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Physics World

Interactions between matter and light in microcavities made of mirrors are fundamentally important for many modern technologies, including lasers. Researchers at the University of Michigan, Ann Arbor, US, have now gained tighter control of these interactions by exploiting a nonlinear effect that occurs in a new kind of hybrid semiconductor made from bilayers of two-dimensional materials. These semiconducting sheets form an egg-carton-like array in which the “pockets” are quantum dots that can be controlled using light, and they could be used to make ultralow-energy switches.

Led by Hui Deng, the researchers made their hybrid semiconductor from flakes of tungsten disulphide (WS₂) and molybdenum diselenide (MoSe₂) just a few atoms thick. In their bulk form, these transition-metal dichalcogenides (TMDCs) act as indirect band-gap semiconductors. When scaled down to a monolayer thickness, however, they behave as direct band-gap semiconductors, capable of efficiently absorbing and emitting light.

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Creating a two-dimensional material, just a few atoms thick, is often an arduous process requiring sophisticated equipment. So scientists were surprised to see 2D puddles emerge inside a three-dimensional superconductor—a material that allows electrons to travel with 100% efficiency and zero resistance—with no prompting.

Within those puddles, superconducting electrons acted as if they were confined inside an incredibly thin, sheet-like plane, a situation that requires them to somehow cross over to another dimension, where different rules of quantum physics apply.

“This is a tantalizing example of emergent behavior, which is often difficult or impossible to replicate by trying to engineer it from scratch,” said Hari Manoharan, a professor at Stanford University and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at the Department of Energy’s SLAC National Accelerator Laboratory, who led the research.

To make some of the most precise measurements we can of the world around us, scientists tend to go small — right down to the atomic scale, using a technique called atom interferometry.

Now, for the first time, scientists have performed this kind of measurement in space, using a sounding rocket specially designed to carry science payloads into low-Earth space.

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Ultralight bosons are hypothetical particles whose mass is predicted to be less than a billionth the mass of an electron. They interact relatively little with their surroundings and have thus far eluded searches to confirm their existence. If they exist, ultralight bosons such as axions would likely be a form of dark matter, the mysterious, invisible stuff that makes up 85 percent of the matter in the universe.

Now, physicists at MIT’s LIGO Laboratory have searched for ultralight bosons using black holes—objects that are mind-bending orders of magnitude more massive than the particles themselves. According to the predictions of quantum theory, a black hole of a certain mass should pull in clouds of ultralight bosons, which in turn should collectively slow down a black hole’s spin. If the particles exist, then all black holes of a particular mass should have relatively low spins.

But the physicists have found that two previously detected black holes are spinning too fast to have been affected by any ultralight bosons. Because of their large spins, the black holes’ existence rules out the existence of ultralight bosons with masses between 1.3×10^-13 electronvolts and 2.7×10^-13 electronvolts—around a quintillionth the mass of an electron.

Extremely precise measurements are possible using atom interferometers that employ the wave character of atoms for this purpose. They can thus be used, for example, to measure the gravitational field of the Earth or to detect gravitational waves. A team of scientists from Germany has now managed to successfully perform atom interferometry in space for the first time—on board a sounding rocket. “We have established the technological basis for atom interferometry on board of a sounding rocket and demonstrated that such experiments are not only possible on Earth, but also in space,” said Professor Patrick Windpassinger of the Institute of Physics at Johannes Gutenberg University Mainz (JGU), whose team was involved in the investigation. The results of their analyses have been published in Nature Communications.

A team of researchers from various universities and research centers led by Leibniz University Hannover launched the MAIUS-1 mission in January 2017. This has since become the first rocket mission on which a Bose-Einstein condensate has been generated in space. This special state of matter occurs when atoms—in this case atoms of rubidium—are cooled to a temperature close to absolute zero, or minus 273 degrees Celsius. “For us, this ultracold ensemble represented a very promising starting point for atom interferometry,” explained Windpassinger. Temperature is one of the determining factors, because measurements can be carried out more accurately and for longer periods at lower temperatures.

A never-before-seen particle has revealed itself in the hot guts of two particle colliders, confirming a half-century-old theory.

Scientists predicted the existence of the particle, known as the odderon, in 1973, describing it as a rare, short-lived conjointment of three smaller particles known as gluons. Since then, researchers have suspected that the odderon might appear when protons slammed together at extreme speeds, but the precise conditions that would make it spring into existence remained a mystery. Now, after comparing data from the Large Hadron Collider (LHC), the 17-mile-long (27 kilometers) ring-shaped atom smasher near Geneva that’s famous for discovering the Higgs boson, and the Tevatron, a now-defunct 3.9-mile-long (6.3 km) American collider that slammed protons and their antimatter twins (antiprotons) together in Illinois until 2011, researchers report conclusive evidence of the odderon’s existence.

At any given time, 1100 tons of microplastic are floating over the western US. New modeling shows the surprising sources of the nefarious pollutant.

If you find yourself in some secluded spot in the American West—maybe Yellowstone, or the deserts of Utah, or the forests of Oregon—take a deep breath and get some fresh air along with some microplastic. According to new modeling, 1100 tons of it is currently floating above the western US. The stuff is falling out of the sky, tainting the most remote corners of North America—and the world. As I’ve said before, plastic rain is the new acid rain.

But where is it all coming from? You’d think it’d be arising from nearby cities—western metropolises like Denver and Salt Lake City. But new modeling published yesterday in the Proceedings of the National Academy of Sciences shows that 84 percent of airborne microplastics in the American West actually comes from the roads outside of major cities. Another 11 percent could be blowing all the way in from the ocean. (The researchers who built the model reckon that microplastic particles stay airborne for nearly a week, and that’s more than enough time for them to cross continents and oceans.)

Continue reading “Plastic Is Falling From the Sky. But Where’s It Coming From?” »

It may be possible in the future to use information technology where electron spin is used to store, process and transfer information in quantum computers. It has long been the goal of scientists to be able to use spin-based quantum information technology at room temperature. A team of researchers from Sweden, Finland and Japan have now constructed a semiconductor component in which information can be efficiently exchanged between electron spin and light at room temperature and above. The new method is described in an article published in Nature Photonics.

It is well known that electrons have a negative charge; they also have another property called spin. This may prove instrumental in the advance of information technology. To put it simply, we can imagine the electron rotating around its own axis, similar to the way in which the Earth rotates around its own axis. Spintronics—a promising candidate for future information technology—uses this quantum property of electrons to store, process and transfer information. This brings important benefits, such as higher speed and lower energy consumption than traditional electronics.

Developments in spintronics in recent decades have been based on the use of metals, and these have been highly significant for the possibility of storing large amounts of data. There would, however, be several advantages in using spintronics based on semiconductors, in the same way that semiconductors form the backbone of today’s electronics and photonics.

Classical hydrodynamics laws can be very useful for describing the behavior of systems composed of many particles (i.e., many-body systems) after they reach a local state of equilibrium. These laws are expressed by so-called hydrodynamical equations, a set of mathematical equations that describe the movement of water or other fluids.

Researchers at Oak Ridge National Laboratory and University of California, Berkeley (UC Berkeley) have recently carried out a study exploring the hydrodynamics of a quantum Heisenberg spin-1/2 chain. Their paper, published in Nature Physics, shows that the spin dynamics of a 1D Heisenberg antiferromagnet (i.e., KCuF₃) could be effectively described by a dynamical exponent aligned with the so-called Kardar-Parisi-Zhang universality class.

“Joel Moore and I have known each other for many years and we both have an interest in quantum magnets as a place where we can explore and test new ideas in physics; my interests are experimental and Joel’s are theoretical,” Alan Tennant, one of the researchers who carried out the study, told Phys.org. “For a long time, we have both been interested in temperature in quantum systems, an area where a number of really new insights have come along recently, but we had not worked together on any projects.”