Field-induced phase transitions and quantum criticality in the honeycomb antiferromagnet ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$

We performed ${}_{23}\mathrm{Na}$ NMR measurements on a single-domain crystal of the Kitaev material ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ 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 $1{/}^{23}{T}_1$ 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.