  Perovskite oxides exhibit a plethora of exceptional electronic properties,
providing the basis for novel concepts of oxide-electronic devices. The
interest in these materials is even extended by the remarkable characteristics
of their interfaces. Studies on single epitaxial connections between the two
wide-bandgap insulators LaAlO3 and SrTiO3 have revealed them to be either
high-mobility electron conductors or insulating, depending on the atomic
stacking sequences. In the latter case they are conceivably positively charged.
For device applications, as well as for basic understanding of the interface
conduction mechanism, it is important to investigate the electronic coupling of
closely-spaced complementary interfaces. Here we report the successful
realization of such electronically coupled complementary interfaces in SrTiO3 -
LaAlO3 thin film multilayer structures, in which the atomic stacking sequence
at the interfaces was confirmed by quantitative transmission electron
microscopy. We found a critical separation distance of 6 perovskite unit cell
layers, corresponding to approximately 2.3 nm, below which a decrease of the
interface conductivity and carrier density occurs. Interestingly, the high
carrier mobilities characterizing the separate electron doped interfaces are
found to be maintained in coupled structures down to sub-nanometer interface
spacing.
