Massive symmetry breaking in LaAlO$_3$/SrTiO$_3$(111) quantum wells: a three-orbital, strongly correlated generalization of graphene

Abstract

Density functional theory calculations with an on-site Coulomb repulsion term (GGA+U method) reveal competing ground states in (111) oriented (LaAlO$_3$)$_M$/(SrTiO$_3$)$_N$ superlattices with n-type interfaces, ranging from spin, orbital polarized, Dirac point Fermi surface to charge ordered flat band phases. These are steered by the interplay of (i) Hubbard U, (ii) SrTiO$_3$ quantum well thickness and (iii) crystal field spitting tied to in-plane strain. In the honeycomb lattice bilayer case 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.