Values Of Coupling Parameter Research Articles

Numerical simulation for isothermal dendritic growth of succinonitrile-acetone alloy

Numerical simulation based on phase field method was developed to describe the solidification of two-dimensional isothermal binary alloys. The evolution of the interface morphology was shown and the effects of phase field parameters were formulated for succinonitrile-acetone alloy. The results indicate that an anti-trapping current(ATC) can suppress many trapped molten packets, which is caused by the thickened interface. With increasing the anisotropy value from 0 to 0.05, a small circular seed grows to develope secondary dendritic, dendritic tip velocity increases monotonically, and the solute accumulation of solid/liquid interface is diminished distinctly. Furthermore, with the increase of the coupling parameter value, the interface becomes unstable and the side branches of crystals appear and grow gradually.

Branch structure of IV-characteristics in the capacitively coupled Josephson junctions model with the diffusion current

We have solved numerically a system of dynamical equations for the gauge-invariant phase differences between superconducting layers for a stack of N intrinsic junctions and obtained a total branch structure in the current–voltage characteristics (IVC) of the stack. The coupling dependence of the branch’s slopes is investigated and demonstrated that the equidistance of the branch structure in capacitively coupled Josephson junctions (CCJJ) model is broken at small values of coupling parameter. Changes in the parameters of the boundary conditions and the use of periodic boundary conditions do not affect this result. In the framework of the CCJJ model with the diffusion current we simulate an experiment and obtain the IV-characteristic with equidistant branch structure at different values of model parameters.

When are projections also embeddings?

We study an autonomous four-dimensional dynamical system used to model certain geophysical processes. This system generates a chaotic attractor that is strongly contracting, with four Lyapunov exponents lambdai that satisfy lambda1+lambda2+lambda3<0 , so the Lyapunov dimension is DL=2+|lambda3|/lambda1<3 in the range of coupling parameter values studied. As a result, it should be possible to find three-dimensional spaces in which the attractors can be embedded so that topological analyses can be carried out to determine which stretching and squeezing mechanisms generate chaotic behavior. We study mappings into R3 to determine which can be used as embeddings to reconstruct the dynamics. We find dramatically different behavior in the two simplest mappings: projections from R4 to R3 . In one case the one-parameter family of attractors studied remains topologically unchanged for all coupling parameter values. In the other case, during an intermediate range of parameter values the projection undergoes self-intersections, while the embedded attractors at the two ends of this range are topologically mirror images of each other.

Vibronic spectra of charge transfer excitons and of mixed charge transfer and Frenkel excitons

The linear absorption spectra in the excitonic and vibronic regions in the case of mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) have been theoretically studied for the exciton parameters of the crystals of MePTCDI and PTCDA. Two coupling parameters for the exciton–phonon coupling are introduced: the FE–phonon coupling and the CTE–phonon coupling. The main features of the vibronics in the absorption spectra are the following: (a) the existence of a doublet structure in the vibronic spectra of CTEs; (b) the vibronic levels of the FE at intermediate values of both coupling parameters are located in the continuum of the many-particle exciton–phonon states which makes its absorption line wide and flat; (c) in the case of strong coupling (coupling constants larger than 1) a doublet of bound states appears above this continuum; (d) in the case of vanishing CTE–phonon coupling the vibronics of the charge transfer excitons practically disappear in the absorption spectra.

Quantum dot potentials: Symanzik scaling, resurgent expansions, and quantum dynamics

This article is concerned with a special class of the ``double-well-like'' potentials that occur naturally in the analysis of finite quantum systems. Special attention is paid, in particular, to the so-called Fokker-Planck potential, which has a particular property: the perturbation series for the ground-state energy vanishes to all orders in the coupling parameter, but the actual ground-state energy is positive and dominated by instanton configurations of the form exp(-a/g), where a is the instanton action. The instanton effects are most naturally taken into account within the modified Bohr-Sommerfeld quantization conditions whose expansion leads to the generalized perturbative expansions (so-called resurgent expansions) for the energy values of the Fokker-Planck potential. Until now, these resurgent expansions have been mainly applied for small values of coupling parameter g, while much less attention has been paid to the strong-coupling regime. In this contribution, we compare the energy values, obtained by directly resumming generalized Bohr-Sommerfeld quantization conditions, to the strong-coupling expansion, for which we determine the first few expansion coefficients in powers of g^(-2/3). Detailed calculations are performed for a wide range of coupling parameters g and indicate a considerable overlap between the regions of validity of the weak-coupling resurgent series and of the strong-coupling expansion. Apart from the analysis of the energy spectrum of the Fokker-Planck Hamiltonian, we also briefly discuss the computation of its eigenfunctions. These eigenfunctions may be utilized for the numerical integration of the (single-particle) time-dependent Schroedinger equation and, hence, for studying the dynamical evolution of the wavepackets in the double-well-like potentials.

Ordering transition and demixing of a binary mixture of thick and thin rodlike molecules in the presence of an external field

The effect of an external field (electric/magnetic) on the phase behavior of the binary mixture of very long thick and thin rodlike particles is studied. Both the thick and thin particles possess positive but different susceptibility anisotropics (Delta alpha). The difference in the extent of interaction between the external field and the two species is varied by means of a coupling parameter (l = Delta alpha(thick)/Delta alpha(thin)). Isotropic-nematic phase transition and demixing phase transitions taking place both in the isotropic and nematic phases are examined as a function of field strength on the level of the second virial theory of Onsager in the range of 0 < l <1. The approximate sixth order Legendre polynomial expansion method is used to represent the excluded volume interaction between the rodlike particles. It is found that the isotropic phase becomes weakly nematic (paranematic) in the presence of external field and the field orients both components in the direction of the field even if the field does not have direct interaction with the thick component (l = 0). Analytical expressions are derived for the external field induced order parameters and birefringence. The increasing field destabilizes both types of demixing transitions (isotropic-isotropic and nematic-nematic) and the paranematic-nematic phase transition. Moreover it induces closed loop immiscibility, and upper and lower critical points terminating the paranematic-nematic phase coexistence may occur for low values of the coupling parameter. It is interesting that while the phase boundaries of the paranematic-paranematic demixing and the paranematic-nematic transitions are very sensitive to the value of the coupling parameter at low pressures, the paranematic-nematic and nematic-nematic phase boundaries are practically independent of the coupling parameter at high pressures.

Equidistance of branch structure in capacitively coupled Josephson junctions model with diffusion current

Branch structure in current–voltage characteristics of intrinsic Josephson junctions of HTSC is studied in the framework of two models: capacitively coupled Josephson junctions (CCJJ) model and CCJJ model with diffusion current (CCJJ + DC). We investigate the coupling dependence of the branch’s slopes and demonstrate that the equidistance of the branch structure in CCJJ model is broken at enough small values of coupling parameter (at α ≪ 1). We show that the inclusion of diffusion in the tunneling current through intrinsic Josephson junctions might restore the equidistance of the branch structure. Change of the current–voltage characteristics in CCJJ + DC model under variation of the coupling and McCumber parameters and effect of boundary conditions on the branch structure is analyzed.

Magnetism of the lanthanides(III) complexes with some polihydroxyflavones

The La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III) and Lu(III) ions form solid complexes with morin and quercetin-5′-sulfonic acid (QSA). The composition and some physicochemical properties of these compounds were described previously. In order to estimate magnetic properties, the oxidation number of metal ions and the nature of the metal–ligand bonding in the investigated rare earth element complexes the magnetic measurements were carried out. On the basis of the magnetic susceptibility ( χ) versus T and the magnetic moments ( μ eff) versus T the values of Curie ( C) and Weiss ( Θ) constants for all examined compounds were determined. Moreover, for Sm(III) and Eu(III) complexes the value of spin-orbit coupling parameter ( λ) was calculated. It was stated that the obtained complexes are paramagnetic, except La(III) and Lu(III), which are diamagnetic. Furthermore, the magnetic properties of Sm(III) and Eu(III) compounds significantly differ from the Curie and the Curie–Weiss laws. The metal–ligand bonding in the investigated rare earth element complexes are most probably electrostatic with small participation of covalent bond.

On diquark clustering in quark—gluon plasma

The possibility that pairs of quarks will form diquark clusters in the regime above deconfinement transition for hadron matter at finite density is revisited. Here we present the results on the diquark-diquark (dq-dq) interaction in the framework of constituent quark model taking account of spin, isospin and color degrees of freedom in the spirit of generalized Pauli principle. By constructing the appropriate spin and color states of the dq-dq clusters we compute the expectation values of the interaction Hamiltonian involving pairwise quark—quark interaction. We find that the effective interaction between two diquark clusters is quite sensitive to different configurations characterized by color and spin states, obtained after the coupling of two diquark states. The value of the coupling parameter for a particular color—spin state, i.e., -3, 1 is compared to the one obtained earlier by Donoghue and Sateesh,Phys. Rev. D38, 360 (1988) based on the effective Φ4-theory. This new value of λ derived for different color-spin dq-dq states, may lead to several important implications in the studies of diquark star and diquark gas.

Test-charge theory for the electric double layer.

We present a model for the ion distribution near a charged surface, based on the response of the ions to the presence of a single test particle. Near an infinite planar surface this model produces the exact density profile in the limits of weak and strong coupling, which correspond to zero and infinite values of the dimensionless coupling parameter. At intermediate values of the coupling parameter our approach leads to approximate density profiles that agree qualitatively with Monte Carlo simulation. For large values of the coupling parameter our model predicts a crossover from exponential to algebraic decay at large distance from the charged plate. Based on the test-charge approach we argue that the exact density profile is described, in this regime, by a modified mean-field equation, which takes into account the interaction of an ion with the ions close to the charged plate.

Attraction of like-charged macroions in the strong-coupling limit.

Like-charged macroions attract each other as a result of strong electrostatic correlations in the presence of multivalent counterions or at low temperatures. We investigate the effective electrostatic interaction between i) two like-charged rods and ii) two like-charged spheres using the recently introduced strong-coupling theory, which becomes asymptotically exact in the limit of large coupling parameter (i.e. for large counterion valency, low temperature, or high surface charge density on macroions). In contrast to previous applications of the strong-coupling theory, we deal with curved surfaces and an additional parameter, referred to as Manning parameter, is introduced, which measures the ratio between the radius of curvature of macroions to the Gouy-Chapman length. This parameter, together with the size of the confining box enclosing the two macroions and their neutralizing counterions, controls the counterion-condensation process that directly affects the effective interactions. For sufficiently large Manning parameters (weakly-curved surfaces), we find a strong long-ranged attraction between two macroions that form a closely-packed bound state with small surface-to-surface separation of the order of the counterion diameter in agreement with recent simulations results. For small Manning parameters (highly-curved surfaces), on the other hand, the equilibrium separation increases and the macroions unbind from each other as the confinement volume increases to infinity. This occurs via a continuous universal unbinding transition for two charged rods at a threshold Manning parameter of Epsilon c = 2/3, while the transition is strongly discontinuous for spheres because of a pronounced potential barrier at intermediate distances. Unlike the cylindrical case, the attractive forces between spheres disappear slowly for increasing confinement volume due to the complete de-condensation of counterions. Scaling arguments suggest that for moderate values of coupling parameter, strong-coupling predictions remain valid for sufficiently small surface-to-surface separations.

Transitions from spatiotemporal chaos to cluster and complete synchronization states in a shift-invariant set of coupled nonlinear oscillators.

We study the spatiotemporal dynamics of a ring of diffusely coupled single-well Duffing oscillators. The transitions from spatiotemporal chaos to cluster and complete synchronization states are particularly investigated, as well as the Hopf bifurcations to instability. It is found that the underlying mechanism of these transitions relies on the motion of the representative points corresponding to the system's nondegenerated spatial transverse Fourier modes in the parametric Strutt diagram. A scaling law is used to demonstrate that the compact interval of the scalar coupling parameter values leading to cluster synchronization broadens in a square-power-like fashion as the number of oscillators is increased. The analytical approach is confirmed by numerical simulations.

A special bifurcation of riddled basin in two coupled skew tent maps

A chaotic synchronized system of coupled skew tent maps is discussed. The locally and globally riddled basins of the chaotic synchronized attractor are studied. It is found that the coupling parameter values corresponding to the locally riddled basin are isolated points embedded in the coupling parameter intervals of the globally riddled basin. This kind of bifurcation is novel and not like the local–global riddling bifurcation in found other references. We believe this bifurcation will be generic in systems, whose chaotic attractors are close to the attractive basins’ boundaries infinitely, despite discrete or differential systems.

Fluxon dynamics in three stacked Josephson junctions

The motion of fluxons of the same polarity in three vertically stacked Josephson junctions is studied. In this configuration the difference between exterior and interior junctions plays a more important role than in other configurations with several interior junctions. Below the Swihart velocity ${c}_{\ensuremath{-}},$ the coupling between junctions leads to a repulsion of the fluxons with the same polarity. Above this critical velocity a fluxon will induce radiation in the neighboring junctions, leading to a bunching of the fluxons in the stacked junctions. Using the Sakai-Bodin-Pedersen model, three coupled perturbed sine-Gordon equations are numerically studied for different values of coupling, damping, and bias parameters. In a narrow range of velocities bunching occurs. Outside this interval the fluxons split and new fluxons may be created. $I\ensuremath{-}V$ characteristics are presented.

Characterization of the α−β Phase Transition in Friedels Salt (Ca2Al(OH)6Cl·2H2O) by Variable-Temperature 27Al MAS NMR Spectroscopy

The first application of variable-temperature 27Al MAS NMR spectroscopy to the satellite transitions in the characterization of a structural phase transition is presented by an investigation of Friedels salt (Ca2Al(OH)6Cl·2H2O) over the temperature range from −121 to 109 °C. Accurate values for the 27Al quadrupole coupling parameters (CQ and ηQ) and the isotropic chemical shifts are obtained from either the manifold of spinning sidebands for the satellite transitions or the line shape observed for the central transition. These data demonstrate that the quadrupole coupling parameters clearly reflect the structural α−β phase transition for Friedels salt at about 34 °C, whereas the isotropic chemical shift is invariant over the studied temperature range. The variations in CQ and ηQ with temperature show a parabolic decrease in CQ and an increase in ηQ with increasing temperature for the α form of Friedels salt, while only a small linear decrease in CQ with increasing temperature and a temperature-independent value for ηQ are observed for the β form. The nonlinear variations of CQ and ηQ with temperature for the α form and their relationships with changes in the structural parameters associated with thermal expansion of the unit cell are discussed. Finally, the abrupt changes in CQ and ηQ at the temperature for the phase transition are investigated by point-monopole calculations of the 27Al electric-field gradient tensor, which strongly suggest that hydrogen bonding plays an important role in the structural changes that occur during the phase transition.

Effect of local Coulomb interactions on the electronic structure and exchange interactions inMn12magnetic molecules

We have studied the effect of local Coulomb interactions on the electronic structure of the molecular magnet Mn12-acetate within the LDA+U approach. The account of the on-site repulsion results in a finite energy gap and an integer value of the molecule's magnetic moment, both quantities being in a good agreement with the experimental results. The resulting magnetic moments and charge states of non-equivalent manganese ions agree very well with experiments. The calculated values of the intramolecular exchange parameters depend on the molecule's spin configuration, differing by 25-30% between the ferrimagnetic ground state and the completely ferromagnetic configurations. The values of the ground-state exchange coupling parameters are in reasonable agreement with the recent data on the magnetization jumps in megagauss magnetic fields. Simple estimates show that the obtained exchange parameters can be applied, at least qualitatively, to the description of the spin excitations in Mn12-acetate.

The influence of surface and interface properties on magnetic characteristics of magnetic triple layers

The system of three magnetic layers indirectly coupled through nonmagnetic spacers is investigated. The Green function method is applied to study magnetisation distribution and thickness dependence of the Curie temperature. It is shown that in the system consisting of very thin films deviation of the Curie temperature from its value calculated for fully coupled system is related to the sum of absolute values of interlayer coupling parameters through both interfaces. Magnetisation profiles are calculated and the role of surface anisotropy and interlayer coupling parameter is discussed.

A novel bimetallic alternating chain: synthesis, crystal structure and magnetic study

A novel one dimensional (1D) bimetallic polymeric complex [ catena-bis(μ- N-(aminoethyl)-3-aminopropanolato)-di-copper(II)-tetracyanonickelate(II)dihydrate] has been synthesized by the reaction of 1:1 mixture of CuCl 2·2H 2O and the hydoxyamine ligand, [ N-(3-hydroxypropyl)ethane-1,2-diamine] with half equivalent of [Ni(CN) 4] −2 in aqueous solution. X-ray quality single crystals were obtained by slow evaporation of the ammoniacal solution of the complex. The single crystal structure reveals that it is an alkoxo bridged Cu(II) dimer linked through [Ni(CN) 4] 2− in trans-fashion resulting in a 1D chain. The properties of the complex have been studied by variable temperature magnetic susceptibility measurements. The susceptibility data fit well with the Cu(lI) dimer equation to give 2 J=−622 cm −1, g=2.05. The very high coupling parameter value clearly indicates the presence of very strong antiferromagnetic interaction between the Cu(II) ions.

Observation of Dominant Ferromagnetic Interaction in Fumarate-Bridged 1-D Polymer of Cu(II)

The reaction of CuCl2·2H2O and Na2C4H2O4 (disodium fumarate) in dilute ammonia solution produces 1-D polymeric chain [Cu(μ-C4H2O4)(NH3)2]n(H2O)n. The crystal structure of the compound has been solved. The compound crystallizes in the monoclinic system, space group C2/m (No. 12), with chemical formula C4H10N2O5Cu, a = 13.955(5) A, b = 7.330(3) A, c = 8.927(2) A, β = 102.39(3)°, Z = 4. The interesting feature of the complex is that one of the carboxylic acid groups acts as a monodentate coordinating ligand whereas the remaining carboxylate moiety acts as a monoatomic bridging bidentate ligand. This unusual coordination behavior of fumarate anion produces a chain with alternate repetition of 4- and 14-member rings. The magnetic properties of the compound have been studied from room temperature to 4 K, which reveal the existence of a dominant ferromagnetic interaction with exchange coupling parameter values of JF = +12 cm-1 and JAF = −3.8 cm-1, using an alternating ferro- and antiferromagnetic chain model for...

Bulk fields in dilatonic and self-tuning flat domain walls

We study the Kaluza-Klein zero modes of massless bulk fields with various spins in the background of dilatonic and self-tuning flat domain walls. We find that the zero modes of all the massless bulk fields in such domain wall backgrounds are normalizable, unlike those in the background of the non-dilatonic domain wall with infinite extra space of Randall and Sundrum. In particular, gravity in the bulk of dilatonic domain walls is effectively compactified to the Einstein gravity with vanishing cosmological constant and nonzero gravitational constant in one lower dimensions for any values of dilaton coupling parameter, provided the warp factor is chosen to decrease on both sides of the domain wall, in which case the tension of the domain wall is positive. However, unexpectedly, for the self-tuning flat domain walls, the cosmological constant of the zero mode effective gravity action in one lower dimensions does not vanish, indicating the need for additional ingredient or modification necessary in cancellation of the unexpected cosmological constant in the graviton zero mode effective action.