Effects of Coupling Between Chiral Vibrations and Spins in Molecular Magnets

Abstract: In single molecular magnets, chiral vibrations carrying vibrational angular momentum ($\hat{L}^{\text{vib}}$) emerge due to the splitting of a doubly degenerate vibrational mode. Here, we identify a new type of effective spin-vibrational coupling responsible for lifting this degeneracy, which can facilitate optically selective excitations. In the presence of an external Zeeman field, this coupling breaks both inversion (in-plane parity) $\mathcal{P}$ and time-reversal $\mathcal{T}$ symmetries, imparting distinct geometric phases to the resulting dressed spin-vibronic states. The wave function of the spin-vibronic state is characterized by a $\pi$-Berry phase, which results in magneto-optical circular dichroism. This framework is validated using density functional theory and multi-reference \emph{ab initio} calculations on the Ce(trenovan) molecular magnet.