Title: Zero-field Skyrmions with a High Topological Number in Itinerant Magnets,
Abstract: Magnetic skyrmions are swirling spin textures with topologically protected
noncoplanarity. Recently, skyrmions with the topological number of unity have
been extensively studied in both experiment and theory. We here show that a
skyrmion crystal with an unusually high topological number of two is stabilized
in itinerant magnets at zero magnetic field. The results are obtained for a
minimal Kondo lattice model on a triangular lattice by an unrestricted
large-scale numerical simulation and variational calculations. We find that the
topological number can be switched by a magnetic field as $2\leftrightarrow
1\leftrightarrow 0$. The skyrmion crystals are formed by the superpositions of
three spin density waves induced by the Fermi surface effect, and hence, the
size of skyrmions can be controlled by the band structure and electron filling.
We also discuss the charge and spin textures of itinerant electrons in the
skyrmion crystals which are directly obtained in our numerical simulations.