平面团簇稳定结构的蒙特卡罗树搜索

以平面团簇为例提出了一种结合结构识别和蒙特卡罗树技术搜索稳定结构的新方法.体系原子之间的相互作用由两类模型势能函数来描述:Lennard-Jones二体势函数与基于Lennard-Jones势的三体势函数.考虑可能的三角晶格碎片作为候选结构,引入编号策略对结构进行快速识别,并运用蒙特卡罗树搜索研究稳定结构随着原子数增大的演化过程;对于能量较低的候选结构,进一步采取局域优化来获得对应体系的稳定结构.计算表明,Lennard-Jones二体势函数对应的三角晶格团簇更稳定;在特定的参数下,三体势函数对应的六角晶格团簇更稳定.结合结构识别和蒙特卡罗树搜索可以对候选结构空间进行高效扫描,在较短时间内更容易搜索到稳定的团簇结构,并可以与第一原理计算结合实现材料的结构预测.     

Variational method for the nonlinear dynamics of an elliptic magnetic stagnation line

The nonlinear evolution of the kink instability of a plasma with an elliptic magnetic stagnation line is studied by means of an amplitude expansion of the ideal magnetohydrodynamic equations. Wahlberg et al. [12] have shown that, near marginal stability, the nonlinear evolution of the stability can be described in terms of a two-dimensional potential U(X,Y), where X and Y represent the amplitudes of the perturbations with positive and negative helical polarization. The potential U(X,Y) is found to be nonlinearly stabilizing for all values of the polarization. In our paper a Lagrangian and an invariant variational principle for two coupled nonlinear ordinal differential equations describing the nonlinear evolution of the stagnation line instability with arbitrary polarization are given. Using a trial function in a rectangular box we find the functional integral. The general case for the two box potential can be obtained on the basis of a different ansatz where we approximate the Jost function by polynomials of order n instead of a piecewise linear function. An example for the second order is given to illustrate the general case. Some considerations concerning solar filaments and filament bands (circular or straight) are indicated as possible applications besides laboratory experiments with cusp geometry corresponding to quadripolar cusp geometries for some clouds and thunderstorms.     

Can X(3872) be a JP=2- Tetraquark State?

In this article, we test the nature of X(3872), which is assumed to be a P-wave [cq]-scalar-diquark [cq]-axial-vector-antidiquark tetraquark state with J^P=2^-. The interpolating current representing the J^P=2^- state is proposed. Technically, contributions of the operators up to dimension six are included in the operator product expansion. The mass obtained for such state is m_2-=(4.38? 0.15) GeV. We conclude that it is impossible to describe the X(3872) structure as J^P=2^- tetraquark state.     

A symmetric positive definite formulation for monolithic fluid structure interaction

In this paper we consider a strongly coupled (monolithic) fluid structure interaction framework for incompressible flow, as opposed to a loosely coupled (partitioned) method. This requires solving a single linear system that combines the unknown velocities of the structure with the unknown pressures of the fluid. In our previous work, we were able to obtain a symmetric formulation of this coupled system; however, it was also indefinite, making it more difficult to solve. In fact in practice there have been cases where we have been unable to invert the system. In this paper we take a novel approach that consists of factoring the damping matrix of deformable structures and show that this can be used to obtain a symmetric positive definite system, at least to the extent that the uncoupled systems were symmetric positive definite. We use a traditional MAC grid discretization of the fluid and a fully Lagrangian discretization of the structures for the sake of exposition, noting that our procedure can be generalized to other scenarios. For the special case of rigid bodies, where there are no internal damping forces, we exactly recover the system of Batty et al. (2007) [4].     

长程表面等离子体的增强效应

研究了双层金属薄膜构型中构型参数对长程表面等离子体的影响,并发现了衰减全反射激发方法下长程表面等离子体的增强效应.以特征矩阵算法为基础,通过数值计算构型的反射谱,研究构型参数的变化对反射谱的影响.发现由于衰减全反射激发方法中耦合器的存在导致的非对称特性,会使双层金属薄膜构型中的长程表面等离子体拥有本征模式特性以外的有趣特性,如长程模式得到增强而另一支受到抑制,从而使能量更为集中在希望被激发的一支.研究结果对非对称激发构型中的长程表面等离子体研究具有启发意义.     

Auxiliary-level-assisted operations with charge qubits in semiconductors

We present a new scheme for rotations of a charge qubit associated with a singly ionized pair of donor atoms in a semiconductor host. The logical states of such a qubit proposed recently by Hollenberg et al. [16] are defined by the lowest two energy states of the remaining valence electron localized around one or another donor. We show that an electron located initially at one donor site can be transferred to another donor site via an auxiliary molecular level formed upon the hybridization of the excited states of two donors. The electron transfer is driven by a single resonant microwave pulse in the case where the energies of the lowest donor states coincide or by two resonant pulses in the case where they differ from each other. Depending on the pulse parameters, various one-qubit operations—including the phase gate, the NOT gate, and the Hadamard gate—can be realized in short times. Decoherence of an electron due to the interaction with acoustic phonons is analyzed and shown to be weak enough for coherent qubit manipulation to be possible, at least in proof-of-principle experiments on one-qubit devices.     

Localized structures in convective experiments

In this work we review localized structures appearing in thermo-convective experiments performed in extended (large “aspect ratio”) fluid layers. After a brief general review (not exhaustive), we focus on some results obtained in pure fluids in a Bénard-Marangoni system with non-homogeneous heating where some structures of this kind appear. The experimental results are compared in reference to the most classical observed in binary mixtures experiments or simulations. In the Bénard-Marangoni experiment we present the stability diagram where localized structures appear and the typical situations where these local mechanisms have been studied experimentally. Some new experimental results are also included. The authors want to honor Prof. H. Brand in his 60th. birthday and to thank him for helpful discussions.     

ON THE RELATION BETWEEN COMPLEX MODES AND WAVE PROPAGATION PHENOMENA

This paper discusses the well-known, but often misunderstood, concept of complex modes of dynamic structures. It shows how complex modes can be interpreted in terms of wave propagation phenomena caused by either localized damping or propagation to the surrounding media. Numerical simulation results are presented for different kinds of structures exhibiting modal and wave propagation characteristics: straight beams, an L-shaped beam, and a three-dimensional frame structure. The input/output transfer relations of these structures are obtained using a spectral formulation known as the spectral element method (SEM). With this method, it is straightforward to use infinite elements, usually known as throw-off elements, to represent the propagation to infinity, which is a possible cause of modal complexity. With the SEM model, the exact dynamic behavior of structures can be investigated. The mode complexity of these structures is investigated. It is shown that mode complexity characterizes a behavior that is half-way between purely modal and purely propagative. A coefficient for quantifying mode complexity is introduced. The mode complexity coefficient consists of the correlation coefficient between the real and imaginary parts of the eigenvector, or of the operational deflection shape (ODS). It is shown that, far from discontinuities, this coefficient is zero in the case of pure wave propagation in which case the plot of the ODS in the complex plane is a perfect circle. In the other extreme situation, a finite structure without damping (or with proportional damping), where the mode shape (or the ODS) is a straight line on the complex plane, has a unitary complexity coefficient. For simple beam structures, it is shown that the mode complexity factor can also be calculated by curve-fitting the mode to an ellipse and computing the ratio of its radii.     

Active control of forced harmonic vibration in finite degree of freedom structures with negligible natural damping

Methods of synthesis for vibration controllers are presented for mechanical structures where the number of actuators equals the number of modes to be controlled and for structures where the number of modes exceeds the number of actuators. Provided an observer can be constructed complete isolation from the disturbance will be possible for the first case even when the dynamics of the structure are not well defined. In the second case complete isolation is not in general possible but a considerable reduction of strain energy can still be achieved. Possible practical limitations are discussed.     

电子结构方法中带结构特征值问题的统一视角

在(相对论)电子结构方法中,四元数矩阵特征值问题和计算激发能的线性响应(Bethe-Salpeter)特征值问题是两个经常出现的结构特征值问题. 尽管前一个问题已被十分仔细地研究,后一个问题在一般形式下,即不假设电子Hessian正定性的复矩阵情况,并没有得到完全的理解. 鉴于它们非常相似的数学结构,本文从一个统一的角度研究了这两个问题,揭示了它们特征向量的“李群”结构,为将来设计对角化算法和数值优化方法提供了一个统一的框架. 利用和处理四元数矩阵特征值问题相同的归约算法,本文给出了表征线性响应问题特征值(实数、纯虚、或复数)的充分必要条件. 这一结果可以看作是实矩阵情况下已知条件的自然推广.     

Multi-gradient pulse investigations of fluid transport in porous media.

NMR pulsed field gradient (PFG) experiments employing the application of n gradient pulses k(1) ellipsis k(n) are discussed in a general way as an n-fold encoding of position at successive times. The experiments are then represented by a sampling of n-dimensional k-space, K(n). Various parameters of motion can be derived from the evolution of correlations within the n-dimensional position (r-)space, R(n), which is the Fourier conjugate space to K(n). A wide class of NMR experiments may be described by this formalism, where the dimension of the experiment is often reduced by imposing conditions to the free variables. This is demonstrated for the case of displacement measurements where the condition summation operatork(i) = 0 is met. The two simplest pulse sequences which allow one to correlate displacements at two different times with each other are presented. While the three-pulse version of SERPENT encodes displacements in two interleaved time intervals Delta(1) and Delta(2), the four-pulse VEXSY experiment includes a mixing time tau(m) in between both encoding intervals Delta. The behaviour of fluid transport subject to external pressure through a model porous system is demonstrated by means of numerical simulations of SERPENT and VEXSY experiments for water flowing through a packed bed of monosized spherical particles. Displacements parallel (Z) and perpendicular (X) to the main flow direction are determined and the 2-D joint probability densities and the conditional probabilities are discussed along with the correlation coefficients related to the displacements at different encoding times. It is shown that all possible correlations between Z and X(2) in VEXSY decay with time constants comparable to the average time needed for a fluid molecule to cover one bead diameter, while a negative correlation is observed between transverse (X) displacements which is explained by molecules flowing along streamlines which follow the circumference of the spherical particles. Correlations for displacements during the different times in SERPENT generally decay much slower and provide complementary information about the evolution of displacements with time.     

Spray diagnostics using an all-solid-state Nd:YAlO<Subscript>3</Subscript> laser and fluorescence tracers in commercial gasoline and diesel fuels

A new diode-pumped Nd:YAlO₃ laser system emitting pulse bursts at 671 nm has made it possible to apply tracer-laser-induced-fluorescence (LIF) techniques for spray diagnostics not only by using model fuels but in real diesel or gasoline fuel sprays. In this work we characterize possible candidates for LIF tracers that can be excited in the wavelength region of 650–680 nm where commercial diesel fuel is transparent. Two fluorescent dyes, Rhodamine 800 and Atto 680, were identified as possible tracers and tested for their relative fluorescence quantum yield and absorption cross section in a diesel fuel environment as well as their behavior at different temperatures. First results of laser-dropsizing experiments with Rhodamine 800 as a fluorescence tracer are presented. PACS 42.62.Fi; 32.50.+d; 42.62.Cf     

Microscopic field in nanocrystalline ferromagnetic metal films and the opportunity for studying it by the μSR method

The relation of the microscopic (local) field in nanocrystalline ferromagnetic metal films to macroscopic characteristics (the external magnetic field, average magnetization, saturation magnetization) is determined for the case where a nanocrystalline ferromagnetic film consists of crystallographically ordered grains separated by disordered regions and where the dimensions of grains along a normal to the film plane are much smaller than those in the film plane. In the case of a strong external field (? ? M), the magnetization direction is determined in grains in the form of oblate ellipsoids for metals with uniaxial or cubic magnetocrystalline anisotropy. Expressions are derived for the spin polarization of an ensemble of rapidly diffusing and nondiffusing muons in nanocrystalline ferromagnetic films. It is shown that experiments with “slow” positive muons make it possible to measure all parameters of such structures and to obtain important information for studying phase transition physics.     

Non-standard Hamiltonian effects on neutrino oscillations

We investigate non-standard Hamiltonian effects on neutrino oscillations, which are effective additional contributions to the vacuum or matter Hamiltonian. Since these effects can enter in either the flavor or mass basis, we develop an understanding of the difference between these bases representing the underlying theoretical model. In particular, the simplest of these effects are classified as “pure” flavor or mass effects, where the appearance of such a “pure” effect can be quite plausible as a leading non-standard contribution from theoretical models. Compared to earlier studies investigating particular effects, we aim for a top–down classification of a possible “new physics” signature at future long-baseline neutrino oscillation precision experiments. We develop a general framework for such effects with two neutrino flavors and discuss the extension to three neutrino flavors, and we demonstrate the challenges for a neutrino factory to distinguish the theoretical origin of these effects with a numerical example as well. We find how the precision measurements of neutrino oscillation parameters can be altered by non-standard effects alone (not including non-standard interactions in the creation and detection processes) and that the non-standard effects on Hamiltonian level can be distinguished from other non-standard effects (such as neutrino decoherence and decay) if we consider the specific imprint of the effects on the energy spectra of several different oscillation channels at a neutrino factory.     

Applications of focused ion beam for preparation of specimens of ancient ceramic for electron microscopy and synchrotron X-ray studies

In this paper the capabilities of FIB systems as a tool for TEM studies of ancient pottery are explored, especially when the amount of available material is very limited and when, for instance, there is stringent demand for very accurate location of the electron-transparent area as is the case for investigation of outer surface layers, such as slips and patinas. The advantages of the two main FIB milling techniques (H-bar and Lift-out) are discussed in detail and illustrated through the study of metallic lustre decorations and a particular type of Roman Terra Sigillata coating. The H-bar technique is ideal for investigations where the features of interest are near the edges of a ceramic fragment. A significantly large area of surface decoration can be studied without any restriction on the size and the shape of fragment. On the other hand, the Lift-out technique is very powerful for extracting TEM membranes far from the edges. An added advantage of this technique is that the thickness of the foil is very uniform and that allows large tilts and makes it possible to obtain electron diffraction patterns of several zones axes from the same crystal, making crystallographic phase identification easier and precise, and identification of complex structures possible. We also show that the FIB system can be used to deposit very precise registration marks, allowing an experimenter to correlate results from TEM measurements with other complementary techniques, such as synchrotron based microdiffraction and μXANES. Combination of these complementary techniques is becoming a very powerful approach to probe the chemical and morphological microstructure of heterogeneous and complex material from the nanometre to millimetre scale.     

The Random Plots Graph Generation Model for Studying Systems with Unknown Connection Structures

We consider the problem of modeling complex systems where little or nothing is known about the structure of the connections between the elements. In particular, when such systems are to be modeled by graphs, it is unclear what vertex degree distributions these graphs should have. We propose that, instead of attempting to guess the appropriate degree distribution for a poorly understood system, one should model the system via a set of sample graphs whose degree distributions cover a representative range of possibilities and account for a variety of possible connection structures. To construct such a representative set of graphs, we propose a new random graph generator, Random Plots, in which we (1) generate a diversified set of vertex degree distributions and (2) target a graph generator at each of the constructed distributions, one-by-one, to obtain the ensemble of graphs. To assess the diversity of the resulting ensembles, we (1) substantialize the vague notion of diversity in a graph ensemble as the diversity of the numeral characteristics of the graphs within this ensemble and (2) compare such formalized diversity for the proposed model with that of three other common models (Erdős–Rényi–Gilbert (ERG), scale-free, and small-world). Computational experiments show that, in most cases, our approach produces more diverse sets of graphs compared with the three other models, including the entropy-maximizing ERG. The corresponding Python code is available at GitHub.     

Ballistic electron transport in AlAs quantum wells

We report the observation of commensurability oscillations in an AlAs two-dimensional electron system where two conduction-band valleys with elliptical in-plane Fermi contours are occupied. The Fourier power spectrum of the oscillations shows two frequency components consistent with those expected for the Fermi contours of the two valleys. From an analysis of the spectra we deduce m(l)/m(t)=5.2+/-0.5 for the ratio of the longitudinal and transverse electron effective masses, a fundamental parameter that cannot be directly measured from other transport experiments.     

The influence of contacts on the VA-characteristics of thin-layfr systems with amorphous semiconductors

In the paper the VA-characteristics of a sandwich-type thin-layer system with amorphous semiconductor are discussed, where ohmic and non-ohmic contacts are considered. The VA-characteristics are derived which, in the case of ohmic contacts, lead to a linear dependence of the current on voltage and, in the case of non-ohmic contacts (where barriers are supposed), to a nonlinear dependence, so that VA-characteristics with negative resistance are possible as well. These VA-characteristics are compared with the experimental ones, measured on thin-layer metalamorphous Si-metal and metal-amorphous CdTe-metal structures.     

Study of the structures of four-quark states in terms of the Born-Oppenheimer approximation

In this work,we use the Born-Oppenheimer approximation,where the potential between atoms can be approximated as a function of distance between the two nuclei,to study the four-quark bound states.By this approximation,Heitler and London calculated the spectrum of the hydrogen molecule,which includes two protons(heavy)and two electrons(light).Generally,the observed exotic mesons Zb(10610),Zb(10650),Zc(3900)and Zc(4020)(Zc(4025))may be molecular states made of two physical mesons and/or diquark-anti-diquark structures.Analogous to the Heitler-London method for calculating the mass of the hydrogen molecule,we investigate whether there exist energy minima for these two structures.Contrary to the hydrogen molecule case where only the spin-triplet possesses an energy minimum,there exist minima for both of these states.This implies that both molecule and tetraquark states can be stable objects.Since they have the same quantum numbers,however,the two states may mix to result in the physical states.A consequence would be that partner exotic states co-existing with Zb(10610),Zb(10650),Zc(3900)and Zc(4020)(Zc(4025))are predicted and should be experimentally observed.