Analysis of magnetic states in Kitaev candidate materials RuX₃ (X = Cl, Br) using a localized effective model based on first-principles calculations

Mott insulators exhibit emergent spin and/or orbital degrees of freedom, leading to various intriguing physical phenomena driven by quantum effects, such as excitonic insulators and spin liquids. For instance, spin-orbital Mott insulators RuX₃ (X = Cl, Br) have attracted considerable attention as candidate materials for Kitaev spin liquids, and extensive studies have been conducted on changes in their physical properties induced by halogen substitution. In addition, a spin liquid state under a 100 T-class out-of-plane magnetic field has been reported for RuCl₃. To understand such phenomena theoretically, a quantitative analysis of these materials requires investigating multiorbital Hubbard models that reflect their band structure. However, this is challenging due to the high computational cost, necessitating a change of approach.

In this work, we developed a numerical method to overcome this difficulty by employing localized effective models based on first-principles calculations. Thus far, we have established a framework of localized effective models constructed from Quantum ESPRESSO, Wannier90, and RESPACK. We then analyze the model using flavor-wave theory, a generalization of spin-wave theory. In this talk, we will discuss a change of the exchange interactions and g-factors derived from halogen substitution and present results on magnetic-field responses analyzed within the flavor-wave theory. Our framework provides a comprehensive approach for analyzing multiorbital systems such as van der Waals compounds by integrating mean-field, classical, and flavor-wave theories.