Abstract
We show that the mesoscopic incommensurate $\mathbb{Z}_2$ vortex crystals proposed for layered triangular anisotropic magnets can be most saliently identified by two distinctive signatures in dynamical spin response experiments: The presence of pseudo-Goldstone `phonon' modes at low frequencies $\omega$, associated with the collective vibrations of the vortex cores, and a characteristic multi-scattered intensity profile at higher $\omega$, arising from a large number of Bragg reflections and magnon bandgaps. These are direct fingerprints of the large vortex sizes and magnetic unit cells and the solitonic spin profile around the vortex cores.
Highlights
A significant experimental and theoretical effort has been devoted to the understanding of correlated electron systems with 4d and 5d transition-metal ions, characterized by effective Jeff = 1/2 pseudospins, edge-sharing oxygen octahedra, and tricoordinated lattice geometries [1,2,3,4,5,6]
The prediction [33] that the coplanar 120◦ order of triangular Heisenberg antiferromagnets becomes immediately unstable under an infinitesimal Kitaev anisotropy, giving way to mesoscopic Z2 vortex crystals, has triggered a significant interest in the community [5,34,35,36,37,38,41,67], and remains to be explored and verified experimentally
The Z2 vortex crystals can be detected by small-angle neutron or x-ray scattering methods, in analogy to onedimensional (1D) soliton lattices in modulated antiferromagnets or skyrmion lattices in chiral ferromagnetic helimagnets
Summary
Such vortices have been known [42] to be present in triangular HAFs as topological excitations, but here the bond-dependent anisotropy condenses them in the ground state via a commensurate-incommensurate (C-IC) nucleation mechanism [43,44,45,46] This is akin to the formation of magnetic domains [43], Abrikosov vortices [47,48,49], blue phases in cholesteric liquid crystals [50], skyrmions in chiral helimagnets [51,52,53,54,55,56,57,58], and other systems [59,60,61,62,63]. For small |K|/J, d ∝ J/|K|, i.e., the distance between vortex cores goes to infinity at the HAF point, and the transition between the 120◦ order and the Z2VC phase is of the C-IC nucleation type [43,44,45,46]
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