Hard x-ray photoemission studies of Ca${}_3$(Ru${}_{1-x}$Ti${}_x$)${}_2$O${}_7$ with $x$ $=$ 0, 0.003, and 0.005 have been carried out to clarify the origin of the extremely high sensitivity of the metal-insulator (M-I) transition to the amount of Ti substitution. All the compounds show an antiferromagnetic transition followed by a structural transition at a temperature $T_{\mathrm{S}}$, with decreasing temperature. For $x$ > 0, the structural transition is accompanied with a drastic M-I transition and the intensities of the screened peaks in the Ru 3$d$ core-level spectra are significantly reduced below $T_{\mathrm{S}}$. The spectral weight of the valence band is concurrently shifted toward the higher binding energy side by an order of 2 eV, which shows that a strong electron correlation plays an essential role for the M-I transition at $T_{\mathrm{S}}$ for $x$ > 0. These results provide strong evidence that Mott transition is induced by a minute amount of the Ti substitution in Ca${}_3$Ru${}_2$O${}_7$.

• Received 7 March 2013

DOI:https://doi.org/10.1103/PhysRevB.87.241107