Quantum entanglement in strongly correlated electron systems often leads to
exotic elementary excitations. Quantum spin liquids (QSLs) provide a
paradigmatic example, where the elementary excitations are described by
fractional quasiparticles such as spinons. However, such fractional
quasiparticles behave differently from electrons, making their experimental
identification challenging. Here, we theoretically investigate the spin Seebeck
effect, which is a thermoelectric response via a spin current, as an efficient
probe of the fractional quasiparticles in QSLs, focusing on the Kitaev
honeycomb model. By comprehensive studies using the real-time dynamics, the
perturbation theory, and the linear spin-wave theory based on the tunnel
spin-current theory, we find that the spin current is induced by thermal
gradient in the Kitaev spin liquid, via the low-energy fractional Majorana
excitations. This underscores the ability of Majorana fermions to carry spin
current, despite lacking spin angular momentum. Furthermore, we find that the
induced spin current changes its sign depending on the sign of the Kitaev
interaction, indicating that the Majorana fermions contribute to the spin
current with (up-)down-spin like nature when the exchange coupling is
(anti)ferromagnetic. Thus, in contrast to the negative spin current already
found in a one-dimensional QSL, our finding reveals that the spin Seebeck
effect can exhibit either positive or negative signals, contingent upon the
nature of fractional excitations in the QSLs. We also clarify contrasting
field-angle dependence between the Kitaev spin liquid in the low-field limit
and the high-field ferromagnetic state, which is useful for the experimental
identification. Our finding suggests that the spin Seebeck effect could be used
not only to detect fractional quasiparticles emerging in QSLs but also to
generate and control them.Comment: 16 pages, 10 figure