  The perovskite SrTiO$_3$-LaAlO$_3$ structure has advanced to a model system
to investigate the rich electronic phenomena arising at polar interfaces. Using
first principles calculations and transport measurements we demonstrate that an
additional SrTiO$_3$ capping layer prevents structural and chemical
reconstruction at the LaAlO$_3$ surface and triggers the electronic
reconstruction at a significantly lower LaAlO$_3$ film thickness than for the
uncapped systems. Combined theoretical and experimental evidence (from
magnetotransport and ultraviolet photoelectron spectroscopy) suggests two
spatially separated sheets with electron and hole carriers, that are as close
as 1 nm.
