Physics News and Discussions

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New theory for detection of terahertz electromagnetic waves gives hope for advances in IT and medicine
https://phys.org/news/2022-09-theory-te ... icine.html
by Michael Hallermayer, Universität Augsburg
Detecting electromagnetic waves in the terahertz frequency range remains a challenging problem. Researchers from the University of Cambridge, together with physicists from the University of Augsburg, have recently discovered a new physical effect which could change that. In a new study, the scientists are now developing a theory explaining the mechanism behind it. Their findings make it possible to construct small, inexpensive, and highly sensitive terahertz detectors. These could be used, for example, in medical diagnostics, for contactless security checks, or for faster wireless data transmission. The results of the new theory have been published in the journal Physical Review B.

When X-rays or UV rays fall on a metallic surface, they knock electrons out of the material. This "photoelectric effect" can form the basis for detectors that detect the presence of electromagnetic waves.
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Physicists discover new rule for orbital formation in chemical reactions
https://phys.org/news/2022-09-physicist ... tions.html
by Forschungszentrum Juelich
Squeaky, cloudy or spherical—electron orbitals show where and how electrons move around atomic nuclei and molecules. In modern chemistry and physics, they have proven to be a useful model for quantum mechanical description and prediction of chemical reactions. Only if the orbitals match in space and energy can they be combined—this is what happens when two substances react with each other chemically. In addition, there is another condition that must be met, as researchers at Forschungszentrum Jülich and the University of Graz have now discovered: The course of chemical reactions also appears to be dependent on the orbital distribution in momentum space. The results were published in the journal Nature Communications.

Chemical reactions are ultimately nothing more than the formation and breakdown of electron bonds, which can also be described as orbitals. The so-called molecular orbital theory thus makes it possible to predict the path of chemical reactions. Chemists Kenichi Fukui and Roald Hoffmann received the Nobel Prize in 1981 for greatly simplifying the method, which led to its widespread use and application.

"Usually, the energy and location of electrons are analyzed. However, using the photoemission tomography method, we looked at the momentum distribution of the orbitals," explains Dr. Serguei Soubatch. Together with his colleagues at the Peter Grünberg Institute (PGI-3) in Jülich and the University of Graz in Austria, he adsorbed various types of molecules on metal surfaces in a series of experiments and mapped the measured momentum in the so-called momentum space.
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Researchers succeed in coupling two types of electron-hole pairs
https://phys.org/news/2022-09-coupling- ... pairs.html
by University of Basel
Two-dimensional van der Waals materials have been the focus of work by numerous research groups for some time. Standing just a few atomic layers thick, these structures are produced in the laboratory by combining atom-thick layers of different materials (in a process referred to as "atomic Lego"). Interactions between the stacked layers allow the heterostructures to exhibit properties that the individual constituents lack.

Two-layered molybdenum disulfide is one such van der Waals material, in which electrons can be excited using a suitable experimental setup. These negatively charged particles then leave their position in the valence band, leaving behind a positively charged hole, and enter the conduction band. Given the different charges of electrons and holes, the two are attracted to one another and form what is known as a quasiparticle. The latter is also referred to as an electron-hole pair, or exciton, and can move freely within the material.

In two-layered molybdenum disulfide, excitation with light produces two different types of electron-hole pairs: intralayer pairs, in which the electron and hole are localized in the same layer of the material, and interlayer pairs, whose hole and electron are located in different layers and are therefore spatially separate from one another.
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Theoretical physicists argue that black holes admit vortex structures
https://phys.org/news/2022-09-theoretic ... ortex.html
by Ingrid Fadelli , Phys.org
Black holes are astronomical objects with extremely strong gravitational pulls from which not even light can escape. While the idea of bodies that would trap light has been around since the 18th century, the first direct observation of black holes took place in 2015.

Since then, physicists have conducted countless theoretical and experimental studies aimed at better understanding these fascinating cosmological objects. This had led to many discoveries and theories about the unique characteristics, properties, and dynamics of black holes.

Researchers at Ludwig-Maximilians-Universität and Max-Planck-Institut für Physik have recently carried out a theoretical study exploring the possible existence of vortices in black holes. Their paper, published in Physical Review Letters, shows that black holes should theoretically be able to admit vortex structures.

"Recently, a new quantum framework for black holes, namely in terms of Bose-Einstein condensates of gravitons (the quanta of gravity itself), has been introduced," Florian Kühnel, one of the researchers who carried out the study, told Phys.org. "Up until our article was published, rotating black holes have not been thoroughly studied within this framework. However, they might not only exist, but also be the rule rather than the exception."
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MICROSCOPE mission presents most precise test of general relativity's Weak Equivalence Principle
https://phys.org/news/2022-09-microscop ... -weak.html
by American Physical Society

In new studies published in Physical Review Letters and a special issue of Classical and Quantum Gravity on September 14, a team of researchers present the most precise test yet of the Weak Equivalence Principle, a key component of the theory of general relativity. The report describes the final results from the MICROSCOPE mission, which tested the principle by measuring accelerations of free-falling objects in a satellite orbiting Earth. The team found that the accelerations of pairs of objects differed by no more than about one part in 1015 ruling out any violations of the Weak Equivalence Principle or deviations from the current understanding of general relativity at that level.

"We have new and much better constraints for any future theory, because these theories must not violate the equivalence principle at this level," says Gilles Métris, a scientist at Côte d'Azur Observatory and member of the MICROSCOPE team.
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University of Michigan's ZEUS will be most powerful laser in US
https://phys.org/news/2022-09-universit ... laser.html
by MIKE HOUSEHOLDER

A newly constructed University of Michigan facility that will be home to the most powerful laser in the United States is hosting its first experiment this week as the nation seeks to become competitive again in the realm of high-power laser facilities.

The experiment will be conducted at ZEUS—short for Zettawatt-Equivalent Ultrashort pulse laser System—by researchers from the University of California, Irvine. They traveled to Ann Arbor as part of their study of extremely intense interactions of light and matter, and how such interactions can be harnessed to shrink particle accelerators.

At the height of its power, ZEUS will be a 3-petawatt laser.

Three petawatts is "3 with 15 zeroes after it," said Louise Willingale, an associate professor of electrical engineering and computer science at Michigan.

And "3 petawatts is 3,000 times more powerful than the U.S. power grid," she said.

Michigan was awarded $18.5 million by the National Science Foundation to establish ZEUS as a federally funded international user facility.
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New multi-channel visible light communication system uses single optical path
https://phys.org/news/2022-09-multi-cha ... -path.html
by Optica
Researchers have demonstrated a new visible light communication system that uses a single optical path to create a multi-channel communication link over the air. This approach could be used as a backup communication link or for connecting Internet of Things devices.

"Today's free-space optical communication systems typically use two separate links with separate optical paths to establish two channels," said research team leader Yongjin Wang from Nanjing University of Posts and Telecommunications in China. "This new communication mode can save half the channel space, cost and power by using a single link."

The researchers describe their new approach in the journal Optics Letters. It is based on devices called multiple quantum well (MQW) III-nitride diodes that can emit and detect light at the same time.

"This technique could enable light-based communication functions to be highly integrated onto a chip, which could also be used to reduce the size of circuit boards, making them cheaper and more portable," said Wang. "Eventually we would like to develop a photonic CPU based on this communication mode."
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Feeling out of equilibrium in a dual geometric world: A novel theory for nonlinear dissipative phenomena
https://phys.org/news/2022-09-equilibri ... heory.html
by University of Tokyo
Losing energy is rarely a good thing, but now, researchers in Japan have shown how to extend the applicability of thermodynamics to systems that are not in equilibrium. By encoding the energy dissipation relationships in a geometric way, they were able to cast the physical constraints in a generalized geometric space. This work may significantly improve our understanding of chemical reaction networks, including those that underlie the metabolism and growth of living organisms.

Thermodynamics is the branch of physics dealing with the processes by which energy is transferred between entities. Its predictions are crucial for both chemistry and biology when determining if certain chemical reactions, or interconnected networks of reactions, will proceed spontaneously. However, while thermodynamics tries to establish a general description of macroscopic systems, often we encounter difficulties in working on the system out of equilibrium. Successful attempts to extend the framework to nonequilibrium situations have usually been limited only to specific systems and models.
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Picotesla magnetometry of microwave fields with diamond sensors
https://phys.org/news/2022-09-picotesla ... amond.html
by Thamarasee Jeewandara , Phys.org

Microwave field sensors are important in practice for a variety of applications across astronomy and communication engineering. The nitrogen vacancy center in diamond allows magnetometric sensitivity, stability and compatibility with ambient conditions. Despite that, the existing nitrogen vacancy center-based magnetometers have limited sensitivity in the microwave band.

In a new report now published in Science Advances, Zeching Wang and a team of scientists at the University of Science and Technology of China, presented a continuous, heterodyne detection scheme to improve the sensor's response to weak microwaves in the absence of spin controls. The team achieved a sensitivity of 8.9 pTHz-1/2 for microwaves via an ensemble of nitrogen vacancy centers within a specified sensor volume. The work can benefit practical applications of diamond-based microwave sensors.

Advanced applications of microwave sensing

The sensitivity of most modern applications that range from wireless communication to electron paramagnetic resonance and astronomical observations can be improved via advances in microfield detection methods. Researchers have already developed a variety of quantum sensors in the past decade with enhanced capabilities. Among them, the nitrogen vacancy center is identified by its unique properties for on-chip detection, although it suffers from relatively low sensitivity. Scientists can use nitrogen vacancy ensembles to substantially improve the sensitivity of the diamond magnetometer.

In this work, Wang and others proposed a continuous heterodyne detection scheme to improve the sensor's response to weak microwave fields by introducing a moderate and slightly detuned auxiliary microwave. The outcome made the scheme applicable to larger diamond sensors with improved sensitivity with great practical benefits.
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Compact electron accelerator reaches new speeds with nothing but light
https://phys.org/news/2022-09-compact-electron.html
by Dina Genkina, University of Maryland
Scientists harnessing precise control of ultrafast lasers have accelerated electrons over a 20-centimeter stretch to speeds usually reserved for particle accelerators the size of 10 football fields.

A team at the University of Maryland (UMD) headed by Professor of Physics and Electrical and Computer Engineering Howard Milchberg, in collaboration with the team of Jorge J. Rocca at Colorado State University (CSU), achieved this feat using two laser pulses sent through a jet of hydrogen gas. The first pulse tore apart the hydrogen, punching a hole through it and creating a channel of plasma. That channel guided a second, higher power pulse that scooped up electrons out of the plasma and dragged them along in its wake, accelerating them to nearly the speed of light in the process.

With this technique, the team accelerated electrons to almost 40% of the energy achieved at massive facilities like the kilometer-long Linac Coherent Light Source (LCLS), the accelerator at SLAC National Accelerator Laboratory. The paper was accepted to the journal Physical Review X on August 1, 2022.

"This is the first multi-GeV electron accelerator powered entirely by lasers," says Milchberg, who is also affiliated with the Institute of Research Electronics and Applied Physics at UMD. "And with lasers becoming cheaper and more efficient, we expect that our technique will become the way to go for researchers in this field."
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