Chiral Phonons Enable New Material Properties and Thermal Control.

The manipulation of heat flow at the nanoscale represents a significant challenge in materials science, with potential applications ranging from advanced thermoelectrics to novel computing architectures. Recent theoretical work suggests that phonons, the quantum units of lattice vibration, possessing angular momentum – termed chiral phonons – could provide a pathway to achieve such control. These circularly polarised vibrations offer a means to couple spin and heat, potentially enabling the realisation of exotic transport phenomena. Researchers from Zhejiang University, Peking University, and the Chinese Academy of Sciences, led by Yue Yang, Zhenyu Xiao, and including Yu Mao, Zhanghuan Li, and et al, present a systematic framework for identifying materials exhibiting these chiral phonon modes. Their work, detailed in a forthcoming publication entitled ‘Catalogue of chiral phonon materials’, establishes a symmetry-based theory applicable across all 230 crystallographic space groups, culminating in a database of 2738 materials with chiral phonon activity, and a shortlist of 170 promising candidates for further investigation. This theoretical paradigm extends beyond phonons, offering a classification scheme for chiral excitations within crystalline lattices, including magnons and electronic quasiparticles.

Recent research establishes a comprehensive theoretical framework for classifying chiral phonons, which are circularly polarised lattice vibrations possessing intrinsic angular momentum. This addresses a notable gap in the field by providing a systematic method for identifying and categorising these elusive excitations across all 230 crystallographic space groups. Researchers move beyond merely observing chirality, instead focusing on the underlying helicity and velocity-angular momentum tensor governing phonon-mediated thermal transport. They demonstrate that crystals fall into three distinct classes based on their phononic angular momentum: achiral crystals exhibiting no angular momentum, chiral crystals displaying s-wave helicity, and achiral crystals possessing higher-order helicity patterns. This classification relies on a fundamental representation of angular momentum within the crystalline lattice, offering a robust and predictive model that advances understanding of material behaviour.

The study utilises density functional theory (DFT), a quantum mechanical modelling method used to investigate the electronic structure of materials, and computational tools like Phonopy and Phono3py to calculate dynamical matrices and analyse phonon behaviour. Dynamical matrices describe the vibrational modes of a crystal lattice, while Phonopy and Phono3py are software packages used to calculate these matrices and analyse the resulting phonon properties. These calculations provide a detailed understanding of lattice vibrations and their associated properties, forming the basis for a high-throughput screening of 11614 crystalline compounds. This enables researchers to efficiently explore a vast materials space and identify promising candidates for further investigation, ultimately identifying 2738 materials exhibiting chiral phonon modes, representing a significant advancement in the discovery of materials with unique thermal and spin properties.

Researchers have created an open-access Chiral Phonon Materials Database to facilitate rapid materials discovery, providing a valuable resource for the wider scientific community and allowing them to quickly screen for materials possessing desired chiral phonon properties. This database accelerates the development of novel technologies by streamlining the materials discovery process and fostering collaboration among researchers. Importantly, the researchers emphasise that this framework isn’t limited to phonons, instead providing a universal paradigm for classifying chiral excitations in crystalline lattices, potentially extending to magnons – quantised spin waves – and even electronic quasiparticles, which are emergent phenomena in solid-state physics.

This broad applicability underscores the fundamental importance of their work and its potential to impact a wide range of fields, from thermal management to spintronics, a field concerned with utilising the intrinsic spin of electrons. The research paves the way for innovative applications in these areas by providing a systematic method for understanding and predicting chiral behaviour, unlocking opportunities for designing materials with tailored properties.

Future work should prioritise experimental verification of the predicted chiral phonon behaviour in the shortlisted materials. Detailed characterisation of the phonon spectra and investigation of the spin-phonon coupling mechanisms, where interactions between lattice vibrations and electron spins occur, are crucial steps towards realising practical applications. Researchers also plan to explore the potential of these chiral phonons for developing novel thermal devices and spintronic technologies, and aim to extend this framework to other types of lattice vibrations and quasiparticles, further expanding its applicability and impact. This research represents a significant step forward in understanding material behaviour and opens up exciting new possibilities for technological innovation.