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In the attempt to understand the mechanism which leads to high-temperature superconductivity in the cuprates it is widely accepted as the least common denominator of this material class that they are doped Mott (or charge-transfer) insulators. Hence, the question how the electronic structure of a Mott insulator develops upon doping is of fundamental interest. TiOCl is a prototypical quasi-one-dimensional Mott insulator with a d1 configuration. In this respect it represents the electron analogue to the almost full d shell with one single hole in the cuprates. We have doped TiOCl by alkali metal evaporation and studied the electronic structure by photoemission spectroscopy. Surprisingly we do not detect a metallic phase at any doping level signalled by the vanishing spectral weight at the chemical potential and the absence of a Fermi edge. These findings are at variance with theoretical expectations within simple one- or multi-orbital Mott-Hubbard scenarios. Against the background of recent molecular dynamics studies combined with density functional calculations for Na doped TiOCl, we interpret our observations in an alloy Mott Hubbard picture. An additional alloy band is induced between the original lower and upper Hubbard bands of the undoped compound which is formed by the 3d states of the Ti sites in the closest vicinity of the alkali metal ions. Due to the lowering of the 3d states at these sites, the electrons donated by the alkali metal atoms to the Ti 3d shell are trapped resulting in a situation where the original lower Hubbard band corresponds exclusively to the singly occupied and the alloy band to the doubly occupied sites. The finite excitation gap between the alloy band and the upper Hubbard band is then responsible for the insulating nature at all doping levels in full quantitative agreement between experiment and theory. In this picture also the transfer of spectral weight between the original lower Hubbard band and the additional spectral weight upon doping, which indicates correlated bands, is reconciled. Thus we identify alkali-metal doped TiOCl as an alloy Mott insulator. |
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