The effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system

We investigate the effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system by applying the concept of negativity. It is found that the nonlinear couplings favor the thermal entanglement creating. Only when the nonlinear couplings ∣K∣ are larger than a certain critical value does the entanglement exist. The dependence of the thermal entanglement in this system on the magnetic field and temperature is also presented. The critical magnetic field increases with the increasing nonlinear couplings constant ∣K∣. And for a fixed nonlinear couplings constant, the critical temperature is independent of the magnetic field B.     

Multi-pomeron couplings to external particles in reggeon field theory

We show that, contrary to previous assertions, multi-pomeron couplings to the external particles are equally as important as single couplings in the asymptotic behaviour of the amplitude in the reggeon calculus. Our arguments require a careful discussion of the relation of the reggeon calculus to a renormalizable field theory and of the renormalization of composite operators in reggeon field theory.     

A simulation algorithm based on Bloch equations and product operator matrix: application to dipolar and scalar couplings

A product operator matrix is proposed to describe scalar couplings in liquid NMR. Combination of the product operator matrix and non-linear Bloch equations is employed to describe effects of chemical shift, translational diffusion, dipolar field, radiation damping, and relaxation in multiple spin systems with both scalar and dipolar couplings. A new simulation algorithm based on this approach is used to simulate NMR signals from dipolar field effects in the presence of scalar couplings. Several typical coupled spin systems with both intra-molecular scalar couplings and inter-molecular dipolar couplings are simulated. Monte Carlo methods are incorporated into simulations as well to analyze diffusion process in these complicated spin systems. The simulated results of diffusion and relaxation parameters and 2D NMR spectra are coincident with the experimental measurements, and agree with theoretical predictions as well. The simulation algorithm presented herein therefore provides a convenient means for designing pulse sequences and quantifying experimental results in complex coupled spin systems.     

Effects of inhomogeneous couplings between atoms and acavity field on entanglement dynamics

Based on the concept of concurrence, we have investigated the entanglement dynamics of two two-level atoms coupled to a single-mode cavity field with inhomogeneous couplings. We find that, for some initial states, the inhomogeneous couplings not only induce but also enhance the entanglement in the process of its evolution. In addition, considering the intrinsic decoherence proposed by Milburn, we also find that a proper value of inhomogeneous couplings can enhance the stationary entanglement, and as a result, the destructive effect of intrinsic decoherence on entanglement can be moderated by the inhomogeneous couplings.     

超短激光脉冲中时空耦合效应的研究进展

概述了超短激光脉冲中时空耦合效应的研究进展。将时空耦合效应分为一阶时空耦合和高阶时空耦合两大类,并详细介绍了两类时空耦合效应的研究历史和现状。其中一阶时空耦合效应相对较为简单,对其的理论及实验研究也较为充分。相比一阶时空耦合效应,高阶时空耦合效应的起源更广泛、种类更丰富、时空结构更复杂。目前,对高阶时空耦合效应的研究也刚处于起步阶段。最后,展望了该领域的研究趋势。     

Universal scaling of wave propagation failure in arrays of coupled nonlinear cells

We study the onset of the propagation failure of wave fronts in systems of coupled cells. We introduce a new method to analyze the scaling of the critical external field at which fronts cease to propagate, as a function of intercellular coupling. We find the universal scaling of the field throughout the range of couplings and show that the field becomes exponentially small for large couplings. Our method is generic and applicable to a wide class of cellular dynamics in chemical, biological, and engineering systems. We confirm our results by direct numerical simulations.     

Note on the stability in affine Toda field theories

We show that certain vanishing couplings in the Â Ê affine Toda field theories remain vanishing even after higher order corrections are included. This is a requisite property for the lagrangian formulation of the theory. We develop a new perturbative formulation and treat affine Toda field theories as a massless theory with exponential interaction terms. We show that the Dynkin automorphism of the Lie algebra leads to the non-renormalization of the vanishing couplings.     

Anomalies in Ising metamagnets

Ising metamagnets in a field with weak ferromagnetic intralayer interactions and highly coordinated antiferromagnetic interlayer couplings are studied using mean field theory and Monte Carlo simulations. In the antiferromagnetic phase anomalies in the magnetization and the specific heat are observed, reflecting the competing ordering tendencies of the external field and the interlayer couplings. Results are compared to recent experimental findings on FeBr₂.     

Doubly coupled matter fields in massive bigravity

In the context of massive(bi-)gravity,non-minimal matter couplings have been proposed.These couplings are special in the sense that they are free of the Boulware-Deser ghost below the strong coupling scale and can be used consistently as an effective field theory.Furthermore,they enrich the phenomenology of massive gravity.We consider these couplings in the framework of bimetric gravity and study the cosmological implications for background and linear tensor,vector,and scalar Previous works have investigated special branches of solutions.Here we perform a complete perturbation analysis for the general background equations of motion,completing previous analyses.     

Splitting of satellites in a time-dependent Kondo model

A time-dependent Kondo model, where both the voltage and the Kondo couplings oscillate on time, is considered. The bosonization technique at the Toulouse limit is applied to study the conductance and the magnetic susceptibility. It is shown that in addition to the satellites of the Kondo peak that appear in the conductance and the susceptibility as a function of the magnetic field or dc voltage, when the voltage oscillates, these satellites further split when the Kondo couplings also oscillate on time. The distance of the satellite splitting solely depends on the ratio between the oscillation frequencies of the voltage and of the Kondo couplings. When the Kondo couplings oscillate more rapidly than the voltage, the distance of the satellite peaks can be smaller than the voltage oscillation frequency.     

尖晶石氧化物CoCr2O4的多铁性研究

为深入理解化合物CoCr₂O₄奇特的多铁性行为,结合尖晶石晶格结构特点,考虑近邻A-A、A-B、B-B及B位离子间次近邻交换耦合的影响,构建经典海森堡自旋模型对磁致铁电行为进行描述,并采用蒙特卡罗模拟对模型进行求解.重点考察不同磁交换耦合作用下,外磁场对体系磁化行为和电极化行为的调控.结果表明：子晶格B1离子贡献了体系的宏观磁化强度和电极化强度;近邻A-A和B位次近邻交换耦合参数的改变对子晶格A位磁性离子的磁电行为没有影响,但对B位离子的磁化强度和电极化强度有显著的调制作用,尤其是来源于子晶格B1的电极化强度对B位次近邻交换耦合参数的改变极为敏感;这些结果反映了立方尖晶石磁结构中A位和B位磁性离子环境及分布对称性的差异.     

不同耦合下混沌神经元网络的同步

采用Hindmarsh-Rose神经元动力学模型, 对二维点阵上的神经元网络的同步进行了研究. 为了解不同耦合对网络同步的影响, 提出了一般反馈耦合、分层反馈耦合和分层局域平均场反馈耦合三种方案.研究表明:在耦合强度较小的近邻耦合下, 一般反馈耦合不能使网络达到完全同步, 而分层反馈耦合和分层局域平均场反馈耦合可以使网络出现局部同步和全局同步. 不同形式的耦合会导致网络出现不同的斑图, 随着耦合强度的增大, 网络从不同步到同步的过程也不相同, 一般反馈耦合和分层反馈耦合网络是突然出现全局同步, 同步之前网络出现非周期性的相干斑图; 对于分层局域平均场反馈耦合网络, 同层神经元之间先出现从簇放电同步到同步的转变, 形成靶波, 然后同步区由中心向外逐渐扩大, 最终达到网络的全局同步. 这些结果表明, 只有适当的耦合才能实现信号的无损耗的传递. 此外我们发现分层局域平均场反馈耦合可以促进网络的同步.     

Enhancement of next-nearest-neighbouring entanglement in quantum Ising spin chain

Using the method of the Jordan--Wigner transformation for solving different spin--spin correlation functions, we have investigated the generation of next-nearest-neighbouring entanglement in a one-dimensional quantum Ising spin chain with the Gaussian distribution impurities of exchange couplings and external magnetic fields taken into account. The maximal value of entanglement between the next-nearest-neighbouring qubits in the transverse Ising model was analysed in detail by varying the effectively controlled parameters such as interchange coupling, magnetic field and the system impurity. For such systems, where both exchange couplings and external magnetic field disorder appear, we show that it is possible to achieve next-nearest-neighbouring entanglement better than the previously discussed pure Ising spin chain case. We also show that the Gaussian distribution impurity can induce next-nearest-neighbouring entanglement, which can be used as a means to characterize quantum phase transition.     

Direct couplings of mimetic dark matter and their cosmological effects

The original mimetic model was proposed to take the role of dark matter. In this paper we consider possible direct interactions of mimetic dark matter with other matter in the universe, especially standard model particles such as baryons and photons. By imposing shift symmetry, the mimetic dark matter field can only have derivative couplings. We discuss the possibilities of generating baryon number asymmetry and cosmic birefringence in the universe based on the derivative couplings of mimetic dark matter to baryons and photons.     

Tractable approximations for probabilistic models: the adaptive Thouless-Anderson-Palmer mean field approach

We develop an advanced mean field method for approximating averages in probabilistic data models that is based on the Thouless-Anderson-Palmer (TAP) approach of disorder physics. In contrast to conventional TAP, where the knowledge of the distribution of couplings between the random variables is required, our method adapts to the concrete couplings. We demonstrate the validity of our approach, which is so far restricted to models with nonglassy behavior, by replica calculations for a wide class of models as well as by simulations for a real data set.     

非均匀磁场和杂质磁场对自旋1系统量子关联的影响

通过负值度和测量诱导的扰动, 研究了非均匀磁场和杂质磁场对自旋为1的Heisenberg系统量子关联的影响. 研究发现非均匀磁场的增加会降低纠缠, 但也可用来产生纠缠, 并且会提高临界非线性作用K_c的值, 测量诱导的扰动的临界磁场要高于负值度的临界磁场, 而且测量诱导的扰动不会随着非线性作用|K| 的减小而消失, 它能全面反映量子关联的存在. 研究还发现, 不同杂质磁场对测量诱导的扰动的影响彼此间无交叉. 杂质磁场下, 相互作用|J| 必须小于非线性作用|K| 才会有纠缠存在, 但是测量诱导的扰动却可以在相互作用|J| 大于非线性作用|K| 时依然存在, |J| 与|K| 相同时只是测量诱导的扰动的最小取值点. 此外, 系统粒子数目对量子关联也具有重要影响.     

Entanglement of Non-symmetric Two Atomic Qubits Induced by a Coherent State Field

The entanglement of two atomic qubits, which are coupled to a coherent state field with different couplings, is studied. The dynamical evolution of the concurrence, which measures the degree of the entanglement between the two qubits, is plotted versus the scaled time gt. It is found that the two qubits can be entangled by the coherent state field even when they are initially prepared in the most mixed state, and for very weak field, the most mixed state can be more easily entangled than some pure states. It is also found that the entanglement between the qubits sensitively depends on the relative difference of the atomic couplings and the mean photon number of the field.     

Monte Carlo study of the magnetic properties and magnetocaloric effect of an AFM/FM BiFeO_3/Co bilayer

The magnetic properties and magnetocaloric effect of an antiferromagnetic/ferromagnetic(AFM/FM)BiFeO_3/Co bilayer with mixed-spin(5/2,3/2) have been studied based on Monte Carlo simulation.The magnetization, susceptibility, and critical temperature are investigated under various exchange couplings and an external magnetic field. In particular, the influence of exchange couplings and an external magnetic field on the magnetic entropy change, adiabatic temperature change, and the relative cooling power(RCP) are studied. The simulation results indicated that the decrease of the exchange coupling and the increase of external magnetic fields can cause an increase of magnetic entropy change, adiabatic temperature change, and RCP. In addition, the hysteresis loops of the system are presented for different exchange couplings and temperatures.     

Critical behaviour of the local magnetic susceptibility in a ferromagnetic film

The nearest-neighbour Ising model of a ferromagnetic film in which couplings between surface spins may differ from couplings between remaining spins is considered. Using the mean-field approximation, the local magnetic susceptibility defined as the derivative of the local magnetization with respect to the external uniform magnetic field is obtained. The behaviour of the local magnetic susceptibility near the ordinary, surface-bulk and surface phase transitions and in a range of temperatures where physical quantities have pseudocritical behaviour is discussed. The critical behaviour of the local magnetic susceptibility in a three-dimensional semi-infinite model is also given for comparison.     

Questioning the equivalence principle

The Equivalence Principle (EP) is not one of the ‘universal’ principles of physics (like the action principle). It is a heuristic hypothesis which was introduced by Einstein in 1907, and used by him to construct his theory of general relativity. In modern language, the (Einsteinian) EP consists in assuming that the only long-range field with gravitational-strength couplings to matter is a massless spin-2 field. Modern unification theories, and notably string theory, suggest the existence of new fields (in particular, scalar fields: ‘dilaton’ and ‘moduli’) with gravitational-strength couplings. In most cases the couplings of these new fields ‘violate’ the EP. If the field is long-ranged, these EP violations lead to many observable consequences (variation of ‘constants’, non-universality of free fall, relative drift of atomic clocks, etc.). The best experimental probe of a possible violation of the EP is to compare the free-fall acceleration of different materials.