Direct observation of the proliferation of ferroelectric loop domains and vortex-antivortex pairs

We discovered stripe patterns of trimerization-ferroelectric domains in hexagonal REMnO(3) (RE=Ho,···,Lu) crystals (grown below ferroelectric transition temperatures (T(c)), reaching up to 1435 °C), in contrast with the vortex patterns in YMnO(3). These stripe patterns roughen with the appearance of numerous loop domains through thermal annealing just below T(c), but the stripe domain patterns turn to vortex-antivortex domain patterns through a freezing process when crystals cross T(c) even though the phase transition appears to not be Kosterlitz-Thouless-type. The experimental systematics are compared with the results of our six-state clock model simulation and also the Kibble-Zurek mechanism for trapped topological defects.     

Conformal mapping for the Helmholtz equation: acoustic wave scattering by a two dimensional inclusion with irregular shape in an ideal fluid

The complex variables method with mapping function was extended to solve the linear acoustic wave scattering by an inclusion with sharp/smooth corners in an infinite ideal fluid domain. The improved solutions of Helmholtz equation, shown as Bessel function with mapping function as the argument and fractional order Bessel function, were analytically obtained. Based on the mapping function, the initial geometry as well as the original physical vector can be transformed into the corresponding expressions inside the mapping plane. As all the physical vectors are calculated in the mapping plane (η,η), this method can lead to potential vast savings of computational resources and memory. In this work, the results are validated against several published works in the literature. The different geometries of the inclusion with sharp corners based on the proposed mapping functions for irregular polygons are studied and discussed. The findings show that the variation of angles and frequencies of the incident waves have significant influence on the bistatic scattering pattern and the far-field form factor for the pressure in the fluid.     

Ferroelectricity induced by acentric spin-density waves in YMn2O5

The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition.     

Pipecolic acid-catalyzed direct asymmetric mannich reactions

[STRUCTURE: SEE TEXT] Mannich reactions between aldehydes and N-p-methoxyphenyl-protected alpha-imino ethyl glyoxylate have been performed using (S)-pipecolic acid as catalyst. The reactions give both syn- and anti-products (dr=1.4-2:1) with high enantioselectivities (>98% ee). In contrast, (S)-proline-catalyzed reactions give mainly syn-products with high enantioselectivities. Computational studies reveal that the energetic preference between the transition structures involving the s-cis-enamine and the s-trans-enamine is smaller for the pipecolic acid as compared to proline, yielding the (2S,3R)-anti and the (2S,3S)-syn Mannich product in nearly equal amounts.     

Effect of composition and deposition temperature on the characteristics of Ga doped ZnO thin films

ZnO films doped with Ga (GZO) of varying composition were prepared on Corning glass substrate by radio frequency magnetron sputtering at various deposition temperatures of room temperature, 150, 250 and 400 °C, and their temperature dependent photoelectric and structural properties were correlated with Ga composition. With increasing deposition temperature, the Ga content, at which the lowest electrical resistivity and the best crystallinity were observed, decreased. Films with optimal electrical resistivity of 2-3 × 10⁻⁴ Ω cm and with good crystallinity were obtained in the substrate temperature range from 150 to 250 °C, and the corresponding C_Ga/(C_Ga + C_Zn) atomic ratio was about 0.049. GZO films grown at room temperature had coarse columnar structure and low optical transmittance, while films deposited at 400 °C yielded the highest figure of merit (FOM) due to very low optical absorption despite rather moderate electrical resistivity slightly higher than 4 × 10⁻⁴ Ω cm. The optimum Ga content at which the maximum figure of merit was obtained decreased with increasing deposition temperature.     

Area-preserving approximations of polygonal paths

Let P be an x-monotone polygonal path in the plane. For a path Q that approximates P let W_A(Q) be the area above P and below Q, and let W_B(Q) be the area above Q and below P. Given P and an integer k, we show how to compute a path Q with at most k edges that minimizes W_A(Q)+W_B(Q). Given P and a cost C, we show how to find a path Q with the smallest possible number of edges such that W_A(Q)+W_B(Q)C. However, given P, an integer k, and a cost C, it is NP-hard to determine if a path Q with at most k edges exists such that max{W_A(Q),W_B(Q)}C. We describe an approximation algorithm for this setting. Finally, it is also NP-hard to decide whether a path Q exists such that |W_A(Q)−W_B(Q)|=0. Nevertheless, in this error measure we provide an algorithm for computing an optimal approximation up to an additive error.     

The Voronoi Diagram of Curved Objects

Voronoi diagrams of curved objects can show certain phenomena that are often considered artifacts: The Voronoi diagram is not connected; there are pairs of objects whose bisector is a closed curve or even a two-dimensional object; there are Voronoi edges between different parts of the same site (so-called self-Voronoi-edges); these self-Voronoi-edges may end at seemingly arbitrary points not on a site, and, in the case of a circular site, even degenerate to a single isolated point. We give a systematic study of these phenomena, characterizing their differential-geometric and topological properties. We show how a given set of curves can be refined such that the resulting curves define a “well-behaved” Voronoi diagram. We also give a randomized incremental algorithm to compute this diagram. The expected running time of this algorithm is O(n log n).     

Synthesis and characterizations of silylated polyurethane from methyl ethyl ketoxime-blocked polyurethane dispersion

Water-dispersible blocked polyurethane dispersions (BPUD) were synthesized by prepolymer mixing process using toluene 2,4-diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI), poly(tetramethylene glycol) (PTMG), dimethylol propionic acid (DMPA), and methyl ethyl ketoxime (MEKO). The particle size, viscosity, pH and storage stability of the BPUDs were studied and compared. The aqueous dispersions were characterized by FT-IR, GPC, DSC and TGA techniques. De-blocking temperatures of the BPUDs were measured and end-capped with phenylamino propyl trimethoxysilane (PAPTMS) at different de-blocking temperatures. The thermal analysis revealed that both MDI- and TDI-based BPUDs started to de-block at about 60–85 °C. The average molecular weights of the MDI-BPUDs were higher than that of the TDI-BPUDs due to the high reactivity of MDI. It was noticed that the tensile strength increased and elongation at break decreased in the silylated BPUD compared to pure BPUDs, which confirmed that the BPUDs were de-blocked and end-capped with PAPTMS. The T_g values of the silylated BPUD were higher than the BPUD and pure PTMG as well as thermal stability. Storage stability results showed that all BPUDs containing PAPTMS were stable. Water and xylene resistance tests and gel content studies confirmed that silylated PU cross-linked well after silylation of blocked PUDs.     

Finding a Guard that Sees Most and a Shop that Sells Most

We present a near-quadratic time algorithm that computes a point inside a simple polygon P in the plane having approximately the largest visibility polygon inside P, and a near-linear time algorithm for finding the point that will have approximately the largest Voronoi region when added to an n-point set in the plane. We apply the same technique to find the translation that approximately maximizes the area of intersection of two polygonal regions in near-quadratic time, and the rigid motion doing so in near-cubic time.