Massive Symmetry Breaking in LaAlO_3/SrTiO_3(111) Quantum Wells: A Three-Orbital Strongly Correlated Generalization of Graphene

摘要

Density functional theory calculations with an on-site Coulomb repulsion term reveal competing ground states in (111)-oriented (LaAlO_3)_M/(SrTiO_3)_N superlattices with n-type interfaces, ranging from spin, orbitally polarized (with selective e'_g, a_(1g), or d_(xy) occupation), Dirac point Fermi surface, to charge-ordered flat band phases. These phases are steered by the interplay of (ⅰ) Hubbard U, (ⅱ) SrTiO_3 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 SrTiO_3 quantum well thickness an insulator-to-metal transition occurs.