Electrons accelerated to relativistic speeds in a dielectric material can produce bursts of x rays, similar to a phenomenon found in thunderstorms.
Category: materials
Scientists create a hexagonal diamond that could be even harder than the real thing
To misquote a famous song, “Diamonds are industry’s best friend.” Cubic diamond is the hardest mineral on Earth and is used in everything from precision cutting tools to high-performance semiconductors as well as expensive jewelry. But there is a rare and potentially tougher form called hexagonal diamond (HD), which has long been the subject of theories and debate over its actual existence. But now researchers from China claim to have created this elusive form of carbon in the lab.
Hexagonal diamond (also known as lonsdaleite) is usually found at sites of meteorite impacts. But because the quantities are so small and mixed with minerals, some scientists doubted it was a distinct material. In a paper published in the journal Nature, researchers describe how they made a bulk piece of pure HD using extreme pressure and heat.
Electric field tunes vibrations to ease heat transfer
New research from the Department of Energy’s Oak Ridge National Laboratory, in collaboration with The Ohio State University and Amphenol Corporation, challenges conventional understanding about controlling heat flow in solid materials. The study, published in PRX Energy, shows that applying an electric field to a ceramic material changes how phonons (tiny vibrations that carry heat) behave.
Phonons with atoms moving along the field direction (poling direction) last longer than those with atoms moving perpendicular to the field. As a result, the material conducts heat almost three times more efficiently along the field direction than in perpendicular directions. This promising approach could lead to new solid-state devices that control heat flow in everyday technologies.
“Being able to control both how fast and in what manner heat flows could lead to devices that manage thermal energy far more efficiently,” said Puspa Upreti, an ORNL postdoctoral research associate.
Möbius-inspired surface controls light in two directions
Light is an unusually rich carrier of information. Its direction of travel, wavelength, and polarization can all be used to encode signals or images. Yet controlling these properties independently remains difficult, especially when light can enter a device from either side.
In most optical materials—and even in many metasurfaces—the laws of reciprocity and time-reversal symmetry tightly link how a device behaves for forward and backward illumination. As a result, truly different responses in the two directions are hard to achieve in a compact optical element.
The challenge grows sharper when polarization is included. Many metasurfaces work only with simple polarization states, such as horizontal and vertical or left-and right-circular polarization.
Hidden atomic dichotomy drives superconductivity in ultra-thin compound
Physicists in China have unveiled new clues to the origins of high-temperature superconductivity in an iron-based material just a single unit-cell thick. Led by Qi-Kun Xue and Lili Wang at Tsinghua University, the team’s experiments show that the effect emerges through a striking dichotomy between two atomic “sublattices” in the material—offering deeper insight into how superconductivity arises. Their results are published in Physical Review Letters.
When cooled below its critical temperature, a superconductor allows electrical currents to flow with virtually zero resistance. While most superconductors discovered so far have critical temperatures close to absolute zero, recent decades have seen the discovery of increasingly advanced materials that host the effect at ever higher temperatures, making them far easier to implement for practical applications.
In 2012, superconductivity was discovered in a single-unit-cell-thick layer of iron selenide (FeSe), consisting of a Se–Fe–Se trilayer only 0.55 nm thick. However, it remained unclear how such a strong superconducting effect could emerge in such an ultrathin system.