Doennig, D.; Pickett, Warren E.; Pentcheva, Rossitza (2013): Massive symmetry breaking in LaAlO3/SrTiO3(111) quantum wells: A three-orbital strongly correlated generalization of graphene. In: Physical Review Letters, Vol. 111, Nr. 12
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Density functional theory calculations with an on-site Coulomb repulsion term reveal competing ground states in (111)-oriented (LaAlO3) M/(SrTiO3)N superlattices with n-type interfaces, ranging from spin, orbitally polarized (with selective eg′, a1g, or dxy occupation), Dirac point Fermi surface, to charge-ordered flat band phases. These phases are steered by the interplay of (i) Hubbard U, (ii) SrTiO3 quantum well thickness, and (iii) crystal field splitting tied to in-plane strain. In the honeycomb lattice bilayer N=2 under tensile strain, inversion symmetry breaking drives the system from a ferromagnetic Dirac point (massless Weyl semimetal) to a charge-ordered multiferroic (ferromagnetic and ferroelectric) flat band massive (insulating) phase. With increasing SrTiO3 quantum well thickness an insulator-to-metal transition occurs.