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Register a new userAtomic layer engineering of perovskite oxides for chemically sharp heterointerfaces
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Advances in synthesis techniques and materials understanding have given rise to oxide heterostructures with intriguing physical phenomena that cannot be found in their constituents. In these structures, precise control of interface quality, including oxygen stoichiometry, is critical for unambiguous tailoring of the interfacial properties, with deposition of the first monolayer being the most important step in shaping a well-defined functional interface. Here, we studied interface formation and strain evolution during the initial growth of LaAlO3 on SrTiO3 by pulsed laser deposition, in search of a means for controlling the atomic-sharpness of the interfaces. Our experimental results show that growth of LaAlO3 at a high oxygen pressure dramatically enhances interface abruptness. As a consequence, the critical thickness for strain relaxation was increased, facilitating coherent epitaxy of perovskite oxides. This provides a clear understanding of the role of oxygen pressure during the interface formation, and enables the synthesis of oxide heterostructures with chemically-sharper interfaces.
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Several challenges in designing an operational Skyrmion racetrack memory are well-known. Among those challenges, a few contradictions can be identified if researchers were to rely only on metallic materials. Hence, expanding the exploration on Skyrmion Physics into oxide materials is essential to bridge the contradicting gap. In this topical review, we first briefly revise the theories and criteria involved in stabilizing and manipulating Skymions, followed by studying the behaviors of dipolar-stabilized magnetic bubbles. Next, we explore the properties of multiferroic Skyrmions with magnetoelectric coupling, which can only be stabilized in Cu$_2$OSeO$_3$ thus far, as well as the rare bulk Neel-type Skyrmions in some polar materials. As an interlude section, we review the theory of Anomalous (AHE) and Topological Hall Effect (THE), before going through the recent progress of THE in oxide thin films. The debate about an alternative interpretation is also discussed. Finally, this review ends with future outlooks about the promising strategies of using interfacial charge-transfer and (111)-orientation of perovskites to benefit the field of Skyrmion research.
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 intriguing nucleation behavior takes place during the initial stabilization of a hexagonal perovskite 9R BaRuO3 (BRO) thin film on a (111) SrTiO3 (STO) substrate. We use high-resolution high-angle annular dark field scanning transmission electron microscopy in combination with geometrical phase analysis to understand the local strain relaxation behavior. We find that nano-scale strained layers, composed of different RuO6 octahedral stacking, are initially formed at the interface, followed by a relaxed single crystal9R BRO thin film. Within the interface layer, hexagonal BROs are nucleated on the STO (111) substrate by both corner- and face-sharing. More interestingly, we find that the boundaries between the differently-stacked nucleation layers, i.e. heterostructural boundaries facilitates strain relaxation, in addition to the formation of conventional misfit dislocations evolving from homostructural boundaries. Our observations reveal an important underlying mechanism to understand the thin-film epitaxy and strain accommodation in hexagonal perovskites.
We have synthesized and investigated the heterointerfaces of KTaO$_3$ (KTO) and GdScO$_3$ (GSO), which are both polar complex-oxides along the pseudo-cubic [001] direction. Since their layers have the same, conflicting net charges at interfaces, i.e. KO(-1)/ScO$_2$(-1) or TaO$_2$(+1)/GdO(+1), forming the heterointerface of KTO/GSO should be forbidden due to strong Coulomb repulsion, the so-called $textit{polarity conflict}$. However, we have discovered that atomic reconstruction occurs at the heterointerfaces between KTO thin-films and GSO substrates, which effectively alleviates the polarity conflict without destroying the hetero-epitaxy. Our result demonstrates one of the important ways to create artificial heterostructures from polar complex-oxides.
We investigate the topological properties of Floquet-engineered twisted bilayer graphene above the magic angle driven by circularly polarized laser pulses. Employing a full Moire-unit-cell tight-binding Hamiltonian based on first-principles electronic structure we show that the band topology in the bilayer, at twisting angles above 1.05$^circ$, essentially corresponds to the one of single-layer graphene. However, the ability to open topologically trivial gaps in this system by a bias voltage between the layers enables the full topological phase diagram to be explored, which is not possible in single-layer graphene. Circularly polarized light induces a transition to a topologically nontrivial Floquet band structure with the Berry curvature of a Chern insulator. Importantly, the twisting allows for tuning electronic energy scales, which implies that the electronic bandwidth can be tailored to match realistic driving frequencies in the ultraviolet or mid-infrared photon-energy regimes. This implies that Moire superlattices are an ideal playground for combining twistronics, Floquet engineering, and strongly interacting regimes out of thermal equilibrium.
We show that the growth of the heterostructure LaGaO3/SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3/SrTiO3 and a superconducting transition is found. The results open the route to widening the field of polar-non polar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.
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