Chiral charge density wave (chiral CDW) is a correlated electronic state that typically emerges at low temperatures, characterized by periodic modulation of charge density in space, accompanied by lattice structural distortions that break mirror and inversion symmetries. This state is believed to be associated with various intriguing physical phenomena, such as the nonlocal Hall effect, chiral Cooper pairs, and axion insulator states. However, to date, materials exhibiting chiral charge density wave states are exceedingly rare, and the underlying theoretical mechanisms remain poorly understood. This has significantly limited the in-depth exploration of this intriguing state of matter. 

A research team led by Associate Professor Zhang Tiantian from the Institute of Theoretical Physics, Chinese Academy of Sciences, combined theoretical analysis and first-principles calculations to propose that the softening of chiral phonons, induced by electro-acoustic coupling interactions, could be a potential mechanism for triggering chiral charge density waves (chiral CDWs). Additionally, by incorporating the multi-phonon thermal diffusion effect into their calculations of X-ray diffraction, they suggested that the anisotropy of X-ray diffraction peaks could serve as a possible method for detecting chiral CDWs. Chiral phonons are a unique type of lattice excitation characterized by the helical vibration of atoms propagating in a specific direction. These phonon modes lack both mirror and inversion symmetry. When the electron-phonon coupling strength associated with such modes becomes sufficiently strong to induce their softening, it results in helical lattice distortions and charge density modulations, leading to the formation of a chiral charge density wave (chiral CDW) state. The team computed the phonon spectrum of monolayer TiSe₂ and observed that the softening of non-chiral phonons does not lead to the formation of a chiral CDW state. In contrast, the softening of chiral phonons results in the emergence of chiral CDWs, providing numerical confirmation of the theoretical predictions. Building on this theory, the team proposed that the nonlinear electro-/magnetostrictive effect could serve as an effective mechanism for regulating chiral CDWs. Additionally, by incorporating multi-phonon thermal scattering effects, the team conducted more realistic X-ray diffraction simulations. They discovered that for chiral CDW states, the anisotropy of X-ray diffraction spots also breaks mirror symmetry. This finding demonstrates that diffraction peak anisotropy can serve as a reliable method for detecting chiral CDWs.

This study provides a mechanism for the chiral charge density wave phase transition and a novel detection method, laying a solid foundation for the exploration of chiral charge density waves and related phenomena.. This article was recently published in npj Computational Materials 10, 264 (2024).