  A combined experimental and theoretical investigation of the electronic
structure of the archetypal oxide heterointerface system LaAlO3 on SrTiO3 is
presented. High-resolution, hard x-ray photoemission is used to uncover the
occupation of Ti 3d states and the relative energetic alignment - and hence
internal electric fields - within the LaAlO3 layer. Firstly, the Ti 2p core
level spectra clearly show occupation of Ti 3d states already for two unit
cells of LaAlO3. Secondly, the LaAlO3 core levels were seen to shift to lower
binding energy as the LaAlO3 overlayer thickness, n, was increased - agreeing
with the expectations from the canonical electron transfer model for the
emergence of conductivity at the interface. However, not only is the energy
offset of only 300meV between n=2 (insulating interface) and n=6 (metallic
interface) an order of magnitude smaller than the simple expectation, but it is
also clearly not the sum of a series of unit-cell by unit-cell shifts within
the LaAlO3 block. Both of these facts argue against the simple charge-transfer
picture involving a cumulative shift of the LaAlO3 valence bands above the
SrTiO3 conduction bands, resulting in charge transfer only for n>3. Turning to
the theoretical data, our density functional simulations show that the presence
of oxygen vacancies at the LaAlO3 surface at the 25% level reverses the
direction of the internal field in the LaAlO3. Therefore, taking the
experimental and theoretical results together, a consistent picture emerges for
real-life samples in which nature does not wait until n=4 and already for n=2,
mechanisms other than internal-electric-field-driven electron transfer from
idealized LaAlO3 to near-interfacial states in the SrTiO3 substrate are active
in heading off the incipient polarization catastrophe that drives the physics
in these systems.
