LaO Layers Research Articles

Magnetic proximity effect in LaO/EuO heterostructures: A first-principles study

The heterostructures constructed with superconductor and ferromagnet (S/F) have attracted intensive attention because of their remarkable properties such as spin-triplet pairing at the interface due to the magnetic proximity effect (MPE). Recently, the heteroepitaxial thin films formed by rock-salt rare earth monoxides LaO and EuO were realized in experiments [Kaminaga et al., Chem. Lett. 48, 1244 (2019)], in which LaO is a conventional superconductor with the superconducting ${T}_{c}\ensuremath{\sim}5$ K and EuO is a ferromagnetic insulator with a Curie temperature of 69 K. To elucidate the MPE in the LaO/EuO heterostructure, we have studied its electronic and magnetic properties based on first-principles calculations. The spin splitting of the surface bands as well as the nonzero local magnetic moments on La atoms confirm the existence of MPE. In addition, the electron gases accumulated at the interface and on top of the EuO layer are also spin-polarized. Interestingly, we find that the spin polarization spreads throughout the LaO layers, in which the electron gases in the interstitial regions play an important role. Furthermore, we demonstrate that the net magnetic moment of the LaO layers in the LaO/EuO heterostructure can be effectively modulated by means of an external electric field. These results not only deepen the understanding of the MPE in the LaO/EuO heterostructures, but also suggest that LaO/EuO can serve as a promising platform for studying the Majorana bound states and may apply in the voltage-controlled superconducting switch.

Alternation of Magnetic Anisotropy Accompanied by Metal-Insulator Transition in Strained Ultrathin Manganite Heterostructures

Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, using x-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy (PES). We successfully detect the clear change of the magnetic behavior of the Mn ions concomitant with the thickness-dependent metal-insulator transition (MIT). Our results suggest that double-exchange interaction induces the ferromagnetism in the metallic LSMO film under tensile strain caused by the SrTiO$_3$ substrate, while superexchange interaction determines the magnetic behavior in the insulating LSMO film under compressive strain originating from the top LAO layer. Based on those findings, the formation of a magnetic dead layer near the LAO/LSMO interface is attributed to competition between the superexchange interaction via Mn 3$d_{3z^2-r^2}$ orbitals under compressive strain and the double-exchange interaction via the 3$d_{x^2-y^2}$ orbitals.

Ruddlesden-Popper type La2NiO4+δ oxide coated by Ag nanoparticles as an outstanding anion intercalation cathode for hybrid supercapacitors

Electrode materials play the important role in electrochemical capacitors (ECs). Ruddlesden-Popper type La2NiO4+δ oxides were investigated as an anion intercalation cathode. It has a special crystal structure, which leads to an interesting oxygen transportation. Ag shows significant conductivity, fast rates of oxide ion dissociation and diffusion, a relatively low price, and high oxygen solubility. La2NiO4+δ oxide was synthesized by citrate method, and then was coated by Ag nanoparticles (La2NiO4+δ@Ag) to further improve the cycling stability and capacity. XRD, SEM, TEM and XPS were used to characterize the phase composition, microstructure and surface chemistry. The electrochemical performances in 1 M KOH electrolyte at room temperature were investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests. The capacity at 1 mV s−1 of La2NiO4+δ@Ag is 466.4 C g−1, and the average efficiency at 10 A g−1 approximates to 93% for 10,000 cycles, showing superior performance stability. Oxygen intercalation, migration and extrusion mechanisms of the La2NiO4+δ@Ag are related to oxygen transportation in the perovskite LaNiO3 layers and the rock salt LaO layers. The superior cycling stability and electrode performance can be ascribed to Ruddlesden-Popper structure of La2NiO4+δ, the role of Ag and the effective unitization of redox process. The assembled La2NiO4+δ@Ag//AC hybrid supercapacitor device shows an excellent energy density of 44.7 Wh kg−1 at a power density of 800 W kg−1.

Strain-induced conductivity accelerated recoveries in LaAlO3/SrTiO3 heterostructure with millimeter scale.

The transport and magnetic properties of LaAlO3/SrTiO3 (LAO/STO) heterostructure have been studied during cooling and warming. The strain gradient perpendicular to the surface of the heterostructure increases with the thickness of LAO film. The conductivity accelerated recoveries (CAR) are found at 80 K and 176 K in the interface of LAO/STO sample with millimeter scale, and are more obvious for thicker LAO layers during warming. These two recovering temperatures correspond to the migrating energies of oxygen single vacancy and divacancy trapped by polarized domain walls, separately. This indicated that domain walls diffuse along the longitudinal direction and expand to larger area due the strain gradient perpendicular to the interface. The stable and precise accelerating recovering temperatures make the sample at a larger scale a potential widely applied temperature standard reference. The magnetization measurements reveal the coexistence of paramagnetic and diamagnetic in the LAO/STO samples at whole temperature from 2 K to 300 K. The abnormal electric resistance rise is observed with the decreasing temperature below 25 K for the samples of 7 and 15 LAO layers. This anomaly is attributed to the Kondo effect below 25 K and weak anti-localization below 5 K due to the weightier paramagnetic content. The larger diamagnetic content suppresses these contributions in 25 LAO layers sample. This work provided an insightful view that the strain modified structure domain leads to the enhancement of CAR effect, which helps to achieve a better understanding of domain related physics in the LAO/STO system.

First-principles study on the electronic structure and optical properties of La0.75Sr0.25MnO3-σ materials with oxygen vacancies defects

The electronic structure and optical properties of La0.75Sr0.25MnO3-σ (LSMO3-σ) materials with 1 × 1 × 4 orthorhombic perovskite structure were performed by first-principles calculation. The structural changing of LSMO3 (ideal structure, σ = 0) was not obvious under generalized gradient approximation (GGA) and GGA + U arithmetic. On the contrary, the structural changing of LSMO3-σ (σ = 0.25, with oxygen vacancies defects in the z = 0, c/8, c/6, c/4, and c/2) with GGA + U were more obvious than the result of ideal. This structural distortion induced distinct changing in density of states (DOS) for LSMO3-σ materials. Oxygen vacancy defects caused a shift of the total density of states (TDOS) features toward low binding energies and LSMO3-σ keep half-metal properties as well as LSMO3 ideal structure. In addition, the hybridization between the Mn-eg and O-2p orbital was weakened and the partial density of states (PDOS) of Mn indicated a strong d-d orbital interaction. By the result of oxygen vacancy formation energy, oxygen vacancy defects can be more easily formed in La-O layers (z = 0 and c/6) to compare with other layers (z = c/8, c/4 and c/2). The calculation result of optical properties suggested that the ideal LSMO could be produced strong absorption in the range of ultraviolet and visible light, while the LSMO3-σ with oxygen vacancies defects were presented weak absorption in the range of visible light.

Pressure induced superconductivity in very lightly doped LaFeAsO0.975F0.025

We have investigated whether or not superconductivity is induced by the application of pressure in very lightly F-doped LaFeAsO1−xFx, which shows spin density wave (SDW) state at ambient pressure, through the measurements of DC magnetization and electrical resistivity under pressure using pulse current sintered (PCS) high density polycrystalline specimens. It has been confirmed that the specimens with x = 0.025 shows superconductivity with Tcdia∼15 K under pressure above ∼ 1.3GPa. The pressure induced superconductivity can be explained by the lattice compression along c-axis, which enhances the electron doping from LaO layers to FeAs layers.

Theoretical Study of Oxygen Vacancy Formation at LSC/GDC Interface

Introduction Doped lanthanum cobaltite and its derivatives are used as cathode materials in solid oxide fuel cell (SOFC). For a high catalytic activity, the improvement of properties governing the cathode performance is important. It is generally regarded that one of the important factors is the electronic structure of cathode material. Recently, the influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was reported [1]. In addition, the heterostructured oxide interface ((La,Sr)2CoO4±δ/La1-xSrxCoO3-δ (LSC)) and thin-film heterostructure interface (La0.6Sr0.4Co0.2Fe0.8O3-δ/gadolinium-doped CeO2 (GDC)) enhanced the surface oxygen exchange kinetics and ORR activity [2,3]. These results indicate the possibility that the structural change near interface changes the electronic structure of cathode materials. Detailed understandings of electronic structure of doped lanthanum cobaltite near the interface is important to design the high catalytic cathode materials. In this study, we analyzed the stability of oxygen vacancy in LSC of LSC/GDC interface by using the density functional theory (DFT) calculation. Computational Details We used LaO-terminated (001) surface of cubic LaCoO3 crystal structure. Half of La atoms were substituted by Sr atoms in LaO layers in LSC slab model (La6Sr6Co8O28). As a surface of CeO2, we used (111) surface model (Ce14O28) from the surface stability. GDC surface model was prepared based on the previous DFT calculation [4]. The interface of LSC and GDC was modeled from CoO2-terminated LSC(001) and GDC(110) surfaces. The initial spin configuration of Co3+ ion was set as intermediate spin state. GGA-PAW potentials were applied with cutoff energy of 600 eV and 1×1×1 k-points. We employed GGA+U to improve the description of Co 3d and Ce 4f electrons. All DFT calculations were performed by using VASP. Results and Discussion We first analyzed the charge distribution near LSC/GDC interface. In case of LSC/CeO2 interface, the electron transferred from CeO2 to LSC. We clearly observed the larger electron transfer from GDC to LSC. These results indicate that the LSC region near CeO2 or GDC interface shows the negatively charged structure. We analyzed the oxygen vacancy formation energy in LSC. The oxygen vacancy formation energy from surface at LSC/GDC interface was more stable than that of LSC model. Although the oxygen vacancy formation energy from 2ndlayer in LSC model was stable, unstable oxygen vacancy formation energy was obtained in the LSC/GDC interface model. One of the reasons of oxygen vacancy formation energy difference between LSC and LSC/GDC interface is an effect of electron transfer from GDC to LSC. The atomic position change at interface is also influenced the oxygen vacancy formation energy. To see the relation between oxygen vacancy formation energy and distance from interface, we prepared larger LSC/GDC interface models. Details are discussed in the presentation. Acknowledgement This work was supported by CREST, Japan Science and Technology Agency. All calculations were performed on the HA8000 computer systems in Research Institute for Information Technology, Kyushu University. The activities of Advanced Automotive Research Collaborative Laboratory in Hiroshima University are supported by Mazda Corporation. The activities of INAMORI Frontier Research Center in Kyushu University are supported by Kyocera Corporation.

Surface atomic structure of epitaxialLaNiO3thin films studied byin situLEED-I(V)

We report in situ low-energy electron diffraction intensity versus voltage [LEED-$I$($V$)] studies of the surface atomic structure of epitaxially grown ${(001)}_{\text{pc}}$-oriented $(\text{pc}=\text{pseudocubic})$ thin films of the correlated $3d$ transition-metal oxide ${\mathrm{LaNiO}}_{3}$. Our analysis indicates the presence of large out-of-plane bucklings of the topmost LaO layers but only minor bucklings of the topmost ${\mathrm{NiO}}_{2}$ layers, in close agreement with earlier surface x-ray diffraction data. In view of materials design approaches that have suggested using atomic-scale polar structural distortions to produce designer electronic structures in artificial nickelate heterostructures, we propose that the broken inversion symmetry inherent to the surface of rare-earth nickelate thin films offers a similar opportunity to study the coupling between atomic structure and electronic properties in this family of materials.

Cationic Redistribution at Epitaxial Interfaces in Superconducting Two-Dimensionally Doped Lanthanum Cuprate Films.

The exploration of interface effects in complex oxide heterostructures has led to the discovery of novel intriguing phenomena in recent years and has opened the path toward the precise tuning of material properties at the nanoscale. One recent example is space-charge superconductivity. Among the complex range of effects which may arise from phase interaction, a crucial role is played by cationic intermixing, which defines the final chemical composition of the interface. In this work, we performed a systematic study on the local cationic redistribution of two-dimensionally doped lanthanum cuprate films grown by oxide molecular beam epitaxy, in which single LaO layers in the epitaxial crystal structure were substituted by layers of differently sized and charged dopants (Ca, Sr, Ba, and Dy). In such a model system, in which the dopant undergoes an asymmetric redistribution across the interface, the evolution of the cationic concentration profile can be effectively tracked by means of atomically resolved imaging and spectroscopic methods. This allowed for the investigation of the impact of the dopant chemistry (ionic size and charge) and of the growth conditions (temperature) on the final superconducting and structural properties. A qualitative model for interface cationic intermixing, based on thermodynamic considerations, is proposed. This work highlights the key role which cationic redistribution may have in the definition of the final interface properties and represents a further step forward the realization of heterostructures with improved quality.

Influence of quantum confinement and strain on orbital polarization of four-layerLaNiO3superlattices: A DFT+DMFT study

Atomically precise superlattices involving transition metal oxides provide a unique opportunity to engineer correlated electron physics using strain (modulated by choice of substate) and quantum confinement (controlled by layer thickness). Here we use the combination of density functional theory and dynamical mean field theory (DFT+DMFT) to study Ni E$_g$ $d$-orbital polarization in strained LaNiO$_3$/LaAlO$_3$ superlattices consisting of four layers of nominally metallic NiO$_2$ and four layers of insulating AlO$_2$ separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analyzed in terms of structural, quantum confinement, and correlation effects. The effect of strain is determined from the dependence of the results on the Ni-O bond-length ratio and the octahedral rotation angles; quantum confinement is studied by comparison to bulk calculations with similar degrees of strain; correlation effects are inferred by varying interaction parameters within our DFT+DMFT calculations. The calculated dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent X-ray absorption spectroscopy and resonant reflectometry data. However, interesting differences of detail are found between theory and experiment. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk LaNiO$_3$ and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk LaNiO$_3$.

Tunable one-dimensional electron gas carrier densities at nanostructured oxide interfaces

The emergence of two-dimensional metallic states at the LaAlO3/SrTiO3 (LAO/STO) heterostructure interface is known to occur at a critical thickness of four LAO layers. This insulator to-metal transition can be explained through the “polar catastrophe” mechanism arising from the divergence of the electrostatic potential at the LAO surface. Here, we demonstrate that nanostructuring can be effective in reducing or eliminating this critical thickness. Employing a modified “polar catastrophe” model, we demonstrate that the nanowire heterostructure electrostatic potential diverges more rapidly as a function of layer thickness than in a regular heterostructure. Our first-principles calculations indicate that for nanowire heterostructures a robust one-dimensional electron gas (1DEG) can be induced, consistent with recent experimental observations of 1D conductivity at LAO/STO steps. Similar to LAO/STO 2DEGs, we predict that the 1D charge density decays laterally within a few unit cells away from the nanowire; thus providing a mechanism for tuning the carrier dimensionality between 1D and 2D conductivity. Our work provides insight into the creation and manipulation of charge density at an oxide heterostructure interface and therefore may be beneficial for future nanoelectronic devices and for the engineering of novel quantum phases.

Effects of structural relaxation, interdiffusion, and surface termination on two-dimensional electron gas formation at theLaAlO3/SrTiO3(001) interface

First-principles (generalized gradient approximation) calculations are presented for symmetric ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}/{\mathrm{LaAlO}}_{3}$ (001) orientation slab models with varying thickness (3, 4, 5 unit cells) of the ${\mathrm{LaAlO}}_{3}$ (LAO) layers. The buckling of the layers and their effect on the slope of the layer-averaged electrostatic potential and layer-projected densities of states are studied. We find the buckling of the LAO layers to increase from the interface toward the surface, while the buckling of the ${\mathrm{AlO}}_{2}$ layers decreases toward the surface. The critical layer thickness for obtaining electrons in the Ti-$d$ band of the ${\mathrm{SrTiO}}_{3}$ (STO) is determined to be 4 layers within this model. Beyond this point, the sloped potential is confined to the 4 layers of LAO nearest to the interface. The electrons in the Ti-$d$ states extend throughout the 5.5-layer-thick STO region of our calculation. The sheet charge density of electrons in the STO conduction band is determined and found to be of order $(1--3)\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}e/{\mathrm{cm}}^{2}$, in fair agreement with experimental values and an order of magnitude smaller than required by the polar discontinuity model. We also find still a significant change in the sheet density between the 4-LAO layer and 5-LAO layer model. It results in only ${d}_{xy}$-like states being occupied for the 4-LAO layer case but other ${t}_{2g}$ bands becoming occupied for the 5-LAO layer case. The effects of H adsorption on surface O and OH adsorption on the surface Al are investigated for a model with $1/8$ coverage of H and $1/4$ coverage of OH. The former leads to electron doping of the STO layer while the latter leads to a p-type surface. When both together are present, they cancel each other. For high H coverage, we find that only a certain fraction of the electrons donated by H can be accommodated at the interface while the remaining go to the surface and lead to a reversal of the slope of the potential in the LAO region. The addition of 25% Ti on Al sites into the first layer of LAO already leads to a cancellation of the field in the LAO layer. It does not lead to ${\mathrm{Ti}}^{3+}$ embedded in the LAO site but rather the Ti donates its additional electron to the interface two-dimensional electron gas (2DEG) confined to the STO ${\mathrm{TiO}}_{2}$ layers. A swap of Al with Ti in the layers closest to the interface does not produce a 2DEG because the Al in the ${\mathrm{TiO}}_{2}$ interface layer provides holes compensating the electron doping from the Ti. Interdiffusion of Sr and La between the layers nearest to the interface does not lead to a 2DEG. These species are just electron donors in their own respective materials. In a swap compensating dipoles result from the different nuclear charges but the electronic states near the gap are not affected. Thus no 2DEG formation occurs. On the other hand a ${\mathrm{Sr}}_{\mathrm{La}}$ placed in the middle of the LAO layer is found to facilitate electron transfer from the surface to the interface and could lead to a 2DEG. However, the latter had only a small sheet density.

Theoretical Study on Oxygen Dissociation Reaction on LaSrCoO3

Introduction Doped lanthanum cobaltite and its derivatives are used as cathode materials in solid oxide fuel cell (SOFC). For a high catalytic activity, the improvement of properties governing the cathode performance is important. It is generally regarded that one of the important factors is electronic structure of cathode material. It is known that the Co ion in doped lanthanum cobaltite shows three spin states, such as low spin (LS), intermediate spin (IS), and high spin (HS). Hong et al. pointed out that the change of electronic structure due to the difference of spin states affect the formation energy of oxygen vacancy in LaCoO3 bulk system. Although it is expected that the high catalytic materials for oxygen adsorption and dissociation on oxide surface are proposed by the controlling the electronic structure of doped lanthanum cobaltite, the effect of electronic structure due to the difference of spin states of Co ion is unclear for the surface reactions on doped lanthanum cobaltite. In this study, we analyzed the stability of oxygen vacancy and surface reaction of oxygen under the different spin states of Co ion in Sr doped LaCoO3by using density functional theory (DFT) calculation. Computational Details We used LaSrO-terminated (001) surface of cubic LaSrCoO3 crystal structure. Half of La atoms were substituted by Sr atoms in LaO layers to analyze the effect of segregation of Sr in La6Sr6Co8O28 slab model. GGA-PBE with PAW potentials were applied with cutoff energy of 600 eV and 2x2x1 k-points. The initial spin configuration was constrained, such as LS, IS, and HS states, to obtain the different electronic structures. All DFT calculations were performed by using VASP. Results and Discussion We first analyzed the oxygen vacancy formation energy on the surface of La6Sr6Co8O28. The vacancy formation energies of one oxygen on surface under the LS, IS, and HS states were 0.92, 1.03, and 0.95 eV, respectively. The vacancy formation energies of two oxygen on surface under the LS, IS, and HS states were 3.27, 2.36, and 1.90 eV, respectively. The oxygen vacancy formation became unstable when the number of oxygen vacancy increases in all spin states. The oxygen vacancy formation energy for HS was the smallest compared with LS and IS states for two oxygen vacancy systems. We found that the difference of spin state of Co ion affects the oxygen vacancy formation energy on surface of LaSrCoO3. In addition, we analyzed the stability of oxygen vacancy under the different Sr configurations to understand the effect Sr segregation. Details are discussed in the presentation. Acknowledgement The activities of INAMORI Frontier Research Center are supported by KYOCERA Corporation. All calculations are performed on the HA8000 computer systems in Research Institute for Information Technology, Kyushu University. Reference [1] W. T. Hong, M. Gadre, Y. -L. Lee, M. D. Biegalski, H. M. Christen, D. Morgan, and Y. Shao-Horn, J. Phys. Chem. Lett., 4, 2493 (2013).

A Study of Superconducting La<sub>2</sub>CuO<sub>4</sub> via Generalized BCS Equations Incorporating Chemical Potential

We address the Tc (s) and multiple gaps of La2CuO4 (LCO) via generalized BCS equations incorporating chemical potential. Appealing to the structure of the unit cell of LCO, which comprises sub- lattices with LaO and OLa layers and brings into play two Debye temperatures, the concept of itinerancy of electrons, and an insight provided by Tacon et al.’s recent experimental work concerned with YBa2Cu3O6.6 which reveals that very large electron-phonon coupling can occur in a very narrow region of phonon wavelengths, we are enabled to account for all values of its gap-to-Tc ratio (2Δ0/kBTc), i.e., 4.3, 7.1, ≈8 and 9.3, which were reported by Bednorz and Müller in their Nobel lecture. Our study predicts carrier concentrations corresponding to these gap values to lie in the range 1.3 × 1021 - 5.6 × 1021 cm-3, and values of 0.27 - 0.29 and 1.12 for the gap-to-Tc ratios of the smaller gaps.

Anisotropy and high thermopower of LaOBiS2

Transport properties of LaOBiS2 are studied using the Boltzmann theory in the constant scattering time approximation, the input to which are derived from the ab-initio calculations with spin–orbit interactions included. By virtue of the materials chemical bonding and band structure, strong anisotropy in the transport is observed, which also have non-symmetric behavior with respect to carrier type. With thermopower being in few hundreds μV/K, and LaO layers being charge inactive, it could be well anticipated that the thermal conductivity would be significantly suppressed along the crystallographic c-axis. We argue LaOBiS2 as a potential oxide thermoelectric material for high temperature applications. Further optimization of the materials properties in terms of “p” and “n” type conduction could also be accomplished via Sr and F doping, respectively, following the discovery of its isoelectronic counterpart SrFBiS2.

Superconductivity in a new layered bismuth oxyselenide: LaO0.5F0.5BiSe2

We report superconductivity at Tc ≈ 2.6 K in a new layered bismuth oxyselenide LaO0.5F0.5BiSe2 with the ZrCuSiAs-type structure composed of alternating superconducting BiSe2 and blocking LaO layers. The superconducting properties of LaO0.5F0.5BiSe2 were investigated by means of dc magnetization, resistivity and muon-spin rotation experiments, revealing the appearance of bulk superconductivity with a rather large superconducting volume fraction of ≈ 70% at 1.8 K.

Magnetic and electronic properties of (LaMnO3)5(SrMnO3)5 superlattice revealed by resonant soft X-ray scattering

We have investigated the magnetic and electronic properties of (LaMnO3)5(SrMnO3)5 superlattice fabricated on a lattice-matched La0.3Sr0.7Al0.65Ta0.35O3 substrate by resonant soft X-ray scattering. The temperature- and Q-dependences of the energy spectra were measured around the Mn L2,3-absorption edges. We successfully observed not only a signal that reflects the valence modulation of Mn ion, but also a magnetic signal reflecting ferromagnetism. Compared with the calculated models, we concluded that the ferromagnetism mainly originates from the magnetic moments at the Mn sites sandwiched by LaO layers.

Oxygen-vacancy-induced polar behavior in (LaFeO3)2/(SrFeO3) superlattices.

Complex oxides displaying ferroelectric and/or multiferroic behavior are of high fundamental and applied interest. In this work, we show that it is possible to achieve polar order in a superlattice made up of two nonpolar oxides by means of oxygen vacancy ordering. Using scanning transmission electron microscopy imaging, we show the polar displacement of magnetic Fe ions in a superlattice of (LaFeO3)2/(SrFeO3) grown on a SrTiO3 substrate. Using density functional theory calculations, we systematically study the effect of epitaxial strain, octahedral rotations, and surface terminations in the superlattice and find them to have a negligible effect on the antipolar displacements of the Fe ions lying in between SrO and LaO layers of the superlattice (i.e., within La0.5Sr0.5FeO3 unit cells). The introduction of oxygen vacancies, on the other hand, triggers a polar displacement of the Fe ions. We confirm this important result using electron energy loss spectroscopy, which shows partial oxygen vacancy ordering in the region where polar displacements are observed and an absence of vacancy ordering outside of that area.

Dominant role of oxygen vacancies in electrical properties of unannealed LaAlO3/SrTiO3 interfaces

We report that in unannealed LaAlO3/SrTiO3 (LAO/STO) heterostructures the critical thickness for the appearance of the two-dimensional electron gas can be less than 4 unit cell, the interface is conducting even for STO substrates with mixed terminations and the low-temperature resistance upturn in LAO/STO heterostructures with thick LAO layers strongly depends on laser fluence. Our experimental results provide fundamental insights into the different roles played by oxygen vacancies and polarization catastrophe in the two-dimensional electron gas in crystalline LAO/STO heterostructures.

Effect of cationic order–disorder on the transport properties of LaBaCo2O6–δand La0.5Ba0.5CoO3–δperovskites

A-site cationic ordered LaBaCo2O6−δand disordered La0.5Ba0.5CoO3−δperovskite phases were obtained by solid state reaction. Their structural properties were studied at room temperature and 673 K, by combining powder diffraction techniques, X-ray diffraction and neutron powder diffraction with an independent determination of the oxygen content of the samples by thermogravimetry. La0.5Ba0.5CoO3−δexhibits cubic symmetry with cations randomly distributed, whereas LaBaCo2O6−δshows tetragonal symmetry with the La3+and Ba2+ions distributed in alternating layers. The diffraction data were analyzed using the Rietveld method and different structural and microstructural models. Bond valence and Fourier methods were used to determine bond distances and neutron/electron density maps. LaBaCo2O6−δexhibits a higher concentration of oxygen vacancies than La0.5Ba0.5CoO3−δ, because the O atom is weakly bonded to the LaO layers. The anisotropic atomic displacement and the neutron density distribution suggest a two-dimensional O-migration path for LaBaCo2O6−δand a three-dimensional path for La0.5Ba0.5CoO3−δ.The mechanism of electrical conductivity isviaelectron holes with high mobilities (μLa0.5Ba0.5CoO3−δ= 2.49 cm2 V−1 s−1and μLaBaCo2O6−δ= 1.48 cm2 V−1 s−1at room temperature) and low activation energy (EaLaBaCo2O6−δ = 0.019 eV andEaLa0.5Ba0.5CoO3−δ= 0.030 eV). It has also been found that the higher electronic and ionic conductivities in La0.5Ba0.5CoO3−δcompared to those in LaBaCo2O6−δare due to the higher dimensionality of transport and to greater overlapping between the Co 3dand O 2porbitals.