Recently, α-RuCl₃ has been extensively studied because of potential bond-dependent Kitaev magnetic exchange interactions and the resulting quantum spin liquid phase that can be realized therein. The covalency between Ru 4d- and Cl p-orbitals is crucial for inducing large Kitaev interactions in this compound, therefore replacing Cl with heavier halogen elements such as Br or I appears to be a promising method for further promoting the Kitaev interaction. There have been several reports on synthesis of α-RuBr₃ and α-RuI₃, which are expected to host the same spin-orbit-entangled orbitals and Kitaev exchange interactions with α-RuCl₃. This study investigated electronic structures of α-RuCl₃, α-RuBr₃, and α-RuI₃ via comparisons, focusing on the cooperation of the spin-orbit coupling and on-site Coulomb repulsions to realize the spin-orbit-entangled pseudospin-1/2 at Ru sites. In addition, magnetic exchange interactions of all three compounds were estimated, thereby demonstrating that α-RuBr₃ can be promising candidates for realizing Kitaev spin liquid phases in solid-state systems.

Keywords: Kitaev magnetism, Layered transition metal halides, Spin-orbit coupling, Strongly correlated electron systems, Frustrated magnetism, Density functional theory