  Understanding and manipulating properties emerging at a surface or an
interface require a thorough knowledge of structure-property relationships. We
report a study of a prototype oxide system, La2/3Sr1/3MnO3 grown on
SrTiO3(001), by combining in-situ angle-resolved x-ray photoelectron
spectroscopy, ex-situ x-ray diffraction, and scanning transmission electron
microscopy/spectroscopy with electric transport measurements. We find that
La2/3Sr1/3MnO3 films thicker than 20 unit cells (u.c.) exhibit a universal
behavior with no more than one u.c. intermixing at the interface but at least 3
u.c. of Sr segregation near the surface which is (La/Sr)O terminated. The
conductivity vs film thickness shows the existence of nonmetallic layers with
thickness ~ 6.5 +/- 0.9 u.c., which is independent of film thickness but mainly
relates to the deviation of Sr concentration near the surface region. Below 20
u.c., the surface of the films appears mixed (La/Sr)O with MnO2 termination.
Decreasing film thickness to less than 10 u.c. leads to the enhanced deviation
of chemical composition in the films and eventually drives the film insulating.
Our observation offers a natural explanation for the thickness-driven
metal-nonmetal transition in thin films based on the variation of film
stoichiometry.
