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Magnetic anisotropy in atomically thin correlated heterostructures is essential for exploring quantum magnetic phases for next-generation spintronics. Whereas previous studies have mostly focused on van der Waals systems, here, we investigate the impact of dimensionality of epitaxially-grown correlated oxides down to the monolayer limit on structural, magnetic, and orbital anisotropies. By designing oxide superlattices with a correlated ferromagnetic SrRuO3 and nonmagnetic SrTiO3 layers, we observed modulated ferromagnetic behavior with the change of the SrRuO3 thickness. Especially, for three-unit-cell-thick layers, we observe a significant 1,500% improvement of coercive field in the anomalous Hall effect, which cannot be solely attributed to the dimensional crossover in ferromagnetism. The atomic-scale heterostructures further reveal the systematic modulation of anisotropy for the lattice structure and orbital hybridization, explaining the enhanced magnetic anisotropy. Our findings provide valuable insights into engineering the anisotropic hybridization of synthetic magnetic crystals, offering a tunable spin order for various applications., Comment: 23 pages

In situ reflective high-energy electron diffraction (RHEED) is widely used to monitor the surface crystalline state during thin-film growth by molecular beam epitaxy (MBE) and pulsed laser deposition. With the recent development of machine learning (ML), ML-assisted analysis of RHEED videos aids in interpreting the complete RHEED data of oxide thin films. The quantitative analysis of RHEED data allows us to characterize and categorize the growth modes step by step, and extract hidden knowledge of the epitaxial film growth process. In this study, we employed the ML-assisted RHEED analysis method to investigate the growth of 2D thin films of transition metal dichalcogenides (ReSe2) on graphene substrates by MBE. Principal component analysis (PCA) and K-means clustering were used to separate statistically important patterns and visualize the trend of pattern evolution without any notable loss of information. Using the modified PCA, we could monitor the diffraction intensity of solely the ReSe2 layers by filtering out the substrate contribution. These findings demonstrate that ML analysis can be successfully employed to examine and understand the film-growth dynamics of 2D materials. Further, the ML-based method can pave the way for the development of advanced real-time monitoring and autonomous material synthesis techniques., Comment: 21 pages, 4 figures

Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, we report deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO$_{3}$, which boosts square-tensile-strain to BaTiO$_{3}$ and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti$^{3+}$ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, we experimentally demonstrate that three electrically controlled polar-ordering states lead to switchable and non-volatile dielectric states for application of non-destructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates., Comment: According to the Copyright Policy, the submission version (before peer-review and revision)

Chiral symmetry breaking of phonons plays an essential role in emergent quantum phenomena owing to its strong coupling to spin degree of freedom. However, direct experimental evidence of the chiral phonon-spin coupling is lacking. In this study, we report a chiral phonon-mediated interlayer exchange interaction in atomically controlled ferromagnetic metal (SrRuO3)-nonmagnetic insulator (SrTiO3) heterostructures. Owing to the unconventional interlayer exchange interaction, we have observed rotation of magnetic moments as a function of nonmagnetic insulating spacer thickness, resulting in a spin spiral state. The chiral phonon-spin coupling is further confirmed by phonon Zeeman effects. The existence of the chiral phonons and their interplay with spins along with our atomic-scale heterostructure approach open a window to unveil the crucial roles of chiral phonons in magnetic materials., Comment: 40 pages

The properties of metal-semiconductor junctions are often unpredictable because of non-ideal interfacial structures, such as interfacial defects or chemical reactions introduced at junctions. Black phosphorus (BP), an elemental two-dimensional (2D) semiconducting crystal, possesses the puckered atomic structure with high chemical reactivity, and the establishment of a realistic atomic-scale picture of BP's interface toward metallic contact has remained elusive. Here we examine the interfacial structures and properties of physically-deposited metals of various kinds on BP. We find that Au, Ag, and Bi form single-crystalline films with (110) orientation through guided van der Waals epitaxy. Transmission electron microscopy and X-ray photoelectron spectroscopy confirm that atomically sharp van der Waals metal-BP interfaces forms with exceptional rotational alignment. Under a weak metal-BP interaction regime, the BP's puckered structure play an essential role in the adatom assembly process and can lead to the formation of a single crystal, which is supported by our theoretical analysis and calculations. The experimental survey also demonstrates that the BP-metal junctions can exhibit various types of interfacial structures depending on metals, such as the formation of polycrystalline microstructure or metal phosphides. This study provides a guideline for obtaining a realistic view on metal-2D semiconductor interfacial structures, especially for atomically puckered 2D crystals., Comment: 27 pages, 5 figures

Nanoscale layered ferromagnets have demonstrated fascinating two-dimensional magnetism down to atomic layers, providing a peculiar playground of spin orders for investigating fundamental physics and spintronic applications. However, strategy for growing films with designed magnetic properties is not well established yet. Herein, we present a versatile method to control the Curie temperature (T_{C}) and magnetic anisotropy during growth of ultrathin Cr_{2}Te_{3} films. We demonstrate increase of the TC from 165 K to 310 K in sync with magnetic anisotropy switching from an out-of-plane orientation to an in-plane one, respectively, via controlling the Te source flux during film growth, leading to different c-lattice parameters while preserving the stoichiometries and thicknesses of the films. We attributed this modulation of magnetic anisotropy to the switching of the orbital magnetic moment, using X-ray magnetic circular dichroism analysis. We also inferred that different c-lattice constants might be responsible for the magnetic anisotropy change, supported by theoretical calculations. These findings emphasize the potential of ultrathin Cr_{2}Te_{3} films as candidates for developing room-temperature spintronics applications and similar growth strategies could be applicable to fabricate other nanoscale layered magnetic compounds., Comment: 30 pages, 6 figures, accepted in ACS Applied Nano Materials

Growth of two-dimensional van der Waals layered single-crystal (SC) films is highly desired to manifest intrinsic material sciences and unprecedented devices for industrial applications. While wafer-scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) film has not been established to date. Here, we report the SC growth of TMdC monolayers in a centimeter scale via atomic sawtooth gold surface as a universal growth template. Atomic tooth-gullet surface is constructed by the one-step solidification of liquid gold, evidenced by transmission-electron-microscopy. Anisotropic adsorption energy of TMdC cluster, confirmed by density-functional calculations, prevails at the periodic atomic-step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of Miller indices. Growth using atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS2, WSe2, MoS2, MoSe2/WSe2 heterostructure, and W1-xMoxS2 alloy. Our strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures in a wafer scale, to further facilitate the applications of TMdCs in post silicon technology., Comment: The supporting information is included in the manuscript file

Lead-free double perovskite halides are emerging optoelectronic materials that are alternatives to lead-based perovskite halides. Recently, single-crystalline double perovskite halides were synthesized, and their intriguing functional properties were demonstrated. Despite such pioneering works, lead-free double perovskite halides with better crystallinity are still in demand for applications to novel optoelectronic devices. Here, we realized highly crystalline Cs2AgBiBr6 single crystals with a well-defined atomic ordering on the microscopic scale. We avoided the formation of Ag vacancies and the subsequent secondary Cs3Bi2Br9 by manipulating the initial chemical environments in hydrothermal synthesis. The suppression of Ag vacancies allows us to reduce the trap density in the as-grown crystals and to enhance the carrier mobility further. Our design strategy is applicable for fabricating other lead-free halide materials with high crystallinity.

Bonding geometry engineering of metal-oxygen octahedra is a facile way of tailoring various functional properties of transition metal oxides. Several approaches, including epitaxial strain, thickness, and stoichiometry control, have been proposed to efficiently tune the rotation and tilting of the octahedra, but these approaches are inevitably accompanied by unnecessary structural modifications such as changes in thin-film lattice parameters. In this study, we propose a method to selectively engineer the octahedral bonding geometries, while maintaining other parameters that might implicitly influence the functional properties. A concept of octahedral tilt propagation engineering has been developed using atomically designed SrRuO3/SrTiO3 superlattices. In particular, the propagation of RuO6 octahedral tilting within the SrRuO3 layers having identical thicknesses was systematically controlled by varying the thickness of adjacent SrTiO3 layers. This led to a substantial modification in the electromagnetic properties of the SrRuO3 layer, significantly enhancing the magnetic moment of Ru. Our approach provides a method to selectively manipulate the bonding geometry of strongly correlated oxides, thereby enabling a better understanding and greater controllability of their functional properties., Comment: 27 pages, 4 figures, 6 supplementary figures

Phosphorene, a monolayer of black phosphorus (BP), is an elemental two-dimensional material with interesting physical properties, such as high charge carrier mobility and exotic anisotropic in-plane properties. To fundamentally understand these various physical properties, it is critically important to conduct an atomic-scale structural investigation of phosphorene, particularly regarding various defects and preferred edge configurations. However, it has been challenging to investigate mono- and few-layer phosphorene because of technical difficulties arising in the preparation of a high-quality sample and damages induced during the characterization process. Here, we successfully fabricate high-quality monolayer phosphorene using a controlled thinning process with transmission electron microscopy, and subsequently perform atomic-resolution imaging. Graphene protection suppresses the e-beam-induced damage to multi-layer BP and one-side graphene protection facilitates the layer-by-layer thinning of the samples, rendering high-quality monolayer and bilayer regions. We also observe the formation of atomic-scale crystalline edges predominantly aligned along the zigzag and (101) terminations, which is originated from edge kinetics under e-beam-induced sputtering process. Our study demonstrates a new method to image and precisely manipulate the thickness and edge configurations of air-sensitive two-dimensional materials., Comment: 21 pages, 5 figures

Artificial crystals synthesized by atomic-scale epitaxy provides the ability to control the dimensions of the quantum phases and associated phase transitions via precise thickness modulation. In particular, reduction in dimensionality via quantized control of atomic layers is a powerful approach to revealing hidden electronic and magnetic phases. Here, we demonstrate a dimensionality-controlled and induced metal-insulator transition (MIT) in atomically designed superlattices by synthesizing a genuine two dimensional (2D) SrRuO3 crystal with highly suppressed charge transfer. The tendency to ferromagnetically align the spins in SrRuO3 layer diminishes in 2D as the interlayer exchange interaction vanishes, accompanying the 2D localization of electrons. Furthermore, electronic and magnetic instabilities in the two SrRuO3 unit cell layers induce a thermally-driven MIT along with a metamagnetic transition., Comment: 31 pages, 3 figures, 13 supplementary figures

It is demonstrated that the electric dipole layer due to the overlapping of electron wavefunctions at metal/graphene contact results in negative Fermi-level pinning effect on the region of GaAs surface with low interface-trap density in metal/graphene/n-GaAs(001) junction. The graphene interlayer takes a role of diffusion barrier preventing the atomic intermixing at interface and preserving the low interface-trap density region. The negative Fermi-level pinning effect is supported by the Schottky barrier decreasing as metal work-function increasing. Our work shows that the graphene interlayer can invert the effective work-function of metal between $high$ and $low$, making it possible to form both Schottky and Ohmic-like contacts with identical (particularly $high$ work-function) metal electrodes on a semiconductor substrate possessing low surface-state density., Comment: 19 pages, 10 figures

Despite a rich choice of two-dimensional materials, which exists these days, heterostructures, both vertical (van der Waals) and in-plane, offer an unprecedented control over the properties and functionalities of the resulted structures. Thus, planar heterostructures allow p-n junctions between different two-dimensional semiconductors and graphene nanoribbons with well-defined edges; and vertical heterostructures resulted in the observation of superconductivity in purely carbon-based systems and realisation of vertical tunnelling transistors. Here we demonstrate simultaneous use of in-plane and van der Waals heterostructures to build vertical single electron tunnelling transistors. We grow graphene quantum dots inside the matrix of hexagonal boron nitride, which allows a dramatic reduction of the number of localised states along the perimeter of the quantum dots. The use of hexagonal boron nitride tunnel barriers as contacts to the graphene quantum dots make our transistors reproducible and not dependent on the localised states, opening even larger flexibility when designing future devices.

The direction of ferroelectric polarization is prescribed by the symmetry of the crystal structure. Therefore, rotation of the polarization direction is largely limited, despite the opportunity it offers in understanding important dielectric phenomena such as piezoelectric response near the morphotropic phase boundaries and practical applications such as ferroelectric memory. In this study, we report the observation of continuous rotation of ferroelectric polarization in order-disorder type LiNbO3 thin films. The spontaneous polarization could be tilted from an out-of-plane to an in-plane direction in the thin film by controlling the Li vacancy concentration within the hexagonal lattice framework. Partial inclusion of monoclinic-like phase is attributed to the breaking of macroscopic inversion symmetry along different directions and the emergence of ferroelectric polarization along the in-plane direction., Comment: 38 pages, 12 figures

Black phosphorus (BP) has recently emerged as an alternative 2D semiconductor owing to its fascinating electronic properties such as tunable bandgap and high charge carrier mobility. The structural investigation of few-layer BP, such as identification of layer thickness and atomic-scale edge structure, is of great importance to fully understand its electronic and optical properties. Here we report atomic-scale analysis of few-layered BP performed by aberration corrected transmission electron microscopy (TEM). We establish the layer-number-dependent atomic resolution imaging of few-layer BP via TEM imaging and image simulations. The structural modification induced by the electron beam leads to revelation of crystalline edge and formation of BP nanoribbons. Atomic resolution imaging of BP clearly shows the reconstructed zigzag (ZZ) edge structures, which is also corroborated by van der Waals first principles calculations on the edge stability. Our study on the precise identification of BP thickness and atomic-resolution imaging of edge structures will lay the groundwork for investigation of few-layer BP, especially BP in nanostructured forms., Comment: 20 pages, 6 figures