Mean-Field Study of Charge, Spin, and Orbital Orderings in Triangular-Lattice Compounds ANiO2 (A = Na, Li, Ag)

We present our theoretical results on the ground states in the layered triangular-lattice compounds ANiO(2) (A = Na, Li, Ag). To describe the interplay between charge, spin, orbital, and lattice degrees of freedom in these materials, we study a doubly degenerate Hubbard model with electron-phonon couplings by the Hartree-Fock approximation combined with the adiabatic approximation. In a weakly correlated region, we find a metallic state accompanied by root 3 x root 3 charge ordering. On the other hand, we obtain an insulating phase with spin-ferro and orbital-ferro ordering in a wide range from intermediate to strong correlation. These phases share many characteristics with the low-temperature states of AgNiO2 and NaNiO2, respectively. The charge-ordered metallic phase is stabilized by a compromise between Coulomb repulsions and effective attractive interactions originating from the breathing-type electron-phonon coupling as well as the Hund's-rule coupling. The spin-orbital-ordered insulating phase is stabilized by the cooperative effect of electron correlations and the Jahn-Teller coupling, while the Hund'-rule coupling also plays a role in the competition with other orbital-ordered phases. The results suggest a unified way of understanding a variety of low-temperature phases in ANiO(2). We also discuss a keen competition among different spin-orbital-ordered phases in relation to the puzzling behavior observed in LiNiO2.