Shell-model analysis of helion and triton inelastic scattering from90Zr including core polarization effects

A microscopic analysis of the inelastic scattering of helions and tritons from⁹⁰Zr is performed using the shell-model wavefunctions and phenomenological effective interactions. Effects of the core polarization are studied assuming a collective model for the core of closed shells. Core coupling parametersA _L , are obtained from the measured electromagnetic transition rate,B(EL). For those transitions for which no meauredB(EL) values are available,A _L values which fit the data have been extracted. The radial forms of both the Gaussian and Yukawa interactions were studied. Effects of the non-locality of the bound states on the predicted cross sections are found to be small. As in the case of proton inelastic scattering, it is found that the valence (direct) contributions are small and the transitions are dominated by core polarization. The strength of the effective interaction required to fit the data is reduced by a factor of two or more when the core polarization contributions are included.     

Adatom dipole moments on metals and their interactions

An expression is derived for the dipole moment μ, associated with an arbitrary adatom on an arbitrary metal surface, in terms of the differential work done in moving the adatom from the interior of the metal to its actual position in the presence and absence, respectively, of small, asymptotically uniform electric field on the exterior of the metal. With the aid of this expression it is shown that the non-oscillatory part of the interaction energy of two dipoles μ_A and μ_B, situated on the surface and separated by a large distance v, is given by U_AB = 2μ_Aμ_Bv^?3. This exceeds by a factor of 2 the interaction energy of two dipoles in the same relative configuration but in a vacuum.     

Magnetic properties of the mixed spin-5/2 and spin-3/2 Blume-Capel Ising system on the two-fold Cayley tree

We use exact recursion relations to study the magnetic properties of the half-integer mixed spin-5/2 and spin-3/2 Blume-Capel Ising ferromagnetic system on the two-fold Cayley tree that consists of two sublattices A and B. Two positive crystal-field interactions Δ₁ and Δ₂ are considered for the sublattice with spin-5/2 and spin-3/2 respectively. For different coordination numbers q of the Cayley tree sites, the phase diagrams of the model are presented with a special emphasis on the case q = 3, since other values of q reproduce similar results. First, the T = 0 phase diagram is illustrated in the (D _A = Δ₁/J,D _B = Δ₂/J) plane of reduced crystal-field interactions. This diagram shows triple points and coexistence lines between thermodynamically stable phases. Secondly, the thermal variation of the magnetization belonging to each sublattice for some coordination numbers q are investigated as well as the Helmoltz free energy of the system. First-order and second-order phase transitions are found. The second-order phase transitions become sharper and sharper when D _A or D _B increases. The first-order transitions only exist for some appropriate non-zero values of D _A and/or D _B . The corresponding transition lines never connect to the second-order transition lines. Thus, the non-existence of tricritical points remains one of the key features of the present model. The magnetic exponent β ₀ of the model is estimated and found to be ¼ at small values of D _A = D _B = D and β ₀ = ½ at large values of D. At intermediate values of D, there is a crossover region where the magnetic exponent displays interesting behaviours.     

星形两嵌段共聚高分子薄膜微相分离的耗散粒子动力学模拟

利用耗散粒子动力学模拟研究了两类典型的星形两嵌段共聚高分子薄膜的微相分离行为. 结果表明,(A_x)₄(B_y)₄体系的相图较(A_xB_y)₄体系具有明显的对称性,且前者比后者更易发生相分离,与体相中的情况相近,并可归因于两类星形高分子在结构上的差异;组成相同的星形共聚高分子体系在体相与薄膜中所形成的介观结构之间存在对应关系;减小薄膜厚度与加大A-B组分间排斥强度均有利于增强所形成结构的有序性. 同样是缘于分子结构差异,两类星形高分 子薄膜中的均方回旋半径随体系组分分率的变化规律几乎相反.     

How chaotic is a state of a quantum system?

A concept related to the entropy is studied. Let A and B be two density matrices, with eigenvalues a₁, a₂,… and b₁, b₂,…, arranged in decreasing order and repeated according to multiplicity. Then A is said to be “more mixed”, or “more chaotic”, than B, if a₁?b₁, a₁+a₂?b₁+b₂,…,a₁+…+a_m?b₁+…+b_m,…; It turns out that if A is more mixed than B, then the entropy of A is larger than the entropy of B. However, more generally, let v be an arbitrary concave function, ?0, and vanishing at 0. Then, if A is more mixed than B, trv(A)?trv(B). It is shown that also the converse is true. Furthermore, a variety of other characterizations of the relation “A is more mixed than B” is obtained, and several applications to quantum statistical mechanics are given.     

Ecological interaction and phylogeny, studying functionality on composed networks

We study a class of composed networks that are formed by two tree networks, T_P and T_A, whose end points touch each other through a bipartite network B_PA. We explore this network using a functional approach. We are interested in how much the topology, or the structure, of T_X (X=A or P) determines the links of B_PA. This composed structure is a useful model in evolutionary biology, where T_P and T_A are the phylogenetic trees of plants and animals that interact in an ecological community. We make use of ecological networks of dispersion of fruits, which are formed by frugivorous animals and plants with fruits; the animals, usually birds, eat fruits and disperse their seeds. We analyse how the phylogeny of T_X determines or is correlated with B_PA using a Monte Carlo approach. We use the phylogenetic distance among elements that interact with a given species to construct an index κ that quantifies the influence of T_X over B_PA. The algorithm is based on the assumption that interaction matrices that follows a phylogeny of T_X have a total phylogenetic distance smaller than the average distance of an ensemble of Monte Carlo realisations. We find that the effect of phylogeny of animal species is more pronounced in the ecological matrix than plant phylogeny.     

Prediction of amorphous alloy formation by ion beam mixing

In order to predict the glass forming ability of binary alloys produced by ion beam mixing, a two dimensional map is constructed. The two coordinates of this map are the ratio of atomic radii r_A/r_B, and the heat of formation ΔH of an equiatomic compound, calculated using Miedema's model. In this map, binary systems which form in the amorphous phase are clearly separated from the others. The same map is valid both for metal-metal and metal-semiconductor binaries.     

The Mössbauer effect and magnetic phase transitions

The Mössbauer effect provides a direct method for identifying the spin axis in magnetic crystals and observing magnetic phase transitions. The order of the transition may be inferred from the Mössbauer spectrum. Phase changes can occur as a function of temperature (e.g. when the anisotropy fieldB _A changes sign) or as a function of applied magnetic field. In an antiferromagnet a field ?(2B _E B _A)^1/2 along the spin axis whereB _E is the exchange field causes the spin-flop transition which is normally first order (sharp) whereas the transition to the paramagnetic phase which occurs at higher fields?2B _E is second order (continuous). In quasi-one-dimensional crystals Mössbauer spectra show that the spin-flop transition is first order locally but occurs over a range of fields throughout the crystal, so that the first order character is masked in a conventional magnetization measurement. In fields applied at a finite angle>B _A/2B _E to the spin axis the transition becomes second order, i.e. a continuous rotation of the spins occurs. In canted antiferromagnets (or weak ferromagnets) the spin-flop transition is also continuous; in addition a “screw” re-orientation may be induced by fields applied perpendicular to the spin axis and arises from antisymmetric exchange. For crystals with lowT _N the hyperfine field changes when a magnetic field is applied and has a minimum at a phase transition; this may be used to map out the magnetic phase diagram.     

Phase diagram calculations in glass-forming systems—I: Theory

The theory of regular associated solutions is assumed to deal with the formation of molecular species or chemical complexes of general formula A_pB_q, in a liquid phase. Two approximate treatments are developed, depending on the association strength, in order to treat binary and ternary systems. The solid-liquid equilibrium equations for a ternary system containing A_pB_q and A_rC_s complexes are calculated. The success of these treatments for glass-forming liquids is discussed.     

Superconducting critical temperature, Tc, and phase equilibrium diagram of A3B β-W) type compounds

A correlation between the superconducting critical temperature, T_c, and solid solution range (SSR) appearing in the phase equilibrium diagram of A₃B (β-W) type compounds has been observed. Among the compounds of incongruent melting (peritectic or peritectoid) type, the T_c is directly proportional to the solid solution range. Further, in the low temperature range, the solid solution range invariably is found on the A element-rich side of the stoichiometric composition (75 at. % A: 25 at. % B), suggesting a fundamental importance in the “A-chain integrity,” viz., an A element can substitute for a B element which is not a part of the chain, but a B element cannot replace an A element which forms a part of the chain) which is believed to be a key to superconductivity.     

The equilibrium states of A1?xB1+x binary alloys in the hard-sphere and pair-binding model

The equilibrium ordering states of A _1−x B _1+x alloys are investigated analytically and by computer simulation with the hard-sphere and pair-binding model. The alloy energies are calculated for the case where an excess of the B component forms a solid solution or precipitates as pure phase particles. Criteria for pair-binding energies are obtained that define the type of the resulting alloy. The analytical results are presented for an alloy of arbitrary dimensionality and for any number of coordination spheres involved in interatomic interactions. Illustrating examples are given for B2 and L1₀ superstructures and for a two-dimensional square lattice in view of the atomic interaction in the first two coordination spheres.     

Theory of chemical equilibrium in a lattice

The chemical equilibrium is studied for the reactionA+B?C, assuming that, initially, the particlesB form a lattice and the particlesA are statistically distributed on interstices. A mass action law is derived which defines the numbersn _A, n_B, n_C of particlesA, B,C in the chemical equilibrium assuming the initial distribution to be known. It predicts a considerably larger numbern _C of fused particlesC compared to the mass action law for the gaseous phase. The result holds for an ordinary as well as for a nuclear lattice. Its possible relevance for the production of proton-rich isotopes in the universe is discussed.     

Structural phase transition in mixed IV–VI compounds

On the basis of a linear chain model and CPA, structural phase transition in mixed IV–VI compound A_χB_1?χTe (where A and B stand for Pb, Sn or Ge) is discussed. It is suggested that observed non-linear nature of T_c?χ relation may be attributed to the change in the electronic state with χ.     

Commitment Versus Persuasion in the Three-Party Constrained Voter Model

In the framework of the three-party constrained voter model, where voters of two radical parties (A and B) interact with “centrists” (C and C _ζ ), we study the competition between a persuasive majority and a committed minority. In this model, A’s and B’s are incompatible voters that can convince centrists or be swayed by them. Here, radical voters are more persuasive than centrists, whose sub-population comprises susceptible agents C and a fraction ζ of centrist zealots C _ζ . Whereas C’s may adopt the opinions A and B with respective rates 1+δ _A and 1+δ _B (with δ _A ≥δ _B >0), C _ζ ’s are committed individuals that always remain centrists. Furthermore, A and B voters can become (susceptible) centrists C with a rate 1. The resulting competition between commitment and persuasion is studied in the mean field limit and for a finite population on a complete graph. At mean field level, there is a continuous transition from a coexistence phase when ζ<Δ _c =δ _A /(1+δ _A ) to a phase where centrism prevails when ζ≥Δ _c . In a finite population of size N, demographic fluctuations lead to centrism consensus and the dynamics is characterized by the mean consensus time τ. Because of the competition between commitment and persuasion, here consensus is reached much slower (ζ<Δ _c ) or faster (ζ≥Δ _c ) than in the absence of zealots (when τ～N). In fact, when ζ<Δ _c and there is an initial minority of centrists, the mean consensus time grows as τ～N ^?1/2 e ^Nγ , with N?1 and . The dynamics is thus characterized by a metastable state where the most persuasive voters and centrists coexist when δ _A >δ _B , whereas all species coexist when δ _A =δ _B . When ζ≥Δ _c and the initial density of centrists is low, one finds τ～lnN (when N?1). Our analytical findings are corroborated by stochastic simulations.     

Systematic Density Expansion of the Lyapunov Exponents for a Two-Dimensional Random Lorentz Gas

We study the Lyapunov exponents of a two-dimensional, random Lorentz gas at low density. The positive Lyapunov exponent may be obtained either by a direct analysis of the dynamics, or by the use of kinetic theory methods. To leading orders in the density of scatterers it is of the form A ₀ñln ñ+B ₀ñ, where A ₀ and B ₀ are known constants and ñ is the number density of scatterers expressed in dimensionless units. In this paper, we find that through order (ñ²), the positive Lyapunov exponent is of the form A ₀ñln ñ+B ₀ñ+A ₁ñ²ln ñ +B ₁ñ². Explicit numerical values of the new constants A ₁ and B ₁ are obtained by means of a systematic analysis. This takes into account, up to O(ñ²), the effects of all possible trajectories in two versions of the model; in one version overlapping scatterer configurations are allowed and in the other they are not.     

Spontaneous breaking of molecular identity and phase diagrams of thermally reversible association systems with alternating molecules

The global phase behavior of a mixture of molecules A _f and B _f, each containing f functional groups of, respectively, types A and B capable of forming thermally reversible chemical bonds, is considered. Contrary to the traditional approach based on the consideration of an infinite cluster of labile bonds that appears in such systems on the Bethe lattice (i.e., in the Cayley tree approximation) under certain conditions, we additionally take into account the contribution to the thermodynamics from the cluster fragments forming mesoscopic cycles. It is shown, within the framework of the suggested mesoscopic cyclization approximation, which is based on the concept of spontaneous breaking of molecular identity upon the formation of an infinite cluster, that this contribution is finite. Phase diagrams are constructed for the systems considered. The presence of a point of equal concentrations, where two liquid phases coexist and one of them contains an infinite cluster of thermally reversible bonds, is the specific feature of the phase diagrams in the approximation suggested.     

Raman scattering from soft phonons in NdP5O14

Raman spectra of NdP₅O₁₄ were recorded between 4.2 and 600 K and analyzed. The monoclinic-orthorhombic phase transition temperature was determined to be 420 K by measuring the energy of an A_g-B_2g and a B_g-B_3g soft optic phonon mode as a function of temperature (symmetries refer to the monoclinic and orthorhombic phase, respectively). Coupling of these optic phonons to the soft acoustical phonon which is associated with the phase transition is discussed.     

Stress-optical studies of VI B transition metal dichalcogenides

The effect of a uniaxial stress on the excitonic optical spectra are studied for MoS₂, WS₂, MoSe₂ and WSe₂. Stress dichroism appears in the A′, B′ excitons in diselenides, while it is absent in the A, B excitons in the four compounds. The A′, B′ excitons shift oppositely to the A, B excitons, indicating that A, B and A′, B′ are not pair excitons split by interlayer interaction.     

The Tc-SSR correlation in A3B(A15-type) compounds

The 28A₃B(A15-type) compounds, for which both the phase equilibrium diagram and the superconducting critical temperature, T_c, are known, can be divided into two groups. The first group consists of compounds whose SSR (solid solution range) is either zero or extends only to the A-rich side of the stoichiometric composition. The second group consists of the compounds whose SSR extends to both sides or only to the B-rich side. The first group of compounds generally has high T_c's and follow the T_c-SSR correlation proposed earlier.Both the grouping and the T_c-SSR correlation are directly relatable to “A-chain integrity” and thus lend further support to the thesis that “A-chain integrity” is of fundamental importance in the superconductivity of these compounds.     

Electronic structure and conduction properties of copolymers of silole based donor-acceptor polymers: effect of multi-neighbour interaction

Using the ab initio band structure results of two novel silole based donor-acceptor polymers PSICF (A)_x and PSICN (B)_x, the electronic structures and conduction properties of their various quasi-one-dimensional superlattices (copolymers) (A_mB_n)_x, belonging to the class of type-II staggered superlattices, have been investigated using negative factor counting method taking into account multi-neighbour interaction. Both PSICF and PSICN consist of a bicyclopentadisilole unit bridged by an electron-accepting group Y (Y=CCF₂ in PSICF and Y=CC(CN)₂ in PSICN). The trends in the electronic structures and conduction properties of these copolymers (A_mB_n)_x as a function of the (i) block sizes m and n; (ii) composition (m/n) and (iii) arrangement of blocks (periodic or aperiodic) in the copolymer chain are discussed. The results obtained are important guidelines for designing copolymers with tailor-made conduction properties.