LAYERED RUTHENIUM OXIDES - FROM BAND METAL TO MOTT INSULATOR

We present results of the first optical and angle-resolved photoemissi on study on a layered ruthenium oxide system with various Ca/Sr substi tution levels. Using two-plane (ST1-xCax)(3)RU2O7 and one-plane Ca2RuO 4, crystals we were able to study evolution of the electronic properti es in a range from a band metal (Sr3RU2O7) to a ''bad'' metal (Ca3Ru2O 7) to a Mott-Hubbard insulator (Ca2RuO4). Apart from a Mott-Hubbard me tal-insulator transition (MIT), we have uncovered a qualitative change of the electronic properties at a critical x = 0.33. We suggest that the latter is a result of a quantum phase transition into an antiferro magnetic phase that precedes the real Mott-Hubbard MIT. [S0031-9007(98 )07196-8].