Strain-driven formation of epitaxial nanostructures in brownmillerite strontium cobaltite thin films.

Nanostructured materials can display unique physical properties and are of particular interest for their new functionalities. Epitaxial growth is a promising approach for the controlled synthesis of nanostructures with desired structures and crystallinity. SrCoOx is a particularly intriguing material owing to a topotactic phase transition between an antiferromagnetic insulating brownmillerite SrCoO2.5 (BM-SCO) phase and a ferromagnetic metallic perovskite SrCoO3-δ (P-SCO) phase depending on the oxygen concentration. Here, we present the formation and control of epitaxial BM-SCO nanostructures by substrate-induced anisotropic strain. Perovskite substrates with a (110)-orientation and which allow for compressive strain result in the creation of BM-SCO nanobars, while (111)-oriented substrates give rise to the formation of BM-SCO nanoislands. We have found that substrate-induced anisotropic strain coupled with the orientation of crystalline domains determines the shape and facet of the nanostructures, while their size can be tuned by the degree of strain. Moreover, the nanostructures can be transformed between antiferromagnetic BM-SCO and ferromagnetic P-SCO via ionic liquid gating. Thus, this study provides insights into the design of epitaxial nanostructures whose structure and physical properties can be readily controlled.

Li iontronics in single-crystalline T-Nb2O5 thin films with vertical ionic transport channels.

The niobium oxide polymorph T-Nb2O5 has been extensively investigated in its bulk form especially for applications in fast-charging batteries and electrochemical (pseudo)capacitors. Its crystal structure, which has two-dimensional (2D) layers with very low steric hindrance, allows for fast Li-ion migration. However, since its discovery in 1941, the growth of single-crystalline thin films and its electronic applications have not yet been realized, probably due to its large orthorhombic unit cell along with the existence of many polymorphs. Here we demonstrate the epitaxial growth of single-crystalline T-Nb2O5 thin films, critically with the ionic transport channels oriented perpendicular to the film's surface. These vertical 2D channels enable fast Li-ion migration, which we show gives rise to a colossal insulator-metal transition, where the resistivity drops by 11 orders of magnitude due to the population of the initially empty Nb 4d0 states by electrons. Moreover, we reveal multiple unexplored phase transitions with distinct crystal and electronic structures over a wide range of Li-ion concentrations by comprehensive in situ experiments and theoretical calculations, which allow for the reversible and repeatable manipulation of these phases and their distinct electronic properties. This work paves the way for the exploration of novel thin films with ionic channels and their potential applications.

Zero-field polarity-reversible Josephson supercurrent diodes enabled by a proximity-magnetized Pt barrier.

Simultaneous breaking of inversion and time-reversal symmetries in a conductor yields a non-reciprocal electronic transport1-3, known as the diode or rectification effect, that is, low (ideally zero) conductance in one direction and high (ideally infinite) conductance in the other. So far, most of the diode effects observed in non-centrosymmetric polar/superconducting conductors4-7 and Josephson junctions8-10 require external magnetic fields to break the time-reversal symmetry. Here we report zero-field polarity-switchable Josephson supercurrent diodes, in which a proximity-magnetized Pt layer by ferrimagnetic insulating Y3Fe5O12 serves as the Rashba(-type) Josephson barrier. The zero-field diode efficiency of our proximity-engineered device reaches up to ±35% at 2 K, with a clear square-root dependence on temperature. Measuring in-plane field-strength/angle dependences and comparing with Cu-inserted control junctions, we demonstrate that exchange spin-splitting11-13 and Rashba(-type) spin-orbit coupling13-15 at the Pt/Y3Fe5O12 interface are key for the zero-field giant rectification efficiency. Our achievement advances the development of field-free absolute Josephson diodes.

Long-range supercurrents through a chiral non-collinear antiferromagnet in lateral Josephson junctions.

The proximity-coupling of a chiral non-collinear antiferromagnet (AFM)1-5 with a singlet superconductor allows spin-unpolarized singlet Cooper pairs to be converted into spin-polarized triplet pairs6-8, thereby enabling non-dissipative, long-range spin correlations9-14. The mechanism of this conversion derives from fictitious magnetic fields that are created by a non-zero Berry phase15 in AFMs with non-collinear atomic-scale spin arrangements1-5. Here we report long-ranged lateral Josephson supercurrents through an epitaxial thin film of the triangular chiral AFM Mn3Ge (refs. 3-5). The Josephson supercurrents in this chiral AFM decay by approximately one to two orders of magnitude slower than would be expected for singlet pair correlations9-14 and their response to an external magnetic field reflects a clear spatial quantum interference. Given the long-range supercurrents present in both single- and mixed-phase Mn3Ge, but absent in a collinear AFM IrMn16, our results pave a way for the topological generation of spin-polarized triplet pairs6-8 via Berry phase engineering15 of the chiral AFMs.

Electron-hole separation in ferroelectric oxides for efficient photovoltaic responses.

Despite their potential to exceed the theoretical Shockley-Queisser limit, ferroelectric photovoltaics (FPVs) have performed inefficiently due to their extremely low photocurrents. Incorporating Bi2FeCrO6 (BFCO) as the light absorber in FPVs has recently led to impressively high and record photocurrents [Nechache R, et al. (2015) Nat Photonics 9:61-67], which has revived the FPV field. However, our understanding of this remarkable phenomenon is far from satisfactory. Here, we use first-principles calculations to determine that such excellent performance mainly lies in the efficient separation of electron-hole (e-h) pairs. We show that photoexcited electrons and holes in BFCO are spatially separated on the Fe and Cr sites, respectively. This separation is much more pronounced in disordered BFCO phases, which adequately explains the observed exceptional PV responses. We further establish a design strategy to discover next-generation FPV materials. By exploring 44 additional Bi-based double-perovskite oxides, we suggest five active-layer materials that offer a combination of strong e-h separations and visible-light absorptions for FPV applications. Our work indicates that charge separation is the most important issue to be addressed for FPVs to compete with conventional devices.

Metal Nanoparticle Exsolution on a Perovskite Stannate Support with High Electrical Conductivity.

In situ exsolution of metal nanoparticles (NPs) is emerging as an alternative technique to deliver thermally stable and evenly dispersed metal NPs, which exhibit excellent adhesion with conducting perovskite oxide supports. Here we provide the first demonstration that Ni metal NPs with high areal density (∼175 µm-2) and fine size (∼38.65 nm) are exsolved from an A-site-deficient perovskite stannate support (La0.2Ba0.7Sn0.9Ni0.1O3-δ (LBSNO)). The NPs are strongly anchored and impart coking resistance, and the Ni-exsolved stannates show exceptionally high electrical conductivity (∼700 S·cm-1). The excellent conductivity is attributed to conduction between delocalized Sn 5s orbitals along with structural improvement toward ABO3 stoichiometry in the stannate support. We also reveal that experimental conditions with strong interaction must be optimized to obtain Ni exsolution without degrading the perovskite stannate framework. Our finding suggests a unique process to induce the formation of metal NPs embedded in stannate with excellent electrical properties.

Facet-Dependent in Situ Growth of Nanoparticles in Epitaxial Thin Films: The Role of Interfacial Energy.

Nucleation of nanoparticles using the exsolution phenomenon is a promising pathway to design durable and active materials for catalysis and renewable energy. Here, we focus on the impact of surface orientation of the host lattice on the nucleation dynamics to resolve questions with regards to "preferential nucleation sites". For this, we carried out a systematic model study on three differently oriented perovskite thin films. Remarkably, in contrast to the previous bulk powder-based study suggesting that the (110)-surface is a preferred plane for exsolution, we identify that other planes such as (001)- and (111)-facets also reveal vigorous exsolution. Moreover, particle size and surface coverage vary significantly depending on the surface orientation. Exsolution of (111)-oriented film produces the largest number of particles, the smallest particle size, the deepest embedment, and the smallest and most uniform interparticle distance among the oriented films. Based on classic nucleation theory, we elucidate that the differences in interfacial energies as a function of substrate orientation play a crucial role in controlling the distinct morphology and nucleation behavior of exsolved nanoparticles. Our finding suggests new design principles for tunable solid-state catalyst or nanoscale metal decoration.

Low-Temperature Solid-State Synthesis of High-Purity BiFeO3 Ceramic for Ferroic Thin-Film Deposition.

The synthesis of high-purity BiFeO3 (BFO) ceramic by solid-state reaction is known to be very difficult due to inevitable formation of the secondary phases, mostly mullite-type Bi2Fe4O9 and sillenite-type Bi25FeO39. In particular, it is very difficult to completely remove the Bi-deficient Bi2Fe4O9 phase from sintered ceramic BFO targets. This problem consequently leads to the difficulty of fabricating high-quality BFO thin films using these sintered targets. Herein, we introduce a simple but effective low-temperature processing scheme for removing impurity phases in which optimized processing conditions are obtained by chemically correlating the first calcination step with the subsequent leaching and sintering steps. More specifically, we suitably avoid the formation of the high-temperature-stable Bi2Fe4O9 phase by performing the calcination at significantly low temperatures (between 650 and 675 °C) with Bi-excess starting powders. We have then fabricated epitaxially grown BFO thin films using these phase-pure ceramic targets and consequently achieved high-quality ferroelectricity and switchable photovoltaic responses. On the basis of the present experimental observations, we suggest that a low impurity concentration in the sintered BFO ceramic target, even with a low relative density, is advantageous for high-quality thin-film fabrication.

Evaluation of the effects and adverse drug reactions of low-dose dexamethasone premedication with weekly docetaxel.

PURPOSE: A weekly docetaxel regimen had comparable efficacy with a tri-weekly schedule and caused significantly less severe neutropenia and febrile neutropenia. Therefore, a weekly docetaxel regimen has become increasingly common in cancer treatment. Premedication with corticosteroids can effectively prevent or reduce the severity of hypersensitivity and fluid retention. However, no recommended steroid dosage for a weekly docetaxel regimen has been established to date. The aim of this study is to compare the efficacy and complications of two different weekly docetaxel premedication protocols. METHODS: We retrospectively compared the hypersensitivity, hyperglycemia, and infection incidence associated with two weekly docetaxel premedication protocols. The control group (dexamethasone 10 mg intravenously and 4 mg orally every 12 h for four doses, starting 1 h before docetaxel administration) patients started weekly docetaxel chemotherapy between May 2012 and April 2013 at Seoul National University Hospital, and the experimental group (dexamethasone 10 mg intravenously 1 h prior to each docetaxel administration) patients started weekly docetaxel chemotherapy between May 2013 and April 2014. RESULTS: In total, 109 patients in the control group and 97 patients in the experimental group were included in this study, and there were no statistically significant differences in baseline characteristics between the two groups. The incidence of hypersensitivity and hyperglycemia were similar, but infections were observed significantly less in the experimental group (p = 0.020, OR = 0.408, 0.0190-0.0879). CONCLUSIONS: A low-dose dexamethasone premedication protocol has comparable efficacy in the prevention of docetaxel hypersensitivity with fewer infection complications. Therefore, we recommend a low-dose dexamethasone premedication protocol for weekly docetaxel regimens.

All-Oxide Metasurfaces Formed by Synchronized Local Ionic Gating.

Ionic gating of oxide thin films has emerged as a novel way of manipulating the properties of thin films. Most studies are carried out on single devices with a three-terminal configuration, but, by exploring the electrokinetics during the ionic gating, such a configuration with initially insulating films leads to a highly non-uniform gating response of individual devices within large arrays of the devices. It is shown that such an issue can be circumvented by the formation of a uniform charge potential by the use of a thin conducting underlayer. This synchronized local ionic gating allows for the simultaneous manipulation of the electrical, magnetic, and/or optical properties of large arrays of devices. Designer metasurfaces formed in this way from SrCoO2.5 thin films display an anomalous optical reflection of light that relies on the uniform and coherent response of all the devices. Beyond oxides, almost any material whose properties can be controlled by the addition or removal of ions via gating can form novel metasurfaces using this technique. These findings provide insights into the electrokinetics of ionic gating and a wide range of applications using synchronized local ionic gating.

Immunostimulatory activity of Lactococcus lactis LM1185 isolated from Hydrangea macrophylla.

The lactic acid bacteria, Lactococcus lactis subsp. lactis LM1185 was isolated from Hydrangea macrophylla. Strain LM1185 showed 50.5% of acid tolerance at pH 2.5 for 2 h and 30.4% of 0.3% (w/v) bile salt tolerance for 24 h. The antioxidant activity of this strain was measured at 99.4% of 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) radical scavenging activity. When RAW 264.7 macrophage cells were treated with strain LM1185, there was no observed cytotoxicity. This strain showed high nitric oxide production and mRNA expression levels of cytokines such as tumor necrosis factor-α and inducible nitric oxide synthase (iNOS). The nuclear factor-kB signaling pathway was activated by this strain resulting in the production of iNOS and cyclooxygenase-2 determined by western blotting. The present results indicated that L. lactis subsp. lactis LM1185 could be used as potential probiotics and may play a crucial role in the immunostimulatory effect on macrophages. Supplementary Information: The online version contains supplementary material available at 10.1007/s10068-022-01199-5.

Reversal of Anomalous Hall Effect and Octahedral Tilting in SrRuO3 Thin Films via Hydrogen Spillover.

The perovskite SrRuO3 (SRO) is a strongly correlated oxide whose physical and structural properties are strongly intertwined. Notably, SRO is an itinerant ferromagnet that exhibits a large anomalous Hall effect (AHE) whose sign can be readily modified. Here, a hydrogen spillover method is used to tailor the properties of SRO thin films via hydrogen incorporation. It is found that the magnetization and Curie temperature of the films are strongly reduced and, at the same time, the structure evolves from an orthorhombic to a tetragonal phase as the hydrogen content is increased up to ≈0.9 H per SRO formula unit. The structural phase transition is shown, via in situ crystal truncation rod measurements, to be related to tilting of the RuO6 octahedral units. The significant changes observed in magnetization are shown, via density functional theory (DFT), to be a consequence of shifts in the Fermi level. The reported findings provide new insights into the physical properties of SRO via tailoring its lattice symmetry and emergent physical phenomena via the hydrogen spillover technique.

Shape-memory effect in twisted ferroic nanocomposites.

The shape recovery ability of shape-memory alloys vanishes below a critical size (~50 nm), which prevents their practical applications at the nanoscale. In contrast, ferroic materials, even when scaled down to dimensions of a few nanometers, exhibit actuation strain through domain switching, though the generated strain is modest (~1%). Here, we develop freestanding twisted architectures of nanoscale ferroic oxides showing shape-memory effect with a giant recoverable strain (>8%). The twisted geometrical design amplifies the strain generated during ferroelectric domain switching, which cannot be achieved in bulk ceramics or substrate-bonded thin films. The twisted ferroic nanocomposites allow us to overcome the size limitations in traditional shape-memory alloys and open new avenues in engineering large-stroke shape-memory materials for small-scale actuating devices such as nanorobots and artificial muscle fibrils.

Chiral antiferromagnetic Josephson junctions as spin-triplet supercurrent spin valves and d.c. SQUIDs.

Spin-triplet supercurrent spin valves are of practical importance for the realization of superconducting spintronic logic circuits. In ferromagnetic Josephson junctions, the magnetic-field-controlled non-collinearity between the spin-mixer and spin-rotator magnetizations switches the spin-polarized triplet supercurrents on and off. Here we report an antiferromagnetic equivalent of such spin-triplet supercurrent spin valves in chiral antiferromagnetic Josephson junctions as well as a direct-current superconducting quantum interference device. We employ the topological chiral antiferromagnet Mn3Ge, in which the Berry curvature of the band structure produces fictitious magnetic fields, and the non-collinear atomic-scale spin arrangement accommodates triplet Cooper pairing over long distances (>150 nm). We theoretically verify the observed supercurrent spin-valve behaviours under a small magnetic field of <2 mT for current-biased junctions and the direct-current superconducting quantum interference device functionality. Our calculations reproduce the observed hysteretic field interference of the Josephson critical current and link these to the magnetic-field-modulated antiferromagnetic texture that alters the Berry curvature. Our work employs band topology to control the pairing amplitude of spin-triplet Cooper pairs in a single chiral antiferromagnet.

Giant Spin Hall Effect and Spin-Orbit Torques in 5d Transition Metal-Aluminum Alloys from Extrinsic Scattering.

The generation of spin currents from charge currents via the spin Hall effect (SHE) is of fundamental and technological interest. Here, some of the largest SHEs yet observed via extrinsic scattering are found in a large class of binary compounds formed from a 5d element and aluminum, with a giant spin Hall angle (SHA) of ≈1 in the compound Os22 Al78 . A critical composition of the 5d element is found at which there is a structural phase boundary between poorly and highly textured crystalline material, where the SHA exhibits its largest value. Furthermore, a systematic increase is found in the spin Hall conductivity (SHC) and SHA at this critical composition as the atomic number of the 5d element is systematically increased. This clearly shows that the SHE and SHC are derived from extrinsic scattering mechanisms related to the potential mismatch between the 5d element and Al. These studies show the importance of extrinsic mechanisms derived from potential mismatch as a route to obtaining large spin Hall angles with high technological impact. Indeed, it is demonstrated that a state-of-the-art racetrack device has a several-fold increased current-induced domain wall efficiency using these materials as compared to prior-art materials.

Anti-phase boundary accelerated exsolution of nanoparticles in non-stoichiometric perovskite thin films.

Exsolution of excess transition metal cations from a non-stoichiometric perovskite oxide has sparked interest as a facile route for the formation of stable nanoparticles on the oxide surface. However, the atomic-scale mechanism of this nanoparticle formation remains largely unknown. The present in situ scanning transmission electron microscopy combined with density functional theory calculation revealed that the anti-phase boundaries (APBs) characterized by the a/2 < 011> type lattice displacement accommodate the excess B-site cation (Ni) through the edge-sharing of BO6 octahedra in a non-stoichiometric ABO3 perovskite oxide (La0.2Sr0.7Ni0.1Ti0.9O3-δ) and provide the fast diffusion pathways for nanoparticle formation by exsolution. Moreover, the APBs further promote the outward diffusion of the excess Ni toward the surface as the segregation energy of Ni is lower at the APB/surface intersection. The formation of nanoparticles occurs through the two-step crystallization mechanism, i.e., the nucleation of an amorphous phase followed by crystallization, and via reactive wetting on the oxide support, which facilitates the formation of a stable triple junction and coherent interface, leading to the distinct socketing of nanoparticles to the oxide support. The atomic-scale mechanism unveiled in this study can provide insights into the design of highly stable nanostructures.

Control of Oxygen Vacancy Ordering in Brownmillerite Thin Films via Ionic Liquid Gating.

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film's surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.

Role of Two-Dimensional Ising Superconductivity in the Nonequilibrium Quasiparticle Spin-to-Charge Conversion Efficiency.

Nonequilibrium studies of two-dimensional (2D) superconductors (SCs) with Ising spin-orbit coupling are prerequisite for their successful application to equilibrium spin-triplet Cooper pairs and, potentially, Majorana Fermions. By taking advantage of the recent discoveries of 2D SCs and their compatibility with any other materials, we fabricate here nonlocal magnon devices to examine how such 2D Ising superconductivity affects the conversion efficiency of magnon spin to quasiparticle charge in superconducting flakes of 2H-NbSe2 transferred onto ferrimagnetic insulating Y3Fe5O12. Comparison with a reference device based on a conventionally paired superconductor shows that the Y3Fe5O12-induced in-plane (IP) exchange spin-splitting in the NbSe2 flake is hindered by its inherent out-of-plane (OOP) spin-orbit field, which, in turn, limits the transition-state enhancement of the spin-to-charge conversion efficiency. Our out-of-equilibrium study highlights the significance of symmetry matching between underlying Cooper pairs and exchange-induced spin-splitting for the giant transition-state spin-to-charge conversion and may have implications toward proximity-engineered spin-polarized triplet pairing via tuning the relative strength of IP exchange and OOP spin-orbit fields in ferromagnetic insulator/2D Ising SC bilayers.

Directional ionic transport across the oxide interface enables low-temperature epitaxy of rutile TiO2.

Heterogeneous interfaces exhibit the unique phenomena by the redistribution of charged species to equilibrate the chemical potentials. Despite recent studies on the electronic charge accumulation across chemically inert interfaces, the systematic research to investigate massive reconfiguration of charged ions has been limited in heterostructures with chemically reacting interfaces so far. Here, we demonstrate that a chemical potential mismatch controls oxygen ionic transport across TiO2/VO2 interfaces, and that this directional transport unprecedentedly stabilizes high-quality rutile TiO2 epitaxial films at the lowest temperature (≤ 150 °C) ever reported, at which rutile phase is difficult to be crystallized. Comprehensive characterizations reveal that this unconventional low-temperature epitaxy of rutile TiO2 phase is achieved by lowering the activation barrier by increasing the "effective" oxygen pressure through a facile ionic pathway from VO2-δ sacrificial templates. This discovery shows a robust control of defect-induced properties at oxide interfaces by the mismatch of thermodynamic driving force, and also suggests a strategy to overcome a kinetic barrier to phase stabilization at exceptionally low temperature.