Truly chiral phonons observed for the first time in three-dimensional materials

Chirality is the breaking of reflection and inversion symmetries. Simply put, it’s when mirror images of an object cannot be superimposed on each other. A common example is your two hands – even though they mirror each other, they can never overlap. Chirality occurs at all levels in nature and is ubiquitous.

In addition to static chirality, chirality can also occur due to dynamic motion, including rotation. With this in mind, we can distinguish true and false chirality. A system is truly chiral if – in translation – the space inversion does not equal a time inversion combined with an appropriate 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 chiral phonons discovered are atomic movements in rotation, but not in propagation. But, truly chiral phonons would be atomic motions that are both rotating and propagating, and these have never been observed in massive three-dimensional (3D) systems.

Now, a team of researchers led by scientists from the Tokyo Institute of Technology (Tokyo Tech) have identified truly chiral phonons, both theoretically and experimentally. Their work is published in Natural Physics. The team, led by Professor Takuya Satoh from Tokyo Tech’s Department of Physics, observed 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. Thus, the use of circularly polarized light, which has its own laterality (i.e., right-handed or left-handed), is essential. Dynamic chiral structures can be mapped using pseudo-angular momentum (PAM). Pseudo-momentum and PAM come from the phase factors acquired by discrete translation and rotational symmetry operations, respectively,” explains the Professor Satho.

The researchers’ new experimental approach also allowed them to probe fundamental traits of PAM. They discovered that the law of conservation of PAM, one of the key laws of physics, applies between circularly polarized photons and chiral phonons.

“Our work also provides an optical method to identify the sensitivity of chiral materials using PAM. Namely, we can determine the sensitivity of materials with better resolution than X-ray diffraction (XRD). requires a sufficiently large crystal, is invasive, and can be destructive. Circularly polarized Raman scattering, on the other hand, has allowed us to determine the chirality of structures that XRD could not, without contact and in a non-destructive way,” concludes Professor Satoh.

This study is the first to identify truly chiral phonons in 3D materials, which are clearly distinct from those previously observed in 2D hexagonal systems. The insights gained here could drive further research into developing ways to transfer PAM from photons to electron spins via chiral phonon propagation in future devices. Moreover, this approach allows the determination of the true chirality of a crystal in an improved way, providing a critical new tool for experimenters and researchers.