Recent results on Kitaev interactions in Co based magnets.*

Kitaev quantum spin liquids (QSLs) are exotic states of matter that are predicted to host Majorana fermions and gauge flux excitations. However, so far all known Kitaev QSL candidates are known to have appreciable non-Kitaev interactions that pushes these systems far from the QSL regime.   Co based magnets have been proposed to be perhaps a more ideal platform for realizing Kitaev QSLs.   In this talk I will show evidence for a Kitaev interactions in both the quasi-one-dimensional ferromagnet CoNb₂O₆ as well as the hexagonal magnet BaCo₂(AsO₄).   Although it is usually believed to be the best material realization of a 1D Ising chain, by combining terahertz spectroscopy and calculations we have shown that CoNb₂O₆ is well described by a model with bond-dependent interactions.  We call this model 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 behavior as a function of field.   Most recently, we have shown also that the honeycomb cobalt-based Kitaev QSL candidate, BaCo₂(AsO₄)₂, has dominant Kitaev interactions. Due to only small non-Kitaev terms a magnetic continuum consistent with Majorana fermions and the existence of a Kitaev QSL can be induced by a small out-of-plane-magnetic field.  Our results demonstrate BaCo₂(AsO₄)₂, as a far more ideal version of Kitaev QSL compared with other candidates.