Soft x-ray absorption study of the doped Mott insulator Y_1-xCa_xTiO₃

Metal-insulator transitions are ubiquitous in condensed matter physics and have been the subject of decades of extensive research. Rare-earth titanates are Mott-Hubbard insulators which exhibit such transitions into a metallic phase upon charge carrier doping. Depending on the rare-earth ion, the doping at which the transition occurs varies widely; La_1-xSr_xTiO₃ requires as little as x = 0.05 to become metallic, whereas Y_1-xCa_xTiO₃ turns metallic only at x = 0.4. YTiO₃ is particularly interesting due to this enhanced stability of the insulating state against charge-carrier doping. We report a detailed X-ray absorption spectroscopic (XAS) study of the electronic structure across the insulator-metal transition in Y_1-xCa_xTiO₃. From Ti L-edge XAS, we find that Ca-doping is associated with a proportional increase in the volume fraction of the Ti⁴⁺ content. O K-edge XAS reveals that the insulator-metal transition is driven by a formation of in-gap states of mixed O 2p and Ti 3d character, which increase in spectral intensity with increasing doping as well as decreasing temperature, eventually leading to the closing of the charge gap.