Impact of disorder on dynamics and ordering in the honeycomb lattice iridate Na$_2$IrO$_3$

TL;DR

This study investigates how disorder affects magnetic properties and excitations in Na$_2$IrO$_3$, revealing inhomogeneities and gapless excitations that influence the material's spin-liquid behavior.

Contribution

It provides experimental evidence of inhomogeneities and disorder effects on the magnetic dynamics in Na$_2$IrO$_3$ using $^{23}$Na NMR, highlighting their impact on spin-liquid characteristics.

Findings

Inhomogeneities are present at low temperatures.

Spin-lattice relaxation rate diverges at $T_N$.

Gapless excitations observed in the ordered state.

Abstract

Kitaev's honeycomb spin-liquid model and its proposed realization in materials such as $\alpha$-RuCl$_3$, Li$_2$IrO$_3$ and Na$_2$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 $^{23}$Na nuclear magnetic resonance to probe both static and dynamical magnetic properties in single crystal Na$_2$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.