Asymmetry Of Coupling Research Articles

Thermoelectric transport through a quantum dot with a magnetic impurity

We study the thermoelectric effect in a small quantum dot with a magnetic impurity in the Coulomb blockade regime. The electrical conductance, thermal conductance, thermopower, and the thermoelectrical figure of merit (FOM) are calculated by using Green's function method. It is found that the peaks in the electrical conductance are split by the exchange coupling between the electron entering into the dot and the magnetic impurity inside the dot, accompanied by the decrease in the height of peaks. As a result, the resonances in the thermoelectric quantities, such as the thermal conductance, thermopower, and the FOM, are all split, opening some effective new working regions. Despite of the significant reduction in the height of the electrical conductance peaks induced by the exchange coupling, the values of the FOM and the thermopower can be as large as those in the case of zero exchange coupling. We also find that the thermoelectric efficiency, characterized by the magnitude of the FOM, can be enhanced by adjusting the left—right asymmetry of the electrode-dot coupling or by optimizing the system's temperature.

Estimating coupling directions in the cardiorespiratory system using recurrence properties

The asymmetry of coupling between complex systems can be studied by conditional probabilities of recurrence, which can be estimated by joint recurrence plots. This approach is applied for the first time on experimental data: time series of the human cardiorespiratory system in order to investigate the couplings between heart rate, mean arterial blood pressure and respiration. We find that the respiratory system couples towards the heart rate, and the heart rate towards the mean arterial blood pressure. However, our analysis could not detect a clear coupling direction between the mean arterial blood pressure and respiration.

Intrinsic spin Hall effect at asymmetric oxide interfaces: Role of transverse wave functions

An asymmetric triangular potential well provides the simplest model for the confinement of mobile electrons at the interface between two insulating oxides, such as LaAlO${}_{3}$ and SrTiO${}_{3}$ (LAO/STO). These electrons have been recently shown to exhibit a large spin-orbit coupling of the Rashba type, i.e., linear in the in-plane momentum. In this paper we study the intrinsic spin Hall effect due to Rashba coupling in an asymmetric triangular potential well. This is the minimal model that captures the asymmetry of the spin-orbit coupling on opposite sides of the interface. Besides splitting each subband into two branches of opposite chirality, the spin-orbit interaction causes the transverse wave function (i.e., the wave function in the $z$ direction, perpendicular to the plane of the quantum well) to depend on the in-plane wave vector $\mathbf{k}$. At variance with the standard Rashba model, the triangular well supports a nonvanishing intrinsic spin Hall conductivity, which is proportional to the square of the spin-orbit coupling constant and, in the limit of low carrier density, depends only on the effective mass renormalization associated with the $\mathbf{k}$ dependence of the transverse wave functions. The origin of the effects lies in the nonvanishing matrix elements of the spin current between subbands corresponding to different states of quantized motion perpendicular to the plane of the well.

Phase-lag return mappings for a 3 cell multifunctional central pattern generator

We describe and expand on a novel computational approach to reduce detailed models of central pattern generation to equationless return mapping for the phase lags between the constituting bursting interneurons [1]. Such mappings are then studied geometrically as the model parameters, including coupling properties of inhibitory and excitatory synapses, or external inputs are varied. Bifurcations of the fixed points and invariant circles of the mappings corresponding to various types of rhythmic activity are examined. These changes uncover possible biophysical mechanisms for control and modulation of motor-pattern generation. Our analysis does not require knowledge of the equations that model the system, and so provides a powerful new approach to studying detailed models, applicable to a variety of biological phenomena beyond motor control. Motifs of three coupled cells are a common network configuration including models of biological central pattern generators. We demonstrate our technique on a motif of three reciprocally coupled, inhibitory and excitatory, cells that is able to produce multiple patterns of bursting rhythms. In particular, we examine the qualitative geometric structure of two-dimensional maps for phase lag between the cells. This reveals the organizing centers of emergent polyrhythmic patterns and their bifurcations, as the asymmetry of the synaptic coupling is varied. The presence of multistability and the types of attractors in the network are shown to be determined by the duty cycle of bursting, as well as coupling interactions. Figure 1 (A). We defined phase-lags for a 3 cell CPG model on a 2D torus. (B) A “flattened” phase-lag mapping showing convergence to 5 distinct attractors. Each stable fixed point represents a rhythmic output of the CPG

Quadrupole relaxation enhancement—application to molecular crystals

A general theory of field dependent spin-lattice relaxation for nuclei of the spin quantum number 1/2 ( 1H, 19F, 13C) caused by dipole–dipole interactions with neighboring quadrupolar nuclei (nuclei possessing a quadrupolar moment) is presented. The theory is valid for arbitrary motional conditions and should be treated as a quadrupolar counterpart of the paramagnetic relaxation enhancement theory. When the energy level splitting of the dipolar spin ( I=1/2) matches one of the transition frequencies of the quadrupolar nuclei one can observe a local enhancement of the dipolar spin relaxation (referred to as “quadrupolar peaks”). To see such effects the dynamics modulating the spin interactions has to be relatively slow. This brings the system beyond the validity range of perturbation approaches and requires the stochastic Liouville equation to be applied. The presented theory describes the quadrupolar relaxation enhancement (QRE) for an arbitrary spin quantum number of the quadrupolar nuclei and includes the asymmetry of the quadrupolar coupling. It has been applied to interpret the shape of magnetization curves (amplitude of 1H magnetization versus magnetic field) for the molecular crystal [C 3N 2H 5] 6[Bi 4Br 18] ([C 3N 2H 5]—imidazolium). The magnetization curves show several dips (local minima) attributed to 1H– 14N quadrupolar relaxation enhancement effects. In addition, as a limiting case a perturbation approach to QRE has been presented and its validity conditions have been discussed.

Models of Multifunctional Central Pattern Generators: Polyrhythmic Bursting

AbstractWe demonstrate a motif of three reciprocally inhibitory cells that is able to produce multiple patterns of bursting rhythms. Through the examination of the qualitative geometric structure of two-dimensional maps for phase lag between the cells we reveal the organizing centers of emergent polyrhythmic patterns and their bifurcations, as the asymmetry of the synaptic coupling is varied. The presence of multistability and the types of attractors in the network are shown to be determined by the duty cycle of bursting. This analysis does not require knowledge of the equations that model the system, and so provides a powerful new approach to studying regulatory networks. Thus, the approach is applicable to a variety of biological phenomena beyond motor control.

Tunable electron counting statistics in a single-molecule magnet

Based on an efficient particle-number-resolved quantum master equation, we study the full counting statistics of electron transport through a single-molecule magnet (SMM) weakly coupled to two metallic electrodes. It is demonstrated that the internal level structure of the SMM and the left-right asymmetry of the SMM-electrode coupling play a crucial role in the super-Poissonian statistics of electron transport. In particular, above the sequential tunneling threshold the shot noise depends not only on the gate voltage by which the internal level structure of the SMM can be tuned but also on the left-right asymmetry of the SMM-electrode coupling. Moreover, it was found that the temperature dependence of super-Poissonian shot noise also depends on the left-right asymmetry of the SMM-electrode coupling. The occurrence-mechanism of super-Poissonian shot noise can be qualitatively attributed to the competition between fast and slow transport channels.

Calculating the energy spectrum and electronic structure of two holes in a pair of strained Ge/Si coupled quantum dots

We theoretically investigate the energy spectrum and electronic structure of two vertically stacked Ge/Si quantum dots loaded with a pair of interacting holes, and study the dependence of basic physical characteristics on dot size and interdot separation. In particular, we focus on the splitting of the lowest spin singlet and triplet states and spatial correlations between holes. Here the term ``spin'' is rather an index for two Kramers degenerate states. The two holes are treated by six-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ calculations combined with the configuration-interaction model taking into account the realistic situation when both Si matrix and Ge nanoclusters are inhomogeneously strained. It is shown that asymmetry of strain distribution and spin-orbit coupling of the valence band introduce characteristic features in both single- and many-particle hole states, which are not captured by the usual one-band heavy-hole approximation. We find a level anticrossing between the lowest singlet and triplet states with a zero anticrossing energy gap as a function of interdot spacing. It is demonstrated that the level anticrossing between many-particle states is accompanied by a level crossing between single-particle bonding and antibonding molecular orbitals. We argue that both phenomena of level crossing and anticrossing originate from the asymmetry-induced localization of single-particle hole wave functions on different dots. Above a certain dot separation, the Mott-type delocalized-to-localized transition for the many-particle hole states is observed. We show that the transition is caused by the combined action of correlations and strain asymmetry.

Noise spectra of a biased quantum dot

The noise spectra associated with correlations of the current through a single level quantum dot, and with the charge fluctuations on the dot, are calculated for a finite bias voltage. The results turn out to be sensitive to the asymmetry of the dot's coupling to the two leads. At zero temperature, both spectra exhibit two or four steps (as a function of the frequency), depending on whether the resonant level lies outside or within the range between the chemical potentials on the two leads. In addition, the low-frequency shot noise exhibits dips in the charge noise, and dips, peaks, and discontinuities in the derivative of the current noise. In spite of some smearing, several of these features persist at finite temperatures, where a peak can also turn into a dip.

Synchronization in complex dynamical networks with nonsymmetric coupling

Based on the work of Nishikawa and Motter, who have extended the well-known master stability framework to include non-diagonalizable cases, we develop another extension of the master stability framework to obtain criteria for global synchronization. Several criteria for global synchronization are provided which generalize some previous results. The Jordan canonical transformation method is used in stead of the matrix diagonalization method. Especially, we show clearly that, the synchronizability of a dynamical network with nonsymmetric coupling is not always characterized by its second-largest eigenvalue, even though all the eigenvalues of the nonsymmetric coupling matrix are real. Furthermore, the effects of the asymmetry of coupling on synchronizability of networks with different structures are analyzed. Numerical simulations are also done to illustrate and verify the theoretical results on networks in which each node is a dynamical limit cycle oscillator consisting of a two-cell cellular neural network.

Microscopic Theory of Coupled Quantum Well Structures in Photovoltaics

AbstractWe use our recently developed microscopic approach to a quantum theory of photovoltaic processes in nanostructures [1]to investigate geometry effects in quantum well photovoltaics, focussing on the role of asymmetry and inter-well coupling in double quantum well (DQW) systems. For that purpose, the IV and power characteristics for DQW systems with different asymmetry and degree of coupling are calculated numerically in the vicinity of the maximum power point. In order to assess the transport properties of a specific QW structure, we isolate these from the effects of absorption by normalizing to the absorptivity. The results obtained from this procedure reveal the escape regime dominating at room temperature and confirm previous experimental observations.

Multidimensional wave packet dynamics in polypeptides: Coupled amide-exciton chain-vibrational motion in an α-helix

Wave packet dynamics in eight dimensions and for eight coupled “diabatic” states are discussed in using the multi configuration time-dependent Hartree method. For this purpose a reduced dimensionality model for an α-helical polypeptide is used which combines intra amide unit high frequency vibrations with the low frequency longitudinal vibrations of the units along a single chain of hydrogen bonds. The model comprises the possibility to form delocalized vibrational Frenkel exciton states as well as to localize them by an appropriate chain compression. As it has been already shown [D.V. Tsivlin, H.-D. Meyer, V. May, J. Chem. Phys. 124 (2006) 134907; D.V. Tsivlin, V. May, J. Chem. Phys. 125 (2006) 224902] the used model is also ready to give a satisfactory reproduction of infrared transient absorption spectra taken either in the absorption range of CO amide vibrations or NH amide vibrations. Wave packet dynamics following special initial states are discussed based on the reduced single chain coordinate probability distribution as well as on the expectation values of the chain coordinates themselves. The high frequency excitation energy transfer through the chain is found to be anisotropic due to the intrinsic asymmetry of the exciton chain vibrational coupling. It is accompanied by a displacement impulse of the amide units.

Giant Optical Activity in Quasi-Two-Dimensional Planar Nanostructures

We examine the spectral dependence in the visible frequency range of the polarization rotation of two-dimensional gratings consisting of chiral gold nanostructures with subwavelength features. The gratings, which do not diffract, are shown to exhibit giant specific rotation (approximately 10(4) degrees/mm) of polarization in direct transmission at normal incidence. The rotation is the same for light incident on the front and back sides of the sample. Such reciprocity indicates three dimensionality of the structure arising from the asymmetry of light-plasmon coupling at the air-metal and substrate-metal interfaces. The structures thus enable polarization control with quasi-two-dimensional planar objects. However, in contradiction with recently suggested interpretation of experiments on larger scale but otherwise similar structures, the observed polarization phenomena violate neither reciprocity nor time-reversal symmetry.

Spatially resolved measurements in r.f. capacitive discharges in argon and nitrogen

Plasma potential, electron concentration and energy distribution of ions collected by grounded surface in contact with plasma were measured in argon and nitrogen capacitively coupled r.f. discharges. Both diagnostics tools used, Langmuir probe and Balzers Plasma Process Monitor (PPM 421), were oriented parallel to the movable electrodes allowing a perpendicular spatially resolved measurements. The asymmetry of capacitive coupling was estimated from the plasma potential and d.c. self-bias. It increased for increasing power since the discharge extended towards the grounded PPM orifice, its tubular housing as well as chamber walls. The IEDs measured at the high power with PPM therefore reflected the situation at the grounded electrode. It exhibited only one peak which was typically found for low voltage modulated r.f. sheaths. The mean energy of Ar+ and N2+ ions was about 92% of the energy related to the plasma potential. Moreover, low energy electron tail appeared in their IEDs. These both facts are explained by collisions in the sheath. Since the mean free path of N+ ions is about one order higher we did not observed effect of collisions in their IEDs. The ion mean energies decreased towards the grounded electrodes in agreement with the plasma potential. The ion and electron concentrations showed similar profiles with the maximum about 12 mm above the grounded electrode.

Nonideal quantum detectors in Bayesian formalism

The Bayesian formalism for a continuous measurement of solid-state qubits is derived for a model which takes into account several factors of the detector nonideality. In particular, we consider additional classical output and backaction noises (with finite correlation), together with quantum-limited output and backaction noises, and take into account possible asymmetry of the detector coupling. The formalism is first derived for a single qubit and then generalized to the measurement of entangled qubits.

Nonlinear transport through a nanoscale molecule.

A simple model of electrical transport through a metal-aromatic molecule-metal system that includes charging effects as well as aspects of the electronic structure of the molecule, is presented. The interplay of a large charging energy and an asymmetry of the metal-molecule coupling can lead to various effects in nonlinear electrical transport. In particular, strong negative differential conductance is observed under certain conditions.

Effect of asymmetry on the loss of chaos synchronization.

We investigate the effect of asymmetry of coupling on the bifurcation mechanism for the loss of synchronous chaos in coupled systems. It is found that only when the symmetry-breaking pitchfork bifurcations take part in the process of the synchronization loss for the case of symmetric coupling, the asymmetry changes the bifurcation scenarios of the desynchronization. For the case of weak coupling, pitchfork bifurcations of asynchronous periodic saddles are replaced by saddle-node bifurcations, while for the case of strong coupling, pitchfork bifurcations of synchronous periodic saddles transform to transcritical bifurcations. The effects of the saddle-node and transcritical bifurcations for the weak asymmetry are similar to those of the pitchfork bifurcations for the symmetric-coupling case. However, with increasing the "degree" of the asymmetry, their effects change qualitatively, and eventually become similar to those for the extreme case of unidirectional asymmetric coupling.

Cobalt-59 chemical shift and quadrupolar tensors of simple octahedral cobalt(III) complexes

Analysis of the solid-state powder59Co NMR spectra of ten simple inorganic cobalt(III) complexes at 11.75, and in most cases, 4.7 T have permitted the assignment of specific ligand planes to ranges of values of the observed chemical shift principal components. The relevant chemical shift components were determined from the simulations of the powder line shapes. These simulations also provided the relative orientations of the chemical shift (CS) and electric field gradient (efg) tensors, as well as magnitude and asymmetry of the59Co quadrupolar coupling. Using symmetry arguments and ab initio calculations, as appropriate or necessary, the orientations of the efg tensors in the molecular frame were deduced. This allowed the determination of the CS tensors in the molecular frame and thus assignment of the ligand planes responsible for the observed values of chemical shifts.Key words: cobalt, chemical shift, quadrupolar coupling, solid state NMR.

Asymmetry of two-wave coupling in cubic photorefractive crystals

Analysis of two-wave coupling in cubic crystal classes 23 and 4¯3m is presented for arbitrary orientation of the grating vector and the applied electric field. We show that the most efficient two-wave coupling is achieved when both the grating vector and the external electric field are parallel to the 〈1̅11〉 axis in the (110) crystal cut. However, beam coupling is asymmetric in this geometry: The enhancement coefficient may differ by orders of magnitude whether interacting beams propagate in the 〈110〉 direction or backwards. Because of this asymmetry, the far-field fanning patterns of photorefractive Bi12TiO20 crystals strongly depend on the direction of the pump-beam propagation. A theoretical model is able to explain main features of the observed fanning patterns if the piezoelectric and photoelastic effects are incorporated into the system of vectorial coupled-wave equations.

Shallow-trap-induced positive absorptive two-beam coupling ‘‘gain’’ and light-induced transparency in nominally undoped barium titanate

We use the asymmetry of beam coupling with respect to the orientation of the polar axis in a nominally undoped barium titanate crystal to determine the electro-optic and absorptive ‘‘gain’’ in the usual beam-coupling geometry. For small grating wave vectors, the electro-optic coupling vanishes but the absorptive coupling remains finite and positive. Positive absorptive coupling at small grating wave vectors is correlated with the light-induced transparency of the crystal described herein. The intensity and grating wave vector dependence of the electro-optic and absorptive coupling, and the light-induced transparency are consistent with a model incorporating deep and shallow levels.