#excitons

A superfluid freezes and breaks the rules of physics

In a sheet of graphene, a superfluid has finally frozen—revealing a long-mysterious quantum state that blurs the line between solid and liquid.

When everyday matter is cooled, it follows a familiar path. A gas becomes a liquid, and with further cooling, that liquid turns into a solid. Quantum matter does not always follow these rules. More than a century ago, scientists discovered that helium behaves in an unexpected way at extremely low temperatures. Instead of freezing, it transforms from a normal gas into a superfluid, a rare state of matter that flows without any resistance and displays strange behaviors, including creeping up the sides of containers. Physicists have long wondered what would happen if a superfluid were cooled even further. Despite decades of effort, that question remained unanswered for nearly 50 years.

MIT physicists and colleagues report new insights into exotic particles key to a form of magnetism that has attracted growing interest because it originates from ultrathin materials only a few atomic layers thick. The work, which could impact future electronics and more, also establishes a new way to study these particles through a powerful instrument at the National Synchrotron Light Source II at Brookhaven National Laboratory. Among their discoveries, the team has identified the microscopic origin of these particles, known as excitons. They showed how they can be controlled by chemically "tuning" the material, which is primarily composed of nickel. Further, they found that the excitons propagate throughout the bulk material instead of being bound to the nickel atoms.

A PHYSICS BREAKTHROUGH COULD LEAD TO A NEW GENERATION OF ADVANCED ELECTRONICS

Alloy-engineered valleytronics: Microscopic mechanism gives scientists precise control over how excitons behave

Scientists have observed a new microscopic mechanism enabling precise control of the magneto-optical properties of excitons in alloys of two-dimensional semiconductors. This discovery opens up tangible prospects for technological applications in devices exploiting valleytronics. The research findings were published in the journal Physical Review Letters. The team includes researchers from the Faculty of Physics at the University of Warsaw, in collaboration with teams from the Wrocław University of Science and Technology, Sapienza University of Rome, University of Central Florida, Laboratoire National des Champs Magnétiques Intenses, National University of Singapore, CNR-IFN, as well as research centers in the Czech Republic (University of Chemistry and Technology, Prague) and Japan (National Institute for Materials Science).

Ordered 'supercrystal' could make lasers faster, smaller and more efficient

An advance from Monash University could pave the way for faster, smaller, and more energy-efficient lasers and other light-based technologies. Engineers have developed a new type of perovskite material arranged into an ordered "supercrystal." In this structure, tiny packets of energy called excitons work together rather than individually, allowing the material to amplify light far more efficiently. The findings, published in Laser & Photonics Reviews, could have applications in communications, sensors, and computing, improving the performance of devices that rely on light, such as sensors in autonomous vehicles, medical imaging, or electronics. Corresponding author Professor Jacek Jasieniak at Monash Materials Science and Engineering highlighted the potential for faster, more energy-efficient optical devices. "What's exciting here is that we're not changing the material itself, but how it's organized. By assembling nanocrystals into an ordered supercrystal, the excitations created by light can cooperate rather than compete, which allows light to be amplified much more efficiently," Professor Jasieniak said.

Next-generation nanoengineered switches can cut heat loss in electronics

Electronic devices lose energy as heat due to the movement of electrons. Now, a breakthrough in nanoengineering has produced a new kind of switch that matches the performance of the best traditional designs while pushing beyond the power-consumption limits of modern electronics. Researchers from the University of Michigan have achieved what scientists have been trying to execute for a long time: designing electronics that harness excitons—pairs of an electron and a corresponding hole (a missing electron) bound together forming a charge-neutral particle—instead of electrons. The newly designed nanoengineered optoexcitonics (NEO) device featured a tungsten diselenide (WSe2) monolayer on a tapered silicon dioxide (SiO2) nanoridge. The switch achieved a 66% reduction in losses compared to traditional switches while surpassing an on–off ratio of 19 dB at room temperature, a performance that rivals the best electronic switches available on the market.

Physicists have discovered an exotic new state of matter that takes the form of a highly ordered crystal of subatomic particles. The new state of matter, called a "bosonic correlated insulator," could lead to the discovery of many new types of exotic materials made from condensed matter, according to the researchers, who detailed their results in a study published May 11 in the journal Science.  Subatomic particles can be separated into two categories: fermions and bosons. The primary differences between the two are how they spin and how they interact with each other.  Fermions, such as electrons and protons, are often thought of as the building blocks of matter because they make up atoms, and are characterized by their half-integer spin. Two identical fermions cannot occupy the same space at the same time. "Bosons can occupy the same energy level; fermions don't like to stay together," study lead author Chenhao Jin, a condensed-matter physicist at the University of California, Santa Barbara, said in a statement. "Together, these behaviors construct the universe as we know it." But there is a case in which two fermions can become a boson: If a negatively charged electron is secured to a positively charged "hole" in a different fermion, it forms a bosonic particle known as an "exciton."  To see how excitons interact with one another, the researchers layered a lattice of tungsten disulfide atop a similar lattice of tungsten diselenide in an overlapping pattern called a moiré. Then, they shined a strong beam of light through the lattices — a method known as "pump-probe spectroscopy." These conditions pushed the excitons together until they were so densely packed that they could no longer move, creating a new symmetrical crystalline state with a neutral charge — a bosonic correlated insulator.