Instability of vicinal crystal surfaces with transparent steps: Transient kinetics and non-local electromigration

The steps at the crystal surfaces could be partially transparent in the sense that the migrating adatoms can cross the steps without visiting a kink position. In the case of significant transparency the velocity of a given step in a given moment is affected by atomic processes which took place at rather distant terraces in rather earlier moments. The reason is that an adatom needs time to cross many terraces before attaching to a kink position. That is why the theory of crystallization with non-local kinetics should account for the time dependence of the adatom concentrations on the terraces. Such a transient growth model (going beyond the quasi-static approximation of the BCF theory of crystal growth) is developed here for the limit of slow kinetics at the steps and fast surface diffusion. This model predicts instability of the vicinal surface (in agreement with the earlier studies) but rather different expression for the wavelength of the most unstable mode, which depends only on the ratio between the step transparency and the step kinetic coefficients.     

Exact equilibrium shapes of Ising crystals on triangular/honeycomb lattices

The anisotropic surface tension for an Ising system below the critical point on a triangular or a honeycomb lattice can be computed through duality. Using the Wulff construction, the equilibrium shape of a crystal (droplet of one phase inside a sea of the other) is found. An exact and simple equation for this shape is derived.     

Equilibrium shape of nickel crystal

The equilibrium shape of pure nickel and the effect of carbon on changes in the equilibrium shape at 1200°C were investigated. A statistical observation on the size-dependent, time-dependent and carbon-induced morphological evolution of crystallites suggested that the equilibrium crystal shape (ECS) of pure nickel is a polyhedron consisting of {111}, {100}, {110} and {210} surfaces. However, crystals with an extensive proportion of {320} surfaces were also frequently observed. The appearance of {320} surfaces was interpreted as kinetically stabilized metastable surfaces, which survived during the thermal equilibrating process, possibly due to a high nucleation energy barrier for their removal. On the other hand, the ECS of pure nickel was observed to change dramatically into a spherical shape with facets of {111}, {100}, {110} and {210} without exception under a carburized atmosphere, which indicates that carbon not only facilitates surface diffusion by which energetically more stable surfaces can be easily developed but also decreases the surface energy anisotropy. Together with X-ray photoelectron spectroscopy studies, it was proposed that the carbon-induced changes in the ECS are possibly due to a solid solution effect, which could lead to a reduction in the binding energy among atoms in the bulk as well as on the surfaces.     

From crystal steps to continuum laws: Behavior near large facets in one dimension

The passage from discrete schemes for surface line defects (steps) to nonlinear macroscopic laws for crystals is studied via formal asymptotics in one space dimension. Our goal is to illustrate by explicit computations the emergence from step motion laws of continuum-scale power series expansions for the slope near the edges of large, flat surface regions (facets). We consider surface diffusion kinetics via the Burton, Cabrera and Frank (BCF) model by which adsorbed atoms diffuse on terraces and attach-detach at steps. Nearest-neighbor step interactions are included. The setting is a monotone train of N steps separating two semi-infinite facets at fixed heights. We show how boundary conditions for the continuum slope and flux, and expansions in the height variable near facets, may emerge from the algebraic structure of discrete schemes as N→∞. Our technique relies on the use of self-similar discrete slopes, conversion of discrete schemes to sum equations, and their reduction to nonlinear integral equations for the continuum-scale slope. Approximate solutions to the continuum equations near facet edges are constructed formally by direct iterations. For elastic-dipole and multipole step interactions, the continuum slope is found in agreement with a previous hypothesis of ‘local equilibrium’.     

1-to-N beam splitter based on photonic crystal branched waveguide structure

A novel beam splitter is proposed based on a two-dimensional (2D) photonic crystal (PC) branched waveguide structure. The beam splitter structure comprises branched waveguide channels and extra dielectric columns. These branched waveguide channels are used to obtain secondary sources and the introduced extra dielectric columns are used to control the phase difference of the secondary point sources. The field distributions of the beam are investigated using the finite-difference time-domain (FDTD). It is found that the number of beams is sensitive to the distance of the waveguide channels ports and extra dielectric columns. By adjusting the positions of the waveguide channels and the parameters of the extra dielectric columns, 1-to-N beam splitters can be realized. These simple, easily fabricated and controllable structures have important potential applications in integrated optical circuits.     

Ultrahigh-Q of the L3 photonic crystal microcavity

In this paper, we theoretically investigate the L3 cavity with three missing holes in the center. They are of great interest for the realization of low threshold laser nanosources and for a strong interaction between the cavity and sources. In order to improve the transmission and Q factor simultaneously of these structure, by reducing unwanted reflection due to mismatch and through minimization of propagation losses, we modified L3 geometry: three missing holes in a line where both lateral displacement of the first hole adjacent to the cavity, d, and their radius, r, were changed. A photonic microcavity with a high Q factor of 1.8741 × 10⁷ and a modal volume V of 0.351 is demonstrated. We demonstrate that the calculated Q factor for the designed cavity increases by a factor of 49 relative to that for a cavity without displaced and reduced air holes, while the modal volume remains almost constant.     

The behavior of lead during the solidification of Zn-0.1Al-0.1Pb coating on batch hot-dipped steel

The typical spangle on batch hot-dipped Zn-0.1Al-0.1Pb alloy coating was investigated. The morphology, distribution, chemical composition and identification of the phases on coating surface were examined by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), respectively. It is found that most of spangles usually exhibit three distinct regions: smooth, feathery and coarse regions, of which lead surface segregation across the coating surface was detected increasingly more significant. Furthermore, lead micro-segregation with the form of tiny orthohexagonal particulates (<0.5 μm), mainly happens in the interdendritic spacing instead of the grain boundary probably owing to no sufficient diffusion. Based on the experimental results and compared to CGL, the solidification sequence of the Zn-0.1Al-0.1Pb coating was analyzed assisted with Zn-Pb phase diagram; the morphology of the spangle is modeled on consideration with the interaction of the thermal condition and the crystallization orientation; the profile of precipitated lead particulates is well explained in the view of crystallography.     

Effects of effective attractive multi-body interaction on quantum phase and transport dynamics of a strongly correlated bosonic gas across the superfluid to Mott insulator transition

We theoretically investigate quantum phases and transport dynamics of ultracold atoms trapped in an optical lattice in the presence of effective multi-body interaction.When a harmonic external potential is added,several interesting phenomena are revealed,such as the broadening and the emergence of a central insulator plateau and the phase transition between superfluid and Mott insulator phase.We also study the transport of the system which runs across the superfluid–insulator transition after ramping up the lattice,and predict a slower relaxation which is attributed to the influence of the multi-body interaction on the mass transport.     

Plane-wave pseudopotential study for the structural stability of Hf: The role of spin-orbit interaction

With the use of density functional theory, first-principles calculation has been performed to investigate the structural stability of Hf. In this context, Hf has several characteristics, including the openness of its omega structure in the sequence hcp→omega→bcc with elevated pressure, its tight-binding d-bands near the Fermi level and the possible requirement of relativistic correction (spin-orbit coupling, SO) for its relatively high atomic number. To decouple these characters and reproduce the equilibrium transformation pressures of hcp→omega as well as omega→bcc, we treat Hf with (I) local density approximation (LDA) without and (II) with SO coupling, (III) generalized gradient approximation (GGA) without and (IV) with SO coupling. Our plane-wave pseudopotential calculation with GGA and SO interaction results in transformation pressures much closer to the experiment compared to the previous linear muffin-tin orbit calculations. Furthermore, we discuss the dependence of lattice parameters and the transformation pressure on SO coupling. The effect of SO on the correlation of d-band occupancy and the structural stability is also revealed.     

Effects of Dzyaloshinskii-Moriya interaction on thermal entanglement and teleportation via a two-qubit Heisenberg XXZ spin chain under external magnetic field

This paper mainly investigates the effects of different Dzyaloshinskii-Moriya (DM) anisotropic antisymmetric interactions on thermal entanglement and teleportation of one-qubit state in both the standard and non-standard protocols as well as the partial teleportation of an entangled state via a two-qubit Heisenberg XXZ spin chain in the presence of external magnetic fields. The dependency of the thermal entanglement and average fidelity on various system parameters is analyzed. The interplay of the different parameters on the teleportation is discussed. The DM interaction is found to be effective for the thermal entanglement in the spin chain both with and without external magnetic fields. However, it turned out to be destructive for the teleportation in the standard protocol, whereas is found constructive for single qubit teleportation when the spin chain with the z-direction parameters is used as the channel in the non-standard protocol. Moreover, the results show that, for teleporting one-qubit state, the antiferromagnetic (AFM) chain is the only qualified candidate in the standard protocol, while both the AFM and ferromagnetic (FM) chains with the parameters along the z-axis are all suitable in the non-standard protocol when the parameters are chosen appropriately. For the partial teleportation of entanglement, both the AFM and FM chains are eligible as long as the appropriate combinations of parameters are chosen. In addition, the comparison of the effects of the same, fixed x- and z-component parameters of the DM interaction (D_x and D_z) on teleportation is presented.     

Studies of the crystal field energy levels and g factors for Ce in LiYF4 crystal

The five observed crystal field energy levels and EPR g factors g_//and g_⊥ for Ce³⁺-doped LiYF₄ crystal are calculated together from a complete diagonalization (of energy matrix) method. In the method, the contributions to g factors of ground Kramers doublet from all the rest doublets within the ground and excited manifolds ²F_5/2 and ²F_7/2 are included. The calculated results show reasonable agreement with the experimental values. The calculations suggest that the crystal field parameter B₂₀ > 0 in LiYF₄: Ce³⁺ crystal. The opinion of the parameter B₂₀ < 0 in the previous paper is not correct. Since this opinion is based on the calculation of g factors using a very simple method where only the contributions to g factors from the doublets within the ground manifold ²F_5/2 are considered, it is suggested that this simple method is not effective in the calculation of g factors for 4f¹ ions in crystals.     

Orientation and structure of triple step staircase on vicinal Si(1 1 1) surfaces

The vicinal Si(1 1 1) surface, inclined towards the direction, was investigated by scanning tunnelling microscopy and spot profile analysing low energy electron diffraction. It has been established that the surface, consisting of regularly spaced triple steps and (1 1 1) terraces with a width equal to that of a single unit cell of the Si(1 1 1)-7 × 7 surface structure, has the (7 7 10) orientation. An atomic model of the triple step is proposed.     

The influence of crystallographic steps on coadsorption: CO---O/Ni(119)

On low index nickel surfaces, repulsive interactions between atomic oxygen and CO drive the phase separation of these species into oxygen-rich and CO-rich islands. Because these adsorbates interact differently with crystallographic steps, the size and the structure of these islands are modified on stepped surfaces. We have monitored coadsorption-induced changes in the distribution of CO with IRRAS, observing six different CO stretching bands which are assigned to distinct local chemisorption environments. When oxygen fully saturates sites along the step edge, the steps are completely blocked from CO adsorption and virtually all the CO population on the terraces shifts from atop to bridge sites. This terrace site shift is similarly accomplished by atomic oxygen chemisorbed at terrace sites. From these coadsorption-induced changes in CO site distributions, constrained by the 10 Å terrace width, we conclude the through-metal O---CO interaction responsible for this CO site shift must be operative over a range of 5 Å. At θ_o = 0.18 ML, when oxygen occupies, but does not fully saturate the step edge, a new CO adsorption site is created, with a characteristic frequency of 1750 cm⁻¹. This new site is assigned to CO bonded to kinks along the step edge based upon its low intensity ( geometric kink density), enhanced binding strength and sensitivity to oxygen coverage. At higher oxygen coverages, compression of the CO adlayer is observed, with CO shifting to asymmetric bridge sites. As saturation coverage is approached, CO occupies weakly bound sites in close proximity ( 3 Å) to O adatoms, with high characteristic frequencies of 2100 cm⁻¹.     

Plasma modification of polypropylene surfaces and its alloying with styrene in situ

The treatment of polypropylene surfaces has been studied by dielectric barrier discharges plasma of Ar. The structure and morphology of polypropylene surfaces of Ar plasma modification are characterized by X-ray photoelectron spectroscopy, Fourier transform infrared spectrometers and scanning electron microscope. The modified by plasma treatment of iPP (isotactic polypropylene) surface properties have been examined in a determination of free radicals. The modified active surfaces of polypropylene can induce grafting copolymerization of styrene onto polypropylene. The structure of grafting copolymer is characterized and the grafting percent of styrene onto polypropylene is calculated. The homopolymer of styrene can be formed under grafting copolymerization of styrene onto polypropylene, which follows that the alloying of polypropylene with styrene is achieved in situ.     

The deformation producing tendency of (d 3/2−f 7/2) empirical effective interaction

The effective matrix elements in (d _3/2−f _7/2)² configuration have been recently defined by Erskineet al. and Sherret al. assuming shell clcsures for S³² and Ca⁴⁰. We have attempted to verify whether this empirically deduced (d _3/2−f _7/2)² interaction permits thed _3/2 shell to remain closed when nucleons are added to Ca⁴⁰. It is found that the Erskine interaction gives rise to ground states of thef _7/2 shell nuclei in which thed _3/2 orbit is completely filled. However that interaction over-binds Ca⁴⁰. If the centroid of the (d _3/2)² interaction is modified to fit the Ca⁴⁰ binding energy, the ground states of Cr⁴⁸ and Fe⁵² become deformed and thed _3/2 orbit is not completely filled.     

Relationship between chemical bond parameters and superconducting temperature in HgBa2Can−1CunO2n+2+δ (n=1, 2, 3, 4)

Bond covalency and valence of elements in HgBa₂Ca_n−1Cu_nO_2n+2+δ (n=1, 2, 3, 4) were calculated and their relationship with T_c was discussed. For both oxygen and argon annealed samples, the results indicated that with the increase of n, the trend of bond covalency of Hg-O and Cu-O was the same or opposite compared with that of superconducting temperature. This may suggest that the magnitudes of Cu-O and Hg-O bond covalency are important in governing the superconducting temperature. For the highest T_c sample, Hg had the lowest valence, implying that lower valence of Hg was preferred in order to produce higher T_c. For fixed n, the valence of Cu in oxygen annealed samples was larger than that in argon annealed samples, indicating that oxygen annealed samples produced more carriers than argon annealed samples.     

One-dimensional optical bright screening photovoltaic photorefractive solitons, soliton pairs and incoherent interaction between them in BaTiO3 crystal

By the use of theory of optical solitons, nonlinear equations governed by screening photorefractive photovoltaic solitons are introduced. By means of numerical methods, intensity profile, refractive index change of the media and also stability under propagation are investigated. Self-bending of solitons under first-order diffusion is also studied. Besides, incoherent coupled bright-bright soliton pairs and incoherent interaction are studied.     

Growth of single crystal PrFeAsO1−y and its characterization

We have successfully grown single crystals of oxygen deficient oxypnictide superconductor PrFeAsO_1−y using high pressure synthesis technique. Typical crystal size is about 600 × 800 × 30 μm³, with its T_c = 44 K. Their resistivity measurements under magnetic field yield the anisotropic factor Γ  5, consistent with previous results on smaller single crystals.     

A Monte Carlo study of the thermal properties of Cu3Au low index surfaces

The thermal dependencies of composition and order of the (111), (100) and (110) Cu₃Au surfaces are studied at the atomic scale by means of Monte Carlo simulations in the “transmutational” ensemble at constant volume. The question addressed is the extent to which such simulations carried on with a model N-body potential designed on the basis of bulk energetic and mechanical properties allow predictions consistent with experimental observations of the surface. Although the currently available experimental data still leave unanswered questions, many of them allow for comparison with modelling. Qualitative agreement is found for temperature dependencies of both surface composition and order, and the simulation results are discussed in detail. Some clear discrepancies are found as well, in particular (but not only) in the case of the (110) surface and its first neighbouring layer. Although the origin of such differences is not yet clear, it is suggested that they may serve to assess and to improve the model in the light of quantitative surface studies.