Tag Archives: #electrons

Catching Electrons in Action in an Antiferromagnetic Nanowire (Physics)

The electron is one of the fundamental particles in nature we read about in school. Its behavior holds clues to new ways to store digital data.

In a study published in Nano Letters, physicists from Michigan Technological University explore alternative materials to improve capacity and shrink the size of digital data storage technologies. Ranjit Pati, professor of physics at Michigan Tech, led the study and explains the physics behind his team’s new nanowire design.

“Thanks to a property called spin, electrons behave like tiny magnets,” Pati said. “Similar to how a bar magnet’s magnetization is dipolar, pointing from south to north, the electrons in a material have magnetic dipole moment vectors that describe the material’s magnetization.”

When these vectors are in random orientation, the material is nonmagnetic. When they are parallel to each other, it’s called ferromagnetism and antiparallel alignments are antiferromagnetism. Current data storage technology is based on ferromagnetic materials, where the data are stored in small ferromagnetic domains. This is why a strong enough magnet can mess up a mobile phone or other electronic storage.

Data Storage Challenges

Depending on the direction of magnetization (whether pointing up or down), data are recorded as bits (either a 1 or 0) in ferromagnetic domains. However, there are two bottlenecks, and both hinge on proximity. First, bring an external magnet too close, and its magnetic field could alter the direction of magnetic moments in the domain and damage the storage device. And, second, the domains each have a magnetic field of their own, so they can’t be too close to each other either. The challenge with smaller, more flexible, more versatile electronics is that they demand devices that make it harder to keep ferromagnetic domains safely apart.

“Ultrahigh-density data packing would be a daunting task with ferromagnetic memory domains,” Pati said. “Antiferromagnetic materials, on the other hand, are free from these issues.”

On their own antiferromagnetic materials aren’t great for electronic devices, but they’re not influenced by outside magnetic fields. This ability to resist magnetic manipulation started getting more attention from the research community and Pati’s team used a predictive quantum many-body theory that considers electron-electron interactions. The team found that chromium-doped nanowires with a germanium core and silicon shell can be an antiferromagnetic semiconductor.

Antiferromagnetism

Several research groups have recently demonstrated manipulation of individual magnetic states in antiferromagnetic materials using electrical current and lasers. They observed spin dynamics in the terahertz frequency — much faster than the frequency used in our current data storage devices. This observation has opened up a plethora of research interests in antiferromagnetism and could lead to faster, higher-capacity data storage.

“In our recent work, we have successfully harnessed the intriguing features of an antiferromagnet into a low-dimensional, complementary metal-oxide compatible semiconductor (CMOS) nanowire without destroying the semiconducting property of the nanowire,” Pati said. “This opens up possibilities for smaller and smarter electronics with higher capacity data storage and manipulation.”

Pati adds that the most exciting part of the research for his team was uncovering the mechanism that dictates antiferromagnetism. The mechanism is called superexchange and it controls the spin of electrons and the antiparallel alignment that makes them antiferromagnetic. In the team’s nanowire, germanium electrons act as a go-between, an exchanger, between unconnected chromium atoms. 

“The interaction between the magnetic states of the chromium atoms is mediated by the intermediate atoms they are bonded to. It is a cooperative magnetic phenomenon,” Pati said. “In a simple way, let us say there are two people A and B: They are far apart and cannot communicate directly. But A has a friend C and B has a friend D. C and D are close friends. So, A and B can interact indirectly through C and D.”

Better understanding how electrons communicate between atomic friends enables more experiments to test the potential of materials like chromium-doped nanowires. Better understanding the germanium-silicon nanowire material’s antiferromagnetic nature is what boosts potential for smaller, smarter, higher capacity electronics.

Featured image: Chromium-doped (purple) nanowires with a germanium (yellow atoms) core and silicon shell (yellow-red connections) can be an antiferromagnetic semiconductor. © MTU


Reference: Sandip Aryal, Durga Paudyal, and Ranjit Pati, “Cr-Doped Ge-Core/Si-Shell Nanowire: An Antiferromagnetic Semiconductor”, Nano Lett. 2021, 21, 4, 1856–1862. Doi: https://doi.org/10.1021/acs.nanolett.0c04971


Provided by Michigan Technological University


About Michigan Technological University

Michigan Technological University is a public research university, home to more than 7,000 students from 54 countries. Founded in 1885, the University offers more than 120 undergraduate and graduate degree programs in science and technology, engineering, forestry, business and economics, health professions, humanities, mathematics, and social sciences. Our campus in Michigan’s Upper Peninsula overlooks the Keweenaw Waterway and is just a few miles from Lake Superior.

New IceCube Detection Proves 60-year-old Theory (Physics)

On December 6, 2016, a high-energy particle called an electron antineutrino was hurtling through space at nearly the speed of light. Normally, the ghostly particle would zip right through the Earth as if it weren’t even there.

But this particle just so happened to smash into an electron deep inside the South Pole’s glacial ice. The collision created a new particle, known as the W boson. That boson quickly decayed, creating a shower of secondary particles.

The whole thing played out in front of the watchful detectors of a massive telescope buried in the Antarctic ice, the IceCube Neutrino Observatory. This enabled IceCube to make the first ever detection of a Glashow resonance event, a phenomenon predicted 60 years ago by Nobel laureate physicist Sheldon Glashow.

This detection provides the latest confirmation of the Standard Model, the name of the particle physics theory explaining the universe’s fundamental forces and particles.

“Finding it wasn’t necessarily a surprise, but that doesn’t mean I wasn’t very happy to see it,” said Claudio Kopper, an associate professor in Michigan State University’s Department of Physics and Astronomy in the College of Natural Science. Kopper and his departmental colleague, assistant professor Nathan Whitehorn, lead IceCube’s Diffuse and Atmospheric Flux Working Group behind the discovery.

The international IceCube Collaboration published this result online on March 11 in the journal Nature.

“Even three years ago, I didn’t think IceCube would be able to make this measurement, or at least as well as we did,” Whitehorn said.

This detection further demonstrates the ability of IceCube, which observes nearly massless particles called neutrinos using thousands of sensors embedded in the Antarctic ice, to do fundamental physics.

Although the Spartans lead the working group, they emphasized that this discovery was a team effort, powered by the paper’s three lead analysts: Lu Lu, an assistant professor at University of Wisconsin–Madison; Tianlu Yuan, an assistant scientist at the Wisconsin IceCube Particle Astrophysics Center, or WIPAC; and Christian Haack, a postdoc at the Technical University of Munich.

“We lead weekly meetings, we talk about how the work is done, we ask hard questions,” said Kopper. “But without the people doing the actual analysis, we wouldn’t have anything.”

“Our job is to be the doubters-in-chief,” Whitehorn said. “The lead authors did a great job convincing everyone that this event was a Glashow resonance.”

The particle physics community has been anticipating such a detection, but Glashow resonance events are extremely rare by nature and technologically challenging to detect.

“When Glashow was a postdoc at Niels Bohr, he could never have imagined that his unconventional proposal for producing the W boson would be realized by an antineutrino from a faraway galaxy crashing into Antarctic ice,” said Francis Halzen, professor of physics at the University of Wisconsin–Madison, the headquarters of IceCube maintenance and operations, and principal investigator of IceCube.

A Glashow resonance event requires an electron antineutrino with a cosmic amount of energy — at least 6.3 peta-electronvolts, or PeV. For comparison, that’s about 1,000 times more energy than that of the most energetic particles produced by the Earth’s most powerful particle accelerators.

Since IceCube started fully operating in 2011, it has detected hundreds of high-energy neutrinos from space. Yet the neutrino in December 2016 was only the third with an energy higher than 5 PeV.

And simply having a high-energy neutrino is not sufficient to detect a Glashow resonance event. The neutrino then has to interact with matter, which is not a guarantee. But IceCube encompasses quite a bit of matter in the form of Antarctic ice.

IceCube sits on the South Pole, waiting to see particles from the cosmos. Credit: Yuya Makino, IceCube/NSF

The observatory’s detector array has been built into the ice, spanning nearly 250 acres with sensors reaching up to about a mile deep. All told, IceCube boasts a cubic kilometer of coverage, watching over a billion metric tons of extremely clear ice.

That’s what it takes to detect neutrinos, along with a team of scientists who have the skill and determination to spot rare events.

IceCube’s more than 5,000 detectors take in a tremendous firehose of light, Whitehorn said. Detecting the Glashow resonance meant researchers had to pick out a handful of telltale photons, individual particles of light, from that firehose spray.

“This is some of the most impressive technical work I’ve ever seen,” Whitehorn said, calling the team unstoppable over the years-long effort to confirm this was a Glashow resonance event.

Making the work even more impressive was the fact that the lead authors — Lu, Yuan and Haack — were in three countries on three different continents during the analysis. Lu was a postdoc at Chiba University in Japan, Yuan was at WIPAC in the U.S. and Haack was a doctoral student at Rheinisch-Westfälische Technische Hochschule Aachen University in Germany.

“It was amazing to me just seeing that that is possible,” Kopper said.

But this is very much in keeping with the ethos of IceCube, an observatory built on international collaboration. IceCube is operated by a group of scientists, engineers and staff from 53 institutions in 12 countries, together known as the IceCube Collaboration. The project’s headquarters is WIPAC, a research center of UW–Madison in the United States.

Flags outside of IceCube represent the international collaboration of the project. Credit: Yuya Makino, IceCube/NSF

To confirm the detection and usher in a new chapter of neutrino astronomy, the IceCube Collaboration is working to detect more Glashow resonances. And they need IceCube-Gen2, a proposed expansion of the IceCube detector, to make it happen.

“We already know that the astrophysical spectrum does not end at 6 PeV,” Lu said. “The key is to detect more Glashow resonance events and to identify the sources that accelerate those antineutrinos. IceCube-Gen2 will be key to making such measurements in a statistically significant way.”

Glashow himself echoed that sentiment about validation. “To be absolutely sure, we should see another such event at the very same energy as the one that was seen,” said Glashow, now an emeritus professor of physics at Boston University. “So far there’s one, and someday there will be more.”

The IceCube Neutrino Observatory is funded primarily by the National Science Foundation and is operated by a team headquartered at the University of Wisconsin–Madison. IceCube’s research efforts, including critical contributions to the detector operation, are funded by agencies in Australia, Belgium, Canada, Denmark, Germany, Japan, New Zealand, Republic of Korea, Sweden, Switzerland, the United Kingdom, and the United States. IceCube construction was also funded with significant contributions from the National Fund for Scientific Research — the FNRS and FWO — in Belgium; the Federal Ministry of Education and Research and the German Research Foundation in Germany; the Knut and Alice Wallenberg Foundation, the Swedish Polar Research Secretariat, and the Swedish Research Council in Sweden; and the Department of Energy and the University of Wisconsin–Madison Research Fund in the U.S.

Featured image: A visualization of the Glashow resonance event detected by IceCube. The event was nicknamed “Hydrangea.” Credit: IceCube Collaboration


Reference: The IceCube Collaboration., Aartsen, M.G., Abbasi, R. et al. Detection of a particle shower at the Glashow resonance with IceCube. Nature 591, 220–224 (2021). DOI : https://dx.doi.org/10.1038/s41586-021-03256-1)


Provided by MSU

Terahertz Waves From Electrons Oscillating in Liquid Water (Physics)

Ionization of water molecules by light generates free electrons in liquid water. After generation, the so-called solvated electron is formed, a localized electron surrounded by a shell of water molecules. In the ultrafast localization process, the electron and its water shell display strong oscillations, giving rise to terahertz emission for tens of picoseconds.

Ionization of atoms and molecules by light is a basic physical process generating a negatively charged free electron and a positively charged parent ion. If one ionizes liquid water, the free electron undergoes a sequence of ultrafast processes by which it loses energy and eventually  localizes at a new site in the liquid, surrounded by a water shell [Fig. 1]. The localization process includes a reorientation of water molecules at the new site, a so-called solvation process, in order to minimize the electric interaction energy between the electron and the water dipole moments. The localized electron obeys the laws of quantum mechanics and displays discrete energy levels. Electron localization occurs in the subpicosecond time range (1 ps = 10-12 s = a millionth of a millionth of a second) and is followed by dissipation of excess energy into the liquid.

Researchers at the Max-Born-Institute have now observed radiation in the terahertz range  (1 THz = 1012 Hz = 1012 oscillations per second) which is initiated during the electron localization process. As they report in the recent issue of Physical Review Letters, Vol. 126, 097401 (2021), the THz emission can persist for up to 40 ps, i.e., much longer than the localization process itself. It displays a frequency between 0.2 and 1.5 THz, depending on the electron concentration in the liquid.

The emitted THz waves originate from oscillations of the solvated electrons and their water shells. The oscillation frequency is determined by the local electric field the liquid environment exerts on this quantum system. Adding hydrated electrons to the liquid changes the local field and, thus, induces a change of oscillation frequency with electron concentration. Most surprising is the comparably weak damping of the oscillations which points to a weak interaction with the fluctuating larger environment in the liquid and a longitudinal character of the underlying electron and water motions.

The new experimental results are accounted for by a theoretical model based on a polaron picture as explained in Fig. 1. The polaron is an excitation which includes coupled motions of the electron and the water shell at low frequency. Due to such internal oscillations of charge, the hydrated electron radiates a THz wave. The weak damping of this wave allows for a manipulation of the emission, e.g., by interaction of the hydrated electron with a sequence of ultrashort light pulses.

Featured image: Cartoon of an oscillating polaron in liquid water: (a) Schematic network of hydrogen-bonded water molecules of neat water (red: oxygen atoms, green: hydrogen atoms). (b) Electron solvated in water (yellow-red cloud). The electron attracts the hydrogen atoms of water molecules, thereby polarizing its environment of water molecules and generating a self-consistent potential trap for the electron. The electron solvated this way represents an elementary quantum system (c) A possible elementary excitation is a combined motion of the electron and the water shell, a so-called polaron. The polaron can be connected with an oscillation of the size of the quantum system (panels (b) and (c)), changing the strength of the overall electric polarization originating from the water molecules. (d) The oscillating electric polarization emits an electric field Eosc(τ) which is plotted as a function of time τ and represents the quantity observed experimentally.


Original publication

A. GhalgaouiB. P. FingerhutK. ReimannT. ElsaesserM. Woerner, “Terahertz Polaron Oscillations of Electrons Solvated in Liquid Water”, Phys. Rev. Lett. 126, 097401 (2021) (Editor’s suggestion) URL, DOI or PDF


Provided by MBI Berlin

Ultrafast Electron Dynamics in Space and Time (Quantum)

“For decades, chemistry has been governed by two ambitions goals,” says Professor Stefan Tautz, head of the Quantum Nanoscience subinstitute at Forschungszentrum Jülich. “One of these is understanding chemical reactions directly from the spatial distribution of electrons in molecules, while the other is tracing electron dynamics over time during a chemical reaction.” Both of these goals have been achieved in separate ground-breaking discoveries in chemistry: frontier molecular orbital theory explained the role of the electron distribution in molecules during chemical reactions, while femtosecond spectroscopy made it possible to observe transition states in reactions. “It has long been a dream of physical chemistry to combine these two developments and to then trace electrons in a chemical reaction in time and space.”

The scientists have now come a huge step closer to achieving this goal: they observed electron transfer processes at a metal–molecule interface in space and time. Such interfaces are the focus of research in the German Research Foundation’s Collaborative Research Centre 1083 at Philipps-Universität Marburg, and it was experiments conducted here that lead to today’s publication. “Interfaces initially appear to be no more than two layers side by side, whereas they are in fact the place where the functions of materials come into being. They therefore play a decisive role in technological applications,” says Ulrich Höfer, professor of experimental physics at Philipps-Universität Marburg and collaborative research centre spokesman. In organic solar cells, for example, combining different materials at an interface improves the splitting of the states excited by incident light, thus allowing electricity to flow. Interfaces also play a key role in organic light-emitting diode (OLED) displays used in smartphones, for example.

The experimental approach used by the scientists is based on a breakthrough made a few years ago in molecular spectroscopy: photoemission orbital tomography, which itself is based on the well-known photoelectric effect. “Here, a layer of molecules on a metal surface is bombarded with photons, or particles of light, which excites the electrons and causes them to be released,” says Professor Peter Puschnig from the University of Graz. “These released electrons do not simply fly around in space, but – and this is the decisive point – based on their angular distribution and energy distribution, they provide a good indication of the spatial distribution of electrons in molecular orbitals.”

“The key result of our work is that we can image the orbital tomograms with ultrahigh resolution over time,” says Dr. Robert Wallauer, group leader and research assistant at Philipps-Universität Marburg. To do so, the scientists not only used special lasers with ultrashort pulses in the femtosecond range to excite the electrons in the molecules; they also used a novel impulse microscope which simultaneously measured the direction and energy of the electrons released with very high sensitivity. One femtosecond is 10-15 seconds – a millionth of a billionth of a second. In relation to a second, this is as little as a second in relation to 32 million years. Such short pulses are like a kind of strobe light and can be used to break down fast processes into individual images. This enabled the researchers to trace the electron transfer as if in slow motion. “This allowed us to spatially trace the electron excitation pathways almost in real time,” says Tautz. “In our experiment, an electron was first excited from its initial state into an unoccupied molecular orbital by a first laser pulse before a second laser pulse enabled it to finally reach the detector. Not only could we observe this process in detail over time, but the tomograms also allowed us to clearly trace where the electrons came from.”

“We believe that our findings represent a crucial breakthrough towards the goal of tracing electrons through chemical reactions in space and time,” says Ulrich Höfer. “In addition to the fundamental insights into chemical reactions and electron transfer processes, these findings will also have very practical implications. They open up countless possibilities for the optimization of interfaces and nanostructures and the resulting processors, sensors, displays, organic solar cells, catalysts, and potentially even applications and technologies we haven’t even thought of yet.”

Featured image: The scientists tracked the orbital tomograms with ultrahigh resolution through time. For this purpose, the electrons in the molecules were excited into a different orbital with femtosecond laser pulses. © Philipps-Universität Marburg / Till Schürmann


Original publication: Tracing orbital images on ultrafast time scales, by R. Wallauer, M. Raths, K. Stallberg, L. Münster, D. Brandstetter, X. Yang, J. Güdde, P. Puschnig, S. Soubatch, C. Kumpf, F. C. Bocquet, F. S. Tautz, U. Höfer, Science (first release, publishes online 18 February 2018), DOI: 10.1126/science.abf3286


Provided by FZ-Juelich

How Do Electrons Close to Earth Reach Almost the Speed of Light? (Planetary Science)

New study found that electrons can reach ultra-relativistic energies for very special conditions in the magnetosphere when space is devoid of plasma.

Recent measurements from NASA’s Van Allen Probes spacecraft showed that electrons can reach ultra-relativistic energies flying at almost the speed of light. Hayley Allison, Yuri Shprits and collaborators from the German Research Centre for Geosciences have revealed under which conditions such strong accelerations occur. They had already demonstrated in 2020 that during solar storm plasma waves play a crucial role for that. However, it was previously unclear why such high electron energies are not achieved in all solar storms. In the journal Science Advances, Allison, Shprits and colleagues now show that extreme depletions of the background plasma density are crucial.

Ultra-relativistic electrons in space

At ultra-relativistic energies, electrons move at almost the speed of light. Then the laws of relativity become most important. The mass of the particles increases by a factor ten, time is slowing down, and distance decreases. With such high energies, charged particles become most dangerous to even the best protected satellites. As almost no shielding can stop them, their charge can destroy sensitive electronics. Predicting their occurrence – for example, as part of the observations of space weather practised at the GFZ – is therefore very important for modern infrastructure.

To investigate the conditions for the enormous accelerations of the electrons, Allison and Shprits used data from a twin mission, the “Van Allen Probes”, which the US space agency NASA had launched in 2012. The aim was to make detailed measurements in the radiation belt, the so-called Van Allen belt, which surrounds the Earth in a donut shape in terrestrial space. Here – as in the rest of space – a mixture of positively and negatively charged particles forms a so-called plasma. Plasma waves can be understood as fluctuations of the electric and magnetic field, excited by solar storms. They are an important driving force for the acceleration of electrons.

Data analysis with machine learning

During the mission, both solar storms that produced ultra-relativistic electrons and storms without this effect were observed. The density of the background plasma turned out to be a decisive factor for the strong acceleration: electrons with the ultra-relativistic energies were only observed to increase when the plasma density dropped to very low values of only about ten particles per cubic centimetre, while normally such density is five to ten times higher.

Using a numerical model that incorporated such extreme plasma depletion, the authors showed that periods of low density create preferential conditions for the acceleration of electrons – from an initial few hundred thousand to more than seven million electron volts. To analyse the data from the Van Allen probes, the researchers used machine learning methods, the development of which was funded by the GEO.X network. They enabled the authors to infer the total plasma density from the measured fluctuations of electric and magnetic field.

The crucial role of plasma

“This study shows that electrons in the Earth’s radiation belt can be promptly accelerated locally to ultra-relativistic energies, if the conditions of the plasma environment – plasma waves and temporarily low plasma density – are right. The particles can be regarded as surfing on plasma waves. In regions of extremely low plasma density they can just take a lot of energy from plasma waves. Similar mechanisms may be at work in the magnetospheres of the outer planets such as Jupiter or Saturn and in other astrophysical objects”, says Yuri Shprits, head of the GFZ section Space physics and space weather and Professor at University of Potsdam.

“Thus, to reach such extreme energies, a two-stage acceleration process is not needed, as long assumed – first from the outer region of the magnetosphere into the belt and then inside. This also supports our research results from last year,” adds Hayley Allison, PostDoc in the Section Space physics and space weather.

Featured image: The contours in color show the intensities of the radiation belts. Grey lines show the trajectories of the relativistic electrons in the radiation belts. Concentric circular lines show the trajectory of scientific satellites traversing this dangerous region in space. © Ingo Michaelis and Yuri Shprits, GFZ


Reference: Hayley J. Allison, Yuri Y. Shprits, Irina S. Zhelavskaya, Dedong Wang and Artem G. Smirnov, “Gyroresonant wave-particle interactions with chorus waves during extreme depletions of plasma density in the Van Allen radiation belts”, Science Advances 29 Jan 2021: Vol. 7, no. 5, eabc0380
DOI: 10.1126/sciadv.abc0380 https://advances.sciencemag.org/content/7/5/eabc0380


Provided by University of Potsdam

Navigation by Atom – Coming to a Vehicle Near You (Physics)

Instruments normally found in physics labs are making their way into everyday applications. Institute scientists have greatly expanded these instruments’ capabilities.

Satellite-based GPS is a wondrous invention, but even GPS has its limitations – such as in navigating underwater or when one of the orbiting satellites malfunctions. GPS, like atlases and paper maps, works on parameters of distance and height, but there are other ways to measure locations and routes – for example, by charting the Earth’s gravitational field or, in the near future, by basing very small guidance systems on the trajectories of atoms. Such atomic systems could be used for navigation without GPS.

These atomic systems, known as cold-atom interferometers, work on a well-known quantum principle. Quantum particles like photons or electrons can act as waves; as waves they split into two in a device like an interferometer and then recombine, the patterns of interference between the two waves revealing information about that particle’s trajectory. Creating interferometers that work with atomic matter waves is significantly more challenging than with light, because to perform properly as quantum particles, atoms must be cooled to temperatures of just a few millionths of a degree above absolute zero. Nonetheless, researchers in several leading labs around the globe have been developing the technology, in part as a highly accurate means of navigation. The method can also measure slight changes in gravitational forces acting on the atom.

A cloud of cold atoms filmed at 0.1-second intervals in free-fall. The atoms’ motion is parabolic, due to gravity, while the cloud expands radially due to its finite (non-zero) temperature. As they fall, they are interrogated by three laser pulses, at the beginning, middle and end of their trajectory © Weizmann Institute of Science

Weizmann Institute of Science researchers recently demonstrated a way to make this technology even more useful, by greatly extending the range of the measurements it can perform. Chen Avinadav and Dr. Dimitry Yankelev in the groups of Dr. Ofer Firstenberg and Prof. Nir Davidson, all in the Physics of Complex Systems Department, led this research.

Interferometers are generally highly sensitive, but they are but limited in range. Measuring with an instrument like an interferometer has the same limitations as weighing something on a mechanical scale: There is always a trade-off between the range of measurement and the sensitivity. Your bathroom scale, for example, has a range of around 100 kilograms, but it cannot tell if your weight has changed by a few grams. Your kitchen scale, on the other hand, has a range of around a kilogram but is sensitive down to a gram or less.

(l-r) Prof. Nir Davidson, Chen Avinadav, Dr. Ofer Firstenberg and Dr. Dimitry Yankelev © WIS

The Institute researchers found a way around this limitation for atomic interferometers by performing several measurements at slightly different scales. When these are combined, the interfering waves generate a so-called “moire” pattern, which extends the range of the measurement many times over. In the present study, the research team succeeded in building on their previous improvements in obtaining moire measurements, as well as reducing the noise inherent in the process. They developed a system based on simultaneous measurements of two scales in interferometer setups that were identical in all other ways.

The range of measurement could be extended up to a thousand times over, with almost no sacrifice in the sensitivity.

When two or three such measurements were taken in quick succession, the range of measurement could be extended up to a thousand times over, with almost no sacrifice in the sensitivity of those measurements. And this might be only the beginning, says Firstenberg. The two labs are planning future experiments with even more scales, to see if it is possible to extend the range even further.

Such atomic interferometers are touted for two main applications: inertial navigation, which works by dead reckoning from a starting point, and gravitational mapping which, among other things, can assist in the search for natural resources. The new technique could represent a significant advance in the efforts to create everyday applications in the field.

The moire effect: two nearly-identical fine scales provide for a high measurement sensitivity, while the coarse scale generated in their overlap increases the measurement’s range © WIS

Prof. Nir Davidson is Head of the André Deloro Institute for Advanced Research in Space and Optics; and Head of the Center for Experimental Physics. His research is also supported by the Veronika A. Rabl Physics Discretionary Fund; Dana and Yossie Hollander; the Norman E. Alexander Family M Foundation; and Paul and Tina Gardner. Prof. Davidson is the incumbent of the Peter and Carola Kleeman Professorial Chair of Optical Sciences.  Dr. Ofer Firstenberg’s research is supported by the Laboratory in Memory of Leon and Blacky Broder.


Reference: Dimitry Yankelev, Chen Avinadav, Nir Davidson, Ofer Firstenberg, “Atom interferometry with thousand-fold increase in dynamic range”, Science Advances  04 Nov 2020: Vol. 6, no. 45, eabd0650 DOI: 10.1126/sciadv.abd0650 https://advances.sciencemag.org/content/6/45/eabd0650


Provided by Weizmann Institute of Science

Clocking the Movement of Electrons Inside an Atom (Physics)

New technique delivers resolution improvement in ultrafast processes.

Ultrafast science is pursued at the Technical University of Munich (TUM). An international consortium of scientists, initiated by Reinhard Kienberger, Professor of Laser and X-ray Physics several years ago, has made significant measurements in the femtosecond range at the U.S. Stanford Linear Accelerator Center (SLAC).

The inherent delay between the emission of the two types of electron leads to a characteristic ellipse in the analysed data. In principle, the position of individual data points around the ellipse can be read like the hands of a clock to reveal the precise timing of the dynamical processes. © Daniel Haynes / Jörg Harms

X-ray free-electron lasers (XFELs) have delivered intense, ultrashort X-ray pulses in the femtosecond range for over a decade. A femtosecond is equivalent to a millionth of a billionth of a second.

One of the most promising applications of XFELs is in biology, where researchers can capture images down to the atomic scale even before the radiation damage destroys the sample. In physics and chemistry, these X-rays can also shed light on the fastest processes occurring in nature with a shutter speed lasting only one femtosecond.

Measurements on miniscule timescales are particularly difficult

However, on these miniscule timescales, it is extremely difficult to synchronize the X-ray pulse that sparks a reaction in the sample on the one hand and the laser pulse which ‘observes’ it on the other. This problem is called timing jitter, and it is a major hurdle in ongoing efforts to perform time-resolved experiments at XFELs with ever-shorter resolution.

Now, a large international research team has developed a method to get around this problem at XFELs and demonstrated its efficacy by measuring a fundamental decay process in neon gas.

Electrons accelerated by SLAC’s linear accelerator ender the LCLS undulator hall and run a gauntlet of 32 powerful undulators. Each undulator contains 224 magnets whose alternating poles force the electrons to zigzag violently and radiate X-rays. By the time they leave the undulator hall, the X-ray laser pulses are a billion times brighter than beams from traditional synchrotron X-ray sources, opening a new realm of possible experiments and discoveries. Image: Christopher Smith/SLAC National Accelerator Laboratory

Good timing can avoid radiation damage

Many biological systems – and some non-biological ones – suffer damage when they are excited by an X-ray pulse from an XFEL. One of the causes of damage is the process known as Auger decay. The X-ray pulse ejects photoelectrons from the sample, leading to their replacement by electrons in outer shells. As these outer electrons relax, they release energy which can later induce the emission of another electron, known as an Auger electron.

Radiation damage is caused by both the intense X-rays and the continued emission of Auger electrons, which can rapidly degrade the sample. Timing this decay would help to evade radiation damage in experiments studying different molecules. In addition, Auger decay is a key parameter in studies of exotic, highly excited states of matter, which can only be investigated at XFELs.

Research team delivers pioneering and highly accurate approach

To chart Auger decay the scientists used a technique dubbed self-referenced attosecond streaking, which is based on mapping the electrons in thousands of images and deducing when they were emitted based on global trends in the data.

For the first application of their method, the team used neon gas, where the decay timings have been inferred in the past. After exposing both photoelectrons and Auger electrons to an external ‘streaking’ laser pulse, the researchers determined their final kinetic energy in each of tens of thousands of individual measurements.

“Crucially, in each measurement, the Auger electrons always interact with the streaking laser pulse slightly later than the photoelectrons displaced initially, because they are emitted later,“ says Prof. Reinhard Kienberger, who helped to develop the experiment’s design. “This constant factor forms the foundation of the technique.” By combining so many individual observations, the team was able to construct a detailed map of the physical process, and thereby determine the characteristic time delay between the photo- and Auger emission.

Streaking method leads to success

The required high time resolution is made possible by the so-called streaking method. “This technique is successfully applied in our laboratory. In several preliminary papers of our group, we have performed time-resolved measurements on free-electron lasers using the streaking method,” says TUM PhD student Albert Schletter, co-author of the publication. “Using this method, we were able to measure the delay between X-ray ionization and Auger emission in neon gases with the highest precision,” explains lead author Dan Haynes of Hamburg’s Max Planck Institute for the Structure and Dynamics of Matter.

The researchers are hopeful that self-referenced streaking will have a broader impact in the field of ultrafast science. “Self-referenced streaking may facilitate a new class of experiments benefitting from the flexibility and extreme intensity of XFELs without compromising on time resolution,” adds co-author Markus Wurzer, who is a PhD student of Prof. Kienberger.

Publications:

D. C. Haynes, M. Wurzer, A. Schletter, A. Al-Haddad, C. Blaga, C. Bostedt, J. Bozek10, M. Bucher, A. Camper, S. Carron, R. Coffee, J. T. Costello, L. F. DiMauro, Y. Ding, K. Ferguson, I. Grguraš, W. Helml, M. C. Hoffmann, M. Ilchen, S. Jalas, N. M. Kabachnik, A. K. Kazansky, R. Kienberger4 A. R. Maier, T. Maxwell, T. Mazza, M. Meyer, H. Park, J. Robinson, C. Roedig, H. Schlarb, R. Singla, F. Tellkamp, K. Zhang, G. Doumy, C. Behrens, A. L. Cavalieri:
Clocking Auger electrons. In: Nature Physics. https://www.nature.com/articles/s41567-020-01111-0
DOI: 10.1038/s41567-020-01111-0

Provided by Technical University of Munich

Yes to Breaking the Speed-of-Light Barrier (Physics / Quantum)

It could happen if the tunnel is long enough, but the chances are basically zero.

How much time does it take to send a package from New York to Tel Aviv, and how does that compare with sending an email from one side of the Weizmann Institute of Science campus to the other? Now shrink the package down to the size of one of the electrons making up the email and put up an impenetrable barrier over the ocean. The “package” electron could make the crossing faster, even breaking the light “speed limit.”  Prof. Eli Pollak, together with postdoctoral fellow Dr. Tom Rivlin, both of the Weizmann Institute’s Chemical and Biological Physics Department, and Prof. Randall Dumont of McMaster University in Canada, recently provided theoretical support for this idea.

(l-r) Prof. Eli Pollak and Dr. Tom Rivlin © Weizmann Institute of Science

Impenetrable barriers are integral to one of the more fascinating quantum phenomena: tunneling. For around ninety years, researchers have been studying the way that quantum particles are able to pass through such barriers. In the 1960s, Thomas Hartman added a strange twist to tunneling: He showed that tunneling takes a fixed amount of time, no matter what length “tunnel” the particle transverses. That means that a particle could speed up while tunneling, and to make its goal in that fixed amount of time, it might even surpass the speed of light – if the tunnel is long enough or the barrier thick enough. Of course, this idea does not fit with the precepts of relativity – neither special nor general relativity – which are quite strict in insisting that particles cannot exceed the speed of light. Still, most researchers were not overly concerned with the results of this study, since it was already known that quantum mechanics and the physics of relativity do not jibe in many ways.

Therefore, so the thinking has been, this is another apparent anomaly that will work itself out once we figure out how to reconcile relativity with quantum mechanics. Pollak and his colleagues recently developed a new calculation of Hartman’s idea, basing it on equations for quantum behavior first developed by Paul Dirac, which enable one to perform quantum mechanical calculations that are consistent with special relativity.

Their results support the scenario in which a particle is traveling in a tunnel and the timing of the process is a constant, independent of the length of the tunnel (or the thickness of the barrier). Theoretically, if the barrier is very long, the particle may reach the end of the tunnel faster than if it had just flown to the same destination in open space with no barrier in the way. And if the particle normally travels near the speed of light, then a tunneling particle would be able to get there faster than the speed of light.

Back to those emails: Could this finding enable us to send our information faster than the speed of light? The answer, to the likely disappointment of some and relief of others, is no. Prof. Pollak: “The chances of a particular particle tunneling are quite small, and those chances decrease exponentially as the length of the tunnel or thickness of the barrier increases. So the odds of the particle carrying our information making that faster-than-light trip through the tunnel are basically zero. For now, we’ll have to content ourselves with the speeds of the existing options for sending packages and information, even if they do move slower than the speed of light.”

Reference: Randall S Dumont, Tom Rivlin and Eli Pollak, “The relativistic tunneling flight time may be superluminal, but it does not imply superluminal signaling”, New Journal of Physics, Volume 22, September 2020. https://iopscience.iop.org/article/10.1088/1367-2630/abb515/meta

Provided by Weizmann Institute of Science

Entangling Electrons With Heat (Quantum)

Quantum entanglement is key for next-generation computing and communications technology, Aalto researchers can now produce it using temperature differences.

A joint group of scientists from Finland, Russia, China and the USA have demonstrated that temperature difference can be used to entangle pairs of electrons in superconducting structures. The experimental discovery, published in Nature Communications, promises powerful applications in quantum devices, bringing us one step closer towards applications of the second quantum revolution.

False-colour electron microscope image of the sample, the green layers are graphene on top of the grey superconductor. The blue metal electrodes are used to extract the entangled electrons © Aalto University

The team, led by Professor Pertti Hakonen from Aalto University, has shown that the thermoelectric effect provides a new method for producing entangled electrons in a new device. “Quantum entanglement is the cornerstone of the novel quantum technologies. This concept, however, has puzzled many physicists over the years, including Albert Einstein who worried a lot about the spooky interaction at a distance that it causes”, says Prof. Hakonen.

In quantum computing, entanglement is used to fuse individual quantum systems into one, which exponentially increases their total computational capacity. “Entanglement can also be used in quantum cryptography, enabling the secure exchange of information over long distances”, explains Prof. Gordey Lesovik, from the Moscow Institute of Physics and Technology, who has acted several times as a visiting professor at Aalto University School of Science. Given the significance of entanglement to quantum technology, the ability to create entanglement easily and controllably is an important goal for researchers.

The researchers designed a device where a superconductor was layered withed graphene and metal electrodes. “Superconductivity is caused by entangled pairs of electrons called “cooper pairs”. Using a temperature difference, we cause them to split, with each electron then moving to different normal metal electrode,” explains doctoral candidate Nikita Kirsanov, from Aalto University. “The resulting electrons remain entangled despite being separated for quite long distances.”

Along with the practical implications, the work has significant fundamental importance. The experiment has shown that the process of Cooper pair splitting works as a mechanism for turning temperature difference into correlated electrical signals in superconducting structures. The developed experimental scheme may also become a platform for original quantum thermodynamical experiments.

The work was carried out using the OtaNano research infrastructure. OtaNano provides state-of-the-art working environment and equipment for nanoscience and -technology, and quantum technologies research in Finland. OtaNano is operated by Aalto University and VTT, and is available for academic and commercial users internationally. To find out more, visit their website. The work was supported by funded from QTF (Academy of Finland CoE). Gordey Lesovik’s visiting professorship funding came fro Aalto University School of Science and Zhenbing Tan’s post doctoral grant came from the Academy of Finland.

Reference: Tan, Z.B., Laitinen, A., Kirsanov, N.S. et al. Thermoelectric current in a graphene Cooper pair splitter. Nat Commun 12, 138 (2021). https://www.nature.com/articles/s41467-020-20476-7 https://doi.org/10.1038/s41467-020-20476-7

Provided by Aalto University