When semiconductors stick together, materials go quantum

The twist angle formed between atomically thin layers of tungsten disulfide and tungsten diselenide acts as a "tuning knob," turning ordinary semiconductors into an exotic quantum material.

A team of researchers led by the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has developed a simple method that could turn ordinary semiconducting materials into quantum machines -- superthin devices marked by extraordinary electronic behavior. Such an advancement could help to revolutionize a number of industries aiming for energy-efficient electronic systems -- and provide a platform for exotic new physics.

The study describing the method, which stacks together 2D layers of tungsten disulfide and tungsten diselenide to create an intricately patterned material, or superlattice, was published online recently in the journal Nature.

"This is an amazing discovery because we didn't think of these semiconducting materials as strongly interacting," said Feng Wang, a condensed matter physicist with Berkeley Lab's Materials Sciences Division and professor of physics at UC Berkeley. "Now this work has brought these seemingly ordinary semiconductors into the quantum materials space."

Two-dimensional (2D) materials, which are just one atom thick, are like nanosized building blocks that can be stacked arbitrarily to form tiny devices. When the lattices of two 2D materials are similar and well-aligned, a repeating pattern called a moiré superlattice can form.

For the past decade, researchers have been studying ways to combine different 2D materials, often starting with graphene -- a material known for its ability to efficiently conduct heat and electricity. Out of this body of work, other researchers had discovered that moiré superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at just the right angle.

The new study, led by Wang, used 2D samples of semiconducting materials -- tungsten disulfide and tungsten diselenide -- to show that the twist angle between layers provides a "tuning knob" to turn a 2D semiconducting system into an exotic quantum material with highly interacting electrons.

Entering a new realm of physics

Co-lead authors Chenhao Jin, a postdoctoral scholar, and Emma Regan, a graduate student researcher, both of whom work under Wang in the Ultrafast Nano-Optics Group at UC Berkeley, fabricated the tungsten disulfide and tungsten diselenide samples using a polymer-based technique to pick up and transfer flakes of the materials, each measuring just tens of microns in diameter, into a stack.

They had fabricated similar samples of the materials for a previous study, but with the two layers stacked at no particular angle. When they measured the optical absorption of a new tungsten disulfide and tungsten diselenide sample for the current study, they were taken completely by surprise.

The absorption of visible light in a tungsten disulfide/tungsten diselenide device is largest when the light has the same energy as the system's exciton, a quasiparticle that consists of an electron bound to a hole that is common in 2D semiconductors. (In physics, a hole is a currently vacant state that an electron could occupy.)

For light in the energy range that the researchers were considering, they expected to see one peak in the signal that corresponded to the energy of an exciton.

Instead, they found that the original peak that they expected to see had split into three different peaks representing three distinct exciton states.

What could have increased the number of exciton states in the tungsten disulfide/tungsten device from one to three? Was it the addition of a moiré superlattice?

To find out, their collaborators Aiming Yan and Alex Zettl used a transmission electron microscope (TEM) at Berkeley Lab's Molecular Foundry, a nanoscale science research facility, to take atomic-resolution images of the tungsten disulfide/tungsten diselenide device to check how the materials' lattices were aligned.

The TEM images confirmed what they had suspected all along: the materials had indeed formed a moiré superlattice. "We saw beautiful, repeating patterns over the entire sample," said Regan. "After comparing this experimental observation with a theoretical model, we found that the moiré pattern introduces a large potential energy periodically over the device and could therefore introduce exotic quantum phenomena."

The researchers next plan to measure how this new quantum system could be applied to optoelectronics, which relates to the use of light in electronics; valleytronics, a field that could extend the limits of Moore's law by miniaturizing electronic components; and superconductivity, which would allow electrons to flow in devices with virtually no resistance.

Also contributing to the study were researchers from Arizona State University and the National Institute for Materials Science in Japan.

The work was supported by the DOE Office of Science. Additional funding was provided by the National Science Foundation, the Department of Defense, and the Elemental Strategy Initiative conducted by MEXT, Japan, and JSPS KAKENHI. The Molecular Foundry is a DOE Office of Science user facility.

Stars exploding as supernovae lose their mass to companion stars during their lives

A massive star evolving and becoming a red supergiant, and finally exploding as a supernova. A binary companion may strip the star's hydrogen away (producing supernova type IIb/Ib), and for a more massive star the stellar wind expels the remaining helium layer (producing supernova type Ic).

Stars over eight times more massive than the Sun end their lives in supernovae explosions. The composition of the star influences what happens during the explosion.

A considerable number of massive stars have a close companion star. Led by researchers at Kyoto University, a team of international researchers observed that some stars exploding as supernovae may release part of their hydrogen layers to their companion stars before the explosion.

"In a binary star system, the star can interact with the companion during its evolution. When a massive star evolves, it swells to become a red supergiant star, and the presence of a companion star may disrupt the outer layers of this supergiant star, which is rich in hydrogen. Therefore, binary interaction may remove the hydrogen layer of the evolved star either partially or completely," says Postdoctoral Researcher Hanindyo Kuncarayakti from the Department of Physics and Astronomy at the University of Turku in Finland and the Finnish Centre for Astronomy with ESO. Kuncarayakti is a member of the researcher team that made the observations.

As the star has released a significant part of its hydrogen layer due to the close companion star, its explosion can be observed as a type Ib or IIb supernova.

A star more massive explodes as a type Ic supernova after having lost its helium layer, too, due to the so-called stellar winds. Stellar winds are massive streams of energetic particles from the surface of the star that may remove the helium layer below the hydrogen layer.

"However, the companion star does not have a significant role in what happens to the exploding star's helium layer. Instead, stellar winds play a key role in the process as their intensity is dependent on the star's own initial mass. According to theoretical models and our observations, the effects of stellar winds on the mass loss of the exploding star are significant only for stars above a certain mass range," says Kuncarayakti.

The research group's observations show that the so-called hybrid mechanism is a potential model in describing the evolution of massive stars. The hybrid mechanism indicates that during its lifespan, the star may gradually lose part of its mass both to its companion star as a result of interaction as well as due to stellar winds.

"By observing stars dying as supernovae and the phenomena within, we can improve our understanding on massive star evolution. However, our understanding of massive star evolution is still far from complete," states Professor Seppo Mattila from the Department of Physics and Astronomy at the University of Turku.

'Goldilocks' stars may be 'just right' for finding habitable worlds

Artist's concept of a planet orbiting in the habitable zone of a K star.

Scientists looking for signs of life beyond our solar system face major challenges, one of which is that there are hundreds of billions of stars in our galaxy alone to consider. To narrow the search, they must figure out: What kinds of stars are most likely to host habitable planets?

A new study finds a particular class of stars called K stars, which are dimmer than the Sun but brighter than the faintest stars, may be particularly promising targets for searching for signs of life.

Why? First, K stars live a very long time -- 17 billion to 70 billion years, compared to 10 billion years for the Sun -- giving plenty of time for life to evolve. Also, K stars have less extreme activity in their youth than the universe's dimmest stars, called M stars or "red dwarfs."

M stars do offer some advantages for in the search for habitable planets. They are the most common star type in the galaxy, comprising about 75 percent of all the stars in the universe. They are also frugal with their fuel, and could shine on for over a trillion years. One example of an M star, TRAPPIST-1, is known to host seven Earth-size rocky planets.

But the turbulent youth of M stars presents problems for potential life. Stellar flares -- explosive releases of magnetic energy -- are much more frequent and energetic from young M stars than young Sun-like stars. M stars are also much brighter when they are young, for up to a billion years after they form, with energy that could boil off oceans on any planets that might someday be in the habitable zone.

"I like to think that K stars are in a 'sweet spot' between Sun-analog stars and M stars," said Giada Arney of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Arney wanted to find out what biosignatures, or signs of life, might look like on a hypothetical planet orbiting a K star. Her analysis is published in the Astrophysical Journal Letters.

Scientists consider the simultaneous presence of oxygen and methane in a planet's atmosphere to be a strong biosignature because these gases like to react with each other, destroying each other. So, if you see them present in an atmosphere together, that implies something is producing them both quickly, quite possibly life, according to Arney.

However, because planets around other stars (exoplanets) are so remote, there needs to be significant amounts of oxygen and methane in an exoplanet's atmosphere for it to be seen by observatories at Earth. Arney's analysis found that the oxygen-methane biosignature is likely to be stronger around a K star than a Sun-like star.

Arney used a computer model that simulates the chemistry and temperature of a planetary atmosphere, and how that atmosphere responds to different host stars. These synthetic atmospheres were then run through a model that simulates the planet's spectrum to show what it might look like to future telescopes.

"When you put the planet around a K star, the oxygen does not destroy the methane as rapidly, so more of it can build up in the atmosphere," said Arney. "This is because the K star's ultraviolet light does not generate highly reactive oxygen gases that destroy methane as readily as a Sun-like star."

This stronger oxygen-methane signal has also been predicted for planets around M stars, but their high activity levels might make M stars unable to host habitable worlds. K stars can offer the advantage of a higher probability of simultaneous oxygen-methane detection compared to Sun-like stars without the disadvantages that come along with an M star host.

Additionally, exoplanets around K stars will be easier to see than those around Sun-like stars simply because K stars are dimmer. "The Sun is 10 billion times brighter than an Earthlike planet around it, so that's a lot of light you have to suppress if you want to see an orbiting planet. A K star might be 'only' a billion times brighter than an Earth around it," said Arney.

Arney's research also includes discussion of which of the nearby K stars may be the best targets for future observations. Since we don't have the ability to travel to planets around other stars due to their enormous distances from us, we are limited to analyzing the light from these planets to search for a signal that life might be present. By separating this light into its component colors, or spectrum, scientists can identify the constituents of a planet's atmosphere, since different compounds emit and absorb distinct colors of light.

"I find that certain nearby K stars like 61 Cyg A/B, Epsilon Indi, Groombridge 1618, and HD 156026 may be particularly good targets for future biosignature searches," said Arney.

Listening to quantum radio

This quantum chip (1x1 cm big) allows the researchers to listen to the smallest radio signal allowed by quantum mechanics.

Researchers at Delft University of Technology have created a quantum circuit that enables them to listen to the weakest radio signal allowed by quantum mechanics. This new quantum circuit opens the door to possible future applications in areas such as radio astronomy and medicine (MRI). It also enables researchers to do experiments that can shed light on the interplay between quantum mechanics and gravity.

We have all been annoyed by weak radio signals at some point in our lives: our favourite song in the car turning to noise, being too far away from our wifi router to check our email. Our usual solution is to make the signal bigger, for instance by picking a different radio station or by moving to the other side of the living room. What if, however, we could just listen more carefully?

Weak radio signals are not just a challenge for people trying to find their favourite radio station, but also for magnetic resonance imaging (MRI) scanners at hospitals, as well as for the telescopes scientists use to peer into space.

In a quantum 'leap' in radio frequency detection, researchers in the group of Prof. Gary Steele in Delft demonstrated the detection of photons or quanta of energy, the weakest signals allowed by the theory of quantum mechanics.

Quantum chunks

One of the strange predictions of quantum mechanics is that energy comes in tiny little chunks called 'quanta'. What does this mean? "Say I am pushing a kid on a swing," lead researcher Mario Gely said. "In the classical theory of physics, if I want the kid to go a little bit faster I can give them a small push, giving them more speed and more energy. Quantum mechanics says something different: I can only increase the kid's energy one 'quantum step' at a time. Pushing by half of that amount is not possible."

For a kid on a swing these 'quantum steps' are so tiny that they are too small to notice. Until recently, the same was true for radio waves. However, the research team in Delft developed a circuit that can actually detect these chunks of energy in radio frequency signals, opening up the potential for sensing radio waves at the quantum level.

From quantum radio to quantum gravity?

Beyond applications in quantum sensing, the group in Delft is interested in taking quantum mechanics to the next level: mass. While the theory of quantum electromagnetism was developed nearly 100 years ago, physicists are still puzzled today on how to fit gravity into quantum mechanics.

"Using our quantum radio, we want to try to listen to and control the quantum vibrations of heavy objects, and explore experimentally what happens when you mix quantum mechanics and gravity," Gely said. "Such experiments are hard, but if successful we would be able to test if we can make a quantum superposition of space-time itself, a new concept that would test our understanding of both quantum mechanics and general relativity."

Sleep tight! Researchers identify the beneficial role of sleep

Simultaneous imaging of chromosome dynamics (red) and neuronal activity (green) in live zebrafish.

Why do animals sleep? Why do humans "waste" a third time of their lives sleeping? Throughout evolution sleep has remained universal and essential to all organisms with a nervous system, including invertebrates such as flies, worms, and even jellyfish. But the reason why animals sleep -- despite the continuous threat of predators -- still remains a mystery, and is considered among the biggest unanswered questions in life sciences.

In a new study, published today in the journal Nature Communications, researchers at Bar-Ilan University in Israel reveal a novel and unexpected function of sleep that they believe could explain how sleep and sleep disturbances affect brain performance, aging and various brain disorders.

Using 3D time-lapse imaging techniques in live zebrafish, the researchers were able to define sleep in a single chromosome resolution and show, for the first time, that single neurons require sleep in order to perform nuclear maintenance.

DNA damage can be caused by many processes including radiation, oxidative stress, and even neuronal activity. DNA repair systems within each cell correct this damage. The current work shows that during wakefulness, when chromosome dynamics are low, DNA damage consistently accumulates and can reach unsafe levels.

The role of sleep is to increase chromosome dynamics, and normalize the levels of DNA damage in each single neuron. Apparently, this DNA maintenance process is not efficient enough during the online wakefulness period and requires an offline sleep period with reduced input to the brain in order to occur. "It's like potholes in the road," says Prof. Lior Appelbaum, of Bar-Ilan University's Mina and Everard Goodman Faculty of Life Sciences and Gonda (Goldschmied) Multidisciplinary Brain Research Center, who led the study. "Roads accumulate wear and tear, especially during daytime rush hours, and it is most convenient and efficient to fix them at night, when there is light traffic."

Appelbaum calls the accumulation of DNA damage the "price of wakefulness." He and his doctoral student David Zada, first author of the study, as well as co-authors, Dr. Tali Lerer-Goldshtein, Dr. Irina Bronshtein, and Prof. Yuval Garini, hypothesized that sleep consolidates and synchronizes nuclear maintenance within individual neurons, and set out to confirm this theory.

Their discovery was achieved thanks to the characteristics of the zebrafish model. With their absolute transparency, and a brain very similar to humans, zebrafish are a perfect organism in which to study single cell within a live animal under physiological conditions. Using a high resolution microscope, the movement of DNA and nuclear proteins within the cell -- inside the fish -- can be observed while the fish are awake and asleep. The researchers were particularly surprised to find that chromosomes are more active at night, when the body rests, but this increased activity enables the efficiency of the repair to DNA damage.

The results establish chromosome dynamics as a potential marker for defining single sleeping cells and propose that the restorative function of sleep is nuclear maintenance. "We've found a causal link between sleep, chromosome dynamics, neuronal activity, and DNA damage and repair with direct physiological relevance to the entire organism," says Prof. Appelbaum. "Sleep gives an opportunity to reduce DNA damage accumulated in the brain during wakefulness."

"Despite the risk of reduced awareness to the environment, animals -- ranging from jellyfish to zebrafish to humans -- have to sleep to allow their neurons to perform efficient DNA maintenance, and this is possibly the reason why sleep has evolved and is so conserved in the animal kingdom," concludes Prof. Appelbaum.