We report the epitaxial thin film synthesis of SrCu$_3$O$_4$ with infinitely stacked Cu$_3$O$_4$ layers composed of edge-sharing CuO$_4$ square-planes, using molecular beam epitaxy. Experimental and theoretical characterizations showed that this material is a metastable phase that can exist by applying tensile biaxial strain from the (001)-SrTiO$_3$ substrate. SrCu$_3$O$_4$ shows an insulating electrical resistivity in accordance with the Cu$^{2+}$ valence state revealed X-ray photoelectron spectroscopy. First-principles calculations also indicated that the unoccupied $d_{3z^2-r^2}$ band becomes substantially stabilized owing to the absence of apical anions, in contrast to $A_2$Cu$_3$O$_4$Cl$_2$ ($A = $Sr, Ba) with an $A_2$Cl$_2$ block layer and therefore a trans-CuO$_4$Cl$_2$ octahedron. These results suggest that SrCu$_3$O$_4$ is a suitable parent material for electron-doped superconductivity based on the Cu$_3$O$_4$ plane.
The unconventional Si-compatible ferroelectricity in hafnia-based systems, which becomes robust only at nanoscopic sizes, has attracted a lot of interest. While a metastable polar orthorhombic (o-) phase (Pca21) is widely regarded as the responsible phase for ferroelectricity, a higher energy polar rhombohedral (r-) phase is recently reported on epitaxial HfZrO4 (HZO) films grown on (001) SrTiO3 (R3m or R3), (0001) GaN (R3), and Si (111). Armed with results on these systems, here we report a systematic study leading towards identifying comprehensive global trends for stabilizing r-phase polymo…
Epitaxial stabilization of pulsed laser deposited Sr$_{n+1}$Ir$_n$O$_{3n+1}$ thin films: entangled effect of growth dynamics and strain
Ir $^{4+}$层状氧化物中电子关联、晶场分裂和自旋-轨道耦合的微妙平衡可以产生新的电子和磁性相。这一领域的实验进展依赖于这些氧化物的外延膜的合成。然而，具有优异结构质量的层状虹彩晶体的生长是一个巨大的实验挑战。在这里，我们选择性地生长高质量的单相薄膜的Sr $_2$ IrO $_4$，Sr $_3$ Ir $_2$ O $_7$，和SrIrO $_3$在各种衬底上从一个单一的Sr $_3$ Ir $_2$ O $_7$目标通过调整背景氧压力和外延应变。我们证明了薄膜沉积过程中生长动力学和应变之间复杂的相互作用。这种相互作用导致在相同的生长条件下生长在不同衬底上的膜中的不同相的稳定，这不能用简单的动力学模型来解释。我们进一步研究了这三个阶段的热电性能，并提出，弱本地化是负责低温激活电阻率观察SrIrO $_3$压缩应变下。
Doped HfO2 and HfO2-ZrO2 compounds are gaining significant interest thanks to their ferroelectric properties in ultrathin films. Here, we show that ZrO2 could be a playground for doping and strain engineering to increase the thickness in epitaxial thin films. Based on surface energy considerations supported by ab initio calculations, we show that a pure ZrO2 epitaxial thin film exhibits a ferroelectric rhombohedral phase (r-phase, with R3m space group) more stable than for the Hf0.5Zr0.5O2 (HZO) and pure HfO2 cases. In particular, for a thickness up to 37 nm we experimentally evidence a single…
Hafnia (HfO$_2$)-based thin films have promising applications in nanoscale electronic devices due to their robust ferroelectricity and integration with silicon. However, HfO$_2$ has various stable and metastable polymorphs with quite similar structures and energies. Identifying and stabilizing the ferroelectric functional phases of HfO$_2$ have attracted intensive research interest in recent years. In this work, first-principles calculations on (111)-oriented HfO$_2$ are used to discover that imposing an in-plane shear strain on the tetragonal phase induces a nonpolar to polar phase transition…
Thin films of perovskite Ruthenates of the general formula ARuO3 (A = Ca and Sr) are versatile electrical conductors for viable oxide electronics. They are also scientifically intriguing, as they exhibit non-trivial electromagnetic ground states depending on the A-site element. Among them, realization of the cubic perovskite (3C) BaRuO3 thin film has been a challenge so far, because the 3C phase is metastable with the largest formation energy among the various polymorph phases of BaRuO3. In this study, we successfully prepared 3C BaRuO3 thin films employing epitaxial stabilization. The 3C BaRu…
Piezoelectrics interconvert mechanical energy and electric charge and are widely used in actuators and sensors. The best performing materials are ferroelectrics at a morphotropic phase boundary (MPB), where several phases can intimately coexist. Switching between these phases by electric field produces a large electromechanical response. In the ferroelectric BiFeO$_3$, strain can be used to create an MPB-like phase mixture and thus to generate large electric field dependent strains. However, this enhanced response occurs at localized, randomly positioned regions of the film, which potentially …
Computationally guided high-throughput synthesis is used to explore the Zn-V-N phase space, resulting in the synthesis of a novel ternary nitride Zn$_2$VN$_3$. Following a combinatorial PVD screening, we isolate the phase and synthesize polycrystalline Zn$_2$VN$_3$ thin films with wurtzite structure on conventional borosilicate glass substrates. In addition, we demonstrate that cation-disordered, but phase-pure (002)-textured, Zn$_2$VN$_3$ thin films can be grown using epitaxial stabilization on α-Al2O3 (0001) substrates at remarkably low growth temperatures well below 200 °C. The structural p…
Stabilization of $\varepsilon$-Fe$_2$O$_3$ epitaxial layer on MgO(111)/GaN via an intermediate $γ$-Fe$_2$O$_3$-phase
在目前的研究中，我们已经证明了外延稳定的亚稳磁硬$\varepsilon$ -Fe $_2$ O $_3$相上的薄MgO（111）缓冲层生长到GAN（0001）表面上的顶部。在Fe $_2$ O $_3$和GAN之间引入4 nm厚的MgO缓冲层的主要目的是阻止Ga热迁移到氧化铁层中。虽然这种迁移和正交GaFeO $_3$的连续形成被认为是早期的同构$\varepsilon$ -Fe $_2$ O $_3$的成核的潜在触发，本工作表明，即使在MgO覆盖的GAN上，通过脉冲激光沉积生长单晶均匀的铁氧体薄膜也是可能的。通过电子衍射和X射线衍射研究了MgO / GAN上60 nm厚的Fe $_2$ O $_3$层的结构特性，两者都表明在生长$\varepsilon$ -Fe $_2$ O $_3$之前先形成了一薄层$γ$ -Fe $_2$ O $_3$ .的存在下的磁性硬铁氧体的温度依赖性的磁力测量独立确认。深度分辨X射线衍射和极化中子反射谱分析表明，界面处10. 0 nm的氧化铁层比$\varepsilon$ -Fe $_2$ O $_3$膜的主体积密度低，磁化强度高.通过拟合反射数据得到的密度和磁矩深度分布与MgO和$\varepsilon$-Fe $_2$ O $_3$之间磁降解的$γ$-Fe $_2$ O $_3$过渡层的存在很好地一致。磁电$\varepsilon$ -和自旋热电…
Epitaxial orthorhombic Hf0.5Zr0.5O2 (HZO) films on La0.67Sr0.33MnO3 (LSMO) electrodes show robust ferroelectricity, with high polarization, endurance and retention. However, no similar results have been achieved using other perovskite electrodes so far. Here, LSMO and other perovskite electrodes are compared. A small amount of orthorhombic phase and low polarization is found in HZO films grown on La-doped BaSnO3 and Nb-doped SrTiO3, while null amounts of orthorhombic phase and polarization are detected in films on LaNiO3 and SrRuO3. The critical effect of the electrode on the stabilized phases…
Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain
无稀土合金是永磁体研究的焦点，因为当今常规磁体所需的元素的可及性有限。四方应变铁钴（Fe-Co）由于理论计算而作为有希望的候选者引起了人们的极大兴趣。然而，在实验中，所施加的应变随着膜厚度的增加而迅速松弛，并且阻碍了强磁晶各向异性的稳定。在我们的研究中，我们表明，已经2原子%的碳大大减少晶格弛豫，导致形成自发应变相与3%的tetraorbic失真。在这些应变（Fe $_{0.4}$ Co $_{0.6}$）$_{0.98}$ C $_{0.02}$薄膜中，观察到大于0.4 MJ/m $^3$的磁晶各向异性，同时保持2.1 T的大极化。与二元Fe-Co相比，这是内禀磁性的显著改善。在本文中，我们将我们的实验工作与应变Fe-Co-C的理论研究联系起来，并发现非常一致。
The growth window of epitaxial Hf0.5Zr0.5O2 is established taking into account the main ferroelectric properties that films have to present simultaneously: high remanent polarization, low fatigue and long retention. Defects in the film and imprint field depend on deposition temperature and oxygen pressure, with an impact on fatigue and retention, respectively. Fatigue increases with substrate temperature and pressure, and retention is short if low temperature is used. The growth window of epitaxial stabilization of ferroelectric Hf0.5Zr0.5O2 is narrower when all major ferroelectric properties …
In the quest for quantum spin liquids, thin films are expected to open the way for the control of intricate magnetic interactions in actual materials by exploiting epitaxial strain and two-dimensionality. However, materials compatible with conventional thin-film growth methods have largely remained undeveloped. As a promising candidate towards the materialization of quantum spin liquids in thin films, we here present a robust ilmenite-type oxide with a honeycomb lattice of edge-sharing IrO$_6$ octahedra artificially stabilized by superlattice formation with an ilmenite-type antiferromagnetic o…
Gilbert Damping Parameters of Epitaxially-Stabilized Iron Gallium Thin Films from Ferromagnetic Resonance
The Ferroelectronics Laboratory, University of Michigan（密歇根大学铁电实验室）；Imec（imec研究所）；Department of Materials Science and Engineering, University of Wisconsin-Madison（威斯康星大学麦迪逊分校材料科学与工程系）
In situ growth of pyrochlore iridate thin films has been a long-standing challenge due to the low reactivity of Ir at low temperatures and the vaporization of volatile gas species such as IrO3(g) and IrO2(g) at high temperatures and high oxygen partial pressures. To address this challenge, we combine thermodynamic analysis of the Pr-Ir-O2 system with experimental results from the conventional physical vapor deposition (PVD) technique of co-sputtering. Our results indicate that only high growth temperatures yield films with crystallinity sufficient for utilizing and tailoring the desired topolo…
Synthesis of epitaxial TaO$_2$ thin films on Al$_2$O$_3$ by suboxide molecular-beam epitaxy and thermal laser epitaxy
Cornell University（康奈尔大学）；Platform for the Accelerated Realization, Analysis, and Discovery of Interface Materials (PARADIM), Cornell University（界面材料加速实现、分析和发现平台 (PARADIM)，康奈尔大学）；Kavli Institute at Cornell for Nanoscale Science（康奈尔卡弗里纳米科学研究所）；Department of Materials Science and Engineering, Cornell University（康奈尔大学材料科学与工程系）；School of Applied and Engineering Physics, Cornell University（康奈尔大学应用工程物理学院）
Manipulation of octahedral distortion at atomic length scale is an effective means to tune the physical ground states of functional oxides. Previous work demonstrates that epitaxial strain and film thickness are variable parameters to modify the octahedral rotation and tilt. However, selective control of bonding geometry by structural propagation from adjacent layers is rarely studied. Here we propose a new route to tune the ferromagnetic response in SrRuO3 (SRO) ultrathin layers by oxygen coordination of adjacent SrCuO2 (SCO) layers. The infinite-layered CuO2 in SCO exhibits a structural tran…
AB$_2$O$_4$ normal spinels with a magnetic B site can host a variety of magnetic and orbital frustrations leading to spin-liquid phases and field-induced phase transitions. Here we report the first epitaxial growth of (111)-oriented MgCr$_2$O$_4$ thin films. By characterizing the structural and electronic properties of films grown along (001) and (111) directions, the influence of growth orientation has been studied. Despite distinctly different growth modes observed during deposition, the comprehensive characterization reveals no measurable disorder in the cation distribution nor multivalency…
Ultra-wide bandgap semiconductors are ushering in the next generation of high power electronics. The correct crystal orientation can make or break successful epitaxy of such semiconductors. Here it is discovered that single-crystalline layers of $α$-(AlGa)$_2$O$_3$ alloys spanning bandgaps of 5.4 - 8.6 eV can be grown by molecular beam epitaxy. The key step is found to be the use of m-plane sapphire crystal. The phase transition of the epitaxial layers from the $α$- to the narrower bandgap $β$-phase is catalyzed by the c-plane of the crystal. Because the c-plane is orthogonal to the growth fro…
Control of polarization and polar chiral textures in BiFeO$_3$ by epitaxial strain and interfacial chemistry
Laboratoire Albert Fert, CNRS, Thales, Université Paris–Saclay（阿尔贝·费尔特实验室，法国国家科学研究中心，泰雷兹，巴黎-萨克雷大学）；Department of Materials, ETH Zurich（苏黎世联邦理工学院材料系）；Université Grenoble Alpes, CNRS, Institut Néel（格勒诺布尔阿尔卑斯大学，法国国家科学研究中心，内尔研究所）；Electron Microscopy Center, Empa（电子显微镜中心，瑞士联邦材料科技与测试研究院）
Hexagonal perovskites are an attractive group of materials due to their various polymorph phases and rich structure-property relationships. BaRuO3 (BRO) is a prototypical hexagonal perovskite, in which the electromagnetic properties are significantly modified depending on its atomic structure. Whereas thin-film epitaxy would vastly expand the application of hexagonal perovskites by epitaxially stabilizing various metastable polymorphs, the atomic structure of epitaxial hexagonal perovskites, especially at the initial growth stage, has rarely been investigated. In this study, we show that an in…
Sub-unit cell engineering of CrVO$_3$ superlattice thin films
KU Leuven（荷语鲁汶大学）；ETH Zurich（苏黎世联邦理工学院）；University of Antwerp（安特卫普大学）；Instituto de Cerámica y Vidrio - Consejo Superior de Investigaciones Cientificas (ICV-CSIC)（陶瓷与玻璃研究所-西班牙国家研究委员会）；Sunchon National University（顺天国立大学）
