells and so-called invisibility cloaks. They are also used to trap light in extremely small volumes and to process optical information. In addition, the ability to tightly control their emission properties makes them attractive for advanced applications such as nonlinear processors for quantum computing and memories that store information encoded as light.

    To date, all these applications have been static, because the structure of the crystals (and thus the path of the light transported within them) is fixed. New functionalities should be possible, however, if light can be controllably steered anywhere inside the crystals, beyond the depth set by Bragg interference.

    “This depth is known as the Bragg length and is determined by the intentionally introduced periodic structural order in the crystal when it is fabricated,” explains study lead author Ravitej Uppu. “Disorder arising from unavoidable imperfections in the nanofabrication process, however, produces channels that penetrate deep into the crystal and through which the trajectory of incoming light waves can be deviated. These channels are usually detrimental for applications because they allow a small fraction of waves to ‘get out of control’ and randomly scatter into the crystal.”

    Light-steering demonstration

    Led by Willem Vos of the University of Twente, Uppu and colleagues have now turned these channels and the fact that light waves can travel through them into an advantage. They did this by shaping the wavefronts of light waves so that they selectively couple to these channels, thus allowing the waves to travel much further into the crystal. What is more, by programming the wavefronts correctly, they could interfere the waves such that their intensity concentrates at a single location deep inside the crystals.

    In their work, published in Physical Review Letters, the researchers studied light propagation in two-dimensional photonic crystals consisting of large periodic arrays of pores (about 6 microns deep) etched in a silicon wafer. They began by directing unstructured, random, plane light waves onto the crystals and imaging the light that leaks through the structures’ top surface. This leaked light revealed the energy density of light at any given position inside the crystals, and as the researchers expected, they saw hardly any sign that light had penetrated the crystal at all. They confirmed this result by showing that 95% of the incident light was reflected.

    Eight times the Bragg length

    The researchers then repeated their experiment using light waves with wavefronts shaped using a device known as a spatial light modulator. By programming the shapes, they managed to steer the waves into otherwise forbidden gaps in the crystal, travelling up to eight times the Bragg length. Focusing this light allowed them to create a bright spot that is up to 100 times more intense compared to that created by unshaped wavefronts.

    Members of the team say they now plan to extend their experiments to 3D photonic band gap crystals, where they “eagerly expect” to see additional phenomena such as Anderson localization of light. “Such 3D control of light transport could be exploited for exotic light hopping across a lattice of cavities inside these crystals,” Uppu tells Physics World. “The combination of reconfigurable light transport and cavities could potentially allow us to realize nonlinear quantum operations for quantum computing.”

    And that is not all. Since the observed phenomena are essentially exploiting wave interference, the team is confident that their results can be generalized to electron waves, magnetic spin waves or even sound waves. Indeed, Uppu notes that other researchers have recently made considerable advances in the latter two fields, so the required spatial shaping of these waves should be feasible.

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    a href=”http://www.stemedcoalition.org/”>Stem Education Coalition

    PhysicsWorld(UK) is a publication of the Institute of Physics(UK). The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
  • richardmitnick 2:48 pm on April 24, 2021 Permalink | Reply
    Tags: "How the Daya Bay experiment helped China build a neutrino legacy", , Institute of High Energy Physics-Chinese Academy of Sciences [中国科学院](CN), , physicsworld.com (UK)   

    From physicsworld.com (UK) : “How the Daya Bay experiment helped China build a neutrino legacy” 

    From physicsworld.com (UK)

    21 Apr 2021

    Ling Xin examines the legacy of the recently closed Daya Bay Reactor Neutrino Experiment on neutrino physics, US–China collaborations and future neutrino facilities.

    In an underground laboratory near Shenzhen, southern China, officials gathered on 12 December 2020 to say goodbye to a decade-old experiment that not only unveiled secrets of the neutrino but also fostered China–US scientific collaboration. A little after 10.30 a.m., Yifang Wang from the Institute of High Energy Physics-Chinese Academy of Sciences [中国科学院](CN), pressed a red button that stopped the Daya Bay Reactor Neutrino Experiment from taking data. A few minutes later, covers were removed and four massive, cylindrical tanks appeared in a pool filled with highly purified water.

    “Today we are here to celebrate the completion of the Daya Bay Reactor Neutrino Experiment, which has fulfilled all its missions,” noted Jun Cao, co-spokesperson of the collaboration, during the ceremony. Only a small audience was present due to coronavirus restrictions, but 1.7 million people joined online to see the experiment come to an end. Among them was Kam-Biu Luk, a particle physicist from the University of California at Berkeley (US) and DOE’s Lawrence Berkeley National Laboratory (US) and the experiment’s US spokesperson, who watched the livestream from his home in California. “I’ve worked on a number of experiments in my life,” he told Physics World, “but Daya Bay has achieved so much that it is extremely rewarding. This is certainly a happy ending for all of us.”

    Early days

    Born in nuclear interactions, neutrinos are extremely light and hard to catch, yet they are everywhere around us. They come in three types – electron, muon and tau – that morph into each other as they travel near the speed of light. Thanks to large-scale neutrino detectors in Japan, the US, Canada and other countries, by the early 2000s physicists had a good idea about how electron neutrinos transform into muon and tau neutrinos (as in solar neutrino oscillation) and how muon neutrinos transform into tau neutrinos (as in atmospheric neutrino oscillation). However, the case of electron–muon oscillation – the last missing piece in the puzzle of neutrino oscillations and dictated by the parameter “theta-13” – remained unclear.

    Some scientists proposed using nuclear reactors to study this neutrino oscillation, since reactors are well-understood neutrino sources, and Luk realized it could be the best way to solve the theta-13 problem. He started searching for potential sites in Japan, South Korea and the US. Originally from Hong Kong, Luk also knew about the Daya Bay nuclear power plant and added it to his list.

    Daya Bay stood out in many ways, not least because the Daya Bay and Ling Ao reactors are powerful enough to produce a large number of antineutrinos. The site is also next to a mountain range, making the construction and shielding of cosmic rays much easier. Given that the most efficient scheme to infer theta-13 was to compare antineutrino events at the near and far sites, the team planned eight detectors, four placed between 300 and 500 m from the reactors – dubbed “near detectors” – and four positioned 2 km away.

    Daya Bay remains the largest collaboration between China and the US in basic research and has benefited scientists from both sides

    Luk opted for Daya Bay and in late 2003 contacted IHEP for potential collaboration. Despite a lack of neutrino researchers in China, Wang, who was leading the institute’s experimental department, knew that it was an opportunity not to be missed and began searching for funding and people. The idea also quickly won support from the US’s Department of Energy, which later contributed about one third of the total cost, with Cao among the first to join. Wrapping up his postdoctoral research in the US at DOE’s Fermi National Accelerator Laboratory (US), he immediately got down to basic design issues such as the shape of the detectors and the development of the liquid scintillator, which was done together with collaborators at DOE’s Brookhaven National Laboratory (US).

    Students also got involved in the project. They included Liangjian Wen, who was studying nuclear physics at the University of Science and Technology [中国科学技术大学] (CN) at Chinese Academy of Sciences [中国科学院](CN) in Hefei, and came to IHEP Beijing to work on his undergraduate project. Inexperienced with building detectors, he was asked to develop the reflecting panels placed at the top and bottom inside the detector, a technique never used in similar experiments before. “The panels can reflect photons to the side, so we got to use fewer photomultiplier tubes and save about 20 million yuan,” says Wen. Doing everything from scratch, Wen learned what materials to use for the supporting structure, how to apply the reflecting film between the panels, and how to assemble the panels with high precision. “We made it in the end,” he adds. “The reflecting panels gave the detectors a simpler structure and better performance.”

    Surprising findings

    Daya Bay began taking data on 24 December 2011, when only six of the eight detectors were in place. Researchers were quick to remove noise signals and identify something indicative from data collected within the first few days. Cao remembers vividly how they worked late into the night, had lots of meetings and used a variety of cross-checking methods to make sure the results were correct. Then on 8 March 2012 the collaboration announced its groundbreaking findings on theta-13 at a press conference in Beijing.

    Based on tens of thousands of antineutrino events observed, about 6% of the reactor’s antineutrinos transformed into other types of neutrinos on their way from the reactors to the far site. The transformation rate was surprisingly large, allowing Wang to announce the discovery of a new type of neutrino oscillation. For Cao, it was a wonderful surprise given it only took 55 days to get a definitive answer to the critically important theta-13 problem, the value of which turned out to be much larger than expected. In the eight years that followed, as the team collected and analysed more data, the measurement precision of theta-13 improved by sixfold to 3.4%, a milestone no other experiment is expected to surpass in the next 20 years.

    Besides theta-13, the experiment also made other important findings. For example, it strongly challenged the assumption that a fourth type of neutrino, the sterile neutrino, exists. Observations at the near detector clearly showed that the reactors gave off far fewer antineutrinos than predicted – potentially because some had morphed into sterile neutrinos. To clarify the case, the team made separate measurements on uranium and plutonium, two major reactor fuel components and antineutrino sources. They found that the modelling and observation matched well for plutonium but there was a major discrepancy with uranium. “This largely ruled out the theory of explaining the deficit with sterile neutrinos,” says Wang. “If sterile neutrinos did exist, they should have acted on plutonium and uranium the same way.”

    The Daya Bay legacy

    Daya Bay remains the largest collaboration between China and the US in basic research and has benefited scientists from both sides. For China, the neutrino research team has grown from a small number of people in the early 2000s to about 100 today. For the US, the Daya Bay experiment turned out to be much cheaper and quicker than if the US had done the experiment alone.

    Since the shutdown ceremony, the eight detectors have been taken apart, with some components such as the electronics being reused in the Jiangmen Underground Neutrino Observatory JUNO [地下天文台] (CN) – China’s next major neutrino experiment.

    Other parts have been donated to overseas experiments, including 32 tonnes of gadolinium-doped liquid scintillator and 50 tonnes of undoped liquid scintillator to a Japanese experiment called JSNS2.

    The rest of the experiment will be given to schools for educational or outreach use. The main laboratory hall, meanwhile, will be repurposed into an exhibition facility about the experiment. The team will also continue to analyse the complete dataset, which will take another year or two to complete.

    IHEP researchers are working hard to make sure that JUNO will be up and running by the end of 2022. It will seek to work out the mass ordering of different types of neutrinos, which will help other upcoming neutrino facilities such as the Deep Underground Neutrino Experiment in the US and the Hyper-Kamiokande neutrino observatory in Japan to examine their absolute values as well as possibly reveal why the universe is made up of matter instead of antimatter.

    “These questions will keep particle physicists fully occupied for a few decades from now,” says Luk.

    Researchers are also developing crucial technologies for a second phase of JUNO, which will conduct a neutrino-less double-beta decay experiment to study whether neutrinos are their own antiparticles and seek to measure the absolute masses of neutrinos. Yet Cao does not feel sorry to witness the end of Daya Bay. “On the contrary, we yearn for tomorrow,” he says, “to reveal more unknowns in neutrino physics with the new generation of experiments.”

    See the full article here .


    five-ways-keep-your-child-safe-school-shootings
    Please help promote STEM in your local schools.

    Stem Education Coalition

    PhysicsWorld(UK) is a publication of the Institute of Physics(UK). The Institute of Physics is a leading scientific society. We are a charitable organisation with a worldwide membership of more than 50,000, working together to advance physics education, research and application.

    We engage with policymakers and the general public to develop awareness and understanding of the value of physics and, through IOP Publishing, we are world leaders in professional scientific communications.
    IOP Institute of Physics

     
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