Mott transition controlled by lattice-orbital coupling in double layer ruthenates

We have investigated unusual phase transitions triggered by chemical doping in Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$. Our experiments show a few percent doping of Mn (\textgreater 4{\%}) can switch the quasi-two-dimensional metallic state with the antiferromagnetic order (AFM-b) comprised of ferromagnetic (FM) bilayers of Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$ to a Mott insulating state with the nearest-neighbor antiferromagnetic order (G-AFM), while Fe doping cannot realize such a Mott transition, but leads to a localized state with the AFM-b order. Combined with first-principles calculations, we find that the lattice-orbital coupling (LOC) plays a critical role in driving the Mott transition caused by Mn doping and the Mott transition temperature $T_{\mathrm{MIT}}$ is strikingly dependent on the structural parameter $c/a$ at the temperatures far above $T_{\mathrm{MIT}}$. Such LOC-assisted Mott transition mechanism, which also accounts for the previously-reported Mott transition induced by Ti doping in Ca$_{\mathrm{3}}$Ru$_{\mathrm{2}}$O$_{\mathrm{7}}$, forms a clear contrast with the Mott transition mechanism controlled by band filling in 3$d$ strongly correlated systems. Our findings advance the understanding of how exotic properties of 4$d$ correlated systems are governed by the complex interplay between charge, spin, lattice and orbital degrees of freedom.