The electronic structure and morphology of ultrathin MgO films epitaxially grown on Ag(001) were investigated using low-temperature scanning tunneling spectroscopy and scanning tunneling microscopy. Layer-resolved differential conductance (dI/dU) measurements reveal that, even at a film thickness of three monolayers, a band gap of about 6 eV is formed corresponding to that of the MgO(001) single-crystal surface. This finding is confirmed by layer-resolved calculations of the local density of states based on density functional theory.

Scanning tunneling microscopy at low temperature reveals that upon dissociative adsorption of oxygen on Ag(001) “hot” adatoms have separated in a far-ranged transient motion to two different intrapair distances around 2 and 4 nm, corresponding to 7 and 14 surface lattice constants, respectively. Manipulation experiments on oxygen atoms displaying different contrasts in the images and model calculations suggest that the transient motion ends up not only in the stable fourfold hollow site but also in two metastable sites.

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The electronic structure of the Kondo insulator Ce3Bi4Pt3 has been studied by high-resolution photoelectron spectroscopy. Indications of the unusual electronic structure are observed: the 4f derived peak near the Fermi level is located at a binding energy of 20 meV and is not cut-off by the Fermi edge as in conventional Kondo systems; for low photon energies, where the 4f contribution is negligible and the conduction band states are probed, a loss of spectral intensity in the same energy range is observed. These observations are related to the opening of a gap at low temperature, confirming the picture that hybridisation between a localised 4f state and the conduction band is responsible for the insulating character of this material.

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