We have investigated unusual phase transitions that were triggered by chemical doping in $\mathrm{C}{\mathrm{a}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$. Our experiments showed that doping with a few percent of Mn ($>4\%$) can change the quasi-two-dimensional metallic state of $\mathrm{C}{\mathrm{a}}_3\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ 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 $T_{\mathrm{MIT}}$ is found to be predetermined by a structural parameter denoted by $c/\sqrt{ab}$ at temperatures far above Néel temperature $T_{\mathrm{N}}$. 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.

• Received 29 September 2016
• Revised 25 August 2017

DOI:https://doi.org/10.1103/PhysRevB.96.205105