Topological phonons have been discovered in graphene. Free Technical Library News

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Topological phonons discovered in graphene

02.10.2023

Chinese scientists have made a significant discovery by discovering topological phonons in graphene. To study phonon spectra across the entire two-dimensional Brillouin zone, they used high-resolution electron microscopy, analyzing the characteristic electron energy losses.

Phonons play an important role in the properties of crystalline materials, including thermal and mechanical characteristics, as well as their electronic properties. Of particular interest are topological phonons that arise when phonon branches with different parameters intersect. However, the study of such conditions is a relatively new area of research.

Until recently, topological phonons were not detected in two-dimensional materials such as graphene due to the need for very high resolution, ranging from 0,1 to 10 millielectronvolts. This resolution has been difficult to achieve with traditional methods such as X-ray or neutron diffraction, making the experimental study of topological phonon states challenging.

Chinese scientists from the Beijing National Condensed Matter Physics Laboratory and other institutes used high-resolution electron microscopy to probe phonon structures in graphene. This allowed them to obtain high-resolution phonon spectra across the entire two-dimensional Brillouin zone and identify several topological phonons.

Scientists note that in periodic crystals, such as the hexagonal lattice of graphene, topological characteristics are determined by the symmetries of the crystal lattice. These symmetries determine possible topological structures of phonons, such as Dirac phonons and ring node phonons. Computer simulations predicted the presence of two types of ring node phonons and four types of Dirac phonons in graphene.

The scientists conducted an experimental study on a single-layer graphene sample using a lens system, an electron monochromator and an electron analyzer. Their experimental results matched computer simulation predictions, and they were able to determine the three-dimensional structure of topological phonon states in momentum and energy space.

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Chemists have been struggling with this problem for a long time and have made some progress in recent years. The best results were obtained by Japanese researchers at the beginning of the 2003st century. They created a mat of plastic fibers impregnated with molecules that both bound the fibers and absorbed the uranium. In 100, with the help of such a mat, the Japanese "got" a kilogram of uranium out of the water. Such carpets up to 200 meters long can be lowered to a depth of XNUMX meters. They are taken out of the water, washed with an acid solution to release the uranium, and put back into the water.

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