Crystal-facet-dependent surface transformation dictates the oxygen evolution reaction activity in lanthanum nickelate.

Electrocatalysts are the cornerstone in the transition to sustainable energy technologies and chemical processes. Surface transformations under operation conditions dictate the activity and stability. However, the dependence of the surface structure and transformation on the exposed crystallographic facet remains elusive, impeding rational catalyst design.

High Chern numbers in a perovskite-derived dice lattice (LaXO)/(LaAlO)(111) with X = Ti, Mn and Co.

The dice lattice, containing a stack of three triangular lattices, has been proposed to exhibit nontrivial flat bands with nonzero Chern numbers, but unlike the honeycomb lattice it is much less studied. By employing density-functional theory (DFT) calculations with an on-site Coulomb repulsion term, we explore systematically the electronic and topological properties of (LaXO)/(LaAlO)(111) superlattices with X = Ti, Mn and Co, where a LaAlO trilayer spacer confines the LaXO (LXO) dice lattice. In the absence of spin-orbit coupling (SOC) with symmetry constrained to P3, the ferromagnetic (FM) phase of the LXO(111) trilayers exhibits a half-metallic band structure with multiple Dirac crossings and coupled electron-hole pockets around the Fermi energy.

Resolving the polar interface of infinite-layer nickelate thin films.

Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and d electron filling, however, superconductivity in nickelates has thus far only been stabilized in thin-film geometry, raising questions about the polar interface between substrate and thin film. Here we conduct a detailed experimental and theoretical study of the prototypical interface between NdSrNiO and SrTiO.

Synergistic Effects of Co and Fe on the Oxygen Evolution Reaction Activity of LaCo Fe O.

In a combined experimental and theoretical study we assess the role of Co incorporation on the OER activity of LaCo Fe O . Phase pure perovskites were synthesized up to in 0.025/0.

Surface oxygen Vacancies on Reduced Co O (100): Superoxide Formation and Ultra-Low-Temperature CO Oxidation.

The activation of molecular oxygen is a fundamental step in almost all catalytic oxidation reactions. We have studied this topic and the role of surface vacancies for Co O (100) films with a synergistic combination of experimental and theoretical methods. We show that the as-prepared surface is B-layer terminated and that mild reduction produces oxygen single and double vacancies in this layer.

Extreme tensile strain states in LaCaMnO membranes.

A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites, the balance between metallic and insulating phases can be tuned by the lattice; extending the range of lattice control would enhance the ability to access other phases. We stabilized uniform extreme tensile strain in nanoscale LaCaMnO membranes, exceeding 8% uniaxially and 5% biaxially.

Influence of 3d, 4d, and 5d dopants on the oxygen evolution reaction at α-FeO(0001) under dark and illumination conditions.

Using density functional theory+U (DFT+U) calculations, we explore the effect of dopants on the performance of α-FeO(0001) as an anode material for the oxygen evolution reaction (OER). Systematic screening of 3d, 4d, and 5d transition metal dopants indicates general trends with dopant band filling and allows us to identify the most efficient dopants with respect to the overpotential and relate those to the solution energy and electronic properties. Different conditions (electrochemical vs photoelectrochemical) are accounted for by considering hydroxylated, hydrated, and oxygenated terminations.

How photocorrosion can trick you: a detailed study on low-bandgap Li doped CuO photocathodes for solar hydrogen production.

The efficiency of photoelectrochemical tandem cells is still limited by the availability of stable low band gap electrodes. In this work, we report a photocathode based on lithium doped copper(ii) oxide, a black p-type semiconductor. Density functional theory calculations with a Hubbard U term show that low concentrations of Li (LiCuO) lead to an upward shift of the valence band maximum that crosses the Fermi level and results in a p-type semiconductor.

Chern and Z topological insulating phases in perovskite-derived 4d and 5d oxide buckled honeycomb lattices.

Based on density functional theory calculations including a Coulomb repulsion parameter U, we explore the topological properties of (LaXO)/(LaAlO) (111) with X = 4d and 5d cations. The metastable ferromagnetic phases of LaTcO and LaPtO with preserved P321 symmetry emerge as Chern insulators (CI) with C = 2 and 1 and band gaps of 41 and 38 meV at the lateral lattice constant of LaAlO, respectively. Berry curvatures, spin textures as well as edge states provide additional insight into the nature of the CI states.

Viewpoint: Atomic-Scale Design Protocols toward Energy, Electronic, Catalysis, and Sensing Applications.

Nanostructured materials are essential building blocks for the fabrication of new devices for energy harvesting/storage, sensing, catalysis, magnetic, and optoelectronic applications. However, because of the increase of technological needs, it is essential to identify new functional materials and improve the properties of existing ones. The objective of this Viewpoint is to examine the state of the art of atomic-scale simulative and experimental protocols aimed to the design of novel functional nanostructured materials, and to present new perspectives in the relative fields.

Electronic Structure of a Graphene-like Artificial Crystal of NdNiO.

Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO confined between two 4-unit cell-thick layers of insulating LaAlO.

Modulations in martensitic Heusler alloys originate from nanotwin ordering.

Heusler alloys exhibiting magnetic and martensitic transitions enable applications like magnetocaloric refrigeration and actuation based on the magnetic shape memory effect. Their outstanding functional properties depend on low hysteresis losses and low actuation fields. These are only achieved if the atomic positions deviate from a tetragonal lattice by periodic displacements.

Nonzero Berry phase in quantum oscillations from giant Rashba-type spin splitting in LaTiO/SrTiO heterostructures.

The manipulation of the spin degrees of freedom in a solid has been of fundamental and technological interest recently for developing high-speed, low-power computational devices. There has been much work focused on developing highly spin-polarized materials and understanding their behavior when incorporated into so-called spintronic devices. These devices usually require spin splitting with magnetic fields.

Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate.

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, NdNiO_{3}, combined with a dielectric spacer, LaAlO_{3}, has been layered along the pseudocubic [111] direction. The resulting artificial graphenelike Mott crystal with magnetic 3d electrons has antiferromagnetic correlations.

Electrostatic doping as a source for robust ferromagnetism at the interface between antiferromagnetic cobalt oxides.

Polar oxide interfaces are an important focus of research due to their novel functionality which is not available in the bulk constituents. So far, research has focused mainly on heterointerfaces derived from the perovskite structure. It is important to extend our understanding of electronic reconstruction phenomena to a broader class of materials and structure types.

Control of orbital reconstruction in (LaAlO3)M/(SrTiO3)N(001) quantum wells by strain and confinement.

The diverse functionality emerging at oxide interfaces calls for a fundamental understanding of the mechanisms and control parameters of electronic reconstructions. Here, we explore the evolution of electronic phases in (LaAlO3)M/(SrTiO3)N (001) superlattices as a function of strain and confinement of the SrTiO3 quantum well. Density functional theory calculations including a Hubbard U term reveal a charge ordered Ti(3+) and Ti(4+) state for N = 2 with an unanticipated orbital reconstruction, displaying alternating d(xz) and d(yz) character at the Ti(3+) sites, unlike the previously reported d(xy) state, obtained only for reduced c-parameter at a(STO).

Suppression of the critical thickness threshold for conductivity at the LaAlO3/SrTiO3 interface.

Perovskite materials engineered in epitaxial heterostructures have been intensely investigated during the last decade. The interface formed by an LaAlO3 thin film grown on top of a TiO2-terminated SrTiO3 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 LaAlO3 thickness threshold of four unit cells.

Massive symmetry breaking in LaAlO3/SrTiO3(111) quantum wells: a three-orbital strongly correlated generalization of graphene.

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.

Termination control of electronic phases in oxide thin films and interfaces: LaAlO3/SrTiO3(001).

A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO(3) and SrTiO(3) such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO(3) films on SrTiO(3)(001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects.

Ab initio electronic structures of rhombohedral and cubic HgXO3 (X = Ti, Pb).

First-principles calculations were performed for orthorhombic HgO, rhombohedral and cubic phases of HgTiO(3) (HTO) and HgPbO(3) (HPO). The calculations show that in the rhombohedral phase HTO is a direct gap insulator with a gap of ∼1.6 eV.

Electronic phenomena at complex oxide interfaces: insights from first principles.

Oxide interfaces have attracted considerable attention in recent years due to the emerging novel behavior which does not exist in the corresponding bulk parent compounds. This opens possibilities for future applications in oxide-based electronics and spintronics devices. Among the different materials combinations, heterostructures containing the two simple band insulators LaAlO(3) and SrTiO(3) have advanced to a model system exhibiting unanticipated properties ranging from conductivity, to magnetism, even to superconductivity.

Parallel electron-hole bilayer conductivity from electronic interface reconstruction.

The perovskite SrTiO3-LaAlO3 structure has advanced to a model system to investigate the rich electronic phenomena arising at polar oxide interfaces. Using first principles calculations and transport measurements we demonstrate that an additional SrTiO3 capping layer prevents atomic reconstruction at the LaAlO3 surface and triggers the electronic reconstruction at a significantly lower LaAlO3 film thickness than for the uncapped systems. Combined theoretical and experimental evidence (from magnetotransport and ultraviolet photoelectron spectroscopy) suggests two spatially separated sheets with electron and hole carriers, that are as close as 1 nm.

Partial dissociation of water on Fe3O4(001): adsorbate induced charge and orbital order.

The interaction of water with Fe3O4(001) is studied by density functional theory calculations including an on-site Coulomb term. For isolated molecules, dissociative adsorption is strongly promoted at surface defect sites, while at higher coverages a hydrogen-bonded network forms with alternating molecular and dissociated species. This mixed adsorption mode and a suppression of the (square root of 2 x square root of 2)R45 degrees reconstruction are confirmed by a quantitative low energy electron diffraction analysis.

Avoiding the polarization catastrophe in LaAlO3 overlayers on SrTiO3(001) through polar distortion.

A pronounced uniform polar distortion extending over several unit cells enables thin LaAlO3 overlayers on SrTiO3(001) to counteract the charge dipole and thereby neutralize the "polarization catastrophe" that is suggested by simple ion counting. This unanticipated mechanism, obtained from density functional theory calculations, allows several unit cells of the LaAlO3 overlayer to remain insulating (hence, fully ionic). The band gap of the system, defined by occupied O 2p states at the surface and unoccupied Ti 3d states at the interface in some cases approximately 20 A distant, decreases with increasing thickness of the LaAlO3 film before an insulator-to-metal transition and a crossover to an electronic reconstruction occurs at around five monolayers of LaAlO3.

Electronic structure and magnetism of EuTiO(3): a first-principles study.

Density-functional theory calculations were carried out for the multiferroic EuTiO(3) using the LDA+U approach. Total-energy calculations for ferromagnetic (F), and antiferromagnetic A-, C-, and G-type arrangements in the cubic phase shows that the ground-state magnetic configuration is G-type antiferromagnetic for U≤6 eV and ferromagnetic for U≥7 eV. Values of first- and second-neighbour exchange integrals have been calculated by mapping the energy difference between the different magnetic configurations to a Heisenberg Hamiltonian.