Growth and atomically resolved polarization mapping of ferroelectric Bi2WO6Bi_2WO_6 thin film

Aurivillius ferroelectric $Bi_2WO_6$ (BWO) encompasses a broad range of functionalities, including robust fatigue-free ferroelectricity, high photocatalytic activity, and ionic conductivity. Despite these promising characteristics, an in-depth study on the growth of BWO thin films and ferroelectric characterization, especially at the atomic scale, is still lacking. Here, we report pulsed laser deposition (PLD) of BWO thin films on (001) $SrTiO_3$ substrates and characterization of ferroelectricity using the scanning transmission electron microscopy (STEM) and piezoresponse force microscopy (PFM) techniques. We show that the background oxygen gas pressure used during PLD growth mainly determines the phase stability of BWO films, whereas the influence of growth temperature is comparatively minor. Atomically resolved STEM study of a fully strained BWO film revealed collective in-plane polar off-centering displacement of W atoms. We estimated the spontaneous polarization value based on polar displacement mapping to be about 54 $\pm$ 4 ${\mu}C cm^{-2}$, which is in good agreement with the bulk polarization value. Furthermore, we found that pristine film is composed of type-I and type-II domains, with mutually orthogonal polar axes. Complementary PFM measurements further elucidated that the coexisting type-I and type-II domains formed a multidomain state that consisted of 90$\deg$ domain walls (DWs) alongside multiple head-to-head and tail-to-tail 180$\deg$ DWs. Application of an electrical bias led to in-plane 180$\deg$ polarization switching and 90$\deg$ polarization rotation, highlighting a unique aspect of domain switching, which is immune to substrate-induced strain.Comment: This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Electronic Materials, \copyright American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://pubs.acs.org/doi/full/10.1021/acsaelm.1c00005 .This submission contains 34 page

Ultrathin Magnesium-based Coating as an Efficient Oxygen Barrier for Superconducting Circuit Materials

Scaling up superconducting quantum circuits based on transmon qubits necessitates substantial enhancements in qubit coherence time. Among the materials considered for transmon qubits, tantalum (Ta) has emerged as a promising candidate, surpassing conventional counterparts in terms of coherence time. However, the presence of an amorphous surface Ta oxide layer introduces dielectric loss, ultimately placing a limit on the coherence time. In this study, we present a novel approach for suppressing the formation of tantalum oxide using an ultrathin magnesium (Mg) capping layer deposited on top of tantalum. Synchrotron-based X-ray photoelectron spectroscopy (XPS) studies demonstrate that oxide is confined to an extremely thin region directly beneath the Mg/Ta interface. Additionally, we demonstrate that the superconducting properties of thin Ta films are improved following the Mg capping, exhibiting sharper and higher-temperature transitions to superconductive and magnetically ordered states. Based on the experimental data and computational modeling, we establish an atomic-scale mechanistic understanding of the role of the capping layer in protecting Ta from oxidation. This work provides valuable insights into the formation mechanism and functionality of surface tantalum oxide, as well as a new materials design principle with the potential to reduce dielectric loss in superconducting quantum materials. Ultimately, our findings pave the way for the realization of large-scale, high-performance quantum computing systems

Oxygen vacancy-induced topological nanodomains in ultrathin ferroelectric films

Oxygen vacancy in oxide ferroelectrics can be strongly coupled to the polar order via local strain and electric fields, thus holding the capability of producing and stabilizing exotic polarization patterns. However, despite intense theoretical studies, an explicit microscopic picture to correlate the polarization pattern and the distribution of oxygen vacancies remains absent in experiments. Here we show that in a high-quality, uniaxial ferroelectric system, i.e., compressively strained BaTiO3 ultrathin films (below 10 nm), nanoscale polarization structures can be created by intentionally introducing oxygen vacancies in the film while maintaining structure integrity (namely no extended lattice defects). Using scanning transmission electron microscopy, we reveal that the nanodomain is composed of swirling electric dipoles in the vicinity of clustered oxygen vacancies. This finding opens a new path toward the creation and understanding of the long-sought topological polar objects such as vortices and skyrmions.11Nsciescopu

Nanoscale Enhancement of the Local Optical Conductivity near Cracks in Metallic SrRuO3 Film

Cracking has been recognized as a major obstacle degrading material properties, including structural stability, electrical conductivity, and thermal conductivity. Recently, there have been several reports on the nanosized cracks (nanocracks), particularly in the insulating oxides. In this work, we comprehensively investigate how nanocracks affect the physical properties of metallic SrRuO3 (SRO) thin films. We grow SRO/SrTiO3 (STO) bilayers on KTaO3 (KTO) (001) substrates, which provide +1.7% tensile strain if the SRO layer is grown epitaxially. However, the SRO/STO bilayers suffer from the generation and propagation of nanocracks, and then, the strain becomes inhomogeneously relaxed. As the thickness increases, the nanocracks in the SRO layer become percolated, and its dc conductivity approaches zero. Notably, we observe an enhancement of the local optical conductivity near the nanocrack region using scanning-type near-field optical microscopy. This enhancement is attributed to the strain relaxation near the nanocracks. Our work indicates that nanocracks can be utilized as promising platforms for investigating local emergent phenomena related to strain effects.11Nsciescopu

Experimental realization of atomically flat and AlO2-terminated LaAlO3(001) substrate surfaces

Oxide single-crystal substrates with atomically smooth and chemically uniform surfaces are indispensable for constructing sharp epitaxial heterointerfaces and investigating emergent interfacial physical phenomena. Here, we report a simple method to realize atomically flat and AlO2-terminated LaAlO3 (001) [LAO(001)] substrate surfaces. So far, the LAO(001) substrate has been utilized as a structural template for the epitaxial growth of a variety of oxide films. However, well-established methods for achieving atomically flat, singly terminated LAO(001) surfaces have rarely been reported. This is mainly due to the unstable charged surfaces of LaO+ or AlO2-, which hinders simultaneous stabilizations of atomic-scale smoothness and single termination. To overcome this problem, we combined thermal annealing and subsequent deionized water leaching to treat the LAO(001) surface. We used atomic force microscopy to investigate the evolution of the LAO(001) surface during the water leaching and confirmed the atomically flat surface of the 120-min-water-leached sample. We further demonstrated the uniform AlO2 termination of the LAO(001) surface via coaxial impact-collision ion scattering spectroscopy. Using the treated substrates, we are able to grow perovskite oxide films (i.e., SrRuO3) on the LAO(001) substrate with atomically sharp heterointerfaces. Our paper provides an effective means for controlling the surface and interface of transition-metal oxide heterostructures at the atomic scale. © 2019 American Physical Societ

In Situ Cryogenic HAADF-STEM Observation of Spontaneous Transition of Ferroelectric Polarization Domain Structures at Low Temperatures

© 2021 The Authors. Published by American Chemical Society.Precise determination of atomic structures in ferroelectric thin films and their evolution with temperature is crucial for fundamental study and design of functional materials. However, this has been impeded by the lack of techniques applicable to a thin-film geometry. Here we use cryogenic scanning transmission electron microscopy (STEM) to observe the atomic structure of a BaTiO3 film on a (111)-SrTiO3 substrate under varying temperatures. Our study explicitly proves a structure transition from a complex polymorphic nanodomain configuration at room temperature transitioning to a homogeneous ground-state rhombohedral structure of BaTiO3 below ∼250 K, which was predicted by phase-field simulation. More importantly, another unexpected transition is revealed, a transition to complex nanodomains below ∼105 K caused by an altered mechanical boundary condition due to the antiferrodistortive phase transition of the SrTiO3 substrate. This study demonstrates the power of cryogenic STEM in elucidating structure-property relationships in numerous functional materials at low temperatures.11Nsciescopu

Coherent-strained superconducting BaPb1-xBixO3 thin films by interface engineering

The bismuthate superconductor BaPb1-xBixO3 and its bismuthate heterostructure have gained much attention due to their potential applications, such as topologically protected quantum devices. To fabricate BaPb1-xBixO3-based junctions or devices with atomically sharp interfaces, researchers have long searched for innovative methods to achieve coherent-strained superconducting BaPb1-xBixO3 films. However, the large lattice mismatches between BaPb1-xBixO3 and widely used perovskite substrates have been huge obstacles hindering the achievement of coherent-strained superconducting films. Here, we successfully fabricated coherent-strained superconducting BaPb1-xBixO3 films on SrTiO3 substrates by inserting BaCeO3/BaZrO3 buffer layers. By performing both grazing-incidence in-plane x-ray diffraction and transmission electron microscopy, we demonstrated that without buffer layers the BaPb1-xBixO3 films exhibited fully relaxed structures with a reconstructed interface layer. With buffer layers, the superconducting transition temperatures of coherent-strained BaPb1-xBixO3 films were higher than that of relaxed films. Based on these interface-engineering results, this paper provides opportunities for exploring the emergent properties of bismuthate-based superconducting devices, such as quantum computing circuits. © 2019 American Physical Society11sciescopu

Oxygen Vacancy Engineering for Highly Tunable Ferromagnetic Properties: A Case of SrRuO(3)Ultrathin Film with a SrTiO(3)Capping Layer

© 2020 Wiley-VCH GmbH. Oxide heterostructures have great potential for spintronics applications due to their well-defined heterointerfaces and vast functionalities. To integrate such compelling features into practical spintronics devices, effective control of the magnetic switching behavior is key. Here, continuous control of the magnetic coercive field in SrTiO3/SrRuO(3)ultrathin heterostructures is achieved by oxygen vacancy (V-O) engineering. Pulsed laser deposition of an oxygen-deficient SrTiO(3)capping layer can trigger V(O)migration into the SrRuO(3)layer while avoiding the formation of Ru vacancies. Moreover, by varying the thickness and growth conditions of the SrTiO(3)capping layer, the value of the coercive field (H-C) in the ferromagnetic SrRuO(3)layer can be continuously tuned. The maximum enhancement ofH(C)at 5 K is 3.2 T. Such a wide-range tunability ofH(C)may originate from a V-O-induced enhancement of perpendicular magnetic anisotropy and domain wall pinning. This study offers effective approaches for controlling physical properties of oxide heterostructures via V(O)engineering, which may facilitate the development of oxide-based functional devices11sciescopu

Strain engineering of the magnetic multipole moments and anomalous Hall effect in pyrochlore iridate thin films

ⓒ 2020 American Association for the Advancement of Science The recent observation of the anomalous Hall effect (AHE) without notable magnetization in antiferromagnets has suggested that ferromagnetic ordering is not a necessary condition. Thus, recent theoretical studies have proposed that higher-rank magnetic multipoles formed by clusters of spins (cluster multipoles) can generate the AHE without magnetization. Despite such an intriguing proposal, controlling the unconventional AHE by inducing these cluster multipoles has not been investigated. Here, we demonstrate that strain can manipulate the hidden Berry curvature effect by inducing the higher-rank cluster multipoles in spin-orbit-coupled antiferromagnets. Observing the large AHE on fully strained antiferromagnetic Nd2Ir2O7 thin films, we prove that strain-induced cluster T-1-octupoles are the only source of observed AHE. Our results provide a previously unidentified pathway for generating the unconventional AHE via strain-induced magnetic structures and establish a platform for exploring undiscovered topological phenomena via strain in correlated materials11sci

Engineering structural homogeneity and magnetotransport in strained Nd2Ir2O7 films

The 5d pyrochlore iridate family (R2Ir2O7, where R is a rare earth ion) has garnered significant attention due to its topological properties, such as Weyl semimetallic phases and axion insulator. However, the investigation of these properties has been impeded by severe iridium loss during growth, which results in the formation of defects and impurities. Herein, we demonstrate a method for controlling impurities and defects in strained Nd2Ir2O7 (NIO-227) films by compensating for iridium loss during growth. By increasing the amount of IrO2 target ablated, we enhance the morphological quality and electrical transport properties of the fabricated films. Furthermore, our results show that the anomalous Hall effects of the films have a strong dependency on the amount of IrO2 target ablated, which is attributed to the structural inhomogeneity in the NIO-227 films. Our work provides a way to control defects and impurities and would promote the investigation of topological phases in the family R2Ir2O711Nsciescopu