  The Kitaev model offers a platform for quantum spin liquids (QSLs) with
fractional excitations, itinerant Majorana fermions and localized fluxes. Since
these fractional excitations could be utilized for quantum computing, how to
create, observe, and control them through the spin degree of freedom is a
central issue. Here, we study dynamical spin transport in a wide range of
frequency for the Kitaev-Heisenberg model, by applying an AC magnetic field to
an edge of the system. We find that, in the Kitaev QSL phase, spin
polarizations at the other edge are resonantly induced in a specific spin
component, even though the static spin correlations are vanishingly small. This
interedge spin resonance appears around the input frequency over the broad
frequency range. Comparing with the dynamical spin correlations, we clarify
that the resonance is governed by the itinerant Majorana fermions with a broad
continuum excitation spectrum, which can propagate over long distances,
although it vanishes for the pure Kitaev model because of accidental degeneracy
and requires weak Heisenberg interactions. We also find that the spin
polarizations in the other spin components are weakly induced at an almost
constant frequency close to the excitation gap of the localized fluxes,
irrespective of the input frequency. These results demonstrate that the
dynamical spin transport is a powerful probe of the fractional excitations in
the Kitaev QSL. Possible experimental realization of the interedge spin
resonance is discussed.
