Ultrafast reflectivity dynamics in the honeycomb iridates

The combination of strong spin orbit coupling and Mott physics in the iridium oxides produces a variety of interesting phenomena. In the A$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ compounds, this is thought to give rise to spin-anisotropic magnetic interactions described by the Kitaev model. While Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ displays simple zigzag antiferromagnetism, the complex, incommensurate spiral magnetic order observed in $\gamma $-Li$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ suggests that Kitaev exchange is the dominant spin interaction in this system. In this work, we performed ultrafast pump-probe reflectivity measurements on single crystals of Na$_{\mathrm{2}}$IrO$_{\mathrm{3\thinspace }}$ and $\gamma $-Li$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ using nJ pulses at 1.5 eV photon energy. At high temperatures, we observe a reflectivity transient corresponding to electronic heating and excitation across the Mott gap which is isotropic with respect to probe polarization. In $\gamma $-Li$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ , a small anisotropy emerges upon cooling close to the Ne\'{e}l transition at 38 K, followed by an abrupt onset of a long lived, highly anisotropic photo-induced increase in the reflectivity at T$_{\mathrm{N}}$. The temperature dependent dynamics of this signal indicate that it corresponds to non-thermal destruction of the magnetic order. Although similar reflectivity dynamics are observed at low temperature in Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$, there are no clear features related to the Ne\'{e}l transition at 16 K.