Electronic structure of La1-xSrxMnO3 studied by photoemission and x-ray-absorption spectroscopy

Fully Magnetically Polarized Ultrathin La0.8Sr0.2MnO3 Films

We report the observation of fully magnetically polarized ultrathin La0.8Sr0.2MnO3 films by using LaMnO3 and La0.45Sr0.55MnO3 buffer layers grown epitaxially on SrTiO3(001) substrates by molecular beam epitaxy. Specifically, we show that La0.8Sr0.2MnO3 films grown on 12-unit-cell LaMnO3 have bulk-like magnetic moments starting from a single unit cell thickness, while for the 15-unit-cell La0.45Sr0.55MnO3 buffer layer, the La0.8Sr0.2MnO3 transitions from an antiferromagnetic state to a fully spin-polarized ferromagnetic state at 4 unit cells. The magnetic results are confirmed by X-ray magnetic circular dichroism, while linear dichroic measurements carried out for the La0.8Sr0.2MnO3/La0.45Sr0.55MnO3 series show the presence of an orbital reorganization at the transition from the antiferromagnetic to ferromagnetic state corresponding to a change from a preferred in-plane orbital hole occupancy, characteristic of the A-type antiferromagnetic state of La0.45Sr0.55MnO3, to preferentially out of plane. We interpret our findings in terms of the different electronic charge transfers between the adjacent layers, confined to the unit cell in the case of insulating LaMnO3 and extended to a few unit cells in the case of conducting La0.45Sr0.55MnO3. Our work demonstrates an approach to growing ultrathin mixed-valence manganite films that are fully magnetically polarized from the single unit cell, paving the way to fully exploring the unique electronic properties of this class of strongly correlated oxide materials.

Electronic correlations in epitaxial CrN thin film

Chromium nitride (CrN) spurred enormous interest due to its coupled magnetostructural and unique metal-insulator transition. The underneath electronic structure of CrN remains elusive. Herein, the electronic structure of epitaxial CrN thin film has been explored by employing resonant photoemission spectroscopy (RPES) and X-ray absorption near edge spectroscopy study in combination with the first-principles calculations. The RPES study indicates the presence of a charge-transfer screened 3[Formula: see text] ([Formula: see text]: hole in the N-2[Formula: see text]) and 3[Formula: see text] final-states in the valence band regime. The combined experimental electronic structure along with the orbital resolved electronic density of states from the first-principles calculations reveals the presence of Cr(3[Formula: see text])-N(2[Formula: see text]) hybridized (3[Formula: see text]) states between lower Hubbard (3[Formula: see text]) and upper Hubbard (3[Formula: see text]) bands with onsite Coulomb repulsion energy (U) and charge-transfer energy ([Formula: see text]) estimated as [Formula: see text] 4.5 and 3.6 eV, respectively. It verifies the participation of ligand (N-2[Formula: see text]) states in low energy charge fluctuations and provides concrete evidence for the charge-transfer ([Formula: see text]U) insulating nature of CrN thin film.

Investigation of thermal control in phase-changing ABO3 perovskites via first-principles predictions: general mechanism of solar absorptivity

The fundamental mechanism of solar absorbance during the phase-change process is investigated in ABO₃ perovskites based on first-principles predictions. A Gaussian-like relationship between the solar absorbance and band gaps is established, which follows the Shockley-Queisser limiting efficiency. For ABO₃ perovskites with bandgaps of E_g > 3.5 eV, a low solar absorbance is obtained, whereas a high solar absorbance is obtained for ABO₃ perovskites, with band gaps ranging from 0.25 to 2.2 eV. The relationship between the orbital character of the density of states (DOS) and the absorption spectra reveals that ABO₃ perovskites with magnetic (strongly interacting) and distorted crystal structures always exhibit a higher solar absorptivity. In contrast, non-magnetic and cubic ABO₃ perovskites always exhibit a lower solar absorptivity. Moreover, the tunable solar absorptivity always undergoes a phase change from cubic to large distorted crystal structures in ABO₃ perovskites with strong interactions. These results can be attributed to a rich structural, electronic, and magnetic phase diagram resulting from the strong interplay between the lattice, spin, and orbital degrees of freedom, which induce highly tunable optical characteristics in the phase-change process. The findings presented in this study are critical for the development of ABO₃ perovskite-based smart thermal control materials in the spacecraft field.

Potential of AMnO3 (A=Ca, Sr, Ba, La) as Active Layer in Inorganic Perovskite Solar Cells

Inorganic perovskite CaMnO3 ${{}_{3}}$ was proposed as a substitution for the TiO2 ${{}_{2}}$ anatase in electron transport layers of solar cells containing the hybrid perovskite CH3 ${{}_{3}}$ NH3 ${{}_{3}}$ PbI3 ${{}_{3}}$ based on increased mobility of electrons and better optical matching. Due to a suitable band gap concerning the absorption of sunlight, we investigate the potential of CaMnO3 ${{}_{3}}$ and similar manganite perovskites, where Ca is replaced by either Sr, Ba or La, as an absorber layer in inorganic perovskite solar cells. In this study, we have used optical measurements on the synthesized AMnO3 ${{}_{3}}$ (A=Ca, Sr, Ba, La) samples to aid density functional theory calculations (DFT) in order to accurately simulate the electronic and optical properties of AMnO3 ${{}_{3}}$ compounds and gauge their potential for the role of absorber layer. Both experimental measurements and theoretical calculations show suitable band gap of 1.1-1.5 eV, depending on the compound, and absorption coefficients of the order of10 5 ${{10}^{5}}$ cm- 1 ${{}^{-1}}$ in the visible part of the spectrum.

Existence of La-site antisite defects in [Formula: see text] ([Formula: see text], Fe, and Co) predicted with many-body diffusion quantum Monte Carlo

The properties of [Formula: see text] (M: 3d transition metal) perovskite crystals are significantly dependent on point defects, whether introduced accidentally or intentionally. The most studied defects in La-based perovskites are the oxygen vacancies and doping impurities on the La and M sites. Here, we identify that intrinsic antisite defects, the replacement of La by the transition metal, M, can be formed under M-rich and O-poor growth conditions, based on results of an accurate many-body ab initio approach. Our fixed-node diffusion Monte Carlo (FNDMC) calculations of [Formula: see text] ([Formula: see text], Fe, and Co) find that such antisite defects can have low formation energies and are magnetized. Complementary density functional theory (DFT)-based calculations show that Mn antisite defects in [Formula: see text] may cause the p-type electronic conductivity. These features could affect spintronics, redox catalysis, and other broad applications. Our bulk validation studies establish that FNDMC reproduces the antiferromagnetic state of [Formula: see text], whereas DFT with PBE (Perdew-Burke-Ernzerhof), SCAN (strongly constrained and appropriately normed), and the LDA+U (local density approximation with Coulomb U) functionals all favor ferromagnetic states, at variance with experiment.

(LaCrO3) m /SrCrO3 superlattices as transparent p-type semiconductors with finite magnetization

The electronic and magnetic properties of (LaCrO₃) _m /SrCrO₃ superlattices are investigated using first principles calculations. We show that the magnetic moments in the two CrO₂ layers sandwiching the SrO layer compensate each other for even m but give rise to a finite magnetization for odd m, which is explained by charge ordering with Cr³⁺ and Cr⁴⁺ ions arranged in a checkerboard pattern. The Cr⁴⁺ ions induce in-gap hole states at the interface, implying that the transparent superlattices are p-type semiconductors. The availability of transparent p-type semiconductors with finite magnetization enables the fabrication of transparent magnetic diodes and transistors, for example, with a multitude of potential technological applications.

Triple Excitations in Green's Function Coupled Cluster Solver for Studies of Strongly Correlated Systems in the Framework of Self-Energy Embedding Theory

Embedding theories became important approaches used for accurate calculations of both molecules and solids. In these theories, a small chosen subset of orbitals is treated with an accurate method, called an impurity solver, capable of describing higher correlation effects. Ideally, such a chosen fragment should contain multiple orbitals responsible for the chemical and physical behavior of the compound. Handling a large number of chosen orbitals presents a very significant challenge for the current generation of solvers used in the physics and chemistry community. Here, we develop a Green's function coupled cluster singles doubles and triples (GFCCSDT) solver that can be used for a quantitative description in both molecules and solids. This solver allows us to treat orbital spaces that are inaccessible to other accurate solvers. At the same time, GFCCSDT maintains high accuracy of the resulting self-energy. Moreover, in conjunction with the GFCCSD solver, it allows us to test the systematic convergence of computational studies. Developing the CC family of solvers paves the road to fully systematic Green's function embedding calculations in solids. In this paper, we focus on the investigation of GFCCSDT self-energies for a strongly correlated problem of SrMnO₃ solid. Subsequently, we apply this solver to solid MnO showing that an approximate variant of GFCCSDT is capable of yielding a high accuracy orbital resolved spectral function.

Electronic Properties of Fully Strained La1-x Sr x MnO3 Thin Films Grown by Molecular Beam Epitaxy (0.15 ≤ x ≤ 0.45)

The structural, electronic, and magnetic properties of Sr-hole-doped epitaxial La_1-x Sr _x MnO₃ (0.15 ≤ x ≤ 0.45) thin films deposited using the molecular beam epitaxy technique on 4° vicinal STO (001) substrates are probed by the combination of X-ray diffraction and various synchrotron-based spectroscopy techniques. The structural characterizations evidence a significant shift in the LSMO (002) peak to the higher diffraction angles owing to the increase in Sr doping concentrations in thin films. The nature of the LSMO Mn mixed-valence state was estimated from X-ray photoemission spectroscopy together with the relative changes in the Mn L_2,3 edges observed in X-ray absorption spectroscopy (XAS), both strongly affected by doping. CTM4XAS simulations at the XAS Mn L_2,3 edges reveal the combination of epitaxial strain, and different MnO₆ crystal field splitting give rise to a peak at ∼641 eV. The observed changes in the occupancy of the e_g and the t_2g orbitals as well as their binding energy positions toward the Fermi level with hole doping are discussed. The room-temperature magnetic properties were probed at the end by circular dichroism.

Robust electronic and tunable magnetic states in Sm2 NiMnO6ferromagnetic insulator

Ferromagnetic insulators (FM-Is) are the materials of interest for the new generation quantum electronic applications. Here, we have investigated the physical observables depicting FM-I ground states in epitaxial Sm₂NiMnO₆(SNMO) double perovskite thin films fabricated under different conditions to realize the different level of Ni/Mn anti-site disorders (ASDs). The presence of ASDs immensely influence the characteristic magnetic and anisotropy behaviors in SNMO system by introducing short scale antiferromagnetic interactions in predominant long range FM ordered host matrix. Charge disproportion between cation sites, in the form of Ni²⁺+ Mn⁴⁺→ Ni³⁺+ Mn³⁺, causes mixed valency in both Ni and Mn species, which is found insensitive to ASD concentrations. Temperature dependent photo emission, photo absorption measurements duly combined with cluster model configuration interaction simulations, suggest that the eigenstates of Ni and Mn cations can be satisfactorily described as a linear combination of the unscreeneddⁿand screeneddn+1L̲(L̲: O 2phole) states. The electronic structure across the Fermi level (E_F) exhibits closely spaced Ni 3d, Mn 3dand O 2pstates. From occupied and unoccupied bands, estimated values of the Coulomb repulsion energy (U) and ligand to metal charge transfer energy (Δ), indicate charge transfer insulating nature, where remarkable modification in Ni/Mn 3d-O 2phybridization takes place across the FM transition temperature. Existence of ASD broadens the Ni, Mn 3dspectral features, whereas the spectral positions are found to be unaltered. Hereby, present work demonstrates SNMO thin film as a FM-I system, where the FM state can be tuned by manipulating ASD in the crystal structure, while the I state remains intact.

Octahedral rotations trigger electronic and magnetic transitions in strontium manganate under volume expansion

The perovskite structure of manganate yields a series of intriguing physical properties. Based on the results of first-principles calculations, strontium manganate appears to undergo a magnetic phase transition and a metal-insulator transition-from antiferromagnetic insulator to ferromagnetic metal and then to ferromagnetic insulator-under isotropic volume expansion combined with oxygen octahedral distortions. Interestingly, the results show that increasing the Mn-O bond length and adding rotation of the oxygen octahedra can soften the breathing distortion and account for the insulator phase. We further build a simple model to explain such transitions. Due to electron transfer and the favoring of a hole state of ligandporbitals, the electron state transfer from2(t2g3)to2(eg1+t2g2)and then tot2g3eg1+(t2g3L̲1). Such rearrangement of charges is responsible for the transitions of its magnetic order and electronic structure. Furthermore, we calculate spin susceptibility under the bare conditions and random phase approximation. The magnetic order of the intermediate metal state of itinerant electrons behaves as a ferromagnetic.

Mixed Mott-Hubbard and charge transfer nature of 4H-SrMnO3thin film on Si (100)

Room temperature electronic structure of polycrystalline 4H-SrMnO₃thin film grown on Si (100) substrate has been studied using resonance photo emission spectroscopy and soft x-ray absorption spectroscopy measurements. Presence of charge transfer screen Mn 3dⁿLfinal state along with the 3d^n-1final state at the valence band edge of 4H-SrMnO₃thin film confirms that the ground state is strongly mixed between Mn 3dand O 2pstates. The estimated equivalent values of on-site Coulomb interaction energy (U) and O 2pto Mn 3d- charge transfer energy (Δ) (U≈ Δ ≈ 4.8 eV) from the combination of occupied and unoccupied spectra further confirm the intermediate Mott-Hubbard and charge transfer insulator nature of 4H-SrMnO₃film. Despite having similar Mn 4+ valence state in 4H-SrMnO₃and cubic SrMnO₃, 4H phase is observed to reveal much higher band gap ∼1.5 eV than the cubic phase (0.3 eV), which arises due to different MnO₆octahedra environment.

Porous calcium–manganese oxide/carbon nanotube microspheres as efficient oxygen reduction catalysts for rechargeable zinc–air batteries

CaMnO_3−δ/carbon nanotubes show excellent oxygen reduction reaction activity benefitting from their unique porous structure and synergistic coupling between CaMnO_3−δ and carbon nanotubes.

Interface Engineered Room-Temperature Ferromagnetic Insulating State in Ultrathin Manganite Films

Ultrathin epitaxial films of ferromagnetic insulators (FMIs) with Curie temperatures near room temperature are critically needed for use in dissipationless quantum computation and spintronic devices. However, such materials are extremely rare. Here, a room-temperature FMI is achieved in ultrathin La0.9Ba0.1MnO3 films grown on SrTiO3 substrates via an interface proximity effect. Detailed scanning transmission electron microscopy images clearly demonstrate that MnO6 octahedral rotations in La0.9Ba0.1MnO3 close to the interface are strongly suppressed. As determined from in situ X-ray photoemission spectroscopy, O K-edge X-ray absorption spectroscopy, and density functional theory, the realization of the FMI state arises from a reduction of Mn eg bandwidth caused by the quenched MnO6 octahedral rotations. The emerging FMI state in La0.9Ba0.1MnO3 together with necessary coherent interface achieved with the perovskite substrate gives very high potential for future high performance electronic devices.

Laplacian free and asymptotic corrected semilocal exchange potential applied to the band gap of solids

It is well known that the modified semilocal exchange potentials explicitly designed for the study of solid-state band gaps are very successful in describing these properties. These exchange potentials are in principle designed either from a spherically averaged exchange hole or by satisfying the exact asymptotic conditions. In this present attempt, we use the recently developed novel technique of density matrix expansion to construct the model exchange hole potential. The proposed exchange hole potential is free from the Laplacian of density and generalized through the coordinate transformation. An improvement in the exchange energies of atoms using this potential is shown. The salient feature of the proposed semilocal potential is that it can be used within generalized Kohn-Sham formalism because of its Laplacian free representation. This modified potential is used in the framework of TBMBJ [Phys. Rev. Lett., 2009, 102, 226401] to calculate the band gaps of materials. The comparison and assessment of the newly constructed Laplacian free, asymptotically corrected semilocal potential to address the band gap problem show good agreement with the experimental band gaps and diversify the studies done in the same direction.

Mn 2p resonant X-ray emission clarifies the redox reaction and charge-transfer effects in LiMn2O4

High-energy-resolution soft X-ray emission spectroscopy (XES) was applied to understand the changes in the electronic structure of LiMn2O4 upon Li-ion extraction/insertion. Mn 2p-3d-2p resonant XES spectra were analyzed by configuration-interaction full-multiplet (CIFM) calculations, which reproduced both dd and charge-transfer (CT) excitations. From the resonant XES spectra it is found that Mn3+ and Mn4+ coexist in the initial state, while this changes into Mn4+ in the charged-state. For the discharged-state, the Mn3+ component appears again although the dd excitations are slightly modified from those for the initial state. Furthermore, negative CT energy is expected for the Mn4+ configuration, which suggests very strong hybridization between the Mn 3d and O 2p orbitals. The large difference in the CT effect between the Mn4+ and Mn3+ states should give mechanical stress to the Mn-O bond during charge-discharge cycling, leading to capacity fading.

Atomic replacement effects on the band structure of doped perovskite thin films

The potential applications of perovskite manganite R_1-xA_xMnO₃ (R = rare earth element; A = Sr, Ca) thin films have been continuously explored due to their multi-functional properties. In particular, the optimally hole-doped La_0.67Ca_0.33MnO₃ thin film demonstrates a colossal magneto-resistance that is beneficial to the performance of spintronic devices. To understand the effect of R and A ions on the material properties, we systematically measure the resistivity, magnetization, and electronic energy states for three optimally hole-doped R_0.67A_0.33MnO₃ thin films with R = La, Sm and A = Sr, Ca. Various energy parameters are derived based on the X-ray absorption and X-ray photoelectron spectra, including the band gap, the charge frustration energy and the magnetic exchange energy. It is interesting to find that the replacement of La with Sm is more effective than that of Sr with Ca in terms of tuning the electrical property, the Curie temperature, and the band gap. The strain-induced reduction of the O 2p- Mn 3d hybridization and the interplay of R/A site disorder and strain effect are discussed. The results of this study provide useful information for the band design of perovskite oxide films.

BEEF-vdW+U method applied to perovskites: thermodynamic, structural, electronic, and magnetic properties

The recently developed BEEF-vdW exchange-correlation method provides a reasonably reliable description of both long-range van der Waals interactions and short-range covalent bonding between molecules and surfaces. However, this method still suffers from the excessive electron delocalization that is connected with the self-interaction error and, consequently, the calculated chemical and physical properties such as formation energy and band gap deviate markedly from the experimental values, especially when strongly correlated systems are under investigation. In this contribution, BEEF-vdW+U calculations have been performed to study the thermodynamic, structural, electronic, and magnetic properties of La-based perovskites. An effective interaction parameter [Formula: see text] and an energy adjustment [Formula: see text] are determined simultaneously by a mixing GGA and GGA+U method, where the enthalpy or Gibbs free energy of formation of oxides containing a transition metal in different oxidation states are fitted to available experimental data. The [Formula: see text] is found to have its origin in the fact that the GGA+U method gives rise to the offsets in the total energy that include not only the desired physical correction but also an arbitrary contribution. Calculated results indicate that the BEEF-vdW method provides a more accurate description of the bonding in the O₂ molecule than the PBE method and has generally smaller [Formula: see text] values for the 3d-block transition metals, thereby giving rise to band gaps and magnetic moments that are in better agreement with the experimentally measured values.

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr₂MnNiO₆ was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+  and 2+  states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni²⁺ and Mn⁴⁺ ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U  -  J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p  -  d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.

Tailoring manganese oxide with atomic precision to increase surface site availability for oxygen reduction catalysis

Controlling the structure of catalysts at the atomic level provides an opportunity to establish detailed understanding of the catalytic form-to-function and realize new, non-equilibrium catalytic structures. Here, advanced thin-film deposition is used to control the atomic structure of La_2/3Sr_1/3MnO₃, a well-known catalyst for the oxygen reduction reaction. The surface and sub-surface is customized, whereas the overall composition and d-electron configuration of the oxide is kept constant. Although the addition of SrMnO₃ benefits the oxygen reduction reaction via electronic structure and conductivity improvements, SrMnO₃ can react with ambient air to reduce the surface site availability. Placing SrMnO₃ in the sub-surface underneath a LaMnO₃ overlayer allows the catalyst to maintain the surface site availability while benefiting from improved electronic effects. The results show the promise of advanced thin-film deposition for realizing atomically precise catalysts, in which the surface and sub-surface structure and stoichiometry are tailored for functionality, over controlling only bulk compositions.

Controlling structures at the atomic level provides an opportunity to design and understand catalysts. Here the authors use thin-film deposition to fabricate perovskite heterostructures in a non-equilibrium manner to assess the effects on electrocatalytic activity for oxygen reduction.

Atomically Resolved Electronic States and Correlated Magnetic Order at Termination Engineered Complex Oxide Heterointerfaces

We map electronic states, band gaps, and interface-bound charges at termination-engineered BiFeO₃/La_0.7Sr_0.3MnO₃ interfaces using atomically resolved cross-sectional scanning tunneling microscopy. We identify a delicate interplay of different correlated physical effects and relate these to the ferroelectric and magnetic interface properties tuned by engineering the atomic layer stacking sequence at the interfaces. This study highlights the importance of a direct atomically resolved access to electronic interface states for understanding the intriguing interface properties in complex oxides.

Assessing the performance of self-consistent hybrid functional for band gap calculation in oxide semiconductors

In this paper we assess the predictive power of the self-consistent hybrid functional scPBE0 in calculating the band gap of oxide semiconductors. The computational procedure is based on the self-consistent evaluation of the mixing parameter α by means of an iterative calculation of the static dielectric constant using the perturbation expansion after discretization method and making use of the relation [Formula: see text]. Our materials dataset is formed by 30 compounds covering a wide range of band gaps and dielectric properties, and includes materials with a wide spectrum of applications such as thermoelectrics, photocatalysis, photovoltaics, transparent conducting oxides, and refractory materials. Our results show that the scPBE0 functional provides better band gaps than the non self-consistent hybrids PBE0 and HSE06, but scPBE0 does not show significant improvement on the description of the static dielectric constants. Overall, the scPBE0 data exhibit a mean absolute percentage error of 14% (band gaps) and 10% ([Formula: see text]). For materials with weak dielectric screening and large excitonic biding energies scPBE0, unlike PBE0 and HSE06, overestimates the band gaps, but the value of the gap becomes very close to the experimental value when excitonic effects are included (e.g. for SiO₂). However, special caution must be given to the compounds with small band gaps due to the tendency of scPBE0 to overestimate the dielectric constant in the proximity of the metallic limit.

Charge Transfer in Iridate-Manganite Superlattices

Charge transfer in superlattices consisting of SrIrO₃ and SrMnO₃ is investigated using density functional theory. Despite the nearly identical work function and nonpolar interfaces between SrIrO₃ and SrMnO₃, rather large charge transfer was experimentally reported at the interface between them. Here, we report a microscopic model that captures the mechanism behind this phenomenon, providing a qualitative understanding of the experimental observation. This leads to unique strain dependence of such charge transfer in iridate-manganite superlattices. The predicted behavior is consistently verified by experiment with soft X-ray and optical spectroscopy. Our work thus demonstrates a new route to control electronic states in nonpolar oxide heterostructures.

Oxygen electrode reactions of doped BiFeO3 materials for low and elevated temperature fuel cell applications

Bi_0.6Ca_0.4FeO₃ demonstrates potential as an oxygen electrode material (for oxygen evolution and reduction reactions) for operation at room and elevated temperatures.

Modelling oxygen defects in orthorhombic LaMnO3 and its low index surfaces

The findings of this work represent the first comprehensive study of the formation of oxygen defects in bulk orthorhombic LaMnO₃ and at its low index surfaces.

Electronic structure origin of conductivity and oxygen reduction activity changes in low-level Cr-substituted (La,Sr)MnO3

The electronic structure of the (La(0.8)Sr(0.2))(0.98)Mn(1-x)Cr(x)O3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La,Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t(2g) ↑ state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t(2g) ↑ state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La,Sr)MnO3 is presented.

Reversible Luminescence Modulation upon Photochromic Reactions in Rare-Earth Doped Ferroelectric Oxides by in Situ Photoluminescence Spectroscopy

Reversible luminescence photoswitching upon photochromic reactions with excellent reproducibility is achieved in a new inorganic luminescence material: Na(0.5)Bi(2.5)Nb2O9: Pr(3+) (NBN:Pr) ferroelectric oxides. Upon blue light (452 nm) or sunlight irradiation, the material exhibits a reversible photochromism (PC) performance between dark gray and green color by alternating visible light and thermal stimulus without inducing any structure changes and is accompanied by a red emission at 613 nm. The coloration and decoloration process can be quantitatively evaluated by in situ photoluminescence spectroscopy. Meanwhile, the luminescence emission intensity based on PC reactions is effectively tuned by changing irradiation time and excitation wavelength, and the degree of luminescence modulation has no significant degradation after several periods, showing very excellent reproducibility. On the basis of the luminescence modulation behavior, a double-exponential relaxation model is proposed, and a combined equation is adopted to well describe the luminescence response to light irradiation, being in agreement with the experimental data.

Highly Active Epitaxial La(1-x)Sr(x)MnO3 Surfaces for the Oxygen Reduction Reaction: Role of Charge Transfer

Most studies of oxide catalysts for the oxygen reduction reaction (ORR) use oxide powder, where the heterogeneity of exposed surfaces and the composite nature of electrodes limit fundamental understanding of the reaction mechanism. We present the ORR activity of epitaxially oriented La(1-x)Sr(x)MnO3 surfaces and investigate, by varying Sr substitution, the relationship between the role of charge transfer and catalytic activity in an alkaline environment. The activity is greatest for La(1-x)Sr(x)MnO3 with 33% Sr, containing mixed Mn(3+/4+), and the (110) and (111) orientations display comparable activities to that of the (001). Electrochemical measurements using the facile redox couple [Fe(CN)6](3-/4-) illustrate that increasing ORR activity trends with faster charge-transfer kinetics, indicating the importance of facile charge transfer at the oxide/water interface and mixed Mn valence in promoting ORR kinetics.

Electronic conduction in La-based perovskite-type oxides

A systematic study of La-based perovskite-type oxides from the viewpoint of their electronic conduction properties was performed. LaCo0.5Ni0.5O3±δ was found to be a promising candidate as a replacement for standard metals used in oxide electrodes and wiring that are operated at temperatures up to 1173 K in air because of its high electrical conductivity and stability at high temperatures. LaCo0.5Ni0.5O3±δ exhibits a high conductivity of 1.9 × 103 S cm-1 at room temperature (R.T.) because of a high carrier concentration n of 2.2 × 1022 cm-3 and a small effective mass m∗ of 0.10 me. Notably, LaCo0.5Ni0.5O3±δ exhibits this high electrical conductivity from R.T. to 1173 K, and little change in the oxygen content occurs under these conditions. LaCo0.5Ni0.5O3±δ is the most suitable for the fabrication of oxide electrodes and wiring, though La1-x Sr x CoO3±δ and La1-x Sr x MnO3±δ also exhibit high electronic conductivity at R.T., with maximum electrical conductivities of 4.4 × 103 S cm-1 for La0.5Sr0.5CoO3±δ and 1.5 × 103 S cm-1 for La0.6Sr0.4MnO3±δ because oxygen release occurs in La1-x Sr x CoO3±δ as elevating temperature and the electrical conductivity of La0.6Sr0.4MnO3±δ slightly decreases at temperatures above 400 K.

Reversible photoresponsive switching in Bi2.5Na0.5Nb2O9-based luminescent ferroelectrics

Reversible luminescence modulation upon photochromic reactions with excellent reproducibility is achieved from Eu³⁺ doped Bi_2.5Na_0.5Nb₂O₉ multifunctional ferroelectric oxides.

Hybrid functionals applied to perovskites

After being used for years in the chemistry community to describe molecular properties, hybrid functionals have been increasingly and successfully employed for a wide range of solid state problems which are not accurately accessible by standard density functional theory. In particular, the upsurge of interest in transition metal perovskite-based compounds, motivated by their technological relevance and functional ductility, has incentivized the use of hybrid functionals for realistic applications, as hybrid functionals appear to be capable of capturing the complex correlated physics of this class of oxide material, characterized by a subtle coupling between several competing interactions (lattice, orbital, spin). Here we present a map of recent applications of hybrid functionals to perovskites, aiming to cover an ample spectra of cases, including the 'classical' 3d compounds (manganites, titanates, nickelates, ferrites, etc.), less conventional examples from the the 4d (technetiates) and 5d (iridates) series, and the (non-transition metal) sp perovskite BaBiO3. We focus our attention on the technical aspects of the hybrid functional formalism, such as the role of the mixing and (for range-separated hybrids) screening parameters, and on an extended array of physical phenomena: pressure- and doping-induced insulator-to-metal and structural phase transitions, multiferroism, surface and interface effects, charge ordering and localization effects, and spin-orbit coupling.

The role of the hybridization between Mn 3d and O 2p orbitals in the existence of the Griffiths phase in La0.85Ca0.15MnO3

Favourable conditions for the existence of the Griffiths phase in the La0.85Ca0.15MnO3 compound are experimentally investigated in terms of electronic and lattice structure by temperature-dependent x-ray absorption spectroscopy, valence band photoemission spectroscopy, and x-ray diffraction experiments. The chemical shifts of Mn L-edge and O K-edge x-ray absorption lines in the Griffiths phase are understood to be related to the hybridization between Mn 3d and O 2p states instead of the variation of Mn valence states. Valence band spectra also indicate that the hybridization of O 2p with Mn 3d is enhanced in the Griffiths phase. From a 2D diluted Ising ferromagnet model, this hybridization between Mn 3d and O 2p orbitals surely enhances the Griffiths phase feature.

Analysis of electronic structure and its effect on magnetic properties in (001) and (110) oriented La0.7Sr0.3MnO3 thin films

Epitaxial thin films of half-metallic oxide La0.7Sr0.3MnO3 (LSMO) have been grown in two crystalline orientations, one with the c-axis out-of-plane, the (001) orientation, and one with the c-axis in-plane, the (110) orientation. For the (110) oriented growth, there is no polar discontinuity at the substrate-film interface and hence no dead layer formation, which improves ferromagnetic interaction in the LSMO, making it highly attractive for spintronic applications. In our experiments, with x-ray diffraction, x-ray photoelectron spectroscopy and magnetic measurements, we have demonstrated that in the (110) oriented LSMO the lattice is more relaxed, leading to less deformation of electronic density around the La atom or in the MnO6 octahedra. This improved crystal and electronic structure improves the ferromagnetic properties of the films, making the Curie temperature higher by almost 15 K, which is of potential interest for spintronics. However, substrate strain induced magnetic anisotropy causes domain formation with out-of-plane components in these films, which poses some concern for practical spintronic devices.

Maximally localized Wannier functions in LaMnO3 within PBE + U, hybrid functionals and partially self-consistent GW: an efficient route to construct ab initio tight-binding parameters for eg perovskites

Using the newly developed VASP2WANNIER90 interface we have constructed maximally localized Wannier functions (MLWFs) for the e(g) states of the prototypical Jahn-Teller magnetic perovskite LaMnO(3) at different levels of approximation for the exchange-correlation kernel. These include conventional density functional theory (DFT) with and without the additional on-site Hubbard U term, hybrid DFT and partially self-consistent GW. By suitably mapping the MLWFs onto an effective e(g) tight-binding (TB) Hamiltonian we have computed a complete set of TB parameters which should serve as guidance for more elaborate treatments of correlation effects in effective Hamiltonian-based approaches. The method-dependent changes of the calculated TB parameters and their interplay with the electron-electron (el-el) interaction term are discussed and interpreted. We discuss two alternative model parameterizations: one in which the effects of the el-el interaction are implicitly incorporated in the otherwise 'noninteracting' TB parameters and a second where we include an explicit mean-field el-el interaction term in the TB Hamiltonian. Both models yield a set of tabulated TB parameters which provide the band dispersion in excellent agreement with the underlying ab initio and MLWF bands.

Evidence for a crucial role played by oxygen vacancies in LaMnO3 resistive switching memories

LaMnO(3) (LMO) films are deposited on SrTiO(3):Nb (0.8 wt%) substrates under various oxygen pressures to obtain different concentrations of oxygen vacancies in the films. The results of X-ray diffraction verify that with a decrease of the oxygen pressure, the c-axis lattice constant of the LMO films becomes larger, owing to an increase of the oxygen vacancies. Aberration-corrected annular-bright-field scanning transmission electron microscopy with atomic resolution and sensitivity for light elements is used, which clearly shows that the number of oxygen vacancies increases with the decrease of oxygen pressure during fabrication. Correspondingly, the resistive switching property becomes more pronounced with more oxygen vacancies in the LMO films. Furthermore, a numerical model based on the modification of the interface property induced by the migration of oxygen vacancies in these structures is proposed to elucidate the underlying physical origins. The calculated results are in good agreement with the experimental data, which reveal from a theoretical point of view that the migration of oxygen vacancies and the variation of the Schottky barrier at the interface with applied bias dominate the resistive switching characteristic. It is promising that the resistive switching property in perovskite oxides can be manipulated by controlling the oxygen vacancies during fabrication or later annealing in an oxygen atmosphere.

Structural and thermal properties of LaMnO3 from neutron diffraction and first principles studies

Neutron diffraction experiments have been performed on powder samples of LaMnO(3) below and above the Jahn-Teller transition temperature of 750 K. Experimental investigations are assisted by density functional theory calculations. Theoretical studies are carried out for the orbitally ordered state of LaMnO(3) which allows one to compare the behavior of the orbitally ordered and disordered structures as a function of temperature. The temperature dependences of the structural parameters characterizing the Jahn-Teller distortions are reported and discussed. A gradual departure of the experimental data from theoretical predictions is observed above 650 K. In this range of temperatures, anions surrounding the Jahn-Teller active cations perform more isotropic thermal motion. The onset of structural phase transition induces a reduction of the crystal volume by about 0.4% which follows from the structural transformations yielding more regular oxygen octahedra formed above the phase transformation. It is found that above the Jahn-Teller transition the distortions of the MnO(6) octahedra are not completely removed. The non-vanishing distortions are accompanied by the lifted degeneracy of the Mn e(g) states. Weak residual distortions can be assigned to the short-range orbital order that persists within a local scale but it seems quenched on average giving rise to a disappearance of the long-range order coherency of the Jahn-Teller effect.

Jahn-Teller distortions and the magnetic order in the perovskite manganites

We introduce an effective model for e(g) electrons to describe three-dimensional perovskite (La(1 - x)Sr(x)MnO(3) and La(1 - x)Ca(x)MnO(3)) manganites and study the magnetic and orbital order on a 4 × 4 × 4 cluster using correlated wavefunctions. The model includes the kinetic energy, and on-site Coulomb interactions for e(g) electrons, antiferromagnetic superexchange interaction between S = 3/2 core spins, and the coupling between e(g) electrons and Jahn-Teller modes. The model reproduces the experimentally observed magnetic order: (i) an A-type antiferromagnetic phase in the undoped insulator LaMnO(3), with alternating e(g) orbitals and with small Jahn-Teller distortions, changing to a conducting phase at 32 GPa pressure, and (ii) ferromagnetic order in one-eighth-doped La(7/8)Sr(1/8)MnO(3) and in quarter-doped La(3/4)Sr(1/4)MnO(3) compounds. For half-doped La(1/2)Ca(1/2)MnO(3) one finds a competition between a ferromagnetic conductor and the CE insulating phase; the latter is stabilized by the Jahn-Teller coupling being two times larger than for the strontium-doped compound. Altogether, there is a subtle balance between all Hamiltonian parameters and the phase diagram is quite sensitive to the precise values they take.

Anisotropic magnetoresistance in manganites: experiment and theory

We present measurements of the anisotropic magnetoresistance (AMR) of La(0.75)Sr(0.25)MnO(3) films deposited on (001) SrTiO(3) substrates, and a model that describes the experimental results. The model, based on the electronic structure of manganites plus the spin-orbit coupling, correctly accounts for the dependence of the AMR on the direction of the current to the crystalline axes. We measure an AMR of the order of 10(-3) for the current I parallel to the [100] axis of the crystal and vanishing AMR for I <parallel> , in agreement with the model predictions. Further, we calculate the planar Hall effect and show its connection to AMR.

Origin of Jahn-Teller distortion and orbital order in LaMnO3

The origin of the cooperative Jahn-Teller distortion and orbital order in LaMnO3 is central to the physics of the manganites. The question is complicated by the simultaneous presence of tetragonal and GdFeO3-type distortions and the strong Hund's rule coupling between e{g} and t{2g} electrons. To clarify the situation we calculate the transition temperature for the Kugel-Khomskii superexchange mechanism by using the local density approximation+dynamical mean-field method, and disentangle the effects of superexchange from those of lattice distortions. We find that superexchange alone would yield T{KK} approximately 650 K. The tetragonal and GdFeO3-type distortions, however, reduce T{KK} to approximately 550 K. Thus electron-phonon coupling is essential to explain the persistence of local Jahn-Teller distortions to greater than or approximately 1150 K and to reproduce the occupied orbital deduced from neutron scattering.

Interplay between the orbital and magnetic order in monolayer La(1-x)Sr(1+x)MnO(4) manganites

A two-dimensional model which describes e(g) electrons in a monolayer of an undoped and half doped manganite La(1-x)Sr(1+x)MnO(4) is studied using correlated wavefunctions. The effective Hamiltonian takes into account the kinetic energy, the crystal field splitting between x(2)-y(2) and 3z(2)-r(2) orbitals, and on-site Coulomb interactions for e(g) electrons. They interact with S = 3/2 spins due to t(2g) electrons, which are treated as frozen core spins. Furthermore, the model includes antiferromagnetic superexchange interaction between core spins, and the coupling between e(g) electrons and Jahn-Teller modes. The model reproduces the antiferromagnetic order in the undoped LaSrMnO(4) compound, with occupied 3z(2)-r(2) orbitals and elongated MnO(6) octahedra along the direction perpendicular to the Mn-O plane. In half doped La(0.5)Sr(1.5)MnO(4) manganite one finds robust chequerboard-like charge order using realistic parameters. However, the experimentally observed CE phase is more difficult to stabilize, and we discuss the necessary conditions to obtain it within the present model. Altogether, we conclude that the Jahn-Teller effect plays a crucial role in the entire regime of doping.

Electronic Structure of A- and B-Site Doped Lanthanum Manganites: A Combined X-ray Spectroscopic Study

The electronic properties of a series of colossal magnetoresistance (CMR) compounds, namely LaMnO3, La(1-x)Ba(x)(MnO3 (0.2 < or = x < or = 0.55), La(0.76)Ba(0.24)Mn(0.84)Co(0.16)O3, and La(0.76)Ba(0.24)Mn(0.78)Ni(0.22)O3, have been investigated in a detailed spectroscopic study. A combination of X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) was used to reveal a detailed picture of the electronic structure in the presence of Ba, Co, and Ni doping in different concentrations. The results are compared with available theory. The valence band of La(1-x)()Ba(x)MnO3 (0 < or = x < or = 0.55) is dominated by La 5p, Mn 3d, and O 2p states, and strong hybridization between Mn 3d and O 2p states is present over the whole range of Ba concentrations. Co-doping at the Mn site leads to an increased occupancy of the e(g) states near the Fermi energy and an increase in the XPS valence band intensity between 0.5 and 5 eV, whereas the Ni-doped sample shows a lower density of occupied states near the Fermi energy. The Ni d states are located in a band spanning the energy range of 1.5-5 eV. XAS spectra indicate that the hole doping leads to mixed Mn 3d-O 2p states. Furthermore, RIXS at the Mn L edge has been used to probe d-d transitions and charge-transfer excitations in La(1-x)Ba(x)MnO3.

Nature of the well screened state in hard X-ray Mn 2p core-level photoemission measurements of La1-xSrxMnO3 films

Using hard x-ray (HX; hnu=5.95 keV) synchrotron photoemission spectroscopy (PES), we study the intrinsic electronic structure of La(1-x)Sr(x)MnO(3) (LSMO) thin films. Comparison of Mn 2p core-levels with soft x-ray (SX; hnu approximately 1000 eV) PES shows a clear additional well-screened feature only in HX PES. Takeoff-angle dependent data indicate its bulk (> or =20 A) character. The doping and temperature dependence track the ferromagnetism and metallicity of the LSMO series. Cluster model calculations including charge transfer from doping-induced states show good agreement, confirming this picture of bulk properties reflected in Mn 2p core-levels using HX PES.