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The honeycomb Kitaev spin model provides a quantum spin liquid in the groundstate, where the spin excitations are fractionalized into itinerant andlocalized Majorana fermions; the former spectrum has a broad continuum rangingup to a high energy, while the latter has a sharp peak at a low energy. Despitetremendous efforts, it remains elusive to clearly identify these distinctMajorana excitations in experiments. Here we show their manifestation in thetime evolution after quenching the magnetic field, by using the time-dependentMajorana mean-field theory for both the ferromagnetic and antiferromagneticKitaev models. We find that the transient spin dynamics from the quantum spinliquid states is qualitatively different from the conventional spin precessionsby the quench from the high-field forced-ferromagnetic state. We obtainpeculiar time evolutions with distinct time scales, i.e., short-time decay ofhigh-energy components associated with the itinerant Majorana excitations, andlong-lived excitations at a low energy by the localized ones. These peculiarbehaviors are caused by the energy transfer between the two Majoranaquasiparticles after the field quench. Moreover, we find that the Majoranasemimetal with the point nodes in equilibrium turns into a Majorana metal withthe transient "Fermi surfaces" by the energy transfer. In particular, for thequench from the intermediate-field quantum spin liquid in the antiferromagneticKitaev model, the Fermi surfaces change their topology in the time evolution,which is regarded as a dynamical version of the Majorana "Lifshitz transition".Our results unveil that the real-time dynamics provides another route to notonly the identification of the fractional Majorana excitations in candidatematerials of Kitaev magnets but also unprecedented quantum phases that cannotbe stabilized as the equilibrium states.

Nonequilibrium Majorana Dynamics by Quenching a Magnetic Field in Kitaev Spin Liquids