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-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 )not equal 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.