Field-induced phase transitions and quantum criticality in the honeycomb antiferromagnet Na3Co2SbO6

We performed ^23Na NMR measurements on a single-domain crystal of the Kitaev material Na₃Co₂SbO₆ with magnetic field applied along the crystalline a axis. A positive Curie-Weiss constant is obtained from the NMR Knight shift, which suggests the existence of ferromagnetic exchange couplings. The antiferromagnetic ordering is found to be suppressed at a field of 1. 9 T. Inside the ordered phase, our data reveal two additional phase transitions. At 1. 9 T the spin-lattice relaxation rate 1/^23T₁ establishes a quantum critical behavior at high temperatures. However, at low temperatures a gapped behavior is observed at the ``critical'' field, which suggests a weak first-order transition instead and a possible field-induced quantum spin liquid. Our results reveal complex microscopic interactions in the system that may help to search for possible quantum spin liquids.