Fig. 3

Time evolution of the spin driven by the E″ mode. a, b Model Hamiltonian calculation of the spin trajectory starting at t=0 with the spin-down state shown in Fig. 1a with a a linearly polarized and b a circularly polarized in-plane magnetic field. c, d The Floquet eigenstate on the Bloch sphere for c the model Hamiltonian with the circularly polarized in-plane magnetic field and d the full TDDFT solution with the circularly polarized E″ phonon. The inset of c depicts schematically the two Floquet eigenstates, \({\varPsi} _{{{\alpha }}_{{ + }}}\) and \({\varPsi} _{{\mathrm{\alpha }}_ - }\) described in the main text. In a–c the time scale is presented in units of the E″ phonon period, i.e., T=2π/ω_ph=122 fs