J_eff=1/2 states, Kitaev exchange, and proximate spin liquid in Na₂IrO₃/α-RuCl₃
In Na₂IrO₃ and α-RuCl₃, strong spin-orbit coupling (λ ≈ 0.5 eV) splits the t₂g d-states into J_eff=1/2 (upper) and J_eff=3/2 (lower) bands. With one electron in J_eff=1/2 and moderate U, a spin-orbit-assisted Mott insulator forms. The bond-directional Kitaev exchange K (from edge-sharing octahedra) competes with Heisenberg J, placing these materials near a quantum spin liquid phase with Majorana excitations.