Spin disorder state induced by Mg$^{2+}$ doping in a Kitaev material Na$_{3}$Co$_{2}$SbO$_{6}$

Due to the dominant Kitaev exchange interactions, the cobaltate, Na$_{3}$Co$_{2}$SbO$_{6}$, has been considered to be approximate to the Kitaev quantum spin liquid (QSL). Here, we investigate both magnetic dilution and chemical pressure effects of Na$_{3}$Co$_{2}$SbO$_{6}$ by the substitutions of Mg$^{2+}$ for Co$^{2+}$ through the structural, optical, magnetic and thermodynamic measurements. No structural transition has been observed, and the bandgaps remain constant in all doping levels. Combining with the magnetic and thermodynamic measurements, we find that the antiferromagnetic transition temperature is continuously suppressed with increasing Mg doping levels and completely disappears at $x=0.2$. Interestingly, when the doping level $x$ is larger than 0.2, neither long-range magnetic order nor spin glass state has been detected, and the specific heat has a residual linear term at zero field. All features indicate that Na$_{3}$(Co$_{2-x}$Mg$_{x}$)SbO$_{6}$ system enters into a novel spin disorder (NSD) state.