Connectivity of growing networks with link constraints

We propose a growing network model with link constraint, in which new nodes are continuously introduced into the system and immediately connected to preexisting nodes, and any arbitrary node cannot receive new links when it reaches a maximum number of links k_m. The connectivity of the network model is then investigated by means of the rate equation approach. For the connection kernel A(k)=k^γ, the degree distribution n_k takes a power law if γ≥1 and decays stretched exponentially if 0≤γ< 1. We also consider a network system with the connection kernel A(k)=k^α(k_m-k)^β. It is found that n_k approaches a power law in the α> 1 case and has a stretched exponential decay in the 0≤α< 1 case, while it can take a power law with exponential truncation in the special α=β=1 case. Moreover, n_k may have a U-type structure if α> β.     

Spin dynamics of Rashba-Dresselhaus two-dimensional electron systems with electron-electron interactions

Spin dynamics of Rashba-Dresselhaus two-dimensional electron systems is studied by taking account of electron-electron interactions under the D’yakonov-Perel’ mechanism. The diffusion equations for charge and spin densities are obtained through decoupling of the interactions using the auxiliary Bose field. We show that the electron-electron interaction has no effect on the infinite spin lifetime when the Rashba and Dresselhaus coupling constants satisfy the condition α = ±β. If the general condition α≠±β is satisfied, the spin lifetime is finite and enhanced by the electron-electron interaction with the increment of the temperature in the ballistic regime. The increasing amplitude of the spin lifetime depends on the ratio of the temperature to the Fermi temperature.     

On bound state computations in three- and four-electron atomic systems

A variational approach is developed for bound state calculations in three- and four-electron atomic systems. This approach can be applied to determine, in principle, an arbitrary bound state in three- and four-electron ions and atoms. Our variational wave functions are constructed from four- and five-body Gaussoids that respectively depend on six (r ₁₂, r ₁₃, r ₁₄, r ₂₃, r ₂₄, r ₃₄) and ten (r ₁₂, r ₁₃, r ₁₄, r ₁₅, r ₂₃, r ₂₄, r ₂₅, r ₃₄, r ₃₅ and r ₄₅) relative coordinates. The approach allows operating with the more than one electron spin functions. In particular, the trial wave functions for the ¹ S states in four-electron atomic systems include the two independent spin functions χ₁ = αβαβ + βαβα − βααβ − αββα and χ₂ = 2ααββ + 2ββαα − βααβ − αββα − βαβα − αβαβ. We also discuss the construction of variational wave functions for the excited 2³ S states in four- electron atomic systems.     

Electrical generation of pure spin current with oscillating spin-orbit interaction

We propose an electrical scheme for the generation of a pure spin current without a charge current in a two-terminal device, which consists of a scattering region of a two-dimensional electron gas (2DEG) with Rashba (R) and/or Dresselhaus (S) spin-orbit interaction (SOI) and two normal leads. The SOI is modulated by a time-dependent gate voltage to pump a spin current. Based on a tight-binding model and the Keldysh Green’s function technique, we obtain the analytical expression of the spin current. It is shown that a pure spin current can be pumped out, and its magnitude could be modulated by device parameters such as the oscillating frequency of the SOI, as well as the SOI strength. Moreover, the spin polarisation direction of the spin current could also be tuned by the strength ratio between RSOI and DSOI. Our proposal provides not only a fully electrical means to generate a pure spin current but also a way to control the spin polarisation direction of the generated spin current.     

Weak chaos and metastability in a symplectic system of many long-range-coupled standard maps

We introduce, and numerically study, a system of N symplectically and globally coupled standard maps localized in a d=1 lattice array. The global coupling is modulated through a factor r^-α, being r the distance between maps. Thus, interactions are long-range (nonintegrable) when 0≤α≤1, and short-range (integrable) when α>1. We verify that the largest Lyapunov exponent λ_M scales as λ_M ∝ N^-κ(α), where κ(α) is positive when interactions are long-range, yielding weak chaos in the thermodynamic limit N↦∞ (hence λ_M→0). In the short-range case, κ(α) appears to vanish, and the behaviour corresponds to strong chaos. We show that, for certain values of the control parameters of the system, long-lasting metastable states can be present. Their duration t_c scales as t_c ∝N^β(α), where β(α) appears to be numerically in agreement with the following behavior: β>0 for 0 ≤α< 1, and zero for α≥1. These results are consistent with features typically found in nonextensive statistical mechanics. Moreover, they exhibit strong similarity between the present discrete-time system, and the α-XY Hamiltonian ferromagnetic model.     

Optical characteristics of highly ordered gallium oxide nano-islands on GaAs

A periodic array of Ga oxide islands was obtained by annealing the highly ordered Ga nano-droplets on GaAs surface at 400°C under an oxygen atmosphere for 7 hours. These Ga oxides are a mixture of α-Ga₂O₃ and β-Ga₂O₃ confirmed by Raman spectroscopy study. Enhanced optical transmission of GaAs with such ordered Ga oxide nano-islands was obtained. Both dielectric and dimensional confinement effects were considered in analysis of the electromagnetic characteristics of the nanostructured materials. Finite-difference time-domain method was used to numerically study the light transmission through the patterned Ga oxide on GaAs surface. Based on the calculated results, the light transmission enhancement is attributed to the formation of the ordered nano Ga oxides.     

Modeling the atomic and electronic structure of diamond nanocrystals containing [NV]<Superscript>−</Superscript> centers by the density functional method

We have used the density functional method to model the atomic and electronic structure of diamond nanocrystals passivated by hydrogen atoms and either not containing defects or containing a single [NV]⁻ center. We have shown that in all cases, after relaxation the nanocrystals are formed as diamond-like structures. We have studied the features of the electronic structure of the nanocrystals. We have analyzed in detail the mechanism for the formation of energy levels in the bandgap due to [NV]⁻ centers. We have established that the optical absorption and fluorescence spectra for the [NV]⁻ centers are mainly associated with transitions of electrons between the highest occupied β orbitals (projection of the electron spin equal to +1/2) and lower unoccupied α orbitals (projection of the electron spin equal to −1/2). The results on the localization and energy position of the states in the bandgap match data obtained for the [NV]⁻ center in bulk diamond. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 86–92, January–February, 2007.     

Time-resolved,stress-induced,and anisotropic phase transformation of a piezoelectric polymer

We investigate the time-dependent and anisotropic phase transformation of poly (vinylidene difluoride) (PVDF) under bending. Using combined techniques of an atomic force microscope and a Fourier transform infrared spectroscope, observation of surface morphology and phase transformation in time was made. Results showed that bending stress induces the transformation of amorphous, α,β, and γ crystalline phases. Specifically, the amorphous phase was transformed into the β phase when the bending force was applied. In addition, the transformation observed was time and direction dependent. The anisotropic behavior observed brings insights into the origin of the piezoelectricity of PVDF.     

Two-particle dispersion in model two-dimensional velocity fields

We consider two-particle dispersion in a velocity field, where the relative two-point velocity scales according to v ²(r) ∝r ^α and the corresponding correlation time scales as τ(r) ∝r ^β, and fix α = 2/3, as typical for turbulent flows. We show that two generic types of dispersion behavior arize: For α/2 + β < 1 the correlations in relative velocities decouple and the diffusion approximation holds. In the opposite case, α/2 + β > 1, the relative motion is strongly correlated. The case of Kolmogorov flows corresponds to a marginal, nongeneric situation. In this case, depending on the particular parameters of the flow, the dispersion behavior can be rather diffusive or rather ballistic. Received 13 March 2001     

Geometric entanglement in valance-bond-solid state

Multipartite entanglement, measured by the geometric entanglement (GE), is discussed for integer spin Valance-Bond-Solid (VBS) state respectively with periodic boundary condition (PBC) and open boundary condition (OBC) in this paper. The optimization in the definition of geometric entanglement can be reduced greatly by exploring the symmetry of VBS state, and then the fully separable state can be determined explicitly. Numerical evaluation for GE by the random simulation is also implemented in order to demonstrate the validity of the reductions. Our calculations show that GE is saturated by a finite value with the increment of particle number, that means that the total entanglement for VBS state would be divergent under the thermodynamic limit. Moreover it is found that the scaling behavior of GE with spin number s is fitted as α log(s + β/s + γ)+δ, in which the values of the parameters α, β, γ, σ are only dependent on the parity of spin s. A comparison with entanglement entropy of VBS state is also made, in order to demonstrate the essential differences between multipartite and bipartite entanglement in this model.     

The X-ray fluorescence cross-section for bromide and iodide compounds

The K_α and K_β X-ray fluorescence cross-sections for the bromide and the iodide compounds were measured by a high-resolution Si(Li) X-ray detector. The vacancies were produced by heavily filtered ²⁴¹Am gamma rays. We found that K_α and K_β X-ray fluorescence cross-sections are changed by chemical effect for different Br and I compounds. Experimental results were compared with the calculated values of Br and I elements. Received 15 May 2001 and Received in final form 8 July 2001     

Effect of the ceramic grain size and concentration on?the?dynamical mechanical and dielectric behavior of?poly(vinilidene fluoride) Pb(Zr0.53Ti0.47)O3 composites

In this work, poly(vinilidene fluoride)/Pb(Zr_0.53Ti_0.47)O₃([PVDF]_1−x /[PZT] _x ) composites of volumetric fractions x and (0–3) type connectivity were prepared in the form of thin films. PZT powder of crystallite size of 0.84, 1.68, and 2.35 μm in different amounts of PZT (10, 20, 30, and 40%) was mixed with the polymeric matrix. The crystalline phase of the polymeric matrix was the nonpolar α-phase and the polar β-phase. Dielectric and dynamic mechanical (DMA) measurements were performed to these composites in order to evaluate the influence of particle size and the amount of PZT filler with respect to the PVDF matrix. The inclusion of ceramic particles in the PVDF polymer matrix increases the complex dielectric constant and dynamical mechanical response of the composites. A similar behavior is observed for the α- or β-phase of the polymeric matrix indicating that the PVDF polymer matrix is not particularly relevant for the composite behavior. On the other hand, ceramic size and especially content play the major role in the increase of the dielectric response and the room temperature storage modulus. In particular, the storage modulus increases with increasing PZT concentration, but this increase is more pronounced, in terms of maximum value, for the sample with 2.35 μm particle size; DMA reveals two main relaxations in the analyzed samples. A low-temperature process maximum at ca. −40°C, usually labeled by β or α _a associated to the T _g of the polymer and the α-relaxation at temperatures above 30°C. The β-relaxation is also observed in the dielectric measurements. The models used to asses the dielectric behavior of the samples with increasing PZT concentration indicate that the particle–matrix interaction plays a relevant role, as well as the particle asymmetry and relative orientation, being the Yamada model the most appropriate to describe the composite behavior. An erratum to this article can be found at     

Current-induced domain wall motion with adiabatic and nonadiabatic spin torques in magnetic nanowires

We investigate current-driven domain wall (DW) propagation in magnetic nanowires in the framework of the modified Landau-Lifshitz-Gilbert equation with both adiabatic and nonadiabatic spin torque (AST and NAST) terms. By employing a simple analytical model, we can demonstrate the essential physics that any small current density can drive the DW motion along a uniaxial anisotropy nanowire even in absence of NAST, while a critical current density threshold is required due to intrinsic anisotropy pinning in a biaxial nanowire without NAST. The DW motion along the uniaxial wire corresponds to the asymptotical DW oscillation solution under high field/current in the biaxial wire case. The current-driven DW velocity weakly depends on the NAST parameter β in a uniaxial wire and it is similar to the β = α case (α: damping) in the biaxial wire. Apart from that, we discuss the rigid DW motion from both the energy and angular momentum viewpoints and point out some physical relations in between. We also propose an experimental scheme to measure the spin current polarization by combining both field- and current-driven DW motion in a usual flat (biaxial) nanowire.     

The effect of α, β crystalline structure on the mechanical properties of polypropylene

In order to study the effect of the α,β crystalline structure of polypropylene (PP) on its mechanical properties, it is necessary to prepare samples with variable α/β-phase content but with constant crystallinity and constant spherulite size. With this objective, heat treatment was first defined to be applied to an isotactic PP containing a β nucleating agent in order to achieve these conditions. Then study of the effect of the β-phase content on the tensile properties and fracture behaviour has been done at room temperature. The mechanical properties at fracture were assessed by three-point bending tests and were analysed on the basis of the “Essential Work of Fracture” (EWF). The results show that the elongation at fracture under tensile stress and the “near” Plane-Strain Essential Work of Fracture, w _Ie, increase substantially with the β-phase content. Besides, Young's modulus and the yield stress in tensile tests decrease slowly with the β-phase content. Finally, these results are analysed taking account the differences in structure of the α and β spherulites. Received 18 September 2000 and Received in final form 19 December 2000     

Analytic Perturbation Theory for Bound States in the Transfer Matrix Spectrum of Weakly Correlated Lattice Ferromagnetic Spin Systems

We consider general d-dimensional lattice ferromagnetic spin systems with nearest neighbor interactions in the high temperature region (β≪1). Each model is characterized by a single site apriori spin distribution taken to be even. We also take the parameter α=〈s ⁴〉−3〈s ²〉²>0, i.e. in the region which we call Gaussian subjugation, where 〈s ^k 〉 denotes the kth moment of the apriori distribution. Associated with the model is a lattice quantum field theory known to contain a particle of asymptotic mass −lnβ and a bound state below the two-particle threshold. We develop a β analytic perturbation theory for the binding energy of this bound state. As a key ingredient in obtaining our result we show that the Fourier transform of the two-point function is a meromorphic function, with a simple pole, in a suitable complex spectral parameter and the coefficients of its Laurent expansion are analytic in β.     

Static and dynamic properties of supercooled thin polymer films

The dynamic and static properties of a supercooled (non-entangled) polymer melt are investigated via molecular-dynamics (MD) simulations. The system is confined between two completely smooth and purely repulsive walls. The wall-to-wall separation (film thickness), D, is varied from about 3 to about 14 times the bulk radius of gyration. Despite the geometric confinement, the supercooled films exhibit many qualitative features which were also observed in the bulk and could be analyzed in terms of mode-coupling theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time-temperature superposition property of the late time α-process and the space-time factorization of the scattering function on the intermediate time scale of the MCT β-process. An analysis of the temperature dependence of the α-relaxation time suggests that the critical temperature, T _c, of MCT decreases with D. If the confinement is not too strong ( D≥10monomer diameter), the static structure factor of the film coincides with that of the bulk when compared for the same distance, T - T _c(D), to the critical temperature. This suggests that T - T _c(D) is an important temperature scale of our model both in the bulk and in the films. Received 12 September 2001     

High-temperature behaviour of average structure and vibrational density of states in the ternary superionic compound AgCuSe

Results of simultaneous thermal analysis (STA), synchrotron powder diffraction (in the range 300–973 K) and inelastic neutron scattering (at 285 and 505 K) on non-superionic β- and superionic α-AgCuSe are reported. The sample is stable in argon on heating. The volume change at the superionic phase transition is about 5%. A model for the average structure of α-AgCuSe is proposed. No anomalies in the temperature dependence of the parameters of the average structure were revealed. Ionic conductivity in α-AgCuSe can originate from cation jumps in “skewed” 〈100 〉 directions between nearest-neighbour tetrahedral sites via the peripheries of the octahedral cavities. A correlation between the temperature dependence of the cation redistribution in α-AgCuSe and the temperature dependence of the ionic conductivity is supposed. Various contributions (anharmonic effects, time-average static disorder and phonon-phonon scattering) to the widths of individual phonons upon temperature increase lead to pronounced changes in the neutron-weighted densities of states of β- and α-AgCuSe and accompany the superionic phase transition as well.     

Transitions between secondary structures in isolated polyalanines

Monte Carlo simulations of gas-phase polyalanine peptides have been carried out with the Amber ff96 force field. A low-temperature structural transition takes place between the α-helix stable conformation and β-sheet structures, followed by the unfolding phase change. The transition state ensembles connecting the helix and sheet conformations are investigated by sampling the energy landscape along specific geometric order parameters as putative reaction coordinates, namely the electric dipole μ, the end-to-end distance d, and the gyration radius R_g. By performing series of shooting trajectories, the committor probabilities and their distributions are obtained, revealing that only the electric dipole provides a satisfactory transition coordinate for the α↔β interconversion. The nucleus at the transition is found to have a high helical content.     

Universality in diffusion front growth dynamics

We have studied the scaling properties of diffusion fronts by numerical calculations based on the mean field approach in the context of a lattice gas model, performed in a triangular lattice. We find that the height-height correlation function scales with time t and length l as C(l, t) ≈l ^α f (t/l ^α/β) with α = 0.62±0.01 and β = 0.39±0.02. These exponent values are identical to those characterising the roughness of the diffusion fronts evolving through a square lattice [1,2], thus confirming their universality. Received 14 November 2001 / Received in final form 20 April 2002 Published online 31 July 2002     

Universality of the REM for Dynamics of Mean-Field Spin Glasses

We consider a version of Glauber dynamics for a p-spin Sherrington– Kirkpatrick model of a spin glass that can be seen as a time change of simple random walk on the N-dimensional hypercube. We show that, for all p ≥ 3 and all inverse temperatures β > 0, there exists a constant γ _{β ,p}  > 0, such that for all exponential time scales, exp(γ N), with γ < γ _{β ,p} , the properly rescaled clock process (time-change process) converges to an α-stable subordinator where α = γ/β ² < 1. Moreover, the dynamics exhibits aging at these time scales with a time-time correlation function converging to the arcsine law of this α-stable subordinator. In other words, up to rescaling, on these time scales (that are shorter than the equilibration time of the system) the dynamics of p-spin models ages in the same way as the REM, and by extension Bouchaud’s REM-like trap model, confirming the latter as a universal aging mechanism for a wide range of systems. The SK model (the case p = 2) seems to belong to a different universality class.