Mott transition extremely sensitive to impurities in Ca3Ru2O7 revealed by hard x-ray photoemission studies

Hard x-ray photoemission studies of Ca-3(Ru1-xTix)(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-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 3d core-level spectra are significantly reduced below T-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-S for x > 0. These results provide strong evidence that Mott transition is induced by a minute amount of the Ti substitution in Ca3Ru2O7.