Mott physics and band topology in materials with strong spin-orbit interaction

Recent theory and experiment have revealed that strong spin-orbit coupling can have marked qualitative effects on the band structure of weakly interacting solids, leading to a distinct phase of matter, the topological band insulator. We show that spin-orbit interaction also has quantitative and qualitative effects on the correlation-driven Mott insulator transition. Taking Ir-based pyrochlores as a specific example, we predict that for weak electron-electron interaction Ir electrons are in metallic and topological band insulator phases at weak and strong spin-orbit interaction, respectively. We show that by increasing the electron-electron interaction strength, the effects of spin-orbit coupling are enhanced. With increasing interactions, the topological band insulator is transformed into a 'topological Mott insulator' phase having gapless surface spin-only excitations. The proposed phase diagram also includes a region of gapless Mott insulator with a spinon Fermi surface, and a magnetically ordered state at still larger electron-electron interaction.