In- and out-of-plane field induced quantum spin-liquid states in a more ideal Kitaev material: BaCo$_2$(AsO$_4$)$_2$

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. Using time-domain terahertz spectroscopy (TDTS) we show that the honeycomb cobalt-based Kitaev QSL candidate, BaCo$_2$(AsO$_4$)$_2$, 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 4 T out-of-plane-magnetic field. Applying an even smaller in-plane magnetic field $sim$ 0.5 T suppresses the effects of the non-Kitaev interactions and gives rise to a field induced intermediate state also consistent with a QSL. These results may have fundamental impact for realizing quantum computation. Our results demonstrate BaCo$_2$(AsO$_4$)$_2$ as a far more ideal version of Kitaev QSL compared with other candidates.