Mott Transition Controlled By Lattice-Orbital Coupling In 3d-Metal-Doped Double-Layer Ruthenates

We have investigated unusual phase transitions that were triggered by chemical doping in Ca3Ru2O7. Our experiments showed that doping with a few percent of Mn (>4%) can change the quasi-two-dimensional metallic state of Ca3Ru2O7 into a Mott insulating state with a G-type antiferromagnetic order, but this Mott state cannot be induced by Fe doping. By combining these results with first-principles calculations, we show that lattice-orbital coupling (LOC) plays an important role in the Mott transition. Interestingly, the transition temperature T-MIT is found to be predetermined by a structural parameter denoted by c/root ab at temperatures far above Neel temperature T-N. This LOC-assisted Mott transition clearly contrasts with the band-filling picture. It is addressed that this type of Mott transition originates in the strong scattering centers formed by specific 3d dopants. The dopant-scattering picture is then applied to explain the puzzling doping effects that occur in other ruthenates and 3d oxides. Our findings will advance the general understanding of how the unusual properties of 4d correlated systems are governed by the complex interplay that occurs among the charge, spin, lattice, and orbital degrees of freedom.