Abstract The breaking of inversion symmetry dictates the emergence of electric polarization, whose topological states in superlattices and bulks have received tremendous attention for their intriguing physics brought for novel device design. However, as for substrate oxides such as LaAlO 3 , KTaO 3 , R ScO 3 ( R = rare earth element), their centrosymmetric trivial attributes make their functionality poorly explored. Here, the discovery of nanoscale thickness gradient‐induced nonpolar‐to‐polar phase transition in band insulator DyScO 3 is reported by using atomic resolution transmission electron microscopy. As the free‐standing specimen reduces to a critical thickness ≈5 nm, its inversion symmetry is spontaneously broken by surface charge transfer, which gives rise to asymmetric Dy atomic displacements and ferrodistortive octahedral order, as substantiated by the first‐principles calculations. Apart from the observation of migratable polar vortex structures, the switchable electric polarization by applied electric field is demonstrated by the piezoresponse force microscopy experiments. Given the decisive role of critical size in generating ferroelectricity, a concept of “inverse size‐scaling ferroelectric” is proposed to define a class of such materials. Distinct from the proper and improper ferroelectrics, the findings offer a new platform to explore novel low‐dimensional ferroelectrics and device applications in the future.
Owing to their excellent mixed-ionic and electronic conductivity, fast oxygen kinetics, and cost efficiency, layered oxygen-deficient perovskite oxides hold great potential as highly efficient cathodes for solid oxide fuel cells and anodes for water oxidation. Under working conditions, cation ordering is believed to substantially enhance oxygen diffusion while maintaining structural stability owing to the formation of double perovskite (DP), thus attracting extensive research attention. In contrast, the incorporation of oxygen vacancies and the associated vacancy ordering have rarely been studied at the atomic scale, despite their decisive roles in regulating the electronic and spin structures as well as in differentiating the crystal structure from DP. Here, atomic-resolution transmission electron microscopy is used to directly image oxygen vacancies and measure their concentration in (Pr,Ba)CoO3-δ films grown on SrTiO3 substrates. We find that accompanied by the presence of oxygen vacancy ordering at Co–O planes, the A–O (A = Pr/Ba) planes also exhibit a breathing-like lattice modulation. Specifically, as confirmed by first-principle calculations, the AO–AO interplanar spacings are found to be linearly correlated with the vacancy concentration in the enclosing Co–O planes. On this basis, potential consequences of oxygen occupancy for the catalytic properties of structurally pure PBCO phases are discussed. Through establishing a simple correlation of oxygen concentration with the easily achievable lattice measurement, our results pave a way for better understanding the structure–performance relationship of oxygen-deficient complex cobaltites used for electrocatalysis.
We report the multiferroic properties of 12R-type hexagonal Ba(Ti1/3Mn2/3)O3-δ found in Mn-doped BaTiO3 series samples. Hysteresis measurements reveal the coexistence of weak ferromagnetism and ferroelectricity at room temperature. Furthermore, frequency-driven dynamic ferroelectric phase transition is disclosed around a critical frequency of 220 Hz. Analyses on the dielectric relaxation, leakage current, crystal structure, and magnetic susceptibility lead us to conclude that the response of polarons dominates the observed physical properties, and the dynamic phase transition may ascribe to the response mode changes of the localized electrons. More importantly, we figure out the crucial factors leading to difference of the ferroelectric and magnetic properties of the 12R-type Ba(Ti1/3Mn2/3)O3-δ samples from that of the 6H-type Ba(Ti1-xMx)O3-δ (M = Fe, Mn) samples.
Abstract DEM (discrete element method) is an effective method to study the dynamics of ballast track. The DEM assumes that the ballast is composed of spheres and the optimal combination of spheres in single ballast is the key to study ballast tracks by using DEM. A track ballast model using DEM is established with accurate ballast profile that is filled with spheres obtained by using a binocular 3D scanner and the Bubble Pack algorithm; In this paper, a direct shear test is adopted to study mechanical properties of the track ballast filled differently. The optimal sphere filling method is chosen using a ballast model based on DEM. The results show that: the volume filling rate ω of the ballast is negatively correlated with the radius ratio r of the small ball and the large ball that are adjacent, and positively correlated with the overlap angle ϕ of the two adjacent balls; with the same volume filling rate, the smaller ratio r is, the greater number of spheres is, and the closer the maximum shear stress of numerical calculation is to the measured level; for general engineering calculations, the optimal combination with a volume filling rate of 0.8 should be used for calculation.
Abstract In contrast to the flexible rotation of magnetization direction in ferromagnets, the spontaneous polarization in ferroelectric materials is highly confined along the symmetry-allowed directions. Accordingly, chirality at ferroelectric domain walls was treated only at the theoretical level and its real appearance is still a mystery. Here we report a Néel-like domain wall imaged by atom-resolved transmission electron microscopy in Ti-rich ferroelectric Pb(Zr 1− x Ti x )O 3 crystals, where nanometre-scale monoclinic order coexists with the tetragonal order. The formation of such domain walls is interpreted in the light of polarization discontinuity and clamping effects at phase boundaries between the nesting domains. Phase-field simulation confirms that the coexistence of both phases as encountered near the morphotropic phase boundary promotes the polarization to rotate in a continuous manner. Our results provide a further insight into the complex domain configuration in ferroelectrics, and establish a foundation towards exploring chiral domain walls in ferroelectrics.
Abstract Proximity‐induced superconductivity in hybrid devices of topological insulators and superconductors offers a promising platform for the pursuit of elusive topological superconductivity and its anticipated applications, such as fault‐tolerant quantum computing. To study and harness such hybrid devices, a key challenge is the realization of highly functional material interfaces with a suitable superconductor featuring 2‐periodic parity‐conserving transport to ensure a superconducting hard‐gap free of unpaired electrons, which is important for Majorana physics. A superconductor well‐known for this characteristic is Al, however, its direct integration into devices based on tetradymite topological insulators has so far been found to yield non‐transparent interfaces. By focusing on Bi 2 Te 3 ‐Al heterostructures, this study identifies detrimental interdiffusion processes at the interface through atomically resolved structural and chemical analysis, and showcases their mitigation by leveraging different interlayers – namely Nb, Ti, Pd, and Pt – between Bi 2 Te 3 and Al. Through structural transformation of the interlayer materials (X) into their respective tellurides (XTe 2 ) atomically‐sharp epitaxial interfaces are engineered and further characterized in low‐temperature transport experiments on Al‐X‐Bi 2 Te 3 ‐X‐Al Josephson junctions and in complementary density functional theory calculations. By demonstrating functional interfaces between Bi 2 Te 3 and Al, this work provides key insights and paves the way for the next generation of sophisticated topological devices.
The proximity effect at a highly transparent interface of an s-wave superconductor (S) and a topological insulator (TI) provides a promising platform to create Majorana zero modes in artificially designed heterostructures. However, structural and chemical issues pertinent to such interfaces have been poorly explored so far. Here, we report the discovery of Pd diffusion-induced polarization at interfaces between superconductive Pd1+x(Bi0.4Te0.6)2 (xPBT, 0 ≤ x ≤ 1) and Pd-intercalated Bi2Te3 by using atomic-resolution scanning transmission electron microscopy. Our quantitative image analysis reveals that nanoscale lattice strain and QL polarity synergistically suppress and promote Pd diffusion at the normal and parallel interfaces, formed between Te–Pd–Bi triple layers (TLs) and Te–Bi–Te–Bi–Te quintuple layers (QLs), respectively. Further, our first-principles calculations unveil that the superconductivity of the xPBT phase and topological nature of the Pd-intercalated Bi2Te3 phase are robust against the broken inversion symmetry. These findings point out the necessity of considering the coexistence of electric polarization with superconductivity and topology in such S–TI systems.
Hexagonal Ba(Ti1−xFex)O3−δ (x = 1/6, 1/3) ceramics treated with post annealing are specifically synthesized to explore the origin mechanism of the unusual ferromagnetism in the doped system. X-ray diffraction refinements and transmission electron microscope experiments reveal that their structures are incommensurately modulated owing to simultaneous oxygen vacancies at both O1 and O2 sites. Consequently, coexisting weak ferrielectricity and weak ferromagnetism are presented at room temperature. Analysis on their leakage current plot reveals that their conduction follows trap-filled limit model. In combination with the magnetism studies on 5 mol% Fe-doped crystals [Phys. Rev. B 83, 144407 (2011)], the reduction of ferromagnetism with an increase of conductivity suggests that dynamic exchanges of trapped electrons among the bound magnetic polarons attribute to the intrinsic ferromagnetism.
Taking arch bridges, including deck, half-through, and through arch bridges (short for DAB, HTAB, and TAB) as examples, mechanics analysis models of longitudinal interaction between continuously welded rails (short for CWRs) and arch bridges are established. Based on the finite element method (FEM), the longitudinal interaction calculation software of CWR on arch bridges has been developed. Focusing on an HTAB, the tension, compression, and deflection conditions are calculated and analyzed. The results show that the mechanics analysis models of three types of arch bridges can truly reflect the real state of the structure; the calculation software can be used for systematic research of the CWR on arch bridge; as for HTAB, temperature difference of arch rib has a small effect on rail tension/compression, and arch bridge can be simplified as a continuous beam for rail tension/compression additional force calculation; in calculation of deflection conditions of HTAB, it is suggested that train loads are arranged on half span and full span and take the direction of load entering bridge into account. Additionally, the deflection additional force variation of CFST basket handle arch bridge is different from that of ordinary bridge.
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