Continuum modeling of the equilibrium and stability of animal flocks

Groups of animals often tend to arrange themselves in flocks that have characteristic spatial attributes and temporal dynamics. Using a dynamic continuum model for a flock of individuals, we find equilibria of finite spatial extent where the density goes continuously to zero at a well-defined flock edge, and we discuss conditions on the model that allow for such solutions. We also demonstrate conditions under which, as the flock size increases, the interior density in our equilibria tends to an approximately uniform value. Motivated by observations of starling flocks that are relatively thin in a direction transverse to the direction of flight, we investigate the stability of infinite, planar-sheet flock equilibria. We find that long-wavelength perturbations along the sheet are unstable for the class of models that we investigate. This has the conjectured consequence that sheet-like flocks of arbitrarily large transverse extent relative to their thickness do not occur. However, we also show that our model admits approximately sheet-like, ‘pancake-shaped’, three-dimensional ellipsoidal equilibria with definite aspect ratios (transverse length-scale to flock thickness) determined by anisotropic perceptual/response characteristics of the flocking individuals, and we argue that these pancake-like equilibria are stable to the previously mentioned sheet instability.     

Amorphization of oxides under irradiation with fast neutrons

A variant of the solid-state radiation amorphization as a result of accumulation of the critical concentration of defects in the crystal has been considered using the example of oxides with the garnet and perovskite structures irradiated by fast neutrons. It has been shown that such defects can be antisite defects, the formation of which leads to considerable static displacements from the equilibrium sites of nearest ions and, consequently, to the loss of stability of the crystalline structure. The dependences of the root-mean-square displacements of oxygen ions on the concentration of the antisite defects are constructed based on the analysis of the experimental data. It has been established that the so-called critical concentrations of antisite defects, at which the spontaneous amorphization occurs, differ for oxides with the garnet and perovskite structures. As the criterion of the spontaneous radiation amorphization, it is proposed to consider the critical static displacement of the ions, which is identical for studied oxides and equal to ～0.28 Å, or ～0.14 in fractions of interatomic distances, which is close to the well-known Lindemann melting criterion.     

Pattern formation in a fractional reaction–diffusion system

We investigate pattern formation in a fractional reaction–diffusion system. By the method of computer simulation of the model of excitable media with cubic nonlinearity we are able to show structure formation in the system with time and space fractional derivatives. We further compare the patterns obtained by computer simulation with those obtained by simulation of the similar system without fractional derivatives. As a result, we are able to show that nonlinearity plays the main role in structure formation and fractional derivative terms change the transient dynamics. So, when the order of time derivative increases and approaches the value of 1.5, the special structure formation switches to homogeneous oscillations. In the case of space fractional derivatives, the decrease of the order of these derivatives leads to more contrast dissipative structures. The variational principle is used to find the approximate solution of such fractional reaction–diffusion model. In addition, we provide a detailed analysis of the characteristic dissipative structures in the system under consideration.     

Formation of extended straight molecular chains by pairing of thymine molecules on the Ag–Si(111) surface

Here, we report on the assembly of thymine molecules into extended straight chains of single and multiple dimer rows on a Si(111)/Ag R30° surface. Using variable temperature scanning tunneling microscopy, we follow the nucleation process and formation of self-assembled structures. Submonolayer coverages at 120 K are disordered and exhibit a high density of thymine dimers. Upon annealing the dimers assemble into extended dimer chains along three equivalent high-symmetry surface directions. At low coverages single dimer rows are favored. At increased coverage chains with multiple dimer rows are observed, with a preference to an even multiplicity. We show that a complex cross-talk between H-bond thymine–thymine interactions and commensurization of dimer chains to the Ag/Si surface leads to this specific layout.     

Estimation of dynamic structural displacements using fiber Bragg grating strain sensors

This study investigates dynamic structural displacements estimation using displacement-strain relationship and measured strain data. Strain signals are obtained from fiber Bragg grating (FBG) sensors that have an excellent multiplexing ability. Vibration experiments were performed with aluminum and acryl beam specimens. The beam structures were subjected to various loading conditions, and deformed shapes were reconstructed by using strain signals. The estimated displacements show good agreements with those measured directly from laser displacement sensors. In present study, it is confirmed structural displacements can be estimated using strain data without displacement measurement.     

Tunable complex photonic chiral lattices by reconfigurable optical phase engineering

We present a novel single-step fabrication approach, based on optical phase engineering, for tunable complex photonic chiral lattices of diverse geometries in a large area. By means of engineered reconfigurable phase patterns, we computationally simulate as well as experimentally investigate these complex structures. We show the generation of both periodic right- and left-handed chiral structures as well as photonic transversely quasi-crystallographic chiral structures. These complex chiral lattices are also demonstrated in a photorefractive material, and the lattice formation is analyzed by plane-wave-guided imaging as well as diffraction pattern imaging. Furthermore, complex photonic chiral structures with engineered tunable relative phase shifts between adjacent spiral units realizable in a single step are explored and analyzed.     

Recording receptance of flat plates by holographic interferometry

In the study of mechanical vibrations of structures, the complex ratio of the displacement of the structure at a point divided by the force input to the structure at the same point or a different point is called receptance. This ratio is usually recorded in magnitude and phase by using piezoelectric transducers attached to the structure. This paper describes a study of the feasibility of recording the displacement of a square cantilevered plate not just at one point but over the entire plate surface, by using holographic interferometry. For purposes of comparison of the two techniques a piezoelectric transducer was attached to a free corner of the plate and the displacements at that point were recorded over the frequency range of interest. Although the dynamic range of the holographic technique was found to be somewhat limited, it was found that it is quite feasible to calculate at least the magnitude of the receptance by recording displacements holographically. Since the holographic fringe formation is an integrated effect in time, it was not possible to record the phase by holographic means. The displacements recorded at the corner of the plate by the two methods agreed very well.     

Gregarious versus individualistic behavior in Vicsek swarms and the onset of first-order phase transitions

The standard Vicsek model (SVM) is a minimal non-equilibrium model of self-propelled particles that appears to capture the essential ingredients of critical flocking phenomena. In the SVM, particles tend to align with each other and form ordered flocks of collective motion; however, perturbations controlled by a noise term lead to a noise-driven continuous order–disorder phase transition. In this work, we extend the SVM by introducing a parameter α$\alpha$ that allows particles to be individualistic instead of gregarious, i.e. to choose a direction of motion independently of their neighbors. By focusing on the small-noise regime, we show that a relatively small probability of individualistic motion (around 10%$10\%$) is sufficient to drive the system from a Vicsek-like ordered phase to a disordered phase. Despite the fact that the α$\alpha$-extended model preserves the O(n)$O\left(n\right)$ symmetry and the interaction range, as well as the dimensionality of the underlying SVM, this novel phase transition is found to be discontinuous (first order), an intriguing manifestation of the richness of the non-equilibrium flocking/swarming phenomenon.     

On the geometric structure of nonstationary multisoliton vacuum metrics with real pole trajectories on a general background

We present an analysis of the geometric structure of nonstationary multisoliton vacuum metrics obtained from a general background using the inverse scattering method of Belinsky and Zakharov. In the case of real pole trajectories, in general the algorithm leads, for ann-soliton metric, ton+1 disjoint coordinate patches. We show, by an explicit construction, that each coordinate patch can be smoothly extended to a separate spacetime manifold. As a result we find that there are no shock fronts associated with the pole trajectories and that, contrasting with the situation for pairs of complex pole, there are no structures that can be easily identified with the region of interaction of a given number of solitons.CONICET research member.     

A Drive towards Thermodynamic Efficiency for Dissipative Structures in Chemical Reaction Networks

Dissipative accounts of structure formation show that the self-organisation of complex structures is thermodynamically favoured, whenever these structures dissipate free energy that could not be accessed otherwise. These structures therefore open transition channels for the state of the universe to move from a frustrated, metastable state to another metastable state of higher entropy. However, these accounts apply as well to relatively simple, dissipative systems, such as convection cells, hurricanes, candle flames, lightning strikes, or mechanical cracks, as they do to complex biological systems. Conversely, interesting computational properties—that characterize complex biological systems, such as efficient, predictive representations of environmental dynamics—can be linked to the thermodynamic efficiency of underlying physical processes. However, the potential mechanisms that underwrite the selection of dissipative structures with thermodynamically efficient subprocesses is not completely understood. We address these mechanisms by explaining how bifurcation-based, work-harvesting processes—required to sustain complex dissipative structures—might be driven towards thermodynamic efficiency. We first demonstrate a simple mechanism that leads to self-selection of efficient dissipative structures in a stochastic chemical reaction network, when the dissipated driving chemical potential difference is decreased. We then discuss how such a drive can emerge naturally in a hierarchy of self-similar dissipative structures, each feeding on the dissipative structures of a previous level, when moving away from the initial, driving disequilibrium.     

基于单细胞拉曼技术鉴定非结核分枝杆菌的方法研究

非结核分枝杆菌（NTM）是除结核分枝杆菌复合群（MTC）和麻风分支杆菌以外的分枝杆菌总称。近年来NTM导致人类感染的发病率不断上升,其感染的临床症状与MTC感染极为相似,但两者治疗方案却存在差异,临床亟须快速、准确的鉴定方法用于诊断NTM感染。单细胞拉曼光谱技术（SCRS）具有非标记、免培养、快速、准确、低成本等优势。据此,我们提出了一种基于显微共聚焦单细胞拉曼光谱技术鉴定NTM的方法。通过对临床常见的六种NTM（脓肿分枝杆菌、戈登分枝杆菌、偶发分枝杆菌、土分枝杆菌、鸟分枝杆菌以及堪萨斯分枝杆菌）的拉曼光谱进行处理比较,并结合峰位注释进行分析。采用无监督低维可视化的t-分布式随机邻域嵌入方法展示六种NTM的拉曼数据结构,证明其数据在低维空间上的可分性后,比较分类中常用的六种分类器[支持向量机分析(SVM)、K最近邻分类算法(KNN)、偏最小二乘判别分析(PLS-DA)、随机森林(RF)、线性判别分析(LDA)、XG Boost]的效果。SVM和LDA在NTM分类中效果最好,分别达到了99.4%和98.8%的测试准确率;SVM仅对于堪萨斯分枝杆菌（97.96%,48/49）的分类准确性略低,其余均为100%;LDA对于脓肿分枝杆菌（95.65%,22/23）和戈登分枝杆菌（96.30%,26/27）,其余也均为100%。因此,单细胞拉曼检测结合SVM分类器为NTM快速准确鉴定提供了富有潜力的新工具。     

Structure and polarization in the high Tc ferroelectric Bi(Zn,Ti)O3-PbTiO3 solid solutions

Theoretical ab initio and experimental methods are used to investigate the [Bi(Zn1/2Ti1/2)O3]x[PbTiO3]1-x solid solution. We find that hybridization between Zn 4s and 4p and O 2p orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to unusually large polarization, strong tetragonality, and an elevated ferroelectric to paraelectric phase transition temperature.     

Nanoscale self-organization using standing surface acoustic waves

The diffusion of an adatom on a substrate submitted to a standing surface acoustic wave is theoretically studied. By performing large scale molecular dynamic simulations, we show that the wave dynamically structures the substrate by encouraging the presence of the adatom in the vicinity of the maximum displacements of the substrate. Using an analytical model, we explain this feature introducing an effective potential induced by the wave. Applied in an atomic deposition experiment, this dynamic structuring process should govern the nucleation sites distribution opening the route to accurately control the self-organization process at the nanoscale.     

Predictions of pulsed field gradient NMR echo-decays for molecules diffusing in various restrictive geometries. Simulations of diffusion propagators based on a finite element method

Pulsed field gradient NMR diffusometry is a promising tool for investigating structures of porous material through determinations of dynamic displacements of molecules in porous systems. A problem with this approach is the lack of closed analytical expressions for echo-decays in anything but idealized pore geometries. We present here an approach based on calculating the appropriate diffusion propagator by means of finite element calculations. The suggested method is quite general, and can be applied to arbitrary porous systems. The protocol for the calculations is outlined and we show results from some different cases: diffusion in confined geometries and in systems that are spatially inhomogeneous with respect to concentration.     

Digital Speckle Photography of Subpixel Displacements of Speckle Structures Based on Analysis of Their Spatial Spectra

We have investigated the capabilities of the method of digital speckle interferometry for determining subpixel displacements of a speckle structure formed by a displaceable or deformable object with a scattering surface. An analysis of spatial spectra of speckle structures makes it possible to perform measurements with a subpixel accuracy and to extend the lower boundary of the range of measurements of displacements of speckle structures to the range of subpixel values. The method is realized on the basis of digital recording of the images of undisplaced and displaced speckle structures, their spatial frequency analysis using numerically specified constant phase shifts, and correlation analysis of spatial spectra of speckle structures. Transformation into the frequency range makes it possible to obtain quantities to be measured with a subpixel accuracy from the shift of the interference-pattern minimum in the diffraction halo by introducing an additional phase shift into the complex spatial spectrum of the speckle structure or from the slope of the linear plot of the function of accumulated phase difference in the field of the complex spatial spectrum of the displaced speckle structure. The capabilities of the method have been investigated in natural experiment.     

Structural transformations in carbon nanoparticles induced by electron irradiation

This paper reviews the electron-irradiation effects in graphitic nanoparticles. Irradiation-induced atomic displacements cause structural defects in graphite lattice forming the basis of carbon nanoparticles such as nanotubes or carbon onions. Defects of the type of non-six-membered rings induce topological alterations of graphene layers. The generation of curvature under electron irradiation leads to the formation of new structures, such as spherical carbon onions or coalescent nanotubes. At high temperatures, the self-compression of carbon onions can promote the nucleation of diamond cores or phase transformations of foreign materials that are encapsulated by onionlike graphitic shells. Under the nonequilibrium conditions of intense irradiation, the phase equilibrium between graphite and diamond can be reversed. It is shown that graphite can be transformed into diamond even if no external pressure is applied. All electron-irradiation and electron-microscopic studies described here were carried out using in situ transmission electron microscopy.     

低维铁电材料研究进展

铁电材料是一类重要的功能材料,铁电元件的小型化、集成化是当今铁电材料发展的一大趋势.但是尺寸效应、表面效应等的存在制约了传统块体铁电材料在纳米尺度下的应用,因而低维度纳米材料中的铁电性能研究成为当前材料科学领域的研究热点之一.本文综述了近年来理论和实验上关于低维铁电材料的探索,包括二维范德瓦耳斯层状铁电材料、共价功能化低维铁电材料、低维钙钛矿材料、外界调控以及二维"铁电金属"等材料的理论预言与实验铁电性的观测;也提出一些物理新机制来解释低维下的铁电性;最后对该领域今后的发展进行了展望.     

A quantum field theory of localized and resonant modes in 3D nonlinear lattices at finite temperatures I

The effective interaction Hamiltonian in 3D nonlinear lattices is established taking into account the repetitions of the up and down transition of an atom between two levels at the same site. The effective interaction Hamiltonian leads to the Heisenberg equation for phonon operators, which yields the conventional dynamical equation for displacements of atoms in 3D nonlinear lattices in the tree approximation by the boson transformation method. Making the one-loop approximation to the nonlinear potential in the Heisenberg equation, we obtain a dynamical equation with a self-consistent potential created by a localized or a resonant mode. In paper II, we show that the dynamical equation yields solutions for localized and resonant modes at finite temperatures.     

Size-dependent effects on electrical contacts to nanotubes and nanowires

Metal-semiconductor contacts play a key role in electronics. Here we show that for quasi-one-dimensional (Q1D) structures such as nanotubes and nanowires, side contact with the metal only leads to weak band realignment, in contrast with bulk metal-semiconductor contacts. Schottky barriers are much reduced compared with the bulk limit, and should facilitate the formation of good contacts. However, the conventional strategy of heavily doping the semiconductor to obtain Ohmic contacts breaks down as the nanowire diameter is reduced. The issue of Fermi level pinning is also discussed, and it is demonstrated that the unique density of states of Q1D structures makes them less sensitive to this effect. Our results agree with recent experimental work, and should apply to a broad range of Q1D materials.     

First-principles study of the electronic and optical properties of the （Y,N）-codoped anatase TiO2 photocatalyst

First-principles plane-wave pseudopotential calculations are performed to study the geometrical structures,formation energies,and electronic and optical properties of Y-doped,N-doped,and(Y,N)-codoped TiO 2.The calculated results show that Y and N codoping leads to lattice distortion,easier separation of photogenerated electron-hole pairs and band gap narrowing.The optical absorption spectra indicate that an obvious red-shift occurs upon Y and N codoping,which enhances visible-light photocatalytic activity.