Superconducting Boride Nb2IrB2 with Variable and Tunable Stacking Behaviors of Two-Dimensional [Nb-Ir-Nb] Triangular Lattices.

In structures with special geometry lattices, variations in stacking sequences are ubiquitous, yielding many novel structures and functionalities. Despite a wealth of intriguing properties and wide-ranging applications, there remains a considerable gap in understanding the correlation between special geometry lattices and functionalities in borides. Here, we design and synthesize a new superconducting boride Nb2IrB2, with a body-centered orthorhombic structure, consisting of alternating two-dimensional [Nb-Ir-Nb] triple-triangular-lattice-layers and B fragment layers. Advanced aberration-corrected scanning transmission electron microscopy observations show variable stacking configurations between [Nb-Ir-Nb] triple-triangular-lattice layers that can be tuned through synthesis conditions. Density functional theory calculations reveal that the coherent low-energy boundary interface plane of {101} between [11̅1] and [010] domains is responsible for the variable stacking behaviors. Energetically favorable structures are thereby reasonably proposed, based on nanoscale imperfect structure units. These findings provide valuable insights for designing and exploring new structures and functionalities within boride systems involving special geometry lattices.

Unraveling the Oxygen Vacancy-Performance Relationship in Perovskite Oxides at Atomic Precision via Precise Synthesis.

Understanding the fundamental effect of the oxygen vacancy atomic structure in perovskite oxides on catalytic properties remains challenging due to diverse facets, surface sites, defects, etc. in traditional powder catalysts and the inherent structural complexity. Through quantitative synthesis of tetrahedral (LaCoO2.5-T), pyramidal (LaCoO2.5-P), and octahedral (LaCoO3) epitaxial thin films as model catalysts, we demonstrate the reactivity orders of active-site geometrical configurations in oxygen-deficient perovskites during the CO oxidation model reaction: CoO4 tetrahedron > CoO6 octahedron > CoO5 pyramid. Ambient-pressure Co L-edge and O K-edge XAS spectra clarify the dynamic evolutions of active-site electronic structures during realistic catalytic processes and highlight the important roles of defect geometrical structures. In addition, in situ XAS and resonant inelastic X-ray scattering spectra and density functional theory calculations directly reveal the nature of high reactivity for CoO4 sites and that the derived shallow-acceptor defect levels in the band structure facilitate the adsorption and activation of reactive gases, resulting in more than 23-fold enhancement for catalytic reaction rates than CoO5 sites.

Superfine Nanodomain Engineering Unleashing Ferroelectricity in Incipient Ferroelectrics.

Incipient ferroelectrics have emerged as an attractive class of functional materials owing to their potential to be engineered for exotic ferroelectric behavior, holding great promise for expanding the ferroelectric family. However, thus far, their artificially engineered ferroelectricity has fallen far short of rivaling classic ferroelectrics. In this study, we address this challenge by developing a superfine nanodomain engineering strategy. By applying this approach to representative incipient ferroelectric of SrTiO3-based films, we achieve unprecedentedly strong ferroelectricity, not only surpassing previous records for incipient ferroelectrics but also being comparable to classic ferroelectrics. The remanent polarization of the thin film reaches up to 17.0 µC cm-2 with an ultrahigh Curie temperature of 973 K. Atomic-scale investigations elucidate the origin of this robust ferroelectricity in the emergent high-density superfine nanodomains spanning merely 3-10 unit cells. Combining experimental results with theoretical assessments, we unveil the underlying mechanism, where the intentionally introduced diluted foreign Fe element creates a deeper Landau energy well and promotes a short-range ordering of polarization. Our developed strategy significantly streamlines the design of unconventional ferroelectrics, providing a versatile pathway for exploring new and superior ferroelectric materials.

Stereointerface Structure Drives Ferroelectricity in BaZrO3 Films.

Interfacial strain engineering can induce structural transformation and introduce new physical properties into materials, which is an effective method to prepare new multifunctional materials. However, interfacial strain has a limited spatial impact size. For example, in 2D thin films, the critical thickness of biaxial strain is typically less than 20 nm, which is not conducive to the maintenance of a strained structure and properties in thick film materials. The construction of a 3D interface can solve this problem. The large lattice mismatch between the BaZrO3 thin film and the substrate can induce the out-of-phase boundary (OPB) structure, which can extend along the thickness direction with the stacking of atoms. The lattice distortion at the OPB structure can provide a clamping effect for each layer of atoms, thus expanding the spatial influence range of biaxial strain. As a result, the uniform in-plane strain distribution and strain-induced ferroelectricity (Pr = 13 µC/cm2) are maintained along the thickness direction in BaZrO3 films.

Significantly Enhanced Room-Temperature Ferromagnetism in Multiferroic EuFeO3-δ Thin Films.

Regulating the magnetic properties of multiferroics lays the foundation for their prospective application in spintronic devices. Single-phase multiferroics, such as rare-earth ferrites, are promising candidates; however, they typically exhibit weak magnetism at room temperature (RT). Here, we significantly boosted the RT ferromagnetism of a representative ferrite, EuFeO3, by oxygen defect engineering. Polarized neutron reflectometry and magnetometry measurements reveal that saturation magnetization reaches 0.04 µB/Fe, which is approximately 5 times higher than its bulk phase. Combining the annular bright-field images with theoretical assessment, we unravel the underlying mechanism for magnetic enhancement, in which the decrease in Fe-O-Fe bond angles caused by oxygen vacancies (VO) strengthens magnetic interactions and tilts Fe spins. Furthermore, the internal relationship between magnetism and VO was established by illustrating how the magnetic structure and magnitude change with VO configuration and concentration. Our strategy for regulating magnetic properties can be applied to numerous functional oxide materials.

Photoinduced Phase Transition in Infinite-Layer Nickelates.

The quantum phase transition caused by regulating the electronic correlation in strongly correlated quantum materials has been a research hotspot in condensed matter science. Herein, a photon-induced quantum phase transition from the Kondo-Mott insulating state to the low temperature metallic one accompanying with the magnetoresistance changing from negative to positive in the infinite-layer NdNiO2 films is reported, where the antiferromagnetic coupling among the Ni1+ localized spins and the Kondo effect are effectively suppressed by manipulating the correlation of Ni-3d and Nd-5d electrons under the photoirradiation. Moreover, the critical temperature Tc of the superconducting-like transition exhibits a dome-shaped evolution with the maximum up to ≈42 K, and the electrons dominate the transport process proved by the Hall effect measurements. These findings not only make the photoinduction a promising way to control the quantum phase transition by manipulating the electronic correlation in Mott-like insulators, but also shed some light on the possibility of the superconducting in electron-doped nickelates.

High-Temperature Ferroic Glassy States in SrTiO_{3}-Based Thin Films.

Disordered ferroics hold great promise for next-generation magnetoelectric devices because their lack of symmetry constraints implies negligible hysteresis with low energy costs. However, the transition temperature and the magnitude of polarization and magnetization are still too low to meet application requirements. Here, taking the prototype perovskite of SrTiO_{3} as an instance, we realize a coexisting spin and dipole reentrant glass states in SrTiO_{3} homoepitaxial films via manipulation of local symmetry. Room-temperature saturation magnetization and spontaneous polarization reach ∼ 10 emu/cm^{3} and ∼ 25 µC/cm^{2}, respectively, with high transition temperatures (101 K and 236 K for spin and dipole glass temperatures and 556 K and 1100 K for Curie temperatures, respectively). Our atomic-scale investigation points out an underlying mechanism, where the Ti/O-defective unit cells break the local translational and orbital symmetry to drive the formation of unusual slush states. This study advances our understanding of the nature of the intricate couplings of ferroic glasses. Our approach could be applied to numerous perovskite oxides for the simultaneous control of the local magnetic and polar orderings and for the exploration of the underlying physics.

Protonation-Induced Colossal Chemical Expansion and Property Tuning in NdNiO3 Revealed by Proton Concentration Gradient Thin Films.

Protonation can be used to tune diverse physical and chemical properties of functional oxides. Although protonation of nickelate perovskites has been reported, details on the crystal structure of the protonated phase and a quantitative understanding of the effect of protons on physical properties are still lacking. Therefore, in this work, we select NdNiO3 (NNO) as a model system to understand the protonation process from pristine NNO to protonated HxNdNiO3 (H-NNO). We used a reliable electrochemical method with well-defined reference electrode to trigger the protonation-induced phase transition. We found that the protonated H-NNO phase showed a colossal ∼13% lattice expansion caused by a large tilt of NiO6 octahedra and displacement of Nd cations. Importantly, we further designed a novel device configuration to induce a gradient of proton concentration into a single NNO thin film to establish a quantitative correlation between the proton concentration and the lattice constant and transport property of H-NNO.

Room-Temperature Ferromagnetism at an Oxide-Nitride Interface.

Heterointerfaces have led to the discovery of novel electronic and magnetic states because of their strongly entangled electronic degrees of freedom. Single-phase chromium compounds always exhibit antiferromagnetism following the prediction of the Goodenough-Kanamori rules. So far, exchange coupling between chromium ions via heteroanions has not been explored and the associated quantum states are unknown. Here, we report the successful epitaxial synthesis and characterization of chromium oxide (Cr_{2}O_{3})-chromium nitride (CrN) superlattices. Room-temperature ferromagnetic spin ordering is achieved at the interfaces between these two antiferromagnets, and the magnitude of the effect decays with increasing layer thickness. First-principles calculations indicate that robust ferromagnetic spin interaction between Cr^{3+} ions via anion-hybridization across the interface yields the lowest total energy. This work opens the door to fundamental understanding of the unexpected and exceptional properties of oxide-nitride interfaces and provides access to hidden phases at low-dimensional quantum heterostructures.

Dynamics of Anisotropic Oxygen-Ion Migration in Strained Cobaltites.

Orientation control of the oxygen vacancy channel (OVC) is highly desirable for tailoring oxygen diffusion as it serves as a fast transport channel in ion conductors, which is widely exploited in solid-state fuel cells, catalysts, and ion-batteries. Direct observation of oxygen-ion hopping toward preferential vacant sites is a key to clarifying migration pathways. Here we report anisotropic oxygen-ion migration mediated by strain in ultrathin cobaltites via in situ thermal activation in atomic-resolved transmission electron microscopy. Oxygen migration pathways are constructed on the basis of the atomic structure during the OVC switching, which is manifested as the vertical-to-horizontal OVC switching under tensile strain but the horizontal-to-diagonal switching under compression. We evaluate the topotactic structural changes to the OVC, determine the crucial role of the tolerance factor for OVC stability, and establish the strain-dependent phase diagram. Our work provides a practical guide for engineering OVC orientation that is applicable to ionic-oxide electronics.

Dimensional Control of Octahedral Tilt in SrRuO3 via Infinite-Layered Oxides.

Manipulation of octahedral distortion at atomic scale is an effective means to tune the ground states of functional oxides. Previous work demonstrates that strain and film thickness are variable parameters to modify the octahedral parameters. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetism in SrRuO3 (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO2 (SCO) layers. The infinite-layered CuO2 exhibits a structural transformation from "planar-type" to "chain-type" with reduced film thickness. Two orientations dramatically modify the polyhedral connectivity at the interface, thus altering the octahedral distortion of SRO. The local structural variation changes the spin state of Ru and orbital hybridization strength, leading to a significant change in the magnetoresistance and anomalous Hall resistivity. These findings could launch investigations into adaptive control of functionalities in quantum oxide heterostructures using oxygen coordination.

Understanding the Electronic Structure Evolution of Epitaxial LaNi1-xFexO3 Thin Films for Water Oxidation.

Rare earth nickelates including LaNiO3 are promising catalysts for water electrolysis to produce oxygen gas. Recent studies report that Fe substitution for Ni can significantly enhance the oxygen evolution reaction (OER) activity of LaNiO3. However, the role of Fe in increasing the activity remains ambiguous, with potential origins that are both structural and electronic in nature. On the basis of a series of epitaxial LaNi1-xFexO3 thin films synthesized by molecular beam epitaxy, we report that Fe substitution tunes the Ni oxidation state in LaNi1-xFexO3 and a volcano-like OER trend is observed, with x = 0.375 being the most active. Spectroscopy and ab initio modeling reveal that high-valent Fe3+δ cationic species strongly increase the transition-metal (TM) 3d bandwidth via Ni-O-Fe bridges and enhance TM 3d-O 2p hybridization, boosting the OER activity. These studies deepen our understanding of structural and electronic contributions that give rise to enhanced OER activity in perovskite oxides.

Uniaxial Strain-Controlled Ground States in Manganite Films.

Strongly correlated perovskite oxides exhibit a plethera of intriguing phenomena and stimulate a great potential for multifunctional device applications. Utilizing tunable uniaxial strain, rather than biaxial or anisotropic strain, delivered from the crystallography of a single crystal substrate to modify the ground state of strongly correlated perovskite oxides has rarely been addressed for phase-space control. Here, we show that the physical properties of La2/3Ca1/3MnO3 (LCMO) films are remarkably different depending on the crystallographic orientations of the orthorhombic NdGaO3 (NGO) substrates. More importantly, the antiferromagnetic charge-ordered insulating (COI) phase induced in the (100) or (001)-oriented LCMO films can be dramatically promoted (or suppressed) by a uniaxial tensile (or compressive) bending stress along the in-plane [010] direction. By contrast, the COI phase is nearly unaffected along the other transverse in-plane directions. Results from scanning transmission electron microscopy reveal that the (100)- or (001)-oriented LCMO films are uniaxially tensile strained along the [010] direction, while the LCMO/NGO(010) and LCMO/NGO(110) films remaining as a bulklike ferromagnetic metallic state exhibit a different strain state. Density functional theory calculations further reveal that the cooperatively increased Jahn-Teller distortion and charge ordering may be indispensible for the inducing and promoting of the COI phase. These findings provide a path to understand the correlation between local and extended structural distortions imparted by coherent epitaxy and the electronic states for quantum phase engineering.

Designing Morphotropic Phase Composition in BiFeO3.

In classical morphotropic piezoelectric materials, rhombohedral and tetragonal phase variants can energetically compete to form a mixed phase regime with improved functional properties. While the discovery of morphotropic-like phases in multiferroic BiFeO3 films has broadened this definition, accessing these phase spaces is still typically accomplished through isovalent substitution or heteroepitaxial strain which do not allow for continuous modification of phase composition postsynthesis. Here, we show that it is possible to use low-energy helium implantation to tailor morphotropic phases of epitaxial BiFeO3 films postsynthesis in a continuous and iterative manner. Applying this strain doping approach to morphotropic films creates a new phase space based on internal and external lattice stress that can be seen as an analogue to temperature-composition phase diagrams of classical morphotropic ferroelectric systems.

Exploiting Symmetry Mismatch to Control Magnetism in a Ferroelastic Heterostructure.

In the bulk, LaCoO_{3} (LCO) is a paramagnet, yet the low-temperature ferromagnetism (FM) is observed in tensile strained thin films, and its origin remains unresolved. Here, we quantitatively measured the distribution of atomic density and magnetization in LCO films by polarized neutron reflectometry (PNR) and found that the LCO layers near the heterointerfaces exhibit a reduced magnetization but an enhanced atomic density, whereas the film's interior (i.e., its film bulk) shows the opposite trend. We attribute the nonuniformity to the symmetry mismatch at the interface, which induces a structural distortion related to the ferroelasticity of LCO. This assertion is tested by systematic application of hydrostatic pressure during the PNR experiments. The magnetization can be controlled at a rate of -20.4% per GPa. These results provide unique insights into mechanisms driving FM in strained LCO films while offering a tantalizing observation that tunable deformation of the CoO_{6} octahedra in combination with the ferroelastic order parameter.

Reversible Control of Interfacial Magnetism through Ionic-Liquid-Assisted Polarization Switching.

The ability to control magnetism of materials via electric field enables a myriad of technological innovations in information storage, sensing, and computing. We use ionic-liquid-assisted ferroelectric switching to demonstrate reversible modulation of interfacial magnetism in a multiferroic heterostructure composed of ferromagnetic (FM) La0.8Sr0.2MnO3 and ferroelectric (FE) PbZr0.2Ti0.8O3. It is shown that ionic liquids can be used to persistently and reversibly switch a large area of a FE film. This is a prerequisite for polarized neutron reflectometry (PNR) studies that are conducted to directly probe magnetoelectric coupling of the FE polarization to the interfacial magnetization.

Magnon Mode Selective Spin Transport in Compensated Ferrimagnets.

We investigate the generation of magnonic thermal spin currents and their mode selective spin transport across interfaces in insulating, compensated ferrimagnet/normal metal bilayer systems. The spin Seebeck effect signal exhibits a nonmonotonic temperature dependence with two sign changes of the detected voltage signals. Using different ferrimagnetic garnets, we demonstrate the universality of the observed complex temperature dependence of the spin Seebeck effect. To understand its origin, we systematically vary the interface between the ferrimagnetic garnet and the metallic layer, and by using different metal layers we establish that interface effects play a dominating role. They do not only modify the magnitude of the spin Seebeck effect signal but in particular also alter its temperature dependence. By varying the temperature, we can select the dominating magnon mode and we analyze our results to reveal the mode selective interface transmission probabilities for different magnon modes and interfaces. The comparison of selected systems reveals semiquantitative details of the interfacial coupling depending on the materials involved, supported by the obtained field dependence of the signal.

Length Scale of the Spin Seebeck Effect.

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to 50 µm at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found.

Ferroelectric Order Evolution in Freestanding PbTiO3 Films Monitored by Optical Second Harmonic Generation.

The demand for low-dimensional ferroelectric devices is steadily increasing, however, the thick substrates in epitaxial films impede further size miniaturization. Freestanding films offer a potential solution by eliminating substrate constraints. Nevertheless, it remains an ongoing challenge to improve the stability in thin and fragile freestanding films under strain and temperature. In this work, the structure and ferroelectric order of freestanding PbTiO3 (PTO) films are investigated under continuous variation of the strain and temperature using nondestructive optical second harmonic generation (SHG) technique. The findings reveal that there are both out-of-plane and in-plane domains with polarization along out-of-plane and in-plane directions in the orthorhombic-like freestanding PTO films, respectively. In contrast, only out-of-plane domains are observed in the tetragonal epitaxial PTO films. Remarkably, the ferroelectricity of freestanding PTO films is strengthened under small uniaxial tensile strain from 0% up to 1.66% and well-maintained under larger biaxial tensile strain up to 2.76% along the [100] direction and up to 4.46% along the [010] direction. Moreover, a high Curie temperature of 630 K is identified in 50 nm thick freestanding PTO films by wide-temperature-range SHG. These findings provide valuable understanding for the development of the next-generation electronic nanodevices with flexibility and thermostability.