Kitaev's honeycomb spin-liquid model and its proposed realization in materials such as $\alpha \text{−}{\mathrm{RuCl}}_3$, ${\mathrm{Li}}_2{\mathrm{IrO}}_3$, and ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ continue to present open questions about how the dynamics of a spin liquid are modified in the presence of non-Kitaev interactions as well as the presence of inhomogeneities. Here we use ${}^{23}\mathrm{Na}$ nuclear magnetic resonance to probe both static and dynamical magnetic properties in single-crystal ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. We find that the NMR shift follows the bulk susceptibility above 30 K but deviates from it below; moreover below $T_N$ the spectra show a broad distribution of internal magnetic fields. Both of these results provide evidence for inequivalent magnetic sites at low temperature, suggesting inhomogeneities are important for the magnetism. The spin-lattice relaxation rate is isotropic and diverges at $T_N$, suggesting that the Kitaev cubic axes may control the critical quantum spin fluctuations. In the ordered state, we observe gapless excitations, which may arise from site substitution, emergent defects from milder disorder, or possibly be associated with nearby quantum paramagnetic states distinct from the Kitaev spin liquid.

• Received 20 September 2019
• Accepted 22 January 2020

DOI:https://doi.org/10.1103/PhysRevB.101.081101