Contrasting electronic states of RuI3 and RuCl3

The spin-orbital entangled states are of great interest as they hold exotic phases and intriguing properties. Here we use first-principles calculations to investigate the electronic and magnetic properties of ${\mathrm{RuI}}_{3}$ and ${\mathrm{RuCl}}_{3}$ in both bulk and monolayer cases. Our results show that ${\mathrm{RuI}}_{3}$ bulk is a paramagnetic metal, which is in agreement with recent experiments. We find that the ${\mathrm{Ru}}^{3+}$ ion of ${\mathrm{RuI}}_{3}$ is in the spin-orbital entangled ${j}_{\mathrm{eff}}=\frac{1}{2}$ state. More interestingly, a metal-insulator transition occurs from ${\mathrm{RuI}}_{3}$ bulk to monolayer, and this is mainly due to the band narrowing with the decreasing lattice dimensionality and to the Ru-I hybridization altered by the I $5p$ spin-orbit coupling. In contrast, ${\mathrm{RuCl}}_{3}$ bulk and monolayer both show Mott-insulating behavior, the ${\mathrm{Ru}}^{3+}$ ion is in the formal $S=\frac{1}{2}$ and $L=1$ state with a large in-plane orbital moment, and this result well explains the experimental large effective magnetic moment of ${\mathrm{RuCl}}_{3}$ and the strong in-plane magnetization. The present work demonstrates the contrasting spin-orbital states and the varying properties of ${\mathrm{RuI}}_{3}$ and ${\mathrm{RuCl}}_{3}$.