Nov 22, 2022
Truly Chiral Phonons Observed in Three-Dimensional Materials for the First Time
Chirality can be true or false, depending on the symmetry changes during dynamic propagation. Crystal lattice vibrations, called phonons, can also display chirality as has been shown in some two-dimensional structures. However, truly chiral phonons have never been observed in three-dimensional systems—that is, until now. Researchers from Osaka Metropolitan University and their colleagues have identified these for the first time, in bulk cinnabar. Utilizing pseudo-angular momentum, their method can be used to identify the chirality in crystalline structures. Their findings were published in Nature Physics.
Truly chiral phonons—i.e., rotating and propagating atomic motions seen in a crystal lattice—have never been observed in a bulk 3D material. However, now, researchers have identified them in cinnabar.
Chirality is the breaking of reflection and inversion symmetries. Simply put, it is when an object’s mirror images cannot be superimposed over each other. A common example is your two hands—while mirror images of each other, they can never overlap. Chirality appears at all levels in nature and is ubiquitous. In addition to static chirality, chirality can also occur due to dynamic motion such as rotation. With this in mind, we can distinguish true and false chirality. A system is truly chiral if, when translating, space inversion does not equate to time reversal combined with a proper spatial rotation.
Phonons are quanta (or small packets) of energy associated with the vibration of atoms in a crystal lattice. Recently, phonons with chiral properties have been theorized and experimentally discovered in two-dimensional (2D) materials such as tungsten diselenide. The discovered chiral phonons are rotating—yet not propagating—atomic motions. But truly chiral phonons would be atomic motions that are both rotating and propagating, and these have never been observed in three-dimensional (3D) bulk systems.
Now, the research team including Professor Yoshihiko Togawa, from the Department of Physics and Electronics at Osaka Metropolitan University, and Professor Takuya Satoh, from the Department of Physics at the Tokyo Institute of Technology, has identified truly chiral phonons, both theoretically and experimentally. The team observed the chiral phonons in cinnabar (α-HgS). This was achieved using a combination of first-principles calculations and an experimental technique called circularly polarized Raman scattering. “Chiral structures can be probed using chiral techniques,” explains Professor Satoh. “So, using circularly polarized light, which has its own handedness (i.e., right-handed or left-handedness), is critical. Dynamic chiral structures can be mapped using pseudo-angular momentum (PAM). Pseudo-momentum and PAM originate from the phase factors acquired by discrete translation and rotation symmetry operations, respectively.”
The researchers’ novel experimental approach also allowed them to probe the fundamental traits of PAM. They found that the law of the conservation of PAM—one of the key laws of physics—holds between circularly polarized photons and chiral phonons. Professor Satoh explains, “Our work also provides an optical method to identify the handedness of chiral materials using PAM. Namely, we can determine the handedness of materials with better resolution than x-ray diffraction (XRD) can achieve. Moreover, XRD requires a large-enough crystal, is invasive, and can be destructive. Circularly polarized Raman scattering, unlike XRD, allowed us to determine the chirality of structures in a non-contact and non-destructive manner.”
“This study results enable us to explore a new direction of chirality-induced phenomena,” concludes Professor Togawa. “The more we delve into the concept of chirality, the more appealing it becomes, giving us a novel perspective to deepen our scientific understanding.”
This study is the first to identify truly chiral phonons in 3D materials, which are clearly distinct from chiral phonons seen previously in 2D hexagonal systems. The learnings gained here could drive new research into developing ways for transferring the PAM from photons to electron spins via propagating chiral phonons in future devices. Furthermore, this approach enables the determination of the true chirality of a crystal in an improved manner, providing a new critical tool for experimentalists and researchers.
This study was supported financially by the Japan Society for the Promotion of Science KAKENHI (grant nos. JP19H01828, JP19H05618, JP19K21854, JP21H01032, and JP22H01154) and the Frontier Photonic Sciences Project of the National Institutes of Natural Sciences (grant nos. 01212002 and 01213004).
Journal: Nature Physics
Title: Truly chiral phonons in α-HgS
Author: Kyosuke Ishito, Huiling Mao, Yusuke Kousaka, Yoshihiko Togawa, Satoshi Iwasaki, Tiantian Zhang, Shuichi Murakami, Jun-ichiro Kishine, and Takuya Satoh
Publication date: October 31, 2022
Nature Physics (Nature Portfolio Webpage)
Contribution to SDGs
GOAL 9: INDUSTRY, INNOVATION, AND INFRASTRUCTURE
Graduate School of Engineering
Prof. Yoshihiko Togawa
E-mail ytogawa [at]omu.ac.jp
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