Two-component electronic phase separation in the doped Mott insulator Y1-xCaxTiO3

One of the major puzzles in condensed matter physics has been the observation of a Mott-insulating state away from half-filling. Several theoretical proposals aimed to elucidate this phenomenon have been put forth, a notable one being phase separation and an associated percolation-induced Mott insulator-metal transition. In the present work we study the prototypical doped Mott-insulating rare-earth titanate YTiO3, in which the insulating state survives up to a large hole concentration of 35%. Single crystals of Y1-xCaxTiO3 with 0 <= x <= 0.5, spanning the insulator-metal transition, are grown and investigated. Using x-ray absorption spectroscopy, a powerful technique capable of probing element-specific electronic states, we find that the primary effect of hole doping is to induce electronic phase separation into hole-rich and hole-poor regions. The data reveal the formation of electronic states within the Mott-Hubbard gap, near the Fermi level, which increase in spectral weight with increasing doping. From a comparison with DFT+U calculations, we infer that the hole-poor and hole-rich components have charge densities that correspond to the insulating x = 0 and metallic x similar to 0.5 states, respectively, and that the new electronic states arise from the metallic component. Our results indicate that the doping-induced insulator-metal transition in Y1-xCaxTiO3 is indeed percolative in nature, and thus of inherent first-order character.