  Topological transports of fermions are governed by the Chern numbers of the
energy bands lying below the Fermi energy. For bosons, e.g. phonons and magnons
in a crystal, topological transport is dominated by the Chern number of the
lowest energy band when the band gap is comparable to the thermal energy. Here,
we demonstrate the presence of topological transport by bosonic magnons in a
lattice of magnetic skyrmions - topological defects formed by a vortex-like
texture of spins. We find a distinct thermal Hall signal in the magnetic
skyrmion phase of an insulating polar magnet GaV4Se8, identified as the
topological thermal Hall effect of magnons governed by the Chern number of the
lowest energy band of the magnons in a triangular lattice of magnetic
skyrmions. Our findings lay a foundation for studying topological phenomena of
other bosonic excitations through thermal Hall probe.
