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One of the major puzzles in condensed matter physics has been the observationof a Mott-insulating state away from half-filling. The filling-controlled Mottinsulator-metal transition, induced via charge-carrier doping, has beenextensively researched, but its governing mechanisms have yet to be fullyunderstood. Several theoretical proposals aimed to elucidate the nature of thetransition have been put forth, a notable one being phase separation and anassociated percolation-induced transition. In the present work, we study theprototypical doped Mott-insulating rare-earth titanate YTiO$_3$, in which theinsulating state survives up to a large hole concentration of 35%. Singlecrystals of Y$_{1-x}$Ca$_x$TiO$_3$ with $0 \leq x \leq 0.5$, spanning theinsulator-metal transition, are grown and investigated. Using x-ray absorptionspectroscopy, a powerful technique capable of probing element-specificelectronic states, we find that the primary effect of hole doping is to induceelectronic phase separation into hole-rich and hole-poor regions. The datareveal the formation of electronic states within the Mott-Hubbard gap, near theFermi level, which increase in spectral weight with increasing doping. From acomparison with DFT+$U$ calculations, we infer that the hole-poor and hole-richcomponents have charge densities that correspond to the Mott-insulating $x = 0$and metallic $x \sim 0.5$ states, respectively, and that the new electronicstates arise from the metallic component. Our results indicate that thehole-doping-induced insulator-metal transition in Y$_{1-x}$Ca$_x$TiO$_3$ isindeed percolative in nature, and thus of inherent first-order character.

Two-component electronic phase separation in the doped Mott insulator Y$_{1-x}$Ca$_{x}$TiO$_{3}$