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Emergent skyrmion-based chiral order in zero-field Heisenberg antiferromagnets on the breathing kagome lattice

We show that classical Heisenberg antiferromagnets on the breathing kagome lattice can be a platform to realize a zero-field topological order of the scalar spin chirality which can be viewed as a miniature skyrmion crystal (SkX) of discrete form with a small number of spins in its magnetic unit cell. In the model, a third nearest-neighbor (NN) antiferromagnetic interaction along the bond direction $J_3$ and the breathing bond-alternation characterized by the ratio of the NN interaction for large triangles to that for small ones, $J_1'/J_1$, are essential. It is found by means of Monte Carlo simulations that a commensurate triple-${\bf Q}$ state appearing for relatively strong $J_3$ at zero field is the noncoplanar state with the SkX structure in the breathing case of $J_1'/J_1 \neq 1$, while in the uniform case of $J_1'/J_1 =1$, it is a collinear state favored by thermal fluctuations. The origin of this chiral order and experimental implications of our result are also discussed.