Relative Coupling Strength Research Articles

Detecting common-cause relationships with directional information transfer

Abstract Correlations between sedimentary rock and fossil records may involve a combination of rock-record sampling bias and common response to external forcing. Quantifying their relative importance from incomplete and uncertain proxy data is not trivial given the potential complexity of interactions among the underlying processes. This paper shows how a non-parametric method can be used to detect causal interactions directly from incomplete and irregular time series, by quantifying directional information transfer between variables. A numerical experiment illustrates how estimates of the relative strength, scale, and directionality of coupling can correctly distinguish a common-cause variable from a spurious relationship, even in cases where correlations are misleading. With a joint analysis of Phanerozoic rock and fossil records pending, the method is applied to oxygen, carbon, and sulphur stable isotope records from marine carbonates, identifying complex interactions between climatic changes and the cycling of carbon and sulphur over the Phanerozoic.

Near-maximal mixing of scalar gluonium and quark mesons: A Gaussian sum-rule analysis

Gaussian QCD sum-rules are ideally suited to the study of mixed states of gluonium (glueballs) and quark ( q q ¯ ) mesons because of their capability to resolve widely-separated states of comparable strength. The analysis of the Gaussian QCD sum-rules (GSRs) for all possible two-point correlation functions of gluonic and non-strange ( I = 0 ) quark scalar ( J P C = 0 + + ) currents is discussed. For the non-diagonal sum-rule of gluonic and q q ¯ currents we show that perturbative and gluon condensate contributions are chirally suppressed compared to non-perturbative effects of the quark condensate, mixed condensate, and instantons, implying that the mixing of quark mesons and gluonium is of non-perturbative origin. The independent predictions of the masses and relative coupling strengths from the non-diagonal and the two diagonal GSRs are remarkably consistent with a scenario of two states with masses of approximately 1 GeV and 1.4 GeV that couple to significant mixtures of quark and gluonic currents. The mixing is nearly maximal with the heavier mixed state having a slightly larger coupling to gluonic currents than the lighter state.

Off-resonant quantum Zeno and anti-Zeno effects on entanglement

We examine the appearance of Zeno and anti-Zeno effects (Misra and Sudarshan 1977J. Math. Phys.18 756; Kofman and Kurizki 2000 Nature405 546; Kofman and Kurizki 1996 Phys. Rev. A 54 3750; Facchi et al 2001 Phys. Rev. Lett.86 2699) in the entanglement dynamics of two qubits off-resonantly coupled to the same lossy cavity when the unitary evolution of the system is interrupted by repeated projective measurements. We describe in detail these quantum effects by comparing the measurement-induced coarse-grained dynamics to the entanglement evolution in the absence of measurements in several scenarios (Francicaet al 2009 Phys. Rev. A 79 032310). In particular, we examine the strong and weak coupling regimes, the role of the relative coupling strengths between the two qubits and the reservoir and the effect of the detuning from the main cavity frequency. We show that the anti-Zeno effect can occur in the entanglement dynamics when the qubit frequencies are detuned from the main reservoir frequency. Furthermore, we find that Zeno and anti-Zeno effects can even appear sequentially many times as a function of the interval between the measurements. Finally, we show that, in the off-resonant regime, we can preserve the entanglement using the quantum Zeno effect more efficiently than in the resonant limit (Maniscalco et al 2008Phys. Rev. Lett.100 090503), even if, in this case, no sub-radiant state exists.

Energy levels of few-electron quantum dots imaged and characterized by atomic force microscopy

Strong confinement of charges in few-electron systems such as in atoms, molecules, and quantum dots leads to a spectrum of discrete energy levels often shared by several degenerate states. Because the electronic structure is key to understanding their chemical properties, methods that probe these energy levels in situ are important. We show how electrostatic force detection using atomic force microscopy reveals the electronic structure of individual and coupled self-assembled quantum dots. An electron addition spectrum results from a change in cantilever resonance frequency and dissipation when an electron tunnels on/off a dot. The spectra show clear level degeneracies in isolated quantum dots, supported by the quantitative measurement of predicted temperature-dependent shifts of Coulomb blockade peaks. Scanning the surface shows that several quantum dots may reside on what topographically appears to be just one. Relative coupling strengths can be estimated from these images of grouped coupled dots.

Direct Evidence for Nitrogen Ligation to the High Stability Semiquinone Intermediate in Escherichia coli Nitrate Reductase A

The membrane-bound heterotrimeric nitrate reductase A (NarGHI) catalyzes the oxidation of quinols in the cytoplasmic membrane of Escherichia coli and reduces nitrate to nitrite in the cytoplasm. The enzyme strongly stabilizes a menasemiquinone intermediate at a quinol oxidation site (Q(D)) located in the vicinity of the distal heme b(D). Here molecular details of the interaction between the semiquinone radical and the protein environment have been provided using advanced multifrequency pulsed EPR methods. (14)N and (15)N ESEEM and HYSCORE measurements carried out at X-band ( approximately 9.7 GHz) on the wild-type enzyme or the enzyme uniformly labeled with (15)N nuclei reveal an interaction between the semiquinone and a single nitrogen nucleus. The isotropic hyperfine coupling constant A(iso)((14)N) approximately 0.8 MHz shows that it occurs via an H-bond to one of the quinone carbonyl group. Using (14)N ESEEM and HYSCORE spectroscopies at a lower frequency (S-band, approximately 3.4 GHz), the (14)N nuclear quadrupolar parameters of the interacting nitrogen nucleus (kappa = 0.49, eta = 0.50) were determined and correspond to those of a histidine N(delta), assigned to the heme b(D) ligand His-66 residue. Moreover S-band (15)N ESEEM spectra enabled us to directly measure the anisotropic part of the nitrogen hyperfine interaction (T((15)N) = 0.16 MHz). A distance of approximately 2.2 Abetween the carbonyl oxygen and the nitrogen could then be calculated. Mechanistic implications of these results are discussed in the context of the peculiar properties of the menasemiquinone intermediate stabilized at the Q(D) site of NarGHI.

The entanglement between two isolated atoms in the double mode–mode competition model

Extending the double Jaynes–Cummings model to a more complicated case where the mode–mode competition is considered, we investigate the entanglement character of two isolated atoms by means of concurrence, and discuss the dependence of atom–atom entanglement on the different initial state and the relative coupling strength between the atom and the corresponding cavity field. The results show that the amplitude and the period of the atom–atom entanglement evolution can be controlled by the choice of initial state and relative coupling strength, respectively. We find that the phenomenon of entanglement sudden death (ESD) is sensitive to the initial conditions. The length of the time interval for zero entanglement depends not only on the initial degree of entanglement between two atoms but also on the relative coupling strength of atom–field interaction. The ESD effect can be weakened by enhancing the mode–mode competition between the three- and single-photon processes.

Grouping decomposition under constraints for design/build life cycle in project delivery system

The design/build project delivery system has been adopted in the construction industry for many years. The numerous iterations of activities (tasks) can result in a hierarchical structure that reflects the life cycle of this type of delivery system. The Design Structure Matrix (DSM) approach has been used for simplifying complex project activities by clarifying the relationships between activities through grouping. However, if there are a large number of coupled activities it can increase the design/build cycle time and it is difficult to apply grouping decomposition. This paper presents the Grouping Decomposition (GD) model that decomposes a large number of activities through grouping. It is used together with the K-means clustering algorithm which quantifies the relative coupling strength of the design/build activities. An example is proposed that demonstrates the GD modelling method. The results are compared with different grouping methods. These show that the proposed GD model performs well in partitioning and extremely well in clustering the coupled activities.

Information Entropy Squeezing for a Atom in Mode-Mode Competition System

The entropy squeezing properties for a two-level atom interacting with a two-mode field via two different competing transitions are investigated from a quantum information point of view. The influences of the initial state of the system and the relative coupling strength between the atom and the field on the atomic information entropy squeezing are discussed. Our results show that the squeezed direction and the frequency of the information entropy squeezing can be controlled by choosing the phase of the atom dipole and the relative competing strength of atom-field, respectively. We find that, under the same condition, no atomic variance squeezing is predicted from the HUR while optimal entropy squeezing is obtained from the EUR, so the quantum information entropy is a remarkable precision measure for the atomic squeezing in the considered system.

Quantum entanglement in the mode-mode competition system

Considering a two-level atom interacting with the competing two-mode field, this paper investigates the entanglement between the two-level atom and the two-mode field by using the quantum reduced entropy, and that between the two-mode field by using the quantum relative entropy of entanglement. It shows that the two kinds of entanglement are dependent on the relative coupling strength of atom-field and the atomic distribution, and exhibit the periodical evolution. The maximal atom–field entanglement state can be prepared via the appropriate selection of system parameters and interaction time.

Adiabatic charge and spin pumping through quantum dots with ferromagnetic leads

We study the adiabatic pumping of electrons through quantum dots attached to ferromagnetic leads. Hereby, we make use of a real-time diagrammatic technique in the adiabatic limit that takes the strong Coulomb interaction in the dot into account. We analyze the degree of spin polarization of electrons pumped from a ferromagnet through the dot to a nonmagnetic lead (N-dot-F) as well as the dependence of the pumped charge on the relative leads' magnetization orientations for a spin-valve (F-dot-F) structure. For the former case, we find that, depending on the relative coupling strength to the leads, spin and charge can, on average, be pumped in opposite directions. For the latter case, we find an angular dependence of the pumped charge, which becomes more and more anharmonic for large spin polarization in the leads.

Propagation in a directional coupler of parallel microring coupled-resonator optical waveguides

The fundamental concept of coupled-resonator optical waveguides (CROWs) is for the first time taken a step farther to propose the more complicated case of coupled-resonator directional couplers, by analyzing the phenomenon of propagation in two parallel chains of microring resonators. Using the method of transfer matrices it is shown that the infinite coupler supports even and odd supermodes that satisfy frequency-shifted replicas of the familiar CROW dispersion equation as well as modified periodic Floquet conditions. Depending on the relative coupling strengths between neighboring resonators in the same chain and between the two chains, the frequency bands of the supermodes can overlap or be completely separated, resulting, in the latter case, to a complete bandgap around the central resonance frequency of the microrings. Three frequency windows, corresponding to totally different properties of directional coupling, are defined through this band-overlap mechanism along the increased total bandwidth of the coupler. These predictions are fully confirmed in the response of the realistic finite-length version of the considered coupler which is also analyzed.

Spin relaxation at the singlet-triplet crossing in a quantum dot

We study spin relaxation in a two-electron quantum dot in the vicinity of the singlet-triplet crossing. The spin relaxation occurs due to a combined effect of the spin-orbit, Zeeman, and electron-phonon interactions. The singlet-triplet relaxation rates exhibit strong variations as a function of the singlet-triplet splitting. We show that the Coulomb interaction between the electrons has two competing effects on the singlet-triplet spin relaxation. One effect is to enhance the relative strength of spin-orbit coupling in the quantum dot, resulting in larger spin-orbit splittings and thus in a stronger coupling of spin to charge. The other effect is to make the charge density profiles of the singlet and triplet look similar to each other, thus diminishing the ability of charge environments to discriminate between singlet and triplet states. We thus find essentially different channels of singlet-triplet relaxation for the case of strong and weak Coulomb interactions. Finally, for the linear in momentum Dresselhaus and Rashba spin-orbit interactions, we calculate the singlet-triplet relaxation rates to leading order in the spin-orbit interaction and find that they are proportional to the second power of the Zeeman energy, in agreement with recent experiments on triplet-to-singlet relaxation in quantum dots.

An improved analysis of the SSHI interface in piezoelectric energy harvesting

This paper provides an analysis for the performance evaluation of a piezoelectric energyharvesting system using the synchronized switch harvesting on inductor (SSHI) electronicinterface. In contrast with estimates based on a variety of approximations in the literature,an analytic expression of harvested power is derived explicitly and validated numerically forthe SSHI system. It is shown that the electrical response using an ideal SSHI interface issimilar to that using the standard interface in a strongly coupled electromechanicalsystem operated at short circuit resonance. On the other hand, if the SSHI circuitis not ideal, the performance degradation is evaluated and classified accordingto the relative strength of coupling. It is found that the best use of the SSHIharvesting circuit is for systems in the mid-range of electromechanical coupling. Thedegradation in harvested power due to the non-perfect voltage inversion is notpronounced in this case, and a new finding shows that the reduction in power is muchless sensitive to frequency deviations than that using the standard technique.

Tuning the Kondo Effect with a Mechanically Controllable Break Junction

We study electron transport through C(60) molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and the height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The linear conductance as a function of T/T(K) agrees with the scaling function expected for the spin-1/2 Kondo problem. We are also able to tune finite-bias Kondo features which appear at the energy of the first C(60) intracage vibrational mode.

Atom–photon entanglement in the system with competingk-photon andl-photon transitions

We have investigated the evolution of the atomic quantum entropy and the entanglement of atom–photon in the system with competing k-photon and l-photon transitions by means of fully quantum theory, and examined the effects of competing photon numbers (k and l), the relative coupling strength between the atom and the two-mode field (λ/g), and the initial photon number of the field on the atomic quantum entropy and the entanglement of atom–photon. The results show that the multiphoton competing transitions or the large relative coupling strength can lead to the strong entanglement between atoms and photons. The maximal atom–photon entanglement can be prepared via the appropriate selection of system parameters and interaction time.

Theoretical study of the structure and electronic spectra of fully protonated emeraldine oligomers

AbstractPolyaniline (PANI) is one of the most studied conducting polymers. Obtained in its conducting form (known as “emeraldine salt”) by chemical or electrochemical oxidation of aniline in aqueous acidic medium, this polymer manifests an array of attractive properties. Nevertheless, these properties still need to be described at the molecular level. Intense theoretical investigations during the past few years aim at explaining the chain organization, conductivity mechanism, and other structural and spectral characteristics. Most studies adopt simplified models in which hydration effect is underestimated, since all simulations are performed either in vacuum or in the presence of a limited number of water molecules. The present computational study sheds light on the molecular organization of a number of model PANI hydrated clusters with different alignment and multiplicity, which can explain the experimentally recorded UV/VIS spectra. The influence of hydration and interaction with adjacent oligomers is estimated. Short‐chain doubly protonated emeraldine oligomers are used as model systems. The calculations are performed at the semi‐empirical (AM1) and/or molecular mechanics (AMBER96) level. Proper configurations of the clusters are selected using Monte Carlo simulations. Electron correlation (CIS) is accounted for upon evaluation of the absorption spectra of the clusters. The relative strength of the interchain coupling is estimated by simulation of PANI clusters consisting of two PANI tetramers in water. Comparison to experimental results is made. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Diffusion of Calcium and Metabolites in Pancreatic Islets: Killing Oscillations with a Pitchfork

Cell coupling is important for the normal function of the β-cells of the pancreatic islet of Langerhans, which secrete insulin in response to elevated plasma glucose. In the islets, electrical and metabolic communications are mediated by gap junctions. Although electrical coupling is believed to account for synchronization of the islets, the role and significance of diffusion of calcium and metabolites are not clear. To address these questions we analyze two different mathematical models of islet calcium and electrical dynamics. To study diffusion of calcium, we use a modified Morris-Lecar model. Based on our analysis, we conclude that intercellular diffusion of calcium is not necessary for islet synchronization, at most supplementing electrical coupling. Metabolic coupling is investigated with a recent mathematical model incorporating glycolytic oscillations. Bifurcation analysis of the coupled system reveals several modes of behavior, depending on the relative strength of electrical and metabolic coupling. We find that whereas electrical coupling always produces synchrony, metabolic coupling can abolish both oscillations and synchrony, explaining some puzzling experimental observations. We suggest that these modes are generic features of square-wave bursters and relaxation oscillators coupled through either the activation or recovery variable.

A technique for distinguishing dynamical species in the temperature time series of Yangtze River delta

The conditional entropy of two symbolic sequences which are encoded faithfully from two different time series is minimized at the zero relative shift of the two time series if the two time series have their origins in the same underlying dynamics. This technique is also useful in studying the delayed coupling between different time series. Applications are made to the temperature data derived from five meteorological stations in China, namely Shanghai, Nantong, Changzhou, Nanjing and Hangzhou in order to test the commonality of their underlying dynamics, obtain the relative strength of the dynamics coupling between different variables, and extract the implicit delayed information in the dynamics.

Two-step depinning and reentrant superconducting phase of moving vortex lattice in disordered superconductors

We developed a 3D flux_line lattice model with magnetic interactions between intraplane and interplane vortices to simulate the transport properties and the dynamic behavior of disordered anisotropic superconductors. We observed a double peak in the differential resistivity as the driving current increases, which corresponds to a two-step depinning in the vortex motion. Between the two peaks we also observed a reentering pinning phase. We also show that the transition from 2D plastic flow to 3D elastic flow of vortex motion is a recoupling transition which is associated with the double peak in differential resistivity. The increase of critical current of 3D-2D transition with the increasing of magnetic field (decreasing of relative interlayer coupling strength), indicates of a second peak effect.

Magnetic susceptibility of quasi-one-dimensional Ising superantiferromagnets FeTAC and MCl2 · 2NC5H5 (M = Co, Fe): Approximation with L × ∞ and L × L × ∞ chain clusters

The temperature dependence of the zero-field susceptibilities of 2D and 3D Ising lattices with anisotropic coupling is analyzed. Infinite 2D and 3D lattices are approximated, respectively, by ensembles of independent L x oo and L x L x oo chain clusters that are infinitely long in the strong-coupling (J) direction. This approach is used as a basis for a quantitative description of available experimental data on the magnetic susceptibilities of the 2D anisotropic Ising magnet [(CH_3)_3NH]FeCl_3*2H_2O (FeTAC) and the quasi-one-dimensional 3D magnets CoCl_2*2NC_5H_5 and FeCl_2*2NC_5H_5 in the entire experimental temperature range. A method is proposed for determining the relative interchain coupling strength J'/J from the maximum susceptibility value, which improves the accuracy of estimates for J'/J by more than an order of magnetude.