Phase transitions and phase diagram of the ferroelectric perovskite NBT-BT by anelastic and dielectric measurements

The complex elastic compliance and dielectric susceptibility of (Na_{0.5}Bi_{0.5})_{1-x}Ba_{x}TiO_{3} (NBT-BT) have been measured in the composition range between pure NBT and the morphotropic phase boundary included, 0 <= x <= 0.08. The compliance of NBT presents sharp peaks at the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the tetragonal region of the phase diagram, up to now impossible due to the strong lattice disorder and small distortions and polarizations involved. In spite of ample evidence of disorder and structural heterogeneity, the R-T transition remains sharp up to x = 0.06, whereas the T-C transition merges into the diffuse and relaxor-like transition associated with broad maxima of the dielectric and elastic susceptibilities. An attempt is made at relating the different features in the anelastic and dielectric curves to different modes of octahedral rotations and polar cation shifts. The possibility is also considered that the cation displacements locally have monoclinic symmetry, as for PZT near the morphotropic phase boundary.Comment: 11 pages, 9 figures, submitted to Phys. Rev.

On stress/strain state in a rotating disk

In the framework of mechanics of continuum bodies, the problem of stress/strain state in a high-speed rotating disk of constant thickness has been considered. The material of the disk is assumed to be homogeneous, elastic/perfectly-plastic. In the plastic zone, the stresses and plastic strains are related by some associated law similar to the one employed in deformation theory of plasticity. The general algorithm of the solution covers any smooth plasticity function. At some steps of the algorithm, it is possible to get analytical expressions, particularly, for the quadratic Mises yield criterion. For the given model, the notion of control parameters (external and internal) has been introduced. The allowable boundaries of external parameters have been defined as well. For some states of the disk, the coherent values of external parameters have been obtained. The results are represented graphically to show various states of the disk. The usage of piecewise plasticity functions has been briefly discussed. The results obtained can be used in preliminary engineering design and related numerical codes.info:eu-repo/semantics/publishedVersio

Use of accelerated helium-3 ions for determining oxygen and carbon impurities in some pure materials

Methods are developed for the determination of O impurity in Be and Si carbide and concurrent determination of C and O impurities in Si and W by irradiation with accelerated He-3 ions and subsequent activity measurements of C-11 and F-18 formed from C and O with the aid of a gamma-gamma coincidence spectrometer. Techniques for determining O in Ge and Ga arsenide with radiochemical separation of F-18 are also described

Speeding Strings

There is a class of single trace operators in ${\cal N}=4$ Yang-Mills theory which are related by the AdS/CFT correspondence to classical string solutions. Interesting examples of such solutions corresponding to periodic trajectories of the Neumann system were studied recently. In our paper we study a generalization of these solutions. We consider strings moving with large velocities. We show that the worldsheet of the fast moving string can be considered as a perturbation of the degenerate worldsheet, with the small parameter being the relativistic factor $\sqrt{1-v^2}$. The series expansion in this relativistic factor should correspond to the perturbative expansion in the dual Yang-Mills theory. The operators minimizing the anomalous dimension in the sector with given charges correspond to periodic trajectories in the mechanical system which is closely related to the product of two Neumann systems.Comment: v3: added a reference to the earlier wor

Eocene–early Oligocene climate and vegetation change in southern China: Evidence from the Maoming Basin

Although the Eocene-Oligocene climate transition marks a critical point in the development of the ‘icehouse’ global climate of the present little is known about this important change in the terrestrial realm at low latitudes. Our palynological study of the Shangcun Formation shows it to be early Oligocene in age: palyno-assemblages in the lower part of the formation indicate a cool interval dominated by conifer pollen in the earliest Oligocene followed by a warmer regime in the second half of the early Oligocene. To quantify middle Eocene to late early Oligocene climate conditions at low (~ 20°N) palaeolatitudes in southern Asia several thousand leaf fossil specimens from the Maoming Basin, southern China, were subjected to a multivariate (CLAMP) analysis of leaf form. For terrestrial palaeoclimate comparisons to be valid the palaeoaltitude at which the proxy data are obtained must be known. We find that leaves preserved in the Youganwo (middle Eocene), Huangniuling (late Eocene) and Shangcun (early Oligocene) formations were likely to have been deposited well above sea level at different palaeoelevations. In the Youganwo Formation fine-grained sediments were deposited at an altitude of ~ 1.5 km, after which the basin dropped to ~ 0.5 km by the time the upper Huangniuling sediments were deposited. The basin floor then rose again by 0.5 km reaching an altitude of approximately 1 km in which the Shangcun Formation fine-grained sediments were accumulated. Within the context of these elevation changes the prevailing climates experienced by the Youganwo, Lower Huangniuling, Upper Huangniuling and Shangcun fossil floras were humid subtropical with hot summers and warm winters, but witnessed a progressive increase in rainfall seasonality. By the early Oligocene rainfall seasonality was similar to that of the modern monsoonal climate of Guangdong Province, southern China. All floras show leaf physiognomic spectra most similar to those growing under the influence of the modern Indonesia-Australia Monsoon, but with no evidence of any adaptation to today's South or East Asia Monsoon regimes. The Upper Huangniuling Flora, rich in dipterocarp plant megafossils, grew in the warmest conditions with the highest cold month mean temperature and at the lowest altitude

Folded Three-Spin String Solutions in AdS_5 x S^5

We construct a spinning closed string solution in AdS_5 x S^5 which is folded in the radial direction and has two equal spins in AdS_5 and a spin in S^5. The energy expression of the three-spin solution specified by the folding and winding numbers for the small S^5 spin shows a logarithmic behavior and a one-third power behavior of the large total AdS_5 spin, in the long string and in the short string located near the boundary of AdS_5 respectively. It exhibits the non-regular expansion in the 't Hooft coupling constant, while it takes the regular one when the S^5 spin becomes large.Comment: 14 pages, LaTeX, no figures, a reference adde

The preventive destruction of a hazardous asteroid

One means of countering a hazardous asteroid is discussed: destruction of the object using a nuclear charge. Explosion of such an asteroid shortly before its predicted collision would have catastrophic consequences, with numerous highly radioactive fragments falling onto the Earth. The possibility of exploding the asteroid several years before its impact is also considered. Such an approach is made feasible because the vast majority of hazardous objects pass by the Earth several times before colliding with it. Computations show that, in the 10 years following the explosion, only a negligible number of fragments fall onto the Earth, whose radioactivity has substantially reduced during this time. In most cases, none of these fragments collides with the Earth. Thus, this proposed method for eliminating a threat from space is reasonable in at least two cases: when it is not possible to undergo a soft removal of the object from the collisional path, and to destroy objects that are continually returning to near-Earth space and require multiple removals from hazardous orbits

Two-step recommendations: contrast analysis and matrix factorization techniques

In this paper we present a two-step recommendation model based on Contrast Analysis and Matrix Factorization techniques which mutually complement each other. We also provide a brief overview of different Matrix Factorization approaches.У даній статті представлена двокрокова модель рекомендаційної системи, що використовує взаємодоповнюючим чином техніки контрастного аналізу та матричної факторизації. Також наданий короткий огляд варіацій методу матричної факторизації.В данной статье представлена двушаговая модель рекомендационной системы, которая использует взаимодополняющим образом техники контрастного анализа и матричной факторизации. Также приведен краткий обзор вариаций метода матричной факторизации

Study of Lead-Free Perovskite Solar Cells at Elevated Temperatures and UV Irradiatio

In the present work, the change of the electrical performance is investigated for two lead-free perovskite materials with novel iodide- based and bromide-based compositions under different exploitation conditions, such as light-induced stress, elevated temperatures and ultraviolet light exposure. The charge transport properties are studied in more detail by spectroscopic methods for the cell with the iodide layer due to its greater stability, aiming to understand the degradation mechanism. The results show that this perovskite exhibited excellent stability at UV exposure and acceptable stability at continuous illumination at 600 nm. The device is stable up to 55 °C, when the photovoltage drops. Beyond this threshold temperature, a phase change transition occurs related to traps formation and charge carriers escaping, which affects the photovoltage and it slightly increases

Spinning and rotating strings for N=1 SYM theory and brane constructions

We obtain spinning and rotating closed string solutions in AdS_5 \times T^{1,1} background, and show how these solutions can be mapped onto rotating closed strings embedded in configurations of intersecting branes in type IIA string theory. Then, we discuss spinning closed string solutions in the UV limit of the Klebanov-Tseytlin background, and also properties of classical solutions in the related intersecting brane constructions in the UV limit. We comment on extensions of this analysis to the deformed conifold background, and in the corresponding intersecting brane construction, as well as its relation to the deep IR limit of the Klebanov-Strassler solution. We briefly discuss on the relation between type IIA brane constructions and their related M-theory descriptions, and how solitonic solutions are related in both descriptions.Comment: 35 pages. Dedicated to the memory of Ian I. Kogan. References adde