Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces

Emergence of Interfacial Magnetism in Strongly-Correlated Nickelate-Titanate Superlattices

Strongly-correlated transition-metal oxides are widely known for their various exotic phenomena. This is exemplified by rare-earth nickelates such as LaNiO3, which possess intimate interconnections between their electronic, spin, and lattice degrees of freedom. Their properties can be further enhanced by pairing them in hybrid heterostructures, which can lead to hidden phases and emergent phenomena. An important example is the LaNiO3/LaTiO3 superlattice, where an interlayer electron transfer has been observed from LaTiO3 into LaNiO3 leading to a high-spin state. However, macroscopic emergence of magnetic order associated with this high-spin state has so far not been observed. Here, by using muon spin rotation, x-ray absorption, and resonant inelastic x-ray scattering, direct evidence of an emergent antiferromagnetic order with high magnon energy and exchange interactions at the LaNiO3/LaTiO3 interface is presented. As the magnetism is purely interfacial, a single LaNiO3/LaTiO3 interface can essentially behave as an atomically thin strongly-correlated quasi-2D antiferromagnet, potentially allowing its technological utilization in advanced spintronic devices. Furthermore, its strong quasi-2D magnetic correlations, orbitally-polarized planar ligand holes, and layered superlattice design make its electronic, magnetic, and lattice configurations resemble the precursor states of superconducting cuprates and nickelates, but with an S→1 spin state instead.

Charge-Transfer-Induced Interfacial Ferromagnetism in Ferromagnet-Free Oxide Heterostructures

Due to the strong interlayer coupling between multiple degrees of freedom, oxide heterostructures have demonstrated exotic properties that are not shown by their bulk counterparts. One of the most interesting properties is ferromagnetism at the interface formed between "nonferromagnetic" compounds. Here we report on the interfacial ferromagnetic phase induced in the superlattices consisting of the two paramagnetic oxides CaRuO3 (CRO) and LaNiO3 (LNO). By varying the sublayer thickness in the superlattice period, we demonstrate that the ferromagnetic order has been established in both CaRuO3 and LaNiO3 sublayers, exhibiting an identical Curie temperature of ∼75 K. The X-ray absorption spectra suggest a strong charge transfer from Ru to Ni at the interface, triggering superexchange interactions between Ru/Ni ions and giving rise to the emergent ferromagnetic phase. Moreover, the X-ray linear dichroism spectra reveal the preferential occupancy of the d3z2-r2 orbital for the Ru ions and the dx2-y2 orbital for the Ni ions in the heterostructure. This leads to different magnetic anisotropy of the superlattices when they are dominated by CRO or LNO sublayers. This work clearly demonstrates a charge-transfer-induced interfacial ferromagnetic phase in the whole ferromagnet-free oxide heterostructures, offering a feasible way to tailor oxide materials for desired functionalities.

Ferromagnetic ordering correlated strong metal-oxygen hybridization for superior oxygen reduction reaction activity

The efficiency of transition-metal oxide materials toward oxygen-related electrochemical reactions is classically controlled by metal-oxygen hybridization. Recently, the unique magnetic exchange interactions in transition-metal oxides are proposed to facilitate charge transfer and reduce activation barrier in electrochemical reactions. Such spin/magnetism-related effects offer a new and rich playground to engineer oxide electrocatalysts, but their connection with the classical metal-oxygen hybridization theory remains an open question. Here, using the MnxVyOz family as a platform, we show that ferromagnetic (FM) ordering is intrinsically correlated with the strong manganese (Mn)-oxygen (O) hybridization of Mn oxides, thus significantly increasing the oxygen reduction reaction (ORR) activity. We demonstrate that this enhanced Mn-O hybridization in FM Mn oxides is closely associated with the generation of active Mn sites on the oxide surface and obtaining favorable reaction thermodynamics under operating conditions. As a result, FM-Mn2V2O7 with a high degree of Mn-O hybridization achieves a record high ORR activity. Our work highlights the potential applications of magnetic oxide materials with strong metal-oxygen hybridization in energy devices.

Colossal Dielectric Perovskites of Calcium Copper Titanate (CaCu3 Ti4 O12 ) with Low-Iridium Dopants Enables Ultrahigh Mass Activity for the Acidic Oxygen Evolution Reaction

Oxygen evolution reaction (OER) under acidic conditions becomes of significant importance for the practical use of a proton exchange membrane (PEM) water electrolyzer. In particular, maximizing the mass activity of iridium (Ir) is one of the maiden issues. Herein, the authors discover that the Ir-doped calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO) perovskite exhibits ultrahigh mass activity up to 1000 A g_Ir ^-1 for the acidic OER, which is 66 times higher than that of the benchmark catalyst, IrO₂ . By substituting Ti with Ir in CCTO, metal-oxygen (M-O) covalency can be significantly increased leading to the reduced energy barrier for charge transfer. Further, highly polarizable CCTO perovskite referred to as "colossal dielectric", possesses low defect formation energy for oxygen vacancy inducing a high number of oxygen vacancies in Ir-doped CCTO (Ir-CCTO). Electron transfer occurs from the oxygen vacancies and Ti to the substituted Ir consequentially resulting in the electron-rich Ir and -deficient Ti sites. Thus, favorable adsorptions of oxygen intermediates can take place at Ti sites while the Ir ensures efficient charge supplies during OER, taking a top position of the volcano plot. Simultaneously, the introduced Ir dopants form nanoclusters at the surface of Ir-CCTO, which can boost catalytic activity for the acidic OER.

Identification of the Origin for Reconstructed Active Sites on Oxyhydroxide for Oxygen Evolution Reaction

The regulation of atomic and electronic structures of active sites plays an important role in the rational design of oxygen evolution reaction (OER) catalysts toward electrocatalytic hydrogen generation. However, the precise identification of the active sites for surface reconstruction behavior during OER remains elusive for water-alkali electrolysis. Herein, irreversible reconstruction behavior accompanied by copper dynamic evolution for cobalt iron layered double hydroxide (CoFe LDH) precatalyst to form CoFeCuOOH active species with high-valent Co species is reported, identifying the origin of reconstructed active sites through operando UV-Visible (UV-vis), in situ Raman, and X-ray absorption fine-structure (XAFS) spectroscopies. Density functional theory analysis rationalizes this typical electronic structure evolution causing the transfer of intramolecular electrons to form ligand holes, promoting the reconstruction of active sites. Specifically, unambiguous identification of active sites for CoFeCuOOH is explored by in situ ¹⁸ O isotope-labeling differential electrochemical mass spectrometry (DEMS) and supported by theoretical calculation, confirming mechanism switch to oxygen-vacancy-site mechanism (OVSM) pathway on lattice oxygen. This work enables to elucidate the vital role of dynamic active-site generation and the representative contribution of OVSM pathway for efficient OER performance.

Selectively Controlled Ferromagnets by Electric Fields in van der Waals Ferromagnetic Heterojunctions

Charge transfer plays a key role at the interfaces of heterostructures, which can affect electronic structures and ultimately the physical properties of the materials. However, charge transfer is difficult to manipulate externally once the interface is formed. The recently discovered van der Waals ferromagnets with atomically sharp interfaces provided a perfect platform for the electrical control of interfacial charge transfer. Here, we report magnetoresistance experiments revealing electrically tunable charge transfer in Fe₃GeTe₂/Cr₂Ge₂Te₆/Fe₃GeTe₂ all-magnetic van der Waals heterostructures, which can be exploited to selectively modify the switching fields of the top or bottom Fe₃GeTe₂ electrodes. The directional charge transfer from metallic Fe₃GeTe₂ to semiconducting Cr₂Ge₂Te₆ is revealed by first-principles calculations, which remarkably modifies the magnetic anisotropy energy of Fe₃GeTe₂, leading to the dramatically suppressed coercivity. The electrically selective control of magnetism demonstrated in this study could stimulate the development of spintronic devices based on van der Waals magnets.

Dimensionality-Controlled Evolution of Charge-Transfer Energy in Digital Nickelates Superlattices

Fundamental understanding and control of the electronic structure evolution in rare-earth nickelates is a fascinating and meaningful issue, as well as being helpful to understand the mechanism of recently discovered superconductivity. Here the dimensionality effect on the ground electronic state in high-quality (NdNiO₃ ) _m /(SrTiO₃ )₁ superlattices is systematically studied through transport and soft X-ray absorption spectroscopy. The metal-to-insulator transition temperature decreases with the thickness of the NdNiO₃ slab decreasing from bulk to 7 unit cells, then increases gradually as m further reduces to 1 unit cell. Spectral evidence demonstrates that the stabilization of insulating phase can be attributed to the increase of the charge-transfer energy between O 2p and Ni 3d bands. The prominent multiplet feature on the Ni L₃ edge develops with the decrease of NdNiO₃ slab thickness, suggesting the strengthening of the charge disproportionate state under the dimensional confinement. This work provides convincing evidence that dimensionality is an effective knob to modulate the charge-transfer energy and thus the collective ground state in nickelates.

Topological Hall effect in SrRuO3thin films and heterostructures

Transition metal oxides hold a wide spectrum of fascinating properties endowed by the strong electron correlations. In 4dand 5doxides, exotic phases can be realized with the involvement of strong spin-orbit coupling (SOC), such as unconventional magnetism and topological superconductivity. Recently, topological Hall effects (THEs) and magnetic skyrmions have been uncovered in SrRuO₃thin films and heterostructures, where the presence of SOC and inversion symmetry breaking at the interface are believed to play a key role. Realization of magnetic skyrmions in oxides not only offers a platform to study topological physics with correlated electrons, but also opens up new possibilities for magnetic oxides using in the low-power spintronic devices. In this review, we discuss recent observations of THE and skyrmions in the SRO film interfaced with various materials, with a focus on the electric tuning of THE. We conclude with a discussion on the directions of future research in this field.

Recent advances in the heteroatom doping of perovskite oxides for efficient electrocatalytic reactions

Perovskite-type transition metal oxides have emerged as promising electrocatalysts for various electrocatalytic reactions owing to their low cost, compositional tunability and high stability. However, insufficient electrocatalytic activities of pristine perovskite oxides hinder their pathway towards real-world applications. The incorporation of heteroatoms into perovskite oxide structures has been regarded as an efficient way to improve the electrocatalytic performance. This minireview summarizes the recent advances in the heteroatom doping of perovskite oxides as efficient electrocatalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). These heteroatom doping strategies are classified based on various types of doping sites. The mechanisms of improved electrocatalytic activities are discussed in detail within different doping sites and various kinds of dopants. Finally, the remaining challenges and perspectives are outlined for future developments of perovskite oxide-based catalysts.

Revealing the Correlation of OER with Magnetism: A New Descriptor of Curie/Neel Temperature for Magnetic Electrocatalysts

Developing accurate descriptors for oxygen evolution reaction (OER) is of great significance yet challenging, which roots in and also boosts the understanding of its intrinsic mechanisms. Despite various descriptors are reported, it still has limitations in the facile prediction, given that complicated analytical techniques as well as time-consuming modeling and calculations are indispensable. In the present work, strong correlation of magnetic property with OER performance is revealed by in-depth investigations on the crystal and electronic structures. A facile descriptor of Curie/Neel temperature (T_C/N ) is developed for La_{2- x} Sr_x Co₂ O_{6- δ} perovskite oxides, based on the inference that both magnetism and OER are rooted in the electron exchange interaction. Specifically, both the T_C/N and OER activity are proportional to the degree of p-d orbital hybridization, which increases with enlarged bond angle of Co─O─Co and/or increased oxidation of Co. This finding reveals that T_C/N from magnetic characterizations is an effective descriptor in designing novel OER electrocatalysts, and interdisciplinary researches are advantageous for revealing the controversial mechanisms of OER process.

Metal Substitution Steering Electron Correlations in Pyrochlore Ruthenates for Efficient Acidic Water Oxidation

Exploring the advanced oxygen evolution reaction (OER) electrocatalysts is highly desirable toward sustainable energy conversion and storage, yet improved efficiency in acidic media is largely hindered by its sluggish reaction kinetics. Herein, we rationally manipulate the electronic states of the strongly electron correlated pyrochlore ruthenate Y₂Ru₂O₇ alternative through partial A-site substitution of Sr²⁺ for Y³⁺, efficiently improving its intrinsic OER activity. The optimized Y_1.7Sr_0.3Ru₂O₇ candidate observes a highly intrinsic mass activity of 1018 A g_Ru^-1 at an overpotential of 300 mV with excellent durability in 0.5 M H₂SO₄ electrolyte. Combining synchrotron-radiation X-ray spectroscopic investigations with theoretical simulations, we reveal that the electron correlations in the Ru 4d band are weakened through coordinatively geometric regulation and charge redistribution by the exotic Sr²⁺ cation, enabling the delocalization of Ru 4d electrons via an insulator-to-metal transition. The induced Ru-O covalency promotion and band alignment rearrangement decreases the charge transfer energy to accelerate interfacial charge transfer kinetics. Meanwhile, the chemical affinity of oxygen intermediates is also rationalized to weaken the metal-oxygen binding strength, thus lowering the energy barrier of the overall reaction. This work offers fresh insights into designing advanced solid-state electrocatalysts and underlines the versatility of electronic structure manipulation in tuning catalytic activity.

Gate-tuned anomalous Hall effect driven by Rashba splitting in intermixed LaAlO3/GdTiO3/SrTiO3

The Anomalous Hall Effect (AHE) is an important quantity in determining the properties and understanding the behaviour of the two-dimensional electron system forming at the interface of SrTiO3-based oxide heterostructures. The occurrence of AHE is often interpreted as a signature of ferromagnetism, but it is becoming more and more clear that also paramagnets may contribute to AHE. We studied the influence of magnetic ions by measuring intermixed LaAlO3/GdTiO3/SrTiO3 at temperatures below 10 K. We find that, as function of gate voltage, the system undergoes a Lifshitz transition while at the same time an onset of AHE is observed. However, we do not observe clear signs of ferromagnetism. We argue the AHE to be due to the change in Rashba spin-orbit coupling at the Lifshitz transition and conclude that also paramagnetic moments which are easily polarizable at low temperatures and high magnetic fields lead to the presence of AHE, which needs to be taken into account when extracting carrier densities and mobilities.

Three dimensional band-filling control of complex oxides triggered by interfacial electron transfer

The d-band-filling of transition metals in complex oxides plays an essential role in determining their structural, electronic and magnetic properties. Traditionally, at the oxide heterointerface, band-filling control has been achieved via electrostatic modification in the structure of field-effect transistors or electron transfer, which is limited to the quasi-two-dimension at the interface. Here we report a three-dimensional (3D) band-filling control by changing the local lattice coordination in a designed oxide heterostructure. At the LaCoO₃/LaTiO₃ heterointerface, due to the Fermi level mismatch, electrons transfer from LaTiO₃ to LaCoO₃. This triggers destabilisation of the CoO₆ octahedrons, i.e. the formation of lattice configurations with a reduced Co valence. The associated oxygen migration results in the 3D topotactic phase transition of LaCoO₃. Tuned by the thickness of LaTiO₃, different crystalline phases and band-fillings of Co occur, leading to the emergence of different magnetic ground states.

Near-Atomic-Scale Mapping of Electronic Phases in Rare Earth Nickelate Superlattices

Nanoscale mapping of the distinct electronic phases characterizing the metal-insulator transition displayed by most of the rare-earth nickelate compounds is fundamental for discovering the true nature of this transition and the possible couplings that are established at the interfaces of nickelate-based heterostructures. Here, we demonstrate that this can be accomplished by using scanning transmission electron microscopy in combination with electron energy-loss spectroscopy. By tracking how the O K and Ni L edge fine structures evolve across two different NdNiO₃/SmNiO₃ superlattices, displaying either one or two metal-insulator transitions depending on the individual layer thickness, we are able to determine the electronic state of each of the individual constituent materials. We further map the spatial configuration associated with their metallic/insulating regions, reaching unit cell spatial resolution. With this, we estimate the width of the metallic/insulating boundaries at the NdNiO₃/SmNiO₃ interfaces, which is measured to be on the order of four unit cells.

Hole-Trapping-Induced Stabilization of Ni4 + in SrNiO3 /LaFeO3 Superlattices

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO₃ )_m /(LaFeO₃ )_n superlattices by tuning n and m. The Goldschmidt tolerance constraints are lifted, and SrNi⁴⁺ O₃ with holes on adjacent O anions is stabilized as a perovskite at the single-unit-cell level (m = 1). Holding m = 1, spectroscopy reveals that the n = 1 superlattice contains Ni³⁺ and Fe⁴⁺ , whereas Ni⁴⁺ and Fe³⁺ are observed in the n = 5 superlattice. It is revealed that the B-site cation valences can be tuned by controlling the magnitude of the FeO₆ octahedral rotations, which, in turn, determine the energy balance between Ni³⁺ /Fe⁴⁺ and Ni⁴⁺ /Fe³⁺ , thus controlling emergent electrical properties such as the band alignment and resulting hole confinement. This approach can be extended to other systems for synthesizing novel, metastable layered structures with new functionalities.

Perovskite oxides as transparent semiconductors: a review

Traditional transparent conducting oxides (TCOs) have been widely used for various optoelectronic applications, but have the trade-off between conductivity and transmittance. Recently, perovskite oxides, with structural and chemical stability, have exhibited excellent physical properties as new TCOs. We focus on SrVO3-based perovskites with a high carrier concentration and BaSnO3-based perovskites with a high mobility for n-type TCOs. In addition, p-type perovskites are discussed, which can serve as potential future options to couple with n-type perovskites to design full perovskite based devices.

Machine learning approaches for ELNES/XANES

Materials characterization is indispensable for materials development. In particular, spectroscopy provides atomic configuration, chemical bonding and vibrational information, which are crucial for understanding the mechanism underlying the functions of a material. Despite its importance, the interpretation of spectra using human-driven methods, such as manual comparison of experimental spectra with reference/simulated spectra, is becoming difficult owing to the rapid increase in experimental spectral data. To overcome the limitations of such methods, we develop new data-driven approaches based on machine learning. Specifically, we use hierarchical clustering, a decision tree and a feedforward neural network to investigate the electron energy loss near edge structures (ELNES) spectrum, which is identical to the X-ray absorption near edge structure (XANES) spectrum. Hierarchical clustering and the decision tree are used to interpret and predict ELNES/XANES, while the feedforward neural network is used to obtain hidden information about the material structure and properties from the spectra. Further, we construct a prediction model that is robust against noise by data augmentation. Finally, we apply our method to noisy spectra and predict six properties accurately. In summary, the proposed approaches can pave the way for fast and accurate spectrum interpretation/prediction as well as local measurement of material functions.

Engineering two-dimensional metal oxides via surface functionalization for biological applications

Schematic illustration of 2D MO nanosheets for applications in biosystems.

Tuning the Electronic Structure of LaNiO3 through Alloying with Strontium to Enhance Oxygen Evolution Activity

The perovskite oxide LaNiO3 is a promising oxygen electrocatalyst for renewable energy storage and conversion technologies. Here, it is shown that strontium substitution for lanthanum in coherently strained, epitaxial LaNiO3 films (La1- x Sr x NiO3) significantly enhances the oxygen evolution reaction (OER) activity, resulting in performance at x = 0.5 comparable to the state-of-the-art catalyst Ba0.5Sr0.5Co0.8Fe0.2O3- δ . By combining X-ray photoemission and X-ray absorption spectroscopies with density functional theory, it is shown that an upward energy shift of the O 2p band relative to the Fermi level occurs with increasing x in La1- x Sr x NiO3. This alloying step strengthens Ni 3d-O 2p hybridization and decreases the charge transfer energy, which in turn accounts for the enhanced OER activity.

Anion-mediated electronic effects in reducible oxides: Tuning the valence band of ceria via fluorine doping

Combining experimental spectroscopy and hybrid density functional theory calculations, we show that the incorporation of fluoride ions into a prototypical reducible oxide surface, namely, ceria(111), can induce a variety of nontrivial changes to the local electronic structure, beyond the expected increase in the number of Ce³⁺ ions. Our resonant photoemission spectroscopy results reveal new states above, within, and below the valence band, which are unique to the presence of fluoride ions at the surface. With the help of hybrid density functional calculations, we show that the different states arise from fluoride ions in different atomic layers in the near surface region. In particular, we identify a structure in which a fluoride ion substitutes for an oxygen ion at the surface, with a second fluoride ion on top of a surface Ce⁴⁺ ion giving rise to F 2p states which overlap the top of the O 2p band. The nature of this adsorbate F^--Ce⁴⁺ resonant enhancement feature suggests that this bond is at least partially covalent. Our results demonstrate the versatility of anion doping as a potential means of tuning the valence band electronic structure of ceria.

Modulation of New Excitons in Transition Metal Dichalcogenide-Perovskite Oxide System

The exciton, a quasi-particle that creates a bound state of an electron and a hole, is typically found in semiconductors. It has attracted major attention in the context of both fundamental science and practical applications. Transition metal dichalcogenides (TMDs) are a new class of 2D materials that include direct band-gap semiconductors with strong spin-orbit coupling and many-body interactions. Manipulating new excitons in semiconducting TMDs could generate a novel means of application in nanodevices. Here, the observation of high-energy excitonic peaks in the monolayer-MoS2 on a SrTiO3 heterointerface generated by a new complex mechanism is reported, based on a comprehensive study that comprises temperature-dependent optical spectroscopies and first-principles calculations. The appearance of these excitons is attributed to the change in many-body interactions that occurs alongside the interfacial orbital hybridization and spin-orbit coupling brought about by the excitonic effect propagated from the substrate. This has further led to the formation of a Fermi-surface feature at the interface. The results provide an atomic-scale understanding of the heterointerface between monolayer-TMDs and perovskite oxide and highlight the importance of spin-orbit-charge-lattice coupling on the intrinsic properties of atomic-layer heterostructures, which open up a way to manipulate the excitonic effects in monolayer TMDs via an interfacial system.

Double-Exchange Magnetic Interactions in High-Temperature Ferromagnetic Iron Chalcogenide Monolayers

Smythite ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub><mml:mrow><mml:mi>F</mml:mi> <mml:mi>e</mml:mi></mml:mrow> <mml:mn>3</mml:mn></mml:msub> <mml:msub><mml:mi>S</mml:mi> <mml:mn>4</mml:mn></mml:msub> </mml:mrow> </mml:math> ) is an iron-based chalcogenide with a lamellar structure, different from the compositionally identical mineral greigite. Owing to their natural abundance, such transition metal chalcogenides are promising materials for low-cost spintronic-based devices. Herein, we discuss the charge transfer processes and complex magnetic ordering in a two-dimensional (2D) smythite lattice. We find that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>F</mml:mi> <mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> <mml:mo>/</mml:mo> <mml:mi>F</mml:mi> <mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow> <mml:mrow><mml:mn>3</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> </mml:math> redox couple and complex magnetic ordering are governing factors in the charge transfer processes. A very strong ferromagnetic in-lattice coupling is also observed, which is attributed to the presence of three Fe-centres. To describe the magnetic behaviour molecular and periodic approaches have been considered. We found a substantial increase in Curie temperature with applied mechanical stress due to opening of the double exchange interaction angle. We also observe an in-plane Jahn-Teller distortion, which is further confirmed by the spin-orbit counter plot. Our study thus provides an insight into the double exchange mechanism favoured by the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>F</mml:mi> <mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> <mml:mo>/</mml:mo> <mml:mi>F</mml:mi> <mml:msup><mml:mrow><mml:mi>e</mml:mi></mml:mrow> <mml:mrow><mml:mn>3</mml:mn> <mml:mo>+</mml:mo></mml:mrow> </mml:msup> </mml:mrow> </mml:math> redox couple and results in a strong ferromagnetic ordering.

Interfacial Engineering of Ferromagnetism in Epitaxial Manganite/Ruthenate Superlattices via Interlayer Chemical Doping

Interfacial charge transfer and structural proximity effects are the two essential routes to trigger and tune numerous functionalities of perovskite oxide heterostructures. However, the cooperation and competition of these two interfacial effects in one epitaxial system have not been fully understood. Herein, we fabricate a series of La_0.67Ca_0.33MnO₃/CaRuO₃ superlattices and introduce various chemical doping in the nonmagnetic CaRuO₃ interlayers. We found that Ti, Sr, and La doping in the CaRuO₃ layer can effectively tune the interfacial charge transfer and octahedral rotation, thus modulating the ferromagnetism of the superlattices. Specifically, the B-site Ti doping depletes the Ru 4d band and suppresses the interfacial charge transfer, leading to a decay of ferromagnetic Curie temperature ( T_C). In contrast, the A-site Sr doping maintains a sizable charge transfer and meanwhile suppresses the octahedral rotation, which facilitates ferromagnetism and significantly enhances the T_C up to 291 K. The La doping turns out to localize the itinerant electrons in the CaRuO₃ layer, which suppresses both the interfacial charge transfer and ferromagnetism. The observed intriguing interfacial engineering of magnetism would pave a new way to understand the collective effects of interfacial charge transfer and structural proximity on the physical properties of oxide heterostructures.

Evolution of Oxygen-Metal Electron Transfer and Metal Electronic States During Manganese Oxide Catalyzed Water Oxidation Revealed with In Situ Soft X-Ray Spectroscopy

Manganese oxide (MnO_x ) electrocatalysts are examined herein by in situ soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) during the oxidation of water buffered by borate (pH 9.2) at potentials from 0.75 to 2.25 V vs. the reversible hydrogen electrode. Correlation of L-edge XAS data with previous mechanistic studies indicates Mn^IV is the highest oxidation state involved in the catalytic mechanism. MnO_x is transformed into birnessite at 1.45 V and does not undergo further structural phase changes. At potentials beyond this transformation, RIXS spectra show progressive enhancement of charge transfer transitions from oxygen to manganese. Theoretical analysis of these data indicates increased hybridization of the Mn-O orbitals and withdrawal of electron density from the O ligand shell. In situ XAS experiments at the O K-edge provide complementary evidence for such a transition. This step is crucial for the formation of O₂ from water.

Large orbital polarization in nickelate-cuprate heterostructures by dimensional control of oxygen coordination

Artificial heterostructures composed of dissimilar transition metal oxides provide unprecedented opportunities to create remarkable physical phenomena. Here, we report a means to deliberately control the orbital polarization in LaNiO₃ (LNO) through interfacing with SrCuO₂ (SCO), which has an infinite-layer structure for CuO₂. Dimensional control of SCO results in a planar-type (P–SCO) to chain-type (C–SCO) structure transition depending on the SCO thickness. This transition is exploited to induce either a NiO₅ pyramidal or a NiO₆ octahedral structure at the SCO/LNO interface. Consequently, a large change in the Ni d orbital occupation up to ~30% is achieved in P–SCO/LNO superlattices, whereas the Ni e_g orbital splitting is negligible in C–SCO/LNO superlattices. The engineered oxygen coordination triggers a metal-to-insulator transition in SCO/LNO superlattices. Our results demonstrate that interfacial oxygen coordination engineering provides an effective means to manipulate the orbital configuration and associated physical properties, paving a pathway towards the advancement of oxide electronics.

In correlated materials, physical properties depend on orbital occupancy and polarization. Here, a way to control the oxygen coordination via dimensionality of superlattices is presented that results in the change of orbital occupancy by 30%, which is larger than what has been achieved by other methods.

Determination of the local structure of Sr2-xMxIrO4 (M = K, La) as a function of doping and temperature

The local structure of correlated spin-orbit insulator Sr2-xMxIrO4 (M = K, La) has been investigated by Ir L3-edge extended X-ray absorption fine structure measurements. The measurements were performed as a function of temperature for different dopings induced by substitution of Sr with La or K. It is found that Ir-O bonds have strong covalency and they hardly show any change across the Néel temperature. In the studied doping range, neither Ir-O bonds nor their dynamics, measured by their mean square relative displacements, show any appreciable change upon carrier doping, indicating the possibility of nanoscale phase separation in the doped system. On the other hand, there is a large increase of the static disorder in Ir-Sr correlation, larger for K doping than La doping. Similarities and differences with respect to the local lattice displacements in cuprates are briefly discussed.

Strain-Enhanced Charge Transfer and Magnetism at a Manganite/Nickelate Interface

The strain effect on charge transfer in correlated oxide La_0.8Sr_0.2MnO₃/NdNiO₃ (LSMO/NNO) heterostructures is investigated. This is achieved by carefully tailoring the strain on the two layers using various substrates. In contrast to bare LSMO films, the strain dependence of the enhanced magnetic moment of the LSMO/NNO bilayers strongly suggests that the charge transfer can be controlled via strain engineering in complex oxide heterostructures. Furthermore, our study also reveals that the coercive field, exchange bias, and conductivity are dramatically affected by the strain-modulated charge transfer in LSMO/NNO heterostructures. Our work thus points out a new path to control electronic states in oxide heterostructures to advance the use of interfaces in oxide-based electronics.

Metal-insulator-transition engineering by modulation tilt-control in perovskite nickelates for room temperature optical switching

Correlated transition metal oxide perovskites receive a lot of attention due to their unique physical properties, which are largely driven by distortion of the BO₆ octahedral network. In bulk, the control of the octahedral network is normally obtained by cation substitutions in a random alloy. Similar to the charge donors in semiconductors, cation substitutions will introduce scattering and disorder. The development of artificial heterostructures offers unprecedented opportunities to lattice engineering to achieve desired properties. In this work, we demonstrated a structural analogue of modulation doping in nickelate heterostructures through the interfacial transfer of tilt patterns. Modulation tilt control was used to remotely control the Ni–O bonds in the compound SmNiO₃ and thereby its critical temperature for optimal optical switching application.

In transition metal perovskites ABO₃, the physical properties are largely driven by the rotations of the BO₆ octahedra, which can be tuned in thin films through strain and dimensionality control. However, both approaches have fundamental and practical limitations due to discrete and indirect variations in bond angles, bond lengths, and film symmetry by using commercially available substrates. Here, we introduce modulation tilt control as an approach to tune the ground state of perovskite oxide thin films by acting explicitly on the oxygen octahedra rotation modes—that is, directly on the bond angles. By intercalating the prototype SmNiO₃ target material with a tilt-control layer, we cause the system to change the natural amplitude of a given rotation mode without affecting the interactions. In contrast to strain and dimensionality engineering, our method enables a continuous fine-tuning of the materials’ properties. This is achieved through two independent adjustable parameters: the nature of the tilt-control material (through its symmetry, elastic constants, and oxygen rotation angles), and the relative thicknesses of the target and tilt-control materials. As a result, a magnetic and electronic phase diagram can be obtained, normally only accessible by A-site element substitution, within the single SmNiO₃ compound. With this unique approach, we successfully adjusted the metal–insulator transition (MIT) to room temperature to fulfill the desired conditions for optical switching applications.

A Living-Dead Magnetic Layer at the Surface of Ferrimagnetic DyTiO3 Thin Films

When ferromagnetic films become ultrathin, key properties such as the Curie temperature and the saturation magnetization are usually depressed. This effect is thoroughly investigated in magnetic oxides such as half-metallic manganites, but much less in ferrimagnetic insulating perovskites such as rare-earth titanates RTiO3 , despite their appeal to design correlated 2D electron gases. Here, the magnetic properties of epitaxial DyTiO3 thin films are reported. While films thicker than about 50 nm show a bulk-like response, at low thickness a surprising increase of the saturation magnetization is observed. This behavior is described using a classical model of a "dead layer" but assuming that this layer is actually "living," that is, it responds to the magnetic field with a strong paramagnetic susceptibility. Through depth-dependent X-ray absorption and photoemission spectroscopy, it is shown that the "living-dead layer" corresponds to surface regions where magnetic (S = 1/2) Ti3+ ions are replaced by nonmagnetic Ti4+ ions. Hysteresis cycles at the Dy M 5 and Ti L 3 edges indicate that the surface Ti4+ ions decouple the Dy3+ ions, thus unleashing their strong paramagnetic response. Finally, it is shown how capping the DyTiO3 film can help increase the Ti3+ content near the surface and thus recover a better ferrimagnetic behavior.

Rare-earth nickelates RNiO3: thin films and heterostructures

This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the chemical formula RNiO₃ where R is a trivalent rare-earth R  =  La, Pr, Nd, Sm, …, Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron-lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film, multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices.

Oxygen Diode Formed in Nickelate Heterostructures by Chemical Potential Mismatch

Deliberate control of oxygen vacancy formation and migration in perovskite oxide thin films is important for developing novel electronic and iontronic devices. Here, it is found that the concentration of oxygen vacancies (V_O ) formed in LaNiO₃ (LNO) during pulsed laser deposition is strongly affected by the chemical potential mismatch between the LNO film and its proximal layers. Increasing the V_O concentration in LNO significantly modifies the degree of orbital polarization and drives the metal-insulator transition. Changes in the nickel oxidization state and carrier concentration in the films are confirmed by soft X-ray absorption spectroscopy and optical spectroscopy. The ability to unidirectional-control the oxygen flow across the heterointerface, e.g., a so-called "oxygen diode", by exploiting chemical potential mismatch at interfaces provides a new avenue to tune the physical and electrochemical properties of complex oxides.

Structurally triggered metal-insulator transition in rare-earth nickelates

Rare-earth nickelates form an intriguing series of correlated perovskite oxides. Apart from LaNiO₃, they exhibit on cooling a sharp metal-insulator electronic phase transition, a concurrent structural phase transition, and a magnetic phase transition toward an unusual antiferromagnetic spin order. Appealing for various applications, full exploitation of these compounds is still hampered by the lack of global understanding of the interplay between their electronic, structural, and magnetic properties. Here we show from first-principles calculations that the metal-insulator transition of nickelates arises from the softening of an oxygen-breathing distortion, structurally triggered by oxygen-octahedra rotation motions. The origin of such a rare triggered mechanism is traced back in their electronic and magnetic properties, providing a united picture. We further develop a Landau model accounting for the metal-insulator transition evolution in terms of the rare-earth cations and rationalizing how to tune this transition by acting on oxygen rotation motions.

Applications of rare-earth nickelates are hampered by lack of global understanding of the interplay among various degrees of freedom. Here, Mercy et al. propose that the metal-insulator transition of nickelates arises from the softening of an oxygen breathing distortion, providing a united picture of electronic, structural and magnetic properties.

Design of new Mott multiferroics via complete charge transfer: promising candidates for bulk photovoltaics

Optimal materials to induce bulk photovoltaic effects should lack inversion symmetry and have an optical gap matching the energies of visible radiation. Ferroelectric perovskite oxides such as BaTiO₃ and PbTiO₃ exhibit substantial polarization and stability, but have the disadvantage of excessively large band gaps. We use both density functional theory and dynamical mean field theory calculations to design a new class of Mott multiferroics–double perovskite oxides A₂VFeO₆ (A = Ba, Pb, etc). While neither perovskite AVO₃ nor AFeO₃ is ferroelectric, in the double perovskite A₂VFeO₆ a ‘complete’ charge transfer from V to Fe leads to a non-bulk-like charge configuration–an empty V-d shell and a half-filled Fe-d shell, giving rise to a polarization comparable to that of ferroelectric ATiO₃. Different from nonmagnetic ATiO₃, the new double perovskite oxides have an antiferromagnetic ground state and around room temperatures, are paramagnetic Mott insulators. Most importantly, the V d⁰ state significantly reduces the band gap of A₂VFeO₆, making it smaller than that of ATiO₃ and BiFeO₃ and rendering the new multiferroics a promising candidate to induce bulk photovoltaic effects.

Charge transfer driven emergent phenomena in oxide heterostructures

Complex oxides exhibit many intriguing phenomena, including metal-insulator transition, ferroelectricity/multiferroicity, colossal magnetoresistance and high transition temperature superconductivity. Advances in epitaxial thin film growth techniques enable us to combine different complex oxides with atomic precision and form an oxide heterostructure. Recent theoretical and experimental work has shown that charge transfer across oxide interfaces generally occurs and leads to a great diversity of emergent interfacial properties which are not exhibited by bulk constituents. In this report, we review mechanisms and physical consequence of charge transfer across interfaces in oxide heterostructures. Both theoretical proposals and experimental measurements of various oxide heterostructures are discussed and compared. We also review the theoretical methods that are used to calculate charge transfer across oxide interfaces and discuss the success and challenges in theory. Finally, we present a summary and perspectives for future research.

Quantitative Determination on Ionic-Liquid-Gating Control of Interfacial Magnetism

Ionic-liquid gating on a functional thin film with a low voltage has drawn a lot of attention due to rich chemical, electronic, and magnetic phenomena at the interface. Here, a key challenge in quantitative determination of voltage-controlled magnetic anisotropy (VCMA) in Au/[DEME]⁺ [TFSI]^- /Co field-effect transistor heterostructures is addressed. The magnetic anisotropy change as response to the gating voltage is precisely detected by in situ electron spin resonance measurements. A reversible change of magnetic anisotropy up to 219 Oe is achieved with a low gating voltage of 1.5 V at room temperature, corresponding to a record high VCMA coefficient of ≈146 Oe V^-1 . Two gating effects, the electrostatic doping and electrochemical reaction, are distinguished at various gating voltage regions, as confirmed by X-ray photoelectron spectroscopy and atomic force microscopy experiments. This work shows a unique ionic-liquid-gating system for strong interfacial magnetoelectric coupling with many practical advantages, paving the way toward ion-liquid-gating spintronic/electronic devices.

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.

Interfacial Multiferroics of TiO2/PbTiO3 Heterostructure Driven by Ferroelectric Polarization Discontinuity

Novel phenomena appear when two different oxide materials are combined together to form an interface. For example, at the interface of LaAlO₃/SrTiO₃, two-dimensional conductive states form to avoid the polar discontinuity, and magnetic properties are found at such an interface. In this work, we propose a new type of interface between two nonmagnetic and nonpolar oxides that could host a magnetic state, where it is the ferroelectric polarization discontinuity instead of the polar discontinuity that leads to the charge transfer, forming the interfacial magnetic state. As a concrete example, we investigate by first-principles calculations the heterostructures made of ferroelectric perovskite oxide PbTiO₃ and nonferroelectric polarized oxide TiO₂. We show that charge is transferred to the interfacial layer forming an interfacial ferromagnetic ordering that may persist up to room temperature. Especially, the strong coupling between bulk ferroelectric polarization and interface ferromagnetism represents a new type of magnetoelectric effect, which provides an ideal platform for exploring the intriguing interfacial multiferroics. The findings here are important not only for fundamental science but also for promising applications in nanoscale electronics and spintronics.

Interfacial Control of Ferromagnetism in Ultrathin La0.67Ca0.33MnO3 Sandwiched between CaRu1-xTixO3 (x = 0-0.8) Epilayers

Controlling functionalities in oxide heterostructures remains challenging for the rather complex interfacial interactions. Here, by modifying the interface properties with chemical doping, we achieve a nontrivial control over the ferromagnetism in ultrathin La_0.67Ca_0.33MnO₃ (LCMO) layer sandwiched between CaRu_1-xTi_xO₃ [CRTO(x)] epilayers. The Ti doping suppresses the interfacial electron transfer from CRTO(x) to LCMO side; as a result, a steadily decreased Curie temperature with increasing x, from 262 K at x = 0 to 186 K at x = 0.8, is observed for the structures with LCMO fixed at 3.2 nm. Moreover, for more insulating CRTO(x ≥ 0.5), the electron confinement induces an interfacial Mn-e_g(x²-y²) orbital order in LCMO which further attenuates the ferromagnetism. Also, in order to characterize the heterointerfaces, for the first time the doping- and thickness-dependent metal-insulator transitions in CRTO(x) films are examined. Our results demonstrate that the LCMO/CRTO(x) heterostructure could be a model system for investigating the interfacial multiple interactions in correlated oxides.

Orbital Engineering in Nickelate Heterostructures Driven by Anisotropic Oxygen Hybridization rather than Orbital Energy Levels

Resonant inelastic x-ray scattering is used to investigate the electronic origin of orbital polarization in nickelate heterostructures taking LaTiO_{3}-LaNiO_{3}-3×(LaAlO_{3}), a system with exceptionally large polarization, as a model system. We find that heterostructuring generates only minor changes in the Ni 3d orbital energy levels, contradicting the often-invoked picture in which changes in orbital energy levels generate orbital polarization. Instead, O K-edge x-ray absorption spectroscopy demonstrates that orbital polarization is caused by an anisotropic reconstruction of the oxygen ligand hole states. This provides an explanation for the limited success of theoretical predictions based on tuning orbital energy levels and implies that future theories should focus on anisotropic hybridization as the most effective means to drive large changes in electronic structure and realize novel emergent phenomena.

Superconductor to Mott insulator transition in YBa2Cu3O7/LaCaMnO3 heterostructures

The superconductor-to-insulator transition (SIT) induced by means such as external magnetic fields, disorder or spatial confinement is a vivid illustration of a quantum phase transition dramatically affecting the superconducting order parameter. In pursuit of a new realization of the SIT by interfacial charge transfer, we developed extremely thin superlattices composed of high Tc superconductor YBa2Cu3O7 (YBCO) and colossal magnetoresistance ferromagnet La0.67Ca0.33MnO3 (LCMO). By using linearly polarized resonant X-ray absorption spectroscopy and magnetic circular dichroism, combined with hard X-ray photoelectron spectroscopy, we derived a complete picture of the interfacial carrier doping in cuprate and manganite atomic layers, leading to the transition from superconducting to an unusual Mott insulating state emerging with the increase of LCMO layer thickness. In addition, contrary to the common perception that only transition metal ions may respond to the charge transfer process, we found that charge is also actively compensated by rare-earth and alkaline-earth metal ions of the interface. Such deterministic control of Tc by pure electronic doping without any hindering effects of chemical substitution is another promising route to disentangle the role of disorder on the pseudo-gap and charge density wave phases of underdoped cuprates.

Predictive Control over Charge Density in the Two-Dimensional Electron Gas at the Polar-Nonpolar NdTiO_{3}/SrTiO_{3} Interface

Through systematic control of the Nd concentration, we show that the carrier density of the two-dimensional electron gas (2DEG) in SrTiO_{3}/NdTiO_{3}/SrTiO_{3}(001) can be modulated over a wide range. We also demonstrate that the NdTiO_{3} in heterojunctions without a SrTiO_{3} cap is degraded by oxygen absorption from air, resulting in the immobilization of donor electrons that could otherwise contribute to the 2DEG. This system is, thus, an ideal model to understand and control the insulator-to-metal transition in a 2DEG based on both environmental conditions and film-growth processing parameters.

Investigation of the transport, structural and mechanical properties of half-metallic REMnO3 (RE = Ce and Pr) ferromagnets

Systematic investigation of the ground state structure, elastic and transport properties, of perovskite oxides REMnO₃ (RE = Ce and Pr) has been carried out by first principles calculations. The half-metallicity and ferromagnetism is well explained.