Layer-resolved electronic structure of transition-metal oxide films: A scanning tunneling spectroscopy and density functional theory study

Widdra, Wolf1; Grosser, Stephan1; Hagendorf , Christian1; Daene, Markus1; Hergert, Wolfram1; Ernst, Artur1; Lüders, M2; Szotek, Z2; Temmerman, W.M.2 1Germany; 2United Kingdom

Scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) have been applied to study the local electronic and geometric structure of NiO and CoO thin films on Ag(001). Upon metal deposition in an O2 atmosphere and subsequent annealing of the films, ordered (001)-oriented monoxide films have been grown. Differential conductance spectra for structurally well-characterized oxide islands up to ten monolayer thickness have been determined by STS in the range of -6 to 10 eV. They exhibit distinct layer-dependent differences in the electronic structure for the occupied as well as unoccupied states. The region of the unoccupied states up to 4 eV above EF is dominated by the local density of states within the oxide film. Especially for 2 ML islands and thicker films strong and well-resolved unoccupied metal 3d-states are found. At energies above 4.5 eV, the experimental spectra exhibit strong field emission resonances which are characteristically modified by the layer-dependent oxide band structure at higher energies. These results will be compared with DFT calculations for NiO layers of various thickness on a Ag(001) substrate considering also different magnetic ordering within the NiO film. The strong electron correlation effects in the oxide are taken into account via self-interaction corrections. The calculated layer-dependent evolution of the NiO electronic structure as well as the appearance of a Ni3d-derived surface state at a critical film thickness or 3 ML compare well with the experimental results.

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