Relative Coupling Strength Research Articles

Pulse retrieval and soliton formation in a nonstandard scheme for dynamic electromagnetically induced transparency

We examine in detail an alternative method of retrieving the information written into an atomic ensemble of three-level atoms using electromagnetically induced transparency. We find that the behavior of the retrieved pulse is strongly influenced by the relative atom-light coupling strengths of the two relevant transitions. When the individual atom-light coupling strength for the retrieval beam is the stronger of the two transitions, regeneration of the stored pulse is possible. Otherwise, we show the retrieval process can lead to creation of solitonlike pulses.

Transmission probability of the two-mode mazer with injected atomic coherence

The transmission probability of the two-mode mazer injected with V-type three-level atoms is studied and the effects of the atomic coherence on it are examined. It is shown that the atomic coherence can affect the transmission probability. In the plots of the atomic transmission probability versus the dimensionless centre-of-mass momentum, there are resonance peaks and non-resonance platforms. The heights of these resonance peaks and non-resonance platforms can be adjusted by the atomic coherence parameter and by the relative coupling strength of the two transition channels.

Orbital-only models: ordering and excitations

We consider orbital-only models in Mott insulators, where the orbital–orbital interactions are either due to Jahn–Teller distortions or due to the Kugel–Khomskii superexchange. This leads to highly anisotropic and frustrated orbital Hamiltonians. For two-fold degenerate eg systems, both types of orbital interactions lead to the same form of the Hamiltonian—the 120° model. In both cases, the predicted symmetry of the orbital ordering is the same, although different from the one observed experimentally. The orbital operators that appear in the two kinds of orbital-only Hamiltonians are different. In the case of superexchange, the orbital degrees of freedom are represented by quantum pseudo-spin 1/2 operators. But when the interactions are Jahn–Teller mediated and the coupling with the lattice is strong, the orbital operators are essentially classical pseudospins. Thus as a function of the relative coupling strengths, a quantum-to-classical crossover is expected. For three-fold degenerate t2g orbitals, the Jahn–Teller coupling gives rise to a particular type of orbital compass models. We point out that fluctuations—whether due to quantum effects or finite temperature—are of prime importance for ordering in the 120° and orbital compass models. The fluctuations generally generate a gap in the orbital excitation spectrum. These orbital excitations—orbitons—are hybrid excitations that carry both a lattice Jahn–Teller and a magnetic Kugel–Khomskii character.

Boundary and impurity effects on the entanglement of Heisenberg chains.

We study entanglement of a pair of qubits and the bipartite entanglement between the pair and the rest within open-ended Heisenberg XXX and XY models. The open boundary condition leads to strong oscillations of entanglements with a two-site period, and the two kinds of entanglements are 180 degrees out of phase with each other. The mean pairwise entanglement and ground-state energy per site in the XXX model are found to be proportional to each other. We study the effects of a single bulk impurity on entanglement, and find that there exist threshold values of the relative coupling strength between the impurity and its nearest neighbors, after which the impurity becomes pairwise entangled with its nearest neighbors.

Analysis of the vibronic fine structure in circularly polarized emission spectra from chiral molecular aggregates.

Using a Frenkel-exciton model, the degree of circular polarization of the luminescence (g(lum)) from one-dimensional, helical aggregates of chromophoric molecules is investigated theoretically. The coupling between the electronic excitation and a local, intramolecular vibrational mode is taken into account. Analytical expressions for the fluorescence band shape and g(lum) are presented for the case of strong and weak electronic coupling between the chromophoric units. Results are compared to those from numerical calculations obtained using the three particle approximation. g(lum) for the 0-0 vibronic band is found to be independent of the relative strength of electronic coupling between chromophores and excitation-vibration coupling. It depends solely on the number of coherently coupled molecules. In contrast, for the higher vibronic transitions[g(lum)] decreases with decreasing strength of the electronic coupling. In the limit of strong electronic coupling, [g(lum)] is almost constant throughout the series of vibronic transitions but for weak coupling [g(lum)] becomes vanishingly small for all vibronic transitions except for the 0-0 transition. The results are interpreted in terms of dynamic localization of the excitation during the zero point vibrational motion in the excited state of the aggregate. It is concluded that circular polarization measurements provide an independent way to determine the coherence size and bandwidth of the lowest exciton state for chiral aggregates.

Symbol sequence analysis of climatic time signals

The conditional entropy of two symbolic sequences encoded faithfully from different chaotic time signal data is minimised at zero relative shift of the two signals if the signals have their origin in same underlying dynamics. We show that this minimum in conditional entropies E( Z/ X) and E( X/ Z) obtained after suitably ‘coarse–graining’ the time signals of, say, variables X and Z have marked differences depending upon the degree of dynamical coupling of the variables in the model. This technique has also been shown to be useful in studying delayed dependences of time signals. Application is made to climate data of four meteorological stations in India, namely New Delhi, Jaipur, Sundernagar and Chennai in order to determine (a) the commonality of underlying dynamics, (b) relative strength of dynamical coupling of different variables and (c) the delay implicit in the dynamics. The method appears robust to measurement noise.

Three numerical puzzles and the top quark’s chiral weak moment

Versus the standard model's t --> W b decay helicity amplitudes, three numerical puzzles occur at the 0.1 % level when one considers the amplitudes in the case of an additional (f_M + f_E) coupling of relative strength 53 GeV. The puzzles are theoretical ones which involve the t --> W b decay helicity amplitudes in the two cases, the relative strength of this additional coupling, and the observed masses of these three particles. A deeper analytic realization is obtained for two of them. Equivalent realizations are given for the remaining one. An empirical consequence of these analytic realizations is that it is important to search for effects of a large chiral weak-moment of the top-quark, the effective mass-scale is about 53 GeV. A full theoretical resolution would include relating the origin of such a chiral weak-moment and the mass generation of the top-quark, the W-boson, and probably the b-quark.

Orbital correlations in doped manganites

We review our recent x-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable from the undoped structure with long range orbital order at low temperatures. For dopings 0.3<x<0.5, we find scattering consistent with a charge and orbitally ordered CE-type structure. While in each case the charge order peaks are resolution limited, the orbital order exhibits only short range correlations. We report the doping dependence of the correlation length and discuss the connection between the orbital correlations and the finite magnetic correlation length observed on the Mn^3+ sublattice with neutron scattering techniques. The physical origin of these domains, which appear to be isotropic, remains unclear. We find that weak orbital correlations persist well above the phase transitions, with a correlation length of 1-2 lattice constants at high temperatures. Significantly, we observe similar correlations at high temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered ground state, and we conclude that such correlations are robust to variations in the relative strength of the electron-phonon coupling.

Correlated polarons in dissimilar perovskite manganites

We report x-ray scattering studies of broad peaks located at (0.5 0 0)/(0 0.5 0)-type wave vectors in the paramagnetic insulating phases of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ and ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}.$ We interpret the scattering in terms of correlated polarons and measure isotropic correlation lengths of 1-2 lattice constants in both samples. Remarkably, the size of these correlated polarons remains constant over the entire temperature range investigated. In ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3},$ this range extends up to \ensuremath{\sim}400 K, at which temperature the peaks are observed to disappear. Based on the wavevector, the correlated polarons are found to be consistent with a CE-type structure. Differences in behavior between the samples arise as they are cooled through their respective transition temperatures and become ferromagnetic metallic $({\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3})$ or charge and orbitally ordered insulating $({\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}).$ Since the primary difference between the two samples is the trivalent cation size, these results illustrate the robust nature of the correlated polarons to variations in the relative strength of the electron-phonon coupling, in contrast to the sensitivity of the low-temperature ground state to such variations.

On the nonlinear turbulent dynamics of shear-flow decorrelation and zonal flow generation

Sheared flows, thought to be generated by turbulence in magnetized fusion plasmas, are predicted to mediate the transport of mass, momentum, and heat across the shear flow region. In this paper we show that an examination of three-wave coupling processes using the bispectrum and bicoherence of turbulent fields provides an experimentally accessible test of the turbulence-generated shear flow hypothesis. Results from the Continuous Current Tokamak (CCT), Princeton Beta Experiment–Modified (PBX–M), and DIII–D tokamaks indicate that the relative strength of three-wave coupling increases during low-mode to high-mode (L–H) transitions and that this increase is localized to the region of strong flow and strong flow shear. These results appear to be qualitatively consistent with the turbulence-generated shear flow hypothesis.

Static and Dynamic Coupling Transitions of Vortex Lattices in Disordered Anisotropic Superconductors

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current reminiscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV/dI.

Domain formation in Fe–N multilayers

We prepared a series of sputtered FeN single layer films, and bilayer and multilayer films laminated with alumina, in which both the Fe–N thickness and the alumina interlayer thickness varied. The films were characterized by quantitative magneto-optical Kerr-microscopy and inductive measurements. We found a strong influence of interlayer coupling and FeN thickness on the domain formation in these layers. For single layers a transition from cross-tie walls to Bloch-type walls for increasing ferromagnetic layer thickness was observed. The multilayer films displayed a wide range of different types of coupled Neel walls depending on interlayer thickness. These include superimposed, compensated, and 360° walls. The change in domain wall structure strongly influences the coercivity of the thin film systems. The relative coupling strength between the magnetic films is calculated from the domain wall width of the superimposed Neel walls. Magnetic in-plane deviation strongly depends on the sign of magnetostriction. The observed magnetic dispersion leads to an increase in the minimum magnetic switching time. The observed micromagnetic structures result in unwanted irregular domain patterns in structured samples. Both are indicating limitations for applications in thin film recording heads.

Higgs boson decays to τ-pairs in the s-channel at a muon collider

We study the observability of the τ − τ + decay mode of a Higgs boson produced in the s-channel at a muon collider. We find that the spin correlations of the τ − τ + in τ→ πν τ , ρν τ decays are discriminative between the Higgs boson signal and the Standard Model background. Observation of the predicted distinctive distribution can confirm the spin-0 nature of the Higgs resonance. The relative coupling strength of the Higgs boson to b and τ can also be experimentally determined.

The Influence of Vibrational Excitation of NO+(X1Σ+ : v″) on the Formation of NO* by the NO+ + C6F6− Neutralization Reaction at Thermal Energy

Abstract The NO(A2Σ+-X2Πr, C2Πr-X2Πr, D2Σ+-X2Πr) emissions resulting from the mutual ion–ion neutralization reaction between NO+(X1Σ+) and C6F6− have been measured at various source gas pressures of NO+ in a flowing afterglow in order to examine the influence of vibrational excitation in NO+(X : v″). N2O was used as a source gas of NO+(X). The NO(A-X, C-X, D-X) emissions from only the v′ = 0 level were observed at high N2O pressures above ca. 60 mTorr (1 Torr = 133.322 Pa), where only NO+(X : v″ = 0) was present. On the other hand, the NO(A-X) emission from v′ = 1, 2 and the NO(D-X) emission from v′ = 1 appeared at low N2O pressures below ca. 60 mTorr, where NO+(X : v″ &amp;gt; 0) was present. It was found that vibrational excitation in NO+(X : v″) results in vibrational excitation of the product NO(A, D : v′ = v″) molecule because of favorable Franck–Condon factors for the NO+(X : v″) → NO(A, D : v′ = v″) neutralization. The fact that the vibrational excitation of NO(D) is slightly higher than that of NO(A) indicated that the relative coupling strength between NO+(X : v″ &amp;gt; 0) + C6F6− and NO(D : v′ = v″) + C6F6 to that between NO+(X : v″ = 0) + C6F6− and NO(D : v′ = 0) + C6F6 is larger than that in the corresponding processes leading to NO(A : v′ = v″). The lack of vibrational excitation of NO(C) was explained by the predissociation of NO(C : v′ &amp;gt; 0).

Spectral analysis of oscillatory neural circuits

Oscillatory dynamics are found at all levels of the nervous system. The goal of our current research on the control of rhythmic motor output by the lamprey spinal cord is to determine the features of neuronal coupling that lead to stable oscillatory activity and precisely-controlled intersegmental phase. Since our experimental manipulations can greatly increase the variability of the ventral root bursting pattern, it is important for us to employ a data analysis method which remains valid independent of this variability. Traditional analysis approaches which rely on identification of burst event times do not generally satisfy this requirement. In this paper, we illustrate the application of a straightforward statistically-based method for determining important parameters of oscillatory motor circuits using Fourier spectral analysis of spike trains. The frequency, phase, and their variabilities can be quantified; and the relative strength of coupling between different parts of the circuit can be tested for statistical significance. The approach we adopt is highly convenient for neuroscientists who study oscillatory systems as it operates directly on trains of action potentials stored as lists of event times (point-processes). Basic concepts and practical issues concerning use of Fourier analysis are discussed.

Effect of interfacial roughness on exchange coupling (invited) (abstract)

The magnitude and the direction of the oscillatory exchange coupling in multilayer structures depend sensitively on the atomic scale ordering of the layers. Disorder in the form of substrate roughness, interlayer thickness fluctuations, or interfacial interdiffusion can result in reduced bilinear exchange coupling, the disappearance of short period coupling oscillations, and the increase or decrease of the relative strength of the biquadratic coupling. We have investigated the effect of roughness by growing nearly perfect multilayer Fe/Cr and Fe/Au structures on single crystal Fe whisker substrates. The disorder in these structures was measured using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED), while the coupling directions and magnitudes were measured using scanning electron microscopy with polarization analysis and magneto-optic Kerr microscopy. For the nearly perfect trilayer structures, the periodicity and the magnitude of the oscillatory coupling are in agreement with theoretical predictions based on spacer layer Fermi surface properties. For rougher multilayer systems the exchange coupling can be modeled using structural information from STM and RHEED measurements.

Influence of the strongly dispersive transverse acoustic phonon mode on the elastic constants C11 and C′ 66 of α-quartz below 300 K

From measurements of the velocities of longitudinal and transverse ultrasonic waves along the x- and ac-axes, respectively, the temperature dependence of the elastic constants C 11 and C′ 66 of synthetic quartz crystals has been determined in the temperature range 4–300 K. Analysis of the experimental results allows an estimate to be made of the relative strength of the anharmonic coupling of the ultrasonic waves to the strongly dispersive transverse acoustic mode.

A Monte Carlo study of the temperature dependence of magnetic order on ferromagnetic and antiferromagnetic surfaces: Implications for spin-polarized photoelectron diffraction (abstract)

We have used Ising-model Monte Carlo calculations to study the magnetic order near cubic ferromagnetic and antiferromagnetic surfaces. The antiferromagnets were studied with and without frustrated next-nearest-neighbor interactions. Intralayer and interlayer spin-spin correlation functions have been calculated as a function of the relative coupling strength in the surface layer. If this coupling strength is more than a few times the bulk value, a distinct surface phase transition is observed at temperatures TC,surf or TN,surf that can be significantly above the corresponding bulk values, TC,bulk or TN,bulk. These calculations suggest that previous spin-polarized photoelectron diffraction measurements on antiferromagnetic KMnF3(110) and MnO(001) could in fact have been observing such surface transitions at TN,surf values of 2.7 and 4.5 times TN,bulk, respectively.

Electron transport and phonon coupling in K3C60.

The temperature dependence of the resistivity of single-crystal ${\mathrm{K}}_{3}$${\mathrm{C}}_{60}$ below 260 K of Crespi et al. is analyzed utilizing the results of Raman scattering. It is found that coupling to the two lowest intramolecular ${\mathit{H}}_{\mathit{g}}$ modes gives an excellent fit to the temperature variation of the resistivity with the inclusion of weak lower frequency acoustic or librational scattering. The relative coupling strength of ${\mathit{H}}_{\mathit{g}}$(1) to ${\mathit{H}}_{\mathit{g}}$(2) is found to be similar to that estimated from Raman studies of thin films. The weaker low-frequency contribution is found to have a value consistent with neutron-scattering studies.

WW scattering with scalar and vector resonances.

We derive partial wave amplitudes for the scattering of longitudinal [ital W]'s interacting strongly at TeV energies, with resonances in the scalar and/or vector channels. The amplitudes are written in a simple analytic form which satisfies unitarity, analyticity, and approximate crossing, and can accommodate the variation of scalar and vector masses and their relative coupling strengths. These amplitudes provide a convenient parametrization for phenomenological studies of the strong [ital WW] scattering.