  The oxygen stoichiometry has a large influence on the physical and chemical
properties of complex oxides. Most of the functionality in e.g. catalysis and
electrochemistry depends in particular on control of the oxygen stoichiometry.
In order to understand the fundamental properties of intrinsic surfaces of
oxygen-deficient complex oxides, we report on in situ temperature dependent
scanning tunnelling spectroscopy experiments on pristine oxygen deficient,
epitaxial manganite films. Although these films are insulating in subsequent ex
situ in-plane electronic transport experiments at all temperatures, in situ
scanning tunnelling spectroscopic data reveal that the surface of these films
exhibits a metal-insulator transition (MIT) at 120 K, coincident with the onset
of ferromagnetic ordering of small clusters in the bulk of the oxygen-deficient
film. The surprising proximity of the surface MIT transition temperature of
nonstoichiometric films with that of the fully oxygenated bulk suggests that
the electronic properties in the surface region are not significantly affected
by oxygen deficiency in the bulk. This carries important implications for the
understanding and functional design of complex oxides and their interfaces with
specific electronic properties for catalysis, oxide electronics and
electrochemistry.
