Chiral materials, such as helical crystals, exhibit unique symmetries that give rise to physical phenomena that are not observed in achiral (nonchiral) systems. One of the most intriguing topics in recent years has been the emergence of chiral phonons: the quanta of lattice wave in which the rotational motion of atoms propagates with a non-zero wave number along the rotational axis. This definition of chiral phonon indicates emergence of a nonzero inner product between the wave vector and mechanical angular momentum vector. Chiral phonons are of great interest because they possess angular momentum, which is expected to cause novel quantum processes.

This study addresses fundamental questions in condensed matter physics: how do electrons interact with chiral phonons in a chiral crystal, and what quantities govern their interaction?

To address this, the authors constructed a microscopic theoretical framework that explicitly describes the wavefunctions of chiral phonons propagating along the screw axes of helical crystals. Importantly, this study uses group representation theory to capture not only the usual crystal momentum (CM) associated with translational symmetry but also a lesser-known but crucial conserved quantity: crystal angular momentum (CAM), which arises from the discrete rotational symmetry of the crystal.

A key result is that the interaction vertex, which describes how electrons and phonons couple, must conserve both CM and CAM. These results represent a significant advance beyond the conventional framework, in which electrons couple only to longitudinal phonons and only CM conservation is required. In contrast, this work shows that electrons can also couple to transverse chiral phonons and that this coupling is mediated by the conservation of CAM.

Moreover, the authors introduce a phononic analogue of “Zilch”, which is a time-even pseudoscalar quantity originally introduced in electromagnetism to quantify the chirality of light. Although phononic Zilch is not a conserved quantity, its behavior is closely correlated with CAM and may offer a means by which to characterize the chirality of lattice vibrations.

This theoretical framework paves the way for understanding and controlling angular momentum transfer in chiral crystals. Moreover, these results open exciting possibilities, such as electrically driven phonon angular momentum and even spin-polarization via angular momentum transfer, with potential applications in next-generation spintronic devices. This study deepens our understanding of the fundamental quantum processes in chiral systems and sets the stage for future developments of research on quantum chiral materials.

(Written by J. Kishine on behalf of all the authors)

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