  The diverse functionality emerging at oxide interfaces calls for a
fundamental understanding of the mechanisms and control parameters of
electronic reconstructions. Here, we explore the evolution of electronic phases
in (LaAlO3)M/(SrTiO3)N(001) superlattices as a function of strain and
confinement of the SrTiO3 quantum well. Density functional theory calculations
including a Hubbard U term reveal a charge ordered Ti3+ and Ti4+ state for N=2
with an unanticipated orbital reconstruction, displaying alternating dxz and
dyz character at the Ti3+ sites, unlike the previously reported dxy state,
obtained only for reduced c-parameter at aSTO. At aLAO c-compression leads to a
Dimer-Mott insulator with alternating dxz, dyz sites and an almost zero band
gap. Beyond a critical thickness of N=3 (aSTO) and N=4 (aLAO) an
insulator-to-metal transition takes place, where the extra e/2 electron at the
interface is redistributed throughout the STO slab with a dxy interface orbital
occupation and a mixed dxz + dyz occupation in the inner layers. Chemical
variation of the SrTiO3 counterpart (LaAlO3 vs. NdGaO3) proves that the
significant octahedral tilts and distortions in the STO quantum well are
induced primarily by the electrostatic doping at the polar interface and not by
variation of the STO counterpart.
