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Dissipationless mechanism of skyrmion Hall current in double-exchange ferromagnets

We revisit a theory of skyrmion transport in ferromagnets. On a basis of an effective U(1) gauge theory for spin-chirality fluctuations in double-exchange ferromagnets, we derive an expression for the velocity of a skyrmion core driven by the dc electric field. We find that mutual feedback effects between conduction electrons and localized spins give rise to Chern-Simons terms, suggesting a dissipationless mechanism for the skyrmion Hall current. A conventional description of the current-induced skyrmion motion, appearing through the spin transfer torque and scattering events, is reproduced in a certain limit of our description, where the Chern-Simons terms are not fully incorporated. Our theory is applicable to not only metallic but also insulating systems, where the purely topological and dissipationless skyrmion Hall current can be induced in the presence of an energy gap.