In-Situ Atomic-Scale Observation of Brownmillerite to Ruddlesden-Popper Phase Transition Tuned by Epitaxial Strain in Cobaltites

Phase transitions involving oxygen ion extraction within the framework of the crystallographic relevance have been widely exploited for sake of superconductivity,ferromagnetism,and ion conductivity in perovskiterelated oxides.However,atomic-scale pathways of phase transitions and ion extraction threshold are inadequately understood.Here we investigate the atomic structure evolution of LaCoO_(3) films upon oxygen extraction and subsequent Co migration,focusing on the key role of epitaxial strain.The brownmillerite to Ruddlesden-Popper phase transitions are discovered to stabilize at distinct crystal orientations in compressive-and tensile-strained cobaltites,which could be attributed to in-plane and out-of-plane Ruddlesden-Popper stacking faults,respectively.A two-stage process from exterior to interior phase transition is evidenced in compressive-strained LaCoO_(2.5),while a single-step nucleation process leaving bottom layer unchanged in tensile-strained situation.Strain analyses reveal that the former process is initiated by an expansion in Co layer at boundary,whereas the latter one is associated with an edge dislocation combined with antiphase boundary.These findings provide a chemomechanical perspective on the structure regulation of perovskite oxides and enrich insights into strain-dependent phase diagram in epitaxial oxides films.

Strong Anisotropic Order Parameters at All-Nitride Ferromagnet/Superconductor Interfaces

Proximity effects between superconductors and ferromagnets(SC/FM)hold paramount importance in comprehending the spin competition transpiring at their interfaces.This competition arises from the interplay between Cooper pairs and ferromagnetic exchange interactions.The proximity effects between transition metal nitrides(TMNs)are scarcely investigated due to the formidable challenges of fabricating high-quality SC/FM interfaces.We fabricated heterostructures comprising SC titanium nitride(TiN)and FM iron nitride(Fe_(3)N)with precise chemical compositions and atomically well-defined interfaces.The magnetoresistance of Fe_(3)N/TiN heterostructures shows a distinct magnetic anisotropy and strongly depends on the external perturbations.Moreover,the superconducting transition temperatureT_(C) and critical field of TiN experience notable suppression when proximity to Fe_(3)N.We observe the intriguing competition of interfacial spin orientations near𝑇T_(C)(∼1.25 K).These findings not only add a new materials system for investigating the interplay between superconductor and ferromagnets,but also potentially provide a building block for future research endeavors and applications in the realms of superconducting spintronic devices.

High-performance synaptic transistors for neuromorphic computing

The further development of traditional von Neumann-architecture computers is limited by the breaking of Moore’s law and the von Neumann bottleneck, which make them unsuitable for future high-performance artificial intelligence (AI)systems. Therefore, new computing paradigms are desperately needed. Inspired by the human brain, neuromorphic computing is proposed to realize AI while reducing power consumption. As one of the basic hardware units for neuromorphic computing, artificial synapses have recently aroused worldwide research interests. Among various electronic devices that mimic biological synapses, synaptic transistors show promising properties, such as the ability to perform signal transmission and learning simultaneously, allowing dynamic spatiotemporal information processing applications. In this article, we provide a review of recent advances in electrolyte-and ferroelectric-gated synaptic transistors. Their structures, materials,working mechanisms, advantages, and disadvantages will be presented. In addition, the challenges of developing advanced synaptic transistors are discussed.

A synaptic transistor with NdNiO3

Recently, neuromorphic devices for artificial intelligence applications have attracted much attention. In this work, a three-terminal electrolyte-gated synaptic transistor based on NdNiO3 epitaxial films, a typical correlated electron material, is presented. The voltage-controlled metal-insulator transition was achieved by inserting and extracting H+ ions in the NdNiO3 channel through electrolyte gating. The non-volatile conductance change reached 104 under a 2 V gate voltage. By manipulating the amount of inserted protons, the three-terminal NdNiO3 artificial synapse imitated important synaptic functions, such as synaptic plasticity and spike-timing-dependent plasticity. These results show that the correlated material NdNiO3 has great potential for applications in neuromorphic devices.

Photon-interactions with perovskite oxides

Photons with variable energy, high coherency, and switchable polarization provide an ideal tool-kits for exploring the cutting-edge scientific questions in the condensed matter physics and material sciences. Over decades, extensive researches in the sample fabrication and excitation have employed the photon as one of the important means to synthesize and explore the low-dimensional quantum materials. In this review, we firstly summarize the recent progresses of the state-of-the-art thin-film deposition methods using excimer pulsed laser, by which syntactic oxides with atomic-unit-cell-thick layers and extremely high crystalline quality can be programmatically fabricated. We demonstrate that the artificially engineered oxide quantum heterostructures exhibit the unexpected physical properties which are absent in their parent forms. Secondly, we highlight the recent work on probing the symmetry breaking at the surface/interface/interior and weak couplings among nanoscale ferroelectric domains using optical second harmonic generation. We clarify the current challenges in the insitu characterizations under the external fields and large-scale imaging using optical second harmonic generation. The improvements in the sample quality and the non-contact detection technique further promote the understanding of the mechanism of the novel properties emerged at the interface and inspire the potential applications, such as the ferroelectric resistive memory and ultrahigh energy storage capacitors.

Two-Dimensional Electron Gas with High Mobility Forming at BaO/SrTiO_(3) Interface

Two-dimensional electron gas(2DEG)with high electron mobility is highly desired to study the emergent properties and to enhance future device performance.Here we report the formation of 2DEG with high mobility at the interface between rock-salt Ba O and perovskite Sr Ti O_(3).The interface consists of the ionically compensated Ba O_(1-δ) layer and the electronically compensated Ti O_(2) layer,which is demonstrated as a perfect interface without lattice mismatch.The so-formed interface features metallic conductivity with ultralow square resistance of7.3×10^(-4)Ω/□at 2 K and high residual resistance ratios R_(300 K)/R_(2 K) up to 4200.The electron mobility reaches69000 cm^(2)·V^(-1)·s^(-1)at 2 K,leading to Shubnikov–de Haas oscillations of resistance.Density functional theory calculations reveal that the effective charge transfers from Ba O to the Ti 3d_(xy) orbital occur at the interface,leading to the conducting Ti O_(2) layer.Our work unravels that Ba O can adapt itself by removing oxygen to minimize the lattice mismatch and to provide substantial carriers to Sr Ti O_(3),which is the key to forming 2DEGs with high mobility at the interfaces.

Lateral strain tailoring in manganite homostructures assisted by atomic-flat freestanding membranes

Complex oxide thin films exhibit intriguing phenomena due to the coupling between multiple degrees of freedom through interfacial structural engineering.Atomic tailoring of structural parameters determines unique band structure and phonon modes,regulating emergent magnetic and electrical properties of oxide films.However,the construction of different strained and oriented domains in one intact oxide thin film is impossible using conventional means.Here we report the fabrication and quantitative structural analysis of La_(0.7)Sr_(0.3)MnO_(3)(LSMO)homostructures assisted by atomic-flat freestanding membranes.Pristine substrates and suspended membranes regulate the epitaxial strain and orientation of subsequently grown films.Our results demonstrate an ultrathin transition layer(~4 atomic layers)between freestanding membranes and LSMO films is formed due to the strain relaxation.This work offers a simple and scalable methodology for fabricating unprecedented innovative functional oxide homostructures through artificially controlled synthesis routes.

Improved multiferroic in EuTiO_(3) films by interphase strain engineering

Interphase strain engineering provides a unique methodology to significantly modify the lattice structure across a single film,enabling the emergence and manipulation of novel functionalities that are inaccessible in the context of traditional strain engineering methods.In this work,by using the interphase strain,we achieve a ferromagnetic state with enhanced Curie temperature and a room-temperature polar state in EuO secondary phase-tunned EuTiO_(3) thin films.A combination of atomic-scale electron microscopy and synchrotron X-ray spectroscopy unravels the underlying mechanisms of the ferroelectric and ferromagnetic properties enhancement.Wherein,the EuO secondary phase is found to be able to dramatically distort the TiO_6 octahedra,which favors the non-centrosymmetric polar state,weakens antiferromagnetic Eu-Ti-Eu interactions,and enhances ferromagnetic Eu-O-Eu interactions.Our work demonstrates the feasibility and effectiveness of interphase strain engineering in simultaneously promoting ferroelectric and ferromagnetic performance,which would provide new thinking on the property regulation of numerous strongly correlated functional materials.

In situ observation of atomic movement in a ferroelectric film under an external electric field and stress

Atomic movement under application of external stimuli （i.e., electric field or mechanical stress） in oxide materials has not been observed due to a lack of experimental methods but has been well known to determine the electric polarization. Here, we investigated atomic movement arising from the ferroelectric response of BiFeO3 thin films under the effect of an electric field and stress in real time using a combination of switching spectroscop）6 time-resolved X-ray microdiffraction, and in situ stress engineering. Under an electric field applied to a BiFeO3 film, the hysteresis loop of the reflected X-ray intensity was found to result from the opposing directions of displaced atoms between the up and down polarization states. An additional shift of atoms arising from the linearly increased dielectric component of the polarization in BiFeO3 was confirmed through gradual reduction of the diffracted X-ray intensity. The electric-field- induced displacement of oxygen atoms was found to be larger than that of Fe atom for both ferroelectric switching and increase of the polarization. The effect of external stress on the BiFeO3 thin film, which was controlled by applying an electric field to the highly piezoelectric substrate, showed smaller atomic shifts than for the case of applying an electric field to the film, despite the similar tetragonality.

Strain-mediated insulator-metal transition in topotactically hydro-reduced SrFeO_(2)

Controlling oxygen redox reactions in transition metal oxides offers an attractive route to tune their physical properties;a topotactic structural transformation from their parent phases effectively modifies the electronic state. In this work, infinitelayered SrFeO_(2) thin films were produced from brownmillerite SrFeO_(2.5) via low-temperature hydro-reduction. After the structural transition, their out-of-plane lattice constants dramatically shrank by 12%;tensilely strained samples exhibited metallic character, whereas the compressively strained ones maintained the insulating behavior of their bulk form. According to X-ray linear dichroism results, this strain-mediated electronic anisotropy may be attributed to electron redistribution within degenerated orbitals. This suggests a possible mechanism for the metallic conductivity of infinite-layered SrFeO_(2), giving a hint for understanding emergent quantum phenomena, such as the recently discovered superconductivity in nickelates, and stimulating various applications, including in ionic conductivity and oxygen catalysis.

Surface protonation and oxygen evolution activity of epitaxial La(1-x)SrxCoO3 thin films

As an alternative electrode material,transition metal oxides are promising candidates due to multivalent nature and oxygen vacancies present in the structure with facilitate redox reactions.The aim of this study is to explore the intrinsic mechanism of oxygen evolution reaction(OER)using two-dimensional thin film La1-xSrxCoO3 electrode as a model.Herein,we report a planar two-dimensional model La1-xSrxCoO3 electrode grown on a Nb-SrTiO3 single-crystal substrate via pulsed laser deposition.The two-dimensional La1-xSrxCoO3 films offer different oxygen evolution activities at different pH electrolyte solutions.The mechanisms behind the variations of the oxygen evolution activity were discussed after comparing the oxygen evolution activity before and after treatments of the electrodes and measurements by various test methods.The results of this study offer a promising,low-cost electrode material for the efficient OER and a sustainable production of hydrogen fuel.