Simple Coupling Model Research Articles

Raman characterization of single defect layers embedded in finite superlattices

Single defect layers embedded in finite, nominally periodic superlattices have been examined using Raman scattering. The spectrum of zone-folded acoustic phonons exhibits defect-associated peaks due to the broken symmetry of the lattice. The frequencies and intensities of these additional modes can be qualitatively estimated using a simple photoelastic coupling model. Shifts in the defect mode frequencies are observed when the spatial location of the defect layer is moved from the substrate to the air interface.

The Role of the Western Boundary in the ENSO Cycle: Experiments with Coupled Models

Abstract Coupled models of the Pacific ocean–atmosphere system have been shown to produce oscillations in the model coupled system that resemble the observed El Nino–Southern Oscillation (ENSO) cycle in many respects. The tendency for the coupled models to oscillate is due to delayed negative feedback resulting from the reflection of Rossby wave activity from the western boundary. In the real ocean the reflection process is difficult to observe. Furthermore, among other potential complications affecting the generation and propagation baroclinic waves, the real western boundary is ragged with unknown reflective characteristics rather than the efficiently reflecting boundaries found in the oceanic components of the simple coupled models that produce ENSO-like oscillations. Thus, the validity of the delayed negative feedback–coupled oscillator concept remains open to question. We present the results of experiments with two coupled models, one simple and one more complex, in which realistic western equatorial...

Theory of resonance Raman scattering and fluorescence from strongly vibronically coupled excited states of polyatomic molecules

A theoretical description of secondary emission from complex absorption bands of isolated polyatomic molecules is developed. The strong non-Born–Oppenheimer coupling associated with conical intersections of the multidimensional excited-state potential-energy surfaces is included in a fully microscopic manner by solving the time-dependent Schrödinger equation for appropriate model systems incorporating the most relevant electronic states and vibrational modes. The effect of the large number of remaining vibrational modes and of the weaker coupling with additional electronic states is modeled by phenomenological relaxation terms (lifetime broadening and pure dephasing) in the framework of the density-matrix formalism. Explicit eigenstate-free expressions for absorption, resonance Raman, and fluorescence spectra are derived via density-matrix perturbation theory. The computational feasibility of the resulting mixed microscopic/phenomenological theory is demonstrated for a simple three-mode model of the vibronic coupling of the S1(nπ*) and S2(ππ*) states of pyrazine. The effect of excited-state vibronic coupling and ultrafast S2→S1 internal conversion on resonance Raman and fluorescence spectra is analyzed on the basis of these model calculations.

A simple model for translational-rotational coupling

We studied a simple model for translational-rotational coupling. Our system is made up from four harmonic oscillators driven by a stochastic force and hindered by friction, which move on the four sides of a square. In the center of the square is a rotating dumbbell molecule, which is coupled to the four oscillators via an attractiver−1 potential. We show details of the dynamics of the system, which indicate that jumps do not always happen through thermally activated crossing of potential barriers but through dynamical deformation of the surrounding.

"Missing" 19/2(+) states in 141Pr and 143Pm.

A simple weak coupling model previously applied to the analysis of the yrast spectrum of $^{143}\mathrm{Nd}$ is extended to yrast spectra of odd-A N=82 and 83 isotopes of Z=58--61 nuclei. The model generally provides a good description of these spectra; however, yrast 19/${2}^{+}$ states expected to arise in $^{141}\mathrm{Pr}$ and $^{143}\mathrm{Pm}$ from the coupling of \ensuremath{\pi}${\mathit{g}}_{7/2}$ holes to the ${6}_{1}^{+}$ states of the even-even cores are not observed experimentally. The absence of these states can be explained using a simple microscopic argument.

A simplified approach to calculating current distribution in parallel power buses

A practical method of determining the conditions for current sharing amongst paralleled power buses is described. A simple coupled circuit model which includes the effects of externally added series resistance is used. A dimensionless factor (K) is introduced for purposes of identifying the conditions for current sharing. A critical selection of geometrical layouts is considered for determining a lower bound on K. For all practical cases it was found that a value greater than 10 would give rise to uniform current sharing. On the other hand, for a more optimal utilization of space it was found that the cable layouts could be optimized in several ways so as to maintain current sharing. Individual applications could resort to the use of specific geometrical layouts based on the requirements for allocated space, external magnetic field strengths, forces between wires, and maintainability. A concentric arrangement of alternating polarities offers the best utilization of space and has the lowest external fields. >

Aspects of dissipative electronic and vibrational dynamics of strongly vibronically coupled systems

The intramolecular non–Born–Oppenheimer quantum dynamics on conically intersecting potential-energy surfaces is analyzed on the basis of exact (numerical) time-dependent quantum calculations for two representative two-state three-mode vibronic-coupling models. A compact description of the time-dependent dynamics in terms of reduced density matrices of the electronic and vibrational subsystems is introduced. Results are presented for the time evolution of electronic and vibrational coherences, populations, as well as subsystem entropies. It is found that such simple two-state three-mode vibronic coupling models exhibit a rich variety of dissipative phenomena on femtosecond time scales. The numerical results reveal an interesting interplay of driven electronic surface-hopping processes and dephasing of coherent vibrational motion which is presumably a generic feature of ultrafast internal conversion processes in polyatomic molecules.

A simple ocean-atmosphere coupled model for the origin of a warm El Niño Southern Oscillation event

A simple ocean-atmosphere coupled model is developed to investigate the evolution of a warm El Niño event in the tropical Pacific Ocean. In the absence of climatological winds the model readily evolves a realistic eastward-propagating structure provided that warm sea surface temperature (sst) covers the entire equatorial ocean uniformly. The final state of the model is similar to the mature phase of El Niño. In the presence of realistic climatological winds, however, the model behaves in a different way. It remains in a cold La Nina phase without showing any significant eastward propagation of an air-sea coupled coherent structure. Based on these model results and recent data analyses on the tropospheric quasi-biennial oscillation, it is argued that the Asian summer monsoon before and after the warm phase of El Niño Southern Oscillation is possible.

Interchain coupling in trans-polyacetylene.

The three-dimensional structure of crystalline trans-polyacetylene is examined and a simple model for interchain coupling is derived. The structure of the ground state is analyzed and parameters for parallel versus antiparallel ordering are contrasted. Consequences for the three-dimensional structure of different polymers are discussed.

A Simple Steady-State Coupled Ice-Ocean Model, with Application to the Greenland-Norwegian Sea

Abstract A thermodynamic reduced-gravity ocean model forced by the steady-state surface wind stress and a Haney-type heat flux was used to determine the climatological ice-edge position, ice thickness, ocean circulation, and upper-ocean temperature in a high-latitude meridional channel. The ice model used is purely thermodynamic; however, a parameterization is used to allow the surface wind stress to be transmitted to the water below the ice. The temperature distribution of the upper ocean is specified along the southern zonal boundary of the model domain, and the heat equation is integrated from this boundary poleward along streamlines for the mass transport. As a column of warm water moves poleward, its temperature tends to decrease since the air temperature monotonically decreases to the north. At high latitudes the steady-state heat balance between horizontal advection and cooling to the atmosphere can no longer hold in an ice-free ocean. Thus, an ice layer forms to insulate the ocean from the very co...

PHASE TRANSITIONS IN COUPLED SPIN SYSTEMS

A simple coupled spin model is presented to describe the phase diagram of plastic crystals. Translational and rotational degrees of freedom are associated respectively with a lattice-gaz Hamiltonian and a q-Potts Hamiltonian. A tetralinear coupling between rotations and translations is considered. Effects of adding random fields which couple solely to the Potts variables are studied.

Auroral E-fields from DE-1 and -2 at magnetically conjugate points

The results of studies of the electric field measurements from the two Dynamics Explorer satellites are reviewed. There are times at which DE-2 measured electric fields at the auroral ionosphere while DE-1 obtained nearly simultaneous measurement at a higher altitude on the same magnetic field lines. When these electric fields are mapped to a common altitude it is found that the large-scale electric fields are similar, but the small-scale electric fields have a larger amplitude at the higher altitude. Through the use of Fourier analysis it is shown that these results are evidence for a magnetic field-aligned potential drop between the two satellites. While these results were made possible by the satellites' altitude separation, it is also possible to take advantage of the fact that the satellites may pass through the same geomagnetic location with a time separation. This was useful for confirming that sinusoidal electric field oscillations which were detected near one conjunction were due to spatial rather than temporal variations, since similar oscillations were found at the same location at different times. The observed spatial oscillations can be explained by a simple model of magnetosphere-ionosphere coupling.

Large deflection behavior of circular quasi-isotropic laminates under point loading

Eight-ply quasi-isotropic circular composite plates of Thornel 300 graphite in Narmco 5208 epoxy resin (T300/5208) were analyzed to obtain the large deformation behavior under low-velocity impact-type point loads. A simple plate-membrane coupling model was developed. The impact-type point loads were replaced by equivalent quasistatic point loads. The plate-membrane coupling model was used to obtain the large deformation shapes for the thin circular composite laminates. The analyses indicated that the large deformation shapes of the composite plates under point loads vary with the center point displacements, and hence are different for different load levels. Quasi-isotropic plates were analyzed by replacing anisotropic bending stiffness components with the equivalent flexural stiffness for the isotropic plates. The plate-membrane coupling model was verified by conducting a series of tests on clamped, circular quasi-isotropic laminates. Deflected shapes and loaddisplacement curves for the thin composite plates were experimentally obtained. These deflected shapes and load-displacement curves agreed well with the analytical results obtained using the plate-membrane coupling model.

On the recent development of simple, coupled ocean-atmosphere models of ENSO

Based on simple model results, I describe the recent progress in our understanding of physical mechanisms directly associated with El Nino-Southern Oscillation (ENSO) phenomena. In particular, I extract two extremes from recent simple coupled models in order to interprete two complementary phases of interannual ENSO events. I also discuss what is most necessary for more complete model studies useful to improve forecasting skills.

Unstable and Damped Equatorial Modes in Simple Coupled Ocean-Atmosphere Models

Abstract Free equatorial modes for several simple coupled ocean-atmosphere models are determined. They are found to include unstable and damped modes of large zonal scale and long period. The influence of ocean thermo-dynamics on unstable modal behavior is systematically explored. The Model I ocean features a local thermal equilibrium. In Model II, linearized temperature advection is the sole ocean thermal process. The Model III ocean features both advective and local thermal processes, while that of Model IV features only local thermal processes. The ocean and atmosphere are each represented by linear shallow water equations on the equatorial β-plane, and are linked by traditional couplings. A finite difference method with variable resolution is used to find eigenvalues and eigenvectors of the coupled systems. Key results are checked via a series method. Ocean modes are influenced most strongly by coupling, and are damped or destabilized depending on the configuration of induced atmospheric motion relati...

Electronic and vibrational IR MCD in the Creutz-Taube complex

The Creutz-Taube complex was analysed in terms of a simple electronic coupling model and predictions of electronic transitions in the mid-IR (1800–4000 cm −1) were made. Low-temperature MCD measurements in this region show broad features around 2000 and 4000 cm −1 and sharp features near 3200 and 4800 cm −1. Deuteration of the ammine protons indicates that the sharper features are vibrational MCD of anomalous intensity and the broader features are electronic with substantial vibronically induced intensity in the 4000 cm −1 region. Measurements were made by incorporating the complex in a modified PTFE (nafion) film. Thin nafion films are transparent throughout the near- and mid-IR regions and are optically isotropic and transparent. The spectra appear to be the first C-term IR MCD spectra.

Nonperturbative theory of temperature-dependent optical dephasing in crystals. II. Pseudolocal phonons

We continue our discussion of the nonperturbative theory of zero-phonon impurity lines in crystals formulated by Osad’ko and more recently by us. In Paper I of this series we evaluated our general results for the linewidth and shift for models of electronic coupling to both acoustic and optical phonons. Here we evaluate our general results for a simple model of electronic coupling to a pseudolocal phonon. The pseudolocal phonon is characterized by its frequency and lifetime in both the ground and excited electronic states. In addition to providing exact analytic expressions (reduced to quadrature) for the width and shift, we derive an approximate analytic expression for the linewidth that is bi-Arrhenius, with activation energies corresponding to the ground and excited state pseudolocal phonon frequencies. We discuss how our theoretical approach and results are related to the exchange theory of Harris, the optical Redfield theory of deBree and Wiersma, and the perturbation theory of Small and Jones and Zewail. In Paper III of this series, we compare the results of this paper and of Paper I to experiment.

Adiabatic and self-consistent-field approximations for coupled vibrations: A simple two-mode comparison

A simple coupled harmonic oscillator model is used to test the SCF and adiabatic approximations for coupled-mode systems. Both approximations are quite good; both fail when the unperturbed frequencies of the two modes approach degeneracy. While for two-mode systems the approximations are roughly comparable, for real molecules, the more easily applied SCF is preferable. Semiclassical quantization may be used with both approximations.

Seismic coupling and uncoupling at subduction zones

Seismic coupling has been used as a qualitative measure of the “interaction” between the two plates at subduction zones. Kanamori (1971) introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern Pacific subduction zones. A quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables: age of the subducting lithosphere and convergence rate. The largest earthquakes occur in zones with young lithosphere and fast convergence rates, while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes. Results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane (asperities are strong regions that resist the motion between the two plates). The zones with the largest earthquakes have very large asperities, while the zones with smaller earthquakes have small scattered asperities. This observation can be translated into a simple model of seismic coupling, where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. While the variation in asperity size is used to establish a connection between earthquake size and tectonic stress, it also implies that plate age and rate affect the asperity distribution. Plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the “preferred trajectory” (i.e. the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity). Indirect influences are many, including oceanic plate topography and the amount of subducted sediments. All subduction zones are apparently uncoupled below a depth of about 40 km, and we propose that the basalt to eclogite phase change in the down-going oceanic crust may be largely responsible. This phase change should start at a depth of 30–35 km, and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change.

Magnetic interactions in clusters studied by neutron inelastic scattering

Inelastic neutron scattering (INS) measurements have been performed on Ba 3CaRu 2O 9 and Cs 4CdNi 3F 12 compounds. The transitions between split levels of the electronic ground state have been observed. The values measured for the exchange parameters are close to those obtained from magnetic susceptibility measurements. Calculations based on a simple spin-spin coupling model are in good agreement with the transitions observed by INS.