Nonuniform Coupling Research Articles

Entanglement Dynamics of Two Spin Qubits in a Spin Environment with Nonuniform Coupling

We investigate the entanglement dynamics of a two-spin-qubit system coupled to a spin environment with nonuniform coupling through using the time-dependent density-matrix renormalization group method. We show that the entanglement generation and decay depend on the number of environment spins, the coupling strength between the central spin system and the environment, and the initial state of the central spin system.

MAGNETOELECTRIC PONDEROMOTIVE FORCE

The dynamics of a system consisting of a matter continuum with a weak linear magnetoelectric coupling interacting with electromagnetic fields is examined on a local scale in a nonrelativistic limit. A consistent expression for the internal energy of the system is derived. The internal energy density and the continuity equation for the momentum lead to the derivation of ponderomotive forces. A nonuniform magnetoelectric coupling generates a "magnetoelectric" ponderomotive force that could be distinguished from the purely electric or magnetic ponderomotive forces by applying alternating electric and magnetic fields at distinct frequencies.

Non-skew-symmetric classical r-matrices and integrable “[formula omitted]” proton–neutron BCS models

We study integrable cases of the pairing BCS Hamiltonians containing several types of fermions and possessing non-uniform coupling constants. We prove that there exist three classes of such the integrable models associated with “Z2-graded” non-skew-symmetric classical r-matrices with spectral parameters and Lie algebras gl(2m), sp(2m) and so(2m), respectively. The proposed models are higher rank generalizations of the so-called “px+ipy” one-type fermion (m=1) BCS model. In the partial case of two types of fermions (m=2) the obtained models may be interpreted as N=Z, “px+ipy” proton–neutron integrable models. In particular, in the case of sp(4) we obtain the “px+ipy”-analogue of the famous integrable proton–neutron model of Richardson. We find the spectrum of the constructed Hamiltonians in terms of solutions of Bethe-type equations.

Research on the Grinding Mechanism Based on the Cutting Simulation

Grinding of metals is a complex material removal operation. Research on cutting process of a single abrasive grain is the basis of further understanding of grinding mechanism. In this investigation, the simulation and analysis for the non-uniform thermo-mechanical coupling intense stress fields in cutting zones of a single abrasive with negative rake are conducted by means of the FEM techniques. The cutting forces, the cutting temperature distribution and the strain rate in cutting zone are numerically demonstrated. Grinding mechanics are analyzed from microscopic view according to the simulation results. Research results facilitate a better understanding on the mechanics of grinding.

Regional increase in extracellular potassium can be arrhythmogenic due to nonuniform muscle contraction in rat ventricular muscle

In the ischemic myocardium, extracellular potassium ([K(+)](o)) increases to ≥20 mmol/l. To determine how lethal arrhythmias occur during ischemia, we investigated whether the increased spatial pattern of [K(+)](o), i.e., a regional or a global increase, affects the incidence of arrhythmias. Force, sarcomere length, membrane potential, and nonuniform intracellular Ca(2+) ([Ca(2+)](i)) were measured in rat ventricular trabeculae. A "regional" or "global" increase in [K(+)](o) was produced by exposing a restricted region of muscle to a jet of 30 mmol/l KCl or by superfusing trabeculae with a solution containing 30 mmol/l KCl, respectively. The increase in [Ca(2+)](i) (Ca(CW)) during Ca(2+) waves was measured (24°C, 3.0 mmol/l [Ca(2+)](o)). A regional increase in [K(+)](o) caused nonuniform [Ca(2+)](i) and contraction. In the presence of isoproterenol, the regional increase in [K(+)](o) induced sustained arrhythmias in 10 of 14 trabeculae, whereas the global increase did not induce such arrhythmias. During sustained arrhythmias, Ca(2+) surged within the jet-exposed region. In the absence of isoproterenol, the regional increase in [K(+)](o) increased Ca(CW), whereas the global increase decreased it. This increase in Ca(CW) with the regional increase in [K(+)](o) was not suppressed by 100 μmol/l streptomycin, whereas it was suppressed by 1) a combination of 10 μmol/l cilnidipine and 3 μmol/l SEA0400; 2) 20 mmol/l 2,3-butanedione monoxime; and 3) 10 μmol/l blebbistatin. A regional but not a global increase in [K(+)](o) induces sustained arrhythmias, probably due to nonuniform excitation-contraction coupling. The same mechanism may underlie arrhythmias during ischemia.

One-dimensional Kinetic Ising Model with Nonuniform Coupling Constants

A nonuniform extension of the Glauber model on a one-dimensional lattice with boundaries is investigated. Based on detailed balance, reaction rates are proposed for the system. The static behavior of the system is investigated. It is shown that there are cases where the system exhibits a static phase transition, which is a change of behavior of the static profile of the expectation values of the spins near end points.

Graphene p-n junctions with nonuniform Rashba spin-orbit coupling

Linear conductance of graphene-based p-n junctions with Rashba spin-orbit coupling is considered theoretically. A square potential step is used to model the junctions, while the coupling is introduced in terms of the Kane-Mele model (C. L. Kane and E. J. Mele, Phys. Rev. Lett. 95, 226801 (2005)). The main objective is a description of electronic transport in junctions where Rashba parameter is nonuniform. Such a nonuniformity can appear when graphene is asymmetrically covered with atomic layers, or when Rashba coupling is strongly dependent on electric field. It is shown that conductance is significantly modified by the considered nonuniformity, which is most clearly manifested by an anomalous minimum at a certain potential step height.

Spin Currents Induced by Nonuniform Rashba-Type Spin–Orbit Field

We study the spin relaxation torque in nonmagnetic or ferromagnetic metals with nonuniform spin-orbit coupling within the Keldysh Green's function formalism. In non-magnet, the relaxation torque is shown to arise when the spin-orbit coupling is not uniform. In the absence of an external field, the spin current induced by the relaxation torque is proportional to the vector chirality of Rashba-type spin-orbit field (RSOF). In the presence of an external field, on the other hand, spin relaxation torque arises from the coupling of the external field and vector chirality of RSOF. Our result indicates that spin-sink or source effects are controlled by designing RSOF in junctions.

The Kamchatka subduction zone: Seismotectonic regionalization and geodynamics

Seismotectonic regionalization of the Kamchatka subduction zone was carried out by retrospective analysis of the temporal sequence and locations of earthquake occurrence and an examination of relationships between the earthquake hypocenters and morphostructures in the continental slope of eastern Kamchatka. Ten segments separated with earthquake-generating strike-slip faults have been identified in the overthrusting (overhanging) margin of the Sea-of-Okhotsk plate in the zone where the Pacific and the Sea-of-Okhotsk plates interact orthogonally. Two to three earthquake-generating thrust blocks have been identified within these segments. This type of subduction is consistent with the keyboard-block model of L.I. Lobkovskii and B.V. Baranov. We put forward a model involving segmentation and generation of thrust blocks due to nonuniform coupling between the subducted Pacific plate and the overhanging Sea-of-Okhotsk plate. According to this model, both segmentation and the formation of thrust blocks are caused by nonuniform plate coupling due to unevenness in the relief of the plunging plate. The thrusts have relief expression as underwater highs and terraces, which indicate that a tsunami-generating earthquake can occur at this location. The highest rate of occurrence for magnitude 7 or greater earthquakes is found at the sharp bend of the Pacific plate, where the subduction angle is 10°–12° instead of 50°–51°, corresponding to a frontal (tectonic) arc, which can be traced by a positive free-air gravity anomaly and by an isostatic anomaly.

Effect of fenoterol stereochemistry on the β2 adrenergic receptor system: Ligand‐directed chiral recognition

The β(2) adrenergic receptor (β(2)-AR) is a model system for studying the ligand recognition process in G protein-coupled receptors. Fenoterol (FEN) is a β(2)-AR selective agonist that has two centers of chirality and exists as four stereoisomers. Radioligand binding studies determined that stereochemistry greatly influences the binding affinity. Subsequent Van't Hoff analysis shows very different thermodynamics of binding depending on the stereoconfiguration of the molecule. The binding of (S,x')-isomers is almost entirely enthalpy controlled whereas binding of (R,x')-isomers is purely entropy driven. Stereochemistry of FEN molecule also affects the coupling of the receptor to different G proteins. In a rat cardiomyocyte contractility model, (R,R')-FEN was shown to selectively activate G(s) protein signaling while the (S,R')-isomer activated both G(i) and G(s) protein. The overall data demonstrate that the chirality at the two chiral centers of the FEN molecule influences the magnitude of binding affinity, thermodynamics of local interactions within the binding site, and the global mechanism of β(2)-AR activation. Differences in thermodynamic parameters and nonuniform G-protein coupling suggest a mechanism of chiral recognition in which observed enantioselectivities arise from the interaction of the (R,x')-FEN stereoisomers with a different receptor conformation than the one with which the (S,x')-isomer interacts.

Dynamical phase transition in one-dimensional kinetic Ising model with nonuniformcoupling constants

An extension of the kinetic Ising model with nonuniform coupling constants on aone-dimensional lattice with boundaries is investigated, and the relaxation of such a systemtowards its equilibrium is studied. Using a transfer matrix method, it is shown that thereare cases where the system exhibits a dynamical phase transition. There may be twophases, the fast phase and the slow phase. For some region of the parameterspace, the relaxation time is independent of the reaction rates at the boundaries.Changing continuously the reaction rates at the boundaries, however, there is a pointwhere the relaxation time begins changing as a continuous (nonconstant) functionof the reaction rates at the boundaries, so that at this point there is a jump inthe derivative of the relaxation time with respect to the reaction rates at theboundaries.

Nonuniform state-space reconstruction and coupling detection

We investigate the state space reconstruction from multiple time series derived from continuous and discrete systems and propose a method for building embedding vectors progressively using information measure criteria regarding past, current, and future states. The embedding scheme can be adapted for different purposes, such as mixed modeling, cross-prediction and Granger causality. In particular, we apply this method in order to detect and evaluate information transfer in coupled systems. As a practical application, we investigate in records of scalp epileptic EEG the information flow across brain areas.

Achieving Highly Localized Effective Magnetic Fields With Non-Uniform Rashba Spin-Orbit Coupling for Tunable Spin Current in Metal/Semiconductor/Metal Structures

We theoretically study the spin-dependent transport of conduction electrons across typical metal/semiconductor (SC)/metal structures, where the SC channel exhibits Rashba spin-orbit coupling (SOC) and the metal contacts do not. The spatial discontinuity of the Rashba SOC is shown to result in highly localized, effective magnetic field barriers at the device interfaces. As a result, electrons with oppositely polarized spins along the injection direction are found to be transmitted with different probabilities, resulting in a finite spin polarization. The value of the spin polarization depends sensitively on the Rashba SOC strength within the SC channel, which is well known to be adjustable by an applied gate bias. Thus the proposed structure could be useful as a tunable source of spin-polarized current in spintronic applications.

Integrable modifications of Dicke and Jaynes–Cummings models, Bose–Hubbard dimers and classical r-matrices

We consider quantum integrable systems associated with non-skew-symmetric sl(2)-valued classical r-matrices. For a special class of such r-matrices we construct a one-parametric family of integrable modifications of the ‘two-level one-mode’ Jaynes–Cummings–Dicke Hamiltonians, with non-uniform coupling constants, containing additional Kerr- and Stark-type nonlinearities. We also construct a family of integrable Bose–Hubbard-type dimers and a family of integrable models that unifies Bose–Hubbard- and Jaynes–Cummings–Dicke-type models and may be called a ‘two-level, two-mode’ Jaynes–Cummings–Dicke model or a ‘spin generalization’ of a Bose–Hubbard dimer. We diagonalize the constructed models with the help of the algebraic Bethe ansatz technique in any irreducible representation of the sl(2)⊕N spin algebra.

Universal characteristics of nonuniform quasi–two-dimensional cold atom systems

We study the nonuniform potential effect in a quasi--two-dimensional (2D) Bose gas system at a finite temperature by numerically solving the projected Gross-Pitaevskii equation. The gradual emergence of superfluidity is confirmed by calculating the moment of inertia as the temperature decreases. Reflecting the nonuniformity of the system, the long-distance decay of a phase-correlation function is characterized by a local effective exponent, $\ensuremath{\eta}(r)$, which depends on the local superfluid density. This ``local-density approximation'' is endorsed by the 2D Gaussian field theory with the nonuniform coupling constant. Indeed, the effective exponent turns out to be close to $1/4$ at the boundary of the phase-coherent region.

Vertical coupling in multiple stacks quantum-dot semiconductor optical amplifiers

The characteristics of vertically coupled multiple stacks quantum-dot (QD) semiconductor optical amplifiers (SOAs) are studied taking into account the effect of carrier coupling between adjacent stacks. We find that QD-SOAs that consist of a few numbers of stacks provide higher optical gain at lower applied current. Our analysis shows that the optical gain of the amplifier can be considerably enhanced due to short tunnelling lifetimes between adjacent stacks. Also, our analysis reveals that short coupling lifetimes can significantly reduce the transparency current and improve the uniformity of the active region since the net coupling rate is a linear function of the injection rate. Long coupling lifetime, on the other hand, produces non-linear and non-uniform coupling rates, which reduce the optical gain and increase the transparency current of the amplifier. We also find that the degradation in the unsaturated optical gain due to long coupling lifetimes is more severe in p-type doped QD-SOA compared with un-doped QD-SOA.

Development of ultra wideband, high efficiency, distributed power amplifiers using discrete GaN HEMTs

This study describes the design and performance of a discrete ultra wideband GaN HEMT distributed power amplifier (DPA) with over 5 W (37 dBm) output power and a PAE exceeding 27% in the 0.02–3 GHz frequency range. The implemented DPA design is comprised of three discrete GaN HEMT devices. Its performance was enhanced using tapered drain lines and non-uniform gate capacitive coupling. The design methodology is based on both small and large signal analysis using harmonic balance technique, and their associated predicted and experimental results are discussed here in detail.

Sarcomere Mechanics in Uniform and Nonuniform Cardiac Muscle

Starling's law and the end-systolic pressure-volume relationship (ESPVR) reflect the effect of sarcomere length (SL) on the development of stress (sigma) and shortening by myocytes in the uniform ventricle. We show here that tetanic contractions of rat cardiac trabeculae exhibit a sigma-SL relationship at saturating [Ca2+] that depends on sarcomere geometry in a manner similar to that of skeletal sarcomeres and the existence of opposing forces in cardiac muscle shortened below slack length. The sigma-SL -[Ca2+](free) relationships (sigma-SL-Ca relationships) at submaximal [Ca2+] in intact and skinned trabeculae were similar, although the sensitivity for Ca2+ of intact muscle was higher. We analyzed the mechanisms underlying the sigma-SL-Ca relationship by using a kinetic model assuming that the rates of Tn-C Ca2+ binding and/or cross-bridge (XB) cycling are determined by either the SL, [Ca2+], or sigma. We analyzed the correlation between the model results and steady-state sigma measurements at varied SL at [Ca2+] from skinned rat cardiac trabeculae to test the hypotheses that the dominant feedback mechanism is SL-, sigma-, or [Ca2+]-dependent, and that the feedback mechanism regulates Tn-C Ca2+ affinity, XB kinetics, or the unitary XB force. The analysis strongly suggests that the feedback of the number of strong XBs to cardiac Tn-C Ca2+ affinity is the dominant mechanism regulating XB recruitment. Using this concept in a model of twitch-sigma accurately reproduced the sigma-SL-Ca relationship and the time courses of twitch sigma and the intracellular [Ca2+]i. The foregoing concept has equally important repercussions for the nonuniformly contracting heart, in which arrhythmogenic Ca2+ waves arise from weakened areas in the cardiac muscle. These Ca2+ waves can reversibly be induced with nonuniform excitation-contraction coupling (ECC) by the cycle of stretch and release in the border zone between the damaged and intact regions. Stimulus trains induced propagating Ca2+ waves and reversibly induced arrhythmias. We hypothesize that rapid force loss by the sarcomeres in the border zone during relaxation causes Ca2+ release from Tn-C and initiates Ca2+ waves propagated by the sarcoplasmic reticulum (SR). Modeling of the response of the cardiac twitch to rapid force changes using the feedback concept uniquely predicts the occurrence of [Ca2+]i transients as a result of accelerated Ca2+ dissociation from Tn-C. These results are consistent with the hypothesis that a force feedback to Ca2+ binding by Tn-C is responsible for Starling's law and the ESPVR in the uniform myocardium and leads to a surge of Ca2+ released by the myofilaments during relaxation in the nonuniform myocardium, which initiates arrhythmogenic propagating Ca2+ release by the SR.

Abstract 1351: Ca 2+ Dissociated from Myofilaments Enhances Delayed Afterdepolarizations by Triggering Ca 2+ Waves in Rat Cardiac Muscle

Background: Non-uniform excitation-contraction coupling (ECC) causes arrhythmias by triggering Ca 2+ waves from the border zone (BZ) between contracting and stretched regions. To elucidate the triggering mechanism of Ca 2+ waves under the condition of non-uniform ECC, we examined the role of Ca 2+ dissociated from myofilaments in delayed afterdepolarizations (DADs) and Ca 2+ waves. Methods: Force was measured in 25 rat trabeculae with a strain gauge, sarcomere length with a laser diffraction technique, membrane potential with a ultracompliant microelectrode, and [Ca 2+ ] i with microinjected fura-2 and a CCD camera. To produce non-uniform ECC, a restricted region (~300 μm length) of muscle was exposed to a jet of Hepes solution containing 20 mM 2,3-butanedione monoxime (BDM), causing sarcomere stretch inside the region during twitches. DADs were induced by 7.5 s-2 Hz stimulus trains in the presence or absence of the BDM jet. Streptomycin (100 μM) was then added to eliminate the effect of stretch-activated channels. To modulate myofilament-Ca 2+ affinity inside the BDM jet region, muscle length was varied transiently from 2.0 to 2.1 or 2.2 μm during a twitch by the last stimulus of the trains (24 °C, [Ca 2+ ] o =2.0 mM). Results: The regional exposure of muscle to the BDM jet reduced the twitch force by 41 %, but enhanced DADs (n=6, P<0.05) and aftercontractions (n=6, P<0.05) and occasionally induced action potentials. Streptomycin did not suppress the significant enhancement of DADs and aftercontractions caused by the BDM jet (n=6). Stretch of sarcomere length for 210 ms increased the velocity of Ca 2+ waves and the amplitude of aftercontractions in proportion to the increase in force during the stretch (n=12, r=0.90, P<0.01). In the presence of 1 mM caffeine, quick return of muscle length after the stretch for 400 ms induced a small and quick increase in [Ca 2+ ] i at BZ, whose amplitude was correlated with the change in force during the stretch (n=15, r=0.76, P<0.01). Conclusions: These results suggest that Ca 2+ dissociated from myofilaments due to quick release of active sarcomere enhances DADs by triggering Ca 2+ -induced Ca 2+ release in the BZ and thereby induces triggered arrhythmias. Thus, non-uniform ECC can be arrhythmogenic by the dissociated Ca 2+ in the BZ.

Micromagnetic Investigation of Precession Dynamics in Magnetic Nanopillars

This paper interprets and reproduces, by means of full micromagnetic simulations, the pioneering experimental data on magnetization dynamics driven by spin polarized current of the experiment by Kiselev The effect of the spatial dependence of the polarization function together with either nonuniform magnetostatic coupling from the fixed layer and classical Ampere field are shown to play a fundamental role in the magnetization dynamics. A detailed study of the stable magnetization self-oscillations shows that for high field and high current regimes, the dynamics is localized in the sides of the structure, where the energy dissipated by damping and the energy provided by the spin flow compensate exactly.