SPICE Workshop on Chiral Phonons, July 29th - 31st 2025
Shizeng Lin
Recent experimental observations have demonstrated surprisingly large phonon magnetic moments, on the order of the Bohr magneton, in doped Dirac materials such as graphene and Cd3As2. We present a comprehensive theoretical framework elucidating the origin of these giant phonon magnetic moments through the lens of emergent gauge and gravitational fields arising from electron-phonon interactions. Utilizing the concept of Dirac fermions in curved spacetime, we categorize electron-phonon couplings into distinct angular momentum channels: an emergent gauge field in the l=1 channel, and a gravitational-like frame field in the l=0, 2 channels. The gauge field coupling directly relates phonon magnetic moments to the electronic Hall conductivity, while the frame field coupling provides a novel connection to electron Hall viscosity. We further demonstrate, through a combined approach of first-principles calculations, tight-binding modeling, and quantum field theory, that optical phonon modes (both Raman and infrared-active) in graphene and Cd3As2 exhibit substantial magnetic moments consistent with recent experiments. Our findings not only explain existing experimental results but also propose optical phonons as a powerful probe for measuring elusive electronic transport coefficients such as Hall viscosity.
Reference:
1. Wenqin Chen, Xiao-Wei Zhang, Ying Su, Ting Cao, Di Xiao, and Shi-Zeng Lin, Phys. Rev. B 111, 035126 (2025)
2. Wenqin Chen, Xiao-Wei ZhangTing Cao Shi-Zeng Lin and Di Xiao, in preparation.