📝 SummarXiv

Abstract

Based on the molecular Berry curvature (MBC) framework, we develop an \textit{ab initio} algorithm to capture the quantitative effects of magnetic order on lattice dynamics. Using the ferromagnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ as a prototype, we show that electronic-order-driven phonon symmetry breaking requires spin-orbit coupling (SOC) and leads to an MBC term that breaks both time-reversal ($\mathcal{T}$) and mirror symmetries. We demonstrate that mirror-symmetry breaking is essential to account for the experimentally observed phonon splitting, $\mathcal{T}$-breaking alone is insufficient. The MBC is widely distributed across the Brillouin zone, giving rise to significant off-$\Gamma$ effects. Our results agree well with experiments and establish a framework for predicting large phonon magnetism in magnetic materials with strong spin-orbit coupling and electron-phonon coupling. This work also suggests new avenues for controlling non-reciprocal phonon transport.