2-D Atoms Do the Twist

September 18, 2019

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Waves of heat, called phonons, cause atoms to rotate in a certain direction. Selenium atoms (yellow) collectively go through a clockwise circular atomic motion while the tungsten atoms (blue) don’t move.
Waves of heat, called phonons, cause atoms to rotate in a certain direction. Selenium atoms (yellow) collectively go through a clockwise circular atomic motion while the tungsten atoms (blue) don’t move.
Image courtesy of Lawrence Berkeley National Laboratory

The Science

It is well known that atoms in a crystal vibrate when heated, creating collective excitations. This leads to the propagation of vibrational waves (phonons), which are similar to the ripples seen on the surface of water. In the study, scientists demonstrated, for the first time, an intrinsically rotating form of motion for the atoms in a crystal. The observations were on collective excitations of a single molecular layer of tungsten diselenide. Whether the rotation is clockwise or counter-clockwise depends on the wave’s propagation direction. The observation means that there is a chirality associated with the phonon transport.

The Impact

In this study, researchers have observed, for the first time, that atoms rotate clockwise or counter-clockwise when they transport phonons. This rotation could become the building block for energy-efficient information tech. It could also enable the development of molecular-scale rotors to drive microscopic motors and machines.

Summary

Chirality associated with the transport of electrons controls important electrical phenomena in materials. In addition, scientists discovered that phonons could also have a chirality associated with their movement in a solid, which can influence the properties of the material. The phenomenon was first observed in laser-excited tungsten diselenide monolayers. In their measurements, the scientists used a combination of two ultrafast laser pulses to optically excite the crystal and generate phonons propagating in a particular direction. At the same time, they observed a clear difference in absorption between left- and right-circularly polarized infrared light, which confirmed the chiral nature of the phonons. Theoretical predictions provide an understanding of the phenomena: in one phonon mode, selenium atoms collectively go through a clockwise circular atomic motion, while tungsten atoms do not move. In a different mode, the collective circular motion of the tungsten atoms is clockwise, and the selenium atoms’ circular motion is counter-clockwise. Learning to control this circular motion could lead to the development of exotic electronics and new capabilities for energy-efficient information processing.

Contact

Xiang Zhang
University of California, Berkeley
Lawrence Berkeley National Laboratory
xzhang@me.berkeley.edu

Robert Kaindl
University of California, Berkeley
Lawrence Berkeley National Laboratory
mrakaindl@lbl.gov

Funding

Department of Energy, Office of Science, Basic Energy Sciences (sample preparation, theory and data analysis, optical design and measurement, mid-infrared frequency conversion); China Scholarship Council (scholarship); National Natural Science Foundation of China (Nanjing Normal University); and King Abdullah University of Science and Technology (monolayer WSe2 synthesis) funded the research.

Publications

H. Zhu, J. Yi, M.Y. Li, J. Xiao, L. Zhang, C.W. Yang, R.A. Kaindl, L.J. Li, Y. Wang, and X. Zhang, “Observation of chiral phonons.” Science 359, 579 (2018). [DOI: 10.1126/science.aar2711]

Related Links

Phys.org news article: Scientists discover chiral phonons in a 2-D semiconductor crystal

Berkeley Lab news release: Scientists discover ‘chiral phonons’—Atomic rotations in a 2-D semiconductor crystal

R&D Magazine news article: Scientists discover 'chiral phonons'—Atomic rotations in a 2D semiconductor crystal

Science Daily news article: Scientists discover 'chiral phonons'—Atomic rotations in a 2-D semiconductor crystal

Newswise article: Scientists discover 'chiral phonons'—Atomic rotations in a 2-D semiconductor crystal