  Using spin-assisted ab-initio random structure searches, we explore an
exhaustive quantum phase diagram of archetypal interfaced Mott insulators, i.e.
lanthanum-iron and lanthanum-titanium oxides. In particular, we report that the
charge transfer induced by the interfacial electronic reconstruction stabilises
a high spin ferrous Fe2+ state. We provide a pathway to control the strength of
correlation in this electronic state by tuning the epitaxial strain, yielding a
manifold of quantum electronic phases, i.e. Mott-Hubbard, charge transfer and
Slater insulating states. Furthermore we report that the electronic
correlations are closely related to the structural oxygen octahedral rotations,
whose control is able to stabilise the low spin state of Fe2+ at low pressure
previously observed only under the extreme high pressure conditions in the
Earth's lower mantle. Thus we provide avenues for magnetic switching via THz
radiations which have crucial implications for next generation of spintronics
technologies.
