  Heterostructures and superlattices consisting of a prototype Mott insulator,
GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and
show intrinsic electronic reconstruction, approximately 1/2 electron per
surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities
in all structures containing more than one unit cell of SrTiO3 are independent
of layer thicknesses and growth sequences, indicating that the mobile carriers
are in a high concentration, two-dimensional electron gas bound to the
interface. These carrier densities closely meet the electrostatic requirements
for compensating the fixed charge at these polar interfaces. Based on the
experimental results, insights into interfacial band alignments, charge
distribution and the influence of different electrostatic boundary conditions
are obtained.
