A set of broken symmetry two-dimensional ground states are predicted in (111)-oriented (LaNiO3)N /(LaAlO3)M (N /M ) superlattices, based on density functional theory (DFT) calculations including a Hubbard U term. An unanticipated Jahn-Teller distortion with d z 2 orbital polarization and a FM Mott insulating (and multiferroic) phase emerges in the double perovskite (1/1), that shows strong susceptibility to strain-controlled orbital engineering. The LaNiO3 bilayer with graphene topology has a switchable multiferroic (ferromagnetic (FM) and ferroelectric) insulating ground state with inequivalent Ni sites. Beyond N = 3 the confined LaNiO3 slab undergoes a metal-to-insulator transition through a half-semimetallic phase with conduction originating from the interfaces. Antiferromagnetic arrangements allow combining motifs of the bilayer and single trigonal layer band structures in designed artificial mixed phases.PACS numbers: 73.21. Fg, 73.22.Gk, 75.70.Cn Rare earth nickelates RNiO 3 (RNO), with formal d 7 configuration, exhibit intriguing properties, e.g. a temperature-driven metal-to-insulator transition (MIT), related to the strongly distorted perovskite structure and the size of rare earth ion R 1,2 . The origin of MIT is strongly debated: instead of the Jahn-Teller (JT) distortion that one may expect of an e 1 g ion, charge order 3 , a site-selective Mott transition 4 or a prosaic order-disorder origin 5 have been discussed. Recently, LaNiO 3 (LNO), the only RNO representative that remains metallic at all temperatures 6 , has been in the spotlight of research, due to the proposal that a cuprate-like behavior can be stabilized when confined in a superlattice (SL) with a band insulator, e.g. LaAlO 3 (LAO)7 . However, despite intensive efforts the selective d x 2 −y 2 or d z 2 orbital polarization as a function of strain could only partially be realized 8 . Instead, DFT studies on (001) SLs indicate that both e g states contribute to the Fermi surface 9-11 . Nevertheless, these (001) SLs have proven to be a fruitful playground to explore lowdimensional phenomena such as a MIT due to confinement and Coulomb interaction [11][12][13][14][15][16] . The possibility of topologically nontrivial behavior is currently shifting the interest from the much studied (001) stacking of AO/BO 2 planes to the (111)-perovskite superlattices with a B/AO 3 sequence. Theoretical work has concentrated on the LNO bilayer sandwiched between LAO, where two triangular NiO 6 octahedron layers form a buckled honeycomb lattice. Model Hamiltonian studies together with DFT calculations 17-20 have shown topological phases with a set of four symmetric (around band center) bands, two flat and two crossing, forming a Dirac point (DP) at K, with quadratic band touching points at Γ. First experiments 21 report the growth of (LNO) N /(LAO) M (111) superlattices on mixed-terminated LAO(111) surfaces with sheet resistance and activated transport, characteristic more of semiconductors than the predicted Dirac-point semimetals [17][18][19] , necessitat...

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.

Perovskite materials engineered in epitaxial heterostructures have been intensely investigated during the last decade. The interface formed by an LaAlO 3 thin film grown on top of a TiO 2 -terminated SrTiO 3 substrate hosts a two-dimensional electronic system and has become the prototypical example of this field. Although controversy exists regarding some of its physical properties and their precise origin, it is universally found that conductivity only appears beyond an LaAlO 3 thickness threshold of four unit cells. Here, we experimentally demonstrate that this critical thickness can be reduced to just one unit cell when a metallic film of cobalt is deposited on top of LaAlO 3 . First-principles calculations indicate that Co modifies the electrostatic boundary conditions and induces a charge transfer towards the Ti 3d bands, supporting the electrostatic origin of the electronic system at the LaAlO 3 /SrTiO 3 interface. Our results expand the interest of this low-dimensional oxide system from in-plane to perpendicular transport and to the exploration of elastic and inelastic tunnel-type transport of (spin-polarized) carriers.

Perovskite bilayers with (111)-orientation combine a honeycomb lattice as a key feature with the strongly correlated, multiorbital nature of electrons in transition metal oxides. In a systematic DFT+U study of (111)-oriented (LaXO3)2/(LaAlO3)4 superlattices, we establish trends in the evolution of ground states versus band filling in (111)-oriented (LaXO3)2/(LaAlO3)4 superlattices, with X spanning the entire 3d transition metal series. The competition between local quasi-cubic and global triangular symmetry triggers unanticipated broken symmetry phases, with mechanisms ranging from Jahn-Teller distortions, to charge-, spin-, and orbital-ordering. LaMnO3, where spinorbit coupling opens a sizable gap in the Dirac-point Fermi surface, emerges as a topological Chern insulator.