Perovskite (LaXO3)2 /(LaAlO3)4 (111) superlattices with X spanning the entire 3d transition-metal series combine the strongly correlated, multiorbital nature of electrons in transition-metal oxides with a honeycomb lattice as a key feature. Based on density functional theory calculations including strong interaction effects, we establish trends in the evolution of electronic states as a function of several control parameters: band filling, interaction strength, spin-orbit coupling (SOC), and lattice instabilities. Competition between local pseudocubic and global trigonal symmetry as well as the additional flexibility provided by the magnetic and spin degrees of freedom of 3d ions lead to a broad array of distinctive broken-symmetry ground states not accessible for the (001)-growth direction, offering a platform to design two-dimensional electronic functionalities. Constraining the symmetry between the two triangular sublattices causes X = Mn, Co, and Ti to emerge as Chern insulators driven by SOC. For X=Mn we illustrate how interaction strength and lattice distortions can tune these systems between a Dirac semimetal, a Chern and a trivial Mott insulator.
Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate
S. Middey, D. Meyers, D. Doennig, M. Kareev, X. Liu, Y. Cao, Zhenzhong Yang, Jinan Shi, Lin Gu, P. J. Ryan, R. Pentcheva, J. W. Freeland, and J. Chakhalian
Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of the material properties, which, along with the interactions, define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate, NdNiO3, combined with a dielectric spacer, LaAlO3, has been layered along the pseudocubic  direction. The resulting artificial graphenelike Mott crystal with magnetic 3d electrons has antiferromagnetic correlations. In addition, a combination of resonant X-ray linear dichroism measurements and ab initio calculations reveal the presence of an ordered orbital pattern, which is unattainable in either bulk nickelates or nickelate based heterostructures grown along the  direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering.
Control of orbital reconstruction in (LaAlO3)M/(SrTiO3)N(001) quantum wells by strain and confinement
DFT+U calculations, performed on (001)-oriented (LAO)M/(STO)N and (NGO)M/(STO)N superlattices with n-type interfaces show a rich set of orbital reconstructions depending on the STO quantum well thickness and c/a ratio. A central finding is the pronounced enhancement of octahedral tilts in the STO quantum well that are not present in the bulk and can be unambiguously associated with the electrostatic doping of the polar n-type interface in these systems. Together with the exotic electronic states found in (111)-oriented (LAO)M/(STO)N SLs, the results demonstrate how strain and the thickness of the STO quantum well can be used to engineer the orbital reconstruction and insulator-to-metal transitions in oxide superlattices.
Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides
Zi-An Li, N. Fontaíña-Troitiño, A. Kovács, S. Liébana-Viñas, M. Spasova, R. E. Dunin-Borkowski, M. Müller, D. Doennig, R. Pentcheva, M. Farle and V. Salgueiriño
Polar discontinuities at oxide interfaces may be a source of novel functionality which is not available in the bulk constituents. While most of the research so far has focused on heterointerfaces derived from the perovskite structure. H, here we report from high-resolution transmission electron microscopy and quantitative magnetometry a robust – above room temperature (Curie temperature TC>300 K) – stable ferromagnetically coupled interface layer between the antiferromagnetic rocksalt CoO core and a 2–4 nm thick antiferromagnetic spinel Co3O4 surface layer in octahedron-shaped nanocrystals. Density functional theory calculations with an on-site Coulomb repulsion parameter identify the origin of the experimentally observed ferromagnetic phase as a charge transfer process (partial reduction) of Co3+ to Co2+ at the CoO/Co3O4 interface, with Co2+ being in the low spin state, unlike the high spin state of its counterpart in CoO. This finding may serve as a guideline for designing new functional nanomagnets and/or catalysts based on oxidation resistant antiferromagnetic transition metal oxides.
Massive Symmetry Breaking in Quantum Wells: A Three-Orbital Strongly Correlated Generalization of Graphene
In contrast to the much studied (001)-oriented perovskite superlattices,systems with (111)-orietation promise to host even more exotic electronicstates due to their distinct topology: For example, a bilayer of octahedrally coordianted B-sites of the ABO3 structure forms a buckled honeycomb lattice. Material-specific density functional theory calculations with an on-site Coulomb repulsion term (DFT+U) are used to explore the role of confinement, symmetry breaking, polarity mismatch and strain in the emergence of novel electronic phases. The results illuminate a rich set of competing ground states in polar (LaAlO3)𝑀/(SrTiO3)𝑁(111) and non-polar (LaNiO3)𝑁/ (LaAlO3)𝑀(111) superlattices, ranging from spin-polarized, Dirac-point Fermi surfaces protected by lattice symmetry to charge-ordered Mott or Peierls insulating phases. Analogous to the (001) counterparts, orbital reconstructions and metal-to-insulator transitions depend critically on the thickness of the quantum well 𝑁 and in-plane strain, thus opening avenues for engineering properties at the nanoscale.