Duality and domain wall dynamics in a twisted Kitaev chain

The Ising chain in a transverse field is a paradigmatic model for a host of physical phenomena, including spontaneous symmetry breaking, quantum criticality and duality. Although the quasi-one-dimensional ferromagnet CoNb₂O₆ has been regarded as the Ising chain’s best material realization, it exhibits substantial deviations from ideality. By combining terahertz spectroscopy and calculations, we show that CoNb₂O₆ is in fact described by a different model with bond-dependent interactions, which we call the ‘twisted Kitaev chain’, as these interactions are similar to those of the honeycomb Kitaev spin liquid. The ferromagnetic ground state of CoNb₂O₆ arises from the compromise between two axes. Owing to this frustration, even at zero field domain walls have quantum motion, which is described by the celebrated Su–Schriefer–Heeger model of polyacetylene and shows rich behaviour as a function of field. Nevertheless, close to the critical field, this model enters a universal regime in the Ising universality class. We observe that the excitation gap in the ferromagnet closes at a rate twice that of the paramagnet. This universal ratio originates in the Kramers–Wannier duality between domain walls and spin flips, and in the topological conservation of domain wall parity. Our work also shows that Co²⁺ magnets are fertile ground in the search for quantum spin liquids.